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draft-ietf-pcn-architecture-10.txt --> draft-ietf-pcn-architecture-11.txt

Congestion and Pre-Congestion Philip. Eardley (Editor)
Notification Working Group BT
Internet-Draft March 16, April 7, 2009
Intended status: Informational
Expires: September 17, October 9, 2009

Pre-Congestion Notification (PCN) Architecture
draft-ietf-pcn-architecture-10
draft-ietf-pcn-architecture-11

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Abstract

This document describes a general architecture for flow admission and
termination based on pre-congestion information in order to protect
the quality of service of established inelastic flows within a single
DiffServ
Diffserv domain.

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Status

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 4
1.1. Applicability Overview of PCN . . . . . . . . . . . . . . . . . . 5 . . . 4
1.2. Example use case for PCN . . . . . . . . . . . . . . . . 6 4
1.3. Applicability of PCN . . . . . . . . . . . . . . . . . . 8
1.4. Documents about PCN . . . . . . . . . . . . . . . . . . . 9
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. High-level functional architecture . . . . . . . . . . . . . . 12
3.1. Flow admission . . . . . . . . . . . . . . . . . . . . . 14
3.2. Flow termination . . . . . . . . . . . . . . . . . . . . 15
3.3. Flow admission and/or flow termination when there are
only two PCN encoding states . . . . . . . . . . . . . . 16
3.4. Information transport . . . . . . . . . . . . . . . . . . 17
3.5. PCN-traffic . . . . . . . . . . . . . . . . . . . . . . . 17
3.6. Backwards compatibility . . . . . . . . . . . . . . . . . 18
4. Detailed Functional architecture . . . . . . . . . . . . . . . 18 19
4.1. PCN-interior-node functions . . . . . . . . . . . . . . . 19 20
4.2. PCN-ingress-node functions . . . . . . . . . . . . . . . 20 21
4.3. PCN-egress-node functions . . . . . . . . . . . . . . . . 20 22
4.4. Admission control functions . . . . . . . . . . . . . . . 21 22
4.5. Flow termination functions . . . . . . . . . . . . . . . 22 23
4.6. Addressing . . . . . . . . . . . . . . . . . . . . . . . 22 24
4.7. Tunnelling . . . . . . . . . . . . . . . . . . . . . . . 23 24
4.8. Fault handling . . . . . . . . . . . . . . . . . . . . . 25 26
5. Operations and Management . . . . . . . . . . . . . . . . . . 25 26
5.1. Configuration Operations and Management . . . . . . . . . 25 27
5.1.1. System options . . . . . . . . . . . . . . . . . . . . 26 27
5.1.2. Parameters . . . . . . . . . . . . . . . . . . . . . . 27 28
5.2. Performance & Provisioning Operations and Management . . 28 30
5.3. Accounting Operations and Management . . . . . . . . . . 30 31
5.4. Fault Operations and Management . . . . . . . . . . . . . 30 31

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5.5. Security Operations and Management . . . . . . . . . . . 31 32
6. IANA Considerations Applicability of PCN . . . . . . . . . . . . . . . . . . . . . 31
7. Security considerations 33
6.1. Benefits . . . . . . . . . . . . . . . . . . . . 32
8. Conclusions . . . . 33
6.2. Deployment scenarios . . . . . . . . . . . . . . . . . . 35
6.3. Assumptions and constraints on scope . . . . 33
9. Acknowledgements . . . . . . 36
6.3.1. Assumption 1: Trust and support of PCN -
controlled environment . . . . . . . . . . . . . . . . 37
6.3.2. Assumption 2: Real-time applications . 33
10. Comments Solicited . . . . . . . . 37
6.3.3. Assumption 3: Many flows and additional load . . . . . 38
6.3.4. Assumption 4: Emergency use out of scope . . . . . . . 38
6.4. Challenges . . 33
11. Changes . . . . . . . . . . . . . . . . . . . . . 39
7. IANA Considerations . . . . . . 34
11.1. Changes from -098 to -10 . . . . . . . . . . . . . . . 41
8. Security considerations . 34
11.2. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 34
11.3. Changes from -07 to -08 . 41
9. Conclusions . . . . . . . . . . . . . . . . 34
11.4. Changes from -06 to -07 . . . . . . . . . 42
10. Acknowledgements . . . . . . . . 35
11.5. Changes from -05 to -06 . . . . . . . . . . . . . . . 42
11. Comments Solicited (to be removed by RFC Editor) . . 35
11.6. Changes from -04 to -05 . . . . . 43
12. Changes (to be removed by RFC Editor) . . . . . . . . . . . . 36
11.7. 43
12.1. Changes from -03 -10 to -04 -11 . . . . . . . . . . . . . . . . . 37
11.8. 43
12.2. Changes from -02 -09 to -03 -10 . . . . . . . . . . . . . . . . . 37
11.9. 44
12.3. Changes from -01 -08 to -02 -09 . . . . . . . . . . . . . . . . . 38
11.10. 44
12.4. Changes from -00 -07 to -01 . . . . . . . . -08 . . . . . . . . . 39

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12. Appendix A: Applicability of PCN . . . . . . . . 44
12.5. Changes from -06 to -07 . . . . . . . 41
12.1. Benefits . . . . . . . . . . 45
12.6. Changes from -05 to -06 . . . . . . . . . . . . . . 41
12.2. Deployment scenarios . . . 45
12.7. Changes from -04 to -05 . . . . . . . . . . . . . . . 42
12.3. Assumptions and constraints on scope . . 46
12.8. Changes from -03 to -04 . . . . . . . . 44
12.3.1. Assumption 1: Trust and support of PCN -
controlled environment . . . . . . . . . 46
12.9. Changes from -02 to -03 . . . . . . . 45
12.3.2. Assumption 2: Real-time applications . . . . . . . . . 45
12.3.3. Assumption 3: Many flows and additional load . 47
12.10. Changes from -01 to -02 . . . . 46
12.3.4. Assumption 4: Emergency use out of scope . . . . . . . 46
12.4. Challenges . . . . . . 48
12.11. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 46 49
13. Appendix B: Possible future work items . . . . . . . . References . . . . 49
13.1. Benefits . . . . . . . . . . . . . . . . . . . . . . 51
13.1. Normative References . . 49
13.2. Probing . . . . . . . . . . . . . . . . 51
13.2. Informative References . . . . . . . . . 52
13.2.1. Introduction . . . . . . . . 51
Appendix A. Possible future work items . . . . . . . . . . . . . 52
13.2.2. 55
A.1. Probing functions . . . . . . . . . . . . . . . . . . 53
13.2.3. Discussion of rationale for probing, its downsides
and open issues . . . . . . . . . . . . . . . . . . . 54
14. References . . . 57
A.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . 57
A.1.2. Probing functions . . 56
14.1. Normative References . . . . . . . . . . . . . . . . 58
A.1.3. Discussion of rationale for probing, its downsides
and open issues . . 56
14.2. Informative References . . . . . . . . . . . . . . . . . 56 58
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 61

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1. Introduction

1.1. Applicability Overview of PCN

The objective of the Pre-Congestion Notification (PCN) mechanisms is to protect the
quality of service (QoS) of inelastic flows within a DiffServ
domain [RFC2475], Diffserv domain,
in a simple, scalable and robust fashion. Two mechanisms are defined to protect the QoS of established inelastic
flows within a single DiffServ domain, where all boundary and
interior nodes are PCN-enabled and are trusted for correct PCN
operation. Flow used:
admission control determines control, to decide whether to admit or block a new flow
should be admitted,
request, and (in abnormal circumstances) flow termination to decide
whether to terminate some of the existing flows. To achieve this,
the overall rate of PCN traffic is metered on every link in order the
domain, and PCN packets are appropriately marked when certain
configured rates are exceeded. These configured rates are below the
rate of the link thus providing notification to protect boundary nodes about
overloads before any congestion occurs (hence "pre-congestion
notification"). The level of marking allows boundary nodes to make
decisions about whether to admit or terminate.

Within a PCN-domain, PCN-traffic is forwarded in a prioritised
Diffserv traffic class. Every link in the QoS PCN-domain is configured
with two rates (PCN-threshold-rate and PCN-excess-rate). If the
overall rate of existing PCN-flows PCN-traffic on a link exceeds a configured rate, then
a PCN-interior-node marks PCN-packets appropriately. The PCN-egress-
nodes use this information to make admission control and flow
termination decisions. Flow admission control determines whether a
new flow can be admitted without any impact, in normal circumstances. circumstances,
on the QoS of existing PCN-flows. However, in abnormal
circumstances, for instance a disaster affecting multiple nodes and
causing traffic re-
routes, re-routes, then the QoS on existing PCN-flows may
degrade even though care was exercised when admitting those flows.
The flow termination mechanism removes enough sufficient traffic in order to
protect the QoS of the remaining PCN-flows.

Compared with alternative QoS mechanisms, PCN has certain advantages All PCN-boundary-nodes
and disadvantages that will make it appropriate in particular
scenarios. For example, compared with hop-by-hop IntServ [RFC1633],
PCN only requires per flow state at the PCN-ingress-nodes. Compared
with the DiffServ architecture [RFC2475], an operator needs to be
less accurate and/or conservative in its prediction of the traffic
matrix. The DiffServ architecture's traffic conditioning agreements PCN-interior-nodes are static PCN-enabled and coarse; they are defined at subscription time, and trusted for correct
PCN operation. PCN-ingress-nodes police arriving packets to check
that they are used to limit the total traffic at each ingress part of the
domain regardless of the egress for the traffic. On the other hand, an admitted PCN-flow that keeps within its
agreed flowspec, and hence they maintain per flow state. PCN-
interior-nodes meter all PCN firstly uses traffic, and hence do not need to
maintain any per flow state. Decisions about flow admission control based on measurements of the
current conditions between the specific and
termination are made for a particular pair of PCN-boundary-nodes, and secondly, in
hence PCN-egress-nodes must be able to identify which PCN-ingress-
node sent each PCN-packet.

1.2. Example use case of a disaster, for PCN protects the

This section outlines an end-to-end QoS of most
flows by terminating a few selected ones.

PCN's admission control is a measurement-based mechanism. Hence it
assumes scenario that uses the present is a reasonable prediction of the future: PCN
mechanisms within one domain. The parts outside the network conditions PCN-domain are measured at the time
out of a new flow
request, scope for PCN, but the actual network performance must are included to help clarify how PCN could
be acceptable during used. Note that the call some time later. Hence PCN section is unsuitable in several
circumstances:

o If the source adapts its bit rate dependent on the level of pre-
congestion, because then the aggregate traffic might become
unstable. The assumption only an example - in this document is that PCN-packets
come from real time applications generating inelastic traffic,
such as the Controlled Load Service, [RFC2211]. particular

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o If a potential bottleneck link has capacity

there are other possibilities (see later) for only a few flows,
because then a new flow can move a link directly from no pre-
congestion to being so overloaded that it has to drop packets.
The assumption in this document is that this isn't a problem.

o If there is the danger of a "flash crowd" in which many admission
requests arrive within the reaction time of PCN's admission
mechanism, because then they all might get admitted and so
overload the network. The assumption in this document is that, if
it is necessary, then flash crowds are limited in some fashion
beyond the scope of this document, for instance by rate limiting
QoS requests.

The applicability of PCN is discussed further in Appendix A.

1.2. Example use case for PCN

This section outlines an end-to-end QoS scenario that uses the PCN
mechanisms within one domain. The parts outside the PCN-domain are
out of scope for PCN, but are included to help clarify how PCN could
be used. Note that the section is only an example - in particular
there are other possibilities (see later) for how the PCN-boundary-
nodes perform admission control and flow termination.

As how the PCN-boundary-
nodes perform admission control and flow termination.

As a fundamental building block, each link of the PCN-domain operates
two PCN-marking behaviours
a [PCN08-2] (Figure 1):

o Threshold marking, meter and marker, which marks all PCN-packets if the PCN
traffic rate is greater than a first configured rate, the PCN-threshold-
rate. PCN-
threshold-rate. The admission control mechanism limits the PCN-traffic PCN-
traffic on each link to *roughly* its PCN-threshold-rate.

o Excess traffic marking, meter and marker, which marks a proportion of PCN-packets, PCN-
packets, such that the amount marked equals the traffic rate in
excess of a second configured rate, the PCN-excess-rate. The flow
termination mechanism limits the PCN-traffic on each link to
*roughly* its PCN-excess-rate.

Overall the aim is to give an "early warning" of potential congestion
before there is any significant build-up of PCN-packets in the queue
on the link; we term this "pre-congestion notification" by analogy
with ECN (Explicit Congestion Notification, [RFC3168]). Note that
the link only meters the bulk PCN-traffic (and not per flow).

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Figure 1: Example of how the PCN admission control and flow
termination mechanisms operate as the rate of PCN-traffic increases.

The two forms of PCN-marking are indicated by setting of the ECN and
DSCP (Differentiated Services Codepoint [RFC2474]) fields to known
values, which are configured for the domain. Thus the PCN-egress-
nodes can monitor the PCN-markings in order to measure the severity
of pre-congestion. In addition, the PCN-ingress-nodes need to set
the ECN and DSCP fields to that configured for an unmarked PCN-
packet, and the PCN-egress-nodes need to revert to values appropriate
outside the PCN-domain.

For admission control, we assume end-to-end RSVP signalling (Resource
Reservation Protocol) [RFC2205]) in this example. The PCN-domain is
a single RSVP hop. The PCN-domain operates DiffServ, Diffserv, and we assume
that PCN-traffic is scheduled with the expedited forwarding (EF) per-
hop behaviour, [RFC3246]. Hence the overall solution is in line with
the "IntServ over DiffServ" Diffserv" framework defined in [RFC2998], as shown
in Figure 2.

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___ ___ _______________________________________ ____ ___
| | | | | PCN- PCN- PCN- | | | | |
| | | | |ingress interior egress| | | | |
| | | | | -node -nodes -node | | | | |
| | | | |-------+ +-------+ +-------+ +------| | | | |
| | | | | PCN- | | PCN- PCN | | PCN | | PCN- | | | | |
| |..| |..|Ingress|..|meter &|..|meter &|..|Egress|..| |..| |
| |..| |..|marking|..|marking|..|marking|..| Meter|..| |..|Policer|..|marker |..|marker |..|Meter |..| |..| |
| | | | |-------+ +-------+ +-------+ +------| | | | |
| | | | | \ / | | | | |
| | | | | \ / | | | | |
| | | | | \ PCN-feedback-information / | | | | |
| | | | | \ (for admission control) / | | | | |
| | | | | --<-----<----<----<-----<-- | | | | |
| | | | | PCN-feedback-information | | | | |
| | | | | (for flow termination) | | | | |
|___| |___| |_______________________________________| |____| |___|

Sx Access PCN-domain Access Rx
End Network Network End
Host Host
<---- signalling across PCN-domain--->
(for admission control & flow termination)

<-------------------end-to-end QoS signalling protocol--------------->

Figure 2: Example of possible overall QoS architecture

A source wanting to start a new QoS flow sends an RSVP PATH message.
Normal hop-by-hop IntServ [RFC1633] is used outside the PCN-domain
(we assume successfully). The PATH message travels across the PCN-
domain; the PCN-egress-node reads the PHOP object to discover the
specific PCN-ingress-node for this flow. The RESV message travels
back from the receiver, and triggers the PCN-egress-node to check
what fraction of the PCN-traffic, from the relevant PCN-ingress-node,
is currently being threshold-marked. It adds an object with this
information onto the RESV message, and hence the PCN-ingress-node
learns about the level of pre-congestion on the path. If this level
is below some threshold, then the PCN-ingress-node admits the new
flow into the PCN-domain. The RSVP message triggers the PCN-ingress-
node to install two normal IntServ items: five-tuple information, so
that it can subsequently identify data packets that are part of a
previously admitted PCN-flow; and a traffic profile, so that it can
police the flow to within its contract. Similarly, the RSVP message
triggers the PCN-egress-node to install five-tuple and PHOP
information, so that it can identify packets as part of a flow from a
specific PCN-ingress-node.

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The flow termination mechanism may happen when some abnormal
circumstances causes a link to become so pre-congested that it
excess-traffic-marks (and perhaps also drops) PCN-packets. In this
example, when a PCN-egress-node observes such a packet it then, with
some probability, terminates this PCN-flow; the probability is
configured low enough to avoid over-termination and high enough to
ensure rapid termination of enough flows. It also informs the
relevant PCN-ingress-node, so it can block any further traffic on the
terminated flow.

1.3. Documents about PCN

The purpose Applicability of this document is to describe a general architecture
for flow admission PCN

Compared with alternative QoS mechanisms, PCN has certain advantages
and termination based on (pre-) congestion
information disadvantages that will make it appropriate in order to protect the quality of service of flows
within a DiffServ domain. This document describes the particular
scenarios. For example, compared with hop-by-hop IntServ [RFC1633],
PCN
architecture only requires per flow state at a high level (Section 3) and in more detail (Section
4). It also defines some terminology, and considerations about
operations and management, and security. Appendix A considers the
applicability of PCN PCN-ingress-nodes. Compared
with the Diffserv architecture [RFC2475], an operator needs to be
less accurate and/or conservative in more detail, covering its benefits,
deployment scenarios, assumptions and potential challenges. Appendix
B covers some potential future work items.

Aspects prediction of PCN the traffic
matrix. The Diffserv architecture's traffic conditioning agreements
are also documented elsewhere:

o Marking behaviour: threshold marking static and excess traffic marking coarse; they are standardised in [PCN08-2].

o Encoding: the "baseline" encoding is described in [PCN08-1], which
standardises two PCN encoding states (PCN-marked defined at subscription time, and not PCN-
marked), whilst (experimental) extensions
they are used (for instance) to limit the baseline encoding
can provide three encoding states (threshold-marked, excess-
traffic-marked, not PCN-marked, or perhaps further encoding states
as suggested in [Westberg08]). Section 3.6 considers backwards
compatability total traffic at each
ingress of PCN encoding with ECN.

o PCN-boundary-node behaviour: how the PCN-boundary-nodes convert domain regardless of the PCN-markings into decisions about flow egress for the traffic. On
the other hand, PCN firstly uses admission and flow
termination. The concept is that control based on
measurements of the standardised marking
behaviours allow several possible PCN-boundary-node behaviours,
which are described in Informational documents. A number current conditions between the specific pair of
possibilities are outlined in this document; detailed descriptions
PCN-boundary-nodes, and comparisons are secondly, in [Charny07-1] and [Menth08-3].

o Signalling between PCN-boundary-nodes: Signalling case of a disaster, PCN protects
the QoS of most flows by terminating a few selected ones.

PCN's admission control is needed to
transport PCN-feedback-information between a measurement-based mechanism. Hence it
assumes that the PCN-boundary-nodes
(in present is a reasonable prediction of the example above, this future:
the network conditions are measured at the time of a new flow
request, but the actual network performance must be acceptable during
the call some time later. Hence PCN is unsuitable in several
circumstances:

o If the fraction source adapts its bit rate dependent on the level of pre-
congestion, because then the aggregate traffic might become
unstable. The assumption in this document is that PCN-packets
come from real time applications generating inelastic traffic, between
such as the pair of PCN-boundary-nodes, Controlled Load Service, [RFC2211].

o If a potential bottleneck link has capacity for only a few flows,
because then a new flow can move a link directly from no pre-
congestion to being so overloaded that is PCN-marked). it has to drop packets.
The exact assumption in this document is that this isn't a problem.

o If there is the danger of a "flash crowd" in which many admission
requests arrive within the reaction time of PCN's admission
mechanism, because then they all might get admitted and so

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details vary for different PCN-boundary-node behaviours, and so
should be described in those documents. It may require an
extension to

overload the signalling protocol - standardisation network. The assumption in this document is out of that, if
it is necessary, then flash crowds are limited in some fashion
beyond the scope of the PCN WG.

o The interface this document, for instance by which the PCN-boundary-nodes learn identification
information rate limiting
QoS requests.

The applicability of PCN is discussed further in Section 6.

1.4. Documents about the admitted flows: the exact requirements vary
for different PCN-boundary-node behaviours and PCN

The purpose of this document is to describe a general architecture
for different
signalling protocols, flow admission and so should be described termination based on (pre-) congestion
information in those
documents. They will be similar order to those described in protect the example
above - a PCN-ingress-node needs to be able to identify that a
packet is part quality of service of flows
within a previously admitted flow (typically from its
five-tuple) and each PCN-boundary-node needs to be able to
identify the other PCN-boundary-node for Diffserv domain. This document describes the flow.

2. Terminology

o PCN-domain: a PCN-capable domain; PCN
architecture at a contiguous set of PCN-enabled
nodes that perform DiffServ scheduling [RFC2474]; the complete set
of PCN-nodes whose PCN-marking can high level (Section 3) and in principle influence
decisions more detail (Section
4). It also defines some terminology, and considerations about flow admission
operations and termination for the PCN-domain,
including management, and security. Section 6 considers the PCN-egress-nodes, which measure these PCN-marks.

o PCN-boundary-node: a PCN-node that connects one PCN-domain to a
node either
applicability of PCN in another PCN-domain or in a non PCN-domain. more detail, covering its benefits,
deployment scenarios, assumptions and potential challenges. The
Appendix covers some potential future work items.

Aspects of PCN are also documented elsewhere:

o PCN-interior-node: a node in a PCN-domain that Metering and marking: [PCN08-2] standardises threshold metering
and marking, and excess traffic metering and marking. A PCN-
packet may be marked, depending on the metering results.

o Encoding: the "baseline" encoding is described in [PCN08-1], which
standardises two PCN encoding states (PCN-marked and not a PCN-
boundary-node.

o PCN-node: a PCN-boundary-node
marked), whilst (experimental) extensions to the baseline encoding
can provide three encoding states (threshold-marked, excess-
traffic-marked, not PCN-marked, or a PCN-interior-node

o PCN-egress-node: a PCN-boundary-node in its role in handling
traffic perhaps further encoding states
as it leaves a PCN-domain.

o PCN-ingress-node: a PCN-boundary-node in its role suggested in handling
traffic as it enters a PCN-domain.

o PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic [Westberg08]). Section 3.6 considers backwards
compatability of
different DiffServ behaviour aggregates (BAs) [RFC2474]. The
PCN-BA uses the PCN mechanisms to carry PCN-traffic and encoding with ECN.

o PCN-boundary-node behaviour: how the
corresponding packets are PCN-packets. The same network will
carry traffic of other DiffServ BAs. PCN-boundary-nodes convert
the PCN-markings into decisions about flow admission and flow
termination, as described in Informational documents. The PCN-BA concept
is distinguished that the standardised metering and marking by a combination PCN-nodes allows
several possible PCN-boundary-node behaviours. A number of the DiffServ codepoint (DSCP)
possibilities are outlined in this document; detailed descriptions
and ECN fields. comparisons are in [Charny07-1] and [Menth08-3].

o PCN-flow: Signalling between PCN-boundary-nodes: Signalling is needed to
transport PCN-feedback-information between the unit of PCN-traffic that PCN-boundary-nodes
(in the PCN-boundary-node
admits (or terminates); example above, this is the unit could fraction of traffic, between
the pair of PCN-boundary-nodes, that is PCN-marked). The exact
details vary for different PCN-boundary-node behaviours, and so
should be a single microflow (as
defined described in [RFC2474]) or some identifiable collection of those documents. It may require an

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microflows.

o Ingress-egress-aggregate: The collection

extension to the signalling protocol - standardisation is out of PCN-packets from all
PCN-flows that travel in one direction between a specific pair
scope of
PCN-boundary-nodes.

o PCN-threshold-rate: a reference rate configured for each link in the PCN-domain, PCN WG.

o The interface by which is lower than the PCN-excess-rate. It is
used by a marking behaviour that determines whether a packet PCN-boundary-nodes learn identification
information about the admitted flows: the exact requirements vary
for different PCN-boundary-node behaviours and for different
signalling protocols, and so should be PCN-marked with a first encoding, "threshold-marked".

o PCN-excess-rate: a reference rate configured for each link described in those
documents. They will be similar to those described in the
PCN-domain, which is higher than the PCN-threshold-rate. It is
used by example
above - a marking behaviour PCN-ingress-node needs to be able to identify that determines whether a
packet
should be PCN-marked with a second encoding, "excess-traffic-
marked".

o Threshold-marking: a PCN-marking behaviour with the objective that
all PCN-traffic is marked if part of a previously admitted flow (typically from its
five-tuple) and each PCN-boundary-node needs to be able to
identify the PCN-traffic exceeds other PCN-boundary-node for the PCN-
threshold-rate. flow.

2. Terminology

o Excess-traffic-marking: PCN-domain: a PCN-marking behaviour with the objective
that the amount PCN-capable domain; a contiguous set of PCN-traffic PCN-enabled
nodes that perform Diffserv scheduling [RFC2474]; the complete set
of PCN-nodes that in principle can, through PCN-marking packets,
influence decisions about flow admission and termination for the
PCN-domain; the PCN-domain includes the PCN-egress-nodes, which
measure these PCN-marks, and the PCN-ingress-nodes.

o PCN-boundary-node: a PCN-node that connects one PCN-domain to a
node either in another PCN-domain or in a non PCN-domain.

o PCN-interior-node: a node in a PCN-domain that is PCN-marked is equal not a PCN-
boundary-node.

o PCN-node: a PCN-boundary-node or a PCN-interior-node

o PCN-egress-node: a PCN-boundary-node in its role in handling
traffic as it leaves a PCN-domain.

o PCN-ingress-node: a PCN-boundary-node in its role in handling
traffic as it enters a PCN-domain.

o PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of
different Diffserv behaviour aggregates (BAs) [RFC2474]. The
PCN-BA uses the PCN mechanisms to carry PCN-traffic and the
amount
corresponding packets are PCN-packets. The same network will
carry traffic of other Diffserv BAs. The PCN-BA is distinguished
by a combination of the Diffserv codepoint (DSCP) and ECN fields.

o PCN-flow: the unit of PCN-traffic that exceeds the PCN-excess-rate. PCN-boundary-node
admits (or terminates); the unit could be a single microflow (as
defined in [RFC2474]) or some identifiable collection of
microflows.

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o Pre-congestion: a condition of a link within a PCN-domain such
that the PCN-node performs PCN-marking, in order to provide an
"early warning" of potential congestion before there is any
significant build-up of PCN-packets in the real queue. (Hence, by
analogy with ECN we call our mechanism Pre-Congestion
Notification.)

o PCN-marking: the process of setting the header in a PCN-packet
based on defined rules, in reaction to pre-congestion; either
threshold-marking or excess-traffic-marking.

o PCN-colouring: PCN-threshold-rate: a reference rate configured for each link in
the process of setting PCN-domain, which is lower than the header in a PCN-packet
by a PCN-boundary-node; performed PCN-excess-rate. It is
used by a PCN-ingress-node so metering behaviour that
PCN-nodes can easily identify PCN-packets; performed by determines whether a PCN-
egress-node so that the header packet
should be PCN-marked with a first encoding, "threshold-marked".

o Threshold-metering: a metering behaviour that, if the PCN-traffic
exceeds the PCN-threshold-rate, indicates that all PCN-traffic is
to be threshold-marked.

o Threshold-marking: the setting of the header in a PCN-packet to a
specific encoding, based on indications from the threshold-meter.

o PCN-excess-rate: a reference rate configured for each link in the
PCN-domain, which is higher than the PCN-threshold-rate. It is
used by a metering behaviour that determines whether a packet
should be PCN-marked with a second encoding, "excess-traffic-
marked".

o Excess-traffic-metering: a metering behaviour that, if the PCN-
traffic exceeds the PCN-excess-rate, indicates that the amount of
PCN-traffic to be PCN-marked is equal to the amount in excess of
the PCN-excess-rate.

o Excess-traffic-marking: the setting of the header in a PCN-packet
to a specific encoding, based on indications from the excess-
traffic-meter.

o PCN-colouring: the process of setting the header in a PCN-packet
by a PCN-boundary-node; performed by a PCN-ingress-node so that
PCN-nodes can easily identify PCN-packets; performed by a PCN-
egress-node so that the header is appropriate for nodes beyond the
PCN-domain.

o Ingress-egress-aggregate: The collection of PCN-packets from all
PCN-flows that travel in one direction between a specific pair of
PCN-boundary-nodes.

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o PCN-feedback-information: information signalled by a PCN-egress-
node to a PCN-ingress-node (or a central control node), which is
needed for the flow admission and flow termination mechanisms.

o PCN-admissible-rate: the rate of PCN-traffic on a link up to which
PCN admission control should accept new PCN-flows.

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o PCN-supportable-rate: the rate of PCN-traffic on a link down to
which PCN flow termination should, if necessary, terminate already
admitted PCN-flows.

3. High-level functional architecture

The high-level approach is to split functionality between:

o PCN-interior-nodes 'inside' the PCN-domain, which monitor their
own state of pre-congestion and mark PCN-packets as appropriate.
They are not flow-aware, nor aware of ingress-egress-aggregates.
The functionality is also done by PCN-ingress-nodes for their
outgoing interfaces (ie those 'inside' the PCN-domain).

o PCN-boundary-nodes at the edge of the PCN-domain, which control
admission of new PCN-flows and termination of existing PCN-flows,
based on information from PCN-interior-nodes. This information is
in the form of the PCN-marked data packets (which are intercepted
by the PCN-egress-nodes) and not signalling messages. Generally
PCN-ingress-nodes are flow-aware.

The aim of this split is to keep the bulk of the network simple,
scalable and robust, whilst confining policy, application-level and
security interactions to the edge of the PCN-domain. For example the
lack of flow awareness means that the PCN-interior-nodes don't care
about the flow information associated with PCN-packets, nor do the
PCN-boundary-nodes care about which PCN-interior-nodes its ingress-
egress-aggregates traverse.

In order to generate information about the current state of the PCN-
domain, each PCN-node PCN-marks packets if it is "pre-congested".
Exactly when a PCN-node decides if it is "pre-congested" (the
algorithm) and exactly how packets are "PCN-marked" (the encoding)
will be defined in separate standards-track documents, but at a high
level it is as follows:

o the algorithms: a PCN-node meters the amount of PCN-traffic on
each one of its outgoing (or incoming) links. The measurement is
made as an aggregate of all PCN-packets, and not per flow. There
are two algorithms, one for threshold-marking threshold-metering and one for excess-
traffic-marking.

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traffic-metering. The meters trigger PCN-marking as necessary.

o the encoding(s): a PCN-node PCN-marks a PCN-packet by modifying a
combination of the DSCP and ECN fields. In the "baseline"
encoding [PCN08-1], the ECN field is set to 11 and the DSCP is not
altered. Extension encodings may be defined that, at most, use a
second DSCP (eg as in [Moncaster08]) and/or set the ECN field to

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values other than 11 (eg as in [Menth08-2]).

In a PCN-domain the operator may have two or three encoding states
available. The baseline encoding provides two encoding states (not
PCN-marked, PCN-marked), whilst extended encodings can provide three
encoding states (not PCN-marked, threshold-marked, excess-traffic-
marked).

An operator may choose to deploy either admission control or flow
termination or both. Although designed to work together, they are
independent mechanisms, and the use of one does not require or
prevent the use of the other. Three encoding states naturally allows
both flow admission and flow termination. If there are only two
encoding states, then there are several options - see Section 3.3.

The PCN-boundary-nodes monitor the PCN-marked packets in order to
extract information about the current state of the PCN-domain. Based
on this monitoring, a distributed decision is made about whether to
admit a prospective new flow or whether to terminate existing
flow(s). Sections 4.4 and 4.5 mention various possibilities for how
the functionality could be distributed.

PCN-metering and PCN-marking needs to be configured on all
(potentially pre-congested) links in the PCN-domain to ensure that
the PCN mechanisms protect all links. The actual functionality can
be configured on the outgoing or incoming interfaces of PCN-nodes -
or one algorithm could be configured on the outgoing interface and
the other on the incoming interface. The important point is that a
consistent choice is made across the PCN-domain to ensure that the
PCN mechanisms protect all links. See [PCN08-2] for further
discussion.

The objective of threshold-marking, as triggerd by the threshold-marking algorithm threshold-
metering algorithm, is to threshold-mark all PCN-packets whenever the
rate of PCN-packets is greater than some configured rate, the PCN-threshold-rate. PCN-
threshold-rate. The objective of the
excess-traffic-marking algorithm excess-traffic-metering, as
triggered by the excess-traffic-marking algorithm, is to excess-traffic-mark PCN-
packets excess-
traffic-mark PCN-packets at a rate equal to the difference between
the bit rate of PCN-packets and some configured rate, the PCN-excess-rate. PCN-excess-
rate. Note that this description reflects the overall intent of the algorithm
algorithms rather than its their instantaneous behaviour, since the rate

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measured at a particular moment depends on the detailed algorithm,
its implementation, and the traffic's variance as well as its rate
(eg marking may well continue after a recent overload even after the
instantaneous rate has dropped). The algorithms are specified in
[PCN08-2].

Admission and termination approaches are detailed and compared in
[Charny07-1] and [Menth08-3]. The discussion below is just a brief
summary. Sections 3.1 and 3.2 assume there are three encoding states

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available, whilst Section 3.3 assumes there are two encoding states
available.

From the perspective of the outside world, a PCN-domain essentially
looks like a DiffServ domain. Diffserv domain, but without the Diffserv architecture's
traffic conditioning agreements. PCN-traffic is either transported
across it transparently or policed at the PCN-ingress-node (ie
dropped or carried at a lower QoS). One difference is that PCN-
traffic has better QoS guarantees than normal DiffServ Diffserv traffic,
because the PCN mechanisms better protect the QoS of admitted flows.
Another difference may occur in the rare circumstance when there is a
failure: on the one hand some PCN-flows may get terminated, but on
the other hand other flows will get their QoS restored. Non PCN-
traffic is treated transparently, ie the PCN-domain is a normal
DiffServ
Diffserv domain.

3.1. Flow admission

The objective of PCN's flow admission control mechanism is to limit
the PCN-traffic on each link in the PCN-domain to *roughly* its PCN-
admissible-rate, by admitting or blocking prospective new flows, in
order to protect the QoS of existing PCN-flows. With three encoding
states available, the PCN-threshold-rate is configured by the
operator as equal to the PCN-admissible-rate on each link. It is set
lower than the traffic rate at which the link becomes congested and
the node drops packets.

Exactly how the admission control decision is made will be defined
separately in informational documents. This document describes two
approaches (others might be possible):

o the PCN-egress-node measures (possibly as a moving average) the
fraction of the PCN-traffic that is threshold-marked. The
fraction is measured for a specific ingress-egress-aggregate. If
the fraction is below a threshold value then the new flow is
admitted, and if the fraction is above the threshold value then it
is blocked. The fraction could be measured as an EWMA
(exponentially weighted moving average), which has sometimes been
called the "congestion level estimate".

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o the PCN-egress-node monitors PCN-traffic and if it receives one
(or several) threshold-marked packets, then the new flow is
blocked, otherwise it is admitted. One possibility may be to
react to the marking state of an initial flow set-up packet (eg
RSVP PATH). Another is that after one (or several) threshold-
marks then all flows are blocked until after a specific period of
no congestion.

Note that the admission control decision is made for a particular

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pair of PCN-boundary-nodes. So it is quite possible for a new flow
to be admitted between one pair of PCN-boundary-nodes, whilst at the
same time another admission request is blocked between a different
pair of PCN-boundary-nodes.

3.2. Flow termination

The objective of PCN's flow termination mechanism is to limit the
PCN-traffic on each link to *roughly* its PCN-supportable-rate, by
terminating some existing PCN-flows, in order to protect the QoS of
the remaining PCN-flows. With three encoding states available, the
PCN-excess-rate is configured by the operator as equal to the PCN-
supportable-rate on each link. It may be set lower than the traffic
rate at which the link becomes congested and the node drops packets.

Exactly how the flow termination decision is made will be defined
separately in informational documents. This document describes
several approaches (others might be possible):

o In one approach the PCN-egress-node measures the rate of PCN-
traffic that is not excess-traffic-marked, which is the amount of
PCN-traffic that can actually be supported, and communicates this
to the PCN-ingress-node. Also the PCN-ingress-node measures the
rate of PCN-traffic that is destined for this specific PCN-egress-
node. The difference represents the excess amount that should be
terminated.

o Another approach instead measures the rate of excess-traffic-
marked traffic and terminates this amount of traffic. This
terminates less traffic than the previous bullet if some nodes are
dropping PCN-traffic.

o Another approach monitors PCN-packets and terminates some of the
PCN-flows that have an excess-traffic-marked packet. (If all such
flows were terminated, far too much traffic would be terminated,
so a random selection needs to be made from those with an excess-
traffic-marked packet, [Menth08-1].)

Since flow termination is designed for "abnormal" circumstances, it

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is quite likely that some PCN-nodes are congested and hence packets
are being dropped and/or significantly queued. The flow termination
mechanism must accommodate this.

Note also that the termination control decision is made for a
particular pair of PCN-boundary-nodes. So it is quite possible for
PCN-flows to be terminated between one pair of PCN-boundary-nodes,
whilst at the same time none are terminated between a different pair
of PCN-boundary-nodes.

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3.3. Flow admission and/or flow termination when there are only two PCN
encoding states

If a PCN-domain has only two encoding states available (PCN-marked
and not PCN-marked), ie it is using the baseline encoding [PCN08-1],
then an operator has three options (others might be possible):

o admission control only: PCN-marking means threshold-marking, ie
only the threshold-marking threshold-metering algorithm writes PCN-marks. triggers PCN-marking. Only
PCN admission control is available.

o flow termination only: PCN-marking means excess-traffic-marking,
ie only the excess-traffic-marking excess-traffic-metering algorithm writes PCN-marks. triggers PCN-
marking. Only PCN termination control is available.

o both admission control and flow termination: only the excess-
traffic-marking
traffic-metering algorithm writes PCN-marks, triggers PCN-marking, however the
configured rate (PCN-excess-rate) is set equal to the PCN-admissible-rate, PCN-
admissible-rate, as shown in Figure 3. [Charny07-2] describes how
both admission control and flow termination can be triggered in
this case and also gives some of the pros and cons of this
approach. The main downside is that admission control is less
accurate.

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Figure 3: Schematic of how the PCN admission control and flow
termination mechanisms operate as the rate of PCN-traffic increases,
for a PCN-domain with two encoding states and using the approach of
[Charny07-2]. Note: U is a global parameter for all links in the

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PCN-domain.

3.4. Information transport

The transport of pre-congestion information from a PCN-node to a PCN-
egress-node is through PCN-markings in data packet headers, ie "in-
band": no signalling protocol messaging is needed. Signalling is
needed to transport PCN-feedback-information, for example to convey
the fraction of PCN-marked traffic from a PCN-egress-node to the
relevant PCN-ingress-node. Exactly what information needs to be
transported will be described in future documents about possible
boundary mechanisms. The signalling could be done by an extension of
RSVP or NSIS, for instance; [Lefaucheur06] describes the extensions
needed for RSVP.

3.5. PCN-traffic

The following are some high-level points about how PCN works:

o There needs to be a way for a PCN-node to distinguish PCN-traffic
from other traffic. This is through a combination of the DSCP
field and/or ECN field.

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o It is not advised to have non PCN-traffic that competes for the
same capacity as PCN-traffic but, if there is such traffic, there
needs to be a mechanism to limit it. "Capacity" means the
forwarding bandwidth on a link; "competes" means that non PCN-
packets will delay PCN-packets in the queue for the link. Hence
more non PCN-traffic results in poorer QoS for PCN. Further, the
unpredictable amount of non PCN-traffic makes the PCN mechanisms
less accurate and so reduces PCN's ability to protect the QoS of
admitted PCN-flows

o Two examples of such non PCN-traffic (ie that competes for the
same capacity as PCN-traffic) are:

1. traffic that is priority scheduled over PCN (perhaps a particular
application or an operator's control messages).

2. traffic that is scheduled at the same priority as PCN (for
example if the Voice-Admit codepoint is used for PCN-traffic
[PCN08-1] and there is non-PCN voice-admit traffic in the PCN-
domain).

o If there is such non PCN-traffic (ie that competes for the same
capacity as PCN-traffic), then PCN's mechanisms should take
account of it, in order to improve the accuracy of the decision
about whether to admit (or terminate) a PCN-flow. For example,

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one mechanism is that such non PCN-traffic contributes to the PCN
meters (ie is metered by the threshold-marking and excess-traffic-
marking algorithms).

o There will be non PCN-traffic that doesn't compete for the same
capacity as PCN-traffic, because it is forwarded at lower
priority. Hence it shouldn't contribute to the PCN meters.
Examples are best effort and assured forwarding traffic. However,
a PCN-node should dedicate some capacity to lower priority traffic
so that it isn't starved.

o The document assumes that the PCN mechanisms are applied to a
single behaviour aggregate in the PCN-domain. However, it would
also be possible to apply them independently to more than one
behaviour aggregate, which are distinguished by DSCP.

3.6. Backwards compatibility

PCN specifies semantics for the ECN field that differ from the
default semantics of [RFC3168]. A particular PCN encoding scheme
needs to describe how it meets the guidelines of BCP 124 [RFC4774]
for specifying alternative semantics for the ECN field. In summary
the approach is to:

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o use a DSCP to allow PCN-nodes to distinguish PCN-traffic that uses
the alternative ECN semantics;

o define these semantics for use within a controlled region, the
PCN-domain;

o take appropriate action if ECN capable, non-PCN traffic arrives at
a PCN-ingress-node with the DSCP used by PCN.

For the baseline encoding [PCN08-1], the 'appropriate action' is to
block ECN-capable traffic that uses the same DSCP as PCN from
entering the PCN-domain directly. Blocking means it is dropped or
downgraded to a lower priority behaviour aggregate, or alternatively
such traffic may be tunnelled through the PCN-domain. The reason
that 'appropriate action' is needed is that the PCN-egress-node
clears the ECN field to 00.

Extended encoding schemes may need to take different 'appropriate
action'.

4. Detailed Functional architecture

This section is intended to provide a systematic summary of the new
functional architecture in the PCN-domain. First it describes

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functions needed at the three specific types of PCN-node; these are
data plane functions and are in addition to their normal router
functions. Then it describes further functionality needed for both
flow admission control and flow termination; these are signalling and
decision-making functions, and there are various possibilities for
where the functions are physically located. The section is split
into:

1. functions needed at PCN-interior-nodes

2. functions needed at PCN-ingress-nodes

3. functions needed at PCN-egress-nodes

4. other functions needed for flow admission control

5. other functions needed for flow termination control

Note: Probing is covered in the Appendix.

The section then discusses some other detailed topics:

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1. addressing

2. tunnelling

3. fault handling

4.1. PCN-interior-node functions

Each link of the PCN-domain is configured with the following
functionality:

o Behaviour aggregate classification - determine whether an incoming
packet is a PCN-packet or not.

o Meter PCN-meter - measure the 'amount of PCN-traffic'. The measurement
is made as an aggregate of all PCN-packets, on the overall PCN-traffic, and not per flow.

o PCN-mark - algorithms Algorithms
determine whether to PCN-mark PCN-packets
and what packet encoding is used.

The functions are defined in [PCN08-2] and the baseline encoding in
[PCN08-1] (extended encodings are indicate to the PCN-marking functionality
that packets should be defined in other documents).

Eardley (Editor) PCN-marked.

o PCN-mark - as triggered by indications from the PCN-meter
functionality, if necessary PCN-mark packets wth the appropiate
encoding.

o Drop - if the queue overflows then naturally packets are dropped.
In addition, the link may be configured with a maximum rate for
PCN-traffic (below the physical link rate), above which PCN-
packets are dropped.

The functions are defined in [PCN08-2] and the baseline encoding in
[PCN08-1] (extended encodings are to be defined in other documents).

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+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+----------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream | | | | | | | Stream
+----------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+

Figure 4: Schematic of PCN-interior-node functionality

4.2. PCN-ingress-node functions

Each ingress link of the PCN-domain is configured with the following
functionality:

o Packet classification - determine whether an incoming packet is
part of a previously admitted flow, by using a filter spec (eg
DSCP, source and destination addresses, port numbers, and
protocol).

o Traffic conditioning Police - police, by dropping, any packets received with a DSCP
indicating PCN transport that do not belong to an admitted flow.
(A prospective PCN-flow that is rejected could be blocked or
admitted into a lower priority behaviour aggregate.) Similarly,
police packets that are part of a previously admitted flow, to
check that the flow keeps to the agreed rate or flowspec (eg
[RFC1633] for a microflow and its NSIS equivalent).

o PCN-colour - set the DSCP and ECN fields appropriately for the
PCN-domain, for example as in [PCN08-1].

o Meter - some approaches to flow termination require the PCN-
ingress-node to measure the (aggregate) rate of PCN-traffic
towards a particular PCN-egress-node.

The first two are policing functions, needed to make sure that PCN-
packets admitted into the PCN-domain belong to a flow that has been
admitted and to ensure that the flow keeps to the flowspec agreed (eg
doesn't exceed an agreed maximum rate and is inelastic traffic).
Installing the filter spec will typically be done by the signalling
protocol, as will re-installing the filter, for example after a re-

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route that changes the PCN-ingress-node (see [Briscoe06] for an
example using RSVP). PCN-colouring allows the rest of the PCN-domain
to recognise PCN-packets.

4.3. PCN-egress-node functions

Each egress link of the PCN-domain is configured with the following
functionality:

o Packet classify - determine which PCN-ingress-node a PCN-packet
has come from.

o Meter - "measure PCN-traffic" or "monitor PCN-marks".

o PCN-colour - for PCN-packets, set the DSCP and ECN fields to the
appropriate values for use outside the PCN-domain.

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The metering functionality of course depends on whether it is
targeted at admission control or flow termination. Alternatives
involve the PCN-egress-node "measuring" as an aggregate (ie not per
flow) all PCN-packets from a particular PCN-ingress-node, or
"monitoring" the PCN-traffic and reacting to one (or several) PCN-
marked packets. For PCN-colouring, [PCN08-1] specifies that the PCN-
egress-node re-sets the ECN field to 00; other encodings may define
different behaviour.

4.4. Admission control functions

As well as the functions covered above, other specific admission
control functions need to be performed (others might be possible):

o Make decision about admission - based on the output of the PCN-
egress-node's PCN meter function. In the case where it "measures
PCN-traffic", PCN-
traffic", the measured traffic on the ingress-egress-aggregate is
compared with some reference level. In the case where it
"monitors PCN-marks", then the decision is based on whether one
(or several) packets is (are) PCN-marked or not (eg the RSVP PATH
message). In either case, the admission decision also takes
account of policy and application layer requirements [RFC2753].

o Communicate decision about admission - signal the decision to the
node making the admission control request (which may be outside
the PCN-domain), and to the policer (PCN-ingress-node function)
for enforcement of the decision.

There are various possibilities for how the functionality could be
distributed (we assume the operator would configure which is used):

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o The decision is made at the PCN-egress-node and the decision
(admit or block) is signalled to the PCN-ingress-node.

o The decision is recommended by the PCN-egress-node (admit or
block) but the decision is definitively made by the PCN-ingress-
node. The rationale is that the PCN-egress-node naturally has the
necessary information about PCN-marking the amount of PCN-marks on the ingress-egress-
aggregate,
ingress-egress-aggregate, but the PCN-ingress-node is the policy
enforcement point [RFC2753], which polices incoming traffic to
ensure it is part of an admitted PCN-flow.

o The decision is made at the PCN-ingress-node, which requires that
the PCN-egress-node signals PCN-feedback-information to the PCN-
ingress-node. For example, it could signal the current fraction
of PCN-traffic that is PCN-marked.

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o The decision is made at a centralised node (see Appendix B). Appendix).

Note: Admission control functionality is not performed by normal PCN-
interior-nodes.

4.5. Flow termination functions

As well as the functions covered above, other specific termination
control functions need to be performed (others might be possible):

o PCN-meter at PCN-egress-node - similarly to flow admission, there
are two types of possibilities: to "measure PCN-traffic" on the
ingress-egress-aggregate, and to "monitor PCN-marks" and react to
one (or several) PCN-marks.

o (if required) PCN-meter at PCN-ingress-node - make "measurements
of PCN-traffic" being sent towards a particular PCN-egress-node;
again, this is done for the ingress-egress-aggregate and not per
flow.

o (if required) Communicate PCN-feedback-information to the node
that makes the flow termination decision. For example, as in
[Briscoe06], communicate the PCN-egress-node's measurements to the
PCN-ingress-node.

o Make decision about flow termination - use the information from
the PCN-meter(s) to decide which PCN-flow or PCN-flows to
terminate. The decision takes account of policy and application
layer requirements [RFC2753].

o Communicate decision about flow termination - signal the decision
to the node that is able to terminate the flow (which may be

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outside the PCN-domain), and to the policer (PCN-ingress-node
function) for enforcement of the decision.

There are various possibilities for how the functionality could be
distributed, similar to those discussed above in the Admission
control section.

Note: Flow termination functionality is not performed by normal PCN-
interior-nodes.

4.6. Addressing

PCN-nodes may need to know the address of other PCN-nodes. Note: in
all cases PCN-interior-nodes don't need to know the address of any
other PCN-nodes (except as normal their next hop neighbours, for
routing purposes).

The PCN-egress-node needs to know the address of the PCN-ingress-node
associated with a flow, at a minimum so that the PCN-ingress-node can

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be informed to enforce the admission decision (and any flow
termination decision) through policing. There are various
possibilities for how the PCN-egress-node can do this, ie associate
the received packet to the correct ingress-egress-aggregate. It is
not the intention of this document to mandate a particular mechanism.

o The addressing information can be gathered from signalling. For
example, regular processing of an RSVP PATH message, as the PCN-
ingress-node is the previous RSVP hop (PHOP) ([Lefaucheur06]). Or
the PCN-ingress-node could signal its address to the PCN-egress-
node.

o Always tunnel PCN-traffic across the PCN-domain. Then the PCN-
ingress-node's address is simply the source address of the outer
packet header. The PCN-ingress-node needs to learn the address of
the PCN-egress-node, either by manual configuration or by one of
the automated tunnel endpoint discovery mechanisms (such as
signalling or probing over the data route, interrogating routing
or using a centralised broker).

4.7. Tunnelling

Tunnels may originate and/or terminate within a PCN-domain (eg IP
over IP, IP over MPLS). It is important that the PCN-marking of any
packet can potentially influence PCN's flow admission control and
termination - it shouldn't matter whether the packet happens to be
tunnelled at the PCN-node that PCN-marks the packet, or indeed
whether it's decapsulated or encapsulated by a subsequent PCN-node.
This suggests that the "uniform conceptual model" described in

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[RFC2983] should be re-applied in the PCN context. In line with this
and the approach of [RFC4303] and [Briscoe08-2], the following rule
is applied if encapsulation is done within the PCN-domain:

o any PCN-marking is copied into the outer header

Note: A tunnel will not provide this behaviour if it complies with
[RFC3168] tunnelling in either mode, but it will if it complies with
[RFC4301] IPSec tunnelling.

Similarly, in line with the "uniform conceptual model" of [RFC2983],
the "full-functionality option" of [RFC3168], and [RFC4301], the
following rule is applied if decapsulation is done within the PCN-
domain:

o if the outer header's marking state is more severe then it is
copied onto the inner header.

Note: the order of increasing severity is: not PCN-marked; threshold-

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marking; excess-traffic-marking.
marked; excess-traffic-marked.

An operator may wish to tunnel PCN-traffic from PCN-ingress-nodes to
PCN-egress-nodes. The PCN-marks shouldn't be visible outside the
PCN-domain, which can be achieved by the PCN-egress-node doing the
PCN-colouring function (Section 4.3) after all the other (PCN and
tunnelling) functions. The potential reasons for doing such
tunnelling are: the PCN-egress-node then automatically knows the
address of the relevant PCN-ingress-node for a flow; even if ECMP is
running, all PCN-packets on a particular ingress-egress-aggregate
follow the same path. (ECMP: Equal Cost Multi-Path, Section 12.4.) 6.4.)
But it also has drawbacks, for example the additional overhead in
terms of bandwidth and processing, and the cost of setting up a mesh
of tunnels between PCN-boundary-nodes (there is an N^2 scaling
issue).

Potential issues arise for a "partially PCN-capable tunnel", ie where
only one tunnel endpoint is in the PCN domain:

1. The tunnel originates outside a PCN-domain and ends inside it.
If the packet arrives at the tunnel ingress with the same
encoding as used within the PCN-domain to indicate PCN-marking,
then this could lead the PCN-egress-node to falsely measure pre-
congestion.

2. The tunnel originates inside a PCN-domain and ends outside it.
If the packet arrives at the tunnel ingress already PCN-marked,
then it will still have the same encoding when it's decapsulated
which could potentially confuse nodes beyond the tunnel egress.

In line with the solution for partially

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In line with the solution for partially capable DiffServ Diffserv tunnels in
[RFC2983], the following rules are applied:

o For case (1), the tunnel egress node clears any PCN-marking on the
inner header. This rule is applied before the 'copy on
decapsulation' rule above.

o For case (2), the tunnel ingress node clears any PCN-marking on
the inner header. This rule is applied after the 'copy on
encapsulation' rule above.

Note that the above implies that one has to know, or determine, the
characteristics of the other end of the tunnel as part of
establishing it.

Tunnelling constraints were a major factor in the choice of the
baseline encoding. As explained in [PCN08-1], with current
tunnelling endpoints only the 11 codepoint of the ECN field survives

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decapsulation, and hence the baseline encoding only uses the 11
codepoint to indicate PCN-marking. Extended encoding schemes need to
explain their interactions with (or assumptions about) tunnelling. A
lengthy discussion of all the issues associated with layered
encapsulation of congestion notification (for ECN as well as PCN) is
in [Briscoe08-2].

4.8. Fault handling

If a PCN-interior-node (or one of its links) fails, then lower layer
protection mechanisms or the regular IP routing protocol will
eventually re-route around it. If the new route can carry all the
admitted traffic, flows will gracefully continue. If instead this
causes early warning of pre-congestion on the new route, then
admission control based on pre-congestion notification will ensure
new flows will not be admitted until enough existing flows have
departed. Re-routing may result in heavy (pre-)congestion, when the
flow termination mechanism will kick in.

If a PCN-boundary-node fails then we would like the regular QoS
signalling protocol to be responsible for taking appropriate action.
As an example [Briscoe08-2] considers what happens if RSVP is the QoS
signalling protocol.

5. Operations and Management

This Section considers operations and management issues, under the
FCAPS headings: the Operations and Management of Faults,
Configuration, Accounting, Performance and Security. Provisioning is

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discussed with performance.

5.1. Configuration Operations and Management

Threshold-marking

Threshold-metering and excess-traffic-marking -marking and excess-traffic-metering and
-marking are standardised in [PCN08-2]. However, more diversity in
PCN-boundary-node behaviours is expected, in order to interface with
diverse industry architectures. It may be possible to have different PCN-boundary-
node
PCN-boundary-node behaviours for different ingress-egress-aggregates
within the same PCN-domain.

PCN marking metering behaviour (threshold-marking, excess-traffic-marking) is enabled on either the egress or the ingress
interfaces of PCN-
nodes. PCN-nodes. A consistent choice must be made across the
PCN-domain to ensure that the PCN mechanisms protect all links.

PCN configuration control variables fall into the following
categories:

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o system options (enabling or disabling behaviours)

o parameters (setting levels, addresses etc)

One possibility is that all configurable variables sit within an SNMP
management framework [RFC3411], being structured within a defined
management information base (MIB) on each node, and being remotely
readable and settable via a suitably secure management protocol
(SNMPv3).

Some configuration options and parameters have to be set once to
'globally' control the whole PCN-domain. Where possible, these are
identified below. This may affect operational complexity and the
chances of interoperability problems between equipment from different
vendors.

It may be possible for an operator to configure some PCN-interior-
nodes so that they don't run the PCN mechanisms, if it knows that
these links will never become (pre-)congested.

5.1.1. System options

On PCN-interior-nodes there will be very few system options:

o Whether two PCN-markings (threshold-marked and excess-traffic-
marked) are enabled or only one. Typically all nodes throughout a
PCN-domain will be configured the same in this respect. However,
exceptions could be made. For example, if most PCN-nodes used
both markings, but some legacy hardware was incapable of running
two algorithms, an operator might be willing to configure these

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legacy nodes solely for excess-traffic-marking to enable flow
termination as a back-stop. It would be sensible to place such
nodes where they could be provisioned with a greater leeway over
expected traffic levels.

o In the case where only one PCN-marking is enabled, all nodes must
be configured to generate PCN-marks from the same meter (ie either
the threshold meter or the excess traffic meter).

PCN-boundary-nodes (ingress and egress) will have more system
options:

o Which of admission and flow termination are enabled. If any PCN-
interior-node is configured to generate a marking, all PCN-
boundary-nodes must be able to interpret that marking (which
includes understanding, in a PCN-domain that uses only one type of
PCN-marking, whether they are generated by PCN-interior-nodes'
threshold meters or the excess traffic meters). Therefore all

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PCN-boundary-nodes must be configured the same in this respect.

o Where flow admission and termination decisions are made: at PCN-
ingress-nodes or at PCN-egress-nodes (or at a centralised node,
see Appendix B). Appendix). Theoretically, this configuration choice could be
negotiated for each pair of PCN-boundary-nodes, but we cannot
imagine why such complexity would be required, except perhaps in
future inter-domain scenarios.

o How PCN-markings are translated into admission control and flow
termination decisions (see Section 3.1 and Section 3.2).

PCN-egress-nodes will have further system options:

o How the mapping should be established between each packet and its
aggregate, eg by MPLS label, by IP packet filter spec; and how to
take account of ECMP.

o If an equipment vendor provides a choice, there may be options to
select which smoothing algorithm to use for measurements.

5.1.2. Parameters

Like any DiffServ Diffserv domain, every node within a PCN-domain will need to
be configured with the DSCP(s) used to identify PCN-packets. On each
interior link the main configuration parameters are the PCN-
threshold-rate and PCN-excess-rate. A larger PCN-threshold-rate
enables more PCN-traffic to be admitted on a link, hence improving
capacity utilisation. A PCN-excess-rate set further above the PCN-
threshold-rate allows greater increases in traffic (whether due to

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natural fluctuations or some unexpected event) before any flows are
terminated, ie minimises the chances of unnecessarily triggering the
termination mechanism. For instance, an operator may want to design
their network so that it can cope with a failure of any single PCN-
node without terminating any flows.

Setting these rates on first deployment of PCN will be very similar
to the traditional process for sizing an admission controlled
network, depending on: the operator's requirements for minimising
flow blocking (grade of service), the expected PCN traffic load on
each link and its statistical characteristics (the traffic matrix),
contingency for re-routing the PCN traffic matrix in the event of
single or multiple failures, and the expected load from other classes
relative to link capacities [Menth07]. But once a domain is in
operation, a PCN design goal is to be able to determine growth in
these configured rates much more simply, by monitoring PCN-marking
rates from actual rather than expected traffic (see Section 5.2 on
Performance & Provisioning).

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Operators may also wish to configure a rate greater than the PCN-
excess-rate that is the absolute maximum rate that a link allows for
PCN-traffic. This may simply be the physical link rate, but some
operators may wish to configure a logical limit to prevent starvation
of other traffic classes during any brief period after PCN-traffic
exceeds the PCN-excess-rate but before flow termination brings it
back below this rate.

Threshold-marking

Threshold-metering requires a threshold token bucket depth to be
configured, excess-traffic-marking excess-traffic-metering needs a value for the MTU
(maximum size of a PCN-packet on the link) and both require setting a
maximum size of their token buckets. It will be preferable for there
to be rules to set defaults for these parameters, but then allow
operators to change them, for instance if average traffic
characteristics change over time.

The PCN-egress-node may allow configuration of the following:

o how it smooths metering of PCN-markings (eg EWMA parameters)

Whichever node makes admission and flow termination decisions will
contain algorithms for converting PCN-marking levels into admission
or flow termination decisions. These will also require configurable
parameters, for instance:

o an admission control algorithm that is based on the fraction of
marked packets will at least require a marking threshold setting
above which it denies admission to new flows;

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o flow termination algorithms will probably require a parameter to
delay termination of any flows until it is more certain that an
anomalous event is not transient;

o a parameter to control the trade-off between how quickly excess
flows are terminated, and over-termination.

One particular approach, [Charny07-2] would require a global
parameter to be defined on all PCN-nodes, but only needs one PCN
marking rate to be configured on each link. The global parameter is
a scaling factor between admission and termination (the PCN-traffic
rate on a link up to which flows are admitted vs the rate above which
flows are terminated). [Charny07-2] discusses in full the impact of
this particular approach on the operation of PCN.

5.2. Performance & Provisioning Operations and Management

Monitoring of performance factors measurable from *outside* the PCN
domain will be no different with PCN than with any other packet-based

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flow admission control system, both at the flow level (blocking
probability, etc) and the packet level (jitter [RFC3393], [Y.1541],
loss rate [RFC4656], mean opinion score [P.800], etc). The
difference is that PCN is intentionally designed to indicate
*internally* which exact resource(s) are the cause of performance
problems and by how much.

Even better, PCN indicates which resources will probably cause
problems if they are not upgraded soon. This can be achieved by the
management system monitoring the total amount (in bytes) of PCN-
marking generated by each queue over a period. Given possible long
provisioning lead times, pre-congestion volume is the best metric to
reveal whether sufficient persistent demand has occurred to warrant
an upgrade. Because, even before utilisation becomes problematic,
the statistical variability of traffic will cause occasional bursts
of pre-congestion. This 'early warning system' decouples the process
of adding customers from the provisioning process. This should cut
the time to add a customer when compared against admission control
provided over native DiffServ Diffserv [RFC2998], because it saves having to
verify the capacity planning process before adding each customer.

Alternatively, before triggering an upgrade, the long term pre-
congestion volume on each link can be used to balance traffic load
across the PCN-domain by adjusting the link weights of the routing
system. When an upgrade to a link's configured PCN-rates is
required, it may also be necessary to upgrade the physical capacity
available to other classes. But usually there will be sufficient
physical capacity for the upgrade to go ahead as a simple
configuration change. Alternatively, [Songhurst06] describes an

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adaptive rather than preconfigured system, where the configured PCN-
threshold-rate is replaced with a high and low water mark and the
marking algorithm automatically optimises how physical capacity is
shared using the relative loads from PCN and other traffic classes.

All the above processes require just three extra counters associated
with each PCN queue: threshold-markings, excess-traffic-markings and
drop. Every time a PCN packet is marked or dropped its size in bytes
should be added to the appropriate counter. Then the management
system can read the counters at any time and subtract a previous
reading to establish the incremental volume of each type of
(pre-)congestion. Readings should be taken frequently, so that
anomalous events (eg re-routes) can be distinguished from regular
fluctuating demand if required.

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5.3. Accounting Operations and Management

Accounting is only done at trust boundaries so it is out of scope of
this document, which is confined to intra-domain issues. Use of PCN
internal to a domain makes no difference to the flow signalling
events crossing trust boundaries outside the PCN-domain, which are
typically used for accounting.

5.4. Fault Operations and Management

Fault Operations and Management is about preventing faults, telling
the management system (or manual operator) that the system has
recovered (or not) from a failure, and about maintaining information
to aid fault diagnosis.

Admission blocking and particularly flow termination mechanisms
should rarely be needed in practice. It would be unfortunate if they
didn't work after an option had been accidentally disabled.
Therefore it will be necessary to regularly test that the live system
works as intended (devising a meaningful test is left as an exercise
for the operator).

Section 4 describes how the PCN architecture has been designed to
ensure admitted flows continue gracefully after recovering
automatically from link or node failures. The need to record and
monitor re-routing events affecting signalling is unchanged by the
addition of PCN to a DiffServ Diffserv domain. Similarly, re-routing events
within the PCN-domain will be recorded and monitored just as they
would be without PCN.

PCN-marking does make it possible to record 'near-misses'. For
instance, at the PCN-egress-node a 'reporting threshold' could be set
to monitor how often - and for how long - the system comes close to

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triggering flow blocking without actually doing so. Similarly,
bursts of flow termination marking could be recorded even if they are
not sufficiently sustained to trigger flow termination. Such
statistics could be correlated with per-queue counts of marking
volume (Section 5.2) to upgrade resources in danger of causing
service degradation, or to trigger manual tracing of intermittent
incipient errors that would otherwise have gone unnoticed.

Finally, of course, many faults are caused by failings in the
management process ('human error'): a wrongly configured address in a
node, a wrong address given in a signalling protocol, a wrongly
configured parameter in a queueing algorithm, a node set into a
different mode from other nodes, and so on. Generally, a clean
design with few configurable options ensures this class of faults can
be traced more easily and prevented more often. Sound management

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practice at run-time also helps. For instance: a management system
should be used that constrains configuration changes within system
rules (eg preventing an option setting inconsistent with other
nodes); configuration options should also be recorded in an offline
database; and regular automatic consistency checks between live
systems and the database should be performed. PCN adds nothing
specific to this class of problems.

5.5. Security Operations and Management

Security Operations and Management is about using secure operational
practices as well as being able to track security breaches or near-
misses at run-time. PCN adds few specifics to the general good
practice required in this field [RFC4778], other than those below.
The correct functions of the system should be monitored (Section 5.2)
in multiple independent ways and correlated to detect possible
security breaches. Persistent (pre-)congestion marking should raise
an alarm (both on the node doing the marking and on the PCN-egress-
node metering it). Similarly, persistently poor external QoS metrics
(such as jitter or mean opinion score) should raise an alarm. The
following are examples of symptoms that may be the result of innocent
faults, rather than attacks, but until diagnosed they should be
logged and trigger a security alarm:

o Anomalous patterns of non-conforming incoming signals and packets
rejected at the PCN-ingress-nodes (eg packets already marked PCN-
capable, or traffic persistently starving token bucket policers).

o PCN-capable packets arriving at a PCN-egress-node with no
associated state for mapping them to a valid ingress-egress-
aggregate.

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o A PCN-ingress-node receiving feedback signals about the pre-
congestion level on a non-existent aggregate, or that are
inconsistent with other signals (eg unexpected sequence numbers,
inconsistent addressing, conflicting reports of the pre-congestion
level, etc).

o Pre-congestion marking arriving at a PCN-egress-node with
(pre-)congestion markings focused on particular flows, rather than
randomly distributed throughout the aggregate.

6. IANA Considerations

This memo includes no request to IANA.

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7. Security considerations

Security considerations essentially come from

6.1. Benefits

The key benefits of the Trust Assumption
(Section 12.3.1), ie PCN mechanisms are that all PCN-nodes they are PCN-enabled and are
trusted for truthful PCN-marking and transport. PCN splits
functionality between PCN-interior-nodes and PCN-boundary-nodes, and
the security considerations are somewhat different for each, mainly
because PCN-boundary-nodes are flow-aware simple,
scalable, and PCN-interior-nodes are
not. robust because:

o Because Per flow state is only required at the PCN-boundary-nodes are flow-aware, they are trusted to
use that awareness correctly. The degree of trust PCN-ingress-nodes
("stateless core"). This is required
depends on for policing purposes (to
prevent non-admitted PCN traffic from entering the kinds of decisions they have to make PCN-domain) and the kinds
so on. It is not generally required that other network entities
are aware of information individual flows (although they need to make them. There is nothing specific
to PCN. may be in particular
deployment scenarios).

o Admission control is resilient: with PCN QoS is decoupled from the
routing system. Hence in general admitted flows can survive
capacity, routing or topology changes without additional
signalling. The PCN-ingress-nodes police packets to ensure a PCN-flow sticks
within its agreed limit, and to ensure that only PCN-flows PCN-admissible-rate on each link can be chosen
small enough that
have been admitted contribute PCN-traffic into the PCN-domain.
The policer must drop (or perhaps downgrade to traffic can still be carried after a different DSCP)
any PCN-packets received that are outside this remit.
rerouting in most failure cases [Menth07]. This is
similar an important
feature as QoS violations in core networks due to link failures
are more likely than QoS violations due to increased traffic
volume [Iyer03].

o The PCN-metering behaviours only operate on the existing IntServ behaviour. Between them the overall PCN-
boundary-nodes must encircle
traffic on the PCN-domain, otherwise PCN-packets
could enter the PCN-domain without being subject link, not per flow.

o The information of these measurements is signalled to admission
control, which would potentially destroy the QoS of existing
flows.

o PCN-interior-nodes are not flow-aware. This prevents some
security attacks where an attacker targets specific flows PCN-
egress-nodes by the PCN-marks in the
data plane - for instance packet headers, ie [Style]
"in-band". No additional signalling protocol is required for DoS or eavesdropping.

o The PCN-boundary-nodes rely on correct PCN-marking by
transporting the PCN-
interior-nodes. For instance a rogue PCN-interior-node could PCN-
mark all packets so that PCN-marks. Therefore no flows were admitted. Another
possibility is that it doesn't PCN-mark any packets, even when it secure binding is pre-congested. More subtly, the rogue PCN-interior-node could
perform these attacks selectively on particular flows, or it could
PCN-mark the correct fraction overall, but carefully choose which
flows it marked.

o The PCN-boundary-nodes should be able to deal with DoS attacks
required between data packets and
state exhaustion attacks based on fast changes in per flow
signalling. separate congestion messages.

o The signalling between PCN-egress-nodes make separate measurements, operating on the PCN-boundary-nodes must be protected
aggregate PCN-traffic from attacks. For example the recipient needs to validate that each PCN-ingress-node, ie not per flow.
Similarly, signalling by the message PCN-egress-node of PCN-feedback-
information (which is indeed from the node that claims to have sent it.
Possible measures include digest authentication used for flow admission and protection termination

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against replay and man-in-the-middle attacks. For

decisions) is at the specific
protocol RSVP, hop-by-hop authentication granularity of the ingress-egress-aggregate.
An alternative approach is in [RFC2747], that the PCN-egress-nodes monitor the
PCN-traffic and
[Behringer07] may also be useful.

Operational security advice signal PCN-feedback-information (which is given in Section 5.5.

8. Conclusions

The document describes a general architecture used for
flow admission and termination based on pre-congestion information in order to protect decisions) at the quality of service granularity of established inelastic flows within
one (or a single
DiffServ domain. few) PCN-marks.

o The main topic admitted PCN-load is controlled dynamically. Therefore it
adapts as the functional architecture. It traffic matrix changes, and also mentions if the network
topology changes (eg after a link failure). Hence an operator can
be less conservative when deploying network capacity, and less
accurate in their prediction of the PCN-traffic matrix.

o The termination mechanism complements admission control. It
allows the network to recover from sudden unexpected surges of
PCN-traffic on some links, thus restoring QoS to the remaining
flows. Such scenarios are expected to be rare but not impossible.
They can be caused by large network failures that redirect lots of
admitted PCN-traffic to other topics like links, or by malfunction of the assumptions
measurement-based admission control in the presence of admitted
flows that send for a while with an atypically low rate and open issues.

9. Acknowledgements

This document is then
increase their rates in a revised version of correlated way.

o Flow termination can also enable an earlier individual draft
authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R. Geib, G.
Karagiannis, M. Menth, T. Tsou. They are therefore contributors operator to
this document.

Thanks be less
conservative when deploying network capacity. It is an
alternative to those who have made comments on this document: Lachlan
Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake, Ron
Bonica, Scott Bradner, Bob Briscoe, Ross Callon, Jason Canon, Ken
Carlberg, Anna Charny, Joachim Charzinski, Andras Csaszar, Francis
Dupont, Lars Eggert, Pasi Eronen, Ruediger Geib, Wei Gengyu, Robert
Hancock, Fortune Huang, Christian Hublet, Cullen Jennings, Ingemar
Johansson, Georgios Karagiannis, Hein Mekkes, Michael Menth, Toby
Moncaster, Dan Romascanu, Daisuke Satoh, Ben Strulo, Tom Taylor,
Hannes Tschofenig, Tina Tsou, David Ward, Lars Westberg, Magnus
Westerlund, Delei Yu. Thanks running links at low utilisation in order to Bob Briscoe who extensively revised
the Operations and Management section.
protect against link or node failures. This document is especially the result
case with SRLGs (shared risk link groups, which are links that
share a resource, such as a fibre, whose failure affects all those
links [RFC4216]). Fully protecting traffic against a single SRLG
failure requires low utilisation (~10%) of discussions in the PCN WG and
forerunner activity in link bandwidth on
some links before failure [Charny08].

o The PCN-supportable-rate may be set below the TSVWG. A number of previous drafts were
presented to TSVWG; their authors were: B, Briscoe, P. Eardley, D.
Songhurst, F. Le Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley,
G. Karagiannis, A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G.
Liu, A. Bhargava.

10. Comments Solicited

Comments and questions are encouraged and very welcome. They maximum rate that
PCN-traffic can be
addressed transmitted on a link, in order to trigger
termination of some PCN-flows before loss (or excessive delay) of
PCN-packets occurs, or to keep the IETF PCN working group mailing list <pcn@ietf.org>.

Eardley (Editor) maximum PCN-load on a link
below a level configured by the operator.

o Provisioning of the network is decoupled from the process of
adding new customers. By contrast, with the Diffserv architecture
[RFC2475] operators rely on subscription-time Service Level
Agreements, which statically define the parameters of the traffic
that will be accepted from a customer, and so the operator has to
verify provision is sufficient each time a new customer is added
to check that the Service Level Agreement can be fulfilled. A
PCN-domain doesn't need such traffic conditioning.

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11. Changes

11.1. Changes from -098 to -10

Changes to deal with IESG comments:

o New introduction

6.2. Deployment scenarios

Operators of networks will want to provide gentler introduction for use the PCN
novice: quick summary of PCN's applicability; quick example of mechanisms in various
arrangements, for instance depending on how
it they are performing
admission control outside the PCN-domain (users after all hangs together in one end-to-end qos scenario; quick
summary of are
concerned about QoS end-to-end), what their particular goals and
assumptions are, how many PCN "documentation"

o OAM changed encoding states are available, and so
on.

A PCN-domain may have three encoding states (or pedantically, an
operator may choose to Operations use up three encoding states for PCN): not
PCN-marked, threshold-marked, excess-traffic-marked. Then both PCN
admission control and Management

o Processed some of the minor suggestions flow termination can be supported. As
illustrated in Figure 1, admission control accepts new flows until
the Gen-ART Review by
Francis Dupont

o Two wording tweaks in Sections 3.2 & 3.4 (as agreed PCN-traffic rate on mailing
list)

o Updated boilerplate. this draft may include material pre- Nov 10
2008 blah.

11.2. Changes from -08 to -09

Small changes the bottleneck link rises above the PCN-
threshold-rate, whilst if necessary the flow termination mechanism
terminates flows down to deal with WG Chair comments:

o tweak language the PCN-excess-rate on the bottleneck link.

On the other hand, a PCN-domain may have two encoding states (as in various places
[PCN08-1]) (or pedantically, an operator may choose to make it more RFC-like and less
that of a scholarly work, use up two
encoding states for instance from "we propose" to "this
document describes"

o tweak language PCN): not PCN-marked, PCN-marked. Then there are
three possibilities, as discussed in various places to make it a stand alone
architecture document rather than a discussion of the PCN WG. Now
only mentions WG at start of Annex.

o References: IDs are no longer referenced to following paragraphs (see
also Section 3.3).

o References: removed some
admission control mechanism will prevent admission of less important references to IDs

11.3. Changes from -07 to -08

Small changes from second WG last call:

o Section 2: added definition for PCN-admissible-rate and PCN-
supportable-rate. Small changes to new flows that
use these terms as follows:
Section 3, bullets 2 & 9; S6.1 para 1; S6.2 para1; S6.3 bullet 3;
added the affected links. So the PCN-domain will naturally return to Figs 1 & 2.
normal operation, but with reduced capacity. The drawback of this
approach would be that, until sufficient sessions have ended to
relieve the congestion, all PCN-flows as well as lower priority
services will be adversely affected.

Second, an operator could just rely for admission control on
statically provisioned capacity per PCN-ingress-node (regardless of
the PCN-egress-node of a flow), as is typical in the hose model of
the Diffserv architecture [RFC2475]. Such traffic conditioning
agreements can lead to focused overload: many flows happen to focus
on a particular link and then all flows through the congested link
fail catastrophically. PCN's flow termination mechanism could then
be used to counteract such a problem.

Third, both admission control and flow termination can be triggered
from the single type of PCN-marking; the main downside is that
admission control is less accurate [Charny07-2]. This possibility is
illustrated in Figure 3.

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o added

Within the phrase "(others might be possible") before PCN-domain there is some flexibility about how the list of
approaches
decision making functionality is distributed. These possibilities
are outlined in Section 6.3, 7.4 & 7.5.

o added references to RFC2753 (A framework for policy-based
admission control) 4.4 and also discussed elsewhere, such as in S7.4 & S7.5.

o throughout, updated references now that marking behaviour &
baseline encoding
[Menth08-3].

The flow admission and termination decisions need to be enforced
through per flow policing by the PCN-ingress-nodes. If there are WG drafts.

o a few typos corrected

11.4. Changes from -06 to -07

References re-formatted to pass ID nits. No other changes.

11.5. Changes from -05 to -06

Minor clarifications throughout, the least insignificant are as
follows:

o Section 1: added
several PCN-domains on the end-to-end path, then each needs to police
at its PCN-ingress-nodes. One exception is if the list of encoding states in an 'extended'
scheme: "or perhaps further encoding states as suggested in
draft-westberg-pcn-load-control"

o Section 2: added definition for PCN-colouring (to clarify that operator runs both
the
term is used consistently differently from 'PCN-marking')

o Section 6.1 access network (not a PCN-domain) and 6.2: added "(others might be possible)" before the
list of high level approaches for making core network (a PCN-
domain); per flow admission
(termination) decisions.

o Section 6.2: corrected a significant typo in 2nd bullet (more ->
less)

o Section 6.3: corrected a couple of significant typos in Figure 2

o Section 6.5 (PCN-traffic) re-written for clarity. Non PCN-traffic
contributing to PCN meters is now given as an example (there may policing could be cases where don't need to meter it).

o Section 7.7: added devolved to the text about encapsulation being access network
and not done
within at the PCN-domain: "Note: A tunnel will not provide this
behaviour if it complies with [RFC3168] tunnelling in either mode,
but it will if it complies with [RFC4301] IPSec tunnelling."

o Section 7.7: added mention of [RFC4301] PCN-ingress-node. Note: to aid readability, the text about
decapsulation being
rest of this draft assumes that policing is done within by the PCN-domain.

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o Section 8: deleted admission control has to fit with the text about design goals, since this overall approach to
admission control. For instance [Briscoe06] describes the case where
RSVP signalling runs end-to-end. The PCN-domain is
already covered adequately earlier eg a single RSVP
hop, ie only the PCN-boundary-nodes process RSVP messages, with RSVP
messages processed on each hop outside the PCN-domain, as in S3.

o Section 11: replaced IntServ
over Diffserv [RFC2998]. It would also be possible for the last sentence of bullet 1 RSVP
signalling to be originated and/or terminated by "There proxies, with
application-layer signalling between the end user and the proxy (eg
SIP signalling with a home hub). A similar example would use NSIS
signalling instead of RSVP. (NSIS: Next Steps in Signalling,
[RFC3726].)

It is
nothing specific possible that a user wants its inelastic traffic to PCN."

o Appendix: added use the PCN
mechanisms but also react to open issues: possibility of automatically and
periodically probing.

o References: Split out Normative references (RFC2474 & RFC3246).

11.6. Changes from -04 ECN marking outside the PCN-domain
[Sarker08]. Two possible ways to -05

Minor nits removed as follows:

o Further minor changes do this are to reflect tunnel all PCN-
packets across the PCN-domain, so that baseline the ECN marks are carried
transparently across the PCN-domain, or to use an encoding like
[Moncaster08]. Tunnelling is
consensus, standards track document, whilst there can be
(experimental track) encoding extensions

o Traffic conditioning updated to reflect discussions discussed further in Dublin,
mainly that PCN-interior-nodes don't police PCN-traffic (so
deleted bullet Section 4.7.

Some further possible deployment models are outlined in S7.1) the Appendix.

6.3. Assumptions and that it constraints on scope

The scope is not advised to have non
PCN-traffic that shares restricted by the same capacity (on a link) as PCN-
traffic (so added bullet following assumptions:

1. these components are deployed in S6.5)

o Probing moved into Appendix A and deleted the 'third viewpoint'
(admission control based on the marking of a single packet like an
RSVP PATH message) - since this isn't really probing, Diffserv domain, within
which all PCN-nodes are PCN-enabled and in any
case is already mentioned in S6.1.

o Minor changes to S9 Operations are trusted for truthful
PCN-marking and management - mainly to reflect
that consensus on marking behaviour has simplified things so eg
there transport

2. all flows handled by these mechanisms are fewer parameters to configure.

o A few terminology-related errors expunged, inelastic and two pictures added
to help.

o Re-phrased the claim about the natural decision point in S7.4

o Clarified that extended encoding schemes need
constrained to explain their
interactions with (or assumptions about) tunnelling (S7.7) and how
they meet the guidelines of BCP124 (S6.6)

o Corrected the third bullet in S6.2 (to reflect consensus about
PCN-marking) a known peak rate through policing or shaping

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11.7. Changes from -03 to -04

o Minor changes throughout to reflect

3. the consensus call about number of PCN-flows across any potential bottleneck link is
sufficiently large that stateless, statistical mechanisms can be
effective. To put it another way, the aggregate bit rate of PCN-
marking (as reflected in [PCN08-2]).

o Minor changes throughout
traffic across any potential bottleneck link needs to be
sufficiently large relative to reflect the current decisions about
encoding (as reflected in [PCN08-1] and [Moncaster08]).

o Introduction: re-structured to create new sections on Benefits,
Deployment scenarios and Assumptions.

o Introduction: Added pointers to other PCN documents.

o Terminology: changed PCN-lower-rate to PCN-threshold-rate and PCN-
upper-rate to PCN-excess-rate; excess-rate-marking to excess-
traffic-marking.

o Benefits: maximum additional bit rate
added bullet about SRLGs.

o Deployment scenarios: new section combining material from various
places within by one flow. This is the document.

o S6 (high level functional architecture): re-structured and edited
to improve clarity, and reflect basic assumption of measurement-
based admission control.

4. PCN-flows may have different precedence, but the latest PCN-marking applicability of
the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.)
is out of scope.

6.3.1. Assumption 1: Trust and
encoding drafts.

o S6.4: added claim support of PCN - controlled environment

It is assumed that the most natural place to make an admission
decision PCN-domain is a PCN-egress-node.

o S6.5: updated the bullet about non-PCN-traffic that uses controlled environment, ie all
the same
DSCP as PCN-traffic.

o S6.6: added nodes in a section about backwards compatibility with respect
to [RFC4774].

o Appendix A: added bullet about end-to-end PCN.

o Probing: moved to Appendix B. PCN-domain run PCN and are trusted. There are several
reasons this assumption:

o Other minor clarifications, typos etc.

11.8. Changes from -02 The PCN-domain has to -03

o Abstract: Clarified be encircled by removing the term 'aggregated'. Follow-up
clarifications later in draft: S1: expanded PCN-egress-nodes
bullet a ring of PCN-boundary-
nodes, otherwise traffic could enter a PCN-BA without being
subject to mention case where admission control, which would potentially degrade the PCN-feedback-information is about
one (or
QoS of existing PCN-flows.

o Similarly, a few) PCN-marks, rather than aggregated information; S3
clarified PCN-meter; S5 minor changes; conclusion.

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o S1: added a paragraph about how the PCN-domain looks PCN-boundary-node has to trust that all the
outside world (essentially it looks like a DiffServ domain).

o S2: tweaked the PCN-nodes
mark PCN-traffic terminology bullet: changed PCN
traffic classes consistently. A node not performing PCN-marking
wouldn't be able to PCN behaviour aggregates, alert when it suffered pre-congestion, which
potentially would lead to be more in line
with traditional DiffServ jargon (-> follow-up changes later in
draft); included a definition of too many PCN-flows (and corrected being admitted (or
too few being terminated). Worse, a couple
of 'PCN microflows' to 'PCN-flows' later rogue node could perform
various attacks, as discussed in draft)

o S3.5: added possibility the Security Considerations
section.

One way of downgrading to best effort, where assuring the above two points is that the entire PCN-
packets arrive at PCN-ingress-node already ECN marked (CE or ECN
nonce)

o S4: added note about whether talk about PCN operating on an
interface or on
domain is run by a link. In S8.1 (OAM) mentioned single operator. Another possibility is that PCN
functionality needs
there are several operators that trust each other in their handling
of PCN-traffic.

Note: All PCN-nodes need to be configured consistently on either the
ingress or the egress trustworthy. However if it is known
that an interface cannot become pre-congested then it is not strictly
necessary for it to be capable of PCN-nodes PCN-marking. But this must be
known even in a PCN-domain.

o S5.2: clarified that signalling protocol installs flow filter spec
at PCN-ingress-node (& updates unusual circumstances, eg after possible re-route)

o S5.6: addressing: clarified

o S5.7: added tunnelling issue of N^2 scaling if you set up a mesh
of tunnels between PCN-boundary-nodes

o S7.3: Clarified the "third viewpoint" failure of probing (always probe).

o S8.1: clarified that SNMP is only an example; added note that an
operator may be able to not run PCN on some PCN-interior-nodes, if
it knows
links.

6.3.2. Assumption 2: Real-time applications

It is assumed that these links will never become (pre-)congested; added
note any variation of source bit rate is independent of
the level of pre-congestion. We assume that it may be possible to have different PCN-boundary-node
behaviours for different ingress-egress-aggregates within PCN-packets come from
real time applications generating inelastic traffic, ie sending
packets at the same
PCN-domain.

o Appendix: Created an Appendix about "Possible work items beyond rate the scope codec produces them, regardless of the current PCN WG Charter". Material moved from
near start of S3 and elsewhere throughout draft. Moved text about
centralised decision node to Appendix.

o Other minor clarifications.

11.9. Changes from -01 to -02

o S1: Benefits: provisioning bullet extended to stress that PCN does
not use RFC2475-style traffic conditioning.

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o S1: Deployment models: mentioned, as variant

availability of PCN-domain
extending to end nodes, that may extend capacity [RFC4594]. For example, voice and video
requiring low delay, jitter and packet loss, the Controlled Load
Service, [RFC2211], and the Telephony service class, [RFC4594]. This
assumption is to LAN edge switch.

o S3.1: Trust Assumption: added note about not needing PCN-marking
capability if help focus the effort where it looks like PCN would
be most useful, ie the sorts of applications where per flow QoS is a
known that an interface cannot become pre-congested.

o S4: now divided into sub-sections

o S4.1: Admission control: added second proposed method for how requirement. In other words we focus on PCN providing a
benefit to
decide inelastic traffic (PCN may or may not provide a benefit to block new flows (PCN-egress-node receives one (or
several) PCN-marked packets).

o S5: Probing sub-section removed. Material now in new S7.

o S5.6: Addressing: clarified how PCN-ingress-node can discover
address
other types of PCN-egress-node

o S5.6: Addressing: centralised node case, added traffic).

As a consequence, it is assumed that PCN-ingress-
node may need PCN-metering and PCN-marking is
being applied to know address of PCN-egress-node

o S5.8: Tunnelling: added case of "partially PCN-capable tunnel" traffic scheduled with the expedited forwarding per-
hop behaviour, [RFC3246], or a per-hop behaviour with similar
characteristics.

6.3.3. Assumption 3: Many flows and
degraded bullet additional load

It is assumed that there are many PCN-flows on this any bottleneck link in S6 (Open Issues)

o S7: Probing: new section. Much more comprehensive than old S5.5.

o S8: Operations and Management: substantially revised.

o other minor changes not affecting semantics

11.10. Changes from -00
the PCN-domain (or, to -01

In addition put it another way, the aggregate bit rate of
PCN-traffic across any potential bottleneck link is sufficiently
large relative to clarifications and nit squashing, the main changes
are:

o S1: Benefits: maximum additional bit rate added by one about provisioning (and contrast with
DiffServ SLAs)

o S1: Benefits: clarified PCN-
flow). Measurement-based admission control assumes that the objective present
is also a reasonable prediction of the future: the network conditions are
measured at the time of a new flow request, however the actual
network performance must be acceptable during the call some time
later. One issue is that if there are only a few variable rate
flows, then the aggregate traffic level may vary a lot, perhaps
enough to stop PCN- cause some packets being significantly delayed (previously only mentioned not
dropping packets)

o S1: Deployment models: added one where policing to get dropped. If there are many flows
then the aggregate traffic level should be statistically smoothed.
How many flows is done at ingress enough depends on a number of access network factors such as the
variation in each flow's rate, the total rate of PCN-traffic, and not at ingress the
size of PCN-domain (assume trust the "safety margin" between networks)

o S1: Deployment models: corrected MPLS-TE the traffic level at which we
start admission-marking and at which packets are dropped or
significantly delayed.

No explicit assumptions are made about how many PCN-flows are in each
ingress-egress-aggregate. Performance evaluation work may clarify
whether it is necessary to MPLS make any additional assumption on
aggregation at the ingress-egress-aggregate level.

6.3.4. Assumption 4: Emergency use out of scope

PCN-flows may have different precedence, but the applicability of the
PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc) is out
of scope of this document.

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o S2: Terminology: adjusted definition of PCN-domain

o S3.5: Other assumptions: corrected, so that two assumptions (PCN-
nodes not performing ECN

6.4. Challenges

Prior work on PCN and PCN-ingress-node discarding arriving
CE packet) only apply if the similar mechanisms has thrown up a number of
considerations about PCN's design goals (things PCN WG decides should be good
at) and some issues that have been hard to encode PCN-marking solve in the ECN-field.

o S4 & S5: changed PCN-marking algorithm to marking behaviour

o S4: clarified a fully
satisfactory manner. Taken as a whole it represents a list of trade-
offs (it is unlikely that PCN-interior-node functionality applies for
each outgoing interface, they can all be 100% achieved) and added clarification: "The
functionality is also done by PCN-ingress-nodes for their outgoing
interfaces (ie those 'inside' the PCN-domain)."

o S4 (near end): altered perhaps
as evaluation criteria to say that a PCN-node "should" dedicate help an operator (or the IETF) decide
between options.

The following are open issues. They are mainly taken from
[Briscoe06], which also describes some capacity to lower priority traffic so possible solutions. Note that it isn't starved
(was "may")
some may be considered unimportant in general or in specific
deployment scenarios or by some operators.

NOTE: Potential solutions are out of scope for this document.

o S5: clarified to say that PCN functionality ECMP (Equal Cost Multi-Path) Routing: The level of pre-congestion
is done on an
'interface' (rather than measured on a 'link')

o S5.2: deleted erroneous mention specific ingress-egress-aggregate. However, if
the PCN-domain runs ECMP, then traffic on this ingress-egress-
aggregate may follow several different paths - some of service level agreement

o S5.5: Probing: re-written, especially to distinguish probing to
test the ingress-egress-aggregate from probing to test paths
could be pre-congested whilst others are not. There are three
potential problems:

1. over-admission: a
particular ECMP path.

o S5.7: Addressing: added mention of probing; added that in the case
where traffic new flow is always tunnelled across admitted (because the PCN-domain, add a
note pre-
congestion level measured by the PCN-egress-node is
sufficiently diluted by unmarked packets from non-congested
paths that he PCN-ingress-node needs to know a new flow is admitted), but its packets travel
through a pre-congested PCN-node.

2. under-admission: a new flow is blocked (because the address of pre-
congestion level measured by the
PCN-egress-node.

o S5.8: Tunnelling: re-written, especially to provide a clearer
description of copying on tunnel entry/exit, PCN-egress-node is
sufficiently increased by adding explanation
(keeping tunnel encaps/decaps and PCN-marking orthogonal),
deleting one bullet ("if PCN-marked packets from pre-
congested paths that a new flow is blocked), but its packets
travel along an uncongested path.

3. ineffective termination: a flow is terminated, but its path
doesn't travel through the inner header's marking state (pre-)congested router(s). Since
flow termination is more
sever then it a 'last resort', which protects the
network should over-admission occur, this problem is preserved" - shouldn't happen), and better
referencing of probably
more important to solve than the other IETF documents. two.

o S6: Open issues: stressed that "NOTE: Potential solutions are out
of scope for this document" ECMP and edited a couple of sentences that
were close to solution space.

o S6: Open issues: added one about scenarios with only one tunnel
endpoint signalling: It is possible that, in a PCN-domain running
ECMP, the PCN domain .

o S6: Open issues: ECMP: added under-admission signalling packets (eg RSVP, NSIS) follow a different
path than the data packets, which could matter if the signalling
packets are used as another potential
risk probes. Whether this is an issue depends on
which fields the ECMP algorithm uses; if the ECMP algorithm is
restricted to the source and destination IP addresses, then it

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o S6: Open issues: added one about "Silent at start"

o S10: Conclusions:

will not be an issue. ECMP and signalling interactions are a small conclusions section added

12. Appendix A: Applicability of PCN

12.1. Benefits

We believe that the key benefits
specific instance of a general issue for non-traditional routing
combined with resource management along a path [Hancock02].

o Tunnelling: There are scenarios where tunnelling makes it
difficult to determine the PCN mechanisms described path in
this document are that they are simple, scalable, the PCN-domain. The problem,
its impact, and robust because: the potential solutions are similar to those for
ECMP.

o Per flow state is Scenarios with only required at one tunnel endpoint in the PCN-ingress-nodes
("stateless core"). This is required for policing purposes (to
prevent non-admitted PCN traffic domain may make
it harder for the PCN-egress-node to gather from entering the PCN-domain) and
so on. It is not generally required that other network entities
are aware signalling
messages (eg RSVP, NSIS) the identity of individual flows (although they may be in particular
deployment scenarios).

o Bi-Directional Sessions: Many applications have bi-directional
sessions - hence there are two microflows that admitted traffic can still should be carried after a
rerouting in most failure cases [Menth07]. This is an important
feature admitted
(or terminated) as QoS violations a pair - for instance a bi-directional voice
call only makes sense if microflows in core networks due to link failures both directions are more likely than QoS violations due to increased traffic
volume [Iyer03].

o The PCN-marking behaviours only operate on the overall PCN-traffic
on
admitted. However, the link, not per flow.

o The information PCN mechanisms concern admission and
termination of a single flow, and coordination of these measurements is signalled to the PCN-
egress-nodes by the PCN-marks in the packet headers, ie [Style]
"in-band". No additional signalling protocol decision for
both flows is required a matter for
transporting the PCN-marks. Therefore no secure binding is
required between data packets signalling protocol and separate congestion messages. out of
scope of PCN. One possible example would use SIP pre-conditions.
However, there are others.

o The PCN-egress-nodes make separate measurements, operating Global Coordination: PCN makes its admission decision based on the
aggregate PCN-traffic
PCN-markings on a particular ingress-egress-aggregate. Decisions
about flows through a different ingress-egress-aggregate are made
independently. However, one can imagine network topologies and
traffic matrices where, from each PCN-ingress-node, ie not per flow.
Similarly, signalling by a global perspective, it would be
better to make a coordinated decision across all the PCN-egress-node of PCN-feedback-
information (which is used ingress-
egress-aggregates for flow admission and termination
decisions) is at the granularity whole PCN-domain. For example, to block
(or even terminate) flows on one ingress-egress-aggregate so that
more important flows through a different ingress-egress-aggregate
could be admitted. The problem may well be relatively
insignificant.

o Aggregate Traffic Characteristics: Even when the number of flows
is stable, the ingress-egress-aggregate.
An alternative approach traffic level through the PCN-domain will vary
because the sources vary their traffic rates. PCN works best when
there is that not too much variability in the PCN-egress-nodes monitor total traffic level at a
PCN-node's interface (ie in the
PCN-traffic aggregate traffic from all
sources). Too much variation means that a node may (at one
moment) not be doing any PCN-marking and signal PCN-feedback-information (which then (at another moment)
drop packets because it is used for
flow overloaded. This makes it hard to tune
the admission and termination decisions) control scheme to stop admitting new flows at the granularity of
one (or a few) PCN-marks.
right time. Therefore the problem is more likely with fewer,
burstier flows.

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o The admitted PCN-load is controlled dynamically. Therefore it
adapts as the traffic matrix changes, Flash crowds and also if the network
topology changes (eg after Speed of Reaction: PCN is a link failure). Hence measurement-based
mechanism and so there is an operator can
be less conservative when deploying network capacity, inherent delay between packet marking
by PCN-interior-nodes and less
accurate in their prediction any admission control reaction at PCN-
boundary-nodes. For example, potentially if a big burst of the PCN-traffic matrix.

o The termination mechanism complements
admission control. It
allows the network to recover from sudden unexpected surges requests occurs in a very short space of
PCN-traffic on some links, thus restoring QoS to the remaining
flows. Such scenarios are expected to be rare but not impossible.
They can be caused time (eg
prompted by large network failures that redirect lots of a televote), they could all get admitted PCN-traffic before enough
PCN-marks are seen to block new flows. In other links, or by malfunction words, any
additional load offered within the reaction time of the
measurement-based admission control in mechanism
must not move the presence of admitted
flows that send for a while with an atypically low rate and then
increase their rates in PCN-domain directly from a correlated way.

o Flow termination can also enable an operator no congestion state
to be less
conservative when deploying network capacity. It is overload. This 'vulnerability period' may have an
alternative to running links impact at low utilisation in order to
protect against link or node failures. This is especially the
case with SRLGs (shared risk link groups, which are links that
share a resource, such as a fibre, whose failure affects all those
links [RFC4216]). Fully protecting traffic against a single SRLG
failure requires low utilisation (~10%) of
the link bandwidth on
some links before failure [Charny08].

o The PCN-supportable-rate may signalling level, for instance QoS requests should be set below the maximum rate that
PCN-traffic can be transmitted on a link, in order
limited to trigger
termination of some PCN-flows before loss (or excessive delay) bound the number of
PCN-packets occurs, or requests able to keep arrive within the maximum PCN-load on
vulnerability period.

o Silent at start: after a link
below a level configured by successful admission request the operator.

o Provisioning of source
may wait some time before sending data (eg waiting for the network called
party to answer). Then the risk is decoupled that, in some circumstances,
PCN's measurements underestimate what the pre-congestion level
will be when the source does start sending data.

7. IANA Considerations

This memo includes no request to IANA.

8. Security considerations

Security considerations essentially come from the process of
adding new customers. By contrast, with Trust Assumption
(Section 6.3.1), ie that all PCN-nodes are PCN-enabled and are
trusted for truthful PCN-metering and PCN-marking. PCN splits
functionality between PCN-interior-nodes and PCN-boundary-nodes, and
the DiffServ architecture
[RFC2475] operators rely on subscription-time Service Level
Agreements, which statically define security considerations are somewhat different for each, mainly
because PCN-boundary-nodes are flow-aware and PCN-interior-nodes are
not.

o Because the parameters PCN-boundary-nodes are flow-aware, they are trusted to
use that awareness correctly. The degree of trust required
depends on the traffic
that will be accepted from a customer, kinds of decisions they have to make and so the operator has kinds
of information they need to
verify provision make them. There is sufficient each time nothing specific
to PCN.

o The PCN-ingress-nodes police packets to ensure a new customer is added PCN-flow sticks
within its agreed limit, and to check ensure that only PCN-flows that
have been admitted contribute PCN-traffic into the Service Level Agreement can be fulfilled. A
PCN-domain doesn't need such traffic conditioning.

12.2. Deployment scenarios

Operators of networks will want PCN-domain.
The policer must drop (or perhaps downgrade to use the PCN mechanisms in various
arrangements, for instance depending on how they a different DSCP)
any PCN-packets received that are performing
admission control outside this remit. This is
similar to the PCN-domain (users after all are
concerned about QoS end-to-end), what their particular goals and
assumptions are, how many PCN encoding states are available, and so existing IntServ behaviour. Between them the PCN-

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on.

boundary-nodes must encircle the PCN-domain, otherwise PCN-packets
could enter the PCN-domain may have three encoding states (or pedantically, an
operator may choose without being subject to use up three encoding states for PCN): not
PCN-marked, threshold-marked, excess-traffic-marked. Then both PCN
admission control and flow termination can be supported. As
illustrated in Figure 1, admission control accepts new
control, which would potentially destroy the QoS of existing
flows.

o PCN-interior-nodes are not flow-aware. This prevents some
security attacks where an attacker targets specific flows until in the PCN-traffic rate
data plane - for instance for DoS or eavesdropping.

o The PCN-boundary-nodes rely on the bottleneck link rises above correct PCN-marking by the PCN-
threshold-rate, whilst if necessary the flow termination mechanism
terminates
interior-nodes. For instance a rogue PCN-interior-node could PCN-
mark all packets so that no flows down to were admitted. Another
possibility is that it doesn't PCN-mark any packets, even when it
is pre-congested. More subtly, the PCN-excess-rate rogue PCN-interior-node could
perform these attacks selectively on particular flows, or it could
PCN-mark the bottleneck link.

On the other hand, a PCN-domain may have two encoding states (as in
[PCN08-1]) (or pedantically, an operator may correct fraction overall, but carefully choose which
flows it marked.

o The PCN-boundary-nodes should be able to use up two
encoding states for PCN): not PCN-marked, PCN-marked. Then there are
three possibilities, as discussed deal with DoS attacks and
state exhaustion attacks based on fast changes in per flow
signalling.

o The signalling between the following paragraphs (see
also Section 3.3).

First, an operator could just use PCN's admission control, solving
heavy congestion (caused by re-routing) by 'just waiting' - as
sessions end, PCN-traffic naturally reduces, and meanwhile PCN-boundary-nodes must be protected
from attacks. For example the
admission control mechanism will prevent admission of new flows recipient needs to validate that
use
the affected links. So message is indeed from the PCN-domain will naturally return node that claims to
normal operation, but with reduced capacity. The drawback of this
approach would be that, until sufficient sessions have ended to
relieve sent it.
Possible measures include digest authentication and protection
against replay and man-in-the-middle attacks. For the congestion, all PCN-flows as well as lower priority
services will specific
protocol RSVP, hop-by-hop authentication is in [RFC2747], and
[Behringer07] may also be adversely affected.

Second, an operator could just rely useful.

Operational security advice is given in Section 5.5.

9. Conclusions

The document describes a general architecture for flow admission control and
termination based on
statically provisioned capacity per PCN-ingress-node (regardless of pre-congestion information in order to protect
the PCN-egress-node quality of service of established inelastic flows within a flow), as single
Diffserv domain. The main topic is typical in the hose model of functional architecture. It
also mentions other topics like the DiffServ architecture [RFC2475]. Such traffic conditioning
agreements can lead to focused overload: many flows happen to focus
on a particular link assumptions and then all flows through the congested link
fail catastrophically. PCN's flow termination mechanism could then
be used to counteract such a problem.

Third, both admission control and flow termination can be triggered
from the single type of PCN-marking; the main downside is that
admission control is less accurate [Charny07-2]. open issues.

10. Acknowledgements

This possibility is
illustrated in Figure 3.

Within the PCN-domain there is some flexibility about how the
decision making functionality document is distributed. These possibilities a revised version of an earlier individual draft
authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R. Geib, G.
Karagiannis, M. Menth, T. Tsou. They are outlined in Section 4.4 and also discussed elsewhere, such as in
[Menth08-3].

The flow admission and termination decisions need therefore contributors to be enforced
through per flow policing by the PCN-ingress-nodes. If there are
this document.

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several PCN-domains

Thanks to those who have made comments on the end-to-end path, then each needs this document: Lachlan
Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake, Ron
Bonica, Scott Bradner, Bob Briscoe, Ross Callon, Jason Canon, Ken
Carlberg, Anna Charny, Joachim Charzinski, Andras Csaszar, Francis
Dupont, Lars Eggert, Pasi Eronen, Adrian Farrel, Ruediger Geib, Wei
Gengyu, Robert Hancock, Fortune Huang, Christian Hublet, Cullen
Jennings, Ingemar Johansson, Georgios Karagiannis, Hein Mekkes,
Michael Menth, Toby Moncaster, Dimitri Papadimitriou, Dan Romascanu,
Daisuke Satoh, Ben Strulo, Tom Taylor, Hannes Tschofenig, Tina Tsou,
David Ward, Lars Westberg, Magnus Westerlund, Delei Yu. Thanks to police
at its PCN-ingress-nodes. One exception
Bob Briscoe who extensively revised the Operations and Management
section.

This document is if the operator runs both result of discussions in the access network (not a PCN-domain) PCN WG and
forerunner activity in the core network (a PCN-
domain); per flow policing could be devolved TSVWG. A number of previous drafts were
presented to the access network TSVWG; their authors were: B, Briscoe, P. Eardley, D.
Songhurst, F. Le Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley,
G. Karagiannis, A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G.
Liu, A. Bhargava.

11. Comments Solicited (to be removed by RFC Editor)

Comments and not done at the PCN-ingress-node. Note: questions are encouraged and very welcome. They can be
addressed to aid readability, the
rest of this draft assumes that policing is done by the PCN-ingress-
nodes. IETF PCN admission control has working group mailing list <pcn@ietf.org>.

12. Changes (to be removed by RFC Editor)

12.1. Changes from -10 to fit -11

Changes to deal with the overall approach IESG comments from routing area review:

o Small clarifications to
admission control. For instance [Briscoe06] describes Introduction

o the case where
RSVP signalling runs end-to-end. The PCN-domain is a single RSVP
hop, ie term "marking" now only used to refer only to setting the PCN-boundary-nodes process RSVP messages, with RSVP
messages processed on each hop outside the PCN-domain,
codepoint (not as in IntServ
over DiffServ [RFC2998]. It would also be possible a shorthand for the RSVP
signalling to be originated and/or terminated by proxies, with
application-layer signalling between the end user 'metering and setting the proxy (eg
SIP signalling with a home hub). A similar example would use NSIS
signalling instead
codepoint')

o Added Figure 4 (Schematic of RSVP. (NSIS: Next Steps in Signalling,
[RFC3726].)

It is possible that a user wants its inelastic traffic to use PCN-interior-node functionality)
(from [PCN08-2]

o Appendix A brought back into the main body.

o Other minor clarifications

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mechanisms but also react Architecture April 2009

12.2. Changes from -09 to ECN marking outside the PCN-domain
[Sarker08]. Two possible ways -10

Changes to do this are deal with IESG comments:

o New introduction to tunnel all PCN-
packets across the PCN-domain, so that the ECN marks are carried
transparently across provide gentler introduction for the PCN-domain, or to use an encoding like
[Moncaster08]. Tunnelling is discussed further PCN
novice: quick summary of PCN's applicability; quick example of how
it all hangs together in Section 4.7.

Some further possible deployment models are outlined one end-to-end qos scenario; quick
summary of PCN "documentation"

o OAM changed to Operations and Management

o Processed some of the minor suggestions in the Appendix.

12.3. Assumptions and constraints on scope

The scope is restricted Gen-ART Review by the following assumptions:

1. these components are deployed
Francis Dupont

o Two wording tweaks in a single DiffServ domain, within
which all PCN-nodes are PCN-enabled Sections 3.2 & 3.4 (as agreed on mailing
list)

o Updated boilerplate. this draft may include material pre- Nov 10
2008 blah.

12.3. Changes from -08 to -09

Small changes to deal with WG Chair comments:

o tweak language in various places to make it more RFC-like and are trusted less
that of a scholarly work, for truthful
PCN-marking and transport

2. all flows handled by these mechanisms are inelastic and
constrained instance from "we propose" to a known peak rate through policing or shaping

3. the number of PCN-flows across any potential bottleneck link is
sufficiently large that stateless, statistical mechanisms can be
effective. To put "this
document describes"

o tweak language in various places to make it another way, a stand alone
architecture document rather than a discussion of the aggregate bit rate PCN WG. Now
only mentions WG at start of Annex.

o References: IDs are no longer referenced to by the draft name

o References: removed some of less important references to IDs

12.4. Changes from -07 to -08

Small changes from second WG last call:

o Section 2: added definition for PCN-admissible-rate and PCN-
traffic across any potential bottleneck link needs
supportable-rate. Small changes to be
sufficiently large relative use these terms as follows:
Section 3, bullets 2 & 9; S6.1 para 1; S6.2 para1; S6.3 bullet 3;
added to the maximum additional bit rate Figs 1 & 2.

o added by one flow. This is the basic assumption phrase "(others might be possible") before the list of measurement-
based admission control.
approaches in Section 6.3, 7.4 & 7.5.

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4. PCN-flows may have different precedence, but the applicability of
the PCN mechanisms

o added references to RFC2753 (A framework for emergency use (911, GETS, WPS, MLPP, etc.)
is out of scope.

12.3.1. Assumption 1: Trust and support of PCN - controlled environment

We assume that the PCN-domain is a controlled environment, ie all the
nodes policy-based
admission control) in a PCN-domain run PCN and are trusted. There are several
reasons this assumption: S7.4 & S7.5.

o The PCN-domain has to be encircled by a ring of PCN-boundary-
nodes, otherwise traffic could enter a PCN-BA without being
subject to admission control, which would potentially degrade the
QoS of existing PCN-flows. throughout, updated references now that marking behaviour &
baseline encoding are WG drafts.

o Similarly, a PCN-boundary-node has few typos corrected

12.5. Changes from -06 to trust that all the PCN-nodes
mark PCN-traffic consistently. A node not performing PCN-marking
wouldn't be able -07

References re-formatted to alert when it suffered pre-congestion, which
potentially would lead pass ID nits. No other changes.

12.6. Changes from -05 to too many PCN-flows being admitted (or
too few being terminated). Worse, a rogue node could perform
various attacks, -06

Minor clarifications throughout, the least insignificant are as discussed in
follows:

o Section 1: added to the Security Considerations
section.

One way list of assuring the above two points is encoding states in an 'extended'
scheme: "or perhaps further encoding states as suggested in
draft-westberg-pcn-load-control"

o Section 2: added definition for PCN-colouring (to clarify that the entire PCN-
domain
term is run by used consistently differently from 'PCN-marking')

o Section 6.1 and 6.2: added "(others might be possible)" before the
list of high level approaches for making flow admission
(termination) decisions.

o Section 6.2: corrected a single operator. Another possibility is that
there are several operators that trust each other significant typo in their handling 2nd bullet (more ->
less)

o Section 6.3: corrected a couple of PCN-traffic.

Note: All PCN-nodes need significant typos in Figure 2

o Section 6.5 (PCN-traffic) re-written for clarity. Non PCN-traffic
contributing to be trustworthy. However if it PCN meters is known
that now given as an interface cannot become pre-congested then it is not strictly
necessary for it to be capable of PCN-marking. But this must example (there may
be
known even in unusual circumstances, eg after the failure of some
links.

12.3.2. Assumption 2: Real-time applications

We assume that any variation of source bit rate is independent of the
level of pre-congestion. We assume that PCN-packets come from real
time applications generating inelastic traffic, ie sending packets at
the rate cases where don't need to meter it).

o Section 7.7: added to the codec produces them, regardless of text about encapsulation being done
within the availability PCN-domain: "Note: A tunnel will not provide this
behaviour if it complies with [RFC3168] tunnelling in either mode,
but it will if it complies with [RFC4301] IPSec tunnelling."

o Section 7.7: added mention of
capacity [RFC4594]. For example, voice and video requiring low
delay, jitter and packet loss, the Controlled Load Service,
[RFC2211], and the Telephony service class, [RFC4594]. This
assumption is [RFC4301] to help focus the effort where it looks like PCN would
be most useful, ie text about
decapsulation being done within the sorts of applications where per flow QoS PCN-domain.

o Section 8: deleted the text about design goals, since this is a
known requirement. In other words we focus on PCN providing a
benefit to inelastic traffic (PCN may or may not provide a benefit to
other types of traffic).
already covered adequately earlier eg in S3.

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As a consequence, it is assumed that PCN-marking

o Section 11: replaced the last sentence of bullet 1 by "There is being applied
nothing specific to
traffic scheduled with the expedited forwarding per-hop behaviour,
[RFC3246], or a per-hop behaviour with similar characteristics.

12.3.3. Assumption 3: Many flows PCN."

o Appendix: added to open issues: possibility of automatically and additional load

We assume
periodically probing.

o References: Split out Normative references (RFC2474 & RFC3246).

12.7. Changes from -04 to -05

Minor nits removed as follows:

o Further minor changes to reflect that baseline encoding is
consensus, standards track document, whilst there are many PCN-flows on any bottleneck link in the
PCN-domain (or, can be
(experimental track) encoding extensions

o Traffic conditioning updated to put reflect discussions in Dublin,
mainly that PCN-interior-nodes don't police PCN-traffic (so
deleted bullet in S7.1) and that it another way, the aggregate bit rate of PCN-
traffic across any potential bottleneck link is sufficiently large
relative not advised to the maximum additional bit rate added by one PCN-flow).
Measurement-based admission control assumes have non
PCN-traffic that shares the present is same capacity (on a
reasonable prediction of the future: link) as PCN-
traffic (so added bullet in S6.5)

o Probing moved into Appendix A and deleted the network conditions are
measured at 'third viewpoint'
(admission control based on the time marking of a new flow request, however the actual
network performance must be acceptable during the call some time
later. One issue single packet like an
RSVP PATH message) - since this isn't really probing, and in any
case is already mentioned in S6.1.

o Minor changes to S9 Operations and management - mainly to reflect
that if consensus on marking behaviour has simplified things so eg
there are only a fewer parameters to configure.

o A few variable rate
flows, then the aggregate traffic level may vary a lot, perhaps
enough to cause some packets terminology-related errors expunged, and two pictures added
to get dropped. If there are many flows
then help.

o Re-phrased the aggregate traffic level should be statistically smoothed.
How many flows is enough depends on a number of factors such as claim about the
variation natural decision point in each flow's rate, the total rate of PCN-traffic, S7.4

o Clarified that extended encoding schemes need to explain their
interactions with (or assumptions about) tunnelling (S7.7) and how
they meet the
size guidelines of BCP124 (S6.6)

o Corrected the "safety margin" between the traffic level at which we
start admission-marking and at which packets are dropped or
significantly delayed.

We do not make explicit assumptions on how many PCN-flows are third bullet in each
ingress-egress-aggregate. Performance evaluation work may clarify
whether it is necessary S6.2 (to reflect consensus about
PCN-marking)

12.8. Changes from -03 to make any additional assumption on
aggregation at the ingress-egress-aggregate level.

12.3.4. Assumption 4: Emergency use out of scope

PCN-flows may have different precedence, but the applicability of -04

o Minor changes throughout to reflect the
PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc) is out
of scope of this document.

12.4. Challenges

Prior work on PCN and similar mechanisms has thrown up a number of
considerations consensus call about PCN's design goals (things PCN-
marking (as reflected in [PCN08-2]).

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at) and some issues that have been hard Architecture April 2009

o Minor changes throughout to solve in a fully
satisfactory manner. Taken as a whole it represents a list of trade-
offs (it is unlikely that they can all be 100% achieved) and perhaps
as evaluation criteria to help an operator (or reflect the IETF) decide
between options.

The following are open issues. They are mainly taken from
[Briscoe06], which also describes some possible solutions. Note that
some may be considered unimportant in general or current decisions about
encoding (as reflected in specific

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deployment [PCN08-1] and [Moncaster08]).

o Introduction: re-structured to create new sections on Benefits,
Deployment scenarios or by some operators.

NOTE: Potential solutions are out of scope for this and Assumptions.

o Introduction: Added pointers to other PCN documents.

o Terminology: changed PCN-lower-rate to PCN-threshold-rate and PCN-
upper-rate to PCN-excess-rate; excess-rate-marking to excess-
traffic-marking.

o Benefits: added bullet about SRLGs.

o Deployment scenarios: new section combining material from various
places within the document.

o ECMP (Equal Cost Multi-Path) Routing: The S6 (high level of pre-congestion
is measured on a specific ingress-egress-aggregate. However, if functional architecture): re-structured and edited
to improve clarity, and reflect the PCN-domain runs ECMP, then traffic on this ingress-egress-
aggregate may follow several different paths - some of latest PCN-marking and
encoding drafts.

o S6.4: added claim that the paths
could be pre-congested whilst others are not. There are three
potential problems:

1. over-admission: a new flow most natural place to make an admission
decision is admitted (because a PCN-egress-node.

o S6.5: updated the pre-
congestion level measured by bullet about non-PCN-traffic that uses the PCN-egress-node is
sufficiently diluted by unmarked packets from non-congested
paths that a new flow is admitted), but its packets travel
through a pre-congested PCN-node.

2. under-admission: same
DSCP as PCN-traffic.

o S6.6: added a new flow is blocked (because the pre-
congestion level measured section about backwards compatibility with respect
to [RFC4774].

o Appendix A: added bullet about end-to-end PCN.

o Probing: moved to Appendix B.

o Other minor clarifications, typos etc.

12.9. Changes from -02 to -03

o Abstract: Clarified by removing the PCN-egress-node is
sufficiently increased by PCN-marked packets from pre-
congested paths that a new flow is blocked), but its packets
travel along an uncongested path.

3. ineffective termination: a flow is terminated, but its path
doesn't travel through term 'aggregated'. Follow-up
clarifications later in draft: S1: expanded PCN-egress-nodes
bullet to mention case where the (pre-)congested router(s). Since
flow termination PCN-feedback-information is about
one (or a 'last resort', which protects the
network should over-admission occur, this problem is probably
more important to solve few) PCN-marks, rather than the other two. aggregated information; S3
clarified PCN-meter; S5 minor changes; conclusion.

o ECMP and signalling: It is possible that, in a PCN-domain running
ECMP, the signalling packets (eg RSVP, NSIS) follow S1: added a different
path than the data packets, which could matter if the signalling
packets are used as probes. Whether this is an issue depends on
which fields the ECMP algorithm uses; if paragraph about how the ECMP algorithm is
restricted PCN-domain looks to the source and destination IP addresses, then
outside world (essentially it
will not be an issue. ECMP and signalling interactions are a
specific instance of looks like a general issue for non-traditional routing
combined with resource management along a path [Hancock02].

o Tunnelling: There are scenarios where tunnelling makes it
difficult to determine the path in the PCN-domain. The problem,
its impact, and the potential solutions are similar to those for
ECMP.

o Scenarios with only one tunnel endpoint in the PCN domain may make
it harder for the PCN-egress-node to gather from the signalling
messages (eg RSVP, NSIS) the identity of the PCN-ingress-node. Diffserv domain).

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o Bi-Directional Sessions: Many applications have bi-directional
sessions - hence there are two microflows that should be admitted
(or terminated) as a pair - for instance a bi-directional voice
call only makes sense if microflows in both directions are
admitted. However, S2: tweaked the PCN-traffic terminology bullet: changed PCN mechanisms concern admission and
termination of
traffic classes to PCN behaviour aggregates, to be more in line
with traditional Diffserv jargon (-> follow-up changes later in
draft); included a single flow, and coordination definition of the decision for
both flows is PCN-flows (and corrected a matter for the signalling protocol and out couple
of
scope 'PCN microflows' to 'PCN-flows' later in draft)

o S3.5: added possibility of PCN. One possible example would use SIP pre-conditions.
However, there are others. downgrading to best effort, where PCN-
packets arrive at PCN-ingress-node already ECN marked (CE or ECN
nonce)

o Global Coordination: S4: added note about whether talk about PCN makes its admission decision based operating on
PCN-markings an
interface or on a particular ingress-egress-aggregate. Decisions
about flows through a different ingress-egress-aggregate are made
independently. However, one can imagine network topologies and
traffic matrices where, from a global perspective, it would be
better link. In S8.1 (OAM) mentioned that PCN
functionality needs to make a coordinated decision across all be configured consistently on either the ingress-
egress-aggregates for
ingress or the whole egress interface of PCN-nodes in a PCN-domain. For example, to block
(or even terminate) flows on one ingress-egress-aggregate so

o S5.2: clarified that
more important flows through signalling protocol installs flow filter spec
at PCN-ingress-node (& updates after possible re-route)

o S5.6: addressing: clarified

o S5.7: added tunnelling issue of N^2 scaling if you set up a different ingress-egress-aggregate
could be admitted. The problem may well be relatively
insignificant. mesh
of tunnels between PCN-boundary-nodes

o Aggregate Traffic Characteristics: Even when S7.3: Clarified the number "third viewpoint" of flows
is stable, the traffic level through the PCN-domain will vary
because the sources vary their traffic rates. PCN works best when
there probing (always probe).

o S8.1: clarified that SNMP is not too much variability in the total traffic level at a
PCN-node's interface (ie in the aggregate traffic from all
sources). Too much variation means only an example; added note that a node an
operator may (at one
moment) not be doing any PCN-marking and then (at another moment)
drop packets because able to not run PCN on some PCN-interior-nodes, if
it is overloaded. This makes knows that these links will never become (pre-)congested; added
note that it hard may be possible to tune have different PCN-boundary-node
behaviours for different ingress-egress-aggregates within the admission control scheme to stop admitting new flows at same
PCN-domain.

o Appendix: Created an Appendix about "Possible work items beyond
the
right time. Therefore the problem is more likely with fewer,
burstier flows.

o Flash crowds and Speed scope of Reaction: the current PCN is a measurement-based
mechanism and so there is an inherent delay between packet marking
by PCN-interior-nodes and any admission control reaction at PCN-
boundary-nodes. For example, potentially if a big burst of
admission requests occurs in a very short space WG Charter". Material moved from
near start of time (eg
prompted by a televote), they could all get admitted before enough
PCN-marks are seen S3 and elsewhere throughout draft. Moved text about
centralised decision node to block new flows. In other words, any
additional load offered within the reaction time of the mechanism
must not move the PCN-domain directly Appendix.

o Other minor clarifications.

12.10. Changes from a no congestion state -01 to overload. This 'vulnerability period' may have an impact at
the signalling level, for instance QoS requests should be rate
limited -02

o S1: Benefits: provisioning bullet extended to bound the number stress that PCN does
not use RFC2475-style traffic conditioning.

o S1: Deployment models: mentioned, as variant of requests able PCN-domain
extending to arrive within the
vulnerability period. end nodes, that may extend to LAN edge switch.

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o Silent at start: after a successful admission request the source
may wait some time before sending data (eg waiting S3.1: Trust Assumption: added note about not needing PCN-marking
capability if known that an interface cannot become pre-congested.

o S4: now divided into sub-sections

o S4.1: Admission control: added second proposed method for the called
party how to answer). Then the risk is that,
decide to block new flows (PCN-egress-node receives one (or
several) PCN-marked packets).

o S5: Probing sub-section removed. Material now in some circumstances,
PCN's measurements underestimate what the pre-congestion level
will be when the source does start sending data.

13. Appendix B: Possible future work items

13.1. Benefits

We believe new S7.

o S5.6: Addressing: clarified how PCN-ingress-node can discover
address of PCN-egress-node

o S5.6: Addressing: centralised node case, added that the key benefits PCN-ingress-
node may need to know address of the PCN mechanisms described in PCN-egress-node

o S5.8: Tunnelling: added case of "partially PCN-capable tunnel" and
degraded bullet on this document are that they are simple, scalable, in S6 (Open Issues)

o S7: Probing: new section. Much more comprehensive than old S5.5.

o S8: Operations and robust because: Management: substantially revised.

o Per flow state is only required at the PCN-ingress-nodes
("stateless core"). This is required for policing purposes (to
prevent non-admitted PCN traffic other minor changes not affecting semantics

12.11. Changes from entering the PCN-domain) -00 to -01

In addition to clarifications and
so on. It nit squashing, the main changes
are:

o S1: Benefits: added one about provisioning (and contrast with
Diffserv SLAs)

o S1: Benefits: clarified that the objective is also to stop PCN-
packets being significantly delayed (previously only mentioned not generally required that other
dropping packets)

o S1: Deployment models: added one where policing is done at ingress
of access network entities
are aware and not at ingress of individual flows (although they may be in particular
deployment scenarios). PCN-domain (assume trust
between networks)

o Admission control is resilient: with PCN QoS is decoupled from the
routing system. Hence in general admitted flows can survive
capacity, routing or topology changes without additional
signalling. The PCN-admissible-rate on each link can be chosen
small enough that admitted traffic can still be carried after a
rerouting in most failure cases [Menth07]. This is an important
feature as QoS violations in core networks due to link failures
are more likely than QoS violations due S1: Deployment models: corrected MPLS-TE to increased traffic
volume [Iyer03].

o The PCN-marking behaviours only operate on the overall PCN-traffic
on the link, not per flow. MPLS

o The information S2: Terminology: adjusted definition of these measurements is signalled to the PCN-
egress-nodes by the PCN-marks in the packet headers, ie [Style]
"in-band". No additional signalling protocol is required for
transporting the PCN-marks. Therefore no secure binding is
required between data packets and separate congestion messages. PCN-domain

o The PCN-egress-nodes make separate measurements, operating on the
aggregate PCN-traffic from each PCN-ingress-node, ie not per flow.
Similarly, signalling by the PCN-egress-node of PCN-feedback-
information (which is used for flow admission and termination
decisions) is at the granularity of the ingress-egress-aggregate.
An alternative approach is S3.5: Other assumptions: corrected, so that the PCN-egress-nodes monitor the
PCN-traffic and signal PCN-feedback-information (which is used for
flow admission two assumptions (PCN-
nodes not performing ECN and termination decisions) at the granularity of PCN-ingress-node discarding arriving

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one (or a few) PCN-marks.

o The admitted PCN-load is controlled dynamically. Therefore it
adapts as the traffic matrix changes, and also

CE packet) only apply if the network
topology changes (eg after a link failure). Hence an operator can
be less conservative when deploying network capacity, and less
accurate PCN WG decides to encode PCN-marking
in their prediction of the PCN-traffic matrix. ECN-field.

o The termination mechanism complements admission control. It
allows S4 & S5: changed PCN-marking algorithm to marking behaviour

o S4: clarified that PCN-interior-node functionality applies for
each outgoing interface, and added clarification: "The
functionality is also done by PCN-ingress-nodes for their outgoing
interfaces (ie those 'inside' the network PCN-domain)."

o S4 (near end): altered to recover from sudden unexpected surges of
PCN-traffic on say that a PCN-node "should" dedicate
some links, thus restoring QoS to the remaining
flows. Such scenarios are expected capacity to be rare but not impossible.
They can be caused by large network failures lower priority traffic so that redirect lots of
admitted PCN-traffic it isn't starved
(was "may")

o S5: clarified to other links, or by malfunction of the
measurement-based admission control in the presence of admitted
flows say that send for a while with PCN functionality is done on an atypically low rate and then
increase their rates in
'interface' (rather than on a correlated way. 'link')

o Flow termination can also enable an operator S5.2: deleted erroneous mention of service level agreement

o S5.5: Probing: re-written, especially to be less
conservative when deploying network capacity. It is an
alternative distinguish probing to running links at low utilisation in order
test the ingress-egress-aggregate from probing to
protect against link or node failures. This is especially test a
particular ECMP path.

o S5.7: Addressing: added mention of probing; added that in the case with SRLGs (shared risk link groups, which are links that
share a resource, such as a fibre, whose failure affects all those
links [RFC4216]). Fully protecting
where traffic against is always tunnelled across the PCN-domain, add a single SRLG
failure requires low utilisation (~10%)
note that he PCN-ingress-node needs to know the address of the link bandwidth on
some links before failure [Charny08].
PCN-egress-node.

o The PCN-supportable-rate may be set below the maximum rate that
PCN-traffic can be transmitted on a link, in order S5.8: Tunnelling: re-written, especially to trigger
termination of some PCN-flows before loss (or excessive delay) provide a clearer
description of
PCN-packets occurs, or to keep the maximum PCN-load copying on a link
below a level configured tunnel entry/exit, by the operator.

o Provisioning of the network is decoupled from the process of adding new customers. By contrast, with the DiffServ architecture
[RFC2475] operators rely on subscription-time Service Level
Agreements, which statically define the parameters of the traffic
that will be accepted from a customer, explanation
(keeping tunnel encaps/decaps and so PCN-marking orthogonal),
deleting one bullet ("if the operator has to
verify provision inner header's marking state is sufficient each time a new customer more
sever then it is added
to check that the Service Level Agreement can be fulfilled. A
PCN-domain doesn't need such traffic conditioning.

This section mentions some topics that are outside the PCN WG's
current charter, but which have been mentioned as areas preserved" - shouldn't happen), and better
referencing of interest.
They might be work items for: the PCN WG after a future re-
chartering; some other IETF WG; another standards body; an operator-
specific usage documents.

o S6: Open issues: stressed that is not standardised. "NOTE: Potential solutions are out
of scope for this document" and edited a couple of sentences that
were close to solution space.

o S6: Open issues: added one about scenarios with only one tunnel
endpoint in the PCN domain .

o S6: Open issues: ECMP: added under-admission as another potential
risk

o S6: Open issues: added one about "Silent at start"

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NOTE: it should be crystal clear that this section discusses
possibilities only.

The first set of possibilities relate to the restrictions described
in Section 12.3:

o a single PCN-domain encompasses several autonomous systems that do
not trust each other, perhaps by using a mechanism like re-PCN,
[Briscoe08-1].

o not all the nodes run PCN. For example, the PCN-domain is a
multi-site enterprise network. The sites are connected by S10: Conclusions: a VPN
tunnel; although PCN doesn't operate inside the tunnel, the PCN
mechanisms still work properly because small conclusions section added

13. References

13.1. Normative References

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the good QoS on Differentiated Services Field (DS
Field) in the
virtual link (the tunnel). Another example is that PCN is
deployed on IPv4 and IPv6 Headers", RFC 2474,
December 1998.

[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.

13.2. Informative References

[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the general Internet (ie widely but not universally
deployed).

o applying the PCN mechanisms to other types of traffic, ie beyond
inelastic traffic. For instance, applying the PCN mechanisms to
traffic scheduled with the Assured Forwarding per-hop behaviour.
One example could be flow-rate adaptation by elastic applications
that adapt according to the pre-congestion information.

o the aggregation assumption doesn't hold, because the link capacity
is too low. Measurement-based admission control is less accurate,
with a greater risk of over-admission for instance.

o the applicability of PCN mechanisms for emergency use (911, GETS,
WPS, MLPP, etc.)

Other possibilities include:

o Probing. This is discussed in Section 13.1 below.

o The PCN-domain extends to the end users. The scenario is
described in [Babiarz06]. The end users need to be trusted to do
their own policing. If there is sufficient traffic, then the
aggregation assumption may hold. A variant is that the PCN-domain
extends out as far as the LAN edge switch.

o indicating pre-congestion through signalling messages rather than
in-band (in the form of PCN-marked packets)

o the decision-making functionality is at a centralised node rather
than at the PCN-boundary-nodes. This requires that the PCN-
egress-node signals PCN-feedback-information to the centralised
node, and that the centralised node signals to the PCN-ingress-

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node the decision about admission (or termination). It may need
the centralised node and the PCN-boundary-nodes to be configured
with each other's addresses. The centralised case is described
further in [Tsou08].

o Signalling extensions for specific protocols (eg RSVP, NSIS). For
example: the details of how the signalling protocol installs the
flowspec at the PCN-ingress-node for an admitted PCN-flow; and how
the signalling protocol carries the PCN-feedback-information.
Perhaps also for other functions such as: coping with failure of a
PCN-boundary-node ([Briscoe06] considers what happens if RSVP is
the QoS signalling protocol); establishing a tunnel across the
PCN-domain if it is necessary to carry ECN marks transparently.

o Policing by the PCN-ingress-node may not be needed if the PCN-
domain can trust that the upstream network has already policed the
traffic on its behalf.

o PCN for Pseudowire: PCN may be used as a congestion avoidance
mechanism for edge to edge pseudowire emulations [PWE3-08].

o PCN for MPLS: [RFC3270] defines how to support the DiffServ
architecture in MPLS networks (Multi-protocol label switching).
[RFC5129] describes how to add PCN for admission control of
microflows into a set of MPLS aggregates. PCN-marking is done in
MPLS's EXP field (which [MPLS08] re-names the Class of Service
(CoS) field).

o PCN for Ethernet: Similarly, it may be possible to extend PCN into
Ethernet networks, where PCN-marking is done in the Ethernet
header. NOTE: Specific consideration of this extension is outside
the IETF's remit.

13.2. Probing

13.2.1. Introduction

Probing is a potential mechanism to assist admission control.

PCN's admission control, as described so far, is essentially a
reactive mechanism where the PCN-egress-node monitors the pre-
congestion level for traffic from each PCN-ingress-node; if the level
rises then it blocks new flows on that ingress-egress-aggregate.
However, it's possible that an ingress-egress-aggregate carries no
traffic, and so the PCN-egress-node can't make an admission decision
using the usual method described earlier.

One approach is to be "optimistic" and simply admit the new flow.

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However it's possible to envisage a scenario where the traffic levels
on other ingress-egress-aggregates are already so high that they're
blocking new PCN-flows, and admitting a new flow onto this 'empty'
ingress-egress-aggregate adds extra traffic onto a link that is
already pre-congested - which may 'tip the balance' so that PCN's
flow termination mechanism is activated or some packets are dropped.
This risk could be lessened by configuring on each link sufficient
'safety margin' above the PCN-threshold-rate.

An alternative approach is to make PCN a more proactive mechanism.
The PCN-ingress-node explicitly determines, before admitting the
prospective new flow, whether the ingress-egress-aggregate can
support it. This can be seen as a "pessimistic" approach, in
contrast to the "optimism" of the approach above. It involves
probing: a PCN-ingress-node generates and sends probe packets in
order to test the pre-congestion level that the flow would
experience.

One possibility is that a probe packet is just a dummy data packet,
generated by the PCN-ingress-node and addressed to the PCN-egress-
node.

13.2.2. Probing functions

The probing functions are:

o Make decision that probing is needed. As described above, this is
when the ingress-egress-aggregate (or the ECMP path - Section
12.4) carries no PCN-traffic. An alternative is always to probe,
ie probe before admitting every PCN-flow.

o (if required) Communicate the request that probing is needed - the
PCN-egress-node signals to the PCN-ingress-node that probing is
needed

o (if required) Generate probe traffic - the PCN-ingress-node
generates the probe traffic. The appropriate number (or rate) Architecture: an Overview",
RFC 1633, June 1994.

[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.

[RFC2211] Wroclawski, J., "Specification of
probe packets will depend on the PCN-marking algorithm; Controlled-Load
Network Element Service", RFC 2211, September 1997.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for
example an excess-traffic-marking algorithm generates fewer PCN-
marks than a threshold-marking algorithm, Differentiated
Services", RFC 2475, December 1998.

[RFC2747] Baker, F., Lindell, B., and so will need more
probe packets.

o Forward probe packets - as far as PCN-interior-nodes are
concerned, probe packets are handled the same as (ordinary data)
PCN-packets, in terms of routing, scheduling M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.

[RFC2753] Yavatkar, R., Pendarakis, D., and PCN-marking.

o Consume probe packets - the PCN-egress-node consumes probe packets
to ensure that they don't travel beyond the PCN-domain. R. Guerin, "A Framework
for Policy-based Admission Control", RFC 2753,
January 2000.

[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, October 2000.

[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework for Integrated Services Operation
over Diffserv Networks", RFC 2998, November 2000.

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13.2.3. Discussion

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of rationale Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.

[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.

[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for probing, its downsides IP Performance Metrics (IPPM)", RFC 3393,
November 2002.

[RFC3411] Harrington, D., Presuhn, R., and open
issues

It is an unresolved question whether probing is really needed, but
two viewpoints have been put forward as to why it is useful. The
first is perhaps B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.

[RFC3726] Brunner, M., "Requirements for Signaling Protocols",
RFC 3726, April 2004.

[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the most obvious: there is no PCN-traffic on
Internet Protocol", RFC 4301, December 2005.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.

[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
August 2006.

[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.

[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
ingress-egress-aggregate. The second assumes that multipath routing
ECMP is running
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.

[RFC4778] Kaeo, M., "Operational Security Current Practices in the PCN-domain. We now consider each
Internet Service Provider Environments", RFC 4778,
January 2007.

[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion

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Marking in turn.

The first viewpoint assumes the following:

o There is no PCN-traffic on the ingress-egress-aggregate (so a
normal admission decision cannot be made).

o Simply admitting the new flow has a significant risk of leading MPLS", RFC 5129, January 2008.

[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to
overload: packets dropped or flows terminated.

On the former bullet, [Eardley07] suggests that, during the future
busy hour "Traffic
Class" Field", RFC 5462, February 2009.

[P.800] "Methods for subjective determination of a national network with about 100 PCN-boundary-nodes,
there are likely to be significant numbers transmission
quality", ITU-T Recommendation P.800, August 1996.

[Y.1541] "Network Performance Objectives for IP-based Services",
ITU-T Recommendation Y.1541, February 2006.

[PCN08-1] "Baseline Encoding and Transport of aggregates with very
few flows under nearly all circumstances.

The latter bullet could occur if new flows start on many Pre-Congestion
Information (work in progress)", Oct 2008.

[PCN08-2] "Metering and marking behaviour of the empty
ingress-egress-aggregates, which together overload a link PCN-nodes (work in the PCN-
domain. To be
progress)", Oct 2008.

[PWE3-08] "Pseudowire Congestion Control Framework (work in
progress)", May 2008.

[Babiarz06]
"SIP Controlled Admission and Preemption (work in
progress)", Oct 2006.

[Behringer07]
"Applicability of Keying Methods for RSVP Security (work
in progress)", Nov 2007.

[Briscoe06]
"An edge-to-edge Deployment Model for Pre-Congestion
Notification: Admission Control over a problem this would probably have to happen Diffserv Region
(work in a
short time period (flash crowd) because, after the reaction time progress)", October 2006.

[Briscoe08-1]
"Emulating Border Flow Policing using Re-PCN on Bulk Data
(work in progress)", Sept 2008.

[Briscoe08-2]
"Tunnelling of
the system, other (non-empty) ingress-egress-aggregates that pass
through the link will measure pre-congestion and so block new flows.
Also, flows naturally end anyway.

The downsides Congestion Notification (work in
progress)", July 2008.

[Charny07-1]
"Comparison of probing Proposed PCN Approaches (work in
progress)", November 2007.

[Charny07-2]
"Pre-Congestion Notification Using Single Marking for this viewpoint are:

o Probing adds delay

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Admission and Termination (work in progress)",
November 2007.

[Charny07-3]
"Email to the admission control process.

o Sufficient probing traffic has PCN WG mailing list", November 2007, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg00871.html>.

[Charny08]
"Email to be generated PCN WG mailing list", March 2008, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg01359.html>.

[Eardley07]
"Email to test the pre-
congestion level PCN WG mailing list", October 2007, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg00831.html>.

[Hancock02]
"Slide 14 of the ingress-egress-aggregate. But the probing
traffic itself may cause pre-congestion, causing other PCN-flows 'NSIS: An Outline Framework for QoS
Signalling'", May 2002, <http://www-nrc.nokia.com/sua/
nsis/interim/nsis-framework-outline.ppt>.

[Iyer03] "An approach to be blocked or even terminated - and in the flash crowd scenario
there will be probing on many ingress-egress-aggregates.

The second viewpoint applies in the case where there is multipath
routing (ECMP) in the PCN-domain. Note that ECMP is often used on
core networks. There are two possibilities:

(1) If admission control is based alleviate link overload as observed on measurements an
IP backbone", IEEE INFOCOM , 2003,
<http://www.ieee-infocom.org/2003/papers/10_04.pdf>.

[Lefaucheur06]
"RSVP Extensions for Admission Control over Diffserv using
Pre-congestion Notification (PCN) (work in progress)",
June 2006.

[Menth07] "PCN-Based Resilient Network Admission Control: The Impact
of the ingress-
egress-aggregate, then the viewpoint that probing is useful assumes: a Single Bit"", Technical Report , 2007, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth07-PCN-Config.pdf>.

[Menth08-1]
"Edge-Assisted Marked Flow Termination (work in
progress)", February 2008.

[Menth08-2]
"PCN Encoding for Packet-Specific Dual Marking (PSDM)
(work in progress)", July 2008.

[Menth08-3]
"PCN-Based Admission Control and Flow Termination", 2008,
<http://www3.informatik.uni-wuerzburg.de/staff/menth/
Publications/Menth08-PCN-Comparison.pdf>.

[Moncaster08]

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o there's a significant chance that the traffic is unevenly balanced
across

"A three state extended PCN encoding scheme (work in
progress)", June 2008.

[Sarker08]
"Usecases and Benefits of end to end ECN support in PCN
Domains (work in progress)", November 2008.

[Songhurst06]
"Guaranteed QoS Synthesis for Admission Control with
Shared Capacity", BT Technical Report TR-CXR9-2006-001,
Feburary 2006, <http://www.cs.ucl.ac.uk/staff/B.Briscoe/
projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf>.

[Style] "Guardian Style", Note: This document uses the ECMP paths,
abbreviations 'ie' and hence there's a significant risk 'eg' (not 'i.e.' and 'e.g.'), as in
many style guides, eg, 2007,
<http://www.guardian.co.uk/styleguide/>.

[Tsou08] "Applicability Statement for the Use of
admitting Pre-Congestion
Notification in a flow Resource-Controlled Network (work in
progress)", November 2008.

[Westberg08]
"LC-PCN: The Load Control PCN Solution (work in
progress)", November 2008.

Appendix A. Possible future work items

This section mentions some topics that should are outside the PCN WG's
current charter, but which have been mentioned as areas of interest.
They might be blocked (because it follows work items for: the PCN WG after a future re-
chartering; some other IETF WG; another standards body; an
ECMP path operator-
specific usage that is pre-congested) or blocking a flow that not standardised.

NOTE: it should be
admitted.

o Note: [Charny07-3] suggests unbalanced traffic is quite possible,
even with quite a large number crystal clear that this section discusses
possibilities only.

The first set of flows on a PCN-link (eg 1000)
when Assumption 3 (aggregation) is likely possibilities relate to be satisfied.

(2) If admission control is based on measurements of pre-congestion
on specific ECMP paths, then the viewpoint that probing is useful
assumes:

o There is no PCN-traffic on the ECMP path on which to base an
admission decision. restrictions described
in Section 6.3:

o Simply admitting the new flow has a significant risk of leading to
overload.

o The PCN-egress-node can match single PCN-domain encompasses several autonomous systems that do
not trust each other, perhaps by using a packet to an ECMP path. mechanism like re-PCN,
[Briscoe08-1].

o Note: This is similar to not all the first viewpoint and so similarly
could occur in nodes run PCN. For example, the PCN-domain is a flash crowd if
multi-site enterprise network. The sites are connected by a new flow starts more-or-less
simultaneously on many of VPN
tunnel; although PCN doesn't operate inside the empty ECMP paths. Because there are
several (sometimes many) ECMP paths between each pair of PCN-
boundary-nodes, it's presumably more likely that an ECMP path is
'empty' than an ingress-egress-aggregate is. To constrain tunnel, the
number of ECMP paths, a few tunnels could be set-up between each
pair PCN

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mechanisms still work properly because of PCN-boundary-nodes. Tunnelling also solves the issue in good QoS on the bullet immediately above (which
virtual link (the tunnel). Another example is otherwise hard because an
ECMP routing decision that PCN is made independently
deployed on each node).

The downsides of probing for this viewpoint are: the general Internet (ie widely but not universally
deployed).

o Probing adds delay applying the PCN mechanisms to other types of traffic, ie beyond
inelastic traffic. For instance, applying the admission control process.

o Sufficient probing traffic has PCN mechanisms to
traffic scheduled with the Assured Forwarding per-hop behaviour.
One example could be generated flow-rate adaptation by elastic applications
that adapt according to test the pre-
congestion level of pre-congestion information.

o the ECMP path. But there's aggregation assumption doesn't hold, because the link capacity
is too low. Measurement-based admission control is less accurate,
with a greater risk that of over-admission for instance.

o the
probing traffic itself may cause pre-congestion, causing other
PCN-flows to be blocked or even terminated. applicability of PCN mechanisms for emergency use (911, GETS,
WPS, MLPP, etc.)

Other possibilities include:

o Probing. This is discussed in Section A.1 below.

o The PCN-egress-node needs PCN-domain extends to consume the probe packets end users. The scenario is
described in [Babiarz06]. The end users need to ensure
they don't travel beyond be trusted to do
their own policing. If there is sufficient traffic, then the PCN-domain, since they might confuse
aggregation assumption may hold. A variant is that the PCN-domain
extends out as far as the LAN edge switch.

o indicating pre-congestion through signalling messages rather than
in-band (in the form of PCN-marked packets)

o the destination end node. This decision-making functionality is non-trivial, since probe
packets are addressed to the destination end node, in order to
test at a centralised node rather
than at the relevant ECMP path (ie they are not addressed to PCN-boundary-nodes. This requires that the PCN-
egress-node, unlike
egress-node signals PCN-feedback-information to the first viewpoint above).

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The open issues associated with this viewpoint include:

o What rate centralised
node, and pattern of probe packets does the PCN-ingress-node
need to generate, so that there's enough traffic the centralised node signals to make the PCN-ingress-
node the decision about admission decision?

o What difficulty does (or termination). It may need
the delay (whilst probing is done), centralised node and
possible packet drops, cause applications?

o Can the delay PCN-boundary-nodes to be alleviated by automatically and periodically
probing on the ingress-egress-aggregate? Or does this add too
much overhead?

o Are there other ways of dealing configured
with the flash crowd scenario?
For instance, by limiting the rate at which new flows are
admitted; or perhaps by a PCN-egress-node blocking new flows on
its empty ingress-egress-aggregates when its non-empty ones are
pre-congested.

o (Second viewpoint only) How does the PCN-egress-node disambiguate
probe packets from data packets (so it can consume the former)? each other's addresses. The PCN-egress-node must match centralised case is described
further in [Tsou08].

o Signalling extensions for specific protocols (eg RSVP, NSIS). For
example: the characteristic setting details of
particular bits in how the probe packet's header or body - but these
bits must not be used by any PCN-interior-node's ECMP algorithm.
In signalling protocol installs the general case this isn't possible, but it should be possible
for a typical ECMP algorithm (which examines:
flowspec at the source and
destination IP addresses PCN-ingress-node for an admitted PCN-flow; and port numbers, how
the signalling protocol ID, and carries the DSCP).

14. References

14.1. Normative References

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition PCN-feedback-information.
Perhaps also for other functions such as: coping with failure of a
PCN-boundary-node ([Briscoe06] considers what happens if RSVP is
the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.

[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.

14.2. Informative References

[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in QoS signalling protocol); establishing a tunnel across the Internet Architecture: an Overview",
PCN-domain if it is necessary to carry ECN marks transparently.

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RFC 1633, June 1994.

[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.

[RFC2211] Wroclawski, J., "Specification of

o Policing by the Controlled-Load
Network Element Service", RFC 2211, September 1997.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.

[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.

[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework PCN-ingress-node may not be needed if the PCN-
domain can trust that the upstream network has already policed the
traffic on its behalf.

o PCN for Policy-based Admission Control", RFC 2753,
January 2000.

[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, October 2000.

[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework Pseudowire: PCN may be used as a congestion avoidance
mechanism for Integrated Services Operation
over edge to edge pseudowire emulations [PWE3-08].

o PCN for MPLS: [RFC3270] defines how to support the Diffserv Networks", RFC 2998, November 2000.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
architecture in MPLS networks (Multi-protocol label switching).
[RFC5129] describes how to add PCN for admission control of Explicit Congestion Notification (ECN)
microflows into a set of MPLS aggregates. PCN-marking is done in
MPLS's EXP field (which [RFC5462] re-names the Class of Service
(CoS) field).

o PCN for Ethernet: Similarly, it may be possible to IP",
RFC 3168, September 2001.

[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., extend PCN into
Ethernet networks, where PCN-marking is done in the Ethernet
header. NOTE: Specific consideration of this extension is outside
the IETF's remit.

A.1. Probing

A.1.1. Introduction

Probing is a potential mechanism to assist admission control.

PCN's admission control, as described so far, is essentially a
reactive mechanism where the PCN-egress-node monitors the pre-
congestion level for traffic from each PCN-ingress-node; if the level
rises then it blocks new flows on that ingress-egress-aggregate.
However, it's possible that an ingress-egress-aggregate carries no
traffic, and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.

[RFC3393] Demichelis, C. so the PCN-egress-node can't make an admission decision
using the usual method described earlier.

One approach is to be "optimistic" and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002.

[RFC3411] Harrington, D., Presuhn, R., simply admit the new flow.
However it's possible to envisage a scenario where the traffic levels
on other ingress-egress-aggregates are already so high that they're
blocking new PCN-flows, and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.

[RFC3726] Brunner, M., "Requirements for Signaling Protocols",
RFC 3726, April 2004. admitting a new flow onto this 'empty'
ingress-egress-aggregate adds extra traffic onto a link that is
already pre-congested - which may 'tip the balance' so that PCN's
flow termination mechanism is activated or some packets are dropped.
This risk could be lessened by configuring on each link sufficient
'safety margin' above the PCN-threshold-rate.

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[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.

[RFC4301] Kent, S.

probing: a PCN-ingress-node generates and K. Seo, "Security Architecture for sends probe packets in
order to test the
Internet Protocol", RFC 4301, December 2005.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.

[RFC4594] Babiarz, J., Chan, K., pre-congestion level that the flow would
experience.

One possibility is that a probe packet is just a dummy data packet,
generated by the PCN-ingress-node and F. Baker, "Configuration
Guidelines addressed to the PCN-egress-
node.

A.1.2. Probing functions

The probing functions are:

o Make decision that probing is needed. As described above, this is
when the ingress-egress-aggregate (or the ECMP path - Section 6.4)
carries no PCN-traffic. An alternative is always to probe, ie
probe before admitting every PCN-flow.

o (if required) Communicate the request that probing is needed - the
PCN-egress-node signals to the PCN-ingress-node that probing is
needed

o (if required) Generate probe traffic - the PCN-ingress-node
generates the probe traffic. The appropriate number (or rate) of
probe packets will depend on the PCN-metering algorithm; for DiffServ Service Classes", RFC 4594,
August 2006.

[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J.,
example an excess-traffic-metering algorithm triggers fewer PCN-
marks than a threshold-metering algorithm, and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.

[RFC4774] Floyd, S., "Specifying Alternate Semantics for so will need more
probe packets.

o Forward probe packets - as far as PCN-interior-nodes are
concerned, probe packets are handled the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.

[RFC4778] Kaeo, M., "Operational Security Current Practices in
Internet Service Provider Environments", RFC 4778,
January 2007.

[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking same as (ordinary data)
PCN-packets, in MPLS", RFC 5129, January 2008.

[P.800] "Methods for subjective determination terms of transmission
quality", ITU-T Recommendation P.800, August 1996.

[Y.1541] "Network Performance Objectives routing, scheduling and PCN-marking.

o Consume probe packets - the PCN-egress-node consumes probe packets
to ensure that they don't travel beyond the PCN-domain.

A.1.3. Discussion of rationale for IP-based Services",
ITU-T Recommendation Y.1541, February 2006.

[MPLS08] "Multi-Protocol Label Switching (MPLS) label stack entry:
"EXP" field renamed probing, its downsides and open
issues

It is an unresolved question whether probing is really needed, but
two viewpoints have been put forward as to "Traffic Class" field (work why it is useful. The
first is perhaps the most obvious: there is no PCN-traffic on the
ingress-egress-aggregate. The second assumes that multipath routing
ECMP is running in
progress)", Dec 2008.

[PCN08-1] "Baseline Encoding and Transport the PCN-domain. We now consider each in turn.

The first viewpoint assumes the following:

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o There is no PCN-traffic on the ingress-egress-aggregate (so a
normal admission decision cannot be made).

o Simply admitting the new flow has a significant risk of Pre-Congestion
Information (work leading to
overload: packets dropped or flows terminated.

On the former bullet, [Eardley07] suggests that, during the future
busy hour of a national network with about 100 PCN-boundary-nodes,
there are likely to be significant numbers of aggregates with very
few flows under nearly all circumstances.

The latter bullet could occur if new flows start on many of the empty
ingress-egress-aggregates, which together overload a link in the PCN-
domain. To be a problem this would probably have to happen in progress)", Oct 2008.

[PCN08-2] "Marking behaviour a
short time period (flash crowd) because, after the reaction time of PCN-nodes (work in progress)",
Oct 2008.

[PWE3-08] "Pseudowire Congestion Control Framework (work in
progress)", May 2008.

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[Babiarz06]
"SIP Controlled Admission
the system, other (non-empty) ingress-egress-aggregates that pass
through the link will measure pre-congestion and Preemption (work in
progress)", Oct 2006.

[Behringer07]
"Applicability so block new flows.
Also, flows naturally end anyway.

The downsides of Keying Methods for RSVP Security (work
in progress)", Nov 2007.

[Briscoe06]
"An edge-to-edge Deployment Model probing for Pre-Congestion
Notification: Admission Control over a DiffServ Region
(work this viewpoint are:

o Probing adds delay to the admission control process.

o Sufficient probing traffic has to be generated to test the pre-
congestion level of the ingress-egress-aggregate. But the probing
traffic itself may cause pre-congestion, causing other PCN-flows
to be blocked or even terminated - and in progress)", October 2006.

[Briscoe08-1]
"Emulating Border Flow Policing using Re-PCN the flash crowd scenario
there will be probing on Bulk Data
(work many ingress-egress-aggregates.

The second viewpoint applies in progress)", Sept 2008.

[Briscoe08-2]
"Layered Encapsulation of Congestion Notification (work the case where there is multipath
routing (ECMP) in
progress)", July 2008.

[Charny07-1]
"Comparison the PCN-domain. Note that ECMP is often used on
core networks. There are two possibilities:

(1) If admission control is based on measurements of Proposed PCN Approaches (work in
progress)", November 2007.

[Charny07-2]
"Pre-Congestion Notification Using Single Marking for
Admission the ingress-
egress-aggregate, then the viewpoint that probing is useful assumes:

o there's a significant chance that the traffic is unevenly balanced
across the ECMP paths, and Termination (work in progress)",
November 2007. hence there's a significant risk of
admitting a flow that should be blocked (because it follows an
ECMP path that is pre-congested) or blocking a flow that should be
admitted.

o Note: [Charny07-3]
"Email to PCN WG mailing list", November 2007, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg00871.html>.

[Charny08]
"Email to PCN WG mailing list", March 2008, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg01359.html>.

[Eardley07]
"Email to PCN WG mailing list", October 2007, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg00831.html>.

[Hancock02]
"Slide 14 suggests unbalanced traffic is quite possible,
even with quite a large number of 'NSIS: An Outline Framework for QoS
Signalling'", May 2002, <http://www-nrc.nokia.com/sua/
nsis/interim/nsis-framework-outline.ppt>.

[Iyer03] "An approach flows on a PCN-link (eg 1000)
when Assumption 3 (aggregation) is likely to alleviate link overload as observed be satisfied.

(2) If admission control is based on an measurements of pre-congestion
on specific ECMP paths, then the viewpoint that probing is useful

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IP backbone", IEEE INFOCOM , 2003,
<http://www.ieee-infocom.org/2003/papers/10_04.pdf>.

[Lefaucheur06]
"RSVP Extensions for Admission Control over Diffserv using
Pre-congestion Notification (PCN) (work in progress)",
June 2006.

[Menth07] "PCN-Based Resilient Network Admission Control:

assumes:

o There is no PCN-traffic on the ECMP path on which to base an
admission decision.

o Simply admitting the new flow has a significant risk of leading to
overload.

o The Impact PCN-egress-node can match a packet to an ECMP path.

o Note: This is similar to the first viewpoint and so similarly
could occur in a flash crowd if a new flow starts more-or-less
simultaneously on many of the empty ECMP paths. Because there are
several (sometimes many) ECMP paths between each pair of PCN-
boundary-nodes, it's presumably more likely that an ECMP path is
'empty' than an ingress-egress-aggregate is. To constrain the
number of ECMP paths, a Single Bit"", Technical Report , 2007, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth07-PCN-Config.pdf>.

[Menth08-1]
"Edge-Assisted Marked Flow Termination (work few tunnels could be set-up between each
pair of PCN-boundary-nodes. Tunnelling also solves the issue in
progress)", February 2008.

[Menth08-2]
"PCN Encoding
the bullet immediately above (which is otherwise hard because an
ECMP routing decision is made independently on each node).

The downsides of probing for Packet-Specific Dual Marking (PSDM)
(work in progress)", July 2008.

[Menth08-3]
"PCN-Based Admission Control and Flow Termination", 2008,
<http://www3.informatik.uni-wuerzburg.de/staff/menth/
Publications/Menth08-PCN-Comparison.pdf>.

[Moncaster08]
"A three state extended PCN encoding scheme (work in
progress)", June 2008.

[Sarker08]
"Usecases and Benefits this viewpoint are:

o Probing adds delay to the admission control process.

o Sufficient probing traffic has to be generated to test the pre-
congestion level of the ECMP path. But there's the risk that the
probing traffic itself may cause pre-congestion, causing other
PCN-flows to be blocked or even terminated.

o The PCN-egress-node needs to consume the probe packets to ensure
they don't travel beyond the PCN-domain, since they might confuse
the destination end node. This is non-trivial, since probe
packets are addressed to the destination end ECN support in PCN
Domains (work node, in progress)", November 2008.

[Songhurst06]
"Guaranteed QoS Synthesis for Admission Control order to
test the relevant ECMP path (ie they are not addressed to the PCN-
egress-node, unlike the first viewpoint above).

The open issues associated with
Shared Capacity", BT Technical Report TR-CXR9-2006-001,
Feburary 2006, <http://www.cs.ucl.ac.uk/staff/B.Briscoe/
projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf>.

[Style] "Guardian Style", Note: This document uses this viewpoint include:

o What rate and pattern of probe packets does the
abbreviations 'ie' PCN-ingress-node
need to generate, so that there's enough traffic to make the
admission decision?

o What difficulty does the delay (whilst probing is done), and 'eg' (not 'i.e.'
possible packet drops, cause applications?

o Can the delay be alleviated by automatically and 'e.g.'), as in
many style guides, eg, 2007,
<http://www.guardian.co.uk/styleguide/>.

[Tsou08] "Applicability Statement for periodically
probing on the Use of Pre-Congestion
Notification in a Resource-Controlled Network (work in
progress)", November 2008. ingress-egress-aggregate? Or does this add too

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[Westberg08]
"LC-PCN:

much overhead?

o Are there other ways of dealing with the flash crowd scenario?
For instance, by limiting the rate at which new flows are
admitted; or perhaps by a PCN-egress-node blocking new flows on
its empty ingress-egress-aggregates when its non-empty ones are
pre-congested.

o (Second viewpoint only) How does the PCN-egress-node disambiguate
probe packets from data packets (so it can consume the former)?
The Load Control PCN Solution (work PCN-egress-node must match the characteristic setting of
particular bits in
progress)", November 2008. the probe packet's header or body - but these
bits must not be used by any PCN-interior-node's ECMP algorithm.
In the general case this isn't possible, but it should be possible
for a typical ECMP algorithm (which examines: the source and
destination IP addresses and port numbers, the protocol ID, and
the DSCP).

Author's Address

Philip Eardley
BT
B54/77, Sirius House Adastral Park Martlesham Heath
Ipswich, Suffolk IP5 3RE
United Kingdom

Email: philip.eardley@bt.com

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