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draft-ietf-mpls-tp-gach-gal-01.txt --> draft-ietf-mpls-tp-gach-gal-02.txt

MPLS Working Group M. Bocci, Ed.
Internet-Draft M. Vigoureux, Ed.
Updates: 3032, 4385, 5085 Alcatel-Lucent
(if approved) G. Swallow
Intended status: Standards Track D. Ward
Expires: July 10, August 27, 2009 S. Bryant
Cisco
R. Aggarwal
Juniper Networks
January 6,
February 23, 2009

MPLS Generic Associated Channel
draft-ietf-mpls-tp-gach-gal-01 header
draft-ietf-mpls-tp-gach-gal-02

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to this document.

Abstract

This document generalises the applicability of the pseudowire (PW)
Associated Channel Header (ACH), enabling the realization of a
control channel associated to MPLS Label Switched Paths (LSP), MPLS
pseudowires (PW) (LSPs) and
MPLS Sections. Sections in addition to MPLS pseudowires. In order to identify
the presence of this Associated Channel Header in the Generic ACH (G-ACH), label stack,
this document also assigns of one of the reserved MPLS label values to
the 'Generic Generic Associated channel header Label (GAL)', (GAL), to be used as a label
based exception mechanism.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Contributing Authors . . . . . . . . . . . . . . . . . . . 5
1.2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 6
2. Generic Associated Channel Header . . . . . . . . . . . . . . 6
2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Allocation of Channel Types . . . . . . . . . . . . . . . 7
3. ACH TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. ACH TLV Payload Structure . . . . . . . . . . . . . . . . 8
3.2. ACH TLV Header . . . . . . . . . . . . . . . . . . . . . . 8
3.3. ACH TLV Object . . . . . . . . . . . . . . . . . . . . . . 9
4. Generalised Exception Mechanism . . . . . . . . . . . . . . . 7
3.1. 9
4.1. Relationship with Existing MPLS OAM Alert Mechanisms . . . 8
3.2. 10
4.2. GAL Applicability and Usage . . . . . . . . . . . . . . . 8
3.2.1. 10
4.2.1. GAL Processing . . . . . . . . . . . . . . . . . . . . 8
3.3. 10
4.3. Relationship wth RFC 3429 . . . . . . . . . . . . . . . . 11
4. 13
5. Compatability . . . . . . . . . . . . . . . . . . . . . . . . 11
5. 13
6. Congestion Considerations . . . . . . . . . . . . . . . . . . 12
6. 14
7. Security Consderations . Considerations . . . . . . . . . . . . . . . . . . . 12
7. 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. 16
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. 16
10.2. Informative References . . . . . . . . . . . . . . . . . . 14 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 17

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1. Introduction

There is a need for Operations, Administration and Maintenance (OAM)
mechanisms that can be used for edge-to-edge (i.e. between
originating fault detection, diagnostics,
maintenance and terminating LSRs or T-PEs) other functions on a PW and segment (e.g. an LSP. These functions
can be used between any two LSRs Label Edge Routers (LERs) / Label
Switching Router (LSRs) or T-PEs/S-PEs Terminating Provider Edge routers (T-PEs)
/ Switching Provider Edge routers (S-PEs) along the path of a an LSP or
PW [15]) fault
detection, diagnostics, maintenance and other functions for a PW and
a LSP. respectively [15]. Some of these functions can be supported using
existing tools such as
VCCV Virtual Circuit Connectivity Verification
(VCCV) [2], BFD [3], or Bidirectional Forwarding Detection for MPLS LSPs (BFD-
MPLS)[3], LSP-Ping [4]. [4], or BFD-VCCV [5]. However, a requirement has
been indicated to augment the this set of maintenance functions, in
particular
where when MPLS networks are used for packet transport services
and transport network operations [16]. Examples of these functions
include performance monitoring, automatic protection switching, and
support for management and signaling communication channels. These
tools must MUST be applicable to, and function in essentially the same
manner (from an operational point of view) on both MPLS PWs PWs, MPLS LSPs and
MPLS LSPs. Sections. They must MUST also operate in-band on the PW or LSP such
that they do not depend on PSN
routing, Packet Switched Network (PSN) routing or
on user data traffic or ultimately traffic, and MUST also not depend on PSN or other dynamic control
plane functions.

Virtual Circuit Connectivity Verification (VCCV)

VCCV can use an
associated channel Associated Channel Header (ACH) to provide a PW-
associated control channel between a PW's
ingress and egress points and end points, over which OAM
and other control messages can be exchanged. In this document, we propose a generic
associated channel header (G-ACH) This document
generalises the use of the ACH to enable the same associated control
channel mechanism to be used for MPLS Sections, LSPs and PWs. The
associated control channel header (ACH) thus generalized is known as the Generic
Associated Channel (G-ACh). The ACH, specified in RFC 4385 [5] is [6], may
be used with additional code points to support additional MPLS
maintenance
functions. functions on the G-ACh.

Generalizing the ACH associated control channel mechanism to MPLS LSPs and MPLS
Sections also requires a method to identify that a packet contains a G-ACH an
ACH followed by a non-service payload. This Therefore, this document therefore also
defines a label based exception mechanism (the Generic Associated channel
header Label, or GAL) that serves to inform an
LSR (or LER) that a packet that it receives on an LSP or section Section belongs
to an associated channel. control channel for that LSP or Section.

RFC 4379 [4] and BFD for MPLS LSPs BFD-MPLS [3] have defined define alert mechanisms that enable a an
MPLS LSR to identify and process MPLS OAM packets when
the OAM packets these are
encapsulated in an IP header. These alert mechanisms are based on TTL
MPLS or PW label Time to Live (TTL) expiration and/or on the use of
an IP destination address in the range 127/8. These mechanisms are
the default mechanisms for identifying MPLS OAM packets when the OAM packets are
encapsulated in an IP header. However it may not always be possible

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to use these mechanisms in some MPLS applications, (e.g. MPLS-TP
[15]) e.g. MPLS
Transport Profile (MPLS-TP) [15], particularly when IP based
demultiplexing cannot be used. This document proposes an OPTIONAL defines a mechanism
that is RECOMMENDED for identifying and demultiplexing encapsulating MPLS OAM and
other maintenance

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[4] and [3] are not available. This mechanism MAY be used in
addition to IP-based mechanisms.

The G-ACH and GAL mechanisms are mechanism is defined to work together. together with the ACH for LSPs
and MPLS Sections.

Note that, in this document, maintenace maintenance functions and packets should
be understood in the broad sense, that sense. That is, as a set of FCAPS maintenance and
management mechanisms that include OAM, Automatic Protection
Switching (APS), Signalling Communication Channel (SCC) and
Management Communication Channel (MCC) messages.

Note

Also note that the GAL and G-ACH ACH are applicable to MPLS in general.
Their applicability to specific applications of MPLS is outside the
scope of this document. For example, the applicability of the GAL and G-ACH to
MPLS-TP is described in [15] and [17].

1.1. Contributing Authors

The editors gratefully acknowledge the contibution contributions of Stewart Bryant, Sami Boutros,
Italo Busi, Marc Lasserre, and Lieven Levrau. Levrau and Siva Sivabalan

1.2. Objectives

This document proposes defines a mechanism that provides a solution to provide for the
extended maintenance needs of emerging applications for MPLS. It
creates a generic control channel identification mechanism that may be applied to all
MPLS LSPs, LSPs and Sections, while maintaining compatibility with the PW
associated channel header (ACH) . channel. It also normalizes normalises the use of the ACH for PWs in
a transport context.

1.3. Scope

This document context, and defines the encapsulation header for a label based exception mechanism to
alert LERs/LSRs of the presence of an ACH after the bottom of the
stack.

1.3. Scope

This document defines the encapsulation header for LSP, MPLS Section
and PW associated channel messages.

It does not define how associated control channel capabilities are
signaled or negotiated between LSRs LERs/LSRs or PEs, or the operation of
various OAM
functions, nor how the messages transmitted on the associated
channel. functions.

This document does not deprecate existing MPLS and PW OAM mechanisms.

1.4. Terminology

G-ACH: Generic Associated Channel Header

GAL: Generic Associated Channel Header Label

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MPLS Section: A network segment between two LSRs that are immediately
adjacent at the MPLS layer

1.4. Terminology

ACH: Associated Channel Header

G-ACh: Generic Associated Channel

GAL: G-ACh Label

Maintenance Packet: packet: Any packet containing a message belonging to a
maintenace
maintenance protocol that is carried on a PW, LSP or MPLS Section
associated control channel. Examples of such maintenance protocols
include OAM functions, signaling communications or management
communications.

The terms 'Section' and 'Concatenated Segment' are defined in [17].

2. Generic Associated Channel Header

VCCV [2] defines three MPLS Control Channel (CC) Types that may be
used to multiplex exchange OAM messages onto through a PW: CC Type 1 uses an
associated channel header ACH and
is referred to as "In-band VCCV"; CC Type 2 uses the MPLS Router
Alert Label to indicate VCCV packets and is referred to as "Out of
Band VCCV"; CC Type 3 uses the TTL to force the packet to be
processed by the targeted router control plane and is referred to as
"MPLS PW Label with TTL == 1".

2.1. Definition

The use of the CC Type 1, currently previously limited to MPLS PWs, is here extended
to also apply to MPLS LSPs as well as and to MPLS Sections. This
associated channel header is called Note that for PWs, the Generic Associated Channel
Header (G- ACH). The PWE3
control word [6] MUST be present in the encapsulation of user packets
when the G-ACH ACH is used to demultiplex realize the associated channel packet on a PW. control channel.

The CC Type 1 control channel header is depicted in figure below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version|A| 1|Version| Reserved | Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Generic Associated Channel Header

In the above figure, the first nibble is set to 0001b to indicate a
control channel associated with a PW, a an LSP or a Section. The
Version field is set to 0, as specified in RFC 4385 [5]. This draft allocates Bit
8 of the ACH to the ACH TLV bit. This bit is set to 1 to indicate
that an object defined in the ACH TLV registry immediately follows
the G-ACH, otherwise it is set to 0. [6]. Bits 8 to
14 of the G-ACH are reserved and MUST be set to 0.. 0 and ignored on

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reception.

Note that VCCV also includes mechanisms for negotiating the Control
Channel and Connectivity Verification (i.e. OAM functions) Types
between PEs. It is anticipated that similar mechanisms will be
applied to existing MPLS LSPs. Such application will require further

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specification. However, such specification is beyond the scope of
this document.

2.1.

2.2. Allocation of Channel Types

The Channel Type field indicates the type of message carried on the
associated control channel e.g. IPv4 or IPv6 if IP demultiplexing is
used for messages sent on the G-ACH, associated control channel, or OAM or
other FCAPS maintenance function if IP demultiplexing is not used. For G-ACH
associated control channel packets where IP is not used as the
multiplexer, the Channel Type SHOULD indicate the specific
maintenance protocol carried in the associated control channel.

Values for the Channel Type field currently used for VCCV are
specified elsewhere, e.g. in RFC 4446 [6]. The [7]and RFC 4385[6] .
Additional Channel Type values and the associated maintenance
functionality of any additional
channel types will be defined in another document. other documents. Each associated
channel document
specifying a protocol solution document must relying on the ACH MUST also specify
the value to use for
any additional channel types. applicable Channel Type field value.

Note that these values are allocated from the PW Associated Channel
Type registry, but this document modifies the existing policy to
accomodate
accommodate a level of experimentation. See Section 7 8 for further
details.

3. Generalised Exception Mechanism

The above mechanism enables the multiplexing of various maintenace
packets onto a PW, LSP ACH TLVs

In some applications of the "In-band VCCV" associated control channel
it is necessary to include one or Section and provides more ACH TLVs to provide additional
context information on to the
type maintenance packet. One use of function being performed. In these ACH
TLVs might be to identify the case source and/or intended destination of a PW,
the associated control channel maintenance message. However, the use
of this construct is not limited to providing addressing information
nor is the applicability restricted to transport network
applications.

If the maintenance message MAY be preceded by one or more ACH TLVs,
then this MUST be explicitly specified in the definition of an ACH
Channel Type. If the ACH Channel Type definition does state that one
or more ACH TLVs MAY precede the maintenance message, an ACH TLV
Header MUST follow the ACH. If no ACH TLVs are required in a

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specific associated control word channel packet, but the Channel Type
nevertheless defines that ACH TLVs MAY be used, an ACH TLV Header
MUST be present but with a length field set to zero to indicate that
no ACH TLV follow this header.

If a channel type specification does not explicitly specify that ACH
TLVs MAY be used, then an ACH TLV Header MUST NOT be used.

3.1. ACH TLV Payload Structure

This section defines and describes the structure of an ACH payload
when an ACH TLV Header is negotiated present. The structure of ACH TLVs that
MAY follow an ACH TLV Header is defined and described in the
following sections.

The following figure (Figure 2) shows the structure of a G-ACh packet
payload.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ zero or configured at more ACH TLVs ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Maintenance Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: ACH TLV Payload Structure

3.2. ACH TLV Header

The ACH TLV Header defines the time length of the PW
establishment. A special case set of ACH TLVs that
follow.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: ACH TLV Header

The length field specifies the length in octets of the control word (the G-ACH) complete set

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of TLVs including TLVs that follow the ACH TLV header. A length of
zero indicates that no ACH TLV follow this header.

The reserved field is
used to identify packets belonging for future use and MUST be set to zero on
transmission and ignored on reception.

3.3. ACH TLV Object

An ACH TLV consists of a PW associated channel.

Generalizing 16-bit Type field, followed by a 16-bit
Length field which specifies the number of octets of the Value field
which follows the Length field. This 32-bit word is followed by zero
or more octets of Value information. The format and semantics of the
value information are defined by the TLV Type as recorded in the TLV
Type registry. See Section 8 for further details. Note that the
Value field of ACH TLVs MAY contain sub-TLV objects.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Value ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: ACH TLV Format

4. Generalised Exception Mechanism

Generalizing the associated channel mechanism to MPLS LSPs and MPLS Sections
also requires a method to identify that a packet contains a G-ACH an ACH
followed by a non-service payload. This document specifies that a
label be is used for that purpose and calls this special label the 'Generic Associated channel
header G-ACh
Label (GAL)'. (GAL). One of the reserved label values defined in RFC 3032 [7]
[8] is assigned for this purpose. The value of the label is to be
allocated by IANA; this document suggests the value 13.

The GAL provides a generalised an alert based exception mechanism to:

o Differentiate differentiate specific packets (e.g. those containing OAM maintenance messages) from
others, such as normal user-plane ones,

o Indicate indicate that the Generic Associated Channel Header (G-ACH) ACH appears immediately after the bottom of the
label stack.

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The 'Generic Associated channel header Label (GAL)' GAL MUST only be used where both of these purposes are applicable.

3.1. apply.

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4.1. Relationship with Existing MPLS OAM Alert Mechanisms

RFC 4379 [4] and BFD for MPLS LSPs BFD-MPLS [3] have defined alert mechanisms that
enable a MPLS LSR to identify and process MPLS OAM packets when the
OAM packets are encapsulated in an IP header. These alert mechanisms
are based on TTL expiration and/or use an IP destination address in
the range 127/8.

These alert mechanisms SHOULD preferably be used in non MPLS-TP
environments. The environments,
although the mechanism defined in this document MAY also be used.

3.2.

4.2. GAL Applicability and Usage

The 'Generic Associated channel header Label (GAL)' GAL MUST only be used with Label Switched Paths (LSPs), with their associated Tandem
Connection Monitoring Entities (see [17] for definitions LSPs, concatenated segments of TCMEs) LSPs,
and with MPLS Sections.

The GAL applies to both P2P and P2MP LSPs, unless otherwise stated.

In MPLS-TP, the GAL MUST always be at the bottom of the label stack
(i.e. S bit set to 1). However, in other MPLS environments, this
document places no restrictions on where the GAL may appear within
the label stack.

The GAL MUST NOT appear in the label stack when transporting normal
user-plane packets. Furthermore, when present, the GAL MUST only
appear once in the label stack for packets on the generic associated channel.

3.2.1. stack.

4.2.1. GAL Processing

The Traffic Class (TC) field (formerly known as the EXP field) of the
label stack entry containing the GAL follows the definition and
processing rules specified and referenced in [8]. [9].

The Time-To-Live (TTL) field of the label stack entry that contains
the GAL follows the definition and processing rules specified in [9].

3.2.1.1.
[10].

4.2.1.1. MPLS Section Label Switched Paths and Segments

The following figure (Figure 2) 5) depicts two MPLS LERs (A and D) and two
LSRs immediately
adjacent at the MPLS layer.

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switched in B and C.

+---+ +---+ +---+ +---+
| A |-------------| Z B |-------------| C |-------------| D |
+---+ +---+ +---+ +---+

Figure 2: Maintenance 5: MPLS-TP maintenance over a LSP

In this example, a G-ACh exists on an MPLS Section Associated Channel

With regards to the MPLS Section, both LSP that extends between LERs are Maintenance End
Points (see [17] for definitions of MEPs). A

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and D, via LSRs B and C. Only these nodes may insert, extract or
process packets on this G-ACh.

The following figure (Figure 3) 6) depicts the format of a labelled OAM
packet on an associated channel MPLS-TP
maintenance message when used for MPLS Section
maintenance. an LSP.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generic-ACH ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. ACH TLV Header (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Zero or more ACH TLVs ~
~ (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Maintenance Message .
. ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Maintenance Packet Format 6: MPLS-TP maintenance message format for a LSP

Note that it is possible that the LSP may be tunnelled in another LSP
(e.g. if a MPLS Section Tunnel exists between B and C), and as such other
labels may be present in the label stack.

To send a MPLS-TP maintenance packet message on an associated channel of the
MPLS Section, LSP associated control channel,
the head-end LSR LER (A) of the MPLS Section generates a maintenance packet message, to which it MAY
prepended an ACH TLV header and appropriate ACH TLVs, and with a G-ACH ACH
to which it pushes a GAL. GAL and finally the LSP label.

o The TTL field of the GAL SHOULD MUST be set to at least 1. The exact
value of the TTL is application specific.

o The S bit of the GAL MUST be set according to 1 its position in MPLS-TP. the
label stack.

o The setting of the TC field is application specific.

The maintenance packet, message, the G-ACH and ACH or the GAL SHOULD NOT be modified
towards the tail-end LSR (Z). targeted destination. Upon reception of the labelled
packet, the tail-end LSR (Z), targeted destination, after having checked both the LSP

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label and GAL fields, SHOULD pass the whole packet maintenance message to
the appropriate processing entity.

3.2.1.2. Label Switched Paths

4.2.1.2. MPLS Section

The following figure (Figure 4) 7) depicts four LSRs. A LSP is
established from A to D and switched in B and C.

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+---+ +---+ an example of a MPLS Section.

+---+ +---+
| A |-------------| B |-------------| C |-------------| D Z |
+---+ +---+ +---+ +---+

Figure 4: 7: Maintenance over an LSP Associated Channel

LERs MPLS Section

With regard to the MPLS Section, a G-ACh exists between A and D are Maintenance End Points (MEPs) with respect to this
LSP. Furthermore, LSRs B Z. Only
A and C could also be Maintenance
Intermediate Points (MIPs) with respect to Z can insert, extract or process packets on this LSP (see [17] for
definitions of MEPs and MIPs). G-ACh.

The following figure (Figure 5) 8) depicts the format of a labelled maintenance packet
message when used for a MPLS-TP LSP. MPLS Section. The GAL MAY provide the
exception mechanism for a control channel in its own right without
being associated with a specific LSP, thus providing maintenance
related communications across a specific link interconnecting two
LSRs. In this case, the GAL is the only label in the stack.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP Label GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL ACH | TC |S| TTL
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generic-ACH ~
~ Zero or more ACH TLVs ~
~ (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. ~
~ Maintenance Message .
. ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: 8: Maintenance Packet Format message format for MPLS-TP LSP

Note that it is possible that the LSP MAY also be tunnelled in
another LSP (e.g. if a MPLS Tunnel exists between B and C), and as
such other labels MAY be present above it in the label stack. Section

To send a maintenance packet message on the LSP a control channel associated channel, to the head-
end
Section, the head-end LSR (A) of the Section generates a OAM message maintenance
message, to which it MAY prepend an ACH TLV Header and appropriate
ACH TLVs, and with a G-ACH on ACH to which it first pushes a GAL.

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o The TTL field of the GAL SHOULD GAL MUST be set to at least 1. The exact
value of the TTL is application specific.

o The S bit of the GAL MUST be set according to its position in the
label stack. For MPLS Sections, the S bit MUST be set to 1.

o The S bit setting of the GAL SHOULD be set to 1 in MPLS-TP. TC field is application specific.

The maintenance message, the G-ACH or ACH and the GAL SHOULD NOT be modified
towards the targeted destination. tail-end LSR (Z). Upon reception of the labelled packet,
the targeted destination, tail-end LSR (Z), after having checked both the LSP
label and GAL fields, SHOULD
pass the whole packet to the appropriate processing entity.

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3.2.1.3. Tandem Connection Monitoring Entity

Tandem Connection Monitoring will be specified in a separate
document.

3.3.

4.3. Relationship wth RFC 3429

RFC 3429 [18] describes the assignment of one of the reserved label
values, defined in RFC 3032 [7], [8], to the 'OAM Alert Label' that is
used by user-plane MPLS OAM functions for the identification of MPLS
OAM packets. The value of 14 is used for that purpose.

Both this document and RFC 3429 [18] therefore describe the
assignment of reserved label values for similar purposes. The
rationale for the assignment of a new reserved label can be
summarized as follows:

o Unlike the mechanisms described and referenced in RFC 3429 [18],
MPLS-TP OAM packet payloads maintenance messages will not reside immediately after the
GAL but instead behind the G-ACH, ACH, which itself resides immediately after the
bottom of the label stack when the GAL is present. stack. This ensures that OAM OAM, using the generic associated channel
G-ACh, complies with RFC 4928 [10]. [11].

o The set of maintenance functions potentially operated in the
context of the generic associated channel G-ACh is wider than the set of OAM functions
referenced in RFC 3429 [18].

o It has been reported that there are existing implementations and
running deployments using the 'OAM Alert Label' as described in
RFC 3429 [18]. It is therefore not possible to modify the 'OAM
Alert Label' allocation, purpose or usage. Nevertheless, it is
RECOMMENDED by this document that no further OAM extensions based
on 'OAM Alert Label' (Label 14) usage be specified or developed.

5. Compatability

Procedures for handling a packet received with an invalid incoming
label are specified in RFC 3031[12].

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An LER, LSR or PE MUST discard received G-ACH associated channel packets on
which all of the MPLS or PW labels have been popped if it any one of the
following conditions is not G-
ACH capable, if it true:

o It is not capable of processing packets on the Channel Type
indicated G-ACH channel, or if it by the ACH of the received packet.

o It has not, through means outside the scope of this document,
indicated to the sending LSR, LER or PE that it will process G-ACH
associated channel packets received on the Channel Type indicated channel. by the
ACH of the received packet.

o If the ACH was indicated by the presence of a GAL, and the first
nibble of the ACH of the received packet is not 0b0001.

o The
LER, LSR or PE ACH version is not recognised.

In addition, it MAY increment an error counter and MAY also
optionally issue a system and/or SNMP notification.

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6. Congestion Considerations

The congestion considerations detailed in RFC 5085 [2] apply.
Further generic associated channel-specific congestion considerations
will be detailed in a future revision of this document.

7. Security Consderations Considerations

The security considerations detailed for the associated control channel are
described in RFC 5085 [2], the MPLS
architecture [11], 4385[6]. Further security considerations MUST be
described in the PWE3 architecture [12] and relevant associated channel type specification.

RFC 5085 [2] provides data plane related security considerations.
These also apply to a G-ACh, whether the MPLS-TP
framework [15] apply.

7. alert mechanism uses a GAL
or only an ACH.

8. IANA Considerations

This document requests that IANA allocates a Label label value, to the
'Generic Associated channel header Label (GAL)', GAL,
from the pool of reserved labels, and suggests this value to be 13.

Channel Types for the Generic Associated Channel Header are allocated from
the IANA PW Associated Channel Type registry [6]. [7]. The PW Associated
Channel Type registry is currently allocated based on the IETF
consensus process, described in[13]. in [13]. This allocation process was
chosen based on the consensus reached in the PWE3 working group that
pseudowire associated channel mechanisms should be reviewed by the

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IETF and only those that are consistent with the PWE3 architecture
and requirements should be allocated a code point.

However, a requirement has emerged (see [16]) to allow for
optimizations or extensions to OAM and other control protocols
running in an associated channel to be experimented with without resorting
to the IETF standards process, by supporting experimental code
points. This would prevent code points used for such functions from
being used from the range allocated through the IETF standards and
thus protects an installed base of equipment from potential
inadvertent overloading of code points. In order to support this
requirement, this document requests that the code-point code point allocation
scheme for the PW Associated Channel Type be changed as follows:

0 - 32751 : IETF Consensus

32752 - 32767 : Experimental

Code points in the experimental range MUST be used according to the
guidelines of RFC 3692 [14]. Experimental OAM functions MUST be
disabled by default. The channel type Channel Type value used for a given
experimental OAM function MUST be configurable, and care MUST be

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taken to ensure that different OAM functions that are not
interoperable inter-
operable are configured to use different channel type Channel Type values.

The PW Associated Channel Type registry needs to be updated to
include a column indicating whether the ACH is followed by a ACH TLV
header (Yes/No). There are two ACH Channel Type code-points
currently assigned and in both cases no ACH TLV header is used. Thus
the new format of the PW Channel Type registry is:

Registry:
Value Description TLV Follows Reference
----- ---------------------------- ----------- ---------
0x21 ACH carries an IPv4 packet No [RFC4385]
0x57 ACH carries an IPv6 packet No [RFC4385]

Figure 9: PW Channel Type registry

IANA is requested create a new registry called the Associated Channel
TLV Registry. The allocation policy for this registry is IETF
consensus. This registry MUST record the following information.
There are no initial entries.

Name Type Length Description Reference
(octets)

Figure 10: PW ACH TLV registry

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9. Acknowledgements

The authors would like to thank all members of the teams (the Joint
Working Team, the MPLS Interoperability Design Team in IETF and the
T-MPLS Ad Hoc Group
MPLS-TP Ad-Hoc Team in ITU-T) involved in the definition and
specification of MPLS Transport Profile.

10. References

9.1.

10.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
Connectivity Verification (VCCV): A Control Channel for
Pseudowires", RFC 5085, December 2007.

[3] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, "BFD
For MPLS LSPs", draft-ietf-bfd-mpls-07 (work in progress),
June 2008.

[4] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label
Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.

[5] Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
Detection (BFD) for the Pseudowire Virtual Circuit
Connectivity Verification (VCCV)", draft-ietf-pwe3-vccv-bfd-03
(work in progress), February 2009.

[6] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use
over an MPLS PSN", RFC 4385, February 2006.

[6]

[7] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

[7]

[8] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci,
D., Li, T., and A. Conta, "MPLS Label Stack Encoding",
RFC 3032, January 2001.

[8]

[9] Andersson, L. and R. Asati, "Multi-Protocol Label Switching
(MPLS) label stack entry: "EXP" field renamed to "Traffic
Class" field", draft-ietf-mpls-cosfield-def-08 (work in
progress), December 2008.

[9]

[10] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,

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Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
January 2003.

[10]

[11] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal Cost
Multipath Treatment in MPLS Networks", BCP 128, RFC 4928,
June 2007.

[11]

[12] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.

[12] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC 3985, March 2005.

[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998. 5226, May 2008.

[14] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.

9.2.

10.2. Informative References

[15] Bocci, M., Bryant, S., and L. Levrau, "A Framework for MPLS in
Transport Networks", draft-ietf-mpls-tp-framework-00 (work in
progress), November 2008.

[16] Vigoureux, M., Ward, D., and M. Betts, "Requirements for OAM in
MPLS Transport Networks",
draft-ietf-mpls-tp-oam-requirements-00 (work in progress),
December 2008.

[17] Busi, I. and B. Niven-Jenkins, "MPLS-TP OAM Framework B., Brungard, D., Betts, M., Sprecher, N., and
Overview", draft-busi-mpls-tp-oam-framework-00
S. Ueno, "MPLS-TP Requirements",
draft-ietf-mpls-tp-requirements-04 (work in progress), October 2008.
February 2009.

[18] Ohta, H., "Assignment of the 'OAM Alert Label' for
Multiprotocol Label Switching Architecture (MPLS) Operation and
Maintenance (OAM) Functions", RFC 3429, November 2002.

Authors' Addresses

Matthew Bocci (editor)
Alcatel-Lucent
Voyager Place, Shoppenhangers Road
Maidenhead, Berks SL6 2PJ
UK

Email: matthew.bocci@alcatel-lucent.com

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Internet-Draft GACH G-ACh and GAL January February 2009

Martin Vigoureux (editor)
Alcatel-Lucent
Route de Villejust
Nozay, 91620
France

Email: martin.vigoureux@alcatel-lucent.com

George Swallow
Cisco

Email: swallow@cisco.com

David Ward
Cisco

Email: dward@cisco.com

Stewart Bryant
Cisco

Email: stbryant@cisco.com

Rahul Aggarwal
Juniper Networks

Email: rahul@juniper.net

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