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Internet-Draft Grenville Armitage
Bellcore
February 4th,
May 31st, 1995
Support for Multicast over UNI 3.1 based ATM Networks.
<draft-ietf-ipatm-ipmc-04.txt>
<draft-ietf-ipatm-ipmc-05.txt>
Status of this Memo
This document was submitted to the IETF IP over ATM WG. Publication
of this document does not imply acceptance by the IP over ATM WG of
any ideas expressed within. Comments should be submitted to the ip-
atm@matmos.hpl.hp.com mailing list.
Distribution of this memo is unlimited.
This memo is an internet draft. Internet Drafts are working documents
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Internet Draft.
Abstract
Mapping the connectionless IP multicast service over the connection
oriented ATM services provided by UNI 3.1 is a non-trivial task. This
memo describes a mechanism to support the multicast needs of Layer 3
protocols in general, and describes its application to IP
multicasting in particular.
ATM based IP hosts and routers use a Multicast Address Resolution
Server (MARS)
architecture that allows ATM based IP hosts to support RFC 1112 style Level 2 IP multicast using over the
ATM Forum's UNI 3.1 point to multipoint connection service. It also describes how this
architecture can be generalized to support other protocols wishing A single
endpoint interface behaviour is described, along with two levels of
MARS - Class I and Class II. The Class I MARS service supports layer
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3 multicasting using meshes of VCs. The Class II MARS adds the
ability to
multicast over UNI 3.1 based use ATM service. level multicast servers to support distribution of
layer 3 packets.
[Editorial note: The differences between this This version has been substantially restructured
from ipmc-04 in an attempt to group related topics together in a
more logical fashion. Additions and 03.txt modifications to the actual
protocol are substantial generally in accordance with the area set of multicast server support. This
impacts on Chapter 8, proposed
changes published and anything referring updated during the March to MARS_MSERV. Two
control VCs have been identified and named, two sequence numbers
are now used, and three major appendices have been added
discussing issues that cannot at this May time be standardized. The
MARS_JOIN/LEAVE message format has been extended by 32 bits, period.
Section 5.4 is a notable exception to this, and to a lesser extent
so is section 5.3. Other tweaks were added as inspiration took me
during the rewrite session.]
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modified to support multiple address groups. Scattered
editorial/clarificatory changes have been made to the rest of the
document. Editorial notes will be removed.]
Contents.
1. Introduction.
1.1 The Multicast support allows a source host or protocol entity to send a
packet to multiple destinations simultaneously using a single, local
'transmit' operation. This facility is utilized by network layer
protocols such IP. Most models, like the one described in RFC 1112
[1] for IP multicasting, assume sources may send their packets to an
abstract 'multicast group addresses'. Link layer support for such an
abstraction is assumed to exist, and is provided by technologies such
as Ethernet. Address Resolution Server (MARS).
1.2 The ATM is being utilized as a new link layer technology to level multicast Cluster.
1.3 Document overview.
2. The IP multicast service model.
3. UNI 3.1 support a
variety of protocols, including IP. With RFC 1483 [2] the IETF
defined a multiprotocol mechanism for encapsulating and transmitting
packets using AAL5 over ATM Virtual Channels (VCs). However, the ATM
Forum's currently published signalling specification (UNI 3.0 [4], intra-cluster multicasting.
3.1 VC meshes.
3.2 Multicast Servers.
3.3 Tradeoffs.
3.4 Interaction with additions for local UNI 3.1 released in late 1994) does not provide signalling entity.
4. Overview of the multicast address abstraction. Unicast connections are supported
by point to point, bidirectional VCs. Multicasting is supported
through point to multipoint VCs. The key limitation is that MARS.
5. Endpoint (MARS client) interface behaviour.
5.1 Transmit side behaviour.
5.1.1 Retrieving Group Membership from the
sender must have prior knowledge of each intended recipient, MARS.
5.1.2 MARS_REQUEST, MARS_MULTI, and
explicitly establish a VC with itself as MARS_NAK messages.
5.1.3 Establishing the root node and outgoing multipoint VC.
5.1.4 Monitoring updates on ClusterControlVC.
5.1.4.1 Updating the
recipients as active VCs.
5.1.4.2 Tracking the Cluster Sequence Number.
5.1.5 Revalidating a VC's leaf nodes.
The main goal of this document
5.1.5.1 When leaf node drops itself.
5.1.5.2 When a jump is to define an address registration
and distribution mechanism that allows UNI 3.1 based networks to
support detected in the multicast service of protocols such as IP. The second
goal is to define specific endpoint behaviour and management of point
to multipoint VCs. As the IETF is currently in the forefront of
using wide area multicasting this document's descriptions will focus
on IP version 4 (IPv4). A final chapter will note the more general
application of the architecture.
The Multicast Address Resolution Server (MARS), a distant relative CSN.
5.2. Receive side behaviour.
5.2.1 Format of the ATM ARP Server introduced in RFC 1577 [3], acts as a registry MARS_JOIN and MARS_LEAVE Messages.
5.2.1.1 Important IPv4 default values.
5.2.2 Retransmission of
multicast group membership. MARS messages, based on the ATM ARP
format, support MARS_JOIN and MARS_LEAVE messages.
5.2.3 Registering with the distribution of multicast MARS.
5.3 Support for Layer 3 group membership
information between management.
5.4 Support for redundant/backup MARS and hosts or endpoints. Endpoint address
resolution entities query the entities.
5.4.1 First response to MARS when a multicast group address
needs problems.
5.4.2 Connecting to be resolved. The actual mechanism a backup MARS.
5.4.3 Dynamic backup lists, and soft redirects.
5.5 LLC/SNAP encapsulations for multicasting data
packets may be through meshes of point transmit and receive.
6. The MARS in greater detail.
6.1 Class I MARS requirements.
6.2 Class II MARS requirements.
6.2.1 Class II MARS response to multipoint VCs, or the use
of a MARS_REQUEST.
6.2.2 MARS_MSERV and MARS_UNSERV messages.
6.2.3 Registering a Multicast Servers. To provide Server (MCS).
6.2.4 Class II response to MARS_JOIN and MARS_LEAVE.
6.2.5 Sequence numbers for asynchronous notification of
group membership changes the ServerControlVC traffic.
6.3 Why global sequence numbers?
6.4 Redundant/Backup MARS manages two point to multipoint VCs Architectures.
7. How an MCS utilises a Class II MARS.
8. Support for IP multicast routers.
8.1 Forwarding into a Cluster.
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- one out to all endpoints desiring multicast support, and
8.2 Joining in 'promiscuous' mode.
8.3 Forwarding across the other cluster.
8.4 Joining in 'semi-promiscous' mode.
8.5 An alternative to all multicast servers registered as providing support to any
multicast groups. The choice of mesh or multicast server is
configurable on a group by group basis.
The numerical size IGMP Queries.
9. Multiprotocol applications of link layer multicast groups will be constrained
by practical concerns such as limited VC support within endpoint ATM
interfaces. Each the MARS manages a 'cluster' and MARS clients.
10. Key Decisions and open issues.
Acknowledgments
Appendix A. Hole punching algorithms for Class II MARS messages.
Appendix B. Minimising the impact of ATM-attached endpoints.
A cluster IGMP in IPv4 environments.
Appendix C. Further comments on 'Clusters'.
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1. Introduction.
Multicasting is defined as the process whereby a set of endpoints willing to be grouped
together as link layer members of multicast groups. It is assumed
that specially configured routers are used source host or protocol entity
sends a packet to pass multicast traffic
between clusters. This document explicitly avoids specifying the
nature of inter-cluster multicast routing protocols. multiple destinations simultaneously using a
single, local 'transmit' operation. The mapping more familiar cases of clusters
Unicasting and Broadcasting may be considered to other constrained sets be special cases of endpoints (such
as Logical IP Subnets) is left
Multicasting (with the packet delivered to one destination, or 'all'
destinations, respectively).
Most network administrators. A simple
approach layer models, like the one described in overlaid RFC 1112 [1] for
IP environments would be multicasting, assume sources may send their packets to an abstract
'multicast group addresses'. Link layer support for each LIS such an
abstraction is assumed to be
served exist, and is provided by a separate MARS, with the cluster being built from the LIS
members. IP multicast routers would interconnect each LIS technologies such
as they do
with conventional subnets. However, there Ethernet.
ATM is no requirement that being utilized as a
cluster be limited new link layer technology to support a single LIS.
Section 2 provides an overview
variety of IP multicast and what protocols, including IP. With RFC 1112
required from Ethernet. Section 3 outlines 1483 [2] the set of generic
functions that should be available to clients of IETF
defined a local host's multiprotocol mechanism for encapsulating and transmitting
packets using AAL5 over ATM Virtual Channels (VCs). However, the ATM
Forum's currently published signalling specification (UNI 3.0 [4],
with additions for UNI 3.1 signalling service. Section 4 specifies released in late 1994) does not provide
the encapsulation to be
used for MARS messages and multicast packet traffic. address abstraction. Unicast connections are supported
by point to point, bidirectional VCs. Multicasting is supported
through point to multipoint VCs. The basic
behaviour for key limitation is that the sending side
sender must have prior knowledge of an interface is described in
section 5, each intended recipient, and
explicitly establish a VC with section 6 covering itself as the mechanism whereby a host joins root node and leaves multicast groups. Sections 7 covers the way in which hosts
respond to dynamic
recipients as the leaf nodes.
This document has two broad goals:
Define a group address registration and membership changes. Configuring distribution
mechanism that allows UNI 3.1 based networks to support the use
multicast service of
Multicast Servers is covered in section 8. Support protocols such as IP.
Define specific endpoint behaviour for multicast
routers is described in section 9, and section 10 explains the
features included managing point to improve the reliability
multipoint VCs to achieve efficient multicasting of layer 3
packets.
As the membership
management mechanisms. Section 11 discusses IETF is currently in the application forefront of using wide area
multicasting this
document beyond IP. Section 12 is a summary of the documents key
points.
The appendices provide discussion document's descriptions will often focus on issues that arise out the
implementation IP
service model of this memo. Appendix RFC 1112. A discusses MARS and endpoint
algorithms for parsing MARS messages. Appendix B describes the
particular problems introduced by final chapter will note the current IGMP paradigms, and
possible interim work-arounds. Finally, Appendix C covers
multiprotocol application of the various
designs that are possible for multicast server support within
clusters. architecture.
This document assumes an understanding avoids discussion of concepts explained one highly non-trivial aspect of
using ATM - the specification of QoS for VCs being established in
greater detail
response to higher layer needs. Research in RFC 1112, RFC 1577, UNI 3.1, this area is still very
formative, and <draft-ietf- so it is assumed that future documents will further
clarify the mapping of QoS requirements to VC establishment. The
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ipatm-sig-02.txt>.
2. Review of RFC 1112 and IP Multicast over Ethernet.
Under IP version 4 (IPv4) ddresses in
default at this time is that VCs SHOULD be established with a request
for Unspecified Bit Rate (UBR) service (as typified by the range IETF's use
of 224.0.0.0 and
239.255.255.255 are termed 'Class D' or 'multicast group' addresses.
In VCs for unicast IP, described in RFC 1112 the behaviour 1755 [6]).
1.1 The Multicast Address Resolution Server (MARS).
The Multicast Address Resolution Server (MARS) is a superset of the transmit and receive sides are quite
independent, making the concept of being
ATM ARP Server introduced in RFC 1577 [3]. It acts as a 'member' of an IP registry,
associating layer 3 multicast group imprecise at identifiers with the link layer interface.
The interface must ATM
interfaces representing the group's members. MARS messages, based on
the ATM ARP format, support the transmission distribution of IP packets to an IP multicast group address, whether
membership information between MARS and endpoints (hosts or not routers).
Endpoint address resolution entities query the node considers itself MARS when a
'member' of that group. Consequently, group membership is effectively
irrelevant layer 3
address needs to be resolved to the transmit side set of ATM endpoints making up
the link group at any one time. Endpoints keep the MARS informed when they
need to join or leave particular layer interfaces. No
address resolution is required 3 groups. To provide for
asynchronous notification of group membership changes the MARS
manages a point to transmit multipoint VC out to all endpoints desiring
multicast support
Valid arguments can be made for two different approaches to ATM level
multicasting of layer 3 packets - an algorithmic
mapping from IP through meshes of point to
multipoint VCs, or ATM level multicast address servers (MCS). Two classes of
MARS are described - Class I (allowing VC meshes to Ethernet support layer 3
traffic), and Class II (which allows either VC meshes or MCSs to be
assigned for use on a per-group basis).
1.2 The ATM level multicast address Cluster.
Each MARS manages a 'cluster' of ATM-attached endpoints. A Cluster is
performed locally before the packet
defined as
The set of ATM interfaces chosen to participate in direct ATM
connections to achieve multicasting of AAL_SDUs between
themselves.
In practice, a Cluster is sent out the local interface
in set of endpoints that choose to use the
same 'send MARS to register their memberships and forget' manner as a unicast receive their updates
from.
By implication of this definition, traffic between interfaces
belonging to different Clusters passes through an inter-cluster
device. (In the IP packet.
Joining and Leaving world an inter-cluster device would be an IP
multicast group is more explicit on the
receive side - router with logical interfaces into each Cluster.) This
document explicitly avoids specifying the primitives JoinLocalGroup and LeaveLocalGroup
affecting what groups the local link layer interface should accept
packets from. When the IP layer wants nature of inter-cluster
(layer 3) routing protocols.
The mapping of clusters to receive packets from a
group, it issues JoinLocalGroup. When it no longer wants other constrained sets of endpoints (such
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as unicast Logical IP Subnets) is left to receive
packets, it issues LeaveLocalGroup. each network administrator.
A key point simple approach in overlaid IP environments would be for each LIS
to note is that
changing state is be served by a local issue, it has no affect on other hosts
attached to separate MARS, with the Ethernet.
IGMP is defined in RFC 1112 to support cluster being built from
the LIS members. IP multicast routers attached
to a given subnet. Hosts issue IGMP Report messages when would interconnect each LIS as
they perform do with conventional subnets. However, there is no requirement
that a JoinLocalGroup, or in response cluster be limited to a single LIS.
1.3 Document overview.
This document assumes an IP multicast router sending understanding of concepts explained in
greater detail in RFC 1112, RFC 1577, UNI 3.1, and RFC 1755 [6].
Section 2 provides an
IGMP Query. By periodically transmitting queries overview of IP multicast routers
are able to identify and what IP multicast groups have non-zero
membership on a given subnet.
A specific IP RFC 1112
required from Ethernet.
Section 3 describes in more detail the multicast address, 224.0.0.1, is allocated for support services
offered by UNI 3.1, and outlines the
transmission of IGMP Query messages. All IP differences between VC meshes
and multicast hosts must
issue JoinLocalGroup servers (MCSs) as mechanisms for 224.0.0.1 during their initialisation. Each
host keeps distributing packets
to multiple destinations.
Section 4 provides an overview of the MARS and its relationship to
ATM endpoints. This section also discusses the encapsulation of MARS
control messages, and some encapsulation issues for data traffic.
Section 5 substantially defines the entire cluster member endpoint
behaviour, on both receive and transmit sides. This includes both
normal operation and error recovery.
Section 6 summarises the requirements of a list Class I MARS, and provides
a detailed description of IP the Class II MARS.
Section 7 looks at how a multicast groups it has been JoinLocalGroup'd
to. When server (MCS) interacts with a router issues an IGMP Query on 224.0.0.1 each host begins
to send IGMP Reports for each
Class II MARS.
Section 8 discusses how IP multicast routers may make novel use of
promiscuous and semi-promiscuous group it joins. Also discussed is a member of. IGMP Reports
are sent
mechanism designed to reduce the group address, not 224.0.0.1, "so that other members amount of IGMP traffic issued by
routers.
Section 9 discusses how this document applies in the same group on the same network can overhear more general
(non-IP) case.
Section 10 summarises the Report" key proposals, and
not bother sending one of their own. IP multicast routers conclude identifies areas for
future research that a group has no members are generated by this MARS architecture.
The appendices provide discussion on issues that arise out the
implementation of this memo. Appendix A discusses MARS and endpoint
algorithms for parsing MARS messages. Appendix B describes the subnet when IGMP Queries no longer
elict associated replies.
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3. Multicast support under UNI 3.1.
This document will describe its operation in terms of 'generic'
functions that should be available to clients
particular problems introduced by the current IGMP paradigms, and
possible interim work-arounds. Finally, Appendix C discusses the use
of a UNI 3.1 signalling
entity 'clusters' in a given ATM endpoint. The ATM model broadly describes 'AAL
Users' as any entity that establishes and manages VCs and underlying
AAL further detail.
2. Summary of the IP multicast service to exchange data. An model.
Under IP over ATM interface is a form version 4 (IPv4), addresses in the range of
'AAL User' (either directly, when VC multiplexing is used, 224.0.0.0 and
239.255.255.255 are termed 'Class D' or
indirectly, when LLC/SNAP encpasulation is used).
The most fundamental limitations 'multicast group' addresses.
These abstractly represent all the IP hosts in the Internet (or some
constrained subset of UNI 3.1's multicast support are:
Only point the Internet) who have decided to multipoint, unidirectional VCs may be established.
Only 'join' the root node of
specified group.
RFC1112 requires that a given VC may add or remove leaf nodes.
Within these constraints, multicast group members can communicate by multicast-capable IP interface must support
the use transmission of IP packets to an IP multicast meshes, group address,
whether or multicast servers. With a mesh each
transmitting host is not the Root of node considers itself a point to multipoint VC 'member' of that has
every other host in the group as a Leaf. The Multicast Server model
has every group.
Consequently, group member send their packets directly membership is effectively irrelevant to a 'server'
entity somewhere on the ATM cloud, which then retransmits copies to
all other members.
This document defines
transmit side of the MARS-Endpoint signalling link layer interfaces. When Ethernet is used as
the link layer (the example used in RFC1112), no address resolution
is required to
support both mechanisms. Issues relating transmit packets. An algorithmic mapping from IP
multicast address to the architecture,
operation, and management of Ethernet multicast servers are discussed in
Appendix C.
The following generic signalling functions are presumed to be
available to address is performed locally
before the packet is sent out the local AAL Users:
L_CALL_RQ - Establish interface in the same 'send
and forget' manner as a unicast VC to a specific endpoint.
L_MULTI_RQ - Establish IP packet.
Joining and Leaving an IP multicast VC to a specific endpoint.
L_MULTI_ADD - Add new leaf node to previously established VC.
L_MULTI_DROP - Remove specific leaf node from established VC.
L_RELEASE group is more explicit on the
receive side - Release unicast VC, or all Leaves of a multicast VC.
The signalling exchanges and local information passed between AAL
User and UNI 3.1 signalling entity with these functions is currently
beyond the scope of this document.
The following indications are assumed to be available to AAL Users,
generated by by primitives JoinLocalGroup and LeaveLocalGroup
affecting what groups the local UNI 3.1 signalling entity:
L_ACK - Succesful completion of link layer interface should accept
packets from. When the IP layer wants to receive packets from a request
group, it issues JoinLocalGroup. When it no longer wants to signalling
entity.
L_REMOTE_CALL - receive
packets, it issues LeaveLocalGroup. A new VC key point to note is that
changing state is a local issue, it has been established no affect on other hosts
attached to the AAL User.
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ERR_L_RQFAILED - A remote ATM endpoint rejected an L_CALL_RQ,
L_MULTI_RQ, or L_MULTI_ADD.
ERR_L_RELEASE - A remote ATM endpoint has elected Ethernet.
IGMP is defined in RFC 1112 to terminate a
pre-existing VC.
The signalling exchanges and local information passed between AAL
User support IP multicast routers attached
to a given subnet. Hosts issue IGMP Report messages when they perform
a JoinLocalGroup, or in response to an IP multicast router sending an
IGMP Query. By periodically transmitting queries IP multicast routers
are able to identify what IP multicast groups have non-zero
membership on a given subnet.
A specific IP multicast address, 224.0.0.1, is allocated for the
transmission of IGMP Query messages. All IP multicast hosts must
issue JoinLocalGroup for 224.0.0.1 during their initialisation. Each
host keeps a list of IP multicast groups it has been JoinLocalGroup'd
to. When a router issues an IGMP Query on 224.0.0.1 each host begins
to send IGMP Reports for each group it is a member of. IGMP Reports
are sent to the group address, not 224.0.0.1, "so that other members
of the same group on the same network can overhear the Report" and
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not bother sending one of their own. IP multicast routers conclude
that a group has no members on the subnet when IGMP Queries no longer
elict associated replies.
3. UNI 3.1 signalling entity with these support for intra-cluster multicasting.
This document will describe its operation in terms of 'generic'
functions is currently
beyond the scope that should be available to clients of this document. a UNI 3.1 defines two signalling
entity in a given ATM address formats - E.164 and ISO NSAP. In UNI
3.1 endpoint. The ATM model broadly describes an 'ATM Number' is
'AAL User' as any entity that establishes and manages VCs and
underlying AAL services to exchange data. An IP over ATM interface is
a form of 'AAL User' (although the default LLC/SNAP encapsulation
mode specified in RFC1755 really requires that an 'LLC entity' is the
AAL User, which in turn supports the IP/ATM interface).
The most fundamental limitations of UNI 3.1's multicast support are:
Only point to multipoint, unidirectional VCs may be established.
Only the root (source) node of a given VC may add or remove leaf
nodes.
Leaf nodes are identified by their unicast ATM addresses. UNI 3.1
defines two ATM address formats - native E.164 and NSAP (although it
must be stressed that the NSAP address is so called because it uses
the NSAP format - an ATM endpoint is NOT a Network layer termination
point). In UNI 3.1 an 'ATM Number' is the primary identification of
an ATM endpoint, and it may use either format. Under some
circumstances an ATM endpoint must be identified by both a native
E.164 address (identifying the attachment point of a private network
to a public network), and an NSAP address ('ATM Subaddress')
identifying the final endpoint within the private network. For the
rest of this document the term will be used to mean either a single
'ATM Number' or an 'ATM Number' combined with an 'ATM Subaddress'.
3.1 VC meshes.
The most fundamental approach to intra-cluster multicasting is the
multicast VC mesh. Each source establishes its own independent point
to multipoint VC (a single multicast tree) to the set of leaf nodes
(destinations) that it has been told are members of the group it
wishes to send packets to.
Interfaces that are both senders and group members (leaf nodes) to a
given group will originate one point to multipoint VC, and terminate
one VC for every other active sender to the group. This criss-
crossing of VCs across the ATM network gives rise to the name 'VC
mesh'.
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3.2 Multicast Servers.
An alternative model has each source establish a VC to an
intermediate node - the multicast server (MCS). The multicast server
itself establishes and manages a point to multipoint VC out to the
actual desired destinations.
The MCS reassembles AAL_SDUs arriving on all the incoming VCs, and
then queues them for transmission on its single outgoing point to
multipoint VC. (Reassembly of incoming AAL_SDUs is required at the
multicast server as AAL5 does not support cell level multiplexing of
different AAL_SDUs on a single outgoing VC.)
The leaf nodes of the multicast server's point to multipoint VC must
be established prior to packet transmission, and the multicast server
requires an external mechanism to identify them. A side-effect of
this method is that ATM interfaces that are both sources and group
members will receive copies of their own packets back from the MCS
(An alternative method is for the multicast server to explicitly
retransmit packets on individual VCs between itself and group
members. A benefit of this second approach is that the multicast
server can ensure that sources do not receive copies of their own
packets.)
An MCS does NOT pay any attention to the contents of each AAL_SDU. It
is purely an AAL/ATM level device.
3.3 Tradeoffs.
Arguments over the relative merits of VC meshes and multicast servers
have raged for some time. Ultimately the choice depends on the
relative trade-offs a system administrator must make between
throughput, latency, congestion, and resource consumption. Even
criteria such as latency can mean different things to different
people - is it end to end packet time, or the time it takes for a
group to settle after a membership change? The final choice depends
on the characteristics of the applications generating the multicast
traffic.
If we focussed on the data path we might prefer the VC mesh because
it lacks the obvious single congestion point of an MCS. Throughput
is likely to be higher, and end to end latency lower, because the
mesh lacks the intermediate AAL_SDU reassembly that must occur in
MCSs. The underlying ATM signalling system also has greater
opportunity to ensure optimal branching points at ATM switches along
the multicast trees originating on each source.
However, resource consumption will be higher. Every group member's
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ATM interface must terminate a VC per sender (consuming on-board
memory for state information, instance of an AAL service, and
buffering in accordance with the vendors particular architecture). On
the contrary, with a multicast server only 2 VCs (one out, one in)
are required, independent of the number of senders. The allocation of
VC related resources is also lower within the ATM cloud when using a
multicast server. These points may be considered to have merit in
environments where VCs across the UNI or within the ATM cloud are
valuable (e.g. the ATM provider charges on a per VC basis), or AAL
contexts are limited in the ATM interfaces of endpoints (many current
implementations allow only 2k, 1k, or less).
If we focus on the signalling load then MCSs have the advantage when
faced with dynamic sets of receivers. Every time the membership of a
multicast group changes (a leaf node needs to be added or dropped),
only a single point to multipoint VC needs to be modified when using
an MCS. This generates a single signalling event across the MCS's
UNI. However, when membership change occurs in a VC mesh, signalling
events occur at the UNIs of every traffic source - the transient
signalling load scales with the number of sources. This has obvious
ramifications if you define latency as the time for a group's
connectivity to stabilise after change (especially as the number of
senders increases).
Finally, as noted above, MCSs introduce a 'reflected packet' problem,
which requires additional per-AAL_SDU information to be carried in
order for layer 3 sources to detect their own AAL_SDUs coming back.
The Class II MARS allows system administrators to utilize either
approach on a group by group basis.
3.4 Interaction with local UNI 3.1 signalling entity.
The following generic signalling functions are presumed to be
available to local AAL Users:
L_CALL_RQ - Establish a unicast VC to a specific endpoint.
L_MULTI_RQ - Establish multicast VC to a specific endpoint.
L_MULTI_ADD - Add new leaf node to previously established VC.
L_MULTI_DROP - Remove specific leaf node from established VC.
L_RELEASE - Release unicast VC, or all Leaves of a multicast VC.
The signalling exchanges and local information passed between AAL
User and UNI 3.1 signalling entity with these functions are outside
the scope of this document.
The following indications are assumed to be available to AAL Users,
generated by by the local UNI 3.1 signalling entity:
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L_ACK - Succesful completion of a local request.
L_REMOTE_CALL - A new VC has been established to the AAL User.
ERR_L_RQFAILED - A remote ATM endpoint rejected an L_CALL_RQ,
L_MULTI_RQ, or L_MULTI_ADD.
ERR_L_RELEASE - A remote ATM endpoint terminated an existing VC.
The signalling exchanges and local information passed between AAL
User and UNI 3.1 signalling entity with these functions are outside
the scope of this document.
4. Overview of the MARS.
The MARS may reside within any ATM endpoint that is directly
addressable by the endpoints it is serving. Endpoints wishing to join
a multicast cluster must be configured with the ATM address of the
node on which the cluster's MARS resides. (Section 5.4 describes how
backup MARSs may be added to support the activities of a cluster.
References to 'the MARS' in following sections will be assumed to
mean the acting MARS for the cluster.)
Architecturally the MARS is an evolution of the RFC 1577 ARP Server.
Whilst the ARP Server keeps a table of {IP,ATM} address pairs for all
IP endpoints in an LIS, the MARS keeps extended tables of {layer 3
address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either be
configured with certain mappings, or dynamically 'learn' mappings.
The format of the {layer 3 address} field is generally not
interpreted by the MARS (except for a few special cases, described
later).
A single MARS may not support more than one cluster (by definition).
However, a single ATM node may support multiple logical MARSs, each
of which support a separate cluster. The restriction is that each
MARS has a unique ATM address (e.g. a different SEL field in the NSAP
address of the node on which the multiple MARSs reside)
Two classes of MARS are defined in this memo - Class I (with the
minimum support required to enable multicasting using VC meshes), and
Class II (Class I + extensions to support the introduction of MCSs).
Both Class I and Class II MARS distributes group membership
information to cluster members over a point to multipoint VC known as
the ClusterControlVC. A Class II MARS also establishes a separate
point to multipoint VC out to registered MCSs, known as the
ServerControlVC. All cluster members are leaf nodes of
ClusterControlVC. All registered multicast servers are leaf nodes of
ServerControlVC (described further in section 6).
The MARS message format is an extension of the ATM ARP message
format. By default all MARS messages MUST be LLC/SNAP encapsulated
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in accordance with RFC 1483, using the same encapsulation as ATM ARP:
[0xAA-AA-03][0x00-00-00][0x08-06][MARS message]
(LLC) (OUI) (PID)
The choice of common encapsulation and message format means that MARS
and ARP Server functionality may be implemented within a common
entity if a network designer so chooses.
Finally, the MARS does NOT take part in the actual multicasting of
layer 3 data packets.
5. Endpoint (MARS client) interface behaviour.
This section describes in detail the operation of what might best be
thought of as a 'shim layer', sitting between your layer 3 protocol's
link layer interface and the underlying UNI 3.1 service. An endpoint
in this context can be a host or a router - any entity that requires
a generic 'layer 3 over ATM' interface to support layer 3 multicast.
It is broken into two key subsections - one for the transmit side,
and one for the receive side.
Multiple logical ATM interfaces may be supported by a single physical
ATM interface (for example, using different SEL values in the NSAP
formatted address assigned to the physical ATM interface). Therefore
implementors MUST allow for multiple independent 'layer 3 over ATM'
interfaces too, each with its own configured MARS (or table of MARSs,
as discussed in section 5.4), and ability to be attached to the same
or different clusters.
The primary signalling paths between a MARS client (managing an
endpoint) and their associated MARS is a transient point to point,
bidirectional VC. This VC is established by the MARS client, and is
used to send queries to, and receive replies from, the MARS. It has
an associated idle timer, and is dismantled if not used for a
configurable period of time. The minimum suggested value for this
time is 1 minute, and the RECOMMENDED default is 20 minutes. Where
the MARS and ARP Server are co-resident, this VC may be used for both
ATM ARP traffic and MARS traffic.
Most of this specification is concerned with managing and
distributing information that allows the establishment of VCs for
actually carrying layer 3 data packets. The actual format of the data
carried on these VCs is almost completely outside the scope of this
specification. However, when using MCSs (described in section 3)
endpoints need to filter out the reflected packets that can occur.
The solution to this problem in a general way requires the use of
additional per-packet encapsulation. This is discussed in section 5.5
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MARS messages contain variable length address fields. In all cases
null addresses MUST be encoded as zero length, and have no space
allocated in the message. Addresses with non-zero length, but zero
value can have specific meanings to the MARS, and MUST NOT be used in
any other fashion.
5.1 Transmit side behaviour.
The following description will often be in terms of an IP/ATM
interface that is capable of transmitting packets to a Class D
address at any time, without prior warning. It should be trivial for
an implementor to generalise this behaviour to the requirements of
another layer 3 data protocol.
When a packet arrives for transmission, and there is no outgoing VC
already marked as serving the packet's multicast destination address,
the MARS is queried for the set of ATM endpoints currently making up
the multicast group.
The query is executed by issuing a MARS_REQUEST. The MARS_REQUEST
message is formatted as an ATM ARP_REQUEST (RFC 1577) with operation
type code (ar$op field) of 11 (decimal). The reply from the MARS may
take one of two forms:
MARS_MULTI - Sequence of MARS_MULTI messages returning the set of
ATM endpoints that are to be leaf nodes of the
outgoing VC.
MARS_NAK - No mapping found, group is empty.
5.1.1 Retrieving Group Membership from the MARS.
If the MARS had no mapping for the desired Class D address a MARS_NAK
will be returned. In this case the IP packet MUST be discarded
silently. If a match is found in the MARS's tables it proceeds to
return addresses ATM.1 through ATM.n in a sequence of one or more
MARS_MULTIs. A simple mechanism is used to detect and recover from
loss of MARS_MULTI messages.
Each MARS_MULTI carries a boolean field x, and a 15 bit integer field
y - expressed as MARS_MULTI(x,y). Field y acts as a sequence number,
starting at 1 and incrementing for each MARS_MULTI sent. Field x
acts as an 'end of reply' marker. When x == 1 the MARS response is
considered complete.
In addition, each MARS_MULTI may carry multiple ATM addresses from
the set {ATM.1, ATM.2, .... ATM.n}. A MARS MUST minimise the number
of MARS_MULTIs transmitted by placing as many group member's
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addresses in a single MARS_MULTI as possible. The limit on the length
of an individual MARS_MULTI message MUST be the MTU of the underlying
VC.
Assume n ATM addresses must be returned, each MARS_MULTI is limited
to only p ATM addresses, and p << n. This would require a sequence of
k MARS_MULTI messages (where k = (n/p)+1, using integer arithmetic),
transmitted as follows:
MARS_MULTI(0,1) carries back {ATM.1 ... ATM.p}
MARS_MULTI(0,2) carries back {ATM.(p+1) ... ATM.(2p)}
[.......]
MARS_MULTI(1,k) carries back { ... ATM.n}
If k == 1 then only MARS_MULTI(1,1) is sent.
Typical failure mode will be losing one or more of MARS_MULTI(0,1)
through MARS_MULTI(0,k-1). This is detected when y jumps by more than
one between consecutive MARS_MULTI's. An alternative failure mode is
losing MARS_MULTI(1,k). A timer MUST be implemented to flag the
failure of the last MARS_MULTI to arrive. A default value of 10
seconds is suggested.
If a 'sequence jump' is detected, the host MUST wait for the
MARS_MULTI(1,k), discard all results, and repeat the MARS_REQUEST.
If a timeout occurs, the host MUST discard all results, and repeat
the MARS_REQUEST.
(Corruption of cell contents will lead to loss of a MARS_MULTI
through AAL5 CPCS_PDU reassembly failure, which will be detected
through the mechanisms described above.)
If the MARS is managing a cluster of endpoints spread across
different but directly accessible ATM networks it will not be able to
return all the group members in a single MARS_MULTI. The MARS_MULTI
message format allows for either E.164, ISO NSAP, or (E.164 + NSAP)
to be returned as ATM addresses. However, each MARS_MULTI message may
only return ATM addresses of the same type and length. The returned
addresses MUST be grouped according to type (E.164, ISO NSAP, or
both) and returned in a sequence of separate MARS_MULTI parts.
5.1.2 MARS_REQUEST, MARS_MULTI, and MARS_NAK messages.
MARS_REQUEST is an RFC1577 ATM ARP_REQUEST, but with an 'operation
type value' of 11 (decimal). The multicast address being resolved is
placed into the the target protocol address field (ar$tpa). The
target hardware address is set to null (ar$thtl and ar$tstl both
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zero). The hardware type (ar$hrd) is set to 19 (decimal), and in IP
environments the protocol type is 2048 (decimal). Section 6.6 of RFC
1577 should be consulted for specific details and coding of the
ar$shtl and ar$sstl fields.
MARS_NAK is the MARS_REQUEST returned with operation type value of 16
(decimal). All other fields should be left unchanged from the
MARS_REQUEST.
The MARS_MULTI message is identified by an 'operation type value' of
12 (decimal). The message format is:
Data:
ar$hrd 16 bits Hardware type ( 19 decimal, 0x13 hex)
ar$pro 16 bits Protocol type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_MULTI)
ar$spln 8 bits Length of source protocol address (s)
ar$thtl 8 bits Type & length of target ATM number (x)
ar$tstl 8 bits Type & length of target ATM subaddress (y)
ar$tpln 8 bits Length of target multicast group address (z)
ar$tnum 16 bits Number of target ATM addresses returned (N).
ar$seqxy 16 bits Boolean flag x and sequence number y.
ar$msn 32 bits MARS Sequence Number.
ar$sha qoctets source ATM number
ar$ssa roctets source ATM subaddress
ar$spa soctets source protocol address
ar$tpa zoctets target multicast group address
ar$tha.1 xoctets target ATM number 1
ar$tsa.1 yoctets target ATM subaddress 1
ar$tha.2 xoctets target ATM number 2
ar$tsa.2 yoctets target ATM subaddress 2
[.......]
ar$tha.N xoctets target ATM number N
ar$tsa.N yoctets target ATM subaddress N
ar$seqxy is coded with flag x in the leading bit, and sequence number
y coded as an unsigned integer in the remaining 15 bits.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x| y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ar$tnum is an unsigned integer indicating how many pairs of
{ar$tha,ar$tsa} (i.e. how many group member's ATM addresses) are
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present in the message. ar$msn is an unsigned 32 bit number filled in
by the MARS before transmitting each MARS_MULTI. Its use is described
further in section 5.1.4. Section 6.6 of RFC 1577 should be consulted
for specific details and coding of all other fields.
As an example, assume we have a multicast cluster using 4 byte
protocol addresses, 20 byte ATM numbers, and 0 byte ATM subaddresses.
For n group members in a single MARS_MULTI we require a (44 + 20n)
byte message. If we assume the default MTU of 9180 bytes, we can
return a maximum of 456 group member's addresses in a single
MARS_MULTI.
5.1.3 Establishing the outgoing multipoint VC.
Following the completion of the MARS_MULTI reply the endpoint may
establish a new point to multipoint VC, or reuse an existing one.
If establishing a new VC, an L_MULTI_RQ is issued for ATM.1, followed
by an L_MULTI_ADD for every member of the set {ATM.2, ....ATM.n}
(assuming the set is non-null). The packet is then transmitted over
the newly created VC just as it would be for a unicast VC.
After transmitting the packet, the local interface holds the VC open
and marks it as the active path out of the host for any subsequent IP
packets being sent to that Class D address.
When establishing a new multicast VC it is possible that one or more
L_MULTI_RQ or L_MULTI_ADD may fail. The UNI 3.1 failure cause must
be returned in the ERR_L_RQFAILED signal from the local signalling
entity to the AAL User. If the failure cause is not 49 (Quality of
Service unavailable) or 51 (user cell rate not available), the
endpoint's ATM address is dropped from the set {ATM.1, ATM.2, ...,
ATM.n} returned by the MARS. Otherwise, the L_MULTI_RQ or
L_MULTI_ADD should be reissued after a delay of 10 to 20 seconds. If
the request fails again, another request should be issued after twice
the previous delay has elapsed. This process should be continued
until the call succeeds or the multipoint VC gets released.
If the initial L_MULTI_RQ fails for ATM.1, and n is greater than 1
(i.e. the returned set of ATM addresses contains 2 or more addresses)
a new L_MULTI_RQ should be immediately issued for the next ATM
address in the set. This procedure is repeated until an L_MULTI_RQ
succeeds, as no L_MULTI_ADDs may be issued until an initial outgoing
VC is established.
Each ATM address for which an L_MULTI_RQ failed with cause 49 or 51
MUST be tagged rather than deleted. An L_MULTI_ADD is issued for
these tagged addresses using the random delay procedure outlined
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above.
The VC MAY be considered 'up' before failed L_MULTI_ADDs have been
successfully re-issued. An endpoint MAY implement a concurrent
mechanism that allows data to start flowing out the new VC even while
failed L_MULTI_ADDs are being re-tried. (The alternative of waiting
for each leaf node to accept the connection could lead to significant
delays in transmitting the first packet.)
Each VC MUST have a configurable inactivity timer associated with it.
If the timer expires, an L_RELEASE is issued for that VC, and the
Class D address is no longer considered to have an active path out of
the local host. The timer SHOULD be no less than 1 minute, and a
default of 20 minutes is RECOMMENDED. Choice of specific timer
periods is beyond the scope of this document.
VC consumption may also be reduced by endpoints noting when a new
group's set of {ATM.1, ....ATM.n} matches that of a pre-existing VC
out to another group. With careful local management, and assuming the
QoS of the existing VC is sufficient for both groups, a new pt to mpt
VC may not be necessary. Under certain circumstances endpoints may
decide that it is sufficient to re-use an existing VC whose set of
leaf nodes is a superset of the new group's membership (in which case
some endpoints will receive multicast traffic for a layer 3 group
they haven't joined, and must filter them above the ATM interface).
Algorithms for performing this type of optimization are not discussed
here, and are not required for conformance with this memo.
5.1.4 Monitoring updates on ClusterControlVC.
Once a new VC has been established, the transmit side of the cluster
member's interface needs to monitor subsequent group changes - adding
or dropping leaf nodes as appropriate. This is achieved by watching
for MARS_JOIN and MARS_LEAVE messages from the MARS itself. These
messages are described in detail in section 5.2 - at this point it is
sufficient to note that they carry:
- The ATM address of a node joining or leaving a group.
- The layer 3 address of the group(s) being joined or left.
- A Cluster Sequence Number (CSN) from the MARS.
MARS_JOIN and MARS_LEAVE messages arrive at each cluster member
across ClusterControlVC. MARS_JOIN or MARS_LEAVE messages that simply
confirm information already held by the cluster member are used to
track the Cluster Sequence Number, but are otherwise ignored.
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5.1.4.1 Updating the active VCs.
If a MARS_JOIN is seen that refers to (or encompasses) a group for
which the transmit side already has a VC open, the new member's ATM
address is extracted and an L_MULTI_ADD issued locally. This ensures
that endpoints already sending to a given group will immediately add
the new member to their list of recipients.
If a MARS_LEAVE is seen that refers to (or encompasses) a group for
which the transmit side already has a VC open, the old member's ATM
address is extracted and an L_MULTI_DROP issued locally. This ensures
that endpoints already sending to a given group will immediately drop
the old member from their list of recipients. When the last leaf of a
VC is dropped, the VC is closed completely and the affected group no
longer have a path out of the local endpoint (the next outbound
packet to that group's address will trigger the creation of a new VC,
as described in sections 5.1.1 to 5.1.3).
In an IPv4 environment any endpoint leaving 224.0.0.1 is assumed to
be ceasing support for IP multicast operation. If a MARS_LEAVE is
seen that refers to group 224.0.0.1 then the ATM address of the
endpoint specified in the message MUST be removed from every
multipoint VC on which it is listed as a leaf node.
The transmit side of the interface MUST NOT shut down an active VC to
a group for which the receive side has just executed a
LeaveLocalGroup. This behaviour is consistent with the model of
hosts transmitting to groups regardless of their own membership
status.
If a MARS_JOIN or MARS_LEAVE arrives with ar$pnum == 0 it carries no
<min,max> pairs, and is only used for tracking the CSN (and possibly
for confirming the transmission of the local cluster member's own
MARS_JOIN or MARS_LEAVE, as described in section 5.2.2).
5.1.4.2 Tracking the Cluster Sequence Number.
It is important that endpoints do not miss group membership updates
issued by the MARS over ClusterControlVC. However, this will happen
from time to time. The Cluster Sequence Number is carried as an
unsigned 32 bit value in the ar$msn field of many MARS messages
(except for MARS_REQUEST and MARS_NAK). It increments once for every
transmission the MARS makes on ClusterControlVC, regardless of
whether the transmission represents a change in the MARS database or
not. By tracking this counter, cluster members can determine whether
they have missed a previous message on ClusterControlVC, and possibly
a membership change. This is then used to trigger revalidation
(described in section 5.1.5).
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The current CSN is copied into the ar$msn field of MARS messages
being sent to cluster members, whether out ClusterControlVC or on an
point to point VC.
Calculations on the sequence numbers MUST be performed as unsigned 32
bit arithmetic, to ensure no glitches when the counters roll over.
Every cluster member keeps its own 32 bit Host Sequence Number (HSN)
to track the MARS's sequence number. Whenever a message is received
that carries an ar$msn field the following processing is performed:
Seq.diff = ar$msn - HSN
ar$msn -> HSN
{...process MARS message as appropriate...}
if ((Seq.diff != 1) && (Seq.diff != 0))
then {...revalidate group membership information...}
The basic result is that the cluster member attempts to keep locked
in step with membership changes noted by the MARS. If it ever detects
that a membership change occurred (in any group) without it noticing,
it re-validates the membership of all groups it currently has
multicast VCs open to.
The ar$msn value in an individual MARS_MULTI is not used to update
the HSN until all parts of the MARS_MULTI (if more than 1) have
arrived. However, the ar$msn field in consecutive messages of a
multi-part MARS_MULTI MUST be constant. If the ar$msn field changes
before the MARS_MULTI is completely received, then the entire
MARS_MULTI MUST be discarded at the completion of the response, and
the MARS_REQUEST re-issued.
The MARS is free to choose an initial value of CSN. When a new
cluster member starts up it should initialise HSN to zero. When the
cluster member sends the MARS_JOIN to register (described later), the
HSN will be correctly updated to the current CSN value when the
endpoint receives the copy of its MARS_JOIN back from the MARS.
5.1.5 Revalidating a VC's leaf nodes.
Certain events may inform a cluster member that it has incorrect
information about the sets of leaf nodes it should be sending to. If
an error occurs on a VC associated with a particular group, the
cluster member initiates revalidation procedures for that specific
group. If a jump is detected in the Cluster Sequence Number, this
initiates revalidation of all groups to which the cluster member
currently has open point to multipoint VCs.
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Each open and active multipoint VC has a flag associated with it
called 'VC_revalidate'. This flag is checked everytime a packet is
queued for transmission on that VC. If the flag is false, the packet
is transmitted and no further action is required.
However, if the VC_revalidate flag is true then the packet is
transmitted and a new sequence of events is started locally.
Revalidation begins with re-issuing a MARS_REQUEST for the group
being revalidated. The returned set of members {NewATM.1, NewATM.2,
.... NewATM.n} is compared with the set already held locally.
L_MULTI_DROPs are issued on the group's VC for each node that appears
in the original set of members but not in the revalidated set of
members. L_MULTI_ADDs are issued on the group's VC for each node that
appears in the revalidated set of members but not in the original set
of members. The VC_revalidate flag is reset when revalidation
concludes for the given group. Implementation specific mechanisms
will be needed to flag the 'revalidation in progress' state.
The key difference between constructing a VC (section 5.1.3) and
revalidating a VC is that packet transmission continues on the open
VC while it is being revalidated. This minimises the disruption to
existing traffic.
The general algorithm for initiating revalidation is:
- When a packet arrives for transmission on a given group,
the groups membership is revalidated if VC_revalidate == TRUE.
Revalidation resets VC_revalidate.
- When an event occurs that demands revalidation, every
group has its VC_revalidate flag set TRUE at a random time
between 1 and 10 seconds.
Benefit: Revalidation of active groups occurs quickly, and
essentially idle groups are revalidated as needed. Randomly
distributed setting of VC_revalidate flag improves chances of
staggered revalidation requests from senders when a sequence number
jump is detected.
5.1.5.1 When leaf node drops itself.
During the life of a multipoint VC an ERR_L_RELEASE may be received
indicating that a leaf node has terminated its participation at the
ATM level. The ATM endpoint associated with the ERR_L_RELEASE MUST be
removed from the locally held set {ATM.1, ATM.2, .... ATM.n}
associated with the primary identification VC.
After a random period of an ATM endpoint, time between 1 and it may use either format. Under some circumstances an ATM
endpoint must 10 seconds the
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VC_revalidate flag associated with that VC MUST be identified by both an E.164 address (identifying set true.
5.1.5.2 When a jump is detected in the
attachment point CSN.
Section 5.1.4.2 describes how a CSN jump is detected. If a CSN jump
is detected upon receipt of a private network to MARS_JOIN or a public network), MARS_LEAVE then every
outgoing multicast VC MUST have its VC_revalidate flag set true at
some random interval between 1 and an
ISO NSAP address ('ATM Subaddress') identifying the final endpoint
within 10 seconds from when the private network. For CSN jump
was detected.
The only exception to this rule is if a sequence number jump is
detected during the rest establishment of a new group's VC (i.e. a
MARS_MULTI reply was correctly received, but its ar$msn indicated
that some previous MARS traffic had been missed on ClusterControlVC).
In this document case every open VC, EXCEPT the term
'ATM Address' will be used to mean either a single 'ATM Number' or an
'ATM Number' combined with an 'ATM Subaddress'.
4. Overview of one just established, MUST
have its VC_revalidate flag set true at some random interval between
1 and 10 seconds from when the Multicast Address Resolution Server.
The MARS may reside within any ATM endpoint that CSN jump was detected. (The VC being
established at the time is directly
addressable by considered already validated.)
5.2. Receive side behaviour.
A cluster member is a 'group member' (in the endpoints sense that it is serving. Endpoints wishing to join receives
packets directed at a given multicast cluster must be configured with the group) when its ATM address of
appears in the
node on which MARS's table entry for the cluster's MARS resides. This group's multicast address.
A key function within each cluster is the cluster's
Primary MARS. If a distribution of group
membership information from the MARS to cluster is members.
An endpoint may wish to be served by 'join a backup MARS,
endpoints are configured with the ATM address group' in response to a local, higher
level request for membership of a Secondary MARS.
Section 10 will discuss the relationship between group, or because the Primary MARS and
Secondary MARS during failure conditions. Although a Secondary MARS
is optional, endpoint implementations must be capable of utilizing
them as described in section 10. References
supports a layer 3 multicast forwarding engine that requires the
ability to 'the MARS' 'see' intra-cluster traffic in
following sections will be assumed order to forward it.
Two messages support these requirements - MARS_JOIN and MARS_LEAVE.
These are sent to mean the acting MARS for the
cluster.
Architecturally by endpoints when the MARS local layer 3/ATM
interface is similar requested to join or leave a multicast group. The MARS
propagates these messages back out over ClusterControlVC, to ensure
the RFC 1577 ARP Server,
although there is little overlap between the information they manage.
Whilst knowledge of the ARP Server keeps group's membership change is distributed in a table
timely fashion to other cluster members.
Certain models of {IP,ATM} address pairs for all
IP layer 3 endpoints in the LIS, the MARS keeps extended tables of {multicast
address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either (e.g. IP multicast routers)
expect to be
configured with certain mappings, or dynamically 'learn' mappings.
The MARS distributes group membership information able to receive packet traffic 'promiscuously' across
all groups. This functionality may be emulated by allowing routers
to cluster request that the MARS returns them as 'wild card' members
over of all
Class D addresses. However, a point to multipoint VC known as problem inherent in the ClusterControlVC. When
supporting multicast servers within current ATM
model is that a cluster, completely promiscuous router may exhaust the MARS also
establishes local
reassembly resources in its ATM interface. MARS_JOIN supports a separate point
generalisation to multipoint VC known as the
ServerControlVC. All cluster members are leaf nodes notion of
ClusterControlVC. All registered multicast servers are leaf nodes 'wild card' entries, enabling routers
to limit themselves to 'blocks' of
ServerControlVC (Section 8 will discuss the use Class D address space. Use of ServerControlVC).
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The MARS message format
this facility is an extension of the ATM ARP message
format. By default all MARS messages MUST be LLC/SNAP encapsulated described in accordance with RFC 1483, using the same encapsulation greater detail in Section 8.
A block can be as small as 1 (a single group) or as large as ATM ARP:
LLC = 0xAA-AA-03
OUI = 0x00-00-00
PID = 0x08-06
The default for data traffic carried on point to multipoint VCs is
LLC/SNAP encapsulation with a header appropriate to the protocol
being carried. For IP traffic this
entire multicast address space (e.g. default IPv4 'promiscuous'
behaviour). A block is defined in RFC 1483 as:
LLC = 0xAA-AA-03
OUI = 0x00-00-00
PID = 0x08-00
The choice of common encapsulation and message format means that MARS as all addresses between, and ARP Server functionality may be implemented within
inclusive of, a common
entity if <min,max> address pair. A MARS_JOIN or MARS_LEAVE may
carry multiple <min,max> pairs.
Cluster members MUST provide ONLY a network designer so chooses.
5. Transmitting single <min,max> pair in each
JOIN/LEAVE message they issue. However, they MUST be able to Multicast groups.
[Editorial note: This section has discarded the MARS_MSERV
function of version ipmc-03.txt. MARS_MSERV is now used process
multiple <min,max> pairs in an
entirely different fashion. Endpoint JOIN/LEAVE messages when performing VC
management is now
entirely independent of whether the group is mesh or mc server
supported.]
The following description will be as described in terms of an IP/ATM interface section 5.1.4 (the interpretation being
that is capable of transmitting packets the join/leave operation applies to all addresses in range from
<min> to <max> inclusive, for every <min,max> pair).
In RFC1112 environments a Class D address at any
time, without prior warning.
When a packet arrives MARS_JOIN for transmission, and there a single group is no outgoing VC
already marked as serving the packet's multicast destination address, triggered
by a JoinLocalGroup signal from the MARS is queried IP layer. A MARS_LEAVE for the set of ATM endpoints currently making up
the multicast group.
The query a
single group is executed triggered by issuing a MARS_REQUEST. The MARS_REQUEST
message is formatted as an ATM ARP_REQUEST with type code of 11
(decimal). The reply LeaveLocalGroup signal from the MARS IP
layer.
Cluster members with special requirements (e.g. multicast routers)
may take one of two forms:
MARS_MULTI - Sequence issue MARS_JOINs and MARS_LEAVEs specifying a block of MARS_MULTI messages return the set multicast
group addresses.
An endpoint MUST register with a MARS in order to become a member of
endpoints
a cluster and be added as a leaf to ClusterControlVC. Registration
is covered in section 5.2.3.
Finally, the group.
MARS_NAK - No mapping found, group is empty.
The request/response traffic endpoint MUST occur on be capable of terminating unidirectional
VCs (i.e. act as a leaf node of a UNI 3.1 point to point VC
established by multipoint VC).
RFC 1755 describes the host information required to terminate VCs carrying
LLC/SNAP encapsulated traffic (discussed further in section 5.5).
5.2.1 Format of the MARS. Where MARS_JOIN and MARS_LEAVE Messages.
The MARS_JOIN message is indicated by an operation type value of 14
(decimal). MARS_LEAVE has the MARS same format and ARP Server operation type value of
15 (decimal). The message format is:
Data:
ar$hrd 16 bits Hardware type (19 decimal)
ar$pro 16 bits Protocol type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_JOIN or MARS_LEAVE)
ar$spln 8 bits Length of source protocol address (s)
ar$tpln 8 bits Length of multicast group address (z)
ar$pnum 16 bits Number of multicast group address pairs (N)
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are co-resident, this VC may be shared between ATM ARP traffic
ar$resv 16 bits ar$layer3grp flag, and 15 bits reserved.
ar$cmi 16 bits Cluster Member ID
ar$msn 32 bits MARS traffic.
5.1 Retrieving Group Membership from the MARS.
If the MARS had no mapping for the desired Class D address a MARS_NAK
will be returned. In this case the IP packet MUST be discarded
silently. If a match is found in the MARS's tables it proceeds to
return addresses ATM.1 through ATM.n in a sequence of one Sequence Number.
ar$sha qoctets source ATM number (E.164 or more
MARS_MULTIs. A simple mechanism is used to detect and recover from
loss of MARS_MULTI messages.
Each MARS_MULTI carries a new boolean field x, and a 15 bit integer
field y ATM Forum NSAPA).
ar$ssa roctets source ATM subaddress (ATM Forum NSAPA).
ar$spa soctets source protocol address
ar$min.1 zoctets Minimum multicast group address - expressed as MARS_MULTI(x,y). Field y acts as a sequence
number, starting at 1 and incrementing pair.1
ar$max.1 zoctets Maximum multicast group address - pair.1
[.......]
ar$min.N zoctets Minimum multicast group address - pair.N
ar$max.N zoctets Maximum multicast group address - pair.N
Refer to RFC 1577, section 6.6 for each MARS_MULTI sent.
Field x acts as an 'end of reply' marker. When x == 1 the MARS
response is considered complete.
In addition, each MARS_MULTI may carry multiple ATM addresses from coding of the set {ATM.1, ATM.2, .... ATM.n}. A MARS MUST minimise ar$shtl and
ar$sstl fields. ar$spln indicates the number of MARS_MULTIs transmitted by placing as many group member's
addresses bytes in a single MARS_MULTI as possible. The limit on MARS_MULTI
message length MUST be the MTU of the underlying VC.
Assume n ATM addresses must be returned, each MARS_MULTI is limited
to only p ATM addresses, source
endpoint's protocol address, and p << n. This would require a sequence of
k MARS_MULTI messages (where k = (n/p)+1, using integer arithmetic),
transmitted as follows:
MARS_MULTI(0,1) carries back {ATM.1 ... ATM.p}
MARS_MULTI(0,2) carries back {ATM.(p+1) ... ATM.(2p)}
[.......]
MARS_MULTI(1,k) carries back { ... ATM.n}
If k == 1 then only MARS_MULTI(1,1) is sent.
Typical failure mode interpreted in the context of the
protocol indicated by the ar$pro field. (e.g. in IPv4 environments
ar$pro will be losing one or more of MARS_MULTI(0,1)
through MARS_MULTI(0,k-1). This 0x800, ar$spln is detected when y jumps by more than
one between consecutive MARS_MULTI's. An alternative failure mode 4, and ar$tpln is
losing MARS_MULTI(1,k). A timer 4.)
The ar$resv field contains a flag - ar$layer3grp - in its most
significant bit, and 15 unused bits which MUST be implemented to zero. This flag is
to allow the
failure of the last MARS_MULTI MARS to arrive. A default value of 10
seconds is suggested.
If a 'sequence jump' is detected, provide the host MUST wait 'short cut' group membership
information described further in section 5.3. The rules for its use
are:
ar$layer3grp MUST be set when the
MARS_MULTI(1,k), discard all results, and repeat cluster member is issuing the MARS_REQUEST.
If
MARS_JOIN a timeout occurs, the host MUST discard all results, and repeat the MARS_REQUEST.
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Corruption result of cell contents will lead to loss a layer 3 multicast group being
explicitly joined. (e.g. as a result of a MARS_MULTI through
AAL5 CPCS_PDU reassembly failure, which will JoinHostGroup operation
in an RFC1112 compliant host).
The flag MUST be detected through reset in each MARS_JOIN if the
mechanisms described above.
If MARS_JOIN is
simply the MARS local ip/atm interface registering to receive traffic
on that group for its own reasons.
The flag is managing a cluster of endpoints spread across
different but directly accessible ATM networks it will not ignored and MUST be able to
return all treated as reset by the group members in MARS for
any MARS_JOIN that specifies a block covering more than a single MARS_MULTI. The MARS_MULTI
group (e.g. a block join from a router ensuring their forwarding
engines 'see' all traffic).
ar$pnum indicates how many <min,max> pairs are included in the
message. This field must always be 1 when the message format allows for either E.164, ISO NSAP, or (E.164 + NSAP)
to is sent from a
cluster member. (It will be unchanged when returned as ATM addresses. However, each MARS_MULTI message by a Class I
MARS. A Class II MARS may
only return ATM addresses of the same type. a MARS_JOIN or MARS_LEAVE with any
ar$pnum value, including zero. This will be explained futher in
section 6.2.4.)
The returned addresses ar$cmi field SHOULD be zeroed by cluster members, and is used by
the MARS during cluster member registration, described in section
5.2.3.
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ar$msn MUST be grouped according zero when transmitted by an endpoint. It is set to type (E.164, ISO NSAP, the
current value of the Cluster Sequence Number by the MARS when the
MARS_JOIN or both) and
returned MARS_LEAVE is retransmitted. Its use has been described
in a sequence section 5.1.4.
To simplify construction and parsing of separate MARS_MULTI parts.
5.2 MARS_REQUEST, MARS_MULTI, MARS_MSERV, MARS_JOIN and MARS_NAK formats.
MARS_REQUEST is based MARS_LEAVE
messages, the following restrictions are imposed on an ATM ARP_REQUEST, but the <min,max>
pairs:
Assume max(N) is the <max> field from the Nth <min,max> pair.
Assume min(N) is the <min> field from the Nth <min,max> pair.
Assume a join/leave message arrives with K <min,max> pairs.
The following must hold:
max(N) < min(N+1) for 1 <= N < K
max(N) >= min(N) for 1 <= N <= K
In plain english, the set must specify an 'operation
type value' ascending sequence of 11 (decimal). The multicast
address being resolved is
placed into blocks. The definition of "greater" or "less than" may be
protocol specific. In IPv4 environments the addresses are treated as
32 bit, unsigned binary values (most significant byte first).
5.2.1.1 Important IPv4 default values.
The JoinLocalGroup and LeaveLocalGroup operations are only valid for
a single group. For any arbitrary group address X the associated
MARS_JOIN or MARS_LEAVE MUST specify a single pair <X, X>. In general
the target protocol address field (ar$tpa). The
hardware type (ar$hrd) is ar$layer3grp flag MUST be set under these circumstances.
A router choosing to 19 (decimal), and behave strictly in accordance with RFC1112 MUST
specify the entire Class D space. The associated MARS_JOIN or
MARS_LEAVE MUST specify a single pair <224.0.0.0, 239.255.255.255>.
Whenever a router issues a MARS_JOIN only in order to forward IP environments
traffic it MUST reset the protocol type ar$layer3grp flag.
The use of alternative <min, max> values by multicast routers is 2048 (decimal).
discussed in Section 6.6 8.
5.2.2 Retransmission of RFC 1577 should
be consulted for specific details MARS_JOIN and coding MARS_LEAVE messages.
Transient problems may result in the loss of messages between the ar$shtl, ar$sstl,
ar$thtl,
MARS and ar$tstl fields.
MARS_NAK is the MARS_REQUEST returned with operation type value of 16
(decimal).
The MARS_MULTI message cluster members
A simple algorithm is identified by an 'operation type value' of
12 (decimal). The message format is:
Data:
ar$hrd 16 bits Hardware type ( 19 decimal, 0x13 hex)
ar$pro 16 bits Protocol type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_MULTI)
ar$spln 8 bits Length of source protocol address (s)
ar$thtl 8 bits Type & length of target ATM number (x)
ar$tstl 8 bits Type & length of target ATM subaddress (y)
ar$tpln 8 bits Length of target multicast group address (z)
ar$tnum 16 bits Number of target ATM addresses returned (N).
ar$seqxy 16 bits Boolean flag x used to solve this problem. Cluster members
retransmit each MARS_JOIN and sequence number y.
ar$msn 32 bits MARS_LEAVE message at regular intervals
until they receive a copy back again, either on ClusterControlVC or
the VC on which they are sending the message. At this point the
local endpoint can be certain that the MARS Sequence Number.
ar$sha qoctets source ATM number
ar$ssa roctets source ATM subaddress
ar$spa soctets source protocol address
ar$tha.1 xoctets target ATM number 1
ar$tsa.1 yoctets target ATM subaddress 1
ar$tpa zoctets target multicast group address received and processed
it.
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ar$tha.2 xoctets target ATM number 2
ar$tsa.2 yoctets target ATM subaddress 2
[.......]
ar$tha.N xoctets target ATM number N
ar$tsa.N yoctets target ATM subaddress N
ar$seqxy
The interval should be no shorter than 5 seconds, and a default value
of 10 seconds is coded with flag x in recommended. After 5 retransmissions the leading bit, attempt
should be flagged locally as a failure. This MUST be considered as a
MARS failure, and sequence number
y coded triggers the MARS reconnection described in section
5.4.
A 'copy' is defined as an unsigned integer seeing a message of the same operation code
containing the local host's identity in the remaining 15 bits.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x| y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ar$tnum source address fields.
The <min,max> pair set is not checked, and does not have to be the
same (this is required to be compatible with the modification that a
Class II MARS may effect on the retransmitted MARS_JOIN or MARS_LEAVE
message).
This algorithm explicitly allows only ONE outstanding MARS_JOIN and
MARS_LEAVE message at a time (although you may have one of both
outstanding).
5.2.3 Registering with the MARS.
To become a cluster member an endpoint must register with the MARS.
This achieves two things - the endpoint is an unsigned integer indicating how many pairs added as a leaf node of
{ar$tha,ar$tsa} (i.e. how many group member's ATM addresses) are
present in
ClusterControlVC, and the message. ar$msn endpoint is an unsigned 32 assigned a 16 bit number filled in
by the MARS before transmitting Cluster
Member Identifier (CMI). The CMI uniquely identifies each MARS_MULTI. Its use endpoint
that is described
further in section 10. Section 6.6 of RFC 1577 should be consulted
for specific details and coding of all other fields.
As attached to the cluster.
Registration with the MARS occurs when an example, assume we have endpoint issues a MARS_JOIN
for a multicast cluster using 4 byte protocol addresses, 20 byte ATM numbers, and 0 byte ATM subaddresses.
For n specific multicast group members address.
In IPv4 environments an endpoint (whether in a single MARS_MULTI we require host or router) MUST
explicitly issue a (44 + 20n)
byte message. If we assume MARS_JOIN for the default MTU of 9180 bytes, we can
return special address "0.0.0.0" in
order to register with the MARS. In other words, a maximum MARS_JOIN with
ar$tpln of 456 group member's 4, and 8 bytes of zero starting at ar$min.1 (equivalent to
the block of <0.0.0.0,0.0.0.0>. This function may be internal to the
IP/ATM driver, and does not require the IP layer to believe it has
'joined' the all-zeroes IP address.
The specific addresses signifying 'registration' for other layer 3
protocols will be defined in a single
MARS_MULTI.
5.3 Establishing subsequent documents.
The cluster member retransmits this MARS_JOIN in accordance with
section 5.2.2 until it confirms that the Multicast VC.
Following MARS has received it.
When the completion of registration MARS_JOIN is returned it contains a non-zero
value in ar$cmi. This value MUST be noted by the MARS_MULTI reply cluster member, and
used whenever circumstances require the cluster member's CMI.
An endpoint may
establish a new point also choose to multipoint VC, or reuse an existing one.
If establishing de-register, using a new VC, an L_MULTI_RQ is issued for ATM.n, followed
by MARS_LEAVE. In an L_MULTI_ADD for every member of the set {ATM.1, ....ATM.(n-1)}
(assuming the set is non-null). The packet is then transmitted over
the newly created VC just as it would be for
IPv4 environment a unicast VC.
After transmitting MARS_LEAVE on the packet, special address of "0.0.0.0"
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would result in the local interface holds MARS dropping the VC open endpoint from ClusterControlVC
and marks it as freeing up its CMI
5.3 Support for Layer 3 group management.
Whilst the active path out intention of the host for any subsequent IP
packets this specification is to be independent of
layer 3 issues, an attempt is being sent made to assist the operation of
layer 3 multicast routing protocols that Class D address.
When establishing need to ascertain if any
groups have members within a new multicast VC cluster.
One example is IP, where IGMP is possible used (as described in section 2)
simply to determine whether any other cluster members are listening
to a group because they have higher layer applications that one or more
returned endpoints want to
receive a group's traffic.
Routers may reject an L_MULTI_RQ or L_MULTI_ADD. If this
occurs then the endpoint's ATM address is dropped from the set
{ATM.1, ATM.2, .... ATM.n} returned by choose to query the MARS, MARS for this information, rather
than multicasting IGMP queries to 224.0.0.1 and incurring the creation
associated cost of
the multipoint setting up a VC continues.
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Multicast VCs have the potential to be expensive all systems in their use of
resources. Therefore each VC MUST have a configurable inactivity
timer associated with it. If the timer expires, an L_RELEASE cluster.
The query is issued for that VC, and by sending a MARS_GROUPLIST_REQUEST to the Class D address MARS.
MARS_GROUPLIST_REQUEST is no longer considered
to have built from a MARS_JOIN, but it has an active path out
operation code of the local host. The timer SHOULD 20 (ar$op = 20). A single <min,max> pair MUST be no
less than 1 minute,
provided (ar$pnum = 1), and a default of 20 minutes is RECOMMENDED.
Choice it specifies the range of specific timer periods groups in which
the querying cluster member is beyond interested.
The response from the scope of this
document.
VC consumption may also be reduced by endpoints noting when MARS is a new
group's set of {ATM.1, ....ATM.n} matches that of MARS_GROUPLIST_REPLY, carrying a pre-existing VC
out to another group. With careful local management, and assuming the
QoS list
of the existing VC multicast groups within the specified <min,max> block that
have Layer 3 members. A group is sufficient for both groups, a new pt to mpt
VC may not be necessary. Algorithms for performing noted in this type list if one or more
of
optimization are not discussed here, and are not required for
conformance with this memo.
Section 7 describes the endpoint's response to group membership
changes while the VC is open. Section 10 describes MARS_JOINs that generated its mapping entry in the mechanism for
ensuring hosts remain up MARS
contained a set ar$layer3grp flag.
MARS_GROUPLIST_REPLYs are transmitted back to date with changes that occur while the VC
is open.
6. Joining and Leaving Multicast Groups.
A querying cluster
member on the VC used to send the MARS_GROUPLIST_REQUEST.
MARS_GROUPLIST_REPLY is a 'group member' (in derived from the sense that MARS_MULTI, it receives
packets directed at the group) when its may have
multiple parts if needed, and is received in a similar manner.
Data:
ar$hrd 16 bits Hardware type ( 19 decimal, 0x13 hex)
ar$pro 16 bits Protocol type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_GROUPLIST_REPLY = 21
decimal)
ar$spln 8 bits Length of source protocol address appears in the
MARS's table entry for the group's (s)
ar$thtl 8 bits Unused - set to zero.
ar$tstl 8 bits Unused - set to zero.
ar$tpln 8 bits Length of target multicast address. A key
requirement within each cluster is the distribution group address (z)
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ar$tnum 16 bits Number of group
membership information between the MARS addresses returned (N).
ar$seqxy 16 bits Boolean flag x and cluster members.
Two new messages sequence number y.
ar$msn 32 bits MARS Sequence Number.
ar$sha qoctets source ATM number (E.164 or ATM Forum NSAPA).
ar$ssa roctets source ATM subaddress (ATM Forum NSAPA).
ar$spa soctets source protocol address
ar$mgrp.1 zoctets Group address 1
[.......]
ar$mgrp.N zoctets Group address N
ar$seqxy is coded as for the MARS_MULTI - multiple
MARS_GROUPLIST_REPLY components are defined: MARS_JOIN transmitted and MARS_LEAVE. These received using
the same algorithm as described in section 5.1.1 for MARS_MULTI. The
only difference is that group address are
sent to being returned rather than
ATM addresses.
As for MARS_MULTIs, if an error occurs in the MARS by endpoints joining or leaving reception of a multicast group.
The MARS propagates these messages back out to multi
part MARS_GROUPLIST_REPLY the cluster over its
ClusterControlVC, to ensure whole thing MUST be discarded and the knowledge
MARS_GROUPLIST_REQUEST re-issued. (This includes the ar$msn value
being constant.)
Note that the ability to generate MARS_GROUPLIST_REQUEST messages,
and receive MARS_GROUPLIST_REPLY messages, is distributed in a timely
fashion. ClusterControlVC not required for
general host interface implementations. It is an outgoing, point optional for interfaces
being implemented to multipoint VC with
each cluster member as a leaf node.
RFC1112 expects that IP support layer 3 multicast routers are capable of behaving
'promiscuously'. This forwarding engines.
However, this functionality may MUST be emulated supported by allowing
routers both Class I and
Class II MARS.
5.4 Support for redundant/backup MARS entities.
Endpoints are assumed to request that have been configured with the MARS returns them as 'wild card' members ATM address of all Class D addresses. However,
at least one MARS. Endpoints MAY choose to maintain a problem inherent table of ATM
addresses, representing alternative MARSs that will be contacted in
the current
ATM model event that normal operation with the original MARS is deemed to
have failed. It is assumed that completely promiscuous behaviour may be wasteful of
reassembly resources on this table orders the router's ATM interface. This document
describes addresses
in descending order of preference.
An endpoint will typically decide there are problems with the MARS
when:
- It fails to establish a generalisation point to point VC to the notion of 'wild card' entries,
enabling routers MARS.
- MARS_REQUESTs fail (section 5.1.1).
- MARS_JOIN/MARS_LEAVEs fail (section 5.2.2).
(If it is able to limit themselves discern which connection represents
ClusterControlVC, it may also use connection failures on this VC to 'blocks' of
indicate problems with the Class D
address space. The application of this facility is described in
greater detail in Section 9. MARS).
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A block can be as small as
5.4.1 First response to MARS problems.
The first response is to assume a transient problem with the MARS
being used at the time. The cluster member should wait a random
period of time between 1 (a single group) or as large as and 10 seconds before attempting to re-
connect and re-register with the
entire Class D address space (default IPv4 'promiscuous' behaviour).
A block MARS. If the registration MARS_JOIN
is defined as all addresses between, and inclusive of, successful then:
The cluster member MUST then proceed to rejoin every group that
its local higher layer protocol(s) have joined. It is recommended
that a
<min,max> address pair. random delay between 1 and 10 seconds be inserted before
attempting each MARS_JOIN.
The key extensions required cluster member MUST initiate the revalidation of every
multicast group it was sending to manage (as though a sequence number
jump had been detected, section 5.1.5).
The rejoin and revalidation procedure must not disrupt the MARS table entries are:
Two new message types:
MARS_JOIN carries one or more <min,max> pairs (specifying one
or more blocks cluster
member's use of groups being joined) multipoint VCs that were already open at the time
of the MARS failure.
If re-registration with the current MARS fails, and a unicast ATM
address (of there are no
backup MARS addresses configured, the node joining).
MARS_LEAVE carries one or more <min,max> pairs (specifying one cluster member MUST wait for at
least 1 minute before repeating the re-registration procedure. It is
RECOMMENDED that the cluster member signals an error condition in
some locally significant fashion.
This procedure may repeat until network administrators manually
intervene or more blocks of groups being left) and a unicast ATM address
(of the node leaving).
When a MARS_JOIN is received by the current MARS it adds the specified ATM
address returns to normal operation.
5.4.2 Connecting to the table entry for the specified multicast group
address(es).
When a MARS_LEAVE is received by backup MARS.
If the re-registration with the current MARS it removes fails, and other MARS
addresses has been configured, the specified
ATM next MARS address from on the ARP entry for list is
chosen to be the specified multicast group
address(es).
MARS_JOIN current MARS, and MARS_LEAVE messages arriving from individual hosts
are processed locally by the MARS and retransmitted on
ClusterControlVC (possibly after modification, as detailed cluster member immediately
restarts the re-registration procedure described in
Section 8).
All endpoints MUST ignore MARS_JOIN or MARS_LEAVE messages section 5.4.1. If
this is succesful the cluster member will resume normal operation
using the new MARS. It is RECOMMENDED that
simply confirm information already held. The MARS retransmits
redundant messages, but otherwise takes no action. Section 7
describes how endpoints utilize retransmitted MARS_JOIN and
MARS_LEAVE messages.
Cluster members MUST only include the cluster member signals
a single <min,max> pair in each
JOIN/LEAVE message they issue. They MUST be able to process
multiple <min,max> pairs warning of this condition in JOIN/LEAVE messages received on
ClusterControlVC from some locally significant fashion.
If the attempt at re-registration with the new MARS (the interpretation being that fails, the
join/leave operation applies to all addresses in range from <min>
to <max> inclusive,
cluster member MUST wait for every <min,max> pair).
In IPv4 environments JoinLocalGroup now results in two messages being
transmitted:
MARS_JOIN, sent over a VC to at least 1 minute before chosing the ARP Server. It identifies
next MARS address in the
single IP group being joined, table and repeating the host's unicast ATM address. procedure. If the
end of the table has been reached, the cluster member starts again at
the top of the table (which should be the original MARS that the
cluster member started with).
In the worst case scenario this will result in cluster members
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An IGMP Report, except for 224.0.0.1 (in accordance with RFC1112).
In IPv4 environments LeaveLocalGroup now results in a MARS_LEAVE
being sent over a VC to the MARS, identifying the IP group being
left, and the host's unicast ATM address.
Endpoints with special requirements (e.g. multicast routers) may
directly issue MARS_JOINs
looping through their table of possible MARS addresses until network
administrators manually intervene.
5.4.3 Dynamic backup lists, and MARS_LEAVEs specifying blocks soft redirects.
To support some level of
multicast group addresses. No IGMP Report is issued for such
operations in IP environments.
An endpoint must register with autoconfiguration, a MARS in order message is defined
that allows the current MARS to become broadcast on ClusterControlVC a member table
of
a backup MARS addresses. When this message is received, cluster and be added as
members that maintain a leaf to ClusterControlVC. Registration
is covered in section 6.2.
6.1 Format list of backup MARS addresses MUST insert
this information at the MARS_JOIN and MARS_LEAVE Messages.
The MARS_JOIN message is indicated by an operation type value top of 14
(decimal). MARS_LEAVE their locally held list (i.e. the
information provided by the MARS has a higher preference than
addresses that may have been manually configured into the same format and operation type value of
15 (decimal). cluster
member).
The message format is:
Data:
ar$hrd 16 bits Hardware type (19 decimal)
ar$pro 16 bits Protocol is MARS_REDIRECT_MAP. It is based on a single MARS_MULTI,
but with an operation type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_JOIN or MARS_LEAVE)
ar$spln 8 bits Length of 22 decimal. The source protocol address (s)
ar$tpln 8 bits Length of multicast group hardware
address (z)
ar$pnum 16 bits Number information MUST be that of multicast group address pairs (N)
ar$resv 16 bits Reserved.
ar$msn 32 bits MARS Sequence Number.
ar$sha qoctets source ATM number (E.164 or ATM Forum NSAPA).
ar$ssa roctets source ATM subaddress (ATM Forum NSAPA).
ar$spa soctets the MARS, and the source protocol
address
ar$min.1 zoctets Minimum multicast group address - pair.1
ar$max.1 zoctets Maximum multicast group mask - pair.1
[.......]
ar$min.N zoctets Minimum multicast group field MUST be null (ar$spln = 0, and no space allocated).
The target protocol address - pair.N
ar$max.N zoctets Maximum multicast group mask - pair.N
Refer MUST be null (ar$tpln = 0, and no space
allocated). If a multi-part MARS_REDIRECT_MAP begins arriving it
should be reassembled and accepted. If a part is lost, the entire
message should simply be discarded.
This message is transmitted regularly by the MARS (it MUST be
transmitted at least every 2 minutes, it is RECOMMENDED that it is
transmitted every 1 minute).
In addition to RFC 1577, section 6.6 for keeping cluster members updated with the coding recommended
list of backup MARSs, the ar$shtl and
ar$sstl fields. For conventional IPv4 environments ar$spln and
ar$tpln are both set MARS_REDIRECT_MAP is used to 4. Note that the message format differs force cluster
members to 'soft redirect' from
ATMARP_REPLY in one MARS to another. If the fields after ar$op. ar$msn is an unsigned 32 bit
number filled first ATM
address contained in by a MARS_REDIRECT_MAP is not the address of the
MARS before re-transmitting currently being used by a MARS_JOIN or
MARS_LEAVE. The originator SHOULD set it cluster member, the cluster member
MUST initiate the following:
- open a point to point VC to zero, although it will be
ignored by the MARS. Its use is described further in section 10.
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A join/leave message carries first ATM address.
- attempt a set {<min,max>, <min,max>, ....,
<min,max>}, with at least one <min,max> pair. ar$pnum indicates how
many pairs are included in registration (e.g. MARS_JOIN for "0.0.0.0").
If the message. To simplify registration succeeds, the cluster member shuts down its point
to point VC to the current MARS (if it had one open), and endhost
interpretation, then
proceeds to use the following restrictions are imposed:
Assume max(N) is newly opened point to point VC as its connection
to the <max> field from 'current MARS'. The cluster member does NOT attempt to rejoin
the Nth <min,max> pair.
Assume min(N) groups it is the <min> field from the Nth <min,max> pair.
Assume a join/leave message arrives with K <min,max> pairs.
The following must hold:
max(N) < min(N+1) for 1 <= N < K
max(N) >= min(N) for 1 <= N <= K
In plain english, the set must specify an ascending sequence of
address blocks. The definition of "greater" member of, or "less than" may be
protocol specific. In IP environments revalidate groups it is currently
sending to.
This is termed a 'soft redirect' because it avoids the addresses are treated as
simple unsigned binary values.
6.1.1 Important IPv4 default values.
The JoinLocalGroup extra
rejoining and LeaveLocalGroup operations revalidation processing that occurs when a MARS failure
is being recovered from. It assumes some external synchronisation
mechanisms exist between the old and new MARS - mechanisms that are only valid for
a single group. For any arbitrary group address X
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outside the associated
MARS_JOIN or MARS_LEAVE MUST specify scope of this specification.
Some level of trust is required before initiating a single pair <X, X>. soft redirect. A router choosing to behave strictly in accordance with RFC1112
cluster member MUST
specify check that the entire Class D space. The associated MARS_JOIN or
MARS_LEAVE MUST specify a single pair <224.0.0.0, 239.255.255.255>.
The use calling party at the other end of alternative <min, max> values
the VC on which the MARS_REDIRECT_MAP arrived (supposedly
ClusterControlVC) is discussed in Section 9.
6.2 Registering with fact the node it trusts as the current MARS.
Two separate signalling paths exist between cluster members
Additional applications of this function are for further study.
5.5 LLC/SNAP encapsulations for transmit and receive.
Network administrators who require only VC mesh support for their
associated MARS. The first is
multicasting would use a transient Class I MARS. In this case the default for
data traffic carried on point to point VC that
cluster members establish multipoint VCs is LLC/SNAP
encapsulation with a header appropriate to the MARS when they need protocol being
carried. For IP traffic this is defined in RFC 1483 as:
[0xAA-AA-03][0x00-00-00][0x08-00][IP packet]
(LLC) (OUI) (PID)
Network administrators who require the ability to issue
MARS_REQUESTs, MARS_JOINs, or MARS_LEAVEs. use MCSs on certain
multicast groups will use a Class II MARS. They will also require
endpoint interfaces that detect and filter out reflected packets.
This VC is used achieved by adding another field of information to the
MARS
encapsulation that is already wrapped around layer 3 data packets.
The information to return MARS_MULTI messages. It has an associated idle timer, be included is the Cluster Member Identifier
(CMI), which is allocated during registration by both Class I and
Class II MARSs (section 5.2.3).
When a packet is transmitted the CMI is dismantled if not used for inserted into the
encapsulation. When a configurable period of time. The
minimum suggested value for this time packet is 1 minute, and received, if the
RECOMMENDED default CMI carried along
with it matches the CMI of the local interface the packet is 20 minutes. simply
dropped.
The second signalling path is ClusterControlVC. Every endpoint
registered as a cluster member recommended encapsulation is:
[Editors note: This is added as a leaf node to this VC,
which exists placeholder for the lifetime results of the MARS. It WG
discussion on the encapsulation options. Check draft-armitage-
ipatm-encaps-01.txt or later version. The WG is used expected to re-
distribute MARS_JOIN and MARS_LEAVE messages received by the MARS
from individual cluster members. Registration come
up with the MARS as a
cluster member occurs when an endpoint issues a MARS_JOIN for a
protocol specific multicast group address. Once some text that will simply be dropped into this occurs the
endpoint is added as section.]
Using a leaf node different LLC/SNAP value to ClusterControlVC. identify packets containing the
CMI allows endpoints to separate and simultaneously support both old
and new encapsulated traffic.
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In IPv4 environments the 'all nodes' Class D address of 224.0.0.1 is
used to register with the MARS. RFC 1112 requires that all hosts
(including routers) that wish to participate in Level 2 IP
multicasting must explicitly issue a JoinLocalGroup for group
224.0.0.1 when they initialise (Level 1 is not supported by this
memo).
6. The JoinLocalGroup to 224.0.0.1 will result MARS in greater detail.
As noted in an MARS_JOIN
being transmitted from the host to the MARS.
If an IPv4 endpoint issues a LeaveLocalGroup for 224.0.0.1 it will
also be considered to have ceased membership overview of section 4, there are two types of all other groups for
which it may have joined. The MARS MUST flush that endpoint's ATM
address from any
defined in this specification. The Class D address entries it appears in. Finally, the
endpoint I MARS is released as a Leaf node from ClusterControlVC.
If superset of the
RFC1577 ARP Server, and is capable of managing clusters where only VC
meshes are used to achieve intra-cluster multicasting.
The Class II MARS receives an ERR_L_RELEASE on ClusterControlVC indicating
that is a cluster member has died, that member's ATM address MUST be
removed from all groups for which it may have joined.
Registration superset of endpoints for other protocols is currently beyond the
scope of this document.
7. Endpoint management of point Class I MARS, with extensions
that allow it to multipoint VCs.
Once a cluster member has transparently introduce multicast servers into the
data paths established a new VC to by endpoints that comply with the members
returned
specifications in a MARS_MULTI response it must:
Monitor traffic on ClusterControlVC for updates section 5. (It is worth noting here that complete
compliance with section 5 includes being able to use the group's
membership.
Revalidate a group's membership if a leaf node releases itself
from new
encapsulation carrying the VC.
7.1 Monitoring updates on ClusterControlVC.
When a cluster member joins or leaves Cluster Member ID. Networks built around a particular multicast group
Class I MARS may choose to initially not fully comply with section 5
in this respect, although it is not sufficient RECOMMENDED that they do.)
The MARS is intended to simply update the be a multiprotocol entity - all its mapping table in
tables and control VCs MUST be managed within the cluster's
MARS. Endpoints context of the
ar$pro field in incoming MARS messages. For example, a MARS supports
completely separate ClusterControlVCs for each layer 3 protocol
(ar$pro type) that it is registering members for. If a MARS receives
messages with an ar$pro type that are already transmitting to it does not support, the multicast
group's members message is
dropped.
6.1 Class I MARS requirements.
A Class I MARS must be informed of understand and/or generate the change so they may add or
remove a leaf node as appropriate. Cluster members track following MARS
messages:
11 MARS_REQUEST
12 MARS_MULTI
14 MARS_JOIN
and
15 MARS_LEAVE
16 MARS_NAK
20 MARS_GROUPLIST_REQUEST
21 MARS_GROUPLIST_REPLY
22 MARS_REDIRECT_MAP
Section 5 covers how these messages retransmitted are used or reacted to by
endpoints within a cluster. This section provides a brief summary of
how the Class I MARS to determine when
another endpoint joins or leaves a group uses or block of groups.
If a MARS_JOIN is seen that refers reacts to (or encompasses) them.
When a group registration MARS_JOIN arrives (e.g. for
which address "0.0.0.0" if
ar$pro = 0x800 [IPv4]) the transmit side already has a VC open, MARS performs the new member's ATM
address is extracted and an L_MULTI_ADD issued locally. This ensures
that hosts already sending following:
- Adds the node to ClusterControlVC.
- Allocates a given group will immediately add new Cluster Member ID (CMI).
- Inserts the new member to their list of recipients. It also ensures that routers
joining a 'block' CMI into the ar$cmi field of groups are added by all endpoints currently the MARS_JOIN.
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sending to groups within
- Retransmits the block. MARS_JOIN back privately.
If the node is already a MARS_LEAVE registered member of the cluster (given the
ar$pro value in the MARS_JOIN) then its CMI is seen that refers simply copied into the
MARS_JOIN, and the MARS_JOIN retransmitted back to (or encompasses) a group for
which the transmit side already has node. A
single node may register multiple times if it supports multiple layer
3 protocols. The retransmitted MARS_JOIN must NOT be sent on
ClusterControlVC. (If a cluster member issues a VC open, MARS_LEAVE for the old member's ATM
registration 'special' address it too is extracted retransmitted privately.)
All other MARS_JOIN and an L_MULTI_DROP issued locally. This ensures
that hosts already sending to a given group will immediately drop the
old member from their list of recipients.
In an IPv4 environment MARS_LEAVE messages are retransmitted on
ClusterControlVC (after successfully performing any endpoint leaving 224.0.0.1 is assumed required database
updates) exactly as they arrived. The MARS retransmits MARS_JOIN and
MARS_LEAVE messages even if they result in no change to the database.
The ar$layer3grp flag (section 5.3) MUST be ceasing support ignored (and treated as
reset) for IP multicast operation. MARS_JOINs specifying more than a single group. If a MARS_LEAVE
MARS_JOIN is
seen received that refers to group 224.0.0.1 then the ATM address of contains more than one <min,max> pair, the
endpoint specified in
MARS MUST ignore the message second and subsequent pairs.
An additional IPv4 specific behaviour exists - if a node issues a
MARS_LEAVE for address "224.0.0.1" (the 'all systems' group) it is
assumed to have ceased multicast support completely. All references
to this node MUST be removed eliminated from every
multipoint VC on which any other IPv4 groups it is listed as a leaf node.
The transmit side
member of in the interface MUST NOT shut down an active VC to
a group for which the receive side has just executed a
LeaveLocalGroup. This behaviour is consistent with database. Finally, the model of
hosts transmitting to groups regardless of their own membership
status.
If a MARS_JOIN or MARS_LEAVE arrives with ar$pnum == 0 it carries no
<min,max> pairs, and endpoint is only used for validation released as described in
section 10.
7.2 Revalidating when a
leaf nodes drop themselves.
During node from ClusterControlVC.
If the life of a multipoint VC MARS receives an ERR_L_RELEASE may be received ERR_L_RELEASE on ClusterControlVC indicating
that a leaf node cluster member has terminated its participation at the
ATM level. The died, that member's ATM endpoint associated with the ERR_L_RELEASE address MUST be
removed from all groups for which it may have joined.
As mentioned in section 4, the locally held set {ATM.1, ATM.2, .... ATM.n}
associated with MARS only needs to interpret the VC.
After
protocol address supplied in MARS messages on a random period of time between 1 and 10 seconds few odd occasions.
In general the endpoint MARS MUST revalidate the associated group's membership by re-issuing a
MARS_REQEUEST. The returned set of members {NewATM.1, NewATM.2, ....
NewATM.n} is compared with treat protocol addresses as arbitrary byte
strings. For example, the set already held locally.
L_MULTI_DROPs MARS MUST NOT apply IPv4 specific 'class'
checks to addresses supplied under ar$pro = 0x800 to see if they
really are issued on the group's VC Class D or not. It is sufficient for each node that appears
in the original set of members but not in MARS to simply
assume that endpoints know how to interpret the revalidated set of
members. L_MULTI_ADDs protocol addresses
that they are issued registering and deregistering mappings for.
A MARS_REDIRECT_MAP message (described in section 5.4.3) MUST be
regularly transmitted on the group's VC for each node ClusterControlVC. It is RECOMMENDED that
appears in the revalidated set of members but not in
this occur every 1 minute, and it MUST occur at least every 2
minutes. If the original set
of members.
8. Configuring for Multicast Servers or Multicast Meshes.
Endpoint's assume that all groups are supported by meshes MARS has no knowledge of point to
multipoint VCs. Under certain circumstances other backup MARSs serving
the consumption of VCs
and AAL resources around cluster, it MUST include its own address as the cluster can make meshes unattractive,
despite their performance advantages. only entry in the
MARS_REDIRECT_MAP message. The design and use of backup MARS protocol provides a entities
is beyond the scope of this specification, and will be covered in
future work.
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mechanism for introducing multicast servers on a per-multicast group
basis, and
The Cluster Sequence Number (CSN) is described in a manner that section 5.1.4, and
is completely transparent to cluster
members.
The multicast server has two key roles:
Providing one (or a limited number of) leaf nodes for outgoing VCs
from cluster members.
Constructing a single point to multipoint VC, with each group
memember as a leaf. This reduces carried in the AAL consumption ar$msn field of MARS messages being sent to one per
group, rather than one per sender per group. cluster
members (either out ClusterControlVC or on an individual VC). The
MARS must keep two sets of mappings for each multicast group
address supported by multicast servers. The original {multicast
address, ATM.1, ATM.2, ... ATM.n} mapping (the 'host map', although
it includes routers) is augmented by increments the CSN every time a parallel {multicast address,
server.1, server.2, .... server.K} mapping (the 'server map'). It message is
assumed that no ATM addresses appear in both the server and host maps
for the same multicast group. Typically K will be 1, but it will be
larger when multiple multicast servers are configured to share sent on
ClusterControlVC. The current CSN is copied into the
data load ar$msn field of a given group.
When the
MARS receives a MARS_REQUEST for a multicast address that
has both host and server maps it generates a response based messages being sent to cluster members, whether out
ClusterControlVC or on a private VC.
A MARS should be carefully designed to minimise the
identity possibility of
the request's source. CSN jumping unecessarily. Under normal operation only cluster
members affected by transient link problems will miss CSN updates and
be forced to revalidate. If the requestor is MARS itself glitches, it will be
innundated with requests for a period as every cluster member of the
server map for the requested group then
attempts to revalidate.
Calculations on the MARS returns CSN MUST be performed as unsigned 32 bit
arithmetic, to ensure no glitches when the contents counters roll over.
(The regular transmission of the host map in MARS_REDIRECT_MAP serves a sequence secondary
purpose of one allowing cluster members to track the CSN, even if they
miss an earlier MARS_JOIN or more MARS_MULTIs. Otherwise MARS_LEAVE.)
One implication of this mechanism is that the MARS returns the contents should serialize
its processing of 'simultaneous' MARS_REQUEST, MARS_JOIN and
MARS_LEAVE messages. Join and Leave operations should be queued
within the server map in a sequence of one
or more MARS_MULTIs. Servers use MARS along with MARS_REQUESTS, and not processed until all
the host map to establish reply packets of a basic
distribution VC for the group. Cluster members will establish
outgoing multipoint VCs to members preceeding MARS_REQUEST have been transmitted.
The transmission of the group's server map, without
being aware that their packets will not MARS_REDIRECT_MAP should also be going directly the
multicast group's members.
The similarly
queued.
6.2 Class II MARS also maintains requirements.
When using the services of a point to multipoint VC out to any multicast
servers it is aware of, called ServerControlVC. This serves an
analogous role Class I MARS, the endpoint behaviour
described in section 5 results in all groups being supported by
meshes of point to ClusterControlVC, allowing multipoint VCs. Section 3 discusses some of the MARS
reasons why network administrators and designers may wish to update utilise
MCSs to achieve their intra-cluster multicasting instead. The Class
II MARS includes all the
servers with group membership changes as they occur.
A set functionality of the Class I, but modifies
its use of four various MARS messages cover to fool endpoints into using MCSs
where needed.
The additional MARS messages supported by a Class II MARS are
primarily associated with iteraction between the current requirements: MARS and the MCSs.
13 MARS_MSERV Register as multicast server for one or more
groups.
17 MARS_UNSERV Deregister as multicast server for one or more
groups.
18 MARS_SJOIN A JOIN message on ServerControlVC.
19 MARS_SLEAVE A LEAVE message on ServerControlVC.
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MARS_SJOIN/SLEAVE
The following MARS messages are identical treated in format to MARS_JOIN/LEAVE, but
have different operation codes so that a node acting as both a
cluster member and multicast server may distinguish between updates
arriving on ServerControlVC and ClusterControlVC.
8.1 Registering and deregistering multicast servers.
MARS_MSERV and MARS_UNSERV are identical to the slightly different
manner:
11 MARS_REQUEST
14 MARS_JOIN message.
MARS_MSERV uses the set {<min,max>, <min,max>, ...., <min,max>} to
specify one or more
15 MARS_LEAVE
A Class II MARS must keep two sets of multicast groups that a multicast server
is willing to support. MARS_UNSERV indicates the set of groups that
the multicast server is no longer willing to support. The operation
code for MARS_MSERV is 11 (decimal), and MARS_UNSERV is 17 (decimal).
When a node registers with MARS_MSERV the MARS adds the new ATM
address to the server maps mappings for each specified group, possibly
constructing a new server map if this is the first multicast server
for the group. If layer 3 group
using MCS support. The original {layer 3 address, ATM.1, ATM.2, ...
ATM.n} mapping (now termed the multicast server is not already a leaf node of
ServerControlVC 'host map', although it includes
routers) is added.
When augmented by a node deregisters with MARS_UNSERV the MARS removes its parallel {layer 3 address, server.1,
server.2, .... server.K} mapping (the 'server map'). It is assumed
that no ATM
address from addresses appear in both the server and host maps for each specified group, deleting the
server map if this was the only server for the
same multicast group.
Both of these messages Typically K will be 1, but it will be larger if
multiple MCSs are sent configured to the support a given group.
The MARS over also maintains a point to point VC,
and echoed on multipoint VC out to any MCSs
registered with it, called ServerControlVC by the MARS (section 10 covers 6.2.3). This
serves an analogous role to ClusterControlVC, allowing the use
of this behaviour). The operation code is then changed MARS to MARS_JOIN
or MARS_LEAVE respectively, and a copy of
update the original message is
transmitted MCSs with group membership changes as they occur. A Class
II MARS MUST also send its regular MARS_REDIRECT_MAP transmissions on
both ServerControlVC and ClusterControlVC.
The
6.2.1 Class II MARS retransmits but otherwise ignores redundant MARS_MSERV and
MARS_UNSERV messages.
It is assumed that at least one server will have registered response to
support a group before MARS_REQUEST.
When the first cluster member joins it. If MARS receives a
MARS_MSERV arrives MARS_REQUEST for a group an address that has a non-null both
host map but no
server map the default response of the MARS will be to drop the
MARS_MSERV without any further action. The originating multicast
server will eventually flag an error when repeated attempts to
register fail.
The opposite situation is where the last or only multicast and server for maps it generates a group deregisters itself while response based on the group still has members. The
default solution identity of
the request's source. If the requestor is a member of the server map
for multicast servers to sever all VCs to which
they are attached as leaf nodes when they deregister, forcing any
active senders to the requested group to revalidate (as described in section
7). Since then the MARS will have deleted the server map, returns the
revalidation will result in contents of the
host map being return, and the group
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reverts to being in a mesh. This shall be sequence of one or more MARS_MULTIs. Otherwise the default mechanism until
future work develops MARS
returns the contents of the server map in a sequence of one or more elegant approach.
Appendix C discusses possible extensions
MARS_MULTIs.
Servers use the host map to allow dynamic transitions
between mesh and multicast server support while establish a group is active.
However, these are not required basic distribution VC for conformance with this memo.
8.2 Handling group membership changes.
The existence of multicast servers supporting some groups but not
others requires the MARS
group. Cluster members will establish outgoing multipoint VCs to intervene in the distribution
members of single
and block join/leave updates to cluster the group's server map, without being aware that their
packets will not be going directly the multicast group's members. The MARS_SJOIN
6.2.2 MARS_MSERV and
MARS_SLEAVE messages MARS_UNSERV messages.
MARS_MSERV and MARS_UNSERV are identical to MARS_JOIN, the MARS_JOIN message.
An MCS uses a MARS_MSERV with operation codes
18 and 19 (decimal) respectively. They exist to allow a node
combining cluster member and <min,max> pair of <X,X> to specify
the multicast server group X that it is willing to support. A single group
MARS_UNSERV indicates the group that the MCS is no longer willing to distinguish between
information arriving on ClusterControlVC
support. The operation code for MARS_MSERV is 13 (decimal), and ServerControlVC.
MARS_UNSERV is 17 (decimal).
When a cluster member an MCS issues MARS_JOIN or MARS_LEAVE for a single
group, MARS_MSERV the MARS checks adds the new ATM address to see if
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the group has an associated server
map.
If map for the specified group does not have group, possibly constructing a new
server map the MARS_JOIN or
MARS_LEAVE if this is retransmitted on ClusterControlVC.
If it does have the first MCS for the group.
When an MCS issues a MARS_UNSERV the MARS removes its ATM address
from the server map two transmissions occur:
A copy is made with type MARS_SJOIN or MARS_SLEAVE as appropriate
and transmitted on ServerControlVC. This allows maps for each specified group, deleting any server
maps that end up being null after the server(s)
supporting operation.
Both of these messages are sent to the group MARS over a point to point VC
(between MCS and MARS). After processing, they are retransmitted on
ServerControlVC to allow other MCSs to note the new member and add it as a leaf node.
The original message's ar$pnum field operation code is set then changed to 0, MARS_JOIN or MARS_LEAVE
respectively, and it another copy of the message is also transmitted back using the VC it arrived on (rather than
ClusterControlVC).
(Section 10 requires
ClusterControlVC. This fools the cluster members have into thinking a mechanism new
leaf node as been added to confirm (or dropped from) the
reception of their message by group specified. The
ar$layer3grp flag MUST be reset for the MARS. For mesh supported groups,
using ClusterControlVC serves dual purpose of providing this
confirmation retransmitted
MARS_JOIN/LEAVE.
The MARS retransmits but otherwise ignores redundant MARS_MSERV and distributing group update information. When using
multicast servers there
MARS_UNSERV messages.
It is no reason for having all assumed that at least one MCS will have MARS_MSERV'ed a group
before the first cluster members
process and discard null join/leave messages on ClusterControlVC).
Receipt of member joins it. If a block join/leave (e.g. from MARS_MSERV arrives for
a router coming on-line)
requires group that has a more complex response. Cluster members must be directly
informed non-null host map but no server map the default
response of which mesh supported groups the block covers. Multicast
servers must also MARS will be informed in case they support one of to silently drop the groups
covered by MARS_MSERV without
any further action. The MCS attempting to support the block being joined.
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eventually flag an error after repeated MARS_MSERVs fail.
The solution is last or only MCS for the MARS a group MAY choose to 'punch holes' in issue a MARS_UNSERV
while the block of
addresses supplied in group still has members. When the join/leave message, creating a set of
<min,max> pairs that excludes those addresses/groups supported MARS_UNSERV is processed
by the
multicast servers. This hole-punched set MARS the 'server map' will be deleted. When the associated
MARS_LEAVE is then sent out issued on ClusterControlVC, ensuring all cluster members with a
VC open to the router is immediately noted by senders MCS for that group will close down the VC (in
accordance with section 5.1.4, since the MCS was their only leaf
node). When cluster members subsequently find they need to any mesh supported groups transmit
packets to the group, they will begin again with the
MARS_REQUEST/MARS_MULTI sequence to establish a new VC. Since the
MARS will have deleted the server map, this will result in the block. The original
MARS_JOIN/LEAVE is then converted host
map being return, and the group reverts to being supported by a MARS_SJOIN/SLEAVE and
transmitted on ServerControlVC. Appendix VC
mesh.
A discusses some algorithms clean mechanism for 'hole punching'.
If punching holes in the originally specified block leaves reverse process - transitioning a null
set, the ar$pnum field is set group
from a VC mesh to zero before sending the modified
MARS_JOIN/LEAVE on ClusterControlVC.
8.3 Multicast server architectures.
Specification of multicast server architectures, and MCS supported while the
synchronisation of multiple multicast servers supporting single
multicast groups, group is beyond the scope of this document and active - is
expected to be the a
subject of further work. Appendix C discusses some
possible approaches.
9. Utilizing blocks for for multicast routers. further study.
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6.2.3 Registering a Multicast routers are required for Server (MCS).
Section 5.2.3 describes how endpoints register as cluster members,
and hence get added as leaf nodes to ClusterControlVC. The same
approach is used to register endpoints that intend to provide MCS
support to a Class II MARS.
Registration with the propagation of MARS occurs when an endpoint issues a
MARS_MSERV for a protocol specific multicast
traffic beyond group address. Upon
registration the constraints of a single cluster. There endpoint is added as a sense leaf node to ServerControlVC.
In IPv4 environments an MCS endpoint MUST explicitly issue a
MARS_MSERV for the special address "0.0.0.0" in which they are multicast servers acting at order to register
with the next higher layer, MARS. In other words, a MARS_MSERV with clusters rather than individual endpoints as their abstract
sources and destinations.
Multicast routers typically participate in higher layer multicast
routing algorithms ar$tpln of 4, and policies that are beyond 8
bytes of zero starting at ar$min.1 (equivalent to the scope block of this
memo (e.g. DVMRP [5]
<0.0.0.0,0.0.0.0>.
The specific addresses signifying 'registration' for other layer 3
protocols will defined in the IPv4 environment).
It is assumed subsequent documents.
The MCS retransmits this MARS_MSERV until it confirms that the multicast routers will be implemented over the
same sort of IP/ATM interface that MARS
has received it (by receiving a multicast host would use. They
will use copy back, in an analogous way to the basic services
mechanism described in the preceeding sections section 5.2.2 for reliably transmitting
MARS_JOINs).
The ar$cmi field in MARS_MSERVs are set to
join and leave multicast groups as necessary, zero by both MCS and will register with MARS.
An MCS may also choose to de-register, using a MARS_UNSERV. In an
IPv4 environment a MARS_UNSERV on the special address of "0.0.0.0"
would result in the MARS as dropping the MCS from ServerControlVC.
Note that multiple logical MCSs may share the same physical ATM
interface, provided that each MCS uses a cluster member.
The rest of this section will assume separate ATM address (e.g. a simple IPv4 scenario where
different SEL field in the
scope NSAP format address). In fact, an MCS may
share the ATM interface of a node that is also a cluster member
(either host or router), provided each logical entity has been limited to a particular LIS that is part
of an overlaid IP network. Not all members different
ATM address.
6.2.4 Class II response to MARS_JOIN and MARS_LEAVE.
The existence of MCSs supporting some groups but not others requires
the LIS are necessarily
registered Class II MARS to modify its distribution of single and block
join/leave updates to cluster members. The Class II MARS also adds
two new messages - MARS_SJOIN and MARS_SLEAVE - for communicating
group changes to MCSs over ServerControlVC.
The MARS_SJOIN and MARS_SLEAVE messages are identical to MARS_JOIN,
with operation codes 18 and 19 (decimal) respectively.
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9.1 Sending to a Group.
If the multicast router needs to transmit a packet to
When a group within
the cluster it opens a VC in the same manner as a normal host would.
Once a VC is open, the router watches for member issues MARS_JOIN and or MARS_LEAVE
messages and responds to them as for a normal host would.
The multicast router's transmit side MUST implement inactivity timers single
group, the MARS checks to shut down idle outgoing VCs, as for normal hosts.
As with normal host, see if the multicast router group has an associated server
map. If the specified group does not need to be a
member of have a group it is sending to.
9.2 Promiscuously Joining Groups.
Once registered and initialised, server map the simplest model of IPv4 multicast
router operation is for it to issue MARS
provides a MARS_JOIN encompassing the
entire Class D address space. In effect it becomes 'promiscuous', as
it will be a leaf node to all present and future multipoint VCs
established to IPv4 groups on the cluster.
How a router chooses which groups to propagate outside the cluster is
beyond the scope of this memo.
Consistent with RFC 1112, IP multicast routers may retain the use of
IGMP Query I service and IGMP Report messages to ascertain group membership.
9.3 Forward Multicast Traffic Across the cluster.
Under some circumstances the cluster may simply be another hop
between IP subnets that have participants in a multicast group.
[LAN.1] ----- IPmcR.1 -- [LIS] -- IPmcR.2 ----- [LAN.2]
LAN.1 and LAN.2 are subnets (such as Ethernet) with attached hosts
that are members of retransmits the MARS_JOIN or
MARS_LEAVE on ClusterControlVC.
However, if a server map exists for the group X.
IPmcR.1 and IPmcR.2 a new set of actions
are multicast routers with interfaces to the LIS. taken.
A traditional solution would be to treat copy of the LIS MARS_JOIN/LEAVE is made with type MARS_SJOIN or
MARS_SLEAVE as a unicast subnet, appropriate, and use tunneling routers. However, this would not allow hosts transmitted on ServerControlVC.
This allows the
LIS MCS(s) supporting the group to participate in note the cross-LIS traffic.
Assume IPmcR.1 new member
and update their data VCs.
The original message's ar$pnum field is receiving packets promiscuously on its LAN.1
interface. Assume further set to 0, and it is configured to propagate multicast
traffic
transmitted back using the VC it arrived on (rather than
ClusterControlVC).
(Section 5.2.2 requires cluster members have a mechanism to all attached interfaces. In this case that means confirm
the LIS. reception of their message by the MARS. For mesh supported
groups, using ClusterControlVC serves dual purpose of providing this
confirmation and distributing group update information. When a packet for group X arrives
is MCS supported, there is no reason for all cluster members to
process null join/leave messages on its LAN.1 interface, IPmcR.1
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simply sends ClusterControlVC, so they are
sent back on the private VC between cluster member and MARS.)
Receipt of a block MARS_JOIN (e.g. from a router coming on-line) or
MARS_LEAVE requires a more complex response. The single <min,max>
block may simultaneously cover VC mesh supported and MCS supported
groups. However, cluster members only need to be informed of the VC
mesh supported groups that the endpoint has joined. Only the MCSs
need to know if the packet endpoint is joining any MCS supported groups.
The solution is to group X modify the MARS_JOIN or MARS_LEAVE that is
retransmitted on ClusterControlVC. The following action is taken:
A copy of the LIS interface MARS_JOIN/LEAVE is made with type MARS_SJOIN or
MARS_SLEAVE as a normal
host would (Issuing MARS_REQUEST for appropriate, and transmitted on ServerControlVC.
This allows the MCS(s) supporting the group X, creating to note the VC,
sending membership
change and update their outgoing point to multipoint VCs.
The <min,max> block supplied in the packet).
Assuming IPmcR.2 initialised itself original MARS_JOIN/LEAVE is
replaced with the MARS as a member 'hole punched' set of zero or more <min,max>
pairs. The 'hole punched' set of <min,max> pairs covers the
entire Class D space, it will have been returned as a member of X
even if no other nodes on address range specified by the LIS were members. All packets for group
X received on IPmcR.2's LIS interface may be retransmitted on LAN.2. original <min,max> pair, but
excludes those addresses/groups supported by MCSs.
If IPmcR.1 the hole-punched set contains 1 or more <min,max> pair, the
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MARS_JOIN/LEAVE is similarly initialised transmitted on ClusterControlVC.
If the reverse process will apply
for multicast traffic from LAN.2 to LAN.1, for any multicast group.
The benefit of this scenario hole-punched set is that cluster members within empty, the LIS
may also join and leave group X at anytime.
9.4 Restricted 'promiscous' Operation.
Both unicast ar$pnum field is set to
zero, and multicast IP routers have a common problem -
limitations on the number of AAL contexts available at their ATM
interfaces. Being 'promiscuous' in MARS_JOIN/LEAVE is transmitted back using the RFC 1112 sense means that VC it
arrived on (rather than ClusterControlVC).
(Appendix A discusses some algorithms for
every M hosts sending to N groups, a multicast router's ATM interface
will have M*N incoming reassembly engines tied up. 'hole punching'.)
It is not hard to envisage situations where a number of multicast
groups are active within assumed that MCSs use the LIS but are not required MARS_SJOINs and MARS_SLEAVEs to
update their own VCs out to be
propagated beyond the LIS itself. An example might be a distributed
simulation system specifically designed actual group's members.
The ar$layer3grp flag is copied over into the messages transmitted by
the MARS.
6.2.5 Sequence numbers for ServerControlVC traffic.
In an analogous fashion to use the high speed IP/ATM
environment. There may be no practical way its traffic could be
utilised Cluster Sequence Number, a Class II
MARS keeps a Server Sequence Number (SSN) that is incremented for
every transmission on 'the other side' ServerControlVC. The current value of the multicast router, yet under SSN
is inserted into the
conventional scheme ar$msn field of every message the router would have MARS issues
that it believes is destined for an MCS. This includes MARS_MULTIs
that are being returned in response to be a leaf to each
participating host anyway.
As this problem occurs at MARS_REQUEST from an MCS,
and MARS_REDIRECT_MAP being sent on ServerControlVC. The MCS must
check the link layer, MARS_REQUESTs source, and if it is worth noting that
'scoping' mechanisms at the IP multicast routing level do not provide a solution.
In this situation registered MCS the network administrator might configure their
multicast routers SSN
is copied into the ar$msn field, otherwise the CSN is copied into the
ar$msn field.
MCSs are expected to exclude sections track and use the SSNs in an analogous manner to
the way endpoints use the CSN in section 5.1 (to trigger revalidation
of the group membership information).
A Class D address space
when issuing MARS_JOIN(s). Multicast groups that will never II MARS should be
propagated beyond carefully designed to minimise the cluster will not have
possibility of the router listed as a
member SSN jumping unecessarily. Under normal operation
only MCSs that are affected by the MARS, transient link problems will miss
ar$msn updates and be forced to revalidate. If the router MARS itself
glitches it will never have be innundated with requests for a period as every
MCS attempts to receive revalidate.
6.3 Why global sequence numbers?
The CSN and
ignore traffic from those groups.
Another scenario involves the product M*N exceeding SSN are global within the capacity context of a
single router's interface (especially if given protocol
(e.g. IP). They count ClusterControlVC and ServerControlVC activity
without reference to the same interface must also
support a unicast IP router service).
A network administrator multicast group(s) involved. This may choose to add a second node, to function be
perceived as a parallel IP limitation, because there is no way for cluster
members or multicast router. Each router would be configured servers to isolate exactly which multicast group
they may have missed an update for. An alternative was to try and
provide a per-group sequence number.
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Unfortunately per-group sequence numbers are not practical. The
current mechanism allows sequence information to be 'promiscuous' over piggy-backed onto
MARS messages already in transit for other reasons. The ability to
specify blocks of multicast addresses with a single MARS_JOIN or
MARS_LEAVE means that a single message can refer to membership change
for multiple groups simultaneously. A single ar$msn field cannot
provide meaningful information about each group's sequence. Multiple
ar$msn fields would have been unwieldy.
Any MARS or cluster member that supports different protocols MUST
keep separate parts of the Class D address space,
thus exposing themselves mapping tables and sequence numbers for each protocol.
6.4 Redundant/Backup MARS Architectures.
If backup MARSs exist for a given cluster then mechanisms are needed
to only part ensure consistency between their mapping tables and those of the VC load. This sharing
would be completely transparent
active, current MARS.
(Cluster members will consider backup MARSs to IP hosts within the LIS.
Restricted promiscuous mode does not break RFC 1112's use of IGMP
Report messages. If the router is exist if they have
been configured to serve with a given block table of Class D MARS addresses, it will receive the IGMP Report. If or the router
is not configured to support regular
MARS_REDIRECT_MAP messages contain a given block, then the existence list of 2 or more addresses.)
The definition of an
IGMP Report for a group in that block MARS-synchronization protocol is irrelevant to the router.
All routers are able to track membership changes through the
MARS_JOIN and MARS_LEAVE traffic anyway.
Mechanisms for establishing these modes of operation are beyond the
current scope of this memo.
10. Robustness of interaction with the MARS.
Transient problems may result in the loss of messages between the
MARS, cluster members, document, and multicast servers. More serious problems
may result in the failure of the MARS itself. There are two problem
scenarios that are addressed. The first is the inability of a cluster
member to send messages to the MARS itself, either through cell loss
on the VC expected to be the MARS, or the cluster member's inability to establish
a VC to subject of
further research work. However, the MARS. following observations may be
made:
The second is with the MARS_JOIN/SJOIN/LEAVE/SLEAVE messages re-
transmitted from the MARS. If a cluster member or multicast server
currently sending MARS_REDIRECT_MAP message exist enable one MARS to a group misses an join update, the newly joined
member misses out on some traffic force
endpoints to move to another MARS (e.g. in the group. If aftermath of a cluster member
or multicast server currently sending MARS
failure, the chosen backup MARS will eventually wish to a group misses a leave
update, hand
control of the cluster member that left will continue over to receive packets
unecessarily.
10.1 Ensuring the main MARS hears you.
A simple algorithm solves the first problem. when it is
functioning properly again).
Cluster members
retransmit MARS_JOIN and MARS_LEAVE MCSs do not need to start up with knowledge of
more than one MARS, provided that MARS correctly issues
MARS_REDIRECT_MAP messages at regular intervals
until they receive a copy back again, either on ClusterControlVC or
the VC on which they are sending the messages. At this point with the
local endpoint can be certain full list of MARSs for that at least
cluster.
Any mechanism for synchronising backup MARSs (and coping with the
aftermath of MARS received it.
Multicast servers retransmit MARS_MSERV and MARS_UNSERV messages at
regular intervals until they receive a copy back on ServerControlVC.
The interval failures) should be no shorter than 5 seconds, and a default value
of 10 seconds is recommended. After 5 retransmissions not require the attempt
should endpoint behaviour
to be flagged locally as modified from what is described in this specification.
7. How an MCS utilises a failure. This should be considered as Class II MARS.
Along the data path the MCS is a MARS failure, and handled as described protocol independent entity, in section 10.2. that
its role is to accept AAL_SDUs from multiple sources and then
transmit them sequentially out a single point to multipoint VC. It
does not look inside the AAL_SDUs at all. However, when an MCS starts
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A 'copy' is defined as seeing a message of the same operation code
containing
up it must register with the local host's identity MARS as described in the source address fields.
The <min,max> pair set is not checked, and does not have section 6.2.3. This
requires it to be the
same (this is required so that cluster members may verify register for a MARS_JOIN
they've sent even if particular protocol (specified in the MARS's hole-punching creates a totally
different set of <min,max> pairs).
10.2 Temporary failure
ar$pro field of the MARS.
Two failure modes indicate problems with MARS_MSERV).
Each MCS MUST terminate unidirectional VCs in the MARS itself:
If same manner as a
cluster member would (e.g. terminate on an ERR_L_RELEASE occurs LLC entity when LLC/SNAP
encapsulation is used, as described in RFC 1755 for unicast
endpoints). This is because the MCS is acting as a surrogate cluster member's attachment to
ClusterControlVC it may be assumed some problem exists with the
MARS.
If
endpoint for the cluster member receives ERR_L_RQFAILED when it attempts senders to
establish a the group.
The MCS manages its outgoing point to point multipoint VC to the MARS in order an analogous
way to send MARS
messages.
The a cluster member should wait a random period of time between 1 and
10 seconds before attempting (as described in section 5.1). MARS_REQUEST
is used by the MCS to re-register with establish the MARS. If initial leaf nodes for the
registration MARS_JOIN is successful (in accordance with section
10.1) then:
The cluster member MUST then proceed MCS's
outgoing point to rejoin every group that
its local higher layer protocol(s) have joined. It is recommended
that a random delay between 1 and 10 seconds be inserted before multipoint VC. After the transmission of each MARS_JOIN.
Finally, using VC is established, the mechanism described MCS
reacts to MARS_SJOINs and MARS_SLEAVEs in section 7, the same way a cluster
member MUST begin revalidating every multicast group it was
sending to.
The rejoin reacts to MARS_JOINs and revalidation procedure must not disrupt MARS_LEAVEs.
The MCS tracks the cluster
member's use of multipoint VCs that were already open at Server Sequence Number from the time ar$msn fields of
messages from the MARS failure.
If the re-registration with the Primary MARS fails, and there is no
configured Secondary MARS, and revalidates its outgoing point to
multipoint VC(s) when a sequence number jump occurs.
The MCS uses the same approach to backup MARSs as a cluster member member,
and tracks MARS_REDIRECT_MAP messages on ServerControlVC in an
analogous manner to cluster members (as described in section 5.4).
An MCS MUST wait NOT share the same ATM address as a cluster member,
although it may share the same physical ATM interface.
8. Support for IP multicast routers.
Multicast routers are required for at least
1 minute before repeating the re-registration procedure. It is
RECOMMENDED that propagation of multicast
traffic beyond the constraints of a single cluster member signals an error condition (inter-cluster
traffic). (There is a sense in
some locally significant fashion.
If which they are multicast servers
acting at the re-registration next higher layer, with the Primary MARS fails, clusters, rather than
individual endpoints, as their abstract sources and a Secondary
MARS has been configured, the Secondary destinations.)
Multicast routers typically participate in higher layer multicast
routing algorithms and Primary MARS addresses policies that are swapped and the cluster member immediately repeats beyond the re-
registration procedure. If scope of this
memo (e.g. DVMRP [5] in the IPv4 environment).
It is succesful assumed that the multicast routers will be implemented over the
same sort of IP/ATM interface that a multicast host would use. Their
IP/ATM interfaces will will register with the MARS as a cluster member
members, joining and leaving multicast groups as necessary. As noted
in section 5, multiple logical 'endpoints' may be implemented over a
single physical ATM interface. Routers use this approach to provide
interfaces into each clusters they will
resume normal operation using the Secondary MARS. It is RECOMMENDED be routing between.
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that the cluster member signals a warning
The rest of this condition in some
locally significant fashion.
If the attempt at re-registration with the Secondary MARS fails, section will assume a simple IPv4 scenario where the
scope of a cluster member MUST wait for at least 1 minute before reverting back has been limited to a particular LIS that is part
of an overlaid IP network. Not all members of the Primary MARS and starting the whole re-registration process
over again. In the worst case scenario this will result in LIS are necessarily
registered cluster members looping between registration attempts with the Primary MARS
and Secondary MARS until network administrators manually intervene.
Multicast servers shall behave (you may have unicast-only hosts in the
LIS).
8.1 Forwarding into a similar manner Cluster.
If the multicast router needs to transmit a packet to a group within
the cluster members
on this issue.
10.3 The MARS Sequence Number.
There is an unsigned 32 bit sequence number identified as ar$msn in
most MARS messages. The following extensions govern its use:
The MARS keeps two independent counters, Cluster Sequence Number
(CSN) IP/ATM interface opens a VC in the same manner as a
normal host would. Once a VC is open, the router watches for
MARS_JOIN and Server Sequence Number (SSN). They are incremented every
time MARS_LEAVE messages and responds to them as a normal
host would.
The multicast router's transmit side MUST implement inactivity timers
to shut down idle outgoing VCs, as for normal hosts.
As with normal host, the multicast router does not need to be a message
member of a group it is sent out ClusterControlVC or ServerControlVC
respectively.
[Editorial note - sending to.
8.2 Joining in ipmc-03.txt 'promiscuous' mode.
Once registered and initialised, the counter was incremented
only when simplest model of IPv4 multicast
router operation is for it to issue a change occurred in MARS_JOIN encompassing the mapping tables. this is
entire Class D address space. In effect it becomes 'promiscuous', as
it will be a
simplification.]
The current CSN is copied into leaf node to all present and future multipoint VCs
established to IPv4 groups on the ar$msn field of MARS messages
being sent cluster.
How a router chooses which groups to propagate outside the cluster members (either out ClusterControlVC or on
an individual VC).
The current SSN is copied into
beyond the ar$msn field scope of MARS this memo.
Consistent with RFC 1112, IP multicast routers may retain the use of
IGMP Query and IGMP Report messages
being sent to ascertain group membership.
However, certain optimisations are possible, and are described in
section 8.5.
8.3 Forwarding across the cluster.
Under some circumstances the cluster may simply be another hop
between IP subnets that have participants in a multicast servers (either out ServerControlVC or on
an individual VC).
Cluster group.
[LAN.1] ----- IPmcR.1 -- [cluster/LIS] -- IPmcR.2 ----- [LAN.2]
LAN.1 and LAN.2 are subnets (such as Ethernet) with attached hosts
that are members of group X.
IPmcR.1 and IPmcR.2 are multicast servers track the increments of CSN
or SSN routers with interfaces to determine if they have missed any update messages.
Calculations on the sequence numbers MUST LIS.
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A traditional solution would be performed to treat the LIS as unsigned 32
bit arithmetic, a unicast subnet,
and use tunneling routers. However, this would not allow hosts on the
LIS to ensure no glitches when participate in the counters roll over.
Every cluster member keeps cross-LIS traffic.
Assume IPmcR.1 is receiving packets promiscuously on its own 32 bit Host Sequence Number (HSN) LAN.1
interface. Assume further it is configured to track propagate multicast
traffic to all attached interfaces. In this case that means the MARS's sequence number. Whenever LIS.
When a MARS_MULTI,
MARS_JOIN, or MARS_LEAVE is received packet for group X arrives on its LAN.1 interface, IPmcR.1
simply sends the following check is then
performed packet to group X on the ar$msn field of LIS interface as a normal
host would (Issuing MARS_REQUEST for group X, creating the VC,
sending the packet).
Assuming IPmcR.2 initialised itself with the new message:
Seq.diff = ar$msn - HSN
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ar$msn -> HSN
{...process MARS message as appropriate...} a member of the
entire Class D space, it will have been returned as a member of X
even if ((Seq.diff != 1) && (Seq.diff != 0))
then {...revalidate no other nodes on the LIS were members. All packets for group membership information...}
X received on IPmcR.2's LIS interface may be retransmitted on LAN.2.
If IPmcR.1 is similarly initialised the reverse process will apply
for multicast traffic from LAN.2 to LAN.1, for any multicast group.
The basic result benefit of this scenario is that the cluster member attempts to keep locked
in step with membership changes noted by the MARS. If it ever detects
that a membership change occurred (in any group) without it noticing,
it re-validates members within the membership of all groups it currently has LIS
may also join and leave group X at anytime.
8.4 Joining in 'semi-promiscous' mode.
Both unicast and multicast VCs open to. Revalidation involves treating each VC as
though an ERR_L_RELEASE was received from IP routers have a leaf node, and executing common problem -
limitations on the procedure described in section 7.
The ar$msn field number of consecutive MARS_MULTIs sent AAL contexts available at their ATM
interfaces. Being 'promiscuous' in response the RFC 1112 sense means that for
every M hosts sending to N groups, a
MARS_REQUEST must be constant. If the ar$msn field changes then all
the messages MUST be discarded at the completion multicast router's ATM interface
will have M*N incoming reassembly engines tied up.
It is not hard to envisage situations where a number of multicast
groups are active within the response, and LIS but are not required to be
propagated beyond the MARS_REQUEST re-issued.
One implication of this mechanism is that LIS itself. An example might be a distributed
simulation system specifically designed to use the MARS should serialize high speed IP/ATM
environment. There may be no practical way its processing of 'simultaneous' MARS_REQUEST, MARS_JOIN and
MARS_LEAVE messages. Join and Leave operations should traffic could be queued
within
utilised on 'the other side' of the MARS along with MARS_REQUESTS, and not processed until all multicast router, yet under the reply packets of a preceeding MARS_REQUEST
conventional scheme the router would have been transmitted.
The MARS is free to choose a value of CSN and SSN. When be a new cluster
member starts up it should initialise HSN leaf to zero. When each
participating host anyway.
As this problem occurs at the cluster
member sends link layer, it is worth noting that
'scoping' mechanisms at the MARS_JOIN IP multicast routing level do not provide
a solution. An IP level scope would still result in the router's ATM
interface receiving traffic on the scoped groups, only to register, drop it.
In this situation the HSN will be correctly set
when it receives a copy network administrator might configure their
multicast routers to exclude sections of its MARS_JOIN from the MARS. If Seq.diff >
1 Class D address space
when the MARS_JOIN returns no action issuing MARS_JOIN(s). Multicast groups that will never be taken anyway, as
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propagated beyond the
host cluster will not have any multicast related VCs established at this
stage.
If a sequence number jump occurs when establishing the router listed as a new group's VC
member, and the cluster member should not revalidate router will never have to receive (and simply ignore)
traffic from those groups.
Another scenario involves the membership product M*N exceeding the capacity of a
single router's interface (especially if the group
it just established. The membership returned in same interface must also
support a unicast IP router service).
A network administrator may choose to add a second node, to function
as a parallel IP multicast router. Each router would be configured to
be 'promiscuous' over separate parts of the MARS_MULTIs that
carried Class D address space,
thus exposing themselves to only part of the new ar$msn field should be considered already validated.
A MARS should VC load. This sharing
would be carefully designed completely transparent to minimise IP hosts within the possibility LIS.
Restricted promiscuous mode does not break RFC 1112's use of
CSN or SSN jumping unecessarily. Under normal operation only hosts
that are affected by transient link problems will miss ar$msn updates
and be forced to revalidate. IGMP
Report messages. If the MARS itself glitches router is configured to serve a given block
of Class D addresses, it will be
innundated with requests for a period as every cluster member
attempts receive the IGMP Report. If the router
is not configured to revalidate.
Multicast servers should utilize support a given block, then the ar$msn fields existence of an
IGMP Report for a group in exactly that block is irrelevant to the
same manner as cluster members. This will enable them router.
All routers are able to track membership changes through the
SSN,
MARS_JOIN and recover from missing any MARS_SJOIN/SLEAVE traffic.
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10.4 Why MARS_LEAVE traffic anyway. (Section 8.5 discusses a Gobal sequence number?
The CSN and SSN are global
better alternative to IGMP within the context of a given protocol
(e.g. IP). They count ClusterControlVC cluster.)
Mechanisms and ServerControlVC activity
without reference to the multicast group(s) involved. This may be
perceived as a limitation, because there is no way reasons for cluster
members or multicast servers to isolate exactly which multicast group
they may have missed an update for. establishing these modes of operation are
beyond the scope of this memo.
8.5 An alternative was to try IGMP Queries.
An unfortunate aspect of IGMP is that it assumes multicasting of IP
packets is a cheap and
provide trivial event at the link layer. As a per-group sequence number.
Unfortunately per-group sequence numbers
consequence, regular IGMP Queries are not practical. The
current mechanism allows sequence information multicasted by routers to be piggy-backed onto
MARS messages already in transit for other reasons. The ability group
224.0.0.1. These queries are intended to
specify blocks trigger IGMP Replies by
cluster members that have layer 3 members of multicast addresses with particular groups.
However, the MARS_GROUPLIST_REQUEST and MARS_GROUPLIST_REPLY messages
were designed to allow routers to avoid actually transmitting IGMP
Queries out into a single MARS_JOIN or
MARS_LEAVE means that cluster.
Whenever the router's forwarding engine wishes to transmit an IGMP
query, a single message MARS_GROUPLIST_REQUEST can refer be sent to membership change
for multiple groups simultaneously. A single ar$msn field cannot
provide meaningful the MARS instead. The
resulting MARS_GROUPLIST_REPLY(s) (described in section 5.3) from the
MARS carry all the information about each group's sequence. Multiple
ar$msn fields that the router would have been unwieldy.
Any MARS or cluster member ascertained
from IGMP replies.
It is RECOMMENDED that supports different protocols MUST
keep separate mapping tables and sequence numbers for each protocol.
10.5 Synchronizing the Primary and Secondary MARS.
If a Secondary multicast routers utilise this MARS exists for a given cluster then some mechanism is
needed service to ensure reasonable consistency between its mapping tables
and those
minimise IGMP traffic within the cluster.
By default a MARS_GROUPLIST_REQUEST SHOULD specify the entire address
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space (e.g. <224.0.0.0, 239.255.255.255> in an IPv4 environment).
However, routers serving part of the Primary MARS, especially as cluster members will
only ever register with one MARS. The inter-server protocol also
needs address space (as described in
section 8.4) MAY choose to cope with post-failure situations where some cluster members
end up registered with issue MARS_GROUPLIST_REQUESTs that specify
only the subset of the address space they are serving.
(On the Primary surface it would also seem useful for multicast routers to
track MARS_JOINs and others MARS_LEAVEs that arrive with the Secondary.
The definition ar$layer3grp
flag set. These might be used in lieu of an inter-server protocol is beyond IGMP Reports, to provide the current
scope of
router with timely indication that a new layer 3 group member exists
within the cluster. However, this document, only works on VC mesh supported
groups, and is expected to be therefore NOT recommended).
Appendix B discusses less elegant mechanisms for reducing the subject impact
of further
work in IGMP traffic within a cluster, on the area.
11. Using assumption that the IP/ATM
interfaces to the cluster are being used by un-optimised IP
multicasting code.
9. Multiprotocol applications of the MARS in non-IP environments.
An and MARS clients.
A deliberate attempt has been made to describe the MARS and
associated mechanisms in a manner independent of a specific higher
layer protocol being run over the ATM cloud. The immediate
application of this document will be in an IPv4 environment, and this
is reflected by the focus of key examples. However, the coding of
each MARS message means that any higher layer protocol identifiable
by a two byte Ethernet Type code can be supported by a MARS.
The 16 bit 'Protocol type' (ar$pro) at the start of each MARS message, message
is taken from the set of Ethernet Type codes. Every MARS MUST
implement entirely separate logical mapping tables and support. Every
cluster
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of the protocol type that the MARS message refers to.
The LLC/SNAP encapsulation specified encapsulations described in section 4 should not be
considered a hinderance in non-IP environments. Experimenters
deploying IPX or AppleTalk over ATM are encouraged 5 similarly allow
multiple protocols to use be identified by the
architecture described use of different values in this document to support possible multicast
needs.
12.
appropriate encapsulation fields.
10. Key Decisions and open issues.
The key decisions this memo proposes:
A Multicast Address Resolution Server (MARS) is proposed to co-
ordinate and distribute mappings of ATM endpoint addresses to
arbitrary higher layer 'multicast group addresses'. The specific
case of IP version 4 IPv4 multicast is used as the example.
Individual multicast groups may be supported by multicast meshes
between group members, or by multicast servers.
The concept of 'clusters' is introduced to define the scope of a
MARS's responsibility, and the set of ATM endpoints willing to
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participate in link level multicasting.
A Class I MARS is described, with the necessary functionality to
support intra-cluster multicasting using VC meshes. A Class II
MARS is described as a superset of the Class I, with additional
functionality required to support intra-cluster multicasting using
either VC meshes or ATM level multicast servers.
MARS message formats and encapsulation allow co-resident MARS and
ATM ARP Server implementations.
New message types: MARS_JOIN, MARS_LEAVE, MARS_REQUEST. Allow
endpoints to join, leave, and request the current membership list
of multicast groups.
New message type: MARS_MULTI. Allows multiple ATM addresses to be
returned by the MARS in response to a MARS_REQUEST.
New message types: MARS_MSERV, MARS_UNSERV. Allow multicast
servers to register and deregister themselves with the MARS.
New message types: MARS_SJOIN, MARS_SLEAVE. Allow MARS to pass on
group membership changes to multicast servers. multicast servers.
New message types: MARS_GROUPLIST_REQUEST, MARS_GROUPLIST_REPLY.
Allow MARS to indicate which groups have actual layer 3 members.
May be used to support IGMP in IPv4 environments, and similar
functions in other environments.
New message type: MARS_REDIRECT_MAP. Allow MARS to specify a set
of backup MARS addresses.
'wild card' MARS mapping table entries possible, where a single
ATM address is simultaneously associated with blocks of multicast
group addresses.
Some
The complete set of messages, and ar$op values, is:
11 MARS_REQUEST
12 MARS_MULTI
13 MARS_MSERV
14 MARS_JOIN
15 MARS_LEAVE
16 MARS_NAK
17 MARS_UNSERV
18 MARS_SJOIN
19 MARS_SLEAVE
20 MARS_GROUPLIST_REQUEST
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21 MARS_GROUPLIST_REPLY
22 MARS_REDIRECT_MAP
A number of issues are left open at this stage, and are likely to be
the subject of on-going research and additional documents that build
upon this one.
The specified endpoint behaviour allows the use of
redundant/backup MARSs within a cluster. However, no
specifications yet exist on how these MARSs co-ordinate amongst
themselves. (The default is to only have not been addressed, although they one MARS per cluster.)
The specified endpoint behaviour and Class II MARS service allows
the use of multiple MCSs per group. However, no specifications
yet exist on how this may be in future
revisions. used, or how these MCSs co-ordinate
amongst themselves. (The default is to only have one MCS per
group.)
The MARS has no mechanism relies on the cluster member dropping off
ClusterControlVC if the cluster member dies. It is not clear if
additional mechanisms are needed to detect and delete 'dead'
cluster members.
If a multicast server attempts to MARS_MSERV for realising an existing VC
mesh supported group, it would be nice to have current senders to
the group migrate their outgoing VCs from the actual cluster members
member leaf nodes to the newly registered multicast server(s). How
this might be achieved, the load this would place on the MARS, and
its scalability, have silently
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died. not yet been considered.
Supporting layer 3 'broadcast' as a special case of multicasting
(where the 'group' encompasses all cluster members) has not been
explicitly discussed.
Supporting layer 3 'unicast' as a special case of multicasting
(where the 'group' is a single cluster member, identified by the
cluster member's unicast protocol address) has not been explicitly
discussed.
The future development of ATM Group Addresses and Leaf Initiated
Join to ATM Forum's UNI specification has not been addressed. The
(However, the problems identified in this memo with respect to VC
scarcity and impact on AAL contexts will not be fixed by such
developments in the signalling protocol. protocol.)
Security Consideration
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Security consideration are not addressed in this memo.
Acknowledgments
The discussions within the IP over ATM Working Group have helped
clarify the ideas expressed in this document. John Moy of Cascade (Cascade
Communications Corp. Corp.) initially suggested the idea of wild-card
entries in the ARP Server. Drew Perkins of Fore Systems (Fore Systems) provided
rigorous and useful critique of early proposed mechanisms for
distributing and validating group membership information. Susan
Symington (and co-workers at MITRE Corp., Don Chirieleison, Rich
Verjinski, and Bill Barns) clearly articulated the need for multicast
server support, proposed a solution, and challenged earlier block
join/leave mechanisms. John Shirron (Fore Systems) provided useful
improvements on my original revalidation procedures. Susan Symington
and Bryan Gleeson (Adaptec) independently championed the need for the
service provided by MARS_GROUPLIST_REQUEST/REPLY.
Author's Address
Grenville Armitage
MRE 2P340, 445 South Street
Morristown, NJ, 07960-6438 07960
USA
Email: gja@thumper.bellcore.com
References
[1] S. Deering, "Host Extensions for IP Multicasting", RFC 1112,
Standford University, August 1989.
[2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaption
Layer 5", RFC 1483, USC/Information Science Institute, July 1993.
[3] Laubach, M., "Classical IP and ARP over ATM", RFC1577, Hewlett-
Packard Laboratories, December 1993
[4] ATM Forum, "ATM User-Network Interface Specification Version
3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993
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[5] D. Waitzman, C. Partridge, S. Deering, "Distance Vector Multicast
Routing Protocol", RFC 1075, November 1988.
[6] M. Perez, F. Liaw, D. Grossman, A. Mankin, E. Hoffman, A. Malis,
"ATM Signaling Support for IP over ATM", Internet Draft, IP over ATM
Working Group, draft-ietf-ipatm-sig-02.txt, November, 1994. RFC 1755, February 1995.
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Appendix A. Parsing Hole punching algorithms for Class II MARS messages.
Implementations are entirely free to comply with the body of this
memo in any way they see fit. This appendix is purely for
clarification.
A smart Class II MARS implementation will might pre-construct a set of <min,max>
pairs (P) that reflects the entire Class D space, excluding any
addresses currently supported by multicast servers. The <min> field
of the first pair MUST be 224.0.0.0, and the <max> field of the last
pair MUST must be 239.255.255.255. The first and last pair may be the
same. This set is updated whenever a multicast server registers or
deregisters.
When the MARS must perform 'hole punching' it might consider the
following algorithm:
Assume the MARS_JOIN/LEAVE received by the MARS from the cluster
member specied the block <Emin, Emax>.
Assume Pmin(N) and Pmax(N) are the <min> and <max> fields from the
Nth pair in the MARS's current set P.
Assume set P has K pairs. Pmin(1) MUST equal 224.0.0.0, and
Pmax(M) MUST equal 239.255.255.255. (If K == 1 then no hole
punching is required).
Execute pseudo-code:
create copy of set P, call it set C.
index1 = 1;
while (Pmax(index1) <= Emin)
index1++;
index2 = K;
while (Pmin(index2) >= Emax)
index2--;
if (index1 > index2)
Exit, as the hole-punched set is null.
if (Pmin(index1) < Emin)
Cmin(index1) = Emin;
if (Pmax(index2) > Emax)
Cmax(index2) = Emax;
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Set C is the required 'hole punched' set of address blocks.
The resulting set C retains all the MARS's pre-constructed 'holes'
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covering the multicast servers, but will have been pruned to cover
the section of the Class D space specified by the originating host's
<Emin,Emax> values.
The host end should keep a table, H, of open VCs in ascending order
of Class D address.
Assume H(x).addr is the Class address associated with VC.x.
Assume H(x).addr < H(x+1).addr.
The pseudo code for updating VCs based on an incoming JOIN/LEAVE
might be:
x = 1;
N = 1;
while (x < no.of VCs open)
{
while (H(x).addr > max(N))
{
N++;
if (N > no. of pairs in JOIN/LEAVE)
return(0);
}
if ((H(x).addr <= max(N) &&
((H(x).addr >= min(N))
perform_VC_update();
x++;
}
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Appendix B. Coping with Minimising the impact of IGMP in IPv4 idiosyncracies. environments.
Implementing any part of this appendix is not required for
conformance with this memo. It is provided solely to document issues
that have been identified.
The intent of section 5.3 5.1 is for cluster members to only have
outgoing point to multipoint VCs when they are actually sending data
to a particular multicast groups. However, in most IPv4 environments
the multicast routers attached to a cluster will periodically issue
IGMP Queries to ascertain if particular groups have members. The
current IGMP specification attempts to avoid having every group
member respond by insisting that each group member wait a random
period, and responding if no other member has responded before them.
The IGMP reply is sent to the multicast address of the group being
queried.
Unfortunately, as it stands the IGMP algorithm will be a nuisance for
cluster members that are essentially passive receivers within a given
multicast group. It is just as likely that a passive member, with no
outgoing VC already established to the group, will decide to send an
IGMP reply - causing a VC to be established were there was no need
for one. This is not a fatal problem for small clusters, but will
seriously impact on the ability of a cluster to scale.
Various solutions exist, providing short and long term solutions
The most obvious solution is for routers to use the problem.
MARS_GROUPLIST_REQUEST and MARS_GROUPLIST_REPLY messages, as
described in section 8.5. This would remove the regular IGMP Queries,
resulting in cluster members only sending an IGMP Report when they
first join a group.
Alternative solutions do exist. One long term solution would be to modify the IGMP reply
algorithm, for example:
If the group member has VC open to the group proceed as per RFC
1112 (picking a random reply delay between 0 and 10 seconds).
If the group member does not have VC already open to the group,
pick random reply delay between 10 and 20 seconds instead, and
then proceed as per RFC 1112.
If even one group member is sending to the group at the time the IGMP
Query is issued then all the passive receivers will find the IGMP
Reply has been transmitted before their delay expires, so no new VC
is required. If all group members are passive at the time of the IGMP
Query then a response will eventually arrive, but 10 seconds later
than under conventional circumstances.
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The preceeding solution requires re-writing existing IGMP code, and
implies the ability of the IGMP entity to ascertain the status of VCs
on the underlying ATM interface. This is not likely to be available
in the short term.
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One short term solution is to provide something like the preceeding
functionality with a 'hack' at the IP/ATM driver level within cluster
members. Arrange for the IP/ATM driver to snoop inside IP packets
looking for IGMP traffic. If an IGMP packet is accepted for
transmission, the IP/ATM driver can buffer it locally if there is no
VC already active to that group. A 10 second timer is started, and if
an IGMP Reply for that group is received from elsewhere on the
cluster the timer is reset. If the timer expires, the IP/ATM driver
then establishes a VC to the group as it would for a normal IP
multicast packet.
Some network implementors may find it advantageous to configure a
multicast server to support the group 224.0.0.1, rather than rely on
a mesh. Given that IP multicast routers regularly send IGMP queries
to this address, a mesh will mean that each router will permanently
consume an AAL context within each cluster member. In clusters served
by multiple routers the VC load within switches in the underlying ATM
network will become a scaling problem.
Finally, if a multicast server is used to support 224.0.0.1, another
ATM driver level hack becomes a possible solution to IGMP Reply
traffic. The ATM driver may choose to grab all outgoing IGMP packets
and send them out on the VC established for sending to 224.0.0.1,
regardless of the Class D address the IGMP message was actually for.
Given that all hosts and routers must be members of 224.0.0.1, the
intended recipients will still receive the IGMP Replies. The negative
impact is that all cluster members will receive the IGMP Replies.
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Appendix C. Issues relating to multicast servers.
Implementing any part of this appendix is not required for
conformance with this memo. It is provided to give some structure to
further research and development Further comments on multicast server support within
clusters.
Various items are not addressed by this memo. They include:
Automatic migration of 'Clusters'.
The cluster members from a mesh to a multicast
server while a group is active.
An elegant mechanism concept was introduced in section 1 for migration of cluster members from
multicast servers back two reasons. The
more well known term of Logical IP Subnet is both very IP specific,
and constrained to a mesh while unicast routing boundaries. As the group is active.
Additional intelligence architecture
described in the MARS to perform load sharing
between multicast servers if this document may be re-used in non-IP environments a
more than one registers for neutral term was needed. As the needs of multicasting are not
always bound by the same
group.
If one or more multicast servers attempt scopes as unicasting, it was not immediately
obvious that apriori limiting ourselves to register for LISs was a group that
already has members, it would win situation
either.
It must be nice to have current senders to the
group migrate stressed that Clusters are purely an administrative being.
You choose their outgoing VCs from size (i.e. the actual cluster members to number of endpoints that register
with the newly registered multicast server(s). One approach might be same MARS) based on your multicasting needs, and the
resource consumption you are willing to
have put up with. The larger the MARS issue a sequence
number of fabricated MARS_JOINs for the ATM attached hosts you require multicast servers, followed by MARS_LEAVEs for each member of the
group's current host map. What load this would place on support for, the MARS, and
its scalability, have
more individual clusters you may choose to establish (along with
multicast routers to provide inter-cluster traffic paths).
Given that not been considered.
An elegant mechanism for the reverse migration might well be based
around the reverse process. Issue MARS_JOINs for all entries in the
host map, then issue MARS_LEAVEs for all remaining entries hosts in the
server map.
In case of groups served by multiple any given LIS may require multicast servers, the current
expectation is
support, it becomes conceivable that each server retrieves the entire group's
membership with MARS_REQUESTs. This memo expects there you might assign a single MARS
to be an
external mechanism for support hosts from across multiple LISs. In effect you have a
cluster covering multiple LISs, and have achieved 'cut through'
routing for multicast servers to synchronize the
load sharing amongst themselves. Whether traffic. Under these circumstances increasing
the MARS should to geographical size of a cluster might be
extended to play considered a part is good thing.
However, practical considerations limit the size of clusters. Having
a subject for further work.
An issue cluster span multiple LISs may not immediately related to the MARS architecture is whether always be a particular 'win'
situation. As the number of multicast server retransmits using a point to multipoint VC out capable hosts in your LISs
increases it becomes more likely that you'll want to
group members, or constrain a set
cluster's size and force multicast traffic to aggregate at multicast
routers scattered across your ATM cloud. (This is especially true
for clusters based on Class I MARSs, as resource consumption of one VC per group member. The first
approach makes better use of
meshes increases rapidly with an increase in the underlying ATM fabric, but data
sources that are also members number of the group will receive copies
senders/group members.)
Finally, multi-LIS clusters require a moderate amount of
their own traffic back. The alternative avoids this problem, but at care when
deploying IP multicast routers. Under the expense of consuming more VCs and bandwidth Classical IP model you need
unicast routers on the path out edges of LISs. Under the MARS architecture you
only need multicast server itself.
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Situations where either issue is a problem should simply revert to
using a multicast mesh between participating endpoints, where routers at the
source never sees copies edges of its own packets, and clusters. If your cluster
spans multiple LISs, then the multicasting
happens within multicast routers will perceive
themselves to have a single interface that is simultaneously attached
to multiple unicast subnets. This situation can work, but may require
some hand configuration of 'default' multicast router behaviour,
depending on the ATM switch fabrics. inter-domain routing protocol you are using.
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