As per Relevance of the word multicast, we have this rfc below:
Network Working Group G.
Request for Comments: 2022
Category: Standards Track November 1996
Support for Multicast over UNI 3.0/3.1 based ATM Networks
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is
Mapping the connectionless IP multicast service over the
oriented ATM services provided by UNI 3.0/3.1 is a non-trivial task
This memo describes a mechanism to support the multicast needs
Layer 3 protocols in general, and describes its application to
multicasting in particular
ATM based IP hosts and routers use a Multicast Address
Server (MARS) to support RFC 1112 style Level 2 IP multicast over
ATM Forum's UNI 3.0/3.1 point to multipoint connection service
Clusters of endpoints share a MARS and use it to track
disseminate information identifying the nodes listed as receivers
given multicast groups. This allows endpoints to establish and
point to multipoint VCs when transmitting to the group
The MARS behaviour allows Layer 3 multicasting to be supported
either meshes of VCs or ATM level multicast servers. This choice
be made on a per-group basis, and is transparent to the endpoints
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Table of
1. Introduction................................................. 4
1.1 The Multicast Address Resolution Server (MARS)............. 5
1.2 The ATM level multicast Cluster............................ 5
1.3 Document overview.......................................... 6
1.4 Conventions................................................ 7
2. The IP multicast service model............................... 7
3. UNI 3.0/3.1 support for intra-cluster multicasting........... 8
3.1 VC meshes.................................................. 9
3.2 Multicast Servers.......................................... 9
3.3 Tradeoffs.................................................. 10
3.4 Interaction with local UNI 3.0/3.1 signalling entity....... 11
4. Overview of the MARS......................................... 12
4.1 Architecture............................................... 12
4.2 Control message format..................................... 12
4.3 Fixed header fields in MARS control messages............... 13
4.3.1 Hardware type.......................................... 14
4.3.2 Protocol type.......................................... 14
4.3.3 Checksum............................................... 15
4.3.4 Extensions Offset...................................... 15
4.3.5 Operation code......................................... 16
4.3.6 Reserved............................................... 16
5. Endpoint (MARS client) interface behaviour................... 16
5.1 Transmit side behaviour.................................... 17
5.1.1 Retrieving Group Membership from the MARS.............. 18
5.1.2 MARS_REQUEST, MARS_MULTI, and MARS_NAK messages........ 20
5.1.3 Establishing the outgoing multipoint VC................ 22
5.1.4 Monitoring updates on ClusterControlVC................. 24
5.1.4.1 Updating the active VCs............................ 24
5.1.4.2 Tracking the Cluster Sequence Number............... 25
5.1.5 Revalidating a VC's leaf nodes......................... 26
5.1.5.1 When leaf node drops itself........................ 27
5.1.5.2 When a jump is detected in the CSN................. 27
5.1.6 'Migrating' the outgoing multipoint VC................. 27
5.2. Receive side behaviour.................................... 29
5.2.1 Format of the MARS_JOIN and MARS_LEAVE Messages........ 30
5.2.1.1 Important IPv4 default values...................... 32
5.2.2 Retransmission of MARS_JOIN and MARS_LEAVE messages.... 33
5.2.3 Cluster member registration and deregistration......... 34
5.3 Support for Layer 3 group management....................... 34
5.4 Support for redundant/backup MARS entities................. 36
5.4.1 First response to MARS problems........................ 36
5.4.2 Connecting to a backup MARS............................ 37
5.4.3 Dynamic backup lists, and soft redirects............... 37
5.5 Data path LLC/SNAP encapsulations.......................... 40
5.5.1 Type #1 encapsulation.................................. 40
5.5.2 Type #2 encapsulation.................................. 41
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
5.5.3 A Type #1 example...................................... 42
6. The MARS in greater detail................................... 42
6.1 Basic interface to Cluster members......................... 43
6.1.1 Response to MARS_REQUEST............................... 43
6.1.2 Response to MARS_JOIN and MARS_LEAVE................... 43
6.1.3 Generating MARS_REDIRECT_MAP........................... 45
6.1.4 Cluster Sequence Numbers............................... 45
6.2 MARS interface to Multicast Servers (MCSs)................. 46
6.2.1 MARS_REQUESTs for MCS supported groups................. 47
6.2.2 MARS_MSERV and MARS_UNSERV messages.................... 47
6.2.3 Registering a Multicast Server (MCS)................... 49
6.2.4 Modified response to MARS_JOIN and MARS_LEAVE.......... 49
6.2.5 Sequence numbers for ServerControlVC traffic........... 51
6.3 Why global sequence numbers?............................... 52
6.4 Redundant/Backup MARS Architectures........................ 52
7. How an MCS utilises a MARS................................... 53
7.1 Association with a particular Layer 3 group................ 53
7.2 Termination of incoming VCs................................ 54
7.3 Management of outgoing VC.................................. 54
7.4 Use of a backup MARS....................................... 54
8. Support for IP multicast routers............................. 54
8.1 Forwarding into a Cluster.................................. 55
8.2 Joining in 'promiscuous' mode.............................. 55
8.3 Forwarding across the cluster.............................. 56
8.4 Joining in 'semi-promiscous' mode.......................... 56
8.5 An alternative to IGMP Queries............................. 57
8.6 CMIs across multiple interfaces............................ 58
9. Multiprotocol applications of the MARS and MARS clients...... 59
10. Supplementary parameter processing.......................... 60
10.1 Interpreting the mar$extoff field......................... 60
10.2 The format of TLVs........................................ 60
10.3 Processing MARS messages with TLVs........................ 62
10.4 Initial set of TLV elements............................... 62
11. Key Decisions and open issues............................... 62
Security Considerations......................................... 65
Acknowledgments................................................. 65
Author's Address................................................ 65
References...................................................... 66
Appendix A. Hole punching algorithms............................ 67
Appendix B. Minimising the impact of IGMP in IPv4 environments.. 69
Appendix C. Further comments on 'Clusters'...................... 71
Appendix D. TLV list parsing algorithm.......................... 72
Appendix E. Summary of timer values............................. 73
Appendix F. Pseudo code for MARS operation...................... 74
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
1. Introduction
Multicasting is the process whereby a source host or protocol
sends a packet to multiple destinations simultaneously using
single, local 'transmit' operation. The more familiar cases
Unicasting and Broadcasting may be considered to be special cases
Multicasting (with the packet delivered to one destination, or 'all
destinations, respectively).
Most network layer models, like the one described in RFC 1112 [1]
IP multicasting, assume sources may send their packets to
'multicast group addresses'. Link layer support for such
abstraction is assumed to exist, and is provided by technologies
as Ethernet
ATM is being utilized as a new link layer technology to support
variety of protocols, including IP. With RFC 1483 [2] the
defined a multiprotocol mechanism for encapsulating and
packets using AAL5 over ATM Virtual Channels (VCs). However, the
Forum's currently published signalling specifications (UNI 3.0 [8]
and UNI 3.1 [4]) does not provide the multicast address abstraction
Unicast connections are supported by point to point,
VCs. Multicasting is supported through point to
unidirectional VCs. The key limitation is that the sender must
prior knowledge of each intended recipient, and explicitly
a VC with itself as the root node and the recipients as the
nodes
This document has two broad goals
Define a group address registration and membership
mechanism that allows UNI 3.0/3.1 based networks to support
multicast service of protocols such as IP
Define specific endpoint behaviours for managing point
multipoint VCs to achieve multicasting of layer 3 packets
As the IETF is currently in the forefront of using wide
multicasting this document's descriptions will often focus on
service model of RFC 1112. A final chapter will note
multiprotocol application of the architecture
This document avoids discussion of one highly non-trivial aspect
using ATM - the specification of QoS for VCs being established
response to higher layer needs. Research in this area is still
formative [7], and so it is assumed that future documents
clarify the mapping of QoS requirements to VC establishment.
default at this time is that VCs are established with a request
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Unspecified Bit Rate (UBR) service, as typified by the IETF's use
VCs for unicast IP, described in RFC 1755 [6].
1.1 The Multicast Address Resolution Server (MARS).
The Multicast Address Resolution Server (MARS) is an extended
of the ATM ARP Server introduced in RFC 1577 [3]. It acts as
registry, associating layer 3 multicast group identifiers with
ATM interfaces representing the group's members. MARS
support the distribution of multicast group membership
between MARS and endpoints (hosts or routers). Endpoint
resolution entities query the MARS when a layer 3 address needs to
resolved to the set of ATM endpoints making up the group at any
time. Endpoints keep the MARS informed when they need to join
leave particular layer 3 groups. To provide for
notification of group membership changes the MARS manages a point
multipoint VC out to all endpoints desiring multicast
Valid arguments can be made for two different approaches to ATM
multicasting of layer 3 packets - through meshes of point
multipoint VCs, or ATM level multicast servers (MCS). The
architecture allows either VC meshes or MCSs to be used on a per
group basis
1.2 The ATM level multicast Cluster
Each MARS manages a 'cluster' of ATM-attached endpoints. A Cluster
defined
The set of ATM interfaces choosing to participate in direct
connections to achieve multicasting of AAL_SDUs
themselves
In practice, a Cluster is the set of endpoints that choose to use
same MARS to register their memberships and receive their
from
By implication of this definition, traffic between
belonging to different Clusters passes through an inter-
device. (In the IP world an inter-cluster device would be an
multicast router with logical interfaces into each Cluster.)
document explicitly avoids specifying the nature of inter-
(layer 3) routing protocols
The mapping of clusters to other constrained sets of endpoints (
as unicast Logical IP Subnets) is left to each network administrator
However, for the purposes of conformance with this document
administrators MUST ensure that each Logical IP Subnet (LIS)
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
served by a separate MARS, creating a one-to-one mapping
cluster and unicast LIS. IP multicast routers then interconnect
LIS as they do with conventional subnets. (Relaxation of
restriction MAY only occur after future research on the
between existing layer 3 multicast routing protocols and
subnet boundaries.)
The term 'Cluster Member' will be used in this document to refer
an endpoint that is currently using a MARS for multicast support
Thus potential scope of a cluster may be the entire membership of
LIS, while the actual scope of a cluster depends on which
are actually cluster members at any given time
1.3 Document overview
This document assumes an understanding of concepts explained
greater detail in RFC 1112, RFC 1577, UNI 3.0/3.1, and RFC 1755 [6].
Section 2 provides an overview of IP multicast and what RFC 1112
required from Ethernet
Section 3 describes in more detail the multicast support
offered by UNI 3.0/3.1, and outlines the differences between
meshes and multicast servers (MCSs) as mechanisms for
packets to multiple destinations
Section 4 provides an overview of the MARS and its relationship
ATM endpoints. This section also discusses the encapsulation
structure of MARS control messages
Section 5 substantially defines the entire cluster member
behaviour, on both receive and transmit sides. This includes
normal operation and error recovery
Section 6 summarises the required behaviour of a MARS
Section 7 looks at how a multicast server (MCS) interacts with
MARS
Section 8 discusses how IP multicast routers may make novel use
promiscuous and semi-promiscuous group joins. Also discussed is
mechanism designed to reduce the amount of IGMP traffic issued
routers
Section 9 discusses how this document applies in the more
(non-IP) case
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Section 10 summarises the key proposals, and identifies areas
future research that are generated by this MARS architecture
The appendices provide discussion on issues that arise out of
implementation of this document. Appendix A discusses MARS
endpoint algorithms for parsing MARS messages. Appendix B
the particular problems introduced by the current IGMP paradigms,
possible interim work-arounds. Appendix C discusses the 'cluster
concept in further detail, while Appendix D briefly outlines
algorithm for parsing TLV lists. Appendix E summarises various
values used in this document, and Appendix F provides
pseudo-code for a MARS entity
1.4 Conventions
In this document the following coding and packet representation
are used
All multi-octet parameters are encoded in big-endian form (i.e
the most significant octet comes first).
In all multi-bit parameters bit numbering begins at 0 for
least significant bit when stored in memory (i.e. the n'th bit
weight of 2^n).
A bit that is 'set', 'on', or 'one' holds the value 1.
A bit that is 'reset', 'off', 'clear', or 'zero' holds the
0.
2. Summary of the IP multicast service model
Under IP version 4 (IPv4), addresses in the range between 224.0.0.0
and 239.255.255.255 (224.0.0.0/4) are termed 'Class D' or '
group' addresses. These abstractly represent all the IP hosts in
Internet (or some constrained subset of the Internet) who
decided to 'join' the specified group
RFC1112 requires that a multicast-capable IP interface must
the transmission of IP packets to an IP multicast group address
whether or not the node considers itself a 'member' of that group
Consequently, group membership is effectively irrelevant to
transmit side of the link layer interfaces. When Ethernet is used
the link layer (the example used in RFC1112), no address
is required to transmit packets. An algorithmic mapping from
multicast address to Ethernet multicast address is performed
before the packet is sent out the local interface in the same '
and forget' manner as a unicast IP packet
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Joining and Leaving an IP multicast group is more explicit on
receive side - with the primitives JoinLocalGroup and
affecting what groups the local link layer interface should
packets from. When the IP layer wants to receive packets from
group, it issues JoinLocalGroup. When it no longer wants to
packets, it issues LeaveLocalGroup. A key point to note is
changing state is a local issue, it has no effect on other
attached to the Ethernet
IGMP is defined in RFC 1112 to support IP multicast routers
to a given subnet. Hosts issue IGMP Report messages when they
a JoinLocalGroup, or in response to an IP multicast router sending
IGMP Query. By periodically transmitting queries IP multicast
are able to identify what IP multicast groups have non-
membership on a given subnet
A specific IP multicast address, 224.0.0.1, is allocated for
transmission of IGMP Query messages. Host IP layers issue
JoinLocalGroup for 224.0.0.1 when they intend to participate in
multicasting, and issue a LeaveLocalGroup for 224.0.0.1 when they'
ceased participating in IP multicasting
Each host keeps a list of IP multicast groups it has
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
of. IGMP Reports are sent to the group address, not 224.0.0.1, "
that other members of the same group on the same network can
the Report" and not bother sending one of their own. IP
routers conclude that a group has no members on the subnet when
Queries no longer elicit associated replies
3. UNI 3.0/3.1 support for intra-cluster multicasting
For the purposes of the MARS protocol, both UNI 3.0 and UNI 3.1
provide equivalent support for multicasting. Differences between
3.0 and UNI 3.1 in required signalling elements are covered in
1755.
This document will describe its operation in terms of 'generic
functions that should be available to clients of a UNI 3.0/3.1
signalling entity in a given ATM endpoint. The ATM model
describes an 'AAL User' as any entity that establishes and
VCs and underlying AAL services to exchange data. An IP over
interface is a form of 'AAL User' (although the default LLC/
encapsulation mode specified in RFC1755 really requires that an '
entity' is the AAL User, which in turn supports the IP/
interface).
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The most fundamental limitations of UNI 3.0/3.1's multicast
are
Only point to multipoint, unidirectional VCs may be established
Only the root (source) node of a given VC may add or remove
nodes
Leaf nodes are identified by their unicast ATM addresses.
3.0/3.1 defines two ATM address formats - native E.164 and
(although it must be stressed that the NSAP address is so
because it uses the NSAP format - an ATM endpoint is NOT a
layer termination point). In UNI 3.0/3.1 an 'ATM Number' is
primary identification of an ATM endpoint, and it may use
format. Under some circumstances an ATM endpoint must be
by both a native E.164 address (identifying the attachment point of
private network to a public network), and an NSAP address ('
Subaddress') identifying the final endpoint within the
network. For the rest of this document the term will be used to
either a single 'ATM Number' or an 'ATM Number' combined with an '
Subaddress'.
3.1 VC meshes
The most fundamental approach to intra-cluster multicasting is
multicast VC mesh. Each source establishes its own independent
to multipoint VC (a single multicast tree) to the set of leaf
(destinations) that it has been told are members of the group
wishes to send packets to
Interfaces that are both senders and group members (leaf nodes) to
given group will originate one point to multipoint VC, and
one VC for every other active sender to the group. This criss
crossing of VCs across the ATM network gives rise to the name '
mesh'.
3.2 Multicast Servers
An alternative model has each source establish a VC to
intermediate node - the multicast server (MCS). The multicast
itself establishes and manages a point to multipoint VC out to
actual desired destinations
The MCS reassembles AAL_SDUs arriving on all the incoming VCs,
then queues them for transmission on its single outgoing point
multipoint VC. (Reassembly of incoming AAL_SDUs is required at
multicast server as AAL5 does not support cell level multiplexing
different AAL_SDUs on a single outgoing VC.)
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
The leaf nodes of the multicast server's point to multipoint VC
be established prior to packet transmission, and the multicast
requires an external mechanism to identify them. A side-effect
this method is that ATM interfaces that are both sources and
members will receive copies of their own packets back from the
(An alternative method is for the multicast server to
retransmit packets on individual VCs between itself and
members. A benefit of this second approach is that the
server can ensure that sources do not receive copies of their
packets.)
The simplest MCS pays no attention to the contents of each AAL_SDU
It is purely an AAL/ATM level device. More complex MCS
(where a single endpoint serves multiple layer 3 groups)
possible, but are beyond the scope of this document. More
discussion is provided in section 7.
3.3 Tradeoffs
Arguments over the relative merits of VC meshes and multicast
have raged for some time. Ultimately the choice depends on
relative trade-offs a system administrator must make
throughput, latency, congestion, and resource consumption.
criteria such as latency can mean different things to
people - is it end to end packet time, or the time it takes for
group to settle after a membership change? The final choice
on the characteristics of the applications generating the
traffic
If we focussed on the data path we might prefer the VC mesh
it lacks the obvious single congestion point of an MCS.
is likely to be higher, and end to end latency lower, because
mesh lacks the intermediate AAL_SDU reassembly that must occur
MCSs. The underlying ATM signalling system also has
opportunity to ensure optimal branching points at ATM switches
the multicast trees originating on each source
However, resource consumption will be higher. Every group member'
ATM interface must terminate a VC per sender (consuming on-
memory for state information, instance of an AAL service,
buffering in accordance with the vendors particular architecture).
the contrary, with a multicast server only 2 VCs (one out, one in
are required, independent of the number of senders. The allocation
VC related resources is also lower within the ATM cloud when using
multicast server. These points may be considered to have merit
environments where VCs across the UNI or within the ATM cloud
valuable (e.g. the ATM provider charges on a per VC basis), or
contexts are limited in the ATM interfaces of endpoints
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
If we focus on the signalling load then MCSs have the advantage
faced with dynamic sets of receivers. Every time the membership of
multicast group changes (a leaf node needs to be added or dropped),
only a single point to multipoint VC needs to be modified when
an MCS. This generates a single signalling event across the MCS'
UNI. However, when membership change occurs in a VC mesh,
events occur at the UNIs of every traffic source - the
signalling load scales with the number of sources. This has
ramifications if you define latency as the time for a group'
connectivity to stabilise after change (especially as the number
senders increases).
Finally, as noted above, MCSs introduce a 'reflected packet' problem
which requires additional per-AAL_SDU information to be carried
order for layer 3 sources to detect their own AAL_SDUs coming back
The MARS architecture allows system administrators to utilize
approach on a group by group basis
3.4 Interaction with local UNI 3.0/3.1 signalling entity
The following generic signalling functions are presumed to
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
User and UNI 3.0/3.1 signalling entity with these functions
outside the scope of this document
The following indications are assumed to be available to AAL Users
generated by the local UNI 3.0/3.1 signalling entity
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_DROP - A remote ATM endpoint dropped off an existing VC
ERR_L_RELEASE - An existing VC was terminated
The signalling exchanges and local information passed between
User and UNI 3.0/3.1 signalling entity with these functions
outside the scope of this document
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
4. Overview of the MARS
The MARS may reside within any ATM endpoint that is
addressable by the endpoints it is serving. Endpoints wishing to
a multicast cluster must be configured with the ATM address of
node on which the cluster's MARS resides. (Section 5.4 describes
backup MARSs may be added to support the activities of a cluster
References to 'the MARS' in following sections will be assumed
mean the acting MARS for the cluster.)
4.1 Architecture
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
IP endpoints in an LIS, the MARS keeps extended tables of {layer 3
address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either
configured with certain mappings, or dynamically 'learn' mappings
The format of the {layer 3 address} field is generally
interpreted by the MARS
A single ATM node may support multiple logical MARSs, each of
support a separate cluster. The restriction is that each MARS has
unique ATM address (e.g. a different SEL field in the NSAP address
the node on which the multiple MARSs reside). By definition a
instance of a MARS may not support more than one cluster
The MARS distributes group membership update information to
members over a point to multipoint VC known as the ClusterControlVC
Additionally, when Multicast Servers (MCSs) are being used it
establishes a separate point to multipoint VC out to registered MCSs
known as the ServerControlVC. All cluster members are leaf nodes
ClusterControlVC. All registered multicast servers are leaf nodes
ServerControlVC (described further in section 6).
The MARS does NOT take part in the actual multicasting of layer 3
data packets
4.2 Control message format
By default all MARS control messages MUST be LLC/SNAP
using the following codepoints
[0xAA-AA-03][0x00-00-5E][0x00-03][MARS control message
(LLC) (OUI) (PID
(This is a PID from the IANA OUI.)
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
MARS control messages are made up of 4 major components
[Fixed header][Mandatory fields][Addresses][Supplementary TLVs
[Fixed header] contains fields indicating the operation
performed and the layer 3 protocol being referred to (e.g IPv4, IPv6,
AppleTalk, etc). The fixed header also carries checksum information
and hooks to allow this basic control message structure to be re-
by other query/response protocols
The [Mandatory fields] section carries fixed width parameters
depend on the operation type indicated in [Fixed header].
The following [Addresses] area carries variable length fields
source and target addresses - both hardware (e.g. ATM) and layer 3
(e.g. IPv4). These provide the fundamental information that
registrations, queries, and updates use and operate on. For the
protocol fields in [Fixed header] indicate how to interpret
contents of [Addresses].
[Supplementary TLVs] represents an optional list of TLV (type
length, value) encoded information elements that may be appended
provide supplementary information. This feature is described
further detail in section 10.
MARS messages contain variable length address fields. In all
null addresses SHALL be encoded as zero length, and have no
allocated in the message
(Unique LLC/SNAP encapsulation of MARS control messages means
and ARP Server functionality may be implemented within a
entity, and share a client-server VC, if the implementor so chooses
Note that the LLC/SNAP codepoint for MARS is different to
codepoint used for ATMARP.)
4.3 Fixed header fields in MARS control messages
The [Fixed header] has the following format
Data
mar$afn 16 bits Address Family (0x000F).
mar$pro 56 bits Protocol Identification
mar$hdrrsv 24 bits Reserved. Unused by MARS control protocol
mar$chksum 16 bits Checksum across entire MARS message
mar$extoff 16 bits Extensions Offset
mar$op 16 bits Operation code
mar$shtl 8 bits Type & length of source ATM number. (r
mar$sstl 8 bits Type & length of source ATM subaddress. (q
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
mar$shtl and mar$sstl provide information regarding the source'
hardware (ATM) address. In the MARS protocol these fields are
present, as every MARS message carries a non-null source ATM address
In all cases the source ATM address is the first variable
field in the [Addresses] section
The other fields in [Fixed header] are described in the
subsections
4.3.1 Hardware type
mar$afn defines the type of link layer addresses being carried.
value of 0x000F SHALL be used by MARS messages generated
accordance with this document. The encoding of ATM addresses
subaddresses when mar$afn = 0x000F is described in section 5.1.2.
Encodings when mar$afn != 0x000F are outside the scope of
document
4.3.2 Protocol type
The mar$pro field is made up of two subfields
mar$pro.type 16 bits Protocol type
mar$pro.snap 40 bits Optional SNAP extension to protocol type
The mar$pro.type field is a 16 bit unsigned integer representing
following number space
0x0000 to 0x00FF Protocols defined by the equivalent NLPIDs
0x0100 to 0x03FF Reserved for future use by the IETF
0x0400 to 0x04FF Allocated for use by the ATM Forum
0x0500 to 0x05FF Experimental/Local use
0x0600 to 0xFFFF Protocols defined by the equivalent Ethertypes
(based on the observations that valid Ethertypes are never
than 0x600, and NLPIDs never larger than 0xFF.)
The NLPID value of 0x80 is used to indicate a SNAP encoded
is being used to encode the protocol type. When mar$pro.type == 0x80
the SNAP extension is encoded in the mar$pro.snap field. This
termed the 'long form' protocol ID
If mar$pro.type != 0x80 then the mar$pro.snap field MUST be zero
transmit and ignored on receive. The mar$pro.type field
identifies the protocol being referred to. This is termed the '
form' protocol ID
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
In all cases, where a protocol has an assigned number in
mar$pro.type space (excluding 0x80) the short form MUST be used
transmitting MARS messages. Additionally, where a protocol has
short and long forms of identification, receivers MAY choose
recognise the long form
mar$pro.type values other than 0x80 MAY have 'long forms' defined
future documents
For the remainder of this document references to mar$pro SHALL
interpreted to mean mar$pro.type, or mar$pro.type in combination
mar$pro.snap as appropriate
The use of different protocol types is described further in
9.
4.3.3 Checksum
The mar$chksum field carries a standard IP checksum calculated
the entire MARS control message (excluding the LLC/SNAP header).
field is set to zero before performing the checksum calculation
As the entire LLC/SNAP encapsulated MARS message is protected by
32 bit CRC of the AAL5 transport, implementors MAY choose to
the checksum facility. If no checksum is calculated these bits
be reset before transmission. If no checksum is performed
reception, this field MUST be ignored. If a receiver is capable
validating a checksum it MUST only perform the validation when
received mar$chksum field is non-zero. Messages arriving
mar$chksum of 0 are always considered valid
4.3.4 Extensions Offset
The mar$extoff field identifies the existence and location of
optional supplementary parameters list. Its use is described
section 10.
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4.3.5 Operation code
The mar$op field is further subdivided into two 8 bit fields -
mar$op.version (leading octet) and mar$op.type (trailing octet).
Together they indicate the nature of the control message, and
context within which its [Mandatory fields], [Addresses],
[Supplementary TLVs] should be interpreted
mar$op.
0 MARS protocol defined in this document
0x01 - 0xEF Reserved for future use by the IETF
0xF0 - 0xFE Allocated for use by the ATM Forum
0xFF Experimental/Local use
mar$op.
Value indicates operation being performed, within context
the control protocol version indicated by mar$op.version
For the rest of this document references to the mar$op value SHALL
taken to mean mar$op.type, with mar$op.version = 0x00. The
used in this document are summarised in section 11.
(Note this number space is independent of the ATMARP operation
number space.)
4.3.6 Reserved
mar$hdrrsv may be subdivided and assigned specific meanings for
control protocols indicated by mar$op.version != 0.
5. Endpoint (MARS client) interface behaviour
An endpoint is best thought of as a 'shim' or 'convergence' layer
sitting between a layer 3 protocol's link layer interface and
underlying UNI 3.0/3.1 service. An endpoint in this context can
in a host or a router - any entity that requires a generic 'layer 3
over ATM' interface to support layer 3 multicast. It is broken
two key subsections - one for the transmit side, and one for
receive side
Multiple logical ATM interfaces may be supported by a single
ATM interface (for example, using different SEL values in the
formatted address assigned to the physical ATM interface).
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
or different clusters
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The initial signalling path between a MARS client (managing
endpoint) and its associated MARS is a transient point to point
bidirectional VC. This VC is established by the MARS client, and
used to send queries to, and receive replies from, the MARS. It
an associated idle timer, and is dismantled if not used for
configurable period of time. The minimum suggested value for
time is 1 minute, and the RECOMMENDED default is 20 minutes. (
the MARS and ARP Server are co-resident, this VC may be used for
ATM ARP traffic and MARS control traffic.)
The remaining signalling path is ClusterControlVC, to which the
client is added as a leaf node when it registers (described
section 5.2.3).
The majority of this document covers the distribution of
allowing endpoints to establish and manage outgoing point
multipoint VCs - the forwarding paths for multicast traffic
particular multicast groups. The actual format of the AAL_SDUs
on these VCs is almost completely outside the scope of
specification. However, endpoints are not expected to know
their forwarding path leads directly to a multicast group's
or to an MCS (described in section 3). This requires additional per
packet encapsulation (described in section 5.5) to aid in the
detection of reflected AAL_SDUs
5.1 Transmit side behaviour
The following description will often be in terms of an IPv4/
interface that is capable of transmitting packets to a Class
address at any time, without prior warning. It should be trivial
an implementor to generalise this behaviour to the requirements
another layer 3 data protocol
When a local Layer 3 entity passes down a packet for transmission
the endpoint first ascertains whether an outbound path to
destination multicast group already exists. If it does not, the
is queried for a set of ATM endpoints that represent an
forwarding path. (The ATM endpoints may represent the actual
members within the cluster, or a set of one or more MCSs.
endpoint does not distinguish between either case. Section 6.2
describes the MARS behaviour that leads to MCSs being supplied as
forwarding path for a multicast group.)
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The query is executed by issuing a MARS_REQUEST. The reply from
MARS may take one of two forms
MARS_MULTI - Sequence of MARS_MULTI messages returning the set
ATM endpoints that are to be leaf nodes of
outgoing point to multipoint VC (the
path).
MARS_NAK - No mapping found, group is empty
The formats of these messages are described in section 5.1.2.
Outgoing VCs are established with a request for Unspecified Bit
(UBR) service, as typified by the IETF's use of VCs for unicast IP
described in RFC 1755 [6]. Future documents may vary this
and allow the specification of different ATM traffic parameters
locally configured information or parameters obtained through
external means
5.1.1 Retrieving Group Membership from the MARS
If the MARS had no mapping for the desired Class D address a MARS_
will be returned. In this case the IP packet MUST be
silently. If a match is found in the MARS's tables it proceeds
return addresses ATM.1 through ATM.n in a sequence of one or
MARS_MULTIs. A simple mechanism is used to detect and recover
loss of MARS_MULTI messages
(If the client learns that there is no other group member in
cluster - the MARS returns a MARS_NAK or returns a MARS_MULTI
the client as the only member - it MUST delay sending out a
MARS_REQUEST for that group for a period no less than 5 seconds
no more than 10 seconds.)
Each MARS_MULTI carries a boolean field x, and a 15 bit integer
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
acts as an 'end of reply' marker. When x == 1 the MARS response
considered complete
In addition, each MARS_MULTI may carry multiple ATM addresses
the set {ATM.1, ATM.2, .... ATM.n}. A MARS MUST minimise the
of MARS_MULTIs transmitted by placing as many group members
addresses in a single MARS_MULTI as possible. The limit on the
of an individual MARS_MULTI message MUST be the MTU of the
VC
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For example, assume n ATM addresses must be returned, each MARS_
is limited to only p ATM addresses, and p << n. This would require
sequence of k MARS_MULTI messages (where k = (n/p)+1, using
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
one between consecutive MARS_MULTI's. An alternative failure mode
losing MARS_MULTI(1,k). A timer MUST be implemented to flag
failure of the last MARS_MULTI to arrive. A default value of 10
seconds is RECOMMENDED
If a 'sequence jump' is detected, the host MUST wait for
MARS_MULTI(1,k), discard all results, and repeat the MARS_REQUEST
If a timeout occurs, the host MUST discard all results, and
the MARS_REQUEST
A final failure mode involves the MARS Sequence Number (described
section 5.1.4.2 and carried in each part of a multi-part MARS_MULTI).
If its value changes during the reception of a multi-part MARS_
the host MUST wait for the MARS_MULTI(1,k), discard all results,
repeat the MARS_REQUEST
(Corruption of cell contents will lead to loss of a MARS_
through AAL5 CPCS_PDU reassembly failure, which will be
through the mechanisms described above.)
If the MARS is managing a cluster of endpoints spread
different but directly accessible ATM networks it will not be able
return all the group members in a single MARS_MULTI. The MARS_
message format allows for either E.164, ISO NSAP, or (E.164 + NSAP
to be returned as ATM addresses. However, each MARS_MULTI message
only return ATM addresses of the same type and length. The
addresses MUST be grouped according to type (E.164, ISO NSAP,
both) and returned in a sequence of separate MARS_MULTI parts
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5.1.2 MARS_REQUEST, MARS_MULTI, and MARS_NAK messages
MARS_REQUEST is shown below. It is indicated by an 'operation
value' (mar$op) of 1.
The multicast address being resolved is placed into the the
protocol address field (mar$tpa), and the target hardware address
set to null (mar$thtl and mar$tstl both zero).
In IPv4 environments the protocol type (mar$pro) is 0x800 and
target protocol address length (mar$tpln) MUST be set to 4.
source fields MUST contain the ATM number and subaddress of
client issuing the MARS_REQUEST (the subaddress MAY be null).
Data
mar$afn 16 bits Address Family (0x000F).
mar$pro 56 bits Protocol Identification
mar$hdrrsv 24 bits Reserved. Unused by MARS control protocol
mar$chksum 16 bits Checksum across entire MARS message
mar$extoff 16 bits Extensions Offset
mar$op 16 bits Operation code (MARS_REQUEST = 1)
mar$shtl 8 bits Type & length of source ATM number. (r
mar$sstl 8 bits Type & length of source ATM subaddress. (q
mar$spln 8 bits Length of source protocol address (s
mar$thtl 8 bits Type & length of target ATM number (x
mar$tstl 8 bits Type & length of target ATM subaddress (y
mar$tpln 8 bits Length of target group address (z
mar$pad 64 bits Padding (aligns mar$sha with MARS_MULTI).
mar$sha roctets source ATM
mar$ssa qoctets source ATM
mar$spa soctets source protocol
mar$tpa zoctets target multicast group
mar$tha xoctets target ATM
mar$tsa yoctets target ATM
Following the RFC1577 approach, the mar$shtl, mar$sstl, mar$thtl
mar$tstl fields are coded as follows
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|0|x| length |
+-+-+-+-+-+-+-+-+
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The most significant bit is reserved and MUST be set to zero.
second most significant bit (x) is a flag indicating whether the
address being referred to is in
- ATM Forum NSAPA format (x = 0).
- Native E.164 format (x = 1).
The bottom 6 bits is an unsigned integer value indicating the
of the associated ATM address in octets. If this value is zero
flag x is ignored
The mar$spln and mar$tpln fields are unsigned 8 bit integers,
the length in octets of the source and target protocol address
respectively
MARS packets use true variable length fields. A null (non-existant
address MUST be coded as zero length, and no space allocated for
in the message body
MARS_NAK is the MARS_REQUEST returned with operation type value of 6.
All other fields are left unchanged from the MARS_REQUEST (e.g.
not transpose the source and target information. In all cases
clients use the source address fields to identify their own
coming back).
The MARS_MULTI message is identified by an mar$op value of 2.
message format is
Data
mar$afn 16 bits Address Family (0x000F).
mar$pro 56 bits Protocol Identification
mar$hdrrsv 24 bits Reserved. Unused by MARS control protocol
mar$chksum 16 bits Checksum across entire MARS message
mar$extoff 16 bits Extensions Offset
mar$op 16 bits Operation code (MARS_MULTI = 2).
mar$shtl 8 bits Type & length of source ATM number. (r
mar$sstl 8 bits Type & length of source ATM subaddress. (q
mar$spln 8 bits Length of source protocol address (s
mar$thtl 8 bits Type & length of target ATM number (x
mar$tstl 8 bits Type & length of target ATM subaddress (y
mar$tpln 8 bits Length of target group address (z
mar$tnum 16 bits Number of target ATM addresses returned (N
mar$seqxy 16 bits Boolean flag x and sequence number y
mar$msn 32 bits MARS Sequence Number
mar$sha roctets source ATM
mar$ssa qoctets source ATM
mar$spa soctets source protocol
mar$tpa zoctets target multicast group
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mar$tha.1 xoctets target ATM number 1
mar$tsa.1 yoctets target ATM subaddress 1
mar$tha.2 xoctets target ATM number 2
mar$tsa.2 yoctets target ATM subaddress 2
[.......]
mar$tha.N xoctets target ATM number
mar$tsa.N yoctets target ATM subaddress
The source protocol and ATM address fields are copied directly
the MARS_REQUEST that this MARS_MULTI is in response to (not the
itself).
mar$seqxy is coded with flag x in the leading bit, and
number y coded as an unsigned integer in the remaining 15 bits
| 1st octet | 2nd octet |
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x| y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
mar$tnum is an unsigned integer indicating how many pairs
{mar$tha,mar$tsa} (i.e. how many group member's ATM addresses)
present in the message. mar$msn is an unsigned 32 bit number
in by the MARS before transmitting each MARS_MULTI. Its use
described further in section 5.1.4.
As an example, assume we have a multicast cluster using 4
protocol addresses, 20 byte ATM numbers, and 0 byte ATM subaddresses
For n group members in a single MARS_MULTI we require a (60 + 20n
byte message. If we assume the default MTU of 9180 bytes, we
return a maximum of 456 group member's addresses in a
MARS_MULTI
5.1.3 Establishing the outgoing multipoint VC
Following the completion of the MARS_MULTI reply the endpoint
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,
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
the newly created VC just as it would be for a unicast VC
After transmitting the packet, the local interface holds the VC
and marks it as the active path out of the host for any subsequent
packets being sent to that Class D address
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When establishing a new multicast VC it is possible that one or
L_MULTI_RQ or L_MULTI_ADD may fail. The UNI 3.0/3.1 failure
must be returned in the ERR_L_RQFAILED signal from the
signalling entity to the AAL User. If the failure cause is not 49
(Quality of Service unavailable), 51 (user cell rate not available -
UNI 3.0), 37 (user cell rate not available - UNI 3.1), or 41
(Temporary failure), the endpoint's ATM address is dropped from
set {ATM.1, ATM.2, ..., ATM.n} returned by the MARS. Otherwise,
L_MULTI_RQ or L_MULTI_ADD should be reissued after a random delay
5 to 10 seconds. If the request fails again, another request
be issued after twice the previous delay has elapsed. This
should be continued until the call succeeds or the multipoint VC
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
address in the set. This procedure is repeated until an L_MULTI_
succeeds, as no L_MULTI_ADDs may be issued until an initial
VC is established
Each ATM address for which an L_MULTI_RQ failed with cause 49, 51,
37, or 41 MUST be tagged rather than deleted. An L_MULTI_ADD
issued for these tagged addresses using the random delay
outlined above
The VC MAY be considered 'up' before failed L_MULTI_ADDs have
successfully re-issued. An endpoint MAY implement a
mechanism that allows data to start flowing out the new VC even
failed L_MULTI_ADDs are being re-tried. (The alternative of
for each leaf node to accept the connection could lead to
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
Class D address is no longer considered to have an active path out
the local host. The timer SHOULD be no less than 1 minute, and
default of 20 minutes is RECOMMENDED. Choice of specific
periods is beyond the scope of this document
VC consumption may also be reduced by endpoints noting when a
group's set of {ATM.1, ....ATM.n} matches that of a pre-existing
out to another group. With careful local management, and assuming
QoS of the existing VC is sufficient for both groups, a new pt to
VC may not be necessary. Under certain circumstances endpoints
decide that it is sufficient to re-use an existing VC whose set
leaf nodes is a superset of the new group's membership (in which
some endpoints will receive multicast traffic for a layer 3
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
they haven't joined, and must filter them above the ATM interface).
Algorithms for performing this type of optimization are not
here, and are not required for conformance with this document
5.1.4 Tracking subsequent group updates
Once a new VC has been established, the transmit side of the
member's interface needs to monitor subsequent group changes -
or dropping leaf nodes as appropriate. This is achieved by
for MARS_JOIN and MARS_LEAVE messages from the MARS itself.
messages are described in detail in section 5.2 - at this point it
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
across ClusterControlVC. MARS_JOIN or MARS_LEAVE messages that
confirm information already held by the cluster member are used
track the Cluster Sequence Number, but are otherwise ignored
5.1.4.1 Updating the active VCs
If a MARS_JOIN is seen that refers to (or encompasses) a group
which the transmit side already has a VC open, the new member's
address is extracted and an L_MULTI_ADD issued locally. This
that endpoints already sending to a given group will immediately
the new member to their list of recipients
If a MARS_LEAVE is seen that refers to (or encompasses) a group
which the transmit side already has a VC open, the old member's
address is extracted and an L_MULTI_DROP issued locally. This
that endpoints already sending to a given group will immediately
the old member from their list of recipients. When the last leaf of
VC is dropped, the VC is closed completely and the affected group
longer has a path out of the local endpoint (the next outbound
to that group's address will trigger the creation of a new VC,
described in sections 5.1.1 to 5.1.3).
The transmit side of the interface MUST NOT shut down an active VC
a group for which the receive side has just executed
LeaveLocalGroup. (This behaviour is consistent with the model
hosts transmitting to groups regardless of their own
status.)
If a MARS_JOIN or MARS_LEAVE arrives with mar$pnum == 0 it carries
pairs, and is only used for tracking the CSN
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5.1.4.2 Tracking the Cluster Sequence Number
It is important that endpoints do not miss group membership
issued by the MARS over ClusterControlVC. However, this will
from time to time. The Cluster Sequence Number is carried as
unsigned 32 bit value in the mar$msn field of many MARS
(except for MARS_REQUEST and MARS_NAK). It increments once for
transmission the MARS makes on ClusterControlVC, regardless
whether the transmission represents a change in the MARS database
not. By tracking this counter, cluster members can determine
they have missed a previous message on ClusterControlVC, and
a membership change. This is then used to trigger
(described in section 5.1.5).
The current CSN is copied into the mar$msn field of MARS
being sent to cluster members, whether out ClusterControlVC or on
point to point VC
Calculations on the sequence numbers MUST be performed as unsigned 32
bit arithmetic
Every cluster member keeps its own 32 bit Host Sequence Number (HSN
to track the MARS's sequence number. Whenever a message is
that carries an mar$msn field the following processing is performed
Seq.diff = mar$msn -
mar$msn ->
{...process MARS message as appropriate...}
if ((Seq.diff != 1) && (Seq.diff != 0))
then {...revalidate group membership information...}
The basic result is that the cluster member attempts to keep
in step with membership changes noted by the MARS. If it ever
that a membership change occurred (in any group) without it noticing
it re-validates the membership of all groups it currently
multicast VCs open to
The mar$msn value in an individual MARS_MULTI is not used to
the HSN until all parts of the MARS_MULTI (if more than 1)
arrived. (If the mar$msn changes the MARS_MULTI is discarded,
described in section 5.1.1.)
The MARS is free to choose an initial value of CSN. When a
cluster member starts up it should initialise HSN to zero. When
cluster member sends the MARS_JOIN to register (described later),
HSN will be correctly updated to the current CSN value when
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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
information about the sets of leaf nodes it should be sending to.
an error occurs on a VC associated with a particular group,
cluster member initiates revalidation procedures for that
group. If a jump is detected in the Cluster Sequence Number,
initiates revalidation of all groups to which the cluster
currently has open point to multipoint VCs
Each open and active multipoint VC has a flag associated with
called 'VC_revalidate'. This flag is checked everytime a packet
queued for transmission on that VC. If the flag is false, the
is transmitted and no further action is required
However, if the VC_revalidate flag is true then the packet
transmitted and a new sequence of events is started locally
Revalidation begins with re-issuing a MARS_REQUEST for the
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
in the original set of members but not in the revalidated set
members. L_MULTI_ADDs are issued on the group's VC for each node
appears in the revalidated set of members but not in the original
of members. The VC_revalidate flag is reset when
concludes for the given group. Implementation specific
will be needed to flag the 'revalidation in progress' state
The key difference between constructing a VC (section 5.1.3)
revalidating a VC is that packet transmission continues on the
VC while it is being revalidated. This minimises the disruption
existing traffic
The 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,
group has its VC_revalidate flag set TRUE at a random
between 1 and 10 seconds
Benefit: Revalidation of active groups occurs quickly,
essentially idle groups are revalidated as needed.
distributed setting of VC_revalidate flag improves chances
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
staggered revalidation requests from senders when a sequence
jump is detected
5.1.5.1 When leaf node drops itself
During the life of a multipoint VC an ERR_L_DROP may be
indicating that a leaf node has terminated its participation at
ATM level. The ATM endpoint associated with the ERR_L_DROP MUST
removed from the locally held set {ATM.1, ATM.2, .... ATM.n
associated with the VC
After a random period of time between 1 and 10 seconds
VC_revalidate flag associated with that VC MUST be set true
If an ERR_L_RELEASE is received then the entire set {ATM.1, ATM.2,
.... ATM.n} is cleared and the VC is considered to be completely
down. Further packet transmission to the group served by this VC
result in a new VC being established as described in section 5.1.3.
5.1.5.2 When a jump is detected in the CSN
Section 5.1.4.2 describes how a CSN jump is detected. If a CSN
is detected upon receipt of a MARS_JOIN or a MARS_LEAVE then
outgoing multicast VC MUST have its VC_revalidate flag set true
some random interval between 1 and 10 seconds from when the CSN
was detected
The only exception to this rule is if a sequence number jump
detected during the establishment of a new group's VC (i.e.
MARS_MULTI reply was correctly received, but its mar$msn
that some previous MARS traffic had been missed on ClusterControlVC).
In this case every open VC, EXCEPT the one just establis