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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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
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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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
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 established,
have its VC_revalidate flag set true at some random interval
1 and 10 seconds from when the CSN jump was detected. (The VC
established at the time is considered already validated.)
5.1.6 'Migrating' the outgoing multipoint
In addition to the group tracking described in section 5.1.4,
transmit side of a cluster member must respond to 'migration
requests by the MARS. This is triggered by the reception of
MARS_MIGRATE message from ClusterControlVC. The MARS_MIGRATE
is shown below, with an mar$op code of 13.
Data
mar$afn 16 bits Address Family (0x000F).
mar$pro 56 bits Protocol Identification
mar$hdrrsv 24 bits Reserved. Unused by MARS control protocol
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
mar$chksum 16 bits Checksum across entire MARS message
mar$extoff 16 bits Extensions Offset
mar$op 16 bits Operation code (MARS_MIGRATE = 13).
mar$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$resv 16 bits Reserved
mar$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
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
A migration is requested when the MARS determines that it no
wants cluster members forwarding their packets directly to the
addresses it had previously specified (through MARS_REQUESTs
MARS_JOINs). When a MARS_MIGRATE is received each cluster member
perform the following steps
Close down any existing outgoing VC associated with the
carried in the mar$tpa field (L_RELEASE), or dissociate the
from any outgoing VC it may have been sharing (as described
section 5.1.3).
Establish a new outgoing VC for the specified group, using
algorithm described in section 5.1.3 and taking the set of
addresses supplied in the MARS_MIGRATE as the group's new set
members {ATM.1, .... ATM.n}.
The MARS_MIGRATE carries the new set of members {ATM.1, .... ATM.n
in a single message, in similar manner to a single part MARS_MULTI
As with other messages from the MARS, the Cluster Sequence
carried in mar$msn is checked as described in section 5.1.4.2.
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
5.2. Receive side behaviour
A cluster member is a 'group member' (in the sense that it
packets directed at a given multicast group) when its ATM
appears in the MARS's table entry for the group's multicast address
A key function within each cluster is the distribution of
membership information from the MARS to cluster members
An endpoint may wish to 'join a group' in response to a local,
level request for membership of a group, or because the
supports a layer 3 multicast forwarding engine that requires
ability to 'see' intra-cluster traffic in order to forward it
Two messages support these requirements - MARS_JOIN and MARS_LEAVE
These are sent to the MARS by endpoints when the local layer 3/
interface is requested to join or leave a multicast group. The
propagates these messages back out over ClusterControlVC, to
the knowledge of the group's membership change is distributed in
timely fashion to other cluster members
Certain models of layer 3 endpoints (e.g. IP multicast routers
expect to be able to receive packet traffic 'promiscuously'
all groups. This functionality may be emulated by allowing
to request that the MARS returns them as 'wild card' members of
Class D addresses. However, a problem inherent in the current
model is that a completely promiscuous router may exhaust the
reassembly resources in its ATM interface. MARS_JOIN supports
generalisation to the notion of 'wild card' entries, enabling
to limit themselves to 'blocks' of the Class D address space. Use
this facility is described in greater detail in Section 8.
A block can be as small as 1 (a single group) or as large as
entire multicast address space (e.g. default IPv4 'promiscuous
behaviour). A block is defined as all addresses between,
inclusive of, a address pair. A MARS_JOIN or MARS_LEAVE
carry multiple pairs
Cluster members MUST provide ONLY a single pair in
JOIN/LEAVE message they issue. However, they MUST be able to
multiple pairs in JOIN/LEAVE messages when performing
management as described in section 5.1.4 (the interpretation
that the join/leave operation applies to all addresses in the
from to inclusive, for every pair).
In RFC1112 environments a MARS_JOIN for a single group is
by a JoinLocalGroup signal from the IP layer. A MARS_LEAVE for
single group is triggered by a LeaveLocalGroup signal from the
layer
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Cluster members with special requirements (e.g. multicast routers
may issue MARS_JOINs and MARS_LEAVEs specifying a single block of 2
or more multicast group addresses. However, a cluster member
NOT issue such a multi-group block join for an address range fully
partially overlapped by multi-group block join(s) that the
member has previously issued and not yet retracted. A cluster
MAY issue combinations of single group MARS_JOINs that overlap with
multi-group block MARS_JOIN
An endpoint MUST register with a MARS in order to become a member
a cluster and be added as a leaf to ClusterControlVC.
is covered in section 5.2.3.
Finally, the endpoint MUST be capable of terminating
VCs (i.e. act as a leaf node of a UNI 3.0/3.1 point to multipoint VC
with zero bandwidth assigned on the return path). RFC 1755
the signalling information required to terminate VCs
LLC/SNAP encapsulated traffic (discussed further in section 5.5).
5.2.1 Format of the MARS_JOIN and MARS_LEAVE Messages
The MARS_JOIN message is indicated by an operation type value of 4.
MARS_LEAVE has the same format and operation type value of 5.
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_JOIN or MARS_LEAVE).
mar$shtl 8 bits Type & length of source ATM number. (r
mar$sstl 8 bits Type & length of source ATM subaddress. (q
mar$spln 8 bits Length of source protocol address (s
mar$tpln 8 bits Length of group address (z
mar$pnum 16 bits Number of group address pairs (N
mar$flags 16 bits layer3grp, copy, and register flags
mar$cmi 16 bits Cluster Member
mar$msn 32 bits MARS Sequence Number
mar$sha roctets source ATM number
mar$ssa qoctets source ATM subaddress
mar$spa soctets source protocol
mar$min.1 zoctets Minimum multicast group address - pair.1
mar$max.1 zoctets Maximum multicast group address - pair.1
[.......]
mar$min.N zoctets Minimum multicast group address - pair.
mar$max.N zoctets Maximum multicast group address - pair.
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mar$spln indicates the number of bytes in the source endpoint'
protocol address, and is interpreted in the context of the
indicated by the mar$pro field. (e.g. in IPv4 environments mar$
will be 0x800, mar$spln is 4, and mar$tpln is 4.)
The mar$flags field contains three flags
Bit 15 - mar$flags.layer3grp
Bit 14 - mar$flags.copy
Bit 13 - mar$flags.register
Bit 12 - mar$flags.punched
Bit 0-7 - mar$flags.sequence
Bits 8 to 11 are reserved and MUST be zero
mar$flags.sequence is set by cluster members, and MUST always
passed on unmodified by the MARS when retransmitting MARS_JOIN
MARS_LEAVE messages. It is source specific, and MUST be ignored
other cluster members. Its use is described in section 5.2.2.
mar$flags.punched MUST be zero when the MARS_JOIN or MARS_LEAVE
transmitted to the MARS. Its use is described in section 5.2.2
section 6.2.4.
mar$flags.copy MUST be set to 0 when the message is being sent from
MARS client, and MUST be set to 1 when the message is being sent
a MARS. (This flag is intended to support integrating the
function with one of the MARS clients in your cluster.
destination of an incoming MARS_JOIN can be determined from
value.)
mar$flags.layer3grp allows the MARS to provide the group
information described further in section 5.3. The rules for its
are
mar$flags.layer3grp MUST be set when the cluster member is
the MARS_JOIN as the result of a layer 3 multicast group
explicitly joined. (e.g. as a result of a JoinHostGroup
in an RFC1112 compliant host).
mar$flags.layer3grp MUST be reset in each MARS_JOIN if
MARS_JOIN is simply the local ip/atm interface registering
receive traffic on that group for its own reasons
mar$flags.layer3grp is ignored and MUST be treated as reset by
MARS for any MARS_JOIN that specifies a block covering more than
single group (e.g. a block join from a router ensuring
forwarding engines 'see' all traffic).
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mar$flags.register indicates whether the MARS_JOIN or MARS_LEAVE
being used to register or deregister a cluster member (described
section 5.2.3). When used to join or leave specific groups
mar$register flag MUST be zero
mar$pnum indicates how many pairs are included in
message. This field MUST be 1 when the message is sent from a
member. A MARS MAY return a MARS_JOIN or MARS_LEAVE with any mar$
value, including zero. This will be explained futher in
6.2.4.
The mar$cmi field MUST be zeroed by cluster members, and is used
the MARS during cluster member registration, described in
5.2.3.
mar$msn MUST be zero when transmitted by an endpoint. It is set
the current value of the Cluster Sequence Number by the MARS when
MARS_JOIN or MARS_LEAVE is retransmitted. Its use has been
in section 5.1.4.
To simplify construction and parsing of MARS_JOIN and MARS_
messages, the following restrictions are imposed on the
pairs
Assume max(N) is the field from the Nth pair
Assume min(N) is the field from the Nth pair
Assume a join/leave message arrives with K pairs
The following must hold
max(N) < min(N+1) for 1 <= N <
max(N) >= min(N) for 1 <= N <=
In plain language, the set must specify an ascending sequence
address blocks. The definition of "greater" or "less than" may
protocol specific. In IPv4 environments the addresses are treated
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
a single group. For any arbitrary group address X the
MARS_JOIN or MARS_LEAVE MUST specify a single pair .
mar$flags.layer3grp MUST be set under these circumstances
A router choosing to behave strictly in accordance with RFC1112
specify the entire Class D space. The associated MARS_JOIN
MARS_LEAVE MUST specify a single pair <224.0.0.0, 239.255.255.255>.
Whenever a router issues a MARS_JOIN only in order to forward
traffic it MUST reset mar$flags.layer3grp
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
The use of alternative values by multicast routers
discussed in Section 8.
5.2.2 Retransmission of MARS_JOIN and MARS_LEAVE messages
Transient problems may result in the loss of messages between
MARS and cluster
A simple algorithm is used to solve this problem. Cluster
retransmit each MARS_JOIN and MARS_LEAVE message at regular
until they receive a copy back again, either on ClusterControlVC
the VC on which they are sending the message. At this point
local endpoint can be certain that the MARS received and
it
The interval should be no shorter than 5 seconds, and a default
of 10 seconds is recommended. After 5 retransmissions the
should be flagged locally as a failure. This MUST be considered as
MARS failure, and triggers the MARS reconnection described in
5.4.
A 'copy' is defined as a received message with the following
matching a previously transmitted MARS_JOIN/LEAVE
- mar$
- mar$flags.
- mar$flags.
- mar$
- Source ATM
- First
In addition, a valid copy MUST have the following field values
- mar$flags.punched = 0
- mar$flags.copy = 1
The mar$flags.sequence field is never modified or checked by a MARS
Implementors MAY choose to utilize locally significant
number schemes, which MAY differ from one cluster member to the next
In the absence of such schemes the default value
mar$flags.sequence MUST be zero
Careful implementations MAY have more than one
MARS_JOIN/LEAVE outstanding at a time
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
5.2.3 Cluster member registration and deregistration
To become a cluster member an endpoint must register with the MARS
This achieves two things - the endpoint is added as a leaf node
ClusterControlVC, and the endpoint is assigned a 16 bit
Member Identifier (CMI). The CMI uniquely identifies each
that is attached to the cluster
Registration with the MARS occurs when an endpoint issues a MARS_
with the mar$flags.register flag set to one (bit 13 of the mar$
field).
The cluster member MUST include its source ATM address, and
choose to specify a null source protocol address when registering
No protocol specific group addresses are included in a
MARS_JOIN
The cluster member retransmits this MARS_JOIN in accordance
section 5.2.2 until it confirms that the MARS has received it
When the registration MARS_JOIN is returned it contains a non-
value in mar$cmi. This value MUST be noted by the cluster member,
used whenever circumstances require the cluster member's CMI
An endpoint may also choose to de-register, using a MARS_LEAVE
mar$flags.register set. This would result in the MARS dropping
endpoint from ClusterControlVC, removing all references to the
in the mapping database, and freeing up its CMI
As for registration, a deregistration request MUST include
correct source ATM address for the cluster member, but MAY choose
specify a null source protocol address
The cluster member retransmits this MARS_LEAVE in accordance
section 5.2.2 until it confirms that the MARS has received it
5.3 Support for Layer 3 group management
Whilst the intention of this specification is to be independent
layer 3 issues, an attempt is being made to assist the operation
layer 3 multicast routing protocols that need to ascertain if
groups have members within a cluster
One example is IP, where IGMP is used (as described in section 2)
simply to determine whether any other cluster members are
to a group because they have higher layer applications that want
receive a group's traffic
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Routers may choose to query the MARS for this information,
than multicasting IGMP queries to 224.0.0.1 and incurring
associated cost of setting up a VC to all systems in the cluster
The query is issued by sending a MARS_GROUPLIST_REQUEST to the MARS
MARS_GROUPLIST_REQUEST is built from a MARS_JOIN, but it has
operation code of 10. The first pair will be used by
MARS to identify the range of groups in which the querying
member is interested. Any additional pairs will be ignored
A request with mar$pnum = 0 will be ignored
The response from the MARS is a MARS_GROUPLIST_REPLY, carrying a
of the multicast groups within the specified block
have Layer 3 members. A group is noted in this list if one or
of the MARS_JOINs that generated its mapping entry in the
contained a set mar$flags.layer3grp flag
MARS_GROUPLIST_REPLYs are transmitted back to the querying
member on the VC used to send the MARS_GROUPLIST_REQUEST
MARS_GROUPLIST_REPLY is derived from the MARS_MULTI but with mar$op =
11. It may have multiple parts if needed, and is received in
similar manner to a MARS_MULTI
Data
mar$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$op 16 bits Operation code (MARS_GROUPLIST_REPLY).
mar$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$thtl 8 bits Unused - set to zero
mar$tstl 8 bits Unused - set to zero
mar$tpln 8 bits Length of target group address (z
mar$tnum 16 bits Number of group addresses returned (N).
mar$seqxy 16 bits Boolean flag x and sequence number y
mar$msn 32 bits MARS Sequence Number
mar$sha roctets source ATM number
mar$ssa qoctets source ATM subaddress
mar$spa soctets source protocol
mar$mgrp.1 zoctets Group address 1
[.......]
mar$mgrp.N zoctets Group address
mar$seqxy is coded as for the MARS_MULTI -
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MARS_GROUPLIST_REPLY components are transmitted and received
the same algorithm as described in section 5.1.1 for MARS_MULTI.
only difference is that protocol addresses are being returned
than ATM addresses
As for MARS_MULTIs, if an error occurs in the reception of a
part MARS_GROUPLIST_REPLY the whole thing MUST be discarded and
MARS_GROUPLIST_REQUEST re-issued. (This includes the mar$msn
being constant.)
Note that the ability to generate MARS_GROUPLIST_REQUEST messages
and receive MARS_GROUPLIST_REPLY messages, is not required
general host interface implementations. It is optional for
being implemented to support layer 3 multicast forwarding engines
However, this functionality MUST be supported by the MARS
5.4 Support for redundant/backup MARS entities
Endpoints are assumed to have been configured with the ATM address
at least one MARS. Endpoints MAY choose to maintain a table of
addresses, representing alternative MARSs that will be contacted
the event that normal operation with the original MARS is deemed
have failed. It is assumed that this table orders the ATM
in descending order of preference
An endpoint will typically decide there are problems with the
when
- It fails to establish a point to point VC to the MARS
- MARS_REQUESTs fail (section 5.1.1).
- MARS_JOIN/MARS_LEAVEs fail (section 5.2.2).
- It has not received a MARS_REDIRECT_MAP in the last 4
(section 5.4.3).
(If it is able to discern which connection
ClusterControlVC, it may also use connection failures on this VC
indicate problems with the MARS).
5.4.1 First response to MARS problems
The first response is to assume a transient problem with the
being used at the time. The cluster member should wait a
period of time between 1 and 10 seconds before attempting to re
connect and re-register with the MARS. If the registration MARS_
is successful then
The cluster member MUST then proceed to rejoin every group
its local higher layer protocol(s) have joined. It
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
recommended that a random delay between 1 and 10 seconds
inserted before attempting each MARS_JOIN
The cluster member MUST initiate the revalidation of
multicast group it was sending to (as though a sequence
jump had been detected, section 5.1.5).
The rejoin and revalidation procedure must not disrupt
cluster member's use of multipoint VCs that were already open
the time of the MARS failure
If re-registration with the current MARS fails, and there are
backup MARS addresses configured, the cluster member MUST wait for
least 1 minute before repeating the re-registration procedure. It
RECOMMENDED that the cluster member signals an error condition
some locally significant fashion
This procedure may repeat until network administrators
intervene or the current MARS returns to normal operation
5.4.2 Connecting to a backup MARS
If the re-registration with the current MARS fails, and other
addresses have been configured, the next MARS address on the list
chosen to be the current MARS, and the cluster member
restarts the re-registration procedure described in section 5.4.1.
this is succesful the cluster member will resume normal
using the new MARS. It is RECOMMENDED that the cluster member
a warning of this condition in some locally significant fashion
If the attempt at re-registration with the new MARS fails,
cluster member MUST wait for at least 1 minute before choosing
next MARS address in the table and repeating the procedure. If
end of the table has been reached, the cluster member starts again
the top of the table (which should be the original MARS that
cluster member started with).
In the worst case scenario this will result in cluster
looping through their table of possible MARS addresses until
administrators manually intervene
5.4.3 Dynamic backup lists, and soft redirects
To support some level of autoconfiguration, a MARS message is
that allows the current MARS to broadcast on ClusterControlVC a
of backup MARS addresses. When this message is received,
members that maintain a list of backup MARS addresses MUST
this information at the top of their locally held list (i.e.
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
information provided by the MARS has a higher preference
addresses that may have been manually configured into the
member).
The message is MARS_REDIRECT_MAP. It is based on the MARS_
message, with the following changes
- mar$tpln field replaced by mar$redirf
- mar$spln field reserved
- mar$tpa and mar$spa eliminated
MARS_REDIRECT_MAP has an operation type code of 12 decimal
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_REDIRECT_MAP).
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$tstl 8 bits Type & length of target ATM subaddress (y
mar$redirf 8 bits Flag controlling client redirect behaviour
mar$tnum 16 bits Number of MARS addresses returned (N).
mar$seqxy 16 bits Boolean flag x and sequence number y
mar$msn 32 bits MARS Sequence Number
mar$sha roctets source ATM
mar$ssa qoctets source ATM
mar$tha.1 xoctets ATM number for MARS 1
mar$tsa.1 yoctets ATM subaddress for MARS 1
mar$tha.2 xoctets ATM number for MARS 2
mar$tsa.2 yoctets ATM subaddress for MARS 2
[.......]
mar$tha.N xoctets ATM number for MARS
mar$tsa.N yoctets ATM subaddress for MARS
The source ATM address field(s) MUST identify the originating MARS
A multi-part MARS_REDIRECT_MAP may be transmitted and
using the mar$seqxy field in the same manner as a multi-
MARS_MULTI (section 5.1.1). If a failure occurs during the
of a multi-part MARS_REDIRECT_MAP (a part lost, reassembly timeout
or illegal MARS Sequence Number jump) the entire message MUST
discarded
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
This message is transmitted regularly by the MARS (it MUST
transmitted at least every 2 minutes, it is RECOMMENDED that it
transmitted every 1 minute).
The MARS_REDIRECT_MAP is also used to force cluster members to
from one MARS to another. If the ATM address of the first
contained in a MARS_REDIRECT_MAP table is not the address of
member's current MARS the client MUST 'redirect' to the new MARS.
mar$redirf field controls how the redirection occurs
mar$redirf has the following format
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|x| |
+-+-+-+-+-+-+-+-+
If Bit 7 (the most significant bit) of mar$redirf is 1 then
cluster member MUST perform a 'hard' redirect. Having installed
new table of MARS addresses carried by the MARS_REDIRECT_MAP,
cluster member re-registers with the MARS now at the top of the
using the mechanism described in sections 5.4.1 and 5.4.2.
If Bit 7 of mar$redirf is 0 then the cluster member MUST perform
'soft' redirect, beginning with the following actions
- open a point to point VC to the first ATM address
- attempt a registration (section 5.2.3).
If the registration succeeds, the cluster member shuts down its
to point VC to the current MARS (if it had one open), and
proceeds to use the newly opened point to point VC as its
to the 'current MARS'. The cluster member does NOT attempt to
the groups it is a member of, or revalidate groups it is
sending to
This is termed a 'soft redirect' because it avoids the
rejoining and revalidation processing that occurs when a MARS
is being recovered from. It assumes some external
mechanisms exist between the old and new MARS - mechanisms that
outside the scope of this specification
Some level of trust is required before initiating a soft redirect.
cluster member MUST check that the calling party at the other end
the VC on which the MARS_REDIRECT_MAP arrived (
ClusterControlVC) is in fact the node it trusts as the current MARS
Additional applications of this function are for further study
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
5.5 Data path LLC/SNAP encapsulations
An extended encapsulation scheme is required to support the
of possible reflected packets (section 3.3).
Two LLC/SNAP codepoints are allocated from the IANA OUI space.
support two different mechanisms for detecting reflected packets
They are called Type #1 and Type #2 multicast encapsulations
Type #1
[0xAA-AA-03][0x00-00-5E][0x00-01][Type #1 Extended Layer 3 packet
LLC OUI
Type #2
[0xAA-AA-03][0x00-00-5E][0x00-04][Type #2 Extended Layer 3 packet
LLC OUI
For conformance with this document MARS clients
MUST transmit data using Type #1 encapsulation
MUST be able to correctly receive traffic using Type #1 OR Type #2
encapsulation
MUST NOT transmit using Type #2 encapsulation
5.5.1 Type #1 encapsulation
The Type #1 Extended layer 3 packet carries within it a copy of
source's Cluster Member ID (CMI) and either the 'short form' or '
form' of the protocol type as appropriate (section 4.3).
When carrying packets belonging to protocols with valid short
representations the [Type #1 Extended Layer 3 packet] is encoded as
[pkt$cmi][pkt$pro][Original Layer 3 packet
2octet 2octet N
The first 2 octets (pkt$cmi) carry the CMI assigned when an
registers with the MARS (section 5.2.3). The second 2
(pkt$pro) indicate the protocol type of the packet carried in
remainder of the payload. This is copied from the mar$pro field
in the MARS control messages
When carrying packets belonging to protocols that only have a
form representation (pkt$pro = 0x80) the overhead SHALL be
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
extended to carry the 5 byte mar$pro.snap field (with padding for 32
bit alignment). The encoded form SHALL be
[pkt$cmi][0x00-80][mar$pro.snap][padding][Original Layer 3 packet
2octet 2octet 5 octets 3 octets N
The CMI is copied into the pkt$cmi field of every outgoing Type #1
packet. When an endpoint interface receives an AAL_SDU with
LLC/SNAP codepoint indicating Type #1 encapsulation it compares
CMI field with its own Cluster Member ID for the indicated protocol
The packet is discarded silently if they match. Otherwise the
is accepted for processing by the local protocol entity identified
the pkt$pro (and possibly SNAP) field(s).
Where a protocol has valid short and long forms of identification
receivers MAY choose to additionally recognise the long form
5.5.2 Type #2 encapsulation
Future developments may enable direct multicasting of AAL_SDUs
cluster boundaries. Expanding the set of possible sources in this
may cause the CMI to become an inadequate parameter with which
detect reflected packets. A larger source identification field
be required
The Type #2 Extended layer 3 packet carries within it an 8
source ID field and either the 'short form' or 'long form' of
protocol type as appropriate (section 4.3). The form and content
the source ID field is currently unspecified, and is not relevant
any MARS client built in conformance with this document.
Type #2 encapsulated packets MUST always be accepted and passed up
the higher layer indicated by the protocol identifier
When carrying packets belonging to protocols with valid short
representations the [Type #2 Extended Layer 3 packet] is encoded as
[8 octet sourceID][mar$pro.type][Null pad][Original Layer 3
packet
2octets 2
When carrying packets belonging to protocols that only have a
form representation (pkt$pro = 0x80) the overhead SHALL be
extended to carry the 5 byte mar$pro.snap field (with padding for 32
bit alignment). The encoded form SHALL be
[8 octet sourceID][mar$pro.type][mar$pro.snap][Null pad][Layer 3
packet
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
2octets 5octets 1
(Note that in this case the padding after the SNAP field is 1
rather than the 3 octets used in Type #1.)
Where a protocol has valid short and long forms of identification
receivers MAY choose to additionally recognise the long form
(Future documents may specify the contents of the source ID field
This will only be relevant to implementations sending Type #2
encapsulated packets, as they are the only entities that need to
concerned about detecting reflected Type #2 packets.)
5.5.3 A Type #1 example
An IPv4 packet (fully identified by an Ethertype of 0x800,
requiring 'short form' protocol type encoding) would be
as
[0xAA-AA-03][0x00-00-5E][0x00-01][pkt$cmi][0x800][IPv4 packet
The different LLC/SNAP codepoints for unicast and multicast
transmission allows a single IPv4/ATM interface to support both
demuxing on the LLC/SNAP header
6. The MARS in greater detail
Section 5 implies a lot about the MARS's basic behaviour as
by cluster members. This section summarises the behaviour of the
for groups that are VC mesh based, and describes how a
behaviour changes when an MCS is registered to support a group
The MARS is intended to be a multiprotocol entity - all its
tables, CMIs, and control VCs MUST be managed within the context
the mar$pro field in incoming MARS messages. For example, a
supports completely separate ClusterControlVCs for each layer 3
protocol that it is registering members for. If a MARS
messages with a mar$pro that it does not support, the message
dropped
In general the MARS treats protocol addresses as arbitrary
strings. For example, the MARS will not apply IPv4 specific 'class
checks to addresses supplied under mar$pro = 0x800. It is
for the MARS to simply assume that endpoints know how to
the protocol addresses that they are establishing and
mappings for
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The MARS requires control messages to carry the originator's
in the source ATM address field(s). Messages that arrive with
empty ATM Number field are silently discarded prior to any
processing by the MARS. (Only the ATM Number field needs to
checked. An empty ATM Number field combined with a non-empty
Subaddress field does not represent a valid ATM address.)
(Some example pseudo-code for a MARS can be found in Appendix F.)
6.1 Basic interface to Cluster members
The following MARS messages are used or required by cluster members
1 MARS_
2 MARS_
4 MARS_
5 MARS_
6 MARS_
10 MARS_GROUPLIST_
11 MARS_GROUPLIST_
12 MARS_REDIRECT_
6.1.1 Response to MARS_REQUEST
Except as described in section 6.2, if a MARS_REQUEST arrives
source ATM address does not match that of any registered
member the message MUST be dropped and ignored
6.1.2 Response to MARS_JOIN and MARS_LEAVE
When a registration MARS_JOIN arrives (described in section 5.2.3)
the MARS performs the following actions
- Adds the node to ClusterControlVC
- Allocates a new Cluster Member ID (CMI).
- Inserts the new CMI into the mar$cmi field of the MARS_JOIN
- Retransmits the MARS_JOIN back privately
If the node is already a registered member of the cluster
with the specified protocol type then its existing CMI is
copied into the MARS_JOIN, and the MARS_JOIN retransmitted back
the node. A single node may register multiple times if it
multiple layer 3 protocols. The CMIs allocated by the MARS for
such registration may or may not be the same
The retransmitted registration MARS_JOIN must NOT be sent
ClusterControlVC. If a cluster member issues a
MARS_LEAVE it too is retransmitted privately
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Non-registration MARS_JOIN and MARS_LEAVE messages are ignored
they arrive from a node that is not registered as a cluster member
MARS_JOIN or MARS_LEAVE messages MUST arrive at the MARS
mar$flags.copy set to 0, otherwise the message is silently ignored
All outgoing MARS_JOIN or MARS_LEAVE messages SHALL
mar$flags.copy set to 1, and mar$msn set to the current
Sequence Number for ClusterControlVC (Section 5.1.4.2).
mar$flags.layer3grp (section 5.3) MUST be treated as reset
MARS_JOINs specifying a single pair covering more than
single group. If a MARS_JOIN/LEAVE is received that contains
than one pair, the MARS MUST silently drop the message
If one or more MCSs have registered with the MARS, message
continues as described in section 6.2.4.
The MARS database is updated to add the node to any
group(s) that it was not already considered a member of, and
processing continues as follows
If a single group was being joined or left
mar$flags.punched is set to 0.
If the joining (leaving) node was already (is still) considered
member of the specified group, the message is
privately back to the cluster member. Otherwise the message
retransmitted on ClusterControlVC
If a single block covering 2 or more groups was being joined or left
A copy of the original MARS_JOIN/LEAVE is made. This copy then
its block replaced with a 'hole punched' set of zero
more pairs. The 'hole punched' set of
covers the entire address range specified by the
pair, but excludes those addresses/groups which
joining (leaving) node is already (still) a member of due to
previous single group join
If no 'holes' were punched in the specified block, the
MARS_JOIN/LEAVE is retransmitted out on ClusterControlVC
Otherwise the following occurs
The original MARS_JOIN/LEAVE is transmitted back to the
cluster member unchanged, using the VC it arrived on.
mar$flags.punched field MUST be reset to 0 in this message
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
If the hole-punched set contains 1 or more pair,
copy of the original MARS_JOIN/LEAVE is transmitted
ClusterControlVC, carrying the new list.
mar$flags.punched field MUST be set to 1 in this message. (
mar$flags.punched field is set to ensure the hole-punched
is ignored by the message's source when trying to
received MARS_JOIN/LEAVE messages with ones previously
(section 5.2.2)).
If the MARS receives a deregistration MARS_LEAVE (described
section 5.2.3) that member's ATM address MUST be removed from
groups for which it may have joined, dropped from ClusterControlVC
and the CMI released
If the MARS receives an ERR_L_RELEASE on ClusterControlVC
that a cluster member has disconnected, that member's ATM
MUST be removed from all groups for which it may have joined, and
CMI released
6.1.3 Generating MARS_REDIRECT_MAP
A MARS_REDIRECT_MAP message (described in section 5.4.3) MUST
regularly transmitted on ClusterControlVC. It is RECOMMENDED
this occur every 1 minute, and it MUST occur at least every 2
minutes. If the MARS has no knowledge of other backup MARSs
the cluster, it MUST include its own address as the only entry in
MARS_REDIRECT_MAP message (in addition to filling in the
address fields).
The design and use of backup MARS entities is beyond the scope
this document, and will be covered in future work
6.1.4 Cluster Sequence Numbers
The Cluster Sequence Number (CSN) is described in section 5.1.4,
is carried in the mar$msn field of MARS messages being sent
cluster members (either out ClusterControlVC or on an individual VC).
The MARS increments the CSN after every transmission of a message
ClusterControlVC. The current CSN is copied into the mar$msn
of MARS messages being sent to cluster members, whether
ClusterControlVC or on a private VC
A MARS should be carefully designed to minimise the possibility
the CSN jumping unnecessarily. Under normal operation only
members affected by transient link problems will miss CSN updates
be forced to revalidate. If the MARS itself glitches, it will
innundated with requests for a period as every cluster
attempts to revalidate
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Calculations on the CSN MUST be performed as unsigned 32
arithmetic
One implication of this mechanism is that the MARS should
its processing of 'simultaneous' MARS_REQUEST, MARS_JOIN
MARS_LEAVE messages. Join and Leave operations should be
within the MARS along with MARS_REQUESTS, and not processed until
the reply packets of a preceeding MARS_REQUEST have been transmitted
The transmission of MARS_REDIRECT_MAP should also be
queued
(The regular transmission of MARS_REDIRECT_MAP serves a
purpose of allowing cluster members to track the CSN, even if
miss an earlier MARS_JOIN or MARS_LEAVE.)
6.2 MARS interface to Multicast Servers (MCS).
When the MARS returns the actual addresses of group members,
endpoint behaviour described in section 5 results in all groups
supported by meshes of point to multipoint VCs. However, when
register to support particular layer 3 multicast groups the
modifies its use of various MARS messages to fool endpoints
using the MCS instead
The following MARS messages are associated with interaction
the MARS and MCSs
3 MARS_
7 MARS_
8 MARS_
9 MARS_
The following MARS messages are treated in a slightly
manner when MCSs have registered to support certain group addresses
1 MARS_
4 MARS_
5 MARS_
A MARS must keep two sets of mappings for each layer 3 group
MCS support. The original {layer 3 address, ATM.1, ATM.2, ... ATM.n
mapping (now termed the 'host map', although it includes routers)
augmented by a parallel {layer 3 address, server.1, server.2, ....
server.K} mapping (the 'server map'). It is assumed that no
addresses appear in both the server and host maps for the
multicast group. Typically K will be 1, but it will be larger
multiple MCSs are configured to support a given group
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
The MARS also maintains a point to multipoint VC out to any
registered with it, called ServerControlVC (section 6.2.3).
serves an analogous role to ClusterControlVC, allowing the MARS
update the MCSs with group membership changes as they occur. A
MUST also send its regular MARS_REDIRECT_MAP transmissions on
ServerControlVC and ClusterControlVC
6.2.1 Response to a MARS_REQUEST if MCS is registered
When the MARS receives a MARS_REQUEST for an address that has
host and server maps it generates a response based on the identity
the request's source. If the requestor is a member of the server
for the requested group then the MARS returns the contents of
host map in a sequence of one or more MARS_MULTIs. Otherwise, if
source is a valid cluster member, the MARS returns the contents
the server map in a sequence of one or more MARS_MULTIs. If
source is neither a cluster member, nor a member of the server
for the group, the request is dropped and ignored
Servers use the host map to establish a basic distribution VC for
group. Cluster members will establish outgoing multipoint VCs
members of the group's server map, without being aware that
packets will not be going directly to the multicast group's members
6.2.2 MARS_MSERV and MARS_UNSERV messages
MARS_MSERV and MARS_UNSERV are identical to the MARS_JOIN message
An MCS uses a MARS_MSERV with a pair of to
the multicast group X that it is willing to support. A single
MARS_UNSERV indicates the group that the MCS is no longer willing
support. The operation code for MARS_MSERV is 3 (decimal),
MARS_UNSERV is 7 (decimal).
Both of these messages are sent to the MARS over a point to point
(between MCS and MARS). After processing, they are retransmitted
ServerControlVC to allow other MCSs to note the new node
When registering or deregistering support for specific groups
mar$flags.register flag MUST be zero. (This flag is only one when
MCS is registering as a member of ServerControlVC, as described
section 6.2.3.)
When an MCS issues a MARS_MSERV for a specific group the message
be dropped and ignored if the source has not already registered
the MARS as a multicast server (section 6.2.3). Otherwise, the
adds the new ATM address to the server map for the specified group
possibly constructing a new server map if this is the first MCS
the group
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
If a MARS_MSERV represents the first MCS to register for a
group, and there exists a non null host map serving that
group, the MARS issues a MARS_MIGRATE (section 5.1.6)
ClusterControlVC. The MARS's own identity is placed in the
protocol and hardware address fields of the MARS_MIGRATE. The
address of the MCS is placed as the first and only target
address. The address of the affected group is placed in the
multicast group address field
If a MARS_MSERV is not the first MCS to register for a
group the MARS simply changes its operation code to MARS_JOIN,
sends a copy of the message on ClusterControlVC. This fools
cluster members into thinking a new leaf node has been added to
group specified. In the retransmitted MARS_JOIN mar$flags.layer3
MUST be zero, mar$flags.copy MUST be one, and mar$flags.register
be zero
When an MCS issues a MARS_UNSERV the MARS removes its ATM
from the server maps for each specified group, deleting any
maps that end up being null after the operation
The operation code is then changed to MARS_LEAVE and the MARS sends
copy of the message on ClusterControlVC. This fools the
members into thinking a leaf node has been dropped from the
specified. In the retransmitted MARS_LEAVE mar$flags.layer3grp
be zero, mar$flags.copy MUST be one, and mar$flags.register MUST
zero
The MARS retransmits redundant MARS_MSERV and MARS_UNSERV
directly back to the MCS generating them. MARS_MIGRATE messages
never repeated in response to redundant MARS_MSERVs
The last or only MCS for a group MAY choose to issue a MARS_
while the group still has members. When the MARS_UNSERV is
by the MARS the 'server map' will be deleted. When the
MARS_LEAVE is issued on ClusterControlVC, all cluster members with
VC open to the MCS for that group will close down the VC (
accordance with section 5.1.4, since the MCS was their only
node). When cluster members subsequently find they need to
packets to the group, they will begin again with
MARS_REQUEST/MARS_MULTI sequence to establish a new VC. Since
MARS will have deleted the server map, this will result in the
map being returned, and the group reverts to being supported by a
mesh
The reverse process is achieved through the MARS_MIGRATE message
the first MCS registers to support a group. This ensures
cluster members explicitly dismantle any VC mesh they may have
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up, and re-establish their multicast forwarding path with the MCS
its termination point
6.2.3 Registering a Multicast Server (MCS).
Section 5.2.3 describes how endpoints register as cluster members
and hence get added as leaf nodes to ClusterControlVC. The
approach is used to register endpoints that intend to provide
support
Registration with the MARS occurs when an endpoint issues
MARS_MSERV with mar$flags.register set to one. Upon registration
endpoint is added as a leaf node to ServerControlVC, and
MARS_MSERV is returned to the MCS privately
The MCS retransmits this MARS_MSERV until it confirms that the
has received it (by receiving a copy back, in an analogous way to
mechanism described in section 5.2.2 for reliably
MARS_JOINs).
The mar$cmi field in MARS_MSERVs MUST be set to zero by both MCS
MARS
An MCS may also choose to de-register, using a MARS_UNSERV
mar$flags.register set to one. When this occurs the MARS MUST
all references to that MCS in all servermaps associated with
protocol (mar$pro) specified in the MARS_UNSERV, and drop the
from ServerControlVC
Note that multiple logical MCSs may share the same physical
interface, provided that each MCS uses a separate ATM address (e.g.
different SEL field in the NSAP format address). In fact, an MCS
share the ATM interface of a node that is also a cluster
(either host or router), provided each logical entity has a
ATM address
A MARS MUST be capable of handling a multi-entry servermap. However
the possible use of multiple MCSs registering to support the
group is a subject for further study. In the absence of an
synchronisation protocol a system administrator MUST NOT allow
than one logical MCS to register for a given group
6.2.4 Modified response to MARS_JOIN and MARS_LEAVE
The existence of MCSs supporting some groups but not others
the MARS to modify its distribution of single and block join/
updates to cluster members. The MARS also adds two new messages -
MARS_SJOIN and MARS_SLEAVE - for communicating group changes to
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over ServerControlVC
The MARS_SJOIN and MARS_SLEAVE messages are identical to MARS_JOIN
with operation codes 18 and 19 (decimal) respectively
When a cluster member issues MARS_JOIN or MARS_LEAVE for a
group, the MARS checks to see if the group has an associated
map. If the specified group does not have a server map
continues as described in section 6.1.2.
However, if a server map exists for the group a new set of
are taken
If the joining (leaving) node was not already (is no longer
considered a member of the specified group, a copy of
MARS_JOIN/LEAVE is made with type MARS_SJOIN or MARS_SLEAVE
appropriate, and transmitted on ServerControlVC. This allows
MCS(s) supporting the group to note the new member and
their data VCs
The original message is transmitted back to the source
member unchanged, using the VC it arrived on rather
ClusterControlVC. The mar$flags.punched field MUST be reset to 0
in this message
(Section 5.2.2 requires cluster members have a mechanism to
the reception of their message by the MARS. For mesh
groups, using ClusterControlVC serves dual purpose of providing
confirmation and distributing group update information. When a
is MCS supported, there is no reason for all cluster members
process null join/leave messages on ClusterControlVC, so they
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)
MARS_LEAVE requires a more complex response. The single
block may simultaneously cover mesh supported and MCS
groups. However, cluster members only need to be informed of
mesh supported groups that the endpoint has joined. Only the
need to know if the endpoint is joining any MCS supported groups
The solution is to modify the MARS_JOIN or MARS_LEAVE that
retransmitted on ClusterControlVC. The following action is taken
A copy of the MARS_JOIN/LEAVE is made with type MARS_SJOIN
MARS_SLEAVE as appropriate, with its block replaced
a 'hole punched' set of zero or more pairs. The '
punched' set of pairs covers the entire address
specified by the original pair, but excludes
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addresses/groups which the joining (leaving) node is
(still) a member of due to a previous single group join
Before transmission on the ClusterControlVC, the
MARS_JOIN/LEAVE then has its block replaced with a '
punched' set of zero or more pairs. The 'hole punched
set of pairs covers the entire address range
by the original pair, but excludes
addresses/groups supported by MCSs or which the joining (leaving
node is already (still) a member of due to a previous single
join
If no 'holes' were punched in the specified block, the
MARS_JOIN/LEAVE is re-transmitted out on
unchanged. Otherwise the following occurs
The original MARS_JOIN/LEAVE is transmitted back to the
cluster member unchanged, using the VC it arrived on.
mar$flags.punched field MUST be reset to 0 in this message
If the hole-punched set contains 1 or more pair,
copy of the original MARS_JOIN/LEAVE is transmitted
ClusterControlVC, carrying the new list.
mar$flags.punched field MUST be set to 1 in this message
The mar$flags.punched field is set to ensure the hole-punched
is ignored by the message's source when trying to match
MARS_JOIN/LEAVE messages with ones previously sent (
5.2.2).
(Appendix A discusses some algorithms for 'hole punching'.)
It is assumed that MCSs use the MARS_SJOINs and MARS_SLEAVEs
update their own VCs out to the actual group's members
mar$flags.layer3grp is copied over into the messages transmitted
the MARS. mar$flags.copy MUST be set to one
6.2.5 Sequence numbers for ServerControlVC traffic
In an analogous fashion to the Cluster Sequence Number, the
keeps a Server Sequence Number (SSN) that is incremented after
transmission on ServerControlVC. The current value of the SSN
inserted into the mar$msn field of every message the MARS issues
it believes is destined for an MCS. This includes MARS_MULTIs
are being returned in response to a MARS_REQUEST from an MCS,
MARS_REDIRECT_MAP being sent on ServerControlVC. The MARS must
the MARS_REQUESTs source, and if it is a registered MCS the SSN
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copied into the mar$msn field, otherwise the CSN is copied into
mar$msn field
MCSs are expected to track and use the SSNs in an analogous manner
the way endpoints use the CSN in section 5.1 (to trigger
of group membership information).
A MARS should be carefully designed to minimise the possibility
the SSN jumping unnecessarily. Under normal operation only MCSs
are affected by transient link problems will miss mar$msn updates
be forced to revalidate. If the MARS itself glitches it will
innundated with requests for a period as every MCS attempts
revalidate
6.3 Why global sequence numbers
The CSN and SSN are global within the context of a given
(e.g. IPv4, mar$pro = 0x800). They count ClusterControlVC
ServerControlVC activity without reference to the multicast group(s
involved. This may be perceived as a limitation, because there is
way for cluster members or multicast servers to isolate exactly
multicast group they may have missed an update for. An
was to try and provide a per-group sequence number
Unfortunately per-group sequence numbers are not practical.
current mechanism allows sequence information to be piggy-backed
MARS messages already in transit for other reasons. The ability
specify blocks of multicast addresses with a single MARS_JOIN
MARS_LEAVE means that a single message can refer to membership
for multiple groups simultaneously. A single mar$msn field
provide meaningful information about each group's sequence.
mar$msn fields would have been unwieldy
Any MARS or cluster member that supports different protocols
keep separate 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
to ensure consistency between their mapping tables and those of
active, current MARS
(Cluster members will consider backup MARSs to exist if they
been configured with a table of MARS addresses, or the
MARS_REDIRECT_MAP messages contain a list of 2 or more addresses.)
The definition of an MARS-synchronization protocol is beyond
current scope of this document, and is expected to be the subject
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further research work. However, the following observations may
made
MARS_REDIRECT_MAP messages exist, enabling one MARS to
endpoints to move to another MARS (e.g. in the aftermath of a
failure, the chosen backup MARS will eventually wish to
control of the cluster over to the main MARS when it
functioning properly again).
Cluster members and MCSs do not need to start up with knowledge
more than one MARS, provided that MARS correctly
MARS_REDIRECT_MAP messages with the full list of MARSs for
cluster
Any mechanism for synchronising backup MARSs (and coping with
aftermath of MARS failures) should be compatible with the
member behaviour described in this document
7. How an MCS utilises a MARS
When an MCS supports a multicast group it acts as a proxy
endpoint for the senders to the group. It also behaves in
analogous manner to a sender, managing a single outgoing point
multipoint VC to the real group members
Detailed description of possible MCS architectures are beyond
scope of this document. This section will outline the main issues
7.1 Association with a particular Layer 3 group
When an MCS issues a MARS_MSERV it forces all senders to
specified layer 3 group to terminate their VCs on the supplied
ATM address
The simplest MCS architecture involves taking incoming AAL_SDUs
simply flipping them back out a single point to multipoint VC.
an MCS cannot support more than one group at once, as it has no
to differentiate between traffic destined for different groups
Using this architecture, a physical node would provide MCS
for multiple groups by creating multiple logical instances of
MCS, each with different ATM Addresses (e.g. a different SEL value
the node's NSAPA).
A slightly more complex approach would be to add minimal layer 3
specific processing into the MCS. This would look inside the
AAL_SDUs and determine which layer 3 group they are destined for.
single instance of such an MCS might register its ATM Address
the MARS for multiple layer 3 groups, and manage multiple
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outgoing point to multipoint VCs (one for each group).
When an MCS starts up it MUST register with the MARS as described
section 6.2.3, identifying the protocol it supports with the mar$
field of the MARS_MSERV. This also applies to logical MCSs, even
they share the same physical ATM interface. This is important so
the MARS can react to the loss of an MCS when it drops
ServerControlVC. (One consequence is that 'simple' MCS
end up with one ServerControlVC member per group. MCSs with layer 3
specific processing may support multiple groups while still
registering as one member of ServerControlVC.)
An MCS MUST NOT share the same ATM address as a cluster member
although it may share the same physical ATM interface
7.2 Termination of incoming VCs
An MCS MUST terminate unidirectional VCs in the same manner as
cluster member. (e.g. terminate on an LLC entity when LLC/
encapsulation is used, as described in RFC 1755 for
endpoints.)
7.3 Management of outgoing VC
An MCS MUST establish and manage its outgoing point to multipoint
as a cluster member does (section 5.1).
MARS_REQUEST is used by the MCS to establish the initial leaf
for the MCS's outgoing point to multipoint VC. After the VC
established, the MCS reacts to MARS_SJOINs and MARS_SLEAVEs in
same way a cluster member reacts to MARS_JOINs and MARS_LEAVEs
The MCS tracks the Server Sequence Number from the mar$msn fields
messages from the MARS, and revalidates its outgoing point
multipoint VC(s) when a sequence number jump occurs
7.4 Use of a backup MARS
The MCS uses the same approach to backup MARSs as a cluster
(section 5.4), tracking MARS_REDIRECT_MAP messages
ServerControlVC
8. Support for IP multicast routers
Multicast routers are required for the propagation of
traffic beyond the constraints of a single cluster (inter-
traffic). (In a sense, they are multicast servers acting at the
higher layer, with clusters, rather than individual endpoints,
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their abstract sources and destinations.)
Multicast routers typically participate in higher layer
routing algorithms and policies that are beyond the scope of
memo (e.g. DVMRP [5] in the IPv4 environment).
It is assumed that the multicast routers will be implemented over
same sort of IP/ATM interface that a multicast host would use.
IP/ATM interfaces will register with the MARS as cluster members
joining and leaving multicast groups as necessary. As noted
section 5, multiple logical 'endpoints' may be implemented over
single physical ATM interface. Routers use this approach to
interfaces into each of the clusters they will be routing between
The rest of this section will assume a simple IPv4 scenario where
scope of a cluster has been limited to a particular LIS that is
of an overlaid IP network. Not all members of the LIS are
registered cluster members (you may have unicast-only hosts in
LIS).
8.1 Forwarding into a Cluster
If the multicast router needs to transmit a packet to a group
the cluster its IP/ATM interface opens a VC in the same manner as
normal host would. Once a VC is open, the router watches
MARS_JOIN and MARS_LEAVE messages and responds to them as a
host would
The multicast router's transmit side MUST implement inactivity
to shut down idle outgoing VCs, as for normal hosts
As with normal host, the multicast router does not need to be
member of a group it is sending to
8.2 Joining in 'promiscuous' mode
Once registered and initialised, the simplest model of IPv4
router operation is for it to issue a MARS_JOIN encompassing
entire Class D address space. In effect it becomes 'promiscuous',
it will be a leaf node to all present and future multipoint
established to IPv4 groups on the cluster
How a router chooses which groups to propagate outside the cluster
beyond the scope of this document
Consistent with RFC 1112, IP multicast routers may retain the use
IGMP Query and IGMP Report messages to ascertain group membership
However, certain optimisations are possible, and are described
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section 8.5.
8.3 Forwarding across the cluster
Under some circumstances the cluster may simply be another
between IP subnets that have participants in a multicast group
[LAN.1] ----- IPmcR.1 -- [cluster/LIS] -- IPmcR.2 ----- [LAN.2]
LAN.1 and LAN.2 are subnets (such as Ethernet) with attached
that are members of group X
IPmcR.1 and IPmcR.2 are multicast routers with interfaces to the LIS
A traditional solution would be to treat the LIS as a unicast subnet
and use tunneling routers. However, this would not allow hosts on
LIS to participate in the cross-LIS traffic
Assume IPmcR.1 is receiving packets promiscuously on its LAN.1
interface. Assume further it is configured to propagate
traffic to all attached interfaces. In this case that means the LIS
When a packet for group X arrives on its LAN.1 interface, IPmcR.1
simply sends the packet to group X on the LIS interface as a
host would (Issuing MARS_REQUEST for group X, creating the VC
sending the packet).
Assuming IPmcR.2 initialised itself with the MARS as a member of
entire Class D space, it will have been returned as a member of
even if no other nodes on the LIS were members. All packets for
X received on IPmcR.2's LIS interface may be retransmitted on LAN.2.
If IPmcR.1 is similarly initialised the reverse process will
for multicast traffic from LAN.2 to LAN.1, for any multicast group
The benefit of this scenario is that cluster members within the
may also join and leave group X at anytime
8.4 Joining in 'semi-promiscuous' mode
Both unicast and multicast IP routers have a common problem -
limitations on the number of AAL contexts available at their
interfaces. Being 'promiscuous' in the RFC 1112 sense means that
every M hosts sending to N groups, a multicast router's ATM
will have M*N incoming reassembly engines tied up
It is not hard to envisage situations where a number of
groups are active within the LIS but are not required to
propagated beyond the LIS itself. An example might be a
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simulation system specifically designed to use the high speed IP/
environment. There may be no practical way its traffic could
utilised on 'the other side' of the multicast router, yet under
conventional scheme the router would have to be a leaf to
participating host anyway
As this problem occurs below the IP layer, it is worth noting
'scoping' mechanisms at the IP multicast routing level do not
a solution. An IP level scope would still result in the router's
interface receiving traffic on the scoped groups, only to drop it
In this situation the network administrator might configure
multicast routers to exclude sections of the Class D address
when issuing MARS_JOIN(s). Multicast groups that will never
propagated beyond the cluster will not have the router listed as
member, and the router will never have to receive (and simply ignore
traffic from those groups
Another scenario involves the product M*N exceeding the capacity of
single router's interface (especially if the same interface must
support a unicast IP router service).
A network administrator may choose to add a second node, to
as a parallel IP multicast router. Each router would be configured
be 'promiscuous' over separate parts of the Class D address space
thus exposing themselves to only part of the VC load. This
would be completely transparent to IP hosts within the LIS
Restricted promiscuous mode does not break RFC 1112's use of
Report messages. If the router is configured to serve a given
of Class D addresses, it will receive the IGMP Report. If the
is not configured to support a given block, then the existence of
IGMP Report for a group in that block is irrelevant to the router
All routers are able to track membership changes through
MARS_JOIN and MARS_LEAVE traffic anyway. (Section 8.5 discusses
better alternative to IGMP within a cluster.)
Mechanisms and reasons for establishing these modes of operation
beyond the scope of this document
8.5 An alternative to IGMP Queries
An unfortunate aspect of IGMP is that it assumes multicasting of
packets is a cheap and trivial event at the link layer. As
consequence, regular IGMP Queries are multicasted by routers to
224.0.0.1. These queries are intended to trigger IGMP Replies
cluster members that have layer 3 members of particular groups
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The MARS_GROUPLIST_REQUEST and MARS_GROUPLIST_REPLY messages
designed to allow routers to avoid actually transmitting IGMP
out into a cluster
Whenever the router's forwarding engine wishes to transmit an
query, a MARS_GROUPLIST_REQUEST can be sent to the MARS instead.
resulting MARS_GROUPLIST_REPLY(s) (described in section 5.3) from
MARS carry all the information that the router would have
from IGMP replies
It is RECOMMENDED that multicast routers utilise this MARS service
minimise IGMP traffic within the cluster
By default a MARS_GROUPLIST_REQUEST SHOULD specify the entire
space (e.g. <224.0.0.0, 239.255.255.255> in an IPv4 environment).
However, routers serving part of the address space (as described
section 8.4) MAY choose to issue MARS_GROUPLIST_REQUESTs that
only the subset of the address space they are serving
(On the surface it would also seem useful for multicast routers
track MARS_JOINs and MARS_LEAVEs that arrive with mar$flags.layer3
set. These might be used in lieu of IGMP Reports, to provide
router with timely indication that a new layer 3 group member
within the cluster. However, this only works on VC mesh
groups, and is therefore NOT recommended).
Appendix B discusses less elegant mechanisms for reducing the
of IGMP traffic within a cluster, on the assumption that the IP/
interfaces to the cluster are being used by un-optimised
multicasting code
8.6 CMIs across multiple interfaces
The Cluster Member ID is only unique within the Cluster managed by
given MARS. On the surface this might appear to leave us with
problem when a multicast router is routing between two or
Clusters using a single physical ATM interface. The router
register with two or more MARSs, and thereby acquire two or
independent CMI's. Given that each MARS has no reason to
their CMI allocations, it is possible for a host in one cluster
have the same CMI has the router's interface to another Cluster.
does the router distinguish between its own reflected packets,
packets from that other host
The answer lies in the fact that routers (and hosts)
implement logical IP/ATM interfaces over a single physical
interface. Each logical interface will have a unique ATM Address (eg
an NSAP with different SELector fields, one for each
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interface).
Each logical IP/ATM interface is configured with the address of
single MARS, attaches to only one cluster, and so has only one CMI
worry about. Each of the MARSs that the router is registered
will have been given a different ATM Address (corresponding to
different logical IP/ATM interfaces) in each registration MARS_JOIN
When hosts in a cluster add the router as a leaf node, they'
specify the ATM Address of the appropriate logical IP/ATM
on the router in the L_MULTI_ADD message. Thus, each logical IP/
interface will only have to check and filter on CMIs assigned by
own MARS
In essence the cluster differentiation is achieved by ensuring
logical IP/ATM interfaces are assigned different ATM Addresses
9. Multiprotocol applications of the MARS and MARS clients
A deliberate attempt has been made to describe the MARS
associated mechanisms in a manner independent of a specific
layer protocol being run over the ATM cloud. The
application of this document will be in an IPv4 environment, and
is reflected by the focus of key examples. However, the mar$pro.
and mar$pro.snap fields in every MARS control message allow
higher layer protocol that has a 'short form' or 'long form'
protocol identification (section 4.3) to be supported by a MARS
Every MARS MUST implement entirely separate logical mapping
and support. Every cluster member must interpret messages from
MARS in the context of the protocol type that the MARS message
to
Every MARS and MARS client MUST treat Cluster Member IDs in
context of the protocol type carried in the MARS message or
packet containing the CMI
For example, IPv6 has been allocated an Ethertype of 0x86DD.
means the 'short form' of protocol identification must be used in
MARS control messages and the data path encapsulation (section 5.5).
An IPv6 multicasting client sets the mar$pro.type field of every
message to 0x86DD. When carrying IPv6 addresses the mar$spln
mar$tpln fields are either 0 (for null or non-existent information
or 16 (for the full IPv6 address).
Following the rules in section 5.5, an IPv6 data packet
encapsulated as
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
[0xAA-AA-03][0x00-00-5E][0x00-01][pkt$cmi][0x86DD][IPv6 packet
A host or endpoint interface that is using the same MARS to
multicasting needs of multiple protocols MUST not assume their
will be the same for each protocol
10. Supplementary parameter processing
The mar$extoff field in the [Fixed header] indicates
supplementary parameters are being carried by a MARS control message
This mechanism is intended to enable the addition of
functionality to the MARS protocol in later documents
Supplementary parameters are conveyed as a list of TLV (type, length
value) encoded information elements. The TLV(s) begin on the
32 bit boundary following the [Addresses] field in the MARS
message (e.g. after mar$tsa.N in a MARS_MULTI, after mar$max.N in
MARS_JOIN, etc).
10.1 Interpreting the mar$extoff field
If the mar$extoff field is non-zero it indicates that a list of
or more TLVs have been appended to the MARS message. The first
is found by treating mar$extoff as an unsigned integer
an offset (in octets) from the beginning of the MARS message (the
of the mar$afn field).
As TLVs are 32 bit aligned the bottom 2 bits of mar$extoff are
reserved. A receiver MUST mask off these two bits before
the octet offset to the TLV list. A sender MUST set these two
to zero
If mar$extoff is zero no TLVs have been appended
10.2 The format of TLVs
When they exist, TLVs begin on 32 bit boundaries, are multiples of 32
bits in length, and form a sequential list terminated by a NULL TLV
The TLV structure is
[Type - 2 octets][Length - 2 octets][Value - n*4 octets
The Type subfield indicates how the contents of the Value
are to be interpreted
The Length subfield indicates the number of VALID octets in the
subfield. Valid octets in the Value subfield start immediately
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the Length subfield. The offset (in octets) from the start of
TLV to the start of the next TLV in the list is given by
following formula
offset = (length + 4 + ((4-(length & 3)) % 4))
(where % is the modulus operator
The Value subfield is padded with 0, 1, 2, or 3 octets to ensure
next TLV is 32 bit aligned. The padded locations MUST be set to zero
(For example, a TLV that needed only 5 valid octets of
would be 12 octets long. The Length subfield would hold the value 5,
and the Value subfield would be padded out to 8 bytes. The 5
octets of information begin at the first octet of the
subfield.)
The Type subfield is formatted in the following way
| 1st octet | 2nd octet |
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x | y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The most significant 2 bits (Type.x) determine how a recipient
behave when it doesn't recognise the TLV type indicated by the
14 bits (Type.y). The required behaviours are
Type.x = 0 Skip the TLV, continue processing the list
Type.x = 1 Stop processing, silently drop the MARS message
Type.x = 2 Stop processing, drop message, give error indication
Type.x = 3 Reserved. (currently treat as x = 0)
(The error indication generated when Type.x = 2 SHOULD be logged
some locally significant fashion. Consequential MARS message
in response to such an error condition will be defined in
documents.)
The TLV type space (Type.y) is further subdivided to encourage
outside the IETF
0 Null TLV
0x0001 - 0x0FFF Reserved for the IETF
0x1000 - 0x11FF Allocated to the ATM Forum
0x1200 - 0x37FF Reserved for the IETF
0x3800 - 0x3FFF Experimental use
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10.3 Processing MARS messages with TLVs
Supplementary parameters act as modifiers to the basic
specified by the mar$op field of any given MARS message
If a MARS message arrives with a non-zero mar$extoff field its
list MUST be parsed before handling the MARS message in
with the mar$op value. Unrecognised TLVs MUST be handled as
by their Type.x value
How TLVs modify basic MARS operations will be mar$op and
specific
10.4 Initial set of TLV elements
Conformance with this document only REQUIRES the recognition of
TLV, the Null TLV. This terminates a list of TLVs, and MUST
present if mar$extoff is non-zero in a MARS message. It MAY be
only TLV present
The Null TLV is coded as
[0x00-00][0x00-00]
Future documents will describe the formats, contents,
interpretations of additional TLVs. The minimal parsing
imposed by this document are intended to allow conformant MARS
MARS client implementations to deal gracefully and predictably
future TLV developments
11. Key Decisions and open issues
The key decisions this document proposes
A Multicast Address Resolution Server (MARS) is proposed to co
ordinate and distribute mappings of ATM endpoint addresses
arbitrary higher layer 'multicast group addresses'. The
case of IPv4 multicast is used as the example
The concept of 'clusters' is introduced to define the scope of
MARS's responsibility, and the set of ATM endpoints willing
participate in link level multicasting
A MARS is described with the functionality required to
intra-cluster multicasting using either VC meshes or ATM
multicast servers (MCSs).
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LLC/SNAP encapsulation of MARS control messages allows MARS
ATMARP traffic to share VCs, and allows partially co-resident
and ATMARP entities
New message types
MARS_JOIN, MARS_LEAVE, MARS_REQUEST. Allow endpoints to join
leave, and request the current membership list of
groups
MARS_MULTI. Allows multiple ATM addresses to be returned by
MARS in response to a MARS_REQUEST
MARS_MSERV, MARS_UNSERV. Allow multicast servers to
and deregister themselves with the MARS
MARS_SJOIN, MARS_SLEAVE. Allow MARS to pass on group
changes to multicast servers
MARS_GROUPLIST_REQUEST, MARS_GROUPLIST_REPLY. Allow MARS
indicate which groups have actual layer 3 members. May be
to support IGMP in IPv4 environments, and similar functions
other environments
MARS_REDIRECT_MAP. Allow MARS to specify a set of backup
addresses
MARS_MIGRATE. Allows MARS to force cluster members to
from VC mesh to MCS based forwarding tree in single operation
'wild card' MARS mapping table entries are possible, where
single ATM address is simultaneously associated with blocks
multicast group addresses
For the MARS protocol mar$op.version = 0. The complete set of
control messages and mar$op.type values is
1 MARS_
2 MARS_
3 MARS_
4 MARS_
5 MARS_
6 MARS_
7 MARS_
8 MARS_
9 MARS_
10 MARS_GROUPLIST_
11 MARS_GROUPLIST_
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
12 MARS_REDIRECT_
13 MARS_
A number of issues are left open at this stage, and are likely to
the subject of on-going research and additional documents that
upon this one
The specified endpoint behaviour allows the use
redundant/backup MARSs within a cluster. However,
specifications yet exist on how these MARSs co-ordinate
themselves. (The default is to only have one MARS per cluster.)
The specified endpoint behaviour and MARS service allows the
of multiple MCSs per group. However, no specifications yet
on how this may be used, or how these MCSs co-ordinate
themselves. Until futher work is done on MCS co-
protocols the default is to only have one MCS per group
The MARS relies on the cluster member dropping
ClusterControlVC if the cluster member dies. It is not clear
additional mechanisms are needed to detect and delete 'dead
cluster members
Supporting layer 3 'broadcast' as a special case of
(where the 'group' encompasses all cluster members) has not
explicitly discussed
Supporting layer 3 'unicast' as a special case of
(where the 'group' is a single cluster member, identified by
cluster member's unicast protocol address) has not been
discussed
The future development of ATM Group Addresses and Leaf
Join to ATM Forum's UNI specification has not been addressed
(However, the problems identified in this document with respect
VC scarcity and impact on AAL contexts will not be fixed by
developments in the signalling protocol.)
Possible modifications to the interpretation of the mar$hrdrsv
mar$afn fields in the Fixed header, based on different values
mar$op.version, are for further study
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Security
Security issues are not addressed in this document
The discussions within the IP over ATM Working Group have
clarify the ideas expressed in this document. John Moy (
Communications Corp.) initially suggested the idea of wild-
entries in the ARP Server. Drew Perkins (Fore Systems)
rigorous and useful critique of early proposed mechanisms
distributing and validating group membership information.
Symington (and co-workers at MITRE Corp., Don Chirieleison, and
Barns) clearly articulated the need for multicast server support
proposed a solution, and challenged earlier block join/
mechanisms. John Shirron (Fore Systems) provided useful
on my original revalidation procedures
Susan Symington and Bryan Gleeson (Adaptec) independently
the need for the service provided by MARS_GROUPLIST_REQUEST/REPLY
The new encapsulation scheme arose from WG discussions, captured
Bryan Gleeson in an interim Work in Progress (with Keith
(Cisco), Andy Malis (Ascom Nexion), and Andrew Smith (Bay Networks
as key contributors). James Watt (Newbridge) and Joel
(Newbridge) motivated the development of a more multiprotocol
control message format, evolving it away from its original
roots. They also motivated the development of Type #1 and Type #2
data path encapsulations. Rajesh Talpade (Georgia Tech)
clarify the need for the MARS_MIGRATE function
Maryann Maher (ISI) provided valuable sanity and
checking during the latter stages of the document's development
Finally, Jim Rubas (IBM) supplied the MARS pseudo-code in Appendix
and also provided detailed proof-reading in the latter stages of
document's development
Author's
Grenville
Bellcore, 445 South
Morristown, NJ, 07960
EMail: gja@thumper.bellcore.
Phone: +1 201 829 2635
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[1] Deering, S., "Host Extensions for IP Multicasting", STD 3,
1112, Stanford University, August 1989.
[2] Heinanen, J., "Multiprotocol Encapsulation over ATM
Layer 5", RFC 1483, Telecom Finland, July 1993.
[3] Laubach, M., "Classical IP and ARP over ATM", RFC 1577, Hewlett
Packard Laboratories, December 1993.
[4] ATM Forum, "ATM User Network Interface (UNI)
Version 3.1", ISBN 0-13-393828-X, Prentice Hall, Englewood Cliffs
NJ, June 1995.
[5] Waitzman, D., Partridge, C., and S. Deering, "Distance
Multicast Routing Protocol", RFC 1075, November 1988.
[6] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E.,
A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
February 1995.
[7] Borden, M., Crawley, E., Davie, B., and S. Batsell, "
of Real-time Services in an IP-ATM Network Architecture.", RFC 1821,
August 1995.
[8] ATM Forum, "ATM User-Network Interface Specification
3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993.
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Appendix A. Hole punching algorithms
Implementations are entirely free to comply with the body of
memo in any way they see fit. This appendix is purely
clarification
A MARS implementation might pre-construct a set of
(P) that reflects the entire Class D space, excluding any
currently supported by multicast servers. The field of
first pair MUST be 224.0.0.0, and the field of the last
must be 239.255.255.255. The first and last pair may be the same
This set is updated whenever a multicast server registers
deregisters
When the MARS must perform 'hole punching' it might consider
following algorithm
Assume the MARS_JOIN/LEAVE received by the MARS from the
member specified the block .
Assume Pmin(N) and Pmax(N) are the and fields from
Nth pair in the MARS's current set P
Assume set P has K pairs. Pmin(1) MUST equal 224.0.0.0,
Pmax(M) MUST equal 239.255.255.255. (If K == 1 then no
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
covering the multicast servers, but will have been pruned to
the section of the Class D space specified by the originating host'
values
The host end should keep a table, H, of open VCs in ascending
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/
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. Minimising the impact of IGMP in IPv4 environments
Implementing any part of this appendix is not required
conformance with this document. It is provided solely to
issues that have been identified
The intent of section 5.1 is for cluster members to only
outgoing point to multipoint VCs when they are actually sending
to a particular multicast group. However, in most IPv4
the multicast routers attached to a cluster will periodically
IGMP Queries to ascertain if particular groups have members.
current IGMP specification attempts to avoid having every
member respond by insisting that each group member wait a
period, and responding if no other member has responded before them
The IGMP reply is sent to the multicast address of the group
queried
Unfortunately, as it stands the IGMP algorithm will be a nuisance
cluster members that are essentially passive receivers within a
multicast group. It is just as likely that a passive member, with
outgoing VC already established to the group, will decide to send
IGMP reply - causing a VC to be established where there was no
for one. This is not a fatal problem for small clusters, but
seriously impact on the ability of a cluster to scale
The most obvious solution is for routers to use
MARS_GROUPLIST_REQUEST and MARS_GROUPLIST_REPLY messages,
described in section 8.5. This would remove the regular IGMP Queries
resulting in cluster members only sending an IGMP Report when
first join a group
Alternative solutions do exist. One would be to modify the IGMP
algorithm, for example
If the group member has VC open to the group proceed as per
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,
then proceed as per RFC 1112.
If even one group member is sending to the group at the time the
Query is issued then all the passive receivers will find the
Reply has been transmitted before their delay expires, so no new
is required. If all group members are passive at the time of the
Query then a response will eventually arrive, but 10 seconds
than under conventional circumstances
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The preceding solution requires re-writing existing IGMP code,
implies the ability of the IGMP entity to ascertain the status of
on the underlying ATM interface. This is not likely to be
in the short term
One short term solution is to provide something like the
functionality with a 'hack' at the IP/ATM driver level within
members. Arrange for the IP/ATM driver to snoop inside IP
looking for IGMP traffic. If an IGMP packet is accepted
transmission, the IP/ATM driver can buffer it locally if there is
VC already active to that group. A 10 second timer is started, and
an IGMP Reply for that group is received from elsewhere on
cluster the timer is reset. If the timer expires, the IP/ATM
then establishes a VC to the group as it would for a normal
multicast packet
Some network implementors may find it advantageous to configure
multicast server to support the group 224.0.0.1, rather than rely
a mesh. Given that IP multicast routers regularly send IGMP
to this address, a mesh will mean that each router will
consume an AAL context within each cluster member. In clusters
by multiple routers the VC load within switches in the underlying
network will become a scaling problem
Finally, if a multicast server is used to support 224.0.0.1,
ATM driver level hack becomes a possible solution to IGMP
traffic. The ATM driver may choose to grab all outgoing IGMP
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,
intended recipients will still receive the IGMP Replies. The
impact is that all cluster members will receive the IGMP Replies
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Appendix C. Further comments on 'Clusters'.
The cluster concept was introduced in section 1 for two reasons.
more well known term of Logical IP Subnet is both very IP specific
and constrained to unicast routing boundaries. As the
described in this document may be re-used in non-IP environments
more neutral term was needed. As the needs of multicasting are
always bound by the same scopes as unicasting, it was not
obvious that apriori limiting ourselves to LISs was beneficial in
long term
It must be stressed that Clusters are purely an administrative being
You choose their size (i.e. the number of endpoints that
with the same MARS) based on your multicasting needs, and
resource consumption you are willing to put up with. The larger
number of ATM attached hosts you require multicast support for,
more individual clusters you might choose to establish (along
multicast routers to provide inter-cluster traffic paths).
Given that not all the hosts in any given LIS may require
support, it becomes conceivable that you might assign a single
to support hosts from across multiple LISs. In effect you have
cluster covering multiple LISs, and have achieved 'cut through
routing for multicast traffic. Under these circumstances
the geographical size of a cluster might be considered a good thing
However, practical considerations limit the size of clusters.
a cluster span multiple LISs may not always be a particular 'win
situation. As the number of multicast capable hosts in your
increases it becomes more likely that you'll want to constrain
cluster's size and force multicast traffic to aggregate at
routers scattered across your ATM cloud
Finally, multi-LIS clusters require a degree of care when
IP multicast routers. Under the Classical IP model you need
routers on the edges of LISs. Under the MARS architecture you
need multicast routers at the edges of clusters. If your
spans multiple LISs, then the multicast routers will
themselves to have a single interface that is simultaneously
to multiple unicast subnets. Whether this situation will work
on the inter-domain multicast routing protocols you use, and
multicast router's ability to understand the new relationship
unicast and multicast topologies
In the absence of futher research in this area, networks deployed
conformance to this document MUST make their IP cluster and IP
coincide, so as to avoid these complications
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Appendix D. TLV list parsing algorithm
The following pseudo-code represents how the TLV list
described in section 10 could be handled by a MARS or MARS client
list = (mar$extoff & 0xFFFC);
if (list == 0) exit
list = list + message_base
while (list->Type.y != 0)
{
switch (list->Type.y
{
default
{
if (list->Type.x == 0) break
if (list->Type.x == 1) exit
if (list->Type.x == 2) log-error-and-exit
}
[...other handling goes here..]
}
list += (list->Length + 4 + ((4-(list->Length & 3)) %
4));
}
return
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Appendix E. Summary of timer values
This appendix summarises various timers or limits mentioned in
main body of the document. Values are specified in the
format: [x, y, z] indicating a minimum value of x, a
value of y, and a maximum value of z. A '-' will indicate that
category has no value specified. Values in minutes are followed
'min', values in seconds are followed by 'sec'.
Idle time for MARS - MARS client pt to pt VC
[1 min, 20 min, -]
Idle time for multipoint VCs from client
[1 min, 20 min, -]
Allowed time between MARS_MULTI components
[-, -, 10 sec
Initial random L_MULTI_RQ/ADD retransmit timer range
[5 sec, -, 10 sec
Random time to set VC_revalidate flag
[1 sec, -, 10 sec
MARS_JOIN/LEAVE retransmit interval
[5 sec, 10 sec, -]
MARS_JOIN/LEAVE retransmit limit
[-, -, 5]
Random time to re-register with MARS
[1 sec, -, 10 sec
Force wait if MARS re-registration is looping
[1 min, -, -]
Transmission interval for MARS_REDIRECT_MAP
[1 min, 1 min, 2 min
Limit for client to miss MARS_REDIRECT_MAPs
[-, -, 4 min
Armitage Standards Track [Page 73]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Appendix F. Pseudo code for MARS operation
Implementations are entirely free to comply with the body of
memo in any way they see fit. This appendix is purely for
clarification
A MARS implementation might be built along the lines suggested
this pseudo-code
1.
1.1
Define a server list as the list of leaf
on ServerControlVC
Define a cluster list as the list of leaf
on ClusterControlVC
Define a host map as the list of hosts that
members of a group
Define a server map as the list of hosts (MCSs
that are serving a group
Read config file
Allocate message queues
Allocate internal tables
Set up passive open VC connection
Set up redirect_map timer
Establish logging
1.2 Message
Forever {
If the message has a TLV then {
If TLV is unsupported then {
process as defined in TLV type field
} /* unknown TLV */
} /* TLV present */
Place incoming message in the queue
For (all messages in the queue) {
If the message is not a JOIN/LEAVE/MSERV/UNSERV
mar$flags.register == 1 then {
If the message source is (not a member of server list) &&
(not a member of cluster list) then {
Drop the message silently
}
}
If (mar$pro.type is not supported)
(the ATM source address is missing) then {
Continue
Armitage Standards Track [Page 74]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
}
Determine type of message
If an ERR_L_RELEASE arrives on ClusterControlVC then {
Remove the endpoints ATM address from all
for which it has joined
Release the CMI
Continue
} /* error on CCVC */
Call specific message handling routine
If redirect_map timer pops {
Call MARS_REDIRECT_MAP message handling routine
} /* redirect timer pop */
} /* all msgs in the queue */
} /* forever loop */
2. Message
2.1 Messages
- MARS_
Indicate no MARS_MULTI support of TLV
If the supported TLV is not NULL then {
Indicate MARS_MULTI support of TLV
Process as required
} else { /* TLV NULL */
Indicate message to be sent on Private VC
If the message source is a member of server list then {
If the group has a non-null host map then {
Call MARS_MULTI with the host map for the group
} else { /* no group */
Call MARS_NAK message routine
} /* no group */
} else { /* source is cluster list */
If the group has a non-null server map then {
Call MARS_MULTI with the server map for the group
} else { /* cluster member but no server map */
If the group has a non-null host map then {
Call MARS_MULTI with the host map for the group
} else { /* no group */
Call MARS_NAK message routine
} /* no group */
} /* cluster member but no server map */
} /* source is a cluster list */
} /* TLV NULL */
If a message exists then {
Send message as indicated
}
Armitage Standards Track [Page 75]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Return
- MARS_
Construct a MARS_MULTI for the specified map
If the param indicates TLV support then {
Process the TLV as required
}
Return
- MARS_
If (mar$flags.copy != 0) silently ignore the message
If more than a single pair is specified
silently ignore the message
Indicate message to be sent on private VC
If (mar$flags.register == 1) then {
If the node is already a registered member of the
associated with protocol type then { /*previous register*/
Copy the existing CMI into the MARS_JOIN
} else { /* new register */
Add the node to ClusterControlVC
Add the node to cluster list
mar$cmi = obtain CMI
} /* new register */
} else { /* not a register */
If the group is a duplicate of a previous MARS_JOIN then {
mar$msn = current csn
Indicate message to be sent on Private VC
} else {
Indicate no message to be sent
If the message source is in server map then {
Drop the message silently
} else {
If the first encompasses any group
a server map then {
Call the Modified JOIN/LEAVE Processing routine
} else {
If the MARS_JOIN is for a multi group then {
Call the MultiGroup JOIN/LEAVE Processing Routine
} else {
Indicate message to be sent on ClusterControlVC
} /* not for a multi group */
} /* group not handled by server */
} /* msg src not in server map */
Update internal tables
} /* not a duplicate */
} /* not a register */
Armitage Standards Track [Page 76]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
If a message exists then {
mar$flags.copy = 1.
Send message as indicated
}
Return
- MARS_
If (mar$flags.copy != 0) silently ignore the message
If more than a single pair is specified
silently ignore the message
Indicate message to be sent on ClusterControlVC
If (mar$flags.register == 1) then { /* deregistration */
Update internal tables to remove the member's ATM
from all groups it has joined
Drop the endpoint from ClusterControlVC
Drop the endpoint from cluster list
Release the CMI
Indicate message to be sent on Private VC
} else { /* not a deregistration */
If the group is a duplicate of a previous MARS_LEAVE then {
mar$msn = current csn
Indicate message to be sent on Private VC
} else {
Indicate no message to be sent
If the first encompasses any group
a server map then {
Call the Modified JOIN/LEAVE Processing routine
} else {
If the MARS_LEAVE is for a multi group then {
Call the MultiGroup JOIN/LEAVE Processing Routine
} else {
Indicate message to be sent on ClusterControlVC
}
}
Update internal tables
} /* not a duplicate */
} /* not a deregistration */
If a message exists then {
mar$flags.copy = 1.
Send message as indicated
}
Return
- MARS_
If (mar$flags.register == 1) then { /* server register */
Add the endpoint as a leaf node to ServerControlVC
Armitage Standards Track [Page 77]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
Add the endpoint to the server list
Indicate the message to be sent on Private VC
mar$cmi = 0.
} else { /* not a register */
If the source has not registered then {
Drop and ignore the message
Indicate no message to be sent
} else { /* source is registered */
If MCS is already member of indicated server map {
Indicate message to be sent on Private VC
mar$flags.layer3grp = 0;
mar$flags.copy = 1.
} else { /* New MCS to add. */
Add the server ATM addr to server map for group
Indicate message to be sent on ServerControlVC
Send message as indicated
Make a copy of the message
Indicate message to be sent on ClusterControlVC
If new server map was just created {
Construct MARS_MIGRATE, with MCS as target
} else {
Change the op code to MARS_JOIN
mar$flags.layer3grp = 0.
mar$flags.copy = 1.
} /* new server map */
} /* New MCS to add. */
} /* source is registered */
} /* not a register */
If a message exists then {
Send message as indicated
}
Return
- MARS_
If (mar$flags.register == 1) then { /* deregister */
Remove the ATM addr of the MCS from all server maps
If a server map becomes null then delete it
Remove the endpoint as a leaf of ServerControlVC
Remove the endpoint from server list
Indicate the message to be sent on Private VC
} else { /* not a deregister */
If the source is not a member of server list then {
Drop and ignore the message
Indicate no message to be sent
} else { /* source is registered */
Armitage Standards Track [Page 78]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
If MCS is not member of indicated server map {
Indicate message to be sent on Private VC
mar$flags.layer3grp = 0;
mar$flags.copy = 1.
} else { /* MCS existed, must be removed. */
Remove ATM addr of the MCS from indicated server map
If a server map is null then delete it
Indicate the message to be sent on ServerControlVC
Send message as indicated
Make a copy of the message
Change the op code to MARS_LEAVE
Indicate message (copy) to be sent on ClusterControlVC
mar$flags.layer3grp = 0;
mar$flags.copy = 1.
} /* MCS existed, must be removed. */
} /* source is registered */
} /* not a deregister */
If a message exists then {
Send message as indicated
}
Return
- MARS_
Build command
Return
- MARS_GROUPLIST_
If (mar$pnum != 1) then Return
Call MARS_GROUPLIST_REPLY with the range and output VC
Return
- MARS_GROUPLIST_
Build command for specified range
Indicate message to be sent on specified VC
Send message as indicated
Return
- MARS_REDIRECT_
Include the MARSs own address in the message
If there are backup MARSs then include their addresses
Indicate MARS_REDIRECT_MAP is to be sent on ClusterControlVC
Send message back as indicated
Return
Armitage Standards Track [Page 79]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
3. Send Message
If (the message is going out ClusterControlVC) &&
(a new csn is required) then {
mar$msn = obtain a
}
If (the message is going out ServerControlVC) &&
(a new ssn is required) then {
mar$msn = obtain a
}
Return
4. Number
4.1 Cluster Sequence
Generate the next sequence number
Return
4.2 Server Sequence
Generate the next sequence number
Return
4.3
CMIs are allocated uniquely per registered cluster
within the context of a particular layer 3 protocol type
A single node may register multiple times if it
multiple layer 3 protocols
The CMIs allocated for each such registration may or
not be the same
Generate a CMI for this protocol
Return
5. Modified JOIN/LEAVE
This routine processes JOIN/LEAVE when a server map exists
Make a copy of the message
Change the type of the copy to MARS_SJOIN
If the message is a MARS_LEAVE then {
Change the type of the copy to MARS_SLEAVE
}
mar$flags.copy = 1 (copy).
Hole punch the group by
from the range those groups which the
(leaving) node is already (still) a member
Armitage Standards Track [Page 80]
RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
due to it having previously issued a single
join
Indicate the message to be sent on ServerControlVC
If the message (copy) contains one or more pair {
Send message (copy) as indicated
}
mar$flags.punched = 0 in the original message
Indicate the message to be sent on Private VC
Send message (original) as indicated
Hole punch the group by
from the range those groups that are served by
or which the joining (leaving) node is
(still) a member of due to it having
issued a single group join
Indicate the (original) message to be sent on ClusterControlVC
If (number of holes punched > 0) then { /* punched holes */
In original message do {
mar$flags.punched = 1.
old punched list <- new punched list
}
} /* punched holes */
mar$flags.copy = 1.
Send message as indicated
Return
5.1 MultiGroup JOIN/LEAVE
This routine processes JOIN/LEAVE when a multi group exists
If (mar$flags.layer3grp) {
Ignore this setting, consider it reset
}
mar$flags.copy = 1.
Make a copy of the message
From the copy hole punch the group
excluding from the range those groups that
node has already joined or left
If (number of holes punched > 0) then {
mar$flags.punch = 0 in original message
Indicate original message to be sent on Private VC
Send original message as indicated
mar$flags.punch = 1 in copy message
old group range <- new punched list
Indicate message to be sent on ClusterControlVC
Send copy of message as indicated
} else {
Indicate message to be sent on ClusterControlVC
Send original message as indicated
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RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996
} /* no holes punched */
Return
Armitage Standards Track [Page 82]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
