RFC2022 - Support for Multicast over UNI 3.0/3.1 based ATM Networks

Network Working Group G. Armitage
Request for Comments: 2022 Bellcore
Category: Standards Track November 1996
Support for Multicast over UNI 3.0/3.1 based ATM Networks.
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited
Abstract
Mapping the connectionless IP multicast service over the connection
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 of
Layer 3 protocols in general, and describes its application to IP
multicasting in particular.
ATM based IP hosts and routers use a Multicast Address Resolution
Server (MARS) to support RFC1112 style Level 2 IP multicast over the
ATM Forum"s UNI 3.0/3.1 point to multipoint connection service.
Clusters of endpoints share a MARS and use it to track and
disseminate information identifying the nodes listed as receivers for
given multicast groups. This allows endpoints to establish and manage
point to multipoint VCs when transmitting to the group.
The MARS behaviour allows Layer 3 multicasting to be supported using
either meshes of VCs or ATM level multicast servers. This choice may
be made on a per-group basis, and is transparent to the endpoints.
Table of Contents
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
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
1. Introduction.
Multicasting is the process whereby a source host or protocol entity
sends a packet to multiple destinations simultaneously using a
single, local "transmit" operation. The more familiar cases of
Unicasting and Broadcasting may be considered to be special cases of
Multicasting (with the packet delivered to one destination, or "all"
destinations, respectively).
Most network layer models, like the one described in RFC1112 [1] for
IP multicasting, assume sources may send their packets to abstract
"multicast group addresses". Link layer support for such an
abstraction is assumed to exist, and is provided by technologies such
as Ethernet.
ATM is being utilized as a new link layer technology to support a
variety of protocols, including IP. With RFC1483 [2] the IETF
defined a multiprotocol mechanism for encapsulating and transmitting
packets using AAL5 over ATM Virtual Channels (VCs). However, the ATM
Forum"s currently published signalling 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, bidirectional
VCs. Multicasting is supported through point to multipoint
unidirectional VCs. The key limitation is that the sender must have
prior knowledge of each intended recipient, and eXPlicitly establish
a VC with itself as the root node and the recipients as the leaf
nodes.
This document has two broad goals:
Define a group address registration and membership distribution
mechanism that allows UNI 3.0/3.1 based networks to support the
multicast service of protocols such as IP.
Define specific endpoint behaviours for managing point to
multipoint VCs to achieve multicasting of layer 3 packets.
As the IETF is currently in the forefront of using wide area
multicasting this document"s descriptions will often focus on IP
service model of RFC1112. A final chapter will note the
multiprotocol application of the architecture.
This document avoids discussion of one highly non-trivial ASPect of
using ATM - the specification of QoS for VCs being established in
response to higher layer needs. Research in this area is still very
formative [7], and so it is assumed that future documents will
clarify the mapping of QoS requirements to VC establishment. The
default at this time is that VCs are established with a request for
Unspecified Bit Rate (UBR) service, as typified by the IETF"s use of
VCs for unicast IP, described in RFC1755 [6].
1.1 The Multicast Address Resolution Server (MARS).
The Multicast Address Resolution Server (MARS) is an extended analog
of the ATM ARP Server introduced in RFC1577 [3]. It acts as a
registry, associating layer 3 multicast group identifiers with the
ATM interfaces representing the group"s members. MARS messages
support the distribution of multicast group membership information
between MARS and endpoints (hosts or routers). Endpoint address
resolution entities query the MARS when a layer 3 address needs to be
resolved to the set of ATM endpoints making up the group at any one
time. Endpoints keep the MARS informed when they need to join or
leave particular layer 3 groups. To provide for asynchronous
notification of group membership changes the MARS manages a point to
multipoint VC out to all endpoints desiring multicast support
Valid arguments can be made for two different approaches to ATM level
multicasting of layer 3 packets - through meshes of point to
multipoint VCs, or ATM level multicast servers (MCS). The MARS
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 is
defined as
The set of ATM interfaces choosing to participate in direct ATM
connections to achieve multicasting of AAL_SDUs between
themselves.
In practice, a Cluster is the set of endpoints that choose to use the
same MARS to register their memberships and receive their updates
from.
By implication of this definition, traffic between interfaces
belonging to different Clusters passes through an inter-cluster
device. (In the IP world an inter-cluster device would be an IP
multicast router with logical interfaces into each Cluster.) This
document explicitly avoids specifying the nature of inter-cluster
(layer 3) routing protocols.
The mapping of clusters to other constrained sets of endpoints (such
as unicast Logical IP Subnets) is left to each network administrator.
However, for the purposes of conformance with this document network
administrators MUST ensure that each Logical IP Subnet (LIS) is
served by a separate MARS, creating a one-to-one mapping between
cluster and unicast LIS. IP multicast routers then interconnect each
LIS as they do with conventional subnets. (Relaxation of this
restriction MAY only occur after future research on the interaction
between existing layer 3 multicast routing protocols and unicast
subnet boundaries.)
The term "Cluster Member" will be used in this document to refer to
an endpoint that is currently using a MARS for multicast support.
Thus potential scope of a cluster may be the entire membership of a
LIS, while the actual scope of a cluster depends on which endpoints
are actually cluster members at any given time.
1.3 Document overview.
This document assumes an understanding of concepts explained in
greater detail in RFC1112, RFC1577, UNI 3.0/3.1, and RFC1755 [6].
Section 2 provides an overview of IP multicast and what RFC1112
required from Ethernet.
Section 3 describes in more detail the multicast support services
offered by UNI 3.0/3.1, and outlines the differences between VC
meshes and multicast servers (MCSs) as mechanisms for distributing
packets to multiple destinations.
Section 4 provides an overview of the MARS and its relationship to
ATM endpoints. This section also discusses the encapsulation and
structure of MARS control messages.
Section 5 substantially defines the entire cluster member endpoint
behaviour, on both receive and transmit sides. This includes both
normal operation and error recovery.
Section 6 summarises the required behaviour of a MARS.
Section 7 looks at how a multicast server (MCS) interacts with a
MARS.
Section 8 discusses how IP multicast routers may make novel use of
promiscuous and semi-promiscuous group joins. Also discussed is a
mechanism designed to reduce the amount of IGMP traffic issued by
routers.
Section 9 discusses how this document applies in the more general
(non-IP) case.
Section 10 summarises the key proposals, and identifies areas for
future research that are generated by this MARS architecture.
The appendices provide discussion on issues that arise out of the
implementation of this document. Appendix A discusses MARS and
endpoint algorithms for parsing MARS messages. Appendix B describes
the particular problems introduced by the current IGMP paradigms, and
possible interim work-arounds. Appendix C discusses the "cluster"
concept in further detail, while Appendix D briefly outlines an
algorithm for parsing TLV lists. Appendix E summarises various timer
values used in this document, and Appendix F provides example
pseudo-code for a MARS entity.
1.4 Conventions.
In this document the following coding and packet representation rules
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 the
least significant bit when stored in memory (i.e. the n"th bit has
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 value
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 "multicast
group" addresses. These abstractly represent all the IP hosts in the
Internet (or some constrained subset of the Internet) who have
decided to "join" the specified group.
RFC1112 requires that a multicast-capable IP interface must support
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 the
transmit side of the link layer interfaces. When Ethernet is used as
the link layer (the example used in RFC1112), no address resolution
is required to transmit packets. An algorithmic mapping from IP
multicast address to Ethernet multicast address is performed locally
before the packet is sent out the local interface in the same "send
and forget" manner as a unicast IP packet.
Joining and Leaving an IP multicast group is more explicit on the
receive side - with the primitives JoinLocalGroup and LeaveLocalGroup
affecting what groups the local link layer interface should accept
packets from. When the IP layer wants to receive packets from a
group, it issues JoinLocalGroup. When it no longer wants to receive
packets, it issues LeaveLocalGroup. A key point to note is that
changing state is a local issue, it has no effect on other hosts
attached to the Ethernet.
IGMP is defined in RFC1112 to support IP multicast routers attached
to a given subnet. Hosts issue IGMP Report messages when they perform
a JoinLocalGroup, or in response to an IP multicast router sending an
IGMP Query. By periodically transmitting queries IP multicast routers
are able to identify what IP multicast groups have non-zero
membership on a given subnet.
A specific IP multicast address, 224.0.0.1, is allocated for the
transmission of IGMP Query messages. Host IP layers issue a
JoinLocalGroup for 224.0.0.1 when they intend to participate in IP
multicasting, and issue a LeaveLocalGroup for 224.0.0.1 when they"ve
ceased participating in IP multicasting.
Each host keeps a list of IP multicast groups it has been
JoinLocalGroup"d to. When a router issues an IGMP Query on 224.0.0.1
each host begins to send IGMP Reports for each group it is a member
of. IGMP Reports are sent to the group address, not 224.0.0.1, "so
that other members of the same group on the same network can overhear
the Report" and not bother sending one of their own. IP multicast
routers conclude that a group has no members on the subnet when IGMP
Queries no longer 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 UNI
3.0 and UNI 3.1 in required signalling elements are covered in RFC
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 broadly
describes an "AAL User" as any entity that establishes and manages
VCs and underlying AAL services to exchange data. An IP over ATM
interface is a form of "AAL User" (although the default LLC/SNAP
encapsulation mode specified in RFC1755 really requires that an "LLC
entity" is the AAL User, which in turn supports the IP/ATM
interface).
The most fundamental limitations of UNI 3.0/3.1"s multicast support
are:
Only point to multipoint, unidirectional VCs may be established.
Only the root (source) node of a given VC may add or remove leaf
nodes.
Leaf nodes are identified by their unicast ATM addresses. UNI
3.0/3.1 defines two ATM address formats - native E.164 and NSAP
(although it must be stressed that the NSAP address is so called
because it uses the NSAP format - an ATM endpoint is NOT a Network
layer termination point). In UNI 3.0/3.1 an "ATM Number" is the
primary identification of an ATM endpoint, and it may use either
format. Under some circumstances an ATM endpoint must be identified
by both a native E.164 address (identifying the attachment point of a
private network to a public network), and an NSAP address ("ATM
Subaddress") identifying the final endpoint within the private
network. For the rest of this document the term will be used to mean
either a single "ATM Number" or an "ATM Number" combined with an "ATM
Subaddress".
3.1 VC meshes.
The most fundamental approach to intra-cluster multicasting is the
multicast VC mesh. Each source establishes its own independent point
to multipoint VC (a single multicast tree) to the set of leaf nodes
(destinations) that it has been told are members of the group it
wishes to send packets to.
Interfaces that are both senders and group members (leaf nodes) to a
given group will originate one point to multipoint VC, and terminate
one VC for every other active sender to the group. This criss-
crossing of VCs across the ATM network gives rise to the name "VC
mesh".
3.2 Multicast Servers.
An alternative model has each source establish a VC to an
intermediate node - the multicast server (MCS). The multicast server
itself establishes and manages a point to multipoint VC out to the
actual desired destinations.
The MCS reassembles AAL_SDUs arriving on all the incoming VCs, and
then queues them for transmission on its single outgoing point to
multipoint VC. (Reassembly of incoming AAL_SDUs is required at the
multicast server as AAL5 does not support cell level multiplexing of
different AAL_SDUs on a single outgoing VC.)
The leaf nodes of the multicast server"s point to multipoint VC must
be established prior to packet transmission, and the multicast server
requires an external mechanism to identify them. A side-effect of
this method is that ATM interfaces that are both sources and group
members will receive copies of their own packets back from the MCS
(An alternative method is for the multicast server to explicitly
retransmit packets on individual VCs between itself and group
members. A benefit of this second approach is that the multicast
server can ensure that sources do not receive copies of their own
packets.)
The simplest MCS pays no attention to the contents of each AAL_SDU.
It is purely an AAL/ATM level device. More complex MCS architectures
(where a single endpoint serves multiple layer 3 groups) are
possible, but are beyond the scope of this document. More detailed
discussion is provided in section 7.
3.3 Tradeoffs.
Arguments over the relative merits of VC meshes and multicast servers
have raged for some time. Ultimately the choice depends on the
relative trade-offs a system administrator must make between
throughput, latency, congestion, and resource consumption. Even
criteria such as latency can mean different things to different
people - is it end to end packet time, or the time it takes for a
group to settle after a membership change? The final choice depends
on the characteristics of the applications generating the multicast
traffic.
If we focussed on the data path we might prefer the VC mesh because
it lacks the obvious single congestion point of an MCS. Throughput
is likely to be higher, and end to end latency lower, because the
mesh lacks the intermediate AAL_SDU reassembly that must occur in
MCSs. The underlying ATM signalling system also has greater
opportunity to ensure optimal branching points at ATM switches along
the multicast trees originating on each source.
However, resource consumption will be higher. Every group member"s
ATM interface must terminate a VC per sender (consuming on-board
memory for state information, instance of an AAL service, and
buffering in accordance with the vendors particular architecture). On
the contrary, with a multicast server only 2 VCs (one out, one in)
are required, independent of the number of senders. The allocation of
VC related resources is also lower within the ATM cloud when using a
multicast server. These points may be considered to have merit in
environments where VCs across the UNI or within the ATM cloud are
valuable (e.g. the ATM provider charges on a per VC basis), or AAL
contexts are limited in the ATM interfaces of endpoints.
If we focus on the signalling load then MCSs have the advantage when
faced with dynamic sets of receivers. Every time the membership of a
multicast group changes (a leaf node needs to be added or dropped),
only a single point to multipoint VC needs to be modified when using
an MCS. This generates a single signalling event across the MCS"s
UNI. However, when membership change occurs in a VC mesh, signalling
events occur at the UNIs of every traffic source - the transient
signalling load scales with the number of sources. This has obvious
ramifications if you define latency as the time for a group"s
connectivity to stabilise after change (especially as the number of
senders increases).
Finally, as noted above, MCSs introduce a "reflected packet" problem,
which requires additional per-AAL_SDU information to be carried in
order for layer 3 sources to detect their own AAL_SDUs coming back.
The MARS architecture allows system administrators to utilize either
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 be
available to local AAL Users:
L_CALL_RQ - Establish a unicast VC to a specific endpoint.
L_MULTI_RQ - Establish multicast VC to a specific endpoint.
L_MULTI_ADD - Add new leaf node to previously established VC.
L_MULTI_DROP - Remove specific leaf node from established VC.
L_RELEASE - Release unicast VC, or all Leaves of a multicast VC.
The signalling exchanges and local information passed between AAL
User and UNI 3.0/3.1 signalling entity with these functions are
outside the scope of this document.
The following indications are assumed to be available to AAL Users,
generated by 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 AAL
User and UNI 3.0/3.1 signalling entity with these functions are
outside the scope of this document.
4. Overview of the MARS.
The MARS may reside within any ATM endpoint that is directly
addressable by the endpoints it is serving. Endpoints wishing to join
a multicast cluster must be configured with the ATM address of the
node on which the cluster"s MARS resides. (Section 5.4 describes how
backup MARSs may be added to support the activities of a cluster.
References to "the MARS" in following sections will be assumed to
mean the acting MARS for the cluster.)
4.1 Architecture.
Architecturally the MARS is an evolution of the RFC1577 ARP Server.
Whilst the ARP Server keeps a table of {IP,ATM} address pairs for all
IP endpoints in an LIS, the MARS keeps extended tables of {layer 3
address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either be
configured with certain mappings, or dynamically "learn" mappings.
The format of the {layer 3 address} field is generally not
interpreted by the MARS.
A single ATM node may support multiple logical MARSs, each of which
support a separate cluster. The restriction is that each MARS has a
unique ATM address (e.g. a different SEL field in the NSAP address of
the node on which the multiple MARSs reside). By definition a single
instance of a MARS may not support more than one cluster.
The MARS distributes group membership update information to cluster
members over a point to multipoint VC known as the ClusterControlVC.
Additionally, when Multicast Servers (MCSs) are being used it also
establishes a separate point to multipoint VC out to registered MCSs,
known as the ServerControlVC. All cluster members are leaf nodes of
ClusterControlVC. All registered multicast servers are leaf nodes of
ServerControlVC (described further in section 6).
The MARS 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 encapsulated
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.)
MARS control messages are made up of 4 major components:
[Fixed header][Mandatory fields][Addresses][Supplementary TLVs]
[Fixed header] contains fields indicating the operation being
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-used
by other query/response protocols.
The [Mandatory fields] section carries fixed width parameters that
depend on the operation type indicated in [Fixed header].
The following [Addresses] area carries variable length fields for
source and target addresses - both hardware (e.g. ATM) and layer 3
(e.g. IPv4). These provide the fundamental information that the
registrations, queries, and updates use and operate on. For the MARS
protocol fields in [Fixed header] indicate how to interpret the
contents of [Addresses].
[Supplementary TLVs] represents an optional list of TLV (type,
length, value) encoded information elements that may be appended to
provide supplementary information. This feature is described in
further detail in section 10.
MARS messages contain variable length address fields. In all cases
null addresses SHALL be encoded as zero length, and have no space
allocated in the message.
(Unique LLC/SNAP encapsulation of MARS control messages means MARS
and ARP Server functionality may be implemented within a common
entity, and share a client-server VC, if the implementor so chooses.
Note that the LLC/SNAP codepoint for MARS is different to the
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$sstl 8 bits Type & length of source ATM subaddress. (q)
mar$shtl and mar$sstl provide information regarding the source"s
hardware (ATM) address. In the MARS protocol these fields are always
present, as every MARS message carries a non-null source ATM address.
In all cases the source ATM address is the first variable length
field in the [Addresses] section.
The other fields in [Fixed header] are described in the following
subsections.
4.3.1 Hardware type.
mar$afn defines the type of link layer addresses being carried. The
value of 0x000F SHALL be used by MARS messages generated in
accordance with this document. The encoding of ATM addresses and
subaddresses when mar$afn = 0x000F is described in section 5.1.2.
Encodings when mar$afn != 0x000F are outside the scope of this
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 the
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 smaller
than 0x600, and NLPIDs never larger than 0xFF.)
The NLPID value of 0x80 is used to indicate a SNAP encoded extension
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 is
termed the "long form" protocol ID.
If mar$pro.type != 0x80 then the mar$pro.snap field MUST be zero on
transmit and ignored on receive. The mar$pro.type field itself
identifies the protocol being referred to. This is termed the "short
form" protocol ID.
In all cases, where a protocol has an assigned number in the
mar$pro.type space (excluding 0x80) the short form MUST be used when
transmitting MARS messages. Additionally, where a protocol has valid
short and long forms of identification, receivers MAY choose to
recognise the long form.
mar$pro.type values other than 0x80 MAY have "long forms" defined in
future documents.
For the remainder of this document references to mar$pro SHALL be
interpreted to mean mar$pro.type, or mar$pro.type in combination with
mar$pro.snap as appropriate.
The use of different protocol types is described further in section
9.
4.3.3 Checksum.
The mar$chksum field carries a standard IP checksum calculated across
the entire MARS control message (excluding the LLC/SNAP header). The
field is set to zero before performing the checksum calculation.
As the entire LLC/SNAP encapsulated MARS message is protected by the
32 bit CRC of the AAL5 transport, implementors MAY choose to ignore
the checksum facility. If no checksum is calculated these bits MUST
be reset before transmission. If no checksum is performed on
reception, this field MUST be ignored. If a receiver is capable of
validating a checksum it MUST only perform the validation when the
received mar$chksum field is non-zero. Messages arriving with
mar$chksum of 0 are always considered valid.
4.3.4 Extensions Offset.
The mar$extoff field identifies the existence and location of an
optional supplementary parameters list. Its use is described in
section 10.
4.3.5 Operation code.
The mar$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 the
context within which its [Mandatory fields], [Addresses], and
[Supplementary TLVs] should be interpreted.
mar$op.version
0 MARS protocol defined in this document.
0x01 - 0xEF Reserved for future use by the IETF.
0xF0 - 0xFE Allocated for use by the ATM Forum.
0xFF Experimental/Local use.
mar$op.type
Value indicates operation being performed, within context of
the control protocol version indicated by mar$op.version.
For the rest of this document references to the mar$op value SHALL be
taken to mean mar$op.type, with mar$op.version = 0x00. The values
used in this document are summarised in section 11.
(Note this number space is independent of the ATMARP operation code
number space.)
4.3.6 Reserved.
mar$hdrrsv may be subdivided and assigned specific meanings for other
control protocols indicated by mar$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 the
underlying UNI 3.0/3.1 service. An endpoint in this context can exist
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 into
two key subsections - one for the transmit side, and one for the
receive side.
Multiple logical ATM interfaces may be supported by a single physical
ATM interface (for example, using different SEL values in the NSAP
formatted address assigned to the physical ATM interface). Therefore
implementors MUST allow for multiple independent "layer 3 over ATM"
interfaces too, each with its own configured MARS (or table of MARSs,
as discussed in section 5.4), and ability to be attached to the same
or different clusters.
The initial signalling path between a MARS client (managing an
endpoint) and its associated MARS is a transient point to point,
bidirectional VC. This VC is established by the MARS client, and is
used to send queries to, and receive replies from, the MARS. It has
an associated idle timer, and is dismantled if not used for a
configurable period of time. The minimum suggested value for this
time is 1 minute, and the RECOMMENDED default is 20 minutes. (Where
the MARS and ARP Server are co-resident, this VC may be used for both
ATM ARP traffic and MARS control traffic.)
The remaining signalling path is ClusterControlVC, to which the MARS
client is added as a leaf node when it registers (described in
section 5.2.3).
The majority of this document covers the distribution of information
allowing endpoints to establish and manage outgoing point to
multipoint VCs - the forwarding paths for multicast traffic to
particular multicast groups. The actual format of the AAL_SDUs sent
on these VCs is almost completely outside the scope of this
specification. However, endpoints are not expected to know whether
their forwarding path leads directly to a multicast group"s members
or to an MCS (described in section 3). This requires additional per-
packet encapsulation (described in section 5.5) to aid in the the
detection of reflected AAL_SDUs.
5.1 Transmit side behaviour.
The following description will often be in terms of an IPv4/ATM
interface that is capable of transmitting packets to a Class D
address at any time, without prior warning. It should be trivial for
an implementor to generalise this behaviour to the requirements of
another layer 3 data protocol.
When a local Layer 3 entity passes down a packet for transmission,
the endpoint first ascertains whether an outbound path to the
destination multicast group already exists. If it does not, the MARS
is queried for a set of ATM endpoints that represent an appropriate
forwarding path. (The ATM endpoints may represent the actual group
members within the cluster, or a set of one or more MCSs. The
endpoint does not distinguish between either case. Section 6.2
describes the MARS behaviour that leads to MCSs being supplied as the
forwarding path for a multicast group.)
The query is executed by issuing a MARS_REQUEST. The reply from the
MARS may take one of two forms:
MARS_MULTI - Sequence of MARS_MULTI messages returning the set of
ATM endpoints that are to be leaf nodes of an
outgoing point to multipoint VC (the forwarding
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 Rate
(UBR) service, as typified by the IETF"s use of VCs for unicast IP,
described in RFC1755 [6]. Future documents may vary this approach
and allow the specification of different ATM traffic parameters from
locally configured information or parameters oBTained through some
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_NAK
will be returned. In this case the IP packet MUST be discarded
silently. If a match is found in the MARS"s tables it proceeds to
return addresses ATM.1 through ATM.n in a sequence of one or more
MARS_MULTIs. A simple mechanism is used to detect and recover from
loss of MARS_MULTI messages.
(If the client learns that there is no other group member in the
cluster - the MARS returns a MARS_NAK or returns a MARS_MULTI with
the client as the only member - it MUST delay sending out a new
MARS_REQUEST for that group for a period no less than 5 seconds and
no more than 10 seconds.)
Each MARS_MULTI carries a boolean field x, and a 15 bit integer field
y - expressed as MARS_MULTI(x,y). Field y acts as a sequence number,
starting at 1 and incrementing for each MARS_MULTI sent. Field x
acts as an "end of reply" marker. When x == 1 the MARS response is
considered complete.
In addition, each MARS_MULTI may carry multiple ATM addresses from
the set {ATM.1, ATM.2, .... ATM.n}. A MARS MUST minimise the number
of MARS_MULTIs transmitted by placing as many group members"
addresses in a single MARS_MULTI as possible. The limit on the length
of an individual MARS_MULTI message MUST be the MTU of the underlying
VC.
For example, assume n ATM addresses must be returned, each MARS_MULTI
is limited to only p ATM addresses, and p << n. This would require a
sequence of k MARS_MULTI messages (where k = (n/p)+1, using integer
arithmetic), transmitted as follows:
MARS_MULTI(0,1) carries back {ATM.1 ... ATM.p}
MARS_MULTI(0,2) carries back {ATM.(p+1) ... ATM.(2p)}
[.......]
MARS_MULTI(1,k) carries back { ... ATM.n}
If k == 1 then only MARS_MULTI(1,1) is sent.
Typical failure mode will be losing one or more of MARS_MULTI(0,1)
through MARS_MULTI(0,k-1). This is detected when y jumps by more than
one between consecutive MARS_MULTI"s. An alternative failure mode is
losing MARS_MULTI(1,k). A timer MUST be implemented to flag the
failure of the last MARS_MULTI to arrive. A default value of 10
seconds is RECOMMENDED.
If a "sequence jump" is detected, the host MUST wait for the
MARS_MULTI(1,k), discard all results, and repeat the MARS_REQUEST.
If a timeout occurs, the host MUST discard all results, and repeat
the MARS_REQUEST.
A final failure mode involves the MARS Sequence Number (described in
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_MULTI
the host MUST wait for the MARS_MULTI(1,k), discard all results, and
repeat the MARS_REQUEST.
(Corruption of cell contents will lead to loss of a MARS_MULTI
through AAL5 CPCS_PDU reassembly failure, which will be detected
through the mechanisms described above.)
If the MARS is managing a cluster of endpoints spread across
different but directly Accessible ATM networks it will not be able to
return all the group members in a single MARS_MULTI. The MARS_MULTI
message format allows for either E.164, ISO NSAP, or (E.164 + NSAP)
to be returned as ATM addresses. However, each MARS_MULTI message may
only return ATM addresses of the same type and length. The returned
addresses MUST be grouped according to type (E.164, ISO NSAP, or
both) and returned in a sequence of separate MARS_MULTI parts.
5.1.2 MARS_REQUEST, MARS_MULTI, and MARS_NAK messages.
MARS_REQUEST is shown below. It is indicated by an "operation type
value" (mar$op) of 1.
The multicast address being resolved is placed into the the target
protocol address field (mar$tpa), and the target hardware address is
set to null (mar$thtl and mar$tstl both zero).
In IPv4 environments the protocol type (mar$pro) is 0x800 and the
target protocol address length (mar$tpln) MUST be set to 4. The
source fields MUST contain the ATM number and subaddress of the
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$sha roctets source ATM number
mar$ssa qoctets source ATM subaddress
mar$spa soctets source protocol address
mar$tpa zoctets target multicast group address
mar$tha xoctets target ATM number
mar$tsa yoctets target ATM subaddress
Following the RFC1577 approach, the mar$shtl, mar$sstl, mar$thtl and
mar$tstl fields are coded as follows:
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
0x length
+-+-+-+-+-+-+-+-+
The most significant bit is reserved and MUST be set to zero. The
second most significant bit (x) is a flag indicating whether the ATM
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 length
of the associated ATM address in octets. If this value is zero the
flag x is ignored.
The mar$spln and mar$tpln fields are unsigned 8 bit integers, giving
the length in octets of the source and target protocol address fields
respectively.
MARS packets use true variable length fields. A null (non-existant)
address MUST be coded as zero length, and no space allocated for it
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. do
not transpose the source and target information. In all cases MARS
clients use the source address fields to identify their own messages
coming back).
The MARS_MULTI message is identified by an mar$op value of 2. The
message format is:
Data:
mar$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$sha roctets source ATM number
mar$ssa qoctets source ATM subaddress
mar$spa soctets source protocol address
mar$tpa zoctets target multicast group address
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$tha.N xoctets target ATM number N
mar$tsa.N yoctets target ATM subaddress N
The source protocol and ATM address fields are copied directly from
the MARS_REQUEST that this MARS_MULTI is in response to (not the MARS
itself).
mar$seqxy is coded with flag x in the leading bit, and sequence
number y coded as an unsigned integer in the remaining 15 bits.
1st octet 2nd octet
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
x y
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
mar$tnum is an unsigned integer indicating how many pairs of
{mar$tha,mar$tsa} (i.e. how many group member"s ATM addresses) are
present in the message. mar$msn is an unsigned 32 bit number filled
in by the MARS before transmitting each MARS_MULTI. Its use is
described further in section 5.1.4.
As an example, assume we have a multicast cluster using 4 byte
protocol addresses, 20 byte ATM numbers, and 0 byte ATM subaddresses.
For n group members in a single MARS_MULTI we require a (60 + 20n)
byte message. If we assume the default MTU of 9180 bytes, we can
return a maximum of 456 group member"s addresses in a single
MARS_MULTI.
5.1.3 Establishing the outgoing multipoint VC.
Following the completion of the MARS_MULTI reply the endpoint may
establish a new point to multipoint VC, or reuse an existing one.
If establishing a new VC, an L_MULTI_RQ is issued for ATM.1, followed
by an L_MULTI_ADD for every member of the set {ATM.2, ....ATM.n}
(assuming the set is non-null). The packet is then transmitted over
the newly created VC just as it would be for a unicast VC.
After transmitting the packet, the local interface holds the VC open
and marks it as the active path out of the host for any subsequent IP
packets being sent to that Class D address.
When establishing a new multicast VC it is possible that one or more
L_MULTI_RQ or L_MULTI_ADD may fail. The UNI 3.0/3.1 failure cause
must be returned in the ERR_L_RQFAILED signal from the local
signalling entity to the AAL User. If the failure cause is not 49
(Quality of Service unavailable), 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 the
set {ATM.1, ATM.2, ..., ATM.n} returned by the MARS. Otherwise, the
L_MULTI_RQ or L_MULTI_ADD should be reissued after a random delay of
5 to 10 seconds. If the request fails again, another request should
be issued after twice the previous delay has elapsed. This process
should be continued until the call succeeds or the multipoint VC gets
released.
If the initial L_MULTI_RQ fails for ATM.1, and n is greater than 1
(i.e. the returned set of ATM addresses contains 2 or more addresses)
a new L_MULTI_RQ should be immediately issued for the next ATM
address in the set. This procedure is repeated until an L_MULTI_RQ
succeeds, as no L_MULTI_ADDs may be issued until an initial outgoing
VC is established.
Each ATM address for which an L_MULTI_RQ failed with cause 49, 51,
37, or 41 MUST be tagged rather than deleted. An L_MULTI_ADD is
issued for these tagged addresses using the random delay procedure
outlined above.
The VC MAY be considered "up" before failed L_MULTI_ADDs have been
successfully re-issued. An endpoint MAY implement a concurrent
mechanism that allows data to start flowing out the new VC even while
failed L_MULTI_ADDs are being re-tried. (The alternative of waiting
for each leaf node to accept the connection could lead to significant
delays in transmitting the first packet.)
Each VC MUST have a configurable inactivity timer associated with it.
If the timer expires, an L_RELEASE is issued for that VC, and the
Class D address is no longer considered to have an active path out of
the local host. The timer SHOULD be no less than 1 minute, and a
default of 20 minutes is RECOMMENDED. Choice of specific timer
periods is beyond the scope of this document.
VC consumption may also be reduced by endpoints noting when a new
group"s set of {ATM.1, ....ATM.n} matches that of a pre-existing VC
out to another group. With careful local management, and assuming the
QoS of the existing VC is sufficient for both groups, a new pt to mpt
VC may not be necessary. Under certain circumstances endpoints may
decide that it is sufficient to re-use an existing VC whose set of
leaf nodes is a superset of the new group"s membership (in which case
some endpoints will receive multicast traffic for a layer 3 group
they haven"t joined, and must filter them above the ATM interface).
Algorithms for performing this type of optimization are not discussed
here, and are not required for conformance with this document.
5.1.4 Tracking subsequent group updates.
Once a new VC has been established, the transmit side of the cluster
member"s interface needs to monitor subsequent group changes - adding
or dropping leaf nodes as appropriate. This is achieved by watching
for MARS_JOIN and MARS_LEAVE messages from the MARS itself. These
messages are described in detail in section 5.2 - at this point it is
sufficient to note that they carry:
- The ATM address of a node joining or leaving a group.
- The layer 3 address of the group(s) being joined or left.
- A Cluster Sequence Number (CSN) from the MARS.
MARS_JOIN and MARS_LEAVE messages arrive at each cluster member
across ClusterControlVC. MARS_JOIN or MARS_LEAVE messages that simply
confirm information already held by the cluster member are used to
track the Cluster Sequence Number, but are otherwise ignored.
5.1.4.1 Updating the active VCs.
If a MARS_JOIN is seen that refers to (or encompasses) a group for
which the transmit side already has a VC open, the new member"s ATM
address is extracted and an L_MULTI_ADD issued locally. This ensures
that endpoints already sending to a given group will immediately add
the new member to their list of recipients.
If a MARS_LEAVE is seen that refers to (or encompasses) a group for
which the transmit side already has a VC open, the old member"s ATM
address is extracted and an L_MULTI_DROP issued locally. This ensures
that endpoints already sending to a given group will immediately drop
the old member from their list of recipients. When the last leaf of a
VC is dropped, the VC is closed completely and the affected group no
longer has a path out of the local endpoint (the next outbound packet
to that group"s address will trigger the creation of a new VC, as
described in sections 5.1.1 to 5.1.3).
The transmit side of the interface MUST NOT shut down an active VC to
a group for which the receive side has just executed a
LeaveLocalGroup. (This behaviour is consistent with the model of
hosts transmitting to groups regardless of their own membership
status.)
If a MARS_JOIN or MARS_LEAVE arrives with mar$pnum == 0 it carries no
<min,max> pairs, and is only used for tracking the CSN.
5.1.4.2 Tracking the Cluster Sequence Number.
It is important that endpoints do not miss group membership updates
issued by the MARS over ClusterControlVC. However, this will happen
from time to time. The Cluster Sequence Number is carried as an
unsigned 32 bit value in the mar$msn field of many MARS messages
(except for MARS_REQUEST and MARS_NAK). It increments once for every
transmission the MARS makes on ClusterControlVC, regardless of
whether the transmission represents a change in the MARS database or
not. By tracking this counter, cluster members can determine whether
they have missed a previous message on ClusterControlVC, and possibly
a membership change. This is then used to trigger revalidation
(described in section 5.1.5).
The current CSN is copied into the mar$msn field of MARS messages
being sent to cluster members, whether out ClusterControlVC or on a
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 received
that carries an mar$msn field the following processing is performed:
Seq.diff = mar$msn - HSN
mar$msn -> HSN
{...process MARS message as appropriate...}
if ((Seq.diff != 1) && (Seq.diff != 0))
then {...revalidate group membership information...}
The basic result is that the cluster member attempts to keep locked
in step with membership changes noted by the MARS. If it ever detects
that a membership change occurred (in any group) without it noticing,
it re-validates the membership of all groups it currently has
multicast VCs open to.
The mar$msn value in an individual MARS_MULTI is not used to update
the HSN until all parts of the MARS_MULTI (if more than 1) have
arrived. (If the mar$msn changes the MARS_MULTI is discarded, as
described in section 5.1.1.)
The MARS is free to choose an initial value of CSN. When a new
cluster member starts up it should initialise HSN to zero. When the
cluster member sends the MARS_JOIN to register (described later), the
HSN will be correctly updated to the current CSN value when the
endpoint receives the copy of its MARS_JOIN back from the MARS.
5.1.5 Revalidating a VC"s leaf nodes.
Certain events may inform a cluster member that it has incorrect
information about the sets of leaf nodes it should be sending to. If
an error occurs on a VC associated with a particular group, the
cluster member initiates revalidation procedures for that specific
group. If a jump is detected in the Cluster Sequence Number, this
initiates revalidation of all groups to which the cluster member
currently has open point to multipoint VCs.
Each open and active multipoint VC has a flag associated with it
called "VC_revalidate". This flag is checked everytime a packet is
queued for transmission on that VC. If the flag is false, the packet
is transmitted and no further action is required.
However, if the VC_revalidate flag is true then the packet is
transmitted and a new sequence of events is started locally.
Revalidation begins with re-issuing a MARS_REQUEST for the group
being revalidated. The returned set of members {NewATM.1, NewATM.2,
.... NewATM.n} is compared with the set already held locally.
L_MULTI_DROPs are issued on the group"s VC for each node that appears
in the original set of members but not in the revalidated set of
members. L_MULTI_ADDs are issued on the group"s VC for each node that
appears in the revalidated set of members but not in the original set
of members. The VC_revalidate flag is reset when revalidation
concludes for the given group. Implementation specific mechanisms
will be needed to flag the "revalidation in progress" state.
The key difference between constructing a VC (section 5.1.3) and
revalidating a VC is that packet transmission continues on the open
VC while it is being revalidated. This minimises the disruption to
existing traffic.
The algorithm for initiating revalidation is:
- When a packet arrives for transmission on a given group,
the groups membership is revalidated if VC_revalidate == TRUE.
Revalidation resets VC_revalidate.
- When an event occurs that demands revalidation, every
group has its VC_revalidate flag set TRUE at a random time
between 1 and 10 seconds.
Benefit: Revalidation of active groups occurs quickly, and
essentially idle groups are revalidated as needed. Randomly
distributed setting of VC_revalidate flag improves chances of
staggered revalidation requests from senders when a sequence number
jump is detected.
5.1.5.1 When leaf node drops itself.
During the life of a multipoint VC an ERR_L_DROP may be received
indicating that a leaf node has terminated its participation at the
ATM level. The ATM endpoint associated with the ERR_L_DROP MUST be
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 the
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 shut
down. Further packet transmission to the group served by this VC will
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 jump
is detected upon receipt of a MARS_JOIN or a MARS_LEAVE then every
outgoing multicast VC MUST have its VC_revalidate flag set true at
some random interval between 1 and 10 seconds from when the CSN jump
was detected.
The only exception to this rule is if a sequence number jump is
detected during the establishment of a new group"s VC (i.e. a
MARS_MULTI reply was correctly received, but its mar$msn indicated
that some previous MARS traffic had been missed on ClusterControlVC).
In this case every open VC, EXCEPT the one just established, MUST
have its VC_revalidate flag set true at some random interval between
1 and 10 seconds from when the CSN jump was detected. (The VC being
established at the time is considered already validated.)
5.1.6 "Migrating" the outgoing multipoint VC
In addition to the group tracking described in section 5.1.4, the
transmit side of a cluster member must respond to "migration"
requests by the MARS. This is triggered by the reception of a
MARS_MIGRATE message from ClusterControlVC. The MARS_MIGRATE message
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.
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$sha roctets source ATM number
mar$ssa qoctets source ATM subaddress
mar$spa soctets source protocol address<