Internet-Draft Grenville Armitage
Bellcore
February 4th, 1995
Support for Multicast over UNI 3.1 based ATM Networks.
<draft-ietf-ipatm-ipmc-04.txt>
Status of this Memo
This document was submitted to the IETF IP over ATM WG. Publication
of this document does not imply acceptance by the IP over ATM WG of
any ideas expressed within. Comments should be submitted to the ip-
atm@matmos.hpl.hp.com mailing list.
Distribution of this memo is unlimited.
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Abstract
This memo describes a Multicast Address Resolution Server (MARS)
architecture that allows ATM based IP hosts to support RFC 1112 style
Level 2 IP multicast using the ATM Forum's UNI 3.1 point to
multipoint connection service. It also describes how this
architecture can be generalized to support other protocols wishing to
multicast over UNI 3.1 based ATM service.
[Editorial note: The differences between this version and 03.txt
are substantial in the area of multicast server support. This
impacts on Chapter 8, and anything referring to MARS_MSERV. Two
control VCs have been identified and named, two sequence numbers
are now used, and three major appendices have been added
discussing issues that cannot at this time be standardized. The
MARS_JOIN/LEAVE message format has been extended by 32 bits, and
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modified to support multiple address groups. Scattered
editorial/clarificatory changes have been made to the rest of the
document. Editorial notes will be removed.]
1. Introduction.
Multicast support allows a source host or protocol entity to send a
packet to multiple destinations simultaneously using a single, local
'transmit' operation. This facility is utilized by network layer
protocols such IP. Most models, like the one described in RFC 1112
[1] for IP multicasting, assume sources may send their packets to an
abstract 'multicast group addresses'. Link layer support for such an
abstraction is assumed to exist, and is provided by technologies such
as Ethernet.
ATM is being utilized as a new link layer technology to support a
variety of protocols, including IP. With RFC 1483 [2] the IETF
defined a multiprotocol mechanism for encapsulating and transmitting
packets using AAL5 over ATM Virtual Channels (VCs). However, the ATM
Forum's currently published signalling specification (UNI 3.0 [4],
with additions for UNI 3.1 released in late 1994) does not provide
the multicast address abstraction. Unicast connections are supported
by point to point, bidirectional VCs. Multicasting is supported
through point to multipoint VCs. The key limitation is that 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.
The main goal of this document is to define an address registration
and distribution mechanism that allows UNI 3.1 based networks to
support the multicast service of protocols such as IP. The second
goal is to define specific endpoint behaviour and management of point
to multipoint VCs. As the IETF is currently in the forefront of
using wide area multicasting this document's descriptions will focus
on IP version 4 (IPv4). A final chapter will note the more general
application of the architecture.
The Multicast Address Resolution Server (MARS), a distant relative of
the ATM ARP Server introduced in RFC 1577 [3], acts as a registry of
multicast group membership. MARS messages, based on the ATM ARP
format, support the distribution of multicast group membership
information between MARS and hosts or endpoints. Endpoint address
resolution entities query the MARS when a multicast group address
needs to be resolved. The actual mechanism for multicasting data
packets may be through meshes of point to multipoint VCs, or the use
of Multicast Servers. To provide for asynchronous notification of
group membership changes the MARS manages two point to multipoint VCs
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- one out to all endpoints desiring multicast support, and the other
to all multicast servers registered as providing support to any
multicast groups. The choice of mesh or multicast server is
configurable on a group by group basis.
The numerical size of link layer multicast groups will be constrained
by practical concerns such as limited VC support within endpoint ATM
interfaces. Each MARS manages a 'cluster' of ATM-attached endpoints.
A cluster is defined as a set of endpoints willing to be grouped
together as link layer members of multicast groups. It is assumed
that specially configured routers are used to pass multicast traffic
between clusters. This document explicitly avoids specifying the
nature of inter-cluster multicast routing protocols.
The mapping of clusters to other constrained sets of endpoints (such
as Logical IP Subnets) is left to network administrators. A simple
approach in overlaid IP environments would be for each LIS to be
served by a separate MARS, with the cluster being built from the LIS
members. IP multicast routers would interconnect each LIS as they do
with conventional subnets. However, there is no requirement that a
cluster be limited to a single LIS.
Section 2 provides an overview of IP multicast and what RFC 1112
required from Ethernet. Section 3 outlines the set of generic
functions that should be available to clients of a local host's UNI
3.1 signalling service. Section 4 specifies the encapsulation to be
used for MARS messages and multicast packet traffic. The basic
behaviour for the sending side of an interface is described in
section 5, with section 6 covering the mechanism whereby a host joins
and leaves multicast groups. Sections 7 covers the way in which hosts
respond to dynamic group membership changes. Configuring the use of
Multicast Servers is covered in section 8. Support for multicast
routers is described in section 9, and section 10 explains the
features included to improve the reliability of the membership
management mechanisms. Section 11 discusses the application of this
document beyond IP. Section 12 is a summary of the documents key
points.
The appendices provide discussion on issues that arise out the
implementation of this memo. Appendix A discusses MARS and endpoint
algorithms for parsing MARS messages. Appendix B describes the
particular problems introduced by the current IGMP paradigms, and
possible interim work-arounds. Finally, Appendix C covers the various
designs that are possible for multicast server support within
clusters.
This document assumes an understanding of concepts explained in
greater detail in RFC 1112, RFC 1577, UNI 3.1, and <draft-ietf-
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ipatm-sig-02.txt>.
2. Review of RFC 1112 and IP Multicast over Ethernet.
Under IP version 4 (IPv4) ddresses in the range of 224.0.0.0 and
239.255.255.255 are termed 'Class D' or 'multicast group' addresses.
In RFC 1112 the behaviour of the transmit and receive sides are quite
independent, making the concept of being a 'member' of an IP
multicast group imprecise at the link layer interface.
The 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. 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 affect on other hosts
attached to the Ethernet.
IGMP is defined in RFC 1112 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. All IP multicast hosts must
issue JoinLocalGroup for 224.0.0.1 during their initialisation. Each
host keeps a list of IP multicast groups it has been JoinLocalGroup'd
to. When a router issues an IGMP Query on 224.0.0.1 each host begins
to send IGMP Reports for each group it is a member of. IGMP Reports
are sent to the group address, not 224.0.0.1, "so that other members
of the same group on the same network can overhear the Report" and
not bother sending one of their own. IP multicast routers conclude
that a group has no members on the subnet when IGMP Queries no longer
elict associated replies.
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3. Multicast support under UNI 3.1.
This document will describe its operation in terms of 'generic'
functions that should be available to clients of a UNI 3.1 signalling
entity in a given ATM endpoint. The ATM model broadly describes 'AAL
Users' as any entity that establishes and manages VCs and underlying
AAL service to exchange data. An IP over ATM interface is a form of
'AAL User' (either directly, when VC multiplexing is used, or
indirectly, when LLC/SNAP encpasulation is used).
The most fundamental limitations of UNI 3.1's multicast support are:
Only point to multipoint, unidirectional VCs may be established.
Only the root node of a given VC may add or remove leaf nodes.
Within these constraints, multicast group members can communicate by
the use of multicast meshes, or multicast servers. With a mesh each
transmitting host is the Root of a point to multipoint VC that has
every other host in the group as a Leaf. The Multicast Server model
has every group member send their packets directly to a 'server'
entity somewhere on the ATM cloud, which then retransmits copies to
all other members.
This document defines the MARS-Endpoint signalling required to
support both mechanisms. Issues relating to the architecture,
operation, and management of multicast servers are discussed in
Appendix C.
The following generic signalling functions are presumed to be
available to local AAL Users:
L_CALL_RQ - Establish a unicast VC to a specific endpoint.
L_MULTI_RQ - Establish multicast VC to a specific endpoint.
L_MULTI_ADD - Add new leaf node to previously established VC.
L_MULTI_DROP - Remove specific leaf node from established VC.
L_RELEASE - Release unicast VC, or all Leaves of a multicast VC.
The signalling exchanges and local information passed between AAL
User and UNI 3.1 signalling entity with these functions is currently
beyond the scope of this document.
The following indications are assumed to be available to AAL Users,
generated by by the local UNI 3.1 signalling entity:
L_ACK - Succesful completion of a request to signalling
entity.
L_REMOTE_CALL - A new VC has been established to the AAL User.
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ERR_L_RQFAILED - A remote ATM endpoint rejected an L_CALL_RQ,
L_MULTI_RQ, or L_MULTI_ADD.
ERR_L_RELEASE - A remote ATM endpoint has elected to terminate a
pre-existing VC.
The signalling exchanges and local information passed between AAL
User and UNI 3.1 signalling entity with these functions is currently
beyond the scope of this document.
UNI 3.1 defines two ATM address formats - E.164 and ISO NSAP. In UNI
3.1 an 'ATM Number' is the primary identification of an ATM endpoint,
and it may use either format. Under some circumstances an ATM
endpoint must be identified by both an E.164 address (identifying the
attachment point of a private network to a public network), and an
ISO NSAP address ('ATM Subaddress') identifying the final endpoint
within the private network. For the rest of this document the term
'ATM Address' will be used to mean either a single 'ATM Number' or an
'ATM Number' combined with an 'ATM Subaddress'.
4. Overview of the Multicast Address Resolution Server.
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. This is the cluster's
Primary MARS. If a cluster is to be served by a backup MARS,
endpoints are configured with the ATM address of a Secondary MARS.
Section 10 will discuss the relationship between the Primary MARS and
Secondary MARS during failure conditions. Although a Secondary MARS
is optional, endpoint implementations must be capable of utilizing
them as described in section 10. References to 'the MARS' in
following sections will be assumed to mean the acting MARS for the
cluster.
Architecturally the MARS is similar to the RFC 1577 ARP Server,
although there is little overlap between the information they manage.
Whilst the ARP Server keeps a table of {IP,ATM} address pairs for all
IP endpoints in the LIS, the MARS keeps extended tables of {multicast
address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either be
configured with certain mappings, or dynamically 'learn' mappings.
The MARS distributes group membership information to cluster members
over a point to multipoint VC known as the ClusterControlVC. When
supporting multicast servers within a cluster, the MARS also
establishes a separate point to multipoint VC known as the
ServerControlVC. All cluster members are leaf nodes of
ClusterControlVC. All registered multicast servers are leaf nodes of
ServerControlVC (Section 8 will discuss the use of ServerControlVC).
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The MARS message format is an extension of the ATM ARP message
format. By default all MARS messages MUST be LLC/SNAP encapsulated
in accordance with RFC 1483, using the same encapsulation as ATM ARP:
LLC = 0xAA-AA-03
OUI = 0x00-00-00
PID = 0x08-06
The default for data traffic carried on point to multipoint VCs is
LLC/SNAP encapsulation with a header appropriate to the protocol
being carried. For IP traffic this is defined in RFC 1483 as:
LLC = 0xAA-AA-03
OUI = 0x00-00-00
PID = 0x08-00
The choice of common encapsulation and message format means that MARS
and ARP Server functionality may be implemented within a common
entity if a network designer so chooses.
5. Transmitting to Multicast groups.
[Editorial note: This section has discarded the MARS_MSERV
function of version ipmc-03.txt. MARS_MSERV is now used in an
entirely different fashion. Endpoint VC management is now
entirely independent of whether the group is mesh or mc server
supported.]
The following description will be in terms of an IP/ATM interface
that is capable of transmitting packets to a Class D address at any
time, without prior warning.
When a packet arrives for transmission, and there is no outgoing VC
already marked as serving the packet's multicast destination address,
the MARS is queried for the set of ATM endpoints currently making up
the multicast group.
The query is executed by issuing a MARS_REQUEST. The MARS_REQUEST
message is formatted as an ATM ARP_REQUEST with type code of 11
(decimal). The reply from the MARS may take one of two forms:
MARS_MULTI - Sequence of MARS_MULTI messages return the set of
endpoints in the group.
MARS_NAK - No mapping found, group is empty.
The request/response traffic MUST occur on a point to point VC
established by the host to the MARS. Where the MARS and ARP Server
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are co-resident, this VC may be shared between ATM ARP traffic and
MARS traffic.
5.1 Retrieving Group Membership from the MARS.
If the MARS had no mapping for the desired Class D address a MARS_NAK
will be returned. In this case the IP packet MUST be discarded
silently. If a match is found in the MARS's tables it proceeds to
return addresses ATM.1 through ATM.n in a sequence of one or more
MARS_MULTIs. A simple mechanism is used to detect and recover from
loss of MARS_MULTI messages.
Each MARS_MULTI carries a new boolean field x, and a 15 bit integer
field y - expressed as MARS_MULTI(x,y). Field y acts as a sequence
number, starting at 1 and incrementing for each MARS_MULTI sent.
Field x acts as an 'end of reply' marker. When x == 1 the MARS
response is considered complete.
In addition, each MARS_MULTI may carry multiple ATM addresses from
the set {ATM.1, ATM.2, .... ATM.n}. A MARS MUST minimise the number
of MARS_MULTIs transmitted by placing as many group member's
addresses in a single MARS_MULTI as possible. The limit on MARS_MULTI
message length MUST be the MTU of the underlying VC.
Assume n ATM addresses must be returned, each MARS_MULTI is limited
to only p ATM addresses, and p << n. This would require a sequence of
k MARS_MULTI messages (where k = (n/p)+1, using integer arithmetic),
transmitted as follows:
MARS_MULTI(0,1) carries back {ATM.1 ... ATM.p}
MARS_MULTI(0,2) carries back {ATM.(p+1) ... ATM.(2p)}
[.......]
MARS_MULTI(1,k) carries back { ... ATM.n}
If k == 1 then only MARS_MULTI(1,1) is sent.
Typical failure mode will be losing one or more of MARS_MULTI(0,1)
through MARS_MULTI(0,k-1). This is detected when y jumps by more than
one between consecutive MARS_MULTI's. An alternative failure mode is
losing MARS_MULTI(1,k). A timer MUST be implemented to flag the
failure of the last MARS_MULTI to arrive. A default value of 10
seconds is suggested.
If a 'sequence jump' is detected, the host MUST wait for the
MARS_MULTI(1,k), discard all results, and repeat the MARS_REQUEST.
If a timeout occurs, the host MUST discard all results, and repeat
the MARS_REQUEST.
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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. 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.
5.2 MARS_REQUEST, MARS_MULTI, MARS_MSERV, and MARS_NAK formats.
MARS_REQUEST is based on an ATM ARP_REQUEST, but with an 'operation
type value' of 11 (decimal). The multicast address being resolved is
placed into the the target protocol address field (ar$tpa). The
hardware type (ar$hrd) is set to 19 (decimal), and in IP environments
the protocol type is 2048 (decimal). Section 6.6 of RFC 1577 should
be consulted for specific details and coding of the ar$shtl, ar$sstl,
ar$thtl, and ar$tstl fields.
MARS_NAK is the MARS_REQUEST returned with operation type value of 16
(decimal).
The MARS_MULTI message is identified by an 'operation type value' of
12 (decimal). The message format is:
Data:
ar$hrd 16 bits Hardware type ( 19 decimal, 0x13 hex)
ar$pro 16 bits Protocol type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_MULTI)
ar$spln 8 bits Length of source protocol address (s)
ar$thtl 8 bits Type & length of target ATM number (x)
ar$tstl 8 bits Type & length of target ATM subaddress (y)
ar$tpln 8 bits Length of target multicast group address (z)
ar$tnum 16 bits Number of target ATM addresses returned (N).
ar$seqxy 16 bits Boolean flag x and sequence number y.
ar$msn 32 bits MARS Sequence Number.
ar$sha qoctets source ATM number
ar$ssa roctets source ATM subaddress
ar$spa soctets source protocol address
ar$tha.1 xoctets target ATM number 1
ar$tsa.1 yoctets target ATM subaddress 1
ar$tpa zoctets target multicast group address
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ar$tha.2 xoctets target ATM number 2
ar$tsa.2 yoctets target ATM subaddress 2
[.......]
ar$tha.N xoctets target ATM number N
ar$tsa.N yoctets target ATM subaddress N
ar$seqxy is coded with flag x in the leading bit, and sequence number
y coded as an unsigned integer in the remaining 15 bits.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x| y |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ar$tnum is an unsigned integer indicating how many pairs of
{ar$tha,ar$tsa} (i.e. how many group member's ATM addresses) are
present in the message. ar$msn is an unsigned 32 bit number filled in
by the MARS before transmitting each MARS_MULTI. Its use is described
further in section 10. Section 6.6 of RFC 1577 should be consulted
for specific details and coding of all other fields.
As an example, assume we have a multicast cluster using 4 byte
protocol addresses, 20 byte ATM numbers, and 0 byte ATM subaddresses.
For n group members in a single MARS_MULTI we require a (44 + 20n)
byte message. If we assume the default MTU of 9180 bytes, we can
return a maximum of 456 group member's addresses in a single
MARS_MULTI.
5.3 Establishing the Multicast 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.n, followed
by an L_MULTI_ADD for every member of the set {ATM.1, ....ATM.(n-1)}
(assuming the set is non-null). The packet is then transmitted over
the newly created VC just as it would be for 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 is is possible that one or more
returned endpoints may reject an L_MULTI_RQ or L_MULTI_ADD. If this
occurs then the endpoint's ATM address is dropped from the set
{ATM.1, ATM.2, .... ATM.n} returned by the MARS, and the creation of
the multipoint VC continues.
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Multicast VCs have the potential to be expensive in their use of
resources. Therefore 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. Algorithms for performing this type of
optimization are not discussed here, and are not required for
conformance with this memo.
Section 7 describes the endpoint's response to group membership
changes while the VC is open. Section 10 describes the mechanism for
ensuring hosts remain up to date with changes that occur while the VC
is open.
6. Joining and Leaving Multicast Groups.
A cluster member is a 'group member' (in the sense that it receives
packets directed at the group) when its ATM address appears in the
MARS's table entry for the group's multicast address. A key
requirement within each cluster is the distribution of group
membership information between the MARS and cluster members.
Two new messages are defined: MARS_JOIN and MARS_LEAVE. These are
sent to the MARS by endpoints joining or leaving a multicast group.
The MARS propagates these messages back out to the cluster over its
ClusterControlVC, to ensure the knowledge is distributed in a timely
fashion. ClusterControlVC is an outgoing, point to multipoint VC with
each cluster member as a leaf node.
RFC1112 expects that IP multicast routers are capable of behaving
'promiscuously'. This functionality may be emulated by allowing
routers to request that the MARS returns them as 'wild card' members
of all Class D addresses. However, a problem inherent in the current
ATM model is that completely promiscuous behaviour may be wasteful of
reassembly resources on the router's ATM interface. This document
describes a generalisation to the notion of 'wild card' entries,
enabling routers to limit themselves to 'blocks' of the Class D
address space. The application of this facility is described in
greater detail in Section 9.
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A block can be as small as 1 (a single group) or as large as the
entire Class D address space (default IPv4 'promiscuous' behaviour).
A block is defined as all addresses between, and inclusive of, a
<min,max> address pair.
The key extensions required to manage the MARS table entries are:
Two new message types:
MARS_JOIN carries one or more <min,max> pairs (specifying one
or more blocks of groups being joined) and a unicast ATM
address (of the node joining).
MARS_LEAVE carries one or more <min,max> pairs (specifying one
or more blocks of groups being left) and a unicast ATM address
(of the node leaving).
When a MARS_JOIN is received by the MARS it adds the specified ATM
address to the table entry for the specified multicast group
address(es).
When a MARS_LEAVE is received by the MARS it removes the specified
ATM address from the ARP entry for the specified multicast group
address(es).
MARS_JOIN and MARS_LEAVE messages arriving from individual hosts
are processed locally by the MARS and retransmitted on
ClusterControlVC (possibly after modification, as detailed in
Section 8).
All endpoints MUST ignore MARS_JOIN or MARS_LEAVE messages that
simply confirm information already held. The MARS retransmits
redundant messages, but otherwise takes no action. Section 7
describes how endpoints utilize retransmitted MARS_JOIN and
MARS_LEAVE messages.
Cluster members MUST only include a single <min,max> pair in each
JOIN/LEAVE message they issue. They MUST be able to process
multiple <min,max> pairs in JOIN/LEAVE messages received on
ClusterControlVC from the MARS (the interpretation being that the
join/leave operation applies to all addresses in range from <min>
to <max> inclusive, for every <min,max> pair).
In IPv4 environments JoinLocalGroup now results in two messages being
transmitted:
MARS_JOIN, sent over a VC to the ARP Server. It identifies the
single IP group being joined, and the host's unicast ATM address.
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An IGMP Report, except for 224.0.0.1 (in accordance with RFC1112).
In IPv4 environments LeaveLocalGroup now results in a MARS_LEAVE
being sent over a VC to the MARS, identifying the IP group being
left, and the host's unicast ATM address.
Endpoints with special requirements (e.g. multicast routers) may
directly issue MARS_JOINs and MARS_LEAVEs specifying blocks of
multicast group addresses. No IGMP Report is issued for such
operations in IP environments.
An endpoint must register with a MARS in order to become a member of
a cluster and be added as a leaf to ClusterControlVC. Registration
is covered in section 6.2.
6.1 Format of the MARS_JOIN and MARS_LEAVE Messages.
The MARS_JOIN message is indicated by an operation type value of 14
(decimal). MARS_LEAVE has the same format and operation type value of
15 (decimal). The message format is:
Data:
ar$hrd 16 bits Hardware type (19 decimal)
ar$pro 16 bits Protocol type
ar$shtl 8 bits Type & length of source ATM number (q)
ar$sstl 8 bits Type & length of source ATM subaddress (r)
ar$op 16 bits Operation code (MARS_JOIN or MARS_LEAVE)
ar$spln 8 bits Length of source protocol address (s)
ar$tpln 8 bits Length of multicast group address (z)
ar$pnum 16 bits Number of multicast group address pairs (N)
ar$resv 16 bits Reserved.
ar$msn 32 bits MARS Sequence Number.
ar$sha qoctets source ATM number (E.164 or ATM Forum NSAPA).
ar$ssa roctets source ATM subaddress (ATM Forum NSAPA).
ar$spa soctets source protocol address
ar$min.1 zoctets Minimum multicast group address - pair.1
ar$max.1 zoctets Maximum multicast group mask - pair.1
[.......]
ar$min.N zoctets Minimum multicast group address - pair.N
ar$max.N zoctets Maximum multicast group mask - pair.N
Refer to RFC 1577, section 6.6 for the coding of the ar$shtl and
ar$sstl fields. For conventional IPv4 environments ar$spln and
ar$tpln are both set to 4. Note that the message format differs from
ATMARP_REPLY in the fields after ar$op. ar$msn is an unsigned 32 bit
number filled in by the MARS before re-transmitting a MARS_JOIN or
MARS_LEAVE. The originator SHOULD set it to zero, although it will be
ignored by the MARS. Its use is described further in section 10.
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A join/leave message carries a set {<min,max>, <min,max>, ....,
<min,max>}, with at least one <min,max> pair. ar$pnum indicates how
many pairs are included in the message. To simplify MARS and endhost
interpretation, the following restrictions are imposed:
Assume max(N) is the <max> field from the Nth <min,max> pair.
Assume min(N) is the <min> field from the Nth <min,max> pair.
Assume a join/leave message arrives with K <min,max> pairs.
The following must hold:
max(N) < min(N+1) for 1 <= N < K
max(N) >= min(N) for 1 <= N <= K
In plain english, the set must specify an ascending sequence of
address blocks. The definition of "greater" or "less than" may be
protocol specific. In IP environments the addresses are treated as
simple unsigned binary values.
6.1.1 Important IPv4 default values.
The JoinLocalGroup and LeaveLocalGroup operations are only valid for
a single group. For any arbitrary group address X the associated
MARS_JOIN or MARS_LEAVE MUST specify a single pair <X, X>.
A router choosing to behave strictly in accordance with RFC1112 MUST
specify the entire Class D space. The associated MARS_JOIN or
MARS_LEAVE MUST specify a single pair <224.0.0.0, 239.255.255.255>.
The use of alternative <min, max> values is discussed in Section 9.
6.2 Registering with the MARS.
Two separate signalling paths exist between cluster members and their
associated MARS. The first is a transient point to point VC that
cluster members establish to the MARS when they need to issue
MARS_REQUESTs, MARS_JOINs, or MARS_LEAVEs. This VC is used by the
MARS to return MARS_MULTI messages. 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.
The second signalling path is ClusterControlVC. Every endpoint
registered as a cluster member is added as a leaf node to this VC,
which exists for the lifetime of the MARS. It is used to re-
distribute MARS_JOIN and MARS_LEAVE messages received by the MARS
from individual cluster members. Registration with the MARS as a
cluster member occurs when an endpoint issues a MARS_JOIN for a
protocol specific multicast group address. Once this occurs the
endpoint is added as a leaf node to ClusterControlVC.
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In IPv4 environments the 'all nodes' Class D address of 224.0.0.1 is
used to register with the MARS. RFC 1112 requires that all hosts
(including routers) that wish to participate in Level 2 IP
multicasting must explicitly issue a JoinLocalGroup for group
224.0.0.1 when they initialise (Level 1 is not supported by this
memo). The JoinLocalGroup to 224.0.0.1 will result in an MARS_JOIN
being transmitted from the host to the MARS.
If an IPv4 endpoint issues a LeaveLocalGroup for 224.0.0.1 it will
also be considered to have ceased membership of all other groups for
which it may have joined. The MARS MUST flush that endpoint's ATM
address from any Class D address entries it appears in. Finally, the
endpoint is released as a Leaf node from ClusterControlVC.
If the MARS receives an ERR_L_RELEASE on ClusterControlVC indicating
that a cluster member has died, that member's ATM address MUST be
removed from all groups for which it may have joined.
Registration of endpoints for other protocols is currently beyond the
scope of this document.
7. Endpoint management of point to multipoint VCs.
Once a cluster member has established a new VC to the members
returned in a MARS_MULTI response it must:
Monitor traffic on ClusterControlVC for updates to the group's
membership.
Revalidate a group's membership if a leaf node releases itself
from the VC.
7.1 Monitoring updates on ClusterControlVC.
When a cluster member joins or leaves a particular multicast group it
is not sufficient to simply update the mapping table in the cluster's
MARS. Endpoints that are already transmitting to the multicast
group's members must be informed of the change so they may add or
remove a leaf node as appropriate. Cluster members track MARS_JOIN
and MARS_LEAVE messages retransmitted by the MARS to determine when
another endpoint joins or leaves a group or block of groups.
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 hosts already sending to a given group will immediately add the
new member to their list of recipients. It also ensures that routers
joining a 'block' of groups are added by all endpoints currently
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sending to groups within the block.
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 hosts already sending to a given group will immediately drop the
old member from their list of recipients.
In an IPv4 environment any endpoint leaving 224.0.0.1 is assumed to
be ceasing support for IP multicast operation. If a MARS_LEAVE is
seen that refers to group 224.0.0.1 then the ATM address of the
endpoint specified in the message MUST be removed from every
multipoint VC on which it is listed as a leaf node.
The transmit side of the interface MUST NOT shut down an active VC to
a group for which the receive side has just executed a
LeaveLocalGroup. This behaviour is consistent with the model of
hosts transmitting to groups regardless of their own membership
status.
If a MARS_JOIN or MARS_LEAVE arrives with ar$pnum == 0 it carries no
<min,max> pairs, and is only used for validation as described in
section 10.
7.2 Revalidating when leaf nodes drop themselves.
During the life of a multipoint VC an ERR_L_RELEASE may be received
indicating that a leaf node has terminated its participation at the
ATM level. The ATM endpoint associated with the ERR_L_RELEASE MUST be
removed from the locally held set {ATM.1, ATM.2, .... ATM.n}
associated with the VC.
After a random period of time between 1 and 10 seconds the endpoint
MUST revalidate the associated group's membership by re-issuing a
MARS_REQEUEST. The returned set of members {NewATM.1, NewATM.2, ....
NewATM.n} is compared with 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.
8. Configuring for Multicast Servers or Multicast Meshes.
Endpoint's assume that all groups are supported by meshes of point to
multipoint VCs. Under certain circumstances the consumption of VCs
and AAL resources around the cluster can make meshes unattractive,
despite their performance advantages. The MARS protocol provides a
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mechanism for introducing multicast servers on a per-multicast group
basis, and in a manner that is completely transparent to cluster
members.
The multicast server has two key roles:
Providing one (or a limited number of) leaf nodes for outgoing VCs
from cluster members.
Constructing a single point to multipoint VC, with each group
memember as a leaf. This reduces the AAL consumption to one per
group, rather than one per sender per group.
The MARS must keep two sets of mappings for each multicast group
address supported by multicast servers. The original {multicast
address, ATM.1, ATM.2, ... ATM.n} mapping (the 'host map', although
it includes routers) is augmented by a parallel {multicast address,
server.1, server.2, .... server.K} mapping (the 'server map'). It is
assumed that no ATM addresses appear in both the server and host maps
for the same multicast group. Typically K will be 1, but it will be
larger when multiple multicast servers are configured to share the
data load of a given group.
When the MARS receives a MARS_REQUEST for a multicast address that
has both host and server maps it generates a response based on the
identity of the request's source. If the requestor is a member of the
server map for the requested group then the MARS returns the contents
of the host map in a sequence of one or more MARS_MULTIs. Otherwise
the MARS returns the contents of the server map in a sequence of one
or more MARS_MULTIs. Servers use the host map to establish a basic
distribution VC for the group. Cluster members will establish
outgoing multipoint VCs to members of the group's server map, without
being aware that their packets will not be going directly the
multicast group's members.
The MARS also maintains a point to multipoint VC out to any multicast
servers it is aware of, called ServerControlVC. This serves an
analogous role to ClusterControlVC, allowing the MARS to update the
servers with group membership changes as they occur.
A set of four MARS messages cover the current requirements:
MARS_MSERV Register as multicast server for one or more
groups.
MARS_UNSERV Deregister as multicast server for one or more
groups.
MARS_SJOIN A JOIN message on ServerControlVC.
MARS_SLEAVE A LEAVE message on ServerControlVC.
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MARS_SJOIN/SLEAVE are identical in format to MARS_JOIN/LEAVE, but
have different operation codes so that a node acting as both a
cluster member and multicast server may distinguish between updates
arriving on ServerControlVC and ClusterControlVC.
8.1 Registering and deregistering multicast servers.
MARS_MSERV and MARS_UNSERV are identical to the MARS_JOIN message.
MARS_MSERV uses the set {<min,max>, <min,max>, ...., <min,max>} to
specify one or more sets of multicast groups that a multicast server
is willing to support. MARS_UNSERV indicates the set of groups that
the multicast server is no longer willing to support. The operation
code for MARS_MSERV is 11 (decimal), and MARS_UNSERV is 17 (decimal).
When a node registers with MARS_MSERV the MARS adds the new ATM
address to the server maps for each specified group, possibly
constructing a new server map if this is the first multicast server
for the group. If the multicast server is not already a leaf node of
ServerControlVC it is added.
When a node deregisters with MARS_UNSERV the MARS removes its ATM
address from the server maps for each specified group, deleting the
server map if this was the only server for the group.
Both of these messages are sent to the MARS over a point to point VC,
and echoed on ServerControlVC by the MARS (section 10 covers the use
of this behaviour). The operation code is then changed to MARS_JOIN
or MARS_LEAVE respectively, and a copy of the original message is
transmitted on ClusterControlVC.
The MARS retransmits but otherwise ignores redundant MARS_MSERV and
MARS_UNSERV messages.
It is assumed that at least one server will have registered to
support a group before the first cluster member joins it. If a
MARS_MSERV arrives for a group that has a non-null host map but no
server map the default response of the MARS will be to drop the
MARS_MSERV without any further action. The originating multicast
server will eventually flag an error when repeated attempts to
register fail.
The opposite situation is where the last or only multicast server for
a group deregisters itself while the group still has members. The
default solution is for multicast servers to sever all VCs to which
they are attached as leaf nodes when they deregister, forcing any
active senders to the group to revalidate (as described in section
7). Since the MARS will have deleted the server map, the
revalidation will result in the host map being return, and the group
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reverts to being a mesh. This shall be the default mechanism until
future work develops a more elegant approach.
Appendix C discusses possible extensions to allow dynamic transitions
between mesh and multicast server support while a group is active.
However, these are not required for conformance with this memo.
8.2 Handling group membership changes.
The existence of multicast servers supporting some groups but not
others requires the MARS to intervene in the distribution of single
and block join/leave updates to cluster members. The MARS_SJOIN and
MARS_SLEAVE messages are identical to MARS_JOIN, with operation codes
18 and 19 (decimal) respectively. They exist to allow a node
combining cluster member and multicast server to distinguish between
information arriving on ClusterControlVC and ServerControlVC.
When a cluster member issues MARS_JOIN or MARS_LEAVE for a single
group, the MARS checks to see if the group has an associated server
map.
If the specified group does not have a server map the MARS_JOIN or
MARS_LEAVE is retransmitted on ClusterControlVC.
If it does have a server map two transmissions occur:
A copy is made with type MARS_SJOIN or MARS_SLEAVE as appropriate
and transmitted on ServerControlVC. This allows the server(s)
supporting the group to note the new member and add it as a leaf
node.
The original message's ar$pnum field is set to 0, and it is
transmitted back using the VC it arrived on (rather than
ClusterControlVC).
(Section 10 requires cluster members have a mechanism to confirm the
reception of their message by the MARS. For mesh supported groups,
using ClusterControlVC serves dual purpose of providing this
confirmation and distributing group update information. When using
multicast servers there is no reason for having all cluster members
process and discard null join/leave messages on ClusterControlVC).
Receipt of a block join/leave (e.g. from a router coming on-line)
requires a more complex response. Cluster members must be directly
informed of which mesh supported groups the block covers. Multicast
servers must also be informed in case they support one of the groups
covered by the block being joined.
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The solution is for the MARS to 'punch holes' in the block of
addresses supplied in the join/leave message, creating a set of
<min,max> pairs that excludes those addresses/groups supported by the
multicast servers. This hole-punched set is then sent out on
ClusterControlVC, ensuring the router is immediately noted by senders
to any mesh supported groups in the block. The original
MARS_JOIN/LEAVE is then converted to a MARS_SJOIN/SLEAVE and
transmitted on ServerControlVC. Appendix A discusses some algorithms
for 'hole punching'.
If punching holes in the originally specified block leaves a null
set, the ar$pnum field is set to zero before sending the modified
MARS_JOIN/LEAVE on ClusterControlVC.
8.3 Multicast server architectures.
Specification of multicast server architectures, and the
synchronisation of multiple multicast servers supporting single
multicast groups, is beyond the scope of this document and is
expected to be the subject of further work. Appendix C discusses some
possible approaches.
9. Utilizing blocks for for multicast routers.
Multicast routers are required for the propagation of multicast
traffic beyond the constraints of a single cluster. There is a sense
in which they are multicast servers acting at the next higher layer,
with clusters rather than individual endpoints as their abstract
sources and destinations.
Multicast routers typically participate in higher layer multicast
routing algorithms and policies that are beyond the scope of this
memo (e.g. DVMRP [5] in the IPv4 environment).
It is assumed that the multicast routers will be implemented over the
same sort of IP/ATM interface that a multicast host would use. They
will use the basic services described in the preceeding sections to
join and leave multicast groups as necessary, and will register with
the MARS as a cluster member.
The rest of this section will assume a simple IPv4 scenario where the
scope of a cluster has been limited to a particular LIS that is part
of an overlaid IP network. Not all members of the LIS are necessarily
registered cluster members.
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9.1 Sending to a Group.
If the multicast router needs to transmit a packet to a group within
the cluster it opens a VC in the same manner as a normal host would.
Once a VC is open, the router watches for MARS_JOIN and MARS_LEAVE
messages and responds to them as a normal host would.
The multicast router's transmit side MUST implement inactivity timers
to shut down idle outgoing VCs, as for normal hosts.
As with normal host, the multicast router does not need to be a
member of a group it is sending to.
9.2 Promiscuously Joining Groups.
Once registered and initialised, the simplest model of IPv4 multicast
router operation is for it to issue a MARS_JOIN encompassing the
entire Class D address space. In effect it becomes 'promiscuous', as
it will be a leaf node to all present and future multipoint VCs
established to IPv4 groups on the cluster.
How a router chooses which groups to propagate outside the cluster is
beyond the scope of this memo.
Consistent with RFC 1112, IP multicast routers may retain the use of
IGMP Query and IGMP Report messages to ascertain group membership.
9.3 Forward Multicast Traffic Across the cluster.
Under some circumstances the cluster may simply be another hop
between IP subnets that have participants in a multicast group.
[LAN.1] ----- IPmcR.1 -- [LIS] -- IPmcR.2 ----- [LAN.2]
LAN.1 and LAN.2 are subnets (such as Ethernet) with attached hosts
that are members of 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 the
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 multicast
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
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simply sends the packet to group X on the LIS interface as a normal
host would (Issuing MARS_REQUEST for group X, creating the VC,
sending the packet).
Assuming IPmcR.2 initialised itself with the MARS as a member of the
entire Class D space, it will have been returned as a member of X
even if no other nodes on the LIS were members. All packets for group
X received on IPmcR.2's LIS interface may be retransmitted on LAN.2.
If IPmcR.1 is similarly initialised the reverse process will apply
for multicast traffic from LAN.2 to LAN.1, for any multicast group.
The benefit of this scenario is that cluster members within the LIS
may also join and leave group X at anytime.
9.4 Restricted 'promiscous' Operation.
Both unicast and multicast IP routers have a common problem -
limitations on the number of AAL contexts available at their ATM
interfaces. Being 'promiscuous' in the RFC 1112 sense means that for
every M hosts sending to N groups, a multicast router's ATM interface
will have M*N incoming reassembly engines tied up.
It is not hard to envisage situations where a number of multicast
groups are active within the LIS but are not required to be
propagated beyond the LIS itself. An example might be a distributed
simulation system specifically designed to use the high speed IP/ATM
environment. There may be no practical way its traffic could be
utilised on 'the other side' of the multicast router, yet under the
conventional scheme the router would have to be a leaf to each
participating host anyway.
As this problem occurs at the link layer, it is worth noting that
'scoping' mechanisms at the IP multicast routing level do not provide
a solution.
In this situation the network administrator might configure their
multicast routers to exclude sections of the Class D address space
when issuing MARS_JOIN(s). Multicast groups that will never be
propagated beyond the cluster will not have the router listed as a
member by the MARS, and the router will never have to receive and
ignore traffic from those groups.
Another scenario involves the product M*N exceeding the capacity of a
single router's interface (especially if the same interface must also
support a unicast IP router service).
A network administrator may choose to add a second node, to function
as a parallel IP multicast router. Each router would be configured to
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be 'promiscuous' over separate parts of the Class D address space,
thus exposing themselves to only part of the VC load. This sharing
would be completely transparent to IP hosts within the LIS.
Restricted promiscuous mode does not break RFC 1112's use of IGMP
Report messages. If the router is configured to serve a given block
of Class D addresses, it will receive the IGMP Report. If the router
is not configured to support a given block, then the existence of an
IGMP Report for a group in that block is irrelevant to the router.
All routers are able to track membership changes through the
MARS_JOIN and MARS_LEAVE traffic anyway.
Mechanisms for establishing these modes of operation are beyond the
scope of this memo.
10. Robustness of interaction with the MARS.
Transient problems may result in the loss of messages between the
MARS, cluster members, and multicast servers. More serious problems
may result in the failure of the MARS itself. There are two problem
scenarios that are addressed. The first is the inability of a cluster
member to send messages to the MARS itself, either through cell loss
on the VC to the MARS, or the cluster member's inability to establish
a VC to the MARS.
The second is with the MARS_JOIN/SJOIN/LEAVE/SLEAVE messages re-
transmitted from the MARS. If a cluster member or multicast server
currently sending to a group misses an join update, the newly joined
member misses out on some traffic to the group. If a cluster member
or multicast server currently sending to a group misses a leave
update, the cluster member that left will continue to receive packets
unecessarily.
10.1 Ensuring the MARS hears you.
A simple algorithm solves the first problem. Cluster members
retransmit MARS_JOIN and MARS_LEAVE messages at regular intervals
until they receive a copy back again, either on ClusterControlVC or
the VC on which they are sending the messages. At this point the
local endpoint can be certain that at least the MARS received it.
Multicast servers retransmit MARS_MSERV and MARS_UNSERV messages at
regular intervals until they receive a copy back on ServerControlVC.
The interval should be no shorter than 5 seconds, and a default value
of 10 seconds is recommended. After 5 retransmissions the attempt
should be flagged locally as a failure. This should be considered as
a MARS failure, and handled as described in section 10.2.
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A 'copy' is defined as seeing a message of the same operation code
containing the local host's identity in the source address fields.
The <min,max> pair set is not checked, and does not have to be the
same (this is required so that cluster members may verify a MARS_JOIN
they've sent even if the MARS's hole-punching creates a totally
different set of <min,max> pairs).
10.2 Temporary failure of the MARS.
Two failure modes indicate problems with the MARS itself:
If an ERR_L_RELEASE occurs for the cluster member's attachment to
ClusterControlVC it may be assumed some problem exists with the
MARS.
If the cluster member receives ERR_L_RQFAILED when it attempts to
establish a point to point VC to the MARS in order to send MARS
messages.
The cluster member should wait a random period of time between 1 and
10 seconds before attempting to re-register with the MARS. If the
registration MARS_JOIN is successful (in accordance with section
10.1) then:
The cluster member MUST then proceed to rejoin every group that
its local higher layer protocol(s) have joined. It is recommended
that a random delay between 1 and 10 seconds be inserted before
the transmission of each MARS_JOIN.
Finally, using the mechanism described in section 7, the cluster
member MUST begin revalidating every multicast group it was
sending to.
The rejoin and revalidation procedure must not disrupt the cluster
member's use of multipoint VCs that were already open at the time
of the MARS failure.
If the re-registration with the Primary MARS fails, and there is no
configured Secondary MARS, the cluster member MUST wait for at least
1 minute before repeating the re-registration procedure. It is
RECOMMENDED that the cluster member signals an error condition in
some locally significant fashion.
If the re-registration with the Primary MARS fails, and a Secondary
MARS has been configured, the Secondary and Primary MARS addresses
are swapped and the cluster member immediately repeats the re-
registration procedure. If this is succesful the cluster member will
resume normal operation using the Secondary MARS. It is RECOMMENDED
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that the cluster member signals a warning of this condition in some
locally significant fashion.
If the attempt at re-registration with the Secondary MARS fails, the
cluster member MUST wait for at least 1 minute before reverting back
to the Primary MARS and starting the whole re-registration process
over again. In the worst case scenario this will result in cluster
members looping between registration attempts with the Primary MARS
and Secondary MARS until network administrators manually intervene.
Multicast servers shall behave in a similar manner to cluster members
on this issue.
10.3 The MARS Sequence Number.
There is an unsigned 32 bit sequence number identified as ar$msn in
most MARS messages. The following extensions govern its use:
The MARS keeps two independent counters, Cluster Sequence Number
(CSN) and Server Sequence Number (SSN). They are incremented every
time a message is sent out ClusterControlVC or ServerControlVC
respectively.
[Editorial note - in ipmc-03.txt the counter was incremented
only when a change occurred in the mapping tables. this is a
simplification.]
The current CSN is copied into the ar$msn field of MARS messages
being sent to cluster members (either out ClusterControlVC or on
an individual VC).
The current SSN is copied into the ar$msn field of MARS messages
being sent to multicast servers (either out ServerControlVC or on
an individual VC).
Cluster members and multicast servers track the increments of CSN
or SSN to determine if they have missed any update messages.
Calculations on the sequence numbers MUST be performed as unsigned 32
bit arithmetic, to ensure no glitches when the counters roll over.
Every cluster member keeps its own 32 bit Host Sequence Number (HSN)
to track the MARS's sequence number. Whenever a MARS_MULTI,
MARS_JOIN, or MARS_LEAVE is received the following check is then
performed on the ar$msn field of the new message:
Seq.diff = ar$msn - HSN
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ar$msn -> HSN
{...process MARS message as appropriate...}
if ((Seq.diff != 1) && (Seq.diff != 0))
then {...revalidate group membership information...}
The basic result is that the cluster member attempts to keep locked
in step with membership changes noted by the MARS. If it ever detects
that a membership change occurred (in any group) without it noticing,
it re-validates the membership of all groups it currently has
multicast VCs open to. Revalidation involves treating each VC as
though an ERR_L_RELEASE was received from a leaf node, and executing
the procedure described in section 7.
The ar$msn field of consecutive MARS_MULTIs sent in response to a
MARS_REQUEST must be constant. If the ar$msn field changes then all
the messages MUST be discarded at the completion of the response, and
the MARS_REQUEST re-issued.
One implication of this mechanism is that the MARS should serialize
its processing of 'simultaneous' MARS_REQUEST, MARS_JOIN and
MARS_LEAVE messages. Join and Leave operations should be queued
within the MARS along with MARS_REQUESTS, and not processed until all
the reply packets of a preceeding MARS_REQUEST have been transmitted.
The MARS is free to choose a value of CSN and SSN. When a new cluster
member starts up it should initialise HSN to zero. When the cluster
member sends the MARS_JOIN to register, the HSN will be correctly set
when it receives a copy of its MARS_JOIN from the MARS. If Seq.diff >
1 when the MARS_JOIN returns no action will be taken anyway, as the
host will not have any multicast related VCs established at this
stage.
If a sequence number jump occurs when establishing a new group's VC
the cluster member should not revalidate the membership of the group
it just established. The membership returned in the MARS_MULTIs that
carried the new ar$msn field should be considered already validated.
A MARS should be carefully designed to minimise the possibility of
CSN or SSN jumping unecessarily. Under normal operation only hosts
that are affected by transient link problems will miss ar$msn updates
and be forced to revalidate. If the MARS itself glitches it will be
innundated with requests for a period as every cluster member
attempts to revalidate.
Multicast servers should utilize the ar$msn fields in exactly the
same manner as cluster members. This will enable them to track the
SSN, and recover from missing any MARS_SJOIN/SLEAVE traffic.
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10.4 Why a Gobal sequence number?
The CSN and SSN are global within the context of a given protocol
(e.g. IP). They count ClusterControlVC and ServerControlVC activity
without reference to the multicast group(s) involved. This may be
perceived as a limitation, because there is no way for cluster
members or multicast servers to isolate exactly which multicast group
they may have missed an update for. An alternative was to try and
provide a per-group sequence number.
Unfortunately per-group sequence numbers are not practical. The
current mechanism allows sequence information to be piggy-backed onto
MARS messages already in transit for other reasons. The ability to
specify blocks of multicast addresses with a single MARS_JOIN or
MARS_LEAVE means that a single message can refer to membership change
for multiple groups simultaneously. A single ar$msn field cannot
provide meaningful information about each group's sequence. Multiple
ar$msn fields would have been unwieldy.
Any MARS or cluster member that supports different protocols MUST
keep separate mapping tables and sequence numbers for each protocol.
10.5 Synchronizing the Primary and Secondary MARS.
If a Secondary MARS exists for a given cluster then some mechanism is
needed to ensure reasonable consistency between its mapping tables
and those of the Primary MARS, especially as cluster members will
only ever register with one MARS. The inter-server protocol also
needs to cope with post-failure situations where some cluster members
end up registered with the Primary and others with the Secondary.
The definition of an inter-server protocol is beyond the current
scope of this document, and is expected to be the subject of further
work in the area.
11. Using the MARS in non-IP environments.
An deliberate attempt has been made to describe the MARS and
associated mechanisms in a manner independent of a specific higher
layer protocol being run over the ATM cloud. The immediate
application of this document will be in an IPv4 environment, and this
is reflected by the focus of key examples. However, the coding of
each MARS message means that any higher layer protocol identifiable
by a two byte Ethernet Type code can be supported by a MARS.
The 16 bit 'Protocol type' at the start of each MARS message, taken
from the set of Ethernet Type codes. Every MARS MUST implement
entirely separate logical mapping tables and support. Every cluster
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member must interpret messages from the MARS in the context of the
protocol type that the MARS message refers to.
The LLC/SNAP encapsulation specified in section 4 should not be
considered a hinderance in non-IP environments. Experimenters
deploying IPX or AppleTalk over ATM are encouraged to use the
architecture described in this document to support possible multicast
needs.
12. Key Decisions and open issues.
The key decisions this memo proposes:
A Multicast Address Resolution Server (MARS) is proposed to co-
ordinate and distribute mappings of ATM endpoint addresses to
arbitrary higher layer 'multicast group addresses'. The specific
case of IP version 4 multicast is used as the example.
Individual multicast groups may be supported by multicast meshes
between group members, or by multicast servers. The concept of
'clusters' is introduced to define the scope of a MARS's
responsibility, and the set of ATM endpoints willing to
participate in link level multicasting.
MARS message formats and encapsulation allow co-resident MARS and
ATM ARP Server implementations.
New message types: MARS_JOIN, MARS_LEAVE, MARS_REQUEST. Allow
endpoints to join, leave, and request the current membership list
of multicast groups.
New message type: MARS_MULTI. Allows multiple ATM addresses to be
returned by the MARS in response to a MARS_REQUEST.
New message types: MARS_MSERV, MARS_UNSERV. Allow multicast
servers to register and deregister themselves with the MARS.
New message types: MARS_SJOIN, MARS_SLEAVE. Allow MARS to pass on
group membership changes to multicast servers.
'wild card' MARS mapping table entries possible, where a single
ATM address is simultaneously associated with blocks of multicast
group addresses.
Some issues have not been addressed, although they may be in future
revisions.
MARS has no mechanism for realising cluster members have silently
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died.
The future development of ATM Group Addresses and Leaf Initiated
Join to ATM Forum's UNI specification has not been addressed. The
problems identified in this memo with respect to VC scarcity and
impact on AAL contexts will not be fixed by such developments in
the signalling protocol.
Security Consideration
Security consideration are not addressed in this memo.
Acknowledgments
The discussions within the IP over ATM Working Group have helped
clarify the ideas expressed in this document. John Moy of Cascade
Communications Corp. initially suggested the idea of wild-card
entries in the ARP Server. Drew Perkins of Fore Systems provided
rigorous and useful critique of early proposed mechanisms for
distributing and validating group membership information. Susan
Symington (and co-workers at MITRE Corp., Don Chirieleison, Rich
Verjinski, and Bill Barns) clearly articulated the need for multicast
server support, proposed a solution, and challenged earlier block
join/leave mechanisms.
Author's Address
Grenville Armitage
MRE 2P340, 445 South Street
Morristown, NJ, 07960-6438
USA
Email: gja@thumper.bellcore.com
References
[1] S. Deering, "Host Extensions for IP Multicasting", RFC 1112,
Standford University, August 1989.
[2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaption
Layer 5", RFC 1483, USC/Information Science Institute, July 1993.
[3] Laubach, M., "Classical IP and ARP over ATM", RFC1577, Hewlett-
Packard Laboratories, December 1993
[4] ATM Forum, "ATM User-Network Interface Specification Version
3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993
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[5] D. Waitzman, C. Partridge, S. Deering, "Distance Vector Multicast
Routing Protocol", RFC 1075, November 1988.
[6] M. Perez, F. Liaw, D. Grossman, A. Mankin, E. Hoffman, A. Malis,
"ATM Signaling Support for IP over ATM", Internet Draft, IP over ATM
Working Group, draft-ietf-ipatm-sig-02.txt, November, 1994.
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Appendix A. Parsing MARS messages.
Implementations are entirely free to comply with the body of this
memo in any way they see fit. This appendix is purely for
clarification.
A smart MARS implementation will pre-construct a set of <min,max>
pairs (P) that reflects the entire Class D space, excluding any
addresses currently supported by multicast servers. The <min> field
of the first pair MUST be 224.0.0.0, and the <max> field of the last
pair MUST be 239.255.255.255. The first and last pair may be the
same. This set is updated whenever a multicast server registers or
deregisters.
When the MARS must perform 'hole punching' it might consider the
following algorithm:
Assume the MARS_JOIN/LEAVE received by the MARS from the cluster
member specied the block <Emin, Emax>.
Assume Pmin(N) and Pmax(N) are the <min> and <max> fields from the
Nth pair in the MARS's current set P.
Assume set P has K pairs. Pmin(1) MUST equal 224.0.0.0, and
Pmax(M) MUST equal 239.255.255.255. (If K == 1 then no hole
punching is required).
Execute pseudo-code:
create copy of set P, call it set C.
index1 = 1;
while (Pmax(index1) <= Emin)
index1++;
index2 = K;
while (Pmin(index2) >= Emax)
index2--;
if (Pmin(index1) < Emin)
Cmin(index1) = Emin;
if (Pmax(index2) > Emax)
Cmax(index2) = Emax;
Set C is the required 'hole punched' set of address blocks.
The resulting set C retains all the MARS's pre-constructed 'holes'
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covering the multicast servers, but will have been pruned to cover
the section of the Class D space specified by the originating host's
<Emin,Emax> values.
The host end should keep a table, H, of open VCs in ascending order
of Class D address.
Assume H(x).addr is the Class address associated with VC.x.
Assume H(x).addr < H(x+1).addr.
The pseudo code for updating VCs based on an incoming JOIN/LEAVE
might be:
x = 1;
N = 1;
while (x < no.of VCs open)
{
while (H(x).addr > max(N))
{
N++;
if (N > no. of pairs in JOIN/LEAVE)
return(0);
}
if ((H(x).addr <= max(N) &&
((H(x).addr >= min(N))
perform_VC_update();
x++;
}
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Appendix B. Coping with IPv4 idiosyncracies.
Implementing any part of this appendix is not required for
conformance with this memo. It is provided solely to document issues
that have been identified.
The intent of section 5.3 is for cluster members to only have
outgoing point to multipoint VCs when they are actually sending data
to a particular multicast groups. However, in most IPv4 environments
the multicast routers attached to a cluster will periodically issue
IGMP Queries to ascertain if particular groups have members. The
current IGMP specification attempts to avoid having every group
member respond by insisting that each group member wait a random
period, and responding if no other member has responded before them.
The IGMP reply is sent to the multicast address of the group being
queried.
Unfortunately, as it stands the IGMP algorithm will be a nuisance for
cluster members that are essentially passive receivers within a given
multicast group. It is just as likely that a passive member, with no
outgoing VC already established to the group, will decide to send an
IGMP reply - causing a VC to be established were there was no need
for one. This is not a fatal problem for small clusters, but will
seriously impact on the ability of a cluster to scale.
Various solutions exist, providing short and long term solutions to
the problem. One long term solution would be to modify the IGMP
algorithm, for example:
If the group member has VC open to the group proceed as per RFC
1112 (picking a random reply delay between 0 and 10 seconds).
If the group member does not have VC already open to the group,
pick random reply delay between 10 and 20 seconds instead, and
then proceed as per RFC 1112.
If even one group member is sending to the group at the time the IGMP
Query is issued then all the passive receivers will find the IGMP
Reply has been transmitted before their delay expires, so no new VC
is required. If all group members are passive at the time of the IGMP
Query then a response will eventually arrive, but 10 seconds later
than under conventional circumstances.
The preceeding solution requires re-writing existing IGMP code, and
implies the ability of the IGMP entity to ascertain the status of VCs
on the underlying ATM interface. This is not likely to be available
in the short term.
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One short term solution is to provide something like the preceeding
functionality with a 'hack' at the IP/ATM driver level within cluster
members. Arrange for the IP/ATM driver to snoop inside IP packets
looking for IGMP traffic. If an IGMP packet is accepted for
transmission, the IP/ATM driver can buffer it locally if there is no
VC already active to that group. A 10 second timer is started, and if
an IGMP Reply for that group is received from elsewhere on the
cluster the timer is reset. If the timer expires, the IP/ATM driver
then establishes a VC to the group as it would for a normal IP
multicast packet.
Some network implementors may find it advantageous to configure a
multicast server to support the group 224.0.0.1, rather than rely on
a mesh. Given that IP multicast routers regularly send IGMP queries
to this address, a mesh will mean that each router will permanently
consume an AAL context within each cluster member. In clusters served
by multiple routers the VC load within switches in the underlying ATM
network will become a scaling problem.
Finally, if a multicast server is used to support 224.0.0.1, another
ATM driver level hack becomes a possible solution to IGMP Reply
traffic. The ATM driver may choose to grab all outgoing IGMP packets
and send them out on the VC established for sending to 224.0.0.1,
regardless of the Class D address the IGMP message was actually for.
Given that all hosts and routers must be members of 224.0.0.1, the
intended recipients will still receive the IGMP Replies. The negative
impact is that all cluster members will receive the IGMP Replies.
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Appendix C. Issues relating to multicast servers.
Implementing any part of this appendix is not required for
conformance with this memo. It is provided to give some structure to
further research and development on multicast server support within
clusters.
Various items are not addressed by this memo. They include:
Automatic migration of cluster members from a mesh to a multicast
server while a group is active.
An elegant mechanism for migration of cluster members from
multicast servers back to a mesh while the group is active.
Additional intelligence in the MARS to perform load sharing
between multicast servers if more than one registers for the same
group.
If one or more multicast servers attempt to register for a group that
already has members, it would be nice to have current senders to the
group migrate their outgoing VCs from the actual cluster members to
the newly registered multicast server(s). One approach might be to
have the MARS issue a sequence of fabricated MARS_JOINs for the
multicast servers, followed by MARS_LEAVEs for each member of the
group's current host map. What load this would place on the MARS, and
its scalability, have not been considered.
An elegant mechanism for the reverse migration might well be based
around the reverse process. Issue MARS_JOINs for all entries in the
host map, then issue MARS_LEAVEs for all remaining entries in the
server map.
In case of groups served by multiple multicast servers, the current
expectation is that each server retrieves the entire group's
membership with MARS_REQUESTs. This memo expects there to be an
external mechanism for multiple multicast servers to synchronize the
load sharing amongst themselves. Whether the MARS should to be
extended to play a part is a subject for further work.
An issue not immediately related to the MARS architecture is whether
a multicast server retransmits using a point to multipoint VC out to
group members, or a set of one VC per group member. The first
approach makes better use of the underlying ATM fabric, but data
sources that are also members of the group will receive copies of
their own traffic back. The alternative avoids this problem, but at
the expense of consuming more VCs and bandwidth on the path out of
the multicast server itself.
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Situations where either issue is a problem should simply revert to
using a multicast mesh between participating endpoints, where the
source never sees copies of its own packets, and the multicasting
happens within the ATM switch fabrics.
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