Internet Engineering Task Force                            Talpade and Ammar
INTERNET-DRAFT                               Georgia Institute of Technology
                                                           December 26, 1996

                                                        Expires:  June, 1997

                   Multiple MCS support using an enhanced
                        version of the MARS server.

Status of this Memo

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The basic Multicast Server architecture for layer 3 multicast over ATM has
been described in draft-ietf-ion-marsmcs-01.txt.  It includes a mechanism
for fault tolerance using multiple MCSs.  However no support for sharing
senders/receivers of a group across multiple MCSs has been provided.  This
document aims to satisfy this need by involving an enhanced version of the
MARS in the load sharing and fault tolerance mechanism.  This approach is
an improvement over the previous one as it subverts the need for any
inter-MCS synchronization mechanisms.

INTERNET-DRAFT    <draft-talpade-ion-multiplemcs-01.txt>   December 26, 1996

1 Introduction

A solution to the problem of mapping layer 3 multicast service over the
connection-oriented ATM service provided by UNI 3.0/3.1, has been presented
in [GA96].  A Multicast Address Resolution Server (MARS) is used to
maintain a mapping of layer 3 group addresses to ATM addresses in that
architecture.  Two possible approaches exist for a source to multicast data
to group members (receivers).  It can either get the list of ATM addresses
constituting the group from the MARS, set up a point-to-multipoint virtual
circuit (VC) with the receivers as leaves, and then proceed to send data
out on it (the VC Mesh approach).  Alternatively, the source can make use
of a proxy Multicast Server (MCS). The source transmits data to such an
MCS, which in turn uses a point-to-multipoint VC to get the data to the
group members (the MCS approach).

The MCS approach has been briefly introduced in [GA96].  The basic MCS
architecture, along with MARS-MCS interactions, has been described in
[TA96].  An inter-MCS synchronization protocol based on HELLO messages
([LA96]) is used to support multiple MCSs for fault tolerance.  However no
support is provided for using the multiple MCSs for sharing the
senders/receivers of a group.  [TA96] thus allows atmost one MCS to be
active per group, with one or more MCSs designated as backups.

The possibility of load sharing is an important advantage of using multiple
MCSs.  Experiments ([TA96a] have demonstrated the bottleneck effect that a
single MCS can have on data traffic, which motivates the need for sharing
the senders of a group.  At the same time it is crucial to minimize the
synchronization mechanisms that would be necessary to achieve this goal.
One common feature of mechanisms that offer load sharing is the existence
of an entity which handles allocation of senders/receivers to the
individual MCSs that support a group.  The MARS is a repository of all
relevant information about the cluster (e.g.  senders/receivers of all
groups, MCSs in the cluster, groups supported by each MCS, etc).  It has
access to all the information that an allocation entity might require, and
thus blends naturally into the role.  Using the MARS in this way also
removes the need for any inter-MCS synchronization.  This is because the
allocation and consequent load sharing can take place transparent to the
MCSs supporting a group.  This document provides a means for supporting the
use of multiple MCSs per layer 3 group by involving the MARS in the
allocation mechanism.

The document currently provides an outline of the proposed mechanism, and
does not include a description of the details.  Familiarity of the reader
with the MARS approach as described in [GA96], and the MCS architecture
described in [TA96] is expected.  The main difference between the approach
described in [TA96] and this document is that [TA96] uses an inter-MCS

Talpade and Ammar                                                   [Page 2]

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synchronization mechanism to offer fault tolerance, whereas this document
offers fault tolerance as well as load sharing using the MARS as the
allocation mechanism and does not need any additional inter-MCS protocol.

2 Nonoptimality of using the SCSP approach for multiple MCS support

The Server Cache Synchronization Protocol (SCSP) has recently been proposed
([LA96]) for supporting the use of multiple ATMARP, MARS, NHRP servers.
SCSP essentially provides a means for synchronizing the caches (databases)
of the involved servers.  Thus any one of multiple servers can be used
transparently by clients.  The functionality provided by SCSP may lead to
arguments being made in favour of its use for supporting multiple MCSs
also, and not involve the MARS in the multiple MCS mechanism.  However this
is not an optimal approach, as shall be pointed out in the following

The primary function of the MCS is to be a forwarding entity of data
traffic, and not a repository of control information that can be accessed
by clients.  The opposite is true for the ATMARP, MARS and NHRP servers.
The MCS gets the information it needs from the MARS and not directly from
the clients.  This is another major difference between the MCS and the
other servers.  So an instance of the other servers can receive some
information from client A, with another instance receiving different
information from client B but not from client A. This can lead to
inconsistent caches between the two server instances, which necessitates
the use of SCSP to synchronize their caches.  This is not true for MCSs.
All of them use the MARS for obtaining requisite information, thus getting
it from a consistent source(1).  So there is no need to further synchronize
the MCS caches, thus obviating the need for SCSP.

Even if the MCSs were synchronized using SCSP, an additional entity would
be needed to allocate new senders/receivers.  This allocation entity would
probably be one of the multiple MCSs.  An additional mechanism will thus be
needed to ensure fault tolerance, as the allocation MCS is now a single
point of failure (possibly the Designated Server mechanism described in
[LA96]).  As opposed to this, the MARS is a repository of all the
information that may be needed by an allocation entity.  So it can make the
necessary decisions without needing any additional entity or mechanisms.

Also, using an inter-MCS synchronization protocol like SCSP would mean that
all senders would continue to transmit data to all MCSs, even when the
senders are being shared.  The MCSs would forward data received from
senders supported by it, dropping data received from other senders.  This
is highly inefficient in terms of bandwidth usage and processing.  Using
the MARS server avoids this problem, as the MARS can selectively inform
each sender about the MCS supporting it, thus making the sender transmit

1)this is true even for multiple MARS, as is discussed in section 3.3

Talpade and Ammar                                                   [Page 3]

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data to one MCS only.  Thus using the MARS is more efficient that using
SCSP for supporting multiple MCSs.

3 Overview of the multiple MCS approach

This section provides an overview of the proposed approach.  We do not
describe the details in this version of the draft, but provide a conceptual
introduction to the approach.

3.1 Design goals

   * An important consideration of the approach is to keep the clients and
     the MCSs ignorant of the existence of the allocation mechanism.  This
     simplifies their design and implementation.  It also facilitates
     interoperability between different versions of the clients and MCSs.

   * The MCS should receive and forward data from senders supported by it
     only.  It should not receive, and then have to drop, data from
     unsupported senders.

   * Another design goal is to minimize the complexity and fallibility of
     the allocation mechanism.  Both the above goals are achieved by using
     the MARS for the allocation mechanism.

   * The decision to share senders or receivers (not both) shall be made
     offline, and can be made on a per group basis.

3.2 Architecture

3.2.1 Additional functionality needed in the MARS

The MARS server as defined in [GA96] does not have the capability for
supporting multiple MCSs.  Additional functionality is needed for it to
perform as the allocation mechanism.  This functionality is in terms of
additional processing, and does not involve any new control messages.

   * The MARS should be able to allocate existing senders/receivers to the
     MCSs supporting a group which is transitioning from being VC Mesh based
     to being MCS based.  This allocation should be made such that
     load-sharing is achieved across the MCSs.

     Thus when sharing receivers, the MCSs should be informed of the subset
     of receivers they support in the MARSMULTI messages that the MARS
     sends in response to MARSREQUEST. So each MCS will only be made aware
     of a part of the receiver set, and shall add only those receivers to
     its outgoing point-to-multipoint data VC.

Talpade and Ammar                                                   [Page 4]

INTERNET-DRAFT    <draft-talpade-ion-multiplemcs-01.txt>   December 26, 1996

     When sharing senders, the MARS should allocate the senders such that
     each MCS supports a distinct subset of the sender set.  This can be
     done by having the MARS selectively indicate the MCS that a sender
     should use (possibly by round-robin) in the MARSMULTI message that the
     sender gets in response to a MARSREQUEST.

   * The MARS should maintain information about the current allocation map,
     i.e., information about which sender/receiver has been allocated to
     which MCS. The MARS should ensure that each sender/receiver is
     allocated to one and only one MCS and that all the senders/receivers
     have been allocated.

   * When sharing receivers, the MARS should allocate new receivers to one
     and only one MCS. This involves selectively forwarding MARSSJOINs to
     an MCS, using the point-to-point control VC that exists between each
     MCS and the MARS instead of the ServerControlVC. Similarly, the MARS
     should deallocate a receiver that leaves the group by forwarding the
     MARSSLEAVE on the relevant point-to-point VC only.

   * When sharing senders, the MARS should inform a new sender about exactly
     one MCS. No specific action is needed if an existing sender stops
     transmitting to a group.  An interesting point here is that there is no
     obvious way for the MARS to know about senders that have stopped
     transmitting to a group.  This has implications for maintaining the
     accuracy of the allocation map.  The MARS maintains load sharing across
     the MCSs by basing its allocation decisions on the map.  Hence an
     incorrect map may lead to improper load sharing.  This problem does not
     exist when sharing receivers as they are explicitly registered (using
     MARSJOIN/MARSLEAVE) with the MARS. We explain possible solutions in
     section 3.2.2.

3.2.2 Balancing the load when sharing senders

The MARS can learn about a new sender when it receives a MARSREQUEST
message from it.  However, senders do not explicitly deregister from the
MARS when they stop transmitting to the group.  So the MARS cannot update
its allocation map which may cause it to make incorrect allocation
decisions.  Thus load balancing may not be achieved.

A simplistic solution is to ignore the need for deregistration by the
senders.  This simplicity is ofcourse at the expense of the possibility of
improper load balancing.  However, assuming that all senders have a similar
lifetime, the rate of addition and deletion of senders to an MCSs' set (set
of senders currently supported by an MCS) will remain approximately the
same for all MCSs.

Another solution to the problem is to have the senders explicitly
register/deregister with the MARS. The MARSREQUEST message that a sender
transmits to the MARS can be used for registration, while a new message

Talpade and Ammar                                                   [Page 5]

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(MARSSNDR-LEAVE) can be used for deregistration from the MARS. The MARS can
thus be kept aware of the existing senders.  This solution however involves
changing the client behavior.  So [GA96] based clients will not be
considered by the MARS for load sharing decisions.  Also, compatibility
issues that arise due to existence of different versions of clients will
have to be resolved.

One can avoid changing the client behavior by making the MCS responsible
for deregistering senders.  The MCS terminates a VC from each sender to the
group (or atleast the senders being supported by it).  It is aware of the
state of each of these VCs, and so knows when such a VC is released.
Release of such a VC can be viewed as the sender terminating transmission
to the group.  The MCS can then inform the MARS of this event using a new
message (MARSSNDR-LEAVE), causing the MARS to update its allocation map.

It remains to be seen as to which of the above is a better approach for
solving the problem.

3.2.3 Interactions between the MARS and MCSs

As was indicated in section 3.1, an important goal of this approach is to
not involve the MCSs in the allocation mechanism.  An MCS must remain
oblivious of the existence of other MCSs supporting the same group.  Thus
each MCS is made aware of only the receivers supported by it, when sharing
receivers.  So the MARS-MCS interactions do not change at all from those
described in [TA96].

The only consideration needed in the MCS design is that an MCS should
accept MARSSJOIN and MARS-SLEAVE from the MARS on either the
ServerControlVC or the point-to-point VC. This is because the MARS informs
an MCS of new/leaving group member only if it allocates/has allocated that
member to that MCS.

3.2.4 Interactions between the MARS and clients

The interactions between the MARS and the clients also remain the same.
The only exception in this case is if the explicit deregistration mechanism
explained in section 3.2.2 is adopted for senders.  In that case the
clients will have to send a new message (MARSSNDR-LEAVE) when a group's
outgoing point-to-multipoint data VC is closed.  Closing the VC indicates
that the client is no longer sending to that group, and hence the MARS can
update its allocation map.

Talpade and Ammar                                                   [Page 6]

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3.3 Using multiple MCSs with multiple MARS

The SCSP approach provides a mechanism for supporting multiple MARS in a
cluster.  The multiple MCS mechanism that we have outlined above will work
with multiple MARS also.  In this case, the Designated Server (DS) protocol
described in [LA96] can be used to elect one of the MARS as the allocation
mechanism.  This DS will ensure consistency of the allocation map amongst
the MARS, and allocates senders/receivers amongst the multiple MCSs.  The
DS mechanism also has a recovery mechanism that can be used to elect a new
MARS to function as the DS, in case of failure of the existing one.

4 Summary

We propose a scheme for supporting multiple MCSs per group using the MARS
as the allocation mechanism.  This scheme subverts the need for an
inter-MCS synchronization protocol.  It requires enhancement to the current
MARS server architecture as described in [GA96].  We aim to minimize the
changes required to the client or MCS architecture in this scheme, thus
maintaining interoperability between different versions of clients and

Author's address

Rajesh Talpade - - (404)-894-6737
Mostafa H. Ammar - - (404)-894-3292

College of Computing
Georgia Institute of Technology
Atlanta, GA 30332-0280

Talpade and Ammar                                                   [Page 7]

INTERNET-DRAFT    <draft-talpade-ion-multiplemcs-01.txt>   December 26, 1996


[GA96]   Armitage, G.J., "Support for Multicast over UNI 3.0/3.1 based ATM
         networks", RFC 2022, November 1996.

[BK95]   Birman, A., Kandlur, D., Rubas, J., "An extension to the MARS
         model", Internet Draft, draft-kandlur-ipatm-mars-directvc-00.txt,
         November 1995.
[LM93]   Laubach, M., "Classical IP and ARP over ATM", RFC1577,
         Hewlett-Packard Laboratories, December 1993.

[LA96]   Luciani, J., G. Armitage, and J. Halpern, "Server Cache
         Synchronization Protocol (SCSP)", Internet Draft,
         draft-ietf-ion-scsp-00.txt, December 1996.

[TA96]   Talpade, R., and Ammar, M.H., "Multicast Server Architectures for
         UNI 3.0/3.1 based ATM networks", Internet Draft,
         draft-ietf-ion-marsmcs-01.txt, November 1996.
[TA96a]  Talpade, R., G. Armitage, M. Ammar, "Experience with Architectures
         for Supporting IP Multicast over ATM" , submitted to IEEE ATM '96
         Workshop, San Francisco, August 1996.

Talpade and Ammar                                                   [Page 8]