RFC2149 - Multicast Server Architectures for MARS-based ATM multicasting

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　　Network Working Group R. Talpade
Request for Comments: 2149 M. Ammar
Category: Informational Georgia Institute of Technology
May 1997

Multicast Server Architectures for MARS-based ATM multicasting

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Abstract

A mechanism to support the multicast needs of layer 3 protocols in
general, and IP in particular, over UNI 3.0/3.1 based ATM networks
has been described in RFC2022. Two basic approaches exist for the
intra-subnet (intra-cluster) multicasting of IP packets. One makes
use of a mesh of point to multipoint VCs (the 'VC Mesh' approach),
while the other uses a shared point to multipoint tree rooted on a
Multicast Server (MCS). This memo provides details on the design and
implementation of an MCS, building on the core mechanisms defined in
RFC2022. It also provides a mechanism for using multiple MCSs per
group for providing fault tolerance. This approach can be used with
RFC2022 based MARS server and clients, without needing any change in
their functionality.

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. It can be considered to be an
extended analog of the ATM ARP Server introduced in RFC1577
([ML93]). Hosts in the ATM network use the MARS to resolve layer 3
multicast addresses into corresponding lists of ATM addresses of
group members. Hosts keep the MARS informed when they need to join
or leave a particular layer 3 group.

The 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 AALSDUs 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.

A sender in the cluster has two options for multicasting data to the
group members. 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 group members as leaves, and then
proceed to send data out on it. 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 has been briefly introduced in [GA96]. This memo
presents a detailed description of MCS architecture and proposes a
simple mechanism for supporting multiple MCSs for fault tolerance.
We assume an understanding of the IP multicasting over UNI 3.0/3.1
ATM network concepts described in [GA96], and access to it. This
document is organized as follows. Section 2 presents interactions
with the local UNI 3.0/3.1 signaling entity that are used later in
the document and have been originally described in [GA96]. Section 3
presents an MCS architecture, along with a description of its
interactions with the MARS. Section 4 describes the working of an
MCS. The possibility of using multiple MCSs for the same layer 3
group, and the mechanism needed to support such usage, is described
in section 5. A comparison of the VC Mesh approach and the MCS
approach is presented in Appendix A.

2 Interaction with the local UNI 3.0/3.1 signaling entity

The following generic signaling functions are presumed to be
available to local AAL Users:

LCALL-RQ - Establish a unicast VC to a specific endpoint.
LMULTI-RQ - Establish multicast VC to a specific endpoint.
LMULTI-ADD - Add new leaf node to previously established VC.
LMULTI-DROP - Remove specific leaf node from established VC.
LRELEASE - Release unicast VC, or all Leaves of a multicast VC.

The following indications are assumed to be available to AAL Users,
generated by by the local UNI 3.0/3.1 signaling entity:

LACK - Succesful completion of a local request.
LREMOTE-CALL - A new VC has been established to the AAL User.
ERRL-RQFAILED - A remote ATM endpoint rejected an LCALLRQ,
LMULTIRQ, or L-MULTIADD.
ERRL-DROP - A remote ATM endpoint dropped off an existing VC.
ERRL-RELEASE - An existing VC was terminated.

3 MCS Architecture

The MCS acts as a proxy server which multicasts data received from a
source to the group members in the cluster. All multicast sources
transmitting to an MCS-based group send the data to the specified
MCS. The MCS then forwards the data over a point to multipoint VC
that it maintains to group members in the cluster. Each multicast
source thus maintains a single point-to-multipoint VC to the
designated MCS for the group. The designated MCS terminates one
point-to-multipoint VC from each cluster member that is multicasting
to the layer 3 group. Each group member is the leaf of the point-
to-multipoint VC originating from the MCS.

A brief introduction to possible MCS architectures has been presented
in [GA96]. The main contribution of that document concerning the MCS
approach is the specification of the MARS interaction with the MCS.
The next section lists control messages exchanged by the MARS and
MCS.

3.1 Control Messages exchanged by the MCS and the MARS

The following control messages are exchanged by the MARS and the MCS.

operation code Control Message

1 MARS_REQUEST
2 MARS_MULTI
3 MARS_MSERV
6 MARS_NAK
7 MARS_UNSERV
8 MARS_SJOIN
9 MARS_SLEAVE
12 MARS_REDIRECT_MAP

MARSMSERV and MARS-UNSERV are identical in format to the MARSJOIN
message. MARSSJOIN and MARS-SLEAVE are also identical in format to
MARSJOIN. As such, their formats and those of MARSREQUEST, MARS-
MULTI, MARSNAK and MARSREDIRECT-MAP are described in [GA96]. Their
usage is described in section 4. All control messages are LLC/SNAP
encapsulated as described in section 4.2 of [GA96]. (The "mar$"
notation used in this document is borrowed from [GA96], and indicates
a specific field in the control message.) Data messages are
reflected without any modification by the MCS.

3.2 Association with a layer 3 group

The simplest MCS architecture involves taking incoming AALSDUs from
the multicast sources and sending them out over the point-to-
multipoint VC to the group members. The MCS can service just one
layer 3 group using this design, as it has no way of distinguishing
between traffic destined for different groups. So each layer 3 MCS-
supported group will have its own designated MCS.

However it is desirable in the interests of saving resources to
utilize the same MCS to support multiple groups. This can be done by
adding minimal layer 3 specific processing into the MCS. The MCS can
now look inside the received AALSDUs and determine which layer 3
group they are destined for. A single instance of such an MCS could
register its ATM address with the MARS for multiple layer 3 groups,
and manage multiple point-to-multipoint VCs, one for each group.
This capability is included in the MCS architecture, as is the
capability of having multiple MCSs per group (section 5).

4 Working of MCS

An MCS MUST NOT share its ATM address with any other cluster member
(MARS or otherwise). However, it may share the same physical ATM
interface (even with other MCSs or the MARS), provided that each
logical entity has a different ATM address. This section describes
the working of MCS and its interactions with the MARS and other
cluster members.

4.1 Usage of MARSMSERV and MARS-UNSERV

4.1.1 Registration (and deregistration) with the MARS

The ATM address of the MARS MUST be known to the MCS by out-of-band
means at startup. One possible approach for doing this is for the
network administrator to specify the MARS address at command line
while invoking the MCS. On startup, the MCS MUST open a point-to-
point control VC (MARSVC) with the MARS. All traffic from the MCS to
the MARS MUST be carried over the MARSVC. The MCS MUST register with
the MARS using the MARS-MSERV message on startup. To register, a
MARSMSERV MUST be sent by the MCS to the MARS over the MARSVC. On
receiving this MARS-MSERV, the MARS adds the MCS to the
ServerControlVC. The ServerControlVC is maintained by the MARS with
all MCSs as leaves, and is used to disseminate general control
messages to all the MCSs. The MCS MUST terminate this VC, and MUST
expect a copy of the MCS registration MARSMSERV on the MARS-VC from
the MARS.

An MCS can deregister by sending a MARSUNSERV to the MARS. A copy of
this MARSUNSERV MUST be expected back from the MARS. The MCS will
then be dropped from the ServerControlVC.

No protocol specific group addresses are included in MCS registration
MARSMSERV and MARS-UNSERV. The mar$flags.register bit MUST be set,
the mar$cmi field MUST be set to zero, the mar$flags.sequence field
MUST be set to zero, the source ATM address MUST be included and a
null source protocol address MAY be specified in these MARSMSERV and
MARS-UNSERV. All other fields are set as described in section 5.2.1
of [GA96] (the MCS can be considered to be a cluster member while
reading that section). It MUST keep retransmitting (section 4.1.3)
the MARSMSERV/MARS-UNSERV over the MARSVC until it receives a copy
back.

In case of failure to open the MARSVC, or error on it, the
reconnection procedure outlined in section 4.5.2 is to be followed.

4.1.2 Registration (and deregistration) of layer 3 groups

The MCS can register with the MARS to support particular group(s).
To register groups X through Y, a MARSMSERV with a <min, max> pair of
<X, Y> MUST be sent to the MARS. The MCS MUST expect a copy of the
MARSMSERV back from the MARS. The retransmission strategy outlined in
section 4.1.3 is to be followed if no copy is received.

The MCS MUST similarly use MARSUNSERV if it wants to withdraw support
for a specific layer 3 group. A copy of the group MARSUNSERV MUST be
received, failing which the retransmission strategy in section 4.1.3
is to be followed.

The mar$flags.register bit MUST be reset and the mar$flags.sequence
field MUST be set to zero in the group MARSMSERV and MARS-UNSERV. All
other fields are set as described in section 5.2.1 of [GA96] (the MCS
can be considered to be a cluster member when reading that section).

4.1.3 Retransmission of MARSMSERV and MARS-UNSERV

Transient problems may cause loss of control messages. The MCS needs
to retransmit MARSMSERV/MARS-UNSERV at regular intervals when it does
not receive a copy back from the MARS. This interval should be no
shorter than 5 seconds, and a default value of 10 seconds is
recommended. A maximum of 5 retransmissions are permitted before a
failure is logged. This MUST be considered a MARS failure, which
SHOULD result in the MARS reconnection mechanism described in section
4.5.2.

A "copy" is defined as a received message with the following fields
matching the previously transmitted MARSMSERV/MARS-UNSERV:

- mar$op
- mar$flags.register
- mar$pnum
- Source ATM address
- first <min, max> pair

In addition, a valid copy MUST have the following field values:

- mar$flags.punched = 0
- mar$flags.copy = 1

If either of the above is not true, the message MUST be dropped
without resetting of the MARSMSERV/MARS-UNSERV timer. There MUST be
only one MARSMSERV or MARS-UNSERV outstanding at a time.

4.1.4 Processing of MARSMSERV and MARS-UNSERV

The MARS transmits copies of group MARSMSERV and MARS-UNSERV on the
ServerControlVC. So they are also received by MCSs other than the
originating one. This section discusses the processing of these
messages by the other MCSs.

If a MARSMSERV is seen that refers to a layer 3 group not supported
by the MCS, it MUST be used to track the Server Sequence Number
(section 4.5.1) and then silently dropped.

If a MARSMSERV is seen that refers to a layer 3 group supported by
the MCS, the MCS learns of the existence of another MCS supporting
the same group. This possibility is incorporated (of multiple MCSs
per group) in this version of the MCS approach and is discussed in
section 5.

4.2 Usage of MARSREQUEST and MARS-MULTI

As described in section 5.1, the MCS learns at startup whether it is
an active or inactive MCS. After successful registration with the
MARS, an MCS which has been designated as inactive for a particular
group MUST NOT register to support that group with the MARS. It
instead proceeds as in section 5.4. The active MCS for a group also
has to do some special processing, which we describe in that section.
The rest of section 4 describes the working of a single active MCS,
with section 5 describing the active MCSs actions for supporting
multiple MCSs.

After the active MCS registers to support a layer 3 group, it uses
MARSREQUEST and MARS-MULTI to obtain information about group
membership from the MARS. These messages are also used during the
revalidation phase (section 4.5) and when no outgoing VC exists for a
received layer 3 packet (section 4.3).

On registering to support a particular layer 3 group, the MCS MUST
send a MARSREQUEST to the MARS. The mechanism to retrieve group
membership and the format of MARSREQUEST and MARS-MULTI is described
in section 5.1.1 and 5.1.2 of [GA96] respectively. The MCS MUST use
this mechanism for sending (and retransmitting) the MARSREQUEST and
processing the returned MARSMULTI(/s). The MARS-MULTI MUST be
received correctly, and the MCS MUST use it to initialize its
knowledge of group membership.

On successful reception of a MARSMULTI, the MCS MUST attempt to open
the outgoing point-to-multipoint VC using the mechanism described in
section 5.1.3 of [GA96], if any group members exist. The MCS however
MUST start transmitting data on this VC after it has opened it
successfully with at least one of the group members as a leaf, and
after it has attempted to add all the group members at least once.

4.3 Usage of outgoing point-to-multipoint VC

Cluster members which are sources for MCS-supported layer 3 groups
send (encapsulated) layer 3 packets to the designated MCSs. An MCS,
on receiving them from cluster members, has to send them out over the
specific point-to-multipoint VC for that layer 3 group. This VC is
setup as described in the previous section. However, it is possible
that no group members currently exist, thus causing no VC to be
setup. So an MCS may have no outgoing VC to forward received layer 3
packets on, in which case it MUST initiate the MARSREQUEST and MARS-
MULTI sequence described in the previous section. This new MARSMULTI
could contain new members, whose MARSSJOINs may have been not
received by the MCS (and the loss not detected due to absence of
traffic on the ServerControlVC).

If an MCS learns that there are no group members (MARSNAK received
from MARS), it MUST delay sending out a new MARSREQUEST for that
group for a period no less than 5 seconds and no more than 10
seconds.

Layer 3 packets received from cluster members, while no outgoing
point-to-multipoint VC exists for that group, MUST be silently
dropped after following the guidelines in the previous paragraphs.
This might result in some layer 3 packets being lost until the VC is
setup.

Each outgoing point-to-multipoint VC has a revalidate flag associated
with it. This flag MUST be checked whenever a layer 3 packet is sent
out on that VC. No action is taken if it is not set. If it is set,
the packet is sent out, the revalidation procedure (section 4.5.3)
MUST be initiated for this group, and the flag MUST be reset.

In case of error on a point-to-multipoint VC, the MCS MUST initiate
revalidation procedures for that VC as described in section 4.5.3.
Once a point-to-multipoint VC has been setup for a particular layer 3
group, the MCS MUST hold the VC open and mark it as the outgoing path
for any subsequent layer 3 packets being sent for that group address.
A point-to-multipoint VC MUST NOT have an activity timer associated
with it. It is to remain up at all times, unless the MCS explicitly
stops supporting that layer 3 group, or no more leaves exist on the
VC which causes it to be shut down. The VC is kept up inspite of
non-existent traffic to reduce the delay suffered by MCS supported
groups. If the VC were to be shut down on absence of traffic, the VC
reestablishment procedure (needed when new traffic for the layer 3
group appears) would further increase the initial delay, which can be
potentially higher than the VC mesh approach anyway as two VCs need
to be setup in the MCS case (one from source to MCS, second from MCS
to group) as opposed to only one (from source to group) in the VC
Mesh approach. This approach of keeping the VC from the MCS open
even in the absense of traffic is experimental. A decision either
way can only be made after gaining experience (either through
implementation or simulation) about the implications of keeping the
VC open.

If the MCS supports multiple layer 3 groups, it MUST follow the
procedure outlined in the four previous subsections for each group
that it is an active MCS. Each incoming data AALSDU MUST be examined
for determining its recipient group, before being forwarded onto the
appropriate outgoing point-to-multipoint VC.

4.3.1 Group member dropping off a point-to-multipoint VC

AN ERRL-DROP may be received during the lifetime of a point-to-
multipoint VC indicating that a leaf node has terminated its
participation at the ATM level. The ATM endpoint associated with the
ERRL-DROP MUST be removed from the locally held set associated with
the VC. The revalidate flag on the VC MUST be set after a random
interval of 1 through 10 seconds.

If an ERRL-RELEASE is received for a VC, then the entire set is
cleared and the VC considered to be completely shutdown. A new VC
for this layer 3 group will be established only on reception of new
traffic for the group (as described in section 4.3).

4.4 Processing of MARSSJOIN and MARS-SLEAVE

The MARS transmits equivalent MARSSJOIN/MARS-SLEAVE on the
ServerControlVC when it receives MARSJOIN/MARS-LEAVE from cluster
members. The MCSs keep track of group membership updates through
these messages. The format of these messages are identical to
MARSJOIN and MARS-LEAVE, which are described in section 5.2.1 of
[GA96]. It is sufficient to note here that these messages carry the
ATM address of the node joining/leaving the group(/s), the group(/s)
being joined or left, and a Server Sequence Number from MARS.

When a MARSSJOIN is seen which refers to (or encompasses) a layer 3
group (or groups) supported by the MCS, the following action MUST be
taken. The new member's ATM address is extracted from the MARSSJOIN.
An L-MULTIADD is issued for the new member for each of those referred
groups which have an outgoing point-to-multipoint VC. An LMULTI-RQ is
issued for the new member for each of those refered groups which have
no outgoing VCs.

When a MARSSLEAVE is seen that refers to (or encompasses) a layer 3
group (or groups) supported by the MCS, the following action MUST be
taken. The leaving member's ATM address is extracted. An LMULTI-
DROP is issued for the member for each of the refered groups which
have an outgoing point-to-multipoint VC.

There is a possibility of the above requests (LMULTI-RQ or LMULTIADD
or LMULTI-DROP) failing. The UNI 3.0/3.1 failure cause must be
returned in the ERRL-RQFAILED signal from the local signaling 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 locally held view of the
group by the MCS. Otherwise, the request MUST be re-attempted with
increasing delay (initial value between 5 to 10 seconds, with delay
value doubling after each attempt) until it either succeeds or the
multipoint VC is released or a MARSSLEAVE is received for that group
member. If the VC is open, traffic on the VC MUST continue during
these attempts.

MARSSJOIN and MARS-SLEAVE are processed differently if multiple MCSs
share the members of the same layer 3 group (section 5.4). MARSSJOIN
and MARSSLEAVE that do not refer to (or encompass) supported groups
MUST be used to track the Server Sequence Number (section 4.5.1), but
are otherwise ignored.

4.5 Revalidation Procedures

The MCS has to initiate revalidation procedures in case of certain
failures or errors.

4.5.1 Server Sequence Number

The MCS needs to track the Server Sequence Number (SSN) in the
messages received on the ServerControlVC from the MARS. It is carried
in the mar$msn of all messages (except MARSNAK) sent by the MARS to
MCSs. A jump in SSN implies that the MCS missed the previous
message(/s) sent by the MARS. The MCS then sets the revalidate flag
on all outgoing point-to-multipoint VCs after a random delay of
between 1 and 10 seconds, to avoid all MCSs inundating the MARS
simultaneously in case of a more general failure.

The only exception to the rule is if a sequence number is detected
during the establishment of a new group's VC (i.e. a MARSMULTI 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 being established, MUST have its revalidate
flag set at some random interval between 1 and 10 seconds from the
time the jump in SSN was detected. (The VC being established is
considered already validated in this case).

Each MCS keeps its own 32 bit MCS Sequence Number (MSN) to track the
SSN. Whenever a message is received that carries a mar$msn field,
the following processing is performed:

Seq.diff = mar$msn - MSN

mar$msn -> MSN

(.... process MARS message ....)

if ((Seq.diff != 1) && (Seq.diff != 0))
then (.... revalidate group membership information ....)

The mar$msn value in an individual MARSMULTI is not used to update
the MSN until all parts of the MARSMULTI (if > 1) have arrived. (If
the mar$msn changes during reception of a MARSMULTI series, the
MARS-MULTI is discarded as described in section 5.1.1 of [GA96]).

The MCS sets its MSN to zero on startup. It gets the current value
of SSN when it receives the copy of the registration MARSMSERV back
from the MARS.

4.5.2 Reconnecting to the MARS

The MCSs are assumed to have been configured with the ATM address of
at least one MARS at startup. MCSs MAY choose to maintain a table of
ATM addresses, each address representing alternative MARS which will
be contacted in case of failure of the previous one. This table is
assumed to be ordered in descending order of preference.

An MCS will decide that it has problems communicating with a MARS if:

* It fails to establish a point-to-point VC with the MARS.

* MARSREQUEST generates no response (no MARSMULTI or MARS-NAK
returned).

* ServerControlVC fails.

* MARSMSERV or MARSUNSERV do not result in their respective copies
being
received.

(reconnection as in section 5.4 in [GA96], with MCS-specific actions
used where needed).

4.5.3 Revalidating a point-to-multipoint VC

The revalidation flag associated with a point-to-multipoint VC is
checked when a layer 3 packet is to be sent out on the VC.
Revalidation procedures MUST be initiated for a point-to-multipoint
VC that has its revalidate flag set when a layer 3 packet is being
sent out on it. Thus more active groups get revalidated faster than
less active ones. The revalidation process MUST NOT result in
disruption of normal traffic on the VC being revalidated.

The revalidation procedure is as follows. The MCS reissues a
MARSREQUEST for the VC being revalidated. The returned set of
members is compared with the locally held set; LMULTI-ADDs MUST be
issued for new members, and LMULTI-DROPs MUST be issued for non-
existent ones. The revalidate flag MUST be reset for the VC.

5 Multiple MCSs for a layer 3 group

Having a single MCS for a layer 3 group can cause it to become a
single point of failure and a bottleneck for groups with large
numbers of active senders. It is thus desirable to introduce a level
of fault tolerance by having multiple MCS per group. Support for
load sharing is not introduced in this document as to reduce the
complexity of the protocol.

5.1 Outline

The protocol described in this document offers fault tolerance by
using multiple MCSs for the same group. This is achieved by having a
standby MCS take over from a failed MCS which had been supporting the
group. The MCS currently supporting a group is refered to as the
active MCS, while the one or more standby MCSs are refered to as
inactive MCSs. There is only one active MCS existing at any given
instant for an MCS-supported group. The protocol makes use of the
HELLO messages as described in [LA96].

To reduce the complexity of the protocol, the following operational
guidelines need to be followed. These guidelines need to be enforced
by out-of-band means which are not specified in this document and can
be implementation dependent.

* The set of (one or more) MCSs ("mcslist") that support a
particular IP Multicast group is predetermined and fixed. This
set MUST be known to each MCS in the set at startup, and the
ordering of MCSs in the set is the same for all MCSs in the set.
An implementation of this would be to maintain the set of ATM
addresses of the MCSs in a file, an identical copy of which is
kept at each MCS in the set.

* All MCSs in "mcslist" have to be started up together, with the
first MCS in "mcslist" being the last to be started.

* A failed MCS cannot be started up again.

5.2 Discussion of Multiple MCSs in operation

An MCS on startup determines its position in the "mcslist". If the
MCS is not the first in "mcslist", it does not register for
supporting the group with the MARS. If the MCS is first in the set,
it does register to support the group.

The first MCS thus becomes the active MCS and supports the group as
described in section 4. The active MCS also opens a point-to-
multipoint VC (HelloVC) to the remaining MCSs in the set (the
inactive MCSs). It starts sending HELLO messages on this VC at a
fixed interval (HelloInterval seconds). The inactive MCSs maintain a
timer to keep track of the last received HELLO message. If an
inactive MCS does not receive a message within HelloInterval*
DeadFactor seconds (values of HelloInterval and DeadFactor are the
same at all the MCSs), or if the HelloVC is closed, it assumes
failure of the active MCS and attempts to elect a new one. The
election process is described in section 5.5.

If an MCS is elected as the new active one, it registers to support
the group with the MARS. It also initiates the transmission of HELLO
messages to the remaining inactive MCSs.

5.3 Inter-MCS control messages

The protocol uses HELLO messages in the heartbeat mechanism, and also
during the election process. The format of the HELLO message is
based on that described in [LA96]. The Hello message type code is 5.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Len | Recvr Len | State | Type | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HelloInterval | DeadFactor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Multicast address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender ATM address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver ATM address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sender Len
This field holds the length in octets of the Sender ATM address.

Recvr Len
This field holds the length in octets of the Receiver ATM
address.

State
Currently two states: No-Op (0x00) and Elected (0x01).
It is used by a candidate MCS to indicate if it was successfully
elected.

Type
This is the code for the message type.

HelloInterval
The hello interval advertises the time between sending of
consecutive Hello Messages by an active MCS. If the time between
Hello messages exceeds the HelloInterval then the Hello is to be
considered late by the inactive MCS.

DeadFactor
This is a multiplier to the HelloInterval. If an inactive MCS
does not receive a Hello message within the interval
HelloInterval*DeadFactor from an active MCS that advertised
the HelloInterval then the inactive MCS MUST consider the active
one to have failed.

IP Multicast address
This field is used to indicate the group to associate the HELLO
message with. It is useful if MCSs can support more than one
group.

Sender ATM address
This is the protocol address of the server which is sending the
Hello.

Receiver ATM address
This is the protocol address of the server which is to Reply to
the Hello. If the sender does not know this address then the
sender sets it to zero. (This happens in the HELLO messages sent
from the active MCS to the inactive ones, as they are multicast
and not sent to one specific receiver).

5.4 The Multiple MCS protocol

As is indicated in section 5.1, all the MCSs supporting the same IP
Multicast group MUST be started up together. The set of MCSs
("mcslist") MUST be specified to each MCS in the set at startup.
After registering to support the group with the MARS, the first MCS
in the set MUST open a point-to-multipoint VC (HelloVC) with the
remaining MCSs in the "mcslist" as leaves, and thus assumes the role
of active MCS. It MUST send HELLO messages HelloInterval seconds
apart on this VC. The Hello message sent by the active MCS MUST have
the Receiver Len set to zero, the State field set to "Elected", with
the other fields appropriately set. The Receiver ATM address field
does not exist in this HELLO message. The initial value of
HelloInterval and DeadFactor MUST be the same at all MCSs at startup.
The active MCS can choose to change these values by introducing the
new value in the HELLO messages that are sent out. The active MCS
MUST support the group as described in section 4.

The other MCSs in "mcslist" determine the identity of the first MCS
from the "mcslist". They MUST NOT register to support the group with
the MARS, and become inactive MCSs. On startup, an inactive MCS
expects HELLO messages from the active MCS. The inactive MCS MUST
terminate the HelloVC. A timer MUST be maintained, and if the
inactive MCS does not receive HELLO message from the active one
within a period HelloInterval*DeadFactor seconds, it assumes that the
active MCS died, and initiates the election process as described in
section 5.5. If a HELLO message is received within this period, the
inactive MCS does not initiate any further action, other than
restarting the timer. The inactive MCSs MUST set their values of
HelloInterval and DeadFactor to those specified by the active MCS in
the HELLO messages.

In case of an MCS supporting multiple groups, it MUST register to
support those groups for which it is the first MCS, and MUST NOT
register for other groups. A MARSMSERV with multiple <min, max>
pairs may be used for registering multiple disjoint sets of groups.
Support MUST be provided for the use of a single "mcslist" for more
than one group. This is intended to address the case wherein an MCS
is intended to support multiple groups, with other MCSs acting as
backups. This subverts the need for using a different "mcslist" for
each group being supported by the same set of MCSs.

On failure of the active MCS, a new MCS assumes its role as described
in section 5.5. In this case, the remaining inactive MCSs will
expect HELLO messages from this new active MCS as described in the
previous paragraph.

5.5 Failure handling

5.5.1 Failure of active MCS

The failure of the active MCS is detected by the inactive MCSs if no
HELLO message is received within an interval of
HelloInterval*DeadFactor seconds, or if the HelloVC is closed. In
this case the next MCS in "mcslist" becomes the candidate MCS. It
MUST open a point-to-multipoint VC to the remaining inactive MCSs
(HelloVC) and send a HELLO message on it with the State field set to
No-Op. The rest of the message is formatted as described earlier.

On receiving a HELLO message from a candidate MCS, an inactive MCS
MUST open a point-to-point VC to that candidate. It MUST send a
HELLO message back to it, with the Sender and Receiver fields
appropriately set (not zero), and the State field being No-Op. If a
HELLO message is received by an inactive MCS from a non-candidate
MCS, it is ignored. If no HELLO message is received from the
candidate with the State field set to "Elected" in HelloInterval
seconds, the inactive MCS MUST retransmit the HELLO. If no HELLO
message with State field set to "Elected" is received by the inactive
MCSs within an interval of HelloInterval*DeadFactor seconds, the next
MCS in "mcslist" is considered as the candidate MCS. Note that the
values used for HelloInterval and DeadFactor in the election phase
are the default ones.

The candidate MCS MUST wait for a period of HelloInterval*DeadFactor
seconds for receiving HELLO messages from inactive MCSs. It MUST
transmit HELLO messages with State field set to No-Op at
HelloInterval seconds interval during this period. If it receives
messages from atleast half of the remaining inactive MCSs during this
period, it considers itself elected and assumes the active MCS role.
It then registers to support the group with the MARS, and starts
sending HELLO messages at HelloInterval second intervals with State
field set to "Elected" on the already existing HelloVC. The active
MCS can then alter the HelloInterval and DeadFactor values if
desired, and communicate the same to the inactive MCSs in the HELLO
message.

5.5.2 Failure of inactive MCS

If an inactive MCS drops off the HelloVC, the active MCS MUST attempt
to add that MCS back to the VC for three attempts, spaced
HelloInterval*DeadFactor seconds apart. If even the third attempt
fails, the inactive MCS is considered dead.

An MCS, active or inactive, MUST NOT be started up once it has
failed. Failed MCSs can only be started up by manual intervention
after shutting down all the MCSs, and restarting them together.

5.6 Compatibility with future MARS and MCS versions

Future versions of MCSs can be expected to use an enhanced MARS for
load sharing and fault tolerance ([TA96]). The MCS architecture
described in this document is compatible with the enhanced MARS and
the future MCS versions. This is because the active MCS is the only
one which communicates with the MARS about the group. Hence the
active MCS will only be informed by the enhanced MARS about the
subset of the group that it is to support. Thus MCSs conforming to
this document are compatible with [GA96] based MARS, as well as
enhanced MARS.

6 Summary

This document describes the architecture of an MCS. It also provides
a mechanism for using multiple MCSs per group for providing fault
tolerance. This approach can be used with [GA96] based MARS server
and clients, without needing any change in their functionality. It
uses the HELLO packet format as described in [LA96] for the heartbeat
messages.

7 Acknowledgements

We would like to acknowledge Grenville Armitage (Bellcore) for
reviewing the document and suggesting improvements towards
simplifying the multiple MCS functionalities. Discussion with Joel
Halpern (Newbridge) helped clarify the multiple MCS problem. Anthony
Gallo (IBM RTP) pointed out security issues that are not adequately
addressed in the current document. Arvind Murching (Microsoft)
flagged a potential show stopper in section 4.1.2.

8 Authors' Address

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

Phone: (404)-894-6737
Email: taddy@cc.gatech.edu

Mostafa H. Ammar
College of Computing
Georgia Institute of Technology
Atlanta, GA 30332-0280

Phone: (404)-894-3292
Email: ammar@cc.gatech.edu

9 References

[GA96] Armitage, G.J., "Support for Multicast over UNI 3.0/3.1 based
ATM networks", RFC2022, November 1996.

[BK95] Birman, A., Kandlur, D., Rubas, J., "An extension to the MARS
model", Work in Progress.

[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) - NBMA", Work in Progress.

[TA96] Talpade, R., and Ammar, M.H., "Multiple MCS support using an
enhanced version of the MARS server.", Work in Progress.

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