





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 RFC 2022.  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
   RFC 2022.  It also provides a mechanism for using multiple MCSs per
   group for providing fault tolerance.  This approach can be used with
   RFC 2022 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 RFC 1577
   ([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."




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   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.



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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.








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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.





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   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.





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   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.





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   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.




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   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).




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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.







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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.





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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.



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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.
















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   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.




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   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).




















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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.





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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.






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   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







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   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", RFC 2022, 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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