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rfc:rfc2149

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."

Talpade & Ammar Informational [Page 1] RFC 2149 Multicast Server Architectures May 1997

 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.

Talpade & Ammar Informational [Page 2] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 3] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 4] RFC 2149 Multicast Server Architectures May 1997

 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.

Talpade & Ammar Informational [Page 5] RFC 2149 Multicast Server Architectures May 1997

 A "copy" is defined as a received message with the following fields
 matching the previously transmitted MARSMSERV/MARS-UNSERV:
  1. mar$op
  2. mar$flags.register
  3. mar$pnum
  4. Source ATM address
  5. first <min, max> pair
 In addition, a valid copy MUST have the following field values:
  1. mar$flags.punched = 0
  2. 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.

Talpade & Ammar Informational [Page 6] RFC 2149 Multicast Server Architectures May 1997

 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.

Talpade & Ammar Informational [Page 7] RFC 2149 Multicast Server Architectures May 1997

 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).

Talpade & Ammar Informational [Page 8] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 9] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 10] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 11] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 12] RFC 2149 Multicast Server Architectures May 1997

 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.

Talpade & Ammar Informational [Page 13] RFC 2149 Multicast Server Architectures May 1997

 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).

Talpade & Ammar Informational [Page 14] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 15] RFC 2149 Multicast Server Architectures May 1997

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.

Talpade & Ammar Informational [Page 16] RFC 2149 Multicast Server Architectures May 1997

 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

Talpade & Ammar Informational [Page 17] RFC 2149 Multicast Server Architectures May 1997

 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.

Talpade & Ammar Informational [Page 18]

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