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.
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.
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.
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.
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.
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).
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.
4.1.1 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).
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:
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.
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.
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).
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.
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.
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).
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.