Internet Engineering Task Force Inter-Domain Multicast Routing Working Group INTERNET-DRAFT W. Fenner draft-ietf-idmr-igmp-v2-04.txt Xerox PARC Obsoletes: Appendix I of RFC1112 September 4, 1996 Expires: 1/31/97 Internet Group Management Protocol, Version 2 Status of this Memo This document is an Internet Draft. Internet Drafts are working docu- ments of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts). Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any other Inter- net Draft. Distribution of this document is unlimited. Abstract This draft documents IGMPv2, used by IP hosts to report their mul- ticast group memberships to routers. It replaces Appendix I of RFC1112. IGMPv2 allows group membership termination to be quickly reported to the routing protocol, which is important for high-bandwidth mul- ticast groups and/or subnets with highly volatile group membership. This document is a product of the Inter-Domain Multicast Routing working group within the Internet Engineering Task Force. Comments are soli- cited and should be addressed to the working group's mailing list at idmr@cs.ucl.ac.uk and/or the author(s). Expires January 1997 [Page 1]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 1. Introduction The Internet Group Management Protocol (IGMP) is used by IP hosts to report their multicast group memberships to any immediately-neighboring multicast routers. This memo describes only the use of IGMP between hosts and routers to determine group membership. Routers that are members of multicast groups are expected to behave as hosts as well as routers, and may even respond to their own queries. IGMP may also be used between routers, but such use is not specified here. Like ICMP, IGMP is a integral part of IP. It is required to be imple- mented by all hosts wishing to receive IP multicasts. IGMP messages are encapsulated in IP datagrams, with an IP protocol number of 2. All IGMP messages described in this document are sent with IP TTL 1, and contain the IP Router Alert option [Katz96] in their IP header. All IGMP mes- sages of concern to hosts have the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Max Resp Time | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1.1. Type There are three types of IGMP messages of concern to the host- router interaction: 0x11 = Membership Query 0x16 = Version 2 Membership Report 0x17 = Leave Group There is an additional type of message, for backwards-compatibility with IGMPv1: 0x12 = Version 1 Membership Report 1.2. Max Response Time The Max Response Time field is meaningful only in Membership Query messages, and specifies the maximum allowed time before sending a responding report in units of 1/10 second. In all other messages, it is set to zero by the sender and ignored by receivers. Tuning this setting allows IGMPv2 routers to tune the "leave Expires January 1997 [Page 2]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 latency" of their attached subnets (the time between the moment the last host leaves a group and when the routing protocol is notified that there are no more members). It also allows tuning of the amount of IGMP traffic on a subnet. 1.3. Checksum The checksum is the 16-bit one's complement of the one's complement sum of the IGMP message. For computing the checksum, the checksum field is set to zero. 1.4. Group Address In a Membership Query message, the group address field is set to zero when sending a General Query (used to learn which groups have members on an attached network), and set to the group address being queried when sending a Group-Specific Query (used to learn if a particular group has any members on an attached network). In a Membership Report or Leave Group message, the group address field holds the IP multicast group address of the group being reported or left. 1.5. Other fields Note that IGMP messages may be longer than 8 octets, especially future backwards-compatible versions of IGMP. As long as the Type is one that is recognized, an IGMPv2 implementation should ignore anything past the first 8 octets. 2. Informal Protocol Description Note that defaults for timer values are described later in this docu- ment. Timer and counter names appear in square brackets. Multicast routers use IGMP to learn which groups have members on each of their attached physical networks(*). A multicast router keeps a list of multicast group memberships for each attached network, and a timer for each membership. "Multicast group memberships" means the presence of at _________________________ * The term "interface" is sometimes used in this do- cument to mean "the primary interface on an attached network"; if a router has multiple physical interfaces on a single network this protocol need only run on one of them. Hosts, on the other hand, need to perform their actions on all interfaces that have memberships associated with them. Expires January 1997 [Page 3]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 least one member of a multicast group on a given attached network, not a list of all of the members. With respect to each of its attached net- works, a multicast router may assume one of two roles: Querier or Non- Querier. There is normally only one Querier per physical network. All multicast routers start up as a Querier on each attached network. If a multicast router hears a Query message from a router with a lower IP address, it becomes a Non-Querier on that network. Routers periodically [Query Interval] send a General Query on each attached network for which this router is the Querier, to solicit membership information. On startup, a router should send [Startup Query Count] General Queries spaced closely together [Startup Query Interval] in order to quickly and reliably determine membership information. A General Query is addressed to the all-systems multicast group (224.0.0.1), has a Group Address field of 0, and has a Max Response Time of [Query Response Interval]. When a host receives a General Query, it sets delay timers for each group of which it is a member on the interface from which it received the query. Each timer is set to a different random value(*), up to the requested Max Response Time in the Query packet. When a host receives a Group-Specific Query, it sets a delay timer for the group being queried if it is a member on the interface from which it received the query. If a timer for a group is already running, it is reset only if the requested Max Response Time is less than the remaining value of the run- ning timer. When a group's timer expires, the host multicasts a Version 2 Membership Report to the group, with IP TTL of 1. If the host receives another host's Report (version 1 or 2) while it has a timer running, it stops its timer for the specified group and does not send a Report, in order to suppress duplicate Reports. When a router receives a Report, it adds the group being reported to the list of multicast group memberships on the network on which it received the Report and sets the timer for the membership to the [Group Member- ship Timer]. If no Reports are received for a particular group before this timer has expired, the router assumes that the group has no local members and that it need not forward remotely-originated multicasts for that group onto the attached network. When a host joins a multicast group, it should immediately transmit an unsolicited Version 2 Membership Report for that group, in case it is the first member of that group on the network. To cover the possibility of the initial Membership Report being lost or damaged, it is recom- mended that it be repeated once or twice after short delays [Unsolicited Report Interval]. (A simple way to accomplish this is to send the ini- tial Version 2 Membership Report and then act as if a Group-Specific _________________________ * This random value should use the highest clock granularity available on the host. Expires January 1997 [Page 4]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 Query was received for that group, and set a timer appropriately). When a host leaves a multicast group, if it was the last host to reply to a Query with a Membership Report for that group, it sends a Leave Group message to the all-routers multicast group (224.0.0.2). (*) If it was not the last host to reply to a Query, it may send nothing as there must be another member on the subnet. (+) When a Querier receives a Leave Group message, it sets its query inter- val for that group to a very short value [Last Member Query Interval]. The Querier sets the group membership timer to some multiple [Last Member Query Count] of the query interval and starts sending Group- Specific queries addressed to the group specified in the Leave Group message. These Group-Specific queries must have the Max Response Time set to the query interval. If no Reports are received before the membership timer expires, the routers assume that the group has no local members, as above. 3. Compatibility with IGMPv1 Routers An IGMPv2 host may be placed on a subnet where the Querier router has not yet been upgraded to IGMPv2. The following requirements apply: The IGMPv1 router will send General Queries with the Max Response Time set to 0. This must be interpreted as a value of 100 (10 seconds). The IGMPv1 router expects Version 1 Membership Reports in response to its Queries, and will not pay attention to Version 2 Membership Reports. Therefore, a state variable must be kept for each inter- face, describing whether the multicast Querier on that interface is running IGMPv1 or IGMPv2. This variable must be based upon whether or not an IGMPv1 query was heard in the last [Version 1 Router Present Timeout] seconds, *not* on the type of the last Query heard, as an IGMPv2 router may restart and send a Query before it realizes that it is not the Querier for the subnet. This state variable must be used to decide what type of Membership Reports to _________________________ * Note that a router should also be prepared to ac- cept a Leave Group message sent to the group being left, as one widespread IGMPv2 implementation does this. + This is an optimization to reduce traffic; hosts without sufficient storage to remember whether or not they were the last host to reply may always send a Leave Group message when it leaves a group. Expires January 1997 [Page 5]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 send for unsolicited Membership Reports as well as Membership Reports in response to Queries. An IGMPv2 host may optionally suppress Leave Group messages on a network where the Querier is using IGMPv1. An IGMPv2 router may be placed on a subnet where at least one router on the subnet has not yet been upgraded to IGMPv2. The following require- ments apply: If any IGMPv1 routers are present, the querier must use IGMPv1. Since there was no querier election specification in RFC1112, the rules followed by the IGMPv1 router may not agree with those in this document. In this case, the IGMPv2 router must follow the rules established by the routing protocol for electing a querier. There is not necessarily a way for an IGMPv2 router to know that all IGMPv1 peers have been removed from the network. If this information is available from the routing protocol, then the IGMPv2 router may use that information to know when to switch back to using IGMPv2. Otherwise, switching to IGMPv2 is only done on reboot or explicit configuration. 4. Compatibility with IGMPv1 Hosts An IGMPv2 host may be placed on a subnet where there are hosts that have not yet been upgraded to IGMPv2. The following requirements apply: The host must allow its Membership Report to be suppressed by either a Version 1 Membership Report or a Version 2 Membership Report. An IGMPv2 router may be placed on a subnet where there are hosts that have not yet been upgraded to IGMPv2. The following requirements apply: If a router receives a Version 1 Membership Report, it must set a timer to note that there are version 1 hosts present which are members of the group for which it heard the report. This timer should be the same as the [Group Membership Timer]. If there are version 1 hosts present for a particular group, a router must ignore any Leave Group messages that it receives for that group. 5. Host State Diagram Host behavior is more formally specified by the state transition diagram below. A host may be in one of three possible states with respect to Expires January 1997 [Page 6]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 any single IP multicast group on any single network interface: - "Non-Member" state, when the host does not belong to the group on the interface. This is the initial state for all memberships on all network interfaces; it requires no storage in the host. - "Delaying Member" state, when the host belongs to the group on the interface and has a report delay timer running for that membership. - "Idle Member" state, when the host belongs to the group on the interface and does not have a report delay timer running for that membership. There are five significant events that can cause IGMP state transitions: - "join group" occurs when the host decides to join the group on the interface. It may occur only in the Non-Member state. - "leave group" occurs when the host decides to leave the group on the interface. It may occur only in the Delaying Member and Idle Member states. - "query received" occurs when the host receives either a valid Gen- eral Membership Query message, or a valid Group-Specific Membership Query message. To be valid, the Query message must be at least 8 octets long, and have a correct IGMP checksum. The group address in the IGMP header must either be zero (a General Query) or a valid multicast group address (a Group-Specific Query). A General Query applies to all memberships on the interface from which the Query is received. A Group-Specific Query applies to membership in a single group on the interface from which the Query is received. Queries are ignored for memberships in the Non-Member state. - "report received" occurs when the host receives a valid IGMP Membership Report message (Version 1 or Version 2). To be valid, the Report message must be at least 8 octets long and have a correct IGMP checksum. A Membership Report applies only to the membership in the group identified by the Membership Report, on the interface from which the Membership Report is received. It is ignored for memberships in the Non-Member or Idle Member state. - "timer expired" occurs when the report delay timer for the group on the interface expires. It may occur only in the Delaying Member state. All other events, such as receiving invalid IGMP messages, or IGMP mes- sages other than Query or Report, are ignored in all states. Expires January 1997 [Page 7]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 There are seven possible actions that may be taken in response to the above events: - "send report" for the group on the interface. The type of report is determined by the state of the interface. - "send leave" for the group on the interface. If the interface state says the Querier is running IGMPv1, this action should be skipped. If the flag saying we were the last host to report is cleared, this action should be skipped. - "set flag" that we were the last host to send a report for this group. - "clear flag" since we were not the last host to send a report for this group. - "start timer" for the group on the interface, using a random delay value up to the Max Response Time in the Query. If this is an unsolicited Report, the timer is set to a random delay value up to [Unsolicited Report Interval]. - "reset timer" for the group on the interface to a new value, using a random delay value up to the requested response time in the Query. - "stop timer" for the group on the interface. In all of the following state diagrams, each state transition arc is labeled with the event that causes the transition, and, in parentheses, any actions taken during the transition. Note that the transition is always triggered by the event; even if the action is conditional, the transition still occurs. Expires January 1997 [Page 8]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 ________________ | | | | | | | | --------->| Non-Member |<--------- | | | | | | | | | | | | | |________________| | | | | | leave group | join group | leave group | (stop timer, |(send report, | (send leave if | send leave if | set flag, | flag set) | flag set) | start timer) | ________|________ | ________|________ | |<--------- | | | | | | | |<-------------------| | | | query received | | | Delaying Member | (start timer) | Idle Member | ---->| |------------------->| | | | | report received | | | | | (stop timer, | | | | | clear flag) | | | |_________________|------------------->|_________________| | query received | timer expired | (reset timer if | (send report, | Max Resp Time | set flag) | < current timer) | ------------------- The all-systems group (address 224.0.0.1) is handled as a special case. The host starts in Idle Member state for that group on every interface, never transitions to another state, and never sends a report for that group.(*) In addition, a host may be in one of two possible states with respect to any single network interface: _________________________ * One widely-spread IGMPv2 implementation treats all link-local (224.0.0.X) groups this way. However, this behavior is only specified for the all-systems group. Expires January 1997 [Page 9]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 - "No IGMPv1 Router Present", when the host has not heard an IGMPv1 style query for the [Version 1 Router Present Timeout]. This is the initial state. - "IGMPv1 Router Present", when the host has heard an IGMPv1 style query within the [Version 1 Router Present Timeout]. There are two events that can cause state transitions: - "IGMPv1 query received", when the host receives a query with the Max Response Time field set to 0. - "timer expires", when the timer set to note the presence of an IGMPv1 router expires. And a single action that can be triggered by an event: - "set timer", setting the timer to its maximum value [Version 1 Router Present Timeout] and (re)starting it. ________________ | | | | | No IGMPv1 | | Router | | Present | | | ---->| |---- | | | | | |________________| | timer expires | | IGMPv1 query received | ________________ | (set timer) | | | | | | | | | | | | -----| IGMPv1 |<--- | Router | | Present | | | ---->| |---- | |________________| | | | | IGMPv1 query received | | (set timer) | --------------------------- Expires January 1997 [Page 10]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 6. Router State Diagram Router behavior is more formally specified by the state transition diagrams below. A router may be in one of two possible states with respect to any single attached network: - "Querier", when this router is designated to transmit IGMP Member- ship Queries on this network. - "Non-Querier", when there is another router designated to transmit IGMP membership Queries on this network. The following three events can cause the router to change states: - "query timer expired" occurs when the timer set for query transmis- sion expires. - "query received from a router with a lower IP address" occurs when an IGMP Membership Query is received from a router on the same net- work with a lower IP address. - "other querier present timer expired" occurs when the timer set to note the presence of another querier with a lower IP address on the network expires. There are three actions that may be taken in response to the above events: - "start general query timer" for the attached network. - "start other querier present timer" for the attached network [Other Querier Present Timer]. - "send general query" on the attached network. Expires January 1997 [Page 11]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 -------------------------------- _______|________ gen. query timer | --------- | | expired | | Initial |---------------->| | (send general query, | --------- (send gen. q., | | set gen. q. timer) | set initial gen. q. | |<---------------------- timer) | Querier | | | -----| |<--- | | | | | |________________| | query received from a | | other querier router with a lower | | present timer expired IP address | | (send general query, (set other querier | ________________ | set gen. q. timer) present timer) | | | | | | | | | | | | ---->| Non |---- | Querier | | | | | ---->| |---- | |________________| | | query received from a | | router with a lower IP | | address | | (set other querier | | present timer) | --------------------------- A router should start in the Initial state on all attached networks, and immediately move to Querier state. In addition, to keep track of which groups have members, a router may be in one of four possible states with respect to any single IP multicast group on any single attached network: - "No Members Present" state, when there are no hosts on the network which have sent reports for this multicast group. This is the ini- tial state for all groups on the router; it requires no storage in the router. - "Members Present" state, when there is a host on the network which has sent a Membership Report for this multicast group. Expires January 1997 [Page 12]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 - "Version 1 Members Present" state, when there is an IGMPv1 host on the network which has sent a Version 1 Membership Report for this multicast group. - "Checking Membership" state, when the router has received a Leave Group message but has not yet heard a Membership Report for the multicast group. There are six significant events that can cause router state transi- tions: - "v2 report received" occurs when the router receives a Version 2 Membership Report for the group on the interface. To be valid, the Report message must be at least 8 octets long and must have a correct IGMP checksum. - "v1 report received" occurs when the router receives a Version 1 Membership report for the group on the interface. The same vali- dity requirements apply. - "leave received" occurs when the router receives an IGMP Group Leave message for the group on the interface. To be valid, the Leave message must be at least 8 octets long and must have a correct IGMP checksum. - "timer expired" occurs when the timer set for a group membership expires. - "retransmit timer expired" occurs when the timer set to retransmit a group-specific Membership Query expires. - "v1 host timer expired" occurs when the timer set to note the pres- ence of version 1 hosts as group members expires. There are six possible actions that may be taken in response to the above events: - "start timer" for the group membership on the interface - also resets the timer to its initial value [Group Membership Timer] if the timer is currently running. - "start retransmit timer" for the group membership on the interface [Last Member Query Interval]. - "start v1 host timer" for the group membership on the interface - also resets the timer to its initial value [Group Membership Timer] if the timer is currently running. Expires January 1997 [Page 13]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 - "send group-specific query" for the group on the attached network. - "notify routing +" notify the routing protocol that there are members of this group on this connected network. - "notify routing -" notify the routing protocol that there are no longer any members of this group on this connected network. Expires January 1997 [Page 14]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 The state diagram for a router in Querier state follows: ________________ ----------------------------| |<----------------------------- | | | | | timer expired| |timer expired | | (notify routing -)| No Members |(notify routing -, | | ------->| Present |<--------- clear rxmt tmr) | | | | | | | |v1 report rec'd | | | | | |(notify routing +, | |________________| | --------------- | | start timer, | | | | rexmt timer | | | start v1 host | v2 report received| | | expired | | | timer) | (notify routing +,| | | (send g-s | | | | start timer)| | | query, | | | __________|______ | ________|_|______ st rxmt | | | | |<------------ | | tmr) | | | | | | |<------- | | | | v2 report received | | | | | | (start timer) | | | | | Members Present |<-------------------| Checking | | | ----->| | leave received | Membership | | | | | | (start timer*, | | | | | | | start rexmt timer,| | | | | | | send g-s query) | | | | | --->| |------------------->| | | | | | |_________________| |_________________| | | | |v2 report rec'd | | | | | | |(start timer) | |v1 report rec'd |v1 report rec'd | | | ---------------- |(start timer, |(start timer, | | |v1 host | start v1 host timer) | start v1 host timer) | | |tmr ______________V__ | | | |exp'd | |<---------------------- | | ------| | | | | Version 1 |timer expired | | | Members Present |(notify routing -) | ------->| |------------------------------------------------- | |<-------------------- ------->|_________________| v1 report rec'd | | v2 report rec'd | | (start timer, | | (start timer) | | start v1 host timer) | ----------------- -------------------------- _________________________ * When entering the Checking Membership state, the timer is set to [Last Member Query Interval] * [Last Membership Query Count], not [Group Membership Timer]. Expires January 1997 [Page 15]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 The state diagram for a router in Non-Querier state is similar, but non-Queriers do not send any messages and are only driven by message re- ception. Note that non-Queriers do not care whether a Membership Report message is Version 1 or Version 2. ________________ | | | | timer expired| |timer expired (notify routing -)| No Members |(notify routing -) --------->| Present |<--------- | | | | | | | | | | | | | |________________| | | | | | |report received | | |(notify routing +,| | | start timer) | ________|________ | ________|________ | |<--------- | | | | report received | | | | (start timer) | | | Members Present |<-------------------| Checking | | | g-s query rec'd | Membership | | | (start timer*) | | ---->| |------------------->| | | |_________________| |_________________| | report received | | (start timer) | ----------------- _________________________ * When a non-querier router receives a Group Specific query in the Members Present state, it sets its group membership timer to a multiple [Last Member Query Count] of the Max Response Time in the Group Specific query. It ignores Group Specific queries received in the Checking Membership state. Expires January 1997 [Page 16]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 7. List of timers and default values Note that parentheses are used to group expressions to make the algebra clear. 7.1. Query Interval The Query Interval is the interval between General Queries sent by the Querier. Default: 125 seconds. 7.2. Group Membership Timer The Group Membership Timer is the amount of time that must pass before a multicast router decides there are no more members of a group on a net- work. Default: (twice the Query Interval) plus (one Query Response Interval). 7.3. Other Querier Present Timer The Other Querier Present Timer is the length of time that must pass before a multicast router decides that there is no longer another multi- cast router which should be the querier. Default: (twice the Query Interval) plus (one half of one Query Response Interval). 7.4. Query Response Interval The Max Response Time inserted into the periodic General Queries. Default: 100 (10 seconds) 7.5. Startup Query Interval The Startup Query Interval is the interval between General Queries sent by a Querier on startup. Default: 1/4 the Query Interval. 7.6. Startup Query Count The Startup Query Count is the number of Queries sent out on startup, separated by the Startup Query Interval. Default: 2. 7.7. Last Member Query Interval The Last Member Query Interval is the Max Response Time inserted into Group-Specific Queries sent in response to Leave Group messages, and is also the amount of time between Group-Specific Query messages. Default: 10 (1 second) Expires January 1997 [Page 17]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 7.8. Last Member Query Count The Last Member Query Count is the number of Group-Specific Queries sent before the router assumes there are no local members. Default: 2. 7.9. Unsolicited Report Interval The Unsolicited Report Interval is the time between repetitions of a host's initial report of membership in a group. Default: 10 seconds. 7.10. Version 1 Router Present Timeout The Version 1 Router Present Timeout is how long a host must wait after hearing a Version 1 Query before it may send any IGMPv2 messages. Value: 400 seconds. 8. Security Considerations Security issues are not discussed in this memo. 9. Acknowledgments IGMPv2 was designed by Rosen Sharma and Steve Deering. 10. References Katz96 Katz, D., "IP Router Alert Option," RFC XXXX, Cisco Sys- tems, April 1996. Expires January 1997 [Page 18]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 11. Appendix I - Changes from IGMPv1 The IGMPv1 "Version" and "Type" fields are combined into a single "Type" field. A new IGMP Type is assigned to Version 2 Membership Report messages, so a router may tell the difference between an IGMPv1 and IGMPv2 host report. A new IGMP Type is created for the IGMPv2 Leave Group message. The Membership Query message is changed so that a previously unused field contains a new value, the Max Response Time. The IGMPv2 spec now specifies a querier election mechanism. In IGMPv1, the querier election was left up to the multicast routing protocol, and different protocols used different mechanisms. This could result in more than one querier per network, so the election mechanism has been standardized in IGMPv2. However, this means that care must be taken when an IGMPv2 router is trying to coexist with an IGMPv1 router that uses a different querier election mechanism. In particular, it means that an IGMPv2 router must be able to act as an IGMPv1 router on a par- ticular network if configured to do so. The actions required include: - Set the Max Response Time field to 0 in all queries. - Ignore Leave Group messages. The IGMPv2 spec relaxes the requirements on validity-checking for Membership Queries and Membership Reports. When upgrading an implemen- tation, be sure to remove any checks that do not belong. The IGMPv2 spec requires the presence of the IP Router Alert option [Katz96] in all packets described in this memo. Expires January 1997 [Page 19]
Internet Draft draft-ietf-idmr-igmp-v2-04.txt September 1996 12. Author's Address William C. Fenner Xerox PARC 3333 Coyote Hill Road Palo Alto, CA 94304 Phone: +1 415 812 4816 Email: fenner@parc.xerox.com Expires January 1997 [Page 20]
