【正文】 ode in the received Query message. A system may receive a variety of Queries on different interfaces and of different kinds (., General Queries, GroupSpecific Queries, and Groupand SourceSpecific Queries), each of which may require its own delayed response. Before scheduling a response to a Query, the system must first consider previously scheduled pending responses and in many cases schedule a bined response. Alberto Ornaghi 2022 30 IGMP v3 The protocol (for group members) Action on Reception of a Query Therefore, the system must be able to maintain the following state: ? A timer per interface for scheduling responses to General Queries. ? A pergroup and interface timer for scheduling responses to Group Specific and GroupandSource Specific Queries. ? A pergroup and interface list of sources to be reported in the response to a GroupandSource Specific Query. router IGMP query IGMP report Wait for random interval Alberto Ornaghi 2022 31 IGMP v3 The protocol (for multicast routers) The purpose of IGMP is to enable each multicast router to learn, for each of its directly attached works, which multicast addresses are of interest to the systems attached to those works. IGMP version 3 adds the capability for a multicast router to also learn which *sources* are of interest to neighboring systems, for packets sent to any particular multicast address. The information gathered by IGMP is provided to whichever multicast routing protocol is being used by the router, in order to ensure that multicast packets are delivered to all works where there are interested receivers. NOTE: Multicast routers may also themselves bee members of multicast groups, and therefore also perform the group member part of IGMPv3 Alberto Ornaghi 2022 32 IGMP v3 The protocol (for multicast routers) If a multicast router has more than one interface to the same work, it only needs to operate this protocol over one of those interfaces. On each interface over which this protocol is being run, the router MUST enable reception of multicast address , from all sources. Multicast routers need to know only that *at least one* system on an attached work is interested in packets to a particular multicast address from a particular source。s patibility mode is determined from the Host Compatibility Mode variable which can be in one of three states: IGMPv1, IGMPv2 or IGMPv3. This variable is kept per interface and is dependent on the version of General Queries heard on that interface as well as the Older Version Querier Present timers for the interface. Alberto Ornaghi 2022 42 IGMP v3 Interoperation with older version Multicast Router Behavior In the Presence of Older Version Queriers IGMPv3 routers may be placed on a work where at least one router on the work has not yet been upgraded to IGMPv3. If any older versions of IGMP are present on routers, the querier MUST use the lowest version of IGMP present on the work. When in IGMPv1 mode, routers MUST send Periodic Queries with a Max Resp Code of 0 and truncated at the Group Address field (. 8 bytes long), and MUST ignore Leave Group messages. When in IGMPv2 mode, routers MUST send Periodic Queries truncated at the Group Address field (. 8 bytes long) Alberto Ornaghi 2022 43 IGMP v3 Interoperation with older version Multicast Router Behavior In the Presence of Older Version Group Members IGMPv3 routers may be placed on a work where there are hosts that have not yet been upgraded to IGMPv3. In order to be patible with older version hosts, IGMPv3 routers MUST operate in version 1 and version 2 patibility modes. In order to switch gracefully between versions of IGMP, routers keep an IGMPv1 Host Present timer and an IGMPv2 Host Present timer per group record. Alberto Ornaghi 2022 44 IGMP v3 Interoperation with older version Multicast Router Behavior When Group Compatibility Mode is IGMPv3, a router acts using the IGMPv3 protocol for that group. When Group Compatibility Mode is IGMPv2, a router acts in IGMPv2 patibility mode, treating all IGMPv3 messages mentioning a group as membership in that group, except IS_INC( {} ) and TO_INC( {} ), which are treated as IGMPv2 Leave messages. ALLOW() and BLOCK() messages are ignored. When Group Compatibility Mode is IGMPv1, a router acts in IGMPv1 patibility mode, treating IGMPv3 messages as above but adittionally ignoring IGMPv2 Leave messages. Alberto Ornaghi 2022 45 IGMP v3 Timers Most of these timers are configurable. If nondefault settings are used, they MUST be consistent among all systems. Robustness Variable The Robustness Variable allows tuning for the expected packet loss on a work. If a work is expected to be lossy, the Robustness Variable may be increased. IGMP is robust to (Robustness Variable 1) packet losses. The Robustness Variable MUST NOT be zero, and SHOULD NOT be one. Default: 2 Query Interval The Query Interval is the interval between General Queries sent by the Querier. Default: 125 seconds. Alberto Ornaghi 2022 46 IGMP v3 Timers Query Response Interval The Max Response Time used to calculate the Max Resp Code inserted into the periodic General Queries. Default: 100 (10 seconds) The number of seconds represented by the [Query Response Interval] must be less than the [Query Interval]. Group Membership Interval The Group Membership Interval is the amount of time that must pass before a multicast router decides there are no more members of a group or a particular source on a work. This value MUST be ((the Robustness Variable) * (the Query Interval)) + (one Query Response Interval). Alberto Ornaghi 2