Internet Draft INTERNET-DRAFT Brad Cain, Nortel Networks Steve Deering, Cisco Systems Isidor Kouvelas, Cisco Systems Ajit Thyagarajan, Ericsson Expires December 2000 June 2000 Internet Group Management Protocol, Version 3 <draft-ietf-idmr-igmp-v3-04.txt> STATUS OF THIS MEMO This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as work in progress. The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document specifies Version 3 of the Internet Group Management Protocol, IGMPv3. IGMP is the protocol used by IPv4 systems to report their IP multicast group memberships to neighboring multicast routers. Version 3 of IGMP adds support for "source filtering", that is, the ability for a system to report interest in receiving packets *only* from specific source addresses, or from *all but* specific source addresses, sent to a particular multicast address. That information may be used by multicast routing protocols to avoid delivering multicast packets from specific sources to networks where there are no interested receivers. This document is a product of the Inter-Domain Multicast Routing working group within the Internet Engineering Task Force. Comments are solicited and should be addressed to the working group's mailing list at idmr@cs.ucl.ac.uk and/or the authors. Cain, Deering, Kouvelas, Thyagarajan [Page 1] INTERNET-DRAFT IGMPv3 June 2000 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. Due to the lack of italics, emphasis is indicated herein by bracketing a word or phrase in "*" characters. Table of Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 The API for Requesting IP Multicast Reception . . . . . . . . . . . 3 Multicast Reception State Maintained by Systems . . . . . . . . . . 5 Message Formats . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Description of the Protocol for Group Members . . . . . . . . . . . 18 Description of the Protocol for Multicast Routers . . . . . . . . . 23 Interoperation with Older Versions of IGMP . . . . . . . . . . . . . 33 List of Timers, Counters, and their Default Values . . . . . . . . . 37 Security Considerations . . . . . . . . . . . . . . . . . . . . . . 40 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . 41 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . . 42 Cain, Deering, Kouvelas, Thyagarajan [Page 2] INTERNET-DRAFT IGMPv3 June 2000 1. INTRODUCTION The Internet Group Management Protocol (IGMP) is used by IPv4 systems (hosts and routers) to report their IP multicast group memberships to any neighboring multicast routers. Note that an IP multicast router may itself be a member of one or more multicast groups, in which case it performs both the "multicast router part" of the protocol (to collect the membership information needed by its multicast routing protocol) and the "group member part" of the protocol (to inform itself and other, neighboring multicast routers of its memberships). IGMP is also used for other IP multicast management functions, using message types other than those used for group membership reporting. This document specifies only the group membership reporting functions and messages. This document specifies Version 3 of IGMP. Version 1, specified in [RFC-1112], was the first widely-deployed version and the first version to become an Internet Standard. Version 2, specified in [RFC-2236], added support for "low leave latency", that is, a reduction in the time it takes for a multicast router to learn that there are no longer any members of a particular group present on an attached network. Version 3 adds support for "source filtering", that is, the ability for a system to report interest in receiving packets *only* from specific source addresses, or from *all but* specific source addresses, sent to a particular multicast address. Version 3 is designed to be interoperable with Versions 1 and 2. 2. THE API FOR REQUESTING IP MULTICAST RECEPTION Within an IP system, there is (at least conceptually) an Application Programming Interface or API used by upper-layer protocols or application programs to ask the IP layer to enable and disable reception of packets sent to specific IP multicast addresses. In order to take full advantage of the capabilities of IGMPv3, a system's IP API must support the following operation (or any logical equivalent; for example, see [FILTER-API]): IPMulticastListen ( socket, interface, multicast-address, filter-mode, source-list ) where: "socket" is an implementation-specific parameter used to distinguish among different requesting entities (e.g., programs or processes) within the system; the socket parameter of BSD Unix system calls is a specific example. Cain, Deering, Kouvelas, Thyagarajan [Page 3] INTERNET-DRAFT IGMPv3 June 2000 "interface" is a local identifier of the network interface on which reception of the specified multicast address is to be enabled or disabled. Interfaces may be physical (e.g., an Ethernet interface) or virtual (e.g., the endpoint of a Frame Relay virtual circuit or the endpoint of an IP-in-IP "tunnel"). An implementation may allow a special "unspecified" value to be passed as the interface parameter, in which case the request would apply to the "primary" or "default" interface of the system (perhaps established by system configuration). If reception of the same multicast address is desired on more than one interface, IPMulticastListen is invoked separately for each desired interface. "multicast-address" is the IP multicast address to which the request pertains. If reception of more than one multicast address on a given interface is desired, IPMulticastListen is invoked separately for each desired multicast address. "filter-mode" may be either INCLUDE or EXCLUDE. In INCLUDE mode, reception of packets sent to the specified multicast address is requested *only* from those IP source addresses listed in the source- list parameter. In EXCLUDE mode, reception of packets sent to the given multicast address is requested from all IP source addresses *except* those listed in the source-list parameter. "source-list" is an unordered list of zero or more IP unicast addresses from which multicast reception is desired or not desired, depending on the filter mode. An implementation MAY impose a limit on the size of source lists, but that limit MUST NOT be less than 64 addresses per list. For a given combination of socket, interface, and multicast address, only a single filter mode and source list can be in effect at any one time. However, either the filter mode or the source list, or both, may be changed by subsequent IPMulticastListen requests that specify the same socket, interface, and multicast address. Previous versions of IGMP did not support source filters and had a simpler API consisting of Join and Leave operations to enable and disable reception of a given multicast address (from *all* sources) on a given interface. Those Join and Leave operations are supported by the new API as follows: The Join operation is equivalent to IPMulticastListen ( socket, interface, multicast-address, EXCLUDE, {} ) and the Leave operation is equivalent to: Cain, Deering, Kouvelas, Thyagarajan [Page 4] INTERNET-DRAFT IGMPv3 June 2000 IPMulticastListen ( socket, interface, multicast-address, INCLUDE, {} ) where {} is an empty source list. It is recommended that implementations continue to support the old API, (perhaps as calls on the new API) for compatibility with pre-existing IP multicast applications. 3. MULTICAST RECEPTION STATE MAINTAINED BY SYSTEMS 3.1. Socket State For each socket on which IPMulticastListen has been invoked, the system records the desired multicast reception state for that socket. That state conceptually consists of a set of records of the form: (interface, multicast-address, filter-mode, source-list) The socket state evolves in response to each invocation of IPMulticastListen on the socket, as follows: o If the requested filter mode is INCLUDE *and* the requested source list is empty, then the entry corresponding to the requested interface and multicast address is deleted if present. If no such entry is present, the request is ignored. o If the requested filter mode is EXCLUDE *or* the requested source list is non-empty, then the entry corresponding to the requested interface and multicast address, if present, is changed to contain the requested filter mode and source list. If no such entry is present, a new entry is created, using the parameters specified in the request. 3.2. Interface State In addition to the per-socket multicast reception state, a system must also maintain or compute multicast reception state for each of its interfaces. That state conceptually consists of a set of records of the form: (multicast-address, filter-mode, source-list) This per-interface state is derived from the per-socket state, but may differ from the per-socket state when different sockets have differing filter modes and/or source lists for the same multicast address and Cain, Deering, Kouvelas, Thyagarajan [Page 5] INTERNET-DRAFT IGMPv3 June 2000 interface. For example, suppose one application or process invokes the following operation on socket s1: IPMulticastListen ( s1, i, m, INCLUDE, {a, b, c} ) requesting reception on interface i of packets sent to multicast address m, *only* if they come from source a, b, or c. Suppose another application or process invokes the following operation on socket s2: IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} ) requesting reception on the same interface i of packets sent to the same multicast address m, *only* if they come from sources b, c, or d. In order to satisfy the reception requirements of both sockets, it is necessary for interface i to receive packets sent to m from any one of the sources a, b, c, or d. Thus, in this example, the reception state of interface i for multicast address m has filter mode INCLUDE and source list {a, b, c, d}. After a multicast packet has been accepted from an interface by the IP layer, its subsequent delivery to the application or process listening on a particular socket depends on the multicast reception state of that socket [and possibly also on other conditions, such as what transport- layer port the socket is bound to]. So, in the above example, if a packet arrives on interface i, destined to multicast address m, with source address a, it may be delivered on socket s1 but not on socket s2. Note that IGMP Queries and Reports are not subject to source filtering and must always be processed by hosts and routers. The general rules for deriving the per-interface state from the per- socket state are as follows: For each distinct (interface, multicast- address) pair that appears in any socket state, a per-interface record is created for that multicast address on that interface. Considering all socket records containing the same (interface, multicast-address) pair, o if *any* such record has a filter mode of EXCLUDE, then the filter mode of the interface record is EXCLUDE, and the source list of the interface record is the intersection of the source lists of all socket records in EXCLUDE mode, minus those source addresses that appear in any socket record in INCLUDE mode. For example, if the socket records for multicast address m on interface i are: from socket s1: ( i, m, EXCLUDE, {a, b, c, d} ) from socket s2: ( i, m, EXCLUDE, {b, c, d, e} ) from socket s3: ( i, m, INCLUDE, {d, e, f} ) then the corresponding interface record on interface i is: Cain, Deering, Kouvelas, Thyagarajan [Page 6] INTERNET-DRAFT IGMPv3 June 2000 ( m, EXCLUDE, {b, c} ) o if *all* such records have a filter mode of INCLUDE, then the filter mode of the interface record is INCLUDE, and the source list of the interface record is the union of the source lists of all the socket records. For example, if the socket records for multicast address m on interface i are: from socket s1: ( i, m, INCLUDE, {a, b, c} ) from socket s2: ( i, m, INCLUDE, {b, c, d} ) from socket s3: ( i, m, INCLUDE, {e, f} ) then the corresponding interface record on interface i is: ( m, INCLUDE, {a, b, c, d, e, f} ) In order to bound storage consumption, an implementation MAY impose a limit on the size of the source list in an interface record whose filter mode is INCLUDE, but that limit MUST NOT be less than 64 addresses. If such a limit is imposed, whenever the union of the source lists of all the relevant socket records would exceed that limit, an interface record of the following form would be created, instead: ( m, EXCLUDE, {} ) which enables reception of packets to multicast address m from *all* sources. The above rules for deriving the interface state are (re-)evaluated whenever an IPMulticastListen invocation modifies the socket state by adding, deleting, or modifying a per-socket state record. Note that a change of socket state does not necessarily result in a change of interface state. 4. MESSAGE FORMATS IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol number of 2. Every IGMP message described in this document is sent with an IP Time-to-Live of 1, and carries an IP Router Alert option [RFC-2113] in its IP header. There are two IGMP message types of concern to the IGMPv3 protocol described in this document: Cain, Deering, Kouvelas, Thyagarajan [Page 7] INTERNET-DRAFT IGMPv3 June 2000 Type Number (hex) Message Name ----------------- ------------ 0x11 Membership Query 0x22 Version 3 Membership Report An implementation of IGMPv3 MUST also support the following three message types, for interoperation with previous versions of IGMP (see section 7): 0x12 Version 1 Membership Report [RFC-1112] 0x16 Version 2 Membership Report [RFC-2236] 0x17 Version 2 Leave Group [RFC-2236] Unrecognized message types MUST be silently ignored. Other message types may be used by newer versions or extensions of IGMP, by multicast routing protocols, or for other uses. In this document, unless otherwise qualified, the capitalized words "Query" and "Report" refer to IGMP Membership Queries and IGMP Version 3 Membership Reports, respectively. 4.1. Membership Query Message Membership Queries are sent by IP multicast routers to query the multicast reception state of neighboring interfaces. Queries have the following format: Cain, Deering, Kouvelas, Thyagarajan [Page 8] INTERNET-DRAFT IGMPv3 June 2000 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 = 0x11 | Max Resp Time | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Resv |S| QRV | QQI | Number of Sources (N) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address [1] | +- -+ | Source Address [2] | +- . -+ . . . . . . +- -+ | Source Address [N] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.1.1. Max Resp Time The Max Resp Time field specifies the maximum time allowed before sending a responding report, in units of 1/10 second. Varying this setting allows IGMPv3 routers to tune the "leave latency" (the time between the moment the last host leaves a group and the moment the routing protocol is notified that there are no more members). It also allows tuning of the burstiness of IGMP traffic on a network. 4.1.2. Checksum The Checksum is the 16-bit one's complement of the one's complement sum of the whole IGMP message (the entire IP payload). For computing the checksum, the Checksum field is set to zero. When receiving packets, the checksum MUST be verified before processing a packet. 4.1.3. Group Address The Group Address field is set to zero when sending a General Query, and set to the IP multicast address being queried when sending a Group- Specific Query or Group-and-Source-Specific Query (see section 4.1.9, below). 4.1.4. Resv (Reserved) The Resv field is set to zero on transmission, and ignored on reception. Cain, Deering, Kouvelas, Thyagarajan [Page 9] INTERNET-DRAFT IGMPv3 June 2000 4.1.5. S Flag (Suppress Router-Side Processing) When set to one, the S Flag indicates to any receiving multicast routers that they are to suppress the normal timer updates they perform upon hearing a Query. It does not, however, suppress the querier election or the normal "host-side" processing of a Query that a router may be required to perform as a consequence of itself being a group member. 4.1.6. QRV (Querier's Robustness Variable) If non-zero, the QRV field contains the [Robustness Variable] value used by the querier, i.e., the sender of the Query. If the querier's [Robustness Variable] exceeds 7, the maximum value of the QRV field, the QRV is set to zero. Receivers of a Query adopt the QRV value from the most recently received Query as their own [Robustness Variable] value, unless that most recently received QRV was zero, in which case the receivers use the default [Robustness Variable] value specified in section 8.1. 4.1.7. QQI (Querier's Query Interval) If non-zero, the QQI field contains the [Query Interval] value used by the querier, i.e., the sender of the Query, in units of seconds. If the querier's [Query Interval] exceeds 255 seconds, the maximum value of the QQI field, the QQI is set to zero. Multicast routers that are not the current querier adopt the QQI value from the most recently received Query as their own [Query Interval] value, unless that most recently received QQI was zero, in which case the receiving routers use the default [Query Interval] value specified in section 8.2. 4.1.8. Number of Sources (N) The Number of Sources (N) field specifies how many source addresses are present in the Query. This number is zero in a General Query or a Group-Specific Query, and non-zero in a Group-and-Source- Specific Query. This number is limited by the MTU of the network over which the Query is transmitted. For example, on an Ethernet with an MTU of 1500 octets, the IP header including the Router Alert option consumes 24 octets, and the IGMP fields up to including the Number of Sources (N) field consume 12 octets, leaving 1464 octets for source addresses, which limits the number of source addresses to 366 (1464/4). 4.1.9. Source Address [i] The Source Address [i] fields are a vector of n IP unicast addresses, where n is the value in the Number of Sources (N) field. Cain, Deering, Kouvelas, Thyagarajan [Page 10] INTERNET-DRAFT IGMPv3 June 2000 4.1.10. Additional Data If the Packet Length field in the IP header of a received Query indicates that there are additional octets of data present, beyond the fields described here, IGMPv3 implementations MUST include those octets in the computation to verify the received IGMP Checksum, but MUST otherwise ignore those additional octets. When sending a Query, an IGMPv3 implementation MUST NOT include additional octets beyond the fields described here. 4.1.11. Query Variants There are three variants of the Query message: 1 A "General Query" is sent by a multicast router to learn the complete multicast reception state of the neighboring interfaces (that is, the interfaces attached to the network on which the Query is transmitted). In a General Query, both the Group Address field and the Number of Sources (N) field are zero. 2 A "Group-Specific Query" is sent by a multicast router to learn the reception state, with respect to a *single* multicast address, of the neighboring interfaces. In a Group-Specific Query, the Group Address field contains the multicast address of interest, and the Number of Sources (N) field contains zero. 3 A "Group-and-Source-Specific Query" is sent by a multicast router to learn if any neighboring interface desires reception of packets sent to a specified multicast address, from any of a specified list of sources. In a Group-and-Source-Specific Query, the Group Address field contains the multicast address of interest, and the Source Address [i] fields contain the source address(es) of interest. 4.1.12. IP Destination Addresses for Queries In IGMPv3, General Queries are sent with an IP destination address of 224.0.0.1, the all-systems multicast address. Group-Specific and Group- and-Source-Specific Queries are sent with an IP destination address equal to the multicast address of interest. *However*, a system MUST accept and process any Query whose IP Destination Address field contains *any* of the addresses (unicast or multicast) assigned to the interface on which the Query arrives. 4.2. Version 3 Membership Report Message Version 3 Membership Reports are sent by IP systems to report (to neighboring routers) the current multicast reception state, or changes Cain, Deering, Kouvelas, Thyagarajan [Page 11] INTERNET-DRAFT IGMPv3 June 2000 in the multicast reception state, of their interfaces. Reports 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 = 0x22 | Reserved | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Number of Group Records (M) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Group Record [1] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Group Record [2] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Group Record [M] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Cain, Deering, Kouvelas, Thyagarajan [Page 12] INTERNET-DRAFT IGMPv3 June 2000 where each Group Record has the following internal format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Record Type | Aux Data Len | Number of Sources (N) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Multicast Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address [1] | +- -+ | Source Address [2] | +- -+ . . . . . . . . . +- -+ | Source Address [N] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Auxiliary Data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.2.1. Reserved The Reserved fields are set to zero on transmission, and ignored on reception. 4.2.2. Checksum The Checksum is the 16-bit one's complement of the one's complement sum of the whole IGMP message (the entire IP payload). For computing the checksum, the Checksum field is set to zero. When receiving packets, the checksum MUST be verified before processing a message. 4.2.3. Number of Group Records (M) The Number of Group Records (M) field specifies how many Group Records are present in this Report. 4.2.4. Group Record Each Group Record is a block of fields containing information pertaining to the sender's membership in a single multicast group on the interface from which the Report is sent. Cain, Deering, Kouvelas, Thyagarajan [Page 13] INTERNET-DRAFT IGMPv3 June 2000 4.2.5. Record Type See section 4.2.12, below. 4.2.6. Aux Data Len The Aux Data Len field contains the length of the Auxiliary Data field in this Group Record, in units of 32-bit words. It may contain zero, to indicate the absence of any auxiliary data. 4.2.7. Number of Sources (N) The Number of Sources (N) field specifies how many source addresses are present in this Group Record. 4.2.8. Multicast Address The Multicast Address field contains the IP multicast address to which this Group Record pertains. 4.2.9. Source Address [i] The Source Address [i] fields are a vector of n IP unicast addresses, where n is the value in this record's Number of Sources (N) field. 4.2.10. Auxiliary Data The Auxiliary Data field, if present, contains additional information pertaining to this Group Record. The protocol specified in this document, IGMPv3, does not define any auxiliary data. Therefore, implementations of IGMPv3 MUST NOT include any auxiliary data (i.e., MUST set the Aux Data Len field to zero) in any transmitted Group Record, and MUST ignore any auxiliary data present in any received Group Record. The semantics and internal encoding of the Auxiliary Data field are to be defined by any future version or extension of IGMP that uses this field. 4.2.11. Additional Data If the Packet Length field in the IP header of a received Report indicates that there are additional octets of data present, beyond the last Group Record, IGMPv3 implementations MUST include those octets in the computation to verify the received IGMP Checksum, but MUST otherwise ignore those additional octets. When sending a Report, an IGMPv3 implementation MUST NOT include additional octets beyond the last Group Record. Cain, Deering, Kouvelas, Thyagarajan [Page 14] INTERNET-DRAFT IGMPv3 June 2000 4.2.12. Group Record Types There are a number of different types of Group Records that may be included in a Report message: o A "Current-State Record" is sent by a system in response to a Query received on an interface. It reports the current reception state of that interface, with respect to a single multicast address. The Record Type of a Current-State Record may be one of the following two values: Value Name and Meaning ----- ---------------- 1 MODE_IS_INCLUDE - indicates that the interface has a filter mode of INCLUDE for the specified multicast address. The Source Address [i] fields in this Group Record contain the interface's source list for the specified multicast address, if it is non-empty. 2 MODE_IS_EXCLUDE - indicates that the interface has a filter mode of EXCLUDE for the specified multicast address. The Source Address [i] fields in this Group Record contain the interface's source list for the specified multicast address, if it is non-empty. o A "Filter-Mode-Change Record" is sent by a system whenever a local invocation of IPMulticastListen causes a change of the filter mode (i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to INCLUDE), of the interface-level state entry for a particular multicast address. The Record is included in a Report sent from the interface on which the change occurred. The Record Type of a Filter-Mode-Change Record may be one of the following two values: 3 CHANGE_TO_INCLUDE_MODE - indicates that the interface has changed to INCLUDE filter mode for the specified multicast address. The Source Address [i] fields in this Group Record contain the interface's new source list for the specified multicast address, if it is non-empty. 4 CHANGE_TO_EXCLUDE_MODE - indicates that the interface has changed to EXCLUDE filter mode for the specified multicast address. The Source Address [i] fields in this Group Record contain the interface's new source list for the specified multicast address, if it is non-empty. Cain, Deering, Kouvelas, Thyagarajan [Page 15] INTERNET-DRAFT IGMPv3 June 2000 o A "Source-List-Change Record" is sent by a system whenever a local invocation of IPMulticastListen causes a change of source list that is *not* coincident with a change of filter mode, of the interface-level state entry for a particular multicast address. The Record is included in a Report sent from the interface on which the change occurred. The Record Type of a Source-List-Change Record may be one of the following two values: 5 ALLOW_NEW_SOURCES - indicates that the Source Address [i] fields in this Group Record contain a list of the additional sources that the system wishes to hear from, for packets sent to the specified multicast address. If the change was to an INCLUDE source list, these are the addresses that were added to the list; if the change was to an EXCLUDE source list, these are the addresses that were deleted from the list. 6 BLOCK_OLD_SOURCES - indicates that the Source Address [i] fields in this Group Record contain a list of the sources that the system no longer wishes to hear from, for packets sent to the specified multicast address. If the change was to an INCLUDE source list, these are the addresses that were deleted from the list; if the change was to an EXCLUDE source list, these are the addresses that were added to the list. If a change of source list results in both allowing new sources and blocking old sources, then two Group Records are sent for the same multicast address, one of type ALLOW_NEW_SOURCES and one of type BLOCK_OLD_SOURCES. We use the term "State-Change Record" to refer to either a Filter- Mode- Change Record or a Source-List-Change Record. Unrecognized Record Type values MUST be silently ignored. 4.2.13. IP Destination Addresses for Reports Version 3 Reports are sent with an IP destination address of 224.0.0.22, to which all IGMPv3-capable multicast routers listen. A system that is operating in version 1 or version 2 compatibility modes sends version 1 or version 2 Reports to the multicast group specified in the Group Address field of the Report. In addition, a system MUST accept and process any version 1 or version 2 Report whose IP Destination Address Cain, Deering, Kouvelas, Thyagarajan [Page 16] INTERNET-DRAFT IGMPv3 June 2000 field contains *any* of the addresses (unicast or multicast) assigned to the interface on which the Report arrives. 4.2.14. Notation for Group Records In the rest of this document, we use the following notation to describe the contents of a Group Record pertaining to a particular multicast address: IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x IS_EX ( x ) - Type MODE_IS_EXCLUDE, source addresses x TO_IN ( x ) - Type CHANGE_TO_INCLUDE_MODE, source addresses x TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x where x is either: o a capital letter (e.g., "A") to represent the set of source addresses, or o a set expression (e.g., "A+B"), where "A+B" means the union of sets A and B, "A*B" means the intersection of sets A and B, and "A-B" means the removal of all elements of set B from set A. 4.2.15. Membership Report Size If the set of Group Records required in a Report does not fit within the size limit of a single Report message (as determined by the MTU of the network on which it will be sent), the Group Records are sent in as many Report messages as needed to report the entire set. If a single Group Record contains so many source addresses that it does not fit within the size limit of a single Report message, if its Type is not MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, it is split into multiple Group Records, each containing a different subset of the source addresses and each sent in a separate Report message. If its Type is MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single Group Record is sent, containing as many source addresses as can fit, and the remaining source addresses are not reported; though the choice of which sources to report is arbitrary, it is preferable to report the same set of sources in each subsequent report, rather than reporting different sources each time. Cain, Deering, Kouvelas, Thyagarajan [Page 17] INTERNET-DRAFT IGMPv3 June 2000 5. DESCRIPTION OF THE PROTOCOL FOR GROUP MEMBERS IGMP is an asymmetric protocol, specifying separate behaviors for group members -- that is, hosts or routers that wish to receive multicast packets -- and multicast routers. This section describes the part of IGMPv3 that applies to all group members. (Note that a multicast router that is also a group member performs both parts of IGMPv3, receiving and responding to its own IGMP message transmissions as well as those of its neighbors. The multicast router part of IGMPv3 is described in section 6.) A system performs the protocol described in this section over all interfaces on which multicast reception is supported, even if more than one of those interfaces is connected to the same network. For interoperability with multicast routers running older versions of IGMP, systems maintain a MulticastRouterVersion variable for each interface on which multicast reception is supported. This section describes the behavior of group member systems on interfaces for which MulticastRouterVersion = 3. The algorithm for determining MulticastRouterVersion, and the behavior for versions other than 3, are described in section 7. The all-systems multicast address, 224.0.0.1, is handled as a special case. On all systems -- that is all hosts and routers, including multicast routers -- reception of packets destined to the all-systems multicast address, from all sources, is permanently enabled on all interfaces on which multicast reception is supported. No IGMP messages are ever sent regarding the all-systems multicast address. There are two types of events that trigger IGMPv3 protocol actions on an interface: o a change of the interface reception state, caused by a local invocation of IPMulticastListen. o reception of a Query. (Received IGMP messages of types other than Query are silently ignored, except as required for interoperation with earlier versions of IGMP.) The following subsections describe the actions to be taken for each of these two cases. In those descriptions, timer and counter names appear in square brackets. The default values for those timers and counters are specified in section 8. Cain, Deering, Kouvelas, Thyagarajan [Page 18] INTERNET-DRAFT IGMPv3 June 2000 5.1. Action on Change of Interface State An invocation of IPMulticastListen may cause the multicast reception state of an interface to change, according to the rules in section 3.2. Each such change affects the per-interface entry for a single multicast address. A change of interface state causes the system to immediately transmit a State-Change Report from that interface. The type and contents of the Group Record(s) in that Report are determined by comparing the filter mode and source list for the affected multicast address before and after the change, according to the table below. If no interface state existed for that multicast address before the change (i.e., the change consisted of creating a new per-interface record), or if no state exists after the change (i.e., the change consisted of deleting a per-interface record), then the "non-existent" state is considered to have a filter mode of INCLUDE and an empty source list. Old State New State State-Change Record Sent --------- --------- ------------------------ INCLUDE (A) INCLUDE (B) ALLOW (B-A), BLOCK (A-B) EXCLUDE (A) EXCLUDE (B) ALLOW (A-B), BLOCK (B-A) INCLUDE (A) EXCLUDE (B) TO_EX (B) EXCLUDE (A) INCLUDE (B) TO_IN (B) If the computed source list for either an ALLOW or a BLOCK State-Change Record is empty, that record is omitted from the Report message. To cover the possibility of the State-Change Report being missed by one or more multicast routers, it is retransmitted [Robustness Variable] - 1 more times, at intervals chosen at random from the range (0, [Unsolicited Report Interval]). If more changes to the same interface state entry occur before all the retransmissions of the State-Change Report for the first change have been completed, each such additional change triggers the immediate transmission of a new State-Change Report. The contents of the new transmitted report are calculated as follows. As was done with the first report, the interface state for the affected group before and after the latest change is compared. The report records expressing the difference are built according to the table above. However these records are not transmitted in a message but instead merged with the contents of the pending report, to create the new State- Cain, Deering, Kouvelas, Thyagarajan [Page 19] INTERNET-DRAFT IGMPv3 June 2000 Change report. The rules for merging the difference report resulting from the state change and the pending report are described below. The transmission of the merged State-Change Report terminates retransmissions of the earlier State-Change Reports for the same multicast address, and becomes the first of [Robustness Variable] transmissions of State-Change Reports. Each time a source is included in the difference report calculated above, retransmission state for that source needs to be maintained until [Robustness Variable] State-Change reports have been sent by the host. This is done in order to ensure that a series of successive state changes do not break the protocol robustness. If the interface reception-state change that triggers the new report is a filter-mode change, then the next [Robustness Variable] State-Change Reports will include a Filter-Mode-Change record. This applies even if any number of source-list changes occur in that period. The host has to maintain retransmission state for the group until the [Robustness Variable] State-Change reports have been sent. When [Robustness Variable] State-Change reports with Filter-Mode-Change records have been transmitted after the last filter-mode change, and if source-list changes to the interface reception have scheduled additional reports, then the next State-Change report will include Source-List-Change records. Each time a State-Change Report is transmitted, the contents are determined as follows. If the report should contain a Filter-Mode-Change record, then if the current filter-mode of the interface is INCLUDE, a TO_IN record is included in the report, otherwise a TO_EX record is included. If instead the report should contain Source-List-Change records, an ALLOW and a BLOCK record are included. The contents of these records are built according to the table below. Record Sources included ------ ---------------- TO_IN All in the current interface state that must be forwarded TO_EX All in the current interface state that must be blocked ALLOW All with retransmission state that must be forwarded BLOCK All with retransmission state that must be blocked If the computed source list for either an ALLOW or a BLOCK record is empty, that record is omitted from the State-Change report. Note: When the first State-Change report is sent, the non-existent pending report to merge with, can be treated as a source-change report with empty ALLOW and BLOCK records (no sources have retransmission state). Cain, Deering, Kouvelas, Thyagarajan [Page 20] INTERNET-DRAFT IGMPv3 June 2000 5.2. Action on Reception of a Query When a system receives a Query, it does not respond immediately. Instead, it delays its response by a random amount of time, bounded by the Max Resp Time value in the received Query message. A system may receive a variety of Queries from different interfaces and of different kinds (e.g., General Queries, Group-Specific Queries, and Group-and- Source-Specific 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 combined response. Therefore, the system must be able to maintain the following state: o A timer per interface for scheduling responses to General Queries. o A per-group and interface timer for scheduling responses to Group- Specific and Group-and-Source-Specific Queries. o A per-group list of sources to be reported in the response to a Group- and-Source-Specific Query. When a new Query with the Router-Alert option arrives on an interface, provided the system has state to report, a delay for a response is randomly selected in the range (0, [Max-Resp-Time in Query]). The following rules are then used to determine if a Report needs to be scheduled and the type of Report to schedule: o If there is a pending response to a previous General Query scheduled sooner than the selected delay, no additional response needs to be scheduled. o If the received Query is a General Query, the interface timer is used to schedule a response to the General Query after the selected delay. Any previously pending response to a General Query is canceled. o If the received Query is a Group-Specific Query or a Group-and-Source- Specific Query and there is no pending response to a previous Query for this group, then the group timer is used to schedule a report. If the received Query is a Group-and-Source-Specific Query, the list of queried sources is recorded to be used when generating a response. o If there already is a pending response to a previous Query scheduled for this group, and either the new Query is a Group-Specific Query or the recorded source-list associated with the group is empty, then the group source-list is cleared and a single response is scheduled using the group timer. The new response is scheduled to be sent at the Cain, Deering, Kouvelas, Thyagarajan [Page 21] INTERNET-DRAFT IGMPv3 June 2000 earliest of the remaining time for the pending report and the selected delay. o If the received Query is a Group-and-Source-Specific Query and there is a pending response for this group with a non-empty source-list, then the group source list is augmented to contain the list of sources in the new Query and a single response is scheduled using the group timer. The new response is scheduled to be sent at the earliest of the remaining time for the pending report and the selected delay. When the timer in a pending response record expires, the system transmits, on the associated interface, one or more Report messages carrying one or more Current-State Records (see section 4.2.9(1)), as follows: o If the expired timer is the interface timer (i.e., it is a pending response to a General Query), then one Current-State Record is sent for each multicast address for which the specified interface has reception state, as described in section 3.2. The Current-State Record carries the multicast address and its associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list. Multiple Current-State Records are packed into individual Report messages, to the extent possible. o If the expired timer is a group timer and the list of recorded sources for the that group is empty (i.e., it is a pending response to a Group-Specific Query), then if and only if the interface has reception state for that group address, a single Current-State Record is sent for that address. The Current-Report Record carries the multicast address and its associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list. o If the expired timer is a group timer and the list of recorded sources for that group is non-empty (i.e., it is a pending response to a Group-and-Source-Specific Query), then if and only if the interface has reception state for that group address, the contents of the responding Current-State Record is determined from the interface state and the pending response record, as specified in the following table: set of sources in the interface state pending response record Current-State Record --------------- ----------------------- -------------------- INCLUDE (A) B IS_IN (A*B) EXCLUDE (A) B IS_IN (B-A) Cain, Deering, Kouvelas, Thyagarajan [Page 22] INTERNET-DRAFT IGMPv3 June 2000 If the resulting Current-State Record has an empty set of source addresses, then no response is sent. Finally, after any required Report messages have been generated, the source lists associated with any reported groups are cleared. 6. DESCRIPTION OF THE PROTOCOL FOR MULTICAST ROUTERS The purpose of IGMP is to enable each multicast router to learn, for each of its directly attached networks, which multicast addresses are of interest to the systems attached to those networks. 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 networks where there are interested receivers. This section describes the part of IGMPv3 that is performed by multicast routers. Multicast routers may also themselves become members of multicast groups, and therefore also perform the group member part of IGMPv3, described in section 5. A multicast router performs the protocol described in this section over each of its directly-attached networks. If a multicast router has more than one interface to the same network, 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 224.0.0.22, from all sources (and MUST perform the group member part of IGMPv3 for that address on that interface). Multicast routers need to know only that *at least one* system on an attached network is interested in packets to a particular multicast address from a particular source; a multicast router is not required to keep track of the interests of each individual neighboring system. IGMPv3 is backward compatible with previous versions of the IGMP protocol. In order to remain backward compatible with older IGMP systems, IGMPv3 multicast routers MUST also implement versions 1 and 2 of the protocol (see section 7). 6.1. Conditions for IGMP Queries Multicast routers send General Queries periodically to request group membership information from an attached network. These queries are used to build and refresh the group membership state of systems on attached Cain, Deering, Kouvelas, Thyagarajan [Page 23] INTERNET-DRAFT IGMPv3 June 2000 networks. Systems respond to these queries by reporting their group membership state (and their desired set of sources) with Current-State Group Records in IGMPv3 Membership Reports. As a member of a multicast group, a system may express interest in receiving or not receiving traffic from particular sources. As the desired reception state of a system changes, it reports these changes using Filter-Mode-Change Records or Source-List-Change Records. These records indicate an explicit state change in a group at a system in either the group record's source list or its filter-mode. When a group membership is terminated at a system or traffic from a particular source is no longer desired, a multicast router must query for other members of the group or listeners of the source before deleting the group (or source) and pruning its traffic. To enable all systems on a network to respond to changes in group membership, multicast routers send specific queries. A Group- Specific Query is sent to verify there are no systems that desire reception of the specified group or to "rebuild" the desired reception state for a particular group. Group-Specific Queries are sent when a router receives a State-Change record indicating a system is leaving a group. A Group-and-Source Specific Query is used to verify there are no systems on a network which desire to receive traffic from a set of sources. Group-and-Source Specific Queries list sources for a particular group which have been requested to no longer be forwarded. This query is sent by a multicast router to learn if any systems desire reception of packets to the specified group address from the specified source addresses. Group-and-Source Specific Queries are only sent in response to State-Change Records and never in response to Current-State Records. Section 4.1.8 describes each query in more detail. 6.2. IGMP State Maintained by Multicast Routers Multicast routers implementing IGMPv3 keep state per group per attached network. This group state consists of a filter-mode, a list of sources, and various timers. For each attached network running IGMP, a multicast router records the desired reception state for that network. That state conceptually consists of a set of records of the form: (multicast address, group timer, filter-mode, (source records) ) Each source record is of the form: (source address, source timer) Cain, Deering, Kouvelas, Thyagarajan [Page 24] INTERNET-DRAFT IGMPv3 June 2000 If all sources within a given group are desired, an empty source record list is kept with filter-mode set to EXCLUDE. This means forward all sources for this group. This is the IGMPv3 equivalent to a IGMPv1 or IGMPv2 group join. 6.2.1. Definition of Router Filter-Mode To reduce internal state, IGMPv3 routers keep a filter-mode per group per attached network. This filter-mode is used to condense the total desired reception state of a group to a minimum set such that all systems' memberships are satisfied. This filter-mode may change in response to the reception of particular types of group records or when certain timer conditions occur. In the following sections, we use the term "router filter-mode" to refer to the filter-mode of a particular group within a router. Section 6.4 describes the changes of a router filter-mode per group record received. Conceptually, when a group record is received, the router filter-mode for that group is updated to represent the most number of sources desired with the least amount of state. As a rule, once a group record with a filter-mode of EXCLUDE is received, the router filter-mode for that group will be EXCLUDE. When a router filter-mode for a group is EXCLUDE, the source record list contains two types of sources. The first type is the set of sources which are not being forwarded (i.e. the set that all systems have agreed NOT to forward). The second set is the set which represents conflicts in the desired reception state; this set must be forwarded. Appendix A describes the reasons for keeping this second set when in EXCLUDE mode. When a router filter-mode for a group is INCLUDE, the source record list is the list of sources desired for the group. When a router filter-mode for a group is INCLUDE, the source list is the total desired set of sources for that group. Each source in a source record must be forwarded. Because a reported group record with a filter-mode of EXCLUDE will cause a router to transition its filter-mode for that group to EXCLUDE, a mechanism for transitioning a router's filter-mode back to INCLUDE must exist. If all systems with a group record with filter-mode EXCLUDE mode cease reporting, it is desirable for the router filter-mode for that group to transition back to INCLUDE mode (which will likely cause fewer sources to be forwarded). This transition occurs when the group timer expires and is explained in detail in section 6.5. Cain, Deering, Kouvelas, Thyagarajan [Page 25] INTERNET-DRAFT IGMPv3 June 2000 6.2.2. Definition of Group Timers The group timer is only used when a group is in EXCLUDE mode and it represents the time for the *filter-mode* of the group to expire and switch to INCLUDE mode. We define a group timer as a decrementing timer with a lower bound of zero kept per group per attached network. Group timers are updated according to the types of group records received. A group timer expiring when a router filter-mode for the group is EXCLUDE means there are no listeners on the attached network in EXCLUDE mode. At this point, a router will transition to INCLUDE filter-mode. Section 6.5 describes the actions taken when a group timer expires while in EXCLUDE mode. The following table summarizes the role of the group timer. Section 6.4 describes the details of setting the group timer per type of group record received. Group Filter-Mode Group Timer Value Actions/Comments ----------- ----------------- ---------------- INCLUDE Timer >= 0 All members in INCLUDE mode. EXCLUDE Timer > 0 At least one member in EXCLUDE mode. EXCLUDE Timer == 0 No more listeners to group. If all source timers have expired then delete Group Record. If there are still source record timers running, switch to INCLUDE filter-mode using those source records with running timers as the INCLUDE source record state. 6.2.3. Definition of Source Timers A source timer is kept per source record and is a decrementing timer with a lower bound of zero. Source timers are updated according to the type and filter-mode of the group record received. Source timers are Cain, Deering, Kouvelas, Thyagarajan [Page 26] INTERNET-DRAFT IGMPv3 June 2000 always updated (for a particular group) whenever the source is present in a received record for that group. Section 6.4 describes the setting of source timers per type of group records received. A source record with a running timer with a router filter-mode for the group of INCLUDE means that there is currently one or more systems (in INCLUDE filter-mode) which desire to receive that source. If a source timer expires with a router filter-mode for the group of INCLUDE, the router concludes that traffic from this particular source is no longer desired on the attached network, and deletes the associated source record. Source timers are treated differently when a router filter-mode for a group is EXCLUDE. If a source record has a running timer with a router filter-mode for the group of EXCLUDE, it means that a "conflict" has occurred with respect to desired reception state of that source (i.e. one system desires the source and another one doesn't). Therefore, when a source record has a running timer with a router filter-mode for the group of EXCLUDE it should be forwarded. Appendix A describes the reasons for keeping sources that are being forwarded while in EXCLUDE state. If a source timer expires with a router filter-mode for the group of EXCLUDE, the router stops forwarding traffic from this source onto the network (and takes appropriate actions dependent on the multicast routing protocol). When a router filter-mode for a group is EXCLUDE, source records are only deleted when the group timer expires. Section 6.3 describes the actions that should be taken dependent upon the value of a source timer. 6.3. IGMPv3 Source-Specific Forwarding Rules When a multicast router receives a datagram from a source destined to a particular group, a decision has to be made whether to forward the datagram onto an attached network or not. This decision is first dependent on any multicast routing protocols present. Assuming that there are no other routers downstream (or that downstream routers support source-specific pruning/grafting), the forwarding decision depends on the presence of a group record and/or a source record. If no group record exists, the datagram is not forwarded on the network. If a source record exists which matches the datagrams source address, the source is forwarded according to the router filter-mode for the group and the value of the source timer of the source record. To summarize, the following table describes the forwarding actions for traffic originating from a source destined to a group. It also Cain, Deering, Kouvelas, Thyagarajan [Page 27] INTERNET-DRAFT IGMPv3 June 2000 summarizes the actions taken upon the expiration of a source timer based on the router filter-mode of the group. Group Filter-Mode Source Timer Value Action ----------- ------------------ ------ INCLUDE TIMER > 0 Forward traffic from source INCLUDE TIMER == 0 Stop forwarding traffic from source and remove source record INCLUDE No Source Elements Don't forward source EXCLUDE TIMER > 0 Forward traffic from source EXCLUDE TIMER == 0 Don't forward traffic from source (DO NOT remove record) EXCLUDE No Source Elements Forward traffic from source 6.4. Action on Reception of Reports 6.4.1. Reception of Current-State Records When receiving Current-State Records, a router updates both its group and source timers. In some circumstances, the reception of a type of group record will cause the router filter-mode for that group to change. The table below describes the actions, with respect to state and timers that occur to a router's state upon reception of Current-State Records. The following notation is used to describe the updating of source timers. The notation ( A, B ) will be used to represent the total number of sources for a particular group, where A = set of source records whose source timers > 0 (being forwarded) B = set of source records whose source timers = 0 (not being forwarded) Note that there will only be two sets when a router's filter-mode for a group is EXCLUDE. When a router's filter-mode for a group is INCLUDE, a single set is used to describe the set of sources being forwarded (e.g. simply ( A ) ). Cain, Deering, Kouvelas, Thyagarajan [Page 28] INTERNET-DRAFT IGMPv3 June 2000 In the following tables, abbreviations are used for several variables (all of which are described in detail in section 8). The variable GMI is an abbreviation for the Group Membership Interval which is the time in which group memberships will time out. The variable LMQI is an abbreviation for the Last Member Query Interval (default 1s) which is the Maximum Response Time in Group or Group and Source Specific Queries. Within the "Actions" section of the router state tables, we use the notation 'A=J', which means that the set A of source records should have their source timers set to value J. 'Delete A' means that the set A of source records should be deleted. 'Group Timer=J' means that the Group Timer for the group should be set to value J. Router State Report Rec'd New Router State Actions ------------ ------------ ---------------- ------- INCLUDE (A) IS_IN (B) INCLUDE (A+B) (B)=GMI INCLUDE (A) IS_EX (B) EXCLUDE (A*B,B-A) (B-A)=0 Delete (A-B) Group Timer=GMI EXCLUDE (X,Y) IS_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI EXCLUDE (X,Y) IS_EX (A) EXCLUDE (X*A+(A-Y),Y*A) (A-X-Y)=GMI Delete (X-A) Delete (Y-A) Group Timer=GMI 6.4.2. Reception of Filter-Mode-Change and Source-List-Change Records When a change in the global state of a group occurs in a system, the system sends either a Source-List-Change Record or a Filter-Mode-Change Record for that group. As with Current-State Records, routers must act upon these records and possibly change their own state to reflect the new desired membership state of the network. Routers must query sources that are requested to be no longer forwarded to a group. When a router queries or receives a query for a specific set of sources, it lowers its source timers for those source to a small interval of Last Member Query Interval seconds. If group records are received in response to the queries which express interest in receiving traffic from the queried sources, the corresponding timers are updated. Similarly, when a router queries a specific group, it lowers its group timer for that group to a small interval of Last Member Query Interval seconds. If any group records expressing EXCLUDE mode interest in the Cain, Deering, Kouvelas, Thyagarajan [Page 29] INTERNET-DRAFT IGMPv3 June 2000 group are received within the interval, the group timer for the group is updated and traffic is forwarded without any interruption. During a query period (i.e. Last Member Query Interval seconds), a router continues to forward traffic from the groups or sources that it is querying. It is not until after Last Member Query Interval seconds without receiving a record expressing interest in the queried group or sources that the router may prune the group or sources from the setwork. The following table describes the changes in group state and the action(s) taken when receiving either Filter-Mode-Change or Source-List- Change Records. This table also describes the queries which are sent by the querier when a particular report is received. We use the following notation for describing the queries which are sent. We use the notation 'Q(G) to describe a Group-Specific Query to G. We use the notation 'Q(G,A)' to describe a Group-and-Source Specific Query to G with source-list A. If source-list A is null as a result of the action (e.g. A*B) then no query is sent as a result of the operation. In order to maintain protocol robustness, queries sent by actions in the table below need to be transmitted [Last Member Query Count] times within the [Last Member Query Interval] period. If while scheduling new queries, there are already pending queries to be retransmitted for the same group, the new and pending queries have to be merged. In addition, received host reports for a group with pending queries may affect the contents of those queries. Section 6.6.3 describes the process of building and maintaining the state of pending queries. Cain, Deering, Kouvelas, Thyagarajan [Page 30] INTERNET-DRAFT IGMPv3 June 2000 Router State Report Rec'd New Router State Actions ------------ ------------ ---------------- ------- INCLUDE (A) ALLOW (B) INCLUDE (A+B) (B)=GMI INCLUDE (A) BLOCK (B) INCLUDE (A) Send Q(G,A*B) INCLUDE (A) ALLOW (B), INCLUDE (A+B) (B)=GMI BLOCK (C) Send Q(G,A*C) INCLUDE (A) TO_EX (B) EXCLUDE (A*B,B-A) (B-A)=0 Delete (A-B) Send Q(G,A*B) Group Timer=GMI INCLUDE (A) TO_IN (B) INCLUDE (A+B) (B)=GMI Send Q(G,A-B) EXCLUDE (X,Y) ALLOW (A) EXCLUDE (X+A,Y-A) (A)=GMI EXCLUDE (X,Y) BLOCK (A) EXCLUDE (X+(A-Y),Y) Send Q(G,A-Y) EXCLUDE (X,Y) ALLOW (A), EXCLUDE (X+A+(B-Y),Y-A) (A)=GMI BLOCK (B) Send Q(G,B-Y) EXCLUDE (X,Y) TO_EX (A) EXCLUDE (X*A+(A-Y),Y*A) (A-X-Y)=LMQI Delete (X-A) Delete (Y-A) Send Q(G,A-Y) Group Timer=GMI EXCLUDE (X,Y) TO_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI Send Q(G,X-A) Send Q(G) 6.5. Switching Router Filter-Modes The group timer is used as a mechanism for transitioning the router filter-mode from EXCLUDE to INCLUDE. When a group timer expires with a router filter-mode of EXCLUDE, a router assumes that there are no systems with a *filter-mode* of EXCLUDE present on the attached network. When a router's filter-mode for a group is EXCLUDE and the group timer expires, the router filter-mode for the group transitions to INCLUDE. Cain, Deering, Kouvelas, Thyagarajan [Page 31] INTERNET-DRAFT IGMPv3 June 2000 A router uses source records with running source timers as its state for the switch to a filter-mode of INCLUDE. If there are any source records with source timers greater than zero (i.e. being forwarded), a router switches to filter-mode of INCLUDE using those source records. Source records whose timers are zero (from the previous EXCLUDE mode) are deleted. For example, if a router's state for a group is EXCLUDE(X,Y) and the group timer expires for that group, the router switches to filter-mode of INCLUDE with state INCLUDE(X). 6.6. Action on Reception of Queries 6.6.1. Timer Updates When a router sends or receives a query with a clear Suppress Router- Side Processing flag, it must update its timers to reflect the correct timeout values for the group or sources being queried. The following table describes the timer actions when sending or receiving a Group- Specific or Group-and-Source Specific Query with the Suppress Router- Side Processing flag not set. Query Action ----- ------ Q(G,A) Source Timer for sources in A are lowered to LMQI Q(G) Group Timer is lowered to LMQI 6.6.2. Querier Election IGMPv3 elects a single querier per subnet using the same querier election mechanism as IGMPv2, namely by IP address. When a router receives a query with a lower IP address, it sets the Other-Querier- Present timer to Other Querier Present Interval and ceases to send queries on the network if it was the previously elected querier. After its Other-Querier Present timer expires, it should begin sending General Queries. If a router receives an older version query, it MUST use the oldest version of IGMP on the network. For a detailed description of compatibility issues between IGMP versions see section 7. 6.6.3. Building and Sending Specific Queries Cain, Deering, Kouvelas, Thyagarajan [Page 32] INTERNET-DRAFT IGMPv3 June 2000 6.6.3.1. Building and Sending Group Specific Queries When a table action "Send Q(G)" is encountered, then the group timer must be lowered to LMQI. The router must then immediately send a group specific query as well as schedule [Last Member Query Count - 1] query retransmissions to be sent within [Last Member Query Interval]. When transmitting a group specific query, if the group timer is larger than LMQI, the "Suppress Router-Side Processing" bit is set in the query message. 6.6.3.2. Building and Sending Group and Source Specific Queries When a table action "Send Q(G,X)" is encountered by a querier in the table in section 6.4.2, the following actions must be performed for each of the sources in X of group G, with source timer larger than LMQI: o Set number of retransmissions for each source to [Last Member Query Count]. o Lower source timer to LMQI. The router must then immediately send a group and source specific query as well as schedule [Last Member Query Count -1] query retransmissions to be sent within [Last Member Query Interval]. The contents of these queries are calculated as follows. When building a group and source specific query for a group G, two separate query messages are sent for the group. The first one has the "Suppress Router-Side Processing" bit set and contains all the sources with retransmission state and timers greater than LMQI. The second has the "Suppress Router-Side Processing" bit clear and contains all the sources with retransmission state and timers lower or equal to LMQI. If either of the two calculated messages does not contain any sources, then its transmission is suppressed. Note: If a group specific query is scheduled to be transmitted at the same time as a group and source specific query for the same group, then transmission of the group and source specific message with the "Suppress Router-Side Processing" bit set may be suppressed. 7. INTEROPERATION WITH OLDER VERSIONS OF IGMP IGMP version 3 hosts and routers interoperate with hosts and routers that have not yet been upgraded to IGMPv3. This compatibility is maintained by hosts and routers taking appropriate actions depending on Cain, Deering, Kouvelas, Thyagarajan [Page 33] INTERNET-DRAFT IGMPv3 June 2000 the versions of IGMP operating on hosts and routers within a network. 7.1. Query Version Distinctions The IGMP version of a Membership Query message is determined as follows: IGMPv1 Query: length = 8 octets AND Max Response Time field is zero IGMPv2 Query: length = 8 octets AND Max Response Time field is non-zero IGMPv3 Query: length >= 12 octets Query messages that do not match any of the above conditions (e.g., a Query of length 10 octets) MUST be silently ignored. 7.2. Group Member Behavior 7.2.1. In the Presence of Older Version Queriers In order to be compatible with older version routers, IGMPv3 hosts MUST operate in version 1 and version 2 compatibility modes. IGMPv3 hosts MUST keep state per local interface regarding the compatibility mode of each attached network. A host's compatibility 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. In order to switch gracefully between versions of IGMP, hosts keep both an IGMPv1 Querier Present timer and an IGMPv2 Querier Present timer per interface. IGMPv1 Querier Present is set to Older Version Querier Present Timeout seconds whenever an IGMPv1 Membership Query is received. IGMPv2 Querier Present is set to Older Version Querier Present Timeout seconds whenever an IGMPv2 Membership Query is received. The Host Compatibility Mode of an interface changes whenever an older version query (than the current compatibility mode) is heard or when certain timer conditions occur. When IGMPv1 Querier Present timer expires, a host switches to Host Compatibility mode of IGMPv2 if it has a running IGMPv2 Querier Present timer. If it does not have a running IGMPv2 Querier Present timer then it switches to Host Compatibility of IGMPv3. When IGMPv2 Querier Present timer expires, a host switches to Host Compatibility mode of IGMPv3. Cain, Deering, Kouvelas, Thyagarajan [Page 34] INTERNET-DRAFT IGMPv3 June 2000 The Host Compatibility Mode variable is based on whether an older version query was heard in the last Older Version Querier Present Timeout seconds. The Host Compatibility Mode is set depending on the following: Host Compatibility Mode Timer State ----------------------- ----------- IGMPv3 (default) IGMPv2 Querier Present = 0 and IGMPv1 Querier Present = 0 IGMPv2 IGMPv2 Querier Present > 0 and IGMPv1 Querier Present = 0 IGMPv1 IGMPv1 Querier Present > 0 If a host receives a query which causes its Querier Present timers to be updated and correspondingly its compatibility mode, it should switch compatibility modes immediately. When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3 protocol on that interface. When Host Compatibility Mode is IGMPv2, a host acts in IGMPv2 compatibility mode, using only the IGMPv2 protocol, on that interface. When Host Compatibility Mode is IGMPv1, a host acts in IGMPv1 compatibility mode, using only the IGMPv1 protocol on that interface. An 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). Whenever a host changes its compatability mode, it cancels all its pending response and retransmission timers. 7.2.2. In the Presence of Older Version Group Members An IGMPv3 host may be placed on a network where there are hosts that have not yet been upgraded to IGMPv3. A host MAY allow its IGMPv3 Membership Record to be suppressed by either a Version 1 Membership Report, or a Version 2 Membership Report. 7.3. Multicast Router Behavior 7.3.1. In the Presence of Older Version Queriers IGMPv3 routers may be placed on a network where at least one router on the network has not yet been upgraded to IGMPv3. The following Cain, Deering, Kouvelas, Thyagarajan [Page 35] INTERNET-DRAFT IGMPv3 June 2000 requirements apply: o If any older versions of IGMP are present on routers, the querier MUST use the lowest version of IGMP present on the network. This must be administratively assured; routers that desire to be compatible with IGMPv1 and IGMPv2 MUST have a configuration option to act in IGMPv1 or IGMPv2 compatibility modes. When in IGMPv1 mode, routers MUST send Periodic Queries with a Max Response Time of 0, and MUST ignore Leave Group messages. They SHOULD also warn about receiving an IGMPv2 or IGMPv3 query, although such warnings MUST be rate-limited. o If a router is not explicitly configured to use IGMPv1 and hears an IGMPv1 Query or IGMPv2 Query, it SHOULD log a warning. These warnings MUST be rate-limited. 7.3.2. In the Presence of Older Version Group Members IGMPv3 routers may be placed on a network where there are hosts that have not yet been upgraded to IGMPv3. In order to be compatible with older version hosts, IGMPv3 routers MUST operate in version 1 and version 2 compatibility modes. IGMPv3 routers keep a compatibility mode per group record. A group's compatibility mode is determined from the Group Compatibility Mode variable which can be in one of three states: IGMPv1, IGMPv2 or IGMPv3. This variable is kept per group record and is dependent on the version of Membership Reports heard for that group as well as the Older Version Host Present timer for the group. 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. The IGMPv1 Host Present timer is set to Older Version Host Present Timeout seconds whenever an IGMPv1 Membership Report is received. The IGMPv2 Host Present timer is set to Older Version Host Present Timeout seconds whenever an IGMPv2 Membership Report is received. The Group Compatibility Mode of a group record changes whenever an older version report (than the current compatibility mode) is heard or when certain timer conditions occur. When IGMPv1 Host Present timer expires, a router switches to Group Compatibility mode of IGMPv2 if it has a running IGMPv2 Host Present timer. If it does not have a running IGMPv2 Host Present timer then it switches to Group Compatibility of IGMPv3. When IGMPv2 Host Present timer expires, a router switches to Group Compatibility mode of IGMPv3. The Group Compatibility Mode variable is based on whether an older version report was heard in the last Older Version Host Present Timeout seconds. The Group Compatibility Mode is set depending on the Cain, Deering, Kouvelas, Thyagarajan [Page 36] INTERNET-DRAFT IGMPv3 June 2000 following: Group Compatibility Mode Timer State ------------------------ ----------- IGMPv3 (default) IGMPv2 Host Present = 0 and IGMPv1 Host Present = 0 IGMPv2 IGMPv2 Host Present > 0 and IGMPv1 Host Present = 0 IGMPv1 IGMPv1 Host Present > 0 If a router receives a report which causes its older Host Present timers to be updated and correspondingly its compatibility mode, it SHOULD switch compatibility modes immediately. 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 compatibility mode, using only the IGMPv2 protocol for that group. When Group Compatibility Mode is IGMPv1, a router acts in IGMPv1 compatibility mode, using only the IGMPv1 protocol for that group. 8. LIST OF TIMERS, COUNTERS, AND THEIR DEFAULT VALUES Most of these timers are configurable. If non-default settings are used, they MUST be consistent among all systems on a single link. Note that parentheses are used to group expressions to make the algebra clear. 8.1. Robustness Variable The Robustness Variable allows tuning for the expected packet loss on a network. If a network 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 8.2. Query Interval The Query Interval is the interval between General Queries sent by the Querier. Default: 125 seconds. Cain, Deering, Kouvelas, Thyagarajan [Page 37] INTERNET-DRAFT IGMPv3 June 2000 By varying the [Query Interval], an administrator may tune the number of IGMP messages on the network; larger values cause IGMP Queries to be sent less often. 8.3. Query Response Interval The Max Response Time inserted into the periodic General Queries. Default: 100 (10 seconds) By varying the [Query Response Interval], an administrator may tune the burstiness of IGMP messages on the network; larger values make the traffic lest bursty, as host responses are spread out over a larger interval. The number of seconds represented by the [Query Response Interval] must be less than the [Query Interval]. 8.4. 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 network. This value MUST be ((the Robustness Variable) times (the Query Interval)) plus (one Query Response Interval). 8.5. Other Querier Present Interval The Other Querier Present Interval is the length of time that must pass before a multicast router decides that there is no longer another multicast router which should be the querier. This value MUST be ((the Robustness Variable) times (the Query Interval)) plus (one half of one Query Response Interval). 8.6. 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. 8.7. Startup Query Count The Startup Query Count is the number of Queries sent out on startup, separated by the Startup Query Interval. Default: the Robustness Variable. Cain, Deering, Kouvelas, Thyagarajan [Page 38] INTERNET-DRAFT IGMPv3 June 2000 8.8. 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. It is also the Max Response Time inserted into Group-and-Source-Specific Query messages. Default: 10 (1 second) This value may be tuned to modify the "leave latency" of the network. A reduced value results in reduced time to detect the loss of the last member of a group or source. 8.9. 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. The Last Member Query Count is also the number of Group-and-Source-Specific Queries sent before the router assumes there are no listeners for a particular source. Default: the Robustness Variable. 8.10. Unsolicited Report Interval The Unsolicited Report Interval is the time between repetitions of a host's initial report of membership in a group. Default: 1 second. 8.11. Older Version Querier Present Timeout The Older Version Querier Interval is the time-out for transitioning a host back to IGMPv3 mode once an older version query is heard. When an older version query is received, hosts set their Older Version Querier Present Timer to Older Version Querier Interval. This value MUST be ((the Robustness Variable) times (the Query Interval in the last Query received)) plus (one Query Response Interval). 8.12. Older Host Present Interval The Older Host Present Interval is the time-out for transitioning a group back to IGMPv3 mode once an older version report is sent for that group. When an older version report is received, routers set their Older Host Present Timer to Older Host Present Interval. This value MUST be ((the Robustness Variable) times (the Query Interval)) plus (one Query Response Interval). Cain, Deering, Kouvelas, Thyagarajan [Page 39] INTERNET-DRAFT IGMPv3 June 2000 9. SECURITY CONSIDERATIONS We consider the ramifications of a forged message of each type. 9.1. Query Message A forged Query message from a machine with a lower IP address than the current Querier will cause Querier duties to be assigned to the forger. If the forger then sends no more Query messages, other routers' Other Querier Present timer will time out and one will resume the role of Querier. During this time, if the forger ignores Leave Messages, traffic might flow to groups with no members for up to [Group Membership Interval]. Forged Queries may also cause hosts in exclude mode to create state, leading to potential denial-of-service attacks. Forged Query messages from the local network can be easily traced. There are two measures necessary to defend against externally forged Queries: o Routers SHOULD not forward Queries. This is easier for a router to accomplish if the Query carries the Router-Alert option. o Hosts SHOULD Ignore v3 Queries without the Router-Alert option. 9.2. Report messages A forged Report message may cause multicast routers to think there are members of a group on a network when there are not. Forged Report messages from the local network are meaningless, since joining a group on a host is generally an unprivileged operation, so a local user may trivially gain the same result without forging any messages. Forged Report messages from external sources are more troublesome; there are two defenses against externally forged Reports: o Ignore the Report if you cannot identify the source address of the packet as belonging to a network assigned to the interface on which the packet was received. This solution means that Reports sent by mobile hosts without addresses on the local network will be ignored. o Ignore Report messages without Router Alert options [RFC-2113], and require that routers not forward Report messages. (The requirement is not a requirement of generalized filtering in the forwarding path, since the packets already have Router Alert options in them). This solution breaks backwards compatibility with implementations of earlier versions of this specification which did not require Router Cain, Deering, Kouvelas, Thyagarajan [Page 40] INTERNET-DRAFT IGMPv3 June 2000 Alert. A forged Version 1 Report Message may put a router into "version 1 members present" state for a particular group, meaning that the router will ignore Leave messages. This can cause traffic to flow to groups with no members for up to [Group Membership Interval]. This can be solved by providing routers with a configuration switch to ignore Version 1 messages completely. This breaks automatic compatibility with Version 1 hosts, so should only be used in situations where "fast leave" is critical. 10. ACKNOWLEDGMENTS We would like to thank Bill Fenner, Dave Thaler, Bob Quinn, Sumit Gupta, and Michael Speer for comments and suggestions on this document. Portions of the text of this document were copied from [RFC-1112] and [RFC-2236]. 11. REFERENCES [RFC-1112] Deering, S., "Host Extensions for IP Multicasting", RFC 1112, August 1989. [RFC-2113] Katz, D., "IP Router Alert Option," RFC 2113, April 1996. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP14, March 1997. [RFC-2236] Fenner, W., "Internet Group Management Protocol, Version 2", RFC 2236, November 1997. [FILTER-API] Thaler, D., B. Fenner, and B. Quinn, "Socket Interface Extensions for Multicast Source Filters", Work in progress, draft-ietf-idmr-msf-api-00.txt, February 2000. Cain, Deering, Kouvelas, Thyagarajan [Page 41] INTERNET-DRAFT IGMPv3 June 2000 APPENDIX A. DESIGN RATIONALE A.1 The Need for State-Change Messages IGMPv3 specifies two types of Membership Reports: Current-State and State Change. This section describes the rationale for the need for both these types of Reports. Routers need to distinguish Membership Reports that were sent in response to Queries from those that were sent as a result of a change in interface state. Membership reports that are sent in response to Membership Queries are used mainly to refresh the existing state at the router; they typically do not cause transitions in state at the router. Membership Reports that are sent in response to changes in interface state require the router to take some action in response to the received report (see Section 6.4). The inability to distinguish between the two types of reports would force a router to treat all Membership Reports as potential changes in state and could result in increased processing at the router as well as an increase in IGMP traffic on the network. A.2 Host Suppression In IGMPv1 and IGMPv2, a host would cancel sending a pending membership reports if a similar report was observed from another member on the network. In IGMPv3, this suppression of host membership reports has been removed. The following points explain the reasons behind this decision. 1 Routers may want to track per-host membership status on an interface This allows routers to implement fast leaves (e.g. for layered multicast congestion control schemes) as well as track membership status for possible accounting purposes. 2 Membership Report suppression does not work well on bridged LANs. Many bridges and Layer2/Layer3 switches that implement IGMP snooping do not forward IGMP messages across LAN segments in order to prevent membership report suppression. Removing membership report suppression eases the job of these IGMP snooping devices. 3 By eliminating membership report suppression, hosts have fewer messages to process; this leads to a simpler state machine implementation. Cain, Deering, Kouvelas, Thyagarajan [Page 42] INTERNET-DRAFT IGMPv3 June 2000 4 In IGMPv3, a single membership report now bundles multiple multicast group records to decrease the number of packets sent. In comparison, the previous versions of IGMP required that each multicast group be reported in a separate message. A.3 Switching router filter modes from EXCLUDE to INCLUDE If there exist hosts in both EXCLUDE and INCLUDE modes for a single multicast group in a network, the router must be in EXCLUDE mode as well (see section 6.2.1). In EXCLUDE mode, a router forwards traffic from all sources unless that source exists in the exclusion source list. If all hosts in EXCLUDE mode cease to exist, it would be desirable for the router to switch back to INCLUDE mode seamlessly without interrupting the flow of traffic to existing receivers. One of the ways to accomplish this is for routers to keep track of all sources desired by hosts that are in INCLUDE mode even though the router itself is in EXCLUDE mode. If the group timer now expires in EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on the network (otherwise a membership report from that host would have refreshed the group timer). The router can then switch to INCLUDE mode seamlessly with the list of sources currently being forwarded in its source list. Cain, Deering, Kouvelas, Thyagarajan [Page 43] INTERNET-DRAFT IGMPv3 June 2000 AUTHORS' ADDRESSES Brad Cain Nortel Networks, Inc. 600 Technology Park Billerica, MA 01821 phone: +1-978-916-1316 email: bcain@nortelnetworks.com Steve Deering Cisco Systems, Inc. 170 Tasman Drive San Jose, CA 95134-1706 phone: +1-408-527-8213 email: deering@cisco.com Isidor Kouvelas Cisco Systems, Inc. 170 Tasman Drive San Jose, CA 95134-1706 phone: +1-408-525-0727 email: kouvelas@cisco.com Ajit Thyagarajan Ericsson IP Infrastructure 12120 Plum Orchard Dr. Silver Spring, MD 20904 phone: +1-301-586-8200 email: ajit@torrentnet.com Cain, Deering, Kouvelas, Thyagarajan [Page 44]