INTERNET-DRAFT Dave Marlow March 9, 1994 NSWC-DD Host Group Extensions for CLNP Multicasting draft-ietf-tuba-host-clnp-multicas-00.txt Status of this Memo This document is an Internet-Draft. 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. 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.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, venera.isi.edu, or munnari.oz.au. Abstract This draft provides a specification for multicast extensions to the CLNP protocol similar to those provided to IP by RFC1112. These extensions are intended to provide the mechanisms needed by a host for multicasting in a CLNP based Internet. This draft covers addressing extensions to the CLNP addressing structure, extensions to the CLNP protocol and extensions to the ES-IS protocol. An appendix discusses the differences between IP multicast and the CLNP multicast approach provided in this draft. The multicast extensions to the CLNP addressing structure defines group Network addresses which identify host groups. The multicast extensions to CLNP provides a means for identifying a CLNP packet and provides scope control mechanisms for CLNP multicast packets. The multicast extensions to the ES-IS protocol provide the mechanisms needed for a host to exchange control information with multicast capable routers. These extensions to the ES-IS protocol provide for a host to "announce" which multicast packets are of interest and for a multicast capable router to dynamically "map" group Network addresses to subnetwork addresses. Expires: September 14, 1994 [Page 1] INTERNET DRAFT CLNP Multicasting March 9, 1994 Contents Status of this Memo Abstract 1. Introduction 2. Levels of Conformance 3. Group Network Addresses 4. Model of a CLNP End System Multicast Implementation 5. Extensions to the CLNP Protocol 6. Extensions to the ES-IS Routeing Protocol Appendix A. Considerations for ESGH and MAM PDU destination SNPA address parameters Appendix B. Differences with RFC 1112 Appendix C. Issues Under Study References Author's Address Expires: September 14, 1994 [Page 2] INTERNET DRAFT CLNP Multicasting March 9, 1994 1. Introduction This draft provides a specification for multicast extensions for CLNP in order to provide a CLNP based Internet the capabilities provided for IP by RFC 1112 (Host Extensions for IP Multicasting) [RFC1112]. This draft uses an outline similar to that of RFC 1112. Paraphrasing RFC 1112, "CLNP multicasting is the transmission of a CLNP datagram to a "host group", a set of zero or more End Systems identified by a single group Network address (GNA). A multicast datagram is delivered to all members of its destination host group with the same "best-efforts" reliability as regular unicast CLNP datagrams, i.e., the datagram is not guaranteed to arrive intact at all members of the destination group or in the same order relative to other datagrams. "The membership of a host group is dynamic; that is End Systems may join and leave groups at any time. There is no restrictions on the location or number of members in a host group. An End System may be a member of more than one group at a time. An End System need not be a member of a group to send datagrams to it. "A host group may be permanent or transient. A permanent group has an administratively assigned GNA. It is the address, not the membership of the group, that is permanent; at any time a permanent group may have any number of members, even zero. "Internetwork forwarding of CLNP multicast datagrams is handled by "multicast capable" Intermediate Systems which may be co-resident with, or separate from, unicast capable Intermediate Systems. This draft specifies the extensions required by an End System to make use of CLNP multicast. In addition the requirements placed upon multicast capable Intermediate Systems to exchange information with multicast capable End Systems is specified. No specifications are provided related to the information exchanges between Intermediate Systems to support multicast route selection or multicast Protocol Data Unit (PDU) forwarding. A discussion of multicast route selection and PDU forwarding has been written by Steve Deering [Deering91]. Note that for these multicast extensions to work there must exist an uninterrupted path of multicast capable routers between the End Systems comprising a host group (such paths may utilize tunneling (i.e. unicast CLNP encapsulated paths between multicast capable CLNP routers). In order to support multicast route selection and forwarding for a CLNP based internet additional specifications are needed. Specifications of this type could come in the form of new protocols, extensions to the current CLNP based routing protocols or use of a technique out of the IETF`s Inter-Domain Multicast Routing (IDMR) group. The IDMR group is currently investigating multicast protocols for routers which utilize a router's unicast routing protocols, this approach may extend directly to CLNP routers. While many of the techniques and assumptions of IP multicasting (as discussed in RFC 1112) are used in CLNP multicasting, there are Expires: September 14, 1994 [Page 3] INTERNET DRAFT CLNP Multicasting March 9, 1994 number of differences. Appendix B describes the differences between CLNP multicasting and IP multicasting. This draft describes techniques brought in directly from projects within ISO to incorporate multicast transmission capabilities into CLNP [MULT- AMDS]. This draft is consistent with the current draft specifications of these ISO projects. Key contributions to the techniques described in this draft have been made by a number of individuals within the ANSI X3S3.3 and ISO SC6 Working Group 2 committees. 2. Levels of Conformance There are three levels of conformance for End Systems to this specification: Level 0: no support for CLNP multicasting. There is no requirement for a CLNP End System (or Intermediate System) to support CLNP multicasting. Level 0 hosts should be unaffected by the presence of multicast activity. The destination addresses used in support of multicast transfers, the GNA, should not be enabled by a non-multicast capable End System and the PDUs themselves are marked differently than unicast PDUs and thus should be quietly discarded. Level 1: support for sending but not receiving CLNP multicast PDUs. At this point this level is not defined for CLNP multicast. An End System sourcing multicast PDUs with the current specification at least needs to know whether a multicast Intermediate System is attached to the subnetwork that the multicast PDU is sourced on (i.e. to determine the destination SNPA (subnet) address). Further work on this level of conformance may be undertaken if utility is identified. Level 2: full support for CLNP multicasting. Level 2 allows a host to join and leave host groups as well as send CLNP PDUs to host groups. It requires implementation by the End System of all parts of this specification. 3. Group Network Addresses Individual Network addresses used by CLNP for End System addressing are called Network Service Access Points (NSAPs). RFC 1237 defines the NSAP address for use in the Internet. In order to provide an address for a group of End Systems, this specification does not change the definition of the NSAP address, but adds a new type of identifier - the group Network address - that supports a multicast Network service (i.e., between a single source NSAP, identified by an individual Network address, and a group of destination NSAPs, Expires: September 14, 1994 [Page 4] INTERNET DRAFT CLNP Multicasting March 9, 1994 identified by a group Network address). Host groups are identified by group Network addresses. In the development of multicast address extensions to CLNP, requirements were identified for: (1)"easily distinguishing" group addresses at the Network layer from NSAP addresses; (2)leaving the currently allocated address families unaffected and (3)ensuring that the approach taken would not require the establishment of new addressing authorities. In addition, it was agreed that providing multicast options for all OSI Network layer users was desirable and thus the group Network addressing solution should support options for all address formats covered by ISO/IEC 8348 | Recommendation X.213. The only viable means identified for meeting all requirements was via creating a new set of AFI values with a fixed one-to-one mapping between each of the existing AFI values and a corresponding group AFI value. Group Network addresses are defined by creating a new set of AFI values, one for each existing AFI value, and a fixed one-to-one mapping between each of the existing AFI values and a corresponding group AFI value. The syntax of a group Network address is identical to the syntax of an individual Network address, except that the value of the AFI in an individual Network address may be only one of the values already allocated for individual Network addresses, whereas the value of the AFI in a group Network address may be only one of the values allocated here for group Network addresses. The AFI values allocated for group Network addresses have been chosen in such a way that they do not overlap, in the preferred encoding defined by ISO/IEC 8348 | CCITT Recommendation X.213, with any of the AFI values that have already been allocated for individual Network addresses. 3.1 Definitions group Network address: an address that identifies a set of zero or more Network service access points; these may belong to multiple Network entities, in different End Systems. individual Network address: an address that identifies a single NSAP. 3.2 CLNP Addresses A discussion of the CLNP address format is contained in RFC 1237. The structure of all CLNP addresses is divided into two parts the Initial Domain Part (IDP) and the Domain Specific Part (DSP). The first two octets of the IDP are the Authority and Format Identifier (AFI) field. The AFI has an abstract syntax of two hexadecimal digits with a value in the range of 00 to FF. In addition to identifying the address authority responsible for allocating a particular address and the format of the address, the AFI also identifies whether an address is an individual Network address or a group Network address. There are 90 possible AFI values to support individual Network address allocations. In addition, when the AFI value starts with the value "0" this identifies that the field contains an incomplete individual Network address (i.e. identifies an escape code). Expires: September 14, 1994 [Page 5] INTERNET DRAFT CLNP Multicasting March 9, 1994 Table 1 allocates 90 possible AFI values to support group Network address allocations. In addition if the first two digits of the IDP are hexadecimal FF, this indicates the presence of an incomplete group Network address. The allocation of group addresses is restricted to be only from the AFI values allocated for the assignment of group addresses in Table 1. An addressing authority in allocating either Network addresses or authorizing one or more authorities to allocate addresses, allocates both individual and the corresponding group addresses. Thus each block of addresses allocated by an addressing authority (or its sub-authority) contains a block of individual Network addresses and group Network addresses. The individual and group address block allocated are differentiated by the AFI values used which are related as shown in Table 1. Group Network addresses are only used as the destination address parameter of a CLNP PDU. Source Address parameters are never permitted to be group Network addresses. Table 2 lists the AFI values which have not been assigned, at this time, for the support of neither individual nor group address allocation. Future assignment of these AFI values is possible. Additional information concerning individual Network addresses (i.e. NSAP and NET (Network Entity Titles)) is contained in RFC 1237. Expires: September 14, 1994 [Page 6] INTERNET DRAFT CLNP Multicasting March 9, 1994 TABLE 1 - Relationship of AFI Individual and Group Values ----------------------------------------------------------- |Individual Group | Individual Group | Individual Group | ----------------------------------------------------------- | 0x FF | | | | 10 A0 | 40 BE | 70 DC | | 11 A1 | 41 BF | 71 DD | | 12 A2 | 42 C0 | 72 DE | | 13 A3 | 43 C1 | 73 DF | | 14 A4 | 44 C2 | 74 E0 | | 15 A5 | 45 C3 | 75 E1 | | 16 A6 | 46 C4 | 76 E2 | | 17 A7 | 47 C5 | 77 E3 | | 18 A8 | 48 C6 | 78 E4 | | 19 A9 | 49 C7 | 79 E5 | | 20 AA | 50 C8 | 80 E6 | | 21 AB | 51 C9 | 81 E7 | | 22 AC | 52 CA | 82 E8 | | 23 AD | 53 CB | 83 E9 | | 24 AE | 54 CC | 84 EA | | 25 AF | 55 CD | 85 EB | | 26 B0 | 56 CE | 86 EC | | 27 B1 | 57 CF | 87 ED | | 28 B2 | 58 D0 | 88 EE | | 29 B3 | 59 D1 | 89 EF | | 30 B4 | 60 D2 | 90 F0 | | 31 B5 | 61 D3 | 91 F1 | | 32 B6 | 62 D4 | 92 F2 | | 33 B7 | 63 D5 | 93 F3 | | 34 B8 | 64 D6 | 94 F4 | | 35 B9 | 65 D7 | 95 F5 | | 36 BA | 66 D8 | 96 F6 | | 37 BB | 67 D9 | 97 F7 | | 38 BC | 68 DA | 98 F8 | | 39 BD | 69 DB | 99 F9 | ----------------------------------------------------------- TABLE 2 - AFI values reserved for future allocation -------------- | 1A-1F | | 2A-2F | | 3A-3F | | 4A-4F | | 5A-5F | | 6A-6F | | 7A-7F | | 8A-8F | | 9A-9F | | FA-FE | -------------- Expires: September 14, 1994 [Page 7] INTERNET DRAFT CLNP Multicasting March 9, 1994 4. Model of a CLNP End System Multicast Implementation The use of multicast transmission by a CLNP End System involves extensions to two protocols: CLNP and the ES-IS Routeing Protocol. To provide level 0 service (no support for CLNP multicast), no extensions to the two present protocols are required. In order to support level 2 service (full support for CLNP multicasting), the extensions contained in the following sections are required. Extensions identified for Intermediate Systems are not required (or appropriate) for End Systems. Multicast transmission also requires the use of a group Network address (as previously described) as the destination address parameter. 5. Extensions to the CLNP protocol This section provides extensions to the CLNP Protocol [CLNP] ISO 8473-1, to support multicast transmission. These additions provide procedures for the connectionless transmission of data and control information from one network-entity to one or more peer network- entities. In developing the multicast extensions for CLNP a decision was needed on how to "mark" a packet as multicast (versus the current unicast packets). Such marking is necessary since the forwarding behavior for multicast packets is different (e.g. multiple copies of a packet may need to be forwarded). The two alternatives considered were to mark the packet (via a particular field) or to mark the destination address, in the end both were done. The destination address for a multicast PDU identifies a host group which is of a very different nature than the unicast NSAP address. Rather than changing the nature of NSAP addresses, a new set of addresses were created named group Network addresses which are marked within the first octet (i.e. the AFI field) with values reserved for group Network addresses. Consideration was given to no further marking of the PDU; however, a problem was identified with only using the group Network address to identify multicast packets. Currently routers implementing the IS-IS Intra-Domain protocol as Level 1 routers when receiving a packet with an unknown destination address are permitted to either discard the packet or send it to a Level 2 router. Such actions by non-multicast capable routers to multicast packets can lead to non-deterministic behavior. Level 1 routers upon receiving a packet containing a group Network address might pass the packet up to a Level 2 router (which may or may not be multicast capable) or it might discard it. Depending upon the circumstances this might lead to whole regions missing packets or packet duplication (possibly even explosion). The result was to seek deterministic behavior by non-multicast capable routers by creating a new PDU type (Multicast Data PDU) and inserting into the CLNP reasons for discard: receiving a PDU of unknown type. Note this reason for discard is mandatory on multicast capable and non-multicast capable CLNP implementations. Expires: September 14, 1994 [Page 8] INTERNET DRAFT CLNP Multicasting March 9, 1994 5.1 Definitions multicast: Data transmission to one or more destinations in a selected group in a single service invocation. multicast capable Intermediate System: An Intermediate System which incorporates the multicast features of the Network layer. 5.2 Addresses The destination address parameter of a multicast PDU shall contain a group Network address. The source address parameter shall be an individual Network address. 5.3 Extensions to the current protocol functions In order to support multicast transmissions the following optional CLNP protocol functions will be implemented: 5.3.1 Header Format Analysis function The header format analysis function optionally provides capabilities to Network entities which support multicast transfer to supply applicable PDUs directly to End Systems served by such a Network entity as well as to forward such PDUs on to other Network entities. This optional functionality is realized through a Network entity with multicast capability identifying a PDU as using multicast transfer via the PDU type and the PDU's destination address field. If a Network entity supports multicast transmission, then the header format analysis function shall provide checking to ensure that a PDU does not contain a group Network address in the source address field. Any PDU header analyzed to have a group address in the source address field shall be discarded. 5.3.2 Route PDU functions The route PDU function optionally provides capabilities to Network entities which support multicast transfer for determining multiple Network entities to which a single PDU shall be forwarded to. This may result in multiple invocations of the forward PDU function and hence the need to make multiple copies of the PDU. For PDUs that are received from a different Network entity, the optional functionality for the route PDU function is realized as a result of the header format analysis function's recognition of the PDU as being a multicast PDU. A Network entity originating a multicast PDU should invoke the forward PDU function on one and only one subnetwork to which it is attached and on which it has learned of the existence of at least one multicast capable IS. A Network entity attached to multiple subnetworks should select a subnetwork for which a multicast capable Intermediate System is attached if one exists for the initial sourcing of a PDU. A Network entity attached to multiple subnetworks but has not learned of the existence of any multicast capable IS is permitted to originate a multicast PDU on multiple subnetworks. Expires: September 14, 1994 [Page 9] INTERNET DRAFT CLNP Multicasting March 9, 1994 Editor's Note: The purpose in having an originating Network entity source a multicast PDU on only one subnetwork to which a multicast capable IS is attached is to support the development of multicast routing protocols which will need to determine on which subnetworks a multicast PDU has visited. This behavior is predicated on the assumption that the Intermediate Systems performing multicast forwarding form a connected set. 5.3.3 Forward PDU function This function issues an SN-UNITDATA request primitive, supplying the subnetwork or Subnetwork Dependent Convergence Function (SNDCF) identified by the route PDU function with the protocol data unit as user data to be transmitted, the address information required by that subnetwork or SNDCF to identify the "next" system or systems within the subnetwork-specific addressing domain (this may be one or more Intermediate Systems and/or one or more destination End Systems), and quality of service constraints (if any) to be considered in the processing of the user data. 5.3.4 Discard PDU function Add an additional reason for discard - a PDU is received with an unknown type code. Editor's Note: Comments are requested as to whether this additional reason for discard presents any problem with currently deployed CLNP implementations. 5.3.5 Error reporting function It is important to carefully control the use of the error reporting capability in the case of multicast transfers. The primary concern is to avoid the occurrence of broadcast storms and thus a multicast PDU may not cause the origination of another multicast PDU. This is the primary reason that the source address is not permitted to be a group address. In addition, a multicast PDU with error reporting permitted can result in flooding the source network-entity (as well as the networks used) with Error Report PDUs. While error reports are permitted on multicast PDUs, a PDU with a group Network address in the source address field shall not be responded to with an Error Report. This is to ensure that a multicast PDU does not generate another multicast PDU. If the source address is identified as a group address then an error report PDU shall not be generated and the original PDU shall be discarded. 5.3.6 Source routing functions No source routing capability is provided for multicast PDU transfer. The NS provider shall not accept a multicast PDU with source route parameters. Expires: September 14, 1994 [Page 10] INTERNET DRAFT CLNP Multicasting March 9, 1994 5.4 Scope control function 5.4.1 Overview The scope control function is an option for multicast PDU forwarding only. The scope control function allows the originator to limit the forwarding of the multicast PDU. The scope control function provides the capability to limit the relaying of a particular PDU based on the individual Network addressing hierarchy and/or limit the amount of multicast expansion which can take place. In cases where both forms of scope control are applied to the same PDU, forwarding will cease once either has reached its scope control limit. 5.4.2 Prefix Based Scope Control The prefix based scope control function allows the originator to specify a specific set of address prefixes where the multicast forwarding of a PDU by an Intermediate System occurs only if one of the prefixes matches the Network Entity Title (NET) of the Intermediate System. Prefix based scope control may be selected only by the originator of a PDU. Prefix based scope control is accomplished using one or more address prefixes held in a parameter within the options part of the PDU header. The length of this parameter is determined by the originating network entity, and does not change during the lifetime of a PDU. When an Intermediate System receives a multicast PDU containing a prefix based scope control parameter, forwarding is only performed if every octet of one of the prefixes contained in the prefix based scope control parameter matches that Intermediate System's NET, starting from the beginning of its NET. If no such prefix match exists, the Intermediate System discards the PDU. The error reporting function shall not be invoked upon PDU discard. 5.4.3 Radius Scope Control The radius scope control function allows the originator to specify a maximum logical distance where multicast expansion can occur. It is closely associated with the header format analysis function. Each IS receiving a multicast PDU which is capable of expanding and which contains a Radius Scope Control parameter, decrements the Radius Scope Control field in the PDU by an administratively set amount between 0 and the maximum value of the field. This function determines whether the PDU received may be forwarded or whether its Radius has been reached, in which case it shall be discarded. An Intermediate System shall not forward a multicast PDU containing a Radius Scope Control parameter with a value of 0. The error reporting function shall not be invoked upon PDU discard. 5.4.3.1 Radius Scope Control Example The Radius Scope Control parameter is useful where policies have been established across the potential forwarding path. One possible policy for Internet use is for multicast capable routers to treat Expires: September 14, 1994 [Page 11] INTERNET DRAFT CLNP Multicasting March 9, 1994 this field as a hop count within a domain (decrement by one unit) and for inter-domain routers to either decrement this field to an even multiple of 256 when crossing domains where prior agreements have been made or decrement this field to 0 (i.e. discard the packet) for other domains. 5.5 Structure and Encoding of PDUs Multicast transmission is accomplished via the transfer of Multicast Data (MD) PDUs. The PDU type code for a MD PDU is "1 1 1 0 1". The format of the MD PDU is identical to that of the Data (DT) PDU. The MD and DT PDU may contain the same optional parameters with the following exceptions: (1)The source routing parameter is permitted within DT PDUs but not MD PDUs; and (2)The scope control parameter is permitted within MD PDUs but not DT PDUs. 5.6 Optional parameters for multicast support 5.6.1 Prefix Based Scope Control (this section is extended beyond the ISO draft) The prefix based scope control parameter specifies one or more address prefixes for which Intermediate System forwarding requires a match of one of the contained prefixes with the beginning of the Intermediate System's NET. Parameter Code: 1100 0100 Parameter Length: variable Parameter Value: a concatenation of address prefix entries The parameter value contains an address prefix list. The list consists of variable length address prefix entries. The first octet of each entry gives the length of the address prefix denominated in bits that comprises the remainder of the entry. If the length field does not specify an integral number of octets then the prefix entry is followed by enough trailing zeroes to make the end of the entry fall on an octet boundary. The list must contain at least one entry. The prefix shall end on a boundary that is legal in the abstract syntax of the address family from which it is derived. For example, the encoding of a prefix whose DSP is expressed in decimal syntax must end on a semi-octet boundary, while the encoding of a prefix whose DSP is expressed in binary syntax can end on an arbitrary bit boundary. If the end of the prefix falls within the IDP, then the prefix must end on a semi-octet boundary and must not contain any padding characters. NOTE - The length of the prefix based scope control parameter is determined by the originator of the PDU and is not changed during the lifetime of the PDU. Expires: September 14, 1994 [Page 12] INTERNET DRAFT CLNP Multicasting March 9, 1994 5.6.1.1 Prefix matching A prefix that extends into the DSP shall be compared directly against the encoded NET address, including any padding characters that may be present. A prefix which does not extend into the DSP shall be compared against the derived quantity NET', which is obtained from the NET address by removing all padding characters (as defined by the binary encoding process of ISO 8348). The existence of a match shall be determined as follows: a) If the encoded NET (or NET') contains fewer bits than the pre- fix, then there is no match. b) If the encoded NET (or NET') contains at least as many bits as the prefix, and all bits of the prefix are identical to the corresponding leading bits of the encoded NET (or NET'), there is a match. Otherwise, there is no match. 5.6.2 Radius Scope Control The radius scope control parameter specifies the logical distance that a multicast PDU can be forwarded. Parameter Code: 1100 0110 (THIS CODE MAY CHANGE) Parameter Length: two octets Parameter Value: two octets which represents the remaining distance, that the PDU can be forwarded, in administratively set units. 5.7 Provision of the Underlying Service For a subnetwork that provides an inherent multicast capability, it is the functionality of the SNDCF to provide the mapping between group Network addresses and the corresponding addressing capability of the subnetwork. 5.8 Conformance All of the extensions provided to the functions to support multicast capability are optional. For an End System or Intermediate System which is not multicast capable these extensions are not applicable. An implementation claiming conformance as a multicast capable End System shall meet all of the requirements for an End System which is not multicast capable and also provide all of the multicast exten- sions provided here. An implementation claiming conformance as a mul- ticast capable Intermediate System shall meet all of the requirements for an Intermediate System which is not multicast capable and also provide all of the multicast extensions provided here. Expires: September 14, 1994 [Page 13] INTERNET DRAFT CLNP Multicasting March 9, 1994 6. Extensions to the ES-IS Routeing Protocol This section provides optional extensions to the ES-IS Routeing Pro- tocol [ES-IS], ISO 9542 to support the transfer of multicast PDUs. It is an explicit goal of this specification that ESs and ISs, some of which will have multicast capabilities and some without, will be able to fully function on the same subnetworks. This specification does not change any aspect of a currently defined (i.e. non-multicast) ISO 9542 implementation, it adds new optional functionality not modifying current functionality. Two basic functions are provided: multicast announcement and multicast address mapping. 6.1 Overview of the protocol 6.1.1 Operation of ESs receiving multicast PDUs ESs, upon initialization and periodically thereafter, will construct End System Group Hello (ESGH) PDUs identifying, by particular group Network addresses, the multicast PDUs it wishes to receive. The ES will periodically source (announce) these ESGH PDUs on the subnetwork it wishes to receive these on. Reporting the same group Network address on multiple subnetworks may result in the reception of dupli- cate PDUs. ES-IS operations related to requesting the same group Net- work address on multiple subnetworks are handled totally indepen- dently (e.g. using different logical timers,...). It is permitted for an ES to report a number of group Network addresses in the same ESGH PDU. The only restrictions placed on providing multiple group Net- work addresses within the same ESGH PDU are that all packets requested are to be received on the same subnet, with the same hold- ing time and that the ESGH PDU be of length equal to or less that its maximum packet size constraint. Note each group Network address in the ESGH PDU is paired with its own SNPA (subnetwork point of attach- ment) address. An ES will always have an SNPA address associated with each of its active group Network addresses. An SNPA address is a subnetwork address, in the case of a subnetwork which uses IEEE 802 addresses the SNPA address is a 48 bit IEEE 802 MAC (media access control) address. Of particular interest is the address used to mark the des- tination group. For a subnetwork using IEEE 802 addressing a group SNPA address uses a particular bit position to "mark" group SNPA addresses. Upon initialization the ES may have static SNPA address associations (Pre-configured SNPA addresses). For any group Network address without a Pre-configured SNPA address that the ES wishes to receive, the ES will associate the "All Multicast End System Network Entities" SNPA address. Upon receiving a Multicast Address Mapping (MAM) PDU containing a group Network address that the ES is announcing, the ES will use the SNPA address pairing contained in the MAM PDU for that group Network address. Upon the expiration of the Mapping Holding Timer, the ES shall revert back to associating either the Pre- configured SNPA address if one exists or the "All Multicast End Sys- tem Network Entities" SNPA address for the specific group Network Expires: September 14, 1994 [Page 14] INTERNET DRAFT CLNP Multicasting March 9, 1994 address. While an ES is permitted to listen in on other ESs announce- ments (needed for the damping option), an ES is not permitted to change its group Network address to SNPA address mapping based on the announcement of other ESs. Optionally, the ES may perform damping (resetting a Multicast Announcement Timer corresponding to a particular group Network address) if the conditions necessary to withhold a particular announcement are met. In order to perform damping the following con- ditions must be met: (1)The ES must be processing other ES's announcements; (2)An ESGH PDU is received that identifies the exact same group Network address and SNPA address pairing on a particular subnetwork that this ES is announcing on; (3) The Multicast Holding Timer parameter value in the ESGH PDU received is equal to or greater than the Multicast Holding Timer value, for this subnetwork, that is being used by the ES processing this ESGH PDU. ESs will utilize a local default value for their Multicast Announce- ment Timer to control the period for sending out their ESGH PDUs. The Active Multicast IS, if one exists on a particular subnetwork, will optionally suggest a value for ESs on the subnetwork to use for their Multicast Announcement Timer for a specific group Network address. In order to support the optional damping function, ESs are required to incorporate a 25% jittering to the timer values that they are using. 6.1.2 Operation of ESs sourcing multicast PDUs An ES which is attached to multiple subnetworks when sourcing multi- cast PDUs is required to source only one such PDUs on to a subnetwork to which a multicast IS is attached. No restrictions are placed on a multiply attached ES for which none of the subnetworks to which it is attached to has a multicast capable IS. The ES sourcing multicast PDUs on a subnetwork that has a multicast IS attached always uses the "All Multicast Intermediate System Network entities" as the SNPA des- tination address for all PDUs sourced. For an ES which is not attached to a subnetwork with a multicast capable IS, the ES uses pre-configured multicast address mapping information if available and if there is no pre-configured information maps the "All Multicast End System Network Entities" multi-destination address to the group Net- work address. The ES sourcing multicast PDUs identified by a specific group Network address is not required to be a receiver of such PDUs (and thus is not announcing that particular group Network address). The reason for sourcing packets on a subnetwork with a multicast capable IS attached by multicasting to all multicast capable ISs versus unicasting to one of the multicast capable ISs is to limit the maximum number of transmissions required on any subnetwork to two. In the case of sourcing a packet, one transmission would be used for sourcing the packet to all ISs that might need it and the second, if needed, to multicast the packet to ESs on the subnetwork which belong to the group. If this packet were sourced via a unicast to an IS, then the packet may need to passed on to other ISs on the subnet (depending upon the particular multicast tree) and the same packet may need to be transmitted again to the ESs on the subnetwork which Expires: September 14, 1994 [Page 15] INTERNET DRAFT CLNP Multicasting March 9, 1994 belong to the group. 6.1.3 Operation of the Active Multicast IS The Active Multicast IS listens in on all ESGH PDUs sourced on the subnetwork for which it is serving as the Active Multicast IS. All subnetworks are handled independently (even if multiple subnetworks have the same ESs attached and the IS is serving as the Active Multi- cast IS for these multiple subnetworks). The Active Multicast IS sources MAM PDUs, for all group Network addresses for which it has received ESGH PDUs, on the subnetwork due to the following operational conditions: (1)The IS initializes either as the Active Multicast IS after an election with other multicast capable ISs or initializes believing it is the only multicast capable IS (the determination of such conditions are outside of the scope of this specification); (2)The IS receives an ESGH PDU with a group Net- work address paired to an incorrect SNPA address; (3)The expiration of the IS's Multicast Address Mapping Timer for that group Network address (this is to prevent the expiration of Mapping Holding Timers in ESs and to ensure that ESs which source specific multicast PDUs but are not receivers of such PDUs are using the correct SNPA desti- nation address in such PDUs that they source) or (4)The IS receives a multicast PDU sourced on the subnetwork which used an incorrect des- tination SNPA address (note: this specification does not place any requirement on an IS to listen in on any multicast PDU, with the exception of multicast PDUs addressed to "the All Multicast Inter- mediate System Network Entities" group SNPA address but an IS may receive a PDU with an incorrect destination SNPA address due to other operational actions). Actual relaying of any multicast PDUs sourced on a subnetwork will be the responsibility of the multicast routing protocol used and is out- side the scope of this specification. 6.2 Definitions Active Multicast IS: The one multicast capable IS selected (via means outside of this specification) to source Multicast Address Mapping information on a particular subnetwork. Paired SNPA Address: The SNPA address associated with a particular group Network address on a specific subnetwork. 6.3 Routing information supporting multicast transmission 6.3.1 Multicast Announcement Information An IS should forward a multicast PDU containing a particular destina- tion group Network address onto a subnetwork to which it is attached if and only if one or more of the ESs attached to that subnetwork have declared an interest in receiving multicast PDUs destined for that group Network address. Multicast announcement information enables an IS that supports CLNP multicast to dynamically discover, Expires: September 14, 1994 [Page 16] INTERNET DRAFT CLNP Multicasting March 9, 1994 for each subnetwork to which it is attached, the group Network addresses for which ESs attached to that subnetwork have declared an interest. On a point-to-point subnetwork the multicast announcement information informs the Network entity, in the case where it is attached to an End System, of the group Network addresses for which that End System expects to receive multicast PDUs. On a broadcast subnetwork the multicast announcement information informs the multicast capable Intermediate Systems, of the group Net- work addresses for which ESs attached to that subnetwork expect to receive multicast PDUs. Editor's Note: Intermediate Systems with the optional OSI multicast capabilities do receive information identifying the SNPA address of ESs on the broadcast network that want PDUs with particular group Network addresses as their destination address; however, the critical information is which multicast PDUs are needed, not which ESs need them. 6.3.2 Multicast Address Mapping Information In order to receive multicast PDUs destined for a particular group Network address, an ES may be able to take advantage of an associa- tion of the group Network address with a specific SNPA address. Mul- ticast address mapping information enables an IS to inform ESs that they can receive multicast PDUs destined for a particular group Net- work address on a corresponding specific SNPA address. Multicast address mapping information is not employed on point-to- point subnetworks. Multicast address mapping information is employed on broadcast sub- networks to enable multicast capable Intermediate Systems to inform the multicast capable End Systems that they can receive, on a specific broadcast subnetwork, multicast PDUs destined for a particu- lar group network address on a corresponding specific SNPA address. 6.4 Addresses All exchanges using this protocol are accomplished over a single sub- network. While the control PDU's contain Network addresses (i.e. group Network addresses) actual control PDU transfer is accomplished via Subnetwork based group addresses (i.e. group SNPA addresses). The following group SNPA addresses are used: (1)All Multicast End System Network entities; (2)All Multicast Intermediate System Network enti- ties and (3)a group SNPA address corresponding to a group Network address 6.5 Timers Two additional timers are employed: (1)the Multicast Announcement Timer (MAT) and (2)Multicast Address Mapping Timer (MAMT). Old Expires: September 14, 1994 [Page 17] INTERNET DRAFT CLNP Multicasting March 9, 1994 multicast announcement or multicast address mapping information shall be discarded after the Holding Timer expires to ensure the correct operation of the protocol. 6.5.1 Multicast Announcement Timer The Multicast Announcement Timer is a local timer (i.e. maintained independently by each End System, one timer per group Network address) which assists in performing the Report Multicast Announce- ment function. The timer determines how often an End System reports its desire to receive multicast PDUs with that group Network address as its destination address parameter. Considerations in setting this timer are similar to those described for the Configuration timer in the ES-IS specification. 6.5.2 Multicast Address Mapping Timer The Multicast Address Mapping Timer is a local timer (i.e. maintained independently by an Intermediate System which is actively participat- ing with End Systems to transfer multicast PDUs) which assists in performing the Report Multicast Address Mapping function. The timer determines how often an Intermediate System, actively participating with End Systems for the transfer of multicast PDUs, reports the Mul- ticast Address Mapping for a particular group Network address. The shorter the Multicast Address Mapping Timer, the more quickly End Systems on the subnetwork will become aware of the correct address mapping which may change due to the Intermediate System becoming available or unavailable. There is a trade off between increased responsiveness and increased use of resources in the subnetwork and in the End Systems. 6.6 Extensions to the current protocol functions In order to support multicast transmissions the following optional ES-IS protocol functions will be implemented: 6.6.1 Report Configuration by Intermediate Systems All multicast capable Intermediate Systems on a subnetwork will use the Multicast Capable option in all ISH PDUs that they source. This will provide multicast capable End Systems with a way to determine that a multicast capable Intermediate System is operating on a par- ticular subnetwork. 6.6.2 Query Configuration Note: The Query Configuration function cannot be performed to find the corresponding SNPA address of a group Network address since the addressing information needed is the corresponding group SNPA address and not the SNPA address of a particular End System responding. On a large broadcast subnetwork, many different Configuration Responses could result each incorporating a different End System Address. While it is possible to design a Query Configuration for use with multi- cast, this function does not appear to be required given the use of Expires: September 14, 1994 [Page 18] INTERNET DRAFT CLNP Multicasting March 9, 1994 the "All Multicast End System Network Entities" address for supplying a SNPA address when the group SNPA address is not known. 6.7 Multicast Announcement 6.7.1 Report Multicast Announcement Function by End Systems An End System which needs to receive or continue to receive any mul- ticast PDUs (i.e. PDUs with group Network addresses as their destina- tion address), constructs and transmits ESGH PDUs to inform multicast capable Intermediate Systems of the set of group Network address des- tinations for which it wishes to receive PDUs. This may be done by constructing ESGH PDUs for each group Network address. Alternatively, ESGH PDUs may be constructed which convey information about more than one group Network address at a time, up to the limits imposed by the permitted SNSDU size and the maximum header size of the ESGH PDU. Each ESGH PDU is transmitted by issuing an SN-UNITDATA.Request with the following parameters: SN_Userdata (SNSDU) <- ESGH PDU SN_Destination _Address <- multi-destination address that indicates "All Multicast Intermediate System Network Entities" If an End System supports more than one SNPA, the information about each group Network address desired for receiving on a particular SNPA serving the End System shall be transmitted via that SNPA. It is per- missible for an End System to report group Network addresses on mul- tiple SNPAs; however, duplicate multicast PDUs should be anticipated. The Group Address Pair parameter carries a list of Group Network Addresses, each paired with its associated SNPA address. This infor- mation is used by the Active Multicast IS to determine whether a Mul- ticast Address Mapping PDU should be emitted to update the associa- tion between Group Network Addresses and SNPA addresses. The Holding Time (HT) field is set to approximately twice the ES's Multicast Announcement Timer (MAT) parameter. The value shall be large enough so that even if every other ESGH PDU is discarded (due to lack of resources), or otherwise lost in the subnetwork, the mul- ticast announcement information will still be maintained. The value should be set small enough so that Intermediate Systems resources are not needlessly consumed when the ES no longer wishes to receive PDUs destined to a group Network address. Note: -- When combining multiple group Network addresses in a single ESGH PDU, it should be realized that there is a single Holding Time parameter associated with all of these addresses. 6.7.1.1 Generating Jitter on Multicast Announcement Timers The ES shall apply a 25% jitter to its Multicast Announcement Timer (MAT) parameter. When ESGH PDUs are transmitted as a result of timer expiration, there is a danger that the timers of individual systems Expires: September 14, 1994 [Page 19] INTERNET DRAFT CLNP Multicasting March 9, 1994 may become synchronised. The result of this is that the traffic dis- tribution will contain peaks. Where there are a large number of syn- chronised systems, this can cause overloading of both the transmis- sion medium and the systems receiving the PDUs. In order to prevent this from occurring, all periodic timers, the expiration of which can cause the transmission of PDUs, shall have "jitter" introduced as defined in the following algorithm. Expires: September 14, 1994 [Page 20] INTERNET DRAFT CLNP Multicasting March 9, 1994 CONSTANT Jitter = 25; Resolution = 100; (* The timer resolution in ms *) PROCEDURE Random(max: Integer): Integer; (* This procedure delivers a Uniformly distributed random integer R such that 0 < R