GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1768

Network Working Group D. Marlow Request for Comments: 1768 NSWC-DD Category: Experimental March 1995

            Host Group Extensions for CLNP Multicasting

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

 This memo documents work performed in the TUBA (TCP/UDP over Bigger
 Addresses) working group of IPng area prior to the July 1994 decision
 to utilize SIPP-16 as the basis for IPng.  The TUBA group worked on
 extending the Internet Protocol suite by the use of ISO 8473 (CLNP)
 and its related routing protocols.  This memo describes multicast
 extensions to CLNP and its related routing protocols for Internet
 multicast use.  Publication of this memo does not imply acceptance by
 any IETF Working Group for the ideas expressed within.
 This memo 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 memo 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 memo.

Acknowledgments

 The specification provided here was developed by a number of
 individuals in the IETF TUBA working group as well as the ANSI X3S3.3
 and ISO SC6 WG2 committees.  Key contributions were made by Steve
 Deering, Joel Halpern, Dave Katz and Dave Oran.

Marlow [Page 1] RFC 1768 CLNP Multicasting March 1995

Table of Contents

 1.  Introduction ..........................................  2
 2.  Levels of Conformance..................................  3
 3.  Group Network Addresses................................  4
 4.  Model of a CLNP End System Multicast Implementation....  8
 5.  Extensions to the CLNP Protocol........................  8
 6.  Extensions to the ES-IS Routeing Protocol ............. 15
 7.  Security Considerations ............................... 39
 Appendix A.  Differences with RFC 1112 .................... 40
 Appendix B.  Issues Under Study ........................... 43
 References ................................................ 44
 Author's Address .......................................... 45

1. Introduction

 This memo 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 memo 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 unicast capable Intermediate Systems.
 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

Marlow [Page 2] RFC 1768 CLNP Multicasting March 1995

 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.
 This memo 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
 number of differences. Appendix A describes the differences between
 CLNP multicasting and IP multicasting. This memo describes techniques
 brought in directly from projects within ISO to incorporate multicast
 transmission capabilities into CLNP [MULT-AMDS].

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

Marlow [Page 3] RFC 1768 CLNP Multicasting March 1995

 themselves are marked differently than unicast PDUs and thus should
 be quietly discarded.
 Level 1: support for sending but not receiving CLNP multicast PDUs.
 An End System originating multicast PDUs is required to know whether
 a multicast capable Intermediate System is attached to the
 subnetwork(s) that it originates multicast PDUs (i.e., to determine
 the destination SNPA (subnet) address). An End System with Level 1
 conformance is required to implement all parts of this specification
 except for those supporting only Multicast Announcement.  An End
 System is not required to know the current Multicast Address Mapping
 to start originating multicast PDUs.
 Note: It is possible for End System not implementing Multicast
 Address Mapping to successfully originate multicast PDUs (but with
 the End System knowing of the existence of a multicast capable
 Intermediate System). Such operation may lead to inefficient
 subnetworks use.  Thus when an End System continues (or may continue)
 to originate multicast PDUs destined for the same group,
 implementations are to provide Multicast Address Mapping support.
 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,
 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

Marlow [Page 4] RFC 1768 CLNP Multicasting March 1995

 thus the group Network addressing solution should support options for
 all address formats covered by ISO/IEC 8348 | CCITT 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).
 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

Marlow [Page 5] RFC 1768 CLNP Multicasting March 1995

 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.
 Note: While the format of the Initial Domain Part of a group Network
 address is assigned, the format for the Domain Specific Part of the
 group Network address is specified by an addressing authority and is
 out of the scope of this memo.  While NSAP address assignments are
 typically made to support hierarchical unicast routing, a similar
 consideration for group Network address assignments may not exist.

Marlow [Page 6] RFC 1768 CLNP Multicasting March 1995

       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  |
      -----------------------------------------------------------

Marlow [Page 7] RFC 1768 CLNP Multicasting March 1995

          TABLE 2 - AFI values reserved for future allocation
  1. ————-

| 1A-1F |

                            |    2A-2F   |
                            |    3A-3F   |
                            |    4A-4F   |
                            |    5A-5F   |
                            |    6A-6F   |
                            |    7A-7F   |
                            |    8A-8F   |
                            |    9A-9F   |
                            |    FA-FE   |
                            --------------

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 these two protocols are required. To provide level 1
 service (support for sending but not receiving CLNP multicast PDUs)
 all extensions contained in the following sections are required
 except for those supporting only Multicast Announcement.  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

Marlow [Page 8] RFC 1768 CLNP Multicasting March 1995

 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 that this reason for discard is mandatory on multicast capable
 and non-multicast capable CLNP implementations.

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.

Marlow [Page 9] RFC 1768 CLNP Multicasting March 1995

 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 function

 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 attached to more than one subnetwork
 when originating a multicast PDU is permitted to originate the PDU on
 more than one subnetwork.
 Note: The ES-IS function "Extensions to the ISO CLNP Route Function
 by End Systems" discussed in section 6.10 identifies on which
 subnetworks an End System attached to more than one subnetwork must
 originate multicast PDUs on.
 Note: The purpose in allowing an originating Network entity to
 originate a multicast PDU on multiple subnetworks is to support the
 development of multicast IS-IS 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 in the OSI
 environment 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.

Marlow [Page 10] RFC 1768 CLNP Multicasting March 1995

5.3.4 Discard PDU function

 Add an additional reason for discard - a PDU is received with an
 unknown type code.

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.

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

Marlow [Page 11] RFC 1768 CLNP Multicasting March 1995

 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.  An IS, when it
 decrements the Radius Scope Control field, shall place a value of 0
 into this field if its current value is less than the amount it is to
 decrement by.   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
 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.

Marlow [Page 12] RFC 1768 CLNP Multicasting March 1995

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

 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.

Marlow [Page 13] RFC 1768 CLNP Multicasting March 1995

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
 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
 extensions provided here. An implementation claiming conformance as a

Marlow [Page 14] RFC 1768 CLNP Multicasting March 1995

 multicast 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.

6. Extensions to the ES-IS Routeing Protocol

 This section provides optional extensions to the ES-IS Routeing
 Protocol [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 originate (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 duplicate PDUs. ES-IS operations related to requesting the same
 group Network address on multiple subnetworks are handled totally
 independently (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 Network addresses within the same ESGH PDU are that
 all packets requested are to be received on the same subnet, with the
 same holding time and that the ESGH PDU be of length equal to or less
 that its maximum packet size constraint.  Note that each group
 Network address in the ESGH PDU is paired with its own SNPA
 (subnetwork point of attachment) 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
 destination group.  For a subnetwork using IEEE 802 addressing a
 group SNPA address uses a particular bit position to "mark" group
 SNPA addresses.

Marlow [Page 15] RFC 1768 CLNP Multicasting March 1995

 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 Capable End Systems" 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 Capable
 End Systems" SNPA address for the specific group Network address.
 While an ES is permitted to listen in on other ESs announcements
 (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
 conditions 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
 Announcement Timer to control the period for sending out their ESGH
 PDUs. The Active Multicast IS, if one exists on a particular
 subnetwork, may 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 originating multicast PDUs

 The ES originating multicast packets identified by a specific group
 Network address is not required to be a receiver of such packets (and
 thus is not announcing that particular group Network address).  The
 origination of multicast PDUs involves two differences to the
 origination of unicast PDUs.  The two differences are: (1)The
 mechanism for selecting a destination SNPA address and (2)For End
 Systems attached to more than one subnet, the decision on which
 subnet(s) to originate the PDUs.

Marlow [Page 16] RFC 1768 CLNP Multicasting March 1995

 The destination SNPA address used for originating each multicast
 packet depends on whether there is a multicast capable IS attached to
 the subnetworks. When a multicast capable IS is attached, the
 decision depends on whether there is multicast address mapping
 information available for that subnetwork corresponding to the group
 Network address used as the destination address parameter of the
 multicast packet. When there is a multicast capable IS attached to a
 subnetwork and there is multicast address mapping information
 available corresponding to the group Network address, then the SNPA
 address obtained from the multicast address mapping information is
 used.  Originating multicast packets using the destination SNPA
 address used for receiving such multicast packets ensures that the
 multicast packets will not require additional forwarding on the
 originating subnetwork(s). When there is a multicast capable IS
 attached to a subnetwork but for which there is no multicast address
 mapping information available corresponding to the the group Network
 address, then the SNPA address used is the "All Multicast Capable
 Intermediate Systems" address.
 When there is no multicast capable IS attached to a subnetwork then
 the ES originating a multicast PDU uses pre-configured information if
 it is available or the "All Multicast Capable End Systems" SNPA
 address when no pre-configured information is available.
 ES's attached to more than one subnetwork forward each multicast
 packet that they originate onto every attached subnetwork for which
 the NSAP address being used as the source address of the multicast
 packet is actively being reported through the unicast ES-IS Report
 Configuration function.

6.1.3 Operation of the Active Multicast IS

 The Active Multicast IS listens in on all ESGH PDUs originated 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
 Multicast IS for these subnetworks).
 The Active Multicast IS originates 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;
 Note: The determination of such conditions is outside of the scope of
 this specification;

Marlow [Page 17] RFC 1768 CLNP Multicasting March 1995

 2)   The IS receives an ESGH PDU with a group Network address paired
      to an incorrect SNPA address;
 3)   The expiration of the IS's Multicast Address Mapping Timer for
      that group Network address; or
 Note: This is to prevent the expiration of Mapping Holding Timers in
 ESs.
 4)   The IS receives a multicast PDU originated on the subnetwork
      which used an incorrect destination SNPA address.
 Note: Of particular concern are those multicast packets using the
 "All Multicast Capable Intermediate Systems" SNPA address when
 another SNPA address should have been used.  In addition the
 multicast capable ISs are responsible for listening in on all
 multicast packets using destination SNPA addresses that are contained
 within the current multicast address mapping information.
 As a result of the event driven conditions (i.e., conditions 2 or 4
 above), the Active Multicast IS sends a MAM PDU with direct
 information (i.e., not needing analysis of the Mask parameters).  The
 Active Multicast IS limits the number of MAM PDUs that are sent out
 per unit of time.  Particular MAM PDUs with direct information will
 not be sent more than once per second.  MAM PDU will be sent in
 response to continuing event driven conditions such that events
 occurring greater than 10 seconds after the issuance of such a MAM
 PDU will result in the issuance of another MAM PDU.
 The Active Multicast IS is responsible for forwarding a multicast
 packet back on the subnetwork it was originated when a multicast
 packet used the "All Multicast Capable Intermediate System" SNPA
 address when another SNPA address should have been used.  A packet
 forwarded back onto the subnetwork the multicast packet was
 originated on will be given a CLNP Lifetime of "1" to prevent the
 continued relaying of duplicate packets by the multicast ISs.
 The further relaying of any multicast packet originated on a
 subnetwork is the responsibility of the multicast routing protocol
 used and is outside the scope of this specification.

6.2 Definitions

 Active Multicast IS: The one multicast capable IS selected (via means
 outside of this specification) to originate Multicast Address Mapping
 information on a particular subnetwork.

Marlow [Page 18] RFC 1768 CLNP Multicasting March 1995

 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
 destination 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, 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
 Network addresses for which ESs attached to that subnetwork expect to
 receive multicast PDUs.
 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 packets destined for a particular group
 Network address, an ES may need to associate with the group Network
 address a specific SNPA address.  Multicast address mapping
 information enables an IS to inform ESs that they can receive
 multicast packets destined for a particular group Network address on
 a corresponding specific SNPA address.  In addition, multicast
 address mapping information may provide the specific destination SNPA
 addresses needed by an ES for originating multicast packets.
 Multicast address mapping information is not employed on point-to-
 point subnetworks.

Marlow [Page 19] RFC 1768 CLNP Multicasting March 1995

 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 packets destined for a
 particular group Network address on a corresponding specific SNPA
 address.  In addition multicast address mapping information provides
 the specific destination SNPA address, that corresponds to a
 particular group Network address, for each multicast packet that it
 originates on a specific broadcast subnetwork.

6.4 Addresses

 All exchanges using this protocol are accomplished over a single
 subnetwork. 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 Capable
 End Systems; (2)All Multicast Announcements; (3)All Multicast Capable
 Intermediate Systems and (4)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
 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
 Announcement 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
 participating 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,

Marlow [Page 20] RFC 1768 CLNP Multicasting March 1995

 reports the Multicast 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 shall use
 the Multicast Capable option in all ISH PDUs that they originate.
 This will provide multicast capable End Systems with a way to
 determine that a multicast capable Intermediate System is operating
 on a particular 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
 multicast, this function does not appear to be required given the use
 of the "All Multicast Capable End Systems" 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
 multicast PDUs (i.e., PDUs with group Network addresses as their
 destination address), constructs and transmits ESGH PDUs to inform
 multicast capable Intermediate Systems of the set of group Network
 address destinations 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-

Marlow [Page 21] RFC 1768 CLNP Multicasting March 1995

 UNITDATA.Request with the following parameters:
 SN_Userdata (SNSDU) <- ESGH PDU
 SN_Destination _Address <- multi-destination address that indicates
 "All Multicast Announcements"
 If an End System is attached to more than one subnetwork, the
 information about each group Network address desired for receiving on
 a particular subnetwork serving the End System shall be transmitted
 via that subnetwork. It is permissible for an End System to report
 group Network addresses on multiple subnetworks; 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
 information is used by the Active Multicast IS to determine whether a
 Multicast Address Mapping PDU should be emitted to update the
 association 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
 multicast 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
 may become synchronised. The result of this is that the traffic
 distribution will contain peaks. Where there are a large number of
 synchronised systems, this can cause overloading of both the
 transmission 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.

Marlow [Page 22] RFC 1768 CLNP Multicasting March 1995

         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 <max *)
                 PROCEDURE WaitUntil(time: Integer)
                 (* This procedure waits the specified number of
                 ms and then returns *)
                 PROCEDURE CurrentTime(): Integer
                 (* This procedure returns the current time in ms *)
         PROCEDURE
         DefineJitteredTimer(baseTimeValueInSeconds : Integer;
         expirationAction : Procedure);
         VAR
         baseTimeValue, maximumTimeModifier, waitTime : Integer;
         nextexpiration : Time;
         BEGIN
         baseTimeValue := baseTimeValueInSeconds * 1000 / Resolution;
         maximumTimeModifier := baseTimeValue * Jitter / 100;
         (* Compute maximum possible jitter *)
         WHILE running DO
                 BEGIN
                  (*First compute next expiration time *)
                 randomTimeModifier := Random(maximumTimeModifier);
                 waitTime:= baseTimeValue - randomTimeModifier;
                 nextexpiration := CurrentTime() + waitTime;
                  (* Then perform expiration Action *)
                 expirationAction;
                 WaitUntil(nextexpiration);
         END (* of Loop *)
         END (* of DefineJitteredTimer *)

Marlow [Page 23] RFC 1768 CLNP Multicasting March 1995

 Thus the call "DefineJitteredTimer(HelloTime, SendHelloPDU);" where
 "HelloTime" is 10 seconds, will cause the action "SendHelloPDU" to be
 performed at random intervals of between 7.5 and 10 seconds. The
 essential point of this algorithm is that the value of
 "randomTimeModifier" is randomised within the inner loop. Note that
 the new expiration time is set immediately on expiration of the last
 interval, rather than when the expiration action has been completed.
 The time resolution shall be less than or equal to 100 ms. It is
 recommended to be less than or equal to 10ms. The time resolution is
 the maximum interval than can elapse without there being any change
 in the value of the timer. The periodic transmission period shall be
 random or pseudo-random in the specified range. with uniform
 distribution across similar implementations.
 Note: Applying jitter to the MAT parameter is required in order to
 support the optional Damping function. If no jitter is applied on a
 subnetwork where many ESs are requesting a particular multicast PDU
 it is likely that they will have the same value for their MAT and
 these timers may all become synchronised. Such synchronisation will
 result in peaks in the distribution of traffic as described above.
 The resulting overloading of the transmission medium and the systems
 receiving the PDUs will negate any beneficial use of the Damping
 function (since systems may be attempting to transmit their own ESGH
 PDUs at the time they receive ESGH PDUs originated by other ESs with
 the same group Network address.

6.7.2 Record Multicast Announcement Function

 The Record Multicast Announcement function receives ESGH PDUs,
 extracts the multicast announcement information and updates the
 information in its routing information base.
 The receiving system is not required to process any option fields in
 a received ESGH PDU.
 Note: When a system chooses to process these optional fields, the
 precise actions are not specified by this International Standard.

6.7.2.1 Record Multicast Announcement Function by Intermediate Systems

 On receipt of an ESGH PDU an IS with the optional multicast
 capabilities extracts the configuration information and stores the
 {group Network address, subnetwork} in its routing information base
 replacing any other information for the same entry.

Marlow [Page 24] RFC 1768 CLNP Multicasting March 1995

 The Active Multicast IS upon receipt of an ESGH PDU also extracts the
 Paired SNPA Address parameter corresponding to each group Network
 address in the ESGH PDU. If the Active Multicast IS has a mapping for
 a group Network address carried in the ESGH for which the paired SNPA
 address does not match, the Report Multicast Address Mapping function
 is performed.

6.7.2.2 Optional Damping Function

 An ES with the optional capabilities to support multicast transfer
 may decide to process ESGH PDUs multicast by other End Systems. There
 is potentially some reduction in network traffic by doing this. An ES
 requesting to receive multicast PDUs is permitted to reset its
 Multicast Announcement Timer corresponding to one group Network
 address on one subnetwork upon receiving an ESGH PDU from another ES
 under the following circumstances:
 a)   The {group Network address, paired SNPA address} received on a
      particular subnetwork matches that of the ES processing the ESGH
      PDU for that subnetwork.
 b)   The Holding Timer parameter value in the ESGH PDU received is
      equal to or greater than the Holding Timer value for the, group
      Network address, being used by the ES processing this PDU.

6.7.3 Flush Old Multicast Announcement Function

 The Flush Old Multicast Announcement function is executed to remove
 multicast announcement entries in its routing information base whose
 Holding Timer has expired. When the Holding Timer for a group Network
 address expires, this function removes the corresponding entry from
 the routing information base of the local IS for the corresponding
 subnetwork.

6.8 Multicast Address Mapping

6.8.1 Report Multicast Address Mapping Function by Intermediate Systems

 The Active Multicast Intermediate System constructs a MAM PDU,
 corresponding to a group Network address for which it received via
 the Record Multicast Announcement function, and issues these PDUs
 under the following circumstances:
 a)   The IS initializes either as the Active Multicast IS after an
      election with other multicast capable ISs or initializes after
      determining it is the only multicast capable IS (the
      determination of such conditions are outside of the scope of
      this standard), or

Marlow [Page 25] RFC 1768 CLNP Multicasting March 1995

 b)   The IS receives an ESGH PDU with a group Network address paired
      to an SNPA address other than the SNPA address contained in the
      Active Multicast IS's multicast address mapping information for
      that group Network address, or
 Note: The Active Multicast IS determines which mappings are correct.
 Pre-configured mappings which are used prior to the initialization of
 the Active Multicast IS may be determined to be incorrect by the
 Active Multicast IS.
 c)   The expiration of the IS's Multicast Address Mapping Timer for
      that group Network address.
 Note: This is to prevent the expiration of Holding Timers in ESs.
 d)   The IS receives a multicast PDU originated on the subnetwork
      which used an incorrect destination SNPA address.
 Note: Of particular concern are those multicast packets using the
 "All Multicast Capable Intermediate Systems" SNPA address when
 another SNPA address should have been used.  The Originating
 Subnetwork Forwarding function is performed if this event occurs (see
 section 6.11).
 Note: The multicast capable ISs need to receive multicast packets on
 all SNPA addresses that are contained in the current multicast
 address mapping information for the subnetwork.  The multicast
 capable ISs are not required to receive multicast packets on any SNPA
 addresses other than those contained in the current multicast address
 mapping information and the "All Multicast Capable Intermediate
 Systems" SNPA address.
 Circumstances b) and d) are the event driven conditions for the
 Active Multicast IS to construct and issue a MAM PDU.  The Active
 Multicast IS shall limit the number of MAM PDUs issued per unit of
 time.  MAM PDUs with identical information shall not be issued more
 than once per second.  Event conditions occurring 10 seconds after
 the last issue of an appropriate MAM PDU shall result in the issuance
 of another such MAM PDU.
 The IS serving as the Active Multicast Intermediate System may
 construct a MAM PDU for each group Network address. Alternatively,
 MAM 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 MAM PDU. The
 IS performs all multicast address mapping functions independently for
 each of its subnetworks even if this IS is the Active Multicast IS on
 multiple subnetworks. Each MAM PDU is transmitted by issuing an SN-

Marlow [Page 26] RFC 1768 CLNP Multicasting March 1995

 UNITDATA.Request with the following parameters:
 SN_Userdata (SNSDU) <- MAM PDU
 SN_Destination _Address <- multi-destination address that indicates
 "All Multicast Capable End Systems"
 The Holding Time (HT) field is set to approximately twice the
 Intermediate System's Multicast Address Mapping Timer (MAMT)
 parameter.  This variable shall be set to a value large enough so
 that even if every other MAM PDU, for a particular group Network
 address, is discarded (due to lack of resources), or otherwise lost
 in the subnetwork, the multicast address mapping information will
 still be maintained. The value should be set small enough so that End
 Systems will quickly cease to use the multicast address mappings
 supplied by ISs that have failed.
 Note: -- The Holding Timer parameter value applies to all group
 Network addresses called out in the MAM PDU.
 The Group Address Pair parameter is used to convey the association
 between Group Network Addresses and SNPA addresses.
 Optionally, the Active Multicast IS may include information in the
 MAM PDU indicating a larger population of group Network addresses to
 which the same multicast address mapping information applies. There
 are two optional fields for this purpose: the Group Network Address
 Mask option and the Paired SNPA Address Mask option.
 There are three permitted cases for including or excluding the masks.
 In the first case, both masks are absent. In this case the MAM PDU
 conveys information about one set of enumerated group Network
 addresses only.
 Note: -- Multiple group address pairs may be contained in a single
 MAM PDU.
 In the second case, the MAM PDU contains a Group Network Address Mask
 but no Paired SNPA Address Mask. In this case, the MAM PDU conveys
 information about an equivalence class of group Network addresses.
 The information reveals that multiple group Network addresses are
 mapped to the same SNPA address.
 In the third case, the MAM PDU contains both masks. As in the second
 case, the MAM PDU conveys information about an equivalence class of
 group Network addresses. But in this case, the information reveals
 that the SNPA addresses for the equivalence class of group Network
 address are embedded in the group Network address. In particular the

Marlow [Page 27] RFC 1768 CLNP Multicasting March 1995

 Paired SNPA Address Mask indicates the location of the SNPA address
 in the group Network Address(es).
 The Active Multicast IS shall construct a MAM PDU with direct
 information, not needing analysis of the Mask parameters, in response
 to the occurrence of an event driven condition.  The Active Multicast
 IS may provide additional information in such a MAM PDU via the use
 of Mask parameters.
 An IS may suggest a value for End Systems on the local subnetwork to
 use as their Multicast Announcement Timers, for a specific group
 Network address, by including the Suggested ES Multicast Announcement
 Timer (ESMAT) parameter in the transmitted MAM PDU. Setting this
 parameter permits the Active Multicast IS to influence the frequency
 with which ESs transmit ESGH PDUs.
 Note: If the ESMAT parameter is used, the one value permitted in the
 MAM PDU is suggested for all group Network addresses called out in
 the MAM PDU.

6.8.2 Record Multicast Address Mapping Function by End Systems

 The Record Multicast Address Mapping function receives MAM PDUs,
 extracts the multicast address mapping information and updates the
 information in its routing information base. The receiving system is
 not required to process any option fields in a received MAM PDU with
 the exception of the Suggested ES Multicast Announcement Timer
 (ESMAT) parameter.
 Note: When a system chooses to process these optional fields, the
 precise actions are not specified by this International Standard.
 On receipt of a MAM PDU an ES with the optional multicast
 capabilities extracts the multicast address mapping information and
 stores the {group Network address, paired SNPA address} for a
 particular subnetwork in its routing information base replacing any
 other information for the same group Network address and subnetwork.
 In addition, an ES shall set its Multicast Announcement Timer,
 corresponding to the group Network address for which it is performing
 the Record Multicast Address Mapping function, based on receipt of a
 MAM PDU, corresponding to that group Network address, containing an
 ESMAT parameter.
 Note: While an ES may process ESGH PDUs multicast by other ESs to
 support the optional Damping function, an ES is not permitted to
 change its own mapping due to the mapping found in other ES's ESGH
 PDUs.

Marlow [Page 28] RFC 1768 CLNP Multicasting March 1995

6.8.3 Flush Old Multicast Address Mapping Function by End Systems

 The Flush Old Multicast Address Mapping function is executed to
 remove multicast address mapping entries in its routing information
 base whose corresponding Holding Timer has expired. When such a
 Holding Timer for a multicast address mapping expires, this function
 removes the corresponding entry from its routing information base for
 the corresponding SNPA.

6.9 Paired SNPA Address Selection Function by End Systems

 An End System shall pair each group Network address with an
 associated SNPA address to support receiving (e.g., performing the
 Report Multicast Announcement function) and originating multicast
 PDUs.

6.9.1 Paired SNPA Address Selection for Receiving Multicast PDUs

 An End System always has a paired SNPA address for every active group
 Network address on a particular subnetwork. This mapping is obtained
 by:
 a)   recording a multicast address mapping which is maintaining an
      active holding timer, or if there has been no dynamic
      information received, by
 b)   having pre-configured multicast address mapping information, or
      if neither dynamic nor pre-configured information is available,
      by
 c)   mapping the "All Multicast Capable End Systems" multi-
      destination address to the group Network address.

6.9.2 Paired SNPA Address Selection for Originating Multicast PDUs

 An End System, originating a multicast PDU, pairs a SNPA address to
 the group Network address.  This mapping is obtained in the following
 manner:
 a)   If there is a multicast capable IS reachable on the subnetwork
      then the SNPA address used by an End System originating a multi-
      cast PDU is either the paired SNPA address obtained from the
      multicast address mapping information associated with the group
      Network address in the multicast PDU's Destination address
      parameter or if there is no valid entry for the group Network
      address by using the "All Multicast Capable Intermediate Sys-
      tems" multi-destination address, or if there is no multicast
      capable Intermediate System on the subnetwork, by

Marlow [Page 29] RFC 1768 CLNP Multicasting March 1995

 Note: Multicast address mapping information is valid if the Holding
 Timer associated with it has not expired.
 Note: An ES can determine if a multicast capable IS is reachable on
 the subnetwork by having for that subnetwork either (1)multicast
 address mapping information or (2)routing information received via an
 ISH PDU containing a Multicast Capable optional parameter.  In either
 case the information must be valid (i.e., the Holding Timer for the
 information must not have expired).
 b)   having pre-configured multicast address mapping information, or
      if neither a multicast capable Intermediate System is present on
      the subnetwork nor pre-configured information is available, by
 c)   mapping the "All Multicast Capable End Systems" multi-
      destination address to the group Network address.

6.10 Extensions to the ISO CLNP Route Function by End Systems

 An End System attached to more than one subnetwork shall determine
 when originating a multicast PDU whether to forward this multicast
 PDU to more than one subnetwork or not.  End Systems shall originate
 each multicast PDU on all subnetworks for which the ISO ES-IS
 Configuration function is actively reporting the NSAP address
 contained in the Source Address parameter of the multicast PDU.  As a
 result of this function multiple invocations of the ISO CLNP
 Forwarding function may result when such an ES originates a multicast
 PDU.

6.11 Originating Subnetwork Forwarding Function by Intermediate

      Systems
 The Active Multicast IS upon receiving a multicast PDU originated on
 a subnetwork which used the "All Multicast Capable Intermediate
 Systems" SNPA address when another SNPA address should have been
 used, performs the Originating Subnetwork Forwarding function.  The
 multicast address mapping information defines the correct SNPA
 address pairings for a given subnetwork.  The Originating Subnetwork
 Forwarding function forwards the multicast PDU back on subnetwork it
 was originated on.  In the case that the ES was attached to more than
 one subnetwork and originated the multicast PDU on more than one
 subnetwork, the Active Multicast IS for each subnetwork performs the
 Originating Subnetwork Forwarding function for the subnetwork that
 they are responsible for.
 The Active Multicast IS obtains the contents for the multicast PDU
 for the Originating Subnetwork Forwarding function by using the
 contents of the multicast PDU received with the incorrect destination

Marlow [Page 30] RFC 1768 CLNP Multicasting March 1995

 SNPA address and replacing the original PDU Lifetime field with the
 value one (0000 0001).  The Active Multicast IS performs the ISO 8473
 PDU Composition function and forwards the PDU to the subnetwork that
 the PDU was originated on using the ISO 8473 Forwarding function with
 the correct destination SNPA address.
 Note: The PDU Lifetime field is set to "one" to ensure that ISs
 attached to the originating subnetwork do not forward this PDU on.
 Such ISs should have received the PDU when it was originated since
 this function is only performed in the event of receiving a multicast
 PDU incorrectly addressed to the "All Multicast Capable Intermediate
 Systems" SNPA address.

6.12 Structure and Encoding of PDUs

 The ES-IS multicast control functions are supported via the exchange
 of ESGH and MAM PDUs.  The one exception to this is that a new
 optional parameter, the Multicast Capable parameter, is provided for
 use within the ISH PDU.

6.12.1 PDU Type Codes

 The Multicast Announcement is accomplished via the transfer of End
 System Group Hello (ESGH) PDUs. The PDU type code for an ESGH PDU is
 "0 0 1 0 1". The Multicast Address Mapping (MAM) is accomplished via
 the transfer of Multicast Address Mapping PDUs. The PDU type code for
 a MAM PDU is "0 0 1 1 1".

6.12.2 Hold Time field

 The Holding Time field specifies the maximum time for the receiving
 Network entity to retain the multicast announcement or multicast
 address mapping information contained in the PDU.

6.12.3 Structure of Addressing Parameters

 The ESGH and MAM PDUs carry one or more group Network addresses
 (GNAs) each with their associated Paired SNPA Address (PSA).

6.12.4 Group Address Pair Parameter for ESGH and MAM PDUs

 The Group Address Pair parameter is a list of one or more group
 Network addresses each with their associated Paired SNPA address. The
 group Network address identifies specific multicast PDUs and the
 Paired SNPA address is the SNPA address on which the ES expects to
 receive such multicast PDUs on that subnetwork. It is encoded in the
 ESGH and MAM PDUs as shown in Figure 1.

Marlow [Page 31] RFC 1768 CLNP Multicasting March 1995

                                                            Octet
      ,----------------------------------------------------,
      |        Number of Group Address Pairs               |  10
      |----------------------------------------------------|
      |   Group Network Address Length Indicator (GNAL)    |  11
      |----------------------------------------------------|
      |                                                    |  12
      :           Group Network Address (GNA)              :
      |                                                    |
      |----------------------------------------------------|
      |   Paired SNPA Address Length Indicator (PSAL)      |
      |----------------------------------------------------|
      |                                                    |
      :            Paired SNPA Address (PSA)               :
      |                                                    |
      |----------------------------------------------------|
      |                       GNAL                         |
      |----------------------------------------------------|
      |                                                    |
      :                       GNA                          :
      |                                                    |
      |----------------------------------------------------|
      |                       PSAL                         |
      |----------------------------------------------------|
      |                                                    |
      :                       PSA                          :
      |                                                    | m-1
      '----------------------------------------------------'
      Figure 1 - ESGH and MAM PDUs - - Group Address Pair Parameter

6.12.5 Extensions to the current Option Parameters

 The Security and Priority optional parameters may be carried in a
 ESGH PDU. There is no Security or Priority option for the MAM PDU.

6.12.6 Suggested ES Multicast Announcement Timer

 The ESMAT parameter may appear only in the MAM PDU
 The ESMAT parameter conveys the value that an IS requests the
 receiving ESs to use as their local Multicast Announcement Timer.
 Parameter Code:         1100 0111
 Parameter Length:       two octets
 Parameter Value:        ESMAT in units of seconds.

Marlow [Page 32] RFC 1768 CLNP Multicasting March 1995

6.12.7 Multicast Capable

 The Multicast Capable option may appear only in the ISH PDU
 The Multicast Capable options consists only of a one octet code and a
 one octet parameter length field, there is no parameter field.
 Parameter Code:         1100 1000
 Parameter Length:       zero octets
 Parameter Value:        none (parameter does not exist).

6.12.8 Group Network Address Mask

 The Group Network Address Mask option may only appear in the MAM PDU.
 The Group Network Address Mask parameter indicates that the multicast
 address mapping information applies to a larger population of group
 Network Addresses than the group Network address(es) contained in the
 MAM PDU indicates. When this option is provided in a MAM PDU, the
 masking relationship contained must be valid for all group Network
 addresses contained in this PDU. An End System may ignore this
 parameter.
 The Group Network Address Mask establishes an equivalence class of
 group Network addresses to which the same multicast address mapping
 information applies. To determine whether or not a trial group
 Network address falls within the equivalence class, the ES aligns the
 trial group Network address with the Group Network Address Mask
 padding the latter with trailing zero octets if necessary. If in all
 bit positions where the Group Network Address Mask is "1" the trial
 group Network address matches the Group Network Address field of the
 Group Address Pair parameter of the MAM PDU, then the trial group
 Network address belongs to the equivalence class described by the MAM
 PDU.
 The Group Network Address Mask parameter has additional semantics
 when considered with the Paired SNPA Address Mask parameter.
 Parameter Code:         1110 0011
 Parameter Length:       variable, up to 20 octets
 Parameter Value:        a comparison mask of octets to be
                         aligned with the Group Network Address
                         field of the Group Address Pair
                         parameter of the MAM PDU.

Marlow [Page 33] RFC 1768 CLNP Multicasting March 1995

6.12.9 Paired SNPA Address Mask

 The Paired SNPA Address Mask option may only appear in the MAM PDU.
 When the Paired SNPA Address Mask is present, the equivalence class
 defined by the Group Network Address Mask also has common structure
 below the Group Network Address Mask; i.e., in the portion of the
 group Network address where the Group Network Address Mask is
 logically "0".  The Paired SNPA Address Mask supplies additional
 information about the structure, by indicating certain bit positions
 within the space "below" the Group Network Address Mask.
 Specifically, the Paired SNPA Address Mask indicates the location of
 the Paired SNPA address in the Group Network Address.
 This parameter may appear in a MAM PDU only if the Group Network
 Address Mask is also present. When this option is provided in a MAM
 PDU, the masking relationship contained must be valid for all group
 Network addresses contained in this PDU. An ES receiving such a MAM
 PDU may safely ignore both masks. However (since presence of both
 masks dictates different functional behavior than the presence of the
 Group Network Address Mask alone) an ES shall not ignore one of the
 masks while heeding the other.
 Parameter Code:         1110 0100
 Parameter Length:       variable
 Parameter Value:        a comparison mask of octets to be
                         aligned with the Group Network Address
                         field(s) of the Group Address Pair
                         parameter of the MAM PDU.

6.12.9.1 Mask Parameters Example

 This section provides examples of using the Group Network Address
 Mask and the Paired SNPA Address Mask.  The examples given are for an
 Internet usage of CLNP Multicasting across subnetworks using IEEE 802
 addressing.  For these examples the group Network address format is:
              +-----+----------------------------------------+
              | IDP | Upper DSP | Embedded SNPA address | SEL|
              +-----+-----------+-----------------------+----+
     octets:  |  3  |    10     |           6           |  1 |
              +-----+-----------+-----------------------+----+
 Thus the group Network address used is 20 octets.  For these
 examples, the only field considered is the Embedded SNPA address
 field and its placement within the group Network address.

Marlow [Page 34] RFC 1768 CLNP Multicasting March 1995

 In the first example it is the policy in "this part of the Internet"
 to map the Embedded SNPA address into the IEEE 802 address space
 reserved by IEEE 802 for group addressing using LOCAL assignment,
 this corresponds to all 48 bit values with the two low order bits of
 the first octet set to "11".
 The Active Multicast Intermediate System on this subnetwork may
 construct a MAM PDU to map, for this example, a group Network address
 of {13 octets, 03-00-DA-DA-DA-DA, 1 octet} and a paired SNPA address
 of 03-00-DA-DA-DA-DA.  In addition the Active Multicast Intermediate
 System can include in the MAM PDU a Group Network Address Mask of
 FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-FF-03-00-00-00-00-00-00.
 With this parameter, all group Network addresses which share the
 identical first 13 octet and with "11" in the two low order bits of
 the 14th octet are put in an equivalence class and share the same
 mapping information. If this were the only option present then all of
 these group Network addresses would all have a paired SNPA address of
 03-00-DA-DA-DA-DA.
 In order to map the group Network addresses to the range of IEEE
 addresses of this example, the MAM PDU must also contain a Paired
 SNPA Address Mask.  The Paired SNPA Address Mask identifies where the
 SNPA Address is contained within the group Network addresses (defined
 by the equivalence class formed by the Group Network Address Mask
 within the same PDU).  For this example the Paired SNPA Address Mask
 is 00-00-00-00-00-00-00-00-00-00-00-00-00-FF-FF-FF-FF-FF-FF-00.
 As a second example, all group Network addresses with a specific OUI
 (organizationally unique identifier) using the twenty octet group
 Network address format provided above are mapped to their embedded
 SNPA address.  An OUI is assigned by IEEE 802 and is three octets in
 length.  The OUI is contained in the first three address octets of a
 GLOBALLY assigned IEEE 802 address.  For this example the MAM PDU
 must contain the following:
 1.   A group Network address contained within the MAM PDU with the
      OUI of interest.
 2.   A group Network address Mask of FF-FF-FF-FF-FF-FF-FF-FF-FF-
      FF-FF-FF-FF-FF-FF-FF-00-00-00-00.
 3.   A Paired SNPA Address of 00-00-00-00-00-00-00-00-00-
      00-00-00-00-FF-FF-FF-FF-FF-FF-00.

6.12.10 End System Group Hello (ESGH) PDU

 The ESGH PDU has the format shown in figure 2:

Marlow [Page 35] RFC 1768 CLNP Multicasting March 1995

                                                            Octet
      ,----------------------------------------------------,
      |          Network Layer Protocol Identifier         |  1
      |----------------------------------------------------|
      |                 Length Indicator                   |  2
      |----------------------------------------------------|
      |           Version/Protocol ID Extension            |  3
      |----------------------------------------------------|
      |               reserved (must be zero)              |  4
      |----------------------------------------------------|
      | 0 | 0 | 0 |  Type (00101 = ESGH)                   |  2
      |----------------------------------------------------|
      |                    Holding Time                    | 6,7
      |----------------------------------------------------|
      |                      Checksum                      | 8,9
      |----------------------------------------------------|
      |          Number of Group Address Pairs             |  10
      |----------------------------------------------------|
      |   Group Network Address Length Indicator (GNAL)    |  11
      |----------------------------------------------------|
      |                                                    |  12
      :            Group Network Address (GNA)             :
      |                                                    |
      |----------------------------------------------------|
      |    Paired SNPA Address Length Indicator (PSAL)     |
      |----------------------------------------------------|
      |                                                    |
      :             Paired SNPA Address (PSA)              :
      |                                                    |
      |----------------------------------------------------|
      |                        GNAL                        |
      |----------------------------------------------------|
      |                                                    |
      :                        GNA                         |
      |                                                    |
      |----------------------------------------------------|
      |                        PSAL                        |
      |----------------------------------------------------|
      |                                                    |
      :                        PSA                         :
      |                                                    | m-1
      |----------------------------------------------------|
      |                                                    |  m
      :                       Options                      :
      |                                                    | p-1
      '----------------------------------------------------'
                    Figure 2 - ESGH PDU Format

Marlow [Page 36] RFC 1768 CLNP Multicasting March 1995

6.12.11 Multicast Address Mapping (MAM) PDU

 The MAM PDU has the format shown in figure 3:
                                                            Octet
      ,----------------------------------------------------,
      |       Network Layer Protocol Identifier            |  1
      |----------------------------------------------------|
      |               Length Indicator                     |  2
      |----------------------------------------------------|
      |        Version/Protocol ID Extension               |  3
      |----------------------------------------------------|
      |           reserved (must be zero)                  |  4
      |----------------------------------------------------|
      | 0 | 0 | 0 |  Type (00111 = MAM)                    |  2
      |----------------------------------------------------|
      |                Holding Time                        | 6,7
      |----------------------------------------------------|
      |                  Checksum                          | 8,9
      |----------------------------------------------------|
      |          Number of Group Address Pairs             |  10
      |----------------------------------------------------|
      |  Group Network Address Length Indicator (GNAL)     |  11
      |----------------------------------------------------|
      |                                                    |  12
      :          Group Network Address (GNA)               :
      |                                                    |
      |----------------------------------------------------|
      |     Paired SNPA Address Length Indicator (PSAL)    |
      |----------------------------------------------------|
      |                                                    |
      :              Paired SNPA Address (PSA)             :
      |                                                    |
      |----------------------------------------------------|
      |                       GNAL                         |
      |----------------------------------------------------|
      |                                                    |
      :                       GNA                          :
      |                                                    |
      |----------------------------------------------------|
      |                       PSAL                         |
      |----------------------------------------------------|
      |                                                    |
      :                       PSA                          :
      |                                                    | m-1

Marlow [Page 37] RFC 1768 CLNP Multicasting March 1995

      |----------------------------------------------------|
      |                                                    |  m
      :                      Options                       :
      |                                                    | p-1
      '----------------------------------------------------'
                   Figure 3 - MAM PDU Format

6.13 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. A
 Network entity may choose to be multicast capable, a multicast
 capable Network entity is required to support both multicast
 announcement information and multicast address mapping information.
 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 shall support multicast announcement
 information and shall implement the functions marked as Mandatory (M)
 in column 4 of table 3. A multicast capable End System implementation
 shall also support multicast address mapping information and shall
 implement the functions marked as Mandatory (M) in column 5 of table
 3.
 An implementation claiming conformance as a multicast capable
 Intermediate System shall meet all of the requirements for an
 Intermediate System which is not multicast capable and shall support
 multicast announcement information and shall implement the functions
 marked as Mandatory (M) in column 6 of table 3. A multicast capable
 Intermediate System implementation shall also support multicast
 address mapping information and shall implement the functions marked
 as Mandatory (M) in column 7 of table 3.

Marlow [Page 38] RFC 1768 CLNP Multicasting March 1995

   Table 3 - Static Conformance Requirements for Multicast Capable
   Network Entities
                                                         ES      IS
                                             Clause    --------------
   Label     Function                       Reference  AI  MI  AI  MI
   ------------------------------------------------------------------
   RpMAn     Report Multicast Announcement     6.7.1    M   -   -   -
   RcMAn     Record Multicast Announcement     6.7.2.1  -   -   M   -
   RcDamp    Record Damping                    6.7.2.2  O   -   -   -
   FlMAn     Flush Old Multicast Announcement  6.7.3    O   -   M   -
   RpMAdMa   Report Multicast Address Mapping  6.8.1    -   -   -   M
   MATGn       ESMAT Generation                6.8.1    -   -   -   M
   RcMAdMa   Record Multicast Address Mapping  6.8.2    -   M   -   -
   MATPr       ESMAT Processing                6.8.2    -   M   -   -
   FlMAdMa   Flush Old Multicast Address Map   6.8.3    -   M   -   -
   PSAdSel   Paired SNPA Address Selection     6.9.1    -   M   -   -
   ExtForw   Extensions to CLNP Route Function 6.10     -   M   -   -
   OSuForw   Originating Subnetwork Forwarding 6.11     -   -   -   M
   Key:
   AI = Multicast Announcement information supported
   MI = Multicast Address Mapping information supported
   M = Mandatory;  O = Optional;  - = not applicable

7. Security Considerations

 Security issues are not discussed in this memo.

Marlow [Page 39] RFC 1768 CLNP Multicasting March 1995

Appendix A. Differences with RFC 1112

 This appendix is intended to identify differences between the
 mechanisms defined for CLNP Multicast in this specification and those
 for IP multicast defined in RFC 1112. The work on CLNP Multicast
 followed the work on IP multicast and was explicitly aimed at
 bringing the capabilities described in RFC 1112 into a CLNP context.
 This appendix is intended to provide some background information on
 the difference; however, it is not intended to justify the mechanisms
 selected for CLNP multicast use.
 Static/Dynamic Address Binding of Multicast Datagrams
 IP multicast utilizes a static binding of Class D IP addresses to a
 specific range of IEEE 802 48 bit group addresses. The IEEE 802
 address range that is used is within the address range that IEEE 802
 allocates for "Global" administration and this block of addresses is
 under the control of the Internet Assigned Numbers Authority (IANA)
 which in turn has allocated this block of addresses for use by IP
 multicast.  This scheme is very simple and efficient. Given the use
 of a 32 bit IP address, the lower 23 bits of the Class D address are
 mapped into the lower 23 bits of a 48 bit IEEE 802 address where the
 upper 25 bits are fixed.  Static binding of this form is global in
 scope (all members of a group use the same IEEE 802 address on all
 subnets (at least all that use IEEE 802 addressing).
 CLNP multicast uses a dynamic binding of a group Network address (up
 to 20 bytes) to any subnetwork address. In cases where no multicast
 capable Intermediate Systems are attached to a subnetwork then a
 binding using preconfigured information or the "All Multicast Capable
 End Systems" subnetwork addresses is used. The large GNA provides the
 room to contain a full 48 bit IEEE 802 address if desired.  Mask
 capabilities are optionally provided which allow a multicast capable
 Intermediate System to specify a "static" binding for a particular
 subnetwork. One of the major purposes of providing a dynamic binding
 is to customize a host's subnetwork address usage to the capabilities
 of the attached systems.  There is considerable differences in the
 numbers of group subnetwork addresses that a system can recognize
 using hardware hooks built into the integrated circuits used. For
 example the number of addresses that can be recognized by hardware
 may differ by an attached system depending upon the interface it uses
 (e.g., Ethernet interface and FDDI within the same system may have
 quite different capabilities).  Dynamic binding of this form is local
 in scope (members of a group may use different subnetwork addresses
 (e.g., IEEE 802 addresses) on different subnets).

Marlow [Page 40] RFC 1768 CLNP Multicasting March 1995

 Originating of Multicast Datagrams
 IP multicast originates multicast datagrams directly, where the host
 originating a datagram sends it with the group Subnetwork address as
 its destination. Hosts attached to the network where the datagram is
 originated receive the datagram directly.
 CLNP multicast originates multicast datagrams directly using the
 group's subnetwork address as its destination when multicast address
 mapping information is available.  This case occurs when a multicast
 capable Intermediate System is attached to the subnetwork and a host
 on the subnetwork is announcing an interest in multicast packets
 identified by a particular group Network address.  The Active
 Multicast IS may use MAM PDU mask parameters to provide multicast
 address mapping information for a large number of group Network
 addresses. When there is no multicast address mapping information for
 the particular group Network address on a subnetwork with a multicast
 capable IS attached to it, hosts originate packets using such
 addresses sends to the "All Multicast Capable Intermediate Systems"
 SNPA address.  This case occurs when there are no receivers of such
 multicast packets on the originating subnetwork.  When a multicast
 capable Intermediate System is not attached to a subnetwork, the End
 System may utilize either preconfigured information (which might be a
 direct mapping from a portion of the group Network address) or use
 the "All Multicast Capable End Systems" address.
 Address Binding of Control Packets
 IP multicast sends the control packets related to the IGMP protocol
 on the same subnetwork address that is used by the multicast data
 traffic.
 CLNP multicast sends the control packets related to the ES-IS
 protocol extensions on specific group subnetwork addresses (i.e.,
 "All Multicast Capable End Systems" and "All Multicast Announcements"
 addresses).
 Router Requirements for relaying Multicast Datagrams
 IP multicast requires that a multicast router run in "promiscuous"
 mode where it must receive all multicast datagrams originated on a
 subnetwork regardless of the destination. This is a result of the
 choices selected in the "Originating of Multicast Datagrams" and
 "Address Binding of Control Packets" discussed above.
 CLNP multicast allows a multicast router to limit multicast packet
 reception to only those datagrams sent to the SNPA addresses where
 there is current multicast address mapping information or to the "All

Marlow [Page 41] RFC 1768 CLNP Multicasting March 1995

 Multicast Capable Intermediate Systems" address.  The intention is to
 allow the multicast routers to be in control of the SNPA addresses
 for multicast packets that they need to receive.  This is a result of
 the choices selected in the "Originating of Multicast Datagrams" and
 "Address Binding of Control Packets" discussed above.
 Aggregation of Control Information
 In IP multicast, a host is required to withhold an announcement
 report upon hearing another host reporting a similar interest in a
 particular Class D address on a particular subnetwork. This is an
 option for CLNP multicast (upon hearing interest in a particular
 group Network address on a particular subnetwork). Such reports are
 not combined in IP multicast while CLNP multicast supports providing
 multiple announcements (and address mappings) within a single packet.
 A mask feature for address mappings supports identifying mappings for
 a range of group Network addresses within a single control packet.
 Datagram Scope Control
 IP multicast supports the use of the IP Hop Count as a means to
 support scope control. While not documented in RFC 1112, a technique
 is also being used to use bits within the Class D address to identify
 whether a datagram has single subnetwork, "campus" or global scope.
 CLNP has considerable scope control functionality. While the PDU
 Lifetime field can be employed in a similar way to the IP Hop Count,
 two additional options are available. The Radius scope control
 provides a mechanism for "administratively" setting distance values
 and de-couples the multicast scope control from the PDU lifetime
 function. More importantly, the Prefix based scope control appears to
 provide considerable and flexible functionality that can adjust to
 situations where a known, hierarchical unicast addressing structure
 exists.
 Marking of Multicast Datagrams
 IP multicast marks a multicast PDU via the use of an IP Class D
 address as its destination address parameter. CLNP multicast marks
 both the PDU (a different PDU type) and the destination address
 (i.e., group Network address) parameter.
 Unicast Addressing Differences
 An IP address identifies a specific host interface while a CLNP
 individual Network address (i.e., NSAP address) identifies a
 particular Network entity. This difference has lead to a difference
 with RFC 1112.  IP multicast requires a host which is attached to

Marlow [Page 42] RFC 1768 CLNP Multicasting March 1995

 more than one subnetwork to originate a multicast packet on only one
 subnetwork.  CLNP multicast requires a host which is attached to more
 than one subnetwork to originate a multicast packet on every
 subnetwork that the ISO ES-IS Configuration function is reporting the
 NSAP address contained in the source address parameter of the
 multicast PDU.
 Error Reports
 Error reports sent in response to receiving a multicast PDU are not
 permitted in IP multicast while they are permitted in CLNP multicast.
 Source Routing
 Source routing of multicast PDUs are permitted in IP multicast (but
 at the present time this is discouraged) while they are not permitted
 in CLNP multicast.

Appendix B. Issues Under Study

 This appendix is intended to record the current issues (as discussed
 at the March 1994 TUBA meeting).
 1. Local versus Global address bindings
 The extensions to the ES-IS protocol provide a multicast address
 mapping function which supports dynamically binding a group Network
 address to a subnetwork address.  Concern has been expressed that
 this is an unnecessary feature which complicates the job of network
 administrators without suitable benefit.  A static, global binding of
 group Network addresses to IEEE 802 subnetwork addresses, as is used
 by IP multicast has been suggested.
 The two main reasons that the group Network address to subnetwork
 (IEEE 802) address was made locally configurable were to support
 multicast on subnets with hosts having a mixture of capabilities (as
 to how many multicast subnetwork addresses a host could register to
 receive at a time) and to support multicast on subnets that do not
 use 48 bit IEEE 802 addresses.  Thus it was felt that this should be
 done per subnetwork versus globally.  Even multi-homed hosts with
 subnets that use 802 addresses may have varying capabilities (looking
 at typical Ethernet, FDDI and 802.5 implementations).
 One possible solution is to recommend a direct mapping in any
 Internet use of CLNP multicast on subnets which use IEEE 802
 addressing.  This could be a default for all Internet hosts.  A
 policy would be needed to identify the Internet's group Network
 address format.  Given such a mapping the only operational overhead

Marlow [Page 43] RFC 1768 CLNP Multicasting March 1995

 that would occur is that in the presence of a mapping server (the
 Active Multicast IS), which was supporting this mapping, a MAM PDU
 would periodically be sent with a Group Network Address Mask which
 would identify the direct mapping.
 2.  "Real Time" Scope Control Features
 The scope control features are provided via optional parameters.  Use
 of multicast transfer of audio and video streams may require scope
 control mechanisms which operate very quickly.
 One possible solution is to embed scope control mechanisms into the
 group Network address itself.  For example, a group Network address
 using the "Local" AFI is automatically limited to not cross inter-
 domain borders.  Further, more flexible, address formats may be
 developed.

References

 [Deering91] Deering, S., "Multicast Routing in a Datagram
 Internetwork", PhD thesis, Electrical Engineering Dept., Stanford
 University, December 1991.
 [RFC1112] Deering, S., "Host Extensions for IP Multicasting",
 STD 5, RFC 1112, Stanford University, August 1989.
 [RFC1237] Colella, R., Gardner, E., and R. Callon, "Guidelines for OSI
 NSAP Allocation in the Internet", RFC 1237, NIST, Mitre, DEC, July
 1991.
 [CLNP] Protocol for providing the connectionless-mode network service,
 International Standard 8473-1, Second Edition, ISO/IEC JTC 1,
 Switzerland 1994.  (Available via FTP from
 merit.edu:pub/iso/iso8473part1.ps).
 [ES-IS] End system to Intermediate system routing exchange protocol
 for use in conjunction with the Protocol for providing the
 connectionless-mode network service, International Standard 9542,
 ISO/IEC JTC 1, Switzerland 1987.  (Available via FTP from
 merit.edu:pub/iso/iso9542.ps).
 [MULT-AMDS]: Amendments to ISO standards to support CLNP multicast
 extensions:
 ISO 8348 AM5 Amendment to the Network Service to support Group Network
 Addressing. International Standard ISO 8348 Amendment 5, ISO/IEC JTC
 1, Switzerland 1994.

Marlow [Page 44] RFC 1768 CLNP Multicasting March 1995

 ISO 8473-1 DAM1 - Draft Amendment to the Second Edition of the
 Protocol for providing the connectionless-mode network service [CLNP],
 Multicast Extension, 1993.
 ISO 9542 DAM2 - Draft Amendment to the ES-IS [ES-IS] protocol,
 Addition of connectionless- mode multicast capability, 1993.

Author's Address

 Dave Marlow
 Code B35
 NSWC-DD
 Dahlgren, VA. 22448
 Phone: (703) 663-1675
 EMail: dmarlow@relay.nswc.navy.mil

Marlow [Page 45]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1768.txt · Last modified: 1995/03/02 21:46 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki