Halting the Hacker - A P…uide to Computer Security/Halting the Hacker - A Practical Guide to Computer Security.iso/rfc/rfc1768.txt

Text File | 1997-04-01 | 111KB | 2,524 lines

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

                              --------------
                              |    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: [phone]
   EMail: [e-mail]



































Marlow                                                         [Page 45]