rfc1768.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   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.
























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















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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



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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.



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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.




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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.