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.

   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.




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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;



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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.





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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.




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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,



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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