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.

   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-



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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.






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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 *)





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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.






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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



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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