RFC 1626 (rfc1626) - Page 2 of 5

Default IP MTU for use over ATM AAL5

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RFC 1626          Default IP MTU for use over ATM AAL5          May 1994


Permanent Virtual Circuits

   Implementations which only support Permanent Virtual Circuits (PVCs)
   will (by definition) not implement any ATM signalling protocol.  Such
   implementations shall use the default IP MTU value of 9180 octets
   unless both parties have agreed in advance to use some other IP MTU
   value via some mechanism not specified here.

Switched Virtual Circuits

   Implementations that support Switched Virtual Circuits (SVCs) MUST
   attempt to negotiate the AAL CPCS-SDU size using the ATM signalling
   protocol.  The industry standard ATM signalling protocol uses two
   different parts of the Information Element named "AAL Parameters" to
   exchange information on the MTU over the ATM circuit being setup
   [ATMF93a].  The Forward Maximum CPCS-SDU Size field contains the
   value over the path from the calling party to the called party.  The
   Backwards Maximum CPCS-SDU Size Identifier field contains the value
   over the path from the called party to the calling party.  The ATM
   Forum specifies the valid values of this identifier as 1 to 65535
   inclusive.  Note that the ATM Forum's User-to-Network-Interface (UNI)
   signalling permits the MTU in one direction to be different from the
   MTU in the opposite direction, so the Forward Maximum CPCS-SDU Size
   Identifier might have a different value from the Backwards Maximum
   CPCS-SDU Size Identifier on the same connection.

   If the calling party wishes to use the default MTU it shall still
   include the "AAL Parameters" information element with the default
   values for the Maximum CPCS-SDU Size as part of the SETUP message of
   the ATM signalling protocol [ATMF93b].  If the calling party desires
   to use a different value than the default, it shall include the "AAL
   Parameters" information element with the desired value for the
   Maximum CPCS-SDU Size as part of the SETUP message of the ATM
   Signalling Protocol.  The called party will respond using the same
   information elements and identifiers in its CONNECT message response
   [ATMF93c].

   If the called party receives a SETUP message containing the "Maximum
   CPCS-SDU Size" in the AAL Parameters information element, it shall
   handle the Forward and Backward Maximum CPCS-SDU Size Identifier as
   follows:

    a) If it is able to accept the ATM MTU values proposed by the
       SETUP message, it shall include an AAL Parameters information
       element in its response.  The Forward and Backwards Maximum
       CPCS-SDU Size fields shall be present and their values shall be
       equal to the corresponding values in the SETUP message.




Atkinson
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