RFC 3337 (rfc3337) - Page 2 of 7

Class Extensions for PPP over Asynchronous Transfer Mode Adaptation Layer 2

RFC 3337           Class Extensions for PPP over AAL2      December 2002


   In addition to bandwidth efficiency, real-time applications such as
   voice require low latency.  RFC 2689 [2] describes an architecture
   for providing transport services for real time applications on low
   bit rate links.  The main components of the architecture are: a
   real-time encapsulation format for asynchronous and synchronous low-
   bitrate links, a header compression architecture optimized for real-
   time flows, elements of negotiation protocols used between routers
   (or between hosts and routers), and announcement protocols used by
   applications to allow this negotiation to take place.

   Multi Class Multi Link PPP [3] defines a fragment-oriented solution
   for the real-time encapsulation format part of the architecture
   defined in [2], i.e., for the queues-of-fragments type sender.  As
   described in more detail in the architecture document, a real-time
   encapsulation format is required to guarantee low latency in the
   presence of large non real time packets. For example, a 1500 byte
   packet on a 128 kbit/s ATM virtual circuit makes this link
   unavailable for the transmission of real-time information for about
   100 ms.  This adds a worst-case delay that causes real-time
   applications to operate with round-trip delays that are too high for
   many interactive tasks.  Multi Class Multi Link PPP defines a set of
   extensions of Multi Link PPP [4] that enable the sender to fragment
   the packets of various priorities into multiple classes of fragments,
   allowing high-priority packets to be sent between fragments of lower
   priorities.

   This document defines a set of class extensions to PPP over AAL2 [1]
   that implement equivalent functionality to Multi Class Multi Link PPP
   over a single ATM virtual circuit.  Instead of using Multi Link PPP
   as the basis for fragmentation functionality, this document uses the
   functionality of the Service Specific Segmentation and Reassembly
   Sublayer (SSSAR) [5] that is already required as the basic
   encapsulation format of PPP over AAL2.

   In addition to providing fragmentation, the real time transport
   service must allow high priority fragments to be sent between
   fragments of lower priorities.  This can be accomplished in PPP over
   AAL2 by allowing a single PPP session to span multiple AAL2 CPS [6]
   Channel Identifiers.  Once a PPP session spans multiple AAL2 Channel
   IDs, the Channel ID can be used to identify the class that a fragment
   belongs to.  Fragments belonging to a high priority class can be sent
   using a particular AAL2 Channel ID.  Fragments of lower priority
   classes can be sent using different AAL2 Channel IDs.  Once multiple
   fragment classes are identified using different AAL2 Channel IDs, the
   AAL2 CPS layer can be used to send fragments belonging to a high
   priority class between fragments of lower priorities.





Thompson, et. al.           Standards Track