RFC 3337 (rfc3337) - Page 2 of 7
Class Extensions for PPP over Asynchronous Transfer Mode Adaptation Layer 2
Alternative Format: Original Text Document
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