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
