Class Extensions for PPP over Asynchronous Transfer Mode Adaptation Layer 2
Extracted elements (15)
A 1500-byte packet on a 128 kbit/s ATM virtual circuit blocks the link for about 100 ms, adding unacceptable worst-case latency for interactive real-time applications; multi-class fragmentation solves this by allowing high-priority packets to be sent between lower-priority fragments.
For a simple voice service over low-bandwidth ATM virtual circuits, two classes (one for large data packets, one for real-time voice packets) are sufficient to guarantee low latency; packet size is offered as a simple classifier to distinguish the two classes.
Instead of using Multi Link PPP fragmentation, this document leverages the SSSAR sublayer already required for PPP over AAL2, making the class extensions a natural fit that avoids adding a separate fragmentation mechanism.
The AAL2 CPS Channel ID mechanism is reused to distinguish preemption classes, enabling priority scheduling at the CPS layer without modifying the PPP or SSSAR layers.
Using AAL2 as an adaptation layer for PPP provides bandwidth-efficient transport for small-packet IP applications such as VoIP, which is a primary motivation for defining these class extensions.
The AAL2 CPS specification (ITU-T I.363.2) does not specify a scheduling method for payloads from different Channel IDs; most current AAL2 SAR implementations use fair scheduling, meaning real-time fragments wait at most one non-real-time fragment transmission.
The class extensions defined here implement equivalent functionality to Multi Class Multi Link PPP (RFC 2686) but are specific to PPP over AAL2 deployments; they are not a drop-in replacement for MC-MLPPP on other link types.
Each PPP session MUST have at least as many Channel IDs assigned as there are different classes of preemptible traffic.
Implementations of PPP over AAL2 real-time transport extensions SHOULD implement AAL2 CPS CID schedulers that meet the requirements of multi-class real-time applications.
Packet fragments from each class of traffic MUST be assigned to different AAL2 CPS Channel IDs to enable preemption class identification.
Some real-time applications MAY require a priority-based CID scheduler; others MAY only require a fair or weighted fair CID scheduler.
Various classification methods MAY be used to determine the preemption class of a PPP packet; the architecture document (RFC 2689) describes possible alternatives for implementing a real-time classification scheme.
The AAL2 CPS Channel ID is used to identify the preemption class that a packet fragment belongs to, allowing a single PPP session to span multiple AAL2 Channel IDs corresponding to different traffic classes.
The Service Specific Segmentation and Reassembly Sublayer (SSSAR) fragments PPP packets into 64-byte frames for AAL2 CPS transport. Separate SSSAR instances run per Channel ID, each handling one traffic class.
Operation of this protocol is believed to be no more and no less secure than operation of PPP over AAL2 (RFC 3336); no additional security risks are introduced.