Network Working Group                                        B. Thompson
Request for Comments: 3337                                      T. Koren
Category: Standards Track                                  Cisco Systems
                                                              B. Buffam
                                                        Seaway Networks
                                                          December 2002


                    Class Extensions for PPP over
         Asynchronous Transfer Mode Adaptation Layer 2 (AAL2)

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

  The Point-to-Point Protocol (PPP) over Asynchronous Transfer Mode
  (ATM) Adaptation Layer 2 defines the encapsulation that allows a PPP
  session to be transported over an ATM virtual circuit using the ATM
  Adaptation Layer 2 (AAL2) adaptation layer.  This document defines a
  set of class extensions to PPP over AAL2 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 Segmentation and Reassembly Service Specific Convergence Sublayer
  that is already required as the basic encapsulation format of PPP
  over AAL2.

1. Introduction

  Using AAL2 as an adaptation layer for PPP transport over ATM provides
  a bandwidth efficient transport for IP applications that generate
  small packets.  An example IP application that generates small
  packets is RTP encapsulated voice (Voice over IP).








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  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.





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  The class based extensions to PPP over AAL2 use existing services of
  the AAL2 SSCS and CPS layers already specified in PPP over AAL2.
  Because of this, the extensions described in this document may be
  viewed as a desirable alternative to Multi Class Multi Link PPP in
  providing a class based transport service with PPP over AAL2.

1.1. Specification Language

  The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
  SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
  document, are to be interpreted as described in [7].

2. Requirements

  This document assumes the same service requirements as defined in
  Multi Class Multi Link PPP [3].  The reader is referred to section 2
  of Multi Class Multi Link PPP for the general requirements of a multi
  class fragmentation / preemption service.

3. Class Extensions for PPP over AAL2

  PPP over AAL2 uses the Service Specific Segmentation and Reassembly
  Sublayer (SSSAR) [5] for the AAL type 2.  The SSSAR sub-layer is used
  to segment PPP packets into frames that can be transported using the
  AAL2 CPS.  The SSSAR sub-layer uses different AAL2 UUI code-points to
  indicate whether a segment is the last segment of a packet or not.
  SSSAR provides basic fragmentation functionality for all packets
  encapsulated using PPP over AAL2.  The SSSAR layer fragments all
  packets into 64 byte fragments.

  The AAL2 CPS layer defines a Channel ID that is used to identify
  multiple streams of packets within a single ATM Virtual Circuit.  In
  this document, the AAL2 CPS Channel ID is used to identify the
  preemption class that a packet fragment belongs to.  Since the
  Channel ID is used to identify different preemption classes, packet
  fragments from each class of traffic MUST be assigned to different
  Channel IDs.  In addition, each PPP session MUST have at least as
  many Channel IDs assigned as there are different classes of
  preemptible traffic.

  To allow PPP packets to be assigned to different preemption classes,
  PPP packets must be classified into multiple preemption classes as
  they are fragmented using SSSAR.  Many classification methods may be
  used to determine the class that a particular PPP packet belongs to.
  The architecture document [2] describes possible alternatives that
  MAY be used to implement a real time classification scheme.





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  Once packets have been classified into different preemption classes,
  each class of traffic is then assigned a different Channel ID.  Since
  fragments from each traffic class are now transmitted using separate
  Channel ID, the AAL2 CPS layer can be used to schedule fragments from
  the different classes.  The AAL2 CPS specification [6] does not
  specify a method for scheduling AAL2 CPS payloads from different
  Channel IDs.  The scheduling method required at the AAL2 CPS layer
  depends upon the real time requirements of applications using this
  service.  Some real-time applications MAY require the use of a
  priority based CID scheduler.  Other applications MAY only require a
  fair or weighted fair CID scheduler.  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.

4. Example Implementation: Class Based Extensions for Voice Service

  When PPP over AAL2 is used to transport both voice and non-voice
  packets over low bandwidth ATM virtual circuits, it may be necessary
  to preempt the transmission of a large data packet in order to
  transmit a voice packet with minimal delay.  The example
  implementation described below shows an example of how the class
  extensions for PPP over AAL2 can be used to support a real time voice
  transport service over low bandwidth AAL2 virtual circuits.  To
  guarantee low latency and loss for voice transport, the ATM virtual
  circuit in this example must be provisioned using a real time traffic
  class such as VBRnrt or VBRrt.

  For the simple voice service described above, 2 classes are
  sufficient to guarantee low latency for voice packets.  The PPP over
  AAL2 session in this case can be configured to run across 2 AAL2 CPS
  Channel IDs.  One channel ID is used to transport large data packets
  while the other channel ID is used to transport real time voice
  packets.

  Packets that arrive at the PPP interface must first be classified as
  either belonging to the real time class or belonging to the data
  class.  A simple classifier that can be used to classify packets at
  this layer is packet size.

  Large packets are assigned to the non-real time (or data) traffic
  class and small packets are assigned to the real time traffic class.
  The packet size used to discriminate between real time and non-real
  time packets may vary based on the application and transmission rate
  of the virtual circuit.






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  Once packets have been classified, they are now fragmented using the
  SSSAR layer of PPP over AAL2.  Separate instances of the SSSAR
  fragmentation function run on each of the 2 Channel IDs assigned to
  the PPP session.

  Fragments coming from the SSSAR functions are now scheduled into the
  AAL2 virtual circuit using the AAL2 CPS layer.  Most AAL2 SAR
  implementations currently implement fair scheduling across multiple
  AAL2 Channel IDs.  Since the AAL2 CPS scheduler implements fair
  scheduling, real time fragments will wait for at most one non-real
  time fragment to be transmitted on the AAL2 virtual circuit before
  being scheduled.

5.  Security Considerations

  Operation of this protocol is believed to be no more and no less
  secure than operation of PPP over AAL2 [1].

6. Acknowledgements

  The authors would like to thank James Carlson for his contributions
  to this proposal.

7. References

  [1] Thompson, B., Koren, T. and B. Buffam, "PPP Over Asynchronous
      Transfer Mode Adaptation Layer 2", RFC 3336, December 2002.

  [2] Bormann, C., "Providing Integrated Services over Low-bitrate
      Links", RFC 2689, September 1999.

  [3] Bormann, C., "The Multi-Class Extension to Multi-Link PPP", RFC
      2686 September 1999.

  [4] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti,
      "The PPP Multilink Protocol (MP)", RFC 1990, August 1996.

  [5] International Telecommunications Union, "Segmentation
      and Reassembly Service Specific Convergence Sublayer for the AAL
      type 2", ITU-T Recommendation I.366.1, June 1998.

  [6] International Telecommunications Union, "BISDN ATM Adaptation
      layer specification: Type 2 AAL(AAL2)", ITU-T Recommendation
      I.363.2, September 1997.

  [7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.




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8. Authors' Addresses

  Bruce Thompson
  Cisco Systems, Inc.
  170 West Tasman Drive
  San Jose, CA 95134
  USA

  Phone: +1 408 527-0446
  EMail: [email protected]


  Bruce Buffam
  Seaway Networks
  One Chrysalis Way,
  Suite 300,
  Ottawa, Canada
  K2G-6P9

  Phone: +1 613 723-9161
  EMail: [email protected]


  Tmima Koren
  Cisco Systems, Inc.
  170 West Tasman Drive
  San Jose, CA 95134
  USA

  Phone: +1 408 527-6169
  EMail: [email protected]




















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9.  Full Copyright Statement

  Copyright (C) The Internet Society (2002).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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