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Network Working Group                                        B. ThompsonRequest for Comments: 3337                                      T. KorenCategory: Standards Track                                  Cisco Systems                                                               B. Buffam                                                         Seaway Networks                                                           December 2002Class Extensions for PPP overAsynchronous 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).Thompson, et. al.           Standards Track                     [Page 1]

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                     [Page 2]

RFC 3337           Class Extensions for PPP over AAL2      December 2002   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 tosection 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.Thompson, et. al.           Standards Track                     [Page 3]

RFC 3337           Class Extensions for PPP over AAL2      December 2002   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.Thompson, et. al.           Standards Track                     [Page 4]

RFC 3337           Class Extensions for PPP over AAL2      December 2002   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",RFC2686 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.Thompson, et. al.           Standards Track                     [Page 5]

RFC 3337           Class Extensions for PPP over AAL2      December 20028. Authors' Addresses   Bruce Thompson   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134   USA   Phone: +1 408 527-0446   EMail: brucet@cisco.com   Bruce Buffam   Seaway Networks   One Chrysalis Way,   Suite 300,   Ottawa, Canada   K2G-6P9   Phone: +1 613 723-9161   EMail: bruce@seawaynetworks.com   Tmima Koren   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134   USA   Phone: +1 408 527-6169   EMail: tmima@cisco.comThompson, et. al.           Standards Track                     [Page 6]

RFC 3337           Class Extensions for PPP over AAL2      December 20029.  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.Thompson, et. al.           Standards Track                     [Page 7]