Network Working Group B. Thompson
Request for Comments: 3337 T. Koren
Category: Standards Track Cisco Systems
B. Buffam
Seaway Networks
December 2002
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)
异步传输模式适配层2（AAL2）上PPP的类扩展
Status of this Memo
本备忘录的状况
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.
本文件规定了互联网社区的互联网标准跟踪协议，并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态，请参考当前版本的“互联网官方协议标准”（STD 1）。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The Internet Society (2002). All Rights Reserved.
版权所有（C）互联网协会（2002年）。版权所有。
Abstract
摘要
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.
异步传输模式（ATM）适配层2上的点对点协议（PPP）定义了封装，该封装允许使用ATM适配层2（AAL2）适配层在ATM虚拟电路上传输PPP会话。本文档定义了AAL2上PPP的一组类扩展，这些类扩展实现了单个ATM虚拟电路上多类多链路PPP的等效功能。本文档不使用多链路PPP作为分段功能的基础，而是使用分段和重组特定于服务的聚合子层的功能，该子层已被要求作为AAL2上PPP的基本封装格式。
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).
使用AAL2作为ATM上PPP传输的适配层，为生成小数据包的IP应用程序提供了带宽高效的传输。生成小数据包的示例IP应用程序是RTP封装的语音（IP语音）。
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.
除了带宽效率之外，语音等实时应用程序还需要低延迟。RFC 2689[2]描述了一种为低比特率链路上的实时应用程序提供传输服务的体系结构。该体系结构的主要组成部分包括：异步和同步低比特率链路的实时封装格式、针对实时流优化的报头压缩体系结构、路由器之间（或主机与路由器之间）使用的协商协议元素，以及应用程序用于允许进行此协商的公告协议。
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.
多类多链路PPP[3]为[2]中定义的体系结构的实时封装格式部分定义了面向片段的解决方案，即，针对片段类型发送方的队列。如架构（architecture）文档中更详细地描述的，需要实时封装格式以保证在存在大型非实时数据包时的低延迟。例如，128 kbit/s ATM虚拟电路上的1500字节数据包使该链路在大约100 ms内无法传输实时信息。这增加了最坏情况下的延迟，导致实时应用程序在往返延迟下运行，对于许多交互任务来说，往返延迟过高。多类多链路PPP定义了多链路PPP[4]的一组扩展，使发送方能够将不同优先级的数据包分割成多类数据包，从而允许在低优先级的数据包之间发送高优先级数据包。
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.
本文档定义了AAL2[1]上PPP的一组类扩展，这些类扩展实现了单个ATM虚拟电路上多类多链路PPP的等效功能。本文档不使用多链路PPP作为分段功能的基础，而是使用服务特定分段和重组子层（SSSAR）[5]的功能，该功能已经作为AAL2上PPP的基本封装格式。
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.
除了提供碎片，实时传输服务还必须允许在低优先级的碎片之间发送高优先级的碎片。这可以通过允许单个PPP会话跨越多个AAL2 CP[6]信道标识符在AAL2上的PPP中实现。一旦PPP会话跨越多个AAL2通道ID，通道ID就可以用来标识片段所属的类。属于高优先级类别的片段可以使用特定的AAL2通道ID发送。低优先级类别的片段可以使用不同的AAL2通道ID发送。一旦使用不同的AAL2通道ID识别出多个片段类，AAL2 CPS层可用于在低优先级片段之间发送属于高优先级类的片段。
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.
基于类的AAL2 PPP扩展使用AAL2 PPP中已经指定的AAL2 SSC和CPS层的现有服务。因此，本文档中描述的扩展可被视为多类多链路PPP的理想替代方案，用于通过AAL2上的PPP提供基于类的传输服务。
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].
本文件中出现的关键词必须、不得、必需、应、不应、应、不应、建议、可能和可选时，应按照[7]中的说明进行解释。
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.
本文件假设与多类多链路PPP[3]中定义的服务要求相同。读者可参考多类多链路PPP第2节了解多类碎片/抢占服务的一般要求。
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.
AAL2上的PPP对AAL类型2使用特定于服务的分段和重组子层（SSSAR）[5]。SSSAR子层用于将PPP数据包分割成可以使用AAL2 CP传输的帧。SSSAR子层使用不同的AAL2 UUI代码点来指示一个段是否为数据包的最后一段。SSSAR为使用AAL2上的PPP封装的所有数据包提供基本的分段功能。SSSAR层将所有数据包分割成64字节的片段。
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.
AAL2 CPS层定义了一个通道ID，用于标识单个ATM虚拟电路中的多个数据包流。在本文档中，AAL2 CPS通道ID用于标识数据包片段所属的抢占类。由于通道ID用于标识不同的抢占类，因此必须将来自每类流量的数据包片段分配给不同的通道ID。此外，每个PPP会话必须至少分配相同数量的信道ID，因为存在不同类别的可抢占通信量。
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.
为了允许将PPP数据包分配给不同的抢占类，必须将PPP数据包分类为多个抢占类，因为它们使用SSSAR进行分段。许多分类方法可用于确定特定PPP数据包所属的类别。体系结构文档[2]描述了可用于实现实时分类方案的可能备选方案。
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.
一旦数据包被划分为不同的抢占类别，每个类别的流量就会被分配一个不同的通道ID。由于每个流量类别的片段现在使用单独的通道ID传输，AAL2 CPS层可以用来调度来自不同类别的片段。AAL2 CPS规范[6]未指定从不同信道ID调度AAL2 CPS有效负载的方法。AAL2 CPS层所需的调度方法取决于使用此服务的应用程序的实时要求。某些实时应用程序可能需要使用基于优先级的CID调度器。其他应用程序可能只需要公平或加权公平CID调度程序。通过AAL2实时传输扩展实现PPP应实现满足多类实时应用要求的AAL2 CPS CID调度器。
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.
当AAL2上的PPP用于通过低带宽ATM虚拟电路传输语音和非语音分组时，可能需要抢占大数据分组的传输，以便以最小的延迟传输语音分组。下面描述的示例实现展示了如何使用AAL2上PPP的类扩展来支持低带宽AAL2虚拟电路上的实时语音传输服务的示例。为了保证话音传输的低延迟和低损耗，本例中的ATM虚拟电路必须使用实时通信类（如VBRnrt或VBRrt）进行配置。
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.
对于上面描述的简单语音服务，两个类足以保证语音分组的低延迟。在这种情况下，AAL2会话上的PPP可以配置为跨2个AAL2 CP信道ID运行。一个通道ID用于传输大数据包，而另一个通道ID用于传输实时语音包。
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.
到达PPP接口的数据包必须首先分类为属于实时类或属于数据类。一个简单的分类器可以用来在这一层对数据包进行分类，它是数据包大小。
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.
将大数据包分配给非实时（或数据）流量类别，将小数据包分配给实时流量类别。用于区分实时和非实时分组的分组大小可以根据虚拟电路的应用和传输速率而变化。
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.
一旦对数据包进行了分类，它们现在就使用AAL2上PPP的SSSAR层进行分段。SSSAR分段功能的单独实例在分配给PPP会话的两个通道ID中的每个上运行。
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.
来自SSSAR功能的碎片现在使用AAL2 CPS层调度到AAL2虚拟电路中。目前，大多数AAL2 SAR实现跨多个AAL2通道ID实现公平调度。由于AAL2 CPS调度器实现公平调度，实时片段将在调度之前等待AAL2虚拟电路上最多传输一个非实时片段。
Operation of this protocol is believed to be no more and no less secure than operation of PPP over AAL2 [1].
该协议的操作被认为与AAL2上的PPP操作一样安全[1]。
The authors would like to thank James Carlson for his contributions to this proposal.
作者要感谢James Carlson对本提案的贡献。
[1] Thompson, B., Koren, T. and B. Buffam, "PPP Over Asynchronous Transfer Mode Adaptation Layer 2", RFC 3336, December 2002.
[1] Thompson，B.，Koren，T.和B.Buffam，“异步传输模式适应层2上的PPP”，RFC 3336，2002年12月。
[2] Bormann, C., "Providing Integrated Services over Low-bitrate Links", RFC 2689, September 1999.
[2] Bormann，C.，“通过低比特率链路提供综合服务”，RFC 2689，1999年9月。
[3] Bormann, C., "The Multi-Class Extension to Multi-Link PPP", RFC 2686 September 1999.
[3] Bormann，C.，“多链路PPP的多类扩展”，RFC 2686，1999年9月。
[4] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996.
[4] K.Sklower、Lloyd、B.McGregor、G.Carr、D.和T.Coradetti，“PPP多链路协议（MP）”，RFC 1990，1996年8月。
[5] International Telecommunications Union, "Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2", ITU-T Recommendation I.366.1, June 1998.
[5] 国际电信联盟，“AAL类型2的细分和重组服务特定汇聚子层”，ITU-T建议I.366.1，1998年6月。
[6] International Telecommunications Union, "BISDN ATM Adaptation layer specification: Type 2 AAL(AAL2)", ITU-T Recommendation I.363.2, September 1997.
[6] 国际电信联盟，“BISDN ATM适配层规范：2型AAL（AAL2）”，ITU-T建议I.363.2，1997年9月。
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[7] Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
Bruce Thompson Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA
布鲁斯·汤普森思科系统公司，美国加利福尼亚州圣何塞西塔斯曼大道170号，邮编95134
Phone: +1 408 527-0446
EMail: brucet@cisco.com
Phone: +1 408 527-0446
EMail: brucet@cisco.com
Bruce Buffam Seaway Networks One Chrysalis Way, Suite 300, Ottawa, Canada K2G-6P9
Bruce Buffam Seaway Networks One Chrysalis Way，300套房，加拿大渥太华K2G-6P9
Phone: +1 613 723-9161
EMail: bruce@seawaynetworks.com
Phone: +1 613 723-9161
EMail: bruce@seawaynetworks.com
Tmima Koren Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA
美国加利福尼亚州圣何塞西塔斯曼大道170号，Tmima科伦思科系统有限公司，邮编95134
Phone: +1 408 527-6169
EMail: tmima@cisco.com
Phone: +1 408 527-6169
EMail: tmima@cisco.com
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