Network Working Group S. Singh
Request for Comments: 4454 M. Townsley
Category: Standards Track C. Pignataro
Cisco Systems
May 2006
Network Working Group S. Singh
Request for Comments: 4454 M. Townsley
Category: Standards Track C. Pignataro
Cisco Systems
May 2006
Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
第2层隧道协议版本3（L2TPv3）上的异步传输模式（ATM）
Status of This Memo
关于下段备忘
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 (2006).
版权所有（C）互联网协会（2006年）。
Abstract
摘要
The Layer 2 Tunneling Protocol, Version 3 (L2TPv3) defines an extensible tunneling protocol to transport layer 2 services over IP networks. This document describes the specifics of how to use the L2TP control plane for Asynchronous Transfer Mode (ATM) Pseudowires and provides guidelines for transporting various ATM services over an IP network.
第2层隧道协议版本3（L2TPv3）定义了一个可扩展的隧道协议，用于通过IP网络传输第2层服务。本文档详细介绍了如何将L2TP控制平面用于异步传输模式（ATM）伪线，并提供了通过IP网络传输各种ATM服务的指南。
Table of Contents
目录
1. Introduction ....................................................2
1.1. Abbreviations ..............................................3
1.2. Specification of Requirements ..............................3
2. Control Connection Establishment ................................3
3. Session Establishment and ATM Circuit Status Notification .......4
3.1. L2TPv3 Session Establishment ...............................4
3.2. L2TPv3 Session Teardown ....................................6
3.3. L2TPv3 Session Maintenance .................................6
4. Encapsulation ...................................................6
4.1. ATM-Specific Sublayer ......................................7
4.2. Sequencing .................................................9
5. ATM Transport ...................................................9
5.1. ATM AAL5-SDU Mode .........................................10
5.2. ATM Cell Mode .............................................10
1. Introduction ....................................................2
1.1. Abbreviations ..............................................3
1.2. Specification of Requirements ..............................3
2. Control Connection Establishment ................................3
3. Session Establishment and ATM Circuit Status Notification .......4
3.1. L2TPv3 Session Establishment ...............................4
3.2. L2TPv3 Session Teardown ....................................6
3.3. L2TPv3 Session Maintenance .................................6
4. Encapsulation ...................................................6
4.1. ATM-Specific Sublayer ......................................7
4.2. Sequencing .................................................9
5. ATM Transport ...................................................9
5.1. ATM AAL5-SDU Mode .........................................10
5.2. ATM Cell Mode .............................................10
5.2.1. ATM VCC Cell Relay Service .........................11
5.2.2. ATM VPC Cell Relay Service .........................12
5.2.3. ATM Port Cell Relay Service ........................12
5.3. OAM Cell Support ..........................................12
5.3.1. VCC Switching ......................................12
5.3.2. VPC Switching ......................................13
6. ATM Maximum Concatenated Cells AVP .............................13
7. OAM Emulation Required AVP .....................................14
8. ATM Defects Mapping and Status Notification ....................14
8.1. ATM Alarm Status AVP ......................................14
9. Applicability Statement ........................................15
9.1. ATM AAL5-SDU Mode .........................................16
9.2. ATM Cell Relay Mode .......................................18
10. Congestion Control ............................................20
11. Security Considerations .......................................21
12. IANA Considerations ...........................................21
12.1. L2-Specific Sublayer Type ................................21
12.2. Control Message Attribute Value Pairs (AVPs) .............21
12.3. Result Code AVP Values ...................................22
12.4. ATM Alarm Status AVP Values ..............................22
12.5. ATM-Specific Sublayer Bits ...............................23
13. Acknowledgements ..............................................23
14. References ....................................................23
14.1. Normative References .....................................23
14.2. Informative References ...................................24
5.2.1. ATM VCC Cell Relay Service .........................11
5.2.2. ATM VPC Cell Relay Service .........................12
5.2.3. ATM Port Cell Relay Service ........................12
5.3. OAM Cell Support ..........................................12
5.3.1. VCC Switching ......................................12
5.3.2. VPC Switching ......................................13
6. ATM Maximum Concatenated Cells AVP .............................13
7. OAM Emulation Required AVP .....................................14
8. ATM Defects Mapping and Status Notification ....................14
8.1. ATM Alarm Status AVP ......................................14
9. Applicability Statement ........................................15
9.1. ATM AAL5-SDU Mode .........................................16
9.2. ATM Cell Relay Mode .......................................18
10. Congestion Control ............................................20
11. Security Considerations .......................................21
12. IANA Considerations ...........................................21
12.1. L2-Specific Sublayer Type ................................21
12.2. Control Message Attribute Value Pairs (AVPs) .............21
12.3. Result Code AVP Values ...................................22
12.4. ATM Alarm Status AVP Values ..............................22
12.5. ATM-Specific Sublayer Bits ...............................23
13. Acknowledgements ..............................................23
14. References ....................................................23
14.1. Normative References .....................................23
14.2. Informative References ...................................24
This document describes the specifics of how to use the Layer 2 Tunneling Protocol (L2TP) for Asynchronous Transfer Mode (ATM) Pseudowires, including encapsulation, carrying various ATM services, such as AAL5 SDU, ATM VCC/VPC/Port cell relay over L2TP, and mapping ATM defects to L2TP Set-Link-Info (SLI) messages to notify the peer L2TP Control Connection Endpoint (LCCE).
本文档详细介绍了如何将第2层隧道协议（L2TP）用于异步传输模式（ATM）伪线，包括封装、承载各种ATM服务，如AAL5 SDU、ATM VCC/VPC/端口信元中继，以及将ATM缺陷映射到L2TP设置链路信息（SLI）通知对等L2TP控制连接端点（LCCE）的消息。
Any ATM-specific AVPs or other L2TP constructs for ATM Pseudowire (ATMPW) support are defined here as well. Support for ATM Switched Virtual Path/Connection (SVP/SVC) and Soft Permanent Virtual Path/Connection (SPVP/SPVC) are outside the scope of this document.
这里还定义了支持ATM伪线（ATMPW）的任何特定于ATM的AVP或其他L2TP结构。对ATM交换虚拟路径/连接（SVP/SVC）和软永久虚拟路径/连接（SPVP/SPVC）的支持超出了本文档的范围。
The reader is expected to be very familiar with the terminology and protocol constructs defined in [RFC3931].
读者应非常熟悉[RFC3931]中定义的术语和协议结构。
AIS Alarm Indication Signal ATMPW ATM Pseudowire AVP Attribute Value Pair CC Continuity Check OAM Cell CE Customer Edge HEC Header Error Checksum LAC L2TP Access Concentrator (see [RFC3931]) LCCE L2TP Control Connection Endpoint (see [RFC3931]) MSB Most Significant Byte OAM Operation, Administration, and Maintenance PE Provider Edge PSN Packet Switched Network PWE3 Pseudowire Emulation Edge to Edge RDI Remote Defect Indicator SAR Segmentation and Reassembly SDU Service Data Unit SLI Set-Link-Info, an L2TP control message SVC Switched Virtual Connection SVP Switched Virtual Path SPVC Soft Permanent Virtual Connection SPVP Soft Permanent Virtual Path VC Virtual Circuit VCC Virtual Channel Connection VCI Virtual Channel Identifier VPC Virtual Path Connection VPI Virtual Path Identifier
AIS报警指示信号ATMPW ATM伪线AVP属性值对CC连续性检查OAM信元CE客户边缘HEC报头错误校验和LAC L2TP访问集中器（参见[RFC3931]）LCCE L2TP控制连接端点（参见[RFC3931]）MSB最高有效字节OAM操作、管理、，和维护PE提供商边缘PSN分组交换网络PWE3伪线仿真边缘到边缘RDI远程缺陷指示器SAR分割和重组SDU服务数据单元SLI设置链路信息，L2TP控制消息SVC交换虚拟连接SVP交换虚拟路径SPVC软永久虚拟连接SPVP软永久虚拟路径VC虚拟电路VCC虚拟通道连接VCI虚拟通道标识符VPC虚拟路径连接VPI虚拟路径标识符
In this document, several words are used to signify the requirements of the specification. These words are often capitalized. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
在本文件中，使用了几个词来表示规范的要求。这些词通常大写。本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照[RFC2119]中所述进行解释。
To emulate ATM Pseudowires using L2TP, an L2TP Control Connection as described in Section 3.3 of [RFC3931] MUST be established.
为了使用L2TP模拟ATM伪线，必须建立[RFC3931]第3.3节所述的L2TP控制连接。
The Start-Control-Connection-Request (SCCRQ) and corresponding Start-Control-Connection-Reply (SCCRP) MUST include the supported ATM Pseudowire types (see Section 3.1), in the Pseudowire Capabilities List as defined in Section 5.4.3 of [RFC3931]. This identifies the Control Connection as able to establish L2TP sessions in support of the ATM Pseudowires.
启动控制连接请求（SCCRQ）和相应的启动控制连接回复（SCCRP）必须包括[RFC3931]第5.4.3节中定义的伪线能力列表中支持的ATM伪线类型（见第3.1节）。这将控制连接标识为能够建立L2TP会话以支持ATM伪线。
An LCCE MUST be able to uniquely identify itself in the SCCRQ and SCCRP messages via a globally unique value. By default, this is advertised via the structured Router ID AVP [RFC3931], though the unstructured Hostname AVP [RFC3931] MAY be used to identify LCCEs as well.
LCCE必须能够通过全局唯一值在SCCRQ和SCCRP消息中唯一标识自己。默认情况下，这是通过结构化路由器ID AVP[RFC3931]公布的，尽管非结构化主机名AVP[RFC3931]也可用于标识LCCE。
This section describes how L2TP ATMPWs or sessions are established between two LCCEs. This includes what will happen when an ATM circuit (e.g., AAL5 PVC) is created, deleted, or changes state when circuit state is in alarm.
本节介绍如何在两个LCCE之间建立L2TP ATMPW或会话。这包括创建、删除ATM电路（如AAL5 PVC）或在电路状态处于报警状态时更改状态时将发生的情况。
ATM circuit (e.g., an AAL5 PVC) creation triggers establishment of an L2TP session using three-way handshake described in Section 3.4.1 of [RFC3931]. An LCCE MAY initiate the session immediately upon ATM circuit creation, or wait until the circuit state transitions to ACTIVE before attempting to establish a session for the ATM circuit. It MAY be preferred to wait until circuit status transitions to ACTIVE in order to delay the allocation of resources until absolutely necessary.
ATM电路（如AAL5 PVC）的创建触发使用[RFC3931]第3.4.1节中描述的三方握手建立L2TP会话。LCCE可在ATM电路创建后立即启动会话，或在尝试为ATM电路建立会话之前，等待电路状态转换为活动状态。最好等到电路状态转变为激活状态，以便延迟资源分配，直到绝对必要。
The Circuit Status AVP (see Section 8) MUST be present in the Incoming-Call-Request (ICRQ) and Incoming-Call-Reply (ICRP) messages, and MAY be present in the SLI message for ATMPWs.
电路状态AVP（见第8节）必须出现在来电请求（ICRQ）和来电回复（ICRP）消息中，并且可能出现在ATMPWs的SLI消息中。
The following figure shows how L2TP messages are exchanged to set up an ATMPW after the ATM circuit (e.g., an AAL5 PVC) becomes ACTIVE.
下图显示了在ATM电路（例如AAL5 PVC）激活后，L2TP消息如何交换以建立ATMPW。
LCCE (LAC) A LCCE (LAC) B
------------------ --------------------
LCCE (LAC) A LCCE (LAC) B
------------------ --------------------
ATM Ckt Provisioned
ATM Ckt Provisioned
ATM Ckt ACTIVE
ICRQ (status = 0x03) ---->
ATM Ckt ACTIVE
<----- ICRP (status = 0x03)
L2TP session established
OK to send data into PW
ATM Ckt Provisioned
ATM Ckt Provisioned
ATM Ckt ACTIVE
ICRQ (status = 0x03) ---->
ATM Ckt ACTIVE
<----- ICRP (status = 0x03)
L2TP session established
OK to send data into PW
ICCN ----->
L2TP session established
OK to send data into PW
ICCN ----->
L2TP session established
OK to send data into PW
The following signaling elements are required for the ATMPW establishment.
ATMPW建立需要以下信令元素。
a. Pseudowire Type: One of the supported ATM-related PW types should be present in the Pseudowire Type AVP of [RFC3931].
a. 伪线类型：支持的ATM相关PW类型之一应出现在[RFC3931]的伪线类型AVP中。
0x0002 ATM AAL5 SDU VCC transport 0x0003 ATM Cell transport Port Mode 0x0009 ATM Cell transport VCC Mode 0x000A ATM Cell transport VPC Mode
0x0002 ATM AAL5 SDU VCC传输0x0003 ATM信元传输端口模式0x0009 ATM信元传输VCC模式0x000A ATM信元传输VPC模式
The above cell relay modes can also signal the ATM Maximum Concatenated Cells AVP as described in Section 6.
如第6节所述，上述信元中继模式还可以向ATM最大级联信元AVP发送信号。
b. Remote End ID: Each PW is associated with a Remote End ID akin to the VC-ID in [PWE3ATM]. Two LCCEs of a PW would have the same Remote End ID, and its format is described in Section 5.4.4 of [RFC3931].
b. 远程端ID：每个PW都与一个远程端ID相关联，该ID类似于[PWE3ATM]中的VC-ID。PW的两个LCCE具有相同的远程端ID，其格式见[RFC3931]第5.4.4节。
This Remote End ID AVP MUST be present in the ICRQ in order for the remote LCCE to associate the session to the ATM circuit. The Remote End Identifier AVP defined in [RFC3931] is of opaque form, though ATMPW implementations MAY simply use a 4-octet value that is known to both LCCEs (either by direct configuration or some other means). The exact method of how this value is configured, retrieved, discovered, or otherwise determined at each LCCE is outside the scope of this document.
ICRQ中必须存在该远程端ID AVP，以便远程LCCE将会话与ATM电路相关联。[RFC3931]中定义的远程终端标识符AVP是不透明的形式，尽管ATMPW实现可以简单地使用两个LCCE都知道的4-八位字节值（通过直接配置或其他方式）。如何在每个LCCE配置、检索、发现或以其他方式确定此值的确切方法不在本文档的范围内。
As with the ICRQ, the ICRP is sent only after the ATM circuit transitions to ACTIVE. If LCCE B had not been provisioned yet for the ATM circuit identified in the ICRQ, a Call-Disconnect-Notify (CDN) would have been immediately returned indicating that the circuit either was not provisioned or was not available at this LCCE. LCCE A SHOULD then exhibit a periodic retry mechanism. If so, the period and maximum number of retries MUST be configurable.
与ICRQ一样，ICRP仅在ATM电路转换为活动后发送。如果尚未为ICRQ中标识的ATM电路设置LCCE B，则会立即返回呼叫断开通知（CDN），指示该电路未设置或在此LCCE不可用。然后，LCCE A应显示定期重试机制。如果是这样，则必须配置重试的周期和最大次数。
An implementation MAY send an ICRQ or ICRP before a PVC is ACTIVE, as long as the Circuit Status AVP reflects that the ATM circuit is INACTIVE and an SLI is sent when the ATM circuit becomes ACTIVE (see Section 8).
只要电路状态AVP反映ATM电路处于非活动状态，并且当ATM电路变为活动状态时发送SLI，则实现可以在PVC激活之前发送ICRQ或ICRP（参见第8节）。
The ICCN is the final stage in the session establishment. It confirms the receipt of the ICRP with acceptable parameters to allow bidirectional traffic.
ICCN是会话建立的最后阶段。它用可接受的参数确认收到ICRP，以允许双向通信。
When an ATM circuit is unprovisioned (deleted) at either LCCE, the associated L2TP session MUST be torn down via the CDN message defined in Section 3.4.3 of [RFC3931].
当任一LCCE上的ATM电路未设置（删除）时，必须通过[RFC3931]第3.4.3节中定义的CDN消息中断相关L2TP会话。
All sessions established by a given Control Connection utilize the L2TP Hello facility defined in Section 4.4 of [RFC3931] for session keepalive. This gives all sessions basic dead peer and path detection between LCCEs.
由给定控制连接建立的所有会话都使用[RFC3931]第4.4节中定义的L2TP Hello功能来实现会话保持。这为所有会话提供了基本的死点和LCCE之间的路径检测。
If the control channel utilizing the Hello message is not in-band with data traffic over the PSN, then other method MAY be used to detect the session failure, and it is left for further study.
如果利用Hello消息的控制信道不在通过PSN的数据业务的频带内，则可以使用其他方法来检测会话失败，并留作进一步研究。
ATMPWs over L2TP use the Set-Link-Info (SLI) control message as defined in [RFC3931] to signal ATM circuit status between LCCEs after initial session establishment. This includes ACTIVE or INACTIVE notifications of the ATM circuit, or any other parameters that may need to be shared between the LCCEs in order to provide proper PW emulation.
L2TP上的ATMPWs使用[RFC3931]中定义的Set Link Info（SLI）控制消息在初始会话建立后向LCCE之间的ATM电路状态发送信号。这包括ATM电路的活动或非活动通知，或LCCE之间可能需要共享的任何其他参数，以提供适当的PW仿真。
The SLI message MUST be sent whenever there is a status change that may be reported by any values identified in the Circuit Status AVP. The only exceptions to this are the initial ICRQ, ICRP, and CDN messages, which establish and tear down the L2TP session itself when the ATM circuit is created or deleted. The SLI message may be sent from either LCCE at any time after the first ICRQ is sent (and perhaps before an ICRP is received, requiring the peer to perform a reverse Session ID lookup).
只要电路状态AVP中标识的任何值可能报告状态变化，则必须发送SLI消息。唯一的例外是初始ICRQ、ICRP和CDN消息，它们在创建或删除ATM电路时建立并中断L2TP会话本身。SLI消息可以在发送第一个ICRQ之后（可能在接收ICRP之前，需要对等方执行反向会话ID查找）的任何时间从任一LCCE发送。
The other application of the SLI message is to map the ATM OAM or physical layer alarms into Circuit Status AVP as described in Section 8.
SLI消息的另一个应用是将ATM OAM或物理层警报映射为电路状态AVP，如第8节所述。
This section describes the general encapsulation format for ATM services over L2TP.
本节介绍L2TP上ATM服务的通用封装格式。
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM-Specific Sublayer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM-Specific Sublayer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Service Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: General Format for ATM Encapsulation over L2TPv3 over IP
图1:IP上L2TPv3上ATM封装的通用格式
The PSN Transport header is specific to IP and its underlying transport header. This header is used to transport the encapsulated ATM payload through the IP network.
PSN传输头特定于IP及其底层传输头。此报头用于通过IP网络传输封装的ATM有效负载。
The Session Header is a non-zero 32-bit Session ID with an optional Cookie up to 64-bits. This Session ID is exchanged during session setup.
会话头是一个非零的32位会话ID，具有高达64位的可选Cookie。此会话ID在会话设置期间交换。
The ATM-Specific Sublayer is REQUIRED for AAL5 SDU Mode and OPTIONAL for ATM Cell Mode. Please refer to Section 4.1 for more details.
AAL5 SDU模式需要ATM特定子层，ATM信元模式可选。有关更多详细信息，请参阅第4.1节。
This section defines a new ATM-Specific Sublayer, an alternative to the Default L2-Specific Sublayer as mentioned in Section 4.6 of [RFC3931]. Four new flag bits (T, G, C, and U) are defined that concur with Section 8.2 of [PWE3ATM].
本节定义了一个新的ATM特定子层，它是[RFC3931]第4.6节中提到的默认L2特定子层的替代。定义了符合[PWE3ATM]第8.2节的四个新标志位（T、G、C和U）。
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|B|E|T|G|C|U| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|B|E|T|G|C|U| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ATM-Specific Sublayer Format
图2:ATM特定的子层格式
The meaning of the fields of the ATM-Specific Sublayer is as follows:
ATM特定子层字段的含义如下：
* S bit
* S位
Definition of this bit is as per Section 4.6 of [RFC3931].
该位的定义符合[RFC3931]第4.6节。
* B and E bits
* B和E位
Definitions of these bits are as per Section 5.5 of [L2TPFRAG].
这些位的定义符合[L2TPFRAG]第5.5节。
If these bits are not used as per [L2TPFRAG], they MUST be set to 0 upon transmission and ignored upon reception.
如果这些位未按照[L2TPFRAG]使用，则必须在传输时将其设置为0，并在接收时忽略。
* T (Transport type) bit
* T（传输型）位
Bit (T) of the ATM-Specific Sublayer indicates whether the packet contains an ATM admin cell or an AAL5 payload. If T = 1, the packet contains an ATM admin cell, encapsulated according to the VCC cell relay encapsulation of Section 5.2.
ATM特定子层的位（T）指示数据包是包含ATM管理信元还是AAL5有效负载。如果T=1，则数据包包含一个ATM管理信元，根据第5.2节的VCC信元中继封装进行封装。
If not set, the PDU contains an AAL5 payload. The ability to transport an ATM cell in the AAL5 SDU Mode is intended to provide a means of enabling administrative functionality over the AAL5 VCC (though it does not endeavor to preserve user-cell and admin-cell arrival/transport ordering, as described in Section 9.1).
如果未设置，则PDU包含AAL5有效负载。在AAL5 SDU模式下传输ATM信元的能力旨在提供一种通过AAL5 VCC实现管理功能的方法（尽管它不努力保持用户信元和管理信元到达/传输顺序，如第9.1节所述）。
* G (EFCI) Bit
* G（EFCI）位
The ingress LCCE device SHOULD set this bit to 1 if the Explicit Forward Congestion Indication (EFCI) bit of the final cell of the incoming AAL5 payload is set to 1, or if the EFCI bit of the single ATM cell to be transported in the packet is set to 1. Otherwise, this bit SHOULD be set to 0. The egress LCCE device SHOULD set the EFCI bit of all the outgoing cells that transport the AAL5 payload to the value contained in this field.
如果传入AAL5有效负载的最终信元的显式前向拥塞指示（EFCI）位设置为1，或者如果要在分组中传输的单个ATM信元的EFCI位设置为1，则入口LCCE设备应将该位设置为1。否则，该位应设置为0。出口LCCE设备应将传输AAL5有效负载的所有输出单元的EFCI位设置为该字段中包含的值。
* C (CLP) Bit
* C（CLP）位
The ingress LCCE device SHOULD set this bit to 1 if the Cell Loss Priority (CLP) bit of any of the incoming ATM cells of the AAL5 payload is set to 1, or if the CLP bit of the single ATM cell that is to be transported in the packet is set to 1. Otherwise this bit SHOULD be set to 0. The egress LCCE device SHOULD set the CLP bit of all outgoing cells that transport the AAL5 CPCS-PDU to the value contained in this field.
如果AAL5有效负载的任何传入ATM信元的信元丢失优先级（CLP）位设置为1，或者如果要在分组中传输的单个ATM信元的CLP位设置为1，则入口LCCE设备应将该位设置为1。否则，该位应设置为0。出口LCCE设备应将传输AAL5 CPCS-PDU的所有输出单元的CLP位设置为该字段中包含的值。
* U (Command/Response) Bit
* U（命令/响应）位
When FRF.8.1 Frame Relay / ATM PVC Service Interworking (see [FRF8.1]) traffic is being transported, the CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS-PDU may contain the Frame Relay C/R bit. The ingress LCCE device SHOULD copy this bit to the U bit of the ATM-Specific Sublayer. The egress LCCE device SHOULD copy the U bit to the CPCS-UU Least Significant Bit (LSB) of the AAL5 payload.
当传输FRF.8.1帧中继/ATM PVC业务互通（参见[FRF8.1]）业务时，AAL5 CPCS-PDU的CPCS-UU最低有效位（LSB）可能包含帧中继C/R位。入口LCCE设备应将该位复制到ATM特定子层的U位。出口LCCE设备应将U位复制到AAL5有效负载的CPCS-UU最低有效位（LSB）。
The Sequence Number field is used in sequencing, as described in Section 4.2.
序列号字段用于排序，如第4.2节所述。
In case of a reassembly timeout, the encapsulating LCCE should discard all component cells of the AAL5 frame.
如果重新组装超时，封装LCCE应丢弃AAL5机架的所有组件单元。
An additional enumeration is added to the L2-Specific Sublayer AVP to identify the ATM-Specific Sublayer:
向L2特定子层AVP添加额外的枚举，以识别ATM特定子层：
0 - There is no L2-Specific Sublayer present. 1 - The Default L2-Specific Sublayer (defined in Section 4.6 of [RFC3931]) is used. 2 - The ATM-Specific Sublayer is used.
0-不存在L2特定的子层。1-使用默认的L2特定子层（定义见[RFC3931]第4.6节）。2-使用ATM特定的子层。
The first two values are already defined in the L2TPv3 base specification [RFC3931].
前两个值已在L2TPv3基本规范[RFC3931]中定义。
Data Packet Sequencing MAY be enabled for ATMPWs. The sequencing mechanisms described in [RFC3931] MUST be used to signal sequencing support. ATMPWs over L2TPv3 MUST request the presence of the ATM-Specific Sublayer when sequencing is enabled, and MAY request its presence at all times.
可以为ATMPWs启用数据包排序。必须使用[RFC3931]中描述的排序机制来表示排序支持。L2TPv3上的ATMPWs必须在启用排序时请求ATM特定子层的存在，并且可以随时请求其存在。
There are two encapsulations supported for ATM transport as described below.
ATM传输支持两种封装，如下所述。
The ATM-Specific Sublayer is prepended to the AAL5-SDU. The other cell mode encapsulation consists of the OPTIONAL ATM-Specific Sublayer, followed by a 4-byte ATM cell header and a 48-byte ATM cell-payload.
ATM特定子层在AAL5-SDU之前。另一种信元模式封装包括可选的ATM特定子层，然后是4字节的ATM信元头和48字节的ATM信元有效负载。
In this mode, each AAL5 VC is mapped to an L2TP session. The Ingress LCCE reassembles the AAL5 CPCS-SDU without the AAL5 trailer and any padding bytes. Incoming EFCI, CLP, and C/R (if present) are carried in an ATM-Specific Sublayer across ATMPWs to the egress LCCE. The processing of these bits on ingress and egress LCCEs is defined in Section 4.1.
在此模式下，每个AAL5 VC映射到L2TP会话。入口LCCE在没有AAL5拖车和任何填充字节的情况下重新组装AAL5 CPCS-SDU。传入EFCI、CLP和C/R（如果存在）在ATM特定子层中通过ATMPWs传输到出口LCCE。第4.1节定义了入口和出口LCCE上这些位的处理。
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|T|G|C|U| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| AAL5 CPCS-SDU |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|T|G|C|U| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| AAL5 CPCS-SDU |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ATM AAL5-SDU Mode Encapsulation
图3:ATM AAL5-SDU模式封装
If the ingress LCCE determines that an encapsulated AAL5 SDU exceeds the MTU size of the L2TPv3 session, then AAL5 SDU may be fragmented as per [L2TPFRAG] or underneath the transport layer (IP, etc.). F5 OAM cells that arrive during the reassembly of an AAL5 SDU are sent immediately on the PW followed by the AAL5 SDU payload. In this case, OAM cells' relative order with respect to user data cells is not maintained.
如果入口LCCE确定封装的AAL5 SDU超过L2TPv3会话的MTU大小，则AAL5 SDU可根据[L2TPFRAG]或在传输层（IP等）下分段。AAL5 SDU重新组装期间到达的F5 OAM单元立即发送至PW，随后发送AAL5 SDU有效载荷。在这种情况下，不维护OAM单元相对于用户数据单元的相对顺序。
Performance Monitoring OAM, as specified in ITU-T 610 [I610-1], [I610-2], [I610-3] and security OAM cells as specified in [ATMSEC], should not be used in combination with AAL5 SDU Mode. These cells MAY be dropped at the ingress LCCE because cell sequence integrity is not maintained.
ITU-T 610[I610-1]、[I610-2]、[I610-3]中规定的性能监测OAM和[ATMSEC]中规定的安全OAM单元不应与AAL5 SDU模式结合使用。这些单元可能会在入口LCCE处丢弃，因为单元序列完整性没有得到维护。
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931], Attribute Type 68, MUST be present in the ICRQ messages and MUST include the ATM AAL5 SDU VCC transport PW Type of 0x0002.
[RFC3931]第5.4.4节中定义的伪线类型AVP属性类型68必须出现在ICRQ消息中，并且必须包括0x0002的ATM AAL5 SDU VCC传输PW类型。
In this mode, ATM cells skip the reassembly process at the ingress LCCE. These cells are transported over an L2TP session, either as a single cell or as concatenated cells, into a single packet. Each ATM cell consists of a 4-byte ATM cell header and a 48-byte ATM cell-payload; the HEC is not included.
在此模式下，ATM信元跳过入口LCCE的重新组装过程。这些小区通过L2TP会话传输到单个分组中，可以作为单个小区，也可以作为级联小区。每个ATM信元由一个4字节的ATM信元报头和一个48字节的ATM信元有效载荷组成；港灯不包括在内。
In ATM Cell Mode encapsulation, the ATM-Specific Sublayer is OPTIONAL. It can be included, if sequencing support is required. It is left to the implementation to choose to signal the Default L2- Specific Sublayer or the ATM-Specific Sublayer.
在ATM信元模式封装中，ATM特定子层是可选的。如果需要测序支持，则可以包括它。由实现选择向默认的L2特定子层或ATM特定子层发送信号。
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|x|x|x|x| Sequence Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"
"
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|x|x|x|x| Sequence Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"
"
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ATM Cell Mode Encapsulation
图4:ATM信元模式封装
In the simplest case, this encapsulation can be used to transmit a single ATM cell per Pseudowire PDU. However, in order to provide better Pseudowire bandwidth efficiency, several ATM cells may be optionally encapsulated into a single Pseudowire PDU.
在最简单的情况下，这种封装可用于每个伪线PDU传输单个ATM信元。然而，为了提供更好的伪线带宽效率，可以选择将多个ATM信元封装到单个伪线PDU中。
The maximum number of concatenated cells in a packet is limited by the MTU size of the session and also by the ability of the egress LCCE to process them. For more details about ATM Maximum Concatenated Cells, please refer to Section 6.
分组中连接单元的最大数量受会话的MTU大小以及出口LCCE处理它们的能力的限制。有关ATM最大连接信元的更多详细信息，请参阅第6节。
A VCC cell relay service may be provided by mapping an ATM Virtual Channel Connection to a single Pseudowire using cell mode encapsulation as defined in Section 5.2.
VCC信元中继服务可通过使用第5.2节中定义的信元模式封装将ATM虚拟信道连接映射到单个伪线来提供。
An LCCE may map one or more VCCs to a single PW. However, a service provider may wish to provision a single VCC to a PW in order to satisfy QOS or restoration requirements.
LCCE可将一个或多个VCC映射到单个PW。然而，服务提供商可能希望向PW提供单个VCC，以满足QOS或恢复要求。
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931], Attribute Type 68, MUST be present in the ICRQ messages and MUST include the ATM cell transport VCC Mode PW Type of 0x0009.
[RFC3931]第5.4.4节中定义的伪线类型AVP属性类型68必须存在于ICRQ消息中，并且必须包括ATM信元传输VCC模式PW类型0x0009。
A Virtual Path Connection cell relay service may be provided by mapping an ATM Virtual Path Connection to a single Pseudowire using cell mode encapsulation as defined in Section 5.2.
通过使用第5.2节中定义的信元模式封装将ATM虚拟路径连接映射到单个伪线，可以提供虚拟路径连接信元中继服务。
An LCCE may map one or more VPCs to a single Pseudowire.
LCCE可以将一个或多个VPC映射到单个伪线。
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931], Attribute Type 68, MUST be present in the ICRQ messages and MUST include the ATM cell transport VPC Mode PW Type of 0x000A.
[RFC3931]第5.4.4节中定义的伪线类型AVP属性类型68必须出现在ICRQ消息中，并且必须包括0x000A的ATM信元传输VPC模式PW类型。
ATM port cell relay service allows an ATM port to be connected to another ATM port. All ATM cells that are received at the ingress ATM port on the LCCE are encapsulated as per Section 5.2, into Pseudowire PDU and sent to peer LCCE.
ATM端口信元中继服务允许一个ATM端口连接到另一个ATM端口。根据第5.2节，在LCCE上的入口ATM端口接收的所有ATM信元被封装到伪线PDU中，并发送到对等LCCE。
Each LCCE MUST discard any idle/unassigned cells received on an ATM port associated with ATMPWs.
每个LCCE必须丢弃在与ATMPWs关联的ATM端口上接收的任何空闲/未分配信元。
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931], Attribute Type 68, MUST be present in the ICRQ messages and MUST include the ATM Cell transport Port Mode PW Type of 0x0003.
[RFC3931]第5.4.4节中定义的伪线类型AVP属性类型68必须出现在ICRQ消息中，并且必须包括ATM信元传输端口模式PW类型0x0003。
The OAM cells are defined in [I610-1], [I610-2], [I610-3] and [ATMSEC] can be categorized as follows:
[I610-1]、[I610-2]、[I610-3]和[ATMSEC]中定义的OAM小区可分类如下：
a. Fault Management b. Performance monitoring and reporting c. Activation/deactivation d. System Management (e.g., security OAM cells)
a. 故障管理b。性能监测和报告c。激活/失活d。系统管理（例如，安全OAM单元）
OAM Cells are always encapsulated using cell mode encapsulation, regardless of the encapsulation format used for user data.
OAM单元始终使用单元模式封装，而不管用于用户数据的封装格式如何。
The LCCEs SHOULD be able to pass the F5 segment and end-to-end Fault Management, Resource Management (RM cells), Performance Management, Activation/deactivation, and System Management OAM cells.
LCCE应能够通过F5段和端到端故障管理、资源管理（RM单元）、性能管理、激活/停用和系统管理OAM单元。
F4 OAM cells are inserted or extracted at the VP link termination. These OAM cells are not seen at the VC link termination and are therefore not sent across the PW.
F4 OAM单元在VP链路终端插入或提取。这些OAM单元在VC链路端看不到，因此不会通过PW发送。
The LCCEs MUST be able to pass the F4 segment and end-to-end Fault Management, Resource Management (RM cells), Performance Management, Activation/deactivation, and System Management OAM cells transparently according to [I610-1].
LCCE必须能够根据[I610-1]透明地通过F4段和端到端故障管理、资源管理（RM单元）、性能管理、激活/停用和系统管理OAM单元。
F5 OAM cells are not inserted or extracted at the VP cross-connect. The LCCEs MUST be able to pass the F5 OAM cells transparently across the PW.
F5 OAM单元未在VP交叉连接处插入或提取。LCCE必须能够通过PW透明地传递F5 OAM单元。
The "ATM Maximum Concatenated Cells AVP", Attribute Type 86, indicates that the egress LCCE node can process a single PDU with concatenated cells up to a specified number of cells. An LCCE node transmitting concatenated cells on this PW MUST NOT exceed the maximum number of cells as specified in this AVP. This AVP is applicable only to ATM Cell Relay PW Types (VCC, VPC, Port Cell Relay). This Attribute value may not be same in both directions of the specific PW.
属性类型为86的“ATM最大级联小区AVP”指示出口LCCE节点可以处理具有最多指定数量的级联小区的单个PDU。在此PW上传输级联小区的LCCE节点不得超过此AVP中规定的最大小区数。本AVP仅适用于ATM信元中继PW类型（VCC、VPC、端口信元中继）。该属性值在特定PW的两个方向上可能不相同。
The Attribute Value field for this AVP has the following format:
此AVP的属性值字段具有以下格式：
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Maximum Concatenated Cells|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Maximum Concatenated Cells|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this AVP MAY be set to 0, but MAY vary (see Section 5.2 of [RFC3931]). The length (before hiding) of this AVP is 8.
该AVP可能被隐藏（H位可能为0或1）。该AVP的M位可设置为0，但可能会有所不同（见[RFC3931]第5.2节）。此AVP的长度（隐藏前）为8。
This AVP is sent in an ICRQ, ICRP during session negotiation or via SLI control messages when LCCE changes the maximum number of concatenated cells configuration for a given ATM cell relay circuit.
当LCCE改变给定ATM信元中继电路的最大连接信元数配置时，该AVP在会话协商期间或通过SLI控制消息在ICRQ、ICRP中发送。
This AVP is OPTIONAL. If the egress LCCE is configured with a maximum number of cells to be concatenated by the ingress LCCE, it SHOULD signal this value to the ingress LCCE.
此AVP是可选的。如果出口LCCE配置有由入口LCCE连接的最大单元数，则其应向入口LCCE发送该值的信号。
An "OAM Emulation Required AVP", Attribute Type 87, MAY be needed to signal OAM emulation in AAL5 SDU Mode, if an LCCE cannot support the transport of OAM cells across L2TP sessions. If OAM cell emulation is configured or detected via some other means on one side, the other LCCE MUST support OAM cell emulation as well.
如果LCCE无法支持跨L2TP会话的OAM小区传输，则可能需要属性类型87的“OAM仿真所需AVP”以在AAL5 SDU模式下发出OAM仿真信号。如果一方通过其他方式配置或检测到OAM小区仿真，另一方LCCE也必须支持OAM小区仿真。
This AVP is exchanged during session negotiation (in ICRQ and ICRP) or during the life of the session via SLI control messages. If the other LCCE cannot support the OAM cell emulation, the associated L2TP session MUST be torn down via CDN message with result code 22.
该AVP在会话协商期间（在ICRQ和ICRP中）或会话生命周期内通过SLI控制消息交换。如果其他LCCE无法支持OAM小区模拟，则必须通过CDN消息（结果代码为22）中断相关L2TP会话。
OAM Emulation AVP is a boolean AVP, having no Attribute Value. Its absence is FALSE and its presence is TRUE. This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to 0, but MAY vary (see Section 5.2 of [RFC3931]). The Length (before hiding) of this AVP is 6.
OAM仿真AVP是一个布尔AVP，没有属性值。它的缺席是虚假的，它的存在是真实的。该AVP可能被隐藏（H位可能为0或1）。该AVP的M位应设置为0，但可能有所不同（见[RFC3931]第5.2节）。此AVP的长度（隐藏前）为6。
ATM OAM alarms or circuit status is indicated via the Circuit Status AVP as defined in Section 5.4.5 of [RFC3931]. For reference, usage of this AVP is shown below.
ATM OAM警报或电路状态通过[RFC3931]第5.4.5节中定义的电路状态AVP指示。作为参考，本AVP的使用如下所示。
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Value is a 16-bit mask with the two least significant bits defined, and the remaining bits are reserved for future use. Reserved bits MUST be set to 0 when sending and ignored upon receipt.
该值是一个定义了两个最低有效位的16位掩码，其余位保留供将来使用。发送时保留位必须设置为0，接收时忽略。
The A (Active) bit indicates whether the ATM circuit is ACTIVE (1) or INACTIVE (0).
A（活动）位指示ATM电路是活动（1）还是非活动（0）。
The N (New) bit indicates whether the ATM circuit status indication is for a new ATM circuit (1) or an existing ATM circuit (0).
N（新）位表示ATM电路状态指示是针对新ATM电路（1）还是针对现有ATM电路（0）。
An "ATM Alarm Status AVP", Attribute Type 88, indicates the reason for the ATM circuit status and specific alarm type, if any, to its peer LCCE node. This OPTIONAL AVP MAY be present in the SLI message with the Circuit Status AVP.
属性类型为88的“ATM报警状态AVP”向其对等LCCE节点指示ATM电路状态的原因和特定报警类型（如有）。此可选AVP可能存在于SLI消息中，且电路状态为AVP。
The Attribute Value field for this AVP has the following format:
此AVP的属性值字段具有以下格式：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Circuit Status Reason | Alarm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Circuit Status Reason | Alarm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Circuit Status Reason is a 2-octet unsigned integer, and the Alarm Type is also a 2-octet unsigned integer.
电路状态原因是一个2八位无符号整数，报警类型也是一个2八位无符号整数。
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to 0, but MAY vary (see Section 5.2 of [RFC3931]). The Length (before hiding) of this AVP is 10 octets.
该AVP可能被隐藏（H位可能为0或1）。该AVP的M位应设置为0，但可能有所不同（见[RFC3931]第5.2节）。此AVP的长度（隐藏前）为10个八位字节。
This AVP is sent in the SLI message to indicate additional information about the ATM circuit status.
此AVP在SLI消息中发送，以指示有关ATM电路状态的附加信息。
Circuit Status Reason values for the SLI message are as follows:
SLI消息的电路状态原因值如下：
0 - Reserved 1 - No alarm or alarm cleared (default for Active Status) 2 - Unspecified or unknown Alarm Received (default for Inactive Status) 3 - ATM Circuit received F1 Alarm on ingress LCCE 4 - ATM Circuit received F2 Alarm on ingress LCCE 5 - ATM Circuit received F3 Alarm on ingress LCCE 6 - ATM Circuit received F4 Alarm on ingress LCCE 7 - ATM Circuit received F5 Alarm on ingress LCCE 8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE 9 - ATM Circuit down due to loop-back timeout on ingress LCCE
0-保留1-无报警或报警已清除（默认为活动状态）2-收到未指定或未知报警（默认为非活动状态）3-ATM电路收到入口LCCE 4上的F1报警-ATM电路收到入口LCCE 5上的F2报警-ATM电路收到入口LCCE 6上的F3报警-ATM电路收到入口LCCE 7上的F4报警-ATM电路收到入口LCCE 8上的F5报警-对等LCCE 9上的ATM端口关闭导致ATM电路停机-ATM电路因回环而停机入口LCCE超时
The general ATM Alarm failures are encoded as below:
一般ATM报警故障编码如下：
0 - Reserved 1 - No Alarm type specified (default) 2 - Alarm Indication Signal (AIS) 3 - Remote Defect Indicator (RDI) 4 - Loss of Signal (LOS) 5 - Loss of Pointer (LOP) 6 - Loss of Framer (LOF) 7 - Loopback cells (LB) 8 - Continuity Check (CC)
0-保留1-未指定报警类型（默认）2-报警指示信号（AIS）3-远程缺陷指示器（RDI）4-信号丢失（LOS）5-指针丢失（LOP）6-成帧器丢失（LOF）7-环回单元（LB）8-连续性检查（CC）
The ATM Pseudowire emulation described in this document allows for carrying various ATM services across an IP packet switched network
本文档中描述的ATM伪线仿真允许通过IP分组交换网络承载各种ATM服务
(PSN). These ATM services can be PVC-based, PVP-based, or port-based. In all cases, ATMPWs operate in a point-to-point deployment model.
（PSN）。这些ATM服务可以是基于PVC、基于PVP或基于端口的。在所有情况下，ATMPW都以点对点部署模式运行。
ATMPWs support two modes of encapsulation: ATM AAL5-SDU Mode and ATM Cell Relay Mode. The following sections list their respective characteristics in relationship to the native service.
ATMPWs支持两种封装模式：ATM AAL5-SDU模式和ATM信元中继模式。以下各节列出了它们与本机服务相关的各自特征。
ATMPWs operating in AAL5-SDU Mode only support the transport of PVC-based services. In this mode, the AAL5 CPCS-PDU from a single VCC is reassembled at the ingress LCCE, and the AAL5 CPCS-SDU (i.e., the AAL5 CPCS-PDU without CPCS-PDU Trailer or PAD octets, also referred to as AAL5 CPCS-PDU Payload) is transported over the Pseudowire. Therefore, Segmentation and Reassembly (SAR) functions are required at the LCCEs. There is a one-to-one mapping between an ATM PVC and an ATMPW operating in AAL5-SDU Mode, supporting bidirectional transport of variable length frames. With the exception of optionally transporting OAM cells, only ATM Adaptation Layer (AAL) Type 5 frames are carried in this mode, including multiprotocol over AAL5 packets [RFC2684].
在AAL5-SDU模式下运行的ATMPWs仅支持基于PVC的服务的传输。在此模式下，来自单个VCC的AAL5 CPCS-PDU在入口LCCE处重新组装，AAL5 CPCS-SDU（即，不带CPCS-PDU拖车或PAD八位字节的AAL5 CPCS-PDU，也称为AAL5 CPCS-PDU有效载荷）通过伪线传输。因此，LCCEs需要分段和重组（SAR）功能。在AAL5-SDU模式下运行的ATM PVC和ATMPW之间存在一对一映射，支持可变长度帧的双向传输。除了可选地传输OAM信元之外，在此模式下仅承载ATM适配层（AAL）类型5帧，包括AAL5数据包上的多协议[RFC2684]。
The following considerations stem from ATM AAL5-SDU Mode Pseudowires not transporting the ATM cell headers and AAL5 CPCS-PDU Trailer (see Section 5.1):
以下注意事项源于不传输ATM信元报头和AAL5 CPCS-PDU拖车的ATM AAL5-SDU模式伪线（见第5.1节）：
o An ATMPW operating in AAL5-SDU Mode conveys EFCI and CLP information using the G and C bits in the ATM-Specific Sublayer. In consequence, the EFCI and CLP values of individual ATM cells that constitute the AAL5 frame may be lost across the ATMPW, and CLP and EFCI transparency may not be maintained. The AAL5-SDU Mode does not preserve EFCI and CLP values for every ATM cell within the AAL5 PDU. The processing of these bits on ingress and egress is defined in Section 4.1.
o 在AAL5-SDU模式下运行的ATMPW使用ATM特定子层中的G和C位传送EFCI和CLP信息。因此，构成AAL5帧的各个ATM信元的EFCI和CLP值可能在整个ATMPW中丢失，并且CLP和EFCI透明度可能无法保持。AAL5-SDU模式不会为AAL5 PDU内的每个ATM信元保留EFCI和CLP值。第4.1节定义了这些位在入口和出口上的处理。
o Only the least significant bit (LSB) from the CPCS-UU (User-to-User indication) field in the CPCS-PDU Trailer is transported using the ATM-Specific Sublayer (see Section 4.1). This bit contains the Frame Relay C/R bit when FRF.8.1 Frame Relay / ATM PVC Service Interworking [FRF8.1] is used. The CPCS-UU field is not used in multiprotocol over AAL5 [RFC2684]. However, applications that transfer user to user information using the CPCS-UU octet would fail to operate.
o 只有来自CPCS-PDU拖车中CPCS-UU（用户对用户指示）字段的最低有效位（LSB）使用ATM特定子层传输（见第4.1节）。当使用FRF.8.1帧中继/ATM PVC服务互通[FRF8.1]时，此位包含帧中继C/R位。CPCS-UU字段不用于AAL5上的多协议[RFC2684]。但是，使用CPCS-UU八位字节传输用户到用户信息的应用程序将无法运行。
o The CPI (Common Part Indicator) field in the CPCS-PDU Trailer is also not transported across the ATMPW. This does not affect multiprotocol over AAL5 applications since the field is used for alignment and MUST be coded as 0x00 [RFC2684].
o CPCS-PDU拖车中的CPI（公共零件指示器）字段也不会通过ATMPW传输。这不会影响AAL5上的多协议应用，因为该字段用于对齐，必须编码为0x00[RFC2684]。
o The trailing CRC field in the CPCS-PDU is stripped at the ingress LCCE and not transported over the ATMPW operating in AAL5-SDU Mode. It is in turn regenerated at the egress LCCE. Since the CRC has end-to-end significance, this means that errors introduced in the ATMPW payload during encapsulation or transit across the packet switched network may not be detected. To allow for payload integrity checking transparency on ATMPWs operating in AAL5-SDU Mode using L2TP over IP or L2TP over UDP/IP, the L2TPv3 session can utilize IPsec as specified in Section 4.1.3 of [RFC3931].
o CPCS-PDU中的尾随CRC字段在入口LCCE处剥离，而不是在AAL5-SDU模式下运行的ATMPW上传输。然后在出口LCCE处再生。由于CRC具有端到端的重要性，这意味着在封装或通过分组交换网络传输期间在ATMPW有效载荷中引入的错误可能不会被检测到。为了允许使用L2TP over IP或L2TP over UDP/IP在AAL5-SDU模式下运行的ATMPWs上进行有效负载完整性检查透明度，L2TPv3会话可以按照[RFC3931]第4.1.3节的规定使用IPsec。
Some additional characteristics of the AAL5-SDU Mode are the following:
AAL5-SDU模式的一些附加特性如下：
o The status of the ATM PVC is signaled between LCCEs using the Circuit Status AVP. More granular cause values for the ATM circuit status and specific ATM alarm types are signaled using the ATM Alarm Status AVP (see Section 8.1). Additionally, loss of connectivity between LCCEs can be detected by the L2TPv3 keepalive mechanism (see Section 4.4 in [RFC3931]).
o ATM PVC的状态通过电路状态AVP在LCCE之间发送信号。使用ATM报警状态AVP（见第8.1节）向ATM电路状态和特定ATM报警类型的更精细的原因值发送信号。此外，L2TPv3 keepalive机制可以检测LCCE之间的连接丢失（参见[RFC3931]中的第4.4节）。
o F5 OAM cells' relative order with respect to user data cells may not be maintained. F5 OAM cells that arrive during the reassembly of an AAL5 SDU are sent immediately over the PW and before the AAL5 SDU payload. At egress, these OAM cells are sent before the cells that comprise the AAL5-SDU. Therefore, applications that rely on cell sequence integrity between OAM and user data cells may not work. This includes Performance Monitoring and Security OAM cells (see Section 5.1). In addition, the AAL5-SDU service allows for OAM emulation in which OAM cells are not transported over the ATMPW (see Section 7). This is advantageous for AAL5-SDU Mode ATMPW implementations that do not support cell transport using the T-bit.
o F5 OAM单元相对于用户数据单元的相对顺序可能无法维护。在AAL5 SDU重新组装期间到达的F5 OAM单元立即通过PW发送，并在AAL5 SDU有效负载之前发送。在出口处，这些OAM小区在组成AAL5-SDU的小区之前发送。因此，依赖OAM和用户数据单元之间的单元序列完整性的应用程序可能无法工作。这包括性能监控和安全OAM单元（见第5.1节）。此外，AAL5-SDU服务允许OAM模拟，其中OAM信元不通过ATMPW传输（参见第7节）。这对于不支持使用T位的小区传输的AAL5-SDU模式ATMPW实现是有利的。
o Fragmentation and Reassembly procedures MAY be used for managing mismatched MTUs, as specified in Section 5 of [L2TPFRAG] or in the underlying PSN (IP, etc.) between tunnel endpoints as discussed in Section 4.1.4 of [RFC3931]. Only one of these methods SHOULD be used for a given AAL5-SDU Mode ATMPW. The procedures described in [L2TPFRAG] can be used to support the maximum size of an AAL5 SDU, 2 ^ 16 - 1 (65535) octets. However, relying on fragmentation on the L2TP/IPv4 packet between tunnel endpoints limits the maximum size of the AAL5 SDU
o 如[L2TPFRAG]第5节或[RFC3931]第4.1.4节所述，隧道端点之间的基础PSN（IP等）中规定，碎片和重组程序可用于管理不匹配的MTU。对于给定的AAL5-SDU模式ATMPW，只能使用其中一种方法。[L2TPFRAG]中描述的程序可用于支持AAL5 SDU的最大大小，2^16-1（65535）个八位组。但是，依赖隧道端点之间L2TP/IPv4数据包的碎片限制了AAL5 SDU的最大大小
that can be transported, because the maximum total length of an IPv4 datagram is already 65535 octets. In this case, the maximum AAL5 SDU that can be transported is limited to 65535 minus the encapsulating headers, 24-36 octets for L2TP-over-IPv4 or 36-48 octets for L2TP-over-UDP/IPv4. When the AAL5 payload is IPv4, an additional option is to fragment IP packets before tunnel encapsulation with L2TP/IP (see Section 4.1.4 of [RFC3931]).
这是可以传输的，因为IPv4数据报的最大总长度已经是65535个八位字节。在这种情况下，可传输的最大AAL5 SDU限制为65535减去封装头、IPv4上L2TP的24-36个八位字节或UDP/IPv4上L2TP的36-48个八位字节。当AAL5有效负载为IPv4时，另一个选项是在使用L2TP/IP进行隧道封装之前对IP数据包进行分段（请参见[RFC3931]的第4.1.4节）。
o Sequencing may be enabled on the ATMPW using the ATM-Specific Sublayer Sequence Number field, to detect lost, duplicate, or out-of-order frames on a per-session basis (see Section 4.2).
o 可使用ATM特定子层序列号字段在ATMPW上启用排序，以在每个会话的基础上检测丢失、重复或无序帧（见第4.2节）。
o Quality of Service characteristics such as throughput (cell rates), burst sizes and delay variation can be provided by leveraging Quality of Service features of the LCCEs and the underlying PSN, increasing the faithfulness of ATMPWs. This includes mapping ATM service categories to a compatible PSN class of service.
o 通过利用LCCE和底层PSN的服务质量特性，提高ATMPWs的忠实性，可以提供诸如吞吐量（信元速率）、突发大小和延迟变化等服务质量特性。这包括将ATM服务类别映射到兼容的PSN服务类别。
In this mode, no reassembly takes place at the ingress LCCE. There are no SAR requirements for LCCEs. Instead, ATM-layer cells are transported over the ATMPW. Consequently, all AAL types can be transported over ATMPWs operating in Cell Relay Mode. ATM Cell Relay Pseudowires can operate in three different modes (see Section 5.2): ATM VCC, ATM VPC, and ATM Port Cell Relay Services. The following are some of their characteristics:
在此模式下，入口LCCE处不进行重新组装。LCCE没有SAR要求。取而代之的是，ATM层信元通过ATMPW传输。因此，所有AAL类型都可以通过在小区中继模式下运行的ATMPWs进行传输。ATM信元中继伪线可在三种不同模式下工作（见第5.2节）：ATM VCC、ATM VPC和ATM端口信元中继服务。以下是它们的一些特点：
o The ATM cells transported over Cell Relay Mode ATMPWs consist of a 4-byte ATM cell header and a 48-byte ATM cell-payload (see Section 5.2). The ATM Service Payload of a Cell Relay Mode ATMPW is a multiple of 52 bytes. The Header Error Checksum (HEC) in the ATM cell header containing a Cyclic Redundancy Check (CRC) calculated over the first 4 bytes of the ATM cell header is not transported. Accordingly, the HEC field may not accurately reflect errors on an end-to-end basis; errors or corruption in the 4-byte ATM cell header introduced in the ATMPW payload during encapsulation or transit across the PSN may not be detected. To allow for payload integrity checking transparency on ATMPWs operating in Cell Relay Mode using L2TP over IP or L2TP over UDP/IP, the L2TPv3 session can utilize IPsec as specified in Section 4.1.3 of [RFC3931].
o 通过信元中继模式ATMPWs传输的ATM信元由4字节的ATM信元报头和48字节的ATM信元有效载荷组成（见第5.2节）。信元中继模式ATMPW的ATM服务有效负载是52字节的倍数。ATM信元报头中包含循环冗余校验（CRC）的报头错误校验和（HEC）在ATM信元报头的前4个字节上计算，不传输。因此，HEC字段可能无法准确反映端到端的错误；在封装或通过PSN传输期间，可能检测不到ATMPW有效负载中引入的4字节ATM信元报头中的错误或损坏。为了允许使用L2TP over IP或L2TP over UDP/IP在小区中继模式下运行的ATMPWs上进行有效负载完整性检查透明度，L2TPv3会话可以按照[RFC3931]第4.1.3节的规定使用IPsec。
o ATM PWs operating in Cell Relay Mode can transport a single ATM cell or multiple concatenated cells (see Section 6). Cell concatenation improves the bandwidth efficiency of the ATMPW (by decreasing the overhead) but introduces latency and delay variation.
o 在信元中继模式下运行的ATM PWs可以传输单个ATM信元或多个级联信元（参见第6节）。小区连接提高了ATMPW的带宽效率（通过减少开销），但引入了延迟和延迟变化。
o The status of the ATM PVC is signaled between LCCEs using the Circuit Status AVP. More granular cause values for the ATM circuit status and specific ATM alarm types are signaled using the ATM Alarm Status AVP (see Section 8.1). Additionally, loss of connectivity between LCCEs can be detected by the L2TPv3 keepalive mechanism (see Section 4.4 in [RFC3931]).
o ATM PVC的状态通过电路状态AVP在LCCE之间发送信号。使用ATM报警状态AVP（见第8.1节）向ATM电路状态和特定ATM报警类型的更精细的原因值发送信号。此外，L2TPv3 keepalive机制可以检测LCCE之间的连接丢失（参见[RFC3931]中的第4.4节）。
o ATM OAM cells are transported in the same fashion as user cells, and in the same order as they are received. Therefore, applications that rely on cell sequence integrity between OAM and user data cells are not adversely affected. This includes performance management and security applications that utilize OAM cells (see Section 5.3).
o ATM OAM信元的传输方式与用户信元相同，传输顺序与接收顺序相同。因此，依赖于OAM和用户数据单元之间的单元序列完整性的应用程序不会受到不利影响。这包括使用OAM单元的性能管理和安全应用程序（参见第5.3节）。
o The maximum number of concatenated cells is limited by the MTU size of the session (see Section 5.2 and Section 6). Therefore, Fragmentation and Reassembly procedures are not used for Cell Relay ATMPWs. Concatenating cells to then fragment the resulting packet defeats the purpose of cell concatenation. Concatenation of cells and fragmentation act as inverse functions, with additional processing but null net effect, and should not be used together.
o 连接单元的最大数量受会话的MTU大小限制（参见第5.2节和第6节）。因此，单元中继ATMPWs不使用碎片和重新组装程序。将单元连接起来，然后对结果数据包进行分段，这就违背了单元连接的目的。单元串联和分段作为反向函数，具有附加处理但无净效应，不应一起使用。
o Sequencing may be enabled on the ATMPW to detect lost, duplicate, or out-of-order packets on a per-session basis (see Section 4.2).
o 可在ATMPW上启用排序，以在每个会话的基础上检测丢失、重复或无序的数据包（见第4.2节）。
o Quality of Service characteristics such as throughput (cell rates), burst sizes, and delay variation can be provided by leveraging Quality of Service features of the LCCEs and the underlying PSN, increasing the faithfulness of ATMPWs. This includes mapping ATM service categories to a compatible PSN class of service, and mapping CLP and EFCI bits to PSN classes of service. For example, mapping a Constant Bit Rate (CBR) PVC to a class of service with tight loss and delay characteristics, such as an Expedited Forwarding (EF) Per-Hop Behavior (PHB) if the PSN is an IP DiffServ-enabled domain. The following characteristics of ATMPWs operating in Cell Relay Mode include additional QoS considerations:
o 通过利用LCCE和底层PSN的服务质量特性，提高ATMPW的忠实性，可以提供诸如吞吐量（信元速率）、突发大小和延迟变化等服务质量特性。这包括将ATM服务类别映射到兼容的PSN服务类别，以及将CLP和EFCI位映射到PSN服务类别。例如，对于pHbC服务，具有恒定的比特丢失率（pHbR）的区分服务转发特性。在小区中继模式下运行的ATMPWs的以下特征包括额外的QoS考虑：
- ATM Cell transport VCC Pseudowires allow for mapping multiple ATM VCCs to a single ATMPW. However, a user may
- ATM信元传输VCC伪线允许将多个ATM VCC映射到单个ATMPW。然而，用户可以
wish to map a single ATM VCC per ATMPW to satisfy QoS requirements (see Section 5.2.1).
希望按照ATMPW映射单个ATM VCC，以满足QoS要求（见第5.2.1节）。
- Cell Relay ATMPWs allow for concatenating multiple cells in a single Pseudowire PDU to improve bandwidth efficiency, but may introduce latency and delay variation.
- 小区中继ATMPWs允许在单个伪线PDU中连接多个小区以提高带宽效率，但可能会引入延迟和延迟变化。
As explained in [RFC3985], the PSN carrying the PW may be subject to congestion, with congestion characteristics depending on PSN type, network architecture, configuration, and loading. During congestion the PSN may exhibit packet loss and packet delay variation (PDV) that will impact the timing and data integrity of the ATMPW. During intervals of acute congestion, some Cell Relay ATMPWs may not be able to maintain service. The inelastic nature of some ATM services reduces the risk of congestion because the rates will not expand to consume all available bandwidth, but on the other hand, those ATM services cannot arbitrarily reduce their load on the network to eliminate congestion when it occurs.
如[RFC3985]中所述，承载PW的PSN可能会发生拥塞，其拥塞特性取决于PSN类型、网络架构、配置和负载。在拥塞期间，PSN可能会出现分组丢失和分组延迟变化（PDV），这将影响ATMPW的定时和数据完整性。在急性拥塞间隔期间，某些小区中继ATMPWs可能无法维持服务。某些ATM服务的非弹性性质降低了拥塞风险，因为速率不会扩展到消耗所有可用带宽，但另一方面，这些ATM服务不能在拥塞发生时任意减少其网络负载以消除拥塞。
Whenever possible, Cell Relay ATMPWs should be run over traffic-engineered PSNs providing bandwidth allocation and admission control mechanisms. IntServ-enabled domains providing the Guaranteed Service (GS) or DiffServ-enabled domains using Expedited Forwarding (EF) are examples of traffic-engineered PSNs. Such PSNs will minimize loss and delay while providing some degree of isolation of the Cell Relay ATMPW's effects from neighboring streams.
只要有可能，小区中继ATMPWs应在提供带宽分配和准入控制机制的流量工程PSN上运行。提供保证服务（GS）的支持IntServ的域或使用加速转发（EF）的支持DiffServ的域是流量工程PSN的示例。这样的PSN将最小化损失和延迟，同时提供一定程度的小区中继ATMPW效应与相邻流的隔离。
If the PSN is providing a best-effort service, then the following best-effort service congestion avoidance considerations apply: Those ATMPWs that carry constant bit rate (CBR) and variable bit rate-real time (VBR-rt) services across the PSN will most probably not behave in a TCP-friendly manner prescribed by [RFC2914]. In the presence of services that reduce transmission rate, ATMPWs carrying CBR and VBR-rt traffic SHOULD be halted when acute congestion is detected, in order to allow for other traffic or the network infrastructure itself to continue. ATMPWs carrying unspecified bit rate (UBR) traffic, which are equivalent to best-effort IP service, need not be halted during acute congestion and MAY have cells delayed or dropped by the ingress PE if necessary. ATMPWs carrying variable bit rate-non real time (VBR-nrt) services may or may not behave in a TCP-friendly manner, depending on the end user application, but are most likely safe to continue operating, since the end-user application is expected to be delay-insensitive and may also be somewhat loss-insensitive.
如果PSN提供尽力而为的服务，则以下尽力而为的服务拥塞避免注意事项适用：在PSN上承载恒定比特率（CBR）和可变比特率实时（VBR rt）服务的ATMPW很可能不会以[RFC2914]规定的TCP友好方式运行。在存在降低传输速率的服务的情况下，当检测到严重拥塞时，承载CBR和VBR rt流量的ATMPW应停止，以便允许其他流量或网络基础设施本身继续。承载未指定比特率（UBR）流量的ATMPW相当于尽力而为的IP服务，在急性拥塞期间无需停止，如有必要，可能会导致入口PE延迟或丢弃小区。承载可变比特率非实时（VBR nrt）服务的ATMPW可能以TCP友好的方式运行，也可能不以TCP友好的方式运行，这取决于最终用户应用程序，但最有可能安全地继续运行，因为最终用户应用程序预计会对延迟不敏感，也可能对丢失不敏感。
LCCEs SHOULD monitor for congestion (for example, by measuring packet loss or as specified in Section 6.5 of [RFC3985]) in order to ensure that the ATM service may be maintained. When severe congestion is detected (for example, when enabling sequencing and detecting that the packet loss is higher than a threshold), the ATM service SHOULD be terminated by tearing down the L2TP session via a CDN message. The PW may be restarted by manual intervention, or by automatic means after an appropriate waiting time.
LCCE应监测拥塞情况（例如，通过测量数据包丢失或按照[RFC3985]第6.5节的规定），以确保ATM服务得以维持。当检测到严重拥塞时（例如，当启用排序并检测到数据包丢失高于阈值时），应通过CDN消息中断L2TP会话来终止ATM服务。PW可通过手动干预或在适当的等待时间后通过自动方式重新启动。
ATM over L2TPv3 is subject to the security considerations defined in [RFC3931]. There are no additional considerations specific to carrying ATM that are not present carrying other data link types.
L2TPv3上的ATM应遵守[RFC3931]中定义的安全注意事项。对于承载其他数据链路类型不存在的ATM，没有其他特定的注意事项。
The signaling mechanisms defined in this document rely upon the allocation of the following ATM Pseudowire Types (see Pseudowire Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3 Pseudowire Types in 10.6 of [RFC3931]) by the IANA (number space created as part of publication of [RFC3931]):
本文件中定义的信令机制依赖于IANA（作为[RFC3931]出版物的一部分创建的数字空间）对以下ATM伪线类型的分配（参见[RFC3931]第5.4.3节中定义的伪线能力列表和[RFC3931]第10.6节中定义的L2TPv3伪线类型）：
Pseudowire Types
----------------
Pseudowire Types
----------------
0x0002 ATM AAL5 SDU VCC transport 0x0003 ATM Cell transparent Port Mode 0x0009 ATM Cell transport VCC Mode 0x000A ATM Cell transport VPC Mode
0x0002 ATM AAL5 SDU VCC传输0x0003 ATM信元透明端口模式0x0009 ATM信元传输VCC模式0x000A ATM信元传输VPC模式
This number space is created and maintained per [RFC3931].
此数字空间是根据[RFC3931]创建和维护的。
L2-Specific Sublayer Type
-------------------------
L2-Specific Sublayer Type
-------------------------
2 - ATM L2-Specific Sublayer present
2-存在ATM L2特定子层
This number space is managed by IANA as per [BCP0068].
此数字空间由IANA根据[BCP0068]进行管理。
A summary of the three new AVPs follows:
三个新AVP的摘要如下：
Control Message Attribute Value Pairs
控制消息属性值对
Attribute
Type Description
--------- ----------------------------------
86 ATM Maximum Concatenated Cells AVP
87 OAM Emulation Required AVP
88 ATM Alarm Status AVP
Attribute
Type Description
--------- ----------------------------------
86 ATM Maximum Concatenated Cells AVP
87 OAM Emulation Required AVP
88 ATM Alarm Status AVP
This number space is managed by IANA as per [BCP0068].
此数字空间由IANA根据[BCP0068]进行管理。
A new Result Code value for the CDN message is defined in Section 7. Following is a summary:
CDN消息的新结果代码值在第7节中定义。以下为总结：
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
General Error Codes
一般错误代码
22 - Session not established due to other LCCE cannot support the OAM Cell Emulation
22-由于其他LCCE无法支持OAM小区模拟，因此未建立会话
This is a new registry for IANA to maintain.
这是IANA需要维护的新注册表。
New Attribute values for the ATM Alarm Status AVP in the SLI message are defined in Section 8.1. Additional values may be assigned by Expert Review [RFC2434]. Following is a summary:
第8.1节定义了SLI消息中ATM报警状态AVP的新属性值。专家评审[RFC2434]可指定其他值。以下为总结：
ATM Alarm Status AVP (Attribute Type 88) Values
-----------------------------------------------
ATM Alarm Status AVP (Attribute Type 88) Values
-----------------------------------------------
Circuit Status Reason values for the SLI message are as follows:
SLI消息的电路状态原因值如下：
0 - Reserved 1 - No alarm or alarm cleared (default for Active Status) 2 - Unspecified or unknown Alarm Received (default for Inactive Status) 3 - ATM Circuit received F1 Alarm on ingress LCCE 4 - ATM Circuit received F2 Alarm on ingress LCCE 5 - ATM Circuit received F3 Alarm on ingress LCCE 6 - ATM Circuit received F4 Alarm on ingress LCCE 7 - ATM Circuit received F5 Alarm on ingress LCCE 8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE 9 - ATM Circuit down due to loop-back timeout on ingress LCCE
0-保留1-无报警或报警已清除（默认为活动状态）2-收到未指定或未知报警（默认为非活动状态）3-ATM电路收到入口LCCE 4上的F1报警-ATM电路收到入口LCCE 5上的F2报警-ATM电路收到入口LCCE 6上的F3报警-ATM电路收到入口LCCE 7上的F4报警-ATM电路收到入口LCCE 8上的F5报警-对等LCCE 9上的ATM端口关闭导致ATM电路停机-ATM电路因回环而停机入口LCCE超时
The general ATM Alarm failures are encoded as below:
一般ATM报警故障编码如下：
0 - Reserved 1 - No Alarm type specified (default) 2 - Alarm Indication Signal (AIS) 3 - Remote Defect Indicator (RDI) 4 - Loss of Signal (LOS) 5 - Loss of Pointer (LOP) 6 - Loss of Framer (LOF) 7 - Loopback cells (LB) 8 - Continuity Check (CC)
0-保留1-未指定报警类型（默认）2-报警指示信号（AIS）3-远程缺陷指示器（RDI）4-信号丢失（LOS）5-指针丢失（LOP）6-成帧器丢失（LOF）7-环回单元（LB）8-连续性检查（CC）
This is a new registry for IANA to maintain.
这是IANA需要维护的新注册表。
The ATM-Specific Sublayer contains 8 bits in the low-order portion of the header. Reserved bits may be assigned by IETF Consensus [RFC2434].
ATM特定子层在报头的低阶部分包含8位。保留位可由IETF协商一致[RFC2434]分配。
Bit 0 - Reserved Bit 1 - S (Sequence) bit Bit 2 - B (Fragmentation) bit Bit 3 - E (Fragmentation) bit Bit 4 - T (Transport type) bit Bit 5 - G (EFCI) bit Bit 6 - C (CLP) bit Bit 7 - U (Command/Response) bit
位0—保留位1—S（序列）位2—B（分段）位3—E（分段）位4—T（传输类型）位5—G（EFCI）位6—C（CLP）位7—U（命令/响应）位
Thanks for the contributions from Jed Lau, Pony Zhu, Prasad Yaditi, Durai, and Jaya Kumar.
感谢Jed Lau、Pony Zhu、Prasad Yaditi、Durai和Jaya Kumar的贡献。
Many thanks to Srinivas Kotamraju for editorial review.
非常感谢Srinivas Kotamraju的编辑评论。
Thanks to Shoou Yiu and Fred Shu for giving their valuable time to review this document.
感谢Shoou Yiu和Fred Shu抽出宝贵的时间来审查本文件。
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC3931]Lau，J.，Townsley，M.，和I.Goyret，“第二层隧道协议-版本3（L2TPv3）”，RFC 39312005年3月。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[PWE3ATM] Martini, L., "Encapsulation Methods for Transport of ATM Over MPLS Networks", Work in Progress, September 2005.
[PWE3ATM]Martini，L.，“通过MPLS网络传输ATM的封装方法”，正在进行的工作，2005年9月。
[L2TPFRAG] Malis, A. and M. Townsley, "PWE3 Fragmentation and Reassembly", Work in Progress, November 2005.
[L2TPFRAG]Malis，A.和M.Townsley，“PWE3碎片化和重组”，正在进行的工作，2005年11月。
[FRF8.1] "Frame Relay / ATM PVC Service Interworking Implementation Agreement (FRF 8.1)", Frame Relay Forum 2000.
[FRF8.1]“帧中继/ATM PVC业务互通实施协议（FRF 8.1）”，帧中继论坛2000。
[BCP0068] Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update", BCP 68, RFC 3438, December 2002.
[BCP0068]汤斯利，W.“第二层隧道协议（L2TP）互联网分配号码管理局（IANA）注意事项更新”，BCP 68，RFC 3438，2002年12月。
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[RFC2434]Narten，T.和H.Alvestrand，“在RFCs中编写IANA注意事项部分的指南”，BCP 26，RFC 2434，1998年10月。
[I610-1] ITU-T Recommendation I.610 (1999): B-ISDN operation and maintenance principles and functions
[I610-1]ITU-T建议I.610（1999）：B-ISDN操作和维护原则与功能
[I610-2] ITU-T Recommendation I.610, Corrigendum 1 (2000): B-ISDN operation and maintenance principles and functions (corrigendum 1)
[I610-2]ITU-T建议I.610，勘误表1（2000）：B-ISDN操作和维护原则与功能（勘误表1）
[I610-3] ITU-T Recommendation I.610, Amendment 1 (2000): B-ISDN operation and maintenance principles and functions (Amendment 1)
[I610-3]ITU-T建议I.610，修改件1（2000）：B-ISDN操作和维护原则与功能（修改件1）
[ATMSEC] ATM Forum Specification, af-sec-0100.002 (2001): ATM Security Specification version 1.1
[ATMSEC]ATM论坛规范，af-sec-0100.002（2001）：ATM安全规范版本1.1
[RFC2684] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation over ATM Adaptation Layer 5", RFC 2684, September 1999.
[RFC2684]Grossman，D.和J.Heinanen，“ATM适配层5上的多协议封装”，RFC 2684，1999年9月。
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC3985]Bryant，S.和P.Pate，“伪线仿真边到边（PWE3）架构”，RFC 39852005年3月。
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, September 2000.
[RFC2914]Floyd，S.，“拥塞控制原则”，BCP 41，RFC 2914，2000年9月。
Authors' Addresses
作者地址
Sanjeev Singh Cisco Systems 170 W. Tasman Drive San Jose, CA 95134
Sanjeev Singh Cisco Systems 170 W.塔斯曼大道圣何塞，加利福尼亚州95134
EMail: sanjeevs@cisco.com
EMail: sanjeevs@cisco.com
W. Mark Townsley Cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709
美国北卡罗来纳州三角研究公园14987号吉特克里克路邮政信箱7025号马克·汤斯利思科系统公司
EMail: mark@townsley.net
EMail: mark@townsley.net
Carlos Pignataro Cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709
卡洛斯·皮格纳塔罗思科系统公司，地址：北卡罗来纳州三角研究公园Kit Creek路7025号邮政信箱14987，邮编：27709
EMail: cpignata@cisco.com
EMail: cpignata@cisco.com
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完整版权声明
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Acknowledgement
确认
Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).
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