Network Working Group M. Townsley
Request for Comments: 4591 G. Wilkie
Category: Standards Track S. Booth
S. Bryant
Cisco Systems
J. Lau
July 2006
Network Working Group M. Townsley
Request for Comments: 4591 G. Wilkie
Category: Standards Track S. Booth
S. Bryant
Cisco Systems
J. Lau
July 2006
Frame Relay over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
第2层隧道协议版本3（L2TPv3）上的帧中继
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 a protocol for tunneling a variety of data link protocols over IP networks. This document describes the specifics of how to tunnel Frame Relay over L2TPv3, including frame encapsulation, virtual-circuit creation and deletion, and status change notification.
第2层隧道协议第3版（L2TPv3）定义了一种通过IP网络隧道传输各种数据链路协议的协议。本文档详细介绍了如何通过L2TPv3隧道帧中继，包括帧封装、虚拟电路创建和删除以及状态更改通知。
Table of Contents
目录
1. Introduction ....................................................2
1.1. Abbreviations ..............................................3
1.2. Specification of Requirements ..............................3
2. Control Connection Establishment ................................3
3. PVC Status Notification and Session Establishment ...............3
3.1. L2TPv3 Session Establishment ...............................4
3.2. L2TPv3 Session Teardown ....................................5
3.3. L2TPv3 Session Maintenance .................................5
3.4. Use of the Circuit Status AVP for Frame Relay ..............6
3.5. Frame Relay Header Length AVP ..............................7
4. Encapsulation ...................................................7
4.1. Data Packet Encapsulation ..................................7
4.2. Data Packet Sequencing .....................................9
4.3. MTU Considerations .........................................9
5. Applicability Statement ........................................10
6. Security Considerations ........................................10
7. IANA Considerations ............................................11
7.1. Pseudowire Type ...........................................11
7.2. Result Code AVP Values ....................................11
7.3. Control Message Attribute Value Pairs (AVPs) ..............11
8. Acknowledgements ...............................................11
9. References .....................................................12
9.1. Normative References ......................................12
9.2. Informative References ....................................12
1. Introduction ....................................................2
1.1. Abbreviations ..............................................3
1.2. Specification of Requirements ..............................3
2. Control Connection Establishment ................................3
3. PVC Status Notification and Session Establishment ...............3
3.1. L2TPv3 Session Establishment ...............................4
3.2. L2TPv3 Session Teardown ....................................5
3.3. L2TPv3 Session Maintenance .................................5
3.4. Use of the Circuit Status AVP for Frame Relay ..............6
3.5. Frame Relay Header Length AVP ..............................7
4. Encapsulation ...................................................7
4.1. Data Packet Encapsulation ..................................7
4.2. Data Packet Sequencing .....................................9
4.3. MTU Considerations .........................................9
5. Applicability Statement ........................................10
6. Security Considerations ........................................10
7. IANA Considerations ............................................11
7.1. Pseudowire Type ...........................................11
7.2. Result Code AVP Values ....................................11
7.3. Control Message Attribute Value Pairs (AVPs) ..............11
8. Acknowledgements ...............................................11
9. References .....................................................12
9.1. Normative References ......................................12
9.2. Informative References ....................................12
[RFC3931] defines a base protocol for Layer 2 Tunneling over IP networks. This document defines the specifics necessary for tunneling Frame Relay over L2TPv3. Such emulated circuits are referred to as Frame Relay Pseudowires (FRPWs).
[RFC3931]定义了IP网络上第2层隧道的基本协议。本文件定义了L2TPv3上隧道帧中继的必要细节。这种仿真电路称为帧中继伪线（FRPW）。
Protocol specifics defined in this document for L2TPv3 FRPWs operating in a "virtual circuit-to-virtual circuit" mode include those necessary for frame encapsulation, PVC creation and deletion, and status change notification. Frame Relay traffic may also be transported in a "port-to-port" or "interface-to-interface" fashion using High-Level Data Link Control (HDLC) Pseudowires as defined in [RFC4349]. Support for Switched Virtual Circuits (SVCs) and Switched/Soft Permanent Virtual Circuits (SPVCs) are outside the scope of this document.
本文件中为在“虚拟电路到虚拟电路”模式下运行的L2TPv3 FRPW定义的协议规范包括帧封装、PVC创建和删除以及状态更改通知所需的规范。帧中继通信量也可以使用[RFC4349]中定义的高级数据链路控制（HDLC）伪线以“端口到端口”或“接口到接口”的方式传输。对交换虚拟电路（SVC）和交换/软永久虚拟电路（SPVC）的支持不在本文档的范围内。
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。
FR Frame Relay FRPW Frame Relay Pseudowire LCCE L2TP Control Connection Endpoint (See [RFC3931]) PVC Permanent virtual circuit PW Pseudowire VC Virtual circuit
FR帧继电器FRPW帧继电器伪线LCCE L2TP控制连接端点（参见[RFC3931]）PVC永久虚拟电路PW伪线VC虚拟电路
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。
In order to tunnel a Frame Relay circuit over IP using L2TPv3, an L2TPv3 Control Connection MUST first be established as described in [RFC3931]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP Control Message MUST include the Frame Relay Data Link Connection Identifier (DLCI) PW Type of 0x0001 (see IANA Considerations), 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 to support Frame Relay Pseudowires (FRPWs).
为了使用L2TPv3在IP上传输帧中继电路，必须首先按照[RFC3931]中的说明建立L2TPv3控制连接。L2TPv3 SCCRQ控制消息和相应的SCCRP控制消息必须包括[RFC3931]第5.4.3节中定义的伪线能力列表中的帧中继数据链路连接标识符（DLCI）PW类型0x0001（参见IANA注意事项）。这将控制连接标识为能够建立L2TP会话以支持帧中继伪线（FRPW）。
An LCCE MUST be able to identify itself uniquely in the SCCRQ and SCCRP messages via a globally unique value. By default, this is advertised via the structured Router ID Attribute Value Pairs (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属性值对（AVP）[RFC3931]公布的，尽管非结构化主机名AVP[RFC3931]也可用于识别LCCE。
This section specifies how the status of a PVC is reported between two LCCEs. This includes what should happen when a PVC is created, deleted or when it changes state between ACTIVE and INACTIVE. When emulating a Frame Relay service, if the procedures for PVC status management defined in [Q933] Annex A are being used between an LCCE and the attached Remote System, an LCCE MUST participate in them (see Section 3.3).
本节规定了如何在两个LCCE之间报告PVC的状态。这包括创建、删除PVC或在活动和非活动之间更改状态时应发生的情况。模拟帧中继服务时，如果在LCCE和连接的远程系统之间使用[Q933]附录a中定义的PVC状态管理程序，则LCCE必须参与其中（见第3.3节）。
PVC creation (provisioning) results in establishment of an L2TP session via the standard three-way handshake described in Section 3.4.1 of [RFC3931]. An LCCE MAY initiate the session immediately upon PVC creation or wait until the PVC state transitions to ACTIVE before attempting to establish a session for the PVC. Waiting until the PVC transitions to ACTIVE may be preferred, as it delays allocation of L2TP resources until it is absolutely necessary.
PVC创建（供应）导致通过[RFC3931]第3.4.1节所述的标准三方握手建立L2TP会话。LCCE可在PVC创建后立即启动会话，或在尝试为PVC建立会话之前，等待PVC状态转换为活动状态。等待PVC转换为ACTIVE可能是首选，因为它会延迟L2TP资源的分配，直到绝对必要。
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931], Attribute Type 68, MUST be present in the Incoming-Call-Request (ICRQ) messages and MUST include the Frame Relay DLCI PW Type of 0x0001 for FRPWs.
[RFC3931]第5.4.4节中定义的伪线类型AVP，属性类型68，必须出现在传入呼叫请求（ICRQ）消息中，并且必须包括FRPWs的帧中继DLCI PW类型0x0001。
The Circuit Status AVP (see Section 3.4) MUST be present in the ICRQ and Incoming-Call-Reply (ICRP) messages and MAY be present in the Set Link Info (SLI) message for FRPWs.
电路状态AVP（见第3.4节）必须出现在ICRQ和呼入应答（ICRP）消息中，并且可能出现在FRPWs的Set Link Info（SLI）消息中。
The Frame Relay Header Length AVP (see Section 3.5) MAY be present in the ICRQ and ICRP messages.
ICRQ和ICRP消息中可能存在帧中继头长度AVP（见第3.5节）。
The following is an example of the L2TP messages exchanged for an FRPW that is initiated after a new PVC is provisioned and becomes ACTIVE.
以下是为FRPW交换的L2TP消息示例，该FRPW在新PVC供应并激活后启动。
LCCE (LAC) A LCCE (LAC) B
------------------ ------------------
FR PVC Provisioned
FR PVC Provisioned
FR PVC ACTIVE
LCCE (LAC) A LCCE (LAC) B
------------------ ------------------
FR PVC Provisioned
FR PVC Provisioned
FR PVC ACTIVE
ICRQ (status = 0x03) ---->
ICRQ (status = 0x03) ---->
FR PVC ACTIVE
阻燃聚氯乙烯活性剂
<---- ICRP (status = 0x03)
<---- ICRP (status = 0x03)
L2TP session established, OK to send data into tunnel
L2TP会话已建立，可以将数据发送到隧道中
ICCN ----->
L2TP session established,
OK to send data into tunnel
ICCN ----->
L2TP session established,
OK to send data into tunnel
In the example above, an ICRQ is sent after the PVC is created and becomes ACTIVE. The Circuit Status AVP indicates that this PVC is ACTIVE and New (0x03). The Remote End ID AVP [RFC3931] MUST be
在上述示例中，在PVC创建并激活后发送ICRQ。电路状态AVP指示此PVC处于激活状态且为新状态（0x03）。远程端ID AVP[RFC3931]必须为
present in the ICRQ in order to identify the PVC (together with the identity of the LCCE itself, as defined in Section 2) to associate the L2TP session with. The Remote End ID AVP, defined in [RFC3931], is of opaque form and variable length, though one MUST at a minimum support use of an unstructured four-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.
出现在ICRQ中，以识别与L2TP会话相关联的PVC（以及第2节中定义的LCCE本身的身份）。[RFC3931]中定义的远程端ID AVP为不透明形式且长度可变，但必须至少支持使用两个LCCE已知的非结构化四个八位组值（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 FR PVC transitions to ACTIVE as well. If LCCE B had not been provisioned for the PVC identified in the ICRQ, a Call-Disconnect-Notify (CDN) would have been immediately returned indicating that the circuit was not provisioned or 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FR PVC转换为活动状态后发送。如果没有为ICRQ中标识的PVC配置LCCE B，则会立即返回呼叫断开通知（CDN），表明该LCCE未配置或可用电路。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 PVC is INACTIVE and an SLI is sent when the PVC becomes ACTIVE (see Section 3.3).
只要电路状态AVP反映PVC处于非活动状态，且当PVC处于活动状态时发送SLI，则实施可在PVC激活前发送ICRQ或ICRP（见第3.3节）。
The Incoming-Call-Connected (ICCN) is the final stage in the session establishment, confirming the receipt of the ICRP with acceptable parameters to allow bidirectional traffic.
连接的传入呼叫（ICCN）是会话建立的最后阶段，用可接受的参数确认ICRP的接收，以允许双向通信。
In the event that a PVC is deleted (unprovisioned) at either LCCE, the associated L2TP session MUST be torn down via the CDN message defined in Section 3.4.3 of [RFC3931].
如果在任一LCCE删除（未设置）PVC，则必须通过[RFC3931]第3.4.3节中定义的CDN消息中断相关L2TP会话。
General Result Codes regarding L2TP session establishment are defined in [RFC3931]. Additional Frame Relay result codes are defined as follows:
[RFC3931]中定义了有关L2TP会话建立的一般结果代码。其他帧中继结果代码定义如下：
17: FR PVC was deleted permanently (no longer provisioned) 18: FR PVC has been INACTIVE for an extended period of time 19: Mismatched FR Header Length
17:FR PVC被永久删除（不再配置）18:FR PVC已长时间处于非活动状态19:FR标头长度不匹配
FRPW over L2TP makes use of the SLI control message defined in [RFC3931] to signal Frame Relay link status notifications between LCCEs. This includes ACTIVE or INACTIVE notifications of the VC, and any other parameters that may need to be shared between the tunnel endpoints or LCCEs in order to provide proper PW emulation. The SLI message is a single message that is sent over the L2TP control
L2TP上的FRPW利用[RFC3931]中定义的SLI控制消息向LCCE之间的帧中继链路状态通知发送信号。这包括VC的活动或非活动通知，以及可能需要在隧道端点或LCCE之间共享的任何其他参数，以便提供适当的PW仿真。SLI消息是通过L2TP控件发送的单个消息
channel signalling the state change. Since the message is delivered reliably, there is no additional response or action required of the PW subsystem to ensure that the state change notification was received by the tunnel peer.
发送状态变化信号的通道。由于消息是可靠传递的，因此PW子系统不需要额外的响应或操作来确保隧道对等方收到状态更改通知。
The SLI message MUST be sent any time there is a circuit 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 PVC 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 that the peer to perform a reverse Session ID lookup).
每当电路状态发生变化时，必须发送SLI消息，该变化可能由电路状态AVP中标识的任何值报告。唯一的例外是初始ICRQ、ICRP和CDN消息，它们在创建或删除PVC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，要求对等方执行反向会话ID查找）的任何时间从任一LCCE发送。
An LCCE participating in the procedures for PVC status management defined in [Q933], Annex A, MUST transmit an SLI message including the Circuit Status AVP (see Section 3.4) when it detects a change in the status for a particular local FR PVC (i.e., when it detects a service-affecting condition or the clearing of such a condition). An LCCE receiving an SLI message indicating a change in the status of a particular FRPW SHOULD generate corresponding updates for the FR PVC towards the Remote System, as defined in [Q933], Annex A.
参与[Q933]附录A中规定的PVC状态管理程序的LCCE在检测到特定本地FR PVC的状态变化时（即检测到影响服务的条件或清除此类条件时），必须发送包含电路状态AVP（见第3.4节）的SLI消息。LCCE收到SLI消息，指示特定FRPW状态的变化，应根据[Q933]附录a中的定义，为远程系统的FR PVC生成相应的更新。
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利用[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。
Frame Relay circuit status is reported via the Circuit Status AVP defined in [RFC3931], Attribute Type 71. For reference, this AVP is shown below:
帧中继电路状态通过[RFC3931]属性类型71中定义的电路状态AVP报告。该AVP如下所示，以供参考：
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 reserved for future use. Reserved bits MUST be set to 0 by the sender and ignored by the receiver.
该值是一个16位掩码，定义了两个最低有效位，其余位保留供将来使用。保留位必须由发送方设置为0，并由接收方忽略。
The N (New) bit indicates whether the Circuit Status indication is for a new FR PVC (1) or an existing FR PVC (0).
N（新）位表示电路状态指示是针对新FR PVC（1）还是针对现有FR PVC（0）。
The A (Active) bit indicates whether the FR PVC is ACTIVE (1) or INACTIVE (0).
A（活动）位指示FR PVC是活动（1）还是非活动（0）。
The "Frame Relay Header Length AVP", Attribute type 85, indicates the number of bytes in the Frame Relay header. The two peer LCCEs MUST agree on the length of the Frame Relay header.
“帧中继头长度AVP”（属性类型85）表示帧中继头中的字节数。两个对等LCCE必须就帧中继头的长度达成一致。
This AVP is exchanged during session negotiation (in ICRQ, ICRP). If the other LCCE supports a different Frame Relay header length, the associated L2TP session MUST be torn down via CDN message with result code 19 (see Section 3.2).
此AVP在会话协商期间交换（在ICRQ、ICRP中）。如果另一个LCCE支持不同的帧中继报头长度，则必须通过结果代码为19的CDN消息中断相关L2TP会话（参见第3.2节）。
If the Frame Relay Header Length AVP is not signalled, it MUST be assumed that the peer uses a 2-byte Frame Relay header.
如果帧中继报头长度AVP未发出信号，则必须假设对等方使用2字节帧中继报头。
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：
Frame Relay Header Length (ICRQ, ICRP)
帧中继头长度（ICRQ、ICRP）
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Relay Header Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Relay Header Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Frame Relay Header Length Type is a 2-octet unsigned integer with the following values defined in this document:
帧中继头长度类型为2-octet无符号整数，具有本文档中定义的以下值：
2: Two-octet Frame Relay Header 4: Four-octet Frame Relay Header
2：两个八位字节帧中继头4：四个八位字节帧中继头
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。
The FR PDU is transported in its entirety, excluding the opening and closing High Level Data Link Control (HDLC) flags and the frame check sequence (FCS). Bit stuffing is undone. The L2TPv3 Session Header is that as defined in [RFC3931]. If sequencing or other features require presence of an L2-Specific Sublayer, the Default format defined in Section 4.6 of [RFC3931] MUST be used.
FR PDU整体传输，不包括打开和关闭高级数据链路控制（HDLC）标志和帧检查序列（FCS）。钻头填料松开。L2TPv3会话头是[RFC3931]中定义的会话头。如果序列或其他特征要求存在L2特定子层，则必须使用[RFC3931]第4.6节中定义的默认格式。
The FR header is defined in [Q922]; however, the notation used differs from that used in IETF specifications. For reference, the FR header (referred to as Address Field in [Q922]) in IETF notation is
FR标头在[Q922]中定义；然而，使用的符号与IETF规范中使用的符号不同。作为参考，IETF符号中的FR头（在[Q922]中称为地址字段）为
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hi dlci |C|0|lo dlci|F|B|D|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hi dlci |C|0|lo dlci|F|B|D|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Two-octet FR Header
双八位组FR报头
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hi dlci |C|0| dlci |F|B|D|0| dlci |0| dlci_lo |0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hi dlci |C|0| dlci |F|B|D|0| dlci |0| dlci_lo |0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Four-octet FR Header
四八位组FR报头
C/R (bit 6) FR frame C/R (command/response) bit [Q922].
C/R（第6位）FR帧C/R（命令/响应）位[Q922]。
F - FECN (bit 12): FR FECN (Forward Explicit Congestion Notification) bit [Q922].
F-FECN（位12）：FR FECN（前向显式拥塞通知）位[Q922]。
B - BECN (bit 13):
B-BECN（第13位）：
FR BECN (Backward Explicit Congestion Notification) bit [Q922].
FR BECN（向后显式拥塞通知）位[Q922]。
D - DE (bit 14) FR DE bit indicates the discard eligibility [Q922].
D-DE（位14）FR DE位表示丢弃资格[Q922]。
Usage of the C/R, FECN, BECN, and DE bits is as specified in [Q922].
C/R、FECN、BECN和DE位的使用如[Q922]所述。
The C/R bit is conveyed transparently. Its value MUST NOT be changed by the LCCE.
C/R位的传输是透明的。LCCE不得更改其值。
The FECN bit MAY be set by the LCCE to notify the receiving end-user that the frames it receives have encountered congestion. The end-user may use this indication for destination-controlled transmit rate adjustment. The bit must never be cleared by the LCCE. If the LCCE does not support FECN, it shall pass the bit unchanged.
可由LCCE设置FECN比特以通知接收终端用户其接收的帧已遇到拥塞。最终用户可将该指示用于目的地控制的传输速率调整。LCCE不得清除该位。如果LCCE不支持FECN，则其应通过位不变。
The BECN bit MAY be set by the LCCE to notify the receiving end-user that the frames it transmits may encounter congestion. The end-user may use this indication to adjust its transmit rate. The bit must never be cleared by the LCCE. If the LCCE does not support BECN, it shall pass the bit unchanged.
可由LCCE设置BECN比特以通知接收终端用户其发送的帧可能遇到拥塞。最终用户可使用该指示来调整其传输速率。LCCE不得清除该位。如果LCCE不支持BECN，则应不改变通过位。
The DE bit MAY be set by a policing function on the LCCE to indicate that this frame SHOULD be discarded in preference to other frames in a congestion situation. The bit must never be cleared by the LCCE. If the LCCE does not support DE, it shall pass the bit unchanged.
DE位可由LCCE上的监控功能设置，以指示在拥塞情况下应优先于其他帧丢弃该帧。LCCE不得清除该位。如果LCCE不支持DE，则应不改变通过位。
The encapsulation of Frame Relay frames with the two-octet FR Header is REQUIRED. The encapsulation of Frame Relay frames with the four-octet FR Header is OPTIONAL. The encapsulation of Frame Relay frames with the three-octet FR Header is outside the scope of this document.
需要使用两个八位组FR报头封装帧中继帧。具有四个八位字节FR报头的帧中继帧的封装是可选的。使用三个八位组FR报头封装帧中继帧不在本文档范围内。
Data Packet Sequencing MAY be enabled for FRPWs. The sequencing mechanisms described in [RFC3931] MUST be used for signalling sequencing support. FRPW over L2TP MUST request the presence of the L2TPv3 Default L2-Specific Sublayer when sequencing is enabled and MAY request its presence at all times.
可以为FRPWs启用数据包排序。[RFC3931]中描述的排序机制必须用于发送支持排序的信号。当序列被启用时，L2TP上的FRPW必须请求L2TPv3默认L2特定子层的存在，并且可以随时请求其存在。
If the FRPW is known to be carrying data that does not require packet order be strictly maintained (such as IP), then packet sequencing for the FRPW SHOULD NOT be enabled.
如果已知FRPW携带的数据不需要严格维护数据包顺序（如IP），则不应启用FRPW的数据包顺序。
With L2TPv3 as the tunneling protocol, the packet resulted from the encapsulation is N bytes longer than Frame Relay frame without the opening and closing HDLC flags or FCS. The value of N depends on the following fields:
在L2TPv3作为隧道协议的情况下，封装产生的数据包比帧中继帧长N字节，而不需要打开和关闭HDLC标志或FCS。N的值取决于以下字段：
L2TP Session Header: Flags, Ver, Res 4 octets (L2TPv3 over UDP only) Session ID 4 octets Cookie Size 0, 4, or 8 octets L2-Specific Sublayer 0 or 4 octets (i.e., with sequencing)
L2TP会话头：Flags、Ver、Res 4个八位字节（仅限UDP上的L2TPv3）会话ID 4个八位字节Cookie大小0、4或8个八位字节L2特定子层0或4个八位字节（即，具有序列）
Thus, the range for N in octets is:
因此，八位字节中N的范围为：
N = 4 - 16 L2TPv3 data messages are over IP
N = 16 - 28 L2TPv3 data messages are over UDP
(N does not include the IP header)
N = 4 - 16 L2TPv3 data messages are over IP
N = 16 - 28 L2TPv3 data messages are over UDP
(N does not include the IP header)
The MTU and fragmentation implications resulting from this are discussed in Section 4.1.4 of [RFC3931].
[RFC3931]第4.1.4节讨论了由此产生的MTU和碎片影响。
The Frame Relay PW emulation described in this document allows a service provider to offer a Frame Relay PVC-based service across an IP packet-switched network (PSN). A Frame Relay port-based service can be offered using [RFC4349].
本文档中描述的帧中继PW仿真允许服务提供商跨IP分组交换网络（PSN）提供基于帧中继PVC的服务。可以使用[RFC4349]提供基于帧中继端口的服务。
The FRPW emulation has the following characteristics in relationship to the native service:
FRPW仿真相对于本机服务具有以下特征：
o There is a one-to-one mapping between a Frame Relay PVC and an FRPW, supporting bi-directional transport of variable length frames. The Frame Relay frame is transported in its entirety, including the DLCI and the C/R, FECN, BECN, and DE bits, but excluding the opening and closing flags and the FCS. The egress LCCE re-writes the DLCI and regenerates the FCS.
o 帧中继PVC和FRPW之间存在一对一映射，支持可变长度帧的双向传输。帧中继帧整体传输，包括DLCI和C/R、FECN、BECN和DE位，但不包括打开和关闭标志以及FCS。出口LCCE重新写入DLCI并重新生成FCS。
o Two- and four-octet address fields are supported. The length is negotiated between LCCEs during session establishment (see Section 3.5).
o 支持两个和四个八位字节地址字段。在会话建立期间，LCCE之间协商长度（见第3.5节）。
o The availability or unavailability of the PVC is signalled between LCCEs using the Circuit Status AVP (see Section 3.4). Loss of connectivity between LCCEs can be detected by the L2TPv3 keepalive mechanism (see Section 4.4 in [RFC3931]). These indications can be used to determine the PVC status to be signalled through [Q933] procedures at the Frame Relay interface.
o PVC的可用性或不可用性通过电路状态AVP在LCCE之间发出信号（见第3.4节）。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）。这些指示可用于确定通过帧中继接口处的[Q933]程序发出信号的PVC状态。
o The maximum frame size that can be supported is limited by the PSN MTU, unless fragmentation and reassembly is used (see Section 4.1.4 of [RFC3931]).
o 可支持的最大机架尺寸受PSN MTU的限制，除非使用碎片和重新组装（见[RFC3931]第4.1.4节）。
o Sequencing may be enabled on the FRPW to ensure that frames are delivered in order (see Section 4.2).
o 可在FRPW上启用排序，以确保按顺序交付帧（见第4.2节）。
o Quality of Service characteristics, such as throughput (CIR), committed burst size (bc), excess burst size (be), and priority, can be provided by leveraging Quality of Service features of the LCCEs and the underlying PSN.
o 通过利用LCCE和底层PSN的服务质量特性，可以提供诸如吞吐量（CIR）、承诺突发大小（bc）、超额突发大小（be）和优先级等服务质量特征。
Frame Relay over L2TPv3 is subject to the security considerations defined in [RFC3931]. There are no additional considerations specific to carrying Frame Relay that are not present for carrying other data link types.
L2TPv3上的帧中继受[RFC3931]中定义的安全注意事项的约束。对于承载其他数据链路类型，不存在特定于承载帧中继的附加注意事项。
The following value for the Frame Relay DLCI PW Type (see Pseudowire Capabilities List, as defined in 5.4.3 of [RFC3931], and L2TPv3 Pseudowire Types in 10.6 of [RFC3931]) is allocated by the IANA (number space already created as part of publication of [RFC3931]):
IANA为帧中继DLCI PW类型分配以下值（参见[RFC3931]第5.4.3节中定义的伪线能力列表，以及[RFC3931]第10.6节中定义的L2TPv3伪线类型）（编号空间已作为[RFC3931]发布的一部分创建）：
L2TPv3 Pseudowire Types
-----------------------
L2TPv3 Pseudowire Types
-----------------------
0x0001: Frame Relay DLCI Pseudowire Type
0x0001：帧中继DLCI伪线类型
This number space is managed by IANA as described in Section 2.3 of [RFC3438]. Three new L2TP Result Codes for the CDN message appear in Section 3.2. The following is a summary:
该数字空间由IANA管理，如[RFC3438]第2.3节所述。CDN消息的三个新L2TP结果代码出现在第3.2节中。以下是总结：
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
17: PVC was deleted permanently (no longer provisioned) 18: PVC has been INACTIVE for an extended period of time 19: Mismatched FR Header Length
17:PVC被永久删除（不再配置）18:PVC已长时间处于非活动状态19:FR标头长度不匹配
This number space is managed by IANA as described in Section 2.2 of [RFC3438]. An additional AVP Attribute, specified in Section 3.5, was allocated for this specification:
该数字空间由IANA管理，如[RFC3438]第2.2节所述。为本规范分配了第3.5节规定的附加AVP属性：
Control Message Attribute Value Pairs
-------------------------------------
Control Message Attribute Value Pairs
-------------------------------------
85: Frame Relay Header Length
85：帧中继头长度
The first Frame Relay over L2TP document, "Frame Relay Service Type for L2TP", was published in February of 2001, by Nishit Vasavada, Jim Boyle, Chris Garner, Serge Maskalik, and Vijay Gill. This document is substantially different, but the basic concept of carrying Frame Relay over L2TP is the same.
Ishit Vasavada、Jim Boyle、Chris Garner、Serge Maskalik和Vijay Gill于2001年2月发布了L2TP上的第一个帧中继文件“L2TP的帧中继服务类型”。本文件实质上不同，但在L2TP上承载帧中继的基本概念是相同的。
Thanks to Lloyd Wood for a razor-sharp review.
感谢劳埃德·伍德的锐利评论。
Carlos Pignataro helped with review and editing of the document.
卡洛斯·皮格纳塔罗帮助审查和编辑了该文件。
During IETF Last Call, Mark Lewis provided thorough review and comments.
在IETF的最后一次通话中，Mark Lewis提供了全面的审查和评论。
[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。
[RFC4349] Pignataro, C. and M. Townsley, "High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)", RFC 4349, February 2006.
[RFC4349]Pignataro，C.和M.Townsley，“第2层隧道协议上的高级数据链路控制（HDLC）帧，第3版（L2TPv3）”，RFC 4349，2006年2月。
[RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update", BCP 68, RFC 3438, December 2002.
[RFC3438]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。
[Q922] ITU-T Recommendation Q.922, "ISDN Data Link Layer Specification for Frame Mode Bearer Services", ITU, Geneva, 1992.
[Q922]ITU-T建议Q.922，“帧模式承载业务的ISDN数据链路层规范”，ITU，日内瓦，1992年。
[Q933] ITU-T Recommendation Q.933, "Signalling specifications for frame mode switched and permanent virtual connection control and status monitoring", ITU, Geneva, 2003.
[Q933]ITU-T建议Q.933，“帧模式交换和永久虚拟连接控制和状态监测的信令规范”，ITU，日内瓦，2003年。
Authors' Addresses
作者地址
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
George Wilkie Cisco Systems 96 Commercial Street Edinburgh, EH6 6LX United Kingdom
英国爱丁堡商业街96号George Wilkie Cisco Systems EH6 6LX
EMail: gwilkie@cisco.com
EMail: gwilkie@cisco.com
Skip Booth Cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709
跳过展位思科系统7025 Kit Creek路邮政信箱14987北卡罗来纳州三角研究公园27709
EMail: ebooth@cisco.com
EMail: ebooth@cisco.com
Stewart Bryant Cisco Systems 250 Longwater Ave Green Park Reading RG2 6GB United Kingdom
Stewart Bryant Cisco Systems 250 Longwater Ave Green Park Reading RG2 6GB英国
EMail: stbryant@cisco.com
EMail: stbryant@cisco.com
Jed Lau
刘杰德
EMail: jedlau@gmail.com
EMail: jedlau@gmail.com
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确认
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