Network Working Group B. Thomas
Request for Comments: 3037 Cisco Systems, Inc.
Category: Informational E. Gray
Zaffire, Inc.
January 2001
Network Working Group B. Thomas
Request for Comments: 3037 Cisco Systems, Inc.
Category: Informational E. Gray
Zaffire, Inc.
January 2001
LDP Applicability
LDP适用性
Status of this Memo
本备忘录的状况
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
本备忘录为互联网社区提供信息。它没有规定任何类型的互联网标准。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The Internet Society (2001). All Rights Reserved.
版权所有（C）互联网协会（2001年）。版权所有。
Abstract
摘要
Multiprotocol Label Switching (MPLS) is a method for forwarding packets that uses short, fixed-length values carried by packets, called labels, to determine packet nexthops. A fundamental concept in MPLS is that two Label Switching Routers (LSRs) must agree on the meaning of the labels used to forward traffic between and through them. This common understanding is achieved by using a set of procedures, called a label distribution protocol, by which one LSR informs another of label bindings it has made. This document describes the applicability of a set of such procedures called LDP (for Label Distribution Protocol) by which LSRs distribute labels to support MPLS forwarding along normally routed paths.
多协议标签交换（MPLS）是一种转发数据包的方法，它使用数据包（称为标签）携带的短、固定长度值来确定数据包的下一个路径。MPLS中的一个基本概念是，两个标签交换路由器（LSR）必须就用于在它们之间转发流量的标签的含义达成一致。这一共识是通过使用一组称为标签分发协议的过程实现的，通过该协议，一个LSR通知另一个LSR它已进行的标签绑定。本文档描述了一组称为LDP（用于标签分发协议）的程序的适用性，LSR通过该程序分发标签以支持MPLS沿正常路由路径转发。
A label distribution protocol is a set of procedures by which one Label Switching Router (LSR) informs another of the meaning of labels used to forward traffic between and through them.
标签分发协议是一组过程，通过这些过程，一个标签交换路由器（LSR）通知另一个标签的含义，标签用于在它们之间转发通信量。
The MPLS architecture allows for the possibility of more than a single method for distributing labels, and a number of different label distribution protocols are being standardized. Existing protocols have been extended so that label distribution can be piggybacked on them, and new protocols have been defined for the explicit purpose of distributing labels.
MPLS体系结构允许使用多种方法分发标签，许多不同的标签分发协议正在标准化。现有的协议已经被扩展，以便标签分发可以在它们的基础上进行，并且新的协议已经被定义为明确的分发标签的目的。
This document describes the applicability of the Label Distribution Protocol (LDP), a new protocol for label distribution designed to support label distribution for MPLS forwarding along normally routed paths as determined by destination-based routing protocols. This is sometimes called MPLS hop-by-hop forwarding.
本文档描述了标签分发协议（LDP）的适用性，LDP是一种新的标签分发协议，旨在支持MPLS沿着基于目的地的路由协议确定的正常路由路径转发的标签分发。这有时称为MPLS逐跳转发。
LDP, together with an IP routing plane and software to program ATM switch or Frame Relay switch cross-connect tables, can implement IP in a network of ATM and/or Frame Relay switches without requiring an overlay or the use of ATM-specific or Frame Relay-specific addressing or routing.
LDP与IP路由平面和用于编程ATM交换机或帧中继交换机交叉连接表的软件一起，可以在ATM和/或帧中继交换机网络中实现IP，而无需覆盖或使用特定于ATM或特定于帧中继的寻址或路由。
LDP is also useful in situations that require efficient hop-by-hop routed tunnels, such as MPLS-based VPN architectures [RFC2574] and tunneling between BGP border routers.
LDP在需要高效逐跳路由隧道的情况下也很有用，例如基于MPLS的VPN架构[RFC2574]和BGP边界路由器之间的隧道。
In addition, LDP includes a mechanism that makes it possible to extend it to support MPLS features that go beyond best effort hop-by-hop forwarding.
此外，LDP还包括一种机制，可以对其进行扩展，以支持MPLS功能，而不仅仅是尽力而为的逐跳转发。
As a stand-alone protocol for distributing labels LDP does not rely on the presence of specific routing protocols at every hop along an LSP path in order to establish an LSP. Hence LDP is useful in situations in which an LSP must traverse nodes which may not all support a common piggybacked approach to distributing labels.
作为用于分发标签的独立协议，LDP不依赖于在LSP路径上的每个跳上存在特定路由协议来建立LSP。因此，LDP在LSP必须遍历节点的情况下非常有用，这些节点可能不都支持一种通用的背负式方法来分发标签。
Traffic Engineering [TE] is expected to be an important MPLS application. MPLS support for Traffic Engineering uses explicitly routed LSPs, which need not follow normally-routed (hop-by-hop) paths.
流量工程（TE）有望成为MPLS的一个重要应用。MPLS对流量工程的支持使用显式路由LSP，它不需要遵循正常路由（逐跳）路径。
Explicitly routed LSPs may be setup by CR-LDP [CRLDP-AS], a set of extensions to LDP, or by RSVP-TE [RSVP-TE-AS], a set of extensions to RSVP. There is currently no consensus on which of these protocols is technically superior. Therefore, network administrators should make a choice between the two based upon their needs and particular situation.
显式路由LSP可以由LDP的一组扩展CR-LDP[CRLDP-AS]或RSVP的一组扩展RSVP-TE[RSVP-TE-AS]设置。目前还没有就这些协议中哪一个在技术上更优越达成共识。因此，网络管理员应该根据他们的需要和具体情况在两者之间做出选择。
The "requirement level" [RFC2026] for LDP is:
LDP的“要求级别”[RFC2026]为：
Implementation of LDP is recommended for devices that perform MPLS forwarding along normally routed paths as determined by destination-based routing protocols.
建议对沿基于目的地的路由协议确定的正常路由路径执行MPLS转发的设备实施LDP。
LDP associates a Forwarding Equivalence Class (FEC) [RFC3031] with each label it distributes. Two LSRs which use LDP to exchange FEC-label binding information are known as "LDP Peers", and we speak of there being an "LDP Session" between them.
LDP将转发等价类（FEC）[RFC3031]与其分发的每个标签相关联。使用LDP交换FEC标签绑定信息的两个LSR被称为“LDP对等点”，我们称之为它们之间的“LDP会话”。
LDP uses TCP for session communication. Use of TCP ensures that session messages are reliably delivered, and that distributed labels and state information associated with LSPs need not be refreshed periodically.
LDP使用TCP进行会话通信。TCP的使用确保了会话消息的可靠传递，并且与LSP关联的分布式标签和状态信息不需要定期刷新。
LDP includes a mechanism by which an LSR can discover potential LDP peers. The discovery mechanism makes it unnecessary for operators to explicitly configure each LSR with its LDP peers.
LDP包括LSR可以发现潜在LDP对等点的机制。发现机制使得运营商无需显式配置每个LSR及其LDP对等点。
When an LSR discovers another LSR it follows the LDP session setup procedure to establish an LDP session. By means of this procedure the LSRs establish a session TCP connection and use it to negotiate parameters for the session, such as the label distribution method to be used (see below). After the LSRs agree on the parameters, the session is operational and the LSRs use the TCP connection for label distribution.
当LSR发现另一个LSR时，它遵循LDP会话设置过程来建立LDP会话。通过此过程，LSR建立会话TCP连接，并使用它协商会话参数，如要使用的标签分发方法（见下文）。LSR在参数上达成一致后，会话即可运行，LSR使用TCP连接进行标签分发。
LDP supports two different methods for label distribution. An LSR using Downstream Unsolicited distribution advertises FEC-label bindings to its peers when it is ready to forward packets in the FEC by means of MPLS. An LSR using Downstream on Demand distribution provides FEC-label bindings to a peer in response to specific requests from the peer for a label for the FEC.
LDP支持两种不同的标签分发方法。当LSR准备通过MPLS转发FEC中的数据包时，使用下游非请求分发的LSR向其对等方播发FEC标签绑定。使用下游按需分发的LSR向对等方提供FEC标签绑定，以响应对等方对FEC标签的特定请求。
LDP allows LSRs flexibility in strategies for retaining learned labels. An LSR using liberal label retention stores all labels learned from peers regardless of whether it currently needs them for forwarding, whereas one using conservative label retention stores only labels for which it has immediate use and releases unneeded labels to the peer that advertised them.
LDP允许LSR在保留学习标签的策略上具有灵活性。使用自由标签保留的LSR存储从对等方学习到的所有标签，而不管它当前是否需要它们进行转发，而使用保守标签保留的LSR只存储它可以立即使用的标签，并将不需要的标签发布给发布它们的对等方。
In addition, LDP allows flexibility in strategies for when to advertise FEC-label bindings. An LSR using independent control mode advertises FEC-label bindings to peers whenever it sees fit, whereas one using ordered control advertises bindings only when it has previously received a label for the FEC from the FEC nexthop or it is an MPLS egress for the FEC.
此外，LDP允许在何时公布FEC标签绑定的策略上具有灵活性。使用独立控制模式的LSR在其认为合适时向对等方播发FEC标签绑定，而使用有序控制的LSR仅在其先前从FEC nexthop接收到FEC标签或是FEC的MPLS出口时才播发绑定。
Downstream on Demand distribution with conservative label retention and ordered control is appropriate in situations where labels are a relatively scarce resource that must be conserved, and Downstream
在标签是必须保存的相对稀缺资源的情况下，采用保守的标签保留和有序控制的下游按需分配是合适的，并且
Unsolicited distribution with liberal label retention and independent control is appropriate where labels are plentiful and need not be carefully conserved. However, the protocol permits other combinations of distribution method, label retention mode and control mode, including hybrid variants of them.
在标签丰富且无需仔细保存的情况下，采用自由保留标签和独立控制的主动分发是合适的。然而，该协议允许分配方法、标签保留模式和控制模式的其他组合，包括它们的混合变体。
LDP defines a mechanism for loop detection to protect against forwarding loops in LSPs that traverse non-TTL MPLS clouds; see [RFC3031] for discussion of situations which may benefit from this mechanism. The loop detection mechanism is optional in the sense that it may be disabled by LSR configuration. However, an LDP-compliant LSR must implement it.
LDP定义了循环检测机制，以防止LSP中穿越非TTL MPLS云的转发循环；请参见[RFC3031]了解可能从该机制中受益的情况的讨论。循环检测机制是可选的，因为它可能被LSR配置禁用。然而，符合LDP的LSR必须实现它。
LDP includes an extension mechanism which supports the development of vendor-private and experimental features. This mechanism defines procedures for introducing new types of messages and TLVs, methods an LSR can use for detecting such messages and TLVs, and procedures an LSR must follow when it receives a message or TLV it does not implement. While it is not possible to make every future enhancement backwards compatible, these procedures facilitate the introduction of new capabilities in MPLS networks that include older implementations that do not recognize them.
LDP包括一个扩展机制，支持开发供应商私有和实验特性。该机制定义了引入新类型消息和TLV的过程、LSR可用于检测此类消息和TLV的方法，以及LSR在接收到未实现的消息或TLV时必须遵循的过程。虽然不可能使未来的每个增强都向后兼容，但这些过程有助于在MPLS网络中引入新功能，这些新功能包括不识别它们的旧实现。
The following factors may influence the scalability of LDP implementations:
以下因素可能会影响LDP实现的可伸缩性：
- LDP label distribution is incremental, requiring no periodic refresh of FEC-label bindings.
- LDP标签分发是增量的，不需要定期刷新FEC标签绑定。
- In situations were label resources may be scarce (ATM and Frame Relay links) the use of the Downstream on Demand distribution method with conservative label retention ensures that only those labels required to support normally-routed paths are allocated and distributed.
- 在标签资源可能稀缺的情况下（ATM和帧中继链路），使用具有保守标签保留的下游按需分配方法可确保仅分配和分配支持正常路由路径所需的标签。
- In situations where label resources are not scarce, the use of the Downstream Unsolicited method with liberal label retention ensures that changes in FEC nexthop from one LDP peer to another require no distribution action to update previously distributed labels.
- 在标签资源不稀缺的情况下，使用具有自由标签保留的下游非请求方法可确保FEC nexthop从一个LDP对等点到另一个LDP对等点的更改不需要分发操作来更新先前分发的标签。
- Limitations on the number of TCP connections an LSR supports limit the number of LDP peers the LSR can support.
- 对LSR支持的TCP连接数量的限制限制限制了LSR可以支持的LDP对等点的数量。
- Use of the optional path vector based loop detection mechanism imposes additional memory and processing requirements on an LSR, as well as additional LDP traffic. Both impact scalability.
- 基于可选路径向量的环路检测机制的使用对LSR以及额外LDP通信量施加了额外的内存和处理要求。两者都影响可伸缩性。
LDP defines the optional use of the TCP MD5 Signature Option to protect against the introduction of spoofed TCP segments into LDP session connection streams. LDP use of the TCP MD5 Signature Option is similar to BGP [RFC1771] use of the option specified in [RFC2385].
LDP定义了TCP MD5签名选项的可选使用，以防止将伪造的TCP段引入LDP会话连接流。LDP使用TCP MD5签名选项与BGP[RFC1771]使用[RFC2385]中指定的选项类似。
[CRLDP-AS] J. Ash, M. Girish, E. Gray, B. Jamoussi, G. Wright, "Applicability Statement for CR-LDP", Work in Progress, September 1999.
[CRLDP-AS]J.Ash，M.Girish，E.Gray，B.Jamoussi，G.Wright，“CR-LDP的适用性声明”，正在进行的工作，1999年9月。
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995.
[RFC1771]Rekhter，Y.和T.Li，“边境网关协议4（BGP-4）”，RFC 17711995年3月。
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996.
[RFC2026]Bradner，S.，“互联网标准过程——第3版”，BCP 9，RFC 2026，1996年10月。
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.
[RFC2385]Heffernan，A.，“通过TCP MD5签名选项保护BGP会话”，RFC 2385，1998年8月。
[RFC2547] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March 1999.
[RFC2547]Rosen，E.和Y.Rekhter，“BGP/MPLS VPN”，RFC 2547，1999年3月。
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B. Thomas, "LDP Specification", RFC 3036, January 2001.
[RFC3036]Andersson，L.，Doolan，P.，Feldman，N.，Fredette，A.和B.Thomas，“LDP规范”，RFC 3036，2001年1月。
[RFC3031] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, January 2001.
[RFC3031]Rosen，E.，Viswanathan，A.和R.Callon，“多协议标签交换体系结构”，RFC 30312001年1月。
[RSVP-TE-AS] Awduche, D., Hannan, A. and X. Xiao, "Applicability State for Extensions to RSVP for LSP-Tunnels", Work in Progress, April 2000.
[RSVP-TE-AS]Awduche，D.，Hannan，A.和X.Xiao，“LSP隧道RSVP扩展的适用性状态”，正在进行的工作，2000年4月。
Eric Gray Zaffire, Inc 2630 Orchard Parkway, San Jose, CA 95134-2020
埃里克·格雷·扎菲尔公司，加利福尼亚州圣何塞市乌节大道2630号，邮编95134-2020
Phone: 408-894-7362 EMail: ewgray@mindspring.com
电话：408-894-7362电子邮件：ewgray@mindspring.com
Bob Thomas Cisco Systems, Inc. 250 Apollo Dr. Chelmsford, MA 01824
鲍勃·托马斯·思科系统公司250阿波罗·切姆斯福德博士，马萨诸塞州01824
Phone: 978-244-8078 EMail: rhthomas@cisco.com
电话：978-244-8078电子邮件：rhthomas@cisco.com
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