Network Working Group J.-L. Le Roux, Ed.
Request for Comments: 4105 France Telecom
Category: Informational J.-P. Vasseur, Ed.
Cisco Systems, Inc.
J. Boyle, Ed.
PDNETs
June 2005
Network Working Group J.-L. Le Roux, Ed.
Request for Comments: 4105 France Telecom
Category: Informational J.-P. Vasseur, Ed.
Cisco Systems, Inc.
J. Boyle, Ed.
PDNETs
June 2005
Requirements for Inter-Area MPLS Traffic Engineering
区域间MPLS流量工程的要求
Status of This Memo
关于下段备忘
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 (2005).
版权所有（C）互联网协会（2005年）。
Abstract
摘要
This document lists a detailed set of functional requirements for the support of inter-area MPLS Traffic Engineering (inter-area MPLS TE). It is intended that solutions that specify procedures and protocol extensions for inter-area MPLS TE satisfy these requirements.
本文件列出了支持区域间MPLS流量工程（区域间MPLS TE）的一组详细功能需求。旨在为区域间MPLS TE指定过程和协议扩展的解决方案满足这些要求。
Table of Contents
目录
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Terminology .....................................................3
4. Current Intra-Area Uses of MPLS Traffic Engineering .............4
4.1. Intra-Area MPLS Traffic Engineering Architecture ...........4
4.2. Intra-Area MPLS Traffic Engineering Applications ...........4
4.2.1. Intra-Area Resource Optimization ....................4
4.2.2. Intra-Area QoS Guarantees ...........................5
4.2.3. Fast Recovery within an IGP Area ....................5
4.3. Intra-Area MPLS TE and Routing .............................6
5. Problem Statement, Requirements, and Objectives of Inter-Area ...6
5.1. Inter-Area Traffic Engineering Problem Statement ...........6
5.2. Overview of Requirements for Inter-Area MPLS TE ............7
5.3. Key Objectives for an Inter-Area MPLS-TE Solution ..........8
5.3.1. Preserving the IGP Hierarchy Concept ................8
5.3.2. Preserving Scalability ..............................8
6. Application Scenario.............................................9
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Terminology .....................................................3
4. Current Intra-Area Uses of MPLS Traffic Engineering .............4
4.1. Intra-Area MPLS Traffic Engineering Architecture ...........4
4.2. Intra-Area MPLS Traffic Engineering Applications ...........4
4.2.1. Intra-Area Resource Optimization ....................4
4.2.2. Intra-Area QoS Guarantees ...........................5
4.2.3. Fast Recovery within an IGP Area ....................5
4.3. Intra-Area MPLS TE and Routing .............................6
5. Problem Statement, Requirements, and Objectives of Inter-Area ...6
5.1. Inter-Area Traffic Engineering Problem Statement ...........6
5.2. Overview of Requirements for Inter-Area MPLS TE ............7
5.3. Key Objectives for an Inter-Area MPLS-TE Solution ..........8
5.3.1. Preserving the IGP Hierarchy Concept ................8
5.3.2. Preserving Scalability ..............................8
6. Application Scenario.............................................9
7. Detailed Requirements for Inter-Area MPLS TE ...................10
7.1. Inter-Area MPLS TE Operations and Interoperability ........10
7.2. Inter-Area TE-LSP Signaling ...............................10
7.3. Path Optimality ...........................................11
7.4. Inter-Area MPLS-TE Routing ................................11
7.5. Inter-Area MPLS-TE Path Computation .......................12
7.6. Inter-Area Crankback Routing ..............................12
7.7. Support of Diversely-Routed Inter-Area TE LSPs ............13
7.8. Intra/Inter-Area Path Selection Policy ....................13
7.9. Reoptimization of Inter-Area TE LSP .......................13
7.10. Inter-Area LSP Recovery ..................................14
7.10.1. Rerouting of Inter-Area TE LSPs ..................14
7.10.2. Fast Recovery of Inter-Area TE LSP ...............14
7.11. DS-TE support ............................................15
7.12. Hierarchical LSP Support .................................15
7.13. Hard/Soft Preemption .....................................15
7.14. Auto-Discovery of TE Meshes ..............................16
7.15. Inter-Area MPLS TE Fault Management Requirements .........16
7.16. Inter-Area MPLS TE and Routing ...........................16
8. Evaluation criteria ............................................17
8.1. Performances ..............................................17
8.2. Complexity and Risks ......................................17
8.3. Backward Compatibility ....................................17
9. Security Considerations ........................................17
10. Acknowledgements ..............................................17
11. Contributing Authors ..........................................18
12. Normative References ..........................................19
13. Informative References ........................................19
7. Detailed Requirements for Inter-Area MPLS TE ...................10
7.1. Inter-Area MPLS TE Operations and Interoperability ........10
7.2. Inter-Area TE-LSP Signaling ...............................10
7.3. Path Optimality ...........................................11
7.4. Inter-Area MPLS-TE Routing ................................11
7.5. Inter-Area MPLS-TE Path Computation .......................12
7.6. Inter-Area Crankback Routing ..............................12
7.7. Support of Diversely-Routed Inter-Area TE LSPs ............13
7.8. Intra/Inter-Area Path Selection Policy ....................13
7.9. Reoptimization of Inter-Area TE LSP .......................13
7.10. Inter-Area LSP Recovery ..................................14
7.10.1. Rerouting of Inter-Area TE LSPs ..................14
7.10.2. Fast Recovery of Inter-Area TE LSP ...............14
7.11. DS-TE support ............................................15
7.12. Hierarchical LSP Support .................................15
7.13. Hard/Soft Preemption .....................................15
7.14. Auto-Discovery of TE Meshes ..............................16
7.15. Inter-Area MPLS TE Fault Management Requirements .........16
7.16. Inter-Area MPLS TE and Routing ...........................16
8. Evaluation criteria ............................................17
8.1. Performances ..............................................17
8.2. Complexity and Risks ......................................17
8.3. Backward Compatibility ....................................17
9. Security Considerations ........................................17
10. Acknowledgements ..............................................17
11. Contributing Authors ..........................................18
12. Normative References ..........................................19
13. Informative References ........................................19
The set of MPLS Traffic Engineering components, defined in [RSVP-TE], [OSPF-TE], and [ISIS-TE], which supports the requirements defined in [TE-REQ], is used today by many network operators to achieve major Traffic Engineering objectives defined in [TE-OVW]. These objectives include:
[RSVP-TE]、[OSPF-TE]和[ISIS-TE]中定义的MPLS流量工程组件集支持[TE-REQ]中定义的要求，目前许多网络运营商使用该组件来实现[TE-OVW]中定义的主要流量工程目标。这些目标包括：
- Aggregated Traffic measurement - Optimization of network resources utilization - Support for services requiring end-to-end QoS guarantees - Fast recovery against link/node/Shared Risk Link Group (SRLG) failures
- 聚合流量测量-优化网络资源利用率-支持需要端到端QoS保证的服务-针对链路/节点/共享风险链路组（SRLG）故障的快速恢复
Furthermore, the applicability of MPLS to traffic engineering in IP networks is discussed in [TE-APP].
此外，在[TE-APP]中讨论了MPLS在IP网络流量工程中的适用性。
The set of MPLS Traffic Engineering mechanisms, to date, has been limited to use within a single Interior Gateway Protocol (IGP) area.
迄今为止，MPLS流量工程机制集仅限于在单个内部网关协议（IGP）区域内使用。
This document discusses the requirements for an inter-area MPLS Traffic Engineering mechanism that may be used to achieve the same set of objectives across multiple IGP areas.
本文件讨论了区域间MPLS流量工程机制的要求，该机制可用于跨多个IGP区域实现相同的目标集。
Basically, it would be useful to extend MPLS TE capabilities across IGP areas to support inter-area resources optimization, to provide strict QoS guarantees between two edge routers located within distinct areas, and to protect inter-area traffic against Area Border Router (ABR) failures.
基本上，将MPLS TE能力扩展到IGP区域以支持区域间资源优化、在位于不同区域内的两个边缘路由器之间提供严格的QoS保证以及保护区域间流量不受区域边界路由器（ABR）故障的影响是有用的。
First, this document addresses current uses of MPLS Traffic Engineering within a single IGP area. Then, it discusses a set of functional requirements that a solution must or should satisfy in order to support inter-area MPLS Traffic Engineering. Because the scope of requirements will vary between operators, some requirements will be mandatory (MUST), whereas others will be optional (SHOULD). Finally, a set of evaluation criteria for any solution meeting these requirements is given.
首先，本文档介绍了单个IGP区域内MPLS流量工程的当前用途。然后，讨论了解决方案必须或应该满足的一组功能需求，以支持区域间MPLS流量工程。由于要求的范围因运营商而异，一些要求是强制性的（必须），而其他要求是可选的（应该）。最后，给出了满足这些要求的任何解决方案的一组评估标准。
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].
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。
LSR: Label Switching Router
标签交换路由器
LSP: Label Switched Path
标签交换路径
TE LSP: Traffic Engineering Label Switched Path
TE LSP：流量工程标签交换路径
Inter-area TE LSP: TE LSP whose head-end LSR and tail-end LSR do not reside within the same IGP area or whose head-end LSR and tail-end LSR are both in the same IGP area although the TE-LSP transiting path is across different IGP areas.
区域间TE LSP：其前端LSR和后端LSR不在同一IGP区域内的TE LSP，或其前端LSR和后端LSR都在同一IGP区域内的TE LSP，尽管TE-LSP过渡路径跨越不同的IGP区域。
IGP area: OSPF area or IS-IS level.
IGP区域：OSPF区域或IS-IS级别。
ABR: Area Border Router, a router used to connect two IGP areas (ABR in OSPF, or L1/L2 router in IS-IS).
ABR：区域边界路由器，用于连接两个IGP区域的路由器（OSPF中的ABR，或IS-IS中的L1/L2路由器）。
CSPF: Constraint-based Shortest Path First.
CSPF：基于约束的最短路径优先。
SRLG: Shared Risk Link Group.
SRLG：共享风险链接组。
This section addresses architecture, capabilities, and uses of MPLS TE within a single IGP area. It first summarizes the current MPLS-TE architecture, then addresses various MPLS-TE capabilities, and finally lists various approaches to integrate MPLS TE into routing. This section is intended to help define the requirements for MPLS-TE extensions across multiple IGP areas.
本节介绍单个IGP区域内MPLS TE的体系结构、功能和使用。它首先总结了当前的MPLS-TE体系结构，然后介绍了各种MPLS-TE功能，最后列出了将MPLS-TE集成到路由中的各种方法。本节旨在帮助定义跨多个IGP区域的MPLS-TE扩展的要求。
The MPLS-TE control plane allows establishing explicitly routed MPLS LSPs whose paths follow a set of TE constraints. It is used to achieve major TE objectives such as resource usage optimization, QoS guarantee and fast failure recovery. It consists of three main components:
MPLS-TE控制平面允许建立显式路由的MPLS LSP，其路径遵循一组TE约束。它用于实现资源使用优化、QoS保证和快速故障恢复等主要TE目标。它由三个主要部分组成：
- The routing component, responsible for the discovery of the TE topology. This is ensured thanks to extensions of link state IGP: [ISIS-TE], [OSPF-TE]. - The path computation component, responsible for the placement of the LSP. It is performed on the head-end LSR thanks to a CSPF algorithm, which takes TE topology and LSP constraints as input. - The signaling component, responsible for the establishment of the LSP (explicit routing, label distribution, and resources reservation) along the computed path. This is ensured thanks to RSVP-TE [RSVP-TE].
- 路由组件，负责发现TE拓扑。这得益于链路状态IGP的扩展：[ISIS-TE]，[OSPF-TE]。-路径计算组件，负责LSP的放置。由于采用了CSPF算法，该算法将TE拓扑和LSP约束作为输入，因此可在前端LSR上执行信令组件，负责沿计算路径建立LSP（显式路由、标签分发和资源保留）。由于RSVP-TE[RSVP-TE]，这一点得以确保。
MPLS TE can be used within an area to redirect paths of aggregated flows away from over-utilized resources within a network. In a small scale, this may be done by explicitly configuring a path to be used between two routers. On a grander scale, a mesh of LSPs can be established between central points in a network. LSPs paths can be defined statically in configuration or arrived at by an algorithm that determines the shortest path given administrative constraints such as bandwidth. In this way, MPLS TE allows for greater control over how traffic demands are routed over a network topology and utilize a network's resources.
MPLS TE可在区域内用于重定向聚合流的路径，使其远离网络内过度利用的资源。在小范围内，这可以通过显式配置两个路由器之间使用的路径来实现。在更大的范围内，可以在网络的中心点之间建立LSP网格。LSP路径可以在配置中静态定义，也可以通过在给定管理约束（如带宽）的情况下确定最短路径的算法来实现。通过这种方式，MPLS TE允许更好地控制流量需求如何在网络拓扑上路由并利用网络资源。
Note also that TE LSPs allow measuring traffic matrix in a simple and scalable manner. The aggregated traffic rate between two LSRs is easily measured by accounting of traffic sent onto a TE LSP provisioned between the two LSRs in question.
还请注意，TE LSP允许以简单且可扩展的方式测量流量矩阵。两个LSR之间的聚合流量率很容易通过计算发送到两个LSR之间供应的TE LSP上的流量来测量。
The DiffServ IETF working group has defined a set of mechanisms described in [DIFF-ARCH], [DIFF-AF], and [DIFF-EF] or [MPLS-DIFF], that can be activated at the edge of or over a DiffServ domain to contribute to the enforcement of a QoS policy (or set of policies), which can be expressed in terms of maximum one-way transit delay, inter-packet delay variation, loss rate, etc. Many Operators have some or full deployment of DiffServ implementations in their networks today, either across the entire network or at least at its edge.
DiffServ IETF工作组定义了[DIFF-ARCH]、[DIFF-AF]和[DIFF-EF]或[MPLS-DIFF]中描述的一组机制，这些机制可在DiffServ域的边缘或之上激活，以促进QoS策略（或一组策略）的实施，QoS策略可以用最大单向传输延迟表示，数据包间延迟变化、丢失率等。如今，许多运营商在其网络中部署了部分或全部DiffServ实现，无论是在整个网络中，还是至少在其边缘。
In situations where strict QoS bounds are required, admission control inside the backbone of a network is in some cases required in addition to current DiffServ mechanisms. When the propagation delay can be bounded, the performance targets, such as maximum one-way transit delay, may be guaranteed by providing bandwidth guarantees along the DiffServ-enabled path.
在需要严格QoS边界的情况下，除了当前的DiffServ机制之外，在某些情况下还需要网络主干内的准入控制。当传播延迟可以有界时，可以通过沿启用区分服务的路径提供带宽保证来保证性能目标，例如最大单向传输延迟。
MPLS TE can be simply used with DiffServ: in that case, it only ensures aggregate QoS guarantees for the whole traffic. It can also be more intimately combined with DiffServ to perform per-class of service admission control and resource reservation. This requires extensions to MPLS TE called DiffServ-Aware TE, which are defined in [DSTE-PROTO]. DS-TE allows ensuring strict end-to-end QoS guarantees. For instance, an EF DS-TE LSP may be provisioned between voice gateways within the same area to ensure strict QoS to VoIP traffic.
MPLS TE可以简单地与DiffServ一起使用：在这种情况下，它只能确保整个流量的聚合QoS保证。它还可以与DiffServ更紧密地结合，以执行每类服务的准入控制和资源预留。这需要对MPLS TE进行扩展，称为区分服务感知TE，定义见[DSTE-PROTO]。DS-TE允许确保严格的端到端QoS保证。例如，可以在同一区域内的语音网关之间提供EF DS-TE LSP，以确保对VoIP业务的严格QoS。
MPLS TE allows computing intra-area shortest paths, which satisfy various constraints, including bandwidth. For the sake of illustration, if the IGP metrics reflects the propagation delay, it allows finding a minimum propagation delay path, which satisfies various constraints, such as bandwidth.
MPLS TE允许计算满足各种约束条件（包括带宽）的区域内最短路径。为了说明，如果IGP度量反映了传播延迟，则它允许找到满足各种约束（例如带宽）的最小传播延迟路径。
As quality-sensitive applications are deployed, one of the key requirements is to provide fast recovery mechanisms, allowing traffic recovery to be guaranteed on the order of tens of msecs, in case of network element failure. Note that this cannot be achieved by relying only on classical IGP rerouting.
在部署对质量敏感的应用程序时，关键要求之一是提供快速恢复机制，允许在网元故障的情况下保证流量恢复在数十毫秒左右。请注意，仅依靠经典的IGP重路由无法实现这一点。
Various recovery mechanisms can be used to protect traffic carried onto TE LSPs. They are defined in [MPLS-RECOV]. Protection mechanisms are based on the provisioning of backup LSPs that are used to recover traffic in case of failure of protected LSPs. Among those protection mechanisms, local protection (also called Fast Reroute) is intended to achieve sub-50ms recovery in case of link/node/SRLG
各种恢复机制可用于保护传输到TE LSP上的流量。它们在[MPLS-RECOV]中定义。保护机制基于备份LSP的配置，这些LSP用于在受保护LSP发生故障时恢复通信量。在这些保护机制中，本地保护（也称为快速重路由）旨在在链路/节点/SRLG的情况下实现低于50ms的恢复
failure along the LSP path [FAST-REROUTE]. Fast Reroute is currently used by many operators to protect sensitive traffic inside an IGP area.
LSP路径故障[快速重路由]。快速重路由目前被许多运营商用于保护IGP区域内的敏感流量。
[FAST-REROUTE] defines two modes for backup LSPs. The first, called one-to-one backup, consists of setting up one detour LSP per protected LSP and per element to protect. The second, called facility backup, consists of setting up one or several bypass LSPs to protect a given facility (link or node). In case of failure, all protected LSPs are nested into the bypass LSPs (benefiting from the MPLS label stacking property).
[FAST-REROUTE]为备份LSP定义了两种模式。第一种称为一对一备份，包括为每个受保护的LSP和每个要保护的元素设置一个迂回LSP。第二种称为设施备份，包括设置一个或多个旁路LSP以保护给定的设施（链路或节点）。如果出现故障，所有受保护的LSP都嵌套到旁路LSP中（受益于MPLS标签堆叠属性）。
There are several possibilities for directing traffic into intra-area TE LSPs:
有几种将流量引导到区域内TE LSP的可能性：
1) Static routing to the LSP destination address or any other addresses. 2) IGP routes beyond the LSP destination, from an IGP SPF perspective (IGP shortcuts). 3) BGP routes announced by a BGP peer (or an MP-BGP peer) that is reachable through the TE LSP by means of a single static route to the corresponding BGP next-hop address (option 1) or by means of IGP shortcuts (option 2). This is often called BGP recursive routing. 4) The LSP can be advertised as a link into the IGP to become part of IGP database for all nodes, and thus can be taken into account during SPF for all nodes. Note that, even if similar in concept, this is different from the notion of Forwarding-Adjacency, as defined in [LSP-HIER]. Forwarding-Adjacency is when the LSP is advertised as a TE-link into the IGP-TE to become part of the TE database and taken into account in CSPF.
1) 到LSP目标地址或任何其他地址的静态路由。2） 从IGP SPF（IGP快捷方式）的角度来看，超出LSP目的地的IGP路由。3） BGP对等方（或MP-BGP对等方）宣布的BGP路由，可通过TE LSP通过单个静态路由到达相应的BGP下一跳地址（选项1）或通过IGP快捷方式（选项2）到达。这通常称为BGP递归路由。4） LSP可以作为到IGP的链路进行广告，以成为所有节点的IGP数据库的一部分，因此可以在所有节点的SPF期间考虑LSP。注意，即使在概念上相似，这也不同于[LSP-HIER]中定义的转发邻接概念。转发邻接是指LSP作为TE链路播发到IGP-TE，成为TE数据库的一部分，并在CSPF中予以考虑。
5. Problem Statement, Requirements, and Objectives of Inter-Area MPLS TE
5. 区域间MPLS TE的问题陈述、要求和目标
As described in Section 4, MPLS TE is deployed today by many operators to optimize network bandwidth usage, to provide strict QoS guarantees, and to ensure sub-50ms recovery in case of link/node/SRLG failure.
如第4节所述，目前许多运营商都部署了MPLS TE，以优化网络带宽使用，提供严格的QoS保证，并确保在链路/节点/SRLG故障时进行低于50毫秒的恢复。
However, MPLS-TE mechanisms are currently limited to a single IGP area. The limitation comes more from the Routing and Path computation components than from the signaling component. This is basically because the hierarchy limits topology visibility of head-
然而，MPLS-TE机制目前仅限于单个IGP区域。限制更多地来自路由和路径计算组件，而不是来自信令组件。这基本上是因为层次限制了头部的拓扑可见性-
end LSRs to their IGP area, and consequently head-end LSRs can no longer run a CSPF algorithm to compute the shortest constrained path to the tail-end, as CSPF requires the whole topology to compute an end-to-end shortest constrained path.
将LSR端到其IGP区域，因此前端LSR不能再运行CSPF算法来计算到后端的最短约束路径，因为CSPF需要整个拓扑来计算端到端的最短约束路径。
Several operators have multi-area networks, and many operators that are still using a single IGP area may have to migrate to a multi-area environment, as their network grows and single area scalability limits are approached.
一些运营商拥有多区域网络，许多仍在使用单个IGP区域的运营商可能不得不迁移到多区域环境，因为他们的网络不断增长，单区域可扩展性限制越来越接近。
Thus, those operators may require inter-area traffic engineering to:
因此，这些运营商可能需要区域间交通工程来：
- Perform inter-area resource optimization. - Provide inter-area QoS guarantees for traffic between edge nodes located in different areas. - Provide fast recovery across areas, to protect inter-area traffic in case of link or node failure, including ABR node failures.
- 执行区域间资源优化。-为位于不同区域的边缘节点之间的流量提供区域间QoS保证。-提供跨区域的快速恢复，以在链路或节点故障（包括ABR节点故障）时保护区域间通信。
For instance, an operator running a multi-area IGP may have voice gateways located in different areas. Such VoIP transport requires inter-area QoS guarantees and inter-area fast protection.
例如，运行多区域IGP的运营商可以在不同区域设置语音网关。这种VoIP传输需要区域间QoS保证和区域间快速保护。
One possible approach for inter-area traffic engineering could consist of deploying MPLS TE on a per-area basis, but such an approach has several limitations:
区域间流量工程的一种可能方法可以包括在每个区域的基础上部署MPLS TE，但这种方法有几个局限性：
- Traffic aggregation at the ABR levels implies some constraints that do not lead to efficient traffic engineering. Actually, this per-area TE approach might lead to sub-optimal resource utilization, by optimizing resources independently in each area. What many operators want is to optimize their resources as a whole; in other words, as if there was only one area (flat network). - This does not allow computing an inter-area constrained shortest path and thus does not ensure end-to-end QoS guarantees across areas. - Inter-area traffic cannot be protected with local protection mechanisms such as [FAST-REROUTE] in case of ABR failure.
- ABR级别的流量聚合意味着一些无法实现高效流量工程的约束。实际上，这种按区域TE方法可能会导致次优资源利用率，因为它会独立优化每个区域的资源。许多运营商想要的是从整体上优化他们的资源；换句话说，好像只有一个区域（平面网络）。-这不允许计算区域间受约束的最短路径，因此无法确保跨区域的端到端QoS保证。-在ABR故障的情况下，无法使用本地保护机制（如[FAST-REROUTE]）保护区域间通信。
Therefore, existing MPLS TE mechanisms have to be enhanced to support inter-area TE LSPs.
因此，必须增强现有MPLS-TE机制以支持区域间TE-lsp。
For the reasons mentioned above, it is highly desired to extend the current set of MPLS-TE mechanisms across multiple IGP areas in order to support the intra-area applications described in Section 4 across areas.
出于上述原因，非常希望跨多个IGP区域扩展MPLS-TE机制的当前集合，以便跨区域支持第4节中描述的区域内应用。
The solution MUST allow setting up inter-area TE LSPs; i.e., LSPs whose path crosses at least two IGP areas.
解决方案必须允许设置区域间TE LSP；i、 例如，路径至少穿过两个IGP区域的LSP。
Inter-area MPLS-TE extensions are highly desired in order to provide:
为了提供：
- Inter-area resources optimization. - Strict inter-area QoS guarantees. - Fast recovery across areas, particularly to protect inter-area traffic against ABR failures.
- 区域间资源优化严格的区域间QoS保证。-跨区域快速恢复，特别是保护区域间流量免受ABR故障的影响。
It may be desired to compute inter-area shortest paths that satisfy some bandwidth constraints or any other constraints, as is currently possible within a single IGP area. For the sake of illustration, if the IGP metrics reflects the propagation delay, it may be necessary to be able to find the optimal (shortest) path satisfying some constraints (e.g., bandwidth) across multiple IGP areas. Such a path would be the inter-area path offering the minimal propagation delay.
可能需要计算满足某些带宽约束或任何其他约束的区域间最短路径，正如当前在单个IGP区域内可能的那样。为了说明，如果IGP度量反映了传播延迟，则可能需要能够找到跨多个IGP区域满足某些约束（例如带宽）的最佳（最短）路径。这样的路径将是提供最小传播延迟的区域间路径。
Thus, the solution SHOULD provide the ability to compute inter-area shortest paths satisfying a set of constraints (i.e., bandwidth).
因此，解决方案应提供计算满足一组约束（即带宽）的区域间最短路径的能力。
Any solution for inter-area MPLS TE should be designed with preserving IGP hierarchy concept, and preserving routing and signaling scalability as key objectives.
任何区域间MPLS-TE解决方案的设计都应以保持IGP层次结构概念、保持路由和信令可伸缩性为关键目标。
The absence of a full link-state topology database makes the computation of an end-to-end optimal path by the head-end LSR not possible without further signaling and routing extensions. There are several reasons that network operators choose to break up their network into different areas. These often include scalability and containment of routing information. The latter can help isolate most of a network from receiving and processing updates that are of no consequence to its routing decisions. Containment of routing information MUST not be compromised to allow inter-area traffic engineering. Information propagation for path-selection MUST continue to be localized. In other words, the solution MUST entirely preserve the concept of IGP hierarchy.
由于缺少完整链路状态拓扑数据库，因此在没有进一步信令和路由扩展的情况下，无法通过前端LSR计算端到端最佳路径。网络运营商选择将其网络划分为不同的区域有几个原因。这些通常包括路由信息的可伸缩性和包含性。后者有助于将大部分网络与接收和处理对其路由决策没有影响的更新隔离开来。路由信息的包含不得被破坏，以允许区域间流量工程。路径选择的信息传播必须继续本地化。换句话说，解决方案必须完全保留IGP层次结构的概念。
Achieving the requirements listed in this document MUST be performed while preserving the IGP scalability, which is of the utmost importance. The hierarchy preservation objective addressed in the above section is actually an element to preserve IGP scalability.
必须在保持IGP可扩展性的同时实现本文件中列出的要求，这一点至关重要。上一节中提到的层次结构保留目标实际上是保持IGP可伸缩性的一个要素。
The solution also MUST not increase IGP load unreasonably, which could compromise IGP scalability. In particular, a solution satisfying those requirements MUST not require the IGP to carry some unreasonable amount of extra information and MUST not unreasonably increase the IGP flooding frequency.
解决方案也不能不合理地增加IGP负载，这可能会损害IGP的可扩展性。特别是，满足这些要求的解决方案不得要求IGP携带不合理数量的额外信息，也不得不合理地增加IGP泛洪频率。
Likewise, the solution MUST also preserve scalability of RSVP-TE ([RSVP-TE]).
同样，解决方案还必须保持RSVP-TE（[RSVP-TE]）的可伸缩性。
Additionally, the base specification of MPLS TE is architecturally structured and relatively devoid of excessive state propagation in terms of routing or signaling. Its strength in extensibility can also be seen as an Achilles heel, as there is no real limit to what is possible with extensions. It is paramount to maintain architectural vision and discretion when adapting it for use for inter-area MPLS TE. Additional information carried within an area or propagated outside of an area (via routing or signaling) should be neither excessive, patchwork, nor non-relevant.
此外，MPLS-TE的基本规范在架构上是结构化的，并且在路由或信令方面相对没有过度的状态传播。它在可扩展性方面的优势也可以看作是一个致命弱点，因为扩展的可能性没有真正的限制。在将其用于区域间MPLS TE时，保持体系结构的远见和判断力是至关重要的。在一个区域内承载或在该区域外传播的附加信息（通过路由或信令）不应过多、零散或不相关。
Particularly, as mentioned in Section 5.2, it may be desired for some inter-area TE LSP carrying highly sensitive traffic to compute a shortest inter-area path, satisfying a set of constraints such as bandwidth. This may require an additional routing mechanism, as base CSPF at head-end can no longer be used due to the lack of topology and resource information. Such a routing mechanism MUST not compromise the scalability of the overall system.
特别是，如第5.2节所述，可能需要一些承载高度敏感业务的区域间TE LSP计算最短区域间路径，以满足一组约束，例如带宽。这可能需要额外的路由机制，因为由于缺乏拓扑