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Network Working Group                                          D. AwducheRequest for Comments: 2702                                     J. MalcolmCategory: Informational                                        J. Agogbua                                                                M. O'Dell                                                               J. McManus                                                     UUNET (MCI Worldcom)                                                           September 1999Requirements for Traffic Engineering Over MPLSStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.Abstract   This document presents a set of requirements for Traffic Engineering   over Multiprotocol Label Switching (MPLS). It identifies the   functional capabilities required to implement policies that   facilitate efficient and reliable network operations in an MPLS   domain. These capabilities can be used to optimize the utilization of   network resources and to enhance traffic oriented performance   characteristics.Table of Contents1.0   Introduction .............................................21.1   Terminology ..............................................31.2   Document Organization ....................................32.0   Traffic Engineering ......................................42.1   Traffic Engineering Performance Objectives ...............42.2   Traffic and Resource Control .............................62.3   Limitations of Current IGP Control Mechanisms ............63.0   MPLS and Traffic Engineering .............................73.1   Induced MPLS Graph .......................................9   3.2   The Fundamental Problem of Traffic Engineering Over MPLS .  9   4.0   Augmented Capabilities for Traffic Engineering Over MPLS . 105.0   Traffic Trunk Attributes and Characteristics   ...........105.1   Bidirectional Traffic Trunks .............................115.2   Basic Operations on Traffic Trunks .......................125.3   Accounting and Performance Monitoring ....................12Awduche, et al.              Informational                      [Page 1]

RFC 2702                MPLS Traffic Engineering          September 19995.4   Basic Attributes of Traffic Trunks .......................135.5   Traffic Parameter Attributes  ............................145.6   Generic Path Selection and Management Attributes .........145.6.1 Administratively Specified Explicit Paths ................155.6.2 Hierarchy of Preference Rules for Multi-paths ............155.6.3 Resource Class Affinity Attributes .......................165.6.4 Adaptivity Attribute .....................................175.6.5 Load Distribution Across Parallel Traffic Trunks .........185.7   Priority Attribute .......................................185.8   Preemption Attribute .....................................185.9   Resilience Attribute .....................................195.10  Policing Attribute  ......................................206.0   Resource Attributes ......................................216.1   Maximum Allocation Multiplier ............................216.2   Resource Class Attribute  ................................227.0   Constraint-Based Routing  ................................227.1   Basic Features of Constraint-Based Routing ...............237.2   Implementation Considerations ............................248.0   Conclusion   .............................................259.0   Security Considerations ..................................2610.0  References   .............................................2611.0  Acknowledgments ..........................................2712.0  Authors' Addresses .......................................2813.0  Full Copyright Statement .................................291.0 Introduction   Multiprotocol Label Switching (MPLS) [1,2] integrates a label   swapping framework with network layer routing. The basic idea   involves assigning short fixed length labels to  packets at the   ingress to an MPLS cloud (based on the concept of forwarding   equivalence classes [1,2]). Throughout the interior of the MPLS   domain, the labels attached to packets are used to make forwarding   decisions  (usually without recourse to the original packet headers).   A set of powerful constructs to address many critical issues in the   emerging differentiated services Internet can be devised from this   relatively simple paradigm.  One of the most significant initial   applications of MPLS will be in Traffic Engineering. The importance   of this application is already well-recognized (see [1,2,3]).   This manuscript is exclusively focused on the Traffic Engineering   applications of MPLS. Specifically, the goal of this document is to   highlight the issues and requirements for Traffic Engineering in a   large Internet backbone. The expectation is that the MPLS   specifications, or implementations derived therefrom, will addressAwduche, et al.              Informational                      [Page 2]

RFC 2702                MPLS Traffic Engineering          September 1999   the realization of these objectives.  A description of the basic   capabilities and functionality required of an MPLS implementation to   accommodate the requirements is also presented.   It should be noted that even though the focus is on Internet   backbones, the capabilities described in this document are equally   applicable to Traffic Engineering in enterprise networks. In general,   the capabilities can  be applied to any label switched network under   a single technical administration in which at least two paths exist   between two nodes.   Some recent manuscripts have focused on the considerations pertaining   to Traffic Engineering and Traffic management under MPLS, most   notably the works of Li and Rekhter [3], and others.  In [3], an   architecture is proposed which employs MPLS and RSVP to provide   scalable differentiated services and Traffic Engineering in the   Internet.  The present manuscript complements the aforementioned and   similar efforts.  It reflects the authors' operational experience in   managing a large Internet backbone.1.1 Terminology   The reader is assumed to be familiar with the MPLS terminology as   defined in [1].   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 inRFC 2119 [11].1.2 Document Organization   The remainder of this document is organized as follows:Section 2   discusses the basic functions of Traffic Engineering in the Internet.Section 3, provides an overview of the traffic Engineering potentials   of MPLS. Sections1 to3 are essentially background material.Section4 presents an overview of the fundamental requirements for Traffic   Engineering over MPLS.Section 5 describes the desirable attributes   and characteristics of traffic trunks which are pertinent to Traffic   Engineering.Section 6 presents a set of attributes which can be   associated with resources to constrain the routability of traffic   trunks and LSPs through them.Section 7 advocates the introduction of   a "constraint-based routing" framework in MPLS domains.  Finally,Section 8 contains concluding remarks.Awduche, et al.              Informational                      [Page 3]

RFC 2702                MPLS Traffic Engineering          September 19992.0 Traffic Engineering   This section describes the basic functions of Traffic Engineering in   an Autonomous System in the contemporary Internet. The limitations of   current IGPs with respect to traffic and resource control are   highlighted. This section serves as motivation for the requirements   on MPLS.   Traffic Engineering (TE) is concerned with performance optimization   of operational networks. In general, it encompasses the application   of technology and scientific principles to the measurement, modeling,   characterization, and control of Internet traffic, and the   application of such knowledge and techniques to achieve specific   performance objectives. The aspects of Traffic Engineering that are   of interest concerning MPLS are measurement and control.   A major goal of Internet Traffic Engineering is to facilitate   efficient and reliable network operations while simultaneously   optimizing network resource utilization and traffic performance.   Traffic Engineering has become an indispensable function in many   large Autonomous Systems because of the high cost of network assets   and the commercial and competitive nature of the Internet. These   factors emphasize the need for maximal operational efficiency.2.1 Traffic Engineering Performance Objectives   The key performance objectives associated with traffic engineering   can be classified as being either:    1. traffic oriented or    2. resource oriented.   Traffic oriented performance objectives include the aspects that   enhance the QoS of traffic streams. In a single class, best effort   Internet service model, the key traffic oriented performance   objectives include: minimization of packet loss, minimization of   delay, maximization of throughput, and enforcement of service level   agreements. Under a single class best effort Internet service model,   minimization of packet loss is one of the most important traffic   oriented performance objectives. Statistically bounded traffic   oriented performance objectives (such as peak to peak packet delay   variation, loss ratio, and maximum packet transfer delay) might   become useful in the forthcoming differentiated services Internet.   Resource oriented performance objectives include the aspects   pertaining to the optimization of resource utilization. Efficient   management of network resources is the vehicle for the attainment ofAwduche, et al.              Informational                      [Page 4]

RFC 2702                MPLS Traffic Engineering          September 1999   resource oriented performance objectives. In particular, it is   generally desirable to ensure that subsets of network resources do   not become over utilized and congested while other subsets along   alternate feasible paths remain underutilized. Bandwidth is a crucial   resource in contemporary networks.  Therefore, a central function of   Traffic Engineering is to efficiently manage bandwidth resources.   Minimizing congestion is a primary traffic and resource oriented   performance objective.  The interest here is on congestion problems   that are prolonged rather than on transient congestion resulting from   instantaneous bursts.  Congestion typically manifests under two   scenarios:   1. When network resources are insufficient or inadequate to      accommodate offered load.   2. When traffic streams are inefficiently mapped onto available      resources; causing subsets of network resources to become      over-utilized while others remain underutilized.   The first type of congestion problem can be addressed by either: (i)   expansion of capacity, or (ii) application of classical congestion   control techniques, or (iii) both. Classical congestion control   techniques attempt to regulate the demand so that the traffic fits   onto available resources. Classical techniques for congestion control   include: rate limiting, window flow control, router queue management,   schedule-based control, and others; (see [8] and the references   therein).   The second type of congestion problems, namely those resulting from   inefficient resource allocation, can usually be addressed through   Traffic Engineering.   In general, congestion resulting from inefficient resource allocation   can be reduced by adopting load balancing policies. The objective of   such strategies is to minimize maximum congestion or alternatively to   minimize maximum resource utilization, through efficient resource   allocation. When congestion is minimized through efficient resource   allocation, packet loss decreases, transit delay decreases, and   aggregate throughput increases. Thereby, the perception of network   service quality experienced by end users becomes significantly   enhanced.   Clearly, load balancing is an important network performance   optimization policy. Nevertheless, the capabilities provided for   Traffic Engineering should be flexible enough so that network   administrators can implement other policies which take into account   the prevailing cost structure and the utility or revenue model.Awduche, et al.              Informational                      [Page 5]

RFC 2702                MPLS Traffic Engineering          September 19992.2 Traffic and Resource Control   Performance optimization of operational networks is fundamentally a   control problem. In the traffic engineering process model, the   Traffic Engineer, or a suitable automaton, acts as the controller in   an adaptive feedback control system. This system includes a set of   interconnected network elements, a network performance monitoring   system, and a set of network configuration management tools. The   Traffic Engineer formulates a control policy, observes the state of   the network through the monitoring system, characterizes the traffic,   and applies control actions to drive the network to a desired state,   in accordance with the control policy.  This can be accomplished   reactively by taking action in response to the current state of the   network, or pro-actively by using forecasting techniques to   anticipate future trends and applying action to obviate the predicted   undesirable future states.   Ideally, control actions should involve:   1. Modification of traffic management parameters,   2. Modification of parameters associated with routing, and   3. Modification of attributes and constraints associated with      resources.   The level of manual intervention involved in the traffic engineering   process should be minimized whenever possible.  This can be   accomplished by automating aspects of the control actions described   above, in a distributed and scalable fashion.2.3 Limitations of Current IGP Control Mechanisms   This subsection reviews some of the well known limitations of current   IGPs with regard to Traffic Engineering.   The control capabilities offered by existing Internet interior   gateway protocols are not adequate for Traffic Engineering.  This   makes it difficult to actualize effective policies to address network   performance problems.  Indeed, IGPs based on shortest path algorithms   contribute significantly to congestion problems in Autonomous Systems   within the Internet. SPF algorithms generally optimize based on a   simple additive metric. These protocols are topology driven, so   bandwidth availability and traffic characteristics are not factors   considered in routing decisions. Consequently, congestion frequently   occurs when:Awduche, et al.              Informational                      [Page 6]

RFC 2702                MPLS Traffic Engineering          September 1999   1. the shortest paths of multiple traffic streams converge on      specific links or router interfaces, or   2. a given traffic stream is routed through a link or router      interface which does not have enough bandwidth to accommodate      it.   These scenarios manifest even when feasible alternate paths with   excess capacity exist. It is this aspect of congestion problems (-- a   symptom of suboptimal resource allocation) that Traffic Engineering   aims to vigorously obviate.  Equal cost path load sharing can be used   to address the second cause for congestion listed above with some   degree of success, however it is generally not helpful in alleviating   congestion due to the first cause listed above and particularly not   in large networks with dense topology.   A popular approach to circumvent the inadequacies of current IGPs is   through the use of an overlay model, such as IP over ATM or IP over   frame relay. The overlay model extends the design space by enabling   arbitrary virtual topologies to be provisioned atop the network's   physical topology. The virtual topology is constructed from virtual   circuits which appear as physical links to the IGP routing protocols.   The overlay model provides additional important services to support   traffic and resource control, including: (1) constraint-based routing   at the VC level, (2) support for administratively configurable   explicit VC paths, (3) path compression, (4) call admission control   functions, (5) traffic shaping and traffic policing functions, and   (6) survivability of VCs. These capabilities enable the actualization   of a variety of Traffic Engineering policies. For example, virtual   circuits can easily be rerouted to move traffic from over-utilized   resources onto relatively underutilized ones.   For Traffic Engineering in large dense networks, it is desirable to   equip MPLS with a level of functionality at least commensurate with   current overlay models. Fortunately, this can be done in a fairly   straight forward manner.3.0  MPLS and Traffic Engineering   This section provides an overview of the applicability of MPLS to   Traffic Engineering. Subsequent sections discuss the set of   capabilities required to meet the Traffic Engineering requirements.   MPLS is strategically significant for Traffic Engineering because it   can potentially provide most of the functionality available from the   overlay model, in an integrated manner, and at a lower cost than the   currently competing alternatives. Equally importantly, MPLS offersAwduche, et al.              Informational                      [Page 7]

RFC 2702                MPLS Traffic Engineering          September 1999   the possibility to automate aspects of the Traffic Engineering   function. This last consideration requires further investigation and   is beyond the scope of this manuscript.   A note on terminology: The concept of MPLS traffic trunks is used   extensively in the remainder of this document. According to Li and   Rekhter [3], a traffic trunk is an aggregation of traffic flows of   the same class which are placed inside a Label Switched Path.   Essentially, a traffic trunk is an abstract representation of traffic   to which specific characteristics can be associated. It is useful to   view traffic trunks as objects that can be routed; that is, the path   through which a traffic trunk traverses can be changed. In this   respect, traffic trunks are similar to virtual circuits in ATM and   Frame Relay networks.  It is important, however, to emphasize that   there is a fundamental distinction between a traffic trunk and the   path, and indeed the LSP, through which it traverses. An LSP is a   specification of the label switched path through which the traffic   traverses. In practice, the terms LSP and traffic trunk are often   used synonymously. Additional characteristics of traffic trunks as   used in this manuscript are summarized insection 5.0.   The attractiveness of  MPLS for Traffic Engineering can be attributed   to the following factors: (1) explicit label switched paths which are   not constrained by the destination based forwarding paradigm can be   easily created through manual administrative action or through   automated action by the underlying protocols, (2) LSPs can   potentially be efficiently maintained, (3) traffic trunks can be   instantiated and mapped onto LSPs, (4) a set of attributes can be   associated with traffic trunks which modulate their behavioral   characteristics, (5) a set of attributes can be associated with   resources which constrain the placement of LSPs and traffic trunks   across them, (6) MPLS allows for both traffic aggregation and   disaggregation whereas classical destination only based IP forwarding   permits only aggregation, (7) it is relatively easy to integrate a   "constraint-based routing" framework with MPLS, (8) a good   implementation of MPLS can offer significantly lower overhead than   competing alternatives for Traffic Engineering.   Additionally, through explicit label switched paths, MPLS permits a   quasi circuit switching capability to be superimposed on the current   Internet routing model.  Many of the existing proposals for Traffic   Engineering over MPLS focus only on the potential to create explicit   LSPs. Although this capability is fundamental for Traffic   Engineering, it is not really sufficient.  Additional augmentations   are required to foster the actualization of policies leading to   performance optimization of large operational networks. Some of the   necessary augmentations are described in this manuscript.Awduche, et al.              Informational                      [Page 8]

RFC 2702                MPLS Traffic Engineering          September 19993.1 Induced MPLS Graph   This subsection introduces the concept of an "induced MPLS graph"   which is central to Traffic Engineering in MPLS domains. An induced   MPLS graph is analogous to a virtual topology in an overlay model. It   is logically mapped onto the physical network through the selection   of LSPs for traffic trunks.   An induced MPLS graph consists of a set of LSRs which comprise the   nodes of the graph and a set of LSPs which provide logical point to   point connectivity between the LSRs, and hence serve as the links of   the induced graph. it may be possible to construct hierarchical   induced MPLS graphs based on the concept of label stacks (see [1]).   Induced MPLS graphs are important because the basic problem of   bandwidth management in an MPLS domain is the issue of how to   efficiently map an induced MPLS graph onto the physical network   topology.  The induced MPLS graph abstraction is formalized below.   Let G = (V, E, c) be a capacitated graph depicting the physical   topology of the network. Here, V is the set of nodes in the network   and E is the set of links; that is, for v and w in V, the object   (v,w) is in E if v and w are directly connected under G. The   parameter "c" is a set of capacity and other constraints associated   with E and V. We will refer to G as the "base" network topology.   Let H = (U, F, d) be  the induced MPLS graph, where U is a subset of   V representing the set of LSRs in the network, or more precisely the   set of LSRs that are the endpoints of at least one LSP. Here, F is   the set of LSPs, so that for x and y in U, the object (x, y) is in F   if there is an LSP with x and y as endpoints. The parameter "d" is   the set of demands and restrictions associated with F. Evidently, H   is a directed graph. It can be seen that H depends on the   transitivity characteristics of G.3.2 The Fundamental Problem of Traffic Engineering Over MPLS   There are basically three fundamental problems that relate to Traffic   Engineering over MPLS.   - The first problem concerns how to map packets onto forwarding     equivalence classes.   - The second problem concerns how to map forwarding equivalence     classes onto traffic trunks.   - The third problem concerns how to map traffic trunks onto the     physical network topology through label switched paths.Awduche, et al.              Informational                      [Page 9]

RFC 2702                MPLS Traffic Engineering          September 1999   This document is not focusing on the first two problems listed.   (even-though they are quite important). Instead, the remainder of   this manuscript will focus on the capabilities that permit the third   mapping function to be performed in a manner resulting in efficient   and reliable network operations. This is really the problem of   mapping an induced MPLS graph (H) onto the "base" network topology   (G).4.0 AugmentedCapabilities for Traffic Engineering Over MPLS   The previous sections reviewed the basic functions of Traffic   Engineering in the contemporary Internet. The applicability of MPLS   to that activity was also discussed. The remaining sections of this   manuscript describe the functional capabilities required to fully   support Traffic Engineering over MPLS in large networks.   The proposed capabilities consist of:   1. A set of attributes associated with traffic trunks which      collectively specify their behavioral characteristics.   2. A set of attributes associated with resources which constrain      the placement of traffic trunks through them. These can also be      viewed as topology attribute constraints.   3. A "constraint-based routing" framework which is used to select      paths for traffic trunks subject to constraints imposed by items      1) and 2) above. The constraint-based routing framework does not      have to be part of MPLS. However, the two need to be tightly      integrated together.   The attributes associated with traffic trunks and resources, as well   as parameters associated with routing, collectively represent the   control variables which can be modified either through administrative   action or through automated agents to drive the network to a desired   state.   In an operational network, it is highly desirable that these   attributes can be dynamically modified online by an operator without   adversely disrupting network operations.5.0 Traffic Trunk Attributes and Characteristics   This section describes the desirable attributes which can be   associated with traffic trunks to influence their behavioral   characteristics.Awduche, et al.              Informational                     [Page 10]

RFC 2702                MPLS Traffic Engineering          September 1999   First, the basic properties of traffic trunks (as used in this   manuscript) are summarized below:    - A traffic trunk is an *aggregate* of traffic flows belonging      to the same class. In some contexts, it may be desirable to      relax this definition and allow traffic trunks to include      multi-class traffic aggregates.    - In a single class service model, such as the current Internet,      a traffic trunk could encapsulate all of the traffic between an      ingress LSR and an egress LSR, or subsets thereof.    - Traffic trunks are routable objects (similar to ATM VCs).    - A traffic trunk is distinct from the LSP through which it      traverses. In operational contexts, a traffic trunk can be      moved from one path onto another.    - A traffic trunk is unidirectional.   In practice, a traffic trunk can be characterized by its ingress and   egress LSRs, the forwarding equivalence class which is mapped onto   it, and a set of attributes which determine its behavioral   characteristics.   Two basic issues are of particular significance: (1) parameterization   of traffic trunks and (2) path placement and maintenance rules for   traffic trunks.5.1 Bidirectional Traffic Trunks   Although traffic trunks are conceptually unidirectional, in many   practical contexts, it is useful to  simultaneously instantiate two   traffic trunks with the same endpoints, but which carry packets in   opposite directions. The two traffic trunks are logically coupled   together.  One trunk, called the forward trunk, carries traffic from   an originating node to a destination node. The other trunk, called   the backward trunk, carries traffic from the destination node to the   originating node. We refer to the amalgamation of two such traffic   trunks as one bidirectional traffic trunk (BTT) if the following two   conditions hold:   - Both traffic trunks are instantiated through an atomic action at     one LSR, called the originator node, or through an atomic action     at a network management station.   - Neither of the composite traffic trunks can exist without the     other. That is, both are instantiated and destroyed together.Awduche, et al.              Informational                     [Page 11]

RFC 2702                MPLS Traffic Engineering          September 1999   The topological properties of BTTs should also be considered. A BTT   can be topologically symmetric or topologically asymmetric.  A BTT is   said to be "topologically symmetric" if its constituent traffic   trunks are routed through the same physical path, even though they   operate in opposite directions. If, however, the component traffic   trunks are routed through different physical paths, then the BTT is   said to be "topologically asymmetric."   It should be noted that bidirectional traffic trunks are merely an   administrative convenience. In practice, most traffic engineering   functions can be implemented using only unidirectional traffic   trunks.5.2 Basic Operations on Traffic Trunks   The basic operations on traffic trunks significant to Traffic   Engineering purposes are summarized below.   - Establish: To create an instance of a traffic trunk.   - Activate: To cause a traffic trunk to start passing traffic.     The establishment and activation of a traffic trunk are     logically separate events. They may, however, be implemented     or invoked as one atomic action.   - Deactivate: To cause a traffic trunk to stop passing traffic.   - Modify Attributes: To cause the attributes of a traffic trunk     to be modified.   - Reroute: To cause a traffic trunk to change its route. This     can be done through administrative action or automatically     by the underlying protocols.   - Destroy: To remove an instance of a traffic trunk from the     network and reclaim all resources allocated to it. Such     resources include label space and possibly available bandwidth.   The above are considered the basic operations on traffic trunks.   Additional operations are also possible such as policing and traffic   shaping.5.3 Accounting and Performance Monitoring   Accounting and performance monitoring capabilities are very important   to the billing and traffic characterization functions.  Performance   statistics obtained from accounting and performance monitoringAwduche, et al.              Informational                     [Page 12]

RFC 2702                MPLS Traffic Engineering          September 1999   systems can be used for traffic characterization, performance   optimization, and capacity planning within the Traffic Engineering   realm..   The capability to obtain statistics at the traffic trunk level is so   important that it should be considered an essential requirement for   Traffic Engineering over MPLS.5.4 Basic Traffic Engineering Attributes of Traffic Trunks   An attribute of a traffic trunk is a parameter assigned to it which   influences its behavioral characteristics.   Attributes can be explicitly assigned to traffic trunks through   administration action or they can be implicitly assigned by the   underlying protocols when packets are classified and mapped into   equivalence classes at the ingress to an MPLS domain. Regardless of   how the attributes were originally assigned, for Traffic Engineering   purposes, it should be possible to administratively modify such   attributes.   The basic attributes of traffic trunks  particularly significant for   Traffic Engineering are itemized below.   - Traffic parameter attributes   - Generic Path selection and maintenance attributes   - Priority attribute   - Preemption attribute   - Resilience attribute   - Policing  attribute   The combination of traffic parameters and policing attributes is   analogous to usage parameter control in ATM networks. Most of the   attributes listed above have analogs in well established   technologies.  Consequently, it should be relatively straight forward   to map the traffic trunk attributes onto many existing switching and   routing architectures.   Priority and preemption can be regarded as relational attributes   because they express certain binary relations between traffic trunks.   Conceptually, these binary relations determine the manner in which   traffic trunks interact with each other as they compete for network   resources during path establishment and path maintenance.Awduche, et al.              Informational                     [Page 13]

RFC 2702                MPLS Traffic Engineering          September 19995.5 Traffic parameter attributes   Traffic parameters can be used to capture the characteristics of the   traffic streams (or more precisely the forwarding equivalence class)   to be transported through the traffic trunk. Such characteristics may   include peak rates, average rates, permissible burst size, etc.  From   a traffic engineering perspective, the traffic parameters are   significant because they indicate the resource requirements of the   traffic trunk. This is useful for resource allocation and congestion   avoidance through anticipatory policies.   For the purpose of bandwidth allocation, a single canonical value of   bandwidth requirements can be computed from a traffic trunk's traffic   parameters.  Techniques for performing these computations are well   known. One example of this is the theory of effective bandwidth.5.6 Generic Path Selection and Management Attributes   Generic path selection and management attributes define the rules for   selecting the route taken by a traffic trunk as well as the rules for   maintenance of paths that are already established.   Paths can be computed automatically by the underlying routing   protocols or they can be defined administratively by a network   operator. If there are no resource requirements or restrictions   associated with a traffic trunk, then a topology driven protocol can   be used to select its path. However, if resource requirements or   policy restrictions exist, then a constraint-based routing scheme   should be used for path selection.   InSection 7, a constraint-based routing framework which can   automatically compute paths subject to a set of constraints is   described.  Issues pertaining to explicit paths instantiated through   administrative action are discussed inSection 5.6.1 below.   Path management concerns all aspects pertaining to the maintenance of   paths traversed by traffic trunks.  In some operational contexts, it   is desirable that an MPLS implementation can dynamically reconfigure   itself, to adapt to some notion of change in "system state."   Adaptivity and resilience are aspects of dynamic path management.   To guide the path selection and management process, a set of   attributes are required. The basic attributes and behavioral   characteristics associated with traffic trunk path selection and   management are described in the remainder of this sub-section.Awduche, et al.              Informational                     [Page 14]

RFC 2702                MPLS Traffic Engineering          September 19995.6.1 Administratively Specified Explicit Paths   An administratively specified explicit path for a traffic trunk is   one which is configured through operator action. An administratively   specified path can be completely specified or partially specified. A   path is completely specified if all of the required hops between the   endpoints are indicated. A path is partially specified if only a   subset of intermediate hops are indicated. In this case, the   underlying protocols are required to complete the path. Due to   operator errors, an administratively specified path can be   inconsistent or illogical. The underlying protocols should be able to   detect such inconsistencies and provide appropriate feedback.   A "path preference rule" attribute should be associated with   administratively specified explicit paths.  A path preference rule   attribute is a binary variable which  indicates whether the   administratively configured explicit path is "mandatory" or "non-   mandatory."   If an administratively specified explicit path is selected with a   "mandatory attribute, then that path (and only that path) must be   used. If a mandatory path is topological infeasible (e.g. the two   endpoints are topologically partitioned), or if the path cannot be   instantiated because the available resources are inadequate, then the   path setup process fails. In other words, if a path is specified as   mandatory, then an alternate path cannot be used regardless of   prevailing circumstance.  A mandatory path which is successfully   instantiated is also implicitly pinned. Once the path is instantiated   it cannot be changed except through deletion and instantiation of a   new path.   However, if an administratively specified explicit path is selected   with a "non-mandatory" preference rule attribute value, then the path   should be used if feasible.  Otherwise, an alternate path can be   chosen instead by the underlying protocols.5.6.2 Hierarchy of Preference Rules For Multi-Paths   In some practical contexts, it can be useful to have the ability to   administratively specify a set of candidate explicit paths for a   given traffic trunk and define a hierarchy of preference relations on   the paths. During path establishment, the preference rules are   applied to select a suitable path from the candidate list. Also,   under failure scenarios the preference rules are applied to select an   alternate path from the candidate list.Awduche, et al.              Informational                     [Page 15]

RFC 2702                MPLS Traffic Engineering          September 19995.6.3 Resource Class Affinity Attributes   Resource class affinity attributes associated with a traffic trunk   can be used to specify the class of resources (seeSection 6) which   are to be explicitly included or excluded from the path of the   traffic trunk. These are policy attributes which can be used to   impose additional constraints on the path traversed by a given   traffic trunk.  Resource class affinity attributes for a traffic can   be specified as a sequence of tuples:    <resource-class, affinity>; <resource-class, affinity>; ..   The resource-class parameter identifies a resource class for which an   affinity relationship is defined with respect to the traffic trunk.   The affinity parameter indicates the affinity relationship; that is,   whether members of the resource class are to be included or excluded   from the path of the traffic trunk. Specifically, the affinity   parameter may be a binary variable which takes one of the following   values: (1) explicit inclusion, and (2) explicit exclusion.   If the affinity attribute is a binary variable, it may be possible to   use Boolean expressions to specify the resource class affinities   associated with a given traffic trunk.   If no resource class affinity attributes are specified, then a "don't   care" affinity relationship is assumed to hold between the traffic   trunk and all resources. That is, there is no requirement to   explicitly include or exclude any resources from the traffic trunk's   path. This should be the default in practice.   Resource class affinity attributes are very useful and powerful   constructs because they can be used to implement a variety of   policies. For example, they can be used to contain certain traffic   trunks within specific topological regions of the network.   A "constraint-based routing" framework (seesection 7.0) can be used   to compute an explicit path for a traffic trunk subject to resource   class affinity constraints in the following manner:   1. For explicit inclusion, prune all resources not belonging      to the specified classes prior to performing path computation.   2. For explicit exclusion, prune all resources  belonging to the      specified classes before performing path placement computations.Awduche, et al.              Informational                     [Page 16]

RFC 2702                MPLS Traffic Engineering          September 19995.6.4 Adaptivity Attribute   Network characteristics and state change over time. For example, new   resources become available, failed resources become reactivated, and   allocated resources become deallocated. In general, sometimes more   efficient paths become available.  Therefore, from a Traffic   Engineering perspective, it is necessary to have administrative   control parameters that can be used to specify how traffic trunks   respond to this dynamism. In some scenarios, it might be desirable to   dynamically change the paths of certain traffic trunks in response to   changes in network state. This process is called re-optimization.  In   other scenarios, re-optimization might be very undesirable.   An Adaptivity attribute is a part of the path maintenance parameters   associated with traffic trunks. The adaptivity attribute associated   with a traffic trunk indicates whether the trunk is subject to re-   optimization.  That is, an adaptivity attribute is a binary variable   which takes one of the following values: (1) permit re-optimization   and (2) disable re-optimization.   If re-optimization is enabled, then a traffic trunk can be rerouted   through different paths by the underlying protocols in response to   changes in network state (primarily changes in resource   availability). Conversely, if re-optimization is disabled, then the   traffic trunk is "pinned" to its established path and cannot be   rerouted in response to changes in network state.   Stability is a major concern when re-optimization is permitted. To   promote stability, an MPLS implementation should not be too reactive   to the evolutionary dynamics of the network. At the same time, it   must adapt fast enough so that optimal use can be made of network   assets. This implies that the frequency of re-optimization should be   administratively configurable to allow for tuning.   It is to be noted that re-optimization is distinct from resilience. A   different attribute is used to specify the resilience characteristics   of a traffic trunk (seesection 5.9).  In practice, it would seem   reasonable to expect traffic trunks subject to re-optimization to be   implicitly resilient to failures along their paths. However, a   traffic trunk which is not subject to re-optimization and whose path   is not administratively specified with a "mandatory" attribute can   also be required to be resilient to link and node failures along its   established path   Formally, it can be stated that adaptivity to state evolution through   re-optimization implies resilience to failures, whereas resilience to   failures does not imply general adaptivity through re-optimization to   state changes.Awduche, et al.              Informational                     [Page 17]

RFC 2702                MPLS Traffic Engineering          September 19995.6.5 Load Distribution Across Parallel Traffic Trunks   Load distribution across multiple parallel traffic trunks between two   nodes is an important consideration.  In many practical contexts, the   aggregate traffic between two nodes may be such that no single link   (hence no single path) can carry the load. However, the aggregate   flow might be less than the maximum permissible flow across a "min-   cut" that partitions the two nodes. In this case, the only feasible   solution is to appropriately divide the aggregate traffic into sub-   streams and route the sub-streams through multiple paths between the   two nodes.   In an MPLS domain, this problem can be addressed by instantiating   multiple traffic trunks between the two nodes, such that each traffic   trunk carries a proportion of the aggregate traffic. Therefore, a   flexible means of load assignment to multiple parallel traffic trunks   carrying traffic between a pair of nodes is required.   Specifically, from an operational perspective, in situations where   parallel traffic trunks are warranted,  it would be useful to have   some attribute that can be used to indicate the relative proportion   of traffic to be carried by each traffic trunk. The underlying   protocols will then map the load onto the traffic trunks according to   the specified proportions. It is also, generally desirable to   maintain packet ordering between packets belong to the same micro-   flow (same source address, destination address, and port number).5.7 Priority attribute   The priority attribute defines the relative importance of traffic   trunks.  If a constraint-based routing framework is used with MPLS,   then priorities become very important because they can be used to   determine the order in which path selection is done for traffic   trunks at connection establishment and under fault scenarios.   Priorities are also important in implementations  permitting   preemption because they can be used to impose a partial order on the   set of traffic trunks according to which preemptive policies can be   actualized.5.8 Preemption attribute   The preemption attribute determines whether a traffic trunk can   preempt another traffic trunk from a given path, and whether another   traffic trunk can preempt a specific traffic trunk.  Preemption is   useful for both traffic oriented and resource oriented performanceAwduche, et al.              Informational                     [Page 18]

RFC 2702                MPLS Traffic Engineering          September 1999   objectives. Preemption can used to assure that high priority traffic   trunks can always be routed through relatively favorable paths within   a differentiated services environment.   Preemption can also be used to implement various prioritized   restoration policies following fault events.   The preemption attribute can be used to specify four preempt modes   for a traffic trunk: (1) preemptor enabled, (2) non-preemptor, (3)   preemptable, and (4) non-preemptable. A preemptor enabled traffic   trunk can preempt lower priority traffic trunks designated as   preemptable. A traffic specified as non-preemptable cannot be   preempted by any other trunks, regardless of relative priorities. A   traffic trunk designated as preemptable can be preempted by higher   priority trunks which are preemptor enabled.   It is trivial to see that some of the preempt modes are mutually   exclusive. Using the numbering scheme depicted above, the feasible   preempt mode combinations for a given traffic trunk are as follows:   (1, 3), (1, 4), (2, 3), and (2, 4). The (2, 4) combination should be   the default.   A traffic trunk, say "A", can preempt another traffic trunk, say "B",   only if *all* of the following five conditions hold: (i) "A" has a   relatively higher priority than "B", (ii) "A" contends for a resource   utilized by "B", (iii) the resource cannot concurrently accommodate   "A" and "B" based on certain decision criteria, (iv) "A" is preemptor   enabled, and (v) "B" is preemptable.   Preemption is not considered a mandatory attribute under the current   best effort Internet service model although it is useful. However, in   a differentiated services scenario, the need for preemption becomes   more compelling. Moreover, in the emerging optical internetworking   architectures, where some protection and restoration functions may be   migrated from the optical layer to data network elements (such as   gigabit and terabit label switching routers) to reduce costs,   preemptive strategies can be used to reduce the restoration time for   high priority traffic trunks under fault conditions.5.9 Resilience Attribute   The resilience attribute determines the behavior of a traffic trunk   under fault conditions. That is, when a fault occurs along the path   through which the traffic trunk traverses. The following basic   problems need to be addressed under such circumstances: (1) fault   detection, (2) failure notification, (3) recovery and service   restoration. Obviously, an MPLS implementation will have to   incorporate mechanisms to address these issues.Awduche, et al.              Informational                     [Page 19]

RFC 2702                MPLS Traffic Engineering          September 1999   Many recovery policies can be specified for traffic trunks whose   established paths are impacted by faults. The following are examples   of feasible schemes:   1. Do not reroute the traffic trunk. For example, a survivability      scheme may already be in place, provisioned through an      alternate mechanism, which guarantees service continuity      under failure scenarios without the need to reroute traffic      trunks. An example of such an alternate scheme (certainly      many others exist), is a situation whereby multiple parallel      label switched paths are provisioned between two nodes, and      function in a manner such that failure of one LSP causes the      traffic trunk placed on it to be mapped onto the remaining LSPs      according to some well defined policy.   2. Reroute through a feasible path with enough resources. If none      exists, then do not reroute.   3. Reroute through any available path regardless of resource      constraints.   4. Many other schemes are possible including some which might be      combinations of the above.   A "basic" resilience attribute indicates the recovery procedure to be   applied to traffic trunks whose paths are impacted by faults.   Specifically, a "basic" resilience attribute is a binary variable   which determines whether the target traffic trunk is to be rerouted   when segments of its path fail. "Extended" resilience attributes can   be used to specify detailed actions to be taken under fault   scenarios.  For example, an extended resilience attribute might   specify a set of alternate paths to use under fault conditions, as   well as the rules that govern the relative preference of each   specified path.   Resilience attributes mandate close interaction between MPLS and   routing.5.10 Policing attribute   The policing attribute determines the actions that should be taken by   the underlying protocols when a traffic trunk becomes non-compliant.   That is, when a traffic trunk exceeds its contract as specified in   the traffic parameters.  Generally, policing attributes can indicate   whether a non-conformant traffic trunk is to be rate limited, tagged,   or simply forwarded without any policing action.  If policing is   used, then adaptations of established algorithms such as the ATM   Forum's GCRA [11] can be used to perform this function.Awduche, et al.              Informational                     [Page 20]

RFC 2702                MPLS Traffic Engineering          September 1999   Policing is necessary in many operational scenarios, but is quite   undesirable in some others. In general, it is usually desirable to   police at the ingress to a network (to enforce compliance with   service level agreements) and to minimize policing within the core,   except when capacity constraints dictate otherwise.   Therefore, from a Traffic Engineering perspective, it is necessary to   be able to administratively enable or disable traffic policing for   each traffic trunk.6.0 Resource Attributes   Resource attributes are part of the topology state parameters, which   are used to constrain the routing of traffic trunks through specific   resources.6.1 Maximum Allocation Multiplier   The maximum allocation multiplier (MAM) of a resource is an   administratively configurable attribute which determines the   proportion of the resource that is available for allocation to   traffic trunks.  This attribute is mostly applicable to link   bandwidth. However,  it can also be applied to buffer resources on   LSRs. The concept of MAM is analogous to the concepts of subscription   and booking factors in frame relay and ATM networks.   The values of the MAM can be chosen so that a resource can be under-   allocated or over-allocated. A resource is said  to be under-   allocated if the aggregate demands of all traffic trunks (as   expressed in the trunk traffic parameters) that can be allocated to   it are always less than the capacity of the resource. A resource is   said to be over-allocated if the aggregate demands of all traffic   trunks allocated to it can exceed the capacity of the resource.   Under-allocation can be used to bound the utilization of resources.   However,the situation under MPLS is more complex than in circuit   switched schemes because under MPLS, some flows can be routed via   conventional hop by hop protocols (also via explicit paths) without   consideration for resource constraints.   Over-allocation can be used to take advantage of the statistical   characteristics of traffic in order to implement more efficient   resource allocation policies. In particular, over-allocation can be   used in situations where the peak demands of traffic trunks do not   coincide in time.Awduche, et al.              Informational                     [Page 21]

RFC 2702                MPLS Traffic Engineering          September 19996.2 Resource Class Attribute   Resource class attributes are administratively assigned parameters   which express some notion of "class" for resources. Resource class   attributes can be viewed as "colors" assigned to resources such that   the set of resources with the same "color" conceptually belong to the   same class. Resource class attributes can be used to implement a   variety of policies. The key resources of interest here are links.   When applied to links, the resource class attribute effectively   becomes  an aspect of the "link state" parameters.   The concept of resource class attributes is a powerful abstraction.   From a Traffic Engineering perspective, it can be used to implement   many policies with regard to both traffic and resource oriented   performance optimization. Specifically, resource class attributes can   be used to:   1. Apply uniform policies to a set of resources that do not need      to be in the same topological region.   2. Specify the relative preference of sets of resources for      path placement of traffic trunks.   3. Explicitly restrict the placement of traffic trunks      to  specific subsets of resources.   4. Implement generalized inclusion / exclusion policies.   5. Enforce traffic locality containment policies. That is,      policies    that seek to contain local traffic within      specific topological regions of the network.   Additionally, resource class attributes can be used for   identification purposes.   In general, a resource can be assigned more than one resource class   attribute. For example, all of the OC-48 links in a given network may   be assigned a distinguished resource class attribute. The subsets of   OC-48 links which exist with a given abstraction domain of the   network may be assigned additional resource class attributes in order   to implement specific containment policies, or to architect the   network in a certain manner.7.0 Constraint-Based Routing   This section discusses the issues pertaining to constraint-based   routing in MPLS domains. In contemporary terminology, constraint-   based routing is often referred to as "QoS Routing" see [5,6,7,10].Awduche, et al.              Informational                     [Page 22]

RFC 2702                MPLS Traffic Engineering          September 1999   This document uses the term "constraint-based routing" however,   because it better captures the functionality envisioned, which   generally encompasses QoS routing as a subset.   constraint-based routing enables a demand driven, resource   reservation aware, routing paradigm to co-exist with current topology   driven hop by hop Internet interior gateway protocols.   A constraint-based routing framework uses the following as input:    - The attributes associated with traffic trunks.    - The attributes associated with resources.    - Other topology state information.   Based on this information, a constraint-based routing process on each   node automatically computes explicit routes for each traffic trunk   originating from the node. In this case, an explicit route for each   traffic trunk is a specification of a label switched path that   satisfies the demand requirements expressed in the trunk's   attributes, subject to constraints imposed by resource availability,   administrative policy, and other topology state information.   A constraint-based routing framework can greatly reduce the level of   manual configuration and intervention required to actualize Traffic   Engineering policies.   In practice, the Traffic Engineer, an operator, or even an automaton   will specify the endpoints of a traffic trunk and assign a set of   attributes to the trunk which encapsulate the performance   expectations and behavioral characteristics of the trunk. The   constraint-based routing framework is then expected to find a   feasible path to satisfy the expectations.  If necessary, the Traffic   Engineer or a traffic engineering support system can then use   administratively configured explicit routes to perform fine grained   optimization.7.1 Basic Features of Constraint-Based Routing   A constraint-based routing framework should at least have the   capability to automatically obtain a basic feasible solution to the   traffic trunk path placement problem.   In general, the constraint-based routing problem is known to be   intractable for most realistic constraints. However, in practice, a   very simple well known heuristic (see e.g. [9]) can be used to find a   feasible path if one exists:Awduche, et al.              Informational                     [Page 23]

RFC 2702                MPLS Traffic Engineering          September 1999    - First prune resources that do not satisfy the requirements of      the traffic trunk attributes.    - Next, run a shortest path algorithm on the residual graph.   Clearly, if a feasible path exists for a single traffic trunk, then   the above simple procedure will find it. Additional rules can be   specified to break ties and perform further optimizations.  In   general, ties should be broken so that congestion is minimized.  When   multiple traffic trunks are to be routed, however, it can be shown   that the above algorithm may not always find a mapping, even when a   feasible mapping exists.7.2 Implementation Considerations   Many commercial implementations of frame relay and ATM switches   already support some notion of constraint-based routing. For such   devices or for the novel MPLS centric contraptions devised therefrom,   it should be relatively easy to extend the current constraint-based   routing implementations to accommodate the peculiar requirements of   MPLS.   For routers that use topology driven hop by hop IGPs, constraint-   based routing can be incorporated in at least one of two ways:   1. By extending the current IGP protocols such as OSPF and IS-IS to      support constraint-based routing. Effort is already underway to      provide such extensions to OSPF (see [5,7]).   2. By adding a constraint-based routing process to each router which      can co-exist with current IGPs. This scenario is depicted      in Figure 1.         ------------------------------------------        |          Management Interface            |         ------------------------------------------            |                 |                 |     ------------     ------------------    --------------    |    MPLS    |<->| Constraint-Based |  | Conventional |    |            |   | Routing Process  |  | IGP Process  |     ------------     ------------------    --------------                           |                  |             -----------------------    --------------            | Resource  Attribute   |  | Link State   |            | Availability Database |  | Database     |             -----------------------    --------------    Figure 1. Constraint-Based Routing Process on Layer 3 LSRAwduche, et al.              Informational                     [Page 24]

RFC 2702                MPLS Traffic Engineering          September 1999   There are many important details associated with implementing   constraint-based routing on Layer 3 devices which we do not discuss   here. These include the following:   - Mechanisms for exchange of topology state information     (resource availability information, link state information,     resource attribute information) between constraint-based     routing processes.   - Mechanisms for maintenance of topology state information.   - Interaction between constraint-based routing processes and     conventional IGP processes.   - Mechanisms to accommodate the adaptivity requirements of     traffic trunks.   - Mechanisms to accommodate the resilience and survivability     requirements of traffic trunks.   In summary, constraint-based routing assists in performance   optimization of operational networks by automatically finding   feasible paths that satisfy a set of constraints for traffic trunks.   It can drastically reduce the amount of administrative explicit path   configuration and manual intervention required to achieve Traffic   Engineering objectives.8.0 Conclusion   This manuscript presented a set of requirements for Traffic   Engineering over MPLS. Many capabilities were described aimed at   enhancing the applicability of MPLS to Traffic Engineering in the   Internet.   It should be noted that some of the issues described here can be   addressed by incorporating a minimal set of building blocks into   MPLS, and then using a network management superstructure to extend   the functionality in order to realize the requirements. Also, the   constraint-based routing framework does not have to be part of the   core MPLS specifications. However, MPLS does require some interaction   with a constraint-based routing framework in order to meet the   requirements.Awduche, et al.              Informational                     [Page 25]

RFC 2702                MPLS Traffic Engineering          September 19999.0 Security Considerations   This document does not introduce new security issues beyond those   inherent in MPLS and may use the same mechanisms proposed for this   technology. It is, however, specifically important that manipulation   of administratively configurable parameters be executed in a secure   manner by authorized entities.10.0 References   [1]  Rosen, E., Viswanathan, A. and R. Callon, "A Proposed        Architecture for MPLS", Work in Progress.   [2]  Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow, G.        and A. Viswanathan, "A Framework for Multiprotocol Label        Switching", Work in Progress.   [3]  Li, T. and Y. Rekhter, "Provider Architecture for Differentiated        Services and Traffic Engineering (PASTE)",RFC 2430, October        1998.   [4]  Rekhter, Y., Davie, B., Katz, D., Rosen, E. and  G. Swallow,        "Cisco Systems' Tag Switching Architecture - Overview",RFC2105, February 1997.   [5]  Zhang, Z., Sanchez, C., Salkewicz, B. and E. Crawley "Quality of        Service Extensions to OSPF", Work in Progress.   [6]  Crawley, E., Nair, F., Rajagopalan, B. and H. Sandick, "A        Framework for QoS Based Routing in the Internet",RFC 2386,        August 1998.   [7]  Guerin, R., Kamat, S., Orda, A., Przygienda, T. and D. Williams,        "QoS Routing Mechanisms and OSPF Extensions",RFC 2676, August        1999.   [8]  C. Yang and A. Reddy, "A Taxonomy for Congestion Control        Algorithms in Packet Switching Networks," IEEE Network Magazine,        Volume 9, Number 5, July/August 1995.   [9]  W. Lee, M. Hluchyi, and P. Humblet, "Routing Subject to Quality        of Service Constraints in Integrated Communication Networks,"        IEEE Network, July 1995, pp 46-55.   [10] ATM Forum, "Traffic Management Specification: Version 4.0" April        1996.Awduche, et al.              Informational                     [Page 26]

RFC 2702                MPLS Traffic Engineering          September 199911.0 Acknowledgments   The authors would like to thank Yakov Rekhter for his review of an   earlier draft of this document. The authors would also like to thank   Louis Mamakos and Bill Barns for their helpful suggestions, and   Curtis Villamizar for providing some useful feedback.Awduche, et al.              Informational                     [Page 27]

RFC 2702                MPLS Traffic Engineering          September 199912.0 Authors' Addresses   Daniel O. Awduche   UUNET (MCI Worldcom)   3060 Williams Drive   Fairfax, VA 22031   Phone: +1 703-208-5277   EMail: awduche@uu.net   Joe Malcolm   UUNET  (MCI Worldcom)   3060 Williams Drive   Fairfax, VA 22031   Phone: +1 703-206-5895   EMail: jmalcolm@uu.net   Johnson Agogbua   UUNET  (MCI Worldcom)   3060 Williams Drive   Fairfax, VA 22031   Phone: +1 703-206-5794   EMail: ja@uu.net   Mike O'Dell   UUNET  (MCI Worldcom)   3060 Williams Drive   Fairfax, VA 22031   Phone: +1 703-206-5890   EMail: mo@uu.net   Jim McManus   UUNET  (MCI Worldcom)   3060 Williams Drive   Fairfax, VA 22031   Phone: +1 703-206-5607   EMail: jmcmanus@uu.netAwduche, et al.              Informational                     [Page 28]

RFC 2702                MPLS Traffic Engineering          September 199913.0  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Awduche, et al.              Informational                     [Page 29]