GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3031

Network Working Group E. Rosen Request for Comments: 3031 Cisco Systems, Inc. Category: Standards Track A. Viswanathan

                                                Force10 Networks, Inc.
                                                             R. Callon
                                                Juniper Networks, Inc.
                                                          January 2001
             Multiprotocol Label Switching Architecture

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

 This document specifies the architecture for Multiprotocol Label
 Switching (MPLS).

Table of Contents

 1          Specification  ......................................   3
 2          Introduction to MPLS  ...............................   3
 2.1        Overview  ...........................................   4
 2.2        Terminology  ........................................   6
 2.3        Acronyms and Abbreviations  .........................   9
 2.4        Acknowledgments  ....................................   9
 3          MPLS Basics  ........................................   9
 3.1        Labels  .............................................   9
 3.2        Upstream and Downstream LSRs  .......................  10
 3.3        Labeled Packet  .....................................  11
 3.4        Label Assignment and Distribution  ..................  11
 3.5        Attributes of a Label Binding  ......................  11
 3.6        Label Distribution Protocols  .......................  11
 3.7        Unsolicited Downstream vs. Downstream-on-Demand  ....  12
 3.8        Label Retention Mode  ...............................  12
 3.9        The Label Stack  ....................................  13
 3.10       The Next Hop Label Forwarding Entry (NHLFE)  ........  13
 3.11       Incoming Label Map (ILM)  ...........................  14

Rosen, et al. Standards Track [Page 1] RFC 3031 MPLS Architecture January 2001

 3.12       FEC-to-NHLFE Map (FTN)  .............................  14
 3.13       Label Swapping  .....................................  15
 3.14       Scope and Uniqueness of Labels  .....................  15
 3.15       Label Switched Path (LSP), LSP Ingress, LSP Egress  .  16
 3.16       Penultimate Hop Popping  ............................  18
 3.17       LSP Next Hop  .......................................  20
 3.18       Invalid Incoming Labels  ............................  20
 3.19       LSP Control: Ordered versus Independent  ............  20
 3.20       Aggregation  ........................................  21
 3.21       Route Selection  ....................................  23
 3.22       Lack of Outgoing Label  .............................  24
 3.23       Time-to-Live (TTL)  .................................  24
 3.24       Loop Control  .......................................  25
 3.25       Label Encodings  ....................................  26
 3.25.1     MPLS-specific Hardware and/or Software  .............  26
 3.25.2     ATM Switches as LSRs  ...............................  26
 3.25.3     Interoperability among Encoding Techniques  .........  28
 3.26       Label Merging  ......................................  28
 3.26.1     Non-merging LSRs  ...................................  29
 3.26.2     Labels for Merging and Non-Merging LSRs  ............  30
 3.26.3     Merge over ATM  .....................................  31
 3.26.3.1   Methods of Eliminating Cell Interleave  .............  31
 3.26.3.2   Interoperation: VC Merge, VP Merge, and Non-Merge  ..  31
 3.27       Tunnels and Hierarchy  ..............................  32
 3.27.1     Hop-by-Hop Routed Tunnel  ...........................  32
 3.27.2     Explicitly Routed Tunnel  ...........................  33
 3.27.3     LSP Tunnels  ........................................  33
 3.27.4     Hierarchy: LSP Tunnels within LSPs  .................  33
 3.27.5     Label Distribution Peering and Hierarchy  ...........  34
 3.28       Label Distribution Protocol Transport  ..............  35
 3.29       Why More than one Label Distribution Protocol?  .....  36
 3.29.1     BGP and LDP  ........................................  36
 3.29.2     Labels for RSVP Flowspecs  ..........................  36
 3.29.3     Labels for Explicitly Routed LSPs  ..................  36
 3.30       Multicast  ..........................................  37
 4          Some Applications of MPLS  ..........................  37
 4.1        MPLS and Hop by Hop Routed Traffic  .................  37
 4.1.1      Labels for Address Prefixes  ........................  37
 4.1.2      Distributing Labels for Address Prefixes  ...........  37
 4.1.2.1    Label Distribution Peers for an Address Prefix  .....  37
 4.1.2.2    Distributing Labels  ................................  38
 4.1.3      Using the Hop by Hop path as the LSP  ...............  39
 4.1.4      LSP Egress and LSP Proxy Egress  ....................  39
 4.1.5      The Implicit NULL Label  ............................  40
 4.1.6      Option: Egress-Targeted Label Assignment  ...........  40
 4.2        MPLS and Explicitly Routed LSPs  ....................  42
 4.2.1      Explicitly Routed LSP Tunnels  ......................  42
 4.3        Label Stacks and Implicit Peering  ..................  43

Rosen, et al. Standards Track [Page 2] RFC 3031 MPLS Architecture January 2001

 4.4        MPLS and Multi-Path Routing  ........................  44
 4.5        LSP Trees as Multipoint-to-Point Entities  ..........  44
 4.6        LSP Tunneling between BGP Border Routers  ...........  45
 4.7        Other Uses of Hop-by-Hop Routed LSP Tunnels  ........  47
 4.8        MPLS and Multicast  .................................  47
 5          Label Distribution Procedures (Hop-by-Hop)  .........  47
 5.1        The Procedures for Advertising and Using labels  ....  48
 5.1.1      Downstream LSR: Distribution Procedure  .............  48
 5.1.1.1    PushUnconditional  ..................................  49
 5.1.1.2    PushConditional  ....................................  49
 5.1.1.3    PulledUnconditional  ................................  49
 5.1.1.4    PulledConditional  ..................................  50
 5.1.2      Upstream LSR: Request Procedure  ....................  51
 5.1.2.1    RequestNever  .......................................  51
 5.1.2.2    RequestWhenNeeded  ..................................  51
 5.1.2.3    RequestOnRequest  ...................................  51
 5.1.3      Upstream LSR: NotAvailable Procedure  ...............  52
 5.1.3.1    RequestRetry  .......................................  52
 5.1.3.2    RequestNoRetry  .....................................  52
 5.1.4      Upstream LSR: Release Procedure  ....................  52
 5.1.4.1    ReleaseOnChange  ....................................  52
 5.1.4.2    NoReleaseOnChange  ..................................  53
 5.1.5      Upstream LSR: labelUse Procedure  ...................  53
 5.1.5.1    UseImmediate  .......................................  53
 5.1.5.2    UseIfLoopNotDetected  ...............................  53
 5.1.6      Downstream LSR: Withdraw Procedure  .................  53
 5.2        MPLS Schemes: Supported Combinations of Procedures  .  54
 5.2.1      Schemes for LSRs that Support Label Merging  ........  55
 5.2.2      Schemes for LSRs that do not Support Label Merging  .  56
 5.2.3      Interoperability Considerations  ....................  57
 6          Security Considerations  ............................  58
 7          Intellectual Property  ..............................  58
 8          Authors' Addresses  .................................  59
 9          References  .........................................  59
 10         Full Copyright Statement  ...........................  61

1. Specification

 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 RFC 2119.

2. Introduction to MPLS

 This document specifies the architecture for Multiprotocol Label
 Switching (MPLS).
 Note that the use of MPLS for multicast is left for further study.

Rosen, et al. Standards Track [Page 3] RFC 3031 MPLS Architecture January 2001

2.1. Overview

 As a packet of a connectionless network layer protocol travels from
 one router to the next, each router makes an independent forwarding
 decision for that packet.  That is, each router analyzes the packet's
 header, and each router runs a network layer routing algorithm.  Each
 router independently chooses a next hop for the packet, based on its
 analysis of the packet's header and the results of running the
 routing algorithm.
 Packet headers contain considerably more information than is needed
 simply to choose the next hop.  Choosing the next hop can therefore
 be thought of as the composition of two functions.  The first
 function partitions the entire set of possible packets into a set of
 "Forwarding Equivalence Classes (FECs)".  The second maps each FEC to
 a next hop.  Insofar as the forwarding decision is concerned,
 different packets which get mapped into the same FEC are
 indistinguishable.  All packets which belong to a particular FEC and
 which travel from a particular node will follow the same path (or if
 certain kinds of multi-path routing are in use, they will all follow
 one of a set of paths associated with the FEC).
 In conventional IP forwarding, a particular router will typically
 consider two packets to be in the same FEC if there is some address
 prefix X in that router's routing tables such that X is the "longest
 match" for each packet's destination address.  As the packet
 traverses the network, each hop in turn reexamines the packet and
 assigns it to a FEC.
 In MPLS, the assignment of a particular packet to a particular FEC is
 done just once, as the packet enters the network.  The FEC to which
 the packet is assigned is encoded as a short fixed length value known
 as a "label".  When a packet is forwarded to its next hop, the label
 is sent along with it; that is, the packets are "labeled" before they
 are forwarded.
 At subsequent hops, there is no further analysis of the packet's
 network layer header.  Rather, the label is used as an index into a
 table which specifies the next hop, and a new label.  The old label
 is replaced with the new label, and the packet is forwarded to its
 next hop.
 In the MPLS forwarding paradigm, once a packet is assigned to a FEC,
 no further header analysis is done by subsequent routers; all
 forwarding is driven by the labels.  This has a number of advantages
 over conventional network layer forwarding.

Rosen, et al. Standards Track [Page 4] RFC 3031 MPLS Architecture January 2001

  1. MPLS forwarding can be done by switches which are capable of

doing label lookup and replacement, but are either not capable

       of analyzing the network layer headers, or are not capable of
       analyzing the network layer headers at adequate speed.
  1. Since a packet is assigned to a FEC when it enters the network,

the ingress router may use, in determining the assignment, any

       information it has about the packet, even if that information
       cannot be gleaned from the network layer header.  For example,
       packets arriving on different ports may be assigned to
       different FECs.  Conventional forwarding, on the other hand,
       can only consider information which travels with the packet in
       the packet header.
  1. A packet that enters the network at a particular router can be

labeled differently than the same packet entering the network

       at a different router, and as a result forwarding decisions
       that depend on the ingress router can be easily made.  This
       cannot be done with conventional forwarding, since the identity
       of a packet's ingress router does not travel with the packet.
  1. The considerations that determine how a packet is assigned to a

FEC can become ever more and more complicated, without any

       impact at all on the routers that merely forward labeled
       packets.
  1. Sometimes it is desirable to force a packet to follow a

particular route which is explicitly chosen at or before the

       time the packet enters the network, rather than being chosen by
       the normal dynamic routing algorithm as the packet travels
       through the network.  This may be done as a matter of policy,
       or to support traffic engineering.  In conventional forwarding,
       this requires the packet to carry an encoding of its route
       along with it ("source routing").  In MPLS, a label can be used
       to represent the route, so that the identity of the explicit
       route need not be carried with the packet.
 Some routers analyze a packet's network layer header not merely to
 choose the packet's next hop, but also to determine a packet's
 "precedence" or "class of service".  They may then apply different
 discard thresholds or scheduling disciplines to different packets.
 MPLS allows (but does not require) the precedence or class of service
 to be fully or partially inferred from the label.  In this case, one
 may say that the label represents the combination of a FEC and a
 precedence or class of service.

Rosen, et al. Standards Track [Page 5] RFC 3031 MPLS Architecture January 2001

 MPLS stands for "Multiprotocol" Label Switching, multiprotocol
 because its techniques are applicable to ANY network layer protocol.
 In this document, however, we focus on the use of IP as the network
 layer protocol.
 A router which supports MPLS is known as a "Label Switching Router",
 or LSR.

2.2. Terminology

 This section gives a general conceptual overview of the terms used in
 this document.  Some of these terms are more precisely defined in
 later sections of the document.
    DLCI                      a label used in Frame Relay networks to
                              identify frame relay circuits
    forwarding equivalence class   a group of IP packets which are
                                   forwarded in the same manner (e.g.,
                                   over the same path, with the same
                                   forwarding treatment)
    frame merge               label merging, when it is applied to
                              operation over frame based media, so
                              that the potential problem of cell
                              interleave is not an issue.
    label                     a short fixed length physically
                              contiguous identifier which is used to
                              identify a FEC, usually of local
                              significance.
    label merging             the replacement of multiple incoming
                              labels for a particular FEC with a
                              single outgoing label
    label swap                the basic forwarding operation
                              consisting of looking up an incoming
                              label to determine the outgoing label,
                              encapsulation, port, and other data
                              handling information.
    label swapping            a forwarding paradigm allowing
                              streamlined forwarding of data by using
                              labels to identify classes of data
                              packets which are treated
                              indistinguishably when forwarding.

Rosen, et al. Standards Track [Page 6] RFC 3031 MPLS Architecture January 2001

    label switched hop        the hop between two MPLS nodes, on which
                              forwarding is done using labels.
    label switched path       The path through one or more LSRs at one
                              level of the hierarchy followed by a
                              packets in a particular FEC.
    label switching router    an MPLS node which is capable of
                              forwarding native L3 packets
    layer 2                   the protocol layer under layer 3 (which
                              therefore offers the services used by
                              layer 3).  Forwarding, when done by the
                              swapping of short fixed length labels,
                              occurs at layer 2 regardless of whether
                              the label being examined is an ATM
                              VPI/VCI, a frame relay DLCI, or an MPLS
                              label.
    layer 3                   the protocol layer at which IP and its
                              associated routing protocols operate
                              link layer synonymous with layer 2
    loop detection            a method of dealing with loops in which
                              loops are allowed to be set up, and data
                              may be transmitted over the loop, but
                              the loop is later detected
    loop prevention           a method of dealing with loops in which
                              data is never transmitted over a loop
    label stack               an ordered set of labels
    merge point               a node at which label merging is done
    MPLS domain               a contiguous set of nodes which operate
                              MPLS routing and forwarding and which
                              are also in one Routing or
                              Administrative Domain
    MPLS edge node            an MPLS node that connects an MPLS
                              domain with a node which is outside of
                              the domain, either because it does not
                              run MPLS, and/or because it is in a
                              different domain.  Note that if an LSR
                              has a neighboring host which is not
                              running MPLS, that that LSR is an MPLS
                              edge node.

Rosen, et al. Standards Track [Page 7] RFC 3031 MPLS Architecture January 2001

    MPLS egress node          an MPLS edge node in its role in
                              handling traffic as it leaves an MPLS
                              domain
    MPLS ingress node         an MPLS edge node in its role in
                              handling traffic as it enters an MPLS
                              domain
    MPLS label                a label which is carried in a packet
                              header, and which represents the
                              packet's FEC
    MPLS node                 a node which is running MPLS.  An MPLS
                              node will be aware of MPLS control
                              protocols, will operate one or more L3
                              routing protocols, and will be capable
                              of forwarding packets based on labels.
                              An MPLS node may optionally be also
                              capable of forwarding native L3 packets.
    MultiProtocol Label Switching  an IETF working group and the
                                   effort associated with the working
                                   group
    network layer             synonymous with layer 3
    stack                     synonymous with label stack
    switched path             synonymous with label switched path
    virtual circuit           a circuit used by a connection-oriented
                              layer 2 technology such as ATM or Frame
                              Relay, requiring the maintenance of
                              state information in layer 2 switches.
    VC merge                  label merging where the MPLS label is
                              carried in the ATM VCI field (or
                              combined VPI/VCI field), so as to allow
                              multiple VCs to merge into one single VC
    VP merge                  label merging where the MPLS label is
                              carried din the ATM VPI field, so as to
                              allow multiple VPs to be merged into one
                              single VP.  In this case two cells would
                              have the same VCI value only if they
                              originated from the same node.  This
                              allows cells from different sources to
                              be distinguished via the VCI.

Rosen, et al. Standards Track [Page 8] RFC 3031 MPLS Architecture January 2001

    VPI/VCI                   a label used in ATM networks to identify
                              circuits

2.3. Acronyms and Abbreviations

 ATM                       Asynchronous Transfer Mode
 BGP                       Border Gateway Protocol
 DLCI                      Data Link Circuit Identifier
 FEC                       Forwarding Equivalence Class
 FTN                       FEC to NHLFE Map
 IGP                       Interior Gateway Protocol
 ILM                       Incoming Label Map
 IP                        Internet Protocol
 LDP                       Label Distribution Protocol
 L2                        Layer 2 L3                        Layer 3
 LSP                       Label Switched Path
 LSR                       Label Switching Router
 MPLS                      MultiProtocol Label Switching
 NHLFE                     Next Hop Label Forwarding Entry
 SVC                       Switched Virtual Circuit
 SVP                       Switched Virtual Path
 TTL                       Time-To-Live
 VC                        Virtual Circuit
 VCI                       Virtual Circuit Identifier
 VP                        Virtual Path
 VPI                       Virtual Path Identifier

2.4. Acknowledgments

 The ideas and text in this document have been collected from a number
 of sources and comments received.  We would like to thank Rick
 Boivie, Paul Doolan, Nancy Feldman, Yakov Rekhter, Vijay Srinivasan,
 and George Swallow for their inputs and ideas.

3. MPLS Basics

 In this section, we introduce some of the basic concepts of MPLS and
 describe the general approach to be used.

3.1. Labels

 A label is a short, fixed length, locally significant identifier
 which is used to identify a FEC.  The label which is put on a
 particular packet represents the Forwarding Equivalence Class to
 which that packet is assigned.

Rosen, et al. Standards Track [Page 9] RFC 3031 MPLS Architecture January 2001

 Most commonly, a packet is assigned to a FEC based (completely or
 partially) on its network layer destination address.  However, the
 label is never an encoding of that address.
 If Ru and Rd are LSRs, they may agree that when Ru transmits a packet
 to Rd, Ru will label with packet with label value L if and only if
 the packet is a member of a particular FEC F.  That is, they can
 agree to a "binding" between label L and FEC F for packets moving
 from Ru to Rd.  As a result of such an agreement, L becomes Ru's
 "outgoing label" representing FEC F, and L becomes Rd's "incoming
 label" representing FEC F.
 Note that L does not necessarily represent FEC F for any packets
 other than those which are being sent from Ru to Rd.  L is an
 arbitrary value whose binding to F is local to Ru and Rd.
 When we speak above of packets "being sent" from Ru to Rd, we do not
 imply either that the packet originated at Ru or that its destination
 is Rd.  Rather, we mean to include packets which are "transit
 packets" at one or both of the LSRs.
 Sometimes it may be difficult or even impossible for Rd to tell, of
 an arriving packet carrying label L, that the label L was placed in
 the packet by Ru, rather than by some other LSR.  (This will
 typically be the case when Ru and Rd are not direct neighbors.)  In
 such cases, Rd must make sure that the binding from label to FEC is
 one-to-one.  That is, Rd MUST NOT agree with Ru1 to bind L to FEC F1,
 while also agreeing with some other LSR Ru2 to bind L to a different
 FEC F2, UNLESS Rd can always tell, when it receives a packet with
 incoming label L, whether the label was put on the packet by Ru1 or
 whether it was put on by Ru2.
 It is the responsibility of each LSR to ensure that it can uniquely
 interpret its incoming labels.

3.2. Upstream and Downstream LSRs

 Suppose Ru and Rd have agreed to bind label L to FEC F, for packets
 sent from Ru to Rd.  Then with respect to this binding, Ru is the
 "upstream LSR", and Rd is the "downstream LSR".
 To say that one node is upstream and one is downstream with respect
 to a given binding means only that a particular label represents a
 particular FEC in packets travelling from the upstream node to the
 downstream node.  This is NOT meant to imply that packets in that FEC
 would actually be routed from the upstream node to the downstream
 node.

Rosen, et al. Standards Track [Page 10] RFC 3031 MPLS Architecture January 2001

3.3. Labeled Packet

 A "labeled packet" is a packet into which a label has been encoded.
 In some cases, the label resides in an encapsulation header which
 exists specifically for this purpose.  In other cases, the label may
 reside in an existing data link or network layer header, as long as
 there is a field which is available for that purpose.  The particular
 encoding technique to be used must be agreed to by both the entity
 which encodes the label and the entity which decodes the label.

3.4. Label Assignment and Distribution

 In the MPLS architecture, the decision to bind a particular label L
 to a particular FEC F is made by the LSR which is DOWNSTREAM with
 respect to that binding.  The downstream LSR then informs the
 upstream LSR of the binding.  Thus labels are "downstream-assigned",
 and label bindings are distributed in the "downstream to upstream"
 direction.
 If an LSR has been designed so that it can only look up labels that
 fall into a certain numeric range, then it merely needs to ensure
 that it only binds labels that are in that range.

3.5. Attributes of a Label Binding

 A particular binding of label L to FEC F, distributed by Rd to Ru,
 may have associated "attributes".  If Ru, acting as a downstream LSR,
 also distributes a binding of a label to FEC F, then under certain
 conditions, it may be required to also distribute the corresponding
 attribute that it received from Rd.

3.6. Label Distribution Protocols

 A label distribution protocol is a set of procedures by which one LSR
 informs another of the label/FEC bindings it has made.  Two LSRs
 which use a label distribution protocol to exchange label/FEC binding
 information are known as "label distribution peers" with respect to
 the binding information they exchange.  If two LSRs are label
 distribution peers, we will speak of there being a "label
 distribution adjacency" between them.
 (N.B.: two LSRs may be label distribution peers with respect to some
 set of bindings, but not with respect to some other set of bindings.)
 The label distribution protocol also encompasses any negotiations in
 which two label distribution peers need to engage in order to learn
 of each other's MPLS capabilities.

Rosen, et al. Standards Track [Page 11] RFC 3031 MPLS Architecture January 2001

 THE ARCHITECTURE DOES NOT ASSUME THAT THERE IS ONLY A SINGLE LABEL
 DISTRIBUTION PROTOCOL.  In fact, a number of different label
 distribution protocols are being standardized.  Existing protocols
 have been extended so that label distribution can be piggybacked on
 them (see, e.g., [MPLS-BGP], [MPLS-RSVP-TUNNELS]).  New protocols
 have also been defined for the explicit purpose of distributing
 labels (see, e.g., [MPLS-LDP], [MPLS-CR-LDP].
 In this document, we try to use the acronym "LDP" to refer
 specifically to the protocol defined in [MPLS-LDP]; when speaking of
 label distribution protocols in general, we try to avoid the acronym.

3.7. Unsolicited Downstream vs. Downstream-on-Demand

 The MPLS architecture allows an LSR to explicitly request, from its
 next hop for a particular FEC, a label binding for that FEC.  This is
 known as "downstream-on-demand" label distribution.
 The MPLS architecture also allows an LSR to distribute bindings to
 LSRs that have not explicitly requested them.  This is known as
 "unsolicited downstream" label distribution.
 It is expected that some MPLS implementations will provide only
 downstream-on-demand label distribution, and some will provide only
 unsolicited downstream label distribution, and some will provide
 both.  Which is provided may depend on the characteristics of the
 interfaces which are supported by a particular implementation.
 However, both of these label distribution techniques may be used in
 the same network at the same time.  On any given label distribution
 adjacency, the upstream LSR and the downstream LSR must agree on
 which technique is to be used.

3.8. Label Retention Mode

 An LSR Ru may receive (or have received) a label binding for a
 particular FEC from an LSR Rd, even though Rd is not Ru's next hop
 (or is no longer Ru's next hop) for that FEC.
 Ru then has the choice of whether to keep track of such bindings, or
 whether to discard such bindings.  If Ru keeps track of such
 bindings, then it may immediately begin using the binding again if Rd
 eventually becomes its next hop for the FEC in question.  If Ru
 discards such bindings, then if Rd later becomes the next hop, the
 binding will have to be reacquired.

Rosen, et al. Standards Track [Page 12] RFC 3031 MPLS Architecture January 2001

 If an LSR supports "Liberal Label Retention Mode", it maintains the
 bindings between a label and a FEC which are received from LSRs which
 are not its next hop for that  FEC.  If an LSR supports "Conservative
 Label Retention Mode", it discards such bindings.
 Liberal label retention mode allows for quicker adaptation to routing
 changes, but conservative label retention mode though requires an LSR
 to maintain many fewer labels.

3.9. The Label Stack

 So far, we have spoken as if a labeled packet carries only a single
 label.  As we shall see, it is useful to have a more general model in
 which a labeled packet carries a number of labels, organized as a
 last-in, first-out stack.  We refer to this as a "label stack".
 Although, as we shall see, MPLS supports a hierarchy, the processing
 of a labeled packet is completely independent of the level of
 hierarchy.  The processing is always based on the top label, without
 regard for the possibility that some number of other labels may have
 been "above it" in the past, or that some number of other labels may
 be below it at present.
 An unlabeled packet can be thought of as a packet whose label stack
 is empty (i.e., whose label stack has depth 0).
 If a packet's label stack is of depth m, we refer to the label at the
 bottom of the stack as the level 1 label, to the label above it (if
 such exists) as the level 2 label, and to the label at the top of the
 stack as the level m label.
 The utility of the label stack will become clear when we introduce
 the notion of LSP Tunnel and the MPLS Hierarchy (section 3.27).

3.10. The Next Hop Label Forwarding Entry (NHLFE)

 The "Next Hop Label Forwarding Entry" (NHLFE) is used when forwarding
 a labeled packet.  It contains the following information:
 1. the packet's next hop
 2. the operation to perform on the packet's label stack; this is one
    of the following operations:
    a) replace the label at the top of the label stack with a
       specified new label
    b) pop the label stack

Rosen, et al. Standards Track [Page 13] RFC 3031 MPLS Architecture January 2001

    c) replace the label at the top of the label stack with a
       specified new label, and then push one or more specified new
       labels onto the label stack.
 It may also contain:
    d) the data link encapsulation to use when transmitting the packet
    e) the way to encode the label stack when transmitting the packet
    f) any other information needed in order to properly dispose of
       the packet.
 Note that at a given LSR, the packet's "next hop" might be that LSR
 itself.  In this case, the LSR would need to pop the top level label,
 and then "forward" the resulting packet to itself.  It would then
 make another forwarding decision, based on what remains after the
 label stacked is popped.  This may still be a labeled packet, or it
 may be the native IP packet.
 This implies that in some cases the LSR may need to operate on the IP
 header in order to forward the packet.
 If the packet's "next hop" is the current LSR, then the label stack
 operation MUST be to "pop the stack".

3.11. Incoming Label Map (ILM)

 The "Incoming Label Map" (ILM) maps each incoming label to a set of
 NHLFEs.  It is used when forwarding packets that arrive as labeled
 packets.
 If the ILM maps a particular label to a set of NHLFEs that contains
 more than one element, exactly one element of the set must be chosen
 before the packet is forwarded.  The procedures for choosing an
 element from the set are beyond the scope of this document.  Having
 the ILM map a label to a set containing more than one NHLFE may be
 useful if, e.g., it is desired to do load balancing over multiple
 equal-cost paths.

3.12. FEC-to-NHLFE Map (FTN)

 The "FEC-to-NHLFE" (FTN) maps each FEC to a set of NHLFEs.  It is
 used when forwarding packets that arrive unlabeled, but which are to
 be labeled before being forwarded.

Rosen, et al. Standards Track [Page 14] RFC 3031 MPLS Architecture January 2001

 If the FTN maps a particular label to a set of NHLFEs that contains
 more than one element, exactly one element of the set must be chosen
 before the packet is forwarded.  The procedures for choosing an
 element from the set are beyond the scope of this document.  Having
 the FTN map a label to a set containing more than one NHLFE may be
 useful if, e.g., it is desired to do load balancing over multiple
 equal-cost paths.

3.13. Label Swapping

 Label swapping is the use of the following procedures to forward a
 packet.
 In order to forward a labeled packet, a LSR examines the label at the
 top of the label stack.  It uses the ILM to map this label to an
 NHLFE.  Using the information in the NHLFE, it determines where to
 forward the packet, and performs an operation on the packet's label
 stack.  It then encodes the new label stack into the packet, and
 forwards the result.
 In order to forward an unlabeled packet, a LSR analyzes the network
 layer header, to determine the packet's FEC.  It then uses the FTN to
 map this to an NHLFE.  Using the information in the NHLFE, it
 determines where to forward the packet, and performs an operation on
 the packet's label stack.  (Popping the label stack would, of course,
 be illegal in this case.)  It then encodes the new label stack into
 the packet, and forwards the result.
 IT IS IMPORTANT TO NOTE THAT WHEN LABEL SWAPPING IS IN USE, THE NEXT
 HOP IS ALWAYS TAKEN FROM THE NHLFE; THIS MAY IN SOME CASES BE
 DIFFERENT FROM WHAT THE NEXT HOP WOULD BE IF MPLS WERE NOT IN USE.

3.14. Scope and Uniqueness of Labels

 A given LSR Rd may bind label L1 to FEC F, and distribute that
 binding to label distribution peer Ru1.  Rd may also bind label L2 to
 FEC F, and distribute that binding to label distribution peer Ru2.
 Whether or not L1 == L2 is not determined by the architecture; this
 is a local matter.
 A given LSR Rd may bind label L to FEC F1, and distribute that
 binding to label distribution peer Ru1.  Rd may also bind label L to
 FEC F2, and distribute that binding to label distribution peer Ru2.
 IF (AND ONLY IF) RD CAN TELL, WHEN IT RECEIVES A PACKET WHOSE TOP
 LABEL IS L, WHETHER THE LABEL WAS PUT THERE BY RU1 OR BY RU2, THEN
 THE ARCHITECTURE DOES NOT REQUIRE THAT F1 == F2.  In such cases, we
 may say that Rd is using a different "label space" for the labels it
 distributes to Ru1 than for the labels it distributes to Ru2.

Rosen, et al. Standards Track [Page 15] RFC 3031 MPLS Architecture January 2001

 In general, Rd can only tell whether it was Ru1 or Ru2 that put the
 particular label value L at the top of the label stack if the
 following conditions hold:
  1. Ru1 and Ru2 are the only label distribution peers to which Rd

distributed a binding of label value L, and

  1. Ru1 and Ru2 are each directly connected to Rd via a point-to-

point interface.

 When these conditions hold, an LSR may use labels that have "per
 interface" scope, i.e., which are only unique per interface.  We may
 say that the LSR is using a "per-interface label space".  When these
 conditions do not hold, the labels must be unique over the LSR which
 has assigned them, and we may say that the LSR is using a "per-
 platform label space."
 If a particular LSR Rd is attached to a particular LSR Ru over two
 point-to-point interfaces, then Rd may distribute to Ru a binding of
 label L to FEC F1, as well as a binding of label L to FEC F2, F1 !=
 F2, if and only if each binding is valid only for packets which Ru
 sends to Rd over a particular one of the interfaces.  In all other
 cases, Rd MUST NOT distribute to Ru bindings of the same label value
 to two different FECs.
 This prohibition holds even if the bindings are regarded as being at
 different "levels of hierarchy".  In MPLS, there is no notion of
 having a different label space for different levels of the hierarchy;
 when interpreting a label, the level of the label is irrelevant.
 The question arises as to whether it is possible for an LSR to use
 multiple per-platform label spaces, or to use multiple per-interface
 label spaces for the same interface.  This is not prohibited by the
 architecture.  However, in such cases the LSR must have some means,
 not specified by the architecture, of determining, for a particular
 incoming label, which label space that label belongs to.  For
 example, [MPLS-SHIM] specifies that a different label space is used
 for unicast packets than for multicast packets, and uses a data link
 layer codepoint to distinguish the two label spaces.

3.15. Label Switched Path (LSP), LSP Ingress, LSP Egress

 A "Label Switched Path (LSP) of level m" for a particular packet P is
 a sequence of routers,
                             <R1, ..., Rn>
 with the following properties:

Rosen, et al. Standards Track [Page 16] RFC 3031 MPLS Architecture January 2001

    1. R1, the "LSP Ingress", is an LSR which pushes a label onto P's
       label stack, resulting in a label stack of depth m;
    2. For all i, 1<i<n, P has a label stack of depth m when received
       by LSR Ri;
    3. At no time during P's transit from R1 to R[n-1] does its label
       stack ever have a depth of less than m;
    4. For all i, 1<i<n: Ri transmits P to R[i+1] by means of MPLS,
       i.e., by using the label at the top of the label stack (the
       level m label) as an index into an ILM;
    5. For all i, 1<i<n: if a system S receives and forwards P after P
       is transmitted by Ri but before P is received by R[i+1] (e.g.,
       Ri and R[i+1] might be connected via a switched data link
       subnetwork, and S might be one of the data link switches), then
       S's forwarding decision is not based on the level m label, or
       on the network layer header.  This may be because:
       a) the decision is not based on the label stack or the network
          layer header at all;
       b) the decision is based on a label stack on which additional
          labels have been pushed (i.e., on a level m+k label, where
          k>0).
 In other words, we can speak of the level m LSP for Packet P as the
 sequence of routers:
    1. which begins with an LSR (an "LSP Ingress") that pushes on a
       level m label,
    2. all of whose intermediate LSRs make their forwarding decision
       by label Switching on a level m label,
    3. which ends (at an "LSP Egress") when a forwarding decision is
       made by label Switching on a level m-k label, where k>0, or
       when a forwarding decision is made by "ordinary", non-MPLS
       forwarding procedures.
 A consequence (or perhaps a presupposition) of this is that whenever
 an LSR pushes a label onto an already labeled packet, it needs to
 make sure that the new label corresponds to a FEC whose LSP Egress is
 the LSR that assigned the label which is now second in the stack.

Rosen, et al. Standards Track [Page 17] RFC 3031 MPLS Architecture January 2001

 We will call a sequence of LSRs the "LSP for a particular FEC F" if
 it is an LSP of level m for a particular packet P when P's level m
 label is a label corresponding to FEC F.
 Consider the set of nodes which may be LSP ingress nodes for FEC F.
 Then there is an LSP for FEC F which begins with each of those nodes.
 If a number of those LSPs have the same LSP egress, then one can
 consider the set of such LSPs to be a tree, whose root is the LSP
 egress.  (Since data travels along this tree towards the root, this
 may be called a multipoint-to-point tree.)  We can thus speak of the
 "LSP tree" for a particular FEC F.

3.16. Penultimate Hop Popping

 Note that according to the definitions of section 3.15, if <R1, ...,
 Rn> is a level m LSP for packet P, P may be transmitted from R[n-1]
 to Rn with a label stack of depth m-1.  That is, the label stack may
 be popped at the penultimate LSR of the LSP, rather than at the LSP
 Egress.
 From an architectural perspective, this is perfectly appropriate.
 The purpose of the level m label is to get the packet to Rn.  Once
 R[n-1] has decided to send the packet to Rn, the label no longer has
 any function, and need no longer be carried.
 There is also a practical advantage to doing penultimate hop popping.
 If one does not do this, then when the LSP egress receives a packet,
 it first looks up the top label, and determines as a result of that
 lookup that it is indeed the LSP egress.  Then it must pop the stack,
 and examine what remains of the packet.  If there is another label on
 the stack, the egress will look this up and forward the packet based
 on this lookup.  (In this case, the egress for the packet's level m
 LSP is also an intermediate node for its level m-1 LSP.)  If there is
 no other label on the stack, then the packet is forwarded according
 to its network layer destination address.  Note that this would
 require the egress to do TWO lookups, either two label lookups or a
 label lookup followed by an address lookup.
 If, on the other hand, penultimate hop popping is used, then when the
 penultimate hop looks up the label, it determines:
  1. that it is the penultimate hop, and
  1. who the next hop is.
 The penultimate node then pops the stack, and forwards the packet
 based on the information gained by looking up the label that was
 previously at the top of the stack.  When the LSP egress receives the

Rosen, et al. Standards Track [Page 18] RFC 3031 MPLS Architecture January 2001

 packet, the label which is now at the top of the stack will be the
 label which it needs to look up in order to make its own forwarding
 decision.  Or, if the packet was only carrying a single label, the
 LSP egress will simply see the network layer packet, which is just
 what it needs to see in order to make its forwarding decision.
 This technique allows the egress to do a single lookup, and also
 requires only a single lookup by the penultimate node.
 The creation of the forwarding "fastpath" in a label switching
 product may be greatly aided if it is known that only a single lookup
 is ever required:
  1. the code may be simplified if it can assume that only a single

lookup is ever needed

  1. the code can be based on a "time budget" that assumes that only

a single lookup is ever needed.

 In fact, when penultimate hop popping is done, the LSP Egress need
 not even be an LSR.
 However, some hardware switching engines may not be able to pop the
 label stack, so this cannot be universally required.  There may also
 be some situations in which penultimate hop popping is not desirable.
 Therefore the penultimate node pops the label stack only if this is
 specifically requested by the egress node, OR if the next node in the
 LSP does not support MPLS.  (If the next node in the LSP does support
 MPLS, but does not make such a request, the penultimate node has no
 way of knowing that it in fact is the penultimate node.)
 An LSR which is capable of popping the label stack at all MUST do
 penultimate hop popping when so requested by its downstream label
 distribution peer.
 Initial label distribution protocol negotiations MUST allow each LSR
 to determine whether its neighboring LSRS are capable of popping the
 label stack.  A LSR MUST NOT request a label distribution peer to pop
 the label stack unless it is capable of doing so.
 It may be asked whether the egress node can always interpret the top
 label of a received packet properly if penultimate hop popping is
 used.  As long as the uniqueness and scoping rules of section 3.14
 are obeyed, it is always possible to interpret the top label of a
 received packet unambiguously.

Rosen, et al. Standards Track [Page 19] RFC 3031 MPLS Architecture January 2001

3.17. LSP Next Hop

 The LSP Next Hop for a particular labeled packet in a particular LSR
 is the LSR which is the next hop, as selected by the NHLFE entry used
 for forwarding that packet.
 The LSP Next Hop for a particular FEC is the next hop as selected by
 the NHLFE entry indexed by a label which corresponds to that FEC.
 Note that the LSP Next Hop may differ from the next hop which would
 be chosen by the network layer routing algorithm.  We will use the
 term "L3 next hop" when we refer to the latter.

3.18. Invalid Incoming Labels

 What should an LSR do if it receives a labeled packet with a
 particular incoming label, but has no binding for that label?  It is
 tempting to think that the labels can just be removed, and the packet
 forwarded as an unlabeled IP packet.  However, in some cases, doing
 so could cause a loop.  If the upstream LSR thinks the label is bound
 to an explicit route, and the downstream LSR doesn't think the label
 is bound to anything, and if the hop by hop routing of the unlabeled
 IP packet brings the packet back to the upstream LSR, then a loop is
 formed.
 It is also possible that the label was intended to represent a route
 which cannot be inferred from the IP header.
 Therefore, when a labeled packet is received with an invalid incoming
 label, it MUST be discarded, UNLESS it is determined by some means
 (not within the scope of the current document) that forwarding it
 unlabeled cannot cause any harm.

3.19. LSP Control: Ordered versus Independent

 Some FECs correspond to address prefixes which are distributed via a
 dynamic routing algorithm.  The setup of the LSPs for these FECs can
 be done in one of two ways: Independent LSP Control or Ordered LSP
 Control.
 In Independent LSP Control, each LSR, upon noting that it recognizes
 a particular FEC, makes an independent decision to bind a label to
 that FEC and to distribute that binding to its label distribution
 peers.  This corresponds to the way that conventional IP datagram
 routing works; each node makes an independent decision as to how to
 treat each packet, and relies on the routing algorithm to converge
 rapidly so as to ensure that each datagram is correctly delivered.

Rosen, et al. Standards Track [Page 20] RFC 3031 MPLS Architecture January 2001

 In Ordered LSP Control, an LSR only binds a label to a particular FEC
 if it is the egress LSR for that FEC, or if it has already received a
 label binding for that FEC from its next hop for that FEC.
 If one wants to ensure that traffic in a particular FEC follows a
 path with some specified set of properties (e.g., that the traffic
 does not traverse any node twice, that a specified amount of
 resources are available to the traffic, that the traffic follows an
 explicitly specified path, etc.)  ordered control must be used.  With
 independent control, some LSRs may begin label switching a traffic in
 the FEC before the LSP is completely set up, and thus some traffic in
 the FEC may follow a path which does not have the specified set of
 properties.  Ordered control also needs to be used if the recognition
 of the FEC is a consequence of the setting up of the corresponding
 LSP.
 Ordered LSP setup may be initiated either by the ingress or the
 egress.
 Ordered control and independent control are fully interoperable.
 However, unless all LSRs in an LSP are using ordered control, the
 overall effect on network behavior is largely that of independent
 control, since one cannot be sure that an LSP is not used until it is
 fully set up.
 This architecture allows the choice between independent control and
 ordered control to be a local matter.  Since the two methods
 interwork, a given LSR need support only one or the other.  Generally
 speaking, the choice of independent versus ordered control does not
 appear to have any effect on the label distribution mechanisms which
 need to be defined.

3.20. Aggregation

 One way of partitioning traffic into FECs is to create a separate FEC
 for each address prefix which appears in the routing table.  However,
 within a particular MPLS domain, this may result in a set of FECs
 such that all traffic in all those FECs follows the same route.  For
 example, a set of distinct address prefixes might all have the same
 egress node, and label swapping might be used only to get the the
 traffic to the egress node.  In this case, within the MPLS domain,
 the union of those FECs is itself a FEC.  This creates a choice:
 should a distinct label be bound to each component FEC, or should a
 single label be bound to the union, and that label applied to all
 traffic in the union?
 The procedure of binding a single label to a union of FECs which is
 itself a FEC (within some domain), and of applying that label to all

Rosen, et al. Standards Track [Page 21] RFC 3031 MPLS Architecture January 2001

 traffic in the union, is known as "aggregation".  The MPLS
 architecture allows aggregation.  Aggregation may reduce the number
 of labels which are needed to handle a particular set of packets, and
 may also reduce the amount of label distribution control traffic
 needed.
 Given a set of FECs which are "aggregatable" into a single FEC, it is
 possible to (a) aggregate them into a single FEC, (b) aggregate them
 into a set of FECs, or (c) not aggregate them at all.  Thus we can
 speak of the "granularity" of aggregation, with (a) being the
 "coarsest granularity", and (c) being the "finest granularity".
 When order control is used, each LSR should adopt, for a given set of
 FECs, the granularity used by its next hop for those FECs.
 When independent control is used, it is possible that there will be
 two adjacent LSRs, Ru and Rd, which aggregate some set of FECs
 differently.
 If Ru has finer granularity than Rd, this does not cause a problem.
 Ru distributes more labels for that set of FECs than Rd does.  This
 means that when Ru needs to forward labeled packets in those FECs to
 Rd, it may need to map n labels into m labels, where n > m.  As an
 option, Ru may withdraw the set of n labels that it has distributed,
 and then distribute a set of m labels, corresponding to Rd's level of
 granularity.  This is not necessary to ensure correct operation, but
 it does result in a reduction of the number of labels distributed by
 Ru, and Ru is not gaining any particular advantage by distributing
 the larger number of labels.  The decision whether to do this or not
 is a local matter.
 If Ru has coarser granularity than Rd (i.e., Rd has distributed n
 labels for the set of FECs, while Ru has distributed m, where n > m),
 it has two choices:
  1. It may adopt Rd's finer level of granularity. This would

require it to withdraw the m labels it has distributed, and

       distribute n labels.  This is the preferred option.
  1. It may simply map its m labels into a subset of Rd's n labels,

if it can determine that this will produce the same routing.

       For example, suppose that Ru applies a single label to all
       traffic that needs to pass through a certain egress LSR,
       whereas Rd binds a number of different labels to such traffic,
       depending on the individual destination addresses of the
       packets.  If Ru knows the address of the egress router, and if
       Rd has bound a label to the FEC which is identified by that
       address, then Ru can simply apply that label.

Rosen, et al. Standards Track [Page 22] RFC 3031 MPLS Architecture January 2001

 In any event, every LSR needs to know (by configuration) what
 granularity to use for labels that it assigns.  Where ordered control
 is used, this requires each node to know the granularity only for
 FECs which leave the MPLS network at that node.  For independent
 control, best results may be obtained by ensuring that all LSRs are
 consistently configured to know the granularity for each FEC.
 However, in many cases this may be done by using a single level of
 granularity which applies to all FECs (such as "one label per IP
 prefix in the forwarding table", or "one label per egress node").

3.21. Route Selection

 Route selection refers to the method used for selecting the LSP for a
 particular FEC.  The proposed MPLS protocol architecture supports two
 options for Route Selection: (1) hop by hop routing, and (2) explicit
 routing.
 Hop by hop routing allows each node to independently choose the next
 hop for each FEC.  This is the usual mode today in existing IP
 networks.  A "hop by hop routed LSP" is an LSP whose route is
 selected using hop by hop routing.
 In an explicitly routed LSP, each LSR does not independently choose
 the next hop; rather, a single LSR, generally the LSP ingress or the
 LSP egress, specifies several (or all) of the LSRs in the LSP.  If a
 single LSR specifies the entire LSP, the LSP is "strictly" explicitly
 routed.  If a single LSR specifies only some of the LSP, the LSP is
 "loosely" explicitly routed.
 The sequence of LSRs followed by an explicitly routed LSP may be
 chosen by configuration, or may be selected dynamically by a single
 node (for example, the egress node may make use of the topological
 information learned from a link state database in order to compute
 the entire path for the tree ending at that egress node).
 Explicit routing may be useful for a number of purposes, such as
 policy routing or traffic engineering.  In MPLS, the explicit route
 needs to be specified at the time that labels are assigned, but the
 explicit route does not have to be specified with each IP packet.
 This makes MPLS explicit routing much more efficient than the
 alternative of IP source routing.
 The procedures for making use of explicit routes, either strict or
 loose, are beyond the scope of this document.

Rosen, et al. Standards Track [Page 23] RFC 3031 MPLS Architecture January 2001

3.22. Lack of Outgoing Label

 When a labeled packet is traveling along an LSP, it may occasionally
 happen that it reaches an LSR at which the ILM does not map the
 packet's incoming label into an NHLFE, even though the incoming label
 is itself valid.  This can happen due to transient conditions, or due
 to an error at the LSR which should be the packet's next hop.
 It is tempting in such cases to strip off the label stack and attempt
 to forward the packet further via conventional forwarding, based on
 its network layer header.  However, in general this is not a safe
 procedure:
  1. If the packet has been following an explicitly routed LSP, this

could result in a loop.

  1. The packet's network header may not contain enough information

to enable this particular LSR to forward it correctly.

 Unless it can be determined (through some means outside the scope of
 this document) that neither of these situations obtains, the only
 safe procedure is to discard the packet.

3.23. Time-to-Live (TTL)

 In conventional IP forwarding, each packet carries a "Time To Live"
 (TTL) value in its header.  Whenever a packet passes through a
 router, its TTL gets decremented by 1; if the TTL reaches 0 before
 the packet has reached its destination, the packet gets discarded.
 This provides some level of protection against forwarding loops that
 may exist due to misconfigurations, or due to failure or slow
 convergence of the routing algorithm.  TTL is sometimes used for
 other functions as well, such as multicast scoping, and supporting
 the "traceroute" command.  This implies that there are two TTL-
 related issues that MPLS needs to deal with: (i) TTL as a way to
 suppress loops; (ii) TTL as a way to accomplish other functions, such
 as limiting the scope of a packet.
 When a packet travels along an LSP, it SHOULD emerge with the same
 TTL value that it would have had if it had traversed the same
 sequence of routers without having been label switched.  If the
 packet travels along a hierarchy of LSPs, the total number of LSR-
 hops traversed SHOULD be reflected in its TTL value when it emerges
 from the hierarchy of LSPs.

Rosen, et al. Standards Track [Page 24] RFC 3031 MPLS Architecture January 2001

 The way that TTL is handled may vary depending upon whether the MPLS
 label values are carried in an MPLS-specific "shim" header [MPLS-
 SHIM], or if the MPLS labels are carried in an L2 header, such as an
 ATM header [MPLS-ATM] or a frame relay header [MPLS-FRMRLY].
 If the label values are encoded in a "shim" that sits between the
 data link and network layer headers, then this shim MUST have a TTL
 field that SHOULD be initially loaded from the network layer header
 TTL field, SHOULD be decremented at each LSR-hop, and SHOULD be
 copied into the network layer header TTL field when the packet
 emerges from its LSP.
 If the label values are encoded in a data link layer header (e.g.,
 the VPI/VCI field in ATM's AAL5 header), and the labeled packets are
 forwarded by an L2 switch (e.g., an ATM switch), and the data link
 layer (like ATM) does not itself have a TTL field, then it will not
 be possible to decrement a packet's TTL at each LSR-hop.  An LSP
 segment which consists of a sequence of LSRs that cannot decrement a
 packet's TTL will be called a "non-TTL LSP segment".
 When a packet emerges from a non-TTL LSP segment, it SHOULD however
 be given a TTL that reflects the number of LSR-hops it traversed.  In
 the unicast case, this can be achieved by propagating a meaningful
 LSP length to ingress nodes, enabling the ingress to decrement the
 TTL value before forwarding packets into a non-TTL LSP segment.
 Sometimes it can be determined, upon ingress to a non-TTL LSP
 segment, that a particular packet's TTL will expire before the packet
 reaches the egress of that non-TTL LSP segment.  In this case, the
 LSR at the ingress to the non-TTL LSP segment must not label switch
 the packet.  This means that special procedures must be developed to
 support traceroute functionality, for example, traceroute packets may
 be forwarded using conventional hop by hop forwarding.

3.24. Loop Control

 On a non-TTL LSP segment, by definition, TTL cannot be used to
 protect against forwarding loops.  The importance of loop control may
 depend on the particular hardware being used to provide the LSR
 functions along the non-TTL LSP segment.
 Suppose, for instance, that ATM switching hardware is being used to
 provide MPLS switching functions, with the label being carried in the
 VPI/VCI field.  Since ATM switching hardware cannot decrement TTL,
 there is no protection against loops.  If the ATM hardware is capable
 of providing fair access to the buffer pool for incoming cells
 carrying different VPI/VCI values, this looping may not have any
 deleterious effect on other traffic.  If the ATM hardware cannot

Rosen, et al. Standards Track [Page 25] RFC 3031 MPLS Architecture January 2001

 provide fair buffer access of this sort, however, then even transient
 loops may cause severe degradation of the LSR's total performance.
 Even if fair buffer access can be provided, it is still worthwhile to
 have some means of detecting loops that last "longer than possible".
 In addition, even where TTL and/or per-VC fair queuing provides a
 means for surviving loops, it still may be desirable where practical
 to avoid setting up LSPs which loop.  All LSRs that may attach to
 non-TTL LSP segments will therefore be required to support a common
 technique for loop detection; however, use of the loop detection
 technique is optional.  The loop detection technique is specified in
 [MPLS-ATM] and [MPLS-LDP].

3.25. Label Encodings

 In order to transmit a label stack along with the packet whose label
 stack it is, it is necessary to define a concrete encoding of the
 label stack.  The architecture supports several different encoding
 techniques; the choice of encoding technique depends on the
 particular kind of device being used to forward labeled packets.

3.25.1. MPLS-specific Hardware and/or Software

 If one is using MPLS-specific hardware and/or software to forward
 labeled packets, the most obvious way to encode the label stack is to
 define a new protocol to be used as a "shim" between the data link
 layer and network layer headers.  This shim would really be just an
 encapsulation of the network layer packet; it would be "protocol-
 independent" such that it could be used to encapsulate any network
 layer.  Hence we will refer to it as the "generic MPLS
 encapsulation".
 The generic MPLS encapsulation would in turn be encapsulated in a
 data link layer protocol.
 The MPLS generic encapsulation is specified in [MPLS-SHIM].

3.25.2. ATM Switches as LSRs

 It will be noted that MPLS forwarding procedures are similar to those
 of legacy "label swapping" switches such as ATM switches.  ATM
 switches use the input port and the incoming VPI/VCI value as the
 index into a "cross-connect" table, from which they obtain an output
 port and an outgoing VPI/VCI value.  Therefore if one or more labels
 can be encoded directly into the fields which are accessed by these
 legacy switches, then the legacy switches can, with suitable software
 upgrades, be used as LSRs.  We will refer to such devices as "ATM-
 LSRs".

Rosen, et al. Standards Track [Page 26] RFC 3031 MPLS Architecture January 2001

 There are three obvious ways to encode labels in the ATM cell header
 (presuming the use of AAL5):
    1. SVC Encoding
       Use the VPI/VCI field to encode the label which is at the top
       of the label stack.  This technique can be used in any network.
       With this encoding technique, each LSP is realized as an ATM
       SVC, and the label distribution protocol becomes the ATM
       "signaling" protocol.  With this encoding technique, the ATM-
       LSRs cannot perform "push" or "pop" operations on the label
       stack.
    2. SVP Encoding
       Use the VPI field to encode the label which is at the top of
       the label stack, and the VCI field to encode the second label
       on the stack, if one is present.  This technique some
       advantages over the previous one, in that it permits the use of
       ATM "VP-switching".  That is, the LSPs are realized as ATM
       SVPs, with the label distribution protocol serving as the ATM
       signaling protocol.
       However, this technique cannot always be used.  If the network
       includes an ATM Virtual Path through a non-MPLS ATM network,
       then the VPI field is not necessarily available for use by
       MPLS.
       When this encoding technique is used, the ATM-LSR at the egress
       of the VP effectively does a "pop" operation.
    3. SVP Multipoint Encoding
       Use the VPI field to encode the label which is at the top of
       the label stack, use part of the VCI field to encode the second
       label on the stack, if one is present, and use the remainder of
       the VCI field to identify the LSP ingress.  If this technique
       is used, conventional ATM VP-switching capabilities can be used
       to provide multipoint-to-point VPs.  Cells from different
       packets will then carry different VCI values.  As we shall see
       in section 3.26, this enables us to do label merging, without
       running into any cell interleaving problems, on ATM switches
       which can provide multipoint-to-point VPs, but which do not
       have the VC merge capability.
       This technique depends on the existence of a capability for
       assigning 16-bit VCI values to each ATM switch such that no
       single VCI value is assigned to two different switches.  (If an

Rosen, et al. Standards Track [Page 27] RFC 3031 MPLS Architecture January 2001

       adequate number of such values could be assigned to each
       switch, it would be possible to also treat the VCI value as the
       second label in the stack.)
 If there are more labels on the stack than can be encoded in the ATM
 header, the ATM encodings must be combined with the generic
 encapsulation.

3.25.3. Interoperability among Encoding Techniques

 If <R1, R2, R3> is a segment of a LSP, it is possible that R1 will
 use one encoding of the label stack when transmitting packet P to R2,
 but R2 will use a different encoding when transmitting a packet P to
 R3.  In general, the MPLS architecture supports LSPs with different
 label stack encodings used on different hops.  Therefore, when we
 discuss the procedures for processing a labeled packet, we speak in
 abstract terms of operating on the packet's label stack.  When a
 labeled packet is received, the LSR must decode it to determine the
 current value of the label stack, then must operate on the label
 stack to determine the new value of the stack, and then encode the
 new value appropriately before transmitting the labeled packet to its
 next hop.
 Unfortunately, ATM switches have no capability for translating from
 one encoding technique to another.  The MPLS architecture therefore
 requires that whenever it is possible for two ATM switches to be
 successive LSRs along a level m LSP for some packet, that those two
 ATM switches use the same encoding technique.
 Naturally there will be MPLS networks which contain a combination of
 ATM switches operating as LSRs, and other LSRs which operate using an
 MPLS shim header.  In such networks there may be some LSRs which have
 ATM interfaces as well as "MPLS Shim" interfaces.  This is one
 example of an LSR with different label stack encodings on different
 hops.  Such an LSR may swap off an ATM encoded label stack on an
 incoming interface and replace it with an MPLS shim header encoded
 label stack on the outgoing interface.

3.26. Label Merging

 Suppose that an LSR has bound multiple incoming labels to a
 particular FEC.  When forwarding packets in that FEC, one would like
 to have a single outgoing label which is applied to all such packets.
 The fact that two different packets in the FEC arrived with different
 incoming labels is irrelevant; one would like to forward them with
 the same outgoing label.  The capability to do so is known as "label
 merging".

Rosen, et al. Standards Track [Page 28] RFC 3031 MPLS Architecture January 2001

 Let us say that an LSR is capable of label merging if it can receive
 two packets from different incoming interfaces, and/or with different
 labels, and send both packets out the same outgoing interface with
 the same label.  Once the packets are transmitted, the information
 that they arrived from different interfaces and/or with different
 incoming labels is lost.
 Let us say that an LSR is not capable of label merging if, for any
 two packets which arrive from different interfaces, or with different
 labels, the packets must either be transmitted out different
 interfaces, or must have different labels.  ATM-LSRs using the SVC or
 SVP Encodings cannot perform label merging.  This is discussed in
 more detail in the next section.
 If a particular LSR cannot perform label merging, then if two packets
 in the same FEC arrive with different incoming labels, they must be
 forwarded with different outgoing labels.  With label merging, the
 number of outgoing labels per FEC need only be 1; without label
 merging, the number of outgoing labels per FEC could be as large as
 the number of nodes in the network.
 With label merging, the number of incoming labels per FEC that a
 particular LSR needs is never be larger than the number of label
 distribution adjacencies.  Without label merging, the number of
 incoming labels per FEC that a particular LSR needs is as large as
 the number of upstream nodes which forward traffic in the FEC to the
 LSR in question.  In fact, it is difficult for an LSR to even
 determine how many such incoming labels it must support for a
 particular FEC.
 The MPLS architecture accommodates both merging and non-merging LSRs,
 but allows for the fact that there may be LSRs which do not support
 label merging.  This leads to the issue of ensuring correct
 interoperation between merging LSRs and non-merging LSRs.  The issue
 is somewhat different in the case of datagram media versus the case
 of ATM.  The different media types will therefore be discussed
 separately.

3.26.1. Non-merging LSRs

 The MPLS forwarding procedures is very similar to the forwarding
 procedures used by such technologies as ATM and Frame Relay.  That
 is, a unit of data arrives, a label (VPI/VCI or DLCI) is looked up in
 a "cross-connect table", on the basis of that lookup an output port
 is chosen, and the label value is rewritten.  In fact, it is possible
 to use such technologies for MPLS forwarding; a label distribution
 protocol can be used as the "signalling protocol" for setting up the
 cross-connect tables.

Rosen, et al. Standards Track [Page 29] RFC 3031 MPLS Architecture January 2001

 Unfortunately, these technologies do not necessarily support the
 label merging capability.  In ATM, if one attempts to perform label
 merging, the result may be the interleaving of cells from various
 packets.  If cells from different packets get interleaved, it is
 impossible to reassemble the packets.  Some Frame Relay switches use
 cell switching on their backplanes.  These switches may also be
 incapable of supporting label merging, for the same reason -- cells
 of different packets may get interleaved, and there is then no way to
 reassemble the packets.
 We propose to support two solutions to this problem.  First, MPLS
 will contain procedures which allow the use of non-merging LSRs.
 Second, MPLS will support procedures which allow certain ATM switches
 to function as merging LSRs.
 Since MPLS supports both merging and non-merging LSRs, MPLS also
 contains procedures to ensure correct interoperation between them.

3.26.2. Labels for Merging and Non-Merging LSRs

 An upstream LSR which supports label merging needs to be sent only
 one label per FEC.  An upstream neighbor which does not support label
 merging needs to be sent multiple labels per FEC.  However, there is
 no way of knowing a priori how many labels it needs.  This will
 depend on how many LSRs are upstream of it with respect to the FEC in
 question.
 In the MPLS architecture, if a particular upstream neighbor does not
 support label merging, it is not sent any labels for a particular FEC
 unless it explicitly asks for a label for that FEC.  The upstream
 neighbor may make multiple such requests, and is given a new label
 each time.  When a downstream neighbor receives such a request from
 upstream, and the downstream neighbor does not itself support label
 merging, then it must in turn ask its downstream neighbor for another
 label for the FEC in question.
 It is possible that there may be some nodes which support label
 merging, but can only merge a limited number of incoming labels into
 a single outgoing label.  Suppose for example that due to some
 hardware limitation a node is capable of merging four incoming labels
 into a single outgoing label.  Suppose however, that this particular
 node has six incoming labels arriving at it for a particular FEC.  In
 this case, this node may merge these into two outgoing labels.
 Whether label merging is applicable to explicitly routed LSPs is for
 further study.

Rosen, et al. Standards Track [Page 30] RFC 3031 MPLS Architecture January 2001

3.26.3. Merge over ATM

3.26.3.1. Methods of Eliminating Cell Interleave

 There are several methods that can be used to eliminate the cell
 interleaving problem in ATM, thereby allowing ATM switches to support
 stream merge:
    1. VP merge, using the SVP Multipoint Encoding
       When VP merge is used, multiple virtual paths are merged into a
       virtual path, but packets from different sources are
       distinguished by using different VCIs within the VP.
    2. VC merge
       When VC merge is used, switches are required to buffer cells
       from one packet until the entire packet is received (this may
       be determined by looking for the AAL5 end of frame indicator).
 VP merge has the advantage that it is compatible with a higher
 percentage of existing ATM switch implementations.  This makes it
 more likely that VP merge can be used in existing networks.  Unlike
 VC merge, VP merge does not incur any delays at the merge points and
 also does not impose any buffer requirements.  However, it has the
 disadvantage that it requires coordination of the VCI space within
 each VP.  There are a number of ways that this can be accomplished.
 Selection of one or more methods is for further study.
 This tradeoff between compatibility with existing equipment versus
 protocol complexity and scalability implies that it is desirable for
 the MPLS protocol to support both VP merge and VC merge.  In order to
 do so each ATM switch participating in MPLS needs to know whether its
 immediate ATM neighbors perform VP merge, VC merge, or no merge.

3.26.3.2. Interoperation: VC Merge, VP Merge, and Non-Merge

 The interoperation of the various forms of merging over ATM is most
 easily described by first describing the interoperation of VC merge
 with non-merge.
 In the case where VC merge and non-merge nodes are interconnected the
 forwarding of cells is based in all cases on a VC (i.e., the
 concatenation of the VPI and VCI).  For each node, if an upstream
 neighbor is doing VC merge then that upstream neighbor requires only
 a single VPI/VCI for a particular stream (this is analogous to the
 requirement for a single label in the case of operation over frame
 media).  If the upstream neighbor is not doing merge, then the

Rosen, et al. Standards Track [Page 31] RFC 3031 MPLS Architecture January 2001

 neighbor will require a single VPI/VCI per stream for itself, plus
 enough VPI/VCIs to pass to its upstream neighbors.  The number
 required will be determined by allowing the upstream nodes to request
 additional VPI/VCIs from their downstream neighbors (this is again
 analogous to the method used with frame merge).
 A similar method is possible to support nodes which perform VP merge.
 In this case the VP merge node, rather than requesting a single
 VPI/VCI or a number of VPI/VCIs from its downstream neighbor, instead
 may request a single VP (identified by a VPI) but several VCIs within
 the VP.  Furthermore, suppose that a non-merge node is downstream
 from two different VP merge nodes.  This node may need to request one
 VPI/VCI (for traffic originating from itself) plus two VPs (one for
 each upstream node), each associated with a specified set of VCIs (as
 requested from the upstream node).
 In order to support all of VP merge, VC merge, and non-merge, it is
 therefore necessary to allow upstream nodes to request a combination
 of zero or more VC identifiers (consisting of a VPI/VCI), plus zero
 or more VPs (identified by VPIs) each containing a specified number
 of VCs (identified by a set of VCIs which are significant within a
 VP).  VP merge nodes would therefore request one VP, with a contained
 VCI for traffic that it originates (if appropriate) plus a VCI for
 each VC requested from above (regardless of whether or not the VC is
 part of a containing VP).  VC merge node would request only a single
 VPI/VCI (since they can merge all upstream traffic into a single VC).
 Non-merge nodes would pass on any requests that they get from above,
 plus request a VPI/VCI for traffic that they originate (if
 appropriate).

3.27. Tunnels and Hierarchy

 Sometimes a router Ru takes explicit action to cause a particular
 packet to be delivered to another router Rd, even though Ru and Rd
 are not consecutive routers on the Hop-by-hop path for that packet,
 and Rd is not the packet's ultimate destination.  For example, this
 may be done by encapsulating the packet inside a network layer packet
 whose destination address is the address of Rd itself.  This creates
 a "tunnel" from Ru to Rd.  We refer to any packet so handled as a
 "Tunneled Packet".

3.27.1. Hop-by-Hop Routed Tunnel

 If a Tunneled Packet follows the Hop-by-hop path from Ru to Rd, we
 say that it is in an "Hop-by-Hop Routed Tunnel" whose "transmit
 endpoint" is Ru and whose "receive endpoint" is Rd.

Rosen, et al. Standards Track [Page 32] RFC 3031 MPLS Architecture January 2001

3.27.2. Explicitly Routed Tunnel

 If a Tunneled Packet travels from Ru to Rd over a path other than the
 Hop-by-hop path, we say that it is in an "Explicitly Routed Tunnel"
 whose "transmit endpoint" is Ru and whose "receive endpoint" is Rd.
 For example, we might send a packet through an Explicitly Routed
 Tunnel by encapsulating it in a packet which is source routed.

3.27.3. LSP Tunnels

 It is possible to implement a tunnel as a LSP, and use label
 switching rather than network layer encapsulation to cause the packet
 to travel through the tunnel.  The tunnel would be a LSP <R1, ...,
 Rn>, where R1 is the transmit endpoint of the tunnel, and Rn is the
 receive endpoint of the tunnel.  This is called a "LSP Tunnel".
 The set of packets which are to be sent though the LSP tunnel
 constitutes a FEC, and each LSR in the tunnel must assign a label to
 that FEC (i.e., must assign a label to the tunnel).  The criteria for
 assigning a particular packet to an LSP tunnel is a local matter at
 the tunnel's transmit endpoint.  To put a packet into an LSP tunnel,
 the transmit endpoint pushes a label for the tunnel onto the label
 stack and sends the labeled packet to the next hop in the tunnel.
 If it is not necessary for the tunnel's receive endpoint to be able
 to determine which packets it receives through the tunnel, as
 discussed earlier, the label stack may be popped at the penultimate
 LSR in the tunnel.
 A "Hop-by-Hop Routed LSP Tunnel" is a Tunnel that is implemented as
 an hop-by-hop routed LSP between the transmit endpoint and the
 receive endpoint.
 An "Explicitly Routed LSP Tunnel" is a LSP Tunnel that is also an
 Explicitly Routed LSP.

3.27.4. Hierarchy: LSP Tunnels within LSPs

 Consider a LSP <R1, R2, R3, R4>.  Let us suppose that R1 receives
 unlabeled packet P, and pushes on its label stack the label to cause
 it to follow this path, and that this is in fact the Hop-by-hop path.
 However, let us further suppose that R2 and R3 are not directly
 connected, but are "neighbors" by virtue of being the endpoints of an
 LSP tunnel.  So the actual sequence of LSRs traversed by P is <R1,
 R2, R21, R22, R23, R3, R4>.

Rosen, et al. Standards Track [Page 33] RFC 3031 MPLS Architecture January 2001

 When P travels from R1 to R2, it will have a label stack of depth 1.
 R2, switching on the label, determines that P must enter the tunnel.
 R2 first replaces the Incoming label with a label that is meaningful
 to R3.  Then it pushes on a new label.  This level 2 label has a
 value which is meaningful to R21.  Switching is done on the level 2
 label by R21, R22, R23.  R23, which is the penultimate hop in the
 R2-R3 tunnel, pops the label stack before forwarding the packet to
 R3.  When R3 sees packet P, P has only a level 1 label, having now
 exited the tunnel.  Since R3 is the penultimate hop in P's level 1
 LSP, it pops the label stack, and R4 receives P unlabeled.
 The label stack mechanism allows LSP tunneling to nest to any depth.

3.27.5. Label Distribution Peering and Hierarchy

 Suppose that packet P travels along a Level 1 LSP <R1, R2, R3, R4>,
 and when going from R2 to R3 travels along a Level 2 LSP <R2, R21,
 R22, R3>.  From the perspective of the Level 2 LSP, R2's label
 distribution peer is R21.  From the perspective of the Level 1 LSP,
 R2's label distribution peers are R1 and R3.  One can have label
 distribution peers at each layer of hierarchy.  We will see in
 sections 4.6 and 4.7 some ways to make use of this hierarchy.  Note
 that in this example, R2 and R21 must be IGP neighbors, but R2 and R3
 need not be.
 When two LSRs are IGP neighbors, we will refer to them as "local
 label distribution peers".  When two LSRs may be label distribution
 peers, but are not IGP neighbors, we will refer to them as "remote
 label distribution peers".  In the above example, R2 and R21 are
 local label distribution peers, but R2 and R3 are remote label
 distribution peers.
 The MPLS architecture supports two ways to distribute labels at
 different layers of the hierarchy: Explicit Peering and Implicit
 Peering.
 One performs label distribution with one's local label distribution
 peer by sending label distribution protocol messages which are
 addressed to the peer.  One can perform label distribution with one's
 remote label distribution peers in one of two ways:
    1. Explicit Peering
       In explicit peering, one distributes labels to a peer by
       sending label distribution protocol messages which are
       addressed to the peer, exactly as one would do for local label
       distribution peers.  This technique is most useful when the
       number of remote label distribution peers is small, or the

Rosen, et al. Standards Track [Page 34] RFC 3031 MPLS Architecture January 2001

       number of higher level label bindings is large, or the remote
       label distribution peers are in distinct routing areas or
       domains.  Of course, one needs to know which labels to
       distribute to which peers; this is addressed in section 4.1.2.
       Examples of the use of explicit peering is found in sections
       4.2.1 and 4.6.
    2. Implicit Peering
       In Implicit Peering, one does not send label distribution
       protocol messages which are addressed to one's peer.  Rather,
       to distribute higher level labels to ones remote label
       distribution peers, one encodes a higher level label as an
       attribute of a lower level label, and then distributes the
       lower level label, along with this attribute, to one's local
       label distribution peers.  The local label distribution peers
       then propagate the information to their local label
       distribution peers.  This process continues till the
       information reaches the remote peer.
       This technique is most useful when the number of remote label
       distribution peers is large.  Implicit peering does not require
       an n-square peering mesh to distribute labels to the remote
       label distribution peers because the information is piggybacked
       through the local label distribution peering.  However,
       implicit peering requires the intermediate nodes to store
       information that they might not be directly interested in.
       An example of the use of implicit peering is found in section
       4.3.

3.28. Label Distribution Protocol Transport

 A label distribution protocol is used between nodes in an MPLS
 network to establish and maintain the label bindings.  In order for
 MPLS to operate correctly, label distribution information needs to be
 transmitted reliably, and the label distribution protocol messages
 pertaining to a particular FEC need to be transmitted in sequence.
 Flow control is also desirable, as is the capability to carry
 multiple label messages in a single datagram.
 One way to meet these goals is to use TCP as the underlying
 transport, as is done in [MPLS-LDP] and [MPLS-BGP].

Rosen, et al. Standards Track [Page 35] RFC 3031 MPLS Architecture January 2001

3.29. Why More than one Label Distribution Protocol?

 This architecture does not establish hard and fast rules for choosing
 which label distribution protocol to use in which circumstances.
 However, it is possible to point out some of the considerations.

3.29.1. BGP and LDP

 In many scenarios, it is desirable to bind labels to FECs which can
 be identified with routes to address prefixes (see section 4.1).  If
 there is a standard, widely deployed routing algorithm which
 distributes those routes, it can be argued that label distribution is
 best achieved by piggybacking the label distribution on the
 distribution of the routes themselves.
 For example, BGP distributes such routes, and if a BGP speaker needs
 to also distribute labels to its BGP peers, using BGP to do the label
 distribution (see [MPLS-BGP]) has a number of advantages.  In
 particular, it permits BGP route reflectors to distribute labels,
 thus providing a significant scalability advantage over using LDP to
 distribute labels between BGP peers.

3.29.2. Labels for RSVP Flowspecs

 When RSVP is used to set up resource reservations for particular
 flows, it can be desirable to label the packets in those flows, so
 that the RSVP filterspec does not need to be applied at each hop.  It
 can be argued that having RSVP distribute the labels as part of its
 path/reservation setup process is the most efficient method of
 distributing labels for this purpose.

3.29.3. Labels for Explicitly Routed LSPs

 In some applications of MPLS, particularly those related to traffic
 engineering, it is desirable to set up an explicitly routed path,
 from ingress to egress.  It is also desirable to apply resource
 reservations along that path.
 One can imagine two approaches to this:
  1. Start with an existing protocol that is used for setting up

resource reservations, and extend it to support explicit

       routing and label distribution.
  1. Start with an existing protocol that is used for label

distribution, and extend it to support explicit routing and

       resource reservations.

Rosen, et al. Standards Track [Page 36] RFC 3031 MPLS Architecture January 2001

 The first approach has given rise to the protocol specified in
 [MPLS-RSVP-TUNNELS], the second to the approach specified in [MPLS-
 CR-LDP].

3.30. Multicast

 This section is for further study

4. Some Applications of MPLS

4.1. MPLS and Hop by Hop Routed Traffic

 A number of uses of MPLS require that packets with a certain label be
 forwarded along the same hop-by-hop routed path that would be used
 for forwarding a packet with a specified address in its network layer
 destination address field.

4.1.1. Labels for Address Prefixes

 In general, router R determines the next hop for packet P by finding
 the address prefix X in its routing table which is the longest match
 for P's destination address.  That is, the packets in a given FEC are
 just those packets which match a given address prefix in R's routing
 table.  In this case, a FEC can be identified with an address prefix.
 Note that a packet P may be assigned to FEC F, and FEC F may be
 identified with address prefix X, even if P's destination address
 does not match X.

4.1.2. Distributing Labels for Address Prefixes

4.1.2.1. Label Distribution Peers for an Address Prefix

 LSRs R1 and R2 are considered to be label distribution peers for
 address prefix X if and only if one of the following conditions
 holds:
    1. R1's route to X is a route which it learned about via a
       particular instance of a particular IGP, and R2 is a neighbor
       of R1 in that instance of that IGP
    2. R1's route to X is a route which it learned about by some
       instance of routing algorithm A1, and that route is
       redistributed into an instance of routing algorithm A2, and R2
       is a neighbor of R1 in that instance of A2

Rosen, et al. Standards Track [Page 37] RFC 3031 MPLS Architecture January 2001

    3. R1 is the receive endpoint of an LSP Tunnel that is within
       another LSP, and R2 is a transmit endpoint of that tunnel, and
       R1 and R2 are participants in a common instance of an IGP, and
       are in the same IGP area (if the IGP in question has areas),
       and R1's route to X was learned via that IGP instance, or is
       redistributed by R1 into that IGP instance
    4. R1's route to X is a route which it learned about via BGP, and
       R2 is a BGP peer of R1
 In general, these rules ensure that if the route to a particular
 address prefix is distributed via an IGP, the label distribution
 peers for that address prefix are the IGP neighbors.  If the route to
 a particular address prefix is distributed via BGP, the label
 distribution peers for that address prefix are the BGP peers.  In
 other cases of LSP tunneling, the tunnel endpoints are label
 distribution peers.

4.1.2.2. Distributing Labels

 In order to use MPLS for the forwarding of packets according to the
 hop-by-hop route corresponding to any address prefix, each LSR MUST:
    1. bind one or more labels to each address prefix that appears in
       its routing table;
    2. for each such address prefix X, use a label distribution
       protocol to distribute the binding of a label to X to each of
       its label distribution peers for X.
 There is also one circumstance in which an LSR must distribute a
 label binding for an address prefix, even if it is not the LSR which
 bound that label to that address prefix:
    3. If R1 uses BGP to distribute a route to X, naming some other
       LSR R2 as the BGP Next Hop to X, and if R1 knows that R2 has
       assigned label L to X, then R1 must distribute the binding
       between L and X to any BGP peer to which it distributes that
       route.
 These rules ensure that labels corresponding to address prefixes
 which correspond to BGP routes are distributed to IGP neighbors if
 and only if the BGP routes are distributed into the IGP.  Otherwise,
 the labels bound to BGP routes are distributed only to the other BGP
 speakers.
 These rules are intended only to indicate which label bindings must
 be distributed by a given LSR to which other LSRs.

Rosen, et al. Standards Track [Page 38] RFC 3031 MPLS Architecture January 2001

4.1.3. Using the Hop by Hop path as the LSP

 If the hop-by-hop path that packet P needs to follow is <R1, ...,
 Rn>, then <R1, ..., Rn> can be an LSP as long as:
    1. there is a single address prefix X, such that, for all i,
       1<=i<n, X is the longest match in Ri's routing table for P's
       destination address;
    2. for all i, 1<i<n, Ri has assigned a label to X and distributed
       that label to R[i-1].
 Note that a packet's LSP can extend only until it encounters a router
 whose forwarding tables have a longer best match address prefix for
 the packet's destination address.  At that point, the LSP must end
 and the best match algorithm must be performed again.
 Suppose, for example, that packet P, with destination address
 10.2.153.178 needs to go from R1 to R2 to R3.  Suppose also that R2
 advertises address prefix 10.2/16 to R1, but R3 advertises
 10.2.153/23, 10.2.154/23, and 10.2/16 to R2.  That is, R2 is
 advertising an "aggregated route" to R1.  In this situation, packet P
 can be label Switched until it reaches R2, but since R2 has performed
 route aggregation, it must execute the best match algorithm to find
 P's FEC.

4.1.4. LSP Egress and LSP Proxy Egress

 An LSR R is considered to be an "LSP Egress" LSR for address prefix X
 if and only if one of the following conditions holds:
    1. R has an address Y, such that X is the address prefix in R's
       routing table which is the longest match for Y, or
    2. R contains in its routing tables one or more address prefixes Y
       such that X is a proper initial substring of Y, but R's "LSP
       previous hops" for X do not contain any such address prefixes
       Y; that is, R is a "deaggregation point" for address prefix X.
 An LSR R1 is considered to be an "LSP Proxy Egress" LSR for address
 prefix X if and only if:
    1. R1's next hop for X is R2, and R1 and R2 are not label
       distribution peers with respect to X (perhaps because R2 does
       not support MPLS), or
    2. R1 has been configured to act as an LSP Proxy Egress for X

Rosen, et al. Standards Track [Page 39] RFC 3031 MPLS Architecture January 2001

 The definition of LSP allows for the LSP Egress to be a node which
 does not support MPLS; in this case the penultimate node in the LSP
 is the Proxy Egress.

4.1.5. The Implicit NULL Label

 The Implicit NULL label is a label with special semantics which an
 LSR can bind to an address prefix.  If LSR Ru, by consulting its ILM,
 sees that labeled packet P must be forwarded next to Rd, but that Rd
 has distributed a binding of Implicit NULL to the corresponding
 address prefix, then instead of replacing the value of the label on
 top of the label stack, Ru pops the label stack, and then forwards
 the resulting packet to Rd.
 LSR Rd distributes a binding between Implicit NULL and an address
 prefix X to LSR Ru if and only if:
    1. the rules of Section 4.1.2 indicate that Rd distributes to Ru a
       label binding for X, and
    2. Rd knows that Ru can support the Implicit NULL label (i.e.,
       that it can pop the label stack), and
    3. Rd is an LSP Egress (not proxy egress) for X.
 This causes the penultimate LSR on a LSP to pop the label stack.
 This is quite appropriate; if the LSP Egress is an MPLS Egress for X,
 then if the penultimate LSR does not pop the label stack, the LSP
 Egress will need to look up the label, pop the label stack, and then
 look up the next label (or look up the L3 address, if no more labels
 are present).  By having the penultimate LSR pop the label stack, the
 LSP Egress is saved the work of having to look up two labels in order
 to make its forwarding decision.
 However, if the penultimate LSR is an ATM switch, it may not have the
 capability to pop the label stack.  Hence a binding of Implicit NULL
 may be distributed only to LSRs which can support that function.
 If the penultimate LSR in an LSP for address prefix X is an LSP Proxy
 Egress, it acts just as if the LSP Egress had distributed a binding
 of Implicit NULL for X.

4.1.6. Option: Egress-Targeted Label Assignment

 There are situations in which an LSP Ingress, Ri, knows that packets
 of several different FECs must all follow the same LSP, terminating
 at, say, LSP Egress Re.  In this case, proper routing can be achieved

Rosen, et al. Standards Track [Page 40] RFC 3031 MPLS Architecture January 2001

 by using a single label for all such FECs; it is not necessary to
 have a distinct label for each FEC.  If (and only if) the following
 conditions hold:
    1. the address of LSR Re is itself in the routing table as a "host
       route", and
    2. there is some way for Ri to determine that Re is the LSP egress
       for all packets in a particular set of FECs
 Then Ri may bind a single label to all FECS in the set.  This is
 known as "Egress-Targeted Label Assignment."
 How can LSR Ri determine that an LSR Re is the LSP Egress for all
 packets in a particular FEC?  There are a number of possible ways:
  1. If the network is running a link state routing algorithm, and

all nodes in the area support MPLS, then the routing algorithm

       provides Ri with enough information to determine the routers
       through which packets in that FEC must leave the routing domain
       or area.
  1. If the network is running BGP, Ri may be able to determine that

the packets in a particular FEC must leave the network via some

       particular router which is the "BGP Next Hop" for that FEC.
  1. It is possible to use the label distribution protocol to pass

information about which address prefixes are "attached" to

       which egress LSRs.  This method has the advantage of not
       depending on the presence of link state routing.
 If egress-targeted label assignment is used, the number of labels
 that need to be supported throughout the network may be greatly
 reduced.  This may be significant if one is using legacy switching
 hardware to do MPLS, and the switching hardware can support only a
 limited number of labels.
 One possible approach would be to configure the network to use
 egress-targeted label assignment by default, but to configure
 particular LSRs to NOT use egress-targeted label assignment for one
 or more of the address prefixes for which it is an LSP egress.  We
 impose the following rule:
  1. If a particular LSR is NOT an LSP Egress for some set of

address prefixes, then it should assign labels to the address

       prefixes in the same way as is done by its LSP next hop for
       those address prefixes.  That is, suppose Rd is Ru's LSP next

Rosen, et al. Standards Track [Page 41] RFC 3031 MPLS Architecture January 2001

       hop for address prefixes X1 and X2.  If Rd assigns the same
       label to X1 and X2, Ru should as well.  If Rd assigns different
       labels to X1 and X2, then Ru should as well.
 For example, suppose one wants to make egress-targeted label
 assignment the default, but to assign distinct labels to those
 address prefixes for which there are multiple possible LSP egresses
 (i.e., for those address prefixes which are multi-homed.)  One can
 configure all LSRs to use egress-targeted label assignment, and then
 configure a handful of LSRs to assign distinct labels to those
 address prefixes which are multi-homed.  For a particular multi-homed
 address prefix X, one would only need to configure this in LSRs which
 are either LSP Egresses or LSP Proxy Egresses for X.
 It is important to note that if Ru and Rd are adjacent LSRs in an LSP
 for X1 and X2, forwarding will still be done correctly if Ru assigns
 distinct labels to X1 and X2 while Rd assigns just one label to the
 both of them.  This just means that R1 will map different incoming
 labels to the same outgoing label, an ordinary occurrence.
 Similarly, if Rd assigns distinct labels to X1 and X2, but Ru assigns
 to them both the label corresponding to the address of their LSP
 Egress or Proxy Egress, forwarding will still be done correctly.  Ru
 will just map the incoming label to the label which Rd has assigned
 to the address of that LSP Egress.

4.2. MPLS and Explicitly Routed LSPs

 There are a number of reasons why it may be desirable to use explicit
 routing instead of hop by hop routing.  For example, this allows
 routes to be based on administrative policies, and allows the routes
 that LSPs take to be carefully designed to allow traffic engineering
 [MPLS-TRFENG].

4.2.1. Explicitly Routed LSP Tunnels

 In some situations, the network administrators may desire to forward
 certain classes of traffic along certain pre-specified paths, where
 these paths differ from the Hop-by-hop path that the traffic would
 ordinarily follow.  This can be done in support of policy routing, or
 in support of traffic engineering.  The explicit route may be a
 configured one, or it may be determined dynamically by some means,
 e.g., by constraint-based routing.
 MPLS allows this to be easily done by means of Explicitly Routed LSP
 Tunnels.  All that is needed is:

Rosen, et al. Standards Track [Page 42] RFC 3031 MPLS Architecture January 2001

    1. A means of selecting the packets that are to be sent into the
       Explicitly Routed LSP Tunnel;
    2. A means of setting up the Explicitly Routed LSP Tunnel;
    3. A means of ensuring that packets sent into the Tunnel will not
       loop from the receive endpoint back to the transmit endpoint.
 If the transmit endpoint of the tunnel wishes to put a labeled packet
 into the tunnel, it must first replace the label value at the top of
 the stack with a label value that was distributed to it by the
 tunnel's receive endpoint.  Then it must push on the label which
 corresponds to the tunnel itself, as distributed to it by the next
 hop along the tunnel.  To allow this, the tunnel endpoints should be
 explicit label distribution peers.  The label bindings they need to
 exchange are of no interest to the LSRs along the tunnel.

4.3. Label Stacks and Implicit Peering

 Suppose a particular LSR Re is an LSP proxy egress for 10 address
 prefixes, and it reaches each address prefix through a distinct
 interface.
 One could assign a single label to all 10 address prefixes.  Then Re
 is an LSP egress for all 10 address prefixes.  This ensures that
 packets for all 10 address prefixes get delivered to Re.  However, Re
 would then have to look up the network layer address of each such
 packet in order to choose the proper interface to send the packet on.
 Alternatively, one could assign a distinct label to each interface.
 Then Re is an LSP proxy egress for the 10 address prefixes.  This
 eliminates the need for Re to look up the network layer addresses in
 order to forward the packets.  However, it can result in the use of a
 large number of labels.
 An alternative would be to bind all 10 address prefixes to the same
 level 1 label (which is also bound to the address of the LSR itself),
 and then to bind each address prefix to a distinct level 2 label.
 The level 2 label would be treated as an attribute of the level 1
 label binding, which we call the "Stack Attribute".  We impose the
 following rules:
  1. When LSR Ru initially labels a hitherto unlabeled packet, if

the longest match for the packet's destination address is X,

       and Ru's LSP next hop for X is Rd, and Rd has distributed to Ru
       a binding of label L1 to X, along with a stack attribute of L2,
       then

Rosen, et al. Standards Track [Page 43] RFC 3031 MPLS Architecture January 2001

       1. Ru must push L2 and then L1 onto the packet's label stack,
          and then forward the packet to Rd;
       2. When Ru distributes label bindings for X to its label
          distribution peers, it must include L2 as the stack
          attribute.
       3. Whenever the stack attribute changes (possibly as a result
          of a change in Ru's LSP next hop for X), Ru must distribute
          the new stack attribute.
 Note that although the label value bound to X may be different at
 each hop along the LSP, the stack attribute value is passed
 unchanged, and is set by the LSP proxy egress.
 Thus the LSP proxy egress for X becomes an "implicit peer" with each
 other LSR in the routing area or domain.  In this case, explicit
 peering would be too unwieldy, because the number of peers would
 become too large.

4.4. MPLS and Multi-Path Routing

 If an LSR supports multiple routes for a particular stream, then it
 may assign multiple labels to the stream, one for each route.  Thus
 the reception of a second label binding from a particular neighbor
 for a particular address prefix should be taken as meaning that
 either label can be used to represent that address prefix.
 If multiple label bindings for a particular address prefix are
 specified, they may have distinct attributes.

4.5. LSP Trees as Multipoint-to-Point Entities

 Consider the case of packets P1 and P2, each of which has a
 destination address whose longest match, throughout a particular
 routing domain, is address prefix X.  Suppose that the Hop-by-hop
 path for P1 is <R1, R2, R3>, and the Hop-by-hop path for P2 is <R4,
 R2, R3>.   Let's suppose that R3 binds label L3 to X, and distributes
 this binding to R2.  R2 binds label L2 to X, and distributes this
 binding to both R1 and R4.  When R2 receives packet P1, its incoming
 label will be L2.  R2 will overwrite L2 with L3, and send P1 to R3.
 When R2 receives packet P2, its incoming label will also be L2.  R2
 again overwrites L2 with L3, and send P2 on to R3.
 Note then that when P1 and P2 are traveling from R2 to R3, they carry
 the same label, and as far as MPLS is concerned, they cannot be
 distinguished.  Thus instead of talking about two distinct LSPs, <R1,

Rosen, et al. Standards Track [Page 44] RFC 3031 MPLS Architecture January 2001

 R2, R3> and <R4, R2, R3>, we might talk of a single "Multipoint-to-
 Point LSP Tree", which we might denote as <{R1, R4}, R2, R3>.
 This creates a difficulty when we attempt to use conventional ATM
 switches as LSRs.  Since conventional ATM switches do not support
 multipoint-to-point connections, there must be procedures to ensure
 that each LSP is realized as a point-to-point VC.  However, if ATM
 switches which do support multipoint-to-point VCs are in use, then
 the LSPs can be most efficiently realized as multipoint-to-point VCs.
 Alternatively, if the SVP Multipoint Encoding (section 3.25.2) can be
 used, the LSPs can be realized as multipoint-to-point SVPs.

4.6. LSP Tunneling between BGP Border Routers

 Consider the case of an Autonomous System, A, which carries transit
 traffic between other Autonomous Systems.  Autonomous System A will
 have a number of BGP Border Routers, and a mesh of BGP connections
 among them, over which BGP routes are distributed.  In many such
 cases, it is desirable to avoid distributing the BGP routes to
 routers which are not BGP Border Routers.  If this can be avoided,
 the "route distribution load" on those routers is significantly
 reduced.  However, there must be some means of ensuring that the
 transit traffic will be delivered from Border Router to Border Router
 by the interior routers.
 This can easily be done by means of LSP Tunnels.  Suppose that BGP
 routes are distributed only to BGP Border Routers, and not to the
 interior routers that lie along the Hop-by-hop path from Border
 Router to Border Router.  LSP Tunnels can then be used as follows:
    1. Each BGP Border Router distributes, to every other BGP Border
       Router in the same Autonomous System, a label for each address
       prefix that it distributes to that router via BGP.
    2. The IGP for the Autonomous System maintains a host route for
       each BGP Border Router.  Each interior router distributes its
       labels for these host routes to each of its IGP neighbors.
    3. Suppose that:
       a) BGP Border Router B1 receives an unlabeled packet P,
       b) address prefix X in B1's routing table is the longest match
          for the destination address of P,
       c) the route to X is a BGP route,
       d) the BGP Next Hop for X is B2,

Rosen, et al. Standards Track [Page 45] RFC 3031 MPLS Architecture January 2001

       e) B2 has bound label L1 to X, and has distributed this binding
          to B1,
       f) the IGP next hop for the address of B2 is I1,
       g) the address of B2 is in B1's and I1's IGP routing tables as
          a host route, and
       h) I1 has bound label L2 to the address of B2, and distributed
          this binding to B1.
       Then before sending packet P to I1, B1 must create a label
       stack for P, then push on label L1, and then push on label L2.
    4. Suppose that BGP Border Router B1 receives a labeled Packet P,
       where the label on the top of the label stack corresponds to an
       address prefix, X, to which the route is a BGP route, and that
       conditions 3b, 3c, 3d, and 3e all hold.  Then before sending
       packet P to I1, B1 must replace the label at the top of the
       label stack with L1, and then push on label L2.
 With these procedures, a given packet P follows a level 1 LSP all of
 whose members are BGP Border Routers, and between each pair of BGP
 Border Routers in the level 1 LSP, it follows a level 2 LSP.
 These procedures effectively create a Hop-by-Hop Routed LSP Tunnel
 between the BGP Border Routers.
 Since the BGP border routers are exchanging label bindings for
 address prefixes that are not even known to the IGP routing, the BGP
 routers should become explicit label distribution peers with each
 other.
 It is sometimes possible to create Hop-by-Hop Routed LSP Tunnels
 between two BGP Border Routers, even if they are not in the same
 Autonomous System.  Suppose, for example, that B1 and B2 are in AS 1.
 Suppose that B3 is an EBGP neighbor of B2, and is in AS2.  Finally,
 suppose that B2 and B3 are on some network which is common to both
 Autonomous Systems (a "Demilitarized Zone").  In this case, an LSP
 tunnel can be set up directly between B1 and B3 as follows:
  1. B3 distributes routes to B2 (using EBGP), optionally assigning

labels to address prefixes;

  1. B2 redistributes those routes to B1 (using IBGP), indicating

that the BGP next hop for each such route is B3. If B3 has

       assigned labels to address prefixes, B2 passes these labels
       along, unchanged, to B1.

Rosen, et al. Standards Track [Page 46] RFC 3031 MPLS Architecture January 2001

  1. The IGP of AS1 has a host route for B3.

4.7. Other Uses of Hop-by-Hop Routed LSP Tunnels

 The use of Hop-by-Hop Routed LSP Tunnels is not restricted to tunnels
 between BGP Next Hops.  Any situation in which one might otherwise
 have used an encapsulation tunnel is one in which it is appropriate
 to use a Hop-by-Hop Routed LSP Tunnel.  Instead of encapsulating the
 packet with a new header whose destination address is the address of
 the tunnel's receive endpoint, the label corresponding to the address
 prefix which is the longest match for the address of the tunnel's
 receive endpoint is pushed on the packet's label stack.  The packet
 which is sent into the tunnel may or may not already be labeled.
 If the transmit endpoint of the tunnel wishes to put a labeled packet
 into the tunnel, it must first replace the label value at the top of
 the stack with a label value that was distributed to it by the
 tunnel's receive endpoint.  Then it must push on the label which
 corresponds to the tunnel itself, as distributed to it by the next
 hop along the tunnel.  To allow this, the tunnel endpoints should be
 explicit label distribution peers.  The label bindings they need to
 exchange are of no interest to the LSRs along the tunnel.

4.8. MPLS and Multicast

 Multicast routing proceeds by constructing multicast trees.  The tree
 along which a particular multicast packet must get forwarded depends
 in general on the packet's source address and its destination
 address.  Whenever a particular LSR is a node in a particular
 multicast tree, it binds a label to that tree.  It then distributes
 that binding to its parent on the multicast tree.  (If the node in
 question is on a LAN, and has siblings on that LAN, it must also
 distribute the binding to its siblings.  This allows the parent to
 use a single label value when multicasting to all children on the
 LAN.)
 When a multicast labeled packet arrives, the NHLFE corresponding to
 the label indicates the set of output interfaces for that packet, as
 well as the outgoing label.  If the same label encoding technique is
 used on all the outgoing interfaces, the very same packet can be sent
 to all the children.

5. Label Distribution Procedures (Hop-by-Hop)

 In this section, we consider only label bindings that are used for
 traffic to be label switched along its hop-by-hop routed path.  In
 these cases, the label in question will correspond to an address
 prefix in the routing table.

Rosen, et al. Standards Track [Page 47] RFC 3031 MPLS Architecture January 2001

5.1. The Procedures for Advertising and Using labels

 There are a number of different procedures that may be used to
 distribute label bindings.  Some are executed by the downstream LSR,
 and some by the upstream LSR.
 The downstream LSR must perform:
  1. The Distribution Procedure, and
  1. the Withdrawal Procedure.
 The upstream LSR must perform:
  1. The Request Procedure, and
  1. the NotAvailable Procedure, and
  1. the Release Procedure, and
  1. the labelUse Procedure.
 The MPLS architecture supports several variants of each procedure.
 However, the MPLS architecture does not support all possible
 combinations of all possible variants.  The set of supported
 combinations will be described in section 5.2, where the
 interoperability between different combinations will also be
 discussed.

5.1.1. Downstream LSR: Distribution Procedure

 The Distribution Procedure is used by a downstream LSR to determine
 when it should distribute a label binding for a particular address
 prefix to its label distribution peers.  The architecture supports
 four different distribution procedures.
 Irrespective of the particular procedure that is used, if a label
 binding for a particular address prefix has been distributed by a
 downstream LSR Rd to an upstream LSR Ru, and if at any time the
 attributes (as defined above) of that binding change, then Rd must
 inform Ru of the new attributes.
 If an LSR is maintaining multiple routes to a particular address
 prefix, it is a local matter as to whether that LSR binds multiple
 labels to the address prefix (one per route), and hence distributes
 multiple bindings.

Rosen, et al. Standards Track [Page 48] RFC 3031 MPLS Architecture January 2001

5.1.1.1. PushUnconditional

 Let Rd be an LSR.  Suppose that:
    1. X is an address prefix in Rd's routing table
    2. Ru is a label distribution peer of Rd with respect to X
 Whenever these conditions hold, Rd must bind a label to X and
 distribute that binding to Ru.  It is the responsibility of Rd to
 keep track of the bindings which it has distributed to Ru, and to
 make sure that Ru always has these bindings.
 This procedure would be used by LSRs which are performing unsolicited
 downstream label assignment in the Independent LSP Control Mode.

5.1.1.2. PushConditional

 Let Rd be an LSR.  Suppose that:
    1. X is an address prefix in Rd's routing table
    2. Ru is a label distribution peer of Rd with respect to X
    3. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
       Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
       Rn has bound a label to X and distributed that binding to Rd.
 Then as soon as these conditions all hold, Rd should bind a label to
 X and distribute that binding to Ru.
 Whereas PushUnconditional causes the distribution of label bindings
 for all address prefixes in the routing table, PushConditional causes
 the distribution of label bindings only for those address prefixes
 for which one has received label bindings from one's LSP next hop, or
 for which one does not have an MPLS-capable L3 next hop.
 This procedure would be used by LSRs which are performing unsolicited
 downstream label assignment in the Ordered LSP Control Mode.

5.1.1.3. PulledUnconditional

 Let Rd be an LSR.  Suppose that:
    1. X is an address prefix in Rd's routing table
    2. Ru is a label distribution peer of Rd with respect to X

Rosen, et al. Standards Track [Page 49] RFC 3031 MPLS Architecture January 2001

    3. Ru has explicitly requested that Rd bind a label to X and
       distribute the binding to Ru
 Then Rd should bind a label to X and distribute that binding to Ru.
 Note that if X is not in Rd's routing table, or if Rd is not a label
 distribution peer of Ru with respect to X, then Rd must inform Ru
 that it cannot provide a binding at this time.
 If Rd has already distributed a binding for address prefix X to Ru,
 and it receives a new request from Ru for a binding for address
 prefix X, it will bind a second label, and distribute the new binding
 to Ru.  The first label binding remains in effect.
 This procedure would be used by LSRs performing downstream-on-demand
 label distribution using the Independent LSP Control Mode.

5.1.1.4. PulledConditional

 Let Rd be an LSR.  Suppose that:
    1. X is an address prefix in Rd's routing table
    2. Ru is a label distribution peer of Rd with respect to X
    3. Ru has explicitly requested that Rd bind a label to X and
       distribute the binding to Ru
    4. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
       Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
       Rn has bound a label to X and distributed that binding to Rd
 Then as soon as these conditions all hold, Rd should bind a label to
 X and distribute that binding to Ru.  Note that if X is not in Rd's
 routing table and a binding for X is not obtainable via Rd's next hop
 for X, or if Rd is not a label distribution peer of Ru with respect
 to X, then Rd must inform Ru that it cannot provide a binding at this
 time.
 However, if the only condition that fails to hold is that Rn has not
 yet provided a label to Rd, then Rd must defer any response to Ru
 until such time as it has receiving a binding from Rn.
 If Rd has distributed a label binding for address prefix X to Ru, and
 at some later time, any attribute of the label binding changes, then
 Rd must redistribute the label binding to Ru, with the new attribute.
 It must do this even though Ru does not issue a new Request.

Rosen, et al. Standards Track [Page 50] RFC 3031 MPLS Architecture January 2001

 This procedure would be used by LSRs that are performing downstream-
 on-demand label allocation in the Ordered LSP Control Mode.
 In section 5.2, we  will discuss how to choose the particular
 procedure to be used at any given time, and how to ensure
 interoperability among LSRs that choose different procedures.

5.1.2. Upstream LSR: Request Procedure

 The Request Procedure is used by the upstream LSR for an address
 prefix to determine when to explicitly request that the downstream
 LSR bind a label to that prefix and distribute the binding.  There
 are three possible procedures that can be used.

5.1.2.1. RequestNever

 Never make a request.  This is useful if the downstream LSR uses the
 PushConditional procedure or the PushUnconditional procedure, but is
 not useful if the downstream LSR uses the PulledUnconditional
 procedure or the the PulledConditional procedures.
 This procedure would be used by an LSR when unsolicited downstream
 label distribution and Liberal Label Retention Mode are being used.

5.1.2.2. RequestWhenNeeded

 Make a request whenever the L3 next hop to the address prefix
 changes, or when a new address prefix is learned, and one doesn't
 already have a label binding from that next hop for the given address
 prefix.
 This procedure would be used by an LSR whenever Conservative Label
 Retention Mode is being used.

5.1.2.3. RequestOnRequest

 Issue a request whenever a request is received, in addition to
 issuing a request when needed (as described in section 5.1.2.2).  If
 Ru is not capable of being an LSP ingress, it may issue a request
 only when it receives a request from upstream.
 If Rd receives such a request from Ru, for an address prefix for
 which Rd has already distributed Ru a label, Rd shall assign a new
 (distinct) label, bind it to X, and distribute that binding.
 (Whether Rd can distribute this binding to Ru immediately or not
 depends on the Distribution Procedure being used.)

Rosen, et al. Standards Track [Page 51] RFC 3031 MPLS Architecture January 2001

 This procedure would be used by an LSR which is doing downstream-on-
 demand label distribution, but is not doing label merging, e.g., an
 ATM-LSR which is not capable of VC merge.

5.1.3. Upstream LSR: NotAvailable Procedure

 If Ru and Rd are respectively upstream and downstream label
 distribution peers for address prefix X, and Rd is Ru's L3 next hop
 for X, and Ru requests a binding for X from Rd, but Rd replies that
 it cannot provide a binding at this time, because it has no next hop
 for X, then the NotAvailable procedure determines how Ru responds.
 There are two possible procedures governing Ru's behavior:

5.1.3.1. RequestRetry

 Ru should issue the request again at a later time.  That is, the
 requester is responsible for trying again later to obtain the needed
 binding.  This procedure would be used when downstream-on-demand
 label distribution is used.

5.1.3.2. RequestNoRetry

 Ru should never reissue the request, instead assuming that Rd will
 provide the binding automatically when it is available.  This is
 useful if Rd uses the PushUnconditional procedure or the
 PushConditional procedure, i.e., if unsolicited downstream label
 distribution is used.
 Note that if Rd replies that it cannot provide a binding to Ru,
 because of some error condition, rather than because Rd has no next
 hop, the behavior of Ru will be governed by the error recovery
 conditions of the label distribution protocol, rather than by the
 NotAvailable procedure.

5.1.4. Upstream LSR: Release Procedure

 Suppose that Rd is an LSR which has bound a label to address prefix
 X, and has distributed that binding to LSR Ru.  If Rd does not happen
 to be Ru's L3 next hop for address prefix X, or has ceased to be Ru's
 L3 next hop for address prefix X, then Ru will not be using the
 label.  The Release Procedure determines how Ru acts in this case.
 There are two possible procedures governing Ru's behavior:

5.1.4.1. ReleaseOnChange

 Ru should release the binding, and inform Rd that it has done so.
 This procedure would be used to implement Conservative Label
 Retention Mode.

Rosen, et al. Standards Track [Page 52] RFC 3031 MPLS Architecture January 2001

5.1.4.2. NoReleaseOnChange

 Ru should maintain the binding, so that it can use it again
 immediately if Rd later  becomes Ru's L3 next hop for X.  This
 procedure would be used to implement Liberal Label Retention Mode.

5.1.5. Upstream LSR: labelUse Procedure

 Suppose Ru is an LSR which has received label binding L for address
 prefix X from LSR Rd, and Ru is upstream of Rd with respect to X, and
 in fact Rd is Ru's L3 next hop for X.
 Ru will make use of the binding if Rd is Ru's L3 next hop for X.  If,
 at the time the binding is received by Ru, Rd is NOT Ru's L3 next hop
 for X, Ru does not make any use of the binding at that time.  Ru may
 however start using the binding at some later time, if Rd becomes
 Ru's L3 next hop for X.
 The labelUse Procedure determines just how Ru makes use of Rd's
 binding.
 There are two procedures which Ru may use:

5.1.5.1. UseImmediate

 Ru may put the binding into use immediately.  At any time when Ru has
 a binding for X from Rd, and Rd is Ru's L3 next hop for X, Rd will
 also be Ru's LSP next hop for X.  This procedure is used when loop
 detection is not in use.

5.1.5.2. UseIfLoopNotDetected

 This procedure is the same as UseImmediate, unless Ru has detected a
 loop in the LSP.  If a loop has been detected, Ru will discontinue
 the use of label L for forwarding packets to Rd.
 This procedure is used when loop detection is in use.
 This will continue until the next hop for X changes, or until the
 loop is no longer detected.

5.1.6. Downstream LSR: Withdraw Procedure

 In this case, there is only a single procedure.
 When LSR Rd decides to break the binding between label L and address
 prefix X, then this unbinding must be distributed to all LSRs to
 which the binding was distributed.

Rosen, et al. Standards Track [Page 53] RFC 3031 MPLS Architecture January 2001

 It is required that the unbinding of L from X be distributed by Rd to
 a LSR Ru before Rd distributes to Ru any new binding of L to any
 other address prefix Y, where X != Y.  If Ru were to learn of the new
 binding of L to Y before it learned of the unbinding of L from X, and
 if packets matching both X and Y were forwarded by Ru to Rd, then for
 a period of time, Ru would label both packets matching X and packets
 matching Y with label L.
 The distribution and withdrawal of label bindings is done via a label
 distribution protocol.  All label distribution protocols require that
 a label distribution adjacency be established between two label
 distribution peers (except implicit peers).  If LSR R1 has a label
 distribution adjacency to LSR R2, and has received label bindings
 from LSR R2 via that adjacency, then if adjacency is brought down by
 either peer (whether as a result of failure or as a matter of normal
 operation), all bindings received over that adjacency must be
 considered to have been withdrawn.
 As long as the relevant label distribution adjacency remains in
 place, label bindings that are withdrawn must always be withdrawn
 explicitly.  If a second label is bound to an address prefix, the
 result is not to implicitly withdraw the first label, but to bind
 both labels; this is needed to support multi-path routing.  If a
 second address prefix is bound to a label, the result is not to
 implicitly withdraw the binding of that label to the first address
 prefix, but to use that label for both address prefixes.

5.2. MPLS Schemes: Supported Combinations of Procedures

 Consider two LSRs, Ru and Rd, which are label distribution peers with
 respect to some set of address prefixes, where Ru is the upstream
 peer and Rd is the downstream peer.
 The MPLS scheme which governs the interaction of Ru and Rd can be
 described as a quintuple of procedures: <Distribution Procedure,
 Request Procedure, NotAvailable Procedure, Release Procedure,
 labelUse Procedure>.  (Since there is only one Withdraw Procedure, it
 need not be mentioned.)  A "*" appearing in one of the positions is a
 wild-card, meaning that any procedure in that category may be
 present; an "N/A" appearing in a particular position indicates that
 no procedure in that category is needed.
 Only the MPLS schemes which are specified below are supported by the
 MPLS Architecture.  Other schemes may be added in the future, if a
 need for them is shown.

Rosen, et al. Standards Track [Page 54] RFC 3031 MPLS Architecture January 2001

5.2.1. Schemes for LSRs that Support Label Merging

 If Ru and Rd are label distribution peers, and both support label
 merging, one of the following schemes must be used:
    1. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
       UseImmediate>
       This is unsolicited downstream label distribution with
       independent control, liberal label retention mode, and no loop
       detection.
    2. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
       UseIfLoopNotDetected>
       This is unsolicited downstream label distribution with
       independent control, liberal label retention, and loop
       detection.
    3. <PushConditional, RequestWhenNeeded, RequestNoRetry,
       ReleaseOnChange, *>
       This is unsolicited downstream label distribution with ordered
       control (from the egress) and conservative label retention
       mode.  Loop detection is optional.
    4. <PushConditional, RequestNever, N/A, NoReleaseOnChange, *>
       This is unsolicited downstream label distribution with ordered
       control (from the egress) and liberal label retention mode.
       Loop detection is optional.
    5. <PulledConditional, RequestWhenNeeded, RequestRetry,
       ReleaseOnChange, *>
       This is downstream-on-demand label distribution with ordered
       control (initiated by the ingress), conservative label
       retention mode, and optional loop detection.
    6. <PulledUnconditional, RequestWhenNeeded, N/A, ReleaseOnChange,
       UseImmediate>
       This is downstream-on-demand label distribution with
       independent control and conservative label retention mode,
       without loop detection.

Rosen, et al. Standards Track [Page 55] RFC 3031 MPLS Architecture January 2001

    7. <PulledUnconditional, RequestWhenNeeded, N/A, ReleaseOnChange,
       UseIfLoopNotDetected>
       This is downstream-on-demand label distribution with
       independent control and conservative label retention mode, with
       loop detection.

5.2.2. Schemes for LSRs that do not Support Label Merging

 Suppose that R1, R2, R3, and R4 are ATM switches which do not support
 label merging, but are being used as LSRs.  Suppose further that the
 L3 hop-by-hop path for address prefix X is <R1, R2, R3, R4>, and that
 packets destined for X can enter the network at any of these LSRs.
 Since there is no multipoint-to-point capability, the LSPs must be
 realized as point-to-point VCs, which means that there needs to be
 three such VCs for address prefix X: <R1, R2, R3, R4>, <R2, R3, R4>,
 and <R3, R4>.
 Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is
 implemented using conventional ATM switching hardware (i.e., no cell
 interleave suppression), or is otherwise incapable of performing
 label merging, the MPLS scheme in use between R1 and R2 must be one
 of the following:
    1. <PulledConditional, RequestOnRequest, RequestRetry,
       ReleaseOnChange, *>
       This is downstream-on-demand label distribution with ordered
       control (initiated by the ingress), conservative label
       retention mode, and optional loop detection.
       The use of the RequestOnRequest procedure will cause R4 to
       distribute three labels for X to R3; R3 will distribute 2
       labels for X to R2, and R2 will distribute one label for X to
       R1.
    2. <PulledUnconditional, RequestOnRequest, N/A, ReleaseOnChange,
       UseImmediate>
       This is downstream-on-demand label distribution with
       independent control and conservative label retention mode,
       without loop detection.

Rosen, et al. Standards Track [Page 56] RFC 3031 MPLS Architecture January 2001

    3. <PulledUnconditional, RequestOnRequest, N/A, ReleaseOnChange,
       UseIfLoopNotDetected>
       This is downstream-on-demand label distribution with
       independent control and conservative label retention mode, with
       loop detection.

5.2.3. Interoperability Considerations

 It is easy to see that certain quintuples do NOT yield viable MPLS
 schemes.  For example:
  1. <PulledUnconditional, RequestNever, *, *, *>

<PulledConditional, RequestNever, *, *, *>

       In these MPLS schemes, the downstream LSR Rd distributes label
       bindings to upstream LSR Ru only upon request from Ru, but Ru
       never makes any such requests.  Obviously, these schemes are
       not viable, since they will not result in the proper
       distribution of label bindings.
  1. <*, RequestNever, *, *, ReleaseOnChange>
       In these MPLS schemes, Rd releases bindings when it isn't using
       them, but it never asks for them again, even if it later has a
       need for them.  These schemes thus do not ensure that label
       bindings get properly distributed.
 In this section, we specify rules to prevent a pair of label
 distribution peers from adopting procedures which lead to infeasible
 MPLS Schemes.  These rules require either the exchange of information
 between label distribution peers during the initialization of the
 label distribution adjacency, or a priori knowledge of the
 information (obtained through a means outside the scope of this
 document).
    1. Each must state whether it supports label merging.
    2. If Rd does not support label merging, Rd must choose either the
       PulledUnconditional procedure or the PulledConditional
       procedure.  If Rd chooses PulledConditional, Ru is forced to
       use the RequestRetry procedure.
       That is, if the downstream LSR does not support label merging,
       its preferences take priority when the MPLS scheme is chosen.

Rosen, et al. Standards Track [Page 57] RFC 3031 MPLS Architecture January 2001

    3. If Ru does not support label merging, but Rd does, Ru must
       choose either the RequestRetry or RequestNoRetry procedure.
       This forces Rd to use the PulledConditional or
       PulledUnConditional procedure respectively.
       That is, if only one of the LSRs doesn't support label merging,
       its preferences take priority when the MPLS scheme is chosen.
    4. If both Ru and Rd both support label merging, then the choice
       between liberal and conservative label retention mode belongs
       to Ru.  That is, Ru gets to choose either to use
       RequestWhenNeeded/ReleaseOnChange (conservative) , or to use
       RequestNever/NoReleaseOnChange (liberal).  However, the choice
       of "push" vs. "pull" and "conditional" vs. "unconditional"
       belongs to Rd.  If Ru chooses liberal label retention mode, Rd
       can choose either PushUnconditional or PushConditional.  If Ru
       chooses conservative label retention mode, Rd can choose
       PushConditional, PulledConditional, or PulledUnconditional.
       These choices together determine the MPLS scheme in use.

6. Security Considerations

 Some routers may implement security procedures which depend on the
 network layer header being in a fixed place relative to the data link
 layer header.  The MPLS generic encapsulation inserts a shim between
 the data link layer header and the network layer header.  This may
 cause any such security procedures to fail.
 An MPLS label has its meaning by virtue of an agreement between the
 LSR that puts the label in the label stack (the "label writer"), and
 the LSR that interprets that label (the "label reader").  If labeled
 packets are accepted from untrusted sources, or if a particular
 incoming label is accepted from an LSR to which that label has not
 been distributed, then packets may be routed in an illegitimate
 manner.

7. Intellectual Property

 The IETF has been notified of intellectual property rights claimed in
 regard to some or all of the specification contained in this
 document.  For more information consult the online list of claimed
 rights.

Rosen, et al. Standards Track [Page 58] RFC 3031 MPLS Architecture January 2001

8. Authors' Addresses

 Eric C. Rosen
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA, 01824
 EMail: erosen@cisco.com
 Arun Viswanathan
 Force10 Networks, Inc.
 1440 McCarthy Blvd.
 Milpitas, CA 95035-7438
 EMail: arun@force10networks.com
 Ross Callon
 Juniper Networks, Inc.
 1194 North Mathilda Avenue
 Sunnyvale, CA 94089 USA
 EMail: rcallon@juniper.net

9. References

 [MPLS-ATM]          Davie, B., Lawrence, J., McCloghrie, K., Rekhter,
                     Y., Rosen, E., Swallow, G. and P. Doolan, "MPLS
                     using LDP and ATM VC Switching", RFC 3035,
                     January 2001.
 [MPLS-BGP]          "Carrying Label Information in BGP-4", Rekhter,
                     Rosen, Work in Progress.
 [MPLS-CR-LDP]       "Constraint-Based LSP Setup using LDP", Jamoussi,
                     Editor, Work in Progress.
 [MPLS-FRMRLY]       Conta, A., Doolan, P. and A. Malis, "Use of Label
                     Switching on Frame Relay Networks Specification",
                     RFC 3034, January 2001.
 [MPLS-LDP]          Andersson, L., Doolan, P., Feldman, N., Fredette,
                     A. and B. Thomas, "LDP Specification", RFC 3036,
                     January 2001.

Rosen, et al. Standards Track [Page 59] RFC 3031 MPLS Architecture January 2001

 [MPLS-RSVP-TUNNELS] "Extensions to RSVP for LSP Tunnels", Awduche,
                     Berger, Gan, Li, Swallow, Srinvasan, Work in
                     Progress.
 [MPLS-SHIM]         Rosen, E., Rekhter, Y., Tappan, D., Fedorkow, G.,
                     Farinacci, D. and A. Conta, "MPLS Label Stack
                     Encoding", RFC 3032, January 2001.
 [MPLS-TRFENG]       Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M.
                     and J. McManus, "Requirements for Traffic
                     Engineering Over MPLS", RFC 2702, September 1999.

Rosen, et al. Standards Track [Page 60] RFC 3031 MPLS Architecture January 2001

10. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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.

Rosen, et al. Standards Track [Page 61]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3031.txt · Last modified: 2001/01/04 21:00 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki