Network Working Group                                      Eric C. Rosen
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: October 1999
                                                        Arun Viswanathan
                                                     Lucent Technologies

                                                             Ross Callon
                                               IronBridge Networks, Inc.

                                                              April 1999

               Multiprotocol Label Switching Architecture

                      draft-ietf-mpls-arch-05.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Abstract

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

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Table of Contents

    1          Introduction to MPLS  ...............................   4
    1.1        Overview  ...........................................   4
    1.2        Terminology  ........................................   6
    1.3        Acronyms and Abbreviations  .........................   9
    1.4        Acknowledgments  ....................................  10
    2          MPLS Basics  ........................................  10
    2.1        Labels  .............................................  10
    2.2        Upstream and Downstream LSRs  .......................  11
    2.3        Labeled Packet  .....................................  11
    2.4        Label Assignment and Distribution  ..................  11
    2.5        Attributes of a Label Binding  ......................  12
    2.6        Label Distribution Protocols  .......................  12
    2.7        Unsolicited Downstream vs. Downstream-on-Demand  ....  12
    2.8        Label Retention Mode  ...............................  13
    2.9        The Label Stack  ....................................  13
    2.10       The Next Hop Label Forwarding Entry (NHLFE)  ........  14
    2.11       Incoming Label Map (ILM)  ...........................  15
    2.12       FEC-to-NHLFE Map (FTN)  .............................  15
    2.13       Label Swapping  .....................................  15
    2.14       Scope and Uniqueness of Labels  .....................  16
    2.15       Label Switched Path (LSP), LSP Ingress, LSP Egress  .  17
    2.16       Penultimate Hop Popping  ............................  19
    2.17       LSP Next Hop  .......................................  20
    2.18       Invalid Incoming Labels  ............................  21
    2.19       LSP Control: Ordered versus Independent  ............  21
    2.20       Aggregation  ........................................  22
    2.21       Route Selection  ....................................  24
    2.22       Lack of Outgoing Label  .............................  24
    2.23       Time-to-Live (TTL)  .................................  25
    2.24       Loop Control  .......................................  26
    2.25       Label Encodings  ....................................  27
    2.25.1     MPLS-specific Hardware and/or Software  .............  27
    2.25.2     ATM Switches as LSRs  ...............................  27
    2.25.3     Interoperability among Encoding Techniques  .........  29
    2.26       Label Merging  ......................................  29
    2.26.1     Non-merging LSRs  ...................................  30
    2.26.2     Labels for Merging and Non-Merging LSRs  ............  31
    2.26.3     Merge over ATM  .....................................  32
    2.26.3.1   Methods of Eliminating Cell Interleave  .............  32
    2.26.3.2   Interoperation: VC Merge, VP Merge, and Non-Merge  ..  32
    2.27       Tunnels and Hierarchy  ..............................  33
    2.27.1     Hop-by-Hop Routed Tunnel  ...........................  34
    2.27.2     Explicitly Routed Tunnel  ...........................  34
    2.27.3     LSP Tunnels  ........................................  34

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    2.27.4     Hierarchy: LSP Tunnels within LSPs  .................  35
    2.27.5     Label Distribution Peering and Hierarchy  ...........  35
    2.28       Label Distribution Protocol Transport  ..............  37
    2.29       Why More than one Label Distribution Protocol?  .....  37
    2.29.1     BGP and LDP  ........................................  37
    2.29.2     Labels for RSVP Flowspecs  ..........................  37
    2.29.3     Labels for Explicitly Routed LSPs  ..................  38
    2.30       Multicast  ..........................................  38
    3          Some Applications of MPLS  ..........................  38
    3.1        MPLS and Hop by Hop Routed Traffic  .................  38
    3.1.1      Labels for Address Prefixes  ........................  38
    3.1.2      Distributing Labels for Address Prefixes  ...........  39
    3.1.2.1    Label Distribution Peers for an Address Prefix  .....  39
    3.1.2.2    Distributing Labels  ................................  39
    3.1.3      Using the Hop by Hop path as the LSP  ...............  40
    3.1.4      LSP Egress and LSP Proxy Egress  ....................  41
    3.1.5      The Implicit NULL Label  ............................  41
    3.1.6      Option: Egress-Targeted Label Assignment  ...........  42
    3.2        MPLS and Explicitly Routed LSPs  ....................  44
    3.2.1      Explicitly Routed LSP Tunnels  ......................  44
    3.3        Label Stacks and Implicit Peering  ..................  45
    3.4        MPLS and Multi-Path Routing  ........................  46
    3.5        LSP Trees as Multipoint-to-Point Entities  ..........  46
    3.6        LSP Tunneling between BGP Border Routers  ...........  47
    3.7        Other Uses of Hop-by-Hop Routed LSP Tunnels  ........  49
    3.8        MPLS and Multicast  .................................  49
    4          Label Distribution Procedures (Hop-by-Hop)  .........  50
    4.1        The Procedures for Advertising and Using labels  ....  50
    4.1.1      Downstream LSR: Distribution Procedure  .............  50
    4.1.1.1    PushUnconditional  ..................................  51
    4.1.1.2    PushConditional  ....................................  51
    4.1.1.3    PulledUnconditional  ................................  52
    4.1.1.4    PulledConditional  ..................................  52
    4.1.2      Upstream LSR: Request Procedure  ....................  53
    4.1.2.1    RequestNever  .......................................  53
    4.1.2.2    RequestWhenNeeded  ..................................  53
    4.1.2.3    RequestOnRequest  ...................................  54
    4.1.3      Upstream LSR: NotAvailable Procedure  ...............  54
    4.1.3.1    RequestRetry  .......................................  54
    4.1.3.2    RequestNoRetry  .....................................  54
    4.1.4      Upstream LSR: Release Procedure  ....................  55
    4.1.4.1    ReleaseOnChange  ....................................  55
    4.1.4.2    NoReleaseOnChange  ..................................  55
    4.1.5      Upstream LSR: labelUse Procedure  ...................  55
    4.1.5.1    UseImmediate  .......................................  56
    4.1.5.2    UseIfLoopNotDetected  ...............................  56
    4.1.6      Downstream LSR: Withdraw Procedure  .................  56
    4.2        MPLS Schemes: Supported Combinations of Procedures  .  57

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    4.2.1      Schemes for LSRs that Support Label Merging  ........  57
    4.2.2      Schemes for LSRs that do not Support Label Merging  .  58
    4.2.3      Interoperability Considerations  ....................  59
    5          Security Considerations  ............................  61
    6          Intellectual Property  ..............................  61
    7          Authors' Addresses  .................................  61
    8          References  .........................................  62

1. Introduction to MPLS

1.1. 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.

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   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.

     - 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.

     - 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.

     - 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.

     - 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.

     - 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

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   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.

   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.

   A general discussion of issues related to MPLS is presented in "A
   Framework for Multiprotocol Label Switching" [MPLS-FRMWRK].

1.2. 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.

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     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.

     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

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     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.

     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.

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     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.

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

1.3. 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

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1.4. 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.

2. MPLS Basics

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

2.1. 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.

   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,

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   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.

2.2. 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.

2.3. 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.

2.4. 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.

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2.5. 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.

2.6. 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.

   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], [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.

2.7. 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.

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   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.

2.8. 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.

   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.

2.9. 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.

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   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 2.27).

2.10. 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

            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

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   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".

2.11. 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.

2.12. 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.

   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.

2.13. 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.

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   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.

2.14. 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.

   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:

     - Ru1 and Ru2 are the only label distribution peers to which Rd
       distributed a binding of label value L, and

     - 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

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   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.

2.15. 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:

      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;

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      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.

   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.

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2.16. Penultimate Hop Popping

   Note that according to the definitions of section 2.15, 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:

     - that it is the penultimate hop, and

     - 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
   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:

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     - the code may be simplified if it can assume that only a single
       lookup is ever needed

     - 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 2.14
   are obeyed, it is always possible to interpret the top label of a
   received packet unambiguously.

2.17. 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.

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2.18. 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.

2.19. 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.

   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

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   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.

2.20. 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
   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".

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   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:

     - 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.

     - 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.

   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").

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2.21. 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.

2.22. 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:

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     - If the packet has been following an explicitly routed LSP, this
       could result in a loop.

     - 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.

2.23. 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.

   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

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   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.

2.24. 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
   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].

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2.25. 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.

2.25.1. 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].

2.25.2. 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".

   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-

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         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 2.26, 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
         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.

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2.25.3. 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.

2.26. 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".

   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.

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   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.

2.26.1. 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.

   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

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   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.

2.26.2. 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.

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2.26.3. Merge over ATM

2.26.3.1. 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.

2.26.3.2. 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

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   media). If the upstream neighbor is not doing merge, then the
   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).

2.27. 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".

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2.27.1. 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.

2.27.2. 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.

2.27.3. 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.

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2.27.4. 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>.

   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.

2.27.5. 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 3.6 and 3.7 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

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   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
         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 3.1.2.

         Examples of the use of explicit peering is found in sections
         3.2.1 and 3.6.

      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
         3.3.

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2.28. 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].

2.29. 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.

2.29.1. 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 3.1).  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.

2.29.2. 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.

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2.29.3. 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:

     - Start with an existing protocol that is used for setting up
       resource reservations, and extend it to support explicit routing
       and label distribution.

     - Start with an existing protocol that is used for label
       distribution, and extend it to support explicit routing and
       resource reservations.

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

2.30. Multicast

   This section is for further study

3. Some Applications of MPLS

3.1. 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.

3.1.1. 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.

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3.1.2. Distributing Labels for Address Prefixes

3.1.2.1. 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

      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.

3.1.2.2. 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.

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   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.

3.1.3. 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
   10.2.153/22, 10.2.154/22, 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.

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3.1.4. 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

   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.

3.1.5. 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 3.1.2 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

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      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.

3.1.6. 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
   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:

     - 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.

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     - 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.

     - 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:

     - 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 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

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   will just map the incoming label to the label which Rd has assigned
   to the address of that LSP Egress.

3.2. 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].

3.2.1. 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:

      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.

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3.3. 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:

     - 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

          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

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   become too large.

3.4. 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.

3.5. 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,
   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 2.25.2) can be
   used, the LSPs can be realized as multipoint-to-point SVPs.

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3.6. 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,

            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

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            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:

     - B3 distributes routes to B2 (using EBGP), optionally assigning
       labels to address prefixes;

     - 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.

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

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3.7. 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.

3.8. 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.

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4. 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.

4.1. 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:

     - The Distribution Procedure, and

     - the Withdrawal Procedure.

   The upstream LSR must perform:

     - The Request Procedure, and

     - the NotAvailable Procedure, and

     - the Release Procedure, and

     - 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 4.2, where the
   interoperability between different combinations will also be
   discussed.

4.1.1. 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

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   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.

4.1.1.1. 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.

4.1.1.2. 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.

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   This procedure would be used by LSRs which are performing unsolicited
   downstream label assignment in the Ordered LSP Control Mode.

4.1.1.3. 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

      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.

4.1.1.4. 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

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   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.

   This procedure would be used by LSRs that are performing downstream-
   on-demand label allocation in the Ordered LSP Control Mode.

   In section 4.2, 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.

4.1.2. 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.

4.1.2.1. 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.

4.1.2.2. 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

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   Retention Mode is being used.

4.1.2.3. RequestOnRequest

   Issue a request whenever a request is received, in addition to
   issuing a request when needed (as described in section 4.1.2.2).  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.)

   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.

4.1.3. 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:

4.1.3.1. 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.

4.1.3.2. 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.

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   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.

4.1.4. 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:

4.1.4.1. 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.

4.1.4.2. 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.

4.1.5. 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:

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4.1.5.1. 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.

4.1.5.2. 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.

4.1.6. 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.

   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

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   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.

4.2. 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.

4.2.1. 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.

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      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.

      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.

4.2.2. 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

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   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.

      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.

4.2.3. Interoperability Considerations

   It is easy to see that certain quintuples do NOT yield viable MPLS
   schemes.  For example:

     - <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.

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     - <*, 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 apriori 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.

      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.

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5. 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 such 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.

6. 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.

7. Authors' Addresses

      Eric C. Rosen
      Cisco Systems, Inc.
      250 Apollo Drive
      Chelmsford, MA, 01824
      E-mail: erosen@cisco.com

      Arun Viswanathan
      Lucent Technologies
      101 Crawford Corner Rd., #4D-537
      Holmdel, NJ 07733
      732-332-5163
      E-mail: arunv@dnrc.bell-labs.com

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      Ross Callon
      IronBridge Networks
      55 Hayden Avenue,
      Lexington, MA  02173
      +1-781-372-8117
      E-mail: rcallon@ironbridgenetworks.com

8. References

   [MPLS-ATM] "MPLS using LDP and ATM VC Switching", Davie, Doolan,
   Lawrence, McGloghrie, Rekhter, Rosen, Swallow, work in progress,
   April 1999.

   [MPLS-BGP] "Carrying Label Information in BGP-4", Rekhter, Rosen,
   work in progress, February 1999.

   [MPLS-CR-LDP] "Constraint-Based LSP Setup using LDP", Jamoussi,
   editor, work in progress, March 1999.

   [MPLS-FRMWRK] "A Framework for Multiprotocol Label Switching",
   Callon, Doolan, Feldman, Fredette, Swallow, Viswanathan, work in
   progress, November 1997

   [MPLS-FRMRLY] "Use of Label Switching on Frame Relay Networks",
   Conta, Doolan, Malis, work in progress, November 1998

   [MPLS-LDP], "LDP Specification", Andersson, Doolan, Feldman,
   Fredette, Thomas, work in progress, April 1999.

   [MPLS-RSVP] "Use of Label Switching with RSVP", Davie, Rekhter,
   Rosen, Viswanathan, Srinivasan, work in progress, March 1998.

   [MPLS-RSVP-TUNNELS], "Extensions to RSVP for LSP Tunnels", Awduche,
   Berger, Gan, Li, Swallow, Srinvasan, work in progress, March 1999.

   [MPLS-SHIM] "MPLS Label Stack Encodings", Rosen, Rekhter, Tappan,
   Farinacci, Fedorkow, Li, Conta, work in progress, April 1999.

   [MPLS-TRFENG] "Requirements for Traffic Engineering Over MPLS",
   Awduche, Malcolm, Agogbua, O'Dell, McManus, work in progress, August
   1998.

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