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Versions: (draft-rosen-mpls-arch) 00 01 02 03 04 05 06 RFC 3031

Network Working Group                                      Eric C. Rosen
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: September 1998
                                                        Arun Viswanathan
                                                     Lucent Technologies

                                                             Ross Callon
                                               IronBridge Networks, Inc.

                                                              March 1998


               Multiprotocol Label Switching Architecture


                      draft-ietf-mpls-arch-01.txt

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
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   (US West Coast).


Abstract

   This internet draft specifies the architecture for multiprotocol
   label switching (MPLS). The architecture is based on other label
   switching approaches [2-11] as well as on the MPLS Framework document
   [1].









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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          Outline of Approach  ................................  11
    2.1        Labels  .............................................  11
    2.2        Upstream and Downstream LSRs  .......................  12
    2.3        Labeled Packet  .....................................  12
    2.4        Label Assignment and Distribution; Attributes  ......  12
    2.5        Label Distribution Protocol (LDP)  ..................  13
    2.6        The Label Stack  ....................................  13
    2.7        The Next Hop Label Forwarding Entry (NHLFE)  ........  14
    2.8        Incoming Label Map (ILM)  ...........................  14
    2.9        Stream-to-NHLFE Map (STN)  ..........................  15
    2.10       Label Swapping  .....................................  15
    2.11       Scope and Uniqueness of Labels  .....................  15
    2.12       Label Switched Path (LSP), LSP Ingress, LSP Egress  .  16
    2.13       Penultimate Hop Popping  ............................  18
    2.14       LSP Next Hop  .......................................  19
    2.15       Route Selection  ....................................  20
    2.16       Time-to-Live (TTL)  .................................  21
    2.17       Loop Control  .......................................  22
    2.17.1     Loop Prevention  ....................................  23
    2.17.2     Interworking of Loop Control Options  ...............  25
    2.18       Merging and Non-Merging LSRs  .......................  26
    2.18.1     Stream Merge  .......................................  27
    2.18.2     Non-merging LSRs  ...................................  27
    2.18.3     Labels for Merging and Non-Merging LSRs  ............  28
    2.18.4     Merge over ATM  .....................................  29
    2.18.4.1   Methods of Eliminating Cell Interleave  .............  29
    2.18.4.2   Interoperation: VC Merge, VP Merge, and Non-Merge  ..  29
    2.19       LSP Control: Egress versus Local  ...................  30
    2.20       Granularity  ........................................  32
    2.21       Tunnels and Hierarchy  ..............................  33
    2.21.1     Hop-by-Hop Routed Tunnel  ...........................  33
    2.21.2     Explicitly Routed Tunnel  ...........................  33
    2.21.3     LSP Tunnels  ........................................  33
    2.21.4     Hierarchy: LSP Tunnels within LSPs  .................  34
    2.21.5     LDP Peering and Hierarchy  ..........................  34
    2.22       LDP Transport  ......................................  36
    2.23       Label Encodings  ....................................  36
    2.23.1     MPLS-specific Hardware and/or Software  .............  36
    2.23.2     ATM Switches as LSRs  ...............................  37



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    2.23.3     Interoperability among Encoding Techniques  .........  38
    2.24       Multicast  ..........................................  39
    3          Some Applications of MPLS  ..........................  39
    3.1        MPLS and Hop by Hop Routed Traffic  .................  39
    3.1.1      Labels for Address Prefixes  ........................  39
    3.1.2      Distributing Labels for Address Prefixes  ...........  39
    3.1.2.1    LDP Peers for a Particular Address Prefix  ..........  39
    3.1.2.2    Distributing Labels  ................................  40
    3.1.3      Using the Hop by Hop path as the LSP  ...............  41
    3.1.4      LSP Egress and LSP Proxy Egress  ....................  41
    3.1.5      The POP Label  ......................................  42
    3.1.6      Option: Egress-Targeted Label Assignment  ...........  43
    3.2        MPLS and Explicitly Routed LSPs  ....................  44
    3.2.1      Explicitly Routed LSP Tunnels: Traffic Engineering  .  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          LDP Procedures for Hop-by-Hop Routed Traffic  .......  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  ...................................  53
    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  ....................  54
    4.1.4.1    ReleaseOnChange  ....................................  54
    4.1.4.2    NoReleaseOnChange  ..................................  54
    4.1.5      Upstream LSR: labelUse Procedure  ...................  55
    4.1.5.1    UseImmediate  .......................................  55
    4.1.5.2    UseIfLoopFree  ......................................  55
    4.1.5.3    UseIfLoopNotDetected  ...............................  55
    4.1.6      Downstream LSR: Withdraw Procedure  .................  56
    4.2        MPLS Schemes: Supported Combinations of Procedures  .  56
    4.2.1      TTL-capable LSP Segments  ...........................  57
    4.2.2      Using ATM Switches as LSRs  .........................  57
    4.2.2.1    Without Multipoint-to-point Capability  .............  58
    4.2.2.2    With Multipoint-To-Point Capability  ................  58
    4.2.3      Interoperability Considerations  ....................  59



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    4.2.4      How to do Loop Prevention  ..........................  60
    4.2.5      How to do Loop Detection  ...........................  60
    4.2.6      Security Considerations  ............................  60
    5          Authors' Addresses  .................................  60
    6          References  .........................................  61




1. Introduction to MPLS

1.1. Overview

   In connectionless network layer protocols, as a packet travels from
   one router hop to the next, an independent forwarding decision is
   made at each hop.  Each router runs a network layer routing
   algorithm.  As a packet travels through the network, each router
   analyzes the packet header. The choice of next hop for a packet is
   based on the header analysis and the result 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.  Such
   a set of packets may be called a "stream".

   In conventional IP forwarding, a particular router will typically
   consider two packets to be in the same stream 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 stream.

   In MPLS, the assignment of a particular packet to a particular stream
   is done just once, as the packet enters the network.  The stream to
   which the packet is assigned is encoded with 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".

   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



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   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. If assignment to a stream is based on a "longest match",
   this eliminates the need to perform a longest match computation for
   each packet at each hop; the computation can be performed just once.

   Some routers analyze a packet's network layer header not merely to
   choose the packet's next hop, but also to determine a packet's
   "precedence" or "class of service", in order to apply different
   discard thresholds or scheduling disciplines to different packets.
   MPLS allows the precedence or class of service to be inferred from
   the label, so that no further header analysis is needed; in some
   cases MPLS provides a way to explicitly encode a class of service in
   the "label header".

   The fact that a packet is assigned to a stream just once, rather than
   at every hop, allows the use of sophisticated forwarding paradigms.
   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 point ("policy routing") can be easily made.  In fact,
   the policy used to assign a packet to a stream need not have only the
   network layer header as input; it may use arbitrary information about
   the packet, and/or arbitrary policy information as input.  Since this
   decouples forwarding from routing, it allows one to use MPLS to
   support a large variety of routing policies that are difficult or
   impossible to support with just conventional network layer
   forwarding.

   Similarly, MPLS facilitates the use of explicit routing, without
   requiring that each IP packet carry the explicit route. Explicit
   routes may be useful to support policy routing and traffic
   engineering.

   MPLS makes use of a routing approach whereby the normal mode of
   operation is that L3 routing (e.g., existing IP routing protocols
   and/or new IP routing protocols) is used by all nodes to determine
   the routed path.

   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



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   Framework for Multiprotocol Label Switching" [1].


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.

     aggregate stream          synonym of "stream"

     DLCI                      a label used in Frame Relay networks to
                               identify frame relay circuits

     flow                      a single instance of an application to
                               application flow of data (as in the RSVP
                               and IFMP use of the term "flow")

     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               stream merge, 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 stream, usually of local
                               significance.

     label information base    the database of information containing
                               label bindings

     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 streams of data to be
                               forwarded.





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     label switched hop        the hop between two MPLS nodes, on which
                               forwarding is done using labels.

     label switched path       the path created by the concatenation of
                               one or more label switched hops, allowing
                               a packet to be forwarded by swapping
                               labels from an MPLS node to another MPLS
                               node.

     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 and closed

     loop prevention           a method of dealing with loops in which
                               data is never transmitted over a loop

     label stack               an ordered set of labels

     loop survival             a method of dealing with loops in which
                               data may be transmitted over a loop, but
                               means are employed to limit the amount of
                               network resources which may be consumed
                               by the looping data

     label switched path       The path through one or more LSRs at one
                               level of the hierarchy followed by a
                               stream.

     label switching router    an MPLS node which is capable of
                               forwarding native L3 packets







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     merge point               the node at which multiple streams and
                               switched paths are combined into a single
                               stream sent over a single path.

     Mlabel                    abbreviation for MPLS label

     MPLS core standards       the standards which describe the core
                               MPLS technology

     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 placed in a short MPLS shim
                               header used to identify streams

     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






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     stream                    an aggregate of one or more flows,
                               treated as one aggregate for the purpose
                               of forwarding in L2 and/or L3 nodes
                               (e.g., may be described using a single
                               label). In many cases a stream may be the
                               aggregate of a very large number of
                               flows.  Synonymous with "aggregate
                               stream".

     stream merge              the merging of several smaller streams
                               into a larger stream, such that for some
                               or all of the path the larger stream can
                               be referred to using a single label.

     switched path             synonymous with label switched path

     virtual circuit           a circuit used by a connection-oriented
                               layer 2 technology such as ATM or Frame
                               Relay, requiring the maintenance of state
                               information in layer 2 switches.

     VC merge                  stream merge when it is specifically
                               applied to VCs, specifically so as to
                               allow multiple VCs to merge into one
                               single VC

     VP merge                  stream merge when it is applied to VPs,
                               specifically so as to allow multiple VPs
                               to merge into one single VP. In this case
                               the VCIs need to be unique. 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

   STN                       stream to NHLFE Map



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   IGP                       Interior Gateway Protocol

   ILM                       Incoming Label Map

   IP                        Internet Protocol

   LIB                       Label Information Base

   LDP                       Label Distribution Protocol

   L2                        Layer 2

   L3                        Layer 3

   LSP                       Label Switched Path

   LSR                       Label Switching Router

   MPLS                      MultiProtocol Label Switching

   MPT                       Multipoint to Point Tree

   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


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.






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2. Outline of Approach

   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 stream. The label is based on the stream
   or Forwarding Equivalence Class that a packet is assigned to. The
   label does not directly encode the network layer address.  The choice
   of label depends on the network layer address only to the extent that
   the Forwarding Equivalence Class depends on that address.

   If Ru and Rd are LSRs, and Ru transmits a packet to Rd, they may
   agree to use label L to represent stream S for packets which are sent
   from Ru to Rd.  That is, they can agree to a "mapping" between label
   L and stream S for packets moving from Ru to Rd.  As a result of such
   an agreement, L becomes Ru's "outgoing label" corresponding to stream
   S for such packets; L becomes Rd's "incoming label" corresponding to
   stream S for such packets.

   Note that L does not necessarily correspond to stream S for any
   packets other than those which are being sent from Ru to Rd.  Also, L
   is not an inherently meaningful value and does not have any network-
   wide value; the particular value assigned to L gets its meaning
   solely from the agreement between Ru and Rd.

   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 mapping from label to FEC is
   one-to-one.  That is, in such cases, Rd must not agree with Ru1 to
   use L for one purpose, while also agreeing with some other LSR Ru2 to
   use L for a different purpose.














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2.2. Upstream and Downstream LSRs

   Suppose Ru and Rd have agreed to map label L to stream S, for packets
   sent from Ru to Rd.  Then with respect to this mapping, Ru is the
   "upstream LSR", and Rd is the "downstream LSR".

   The notion of upstream and downstream relate to agreements between
   nodes of the label values to be assigned for packets belonging to a
   particular stream that might be traveling from an upstream node to a
   downstream node. This is independent of whether the routing protocol
   actually will cause any packets to be transmitted in that particular
   direction. Thus, Rd is the downstream LSR for a particular mapping
   for label L if it recognizes L-labeled packets from Ru as being in
   stream S.  This may be true even if routing does not actually forward
   packets for stream S between nodes Rd and Ru, or if routing has made
   Ru downstream of Rd along the path which is actually used for packets
   in stream S.


2.3. Labeled Packet

   A "labeled packet" is a packet into which a label has been encoded.
   The encoding can be done by means of an encapsulation which exists
   specifically for this purpose, or by placing the label in an
   available location in either of the data link or network layer
   headers. Of course, the encoding technique must be agreed to by the
   entity which encodes the label and the entity which decodes the
   label.


2.4. Label Assignment and Distribution; Attributes

   For unicast traffic in the MPLS architecture, the decision to bind a
   particular label L to a particular stream S is made by the LSR which
   is downstream with respect to that mapping.  The downstream LSR then
   informs the upstream LSR of the mapping.  Thus labels are
   "downstream-assigned", and are "distributed upstream".

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








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2.5. Label Distribution Protocol (LDP)

   A Label Distribution Protocol (LDP) is a set of procedures by which
   one LSR informs another of the label/Stream mappings it has made.
   Two LSRs which use an LDP to exchange label/Stream mapping
   information are known as "LDP Peers" with respect to the mapping
   information they exchange; we will speak of there being an "LDP
   Adjacency" between them.

   (N.B.: two LSRs may be LDP Peers with respect to some set of
   mappings, but not with respect to some other set of mappings.)

   The LDP also encompasses any negotiations in which two LDP Peers need
   to engage in order to learn of each other's MPLS capabilities.


2.6. 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".

   IN MPLS, EVERY FORWARDING DECISION IS BASED EXCLUSIVELY ON THE LABEL
   AT THE TOP OF THE STACK.

   Although, as we shall see, MPLS supports a hierarchy, the processing
   of a labeled packet is completely independent of the level of
   hierarchy.  The processing is always based on the top label, without
   regard for the possibility that some number of other labels may have
   been "above it" in the past, or that some number of other labels may
   be below it at present.

   An unlabeled packet can be thought of as a packet whose label stack
   is empty (i.e., whose label stack has depth 0).

   If a packet's label stack is of depth m, we refer to the label at the
   bottom of the stack as the level 1 label, to the label above it (if
   such exists) as the level 2 label, and to the label at the top of the
   stack as the level m label.

   The utility of the label stack will become clear when we introduce
   the notion of LSP Tunnel and the MPLS Hierarchy (sections 2.21.3 and
   2.21.4).







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2.7. 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 data link encapsulation to use when transmitting the packet

      3. the way to encode the label stack when transmitting the packet

      4. 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.

   Note that at a given LSR, the packet's "next hop" might be that LSR
   itself.  In this case, the LSR would need to pop the top level label,
   and then "forward" the resulting packet to itself.  It would then
   make another forwarding decision, based on what remains after the
   label stacked is popped.  This may still be a labeled packet, or it
   may be the native IP packet.

   This implies that in some cases the LSR may need to operate on the IP
   header in order to forward the packet.

   If the packet's "next hop" is the current LSR, then the label stack
   operation MUST be to "pop the stack".


2.8. Incoming Label Map (ILM)

   The "Incoming Label Map" (ILM) is a mapping from incoming labels to
   NHLFEs. It is used when forwarding packets that arrive as labeled
   packets.









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2.9. Stream-to-NHLFE Map (STN)

   The "Stream-to-NHLFE" (STN) is a mapping from stream to NHLFEs. It is
   used when forwarding packets that arrive unlabeled, but which are to
   be labeled before being forwarded.


2.10. Label Swapping

   Label swapping is the use of the following procedures to forward a
   packet.

   In order to forward a labeled packet, a LSR examines the label at the
   top of the label stack. It uses the ILM to map this label to an
   NHLFE.  Using the information in the NHLFE, it determines where to
   forward the packet, and performs an operation on the packet's label
   stack. It then encodes the new label stack into the packet, and
   forwards the result.

   In order to forward an unlabeled packet, a LSR analyzes the network
   layer header, to determine the packet's stream. It then uses the STN
   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.


2.11. Scope and Uniqueness of Labels

   A given LSR Rd may map label L1 to stream S, and distribute that
   mapping to LDP peer Ru1.  Rd may also map label L2 to stream S, and
   distribute that mapping to LDP peer Ru2.  Whether or not L1 == L2 is
   not determined by the architecture; this is a local matter.

   A given LSR Rd may map label L to stream S1, and distribute that
   mapping to LDP peer Ru1.  Rd may also map label L to stream S2, and
   distribute that mapping to LDP 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 S1 == S2.  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:




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     - Ru1 and Ru2 are the only LDP peers to which Rd distributed a
       mapping 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.  When
   these conditions do not hold, the labels must be unique over the LSR
   which has assigned them.

   If a particular LSR Rd is attached to a particular LSR Ru over two
   point-to-point interfaces, then Rd may distribute to Rd a mapping of
   label L to stream S1, as well as a mapping of label L to stream S2,
   S1 != S2, if and only if each mapping 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 mappings of the same label value
   to two different streams.

   This prohibition holds even if the mappings 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.


2.12. 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 stream S"
   if it is an LSP of level m for a particular packet P when P's level m
   label is a label corresponding to stream S.

   Consider the set of nodes which may be LSP ingress nodes for stream
   S.  Then there is an LSP for stream S 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 stream S.







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2.13. Penultimate Hop Popping

   Note that according to the definitions of section 2.11, 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 forward the packet
   based on the information  gained by looking up the label that was at
   the top of the stack.  When the LSP egress  receives the packet, the
   label 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
   every 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 LDP peer.

   Initial LDP negotiations must allow each LSR to determine whether its
   neighboring LSRS are capable of popping the label stack.  A LSR will
   not request an LDP 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 2.11
   are obeyed, it is always possible to interpret the top label of a
   received packet unambiguously.


2.14. 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 stream is the next hop as selected
   by the NHLFE entry indexed by a label which corresponds to that
   stream.

   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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2.15. Route Selection

   Route selection refers to the method used for selecting the LSP for a
   particular stream. 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 the path for a stream. This is the normal mode today with
   existing datagram IP networks. A hop by hop routed LSP refers to an
   LSP whose route is selected using hop by hop routing.

   An explicitly routed LSP is an LSP where, at a given LSR, the LSP
   next hop is not chosen by each local node, but rather is chosen by a
   single node (usually the ingress or egress node of the LSP). 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 allowing
   policy routing and/or facilitating traffic engineering.  With 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 implies that explicit routing with MPLS is
   relatively efficient (when compared with the efficiency of explicit
   routing for pure datagrams).

   For any one LSP (at any one level of hierarchy), there are two
   possible options: (i) The entire LSP may be hop by hop routed from
   ingress to egress; (ii) The entire LSP may be explicit routed from
   ingress to egress. Intermediate cases do not make sense: In general,
   an LSP will be explicit routed specifically because there is a good
   reason to use an alternative to the hop by hop routed path. This
   implies that if some of the nodes along the path follow an explicit
   route but some of the nodes make use of hop by hop routing, then
   inconsistent routing will result and loops (or severely inefficient
   paths) may form.

   For this reason, it is important that if an explicit route is
   specified for an LSP, then that route must be followed. Note that it
   is relatively simple to *follow* an explicit route which is specified
   in a LDP setup.  We therefore propose that the LDP specification
   require that all MPLS nodes implement the ability to follow an
   explicit route if this is specified.

   It is not necessary for a node to be able to create an explicit
   route.  However, in order to ensure interoperability it is necessary



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   to ensure that either (i) Every node knows how to use hop by hop
   routing; or (ii) Every node knows how to create and follow an
   explicit route. We propose that due to the common use of hop by hop
   routing in networks today, it is reasonable to make hop by hop
   routing the default that all nodes need to be able to use.


2.16. 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, or if the
   MPLS labels are carried in an L2 header such as an ATM header or a
   frame relay header.

   If the label values are encoded in a "shim" that sits between the
   data link and network layer headers, then this shim should have a TTL
   field that is initially loaded from the network layer header TTL
   field, is decremented at each LSR-hop, and is copied into the network
   layer header TTL field when the packet emerges from its LSP.

   If the label values are encoded in an L2 header (e.g., the VPI/VCI
   field in ATM's AAL5 header), and the labeled packets are forwarded by
   an L2 switch (e.g., an ATM switch). This implies that unless the data
   link layer itself has a TTL field (unlike ATM), 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".



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   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.


2.17. 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.

   The MPLS architecture will therefore provide a technique for ensuring
   that looping LSP segments can be detected, and a technique for
   ensuring that looping LSP segments are never created.

   All LSRs will be required to support a common technique for loop
   detection.  Support for the loop prevention technique is optional,
   though it is recommended in ATM-LSRs that have no other way to
   protect themselves against the effects of looping data packets.  Use
   of the loop prevention technique, when supported, is optional.




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2.17.1. Loop Prevention

   NOTE: The loop prevention technique described here is being
   reconsidered, and may be changed.

   LSR's maintain for each of their LSP's an LSR id list. This list is a
   list of all the LSR's downstream from this LSR on a given LSP. The
   LSR id list is used to prevent the formation of switched path loops.
   The LSR ID list is propagated upstream from a node to its neighbor
   nodes.  The LSR ID list is used to prevent loops as follows:

   When a node, R, detects a change in the next hop for a given stream,
   it asks its new next hop for a label and the associated LSR ID list
   for that stream.

   The new next hop responds with a label for the stream and an
   associated LSR id list.

   R looks in the LSR id list. If R determines that it, R, is in the
   list then we have a route loop. In this case, we do nothing and the
   old LSP will continue to be used until the route protocols break the
   loop. The means by which the old LSP is replaced by a new LSP after
   the route protocols breathe loop is described below.

   If R is not in the LSR id list, R will start a "diffusion"
   computation [12].  The purpose of the diffusion computation is to
   prune the tree upstream of R so that we remove all LSR's from the
   tree that would be on a looping path if R were to switch over to the
   new LSP.  After those LSR's are removed from the tree, it is safe for
   R to replace the old LSP with the new LSP (and the old LSP can be
   released).

   The diffusion computation works as follows:

   R adds its LSR id to the list and sends a query message to each of
   its "upstream" neighbors (i.e. to each of its neighbors that is not
   the new "downstream" next hop).

   A node S that receives such a query will process the query as
   follows:

     - If node R is not node S's next hop for the given stream, node S
       will respond to node R will an "OK" message meaning that as far
       as node S is concerned it is safe for node R to switch over to
       the new LSP.






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     - If node R is node S's next hop for the stream, node S will check
       to see if it, node S, is in the LSR id list that it received from
       node R.  If it is, we have a route loop and S will respond with a
       "LOOP" message.  R will unsplice the connection to S pruning S
       from the tree.  The mechanism by which S will get a new LSP for
       the stream after the route protocols break the loop is described
       below.

     - If node S is not in the LSR id list, S will add its LSR id to the
       LSR id list and send a new query message further upstream.  The
       diffusion computation will continue to propagate upstream along
       each of the paths in the tree upstream of S until either a loop
       is detected, in which case the node is pruned as described above
       or we get to a point where a node gets a response ("OK" or
       "LOOP") from each of its neighbors perhaps because none of those
       neighbors considers the node in question to be its downstream
       next hop.  Once a node has received a response from each of its
       upstream neighbors, it returns an "OK" message to its downstream
       neighbor.  When the original node, node R, gets a response from
       each of its neighbors, it is safe to replace the old LSP with the
       new one because all the paths that would loop have been pruned
       from the tree.

   There are a couple of details to discuss:

     - First, we need to do something about nodes that for one reason or
       another do not produce a timely response in response to a query
       message.  If a node Y does not respond to a query from node X
       because of a failure of some kind, X will not be able to respond
       to its downstream neighbors (if any) or switch over to a new LSP
       if X is, like R above, the node that has detected the route
       change.  This problem is handled by timing out the query message.
       If a node doesn't receive a response within a "reasonable" period
       of time, it "unsplices" its VC to the upstream neighbor that is
       not responding and proceeds as it would if it had received the
       "LOOP" message.

     - We also need to be concerned about multiple concurrent routing
       updates.  What happens, for example, when a node M receives a
       request for an LSP from an upstream neighbor, N, when M is in the
       middle of a diffusion computation i.e., it has sent a query
       upstream but hasn't received all the responses.  Since a
       downstream node, node R is about to change from one LSP to
       another, M needs to pass to N an LSR id list corresponding to the
       union of the old and new LSP's if it is to avoid loops both
       before and after the transition.  This is easily accomplished
       since M already has the LSR id list for the old LSP and it gets
       the LSR id list for the new LSP in the query message.  After R



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       makes the switch from the old LSP to the new one, R sends a new
       establish message upstream with the LSR id list of (just) the new
       LSP.  At this point, the nodes upstream of R know that R has
       switched over to the new LSP and that they can return the id list
       for (just) the new LSP in response to any new requests for LSP's.
       They can also grow the tree to include additional nodes that
       would not have been valid for the combined LSR id list.

     - We also need to discuss how a node that doesn't have an LSP for a
       given stream at the end of a diffusion computation (because it
       would have been on a looping LSP) gets one after the routing
       protocols break the loop.  If node L has been pruned from the
       tree and its local route protocol processing entity breaks the
       loop by changing L's next hop, L will request a new LSP from its
       new downstream neighbor which it will use once it executes the
       diffusion computation as described above.  If the loop is broken
       by a route change at another point in the loop, i.e. at a point
       "downstream" of L, L will get a new LSP as the new LSP tree grows
       upstream from the point of the route change as discussed in the
       previous paragraph.

     - Note that when a node is pruned from the tree, the switched path
       upstream of that node remains "connected".  This is important
       since it allows the switched path to get "reconnected" to a
       downstream switched path after a route change with a minimal
       amount of unsplicing and resplicing once the appropriate
       diffusion computation(s) have taken place.

   The LSR Id list can also be used to provide a "loop detection"
   capability.  To use it in this manner, an LSR which sees that it is
   already in the LSR Id list for a particular stream will immediately
   unsplice itself from the switched path for that stream, and will NOT
   pass the LSR Id list further upstream.  The LSR can rejoin a switched
   path for the stream when it changes its next hop for that stream, or
   when it receives a new LSR Id list from its current next hop, in
   which it is not contained.  The diffusion computation would be
   omitted.


2.17.2. Interworking of Loop Control Options

   The MPLS protocol architecture allows some nodes to be using loop
   prevention, while some other nodes are not (i.e., the choice of
   whether or not to use loop prevention may be a local decision). When
   this mix is used, it is not possible for a loop to form which
   includes only nodes which do loop prevention. However, it is possible
   for loops to form which contain a combination of some nodes which do
   loop prevention, and some nodes which do not.



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   There are at least four identified cases in which it makes sense to
   combine nodes which do loop prevention with nodes which do not: (i)
   For transition, in intermediate states while transitioning from all
   non-loop-prevention to all loop prevention, or vice versa; (ii) For
   interoperability, where one vendor implements loop prevention but
   another vendor does not; (iii) Where there is a mixed ATM and
   datagram media network, and where loop prevention is desired over the
   ATM portions of the network but not over the datagram portions; (iv)
   where some of the ATM switches can do fair access to the buffer pool
   on a per-VC basis, and some cannot, and loop prevention is desired
   over the ATM portions of the network which cannot.

   Note that interworking is straightforward.  If an LSR is not doing
   loop prevention, and it receives from a downstream LSR a label
   mapping which contains loop prevention information, it (a) accepts
   the label mapping, (b) does NOT pass the loop prevention information
   upstream, and (c) informs the downstream neighbor that the path is
   loop-free.

   Similarly, if an LSR R which is doing loop prevention receives from a
   downstream LSR a label mapping which does not contain any loop
   prevention information, then R passes the label mapping upstream with
   loop prevention information included as if R were the egress for the
   specified stream.

   Optionally, a node is permitted to implement the ability of either
   doing or not doing loop prevention as options, and is permitted to
   choose which to use for any one particular LSP based on the
   information obtained from downstream nodes. When the label mapping
   arrives from downstream, then the node may choose whether to use loop
   prevention so as to continue to use the same approach as was used in
   the information passed to it. Note that regardless of whether loop
   prevention is used the egress nodes (for any particular LSP) always
   initiates exchange of label mapping information without waiting for
   other nodes to act.


2.18. Merging and Non-Merging LSRs

   Merge allows multiple upstream LSPs to be merged into a single
   downstream LSP. When implemented by multiple nodes, this results in
   the traffic going to a particular egress nodes, based on one
   particular stream, to follow a multipoint to point tree (MPT), with
   the MPT rooted at the egress node and associated with the stream.
   This can have a significant effect on reducing the number of labels
   that need to be maintained by any one particular node.

   If merge was not used at all it would be necessary for each node to



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   provide the upstream neighbors with a label for each stream for each
   upstream node which may be forwarding traffic over the link. This
   implies that the number of labels needed might not in general be
   known a priori. However, the use of merge allows a single label to be
   used per stream, therefore allowing label assignment to be done in a
   common way without regard for the number of upstream nodes which will
   be using the downstream LSP.

   The proposed MPLS protocol architecture supports LSP merge, while
   allowing nodes which do not support LSP merge. This leads to the
   issue of ensuring correct interoperation between nodes which
   implement merge and those which do not. 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.


2.18.1. Stream Merge

   Let us say that an LSR is capable of Stream Merge 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. This in effect takes two incoming streams and merges
   them into one. Once the packets are transmitted, the information that
   they arrived from different interfaces and/or with different incoming
   labels is lost.

   Let us say that an LSR is not capable of Stream Merge 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.

   An LSR which is capable of Stream Merge (a "Merging LSR") needs to
   maintain only one outgoing label for each FEC. AN LSR which is not
   capable of Stream Merge (a "Non-merging LSR") may need to maintain as
   many as N outgoing labels per FEC, where N is the number of LSRs in
   the network. Hence by supporting Stream Merge, an LSR can reduce its
   number of outgoing labels by a factor of O(N). Since each label in
   use requires the dedication of some amount of resources, this can be
   a significant savings.


2.18.2. 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



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   use such technologies for MPLS forwarding; LDP can be used as the
   "signalling protocol" for setting up the cross-connect tables.

   Unfortunately, these technologies do not necessarily support the
   Stream Merge capability. In ATM, if one attempts to perform Stream
   Merge, the result may be the interleaving of cells from various
   packets. If cells from different packets get interleaved, it is
   impossible to reassemble the packets. Some Frame Relay switches use
   cell switching on their backplanes. These switches may also be
   incapable of supporting Stream Merge, 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.


2.18.3. Labels for Merging and Non-Merging LSRs

   An upstream LSR which supports Stream Merge needs to be sent only one
   label per FEC. An upstream neighbor which does not support Stream
   Merge 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 Stream Merge, 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 Stream
   Merge, 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 merge, but
   have a limited number of upstream streams which may be merged into a
   single downstream streams. Suppose for example that due to some
   hardware limitation a node is capable of merging four upstream LSPs
   into a single downstream LSP. Suppose however, that this particular
   node has six upstream LSPs arriving at it for a particular stream. In
   this case, this node may merge these into two downstream LSPs
   (corresponding to two labels that need to be obtained from the
   downstream neighbor). In this case, the normal operation of the LDP



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   implies that the downstream neighbor will supply this node with a
   single label for the stream. This node can then ask its downstream
   neighbor for one additional label for the stream, implying that the
   node will thereby obtain the required two labels.

   The interaction between explicit routing and merge is FFS.


2.18.4. Merge over ATM

2.18.4.1. 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

         When VP merge is used, multiple virtual paths are merged into a
         virtual path, but packets from different sources are
         distinguished by using different VCs 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 FFS.

   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.


2.18.4.2. 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



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   with non-merge.

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


2.19. LSP Control: Egress versus Local

   There is a choice to be made regarding whether the initial setup of
   LSPs will be initiated by the egress node, or locally by each
   individual node.

   When LSP control is done locally, then each node may at any time pass
   label bindings to its neighbors for each FEC recognized by that node.



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   In the normal case that the neighboring nodes recognize the same
   FECs, then nodes may map incoming labels to outgoing labels as part
   of the normal label swapping forwarding method.

   When LSP control is done by the egress, then initially only the
   egress node passes label bindings to its neighbors corresponding to
   any FECs which leave the MPLS network at that egress node. Other
   nodes wait until they get a label from downstream for a particular
   FEC before passing a corresponding label for the same FEC to upstream
   nodes.

   With local control, since each LSR is (at least initially)
   independently assigning labels to FECs, it is possible that different
   LSRs may make inconsistent decisions. For example, an upstream LSR
   may make a coarse decision (map multiple IP address prefixes to a
   single label) while its downstream neighbor makes a finer grain
   decision (map each individual IP address prefix to a separate label).
   With downstream label assignment this can be corrected by having LSRs
   withdraw labels that it has assigned which are inconsistent with
   downstream labels, and replace them with new consistent label
   assignments.

   Even with egress control it is possible that the choice of egress
   node may change, or the egress may (based on a change in
   configuration) change its mind in terms of the granularity which is
   to be used. This implies the same mechanism will be necessary to
   allow changes in granularity to bubble up to upstream nodes. The
   choice of egress or local control may therefore effect the frequency
   with which this mechanism is used, but will not effect the need for a
   mechanism to achieve consistency of label granularity. Generally
   speaking, the choice of local versus egress control does not appear
   to have any effect on the LDP mechanisms which need to be defined.

   Egress control and local control can interwork in a very
   straightforward manner (although when both methods exist in the
   network, the overall behavior of the network is largely that of local
   control).  With either approach, (assuming downstream label
   assignment) the egress node will initially assign labels for
   particular FECs and will pass these labels to its neighbors. With
   either approach these label assignments will bubble upstream, with
   the upstream nodes choosing labels that are consistent with the
   labels that they receive from downstream. The difference between the
   two approaches is therefore primarily an issue of what each node does
   prior to obtaining a label assignment for a particular FEC from
   downstream nodes: Does it wait, or does it assign a preliminary label
   under the expectation that it will (probably) be correct?

   Regardless of which method is used (local control or egress control)



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   each node needs to know (possibly by configuration) what granularity
   to use for labels that it assigns. Where egress control is used, this
   requires each node to know the granularity only for streams which
   leave the MPLS network at that node. For local control, in order to
   avoid the need to withdraw inconsistent labels, each node in the
   network would need to be configured consistently to know the
   granularity for each stream. However, in many cases this may be done
   by using a single level of granularity which applies to all streams
   (such as "one label per IP prefix in the forwarding table").

   This architecture allows the choice between local control and egress
   control to be a local matter.  Since the two methods interwork, a
   given LSR need support only one or the other.


2.20. Granularity

   When forwarding by label swapping, a stream of packets following a
   stream arriving from upstream may be mapped into an equal or coarser
   grain stream. However, a coarse grain stream (for example, containing
   packets destined for a short IP address prefix covering many subnets)
   cannot be mapped directly into a finer grain stream (for example,
   containing packets destined for a longer IP address prefix covering a
   single subnet). This implies that there needs to be some mechanism
   for ensuring consistency between the granularity of LSPs in an MPLS
   network.

   The method used for ensuring compatibility of granularity may depend
   upon the method used for LSP control.

   When LSP control is local, it is possible that a node may pass a
   coarse grain label to its upstream neighbor(s), and subsequently
   receive a finer grain label from its downstream neighbor. In this
   case the node has two options: (i) It may forward the corresponding
   packets using normal IP datagram forwarding (i.e., by examination of
   the IP header); (ii) It may withdraw the label mappings that it has
   passed to its upstream neighbors, and replace these with finer grain
   label mappings.

   When LSP control is egress based, the label setup originates from the
   egress node and passes upstream. It is therefore straightforward with
   this approach to maintain equally-grained mappings along the route.









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2.21. 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".


2.21.1. 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.


2.21.2. 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.


2.21.3. 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 becomes
   a stream, and each LSR in the tunnel must assign a label to that
   stream (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.



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   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.


2.21.4. 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.


2.21.5. LDP 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 LDP peer is
   R21.  From the perspective of the Level 1 LSP, R2's LDP peers are R1
   and R3.  One can have LDP 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 LDP
   Peers".  When two LSRs may be LDP peers, but are not IGP neighbors,
   we will refer to them as "Remote LDP Peers".  In the above example,
   R2 and R21 are local LDP peers, but R2 and R3 are remote LDP peers.




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   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 LDP Peers by opening
   LDP connections to them.  One can perform label Distribution with
   one's Remote LDP Peers in one of two ways:

      1. Explicit Peering

         In explicit peering, one sets up LDP connections between Remote
         LDP Peers, exactly as one would do for Local LDP Peers.  This
         technique is most useful when the number of Remote LDP Peers is
         small, or the number of higher level label mappings is large,
         or the Remote LDP 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 have LDP connections to one's
         remote LDP peers, but only to one's local LDP peers.  To
         distribute higher level labels to ones remote LDP peers, one
         encodes the higher level labels as an attribute of the lower
         level labels, and distributes the lower level label, along with
         this attribute, to the local LDP peers. The local LDP peers
         then propagate the information to their peers. This process
         continues till the information reaches remote LDP peers. Note
         that the intermediary nodes may also be remote LDP peers.

         This technique is most useful when the number of Remote LDP
         Peers is large. Implicit peering does not require a n-square
         peering mesh to distribute labels to the remote LDP peers
         because the information is piggybacked through the local LDP
         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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2.22. LDP Transport

   LDP is used between nodes in an MPLS network to establish and
   maintain the label mappings. In order for LDP to operate correctly,
   LDP information needs to be transmitted reliably, and the LDP
   messages pertaining to a particular FEC need to be transmitted in
   sequence.  Flow control is also required, as is the capability to
   carry multiple LDP messages in a single datagram.

   These goals will be met by using TCP as the underlying transport for
   LDP.

   (The use of multicast techniques to distribute label mappings is
   FFS.)


2.23. 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.


2.23.1. 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 generic MPLS encapsulation should contain the following fields:

      1. the label stack,

      2. a Time-to-Live (TTL) field







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      3. a Class of Service (CoS) field

   The TTL field permits MPLS to provide a TTL function similar to what
   is provided by IP.

   The CoS field permits LSRs to apply various scheduling packet
   disciplines to labeled packets, without requiring separate labels for
   separate disciplines.


2.23.2. 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 LDP becomes the ATM "signaling" protocol.  With
         this encoding technique, the ATM-LSRs cannot perform "push" or
         "pop" operations on the label stack.

      2. SVP Encoding

         Use the VPI field to encode the label which is at the top of
         the label stack, and the VCI field to encode the second label
         on the stack, if one is present. This technique some advantages
         over the previous one, in that it permits the use of ATM "VP-
         switching".  That is, the LSPs are realized as ATM SVPs, with
         LDP 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.




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         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, so multipoint-
         to-point VPs can be provided without any cell interleaving
         problems.

         This technique depends on the existence of a capability for
         assigning small unique values to each ATM switch.

   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.  This does presuppose that it be possible to tell,
   when reassembling the ATM cells into packets, whether the generic
   encapsulation is also present.


2.23.3. 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



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   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.


2.24. Multicast

   This section is for further study



3. Some Applications of MPLS

3.1. MPLS and Hop by Hop Routed Traffic

   One use of MPLS is to simplify the process of forwarding packets
   using hop by hop routing.


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 stream
   are just those packets which match a given address prefix in R's
   routing table. In this case, a stream can be identified with an
   address prefix.

   If packet P must traverse a sequence of routers, and at each router
   in the sequence P matches the same address prefix, MPLS simplifies
   the forwarding process by enabling all routers but the first to avoid
   executing the best match algorithm; they need only look up the label.


3.1.2. Distributing Labels for Address Prefixes

3.1.2.1. LDP Peers for a Particular Address Prefix

   LSRs R1 and R2 are considered to be LDP 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




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      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 LDP peers for that
   address prefix are the IGP neighbors.  If the route to a particular
   address prefix is distributed via BGP, the LDP peers for that address
   prefix are the BGP peers.  In other cases of LSP tunneling, the
   tunnel endpoints are LDP peers.


3.1.2.2. Distributing Labels

   In order to use MPLS for the forwarding of normally routed traffic,
   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 an LDP to distribute the
         mapping of a label to X to each of its LDP Peers for X.

   There is also one circumstance in which an LSR must distribute a
   label mapping 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 mapping
         between T 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



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   speakers.

   These rules are intended to indicate which label mappings must be
   distributed by a given LSR to which other LSRs, NOT to indicate the
   conditions under which the distribution is to be made.  That is
   discussed in section 2.19.


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 stream.


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. R1 has an address Y, such that X is the address prefix in R1'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, R2 is a "deaggregation point" for address prefix X.




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   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 R1 and R2 are not LDP 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 POP Label

   The POP 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 mapping of the POP label 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 mapping between the POP label 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 mapping for X, and

      2. when the LDP connection between Ru and Rd was opened, Ru
         indicated that it could support the POP label, and

      3. Rd is an LSP Egress (not proxy egress) for X.

   This causes the penultimate LSR on a LSP to pop the label stack. This
   is quite appropriate; if the LSP Egress is an MPLS Egress for X, then
   if the penultimate LSR does not pop the label stack, the LSP Egress
   will need to look up the label, pop the label stack, and then look up
   the next label (or look up the L3 address, if no more labels are
   present).  By having the penultimate LSR pop the label stack, the LSP
   Egress is saved the work of having to look up two labels in order to
   make its forwarding decision.

   However, if the penultimate LSR is an ATM switch, it may not have the
   capability to pop the label stack.  Hence a POP label mapping 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



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   Egress, it acts just as if the LSP Egress had distributed the POP
   label 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 streams 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 can be used for all such streams;
   it is not necessary to have a distinct label for each stream.  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 streams

   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 stream?  There are a couple 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 stream must leave the routing
       domain or area.

     - It is possible to use LDP 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:




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     - 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
   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
   (i.e., to allow intentional management of the loading of the
   bandwidth through the nodes and links in the network).


3.2.1. Explicitly Routed LSP Tunnels: Traffic Engineering

   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 is known as Traffic Engineering.




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   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 LDP peers. The label mappings they need to exchange are of
   no interest to the LSRs along the tunnel.


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
   mapping, which we call the "Stack Attribute".  We impose the
   following rules:






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     - When LSR Ru initially labels an untagged packet, if the longest
       match for the packet's destination address is X, and R's LSP next
       hop for X is Rd, and Rd has distributed to R1 a mapping of label
       L1 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 mappings for X to its LDP peers,
             it must include L2 as the stack attribute.

          3. Whenever the stack attribute changes (possibly as a result
             of a change in Ru's LSP next hop for X), Ru must distribute
             the new stack attribute.

   Note that although the label value bound to X may be different at
   each hop along the LSP, the stack attribute value is passed
   unchanged, and is set by the LSP proxy egress.

   Thus the LSP proxy egress for X becomes an "implicit peer" with each
   other LSR in the routing area or domain.  In this case, explicit
   peering would be too unwieldy, because the number of peers would
   become too large.


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 mapping 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 mappings 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 mapping to R2.  R2 binds label L2 to X, and distributes this
   mapping 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



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


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:






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            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
               mapping to B1,

            f) the IGP next hop for the address of B2 is I1,

            g) the address of B2 is in B1's and I1's IGP routing tables
               as a host route, and

            h) I1 has bound label L2 to the address of B2, and
               distributed this mapping 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 mappings for
   address prefixes that are not even known to the IGP routing, the BGP
   routers should become explicit LDP peers with each other.











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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 LDP peers. The label mappings 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 mapping 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 mapping 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. LDP Procedures for Hop-by-Hop Routed Traffic

4.1. The Procedures for Advertising and Using labels

   In this section, we consider only label mappings 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.

   There are a number of different procedures that may be used to
   distribute label mappings.  One such procedure is executed by the
   downstream LSR, and the others 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 mapping for a particular address
   prefix to its LDP peers.  The architecture supports four different
   distribution procedures.

   Irrespective of the particular procedure that is used, if a label
   mapping for a particular address prefix has been distributed by a
   downstream LSR Rd to an upstream LSR Ru, and if at any time the



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   attributes (as defined above) of that mapping 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 maps multiple
   labels to the address prefix (one per route), and hence distributes
   multiple mappings.


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 an LDP Peer of Rd with respect to X

   Whenever these conditions hold, Rd must map a label to X and
   distribute that mapping to Ru.  It is the responsibility of Rd to
   keep track of the mappings which it has distributed to Ru, and to
   make sure that Ru always has these mappings.


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 an LDP 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 mapping to Rd.

   Then as soon as these conditions all hold, Rd should map a label to X
   and distribute that mapping to Ru.

   Whereas PushUnconditional causes the distribution of label mappings
   for all address prefixes in the routing table, PushConditional causes
   the distribution of label mappings only for those address prefixes
   for which one has received label mappings from one's LSP next hop, or
   for which one does not have an MPLS-capable L3 next hop.








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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 map a label to X and
         distribute the mapping to Ru

   Then Rd should map a label to X and distribute that mapping to Ru.
   Note that if X is not in Rd's routing table, or if Rd is not an LDP
   peer of Ru with respect to X, then Rd must inform Ru that it cannot
   provide a mapping at this time.

   If Rd has already distributed a mapping for address prefix X to Ru,
   and it receives a new request from Ru for a mapping for address
   prefix X, it will map a second label, and distribute the new mapping
   to Ru.  The first label mapping remains in effect.


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 map a label to X and
         distribute the mapping 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 mapping to Rd,
         or


   Then as soon as these conditions all hold, Rd should map a label to X
   and distribute that mapping 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 mapping
   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 mapping from Rn.



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   If Rd has distributed a label mapping for address prefix X to Ru, and
   at some later time, any attribute of the label mapping changes, then
   Rd must redistribute the label mapping to Ru, with the new attribute.
   It must do this even though Ru does not issue a new Request.

   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 map a label to that prefix and distribute the mapping.  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.


4.1.2.2. RequestWhenNeeded

   Make a request whenever the L3 next hop to the address prefix
   changes, and one doesn't already have a label mapping from that next
   hop for the given address prefix.


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
   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, map it to X, and distribute that mapping.  (Whether
   Rd can distribute this mapping to Ru immediately or not depends on
   the Distribution Procedure being used.)

   This procedure is useful when the LSRs are implemented on
   conventional ATM switching hardware.




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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 mapping for X from Rd, but Rd replies that
   it cannot provide a mapping at this time, 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
   mapping.


4.1.3.2. RequestNoRetry

   Ru should never reissue the request, instead assuming that Rd will
   provide the mapping automatically when it is available.  This is
   useful if Rd uses the PushUnconditional procedure or the
   PushConditional 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 mapping 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 Rd 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 mapping, and inform Rd that it has done so.


4.1.4.2. NoReleaseOnChange

   Ru should maintain the mapping, so that it can use it again
   immediately if Rd later  becomes Ru's L3 next hop for X.







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4.1.5. Upstream LSR: labelUse Procedure

   Suppose Ru is an LSR which has received label mapping 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 mapping if Rd is Ru's L3 next hop for X.  If,
   at the time the mapping is received by Ru, Rd is NOT Ru's L3 next hop
   for X, Ru does not make any use of the mapping at that time.  Ru may
   however start using the mapping 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
   mapping.

   There are three procedures which Ru may use:


4.1.5.1. UseImmediate

   Ru may put the mapping into use immediately.  At any time when Ru has
   a mapping 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.


4.1.5.2. UseIfLoopFree

   Ru will use the mapping only if it determines that by doing so, it
   will not cause a forwarding loop.

   If Ru has a mapping for X from Rd, and Rd is (or becomes) Ru's L3
   next hop for X, but Rd is NOT Ru's current LSP next hop for X, Ru
   does NOT immediately make Rd its LSP next hop.  Rather, it initiates
   a loop prevention algorithm.  If, upon the completion of this
   algorithm, Rd is still the L3 next hop for X, Ru will make Rd the LSP
   next hop for X, and use L as the outgoing label.

   The loop prevention algorithm to be used is still under
   consideration.


4.1.5.3. 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 discard
   packets that would otherwise have been labeled with L and sent to Rd.

   This will continue until the next hop for X changes, or until the



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   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 mapping between label L and address
   prefix X, then this unmapping must be distributed to all LSRs to
   which the mapping was distributed.

   It is desirable, though not required, that the unmapping of L from X
   be distributed by Rd to a LSR Ru before Rd distributes to Ru any new
   mapping of L to any other address prefix Y, where X != Y. If Ru
   learns of the new mapping of L to Y before it learns of the unmapping
   of L from X, and if packets matching both X and Y are forwarded by Ru
   to Rd, then for a period of time, Ru will label both packets matching
   X and packets matching Y with label L.

   The distribution and withdrawal of label mappings is done via a label
   distribution protocol, or LDP. LDP is a two-party protocol. If LSR R1
   has received label mappings from LSR R2 via an instance of an LDP,
   and that instance of that protocol is closed by either end (whether
   as a result of failure or as a matter of normal operation), then all
   mappings learned over that instance of the protocol must be
   considered to have been withdrawn.

   As long as the relevant LDP connection remains open, label mappings
   that are withdrawn must always be withdrawn explicitly.  If a second
   label is bound to an address prefix, the result is not to implicitly
   withdraw the first label, but to map 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 mapping 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



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   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. TTL-capable LSP Segments

   If Ru and Rd are MPLS peers, and both are capable of decrementing a
   TTL field in the MPLS header, then the MPLS scheme in use between Ru
   and Rd must be one of the following:

   <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
   UseImmediate>

   <PushConditional, RequestWhenNeeded, RequestNoRetry, *, *>

   The former, roughly speaking, is "local control with downstream label
   assignment".  The latter is an egress control scheme.


4.2.2. Using ATM Switches as LSRs

   The procedures for using ATM switches as LSRs depends on whether the
   ATM switches can realize LSP trees as multipoint-to-point VCs or VPs.

   Most ATM switches existing today do NOT have a multipoint-to-point
   VC-switching capability.  Their cross-connect tables could easily be
   programmed to move cells from multiple incoming VCs to a single
   outgoing VC, but the result would be that cells from different
   packets get interleaved.

   Some ATM switches do support a multipoint-to-point VC-switching
   capability.  These switches will queue up all the incoming cells from
   an incoming VC until a packet boundary is reached.  Then they will
   transmit the entire sequence of cells on the outgoing VC, without
   allowing cells from any other packet to be interleaved.

   Many ATM switches do support a multipoint-to-point VP-switching
   capability, which can be used if the Multipoint SVP label encoding is
   used.







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4.2.2.1. Without Multipoint-to-point Capability

   Suppose that R1, R2, R3, and R4 are ATM switches which do not support
   multipoint-to-point capability, but are being used as LSRs.  Suppose
   further that the L3 hop-by-hop path for address prefix X is <R1, R2,
   R3, R4>, and that packets destined for X can enter the network at any
   of these LSRs.  Since there is no multipoint-to-point capability, the
   LSPs must be realized as point-to-point VCs, which means that there
   needs to be three such VCs for address prefix X: <R1, R2, R3, R4>,
   <R2, R3, R4>, and <R3, R4>.

   Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is
   implemented using conventional ATM switching hardware (i.e., no cell
   interleave suppression), the MPLS scheme in use between R1 and R2
   must be one of the following:

   <PulledUnconditional, RequestOnRequest, RequestRetry,
   ReleaseOnChange, UseImmediate>

   <PulledConditional, RequestOnRequest, RequestNoRetry,
   ReleaseOnChange, *>

   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.

   The first of these procedures is the "optimistic downstream-on-
   demand" variant of local control.  The second is the "conservative
   downstream-on-demand" variant of local control.

   An egress control scheme which works in the absence of multipoint-
   to-point capability is for further study.


4.2.2.2. With Multipoint-To-Point Capability

   If R1 and R2 are MPLS peers, and either of them is an LSR which is
   implemented using ATM switching hardware with cell interleave
   suppression, and neither is an LSR which is implemented using ATM
   switching hardware that does not have cell interleave suppression,
   then the MPLS scheme in use between R1 and R2 must be one of the
   following;

   <PushConditional, RequestWhenNeeded, RequestNoRetry, *, *>

   <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
   UseImmediate>




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   <PulledConditional, RequestOnRequest, RequestNoRetry,
   ReleaseOnChange, *>

   The first of these is an egress control scheme.  The second is is the
   "downstream" variant of local control.  The third is the
   "conservative downstream-on-demand" variant of local control.


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
       mappings 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 mappings.

     - <*, RequestNever, *, *, ReleaseOnChange>

       In these MPLS schemes, Rd releases mappings 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
       mappings get properly distributed.

   In this section, we specify rules to prevent a pair of LDP peers from
   adopting procedures which lead to infeasible MPLS Schemes.  These
   rules require the exchange of information between LDP peers during
   the initialization of the LDP connection between them.

      1. Each must state whether it is an ATM switch, and if so, whether
         it has cell interleave suppression.

      2. If Rd is an ATM switch without cell interleave suppression, it
         must state whether it intends to use the PulledUnconditional
         procedure or the Pulledconditional procedure.  If the former,
         Ru MUST use the RequestRetry procedure; if the latter, Ru MUST
         use the RequestNoRetry procedure.

      3. If Ru is an ATM switch without cell interleave suppression, it
         must state whether it intends to use the RequestRetry or the
         RequestNoRetry procedure.  If Rd is an ATM switch without cell
         interleave suppression, Rd is not bound by this, and in fact Ru
         MUST adopt Rd's preferences.  However, if Rd is NOT an ATM



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         switch without cell interleave suppression, then if Ru chooses
         RequestRetry, Rd must use PulledUnconditional, and if Ru
         chooses RequestNoRetry, Rd MUST use PulledConditional.

      4. If Rd is an ATM switch with cell interleave suppression, it
         must specify whether it prefers to use PushConditional,
         PushUnconditional, or PulledConditional.  If Ru is not an ATM
         switch without cell interleave suppression, it must then use
         RequestWhenNeeded and RequestNoRetry, or else RequestNever and
         NoReleaseOnChange, respectively.

      5. If Ru is an ATM switch with cell interleave suppression, it
         must specify whether it prefers to use RequestWhenNeeded and
         RequestNoRetry, or else RequestNever and NoReleaseOnChange.  If
         Rd is NOT an ATM switch with cell interleave suppression, it
         must then use either PushConditional or PushUnconditional,
         respectively.


4.2.4. How to do Loop Prevention

   TBD


4.2.5. How to do Loop Detection

   TBD.


4.2.6. Security Considerations

   Security considerations are not discussed in this version of this
   draft.


5. 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



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      E-mail: arunv@dnrc.bell-labs.com

      Ross Callon
      IronBridge Networks
      55 Hayden Avenue,
      Lexington, MA  02173
      +1-781-402-8017
      E-mail: rcallon@ironbridgenetworks.com


6. References

   [1] "A Framework for Multiprotocol Label Switching", R.Callon,
   P.Doolan, N.Feldman, A.Fredette, G.Swallow, and A.Viswanathan, work
   in progress, Internet Draft <draft-ietf-mpls-framework-02.txt>,
   November 1997.

   [2] "ARIS: Aggregate Route-Based IP Switching", A. Viswanathan, N.
   Feldman, R. Boivie, R. Woundy, work in progress, Internet Draft
   <draft-viswanathan-aris-overview-00.txt>, March 1997.

   [3] "ARIS Specification", N. Feldman, A. Viswanathan, work in
   progress, Internet Draft <draft-feldman-aris-spec-00.txt>, March
   1997.

   [4] "Tag Switching Architecture - Overview", Rekhter, Davie, Katz,
   Rosen, Swallow, Farinacci, work in progress, Internet Draft <draft-
   rekhter-tagswitch-arch-00.txt>, January, 1997.

   [5] "Tag distribution Protocol", Doolan, Davie, Katz, Rekhter, Rosen,
   work in progress, Internet Draft <draft-doolan-tdp-spec-01.txt>, May,
   1997.

   [6] "Use of Tag Switching with ATM", Davie, Doolan, Lawrence,
   McGloghrie, Rekhter, Rosen, Swallow, work in progress, Internet Draft
   <draft-davie-tag-switching-atm-01.txt>, January, 1997.

   [7] "Label Switching: Label Stack Encodings", Rosen, Rekhter, Tappan,
   Farinacci, Fedorkow, Li, Conta, work in progress, Internet Draft
   <draft-ietf-mpls-label-encaps-01.txt>, February, 1998.

   [8] "Partitioning Tag Space among Multicast Routers on a Common
   Subnet", Farinacci, work in progress, internet draft <draft-
   farinacci-multicast-tag-part-00.txt>, December, 1996.

   [9] "Multicast Tag Binding and Distribution using PIM", Farinacci,
   Rekhter, work in progress, internet draft <draft-farinacci-
   multicast-tagsw-00.txt>, December, 1996.



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   [10] "Toshiba's Router Architecture Extensions for ATM: Overview",
   Katsube, Nagami, Esaki, RFC 2098, February, 1997.

   [11] "Loop-Free Routing Using Diffusing Computations", J.J. Garcia-
   Luna-Aceves, IEEE/ACM Transactions on Networking, Vol. 1, No. 1,
   February 1993.













































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