rfc3031.txt

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


   MPLS stands for "Multiprotocol" Label Switching, multiprotocol
   because its techniques are applicable to ANY network layer protocol.
   In this document, however, we focus on the use of IP as the network
   layer protocol.

   A router which supports MPLS is known as a "Label Switching Router",
   or LSR.

2.2. Terminology

   This section gives a general conceptual overview of the terms used in
   this document.  Some of these terms are more precisely defined in
   later sections of the document.

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

      forwarding equivalence class   a group of IP packets which are
                                     forwarded in the same manner (e.g.,
                                     over the same path, with the same
                                     forwarding treatment)

      frame merge               label merging, when it is applied to
                                operation over frame based media, so
                                that the potential problem of cell
                                interleave is not an issue.

      label                     a short fixed length physically
                                contiguous identifier which is used to
                                identify a FEC, usually of local
                                significance.


      label merging             the replacement of multiple incoming
                                labels for a particular FEC with a
                                single outgoing label

      label swap                the basic forwarding operation
                                consisting of looking up an incoming
                                label to determine the outgoing label,
                                encapsulation, port, and other data
                                handling information.

      label swapping            a forwarding paradigm allowing
                                streamlined forwarding of data by using
                                labels to identify classes of data
                                packets which are treated
                                indistinguishably when forwarding.



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RFC 3031                   MPLS Architecture                January 2001


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

      label switched path       The path through one or more LSRs at one
                                level of the hierarchy followed by a
                                packets in a particular FEC.

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

      layer 2                   the protocol layer under layer 3 (which
                                therefore offers the services used by
                                layer 3).  Forwarding, when done by the
                                swapping of short fixed length labels,
                                occurs at layer 2 regardless of whether
                                the label being examined is an ATM
                                VPI/VCI, a frame relay DLCI, or an MPLS
                                label.

      layer 3                   the protocol layer at which IP and its
                                associated routing protocols operate
                                link layer synonymous with layer 2

      loop detection            a method of dealing with loops in which
                                loops are allowed to be set up, and data
                                may be transmitted over the loop, but
                                the loop is later detected

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

      label stack               an ordered set of labels

      merge point               a node at which label merging is done

      MPLS domain               a contiguous set of nodes which operate
                                MPLS routing and forwarding and which
                                are also in one Routing or
                                Administrative Domain

      MPLS edge node            an MPLS node that connects an MPLS
                                domain with a node which is outside of
                                the domain, either because it does not
                                run MPLS, and/or because it is in a
                                different domain.  Note that if an LSR
                                has a neighboring host which is not
                                running MPLS, that that LSR is an MPLS
                                edge node.



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RFC 3031                   MPLS Architecture                January 2001


      MPLS egress node          an MPLS edge node in its role in
                                handling traffic as it leaves an MPLS
                                domain

      MPLS ingress node         an MPLS edge node in its role in
                                handling traffic as it enters an MPLS
                                domain

      MPLS label                a label which is carried in a packet
                                header, and which represents the
                                packet's FEC

      MPLS node                 a node which is running MPLS.  An MPLS
                                node will be aware of MPLS control
                                protocols, will operate one or more L3
                                routing protocols, and will be capable
                                of forwarding packets based on labels.
                                An MPLS node may optionally be also
                                capable of forwarding native L3 packets.

      MultiProtocol Label Switching  an IETF working group and the
                                     effort associated with the working
                                     group

      network layer             synonymous with layer 3

      stack                     synonymous with label stack

      switched path             synonymous with label switched path

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

      VC merge                  label merging where the MPLS label is
                                carried in the ATM VCI field (or
                                combined VPI/VCI field), so as to allow
                                multiple VCs to merge into one single VC

      VP merge                  label merging where the MPLS label is
                                carried din the ATM VPI field, so as to
                                allow multiple VPs to be merged into one
                                single VP.  In this case two cells would
                                have the same VCI value only if they
                                originated from the same node.  This
                                allows cells from different sources to
                                be distinguished via the VCI.



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RFC 3031                   MPLS Architecture                January 2001


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

2.3. Acronyms and Abbreviations

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

2.4. Acknowledgments

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

3. MPLS Basics

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

3.1. Labels

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






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RFC 3031                   MPLS Architecture                January 2001


   Most commonly, a packet is assigned to a FEC based (completely or
   partially) on its network layer destination address.  However, the
   label is never an encoding of that address.

   If Ru and Rd are LSRs, they may agree that when Ru transmits a packet
   to Rd, Ru will label with packet with label value L if and only if
   the packet is a member of a particular FEC F.  That is, they can
   agree to a "binding" between label L and FEC F for packets moving
   from Ru to Rd.  As a result of such an agreement, L becomes Ru's
   "outgoing label" representing FEC F, and L becomes Rd's "incoming
   label" representing FEC F.

   Note that L does not necessarily represent FEC F for any packets
   other than those which are being sent from Ru to Rd.  L is an
   arbitrary value whose binding to F is local to Ru and Rd.

   When we speak above of packets "being sent" from Ru to Rd, we do not
   imply either that the packet originated at Ru or that its destination
   is Rd.  Rather, we mean to include packets which are "transit
   packets" at one or both of the LSRs.

   Sometimes it may be difficult or even impossible for Rd to tell, of
   an arriving packet carrying label L, that the label L was placed in
   the packet by Ru, rather than by some other LSR.  (This will
   typically be the case when Ru and Rd are not direct neighbors.)  In
   such cases, Rd must make sure that the binding from label to FEC is
   one-to-one.  That is, Rd MUST NOT agree with Ru1 to bind L to FEC F1,
   while also agreeing with some other LSR Ru2 to bind L to a different
   FEC F2, UNLESS Rd can always tell, when it receives a packet with
   incoming label L, whether the label was put on the packet by Ru1 or
   whether it was put on by Ru2.

   It is the responsibility of each LSR to ensure that it can uniquely
   interpret its incoming labels.

3.2. Upstream and Downstream LSRs

   Suppose Ru and Rd have agreed to bind label L to FEC F, for packets
   sent from Ru to Rd.  Then with respect to this binding, Ru is the
   "upstream LSR", and Rd is the "downstream LSR".

   To say that one node is upstream and one is downstream with respect
   to a given binding means only that a particular label represents a
   particular FEC in packets travelling from the upstream node to the
   downstream node.  This is NOT meant to imply that packets in that FEC
   would actually be routed from the upstream node to the downstream
   node.