IETF Draft               A Framework for MPLS           September 1999

Network Working Group                                       R. Callon
Internet Draft                                    Ironbridge Networks
Expires: March 2000                                         P. Doolan
                                                    Ennovate Networks
                                                           N. Feldman
                                                          A. Fredette
                                                      Nortel Networks
                                                           G. Swallow
                                                        Cisco Systems
                                                       A. Viswanathan
                                                  Lucent Technologies

                                                       September 1999

            A Framework for Multiprotocol Label Switching

Status of this Memo

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

   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 working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document discusses technical issues and requirements for the
   Multiprotocol Label Switching working group. It is the intent of
   this document to produce a coherent description of all significant
   approaches which were and are being considered by the working

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   group. Selection of specific approaches, making choices regarding
   engineering tradeoffs, and detailed protocol specification, are
   outside of the scope of this framework document.


   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, Eric Gray, Jim Luciani, Andy Malis, Rayadurgam
   Ravikanth, Yakov Rekhter, Eric Rosen, Vijay Srinivasan, and Pasi
   Vananen for their inputs and ideas.

1. Introduction and Requirements

1.1 Overview of MPLS

   The primary goal of the MPLS working group is to standardize a
   base technology that integrates the label switching forwarding
   paradigm with network layer routing. This base technology (label
   switching) is expected to improve the price/performance of network
   layer routing, improve the scalability of the network layer, and
   provide greater flexibility in the delivery of (new) routing
   services (by allowing new routing services to be added without a
   change to the forwarding paradigm).

   The initial MPLS effort will be focused on IPv4. However, the core
   technology will be extendible to multiple network layer protocols
   (e.g., Ipv6, IPX, Appletalk, DECnet, CLNP). MPLS is not confined
   to any specific link layer technology, it can work with any media
   over which Network Layer packets can be passed between network
   layer entities.

   MPLS provides connection-oriented (label based) switching based on
   IP routing and control protocols. MPLS may be likened to a 'shim-
   layer' which is used to provide connection services to IP and
   which itself makes use of link-layer services from L2 (e.g. PPP,
   ATM, Ethernet).

   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 provides a simple "core" set of mechanisms
   which can be applied in several ways to provide a rich
   functionality. The core effort includes:

   a) Semantics assigned to a stream label:

     - Labels are associated with specific streams of data.

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   b) Forwarding Methods:

     - Forwarding is simplified by the use of short fixed length
       labels to identify streams.

     - Forwarding may require simple functions such as looking
       up a label in a table, swapping labels, and possibly
       decrementing and checking a TTL.

     - In some cases, MPLS may make direct use of underlying
       layer 2switching, such as is provided by ATM [ATM] or
       Frame Relay [FR] equipment.

   c) Label Distribution Methods:

     - Allow nodes to determine which labels to use for
       specific streams.

     - This may use some sort of control exchange, and/or be
       piggybacked on a routing protocol.

   The MPLS working group will define the procedures and protocols
   used to assign significance to the forwarding labels and to
   distribute that information between cooperating MPLS forwarders.

1.2 Requirements

  - MPLS forwarding MUST simplify packet forwarding in order to
    do the following:

     o lower cost of high speed forwarding
     o improve forwarding performance

  - MPLS core technologies MUST be general with respect to data
    link technologies (ie, work over a very wide range of
    underlying data links). Specific optimizations for
    particular media MAY be considered.

  - MPLS core technologies MUST be compatible with a wide range
    of routing protocols, and MUST be capable of operating
    independently of the underlying routing protocols. It has
    been observed that considerable optimizations can be
    achieved in some cases by small enhancements of existing
    protocols. Such enhancements MAY be considered in the case
    of IETF standard routing protocols, and if appropriate,
    coordinated with the relevant working group(s).

  - Routing protocols which are used in conjunction with MPLS
    might be based on distributed computation. As such, during

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    routing transients, these protocols may compute forwarding
    paths which potentially contain loops. MPLS MUST provide
    protocol mechanisms to either prevent the formation of loops
    and /or contain the amount of (networking) resources that
    can be consumed due to the presence of loops.

  - MPLS forwarding MUST allow "aggregate forwarding" of user
    data; ie, allow streams to be forwarded as a unit and ensure
    that an identified stream takes a single path, where a
    stream may consist of the aggregate of multiple flows of
    user data. MPLS SHOULD provide multiple levels of
    aggregation support (e.g., from individual end to end
    application flows at one extreme, to aggregates of all flows
    passing through a specified switch or router at the other

  - MPLS MUST support operations, administration, and
    maintenance facilities at least as extensive as those
    supported in current IP networks. Current network management
    and diagnostic tools SHOULD continue to work in order to
    provide some backward compatibility. Where such tools are
    broken by MPLS, hooks MUST be supplied to allow equivalent
    functionality to be created.

  - MPLS core technologies MUST work with both unicast and
    multicast streams.

  - The MPLS core specifications MUST clearly state how MPLS
    operates in a hierarchical network.

  - Scalability issues MUST be considered and analyzed during
    the definition of MPLS. Very scaleable solutions MUST be

  - MPLS core technologies MUST be capable of working with O(n)
    streams to switch all best-effort traffic, where n is the
    number of nodes in a MPLS domain. MPLS protocol standards
    MUST be capable of taking advantage of hardware that
    supports stream merging where appropriate. Note that O(n-
    squared) streams or VCs might also be appropriate for use in
    some cases.

  - The core set of MPLS standards, along with existing
    Internet standards, MUST be a self-contained solution. For
    example, the proposed solution MUST NOT require specific
    hardware features that do not commonly exist on network
    equipment at the time that the standard is complete.
    However, the solution MAY make use of additional optional
    hardware features (e.g., to optimize performance).

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  - The MPLS protocol standards MUST support multipath routing
    and forwarding.

  - MPLS MUST be compatible with the IETF Integrated Services
    Model, including RSVP [RFC1663][RFC2205].

  - It MUST be possible for MPLS switches to coexist with non
    MPLS switches in the same switched network. MPLS switches
    SHOULD NOT impose additional configuration on non-MPLS

  - MPLS MUST allow "ships in the night" operation with
    existing layer 2 switching protocols (e.g., ATM Forum
    Signaling) (ie, MPLS must be capable of being used in the
    same network which is also simultaneously operating standard
    layer 2 protocols).

  - The MPLS protocol MUST support both topology-driven and
    traffic/request-driven label assignments.

1.3 Terminology

   aggregate stream

     synonym of "stream"


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


     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 L3 packets which are forwarded in the same
     manner (e.g., over the same path, with the same forwarding
     treatment); a forwarding equivalence class is therefore the
     set of L3 packets which could safely be mapped to the same
     label; note that there may be reasons that packets from a
     single forwarding equivalence class may be mapped to
     multiple labels (e.g., when stream merge is not used)

   frame merge

     stream merge, when it is applied to operation over frame
     based media, so that the potential problem of cell

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     interleave is not an issue


     a short fixed length physically contiguous locally
     significant identifier which is used to identify a stream

   label information base

     the database of information containing label bindings

   label stack

     an ordered set of labels

   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 switching

     a forwarding paradigm allowing streamlined forwarding of
     data by using labels to identify streams of data to be

   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

   label switching router (LSR)

     an MPLS node which is capable of forwarding native L3

   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, or a frame relay DLCI

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   layer 3

     the protocol layer at which IP and its associated routing
     protocols operate

   link layer

     synonymous with layer 2

   loop detection

     a method in which loop setup may occur and data may be
     injected into the loop but a mechanism is provided to
     detect and break such loops

   loop prevention

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

   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

   merge point

     the node at which multiple streams and switched paths are
     combined into a single stream sent over a single path; in
     the case that the multiple paths are not combined prior to
     the egress node, then the egress node becomes the merge

   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

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     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 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

   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 (see LSR)

   MultiProtocol Label Switching

     an IETF working group and the effort associated with the
     working group

   network layer

     synonymous with layer 3

   shortcut VC

     a VC set up as a result of an NHRP query and response


     synonymous with label stack


     an aggregate of one or more flows, treated as one flow 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.

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     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

   VC merge

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

   virtual circuit

     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

   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


     a label used in ATM networks to identify ATM virtual

1.4 Acronyms and Abbreviations

   DLCI            Data Link Circuit Identifier

   FEC             Forwarding Equivalence Class

   ISP             Internet Service Provider

   LIB             Label Information Base

   LDP             Label Distribution Protocol

   L2              Layer 2

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   L3              Layer 3

   LSP             Label Switched Path

   LSR             Label Switching Router

   MPLS            MultiProtocol Label Switching

   MPT             Multipoint to Point Tree

   NHC             Next Hop (NHRP) Client

   NHS             Next Hop (NHRP) Server

   VC              Virtual Circuit

   VCI             Virtual Circuit Identifier

   VPI             Virtual Path Identifier

1.5 Motivation for MPLS

   This section describes the expected and potential benefits of the
   MPLS over existing schemes. Specifically, this section discusses
   the advantages of MPLS over previous methods for building core
   networks (ie, networks for internet service providers or for major
   corporate backbones). The potential advantages of MPLS in campus
   and local area networks are not discussed in this section.

   There are currently two commonly used methods for building core IP
   networks: (i) Networks of datagram routers in which the core of
   the network is based on the datagram routers; (ii) Networks of
   datagram routers operating over an ATM core. In order to describe
   the advantages of MPLS, it is necessary to know which alternate to
   MPLS we are using for the comparison. This section is therefore
   split into two sections: Section 1.5.1 describes the advantages of
   MPLS when compared to a pure datagram routed network. Section
   1.5.2 describes the advantages of MPLS when compared to an IP over
   ATM network.

   This section does not provide a complete list of requirements for
   MPLS. For example, Multipoint to Point Trees are important for
   MPLS to scale. However, datagram forwarding naturally acts in this
   way (since multiple sources are merged automatically), and the ATM
   forum is currently adding support for multipoint to point to the
   ATM standards. The ability to do MPTs is therefore important to
   MPLS, but does not represent an advantage over either datagram
   routing or IP over ATM, and therefore is not mentioned in this

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1.5.1 Benefits Relative to Use of a Router Core Simplified Forwarding

   Label switching allows packet forwarding to be based on an exact
   match for a short label, rather than a longest match algorithm
   applied to a longer address as is required for normal datagram
   forwarding. In addition, the label headers used with MPLS are
   simpler than the headers typically used with datagram protocols
   such as IP. This in turn implies that MPLS allows a much simpler
   forwarding paradigm relative to datagrams, and implies that it is
   easier to build a high speed router using MPLS.

   Whether this simpler forwarding operation will result in
   availability of LSRs which can operate at higher speeds than
   datagram routers is controversial, and probably depends upon
   implementation details. There are some parts of the network, such
   as at hierarchical boundaries, where datagram IP forwarding at
   high speed will be required. This implies that implementation of a
   high speed router is highly desirable. In addition, there are
   currently multiple companies building high speed routers which
   will allow IP packets to be forwarded at very high speed. At
   speeds at least up to OC48, it appears that once the one-time
   engineering is completed, the per-unit cost associated with IP
   forwarding will be a small fraction of the overall equipment cost.

   However, there are also many existing routers which can benefit
   from the simpler forwarding allowed by MPLS. In addition, there
   are some routers being built with implementations that will
   benefit from the simpler forwarding available with MPLS. Efficient Explicit Routing

   Explicit routing (aka Source Routing) is a very powerful technique
   which potentially can be useful for a variety of purposes.
   However, with pure datagram routing the overhead of carrying a
   complete explicit route with each packet is prohibitive. However,
   MPLS allows the explicit route to be carried only at the time that
   the label switched path is set up, and not with each packet. This
   implies that MPLS makes explicit routing practical. This in turn
   implies that MPLS can make possible a number of advanced routing
   features which depend upon explicit routing. Traffic Engineering

   Traffic engineering refers to the process of selecting the paths
   chosen by data traffic in order to balance the traffic load on the
   various links, routers, and switches in the network. Traffic
   engineering is most important in networks where multiple parallel
   or alternate paths are available. The rapid growth in the

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   Internet, and particularly the associated rapid growth in the
   demand for bandwidth, has tended to cause some core networks to
   become increasingly "branchy" in recent years, resulting in an
   increase in the importance of traffic engineering [TRAFENG].

   It is common today, in networks that are running IP over an ATM
   core using PVCs, to manually configure the path of each PVC in
   order to equalize the traffic levels on different links in the
   network. Thus traffic engineering is typically done today in IP
   over ATM networks using manual configuration.

   Traffic engineering is difficult to accomplish with datagram
   routing. Some degree of load balancing can be obtained by
   adjusting the metrics associated with network links. However,
   there is a limit to how much can be accomplished in this way, and
   in networks with a large number of alternative paths between any
   two points balancing of the traffic levels on all links is
   difficult to achieve solely by adjustment of the metrics used with
   hop by hop datagram routing.

   MPLS allows streams from any particular ingress node to any
   particular egress node to be individually identified. MPLS
   therefore provides a straightforward mechanism to measure the
   traffic associated with each ingress node to egress node pair. In
   addition, since MPLS allows efficient explicit routing of Label
   Switched Paths, it is straightforward to ensure that any
   particular stream of data takes the preferred path.

   The hard part of traffic engineering is selection of the method
   used to route each Label Switched Path. There are a variety of
   possible ways to do this, ranging from manual configuration of
   routes, to use of a routing protocol which announces traffic loads
   in the network combined with background recomputation of paths. QoS Routing

   QoS routing refers to a method of routing in which the route
   chosen for a particular stream is chosen in response to the QoS
   required for that stream. In many cases QoS routing needs to make
   use of explicit routing for several reasons:

   In some cases specific bandwidth is likely to be reserved for each
   of many specific streams of data. This implies that the total
   bandwidth of multiple streams may exceed the bandwidth available
   on any particular link, and thus not all streams, even between the
   same ingress and egress nodes, can take the same path. Instead,
   individual streams will need to be individually routed. This is
   somewhat analogous to traffic engineering, but might require
   separation of streams on a finer granularity. Thus explicit
   routing may be needed in order to allow each stream to be

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   individually routed, and to eliminate the need for each switch
   along the path of a stream to compute the route for each stream.

   Consider the case of routing a stream with a specific bandwidth
   requirement: In this case the route chosen will depend upon the
   amount of bandwidth which is requested. For any one given
   bandwidth, it is straightforward to select a path. However there
   are a lot of different levels of bandwidth which could in
   principle be requested. This makes it impractical to precompute
   all possible paths for all possible bandwidths. If the path for a
   particular stream must be computed on demand, then it is
   undesirable to require every LSR on the path to compute the path.
   Instead, it is preferable to have the first node compute the path
   and specify the route to be followed through use of an explicit

   For a variety of reasons the information available for QoS routing
   may in some cases be slightly out of date. This implies that the
   attempt to select a specific path for a QoS-sensitive stream may
   in some cases fail, due to a particular node or link not having
   the required resources available. In these cases it is not in
   general always feasible to tell all other nodes in the network of
   the limited resource in one particular network element. If
   explicit routing is available, then this permits the initial node
   of the stream (the ingress node in MPLS) to be informed that the
   indicated network element is not able to carry the stream,
   allowing an alternate path to be selected. However, in this case
   the node that selects the alternate path has to use explicit
   routing in order to force the stream to follow the alternate path.

   These and similar examples imply that explicit routing is
   necessary in order to do an adequate job of QoS routing. Given
   that MPLS allows efficient explicit routing, it follows that MPLS
   also facilitates QoS routing. Mappings from IP Packet to Forwarding Equivalence Class

   MPLS allows the mapping from IP packet to forwarding equivalence
   class to be performed only once, at the ingress to an MPLS domain.
   This facilitates complex mappings from IP packet to FEC that would
   otherwise be impractical.

   For example, consider the case of provisioned QoS: Some ISPs offer
   a service wherein specific customers subscribe to receive
   differentiated services (e.g., their packets may receive
   preferential forwarding treatment). Mapping of IP packets to the
   service level may require knowing the customer who is transmitting
   the packet, which may in turn require packet filtering based on
   source and destination address, incoming interface, and other
   characteristics. The sheer number of filters that are needed in a

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   moderate sized ISP preclude repetition of the filters at every
   router throughout the network. Also, some information such as
   incoming interface is not available except at the ingress node to
   the network. This implies that the preferred way to offer
   provisioned QoS is to map the packet at the ingress point to the
   preferred QoS level, and then label the packet in some way. MPLS
   offers an efficient method to label the QoS class associated with
   any particular packet.

   Other examples of complex mappings from IP packet to FEC are also
   likely to be determined as MPLS is deployed. Partitioning of Functionality

   Due to the support of the different label granularities, it will
   be possible to hierarchically partition the processing
   functionality to the different network elements, so that the more
   heavy processing takes place on the edges of the network, near the
   customers, and on the core network the processing is as simple as
   possible, e.g. pure label based forwarding.

   AS level aggregations will enable building of the fully switched
   backbone networks and traffic exchange points. Also, it will be
   possible for operators to fully switch the transit traffic
   traveling through the operator's network. Deaggregation will be
   needed for the streams that are destined in the networks connected
   to the MPLS domain, but it shall be noted that this deaggregation
   will only need to perform lookup operations associated with
   finding the label for egress router or interface, e.g. TOS
   information bound to label in source is still valid, and can be
   honored on basis of which label the packet was received in. It
   shall be noted that it is even impossible for the receiving domain
   to do the classification as the original packet classification
   policy is not known by the receiving domain.

   As one example of the improved functional partitioning, consider
   the case of the use of packet filters to map IP packets into a
   substantial number of queues, such that each queue receives
   differentiated services. For example, suppose that a network
   supports individual queuing for on the order of 100 different
   customers, with packets mapped to queues based on the source and
   destination IP address. In this case, with MPLS the packet
   filtering can be done solely on the edge of the network, with the
   packets mapped to labels such that each individual user receives
   separate labels. Thus with MPLS the filtering can be performed at
   the edge only of the network. This allows complex mappings of IP
   packets to forwarding equivalence class. Single Forwarding Paradigm with Service Level Differentiation

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   MPLS can allow a single forwarding paradigm to be used to support
   multiple types of service on the same network.

   Because of the forwarding paradigm, it will be possible to carry
   the different services through the same network elements,
   regardless of the control plane protocols used for the population
   of the LSR's LIB. It is for example possible, in case of ATM based
   switching system to support all the native ATM services, frame
   relay services, and labeled IP services. The simultaneous support
   of multiple service may need partitioning of the label space
   between the services, and shall be supported by the label
   distribution management protocol.

   Non-exhaustive list of examples of the services suitable for
   carrying over LSRs are IP traffic, Frame Relay traffic, ATM
   traffic (in case of cell switching), IP tunneling, VPNs, and other
   datagram protocols.

   Note that MPLS does not necessarily use the same header format
   over all types of media. However, over any particular type of
   media a single header format (at least for the lowest level of the
   Label Stack) should be possible.

1.5.2 Benefits Relative to Use of an ATM or Frame Relay Core

   Note: This section compares MPLS with other methods for
   interconnecting routers over a switched core network. We are not
   considering methods for interconnecting hosts located on virtual
   networks. For example the ATM Forum LANE and MPOA standards
   support virtual networks. MPLS does not directly support virtual
   networks, and should not be compared directly with MPOA or LANE.

   Previously available methods for interconnecting routers in an IP
   over ATM environment make use of either: (i) a full mesh 'n-
   squared' overlay of virtual circuits between n ATM-attached
   routers; (ii) A partial mesh of VCs between routers; or (iii) A
   partial mesh of VCs, plus the use of NHRP to facilitate on demand
   cut-through SVCs. Scaling of the Routing Protocol

   Relative to the interconnection of IP over an ATM core, MPLS
   improves the scaling of routing due to reduced number of peers and
   elimination of the 'n-squared' logical links between routers used
   to operate the routing protocols.

   Because all LSRs will run standard routing protocols, the number
   of the peers routers need to communicate with is reduced to the
   number of LSRs and routers a given LSR is directly connected to,
   instead of having to peer with large numbers of routers at the

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   ends of the switched L2 paths. This benefit is achieved because
   the edge LSRs do not need to peer with every other edge LSR in the
   domain as is the case on a hybrid switch / router network." Common Operation over Packet and Cell media

   MPLS makes use of common methods for routing and forwarding over
   packet and cell media, and potentially allows a common approach to
   traffic engineering, QoS routing, and other aspects of operation.
   For example, this means that the same method for label
   distribution can be used over Frame Relay and ATM media, as well
   as between LSRs using the MPLS Shim Header for forwarding over
   other media (such as PPP links and broadcast LANs).

   Note: There may be some differences with respect to the operation
   of different media. For example, if VP merge is used with ATM
   media (rather than VC merge) then the merge operation may be
   somewhat different than what it would be with packet media or with
   ATM using VC merge. Easier Management

   The use of a common method for label distribution and common
   routing protocols over multiple types of media is expected to
   simplify network management of MPLS networks. Elimination of the 'Routing over Large Clouds' Issue

   MPLS eliminates the need to use NHRP and on-demand cut-through
   SVCs for operation over ATM. This eliminates the latency problem
   associated with cut-through SVCs.

2. Discussion of Core MPLS Components

2.1 The Basic Routing Approach

   Routing is accomplished through the use of standard L3 routing
   protocols, such as OSPF and BGP [RFC1583][RFC1771]. The
   information maintained by the L3 routing protocols is then used to
   distribute labels to neighboring nodes that are used in the
   forwarding of packets as described below. In the case of ATM
   networks, the labels that are distributed are VPI/VCIs and a
   separate protocol (ie, PNNI) is not necessary for the
   establishment of VCs for IP forwarding.

   The topological scope of a routing protocol (ie routing domain)
   and the scope of label switching MPLS-capable nodes may be
   different. For example, MPLS-knowledgeable and MPLS-ignorant
   nodes, all of which are OSPF routers, may be co-resident in an

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   area. In the case that neighboring routers know MPLS, labels can
   be exchanged and used.

   Neighboring MPLS routers may use configured PVCs or PVPs to tunnel
   through non-participating ATM or FR switches.

2.2 Labels

   In addition to the single routing protocol approach discussed
   above, the other key concept in the basic MPLS approach is the use
   of short fixed length labels to simplify user data forwarding.

2.2.1 Label Semantics

   It is important that the MPLS solutions are clear about what
   semantics (ie, what knowledge of the state of the network) is
   implicit in the use of labels for forwarding user data packets or

   At the simplest level, a label may be thought of as nothing more
   than a shorthand for the packet header, in order to index the
   forwarding decision that a router would make for the packet. In
   this context, the label is nothing more than a shorthand for an
   aggregate stream of user data.

   This observation leads to one possible very simple interpretation
   that the "meaning" of the label is a strictly local issue between
   two neighboring nodes. With this interpretation: (i) MPLS could be
   employed between any two neighboring nodes for forwarding of data
   between those nodes, even if no other nodes in the network
   participate in MPLS; (ii) When MPLS is used between more than two
   nodes, then the operation between any two neighboring nodes could
   be interpreted as independent of the operation between any other
   pair of nodes. This approach has the advantage of semantic
   simplicity, and of being the closest to pure datagram forwarding.
   However this approach (like pure datagram forwarding) has the
   disadvantage that when a packet is forwarded it is not known
   whether the packet is being forwarded into a loop, into a black
   hole, or towards links which have inadequate resources to handle
   the traffic flow. These disadvantages are necessary with pure
   datagram forwarding, but are optional design choices to be made
   when label switching is being used.

   There are cases where it would be desirable to have additional
   knowledge implicit in the existence of the label. For example, one
   approach to avoiding loops (see section 4.3) involves signaling
   the label distribution along a path before packets are forwarded
   on that path. With this approach the fact that a node has a label
   to use for a particular IP packet would imply the knowledge that
   following the label (including label switching at subsequent

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   nodes) leads to a non-looping path which makes progress towards
   the destination (something which is usually, but not necessarily
   always true when using pure datagram routing). This would of
   course require some sort of label distribution/setup protocol
   which signals along the path being setup before the labels are
   available for packet forwarding. However, there are also other
   consequences to having additional semantics associated with the
   label: specifically, procedures are needed to ensure that the
   semantics are correct. For example, if the fact that you have a
   label for a particular destination implies that there is a loop-
   free path, then when the path changes some procedures are required
   to ensure that it is still loop free. Another example of semantics
   which could be implicit in a label is the identity of the higher
   level protocol type which is encoded using that label value.

   In either case, the specific value of a label to use for a stream
   is strictly a local issue; however the decision about whether to
   use the label may be based on some global (or at least wider
   scope) knowledge that, for example, the label-switched path is
   loop-free and/or has the appropriate resources.

   A similar example occurs in ATM networks: With standard ATM a
   signaling protocol is used which both reserves resources in
   switches along the path, and which ensures that the path is loop-
   free and terminates at the correct node. Thus implicit in the fact
   that an ATM node has a VPI/VCI for forwarding a particular piece
   of data is the knowledge that the path has been set up

   Another similar example occurs with multipoint to point trees over
   ATM (see section 4.2 below), where the multipoint to point tree
   uses a VP, and cell interleave at merge points in the tree is
   handled by giving each source on the tree a distinct VCI within
   the VP. In this case, the fact that each source has a known
   VPI/VCI to use needs to (implicitly or explicitly) imply the
   knowledge that the VCI assigned to that source is unique within
   the context of the VP.

   In general labels are used to optimize how the system works, not
   to control how the system works. For example, the routing protocol
   determines the path that a packet follows. The presence or absence
   of a label assignment should not affect the path of a L3 packet.
   Note however that the use of labels may make capabilities such as
   explicit routes, loadsharing, and multipath more efficient.

2.2.2 Label Granularity

   Labels are used to create a simple forwarding paradigm. The
   essential element in assigning a label is that the device which
   will be using the label to forward packets will be forwarding all

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   packets with the same label in the same way. If the packet is to
   be forwarded solely by looking at the label, then at a minimum,
   all packets with the same incoming label should be forwarded out
   the same port(s) with the same encapsulation(s), and with the same
   next hop label if any (although the special cases of multipath and
   load sharing are an exception to this rule).

   The term "forwarding equivalence class" is used to refer to a set
   of L3 packets which are all forwarded in the same manner by a
   particular LSR (for example, the IP packets in a forwarding
   equivalence class may be destined for the same egress from an MPLS
   network, and may be associated with the same QoS class). A
   forwarding equivalence class is therefore the set of L3 packets
   which could safely be mapped to the same label. Note that there
   may be reasons that packets from a single forwarding equivalence
   class may be mapped to multiple labels (e.g., when stream merge is
   not used).

   Note that the label could also mean "ignore this label and forward
   based on what is contained within," where within one might find a
   label (if a stack of labels is used) or a layer 3 packet.

   For IP unicast traffic, the granularity of a label allows various
   levels of aggregation in a Label Information Base (LIB). At one
   end of the spectrum, a label could represent a host route (ie the
   full 32 bits of IP address). If a router forwards an entire CIDR
   prefix in the same way, it may choose to use one label to
   represent that prefix. Similarly if the router is forwarding
   several (otherwise unrelated) CIDR prefixes in the same way it may
   choose to use the same label for this set of prefixes. For
   instance all CIDR prefixes which share the same BGP Next Hop could
   be assigned the same label. Taking this to the limit, an egress
   router may choose to advertise all of its prefixes with the same

   By introducing the concept of an egress identifier, the
   distribution of labels associated with groups of CIDR prefixes can
   be simplified. For instance, an egress identifier might specify
   the BGP Next Hop, with all prefixes routed to that next hop
   receiving the label associated with that egress identifier.
   Another natural place to aggregate would be the MPLS egress
   router. This would work particularly well in conjunction with a
   link-state routing protocol, where the association between egress
   router and CIDR prefix is already distributed throughout an area.

   For IP multicast, the natural binding of a label would be to a
   multicast tree, or rather to the branch of a tree which extends
   from a particular port. Thus for a shared tree, the label
   corresponds to the multicast group, (*,G). For (S,G) state, the
   label would correspond to the source address and the multicast

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   A label can also have a granularity finer than a host route. That
   is, it could be associated with some combination of source and
   destination address or other information within the packet. This
   might for example be done on an administrative basis to aid in
   effecting policy. A label could also correspond to all packets
   which match a particular Integrated Services filter specification.

   Labels can also represent explicit routes. This use is
   semantically equivalent to using an IP tunnel with a complete
   explicit route. This is discussed in more detail in section 4.10. Examples of Unicast traffic granularities:

  - PQ (Port Quadruples) same IP source address prefix,
    destination address prefix, TTL, IP protocol and TCP/UDP
    source/destination ports

  - PQT (Port Quadruples with TOS) same IP source address
    prefix, destination address prefix, TTL, IP protocol and
    TCP/UDP source/destination ports and same IP header TOS
    field (including Precedence and TOS bits).

  - HP (Host Pairs) Same specific IP source and destination
    address (32 bit)

  - NP (Network Pairs) Same IP source and destination address
    prefixes (variable length)

  - DN (Destination Network) Same IP destination network
    address prefix (variable length)

  - ER (Egress Router) Same egress router ID (e.g. OSPF)

  - NAS (Next-hop AS) Same next-hop AS number (BGP)

  - DAS (Destination AS) Same destination AS number (BGP) Multicast traffic granularities:

  - SST (Source Specific Tree) Same source address and
    multicast group

  - SMT (Shared Multicast Tree) Same multicast group address

2.2.3 Label Assignment

   Essential to label switching is the notion of binding between a
   label and Network Layer routing (routes). A control component is

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   responsible for creating label bindings, and then distributing the
   label binding information among label switches. Label assignment
   involves allocating a label, and then binding a label to a route.

   Label assignment can be driven by control traffic or by data
   traffic. This is discussed in more detail in section 3.4.

   Control traffic driven label assignment has several advantages, as
   compared to data traffic driven label assignment. For one thing,
   it minimizes the amount of additional control traffic needed to
   distribute label binding information, as label binding information
   is distributed only in response to control traffic, independent of
   data traffic. It also makes the overall scheme independent of and
   insensitive to the data traffic profile/pattern. Control traffic
   driven creation of label binding improves forwarding latency, as
   labels are assigned before data traffic arrives, rather than being
   assigned as data traffic arrives. It also simplifies the overall
   system behavior, as the control plane is controlled solely by
   control traffic, rather than by a mix of control and data traffic.

   There are however situations where data traffic driven label
   assignment is necessary. A particular case may occur with ATM
   without VP or VC merge. In this case in order to set up a full
   mesh of VCs would require n-squared VCs. However, in very large
   networks this may be infeasible. Instead VCs may be setup where
   required for forwarding data traffic. In this case it is generally
   not possible to know a priori how many such streams may occur.

   Label withdrawal is required with both control-driven and data-
   driven label assignment. Label withdrawal is primarily a matter of
   garbage collection, that is collecting up unused labels so that
   they may be reassigned. Generally speaking, a label should be
   withdrawn when the conditions that allowed it to be assigned are
   no longer true. For example, if a label is imbued with extra
   semantics such as loop-free-ness, then the label must be withdrawn
   when those extra semantics cease to hold.

   In certain cases, notably multicast, it may be necessary to share
   a label space between multiple entities. If these sharing
   arrangements are altered by the coming and going of neighbors,
   then labels which are no longer controlled by an entity must be
   withdrawn and a new label assigned.

2.2.4 Label Stack and Forwarding Operations

   The basic forwarding operation consists of looking up the incoming
   label to determine the outgoing label, encapsulation, port, and
   any additional information which may pertain to the stream such as
   a particular queue or other QoS related treatment. We refer to
   this operation as a label swap.

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   When a packet first enters an MPLS domain, the packet is forwarded
   by normal layer 3 forwarding operations with the exception that
   the outgoing encapsulation will now include a label. We refer to
   this operation as a label push. When a packet leaves an MPLS
   domain, the label is removed. We refer to this as a label pop.

   In some situations, carrying a stack of labels is useful. For
   instance both IGP and BGP label could be used to allow routers in
   the interior of an AS to be free of BGP information. In this
   scenario, the "IGP" label is used to steer the packet through the
   AS and the "BGP" label is used to switch between ASes.

   With a label stack, the set of label operations remains the same,
   except that at some points one might push or pop multiple labels,
   or pop & swap, or swap & push.

2.3 Encapsulation

   Label-based forwarding makes use of various pieces of information,
   including a label or stack of labels, and possibly additional
   information such as a TTL field [ENCAP]. In some cases this
   information may be encoded using an MPLS header, in other cases
   this information may be encoded in L2 headers. Note that there may
   be multiple types of MPLS headers. For example, the header used
   over one media type may be different than is used over a different
   media type. Similarly, in some cases the information that MPLS
   makes use of may be encoded in an ATM header. We will use the term
   "MPLS encapsulation" to refer to whatever form is used to
   encapsulate the label information and other information used for
   label based forwarding. The term "MPLS header" will be used where
   this information is carried in some sort of MPLS-specific header
   (ie, when the MPLS information cannot all be carried in a L2
   header). Whether there is one or multiple forms of possible MPLS
   headers is also outside of the scope of this document.

   The exact contents of the MPLS encapsulation is outside of the
   scope of this document. Some fields, such as the label, are
   obviously needed. Some others might or might not be standardized,
   based on further study. An encapsulation scheme may make use of
   the following fields:
     -  label
     -  TTL
     -  class of service
     -  stack indicator
     -  next header type indicator
     -  checksum

   It is desirable to have a very short encapsulation header. For
   example, a four byte encapsulation header adds to the convenience

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   of building a hardware implementation that forwards based on the
   encapsulation header. But at the same time it is tricky assigning
   such a limited number of bits to carry the above listed
   information in an MPLS header. Hence careful consideration must be
   given to the information chosen for an MPLS header.

   A TTL value in the MPLS header may be useful in the same manner as
   it is in IP. Specifically, TTL may be used to terminate packets
   caught in a routing loop, and for other related uses such as
   traceroute. The TTL mechanism is a simple and proven method of
   handling such events. Another use of TTL is to expire packets in a
   network by limiting their "time to live" and eliminating stale
   packets that may cause problems for some of the higher layer
   protocols. When used over link layers which do not provide a TTL
   field, alternate mechanisms will be needed to replace the uses of
   the TTL field.

   A provision for a class of service (COS) field in the MPLS header
   allows multiple service classes within the same label. However,
   when more sophisticated QoS is associated with a label, the COS
   may not have any significance. Alternatively, the COS (like QoS)
   can be left out of the header, and instead propagated with the
   label assignment, but this entails that a separate label be
   assigned to each required class of service. Nevertheless, the COS
   mechanism provides a simple method of segregating flows within a

   As previously mentioned, the encapsulation header can be used to
   derive benefits of tunneling (or stacking).

   The MPLS header must provide a way to indicate that multiple MPLS
   headers are stacked (ie, the "stack indicator"). For this purpose
   a single bit in the MPLS header will suffice. In addition, there
   are also some benefits to indicating the type of the protocol
   header following the MPLS header (ie, the "next header type
   indicator"). One option would be to combine the stack indicator
   and next header type indicator into a single value (ie, the next
   header type indicator could be allowed to take the value "MPLS
   header"). Another option is to have the next header type indicator
   be implicit in the label value (such that this information would
   be propagated along with the label).

   There is no compelling reason to support a checksum field in the
   MPLS header. A CRC mechanism at the L2 layer should be sufficient
   to ensure the integrity of the MPLS header.

3. Observations, Issues and Assumptions

3.1 Layer 2 versus Layer 3 Forwarding

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   MPLS uses L2 switching as a way to provide simple and fast packet
   forwarding capability. One primary reason for the simplicity of L2
   layer switching comes from its short, fixed length labels. A node
   forwarding at L3 must parse a (relatively) large header, and
   perform a longest-prefix match to determine a forwarding path.
   However, when a node performs MPLS label switching, and labels are
   assigned properly, it can do a direct index lookup into its
   forwarding (or in this case, label-switching) table with the short
   header. It is arguably simpler to build label switching hardware
   than it is to build L3 forwarding hardware because the label
   switching function is less complex.

   The relative performance of MPLS switching and L3 forwarding may
   differ considerably between nodes. Some nodes may illustrate an
   order of magnitude difference. Other nodes (for example, nodes
   with more extensive L3 forwarding hardware) may have identical
   performance. However, some nodes may not be capable of doing a L3
   forwarding at all (e.g. some ATM implementations), or have such
   limited capacity as to be unusable at L3. In this situation,
   traffic may be blackholed if no switched path exists. Note that
   delaying route advertisements until a switched path exists for
   associated packets may reduce or eliminate the need to black hole
   these packets.
   On nodes in which L3 forwarding is slower than L2 switching,
   pushing traffic to L3 when no L2 path is available may cause
   congestion. In some cases this could cause data loss (since L3 may
   be unable to keep up with the increased traffic). However, if data
   is discarded, then in general this will cause TCP to backoff,
   which would allow control traffic, traceroute and other network
   management tools to continue to work.

   The MPLS protocol MUST not make assumptions about the forwarding
   capabilities of an MPLS node. Thus, MPLS must propose solutions
   that can leverage the benefits of a node that is capable of L3
   forwarding, but must not mandate the node be capable of such.

   Why We Will Still Need L3 Forwarding:

   MPLS will not, and is not intended to, replace L3 forwarding.
   There is absolutely a need for some systems to continue to forward
   IP packets using normal Layer 3 IP forwarding. L3 forwarding will
   be needed for a variety of reasons, including:
     - For scaling; to forward on a finer granularity than the
       labels can provide
     - For security; to allow packet filtering at firewalls.
     - For forwarding at the initial router (when hosts don't
       do MPLS)

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   Consider a campus network which is serving a small company.
   Suppose that this company makes use of the Internet, for example
   as a method of communicating with customers. A customer on the
   other side of the world has an IP packet to be forwarded to a
   particular system within the company. It is not reasonable to
   expect that the customer will have a label to use to forward the
   packet to that specific system. Rather, the label used for the
   "first hop" forwarding might be sufficient to get the packet
   considerably closer to the destination. However, the granularity
   of the labels cannot be to every host worldwide. Similarly,
   routing used within one routing domain cannot know about every
   host worldwide. This implies that in may cases the labels assigned
   to a particular packet will be sufficient to get the packet close
   to the destination, but that at some points along the path of the
   packet the IP header will need to be examined to determine a finer
   granularity for forwarding that packet. This is particularly
   likely to occur at domain boundaries.

   A similar point occurs at the last router prior to the destination
   host. In general, the number of hosts attached to a network is
   likely to be great enough that it is not feasible to assign a
   separate label to every host. Rather, as least for routing within
   the destination routing domain (or the destination area if there
   is a hierarchical routing protocol in use) a label may be assigned
   which is sufficient to get the packet to the last hop router.
   However, the last hop router will need to examine the IP header
   (and particularly the destination IP address) in order to forward
   the packet to the correct destination host.

   Packet filtering at firewalls is an important part of the
   operation of the Internet. While the current state of Internet
   security may be considerably less advanced than may be desired,
   nonetheless some security (as is provided by firewalls) is much
   better than no security. We expect that packet filtering will
   continue to be important for the foreseeable future. Packet
   filtering requires examination of the contents of the packet,
   including the IP header. This implies that at firewalls the packet
   cannot be forwarded simply by considering the label associated
   with the packet. Note that this is also likely to occur at domain

   Finally, it is very likely that many hosts will not implement
   MPLS. Rather, the host will simply forward an IP packet to its
   first hop router. This first hop router will need to examine the
   IP header prior to forwarding the packet (with or without a

3.2 Scaling Issues

   MPLS scalability is provided by two of the principles of routing.

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   The first is that forwarding follows an inverted tree rooted at a
   destination. The second is that the number of destinations is
   reduced by routing aggregation.

   The very nature of IP forwarding is a merged multipoint-to-point
   tree. Thus, since MPLS mirrors the IP network layer, an MPLS node
   that is capable of merging is capable of creating O(n) switched
   paths which provide network reachability to all "n" destinations.
   The meaning of "n" depends on the granularity of the switched
   paths. One obvious choice of "n" is the number of CIDR prefixes
   existing in the forwarding table (this scales the same as today's
   routing). However, the value of "n" may be reduced considerably by
   choosing switched paths of further aggregation. For example, by
   creating switched paths to each possible egress node, "n" may
   represent the number of egress nodes in a network. This choice
   creates "n" switched paths, such that each path is shared by all
   CIDR prefixes that are routed through the same egress node. This
   selection greatly improves scalability, since it minimizes "n",
   but at the same time maintains the same switching performance of
   CIDR aggregation. (See section 2.2.2 for a description of all of
   the levels of granularity provided by MPLS).

   The MPLS technology must scale at least as well as existing
   technology. For example, if the MPLS technology were to support
   ONLY host-to-host switched path connectivity, then the number of
   switched-paths would be much higher than the number of routing
   table entries.

   There are several ways in which merging can be done in order to
   allow O(n) switches paths to connect n nodes. The merging approach
   used has an impact on the amount of state information, buffering,
   delay characteristics, and the means of control required to
   coordinate the trees. These issues are discussed in more detail in
   section 4.2.

   There are some cases in which O(n-squared) switched paths may be
   used (for example, by setting up a full mesh of point to point
   streams). As label space and the amount of state information that
   can be supported may be limited, it will not be possible to
   support O(n-squared) switched paths in very large networks.
   However, in some cases the use of n-squared paths may even be a
   advantage (for example, to allow load- splitting of individual

   MPLS must be designed to scale for O(n). O(n) scaling allows MPLS
   domains to scale to a very large scale. In addition, if best
   effort service can be supported with O(n) scaling, this conserves
   resources (such as label space and state information) which can be
   used for supporting advanced services such as QoS. However, since
   some switches may not support merging, and some small networks may

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   not require the scaling benefits of O(n), provisions must also be
   provided for a non-merging, O(n-squared) solution.

   Note: A precise and complete description of scaling would consider
   that there are multiple dimensions of scaling, and multiple
   resources whose usage may be considered. Possible dimensions of
   scaling include: (i) the total number of streams which exist in an
   MPLS domain (with associated labels assigned to them); (ii) the
   total number of "label swapping pairs" which may be stored in the
   nodes of the network (ie, entries of the form "for incoming label
   'x', use outgoing label 'y'"); (iii) the number of labels which
   need to be assigned for use over a particular link; (iv) The
   amount of state information which needs to be maintained by any
   one node. We do not intend to perform a complete analysis of all
   possible scaling issues, and understand that our use of the terms
   "O(n)" and "O(n-squared)" is approximate only.

3.3 Types of Streams

   Switched paths in the MPLS network can be of different types:

     -  point-to-point
     -  multipoint-to-point
     -  point-to-multipoint
     -  multipoint-to-multipoint

   Two of the factors that determine which type of switched path is
   used are (i) The capability of the switches employed in a network;
   (ii) The purpose of the creation of a switched path; that is, the
   types of flows to be carried in the switched path. These two
   factors also determine the scalability of a network in terms of
   the number of switched paths in use for transporting data through
   a network.

   The point-to-point switched path can be used to connect all
   ingress nodes to all the egress nodes to carry unicast traffic. In
   this case, since an ingress node has point-to-point connections to
   all the egress nodes, the number of connections in use for
   transporting traffic is of O(n-squared), where n is the number of
   edge MPLS devices. For small networks the full mesh connection
   approach may suffice and not pose any scalability problems.
   However, in large enterprise backbone or ISP networks, this will
   not scale well.

   Point-to-point switched paths may be used on a host-to-host or
   application to application basis (e.g., a switched path per RSVP
   flow). The dedicated point-to-point switched path transports the
   unicast data from the ingress to the egress node of the MPLS
   network. This approach may be used for providing QoS services or
   for best-effort traffic.

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   A multipoint-to-point switched path connects all ingress nodes to
   an single egress node. At a given intermediate node in the
   multipoint-to-point switched path, L2 data units from several
   upstream links are "merged" into a single label on a downstream
   link. Since each egress node is reachable via a single multipoint-
   to-point switched path, the number of switched paths required to
   transport best-effort traffic through a MPLS network is O(n),
   where n is the number of egress nodes.

   The point-to-multipoint switched path is used for distributing
   multicast traffic. This switched path tree mirrors the multicast
   distribution tree as determined by the multicast routing
   protocols. Typically a switch capable of point-to-multipoint
   connection replicates an L2 data unit from the incoming (parent)
   interface to all the outgoing (child) interfaces. Standard ATM
   switches support such functionality in the form of point-to-
   multipoint VCs or VPs.

   A multipoint-to-multipoint switched path may be used to combine
   multicast traffic from multiple sources into a single multicast
   distribution tree. The advantage of this is that the multipoint-to-
   multipoint switched path is shared by multiple sources.
   Conceptually, a form of multipoint-to-multipoint can be thought of
   as follows: Suppose that you have a point to multipoint VC from
   each node to all other nodes. Suppose that any point where two or
   more VCs happen to merge, you merge them into a single VC or VP.
   This would require either coordination of VCI spaces (so that each
   source has a unique VCI within a VP) or VC merge capabilities. The
   applicability of similar concepts to MPLS is FFS.

3.4 Data Driven versus Control Traffic Driven Label Assignment

   A fundamental concept in MPLS is the association of labels and
   network layer routing. Each LSR must assign labels, and distribute
   them to its forwarding peers, for traffic which it intends to
   forward by label switching. In the various contributions that have
   been made so far to the MPLS WG we identify three broad strategies
   for label assignment; (i) those driven by topology based control
   traffic [RFC2105][ARIS][IPNAV]; (ii) Those driven by request based
   control traffic [CR-LDP][RSVP-LSP]; and (iii) those driven by data
   traffic [RFC2098][RFC1953].

   We also note that in actual practice combinations of these methods
   may be employed. One example is that topology based methods for
   best effort traffic plus request based methods for support of

3.4.1 Topology Driven Label Assignment

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   In this scheme labels are assigned in response to normal
   processing of routing protocol control traffic. Examples of such
   control protocols are OSPF and BGP. As an LSR processes OSPF or
   BGP updates it can, as it makes or changes entries in its
   forwarding tables, assign labels to those entries.

   Among the properties of this scheme are:

  - The computational load of assignment and distribution and
    the bandwidth consumed by label distribution are bounded by
    the size of the network.

  - Labels are in the general case preassigned. If a route
    exists then a label has been assigned to it (and
    distributed). Traffic may be label swapped immediately it
    arrives, there is no label setup latency at forwarding time.

  - Requires LSRs to be able to process control traffic load

  - Labels assigned in response to the operation of routing
    protocols can have a granularity equivalent to that of the
    routes advertised by the protocol. Labels can, by this
    means, cover (highly) aggregated routes.

3.4.2 Request Driven Label Assignment

   In this scheme labels are assigned in response to normal
   processing of request based control traffic. Examples of such
   control protocols are RSVP. As an LSR processes RSVP messages it
   can, as it makes or changes entries in its forwarding tables,
   assign labels to those entries.

   Among the properties of this scheme are:

  - The computational load of assignment and distribution and
    the bandwidth consumed by label distribution are bounded by
    the amount of control traffic in the system.

  - Labels are in the general case preassigned. If a route
    exists then a label has been assigned to it (and
    distributed). Traffic may be label swapped immediately it
    arrives, there is no label setup latency at forwarding time.

  - Requires LSRs to be able to process control traffic load

  - Depending upon the number of flows supported, this approach
    may require a larger number of labels to be assigned
    compared with topology driven assignment.

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  - This approach requires applications to make use of request
    paradigm in order to get a label assigned to their flow.

3.4.3 Traffic Driven Label Assignment

   In this scheme the arrival of data at an LSR "triggers" label
   assignment and distribution. Traffic driven approach has the
   following characteristics.

  - Label assignment and distribution costs are a function of
    traffic patterns. In an LSR with limited label space that is
    using a traffic driven approach to amortize its labels over
    a larger number of flows the overhead due to label
    assignment and distribution grows as a function of the
    number of flows and as a function of their "persistence".
    Short lived but recurring flows may impose a heavy control

  - There is a latency associated with the appearance of a
    "flow" and the assignment of a label to it. The documented
    approaches to this problem suggest L3 forwarding during this
    setup phase, this has the potential for packet reordering
    (note that packet reordering may occur with any scheme when
    the network topology changes, but traffic driven label
    assignment introduces another cause for reordering).

  - Flow driven label assignment requires high performance
    packet classification capabilities.

  - Traffic driven label assignment may be useful to reduce
    label consumption (assuming that flows are not close to full

  - If you want flows to hosts, due to limits on label space,
    then traffic based label consumption is probably necessary
    due to the large number of hosts which may occur in a

  - If you want to assign specific network resources to
    specific labels, to be used for support of application
    flows, then again the fine grain associated with labels may
    require data based label assignment.

3.5 The Need for Dealing with Looping

   Routing protocols which are used in conjunction with MPLS will in
   many cases be based on distributed computation. As such, during
   routing transients, these protocols may compute forwarding paths
   which contain loops. For this reason MPLS will be designed with

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   mechanisms to either prevent the formation of loops and /or
   contain the amount of resources that can be consumed due to the
   presence of loops.

   Note that there are a number of different alternative mechanisms
   which have been proposed (see section 4.3). Some of these prevent
   the formation of layer 2 forwarding loops, others allow loops to
   form but minimize their impact in one way or another (e.g., by
   discarding packets which loop, or by detecting and closing the
   loop after a period of time). Generally speaking, there are
   tradeoffs to be made between the amount of looping which might
   occur, and other considerations such as the time to convergence
   after a change in the paths computed by the routing algorithm.

   We are not proposing any changes to normal layer 3 operation, and
   specifically are not trying to eliminate the possibility of
   looping at layer 3. Transient loops will continue to be possible
   in IP networks. Note that IP has a means to limit the damage done
   by looping packets, based on decrementing the IP TTL field as the
   packet is forwarded, and discarding packets whose TTL has expired.
   Dynamic routing protocols used with IP are also designed to
   minimize the amount of time during which loops exist.

   The question that MPLS has to deal with is what to do at L2. In
   some cases L2 may make use of the same method that is used as L3.
   However, other options are available at L2, and in some cases
   (specifically when operating over ATM or Frame Relay hardware) the
   method of decrementing a TTL field (or any similar field) is not

   There are basically two problems caused by packet looping: The
   most obvious problem is that packets are not delivered to the
   correct destination. The other result of looping is congestion.
   Even with TTL decrementing and packet discard, there may still be
   a significant amount of time that packets travel through a loop.
   This can adversely affect other packets which are not looping:
   Congestion due to the looping packets can cause non-looping
   packets to be delayed and/or discarded.

   Looping is particularly serious in (at least) three cases: One is
   when forwarding over ATM. Since ATM does not have a TTL field to
   decrement, there is no way to discard ATM cells which are looping
   over ATM subnetworks. Standard ATM PNNI routing and signaling
   solves this problem by making use of call setup procedures which
   ensure that ATM VCs will never be setup in a loop [PNNI]. However,
   when MPLS is used over ATM subnets, the native ATM routing and
   signaling procedures may not be used for the full L2 path. This
   leads to the possibility that MPLS over ATM might in principle
   allow packets to loop indefinitely, or until L3 routing
   stabilizes. Methods are needed to prevent this problem.

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   Another case in which looping can be particularly unpleasant is
   for multicast traffic. With multicast, it is possible that the
   packet may be delivered successfully to some destinations even
   though copies intended for other destinations are looping. This
   leads to the possibility that huge numbers of identical packets
   could be delivered to some destinations. Also, since multicast
   implies that packets are duplicated at some points in their path,
   the congestion resulting from looping packets may be particularly

   Another unpleasant complication of looping occurs if the
   congestion caused by the loop interferes with the routing
   protocol. It is possible for the congestion caused by looping to
   cause routing protocol control packets to be discarded, with the
   result that the routing protocol becomes unstable. For example
   this could lengthen the duration of the loop.

   In normal operation of IP networks the impact of congestion is
   limited by the fact that TCP backs off (ie, transmits
   substantially less traffic) in response to lost packets. Where the
   congestion is caused by looping, the combination of TTL and the
   resulting discard of looping packets, plus the reduction in
   offered traffic, can limit the resulting impact on the network.
   TCP backoff however does not solve the problem if the looping
   packets are not discarded (for example, if the loop is over an ATM
   subnetwork where TTL is not used).

   The severity of the problem caused by looping may depend upon
   implementation details. Suppose, for instance, that ATM switching
   hardware is being used to provide MPLS switching functions. If the
   ATM hardware has per-VC queuing, and if it is capable of providing
   fair access to the buffer pool for incoming cells based on the
   incoming VC (so that no one incoming VC is allowed to grab a
   disproportionate number of buffers), this looping might not have a
   significant 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 node's total

   Given that MPLS is a relatively new approach, it is possible that
   looping may have consequences which are not fully understood (such
   as looping of LDP control information in cases where stream merge
   is not used).

   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.

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   Methods for dealing with loops are discussed in section 4.3.

3.6 Operations and Management

   Operations and management of networks is critically important.
   This implies that MPLS must support operations, administration,
   and maintenance facilities at least as extensive as those
   supported in current IP networks.

   In most ways this is a relatively simple requirement to meet.
   Given that all MPLS nodes run normal IP routing protocols, it is
   straightforward to expect them to participate in normal IP network
   management protocols.

   There is one issue which has been identified and which needs to be
   addressed by the MPLS effort: There is an issue with regard to
   operation of Traceroute over MPLS networks. Note that other O&M
   issues may be identified in the future.

   Traceroute is a very commonly used network management tool.
   Traceroute is based on use of the TTL field: A station trying to
   determine the route from itself to a specified address transmits
   multiple IP packets, with the TTL field set to 1 in the first
   packet, 2 in the second packet, etc.. This causes each router
   along the path to send back an ICMP error report for TTL exceeded.
   This in turn allows the station to determine the set of routers
   along the route. For example, this can be used to determine where
   a problem exists (if no router responds past some point, the last
   router which responds can become the starting point for a search
   to determine the cause of the problem).

   When MPLS is operating over ATM or Frame Relay networks there is
   no TTL field to decrement (and ATM and Frame Relay forwarding
   hardware does not decrement TTL). This implies that it is not
   straightforward to have Traceroute operate in this environment.

   There is the question of whether we *want* all routers along a
   path to be visible via traceroute. For example, an ISP probably
   doesn't want to expose the interior of their network to a
   customer. However, the issue of whether a network's policy will
   allow the interior of the network to be visible should be
   independent of whether is it possible for some users to see the
   interior of the network. Thus while there clearly should be the
   possibility of using policy mechanisms to block traceroute from
   being used to see the interior of the network, this does not imply
   that it is okay to develop protocol mechanisms which prevent
   traceroute from working.

   There is also the question of whether the interior of a MPLS

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   network is analogous to a normal IP network, or whether it is
   closer to the interior of a layer 2 network (for example, an ATM
   subnet). Clearly IP traceroute cannot be used to expose the
   interior of an ATM subnet. When a packet is crossing an ATM
   subnetwork (for example, between an ingress and an egress router
   which are attached to the ATM subnet) traceroute can be used to
   determine the router to router path, but not the path through the
   ATM switches which comprise the ATM subnet. Note here that MPLS
   forms a sort of "in between" special case:
   Routing is based on normal IP routing protocols, the equivalent of
   call setup (label binding/exchange) is based on MPLS-specific
   protocols, but forwarding is based on normal L2 ATM forwarding.
   MPLS therefore supersedes the normal ATM-based methods that would
   be used to eliminate loops and/or trace paths through the ATM

   It is generally agreed that Traceroute is a relatively "ugly"
   tool, and that a better tool for tracing the route of a packet
   would be preferable. However, no better tool has yet been designed
   or even proposed. Also, however ugly Traceroute may be, it is
   nonetheless very useful, widely deployed, and widely used. In
   general, it is highly preferable to define, implement, and deploy
   a new tool, and to determine through experience that the new tool
   is sufficient, before breaking a tool which is as widely used as

   Methods that may be used to either allow traceroute to be used in
   an MPLS network, or to replace traceroute, are discussed in
   section 4.11.

4. Technical Approaches

4.1 Label Distribution

   A fundamental requirement in MPLS is that an LSR forwarding label
   switched traffic to another LSR apply a label to that traffic
   which is meaningful to the other (receiving the traffic) LSR.
   LSR's could learn about each other's labels in a variety of ways.
   We call the general topic "label distribution".

4.1.1 Explicit Label Distribution

   Explicit label distribution anticipates the specification by MPLS
   of a standard protocol for label distribution. Two of the possible
   approaches (TDP, ARIS [ARIS-PROT]) are oriented toward topology
   driven label distribution. One other approach [FANP], in contrast,
   makes use of traffic driven label distribution. We expect that the
   label distribution protocol [LDP] which emerges from the MPLS WG
   is likely to inherit elements from one or more of the possible

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   Consider LSR A forwarding traffic to LSR B. We call A the upstream
   (wrt to dataflow) LSR and B the downstream LSR. A must apply a
   label to the traffic that B "understands". Label distribution must
   ensure that the "meaning" of the label will be communicated
   between A and B. An important question is whether A or B (or some
   other entity) allocates the label.

   In this discussion we are talking about the allocation and
   distribution of labels between two peer LSRs that are on a single
   segment of what may be a longer path. A related but in fact
   entirely separate issue is the question of where control of the
   whole path resides. In essence there are two models; by analogy to
   upstream and downstream for a single segment we can talk about
   ingress and egress for an LSP (or to and from a label switching
   "domain"). In one model a path is setup from ingress to egress and
   in the other from egress to ingress. Downstream Label Allocation

   "Downstream Label Allocation" refers to a method where the label
   allocation is done by the downstream LSR, ie the LSR that uses the
   label as an index into its switching tables.

   This is, arguably, the most natural label allocation/distribution
   mode for unicast traffic. As an LSR builds its routing tables (we
   consider here control driven allocation of tags) it is free,
   within some limits we will discuss, to allocate labels in any
   manner that may be convenient to the particular implementation.
   Since the labels that it allocates will be those upon which it
   subsequently makes forwarding decisions we assume implementations
   will perform the allocation in an optimal manner. Having allocated
   labels the default behavior is to distribute the labels (and
   bindings) to all peers.

   In some cases (particularly with ATM) there may be a limited
   number of labels which may be used across an interface, and/or a
   limited number of label assignments which may be supported by a
   single device. Operation in this case may make use of "on demand"
   label assignment. With this approach, an LSR may for example
   request a label for a route from a particular peer only when its
   routing calculations indicate that peer to be the new next hop for
   the route. Upstream Label Allocation

   "Upstream Label Allocation" refers to a method where the label
   allocation is done by the upstream LSR. In this case the LSR
   choosing the label (the upstream LSR) and the LSR which needs to

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   interpret packets using the label (the downstream LSR) are not the
   same node. We note here that in the upstream LSR the label at
   issue is not used as an index into the switching tables but rather
   is found as the result of a lookup on those tables.

   The motivation for upstream label allocation comes from the
   recognition that it might be possible to optimize multicast
   machinery in an LSR if it were possible to use the same label on
   all output ports for which a particular multicast packet/cell were
   destined. Upstream assignment makes this possible. Other Label Allocation Methods

   Another option would be to make use of label values which are
   unique within the MPLS domain (implying that a domain-wide
   allocation would be needed). In this case, any stream to a
   particular MPLS egress node could make use of the label of that
   node (implying that label values do not need to be swapped at
   intermediate nodes).

   With this method of label allocation, there is a choice to be made
   regarding the scope over which a label is unique. One approach is
   to configure each node in an MPLS domain with a label which is
   unique in that domain. Another approach is to use a truly global
   identifier (for example the IEEE 48 bit identifier), where each
   MPLS-capable node would be stamped at birth with a truly globally
   unique identifier. The point of this global approach is to
   simplify configuration in each MPLS domain by eliminating the need
   to configure label IDs.

4.1.2 Piggybacking on Other Control Messages

   While we have discussed use of an explicit MPLS LDP we note that
   there are several existing protocols that can be easily modified
   to distribute both routing/control and label information. This
   could be done with any of OSPF, BGP, RSVP and/or PIM. A particular
   architectural elegance of these schemes is that label distribution
   uses the same mechanisms as are used in distribution of the
   underlying routing or control information.

   When explicit label distribution is used, the routing computation
   and label distribution are decoupled. This implies a possibility
   that at some point you may either have a route to a specific
   destination without an associated label, and/or a label for a
   specific destination which makes use of a path which you are no
   longer using. Piggybacking label distribution on the operation of
   the routing protocol is one way to eliminate this decoupling.

   Piggybacking label distribution on the routing protocol introduces
   an issue regarding how to negotiate acceptable label values and

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   what to do if an invalid label is received. This is discussed in
   section 4.1.3.

4.1.3 Acceptable Label Values

   There are some constraints on which label values may be used in
   either allocation mode. Clearly the label values must lie within
   the allowable range described in the encapsulation standards that
   the MPLS WG will produce. The label value used must also, however,
   lie within a range that the peer LSR is capable of supporting. We
   imagine that certain machines, for example ATM switches operating
   as LSRs may, due to operational or implementation restrictions,
   support a label space more limited than that bounded by the valid
   range found in the encapsulation standard. This implies that an
   advertisement or negotiation mechanism for useable label range may
   be a part of the MPLS LDP. When operating over ATM using ATM
   forwarding hardware, due to the need for compatibility with the
   existing use of the ATM VPI/VCI space, it is quite likely that an
   explicit mechanism will be needed for label range negotiation.

   In addition we note that LDP may be one of a number of mechanism
   used to distribute labels between any given pair of LSRs. Clearly
   where such multiple mechanisms exist care must be taken to
   coordinate the allocation of label values. A single label value
   must have a unique meaning to the LSR that distributes it.

   There is an issue regarding how to allow negotiation of acceptable
   label values if label distribution is piggybacked with the routing
   protocol. In this case it may be necessary either to require
   equipment to accept any possible label value, or to configure
   devices to know which range of label values may be selected. It is
   not clear in this case what to do if an invalid label value is
   received as there may be no means of sending a NAK.

   A similar issue occurs with multicast traffic over broadcast
   media, where there may be multiple nodes which receive the same
   transmission (using a single label value). Here again it may be
   "non-trivial" how to allow n-party negotiation of acceptable label

4.1.4 LDP Reliability

   The need for reliable label distribution depends upon the relative
   performance of L2 and L3 forwarding, as well as the relationship
   between label distribution and the routing protocol operation.

   If label distribution is tied to the operation of the routing
   protocol, then a reasonable protocol design would ensure that
   labels are distributed successfully as long as the associated
   route and/or reachability advertisement is distributed

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   successfully. This implies that the reliability of label
   distribution will be the same as the reliability of route

   If there is a very large difference between L2 and L3 forwarding
   performance, then the cost of failing to deliver a label is
   significant. In this case it is important to ensure that labels
   are distributed reliably. Given that LDP needs to operate in a
   wide variety of environments with a wide variety of equipment,
   this implies that it is important for an LDP developed by the MPLS
   WG to ensure reliable delivery of label information.

   Reliable delivery of LDP packets may potentially be accomplished
   either by using an existing reliable transport protocol such as
   TCP, or by specifying reliability mechanisms as part of LDP (for
   example, the reliability mechanisms which are defined in IDRP
   could potentially be "borrowed" for use with LDP).

   TCP supports flow control (in addition to supporting reliable
   delivery of data). Flow control is a desirable feature which will
   be useful for MPLS (as well as other applications making use of a
   reliable transport) and therefore needs to be built into whatever
   reliability mechanism is used for MPLS.

4.1.5 Label Purge Mechanisms

   Another issue to be considered is the "lifetime" of label data
   once it arrives at an LSR, and the method of purging label data.
   There are several methods that could be used either separately, or
   (more likely) in combination.

   One approach is for label information to be timed out. With this
   approach a lifetime is distributed along with the label value. The
   label value may be refreshed prior to timing out. If the label is
   not refreshed prior to timing out it is discarded. In this case
   each lifetime and timer may apply to a single label, or to a group
   of labels (e.g., all labels selected by the same node).

   Similarly, two peer nodes may make use of an MPLS peer keep-alive
   mechanism. This implies exchange of MPLS control packets between
   neighbors on a periodic basis. This in general is likely to use a
   smaller timeout value than label value timers (analogous to the
   fact that the OSPF HELLO interval is much shorter than the OSPF
   LSA lifetime). If the peer session between two MPLS nodes fails
   (due to expiration of the associated timer prior to reception of
   the refresh) then associated label information is discarded.

   If label information is piggybacked on the routing protocol then
   the timeout mechanisms would also be taken from the associated
   routing protocol (note that routing protocols in general have

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   mechanisms to invalidate stale routing information).

   An alternative method for invalidating labels is to make use of an
   explicit label removal message.

4.2 Stream Merging

   In order to scale O(n) (rather than O(n-squared)), MPLS makes use
   of the concept of stream merge. This makes use of multipoint to
   point streams in order to allow multiple streams to be merged into
   one stream.

4.2.1 Types of Stream Merge:

   There are several types of stream merge that can be used,
   depending upon the underlying media.

   When MPLS is used over frame based media merging is
   straightforward. All that is required for stream merge to take
   place is for a node to allow multiple upstream labels to be
   forwarded the same way and mapped into a single downstream label.
   This is referred to as frame merge.

   Operation over ATM media is less straightforward. In ATM, the data
   packets are encapsulated into an ATM Adaptation Layer, say AAL5,
   and the AAL5 PDU is segmented into ATM cells with a VPI/VCI value
   and the cells are transmitted in sequence. It is contingent on ATM
   switches to keep the cells of a PDU (or with the same VPI/VCI
   value) contiguous and in sequence. This is because the device that
   reassembles the cells to re-form the transmitted PDU expects the
   cells to be contiguous and in sequence, as there isn't sufficient
   information in the ATM cell header (unlike IP fragmentation) to
   reassemble the PDU with any cell order. Hence, if cells from
   several upstream link are transmitted onto the same downstream
   VPI/VCI, then cells from one PDU can get interleaved with cells
   from another PDU on the outgoing VPI/VCI, and result in corruption
   of the original PDUs by mis-sequencing the cells of each PDU.

   The most straightforward (but erroneous) method of merging in an
   ATM environment would be to take the cells from two incoming VCs
   and merge them into a single outgoing VCI. If this was done
   without any buffering of cells then cells from two or more packets
   could end up being interleaved into a single AAL5 frame. Therefore
   the problem when operating over ATM is how to avoid interleaving
   of cells from multiple sources.

   There are two ways to solve this interleaving problem, which are
   referred to as VC merge and VP merge.

   VC merge allows multiple VCs to be merged into a single outgoing

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   VC. In order for this to work the node performing the merge needs
   to keep the cells from one AAL5 frame (e.g., corresponding to an
   IP packet) separate from the cells of other AAL5 frames. This may
   be done by performing the SAR function in order to reassemble each
   IP packet before forwarding that packet. In this case VC merge is
   essentially equivalent to frame merge. An alternative is to buffer
   the cells of one AAL5 frame together, without actually
   reassembling them. When the end of frame indicator is reached that
   frame can be forwarded. Note however that both forms of VC merge
   generally require that the entire AAL5 frame be received before
   any cells corresponding to that frame be forwarded. VC merge
   therefore requires capabilities which are generally not available
   in most existing ATM forwarding hardware.

   The alternative for use over ATM media is VP merge. Here multiple
   VPs can be merged into a single VP. Separate VCIs within the
   merged VP are used to distinguish frames (e.g., IP packets) from
   different sources. In some cases, one VP may be used for the tree
   from each ingress node to a single egress node.

4.2.2 Interoperation of Merge Options:

   If some nodes support stream merge, and some nodes do not, then it
   is necessary to ensure that the two types of nodes can
   interoperate within a single network. This affects the number of
   labels that a node needs to send to a neighbor. An upstream LSR
   which supports Stream Merge needs to be sent only one label per
   forwarding equivalence class (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.

   If a particular upstream neighbor does not support stream merge,
   it is not known a priori how many labels it will need. The
   upstream neighbor may need to explicitly ask for labels for each
   FEC. The upstream neighbor may make multiple such requests (for
   one or more labels per request). 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 more labels 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 stream. 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

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   obtained from the downstream neighbor). In this case, the node
   will need to obtain the required two labels.

   The interoperation of the various forms of merging over ATM is
   most easily described by first describing the interoperation of VC
   merge with non-merge.

   In the case where VC merge and non-merge nodes are interconnected
   the forwarding of cells is based in all cases on a VC (ie, 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 outgoing VPI/VCI for a particular FEC (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 it will require a single outgoing VPI/VCI per FEC for
   itself (assuming that it can be an ingress node), plus enough
   outgoing VPI/VCIs to map to incoming 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.

   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).
   Furthermore, suppose that a non-VP-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).

   An alternative method is possible, which requires no support of VP
   switching and VP labels on nodes which do not support VP merge. In
   this method, the VP merge node does not request VPs from the
   downstream node. It does request a number of VPI/VCIs, one per
   source node in the group of nodes which use VP merge.

   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). In addition,
   it may be helpful to allow upstream nodes to request zero of more
   VPs (identified by VPIs). VP merge nodes would therefore request
   one VP, or in the event where this is not supported, the VP merge
   node(s) would request several VCs . VC merge node would request
   only a single VPI/VCI (since they can merge all upstream traffic
   into a single VC). Non-VP-merge nodes would pass on any requests
   that they get from above, plus request a VPI/VCI for traffic that
   they originate (if they can be ingress nodes). However, non-merge
   nodes which can only do VC forwarding (and not VP forwarding) will
   need to know which VCIs are used within each VP in order to

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   install the correct VCs in its forwarding table. This limitation
   is likely to apply to most on-ATM LSRs; most ATM NICs can
   terminate VP connections as numbers individual VC connections. A
   detailed description of how this could work can be found in
   [ATMVP]. Alternatively, the non-VP-merge nodes could issue only VC
   identifiers, as described above.

4.2.3 Coordination of the VCI space with VP Merge:

   VP merge requires that the VCIs be coordinated to ensure
   uniqueness. There are a number of ways in which this may be

  1. Each node may be pre-configured with a unique VCI value
     (or values).

  2. Some one node (most likely they root of the multipoint to
     point tree) may coordinate the VCI values used within the
     VP. A protocol mechanism will be needed to allow this to
     occur. How hard this is to do depends somewhat upon
     whether the root is otherwise involved in coordinating the
     multipoint to point tree. For example, allowing one node
     (such as the root) to coordinate the tree may be useful
     for purposes of coordinating load sharing (see section
     4.10). Thus whether or not the issue of coordinating the
     VCI space is significant or trivial may depend upon other
     design choices which at first glance may have appeared to
     be independent protocol design choices.

  3. Other unique information such as portions of a class B or
     class C address may be used to provide a unique VCI value.

  4. Another alternative is to implement a simple hardware
     extension in the ATM switches to keep the VCI values
     unique by dynamically altering them to avoid collision.

   VP merge makes less efficient use of the VPI/VCI space (relative
   to VC merge). When VP merge is used, the LSPs may not be able to
   transit public ATM networks that don't support SVP.

4.2.4 Buffering Issues Related To Stream Merge:

   There is an issue regarding the amount of buffering required for
   frame merge, VC merge, and VP merge. Frame merge and VC merge
   requires that intermediate points buffer incoming packets until
   the entire packet arrives. This is essentially the same as is
   required in traditional IP routers.

   VP merge allows cells to be transmitted by intermediate nodes as
   soon as they arrive, reducing the buffering and latency at

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   intermediate nodes. However, the use of VP merge implies that
   cells from multiple packets will arrive at the egress node
   interleaved on separate VCIs. This in turn implies that the egress
   node may have somewhat increased buffering requirements. To a
   large extent egress nodes for some destinations will be
   intermediate nodes for other destinations, implying that increase
   in buffers required for some purpose (egress traffic) will be
   offset by a reduction in buffers required for other purposes
   (transit traffic). Also, routers today typically deal with high-
   fanout channelized interfaces and with multi-VC ATM interfaces,
   implying that the requirement of buffering simultaneously arriving
   cells from multiple packets and sources is something that routers
   typically do today. This is not meant to imply that the required
   buffer size and performance is inexpensive, but rather is meant to
   observe that it is a solvable issue.

   ATM equipment provides traffic shaping, in which the ATM cells
   associated with any one particular VC are intentionally not
   transmitted back to back, but rather are spread out over time in
   order to place less short term buffering load on switches. Since
   VC merge requires that all cells associated with a particular
   packet (or a particular AAL5 frame) are buffered before any cell
   from the packet can be transmitted, VC merge defeats much of the
   intent of traffic shaping. An advantage of VP merge is that it
   preserves traffic shaping through ATM switches acting as LSRs.
   While traffic shaping may generally be expected to reduce the
   buffering requirements in ATM switches (whether acting as MPLS
   switches or as native ATM switches), the precise effect of traffic
   shaping has not been studied in the context of MPLS.

4.3 Loop Handling

   Generally, methods for dealing with loops can be split into three
   categories: Loop Survival makes use of methods which minimize the
   impact of loops, for example by limiting the amount of network
   resources which can be consumed by a loop; Loop Detection allows
   loops to be set up, but later detects these loops and eliminates
   them; Loop Prevention provides methods for avoiding setting up L2
   switching in a way which results in a L2 loop.

   Note that we are concerned here only with loops that occur in L2
   switching. Transient loops at L3 will continue to be part of the
   normal IP operation, and will be handled the way that IP has been
   handling loops for years (see section 3.5).

   Loop Survival:

   Loop Survival refers to methods that are used to allow the network
   to operate well even though short term transient loops may be
   formed by the routing protocol. The basic approach to loop

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   survival is to limit the amount of network resources which are
   consumed by looping packets, and to minimize the effect on other
   (non-looping) traffic. Note that loop survival is the method used
   by conventional IP forwarding, and is therefore based on long and
   relatively successful experience in the Internet.

   The most basic method for loop survival is based on the use to a
   TTL (Time To Live) field. The TTL field is decremented at each
   hop. If the TTL field reaches zero, then the packet is discarded.
   This method works well over those media which has a TTL field.
   This explicitly includes L3 IP forwarding. Also, assuming that the
   core MPLS specifications will include definition of a "shim" MPLS
   header for use over those media which do not have their own
   labels, in order to carry labels for use in forwarding of user
   data, the shim header will also include a TTL field.

   However, there is considerable interest in using MPLS over L2
   protocols which provide their own labels, with the L2 label used
   for MPLS forwarding. Specific L2 protocols which offer a label for
   this purpose include ATM and Frame Relay. However, neither ATM nor
   Frame Relay have a TTL field. This implies that this method cannot
   be used when basic ATM or Frame Relay forwarding is being used.

   Another basic method for loop survival is the use of dynamic
   routing protocols which converge rapidly to non-looping paths. In
   some instances it is possible that congestion caused by looping
   data could affect the convergence of the routing protocol (see
   section 3.5). MPLS should be designed to prevent this problem from
   occurring. Given that MPLS uses the same routing protocols as are
   used for IP, this method does not need to be discussed further in
   this framework document.

   Another possible tool for loop survival is the use of fair
   queuing. This allows unrelated flows of user data to be placed in
   different queues. This helps to ensure that a node which is
   overloaded with looping user data can nonetheless forward
   unrelated non-looping data, thereby minimizing the effect that
   looping data has on other data. We cannot assume that fair queuing
   will always be available. In practice, many fair queuing
   implementations merge multiple streams into one queue (implying
   that the number of queues used is less than the number of user
   data flows which are present in the network). This implies that
   any data which happens to be in the same queue with looping data
   may be adversely effected.

   Loop Detection:

   Loop Detection refers to methods whereby a loop may be set up at
   L2, but the loop is subsequently detected. When the loop is
   detected, it may be broken at L2 by dropping the label

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   relationship, implying that packets for a set of destinations must
   be forwarded at L3.

   A possible method for loop detection is based on transmitting a
   "loop detection" control packet (LDCP) along the path towards a
   specified destination whenever the route to the destination
   changes. This LDCP is forwarded in the direction that the label
   specifies, with the labels swapped to the correct next hop value.
   However, normal L2 switching cannot be used because each hop needs
   to examine the packet to check for loops. The LDCP is forwarded
   towards that destination until one of the following happens: (i)
   The LDCP reaches the last MPLS node along the path (ie the next
   hop is either a router which is not participating in MPLS, or is
   the final destination host); (ii) The TTL of the LDCP expires
   (assuming that the control packet uses a TTL, which is optional
   but not absolutely necessary), or (iii) The LDCP returns to the
   node which originally transmitted it. If the latter occurs, then
   the packet has looped and the node which originally transmitted
   the LDCP stops using the associated label, and instead uses L3
   forwarding for the associated destination addresses. One problem
   with this method is that once a loop is detected it is not known
   when the loop clears. One option would be to set a timer, and to
   transmit a new LDCP when the timer expires.

   Loop detection may also be achieved via a Path Vector control
   message. A Path Vector contains a list of the LSRs that label
   distribution Control message has traversed. Each LSR which
   propagates a control packet to either create or modify an LSP adds
   its own unique identifier to the Path Vector list. An LSR that
   receives a message with a Path Vector that contains its own
   identifier detects that the message has traversed a loop.

   An alternate method counts the hops to each egress node, based on
   the routes currently available. Each node advertises its distance
   (in hop counts) to each destination. An egress node advertises the
   destinations that it can reach directly with an associated hop
   count of one. For each destination, a node computes the hop count
   to that destination based on adding one to the hop count
   advertised by its actual next hop used for that destination. When
   the hop count for a particular destination changes, the hop counts
   needs to be readvertised.

   In addition, the first of the loop prevention schemes discussed
   below may be modified to provide loop detection.

   Loop Prevention:

   Loop prevention makes use of methods to ensure that loops are
   never set up at L2. This implies that the labels are not used
   until some method is used to ensure that following the label

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   towards the destination, with associated label swaps at each
   switch, will not result in a loop. Until the L2 path (making use
   of assigned labels) is available, packets are forwarded at L3.

   Loop prevention requires explicit signaling of some sort to be
   used when setting up an L2 stream.

   One method of loop prevention requires that labels be propagated
   starting at the egress switch. The egress switch signals to
   neighboring switches the label to use for a particular
   destination. That switch then signals an associated label to its
   neighbors, etc. The control packets which propagate the labels
   also include the path to the egress (as a list of routerIDs). Any
   looping control packet can therefore be detected and the path not
   set up to or past the looping point.

   During routing changes, a diffusion mechanism may be used to
   prevent the formation of L2 loops. The purpose of the diffusion
   computation is to prune the tree of an LSR that has detected a
   route change for a given FEC, such that all upstream LSR's from
   the tree that would be on a looping path are removed. It is only
   after those LSR's are removed from the tree that it is safe to
   replace the old LSP with the new LSP (and the old LSP can be

   The diffusion mechanism is an extension of the Path Vector
   mechanism. An LSR, D, that detects that the next hop for an FEC
   has changed, transmits a query message with a Path Vector
   containing its unique identifier to its upstream neighbors. An
   LSR, U, that receives such a query will determine if D is the next
   hop for the given FEC. If not, then U may return "OK", meaning
   that as far as node U is concerned it is safe for node D to switch
   over to the new LSP. If node D is the next hop, then node U checks
   the Path Vector to see if its unique identifier is already
   present. If so, then a route loop is detected; in this case, node
   U responds with a "LOOP" message, and node D will prune node U off
   of its tree. If no loop is detected, then node U adds its unique
   identifier to the Path Vector, and propagates the query message to
   each of its upstream neighbors. The diffusion computation
   continues to propagate upstream along each of the paths in the
   tree until an ingress or looping LSR is found. Once an LSR has
   received a response from each of its upstream neighbors, it may
   then return an "OK" message to its downstream neighbor. When the
   original node, node D, receives 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 have looped have been pruned from
   the tree.

   An alternative method of loop prevention is the "colored"
   mechanism. The heart of the Colored Thread (CT) algorithm

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   propagates a procedure that gives a color to each link along the
   LSP in the downstream direction. The color is composed of two
   fixed-length objects; the address of the node that created the
   color and a local identifier that is unique within the creating
   node. A loop-free LSP is established when the node that triggered
   the coloring procedure receives an acknowledgment for the
   procedure from its downstream node. During the coloring procedure,
   a set of attributes (color, hop count, TTL), referred to as a
   thread, is propagated downstream. A node that finds a change in
   the next hop creates a color and passes it on the outgoing link to
   the new next hop. If a node receives a color on an incoming link,
   it either (a) passes the received color or (b) creates a new color
   and passes it, on the outgoing link to the next hop. The coloring
   procedure is propagated downstream until the LSP turns out to be
   loop-free or a loop is found. In case (i), a positive
   acknowledgment (ACK) is returned hop-by-hop to upstream nodes. In
   case (ii), the coloring procedure is stalled and no ACK is
   returned. [LOOP-COLOR]

   Another option is to use explicit routing to set up label
   bindings. This precludes the possibility of looping, since the
   entire path is computed by one node. This also allows non-looping
   paths to be set up provided that the edge switch has a view of the
   topology which is reasonably close to reality (if there are
   operational links which the edge switch doesn't know about, it
   will simply pick a path which doesn't use those links; if there
   are links which have failed but which the edge switch thinks are
   operational, then there is some chance that the setup attempt will
   fail but in this case the attempt can be retried on a separate
   path). Note therefore that non-looping paths can be set up with
   this method in many cases where distributed routing plus hop by
   hop forwarding would not actually result in non-looping paths.
   This method is similar to the method used by standard ATM routing
   to ensure that SVCs are non-looping [PNNI].

   Explicit routing is applicable if it is configured, or if the
   routing protocol gives the edge switch sufficient information to
   set up the explicit route, implying that the protocol must be
   either a link state protocol (such as OSPF) or a path vector
   protocol (such as BGP). This method also requires some overhead
   for the call setup before label-based forwarding can be used. If
   the network topology changes in a manner which breaks the existing
   path, then a new path will need to be explicit routed from the
   edge switch. Due to this overhead this method is probably only
   appropriate if other significant advantages are also going to be
   obtained from having a single node (the edge switch) coordinate
   the paths to be used.

   If label distribution is piggybacked on the routing protocol (see
   section 4.1.2), then loop prevention is only possible if the

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   routing protocol itself does loop prevention.

   What To Do If A Loop Is Detected:

   With all of these schemes, if a loop is known to exist then the L2
   label-swapped path is not set up. This leads to the obvious
   question of what does an MPLS node do when it doesn't have a label
   for a particular destination, and a packet for that destination
   arrives to be forwarded? If possible, the packet is forwarded
   using normal L3 (IP) forwarding. There are two issues that this
   raises: (i) What about nodes which are not capable of L3
   forwarding; (ii) Given the relative speeds of L2 and L3
   forwarding, does this work?

   Nodes which are not capable of L3 forwarding obviously can't
   forward a packet unless it arrives with a label, and the
   associated next hop label has been assigned. Such nodes, when they
   receive a packet for which the next hop label has not been
   assigned, must discard the packet. It is probably safe to assume
   that if a node cannot forward an L3 packet, then it is probably
   also incapable of forwarding an ICMP error report that it
   originates. This implies that the packet will need to be discarded
   in this case.

   In many cases L2 switching will be significantly faster than L3
   forwarding (allowing faster forwarding is a significant motivation
   behind the work on MPLS). This implies that if a node is
   forwarding a large volume of traffic at L2, and a change in the
   routing protocol causes the associated labels to be lost
   (necessitating L3 forwarding), in some cases the node will not be
   capable of forwarding the same volume of traffic at L3. This will
   of course require that packets be discarded. However, in some
   cases only a relatively small volume of traffic will need to be
   forwarded at L3. Thus forwarding at L3 when L2 is not available is
   not necessarily always a problem. There may be some nodes which
   are capable of forwarding equally fast at L2 and L3 (for example,
   such nodes may contain IP forwarding hardware which is not
   available in all nodes). Finally, when packets are lost this will
   cause TCP to backoff, which will in turn reduce the load on the
   network and allow the network to stabilize even at reduced
   forwarding rates until such time as the label bindings can be

   In many cases MPLS may be used for traffic engineering. In these
   cases failure of an LSP may cause packets which would have taken
   that LSP to be forwarded (using L3 forwarding) along paths which
   are not consistent with the traffic engineering solution. This
   could in turn cause congestion. In these cases packets may need to
   be discarded even if the LSRs are capable of full line rate L3
   forwarding. This may cause problems very similar to those

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   discussed in the previous paragraph.

   Note that in most cases loops will be caused either by
   configuration errors, or due to short term transient problems
   caused by the failure of a link. If only one link goes down, and
   if routing creates a normal "tree-shaped" set of paths to any one
   destination, then the failure of one link somewhere in the network
   will effect only one link's worth of data passing through any one
   node in the network. This implies that if a node is capable of
   forwarding one link's worth of data at L3, then in many or most
   cases it will have sufficient L3 bandwidth to handle looping data.

4.4 Interoperation with NHRP

   When label switching is used over ATM, and there exists an LSR
   which is also operating as a Next Hop Client (NHC), the
   possibility of direct interaction arises. That is, could one
   switch cells between the two technologies without reassembly? To
   enable this several important issues must be addressed.

   The encapsulation must be acceptable to both MPLS and NHRP. If
   only a single label is used, then the null encapsulation could be
   used. Other solutions could be developed to handle label stacks.

   NHRP must understand and respect the granularity of a stream.

   Currently NHRP resolves an IP address to an ATM address. The
   response may include a mask indicating a range of addresses.
   However, any VC to the ATM address is considered to be a viable
   means of packet delivery. Suppose that an NHC NHRPs for IP address
   A and gets back ATM address 1 and sets up a VC to address 1. Later
   the same NHC NHRPs for a totally unrelated IP address B and gets
   back the same ATM address 1. In this case normal NHRP behavior
   allows the NHC to use the VC (that was set up for destination A)
   for traffic to B [RFC2332].

   Note: In this section we will refer to a VC set up as a result of
   an NHRP query/response as a shortcut VC.

   If one expects to be able to label switch the packets being
   received from a shortcut VC, then the label switch needs to be
   informed as to exactly what traffic will arrive on that VC and
   that mapping cannot change without notice. Currently there exists
   no mechanism in the defined signaling of an shortcut VC. Several
   means are possible. A binding, equivalent to the binding in LDP,
   could be sent in the setup message. Alternatively, the binding of
   prefix to label could remain in an LDP session (or whatever means
   of label distribution as appropriate) and the setup could carry a
   binding of the label to the VC. This would leave the binding
   mechanism for shortcut VCs independent of the label distribution

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   A further architectural challenge exists in that label switching
   is inherently unidirectional whereas ATM is bi-directional. The
   above binding semantics are fairly straight-forward. However,
   effectively using the reverse direction of a VC presents further

   Label switching must also respect the granularity of the shortcut
   VC. Without VC merge, this means a single label switched flow must
   map to a VC. In the case of VC merge, multiple label switched
   streams could be merged onto a single shortcut VC. But given the
   asymmetry involved, there is perhaps little practical use.

   Another issue is one of practicality and usefulness. What is sent
   over the VC must be at a fine enough granularity to be label
   switched through receiving domain. One potential place where the
   two technologies might come into play is in moving data from one
   campus via the wide-area to another campus. In such a scenario,
   the two technologies would border precisely at the point where
   summarization is likely to occur. Each campus would have a
   detailed understanding of itself, but not of the other campus. The
   wide-area is likely to have summarized knowledge only. But at such
   a point level 3 processing becomes the likely solution.

4.5. Operation in a hierarchy

   MPLS allows hierarchical operation, through use of a label stack.
   This allows MPLS to simultaneously be used for routing at a fine
   grain level (for example, between individual routers within an
   ISP) and at a higher "area by area" or "domain by domain" level.

4.5.1 Example of Hierarchical Operation

   Figure 1 illustrates an example of how MPLS may operate in a
   hierarchy. This example illustrates three transit routing domains
   (Domain #1, #2, and #3). For example, these three domains may
   represent internet service providers. Domain Boundary Routers are
   illustrated in each domain (routers R1 and R2 in domain #1,
   routers R3 and R8 in domain #2, and routers R9 and R10 in domain
   #3. Suppose that these domain boundary routers are operating BGP.

   Internal routers are not illustrated in domains 1 and 3. However,
   internal routers are illustrated within domain #2. In particular,
   the path between routers R3 and R8 follows the internal routers
   R4, R5, R6, and R7 within domain #2.

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   .................    ........................    ................
   .               .    .                      .    .              .
   .               .    .                      .    .              .
   .R1           R2------R3                  R8------R9         R10.
   .               .    . \                 /  .    .              .
   .               .    .  R4---R5---R6---R7   .    .              .
   .               .    .                      .    .              .
   .   Domain#1    .    .       Domain#2       .    .    Domain#3  .
   .................    ........................    ................

            Figure 1: Example of the Use of MPLS in a Hierarchy

   In this example there are two levels of routing taking place. For
   example, OSPF may be used for routing within Domain #2. In this
   case the routers R3, R4, R5, R6, R7, and R8 may be running OSPF
   amongst themselves in order to compute routes within Domain #2.
   The domain boundary routers (R1, R2, R3, R8, R9, and R10) operate
   BGP in order to determine paths between routing domains.

   MPLS allows label forwarding to be done independently at multiple
   levels. In this example, MPLS may be used at the BGP level
   (between routers R1, R2, R3, R8, R9, and R10) and at the OSPF
   level (between routers R4, R5, R6, and R7). Thus when the IP
   packet traverses Domain number 2, it will contain two labels,
   encoded as a "label stack". The higher level label would be used
   between routers R3 and R8. This would be encapsulated inside a
   header specifying a lower level label used within domain 2.

   Consider the forwarding operation that takes place at router R3.
   In this case, R3 will receive a packet from R2 containing a single
   label (the BGP level label). R3 will need to swap BGP level labels
   in order to put the label that R8 expects. R3 will also need to
   add an OSPF-level label, as is expected by R4. R3 therefore
   "pushes down" the BGP level label in the label stack, by adding a
   lower level label. Also note that the actual label switching
   operation performed by R3 can be optimized to allow very simple
   forwarding: R3 receives a single incoming label from R2, and can
   map this label into the new label header to be prepended to the
   packet, it just happens that the new label header to be added by
   R3 contains two labels rather than one.

4.5.2 Components Required for Hierarchical Operation

   In order for MPLS to operate in a hierarchy, there are three
   things which must be accomplished:

   - Hierarchical Label Exchange in LDP
     The Label Distribution Protocol needs to exchange labels at
     each level of the hierarchy. In our example, R3 needs to
     exchange label bindings with R8 for operation at the BGP

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     level. At the same time, R3 needs to exchange label
     bindings with R4 (and R4 needs to exchange label bindings
     with R5) for operation at the OSPF level. The control
     component for hierarchical labeling is essentially the same
     as that for single level tagging, except that labels are
     exchanged not just among physically adjacent LSRs but
     between those switching on the same level in the tag stack.

   - Label Stack
     Multiple labels need to be carried in data packets. For
     example, when a data packet is being carried across domain
     #2, the data packet needs to be encapsulated in a header
     which carries BGP level label, and the resulting packet
     needs to be carried in a header which carries an OSPF level

   - Configuration
     It is necessary for routers to know when hierarchical label
     switching is being used.

4.5.3 Some Restrictions on Use of Hierarchical MPLS

   Consider the example in figure 1. In this case, the BGP-level
   label is encoded by router R1. Label switching is employed for
   packet forwarding at R2, R3, R8, and R9. This is only possible if
   R1 knows the right label to use, implying that the granularity
   used in mapping packets to forwarding equivalence classes is the
   same at routers R2, R3, R8, and R9.

   We can consider some specific examples to illustrate the issue:

   Suppose that the destination host is within domain 3. In this
   case, it is very likely that router R9 will forward the packet
   based on a finer grain than was used previously. For example, a
   relatively short address prefix may be used for advertising the
   addresses reachable in domain 3, while longer (more specific)
   address prefixes may be used for specific areas or subnets within
   domain 3. In this case router R1 may assign a BGP level label to
   the packet, and label based forwarding at the BGP level may be
   used by routers R1, R2, R3, and R8. However, router R9 will need
   to make use of layer 3 forwarding.

   Alternatively, suppose that domain 3 is an Internet Service
   Provider, which offers service to multiple routing domains.
   Suppose that in this case domain 3 makes use of a single CIDR
   address block (based on a single address prefix), with smaller
   address blocks (corresponding to longer address prefixes) assigned
   to each of multiple domains who get their Internet service from
   domain 3. Suppose that the destination for a particular IP packet
   is contained in one of these smaller domains whose addresses are

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   contained in the larger address block assigned to and administered
   by domain 3. Again in this case router R9 will need to make use of
   label based forwarding.

   Let's consider another possible complication: Suppose that router
   R1 is an MPLS node, but that some of the internal routers within
   domain 1 do not know about MPLS. In this case, suppose that R1
   encapsulates an IP packet in an MPLS header in order to carry the
   BGP level label. In this case the non-MPLS-capable routers within
   domain 1 will not know what to do with the MPLS header. This
   implies that MPLS can be used at a higher level (such as between
   the border routers R1 and R2 in our example) only if either the
   lower level routers (such as the routers within domain 1)are also
   using MPLS, or the MPLS header is itself encapsulated within an IP
   header for transmission across the domain.

   These examples imply that there are some cases where IP forwarding
   will be required in a hierarchy. While hierarchical MPLS may be
   useful in many cases, it does not replace layer 3 forwarding.

4.5.4 The Relationship between MPLS hierarchy and Routing Hierarchy Stacked Labels in a Flat Routing Environment

   The label stacking mechanism can be useful in some scenarios
   independent of routing hierarchy.

   The basic concept of stacking is to provide a mechanism to
   segregate streams within a switched path. Under normal operation,
   when packets are encapsulated into a single L2 header, if multiple
   streams are forwarded into a switched path, it will require L3
   processing to segregate a certain stream at the end of the
   switched path. The stacking mechanism provides an easy way to
   maintain the identity of various streams which are merged into a
   single switched path.

   One useful application of this technique is in Virtual Private
   Networks. The packets can be switched both at the ingress and
   egress nodes of the provider network. A packet coming in at one
   end of a customer network contains an encapsulated header with the
   VPN label. At the VPN ingress node, the header is "popped", to
   provide the label for switching through the VPN. Further, this
   header is then "pushed" with an encapsulation of the far end
   customer label. At the VPN egress node, the packet header is
   "popped" again, and the new header provides the label for
   switching through the customer site. This enables one to provide
   customers with benefits of VPN with end-to-end switching for
   optimal performance.

   Another interesting use can be in conjunction with RSVP flows. In

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   RSVP, senders flows can be logically merged under a single
   resource reservation using the Shared and the Wildcard filters.
   The stacking mechanism can be used to merge flows into a single
   label and the shared QoS can be applied to the single label on top
   of the stack. Since sender flows within the merged switched path
   maintain their identity, it is easy to demerge at a downstream
   node without requiring L3 processing of the packets. Another
   similar application can be merging of several premium service
   flows with similar QoS into a single switched path. This helps in
   conserving labels in backbone of a large networks.

   Yet another useful application can be DVMRP tunnels similar in
   concept to the DVMRP tunnels used in the existing Mbone. The
   ingress node to the DVMRP switched tunnels encapsulates the label
   learned from the egress node of the DVMRP tunnel for a particular
   (S,G) pair before forwarding packets into the DVMRP tunnel. The
   egress node of the tunnel just pops the top label and switches the
   packet based on the interior label.

   Note that the use of tunnels can be also quite beneficial in a non-
   hierarchical environment. Take for example the case where a domain
   contains a subset of MPLS nodes. The MPLS egress can advertise
   labels for the routes which are within the domain, but are
   external to the MPLS core. The ingress node can encapsulate
   packets for these destinations within the header for the
   aggregated switched path that crosses the MPLS domain.

   It is not evident if this technique has any useful application in
   a flat routing domain, but can be used in conjunction with
   explicit routing when providing specialized services. The multiple
   levels of encapsulation can also be used like loose source
   routing. Flat labels in a Hierarchical Routing Environment

   It is also possible in some environments to use a single level of
   label in a network using hierarchical routing. This is for example
   possible in the case of a two level OSPF network in which the
   primary purpose of the network is to support external routes.
   Specifically, (depending upon the types of area hierarchy used)
   OSPF allows external routes to be advertised throughout an OSPF
   routing domain, with each external route associated with the
   routerID of the router with reachability to the specific route.
   This implies that it is possible to set up an LSP to every router
   in the routing domain, and then use the LSP for packets destined
   to the associated external routes. Configuration of the Hierarchy

   The possibility of having a variety of different relationships

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   between the routing hierarchy and the MPLS hierarchy leads to an
   obvious question: How is the relationship between the two
   hierarchies to be determined? At first glance it would seem that
   this generality leads to a relatively complex configuration issue,
   and it could be difficult to ensure consistent configuration of
   the network.

   One possible solution is to have the MPLS hierarchy default to
   using the same hierarchy structure as is used for routing, with
   each area and domain boundary (as used by routing) also implying
   an MPLS domain boundary. This would allow the normal default
   operation to conform to the type of operation that we might expect
   to be used in most situations, and would allow a common means of
   interoperation which we would expect all vendors of MPLS compliant
   equipment to support.

4.5.5 Some Advantages of Hierarchical MPLS

   The use of hierarchical MPLS allows the routers internal to a
   transit routing domain to be isolated from the BGP-level routing
   information. In our example network, routers R4, R5, R6, and R7
   can forward packets based solely on the lower level label. These
   internal routers do not need to know anything at all about higher
   level IP routing. Note that this advantage is not available in
   conventional IP forwarding: If the internal routers within a
   routing domain forward IP packets based on the destination IP
   address, then the internal routers need to know which route to use
   for any particular destination IP address. By combining
   hierarchical routing with label stacks MPLS is able to decouple
   the exterior and interior protocols. MPLS switches within a domain
   (interior switches) need only carry the reachability information
   for nodes in the domain. The MPLS border switches for the domain
   still, of course, carry the external routes.

   Use of hierarchical MPLS also extends the simpler forwarding
   offered by MPLS to domain boundary routers.

   MPLS places no bound on the number of labels that may be present
   in a label stack. In principle this means that MPLS can support
   multiple levels of routing hierarchy.

4.6 Interoperation of MPLS systems with "Conventional" ATM

   If we consider the implementation of MPLS on ATM switches we can
   imagine several possibilities.

   We might remove ATM Forum control plane completely. This is the
   approach taken by Ipsilon in their IP Switching approach, and
   allows ATM switches to operate as MPLS LSRs.

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   Alternately, we could build a system that supports a "Ships in the
   night" (SIN) mode of operation where the ATM Forum and MPLS
   control planes both run on the same hardware but are isolated from
   each other, ie, they do not interact. This allows a single device
   to simultaneously operate as both an MPLS LSR and an ATM switch.

   We feel that the MPLS architecture should allow both of these
   models. We note, however, that neither of them addresses the issue
   of operation of MPLS over a public ATM network, ie over a network
   that supports tariffed access to PVCs and ATM Forum SVCs. Because
   public ATM service exists and will, presumably, become more
   pervasive in the future we feel that another model needs to be
   included in the architecture and be supported by MPLS. We call
   this model the "integrated" model. In essence it is the same as
   the SIN model but without the restriction that the two control
   planes are isolated. In the integrated model the MPLS control
   plane is able to use the ATM control plane to setup SVCs as
   needed. An example of this integrated model that allows the
   coexistence and interoperation between ATM and MPLS is the CSR
   proposal from Toshiba.

   Note that there is a distinction relevant to the protocol
   specification process between the SIN and the Integrated approach.
   SIN does not require specification other than to require that it
   be transparent to both the MPLS and ATM control planes (ie neither
   should know of the others existence). Realization of SIN on a
   particular machine is purely an engineering challenge for the
   implementers. The Integrated model on the other hand requires
   specification of procedures for the use of SVCs and association of
   labels with them.

4.7 Multicast

   This section is FFS.

4.8 Multipath

   Many IP routing protocols support the notion of equal-cost
   multipath routes, in which a router maintains multiple next hops
   for one destination prefix when two or more equal-cost paths to
   the prefix exist. There are a few possible approaches for handling
   multipath with MPLS.

   In this discussion we will use the term "multipath node" to mean a
   node which is keeping track of multiple switched paths from itself
   for a single destination.

   The first approach maintains a separate switched path from each
   ingress node via one or more multipath nodes to a merge point.
   This requires MPLS to distinguish the separate switched paths, so

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   that learning of a new switched path is not misinterpreted as a
   replacement of the same switched path. This also requires an
   ingress MPLS node be capable of distributing the traffic among the
   multiple switched paths. This approach preserves switching
   performance, but at a cost of proliferating the number of switched
   paths. For example, each switched path consumes a distinct label.

   The second approach establishes only one switched path from any
   one ingress node to a destination. However, when the paths from
   two different ingress nodes happen to arrive at the same node,
   that node may use different paths for each (implying that the node
   becomes a multipath node). Thus the switched path chosen by the
   multipath node may assign a different downstream path to each
   incoming stream. This conserves switched paths and maintains
   switching performance, but cannot balance loads across downstream
   links as well as the other approaches, even if switched paths are
   selectively assigned. An issue with this approach is that the L2
   path may be different from the normal L3 path, as traffic that
   otherwise would have taken multiple distinct paths is forced onto
   a single path.

   The third approach allows a single stream arriving at a multipath
   node to be split into multiple streams, by using L3 forwarding at
   the multipath node. For example, the multipath node might choose
   to use a hash function on the source and destination IP addresses,
   in order to avoid misordering packets between any one IP source
   and destination. This approach conserves switched paths at the
   cost of switching performance.

4.9 Host Interactions

   There are a range of options for host interaction with MPLS:

   The most straightforward approach is no host involvement. Thus
   host operation may be completely independent of MPLS, rather hosts
   operate according to other IP standards. If there is no host
   involvement then this implies that the first hop requires an L3

   If the host is ATM attached and doing NHRP, then this would allow
   the host to set up a Virtual Circuit to a router. However this
   brings up a range of issues as was discussed in section 4.4
   ("interoperation with NHRP").

   On the ingress side, it is reasonable to consider having the first
   hop LSR provide labels to the hosts, and thus have hosts attach
   labels for packets that they transmit. This could allow the first
   hop LSR to avoid an L3 lookup. It is reasonable here to have the
   host request labels only when needed, rather than require the host
   to remember all labels assigned for use in the network.

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   On the egress side, it is questionable whether hosts should be
   involved. For scaling reasons, it would be undesirable to use a
   different label for reaching each host.

4.10 Explicit Routing

   There are two options for Route Selection: (1) Hop by hop routing,
   and (2) Explicit 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 explicit routing LSP may be chosen
   by configuration, or by an algorithm performed 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).

   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 L3 packet. This implies that explicit
   routing with MPLS is relatively efficient (when compared with the
   efficiency of explicit routing for pure datagrams).

   Explicit routing may be useful for a number of purposes such as
   allowing policy routing and/or facilitating traffic engineering.

4.10.1 Establishment of Point to Point Explicitly Routed LSPs

   In order to establish a point to point explicitly routed LSP, the
   signaling messages used to set up the LSP must contain the
   explicit route. This implies that the LSP is set up in order
   either from the ingress to the egress, or from the egress to the

   One node needs to pick the explicit route: This may be done in at
   least two possible ways: (i) by configuration (eg, the explicit
   route may be chosen by an operator, or by a centralized server of
   some kind); (ii) By use of a routing protocol which allows the
   ingress and/or egress node to know the entire route to be
   followed. This would imply the use of a link state routing
   protocol (in which all nodes know the full topology) or of a path
   vector routing protocol (in which the ingress node is told the
   path as part of the normal operation of the routing protocol).

   Note: The normal operation of path vector routing protocols (such
   as BGP) does not provide the full set of routers along the path.
   This implies that either a partial source route only would be
   provided (implying that LSP setup would use a combination of hop

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   by hop and explicit routing), or it would be necessary to augment
   the protocol in order to provide the complete explicit route.

   In the point to point case, it is relatively straightforward to
   specify the route to use: This is indicated by providing the
   addresses of each LSR on the LSP.

4.10.2 Explicit and Hop by Hop routing: Avoiding Loops

   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 the explicit route is likely to
   follow a path which is inconsistent with the path followed by hop
   by hop routing. If some of the nodes along the path follow an
   explicit route but some of the nodes make use of hop by hop
   routing (and ignore the explicit route), then inconsistent routing
   may result and in some cases loops (or severely inefficient paths)
   may form. This implies that for any one LSP, there are two
   possible options: (i) The entire LSP may be hop by hop routed; or
   (ii) The entire LSP may be explicit routed.

   For this reason, it is important that if an explicit route is
   specified for setting up an LSP, then that route must be followed
   in setting up the LSP.

   There is a related issue when a link or node in the middle of an
   explicitly routed LSP breaks: In this case, the last operating
   node on the upstream part of the LSP will continue receiving
   packets, but will not be able to forward them along the explicitly
   routed LSP (since its next hop is no longer functioning). In this
   case, it is not in general safe for this node to forward the
   packets using L3 forwarding with hop by hop routing (unless loose
   source routing is present). Instead, the packets must be
   discarded, and the upstream partition of the explicitly routed LSP
   must be torn down.

   Where part of an Explicitly Routed LSP breaks, the node which
   originated the LSP needs to be told about this. For robustness
   reasons the MPLS protocol design should not assume that the
   routing protocol will tell the node which originated the LSP. For
   example, it is possible that a link may go down and come back up
   quickly enough that the routing protocol never declares the link
   down. Rather, an explicit MPLS mechanism is needed.

4.10.3 Merge and Explicit Routing

   Explicit Routing is slightly more complex with a multipoint to
   point LSP (ie, in the case that stream merge is used).

   In this case, it is not possible to specify the route for the LSP

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   as a simple list of LSRs (since the LSP does not consist of a
   simple sequence of LSRs). Rather the explicit route must specify a
   tree. There are several ways that this may be accomplished.
   Details are outside the scope of this document.

4.10.4 Using Explicit Routing for Traffic Engineering

   In the Internet today it is relatively common for ISPs to make use
   of a Frame Relay or ATM core, which interconnects a number of IP
   routers. The primary reason for use of a switching (L2) core is to
   make use of low cost equipment which provides very high speed
   forwarding. However, there is another very important reason for
   the use of a L2 core: In order to allow for Traffic Engineering.

   Traffic Engineering (also known as bandwidth management) refers to
   the process of managing the routes followed by user data traffic
   in a network in order to provide relatively equal and efficient
   loading of the resources in the network (ie, to ensure that the
   bandwidth on links and nodes are within the capabilities of the
   links and nodes).

   Some rudimentary level of traffic engineering can be accomplished
   with pure datagram routing and forwarding by adjusting the metrics
   assigned to links. For example, suppose that there is a given link
   in a network which tends to be overloaded on a long term basis.
   One option would be to manually configure an increased metric
   value for this link, in the hopes of moving some traffic onto
   alternate routes. This provides a rather crude method of traffic
   engineering and provides only limited results.

   Another method of traffic engineering is to manually configure
   multiple PVCs across a L2 core, and to adjust the route followed
   by each PVC in an attempt to equalize the load on different parts
   of the network. Where necessary, multiple PVCs may be configured
   between the same two nodes, in order to allow traffic to be split
   between different paths. In some topologies it is much easier to
   achieve efficient non-overlapping or minimally-overlapping paths
   via this method (with manually configured paths) than it would be
   with pure datagram forwarding. A similar ability can be achieved
   with MPLS via the use of manual configuration of the paths taken
   by LSPs.

   A related issue is the decision on where merge is to occur. Note
   that once two streams merge into one stream (forwarded by a single
   label) then they cannot diverge again at that level of the MPLS
   hierarchy (ie, they cannot be bifurcated without looking at a
   higher level label or the IP header). Thus there may be times when
   it is desirable to explicitly NOT merge two streams even though
   they are to the same egress node and FEC. Non-merge may be
   appropriate either because the streams will want to diverge later

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   in the path (for example, to avoid overloading a particular
   downstream link), or because the streams may want to use different
   physical links in the case where multiple slower physical links
   are being aggregated into a single logical link for the purpose of
   IP routing.

   As a network grows to a very large size (on the order of hundreds
   of LSRs), it becomes increasingly difficult to handle the
   assignment of all routes via manual configuration. However,
   explicit routing allows several alternatives:

  1. Partial Configuration: One option is to use
     automatic/dynamic routing for most of the paths through
     the network, but then manually configure some routes. For
     example, suppose that full dynamic routing would result in
     a particular link being overloaded. One of the LSPs which
     uses that link could be selected and manually routed to
     use a different path.

  2. Central Computation: One option would be to provide long
     term network usage information to a single central
     management facility. That facility could then run a global
     optimization to compute a set of paths to use. Network
     management commands can be used to configure LSRs with the
     correct routes to use.

  3. Egress Computation: An egress node can run a computation
     which optimizes the path followed for traffic to itself.
     This cannot of course optimize the entire traffic load
     through the network, but can include optimization of
     traffic from multiple ingress's to one egress. The reason
     for optimizing traffic to a single egress, rather than
     from a single ingress, relates to the issue of when to
     merge: An ingress can never merge the traffic from itself
     to different egresses, but an egress can if desired chose
     to merge the traffic from multiple ingress's to itself.

4.11 TTL and Traceroute

   Traceroute is a useful method which is widely used for management
   of IP networks. It is therefore highly desirable for traceroute
   and TTL to be preserved in networks where MPLS is used. TTL can
   also be useful to minimize the impact of loops (ie, as an aid to
   loop survival).

   In cases where the MPLS shim header is used, and where the IP
   packets are normal Internet packets (ie, not part of a VPN), TTL
   can optionally be handled in a way which is semantically identical
   to operation in native IP networks. The ingress node, when
   encapsulating an IP packet in the MPLS shim header, copies the TTL

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   from the IP header to the MPLS Shim Header. LSRs decrement the
   TTL, and behave as normal IP routers in the case that the TTL
   reaches zero (ie, discard the IP packet and return an ICMP error
   report). Egress routers copy the TTL from the MPLS shim header
   back to the IP header.

   Where multiple MPLS shim headers are used in a label stack, TTL
   can be handled in essentially the same manner. When a LSR pushes a
   new header onto the stack, the TTL is copied from the previous
   shim header to the new header. When an LSR pops a header off of
   the stack, TTL is copied in the other direction.

   Some carriers may choose to avoid exposing the topology (or even
   the diameter) of their networks to customers. One way to do this
   is to treat an entire LSP crossing the carrier network as a single
   hop from the point of view of IP forwarding. In this case the
   ingress router places a value in the TTL field of the shim header
   which is independent of the TTL value found from the IP header.
   Similarly the decapsulating router strips off the MPLS header and
   forwards based on the IP header, but does not copy TTL values.
   Routers which are in the middle of the LSP (neither ingress nor
   egress) decrement the TTL contained in the MPLS shim header, but
   do not return an error report if the TTL is expired.

   There is a problem with the handling of ICMP error reports when
   VPNs are supported using MPLS. In this case, the IP address space
   used in the IP packet (carried over the LSP) might be local to the
   VPN, and therefore might not be understood by the LSR which
   detects that the TTL has reached zero. In addition, core LSRs
   might not necessarily know which LSPs are supporting VPN traffic
   and which are supporting Internet traffic. For this reason in
   networks where VPNs are supported over MPLS, special precautions
   are needed. If the ingress node knows the path of the LSP, then it
   may discard the packet and return an ICMP error report (to the
   VPNs space) if the TTL is less than the length of the LSP.
   Alternatively, the TTL value used in the MPLS header may be
   independent of the TTL value in the IP header, and the entire LSP
   may be treated as a single hop from the perspective of datagram IP
   forwarding. Alternatively, ICMP error reports could be turned off
   in such networks.

   One other potential solution to the ICMP error reporting problem
   is to use "bi-directional" LSPs. In this case, two LSPs may be
   created with the same endpoints, but which carry packets in
   opposite directions. These two LSPs are logically coupled
   together; that is, one LSP carries traffic from an originating
   node to a destination node, while the other carries traffic from
   the destination node to the originating node[TRAFENG]. When a
   packet has to be discarded that had been flowing on the LSP in one
   direction, the error report can be returned on the matching LSP in

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   the other direction. This is true even when the IP address space
   encapsulated inside the LSP is one which the LSR does not
   otherwise understand.

   MPLS may also be used over L2 technologies which do not have TTL
   values (specifically ATM and Frame Relay). In this case, TTL and
   Traceroute may still be supported in some specific situations.

   In our discussion we will assume that the MPLS encapsulation for
   operation of MPLS over ATM and Frame Relay media always use a shim
   header. Thus the packet would consist of an IP packet encapsulated
   inside an MPLS shim header, which would in turn be encapsulated
   for transmission over ATM or Frame Relay (eg, the IP packet and
   MPLS shim header may be encapsulated in an AAL5 frame, which would
   in turn be encapsulated inside ATM cells). If the shim header is
   not used, when manipulations of the TTL in the shim header as
   described below would be replaced by manipulations of the TTL
   inside the IP header.

   The most straightforward case is one where ATM or Frame Relay is
   used for the entire path of the LSP, and where the ingress LSR
   knows the entire path of the LSP (for example, this may occur when
   the LSP is set up based on complete source routing). In this case
   the ingress router decrements the TTL by the length of the LSP. If
   the TTL reaches zero or a negative number, then the IP packet is
   discarded and an ICMP error report is returned by the ingress
   router, but with a source address which indicates the node at
   which the TTL would have expired. In this case in principle the
   TTL which is decremented could be either the one in the IP header
   or the one in the MPLS header. However, it allows more uniform
   operation (compared to other situations) if the TTL in the shim
   header is decremented by the ingress router by the length of the
   path, and then the egress router copies the TTL from the MPLS
   header into the IP packet.

   In some cases the length of the LSP might be known, but not the
   exact identity of the LSRs along the path (eg, the LSP is set up
   via ordered control). In this case the TTL can be decremented as
   above, but if the TTL would expire the packet could be forwarded
   by some "out of band" (control processor to control processor)
   path in order to get the packet to the LSR at which the TTL will
   reach zero.

   There may be cases where part of the LSP traverses ATM or Frame
   Relay links (using an ATM or Frame Relay header), and part
   traverses other media (using the shim header).

   Some of the issues which come up in this situation are best
   illustrated through use of an example. Suppose that in figure 2,
   an LSP goes from R1 to R8. Thus R1 is the ingress LSR, and R8 is

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   the egress LSR for this particular LSP. LSRs R3 and R6 have both
   ATM interfaces and non-ATM interfaces. Thus the MPLS shim header
   is used on the link from R1 to R2, and from R2 to R3. ATM is used
   on the links from R3 to R4, R4 to R5, and R5 to R6. Finally, the
   shim header is again used on the links from R6 to R7, and R7 to

        .                .             .              .
        .                .             .              .
        .R1------R2------R3           R6-----R7-----R8.
        .                . \          /.              .
        .                .  R4------R5 .              .
        .                .             .              .
        .  Shim Header   .     ATM     .  Shim Header .

         Figure 2: LSP spanning ATM and Shim Header Media

   If egress-initiated ordered control is used, then it is possible
   that when the LSP is first set up the signaling protocol could
   keep track of the number of hops to the next LSR that will use a
   shim header (and which therefore understands TTL). In our example
   R3 could therefore know that it is three hops to R6 (which is the
   next router which will use a shim header containing a TTL value).
   R3 can therefore decrement the TTL by the appropriate value (3),
   and return an error report if the TTL will expire.

   If ingress-initiated ordered control or independent control is
   used, then it is not clear how R3 will know the identity of the
   next LSR which understands TTL (ie, will use a shim header instead
   of an ATM or frame relay header). For example, suppose that
   complete explicit routing with ingress control is used. In this
   case R3 will know the complete path to the egress (R8), but will
   not know which downstream links use ATM media and which uses the
   shim header. Thus R3 will know that R6 is a downstream LSR for
   this LSP, but will not know that R6 is the specific LSR which
   removes the packet from the ATM media.

   R6 will forward the packet based on the incoming label implicit in
   the VPI/VCI from the ATM media, plus the existing shim header.
   Thus the TTL used at this point will be based on that received in
   the shim header. This implies that the TTL value in the shim
   header needs to be valid, which in turn implies that R3 needs to
   adjust the TTL value in the shim header to account for the length
   of the path from R3 to R6.

4.12 LSP Control: Ordered versus Independent

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   There is a choice to be made regarding whether the initial setup
   of LSPs will be in an ordered mode, where the LSP is initiated by
   the egress node, or independently by each individual node.

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

   When LSP control is done in an ordered manner, then the egress
   node passes label bindings to its neighbors corresponding to any
   FECs which leave the MPLS network at that egress node. Other nodes
   must 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 independent control, since each LSR is 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

   This may appear to be an advantage of ordered LSP control (since
   with egress control the initial label assignments "bubble up" from
   the egress to upstream nodes, and consistency is therefore easy to
   ensure). However, even with ordered 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 ordered or independent 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.

   Ordered control and independent control can interwork in a very
   straightforward manner: 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.

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   The difference between the two techniques therefore becomes a
   tradeoff between avoiding a short period of initial thrashing on
   startup (in the sense of avoiding the need to withdraw
   inconsistent labels which may have been assigned using local
   control) versus the imposition of a short delay on initial startup
   (while waiting for the initial label assignments to bubble up from
   downstream). The protocol mechanisms which need to be defined are
   the same in either case, and the steady state operation is the
   same in either case.

5. Security

   Security in a network using MPLS should be relatively similar to
   security in a normal IP network.

   Routing in an MPLS network uses precisely the same IP routing
   protocols as are currently used with IP. This implies that route
   filtering is unchanged from current operation. Similarly, the
   security of the routing protocols is not effected by the use of

   Packet filtering also may be done as in normal IP. This will
   require either (i) that label switching be terminated prior to any
   firewalls performing packet filtering (in which case a separate
   instance of label switching may optionally be started after the
   firewall); or (ii) that firewalls "look past the labels", in order
   to inspect the entire IP packet contents. In this latter case note
   that the label may imply semantics greater than that contained in
   the packet header: In particular, a particular label value may
   imply that the packet is to take a particular path after the
   firewall. In environments in which this is considered to be a
   security issue it may be desirable to terminate the label prior to
   the firewall.

   Note that in principle labels could be used to speed up the
   operation of firewalls: In particular, the label could be used as
   an index into a table which indicates the characteristics that the
   packet needs to have in order to pass through the firewall.
   Depending upon implementation considerations matching the contents
   of the packet to the contents of the table may be quicker than
   parsing the packet in the absence of the label.


   [ARCH]      "Multiprotocol Label Switching Architecture", E.
               Rosen, A. Viswanathan, R. Callon, work in progress,
               <draft-ietf-mpls-arch-06.txt>, August 1999.

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   [ARIS]      "ARIS: Aggregate Route-Based IP Switching", A.
               Viswanathan, N. Feldman, R. Boivie, R. Woundy, IBM
               Technical Report TR 29.2353, February 1998.

   [ARIS-PROT] "ARIS Protocol Specification", N. Feldman, A.
               Viswanathan, IBM Technical Report TR 29.2368, March

   [ATM]       "MPLS using LDP and ATM VC Switching", B. Davie, P.
               Doolan, J. Lawrence, K. McGloghrie, Y. Rekhter, E.
               Rosen, G. Swallow, work in progress, Internet Draft
               <draft-ietf-mpls-atm-02.txt>, April 1999.

   [ATMVP]     "MPLS using ATM VP Switching", N. Feldman, B.
               Jamoussi, S. Komandur, A. Viswanathan, T. Worster,
               work in progress, Internet Draft <draft-feldman-mpls-
               atmvp-00.txt>, February, 1999.

   [CR-LDP]    "Constraint-Based LSP Setup using LDP", B. Jamoussi,
               et. al., work in progress, <draft-ietf-mpls-cr-ldp-
               02.txt>, August 1999.

   [ENCAP]     "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D.
               Tappan, D. Farinacci, G. Fedorkow, T. Li, A. Conta,
               work in progress, Internet Draft <draft-ietf-mpls-
               label-encaps-07.txt>, September 1999.

   [FANP]      "Internetworking Based on Cell Switch Router-
               Architecture and Protocol Overview", Y. Katsube, K.
               Nagami, S. Matsuzawa, H. Esaki, Proceedings of the
               IEEE, Vol. 85, No. 12, December, 1997.

   [FR]        "Use of Label Switching on Frame Relay Networks", A.
               Conta, P. Doolan, A. Malis, work in progress, Internet
               Draft <draft-ietf-mpls-fr-03.txt>, November 1998.

   [IPNAV]     "IP Switching for Scalable IP Services", H. Ahmed, R.
               Callon, A. Malis, J. Moy, Proceedings of the IEEE,
               Vol. 85, No. 12, December 1997.

   [LDP]       "LDP Specification", L. Anderson, P. Doolan, N.
               Feldman, A. Fredette, B. Thomas, work in progress,
               <draft-ietf-mpls-ldp-05.txt>, June 1999.

   [LOOP-COLOR]     "MPLS Loop Prevention Mechanism", Y. Ohba, Y.
               Katsube, E. Rosen, P. Doolan, work in progress,
               Internet Draft <draft-ietf-mpls-loop-prevention-
               01.txt>, May 1999.

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   [PNNI]      "ATM Forum Private Network-Network Interface
               Specification, Version 1.0", ATM Forum af-pnni-
               0055.000, March 1996.

   [RFC1583]   "OSPF version 2", J. Moy, RFC 1583, March 1994.

   [RFC1663]   "Integrated Services in the Internet Architecture: an
               Overview", R. Braden et al, RFC 1633, June 1994.

   [RFC1771]   "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter, T.
               Li, RFC1771, March 1995.

   [RFC1953]   "Ipsilon Flow Management Protocol Specification for
               IPv4 Version 1.0", P. Newman et al., RFC 1953, May

   [RFC2098]   "Toshiba's Router Architecture Extensions for ATM:
               Overview", Y. Katsube, K. Nagami, H. Esaki, RFC2098.

   [RFC2105]   "Cisco Systems' Tag Switching Architecture Overview",
               Y. Rekhter, B. Davie, D. Katz, E. Rosen, G. Swallow,
               RFC2105, February, 1997.

   [RFC2205]   "Resource ReSerVation Protocol (RSVP) Version 1
               Functional Specification", R. Braden, L. Zhang, S.
               Berson, S. Herzog, S. Jamin, RFC2205, September 1997.

   [RFC2332]   "NBMA Next Hop Resolution Protocol", J. Luciani, D.
               Katz, D. Piscitello, B. Cole, N. Doraswamy, RFC2332,
               USC/Information Sciences Institute, April 1998

   [RSVP-LSP]  "Extensions to RSVP for LSP Tunnels", D. Awduche, L.
               Berger, D. Gan, T. Li, G. Swallow, V. Srinivasan, work
               in progress, Internet Draft <draft-ietf-mpls-rsvp-lsp-
               tunnel-03.txt>, September 1999.

   [TRAFENG]   "Requirements for Traffic Engineering Over MPLS", D.
               Awduche, J. Malcolm, J. Agogbua, M. O'Dell, J.
               McManus, work in progress, Internet Draft <draft-ietf-
               mpls-traffic-eng-01.txt>, June 1999.

Author's Addresses

        Ross Callon
        IronBridge Networks
        55 Hayden Avenue,
        Lexington, MA 02173

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        Paul Doolan
        Ennovate Networks
        60 Codman Hill Road
        Boxborough, MA 01719

        Nancy Feldman
        IBM Corp.
        30 Saw Mill River Rd.
        Hawthorne NY 10532

        Andre Fredette
        Nortel Networks
        3 Federal Street
        Billerica, MA 01821

        George Swallow
        Cisco Systems, Inc
        250 Apollo Drive
        Chelmsford, MA 01824

        Arun Viswanathan
        Lucent Technologies
        101 Crawford Corner Rd., #4D-537
        Holmdel, NJ 07733

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