MPLS using LDP and ATM VC Switching

Network Working Group                                           B. Davie
Request for Comments: 3035                                   J. Lawrence
Category: Standards Track                                  K. McCloghrie
                                                                E. Rosen
                                                              G. Swallow
                                                     Cisco Systems, Inc.
                                                              Y. Rekhter
                                                        Juniper Networks
                                                               P. Doolan
                                                 Ennovate Networks, Inc.
                                                            January 2001


                  MPLS using LDP and ATM VC Switching

Status of this Memo

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

Copyright Notice

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

Abstract

   The Multiprotocol Label Switching (MPLS) Architecture [1] discusses a
   way in which Asynchronous Transfer Mode (ATM) switches may be used as
   Label Switching Routers.  The ATM switches run network layer routing
   algorithms (such as Open Shortest Path First (OSPF), Intermediate
   System to Intermediate System (IS-IS), etc.), and their data
   forwarding is based on the results of these routing algorithms.  No
   ATM-specific routing or addressing is needed.  ATM switches used in
   this way are known as ATM-LSRs (Label Switching Routers).

   This document extends and clarifies the relevant portions of [1] and
   [2] by specifying in more detail the procedures which to be used when
   distributing labels to or from ATM-LSRs, when those labels represent
   Forwarding Equivalence Classes (FECs, see [1]) for which the routes
   are determined on a hop-by-hop basis by network layer routing
   algorithms.

   This document also specifies the MPLS encapsulation to be used when
   sending labeled packets to or from ATM-LSRs, and in that respect is a
   companion document to [3].

Table of Contents

    1      Introduction  ...........................................   2
    2      Specification of Requirements  ..........................   3
    3      Definitions  ............................................   3
    4      Special Characteristics of ATM Switches  ................   4
    5      Label Switching Control Component for ATM  ..............   5
    6      Hybrid Switches (Ships in the Night)  ...................   5
    7      Use of  VPI/VCIs  .......................................   5
    7.1    Direct Connections  .....................................   6
    7.2    Connections via an ATM VP  ..............................   7
    7.3    Connections via an ATM SVC  .............................   7
    8      Label Distribution and Maintenance Procedures  ..........   7
    8.1    Edge LSR Behavior  ......................................   8
    8.2    Conventional ATM Switches (non-VC-merge)  ...............   9
    8.3    VC-merge-capable ATM Switches  ..........................  11
    9      Encapsulation  ..........................................  12
   10      TTL Manipulation  .......................................  13
   11      Optional Loop Detection: Distributing Path Vectors  .....  15
   11.1    When to Send Path Vectors Downstream  ...................  15
   11.2    When to Send Path Vectors Upstream  .....................  16
   12      Security Considerations  ................................  17
   13      Intellectual Property Considerations  ...................  17
   14      References  .............................................  18
   15      Acknowledgments  ........................................  18
   16      Authors' Addresses  .....................................  18
   17      Full Copyright Statement  ...............................  20

1. Introduction

   The MPLS Architecture [1] discusses the way in which ATM switches may
   be used as Label Switching Routers.  The ATM switches run network
   layer routing algorithms (such as OSPF, IS-IS, etc.), and their data
   forwarding is based on the results of these routing algorithms. No
   ATM-specific routing or addressing is needed.  ATM switches used in
   this way are known as ATM-LSRs.

   This document extends and clarifies the relevant portions of [1] and
   [2] by specifying in more detail the procedures which are to be used
   for distributing labels to or from ATM-LSRs, when those labels
   represent Forwarding Equivalence Classes (FECs, see [1]) for which
   the routes are determined on a hop-by-hop basis by network layer
   routing algorithms.  The label distribution technique described here
   is referred to in [1] as "downstream-on-demand".  This label
   distribution technique MUST be used by ATM-LSRs that are not capable
   of "VC merge" (defined in section 3), and is OPTIONAL for ATM-LSRs
   that are capable of VC merge.

   This document does NOT specify the label distribution techniques to
   be used in the following cases:

      -  the routes are explicitly chosen before label distribution
         begins, instead of being chosen on a hop-by-hop basis as label
         distribution proceeds,

      -  the routes are intended to diverge in any way from the routes
         chosen by the conventional hop-by-hop routing at any time,

      -  the labels represent FECs that consist of multicast packets,

      -  the LSRs use "VP merge".

   Further statements made in this document about ATM-LSR label
   distribution do not necessarily apply in these cases.

   This document also specifies the MPLS encapsulation to be used when
   sending labeled packets to or from ATM-LSRs, and in that respect is a
   companion document to [3].  The specified encapsulation is to be used
   for multicast or explicitly routed labeled packets as well.

   This document uses terminology from [1].

2. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

3. Definitions

   A Label Switching Router (LSR) is a device which implements the label
   switching control and forwarding components described in [1].

   A label switching controlled ATM (LC-ATM) interface is an ATM
   interface controlled by the label switching control component.  When
   a packet traversing such an interface is received, it is treated as a
   labeled packet.  The packet's top label is inferred either from the
   contents of the VCI field or the combined contents of the VPI and VCI
   fields.  Any two LDP peers which are connected via an LC-ATM
   interface will use LDP negotiations to determine which of these cases
   is applicable to that interface.

   An ATM-LSR is a LSR with a number of LC-ATM interfaces which forwards
   cells between these interfaces, using labels carried in the VCI or
   VPI/VCI field, without reassembling the cells into frames before
   forwarding.

   A frame-based LSR is a LSR which forwards complete frames between its
   interfaces.  Note that such a LSR may have zero, one or more LC-ATM
   interfaces.

   Sometimes a single box may behave as an ATM-LSR with respect to
   certain pairs of interfaces, but may behave as a frame-based LSR with
   respect to other pairs.  For example, an ATM switch with an ethernet
   interface may function as an ATM-LSR when forwarding cells between
   its LC-ATM interfaces, but may function as a frame-based LSR when
   forwarding frames from its ethernet to one of its LC-ATM interfaces.
   In such cases, one can consider the two functions (ATM-LSR and
   frame-based LSR) as being coresident in a single box.

   It is intended that an LC-ATM interface be used to connect two ATM-
   LSRs, or to connect an ATM-LSR to a frame-based LSR.  The use of an
   LC-ATM interface to connect two frame-based LSRs is not considered in
   this document.

   An ATM-LSR domain is a set of ATM-LSRs which are mutually
   interconnected by LC-ATM interfaces.

   The Edge Set of an ATM-LSR domain is the set of frame-based LSRs
   which are connected to members of the domain by LC-ATM interfaces.  A
   frame-based LSR which is a member of an Edge Set of an ATM-LSR domain
   may be called an Edge LSR.

   VC-merge is the process by which a switch receives cells on several
   incoming VCIs and transmits them on a single outgoing VCI without
   causing the cells of different AAL5 PDUs to become interleaved.

4. Special Characteristics of ATM Switches

   While the MPLS architecture permits considerable flexibility in LSR
   implementation, an ATM-LSR is constrained by the capabilities of the
   (possibly pre-existing) hardware and the restrictions on such matters
   as cell format imposed by ATM standards.  Because of these
   constraints, some special procedures are required for ATM-LSRs.

   Some of the key features of ATM switches that affect their behavior
   as LSRs are:

      -  the label swapping function is performed on fields (the VCI
         and/or VPI) in the cell header; this dictates the size and
         placement of the label(s) in a packet.

      -  multipoint-to-point and multipoint-to-multipoint VCs are
         generally not supported.  This means that most switches cannot
         support 'VC-merge' as defined above.

      -  there is generally no capability to perform a 'TTL-decrement'
         function as is performed on IP headers in routers.

   This document describes ways of applying label switching to ATM
   switches which work within these constraints.

5. Label Switching Control Component for ATM

   To support label switching an ATM switch MUST implement the control
   component of label switching.  This consists primarily of label
   allocation, distribution, and maintenance procedures.  Label binding
   information is communicated by several mechanisms, notably the Label
   Distribution Protocol (LDP) [2].  This document imposes certain
   requirements on the LDP.

   This document considers only the case where the label switching
   control component uses information learned directly from network
   layer routing protocols.  It is presupposed that the switch
   participates as a peer in these protocols (e.g., OSPF, IS-IS).

   In some cases, LSRs make use of other protocols (e.g., RSVP, PIM,
   BGP) to distribute label bindings.  In these cases, an ATM-LSR would
   need to participate in these protocols.  However, these are not
   explicitly considered in this document.

   Support of label switching on an ATM switch does NOT require the
   switch to support the ATM control component defined by the ITU and
   ATM Forum (e.g., UNI, PNNI).  An ATM-LSR may OPTIONALLY respond to
   OAM cells.

6. Hybrid Switches (Ships in the Night)

   The existence of the label switching control component on an ATM
   switch does not preclude the ability to support the ATM control
   component defined by the ITU and ATM Forum on the same switch and the
   same interfaces.  The two control components, label switching and the
   ITU/ATM Forum defined, would operate independently.

   Definition of how such a device operates is beyond the scope of this
   document.  However, only a small amount of information needs to be
   consistent between the two control components, such as the portions
   of the VPI/VCI space which are available to each component.

7. Use of  VPI/VCIs

   Label switching is accomplished by associating labels with Forwarding
   Equivalence Classes, and using the label value to forward packets,
   including determining the value of any replacement label.  See [1]
   for further details.  In an ATM-LSR, the label is carried in the
   VPI/VCI field, or, when two ATM-LSRs are connected via an ATM
   "Virtual Path", in the VCI field.

   Labeled packets MUST be transmitted using the null encapsulation, as
   defined in Section 6.1 of RFC 2684 [5].

   In addition, if two LDP peers are connected via an LC-ATM interface,
   a non-MPLS connection, capable of carrying unlabelled IP packets,
   MUST be available.  This non-MPLS connection is used to carry LDP
   packets between the two peers, and MAY also be used (but is not
   required to be used) for other unlabeled packets (such as OSPF
   packets, etc.).  The LLC/SNAP encapsulation of RFC 2684 [5] MUST be
   used on the non-MPLS connection.

   It SHOULD be possible to configure an LC-ATM interface with
   additional VPI/VCIs that are used to carry control information or
   non-labelled packets.  In that case, the VCI values MUST NOT be in
   the 0-32 range.  These may use either the null encapsulation, as
   defined in Section 6.1 of RFC 2684 [5], or the LLC/SNAP
   encapsulation, as defined in Section 5.1 of RFC 2684 [5].

7.1. Direct Connections

   We say that two LSRs are "directly connected" over an LC-ATM
   interface if all cells transmitted out that interface by one LSR will
   reach the other, and there are no ATM switches between the two LSRs.

   When two LSRs are directly connected via an LC-ATM interface, they
   jointly control the allocation of VPIs/VCIs on the interface
   connecting them.  They may agree to use the VPI/VCI field to encode a
   single label.

   The default VPI/VCI value for the non-MPLS connection is VPI 0, VCI
   32.  Other values can be configured, as long as both parties are
   aware of the configured value.

   A VPI/VCI value whose VCI part is in the range 0-32 inclusive MUST
   NOT be used as the encoding of a label.

   With the exception of these reserved values, the VPI/VCI values used
   in the two directions of the link MAY be treated as independent
   spaces.

   The allowable ranges of VCIs are communicated through LDP.

7.2. Connections via an ATM VP

   Sometimes it can be useful to treat two LSRs as adjacent (in some
   LSP) across an LC-ATM interface, even though the connection between
   them is made through an ATM "cloud" via an ATM Virtual Path.  In this
   case, the VPI field is not available to MPLS, and the label MUST be
   encoded entirely within the VCI field.

   In this case, the default VCI value of the non-MPLS connection
   between the LSRs is 32.  Other values can be configured, as long as
   both parties are aware of the configured value.  The VPI is set to
   whatever is required to make use of the Virtual Path.

   A VPI/VCI value whose VCI part is in the range 0-32 inclusive MUST
   NOT be used as the encoding of a label.

   With the exception of these reserved values, the VPI/VCI values used
   in the two directions of the link MAY be treated as independent
   spaces.

   The allowable ranges of VPI/VCIs are communicated through LDP.  If
   more than one VPI is used for label switching, the allowable range of
   VCIs may be different for each VPI, and each range is communicated
   through LDP.

7.3. Connections via an ATM SVC

   Sometimes it may be useful to treat two LSRs as adjacent (in some
   LSP) across an LC-ATM interface, even though the connection between
   them is made through an ATM "cloud" via a set of ATM Switched Virtual
   Circuits.

   The current document does not specify the procedure for handling this
   case.  Such procedures can be found in [4].  The procedures described
   in [4] allow a VCID to be assigned to each such VC, and specify how
   LDP can be used used to bind a VCID to a FEC.  The top label of a
   received packet would then be inferred (via a one-to-one mapping)
   from the virtual circuit on which the packet arrived.  There would
   not be a default VPI or VCI value for the non-MPLS connection.

8. Label Distribution and Maintenance Procedures

   This document discusses the use of "downstream-on-demand" label
   distribution (see [1]) by ATM-LSRs.  These label distribution
   procedures MUST be used by ATM-LSRs that do not support VC-merge, and
   MAY also be used by ATM-LSRs that do support VC-merge.  The
   procedures differ somewhat in the two cases, however.  We therefore
   describe the two scenarios in turn.  We begin by describing the
   behavior of members of the Edge Set of an ATM-LSR domain; these "Edge
   LSRs" are not themselves ATM-LSRs, and their behavior is the same
   whether the domain contains VC-merge capable LSRs or not.

8.1. Edge LSR Behavior

   Consider a member of the Edge Set of an ATM-LSR domain.  Assume that,
   as a result of its routing calculations, it selects an ATM-LSR as the
   next hop of a certain FEC, and that the next hop is reachable via a
   LC-ATM interface.  The Edge LSR uses LDP to request a label binding
   for that FEC from the next hop.  The hop count field in the request
   is set to 1 (but see the next paragraph).  Once the Edge LSR receives
   the label binding information, it may use MPLS forwarding procedures
   to transmit packets in the specified FEC, using the specified label
   as an outgoing label.  (Or using the VPI/VCI that corresponds to the
   specified VCID as the outgoing label, if the VCID technique of [4] is
   being used.)

   Note: if the Edge LSR's previous hop is using downstream-on-demand
   label distribution to request a label from the Edge LSR for a
   particular FEC, and if the Edge LSR is not merging the LSP from that
   previous hop with any other LSP, and if the request from the previous
   hop has a hop count of h, then the hop count in the request issued by
   the Edge LSR should not be set to 1, but rather to h+1.

   The binding received by the edge LSR may contain a hop count, which
   represents the number of hops a packet will take to cross the ATM-LSR
   domain when using this label.  If there is a hop count associated
   with the binding, the ATM-LSR SHOULD adjust a data packet's TTL by
   this amount before transmitting the packet.  In any event, it MUST
   adjust a data packet's TTL by at least one before transmitting it.
   The procedures for doing so (in the case of IP packets) are specified
   in section 10.  The procedures for encapsulating the packets are
   specified in section 9.

   When a member of the Edge Set of the ATM-LSR domain receives a label
   binding request from an ATM-LSR, it allocates a label, and returns
   (via LDP) a binding containing the allocated label back to the peer
   that originated the request.  It sets the hop count in the binding to
   1.

   When a routing calculation causes an Edge LSR to change the next hop
   for a particular FEC, and the former next hop was in the ATM-LSR
   domain, the Edge LSR SHOULD notify the former next hop (via LDP) that
   the label binding associated with the FEC is no longer needed.

8.2. Conventional ATM Switches (non-VC-merge)

   When an ATM-LSR receives (via LDP) a label binding request for a
   certain FEC from a peer connected to the ATM-LSR over a LC-ATM
   interface, the ATM-LSR takes the following actions:

      -  it allocates a label,

      -  it requests (via LDP) a label binding from the next hop for
         that FEC;

      -  it returns (via LDP) a binding containing the allocated
         incoming label back to the peer that originated the request.

   For purposes of this procedure, we define a maximum hop count value
   MAXHOP.  MAXHOP has a default value of 255, but may be configured to
   a different value.

   The hop count field in the request that the ATM-LSR sends (to the
   next hop LSR) MUST be set to one more than the hop count field in the
   request that it received from the upstream LSR.  If the resulting hop
   count exceeds MAXHOP, the request MUST NOT be sent to the next hop,
   and the ATM-LSR MUST notify the upstream neighbor that its binding
   request cannot be satisfied.

   Otherwise, once the ATM-LSR receives the binding from the next hop,
   it begins using that label.

   The ATM-LSR MAY choose to wait for the request to be satisfied from
   downstream before returning the binding upstream.  This is a form of
   "ordered control" (as defined in [1] and [2]), in particular
   "ingress-initiated ordered control".  In this case, as long as the
   ATM-LSR receives from downstream a hop count which is greater than 0
   and less than MAXHOP, it MUST increment the hop count it receives
   from downstream and MUST include the result in the binding it returns
   upstream.  However, if the hop count exceeds MAXHOP, a label binding
   MUST NOT be passed upstream.  Rather, the upstream LDP peer MUST be
   informed that the requested label binding cannot be satisfied.  If
   the hop count received from downstream is 0, the hop count passed
   upstream should also be 0; this indicates that the actual hop count
   is unknown.

   Alternatively, the ATM-LSR MAY return the binding upstream without
   waiting for a binding from downstream ("independent" control, as
   defined in [1] and [2]).  In this case, it specifies a hop count of 0
   in the binding, indicating that the true hop count is unknown.  The
   correct value for hop count will be returned later, as described
   below.

   Note that an ATM-LSR, or a member of the edge set of an ATM-LSR
   domain, may receive multiple binding requests for the same FEC from
   the same ATM-LSR.  It MUST generate a new binding for each request
   (assuming adequate resources to do so), and retain any existing
   binding(s).  For each request received, an ATM-LSR MUST also generate
   a new binding request toward the next hop for the FEC.

   When a routing calculation causes an ATM-LSR to change the next hop
   for a FEC, the ATM-LSR MUST notify the former next hop (via LDP) that
   the label binding associated with the FEC is no longer needed.

   When a LSR receives a notification that a particular label binding is
   no longer needed, the LSR MAY deallocate the label associated with
   the binding, and destroy the binding.  In the case where an ATM-LSR
   receives such notification and destroys the binding, it MUST notify
   the next hop for the FEC that the label binding is no longer needed.
   If a LSR does not destroy the binding, it may re-use the binding only
   if it receives a request for the same FEC with the same hop count as
   the request that originally caused the binding to be created.

   When a route changes, the label bindings are re-established from the
   point where the route diverges from the previous route.   LSRs
   upstream of that point are (with one exception, noted below)
   oblivious to the change.

   Whenever a LSR changes its next hop for a particular FEC, if the new
   next hop is reachable via an LC-ATM interface, then for each label
   that it has bound to that FEC, and distributed upstream, it MUST
   request a new label binding from the new next hop.

   When an ATM-LSR receives a label binding for a particular FEC from a
   downstream neighbor, it may already have provided a corresponding
   label binding for this FEC to an upstream neighbor, either because it
   is using independent control, or because the new binding from
   downstream is the result of a routing change.  In this case, unless
   the hop count is 0, it MUST extract the hop count from the new
   binding and increment it by one.  If the new hop count is different
   from that which was previously conveyed to the upstream neighbor
   (including the case where the upstream neighbor was given the value
   'unknown') the ATM-LSR MUST notify the upstream neighbor of the
   change.  Each ATM-LSR in turn MUST increment the hop count and pass
   it upstream until it reaches the ingress Edge LSR.  If at any point
   the value of the hop count equals MAXHOP, the ATM-LSR SHOULD withdraw
   the binding from the upstream neighbor.  A hop count of 0 MUST be
   passed upstream unchanged.

   Whenever an ATM-LSR originates a label binding request to its next
   hop LSR as a result of receiving a label binding request from another
   (upstream) LSR, and the request to the next hop LSR is not satisfied,
   the ATM-LSR SHOULD destroy the binding created in response to the
   received request, and notify the requester (via LDP).

   If an ATM-LSR receives a binding request containing a hop count that
   exceeds MAXHOP, it MUST not establish a binding, and it MUST return
   an error to the requester.

   When a LSR determines that it has lost its LDP session with another
   LSR, the following actions are taken.  Any binding information
   learned via this connection MUST be discarded.  For any label
   bindings that were created as a result of receiving label binding
   requests from the peer, the LSR MAY destroy these bindings (and
   deallocate labels associated with these binding).

   An ATM-LSR SHOULD use 'split-horizon' when it satisfies binding
   requests from its neighbors.  That is, if it receives a request for a
   binding to a particular FEC and the LSR making that request is,
   according to this ATM-LSR, the next hop for that FEC, it should not
   return a binding for that route.

   It is expected that non-merging ATM-LSRs would generally use
   "conservative label retention mode" [1].

8.3. VC-merge-capable ATM Switches

   Relatively minor changes are needed to accommodate ATM-LSRs which
   support VC-merge.  The primary difference is that a VC-merge-capable
   ATM-LSR needs only one outgoing label per FEC, even if multiple
   requests for label bindings to that FEC are received from upstream
   neighbors.

   When a VC-merge-capable ATM-LSR receives a binding request from an
   upstream LSR for a certain FEC, and it does not already have an
   outgoing label binding for that FEC (or an outstanding request for
   such a label binding), it MUST issue a bind request to its next hop
   just as it would do if it were not merge-capable.  If, however, it
   already has an outgoing label binding for that FEC, it does not need
   to issue a downstream binding request.  Instead, it may simply
   allocate an incoming label, and return that label in a binding to the
   upstream requester.  When packets with that label as top label are
   received from the requester, the top label value will be replaced
   with the existing outgoing label value that corresponds to the same
   FEC.

   If the ATM-LSR does not have an outgoing label binding for the FEC,
   but does have an outstanding request for one, it need not issue
   another request.

   When sending a label binding upstream, the hop count associated with
   the corresponding binding from downstream MUST be incremented by 1,
   and the result transmitted upstream as the hop count associated with
   the new binding.  However, there are two exceptions: a hop count of 0
   MUST be passed upstream unchanged, and if the hop count is already at
   MAXHOP, the ATM-LSR MUST NOT pass a binding upstream, but instead
   MUST send an error upstream.

   Note that, just like conventional ATM-LSRs and members of the edge
   set of the ATM-LSR domain, a VC-merge-capable ATM-LSR MUST issue a
   new binding every time it receives a request from upstream, since
   there may be switches upstream which do not support VC-merge.
   However, it only needs to issue a corresponding binding request
   downstream if it does not already have a label binding for the
   appropriate route.

   When a change in the routing table of a VC-merge-capable ATM-LSR
   causes it to select a new next hop for one of its FECs, it MAY
   optionally release the binding for that route from the former next
   hop.  If it doesn't already have a corresponding binding for the new
   next hop, it must request one.  (The choice between conservative and
   liberal label retention mode [1] is an implementation option.)

   If a new binding is obtained, which contains a hop count that differs
   from that which was received in the old binding, then the ATM-LSR
   must take the new hop count, increment it by one, and notify any
   upstream neighbors who have label bindings for this FEC of the new
   value.  Just as with conventional ATM-LSRs, this enables the new hop
   count to propagate back towards the ingress of the ATM-LSR domain.
   If at any point the hop count exceeds MAXHOP, then the label bindings
   for this route must be withdrawn from all upstream neighbors to whom
   a binding was previously provided.  This ensures that any loops
   caused by routing transients will be detected and broken.

9. Encapsulation

   The procedures described in this section affect only the Edge LSRs of
   the ATM-LSR domain.  The ATM-LSRs themselves do not modify the
   encapsulation in any way.

   Labeled packets MUST be transmitted using the null encapsulation of
   Section 6.1 of RFC 2684 [5].

   Except in certain circumstances specified below, when a labeled
   packet is transmitted on an LC-ATM interface, where the VPI/VCI (or
   VCID) is interpreted as the top label in the label stack, the packet
   MUST also contain a "shim header" [3].

   If the packet has a label stack with n entries, it MUST carry a shim
   with n entries.  The actual value of the top label is encoded in the
   VPI/VCI field.  The label value of the top entry in the shim (which
   is just a "placeholder" entry) MUST be set to 0 upon transmission,
   and MUST be ignored upon reception.  The packet's outgoing TTL, and
   its CoS, are carried in the TTL and CoS fields respectively of the
   top stack entry in the shim.

   Note that if a packet has a label stack with only one entry, this
   requires it to have a single-entry shim (4 bytes), even though the
   actual label value is encoded into the VPI/VCI field.  This is done
   to ensure that the packet always has a shim.  Otherwise, there would
   be no way to determine whether it had one or not, i.e., no way to
   determine whether there are additional label stack entries.

   The only ways to eliminate this extra overhead are:

      -  through apriori knowledge that packets have only a single label
         (e.g., perhaps the network only supports one level of label)

      -  by using two VCs per FEC, one for those packets which have only
         a single label, and one for those packets which have more than
         one label

   The second technique would require that there be some way of
   signalling via LDP that the VC is carrying only packets with a single
   label, and is not carrying a shim.  When supporting VC merge, one
   would also have to take care not to merge a VC on which the shim  is
   not used into a VC on which it is used, or vice versa.

   While either of these techniques is permitted, it is doubtful that
   they have any practical utility.  Note that if the shim header is not
   present, the outgoing TTL is carried in the TTL field of the network
   layer header.

10. TTL Manipulation

   The procedures described in this section affect only the Edge LSRs of
   the ATM-LSR domain.  The ATM-LSRs themselves do not modify the TTL in
   any way.

   The details of the TTL adjustment procedure are as follows.  If a
   packet was received by the Edge LSR as an unlabeled packet, the
   "incoming TTL" comes from the IP header.  (Procedures for other
   network layer protocols are for further study.) If a packet was
   received by the Edge LSR as a labeled packet, using the encapsulation
   specified in [3], the "incoming TTL" comes from the entry at the top
   of the label stack.

   If a hop count has been associated with the label binding that is
   used when the packet is forwarded, the "outgoing TTL" is set to the
   larger of (a) 0 or (b) the difference between the incoming TTL and
   the hop count.  If a hop count has not been associated with the label
   binding that is used when the packet is forwarded, the "outgoing TTL"
   is set to the larger of (a) 0 or (b) one less than the incoming TTL.

   If this causes the outgoing TTL to become zero, the packet MUST NOT
   be transmitted as a labeled packet using the specified label.  The
   packet can be treated in one of two ways:

      -  it may be treated as having expired; this may cause an ICMP
         message to be transmitted;

      -  the packet may be forwarded, as an unlabeled packet, with a TTL
         that is 1 less than the incoming TTL; such forwarding would
         need to be done over a non-MPLS connection.

   Of course, if the incoming TTL is 1, only the first of these two
   options is applicable.

   If the packet is forwarded as a labeled packet, the outgoing TTL is
   carried as specified in section 9.

   When an Edge LSR receives a labeled packet over an LC-ATM interface,
   it obtains the incoming TTL from the top label stack entry of the
   generic encapsulation, or, if that encapsulation is not present, from
   the IP header.

   If the packet's next hop is an ATM-LSR, the outgoing TTL is formed
   using the procedures described in this section.  Otherwise the
   outgoing TTL is formed using the procedures described in [3].

   The procedures in this section are intended to apply only to unicast
   packets.

11. Optional Loop Detection: Distributing Path Vectors

   Every ATM-LSR MUST implement, as a configurable option, the following
   procedure for detecting forwarding loops.  We refer to this as the
   LDPV (Loop Detection via Path Vectors) procedure.  This procedure
   does not prevent the formation of forwarding loops, but does ensure
   that any such loops are detected.  If this option is not enabled,
   loops are detected by the hop count mechanism previously described.
   If this option is enabled, loops will be detected more quickly, but
   at a higher cost in overhead.

11.1. When to Send Path Vectors Downstream

   Suppose an LSR R sends a request for a label binding, for a
   particular LSP, to its next hop.  Then if R does not support VC-
   merging, and R is configured to use the LDPV procedure:

      -  If R is sending the request because it is an ingress node for
         that LSP, or because it has acquired a new next hop, then R
         MUST include a path vector object with the request, and the
         path vector object MUST contain only R's own address.

      -  If R is sending the request as a result of having received a
         request from an upstream LSR, then:

         *  if the received request has a path vector object, R MUST add
            its own address to the received path vector object, and MUST
            pass the resulting path vector object to its next hop along
            with the label binding request;

         *  if the received request does not have a path vector object,
            R MUST include a path vector object with the request it
            sends, and the path vector object MUST contain only R's own
            address.

   An LSR which supports VC-merge SHOULD NOT include a path vector
   object in the requests that it sends to its next hop.

   If an LSR receives a label binding request whose path vector object
   contains the address of the node itself, the LSR concludes that the
   label binding requests have traveled in a loop.  The LSR MUST act as
   it would in the case where the hop count exceeds MAXHOP (see section
   8.2).

   This procedure detects the case where the request messages loop
   though a sequence of non-merging ATM-LSRs.

11.2. When to Send Path Vectors Upstream

   As specified in section 8, there are circumstances in which an LSR R
   must inform its upstream neighbors, via a label binding response
   message, of a change in hop count for a particular LSP.  If the
   following conditions all hold:

      -  R is configured for the LDPV procedure,

      -  R supports VC-merge,

      -  R is not the egress for that LSP, and

      -  R is not informing its neighbors of a decrease in the hop
         count,

   then R MUST include a path vector object in the response message.

   If the change in hop count is a result of R's having been informed by
   its next hop, S, of a change in hop count, and the message from S to
   R included a path vector object, then if the above conditions hold, R
   MUST add itself to this object and pass the result upstream.
   Otherwise, if the above conditions hold, R MUST create a new object
   with only its own address.

   If R is configured for the LDPV procedure, and R supports VC merge,
   then it MAY include a path vector object in any label binding
   response message that it sends upstream.  In particular, at any time
   that R receives a label binding response from its next hop, if that
   response contains a path vector, R MAY (if configured for the LDPV
   procedure) send a response to its upstream neighbors, containing the
   path vector object formed by adding its own address to the received
   path vector.

   If R does not support VC merge, it SHOULD NOT send a path vector
   object upstream.

   If an LSR  receives a message from  its next hop, with a  path vector
   object containing its own address, then  LSR  MUST act as it would if
   it received a message with a hop count equal to MAXHOP.

   LSRs which are configured for the LDPV procedure SHOULD NOT store a
   path vector once the corresponding path vector object has been
   transmitted.

   Note that if the ATM-LSR domain consists entirely of non-merging
   ATM-LSRs, path vectors need not ever be sent upstream, since any
   loops will be detected by means of the path vectors traveling
   downstream.

   By not sending path vectors unless the hop count increases, one
   avoids sending them in many situations when there is no loop.  The
   cost is that in some situations in which there is a loop, the time to
   detect the loop may be lengthened.

12. Security Considerations

   The encapsulation and procedures specified in this document do not
   interfere in any way with the application of authentication and/or
   encryption to network layer packets (such as the application of IPSEC
   to IP datagrams).

   The procedures described in this document do not protect against the
   alteration (either accidental or malicious) of MPLS labels.  Such
   alteration could cause misforwarding.

   The procedures described in this document do not enable a receiving
   LSR to authenticate the transmitting LSR.

   A discussion of the security considerations applicable to the label
   distribution mechanism can be found in [2].

13. Intellectual Property Considerations

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

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

14. References

   [1] Rosen, E., Viswanathan, A. and R. Callon "Multi-Protocol Label
       Switching Architecture", RFC 3031, January 2001.

   [2] Andersson L., Doolan P., Feldman N., Fredette A. and R. Thomas,
       "LDP Specification", RFC 3036, January 2001.

   [3] Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D., Fedorkow, G.,
       Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
       January 2001.

   [4] Nagami, K., Demizu N., Esaki H. and P. Doolan, "VCID Notification
       over ATM Link for LDP", RFC 3038, January 2001.

   [5] Grossman, D., Heinanen, J., "Multiprotocol Encapsulation over ATM
       Adaptation Layer 5", RFC 2684, September 1999.

15. Acknowledgments

   Significant contributions to this work have been made by Anthony
   Alles, Fred Baker, Dino Farinacci, Guy Fedorkow, Arthur Lin, Morgan
   Littlewood and Dan Tappan.  We thank Alex Conta for his comments.

16. Authors' Addresses

   Bruce Davie
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   EMail: bsd@cisco.com


   Paul Doolan
   Ennovate Networks Inc.
   60 Codman Hill Rd
   Boxborough, MA 01719

   EMail: pdoolan@ennovatenetworks.com
   Jeremy Lawrence
   Cisco Systems, Inc.
   99 Walker St.
   North Sydney, NSW, Australia

   EMail: jlawrenc@cisco.com


   Keith McCloghrie
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134

   EMail: kzm@cisco.com


   Yakov Rekhter
   Juniper Networks
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089

   EMail: yakov@juniper.net


   Eric Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   EMail: erosen@cisco.com


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

   EMail: swallow@cisco.com

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