rfc3035.txt

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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].



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RFC 3035          MPLS using LDP and ATM VC Switching       January 2001


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.




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RFC 3035          MPLS using LDP and ATM VC Switching       January 2001


   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.



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RFC 3035          MPLS using LDP and ATM VC Switching       January 2001


   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.



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RFC 3035          MPLS using LDP and ATM VC Switching       January 2001


      -  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]