rfc3034.txt

最新的RFC

Network Working Group                                           A. Conta
Request for Comments: 3034                        Transwitch Corporation
Category: Standards Track                                      P. Doolan
                                                                Ennovate
                                                                A. Malis
                                                   Vivace Networks, Inc.
                                                            January 2001


             Use of Label Switching on Frame Relay Networks
                             Specification

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

   This document defines the model and generic mechanisms for
   Multiprotocol Label Switching on Frame Relay networks.  Furthermore,
   it extends and clarifies portions of the Multiprotocol Label
   Switching Architecture described in [ARCH] and the Label Distribution
   Protocol (LDP) described in [LDP] relative to Frame Relay Networks.
   MPLS enables the use of Frame Relay Switches as Label Switching
   Routers (LSRs).

Table of Contents

   1. Introduction................................................2
   2. Terminology.................................................3
   3. Special Characteristics of Frame Relay Switches.............4
   4. Label Encapsulation.........................................5
   5. Frame Relay Label Switching Processing......................6
   5.1  Use of DLCIs..............................................6
   5.2  Homogeneous LSPs..........................................7
   5.3  Heterogeneous LSPs........................................7
   5.4  Frame Relay Label Switching Loop Prevention and Control...7
   5.4.1   FR-LSRs Loop Control - MPLS TTL Processing.............7
   5.4.2   Performing MPLS TTL calculations.......................8
   5.5  Label Processing by Ingress FR-LSRs......................12



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   5.6  Label Processing by Core FR-LSRs.........................12
   5.7  Label Processing by Egress FR-LSRs.......................13
   6.  Label Switching Control Component for Frame Relay.........13
   6.1  Hybrid Switches (Ships in the Night)  ...................14
   7.  Label Allocation and Maintenance Procedures ..............15
   7.1  Edge LSR Behavior........................................15
   7.2  Efficient use of label space-Merging FR-LSRs.............18
   7.3  LDP message fields specific to Frame Relay...............19
   8.  Security Considerations  .................................21
   9.  Acknowledgments  .........................................21
   10. References  ..............................................22
   11. Authors' Addresses  ......................................23
   12. Full Copyright Statement  ................................24

1. Introduction

   The Multiprotocol Label Switching Architecture is described in
   [ARCH].  It is possible to use Frame Relay switches as Label
   Switching Routers.  Such Frame Relay switches run network layer
   routing algorithms (such as OSPF, IS-IS, etc.), and their forwarding
   is based on the results of these routing algorithms.  No specific
   Frame Relay routing is needed.

   When a Frame Relay switch is used for label switching, the top
   (current) label, on which forwarding decisions are based, is carried
   in the DLCI field of the Frame Relay data link layer header of a
   frame.  Additional information carried along with the top (current)
   label, but not processed by Frame Relay switching, along with other
   labels, if the packet is multiply labeled, are carried in the generic
   MPLS encapsulation defined in [STACK].

   Frame Relay permanent virtual circuits (PVCs) could be configured to
   carry label switching based traffic.  The DLCIs would be used as MPLS
   Labels and the Frame Relay switches would become Frame Relay Label
   Switching Routers, while the MPLS traffic would be encapsulated
   according to this specification, and would be forwarded based on
   network layer routing information.

   The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
   SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
   in RFC 2119.

   This document is a companion document to [STACK] and [ATM].








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2. Terminology

   LSR

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

   LC-FR

      A label switching controlled Frame Relay (LC-FR) interface is a
      Frame Relay interface controlled by the label switching control
      component.  Packets traversing such an interface carry labels in
      the DLCI field.

   FR-LSR

      A FR-LSR is an LSR with one or more LC-FR interfaces which
      forwards frames between two such interfaces using labels carried
      in the DLCI field.

   FR-LSR domain

      A FR-LSR domain is a set of FR-LSRs, which are mutually
      interconnected by LC-FR interfaces.

   Edge Set

      The Edge Set of an FR-LSR domain is the set of LSRs, which are
      connected to the domain by LC-FR interfaces.

   Forwarding Encapsulation

      The Forwarding Encapsulation is the type of MPLS encapsulation
      (Frame Relay, ATM, Generic) of a packet that determines the
      packet's MPLS forwarding, or the network layer encapsulation if
      that packet is forwarded based on the network layer (IP,
      etc...)header.

   Input Encapsulation

      The Input Encapsulation is the type of MPLS encapsulation (Frame
      Relay, ATM, Generic) of a packet when that packet is received on
      an LSR's interface, or the network layer (IP, etc...)encapsulation
      if that packet has no MPLS encapsulation.






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RFC 3034            Label Switching with Frame Relay        January 2001


   Output Encapsulation

      The Output Encapsulation is the type of MPLS encapsulation (Frame
      Relay, ATM, Generic) of a packet when that packet is transmitted
      on an LSR's interface, or the network layer (IP,
      etc...)encapsulation if that packet has no MPLS encapsulation.

   Input TTL

      The Input TTL is the MPLS TTL of the top of the stack when a
      labeled packet is received on an LSR interface, or the network
      layer (IP) TTL if the packet is not labeled.

   Output TTL

      The Output TTL is the MPLS TTL of the top of the stack when a
      labeled packet is transmitted on an LSR interface, or the network
      layer (IP) TTL if the packet is not labeled.

   Additionally, this document uses terminology from [ARCH].

3. Special characteristics of Frame Relay Switches

   While the label switching architecture permits considerable
   flexibility in LSR implementation, a FR-LSR is constrained by the
   capabilities of the (possibly pre-existing) hardware and the
   restrictions on such matters as frame format imposed by the
   Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay
   standards [FRF], etc.... Because of these constraints, some special
   procedures are required for FR-LSRs.

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

   -  the label swapping function is performed on fields (DLCI) in the
      frame's Frame Relay data link header; this dictates the size and
      placement of the label(s) in a packet.  The size of the DLCI field
      can be 10 (default) or 23 bits, and it can span two or four bytes
      in the header.

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

   -  congestion control is performed by each node based on parameters
      that are passed at circuit creation.  Flags in the frame headers
      may be set as a consequence of congestion, or exceeding the
      contractual parameters of the circuit.




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RFC 3034            Label Switching with Frame Relay        January 2001


   -  although in a standard switch it may be possible to configure
      multiple input DLCIs to one output DLCI resulting in a
      multipoint-to-point circuit, multipoint-to-multipoint VCs are
      generally not fully supported.

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

4. Label Encapsulation

   By default, all labeled packets should be transmitted with the
   generic label encapsulation as defined in [STACK], using the frame
   relay null encapsulation mechanism:

               0                       1                       (Octets)
              +-----------------------+-----------------------+
   (Octets)0  |                                               |
              /                 Q.922 Address                 /
              /             (length 'n' equals 2 or 4)        /
              |                                               |
              +-----------------------+-----------------------+
           n  |                       .                       |
              /                       .                       /
              /                  MPLS packet                  /
              |                       .                       |
              +-----------------------+-----------------------+

      "n" is the length of the Q.922 Address which can be 2 or 4 octets.

      The Q.922 [ITU] representation of a DLCI (in canonical order  -
      the first bit is stored in the least significant, i.e., the
      right-most bit of a byte in memory) [CANON] is the following:

            7     6     5     4     3     2     1     0      (bit order)
           +-----+-----+-----+-----+-----+-----+-----+-----+
(octet) 0  |            DLCI(high order)       |  0  |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----+
        1  |  DLCI(low order)      |  0  |  0  |  0  |  1  |
           +-----+-----+-----+-----+-----+-----+-----+-----+

              10 bits DLCI










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            7     6     5     4     3     2     1     0      (bit order)
           +-----+-----+-----+-----+-----+-----+-----+-----00
(octet) 0  |            DLCI(high order)       |  0  |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----
        1  |  DLCI                 |  0  |  0  |  0  |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----+
        2  |             DLCI                        |  0  |
           +-----+-----+-----+-----+-----+-----+-----+-----+
        3  |       DLCI (low order)            |  0  |  1  |
           +-----+-----+-----+-----+-----+-----+-----+-----+

              23 bits DLCI

   The use of the frame relay null encapsulation implies that labels
   implicitly encode the network protocol type.

   Rules regarding the construction of the label stack, and error
   messages returned to the frame source are also described in [STACK].

   The generic encapsulation contains "n" labels for a label stack of
   depth "n" [STACK], where the top stack entry carries significant
   values for the EXP, S , and TTL fields [STACK] but not for the label,
   which is rather carried in the DLCI field of the Frame Relay data
   link header encoded in Q.922 [ITU] address format.

5. Frame Relay Label Switching Processing

5.1  Use of DLCIs

   Label switching is accomplished by associating labels with routes and
   using the label value to forward packets, including determining the
   value of any replacement label.  See [ARCH] for further details.  In
   a FR-LSR, the top (current) MPLS label is carried in the DLCI field
   of the Frame Relay data link layer header of the frame.  The top
   label carries implicitly information about the network protocol type.

   For two connected FR-LSRs, a full-duplex connection must be available
   for LDP.  The DLCI for the LDP VC is assigned a value by way of
   configuration, similar to configuring the DLCI used to run IP routing
   protocols between the switches.

   With the exception of this configured value, the DLCI values used for
   MPLS in the two directions of the link may be treated as belonging to
   two independent spaces, i.e., VCs may be half-duplex, each direction
   with its own DLCI.