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Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: June 2000                  Level 3 Communications, LLC.

                                                Dimitri Stratton Vlachos
                                                           Daniel Tappan
                                                           Eric C. Rosen
                                                     Cisco Systems, Inc.

                                                           December 1999


                 Transport of Layer 2 Frames Over MPLS


               draft-martini-l2circuit-trans-mpls-00.txt

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-
   Drafts.

   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 progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This document described a method for transporting the Protocol Data
   Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5, and
   ethernet across an MPLS network.








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Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      Tunnel Labels and VC Labels  ............................   2
    4      Optional Sequencing and/or Padding  .....................   4
    5      Protocol-Specific Issues  ...............................   5
    5.1    Frame Relay  ............................................   5
    5.2    ATM AAL5  ...............................................   5
    5.3    Ethernet  ...............................................   6
    6      LDP  ....................................................   6
    7      Security Considerations  ................................   7
    8      References  .............................................   7
    9      Author Information  .....................................   8





1. 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.


2. Introduction

   In an MPLS network, it is possible to carry the Protocol Data Units
   (PDUs) of layer 2 protocols by prepending an MPLS label stack to
   these PDUs.  This document specifies the necessary label distribution
   and encapsulation procedures for accomplishing this.  We restrict
   discussion to the case of point-to-point transport.  QoS related
   issues are not discussed in this draft.


3. Tunnel Labels and VC Labels

   Suppose it is desired to transport layer 2 PDUs from ingress LSR R1
   to egress LSR R2, across an intervening MPLS network.  We assume that
   there is an LSP from R1 to R2.  That is, we assume that R1 can cause
   a packet to be delivered to R2 by pushing some label onto the packet
   and sending the result to one of its adjacencies.  Call this label
   the "tunnel label", and the corresponding LSP the "tunnel LSP".

   The tunnel LSP merely gets packets from R1 to R2, the corresponding



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   label doesn't tell R2 what to do with the payload, and in fact if
   penultimate hop popping is used, R2 may never even see the
   corresponding label.  (If R1 itself is the penultimate hop, a tunnel
   label may not even get pushed on.)  Thus if the payload is not an IP
   packet, there must be a label, which becomes visible to R2, that
   tells R2 how to treat the received packet.  Call this label the "VC
   label".

   So when R1 sends a layer 2 PDU to R2, it first pushes a VC label on
   its label stack, and then (if R1 is not adjacent to R2) pushes on a
   tunnel label.  The tunnel label gets the MPLS packet from R1 to R2;
   the VC label is not visible until the MPLS packet reaches R2.  R2's
   disposition of the packet is based on the VC label.

   If the payload of the MPLS packet is, for example, an ATM AAL5 PDU,
   the VC label will generally correspond to a particular ATM VC at R2.
   That is, R2 needs to be able to infer from the VC label the outgoing
   interface and the VPI/VCI value for the AAL5 PDU.  If the payload is
   a Frame Relay PDU, then R2 needs to be able to infer from the VC
   label the outgoing interface and the DLCI value.  If the payload is
   an ethernet frame, then R2 needs to be able to infer from the VC
   label the outgoing interface, and perhaps the VLAN identifier.

   Note that the VC label must always be at the bottom of the label
   stack, and the tunnel label, if present, must be immediately above
   the VC label.  Of course, as the packet is transported across the
   MPLS network, additional labels may be pushed on (and then popped
   off) as needed.  Even R1 itself may push on additional labels above
   the tunnel label.  If R1 and R2 are directly adjacent LSRs, then it
   may not be necessary to use a tunnel label at all.

   This document does not specify a method for distributing the tunnel
   label or any other labels that may appear above it on the stack.  Any
   acceptable method of MPLS label distribution will do.

   This document does specify a method for assigning and distributing
   the VC label. Static label assignment MAY be used, and
   implementations SHOULD provide support for this.  If signalling is
   used, the VC label MUST be distributed from R2 to R1 using LDP in the
   downstream unsolicited mode; this requires that an LDP connection be
   created between R1 and R2.

   Note that this technique allows an unbounded number of layer 2 "VCs"
   to be carried together in a single "tunnel".  Thus it scales quite
   well in the network backbone.






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4. Optional Sequencing and/or Padding

   Sometimes it is important to guarantee that sequentiality is
   preserved on a layer 2 virtual circuit.  To accommodate this
   requirement, we provide an optional control word which may appear
   immediately after the label stack and immediately before the layer 2
   PDU.  This control word contains a sequence number. R1 and R2 both
   need to be configured with the knowledge of whether a control word
   will be used for a specific virtual circuit.

   Sometimes it is necessary to transmit a small packet on a medium
   where there is a minimum transport unit larger than the actual packet
   size.  In this case, padding is appended to the packet. When the VC
   label is popped, it may be desirable to remove the padding before
   forwarding the packet.

   To facilitate this, the control word has a length field.  If the
   packet's length (without any padding) is less than 256 bytes, the
   length field MUST be set to the packet's length (without padding).
   Otherwise the length field MUST be set to zero.  The value of the
   length field, if non-zero, can be used to remove any padding.

   The control word is defined as follows:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Reserved   |     Length    |       Sequence Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The first 8 bits are reserved for future use.  They MUST be set to 0
   when transmitting, and MUST be ignored upon receipt. The length byte
   is set as specified above.

   The next 16 bits are the sequence number that is used to guarantee
   ordered packet delivery.  For a given VC label, and a given pair of
   LSRs, R1 and R2, where R2 has distributed that VC label to R1, the
   sequence number is initialized to 0, and is incremented by one for
   each successive packet carrying that VC label which R1 transmits to
   R2.

   The sequence number space is a 16 bit unsigned circular space.  PDUs
   carrying the control word MUST NOT be delivered out of order.   They
   may be discarded or reordered.






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5. Protocol-Specific Issues

5.1. Frame Relay

   A Frame Relay PDU is transported in its entirety, including the Frame
   Relay Header.  The sequencing control word is optional.

   The BCN and FCN signals are carried unchanged across the network in
   the frame relay header.  These signals do not appear in the MPLS
   header, and are unseen by the MPLS network.

   If the MPLS edge LSR detects a service affecting condition as defined
   in [2] Q.933 Annex A.5 sited in IA FRF1.1, it will withdraw the label
   that corresponds to the frame relay DLCI. The Egress side should
   generate the corresponding errors and alarms as defined in [2] on the
   Frame relay VC.

   The ingress LSR MAY consider the DE bit of the Frame Relay header
   when determining the value to be placed in the EXP fields of the MPLS
   label stack.


5.2. ATM AAL5

   Only ATM Adaptation Layer 5 (AAL5) is supported.  A CPCS-PDU is
   transported as a single packet without segmentation.  No AAL5 trailer
   is transported.  The sequencing control word is optional.

   If the edge LSR receives an AIS F5 OAM cell, or fails to receive a
   pre-defined number of the End-to-End loop OAM cells, or the physical
   interface is down, it will withdraw the LSP mapping for the VC
   associated with the failure. The egress LSR will generate AIS F5 OAM,
   or stop returning/forwarding the End-to-End loop OAM Cells to the
   remote destination of the VC associated with the withdrawn LSP.

   The ingress LSR MAY consider the CLP bit when determining the value
   to be placed in the EXP fields of the MPLS label stack.

   The egress LSR MAY consider the value of the EXP field of the VC
   label when determining the value of the ATM CLP bit.











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5.3. Ethernet

   If the VC label corresponds just to an ethernet interface, then the
   ethernet frame is sent in its entirety.  If the VC label corresponds
   to a particular VLAN, however, then the VLAN tag should be removed.


6. LDP

   The VC label bindings are distributed using the LDP downstream
   unsolicited mode described in [1].  The LSRs will establish and LDP
   session using the Extended Discovery mechanism described in [1,
   section 2.4-2.5]

   A new type of FEC TLV, a VC FEC Element is defined as follows:

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | VC tlv        |           VC Type             |  VC ID len    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                     Group ID                                  |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         |                          VC ID                                |
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     - VC Type

       A two octet quantity containing a value which represents the type
       of VC. Assigned Values are:

       1 - Frame Relay DLCI
       2 - ATM PVC
       3 - Ethernet
       4 - Ethernet VLAN
       5 - HDLC
       6 - PPP

     - VC ID length

       Length of the VC ID field in octets. If this value is 0,then it
       references all VCs using the specified Link ID







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     - Group ID

       An arbitrary 32 bit value which represents a group of VCs.

     - VC ID

       Identifies a particular VC.  The interpretation of the identifier
       depends on the VC type:

         * Frame Relay

           A 16-bit DLCI value.

         * ATM

           A 32-bit value representing a 16-bit VPI and a 16-bit VCI.

         * Ethernet

           A port identifier (details TBD).

         * Ethernet VLAN

           A port identifier plus a VLAN tag (details TBD).

         * HDLC

           A port identifier (details TBD).

         * PPP

           A port identifier (details TBD).


7. Security Considerations

   This document does not affect the underlying security issues of MPLS.


8. References

   [1]  "LDP Specification", draft-ietf-mpls-ldp-06.txt, 10/5/99

   [2] ITU-T Recommendation Q.933, and Q.922 Specification for Frame
   Mode Basic call control, ITU Geneva 1995

   [3] "MPLS Label Stack Encoding", draft-ietf-mpls-label-encaps-07.txt,
   9/13/99



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9. Author Information


   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021


   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021


   Dimitri Stratton Vlachos
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: dvlachos@cisco.com


   Dan Tappan
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: tappan@cisco.com


   Eric Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: erosen@cisco.com

















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