Internet Draft






Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: May 2001                                    Giles Heron
                                            Level 3 Communications, LLC.

                                                           Daniel Tappan
                                                           Eric C. Rosen
                                                     Cisco Systems, Inc.

                                                         Steve Vogelsang
                                                            John Shirron
                                                   Laurel Networks, Inc.

                                                         Andrew G. Malis
                                                   Vivace Networks, Inc.

                                                Dimitri Stratton Vlachos
                                                     Mazu Networks, Inc.

                                                           November 2000


                 Transport of Layer 2 Frames Over MPLS


               draft-martini-l2circuit-trans-mpls-04.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
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Abstract

   This document describes methods for transporting the Protocol Data
   Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,
   Ethernet, and providing a SONET circuit emulation service across an
   MPLS network.


Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      Tunnel Labels and VC Labels  ............................   3
    4      Protocol-Specific Issues  ...............................   4
    4.1    Frame Relay  ............................................   4
    4.2    ATM  ....................................................   4
    4.2.1  OAM Cell Support  .......................................   4
    4.2.2  ILMI Support  ...........................................   5
    4.3    HDLC ( Cisco )  .........................................   5
    4.4    PPP  ....................................................   5
    5      LDP  ....................................................   6
    6      Security Considerations  ................................   9
    7      References  .............................................   9
    8      Author Information  .....................................   9




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
   procedures for accomplishing this using the encapsulation methods in
   [7]. We restrict discussion to the case of point-to-point transport.
   QoS related issues are not discussed in this draft.

   An accompanying document [8] also describes a method for transporting
   time division multiplexed (TDM) digital signals (TDM circuit
   emulation) over a packet-oriented MPLS network. The transmission
   system for circuit-oriented TDM signals is the Synchronous Optical
   Network (SONET)[5]/Synchronous Digital Hierarchy (SDH) [6]. To



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   support TDM traffic, which includes voice, data, and private leased
   line service, the MPLS network must emulate the circuit
   characteristics of SONET/SDH payloads. MPLS labels and a new circuit
   emulation header are used to encapsulate TDM signals and provide the
   Circuit Emulation Service over MPLS (CEM). This encapsulation method
   is described in [8].

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
   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. This process
   is unidirectional, and will be repeated independently for
   bidirectional operation. It is REQUIRED to assign the same VC, and
   Group ID for a given circuit in both directions.

   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



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   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 signaling 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.

4. Protocol-Specific Issues

4.1. Frame Relay

   The MPLS edge LSR MAY provide a Frame Relay LMI to the CE device.

   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 MUST withdraw the label
   that corresponds to the frame relay DLCI. The Egress LSR SHOULD
   generate the corresponding errors and alarms as defined in [2] on the
   Frame relay VC.

4.2. ATM

4.2.1. OAM Cell Support

   OAM cells MAY be transported on the VC LSP. A router that does not
   support transport of ATM cells MUST discard incoming MPLS frames on
   an ATM VC LSP that contain a control word with the T bit set. [7] A
   router that supports transport of OAM cells MUST follow the
   procedures outlined in [9] section 8 for mode 0 only in addition to
   the applicable procedures specified in [6].

   A router that does not support transport of OAM cells across an LSP
   MAY provide OAM support on ATM PVCs using the following procedures:

   If an F5 end-to-end OAM cell is received from a VC by a LSR with a
   loopback indication value of 1 and the LSR has a label mapping for
   the VC, the LSR MUST decrement the loopback indication value and loop
   back the cell on the VC. Otherwise the loopback cell MUST be



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   discarded by the LSR.

   The LSR MAY optionally be configured to periodically generate F5
   end-to-end loopback OAM cells on a VC. In this case, the LSR must
   only generate F5 end-to-end loopback cells while a label mapping
   exists for the VC. If the VC label mapping is withdrawn the LSR MUST
   cease generation of F5 end-to-end loopback OAM cells. If the LSR
   fails to receive a response to an F5 end-to-end loopback OAM cell for
   a pre-defined period of time it MUST withdraw the label mapping for
   the VC.

   If an ingress LSR receives an AIS F5 OAM cell, fails to receive a
   pre-defined number of the End-to-End loop OAM cells, or a physical
   interface goes down, it MUST withdraw the label mappings for all VCs
   associated with the failure. When a VC label mapping is withdrawn,
   the egress LSR SHOULD generate AIS F5 OAM cells on the VC associated
   with the withdrawn label mapping.

4.2.2. ILMI Support

   An MPLS edge LSR MAY provide an ATM ILMI to the CE device.

   If an ingress LSR receives an ILMI message indicating that the CE has
   deleted a VC, or if the physical interface goes down, it MUST
   withdraw the label mappings for all VCs associated with the failure.
   When a VC label mapping is withdrawn, the egress LSR SHOULD notify
   its client of this failure by deleting the VC using ILMI.

4.3. HDLC ( Cisco )

   If the MPLS edge LSR detects that the physical link has failed it
   MUST withdraw the label that corresponds to the HDLC link. The Egress
   LSR SHOULD notify the CE device of this failure by using a physical
   layer mechanism to take the link out of service.

4.4. PPP

   If the MPLS edge LSR detects that the physical link has failed it
   MUST withdraw the label that corresponds to the PPP link. The Egress
   LSR SHOULD notify the CE device of this failure by using a physical
   layer mechanism to take the link out of service.










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5. LDP

   The VC label bindings are distributed using the LDP downstream
   unsolicited mode described in [1]. The LSRs will establish an LDP
   session using the Extended Discovery mechanism described in [1,
   section 2.4-2.5], for this purpose a new type of FEC TLV element is
   defined. The FEC element type is 128. [note1]

   The Virtual Circuit FEC TLV 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     |C|         VC Type             |  VC ID len    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Group ID                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                          VC ID                                |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     - VC Type

       A 15 bit quantity containing a value which represents the type of
       VC. Assigned Values are:

               VC Type  Description

               0x0001   Frame Relay DLCI
               0x0002   ATM VCC transport
               0x0003   ATM VPC transport
               0x0004   Ethernet VLAN
               0x0005   Ethernet
               0x0006   HDLC ( Cisco )
               0x0007   PPP
               0x8008   CEM [8]

       The highest order bit is used to flag the presence of a control word as
       follows:
               bit 15 = 1 control word present on this VC.
               bit 15 = 0 no control word present on this VC.

     - VC ID length

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



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     - Group ID

       An arbitrary 32 bit value which represents a group of VCs that is
       used to augment the VC space. This value MUST be user
       configurable.  The group ID is intended to be used as either a
       port index , or a virtual tunnel index. In the latter case a
       switching function at ingress will map a particular circuit from
       a port to a circuit in the virtual tunnel for transport to the
       egress router.

     - VC ID

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

         * Frame Relay

           A 32-bit value representing a 16-bit DLCI value 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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |          Reserved             |             DLCI              |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         * ATM VCC Transport

           A 32-bit value representing a 16-bit VPI, and a 16-bit VCI 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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |             VPI               |              VCI              |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         * ATM VPC Transport

           A 32-bit value containing a 16-bit VPI 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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |             VPI               |            Reserved           |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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         * Ethernet VLAN

           A 32 bit value representing 16bit vlan identifier 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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |           Reserved            |            VLAN ID            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         * Ethernet

           A 32 bit port identifier.

         * HDLC ( Cisco )

           A 32-bit port identifier

         * PPP

           A 32-bit port identifier

         * CEM[8]

           A 32-bit value used 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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |   Reserved    |        Circuit ID         |   Payload Bytes   |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


           Circuit ID:  An assigned number for the SONET circuit being
           transported.

           Payload Bytes(N):  the number of TDM payload bytes contained
           in all packets on the CEM stream, from 48 to 1,023 bytes. All
           of the packets in a given CEM stream have the same number of
           payload bytes. Note that there is a possibility that the
           packet size may exceed the SPE size in the case of an STS-1
           SPE, which could cause two pointers to be needed in the CEM
           header, since the payload may contain two J1 bytes for
           consecutive SPEs. For this reason, the number of payload
           bytes must be less than or equal to 783 for STS-1 SPEs. The
           reserved fields in the above specifications MUST be set to 0
           in the FEC TLV, and ignored when received.




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6. Security Considerations

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

7. References

   [1] "LDP Specification", draft-ietf-mpls-ldp-11.txt ( work in
   progress )

   [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-08.txt
   ( work in progress )

   [4] "IEEE 802.3ac-1998" IEEE standard specification.

   [5] American National Standards Institute, "Synchronous Optical
   Network Formats," ANSI T1.105-1995.

   [6] ITU Recommendation G.707, "Network Node Interface For The
   Synchronous Digital Hierarchy", 1996.

   [7] "Encapsulation Methods for Transport of Layer 2 Frames Over
   MPLS", draft-martini-l2circuit-encap-mpls-00.txt ( Work in progress )

   [8] "SONET/SDH Circuit Emulation Service Over MPLS (CEM)
   Encapsulation", draft-malis-sonet-ces-mpls-01.txt ( Work in progress
   )

   [9] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000.

   [note1] FEC element type 128 is pending IANA approval.

8. Author Information


   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: luca@level3.net









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   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: nna@level3.net


   Giles Heron
   Level 3 Communications
   66 Prescot Street
   London
   E1 8HG
   United Kingdom
   e-mail: giles@level3.net


   Dimitri Stratton Vlachos
   Mazu Networks, Inc.
   125 Cambridgepark Drive
   Cambridge, MA 02140
   e-mail: d@mazunetworks.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


   Steve Vogelsang
   Laurel Networks, Inc.
   2607 Nicholson Rd.
   Sewickley, PA 15143
   e-mail: sjv@laurelnetworks.com








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   John Shirron
   Laurel Networks, Inc.
   2607 Nicholson Rd.
   Sewickley, PA 15143
   e-mail: jshirron@laurelnetworks.com


   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   Phone: +1 408 383 7223
   Email: Andy.Malis@vivacenetworks.com





































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