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RFC 4618 - Encapsulation Methods for Transport of PPP/High-Level

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Network Working Group                                         L. Martini
Request for Comments: 4618                                      E. Rosen
Category: Standards Track                            Cisco Systems, Inc.
                                                                G. Heron
                                                                A. Malis
                                                          September 2006

                Encapsulation Methods for Transport of
       PPP/High-Level Data Link Control (HDLC) over MPLS Networks

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 (2006).


   A pseudowire (PW) can be used to carry Point to Point Protocol (PPP)
   or High-Level Data Link Control (HDLC) Protocol Data Units over a
   Multiprotocol Label Switching (MPLS) network without terminating the
   PPP/HDLC protocol.  This enables service providers to offer
   "emulated" HDLC, or PPP link services over existing MPLS networks.
   This document specifies the encapsulation of PPP/HDLC Packet Data
   Units (PDUs) within a pseudowire.

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................2
   3. Applicability Statement .........................................5
   4. General Encapsulation Method ....................................6
      4.1. The Control Word ...........................................6
      4.2. MTU Requirements ...........................................8
   5. Protocol-Specific Details .......................................9
      5.1. HDLC .......................................................9
      5.2. Frame Relay Port Mode ......................................9
      5.3. PPP .......................................................10
   6. Using an MPLS Label as the Demultiplexer Field .................11
      6.1. MPLS Shim EXP Bit Values ..................................11
      6.2. MPLS Shim S Bit Value .....................................11
   7. Congestion Control .............................................12
   8. IANA Considerations ............................................12
   9. Security Considerations ........................................12
   10. Normative References ..........................................13
   11. Informative References ........................................13

1.  Introduction

   A PPP/HDLC pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs)
   to be carried over an MPLS network.  In addressing the issues
   associated with carrying a PPP/HDLC PDU over an MPLS network, this
   document assumes that a PW has been set up by some means outside the
   scope of this document.  This may be via manual configuration, or
   using a signaling protocol such as that defined in [RFC4447].

   The following figure describes the reference models that are derived
   from [RFC3985] to support the HDLC/PPP PW emulated services.  The
   reader is also assumed to be familiar with the content of the
   [RFC3985] document.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

          |<-------------- Emulated Service ---------------->|
          |                                                  |
          |          |<------- Pseudowire ------->|          |
          |          |                            |          |
          |          |    |<-- PSN Tunnel -->|    |          |
          |          V    V                  V    V          |
          V   AC     +----+                  +----+    AC    V
    +-----+    |     | PE1|==================| PE2|     |    +-----+
    |     |----------|............PW1.............|----------|     |
    | CE1 |    |     |    |                  |    |     |    | CE2 |
    |     |----------|............PW2.............|----------|     |
    +-----+  ^ |     |    |==================|    |     | ^  +-----+
          ^  |       +----+                  +----+     | |  ^
          |  |   Provider Edge 1         Provider Edge 2  |  |
          |  |                                            |  |
    Customer |                                            | Customer
    Edge 1   |                                            | Edge 2
             |                                            |
             |                                            |
       native HDLC/PPP service                   native HDLC/PPP service

       Figure 1.  PWE3 HDLC/PPP interface reference configuration

   This document specifies the emulated PW encapsulation for PPP and
   HDLC; however, quality of service related issues are not discussed in
   this document.  For the purpose of the discussion in this document,
   PE1 will be defined as the ingress router and PE2 as the egress
   router.  A layer 2 PDU will be received at PE1, encapsulated at PE1,
   transported across the network, decapsulated at PE2, and transmitted
   out on an attachment circuit at PE2.

   The following reference model describes the termination point of each
   end of the PW within the PE:

                |                PE                 |
        +---+   +-+  +-----+  +------+  +------+  +-+
        |   |   |P|  |     |  |PW ter|  | PSN  |  |P|
        |   |<==|h|<=| NSP |<=|minati|<=|Tunnel|<=|h|<== From PSN
        |   |   |y|  |     |  |on    |  |      |  |y|
        | C |   +-+  +-----+  +------+  +------+  +-+
        | E |   |                                   |
        |   |   +-+  +-----+  +------+  +------+  +-+
        |   |   |P|  |     |  |PW ter|  | PSN  |  |P|
        |   |==>|h|=>| NSP |=>|minati|=>|Tunnel|=>|h|==> To PSN
        |   |   |y|  |     |  |on    |  |      |  |y|
        +---+   +-+  +-----+  +------+  +------+  +-+
                |                                   |
                        ^        ^          ^
                        |        |          |
                        A        B          C

                       Figure 2.  PW reference diagram

   The PW terminates at a logical port within the PE, defined at point B
   in the above diagram.  This port provides an HDLC Native Service
   Processing function that will deliver each PPP/HDLC packet that is
   received at point A, unaltered, to the point A in the corresponding
   PE at the other end of the PW.

   The Native Service Processing (NSP) function includes packet
   processing that is required for the PPP/HDLC packets that are
   forwarded to the PW termination point.  Such functions may include
   bit stuffing, PW-PW bridging, L2 encapsulation, shaping, and
   policing.  These functions are specific to the native packet
   technology and may not be required for the PW emulation service.

   The points to the left of B, including the physical layer between the
   CE and PE, and any adaptation (NSP) functions between it and the PW
   terminations, are outside of the scope of PWE3 and are not defined

   "PW Termination", between A and B, represents the operations for
   setting up and maintaining the PW, and for encapsulating and
   decapsulating the PPP/HDLC packets as necessary to transmit them
   across the MPLS network.

3.  Applicability Statement

   PPP/HDLC transport over PW service is not intended to emulate the
   traditional PPP or HDLC service perfectly, but it can be used for
   some applications that require PPP or HDLC transport service.

   The applicability statements in [RFC4619] also apply to the Frame
   Relay port mode PW described in this document.

   The following are notable differences between traditional PPP/HDLC
   service, and the protocol described in this document:

   - Packet ordering can be preserved using the OPTIONAL sequence field
     in the control word; however, implementations are not required to
     support this feature.

   - The Quality of Service model for traditional PPP/HDLC links can be
     emulated, however this is outside the scope of this document.

   - A Frame Relay Port mode PW, or HDLC PW, does not process any frame
     relay status messages or alarms as described in [Q922] [Q933].

   - The HDLC Flags are processed locally in the PE connected to the
     attachment circuit.

   The HDLC mode is suitable for port-to-port transport of Frame Relay
   User Network Interface (UNI) or Network Node Interface (NNI) traffic.
   Since all packets are passed in a largely transparent manner over the
   HDLC PW, any protocol that has HDLC-like framing may use the HDLC PW
   mode, including PPP, Frame-Relay, and X.25.  Exceptions include cases
   where direct access to the HDLC interface is required, or modes that
   operate on the flags, Frame Check Sequence (FCS), or bit/byte
   unstuffing that is performed before sending the HDLC PDU over the PW.
   An example of this is PPP Asynchronous-Control-Character-Map (ACCM)

   For PPP, since media-specific framing is not carried, the following
   options will not operate correctly if the PPP peers attempt to
   negotiate them:

   - Frame Check Sequence (FCS) Alternatives

   - Address-and-Control-Field-Compression (ACFC)

   - Asynchronous-Control-Character-Map (ACCM)

   Note, also, that PW LSP Interface MTU negotiation, as specified in
   [RFC4447], is not affected by PPP Maximum Receive Unit (MRU)

   advertisement.  Thus, if a PPP peer sends a PDU with a length in
   excess of that negotiated for the PW tunnel, that PDU will be
   discarded by the ingress router.

4.  General Encapsulation Method

   This section describes the general encapsulation format for PPP and
   HDLC packets over MPLS pseudowires.

    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
   |               PSN Transport Header (As Required)              |
   |                     Pseudowire Header                         |
   |                     Control Word                              |
   |                     PPP/HDLC Service Payload                  |

     Figure 3.  General format for PPP/HDLC encapsulation over PSNs

   The PSN Transport Header depends on the particular tunneling
   technology in use.  This header is used to transport the encapsulated
   PPP/HDLC information through the packet-switched core.

   The Pseudowire Header identifies a particular PPP/HDLC service on a
   tunnel.  In case the of MPLS, the Pseudowire Header is the MPLS label
   at the bottom of the MPLS label stack.

   The Control Word is inserted before the PPP/HDLC service payload.  It
   may contain a length and sequence number.

4.1.  The Control Word

   There are four requirements that may need to be satisfied when
   transporting layer 2 protocols over an MPLS PSN:

   i.    Sequentiality may need to be preserved.

   ii.   Small packets may need to be padded in order to be transmitted
         on a medium where the minimum transport unit is larger than the
         actual packet size.

   iii.  Control bits carried in the header of the layer 2 packet may
         need to be transported.

   iv.   Creating an in-band associated channel for operation and
         maintenance communications.

   The Control Word defined in this section is based on the Generic PW
   MPLS Control Word, as defined in [RFC4385].  It provides the ability
   to sequence individual packets on the PW and avoidance of equal-cost
   multiple-path load-balancing (ECMP) [RFC2992] and enables Operations
   and Management (OAM) mechanisms, including [VCCV].

   [RFC4385] states, "If a PW is sensitive to packet mis-ordering and is
   being carried over an MPLS PSN that uses the contents of the MPLS
   payload to select the ECMP path, it MUST employ a mechanism which
   prevents packet mis-ordering."  This is necessary because ECMP
   implementations may examine the first nibble after the MPLS label
   stack to determine whether the content of the labeled packet is IP.
   Thus, if the PPP protocol number of a PPP packet carried over the PW
   without a control word present begins with 0x4 or 0x6, it could be
   mistaken for an IPv4 or IPv6 packet.  This could, depending on the
   configuration and topology of the MPLS network, lead to a situation
   where all packets for a given PW do not follow the same path.  This
   may increase out-of-order packets on a given PW or cause OAM packets
   to follow a different path from that of actual traffic.

   The features that the control word provides may not be needed for a
   given PPP/HDLC PW.  For example, ECMP may not be present or active on
   a given MPLS network, and strict packet sequencing may not be
   required.  If this is the case, the control word provides little
   value and is therefore optional.  Early PPP/HDLC PW implementations
   have been deployed that do not include a control word or the ability
   to process one if present.  To aid in backwards compatibility, future
   implementations MUST be able to send and receive packets without the
   control word.

   In all cases, the egress PE MUST be aware of whether the ingress PE
   will send a control word over a specific PW.  This may be achieved by
   configuration of the PEs, or by signaling, as defined in [RFC4447].

   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
   |0 0 0 0|0 0 0 0|FRG|   Length  |     Sequence Number           |

                   Figure 4.  MPLS PWE3 control word

   In the above diagram, the first 4 bits are set to 0 in indicate a CW

   The next 4 bits provide space for carrying protocol-specific flags.
   These are not used for HDLC/PPP, and they MUST be set to 0 for
   transmitting and MUST be ignored upon receipt.

   The next 2 bits are defined in [RFC4623].

   The next 6 bits provide a length field, which is used as follows: If
   the packet's length (defined as the length of the layer 2 payload
   plus the length of the control word) is less than 64 bytes, the
   length field MUST be set to the packet's length.  Otherwise, the
   length field MUST be set to zero.  The value of the length field, if
   not zero, is used to remove any padding that may have been added by
   the MPLS network.  If the control word is used and padding was added
   to the packet in transit on the MPLS network, then when the packet
   reaches the egress PE the padding MUST be removed before forwarding
   the packet.

   The next 16 bits provide a sequence number that can be used to
   guarantee ordered packet delivery.  The processing of the sequence
   number field is OPTIONAL.[RFC4385]

   The sequence number space is a 16-bit, unsigned circular space.  The
   sequence number value 0 is used to indicate an unsequenced

   The procedures described in Section 4 of [RFC4385] MUST be followed
   to process the sequence number field.

4.2.  MTU Requirements

   The network MUST be configured with an MTU that is sufficient to
   transport the largest encapsulation packets.  When MPLS is used as
   the tunneling protocol, for example, this is likely to be 12 or more
   bytes greater than the largest packet size.  The methodology
   described in [RFC4623] MAY be used to fragment encapsulated packets
   that exceed the PSN MTU.  However, if [RFC4623] is not used, then if
   the ingress router determines that an encapsulated layer 2 PDU
   exceeds the MTU of the PSN tunnel through which it must be sent, the
   PDU MUST be dropped.

   If a packet is received on the attachment circuit that exceeds the
   interface MTU subTLV value [RFC4447], it MUST be dropped.  It is also
   RECOMMENDED that PPP devices be configured to not negotiate PPP MRUs
   larger than that of the AC MTU.

5.  Protocol-Specific Details

5.1.  HDLC

   HDLC mode provides port-to-port transport of HDLC-encapsulated
   traffic.  The HDLC PDU is transported in its entirety, including the
   HDLC address and control fields, but excluding HDLC flags and the
   FCS.  Bit/Byte stuffing is undone.  If the OPTIONAL control word is
   used, then the flag bits in the control word are not used and MUST be
   set to 0 for transmitting and MUST be ignored upon receipt.

   When the PE detects a status change in the attachment circuit status,
   such as an attachment circuit physical link failure, or if the AC is
   administratively disabled, the PE MUST send the appropriate PW status
   notification message that corresponds to the HDLC AC status.  In a
   similar manner, the local PW status MUST also be reflected in a
   respective PW status notification message, as described in [RFC4447].

   The PW of type 0x0006 "HDLC" will be used to transport HDLC packets.
   The IANA allocation registry of "Pseudowire Type" is defined in the
   IANA allocation document for PWs [RFC4446] along with initial
   allocated values.

5.2.  Frame Relay Port Mode

   Figure 5 illustrates the concept of frame relay port mode or many-
   to-one mapping, which is an OPTIONAL capability.

   Figure 5a shows two frame relay devices physically connected with a
   frame relay UNI or NNI.  Between their two ports, P1 and P2, n frame
   relay Virtual Circuits (VCs) are configured.

   Figure 5b shows the replacement of the physical frame relay interface
   with a pair of PEs and a PW between them.  The interface between a
   Frame Relay (FR) device and a PE is either an FR UNI or an NNI.  All
   FR VCs carried over the interface are mapped into one HDLC PW.  The
   standard frame relay Link Management Interface (LMI) procedures
   happen directly between the CEs.  Thus with port mode, we have many-
   to-one mapping between FR VCs and a PW.

              +------+                          +-------+
              | FR   |                          |   FR  |
              |device|         FR UNI/NNI       | device|
              |    [P1]------------------------[P2]     |
              |      |      carrying n FR VCs   |       |
              +------+                          +-------+

                 [Pn]: A port

                  Figure 5a.  FR interface between two FR devices

                    |                              |
                     +----+                  +----+
   +------+          |    |     One PW       |    |         +------+
   |      |          |    |==================|    |         |      |
   |  FR  |    FR    | PE1| carrying n FR VCs| PE2|    FR   |  FR  |
   |device|----------|    |                  |    |---------|device|
   | CE1  | UNI/NNI  |    |                  |    | UNI/NNI | CE2  |
   +------+          +----+                  +----+         +------+
          |                                                 |
                                  n FR VCs

           Figure 5b.  Pseudowires replacing the FR interface

   FR VCs are not visible individually to a PE; there is no
   configuration of individual FR VC in a PE.  A PE processes the set of
   FR VCs assigned to a port as an aggregate.

   FR port mode provides transport between two PEs of a complete FR
   frame using the same encapsulation as described above for HDLC mode.

   Although frame relay port mode shares the same encapsulation as HDLC
   mode, a different PW type is allocated in [RFC4446]: 0x000F Frame-
   Relay Port mode.

   All other aspects of this PW type are identical to the HDLC PW
   encapsulation described above.

5.3.  PPP

   PPP mode provides point-to-point transport of PPP-encapsulated
   traffic, as specified in [RFC1661].  The PPP PDU is transported in
   its entirety, including the protocol field (whether compressed using
   Protocol Field Compression or not), but excluding any media-specific
   framing information, such as HDLC address and control fields or FCS.

   If the OPTIONAL control word is used, then the flag bits in the
   control word are not used and MUST be set to 0 for transmitting and
   MUST be ignored upon receipt.

   When the PE detects a status change in the attachment circuit (AC)
   status, such as an attachment circuit physical link failure, or if
   the AC is administratively disabled, the PE MUST send the appropriate
   PW status notification message that corresponds to the PPP AC status.
   Note that PPP negotiation status is transparent to the PW and MUST
   NOT be communicated to the remote MPLS PE.  In a similar manner, the
   local PW status MUST also be reflected in a respective PW status
   notification message, as described in [RFC4447].

   A PW of type 0x0007 "PPP" will be used to transport PPP packets.

   The IANA allocation registry of "Pseudowire Type" is defined in the
   IANA allocation document for PWs [RFC4446] along with initial
   allocated values.

6.  Using an MPLS Label as the Demultiplexer Field

   To use an MPLS label as the demultiplexer field, a 32-bit label stack
   entry [RFC3032] is simply prepended to the emulated PW encapsulation
   and thus appears as the bottom label of an MPLS label stack.  This
   label may be called the "PW label".  The particular emulated PW
   identified by a particular label value must be agreed by the ingress
   and egress LSRs, either by signaling (e.g., via the methods of
   [RFC4447]) or by configuration.  Other fields of the label stack
   entry are set as described below.

6.1.  MPLS Shim EXP Bit Values

   If it is desired to carry Quality of Service information, the Quality
   of Service information SHOULD be represented in the EXP field of the
   PW label.  If more than one MPLS label is imposed by the ingress LSR,
   the EXP field of any labels higher in the stack MUST also carry the
   same value.

6.2.  MPLS Shim S Bit Value

   The ingress LSR, PE1, MUST set the S bit of the PW label to a value
   of 1 to denote that the PW label is at the bottom of the stack.

7.  Congestion Control

   As explained in [RFC3985], the PSN carrying the PW may be subject to
   congestion, the characteristics of which are dependent upon PSN type,
   network architecture, configuration, and loading.  During congestion,
   the PSN may exhibit packet loss that will impact the service carried
   by the PPP/HLDC PW.  In addition, since PPP/HDLC PWs carry an
   unspecified type of services across the PSN, they cannot behave in a
   TCP-friendly manner prescribed by [RFC2914].  In the presence of
   services that reduce transmission rate, PPP/HDLC PWs will thus
   consume more than their fair share and SHOULD be halted.

   Whenever possible, PPP/HDLC PWs should be run over traffic-engineered
   PSNs providing bandwidth allocation and admission control mechanisms.
   IntServ-enabled domains providing the Guaranteed Service (GS) or
   DiffServ-enabled domains using EF (expedited forwarding) are examples
   of traffic-engineered PSNs.  Such PSNs will minimize loss and delay
   while providing some degree of isolation of the PPP/HDLC PW's effects
   from neighboring streams.

   The PEs SHOULD monitor for congestion (by using explicit congestion
   notification, [VCCV], or by measuring packet loss) in order to ensure
   that the service using the PPP/HDLC PW may be maintained.  When
   significant congestion is detected, the PPP/HDLC PW SHOULD be
   administratively disabled.  If the PW has been set up using the
   protocol defined in [RFC4447], then procedures specified in [RFC4447]
   for status notification can be used to disable packet transmission on
   the ingress PE from the egress PE.  The PW may be restarted by manual
   intervention, or by automatic means after an appropriate waiting

8.  IANA Considerations

   This document has no new IANA Actions.  All necessary IANA actions
   have already been included in [RFC4446].

9.  Security Considerations

   The PPP and HDLC pseudowire type is subject to all the general
   security considerations discussed in [RFC3985][RFC4447].  This
   document specifies only encapsulations, and not the protocols that
   may be used to carry the encapsulated packets across the MPLS
   network.  Each such protocol may have its own set of security issues,
   but those issues are not affected by the encapsulations specified

10.  Normative References

   [RFC1661]    Simpson, W., "The Point-to-Point Protocol (PPP)", STD
                51, RFC 1661, July 1994.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3032]    Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                Encoding", RFC 3032, January 2001.

   [RFC4385]    Bryant, S., Swallow, G., Martini, L., and D. McPherson,
                "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word
                for Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC4446]    Martini, L., "IANA Allocations for Pseudowire Edge to
                Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [RFC4447]    Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
                Heron, "Pseudowire Setup and Maintenance Using the Label
                Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4619]    Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
                "Encapsulation Methods for Transport of Frame Relay over
                Multiprotocol Label Switching (MPLS) Networks", RFC
                4619, September 2006.

   [RFC4623]    Malis, A. and M. Townsley, "Pseudowire Emulation Edge-
                to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
                August 2006.

11.  Informative References

   [Q922]       ITU-T Recommendation Q.922 Specification for Frame Mode
                Basic call control, ITU Geneva 1995.

   [Q933]       ITU-T Recommendation Q.933 Specification for Frame Mode
                Basic call control, ITU Geneva 2003.

   [RFC2914]    Floyd, S., "Congestion Control Principles", BCP 41, RFC
                2914, September 2000.

   [RFC2992]    Hopps, C., "Analysis of an Equal-Cost Multi-Path
                Algorithm", RFC 2992, November 2000.

   [RFC3985]    Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
                Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.

   [VCCV]       Nadeau, T., et al., "Pseudo Wire Virtual Circuit
                Connection Verification (VCCV)", Work in Progress,
                October 2005.

Contributing Author Information

   Yeongil Seo
   463-1 KT Technology Lab
   Jeonmin-dong Yusung-gu
   Daegeon, Korea

   EMail: syi1@kt.co.kr

   Toby Smith
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205

   EMail: tob@laurelnetworks.com

Authors' Addresses

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112

   EMail: lmartini@cisco.com

   Giles Heron
   Abbey Place
   24-28 Easton Street
   High Wycombe
   HP11 1NT

   EMail: giles.heron@tellabs.com

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719

   EMail: erosen@cisco.com

   Andrew G. Malis
   1415 West Diehl Road
   Naperville, IL  60563

   EMail: Andy.Malis@tellabs.com

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