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Versions: 00 01 02 03 04 RFC 4720

 Internet Draft                                         Andrew G. Malis
 Document: draft-ietf-pwe3-fcs-retention-00.txt                 Tellabs
 Expires:  May 2004                                         David Allan
                                                        Nortel Networks
                                                         Nick Del Regno
                                                          November 2003
                    PWE3 Frame Check Sequence Retention
 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-
    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
    The list of current Internet-Drafts can be accessed at
    The list of Internet-Draft Shadow Directories can be accessed at
    This document defines a mechanism for preserving frame FCS through
    Ethernet, Frame Relay, and HDLC pseudowires.
 Table of Contents
    1. Specification of Requirements.................................2
    2. Overview......................................................2
    3. FCS Retention With MPLS-based Pseudowires.....................3
    4. FCS Retention With L2TPv3-based Pseudowires...................4
    5. Security Considerations.......................................4
    6. IANA Considerations...........................................4
    7. References....................................................5
    8. Authors' Addresses............................................5
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                           PWE3 FCS Retention             November 2003
 1. Specification of Requirements
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    this document are to be interpreted as described in RFC 2119 [6].
 2. Overview
    The specifications for Ethernet, Frame Relay, HDLC, and PPP
    pseudowire encapsulation [1] [2] [3] include a mode of use where
    frames are transparently delivered across the pseudowire without
    any header or other alterations by the pseudowire ingress or egress
    Provider Edge (PE). [Note that this mode is inherent for HDLC and
    PPP Pseudowires.]
    However, these specifications all specify that the original Frame
    Check Sequence (FCS) be removed at ingress and regenerated at
    egress, which means that the frames may be subject to unintentional
    alteration during their traversal of the pseudowire from the
    ingress to the egress PE.  Thus, the pseudowire cannot be
    absolutely guaranteed to be "transparent" in nature.
    To be more precise, pseudowires, as currently defined, leave the
    payload vulnerable to undetected errors caused by the encapsulating
    network. As defined for MPLS, not only can a PW-aware device
    internally corrupt an encapsulated payload, but ANY MPLS LSR in the
    path can corrupt the encapsulated payload. In the event of such
    corruption, there is no way to detect the corruption through
    the path of the pseudowire. Further, because the FCS is calculated
    upon network egress, any corruption will pass transparently through
    ALL Layer 2 switches(Ethernet and Frame Relay) through which the
    packets travel. Only at the endpoint, assuming the corrupted packet
    even reaches the correct endpoint, can the packet be discarded, and
    depending on the contents of the packet, the corruption may not
    ever be detected.
    Not only does the encapsulation technique leave the payload
    unprotected, it also subverts the error checking mechanisms already
    in place in SP and customer networks by calculating FCS on
    questionable data.
    In a perfect network comprising perfect equipment, this is not an
    issue. However, as there is no such thing, it is an issue. SPs
    should have the option of saving overhead by yielding the ability
    to detect faults. Equally, SPs should have the option to sacrifice
    the overhead of carrying the original FCS end-to-end to ensure the
    ability to detect faults in the encapsulating network.
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                           PWE3 FCS Retention             November 2003
    This document defines such a mechanism to allow the ingress PE to
    retain the original frame FCS on ingress to the network, and
    relieves the egress PE of the task of regenerating the FCS.
    This in an OPTIONAL mechanism for pseudowire implementations.  For
    interoperability with systems that do not implement this document,
    the default behavior is that the FCS is removed at the ingress PE
    and regenerated at the egress PE, as specified in [1], [2], and
    Note that this mechanism is not intended to carry errored frames
    through the pseudowire; as usual, the FCS MUST be examined at the
    ingress PE and errored frames MUST be discarded.  The FCS MAY also
    be examined by the egress PE; if this is done, errored frames MUST
    be discarded.  The egress PE MAY also wish to generate an alarm or
    count the number of errored frames.
 3. Signaling FCS Retention With MPLS-based Pseudowires
    When using the signaling procedures in [4], there is a Virtual
    Circuit FEC element parameter ID used to signal the desire to
    retain the FCS when advertising a VC label:
       Parameter   ID Length    Description
            0x??           2    FCS Retention Indicator
      Note: The next currently available parameter number is 0x0A [5].
      This parameter number will be added to [5] if the timing permits
      while it is still a draft, otherwise it will be assigned by IANA.
      This note is to be removed once the assignment has been made.
    The presence of this parameter ID in the VC FEC element indicates
    that the egress PE requests the ingress PE to retain the FCS for
    the VC label being advertised.  It does not obligate the ingress PE
    to retain the FCS; it is simply an indication that the ingress PE
    MAY retain the FCS.  The sender MUST NOT retain the FCS if this
    parameter ID is not present in the VC FEC element.
    Since unknown parameter IDs are silently ignored [4], backwards
    compatibility with systems that do not implement this document is
    provided by requiring that the FCS is retained ONLY if the FCS
    Retention Indicator has been included in the advertisements for
    both directions on a pseudowire.
    If the ingress PE recognizes the FCS Retention Indicator parameter
    ID, but does not wish to retain the FCS, it need only issue its own
    label mapping message for the opposite direction without including
    the FCS Retention Indicator.  This will prevent FCS retention in
    either direction.
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                           PWE3 FCS Retention             November 2003
    If PWE3 signaling [4] is not in use for a pseudowire, then whether
    or not the FCS is to be retained MUST be identically provisioned in
    both PEs at the pseudowire endpoints.  If there is no provisioning
    support for this option, the default behavior is to remove the FCS.
 4. Signaling FCS Retention With L2TPv3-based Pseudowires
    This section will be added in a subsequent revision.
 5. Security Considerations
    This mechanism enhances the data integrity of transparent Ethernet,
    Frame Relay, and HDLC pseudowires, because the original FCS, as
    generated by the Customer Edge (CE), is included in the
    encapsulation.  When the encapsulated payload passes FCS checking
    at the destination CE, it is clear that the payload was not altered
    during its transmission through the network (or at least to the
    accuracy of the original FCS; but that is demonstratably better
    than no FCS at all).
    Of course, nothing comes for free; this requires the additional
    overhead of carrying the original FCS (in general, either two or
    four octets per payload packet).
    This signaling is backwards compatible and interoperable with
    systems that do not implement this document.
 6. Applicability Statement
    In general, this document is intended to further extend the
    applicability of the services defined by [1], [2], and [3] to make
    them more suitable for use in deployments where data integrity is
    an issue (or at least, is as much an issue as in the original
    services that defined the FCS usage in the first place).  There are
    some situations where this extension is not necessary, such as
    where the inner payloads have their own error-checking capabilities
    (such as TCP).  But for inner payloads that do rely on the error-
    detecting capabilities of the link layer (such as SNA), this
    additional protection can be invaluable.
    When pseudowires are being used to connect 802.1 bridges, this
    document allows pseudowires to comply with the requirement that all
    media interconnecting 802.1 bridges have (at least) 32-bit FCS
    Note that this document is one possible alternative for a service
    provider to enhance the end-to-end data integrity of pseudowires.
    Other mechanisms may include the use of end-to-end IPSec between
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                           PWE3 FCS Retention             November 2003
    the PEs, or internal mechanisms in the P routers to assure the
    integrity of packets as they are switched between ingress and
    egress interfaces.  Service providers may wish to compare the
    relative strengths of each approach when planning their pseudowire
    deployments; however, an argument can be made that it may be
    wasteful for a SP to use an end-to-end integrity mechanism that is
    STRONGER than the FCS generated by the source CE and checked by the
    destination CE.
 7. IANA Considerations
    This document requires IANA to allocate a PWE3 Virtual Circuit FEC
    element parameter ID [5].
 8. References
    [1] Martini, L. et al, "Encapsulation Methods for Transport of
        Ethernet Frames Over IP and MPLS Networks", draft-ietf-pwe3-
        ethernet-encap-04.txt, October 2003, work in progress
    [2] Martini, L. et al, "Frame Relay Encapsulation over Pseudo-
        Wires", draft-ietf-pwe3-frame-relay-01.txt, July 2003, work in
    [3] Martini, L. et al, "Encapsulation Methods for Transport of
        PPP/HDLC Frames Over IP and MPLS Networks", draft-ietf-pwe3-
        hdlc-ppp-encap-mpls-00.txt, June 2003, work in progress
    [4] Martini, L. et al, "Transport of Layer 2 Frames Over MPLS",
        draft-ietf-pwe3-control-protocol-04.txt, October 2003, work in
    [5] Martini, L. et al, "IANA Allocations for pseudo Wire Edge to
        Edge Emulation (PWE3)", draft-ietf-pwe3-iana-allocation-02.txt,
        October 2003, work in progress
    [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, March 1997.
 9. Authors' Addresses
    Andrew G. Malis
    90 Rio Robles Dr.
    San Jose, CA 95134
    Email: Andy.Malis@tellabs.com
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                           PWE3 FCS Retention             November 2003
    David Allan
    Nortel Networks
    3500 Carling Ave.
    Ottawa, Ontario, CANADA
    Email: dallan@nortelnetworks.com
    Nick Del Regno
    400 International Parkway
    Richardson, TX 75081
    Email: nick.delregno@mci.com
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