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Versions: (draft-malis-pwe3-fragmentation) 00 01 02 03 04 05 06 07 08 09 10 RFC 4623

 Internet Draft                                         Andrew G. Malis
 Document: draft-ietf-pwe3-fragmentation-08.txt                 Tellabs
 Expires:  August 2005                                 W. Mark Townsley
                                                          Cisco Systems
                                                          February 2005
 
                     PWE3 Fragmentation and Reassembly
 
 IPR Statement
 
    By submitting this Internet-Draft, I certify that any applicable
    patent or other IPR claims of which I am aware have been disclosed,
    or will be disclosed, and any of which I become aware will be
    disclosed, in accordance with RFC 3668.
 
 Status of this Memo
 
    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/1id-abstracts.html
 
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html
 
 Abstract
 
    This document defines a generalized method of performing
    fragmentation for use by Pseudo Wire Emulation Edge to Edge (PWE3)
    protocols and services.
 
 Table of Contents
 
    1. Intellectual Property Statement...............................2
    2. Overview......................................................3
    3. Alternatives to PWE3 Fragmentation/Reassembly.................5
    4. PWE3 Fragmentation With MPLS..................................5
       4.1 Fragment Bit Locations For MPLS...........................6
       4.2 Other Considerations......................................6
    5. PWE3 Fragmentation With L2TP..................................6
       5.1 PW-specific Fragmentation vs. IP fragmentation............7
 
 
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       5.2 Advertising Reassembly Support in L2TP....................7
       5.3 L2TP Maximum Receive Unit (MRU) AVP.......................8
       5.4 L2TP Maximum Reassembled Receive Unit (MRRU) AVP..........8
       5.5 Fragment Bit Locations For L2TPv3 Encapsulation...........9
       5.6 Fragment Bit Locations for L2TPv2 Encapsulation...........9
    6. Security Considerations......................................10
    7. IANA Considerations..........................................10
    8. Acknowledgements.............................................10
    9. Normative References.........................................11
    10. Informative References......................................11
    11. Full Copyright Statement....................................12
    12. Authors' Addresses..........................................12
    13. Appendix A: Relationship Between This Document and RFC 1990.12
 
 
 1. Intellectual Property Statement
 
    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology
    described in this document or the extent to which any license
    under such rights might or might not be available; nor does it
    represent that it has made any independent effort to identify any
    such rights.  Information on the procedures with respect to rights
    in RFC documents can be found in BCP 78 and BCP 79.
 
    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.
 
    The IETF invites any interested party to bring to its attention
    any copyrights, patents or patent applications, or other
    proprietary rights that may cover technology that may be required
    to implement this standard.  Please address the information to the
    IETF at ietf-ipr@ietf.org.
 
 
 
 
 
 
 
 
 
 
 
 
 
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 2. Overview
 
    The Pseudo Wire Emulation Edge to Edge Architecture Document
    [Architecture] defines a network reference model for PWE3:
 
 
           |<-------------- Emulated Service ---------------->|
           |                                                  |
           |          |<------- Pseudo Wire ------>|          |
           |          |                            |          |
           |          |    |<-- PSN Tunnel -->|    |          |
           | PW End   V    V                  V    V  PW End  |
           V Service  +----+                  +----+  Service V
     +-----+    |     | PE1|==================| PE2|     |    +-----+
     |     |----------|............PW1.............|----------|     |
     | CE1 |    |     |    |                  |    |     |    | CE2 |
     |     |----------|............PW2.............|----------|     |
     +-----+  ^ |     |    |==================|    |     | ^  +-----+
           ^  |       +----+                  +----+     | |  ^
           |  |   Provider Edge 1         Provider Edge 2  |  |
           |  |                                            |  |
     Customer |                                            | Customer
     Edge 1   |                                            | Edge 2
              |                                            |
              |                                            |
        native service                               native service
 
                  Figure 1: PWE3 Network Reference Model
 
 
    A Pseudo Wire (PW) payload is normally relayed across the PW as a
    single IP or MPLS Packet Switched Network (PSN) Protocol Data Unit
    (PDU). However, there are cases where the combined size of the
    payload and its associated PWE3 and PSN headers may exceed the PSN
    path Maximum Transmission Unit (MTU). When a packet exceeds the MTU
    of a given network, fragmentation and reassembly will allow the
    packet to traverse the network and reach its intended destination.
 
    Fragmentation is also useful for real-time applications when the
    payload to be transmitted in a PW, such as a low-speed TDM
    multiframe structure, takes too much time to be encapsulated even
    though it may fit within the PW MTU.  In this case, the payload may
    be fragmented for lower-latency transmission.
 
    The purpose of this document is to define a generalized method of
    performing fragmentation for use with all PWE3 protocols and
 
 
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    services. This method should be utilized only in cases where MTU-
    management methods fail. Due to the increased processing overhead,
    fragmentation and reassembly in core network devices should always
    be considered something to avoid whenever possible.
 
    The PWE3 fragmentation and reassembly domain is shown in Figure 2:
 
 
           |<-------------- Emulated Service ---------------->|
           |          |<---Fragmentation Domain--->|          |
           |          ||<------- Pseudo Wire ---->||          |
           |          ||                          ||          |
           |          ||   |<-- PSN Tunnel -->|   ||          |
           | PW End   VV   V                  V   VV  PW End  |
           V Service  +----+                  +----+  Service V
     +-----+    |     | PE1|==================| PE2|     |    +-----+
     |     |----------|............PW1.............|----------|     |
     | CE1 |    |     |    |                  |    |     |    | CE2 |
     |     |----------|............PW2.............|----------|     |
     +-----+  ^ |     |    |==================|    |     | ^  +-----+
           ^  |       +----+                  +----+     | |  ^
           |  |   Provider Edge 1         Provider Edge 2  |  |
           |  |                                            |  |
     Customer |                                            | Customer
     Edge 1   |                                            | Edge 2
              |                                            |
              |                                            |
        native service                               native service
 
              Figure 2: PWE3 Fragmentation/Reassembly Domain
 
 
    Fragmentation takes place in the transmitting PE immediately prior
    to PW insertion, and reassembly takes place in the receiving PE
    immediately after PW extraction.
 
    Since a sequence number is necessary for the fragmentation and
    reassembly procedures, using the Sequence Number field on
    fragmented packets is REQUIRED (see sections 4.1 and 5.5 for the
    location of the Sequence Number fields for MPLS and L2TPv3
    encapsulations respectively).  The order of operation is that first
    fragmentation is performed, and then the resulting fragments are
    assigned sequential sequence numbers.
 
    Depending on the specific PWE3 encapsulation in use, the value 0
    may not be a part of the sequence number space, in which case its
 
 
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    use for fragmentation must follow this same rule - as the sequence
    number is incremented, it skips zero and wraps from 65535 to 1.
    Conversely, if the value 0 is part of the sequence space, then the
    same sequence space is also used for fragmentation and reassembly.
 
 
 3. Alternatives to PWE3 Fragmentation/Reassembly
 
    Fragmentation and reassembly in network equipment generally
    requires significantly greater resources than sending a packet as a
    single unit. As such, fragmentation and reassembly should be
    avoided whenever possible. Ideal solutions for avoiding
    fragmentation include proper configuration and management of MTU
    sizes between the Customer Edge (CE) router, Provider Edge (PE)
    router, and across the PSN, as well as adaptive measures which
    operate with the originating host [e.g. [PATHMTU], [PATHMTUv6]] to
    reduce the packet sizes at the source.
 
    A PE MAY choose to fragment a packet before allowing it to enter a
    PW. For example, if an IP packet arrives from a CE with an MTU
    which will yield a PW packet which is greater than the PW MTU, the
    PE may perform IP fragmentation on the packet. This effectively
    creates two (or more) packets, each carrying an IP fragment, for
    transport individually across the PW. The receiving PE is unaware
    that the originating host did not perform the IP fragmentation, and
    as such does not treat the PW packets in any special way. This
    ultimately has the affect of placing the burden of fragmentation on
    the PE, and reassembly on the IP destination host.
 
 
 4. PWE3 Fragmentation With MPLS
 
    When using the signaling procedures in [MPLS-Control], there is a
    Virtual Circuit FEC element parameter ID used to signal the use of
    fragmentation when advertising a VC label:
 
       Parameter   ID Length    Description
            0x09           2    Fragmentation indicator
 
    The presence of this parameter ID in the VC FEC element indicates
    that the receiver is able to reassemble fragments when the control
    word is in use for the VC label being advertised.  It does not
    obligate the sender to use fragmentation; it is simply an
    indication that the sender MAY use fragmentation.  The sender MUST
    NOT use fragmentation if this parameter ID is not present in the VC
    FEC element.
 
    If [MPLS-Control] signaling is not in use, then whether or not to
    use fragmentation MUST be configured in the sender.
 
 
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 4.1 Fragment Bit Locations For MPLS
 
    MPLS-based PWE3 uses the following control word format [Control-
    Word], with the B and E fragmentation bits identified in position 8
    and 9:
 
 
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Rsvd  | Flags |B|E|   Length  |     Sequence Number           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                    Figure 3: MPLS PWE3 Control Word
 
 
    The B and E bits are defined as follows:
 
    BE
    --
    00 indicates that the entire (un-fragmented) payload is carried
       in a single packet
    01 indicates the packet carrying the first fragment
    10 indicates the packet carrying the last fragment
    11 indicates a packet carrying an intermediate fragment
 
    See Appendix A for a discussion of the derivation of these values
    for the B and E bits.
 
    See section 2 for the description of the use of the Sequence Number
    field.
 
 4.2 Other Considerations
 
    Path MTU [PATHMTU] [PATHMTUv6] may be used to dynamically determine
    the maximum size for fragments. The application of path MTU to MPLS
    is discussed in [LABELSTACK]. The maximum size of the fragments may
    also be configured. The signaled Interface MTU parameter in [MPLS-
    Control] SHOULD be used to set the maximum size of the reassembly
    buffer for received packets to make optimal use of reassembly
    buffer resources.
 
 
 5. PWE3 Fragmentation With L2TP
 
    This section defines the location of the B and E bits for L2TPv3
    [L2TPv3] and L2TPv2 [L2TPv2] headers, as well as the signaling
    mechanism for advertising MRU (Maximum Receive Unit) values and
 
 
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    support for fragmentation on a given PW. As IP is the most common
    PSN used with L2TP, IP fragmentation and reassembly is discussed as
    well.
 
 5.1 PW-specific Fragmentation vs. IP fragmentation
 
    When proper MTU management across a network fails, IP fragmentation
    and reassembly may be used to accommodate MTU mismatches between
    tunnel endpoints. If the overall traffic requiring fragmentation
    and reassembly is very light, or there are sufficient optimized
    mechanisms for IP fragmentation and reassembly available, IP
    fragmentation and reassembly may be sufficient.
 
    When facing a large number of PW packets requiring fragmentation
    and reassembly, a PW-specific method has properties that
    potentially allow for more resource-friendly implementations.
    Specifically, the ability to assign buffer usage on a per-PW basis
    and PW sequencing may be utilized to gain advantage over a general
    mechanism applying to all IP packets across all PWs. Further, PW
    fragmentation may be more easily enabled in a selective manner for
    some or all PWs, rather than enabling reassembly for all IP traffic
    arriving at a given node.
 
    Deployments should avoid a situation which relies upon a
    combination of IP and PW fragmentation and reassembly on the same
    node. Such operation clearly defeats the purpose behind the
    mechanism defined in this document. Care must be taken to ensure
    that the MTU/MRU values are set and advertised properly at each
    tunnel endpoint to avoid this. When fragmentation is enabled within
    a given PW, the DF bit MUST be set on all L2TP over IP packets for
    that PW.
 
    L2TPv3 nodes SHOULD participate in Path MTU [PATHMTU], [PATHMTUv6]
    for automatic adjustment of the PW MTU.
 
 5.2 Advertising Reassembly Support in L2TP
 
    The constructs defined in this section for advertising
    fragmentation support in L2TP are applicable to [L2TPv3] and
    [L2TPv2].
 
    This document defines two new AVPs to advertise maximum receive
    unit values and reassembly support. These AVPs MAY be present in
    the ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, or SLI messages. The most
    recent value received always takes precedence over a previous
    value, and MUST be dynamic over the life of the session if received
    via the SLI message. One of the two new AVPs (MRRU) is used to
    advertise that PWE3 reassembly is supported by the sender of the
    AVP. Reassembly support MAY be unidirectional.
 
 
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 5.3 L2TP Maximum Receive Unit (MRU) AVP
 
        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              MRU              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    MRU (Maximum Receive Unit), attribute number TBD1, is the maximum
    size in octets of a fragmented or complete PW frame, including L2TP
    encapsulation, receivable by the side of the PW advertising this
    value. The advertised MRU does NOT include the PSN header (i.e. the
    IP and/or UDP header). This AVP does not imply that PWE3
    fragmentation or reassembly is supported. If reassembly is not
    enabled or unavailable, this AVP may be used alone to advertise the
    MRU for a complete frame.
 
    All L2TP AVPs have an M (Mandatory) bit, H (Hidden) bit, Length,
    and Vendor ID. This AVP may be hidden (the H bit may be 0 or 1).
    The M bit for this AVP SHOULD be set to 0.  The Length (before
    hiding) is 8. The Vendor ID is the IETF Vendor ID of 0.
 
 5.4 L2TP Maximum Reassembled Receive Unit (MRRU) AVP
 
        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              MRRU             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    MRRU (Maximum Reassembled Receive Unit AVP), attribute number TBD2,
    is the maximum size in octets of a reassembled frame, including any
    PW framing, but not including the L2TP encapsulation or L2-specific
    sublayer. Presence of this AVP signifies the ability to receive PW
    fragments and reassemble them. Packet fragments MUST NOT be sent by
    a peer which has not received this AVP in a control message. If the
    MRRU is present in a message, the MRU AVP MUST be present as well.
 
    All L2TP AVPs have an M (Mandatory) bit, H (Hidden) bit, Length,
    and Vendor ID. This AVP may be hidden (the H bit may be 0 or 1).
    The M bit for this AVP SHOULD be set to 0.  The Length (before
    hiding) is 8. The Vendor ID is the IETF Vendor ID of 0.
 
 
 
 
 
 
 
 
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 5.5 Fragment Bit Locations For L2TPv3 Encapsulation
 
    The B and E bits are defined as bits 2 and 3 in the L2TPv3 default
    L2-specific sublayer as depicted below, using the values defined in
    section 3.1:
 
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |x|S|B|E|x|x|x|x|              Sequence Number                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                      Figure 4: L2TPv3 over IP Header
 
 
    The S bit is as defined in [L2TPv3]. Location of the B and E bits
    for PW-Types which use a variant L2 specific sublayer are outside
    the scope of this document.
 
    When fragmentation is used, an L2-Specific Sublayer with B and E
    bits defined MUST be present in all data packets for a given
    session. Presence of the L2-Specific Sublayer is advertised via AVP
    69, L2-Specific Sublayer AVP, defined in section 5.4.4 of [L2TPv3].
 
    See section 4.1 for the description of the use of the B and E bits.
 
    See section 2 for the description of the use of the Sequence Number
    field.
 
 5.6 Fragment Bit Locations for L2TPv2 Encapsulation
 
    The B and E bits are defined as bits 8 and 9 for the L2TPv2 header
    as depicted below (subject to IANA action), using the values
    defined in section 3.1:
 
 
    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |T|L|x|x|S|x|O|P|B|E|x|x|  Ver  |          Length (opt)         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Tunnel ID           |           Session ID          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Ns (opt)          |             Nr (opt)          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Offset Size (opt)        |    Offset pad... (opt)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                      Figure 5: L2TPv2 over UDP Header
 
 
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 6. Security Considerations
 
    As with any additional protocol construct, each level of complexity
    adds the potential to exploit protocol and implementation errors.
    Implementers should be especially careful of not tying up an
    abundance of resources, even for the most pathological combination
    of packet fragments that could be received. Beyond these issues of
    general implementation quality, there are no known notable security
    issues with using the mechanism defined in this document.  It
    should be pointed out that RFC 1990, on which this document is
    based, and its derivatives have been widely implemented and
    extensively used in the Internet and elsewhere.
 
    [IPFRAG-SEC] and [TINYFRAG] describe potential network attacks
    associated with IP fragmentation and reassembly. The issues
    described in these documents attempt to bypass IP access controls
    by sending various carefully formed "tiny fragments", or by
    exploiting the IP offset field to cause fragments to overlap and
    rewrite interesting portions of an IP packet after access checks
    have been performed. The latter is not an issue with the PW-
    specific fragmentation method described in this document as there
    is no offset field; However, implementations MUST be sure to not
    allow more than one whole fragment to overwrite another in a
    reconstructed frame. The former may be a concern if packet
    filtering and access controls are being placed on tunneled frames
    within the PW encapsulation. To circumvent any possible attacks in
    either case, all filtering and access controls should be applied to
    the resulting reconstructed frame rather than any PW fragments.
 
 
 7. IANA Considerations
 
    This document does not define any new registries for IANA to
    maintain.
 
    This document requires definition of two reserved bits in the
    L2TPv2 [L2TPv2] header. Locations are noted by the "B" and "E" bits
    in section 5.6.
 
    This document requires IANA to assign two new L2TP "Control Message
    Attribute Value Pairs" (TBD1 and TBD2 in this document).
 
 
 8. Acknowledgements
 
    Thanks to Eric Rosen for his review of this document.
 
 
 
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 9. Normative References
 
    [Control-Word] Bryant, S. et al, "PWE3 Control Word for use over an
        MPLS PSN", draft-ietf-pwe3-cw-00.txt, October 2004, work in
        progress
 
    [LABELSTACK] Rosen, E. et al, "MPLS Label Stack Encoding", RFC
        3032, January 2001
 
    [L2TPv2] Townsley, Valencia, Rubens, Pall, Zorn, Palter, "Layer Two
        Tunneling Protocol 'L2TP'", RFC 2661, June 1999
 
    [L2TPv3] Lau, J. et al, "Layer Two Tunneling Protocol (Version 3)
        'L2TPv3'", RFC 3931, November 2004.
 
    [MLPPP] Sklower, K. et al, "The PPP Multilink Protocol (MP)", RFC
        1990, August 1996
 
    [MPLS-Control] Martini, L. et al, "Pseudowire Setup and Maintenance
        using LDP", draft-ietf-pwe3-control-protocol-13.txt, November
        2004, work in progress
 
    [PATHMTU] Mogul, J. C. et al, "Path MTU Discovery", RFC 1191,
        November 1990
 
    [PATHMTUv6] McCann, J. et al, "Path MTU Discovery for IP version
        6", RFC 1981, August 1996
 
 
 10. Informative References
 
    [Architecture] Bryant, S. et al, "PWE3 Architecture", draft-ietf-
        pwe3-arch-07.txt, March 2004, work in progress
 
    [FAST] ATM Forum, "Frame Based ATM over SONET/SDH Transport
        (FAST)", af-fbatm-0151.000, July 2000
 
    [FRF.12] Frame Relay Forum, "Frame Relay Fragmentation
        Implementation Agreement", FRF.12, December 1997
 
    [IPFRAG-SEC] Ziemba, G., Reed, D., Traina, P., "Security
        Considerations for IP Fragment Filtering", RFC 1858, October
        1995
 
    [TINYFRAG] Miller, I., "Protection Against a Variant of the Tiny
        Fragment Attack", RFC 3128, June 2001
 
 
 
 
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 11. Full Copyright Statement
 
    Copyright (C) The Internet Society (2004).  This document is
    subject to the rights, licenses and restrictions contained in BCP
    78 and except as set forth therein, the authors retain all their
    rights.
 
    This document and the information contained herein are provided on
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
    THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
    THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
    ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
    PARTICULAR PURPOSE.
 
 
 12. Authors' Addresses
 
    Andrew G. Malis
    Tellabs
    90 Rio Robles Drive
    San Jose, CA 95134
    Email: Andy.Malis@tellabs.com
 
    W. Mark Townsley
    Cisco Systems
    7025 Kit Creek Road
    PO Box 14987
    Research Triangle Park, NC 27709
    Email: mark@townsley.net
 
 
 13. Appendix A: Relationship Between This Document and RFC 1990
 
    The fragmentation of large packets into smaller units for
    transmission is not new.  One fragmentation and reassembly method
    was defined in RFC 1990, Multi-Link PPP [MLPPP].  This method was
    also adopted for both Frame Relay [FRF.12] and ATM [FAST] network
    technology.  This document adopts the RFC 1990 fragmentation and
    reassembly procedures as well, with some distinct modifications
    described in this appendix.  Familiarity with RFC 1990 is assumed.
 
    RFC 1990 was designed for use in environments where packet
    fragments may arrive out of order due to their transmission on
    multiple parallel links, specifying that buffering be used to place
    the fragments in correct order.  For PWE3, the ability to reorder
    fragments prior to reassembly is OPTIONAL; receivers MAY choose to
    drop frames when a lost fragment is detected. Thus, when the
 
 
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    sequence number on received fragments shows that a fragment has
    been skipped, the partially reassembled packet MAY be dropped, or
    the receiver MAY wish to wait for the fragment to arrive out of
    order.  In the latter case, a reassembly timer MUST be used to
    avoid locking up buffer resources for too long a period.
 
    Dropping out-of-order fragments on a given PW can provide a
    considerable scalability advantage for network equipment performing
    reassembly. If out-of-order fragments are a relatively rare event
    on a given PW, throughput should not be adversely affected by this.
    Note, however, if there are cases where fragments of a given frame
    are received out-or-order in a consistent manner (e.g. a short
    fragment is always switched ahead of a larger fragment) then
    dropping out-of-order fragments will cause the fragmented frame to
    never be received. This condition may result in an effective denial
    of service to a higher-lever application. As such, implementations
    fragmenting a PW frame MUST at the very least ensure that all
    fragments are sent in order from their own egress point.
 
    An implementation may also choose to allow reassembly of a limited
    number of fragmented frames on a given PW, or across a set of PWs
    with reassembly enabled. This allows for a more even distribution
    of reassembly resources, reducing the chance of a single or small
    set of PWs exhausting all reassembly resources for a node. As with
    dropping out-of-order fragments, there are perceivable cases where
    this may also provide an effective denial of service. For example,
    if fragments of multiple frames are consistently received before
    each frame can be reconstructed in a set of limited PW reassembly
    buffers, then a set of these fragmented frames will never be
    delivered.
 
    RFC 1990 headers use two bits which indicate the first and last
    fragments in a frame, and a sequence number.  The sequence number
    may be either 12 or 24 bits in length (from [MLPPP]):
 
                     0             7 8            15
                    +-+-+-+-+-------+---------------+
                    |B|E|0|0|    sequence number    |
                    +-+-+-+-+-------+---------------+
 
                    +-+-+-+-+-+-+-+-+---------------+
                    |B|E|0|0|0|0|0|0|sequence number|
                    +-+-+-+-+-+-+-+-+---------------+
                    |      sequence number (L)      |
                    +---------------+---------------+
 
                    Figure 6: RFC 1990 Header Formats
 
 
 
 
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    PWE3 fragmentation takes advantage of existing PW sequence numbers
    and control bit fields wherever possible, rather than defining a
    separate header exclusively for the use of fragmentation.  Thus, it
    uses neither of the RFC 1990 sequence number formats described
    above, relying instead on the sequence number that already exists
    in the PWE3 header.
 
    RFC 1990 defines a two one-bit fields, a (B)eginning fragment bit
    and an (E)nding fragment bit. The B bit is set to 1 on the first
    fragment derived from a PPP packet and set to 0 for all other
    fragments from the same PPP packet. The E bit is set to 1 on the
    last fragment and set to 0 for all other fragments.  A complete
    unfragmented frame has both the B and E bits set to 1.
 
    PWE3 fragmentation inverts the value of the B and E bits, while
    retaining the operational concept of marking the beginning and
    ending of a fragmented frame. Thus, for PW the B bit is set to 0 on
    the first fragment derived from a PW frame and set to 1 for all
    other fragments derived from the same frame. The E bit is set to 0
    on the last fragment and set to 1 for all other fragments.  A
    complete unfragmented frame has both the B and E bits set to 0. The
    motivation behind this value inversion for the B and E bits is to
    allow complete frames (and particularly, implementations that only
    support complete frames) to simply leave the B and E bits in the
    header set 0.
 
    In order to support fragmentation, the B and E bits MUST be defined
    or identified for all PWE3 tunneling protocols. Sections 4 and 5
    define these locations for PWE3 MPLS [Control-Word], L2TPv2
    [L2TPv2], and L2TPv3 [L2TPv3] tunneling protocols.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Malis, Townsley          Expires August 2005                [Page 14]
 

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