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rfc:rfc4623

Network Working Group A. Malis Request for Comments: 4623 Tellabs Category: Standards Track M. Townsley

                                                         Cisco Systems
                                                           August 2006
             Pseudowire Emulation Edge-to-Edge (PWE3)
                   Fragmentation and Reassembly

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).

Abstract

 This document defines a generalized method of performing
 fragmentation for use by Pseudowire Emulation Edge-to-Edge (PWE3)
 protocols and services.

Malis & Townsley Standards Track [Page 1] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

Table of Contents

 1. Introduction ....................................................3
 2. Conventions Used in This Document ...............................4
 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
    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
    7.1. Control Message Attribute Value Pairs (AVPs) ..............11
    7.2. Default L2-Specific Sublayer Bits .........................11
    7.3. Leading Bits of the L2TPv2 Message Header .................11
 8. Acknowledgements ...............................................11
 9. Normative References ...........................................12
 10. Informative References ........................................12
 Appendix A. Relationship Between This Document and RFC 1990 .......14

Malis & Townsley Standards Track [Page 2] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

1. Introduction

 The Pseudowire Emulation Edge-to-Edge Architecture Document
 [Architecture] defines a network reference model for PWE3:
       |<-------------- Emulated Service ---------------->|
       |                                                  |
       |          |<------- Pseudowire ------->|          |
       |          |                            |          |
       |          |    |<-- 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 Pseudowire (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.
 The purpose of this document is to define a generalized method of
 performing fragmentation for use with all PWE3 protocols and
 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:

Malis & Townsley Standards Track [Page 3] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

       |<-------------- Emulated Service ---------------->|
       |          |<---Fragmentation Domain--->|          |
       |          ||<------- Pseudowire ----->||          |
       |          ||                          ||          |
       |          ||   |<-- 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 encapsulation, and reassembly takes place in the receiving PE
 immediately after PW decapsulation.
 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 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.

2. Conventions Used in This Document

 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 [KEYWORDS].

Malis & Townsley Standards Track [Page 4] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

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 and Provider Edge (PE) router and across
 the PSN, as well as adaptive measures that operate with the
 originating host (e.g., [PATHMTU], [PATHMTUv6]) to reduce the packet
 sizes at the source.
 In some cases, a PE may be able to fragment an IP version 4 (IPv4)
 [RFC791] packet before it enters a PW.  For example, if the PE can
 fragment and forward IPv4 packets with the DF bit clear in a manner
 that is identical to an IPv4 router, it may fragment packets arriving
 from a CE, forwarding the IPv4 fragments with associated framing for
 that attachment circuit (AC) over the PW.  Architecturally, the IPv4
 fragmentation happens before reaching the PW, presenting multiple
 frames to the PW to forward in the normal manner for that PWType.
 Thus, this method is entirely transparent to the PW encapsulation and
 to the remote end of the PW itself.  Packet fragments are ultimately
 reassembled on the destination IPv4 host in the normal way.  IPv6
 packets are not to be fragmented in this manner.

4. PWE3 Fragmentation with MPLS

 When using the signaling procedures in [MPLS-Control], there is a
 Pseudowire Interface Parameter Sub-TLV type used to signal the use of
 fragmentation when advertising a VC label [IANA]:
    Parameter      Length    Description
         0x09           4    Fragmentation indicator
 The presence of this parameter 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 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.

Malis & Townsley Standards Track [Page 5] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Flags |B|E|   Length  |     Sequence Number           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 3: Preferred PW MPLS 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 1 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
 support for fragmentation on a given PW.  As IP is the most common
 PSN used with L2TP, IP PSN fragmentation and reassembly is discussed
 as well.

Malis & Townsley Standards Track [Page 6] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

5.1. PW-Specific Fragmentation vs. IP fragmentation

 When proper MTU management across a network fails, IP PSN
 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 PSN fragmentation and
 reassembly available, IP PSN 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 that uses a combination of IP
 PSN and PW fragmentation and reassembly on the same node.  Such
 operation clearly defeats the purpose behind the mechanism defined in
 this document.  This is especially important for L2TPv3 pseudowires,
 since potentially fragmentation can take place in three different
 places (the IP PSN, the PW, and the encapsulated payload).  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 PSN MTU.  When the payload is IP,
 Path MTU should be used at they payload level as well.

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
 Incoming-Call-Request (ICRQ), Incoming-Call-Reply (ICRP), Incoming-
 Call-Connected (ICCN), Outgoing-Call-Request (OCRQ), Outgoing-Call-
 Reply (OCRP), Outgoing-Call-Connected (OCCN), or Set-Link-Info (SLI)
 messages.  The most recent value received always takes precedence
 over a previous value and MUST be dynamic over the life of the

Malis & Townsley Standards Track [Page 7] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

 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.

5.3. L2TP Maximum Receive Unit (MRU) AVP

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H|0|0|0|0|    Length         |              0                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |              MRU              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 4: L2TP Maximum Receive Unit (MRU) AVP
 MRU (Maximum Receive Unit), attribute number 94, 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.
 This AVP MAY be hidden (the H bit MAY be 0 or 1).  The mandatory (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                   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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H|0|0|0|0|    Length         |              0                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |              MRRU             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 5: L2TP Maximum Reassembled Receive Unit (MRRU) AVP
 MRRU (Maximum Reassembled Receive Unit AVP), attribute number 95, 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 that 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.

Malis & Townsley Standards Track [Page 8] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

 The MRRU SHOULD be used to set the maximum size of the reassembly
 buffer for received packets to make optimal use of reassembly buffer
 resources.
 This AVP MAY be hidden (the H bit MAY be 0 or 1).  The mandatory (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.

5.5. Fragment Bit Locations for L2TPv3 Encapsulation

 The usage of the B and E bits is described in Section 4.1.  For
 L2TPv3 encapsulation, the B and E bits are defined as bits 2 and 3 in
 the leading bits of the Default L2-Specific Sublayer (see Section 7).
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H|0|0|0|0|    Length         |              0                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |x|S|B|E|x|x|x|x|              Sequence Number                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 6: B and E Bits Location in the Default L2-Specific Sublayer
 The S (Sequence) bit is as defined in [L2TPv3].  Location of the B
 and E bits for PW-Types that 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.  The
 presence and format of the L2-Specific Sublayer is advertised via the
 L2-Specific Sublayer AVP, Attribute Type 69, defined in Section 5.4.4
 of [L2TPv3].
 See Section 1 for the description of the use of the Sequence Number
 field.

5.6. Fragment Bit Locations for L2TPv2 Encapsulation

 The usage of the B and E bits is described in Section 4.1.  For
 L2TPv2 encapsulation, the B and E bits are defined as bits 8 and 9 in
 the leading bits of the L2TPv2 header as depicted below (see Section
 7).

Malis & Townsley Standards Track [Page 9] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H|0|0|0|0|    Length         |              0                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |T|L|x|x|S|x|O|P|B|E|x|x|  Ver  |          Length (opt)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 7: B and E bits location in the L2TPv2 Message Header

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 not to 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.
 Note that [IANA] has already allocated the Fragmentation Indicator
 interface parameter, so no further IANA action is required.

Malis & Townsley Standards Track [Page 10] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

 This document requires IANA to assign new values for registries
 already managed by IANA (see Sections 7.1 and 7.2) and two reserved
 bits in an existing header (see Section 7.3).

7.1. Control Message Attribute Value Pairs (AVPs)

 Two additional AVP Attributes are specified in Sections 5.3 and 5.4.
 They are required to be defined by IANA as described in Section 2.2
 of [BCP0068].
 Control Message Attribute Value Pairs
 -------------------------------------
 94 - Maximum Receive Unit (MRU) AVP
 95 - Maximum Reassembled Receive Unit (MRRU) AVP

7.2. Default L2-Specific Sublayer Bits

 This registry was created as part of the publication of [L2TPv3].
 This document defines two reserved bits in the Default L2-Specific
 Sublayer in Section 5.5, which may be assigned by IETF Consensus
 [RFC2434].  They are required to be assigned by IANA.
 Default L2-Specific Sublayer bits - per [L2TPv3]
 ---------------------------------
 Bit 2 - B (Fragmentation) bit
 Bit 3 - E (Fragmentation) bit

7.3. Leading Bits of the L2TPv2 Message Header

 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.
 Leading Bits of the L2TPv2 Message Header - per [L2TPv2, L2TPv3]
 -----------------------------------------
 Bit 8 - B (Fragmentation) bit
 Bit 9 - E (Fragmentation) bit

8. Acknowledgements

 The authors wish to thank Eric Rosen and Carlos Pignataro, both of
 Cisco Systems, for their review of this document.

Malis & Townsley Standards Track [Page 11] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

9. Normative References

 [Control-Word] 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.
 [IANA]         Martini, L., "IANA Allocations for Pseudowire Edge to
                Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
 [KEYWORDS]     Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [LABELSTACK]   Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                Encoding", RFC 3032, January 2001.
 [L2TPv2]       Townsley, W., Valencia, A., Rubens, A., Pall, G.,
                Zorn, G., and B. Palter, "Layer Two Tunneling Protocol
                "L2TP"", RFC 2661, August 1999.
 [L2TPv3]       Lau, J., Townsley, M., and I. Goyret, "Layer Two
                Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
                March 2005.
 [MLPPP]        Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T.
                Coradetti, "The PPP Multilink Protocol (MP)", RFC
                1990, August 1996.
 [MPLS-Control] 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.
 [PATHMTU]      Mogul, J. and S. Deering, "Path MTU discovery", RFC
                1191, November 1990.
 [PATHMTUv6]    McCann, J., Deering, S., and J. Mogul, "Path MTU
                Discovery for IP version 6", RFC 1981, August 1996.

10. Informative References

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

Malis & Townsley Standards Track [Page 12] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

 [BCP0068]      Townsley, W., "Layer Two Tunneling Protocol (L2TP)
                Internet Assigned Numbers Authority (IANA)
                Considerations Update", BCP 68, RFC 3438, December
                2002.
 [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., and P. Traina, "Security
                Considerations for IP Fragment Filtering", RFC 1858,
                October 1995.
 [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26, RFC
                2434, October 1998.
 [RFC791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                September 1981.
 [TINYFRAG]     Miller, I., "Protection Against a Variant of the Tiny
                Fragment Attack (RFC 1858)", RFC 3128, June 2001.

Malis & Townsley Standards Track [Page 13] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

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 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 never
 to 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 that a single or small set
 of PWs will exhaust 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.

Malis & Townsley Standards Track [Page 14] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

 RFC 1990 headers use two bits that 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
 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 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) simply to leave the B and E bits in the
 header set to 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 Standards Track [Page 15] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

Authors' Addresses

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

Malis & Townsley Standards Track [Page 16] RFC 4623 PWE3 Fragmentation and Reassembly August 2006

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Malis & Townsley Standards Track [Page 17]

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