Pseudowire Emulation Edge-to-Edge (PWE3) Fragmentation and Reassembly
RFC 4623

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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.