Mobile IP Traversal of Network Address Translation (NAT) Devices
RFC 3519

Network Working Group                                       H. Levkowetz
Request for Comments: 3519                                   ipUnplugged
Category: Standards Track                                     S. Vaarala
                                                                 Netseal
                                                              April 2003

    Mobile IP Traversal of Network Address Translation (NAT) Devices

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 (2003).  All Rights Reserved.

Abstract

   Mobile IP's datagram tunnelling is incompatible with Network Address
   Translation (NAT).  This document presents extensions to the Mobile
   IP protocol and a tunnelling method which permits mobile nodes using
   Mobile IP to operate in private address networks which are separated
   from the public internet by NAT devices.  The NAT traversal is based
   on using the Mobile IP Home Agent UDP port for encapsulated data
   traffic.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1   Terminology . . . . . . . . . . . . . . . . . . . . . .  2
       1.2   Problem description . . . . . . . . . . . . . . . . . .  3
       1.3   Assumptions . . . . . . . . . . . . . . . . . . . . . .  4
   2.  NAT Traversal Overview. . . . . . . . . . . . . . . . . . . .  5
       2.1   Basic Message Sequence. . . . . . . . . . . . . . . . .  5
   3.  New Message Formats . . . . . . . . . . . . . . . . . . . . .  6
       3.1   UDP Tunnel Request Extension. . . . . . . . . . . . . .  6
             3.1.1 F (Force) Flag. . . . . . . . . . . . . . . . . .  7
             3.1.2 R (Registration through FA Required) flag . . . .  8
             3.1.3 Reserved Fields . . . . . . . . . . . . . . . . .  8
             3.1.4 Encapsulation . . . . . . . . . . . . . . . . . .  8
             3.1.5 Mobile IP Registration Bits . . . . . . . . . . .  9
       3.2   UDP Tunnel Reply Extension. . . . . . . . . . . . . . .  9
             3.2.1 Reply Code. . . . . . . . . . . . . . . . . . . . 10

Levkowetz & Vaarala         Standards Track                     [Page 1]
RFC 3519              NAT Traversal for Mobile IP             April 2003

       3.3   MIP Tunnel Data Message . . . . . . . . . . . . . . . . 10
       3.4   UDP Tunnelling Flag in Agent Advertisements . . . . . . 11
       3.5   New Registration Reply Codes. . . . . . . . . . . . . . 12
   4.  Protocol Behaviour. . . . . . . . . . . . . . . . . . . . . . 12
       4.1   Relation to standard MIP tunnelling . . . . . . . . . . 12
       4.2   Encapsulating IP Headers in UDP . . . . . . . . . . . . 13
       4.3   Decapsulation . . . . . . . . . . . . . . . . . . . . . 15
       4.4   Mobile Node Considerations. . . . . . . . . . . . . . . 15
       4.5   Foreign Agent Considerations. . . . . . . . . . . . . . 16
       4.6   Home Agent Considerations . . . . . . . . . . . . . . . 18
             4.6.1 Error Handling. . . . . . . . . . . . . . . . . . 19
       4.7   MIP signalling versus tunnelling. . . . . . . . . . . . 20
       4.8   Packet fragmentation. . . . . . . . . . . . . . . . . . 21
       4.9   Tunnel Keepalive. . . . . . . . . . . . . . . . . . . . 21
       4.10  Detecting and compensating for loss of NAT mapping. . . 22
       4.11  Co-located registration through FA. . . . . . . . . . . 24
   5.  Implementation Issues . . . . . . . . . . . . . . . . . . . . 24
       5.1   Movement Detection and Private Address Aliasing . . . . 24
       5.2   Mobility Binding Lifetime . . . . . . . . . . . . . . . 25
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . 26
       6.1   Traffic Redirection Vulnerabilities . . . . . . . . . . 27
             6.1.1 Manipulation of the Registration
                   Request Message . . . . . . . . . . . . . . . . . 27
             6.1.2 Sending a Bogus Keepalive Message . . . . . . . . 27
       6.2   Use of IPsec. . . . . . . . . . . . . . . . . . . . . . 28
       6.3   Firewall Considerations . . . . . . . . . . . . . . . . 28
   7.  UNSAF Considerations. . . . . . . . . . . . . . . . . . . . . 28
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
   9.  Intellectual Property Rights. . . . . . . . . . . . . . . . . 30
   10. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 31
   11. Normative References. . . . . . . . . . . . . . . . . . . . . 31
   12. Informative References. . . . . . . . . . . . . . . . . . . . 32