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Network Working Group C. Perkins Request for Comments: 4636 Nokia Research Center Category: Standards Track October 2006

           Foreign Agent Error Extension for Mobile IPv4

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 specifies a new extension for use by Foreign Agents
 operating Mobile IP for IPv4.  Currently, a foreign agent cannot
 supply status information without destroying the ability for a mobile
 node to verify authentication data supplied by the home agent.  The
 new extension solves this problem by making a better place for the
 foreign agent to provide its status information to the mobile node.

Perkins Standards Track [Page 1] RFC 4636 FA Error Extension October 2006

1. Introduction

 This document specifies a new non-skippable extension for use by
 Foreign Agents operating Mobile IP for IPv4 [4].  The new extension
 option allows a foreign agent to supply an error code without
 disturbing the data supplied by the Home Agent within the
 Registration Reply message.  In this way, the mobile node can verify
 that the Registration Reply message was generated by the Home Agent
 even in cases where the foreign agent is required by protocol to
 insert new status information into the Registration Reply message.

2. Terminology

 The keywords "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 [1].  Other
 terminology is used as already defined in [4].

3. FA Error Extension Format

 The format of the FA Error Extension conforms to the Short Extension
 format specified for Mobile IPv4 [4].  The FA Error Extension is not
 skippable.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |   Length      |    Sub-Type   |     Status    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Type

    Length

    Sub-Type

    Status

       A status code used by the foreign agent to supply status
       information to the mobile node.

Perkins Standards Track [Page 2] RFC 4636 FA Error Extension October 2006

4. Operation and Use of the FA Error Extension

 The FA Error Extension is only valid for use within Mobile IPv4
 Registration Reply messages.  The FA Error Extension is not
 skippable.  A mobile node that cannot correctly interpret the
 contents of the FA Error Extension MUST NOT use the care-of address
 provided in the Registration Reply message, until another
 Registration Request message has been sent and a successful
 Registration Reply message received.

 Status codes allowable for use within the FA Error Extension are
 within the range 64-127.  The currently specified codes are as
 follows:

    64 reason unspecified
    65 administratively prohibited
    66 insufficient resources
    68 home agent failed authentication
    71 poorly formed Reply
    77 invalid care-of address
    78 registration timeout

 as defined in RFC 3344 [4] for use by the Foreign Agent.  Status
 codes for use with the FA Error extensions must not be differently
 defined for use in the Code field of Registration Reply messages.

 When a foreign agent appends a FA Error Extension to the Registration
 Reply as received from the Home Agent, it has to update the UDP
 Length field in the UDP header [5] to account for the extra 4 bytes
 of length.

 This document updates the Mobile IP base specification [4] regarding
 the procedures followed by the foreign agent in the case that the
 home agent fails authentication.  Instead of modifying the "status"
 field of the Registration Reply to contain the value 68, now the
 foreign agent should append the Foreign Agent Error Extension
 containing the status value 68.

5. Mobile Node Considerations

 If a mobile node receives a successful Registration Reply (status
 code 0 or 1), with a FA Error Extension indicating that the foreign
 agent is not honoring said Registration Reply, the mobile node SHOULD
 then send a deregistration message to the home agent.  In this way,
 the home agent will not maintain a registration status that is
 inconsistent with the status maintained by the foreign agent.

Perkins Standards Track [Page 3] RFC 4636 FA Error Extension October 2006

6. Foreign Agent Considerations

 When denying a successful Registration Reply, the Foreign Agent
 SHOULD send a Registration Revocation message [2] to the Home Agent
 if a mobility security association exists between them.  For cases
 when the foreign agent does have the required security association,
 this way of informing the home agent does not have the vulnerability
 from detrimental actions by malicious foreign agents, as noted in
 section 8.

7. IANA Considerations

 This specification reserves one number for the FA Error Extension
 (see section 3) from the space of numbers for non-skippable mobility
 extensions (i.e., 0-127) defined in the specification for Mobile IPv4
 [4].

 This specification also creates a new number space of sub-types for
 the type number of this extension.  Sub-type zero is to be allocated
 from this number space for the protocol extension specified in this
 document.  Similar to the procedures specified for Mobile IP [4]
 number spaces, future allocations from this number space require
 expert review [3].

 The status codes that are allowable in the FA Error Extension are a
 subset of the status codes defined in the specification for Mobile
 IPv4 [4].  If, in the future, additional status codes are defined for
 Mobile IPv4, the definition for each new status code must indicate
 whether the new status code is allowable for use in the FA Error
 Extension.

8. Security Considerations

 The extension in this document improves the security features of
 Mobile IPv4 by allowing the mobile node to be assured of the
 authenticity of the information supplied within a Registration
 Request.  Previously, whenever the foreign agent was required to
 provide status information to the mobile node, it could only do so by
 destroying the ability of the mobile device to verify the Mobile-Home
 Authentication Extension data.

 In many common cases, the mobile node will not have a security
 association with the foreign agent that has sent the extension.
 Thus, the mobile node will be unable to ascertain that the foreign
 agent sending the extended Registration Reply message is the same
 foreign agent that earlier received the associated Registration
 Request from the mobile node.  Because of this, a malicious foreign
 agent could cause a mobile node to operate as if the registration had

Perkins Standards Track [Page 4] RFC 4636 FA Error Extension October 2006

 failed, when in fact its home agent and a correctly operating foreign
 agent had both accepted the mobile node's Registration Request.  In
 order to reduce the vulnerability to such maliciously transmitted
 Registration Reply messages with the unauthenticated extension, the
 mobile node MAY delay processing of such denied Registration Reply
 messages for a short while in order to determine whether another
 successful Registration Reply might be received from the foreign
 agent.

9. Acknowledgements

 Thanks to Kent Leung and Henrik Lefkowetz for suggested improvements
 to this specification.

10. Normative References

 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

 [2] Glass, S. and M. Chandra, "Registration Revocation in Mobile
     IPv4", RFC 3543, August 2003.

 [3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
     Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

 [4] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
     2002.

 [5] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
     1980.

Author's Address

 Charles E. Perkins
 Palo Alto Systems Research Lab
 Nokia Research Center
 975 Page Mill Road, Suite 200
 Palo Alto, CA 94304-1003

 Phone: +1 650-496-4402
 Fax:   +1-650-739-0779
 EMail: charles.perkins@nokia.com

Perkins Standards Track [Page 5] RFC 4636 FA Error Extension October 2006

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 Copyright (C) The Internet Society (2006).

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Perkins Standards Track [Page 6]