This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
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 (C) The Internet Society (2006).
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.
This document specifies a new non-skippable extension for use by
Foreign Agents operating Mobile IP for IPv4 . 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.
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 . Other
terminology is used as already defined in .
3. FA Error Extension Format
The format of the FA Error Extension conforms to the Short Extension
format specified for Mobile IPv4 . The FA Error Extension is not
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 |
A status code used by the foreign agent to supply status
information to the mobile node.
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
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  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  to account for the extra 4 bytes
This document updates the Mobile IP base specification  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.
6. Foreign Agent Considerations
When denying a successful Registration Reply, the Foreign Agent
SHOULD send a Registration Revocation message  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
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
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 
number spaces, future allocations from this number space require
expert review .
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 . 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
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
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
Thanks to Kent Leung and Henrik Lefkowetz for suggested improvements
to this specification.
10. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Glass, S. and M. Chandra, "Registration Revocation in Mobile
IPv4", RFC 3543, August 2003.
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
 Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
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
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EID 822 (Verified) is as follows:Section: 99In the header, it does not include any relationship to other RFCs.
Original Text:Corrected Text:
Section 4 of RFC 4636, on page 3, clearly states:
This document updates the Mobile IP base specification  regarding the procedures followed by the foreign agent in the case that the home agent fails authentication. [...]
... and  is RFC 3344.
I expected the line in the RFC heading, and appropriate links in the RFC index.
Has this been omitted by accident, or have there been strong arguments to omit this significant link ? In the former case, can that be corrected 'after the fact' ?