Network Working Group C. Perkins

Network Working Group C. Perkins
Request for Comments: 4721 Nokia Research Center
Obsoletes: 3012 P. Calhoun
Updates: 3344 Cisco Systems, Inc.
Category: Standards Track J. Bharatia
Nortel Networks
January 2007

Mobile IPv4 Challenge/Response Extensions (Revised)

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 IETF Trust (2007).

Abstract

Mobile IP, as originally specified, defines an authentication
extension (the Mobile-Foreign Authentication extension) by which a
mobile node can authenticate itself to a foreign agent.
Unfortunately, that extension does not provide the foreign agent any
direct guarantee that the protocol is protected from replays and does
not allow for the use of existing techniques (such as Challenge
Handshake Authentication Protocol (CHAP)) for authenticating portable
computer devices.

In this specification, we define extensions for the Mobile IP Agent
Advertisements and the Registration Request that allow a foreign
agent to use a challenge/response mechanism to authenticate the
mobile node.

Furthermore, this document updates RFC 3344 by including a new
authentication extension called the Mobile-Authentication,
Authorization, and Accounting (AAA) Authentication extension. This
new extension is provided so that a mobile node can supply
credentials for authorization, using commonly available AAA
infrastructure elements. This authorization-enabling extension MAY
co-exist in the same Registration Request with authentication
extensions defined for Mobile IP Registration by RFC 3344. This
document obsoletes RFC 3012.

Perkins, et al. Standards Track [Page 1]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Table of Contents

1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Mobile IP Agent Advertisement Challenge Extension ...............4
2.1. Handling of Solicited Agent Advertisements .................4
3. Operation .......................................................5
3.1. Mobile Node Processing of Registration Requests ............5
3.2. Foreign Agent Processing of Registration Requests ..........6
3.2.1. Foreign Agent Algorithm for Tracking Used
Challenges .........................................8
3.3. Foreign Agent Processing of Registration Replies ...........9
3.4. Home Agent Processing of Challenge Extensions .............10
3.5. Mobile Node Processing of Registration Replies ............11
4. Mobile-Foreign Challenge Extension .............................11
5. Generalized Mobile IP Authentication Extension .................12
6. Mobile-AAA Authentication Subtype ..............................13
7. Reserved SPIs for Mobile IP ....................................14
8. SPIs for RADIUS AAA Servers ....................................14
9. Configurable Parameters ........................................15
10. Error Values ..................................................16
11. IANA Considerations ...........................................16
12. Security Considerations .......................................17
13. Acknowledgements ..............................................18
14. Normative References ..........................................18
Appendix A. Changes since RFC 3012 ................................20
Appendix B. Verification Infrastructure ...........................21
Appendix C. Message Flow for FA Challenge Messaging with
Mobile-AAA Extension ..................................22
Appendix D. Message Flow for FA Challenge Messaging with
MN-FA Authentication ..................................23
Appendix E. Example Pseudo-code for Tracking Used Challenges ......24

1. Introduction

Mobile IP defines the Mobile-Foreign Authentication extension to
allow a mobile node to authenticate itself to a foreign agent. Such
authentication mechanisms are mostly external to the principal
operation of Mobile IP, since the foreign agent can easily route
packets to and from a mobile node whether or not the mobile node is
reporting a legitimately owned home address to the foreign agent.
Unfortunately, that extension does not provide the foreign agent any
direct guarantee that the protocol is protected from replays and does
not allow for the use of CHAP [RFC1994] for authenticating portable
computer devices. In this specification, we define extensions for
the Mobile IP Agent Advertisements and the Registration Request that
allow a foreign agent to use a challenge/ response mechanism to
authenticate the mobile node. Furthermore, an additional

Perkins, et al. Standards Track [Page 2]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

authentication extension, the Mobile-AAA authentication extension, is
provided so that a mobile node can supply credentials for
authorization using commonly available AAA infrastructure elements.
The foreign agent may be able to interact with an AAA infrastructure
(using protocols outside the scope of this document) to obtain a
secure indication that the mobile node is authorized to use the local
network resources.

1.1. Terminology

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 [RFC2119].

This document uses the term Security Parameters Index (SPI) as
defined in the base Mobile IP protocol specification [RFC3344]. All
SPI values defined in this document refer to values for the SPI as
defined in that specification.

The following additional terminology is used in addition to that
defined in [RFC3344]:

previously used challenge:

The challenge is a previously used challenge if the mobile node
sent the same challenge to the foreign agent in a previous
Registration Request, and if that previous Registration Request
passed all validity checks performed by the foreign agent. The
foreign agent may not be able to keep records for all previously
used challenges, but see Section 3.2 for minimal requirements.

security association:

A "mobility security association", as defined in [RFC3344].

unknown challenge:

Any challenge from a particular mobile node that the foreign agent
has no record of having put either into one of its recent Agent
Advertisements or into a registration reply message to that mobile
node.

unused challenge:

A challenge that has not already been accepted by the foreign
agent from the mobile node in the Registration Request, i.e., a
challenge that is neither unknown nor previously used.

Perkins, et al. Standards Track [Page 3]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

2. Mobile IP Agent Advertisement Challenge Extension

This section defines a new extension to the Router Discovery Protocol
[RFC1256] for use by foreign agents that need to issue a challenge
for authenticating mobile nodes.

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 | Challenge ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1. The Challenge Extension

Type:

24

Length:

The length of the Challenge value in octets; SHOULD be at least 4.

Challenge:

A random value that SHOULD be at least 32 bits.

The Challenge extension, illustrated in Figure 1, is inserted in the
Agent Advertisements by the foreign agent in order to communicate a
previously unused challenge value that can be used by the mobile node
to compute an authentication for its next registration request
message. The challenge is selected by the foreign agent to provide
local assurance that the mobile node is not replaying any earlier
registration request. Eastlake et al. [RFC4086] provides more
information on generating pseudo-random numbers suitable for use as
values for the challenge.

Note that the storage of different Challenges received in Agent
Advertisements from multiple foreign agents is implementation
specific and hence out of scope for this specification.

2.1. Handling of Solicited Agent Advertisements

When a foreign agent generates an Agent Advertisement in response to
a Router Solicitation [RFC1256], some additional considerations come
into play. According to the Mobile IP base specification [RFC3344],
the resulting Agent Advertisement may be either multicast or unicast.

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RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

If the solicited Agent Advertisement is multicast, the foreign agent
MUST NOT generate a new Challenge value and update its window of
remembered advertised Challenges. It must instead re-use the most
recent of the CHALLENGE_WINDOW Advertisement Challenge values
(Section 9).

If the agent advertisement is unicast back to the soliciting mobile
node, it MUST be handled as follows: If the challenge most recently
unicast to the soliciting mobile node has not been previously used
(as defined in Section 1.1), it SHOULD be repeated in the newly
issued unicast agent advertisement. Otherwise, a new challenge MUST
be generated and remembered as the most recent challenge issued to
the mobile node. For further discussion of this, see Section 12.

3. Operation

This section describes modifications to the Mobile IP registration
process [RFC3344] that may occur after the foreign agent issues a
Mobile IP Agent Advertisement containing the Challenge on its local
link. See Appendix C for a diagram showing the canonical message
flow for messages related to the processing of the foreign agent
challenge values.

3.1. Mobile Node Processing of Registration Requests

Retransmission behavior for Registration Requests is identical to
that specified in Mobile IP specification [RFC3344]. A retransmitted
Registration Request MAY use the same Challenge value as given in the
original Registration Request.

Whenever the Agent Advertisement contains the Challenge extension, if
the mobile node does not have a security association with the foreign
agent, then it MUST include the Challenge value in a Mobile-Foreign
Challenge extension to the Registration Request message. If, on the
other hand, the mobile node does have a security association with the
foreign agent, it SHOULD include the Challenge value in its
Registration Request message.

If the mobile node has a security association with the Foreign Agent,
it MUST include a Mobile-Foreign Authentication extension in its
Registration Request message, according to the base Mobile IP
specification [RFC3344]. When the Registration Request contains the
Mobile-Foreign Challenge extension specified in Section 4, the
Mobile-Foreign Authentication MUST follow the Challenge extension in
the Registration Request. The mobile node MAY also include the
Mobile-AAA Authentication extension.

Perkins, et al. Standards Track [Page 5]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

If both the Mobile-Foreign Authentication and the Mobile-AAA
Authentication extensions are present, the Mobile-Foreign Challenge
extension MUST precede the Mobile-AAA Authentication extension, and
the Mobile-AAA Authentication extension MUST precede the Mobile-
Foreign Authentication extension.

If the mobile node does not have a security association with the
foreign agent, the mobile node MUST include the Mobile-AAA
Authentication extension as, defined in Section 6, when it includes
the Mobile-Foreign Challenge extension. In addition, the mobile node
SHOULD include the NAI extension [RFC2794] to enable the foreign
agent to make use of available verification infrastructure that
requires this. The SPI field of the Mobile-AAA Authentication
extension specifies the particular secret and algorithm (shared
between the mobile node and the verification infrastructure) that
must be used to perform the authentication. If the SPI value is
chosen as CHAP_SPI (see Section 9), then the mobile node specifies
CHAP-style authentication [RFC1994] using MD5 [RFC1321].

In either case, the Mobile-Foreign Challenge extension followed by
one of the above specified authentication extensions MUST follow the
Mobile-Home Authentication extension, if present.

A mobile node MAY include the Mobile-AAA Authentication extension in
the Registration Request when the mobile node registers directly with
its home agent (using a co-located care-of address). In this case,
the mobile node uses an SPI value of CHAP_SPI (Section 8) in the
Mobile Node-Authentication, Authorization, and Accounting (MN-AAA)
Authentication extension and MUST NOT include the Mobile-Foreign
Challenge extension. Also, replay protection for the Registration
Request in this case is provided by the Identification field defined
by [RFC3344].

3.2. Foreign Agent Processing of Registration Requests

Upon receipt of the Registration Request, if the foreign agent has
issued a Challenge as part of its Agent Advertisements, and if it
does not have a security association with the mobile node, then the
foreign agent SHOULD check that the Mobile-Foreign Challenge
extension exists, and that it contains a challenge value previously
unused by the mobile node. This ensures that the mobile node is not
attempting to replay a previous advertisement and authentication. In
this case, if the Registration Request does not include a Challenge
extension, the foreign agent MUST send a Registration Reply with the
Code field set to missing_challenge.

Perkins, et al. Standards Track [Page 6]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

If a mobile node retransmits a Registration Request with the same
Challenge extension, and if the foreign agent still has a pending
Registration Request record in effect for the mobile node, then the
foreign agent forwards the Registration Request to the Home Agent
again. The foreign agent SHOULD check that the mobile node is
actually performing a retransmission, by verifying that the relevant
fields of the retransmitted request (including, if present, the
mobile node NAI extension [RFC2794]) are the same as represented in
the visitor list entry for the pending Registration Request (Section
3.7.1 of [RFC3344]). This verification MUST NOT include the
"remaining Lifetime of the pending registration" or the
Identification field, since those values are likely to change even
for requests that are merely retransmissions and not new Registration
Requests. In all other circumstances, if the foreign agent receives
a Registration Request with a Challenge extension containing a
Challenge value previously used by that mobile node, the foreign
agent SHOULD send a Registration Reply to the mobile node, containing
the Code value stale_challenge.

The foreign agent MUST NOT accept any Challenge in the Registration
Request unless it was offered in the last Registration Reply or
unicast Agent Advertisement sent to the mobile node or advertised as
one of the last CHALLENGE_WINDOW (see Section 9) Challenge values
inserted into the immediately preceding Agent Advertisements. If the
Challenge is not one of the recently advertised values, the foreign
Agent SHOULD send a Registration Reply with Code value
unknown_challenge (see Section 10). The foreign agent MUST maintain
the last challenge used by each mobile node that has registered using
any one of the last CHALLENGE_WINDOW challenge values. This last
challenge value can be stored as part of the mobile node's
registration records. Also, see Section 3.2.1 for a possible
algorithm that can be used to satisfy this requirement.

Furthermore, the foreign agent MUST check that there is either a
Mobile-Foreign or a Mobile-AAA Authentication extension after the
Challenge extension. Any registration message containing the
Challenge extension without either of these authentication extensions
MUST be silently discarded. If the registration message contains a
Mobile-Foreign Authentication extension with an incorrect
authenticator that fails verification, the foreign agent MAY send a
Registration Reply to the mobile node with Code value mobile node
failed authentication (see Section 10).

If the Mobile-AAA Authentication extension (see Section 6) is present
in the message, or if a Network Access Identifier (NAI) extension is
included indicating that the mobile node belongs to a different
administrative domain, the foreign agent may take actions outside the
scope of this protocol specification to carry out the authentication

Perkins, et al. Standards Track [Page 7]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

of the mobile node. If the registration message contains a Mobile-
AAA Authentication extension with an incorrect authenticator that
fails verification, the foreign agent MAY send a Registration Reply
to the mobile node with Code value fa_bad_aaa_auth. If the Mobile-
AAA Authentication extension is present in the Registration Request,
the foreign agent MUST NOT remove the Mobile-AAA Authentication
extension and the Mobile-Foreign Challenge extension from the
Registration Request before forwarding to the home agent. Appendix C
provides an example of an action that could be taken by a foreign
agent.

In the event that the Challenge extension is authenticated through
the Mobile-Foreign Authentication extension and the Mobile-AAA
Authentication extension is not present, the foreign agent MAY remove
the Challenge extension from the Registration Request without
disturbing the authentication value used for the computation. If the
Mobile-AAA Authentication extension is present and a security
association exists between the foreign agent and the home agent, the
Mobile-Foreign Challenge extension and the Mobile-AAA Authentication
extension MUST precede the Foreign-Home Authentication extension.

If the foreign agent does remove the Challenge extension and
applicable authentication from the Registration Request message, then
it SHOULD store the Identification field from the Registration
Request message as part of its record-keeping information about the
particular mobile node in order to protect against replays.

3.2.1. Foreign Agent Algorithm for Tracking Used Challenges

If the foreign agent maintains a large CHALLENGE_WINDOW, it becomes
more important for scalability purposes to compare incoming
challenges efficiently against the set of Challenge values that have
been advertised recently. This can be done by keeping the Challenge
values in order of advertisement, and by making use of the mandated
behavior that mobile nodes MUST NOT use Challenge values that were
advertised before the last advertised Challenge value that the mobile
node attempted to use. The pseudo-code in Appendix E accomplishes
this objective. The maximum amount of total storage required by this
algorithm is equal to Size*(CHALLENGE_WINDOW + (2*N)), where N is the
current number of mobile nodes for which the foreign agent is storing
challenge values. Note that whenever the stored challenge value is
no longer in the CHALLENGE_WINDOW, it can be deleted from the foreign
agent's records, perhaps along with all other registration
information for the mobile node if it is no longer registered.

Perkins, et al. Standards Track [Page 8]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

It is presumed that the foreign agent keeps an array of advertised
Challenges, a record of the last advertised challenge used by a
mobile node, and also a record of the last challenge provided to a
mobile node in a Registration Reply or unicast Agent Advertisement.

To meet the security obligations outlined in Section 12, the foreign
agent SHOULD use one of the already stored, previously unused
challenges when responding to an unauthenticated Registration Request
or Agent Solicitation. If none of the already stored challenges are
previously unused, the foreign agent SHOULD generate a new challenge,
include it in the response, and store it in the per-Mobile data
structure.

3.3. Foreign Agent Processing of Registration Replies

The foreign agent SHOULD include a new Mobile-Foreign Challenge
extension in any Registration Reply, successful or not. If the
foreign agent includes this extension in a successful Registration
Reply, the extension SHOULD precede a Mobile-Foreign authentication
extension if present. Suppose that the Registration Reply includes a
Challenge extension from the home agent, and that the foreign agent
wishes to include another Challenge extension with the Registration
Reply for use by the mobile node. In that case, the foreign agent
MUST delete the Challenge extension from the home agent from the
Registration Reply, along with any Foreign-Home authentication
extension, before appending the new Challenge extension to the
Registration Reply.

One example of a situation where the foreign agent MAY omit the
inclusion of a Mobile-Foreign Challenge extension in the Registration
Reply would be when a new challenge has been multicast recently.

If a foreign agent has conditions in which it omits the inclusion of
a Mobile-Foreign Challenge extension in the Registration Reply, it
still MUST respond with an agent advertisement containing a
previously unused challenge in response to a subsequent agent
solicitation from the same mobile node. Otherwise (when the said
conditions are not met), the foreign agent MUST include a previously
unused challenge in any Registration Reply, successful or not.

If the foreign agent does not remove the Challenge extension from the
Registration Request received from the mobile node, then the foreign
agent SHOULD store the Challenge value as part of the pending
registration request list [RFC3344]. Also, if the Registration Reply
coming from the home agent does not include the Challenge extension,
the foreign agent SHOULD NOT reject the registration request. If the
Challenge extension is present in the Registration Reply, it MUST be
the same Challenge value that was included in the Registration Reply

Perkins, et al. Standards Track [Page 9]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

received from the home agent, the foreign agent MUST insert a Foreign
Agent (FA) Error extension with Status value ha_wrong_challenge in
the Registration Reply sent to the mobile node (see Section 10).

A mobile node MUST be prepared to use a challenge from a unicast or
multicast Agent Advertisement in lieu of one returned in a
Registration Reply, and it MUST solicit for one if it has not already
received one either in a Registration Reply or a recent
advertisement.

If the foreign agent receives a Registration Reply with the Code
value ha_bad_aaa_auth, the Registration Reply with this Code value
MUST be relayed to the mobile node. In this document, whenever the
foreign agent is required to reject a Registration Request, it MUST
put the given code in the usual Code field of the Registration Reply,
unless the Registration Reply has already been received from the home
agent. In this case, the foreign agent MUST preserve the value of
the Code field set by the home agent and MUST put its own rejection
code in the Status field of the FA Error extension (defined in
[RFC4636]).

3.4. Home Agent Processing of Challenge Extensions

If the home agent receives a Registration Request with the Mobile-
Foreign Challenge extension and recognizes the extension, the home
agent MUST include the Challenge extension in the Registration Reply.
The Challenge extension MUST be placed after the Mobile-Home
authentication extension, and the extension SHOULD be authenticated
by a Foreign-Home Authentication extension.

The home agent may receive a Registration Request with the Mobile-AAA
Authentication extension. If the Mobile-AAA Authentication extension
is used by the home agent as an authorization-enabling extension and
the verification fails due to an incorrect authenticator, the home
agent MAY reject the Registration Reply with the error code
ha_bad_aaa_auth.

Since the extension type for the Challenge extension is within the
range 128-255, the home agent MUST process such a Registration
Request even if it does not recognize the Challenge extension
[RFC3344]. In this case, the home agent will send a Registration
Reply to the foreign agent that does not include the Challenge
extension.

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RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

3.5. Mobile Node Processing of Registration Replies

A mobile node might receive the error code in the Registration Reply
from the foreign agent as a response to the Registration Request.
The error codes are defined in Section 10.

In any case, if the mobile node attempts to register again after such
an error, it MUST use a new Challenge value in such a registration,
obtained either from an Agent Advertisement, or from a Challenge
extension to the Registration Reply containing the error.

In the co-located care-of address mode, the mobile node receives a
Registration Reply without the Challenge extension and processes the
Registration Reply as specified in [RFC3344]. In this case, when the
mobile node includes the MN-AAA Authentication Extension, the
Challenge value 0 is recommended for the authenticator computation
mentioned in Section 8.

4. Mobile-Foreign Challenge Extension

This section specifies a new Mobile IP Registration extension that is
used to satisfy a Challenge in an Agent Advertisement. The Challenge
extension to the Registration Request message is used to indicate the
challenge that the mobile node is attempting to satisfy.

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 | Challenge ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2. The Mobile-Foreign Challenge Extension

Type:

132 (skippable). (See [RFC3344]).

Length:

Length of the Challenge value.

Challenge:

The Challenge field is copied from the Challenge field found in
the received Challenge extension.

Suppose that the mobile node has successfully registered using one of
the Challenge Values within the CHALLENGE_WINDOW values advertised by

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RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

the foreign agent. In that case, in any new Registration Request the
mobile node MUST NOT use any Challenge Value that was advertised by
the foreign agent before the Challenge Value in the mobile node's
last Registration Request.

5. Generalized Mobile IP Authentication Extension

Several new authentication extensions have been designed for various
control messages proposed for extensions to Mobile IP. A new
authentication extension is required for a mobile node to present its
credentials to any other entity other than the ones already defined;
the only entities defined in the base Mobile IP specification
[RFC3344] are the home agent and the foreign agent. The purpose of
the generalized authentication extension defined here is to collect
together data for all such new authentication applications into a
single extension type with subtypes.

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 | Subtype | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3. The Generalized Mobile IP Authentication Extension

Type:

36 (not skippable). (See [RFC3344]).

Subtype:

A number assigned to identify the kind of endpoints or other
characteristics of the particular authentication strategy.

Length:

4 plus the number of octets in the Authenticator; MUST be at least
20.

SPI:

Security Parameters Index

Perkins, et al. Standards Track [Page 12]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Authenticator:

The variable length Authenticator field

In this document, only one subtype is defined:

1 Mobile-AAA Authentication subtype
(Hashed Message Authentication Code-MD5 (HMAC-MD5))
(see Section 6).

6. Mobile-AAA Authentication Subtype

The Generalized Authentication extension with subtype 1 will be
referred to as a Mobile-AAA Authentication extension. The mobile
node MAY include a Mobile-AAA Authentication extension in any
Registration Request. This extension MAY co-exist in the same
Registration Request with Authentication extensions defined for
Mobile IP Registration ([RFC3344]). If the mobile node does not
include a Mobile-Foreign Authentication extension, then it MUST
include the Mobile-AAA Authentication extension whenever the
Challenge extension is present. If both are present, the Mobile-AAA
Authentication extension MUST precede the Mobile-Foreign
Authentication extension.

If the Mobile-AAA Authentication extension is present, the Mobile-
Home Authentication extension MUST appear prior to the Mobile-AAA
Authentication extension. The corresponding response MUST include
the Mobile-Home Authentication extension and MUST NOT include the
Mobile-AAA Authentication extension.

The default algorithm for computation of the authenticator is HMAC-
MD5 [RFC2104] computed on the following data, in the order shown:

Preceding Mobile IP data || Type, Subtype, Length, SPI

where the Type, Length, Subtype, and SPI are as shown in Section 5.
The Preceding Mobile IP data refers to the UDP payload (the
Registration Request or Registration Reply data) and all prior
extensions in their entirety. The resulting function call, as
described in [RFC2104], would be:

hmac_md5(data, datalen, Key, KeyLength, authenticator);

Each mobile node MUST support the ability to produce the
authenticator by using HMAC-MD5 as shown. Just as with Mobile IP, it
must be possible to configure the use of any arbitrary 32-bit SPI
outside of the SPIs in the reserved range 0-255 for selection of this
default algorithm.

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RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

7. Reserved SPIs for Mobile IP

Mobile IP defines several authentication extensions for use in
Registration Requests and Replies. Each authentication extension
carries a Security Parameters Index (SPI) that should be used to
index a table of security associations. Values in the range 0-255
are reserved for special use. A list of reserved SPI numbers is to
be maintained by IANA at the following URL:

http://www.iana.org/assignments/mobileip-numbers

8. SPIs for RADIUS AAA Servers

Some AAA servers only admit a single security association and thus do
not use the SPI numbers for Mobile IP authentication extensions for
use when determining the security association that would be necessary
for verifying the authentication information included with the
Authentication extension.

SPI number CHAP_SPI (see Section 9) is reserved for indicating the
following procedure for computing authentication data (called the
"authenticator"), which is used by many RADIUS servers [RFC2865]
today.

To compute the authenticator, apply MD5 [RFC1321] computed on the
following data, in the order shown:

High-order octet from Challenge || Key ||

MD5(Preceding Mobile IP data ||

Type, Subtype (if present), Length, SPI) ||

Least-order 237 octets from Challenge

where Type, Length, SPI, and possibly Subtype are the fields of the
authentication extension in use. For instance, all four of these
fields would be in use when SPI == CHAP_SPI is used with the
Generalized Authentication extension. In case of co-located care-of
address, the Challenge value 0 is used (refer to Section 3.5). Since
the RADIUS protocol cannot carry attributes of length greater than
253, the preceding Mobile IP data, type, subtype (if present),
length, and SPI are hashed using MD5. Finally, the least significant
237 octets of the challenge are concatenated. If the challenge has
fewer than 238 octets, this algorithm includes the high-order octet
in the computation twice but ensures that the challenge is used

Perkins, et al. Standards Track [Page 14]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

exactly as is. Additional padding is never used to increase the
length of the challenge; the input data is allowed to be shorter than
237 octets long.

9. Configurable Parameters

Every Mobile IP agent supporting the extensions defined in this
document SHOULD be able to configure each parameter in the following
table. Each table entry contains the name of the parameter, the
default value, and the section of the document in which the parameter
first appears.

+------------------+---------------+---------------------+
| Parameter Name | Default Value | Section of Document |
+------------------+---------------+---------------------+
| CHALLENGE_WINDOW | 2 | 3.2 |
| | | |
| CHAP_SPI | 2 | 8 |
+------------------+---------------+---------------------+

Table 1. Configurable Parameters

Note that CHALLENGE_WINDOW SHOULD be at least 2. This makes it far
less likely that mobile nodes will register using a Challenge value
that is outside the set of values allowable by the foreign agent.

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RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

10. Error Values

Each entry in the following table contains the name of the Code
[RFC3344] to be returned in a Registration Reply, the value for the
Code, and the section in which the error is mentioned in this
specification.

+--------------------+-------+--------------------------+
| Error Name | Value | Section of Document |
+--------------------+-------+--------------------------+
| unknown_challenge | 104 | 3.2 |
| | | |
| mobile node failed | 67 | 3.2; also see [RFC3344] |
| authentication | | |
| | | |
| missing_challenge | 105 | 3.1, 3.2 |
| | | |
| stale_challenge | 106 | 3.2 |
| | | |
| fa_bad_aaa_auth | 108 | 3.2 |
| | | |
| ha_bad_aaa_auth | 144 | 3.4 |
| | | |
| ha_wrong_challenge | 109 | 3.2 |
+--------------------+-------+--------------------------+

Table 2. Error Values

11. IANA Considerations

The following are currently assigned by IANA for RFC 3012 [RFC3012]
and are applicable to this document. IANA has recorded these values
as part of this document.

The Generalized Mobile IP Authentication extension defined in
Section 5 is a Mobile IP registration extension. IANA has
assigned a value of 36 for this extension.

A new number space is to be created for enumerating subtypes of
the Generalized Authentication extension (see Section 5). New
subtypes of the Generalized Authentication extension, other than
the number (1) for the MN-AAA authentication extension specified
in Section 6, must be specified and approved by a designated
expert.

Perkins, et al. Standards Track [Page 16]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

The Mobile Node - Foreign Agent (MN-FA) Challenge extension,
defined in Section 4, is a router advertisement extension as
defined in RFC 1256 [RFC1256] and extended in RFC 3344 [RFC3344].
IANA has assigned a value of 132 for this purpose.

The Code values defined in Section 10 are error codes as defined
in RFC 3344 ([RFC3344]). They correspond to error values
conventionally associated with rejection by the foreign agent
(i.e., values from the range 64-127). The Code value 67 is a
pre-existing value that is to be used in some cases with the
extension defined in this specification. IANA has recorded the
values as defined in Section 10.

A new section for enumerating algorithms identified by specific
SPIs within the range 0-255 has been added by IANA. The CHAP_SPI
number (2) discussed in Section 8 is assigned from this range of
reserved SPI numbers. New assignments from this reserved range
must be specified and approved by the Mobile IP working group.
SPI number 1 should not be assigned unless in the future the
Mobile IP working group decides that SKIP is not important for
enumeration in the list of reserved numbers. SPI number 0 should
not be assigned.

Additionally, the new error codes fa_bad_aaa_auth, ha_bad_aaa_auth,
and ha_wrong_challenge are defined by this document. Among these,
ha_wrong_challenge may appear in the Status code of the FA Error
extension, defined in [RFC4636].

12. Security Considerations

In the event that a malicious mobile node attempts to replay the
authenticator for an old Mobile-Foreign Challenge, the foreign agent
would detect it, since the agent always checks whether it has
recently advertised the Challenge (see Section 3.2). Allowing mobile
nodes with different IP addresses or NAIs to use the same Challenge
value does not represent a security vulnerability, as the
authentication data provided by the mobile node will be computed over
data that is different (at least the mobile node's IP address will
vary).

If the foreign agent chooses a Challenge value (see Section 2) with
fewer than 4 octets, the foreign agent SHOULD include the value of
the Identification field in the records it maintains for the mobile
node. The foreign agent can then determine whether the Registration
messages using the short Challenge value are in fact unique and thus
assuredly not replayed from any earlier registration.

Perkins, et al. Standards Track [Page 17]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Section 8 (SPI For RADIUS AAA Servers) defines a method of computing
the Generalized Mobile IP Authentication extension's authenticator
field, using MD5 in a manner that is consistent with RADIUS
[RFC2865]. The use of MD5 in the method described in Section 8 is
less secure than HMAC-MD5 [RFC2104] and MUST be avoided whenever
possible.

Note that an active attacker may try to prevent successful
registrations by sending a large number of Agent Solicitations or
bogus Registration Requests, each of which could cause the foreign
agent to respond with a fresh challenge, invalidating the challenge
that the MN is currently trying to use. To prevent such attacks, the
foreign agent MUST NOT invalidate previously unused challenges when
responding to unauthenticated Registration Requests or Agent
Solicitations. In addition, the foreign agent MUST NOT allocate new
storage when responding to such messages, as this would also create
the possibility of denial of service.

The Challenge extension specified in this document need not be used
for co-located care-of address mode. In this case, replay protection
is provided by the Identification field in the Registration Request
message [RFC3344].

The Generalized Mobile IP Authentication extension includes a subtype
field that is used to identify characteristics of the particular
authentication strategy. This document only defines one subtype, the
Mobile-AAA Authentication subtype that uses HMAC-MD5. If it is
necessary to move to a new message authentication algorithm in the
future, this could be accomplished by defining a new subtype that
uses a different one.

13. Acknowledgements

The authors would like to thank Pete McCann, Ahmad Muhanna, Henrik
Levkowetz, Kent Leung, Alpesh Patel, Madjid Nakhjiri, Gabriel
Montenegro, Jari Arkko, and other MIP4 WG participants for their
useful discussions.

14. Normative References

[RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
September 1991.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.

[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.

Perkins, et al. Standards Track [Page 18]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.

[RFC2794] Calhoun, P. and C. Perkins, "Mobile IP Network Access
Identifier Extension for IPv4", RFC 2794, March 2000.

[RFC3012] Perkins, C. and P. Calhoun, "Mobile IPv4
Challenge/Response Extensions", RFC 3012, November 2000.

[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.

[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.

[RFC4636] Perkins, C., "Foreign Agent Error Extension for Mobile
IPv4", RFC 4636, October 2006.

Perkins, et al. Standards Track [Page 19]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Appendix A. Changes since RFC 3012

The following is the list of changes from RFC 3012 ([RFC3012]):

o Foreign agent recommended to include a Challenge in every
Registration Reply, so that mobile node can re-register without
waiting for an Advertisement.

o Foreign agent MUST record applicable challenge values used by each
mobile node.

o Mobile node forbidden to use Challenge values which were
advertised previous to the last Challenge value which it had used
for a registration.

o Challenge definitions are cleaned up.

o Programming suggestion added as an appendix.

o HMAC_CHAP_SPI option is added for Generalized Mobile IP
Authentication extension. Upon receipt of HMAC_CHAP_SPI, HMAC-MD5
is used instead of MD5 for computing the authenticator.

o Added fa_bad_aaa_auth and ha_bad_aaa_auth error codes to report
authentication errors caused while processing Mobile-AAA
Authentication extension. Also, added the error code
ha_wrong_challenge to indicate that Challenge value differs in the
Registration Reply received from the home agent compare to the one
sent to the home agent in the Registration Request.

o Processing of the Mobile-AAA Authentication extension is clarified
for the foreign agent and the home agent.

o Co-existence of the Mobile-AAA Authentication extension in the
same Registration Request is made explicit.

o The situation in which the foreign agent sets missing_challenge is
clarified further.

o The use of Mobile-AAA Authentication extension is allowed by the
mobile node with co-located care-of address.

o Added protection against bogus Registration Reply and Agent
Advertisement. Also, the processing of the Challenge is clarified
if it is received in the multicast/unicast Agent Advertisement.

o Added reference of FA Error extension in the References section
and also updated relevant text in section 3.2 and section 11.

Perkins, et al. Standards Track [Page 20]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Appendix B. Verification Infrastructure

The Challenge extensions in this protocol specification are expected
to be useful to help the foreign agent manage connectivity for
visiting mobile nodes, even in situations where the foreign agent
does not have any security association with the mobile node or the
mobile node's home agent. In order to carry out the necessary
authentication, it is expected that the foreign agent will need the
assistance of external administrative systems, which have come to be
called AAA systems. For the purposes of this document, we call the
external administrative support the "verification infrastructure".
The verification infrastructure is described to motivate the design
of the protocol elements defined in this document and is not strictly
needed for the protocol to work. The foreign agent is free to use
any means at its disposal to verify the credentials of the mobile
node. It could, for instance, rely on a separate protocol between
the foreign agent and the Mobile IP home agent and still not require
any modification to the mobile node.

In order to verify the credentials of the mobile node, we assume that
the foreign agent has access to a verification infrastructure that
can return a secure notification to the foreign agent that the
authentication has been performed, along with the results of that
authentication. This infrastructure may be visualized as shown in
Figure 4.

+----------------------------------------------------+
| |
| Verification and Key Management Infrastructure |
| |
+----------------------------------------------------+
^ | ^ |
| | | |
| v | v
+---------------+ +---------------+
| | | |
| foreign agent | | home agent |
| | | |
+---------------+ +---------------+

Figure 4. The Verification Infrastructure

After the foreign agent gets the Challenge authentication, it MAY
pass the authentication to the (here unspecified) infrastructure and
await a Registration Reply. If the Reply has a positive status
(indicating that the registration was accepted), the foreign agent
accepts the registration. If the Reply contains the Code value

Perkins, et al. Standards Track [Page 21]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

BAD_AUTHENTICATION (see Section 10), the foreign agent takes actions
indicated for rejected registrations.

Implicit in this picture is the important observation that the
foreign agent and the home agent have to be equipped to make use of
whatever protocol is required by the challenge verification and key
management infrastructure shown in the figure.

The protocol messages for handling the authentication within the
verification infrastructure and the identity of the agent performing
the verification of the foreign agent challenge are not specified in
this document, as those operations do not have to be performed by any
Mobile IP entity.

Appendix C. Message Flow for FA Challenge Messaging with Mobile-AAA
Extension

MN FA Verification home agent
|<-- Adv+Challenge--| Infrastructure |
| (if needed) | | |
| | | |
|-- RReq+Challenge->| | |
| + Auth.Ext. | | |
| | Auth. Request, incl. | |
| |--- RReq + Challenge --->| |
| | + Auth.Ext | RReq + |
| | |-- Challenge -->|
| | | |
| | | |
| | |<--- RRep ----- |
| | Authorization, incl. | |
| |<-- RRep + Auth.Ext.-----| |
| | | |
|<-- RRep+Auth.Ext--| | |
| + New Challenge | | |

Figure 5. Message Flows for FA Challenge Messaging

In Figure 5, the following informational message flow is illustrated:

1. The foreign agent includes a Challenge Value in a unicast Agent
Advertisement, if needed. This advertisement MAY have been
produced after receiving an Agent Solicitation from the mobile
node (not shown in the diagram).

2. The mobile node creates a Registration Request including the
advertised Challenge Value in the Challenge extension, along with
a Mobile-AAA authentication extension.

Perkins, et al. Standards Track [Page 22]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

3. The foreign agent relays the Registration Request either to the
home agent specified by the mobile node or to its locally
configured Verification Infrastructure (see Appendix B),
according to local policy.

4. The foreign agent receives a Registration Reply with the
appropriate indications for authorizing connectivity for the
mobile node.

5. The foreign agent relays the Registration Reply to the mobile
node, often along with a new Challenge Value to be used by the
mobile node in its next Registration Request message.

Appendix D. Message Flow for FA Challenge Messaging with MN-FA
Authentication

MN FA home agent
|<-- Adv+Challenge--| |
| (if needed) | |
| | |
|-- RReq+Challenge->| |
| + Auth.Ext. | |
| |--- RReq + Challenge --->|
| | + HA-FA Auth.Ext |
| | |
| |<-- RRep + Challenge ----|
| | + HA-FA Auth.Ext |
| | |
|<-- RRep+Auth.Ext--| |
| + New Challenge | |

Figure 6. Message Flows for FA Challenge Messaging with MN-FA
Authentication

In Figure 6, the following informational message flow is illustrated:

1. The foreign agent disseminates a Challenge Value in an Agent
Advertisement, if needed. This advertisement MAY have been
produced after receiving an Agent Solicitation from the mobile
node (not shown in the diagram).

2. The mobile node creates a Registration Request including the
advertised Challenge Value in the Challenge extension, along with
a Mobile-Foreign Authentication extension.

3. The foreign agent relays the Registration Request to the home
agent specified by the mobile node.

Perkins, et al. Standards Track [Page 23]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

4. The foreign agent receives a Registration Reply with the
appropriate indications for authorizing connectivity for the
mobile node.

5. The foreign agent relays the Registration Reply to the mobile
node, possibly along with a new Challenge Value to be used by the
mobile node in its next Registration Request message. If the
Reply contains the Code value ha_bad_aaa_auth (see Section 10),
the foreign agent takes actions indicated for rejected
registrations.

Appendix E. Example Pseudo-Code for Tracking Used Challenges

current_chal := RegistrationRequest.challenge_extension_value
last_chal := mobile_node_record.last_used_adv_chal

if (current_chal == mobile_node_record.RegReply_challenge) {
update (mobile_node_record, current_chal)
return (OK)
}
else if (current_chal "among" VALID_ADV_CHALLENGES[]{
if (last_chal "among" VALID_ADV_CHALLENGES[]) {
if (current_chal is "before" last_chal) {
send_error(STALE_CHALLENGE)
return (FAILURE)
}
else {
update (mobile_node_record, current_chal)
return (OK)
}
}
else {
update (mobile_node_record, current_chal)
return (OK)
}
}
else {
send_error(UNKNOWN_CHALLENGE);
}

Perkins, et al. Standards Track [Page 24]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Authors' Addresses

Charles E. Perkins
Nokia Research Center
Communications Systems Lab
313 Fairchild Drive
Mountain View, California 94043

Phone: +1 650 625-2986
EMail: [email protected]

Pat R. Calhoun
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134

Phone: +1 408-853-5269
EMail: [email protected]

Jayshree Bharatia
Nortel Networks
2221, Lakeside Blvd
Richardson, TX 75082

Phone: +1 972-684-5767
EMail: [email protected]

Perkins, et al. Standards Track [Page 25]

RFC 4721 Mobile IPv4 Challenge/Response Extensions January 2007

Full Copyright Statement

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contained in BCP 78, and except as set forth therein, the authors
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Perkins, et al. Standards Track [Page 26]