Mobile IP Working Group Charles E. Perkins
INTERNET DRAFT Nokia Research Center
1 March 2002 Pat R. Calhoun
Black Storm Networks
Mobile IPv4 Challenge/Response Extensions
draft-ietf-mobileip-rfc3012bis-01.txt
Status of This Memo
This document is a submission by the mobile-ip Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the mobile-ip@sunroof.eng.sun.com mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
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material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at:
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at:
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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, this extension does not provide ironclad replay
protection for the foreign agent, and does not allow for the use
of existing techniques (such as 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.
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Contents
Status of This Memo i
Abstract i
1. Introduction 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. Mobile IP Agent Advertisement Challenge Extension 3
3. Operation 3
3.1. Mobile Node Processing for Registration Requests . . . . 3
3.2. Foreign Agent Processing for Registration Requests . . . 5
3.3. Foreign Agent Processing for Registration Replies . . . . 7
3.4. Home Agent Processing for the Challenge Extensions . . . 7
4. Mobile-Foreign Challenge Extension 8
5. Generalized Mobile IP Authentication Extension 8
6. Mobile-AAA Authentication subtype 9
7. Reserved SPIs for Mobile IP 10
8. SPI For RADIUS AAA Servers 10
9. Configurable Parameters 12
10. Error Values 12
11. IANA Considerations 12
12. Security Considerations 12
13. Acknowledgements 13
A. Changes since Last Revision 15
B. Verification Infrastructure 15
C. Message Flow for FA Challenge Messaging 17
Addresses 18
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1. Introduction
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, this extension does not provide ironclad replay
protection, from the point of view of the foreign agent, and does
not allow for the use of existing techniques (such as CHAP [11]) 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 a use
challenge/response mechanism to authenticate the mobile node.
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 [1].
All SPI values defined in this document refer to values for the
Security Parameter Index, as defined in the base Mobile IP protocol
specification [7].
The following additional terminology
previously used challenge
Any challenge that has been used by the mobile node in a
Registration Request message and accepted by the Foreign
Agent as a valid challenge by relaying or generating a
corresponding Registration Reply message.
stale challenge
Same as "previously used challenge". The Foreign Agent
may not be able to keep records for all stale challenges.
unknown challenge
Any challenge that the foreign agent has no record of
having put either into one of its Agent Advertisements or
a corresponding registration reply message.
unused challenge
A challenge that has not been already accepted by the
Foreign Agent as a valid challenge in a corresponding
Registration Reply message -- i.e., a valid challenge
that is neither unknown nor previously used.
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2. Mobile IP Agent Advertisement Challenge Extension
This section defines a new extension to the Router Discovery
Protocol [4] 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 bytes; 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
the latest 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. [5] provides more information
on generating pseudo-random numbers suitable for use as values for
the challenge.
3. Operation
This section describes modifications to the Mobile IP registration
process which 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 for Registration Requests
Whenever the Agent Advertisement contains the Challenge extension, if
the mobile node does not have a security association with the Foreign
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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 [7]. 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.
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. In addition,
the Mobile Node SHOULD include the NAI extension [2], to enable
the foreign agent to make use of any available verification
infrastructure. 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 [11] using MD5 [10].
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 Registration Reply from the Foreign Agent MAY include a new
Challenge value (see section 3.3). The Mobile Node MAY use either
the value found in the latest Advertisement, or the one found in
the last Registration Reply from the Foreign Agent. This approach
enables the Mobile Node to make use of the challenge without having
to wait for advertisements.
A Mobile Node might receive an UNKNOWN_CHALLENGE error (see
section 9) if it moves to a new Foreign Agent that cannot validate
the challenge provided in the Registration Request. In such
instances, the Mobile Node MUST use a new Challenge value in any new
registration, obtained either from an Agent Advertisement, or from a
Challenge extension to the Registration Reply containing the error.
A Mobile Node that does not include a Challenge when the
Mobile-Foreign Authentication extension is present may receive a
MISSING_CHALLENGE (see section 10) error. In this case, the foreign
agent will not process the request from the mobile node unless the
request contains a valid Challenge.
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A Mobile Node that receives a BAD_AUTHENTICATION Code value (see
section 10) SHOULD include the Mobile-AAA Authentication Extension in
the next Registration Request. This will make it possible for the
Foreign Agent to use its AAA infrastructure in order to authenticate
the Mobile Node.
3.2. Foreign Agent Processing for Registration Requests
Upon receipt of the Registration Request, if the Foreign Agent has
issued a Challenge as part of its Agent Advertisements, and 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. If
the challenge extension is needed and does not exist, the Foreign
Agent MUST send a Registration Reply to the mobile node with the Code
value MISSING_CHALLENGE.
A foreign agent that sends Agent Advertisements containing a
Challenge value MAY send a Registration Reply message with a
MISSING_CHALLENGE error if the mobile node sends a Registration
Request with a Mobile-Foreign Authentication extension without
including a Challenge. In other words, such a foreign agent MAY
refuse to process a Registration Request from the mobile node unless
the request contains a valid Challenge.
If a mobile node retransmits a Registration Request with the same
Challenge extension, and 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 [2]) are the same as represented in the
visitor list entry for the pending Registration Request (section
3.7.1 of [7]). 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 last Registration Reply issued
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to the Mobile Node, or else 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).
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
BAD_AUTHENTICATION (see Section 10).
If the Mobile-AAA Authentication extension (see Section 6) is present
in the message, or if an 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 of the mobile node.
The Foreign Agent MUST NOT remove the Mobile-AAA Authentication
Extension from the Registration Request prior to the completion of
the authentication performed by the AAA infrastructure. The appendix
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, the Foreign Agent MAY
remove the Challenge Extension from the Registration Request without
disturbing the authentication value computed by the Mobile Node for
use by the AAA or the Home Agent. If the Challenge extension is not
removed, it MUST precede the Foreign-Home Authentication extension.
If the Foreign Agent does not remove the Challenge extension, then
the Foreign Agent SHOULD store the Challenge value as part of the
pending registration request list [7]. Also in this case, the
Foreign Agent MUST reject any Registration Reply message coming from
the Home Agent that does not also include the Challenge Extension
with the same Challenge Value that was included in the Registration
Request. The Foreign Agent MUST send the rejected Registration
message to the mobile node, and change the status in the Registration
Reply to the Code value MISSING_CHALLENGE (see section 10).
If the Foreign Agent does remove the Challenge extension and
applicable authentication from the Registration Request message,
then it SHOULD insert the Identification field from the Registration
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Request message along with its record-keeping information about the
particular Mobile Node in order to protect against replays.
3.3. Foreign Agent Processing for Registration Replies
The Foreign Agent MAY include a new 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.
Suppose the Registration Reply includes a Challenge extension from
the Home Agent, and 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.
3.4. Home Agent Processing for the 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.
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 [7].
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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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 [7])
Length Length of the Challenge value
Challenge The Challenge field is copied from the Challenge field
found in the Agent Advertisement Challenge extension
(see section 2).
5. Generalized Mobile IP Authentication Extension
Several new authentication extensions have been designed for various
control messages proposed for extensions to Mobile IP (see, for
example, [8]). 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 [7] are the home agent and the foreign
agent. It is the purpose of the generalized authentication extension
defined here to collect together data for all such new authentication
applications into a single extension type with subtypes.
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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 [7])
Subtype a number assigned to identify the kind of
endpoints or the other characteristics of the
particular authentication strategy
Length 4 plus the number of bytes in the Authenticator;
MUST be at least 20.
SPI Security Parameters Index
Authenticator The variable length Authenticator field
In this document, only one subtype is defined:
1 Mobile-AAA Authentication subtype (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. If the mobile
node does not include a Mobile-Foreign Authentication [7] extension,
then it MUST include the Mobile-AAA Authentication extension whenever
the Challenge extension is present. If the Mobile-AAA Authentication
extension is present, then the Registration Message sent by the
mobile node MUST contain the Mobile-Home Authentication extension [7]
if it shares a security association with the Home Agent. If present,
the Mobile-Home Authentication Extension MUST appear prior to the
Mobile-AAA Authentication extension. The mobile node MAY include a
Mobile-AAA Authentication extension in any Registration Request. The
corresponding response MUST include the Mobile-Home Authentication
Extension, and MUST NOT include the Mobile-AAA Authentication
Extension.
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The default algorithm for computation of the authenticator is
HMAC-MD5 [6] 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 resulting function call, as described in [6], 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,
this default algorithm MUST be able to be configured for selection at
any arbitrary 32-bit SPI outside of the SPIs in the reserved range
0-255.
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) which 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/numbers.html
From that URL, follow the hyperlinks to [M] within the "Directory of
General Assigned Numbers", and subsequently to the specific section
for "Mobile IP Numbers".
8. SPI 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 (see section 7) for
indicating the following procedure for computing authentication
data (called the "authenticator"), which is used by many RADIUS
servers [9] today.
To compute the authenticator, apply MD5 [10] computed on the
following data, in the order shown:
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High-order byte from Challenge || Key ||
MD5(Preceding Mobile IP data ||
Type, Subtype (if present), Length, SPI) ||
Least-order 237 bytes from Challenge
where the 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. Since the RADIUS protocol
cannot carry attributes greater than 253 in size, the preceding
Mobile IP data, type, subtype (if present), length and SPI are hashed
using MD5. Finally, the least significant 237 bytes of the challenge
are concatenated. If the challenge has fewer than 238 bytes, this
algorithm includes the high-order byte in the computation twice, but
ensures that the challenge is used exactly as is. Additional padding
is never used to increase the length of the padding; the input data
is allowed to be shorter than 237 bytes long.
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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(s) of Document
-------------- ------------- ----------------------
CHALLENGE_WINDOW 2 3.2
CHAP_SPI 2 8
10. Error Values
Each entry in the following table contains the name of Code [7] to
be returned in a Registration Reply, the value for the Code, and the
section in which the error is first mentioned in this specification.
Error Name Value Section of Document
---------------------- ----- -------------------
UNKNOWN_CHALLENGE 104 3.2
BAD_AUTHENTICATION 67 3.2 - also see [7]
MISSING_CHALLENGE 105 3.1,3.2
STALE_CHALLENGE 106 3.2
11. IANA Considerations
All protocol values in this specification are to be the same as
defined in RFC 3012 [3].
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, because the
authentication data provided by the mobile node will be computed over
data that is different (at least by the bytes of the mobile nodes' IP
addresses).
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Whenever a Foreign Agent updates a field of the Registration Reply
(as suggested in section 3.2), it invalidates the authentication
data supplied by the Home Agent in the Mobile-Home Authentication
extension to the Registration Reply. Thus, this opens up a security
exposure whereby a node might try to supply a bogus Registration
Reply to a mobile node that causes the mobile node to act as if its
Registration Reply were rejected. This might happen when, in fact, a
Registration Reply showing acceptance of the registration might soon
be received by the mobile node.
If the foreign agent chooses a Challenge value (see section 2) with
fewer than 4 bytes, 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.
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 [9]. The
use of MD5 in the method described in Section 8 is less secure than
HMAC-MD5 [6], and should be avoided whenever possible.
13. Acknowledgements
The authors would like to thank Tom Hiller, Mark Munson, the TIA
TR45-6 WG, Gabriel Montenegro, Vipul Gupta, Pete McCann, Robert
Marks, and Ahmad Muhanna, for their useful discussions. A recent
draft by Mohamed Khalil, Raja Narayanan, Emad Qaddoura, and
Haseeb Akhtar has also suggested the definition of a generalized
authentication extension similar to the specification contained in
section 5.
References
[1] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
[2] P. Calhoun and C. Perkins. Mobile IP Network Access Identifier
Extension for IPv4. Request for Comments (Proposed Standard)
2794, Internet Engineering Task Force, January 2000.
[3] P. Calhoun and C. E. Perkins. Mobile IP Foreign Agent
Challenge/Response Extension. Request for Comments (Proposed
Standard) 3012, Internet Engineering Task Force, December 2000.
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[4] S. Deering. ICMP Router Discovery Messages. Request for
Comments (Proposed Standard) 1256, Internet Engineering Task
Force, September 1991.
[5] D. Eastlake, 3rd, S. Crocker, and J. Schiller. Randomness
Recommendations for Security. Request for Comments
(Informational) 1750, Internet Engineering Task Force, December
1994.
[6] H. Krawczyk, M. Bellare, and R. Canetti. HMAC: Keyed-Hashing
for Message Authentication. Request for Comments
(Informational) 2104, Internet Engineering Task Force,
February 1997.
[7] C. Perkins. IP Mobility Support. Request for Comments
(Proposed Standard) 3220, Internet Engineering Task Force,
December 2001.
[8] C. Perkins and D. Johnson. Route Optimization in Mobile IP
(work in progress). Internet Draft, Internet Engineering Task
Force.
draft-ietf-mobileip-optim-11.txt, September 2001.
[9] C. Rigney, A. Rubens, W. Simpson, and S. Willens. Remote
Authentication Dial In User Service (RADIUS). Request for
Comments (Proposed Standard) 2138, Internet Engineering Task
Force, April 1997.
[10] R. Rivest. The MD5 Message-Digest Algorithm. Request for
Comments (Informational) 1321, Internet Engineering Task Force,
April 1992.
[11] W. Simpson. PPP Challenge Handshake Authentication Protocol
(CHAP). Request for Comments (Draft Standard) 1994, Internet
Engineering Task Force, August 1996.
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A. Changes since Last Revision
Here is a list of the important changes since the previous revision
of this document.
- Changes section added.
- Clarified ordering rules for adding the Challenge extension in
section 3.1.
- Definitions for Stale Challenge, etc. in section 1.1 have been
clarified to avoid misinterpretations that could cause dropped
registrations.
- Used "Code value" field name instead of "error code" to be
compatible with RFC 3220.
- Used Authentication Extension names compatible with RFC 3220.
- Updated document citations.
- Clarified nature of message to Authorization Infrastructure in
appendix C.
- Added message flows to Home Agent in appendix C.
- Specified that NAI has to remain the same in order for a
Registration Request to be considered a retransmission.
- Clarified that padding is NOT added if the input data is shorter
than 237 bytes.
- Replaced IANA Considerations with a statement that the numbers
should stay the same.
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
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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. This could, for instance, rely on a separate protocol
between the foreign agent and the Mobile IP home agent, and still be
completely invisible to the mobile node.
In order to verify the credentials of the mobile node, we imagine
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
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 made available to them by the challenge
verification and key management infrastructure shown in the figure.
The protocol messages for handling the authentication within the
verification infrastructure, and identity of the agent performing the
verification of the Foreign Agent challenge, are not specified in
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this document, because those operations do not have to be performed
by any Mobile IP entity.
C. Message Flow for FA Challenge Messaging
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--| | |
| + Challenge | | |
Figure 5: Message Flows for FA Challenge Messaging
In figure 5, the following 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
an authorization-enabling authentication extension.
3. The foreign agent relays the Registration Request either to
the home agent specified by the mobile node, or else 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 connectvity for the
mobile node.
Perkins, Calhoun Expires 1 September 2002 [Page 17]
Internet Draft Mobile IPv4 Challenge/Response, revised 1 March 2002
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 Reply message.
Addresses
The working group can be contacted via the current chairs:
Basavaraj Patil Phil Roberts
Nokia Megisto Corp.
6000 Connection Dr. Suite 120
20251 Century Blvd
Irving, TX. 75039 Germantown MD 20874
USA USA
Phone: +1 972-894-6709 Phone: +1 847-202-9314
Email: Basavaraj.Patil@nokia.com Email: PRoberts@MEGISTO.com
Questions about this memo can also be directed to the authors:
Charles E. Perkins Pat R. Calhoun
Communications Systems Lab
Nokia Research Center Black Storm Networks
313 Fairchild Drive 250 Cambridge Avenue, Suite 200
Mountain View, California 94043 Palo Alto, California, 94306
USA USA
Phone: +1-650 625-2986 Phone: +1 650-617-2932
EMail: charliep@iprg.nokia.com Email: pcalhoun@diameter.org
Fax: +1 650 625-2502 Fax: +1 720-293-7501
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