Mobile IP Working Group Charles E. Perkins
INTERNET DRAFT Nokia Research Center
22 June 2003 Pat R. Calhoun
Airespace Systems
AAA Registration Keys for Mobile IP
draft-ietf-mobileip-aaa-key-13.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.
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Abstract
AAA servers, such as RADIUS and DIAMETER, are in use within the
Internet today to provide authentication and authorization services
for dial-up computers. Mobile IP requires strong authentication
between the mobile node and its home agent. When the mobile node
shares an AAA Security Association with its home AAA server, however,
it is possible to use that AAA Security Association to create derived
Mobility Security Associations between the mobile node and its home
agent, and again between the mobile node and the foreign agent
currently offering connectivity to the mobile node. This document
specifies extensions to Mobile IP registration messages that can be
used to create Mobility Security Associations between the mobile node
and its home agent, and/or between the mobile node and a foreign
agent.
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1. Introduction
AAA servers, such as RADIUS [14] and DIAMETER [5], are in use within
the Internet today to provide authentication and authorization
services for dial-up computers. Such services are likely to be
valuable for mobile nodes using Mobile IP [13] when the nodes
are attempting to connect to foreign domains with AAA servers.
Requirements for interactions between AAA and Mobile IP are outlined
in RFC 2977 [7]; that document describes an infrastructure which
enables AAA servers to authenticate and authorize network access
requests from mobile nodes. See also appendix C. The Mobile IP
Registration Request is considered to be a request for network
access. It is then possible to augment the functionality of the
Mobile IP mobility agents so that they can translate between Mobile
IP registration messages and the messages used within the AAA
infrastructure, as described in RFC 2977. Mobility agents and
AAA servers that conform to the requirements of RFC 2977 can be
considered as appropriate network entities to support the message
types specified in this document. Please consult RFC 2977 [7] for
further details.
This specification makes use of a single AAA Security Association
to create derivative Mobility Security Associations. A Mobility
Security Association in this specification is a simplex connection
that serves to authenticate MIPv4 control traffic between a MN and HA
and/or a MN and FA. A Mobility Security Association is identified by
the two end points, such as a MN IP address and a HA IP address, and
a SPI. Two nodes may have one or more Mobility Security Associations
established between each other; however, typically there is no reason
to have more than one Mobility Security Association between two
nodes.
This document specifies extensions to Mobile IP registration messages
that can be used to create Mobiity Security Associations between the
MN and FA and/or MN and HA based on the AAA Security Association
between the MN and HAAA. These additional Mobility Security
Associations may then be used in Mobile IP extensions to calculate
the Authentication Data need by authentication extensions used in
Mobile IP control messages. It is assumed that the AAA Security
Association between the MN and its HAAA has been appropriately
configured so that the AAA server has the authorization to provide
key material to be used as the basis for the necessary Mobility
Security Assocation between the MN and its prospective mobility
agents.
AAA servers typically use the Network Access Identifier (NAI) [1] to
uniquely identify the mobile node; the mobile node's home address is
not always necessary to provide that function. Thus, it is possible
for a mobile node to authenticate itself, and be authorized for
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connection to the foreign domain, without having any home address.
However, for Mobile IP to work, the mobile node is required to have a
home address and a Mobility Security Association [13] with its home
agent. When the Mobile IP Registration Reply packet is authenticated
by the MN-AAA Authentication Extension [4], the mobile node can
verify that the key material contained in the extensions were
produced by the AAA server, and thus may be reliably used to create
Mobility Security Associations with the home agent, or alternatively
with the foreign agent.
It is also assumed that the AAA entities involved (i.e., the AAAH,
AAAL, and the AAA interface features of the foreign agents and home
agents) all have means outside of the scope of this document for
exchanging keys. The extensions within this document are intended to
work with any AAA protocol suite that allows for such key exchange,
as long as it satisfies the requirements specified in RFC 2977 [7].
2. 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 [2].
AAA Authentication, Authorization, and Accounting
(see [9]).
AAA entity A network node processing AAA messages according to
the requirements for AAA protocols (see [9]).
AAA security association
A security association between a AAA entity
and another node needing the services of that
AAA entity. In this document all AAA security
associations are between a mobile node and its home
AAA server (AAAH). A mobile node's AAA security
association with its home AAA server (AAAH) may be
based either on the mobile node's IP address or on
its NAI [1].
Key a number, kept secret. Only nodes in possession
of the key have any hope of using the security
transform to obtain correct results.
Key Material
Data used for the purpose of creating a key.
Mobility security association
A Mobility Security Association is a simplex
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connection that serves to authenticate MIPv4 control
traffic such as between a MN and HA and/or a MN and
FA. A Mobility Security Association is identified
by the two end points, such as a MN IP address and
a HA IP address, and a SPI. Two nodes may have one
or more Mobility Security Associations established
between each other; however, typically there is
no reason to have more than one Mobility Security
Association between two nodes.
Registration Key
A key used as the basis of a Mobility Security
Association between a mobile node and a foreign
agent. A registration key is typically only used
once or a very few times, and only for the purposes
of verifying a small volume of Authentication data.
Security Algorithm
A set of rules for using input data and a secret key
for producing data for use in security protocols.
SPI
Security Parameters Index. The SPI is an arbitrary
32-bit value that assists in the identification of
an AAA, IP, or Mobility Security Association.
Other terminology is used as defined in the base Mobile IP
specification [13]. Furthermore, in order to simplify the
discussion, we have used the word "Extension" instead of "Subtype of
the Generalized Extension" in many cases. So, for instance, instead
of using the phrase "The MN-FA Key Material From AAA Subtype of the
Generalized MN-FA Key Reply Extension", we would instead use the
phrase "The MN-FA Key Material From AAA Extension".
3. Overview of Operations with Key Extensions
When a mobile node depends on an AAA infrastructure to obtain
authorization for network connectivity and Mobile IP registration,
it may lack any pre-existing Mobility Security Associations with
either its home agent, or the foreign agent controlling the access to
the foreign network. The extensions defined in this document allow
a AAA entity to supply key material to mobile nodes to be used as
the basis of its Mobility Security Association with mobile agents.
The AAA entity that will act on these extensions is part of the AAA
infrastructure, and is typically identified within the foreign domain
by methods outside the scope of this specification (see appendix C).
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The key material may be requested by the mobile node in new
extensions to Mobile IP Registration Request messages, and supplied
to the mobile node in extensions to the Mobile IP Registration Reply
messages. Alternatively, the AAA server MAY provide unsolicited key
material to mobile nodes; the mobile node MUST then calculate new
keys and update or create its relevant Mobility Security Association.
The method by which key material is supplied to the mobility agents
themselves is out of scope for this document, and would depend on the
particular details of the security architecture for the AAA servers
in the foreign and home domains (see RFC 2977 and appendix C). For
the purposes of this document, we assume that there is a suitable AAA
infrastructure available to the foreign agents, and that the mobile
node does have an AAA Security Association with at least one AAA
server in its home domain.
When a mobile node travels away from home, it may not have a Mobility
Security Association with its home agent, perhaps because it does
not yet have a home address [3]. The protocol and messages in this
document are intended to facilitate the following operations which
may occur between the mobile node, foreign agent, home agent, and AAA
servers in the visited (local) domain (AAAL) and in the home domain
(AAAH). In the following sequence of messages, the only message flows
specified in this document are the Registration Request between the
mobile node and the foreign agent, and Registration Reply between the
foreign agent and the mobile node. The other messages described here
result from the presumed action of the AAA entities as described in
RFC 2977. See also appendix D.
1. If the mobile node does not have a Mobility Security Association
with the foreign agent, it SHOULD include an MN-FA Key Request
extension (see Section 7.1) as part of its Registration Request
that it sends to the Foreign Agent.
2. If the mobile node does not have a Mobility Security Association
with the home agent, it MUST add an MN-HA Key Request extension
(see Section 7.3) as part of its Registration Request that it
sends to the Foreign Agent.
3. If one or more AAA Key Request extensions were added, the
mobile node MUST add the MN-AAA Authentication extension to its
Registration Request after the AAA Key Request extension.
4. By action of the foreign agent, which is presumed to be also a
AAA entity, the mobile node's key requests and authentication
data are transferred to the local AAA server (AAAL), typically
after reformatting to fit into the appropriate AAA messages,
which are out of scope for this document.
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5. After the information within the MN-AAA Authentication extension
is verified by the AAA server in the home domain (AAAH), it then
also generates the Key Material that has been requested by the
mobile node, for the necessary Mobility Security Associations.
6. The respective keys for the Mobility Security Associations are
distributed to the Home Agent and Foreign Agent via the AAA
protocol.
7. The mobile node receives the Registration Reply message from the
Foreign Agent.
8. If a MN-HA Key Material from AAA Key Material extension is
present in the Registration Reply message, then the mobile node
MUST create or update its Mobility Security Association with the
Home Agent indicated in the Registration Reply, using the key
computed from the Key Material in the AAA extension. In this
case, if no Key Material extension is present, the mobile node
MUST discard the Registration Reply. If the mobile node does
not already have a Mobility Security Association with the Home
Agent indicated in the Registration Reply message, and if no Key
Material extension is present, the mobile node MUST discard the
Registration Reply.
9. Using its (perhaps newly created) Mobility Security Association
with the home agent, the mobile node authenticates the
Registration Reply message, by checking the Authentication Data
in the Mobile-Home Authentication extension.
10. If the Registration Reply passes authentication and contains a
MN-FA Key Material From AAA extension (see section 7.2), the
mobile node generates the registration key using the Key Material
provided, according to its AAA security Association with the
AAA. The resulting registration key is used to establish the
mobile node's Mobility Security Association with its foreign
agent, and is used to compute the authentication data used in the
Mobile-Foreign authentication extension.
Any registration reply containing the MN-HA Key Material From AAA
extension MUST also contain a subsequent Mobile Home Authentication
extension, created using the generated MN-HA key. Similarly, a
reply containing the MN-FA Key Material From AAA extension MUST also
contain a subsequent Mobile Foreign Authentication extension, created
using the registration key.
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4. Mobility Security Associations
Mobility Security Associations between Mobile IP entities
(mobile nodes, home agents, foreign agents) contain both the
necessary cryptographic key information, and a way to identify
the cryptographic transform which uses the key to produce the
authentication information which is present in the Mobile-Home
Authentication extension or the Mobile-Foreign Authentication
extension. In order for the mobile node to make use of key material
created by the AAA server, the mobile node also has to be able to
identify and select the appropriate cryptographic transform that uses
the key to produce the authentication.
The transform identifiers are the same as those used in IPsec. They
are tabulated in the list of Authentication Algorithms allowable
as values for the "Attribute Type" (5) (i.e., "Authentication
Algorithm"), one of the classifications in the tabulated
Attribute Types for "IPSEC Security Association Attributes". See
http://www.iana.org/assignments/isakmp-registry for the full listing
of all Attribute Types and other Attributes for IPSEC Security
Associations.
Mobility Security Associations shared between mobile nodes and home
agents also require a replay protection method. The following table
contains the supported replay methods.
Replay Method Name Reference
-------------- ------------ --------------
0,1 Reserved
2 Timestamps RFC 3344 [13]
3 Nonces RFC 3344 [13]
4-65535 Unallocated
5. Key Material Creation and Derivation
This section contains the procedures followed in the creation of
the Key Material by AAA servers, and the key derivation procedures
used by mobile nodes. Note that the AAA servers will also deliver
the keys to the mobility agents (home agent, foreign agent) via the
AAA protocol. AAA servers that follow these procedures will produce
results that can be understood by mobile nodes. The mobility agents
will faithfully transcribe the results into the appropriate Mobile IP
extensions.
The example that follows makes use of HMAC-MD5 [8]. All mobile nodes
and mobility agents implementing Mobile IP [13], and implementing the
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extensions specified in this document, MUST implement HMAC-MD5 [13].
Other cryptographic functions MAY also be used.
The following steps are performed on the AAA server:
1. The AAA server identifies the mobile node. If the Home Address
field of the Registration Request is either zero (0), or all ones
(0xffffffff), then the Mobile Node's NAI is used instead of the
mobile node's home address.
2. The AAA server generates a random [6] value of at least 128 bits
to be used as the Key Material.
3. The AAA server inserts the random value into the Key Reply
extension, in the ``Key Material'' field.
The following steps are performed by the mobile node:
1. Using the Key Material from the extension, the mobile node
calculates
key = HMAC-MD5 (AAA-key, {Key Material || home address})
2. The mobile node creates the Mobility Security Association, using
the key and the other relevant information in the Key Extension.
The secret key used within the HMAC-MD5 computation is indicated by
the AAA Security Association indexed by the AAA SPI, which has been
previously configured as the basis for the AAA Security Association
between the mobile node and the AAA server creating the key material.
6. Generalized Key Request/Reply Extensions
The extensions in this section are Generalized Extensions [13], and
have subtypes as specified in section 7.
6.1. Generalized MN-FA Key Request Extension
Figure 1 illustrates the Generalized MN-FA Key Request Extension.
Type TBD (not skippable) (see [13] and section 9)
Subtype a number assigned to identify the way in
which the Key Request Data is to be used when
generating the registration key
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN-FA Key Request Subtype Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The Generalized Mobile IP MN-FA Key Request Extension
Length The 16-bit Length field indicates the length of
the extension. It is equal to the number of
bytes in the MN-FA Key Request Subtype Data plus
4 (for the Mobile Node SPI field).
Mobile Node SPI The Security Parameters Index that the mobile
node will assign for the Mobility Security
Association created for use with the registration
key.
MN-FA Key Request Subtype Data
Data needed to carry out the creation of the
registration key on behalf of the mobile node.
This field is zero in length and carries no data.
The Generalized MN-FA Key Request Extension defines a set of
extensions, identified by subtype, which may be used by a mobile node
in a Mobile IP Registration Request message to request that some
other entity create a Registration Key for use by the mobile node
with the mobile node's new foreign agent.
6.2. Generalized MN-FA Key Reply Extension
The Generalized MN-FA Key Reply extension supplies a registration key
requested by using one of the subtypes of the Generalized MN-FA Key
Request extension. Figure 2 illustrates the format Generalized MN-FA
Key Reply Extension.
Type TBD (not skippable) (see [13] and section 9)
Subtype a number assigned to identify the way in which the
MN-FA Key Reply Subtype Data is to be decrypted to
obtain the registration key
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN-FA Key Reply Subtype Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: The Generalized Mobile IP MN-FA Key Reply Extension
Length The 16-bit Length field is equal to the number of bytes
in the MN-FA Key Reply Subtype Data.
MN-FA Key Reply Subtype Data
An encoded copy of the registration key, along with any
other information needed by the recipient to create the
designated Mobility Security Association.
For each subtype, the format of the MN-FA Key Reply Subtype Data has
to be separately defined according to the particular method required
to set up the Mobility Security Association.
For the subtypes defined in this document, the MN-FA Key supplied
in the data for a subtype of this extension may come by a request
which was sent using a subtype of the Generalized MN-FA Key Request
Extension. In such cases, the SPI to be used when employing the
Mobility Security Association defined by the registration key is the
same as given in the original request.
Once the mobile node creates the Mobility Security Association with
the foreign agent, by using the transform indexed by the AAA SPI, it
stores that Mobility Security Association indexed by the FA SPI in
its list of Mobile Security Associations.
If the foreign agent receives a Registration Reply that has no MN-FA
Key Reply extension, and if it has no existing Mobility Security
Association with the mobile node, the foreign agent MAY change
the Code value of the Registration Reply to MISSING_MN_FA (see
section 8), effectively causing the registration to fail.
6.3. Generalized MN-HA Key Request Extension
Figure 3 illustrates the Generalized MN-HA Key Request Extension.
Type TBD (not skippable) (see [13] and section 9)
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN-HA Key Request Subtype Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: The Generalized Mobile IP MN-HA Key Request Extension
Subtype a number assigned to identify the way in
which the Key Request Data is to be used when
generating the registration key
Length The 16-bit Length field indicates the length of
the extension. It is equal to the number of
bytes in the MN-HA Key Request Subtype Data plus
4 (for the Mobile Node SPI field).
Mobile Node SPI The Security Parameters Index that the mobile
node will assign for the Mobility Security
Association created for use with the registration
key.
MN-HA Key Request Subtype Data
Data needed to carry out the creation of the
registration key on behalf of the mobile node.
This field is zero in length and carries no data.
The Generalized MN-HA Key Request Extension defines a set of
extensions, identified by subtype, which may be used by a mobile node
in a Mobile IP Registration Request message to request that some
other entity create a MN-HA key for use by the mobile node with the
mobile node's new home agent.
6.4. Generalized MN-HA Key Reply Extension
Type TBD (not skippable) (see [13] and section 9)
Subtype a number assigned to identify the way in which the
MN-HA Key Reply Subtype Data is to be decrypted to
obtain the MN-HA key
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN-HA Key Reply Subtype Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The Generalized Mobile IP MN-HA Key Reply Extension
Length The 16-bit Length field indicates the length of the
extension. It is equal to the number of bytes in the
MN-HA Key Reply Subtype Data plus 4 (for the Lifetime
field).
Lifetime This field indicates the duration of time (in seconds)
for which the MN-HA key is valid.
MN-HA Key Reply Subtype Data
An encrypted copy of the MN-HA key, along with any
other information needed by the mobile node to create
the designated Mobility Security Association with the
home agent.
For each subtype, the format of the MN-HA Key Reply Subtype Data has
to be separately defined according to the particular method required
to set up the Mobility Security Association.
7. Key Request/Reply Subtypes
The extension subtypes in this section are subtypes of the
Generalized Extensions specified in section 6.
7.1. MN-FA Key Request From AAA Subtype
The MN-FA Key Request From AAA subtype data uses subtype 7 of the
Generalized MN-FA Key Request Extension (see section 6.1). The
MN-FA Key Request From AAA extension MUST appear in the Registration
Request before the MN-AAA Authentication extension. The subtype data
field is zero in length.
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7.2. MN-FA Key Material From AAA Subtype
The MN-FA Key Material From AAA extension, shown in figure 5,
uses subtype 1 of the Generalized MN-FA Key Reply Extension (see
section 6.2).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AAA SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FA SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm Identifier | Key Material ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The MN-FA Key Material From AAA Subtype-Specific Data
lifetime This field indicates the duration of time (in seconds)
for which the registration key is valid.
AAA SPI A 32-bit opaque value, indicating the SPI that the
mobile node must use to determine the transform to use
for establishing the Mobility Security Association
between the mobile node and its prospective foreign
agent.
FA SPI The SPI for the Mobility Security Association to the FA
that the mobile node creates using the Key Material
Algorithm Identifier
This field indicates the transform to be used (stored
as part of the Mobility Security Association with
the foreign agent, and selected from among the
values in the "Authentication Algorithm" table
cited in section 4), for future computations of the
Mobile-Foreign Authentication Extension.
Key Material
A random [6] value of at least 128 bits.
The MN-FA Key Material From AAA extension MUST appear in the
Registration Reply before the Mobile-Foreign Authentication
extension. The Key Material is provided by the AAA server for use by
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the mobile node in creating the registration key, which is used to
secure future Mobile IP registrations with the same foreign agent.
7.3. MN-HA Key Request From AAA Subtype
The MN-HA Key Request From AAA subtype data uses subtype 7 of the
Generalized MN-HA Key Request Extension (see section 6.3). The
MN-HA Key Request From AAA extension MUST appear in the Registration
Request before the MN-AAA Authentication extension. The subtype data
field is zero in length.
7.4. MN-HA Key Material From AAA Subtype
The MN-HA Key Material From AAA is subtype 7 of the Generalized MN-HA
Key Reply Extension (see section 6.4).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AAA SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HA SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm Identifier | Replay Method |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Material ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The MN-HA Key Material From AAA Subtype-Specific Data
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AAA SPI A 32-bit opaque value, indicating the SPI that the
mobile node must use to determine the transform to use
for establishing the Mobility Security Association
between the mobile node and its home agent.
HA SPI The SPI for the Mobility Security Association to the HA
that the mobile node creates using the Key Material
Algorithm Identifier
This field indicates the transform to be used for
future computations of the Mobile-Home Authentication
Extension (see section 4)
Replay Method
This field contains the replay method to be used for
future Registration messages (see section 4).
Key Material
A random [6] value of at least 128 bits.
The MN-HA Material Key From AAA subtype-specific data is shown in
figure 6. The Mobile Node calculates the MN-HA key using the Key
Material provided by the AAA server. The calculation proceeds by
using the key shared between the mobile node and the AAA server that
has previously been configured for securing all such communication
requirements with the AAA server which will be contacted within the
AAA infrastructure (see appendix C). The MN-HA key is intended for
use by the mobile node to secure future Mobile IP registrations with
its home agent. The MN-HA Key Material extension MUST appear in the
Registration Reply before the MN-HA Authentication extension.
Once the mobile node creates the MN-HA Key, by using the transform
specified in the AAA SPI, it stores the HA Security Information
indexed by the HA SPI in its list of Mobile Security Associations.
The mobile node uses the Identification field data from the
Registration Request as its initial synchronization data with the
home agent.
8. Error Values
Each entry in the following table contains the name of the Code [13]
value to be returned in a Registration Reply, the value for that
Code, and the section in which the error is first mentioned in this
specification.
Error Name Value Section
---------------------- ----- ---------
MISSING_MN_FA 107 6.2
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9. IANA Considerations
The numbers for the Generalized Extensions in section 6 are
taken from the numbering space defined for Mobile IP registration
extensions defined in RFC 3344 [13] as extended in RFC 2356 [11].
The numbers suggested in this section are already in use by
implementations which have been tested for interoperability.
The number 7, assigned to the MN-FA Key Request From AAA Subtype
extension (see section 7.1), is taken from the numbering space
defined for the Generalized MN-FA Key Request Extension (see
section 6.1).
The number 1, assigned to the MN-FA Key Material From AAA Subtype
extension (see section 7.2), is taken from the numbering space
defined for the Generalized MN-FA Key Reply Extension (see
section 6.2).
The number 7, assigned to the MN-HA Key Request From AAA Subtype
extension (see section 7.4), is taken from the numbering space
defined for the Generalized MN-HA Key Request Extension (see
section 6.3).
The number 7, assigned to the MN-HA Key Material From AAA Subtype
extension (see section 7.4), is taken from the numbering space
defined for the Generalized MN-HA Key Reply Extension (see
section 6.4).
The Code value specified for error MISSING_MN_FA, listed in
section 8, MUST NOT conflict with any other code values listed in
RFC 3344, RFC 3024 [10], or RFC 2356 [11]. This value is to be
taken from the space of error values conventionally associated with
rejection by the foreign agent (i.e., 64-127).
Section 4 introduces the Replay Method Identifier namespace that
requires IANA management. This specification makes use of 1-3;
all other values other than zero (0) or one (1) are available for
assignment, pending review and approval by a Designated Expert [12].
10. Security Considerations
The extensions in this document are intended to provide the
appropriate level of security for Mobile IP entities (mobile node,
foreign agent, and home agent) to calculate the Authentication Data
needed by authentication extensions used with Mobile IP registration
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messages. The Mobility Security Associations resulting from use of
these extensions do not offer any higher level of security than what
is already implicit in use of the AAA Security Association between
the mobile node and the AAAH. In order to deny any adversary the
luxury of unbounded time to analyze and break the secrecy of the AAA
Security Association between the mobile node and the AAA server, that
AAA Security Association MUST be refreshed periodically.
The provisioning and refreshing of the AAA key in the MN and AAA
server is outside the scope of this document. Typical methods for
provisioning and refresh at the MN include the use of https between
the MN and a trusted provisioning server (e.g., over a wireless link
layer). Wireless standards organizations specify the details of the
wireless link operation, including authentication of the MN at the
link layer.
Since the extensions defined in this specification only carries Key
Material, which is used to derive keys, it does not expose any data
that could be used in an attack aimed at recovering the key shared
between the mobile node and the AAA. The authors do not believe this
specification introduces any new security vulnerability.
11. Acknowledgements
Thanks to Fredrik Johansson, Tom Hiller, and the members of the IESG
for their useful comments on this document.
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References
[1] B. Aboba and M. Beadles. The Network Access Identifier.
Request for Comments (Proposed Standard) 2486, Internet
Engineering Task Force, January 1999.
[2] 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.
[3] 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.
[4] 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.
[5] Pat R. Calhoun, John Loughney, E. Guttman, Glen Zorn,
and Jari Arkko. DIAMETER Base Protocol (work in
progress). Internet Draft, Internet Engineering Task
Force. draft-ietf-aaa-diameter-15.txt, October 2002.
[6] D. Eastlake, 3rd, S. Crocker, and J. Schiller. Randomness
Recommendations for Security. Request for Comments
(Informational) 1750, Internet Engineering Task Force, December
1994.
[7] S. Glass, T. Hiller, S. Jacobs, and C. Perkins. Mobile IP
Authentication, Authorization, and Accounting Requirements.
Request for Comments (Proposed Standard) 2977, Internet
Engineering Task Force, October 2000.
[8] H. Krawczyk, M. Bellare, and R. Canetti. HMAC: Keyed-Hashing
for Message Authentication. Request for Comments
(Informational) 2104, Internet Engineering Task Force,
February 1997.
[9] D. Mitton, M. St.Johns, S. Barkley, D. Nelson, B. Patil,
M. Stevens, and B. Wolff. Authentication, Authorization,
and Accounting: Protocol Evaluation. Request for Comments
(Informational) 3127, Internet Engineering Task Force, June
2001.
[10] Editor G. Montenegro. Reverse Tunneling for Mobile IP, revised.
Request for Comments (Proposed Standard) 3024, Internet
Engineering Task Force, January 2001.
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Internet Draft AAA Keys for Mobile IP 22 June 2003
[11] G. Montenegro and V. Gupta. Sun's SKIP Firewall Traversal for
Mobile IP. Request for Comments (Informational) 2356, Internet
Engineering Task Force, June 1998.
[12] T. Narten and H. Alvestrand. Guidelines for Writing an IANA
Considerations Section in RFCs. Request for Comments (Best
Current Practice) 2434, Internet Engineering Task Force, October
1998.
[13] C. Perkins. IP Mobility Support. Request for Comments
(Proposed Standard) 3344, Internet Engineering Task Force,
August 2002.
[14] C. Rigney, A. Rubens, W. Simpson, and S. Willens. Remote
Authentication Dial In User Service (RADIUS). Request for
Comments (Proposed Standard) 2865, Internet Engineering Task
Force, June 2000.
A. Changes Since Previous Revision
- Cleaned up terminology:
* Clarified the use of "security association" throughout the
document as either "IPSec" or "Mobility" or "AAA".
B. Older Changes
In this revision of the document, there have been several major
changes as a result of suggestions received during Last Call.
- Generalized Key Extensions previously specified in another
document have been instead specified in this document in order
that this document can be self-contained and not dependent on the
standardization status of the other document.
- Additional explanation has been included for the purposes of
clarifying the problem space and solution approach.
- An appendix has been added to describe the expected AAA
infrastructure that will produce the keys that are to be
distributed within the extensions specified in this document.
- Ladder diagrams have been included to illustrate the expected
message flows containing the extensions defined in this document.
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- HMAC-MD5 has been mandated for implementation by the mobile node,
for compatibility with RFC 3344 [13]. The example text has been
modified accordingly (see section 5).
- A table of Algorithm Identifiers has been identified as the
numbering space for transform selection when establishing the
Mobility Security Association using the keys distributed with the
extensions in this document. See section 4.
- A terminology section has been added.
- This appendix has been added.
* New terminology entries for "Registration Key", "AAA", "AAA
entity", "Mobility Security Association", "AAA Security
Association",
* All instances of MN-FA key are now called "registration key"
* All instances of the key between mobile node and home agent
are called "MN-HA" key.
- Removed extraneous IANA considerations paragraph for HMAC_MD5
- Removed "Unsolicited" from subtype names
- Changed minimum Key Material length from 64 bits to 128 bits
C. AAA Infrastructure
In this appendix, we attempt to capture the main features of a basic
model for operation of AAA servers that is assumed for understanding
of the use of the Mobile IP registration extensions described in this
document. This information has been adapted from the discussion in
RFC 2977 [7].
Within the Internet, a mobile node belonging to one administrative
domain (called the home domain) often needs to use resources
provided by another administrative domain (called the foreign
domain). A foreign agent that handles the mobile node's Registration
Request is likely to require that the mobile node provide some
credentials that can be authenticated before access to the resources
is permitted. These credentials may be provided as part of the
Mobile-AAA Authentication extension [4], relying on the existence
of an AAA infrastructure such as is described in this section, and
also described in RFC 2977 and RFC 3012 [4]. Such credentials are
typically managed by entities within the mobile node's home domain.
They may be also used for setting up secure communications with the
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mobile node and the foreign agent, or between the mobile node and its
home agent if necessary.
Local Domain Home Domain
+--------------+ +----------------------+
| +------+ | | +------+ |
| | | | | | | |
| | AAAL | | | | AAAH | |
| | +-------------------+ | |
| +---+--+ | | +------+ |
| | | | |
| | | +----------------------+
+------+ | +---+--+ |
| | | | | | MN = mobile node
| MN |- -|- -| FA | | FA = foreign agent
| | | | | | AAAL = local authority
+------+ | +------+ | AAAH = home authority
| |
+--------------+
Figure 7: AAA Servers in Home and Local Domains
The foreign agent often does not have direct access to the data
needed to verify the credentials. Instead, the foreign agent is
expected to consult an authority (typically in the same foreign
domain) in order to request proof that the mobile node has acceptable
credentials. Since the foreign agent and the local authority (AAAL)
are part of the same administrative domain, they are expected to have
established, or be able to establish for the necessary lifetime, a
secure channel for the purposes of exchanging sensitive (access)
information, and keeping it private from (at least) the visiting
mobile node.
The local authority (AAAL) itself may not have enough information
stored locally to carry out the verification for the credentials of
the mobile node. In contrast to the foreign agent, however, the AAAL
is expected to be configured with enough information to negotiate
the verification of mobile node credentials with its home domain.
The home and foreign domains should be configured with sufficient IP
Security Associations and access controls so that they can negotiate
the authorization, and also enable the mobile node to acquire
Mobility Security Associations with the mobility agents within the
foreign domain. For the purposes of the key exchanges specified
within this document, the authorization is expected to depend only
upon secure authentication of the mobile node's credentials.
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Once the authorization has been obtained by the local authority, and
the authority has notified the foreign agent about the successful
negotiation, the foreign agent can deliver the Registration Reply to
the mobile node along with the key material.
In figure 7, there might be many mobile nodes from many different
Home Domains. Each Home Domain provides a AAAH that can check
credentials originating from mobile nodes administered by that Home
Domain. There is a security model implicit in figure 7, and it is
crucial to identify the specific security associations assumed in
the security model. These IP Security Associations are illustrated
in figure 8, and are considered to be relatively long-lived security
associations.
First, it is natural to assume that the mobile node has an AAA
Security Association with the AAAH, since that is roughly what it
means for the mobile node to belong to the home domain.
Second, from the model illustrated in figure 7 it is clear that AAAL
and AAAH have to share an IP Security Association, because otherwise
they could not rely on the authentication results, authorizations,
nor even the accounting data which might be transacted between them.
Requiring such bilateral IP Security Associations is, however, in the
end not scalable; the AAA framework must provide for more scalable
mechanisms, but the methods by which such a broker model is to be
created are out of scope for this document. See RFC 2977 for more
details.
Finally, from figure 7, it is clear that the foreign agent can
naturally share an IP Security Association with the AAAL. This is
necessary in order for the model to work because the foreign agent
has to have a way to find out that it is permissible to allocate
the local resources to the mobile node, and further to transmit any
successful Registration Reply to the mobile node.
Figure 8 illustrates the IP Security Associations we understand from
our proposed model. Note that there may be, by mutual agreement
between AAAL and AAAH, a third party inserted between AAAL and
AAAH to help them arbitrate secure transactions in a more scalable
fashion. The broker model which has been designed to enable such
third-party processing should not have any effect on the Mobile IP
extensions specified in this document, and so no description is
provided here; see RFC 2977 [7] for more details.
Nodes in two separate administrative domains (for instance, AAAH
and AAAL) often must take additional steps to verify the identity
of their communication partners, or alternatively to guarantee
the privacy of the data making up the communication. While these
considerations lead to important security requirements, as mentioned
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+------+ +------+
| | | |
| AAAL +--------------+ AAAH |
| | | |
+---+--+ +--+---+
| |
| |
+---+--+ +--+---+
MN = mobile node | | | |
FA = foreign agent | FA | | MN |
AAAL = local authority | | | |
AAAH = home authority +------+ +------+
Figure 8: IP Security Associations
above in the context of security between servers, we consider the
exact choice of IP Security Associations between the AAA servers to
be beyond the scope of this document. The choices are unlikely to
depend upon Mobile IP, or any specific features of the general model
illustrated in figure 7. On the other hand, the Mobility Security
Associations needed between Mobile IP entities are of central
importance in the design of the key derivation extensions in this
document.
One further detail deserves mention. The Mobility Security
Association to be established between the mobile node and the foreign
agent have to be communicated to the foreign agent as well as to the
mobile node. The way that the key is distributed to the foreign
agent is not relevant to any material in this document, and is
expected to be handled as part of the AAA protocol processing between
the AAAH and AAAL, and the further AAA protocol processing between
the AAAL and the foreign agent. Any method by which the key can be
securely transmitted to the AAAL and then relayed (possibly with
re-encryption) to the foreign agent, is outside the jurisdiction
of any Mobile IP specification, and thus compatible (by reason of
non-interference) with the protocol extensions specified in this
document.
D. Message Flow for Requesting and Receiving Registration Keys
In this section, we show message flows for requesting and receiving
a registration key from the AAA infrastructure, described in
section C. Challenge values, as specified in [4], might be added to
the Advertisement and Registration messages for additional replay
protection, but are not illustrated here.
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Diagram 9 illustrates the message flow for the case when the mobile
node explicitly requests a registration key.
MN FA AAA Infrastructure
| | |
|<--- Advertisement-----| |
| (if needed) | |
| | |
|-- RReq+AAA Key Req.-->| |
| |--- RReq + AAA Key Req.--->|
| | |
| |<--- RRep + AAA Key Rep.---|
|<-- RRep+AAA Key Rep.--| |
| | |
Figure 9: Message Flows for Requesting and
Receiving Registration Keys
In diagram 9, the following message flow is illustrated:
1. The foreign agent disseminates an Agent Advertisement. 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
MN-HA Key Request and/or MN-FA Key Request, as needed, along with
an authorization-enabling authentication extension as required by
Mobile IP [13].
3. The foreign agent relays the Registration Request and/or Key
Request(s) to its locally configured AAA Infrastructure (see
appendix C), according to local policy.
4. The foreign agent receives a AAA Response with the appropriate
indications for authorizing connectivity for the mobile node.
Along with this AAA Response, the foreign agent may also receive
key material by some secure method appropriate for communications
between it and its local AAA infrastructure. At this point if
the foreign agent has not relayed the Registration Request,
it forwards it directly to the Home Agent and waits for a
Registration Reply (not shown in the figure).
5. The foreign agent relays the Registration Reply to the mobile
node, along with the new Key Material extensions to be used by
the mobile node to establish Mobility Security Associations with
the relevant mobility agents (foreign agent and/or home agent).
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Diagram 10 illustrates the message flow for the case when the
mobile node receives an unsolicited registration key from the AAA
Infrastructure.
MN FA AAA Infrastructure
| | |
|<--- Advertisement-----| |
| (if needed) | |
| | |
| ------ RReq --------->| |
| |------- RReq ------------->|
| | |
| |<--- RRep + AAA Key Rep.---|
|<-- RRep+AAA Key Rep.--| |
| | |
Figure 10: Message Flow for Receiving
Unsolicited Registration Keys
In diagram 10, the following message flow is illustrated:
1. The foreign agent disseminates an Agent Advertisement. 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 an
authorization-enabling authentication extension as required by
Mobile IP [13].
3. The foreign agent sends a AAA Request (possibly containing
the Registration Request) to its locally configured AAA
Infrastructure (see appendix C), according to local policy.
4. The foreign agent receives a AAA Response with the appropriate
indications for authorizing connectivity for the mobile node.
Along with this AAA Response, the foreign agent may also receive
key material by some secure method appropriate for communications
between it and its local AAA infrastructure. At this point,
if the foreign agent has not relayed the Registration Request,
it forwards it directly to the Home Agent and waits for a
Registration Reply (not shown in the figure).
5. The foreign agent relays the Registration Reply to the mobile
node, along with the new Key Material extensions to be used by
the mobile node to establish Mobility Security Associations with
the relevant mobility agents (foreign agent and/or home agent).
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Addresses
Questions about this memo can also be directed to the authors:
Charles E. Perkins Pat R. Calhoun
Communications Systems Lab
Nokia Research Center Airespace Networks
313 Fairchild Drive 110 Nortech Parkway
Mountain View, California 94043 San Jose, CA 95134
USA USA
Phone: +1-650 625-2986 Phone: +1 408 635 2000
EMail: charliep@iprg.nokia.com Email: pcalhoun@bstormnetworks.com
Fax: +1 650 625-2502 Fax: +1 408 635 2020
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