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Versions: 00 01 03 04 RFC 2977

Mobile IP Working Group                                         S. Glass
INTERNET DRAFT                                          Sun Microsystems
5 October 1999                                                 S. Jacobs
                                                        GTE Laboratories
                                                              C. Perkins
                                                   Nokia Research Center

  Mobile IP Authentication, Authorization, and Accounting Requirements
                  draft-ietf-mobileip-aaa-reqs-00.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@STANDARDS.NORTELNETWORKS.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
   any time.  It is inappropriate to use Internet-Drafts as reference
   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

   The Mobile IP and AAA working groups are currently looking at
   defining the requirements for Authentication, Authorization, and
   Accounting.  This document contains the requirements which would
   have to be supported by a AAA service to aid in providing Mobile IP
   services.

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1. Introduction

   Clients obtain Internet services by negotiating a point of attachment
   to a "home domain", generally from an ISP, or other organization from
   which service requests are made, and fulfilled.  With the increasing
   popularity of mobile devices, a need has been generated to allow
   users to attach to any domain convenient to their current location.
   In this way, a client needs access to resources being provided by
   an administrative domain different than their home domain (called
   a "foreign domain").  The need for service from a foreign domain
   requires, in many models, Authorization, which leads directly to
   Authentication, and of course Accounting (whence, "AAA").  There
   is some argument which of these leads to, or is derived from the
   others, but there is common agreement that the three AAA functions
   are closely interdependent.

   An agent in a foreign domain, being called on to provide access to a
   resource by a mobile user, is likely to request or require the client
   to provide credentials which can be authenticated before access to
   resources are permitted.  The resource may be as simple as a conduit
   to the Internet, or may be as complex as access to specific private
   resources within the foreign domain.  Credentials can be exchanged
   in many different ways, all of which are beyond the scope of this
   document.  Once authenticated, the mobile user may be authorized to
   access services within the foreign domain.  An accounting of the
   actual resources may then be assembled.

   Mobile IP is a technology that allows a network node ("mobile
   node") to migrate from its "home" network to other networks, either
   within the same administrative domain, or to other administrative
   domains.  The possibility of movement between domains which require
   AAA services has created an immediate demand to design and specify
   AAA protocols.  Once available, the AAA protocols and infrastructure
   will provide the economic incentive for a wide-ranging deployment of
   Mobile IP. This document will identify, describe, and discuss the
   functional and performance requirements that Mobile IP places on AAA
   protocols.

   The formal description of Mobile IP can be found in [13, 11, 12, 15].

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   In this document, we have attempted to exhibit requirements in a
   progressive fashion.  After showing the basic AAA model for Mobile
   IP, we derive requirements as follows:

    -  requirements based on the general model
    -  requirements based on providing IP service for mobile nodes
    -  requirements derived from specific Mobile IP protocol needs

   Then, we exhibit some related AAA models and describe requirements
   derived from the related models.

2. Terminology

   This document frequently uses the following terms in addition to
   those defined in RFC 2002 [13]:

      Accounting   The act of collecting information on resource usage
                   for the purpose of trend analysis, auditing, billing,
                   or cost allocation.

      Administrative Domain
                   An intranet, or a collection of networks,
                   computers, and databases under a common
                   administration.  Computer entities operating in
                   a common administration may be assumed to share
                   administratively created security associations.

      Attendant    A node designed to provide the service interface
                   between a client and the local domain.

      Authentication
                   The act of verifying a claimed identity, in the
                   form of a pre-existing label from a mutually known
                   name space, as the originator of a message (message
                   authentication) or as the end-point of a channel
                   (entity authentication).

      Authorization
                   The act of determining if a particular right, such
                   as access to some resource, can be granted to the
                   presenter of a particular credential.

      Billing      The act of preparing an invoice.

      Broker       An intermediary agent, trusted by two other AAA
                   servers, able to obtain and provide security services
                   from those AAA servers.  For instance, a broker may

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                   obtain and provide authorizations, or assurances that
                   credentials are valid.

      Client       A node wishing to obtain service from an attendant
                   within an administrative domain.

      Foreign Domain
                   An administrative domain, visited by a Mobile IP
                   client, and containing the AAA infrastructure needed
                   to carry out the necessary operations enabling
                   Mobile IP registrations.  From the point of view of
                   the foreign agent, the foreign domain is the local
                   domain.

      Inter-domain Accounting
                   Inter-domain accounting is the collection of
                   information on resource usage of an entity with
                   an administrative domain, for use within another
                   administrative domain.  In inter-domain accounting,
                   accounting packets and session records will typically
                   cross administrative boundaries.

      Intra-domain Accounting
                   Intra-domain accounting is the collection of
                   information on resource within an administrative
                   domain, for use within that domain.  In intra-domain
                   accounting, accounting packets and session records
                   typically do not cross administrative boundaries.

      Local Domain
                   An administrative domain containing the AAA
                   infrastructure of immediate interest to a Mobile IP
                   client when it is away from home.

      Real-time Accounting
                   Real-time accounting involves the processing of
                   information on resource usage within a defined time
                   window.  Time constraints are typically imposed in
                   order to limit financial risk.

      Session record
                   A session record represents a summary of the resource
                   consumption of a user over the entire session.
                   Accounting gateways creating the session record may
                   do so by processing interim accounting events.

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [4].

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3. Basic Model

   In this section, we attempt to capture the main features of a basic
   model for operation of AAA servers that seems to have good support
   within the Mobile IP working group.  Within the Internet, a client
   belonging to one administrative domain (called the home domain) often
   needs to use resources provided by another administrative domain
   (called the foreign domain).  An agent in the foreign domain that
   attends to the client's request (call the agent the "attendant") is
   likely to require that the client provide some credentials that can
   be authenticated before access to the resources is permitted.

                  Local Domain                  Home Domain
                +--------------+           +----------------------+
                |   +------+   |           |   +------+           |
                |   |      |   |           |   |      |           |
                |   | AAAL |   |           |   | AAAH |           |
                |   |      +-------------------+      |           |
                |   +---+--+   |           |   +------+           |
                |       |      |           |                      |
                |       |      |           +----------------------+
     +------+   |   +---+--+   |
     |      |   |   |      |   |       C    =  client
     |   C  |- -|- -|   A  |   |       A    =  attendant
     |      |   |   |      |   |       AAAL =  local authority
     +------+   |   +------+   |       AAAH =  home authority
                |              |
                +--------------+

            Figure 1: AAA Servers in Home and Local Domains

   The attendant often does not have direct access to the data needed
   to complete the transaction.  Instead, the attendant is expected
   to consult an authority (typically in the same foreign domain) in
   order to obtain proof that the client has acceptable credentials.
   Since the attendant and the local authority are part of the
   same administrative domain, they are expected to have security
   relationships that enable them to securely transact information
   locally.

   The local authority (AAAL) itself may not have enough information
   stored locally to carry out the verification for the credentials
   of the client.  In contrast to the attendant, however, the AAAL
   is expected to be configured with enough information to negotiate
   the verification of client credentials with external authorities.
   The local and the external authorities should be configured with

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   sufficient security relationships and access controls so that they,
   possibly without the need for any other AAA agents, can negotiate
   the authorization that may enable the client to have access to the
   requested resources.  In many typical cases, the authorization
   depends only upon secure authentication of the client's credentials.

   Once the authorization has been obtained by the local authority,
   and the authority has notified the attendant about the successful
   negotiation, the attendant can provide the requested resources to the
   client.

   As an example in today's Internet, we can cite the deployment of
   RADIUS [14] to allow mobile computer clients to have access to the
   Internet by way of a local ISP. The ISP wants to make sure that
   the mobile client can pay for the connection.  Once the client
   has provided credentials (e.g., identification, unique data, and
   an unforgeable signature), the ISP checks with the client's home
   authority to verify the signature, and to obtain assurance that the
   client will pay for the connection.  Here, the attendant function can
   be carried out by the NAS, and the local and home authorities can use
   RADIUS servers.  Credentials allowing authorization at one attendant
   SHOULD be unusable in any future negotiations at the same or any
   other attendant.

   In the picture, there might be many attendants for each AAAL, and
   there might be many clients from many different Home Domains.  Each
   Home Domain provides a AAAH that can check credentials originating
   from clients administered by that Home Domain.

   There is a security model implicit in the above figure, and it is
   crucial to identify the specific security associations assumed in the
   security model.

   First, it is natural to assume that the client has a security
   association with the AAAH, since that is roughly what it means for
   the client to belong to the home domain.

   Second, from the model illustrated in figure 1 it is clear that AAAL
   and AAAH have to share a 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 security relationships is, however, in the
   end not scalable; the AAA framework MUST provide for more scalable
   mechanisms, as suggested below in section 6.

   Finally, in the figure, it is clear that the attendant can naturally
   share a security association with the AAAL. This is necessary in
   order for the model to work because the attendant has to be know that
   it is permissible to allocate the local resources to the client.

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   From the description and example above, we can identify several
   requirements.

    -  Each local attendant has to have a security relationship with the
       local AAA server (AAAL)
    -  The local authority has to share, or dynamically establish,
       security relationships with external authorities that are able to
       check client credentials
    -  The attendant has to keep state for pending client requests while
       the local authority contacts the appropriate external authority
    -  Since the mobile node may not necessarily initiate network
       connectivity from within its home domain, it MUST be able to
       provide complete, yet unforgeable credentials without ever having
       been in touch with its home domain.
    -  Since the mobile node's credentials have to remain unforgeable,
       intervening nodes (e.g., neither the attendant or the local
       authority (AAAL) or any other intermediate nodes) MUST NOT be
       able to learn any (secret) information which may enable them to
       reconstruct and reuse the credentials.

   From this last requirement, we can see the reasons for the natural
   requirement that the client has to share, or dynamically establish,
   a security relationship with the external authority in the Home
   Domain.  Otherwise, it is technically infeasible (given the implied
   network topology) for the client to produce unforgeable signatures
   that can be checked by the AAAH. Figure 2 illustrates the natural
   security associations we understand from our proposed model.  Note
   that, according to the discussion in section 6, there may, by mutual
   agreement between AAAL and AAAH, be a third party inserted between
   AAAL and AAAH to help them arbitrate secure transactions in a more
   scalable fashion.

                               +------+              +------+
                               |      |              |      |
                               | AAAL +--------------+ AAAH |
                               |      |              |      |
                               +---+--+              +--+---+
                                   |                    |
                                   |                    |
                               +---+--+              +--+---+
   C    =  client              |      |              |      |
   A    =  attendant           |   A  |              |  C   |
   AAAL =  local authority     |      |              |      |
   AAAH =  home authority      +------+              +------+

                    Figure 2: Security Associations

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   In addition to the requirements listed above, we specify the
   following requirements which derive from operational experience with
   today's roaming protocols.

    -  There are scenarios in which an attendant will have to manage
       requests for many clients at the same time.
    -  The attendant equipment should be as inexpensive as possible,
       since it will be replicated as many times as possible to handle
       as many clients as possible in the foreign domain.
    -  Attendants SHOULD be configured to obtain authorization,
       from a trusted local AAA server (AAAL) for Quality of Service
       requirements placed by the client.
    -  The attendant MUST protect against replay attacks.

   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
   above in the context of security between servers, we consider the
   exact choice of security associations between the AAA servers to be
   beyond the scope of this document.  The choices are unlikely even to
   depend upon any specific features of the general model illustrated
   in figure 1.  On the other hand, the security associations needed
   between Mobile IP entities will be of central importance in the
   design of a suitable AAA infrastructure for Mobile IP. The general
   model shown above is generally compatible with the needs of Mobile
   IP. However, some basic changes are needed in the security model of
   Mobile IP, as detailed in section 5.

   Lastly, recent discussion in the mobile-ip working group has
   identified the following additional requirements:

    -  The local AAA server MUST support anonymous access.
    -  The attendant MUST be able to terminate service to the client
       based on policy determination by the AAA server.

3.1. AAA Protocol Roaming Requirements

   In this section we will detail additional requirements based on
   issues discovered through operational experience of existing roaming
   RADIUS networks.  The AAA protocol MUST satisfy these requirements in
   order for providers to offer a robust service.  These requirements

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   have been identified by TR45.6 as part of their involvement with the
   Mobile IP working group.

    -  Support a reliable AAA transport mechanism.
        *  This transport mechanism will be able indicate to an AAA
           application that a message was delivered to the next peer AAA
           application or that a time out occurred.
        *  Retransmission is controlled by the reliable AAA transport
           mechanism, and not by lower layer protocols such as TCP.
        *  Even if the AAA message is to be forwarded, or the message's
           options or semantics do not conform with the AAA protocol,
           the transport mechanism will acknowledge that the peer
           received the AAA message.
        *  Acknowledgements SHOULD be allowed to be piggybacked in AAA
           messages
    -  Transport a digital certificate in an AAA message, in order
       to minimize the number of round trips associated with AAA
       transactions.  Note:  This requirement applies to AAA
       applications and not mobile stations.  The certificates could
       be used by the foreign and home agents to establish an IPSec
       security association to secure the mobile node's tunneled data.
    -  Provide message integrity and identity authentication on a per
       hop (AAA node) basis.
    -  Support replay protection and optional non-repudiation
       capabilities for all authorization and accounting messages.  The
       AAA protocol must provide the capability for accounting messages
       to be matched with prior authorization messages.
    -  Support accounting via both bilateral arrangements and via broker
       AAA servers providing accounting clearinghouse and reconciliation
       between serving and home networks.  There is an explicit
       agreement that if the private network or home ISP authenticates
       the mobile station requesting service, then the private network
       or home ISP network also agrees to reconcile charges with the
       home service provider or broker.  Real time accounting must be
       supported.

4. Requirements related to basic IP connectivity

   The requirements listed in the previous section pertain to the
   relationships between the functional units, and don't depend on the
   underlying network addressing.  On the other hand, many nodes (mobile
   or merely portable) are programmed to receive some IP-specific
   resources during the initialization phase of their attempt to connect
   to the Internet.

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   We place the following additional requirements on the AAA services in
   order to satisfy such clients.

    -  Either AAA server MUST be able obtain, or to coordinate the
       allocation of, a suitable IP address for the customer.
    -  AAA servers MUST be able to identify the client by some means
       other than its IP address.

   Policy in the home domain may dictate that the home agent instead
   of the AAAH manages the allocation of an IP address for the mobile
   node.  AAA servers MUST be able to coordinate the allocation of an IP
   address for the mobile node at least in this way.

   We propose that an unambiguous identifier, such as the Network Access
   Identifier (NAI) [1, 6], be used for clients.  The NAI could be used
   by any client, whether or not it has an IP address, to identify
   itself for the purposes of obtaining authorizations.  The form of
   the NAI ("user@realm") allows AAAL to easily determine the home
   domain ("realm") for the client.  Both the AAAL and the AAAH can use
   the NAI to keep records indexed by the client's specific identity.
   Therefore, the NAI is well suited for use in the general AAA model
   illustrated in figure 1.

5. AAA for Mobile IP

   Clients using Mobile IP require specific features from the AAA
   services, in addition to the requirements already mentioned in
   connection with the basic AAA functionality and what is needed for
   IP connectivity.  To understand the application of the general model
   for Mobile IP, we consider the mobile node (MN) to be the client in
   figure 1, and the attendant to be the foreign agent (FA). The home
   agent, while important to Mobile IP, is allowed to play a role during
   the initial registration that is subordinate to the role played
   by the AAAH. For application to Mobile IP, we modify the general
   model (as illustrated in figure 3).  After the initial registration,
   the mobile node is authorized to continue using Mobile IP at the
   foreign domain without requiring further involvement by the AAA
   servers.  Thus, the initial registration will probably take longer
   than subsequent Mobile IP registrations.

   In order to reduce this extra time overhead as much as possible, it
   is important to reduce the time taken for communications between
   the AAA servers.  A major component of this communications latency
   is the time taken to traverse the wide-area Internet that is likely
   to separate the AAAL and the AAAH. This leads to a further strong
   motivation for integration of the AAA functions themselves, as
   well as integration of AAA functions with the initial Mobile IP
   registration.  In order to reduce the number of messages that

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   traverse the network for initial registration of a Mobile Node, the
   AAA functions in the visited network (AAAL) and the home network
   (AAAH) need to interface with the foreign agent and the home agent
   to handle the registration message.  Latency would be reduced as a
   result of initial registration being handled in conjunction with
   AAA and the mobile IP mobility agents.  Subsequent registrations,
   however, would be handled according to RFC2002 [13].

   All needed AAA and Mobile IP functions SHOULD be processed during
   a single Internet traversal.  This MUST be done without requiring
   AAA servers to process protocol messages sent to Mobile IP agents.
   The AAA servers MUST identify the Mobile IP agents and security
   associations necessary to process the Mobile IP registration, pass
   the necessary registration data to those Mobile IP agents, and
   remain uninvolved in the routing and authentication processing steps
   particular to Mobile IP registration.

   For Mobile IP, the AAAL and the AAAH servers have the following
   additional general tasks:

    -  enable authentication for Mobile IP registration
    -  authorize the mobile node (once its identity has been
       established) to use at least the set of resources for minimal
       Mobile IP functionality, plus potentially other services
       requested by the mobile node
    -  initiate accounting for service utilization
    -  use AAA protocol extensions specifically for including Mobile
       IP registration messages as part of the initial registration
       sequence to be handled by the AAA servers.

   These tasks, and the resulting more specific tasks to be listed later
   in this section, are beneficially handled and expedited by the AAA
   servers shown in figure 1 because the tasks often happen together,
   and task processing needs access to the same data at the same time.

   In the model in figure 1, the initial AAA transactions are handled
   without needing the home agent, but Mobile IP requires every
   registration to be handled between the home agent (HA) and the
   foreign agent (FA), as shown by the sparse dashed (lower) line in
   figure 3.  This means that during the initial registration, something
   has to happen that enables the home agent and foreign agent to
   perform subsequent Mobile IP registrations.  After the initial
   registration, the AAAH and AAAL in figure 3 would not be needed,
   and subsequent Mobile IP registrations would only follow the lower
   control path between the foreign agent and the home agent.

   Any Mobile IP data that is sent by FA through the AAAL to AAAH MUST
   be considered opaque to the AAA servers.  Authorization data needed
   by the AAA servers then MUST be delivered to them by the foreign

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                  Local Domain                  Home Domain
                +--------------+           +----------------------+
                |   +------+   |           |   +------+           |
                |   |      |   |           |   |      |           |
                |   | AAAL |   |           |   | AAAH |           |
                |   |      +-------------------+      |           |
                |   +---+--+   |           |   +--+---+           |
                |       |      |           |      |               |
                |       |      |           |      |               |
     +------+   |   +---+--+   |           |   +--+---+           |
     |      |   |   |      |   |           |   |      |           |
     |  MN  +- -|- -+  FA  + --  --  --  --  - +  HA  |           |
     |      |   |   |      |   |           |   |      |           |
     +------+   |   +------+   |           |   +------+           |
                |              |           |                      |
                +--------------+           +----------------------+

              Figure 3: AAA Servers with Mobile IP agents

   agent from the data supplied by the mobile node.  The foreign agent
   becomes a translation agent between the Mobile IP registration
   protocol and AAA.

   As mentioned in section 3, nodes in two separate administrative
   domains often must take additional steps to guarantee their security
   and privacy.  In today's Internet, such security measures may be
   provided by using several different algorithms.  Some algorithms rely
   on the existence of a public-key infrastructure [8]; others rely on
   distribution of symmetric keys to the communicating nodes [9].  AAA
   servers SHOULD be able to verify credentials using either style in
   their interactions with Mobile IP entities.

   In order to enable subsequent registrations, the AAA servers MUST
   be able to perform some key distribution during the initial Mobile
   IP registration process from any particular administrative domain.

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   This key distribution MUST be able to provide the following security
   functions:

    -  identify or create a security association between MN and
       home agent (HA); this is required for the MN to produce the
       authentication data for the MN--HA authentication extension,
       which is mandatory on Mobile IP registrations.
    -  identify or create a security association between mobile node and
       foreign agent, for use with subsequent registrations at the same
       foreign agent, so that the foreign agent can continue to obtain
       assurance that the same mobile node has requested the continued
       authorization for Mobile IP services.
    -  identify or create a security association between home agent
       and foreign agent, for use with subsequent registrations at
       the same foreign agent, so that the foreign agent can continue
       to obtain assurance that the same home agent has continued the
       authorization for Mobile IP services for the mobile node.
    -  participate in the distribution of the security association (and
       Security Parameter Index, or SPI) to the Mobile IP entities
    -  The AAA server MUST also be able to validate certificates
       provided by the mobile node and provide reliable indication to
       the foreign agent.
    -  The AAAL SHOULD accept an indication from the foreign agent about
       the acceptable lifetime for its security associations with the
       mobile node and/or the mobile node's home agent.  This lifetime
       for those security associations SHOULD be an integer multiple of
       registration lifetime offered by the foreign agent to the mobile
       node.
    -  The AAA servers SHOULD be able to condition their acceptance of
       a Mobile IP registration authorization depending upon whether
       the registration requires broadcast or multicast service to the
       mobile node tunneled through the foreign agent.

   The lifetime of any security associations distributed by the AAA
   server for use with Mobile IP SHOULD be great enough to avoid
   too-frequent initiation of the AAA key distribution, since each
   invocation of this process is likely to cause lengthy delays between
   registrations [5].  Registration delays in Mobile IP cause dropped
   packets and noticeable disruptions in service.  Note that any key
   distributed by AAAH to the foreign agent and home agent MAY be used
   to initiate Internet Key Exchange (IKE) [7].  Note further that the
   mobile node and home agent may well have a security association
   established that does not depend upon any action by the AAAH.

5.1. Mobile IP with Dynamic IP Addresses

   According to section 4, many people would like their mobile nodes to
   be identified by their NAI, and to obtain a dynamically allocated IP

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   address for use in the foreign domain.  These people may often be
   unconcerned with details about how their computers implement Mobile
   IP, and indeed may not have any knowledge of their home agent or any
   security association except that between themselves and the AAAH (see
   figure 2.

   Mobile IP requires the home address assigned to the mobile node
   belong to the same subnet as the Home Agent providing service to the
   mobile node.  For effective use of IP home addresses, the home AAA
   (AAAH) SHOULD be able to select a home agent for use with the newly
   allocated home address.  In many cases, the mobile node will already
   know the address of its home agent, even if the mobile node does
   not already have an existing home address.  Therefore, the home AAA
   (AAAH) MUST be able to coordinate the allocation of a home address
   with a home agent that might be designated by the mobile node.

   Allocating a home address and a home agent for the mobile would
   provide a further simplification in the configuration needs for the
   client's mobile node.  Currently, in the Proposed Standard Mobile IP
   specification [13] a mobile node has to be configured with a home
   address and the address of a home agent, as well as with a security
   association with that home agent.  In contrast, the proposed AAA
   features would only require the mobile node to be configured with
   its NAI and a secure shared secret for use by the AAAH. The mobile
   node's home address, the address of its home agent, the security
   association between the mobile node and the home agent, and even the
   identity (DNS name or IP address) of the AAAH can all be dynamically
   determined as part of Mobile IP initial registration with the
   mobility agent in the foreign domain (i.e., a foreign agent with AAA
   interface features).

   The reason for all this simplification is that the NAI encodes the
   client's identity as well as the name of the client's home domain;
   this follows existing industry practice for the way NAIs are used
   today (see section 4).  The home domain name is then available for
   use by the local AAA (AAAL) to locate the home AAA serving the
   client's home domain.  In the general model, the AAAL would also have
   to identify the appropriate security association for use with that
   AAAH. Section 6 discusses a way to reduce the number of security
   associations that have to be maintained between pairs of AAA servers
   such as the AAAL and AAAH just described.

5.2. Firewalls and AAA

   Mobile IP has encountered some deployment difficulties related to
   firewall traversal; see for instance [10].  Since the firewall and
   AAA server can be part of the same administrative domain, we propose
   that the AAA server SHOULD be able to issue control messages and keys

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   to the firewall at the boundary of its administrative domain that
   will configure the firewall to be permeable to Mobile IP registration
   and data traffic from the mobile node.

5.3. Mobile IP with Local Home Agents

                +-------------------------+           +--------------+
                |  +------+    +------+   |           |   +------+   |
                |  |      |    |      |   |           |   |      |   |
                |  |  HA  +----+ AAAL |   |           |   | AAAH |   |
                |  |      |    |      +-------------------+      |   |
                |  +-+----+    +---+--+   |           |   +------+   |
                |    |             |      |           |  Home Domain |
                |    |  +- - - - - +      |           +--------------+
     +------+   |  +-+--+-+               |
     |      |   |  |      |               |
     |  MN  +------+  FA  |               |
     |      |   |  |      | Local Domain  |
     +------+   |  +------+               |
                +-------------------------+

                 Figure 4: Home Agent Allocated by AAAL

   In some Mobile IP models, mobile nodes boot on subnets which are
   technically foreign subnets, but the services they need are local,
   and hence communication with the home subnet as if they were residing
   on the home is not necessary.  As long as the mobile node can get an
   address routable from within the current domain (be it publicly, or
   privately addressed) it can use mobile IP to roam around that domain,
   calling the subnet on which it booted its temporary home.  This
   address is likely to be dynamically allocated upon request by the
   mobile node.

   In such situations, when the client is willing to use a dynamically
   allocated IP address and does not have any preference for the
   location of the home network (either geographical or topological),
   the local AAA server (AAAL) may be able to offer this additional
   allocation service to the client.  Then, the home agent will be
   located in the local domain, which is likely to be offer smaller
   delays for new Mobile IP registrations.

   In figure 4, AAAL has received a request from the mobile node to
   allocate a home agent in the local domain.  The new home agent
   receives keys from AAAL to enable future Mobile IP registrations.
   From the picture, it is evident that such a configuration avoids

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   problems with firewall protection at the domain boundaries, such
   as were described briefly in section 5.2.  On the other hand, this
   configuration makes it difficult for the mobile node to receive
   data from any communications partners in the mobile node's home
   administrative domain.  Note that, in this model, the mobile node's
   home address is affiliated with the foreign domain for routing
   purposes, and that any dynamic update to DNS, to associate the mobile
   node's home FQDN with its new IP address, will require insertion of a
   foreign IP address into the home DNS server database.

5.4. Mobile IP with Local Payments

                +-------------------------+
                |  +------+    +------+   |
                |  |      |    |      |   |
                |  |  HA  +----+ AAAL |   |
                |  |      |    |      |   |
                |  +--+---+    +----+-+   |
                |     |             |     |
                |     +- - - - - +  |     |
     +------+   |              +-+--+-+   |
     |      |   |              |      |   |
     |  MN  +- -|- - - - - - - +  FA  |   |
     |      |   | Local Domain |      |   |
     +------+   |              +------+   |
                +-------------------------+

          Figure 5: Local Payment for Local Mobile IP services

   Since the AAAL is expected to be enabled to allocate a local home
   agent upon demand, we can make a further simplification.  In cases
   where the AAAL can manage any necessary authorization function
   locally (e.g., if the client pays with cash or a credit card), then
   there is no need for an AAA protocol or infrastructure to interact
   with the AAAH. The resulting simple configuration is illustrated in
   figure 5.

   In this simplified model, we may consider that the role of the AAAH
   is taken over either by the national government (in the case of a
   cash payment), or by a card authorization service if payment is by
   credit card.  Then, the AAAL expects those external authorities to
   guarantee the value represented by the client's payment credentials
   (cash or credit).  There are likely to be other cases where clients
   are granted access to local resources, or access to the Internet,
   without any charges at all.  Such configurations may be found in

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   airports and other common areas where business clients are likely to
   spend time.  The service provider may find sufficient reward in the
   goodwill of the clients, or from advertisements displayed on Internet
   portals that are to be used by the clients.  In such situations, the
   AAAL SHOULD still allocate a home agent, appropriate keys, and the
   mobile node's home address.

5.5. Fast Handover

   Since the movement from cell to cell may be frequent in Mobile IP
   networks, it is imperative that the latency involved in the handoff
   process be minimized.  When the mobile node enters a new visited
   subnet, it would be desirable for it to provide the previous foreign
   agent's NAI. The new FA can use this information to either contact
   the previous FA to retrieve the KDC session key information, or it
   can attempt to retrieve the keys from the AAAL. If the AAAL cannot
   provide the necessary keying information, the request will have to be
   sent to the mobile node's AAAH to retrieve new keying information.
   After initial authorization, further authorizations SHOULD be done
   locally within the Local Domain.

6. Broker Model

   The picture in Figure 1 shows a configuration in which the local
   and the home authority have to share trust.  This configuration
   causes a quadratic growth in the number of trust relationships as
   the number of AAA authorities (AAAL and AAAH) increases.  This has
   been identified as a problem by the roamops working group [3], and
   any AAA proposal MUST solve this problem.  Using brokers solves
   many of the scalability problems associated with requiring direct
   business/roaming relationships between every two administrative
   domains.  A broker may play the role of a proxy between two
   administrative domains which have security associations with the
   broker, and relay AAA messages back and forth securely.

   Alternatively, a broker may also enable the two domains with which
   it has associations, but the domains themselves do not have a
   direct association, in establishing a security association, thereby
   bypassing the broker for carrying the messages between the domains.
   This may be established by virtue of having the broker relay a
   shared secret key to both the domains that are trying to establish
   secure communication and then have the domains use the keys supplied
   by the broker in setting up a security association.  Since the
   broker is the one who assists the two domains in setting up a secure
   association, the domains can be assured of receiving payments for
   services offered.  This mechanism also reduces latency in the transit

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   of messages between the domains after the broker has completed its
   involvement.

                  Local Domain                        Home Domain
                +--------------+               +----------------------+
                |   +------+   |   +------+    |   +------+           |
                |   |      |   |   |      |    |   |      |           |
                |   | AAAL +-------+ AAAB +--------+ AAAH |           |
                |   |      |   |   |      |    |   |      |           |
                |   +------+   |   +------+    |   +------+           |
                |       |      |               |                      |
                |       |      |               +----------------------+
     +------+   |   +---+--+   |
     |      |   |   |      |   |       C    =  client
     |   C  +- -|- -+   A  |   |       A    =  attendant
     |      |   |   |      |   |       AAAL =  local authority
     +------+   |   +------+   |       AAAH =  home authority
                |              |       AAAB =  broker authority
                +--------------+

                  Figure 6: AAA Servers Using a Broker

   The AAAB in figure 6 is the broker's authority server.  The broker
   acts as a settlement agent, providing security and a central point of
   contact for many service providers and enterprises.

   The AAAB enables the local and home domains to cooperate without
   requiring each of the networks to have a direct business or security
   relationship with all the other networks.  Thus, brokers offer the
   needed scalability for managing trust relationships between otherwise
   independent network domains.  Use of the broker does not preclude
   managing separate trust relationships between domains, but it does
   offer an alternative to doing so.  Just as with the AAAH and AAAL
   (see section 5), data specific to Mobile IP control messages MUST
   NOT be processed by the AAAB. Any credentials or accounting data to
   be processed by the AAAB must be present in AAA message units, not
   extracted from Mobile IP protocol extensions.

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   The following requirements come mostly from [2], which discusses
   use of brokers in the particular case of authorization for roaming
   dial-up users.

    -  allowing management of trust with external domains by way of
       brokered AAA.
    -  accounting reliability.  Accounting data that traverses
       the Internet may suffer substantial packet loss.  Since
       accounting packets may traverse one or more intermediate
       authorization points (e.g., brokers), retransmission is needed
       from intermediate points to avoid long end-to-end delays.
    -  End to End security.  The Local Domain and Home Domain must be
       able to verify signatures within the message, even though the
       message is passed through an intermediate authority server.
    -  Since the AAAH in the home domain MAY be sending sensitive
       information, such as registration keys, the broker MUST be able
       to pass encrypted data between the AAA servers.

   The need for End-to-End security results from the following attacks
   which were identified when brokered operation uses RADIUS [14]
   (see [2] for more information on the individual attacks):

     + Message editing
     + Attribute editing
     + Theft of shared secrets
     + Theft and modification of accounting data
     + Replay attacks
     + Connection hijacking
     + Fraudulent accounting

   These are serious problems which cannot be allowed to persist in any
   acceptable AAA protocol and infrastructure.

7. Security Considerations

   This is a requirements draft for AAA based on Mobile IP. Because AAA
   is security driven, most of this document addresses the security
   considerations AAA MUST make on behalf of Mobile IP. As with any
   security proposal, adding more entities that interact using security
   protocols creates new administrative requirements for maintaining
   the appropriate security associations between the entities.  In the
   case of the AAA services proposed however, these administrative
   requirements are natural, and already well understood in today's
   Internet because of experience with dial up network access.

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8. Acknowledgements

   Thanks to Tom Hiller, Gopal Dommety, and Basavaraj Patil for
   participating in the Mobile IP subcommittee of the aaa-wg which was
   charged with formulating the requirements detailed in this document.
   Thanks to N. Asokan for perceptive comments to the mobile-ip mailing
   list.  Some of the text of this document was taken from a draft
   co-authored by Pat Calhoun.

   The requirements in section 5.5 and section 3.1 were taken from
   a draft submitted by members of the TIA's TR45.6 Working Group.
   We would like to acknowledge the work done by the authors of that
   draft:  Tom Hiller, Pat Walsh, Xing Chen, Mark Munson, Gopal Dommety,
   Sanjeevan Sivalingham, Byng-Keun Lim, Pete McCann, Brent Hirschman,
   Serge Manning, Ray Hsu, Hang Koo, Mark Lipford, Pat Calhoun, Eric
   Jaques, Ed Campbell, and Yingchun Xu.

References

    [1] B. Aboba and M. Beadles.  RFC 2486:  The Network Access
        Identifier, January 1999.  Status:  PROPOSED STANDARD.

    [2] B. Aboba and J. Vollbrecht.  Proxy chaining and policy
        implementation in roaming.  draft-ietf-roamops-auth-10.txt,
        February 1999.  (work in progress).

    [3] B. Aboba and G. Zorn.  RFC 2477:  Criteria for evaluating
        roaming protocols, December 1998.  Status:  INFORMATIONAL.

    [4] S. Bradner.  Key Words for Use in RFCs to Indicate Requirement
        Levels.  RFC 2119, March 1997.

    [5] Ramon Caceres and Liviu Iftode.  Improving the Performance
        of Reliable Transport Protocols in Mobile Computing
        Environments.  IEEE Journal on Selected Areas in Communications,
        13(5):850--857, June 1995.

    [6] Pat R. Calhoun and Charles E. Perkins.  Mobile IP Network
        Address Identifier Extension. draft-ietf-mobileip-mn-nai-04.txt,
        September 1999.  (work in progress).

    [7] D. Harkins and D. Carrel.  RFC 2409:  The Internet Key Exchange
        (IKE), November 1998.  Status:  PROPOSED STANDARD.

    [8] et. al. Housley, R.  Internet Public Key Infrastructure X.509
        Certificate and CRL Profile.  draft-ietf-pkix-ipki-part1-06.txt,
        October 1997.  (work in progress).

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    [9] J. Kohl and C. Newman.  The Kerberos Network Authentication
        Service (V5).  RFC 1510, September 1993.

   [10] G. Montenegro and V. Gupta.  Sun's SKIP Firewall Traversal for
        Mobile IP.  RFC 2356, June 1998.

   [11] Charles Perkins.  IP Encapsulation within IP.  RFC 2003, May
        1996.

   [12] Charles Perkins.  Minimal Encapsulation within IP.  RFC 2004,
        May 1996.

   [13] C. Perkins, Editor.  IP Mobility Support.  RFC 2002, October
        1996.

   [14] C. Rigney, A. Rubens, W. Simpson, and S. Willens.  Remote
        Authentication Dial In User Service (RADIUS).  RFC 2138, April
        1997.

   [15] J. Solomon and S. Glass.  Mobile-IPv4 Configuration Option for
        PPP IPCP.  RFC 2290, February 1998.

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Addresses

   The working group can be contacted via the current chairs:

      Basavaraj Patil                      Phil Roberts
      Nortel Networks Inc.                 Motorola
      2201 Lakeside Blvd.                  1501 West Shure Drive
      Richardson, TX. 75082-4399           Arlington Heights, IL 60004
      USA                                  USA

      Phone:  +1 972-684-1489              Phone:  +1 847-632-3148
      EMail:  bpatil@nortelnetworks.com    EMail:  QA3445@email.mot.com

   Questions about this memo can be directed to:

      Pat R. Calhoun                       Gopal Dommety
      Network and Security Center          IOS Network Protocols
      Sun Microsystems Laboratories        Cisco Systems, Inc.
      15 Network Circle                    170 West Tasman Drive
      Menlo Park, California 94025         San Jose, CA 95134-1706
      USA                                  USA
      Phone:  +1 650-786-7733              Phone:  +1-408-525-1404
      pcalhoun@eng.sun.co                  EMail:  gdommety@cisco.com
      Fax:  +1 650-786-6445                Fax:  +1 408-526-4952

      Steven M. Glass                      Stuart Jacobs
      Sun Microsystems                     Secure Systems Department
      1 Network Drive                      GTE Laboratories
      Burlington, MA                       40 Sylvan Road
      01803                                Waltham, MA 02451-1128
      USA                                  USA
      Phone:  +1-781-442-0504              Phone:  +1 781-466-3076
      EMail:  steven.glass@sun.com         EMail:  sjacobs@gte.com
                                           Fax:  +1 781-466-2838

      Basavaraj Patil                      Charles E. Perkins
      Wireless Technology Labs             Communications Systems Lab
      Nortel Networks                      Nokia Research Center
      2221 Lakeside Blvd.                  313 Fairchild Drive
      Richardson, TX 75082-4399            Mountain View, California
94043
      USA                                  USA
      Phone:  +1 972-684-1489              Phone:  +1-650 625-2986
      EMail:  bpatil@nortelnetworks.com    EMail:
charliep@iprg.nokia.com
      Fax:  +1 972-685-3207                Fax:  +1 650 691-2170

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