





Network Working Group                                          K. Hamzeh
Request for Comments: 2107                         Ascend Communications
Category: Informational                                    February 1997


                Ascend Tunnel Management Protocol - ATMP

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

IESG Note:

   This note documents a private protocol for tunnel management.  This
   protocol is NOT the product of an IETF working group nor is it a
   standards track document. There is ongoing effort in an IETF working
   group which could result in a standards track document which
   specifies a protocol which provides similar functionality.

Abstract

   This document specifies a generic tunnel management protocol that
   allows remote dial-in users to access their home network as if they
   were directly attached to the home network.  The user's client
   software uses an address contained in the home network address space
   for the remote access.  Packets to and from the home network are
   tunneled by the Network Access Server (NAS) to which the user
   connects and a Home Agent (HA) on the user's home network.  This
   allows for the support of access to Virtual Private Networks and also
   allows for the use of protocols other than IP to be carried over the
   tunnel.  An example of how the RADIUS (Remote Authentication Dial In
   User Service) can be used to provide the necessary configuration
   information to support this service is also provided.

1. Introduction

   The Ascend Tunnel Management Protocol (ATMP) is a protocol currently
   being used in Ascend Communication products to allow dial-in client
   software to obtain virtual presence on a user's home network from
   remote locations.  A user calls into a remote NAS but, instead of
   using an address belonging to a network directly supported by the
   NAS, the client software uses an address belonging to the user's
   "Home Network".  This address can be either provided by the client
   software or assigned from a pool of addresses from the Home Network
   address space.  In either case, this address belongs to the Home
   Network and therefore special routing considerations are required in



Hamzeh                       Informational                      [Page 1]

RFC 2107                          ATMP                     February 1997


   order to route packets to and from these clients.  A tunnel between
   the NAS and a special "Home Agent" (HA) located on the Home Network
   is used to carry data to and from the client.

   ATMP currently allows for both IP and IPX protocols to be tunneled
   between the NAS and the HA.  The protocol to be used, the HA to use,
   and other user specific information is provided by some configuration
   mechanism that is beyond the scope of this document.  Appendix A
   illustrates how RADIUS [5] is used to convey this information to the
   NAS.

   The determination of the Home Network address to be used can be
   accomplished in different ways.  It could, for example, be configured
   in the client and negotiated by IPCP (or IPXCP).  Alternatively, it
   could be defined to be an address specific to the given user ID, or
   it could be assigned from a pool of addresses provided by the Home
   Network for the purpose of remote dial-in access.  Again, how this
   address is assigned and how the NAS decides to invoke ATMP for a
   specific call is beyond the scope of this document.

1.1 Protocol Goals and Assumptions

   The ATMP protocol is implemented only by the NAS and HA.  No other
   systems need to be aware of ATMP.  All other systems communicate in
   the normal manner and are unaware that they may be communicating with
   remote clients.  The clients themselves are unaware of ATMP.  It is
   assumed that standard PPP [8] (or SLIP) clients are being used.

   Unlike the mobile-IP protocol [3], ATMP assumes that a single NAS
   will provide the physical connection to a remote client for the
   duration of the session.  The client will not switch between NASes
   expecting to keep the same IP address and all associated sessions
   active during these transitions.  A particular client can be
   registered with a given HA only once at any given time.
   Deregistration with a HA implies loss of all higher layer sessions
   for that client.

   IP multicasting is currently not provided by ATMP.

1.2 Terminology

   The terminology used in this document is similar to that used in
   mobile-IP.  As pointed out in the previous section, however, ATMP
   provides a subset of the functionality provided by mobile-IP and the
   meanings of the various terms used herein have been modified
   accordingly.





Hamzeh                       Informational                      [Page 2]

RFC 2107                          ATMP                     February 1997


      Connection Profile

         A table used to route packets other than by destination
         address.  The Connection Profile is a named entity that
         contains information indicating how packets addressed to it are
         to be routed.  It may be used to route packets to unregistered
         IP addresses and for routing protocols other than IP (e.g.,
         IPX).

      Foreign Agent (FA)

         A routing entity that resides in a NAS on a remote network that
         allows a mobile node to utilize a home network address.  It
         tunnels datagrams to, and detunnels datagrams from, the home
         agent for the given home network.

      Home Address

         An address that is assigned for an extended period of time to a
         mobile node.  It may remain unchanged regardless of where the
         MN is attached to the Internet.  Alternatively, it could be
         assigned from a pool of addresses.  The management of this pool
         is beyond the scope of this document.

      Home Agent (HA)

         A router on a mobile node's home network which tunnels
         datagrams for delivery to, and detunnels datagrams from, a
         mobile node when it is away from home.

      Home Network

         The address space of the network to which a user logically
         belongs.  When a workstation is physically connected to a LAN,
         the LAN address space is the user's home network.  ATMP
         provides for a remote virtual connection to a LAN.

      Mobile Node (MN)

         A host that wishes to use a Home Network address while
         physically connected by a point-to-point link (phone line,
         ISDN, etc.) to a NAS that does not reside on the Home Network.
         Also referred to as the client.

      Mobility Binding

         The association of a Home Address with a Foreign Agent IP
         address and a Tunnel ID.



Hamzeh                       Informational                      [Page 3]

RFC 2107                          ATMP                     February 1997


      Network Access Server (NAS)

         A device providing temporary, on-demand, network access to
         users.  This access is point-to-point using phone or ISDN
         lines.

      Tunnel

         The path followed by a datagram when it is encapsulated.  The
         model is that, while it is encapsulated, a datagram is routed
         to a knowledgeable decapsulation agent, which decapsulates the
         datagram and then correctly delivers it to its ultimate
         destination.  Each mobile node connecting to a home agent does
         so over a unique tunnel, identified by a tunnel identifier
         which is unique to a given FA-HA pair.  A tunnel can carry both
         IP and IPX datagrams simultaneously.

1.3 Protocol Overview

   A mobile node that wishes to use a home address while connected to a
   remote NAS must register with the appropriate home agent.  The
   foreign agent entity of the remote NAS performs this registration on
   behalf of the MN.  Once registered, a tunnel is established between
   the FA and HA to carry datagrams to and from the MN.  While a MN is
   registered with an HA, the HA must intercept any packets destined for
   the MN's home address and forward them via the tunnel to the FA. When
   the FA detects that the MN has disconnected from the NAS, it issues a
   deregister request to the HA.

   Because ATMP allows protocols other than IP to be carried on its
   tunnels and also allows unregistered IP address to be used to provide
   for access to enterprise networks, the HA doesn't necessarily route
   datagrams received from the MN in the conventional manner.  The
   registration request allows for a named "Connection Profile" to be
   specified in the registration request.  This Connection Profile
   contains configuration information that tells the HA where to send
   packets that it receives from the MN.

1.4 Specification Language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

      MUST               This word, or the adjective "required", means
                         that the definition is an absolute requirement
                         of the specification.





Hamzeh                       Informational                      [Page 4]

RFC 2107                          ATMP                     February 1997


      MUST NOT           This phrase means that the definition is an
                         absolute prohibition of the specification.

      SHOULD             This word, or the adjective "recommended",
                         means that, in some circumstances, valid
                         reasons may exist to ignore this item, but
                         the full implications must be understood and
                         carefully weighed before choosing a different
                         course.  Unexpected results may result
                         otherwise.

      MAY                This word, or the adjective "optional", means
                         that this item is one of an allowed set of
                         alternatives.  An implementation which does
                         not include this option MUST be prepared to
                         interoperate with another implementation which
                         does include the option.

      silently discard   The implementation discards the datagram
                         without further processing, and without
                         indicating an error to the sender.  The
                         implementation SHOULD provide the capability of
                         logging the error, including the contents of
                         the discarded datagram, and SHOULD record the
                         event in a statistics counter.

2.0 Protocol Specification

   ATMP defines a set of request and reply messages sent with UDP [4].
   The HA listens on UDP port 5150 [6]) for requests from FA's.  The UDP
   checksum field MUST be computed and verified.  There are 7 different
   ATMP message types represented by the following Type values:

      Message Type            Type code


      Registration Request          1

      Challenge Request             2

      Challenge Reply               3

      Registration Reply            4








Hamzeh                       Informational                      [Page 5]

RFC 2107                          ATMP                     February 1997


      Deregister Request            5

      Deregister Reply              6

      Error Notification            7

2.1 Registration Request

   The FA issues a Registration Request to request the HA to establish a
   mobility binding for the specified MN home address.  The request is
   issued to the HA by the FA upon detecting a MN that wishes to use a
   home address supported by the HA receiving the request.

   IP fields

      Source Address           The IP address of the foreign agent
                               interface from which the request is
                               issued.

      Destination Address      The IP address of the home agent.

   UDP fields:

      Source Port              variable

      Destination Port         5150  (or port number configured in FA
                               for given HA)
























Hamzeh                       Informational                      [Page 6]

RFC 2107                          ATMP                     February 1997


   The UDP header is followed by the ATMP fields shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |      Type     |         Identifier            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Foreign Agent                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Mobile Node                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Mobile Node Mask                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Mobile Node IPX Net                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Mobile Node IPX Station  . . .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                      |           reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Home Network Name    . . .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Version                  The ATMP protocol version.  MUST be 1.

      Type                     1 for Registration Request.

      Identifier               A 16 bit number used to match replies
                               with requests.  A new value should be
                               provided in each new request.
                               Retransmissions of the same request
                               should use the same identifier.

      Foreign Agent            The IP address of the foreign agent
                               issuing the request (typically the same
                               as the UDP source address).

      Mobile Node              The IP address to be used by the mobile
                               node.  This is the mobile node's home
                               address.  This field can be all 0's if
                               IPX is to be tunneled to the mobile node.

      Mobile Node Mask         The network bit mask for the mobile node.
                               Currently this value should be set to all
                               1's.

      Mobile Node IPX Net      The Network portion of the mobile node's
                               IPX address.  This value should be set to
                               all 0's if only IP is to be tunneled.



Hamzeh                       Informational                      [Page 7]

RFC 2107                          ATMP                     February 1997


      Mobile Node IPX Station  The 6 octet value used to represent the
                               station portion of the mobile node's IPX
                               address.  This value should be set to all
                               0's if only IP is to be tunneled instead
                               of IPX.

      Reserved                 This field is for future extensibility
                               and MUST be set to all 0's.

      HN Name                  This is the name of the "Connection
                               Profile" to be used by the home agent to
                               forward all packets received from the
                               mobile node.  This character string is
                               terminated by a NUL character and can be
                               up to 32 characters long, including the
                               NUL terminator.

2.2 Challenge Request

   The Home Agent issues a Challenge Request in response to the receipt
   of a Registration Request from a Foreign Agent.  It is used by the
   Home Agent, in conjunction with the Challenge Reply, to authenticate
   the Foreign Agent.

   IP fields

      Source Address           The IP address of the Home Agent
                               interface from which the request is
                               issued.

      Destination Address      Copied form the Source Address of the
                               Registration Request.

   UDP fields:

      Source Port              variable

      Destination Port         Copied from the Source Port of the
                               Registration Request.












Hamzeh                       Informational                      [Page 8]

RFC 2107                          ATMP                     February 1997


The UDP header is followed by the ATMP fields shown below:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Version    |    Type       |         Identifier            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                         Authenticator                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Result Code           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Version                  The ATMP protocol version.  MUST be 1.

      Type                     2 for Challenge Request

      Identifier               A 16 bit number used to match replies
                               with requests.  A new value should be
                               provided in each new request.
                               Retransmissions of the same request
                               should use the same identifier.

      Authenticator            A series of 16 octet values randomly
                               generated by the Home Agent.  The
                               receiving Foreign Agent is to perform an
                               MD5 [7] hash of these values along with a
                               shared secret.  The resultant digest is
                               returned in the Challenge Reply.  See
                               Sec. 2.3 Retransmissions of the Challenge
                               Request should use the same Authenticator
                               value.

                               A value of all 0's in this field
                               indicates an error occurred with the
                               Registration Request.  The error code
                               will be in the following field.











Hamzeh                       Informational                      [Page 9]

RFC 2107                          ATMP                     February 1997


      Result Code              If non-zero, this value indicates the
                               error condition that occurred.  See Sec.
                               2.8 for a list of Result Code values and
                               their meanings.

                               A non-zero value in this field implies
                               that the Authenticator field will be
                               zero.

2.3 Challenge Reply

   The Foreign Agent issues a Challenge Reply upon receipt of a valid
   Challenge Request (one with a Result Code of 0) from the Home Agent.
   The Foreign Agent uses the randomly generated Authenticator value
   from the Challenge Request along with a shared secret to produce an
   MD5 digest value which is returned to the Home Agent in the Challenge
   Reply.

   IP fields

      Source Address           The IP address of the Foreign Agent
                               interface from which the reply is issued.

      Destination Address      Copied from the Source Address of the
                               Challenge Request.

   UDP fields:

      Source Port              variable

      Destination Port         Copied from the Source Port of the
                               Challenge Request.


   The UDP header is followed by the ATMP fields shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |    Type       |         Identifier            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Reply Length         |       Reply  . . .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








Hamzeh                       Informational                     [Page 10]

RFC 2107                          ATMP                     February 1997


      Version                  The ATMP protocol version.  MUST be 1.

      Type                     3 for Challenge Reply

      Identifier               Copied from the corresponding
                               Deregistration Request.

      Reply Length             This field specifies the length of the
                               challenge reply computation based on the
                               received Authenticator and the shared
                               secret.  For MD5 this length will always
                               be 16.  This field is provided for future
                               extensibility.

      Reply                    This is the computed challenge reply.  It
                               is computed by performing an MD5 message
                               digest computation over the Authenticator
                               value received in the Challenge Request
                               appended with the secret shared between
                               the Foreign Agent and the Home Agent.
                               The digests produced by MD5 are always 16
                               octets long.

2.4 Registration Reply

   A Registration Reply is issued by a Home Agent in reply to a
   Challenge Reply received from a Foreign Agent.  The Registration
   Reply indicates to the Foreign Agent whether the registration was
   accepted by the Home Agent or not.  It also provides a "tunnel ID" to
   uniquely identify the tunnel to be associated with this session.

   The Home Agent calculates the same MD5 hash on the Challenge Request
   Authenticator field and the shared secret.  The resulting digest is
   compared with the Reply value in the Challenge Reply and if it is
   equal, authentication is successful.  Otherwise the registration is
   not accepted and the Foreign Agent is informed by the Result Code of
   the Registration Reply that registration failed due to an
   authentication failure.

   IP fields

      Source Address           The IP address of the Home Agent
                               interface from which the reply is issued.

      Destination Address      Copied from the Source Address of the
                               Challenge Reply.





Hamzeh                       Informational                     [Page 11]

RFC 2107                          ATMP                     February 1997


   UDP fields:

      Source Port              variable

      Destination Port         Copied from the Source Port of the
                               Challenge Reply.


   The UDP header is followed by the ATMP fields shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |    Type       |         Identifier            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Result Code          |         Tunnel ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Version                  The ATMP protocol version.  MUST be 1.

      Type                     4 for Registration Reply

      Identifier               Copied from the corresponding
                               Registration Request.

      Result Code              Specifies the result of the registration
                               and authentication attempt by the Foreign
                               Agent.  Sec. 2.8 for a list of Result
                               Code values and their meanings.

      Tunnel ID                This is the identifier used to indicate a
                               given mobility binding between a given
                               Mobile Node and Home Agent.  This
                               identifier is used to distinguish
                               multiple tunnels between a given Foreign
                               Agent-Home Agent pair.  It is carried in
                               the "key" field of the GRE [1] tunnel
                               packets that ATMP uses as the tunnel
                               protocol.  It is also used in
                               Deregistration Requests and Error
                               Notification messages to indicate the
                               particular mobility binding to which they
                               relate.







Hamzeh                       Informational                     [Page 12]

RFC 2107                          ATMP                     February 1997


2.5 Deregistration Request

   The Deregistration Request is issued by the Foreign Agent to the Home
   Agent to indicate that the specified mobility binding is to be ended.
   This request may result from the Foreign Agent detecting that its
   connection to the Mobile Node has terminated.  It can also be issued
   in response to a detected error condition by the Foreign Agent or
   receipt of an Error Notification message from the Home Agent.

   IP fields

      Source Address           The IP address of the Foreign Agent
                               interface from which the request is
                               issued.

      Destination Address      5150  (or port number configured in FA
                               for given HA)

   UDP fields:

      Source Port              variable

      Destination Port         Copied from the Source Port of the
                               Challenge Reply.


   The UDP header is followed by the ATMP fields shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |    Type       |         Identifier            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tunnel ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Version                  The ATMP protocol version.  MUST be 1.

      Type                     5 for Deregistration Request

      Identifier               A 16 bit number used to match replies
                               with requests.  A new value should be
                               provided in each new request.
                               Retransmissions of the same request
                               should use the same identifier.





Hamzeh                       Informational                     [Page 13]

RFC 2107                          ATMP                     February 1997


      Tunnel ID                Tunnel identifier of the mobility binding
                               to be terminated.

2.6 Deregistration Reply

   The Deregistration Reply is issued by the Home Agent in response to a
   Deregistration Request received from a Foreign Agent.  If the
   Deregistration Request was valid, the Home Agent removes the
   specified mobility binding from its tables and issues an affirmative
   reply.  Otherwise the Home Agent issues a Deregistration Reply with a
   Result Code indicating the reason for failure of the Deregistration
   Request.

   IP fields

      Source Address           The IP address of the Home Agent
                               interface from which the reply is issued.

      Destination Address      Copied from the Source Address of the
                               received Deregistration Request.

   UDP fields:

      Source Port              variable

      Destination Port         Copied from the Source Port of the
                               received Deregistration Request.

   The UDP header is followed by the ATMP fields shown below:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Version    |    Type       |         Identifier            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Result Code          |         Tunnel ID             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Version                  The ATMP protocol version.  MUST be 1.

      Type                     6 for Deregistration Reply

      Identifier               Copied from the corresponding
                               Deregistration Request.






Hamzeh                       Informational                     [Page 14]

RFC 2107                          ATMP                     February 1997


      Result Code              Specifies the result of the registration
                               and authentication attempt by the Foreign
                               Agent.  Sec. 2.8 for a list of Result
                               Code values and their meanings.

      Tunnel ID                Tunnel identifier of the mobility binding
                               specified in the Deregistration Request.

2.7 Error Notification

   This message is sent by either agent to inform the other agent that
   an error condition has occurred.  It provides a last-ditch error
   recovery mechanism that is used for conditions that cannot be
   reported back to the sender by the normal request-reply mechanism,
   such as the receipt of a spontaneously generated reply.

   IP fields

      Source Address           The IP address of the issuing agent
                               interface from which this message is
                               issued.

      Destination Address      The IP address of the Home Agent or
                               Foreign Agent to which this message is to
                               be issued.

   UDP fields:

      Source Port              variable

      Destination Port         If issued to a Home Agent, 5150 (or the
                               port number configured for the given HA).
                               If issued to a Foreign Agent, the source
                               port number saved from the original
                               Registration Request.


   The UDP header is followed by the ATMP fields shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |    Type       |         Identifier            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Result Code          |         Tunnel ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





Hamzeh                       Informational                     [Page 15]

RFC 2107                          ATMP                     February 1997


      Version                  The ATMP protocol version.  MUST be 1.

      Type                     7 for Error Notification

      Identifier               If issued in response to a received reply
                               type message, this value should be copied
                               from the identifier field of the reply.
                               Otherwise the identifier should be the
                               value that would be used for the next
                               generated request.

      Result Code              This indicates the type of error
                               detected.  The possible result codes are
                               defined in Sec. 2.8.

      Tunnel ID                Tunnel identifier of the mobility binding
                               to which this message pertains.  If the
                               Error Notification is being sent in
                               response to an unsolicited reply, the
                               Tunnel ID is copied from the reply.

2.8 Result Codes

   Error Code          Description

   0 NO_ERROR          Successful operation

   1 AUTH_FAILED       Authentication of the Foreign Agent failed.
                       Registration denied.

   2 NOT_ENABLED       The Home Agent is not configured to run ATMP.

   3 TOO_MANY          Too many Mobile Node sessions.  Home Agent is out
                       of resources.

   4 PARAMETER_ERROR   An invalid value was detected in an ATMP message.

   5 INVALID_TUNNEL_ID The Tunnel ID contained in a GRE packet is
                       invalid or the corresponding mobility binding
                       does not exist.  This usually occurs when either
                       the MN or HA has reset.

   6 TIMEOUT           A response to an ATMP request was not received in
                       time.







Hamzeh                       Informational                     [Page 16]

RFC 2107                          ATMP                     February 1997


   7 NET_UNREACHABLE   The Home Network for this mobility binding is not
                       operational (the "Connection Profile" is "down"
                       or is not defined).

   8 GENERAL_ERROR     General Error indication.

2.9 Protocol Operation

   Upon detection of a Mobile Node requiring ATMP service, the NAS
   invokes its ATMP Foreign Agent entity.The FA retrieves configuration
   information for the user involved.This information is obtained in a
   method particular to the NAS and is not specified by this document.
   The information obtained MUST include the Home Agent address and the
   shared secret for this HA.It also MAY include the Home Network Name,
   if a Connection Profile is to be used for this session.

   The FA then sends a Registration Request to the HA informing it that
   an MN wishes to register with it.  The FA then waits for the HA to
   respond with a Challenge Request.  The FA retransmits the
   Registration Request every 2 seconds until it receives the Challenge
   Request.  If, after 10 retransmissions, no Challenge Request is
   received, the FA will terminate the Registration Request, logs the
   registration failure, and disconnects the MN.

   Upon receipt of the Challenge Request, the FA examines the Result
   code.  If it indicates an error condition, the condition is logged
   and the MN is disconnected.  If the result is zero, the FA generates
   a Challenge Reply.  The Challenge Reply is generated by appending the
   Authenticator obtained from the Challenge Request with the shared
   secret (obtained from the configuration data) and then computing the
   MD5 hash of this concatenated string (authenticator + secret).  The
   16 octet hash is then returned in the Challenge Reply for validation
   by the HA.

   Upon receipt of the Challenge Reply from the FA, the HA does the same
   computation of the MD5 hash based on the Challenge Request
   Authenticator and the shared-secret (which it too must be configured
   with).  If this digest matches that provided in the Challenge Reply
   by the FA then the authentication is successful and the registration
   is accepted.  If the authentication fails, or resource limitations
   prohibit the registration attempt, the HA returns a Registration
   Reply with a non-zero result code to the FA.

   If the HA accepts the Challenge Reply from the FA, it assigns a
   Tunnel ID to this session and returns this Tunnel ID in a
   Registration Reply with a zero result code.  This Tunnel ID needs to
   be unique for the FA-HA pair.  The Tunnel ID is used to multiplex and
   demultiplex the packets sent between a given FA-HA pair.



Hamzeh                       Informational                     [Page 17]

RFC 2107                          ATMP                     February 1997


   At the time the HA decides to accept a registration, it creates a
   control block that associates the Tunnel ID with the appropriate
   routing information.  If the Registration Request included a Home
   Network Name, this name is saved in the table and used as the name of
   the Connection Profile for this session.

   Upon receipt of the Registration Reply, the FA examines the result
   code. If it is non-zero, the FA logs the registration failure and
   disconnects the MN.  If it is zero, the FA saves the Tunnel ID in a
   control block associated with the MN session.  The FA and HA are now
   ready to exchange data packets for this MN session.

   On the FA side, all data received from the MN will be encapsulated
   using GRE and sent to the HA with the assigned Tunnel ID included in
   the GRE Key field.  When the HA receives a GRE packet it decapsulates
   it and routes it using the routing information contained in the HA's
   control block associated with this Tunnel ID.

   When the HA receives a packet destined for the MN's Home Address, it
   MUST encapsulate it in a GRE packet and forward it to the appropriate
   FA.  The Tunnel ID is included in the GRE Key field to allow the FA
   to demultiplex the packet.

   When the FA receives a GRE packet, it will examine the Tunnel ID in
   the GRE Key field to see if it matches the Tunnel ID assigned to any
   of the MN's currently connected to the FA.  If so, the packet is
   decapsulated and forwarded to the MN.  If the Tunnel ID doesn't match
   any active MN's, an Error Notification message is issued to the HA
   and the GRE packet is silently discarded.

   When the FA wishes to disconnect the MN from the HA, it issues a
   Deregistration Request.  This request is issued every 2 seconds.  If
   after 10 attempts a Deregistration Reply is not received from the HA,
   an error condition is logged and the MN is disconnected.  Upon
   receipt of a Deregistration Reply from the HA, the FA deallocates the
   Tunnel ID and disconnects the MN.

3.0 Security Considerations

   The Registration function of ATMP is protected by a
   Challenge/Response mechanism similar to CHAP [2]. The Home Agent
   challenges each registration attempt by a Foreign Agent for
   authentication.This authentication requires the configuration of a
   shared secret for each HA/client pair.







Hamzeh                       Informational                     [Page 18]

RFC 2107                          ATMP                     February 1997


4.0 Author's Address

   Kory Hamzeh
   Ascend Communications
   1275 Harbor Bay Parkway
   Alameda, CA 94502

   EMail: kory@ascend.com

5.0 References

   [1]  Hanks, S. Li, T., Farinacci, D., and Traina, P., "Generic
        Routing Encapsulation (GRE)", RFC 1701, October 1994.

   [2]  Lloyd, B., and W. Simpson, "PPP Authentication Protocols",
        RFC 1334, October 1992.

   [3]  Perkins, C., "IP Mobility Support", RFC 2002, October 1996.

   [4]  Postel, J., "User Data Protocol",  STD 6, RFC 768, August 1990.

   [5]  Rigney, C., Rubens, A., Simpson, W., and S. Willens, "Remote
        Authentication Dial In User Service (RADIUS)", RFC 2058,
        January 1997.

   [6]  Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
        RFC 1700, October, 1994.

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

   [8]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, July 1994.


















Hamzeh                       Informational                     [Page 19]

RFC 2107                          ATMP                     February 1997


Appendix A

   Additional RADIUS Attributes for ATMP

   This appendix indicates the RADIUS attributes that have been added by
   Ascend to support ATMP for IP.  Currently these are defined as non-
   vendor-specific attributes but have been included in [5].

       Attribute: "Ascend-Home-Agent-IP-Addr"
       Type:      IP-Address
       Value:     The IP address of the Home Agent

       Attribute: "Ascend-Home-Agent-Password"
       Type:      String
       Value:     Secret shared for this user with HA

       Attribute: "Ascend-Home-Network-Name"
       Type:      String
       Value:     Name of Connection Profile for this session

       Attribute: "Ascend-Home-Agent-UDP-Port"
       Type:      Integer
       Value:     The destination UDP port number for the specified HA

Appendix B

      IPX Operation

      ATMP specifies a mechanism which allows IPX clients to receive
      mobility services from a HA. Section 2 details the protocol used
      to register, deregister, and authenticate a tunnel used for IPX.
      Note that ATMP is based on IP datagrams for the management of
      tunnels and, thus, IPX tunneling with ATMP always requires an
      underlying IP network.

      Each IPX mobile client requires an IPX network number and node
      address pair.  Since IPX does not support a similar facility to
      IP's "host route," an enterprise-unique network number needs to be
      chosen for each home agent. This network number MUST be distinct
      from the IPX network number assigned to any of the home agent's
      LAN interfaces. Each mobile client tunneled to the home agent MUST
      use the same IPX network number.

      For example, consider a home agent which supports two mobile
      clients.  The home agent is on a LAN network with an IPX address
      of AA000001. The home agent's client network may be assigned
      AA000002. The two mobile clients may have addresses
      AA000002:0040F1000001 and AA000002:0040F1000002 respectively.



Hamzeh                       Informational                     [Page 20]

RFC 2107                          ATMP                     February 1997


      IPX node numbers need to be unique on a given network. A mechanism
      must exist to guarantee that for each home agent's network, a
      given mobile client's node address is unique. Several techniques
      may be employed to assure unique node addresses. The current
      implementation of ATMP described in this document relies on RADIUS
      to assign a node address at the foreign agent. The following
      RADIUS attributes are included for IPX operation in addition to
      the attributes described in Appendix A for IP operation:

       Attribute: "Framed-IPX-Network" (See [5] for details).

       Attribute: "Ascend-IPX-Node-Addr"
       Type:      String
       String:    The node address for the mobile client in 12 octet
                  ASCII representing the hexadecimal string. Both
                  lower and upper case characters are permissible.



































Hamzeh                       Informational                     [Page 21]

