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rfc:rfc2107

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]

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