rfc2107.txt

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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



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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.





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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.



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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.





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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








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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)
























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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.



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