[Search] [txt|pdf|bibtex] [Tracker] [Email] [Nits]

Versions: 00 01                                                         
Internet Engineering Task Force                                W. T. Teo
INTERNET DRAFT                                                S. W. Yeow
                                        National University of Singapore
                                                             August 1998

               Reverse Network Address Translators (RAT)
                   <draft-teoyeow-mip-rat-00.txt>

Status of This Memo

   This document is a submission to the MobileIP Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to the mobile-ip@smallworks.com mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (North Europe),
   ftp.nis.garr.it (South Europe), munnari.oz.au (Pacific Rim),
   ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

Abstract

Mobile IP faces difficulties in deployment as all mobile nodes need to
support the protocol. While the implementation and deployment issues
of the Mobile IP home agents do not affect the mobile end-users, the
users are more constrained to the operating systems which support
MobileIP on their mobile nodes.

This document describes an alternative protocol that attempts to
solve the deployment problem. It provides IP reachability for mobile
users through the use of Reverse Network Address Translators (RAT).
This technique leverages on the popularity of Network Address
Translators (NAT) and NAPTs to provide transparent mobility to end
hosts.The protocol's requirements for the mobile node are thus minimal.
Moreover, the RAT protocol is interoperable with the MobileIP base
protocol.

Teo and Yeow             Expires February 1999                 [Page 1]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


1. Introduction

   IP version 4 datagram routing generally depends on the destination
   host's IP address to uniquely identify the host point of attachment
   in the Internet. This implies a host has to be on the network
   indicated by its IP address - the home address in Mobile IP (MIP)
   terminology - to receive packets destined to it.

   This document proposes protocol enhancements to NAT which allow other
   hosts to initiate communication using a mobile node's home address
   when the latter changes location.

   This document does not attempt to describe address translations at a
   RAT device above the network level. RAT only deals with network
   address translation of the IP header. Network Address Port
   Translation (NAPT) and Application Level Gateways (ALGs) can continue
   to operate on top of RAT.

   When a mobile node moves from its home network, by reverse network
   address translation, datagrams destined for its home address can be
   transparently routed to its new location.

   The mobile node will always use a topologically correct IP address as
   its source address. This address will be the home address when the
   mobile node is in its home network. The address will be a care-of
   address assigned by an external mechanism when the mobile node is in
   a foreign network.

   Reverse network address translation is only required when the mobile
   node is not at home and the network sessions are initiated by the
   correspondent nodes, e.g. when the mobile node is an application
   server. For all other situations, the mobile node will communicate
   directly with its correspondent nodes, without the overhead incurred
   by RAT, i.e. the correspondent nodes respond to the mobile node using
   the latter's topologically correct address.

1.1 Goals

   The main motivation for RAT is to facilitate deployment. Common
   network technologies are used where possible to reduce implementation
   effort. Until Mobile IP (MIP) is widely supported on all operating
   platforms for mobile users and MIP home agent's functions are
   available on routers, IP mobility support on the Internet will be
   limited.

   Since NAT routers are already widely deployed and implementations
   with twice NAT [REF 3] are available, to provide RAT capability on
   current NAT [REF 4] devices require minimal enhancements. Networks



Teo and Yeow             Expires Februrary 1999                 [Page 2]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   with NAT installations will be mobile capable by migrating to RAT. By
   depending on currently available network applications for
   registration, little effort is needed on the user's part to gain the
   benefit of mobility.

   By network address translations, RAT provides transparent mobility to
   end hosts. No enhancements to a mobile node's transport and lower
   network layers are necessary.

   The application protocol employed in the registration process is
   flexible and independent of RAT's base protocol. However, for
   interoperability reasons, the control messages between a registration
   server and a RAT device must be followed.

1.2 Applicability

   The protocol does not attempt to maintain transport and higher-layer
   connections when a node changes location. The main function of RAT is
   to allow correspondent nodes to locate a mobile node by its
   permanently assigned home address.

1.3 Deployment Issues

   In a basic setup, to support RAT, a network needs a registration
   server and a RAT device for each physically partitioned subnet.  The
   mobile node does not require foreign networks to support RAT to have
   mobility support. However, the node must be able to acquire a
   topologically correct IP care-of address via any available external
   mechanism in the foreign location.

1.4 Protocol Requirements

   The mobile node must be able to access at least one of its
   registration servers when in a foreign location.

   The RAT device must be able to deliver datagrams to the mobile node's
   foreign location.

   All messages used to inform the registration server as to the mobile
   node current location must be authenticated to protect against remote
   redirection attacks.

1.5 End Host Accessibility

   All correspondent nodes will be able to reach the mobile node if its
   currently assigned IP address is reachable by the RAT device using
   the appropriate routable addresses.




Teo and Yeow             Expires Februrary 1999                 [Page 3]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   Correspondent nodes previously accessible by a mobile node at home
   may not be reachable when the mobile node is in a foreign location.
   This is because the correspondent node's access control list or
   network firewall may deny traffic originating from the mobile node's
   current location. This is not a design flaw since the existing
   security policies should not be circumvented for mobility support.

   Another reason why a mobile node cannot reach certain correspondent
   nodes is when the latter are in a network using private IP addresses
   [REF 2] and the mobile node has moved outside the private network, or
   when the mobile node has moved into a private network without NAT
   support. The protocol should not allow a mobile node to reach these
   correspondent nodes unless the security policies permits.

   Private correspondent nodes can still reach a mobile node outside the
   internal network using RAT. The RAT device may deny the forwarding of
   such datagrams for security reasons and send an ICMP Host-Unreachable
   error to the correspondent node.

1.6 NAT and RAT differences

   Network Address Translation (NAT) is typically used when a network's
   internal IP addresses cannot be used externally. NAT can connect
   separate routing realms with different addressing schemes. It does
   that by translating the network address of datagrams to the
   appropriate routable address in the corresponding routing space.
   Therefore NAT is used when the end hosts are in different routing
   realms. The NAT device must be assigned an address in each of the
   routing realm it connects.

   The purpose of Reverse Network Address Translation (RAT) is
   different. RAT is used even when the end hosts are in the same
   routing space to forward datagrams destined to the mobile node's home
   address to the latter's care-of address. The RAT device only needs to
   be assigned one address if all end hosts are in the same routing
   space.

   Reverse Network Address Translation (RAT) does twice Network Address
   Translation [REF 3] for datagram delivery. In twice NAT, both the
   source and destination addresses are translated. The current reason
   for twice NAT is to connect end hosts that use overlapping address
   space in their home network. A unique intermediate address space is
   used to connect the end hosts i.e. the RAT device becomes the virtual
   sender/receiver of the mobile/correspondent nodes.

   NAT is always needed for communicating hosts in separate routing
   realms regardless of the session direction [REF 3]. RAT is needed
   only when the session is intiated by the correspondent nodes. In



Teo and Yeow             Expires Februrary 1999                 [Page 4]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   traditional NAT [REF 4], translation is initiated by the client nodes
   which the NAT device services. RAT translation is reverse initiated
   by the correspondent nodes and by not the mobile client nodes which
   the RAT device services.

1.7 Mobile IP and RAT comparison

   The Network Address Translator (NAT) and NAPT have become popular
   because of easy deployment. They require no modifications to the
   communicating hosts. Mobile IP however faces difficulties in
   deployment as all mobile nodes need to support the protocol. While
   implementation and deployment of Mobile IP home agents need not be
   concerned with the operating system, mobile users are constrained to
   operating systems which supports Mobile IP. Network adminstration for
   Mobile IP's security authentication and key allocation also requires
   additional configuration tools for the novice user.

   The base protocol of Mobile IP does not support communication across
   different routing realms e.g. between private and public nodes. If
   such mobility support is desired, Mobile IP extension for Private
   Internets Support [8] or Firewall Support for Mobile IP [9] is
   needed. NAT's main function is to allow communication between
   different routing realms. If the current NAT installation already
   support such communication for sessions initiated in any routing
   realm, RAT can provide the mobility support without additional
   enhancements.

   Mobile IP uses IP tunneling to deliver datagrams to a mobile node's
   care-of address. This allows a mobile node to bypass traditional
   firewalls that only filter packets based on the IP tunnel header.
   RAT uses twice NAT/NAPT to deliver datagrams to a mobile node's
   care-of address. Certain applications which embed the end hosts' IP
   addresses in the data payload will not function with NAT/NAPT if
   there are no application layer gateways available to support them.

   Mobile IP specifies the registration message formats and semantics
   for mobile nodes. RAT uses common application protocols supported on
   any network operating systems. The delivery mechanism - twice
   NAT/NAPT - is explicitly separated from the registration mechanism in
   RAT.

   RAT provides limited mobility in comparison to Mobile IP. It does not
   attempt to maintain connection orientated sessions while the mobile
   node moves across multiple networks.

   Where the abovementioned limited mobility and application support is
   sufficient, RAT is much easier as a deployment solution.




Teo and Yeow             Expires Februrary 1999                 [Page 5]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   In Mobile IP, there are 2 main purposes for a mobile node to have a
   fixed IP address - the home IP address :

   1. To enable all correspondent nodes to identify the mobile node with
      a fixed IP address that is unchanged regardless of location.

   2. To retain connection orientated transport protocols, e.g. TCP
      connections, while the mobile node moves across networks.

   The intended function of RAT is to achieve the first purpose. It is
   typically unnecessary for the mobile node in a foreign location to
   use its home IP address as the source IP address when originating a
   datagram.

   Such an approach as defined in MobileIP has two disadvantages :

   1. Datagrams originating from the correspondent node will generally
      need to be routed to the mobile node's home network before they
      are tunnelled to the mobile node care-of address.

   2. If Ingress filtering is deployed at the mobile node's current
      foreign location to filter datagrams with topologically incorrect
      source IP address, bidirectional tunneling is required to bypass
      the Ingress filter.

   Both of the above situations may result in a longer routing path
   between the sender and receiver.

   In RAT, for a correspondent node initiated session, the end hosts'
   routing path is similiar to bidirectional tunneling in Mobile IP.
   This will form a dog-legged route, from the mobile node to RAT device
   to correspondent node and vice versa.

   For communication initiated by the mobile node, since both the end
   hosts' addresses used are topologically correct, standard IP routing
   is sufficient and RAT will not be involved. However, communication
   will fail in situations where the home IP address is necessary e.g.
   firewalls.

2. Terminology

   The document adopts all the terminology defined in "IP Mobility
   Support" [REF 1] and "IP Network Address Translator Terminology and
   Considerations" [REF 3].

   Three new entities are introduced :





Teo and Yeow             Expires Februrary 1999                 [Page 6]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   1. Zero Implementation Mobile Node (0MN)

   Zero Implementation Mobile Node (0MN) identifies a mobile host that
   supports RAT. This is to differentiate the former from the same term
   used in Mobile IP.

   2. Registration Server (RS)

   A registration server is generally located in the mobile node's home
   domain. A 0MN must inform the RS of its new location in a foreign
   network before it can receive datagrams destined for its home
   address. The RS needs to be reachable from the mobile node's current
   location using the available routing mechanisms for the registration
   process to be successful.

   The registration server will interact with a RAT device to set up the
   reverse translation table. The table binds a mobile node's home IP
   address with its care-of address and the binding's lifetime.

   The discovery of a registration server is not specified by the
   protocol and is dependent on the application protocol used in the
   registration process. For example if the registration server uses
   HTTPS for registration, a mobile user may identify the RS by a URL
   address. For the simplest configuration, the RS address can be
   statically configured.

   3. Reverse Network Address Translator (RAT)

   The Reverse Network Address Translator maintains a 0MN's home IP
   address association with a care-of address and the binding's
   lifetime. It uses twice NAT to route datagrams from a 0MN's home
   address to care-of address.

   The RAT device is generally directly connected to the 0MN's home
   network in order to receive datagrams destined for the latter e.g. by
   proxy ARP. The requirement is unnecessary if the RAT device can
   interoperate with the Mobile IP's home agent entity [REF 1]. The home
   agent can then tunnel datagrams destined for the 0MN to the RAT
   device for final delivery to the 0MN's care-of address.

   It is not neccessary for a 0MN to know the identity of the RAT device
   for the protocol to work. For security reasons, only the RS should
   know the RAT devices available in a network.

3. Registration Application Protocol Selection

   The application protocol used by a mobile node to register its
   current care-of address with a registration server is independent of



Teo and Yeow             Expires Februrary 1999                 [Page 7]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   the protocol. However for security reasons the application protocol
   must fulfill the following criteria :

   1. A mechanism to validate both the mobile node/user identity and its
      current location.

      A registration server must never assume the source IP address of a
      registration request is the care-of address of the mobile node.
      Information disclosure could provide the means of hijacking a
      mobile node traffic. Therefore it is recommended that the mobile
      node's care-of address be encrypted. Using a user-centred
      authentication scheme, the mobile node home IP address need not be
      sent during the registration process if the registration server
      maintains a mobile user to mobile node home address association.

   2. A mechanism to confirm the mobile node is still at its current
      registered location.

      The mobile node will need to renew its care-of address by
      re-registration or some similar mechanism, within an appropriate
      lifetime. This is to avoid forwarding datagrams to an old location
      the mobile node has vacated. This renewal request should be
      time-stamped etc to avoid possible replay attacks.

   For easy deployment of RAT as a mobility solution, the application
   protocol used should be widely supported on all operating platforms.
   A good example of an application protocol that meets the above
   security and availability requirements is the Secure Hypertext
   Transfer Protocol (HTTPS), which is HTTP over a Secure Socket Layer
   (SSL).

   A mobile user will only need a World Wide Web (WWW) browser to access
   a WWW server on the registration server. Java applets may be
   downloaded from the registration server to request user
   authentication. With verification of user identity, the applet can
   then transmit time-stamped "keep alive" beacons back to the RS to
   confirm the 0MN location. The messages sent can be encrypted/
   authenticated using a private key the mobile user provided. The
   method of encryption/authentication need not be known to any entity
   except the registration server.

   The specifics of the registration process are beyond the scope of
   this document.

4. Acquiring Care-Of Address

   When a 0MN is in a foreign network and desire mobility support, it
   must acquire a topologically correct IP address in the network.  Any



Teo and Yeow             Expires Februrary 1999                 [Page 8]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   available external mechanism supported by the mobile node can be used
   to acquire a care-of address. Popular protocols available are the
   Dynamic Host Configuration Protocol (DHCP) or the Point to Point
   Protocol (PPP).

5. Control Messages

   Control messages are used to exchange information between the
   registration server and RAT device. It is not recommended that both
   entities reside on the same machine for the following reasons :

   1. The application protocol used for registration may change with the
      introduction of newer technology but the RAT mechanism will remain
      the same.

   2. Address translations incur large overhead in memory and
      computation [refer to Section 9] and dedicated hardware may be
      needed. Administration and installation of myriad feasible
      application protocols on dedicate hardware is not viable.

   3. Failure of the registration server or RAT device will deny
      mobility services. Introducing backup registration servers and
      alternative RAT devices can increase reliability and distribute
      load.

   The control messages are sent with UDP [5] using the well-known port
   number 434 allocated to Mobile IP. New authentication extensions are
   defined to indicate RAT operation. The default authentication
   algorithm uses keyed-MD5 [6] in "prefix+suffix" mode to compute a
   128-bit "message digest" of the control message.

5.1 Reverse Translation Binding (RTB) Request

   A registration server sends a Reverse Translation Binding Request
   (RTBR) to a RAT device to setup the reverse translation table - 0MN's
   home address, care-of address, binding's lifetime.

   The Mobile IP registration request format is used.

   The format is as follows :

   IP fields:

      SA: Typically the interface address from which the registration
          server sends the message.

      DA: Typically that of the RAT device.




Teo and Yeow             Expires Februrary 1999                 [Page 9]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   UDP fields:

      Source Port:       variable

      Destination Port:  434

   The UDP header is followed by the RAT 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |0|B|D|   0     |          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Registration Server                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type     1 (Registration Request)

      B        Broadcast datagrams.  If the 'B' bit is set, the
               registration server requests that the RAT device
               forwards any broadcast datagrams that the 0MN receives on
               the home network.

      D        Decapsulation by RAT device. If the `D` bit is set, the
               RAT device will decapsulate datagrams which are tunneled
               from a Mobile IP's home agent [Refer to Section 8].

      Lifetime
               The number of seconds remaining before the registration
               is considered expired.  A value of zero indicates a
               request for deregistration.  A value of 0xffff indicates
               infinity.

      Home Address
               The IP address of the 0MN.

      Registration Server
               The IP address of the 0MN's registration server.



Teo and Yeow             Expires Februrary 1999                [Page 10]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


      Care-of Address
               The IP address of the 0MN current location.

      Identification
               A 64-bit number, constructed by the registration server
               node, used for matching RTB with RTB Replies, and for
               protecting against replay attacks of RTB messages.

      Extensions
               The fixed portion of the RTB Request is followed by one
               or more of the Extensions. The RAT-Server Authentication
               Extension MUST be included in all RTB Requests.

5.2 Reverse Translation UnBinding (RTU) Request

   When a 0MN is no longer at its registered care-of address, i.e. no
   "keep alive" beacons are sent by the 0MN to the registration server
   or its reverse translation binding's lifetime expires, the
   registration server must send a Reverse Translation UnBinding Request
   to the RAT device to remove the 0MN entry in the reverse translation
   table.

   The format of the RTU request is the same as the RTB request except
   the lifetime field is 0.

5.3 RAT Control Message Reply

   A RAT device returns a Control Reply message to a registration server
   which has sent a RTB or RTU message.

   The Mobile IP registration reply format is used.

   The format of the extension is as follows :

   IP fields:

      SA: Typically copied from the destination address of the RTB or
          RTU Request to which the RAT device is replying.

      DA: Copied from the source address of the RTB or RTU request to
          which the agent is replying

   UDP fields:

      Source Port:           <variable>

      Destination Port:     Copied from the source port of the
                            corresponding RTB or RTU Request



Teo and Yeow             Expires Februrary 1999                [Page 11]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   The UDP header is followed by the RAT 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |           Lifetime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Registration Server                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type     3 (Registration Reply)

      Code     A value indicating the result of the Registration
               Request.  See below for a list of currently defined Code
               values.

      Lifetime
               If the Code field indicates that the registration was
               accepted, the Lifetime field is set to the number of
               seconds remaining before the registration is considered
               expired.  A value of zero indicates that the mobile node
               has been deregistered.  A value of 0xffff indicates
               infinity.  If the Code field indicates that the
               registration was denied, the contents of the Lifetime
               field are unspecified and MUST be ignored on reception.

      Home Address
               The IP address of the 0MN.

      Home Agent
               The IP address of the 0MN's registration server.

      Identification
               A 64-bit number used for matching RTB or RTU Requests
               with RTB or RTU Replies, and for protecting against
               replay attacks of RTB or RTU messages.  The value is
               based on the Identification field from the RTB pr RTU
               Request message from the mobile node, and on the style of
               replay protection used in the security context between
               the registration server and its RAT device (defined by



Teo and Yeow             Expires Februrary 1999                [Page 12]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


               the security association between them, and SPI value in
               the Server-RAT Authentication Extension).

      Extensions
               The fixed portion of the RTB or RTU Reply is followed
               by one or more of the Extensions. The RAT-Server
               Authentication Extension MUST be included in all Control
               Replies returned by the RAT device.

   The following values are defined for use within the Code field.  RTB
   or RTU successful:

        0 registration accepted

   RTB or RTU unsuccessful:

       64 reason unspecified
       65 administratively prohibited
       66 insufficient resources
       68 registration server failed authentication
       69 requested Lifetime too long
       70 poorly formed Request
      128 reason unspecified
      129 administratively prohibited
      130 insufficient resources
      133 registration Identification mismatch
      134 poorly formed Request
      136 unknown registration server address

5.4 RAT-Server Authentication Extension

   A RAT-Server Authentication extension type is defined to indicate
   support for RAT operation.

   The format of the extension is as follows :

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            36

      Length          4 plus the number of bytes in the Authenticator.




Teo and Yeow             Expires Februrary 1999                [Page 13]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


      SPI             Security Parameter Index (4 bytes).  An opaque
                      identifier.

      Authenticator   (variable length)

   If the extension is missing in a RTB or RTU request and the RAT
   device is also a Mobile IP home agent entity, it must process the
   message as a registration request as specified in Mobile IP.

6. Reverse Address Translation

   On successful registration with a registration server, a 0MN is
   associated with the tuple <home address IP, care-of address IP>. For
   any session initiated by a correspondent node, all requests and
   reponse must be routed via the same RAT device.

   Any datagram with a destination address that is a registered 0MN's
   home address in the reverse translation table must be reverse address
   translated. Any reply from the registered 0MN to the RAT device must
   be similarly translated.

   The following example illustrates the operation of a RAT device at
   the network level. Network Address Port Translations and Application
   Layer Gateways' operations (if any) are not illustrated.

   Correspondent Node Address:   137.0.0.10
   Home Network:                 138.0.0.0/24
   0MN home address:             138.0.0.10
   0MN care-of address:          139.0.0.10
   RAT device address:           138.0.0.1

                                       Home Network
                    DA: 138.0.0.10 +--------------------------
     +-------------+SA: 137.0.0.10 |  +------+
     |correspondent|---------------|->| RAT  |
     |     node    |<-----------------|device|
     +-------------+DA: 137.0.0.10 |  +------+
                    SA: 138.0.0.10 +---|---^------------------
                                       |   |
                                       |   |
                         DA: 139.0.0.10|   |DA: 138.0.0.1
                         SA: 138.0.0.1 |   |SA: 139.0.0.10
                                       v   |
                                       +---+
                                       |0MN|
                                       +---+

7. ICMP Error Translation



Teo and Yeow             Expires Februrary 1999                [Page 14]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   When a RAT device receives an ICMP Destination-Unreachable error
   message for datagrams destined to a care-of address in the reverse
   address translation table, the error message should be translated as
   follows :

   IP fields:

      SA: RAT device's outgoing interface address to correspond node
      DA: correspondent node address that initiated the session

   ICMP field:

      Type: 3

   If the ICMP code indicates network unreachable, it should be replaced
   by the corresponding host unreachable number. The IP header embedded
   within the ICMP payload must be similarly modified.

8. Interoperation with MobileIP

   The Registration Server and RAT device may interoperate with Mobile
   IP home agent(s) deployed on the mobile nodes' home network. In such
   an approach, it is not necessary for the RAT device to be directly
   connected to the network(s) of the mobile nodes its services. The RS
   must have the mobile-home security associations [1] of the mobile
   nodes and the RAT device must support IP tunnel decapsulation [7].

   Interoperation will also allow a user whose home network deploys
   Mobile IP home agents but does not have Mobile IP mobile node
   implementations on the preferred operating platform to have mobility
   support.

   In such a scenario, the RS on receiving a valid registration request
   from a mobile node must send a Mobile IP registration request [1] to
   the home agent on behalf of the 0MN. The RS must be able to generate
   the Mobile-Home Authentication extension. The only difference in the
   message format is the care-of address is replaced by a RAT device IP
   address. On receiving a successful registration reply from the home
   agent, the RS must send a RTB request with the `D` - decapsulation -
   bit set to the RAT device. The RS will need to renew the mobility
   binding at the home agent as specified by Mobile IP.

   Datagrams destined for the 0MN's home address will now be IP
   encapsulated and tunneled to the RAT device. The RAT device will
   decapsulate the packet and forward the datagram to the mobile node
   actual location via twice NAT. The RS device should send a
   deregistration request when the 0MN's entry in its reverse address
   translation table is no longer valid.



Teo and Yeow             Expires Februrary 1999                [Page 15]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   For a RAT device that also operates as a NAT router, the mechanism to
   choose whether to decapsulate or translate an IP encapsulated
   datagram is outside the scope of this document. A possible solution
   is to reserve an IP address on the RAT device that must decapsulate
   incoming tunneled packets. The IP address is then used to substitute
   all care-of addresses in Mobile IP registration messages.

   The diagram illustrates datagram delivery with Mobile IP and RAT
   interoperation for sessions initiated by correspondent nodes.

                   2) Datagram is intercepted   3) Datagram is
                      by home agent and            detunneled,
                      is tunneled to the           translated by RAT
                      care-of address.             device and delivered
                                                   to 0MN by standard IP
                                                   routing
                        +-----+          +------+         +---+
                        |home | =======> | RAT  | ------> |0MN|
                        |agent|          |device| <------ +---+
                        +-----+          +------+
        1) Datagram to     ^          /
           mobile node     |        /   4) Datagram is sent back to RAT
           arrives on      |      /        device
           home network    |    /
           via standard    |  |_        5) Datagram is translated by RAT
           IP routing.   +-------------+   device and delivered to
                         |correspondent|   correspondent node via
                         |     node    |   standard IP routing.
                         +-------------+

9. Scalability Issues

   The overhead of maintaining address tables and performing address
   translations is computationally intensive. Each data packet is
   subjected to RAT lookup and modifications. This implies the RAT
   device is a possible bottleneck and point of failure. If there are
   alternative RAT devices, recovery of the reverse translation table
   during a RAT device failure is possible with the information stored
   in the registration server.

10. RAT Limitations

   For any sessions that requires RAT when a mobile node is not at home,
   applications or security mechanisms that fail with NAT/NAPT with no
   available specific application layer gateway (ALG), will similarly
   fail with RAT.

   RAT is subjected to additional limitations listed in [REF 1] when



Teo and Yeow             Expires Februrary 1999                [Page 16]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   address translations are necessary.

11. Current Implementation

   A prototype implementation of RAT by S. W. Yeow is now undergoing
   testing.

12. Security Considerations

   The security considerations described in [REF 1] for all variations
   of NATs are applicable to RAT when address translations are
   necessary.

   A security log must be maintained at the registration server. Each
   registration request <home IP address, care-of address, time> should
   be recorded.

   If simultaneous valid registration requests with different care-of
   addresses from the same mobile node is received, the event MUST be
   logged. The registration server must discard all future registration
   requests from the same mobile node. A registration failure message
   should be sent to the requested care-of address if the application
   protocol supports error handling. The format of the message will be
   dependent on the application protocol used.

   All registration failures MUST be logged. The mobile user should be
   informed the time of the most recent successful/failed registration
   for each new registration attempt if possible.

Acknowledgements

   Many thanks to Dr Y. C. Tay at the National University of Singapore
   for his valuable help in reviewing this document.

   Special thanks to Rhandeev Singh at the National University of
   Singapore for his contribution to the protocol implementation.

   RAT research and development is funded in part by the National
   University of Singapore ARF grant RP960683.

References

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

   [2] Rekhter, Y., Moskowitz, B. Karrenberg, D., G. de Groot, and
       Lear, E. "Address Allocation for Private Internets", RFC 1918,
       February 1996



Teo and Yeow             Expires Februrary 1999                [Page 17]


Internet Draft       <draft-teoyeow-mip-rat-00.txt>          August 1998


   [3] P. Srisuresh, M. Holdrege, "IP Network Address Translator (NAT)
       Terminology and Considerations",
       <draft-ietf-nat-terminology-00.txt> - work in progress, July 1998

   [4] P. Srisuresh, K. Egevang, "Traditional IP Network Address
       Translator (Traditional NAT)",
       <draft-ietf-nat-traditional-00.txt> - work in progress, July 1998

   [5] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980

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

   [7] Perkins, C., "IP Encapsulation within IP", RFC 2003, May 1996

   [8] W. T. Teo, Y. Li, "Mobile IP extension for Private Internets
       Support", <draft-teoyli-mobileip-mvpn-00.txt> - work in progress,
       March 1998

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

Author's Address

   W. T. Teo
   National University of Singapore
   School of Computing
   Lower Kent Ridge Cresent
   Singapore 119260

   EMail: teoweetu@comp.nus.edu.sg

   Phone: (65) - 282 0005

   S. W. Yeow
   National University of Singapore
   School of Computing
   Lower Kent Ridge Cresent
   Singapore 119260

   EMail: yeowshin@comp.nus.edu.sg

   Phone: (65) - 257 3832








Teo and Yeow             Expires Februrary 1999                [Page 18]