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Internet Engineering Task Force                                W. T. Teo
INTERNET DRAFT                                                S. W. Yeow
                                                                R. Singh
                                        National University of Singapore
                                                           February 1999

      IP Relocation through twice Network Address Translators (RAT)

Status of This Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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

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

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

   This document describes a protocol that provides IP reachability for
   mobile users. Like IP Mobility Support, RFC 2002, each mobile node is
   accessible by its home address, regardless of its current location in
   the Internet. When away from home, a mobile node acquires a care-of
   address indicating its current location. It registers the care-of
   address with a registration server (RS). The RS sets up a twice
   network address translation table on a RAT device. The RAT device
   transparently routes datagrams destined for the mobile node's home
   address to the care-of address using twice network address
   translation. The protocol requirements for the mobile node is
   minimal. End-users can have mobility support easily as registration
   is done using widely available applications.









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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 network address
   translators (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.

1.1 IP Reachability Using Twice Network Address Translators

   When a mobile node moves from its home network to a foreign network,
   it acquires a care-of address from the foreign network indicating its
   current location on the Internet. It will use this topologically
   correct IP address as its source address when sending out packets.
   (When the mobile node is at home, the topologically correct IP
   address will be its home address).

   In a foreign network, the mobile node registers the acquired care-of
   address with a registration server (RS) at home. On successful
   registration, the RS sets up a twice network address translation
   entry on a RAT device.

   Once setup, the RAT device will transparently route datagrams
   destined for the mobile node's home address to its registered care-of
   address by twice NAT [refer to Section 6].

   Twice NAT is only required when the mobile node is not at home and
   the network connections are initiated by the correspondent nodes
   (CNs), e.g. when the mobile node is an application server. For all
   other conditions, the mobile node directly communicates with its
   correspondent nodes, without the overhead incurred from network
   address translation, i.e. the correspondent nodes respond to the
   mobile node using the latter's topologically correct address.

1.2 Goals

   The main motivation for RAT is to facilitate deployment of IP
   mobility support. Common network protocols are used to avoid any RAT



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   specific protocol requirements for the mobile node.  effort on the
   mobile node.

   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 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.

   With 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.

1.3 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.

1.4 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.

1.5 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, and accept
   datagrams from, the mobile node's foreign location.

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




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1.6 End Host Accessibility

   The correspondent node does not need to know how to reach the RAT
   device. It sends datagrams to the mobile node's home address, where
   they are intercepted by the RAT device as necessary.

   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.

   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.

2. Terminology

   This document adopts the terminology (e.g. Care-of address,
   Correspondent Node, Foreign Network, Home Network) defined in "IP
   Mobility Support" [REF 1] and "IP Network Address Translator
   Terminology and Considerations" [REF 3].

   In addition, three new entities are introduced :

   1. Zero Implementation Mobile Node (0MN)

   Zero Implementation Mobile Node (0MN) identifies a mobile host that
   uses 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



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   network before it can receive datagrams destined for its home
   address. For the registration process to succeed, the RS needs to be
   reachable from the mobile node's current location using the available
   routing mechanisms.

   The registration server will interact with a RAT device to set up the
   twice network address translation table (RAT table). Each entry in
   the table associates 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. In the simplest configuration, the RS address can be
   statically configured.

   3.  Twice Network Address Translator for Reachability (RAT device)

   The RAT device maintains the association between the home IP address
   of each 0MN, its care-of address and the binding's lifetime. It uses
   twice NAT to route datagrams from a 0MN's home address to its 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 necessary for a 0MN to know the identity of the RAT device
   for the protocol to work. For security as well as scalability, it is
   preferred that the RS and the RAT device are on different hosts and
   only the RS should know the RAT devices available on 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
   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.



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      Information disclosure could provide means of hijacking 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's home IP address need not be sent during
      the registration process if the registration server maintains a
      mobile user to home address association.

   2. A mechanism to confirm that 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 desires mobility support, it
   must acquire a topologically correct IP address in the network.  Any
   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



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   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 translation incurs a 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 dedicated 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 RAT Translation Binding (RTB) Request

   A registration server sends a RAT Translation Binding Request (RTBR)
   to a RAT device to set up the RAT 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.


   UDP fields:

      Source Port:       variable

      Destination Port:  434

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



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    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.

      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



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               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 RAT 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 RAT translation binding's lifetime expires, the registration
   server must send a RAT Translation Unbinding Request to the RAT
   device to remove the 0MN entry in the RAT 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





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




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



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      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.

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




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      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. Twice Network Address Translation in RAT

   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
   reponses must be routed via the same RAT device.

   Any datagram with a destination address that is a registered 0MN's
   home address in the RAT 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

   When a RAT device receives an ICMP Destination-Unreachable error
   message for datagrams destined to a care-of address in the RAT table,



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   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. Comparisons

   8.1 NAT and RAT comparison

   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 realms it connects.

   The purpose of 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.

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



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   which the RAT device services.

   8.2 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.

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




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   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 is not be involved. However, communication will
   fail in situations where the home IP address is necessary e.g.  where
   IP datagrams originating from the care-of address of a 0MN are
   blocked by a firewall, but not those originating from its home
   address.

Interoperation with Mobile IP

   The basic RAT protocol is meant to provide a fast and simple mobility
   solution. Its main advantage is any node can be mobile if desired as
   no RAT-specific protocol support is needed for a 0MN.  To push
   forward the deployment of a Mobile IP infrastruture, RAT is designed
   so that it can be incrementally enhanced to support a gradual
   installation of Mobile IP entities. This is possible mainly because
   RAT adopts the same format and semantics of Mobile IP registration
   messages.  The following section describes how mobility support can



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   migrate from RAT to Mobile IP.

   For the initial adoption of Mobile IP, home agents are deploy instead
   of RAT devices at every network segment. Since the home agents'
   operating platform and hardware do not affect the mobile user, any
   available home agent implementation can be selected. The end result
   is that, instead of one RAT device for every network segment, a home
   agent can intercept the 0MNs' traffic on behalf of the RAT device
   i.e. the RAT device is no longer bound to one home network segment.

   1. Migration Step 1

   To interoperate with home agents, the RAT device will need to add
   support for IP tunnel decapsulation. After accepting a registration
   request from a 0MN, the registration server will now additionally
   send a Mobile IP registration request to the home agent on behalf of
   the 0MN, with the RAT device specified as the tunnel endpoint. Mobile
   traffic delivery is now tunneled from the home agent to the RAT
   device and the RAT device will do address translation to deliver the
   traffic to the 0MN.

   2. Migration Step 2

   The next transition phase is to support IP decapsulation at the
   mobile node. The movement detection and registration process remain
   unchanged, but mobile traffic delivery will no longer require a RAT
   device. The registration server will send Mobile IP registration
   requests to the home agents as Step 1 but now the tunnel endpoint is
   not the RAT device but a mobile node. In this model, seamless
   mobility is now achievable.

   3. Migration Step 3

   The final step to full Mobile IP support will require the mobile node
   to support movement detection and perform registration as specified
   in Mobile IP. In this stage, the registration server is no longer
   required; however the benefits of a central entity for managing
   mobility policies may argue for retaining the registration server. In
   such a configuration, the registration server acts as a trusted
   registration proxy between the mobile node and its home agent. Mobile
   nodes need only know its registration server address. All home
   address to home agent address associations are maintained at the
   registration server, allowing transparent handover to alternative
   home agents in the event of a home agent failure or migration.

   For details on RAT to Mobile IP migration, see [REF 10].

10. Scalability Issues



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

   Furthermore, the RAT device must be able to determine the CNs and
   0MNs association for each IP datagram that it receives, in order to
   perform address translation.

   Address translation can be performed at two levels, and both require
   the RAT device to possess a pool of IP addresses. The scalability
   limitations of the protocol depend on which method is employed.

10.1 RAT with only Twice Network Address Translation

   The RAT device examines the source and destination IP addresses of
   each packet to determine the CN and 0MN association. This is simply
   referred as RAT in the previous sections.

   To eliminate ambiguity, the RAT device must associate a unique IP
   address from its pool of addresses for each CN that is currently
   communicating with a 0MN. This IP address is used by the RAT device
   to determine the end hosts' IP for datagrams between the RAT device
   and registered 0MNs.  The number of CNs that are able to connect to
   0MNs during any period of time is thus limited by the size of the RAT
   device's IP address pool.

   This approach has the advantage of being independent of the protocols
   above the network layer.

10.2 RAT with Twice Network Address and Port Translation

   The RAT device maps an IP datagram to its associated CN and 0MN by
   using three additional fields: the IP protocol type number and the
   transport layer source and destination connection identifiers (e.g.
   TCP port number or ICMP echo request/reply ID field). This is labeled
   as IP Reachability Using Twice Network Address and Port Translation
   (RAPT).

   RAPT does not have the same limitations on scalability as RAT, since
   the limit on the number of concurrent connections possible between
   CNs and 0MNs is much larger in RAPT than in RAT.

   However, RAPT scalability is limited by its inability to support
   protocols that do not employ a transport layer connection identifier.




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11. 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
   address translations are necessary.

12. Current Implementation

   A prototype implementation of RAT based on NAPT is now under
   development at http://cram.comp.nus.edu.sg:8080/cram/rat/. It
   supports TCP, UDP and ICMP. Application-level gateways are required
   for most protocols layered on top of these, including FTP, RTSP and
   HTTP.

   To differentiate CN-to-0MN from 0MN-to-CN datagrams, the source and
   destination IP addresses, port numbers and transport protocol of each
   IP datagram received at the RAT device are examined. The datagrams
   are then translated as illustrated in the following diagram before
   being forwarded by the RAT device:

     Translation of CN-to-0MN and 0MN-to-CN IP datagrams:

      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: 138.0.0.10:23    +--------------------------
     +-------------+ SA: 137.0.0.10:2000  |  +--------+
     |correspondent|----------------------|->| RAT |
     |     node    |<------------------------|device  |
     +-------------+ DA: 137.0.0.10:2000  |  +--------+
                     SA: 138.0.0.10:23    +---|---^------------------
                                              |   |
                                              |   |
                          DA: 139.0.0.10:23   |   |DA: 138.0.0.1:60000
                          SA: 138.0.0.1:60000 |   |SA: 139.0.0.10:23
                                              v   |
                                              +---+
                                              |0MN|
                                              +---+



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13. 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, National University of Singapore, and P.
   Srisuresh, Lucent Technologies, for their valuable help in reviewing
   this document.

   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

   [3] P. Srisuresh, M. Holdrege, "IP Network Address Translator (NAT)
       Terminology and Considerations",
       <draft-ietf-nat-terminology-01.txt> - work in progress, October
       1998

   [4] P. Srisuresh, K. Egevang, "Traditional IP Network Address
       Translator (Traditional NAT)",
       <draft-ietf-nat-traditional-01.txt> - work in progress, November



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       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-01.txt> - work in progress,
       November 1998

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

   [10] R. Singh, Y. C. Tay, W. T. Teo, S. W. Yeow, "RAT: A Quick (And
        Dirty?) Push for Mobility Support", Proceedings of IEEE Workshop
        on Mobile Computing Systems and Application, February 1999
        http://cram.comp.nus.edu.sg:8080/cram/rat/

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

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

   EMail: yeowshin@comp.nus.edu.sg

   R. Singh
   National University of Singapore
   School of Computing
   Lower Kent Ridge Cresent
   Singapore 119260

   EMail: rhandeev@comp.nus.edu.sg





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