Internet Engineering Task Force W. T. Teo INTERNET DRAFT S. W. Yeow National University of Singapore August 1998 Reverse Network Address Translators (RAT) 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 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 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 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 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 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 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 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 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 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 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: Destination Port: Copied from the source port of the corresponding RTB or RTU Request Teo and Yeow Expires Februrary 1999 [Page 11] Internet Draft 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 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 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 . 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 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 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 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 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 August 1998 [3] P. Srisuresh, M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", - work in progress, July 1998 [4] P. Srisuresh, K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", - 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", - 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]