Mobile IP Working Group Charles Perkins INTERNET DRAFT Sun Microsystems 20 November 1997 David B. Johnson Carnegie Mellon University Special Tunnels for Mobile IP draft-ietf-mobileip-spectun-00.txt Status of This Memo This document is a submission by the Mobile IP 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), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document defines actions taken by Mobile IP home agents and foreign agents to try to avoid loss of datagrams sent to an incorrect care-of address, even if a foreign agent has no binding for the mobile node. Perkins and Johnson Expires 20 May 1998 [Page i] Internet Draft Special Tunnels 20 November 1997 1. Introduction The base Mobile IP protocol [3], allows any mobile node to move about, changing its point of attachment to the Internet, while continuing to be identified by its home IP address. An important part of Mobile IP's operation is the maintenance of bindings (care-of address information) associated with the mobile node by the home agent. Route optimization [2] enables other IP nodes (correspondent nodes) to maintain bindings. When bindings indicate care-of addresses that are no longer valid, because of either transient or longer term effects, a foreign agent at a care-of address can receive packets tunneled to a mobile node that is no longer registered with that foreign agent, and for which no additional forwarding information is available. In these circumstances, the base Mobile IP protocol indicates that the tunneled datagrams should be dropped. However, dropping such packets often necessitates retransmissions by higher level protocols, and such retransmissions cause significant performance degradation [1]. With care, it is possible to allow foreign agents to return such decapsulated datagrams to the home agent in an attempt to retry delivery to a more recent care-of address. The means for doing so are specified in this document. Suppose a foreign agent receives a tunneled datagram, but it doesn't have a visitor list entry for the mobile node. Moreover, suppose the foreign agent has no binding cache entry for the destination mobile node. To attempt delivery of the datagram in this case, the node must encapsulate the datagram as a special tunnel datagram (see Section 3), destined to the mobile node. Using a special tunnel allows the home agent to avoid a possible routing loop when a foreign agent has forgotten that it is serving the mobile node, perhaps because the foreign agent has crashed and lost its visitor list state. The special tunnel allows the home agent to see the address of the node that tunneled the datagram, and to avoid tunneling the datagram back to the same node. 2. Terminology This document uses the following terminology, in addition to that used to describe the base Mobile IP protocol: Binding cache A cache of mobility bindings of mobile nodes, maintained by a node for use in tunneling datagrams to those mobile nodes. Perkins and Johnson Expires 20 May 1998 [Page 1] Internet Draft Special Tunnels 20 November 1997 Registration Lifetime The registration lifetime is the time duration for which a binding is valid. The remaining registration lifetime means the amount of time remaining for which a registration lifetime is still valid, at some time after the registration was approved by the home agent. Remaining Registration Lifetime The remaining registration lifetime is the amount of time remaining for which a registration lifetime is still valid, at some time after the registration was approved by the home agent. Special tunnel A method of tunneling a datagram in which the outer destination address when encapsulating the datagram is set equal to the inner destination address (the original destination address of the datagram). A special tunnel is used in Route Optimization for returning a datagram, addressed to a mobile node, to the mobile node's home agent without knowing the home agent's address. 3. Using Special Tunnels Whenever any node receives a tunneled datagram for which it has no visitor list entry for the datagram's destination, the node is not serving the mobile node as a foreign agent, and thus the care-of address used by the tunnel originator is surely incorrect. Thus, the tunneling node has an out-of-date binding cache entry for the destination mobile node. If the node receiving the tunneled datagram has a binding cache entry for the destination, it should re-tunnel the datagram to the care-of address indicated in its binding cache entry. If a foreign agent receiving the tunneled datagram has no binding cache entry for the destination, it cannot re-tunnel the node to its destination. Instead, the foreign agent should forward the datagram to the destination mobile node's home agent, using the special form of tunneling, specified here, called a special tunnel. To tunnel the datagram using a special tunnel, the tunneled datagram's destination address is set equal to the destination address in the tunneled datagram. Thus, both the inner and outer destination addresses are set to the home address of the mobile node. The tunneled datagram will be routed to the mobile node's home network, and then Perkins and Johnson Expires 20 May 1998 [Page 2] Internet Draft Special Tunnels 20 November 1997 intercepted by the mobile node's home agent in the same way as other datagrams addressed to the mobile node. 3.1. Home Agent Handling of Special Tunnels The home agent should then tunnel the datagram to the current care-of address for the mobile node. However, the home agent may not tunnel the datagram to the current care-of address if the special tunnel of the datagram originated at that care-of address, as indicated by the outer source address of the special tunnel. The use of the special tunnel format allows the home agent to identify the node that tunneled the datagram to it (as well as the original sender of the datagram). If the home agent believes that the current care-of address for the mobile node is the same as the source of the special tunnel, then the home agent SHOULD discard the datagram. When that happens, the foreign agent serving the mobile node appears to have lost its entry for the mobile node in its visitor list. For example, the foreign agent may have crashed and rebooted. Otherwise, after tunneling the datagram to the current care-of address for the mobile node, the home agent should notify the source of the special tunnel of the mobile node's current binding, by sending it a Binding Update message. The home agent should also send a Binding Update message [2] to the sender of the original datagram (the inner source address of the tunneled datagram), if it shares a mobility security association with this node. 3.2. Foreign Agents and Special Tunnels When a foreign agent is the endpoint of a tunneled datagram, it examines its visitor list for an entry for the destination mobile node, as in the base Mobile IP protocol. If no visitor list entry is found, the foreign agent examines its binding cache for a cache entry for the destination mobile node. If one is found, the foreign agent re-tunnels the new care-of address indicated in the binding cache entry. In this case, the foreign agent also may infer that the sender of the datagram has an out-of-date binding cache entry for this mobile node, since it otherwise would have tunneled the datagram directly to the correct new care-of address itself. The foreign agent should then send a Binding Warning message to the mobile node's home agent. The foreign agent probably learned the address of the home agent in the Registration Reply message for the mobile node, or a later Binding Update message from which the binding cache entry was created. If the home agent is not known, the mobile node's home address MAY be used. Perkins and Johnson Expires 20 May 1998 [Page 3] Internet Draft Special Tunnels 20 November 1997 If a foreign agent receives a tunneled datagram for a mobile node for which it has no visitor list entry or binding cache entry, the foreign agent should forward the datagram to the mobile node's home agent by sending it as a special tunnel. The home agent will intercept the special tunnel datagram addressed to the mobile node in the same way as any datagram for the mobile node while it is away from home. References [1] Ramon Caceres and Liviu Iftode. Improving the Performance of Reliable Transport Protocols in Mobile Computing Environments. IEEE Journal on Selected Areas in Communications, 13(5):850--857, June 1995. [2] Charles E. Perkins and David B. Johnson. Route Optimization in Mobile-IP. draft-ietf-mobileip-optim-06.txt, July 1997. (work in progress). [3] C. Perkins, Editor. IP Mobility Support. RFC 2002, October 1996. Perkins and Johnson Expires 20 May 1998 [Page 4] Internet Draft Special Tunnels 20 November 1997 Chairs' Addresses The working group can be contacted via the current chairs: Jim Solomon Erik Nordmark Motorola, Inc. Sun Microsystems, Inc. 1301 E. Algonquin Road 901 San Antonio Road Schaumburg, IL 60196 Palo Alto, California 94303 USA USA Phone: +1-847-576-2753 Phone: +1 650 786-5166 Fax: Fax: +1 650 786-5896 E-mail: solomon@comm.mot.com E-mail: nordmark@sun.com Authors' Addresses Questions about this document can also be directed to the authors: Charles E. Perkins David B. Johnson Technology Development Group Computer Science Department Mail Stop MPK15-214 Room 2682 Sun Microsystems, Inc. Carnegie Mellon University 901 San Antonio Road 5000 Forbes Avenue Palo Alto, California 94303 Pittsburgh, PA 15213-3891 USA USA Phone: +1-650-786-6464 Phone: +1-412-268-7399 Fax: +1-650-786-6445 Fax: +1-412-268-5576 E-mail: charles.perkins@Sun.COM E-mail: dbj@cs.cmu.edu Perkins and Johnson Expires 20 May 1998 [Page 5]