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.
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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.
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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addresses that are no longer valid, because of either transient or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.
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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
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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.
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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.
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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
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