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Abstracts of Selected Ph.D. Theses in the Area of Mobile Computing Awarded in 1994

Protocols and Caching Strategies in Support of Internetwork Mobility

Mitchell Paul Tasman

University of Wisconsin - Madison, USA

The full dissertation is available here
This thesis explores the provision of End System (ES) mobility on large, datagram-based, internetworks.

We describe a new type of network layer address, the hybrid address, which contains a Partial Destination Identifier, a Unique ID, and a Location Sequence number. The Partial Destination Identifier, which changes as a mobile ES re-locates between areas of an internetwork, allows for efficient routing, while the Unique ID is used for ES identification at the lowest layers of a routing hierarchy. Finally, the Location Sequence Number, which a mobile ES increments as it relocates between areas, is used to compare the age of multiple addresses for a given mobile ES.

We present a mobility design that incorporates the hybrid address, and is based on the ISO OSI connectionless routing architecture and protocols. Each mobile ES has a home address and a current address. The home address is stored in a global database, and the ES's home area keeps track of the current address. After a mobile ES relocates to a new area, it sends Reconnect messages to the home and previous areas, each of which caches a forwarding pointer for the mobile ES. ESs that communicate with a mobile ES send datagrams directly to the mobile ES. To accomplish this, each ES maintains cache entries for its mobile correspondents. When a datagram containing an out-of-date address is forwarded by an area, a Rewrite message is sent to the source ES, which updates its cache entry for the destination. An ES also updates its cache based on the source addresses of incoming datagrams.

The mobility algorithm has been implemented and tested in a simulation environment, and performs quite well. We also present the results of a study on strategies for caching mobile ES forwarding pointers at Intermediate Systems in the interior of an internetwork, based on the type and contents of transit control messages. Caching at interior Intermediate Systems based on Reconnect messages yields the greatest benefit, for both tree-shaped and general topology internetworks, while caching based on transit Rewrite messages is not recommended.

The ACM Special Interest Group on Mobility of Systems, Users, Data and Computing