| The usual way of installing a new version of a software system is to shut down the running program and then install the new version. This necessitates a sometimes unacceptable delay during which service is denied to the users of the software. An on-line software replacement system replaces parts of the software while it is in execution, thus eliminating the shutdown. While a number of implementations of on-line version change systems have been described in the literature, little investigation has been done on its theoretical aspects. In this paper, we describe a formal framework for studying on-line software version change. We give a general de nition of validity of an on-line change, show that it is in general undecidable and then develop su cient conditions for ensuring validity for a procedural language.

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Abstract-In the Internet, with many competing networks each trying to optimise its own bandwidth, a stub network has limited knowledge about user demands, available network resources and routing policies of other networks. This uncertainty makes the task of interdomain traffic engineering for a stub network very challenging. The basic aim of a stub network connected to multiple ISPs (multihomed) is to load balance its traffic among its various edge links. Our goal in this work is to distribute the incoming traffic of a multihomed stub network among its various edge links. The focus is on networks that primarily download traffic from the Internet. Regulating the incoming traffic is difficult since it will require to influence the behaviour of the remote destinations. We performed a systematic analysis of our problem and showed that even a restricted instance of the problem is NPcomplete. We proposed simple, low-cost route control techniques to load balance traffic by reallocating the routes of outgoing traffic. The techniques were validated using synthetic as well as actual data collected under numerous traffic load conditions. Results show that we can achieve significant improvement in load balancing with minimum traffic re-assignments. Moreover, the proposed techniques neither require any third party assistance nor changes to existing protocols and network setup. This makes our schemes easily deployable in real networks.

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