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The UDP Calculus: Rigorous Semantics for Real Networking

Peter Sewell University of Cambridge Computer Laboratory 4pm Tuesday 6 November Room 2511, JCMB, King's Buildings

Network programming is notoriously hard to understand: one has to deal with a variety of protocols (IP, ICMP, UDP, TCP etc), concurrency, packet loss, host failure, timeouts, the complex sockets interface to the protocols, and subtle portability issues. Moreover, the behavioural properties of operating systems and the network are not well documented.

A few of these issues have been addressed in the process calculus and distributed algorithm communities, but there remains a wide gulf between what has been captured in semantic models and what is required for a precise understanding of the behaviour of practical distributed programs that use these protocols.

In this work we demonstrate (in a preliminary way) that the gulf can be bridged. We present a semantic model for distributed programs that use the standard sockets interface; it covers message loss, host failure and temporary disconnection, and supports reasoning about distributed infrastructure. We consider interaction via the UDP and ICMP protocols.

To do this, it has been necessary to: construct an experimentally-validated post-hoc specification of the UDP/ICMP sockets interface; develop a timed operational semantics with threads, as such programs are typically multithreaded and depend on timeouts; model the behaviour of partial systems, making explicit the interactions that the infrastructure offers to applications; integrate the above with semantics for an executable fragment of a programming language (OCaml) with OS library primitives; and use tool support to manage complexity, mechanizing the model with the HOL theorem prover. We illustrate the whole with some simple network programs, including a module that provides naive heartbeat failure detection.

Joint work with Michael Norrish, Andrei Serjantov, and Keith Wansbrough. Papers describing the work are available here.
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