A Theory of Composing Protocols
Laura Bocchi, Dominic A. Orchard, A. Laura Voinea · The Art Science and Engineering of Programming · 2022
In programming, protocols are everywhere.Protocols describe the pattern of interaction (or communication) between software systems, for example, between a user-space program and the kernel or between a local application and an online service.Ensuring conformance to protocols avoids a significant class of software errors.Subsequently, there has been a lot of work on verifying code against formal protocol specifications.The pervading approaches focus on distributed settings involving parallel composition of processes within a single monolithic protocol description.However we observe that, at the level of a single thread/process, modern software must often implement a number of clearly delineated protocols at the same time which become dependent on each other, e.g., a banking API and one or more authentication protocols.Rather than plugging together modular protocol-following components, the code must re-integrate multiple protocols into a single component.We address this concern of combining protocols via a novel notion of 'interleaving' composition for protocols described via a process algebra.User-specified, domain-specific constraints can be inserted into the individual protocols to serve as 'contact points' to guide this composition procedure, which outputs a single combined protocol that can be programmed against.Our approach allows an engineer to then program against a number of protocols that have been composed (re-integrated), reflecting the true nature of applications that must handle multiple protocols at once.We prove various desirable properties of the composition, including behaviour preservation: that the composed protocol implements the behaviour of both component protocols.We demonstrate our approach in the practical setting of Erlang, with a tool implementing protocol composition that both generates Erlang code from a protocol and generates a protocol from Erlang code.This tool shows that, for a range of sample protocols (including real-world examples), a modest set of constraints can be inserted to produce a small number of candidate compositions to choose from.As we increasingly build software interacting with many programs and subsystems, this new perspective gives a foundation for improving software quality via protocol conformance in a multi-protocol setting.