Extern Objects in P4: an ROHC Header Compression Scheme Case Study

TL;DR

This paper investigates methods to extend P4 language capabilities by implementing an ROHC header compression scheme using extern objects, demonstrating their advantages for target-specific applications and code elegance.

Contribution

It introduces and compares two extension methods for P4, highlighting extern instances as a flexible and elegant solution for complex operations.

Findings

Extern instances are suitable for target-specific applications.

Both methods show similar performance in packet latency.

Externs simplify code and reduce modification risks.

Abstract

P4 is an emergent packet-processing language with which the user can describe how the packets are to be processed in a switching element. This paper presents a way to implement complex operations that are not natively supported in P4. In this work, we explored two different methods to add extensions to P4: i) using new native primitives and ii) using extern instances. As a case study, an ROHC entity was implemented and invoked in a P4 program. The tests showed similar relative performance in both methods in terms of normalized packet latency. However, extern instances appear to be more suitable for target-specific switching applications, where the manufacturer/vendor can specify its own specific operations without changes in the P4 syntax and semantics. Extern instances only require changes in the target-specific backend compiler while keeping the P4 frontend compiler unchanged. The use of externs also results in a more elegant code solution since they are implemented outside the switch-core, thus reducing side effects risks that can be caused by a modification in a switch pipeline implementation.