/p4-guide

Guide to p4lang repositories and some other public info about P4

Primary LanguageP4

Introduction

This repository contains a variety of potentially useful information for those wanting to work with the P4 programming language.

Contents:

  • An overview of the p4lang organization Github repositories

    • includes a small shell script that installs both p4c and behavioral-model projects and their dependencies on an Ubuntu 16.04 Linux machine.
    • A figure with the dependencies between these repositories.

  • A collection of small demo P4 programs, some of them with equivalent versions written in both P4_14 and P4_16 versions of the language.

    • includes a heavily commented P4_16 program, which by reading carefully one may learn some things about the P4_16 language.
    • Each of the demo directories includes its own README.md file with instructions for compiling the program and running it with the simple_switch emulator from the behavior_model repository, including interactively adding table entries to the tables, and send packets using Scapy that will be processed by the emulator.

  • Instructions for setting up several text editing programs for color highlighting P4 programs, and quickly finding definitions for named things like control blocks and tables.

  • 'Cheat sheet' of where P4_16 language constructs are allowed to be used within a program.

  • Some advantages of P4_16 over P4_14:

    • You can write assignments that look like C/C++/Java, rather than modify_field(dst, src); all over the place, and you can have arithmetic expressions on the right-hand side instead of add_to_field/subtract_from_field/etc. This is not additional power in the language, but it is a nice convenience for those familiar with those other languages.
    • Controls could call other controls in P4_14, but there were no parameters or return values. All side effects had to be done via access to global variables. In P4_16, there are no global variables -- you may pass parameters with directionality in, out, inout.
    • Tables must be, and externs may be, defined within the scope of a control, and are then accessible only from that control, which can be useful for keeping the code that accesses those objects closer to it, and knowing where they can be accessed from. Extern are used for things like counters, meters, and registers that were part of the base P4_14 language, but in P4_16 are defined as extern add-ons in the Portable Switch Architecture specification).

  • Disadvantages of P4_16 vs P4_14:

    • Tool and vendor support it not as good for P4_16 as of Jan 2018, but this is gradually changing.