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The P4 Language Speciﬁcation

Version 1.0.3

November 2, 2016

The P4 Language Consortium

1 Introduction

P4 is a declarative language for expressing how packets are processed by the pipeline

of a network forwarding element such as a switch, NIC, router or network function ap-

pliance. It is based upon an abstract forwarding model consisting of a parser and a set

of match

+

action table resources, divided between ingress and egress. The parser iden-

tiﬁes the headers present in each incoming packet. Each match

+

action table performs

a lookup on a subset of header ﬁelds and applies the actions corresponding to the ﬁrst

match within each table. Figure 1 shows this model.

P4 itself is protocol independent but allows for the expression of forwarding plane pro-

tocols. A P4 program speciﬁes the following for each forwarding element.

• Header deﬁnitions: the format (the set of ﬁelds and their sizes) of each header

within a packet.

• Parse graph: the permitted header sequences within packets.

• Table deﬁnitions: the type of lookup to perform, the input ﬁelds to use, the actions

that may be applied, and the dimensions of each table.

• Action deﬁnitions: compound actions composed from a set of primitive actions.

• Pipeline layout and control ﬂow: the layout of tables within the pipeline and the

packet ﬂow through the pipeline.

P4 addresses the conﬁguration of a forwarding element. Once conﬁgured, tables may

be populated and packet processing takes place. These post-conﬁguration operations

are referred to as "run time" in this document. This does not preclude updating a for-

warding element’s conﬁguration while it is running.

1.1 The P4 Abstract Model

The following diagram shows a high level representation of the P4 abstract model.

The P4 machine operates with only a few simple rules.

© 2014-2016, The P4 Language Consortium

1.2 The mTag Example 1 INTRODUCTION

P4 focuses on the speciﬁcation of the parser, match

+

action tables and the control ﬂow

through the pipelines. Programmers control this by writing a P4 program which speci-

ﬁes the switch conﬁguration as shown at the top of Figure 1.

A machine that can run a P4 program is called target. Although a target may directly

execute a P4 program, it is assumed in this document that the program is compiled

into a suitable conﬁguration for the target.

In the current version, P4 does not expose, for example, the functionality of the Queu-

ing Mechanism and does not specify the semantics of the Egress Speciﬁcation beyond

what is mentioned above. Currently they are deﬁned in target speciﬁc input to the

compiler and exposed in conjunction with other interfaces that provide run time sys-

tem management and conﬁguration. Future versions of P4 may expose conﬁguration

of these mechanisms allowing consistent management of such resources from the P4

program.

1.2 The mTag Example

The original P4 paper [1] includes an example called mTag. We use this example through-

out this speciﬁcation as a means of explaining the basic language features as they are

presented. Complete source for this example, including sample run-time APIs, is avail-

able at the P4 web site [2].

We give an overview of the mTag example here. Quoting from the original paper:

Consider an example L2 network deployment with top-of-rack (ToR) switches

at the edge connected by a two-tier core. We will assume the number of

end-hosts is growing and the core L2 tables are overﬂowing. . . . P4 lets us

express a custom solution with minimal changes to the network architec-

ture. . . . The routes through the core are encoded by a 32-bit tag composed

of four single-byte ﬁelds. The 32-bit tag can carry a "source route".... Each

core switch need only examine one byte of the tag and switch on that infor-

mation. [1]

Two P4 programs are deﬁned for this example: One for edge switches (called "ToR"

above) and one for aggregation switches (called "core switches" above). These two pro-

grams share deﬁnitions for packet headers, the parser and actions.

1.3 P4 Abstractions

P4 provides the following abstractions. A P4 program consists of instances of each.

• Header type: A speciﬁcation of ﬁelds within a header.

• Header instance: A speciﬁc instance of a packet header or metadata.

3

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