P416 Language Specification
version 1.2.2
The P4 Language Consortium

Abstract. P4 is a language for programming the data plane of network devices. This document provides a precise definition of the P416 language, which is the 2016 revision of the P4 language (http://​p4.​org). The target audience for this document includes developers who want to write compilers, simulators, IDEs, and debuggers for P4 programs. This document may also be of interest to P4 programmers who are interested in understanding the syntax and semantics of the language at a deeper level.

1. Scope

This specification document defines the structure and interpretation of programs in the P416 language. It defines the syntax, semantic rules, and requirements for conformant implementations of the language.

It does not define:

It is understood that some implementations may be unable to implement the behavior defined here in all cases, or may provide options to eliminate some safety guarantees in exchange for better performance or handling larger programs. They should document where they deviate from this specification.

2. Terms, definitions, and symbols

Throughout this document, the following terms will be used:

All terms defined explicitly in this document should not be understood to refer implicitly to similar terms defined elsewhere. Conversely, any terms not defined explicitly in this document should be interpreted according to generally recognizable sourcese.g., IETF RFCs.

3. Overview


Figure 1. Traditional switches vs. programmable switches.

P4 is a language for expressing how packets are processed by the data plane of a programmable forwarding element such as a hardware or software switch, network interface card, router, or network appliance. The name P4 comes from the original paper that introduced the language, “Programming Protocol-independent Packet Processors,” https://​arxiv.​org/​pdf/​1312.​1719.​pdf. While P4 was initially designed for programming switches, its scope has been broadened to cover a large variety of devices. In the rest of this document we use the generic term target for all such devices.

Many targets implement both a control plane and a data plane. P4 is designed to specify only the data plane functionality of the target. P4 programs also partially define the interface by which the control plane and the data-plane communicate, but P4 cannot be used to describe the control-plane functionality of the target. In the rest of this document, when we talk about P4 as “programming a target”, we mean “programming the data plane of a target”.

As a concrete example of a target, Figure 1 illustrates the difference between a traditional fixed-function switch and a P4-programmable switch. In a traditional switch the manufacturer defines the data-plane functionality. The control-plane controls the data plane by managing entries in tables (e.g. routing tables), configuring specialized objects (e.g. meters), and by processing control-packets (e.g. routing protocol packets) or asynchronous events, such as link state changes or learning notifications.

A P4-programmable switch differs from a traditional switch in two essential ways:

Hence, P4 can be said to be protocol independent, but it enables programmers to express a rich set of protocols and other data plane behaviors.


Figure 2. Programming a target with P4.

The core abstractions provided by the P4 language are:

Figure 2 shows a typical tool workflow when programming a target using P4.

Target manufacturers provide the hardware or software implementation framework, an architecture definition, and a P4 compiler for that target. P4 programmers write programs for a specific architecture, which defines a set of P4-programmable components on the target as well as their external data plane interfaces.

Compiling a set of P4 programs produces two artifacts:

P4 is a domain-specific language that is designed to be implementable on a large variety of targets including programmable network interface cards, FPGAs, software switches, and hardware ASICs. As such, the language is restricted to constructs that can be efficiently implemented on all of these platforms.

Assuming a fixed cost for table lookup operations and interactions with extern objects, all P4 programs (i.e., parsers and controls) execute a constant number of operations for each byte of an input packet received and analyzed. Although parsers may contain loops, provided some header is extracted on each cycle, the packet itself provides a bound on the total execution of the parser. In other words, under these assumptions, the computational complexity of a P4 program is linear in the total size of all headers, and never depends on the size of the state accumulated while processing data (e.g., the number of flows, or the total number of packets processed). These guarantees are necessary (but not sufficient) for enabling fast packet processing across a variety of targets.

P4 conformance of a target is defined as follows: if a specific target T supports only a subset of the P4 programming language, say P4T, programs written in P4T executed on the target should provide the exact same behavior as is described in this document. Note that P4 conformant targets can provide arbitrary P4 language extensions and extern elements.

3.1. Benefits of P4

Compared to state-of-the-art packet-processing systems (e.g., based on writing microcode on top of custom hardware), P4 provides a number of significant advantages:

3.2. P4 language evolution: comparison to previous versions (P4 v1.0/v1.1)


Figure 3. Evolution of the language between versions P414 (versions 1.0 and 1.1) and P416.

Compared to P414, the earlier version of the language, P416 makes a number of significant, backwards-incompatible changes to the syntax and semantics of the language. The evolution from the previous version (P414) to the current one (P416) is depicted in Figure 3. In particular, a large number of language features have been eliminated from the language and moved into libraries including counters, checksum units, meters, etc.

Hence, the language has been transformed from a complex language (more than 70 keywords) into a relatively small core language (less than 40 keywords, shown in Section B) accompanied by a library of fundamental constructs that are needed for writing most P4.

The v1.1 version of P4 introduced a language construct called extern that can be used to describe library elements. Many constructs defined in the v1.1 language specification will thus be transformed into such library elements (including constructs that have been eliminated from the language, such as counters and meters). Some of these extern objects are expected to be standardized, and they will be in the scope of a future document describing a standard library of P4 elements. In this document we provide several examples of extern constructs. P416 also introduces and repurposes some v1.1 language constructs for describing the programmable parts of an architecture. These language constructs are: parser, state, control, and package.

One important goal of the P416 language revision is to provide a stable language definition. In other words, we strive to ensure that all programs written in P416 will remain syntactically correct and behave identically when treated as programs for future versions of the language. Moreover, if some future version of the language requires breaking backwards compatibility, we will seek to provide an easy path for migrating P416 programs to the new version.

4. Architecture Model


Figure 4. P4 program interfaces.

The P4 architecture identifies the P4-programmable blocks (e.g., parser, ingress control flow, egress control flow, deparser, etc.) and their data plane interfaces.

The P4 architecture can be thought of as a contract between the program and the target. Each manufacturer must therefore provide both a P4 compiler as well as an accompanying architecture definition for their target. (We expect that P4 compilers can share a common front-end that handles all architectures). The architecture definition does not have to expose the entire programmable surface of the data planea manufacturer may even choose to provide multiple definitions for the same hardware device, each with different capabilities (e.g., with or without multicast support).

Figure 4 illustrates the data plane interfaces between P4-programmable blocks. It shows a target that has two programmable blocks (#1 and #2). Each block is programmed through a separate fragment of P4 code. The target interfaces with the P4 program through a set of control registers or signals. Input controls provide information to P4 programs (e.g., the input port that a packet was received from), while output controls can be written to by P4 programs to influence the target behavior (e.g., the output port where a packet has to be directed). Control registers/signals are represented in P4 as intrinsic metadata. P4 programs can also store and manipulate data pertaining to each packet as user-defined metadata.

The behavior of a P4 program can be fully described in terms of transformations that map vectors of bits to vectors of bits. To actually process a packet, the architecture model interprets the bits that the P4 program writes to intrinsic metadata. For example, to cause a packet to be forwarded on a specific output port, a P4 program may need to write the index of an output port into a dedicated control register. Similarly, to cause a packet to be dropped, a P4 program may need to set a “drop” bit into another dedicated control register. Note that the details of how intrinsic metadata are interpreted is architecture-specific.


Figure 5. P4 program invoking the services of a fixed-function object.

P4 programs can invoke services implemented by extern objects and functions provided by the architecture. Figure 5 depicts a P4 program invoking the services of a built-in checksum computation unit on a target. The implementation of the checksum unit is not specified in P4, but its interface is. In general, the interface for an extern object describes each operation it provides, as well as their parameter and return types.

In general, P4 programs are not expected to be portable across different architectures. For example, executing a P4 program that broadcasts packets by writing into a custom control register will not function correctly on a target that does not have the control register. However, P4 programs written for a given architecture should be portable across all targets that faithfully implement the corresponding model, provided there are sufficient resources.

4.1. Standard architectures

We expect that the P4 community will evolve a small set of standard architecture models pertaining to specific verticals. Wide adoption of such standard architectures will promote portability of P4 programs across different targets. However, defining these standard architectures is outside of the scope of this document.

4.2. Data plane interfaces

To describe a functional block that can be programmed in P4, the architecture includes a type declaration that specifies the interfaces between the block and the other components in the architecture. For example, the architecture might contain a declaration such as the following:

control MatchActionPipe<H>(in bit<4> inputPort,
                           inout H parsedHeaders,
                           out bit<4> outputPort);

This type declaration describes a block named MatchActionPipe that can be programmed using a data-dependent sequence of match-action unit invocations and other imperative constructs (indicated by the control keyword). The interface between the MatchActionPipe block and the other components of the architecture can be read off from this declaration:

4.3. Extern objects and functions

P4 programs can also interact with objects and functions provided by the architecture. Such objects are described using the extern construct, which describes the interfaces that such objects expose to the data-plane.

An extern object describes a set of methods that are implemented by an object, but not the implementation of these methods (i.e., it is similar to an abstract class in an object-oriented language). For example, the following construct could be used to describe the operations offered by an incremental checksum unit:

extern Checksum16 {
    Checksum16();              // constructor
    void clear();              // prepare unit for computation
    void update<T>(in T data); // add data to checksum
    void remove<T>(in T data); // remove data from existing checksum
    bit<16> get(); // get the checksum for the data added since last clear

5. Example: A very simple switch

As an example to illustrate the features of architectures, consider implementing a very simple switch in P4. We will first describe the architecture of the switch and then write a complete P4 program that specifies the data plane behavior of the switch. This example demonstrates many important features of the P4 programming language.