P4Runtime Specification
version 1.1.0
The P4.org API Working Group
2020-02-03

Abstract. P4 is a language for programming the data plane of network devices. The P4Runtime API is a control plane specification for controlling the data plane elements of a device defined or described by a P4 program. This document provides a precise definition of the P4Runtime API. The target audience for this document includes developers who want to write controller applications for P4 devices or switches.

1. Introduction and Scope

This document is published by the P4.org API Working Group, which was chartered [16] to design and standardize vendor-independent, protocol-independent runtime APIs for P4-defined or P4-described data planes. This document specifies one such API, called P4Runtime. It is meant to disambiguate and augment the programmatic API definition expressed in Protobuf format and available at https://github.com/p4lang/p4runtime/tree/v1.1.0/proto.

1.1. P4 Language Version Applicability

P4Runtime is designed to be implemented in conjunction with the P416 language version or later. P414 programs should be translated into P416 to be made compatible with P4Runtime. This version of P4Runtime utilizes features which are not in P416 1.0, but were introduced in P416 1.1.0 [28]. For this version of P4Runtime, we recommend using P416 1.2.0 [1].

1.2. In Scope

This specification document defines the semantics of P4Runtime messages, whose syntax is defined in Protobuf format. The following are in scope of P4Runtime:

The following are in the scope of this specification document:

1.3. Not In Scope

The following are not in scope of P4Runtime:

The following are not in scope of this specification document:

2. Terms and Definitions

arbitration
Refers to the process through which P4Runtime ensures that at any given time, there is a single master (i.e. a client with write access) for a given role. Also referred to as “master-slave arbitration”.
client
The gRPC client is the software entity which controls the P4 target or device by communicating with the gRPC agent or server. The client may be local (within the device) or remote (for example, an SDN controller).
COS
Class of Service.
device
Synonymous with target, although device usually connotes a physical appliance or other hardware, whereas target can signify hardware or software.
entity
An instantiated P4 program object such as a table or an extern (from PSA or any other architecture).
gRPC
gRPC Remote Procedure Calls, an open-source client-server RPC framework. See [9].
HA
High-Availability. Refers to a redundancy architecture.
Instrumentation
The part of the P4Runtime server which implements the calls to the device or target native “SDK” or backend.
IPC
Inter-Process Communication.
P4 Blob
A more colloquial term for P4 Device Config (Blob = Binary Large Object).
P4 Device Config
The output of the P4 compiler backend, which is included in the Forwarding Pipeline Config. This is opaque, architecture- and target-specific binary data which can be loaded onto the device to change its “program.”
P4Info
Metadata which specifies the P4 entities which can be accessed via P4Runtime. These entities have a one-for-one correspondence with instantiated objects in the P4 source code.
P4RT
Abbreviation for P4Runtime.
Protobuf (Protocol Buffers)
The wire serialization format for P4Runtime. Protobuf version 3 (proto3) is used to define the P4Runtime interface. See [20].
PSA
Portable Switch Architecture [18]; a target architecture that describes common capabilities of network switch devices that process and forward packets across multiple interface ports.
RPC
Remote Procedure Call.
RTT
Round-trip time.
SDN
Software-Defined Networking, an approach to networking that advocates the separation of the control and forwarding planes, as well as the abstraction of the networking infrastructure, in order to promote programmability of the network control. SDN is often associated with OpenFlow, a communications protocol that enables remote control of the network infrastructure through a programmable, centralized network controller.
SDN port
A 32-bit port number defined by a remote Software-Defined Network (SDN) controller. The SDN port number maps to a unique device port id, which may be in a different number space.
server
The gRPC server which accepts P4Runtime requests on the device or target. It uses instrumentation to translate P4Runtime API calls into target-specific actions.
stream
Refers to a gRPC Stream, which is a RPC on which several messages can be sent and received. P4Runtime defines one Stream RPC (StreamChannel), which is a bidirectional stream (both the client and the server can send messages) which is used for packet I/O and master-slave arbitration, among other things.
switch config
Refers to the non-forwarding config (different from the P4 Forwarding Pipeline Config) that is delivered to the switch via a different interface. For example, the switch config may be captured using OpenConfig models and delivered through a gNMI interface.
target
The hardware or software entity which “executes” the P4 pipeline and hosts the P4Runtime Service; often used interchangeably with “device”.
URI
Uniform Resource Identifier; a string of characters designed for unambiguous identification of resources.

3. Reference Architecture

Figure 1 represents the P4Runtime Reference Architecture. The device or target to be controlled is at the bottom, and one or more controllers is shown at the top. A multi-master protocol allows more than one controller to participate, and a role-based arbitration scheme ensures only one controller has write access to each read/write entity, or the pipeline config itself. Any controller may perform read access to any entity or the pipeline config. Later sections describe this in detail. For the sake of brevity, the term controller may refer to one or more controllers.

The P4Runtime API defines the messages and semantics of the interface between the client(s) and the server. The API is specified by the p4runtime.proto Protobuf file, which is available on GitHub as part of the standard [14]. It may be compiled via protoc — the Protobuf compiler — to produce both client and server implementation stubs in a variety of languages. It is the responsibility of target implementers to instrument the server.

Reference implementations of P4 targets supporting P4Runtime, as well as sample clients, may be available on the p4lang/PI GitHub repository [15]. A future goal may be to produce a reference gRPC server which can be instrumented in a generic way, e.g. via callbacks, thus reducing the burden of implementing P4Runtime.

The controller can access the P4 entities which are declared in the P4Info metadata. The P4Info structure is defined by p4info.proto, another Protobuf file available as part of the standard.

The controller can also set the ForwardingPipelineConfig, which amounts to installing and running the compiled P4 program output, which is included in the p4_device_config Protobuf message field, and installing the associated P4Info metadata. Furthermore, the controller can query the target for the ForwardingPipelineConfig to retrieve the device config and the P4Info.

reference-architecture

Figure 1. P4Runtime Reference Architecture.

3.1. Idealized Workflow

In the idealized workflow, a P4 source program is compiled to produce both a P4 device config and P4Info metadata. These comprise the ForwardingPipelineConfig message. A P4Runtime controller chooses a configuration appropriate to a particular target and installs it via a SetForwardingPipelineConfig RPC. Metadata in the P4Info describes both the overall program itself (PkgInfo) as well as all entity instances derived from the P4 program — tables and extern instances. Each entity instance has an associated numeric ID assigned by the P4 compiler which serves as a concise “handle” used in API calls.

In this workflow, P4 compiler backends are developed for each unique type of target and produce P4Info and a target-specific device config. The P4Info schema is designed to be target and architecture-independent, although the specific contents are likely to be architecture-dependent. The compiler ensures the code is compatible with the specific target and rejects code which is incompatible.

In some use cases, it is expected that a controller will store a collection of multiple P4 “packages”, where each package consists of the P4 device config and P4Info, and install them at will onto the target. A controller can also query the ForwardingPipelineConfig from the target via the GetForwardingPipelineRequest RPC. This can be useful to obtain the pipeline configuration from a running device to synchronize the controller to its current state.

3.2. P4 as a Behavioral Description Language

P4 can be considered a behavioral description of a switching device which may or may not execute “P4” natively. There is no requirement that a P4 compiler be used in the production of either the P4 device config or the P4Info. There is no absolute requirement that the target accept a SetForwardingPipelineRequest to change its pipeline “program”, as some devices may be fixed in function, or configured via means other than P4 programs. Furthermore, a controller can run without a P4 source program, since the P4Info file provides all of the information necessary to describe the P4Runtime API messages needed to configure such a device.

While a P4 program does provide a precise description of the data plane behavior, and this can prove invaluable in writing correct control plane software, in some cases it is enough for a control plane software developer to have the control plane API, plus good documentation of the data plane behavior. Some device vendors may wish to keep their P4 source code private. The minimum requirement for the controller and device to communicate properly is a P4Info file that can be loaded by a controller in order to render the correct P4Runtime API.

In such scenarios, it is crucial to have detailed documentation, perhaps included in the P4Info file itself, specifically the metadata in the PkgInfo message as well as the embedded doc fields. Nevertheless, a P4 program which describes the pipeline is ideally available. The contents of the P4Info file will be described in later sections.

3.3. Alternative Workflows

Given the notions above concerning P4 code as behavioral description and P4Info as API metadata, some other workflows are possible. The scenarios below are just examples and actual situations may vary.

3.3.1. P4 Source Available, Compiled into P4Info but not Compiled into P4 Device Config

In this situation, P4 source code is available mainly as a behavioral model and compiled to produce P4Info, but it is not compiled to produce the p4_device_config. The device's configuration might be derived via some other means to implement the P4 source code's intentions. The P4 code, if available, can be studied to understand the pipeline, and the P4Info can be used to implement the control plane.

3.3.2. No P4 Source Available, P4Info Available

In this situation, P4Info is available but no P4 source is available for any number of reasons, the most likely of which are:

  1. The vendor or organization does not wish to divulge the P4 source code, to protect intellectual property or maintain security.

  2. The target was not implemented using P4 code to begin with, although it still obeys the control plane API specified in the P4Info.

As discussed in Section 3.2, in the absence of a P4 program describing the data plane behavior, the detailed knowledge required to write correct control plane code must come from other sources, e.g. documentation.

3.3.3. Partial P4Info and P4 Source are Available

In this situation, a subset of the target's pipeline configuration is exposed as P4 source code and P4Info. The complete device behavior might be expressed as a larger P4 program and P4Info, but these are not exposed to everybody. This limits API access to only certain functions and behaviors. The hidden functions and APIs might be available to select users who would have access to the complete P4Info and possibly P4 source code.

3.3.4. P4Info Role-Based Subsets

In this situation, P4Info is selectively packaged into role-based subsets to allow some controllers access to just the functionality required. For example, a controller may only need read access to statistics counters and nothing more.

3.4. P4Runtime State Across Restarts

All targets support full restarts, where all forwarding state is reset and the P4Runtime server starts with a clean state. Some targets may also support In-Service Software Upgrade (ISSU), where the software on the target can be restarted while traffic is being forwarded. In this case, the P4Runtime server may have the ability to access information from memory before the upgrade.

4. Controller Use-cases

P4Runtime allows for more than one controller. The mechanisms and semantics are described in a later section. Here we present a number of use-cases. Each use-case highlights a particular aspect of P4Runtime's flexibility and is not intended to be exhaustive. Real-world use-cases may combine various techniques and be more complex.

4.1. Single Embedded Controller

Figure 2 shows perhaps the simplest use-case. A device or target has an embedded controller which communicates to an on-board switch via P4Runtime. This might be appropriate for an embedded appliance which is not intended for SDN use-cases.

P4Runtime was designed to be a viable embedded API. Complex controller architectures typically feature multiple processes communicating with some sort of IPC (Inter-Process Communications). P4Runtime is thus both an ideal RPC and an IPC.

single-embedded-controller

Figure 2. Use-Case: Single Embedded Controller

4.2. Single Remote Controller

Figure 3 shows a single remote Controller in charge of the P4 target. In this use-case, the device has no control of the pipeline, it just hosts the server. While this is possible, it is probably more practical to have a hybrid use-case as described in subsequent sections.

single-remote-controller

Figure 3. Use-Case: Single Remote Controller

4.3. Embedded + Single Remote Controller

Figure 4 illustrates the use-case of an embedded controller plus a single remote controller. Both controllers are clients of the single server. The embedded controller is in charge of one set of P4 entities plus the pipeline configuration. The remote controller is in charge of the remainder of the P4 entities. An equally-valid, alternative use-case, could assign the pipeline configuration to the remote controller.

For example, to minimize round-trip times (RTT) it might make sense for the embedded controller to manage the contents of a fast-failover table. The remote controller might manage the contents of routing tables.

embedded-plus-single-remote-controller

Figure 4. Use-Case: Embedded Plus Single Remote Controller

4.4. Embedded + Two Remote Controllers

Figure 5 illustrates the case of an embedded controller similar to the previous use-case, and two remote controllers. One of the remote controllers is responsible for some entities, e.g. routing tables, and the other remote controller is responsible for other entities, perhaps statistics tables. Role-based access divides the ownership.

embedded-plus-two-remote-controllers

Figure 5. Use-Case: Embedded Plus Two Remote Controllers

4.5. Embedded Controller + Two High-Availability Remote Controllers

Figure 6 illustrates a single embedded controller plus two remote controllers in an active-standby HA (High-Availability) configuration. Controller #1 is the active controller and is in charge of some entities. If it fails, Controller #2 takes over and manages the tables formerly owned by Controller #1. The mechanics of HA architectures are beyond the scope of this document, but the P4Runtime multi-master arbitration scheme supports it.

embedded-plus-two-remote-ha-controllers

Figure 6. Use-Case: Embedded Plus Two Remote High-Availability Controllers

5. Master-Slave Arbitration and Controller Replication

The P4Runtime interface allows multiple controllers to be connected to the P4Runtime server running on the device at the same time for the following reasons:

  1. Partitioning of the control plane: Multiple controllers may have orthogonal, non-overlapping, “roles” (or “realms”) and should be able to push forwarding entities simultaneously. The control plane can be partitioned into multiple roles and each role will have a set of controllers, one of which is the master and the rest are slaves. Role definition, i.e. how P4 entities get assigned to each role, is out-of-scope of this document.

  2. Redundancy and fault tolerance: Supporting multiple controllers allows having one or more standby slave controllers. These can already have a connection open, which can help them become master more quickly, especially in the case where the control-plane traffic is in-band and connection setup might be more involved.

To support multiple controllers, P4Runtime uses the streaming channel (available via StreamChannel RPC) for session management. The workflow is described as follows:

gRPC enables the server to identify which client originated each message in the StreamChannel stream. For example, the C++ gRPC library [10] in synchronous mode enables a server process to cause a function to be called when a new client creates a StreamChannel stream. This function should not return until the stream is closed and the server has done any cleanup required when a StreamChannel is closed normally (or broken, e.g. because a client process unexpectedly terminated). Thus the server can easily associate all StreamChannel messages received from the same client, because they are processed within the context of the same function call.

A P4Runtime implementation need not rely on the gRPC library providing information with unary RPC messages that identify which client they came from. Unary RPC messages include requests to write table entries in the data plane, or read state from the data plane, among others described later. P4Runtime relies on clients identifying themselves in every write request, by including the values device_id, role_id, and election_id in all write requests. The server trusts clients not to use a triple of values other than their own in their write requests. gRPC provides authentication methods [8] that should be deployed to prevent untrusted clients from creating channels, and thus from making changes or even reading the state of the server.

5.1. Default Role

A controller can omit the role message in MasterArbitrationUpdate. This implies the “default role”, which corresponds to “full pipeline access”. This also implies that a default role has a role.id of 0 (default). If using a default role, all RPCs from the controller (e.g. Write) must set the role_id to 0.

5.2. Role Config

The role.config field in the MasterArbitrationUpdate message sent by the controller describes the role configuration, i.e. which operations are in the scope of a given role. In particular, the definition of a role may include the following:

An unset role.config implies “full pipeline access” (similar to the default role explained above). In order to support different role definition schemes, role.config is defined as an Any Protobuf message [30]. Such schemes are out-of-scope of this document. When partitioning of the control plane is desired, the P4Runtime client(s) and server need to agree on a role definition scheme in an out-of-band fashion.

It is the job of the P4Runtime server to remember the role.config for every device_id and role_id pair.

5.3. Rules for Handling MasterArbitrationUpdate Messages Received from Controllers

  1. If the MasterArbitrationUpdate message is received for the first time on this particular channel (i.e. for a newly connected controller):

    1. If device_id does not match any of the devices known to the P4Runtime server, the server shall terminate the stream by returning a NOT_FOUND error.

    2. If the election_id is set and is already used by another controller for the same (device_id, role_id), the P4Runtime server shall terminate the stream by returning an INVALID_ARGUMENT error.

    3. If role.config does not match the “out-of-band” scheme previously agreed upon, the server must return an INVALID_ARGUMENT error.

    4. If the number of open streams for the given (device_id, role_id) exceeds the supported limit, the P4Runtime server shall terminate the stream by returning a RESOURCE_EXHAUSTED error.

    5. Otherwise, the controller is added to a list of connected controllers for the given (device_id, role_id) and the server remembers the controllers device_id, role_id and election_id for this gRPC channel. See below for the rules to determine if this controller becomes a master or slave, and what notifications are sent as a consequence.

  2. Otherwise, if the MasterArbitrationUpdate message is received from an already connected controller:

    1. If the device_id does not match the one already assigned to this stream, the P4Runtime server shall terminate the stream by returning a FAILED_PRECONDITION error.

    2. If the role.id does not match the current role_id assigned to this stream, the P4Runtime server shall terminate the stream by returning a FAILED_PRECONDITION error. If the controller wishes to change its role, it must close the current stream channel and open a new one.

    3. If role.config does not match the “out-of-band” scheme previously agreed upon, the server must return an INVALID_ARGUMENT error.

    4. If the election_id is set and is already used by another controller (excluding the controller making the request) for the same (device_id, role_id), the P4Runtime server shall terminate the stream by returning an INVALID_ARGUMENT error.

    5. If the election_id matches the one assigned to this stream:

      1. If the controller for this channel is the master, then the server updates the role.config to the one specified in the MasterArbitrationUpdate. An advisory mastership message is sent to all controllers for this device_id and role_id informing them of the new role.config. Since the format of role.config is out of scope for the P4Runtime specification, the server will send the advisory message for every update, even if the master sets the same role.config as it has before. See the following section for the format of the advisory message.

      2. If the controller is a slave, this is a no-op and the role.config is ignored. No response is sent to any controller.

    6. Otherwise, the server updates the election_id it has stored for this controller. This change might cause a change in mastership (this controller might become master, or the controller might have downgraded itself to a slave, see below), as well as notifications being sent to one or more controllers.

If the MasterArbitrationUpdate is accepted by either of the two steps above (cases 1.5. and 2.6. above), then the server determines if there are changes in mastership. Let election_id_past be the highest election ID the server has ever seen for the given device_id and role_id (including the one of the current master if there is one).

  1. If election_id is greater than or equal to election_id_past, then the controller becomes master. The server updates the role configuration to role.config for the given role.id. Furthermore:

    1. If there was no master for this device_id and role_id before and there are no Write requests still processing from a previous master, then the server immediately sends an advisory notification to all controllers for this device_id and role_id. See the following section for the format of the advisory message.

    2. If there was a previous master or Write requests in flight, then the server carries out the following steps (in this order):

      1. The server stops accepting Write requests from the previous master (if there is one). At this point, the server will reject all Write requests with PERMISSION_DENIED.

      2. The server notifies all controllers other than the new master of the mastership change by sending the advisory notification described in the following section.

      3. The server will finish processing any Write requests that have already started. If there are errors, they are reported as usual to the previous master. If the previous master has already disconnected, any possible errors are dropped and not reported.

      4. The server now accepts the current controller as the new master, thus accepting Write requests from this controller. The server updates the highest election ID (i.e. election_id_past) it has seen for this device_id and role_id to election_id.

      5. The server notifies the new master by sending the advisory message described in the following section.

  2. Otherwise, the controller becomes a slave. If the controller was previously a master (and downgraded itself), then an advisory message is sent to all controllers for this device_id and role_id. Otherwise, the advisory message is only sent to the controller that sent the initial MasterArbitrationUpdate. See the following section for the format of the advisory message.

5.4. Mastership Notifications

For any given device_id and role_id, any time a new master is chosen, a master downgrades its status to a slave, a master disconnects, or the role.config is updated by the master, all controllers for that (device_id, role_id) are informed of this by sending a StreamMessageResponse. The MasterArbitrationUpdate is populated as follows:

Note that on mastership changes with outstanding Write request, some notifications might be delayed, see the previous section for details.

6. The P4Info Message

The purpose of P4Info was described under Reference Architecture. Here we describe the various components.

6.1. Common Messages

These messages appear nested within many other messages.

6.1.1. Documentation Message

Documentation is used to carry both brief and long descriptions of something. Good content within a documentation field is extremely helpful to P4Runtime application developers. 

message Documentation {
  // A brief description of something, e.g. one sentence
  string brief = 1;
  // A more verbose description of something.
  // Multiline is accepted. Markup format (if any) is TBD.
  string description = 2;
}

6.1.2. Preamble Message

The preamble serves as the “descriptor” for each entity and contains the unique instance ID, name, alias, annotations and documentation. 

message Preamble {
  // ids share the same number-space; e.g. table ids cannot overlap with counter
  // ids. Even though this is irrelevant to this proto definition, the ids are
  // allocated in such a way that it is possible based on an id to deduce the
  // resource type (e.g. table, action, counter, ...). This means that code
  // using these ids can detect if the wrong resource type is used
  // somewhere. This also means that ids of different types can be mixed
  // (e.g. direct resource list for a table) without ambiguity. Note that id 0
  // is reserved and means "invalid id".
  uint32 id = 1;
  // fully qualified name of the P4 object, e.g. c1.c2.ipv4_lpm
  string name = 2;
  // an alias (alternative name) for the P4 object, probably shorter than its
  // fully qualified name. The only constraint is for it to be unique with
  // respect to other P4 objects of the same type. By default, the compiler uses
  // the shortest suffix of the name that uniquely identifies the object. For
  // example if the P4 program contains two tables with names s.c1.t and s.c2.t,
  // the default aliases will respectively be c1.t and c2.t. In the future, the
  // P4 programmer may also be able to override the default alias for any P4
  // object (TBD).
  string alias = 3;
  repeated string annotations = 4;
  // Documentation of the entity
  Documentation doc = 5;
}

6.1.3. Annotating P4 Entities with Documentation

P4 entities may be annotated using the following annotations: 

@brief(string...)
@description(string...)

Attaching either or both of these annotations to an entity will generate a P4Info Documentation Message, which in turn will appear in the Preamble Message for the entity.

The P4 compiler should not emit annotation messages in the P4Info for these specific cases; instead, it should generate the Documentation messages as described.

The following example shows documentation annotations for a table entity: 

@brief("Match IPv4 addresses to next-hop MAC and port")
@description("Match IPv4 addresses to next-hop MAC and port. \
Uses LPM match type.")
table my_ipv4_lkup {
  ...
}

6.2. PkgInfo Message

The PkgInfo message contains package-level metadata which describes the overall P4 program itself, as opposed to P4 entities. PkgInfo can be extracted and used to facilitate “browsing” of available P4 programs from a library. Although all fields are technically “optional,” every implementation should include as a minimum the name, version, doc and arch fields. The other fields are recommended to be included. 

// Can be used to manage multiple P4 packages.
message PkgInfo {
  // a definitive name for this configuration, e.g. switch.p4_v1.0
  string name = 1;
  // configuration version, free-format string
  string version = 2;
  // brief and detailed descriptions
  Documentation doc = 3;
  // Miscellaneous metadata, free-form; a way to extend PkgInfo
  repeated string annotations = 4;
  // the target architecture, e.g. "psa"
  string arch = 5;
  // organization which produced the configuration, e.g. "p4.org"
  string organization = 6;
  // contact info for support,e.g. "tech-support@acme.org"
  string contact = 7;
  // url for more information, e.g. "http://support.p4.org/ref/p4/switch.p4_v1.0"
  string url = 8;
}

6.2.1. Annotating P4 code with PkgInfo

A P4 progam's PkgInfo may be declared using one or more of the following annotations, attached to the main block only: 

@pkginfo(key=value)
@pkginfo(key=value[,key=value,...])
@brief("A brief description")
@description("A longer\
description")
@custom_annotation(...)
@another_custom_annotation(...)

Above we see several different types of annotations:

Declaring one or more of these annotations on main will generate a single corresponding PkgInfo message in the P4Info as described in PkgInfo Message.

The following example shows @pkginfo annotations using a mixture of single and multiple key-value pairs. It also shows @brief and @description annotations, plus some additional custom annotations. The well-known annotations will produce corresponding fields inside the PkgInfo message. The custom annotations will be appended to the PkgInfo.annotations list. 

@pkginfo(name="switch.p4",version="2")
@pkginfo(organization="p4.org")
@pkginfo(contact="info@p4.org")
@pkginfo(url="www.p4.org")
@brief("L2/L3 switch")
@description("L2/L3 switch.\
Built for data-center profile.")
@my_annotation1(...) // Not well-known, this will appear in PkgInfo annotations
@my_annotation2(...) // Not well-known, this will appear in PkgInfo annotations
PSA_Switch(IgPipeline, PacketReplicationEngine(), EgPipeline,
           BufferingQueueingEngine()) main;

6.3. ID Allocation for P4Info Objects

P4Info objects receive a unique ID, which is used to identify the object in P4Runtime messages. IDs are 32-bit unsigned integers which are assigned by the compiler during the P4Info generation process. IDs are assigned in such a way that it is possible based on the ID value alone to deduce the type of the object (e.g. table, action, counter, …). The most significant 8 bits of the ID encodes the object type (as per Table 1). The p4info.proto file includes a mapping from object type to 8-bit prefix value, encoded as an enum definition (p4.config.v1.P4Ids.Prefix). These values must be used (e.g. by the compiler) when allocating IDs. The remaining 24 bits must be generated in such a way that the resulting IDs must be globally unique in the scope of the P4Info message. Table 2 shows the ID layout.

8-bit prefix value P4 object type
0x00 Reserved (unspecified)
0x01 Action
0x02 Table
0x03 Value-set
0x04 Controller header (header type with @controller_header annotation)
0x05…0x0f Reserved (for future P4 built-in objects)
0x10 Reserved (start of PSA extern types)
0x11 PSA Action profiles / selectors
0x12 PSA Counter
0x13 PSA Direct counter
0x14 PSA Meter
0x15 PSA Direct meter
0x16 PSA Register
0x17 PSA Digest
0x18…0x7f Reserved (for future PSA extern types)
0x80 Reserved (start of vendor-specific extern types)
0x81…0xfe Vendor-specific extern types
0xff Reserved (max prefix value)
Table 1. Mapping of P4Info object type to 8-bit ID prefix value
MSB bit 31 …….. bit 24 bit 23 ………………….. bit 0 LSB
Object type prefix Generated suffix (e.g. by the compiler)
Table 2. Format of P4Info object IDs

It is possible to statically set the least-significant 24 bits of the ID in the P4 program source by annotating the object with @id (see Table 3). The compiler must honor the @id annotations when generating the P4Info message and must fail the compilation if statically-assigned ID suffixes lead to non-unique IDs (i.e. if the P4 programmer tries to assign the same ID suffix to two different P4 objects of the same type by annotating them with the same @id value). Note that it is not possible for the P4 programmer to change the value of the 8-bit ID prefix, which encodes the object type.

P4 declaration(s) Compiler-allocated ID(s)
@id(0x12ab34) table tA... 0x0212ab34
@id(0x12ab34) table tA... Error(same ID suffixes for 2 objects of the same type)
@id(0x12ab34) table tB...
@id(0x12ab34) table tA... 0x0212ab34
@id(0x12ab34) action act1... 0x0112ab34
Table 3. Example of statically-assigned P4Info object IDs

6.4. P4Info Objects

6.4.1. Table

Table messages are used to specify all possible match-action tables exposed to a control plane. This message contains the following fields:

6.4.2. Action

Action messages are used to specify all possible actions of all match-action tables.

The Action message defines the following fields:

6.4.3. ActionProfile

ActionProfile messages are used to specify all available instances of Action Profile and Action Selector PSA externs.

PSA Action Profiles are used to describe implementations of match-action tables where multiple table entries can share the same action instance. Indeed, differently from a regular match-action table where each entry contains the action specification, when using Action Profile-based tables, the control plane can insert entries pointing to an Action Profile member, where each member then points to an action instance. The control plane is responsible for creating, modifying, or deleting members at runtime.

PSA Action Selectors extend Action Profiles with the capability of bundling together multiple members into groups. Match-action table entries can point to a member or group. When processing a packet, if the table entry points to a group, a dynamic selection algorithm is used to select a member from the group and apply the corresponding action to the packet. The dynamic selection algorithm is typically specified in the P4 program when instantiating the Action Selector, however it is not specified in the P4Info. The control plane is responsible for creating, modifying, or deleting both members and groups at runtime.

While PSA defines Action Profile and Action Selector as two different externs, P4Info uses the same ActionProfile message to describe both.

The ActionProfile message includes the following fields:

6.4.4. Counter & DirectCounter

Counter and DirectCounter messages are used to specify all possible instances of Counter and Direct Counter PSA externs respectively. Both externs are used to represent data plane counters that keep statistics such as the number of packets or bytes. The main difference between (indexed) counters and direct counters is:

Both Counter and DirectCounter messages share the following fields:

For indexed counters, the Counter message contains also a size field, an int64 representing the maximum number of independent values that can be held by this counter array. Conversely, the DirectCounter message contains a direct_table_id field that carries the unit32 identifier of the table to which this direct counter is attached.

For indexed counters, the Counter message contains also an index_type_name field, which indicates whether the index has a user-defined type. This is useful for translation. The underlying built-in type must be a fixed-width unsigned bitstring (bit<W>).

6.4.5. Meter & DirectMeter

Meter and DirectMeter messages are used to specify all possible instances of Meter and Direct Meter PSA externs. Both externs provide mechanism to keep data plane statistics typically used to mark or drop packets that exceed a given packet or bit rate. Similarly to counters, the main difference between (indexed) meters and direct meters is:

Both Meter and DirectMeter messages share the following fields:

For indexed meters, the Meter message contains also a size field, an int64 representing the maximum number of independent cells that can be held by this meter. Conversely, the DirectMeter message contains a direct_table_id field that carries the uint32 identifier of the table to which this direct meter is attached.

For indexed meters, the Meter message contains also an index_type_name field, which indicates whether the index has a user-defined type. This is useful for translation. The underlying built-in type must be a fixed-width unsigned bitstring (bit<W>).

6.4.6. ControllerPacketMetadata

ControllerPacketMetadata messages are used to describe any metadata associated with controller packet-in and packet-out. A packet-in is defined as a data plane packet that is sent by the P4Runtime server to the control plane for further inspection. Similarly, a packet-out is defined as a data packet generated by the control plane and injected in the data plane via the P4Runtime server.

When inspecting a packet-in, the control plane might need to have access to additional information such as the original data plane port where the packet was received, the timestamp when the packet was received, if the packet is a clone, etc. Similarly, when sending a packet-out, the control plane might need to specify additional information used by the device to process the data packet.

Such additional information for packet-in and packet-out can be expressed by means of P4 headers carrying P4 standard annotations @controller_header("packet_in") and @controller_header("packet_out"), respectively. ControllerPacketMetadata messages capture the information contained within these special headers and are needed by the P4Runtime server to process packet-in and packet-out stream messages (see section on Packet I/O stream messages).

A P4Info message can contain at most two ControllerPacketMetadata messages, one describing the packet-in header, and the other the packet-out header. Each message contains the following fields:

As an example, consider the following snippet of a P4 program where controller headers are specified and we show the corresponding ControllerPacketMetadata messages. 

@controller_header("packet_out")
header PacketOut_t {
  bit<9> egress_port; /* suggested port where the packet
                         should be sent */
  bit<8> queue_id;    /* suggested queue ID */
}

@controller_header("packet_in")
header PacketIn_t {
  bit<9> ingress_port; /* data plane port ID where
                          the original packet was received */
  bit<1> is_clone;     /* 1 if this is a clone of the
                          original packet */
}
controller_packet_metadata {
  preamble {
    id: 2868916615
    name: "packet_out"
    annotations: "@controller_header(\"packet_out\")"
  }
  metadata {
    id: 1
    name: "egress_port"
    bitwidth: 9
  }
  metadata {
    id: 2
    name: "queue_id"
    bitwidth: 8
  }
}

controller_packet_metadata {
  preamble {
    id: 2868941301
    name: "packet_in"
    annotations: "@controller_header(\"packet_in\")"
  }
  metadata {
    id: 1
    name: "ingress_port"
    bitwidth: 9
  }
  metadata {
    id: 2
    name: "is_clone"
    bitwidth: 1
  }
}

Note that the use of @controller_header is optional for Packet I/O. The P4 program may define controller headers without this annotation and use them to encapsulate controller packets. However, in this case the client will be responsible for extracting the metadata from the serialized header in packet-in messages and for serializing the metadata when generating packet-out messages.

6.4.7. ValueSet

ValueSet messages are used to specify all possible P4 Parser Value Sets. Parser Value Sets can be used by the control plane to specify runtime matches used by the P4 parser to determine transitions from one state to another. For more information on Parser Value Sets, refer to the P416 specification [37].

The ValueSet message defines the following fields:

According to the P4 specification, the type parameter of a Value Set, which defines the type of the expression that can be matched against the Value Set in a parser transition, and therefore determines the format of the members that can be inserted into the Value Set by the control plane, must be one of bit<W>, tuple, or struct [25]. The rest of this section looks at all 3 of these cases and gives an example ValueSet message when appropriate.

  1. If the type parameter is bit<W>, match will include exactly one MatchField message, with the following fields (if a field is omitted here, it means the default Protobuf value should be used):

    • id: set to 1
    • bitwidth: set to the value of W
    • match_type: set to EXACT 
@id(1) value_set<bit<8> >(4) pvs;
select (hdr.f8) { /* ... */ }
value_sets {
  preamble {
    id: 0x03000001
    name: "pvs"
  }
  match {
    id: 1
    bitwidth: 8
    match_type: EXACT
  }
  size: 4
}

  1. If the type parameter is a tuple, this version of P4Runtime does not support runtime programming of the Value Set. If the P4Info message is generated by a compiler, and the P4 program includes such a Value Set, the compiler must reject the program.

  2. If the type parameter is a struct, this version of P4Runtime requires that all the fields of the struct be of type bit<W> (where W can be different for each field). Otherwise, if the P4Info message is generated by a compiler, the compiler must reject the program. If the Value Set is supported, the match field will include one MatchField message for each field in the struct, with the following fields:

    • id: must be unique with respect to the other match entries. If the P4Info message was generated from a P4 compiler, we recommend that the IDs be assigned incrementally, starting from 1, in the same order as the fields in the P4 struct declaration.
    • name: set to the name of the corresponding struct field.
    • annotations: set to the list of P4 annotations associated with the struct field, except for the @match annotation, if present (see the match field below).
    • bitwidth: set to the value of W for the corresponding struct field.
    • type_name, which indicates whether the struct field has a user-defined type; this is useful for translation.
    • match: by default match_type is set to EXACT; the P4 programmer can specify a different match type by using the @match annotation [25].
    • doc: documentation associated with the struct field. 
struct match_t {
  bit<8> f8;
  @match(ternary) bit<16> f16;
  @match(custom) bit<32> f32;
}
@id(1) value_set<match_t>(4) pvs;
select ({ hdr.f8, hdr.f16, hdr.f32 }) { /* ... */ }
value_sets {
  preamble {
    id: 0x03000001
    name: "pvs"
  }
  match {
    id: 1
    name: "f8"
    bitwidth: 8
    match_type: EXACT
  }
  match {
    id: 2
    name: "f16"
    bitwidth: 16
    match_type: TERNARY
  }
  match {
    id: 3
    name: "f32"
    bitwidth: 32
    other_match_type: "custom"
  }
  size: 4
}

Although not mentioned in the P4 specification, P4Runtime also supports the cases where the Value Set type parameter is a user-defined type that resolves to a bit<W>, or a struct where one or more fields is a user-defined type that resolves to a bit<W>. For each MatchField that corresponds to a user-defined type, the type_name field must be set to the appropriate value (i.e. the name of the type).

6.4.8. Register

Register messages are used to specify all possible instances of Register PSA externs.

Registers are stateful memories that can be read and written by data plane during packet forwarding. The control plane can also access registers at runtime.

The Register message defines the following fields:

6.4.9. Digest

Digest messages are used to specify all possible instances of Packet Digest PSA externs.

A packet digest is a mechanism to efficiently send notifications from the data plane to the control plane. This mechanism differs from packet-in which is generally used to send entire packets (headers plus payload), each one as a separate P4Runtime stream message. A digest for a packet has a size typically much smaller than the packet itself, as it can be used to send only a subset of the headers or P4 metadata associated with the packet. To reduce the rate of messages sent to the control plane, a P4Runtime server can combine digests for multiple packets into larger messages.

The Digest message defines the following fields:

6.4.10. Extern

Extern messages are used to specify all extern instances across all extern types for a non-PSA architecture. This is useful when extending P4Runtime to support a new architecture. Each architecture-specific extern type corresponds to at most one Extern message instance in P4Info. The Extern message defines the following fields:

If the P4 program does not include any instance of a given extern type, the Extern message instance for that type should be omitted from the P4Info.

6.5. Support for Arbitrary P4 Types with P4TypeInfo

See section on Representation of Arbitrary P4 Types.

7. P4 Forwarding-Pipeline Configuration

The ForwardingPipelineConfig captures data needed to realize a P4 forwarding-pipeline and map various IDs passed in P4Runtime entity messages. It is formally called the “Device Configuration” and sometimes also referred to as the “P4 Blob”. It is defined as: 

message ForwardingPipelineConfig {
  config.P4Info p4info = 1;
  bytes p4_device_config = 2;
  message Cookie {
    uint64 cookie = 1;
  }
  Cookie cookie = 3;
}

The p4info field captures the P4 program metadata as described by the P4Info. This message is the output of the P4 compiler and is target-agnostic.

The p4_device_config is opaque binary data which contains the target-specific configuration to realize the P4 program. The P4 program running on a target is changed by loading a new ForwardingPipelineConfig on that target.

The cookie field is opaque data which may be used by a control plane to uniquely identify a forwarding-pipeline configuration among others managed by the same control plane. For example, a controller can compute its value using a hash function over the P4Info and/or target-specific binary data. However, there are no restrictions on how such value is computed, or where this is stored on the target, as long as it is returned with a GetForwardingPipelineConfig RPC. When writing the config via a SetForwardingPipelineConfig RPC, the cookie field is optional. For this reason, the actual value is wrapped in its own message to clearly identify cases where a cookie is not present.

8. General Principles for Message Formatting

8.1. Set / Unset Protobuf Field

In Protobuf version 3 (proto3), the default value for a message field is “unset” [4]. An application, such as the P4Runtime client or server, is able to distinguish between an unset message field and a message field set to its default value. We often use this distinction in P4Runtime and the meaning of a message can vary based on which of its message fields are set. For example, when reading values from an indirect PSA counter using the CounterEntry message, an “unset” index field means that all entries in the counter array should be read and returned to the P4Runtime client (we refer to this as a wildcard read). On the other hand, if the index message field is set, a single entry will be read.

Let's look at the counter example in more details. Based on this specification document, the C++ server code which processes CounterEntry messages may look like this: 

auto *counter_entry = ...
if (counter_entry->has_index()) {
  auto index = counter_entry->index().index();
  read_one_entry(counter_entry->id(), index);
} else {
  read_all_entries(counter_entry->id());
}

  1. Reading a single counter entry at index 0 in the counter array with id <id>:

    • Here is the C++ client code: 
      p4::v1::CounterEntry entry;
entry.set_counter_id(<id>);
entry.mutable_index();
// The above line sets the index field; it is equivalent to:
// auto *index = entry.mutable_index();
// index->set_index(0);
    • Here is the corresponding Protobuf message in text format: 
      counter_id: <id>
index {}
    • Expected behavior: Counter entry at index 0 is read. Notice that the index subfield is missing under the index field message of CounterEntry in the text dump of the message. This is because the subfield is a scalar numeric type and 0 is therefore its default value. Scalar fields with default values are omitted from the textual representation of Protobuf messages.

  2. Reading all counter entries by leaving the index field unset

    • Here is the C++ client code: 
      p4::v1::CounterEntry entry;
entry.set_counter_id(<id>);
    • Here is the corresponding Protobuf message in text format: 
      counter_id: <id>
    • Expected behavior: All counter entries for the provided counter instance are read. Notice that the index message field is unset (default value) and is therefore omitted from the textual representation of the message.

8.2. Read-Write Symmetry

The reads and writes a client issues towards a server should be symmetrical and unambiguous. More specifically, if a client writes a P4 entity and then reads it back then the client should expect that the message it wrote and the message it read should match if the RPCs finished successfully. Consider the following pseudocode as an example: 

intended_value = value

status = server.write(intended_value, p4_entity)
observed_value = server.read(p4_entity)

assert(intended_value == observed_value)

To ensure read-write symmetry, the rest of this document tries to offer canonical representations for various data types, but this principle should be thought of where it falls short. Ensuring this will allow client software to recover programmatically from failures that can affect the switch stack software, communication channel, or the client replicas. If Read RPC returns a semantically-same but syntactically-different response then the client would have to canonicalize the read values to check its internal state, which only pushes the protocol's complexities to the client implementations.

In order to avoid placing too much burden on the P4Runtime server implementation, we do not in general mandate that the order of values in a Protobuf repeated field be preserved. For example, the server is not required to preserve the order of the match fields in a TableEntry message. If there is a specific case for which the order is significant and / or needs to be preserved, it will be explicitly stated in this document. The MessageDifferencer class [35] included in the Protobuf C++ API supports comparing messages while treating repeated fields as sets, so that different orderings of the same elements are considered equal. This method of comparing Protobuf messages may come at a cost in performance.

8.3. Zero as Reserved Value

p4runtime.proto uses proto3 syntax, and so it does not allow not specifying a scalar data type, such as a uint32. Therefore, we usually reserve value 0 for those fields to mean unset. In particular, 0 is not a valid P4 object ID and it is an error to specify 0 for any P4 object ID in a non-read request towards the switch, such as in a WriteRequest or a SetForwardingPipelineConfigRequest.

8.4. Bytestrings

P4Runtime integer values may be too large to fit in Protobuf primitive data types (32-bit and 64-bit words). The P4 language does not put any limit on the size of integer values, whether unsigned (bit<W>) or signed (int<W>), and it is up to the P4 programmer to choose the appropriate sizes. Because of this flexibility, P4Runtime represents P4 integer values as binary strings, using the bytes Protobuf type. The correct bitwidth — as per the P4 program — of each integer variable exposed through P4Runtime is specified in the P4Info message.

The canonical binary string representation uses the shortest string that fits the encoded integer value. This representation achieves three goals:

In particular, the kinds of P4 program updates that this representation facilitates are those where a P4Runtime server and client can continue to transmit P4Runtime messages between them when one has a P4Info file for version A of a P4 program, at the same time that the other has a P4Info file for version B of a P4 program, and those P4 programs differ in the bitwidths of some values of type bit<W> and/or int<W>.

Note that this representation does not make it possible to seamlessly change the type of a value from signed to unsigned, or vice versa. If you attempt to do so, this mechanism can quietly change negative signed values to positive unsigned values, or vice versa. It also limits the magnitude of the values transmitted to those that fit within the smaller of the bitwidths supported by either end of the message transmission. If a message sender attempts to send a value larger than the receiver expects, the receiver will detect it as out of range.

In the P4Runtime API version 1.0, values of table key fields, action parameters, and fields in packet-in and packet-out headers between a device and the controller (see 6.4.6), may not be of type int<W>. The rules for encoding signed values thus only apply to messages of type P4Data (see 8.5.3).

For a value of type bit<W>, the fewest number of bits required to represent the integer value $V > 0$ is the smallest integer $A$ such that $V \leq 2^A -1$.

For a value of type int<W>, the fewest number of bits required to represent the integer value $V \neq 0$ in 2's complement form is the smallest integer $A$ such that $-2^{A-1} \leq V \leq 2^{A-1} - 1$.

As a special case, define that the value $V=0$ requires at least $A=1$ bit to represent, regardless of whether it is signed or unsigned.

The shortest possible binary string for an integer $V$ that needs $A$ bits to represent it is computed as: 

minimum_string_size = floor((A + 7) / 8)

Binary strings with the byte length computed as minimum_string_size promote P4Runtime read-write symmetry in both client-to-server requests and server-to-client replies.

Any additional bits in the bytes sent for an unsigned integer value (type bit<W>) must be 0. If additional bytes are transmitted above the minimum_string_size minimum required, they must be filled with 0.

Any additional bits in the bytes sent for a signed integer value (type int<W>) must be copies of the sign bit, i.e. 0 for non-negative values, or 1 for negative values. If additional bytes are transmitted above the minimum_string_size minimum required, they must be filled with copies of the sign bit, i.e. 0 for non-negative values, or 0xff for negative values. In 2's complement representation, this is called “sign extension”, and leaves the numeric value represented unchanged.

Upon receiving a binary string, the P4Runtime receiver (whether the server or the client) does not impose any restrictions on the length of the string itself. Instead, the receiver verifies that the value encoded by the string fits within the expected type (signed or unsigned) and P4Info-specified bitwidth for the P4 object value.

For a received bitstring expected to fit within a bit<W> type, the value it represents is in range if, after removing all most significant 0 bits, the remaining bitstring's width is W bits or less.

For a received bitstring expected to fit within an int<W> type, the value it represents is in range if, after “undoing sign extension”, the remaining bit string's width is W bits or less. To undo sign extension, start by eliminating the most significant bit, but only if it is equal to the bit that follows it (otherwise removing the most significant bit would change the sign of the value). Repeat that process until either only a single bit remains, or until the two most significant bits are different from each other.

If the string's byte length is zero, the server always rejects the string.

When the server rejects a binary string due to any of the previous criteria, it returns an OUT_OF_RANGE error.

For all binary strings, P4Runtime uses big-endian (i.e. network) byte-order. For signed integer values (int<W> P4 type), P4Runtime uses the same two's complement bitwise representation as P4. Table 4 shows various examples of integer values that the server accepts as valid P4Runtime binary strings according to the criteria in the list above.

P4 type Integer value P4Runtime binary string Read-write symmetry
bit<8> 99 (0x63) \x63 yes
bit<16> 99 (0x63) \x00\x63 no
bit<16> 99 (0x63) \x63 yes
bit<16> 12388 (0x3064) \x30\x64 yes
bit<16> 12388 (0x3064) \x00\x30\x64 no
bit<12> 99 (0x63) \x00\x63 no
bit<12> 99 (0x63) \x63 yes
bit<12> 99 (0x63) \x00\x00\x63 no
int<8> 99 (0x63) \x63 yes
int<8> -99 (-0x63) \x9d yes
int<8> -99 (-0x63) \xff\x9d no
int<12> -739 (-0x2e3) \xfd\x1d yes
int<16> 0 (0x0) \x00\x00 no
int<16> 0 (0x0) \x00 yes
Table 4. Examples of Valid Bytestring Encoding

Table 5 shows some examples of invalid P4Runtime binary strings:

P4 type P4Runtime binary string
bit<8> \x01\x63
bit<8> empty string
bit<16> \x01\x00\x63
bit<12> \x10\x63
bit<12> \x01\x00\x63
bit<12> \x00\x40\x63
int<8> \x00\x9d
int<12> \x8d\x1d
int<16> empty string
Table 5. Examples of Invalid Bytestring Encoding

As the preceding examples illustrate, a P4Runtime server must accept a wide assortment of possible binary string encodings for the same integer value. This requirement addresses P4 program upgrade scenarios where binary string widths can expand or contract. In some P4Runtime environments, the changes cannot be deployed simultaneously to all P4Runtime clients and servers. Given a hypothetical match field type change from bit<8> to bit<9>, a server running the bit<9> version of the P4 program will accept requests from clients that remain on the bit<8> P4Runtime version.

Despite the server's binary string flexibility for P4 program update support, the client and server must both remain aware of the read-write symmetry requirements. As described earlier, read-write symmetry requires that the encoder of a P4Runtime request or reply uses the shortest strings that fit the encoded integer values.

Representation of variable-length integer values (varbit<W> P4 type) is similar to the representation of fixed-width unsigned integers. We use a binary string, whose length is the dynamic-length of the expression. When the value is provided by the P4Runtime client, the server must verify that the length of the binary string is less than the maximum length specified in the P4 program, and return an OUT_OF_RANGE error code otherwise.

8.5. Representation of Arbitrary P4 Types

8.5.1. Problem Statement

The P416 language includes more complex types than just binary strings [3]. Most of these complex data types can be exposed to the control plane through table key expressions, Value Set lookup expressions, Register (PSA extern type) value types, etc. Not supporting these more complex types can be very limiting. Table 6 shows the different P416 types and how they are allowed to be used, as per the P416 specification.

Container type
Element type header header_union struct or tuple
bit<W> allowed error allowed
int<W> allowed error allowed
varbit<W> allowed error allowed
int error error error
void error error error
error error error allowed
match_kind error error error
bool error error allowed
enum allowed1 error allowed
header error allowed allowed
header stack error error allowed
header_union error error allowed
struct error error allowed
tuple error error allowed
Table 6. P4 Type Usage

For example, the following P416 objects involve complex types that need to be exposed in P4Runtime in order to support runtime operations on these objects. 

Digest<tuple<bit<4>, bit<8> > >() digest_complex;
digest_complex.pack({ hdr.ipv4.version, hdr.ipv4.protocol });
// ...
header_union ip_t {
   ipv4_t ipv4;
   ipv6_t ipv6;
}
Register<ip_t, bit<32> >(128) register_ip;

One solution would be to use only binary string (bytes type) in p4runtime.proto and to define a custom serialization format for complex P416 types. The serialization would maybe be trivial for header types but would require some work for header unions, header stacks, etc. For example, in the case of a PSA Register storing header unions, a client reading from that Register would need to receive information about which member header is valid, in addition to the binary contents of this header. Rather than coming-up with a serialization format from scratch, we decided to use a Protobuf representation for all P416 types.

8.5.2. P4 Type Specifications in p4info.proto

In order for the P4Runtime client to generate correctly-formatted messages and for the P4Runtime service implementation to validate them, P4Info needs to specify the type of each P4 expression which is exposed to the control plane. In the Register example above, client and server need to know that each element of the register has type ip_t, which is a header union with 2 possible headers: ipv4 with type ipv4_t and ipv6 with type ipv6_t. Similarly, they need to know the field layout for both of these header types.

To achieve this we introduce 2 main Protobuf messages: P4TypeInfo and P4DataTypeSpec.

P4TypeInfo is a top-level member of P4Info and includes Protobuf maps storing the type specification for all the named types in the P416 program. These named types are struct, header, header_union, enum and serializable_enum; for each of these we have a type specification message, respectively P4StructTypeSpec, P4HeaderTypeSpec, P4HeaderUnionTypeSpec, P4EnumTypeSpec and P4SerializableEnumTypeSpec. We preserve P4 annotations for named types, which is useful to identify well-known headers, such as IPv4 or IPv6. P4TypeInfo also includes the list of parser errors for the program, as a P4ErrorTypeSpec message.

P4DataTypeSpec is meant to be used in P4Info, to specify the expected format of the P4-dependent values being exchanged between the P4Runtime client and server. Each P4DataTypeSpec message corresponds to a compile-time type in the original P416 program (e.g. the type parameter of an extern). This compile-time type is represented as a Protobuf oneof, which can be:

For all P416 compound types (tuple, struct, header, and header_union), the order of members in the repeated field of the Protobuf type specification is guaranteed to be the same as the order of the members in the corresponding P416 declaration. The same goes for the order of members of an enum (serializable or not) or members of error.

8.5.3. P4Data in p4runtime.proto

P4Runtime uses the P4Data message to represent values of arbitrary types. The P4Runtime client must generate correct P4Data messages based on the type specification information included in P4Info. The P4Data message was designed to introduce little overhead compared to using binary strings in the most common case (P416 bit<W> type).

Just like its P4Info counterpart - P4DataTypeSpec -, P4Data uses a Protobuf oneof to represent all possible values.

We define a canonical representation for P4Data messages — therefore guaranteeing read-write symmetry — by introducing the following requirements:

8.5.4. Example

Let's look at the Register example again: 

header_union ip_t {
   ipv4_t ipv4;
   ipv6_t ipv6;
}
Register<ip_t, bit<32> >(128) register_ip;

Here's the corresponding entry in the P4Info message: 

registers {
  preamble {
    id: 369119267
    name: "register_ip"
    alias: "register_ip"
  }
  type_spec {
    header_union {
      name: "ip_t"
    }
  }
  size: 128
}
type_info {
  headers {
    key: "ipv4_t"
    value {
      members {
        name: "version"
        type_spec {

          bit {
            bitwidth: 4
          }
        }
      } # ...
  headers {
    key: "ipv6_t"
    value {
      members {
        name: "version"
        type_spec {
          bit {
            bitwidth: 4
          }
        }
      } # ...
  header_unions {
    key: "ip_t"
    value {
      members {
        name: "ipv4"
        header {
          name: "ipv4_t"
        }
      }
      members {
        name: "ipv6"
        header {
          name: "ipv6_t"
        }
      }
    }
  }
}

Here's a p4.WriteRequest to set the value of register_ip[12]: 

update {
  type: INSERT
  entity {
    register_entry {
      register_id: 369119267
      index {
        index: 12
      }
      data {
        header_union {
          valid_header_name: "ipv4"
          valid_header {
            is_valid: true
            bitstrings: "\x04"
            bitstrings: # ...
          }
        }
      }
    }
  }
}

8.5.5. enum, serializable enum and error

P416 supports 2 different classes of enumeration types: without underlying type (safe enum) and with underlying type (serializable enum or “unsafe” enum) [5]. For enum types with no underlying type — as well as error — there is no integer value associated with each symbolic member entry (whether assigned automatically by the compiler or directly in the P4 source). We therefore use a human-readable string in P4Data to represent enum and error values.

Serializable enum types have an underlying fixed-width unsigned integer representation (bit<W>). All named enum members must be assigned an integer value by the P4 programmer, but not all valid numeric values for the underlying type need to have a corresponding name. P4TypeInfo includes the mapping between entry name and entry value. When providing serializable enum values through P4Data, one must use the assigned integer value (enum_value bytestring field). P4Runtime does not provide a way for the client to use the name — even when the enum member has one — instead of the value, as it makes it easier for the server to respect the read-write symmetry principle.

8.5.6. User-defined types

P416 enables programmers to introduce new types [11]. While similar to typedef, this mechanism introduces in fact a new type, which is not a strict synonym of the original type. It is important to preserve this distinction in the P4Info message, in particular for the purposes of translation. When introducing a new type, the declaration can be annotated with @p4runtime_translation to indicate that the type exposed to the P4Runtime client is different from the original P4 type. One important use-case is for port numbers, whose underlying data plane representation may vary on different targets, but for which it may be convenient to present a unified representation and numbering scheme to the control plane. The @p4runtime_translation annotation can only be used if the underlying P4 built-in type is a fixed-width unsigned bitstring type (bit<W>) and the type exposed to the control plane will also be a fixed-width unsigned bitstring, with a potentially different bitwidth. It takes two parameters: a URI (Uniform Resource Identifier) which uniquely identifies the translation being performed on entities of the new type to the P4Runtime server and the bitwidth of the bitstring type exposed to the control plane. It is recommended that the URI includes at least the P4 architecture name and the type name.

A @p4runtime_translation annotation need not have any effect if it is used to annotate anything in a P4 program other than a type declaration. It is recommended that P4 development tools have an option to issue a warning if such an annotation is used in a part of a P4 program where it has no effect.

User-defined types are specified using the P4NewTypeSpec message, which has the following fields:

For example, an architecture — in this case PSA — may introduce a new type for port numbers: 

@p4runtime_translation("p4.org/psa/v1/PortId_t", 32)
type bit<9> PortId_t;

In this case, the P4Info message would include the following P4TypeInfo message: 

type_info {
  new_types {
    key: "PortId_t"
    value {  # P4NewTypeSpec
      translated_type {  # P4NewTypeTranslation
        uri: "p4.org/psa/v1/PortId_t"
        sdn_bitwidth: 32
      }
    }
  }
}

Note that a P4 compiler may provide a mechanism external to the language to specify if and how a user-defined type is to be translated (e.g. through some configuration file passed on the command-line when invoking the compiler). This mechanism should take precedence over @p4runtime_translation to enable users to overwrite annotations included as part of the P4 architecture definition.

8.5.7. Trade-off for v1.0 Release

For the v1.0 release of P4Runtime, it was decided not to replace occurrences of bytes with P4Data in the p4.v1.FieldMatch message, which is used to represent table and Value Set entries. This is to avoid breaking pre-release implementations of P4Runtime. Similarly it has been decided to keep using bytes to provide action parameter values. However P4Data is used whenever appropriate for PSA externs and we encourage the use of P4Data in architecture-specific extensions.

In order to support translation for action parameters and match fields, we include a type_name field in p4.config.v1.MatchField and p4.config.v1.Action.Param.

9. P4 Entity Messages

P4Runtime covers P4 entities that are either part of the P416 language, or defined as PSA externs. The sections below describe the messages for each supported entity.

9.1. TableEntry

The match-action table is the core packet-processing construct of the P4 language. It consists of a collection of table entries, or flow rules, each mapping a key value to a P4 action along with input values for the action's parameters. Packets are looked-up in the table by matching them against the flow rules. In case of a match, the corresponding action is applied on the packet, otherwise, a default action is applied. The exact behavior of P4 tables is described in the P4 specification.

P4Runtime supports inserting, modifying, deleting and reading table entries with the TableEntry entity, which has the following fields:

The priority field must be set to a non-zero value if the match key includes a ternary match (i.e. in the case of PSA if the P4Info entry for the table indicates that one or more of its match fields has a TERNARY or RANGE match type) or to zero otherwise. A higher priority number indicates that the entry must be given higher priority when performing a table lookup. Clients must allow multiple entries to be added with the same priority value. If a packet can match multiple entries with the same priority, it is not deterministic in the data plane which entry a packet will match. If a client wishes to make the matching behavior deterministic, it must use different priority values for any pair of table entries that the same packet matches.

The match and priority fields are used to uniquely identify an entry within a table. Therefore, these fields cannot be modified after the entry has been inserted and must be provided for MODIFY and DELETE updates. When deleting an entry, these key fields (along with is_default_action) are the only fields considered by the server. All other fields must be ignored, even if they have nonsensical values (such as an invalid action field). In the case of a keyless table (the table has an empty match key), the server must reject all attempts to INSERT a match entry and return an INVALID_ARGUMENT error.

The number of match entries that a table should support is indicated in P4Info (size field of Table message). The guarantees provided to the P4Runtime client are the same as the ones described in the P416 specification for the size property [29]. In particular, some implementations may not be able to always accommodate an arbitrary set of entries up to the requested size, and other implementations may provide the P4Runtime client with more entries than requested. The P4Runtime server must return RESOURCE_EXHAUSTED when a table entry cannot be inserted because of a size limitation. It is recommended that, for the sake of portability, P4Runtime clients do not try to insert additional entries once the size indicated in P4Info has been reached.

9.1.1. Match Format

The bytes fields in the FieldMatch message follow the format described in Bytestrings.

For “don't care” matches, the P4Runtime client must omit the field's entire FieldMatch entry when building the match repeated field of the TableEntry message. This requirement leads to smaller Protobuf messages overall, while enabling a canonical representation for “don't care” matches, which is needed to ensure read-write symmetry. For PSA match types, a “don't care” match for a specific match key field is defined as follows:

Note that there is no “don't care” value for EXACT matches and therefore exact match fields can never be omitted from the TableEntry message.

The following example shows a P4Runtime message that treats a TERNARY field as a “don't care” match. The P4 program defines table t with TERNARY and EXACT fields in its match key: 

table t {
  key = {
    hdr.ipv4.dip: ternary;
    istd.ingress_port: exact;
  }
  actions = {
    drop;
  }
}

In this P4Runtime request, the client omits the table's TERNARY field from the repeated match field to indicate a “don't care” match. As shown below, the match specifies only the EXACT field given by field_id: 2. 

device_id: 3
entities {
  table_entry {
    table_id: 33554439  # Table t's ID.
    match {
      # field_id 1 is not present to use the don't care ternary value.
      field_id: 2
      exact {
        value: "\x20"
      }
    }
    action {
      # Action selection goes here.
    }
  }
}

For every member of the TableEntry repeated match field, field_id must be a valid id for the table, as per the P4Info, and one of the fields in field_match_type must be set. We summarize additional constraints which depend on the match-type in the following list. If any one of them is violated, the P4Runtime server must return an INVALID_ARGUMENT error code. 

assert(BytestringValid(match.exact().value()))
assert(BytestringValid(match.lpm().value()))

pLen = match.lpm().prefix_len()
assert(pLen > 0)

trailing_zeros = countTrailingZeros(match.lpm().value())
assert(trailing_zeros >= field_bits - pLen)
assert(BytestringValid(match.ternary().value()))
assert(BytestringValid(match.ternary().mask()))
assert(match.ternary().value().size() <= match.ternary().mask().size());

value = parseInteger(match.ternary().value())
mask = parseInteger(match.ternary().mask())

assert(mask != 0)

assert(value & mask == value)
assert(BytestringValid(match.range().low()))
assert(BytestringValid(match.range().high()))

low = parseInteger(match.range().low())
high = parseInteger(match.range().high())

assert(low <= high)

assert(low != min_field_value || high != max_field_value)

9.1.2. Action Specification

The TableEntry action field must be set for every INSERT update but may be left unset for MODIFY updates, in which case the action specification for the table entry will not be modified (if a MODIFY update has an unset action field and does not modify any direct resource attached to the table then the MODIFY update is a no-op). Based on the implementation property value of the P4 table, the oneof in the TableAction message will either be:

If the oneof does not match the table description in the P4Info (e.g. the oneof is action_profile_member_id for a direct table), the server must return an INVALID_ARGUMENT error code.

The Action Protobuf message has the following fields:

For indirect tables, if the P4Runtime client provides a member or group id which has not been inserted in the corresponding action profile instance yet, the P4Runtime server must return a NOT_FOUND error code.

9.1.3. Default Entry

According to the P4 specification, the default entry for a table is always set. It can be set at compile-time by the P4 programmer — or defaults to NoAction (which is a no-op) otherwise — and assuming it is not declared as const, can be modified by the P4Runtime client. Because the default entry is always set, we do not allow INSERT and DELETE updates on the default entry and the P4Runtime server must return an INVALID_ARGUMENT error code if the client attempts one.

The default entry is identified by setting the is_default_action boolean field to true. When this flag is set to true, the repeated match field must be empty and the priority field must be set to zero, otherwise the P4Runtime server must return an INVALID_ARGUMENT error code. When performing a MODIFY update on the default entry, the client can either provide a valid action for the table or leave the action field unset, in which case the default entry will be reset to its original value, as defined in the P4 program. When resetting the default entry, its controller_metadata value as well as the configurations for its direct resources will be reset to their defaults. If the default entry is constant (as indicated by the P4 program and the P4Info message), the server must return a PERMISSION_DENIED error code if the client attempts to modify it.

Apart from the above restrictions, the default entry is treated like a regular entry, including with regards to direct resources.

In this P4Runtime release, we have decided to restrict the default entry for indirect tables — tables with an ActionProfile or ActionSelector implementation property — to a constant NoAction action entry, with the hope that it would simplify the implementation of the P4Runtime service.

9.1.4. Constant Tables

Constant tables are defined as tables whose match entries are immutable. They are identified by the is_const_table flag in P4Info.

The only write updates which are allowed for constant tables are MODIFY operations on direct resources, and the default action (assuming the default action itself is not constant). If the P4Runtime client attempts to perform any other kind of write update on a constant table the server must return a PERMISSION_DENIED error. Just like any table entry MODIFY request, the request must specify the match fields and priority to identify the table entry. Because the action of a const entry cannot be modified, the request may not specify an action, even if the action is equal to the existing action. If an action is specified, the switch will return a PERMISSION_DENIED error.

The contents of const tables can be queried by the client through a ReadRequest. When reading static (immutable) entries from a constant table, the following fields must be set by the server: table_id, match, action, is_default_action, and priority (if required). This is in addition to any direct resources that are being queried. Idle timeouts are not supported for static entries. If the table requires a priority value for entries, the server must populate the priority field appropriately, starting at 1 for the lowest priority entry and incrementing the value by 1 for each successive entry. Note that P416 does not support assigning explicit priorities to static entries. When a priority value is required (e.g. for tables including RANGE and / or TERNARY matches in the case of PSA), it is inferred based on the order in which entries appear in the table declaration.

9.1.5. Wildcard Reads

When performing a ReadRequest, the P4Runtime client can select all entries from one or all tables on the target and use several of the TableEntry fields to filter the results, much like when performing a SQL request. For each field that can be used to filter the result, the client may use the default value for the field to act as a wildcard. This default value is zero for scalar fields such as priority and “unset” for message fields such as match. The following fields may be used to select and filter results:

For example, in order to read all entries from all tables from device 3, the client can use the following ReadRequest message. 

device_id: 3
entities {
  table_entry {
    table_id: 0
    priority: 0
    controller_metadata: 0
  }
}

In order to read all entries with priority 11 from a specific table (with id 0x0212ab34) from device 3, the client can use the following ReadRequest message: 

device_id: 3
entities {
  table_entry {
    table_id: 0x0212ab34
    priority: 11
    controller_metadata: 0
  }
}

The canonical representation of “don't care” matches, combined with the ability to do a wildcard read on all table entries by leaving the match field unset, means that there exists a specific ambiguous case in which the same message could be used to either read a single “don't care” entry or to do a wildcard read. If a table has no fields with match kind EXACT, it is possible via P4Runtime to add an entry that is “don't care” for all fields (i.e. has an empty match field) but is not the default entry (i.e. is_default_action is false). When reading this entry from the table, there is no way to read only that entry from the table, because it would require providing an unset match field in the request, which in turn indicates that the client wishes to perform a wildcard read on all non-default entries. Consider the following example which uses a table with a single LPM match: 

table t {
  key = {
    hdr.ipv4.dip: lpm;
  }
  actions = {
    drop; fwd;
  }
}

The following WriteRequest message can be used to add 2 entries: 

device_id: 3
entities {
  table_entry { # don't care entry
    table_id: 0x0212ab34
    # ...
  }
  table entry {
    table_id: 0x0212ab34
    match {
      field_id: 1
      lpm {
        value: 0x0a000000
        prefix_len: 8
      }
    # ...
  }
}

The first entry is a “don't care” entry, while the second one matches all 10.0.0.0/8 addresses. The second entry has higher priority than the first one.

The following ReadRequest message will return all entries in the table, not just the “don't care” entry. 

device_id: 3
entities {
  table_entry {
    table_id: 0x0212ab34
  }
}

This issue also exists for tables with TERNARY and / or RANGE matches. However, in this case the priority is also taken into account for wildcard reads, and because a priority of 0 is not valid, in practice only the entries with the same priority as the “don't care” entry will be returned to the client. If the client uses distinct priority values for all entries — which is strongly recommended to achieve deterministic behavior —, then there is no ambiguity because the wildcard read will actually return a single entry (the “don't care” entry) as long as the priority field is set to the correct value.

9.1.6. Direct Resources

In addition to the DirectCounterEntry and DirectMeterEntry entities, P4Runtime support reading and writing direct resources as part of the TableEntry message. This is convenient for two reasons:

Once the table entry has been inserted, the P4Runtime client is free to use the DirectCounterEntry and DirectMeterEntry messages for read and write operations on DirectCounter and DirectMeter instances. For example, it is usually more convenient as well as more efficient to use DirectCounterEntry to query a counter entry value rather than use TableEntry, assuming the client is not interested in reading other table entry properties as well, such as the controller metadata cookie or the action entry.

The PSA specification states that when a table is assigned a direct resource (meter or counter), this direct resource does not need to be “executed” in every action bound to the table. It is an error to provide a direct resource configuration in a TableEntry message when programming an action that does not execute the direct resource, and the server must return an INVALID_ARGUMENT error code.

We leverage Protobuf's ability to differentiate between set and unset fields to give the P4Runtime client fined-grained control over how direct resources are read and written through the TableEntry message. The list below describes how the server must handle the meter_config and counter_data fields for read and write requests, based on whether the fields are set or not. We do not cover error cases in the list, i.e. we assume that we are dealing with a table which is assigned a direct counter / a direct meter, and that the action being used for the table entry “executes” the direct resource appropriately.

In its default configuration, a meter returns the GREEN color for every packet when it is executed. This default configuration can be achieved by leaving the meter_config field unset when inserting or modifying a table entry. When modifying a table entry, if the P4Runtime client wishes to maintain the same meter configuration, it needs to be provided again in the TableEntry message (i.e. the meter_config field must be set to match the existing configuration).

9.1.7. Idle-timeout

P4Runtime supports idle timeout for table entries. When adding a table entry, the client can specify a Time-To-Live (TTL) value. If at any time during its lifetime, the data plane entry is not “hit” (i.e. not selected by any packet lookup) for a lapse of time greater or equal to its TTL, the P4Runtime should, with best effort, generate a stream notification — using the IdleTimeoutNotification message — to the master client, which can then take action, such as remove the idle table entry.

Two fields of the TableEntry Protobuf message are used to implement idle timeout:

These fields can only be set if idle timeout is supported for the table, as per the P4Info message. If idle timeout is not supported by the table, the P4Runtime server must return an INVALID_ARGUMENT error code if at least one of these conditions is met:

The target should do its best to approximate the idle_timeout_ns value provided by the client. For example, most targets may not be able to accommodate arbitrarily small values of TTL, in which case they should use the smallest value they can support, rather than reject the TableEntry write with an error code. Similarly, each target should do its best to provide reasonably-accurate values for time_since_last_hit.

P4Runtime does not support idle timeout for default entries. When the is_default_action flag is set in a TableEntry message, idle_timeout_ns must be set to 0 (default) and time_since_last_hit must be unset. If the server receives a TableEntry message which violates this, it must return an INVALID_ARGUMENT error.

For more information about idle timeout, in particular regarding IdleTimeoutNotification, please refer to the Table idle timeout notifications section.

9.2. ActionProfileMember & ActionProfileGroup

P4Runtime defines an API for programming a PSA ActionProfile extern using ActionProfileMember messages. A PSA ActionSelector extern can be programmed using both ActionProfileMember and ActionProfileGroup messages. PSA supports tables that can be implemented with an action profile or selector instance. Such tables are referred to as indirect tables, in contrast to direct tables, whose entries are directly bound to an action instance. The following P4 snippet illustrates an indirect table t for L3 routing, implemented with an action selector as. 

ActionSelector(HashAlgorithm.crc32,
               /*size = */ 32w1024,
               /*output_width = */ 32w10) as;

action set_nhop(PortId_t p, EthAddr smac, EthAddr dmac) {
  istd.egress_port = p;
  hdr.ethernet.smac = smac;
  hdr.ethernet.dmac = dmac;
}

table t {
  key = {
    hdr.ipv4.dip: lpm;  // LPM on destination IP address
  }
  actions = {
    set_nhop;
  }
  implementation = as;
}

When programming table t in the example above, a P4Runtime client should specify the TableAction in the TableEntry to be a reference to either an action profile member or group. The reference is a uint32 identifier that uniquely identifies a member or group programmed in the action selector as.

If a table entry in an indirect table with an ActionProfile implementation is hit, then the corresponding table action gives a member id. The member table is looked up with the member id, and the corresponding action specification is used to modify the packet or its metadata.

If a table entry in an indirect table with an ActionSelector implementation is hit, then the corresponding table action gives either a member id or a group id. For a member id, the member table in the selector is looked up, and the corresponding action specification is used to modify the packet or its metadata. For a group id, a hash algorithm, defined in the P4 ActionSelector specification is used to obtain a member id from the set of members in the group. For example, the hash algorithm in the P4 example above is 32-bit CRC. The obtained member id is used to look up the member table in the selector and obtain the action specification, which is then used to modify the packet or its metadata.

9.2.1. Action Profile Member Programming

Action profile members are entries in the ActionProfile or ActionSelector and are referenced by a uint32 identifier that is bound to an action specification. An action profile member for an ActionProfile or ActionSelector extern instance may be bound only to the actions that appear in the actions attribute of the table implemented using the extern instance. If multiple table implementations share an extern instance, then the actions attributes of the tables must have an identical list of P4 actions. The IDs of the tables implemented with a selector will appear in P4Info as part of the ActionProfile message for the selector.

An ActionProfileMember entity update message has the following fields:

An action profile member may be inserted, modified or deleted as per the following semantics.

9.2.2. Action Profile Group Programming

Action profile groups are entries in an ActionSelector and are referenced by a uint32 identifier that is bound to a set of action profile members already programmed in the selector. The action profile members in a group must be bound to actions of the same type.

An ActionProfileGroup entity update message has the following fields:

An action profile group may be inserted, modified or deleted as per the following semantics.

When setting the group membership with INSERT or MODIFY, the members repeated field must not include duplicates, i.e. members with the same member_id. The weight field is used instead to logically “repeat” the member inside the group.

It is explicitly allowed for the same member to be present in multiple groups at the same time. If, as a result, an implementation “stores” the action id and parameters in the target in multiple locations, the server must update all of those locations when a request to modify such a member is made.

9.2.2.1. Rules on Setting max_size

The valid values for max_size depend on the static max_group_size included in the P4Info message:

9.2.3. One Shot Action Selector Programming

P4Runtime supports syntactic sugar to program a table, which is implemented with an action selector, in one shot. One shot means that a table entry, an action profile group, and a set of action profile members can be programmed with a single update message. Using one shots has the advantage that the controller does not need to keep track of group ids and member ids.

One shots are programmed by choosing the ActionProfileActionSet message as the TableAction. The ActionProfileActionSet message consists of a set of ActionProfileAction messages, which in turn have the following fields:

Semantically, one shots are equivalent to programming the table entry, group, and members individually; with the necessary group id and member ids bound to unused ids. An implementation is free to implement one shots in other ways, as long as the implementation matches the above semantics.

To preserve read-write symmetry, an implementation must answer ReadRequests with the original one shot messages. It may not return a desugared version of the one shot message.

For example, consider the action selector table defined here. This table could be programmed with the following one shot update: 

table_entry {
  table_id: 0x0212ab34
  match { /* lpm match */ }
  action {
    action_profile_action_set {
      action_profile_actions {
        action { /* set nexthop 1 */ }
        weight: 1
        watch: 1
      }
      action_profile_actions {
        action { /* set nexthop 2 */ }
        weight: 2
        watch: 2
      }
      action_profile_actions {
        action { /* set nexthop 3 */ }
        weight: 3
        watch: 3
      }
    }
  }
}

Which would be equivalent to the following updates, where GROUP_ID, MEMBER_ID_1, MEMBER_ID_2, and MEMBER_ID_3 are unused ids: 

action_profile_member {
  action_profile_id: 1
  member_id: MEMBER_ID_1
  action { /* set nexthop 1 */ }
}
action_profile_member {
  action_profile_id: 1
  member_id: MEMBER_ID_2
  action { /* set nexthop 2 */ }
}
action_profile_member {
  action_profile_id: 1
  member_id: MEMBER_ID_3
  action {  /* set nexthop 3 */ }
}
action_profile_group {
  action_profile_id: 0x11ab12cd
  group_id: GROUP_ID
  members {
    member_id: MEMBER_ID_1
    weight: 1
    watch: 1
  }
  members {
    member_id: MEMBER_ID_2
    weight: 2
    watch: 2
  }
  members {
    member_id: MEMBER_ID_3
    weight: 3
    watch: 3
  }
}
table_entry {
  table_id: 0x0212ab34
  match { /* lpm match */ }
  action { action_profile_group_id: GROUP_ID }
}

Note that when using the above method (members and groups), the client also needs to use multiple messages to ensure correct ordering between the dependent updates. Members need to be inserted first, then the group needs to be created, and finally the match entry can be inserted. Therefore, 3 distinct WriteRequest batches are required.

It is possible to include several ActionProfileAction messages with the same exact action specification in one ActionProfileActionSet message. However, the P4Runtime client is encouraged not to do so, as the same can be achieved by using the weight field. Note that to preserve read-write symmetry, the server must not coalesce multiple ActionProfileAction messages with the same action specification into one.

All the tables associated with an action selector may either be programmed exclusively with one shots, or exclusively with ActionProfileMember and ActionProfileGroup messages. Programming some entries with one shots, and other entries with ActionProfileMember and ActionProfileGroup messages is not allowed, and the server must return the error code INVALID_ARGUMENT in that case.

A P4Runtime server must support the one shot style of programming tables with an action selector implementation. Support for the ActionProfileMember and ActionProfileGroup style is optional. If ActionProfileMember and ActionProfileGroup are not supported by a server, it must return an UNIMPLEMENTED error for every ActionProfileMember or ActionProfileGroup message that it receives.

9.2.4. Constraints on action selector programming

The PSA specification states that the following features are optional in action selector implementations [21]:

  1. Support for non-empty groups where in the same group, different members are bound to different actions.
  2. Predictable data plane behavior when a matched table entry points to an empty group.

For 1., if a client tries to INSERT or MODIFY a group with members bound to different actions, the server should return UNIMPLEMENTED if not supported by the target. This applies to the one shot style of programming as well. We recommend that control plane implementations take into account this possible limitation and be designed so as not to rely on this feature for the sake of portability. A target with this restriction is also not expected to support modifying the action function of a member which is part of one or more groups and should return UNIMPLEMENTED (modifying the action parameter values must be supported, however).

PSA 1.1 introduces the psa_empty_group_action table property in order to enable the P4 programmer to specify the action to perform on the packet when the matched table entry points to an empty action selector group. This action may be different from the default action, which is performed in case of table miss. psa_empty_group_action is one possible way to achieve property 2. in the list above. We recommend that all P4Runtime implementations support this property. Note that this version of P4Runtime does not provide any mechanism to modify the value of psa_empty_group_action at runtime, so the value will be constant and will either be provided by the P4 programmer or will default to NoAction. Even when psa_empty_group_action is not implemented by the target, P4Runtime does not require the server to return an error code when the client performs an operation which results in an empty group, despite the possibility for undeterministic or target-specific behavior. It is likely that future PSA versions will make the implementation of psa_empty_group_action mandatory and that future P4Runtime versions will provide a mechanism to change the property value dynamically. Note that the discussion above also applies to the one shot style of programming.

The PSA specification includes a discussion on how to implement psa_empty_group_action in software in the P4Runtime server [24].

9.3. CounterEntry & DirectCounterEntry

PSA defines Counters as a mechanism for keeping statistics of bytes and packets. Statistics may be updated as a result of an action associated with a table entry, or a direct invocation such as from a P4 control. The CounterData P4Runtime message can be used for all three types of PSA counters — PACKETS, BYTES and PACKETS_AND_BYTES — and consists of the following fields: 

message CounterData {
  int64 byte_count = 1;
  int64 packet_count = 2;
}

P4Runtime does not distinguish between the different PSA counter types, and allows for simultaneous updates of byte_count and packet_count fields, which is equivalent to specifying the counter type PACKETS_AND_BYTES. Counters may be defined as direct or indirect (indexed) instances.

9.3.1. DirectCounterEntry

A direct counter is a direct resource associated with a TableEntry (see Direct Resources). The counter_data field of the TableEntry message can be used to initialize the counter value at the same time as the table entry is inserted. Once the table entry has been created, the P4Runtime client may modify the associated direct counter entry using the DirectCounterEntry message. Once the table entry is deleted the associated direct counter entry can no longer be accessed. 

message DirectCounterEntry {
  TableEntry table_entry = 1;
  CounterData data = 2;
}

A WriteRequest may only include an Update message of type MODIFY with a DirectCounterEntry, whose fields are specified by the client as follows:

Specifying DirectCounterEntry in an Update message of type INSERT or DELETE is not allowed, and the server must return the error code INVALID_ARGUMENT in that case.

A client may use ReadRequest in two ways to read the contents of a DirectCounter:

9.3.2. CounterEntry

An indirect or indexed counter is not associated with a specific TableEntry and may be updated independently of any action. It may be read or written using the P4Runtime CounterEntry message whose fields are defined as follows: 

message CounterEntry {
  uint32 counter_id = 1;
  Index index = 2;
  CounterData data = 3;
}

The CounterEntry can only be used in a WriteRequest with the MODIFY update type. The P4Runtime server must return an INVALID_ARGUMENT error code for update types INSERT and DELETE. By default all the counter entries in the array have default value 0.

A P4Runtime client may request to read the counter values of one or more indirect counter instances with a ReadRequest by including a CounterEntry entity for each of the instances, specifying the counter_id and index. Wildcard reads are also supported as follows.

9.4. MeterEntry & DirectMeterEntry

Meters are an advanced mechanism for keeping statistics, involving stateful “marking” and usually “throttling” of packets based on configured rates of traffic. The PSA metering function is based on the Two Rate Three Color Marker (trTCM) defined in RFC 2698 [2]. The trTCM meters an arbitrary packet stream using two configured rates — the Peak Information Rate (PIR) and Committed Information Rate (CIR), and their associated burst sizes — and “marks” its packets as GREEN, YELLOW or RED based on the observed rate.

A meter may be configured as a direct or indirect instance, similar to a counter. The MeterConfig P4Runtime message represents meter configuration. 

message MeterConfig {
  int64 cir = 1;  // Committed Information Rate
  int64 cburst = 2;  // Committed Burst Size
  int64 pir = 3;  // Peak Information Rate
  int64 pburst = 4;  // Peak Burst Size
}

9.4.1. DirectMeterEntry

A direct meter is a direct resource associated with a TableEntry (see Direct resources). The meter_config field of the TableEntry message can be used to initialize the meter configuration at the same time as the table entry is inserted. Once the table entry has been created, the P4Runtime client may modify the associated direct meter entry using the DirectMeterEntry message. Once the table entry is deleted the associated direct meter entry can no longer be accessed. 

message DirectMeterEntry {
  TableEntry table_entry = 1;
  MeterConfig config = 2;
}

A WriteRequest may only include an Update message of type MODIFY with a DirectMeterEntry, whose fields are specified by the client as follows:

Specifying DirectMeterEntry in an Update message of type INSERT or DELETE is not allowed, and the server must return the error code INVALID_ARGUMENT in that case.

A client may use ReadRequest in two ways to read a DirectMeter config.

9.4.2. MeterEntry

An indirect or indexed meter is not associated with a specific TableEntry and may be executed independently of any action. Its configuration may be read or written using the P4Runtime MeterEntry message whose fields are defined as follows: 

message MeterEntry {
  uint32 meter_id = 1;
  Index index = 2;
  MeterConfig config = 3;
}

The MeterEntry can only be used in a WriteRequest with the MODIFY update type. The P4Runtime server must return an INVALID_ARGUMENT error code for update types INSERT and DELETE. By default all the meter entries in the array have a default configuration (GREEN for all packets).

A P4Runtime client may request to read the configuration of one or more indirect meter instances with a ReadRequest by including a MeterEntry entity for each of the instances, specifying the meter_id and index. Wildcard reads are also supported as follows:

9.5. PacketReplicationEngineEntry

The PSA Packet Replication Engine (PRE) is an extern that is implicitly instantiated in all PSA programs. The PRE is responsible for implementing multicasting and cloning functionality in the data plane. P4Runtime defines an API to program the PRE with multicast groups and clone sessions to allow replication of data plane packets.

9.5.1. MulticastGroupEntry

Multicasting is achieved in PSA programs by setting the multicast_group ingress output metadata to a non-zero identifier. The number of replicas and their egress ports for the multicast group is programmed at runtime by the client using the MulticastGroupEntry API in P4Runtime. The following P4 program illustrates a possible data plane behavior of multicasting ARP packets in the ingress. Note that the data plane type of the multicast group metadata is 10 bits on the PSA device in this example. 

control arp_multicast(inout H hdr, inout M smeta) {
  apply {
    if (hdr.ethernet.isValid() &&
        hdr.ethernet.eth_type == ETH_TYPE_ARP) {
      smeta.multicast_group = (MulticastGroup_t) 1;
    }
  }
}

At runtime, the client writes the following update in the target (shown in Protobuf text format). 

type: INSERT
entity {
  packet_replication_engine_entry {
    multicast_group_entry {
      multicast_group_id: 1
      replicas { egress_port: 5 instance: 1 }
      replicas { egress_port: 12 instance: 2 }
      replicas { egress_port: 18 instance: 3 }
      replicas { egress_port: 24 instance: 4 }
    }
  }
}

As a result of the above P4Runtime programming, the target device will create four replicas of an ARP packet. These replicas will appear in the egress pipeline as independent packets with egress port set to PSA device port numbers corresponding to SDN port numbers 5, 12, 18 and 24. For more discussion on the translation between SDN ports and PSA device ports, refer to the PSA Metadata Translation section.

The egress packets may be distinguished for further processing in the egress using the instance metadata. Note that a packet may not be both unicast and multicast; if the multicast group is set, it will override the unicast egress port. If the P4 multicast_group metadata is set to a value that is not programmed in the PRE, then the packet is dropped.

A multicast group may be inserted, modified or deleted as per the following semantics.

When reading a multicast group, only multicast_group_id is considered. All other fields in MulticastGroupEntry are ignored. To perform a wildcard Read on all configured multicast group entries, the multicast_group_id field must be set to 0, its default value.

9.5.1.1. Valid Values for multicast_group_id

The PSA specification states that the valid data plane values for multicast group ids (MulticastGroup_t) range from 1 (0 is a special value that indicates no multicast replication is to be performed for a packet) to the maximum value supported by the target [23]. This means that, in the absence of translation, the client must set the multicast_group_id field to a value in this range when inserting a multicast group. However, because P4Runtime reserves 0 as a special wildcard value which is used to read all the multicast groups configured in the target, the multicast_group_id field must never be set to 0 when performing a Write RPC, even when numerical translation is enabled for multicast group ids. In other words, it is not possible to map (using translation) a zero SDN multicast group id value to a non-zero data plane multicast group id value.

9.5.2. CloneSessionEntry

PSA supports cloning of packets in both the ingress and egress pipeline. Ingress cloning creates a mirror of the packet as seen in the beginning of the ingress pipeline, while egress cloning creates a mirror of the packet as seen at the end of the egress pipeline. A packet is cloned in the data plane by setting a clone_session_id identifier and a boolean flag clone in the packet metadata. The clone_session_id serves as a handle to the clone attributes, namely a set replicas of (egress port, instance) pairs to which cloned packets should be sent, a packet length, and class of service. These are programmed at runtime via the P4Runtime CloneSessionEntry API.

The following P4 program illustrates a possible data plane behavior of sending clones of low TTL packets to the CPU for monitoring. Note that the data plane type of the clone session metadata is 10 bits on the PSA device in this example. We assume that the clone_low_ttl control block is applied in the ingress pipeline to create an ingress-to-egress clone. 

control clone_low_ttl(inout H hdr, inout M smeta) {
  apply {
    if (hdr.ipv4.isValid() &&
        hdr.ipv4.ttl <= LOW_TTL_THRESHOLD) {
      smeta.clone_session_id = 10w100;
      smeta.clone = true;
    }
  }
}

At runtime, the client writes the following update in the target (shown in Protobuf text format). 

type: INSERT
entity {
  packet_replication_engine_entry {
    clone_session_entry {
      session_id: 100
      replicas { egress_port: 0xfffffffd instance: 1 } # to CPU
      class_of_service: 2
      packet_length_bytes: 4096
    }
  }
}

As a result of the above P4Runtime programming, the target device will create one replica of a low TTL packet from the ingress to the egress. Note that the clone session ID of the programmed PRE entry is identical to the value used in the data plane in this example (no numerical translation, which is the default for values of type CloneSessionId_t [23]). The clone will be treated for scheduling in the PRE with a class of service value of 2. If the packet is larger than 4096 bytes, it will be truncated to carry at most 4096 bytes.

The cloned replica will appear in the egress pipeline as an independent packet with egress port set to CPU (corresponding to SDN port 0xfffffffd; see Translation of Port Numbers). Note that the egress port must be a 32-bit SDN port number and must refer to a singleton port.

If the clone_session_id data plane metadata is set to a value that is not programmed in the PRE, then no clones are created.

A clone session may be inserted, modified or deleted as per the following semantics:

When reading a clone session, only session_id is considered. All other fields in CloneSessionEntry are ignored. To perform a wildcard Read on all configured clone session entries, the session_id field must be set to 0, its default value. The session_id field can never be equal to 0 in a Write RPC. If it does, the server must return an INVALID_ARGUMENT error.

9.5.2.1. Valid Values for session_id

The PSA specification states that the valid data plane values for clone session ids (CloneSessionId_t) range from 0 to the maximum value supported by the target [23]. Note that unlike for multicast group ids, 0 is a valid data plane value for clone session ids. However, just like for multicast group ids, P4Runtime reserves 0 as a special wildcard value which is used to read all the clone sessions configured in the target. This means that 0 can never be used as a session_id value when inserting a clone session, whether or not numeric translation is enabled for clone session ids. If translation is not enabled, we effectively “lose” one clone session, assuming the target supports 0 as mandated by the PSA specification. If this is an issue (e.g. because the target supports a very limited number of clone sessions), one can enable translation on CloneSessionId_t and map any non-zero SDN session id to the data plane clone session with id 0, then insert a clone session with the chosen SDN session id.

9.6. ValueSetEntry

Parser Value Set is a construct in P4 that is used to support programmability of parser state transitions. A transition select statement in P4 can use a parser Value Set to define a runtime programmable state transition as shown in the example below. A runtime programmable set of TRILL Ethertypes is used to transition the parser state machine to the parse_trill_types state. 

state parse_l2 {
  @id(1) value_set<ETH_TYPE_BITWIDTH>(MAX_TRILL_TYPES) trill_types;
  extract(hdr.ethernet);
  select (hdr.ethernet.eth_type) {
    ETH_TYPE_IPV4: parse_ipv4;
    ETH_TYPE_IPV6: parse_ipv6;
    trill_types:   parse_trill_types;
    _:             reject;
  }
}

The corresponding entry in the P4Info for this Value Set is: 

value_sets {
  preamble {
    id: 0x03000001
    name: "trill_types"
  }
  match {
    id: 1
    bitwidth: <ETH_TYPE_BITWIDTH>
    match_type: EXACT
  }
  size: <MAX_TRILL_TYPES>
}

At runtime, the client writes the following update in the target (shown in Protobuf text format). 

type: MODIFY
entity {
  value_set_entry {
    value_set_id: 0x03000001
    members {
      match {
        field_id: 1
        exact { value: 0x22f3 }
      }
      match {
        field_id: 1
        exact { value: 0x893b }
      }
    }
  }
}

As a result of the above P4Runtime programming, all packets with EtherType values of 0x22f3 and 0x893b will be parsed as per the state machine starting at the parse_trill_types state.

A ValueSetEntry entity update message has the following fields:

A ValueSetEntry may only be modified. If the update type is INSERT or DELETE, the server must return an INVALID_ARGUMENT error. If the update type is MODIFY, the server writes the members given in the repeated field to the Value Set entry indexed by the given value_set_id. The maximum number of matches must not exceed the maximum size given by the size field in P4Info of the Value Set, otherwise the server must return a RESOURCE_EXHAUSTED error. To empty a Value Set (i.e. restore it to its initial state), the P4Runtime client can perform a MODIFY update with an empty members repeated field.

To facilitate read-write symmetry, the server must return an ALREADY_EXISTS error in case of duplicate members. Unlike for match tables, a priority value is not required for ternary and range matches: overlapping entries do not need to be ordered and the parse state transition is determined by whether or not the packet matches at least one entry in the set.

See Appendix A.3 for a more complex Value Set example.

9.7. RegisterEntry

The PSA Register extern is a stateful memory array that can be read and written during packet forwarding. The RegisterEntry P4Runtime entity is used by the client to read and write the contents of a Register instance as part of control plane operations.

RegisterEntry has the following fields:

9.8. DigestEntry

A digest is one mechanism to send a message from the data plane to the control plane. It is traditionally used for MAC address learning: when a packet with an unknown source MAC address is received by the device, the control plane is notified and can populate the L2 forwarding tables accordingly.

The DigestEntry P4Runtime entity is used to configure how the device must generate digest messages. The DigestEntry Protobuf message is not used to carry digest data, which is done on the StreamChannel bidirectional stream using the DigestList (digest data sent by the target to the client) and DigestListAck (digest data acknowledgments sent by the client to the target) Protobuf messages.

In this section, we refer to the data learned by a single data plane call to Digest<T>::pack as a “digest message” and we use “digest list” to designate the list of digest messages bundled by the P4Runtime service in a single DigestList stream message. Note that all the digest messages in a single digest list correspond to the same P4 Digest extern instance. We say that 2 digest messages are “duplicate” if the data emitted by the data plane is exactly the same as per P4 equality rules. We say that 2 digest messages are “distinct” if they are not duplicate.

DigestEntry has the following fields:

Here is the significance of the different Update types for DigestEntry:

A server should buffer digest messages until either:

At which point the server should, with best effort, generate a DigestList stream message with the buffer contents and send it to the master client. All the messages in a digest list must be distinct, which means that duplicates must either be filtered-out directly by the device or in the P4Runtime server software.

To avoid sending duplicate digest messages across different DigestList messages, which could make the channel busy, we define an acknowledgement mechanism through which the master client indicates that it has received the digest list and acted on it. The server must keep a “cache” containing the set of all digest messages that have been sent, but not acknowledged yet by the master client, up-to ack_timeout_ns in the past. The server must delete all cache entries for a given digest list when they are at least ack_timeout_ns old or when a matching DigestListAck message (i.e. with the same digest_id and list_id fields as the DigestList message) is received.

The acknowledgement mechanism described above is not used to implement some sort of reliable transport for digest messages. The loss of digest messages or acknowledgement messages is considered non-critical. The P4Runtime server may drop digest messages if they are generated from the data plane faster than the server software, the channel or the client can handle. P4Runtime does not impose a limit on the number of in-flight, unacknowledged DigestList messages.

When max_timeout_ns is set to 0 and / or max_list_size is set to 1, the server should, with best effort, generate a DigestList message for every digest message generated by the data plane which is not already in the cache. If ack_timeout_ns is set to 0, the cache must always be an empty set. If max_list_size is set to 0, there is no limit on the maximum size of digest lists: the server can use any non-zero value as long as it honors the max_timeout_ns configuration parameter.

The P4Runtime server may empty the digest message cache in case of a client mastership change.

Here is some pseudo-code implementing the handling of digest messages in the P4Runtime server: 

DigestStream stream;
DigestCache cache;
DigestBuffer buffers;

// sends digest list when it is ready
send_buffer(Id digest_id) {
  buffer = buffers[digest_id];
  stream.write(DigestList(buffer));
  cache.merge(buffer);  // updates cache with new digest list
  buffer.clear();
}

// callback which handles data plane digest messages from device
handle_dataplane_digest(Digest msg) {
  digest_id = msg.digest_id();
  buffer = buffers[digest_id];
  if (msg in cache OR msg in buffer) return;
  buffer.enqueue(msg);
  if (buffer.length() < max_list_size(digest_id)) return;
  send_buffer(digest_id);
}

// callback which handles ack messages received on the stream
handle_stream_ack(DigestListAck ack) {
  // clear all cache entries matching the tuple (digest_id, list_id)
  cache.erase( (ack.digest_id(), ack.list_id() )
}

// loop to enforce timeouts
while (true) {
  now = now();
  // check for buffers that need to be sent
  for ((digest_id, buffer) in buffers) {
    if (now - buffer.first_enq_time() >= max_timeout_ns(digest_id))
      send_buffer(buffer_id);
  }
  // check for expired entries in cache
  for ((digest_id, list_id, sent_time) in cache) {
    if (now - sent_time >= ack_timeout_ns(digest_id))
      cache.erase( (digest_id, list_id) );
  }
  sleep(X);
}

9.9. ExternEntry

This is used to support a P4 extern entity that is not part of PSA. It is defined as: 

message ExternEntry {
  uint32 extern_type_id = 1;
  uint32 extern_id = 2;
  google.protobuf.Any entry = 3;
}

Each ExternEntry entity maps to an Extern message in the P4Info and an ExternInstance message within that message. The extern_type_id field must be equal to the one in ExternEntry. The extern_id field must be equal to the ID included in the preamble of the corresponding ExternInstance message.

entry itself is embedded as an Any Protobuf message [30] to keep the protocol extensible. It includes the extern-specific parameters required by the P4Runtime server to perform the read or write operation. The underlying Protobuf message should be defined in a separate architecture-specific Protobuf file. See section on Extending P4Runtime for non-PSA Architectures for more information.

10. Error Reporting Messages

P4Runtime is based on gRPC and all RPCs return a status to indicate success or failure. gRPC supports multiple language bindings; we use C++ binding below to explain how error reporting works in the failure case.

gRPC uses grpc::Status [31] to represent the status returned by an RPC. It has 3 attributes: 

StatusCode code_;
grpc::string error_message_;
grpc::string binary_error_details_;

The code_ represents a canonical error [33] and describes the overall RPC status. The error_message_ is a developer-facing error message, which should be in English. The binary_error_details_ carries a serialized google.rpc.Status message [27] message, which has 3 fields: 

int32 code = 1;  // see code.proto
string message = 2;
repeated google.protobuf.Any details = 3;

The code and message fields must be the same as code_ and error_message_ fields from grpc::Status above. The details field is a list that consists of p4.Error messages that carry error details for individual elements inside batch-request RPCs (e.g. Write and Read). p4.Error includes a canonical error code but also enables different target vendors to additionally express their own error codes in their chosen error-space. This specification document tries to cover all possible generic error cases and to provide the appropriate value for the canonical error code based on best practices [33].

Figure 7 illustrates how these messages fit together.

error-report

Figure 7. P4Runtime Error Report Message Format

gRPC provides utility functions ExtractErrorDetails() and SetErrorDetails() [32] to easily convert between grpc::Status and google.rpc.Status.

Please see sections on individual P4Runtime RPCs for details on how grpc::Status is populated for reporting errors.

Note that P4Runtime also provides a way for the server to asynchronously report errors which occur when processing stream messages from the client. This error-reporting mechanism is orthogonal to the one described in this section, which applies to unary RPCs. See the section on Stream Error Reporting for more information.

11. Atomicity of Individual Write and Read Operations

Each individual entity in a batch is guaranteed to be read or written atomically relative to packet forwarding. For example, for every table data plane apply operation, and every single Write operation on a table that inserts, deletes, or modifies one table entry, the apply operation should behave as if that Write operation has not yet occurred, or as if the Write operation is complete. The P4 program should never behave as if the Write operation is partially complete. These guarantees also apply to extern instances: Read and Write operations on extern entities must execute atomically relative to extern data plane methods.

The atomicity guarantees provided by P4Runtime for individual Read and Write operations are the same as the guarantees required by PSA and are described in details in the PSA specification [22].

The P416 language introduces an @atomic annotation [13], to guarantee atomic data plane execution of entire blocks of P4 code. P4Runtime implementations are required to honor the @atomic annotation for Write operations, as well as non-wildcard Read operations, relative to data plane execution. Consider the following P4 example written for PSA: 

control C1 {
  typedef bit<10> Index_t;
  typedef bit<32> Value_t;
  Register<Value_t, Index_t>(32w1024) r1;

  apply {
    // ...
    @atomic {
        Value_t v = r1.read((Index_t)1);
        v = v + 1;
        r1.write((Index_t)1, v);
    }
  }
}

If a P4Runtime server is processing messages which write to Register r1 at index 1, these writes must not happen between the data plane read and write.

Now let's consider the following example: 

control C1 {
  typedef bit<10> Index_t;
  typedef bit<32> Value_t;
  Register<Value_t, Index_t>(32w1024, (Value_t)0 /* initial value */) r1;

  apply {
    @atomic {
        r1.write((Index_t)1, (Value_t)100);
        r1.write((Index_t)2, (Value_t)100);
    }
    @atomic {
        r1.write((Index_t)1, (Value_t)200);
        r1.write((Index_t)2, (Value_t)200);
    }
  }
}

If a P4Runtime client issues a wildcard Read on Register r1, there is no guarantee that r1[1] == r1[2] in the response, as the read for r1[1] may occur after the data plane executes the first atomic block, but before the second atomic block, and the read for r1[2] may occur after the data plane executes the second atomic block. In other words, the server is explicitly allowed to read r1[1] and r1[2] separately, while allowing the data plane to perform operations on the register between those two reads. The atomicity guarantees for a wildcard read are the same as for the equivalent batch (as one ReadRequest message) of individual read requests. Similar to a batch ReadRequest, a wildcard read of a register can execute the reads of the register array elements (r1[1], r1[2], …) in an arbitrary order relative to each other.

If the @atomic annotation cannot be honored with the above guarantees by the P4Runtime implementation for a P4-programmable target, we expect the P4 compiler to reject the program.

12. Write RPC

The Write RPC updates one or more P4 entities on the target. The request is defined as follows: 

message WriteRequest {
  uint64 device_id = 1;
  uint64 role_id = 2;
  Uint128 election_id = 3;
  repeated Update updates = 4;
  enum Atomicity {
    CONTINUE_ON_ERROR = 0;
    ROLLBACK_ON_ERROR = 1;
    DATAPLANE_ATOMIC = 2;
  }
  Atomicity atomicity = 5;
}

The device_id uniquely identifies the target P4 device. The role_id and election_id define the client role and election-id as described in the Master-Slave Arbitration and Controller Replication section. The server is expected to perform the following checks (in this order) before processing the updates list:

  1. If device_id does not match any of the devices known to the P4Runtime server or if role_id does not match any of the roles for the device, the server must return a NOT_FOUND error.

  2. If the client is not the master for (device_id, role_id) according to the election_id value, the server must return a PERMISSION_DENIED error.

  3. If the Write is attempted before a ForwardingPipelineConfig has been set, the server must return a FAILED_PRECONDITION error.

The updates field is a list of P4 entity updates to be applied. Each update is defined as: 

message Update {
  enum Type {
    UNSPECIFIED = 0;
    INSERT = 1;
    MODIFY = 2;
    DELETE = 3;
  }
  Type type = 1;
  Entity entity = 2;
}

This is modeled as performing an update operation on the given entity against its entity container. The entity container is either a logical table (e.g. CounterEntry) or an actual table (e.g. TableEntry) in the P4 data plane. Each entity in the container is uniquely identified by its key. Please refer to the P4 Entity Messages section for details on what parts of the entity specification make up the key for each P4 entity.

An update can be one of the following types:

If an update is not allowed under the given controller role, the server must return a PERMISSION_DENIED error for this update.

12.1. Batching and Ordering of Updates

P4Runtime supports batching of Write operations. The list of updates in a WriteRequest is referred to as a batch. A batch can consist of arbitrary updates on an arbitrary set of P4 entities. It is not restricted to a particular entity or table (in the case of TableEntry entities).

The P4Runtime server may choose to reorder updates in a batch when processing them, and/or process updates in parallel. Updates across multiple concurrent WriteRequests can also be processed interleaved and/or in parallel. However, the processing of requests must be strictly serializable. That is, given a history $S$ of WriteRequests including the responses to those requests, there must exist an order $L$ for all updates in $S$, such that:

  1. All updates from the same write request must appear as a contiguous subsequence in $L$, but the order within that subsequence can be arbitrary.
  2. For two updates $u_1$ and $u_2$, if the write request containing $u_1$ completed before the write request of $u_2$ was sent, then $u_1$ must appear before $u_2$ in $L$.
  3. Executing all updates in $L$ sequentially must yield the same response for every update as in $S$.
  4. The observable state of the switch after $S$ (e.g., through the Read RPC) is identical to the one obtained by sequentially executing $L$.

The Write RPC demarcates the batch boundary, and can be used to ensure ordering between dependent updates. When the Write RPC returns, it is required that all operations in the batch have been committed to the P4 data plane, with the exception of any operations that return an error status. If two updates from the client depend on each other (e.g. inserting an ActionProfileMember followed by pointing a TableEntry to it) and the updates are not split into separate batches, then the behavior may be non-deterministic. Similarly, clients can invoke multiple outstanding Write RPCs. If the updates across these RPCs have dependencies, the observed behavior may be non-deterministic, as the server can process these RPCs in any arbitrary order, providing strict serializability is enforced. For this reason, most clients are advised to split dependent updates across separate Write calls. Additionally, if the client wants to enforce that batches are applied in a specific order, each Write call should be sent sequentially, waiting for the previous call to be acknowledged before sending the next one.

12.2. Batch Atomicity

A P4Runtime server may arbitrarily reorder messages within a batch. The atomicity semantics of the batch operations are defined by the Atomicity enum. A P4Runtime server is required to support only the modes marked as Required below:

There is no expectation that a given client must always use the same Atomicity enum value. At any given time, the client is free to compose batches and assign atomicity mode as it sees fit. For example, for a set of entities, a client may decide to use DATAPLANE_ATOMIC at one time and default behavior (CONTINUE_ON_ERROR) at other times.

12.3. Error Reporting

Please see section Error Reporting Messages for information on error reporting messages and guidelines. P4Runtime server will populate grpc::Status as follows:

  1. If all batch updates succeeded, set grpc::Status::code_ to OK and do not populate any other field.

  2. If an error is encountered before even trying to attempt individual batch updates, set grpc::Status::code_ that best describes that RPC-wide error. For example, use UNAVAILABLE if the P4Runtime service is not yet ready to handle requests. Set error_message_ to describe the issue. Do not set error_details in this case.

  3. Otherwise, if one or more updates in the batch (WriteRequest.updates) failed, set grpc::Status::code_ to UNKNOWN. For example, one update in the batch may fail with RESOURCE_EXHAUSTED and another with INVALID_ARGUMENT. A p4.Error message is used to capture the status of each and every update in the batch. The number of p4.Error messages packed into google.rpc.Status.details field should therefore always match the number of updates in the WriteRequest, and the order of p4.Error messages must be in the same order as the corresponding updates. If some of the updates were successful, the corresponding p4.Error should set the code to OK and omit other fields. 

# Example of a grpc::Status returned for a Write RPC with a batch of 3 updates.
# The first and third updates encountered an error, while the second update
# succeeded.
code_ = 2  # UNKNOWN
error_message_ = "Write failure."
binary_error_details {
  code: 2  # UNKNOWN
  message: "Write failure."
  details {
    canonical_code: 8  # RESOURCE_EXHAUSTED
    message: "Table is full."
    space: "targetX-psa-vendorY"
    code: 500  # ERR_TABLE_FULL
  }
  details {
    canonical_code: 0  # OK
  }
  details {
    canonical_code: 6  # ALREADY_EXISTS
    message: "Entity already exists."
    space: "targetX-psa-vendorY"
    code: 600  # ERR_ENTITY_ALREADY_EXISTS
  }
}

13. Read RPC

The Read RPC retrieves one or more P4 entities from the P4Runtime server. The request is defined as: 

message ReadRequest {
  uint64 device_id = 1;
  repeated Entity entities = 2;
}

The device_id uniquely identifies the target P4 device. If it does not match any of the devices known to the P4Runtime server, the server must return a NOT_FOUND error. The entities repeated field is a list of P4 entities, each acting as a query filter to be applied to P4 entity containers on the server.

Since ReadRequests do not mutate any state on the switch, they do not require an election_id, and they do not require the presence of an open StreamChannel between the server and client.

The Read response consists of a sequence of messages (a gRPC stream) with each message defined as: 

message ReadResponse {
  repeated Entity entities = 1;
}

The entities repeated field is a list of P4 entities retrieved. The client reads from the returned stream until it is closed by the server when there are no more messages. In case of error, the stream is closed prematurely by the server and the client obtains the error status (in C++ the error status is obtained by calling the Finish() method on the stream object [10]).

13.1. Nomenclature

request
An element of the p4.ReadRequest.entities repeated field.
batch
Refers to the p4.ReadRequest.entities repeated field.

Each request acts as a query filter for that entity type. If a request fully specifies the entity key, the Read operation should retrieve a single P4 entity. Please refer to the P4 Entity Messages section for details on what parts of the entity specification make up the entity key.

13.2. Wildcard Reads

P4Runtime allows wildcard read of P4 entities. A request may omit or use default values for parts of the entity key to achieve wildcard behavior. Please refer to the P4 Entity Messages section for details on what parts of the entity can be wildcarded in a given request.

For example, in a request of type CounterEntry:

To read the entire forwarding state for a given device, the P4Runtime client can generate the following ReadRequest: 

device_id: <ID>
entities {
  extern_entry { }  # read all extern instances for all supported extern types
  table_entry { }  # read all table entries for all tables
  action_profile_member { }  # read all members for all action profiles
  action_profile_group { }  # read all groups for all action profiles
  ...
}

The entity oneof field in the Entity message must always be set, or the server must return an INVALID_ARGUMENT error. In other words, P4Runtime does not support performing a wildcard read on the entire forwarding state by including an empty Entity message in the ReadRequest. The main reason for this decision is to prevent backwards-compatibility issues if new entity types are added to the entity oneof [19].

13.3. Batch Processing

A P4Runtime server may arbitrarily reorder requests within a batch to maximize performance. There is no requirement that a particular entity type request appears only once in the batch.

A P4Runtime server will process the batch as follows:

  1. Lock state (preventing new writes) and validate each request in the batch:

    1. If it is a valid request, perform the read;

      1. If the read was successful, return the entities read in ReadResponse stream.
      2. If the read failed (exception / critical-error), prepare a p4.Error with code set to INTERNAL.

    2. If the request is invalid (invalid-argument, not-supported, etc.), prepare a p4.Error with relevant canonical code to capture the error.

  2. Unlock the state (allowing new writes);

  3. Close the ReadResponse stream and return a grpc::Status as follows:

    1. If no errors were encountered, set code to OK and do not populate any other field.

    2. Otherwise, the overall code should be set to UNKNOWN. See section Error Reporting Messages for information on error reporting messages and guidelines. Assemble a list of p4.Error messages (from step 1 above) such that each element reflects the status of the request in the batch at the same location (1:1 correspondence). This list should be packed into google.rpc.Status.details field. This behavior also matches Write RPC.

13.3.1. Example

If a client asked to read {a,b,c,d} and b and d requests didn't validate, the server will return entities corresponding to a and c, followed by a status {p4.Error(OK), p4.Error(xxx), p4.Error(yyy), p4.Error(OK)} in the details field.

The P4Runtime server is not required to perform any optimization (e.g. merge two requests in the batch if one is a subset of other). As a result of this, it is possible for the ReadResponse to contain the same entity more than once. If performance is a concern, the P4Runtime client should handle this merging.

There is no requirement that each request in the batch will correspond to one ReadResponse message in the stream. The stream-based design for response message is to avoid memory pressure on the P4Runtime server when the Read results in a very large number of entities to be returned. The P4Runtime server is free to break them apart across multiple response messages as it sees fit.

A P4Runtime server must be prepared to handle multiple concurrent Read RPCs. This could be from the same or multiple clients. P4Runtime is based on gRPC which provides a concurrent server design. A server implementation that supports concurrent RPC handlers may choose to maximize performance by using a multi-reader lock (also known as multiple-readers/single-writer lock). Conversely (e.g. in a single-threaded architecture), it may choose to serialize Read RPC processing.

13.4. Parallelism of Read and Write Requests

A P4Runtime server may be implemented to serve at most one ReadRequest or WriteRequest message at a time, sequentially. It may also serve multiple requests in parallel, and it is expected that some client software would be easier to implement with good performance characteristics if a server did so.

For example, imagine a client that wanted to use WriteRequest messages with large batches of insert, modify, and/or delete operations on an IP route table, in order to achieve higher throughput of updates to this table. Such a client might also wish to send WriteRequest messages with only a few updates to an ActionSelector object that controlled which links were in which LAGs, and have those small requests start processing even if there is a large WriteRequest batch currently being processed, but it will not complete for a significant amount of time.

The restrictions on which a client may rely are:

For example, if a P4Runtime server maintained, independently for each device it managed, a separate multi-reader single-writer lock for each stateful object in the P4 program, and before starting the processing of a WriteRequest message it acquired a write lock for each stateful object affected by the WriteRequest, and before starting the processing of a ReadRequest message it acquired a read lock for each stateful object accessed by the ReadRequest, such an implementation meets all of the requirements above.

It is possible to meet the requirements of this specification and perform even more requests in parallel than that example implementation allows, e.g. if the server somehow determined that two WriteRequest messages that inserted entries to the same table could not affect the results of the other, they could also be performed in parallel. It is not required that a P4Runtime server do this, and may be difficult to implement correctly.

14. SetForwardingPipelineConfig RPC

A P4Runtime client may configure the P4Runtime target with a new P4 pipeline by invoking the SetForwardingPipelineConfig RPC. The request is defined as: 

message SetForwardingPipelineConfigRequest {
  enum Action {
    UNSPECIFIED = 0;
    VERIFY = 1;
    VERIFY_AND_SAVE = 2;
    VERIFY_AND_COMMIT = 3;
    COMMIT = 4;
    RECONCILE_AND_COMMIT = 5;
  }
  uint64 device_id = 1;
  uint64 role_id = 2;
  Uint128 election_id = 3;
  Action action = 4;
  ForwardingPipelineConfig config = 5;
}

The server is expected to perform the following checks (in this order) before performing the required action:

  1. If device_id does not match any of the devices known to the P4Runtime server or if role_id does not match any of the roles for the device, the server must return a NOT_FOUND error.

  2. If the client is not the master for (device_id, role_id) according to the election_id value, the server must return a PERMISSION_DENIED error.

The action is the type of configuration action requested, it can be one of:

The config field is a message of type ForwardingPipelineConfig that carries the P4Info, the opaque target-dependent forwarding-pipeline configuration data (e.g. generated by the P4 compiler for the target), and, optionally, the cookie to uniquely identify such configuration. See the Forwarding-Pipeline Configuration section for details.

A P4Runtime server running on a non-programmable device may not support SetForwardingPipelineConfig (e.g. the forwarding-pipeline config is part of the device's software image, or is supplied using a different mechanism). In such cases, the RPC should return an UNIMPLEMENTED error.

15. GetForwardingPipelineConfig RPC

The forwarding-pipeline configuration of the target can be retrieved by invoking the GetForwardingPipelineConfig RPC. The request is defined as: 

message GetForwardingPipelineConfigRequest {
  enum ResponseType {
    ALL = 0;
    COOKIE_ONLY = 1;
    P4INFO_AND_COOKIE = 2;
    DEVICE_CONFIG_AND_COOKIE = 3;
  }
  uint64 device_id = 1;
  ResponseType response_type = 2;
}

The device_id uniquely identifies the target P4 device. A NOT_FOUND error is returned if the device_id is not recognized by the P4Runtime server.

The response_type is used to specify which fields to populate in the response, its value can be one of:

The response contains the ForwardingPipelineConfig for the specified device: 

message GetForwardingPipelineConfigResponse {
  ForwardingPipelineConfig config = 1;
}

If a P4Runtime server is in a state where the forwarding-pipeline config is not known, the top-level config field will be unset in the response. Examples are (i) a server that only allows configuration via SetForwardingPipelineConfig but this RPC hasn't been invoked yet, (ii) a server that is configured using a different mechanism but this configuration hasn't yet occurred.

Once a forwarding-pipeline config is installed on the device (either via SetForwardingPipelineConfig or a different mechanism), some P4Runtime servers may not support retrieval of the target-dependent config, in which case config.p4_device_config will be empty / unset in the response, even if response_type in the request was set to ALL. However, all P4Runtime servers are required to return the P4Info in this scenario. Similarly, if a cookie was present in the SetForwardingPipelineConfig RPC, the same should be returned when reading the config. If the config is installed with a mechanism other than SetForwardingPipelineConfig, the value of config.cookie will be unset.

If a P4Runtime server supports both SetForwardingPipelineConfig as well as returning the p4_device_config, there should be read-write symmetry between SetForwardingPipelineConfig and GetForwardingPipelineConfig RPCs.

16. P4Runtime Stream Messages

16.1. Packet I/O

P4Runtime supports controller packet-in and packet-out by means of PacketIn and PacketOut stream messages, respectively.

PacketIn messages are sent by the P4Runtime server to the client. Conversely, PacketOut messages are sent by the client to the server. Any PacketOut message received by the server from a client which is not allowed to send such messages based on its current role definition must be dropped. The server may also generate a StreamMessageResponse message with the error field set to report the error to the client. See the section on Stream Error Reporting for more information on error.

As introduced in the ControllerPacketMetadata section, such messages can carry arbitrary metadata specified by means of P4 headers annotated with @controller_header. The expected metadata is described in the P4Info using the ControllerPacketMetadata messages.

Both PacketIn and PacketOut stream messages share the same fields and are defined as follows: 

// Packet sent from the controller to the switch.
message PacketOut {
  bytes payload = 1;
  repeated PacketMetadata metadata = 2;
}

// Packet sent from the switch to the controller.
message PacketIn {
  bytes payload = 1;
  repeated PacketMetadata metadata = 2;
}

message PacketMetadata {
  // This refers to Metadata.id coming from P4Info ControllerPacketMetadata.
  uint32 metadata_id = 1;
  bytes value = 2;
}

16.2. Master Arbitration Update

P4Runtime's master arbitration mechanism ensures that only the current master can modify state on the switch, and that the election_id is monotonically increasing. For example, the switch must finish all previous write operations before changing masters, and must only accept write requests from the current master.

As explained earlier in this document, the controller uses the StreamChannel RPC for session management as well as Packet I/O. In fact, before a controller becomes able to do Packet I/O or program any forwarding entry (via Write RPC), it needs to start a controller session and become a “master”. To do so, the controller first opens a bidirectional stream channel to the server via StreamChannel for each device and sends a StreamMessageRequest message. The controller populates the MasterArbitrationUpdate field in this message using its role_id and election_id and the device_id of the device, as explained in detail in the Master-Slave Arbitration and Controller Replication section. For any given (device_id, role_id), the P4Runtime server keeps track of the highest election_id that it has ever received. If a controller's election_id is equal to the highest election_id that the server has ever received, that controller is the master. All other controllers are slaves. Note that it is possible that all controllers are slaves, and that there is no master. There can be at most one master, because for any given (device_id, role_id), each connected controller has a unique election_id.

This invariant must be maintained across in-service software upgrades, and the P4Runtime server must remember the highest election_id after such a restart. However, across a full restart, the election_id must be reset. In fact, a full restart is the only way to reset the election_id.

The MasterArbitrationUpdate message is defined as follows: 

message Role {
  // role_id for this role. Defined offline in agreement across the
  // entire control plane.
  uint64 id = 1;
  // Describes the role configuration.
  google.protobuf.Any config = 2;
}

message MasterArbitrationUpdate {
  // Identifies the device (aka target or node or switching chip).
  uint64 device_id = 1;
  // The role for which the mastership is being arbitrated.
  Role role = 2;
  // The election_id (unique per role).
  Uint128 election_id = 3;
  // Switch populates this with OK for the client that is the master,
  // and with an error status for all other connected clients (at
  // every mastership change). The controller does not populate this
  // field.
  google.rpc.Status status = 4;
}

Note that the status field in the MasterArbitrationUpdate message is not populated by the controller. This field is populated by the P4Runtime server when it sends a StreamMessageResponse message back to the controller, in which it populates the MasterArbitrationUpdate message using the device_id, role, and election_id it previously received from the controller. The server also populates the status field in the MasterArbitrationUpdate according to the rules in an earlier section.

The sender need not specify an election_id. If the election_id is not specified, the sender's election_id is considered lower than any election_id, and the sender will thus never become master. This way, a controller can choose to be a standby controller, in order to avoid master “flapping” (if a standby controller connects to the switch shortly before the actual master controller, therefore becoming master temporarily).

16.3. Digest Messages

See the DigestEntry section.

16.4. Table Idle Timeout Notification

When a table supports idle timeout (as per the P4Info message), the master client can specify a TTL value for each entry in the table (see Idle-timeout section). If the data plane entry is not hit for a lapse of time greater or equal to the TTL, the P4Runtime server should, with best effort, generate an IdleTimeoutNotification message on the StreamChannel bidirectional stream to the master client. The master client can then take the action of its choice, most likely remove the idle entry.

The IdleTimeoutNotification Protobuf message has the following fields:

Because we use a repeated Protobuf field, the P4Runtime server may elect to coalesce several idle timeout notifications in the same IdleTimeoutNotification message if it deems it appropriate. The server should not hold on to individual idle notifications for a significant amount of time just for the sake of coalescing as many as possible in a single message. For example, if the P4Runtime server periodically scans the device for idle data plane entries, we recommend not delaying notifications by more than one scanning interval. The P4Runtime server must not send an IdleTimeoutNotification message with an empty table_entry repeated field.

After generating an idle notification, the P4Runtime server must “reset” the timer for the corresponding entry, which means a new notification will be generated after another TTL if the entry is not hit. As a result, there is no need to guarantee reliable delivery of idle notifications to the master client and the server may drop notifications if they are generated faster than the server software, the channel or the client can handle.

Here is a reasonable pseudo-code implementation for idle timeout for table entries: 

IdleTimeoutStream stream;

scanning_interval = 10ms;

while (true) {
  // iterate over all tables which support idle timeout
  for (table in tables) {
    if (!table.idle_timeout_supported) continue;
    // we coalesce all idle notifications for the same table in one
    // message
    IdleTimeoutNotification msg;
    // read time_since_last_hit from device
    entries = device.load_table_entries_from_hw(table);
    for (entry in entries) {
      if (entry.idle_timeout == 0) continue;  // no TTL
      if (entry.time_since_last_hit < entry.idle_timeout) continue;
      msg.table_entry_add(entry);
      entry.reset_time_since_last_hit();
    }
    if (msg.table_entry_size() == 0) continue;  // no notifications
    msg.set_timestamp(now());
    stream.write(msg);
  }
  sleep(scanning_interval);
}

16.5. Architecture-Specific Notifications

P4Runtime supports streaming arbitrary Protobuf messages between the server and the client on StreamChannel, by including an Any Protobuf field [30] named other in both StreamMessageRequest and StreamMessageResponse. This enables support for architecture-specific externs which require asynchronous streaming of data from the server to the client, much like the PSA Digest extern. See section on Extending P4Runtime for non-PSA Architectures for more information.

16.6. Stream Error Reporting

The P4Runtime server can asynchronously report errors which occur when processing StreamMessageRequest messages, using the error message field (of type StreamError) in StreamMessageResponse. This error reporting is optional and mostly used for debugging purposes. The server may provide an out-of-band mechanism to enable / disable this feature.

The StreamError message has the following fields:

The appropriate canonical error code [33] should be used when populating the canonical_code field. For example:

The server may choose to assign a lower priority to error reporting messages and drop them first if the stream channel comes under heavy load, e.g. because of a burst of PacketIn messages.

Note that master-arbitration errors are never reported using the StreamError message. Invalid MasterArbitrationUpdate messages sent by the client cause the stream to be terminated and the appropriate error code to be returned to the client immediately. See section 5.3.

16.6.1. Examples of StreamError Messages

17. Capabilities RPC

The Capabilities RPC offers a mechanism through which a P4Runtime client can discover the capabilities of the P4Runtime server implementation. At the moment, the CapabilitiesRequest message is empty and the CapabilitiesResponse message only includes the p4runtime_api_version string field. This field must be set to the full semantic version string [26] corresponding to the version of the P4Runtime API implemented by the server, e.g. “1.1.0-rc.1”.

Future versions of P4Runtime may introduce more advanced capability discovery features. For example, P4Runtime supports three atomicity modes for WriteRequest batches, two of them being optional. In the future we may decide to leverage the CapabilitiesResponse message to enable the server to report to the client which subset of these atomicity modes is supported.

The semantic version string included in CapabilitiesResponse can be used by the client to determine which exact feature set is implemented by the server, since minor releases may introduce new functionality. However, because the Capabilities RPC itself was introduced in version 1.1 of P4Runtime, the client should assume that any server which does not implement this RPC (i.e. an UNIMPLEMENTED error is returned by the P4Runtime service) implements an older version of the P4Runtime specification (1.0 or a pre-release version of 1.0).

18. Portability Considerations

18.1. PSA Metadata Translation

The Portable Switch Architecture (PSA) defines standard metadata, whose data plane types are different on different PSA targets. In order to enable uniform programming of multiple PSA targets, a centralized remote controller may define its own types and numbering of such PSA standard metadata [23]. For such metadata, a translation between the controller's metadata values and the corresponding target-specific metadata values is required at runtime. In this section, we will base our discussions on port metadata, although the same translation principles apply to other standard PSA metadata such as class of service.

psa-metadata-translation

Figure 8. P4Runtime Metadata Translation for the Portable Switch Architecture

Figure 8 illustrates a motivating example, where a centralized controller is controlling two P4Runtime targets in a fabric. Switch 1 and Switch 2 use different PSA devices, each defining its own port type and number space. In this example, Switch 1 uses a device with 9-bit space for port numbers, and Switch 2 uses a device with 10-bit space for port numbers. The centralized SDN controller defines an independent 32-bit number space for ports of all targets in its domain. A mapping from the controller's 32 bit port numbers to a target's 9-bit or 10-bit port numbers is input to the switch via the non-forwarding switch config data that is delivered separately to the switch.

18.1.1. Translation of Port Numbers

In order to support the above SDN use case, P4Runtime requires translation of port metadata values between the controller's space and the PSA device's space as needed. Such translation is enabled by identifying a P4 entity (match field, action parameter, controller-header field or other) as being a PSA port metadata type. For this purpose, PSA defines the port metadata field type using special user-defined P4 types, namely PortId_t and PortIdInHeader_t, instead of standard P4 bitstrings. The P4Info entries for all P4 entities whose type is one of the special PSA port types use a controller-defined 32-bit type instead of the data plane bitwidth defined in the P4 program. The following PSA port metadata types are defined in psa.p4 for the PSA device in Switch 1. 

@p4runtime_translation("p4.org/psa/v1/PortId_t", 32)
type PortId_t bit<9>;
@p4runtime_translation("p4.org/psa/v1/PortIdInHeader_t", 32)
type PortIdInHeader_t bit<32>;

The first argument to the @p4runtime_translation annotation is a URI that indicates to the P4Runtime server which numerical mapping — provided by the out-of-band switch configuration mechanism — to use to translate between the SDN value and the data plane value. The second argument is the bitwidth of the SDN representation of the translated entity (32-bit in the case of ports).

An SDN port number of 0 is invalid (while 0 may be a valid device port number depending on the PSA device). A PSA device may define its CPU and recirculation ports in the device-specific port number space. P4Runtime reserves device-independent and controller-specific 32-bit constants for the CPU port and the recirculation port as follows: 

enum SdnPort {
  SDN_PORT_UNSPECIFIED = 0;

  // SDN ports are numbered starting form 1.
  SDN_PORT_MIN = 1;

  // The maximum value of an SDN port (physical or logical).
  SDN_PORT_MAX = 0xfffffeff;

  // Reserved SDN port numbers (0xfffffff0 - 0xffffffff)

  SDN_PORT_RECIRCULATE = 0xfffffffa;
  SDN_PORT_CPU = 0xfffffffd;
}

The switch config will map SDN_PORT_RECIRCULATE and SDN_PORT_CPU — as well as any SDN port number corresponding to a “regular” front-panel port — to the corresponding device-specific values, in order to enable the P4Runtime server to perform the translation.

The sub-sections below detail the translation mechanics for different usage of PSA port types in P4 programs.

18.1.2. Translation of Packet-IO Header Fields

Port type fields can be part of header types. For example, ports may be part of Packet IO headers, as in the following example:. 

@controller_header("packet_out")
header PacketOut_t {
  PortIdInHeader_t egress_port;
}

@controller_header("packet_in")
header PacketIn_t {
  PortIdInHeader_t ingress_port;
}

The header-level annotation @controller_header is a standard P4Runtime annotation that identifies a header type for a controller packet-out or packet-in header. When the P4Runtime server in the target receives a packet-out from the controller over the P4Runtime stream channel, the server will expect a packet-out metadata (egress_port) value of width 32-bit from the given set of SDN port values in the switch config. The server will then translate the SDN port value into the device-specific port value from the mapping provided in the out-of-band switch configuration (the mapping can be identified using the translation URI — first argument to the @p4runtime_translation annotation). Any subsequent reference to the egress_port field in the data plane will use the translated value. PortIdInHeader_t is used in the header definition instead of PortId_t to guarantee byte-aligned headers in case this is required by the target.

A similar reverse translation is required in the P4Runtime server for packets punted from the target to the controller as shown by the packet-in header example above. A packet punted from the target's PSA device will be intercepted by the P4Runtime server before being sent to the controller. The server will first translate the device-specific value of the ingress_port field into the controller-specific 32-bit value given by the port mapping defined in the switch config. The server will then insert the translated controller-specific value in the packet-in metadata fields before sending the packet over the stream channel to the controller.

18.1.3. Translation of Match Fields

Port type entities, particularly ingress and egress port standard metadata, may be used as match fields in a P4 table's match key as shown in the example below: 

table t {
  key = {
    istd.ingress_port: exact;  // PSA standard metadata ingress port
  }
  actions = {
    drop;
  }
}

Table t has an exact match on PSA standard metadata ingress port (istd.ingress_port). Since the field is of type PortId_t, the P4Info representation of the match field will present a 32-bit bitwidth to the controller, regardless of the data plane port type. A P4Runtime write request for a table entry in t from the controller will have the values of the match field set to the controller-specific port value. The P4Runtime server should intercept the write request and use the switch configuration data to translate the SDN port value to respective device-specific value. In the data plane, the packet metadata will carry the device-specific value and, hence, match the right table entry. Similarly, when a read response for table t is returned to the controller, the P4Runtime server should translate the device-specific port values to the corresponding controller-specific values.

Note that it may be infeasible to translate the value-mask pair for ternary matches: LPM, TERNARY or RANGE match kinds. The P4Runtime server may require that for these match kinds the port match be either de facto “exact” (0xffffffff mask for TERNARY, prefix-length of 32 for LPM, or same low and high bounds for RANGE) or “don't care”.

18.1.4. Translation of Action Parameters

PortId_t type parameters can be part of a P4 action definition as shown in the example below: 

action a(PortId_t p) {
  istd.egress_port = p;  // PSA standard metadata egress port
}

table t {
  key = {
    hdr.h.f: exact;
  }
  actions = {
    a;
  }
}

The controller may write entries in table t with action a to set the egress port as shown in the P4 code above. The action parameter p is of type PortId_t, which leads to a 32-bit bitwidth for p being exposed in P4Info. Furthermore, the type will be a signal to the P4Runtime server that translation is required for this parameter. The P4Runtime server will use the switch configuration to translate action parameter values between the controller and the target device.

18.1.5. Port Translation for PSA Extern APIs

The P4Runtime API for action selectors supports specifying a watch field per member in an action profile group that is programmed in a selector. This field is used to implement fast-failover in the target, where the P4Runtime server can locally prune the member from the group if a port is down. This pruning does not require intervention from the controller. Conversely, if the port comes back up, the P4Runtime server can re-enable the member in the group. The watch field is of type uint32 to carry the 32-bit SDN representation of the port being watched. The P4Runtime server will translate the given watch port number into the device-specific data plane port number for implementing the fast-failover functionality on the target device.

The Packet Replication Engine (PRE) API in P4Runtime supports cloning and multicasting to a set of ports. The egress port fields defined in the PRE multicast entry and clone session entry are of type uint32 to carry the 32-bit SDN port number(s). The P4Runtime server will translate these SDN port numbers to device-specific port numbers for multicasting and cloning in the data plane.

18.1.6. Using Port as an Index to a Register, Indirect Counter or Indirect Meter

P4Runtime supports using a translated value (PortId_t or any other translated type for which the underlying built-in type is bit<W>) as an index to a register, indirect counter, or indirect meter. 

Counter<bit<32> /* counter entry type */, PortId_t /* index type */>(
  32w1024, PSA_CounterType_t.PACKETS) counter;
action a(PortId_t p) {
  istd.egress_port = p;  // PSA standard metadata egress port
  counter.count(p);
}

This P4 Counter declaration will translate into the following entry in the P4Info messsage: 

counters {
  preamble {
    id: 0x12000001
    name: "counter"
  }
  spec {
    unit: PACKETS
  }
  index_type_name {
    name: "PortId_t"
  }
}

The controller may read and write counter values from indexed counter counter using SDN port numbers as indices, and not device-specific port numbers. The index_type_name field in the P4Info message is a signal to the P4Runtime server that translation is required.

19. P4Runtime Versioning

P4Runtime follows the Google guidelines for versioning cloud APIs [6]. We use a MAJOR.MINOR.PATCH style version number scheme and we increment the:

The major version number is encoded as the last component of the Protobuf package name for every P4Runtime version, including version 1 (v1), which is why currently the package name for the P4Runtime service is p4.v1 and the package name for P4Info is p4.config.v1. Even though p4 and p4.config are two different Protobuf packages, p4 depends on p4.config and is not meant to be used without it, which is why both packages use the same versioning scheme and the same versioning cadence.

As recommended in [6], we may consider using pre-GA release suffixes (such as alpha or beta) in the Protobuf package name for future major versions, although we have chosen not to do so when developing version 1 (v1).

Within a major version, the API must be evolved in a Protobuf backwards-compatible manner. [7] describes what constitute a backwards-compatible change. We expect MAJOR version bumps to be a rare event.

Note that a P4Runtime server may support multiple major versions of P4Runtime, although a client is expected to use the same version of the P4Runtime service for all its operations with a given device, during the lifetime of its session with the device. A client can check if a major version is supported by attempting to connect to the corresponding service. We may consider including a P4Runtime RPC to query minor + patch version numbers in future releases.

All versions of P4Runtime, including pre-release versions, are tagged in the P4Runtime Github repository [14] and the version label follows semantic versioning rules [26].

20. Extending P4Runtime for non-PSA Architectures

P4Runtime includes native support for PSA programs and in particular support for runtime control of PSA extern instances. While the definition of Protobuf messages for runtime control of non-PSA externs is out-of-scope of this specification, P4Runtime provides an extension mechanism for other architectures, through different hooks in the protocol definition. These hooks are described in various parts of this document and the goal of this section is to offer a comprehensive list of them in a single place.

When extending P4Runtime for a new P4 architecture, one will need to write two additional Protobuf files to extend p4info.proto and p4runtime.proto respectively. We suggest the following Protobuf package names:

We also recommend that the major version number for these packages be the same as the major version number for the P4Runtime version they “extend”.

For the remainder of this section, we will refer to these two files as p4info-ext and p4runtime-ext respectively.

20.1. Extending P4Runtime for Architecture-Specific Externs

Each P4 architecture can define its own set of extern types. Controlling them at runtime requires defining new Protobuf messages in both p4info-ext and p4runtime-ext. To make things more concrete for this section, we will assume that the new architecture we are trying to support in P4Runtime includes the following extern definition, which we will use as a running example: 

// T must be a bit<W> type, it indicates the width of each counter cell
extern MyNewPacketCounter<T> {
  counter(bit<32> size);
  increment(in bit<32> index);
}

20.1.1. Extending the P4Info message

message P4Ids {
  enum Prefix {
    UNSPECIFIED = 0;
    MY_NEW_PACKET_COUNTER = 0x81;
  }
}
message MyNewPacketCounter {
  // corresponds to the T type parameter in the P4 extern definition
  p4.config.v1.P4DataTypeSpec type_spec = 1;
  // constructor argument
  int64 size = 2;
}

20.1.2. Extending the P4Runtime Service

Just like p4info-ext, p4runtime-ext should include a Protobuf message definition for every extern type that can be controlled at runtime. This message should include the extern-specific parameters defining the read or write operation to be performed by the P4Runtime server on the corresponding extern instance. Instances of this architecture-specific message are meant to be embedded in an ExternEntry message generated by the P4Runtime client.

Here is a possible Protobuf message for our MyNewPacketCounter P4 extern: 

// This message enables reading / writing data to the counter at the provided
// index
message MyNewPacketCounter {
  int64 index = 1;
  p4.v1.P4Data data = 2;
}

P4Runtime also supports streaming arbitrary Protobuf messages between the server and the client, by including an Any Protobuf field [30] named other in both p4.v1.StreamMessageRequest and p4.v1.StreamMessageResponse. Architectures that wish to leverage this support should define the appropriate Protobuf messages for this bidirectional streaming in p4runtime-ext and embed instances of these messages in p4.v1.StreamMessageRequest and p4.v1.StreamMessageResponse as appropriate.

20.2. Architecture-Specific Table Extensions

20.2.1. New Match Types

An architecture may introduce new table match types [12]. P4Runtime accounts for this by providing the following hooks:

20.2.2. New Table Properties

An architecture may introduce additional table properties [29]. In some instances, it can be desirable to include the information contained in table properties in P4Info, which is why the p4.config.v1.Table message includes the other_properties Any Protobuf field [30]. At the moment, there is not any mechanism to extend the p4.v1.TableEntry message based on the value of architecture-specific table properties, but we may include on in future versions of the API.

21. Known Limitations of Current P4Runtime Version

22. Security concerns for P4Runtime

Appropriate measures and security best practices need to be in place to protect the P4Runtime server and client, and the communication channel between the two. For example, firewalling and authenticating the incoming connections to the P4Runtime server can prevent a malicious actor from taking over the switch. Similarly, using TLS to authenticate and encrypt the gRPC channel can prevent man-in-the-middle attacks between the server and client.

A. Appendix

A.1. Revision History

A.1.1. Changes in v1.1.0

A.2. P4 Annotations

Table 7 lists P416 annotations introduced primarily for the purpose of adding features for the P4Runtime API.

Annotation Description
@brief See section 6.1.3
@controller_header See section 6.4.6
@description See section 6.1.3
@id See section 6.3
@max_group_size See sections 6.4.3, 9.2.2
@pkginfo See section 6.2.1
@p4runtime_translation See sections 8.5.6, 18.1.1
Table 7. P4 annotations introduced by P4Runtime

A.3. A More Complex Value Set Example

This section includes a more complex Value Set example, with multiple matches of different kinds. 

struct match_t {
  bit<8> f8;
  @match(ternary) bit<16> f16;
  @match(custom) bit<32> f32;
}
@id(1) value_set<match_t>(4) pvs;
select ({ hdr.f8, hdr.f16, hdr.f32 }) { /* ... */ }

This P4 Value Set declaration will translate into the following entry in the P4Info messsage: 

value_sets {
  preamble {
    id: 0x03000001
    name: "pvs"
  }
  match {
    id: 1
    name: "f8"
    bitwidth: 8
    match_type: EXACT
  }
  match {
    id: 2
    name: "f16"
    bitwidth: 16
    match_type: TERNARY
  }
  match {
    id: 3
    name: "f32"
    bitwidth: 32
    other_match_type: "custom"
  }
  size: 4
}

A P4Runtime client can set the membership for this Value Set with WriteRequest messages similar to this one: 

type: MODIFY
entity {
  value_set_entry {
    value_set_id: 0x03000001
    members {
      match {
        field_id: 1
        exact { value: 0xac }
      }
      # match for field_id 2 is missing => don't care match
      match {
        field_id: 3
        other { ... }  # some serialized Any message (architecture-specific)
      }
    }
    members {
      match {
        field_id: 1
        exact { value: 0xdc }
      }
      match {
        field_id: 2
        ternary { value: 0x88 mask: 8f }
      }
      match {
        field_id: 3
        other { ... }  # some serialized Any message (architecture-specific)
      }
    }
  }
}

A.4. Guidelines for Implementations

This section contains practical advice for implementing P4Runtime clients and servers.

A.4.1. gRPC Metadata Maximum Size

In gRPC, the status of a RPC request is sent as metadata, whose size is limited by the grpc.max_metadata_size gRPC channel argument. By default, this limit is 8KB, which can be a problem for the Write P4Runtime RPC. The Write RPC returns an individual error for every item in a batch (see Section 12), which can quickly result in a status size over 8KB. In that case, the gRPC server would not send the status, but instead send a RESOURCE_EXHAUSTED error, without any of the individual errors.

To fix this problem, one can set the grpc.max_metadata_size option on the client channel. This allows the client to receive more than 8KB of metadata, based on the new limit. Note that the gRPC server does not have to change it's limit, as only the receiving side's limit is relevant. The exact limit that is required depends on the maximum batch size, the length of error messages inside every p4.Error, as well as any other metadata that is being sent over the gRPC channel. As a rule of thumb, it might make sense to allow for at least 8192 + MAX_UPDATES_PER_WRITE * 100 bytes of metadata.

For example, in C++, one can create a client channel as follows: 

const int MAX_UPDATES_PER_WRITE = 100;
::grpc::ChannelArguments arguments;
arguments.SetInt(GRPC_ARG_MAX_METADATA_SIZE, 8192 + MAX_UPDATES_PER_WRITE*100);
return grpc::CreateCustomChannel(address, credentials, arguments);

A.4.2. gRPC Server Maximum Receive Message Size

At the time of writing, the default maximum receive message size in gRPC is 4MB — while the default maximum send message size is unlimited. This can be a problem for the SetForwardingPipelineConfig RPC, since for some targets the binary p4_device_config can exceed 4MB, in which case by default the P4Runtime server would return an INVALID_ARGUMENT error. To a lesser extent, this may affect the Write RPC as well, in case of extremely large batches.

To fix this problem, we recommend that vendors implementing a P4Runtime server ensure that the maximum receive message size be large enough to accomodate all possible values of p4_device_config for their target(s). This can be done by setting the grpc.max_receive_message_length when building the gRPC server.

For example, in C++, one can set the maximum receive message size as follows: 

const int MAX_RECEIVE_MESSAGE_SIZE = 128 * 1024 * 1024;  // 128MB
::grpc::ServerBuilder server_builder;
builder.AddListeningPort(/*...*/);
builder.RegisterService(/*...*/);  // register P4Runtime service
builder.SetMaxReceiveMessageSize(MAX_RECEIVE_MESSAGE_SIZE);
builder.BuildAndStart();

On the client side, we recommend that P4Runtime clients do not use Write batches larger than the default maximum receive message size (4MB) — in case the server did not deem necessary to increase the default value —, unless the clients are aware that the server is using a larger maximum receive message size. The gRPC server running the P4Runtime service must not set the maximum receive message size to a value smaller than the default (4MB).

References

[2]“A Two Rate Three Color Marker.” https://​tools.​ietf.​org/​html/​rfc2698.
[7]“Google Cloud APIs Versioning - Backwards-Compatibility.” https://​cloud.​google.​com/​apis/​design/​versioning#​backwards_​compatibility.
[9]“gRPC Main Site.” https://​grpc.​io.
[14]“p4lang/p4Runtime repository:P4Runtime Protobuf Definition Files and Specification.” https://​github.​com/​p4lang/​p4runtime.
[15]“p4lang/PI repository:Legacy Repository for P4Runtime, Includes Reference Implementation.” https://​github.​com/​p4lang/​PI.
[18]“Portable Switch Architecture Specification (v1.1.0).” https://​p4.​org/​p4-​spec/​docs/​PSA-​v1.​1.​0.​html.
[26]“Semantic Versioning.” https://​semver.​org/​.
[34]“The OpenConfig Project.” http://​openconfig.​net.
[36]“The Stratum Project.” https://​stratumproject.​org/​.

1.an enum type used as a field in a header must specify a underlying type and representation for enum elements. ↩