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.
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/v1.0.0/proto.
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 [1].
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:
The following are not in scope of P4Runtime:
The following are not in scope of this specification document:
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.
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.
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.
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.
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.
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.
In this situation, P4Info is available but no P4 source is available for any number of reasons, the most likely of which are:
The vendor or organization does not wish to divulge the P4 source code, to protect intellectual property or maintain security.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
Redundancy and fault tolerance: Supporting multiple controllers allows having one or more standby slave controllers, which take over controlling the devices in case the master controller goes offline.
To support multiple controllers, P4Runtime uses the streaming channel (available
via StreamChannel
RPC) for session management. The workflow is described as
follows:
Each controller instance (e.g. a controller process) can participate in one or
more roles. For each (device_id
, role_id
), the controller receives an
election_id
. This election_id
can be the same for different roles and/or
devices, as long as the tuple (device_id
, role_id
, election_id
) is
unique. For each (device_id
, role_id
) that the controller wishes to
control, it establishes a StreamChannel
with the P4Runtime server
responsible for that device, and sends a MasterArbitrationUpdate
message
containing that tuple of (device_id
, role_id
, election_id
) values. The
P4Runtime server selects a master independently for each (device_id
,
role_id
) pair. The master is the client that has the highest election_id
among all active StreamChannel
connections with the same (device_id
,
role_id
) values. A connection between a controller instance and a device id
— which involves a persistent StreamChannel
— can be referred to as a
P4Runtime client.
Note that the P4Runtime server does not assign a role_id
or election_id
to
any controller. It is up to an arbitration mechanism outside of the server to
decide on the controller roles, and the election_id
values used for each
StreamChannel
. The P4Runtime server only keeps track of the (device_id
,
role_id
, election_id
) of each StreamChannel
that has sent a successful
MasterArbitrationUpdate
message, and maintains the invariant that all such
3-tuples are unique. A server must use all three of these values from a
WriteRequest
message to identify which client is making the WriteRequest
,
not only the election_id
. This enables controllers to re-use the same
numeric election_id
values across different (device_id
, role_id
)
pairs. P4Runtime does not require election_id
values be reused across such
different (device_id
, role_id
) pairs; it allows it.
To start a controller session, a controller first opens a bidirectional stream
channel to the server via the StreamChannel
RPC for each device. This is the
first thing the controller does to identify itself to the P4Runtime server on
the device. This stream will be used for two purposes:
Session management: As soon as the controller opens the stream
channel, it sends a StreamMessageRequest
message to the switch. The
controller populates the MasterArbitrationUpdate
field in this message
using its role_id
and election_id
. Note that the status
field in the
MasterArbitrationUpdate
is not populated by the controller. This field
is populated by the P4Runtime server when it sends a response back to the
client, as explained below.
Streaming of notifications (e.g. digests) and packet I/O: The same streaming channel will be used for streaming notifications, as well as for packet-in and packet-out messages. Note that unless specified otherwise by the role definitions, only the master controller can participate in packet I/O. This feature is explained in more details in the Packet I/O section.
Note that the stream is opened per device. In case a switching platform has
multiple devices (e.g. multi-ASIC line card) which are all controlled via the
same P4Runtime server, it is possible to have different masters for different
devices. In this case, it is the responsibility of the P4Runtime server to
keep track of the master for each device (and role). More specifically, the
P4Runtime server will know which stream corresponds to the master controller
for each pair of (device_id
, role_id
) at any point of time.
The streaming channel between the controller and the server defines the
liveness of the controller session. The controller is considered “offline” or
“dead” as soon as its corresponding stream channel to the switch is
broken, in which case the P4Runtime server quickly sets one of the slave
controllers with the highest election_id
as master.
The mechanism via which the controller receives the P4Runtime server details
which includes the device_id
, ip
and port
, as well as the mechanism via
which it receives the Forwarding Pipeline Config, are implementation specific
and beyond the scope of this specification. Similarly, the mechanism via which
the P4Runtime server receives its switch config (which notably includes the
device_id
) is beyond the scope of this specification. Nevertheless, if the
server details or switch config are transferred via the network, it is
recommended to use TLS or similar encryption and authentication mechanisms to
prevent eavesdropping attacks.
After the controller sends a StreamMessageRequest
message to the P4Runtime
server, the server sends a StreamMessageResponse
message back to the
controller, in which it populates the MasterArbitrationUpdate
. The
controller must populate the device_id
, role
, and election_id
fields. The election_id
field is set to the highest value, i.e. the value
for the current master. The server also populates the status
field in the
MasterArbitrationUpdate
(note that this field is not populated in the
MasterArbitrationUpdate
received by the controller). The value of the status
message is one of the following:
status.code
set to google.rpc.OK
) when the controller is
determined to be the master for a given (device_id
, role_id
).
status.code
set to google.rpc.ALREADY_EXISTS
) when the
controller is determined to be a slave for a given (device_id
,
role_id
).
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.
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.
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:
Write
updates and
receive notification messages (e.g. DigestList
and
IdleTimeoutNotification
).
PacketIn
messages, along with a
filtering mechanism based on the values of the PacketMetadata
fields to
select which PacketIn
messages should be sent to the controller.
PacketOut
messages, along with a
filtering mechanism based on the values of the PacketMetadata
fields to
select which PacketOut
messages are allowed to be sent by the controller.
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 [28]. 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.
MasterArbitrationUpdate
Messages Received from ControllersIf the MasterArbitrationUpdate
message is received for the first time (for
a newly connected controller):
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.
If the election_id
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.
If the max number of clients for the given (device_id
, role_id
)
exceeds the supported limit, the P4Runtime server shall terminate the
stream by returning a RESOURCE_EXHAUSTED
error.
Otherwise, the controller is added to a list of connected controllers for
the given (device_id
, role_id
) and the controller is notified by
sending a StreamMessageResponse
message back to it, as explained
earlier.
If the MasterArbitrationUpdate
message is received from an already
connected controller:
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.
Otherwise, if the role.id
matches the current role_id
assigned to
this stream:
If the election_id
also matches the one assigned to this stream,
the server will accept the (new) role.config
only if this
controller is the current master. If the controller is not a master,
the operation is a no-op.
If the election_id
is already assigned to another controller stream
for the same (device_id
, role_id
), the P4Runtime server shall
terminate the stream by returning an INVALID_ARGUMENT
error.
Otherwise, the P4Runtime server updates the election_id
for this
controller. If this makes the client the new master, the server will
also accept the given role.config
and follow the “mastership change
rules” described in the following section.
Otherwise (i.e. role.id
is different from current role_id
assigned to
this stream), the P4Runtime server moves the controller to the new role.
This controller will then be treated as a new controller for the new
(device_id
, role_id
). The server accepts the given role.config
only
if the client becomes master, in which case the server also follows the
“mastership change rules” described in the following
section.
If role.config
does not match the “out-of-band” scheme previously agreed upon,
the server must return an INVALID_ARGUMENT
error.
“Mastership change” refers to either one of these cases:
A new MasterArbitrationUpdate
is received from an already connected
controller for a given (device_id
, role_id
), which changes the controller
mastership status (the controller becomes master or slave).
A streaming channel for a given master controller breaks, forcing a new master to be elected.
In case of a mastership change, the P4Runtime server shall send the
election_id
of the master to all the connected controllers for a given
(device_id
, role_id
). The StreamMessageResponse
sent back to all the
connected controllers has a MasterArbitrationUpdate
message populated with the
device_id
, role_id
, and election_id
of the master, as well as an OK status
for the master and non-OK status (with ALREADY_EXISTS
error code) for slaves.
The purpose of P4Info was described under Reference Architecture. Here we describe the various components.
These messages appear nested within many other messages.
Documentation
MessageDocumentation
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;
}
Preamble
MessageThe 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;
}
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 {
...
}
PkgInfo
MessageThe 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;
}
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:
@pkginfo
- This is used to populate a specific field within the PkgInfo
message. Multiple @pkginfo
annotations are allowed. For compactness,
multiple key-value pairs can appear in a single @pkginfo
annotation,
separated by commas. Each key must only appear once and the compiler must
reject the program if one appears multiple times. The key
s must be from
among the message fields inside PkgInfo
, for example, name
, version
,
etc. Each key-value pair assigns a value to the corresponding field inside the
single PkgInfo
message for the program's P4Info. One exception is that the
Documentation
field of PkgInfo
must be expressed as individual
@description
and @brief
annotations, see next bullets. The key arch
will
be ignored (with a warning) by the compiler. The value for this should come
from the compiler itself.
@brief
- This will populate the PkgInfo.doc.brief
message field.
@description
- This will populate the PkgInfo.doc.description
message field
@<anything else>
- This will create a PkgInfo.annotation
entry
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;
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) |
MSB bit 31 …….. bit 24 | bit 23 ………………….. bit 0 LSB |
---|---|
Object type prefix | Generated suffix (e.g. by the compiler) |
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
Table messages are used to specify all possible match-action tables exposed to a control plane. This message contains the following fields:
preamble
, a Preamble
message with the ID, name, and alias of this table.
match_fields
, a repeated field of type MatchField
representing the data to
be used to construct the lookup key matched in this table. Each MatchField
message is defined with the following fields:
id, the uint32
identifier of this MatchField
, unique in the scope of
this table. No rules are prescribed on the way MatchField
IDs should be
allocated, as long as two MatchField
of the same table do not have the
same ID. Nonetheless, we recommend that the IDs be assigned incrementally,
starting from 1, in the same order as in the P4 key declaration.
name
, the string representing the name of this MatchField
.
annotations
, a repeated field of strings, each one representing a P4
annotation associated to this match field.
bitwidth
, an int32
value set to the size in bits of this match field.
match
, a oneof
describing the match behavior for this field; it can be
either:
match_type
, an enum field of type MatchType
, which includes all
possible PSA match kinds.
other_match_type
, a string field which can be used to encode any
architecture-specific match type.
doc
, a Documentation
message describing this match field.
type_name
, which indicates whether the match field has a user-defined
type; this is useful for
translation.
action_refs
, a repeated ActionRef
field representing the set of possible
actions for this table. The ActionRef
message is used to reference an action
specified in the same P4Info message and it includes the following fields:
id
, the uint32
identifier of the action.
scope
, an enum value which can take one of three values:
TABLE_AND_DEFAULT
, TABLE_ONLY
and DEFAULT_ONLY
. The scope
of the
action is determined by the use of the P4 standard annotations
@tableonly
and @defaultonly
[17]. TABLE_ONLY
(@tableonly
annotation) means that the action can only appear within
the table, and never as the default action. DEFAULT_ONLY
(@defaultonly
annotation) means that the action can only be used as the
default action. TABLE_AND_DEFAULT
is the default value for the enum and
means that neither annotation was used in P4 and that the action can be
used both within the table and as the default action.
annotations
, a repeated string field, each one representing a P4
annotation associated to the action reference in this table.
const_default_action_id
, if this table has a constant default action, this
field will carry the uint32
identifier of that action, otherwise its value
will be 0. A default action is executed when a matching table entry is not
found for a given packet. Being constant means that the control plane cannot
set a different default action at runtime or change the default action's
arguments.
implementation_id
, the uint32
identifier of the “implementation” of this
table. 0 (default value) means that the table is a regular (direct) match
table. Otherwise, this field will carry the ID of an extern instance specified
in the same P4Info message (e.g. a PSA ActionProfile
or ActionSelector
instance). The table is then referred to as an indirect match table.
direct_resource_ids
, repeated uint32
identifiers for all the direct
resources attached to this table, such as DirectMeter
and DirectCounter
instances, specified in the same P4Info message. In this version of the
P4Runtime specification only one direct resource of each type can be
associated to a table, hence for PSA programs this field is expected to have a
maximum size of 2.
size
, an int64
describing the desired number of table entries that the
target should support for the table. See the “Size” subsection within the
“Table Properties” section of the P416 language specification for details
[27].
idle_timeout_behavior
, which describes the behavior of the data plane when
the idle timeout of a table entry expires (see
Idle-Timeout section). Value can be any of the
IdleTimeoutBehavior
enum:
NO_TIMEOUT
(default value), which means that idle timeout is not
supported for this table.
NOTIFY_CONTROL
, which means that the control plane should be notified of
the expiration of a table entry by means of a notification (see section on
Table Idle Timeout Notifications).
is_const_table
, a boolean flag indicating that the table is filled with
static entries and cannot be modified by the control plane at runtime.
other_properties
, an Any
Protobuf message [28] to embed
architecture-specific table properties [27] which are not part
of the core P4 language or of the PSA architecture.
Action
Action
messages are used to specify all possible actions of all match-action
tables.
The Action
message defines the following fields:
preamble
, a Preamble
message with the ID, name, and alias of this action
params
, a repeated field of Param
messages representing the set of runtime
parameters that should be provided by the control plane when inserting or
modifying a table entry with this action. Each Param
message contains the
following fields:
id
, the uint32
identifier of this parameter. No rules are prescribed
on the way Param
IDs should be allocated, as long as two Param
of the
same action do not have the same ID. Nonetheless, we recommend that the
IDs be assigned incrementally, starting from 1, in the same order as in
the P4 action declaration.
name
, the string representing the name of this parameter.
annotations
, a repeated field of strings, each one representing a P4
annotation associated to this parameter.
bitwidth
, an int32
value set to the size in bits of this parameter.
doc
, which describes this parameter using a Documentation
message.
type_name
, which indicates whether the action parameter has a
user-defined type; this is useful for
translation.
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:
preamble
, a Preamble
message with the ID, name, and alias of this Action
Profile or Selector.
table_ids
, a repeated field of uint32 identifiers used to reference tables
whose implementation uses this Action Profile or Selector.
with_selector
, a boolean flag indicating whether this message describes an
instance of a PSA Action Selector extern.
size
, an int64
representing the maximum number of member entries that the
Action Profile can hold, or, in the case of an Action Selector, the maximum sum
of all member weights across all selector groups.
max_group_size
, an int32
representing the maximum sum of all member
weights within any given selector group, in the case of an Action Selector, or
0 for an Action Profile. PSA programs can use the @max_group_size
annotation
to provide this value for Action Selectors. If the annotation is omitted, the
P4Info field will default to 0.
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:
Indexed counters provide a fixed number of independent counter values, also called cells. Each cell can be read by the control plane using an integer index.
Direct counters are associated a given match-action table, providing as many cells as the number of entries in the table.
Both Counter
and DirectCounter
messages share the following fields:
preamble
, a Preamble
message with the ID, name, and alias of this counter
extern instance.
spec
, a message of of type CounterSpec
used to describe the compile-time
configuration of this counter. Currently, the CounterSpec
message is used to
carry only the counter unit, which can be any of the CounterSpec.Unit
enum
values:
UNSPECIFIED
: reserved value.
BYTES
: byte counter.
PACKETS
: packet counter.
BOTH
: combination of both byte and packet counter.
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>
).
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:
Indexed meters provide a fixed number of independent meter values, also called cells. Each cell can be accessed by the control plane using an integer index, e.g. to set the rate threshold.
Direct meters are associated to match-action tables, providing as many cells as the number of entries in the table.
Both Meter
and DirectMeter
messages share the following fields:
preamble
, a Preamble
message with the ID, name, and alias of this meter
extern instance.
spec
, a message of type MeterSpec
used to describe the capabilities of
this meter extern instance. Currently, the MeterSpec
message is used to
carry only the meter unit, which can be any of the MeterSpec.Unit
enum
values:
UNSPECIFIED
: reserved value.
BYTES
, which signifies that this meter can be configured with rates
expressed in bytes/second.
PACKETS
, for rates expressed in packets/second.
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>
).
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:
preamble
, a Preamble
message where preamble.name
is set to "packet_in"
and "packet_out"
for packet-in and packet-out metadata, respectively.
metadata
, a repeated field of type Metadata
, where each Metadata
message
includes the following fields:
id
, a uint32
identifier of this metadata. No rules are prescribed on
the way metadata IDs should be allocated, as long as two Metadata
of the
same ControllerPacketMetadata
message do not have the same
ID. Nonetheless, 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 header declaration.
name
, a string representation of the name of this metadata. If the
P4Info message was generated from a P4 compiler, then this field is
expected to be set to the name of the P4 controller header field (see
example below).
annotations
, a repeated field of strings, each one representing a P4
annotation associated to this metadata.
bitwidth
, an int32
representing the size in bit of this metadata.
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.
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 [35].
The ValueSet
message defines the following fields:
preamble
, a Preamble
message with the ID, name, and alias of this Value
Set.
match
, a repeated field of MatchField
messages, representing the list of
matches performed when looking up an expression in a Value Set. This
determines the format of the members which can be inserted into the Value Set
by the control plane, similarly to the match_fields
repeated field in the
Table
message.
size
, an int32 representing the maximum number of entries (values) in the
Value Set. It corresponds to the value of the size argument of the P4
value_set
constructor call.
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
[24]. The rest of this section looks at all 3 of
these cases and gives an example ValueSet
message when appropriate.
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
}
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.
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.
match
: by default match_type
is set to EXACT
; the P4 programmer can
specify a different match type by using the @match
annotation
[24].
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).
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:
preamble
, a Preamble
message with the ID, name, and alias of this register
instance.
type_spec
, which specifies the data type stored by this register, expressed
using a P4DataTypeSpec
message (see section on Representation of Arbitrary
P4 Types).
size
, an int32
value representing the total number of independent register
cells available.
index_type_name
, which indicates whether the register 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>
).
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:
preamble
, a Preamble
message with the ID, name, and alias of this digest
instance.
type_spec
, which specifies the data type of an individual digest
notification using a P4DataTypeSpec
message (see section on Representation
of Arbitrary P4 Types).
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:
extern_type_id
, a 32-bit unsigned integer which uniquely identifies the
extern type in the context of the architecture. It must be in the reserved
range [0x81, 0xfe]
. Note that this value does not need
to be unique across all architectures from all organizations, since at any
given time every device managed by a P4Runtime server maps to a single P4Info
message and a single architecture.
extern_type_name
, which specifies the fully-qualified P4 name of the extern
type.
instances
, a repeated field of ExternInstance
Protobuf messages, with each
entry corresponding to a separate P4 instance of the extern. The
ExternInstance
in turn defines the following fields:
preamble
, a Preamble
message with the ID, name, and alias of this digest
instance.
info
, an Any
Protobuf message [28] which is used to embed
arbitrary information specific to the extern instance. Note that the
underlying Protobuf message type for info
should be the same for all
instances of this extern type. That Protobuf message should be defined in a
separate architecture-specific Protobuf file. See section on Extending
P4Runtime for non-PSA Architectures for more
information.
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.
See section on Representation of Arbitrary P4 Types.
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.
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());
}
Reading a single counter entry at index 0 in the counter array with id
<id>
:
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);
counter_id: <id>
index {}
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.
Reading all counter entries by leaving the index
field unset
p4::v1::CounterEntry entry;
entry.set_counter_id(<id>);
counter_id: <id>
index
message field is unset (default
value) and is therefore omitted from the textual representation of the
message.
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 [33] 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.
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
.
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:
It ensures that a properly encoded binary string's integer value conforms to the P4Info-specified bitwidth.
It supports read-write symmetry.
It helps facilitate non-disruptive P4 program updates.
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 is the smallest integer such that .
For a value of type int<W>
, the fewest number of bits required to represent
the integer value in 2's complement form is the smallest integer
such that .
As a special case, define that the value requires at least bit to represent, regardless of whether it is signed or unsigned.
The shortest possible binary string for an integer that needs 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 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 |
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.
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 |
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.
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:
a string representing the name of the type in case of a named type (struct
,
header
, header_union
, enum
, serializable_enum
or user-defined “new”
type
),
an empty Protobuf message for bool
and error
, or
a Protobuf message for other anonymous types (bit<W>
, int<W>
, varbit<W>
,
tuple
or stack). The “binary string” types (bit<W>
, int<W>
, and
varbit<W>
) are grouped together in the P4BitstringLikeTypeSpec
message,
since they are the only sub-types allowed in headers and values with one of
these types are represented similarly in P4Runtime (with the Protobuf bytes
type).
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
.
P4Data
in p4runtime.protoP4Runtime 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:
The order of members
in P4StructLike
and the order of bitstrings
in
P4Header
must match the order in the corresponding p4info.proto type
specification and hence the order in the corresponding P416 type
declaration.
An invalid header is represented by a P4Header
message where the is_valid
field is false and the bitstrings
repeated field is empty.
An invalid header union (i.e. all headers in the union are invalid) is
represented by a P4HeaderUnion
message where the valid_header_name
is the
empty string (default value for the field) and the valid_header
is unset.
The order of entries
in P4HeaderStack
and P4HeaderUnionStack
is from
element at index 0 of the stack to last element of the stack, in ascending
order of index. The length of the entries
field must always be equal to the
compile-time size of the corresponding P4 stack declaration. This size is
included in the P4Info, in the corresponding P4HeaderStackTypeSpec
or
P4HeaderUnionStackTypeSpec
message.
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: # ...
}
}
}
}
}
}
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.
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.
User-defined types are specified using the P4NewTypeSpec
message, which has
the following fields:
representation
, a Protobuf oneof
specifying how values of this type are
exchanged between client and server; it can be either:
original_type
, if and only if no @p4runtime_translation
annotation is
present. It specifies the underlying built-in P4 type for the user-defined
type. If the underlying type used in the P4 type
declaration is itself a
user-defined type, original_type
is obtained by “walking” the chain of
type
declarations recursively until a built-in type (e.g bit<W>
) is
found.
translated_type
, if and only if the P4 type
declaration was annotated
with @p4runtime_translation
. It is of type P4NewTypeTranslation
, which
itself has two fields — uri
and sdn_bitwidth
—, which map to the
two input parameters to the annotation.
annotations
, a repeated field of strings, each one representing a P4
annotation associated to the type declaration.
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.
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
.
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.
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:
table_id
, which identifies the table instance; the table_id
is determined
by the P4Info message.
match
, a repeated field of FieldMatch
messages. Each element in the
repeated field is used to provide a value for the corresponding element in the
key property of the P4 table declaration.
action
, which indicates which of the table's actions to execute in case of
match and with which argument values.
priority
, a 32-bit integer used to order entries when the table's match key
includes a ternary or range match.
controller_metadata
, a 64-bit cookie value which is opaque to the
target. There is no requirement of where this is stored, but it must be
returned by the server along with the rest of the entry when the client
performs a read on the entry.
meter_config
, which is used to read and write the configuration for the
direct meter entry attached to this table entry, if any. See Direct
resources section for more information.
counter_data
, which is used to read and write the value for the direct
counter entry attached to this table entry, if any. See Direct
resources section for more information.
is_default_action
, a boolean flag which indicates whether the table entry is
the default entry for the table. See Default entry
section for more information.
idle_timeout_ns
and time_since_last_hit
, which are two fields used to
implement idle-timeout support for the table, if applicable. See
Idle-timeout section for more information.
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 [27]. 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.
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:
For a TERNARY
match, it is logically equivalent to a mask of zeros.
For an LPM
match, it is logically equivalent to a prefix_len of zero.
For a RANGE
match, it is logically equivalent to a range which includes all
possible values for the field.
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.
EXACT
match
assert(BytestringValid(match.exact().value()))
LPM
match
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)
TERNARY
match
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)
RANGE
match
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)
The TableEntry
action
field must be set for every INSERT
and MODIFY
update, except when resetting the default entry. Based on
the implementation property value of the P4 table, the oneof
in the
TableAction
message will either be:
an Action
specification for direct tables (with no P4 implementation
property)
an action profile member id for indirect tables for which the implementation
property is an action profile with no selector.
an action profile member id or group id for indirect tables for which the
implementation
property is an action profile with selector.
an ActionProfileActionSet
specification for indirect tables for
which the implementation
property is an action profile with
selector. This usage is described in One Shot Action Selector
Programming
If the action
field is not set (and if is_default_action
is false) or 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:
action_id
, which identifies the action instance; the action_id
is
determined by the P4Info message and must match one of the possible action
choices for the table, or the server must return an INVALID_ARGUMENT
error
code. If the client uses a valid action_id
for the table but does not
respect the action scope specified in P4Info (e.g. tries to set a TABLE_ONLY
action as the default action), the server must return a PERMISSION_DENIED
error code.
params
: a repeated Protobuf field of action parameter values, each encoded
as a Param
message. For each parameter, param_id
must be valid for the
action (as per the P4Info) and value must follow the format described in
Bytestrings. The P4Runtime client must provide a valid
value for each parameter of the P4 action; we do not support default values
for action parameters. The server must return an INVALID_ARGUMENT
error code
if a parameter id is missing, if an extra parameter — id not found in the
P4Info — was provided by the client, if a parameter value is missing, or if
the value provided for one of the parameters does not conform to the
Bytestrings format.
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.
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.
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 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. However, the contents of
such tables can be queried by the client through a ReadRequest
. When reading
static (immutable) entries from a constant table, the following fields — and
only these fields —, must be set by the server: table_id
, match
, action
,
priority
and is_default_action
. In particular, we assume that constant
tables cannot be assigned direct resources and idle timeout is 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.
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:
table_id
: If default (0), entries from all tables — including constant
tables — will be selected and no other filter can be used. Otherwise only
the specified table will be considered.
match
: If default (unset), all entries from the specified table will be
considered. Otherwise, results will be filtered based on the provided match
key, which must be a valid match key for the table. The match will be exact,
which means at most one entry will be returned.
action
: If default (unset), all entries from the specified table will be
considered. Otherwise, the client can provide an action_id
(for direct
tables), which will be use to filter table entries. For this P4Runtime
release, this is the only kind of action-based filtering we support: the
client cannot filter based on action parameter values and cannot filter
indirect table entries based on action profile member id / action profile
group id.
priority
: If default (0), all entries from the specified table will be
considered. Otherwise, results will be filtered based on the provided priority
value.
controller_metadata
: If default (0), all entries from the specified table
will be considered. Otherwise, results will be filtered based on the provided
controller_metadata
value.
is_default_action
: If default (false), all non-default entries from the
specified table will be considered. Otherwise, only the default entry will be
considered.
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.
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:
A table entry and its direct resources can be read with a single entity when
doing a Read
RPC call
The initial configuration for an entry's direct resources can be specified when the entry is inserted. This may enable the target to add the table entry and configure the direct resources in an atomic fashion if supported. When the table has a direct meter, this may help guarantee that the lifetime of the meter entry is the same as the lifetime of the table entry, and that there is no time gap during which data plane traffic can “hit” the table entry without executing the appropriate meter entry.
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.
meter_config
field
WriteRequest (INSERT)
WriteRequest (MODIFY)
ReadRequest
meter_config
is unset in the response).
meter_config
is unset in the response. Otherwise, the
response includes the meter entry's configuration that was written by
the client earlier. This respects the “read-write symmetry” principle.
counter_data
field
WriteRequest (INSERT)
WriteRequest (MODIFY)
ReadRequest
counter_data
is unset in the response).
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).
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:
idle_timeout_ns
: the configured TTL for the table entry in nanoseconds. A
value of 0 means that the entry never expires, i.e. no
IdleTimeoutNotification
message will ever be generated for this entry. When
a client reads a TableEntry
, this field will be included in the response and
the value must match exactly the one set by the client when inserting or
modifying the entry.
time_since_last_hit
: a Protobuf message with a single field (elapsed_ns
)
used to indicate the time in nanoseconds elapsed since the last time the
data plane entry was hit. The time_since_last_hit
field must be unset for a
TableEntry
write. When reading a table entry, time_since_last_hit
must be
set in the response if and only if it was set (to an empty message) in the
request. If the field is set in the request, it must be set to the correct
value in the response even if the TTL value for the entry is 0.
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:
idle_timeout_ns
is set to a non-zero value, or
time_since_last_hit
is set
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.
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.
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:
action_profile_id
is the uint32
identifier of the PSA ActionProfile or
ActionSelector extern instance, as defined in P4Info.
member_id
is the uint32
identifier of the action profile member entry being
updated.
action
is the specification of the P4 action instance bound to the action
profile member entry.
An action profile member may be inserted, modified or deleted as per the following semantics.
INSERT
: Add a new member entry bound to an eligible P4 action
specification. The member id must be different from ids of already programmed
entries for that extern, or the server must return an ALREADY_EXISTS
error
code. The action specification must be provided, or the server must return
INVALID_ARGUMENT
. The total number of members should not exceed the maximum
specified in the P4 extern specification as a result of this insertion, or the
server should return RESOURCE_EXHAUSTED
.
MODIFY
: Modify the action specification of an existing member entry. An
entry with the member id must exist, or the server must return NOT_FOUND
,
and the action specification must be provided, or the server must return
INVALID_ARGUMENT
.
DELETE
: Delete the member entry and deallocate the member id. If the member
id is not valid the server must return a NOT_FOUND
error code. The member
must not be part of an action profile group, or the server must return
FAILED_PRECONDITION
. If needed, the action profile group should first be
modified to remove the member from the group. The member must not be
referenced in the table action of any table entry, or the server must also
return FAILED_PRECONDITION
. member_id
is the only field which is
considered when performing a DELETE
and every other field will be ignored.
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:
action_profile_id
is the uint32
identifier of the PSA ActionSelector
extern instance, as defined in P4Info.
group_id
is the uint32
identifier of the action profile group entry being
updated.
members
is a repeated field defining the set of members that are part of the
group. For each member in a group, the controller must define the following
fields:
member_id
for looking up the member table in the selector.
weight
specifying the probability of the member's selection at
runtime. 0 is not a valid weight
value and the server must return
INVALID_ARGUMENT
if the client attempts to use it.
watch
is the controller-defined 32-bit port number that the member's
liveness depends on. At runtime, the member must be excluded from
selection if the watch port is down.
max_size
is the maximum sum of all member weights for the group. This field
is defined when the group is inserted, and must not be changed in a
MODIFY
update. See the subsection below for the rules on setting
max_size
.
An action profile group may be inserted, modified or deleted as per the following semantics.
INSERT
: Add a new group entry bound to a set of existing action profile
members. The group_id
must be different from ids of already programmed
groups for that selector, or the server must return an ALREADY_EXISTS
error
code. All members specified in the group must exist in the selector, or the
server must return NOT_FOUND
. P4Runtime does not explicitly limit the number
of groups, however, such limits may be imposed out-of-band by the target.
MODIFY
: Modify the member set specification of an existing group entry. An
entry with the group_id
must exist, or the server must return
NOT_FOUND
. All members specified in the group entry must exist in the
selector, or the server must return NOT_FOUND
. The value of max_size
must
be identical to the value used when inserting the group, otherwise an
INVALID_ARGUMENT
error is returned.
DELETE
: Delete the group entry and deallocate the group_id
. The group must
not be referenced in the table action of any table entry, or the server must
return a FAILED_PRECONDITION
error code. If the group_id
is invalid, the
server must return NOT_FOUND
. group_id
is the only field which is
considered when performing a DELETE
and every other field will be ignored.
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.
max_size
The valid values for max_size
depend on the static max_group_size
included
in the P4Info message:
If max_group_size
is greater than 0, then max_size
must be greater than 0,
and less than or equal to max_group_size
. We assume that the target can
support selector groups for which the sum of all member weights is up to
max_group_size
, or the P4Runtime server would have rejected the Forwarding
Pipeline Config. If max_size
is greater than max_group_size
, the server
must return INVALID_ARGUMENT
.
Otherwise (i.e. if max_group_size
is 0), the P4Runtime client can set
max_size
to any value greater than or equal to 0.
A max_size
of 0 indicates that the client is not able to specify a maximum
size at group-creation time, and the target should use the maximum value it
can support. If the maximum value supported by the target is exceeded during
a write update (INSERT
or MODIFY
), the target must return a
RESOURCE_EXHAUSTED
error.
If max_size
is greater than 0 and the value is not supported by the
target, the server must return a RESOURCE_EXHAUSTED
error at
group-creation time.
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:
action
is one of the actions specified by the table that is being
programmed.
weight
specifying the probability of the action's selection at runtime. 0 is
not a valid weight
value and the server must return INVALID_ARGUMENT
if
the client attempts to use it. The sum of all weights across all
ActionProfileAction
messages for that ActionProfileActionSet
message must
not exceed the max_group_size
specificed in the P4Info (if greater than 0),
or the server must return INVALID_ARGUMENT
.
watch
is the controller-defined 32-bit port number that the action's
liveness depends on. At runtime, the action must be excluded from selection if
the watch port is down.
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 ReadRequest
s
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.
The PSA specification states that the following features are optional in action selector implementations [20]:
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
[23].
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:
byte_count
is an int64
, corresponding to the number of octets.
packet_count
is an int64
, corresponding to the number of packets.
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.
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:
the table_entry.match
field must match TableEntry.match
of the table entry
to which this direct counter entry is associated. If a matching TableEntry
is not found, the server returns the error code NOT_FOUND
.
data
is used to set the counter value to the value specified by the
client. Note that if this Protobuf field is not set, the counter value is not
modified.
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
:
As a direct resource associated with a table entry, request the server to
return the counter value in the counter_data
field of the TableEntry
message
(see Direct resources).
Explicitly request the counter value by including the DirectCounterEntry
in
the ReadRequest
. The table_entry.match
field must match the TableEntry
whose counter is being read. If no such entry is found, the server returns the
error code NOT_FOUND
.
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:
counter_id
is a uint32
, the unique identifier for the counter.
index
is a Protobuf message that encapsulates an int64
, used to index into
the counter array.
data
is a Protobuf message of type CounterData
, which represents the
counter value.
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.
INSERT
: Server returns the error code INVALID_ARGUMENT
.
MODIFY
: Modify an indirect counter instance whose unique id is counter_id
and array index is specified by index
. The counter value is set to the value
specified by the client in the data
field. Note that the counter value is
not modified if this Protobuf field is not set. If index
is omitted all
counter values in the array will be set to the value provided by the
client. The server must return INVALID_ARGUMENT
for a negative index value
and OUT_OF_RANGE
if the index value exceeds the size of the counter array.
DELETE
: Server returns the error code INVALID_ARGUMENT
.
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.
If the counter_id
field is set to 0 (default), the server returns the
counter values for all indirect counter instances in the ReadResponse
.
If the index
field is not set, the server returns the counter values for all
indirect counters in the array identified by the unique id counter_id
.
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
}
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:
the table_entry.match
field must match the match key of the TableEntry
message used to insert the entry and the associated direct meter entry. The
action field is ignored in this case. If a matching TableEntry
is not found,
the server returns the error code NOT_FOUND
.
config
is used to set the configuration for the meter entry to the value
specified by the client. Note that if this Protobuf field is not set, the
meter config is set to execute the default behavior (GREEN for all packets).
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.
As a direct resource associated with a table entry, request the server to
return the meter config in the meter_config
field of the TableEntry
message (see Direct resources).
Explicitly request the meter configuration by including the DirectMeterEntry
in the ReadRequest
. The table_entry.match
field must match the
TableEntry
whose meter config is being read. If no such entry is found, the
server returns the error code NOT_FOUND
.
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:
meter_id
is a uint32
, the unique identifier for the meter.
index
is a Protobuf message that encapsulates an int64
, used to index into
a meter array.
config
is a Protobuf message of type MeterConfig
, which represents the
meter configuration.
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).
INSERT
: Server returns the error code INVALID_ARGUMENT
.
MODIFY
: Modify an indirect meter instance whose unique id is meter_id
and
array index is specified by index
. The meter is reconfigured using the
config
field specified by the client. Note that the meter configuration is
set to the default behavior (GREEN for all packets) if this Protobuf field is
not set. If the index
field is omitted all meter configurations in the array
will be set to the value provided by the client (or reset to the default value
if config
is unset). The server must return INVALID_ARGUMENT
for a
negative index value and OUT_OF_RANGE
if the index value exceeds the size of
the meter array.
DELETE
: Server returns the error code INVALID_ARGUMENT
.
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:
If the meter_id
field is set to 0 (default), the server returns the
configuration for all indirect meter instances in the ReadResponse
.
If the index
field is not set, the server returns the configuration for all
indirect meters in the array identified by the unique id meter_id
.
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.
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.
INSERT
: Add a new multicast group entry bound to a set of egress ports and
replica IDs. The multicast_group_id
field is a uint32
and must not exceed
the maximum value supported by the target. The PSA specification states that 0
is a special value which indicates that no multicast replication is to be
performed for a packet [22]. Therefore multicast_group_id
must
never be set to 0. If one of these constraints is violated, the P4Runtime
server must return an INVALID_ARGUMENT
error. The replica instance
ID is
also a uint32
, and its value may not exceed the maximum allowed by the
target for the EgressInstance_t
type (0 is allowed), or the server must
return an INVALID_ARGUMENT
error. The egress port must be a 32-bit SDN port
number and must refer to a singleton port. No two replicas may have identical
values of both egress_port
and instance
, or the server must return
INVALID_ARGUMENT
.
MODIFY
: Modify the set of replicas for a given multicast group entry,
indexed by the given multicast_group_id
. Same restrictions as INSERT
apply
here.
DELETE
: Delete the multicast group indexed by the given
multicast_group_id
. The replicas need not be provided for this
operation. Any packets with their multicast_group
metadata in the data plane
set to the deleted multicast_group_id
will be dropped.