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between the control and data planes. Instead, P4 uses the concept of tables to represent forwarding plane state. An interface between the control plane and the various P4 tables must be provided to allow the control plane to inject/modify state in the program. This interface is generally referred to as the
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device, and/or increment a counter. Tables and their associated actions are almost always chained together in sequence to realize the full packet forwarding logic, although in the abstract it is possible to build a single table that includes all the lookup key information and the full output action set.
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The P4 parser is a finite state machine that walks an incoming byte-stream and extracts headers based on the programmed parse graph. A simple example would be a parser that extracts the
Ethernet source and destination and type fields, then performs a further extraction based on the value in the type
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P4 tables contain the state used to forward packets. Tables are composed of lookup keys and a corresponding set of actions and their parameters. A trivial example might be to store a set of destination MAC addresses as the lookup keys, and the corresponding action could set the output port on the
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that maps the P4 source code into a target switch model. The compiler may be embedded in the target device, an externally running software, or even a cloud service. As many of the initial targets for P4 programs were used for simple packet switching it is very common to hear the term "P4 switch"
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and generally consist of things such as copying byte fields from one location to another based on the lookup results on learned forwarding state. P4 addresses only the data plane of a packet forwarding device. It does not specify the control plane nor any exact protocol for communicating state
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Protocol independence and the abstract language model allow for reconfigurability–P4 targets should be able to change the way they process packets (perhaps multiple times) after they are deployed. This capability is traditionally associated with forwarding planes built on
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Header definitions describe packet formats and provide names for the fields within the packet. The language allows customized header names and fields of arbitrary length, although many header definitions use widely known protocol names and fields widths. For example, an
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field by one" or "copy the MAC address from the output port table into the outgoing packet header." P4 defines both standard metadata that must be provided by all targets as well as target-specific metadata, which is provided by the author of specific targets.
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Ethernet header definition might be called “Ethernet” and consist of a 48-bit field named “dest” followed by a 48-bit “src” field, followed by a 16-bit “type” field. The names in a header definition are used later in the P4 program to reference these fields.
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manipulations. In P4 context, metadata is information about a packet that is not directly derived from the parser, such as the input interface that the frame arrived on. English descriptions of an example action might be "decrement the IPv4
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As the language is specifically targeted at packet forwarding applications, the list of requirements or design choices is somewhat specific to those use cases. The language is designed to meet several goals:
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The primary component of a P4 program is a set of user-defined match action tables. P4 treats all match action tables as generic, leaving the user to add their match-action rules via the control plane.
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P4 allows the specification of custom packet header parsing logic including but not limited to parsing typical headers used in the TCP/IP protocol suite and application specific headers.
534:. Conceptually, forwarding network packets or frames can be broken down into a series of table lookups and corresponding header manipulations. In P4 these manipulations are known as
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P4 programs are designed to be implementation-independent: they can be compiled against many different types of execution machines such as general-purpose CPUs,
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paper titled “Programming
Protocol-Independent Packet Processors”—the alliterative name shortens to "P4". The first P4 workshop took place in June 2015 at
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P. Bosshart; D. Daly; G. Gibb; M. Izzard; N. McKeown; J. Rexford; C. Schlesinger; D. Talayco; A. Vahdat; G. Varghese; D. Walker (July 2014).
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P4 is designed to be protocol-independent: the language has no native support for even common protocols such as IP, Ethernet, TCP,
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code, and is maintained by the P4 Project (formerly the P4 Language
Consortium), a not-for-profit organization hosted by the
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in networking devices, such as routers and switches. In contrast to a general purpose language such as
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P4 allows the programmer to maintain state in the form of registers, counters and meters.
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The control flow in P4 determines the relative sequence of tables, and allows for
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625:"P4: Programming Protocol-Independent Packet Processors"
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used, even though "P4 target" is more formally correct.
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P4 programs typically have the following components:
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field (common values might be ipv4, ipv6, or MPLS).
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16:Language for controlling network data forwarding
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53:Learn how and when to remove these messages
360:with a number of constructs optimized for
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162:reliable, independent, third-party sources
198:Learn how and when to remove this message
180:Learn how and when to remove this message
110:Learn how and when to remove this message
603:"P4 Language and Related Specifications"
156:by replacing them with more appropriate
73:This article includes a list of general
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139:too closely associated with the subject
384:P4 was originally described in a 2014
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530:Fundamental to P4 is the concept of
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79:it lacks sufficient corresponding
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34:This article has multiple issues.
563:in P4 describe packet field and
137:may rely excessively on sources
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42:or discuss these issues on the
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759:C programming language family
630:Computer Communication Review
517:Generic match action tables
253:; 11 years ago
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374:Open Networking Foundation
278:; 3 years ago
242:Open Networking Foundation
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364:. P4 is distributed as
358:domain-specific language
526:Match-action processing
362:network data forwarding
94:more precise citations.
532:match-action pipelines
583:conditional execution
433:Protocol independence
370:permissively licensed
456:general-purpose CPUs
339:programming language
754:Computer networking
689:P4 Official Website
656:P4 Official Website
509:Stateful processing
408:Target independence
393:Stanford University
310:Filename extensions
248:First appeared
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710:"P4 Mailing Lists"
460:network processors
652:"1st P4 Workshop"
449:Reconfigurability
418:system(s)-on-chip
346:forwarding planes
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577:Control flow
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712:. July 2015
541:program API
366:open-source
92:introducing
748:Categories
589:References
470:Components
422:P4 targets
356:, P4 is a
230:imperative
154:improve it
144:verifiable
75:references
39:improve it
237:Developer
170:July 2019
158:citations
45:talk page
662:1 August
565:metadata
426:compiler
386:SIGCOMM
222:compiled
217:Paradigm
100:May 2015
716:15 July
636:7 April
561:Actions
556:Actions
536:actions
500:Parsers
486:Headers
380:History
319:Website
297:License
283:2021-05
281: (
256: (
148:neutral
88:improve
694:7 June
608:7 June
547:Tables
399:Design
354:Python
343:packet
304:-style
302:Apache
77:, but
493:802.3
464:ASICs
441:, or
439:VxLAN
414:FPGAs
337:is a
718:2015
696:2022
664:2019
638:2015
610:2022
443:MPLS
326:.org
258:2013
251:2013
146:and
570:TTL
458:or
388:CCR
352:or
314:.p4
160:to
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672:^
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335:P4
324:p4
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Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
