p4 spec v1.1 (1185620)
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The P4 Language SpecificationVersion 1.1.0January 27, 2016The P4 Language Consortium© 2014-2016, The P4 Language ConsortiumCONTENTSCONTENTSContents1 Introduction1.1 The P4 Abstract Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2 The mTag Example . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . .1.3 Specification Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Structure of the P4 Language2.1 Abstractions . . . . . . . . . . . . . . . . . . . . . . . .2.2 Value Specifications . . . . . . . . . . . . . . . . . . .2.3 Types and declarations . .
. . . . . . . . . . . . . . . .2.4 P4 data types . . . . . . . . . . . . . . . . . . . . . . . .2.4.1 Principles . . . . . . . . . . . . . . . . . . . . .2.4.2 Base types . . . . . . . . . . . . . . . . . . . . .2.4.3 Portability . . . . . . . . . .
. . . . . . . . . . .2.4.4 No saturated types . . . . . . . . . . . . . . . .2.4.5 Boolean . . . . . . . . . . . . . . . . . . . . . .2.4.6 Unsigned integers (bit-strings) . . . . . . . . .2.4.7 Signed Integers . . . . . . . . . . . . . . . . . .2.4.8 Dynamically-sized bit-strings . . . . . . . .
. .2.4.9 Infinite-precision integers . . . . . . . . . . . .2.4.10 Integer literal types . . . . . . . . . . . . . . . .2.5 Base type operations . . . . . . . . . . . . . . . . . . .2.5.1 Computations on Boolean values . . . . . . .2.5.2 Operations on unsigned fixed-width integers2.5.3 Operations on signed fixed-width integers . .2.5.4 A note about shifts . . . . . .
. . . . . . . . . .2.5.5 varbit operations . . . . . . . . . . . . . . . .2.5.6 Operations on arbitrary-precision integers . .2.6 Casts . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.6.1 Explicit casts . . . . . . . . . . . . . . . . . . . .2.6.2 Implicit casts . . . . . . . . . . .
. . . . . . . .2.6.3 Illegal expressions . . . . . . . . . . . . . . . .2.7 References . . . . . . . . . . . . . . . . . . . . . . . . .2.8 Expressions . . . . . . . . . . . . . . . . . . . . . . . .2.9 Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . .5577....................................................................................................................................................................................................................................................................................................................88911121213131414141515161617171818192121222223242525263 Headers and Fields3.1 Header Type Declarations .
. . . . . . . . . . . . . . . . . .3.2 Header and Metadata Instances . . . . . . . . . . . . . . .3.2.1 Testing if Header and Metadata Instances are Valid3.2.2 Header Stacks . . . . . . . . . . . . . . . . . . . . . .........................................26262930312........................................................CONTENTSCONTENTS3.3 Header and Field References . . . . . . . .
. . . . . . . . . . . . . . . . . . . 323.4 Field Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 Checksums and Hash-value generators334.1 Checksums . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 345 Parser Specification5.1 Parsed Representation . . . . . . .5.2 Parser Operation . . . . . . . . . .5.3 Value Sets . . . . . . . . . . . . . .5.4 Parser Function BNF . . . . . . . .5.5 The extract Function . . . . . . . .5.6 Parser Exceptions . . . .
. . . . . .5.6.1 Standard Parser Exceptions5.6.2 Default Exception Handling................................................................................................................................................................................................3637373839414142426 Deparsing437 Standard Intrinsic Metadata438 Counters, Meters and Registers468.1 Counters .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478.2 Meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488.3 Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 499 Match+Action Table Overview5010 Actions10.1 Primitive Actions . . . . . . . . . . . . .10.1.1 Parameter Binding . . . . . . . .10.2 Action Definitions . . . . . . . . . . . . .10.2.1 Sequential Execution Semantics5152656667....................................................................................11 Action profile declarations6812 Table Declarations7013 Packet Processing and Control Flow7314 Egress Port Selection, Replication and Queuing7615 Recirculation and Cloning7715.1 Clone . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7715.1.1 Clone to Ingress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7815.1.2 Clone to Egress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783CONTENTSCONTENTS15.1.3 Mirroring . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 7915.2 Resubmit and Recirculate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8116 Extern objects8116.1 Extern types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8216.2 Extern Instances . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 8317 Appendices17.1 Programming Conventions . . . . . . . . . . . . . . . . . .17.2 Revision History . . . . . . . . . . . . . . . . . . . . . . . . .17.2.1 Summary of changes introduced in 1.1.0 . . . . . .17.3 Terminology (Incomplete) . . . . . . . . .
. . . . . . . . . .17.4 Summary of P4 BNF . . . . . . . . . . . . . . . . . . . . . .17.5 P4 Reserved Words . . . . . . . . . . . . . . . . . . . . . . .17.6 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17.6.1 The Annotated mTag Example . . . . . . . . . . . .17.6.2 Adding Hysteresis to mTag Metering with Registers17.6.3 ECMP Selection Example . . . . . . . . . .
. . . . .17.7 Addendum for Version 1.1.0 . . . . . . . . . . . . . . . . . .17.7.1 Architecture-language separation . . . . . . . . . .17.7.2 Targets . . . . . . . . . . . . . . . . . . . . . . . . . .17.7.3 Target Architecture Structure . . . . . . . . . . . . .17.7.4 Target Architecture Selection .
. . . . . . . . . . . .17.7.5 Programmable blocks . . . . . . . . . . . . . . . . .17.7.6 Standard Library . . . . . . . . . . . . . . . . . . . .17.7.7 Standard Switch Architecture . . . . . . . . . . . . .17.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . .4..............................................................................................................................................................................................8484848486869597971121141151151161161171181191211241 INTRODUCTION1IntroductionP4 is a language for expressing how packets are processed by the pipeline of a networkforwarding element such as a switch, NIC, router or network function appliance.
It isbased upon an abstract forwarding model consisting of a parser and a set of match+action table resources, divided between ingress and egress. The parser identifies theheaders present in each incoming packet. Each match+action table performs a lookupon a subset of header fields and applies the actions corresponding to the first matchwithin each table.
Figure 1 shows this model.P4 itself is protocol independent, and hence users can define the forwarding behaviorof their own data-plane protocols in P4. A P4 program specifies the following for eachforwarding element.• Header definitions: the format (the set of fields and their sizes) of each headerwithin a packet.• Parse graph: the permitted header sequences within packets.• Table definitions: the type of lookup to perform, the input fields to use, the actionsthat may be applied, and the dimensions of each table.• Action definitions: compound actions composed from a set of primitive actions.• Pipeline layout and control flow: the layout of tables within the pipeline and thepacket flow through the pipeline.P4 addresses the configuration of a forwarding element.
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