This document provides an overview of the P4 programming language. P4 allows defining the data plane functionality of programmable forwarding elements like software switches, NICs, routers, and network appliances in a protocol and target independent way. The key components of P4 include data declarations to define packet headers and metadata, parser programming to parse packets, packet processing programming using match-action tables, and control functions to define the communication between the data plane and control plane. A P4 program defines the packet parsing and processing logic, while the P4 compiler generates target-specific code that implements this logic on the actual hardware or software target.

1. P4 Language

2. P4 Overview
●
A language for expressing how packets are
processed by the data plane of a programmable
forwarding elements like software switch , NIC ,
router or network appliance.
●
Designed to specify only the data plane
functionality of the target.
●
It also defines the interface by which control
plane and data plane communicates.

3. P4 Language: Objective
●
Protocol Independent
– Define a packet parser
– Define a set of match+action tables
●
Target Independent
– No knowledge of packet­processing device details
– Compiler Driver
●
Reconfigurability
– Allow to change parser and processing

4. P4 Language Components
●
Data Declarations
– Packet Header and Metadata
●
Parser Programming
– Parser Functions
– Checksum Units
●
Packet Flow Programming
– Actions
●
Primitive and compound actions
●
Counters, Meters, Registers
– Tables
●
Match Keys
●
Attributes
– Control Functions

5. P4 Programmable Switch
●
Data Plane Functionality is defined by the P4 program.
●
Control Plane communicate with the data plane using same channels, but the
set of tables and other objects in the data plane are no longer fixed.
– P4 compiler generates the API that the control plane uses to communicate
with the data plane.

6. ROUTER
<L3>
SWITCH
<L2>
How Packet Flows ?
D1
S1
S2
S3
S4
How many times Packet Passed from L2 Layer and L3
Layer ?

7. Encapsulation

8. Let's Make Router
PHY

9. Let's Make Router
PHY
MAC

10. Let's Make Router
PHY
MAC
IP

11. Let's Make Router
PHY
MAC
IP

12. Basic Forwarding: Topology

13. Exercise­1
●
Ipv4 Forwarding
– Key Tasks
●
Update the source and destination MAC addresses
●
Decrement the TTL
● Forward out the correct port

14. Hands­on
● Goto --> ... /tutorials­master/exercises/
●
Create new folder and named it as myip_forward
●
Open any editor and create new p4 program
Example : myip_forward.p4
●
Include core.p4 header file
●
It defines some standard data­types and error codes.
●
For example: declarations of the predefined packet_in and packet_out
extern objects.
● #include <core.4>
●
What is extern in C ?

15. <Include Header Files>
● Goto --> ... /tutorials­master/exercises/
●
Create new folder and named it as myipforward
●
Open any editor and create new p4 program
Example : ip_forward.p4
●
Include core header file
●
It defines some standard data­types and error codes.
●
For example: declarations of the predefined packet_in and packet_out
extern objects.
●
What is extern in C ?
#include<core.4>
●
Include v1model header file
●
For Example:
#include<v1model.p4>

16. Create Packet Headers

17. L2 Header
Filed Name Size(bits)
Source MAC 48
Destination MAC 48
Ethernet Type 16

18. L2 Header
header ethenet_t {
macAddr_t dstAddr;
macAddr_t srcAddr;
bit<16> etherType;
}
typedef bit<48> macAddr_t;
To define the alternative name for
the existing datatype
It is a keyword and hear we are
grouping the Ethernet­II header files

19. Write P4 Program
header ipv4_t {
bit<4> version;
bit<4> ihl;
bit<8> diffserv;
bit<16> totalLen;
bit<16> identification;
bit<3> flags;
bit<13> fragOffset;
bit<8> ttl;
bit<8> protocol;
bit<16> hdrChecksum;
ip4Addr_t srcAddr;
ip4Addr_t dstAddr;
}
typedef bit<32> ipv4Addr_t;
To define the alternative name for
the existing datatype

20. Write P4 Program:Combine Header
struct headers {
ethernet_t ethernet;
ipv4_t ipv4;
}
MAC HeaderIP Header
To group together both these header into one structure

21. Create Parser

22. Write P4 Program: Parser
●
Parser is a function that map packets into
headers and metadata, written in a state
machine style.
●
Every parser has three predefined states
● start
● accept
● reject
●
User can define additional states as well.
●
Each state , execute zero or more statements,
and then transition to another state (loops are
OK)

23. Parse Tree
START

24. Parse Tree
START
ETHERNET

25. Parse Tree
START
ETHERNET
IPv4

26. Parse Tree
START
ETHERNET
ACCEPT
IPv4

27. Parser Model
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT

28. Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT
parser MyParser (packet_in packet ,
out headers hdr,
inout metadata meta ,
inout standard_metadata_t standard_metadata )
Parser Declaration
●
Parser ­­> its a keyword in P4 Language
●
MyParser ­­> Name of Parser
●
(. . . ) ­­> list of parameters

29. Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT
parser MyParser (packet_in packet ,
out headers hdr,
inout metadata meta ,
inout standard_metadata_t standard_metadata )
Parser Declaration
state start {
transition parse_ethernet;
}
●
Each state has a name and body.
●
It consists of a sequence of statements that describe the processing
performed when the parser transitions to that state including:
●
Local variable declarations
●
Assignment statements
●
Method calls, which serve following purpose:
●
Invoking function
●
Invoking methods
●
Transitions to other states

30. Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT
parser MyParser (packet_in packet ,
out headers hdr,
inout metadata meta ,
inout standard_metadata_t standard_metadata )
Parser Declaration
state start {
transition parse_ethernet;
}
parser_ethernet
●
Extract ethernet header
●
Check EtherType field
DEST MAC SRC MAC EtherType
0x0800
0x0806
0x86DD
. . .
. . .

31. Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT
parser MyParser (packet_in packet ,
out headers hdr,
inout metadata meta ,
inout standard_metadata_t standard_metadata )
Parser Declaration
state start {
transition parse_ethernet;
}
parser_ethernet
●
Extract ethernet header
●
Check EtherType field
DEST MAC SRC MAC EtherType
0x0800 : IPv4
0x0806 : ARP
0x86DD : IPv6
. . .
. . .

32. Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT
parser MyParser (packet_in packet ,
out headers hdr,
inout metadata meta ,
inout standard_metadata_t standard_metadata )
Parser Declaration
state start {
transition parse_ethernet;
}
state parse_ethernet {
packet.extract(hdr.ethernet);
transition select(hdr.ethernet.etherType) {
TYPE_IPV4: parse_ipv4;
default: accept;
}
}

33. Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta standard_meta
IN OUT
parser MyParser (packet_in packet ,
out headers hdr,
inout metadata meta ,
inout standard_metadata_t standard_metadata )
Parser Declaration
state start {
transition parse_ethernet;
}
state parse_ethernet {
packet.extract(hdr.ethernet);
transition select(hdr.ethernet.etherType) {
TYPE_IPV4: parse_ipv4;
default: accept;
}
}
state parse_ipv4 {
packet.extract(hdr.ipv4);
transition accept;
}
}

34. Ingress Match/Action
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
standard_meta
standard_meta
IN OUT MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action

35. Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
standard_meta standard_meta
meta
hdr
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop
Table Name: ipv4_lpm

36. S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
Q. Suppose H1 want to send Data to H2.

37. S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
Q. Suppose H1 want to send Data to H2.

38. S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
Q. Suppose H1 want to send Data to H2.
S1­runtime.json

39. Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
standard_meta standard_meta
meta
hdr
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop
Table Name: ipv4_lpm
{
“table”:”MyIngress.ipv4_lpm”,
“match”: {
“hdr.ipv4.dstAddr”: [“10.0.2.2”,32]
},
“action_name”: “MyIngress.ipv4_forward”,
“action_params”:{
“dstAddr”: “00:00:00:02:02:00”,
“port”: 2
}

40. Basic Forwarding
S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
S1­runtime.json S2­runtime.json
S3­runtime.json

41. Basic Forwarding
S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
S1­runtime.json S2­runtime.json
S3­runtime.json

42. Basic Forwarding
S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
S1­runtime.json S2­runtime.json
S3­runtime.json
Table Entries
?????????

43. Basic Forwarding
S1
S3
S2
H1
H2
H3
10.0.1.1 10.0.2.2
10.0.3.3
PORT :1
PORT :2
PORT :3
PORT :2
PORT :1
PORT :3
PORT :2 PORT :3
PORT :1
MAC : 00:00:00:00:03:03
MAC : 00:00:00:00:01:01 MAC : 00:00:00:02:02:00
MAC : 00:00:00:01:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:02:03:00
MAC : 00:00:00:03:03:00
MAC : 00:00:00:01:02:00
MAC : 00:00:00:02:02:00
S1­runtime.json S2­runtime.json
S3­runtime.json
{
“table”:”MyIngress.ipv4_lpm”,
“match”: {
“hdr.ipv4.dstAddr”: [“10.0.3.3”,32]
},
“action_name”: “MyIngress.ipv4_forward”,
“action_params”:{
“dstAddr”: “00:00:00:01:03:00”,
“port”: 2
}

44. Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
standard_meta standard_meta
meta
hdr
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop
Table Name: ipv4_lpm
{
“table”:”MyIngress.ipv4_lpm”,
“default_action”: true,
“action_name”:”MyIngress.drop”,
“action_params”: { }
}

45. Program Execution

46. Execution of P4 Program
mybasic.p4

47. Execution of P4 Program
mybasic.p4
topology.json

48. Execution of P4 Program
mybasic.p4
topology.json

49. Execution of P4 Program
mybasic.p4

50. Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
standard_meta standard_meta
meta
hdr
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop
Table Name: ipv4_lpm

51. Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
standard_meta standard_meta
meta
hdr
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop
Table Name: ipv4_lpm

52. S1
S3
S2
H1 H2
H2

53. S1
S3
S2
H1 H2
H2

54. Ingress Code Block
Ingress Stages
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
standard_meta
standard_meta
meta
hdr
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop

55. Ingress Code Block
Match Action
DST Address * ipv4_forward
* drop
* noAction
default_action drop

56. Parser Program
●
Parser States
●
One start state
●
Userdefine state
●
Two final states named as accept and reject
●
Parser Declarations
●
List of Parameters
●
Local Elements
●
Parser States

57. Parser Program
●
Parser States
●
One start state
●
Userdefine state
●
Two final states named as accept and reject
●
Parser Declarations
●
List of Parameters
●
Local Elements
●
Parser States

58. Create Ethernet Header

59. Simple Switch Architecture

60. P4 Pipeline
Metadata
Parser
Deparser
* Parser
­ Converts packets data into a metadata
* Match+Action Tables
­ Operate on metadata
* Deparser

61. P4 Program Section
typedef bit<9> egressSpec_t;
typedef bit<48> macAddr_t;
typedef bit<32> ip4Addr_t;
.
.
.
parser MyParser(packet_in packet,
out headers hdr,
inout metadata meta,
inout standard_metadata_t
standard_metadata)
.
.
.
control MyIngress(inout headers hdr,
inout metadata meta,
inout standard_metadata_t
standard_metadata)
Metadata
Parser
Deparser

62. PISA: Protocol­Independent Switch Architecture

63. P4­Based Workflow

64. Self Practice
ROUTER
<L3>
SWITCH
<L2>

65. Write P4 Program

66. Load Balancer