P4 foundation

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.

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

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

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.

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

Let's Make Router
PHY
MAC
IP

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

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 ?

<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>

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

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

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

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

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)

Parse Tree
START
ETHERNET
IPv4

Parse Tree
START
ETHERNET
ACCEPT
IPv4

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

Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
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

Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
IN OUT
                 out headers hdr,
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

Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
IN OUT
                 out headers hdr,
Parser Declaration
state start {
    }
parser_ethernet
●
Extract ethernet header
●
Check EtherType field
DEST MAC SRC MAC EtherType
0x0800
0x0806
0x86DD
. . .
. . .

Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
IN OUT
                 out headers hdr,
Parser Declaration
state start {
    }
parser_ethernet
●
Extract ethernet header
●
Check EtherType field
DEST MAC SRC MAC EtherType
0x0800    : IPv4
0x0806    : ARP
0x86DD   : IPv6
. . .
. . .

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

Code Block
START
ETHERNET
ACCEPT
IPv4
MyParser
packet_in
meta meta
hdr
IN OUT
                 out headers hdr,
Parser Declaration
state start {
    }
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;
    }
}

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

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

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.

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

Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
meta
hdr
Match Action
* drop
* noAction
default_action drop
{
“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
}

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

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
S3runtime.json
Table Entries
?????????

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
S3runtime.json
{
“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
}

Ingress Code Block
MyIngress
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
Match
Action
. . . .
. . . .
hdr
meta
meta
hdr
Match Action
* drop
* noAction
default_action drop
{
“default_action”: true,
“action_name”:”MyIngress.drop”,
“action_params”: { }
}

Execution of P4 Program
mybasic.p4

Execution of P4 Program
mybasic.p4
topology.json

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
* drop
* noAction
default_action drop

Ingress Code Block
Match Action
* drop
* noAction
default_action drop

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

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

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

PISA: ProtocolIndependent Switch Architecture

Self Practice
ROUTER
<L3>
SWITCH
<L2>

P4 foundation

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