First acquaintance with P4 language

P4 Way of learning

 Record yourself P4 Some simple things learned during the experiment P4 knowledge 

Concept

• P4 The full name is Programming Protocol-independent Packet Processor, That is, a language for programming protocol independent packet processors
• The so-called irrelevant to the agreement , That is to realize a new network protocol according to the needs of users
• Define the data surface you want , Then through the southbound agreement （openFlow） Add stream table entry

technological process

• P4 The process is shown in the figure below , When packets enter the switch , First, parse the packet header , then ingress To deal with , Finally, the packet is forwarded
• P4 What I write is how to deal with data packets

Package parsing

• Package parsing is The step of extracting the data we want in the data message
  PS：（ The engineer reminded me ） What is extracted is extracted later according to the sequence of messages , A pointer points to the position where the message has been extracted
• The extracted part , It's wrong to extract again
  such as , I had already extracted the packet IPv4 The head of , Contains the purpose IP Address , I want to regard it as four 8 Bit character extraction , You can't write another extraction destination address , I made this mistake

Examples are as follows , demand ： Aim IP The address resolves to four 8 Bit characters
The code is as follows ：

//  modify IPv4 Of header
header ipv4_h {
    
    bit<4> version;
    bit<4> ihl;
    bit<8> diffserv;
    bit<16> total_len;
    bit<16> identification;
    bit<3> flags;
    bit<13> frag_offset;
    bit<8> ttl;
    bit<8> protocol;
    bit<16> hdr_checksum;
    ipv4_addr_t src_addr;
    // ipv4_addr_t dst_addr;
    bit<8> ch1;
    bit<8> ch2; 
    bit<8> ch3; 
    bit<8> ch4;
}

//  Package parsing logic part 
parser SwitchIngressParser(
        packet_in pkt,
        out header_t hdr,
        out metadata_t ig_md,
        out ingress_intrinsic_metadata_t ig_intr_md) {
    

	//  Parser from start The state starts execution and ends , among reject or accept Indicates that the end state has been reached 
	// transition Indicates a transition to another state 
    state start {
    
        pkt.extract(ig_intr_md);
        pkt.advance(PORT_METADATA_SIZE);
        transition parse_ethernet;
    }

    state parse_ethernet {
    
        pkt.extract(hdr.ethernet);
        transition parse_ipv4;
    }

	//  The last statement of the parser state is optional transition sentence , Transfer control to another state , Probably accept  perhaps reject
    state parse_ipv4 {
    
        pkt.extract(hdr.ipv4);
        transition accept;
    }
}

Write a table and look it up

• Write a watch , The table includes the following sections
  • Table name
  • key Value and matching type , The matching types are exact、ternary、lpm Three
  • actions, Include all possible action
  • Define the default action
  • The size of the table

table tableName {
    
        key = {
    
            a : exact;		//  keyword a Is the exact match 
            b : ternary; 	//  keyword b yes ternary type 
        }
        actions = {
    
            get_value;	//  Include all possible actions 
        }
        size=1024;	// The size of the table 
    }

• Look up the table
  • A table can only be checked once , A table corresponds to a match-action unit（ Match the action unit ）
  • To find a table multiple times , It needs to be instantiated many times , That is, write multiple tables with different names
  • Look up the table in apply Carry out inside

apply {
    
		//  To operate 
		//  Look up the table 
        tableName.apply();
    }

• actions
  （ I feel that I can only assign values …）
  • Only straight-line Code
  • There are no conditional statements or expressions

Package package

Not yet understood …

Other

• switch
  about switch sentence , The expression must be t.apply().action_run(), All labels must be table actions t, Or is it default
• select
  Select expression select Compare the extracted header field with a set of constants

Reference resources

• Knowable P4 Learning notes
• Official P4 language norm
• P4 Programming theory and practice —— Theory Chapter
• Article to read P4 Programmable switching chip