p4register

Sample P4 program to show the usage of registers. In particular, in this application a simple TCP netcat communication is assumed on port 4444 (see the parser for more details). If one character is sent by the client to the server, the application replaces that character with another one found/already pushed to the register. The original character, then, will update the register, therefore the server will always be behind the client with one character.

Example

Client@10.0.0.1	Server@10.0.0.2:4444	Register's old value	Register's new value
`# netcat 10.0.0.2 4444`	`# netcat -l -p 4444`	-	-
`a`		-	`a`
`s`	`a`	`a`	`s`
`d`	`s`	`s`	`d`
	`d`	`d`

Additional features

TCP payload, or at least 2 bytes in the beginning of it is being parsed as well (1 byte is the first character, and another additional 1 Byte is the EOL/NL character in netcat)
TCP Checksum recalculation as we modify the payload
Packet headers are sent through different debugging tables to provide a lot of output on the console and make the process easily followable (adapted from my other P4 example code: p4debug - verbosity can be controller by the #defines in the beginning of the source code)

Simplified state machine

TCP payload length is calculated when a TCP packet is parsed/caugth
- if it is 0, nothing happens just port forwarding between the two ports of the switch
- otherwise, we continue parsing
  - if tcp.dstPort is 4444, then it is our netcat communication, so we do extract those 2 bytes from the beginning of the payload
  - otherwise, just port forwarding between the two ports of the switch
in case we have one character caught from the netcat communication, we do as follows:
- read the registers value (empty character in the beginning) into meta.read_from_register
- write the character caugth (hdr.netcat.one_character) into the register.
- write the read register value (meta.read_from_register) into the payload's relevant part (hdr.netcat.one_character)
Since, we modify the TCP payload, we need to recalculate the TCP checksum (this is why we have meta.tcp_metadata, meta.modified, etc.).
- Note that IPv4 Checksum does not need to be recalculated as we do not do any changes in that header (e.g., don't decrease TTL, don't change MAC address).
- See the MyComputeChecksum() for more details about the TCP checksum calculation (for brevity, I did not use the incremental TCP checksum calculation, so each time it is recalculated from scratch - but it's OK, don't worry :)).

Compilation

# p4c-bm2-ss registers.p4 -o registers.json

Execution

# simple_switch --log-console -i 0@eno3 -i 1@eno4 registers.json

Note that my two interfaces were eno3 and eno4, and don't forget the --log-console to have the debugging 'print-outs'.

Reading registers during running (if needed)

# simple_switch_CLI --thrift-port 9090
# register_read r

cslev / p4register