Julia Evans

Day 25: ACK all the things

Today I worked some more on my TCP stack! I read a bit of Stevens’ TCP/IP Illustrated, where I obtained exactly one insight: every TCP packet I send (except for the initial SYN) can have the ACK flag set.

This simplified my packet-sending code a lot – I realized I can construct basically every packet the same way, except for the flags and the payload. Now I can write

def _send_syn(self):
    self.state = "SYN-SENT"

instead of

def send_syn(self):
    syn_pkt = self.ip_header / TCP(dport=self.dest_port, sport=self.src_port, flags="S", seq=self.seq)
    self.state = "SYN-SENT"

Right now the way I’m handling ACKs is basically to send an ACK for every packet I receive. This isn’t really efficient (because if I’m receiving tons of packets really fast it would make sense to acknowledge less often). But it is not incorrect, which is all that I’m going for.

I also decided that I want to be able to be a TCP server as well as a TCP client. So I started writing bind(). Here it is so far:

def bind(self):

Seriously I did not ever think about how bind() works before. It has to manage multiple connections! And keep a queue! What am I supposed to do when people send me packets all at once? Just ignore them until I have time? I don’t get it.

Testing is fantastic. I love testing. I refactored pretty much all the code today in this commit. The code still works, because tests. =D

However writing tests kind of sucks because it takes forever. Next up: writing tests for

  • receiving packets out of order
  • receiving duplicate packets
  • being on the server side of the TCP handshake