Networking

TCP

SYN/ACK Dance

In order to establish a TCP connection, one party must send a SYN packet to the other, receive a SYN-ACK packet back, and finally send an ACK packet.

SYN SYN-ACK ACK

TCP Packet Layout

Size of Packet

TCP vs UDP

In this class, we focus on TCP. Where might UDP be used?

Drfferences

Networking Functions

Socket

socket creates an endpoint for communication and returns a file descriptor that refers to that endpoint.

Bind

bind associates an abstract network socket created with socket to an actual networking interface/address.

Listen

listen marks a socket as “passive”, or ready to accept incoming connections.

Accept

accept accepts an incoming connection on a socket that has been listening.

Connect

connect is used to connect a socket to a given address (which might have a socket listening for connections).

Error checking

Make sure to check errors for every call, networking can fail at any point! All networking functions specify in their manpages what types of errors you might have to deal with.

A few gotchas

Network vs Host Byte Ordering

Byte Ordering

Socket Options

Make sure to set socket options to reuse to enable effective debugging of the server - otherwise, the system won’t immediately reallow you to use a socket once it’s been closed by your program.

Signal Handler Safety

Most server applications are interupted through a signal, but you shouldn’t do all of the cleanup in the signal handler because not every function is signal handler safe (think back to CS233 interrupts). This pattern avoids this issue:

int is_running = 1;
int handler(){
	is_running = 0;
}
int main(){
	while(is_running){ /*...*/};

	close(...);
}

Style

Try to modularize your functions so that everything is not in the main method. This is ideally because we need to tell the system that we are done using shared resources like sockets, and we need to determine when a socket goes “out of scope” – we don’t have RAII like in C++ so we have to determine that ourselves, and modularizing your code makes it easy to see when things are in/out of scope.