Packets sometimes get concatenated

Question

I'm trying to make a simple server/application in Erlang.
My server initialize a socket with gen_tcp:listen(Port, [list, {active, false}, {keepalive, true}, {nodelay, true}]) and the clients connect with gen_tcp:connect(Server, Port, [list, {active, true}, {keepalive, true}, {nodelay, true}]). Messages received from the server are tested by guards such as {tcp, _, [115, 58 | Data]}.

Problem is, packets sometimes get concatenated when sent or received and thus cause unexpected behaviors as the guards consider the next packet as part of the variable.

Is there a way to make sure every packet is sent as a single message to the receiving process?

Answer 1

Plain TCP is a streaming protocol with no concept of packet boundaries (like Alnitak said).

Usually, you send messages in either UDP (which has limited per-packet size and can be received out of order) or TCP using a framed protocol.

Framed meaning you prefix each message with a size header (usualy 4 bytes) that indicates how long the message is.

In erlang, you can add {packet,4} to your socket options to get framed packet behavior on top of TCP.

assuming both sides (client/server) use {packet,4} then you will only get whole messages.

note: you won't see the size header, erlang will remove it from the message you see. So your example match at the top should still work just fine