Messaging over tcp


TCP is streaming protocol. Messages is datagrams.
UDP is datagram protocol. But it is not reliable and not ordered.
Very often we need to send a message over tcp because we need reliability and order preservance over messages.





msg1 = sock.recv()
msg2 = sock.recv()

What will server get? It depends on many factors like MTU, Nahl algorithm and so on.
But usual cases are next:

msg1 = "m"
msg2 = "sg1msg2"

msg1 = "msg1msg"
msg2 = "2"

msg1 = "msg1msg2"
msg2 = // still blocked


The solution is to know where first message ends and second begins.
There are severall working approaches for this problem.

Magic separator

You can use some magic number to separate messages.
For example you have json ascii serialized messages and 0x00("\0") as a separator for your text.

Int prefixed

Int-prefixed is the best and most universal choice if you need to transfer binary(or text) messages.
The solution is next format of sended message

| sizeof(int) bytes | msg_size bytes                          |
| msg_size          | msg_body                                |
| 0x00000010        | 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xde |
|                   | 0xad 0xbe 0xaf 0xb0 0x0b 0xb0 0x0b oxff |

this can be done via next pseudocode

def send_message(buffer, size):
    _sock.send(htons(size), sizeof(size))
    _sock.send(buffer, size)

Note htons - this is Host TO Network Sequence transformation function.
This is done to make big endiand and little endian to be able to communicate each other.

The receiving code is a way more tricky.
You should handle when you got more than one message, situations when you get a half of header,
header and half of a message and so on. For inspiration you can take a look at next implementations.

Use 3rd party library

There are a lot of libraries which can solve transport and message dispatching tasks. Take a look at next projects:

Fixed size messages

One of the less flexible solutions, but the simpliest to implements.
Just set all messages size to be equal. For example 64b. This works great when your messages are simple.