Receiving data in TCP

Question

If i send 1000 bytes in TCP, does it guarantee that the receiver will get the entire 1000 bytes "togther"? or perhaps he will first only get 500 bytes, and later he'll receive the other bytes?

EDIT: the question comes from the application's point of view. If the 1000 bytes are reassembles into a single buffer before they reach the application .. then i don't care if it was fragmented in the way..

Answer 1

The transmission control protocol guarantees successful delivery of all packets by requiring acknowledgment of the successful delivery of each packet to the sender by the receiver. By this definition the receiver will always receive the payload in chunks when the size of the payload exceeds the MTU (maximum transmission unit).

For more information please read Transmission Control Protocol.

Answer 2

As other answers indicated, TCP is a stream protocol -- every byte sent will be received (once and in the same order), but there are no intrinsic "message boundaries" -- whether all bytes are sent in a single .send call, or multiple ones, they might still be received in one or multiple .receive calls.

So, if you need "message boundaries", you need to impose them on top of the TCP stream, IOW, essentially, at application level. For example, if you know the bytes you're sending will never contain a \0, null-terminated strings work fine; various methods of "escaping" let you send strings of bytes which obey no such limitations. (There are existing protocols for this but none is really widespread or widely accepted).

Answer 3

The only thing that the TCP layer guarantees is that the receiver will receive:

• all the bytes transmitted by the sender
• in the same order

There are no guarantees at all about how the bytes might be split up into "packets". All the stuff you might read about MTU, packet fragmentation, maximum segment size, or whatever else is all below the layer of TCP sockets, and is irrelevant. TCP provides a stream service only.

With reference to your question, this means that the receiver may receive the first 500 bytes, then the next 500 bytes later. Or, the receiver might receive the data one byte at a time, if that's what it asks for. This is the reason that the recv() function takes a parameter that tells it how much data to return, instead of it telling you how big a packet is.

Answer 4

The IP packets may get fragmented during retransmission.

So the destination machine may receive multiple packets - which will be reassembled back by TCP/IP stack. Depending on the network API you are using - the data will be given to you either reassembled or in RAW packets.