Are TCP/IP Sockets Atomic?

Question

It is my understanding that a write to a TCP/IP socket will be atomic if the amount of data written is small. By atomic, I mean that the receiver will receive all of the data or none of the data. However, it is not atomic, if the amount of the data written is large. Am I correct? and if so, what counts as large?

Thanks, Bob

Answer 1

No. TCP is a byte-stream protocol. No messages, no datagram-like behaviour.

Answer 2

For UDP, that is true, because all data written by the app is sent out in one UDP datagram.

For TCP, that is not true, unless the application sends only 1 byte of data at a time. A write to a TCP socket will write all of the data to a buffer that is associated with that socket. TCP will then read data from that buffer in the background and send it to the receiver. How much data TCP actually sends in one TCP segment depends on variables of its flow control mechanisms, and other factors, including:

• Receive Window published by the other node (receiver)
• Amount of data sent in previous segments in flight that are not acknowledged yet
• Slow start and congestion avoidance algorithm state
• Negotiated maximum segment size (MSS)

In TCP, you can never assume what the application writes to a socket is actually received in one read by the receiver. Data in the socket's buffer can be sent to the receiver in one or many TCP segments. At any moment when data is made available, the receiver can perform a socket read and return with whatever data is actually available at that moment.

Of course, all sent data will eventually reach the receiver, if there is no failure in the middle preventing that, and if the receiver does not close the connection or stop reading before the data arrives.