Python TCP Duplicate Message

Question

I'm working on an in-house TCP Server, and TCP client. When there is 0% packet loss the Server and Client work fine. However, when I have 20% or more packet loss I am seeing duplicate TCP messages. I am receiving something like this....

Client <-- MessageA -- Server
Client -- MessageB --> Server
Client <-- MessageCMessageA -- Server

Is it possible that MessageA is not completely making it to the Client, it times outs, then TCP resends it, and then the original message makes it which is received at a later time by the Client?

My question is if TCP works like that, and if that's a possible scenario with a network containing 20% packet loss or more.

Barebones of how the client and server are sending/receiving data...

socket.recv(1024)
socket.send(1024)

Assuming that you are using the built in TCP/IP library and assuming that all of the pieces eventually arrive before the connection times out, TCP provides the guarantee that you do not need to worry about duplicates and retransmissions. It's one of the main reasons you would choose TCP over UDP... That and the guarantee that the data is properly ordered. — David Hoelzer
– David Hoelzer, Commented Apr 27, 2016 at 21:15
Ok, thank you. I just wanted to make sure before I started looking for a bug. — Taztingo
– Taztingo, Commented Apr 27, 2016 at 21:34
Perhaps you assumed that your recv call will always return exactly 1,024 bytes? The buffer size is the maximum number of bytes that will be received, not the actual number. Show us your complete code to receive a message. — David Schwartz
– David Schwartz, Commented Apr 28, 2016 at 1:39

jch · Accepted Answer · 2016-04-28 01:29:19Z

1

No, it is not possible. TCP guarantees that it will either deliver data exactly once and in the order in which it was sent, or signal an error to the application. Hence, there is probably a bug in your code. The most likely is that your code fails to deal with partial reads.

When you perform a write or send on a TCP socket, the TCP module will segment your data into as many packets as are needed. In the presence of packet loss, it is possible that some packets have arrived successfully but others must be resent. In that case, the corresponding read or recv will only receive part of the data — the remaining data will arrive in a subsequent read or recv. In other words, TCP does not preserve message boundaries.

Your code is probably interpreting such a split message as multiple messages. Make sure that you accumulate a full message in your buffer before attempting to parse it.

answered Apr 28, 2016 at 1:29

jch

5,66927 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Taztingo Over a year ago

Great explanation.

Collectives™ on Stack Overflow

Python TCP Duplicate Message

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related