How to detect retransmited packets to discard them in TCP Sockets in java

Question

I sometimes receive already received packets (I used sniffer and system ACKs them). Now I read all data (until socket timeout) and then send new request, but this is ugly. I was thinking about using sequence numbers but i didn't find it in Socket interface. Any clues?

Answer 1

No you don't. If the receiving TCP stack misses a packet, it will re-request it, but it can't have delivered the original one to you, because it missed it. And if it gets a packet it has already received, it will drop it.

TCP will deliver all the bytes that are sent, in the order they are sent. Nothing else (well, except some edge cases around disconnects).

Something else is going on.

EDIT:

To be clear, I'm talking about the bytes that are delivered to your application through the socket's InputStream. What happens on the wire is largely irrelevant unless you have some horrific network retransmission problem that you're trying to investigate. And if the receiving stack does get a duplicate packet, it will ACK it, because if it didn't then the sender would re-send it... again.

Answer 2

It sounds like you're trying to account for things that TCP already takes care of. It has sequence numbers built in and will take care of any lost data for you, and from the receiving point you should be waiting until you receive all your expected data, rather than reissuing a request. If you don't want to wait for a response to complete before issuing a new request, consider pipe-lining requests with multiple connections.