CODEWITHSUNDEEP
cnetwork-programmingtcp
Blacklabel
Blacklabel

Reputation: 846

A question on TCP connection buffers

Linux.

I have an ethernet cable between two computers. A simple server-client program in C to transfer a file from client to server. Client reads 100 bytes of data from the file and sends it to the server, and then waits for 2 seconds before sending the next packet.

When the client was sending the data, I pulled out the ethernet cable of the server side, I was expecting some error from the client since the connection is broke. But the client kept on writing the data to the pipe, and the server did not receive anything(but still it is waiting to receive).Client sent the whole file and stopped. Now, again I connect the ethernet cable, the server receives all the data sent by the client. How is this possible? Are the packets stored in some buffer and sent again when the connection is up?

Sorry for making it too long.

Upvotes: 2

Views: 2824

Answers (3)

ninjalj
ninjalj

Reputation: 43688

On Linux, you can see the kernel-maintained send-queue and timeouts by running:

netstat -tanpo

When the connections terminates abruptly, due to an error or a timeout, the client can notice by checking the return values from write(), shutdown() and close().

Upvotes: 1

derobert
derobert

Reputation: 51137

The whole point of TCP is to provide reliable data transfer despite having an unreliable network underneath. Basically, the way it works is TCP only considers data sent after it receives an acknowledgement packet from the remote machine; until then, the kernel stores the data locally. The amount of time it'll be buffered for can be quite long, depending on a bunch of settings in /proc as well as measured network parameters.

edit: An app can check the size of the outstanding send queue by using the SIOCOUTQ ioctl; see the tcp(7) manpage. The same manpage discusses error handling as well.

If you want that data to be lost instead of buffered, use UDP.

Upvotes: 8

Diego Sevilla
Diego Sevilla

Reputation: 29011

Yes. Depending on the configuration, it even may wait some time to see if it can recover the connection. Even, tcp timeouts can be of minutes... Note here for example, the parameter tcp_fin_timeout and other parameters related to timeouts.

Upvotes: 2

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