How can I verify that a TCP packet has received an ACK in C#?

Question

After sending some tcp data with the blocking/non-blocking methods such as:

Socket.Send() or Socket.SendAsync()

How can I know that my data has received an ACK message?

Can .NET know if TCP data has been successfully sent?

Answer 1

You make the other end respond to it.

Even if TCP has Acked it, if the receiving end terminates (for good or bad reasons) before processing the message and acting on it, you still don't know, so the only way to know is for the other end to tell you.

Answer 2

The only way to know for sure is to implement some kind of application-level acknowledgement. The TCP level "ACK" packet is not exposed to the application level at all, so you have to use something more than that.

Answer 3

The TCP layer will keep resending the packet until it receives a successful ACK.

Send will block until this happens - SendAsync will not block, and you can continue processing other stuff while the TCP layer handles sending the packet.

Answer 4

If you are really certain that you need to know the packet level details of your TCP connection, then in addition to creating the TCP socket for sending, you need your application to use the winpcap API to look at the raw traffic as well. You can install a filter to only receive packets relevant to the particular IP,port combination that determines your remote side.

There are a couple of projects out there creating .NET wrappers for libpcap, for example here

Answer 5

"I'm trying to focus on how can you know when your data has been accepted by the other-side of the connection."

I think you need to be aware what type of application layer protocol you are going to implement and what impact this has on application performance.

Take HTTP as an example of a "Streaming like" protocol. A server posts a stream of data to a client. There are no more additional application layer "ACKs" and the server doesn't actually care when and how exactly his stream of data arrives. This is very efficent on high latency links.

Now compare this to SMB! Instead of streaming a file, data is partitioned into blocks. Every successfully transferred block of data is acked on the application layer. This gives you more control, however, it effectively kills the protocol on WAN networks (check out "Bandwidth Delay Product").

Taking this into consideration, you can come up with your own design for your custom protocol.

Answer 6

This information isn't available from .net's class libraries. I had the same kind of considerations when I started working on this port scanner in C#. I have made use of a .NET wrapper for libpcap (after installing the corresponding driver), the SharpPcap (http://sourceforge.net/projects/sharppcap/), in order to get this kind of information. The ACK packets are obtained through SharpPcap's interface (invoking the native libpcap interface's transparently).

My application is NScanner Port Scanner/Sweeper and you can find the code at codeplex, referencing to you my simple usage of the aforementioned library (http://nscanner.codeplex.com/).

I hope I helped.

Answer 7

I recommend using Pcap.Net.

You can easily sniff packets using this library in C# and then easily check the packet values.

You can also easily build and send packets.