Upon receiving a TCP RST packet, will the host drop all the remaining data in the receive buffer that has already been ACKed by the remote host but not read by the application process using the socket?
I'm wondering if it's dangerous to close a socket as soon as I'm not interested in what the other host has to say anymore (e.g. to conserver resources); e.g. if that could cause the other party to lose any data I've already sent, but he has not yet read.
Should RSTs generally be avoided and indicate a complete, bidirectional failure of communication, or are they a relatively safe way to unidirectionally force a connection teardown as in the example above?
I've found some nice explanations of the topic, they indicate that data loss is quite possible in that case:
http://blog.olivierlanglois.net/index.php/2010/02/06/tcp_rst_flag_subtleties
http://blog.netherlabs.nl/articles/2009/01/18/the-ultimate-so_linger-page-or-why-is-my-tcp-not-reliable also gives some more information on the topic, and offers a solution that I've used in my code. So far, I've not seen any RSTs sent by my server application.
Application-level close(2) on a socket does not produce an RST but a FIN packet sent to the other side, which results in normal four-way connection tear-down. RSTs are generated by the network stack in response to packets targeting not-existing TCP connection.
On the other hand, if you close the socket but the other side still has some data to write, its next send(2) will result in EPIPE.
With all of the above in mind, you are much better off designing your own protocol on top of TCP that includes explicit "logout" or "disconnect" message.
Related
I am facing some issues which I need some help on coming with a best way to resolve this.
here is the problem -
I have server code running which has a socket that is listening to accept new incoming connections.
I then attempt to start a client, which also has a socket that is listening to accept new incoming connections.
The client code begins with accepting a new connection on the listening socket file descriptor and gets a new socket file descriptor for I/O.
The server does the same thing and gets a new socket file descriptor for I/O.
Note: The client is not completely up, yet. It needs to receive some bytes from the server and send some before it can start.
I then introduce some packet loss over the TCP/IP network connection. This causes the certain errors (example: the recv() system call in the client process sees no received bytes and then closes the socket connection on the client side and the associated new socket file descriptor is closed.) However, this leaves the client process hanging since there are other descriptors in the FD_SET but none of them are I/O ready. So pselect() keeps returning 0 file descriptors ready for I/O. The client needs to send and receive certain bytes over the connection before it can start up.
My question is more of what should I do here ?
I did research on the SO_KEEPALIVE option when I create the new socket connection during the accept() system call. But I do not think that would resolve my problem here especially if the network packet loss is ongoing.
Should I kill the client process here if I realize there are no file descriptors ready for I/O and never will be ? Is there a better way to approach this ?
If I'm reading the question correctly, the core of the question is: "what should your client program do when a TCP connection that is central to its functionality has been broken?"
The answer to that question is really a matter of preference -- what would you like your client program to do in that case? Or to put it another way, what behavior would your users find most useful?
In many of my own client programs, I have logic included such that if the TCP connection to the server is ever broken, the client will automatically try to create a new TCP connection to the server and thereby recover its connectivity and useful functionality as soon as possible.
The other obvious option would be to just have the client quit when the connection is broken; perhaps with some sort of error indication so that the user will know why the client went away. (perhaps an error dialog that asks if the user would like to try to reconnect?)
SO_KEEPALIVE is probably not going to help you much in this scenario, by the way -- despite its name, its purpose is to help a program discover in a more timely manner that TCP connectivity has been lost, not to try harder to keep a TCP connection from being lost. (And it doesn't even serve that purpose particularly well, since in many TCP stacks only one keepalive packet is sent per hour, or so, which means that even with SO_KEEPALIVE enabled it can be a very long time before your program starts receiving error messages reflecting the loss of network connectivity)
I have a proxy server implemented, after sending the final response to client if I directly close the socket (System.Net.Sockets TCPClient.Client.Close()) then client end receives connection aborted error but instead if I use System.Net.Sockets TCPClient.getStream().Close(), it works successfully.I want to understand what's the difference and why is client side receiving an error in the first scenario?
I would say, that Close of sockets is not trivial operation as most people think :)
First of all, you should understand the how the close should be done correctly. Basically, you have to consider that close is a kind of message like any other message sent out your socket. Or other words close() is an information on the other side of communication that the peer finished some kind of work.
Now the important thing to understand that having a TCP socket you can inform the peer that you finished sending or finished listening.
On this page, you can check out how it works in the background (note that ACK and FIN are IP layer messages so even using plain sockets implementation you will never see them): http://www.tcpipguide.com/free/t_TCPConnectionTermination-2.htm
So now the more practical step. Please consider that you have a client and server. The server needs to receive a message and close the connection. Please consider that client is just going to send a message and then closes the connection. If you will also consider that networks need some time to process your communication, you will realize that if you do it quickly, client will close the connection before server received your message. If you can the TCPClient.Client.Close() client will stop listening for anything (that means also for information about that the server closed the connection). So here comes the TCP stack to play (windows does it for you) - in case you will close this way the socket, TCP stack, needs to inform the server site that whatever server has sent goes to dump. So that's why you have an exception.
So the correct way is to:
inform the server that client finished sending any data (FIN)
wait until server confirms that he knows that client will not send any data (ACK)
now server should inform client that will stop sending data (FIN)
now the client can say - "ok I got it, I will not listen anymore" (ACK)
Anyway, the C# TCPClient seems to hide the logic of the background socket closing routine, but if you will not call the close sequence correct way, you'll end up with errors.
I hope that this little bit long explanation will help you understand how it works in the background and finally let you understand why.
It's also a good way to read more about TCP protocol details if you wish to learn more: http://www.tcpipguide.com/free/t_TCPIPTransmissionControlProtocolTCP.htm
I suppose that in order to close connection, you need to send some special bytes sequence. And looks like it is implemented only by tcpclient library , and not implemented by socket library. Probably something like Eof should be sent.
You may check it by some net traffic utilities like tcpdump.
Good luck!
I'm starting to use golang for a quite amount of time for a project. In my project I have to implement a tcp server which responds to tcp clients. The server has to send a number of messages to a client.
The problem is that when a server writes a message to a client connection, it has to wait until the client has read that message from buffer and then send another message (the server has to wait until the client calls the reader.ReadString('\n') method).
In my server code I wrote:
for {
data := <-client.outgoing
client.writer.WriteString(data + "\n")
client.writer.Flush()
}
but the server sends all the messages to client without waiting for ReadString in client.
How to make server wait until the client read a message and then send the other message?
I think that either the assignment is ambiguous or you're misinterpreting it and solving the XY problem.
The short answer is that you can never know whether the client has read a message just by looking at the TCP conversation. You have to implement this "protocol" in your application.
Here are a few problems:
From your application you don't really have access to what TCP is doing. You get a stream on which you can perform I/O.
The fact that a write to your stream "succeeds" only means that TCP has agreed to try to transport your stuff and has an independent copy. It doesn't say anything about whether the data has been received and it doesn't even mean the data has been even sent
You may find certain mechanisms to peer into TCP's inner workings (such as ioctls, SIOCINQ, SIOCOUTQ or various setsockopts): these won't help
Even if you find out what your TCP is doing, this only tells you what the remote TCP is doing. So if you have full control over your TCP and even see the acknowledgments from the peer, you still don't know what the application is doing. It's very possible the application didn't read the data yet (it might not have requested the data, the TCP might be withholding it in a buffer for some weird reason, the scheduler might not have scheduled the remote process etc.)
Going back to your question, a way to really know whether the remote application has received your message is to have the remote application tell you. This means you have to restructure your protocol to:
Send stuff from the server
Wait for a message from the application telling you it received your stuff
Send more stuff (because you know from point 2 it's safe to do so)
RFC 7230 defines HTTP/1.1 protocol and it has an interesting passage in 6.6, "Connection management. Tear-down":
To avoid the TCP reset problem, servers typically close a connection
in stages. First, the server performs a half-close by closing only the
write side of the read/write connection. The server then continues to
read from the connection until it receives a corresponding close by
the client, or until the server is reasonably certain that its own TCP
stack has received the client's acknowledgement of the packet(s)
containing the server's last response. Finally, the server fully
closes the connection.
Basically it boils down to the following:
shutdown(s, SD_SEND);
while (recv(s, throaway_buffer, throaway_buffer_len, 0) > 0);
closesocket(s);
which is the standard way of doing the graceful shutdown. However, it also acknowledges that a misbehaving client may exist (that keeps sending requests even after receiving a response with Connection: close header), and that the server has to cope with it by resetting the connection after it's sure the client has received the last response.
However, the socket interface doesn't seem to provide the functionality to learn whether all data passed to send have been actually sent and ACK'd by the remote host. Is it actually there? Without it, all I can think about is to set up a timer of sorts, and call recv until either it signals that the remote host has closed connection or the time is out, whichever comes first. But what would be the appropriate timeout? Is 60 seconds okay?
The Sockets interface provides this mean via the little-used and less understood SO_LINGER option. It allows you inter alia to define a timeout during which close() and possibly shutdown() will block while pending data is being sent. It is of little practical use and as I've stated it is rarely used ... at least rarely used correctly.
In TCP/IP sockets, how would the server know that a client is busy and not receiving data ?
My solution:
Use connect(),
I am not sure.
thanks
In TCP/IP sockets, how would the server know that a client is busy and
not receiving data
If a TCP is constantly pushing data that the peer doesn't acknowledge, eventually the send window will fill up. At that point the TCP is going to buffer data to "send later". Eventually the buffer size will be reached and send(2) will hang (something it doesn't usually do).
If send(2) starts hanging it means the peer TCP isn't acknowledging data.
Obviously, even if the peer TCP accepts data it doesn't mean the peer application actually uses it. You could implement your own ACK mechanism on top of TCP, and it's not as unreasonable as it sounds. It would involve having the client send a "send me more" message once in a while.
A client will almost always receive your data, by which I mean the OS will accept the packets and queue them up for reading. If that queue fills up, then the sender will block (TCP, anyways). You can't actually know the activity of the client code. Pretty much your only option is to use timeouts.