now lets take a secnario where we use a locking socket receive and the packet is 5000 bytes
with receivetimeout set to one second
s.SetSocketOption (SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 1000);
int bytes_recevied = 0 ;
byte [] ReceiveBuffer = new byte[8192] ;
try
{
bytes_received = s.Receive(RecevieBuffer) ;
}
catch(SocketException e)
{
if( e.ErrorCode == 10060)
{
Array.Clear(ReceiveBuffer,0,ReceiveBuffer.Length);
}
}
now our secnario dictates that 4000 bytes have gone threw alreadys,the socket is still blocking and some error accured on the receiving end ,
now on the receiving end we would handly dispose of the 4000 bytes by catching the socket ecxecption
is there any guaranty that the socket on the sending end wont thows 1000 bytes that remain
does the sending socket know to truncate them if he wasent disconnected when we attempt to receive again wont they be the first bytes we receive ?
what im asking is :
a) does tcp have some mecanisem that tells the socket to dispose of the rest of them message ?
b) is there a socket flag that we could send or receive with that tells the buffers to dispose of the rest of the message ?
First off, TCP/IP operates on streams, not packets. So you need some kind of message framing in your protocol, regardless of buffer sizes, blocking calls, or MTUs.
Secondly, each TCP connection is independent. The normal design is to close a socket when there's a communications error. A new socket connection can then be established, which is completely independent from the old one.
Related
I have a Socket in a C# application. (the application is acting as the server)
I want to set a timeout on the transmission sending. That if the TCP layer does not get an ack for the data in 10 seconds, the socket should throw and exception and I close the whole connection.
// Set socket timeouts
socket.SendTimeout = 10000;
//Make A TCP Client
_tcpClient = new TcpClient { Client = socket, SendTimeout = socket.SendTimeout };
Then later on in code, I send data to that socket.
/// <summary>
/// Function to send data to the socket
/// </summary>
/// <param name="data"></param>
private void SendDataToSocket(byte[] data)
{
try
{
//Send Data to the connection
_tcpClient.Client.Send(data);
}
catch (Exception ex)
{
Debug.WriteLine("Error writing data to socket: " + ex.Message);
//Close our entire connection
IncomingConnectionManager.CloseConnection(this);
}
}
Now when I test, I let my sockets connect, everything is happy. I then just kill the other socket (no TCP close, just power off the unit)
I try to send a message to it. It doesn't timeout? Even after 60 seconds, it's still waiting.
Have I done something wrong here, or am I misunderstanding the functionality of setting the sockets SendTimeout value?
A socket send() actually does a copy operation of your data into the network´s stack outgoing buffer. If the copy succeeds (i. e there is enough space to receive your data), no error is generated. This does not mean that the other side received it or even that the data went out to the wire.
Any send timeout starts counting when the buffer is full, indicating that the other side is receiving data slower that you are sending it (or, in the extreme case, not receiving anything at all because the cable is broken or it was powered off or crashed without closing its socket properly). If the full buffer persists for timeout seconds, you´ll get an error.
In other words, there is no way to detect an abrupt socket error (like a bad cable or a powered off or crashed peer) other than overfilling the outgoing buffer to trigger a timeout.
Notice that in the case of a graceful shutdown of the peer´s socket, your socket will be aware of it and give you errors if you try to send or receive after the condition was received in your socket, which may be many microseconds after you finished your operation. Again in this case, you have to trigger the error (by sending or receiving), it does not happen by itself.
I am working on client-server appliction in C#. The comunication between them is with TCP sockets. The server listen on specific port for income clients connection. After a new client arrived, his socket being saved in a socket list. I define every new client socket with receive timeout of 1 ms. To receive from the client sockets without blocking my server I use the threadpool like this:
private void CheckForData(object clientSocket)
{
Socket client = (Socket)clientSocket;
byte[] data = new byte[client.ReceiveBufferSize];
try
{
int dataLength = client.Receive(data);
if (dataLength == 0)// means client disconnected
{
throw (new SocketException(10054));
}
else if (DataReceivedEvent != null)
{
string RemoteIP = ((IPEndPoint)client.RemoteEndPoint).Address.ToString();
int RemotePort = ((IPEndPoint)client.RemoteEndPoint).Port;
Console.WriteLine("SERVER GOT NEW MSG!");
DataReceivedEvent(data, new IPEndPoint(IPAddress.Parse(RemoteIP), RemotePort));
}
ThreadPool.QueueUserWorkItem(new WaitCallback(CheckForData), client);
}
catch (SocketException e)
{
if (e.ErrorCode == 10060)//recieve timeout
{
ThreadPool.QueueUserWorkItem(new WaitCallback(CheckForData), client);
}
else if(e.ErrorCode==10054)//client disconnected
{
if (ConnectionLostEvent != null)
{
ConnectionLostEvent(((IPEndPoint)client.RemoteEndPoint).Address.ToString());
DisconnectClient(((IPEndPoint)client.RemoteEndPoint).Address.ToString());
Console.WriteLine("client forcibly disconected");
}
}
}
}
My problem is when sometimes the client send 2 messages one after another, the server doesn't receive the second message. I checked with wireshark and it shows that both of the messages were received and also got ACK.
I can force this problem to occur when I am putting break point here:
if (e.ErrorCode == 10060)//recieve timeout
{
ThreadPool.QueueUserWorkItem(new WaitCallback(CheckForData), client);
}
Then send the two messages from the client, then releasing the breakpoint.
Does anyone met this problem before?
my problem is when sometimes the client send 2 messages one after another, the server doesn't receive the second message
I think it's much more likely that it does receive the second message, but in a single Receive call.
Don't forget that TCP is a stream protocol - just because the data is broken into packets at a lower level doesn't mean that one "send" corresponds to one "receive". (Multiple packets may be sent due to a single Send call, or multiple Send calls may be coalesced into a single packet, etc.)
It's generally easier to use something like TcpClient and treat its NetworkStream as a stream. If you want to layer "messages" on top of TCP, you need to do so yourself - for example, prefixing each message with its size in bytes, so that you know when you've finished receiving one message and can start on the next. If you want to handle this asynchronously, I'd suggest sing C# 5 and async/await if you possibly can. It'll be simpler than dealing with the thread pool explicitly.
Message framing is what you need to do. Here: http://blog.stephencleary.com/2009/04/message-framing.html
if you are new to socket programming, I recommend reading these FAQs http://blog.stephencleary.com/2009/04/tcpip-net-sockets-faq.html
I have the following code
byte[] bytes = Encoding.Default.GetBytes(data);
IAsyncResult res = socket.BeginSend(bytes, 0, bytes.Length, 0, new AsyncCallback(SendCallback), socket);
int waitingCounter = 0;
while (!res.IsCompleted && waitingCounter<10)
{
if (Tracing.TraceInfo) Tracing.WriteLine("Waiting for data to be transmited. Give a timeout of 1 second", _traceName);
Thread.Sleep(1 * 1000);
waitingCounter++;
}
This code has been installed in many machines, but there are some cases where the condition res.IsCompleted takes long to become true.
The reason is related with the network maybe a firewall, proxy? or to the client (too slow) or to the server?
I have not been able to reproduce this scenario.
Edit: I try to reproduce the error by using an asynchronous client and a synchronous server
with the following modifications:
Client=>
while (true) {
Send(client, "This is a test<EOF>");
sendDone.WaitOne();
}
Server=>
while (true){
Console.WriteLine("Waiting for a connection...");
// Program is suspended while waiting for an incoming connection.
Socket handler = listener.Accept();
data = null;
// Show the data on the console.
Console.WriteLine("Text received : {0}", data);
handler.Shutdown(SocketShutdown.Both);
handler.Close();
}
In the second Send(), I get a socket exception. What is normal because the server is not reading the data.
But actually what I want to reproduce is:
Waiting for data to be transmited. Give a timeout of 1 second
Waiting for data to be transmited. Give a timeout of 1 second
Waiting for data to be transmited. Give a timeout of 1 second
Waiting for data to be transmited. Give a timeout of 1 second
Waiting for data to be transmited. Give a timeout of 1 second
As it happens in one of our installations
Edit:
An answer disappeared from this question!!!
Even though BeginSend won't stall your application, it should still have the same constraints as Send. This answer explains why things could go wrong (I've paraphrased):
For your application, the TCP buffer, the network and the local
sending TCP buffer is one big buffer. If the remote application gets
delayed in reading new bytes from its TCP buffer, eventually your
local TCP buffer will end up being (nearly) full. The send won't
complete until the TCP buffer has enough space to store the payload
you're trying to send.
Don't forget when the first check res.IsCompleted, you are always waiting a second for the next check.
I have a socket connection that receives data, and reads it for processing.
When data is not processed/pulled fast enough from the socket, there is a bottleneck at the TCP level, and the data received is delayed (I can tell by the tmestamps after parsing).
How can I see how much TCP bytes are awaiting to be read by the socket ? (via some external tool like WireShark or else)
private void InitiateRecv(IoContext rxContext)
{
rxContext._ipcSocket.BeginReceive(rxContext._ipcBuffer.Buffer, rxContext._ipcBuffer.WrIndex,
rxContext._ipcBuffer.Remaining(), 0, CompleteRecv, rxContext);
}
private void CompleteRecv(IAsyncResult ar)
{
IoContext rxContext = ar.AsyncState as IoContext;
if (rxContext != null)
{
int rxBytes = rxContext._ipcSocket.EndReceive(ar);
if (rxBytes > 0)
{
EventHandler<VfxIpcEventArgs> dispatch = EventDispatch;
dispatch (this, new VfxIpcEventArgs(rxContext._ipcBuffer));
InitiateRecv(rxContext);
}
}
}
The fact is that I guess the "dispatch" is somehow blocking the reception until it is done, ending up in latency (i.e, data that is processed bu the dispatch is delayed, hence my (false?) conclusion that there was data accumulated on the socket level or before.
How can I see how much TCP bytes are awaiting to be read by the socket
By specifying a protocol that indicates how many bytes it's about to send. Using sockets you operate a few layers above the byte level, and you can't see how many send() calls end up as receive() calls on your end because of buffering and delays.
If you specify the number of bytes on beforehand, and send a string like "13|Hello, World!", then there's no problem when the message arrives in two parts, say "13|Hello" and ", World!", because you know you'll have to read 13 bytes.
You'll have to keep some sort of state and a buffer in between different receive() calls.
When it comes to external tools like Wireshark, they cannot know how many bytes are left in the socket. They only know which packets have passed by the network interface.
The only way to check it with Wireshark is to actually know the last bytes you read from the socket, locate them in Wireshark, and count from there.
However, the best way to get this information is to check the Available property on the socket object in your .NET application.
You can use socket.Available if you are using normal Socket class. Otherwise you have to define a header byte which gives number of bytes to be sent from other end.
I'm wondering if in my Async Sockets in c#, receiving 0 bytes in the EndRead call means the server has actually disconnected us?
Many Examples I see suggest that this is the case, but I'm receiving disconnects a lot more frequent that I would be expecting.
Is this code correct? Or does endResult <= 0 not really mean anything about the connection state?
private void socket_EndRead(IAsyncResult asyncResult)
{
//Get the socket from the result state
Socket socket = asyncResult.AsyncState as Socket;
//End the read
int endResult = Socket.EndRead(asyncResult);
if (endResult > 0)
{
//Do something with the data here
}
else
{
//Server closed connection?
}
}
From the docs:
If the remote host shuts down the Socket connection and all available data has been received, the EndRead method completes immediately and returns zero bytes.
So yes, zero bytes indicates a remote close.
0 read length should mean gracefull shutdown. Disconnect throws error (10054, 10053 or 10051).
In practice though I did notice reads complete with 0 length even though the connection was alive, and the only way to handle is to check the socket status on 0 length reads. The situation was as follows: post multiple buffers on a socket for receive. The thread that posted then is trimmed by the pool. The OS notices that the thread that made the requests is gone and it notifies the posted operations with error 995 ERROR_OPERATION_ABORTED, as documented. However what I've found is that when multiple operations are posted (ie. multiple Reads) only the first is notified with error 995, the subsequent are notified with success and 0 length.