I have a server app that listens for connections on port 8888. I am also creating the client application. It is a simple application the only hard thing about it is managing multiple connections. so I just need to send files between computers so the way I do that, I don't know if it is wright but it works maybe you guys can correct me. here is my algorithm when sending the file:
NetworkStream stream = \\ initialize it
while(someCondition)
{
// first I open the file for reading and read chunks of it
byte[] chunk = fileRead(file, indexStart, indexEnd) // I have a similar method this is just to illustate my point
stream.Write(chunk, \\other params)
// since I often send large files it will be nice if I can wait here
// until the stream.Write is done. when debuging this the while loop
// executes several times then it waits.
}
and on the other side I read bytes from that stream and write it to a file.
I also need to wait sometimes because I send multiple files and I want to make sure that the first file has been sent before moving to the next. I know I can solve this by using the stream.Read method once the transfer has been done. and sending data back from the client. but sometimes I believe it will be helpful to know when the stream.write is done.
Edit
ok so based on your answers I can for example send to the client the number of bytes that I am planing to send. and once the client recives that many bytes it means it is done. But my question is if this is efficient. I mean doing something like
on the server:
writing data "sending the file length"
read data "check to see if the client received the length" (expecting a string ok for example)
write data "tel the client the name of the file"
read data "check to see if the client recived the name of the file"
write data "start sending chuncks of the file"
read data "wait until client replies with the string ok for example"
The write is complete when the line
stream.Write(chunk, \\other params)
completes. It's worth noting that this does not imply that the other end has received anything. In fact, immediately subsequent to that line, the data is likely to be in some buffer on the sending machine. That means that it's now out of your control. If you want receipt confirmation, the remote end will have to let you know.
Stream.Write is synchronous, so it will always block your thread until the writing finishes.
Related
I have a problem with writing to a NetworkStream in C#. From MSDN i read:
The Write method blocks until the requested number of bytes is sent or
a SocketException is thrown
Well - in my case, it behaves like an asynchronous method. Thread is not being blocked.
Here is a code sample, to enlighten situation a bit:
TcpClient tcpcl = new TcpClient("192.168.1.128", 1337);
NetworkStream netst = tcpcl.GetStream();
byte[] will_send = File.ReadAllBytes(#"large_file_120_MB.mp4");
Console.WriteLine("Starting transmission...");
netst.Write(will_send, 0, will_send.Length);
Console.WriteLine("File has been sent !");
(... later instructions ...)
Result from console after 1 second of execution:
Starting transmission...
File has been sent !
Second message shows immediately. Later instructions are being executed.
Meanwhile server still receives the file and on its side everything works well. It gets better - if i kill sending program, during transmission, receiving won't stop. Degugger shows clearly that app has ended entirely. However few more megabytes will still be transmitted, until receiving stops completely.
So my question - is there a way to block main thread, until Write method is finished ?
The MSDN description should perhaps be better read as
The Write method blocks until the requested number of bytes is
written to the local network buffer or a SocketException is thrown
i.e. The write will return before the entire file has been successfully received at the other end.
This also means when you close your application anything currently in the network buffer may continue to be sent.
The only way to block the main thread until the entire file has been succesfully received is to potentially use asynchronous sockets and once the send is complete wait until some sort of confirmation is sent by the receiving end, which you would have to implement.
Basically I got a listener that (when it receives a new connection) creates a new socketWorker and assigns the connection to the client to it.
Now if the client sends a huge file(that takes, say 30 seconds to be fully received) and afterwards sends a tiny file of a few bytes the tiny file isn't received until the huge file has been fully received.
This is obviously a bad approach and I wonder how I could do it so the files would be sent simultaneously?
As of now I'm using async methods, every time a file has been fully received BeginReceive() is called again to receive the next file (bad way).
Any way to fix this?
I'd appreciate it!
You'll have to implement multiplexing, like for example SPDY does. This is (basically) done by framing message parts and supplying a stream ID on each frame. This way, multiple streams can be exchanged over a single connection.
Alternatively, you could open one connection per file.
Iam tring to send a file using a socket, but almost always it cuts the file over the transmission, but the socket never sends an exception, it just does not send all the file.
Trying to solve this im sending the lenght of the file as a header to the server, so it could validate if the file is complete, if not it send a signal back asking for retransmition.
This works, but some times it takes up to 250 tries to get the file well, ¿what do you thinks is happening?, I have in mind sending the file in smaller chunks, but what is a good size for the chunk?,
The files that fails to be send are of a size of 80kb, and some times even a file of 20kb fails!.
any tips?
After sending the file data, you can use the Flush() method to force sending of all local buffered data.
When sending, TCP breaks the data up into small packets.
When reading data, the Read() method returns an integer which is the number of bytes actually read.
Always make sure that the amount you intended to read is the amount that was read.
int actuallyRead = stream.Read(bufferToStoreData, 0, bytesToRead);
while (actuallyRead < bytesToRead) {
actuallyRead += stream.Read(bufferToStoreData, actuallyRead, bytesToRead - actuallyRead);
}
See this thread:
Is there a way to block on a socket send() until we get the ack for that packet?
It's the same for C# as for any other socket environment, you will have to do some kind of acknowledgement at the application level if you want to ensure receipt of data.
Sender tells receiver how much data to expect. Receiver reads that many bytes and responds saying "i got it". Sender is then free to close it's connection or go on about it's business. If the Sender closes before receiving the "i got it" from the receiver data likely won't get there.
As to why your app level ack isn't working, post some code and we can take a look at it.
I'm writing a server application for an iPhone application im designing. iPhone app is written in C# (MonoTouch) and the server is written in C# too (.NET 4.0)
I'm using asynchronous sockets for the network layer. The server allows two or more iPhones ("devices") to connect to each other and be able to send data bi-directionally.
Depending on the incoming message, the server either processes the message itself , or relays the data through to the other device(s) in the same group as the sending device. It can make this decision by decoding the header of the packet first, and deciding what type of packet it is.
This is done by framing the stream in a way that the first 8 bytes are two integers, the length of the header and the length of the payload (which can be much larger than the header).
The server reads (asynchronously) from the socket the first 8 bytes so it has the lengths of the two sections. It then reads again, up to the total length of the header section.
It then deserializes the header, and based on the information within, can see if the remaining data (payload) should be forwarded onto another device, or is something that the server itself needs to work with.
If it needs to be forwarded onto another device, then the next step is to read data coming into the socket in chunks of say, 1024 bytes, and write these directly using an async send via another socket that is connected to the recipient device.
This reduces the memory requirements of the server, as i'm not loading in the entire packet into a buffer, then re-sending it down the wire to the recipient.
However, because of the nature of async sockets, I am not guaranteed to receive the entire payload in one read, so have to keep reading until I receive all the bytes. In the case of relaying onto its final destination, this means that i'm calling BeginSend() for each chunk of bytes I receive from the sender, and forwarding that chunk onto the recipient, one chunk at a time.
The issue with this is that because I am using async sockets, this leaves the possibility of another thread doing a similar operation with the same recipient (and therefore same final destination socket), and so it is likely that the chunks coming from both threads will get mixed up and corrupt all the data going to that recipient.
For example: If the first thread sends a chunk, and is waiting for the next chunk from the sender (so it can relay it onwards), the second thread could send one of its chunks of data, and corrupt the first thread's (and the second thread's for that matter) data.
As I write this, i'm just wondering is it as simple as just locking the socket object?! Would this be the correct option, or could this cause other issues (e.g.: issues with receiving data through the locked socket that's being sent BACK from the remote device?)
Thanks in advance!
I was facing a similar scenario a while back, I don't have the complete solution anymore, but here's pretty much what I did :
I didn't use sync sockets, decided to explore the async sockets in C# - fun ride
I don't allow multiple threads to share a single resource unless I really have to
My "packets" were containing information about size, index and total packet count for a message
My packet's 1st byte was unique to signify that it's a start of a message, I used 0xAA
My packets's last 2 bytes were a result of a CRC-CCITT checksum (ushort)
The objects that did the receiving bit contained a buffer with all received bytes. From that buffer I was extracting "complete" messages once the size was ok, and the checksum matched
The only "locking" I needed to do was in the temp buffer so I could safely analyze it's contents between write/read operations
Hope that helps a bit
Not sure where the problem is. Since you mentioned servers, I assume TCP, yes?
A phone needs to communicate some of your PDU to another phone. It connects as a client to the server on the other phone. A socket-pair is established. It sends the data off to the server socket. The socket-pair is unique - no other streams that might be happening between the two phones should interrupt this, (will slow it up, of course).
I don't see how async/sync sockets, assuming implemented correctly, should affect this, either should work OK.
Is there something I cannot see here?
BTW, Maciek's plan to bolster up the protocol by adding an 'AA' start byte is an excellent idea - protocols depending on sending just a length as the first element always seem to screw up eventually and result in a node trying to dequeue more bytes that there are atoms in the universe.
Rgds,
Martin
OK, now I understand the problem, (I completely misunderstood the topology of the OP network - I thought each phone was running a TCP server as well as client/s, but there is just one server on PC/whatever a-la-chatrooms). I don't see why you could not lock the socket class with a mutex, so serializing the messages. You could queue the messages to the socket, but this has the memory implications that you are trying to avoid.
You could dedicate a connection to supplying only instructions to the phone, eg 'open another socket connection to me and return this GUID - a message will then be streamed on the socket'. This uses up a socket-pair just for control and halves the capacity of your server :(
Are you stuck with the protocol you have described, or can you break your messages up into chunks with some ID in each chunk? You could then multiplex the messages onto one socket pair.
Another alternative, that again would require chunking the messages, is introduce a 'control message', (maybee a chunk with 55 at start instead of AA), that contains a message ID, (GUID?), that the phone uses to establish a second socket connection to the server, passes up the ID and is then sent the second message on the new socket connection.
Another, (getting bored yet?), way of persuading the phone to recognise that a new message might be waiting would be to close the server socket that the phone is receiving a message over. The phone could then connect up again, tell the server that it only got xxxx bytes of message ID yyyy. The server could then reply with an instruction to open another socket for new message zzzz and then resume sending message yyyy. This might require some buffering on the server to ensure no data gets lost during the 'break'. You might want to implement this kind of 'restart streaming after break' functionality anyway since phones tend to go under bridges/tunnels just as the last KB of a 360MB video file is being streamed :( I know that TCP should take care of dropped packets, but if the phone wireless layer decides to close the socket for whatever reason...
None of these solutions is particularly satisfying. Interested to see whay other ideas crop up..
Rgds,
Martin
Thanks for the help everyone, i've realised the simpliest approach is to use synchronous send commands on the client, or at least a send command that must complete before the next item is sent. Im handling this with my own send queue on the client, rather than various parts of the app just calling send() when they need to send something.
Despite the documentation, NetworkStream.Write does not appear to wait until the data has been sent. Instead, it waits until the data has been copied to a buffer and then returns. That buffer is transmitted in the background.
This is the code I have at the moment. Whether I use ns.Write or ns.BeginWrite doesn't matter - both return immediately. The EndWrite also returns immediately (which makes sense since it is writing to the send buffer, not writing to the network).
bool done;
void SendData(TcpClient tcp, byte[] data)
{
NetworkStream ns = tcp.GetStream();
done = false;
ns.BeginWrite(bytWriteBuffer, 0, data.Length, myWriteCallBack, ns);
while (done == false) Thread.Sleep(10);
}
public void myWriteCallBack(IAsyncResult ar)
{
NetworkStream ns = (NetworkStream)ar.AsyncState;
ns.EndWrite(ar);
done = true;
}
How can I tell when the data has actually been sent to the client?
I want to wait for 10 seconds(for example) for a response from the server after sending my data otherwise I'll assume something was wrong. If it takes 15 seconds to send my data, then it will always timeout since I can only start counting from when NetworkStream.Write returns - which is before the data has been sent. I want to start counting 10 seconds from when the data has left my network card.
The amount of data and the time to send it could vary - it could take 1 second to send it, it could take 10 seconds to send it, it could take a minute to send it. The server does send an response when it has received the data (it's a smtp server), but I don't want to wait forever if my data was malformed and the response will never come, which is why I need to know if I'm waiting for the data to be sent, or if I'm waiting for the server to respond.
I might want to show the status to the user - I'd like to show "sending data to server", and "waiting for response from server" - how could I do that?
I'm not a C# programmer, but the way you've asked this question is slightly misleading. The only way to know when your data has been "received", for any useful definition of "received", is to have a specific acknowledgment message in your protocol which indicates the data has been fully processed.
The data does not "leave" your network card, exactly. The best way to think of your program's relationship to the network is:
your program -> lots of confusing stuff -> the peer program
A list of things that might be in the "lots of confusing stuff":
the CLR
the operating system kernel
a virtualized network interface
a switch
a software firewall
a hardware firewall
a router performing network address translation
a router on the peer's end performing network address translation
So, if you are on a virtual machine, which is hosted under a different operating system, that has a software firewall which is controlling the virtual machine's network behavior - when has the data "really" left your network card? Even in the best case scenario, many of these components may drop a packet, which your network card will need to re-transmit. Has it "left" your network card when the first (unsuccessful) attempt has been made? Most networking APIs would say no, it hasn't been "sent" until the other end has sent a TCP acknowledgement.
That said, the documentation for NetworkStream.Write seems to indicate that it will not return until it has at least initiated the 'send' operation:
The Write method blocks until the requested number of bytes is sent or a SocketException is thrown.
Of course, "is sent" is somewhat vague for the reasons I gave above. There's also the possibility that the data will be "really" sent by your program and received by the peer program, but the peer will crash or otherwise not actually process the data. So you should do a Write followed by a Read of a message that will only be emitted by your peer when it has actually processed the message.
TCP is a "reliable" protocol, which means the data will be received at the other end if there are no socket errors. I have seen numerous efforts at second-guessing TCP with a higher level application confirmation, but IMHO this is usually a waste of time and bandwidth.
Typically the problem you describe is handled through normal client/server design, which in its simplest form goes like this...
The client sends a request to the server and does a blocking read on the socket waiting for some kind of response. If there is a problem with the TCP connection then that read will abort. The client should also use a timeout to detect any non-network related issue with the server. If the request fails or times out then the client can retry, report an error, etc.
Once the server has processed the request and sent the response it usually no longer cares what happens - even if the socket goes away during the transaction - because it is up to the client to initiate any further interaction. Personally, I find it very comforting to be the server. :-)
In general, I would recommend sending an acknowledgment from the client anyway. That way you can be 100% sure the data was received, and received correctly.
If I had to guess, the NetworkStream considers the data to have been sent once it hands the buffer off to the Windows Socket. So, I'm not sure there's a way to accomplish what you want via TcpClient.
I can not think of a scenario where NetworkStream.Write wouldn't send the data to the server as soon as possible. Barring massive network congestion or disconnection, it should end up on the other end within a reasonable time. Is it possible that you have a protocol issue? For instance, with HTTP the request headers must end with a blank line, and the server will not send any response until one occurs -- does the protocol in use have a similar end-of-message characteristic?
Here's some cleaner code than your original version, removing the delegate, field, and Thread.Sleep. It preforms the exact same way functionally.
void SendData(TcpClient tcp, byte[] data) {
NetworkStream ns = tcp.GetStream();
// BUG?: should bytWriteBuffer == data?
IAsyncResult r = ns.BeginWrite(bytWriteBuffer, 0, data.Length, null, null);
r.AsyncWaitHandle.WaitOne();
ns.EndWrite(r);
}
Looks like the question was modified while I wrote the above. The .WaitOne() may help your timeout issue. It can be passed a timeout parameter. This is a lazy wait -- the thread will not be scheduled again until the result is finished, or the timeout expires.
I try to understand the intent of .NET NetworkStream designers, and they must design it this way. After Write, the data to send are no longer handled by .NET. Therefore, it is reasonable that Write returns immediately (and the data will be sent out from NIC some time soon).
So in your application design, you should follow this pattern other than trying to make it working your way. For example, use a longer time out before received any data from the NetworkStream can compensate the time consumed before your command leaving the NIC.
In all, it is bad practice to hard code a timeout value inside source files. If the timeout value is configurable at runtime, everything should work fine.
How about using the Flush() method.
ns.Flush()
That should ensure the data is written before continuing.
Bellow .net is windows sockets which use TCP.
TCP uses ACK packets to notify the sender the data has been transferred successfully.
So the sender machine knows when data has been transferred but there is no way (that I am aware of) to get that information in .net.
edit:
Just an idea, never tried:
Write() blocks only if sockets buffer is full. So if we lower that buffers size (SendBufferSize) to a very low value (8? 1? 0?) we may get what we want :)
Perhaps try setting
tcp.NoDelay = true