WCF: Message Framing and Custom Channels

Question

I am trying to understand how I would implement message framing with WCF. The goal is to create a server in WCF that can handle proprietary formats over Tcp. I can't use the net.Tcp binding because that is only for SOAP.

I need to write a custom channel that would receive messages in the following format . An example message would be "5 abcde". In particular I am not sure how to do framing in my custom channel.

Here is some sample code

class CustomChannel: IDuplexSessionChannel
{
private class PendingRead
{
 public NetworkStream Stream = null;
 public byte[] Buffer = null;
 public bool IsReading = false;
}

private CommunicationState state = CommunicationState.Closed;
private TcpClient tcpClient = null;
private MessageEncoder encoder = null;
private BufferManager bufferManager = null;
private TransportBindingElement bindingElement = null;
private Uri uri = null;
private PendingRead pendingRead;

public CustomChannel(Uri uri, TransportBindingElement bindingElement, MessageEncoderFactory encoderFactory, BufferManager bufferManager, TcpClient tcpClient)
{
 this.uri = uri;
 this.bindingElement = bindingElement;
 this.tcpClient = tcpClient;
 this.bufferManager = bufferManager;  
 state = CommunicationState.Created;
  }

public IAsyncResult BeginTryReceive(TimeSpan timeout, AsyncCallback callback, object state)
{
 if (this.state != CommunicationState.Opened) return null;

 byte[] buffer = bufferManager.TakeBuffer(tcpClient.Available);
 NetworkStream stream = tcpClient.GetStream();
 pendingRead = new PendingRead { Stream = stream, Buffer = buffer, IsReading = true };
 IAsyncResult result = stream.BeginRead(buffer, 0, buffer.Length, callback, state);
 return result;
}
public bool EndTryReceive(IAsyncResult result, out Message message)
{
 int byteCount =  tcpClient.Client.EndReceive(result);
 string content = Encoding.ASCII.GetString(pendingRead.buffer)

  // framing logic here

  Message.CreateMessage( ... )
}

}

So basically the first time around EndTryReceive could just get a piece of the message from the pending read buffer "5 ab". Then the second time around it could get the rest of the message. The problem is when EndTryReceive gets called the first time, I am forced to create a Message object, this means that there will be a partial Message going up the channel stack.

What I really want to do is to make sure that I have my full message "5 abcde" in the buffer, so that when I construct the message in EndTryReceive it is a full message.

Does anyone have any examples of how they are doing custom framing with WCF?

Thanks, Vadim

Answer 1

Framing at the wire level is not something that the WCF channel model really cares about; it's pretty much up to you to handle it.

What I mean by this is that it is your responsibility to ensure that your transport channel returns "entire" messages on a receive (streaming changes that a bit, but only up to a point).

In your case, it seems you're translating receive operations on your channel directly into receive operations on the underlying socket, and that just won't do, because that won't give you a chance to enforce your own framing rules.

So really, a single receive operation on your channel might very well translate to more than one receive operation on the underlying socket, and that's fine (and you can still do all that async, so it doesn't need to affect that part).

So basically the question becomes: what's your protocol framing model look like? Wild guess here, but it looks like messages are length prefixed, with the length encoded as a decimal string? (looks annoying).

I think your best bet in that case would be to have your transport buffer incoming data (say, up to 64KB of data or whatever), and then on each receive operation check the buffer to see if it contains enough bytes to extract the length of the incoming message. If so, then either read as many bytes as necessary from the buffer, or flush the buffer and read as many bytes from the socket. You'll have to be careful as, depending on how your protocol works, I'm assuming you might end up reading partial messages before you actually need them.