VB.NET Asynchronous sockets packets getting lost

Question

I made a TCP server which communicates with multiple clients at once, but I can't seem to be able to make them stable. When one of the client sends 100 packets to the server, the server receives only a few of them.

Here's the client code in PasteBin. It shows how the client connects to the server and then sends 100 messages in a For loop to the server.

And here's how the server handles the connection. I couldn't paste the full source as it's hundreds of lines long so let me know if it's missing any mandatory parts and I'll upload them as well.

score 1 · Accepted Answer · answered Oct 16 '11 at 15:46

I answered your question on reddit, as well, but figured I'd reply here in case someone else is searching for this kind of thing.

You've got two problems with the way your server handles the reads. First of all, your client object's constructor:

Public Sub New(ByVal client As TcpClient)
    'New client connects
    Me.client = client
    client.GetStream().BeginRead(New Byte() {0}, 0, 0, AddressOf Read, Nothing)
End Sub

BeginRead expects you to be collecting your data in a way that can be passed to the callback method; "Read" in this instance. Typically, that's done by creating a new object to hold the "State" of this async operation, and passing it in as the last parameter to BeginRead. Your method call here is creating a new byte array, but isn't holding a reference to it to pass in as the last argument to the method. That means that the data that gets read by this call will just disappear after being read, since it never gets passed into a method to store it.

Secondly, your Read operation:

Public Sub Read(ByVal ar As IAsyncResult)
    Dim reader As New StreamReader(client.GetStream())
    clientPacket &= reader.ReadLine()
    client.GetStream().BeginRead(New Byte() {0}, 0, 0, AddressOf Read, Nothing)
End Sub

Since you're not passing in the data that was read in the async call, you're creating a new StreamReader here to pull more data in from the client. This is not an asynchronous method call. Your reader.ReadLine() call will block until a newline is encountered, at which point the data will be appended to clientPacket. You then call BeginRead again, with the same issue as noted above, meaning you'll lose more data.

In addition, you're never clearing your AsyncResult objects by invoking EndRead() on the stream, which will eventually result in resource starvation when the CLR runs out of worker threads for your async operations.

Here's an example of the way I'd implement this kind of task. It's in C# since that's what I'm most comfortable with, so sorry about that. ;)

Client Code

Server Code

I hope this helps!

VB.NET Asynchronous sockets packets getting lost

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