TCP Client not receiving all the messages - vb.net

Question

I am trying to write a small client which will listen to a server on a certain port. When I run my code it seems to hang, then it will only give me the first message and not receive anymore after? Im wrecking my brains here. Any help appreciated.
Option Strict On
Imports System.Net
Imports System.Net.Sockets
Imports System.Text
Public Class Form1
'Form Controls.
Private lblPort As New Label
Private txtPort As New TextBox
Private lblIp As New Label
Private txtIp As New TextBox
Private lblSend As New Label
Private txtSend As New TextBox
Private WithEvents btnConnectSendReceive As New Button
Private lblReceived As New Label
Private lvwReceived As New ListView
Private txtBoxrecieved As New RichTextBox
'Global Objects.
Private gSocket As Socket = Nothing
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
'Setup form controls.
With Me
.Text = "SplitPackManagerTest"
.Controls.Add(lblPort)
.Controls.Add(txtPort)
.Controls.Add(lblIp)
.Controls.Add(txtIp)
.Controls.Add(lblSend)
.Controls.Add(txtSend)
.Controls.Add(btnConnectSendReceive)
.Controls.Add(lblReceived)
.Controls.Add(lvwReceived)
.Controls.Add(txtBoxrecieved)
.Height = 600
End With
With lblPort
.Text = "Port:"
.Location = New Point(12, 12)
.AutoSize = True
End With
With txtPort
.Text = "3001" 'Same port that server is listening on.
.Location = New Point(100, 12)
End With
With lblIp
.Text = "IP:"
.Location = New Point(12, 42)
.AutoSize = True
End With
With txtIp
.Text = "127.0.0.1" 'Loop-back IP address (localhost).
.Location = New Point(100, 42)
End With
With lblSend
.Text = "Send:"
.Location = New Point(12, 72)
.AutoSize = True
End With
With txtSend
.Text = Chr(2) & "(login (term-id 2))" & Chr(3)
.Location = New Point(100, 72)
End With
With btnConnectSendReceive
.Text = "Connect"
.Location = New Point(12, 122)
.Width = 260
End With
With lblReceived
.Text = "Received Bytes:"
.Location = New Point(12, 182)
.AutoSize = True
End With
With lvwReceived
.Height = 100
.Dock = DockStyle.Bottom
.View = View.Details
.GridLines = True
.FullRowSelect = True
.MultiSelect = False
.Scrollable = True
.Columns.Add("Dec")
.Columns.Add("Hex")
.Columns.Add("Chr")
For Each vCol As ColumnHeader In .Columns
vCol.Width = CInt(Math.Floor(.Width / .Columns.Count)) - CInt(Math.Floor(30 / .Columns.Count))
Next
End With
With txtBoxrecieved
.Height = 200
.Dock = DockStyle.Bottom
.ScrollBars = RichTextBoxScrollBars.Both
End With
End Sub
Private Function ConnectSendReceive(ByVal pSendData As Byte(), ByVal pIp As String, ByVal pPort As Integer) As Byte()
'Try creating IP endpoint.
If String.IsNullOrEmpty(pIp) Then Return Nothing
If Not IPAddress.TryParse(pIp, Nothing) Then Return Nothing
Dim vIp As IPAddress = IPAddress.Parse(txtIp.Text)
Dim vEndPoint As New IPEndPoint(vIp, CInt(txtPort.Text))
'Timeout will be 0.5 seconds.
Dim vTimeout As Integer = 500000
'For our little example, we expect all messages to be 1024 bytes or below (arbitrary amount).
Dim vMessageLength As Integer = 1002400000
'Remember, when dimensioning arrays, the integer specified is the upper bounds, not the length.
Dim vServerResponse As Byte() = Nothing
'Initiate socket.
Dim gSocket As New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)
'Connect.
Try
gSocket.Connect(vEndPoint)
Catch ex As Exception
Return Nothing
End Try
If Not gSocket.Connected Then Return Nothing
'Send.
'Socket.SendTimeout = vTimeout
gSocket.Send(pSendData)
txtBoxrecieved.AppendText("Sending.. " & txtSend.Text)
'Receive response.
'Socket.ReceiveTimeout = vTimeout
Dim vBuffer(vMessageLength - 1) As Byte
Dim vNumOfBytesReceived As Integer = 0
Try
vNumOfBytesReceived = gSocket.Receive(vBuffer, 0, vMessageLength, SocketFlags.None)
Catch ex As Exception
Return Nothing
End Try
'Return received bytes.
ReDim vServerResponse(vNumOfBytesReceived - 1)
Array.Copy(vBuffer, vServerResponse, vNumOfBytesReceived)
'Disconnect (since we're using a "Using" statement, the socket will be disconnected here without us explicitly doing it).
Return vServerResponse
End Function
Private Sub btnConnectSendReceive_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnConnectSendReceive.Click
'Send message and get response from server.
Dim vServerResponse As Byte() = ConnectSendReceive(Encoding.ASCII.GetBytes(txtSend.Text), txtIp.Text, CInt(txtPort.Text))
'Did we receive a response?
If vServerResponse Is Nothing Then MessageBox.Show("Server not reachable / Received no response from server.") : Exit Sub
'Do something with response.
For Each vByte As Byte In vServerResponse
Dim vLvi As New ListViewItem
With vLvi
'Decimal column.
.Text = vByte.ToString
'Hexidecimal column.
.SubItems.Add(vByte.ToString("X2"))
'Character column.
.SubItems.Add(ChrW(vByte))
End With
With lvwReceived
.Items.Add(vLvi)
.EnsureVisible(.Items.Count - 1)
End With
Next
txtBoxrecieved.AppendText(UnicodeBytesToString(vServerResponse))
End Sub
Private Function UnicodeBytesToString(
ByVal bytes() As Byte) As String
Return System.Text.Encoding.ASCII.GetString(bytes)
End Function
End Class
THanks
David

Answer 1

You need to know what's happening on both sides of this conversation. To do that, you need to listen in with a tool like Wireshark. Download Wireshark and learn to use it.
The reason I tend to agree with the_lotus is that the very large buffer you have allocated (ignoring the likely sizing bug) implies that you are expecting a fairly large response. Packet payloads can be up to 64KB but that's uncommon and even if such a thing occurs the protocol allows for fragmentation en route.
So if the data is split over several packets you'll need to keep reading until you have a complete payload. This may entail looking for boundary markers in a stream, and if there's a possibility of receiving more payloads over the same connection you'll need to preserve the start of the next payload in a way that allows you to accumulate the rest of it.
If you do that you will probably see a complete packet with a partial payload, as described by the_lotus in a comment posted while I was still writing.
I think he's probably right, but he could also be wrong, and a tool like Wireshark will allow you to find out, instead of guessing and hacking your code around in a blind attempt to validate your hypotheses.
The style of I/O that you are using is called polling. Sometimes it's necessary to do it that way, eg embedded systems often don't support anything more sophisticated, there isn't space for it. But on a system with a full .NET framework you really ought to use IOCP. It's not strictly necessary for a single user app, but if you pick up the right habits you won't create scalability problems.
OK, as to why the response is truncated, after examining your code (VB makes my eyes hurt) I'm almost sure the_lotus is right. You are reading exactly once from the socket and then you are closing it without any attempt to check whether you have the entire response or any attempt to read more data.
This looks like a first year university assignment, so presumably you have some sort of specification. To determine whether you have the entire response you need to know either one of the following
an expected payload length
the location and structure of data in the payload telling you how long the remainder is (or sometimes the size of the total packet)
an end of payload marker, this will be a particular sequence of bytes guaranteed not to otherwise occur in the payload.
Some pseudocode
open the connection
while not CheckForCompletePayload(buffer)
read some data and append it to your buffer
close the connection.