Hi,
I have been trying to resolve the following scenario (using the TCP Server
Echo example)
1. The TCP Server Deamon in the echo example runs as normal and accepts a
client connection (such as a client connection via telnet from a different
PC on the network)
2. The client connection is accepted and the TCPEchoThread is started
3. I have changed the code a little, so once the client has sent some data
the server continues to send data to the client until the connection is
stopped by the client, for example:
with sock do
begin
repeat
if terminated then break;
s := RecvPacket(10000);
while lasterror = 0 do
begin
if CanWrite(1) then
SendString('Testing connection...' + #13#10)
else
Writeln('Cannot send');
if lastError<>0 then break;
Sleep(500);
end;
until false;
end;
The code above works fine and when the client connection terminates the
value of sock.lasterror is set and the client thread can terminate.
The problem comes with the following scenario:
1. Instead if stopping the client side application, pull the network cable
from the client PC - this seems to terminate fine and lasterror is
populated on the server side (and client detects the same error)
2. This is the main problem scenario:
- Connect from a client on a virtual pc (using Oracle Virtual Box for
example), and whilst the socket connection is in place, simply disable the
network interface on the virtual pc.
- Observations are:
a. the client side on the virtual pc detects the error and stops
(most of the time)
b. The server side seems to never detect that the client is
disconnected, and continues to send data without lasterror being set at all
and it waits for a very long time before disconnecting finally
c. using a tool such a netstat, it is noted that data continues to
be added to the TCP buffer for the socket connection, which remains in a
ESTABLISHED state
- This same scenario can be simulated across the internet when the
link is unstable / unreliable or routers and switches start failing
I have attempted to resolve this by setting kernel parameters, and various
options available from the documentation. None worked up till now.
So the question is:
A, Using the scenarios above where the socket is being interfered with as
described, how can the server side detect the that the client connection is
actually not there and terminate the client thread?
B. Do I have to set some additional socket options at the server, and if
so, which options needs to be set?
Many thanks in advance!
W
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