Re: Lost UPD Packets with no network

James Carlson <james.d.carlson@xxxxxxx> wrote:
> prabhu.pravin@xxxxxxxxx writes:
>> I use an application which has one module in Windows and another
>> in Unix. The windows and Unix machines are connected via a cross cable.
>> The Windows and Unix modules communicate via UDP packet transfer.
>> My problem is that , even though there is no network involved, i
>> lose some udp packets during transfer, due to which one of my module
>> hangs waiting for data.
>
> Yes, there's a network. That's the wire between the two machines.
> It's not a big network, nor an internetwork, but it's still a network
> connection.
>
>> Any suggestions on why this could be happening?
>
> Sure. The primary reason is that UDP provides no flow control
> whatsoever. If your application overruns either the local network
> stack or (more likely) the capacity of the link, then there'll be
> silent drops.
>
> It's also possible to be hit by electrical noise that garbles the
> packet in flight. Neither Ethernet, nor IP, nor UDP will retransmit a
> packet that fails in this manner. It will just silently disappear.
> Even if you could somehow "fix" the flow control problem described
> above, this issue would make your application unreliable.
>
> If your application uses UDP and doesn't accomodate occasional packet
> loss, reordering, and duplication, then your application isn't coded
> correctly to use UDP. Applications using UDP must do their own error
> control.
>
> If you don't want to do that, then your choices are TCP and (in
> Solaris 10 and up) SCTP.

I also recall the words of a network guru a few years ago, who pointed out
that too many people were using UDP for the speed increase over TCP, and then
building a reliability framework around it in a FAR less efficient manner
than just using TCP in the first place.

If your application requires data to move reliably, then use TCP.

Colin
.