Re: An Ethernet question

[mark_harrison_uk2]

IP is the underlying protocol which handles things like device
addressing. (Hence the term "IP address".) Within the IP header
block, there's a section (8-bits, actually) which determine WHICH
protocol sits on top of IP. The two most common are:

- TCP for connection-based comms
- UDP for datagram-based comms

TCP includes things like packet acknowledgement and sequencing within
the protocol. Hence a programmer writes to a TCP interface, and lets
the compiler/operating system programmers handle those things.

UDP doesn't include those, so if a programmer needs them, (s)he has
to write their own. One advantage of not including them is that it
basically allows broadcast traffic. However, you can equally use UDP
in a directed manner only for a particular IP address.

Most IP stacks will listen for traffic to their own IP address(es),
and traffic for the broadcast address(es) to which their IP address
(es)/subnet mask(s) correspond. They filter out anything else, and
pass the directed/broadcast traffic up to the

It's possible to get a "promiscuous" IP stack which will pass
EVERYTHING that comes in on the IP protocol up. This is useful for
things like network monitors, where you want to have server A monitor
what client B is saying to router C.

This is harder work than it used to be, since in these days of
network switches rather than dumb hubs, the network devices in the
middle try to filter out the traffic - hence you need to be able to
control your switch and tell it not to actually switch, in order to
do some of this.

Most PCs don't have promiscuous stacks on them, hence xAP-over-
ethernet deliberately uses a protocol which will allow ANY device on
a xAP network to listen in on ANY message, whether targetted to it or
not.

That architecture choice meant using our own addressing mechanism,
and using a protocol that allowed broadcast. Hence in the IP world,
the choice of UDP as commonly supported.

Regards,

Mark

--- In <a
href="/group/xAP_developer/post?postID=UUAb8CadhM2ECZ3Ka_YvTKgwJbuHrJjNPxD25nf889vyRnj4GYI7PBnwDRxhX17KhR2-9UofRdvgvMtd-fcaT2Lyr50">xAP_developer@xxxxxxx</a>,
"Ian Bird" <Ian@M...> wrote:
> Hi all
>
> I have been looking into writing my own Ethernet receive etc.
> routines on my AVR to go with the Wiznet hardware. Anyway, since
xAP
> is a broadcast thingy does this mean it uses UDP all the time. From
> what I gather UDP means the IP (source I assume) and message as
well
> as some other stuff is passed to the micro on the end whereas using
> TCP/IP only information for the target IP (assigned to the micro)
> gets through.
>
> I think this means that TCP/IP is a connection based medium and UDP
> is broadcast? Is that correct?
>
> If so, when writing a PC based application do you somehow specify
the
> method used to send a xAP message in the program?
>
> Sorry, no PC experience on this stuff
>
> Thanks
>
> Ian