Dejawin Lost Connection

[Bill Vermillion]

press any key to reboot  -oops- Bob Rasmussen said on Thu, Jul 22 09:35  

> Note to Bill's helpful hint: on Windows, use
>    ping -l <size> <address>

> In testing this, I find that my ping fails for <size> >= 1473.
> What does this mean? Does ping specifically NOT fragment a
> too-long packet?

It's not the progam but the interveneing processes that handle the
fragmentation.   Something in your path isn't handling them
correctly.

> (I am going wireless from a Windows box to a WAP, and then
> wired through my cable modem).

The MTU on an ethernet network is 1472.  

If a packet transits and reaches a router that has a smaller MTU,
the packet will be split and the pieces will be sent along to
the destination and only at the final router will things be
re-assembled.

There is also a bit that can be called 'do not fragment'.  That
means that if a packet is too large for something along the way, it
will stop there, the packet is thrown away, and and ICMP message
is sent back to the source.

Some places filter all ICMP messages in the mistaken belief that
this will protect from attacks.  I've tried to help people with
network problems and when I find they have all that blocked I tell
them I can't help them.

I ran across another site [a computer hw/softare support house no 
less] that blocked all UDP packet in the mistaken belief that it
would keep people from listening to network music broadcasts and
that nothing else would use it.     

Very busy DNS machine may use only UDP - as there is no reason to
have a TCP/IP handshake sequence to make sure the data was
received.  If you think of UDP like the floor of the stock market,
you shout out 'give me the IP of xxx' and you get "its nn.nn.nn.nn"
and thats the end.  If you don't get the address it is YOUR problem
not the servers.

The ping for MS is very different than the Unix pings.

You can use the -f in MS for do not fragment, and then try varying
packet sizes to find 'black hole routers'.  Those are routers
which do NOT return ICMP messages - so you send you data and never
get a response - thus the term 'black hole'.

There is a PMTU discovery - to find the maximum MTU in a path
so that you can send the correct packet size.  MS's workaround
for problem routes is to disable PMTU which then sets all packets
to 576 bytes - which means almost three times the packets used to
send the same amount of data.

But you are at the maximum usually packet size so as long as
nothing tries to send packets larger than 1472 you will be OK.

Bill

-- 
Bill Vermillion - bv @ wjv . com