[HECnet] What use are *you* making of HECnet?

[Peter Lothberg]

Most real core IP networks have MTU=4470 or bigger (10GE 9K), but as soon as you
hit a 100Mbit ethernet thing or so, you loose... 

-P


Paul Koning wrote:
"Jason" == Jason Stevens <neozeed at gmail.com> writes:

As for fragmentation... Now I assume you are talking about the
encapsulation of ethernet packets in UDP packets. Those will be a
bit larger still, and will almost certainly be fragmented when
sent over the internet, yes. I don't see a problem with that. Do
you?


Jason> Well if you were trying to send the whole 1500 bytes of data +
Jason> headers in the UDP packet won't it cut stuff off? 

No, not unless it's either IPv6, or you set the "don't fragment" flag
in the IP header.   IPv4 will fragment oversized packets no matter
what's inside, and indeed this is the only way for random size
UDPgrams to get where they are going.   It should work fine.   Note that
fragmentation is often not all that efficient.   Compared with the
performance of old 10Mb/s DEC Ethernet gear, that's unlikely to be an
issue.

Actually, since there isn't a good way of finding out how large packets 
you can send, fragmentation is almost neccesary. It can occur anywhere 
along the way from source to destination, not only at the start or end.

And fragmentation is actually the most efficient way of getting data 
across. The problem is if one fragment is lost, which will cause 
retransmission of a lot of fragments. That's the reason TCP tries to 
avoid it.
But with large IP packets, you get a lower protocol overhead compared to 
actual data transferred.

	Johnny

-- 
Johnny Billquist                                   || "I'm on a bus
                                                                    ||   on a psychedelic trip
email: bqt at softjar.se                         ||   Reading murder books
pdp is alive!                                         ||   tryin' to stay hip" - B. Idol