Richard A. Steenbergen
ras at e-gerbil.net
Wed May 30 22:05:15 UTC 2001
On Wed, 30 May 2001, Wayne Bouchard wrote:
> Well, the way it oughta work is that the backbone uses the same MTU as
> that of the largest MTU of your endpoints. So, for example, you have a
> buncha hosts on a fddi ring running at 4470, you want to make sure
> those frames don't have to get fragmented inside your network. Idealy,
> all hosts have the same MTU and no one has to worry about that, but in
> practice, it seems to be better to push the fragmentation as close to
> the end user as possible. (That is, if a user on a 1500MTU link makes
> a request to a host on a 4470 link, the response is 4470 up until the
> user's end network.) Of course, path MTU discovery makes this a moot
> point. The conversation will be held in 1500 byte fragments.
Fortunantly hosts on FDDI rings are rare these days, but I'd love to see a
modern analysis of the packet sizes going through the internet (everything
I've seen comes from the days when FDDI roamed the earth).
>From everything I've seen out of IEEE, they continue to view Ethernet as a
"LAN Standard" and don't really want to consider its use in the core, even
for 10GigE. As long as the creation of 99.999% of packets is <= 1500
bytes, and the links which pass packets are equal or greater, noting
really nasty happens. The argument is that "most people won't really
benefit from it, and it will introduce incompatibilities in MTU size, so
why should it be a standard", which misses the potential use in WAN links.
I don't expect to see many hosts w/10GigE cards for a while, but it would
be nice if Path MTU Discovery was a bit better.
Richard A Steenbergen <ras at e-gerbil.net> http://www.e-gerbil.net/ras
PGP Key ID: 0x138EA177 (67 29 D7 BC E8 18 3E DA B2 46 B3 D8 14 36 FE B6)
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