On 22-jul-2005, at 20:20, Stephen Sprunk wrote:
Thinking about this a bit more, this could probably be fairly easy to
achieve by creating a "onlink-MRU" or "interface-MRU" option for ND
Neighbour Advertisements.
If there aren't any big holes in what I'm suggesting, I'm willing to
spend some time co-authoring an Internet draft on this.
I'm sure we can find a nice restaurant or café in Paris to discuss
the matter further. :-)
The hole is that there may be an L2 device in the middle which has
a lower MTU than the end hosts. The neighbor MTU is an upper
bound, but it's not guaranteed to work -- you need to probe to see
what really works.
(Layer 1 devices can also impose MTU limits.)
It's important to keep this simple, unless the IEEE guys also want to
play and add mechanisms to exchange MTU information between switches.
Just like PMTUD, you need to periodically probe and adjust to
changing network conditions, including detecting "black holes".
Fred Baker suggested the host send both minimum MTU (576 for v4 and
1280 for v6) and maximum MTU frames in a given burst and track what
gets through.
I'm not really comfortable with this... It makes more sense to me to
have a router or two, or maybe one or two non-router hosts, send out
"MTU announcements", and other hosts only announce the non-standard
MTU in neighbor advertisements when they recently heard one of those
announcements. When the MTU suddenly decreasees, the announcements
are no longer heard, hosts put 1500 in their neighbor advertisements
and neighbor unreachability detection does the rest. The fact that
ethernet is supposed to have a tree topology makes things slightly
simpler.
The most perverse scenario I can envision is a network where one
host has an MTU of 9k, another has 8k, one network path has 10k,
another path has 3k, and the path varies every few minutes (and
isn't necessarily symmetric).
Real-life ethernet isn't supposed to be like that...
For those who think there isn't a real problem here: it takes a
little over 800 packets per second to saturate a 10 Mbps ethernet
link. At GE speeds that's 80000 packets per second. It is very hard
to achieve decent performance when you have to stop what you're doing
80000 times per second... There is also the environment to consider
because the amount of power switches use is strongly related to the
number of packets that flow through the switch. So increasing the MTU
from (for instance) 1500 to 9000 bytes means it only takes 3 packets
to transfer 18000 bytes (2 data, 1 ack), while it takes (best case)
13 packets at 1500 bytes (12 data, 1 ack) but usually 18 (6 acks).
That saves a LOT of power.
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