Comments on draft-van-beijnum-multi-mtu-00
First, I'd like to say that I agree with the goal of being able to run
mixed-MTU subnets. I'm not yet convinced this draft constitutes a full
and practical solution to the problem.
I have some specific comments below divided by section. A few general
ones first:
One thing that might make the draft easier to read would be some
discussion of how the pieces described in Section 4 fit together to form
a solution to the problem presented.
I think the draft is a bit too focused on Ethernet. For example,
specifying 1500 byte minimum MTUs. A standard in this area should apply
equally to all link types.
The discussion in Section 3 is pretty good, but it might be a bit long
and detailed for this doc.
4.1:
It's not clear what exactly the Allowed MTU and Off-link MTU come from,
and how a router selects these values. Is one of these the router's
interface MRU? Why do you need an off-link MTU? As long as you don't
send packets longer than the router's MRU, then PMTUD will take care of
this.
The whole link speed thing makes me a bit uncomfortable. You definitely
want links with a much slower speed to use smaller MTUs, but it seems
like the end hosts are able to make an appropriate decision.
4.3:
It bothers me having switches send this stuff. It seems like an
architectural violation to have a layer 2 device sending layer 3
information.
Since this is broadcast, a single switch can pull down the MTU for an
entire subnet. Is that a good thing?
These are sent out every 5 minutes. If a new device comes up, it might
be operating for 5 minutes (or MAYbe 60 seconds) without this
information? And, it's not even required for a host to listen to it.
I'm not sure I understand the justification for doing this at all.
4.4:
I think there's some promise in this area, but it's currently
underspecified. This section describes the MTU detection message, but
doesn't really say what to do with it. Do you need to get a reply
before using a larger MTU? If you don't get a reply at a given size,
then what do you do? Fall back to the minimum? Probe at other sizes a
la 4821?
You forbid sending MTU detection messages more often than once per 60
seconds. I don't see this as being practical (say, on a router
interface) where you may need to send/forward packets to large numbers
of hosts that may all need to be probed individually.
Ultimately I would like to see something done in this area. I might
even volunteer to help. :)
Lastly, I apologize for taking so long to send this out. It was sitting
nearly done, but forgotten, in my drafts folder for about a month. Oops..
-John
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