On 27-jul-2007, at 13:05, John Heffner wrote:
(Assuming nothing bad happens to the
network gear by sending oversized frames -- as you mentioned, this has
potential to cause some problems on very old ethernet gear.)
Right, that's why I feel it's very important to have a mechanism that
makes it possible to tell all hosts to reduce their MTU with one
setting in one place, rather than have to do this on all hosts
individually.
(Note, I assume here that MTU==MRU in all cases.) Since the
router does not know the MTU of each host on the subnet, the only
compliant thing for it to do is to use the smallest possible (e.g.,
standard) MTU for that network.
Why is this true for the router but not for other hosts? The issues
are the same, assuming that all hosts (including the one reachable
through the router) implement RFC 4821.
Obviously, that's an "interesting" assumption to make, especially as
there's no obvious way to validate its veracity.
Hosts sending from outside the router
using 4821 will work fine, but any host outside the router with a
larger
MTU (that may be valid on its own network), and still relying on
classical 1122/1982 pmtud, will break.
Right.
In my mind, the question of what the router should do is the most
important in any discussion of mixed-MTU subnets. In order to
generate
the correct ICMP PTB messages, a route with a large-MTU interface on a
mixed-MTU subnet would need to know the individual MTUs of each
host on
that subnet. Neighbor discovery seems like the place you'd want to do
this. However, I think this is a tricky problem, and I'm not sure
your
proposal solves it in full. (Think this over and tell me if I'm wrong
-- I'm not sure I understand your "The MTU detection message"s fully.)
This is extremely simple: put the MTU in neighbor discovery packets
so that nodes learn their neighbor's MTU along with the MAC address.
One thing to note is that if you imagine a world where the vast
majority
of hosts with large MTUs use 4821, it may be operationally
acceptable to
deliberately break classical pmtud by not generating ICMP PTB
messages.
This may not be totally far-fetched, since classical pmtud is so
fragile in practice that large MTUs are currently quite rare. This is
an imperfect solution, but with some luck it may be just good enough.
Why would you want to do that? Even with full RFC 4821 support
regular PMTUD is still extremely useful because it gives you the
exact value of the path MTU immediately after sending an oversized
packet.
Iljitsch
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