> If I hook up two boxes that can do 9000 bytes over ethernet, but the > switch is 1500 bytes only, I had better make sure those two boxes > stick to 1500.
Yes, I know. By "negotiate a larger MTU" I mean not only an initial indication of how much the *neighbor* can handle but also ongoing and continuous attention to how much the *L2 media* can handle.
But unfortunately this isn't something that can easily be determined. Is it ok to just send large probe packets, or can sending packets that are too large be harmful?
> The good thing is that if we allow hosts to discover the maximum > usable MTU between them, we can now use the regular RA MTU option to > broadcast a much smaller MTU for this type of traffic.
But, if there are many routers on the link, what do you do if they aren't all broadcasting a consistent smaller MTU?
Beat the administrator.
RFC 2461 says to accept the most recently heard MTU - is that good enough?
I'd say: trust the router you've chosen as your default gateway. I considered picking the lowest from those advertised, but this would make it trivial to do "MTU reduction attacks".
> Then there would be a new RA option that would function as the
> "maximum MTU": hosts are not allowed to transmit packets larger than
> this. This value must be equal to or higher than the IP over RFC value
> if it's present. There could be a bit in this option that indicates
> "if lower layers tell you it's ok to use this value" or "forget what
> lower layers tell you, this value is the correct one".
I'm afraid I'm not bought into this one as being necessary (yet). We know
from our physical/logical point of attachment what the largest possible
MTU for the attached L2 media is - this is a given.
Yes, but who is "we"? The administrator knows this after perusing the documentation and obviously the box itself knows at some level. We currently don't have any mechanisms to transfer this knowledge to the layer 3 on hosts that are considering using jumboframes.
The IEEE was understandably reluctant to increase the gigabit ethernet MTU beyond 1500 bytes because this would break interoperation with existing 10 and 100 Mbps ethernet. But at some point something has to give, because two hosts that want to utilize 10 gigabit ethernet between them would have to transmit/receive almost a million packets per second with a 1500 byte MTU.
So, to supply a maximum
MTU that is larger than the attached L2 media can support in a single packet
is saying that we expect the sender to do L2 fragmentation locally.
Obviously announcing an MTU that's larger than what layer 2 can support doesn't make sense. But often layer 2 can support a larger MTU than what is specified for that particular layer 2 protocol and/or for IP over that protocol. This "unofficial" MTU is the one we're interested in. (At least when increasing the MTU.)
Is this what you are saying, and is this really desireable? (Maybe; I'm willing to be convinced.)
If layer 2 fragmentation is done there should also be layer 2 reassembly. Then the whole procedure becomes transparent to IP so we don't have a problem. This is what 802.11 does to achieve interference robustness. AAL5 in ATM is basically the same thing.
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