Re: draft-van-beijnum-multi-mtu-05.txt

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David,

>> why couldn't it be done as end-to-end MTU?
>> 
>> the directly connected mechanism would be just to give a hint that
>> there might be nodes here capable of a higher MTU. then do normal MTU
>> probing / detection.
>> 
>> what would be gain be to have a specific protocol to probe directly
>> connected neighbors?
> 
> It can:
> - work (pro-)actively and independent of higher-layer protocols,
>  establishing MTU once

to a large extent we have failed at doing MTU discovery at the network layer.
too hard, couldn't do it, let's make it someone else's problem. ;-)
transport or application that is.

> - contain the different logic to understand that the router's MTU is the
>  upper cap for all destinations behind that router
> - contain the different probing parameters (being more aggressive) that
>  are appropriate for a local network
> - optionally carry extra information to reuse probing results across
>  different addresses that a host has on a segment
> 
> I suppose this can be achieved by using a specialized version of 4821 on
> the link... something like this:
> 
> - host does NS as usual, or catches unsolicited NA
> - NA contains new "MTU discovery" option with contents:
>  - indication of capability of first-hop MTU discovery
>  - known maximum MRU of NA's sender (i.e. upper limit)
>  - link-local address(?) + TCP port number(?) for PLMTUD probing
> - host establishes one or more TCP sessions to that port with the
>  express and only purpose of probing MTU
>  (may as well do it on an existing TCP session if one exists)
> - PLMTUD switches into some "first-hop" special mode (lower timers,
>  parallel operation, different values) to establish and store the value
>  with proper semantics (router case)
> 
> Is that what you have in mind?

I was thinking of something much simpler.
the NS/NA MTU option triggers the stack (transport / application) to use packets up to that MTU.
the transport or application has to do normal PLMTUD.

think of it as if you had a p2p link to the neighbor and that the interface MTU was set to whatever you received in the MTU option from that neighbour.

an on-link router already advertises an MTU in an RA. I'm not sure if it would make sense if it advertised per neighbor MTU options, and how those should be interpreted.

cheers,
Ole