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

[David Lamparter <[e-mail]>]

On Thu, Apr 14, 2016 at 02:30:49PM +0200, [e-mail] wrote:
> David,
> 
> >> In general I'd prefer that we reuse existing mechanisms for path mtu
> >> discovery, and not invent new ones purely for the case of two directly
> >> connected neighbors.
> > 
> > That's actually the crux of it: this isn't Path MTU discovery.  The MTU
> > discovered by this mechanism is the first-hop MTU for a lot of paths in
> > case of a router and it's going to be distinct from the final PMTU
> > towards a particular destination.  Plus, state on this only ever exists
> > for direct neighbors, not for any random internet host.
> 
> 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
- 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?


-David