RE: [RRG] LISP-NERD reachability and MTU detection

["Templin, Fred L" <Fred.L.Templin@boeing.com>]

Iljitsch,

> the maximum packet size the ETR is  
> prepared to receive, possibly (if the ETR supports reassembly) the  
> maximum packet size the ETR is prepared to reassemble from fragments.

I assume you are talking here about the ETR's EMTU_R.
RFC1122 couches EMTU_R in terms of the largest packet
the node can *reassemble*, so it will presumably be at
least as large as the largest packet the node can
receive in one piece.

Adding a means for the ITR to discover the ETR's EMTU_R
is something I have proposed in numerous earlier efforts,
and also something I have considered for sprite-mtu. But
AFAICT, we really don't want the ETR to be reassembling
fragmented outer packets any larger than 1500 bytes;
instead, the ITR should send packets larger than 1500
bytes in one piece and/or send back a PTB if they are
too big.

So, IMHO all that needs to be known about the ETR is the
binary as to whether it can reassemble up to 1500 bytes
or not. If we say that all ETR's must be able to
reassemble up to 2KB (enough to cover the 1500 byte
packet plus any additional encapsulation overhead)
then maybe there isn't all that much to be gained by
an explicit EMTU_R discovery exchange?

Fred
fred.l.templin@boeing.com

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