RE: [pmtud] I-D ACTION:draft-savola-mtufrag-network-tunneling-00.txt(fwd)

[Mika.Joutsenvirta at nokia.com]

Hi Pekka,

I would like to make a note of following scenario.
This is applicaple to IPv6 and if DF bit is set 
also to IPv4.

In host-to-router case, where application wants to 
send > MTU UDP packet to server behind router. 
In common case IP stack generates UDP packet and then
starts to perform tunneling. If host now follows rules
from 3.1, it first encapsulates (IP-in-IP, IPSec, etc.)
this big UDP packet and then fragments it to suitable
pieces. Once router receives the fragments, it needs to
do reassembly and then decapsulation. After that the 
original UDP packet should be forwarded to server. But
in this scenario inner UDP packet was not fragemented and
the size of it is > MTU. So in IPv6, only thing that router
can do is to send "Packet Too Big" ICMP. But this does not
help as the fragmentation is done only for outer packet. 

So it should be stated more clearly that in host-to-router
case, inner packet must be fragmented to suitable pieces before
encapsulation. Otherwise router drops the packets. 

Mika


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