Evolution of the IP model - ICMP and MTUs

[Rémi Després <remi.despres@free.fr>]

Dave,

I find your work on the IP model quite interesting.
To answer your invitation to send comments, one remark.

It concerns Path Maximum Transmission Unit Discovery (PMTUD), and more
precisely its relationship to ICMP and Fragmentation.

- In the original model, senders of datagrams with the DF bit set (Don't
Fragment) received  no information back, when these packets were
discarded on their way because of their being "too big", about what size
would have fit.
- In 1990, the Next-Hop MTU information was added to Datagram Too Big
ICMP message (RFC 1191).
- Thus, hosts have a chance to quickly discover exact path MTUs. (They
have only "a chance" because: (1) ICMP messages, like any other packets,
are returned in best effort mode; (2) some routers may have not
implemented Next-Hop MTUs; (3) some firewalls filter ICMP packets.)
- The upper layer TCPs can now take advantage of this IP-layer provided
information to adjust their  Maximum Segment Size MSS (sec. 3.1 of RFC
1191).

IMHO, this evolution would be worth noting in a next version of your draft.


Regards,

Rémi