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Re: Evolution of the IP model - ICMP and MTUs

Christian Huitema  - Le 8/15/08 10:14 PM :

If you want to document the evolution, you have to be complete.

1) In the original model, senders of datagrams with the DF bit set
 (Don't Fragment) received  no information back.
2) In 1990, the Next-Hop MTU information was added to Datagram Too Big
 ICMP message (RFC 1191).
        Hosts have a chance to discover the real MTU in the path using ICMP
3) Around 1995, firewalls started to drop all ICMP by default
        Hosts that rely on ICMP to discover PMTU observe terrible performance
4) Around 2000, broadband connections start being equipped with tiny "home
   routers" whose NAT function does a pretty bad job at reassembling IP packets
        Hosts that send packets too large observe terrible performance, and they
        are in a bind since PMTU discovery does not work well.
5) By 2008, the IETF might recognize that firewalls are here to stay,
   that we could just as well forget about ICMP, but that we really
   need another solution.
- Well, I don't "want" anything here. Just trying to answer Dave's invitation to comment. 
- Please note that I did mention that "some firewalls filter ICMP packets".
- IMU, your more detailed comments are worth being included too (except 5., wich is a comment about the future). - In particular a brief summary of what you mean by "a pretty bad job at reassembling packets" should IMO have its place in Dave's document, its scope being the evolution of the IP model. 

Regards,

Rémi Després