RE: Evolution of the IP model - ICMP and MTUs

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

1.5) In 1987, "Fragmentation Considered Harmful".

Fred 

>-----Original Message-----
>From: Christian Huitema [mailto:huitema@windows.microsoft.com] 
>Sent: Friday, August 15, 2008 1:14 PM
>To: Rémi Després; Dave Thaler; v6ops
>Subject: RE: Evolution of the IP model - ICMP and MTUs
>
>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.
>
>
>
>
>> -----Original Message-----
>> From: owner-v6ops@ops.ietf.org [mailto:owner-v6ops@ops.ietf.org] On
>> Behalf Of Rémi Després
>> Sent: Thursday, August 14, 2008 11:40 PM
>> To: Dave Thaler; v6ops
>> Subject: Evolution of the IP model - ICMP and MTUs
>>
>> 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
>>
>>
>>
>>
>
>