Re: [v6ops] proposed TCP MSS text for rfc6204bis

[Mark Andrews <[e-mail]>]

In message <[e-mail]>, Brian E Carpenter writes:
> On 2012-05-19 22:26, Randy Bush wrote:
> >> If you want to send packets of arbitrary size, in any environment
> >> where PMTUD is impossible or fails, won't you need to always include
> >> a fragmentation header in every packet greater than 1280?
> > 
> > see discussion of jumbo frames, commonly 4k or 9k, between consenting
> > adults on known links
> 
> Yes indeed, but that isn't the general case. Across the open Internet,
> I think we have the situation I described.
> 
> On 2012-05-19 21:16, Gert Doering wrote:
> 
> >>> UDP packets larger than 1280 bytes
> >> > Don't do that!
> > 
> > Tell that to the DNS people.  They seem to really like not-using-TCP.
> 
> Yes, but I understand that DNSSEC more or less dooms that plan anyway.
> 
> However, I thinks it's true that the only fail-safe solution is to
> include a frag header if you need to send UDP >1280.
> 
>    Brian

For DNS we just fragment at 1280 using IPV6_USE_MIN_MTU.   We were
thinking about this back in 1998 (draft-ietf-ipngwg-bsd-frag-00.txt)
which was rolled into the advanced socket api.  It took a few more
years than I would have liked to become RFC and for implementations
to be available.  EDNS was already being developed back then and
it was obvious that PMTUD wouldn't work for large nameservers even
if they got the ICMPv6 PTBs.

For DNS there is little to be gained by trying to send any bigger
packets.

YMMV for other UDP based protocols.

Mark

>     Brian
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Mark Andrews, ISC
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