Re: [DNSOP] WGLC for draft-ietf-dnsop-avoid-fragmentation

[Andrew McConachie <[e-mail]>]

> Path MTU discovery remains widely undeployed due to
>    security issues, and IP fragmentation has exposed weaknesses in
>    application protocols.

PMTUD doesn’t work through NAT and that’s probably the main reason 
why it doesn’t work on the Internet. I think that’s less of a 
security issue than just a general issue with PMTUD not working on the 
modern Internet.

> Currently, DNS is known to be the largest
>    user of IP fragmentation.

Compared to what? I would just drop this sentence because it doesn’t 
add anything to the document and it’s trying to make a point that 
doesn’t need to be made.

>  Most of the Internet and especially the inner core has an MTU of
>  at least 1500 octets.  Maximum DNS/UDP payload size for IPv6 on
>  MTU 1500 ethernet is 1452 (1500 minus 40 (IPv6 header size) minus
>  8 (UDP header size)).  To allow for possible IP options and
>  distant tunnel overhead, authors' recommendation of default
>  maximum DNS/UDP payload size is 1400.

Before I was interested in the DNS I worked for an ethernet switch 
vendor for 8 years, and I often find the way MTU gets talked about in 
IETF documents simply weird. MTU is a measurement of maximum frame size 
for a network segment starting at Layer 2. Yet there’s no discussion 
of layer 2 here. The discussion starts at layer 3 and because of that 
the math doesn’t make any sense to me.

Is there just an assumption that layer 2 will consume 18 bytes? 
(6+6+2+4) (DA+SA+ET+FCS) Can we state this assumption in the document? 
As I read it now it’s not clear how many bytes are assumed to be 
consumed by layer 2 headers.

I said many of these same things in a mail to this list on August 12, 
2020 but never received a response.

Thanks,
Andrew

On 26 Jul 2022, at 23:13, Suzanne Woolf wrote:

> Dear colleagues,
>
>
> This message starts the Working Group Last Call for 
> draft-ietf-dnsop-avoid-fragmentation 
> (https://datatracker.ietf.org/doc/draft-ietf-dnsop-avoid-fragmentation/). 
> The requested status is BCP.
>
> Since we're starting the Last Call during the IETF week, and many 
> folks are on holidays in the next few weeks, the WGLC will end in 
> three weeks (instead of the usual two), on August 16.
>
> Substantive comments to the list, please. It’s fine for minor edits 
> to go direct to the authors. We need to hear positive support to 
> advance it, or your comments on what still needs to be done.
>
>
>
> Thanks,
> Suzanne
> For the chairs
>
>
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