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Re: [Int-area] Stateless devices and IP fragmentation

Tim Chown <Tim.Chown@jisc.ac.uk> Wed, 21 November 2018 16:52 UTC

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From: Tim Chown <Tim.Chown@jisc.ac.uk>
To: Ron Bonica <rbonica@juniper.net>
CC: Brian E Carpenter <brian.e.carpenter@gmail.com>, Tom Herbert <tom@herbertland.com>, Joe Touch <touch@strayalpha.com>, int-area <int-area@ietf.org>
Thread-Topic: [Int-area] Stateless devices and IP fragmentation
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Date: Wed, 21 Nov 2018 16:52:07 +0000
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Subject: Re: [Int-area] Stateless devices and IP fragmentation
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Hi,

> On 21 Nov 2018, at 13:40, Ron Bonica <rbonica@juniper.net> wrote:
> 
> Brian,
> 
> Fair enough. I have worked the 1280 byte requirement into Section 7.4. New text is included below.
> 
>                                                       Ron
> 
> 7.4.  For Network Operators
> 
>   As per RFC 4890, network operators MUST NOT filter ICMPv6 PTB
>   messages unless they are known to be forged or otherwise
>   illegitimate.  As stated in Section 4.6, filtering ICMPv6 PTB packets
>   causes PMTUD to fail.  Operators MUST ensure proper PMTUD operation
>   in their network, including making sure the network generates PTB
>   packets when dropping packets too large compared to outgoing
>   interface MTU.  Many upper-layer protocols rely on PMTUD.
> 
>   As per RFC 8200, network operators MUST NOT deploy IPv6 links whose
>   MTU is less than 1280 bytes.
> 
>   Network operators SHOULD NOT filter IP fragments if they originated
>   at a domain name server or are destined for a domain name server.

There is some similar-ish text in the RFC6434-bis document (in AUTH48 now), see https://tools.ietf.org/html/draft-ietf-6man-rfc6434-bis-09#section-5.7.1

The flip side of your last para is what we say there:

"While an IPv6 link MTU can be set to 1280 bytes, it is recommended
   that for IPv6 UDP in particular, which includes DNS operation, the
   sender use a large MTU if they can, in order to avoid gratuitous
   fragmentation-caused packet drops."

Tim

> 
> 
> 
> 
> 
>> So: I think the document should say much more emphatically that there is no
>> exception to the 1280 requirement for every IPv6 hop.
>> It's mentioned briefly but maybe it should be underlined three times :-).
>> 
>> Amd maybe, since the draft is aimed at BCP, we could even go so far as to
>> change this assumption into a SHOULD, if not a MUST:
>> 
> decisions.
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