Re: Benoit Claise's Discuss on draft-ietf-6man-rfc1981bis-06: (with DISCUSS and COMMENT)

[Bob Hinden <[e-mail]>]

Benoit,

Comments below on your Discuss.

Bob


> On May 11, 2017, at 3:45 AM, Benoit Claise <[e-mail]> wrote:
> 
> Benoit Claise has entered the following ballot position for
> draft-ietf-6man-rfc1981bis-06: Discuss
> 
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> 
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
> 
> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-6man-rfc1981bis/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
>  Nodes not implementing Path MTU Discovery MUST use the IPv6 minimum
>   link MTU defined in [I-D.ietf-6man-rfc2460bis] as the maximum packet
>   size.
> 
> I searched for "IPv6 minimum link MTU" in draft-ietf-6man-rfc2460bis-09,
> and could not find that term.

The text in rfc2460bis (unchanged from rfc2460) is:

5.  Packet Size Issues

   IPv6 requires that every link in the internet have an MTU of 1280
   octets or greater.  On any link that cannot convey a 1280-octet
   packet in one piece, link-specific fragmentation and reassembly must
   be provided at a layer below IPv6.

   Links that have a configurable MTU (for example, PPP links [RFC1661])
   must be configured to have an MTU of at least 1280 octets; it is
   recommended that they be configured with an MTU of 1500 octets or
   greater, to accommodate possible encapsulations (i.e., tunneling)
   without incurring IPv6-layer fragmentation.

rfc1981bis (and rfc1981) doesn’t use "IPv6 minimum link MTU” as a defined term (otherwise it would be in Section 2. “Terminology”.  It’s just words describing the concept in Section 5 of rfc2460bis (as shown above).


> Even unlikely at this point in the IPv6 implementation cycle, we don't
> want readers to believe that they should look at the minimum of the
> device IPv6 MTU link(s).
> Proposal: define "IPv6 minimum link MTU" as 1280 octets in 2460bis, or in
> both documents.

I think rfc1981bis (and rfc1981) are correct about pointing to rfc2460bis (and rfc2460).  Having to change it in two places would make it much harder to change it in the future.

> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> In this document, I see:
> 
>   IPv6 nodes SHOULD implement Path MTU Discovery in order to discover
>   and take advantage of paths with PMTU greater than the IPv6 minimum
>   link MTU [I-D.ietf-6man-rfc2460bis].  A minimal IPv6 implementation
>   (e.g., in a boot ROM) may choose to omit implementation of Path MTU
>   Discovery.
> 
> In draft-ietf-6man-rfc2460bis-09:
>   It is strongly recommended that IPv6 nodes implement Path MTU
>   Discovery [RFC1981], in order to discover and take advantage of path
>   MTUs greater than 1280 octets.  However, a minimal IPv6
>   implementation (e.g., in a boot ROM) may simply restrict itself to
>   sending packets no larger than 1280 octets, and omit implementation
>   of Path MTU Discovery.
> 
> So a SHOULD in one document versus "strongly recommended" in the other.
> 
> We should reconcile the two texts.
> Note: may and may are consistent.
> 
> 
> ICMPv6 PTB => ICMPv6 Packet to Big (PTB)
> 
>