Masataka,
Thanks for your thoughtful review. Would your concern be addressed if we added the text below?
Ron
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2.2.1. RFC 2460 Compliance
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The solution described above is widely-deployed on the Internet in its default configuration.
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NEW>
The solution described above is widely-deployed on the Internet in its default configuration. However, the default configuration is not always appropriate for GRE tunnels that carry IPv6.
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.
Therefore, the default configuration is appropriate for tunnels that carry IPv6 only if the network is engineered so that the GMTU is guaranteed to be 1280-bytes or greater. In all other scenarios, a non-default configuration is required.
In the non-default configuration, when the GRE ingress router receives a packet lager than the GMTU, the GRE ingress router encapsulates the entire packet in a single GRE and delivery header. It then fragments the delivery header and sends the resulting fragments to the GRE egress, where they are reassembled.
<NEW
> -----Original Message-----
> From: ietf [mailto:ietf-bounces@xxxxxxxx] On Behalf Of Masataka Ohta
> Sent: Friday, April 10, 2015 8:26 AM
> To: ietf@xxxxxxxx
> Cc: int-area@xxxxxxxx
> Subject: Re: Last Call: <draft-ietf-intarea-gre-mtu-02.txt> (A Widely-
> Deployed Solution To The Generic Routing Encapsulation (GRE)
> Fragmentation Problem) to Informational RFC
>
> As the draft says;
>
> o When the GRE ingress node receives a non-fragmentable packet with
> length greater than the GMTU, it discards the packet and send an
> ICMP PTB message to the packet's source.
>
> the draft should clearly state that, if GMTU<1280B, it is a violation of the
> following requirement of RFC2460:
>
> 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.
>
> and that 1280B IPv6 packets can not be carried over IPv6 with the default GRE
> configuration.
>
> It is especially so, because, according to the draft:
>
> Typically, GRE ingress nodes further refine their GMTU estimate by
> executing PMTUD procedures. However, if an implementation supports
> PMTUD for GRE tunnels, it also includes a configuration option that
> disables PMTUD. This configuration option is required to mitigate
> certain denial of service attacks (see Section 5).
>
> PMTUD is often turned off and, then, RFC2460 requires GMTU<1280B.
>
> Also, I think the paragraph above is not very honest on the reason why
> PMTUD is often turned off.
>
> Masataka Ohta