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