OPINION/BELIEF:
I concur with Fred Baker's analysis of "SHOULD" versus "MUST".
I think the requirement that a packet that violates the
proposed oversized-heacer-chain rule be dropped "silently"
is too strong and lacks operational flexibility.
A device ought to be allowed, but not required, to send
an ICMPv6 Parameter Problem message if/when it drops
such a packet.
In some situations, it might be desirable for a device
to drop the packet AND also generate an ICMPv6
"Parameter Problem" message for the packet originator.
ACTIONABLE SUGGESTIONS:
So....
1) I'd prefer this draft say that such illegal packets MUST
be dropped, but then also say that the device dropping the
packet MAY send an ICMPv6 Parameter Problem control message
back to the (alleged) sending node. A brief sentence
suggesting, but not requiring, that implementations make
the sending of the ICMPv6 message configurable also would be
sensible.
and
2) For the situation where an IPv6 packet is dropped for this
specific reason, I'd suggest that the "Reason Code" registry
for an ICMPv6 "Type 4 - Parameter Problem" message be updated
with a new focused reason code defined. As a straw man,
I'd propose adding this new entry to that registry,
as part of this proposal:
CODE NAME/DESCRIPTION
3 IPv6 Initial Fragment missing upper-layer protocol header
Of course, this means an "IANA Considerations" section
update for the I-D would be in order.
and
3) I'd also suggest a sentence or two clarifying that the
ICMPv6 "Parameter Problem" with the new Reason Code
is the appropriate message to send -- if the device dropping
the packet with oversized-header-chain sends an ICMPv6
message. One would hope this would be obvious, but being
very clear is good.
and
4) Security Considerations ought to note the importance of
deploying Source Address Forgery Prevention filters, in
order to reduce the threat of an adversary forging an
illegal packet with contains a victim/target's IP address
in the Source IPv6 Address field of the illegal packet.
[RFC-2827, RFC-3704]
Yours,
Ran
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