Re: I-D ACTION:draft-rahman-rtg-router-alert-dangerous-00.txt

Hi Adrian,

comments inline.

Adrian Farrel wrote:
> It is not a question of the Value field.
> 
> Suppose the Value field is set to zero. That means "routers shall
> examine this packet further".

IMHO this is a strange definition/use causing all the problems you are
describing in the text below. The RAO should allow for easier
interception of control/signaling packets while allowing to leave
packets in the fast path, i.e. forward them, if they are not of
interest.
If a router intercepts only those packets matching specific
RAO values situations could not occur where there is no suitable
application/protocol entity for further processing. As soon as
an application registers for specific RAO value(s), it will get
such packets. Thus, I think that the concept of having a general
value for "intercept and examine" is really counter-productive
(would be nearly the same as passing every packet with an RAO up).
So IMHO the RAO value should be the demultiplexer already.

> It is clear what they do if they examine the packet and decide it is for
> local processing (for example, they look at the IP Protocol field and
> see the payload is RSVP, and there is a local RSVP stack enabled on the
> interface).
> 
> It is unclear what they do if the payload is STII and there is no local
> stack.
> Router alert says "pass it up for local processing" but there is no-one
> to pass it to.
> Should it be dropped, forwarded on the slow path, or forwarded on the
> fast path?
> 
> The only evidence we have for the "correct" behavior is in 2205 where
> packets are send with RA and are expected to be able to pass through
> non-RSVP clouds.
> 
> Even the "silently ignore" for unrecognized Value fields is ambiguous.
> Does that mean drop the packet (ignore the packet) or forward it (ignore
> the RA option)?
> 
> Lastly, it is really unclear what should be done with a packet that
> carries RA and a protocol that *is* active on the router but which is
> not expecting router alert packets. For example, UDP. Should the packets
> be delivered there and then dropped (causing unnecessary processing) or
> should the protocol be expected to register as a router alert consumer
> with the fast path (requiring specifics of the implementation)?

I would say the latter. If a router only intercepts packets containing
RAO values of registered protocols, it should forward all other
packets. If the general purpose value of 0 (only for IPv4 RAO) is
avoided, there will be also no attacking problem if they router
only passes those packets up into the slow path that carry a locally
registered RAO value. From an NSIS perspective it would be optimal
if only those packets are intercepted that contain specific RAO values
whereas all other RAO packets remain in the fast path. This is also
important for scalability reasons, e.g., intra-domain aggregation
concepts...

Regards,
 Roland