Re: [core] Chair's review of draft-ietf-core-hop-limit-02.txt

[Klaus Hartke <[e-mail]>]

Mohamed Boucadair wrote:
> Carsten Bormann wrote:
> > 2. I don't understand the rule about using cached copies.  As a
> >    general observation, rules like this should not simply be stated
> >    without a rationale why they are the way they are.

I assume you're referring to this rule:

   A CoAP proxy which understands the Hop-Limit option MUST NOT use a
   stored response unless the value of the Hop-Limit option in the
   presented request is less than or equal to the value of the Hop-Limit
   option in the request used to obtain the stored response.

(This matches the language in [1].)

Let's say proxy receives a request and has a matching Hop Limit
Reached error response in its cache. If the hop limit in the request
is equal to the hop limit in the request for the stored response, then
the stored error response can be returned. If the hop limit is less,
then the stored error response can also be returned. (If 6 hops are
not enough to reach the server, then 3 hops are also not enough.) If
the hop limit is greater, then the stored error response cannot be
returned. (The server might not be reachable in 6 hops, but it might
be reachable in 7 hops.) (This assumes that the number of hops to
reach a server doesn't change. If it changes, then a stored error
response might be served incorrectly until its max-age expires.)

I have to admit that I never checked if this also makes sense for
stored responses that are not a Hop Limit Reached error response...

Klaus

[1] https://tools.ietf.org/html/rfc7252#section-5.6