Re: [TLS] Éric Vyncke's No Objection on draft-ietf-tls-dtls-connection-id-11: (with COMMENT)

[Benjamin Kaduk <[e-mail]>]

Hi Éric,

Trimming heavily, as several other responses have landed already...

On Mon, Apr 19, 2021 at 12:47:51AM -0700, Éric Vyncke via Datatracker wrote:
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
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> 
> -- Section 3 --
> While I am not a DTLS expert, I find this section quite difficult to understand
> the reasoning behind the specification as little explanations are given about,
> e.g, what is the motivation of "A zero-length value indicates that the server
> will send with the client's CID but does not wish the client to include a CID."

We had a lot of discussion in the WG about zero-length CIDs and the various
edge cases that can arise.  (You might imagine, for example, wanting to use
the new record format and get content-type confidentiality even when a CID
is not needed for routing messages.)  We ended up concluding that it's
simpler and safer to do what's described now -- no CID means the RFC 6347
record format.  But the discussion in the document may not have captured
the full extent of the WG discussion, so we can take a look at whether
there is more to be said.

The short answer is that this is a typical mechanism where each part tells
the other how to contact them, but in order to confirm the negotiation to
use the feature, we need a signal for "I don't need a CID but I support
CIDs", and the zero-length field in the handshake packets plays that role
quite naturally.

-Ben