Smith,
I agree with Ira - TLS version numbers, errors, etc. belong at the TLS level and not in IPP.
I've included a few specific comments inline below...
> On Jul 27, 2018, at 4:25 PM, Kennedy, Smith (Wireless & Standards Architect) <smith.kennedy at hp.com> wrote:
> ...
> None of these seem to cover a lower-level protocol negotiation level failure. Do we need to add a new one for TLS version negotiation failure?
I don't think so. If the Client and Printer cannot negotiate a common supported version of TLS, the connection will fail and the Printer won't have the opportunity to tell the Client why in an IPP response since the request will not have been received.
> The Client can learn the Printer's maximum TLS version via the "TLS" DNS-SD TXT record key (5100.14 section 4.2.3.4). The "uri-security-supported" attribute simply uses 'tls' but lists no version (which troubles me because DNS-SD shouldn't be more descriptive than IPP).
While the TLS key does provide slightly more information, I don't think this is critical since a) most/all? clients look for _ipps advertisements these days to determine TLS support and b) Clients do not trust such information for downgrading to a lower version of TLS thanks to the SSL3 and TLS/1.0 downgrade attacks.
At best the information might allow a Client to hide printers that (for example) only support TLS/1.0, or treat such printers as insecure after a suitably ominous warning to the user...
_________________________________________________________
Michael Sweet, Senior Printing System Engineer
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.pwg.org/pipermail/ipp/attachments/20180729/74fde176/attachment.html>