Re: [core] Editors' draft of changes to draft-ietf-core-coap-tcp-tls-08 after IESG input

[Bill Silverajan <[e-mail]>]

Hi Carsten,

On 17/06/17 15:32, Carsten Bormann wrote:
> Hi Bill,
>
>> On Jun 17, 2017, at 12:07, Bill Silverajan <[e-mail]> wrote:
>>
>> Hi,
>>
>>>> It would be good to know what the actual assumptions are.
>>>> CoAP clients do trial-and-error until they get a response?
>>>> All CoAP endpoints are expected to implement all transport and the pick of transport is just because of the middlebox baggage?
>> In discouraging the use of several URI schemes for CoAP, one review recommended a series of steps, in which CoAP over UDP is MTI: Always try UDP first, and if no response is forthcoming, to then resort to TCP. Port 80/443 would always default to CoAP over ws or wss without needing to engage UDP initially. Obviously this has repercussions regarding protocol and application logic.
> Right.  It also does not work for one of the main applications of CoAP-TCP: traversal of NATs with short UDP binding lifetimes.  The probe will always work through such a NAT, but then the observation relationships won’t, because the UDP binding is gone by the time the notification needs to be transferred.
>
> (Of course, a server can work around this issue by providing a URI with an IP address that listens on TCP only, so the UDP probe fails.  This likely outcome is wrong on so many levels…)
>
> […]
As a sort of last-resort measure, I can envisage working out a 
server-initiated protocol negotiation mechanism requiring the client to 
switch transports, or to a lesser extent, presenting a preference order 
for the alternate transport that should be subsequently used.
>> Conversely, it must also be recognised that right now, some mechanisms are bound to the "coap" and "coaps" URI schemes, such as using ".well-known/". Minting new CoAP URI schemes would also require extending this to support them.
> Not sure I get this point.  We did add well-known support for the new URI schemes in -07.
Yes, -07 did support it. My (fairly trivial) point was aimed at possible 
provisional registrations made by implementers for convenience.

Regards,
Bill