Subject: [TLS] cTLS transport question

From: Thomas Fossati <[email-masked]>
Date: Thu, 25 Jul 2019 16:04:05 +0000

Thanks for presenting this work.  I really like this and I think
it'd be really useful for the use cases we have (IoT, M2M).

One comment: from a quick skimming of the draft, I'm not sure I
understand what the stated expectations on the transport layer are?

Since it's cTLS and not cDTLS I'd have thought it's the same as TLS
(in-order & reliable) but then I got confused reading section 3.2 [1]:

  "The CTLS Record Layer assumes that records are externally framed
   (i.e., that the length is already known because it is carried in
   a UDP datagram or the like)"

On Jabber Ben suggested that one could put CoAP between UDP and cTLS to
get in-order & reliable delivery with a datagram transport, but then I'm
not sure what the advantage would be since we'd be trading 2 bytes of
TLSPlaintext.length with 4 bytes (minimum) of CoAP header?

From the Jabber discussion, my current understanding is that there are 2
assumptions: a) the transport is in-order & reliable, and 2) there is
one TLS record per transport layer packet.

Is that the correct interpretation?  If it is, is it not too restrictive
a requirement?

Cheers, thanks

[1] https://tools.ietf.org/html/draft-rescorla-tls-ctls-02#section-3.2

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From: Eric Rescorla <[email-masked]>
Date: Mon, 29 Jul 2019 05:09:43 -0700

> TLSPlaintext.length with 4 bytes (minimum) of CoAP header?

The idea here was that you might be in a situation where you had external
framing
anyway. I'm sure we can figure out some way to make that optional.

-Ekr

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