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Re: [Lwip] [core] Issues of CoAP with DTLS

"Fossati, Thomas (Nokia - GB/Cambridge, UK)" <thomas.fossati@nokia.com> Fri, 18 August 2017 10:06 UTC

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From: "Fossati, Thomas (Nokia - GB/Cambridge, UK)" <thomas.fossati@nokia.com>
To: Hannes Tschofenig <hannes.tschofenig@gmx.net>, Zhen Cao <zhencao.ietf@gmail.com>, "draft-ietf-lwig-coap@ietf.org" <draft-ietf-lwig-coap@ietf.org>, "lwip@ietf.org" <lwip@ietf.org>, "core@ietf.org WG" <core@ietf.org>
CC: "Fossati, Thomas (Nokia - GB/Cambridge, UK)" <thomas.fossati@nokia.com>
Thread-Topic: [core] [Lwip] Issues of CoAP with DTLS
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Date: Fri, 18 Aug 2017 10:05:54 +0000
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Subject: Re: [Lwip] [core] Issues of CoAP with DTLS
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Hi Zhen,

If you are interested in the f2f discussion that Hannes mentioned, see [1].

Cheers, t

[1] https://youtu.be/ms-0PlY1R-8?t=2534


On 18/08/2017, 10:37, "core on behalf of Hannes Tschofenig" <core-bounces@ietf.org on behalf of hannes.tschofenig@gmx.net> wrote:

Hi Zhen,

a few people have pointed out this problem and hence a solution will be
worked on (as agreed at the last TLS WG meeting).

Ciao
Hannes

On 08/18/2017 11:05 AM, Zhen Cao wrote:
> Hi Authors of draft-lwig-coap,
> 
> Thank you for the draft.  I have a question related to CoAP-over-DTLS.
> Section 5.4 of the draft
> (https://tools.ietf.org/html/draft-ietf-lwig-coap-04#section-5.4) has
> some discussion over the problem, it however does not help with the
> case below.
> 
> Say, the client and server is talking over a CoAP-DTLS session with a
> NAT between.   Then the NAT session expires because of an idle period
> when no traffic re-enforce the NAT state.  Assume afterwards the
> client would like to send a new CoAP-CON message towards the server.
> With the NAT outgoing <address port> pair changed, the server will not
> be able to resume the previous DTLS session and will discard this
> message.  Sad though, it is not that serious because NAT problems is
> everywhere.
> 
> What's worse is however, under such scenario, the client is unclear if
> it needs to retransmit the CoAP-over-DTLS message or re-negotiate a
> new DTLS (isn't it? because it does not know whether it is a network
> issue or DTLS failure).   If it takes it as a network issue, it will
> keep trying to retransmit the CoAP-CON, until it reaches the retry
> limit (4 defined in RFC7252). This is very costly because of the
> exponential backoff may sum to more than 10s.  In this case, it will
> be more efficient in this case to have the client re-negotiates the
> DTLS with server immediately.
> 
> a) So my first question will be :
> Is this an issue with the current implementation and shall we make
> some recommendations?
> 
> b) With regards to a better solution,
> draft-fossati-tls-iot-optimizations-00 will be a direct solution by
> including a connection ID in the DTLS record layer,  but I do not know
> why this draft expires.   @Hannes, could you help me with some
> background.
> 
> Many thanks for discussion.
> 
> BR,
> Zhen
> 
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