ST87M01 - DTLS persistent Socket during PSM?

[GeoP]

Hi all, 
for minimum wake time I want to use UDP with DTLS.

However, I wonder if it is OK to keep the Socket open for a longer time (not closing it ever)? Even during PSM?

Opening the DTLS-Sockets costs 2 seconds (handshake) and energy, for a Raw-Socket it is almost zero

DTLS: Init->Wait->Use
[000000]> AT#SOCKETCREATE=5,0,UDP,0,60,60,9,0,0
[000116]< #SOCKETCREATE: 0
[000118]< OK
[000000]> AT#DTLSCONNECT=5,0,xxx.xxx.xxx.xxx,pppp
[002011]< #TLS: 3,0x00A8,2048,0
[002012]< OK
[000000]> AT#IPSENDUDP=5,0,xxx.xxx.xxx.xxx,pppp…


Raw
[000011]< #SOCKETCREATE: 0
[000012]< OK
[000000]> AT#IPSENDUDP=5,0,xxx.xxx.xxx.xxx,pppp…

Thanks for any helpful infos,
Jo

[Didier HERROUIN]

Dear GeoP,

• While the module is active and not in PSM : you can keep a socket open for a long time if your use case and network timers allow it.
• Across PSM : sockets are effectively not preserved.  We recommend to re‑open them on each wake‑up, send/receive what you need, then close them before re‑entering PSM.
  You can find a DTLS handshake and session example in the Application Note : “ST87MXX_TLS_Application_Note.pdf” (Note: All the Application Notes are copied on your PC when you install the GUI)

Regards,
Didier

[GeoP]

Dear Didier,

thank you very much for the clear explanation and for pointing me to the TLS Application Note.

This confirms that, for our PSM-based ultra-low-power use case, we cannot keep the DTLS socket alive between wake-up cycles. Reopening the socket and performing a DTLS handshake on every wake-up would add considerable latency and energy overhead compared with the single measurement datagram we want to transmit.

Our intended communication pattern is similar to the LoRaWAN philosophy: one uplink packet and, only when required, a maximum of one downlink response per wake-up cycle. We will therefore most likely use raw UDP with authenticated encryption at the application layer for this particular operating mode.

One final question: can the session resumption described in the TLS Application Note still be used after PSM when a new socket is created, or is it only useful while the module remains active?

By the way, the more I work with the ST87M01, the more I like it - it really is a great product! (Although the STMxx GUI stll is not running stable on my PC with latest .NET.. ;.)

Thanks again and best regards,
Jo

[Didier HERROUIN]

Dear GeoP,

The resumption feature is precisely done for that : you open a socket, you send/receive the data you need, you close it and then, you can re-open the socket faster, using the resumption option. 
Nevertheless, be aware that the session is saved by the server : this one can request a complete handshake whenever it wants, after 1 day or after several sessions resumptions. The node is not the master.

Regards,
Didier

[GeoP]

Hi Didier,

Ah, thanks for the clarification.  

Thanks again and best regards,
Jo