How big is the DTLS 1.2 CID message header for CoAP messages?

Question

I'm using the Cloud Multi-Service app on nRF9160 to test the CoAP DTLS 1.2 CID support while connected to nRF Cloud to evaluate, and want to check the data size for messages sent to help estimate total data use over time. In coap_client.c I can see the size of the CoAP message layer sent and received, but I need to know how big the underlying DTLS overheads are. 1) In DTLS 1.2. CID - How big is the CID used by the nRF SDK 2.6.1 + Modem v1.3.6? 
 2) Using AT%XCONNSTAT to help check the message overheads for a CoAP POST (after DTLS has already connected) 
 Using AT%XCONNSTAT? to check the modem states (with AT%XCONNSTAT=1 just before temperature data sent) returns: 
 %XCONNSTAT: 0,0,0,0,139,97 - where: max_packet = 139, min_packet = 97 
 Where the coap POST (confirmable) len = 51 (for header_len=12(token=8), options len=11, data len=28). 
 Does this mean the total data pkt overhead = 139 - 51 = 88 bytes for this POST? And 97 bytes is the size for the Coap Ack (including overheads) ? 3) After a Google search on DTLS overheads - I had expected total data overheads for DTLS 1.2 CID + UDP/IP to be approx = ~57 bytes if CID length is say 16 bytes long. 
 Assuming IP(20) + UDP(8) + DTLS(13) + CID (16) = 57. So I'm not sure if the results from XCONNSTAT implies a much bigger CID length is being used? 
 
 In conclusion - Can you please confirm the actual data overheads for DTLS 1.2 CID with nRF SDK. This will be very helpful for estimating data use over time

Achim Kraus · Answer

> IP(20) + UDP(8) + DTLS(13) + CID (16) 
 If you want to "save" data, I would not use a 16 bytes CID (but that's the server's configuration, e.g. "coap.nrfcloud.com" uses 10 bytes). 
 The DTLS 1.2 CID record does not only add the DTLS 1.2 header with the CID, it also adds the explicit part of the "nonce" (usually 8 bytes) and 8, 16, 32 or <what ever> bytes MAC (depending on the negotiated cipher suite). There is also some CoAP header (variable, depending on the options as path or query or), therefore, overall, I usually consider 100 bytes overhead per message (that includes then also the handshake). e2e security isn't for free. e.g. the "explicit nonce" is used to ensure, that the same data doesn't result in the same encrypted message. The MAC is used, to protect the appl. layer (e.g. CoAP) from "noise by an attacker". Other techniques may come with an other balance here, but that usually only effects applications, which are using very small application messages. 
 To be frank, in my cases, I more afraid, that a MVNO disables my SIM because of the very low usage than in a too expensive bill for the extra 100 bytes.

How big is the DTLS 1.2 CID message header for CoAP messages?

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