Expand text in OSCORE context configuration section on the use of Sender Context when sending

Tag: WGLC

Göran Selander wrote (https://mailarchive.ietf.org/arch/msg/6tisch/fWK4i3bs3r4VmFCkSoGzgtlWSuo):

"This OSCORE security context is used for initial joining of the (6LBR) pledge, where the (6LBR) pledge acts as a CoAP client, as well as for any later parameter updates, where the JRC acts as a CoAP client and the joined node as a CoAP server, as discussed in Section 9.2. A (6LBR) pledge is expected to have exactly one OSCORE security context with the JRC."

This is correct, not specific to this application of OSCORE, but may be good for explaining how it works. In the spirit of explaining OSCORE and not specific to this application, you could emphasize that the same Sender Context is always used when sending, independently of if the node is CoAP client or CoAP server, so that role switching between client and server must not change the Sender / Recipient Context in a given endpoint. This may be added here, in the security considerations section, or in both.

Good point, I think this text fits well within the current section on the OSCORE context.

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