[core] Comments and questions on draft-jarvinen-core-fasor-02

[Christer Holmberg <[e-mail]>]

Hi,

I took a look at the FASOR draft, and I have a few comments and questions.


Q1:

Section 4.4. says:

   "The Retransmission Count Option is used to distinguish whether an
   Acknowledgement message arrives for the original transmission or one
   of the retransmissions of a Confirmable message."

- I assume this is achieved by, once it has been confirmed that the receiver supports the option, incrementing the counter value in each retransmitted message. However, I cannot find that described anywhere. The text only says that the value of the original transmission is zero.

- Does CoAP allow changing option values in retransmissions? I cannot find any text about that in RFC 7252 that forbids it, but my general assumption is that retransmitted messages are identical.


Q2:

Section 4.4. says:

   "However, the Retransmission Count Option cannot be used with an Empty
   Acknowledgement (or Reset) message because the CoAP protocol
   specification [RFC7252] does not allow adding options to an Empty
   message.  Therefore, Retransmission Count Option is useful only for
   the common case of Piggybacked Response."

- This means that the count option cannot be used when the receiver is a proxy, since a proxy will typically send an empty acknowledgement when it receives a message.


Q3:

Section 4.4 says:

   "The original transmission of a request is indicated with the number
   0, except when sending the first request to a new destination
   endpoint.  The first original transmission of the request to a new
   endpoint carries the number 255 (0xFF) and is interpreted the same as
   an original transmission carrying the number 0."

- What is meant by "new" endpoint? Does the sender have to remember the endpoints it has previously communicated with? If so, for how long?


Q4:

Section 4.5 describes how, as an alternative solution, the Token can be used to exchange count information.

- How would that work? Would you use different Token values in each retransmission? Again, I am not sure whether CoAP explicitly forbids that, but I assume that is not the intention.


Regards,

Christer