You only need to use a salt if you need domain separation or randomness extraction (as you can see from the comments in the randomness extraction answer, it is questionable as to whether a salt is actually even needed for randomness extraction).

The info parameter will contain strings as defined in your protocol that will allow different keys to be derived from the same initial keying material at the HKDF-Expand step.

You only need to use a salt if you need domain separation or randomness extraction (as you can see from the comments in the randomness extraction answer, it is questionable as to whether a salt is actually even needed for randomness extraction).

The info parameter will contain strings as defined in your protocol that will allow different keys to be derived from the same initial keying material at the HKDF-Expand step. Therefore, since the choice of info parameter values used to generate each key will be defined in your protocol, you should not be transmitting them.