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I've understood masterSlaveDetermination procedure goes as follows: 1. EP1 sends mSD message with a value specific to it and also the random number. 2. EP2 sends back an mSDAck with slave/master info (after comparing its own and EP1's related info). 3. EP1 yet sends mSDAck to EP2 with slave/master info.

Question: Could someone try to explain the reasoning behind the "third message", that is, what is it needed for?

Step 3 is needed because EP2 needs to know if the result of the present instance of MSD procedure is known to EP1. EP2 can not initiate those procedures which rely on the knowledge of the MSD result, until it has received confirmation that EP1 terminal also has knwoledge of the result of the current MSD Preocedure. That's why Step 3 is required. regards Manoj.