MRAS - Media Relay Authentication Services, how does client obtains it ?

ICE - Interactive Connectivity Establishment, is a protocol used by Lync client, which uses STUN and TURN 

STUN - Protocol is used to find out the client side public IP, its sent by the client and the IP address discovered  will be listed as SRFLX in the candidates list 

TURN - Protocol is used to allocate a port  on AV Edge on which the client can connect, RELAY will have the AV Edge address 

When the client signs in it will send a SUBSCRIBE packet  

200 OK will be returned from the server which will have the in-band provisioning which includes Policies, Contact List, Presence subscription,  

Location Profile, and MRAS URI i.e. FQDN of the Edge server. 

Now Client will send the SERVICE Request to the MRAS URI, which goes to the FE and FE will talk to the AV Edge and get the encrypted User name  

and password to the client to use the AV Edge service. 

Note - MRAS always happens by the internal NIC of the Edge server 

Candidate List Population 

Lync client is an ICE client,  Clients about to get in a SIP conversation will exchange will do: 

Address Discovery - Local IP (HOST), SRFLEX (Public IP behind which the Lync client or this can be Public IP of the EDGE),  

RLY (IP of the internal NIC of the EDGE) 

Address Exchange - after discovering their address they will exchange the IP address discovered by then which will have the  

(HOST, RELAY and SRFLEX) it with the other partner 

Connectivity Establishment  - Check for the connectivity between the IP address Exchanged  

Candidate Promotion - The Final IP address via which the media will flow i.e. a=remote-candidate