Which DNS SRV must the Expressway-E resolve when routing an outbound call to Microsoft clients on Lync or Skype for Business?
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A. B. C. D. E.C.
When routing an outbound call to Microsoft clients on Lync or Skype for Business, the Expressway-E needs to resolve the DNS SRV record _sipfederationtls._tcp.domain.com. Therefore, the correct answer is D.
Explanation:
In the context of SIP (Session Initiation Protocol), DNS SRV (Domain Name System Service Resource Record) is a mechanism for determining the domain and port to be used for establishing a SIP session with a remote endpoint.
Microsoft clients on Lync or Skype for Business use the SIP federation protocol to connect with external entities. The SIP federation protocol uses the Transport Layer Security (TLS) protocol to secure communication between the Microsoft client and the external entity.
The DNS SRV record _sipfederationtls._tcp.domain.com is used by the Expressway-E to locate the external server that can handle the TLS connection for the SIP federation protocol. This record indicates the server's FQDN (Fully Qualified Domain Name) and port number, which is typically 5061.
Therefore, when routing an outbound call to Microsoft clients on Lync or Skype for Business, the Expressway-E must resolve the DNS SRV record _sipfederationtls._tcp.domain.com to establish a secure SIP session with the external entity.