Server initiates a SIP session with “” by contacting IP Address “213.165.99.202” Server initiates an SMTP session with “” by contacting IP Address “213.165.99.201”
Server contacts the host providing the service for the destination domain to deliver the message Server makes a DNS search for an A record for “” to retrieve the IP Address assigned to this FQDN. Server identifies the IP Address for the host which provides the required service Server retrieves the FQDN for the SIP Server for “” and listening port number (unlike SMTP, this is variable for SIP) with the highest priority and weight from step 3. Server retrieves the FQDN for the Mail Exchanger (MX) for “” with the highest priority from step 3. Server i dentifies which host provides the requested service SRV records are non-service-specific, so the “_sip._udp” and “_sip._tcp” prefixes are what identify the service Server makes a DNS search for an SRV record for “_sip._” or “_sip._”. The MX record is a service specific record for “Mail eXchanger” used for mail communication via SMTP (Simple Mail Transfer Protocol). Server does a DNS search for an MX record for “”. Server i dentifies whether the destination domain provides the requested service Server identifies destination domain “”, as being non-local Server i dentifies destination domain as non-local Ĭlient delivers a message to Server for forwarding – destination “ ”Ĭlient connects to SMTP Server (directly) and delivers a message to an unknown (non-local) user “ ”Ĭlient connects to a SIP Server (directly) and delivers a message to an unknown (non-local) user “ ” To simplify the understanding of the mechanisms behind Direct SIP Calls, a quick overview of email delivery is useful in drawing parallels with the mechanisms for d irect SIP c alls. Th e same address however does not mean that the m ail s erver will be required to also handle SIP c alls, but that the same format can be used to address different services, processed by different hosts or services. It is possible and probably desirable for a user’s Email a ddress and SIP a ddress to be the same. SIP a ddresses have exactly the same structure as Email a ddresses. SMTP and SIP Delivery Mechanism Comparison Alternatively, if a user calls “ ” from a SIP phone registered to an IP PBX or VoIP provider that supports direct SIP, when the SIP server receives the SIP “ INVITE ” message to initiate the call, it recognizes that the domain “ ” is not local for the SIP server itself and makes the appropriate DNS (Domain Name System) queries to identify the SIP Server responsible for the domain “ ”, and then initiates a SIP session with this SIP server. Once the SIP server’s IP Address is resolved, the caller’s soft phone initiates a SIP session with the SIP server responsible for the domain “ ”.
If for example, a user calls “ ” from a stand alone SIP phone, the caller’s soft phone makes appropriate DNS queries to resolve the address of the SIP server responsible for SIP - based VoIP calls to “ ”. A caller can call a callee from either a stand alone SIP phone which is not registered to any SIP server, to a VoIP (Voice-over-IP) provider, or from a SIP phone registered to an IP PBX (Private Branch Exchange), which supports direct SIP. The way direct SIP calls are initiated is very similar to the way sending an email works.
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