In this case, the SIP messages have been heavily abbreviated to show the flow of traffic.Īlthough the diagram here shows that the proxies do not participate in the SIP protocol once Alice acknowledges that Bob has picked up the phone, not every call will work that way. The diagram below shows a SIP dialog involving two parties (Alice and Bob) and their SIP proxy servers, the Atlanta and Biloxi. Because phones generally have numeric keypads, the phone is responsible for translating what you dial (such as extension 4094) into a SIP URL (such as You can learn more about how SIP URIs, traditional telephone numbers, DNS, and IP addresses all interact in our white paper on "ENUM." In SIP, an end station has a SIP URI (a form of URL) that identifies it and is used in the SIP protocol. We normally think in telephony of phones as having numeric addresses. The job of the SIP proxy is to know what "4094" really means: Is that a phone? An auto-attendant system? Perhaps several phones, all to be rung at once? Although the phone could have some magical way of translating that 4094 into an IP address or location, it typically will simply send its call request to its own SIP proxy. As an example, a SIP phone might want to make a call to another phone at extension 4094. SIP proxies are very helpful in offloading tasks and simplifying implementation of end station telephones. Note the SIP proxy only participates in the SIP messages-once the call is set up, the phones send their voice traffic directly to each other without involving the proxy. This minimizes overhead, thereby speeding performance.Īlthough two SIP devices can talk directly to each other, they generally will use an intermediary system that acts as a SIP proxy.
The most common implementations, though, use IPv4 and UDP. SIP can run over IPv4 and IPv6 and it can use either TCP or UDP. Nevertheless, it has enough powerful features to model the behavior of a very complex traditional telephone PBX. Contrasted with H.323, SIP is an exceedingly simple protocol. This makes it easy to debug because the messages are easy to construct (if you're a developer) and easy to see (if you're a network manager). One of the wonderful things about SIP is that it is a text-based protocol modeled on the request/response model used in HTTP. SIP is used to transfer calls, terminate calls, and change call parameters in mid-session (such as adding a 3-way conference). SIP tells the end point that its phone should be “ringing ” SIP is used to agree on session attributes used by the calling and called party. SIP is used by end points to negotiate media capabilities, such as agreeing on a mutually supported voice codec. SIP is used by end points to determine whether they will “answer” a call. The table below shows the five major functions within SIP from a VoIP point of view.Įnd points (telephones) notify SIP proxies of their location SIP determines which end points will participate in a call. SIP also has great breadth as it does more than just handle call setup. In this document, we'll focus on SIP's capabilities for VoIP, and how it sets up calls that then use RTP (the Real-time Transport Protocol) to actually send the voice data between phones. For example, in addition to simple telephone calls, SIP can also be used to set up video and audio multicast meetings, or instant messaging conferences. It was designed to be a general-purpose way to set up real-time multimedia sessions between groups of participants. SIP is a very flexible protocol that has great depth. You may have also heard of H.323, an ITU protocol with similar function. In the world of VoIP, SIP is a call setup protocol that operates at the application layer. Once the call has been setup, the phones enter a "data transfer" phase of the call using an entirely different family of protocols to actually move the voice packets between the two phones. The first phase is "call setup," and includes all of the details needed to get two telephones talking. In IP and traditional telephony, network engineers have always made a clear distinction between two different phases of a voice call.
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