What Is a SIP Proxy? How Does a SIP Server Work?

October 4, 2019 8 min read

Gaetano DiNardi

Gaetano DiNardi

SIP Proxy Server - Featured Image

If you’ve used VoIP before, you may have heard the terms SIP and SIP proxy. But what do these terms mean?
SIP stands for Session Initiation Protocol. It brings together many of the ‘building blocks’ needed to make phone calls via an internet connection (aka VoIP calls). Through SIP, a connection forms between endpoints.
This connection enables the transmission of voice and video data. Everyone connected can hear and take part in the same call.
A SIP proxy receives and processes SIP requests from a redirect server or software. (Like when you type in the domain name of a web page or want to open a file).
This article will explain precisely how a SIP proxy works, along with:

Components of a SIP Proxy Server

SIP Trunking in a VoIP Business Phone System
These two technologies are the heart of the system, responsible for call routing as well as a bunch of other options. An example could be alerting operators that a caller is on hold and redirecting them according to call handling rules.
The SIP proxy server doesn’t just allow you to send and receive voice calls using VoIP. It does the same for instant messaging on mobile platforms like Android, video conferencing, and load balancing.
SIP: Video, Messaging, and Voice - Venn Diagram - SIP Proxy
You could describe it as a ‘Highway Code.’ It provides the rules that make sure traffic flows freely through and between networks.
The result is seamless, unified communications.
Related: PRI vs SIP – Which Is Right for Your Business?

Ensuring Common Standards

Development and oversight of the common standards needed for SIP go through the Internet Engineering Task Force (IETF). This agency has helped ensure that all communication using voice, video, and other media is of the highest quality.
This has helped make VoIP calling popular, not only in terms of quality, but also price. Like sending an email, voice and video calls between SIP users cost next to nothing, no matter where you are.
Unlike a phone number, there is also no charge for getting a SIP address.
Most SIP addresses connect to a unique phone number. That’s because it’s a numbering system that is more readily accepted and understood (a hangover from the PSTN days).
So each user on a SIP network can have a phone number that lets them make and receive direct calls.
Because it is free and easily scaled, SIP is becoming the ideal solution for businesses looking to grow.
SIP Statistic: 90% of SIP users are satisfied with its audio quality.
It also means businesses can use these systems in a package. It’s a simple matter when using a hosted cloud PBX system, like the one offered by us here at Nextiva.
That means your business or organization doesn’t have to worry about maintaining its PBX – plus you save on the hardware and maintenance costs.
Instead, the phone on your team’s desks plugs into a router. That router connects to an IP-PBX server, which lets you access all the system features.

VoIP Features

VoIP features include, but are not limited to:

Head to the Nextiva VoIP features page for the full list of VoIP features.

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What are SIP and VoIP? How Are They Different?

While VoIP is the most common SIP technology, SIP extends beyond office phones. It’s a part of all digital media. SIP is capable of handling everything from instant messaging to video and other multimedia.
SIP opens up a new world of possibilities for sending different types of media with your internet connection from port numbers to video conferencing.
SIP messages bring you the benefits of VoIP. This includes the ability to communicate through more than just voice calls.
You can make SIP requests more varied and much more closely tailored to the kinds of messages your customer base prefers to receive.
Related: What is SIP Trunking and How Does it Work?

What Does a SIP Server Do?

What does a SIP Server Do? SIP Network Diagram (SIP Proxy)
A SIP server is an important part of any PBX (private branch exchange) network. It is a facilitator of all elements which make up communications between two or more endpoints.
It’s like a signalman on the railway who makes sure that all the points are set in the right order. If the signals are at ‘green’ along the route of the message, it can transmit.
Once a communication session ends, the SIP server ensures that the line is clear and ready for the next call or message.
It’s like when someone calls your phone number (signaling that they want to start a conversation. This process creates a connection between two networks when two or more people want to communicate.
The SIP proxy server is also known as the ‘decoder’ which ensures that any message, in whatever format, is received. Once the connection establishes, the server takes action (i.e., placing a caller on hold or transferring them to another extension).
Finally, once a call is complete, the SIP server ensures that the session ends correctly.
Related: 12 Surprising Reasons Why You Need SIP Trunking

How Messages Are Sent

For large amounts of data to send almost instantaneously, each message divides into several packets.
Each packet contains a portion of the original message. Like a 1,000-piece puzzle, once these packets reach their intended destination, they are re-assembled in their proper, understandable order.
In preparation for this packet transfer, a SIP server will first send signaling packets. Once the receiving network has acknowledged these signaling packets, the system knows it can proceed to transfer the intended message.

How SIP Servers Help Keep a VoIP Network Secure

SIP proxies start by checking a caller’s identity to keep messages safe before and during transfer. In VoIP, this information goes through the IP address.
Ensuring that the data goes to a safe IP is only the beginning.
Nowadays, each SIP server is also required to use a form of Message Digest (MD) Authentication.
This authentication process is now in its fifth generation and currently goes by the acronym MD5.
It works by a SIP proxy server challenging the identity of a SIP user agent and converting a message of any length into a random alphanumeric code.
The diagram here explains how MD5 generates its codes – a process known as ‘hashing.’
At the bottom, you can see examples of its unique output to code and authenticate every message:
MD5 Hashing Process Example
As you can see, MD5 adds a layer of VoIP security by encrypting bits of data.
If networks need additional security beyond MD5, they can then opt for what’s called a Secure Multipurpose Internet Mail Extension (S/MIME).
An S/MIME goes beyond the industry standard by encrypting all data in each message.

Technical Aspects of SIP Proxies: Modes of Operation

A SIP proxy server will operate in one of two modes (depending on its level of sophistication):

  • Stateless
  • Stateful

1. Stateless – What is a Stateless SIP Proxy?

A basic SIP server is ‘stateless.’
Stateless SIP Proxies receive and transmit information needed to do their job, but don’t keep any record of it. Once you’ve connected to make a call or to the person receiving your message, the stateless SIP server’s job is complete.
Diagram of a SIP Stateless SIP Proxy Servers
Because of the simplicity of a stateless proxy server, it is easy for businesses to scale and upgrade.
The server also operates at a slightly faster speed because it only has to worry about a limited number of functions.
Though the difference in speed is often negligible, this is important to note if you find your current system struggling to keep up with your business needs.
Related: What Is Session Initiation Protocol (SIP) & How Does It Work?

2. Stateful – What is a Stateful SIP Proxy?

A more sophisticated form of a SIP server is also available, known as a ‘stateful’ server.
This not only transmits information but stores it to access later. For example, if your first attempt to connect is unsuccessful, it can hold onto the call or message and keep re-trying to establish the connection.
Because it remembers information about each action, a stateful SIP server can also pick up a request message again and re-route it through another part of a network.
Diagram of a Stateful SIP Proxy Servers
A common example of this is having ‘day’ and ‘night’ modes on a phone system. Stateful SIP proxies make it so that businesses can route calls to different devices, departments, or offices depending on the time of day.
Someone could call at 2 pm and speak with a support representative in California. Then when they call again at 10 pm, they’ll be routed to someone in Ukraine.

Key Benefits of a SIP Proxy

While its main job is to keep a telecoms network running smoothly, a SIP proxy server offers several key benefits, beyond those outlined above

1) Keeping Your Network Secure

A SIP proxy server will stop hackers from hijacking a SIP proxy server and getting access to free voice calls or other communications.
A proxy server will disconnect anyone who tries to use SIP calls without enough credit to pay for them or authorization to use the service. Or it can check that a caller has the credit needed, and, if not, disconnect them.
It is a gift to network security.

2) Easy Call Forwarding

SIP proxies can also forward inbound calls to several SIP devices, enabling them to ring on any number of SIP phones.
In a busy call center, for example, this can be a boon. It ensures a quick response to inbound calls. It can also send a call to a list of phones in sequence and ring each one until it’s answered.
Using a stateful SIP proxy, you can save all your calls, messages, and files. You can access or resend them later if they don’t get through the first time.
You also maintain a record of all calls made. You can also use IP PBX to allow all users to share several external phone lines.
If having a full record of VoIP conversations is important for your business, you should get a stateful SIP server.
Related: 12 Eye-Opening SIP Trunking Benefits You Probably Didn’t Know

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In Conclusion

A SIP proxy server keeps calls flowing smoothly, ensuring that your VoIP call traffic operates efficiently. It enables you to cut costs and easily scale as your business continues to grow.
If you aren’t already taking advantage of VoIP, get in touch! Whether you are just getting started or already have offices across the globe, we can help you determine which solution will fit your business needs.
Chat with us on our website or visit our VoIP solutions page to learn more about Nextiva’s business phone service.

Gaetano DiNardi


Gaetano DiNardi

Gaetano DiNardi led demand generation at Nextiva and has a track record of success working with brands like Major League Baseball, Pipedrive, Sales Hacker, and Outreach.io. Outside of marketing, Gaetano is an accomplished music producer and songwriter. He’s worked with major artists like Fat Joe, Shaggy, and loves making music to stay turbocharged.

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