What is a SIP Proxy Server?

A Session Initiation Protocol (SIP) proxy server manages and directs all SIP-based communication packets, like telephony, allowing users to communicate over the network.

A SIP proxy server is a robust system for authenticating, encrypting, and delivering voice, video, and messaging communication.

SIP servers are a critical component of the Private Branch Exchange (PBX), enabling communication between users regardless of their location. We’ll explain more about a SIP proxy server’s role and why it is essential for your business.

Read on to learn more about SIP, SIP proxy servers, and
other key insights to help you implement best practices for your business communications

How Does a SIP Server Work?

SIP is an application-layer control protocol that enables the creation, modification, and termination of a communication session for VoIP, multimedia distribution, and multimedia conferences.

The SIP proxy server works as a middleman for all SIP communication. It transmits communication and ends communication through stateless and stateful servers:

  • Stateless Server: This type of SIP server passes SIP communication packets without storing information. It will route appropriately based on request but then immediately forget what was requested.
  • Stateful Server: This type of SIP server stores and tracks all requests and responses for monitoring, reporting, and troubleshooting.

Essentially, the proxy server’s main functionality is to create and terminate multimedia communications by:

  • Determining the user’s location and system of communication
  • Evaluating user availability for engaging in communication
  • Determining the media and its parameters to be used for communication
  • Setting up the session between the users
  • Managing the communications session

While a SIP proxy server is essential to a business’ communication network, it is just one component of a larger communication system, like a PBX.

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Different Types of SIP Proxy Servers

While some communication systems may only have one proxy server, larger systems may utilize specific SIP proxy servers to handle particular functions. For large enterprises, this is a must to handle all the traffic on the communications network. These types of servers are:

  • Registrar Server: This proxy server handles all user authentication and can route all traffic to a single IP address or to a dynamic domain address that can change.
  • Redirect Server: This proxy server works in conjunction with a registrar server. It receives session requests from the registrar server database, locates the target user, and routes the session to the destination.
  • Location Server: The location server works with the redirect server to locate the user based on the redirect request.

IETF Standards

The Internet Engineering Task Force (IETF) develops and oversees common standards and regulations to ensure SIP communication complies with regulation quality. They also have a charter group tasked with developing and improving the SIP protocol and providing valuable information to the Internet community about the technology.

Benefits of a SIP Proxy

Having a SIP proxy server has several key benefits for an organization:

  • Network Security: Like a firewall, a SIP proxy server can detect and keep hackers from accessing free communication services or hijacking network communication. The server will disconnect unauthorized users and authenticate users that are allowed to communicate.
  • Simplified Call Forwarding: This is particularly beneficial for larger enterprises because the server can manage all the extensions for each user and efficiently manage and forward the inbound calls.
  • Communication Tracking: A stateful proxy server can monitor, save, and report on all communication sessions.
  • Smooth Communication Management: The proxy server leverages a robust cache system to manage, sort, and direct communication traffic creating a smooth communication system.

Are you looking to add a SIP solution to your communications network? Learn more about Momentum’ SIP Trunking solution.

SIP vs. VoIP: How They’re Different

VoIP is a popular SIP technology, but the main functionality of VoIP deals primarily with voice communication over a network. This can include:

  • Call logging/parking/forwarding
  • Automated greetings
  • Advanced call analytics

VoIP is an excellent solution for enterprises because it can:

  • Reduce costs
  • Increase accessibility (especially for international communication)
  • Scale
  • Offer more flexibility
  • Supports multitasking

What differentiates a SIP proxy server from VoIP is that it can extend beyond phone communication. SIP enables all digital media, including instant messaging, video, and other multimedia mediums. SIP allows you to vary your communication based on your customers’ preferences.

Why SIP Servers Are Integral to VoIP Network Security


Your communication network needs a SIP server as an integral part of the system’s network security to enable VoIP. To protect VoIP, the SIP proxy servers authenticate the caller’s IP address using Digest Authentication, which is a form of the cryptographic hash algorithm, Message Digest Algorithm 5 (MD5).

As an extra authentication measure, MD5 challenges the SIP user agent’s identity and encrypts the message into random alphanumeric code, adding another layer of VoIP security. For enterprises that need additional message security, they can add Secure Multipurpose Internet Mail Extension (S/MIME).

Disadvantages of a SIP Proxy Server

SIP proxy servers are critical, but they do pose a few disadvantages:

  • The service provider can view SIP proxy server data: A couple of free proxy server solutions are available, but this poses a security threat for sensitive data. You’re better off having a dedicated proxy server or paying for the proxy server service because then it will have security checks in place from the service provider.
  • The proxy server using TLS as a transport protocol can reduce performance: While TLS is a robust transport protocol for VoIP security, it can reduce performance by a factor of 17 compared to SIP-over-UDP. If security is of utmost importance, you’ll need to weigh it against performance needs.

A SIP Proxy Server is a Critical Component of Your Communication Network

Overall a SIP proxy server provides essential security, management, and performance for your communication network for VoIP, multimedia, and messaging. For enterprises that want to monitor and track communication, a SIP stateful proxy server is critical for reporting.

Momentum Communication is a global leader in cost-effective communication services and solutions for SMBs and SMEs. We offer secure, seamless cloud PBX, SIP trunking, and direct routing services to streamline your business’s communication strategy.

Need an intuitive, streamlined communications network? Contact Momentum to customize a solution for your business.

Related Link: FAQ: SIP Trunking Service

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