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When you participate in a VoIP call or video conference, you are probably using a SIP proxy server. SIP proxy servers handle all the data transactions between endpoints during SIP session setup, which allows call participants to connect for a VoIP call.

This article will cover SIP proxy servers in detail–discussing how they work and their types, benefits, and features.


What is SIP?

Session Initiation Protocol (SIP) is a signaling protocol that guides software systems to initiate, modify, and terminate real-time media sessions like voice calls, video conferencing, and messaging. SIP dictates the rules and systems that guide user devices through setting up, maintaining, and ultimately ending the media session. The most commonly used signaling protocol in VoIP, SIP works in conjunction with several other protocols that handle other aspects of the media session.

When a user wants to begin a media session, such as a VoIP call, their device and software application coordinate details with the other participants’ devices, which are called endpoints. This coordination process requires several steps–such as authenticating and locating other users, ringing their devices, and agreeing upon the codecs and media the session will feature. SIP provides guidelines that give software systems and endpoints an organized and consistent way to move through this setup process and get the session started. Once the session has begun, other protocols take over for SIP and handle the active media session.


How SIP Works: Servers, Requests, and Messages

Callers communicate by sending requests to servers–such as proxy servers, data centers, and registration servers–which forward the request to the recipient’s endpoints. In response, servers provide messages containing information and receipts. Each endpoint in a SIP exchange acts as both a caller and a recipient, respectively called clients and servers.

SIP accomplishes the following capabilities:

  • Registration: SIP authenticates user identities, locates each user’s IP address, and sends this information to each endpoint so that users can send media directly to each other
  • Invitation and ringing: SIP allows endpoints to invite other endpoints to the session, ringing each contact until they answer
  • Session details: SIP messages include Session Description Protocol (SDP) messages that coordinate details about the session–including which codecs it will use, which types of media the session will stream, and formatting details
  • Modification: SIP requests and messages can modify the session by adding or removing users and media streams
  • Messaging: SIP messages deliver text and chat during real-time media sessions

SIP Protocol Messaging

What is a SIP Proxy Server?

A SIP proxy server is an intermediary program that forwards, requests, and receives data from SIP endpoints. During a SIP session, each endpoint is assigned a proxy server that handles SIP data transactions on the endpoint’s behalf–registering and locating other users, inviting participants, connecting and starting the call, and modifying the session when needed. Proxy servers not only interact with each other on behalf of users, but they interact with other servers–like location and registrar servers–which are essential in establishing a VoIP call.

Each SIP session utilizes SIP proxy servers, which are assigned by the provider to each endpoint during a call. SIP proxies authenticate users, confirm each participant’s IP address, add new users to a call, and ensure that user requests and data packets reach the intended destination. Proxy servers also track packet delivery, which provides billing receipts and generates reliable Quality of Service (QoS) reports for connection troubleshooting.


How a SIP Proxy Server Works

SIP proxy servers forward and receive requests and data messages to user endpoints or other servers. Each end user in a SIP session typically has their own proxy server, which connects them to VoIP data servers and other end users’ proxies. Put simply, SIP proxy servers act as middlemen that carry out the data exchange between endpoints during a SIP session.

SIP media sessions require that all involved endpoints exchange data with each other, which they achieve through backend requests and messages. However, users’ devices don’t interact directly with each other. Instead, each endpoint uses a SIP proxy server to handle all data routing for the SIP session. Proxy servers send and receive data with SIP servers, registration servers, data centers, and other proxy servers.

Here’s a more detailed breakdown of how proxy servers work:

  1. Caller makes a request on a SIP app: When an end user wants to take a SIP action, such as making a call on a VoIP app, their device–or endpoint–makes a backend request and labels it with its intended destination. The SIP app assigns a proxy server to any endpoint involved in a VoIP call.
  2. Caller’s proxy server confirms the request’s destination: The caller’s proxy server receives the request and verifies the data packet’s destination. To do this, the proxy server makes a request to another server–such as a registrar, redirect, or location server–to confirm the destination’s IP address. If the destination is invalid, stateful proxy servers can modify the packet header so it reaches the intended endpoint.
  3. Caller’s proxy server forwards the request to the destination: Once the SIP proxy confirms the destination’s identity and location, it forwards the packet to the receiving endpoint’s proxy server
  4. Caller’s proxy sends a receipt: When a proxy server receives or sends a request, it sends a transaction receipt back to the initial client. These receipts track information about the call state, quality, and data delivery.
  5. Recipient’s server responds with a message: When the recipient’s proxy server receives the request from the caller’s proxy, the recipient’s server responds with a message and data.
  6. Caller’s proxy server receives the message and forwards it to the caller’s endpoint: The caller’s proxy server receives the data answering its request, and it forwards this data back to the original caller’s endpoint
  7. Process repeats throughout the call: This process occurs in nanoseconds and repeats whenever any SIP user makes a request–invite a new user, send a message, route a VoIP call, end the call, and more. Users frequently switch between the role of caller and recipient, also called client and server, depending on whether they’re requesting or providing data.

SIP Proxy Server Dialogue

What Does a SIP Proxy Server Do?

SIP proxy servers send and receive data messages that enable all SIP activities, which initiate and modify real-time media sessions. This means that proxy servers locate each call participant’s IP address, send invites, transmit essential call formatting details, connect the call, deliver text messages during the call, route calls when necessary, and ultimately terminate the call when it’s over.


Types of SIP Proxy Servers

There are two main types of SIP proxy servers, stateless and stateful servers, which vary by speed and function. Redirect servers are not proxy servers but are worth mentioning because they work closely with proxies.


Stateless SIP Proxy Server

Stateless SIP proxy servers rapidly forward data packets between endpoints without recording the state of the transaction or packet delivery. Stateless proxies simply forward requests to their destination, without checking the packet header, modifying the message, or providing any receipts back to the client who sent the request.

Stateless proxies demand less memory and CPU than stateful proxies, and they can quickly transmit many data requests. This makes stateless proxies effective for high-volume local networks, or interactions that require large amounts of concurrent data exchange.


Stateful SIP Proxy Server

A stateful server scans and records information about each request, data packet, and message it sends. It uses this information about the transaction’s “state” to learn and store information about users, sessions, and servers. Stateful servers can forward packets to multiple destinations and learn the most reliable servers and routes, providing quicker and better data for subsequent actions.

Stateful proxies log call information, enabling users to revisit transaction histories in case there’s an issue with packet delivery, or a call quality issue like latency or jitter. In case one user drops from the call, stateful proxy servers can re-connect that user back to the original session without having to create a new one. Stateful proxy servers offer reliable data transmission and useful records of transaction states. However, they are more expensive than stateless servers because stateful proxies demand more memory and CPU.


Redirect Server

Redirect servers receive all proxy server requests and inform the proxy if the destination’s IP address has changed. The proxy server then uses this information to adjust the packet header and send the request to the correct destination.

A redirect server is not a proxy server itself but processes each proxy server request.


Features of a SIP Proxy Server

SIP proxy servers support the following features and capabilities:

  • Call and Packet Routing: Proxy servers ensure that backend requests, audio data, and media calls reach the correct endpoint quickly. They first authorize the appropriate IP address to send data, confirm its location, and begin the back-and-forth data transaction between client and server. This connection is critical for call routing, which connects callers to the right call recipient.
  • Authentication: To establish a VoIP connection, SIP proxies send requests to a SIP registrar server to retrieve each call recipient’s IP address and verify the endpoint’s identity. This provides security for the call and ensures that callers connect to the right endpoints.
  • Authorization: Stateful SIP proxies verify that each data packet travels to the right destination, and correct the packet or destination if something is wrong. Proxy servers check each data packet’s header and then confirm with the redirect server to ensure the packet’s intended destination is still correct. If the recipient’s IP address has changed or the packet has an incorrect header, the stateful proxy can modify the header or destination to route data and requests to the correct call recipient.
  • Modify SIP Messages: Stateful SIP proxies can modify packet headers and addresses to navigate requests and messages to their endpoint. Some PBX systems don’t use compatible address formatting, in which case proxy servers can alter data packets’ headers and addresses to allow the messages to reach the intended user.
  • Security: SIP proxy servers act as a gateway, or firewall, between a local network and the internet. Proxies examine incoming data transaction headers to ensure they’re from the appropriate client, before allowing the data to reach the local network and endpoint. SIP proxies provide security by protecting the local network from data from unverified sources.
  • Media Session Setup: SIP proxy servers send data requests and messages to each other to establish the media session between peers. Once SIP proxies have set up the session, real-time messaging protocol (RTP) can facilitate direct peer-to-peer (P2P) media streaming between call parties.
  • Load Balancing: SIP proxy servers manage inbound and outbound data for endpoints within a local network. Proxy servers can distribute inbound requests and data demands evenly among multiple servers, balancing the bandwidth load so that each user retains better call quality.


Benefits of Using a SIP Proxy

All SIP interactions use proxy servers to intermediate communications between endpoints. While your local network may utilize a personal proxy server, the SIP provider will also assign a proxy server to manage SIP transactions for your device.

SIP proxy servers offer the following benefits:

  • Better call quality: Stateful SIP proxy servers check data packet headers and verify endpoint P addresses to ensure that packets reach the right destination. SIP proxies can modify packets to help them reach the intended endpoint, delivering more data and protecting calls from packet loss, jitter, and latency.
  • Quick call rejoining: Stateful SIP proxies verify and record each user’s information for a call. If one user drops out of a phone call or video meeting, they can quickly rejoin the same call. The stateful proxy’s memory allows this to happen without having to re-initiate the connection.
  • Quality of Service troubleshooting: Stateful SIP proxies track the transaction state and delivery status for data during a media session. This data informs Quality of Service (QoS) reports that provide feedback on connection quality, enabling IT teams to troubleshoot poor connections or malfunctioning devices.
  • Security: SIP proxies interact with location and registrar servers to authenticate user IP addresses, which ensures that only verified and invited users are allowed on the call. If a malicious user does manage to access the call, proxy servers provide a layer of anonymity that protects a user’s actual IP address from the attacker.
  • Call routing: A SIP proxy server quickly locates and authenticates all users involved with a call, repeating this process when a user transfers a call or your business phone system routes an inbound call. Therefore, SIP proxies enable quick and reliable call routing.
  • Multichannel communications: SIP proxy servers initiate multiple types of UCaaS communications, including VoIP, video conferencing, and chat messaging


Why SIP is Important in VoIP Communications

Session Initiation Protocol is responsible for setting up, initiating, modifying and terminating VoIP calls. While other protocols like H.323 offer similar functionality, SIP is the most popular signaling protocol for VoIP call initiation because it’s reliable and easy for developers to use. SIP is responsible for authenticating users, inviting call participants, coordinating the types of media and formatting, and adding or removing users from the call.

Most cloud phone systems use SIP, which works alongside other application-layer protocols like Real-Time Messaging Protocol (RTP) and Session Description Protocol.