VoIP Insider

What Is SIP Proxy Server?

SIP Proxy Server, or sometimes called SIP Proxy or SIP Server, acts like intermediary or facilitator among SIP devices assisting point-to-point communication from picking up to dropping of calls. However, SIP Proxy’s job is more than just a go-between.

To make it easier to understand, we can think of SIP Proxy Server as a phone call manager and security guard. Now, let’s learn what its duty is in each role.

Phone Call Manager

Assist call processing in the SIP network to set up calls between SIP devices and implement provider call-routing policies.
Control call rules such as arranging simultaneous or sequential phone rings and the time length before the call being transferred to another destination like voice mailbox or another extension.
In some circumstance, a SIP Proxy Server even interprets or rewrites certain parts of the request messages before forwarding them.

Security Guard

Grant or deny access to a network resource. A SIP Proxy Server first ensures that a user is the correct person and then based on that, it determines the user’s identity and authorizes the user to access various resources.
Enhance network security. For example: A SIP Proxy Server checks the caller’s identity and credit before the caller can connect to a destination that costs money such as PSTN network or foreign locations.

Two Types of SIP Proxy Servers

Based on how the SIP Proxy Servers handle the information they receive, there are two main types of SIP Proxy Server performing different functions to fit users’ needs in different situations.

Stateless Proxy Server

Stateless Proxy Servers simply pass along the messages they receive to the assigned destination. They don’t store any information of the call and transaction.

Since they don’t need to spend the extra time and energy to remember the SIP requests and messages that they have forwarded, Stateless Proxy Servers usually work very fast and efficient. Therefore, Stateless Proxy Server works well for network hubs.

Stateful Proxy Server

Stateful Proxy Servers, on the other hand, track everything from SIP requests, messages, to responses. This function allows you to restore the information you need in the future and also retransmit the request if the servers haven’t received a response they were waiting for.