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SIP Proxy Server

by OnSIP

Learn about SIP servers as defined by RFC 3261, including the SIP proxy, registrar, and redirect server.

A SIP proxy – sometimes also referred to as a SIP server or SIP proxy server – is mainly used by a SIP network to do call processing, but that isn’t its only function. In addition to managing the set-up of calls between SIP devices and controlling call routing, a SIP proxy may also perform other tasks such as authorization, network access control and even the handling of network security in some instances.

SIP Servers

While SIP user-agents (endpoints like VoIP phones) can communicate directly without any additional SIP intermediaries, from a practical standpoint, SIP servers are required to facilitate end-to-end communication when utilizing SIP as a public service.

RFC 3261 defines these server elements:

  1. A proxy, or proxy server, "is an intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to another entity "closer" to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a user is allowed to make a call). A proxy interprets, and, if necessary, rewrites specific parts of a request message before forwarding it."
  2. A registrar is a server that accepts REGISTER requests and places the information it receives in those requests into the location service for the domain it handles.
  3. A redirect server is a user agent server that generates 3xx responses to requests it receives, directing the client to contact an alternate set of URIs.The redirect server allows SIP Proxy Servers to direct SIP session invitations to external domains.

The RFC specifies: "It is an important concept that the distinction between types of SIP servers is logical, not physical."

OnSIP offers free SIP Proxy and Registrar Service

As a provider of strictly standards based hosted IP communications systems, we are in a great position to offer a suite of unique services at no cost to our customers.

  • Create an unlimited number of SIP addresses for free
  • Create users for free
  • Free call routing to SIP addresses
  • Free calling to any SIP address on the Internet
  • Free call routing to multiple SIP devices
  • Access to web 2.0 administrative portal to for easy account management
  • Free access to the OnSIP app, a softphone for Mac, Windows, Chrome, and Firefox
  • Use any SIP endpoint (deskphone, softphone, smartphone app, you name it)

All of this we are able to offer completely free. No strings attached, no hidden fees, no “Oh—I forget to mention that you have to buy this package for $ XX to actually use this.”

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Our Philosophy

Our company philosophy has always been focused on providing the best VoIP solution for our customer. We believe an open platform supporting the de facto standard protocol for establishing, conducting and ending VoIP calls on the internet, SIP, provides that solution. In our eyes, SIP is to VoIP as SMTP is to e-mail.

The analogy doesn’t stop there.

Hosted VoIP is evolving in the same fashion as as email. Not long ago email existed in semi-isolated and poorly interconnect islands (university email being a notable exception). There was a time when AOL users could only send email to other AOL users. It was not until 1993 that AOL decided to give all their users aol.com email addresses. By the late 1990’s open domain based SMTP email via the internet was widespread. VoIP is following a similar path.

Nowadays, e-mail is generally handled one of two ways. Groups can invest in their own private email servers (i.e Exchange) or choose to go with a hosted email provider like Google. Either way, the days when you could only send emails to people with your domain name are long gone.

VoIP has not had as much time to develop, but we anticipate that it will one day be as widely accepted as e-mail and we’re taking steps to prepare for that future: with OnSIP SIP service, users can make / receive VoIP calls to / from any SIP address on the Internet free of charge. OnSIP is currently home to thousands of SIP addresses. There are currently more than a billion email addresses on the internet. How many SIP addresses will there be?

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