Learning Guide: SIP

In this Learning Guide, you'll find out what SIP is, how it works, which applications are SIP friendly, what compatibility and interoperability issues surround SIP and more.

SIP (Session Initiation Protocol)
In this learning guide, you'll find out what SIP is, how it works, which applications are SIP friendly, what compatibility and interoperability issues surround SIP and more.

TABLE OF CONTENTS 
   Introduction to SIP 
   How does SIP work? 
   Securing SIP
   SIP vs. H.323
   Interoperability
   SIP killer apps: Instant messaging and presence
   SIP working groups

Introduction to SIP 
 

SIP was created in 1996 using the KISS (Keep it Simple Stupid) principal. SIP provides a means to invite users to participate in multimedia multicast transmissions and runs on the Mbone, or multicast backbone. Since its introduction to corporate communications, SIP has energetically evolved, proving useful for many innovative tasks – so many in fact, it's a challenge to keep up with this so-called simple protocol. No worries, though. We've distilled this highly successful call control signaling protocol down to its essence. Take a SIP and enjoy!

How does SIP work? 
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In a SIP transaction, messages (either requests or responses) are sent between devices that use either UDP or TCP or other transport protocols. These requests or responses are made of up two parts: a set of headers and one or more message bodies.

When a SIP request (or INVITE) is sent, it is sent to the intended party's (or User Agent's) SIP address. SIP addresses are actually URLs and look very much like e-mail addresses. Before a message is delivered to the receiving UA, it is first sent to a proxy server, which routes and delivers the message to the receiving UA. The response from the receiving UA is then sent back to the initiating UA via proxy servers.

While SIP doesn't actually define what a session is, it does provide a description of the session in which the user is being invited. Find out more about how SIP works here:

Securing SIP 
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According to David Endler, the first chairman of the newly created VoIP Security Alliance , it's not VoIP protocols that are vulnerable, it is how vendors choose to implement them that negatively impacts security. After all, SIP was developed on the premise that it would run in trusted environments. SIP can actually enhance security through management of connection rights. Uncover how SIP can help, and possibly hinder, communication security here:

SIP vs. H.323 
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H.323 and SIP were born out of necessity: the demand from both the telecommunications community and the Internet community to have a protocol that met their respective needs. Fast forward a few years to today where the lines that once separated telecommunications and Internet communications have almost vanished. H.323 and SIP are still compared and do compete with each other for VoIP services, but SIP is winning race for the title of accepted VoIP standard. Learn more about the differences between H.323 and SIP and the advantages and disadvantages of each:

Interoperability 
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When the notion of SIP was first tossed around at the Internet Engineering Task Force IETF, interoperability topped the design agenda, second only to scalability. Now, after a few years of brewing and maturing, most industry experts will argue that SIP has become the basis of interoperable enterprise-level VoIP. Vendors are also showing their enthusiasm and support for SIP interoperability by regularly participating in SIPIT interoperability test events. Find out more about SIP interoperability issues here:

SIP killer apps: Instant messaging and presence 
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If a killer VoIP app exists, instant messaging and presence would be arguably be the two top contenders vying for the title. Through SIMPLE (SIP Instant Messaging and Presence Leveraging Extensions), buddy lists and "away" or "online" messages have become an integral part of our day-to-day business correspondence. Find out if these new trends will ease communication roadblocks or if they are more trouble than they are worth:

SIP working groups 
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This was first published in May 2005
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