[asterisk-users] what is a SIP invite, and who can issue them?

Joshua Colp jcolp at digium.com
Wed Jun 29 04:38:53 CDT 2016

Thufir wrote:
> I don't understand what a SIP invite is.  Certainly it's explained as:
> "This article explains the main fields included in a SIP INVITE, which
> is sent to set-up a VoIP call. A SIP INVITE message contains typically
> between 4 and 6 header entries with contact information inside them."
> http://www.3cx.com/blog/voip-howto/sip-invite-header-fields/
> The article enumerates the headers and explains them.  But what sends
> the invite?  Asterisk?  A soft-phone?
> I found sample config's to send an invite with Asterisk but no other
> method was given.  Can only Asterisk send an invite?  Why?  The article
> says that it's sent "to set-up a VoIP call," so presumably any
> reasonable soft-phone sends these invites as a normal process.
> That's all well and good, but how do send an actual invite and get a
> response?  This can only be done through Asterisk?

An INVITE is a request to set up a session, commonly referred to as a 
call. Anything supporting SIP to establish calls uses INVITE to do so. 
It's equivalent to picking up the phone and dialing a number.

In the case of chan_sip the sip.conf file is used to configure the 
settings for making the call, and extensions.conf is the logic which 
describes when to place an outgoing call when we get an incoming call.

Whether you must use Asterisk is really up to your experience, what you 
are trying to do, and whether other things can do what you need.

Joshua Colp
Digium, Inc. | Senior Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - US
Check us out at: www.digium.com & www.asterisk.org

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