[Asterisk-Users] Re: Advantage of IAX2 to SIP?

[Randy Bush]

> Some - few - providers are using IAX2 as a protocol. Most are using SIP. 
> I know that there are advantages of IAX2 regarding multiple connections. 
> But beside this I'm asking myself (and you all) why I should prefer IAX2 
> when my SIP connection is working.

some discussion of this a few months back produced the following
excellent precis

From: Mark Spencer <markster at digium.com>
To: <asterisk-users at lists.digium.com>
Subject: Re: Re: iax or sip
Date: Mon, 5 Jul 2004 18:59:52 -0500 (CDT)

Okay, setting aside conspiracy theories, trolling, flaming, etc, let me
summarize some differences between SIP and IAX, and it might help you
make a decision about what is best for you.

1) IAX is more efficient on the wire than RTP for *any* number of calls,
*any* codec.  The benefit is anywhere from 2.4k for a single call to
approximately trippling the number of calls per megabit for G.729 when
measured to the MAC level when running trunk mode.

2) IAX is information-element encoded rather than ASCII encoded.  This
makes implementations substantially simpler and more robust to buffer
overrun attacks since absolutely no text parsing or interpretation is
required.  The IAXy runs its entire IP stack, IAX stack, TDM interface,
echo canceller, and callerid generation in 4k of heap and stack and 64k of
flash.  Clearly this demonstrates the implementation efficiency of its
design.  The size of IAX signalling packets is phenomenally smaller than
those of SIP, but that is generally not a concern except with large
numbers of clients frequently registering.  Generally speaking, IAX2 is
more efficient in its encoding, decoding and verifying information, and it
would be extremely difficult for an author of an IAX implementation to
somehow be incompatible with another implementation since so little is
left to interpretation.

3) IAX has a very clear layer2 and layer3 separation, meaning that both
signalling and audio have defined states, are robustly transmitted in a
consistant fashion, and that when one end of the call abruptly disappears,
the call WILL terminate in a timely fashion, even if no more signalling
and/or audio is received.  SIP does not have such a mechanism, and its
reliability from a signalling perspective is obviously very poor and
clumsy requiring additional standards beyond the core RF3261.

4) IAX's unified signalling and audio paths permit it to transparently
navigate NAT's and provide a firewal administrator only a *single* port to
have to open to permit its use.  It requires an IAX client to know
absolutely nothing about the network that it is on to operate.  More
clearly stated, there is *never* a situation that can be created with a
firewall in which IAX can complete a call and not be able to pass audio
(except of course if there was insufficient bandwidth).

5) IAX's authenticated transfer system allows you to transfer audio and
call control off a server-in-the-middle in a robust fashion such that if
the two endpoints cannot see one another for any reason, the call
continues through the central server.

6) IAX clearly separates Caller*ID from the authentication mechanism of
the user.  SIP does not have a clear method to do this unless
Remote-Party-ID is used.

7) SIP is an IETF standard.  While there is some fledgling documentation
courtesy Frank Miller, IAX is not a published standard at this time.

8) IAX allows an endpoint to check the validity of a phone number to know
whether the number is complete, may be complete, or is complete but could
be longer.  There is no way to completely support this in SIP.

9) IAX always sends DTMF out of band so there is never any confusion about
what method is used.

10) IAX support transmission of language and context, which are useful in
an Asterisk environment.  That's pretty much all that comes to mind at the
moment.

Mark