Page 216 - Asterisk™: The Future of Telephony

single datagram header, that will lower the overhead associated with individual
channels. This helps to lower latency and reduce the processing power and bandwidth
required, allowing the protocol to scale much more easily with a large number of active
channels between endpoints. If you have a large quantity of IP calls to pass between
two endpoints, you should take a close look at IAX trunking.

Future
Since IAX was optimized for voice, it has received some criticism for not better sup-
porting video—but in fact, IAX holds the potential to carry pretty much any media
stream desired. Because it is an open protocol, future media types are certain to be
incorporated as the community desires them.

Security considerations
IAX includes the ability to authenticate in three ways: plain text, MD5 hashing, and
RSA key exchange. This, of course, does nothing to encrypt the media path or headers
between endpoints. Many solutions include using a Virtual Private Network (VPN)
appliance or software to encrypt the stream in another layer of technology, which re-
quires the endpoints to pre-establish a method of having these tunnels configured and
operational. However, IAX is now also able to encrypt the streams between endpoints
with dynamic key exchange at call setup (using the configuration option encryp
tion=aes128), allowing the use of automatic key rollover.

IAX and NAT
The IAX2 protocol was deliberately designed to work from behind devices performing
NAT. The use of a single UDP port for both signaling and transmission of media also
keeps the number of holes required in your firewall to a minimum. These considerations
have helped make IAX one of the easiest protocols (if not the easiest) to implement in
secure networks.

SIP

The Session Initiation Protocol (SIP) has taken the telecommunications industry by
storm. SIP has pretty much dethroned the once-mighty H.323 as the VoIP protocol of
choice—certainly at the endpoints of the network. The premise of SIP is that each end
of a connection is a peer; the protocol negotiates capabilities between them. What
makes SIP compelling is that it is a relatively simple protocol, with a syntax similar to
that of other familiar protocols such as HTTP and SMTP. SIP is supported in Asterisk
with the chan_sip.so module. ‖

History

SIP was originally submitted to the Internet Engineering Task Force (IETF) in February
of 1996 as “draft-ietf-mmusic-sip-00.” The initial draft looked nothing like the SIP we

188 | Chapter 8: Protocols for VoIP