Page 217 - Asterisk™: The Future of Telephony

SIP signaling

RTP media

Figure 8-1. The SIP trapezoid
know today and contained only a single request type: a call setup request. In March of
1999, after 11 revisions, SIP RFC 2543 was born.
At first, SIP was all but ignored, as H.323 was considered the protocol of choice for
VoIP transport negotiation. However, as the buzz grew, SIP began to gain popularity,
and while there may be a lot of different factors that accelerated its growth, we’d like
to think that a large part of its success is due to its freely available specification.
SIP is an application-layer signaling protocol that uses the well-known port 5060 for
communications. SIP can be transported with either the UDP or TCP transport-layer
protocols. Asterisk does not currently have a TCP implementation for transporting SIP
messages, but it is possible that future versions may support it (and patches to the code
base are gladly accepted). SIP is used to “establish, modify, and terminate multimedia
sessions such as Internet telephony calls.” #

SIP does not transport media between endpoints.
RTP is used to transmit media (i.e., voice) between endpoints. RTP uses high-num-
bered, unprivileged ports in Asterisk (10,000 through 20,000, by default).
A common topology to illustrate SIP and RTP, commonly referred to as the “SIP tra-
pezoid,” is shown in Figure 8-1. When Alice wants to call Bob, Alice’s phone contacts
her proxy server, and the proxy tries to find Bob (often connecting through his proxy).
Once the phones have started the call, they communicate directly with each other (if
possible), so that the data doesn’t have to tie up the resources of the proxy.
SIP was not the first, and is not the only, VoIP protocol in use today (others include H.
323, MGCP, IAX, and so on), but currently it seems to have the most momentum with
hardware vendors. The advantages of the SIP protocol lie in its wide acceptance and
architectural flexibility (and, we used to say, simplicity!).

‖ Having just called SIP simple, it should be noted that it is by no means lightweight. It has been said that if
one were to read all of the IETF RFCs that are relevant to SIP, one would have more than 3,000 pages of
reading to do. SIP is quickly earning a reputation for being far too bloated, but that does nothing to lessen
its popularity.
# RFC 3261, SIP: Session Initiation Protocol, p. 9, Section 2.

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