Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 3.3 Internet model

The Internet is characterized mainly by the following:

■■ Datagram-oriented instead of circuits.

■■ No single point of failure.

■■ End-to-end transparency for applications (see below for how this has

degraded).

■■ End-to-end control.

■■ End-to-end security.

■■ Complete control by users over the applications and selection of

services.

Internet standards are focused on protocols and not on interfaces, specifying

only how the devices communicate across the Net.

The end-to-end control design of the Internet cannot be always maintained,

because of the loss of Internet transparency from various developments such

as NAT and other devices. This problem is discussed in RFC 2775 [4]. Besides

NAT, for SIP, the so-called Session Border Controllers (SBC) or back-to-back

user agents (B2BUA) discussed in Chapter 10, “NAT and Firewall Traversal”

are also destroying Internet transparency.

The Internet Backbone Architecture

One of the key advantages of the Internet is complete independence of the

applications from the network, except for such performance metrics as band-

width and availability of service. Packet loss, delay, and jitter on the Internet

have improved to such a degree as to be negligible for voice in most parts of

the world [5]. This is shown in Figure 3.4.

USER USER

Elective

No single point of failure Server

All services enabled by

protocols: From ftp to web

User has control of all

applications and choice

of servers

Elective

Server

Internet

“Dumb Network”

R R

R R

44 Chapter 3