Wide Web is that the various parts should be designed and implemented inde-
pendently of each other, and yet still interoperate flawlessly. This is experi-
enced daily by everyone using file transfer, e-mail, the web, or any other
standards-compliant applications over the Internet, on a truly global scale.Is XML the Presentation Layer of the
Internet Protocol Architecture?
As will be seen in Chapter 13, “Presence and Instant Messaging,” the new pre-
ferred data format for most SIP related protocols is XML [8]. XML has become
the design choice by default for Internet application data and seems to be the
equivalent of a presentation layer in the Internet protocol architecture.
By similar arguments, it appears that SIP and Real Time Streaming Protocol
(RTSP) are the de facto session layer for the Internet.Middle-Age Symptoms of the Internet
The classical model of the Internet protocol stack has an hourglass architecture
with the following three parts:- The base of the hourglass—Various Layer 2 link protocols such as Ether-
net, SONET, cable, and DSL link layers, as well as many types of radio
links. - The slim middle (waistline)—One single Layer 3 protocol (IP).
- The upper part of the hourglass—Several Layer 4 transport protocols (such
as User Datagram Protocol [UDP] and Transmission Control Protocol
[TCP]) and many application Layer 5 protocols (such as FTP, SMTP,
HTTP, and SIP).
The maturity of the Internet has led to some symptoms of middle-age [9],
such as the thickening of the waistline and other symptoms, including the
following:
■■ More functionality—The push for QoS requires more functionality from
the underlying Layer 2 networks resulting in additional complexity.
■■ Layer splitting—A new Layer 2.5 is emerging with the use of MPLS and
L2TP used for certain virtual private networks (VPN).
■■ Layer violations— Putting various functions into the IP layer, such as
packet inspection for security.50 Chapter 3