Some Alternative Protocols
Although it almost seems like it, the TCP/IP protocols are not the only ones in existence and in the long run may not even be the
most successful. There are many protocols occupying significant niches, such as
Firewire
USB
Bluetooth
WiFi
Thre is active work continuing on many other protocols, even quite bizarre ones such as those for the "internet in space."
The focus in this book will be on the TCP/IP, but you should be aware of these other ones.
1.3 Networking
A network is a communications system for connecting end systems called hosts. The mechanisms of connection might be copper
wire, ethernet, fibre optic or wireless, but that won't concern us here. A local area network (LAN) connects computers that are
close together, typically belonging to a home, small organisation or part of a larger organisation.
A Wide Area Network (WAN) connects computers across a larger physical area, such as between cities. There are other types as
well, such as MANs (Metropolitan Area Network), PANs (Personal Are Networks) and even BANs (Body Are Network).
An internet is a connection of two or more distinct networks, typically LANs or WANs. An intranet is an internet with all networks
belonging to a single organisation.
There are significant differences between an internet and an intranet. Typically an intranet will be under a single administrative
control, which will impose a single set of coherent policies. An internet on the other hand will not be under the control of a single
body, and the controls exercised over different parts may not even be compatable.
A trivial example of such differences is that an intranet will often be restricted to computers by a small number of vendors running a
standardised version of a particular operating system. On the other hand, an internet will often have a smorgasborg of different
computers and operating systems.
The techniques of this book will be applicable to internets. They will also be valid for intranets, but there you will also find
specialised, non-portable systems.
And then there is the "mother" of all internets: The Internet. This is just a very, very large internet that connects us to Google, my
computer to your computer and so on.
1.4 Gateways
A gateway is a generic term for an entity used to connect two or more networks. A repeater operates at the physical level copies
the information from one subnet to another. A bridge operates at the data link layer level and copies frames between networks. A
router operates at the network level and not only moves information between networks but also decides on the route.
1.5 Packet encapsulation
The communication between layers in either the OSI or the TCP/IP stacks is done by sending packets of data from one layer to
the next, and then eventually across the network. Each layer has administrative information that it has to keep about its own layer.
It does this by adding header information to the packet it receives from the layer above, as the packet passes down. On the
receiving side, these headers are removed as the packet moves up.
For example, the TFP (Trivial File Transfer Protocol) moves files from one computer to another. It uses the UDP protocol on top of
the IP protocol, which may be sent over Ethernet. This looks like:
tiankj2013-09-29非常清楚，谢谢分享
