TCP/IP Protocols
2.2.1 TCP/IP Protocols
In this part the infrastructures of TCP/IP are briefly introduced and made a lead-in to the security problems on the Internet. TCP/IP has become popular partly because it can be used in various channels and protocols. To be exact, TCP/IP is a set of protocols including TCP and IP, UDP (User Datagram Protocol), ICMP (Internet Control Message Protocol) and some other protocols, as illustrated in Fig. 2.2.
OSI Model
Layer
The TCP/IP model
No.
(RFC 1122)
Application layer
Presentation layer 6 Application layer
Session layer
Transport layer
4 Transport layer
Network layer
3 Internet layer
Data link layer
Link layer
Physical layer
Figure 2.2 TCP/IP infrastructure Different from OSI reference model, TCP/IP model focuses more on the data
transmission between interconnected equipments, rather than strict function classification. It has achieved this goal by explaining the importance of the function architecture, which allows wide room for designers to execute the protocols. Thus OSI is more suitable to explain the Internet communication mechanisms, while TCP/IP has become the market standard of Internet protocols. TCP/IP is more flexible than the OSI model, as illustrated in Fig. 2.2.
TCP/IP enables the network interconnection between different platforms. For instance, a Windows NT network can include workstations of UNIX and Macintosh, or even UNIX or Macintosh network. The features of TCP/IP are as follows:
(1) a sound recovery mechanism (2) entering the network without interrupting the existing service (3) efficient error processing (4) platform independence (5) low cost The original aim of TCP/IP is related to national defense, so the features listed
above are actually the design requirements of TCP/IP. “A sound recovery mechanism” is based on the following idea. When the network is being intruded or destroyed, the rest part of it will still be able to work. The same consideration
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applies to new service addition mechanism. “Efficient error processing” is based on the following occasion: if the message gets lost at a certain router, there should
be a mechanism to enable it to reach its destination via another router. Platform independence means the network and workstations can be any combination of Windows, UNIX, Macintosh and any other platforms. TCP/IP is highly efficient because of its low cost. Performance is the key to any network. No other protocols can match TCP/IP in speed and simplicity.
The data flow of the network is co-managed by TCP and IP. IP constantly puts the messages on the Internet, and TCP takes charge in ensuring the arrival of the messages. TCP does the following jobs:
(1) handshake process (2) message management (3) flow management (4) error checking and processing TCP/IP provides an environment for the operations mentioned above. TCP/IP
is composed of four layers, different from the seven-layer model of OSI. These four layers include:
(1) Application layer (2) Transport layer (3) Network layer (4) Link layer TCP/IP differs from OSI primarily in the aspects below: the transport layer
does not guarantee the transport of any moment. TCP/IP provides UDP for users, which is an easier protocol. In UDP, all layers in the TCP/IP execute specific jobs or applications.
(1) Application layer The application layer includes SMTP, FTP, NFS, NIS, LPD, Telnet and
Remote Login, which are familiar to most Internet users. (2) Transport layer The transport layer includes UDP and TCP. UDP rarely checks, and TCP
guarantees the transport. (3) Network layer The network layer is composed of ICMP, IP, IGMP, RIP, OSPF and EGP used
for routing. Users are unnecessarily concerned about these, since they are very fundamental.
(4) Link layer The link layer includes ARP and RARP, in charge of message transport.