1.2 TCP/IP

With a few exceptions, the TCP/IP family does not deal with the physical or link layers. In practice, Internet protocols often use protocols that adhere to the ISO OSI standards for the physical and link layers.

What is the correlation between the ISO OSI protocols and TCP/IP? Each group of protocols has its definition of its own layers as well as the protocols used on these layers. Generally speaking, ISO OSI protocols and TCP/IP are incompatible. In practice, ISO OSI-compliant communication appliances need to be used for transferring IP datagrams, or on the other hand, services based on ISO OSI need to be provided via the Internet.

1.2.1 Internet Protocol

Internet Protocol (IP) basically corresponds to the network layer. IP is used for transmitting IP datagrams between remote computers. Each IP datagram header contains the destination address, which is the complete routing information used for delivering the IP datagram to its destination. Therefore, the network can only transmit each datagram individually. IP datagrams of one session can be transmitted through different paths and can thus be received by the destination in a different order than they were sent.

Each network interface on the large Internet network has one or more IP address that is unique worldwide. (One network interface can have several IP addresses, but one IP address cannot be used by many network interfaces.) The Internet is composed of individual networks that are interconnected via routers. Routers are also referred to as gateways in old literature.

1.2.2 TCP and UDP

TCP and UDP correspond to the transportation layer. TCP transports data using TCP segments that are addressed to individual applications. UDP transports data using UDP datagrams.

TCP and UDP arrange a connection between applications that run on remote computers. TCP and UDP can also facilitate communication between processes running on the same computer, but this is not very interesting for our purposes.

The difference between TCP and UDP is that TCP is a connection-oriented service—the destination confirms the data received. If some data (TCP segments) gets lost, the destination requests a retransmission of the lost data. UDP transports data using datagrams (the delivery is not guaranteed). In other words, the source party sends the datagram without worrying about whether it has been received. UDP is connectionless-oriented service.

The port is used as the address. To understand the difference between an IP address and port number, think of it as a mailing address. The IP address corresponds to the address of a house, while the port tells you the name of the person that should receive the letter.

TCP is described in Chapter 9 and UDP in Chapter 10.

1.2.3 Application Protocols

Application protocols correspond to several ISO OSI layers. The session, presentation, and application ISO OSI layers are reduced to one TCP/IP application layer.

The absence of a presentation layer is made up for by introducing specialized presentation-application protocols such as SSL and S/MINE that specialize in securing data or the Virtual Terminal and ASN.1 protocols that are designed for presenting data. The Virtual Terminal protocol (not to be confused with the ISO OSI protocol of the same name) specifies the network data presentation for character-oriented network protocols (Telnet, FTP, SMTP, and, partly, HTTP). Similarly, ASN.1 is often used for binary-oriented network transport. ASN.1 (including BER or DER encoding) was initially used by SNMP, but today it is also used by S/MINE.

There are many different application protocols. For practical purposes, they can be divided into two groups:

• User protocols utilized by user applications (HTTP, SMTP, Telnet, FTP, IMAP, PIP3, and so on).
• Service protocols, i.e., the protocols that ordinary Internet users rarely encounter. These protocols make sure the Internet functions correctly. For example, these could be routing protocols that are used for mutual communication by routers to correctly set their routing tables. Another example is SNMP usage in network administration.

Figure 1.11: Some protocols of the TCP/IP family