Introducing the TCP/IP Protocol Suite

This section presents an in-depth introduction to the protocols that are included in TCP/IP. Although the information is conceptual, you should learn the names of the protocols. You should also learn what each protocol does.

“TCP/IP” is the acronym that is commonly used for the set of network protocols that compose the Internet Protocol suite. Many texts use the term “Internet” to describe both the protocol suite and the global wide area network. In this book, “TCP/IP” refers specifically to the Internet protocol suite. “Internet” refers to the wide area network and the bodies that govern the Internet.

To interconnect your TCP/IP network with other networks, you must obtain a unique IP address for your network. At the time of this writing, you obtain this address from an Internet service provider (ISP).

If hosts on your network are to participate in the Internet Domain Name System (DNS), you must obtain and register a unique domain name. The InterNIC coordinates the registration of domain names through a group of worldwide registries. For more information on DNS, refer to System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP).

Protocol Layers and the Open Systems Interconnection Model

Most network protocol suites are structured as a series of layers, sometimes collectively referred to as a protocol stack. Each layer is designed for a specific purpose. Each layer exists on both the sending and receiving systems. A specific layer on one system sends or receives exactly the same object that another system's peer process sends or receives. These activities occur independently from activities in layers above or below the layer under consideration. In essence, each layer on a system acts independently of other layers on the same system. Each layer acts in parallel with the same layer on other systems.

OSI Reference Model

Most network protocol suites are structured in layers. The International Organization for Standardization (ISO) designed the Open Systems Interconnection (OSI) Reference Model that uses structured layers. The OSI model describes a structure with seven layers for network activities. One or more protocols is associated with each layer. The layers represent data transfer operations that are common to all types of data transfers among cooperating networks.

The OSI model lists the protocol layers from the top (layer 7) to the bottom (layer 1). The following table shows the model.

Table 1-1 Open Systems Interconnection Reference Model

Layer No.	Layer Name	Description
7	Application	Consists of standard communication services and applications that everyone can use.
6	Presentation	Ensures that information is delivered to the receiving system in a form that the system can understand.
5	Session	Manages the connections and terminations between cooperating systems.
4	Transport	Manages the transfer of data. Also assures that the received data are identical to the transmitted data.
3	Network	Manages data addressing and delivery between networks.
2	Data link	Handles the transfer of data across the network media.
1	Physical	Defines the characteristics of the network hardware.

The OSI model defines conceptual operations that are not unique to any particular network protocol suite. For example, the OSI network protocol suite implements all seven layers of the OSI model. TCP/IP uses some of OSI model layers. TCP/IP also combines other layers. Other network protocols, such as SNA, add an eighth layer.

TCP/IP Protocol Architecture Model

The OSI model describes idealized network communications with a family of protocols. TCP/IP does not directly correspond to this model. TCP/IP either combines several OSI layers into a single layer, or does not use certain layers at all. The following table shows the layers of the Solaris implementation of TCP/IP. The table lists the layers from the topmost layer (application) to the bottommost layer (physical network).

Table 1-2 TCP/IP Protocol Stack

OSI Ref. Layer No.	OSI Layer Equivalent	TCP/IP Layer	TCP/IP Protocol Examples
5,6,7	Application, session, presentation	Application	NFS, NIS, DNS, LDAP, telnet, ftp, rlogin, rsh, rcp, RIP, RDISC, SNMP, and others
4	Transport	Transport	TCP, UDP, SCTP
3	Network	Internet	IPv4, IPv6, ARP, ICMP
2	Data link	Data link	PPP, IEEE 802.2
1	Physical	Physical network	Ethernet (IEEE 802.3), Token Ring, RS-232, FDDI, and others

The table shows the TCP/IP protocol layers and the OSI model equivalents. Also shown are examples of the protocols that are available at each level of the TCP/IP protocol stack. Each system that is involved in a communication transaction runs a unique implementation of the protocol stack.

Physical Network Layer

The physical network layer specifies the characteristics of the hardware to be used for the network. For example, physical network layer specifies the physical characteristics of the communications media. The physical layer of TCP/IP describes hardware standards such as IEEE 802.3, the specification for Ethernet network media, and RS-232, the specification for standard pin connectors.

Data-Link Layer

The data-link layer identifies the network protocol type of the packet, in this instance TCP/IP. The data-link layer also provides error control and “framing.” Examples of data-link layer protocols are Ethernet IEEE 802.2 framing and Point-to-Point Protocol (PPP) framing.

Internet Layer

The Internet layer, also known as the network layer or IP layer, accepts and delivers packets for the network. This layer includes the powerful Internet Protocol (IP), the Address Resolution Protocol (ARP), and the Internet Control Message Protocol (ICMP).

IP Protocol

The IP protocol and its associated routing protocols are possibly the most significant of the entire TCP/IP suite. IP is responsible for the following:

• IP addressing – The IP addressing conventions are part of the IP protocol. Designing an IPv4 Addressing Scheme introduces IPv4 addressing and IPv6 Addressing Overview introduces IPv6 addressing.

• Host-to-host communications – IP determines the path a packet must take, based on the receiving system's IP address.

• Packet formatting – IP assembles packets into units that are known as datagrams. Datagrams are fully described in Internet Layer: Where Packets Are Prepared for Delivery.

• Fragmentation – If a packet is too large for transmission over the network media, IP on the sending system breaks the packet into smaller fragments. IP on the receiving system then reconstructs the fragments into the original packet.

The Solaris OS supports both IPv4 and IPv6 addressing formats, which are described in this book. To avoid confusion when addressing the Internet Protocol, one of the following conventions is used:

• When the term “IP” is used in a description, the description applies to both IPv4 and IPv6.

• When the term “IPv4” is used in a description, the description applies only to IPv4.

• When the term “IPv6” is used in a description, the description applies only to IPv6.

ARP Protocol

The Address Resolution Protocol (ARP) conceptually exists between the data-link and Internet layers. ARP assists IP in directing datagrams to the appropriate receiving system by mapping Ethernet addresses (48 bits long) to known IP addresses (32 bits long).

ICMP Protocol

The Internet Control Message Protocol (ICMP) detects and reports network error conditions. ICMP reports on the following:

• Dropped packets – Packets that arrive too fast to be processed

• Connectivity failure – A destination system cannot be reached

• Redirection – Redirecting a sending system to use another router

Chapter 8, Administering a TCP/IP Network (Tasks) contains more information on the Solaris OS commands that use ICMP for error detection.

Transport Layer

The TCP/IP transport layer ensures that packets arrive in sequence and without error, by swapping acknowledgments of data reception, and retransmitting lost packets. This type of communication is known as end-to-end. Transport layer protocols at this level are Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). TCP and SCTP provide reliable, end-to-end service. UDP provides unreliable datagram service.

TCP Protocol

TCP enables applications to communicate with each other as though they were connected by a physical circuit. TCP sends data in a form that appears to be transmitted in a character-by-character fashion, rather than as discrete packets. This transmission consists of the following:

• Starting point, which opens the connection

• Entire transmission in byte order

• Ending point, which closes the connection.

TCP attaches a header onto the transmitted data. This header contains many parameters that help processes on the sending system connect to peer processes on the receiving system.

TCP confirms that a packet has reached its destination by establishing an end-to-end connection between sending and receiving hosts. TCP is therefore considered a “reliable, connection-oriented” protocol.

SCTP Protocol

SCTP is a reliable, connection-oriented transport layer protocol that provides the same services to applications that are available from TCP. Moreover, SCTP can support connections between systems that have more than one address, or multihomed. The SCTP connection between sending and receiving system is called an association. Data in the association is organized in chunks. Because SCTP supports multihoming, certain applications, particularly applications used by the telecommunications industry, need to run over SCTP, rather than TCP.

UDP Protocol

UDP provides datagram delivery service. UDP does not verify connections between receiving and sending hosts. Because UDP eliminates the processes of establishing and verifying connections, applications that send small amounts of data use UDP.

Application Layer

The application layer defines standard Internet services and network applications that anyone can use. These services work with the transport layer to send and receive data. Many application layer protocols exist. The following list shows examples of application layer protocols:

• Standard TCP/IP services such as the ftp, tftp, and telnet commands

• UNIX “r” commands, such as rlogin and rsh

• Name services, such as NIS and the domain name system (DNS)

• Directory services (LDAP)

• File services, such as the NFS service

• Simple Network Management Protocol (SNMP), which enables network management

• Router Discovery Server protocol (RDISC) and Routing Information Protocol (RIP) routing protocols

Standard TCP/IP Services

• FTP and Anonymous FTP – The File Transfer Protocol (FTP) transfers files to and from a remote network. The protocol includes the ftp command and the in.ftpd daemon. FTP enables a user to specify the name of the remote host and file transfer command options on the local host's command line. The in.ftpd daemon on the remote host then handles the requests from the local host. Unlike rcp, ftp works even when the remote computer does not run a UNIX based operating system. A user must log in to the remote system to make an ftp connection, unless the remote system has been configured to allow anonymous FTP.

  You can obtain an enormous amount of material from anonymous FTP servers that are connected to the Internet. Universities and other institutions set up these servers to offer software, research papers, and other information to the public domain. When you log in to this type of server, you use the login name anonymous, hence the term “anonymous FTP server.”

  Using anonymous FTP and setting up anonymous FTP servers is outside the scope of this manual. However, many books, such as The Whole Internet User's Guide & Catalog, discuss anonymous FTP in detail. Instructions for using FTP are in System Administration Guide: Network Services. The ftp(1) man page describes all ftp command options that are invoked through the command interpreter. The ftpd(1M) man page describes the services that are provided by the in.ftpd daemon.

• Telnet – The Telnet protocol enables terminals and terminal-oriented processes to communicate on a network that runs TCP/IP. This protocol is implemented as the telnet program on local systems and the in.telnetd daemon on remote machines. Telnet provides a user interface through which two hosts can communicate on a character-by-character or line-by-line basis. Telnet includes a set of commands that are fully documented in the telnet(1) man page.

• TFTP – The Trivial File Transfer Protocol (tftp) provides functions that are similar to ftp, but the protocol does not establish ftp's interactive connection. As a result, users cannot list the contents of a directory or change directories. A user must know the full name of the file to be copied. The tftp(1)man page describes the tftp command set.

UNIX “r” Commands

The UNIX “r” commands enable users to issue commands on their local machines that run on the remote host. These commands include the following:

• rcp

• rlogin

• rsh

Instructions for using these commands are in the rcp(1), rlogin(1), and rsh(1) man pages.

Name Services

The Solaris OS provides the following name services:

• DNS – The domain name system (DNS) is the name service provided by the Internet for TCP/IP networks. DNS provides host names to the IP address service. DNS also serves as a database for mail administration. For a complete description of this service, see System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP). See also the resolver(3RESOLV) man page.

• /etc files – The original host-based UNIX name system was developed for standalone UNIX machines and then adapted for network use. Many old UNIX operating systems and computers still use this system, but it is not well suited for large complex networks.

• NIS – Network Information Service (NIS) was developed independently of DNS and has a slightly different focus. Whereas DNS focuses on making communication simpler by using machine names instead of numerical IP addresses, NIS focuses on making network administration more manageable by providing centralized control over a variety of network information. NIS stores information about machine names and addresses, users, the network itself, and network services. NIS name space information is stored in NIS maps. For more information on NIS Architecture and NIS Administration, see System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP).

Directory Service

The Solaris OS supports LDAP (Lightweight Directory Access Protocol) in conjunction with the Sun Open Net Environment (Sun ONE) Directory Server, as well as other LDAP directory servers. The distinction between a name service and a directory service is in the differing extent of functionality. A directory service provides the same functionality of a naming service, but provides additional functionalities as well. See System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP).

File Services

The NFS application layer protocol provides file services for the Solaris OS. You can find complete information about the NFS service in System Administration Guide: Network Services.

Network Administration

The Simple Network Management Protocol (SNMP) enables you to view the layout of your network and the status of key machines. SNMP also enables you to obtain complex network statistics from software that is based on a graphical user interface (GUI). Many companies offer network management packages that implement SNMP.

Routing Protocols

The Routing Information Protocol (RIP) and the Router Discovery Server Protocol (RDISC) are two routing protocols for TCP/IP networks. They are described in Routing Protocols in the Solaris OS.