The OSI model describes an idealized network communications with a family of protocols. TCP/IP does not correspond to this model directly. 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 lowest (physical network).

Table 2–2 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, telnet, ftp, rlogin, rsh, rcp, RIP, RDISC, SNMP, and others
4	Transport	Transport	TCP, UDP
3	Network	Internet	IP, ARP, ICMP
2	Data link	Data link	PPP, IEEE 802.2
1	Physical	Physical network	Ethernet (IEEE 802.3) Token Ring, RS-232, others

The table shows the TCP/IP protocol layers. Also shown are the OSI Model equivalents with examples of the protocols that are available at each level of the TCP/IP protocol stack. Each host 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

This layer, also known as the network 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. Chapter 3, Planning Your TCP/IP Network (Task) describes IPv4 addressing in detail and Chapter 14, IPv6 (Overview) describes IPv6 addressing in detail.
• Host-to-host communications – IP determines the path a packet must take, based on the receiving host’s IP address.
• Packet formatting – IP assembles packets into units that are known as IP datagrams. Datagrams are fully described in Internet Layer.
• Fragmentation – If a packet is too large for transmission over the network media, IP on the sending host breaks the packet into smaller fragments. IP on the receiving host then reconstructs the fragments into the original packet.

Previous releases of the Solaris operating environment implement version 4 of the Internet Protocol, which is abbreviated as IPv4. However, because of the rapid growth of the Internet, a new Internet Protocol was created. The new protocol increases address space. This new version, known as version 6, is abbreviated as IPv6. The Solaris operating environment supports both versions, 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 host by mapping Ethernet addresses (48 bits long) to known IP addresses (32 bits long).

ICMP Protocol

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 host that cannot be reached)
• Redirection – Redirecting a sending host to use another router

The ping Command contains more information on the operating system commands that use ICMP for error detection.

Transport Layer

The TCP/IP transport layer protocols ensure 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) and User Datagram Protocol (UDP).

TCP Protocol

TCP enables applications to communicate with each other as though 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 a starting point, which opens the connection, the entire transmission in byte order, and an ending point, which closes the connection.

TCP attaches a header onto the transmitted data. This header contains a large number of parameters that help processes on the sending machine connect to peer processes on the receiving machine.

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.

UDP Protocol

UDP, the other transport layer protocol, 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 rather than TCP.

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 domain name system (DNS)
• File services, such as the NFS service
• Simple Network Management Protocol (SNMP), which enables network management
• RIP and RDISC 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 (local machine) and the in.ftpd daemon (remote machine). 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 computer to make an ftp connection unless the remote computer has been configured to allow anonymous FTP.

You can now obtain an enormous amount of materials 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 servers.”

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 to reach standard machines are in System Administration Guide: Resource Management and 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 daemon in.ftpd.

• 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 program telnet (on local machines) and the daemon in.telnetd (on remote machines). Telnet provides a user interface through which two hosts can communicate on a character-by-character or line-by-line basis. The application 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 telnet(1) man page describes the tftp command set.