TCP/IP

The TCP/IP (Transmission Control Protocol/Internet Protocol) suite, included with the SCO OpenServer Desktop and Enterprise systems, provides the basis for many useful services, including electronic mail, file transfer, remote login, network monitoring, and others, described in ``Administering TCP/IP''.

TCP/IP is a set of protocols and programs used to interconnect computer networks and to route traffic among different types of computers. These protocols describe allowable data formats, error handling, message passing, and communication standards. Computer systems that use TCP/IP speak a common language, regardless of hardware or operating system differences.

Many large networks conform to these protocols, including the Internet. Thousands of computers at universities, government agencies, and corporations are connected to a network that follows the TCP/IP protocols. Any machine on the Internet can communicate with any other. Machines on the Internet are referred to as hosts or nodes and are defined by their Internet (or IP) address.

TCP/IP provides:

• Remote file transfer and copy via ftp(TC) and rcp(TC).

• Remote login capability via rlogin(TC) and telnet(TC).

• Remote shell and command execution via rcmd(TC).

• Remote file distribution via rdist(TC).

• Serial line communications via the Point-to-Point Protocol (PPP) or the Serial Line Internet Protocol (SLIP).

• Distributed and local host naming services via the Berkeley Internet Name Domain (BIND) name server or /etc/hosts .

• Remote hostname lookup via nslookup(TC)

• Time synchronization via the Time Synchronization Protocol (TSP) or Network Time Protocol (NTP).

• Gateway and router support via gated(ADMN), irdd(ADMN), and routed(ADMN).

• Network management capability via the Simple Network Management Protocol.

• Remote printing capabilities via Remote Line Printing (RLP).

• The ability to run X and X clients, such as scocal and scomail, over the network.

TCP/IP Protocols

TCP/IP is made up of three basic protocols -- Internet Protocol (IP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP) -- as well as a variety of others that run over the IP protocol stack.

• The IP protocol splits and forwards packets to the appropriate computer.

• The TCP protocol provides security and reliability to the IP protocol.

• The UDP protocol is a low-security alternative to TCP.

• Other protocols facilitate name services, mail handling, network management, and other useful services.

The Internet Protocol (IP)

The Internet Protocol defines a data delivery system wherein the sending and receiving machines are not necessarily directly connected. IP splits data into packets of a given size, which are then forwarded to the receiving machine via the network. These individual packets of data (often called ``datagrams'') are routed through different machines on the Internet to the destination network and receiving machine. A particular set of data, such as a file, can be broken up into several datagrams that are sent separately.

A datagram consists of header information and a data segment. The header contains information about routing and processing the datagram. Datagrams can be further fragmented into smaller pieces, depending on the physical requirements of the networks they cross. For example, when a gateway sends a datagram to a network that cannot accommodate the datagram as a single packet, the datagram must be split into pieces that are small enough for transmission. The datagram fragment headers contain the information necessary to reassemble the fragments into the complete datagram. Fragments do not necessarily arrive in order; the software module implementing the IP protocol on the destination machine must reassemble the fragments into the original datagram. If any fragments are lost, the entire datagram is discarded.

See also:

• inet(ADMP)

• ip(ADMP)

The Transmission Control Protocol (TCP)

The Transmission Control Protocol works with IP to provide reliable delivery. It provides a means to ensure that the various datagrams making up a message are reassembled in the correct order at their final destination and that any missing datagrams are re-sent until they are correctly received.

The primary purpose of TCP is to avoid the loss, damage, duplication, delay, or misordering of packets that can occur under IP. When IP forwards datagrams, individual datagrams may or may not arrive, and they probably will not arrive in the order in which they were sent. TCP adds the reliability that IP lacks. Also, security provisions such as limiting user access to certain machines can be implemented through TCP.

TCP provides reliability by using checksums (error detection codes) on the data, sequence numbers in the TCP header, positive acknowledgment of data received, and retransmission of unacknowledged data.

See also:

• ifconfig(ADMN)

• inet(ADMP)

• tcp(ADMN)

• tcp(ADMP)

The User Datagram Protocol (UDP)

The User Datagram Protocol provides data transfer without many of the reliable delivery capabilities of TCP. UDP is less CPU-intensive than TCP and is useful when guaranteed data delivery is not of paramount importance.

See also:

• udp(ADMP)

Other TCP/IP protocols

Address Resolution Protocol (ARP)

Translates between DARPA Internet and Ethernet addresses. See arp(ADMN) and arp(ADMP).

Internet Control Message Protocol (ICMP)

Handles error-message and protocol control for TCP/IP. See icmp(ADMP).

Point-to-Point Protocol (PPP)

Provides both synchronous and asynchronous network connections over a serial line. See ppp(ADMP) and ``Configuring the Point-to-Point Protocol (PPP)''.

Reverse Address Resolution Protocol (RARP)

Translates between Ethernet and DARPA Internet addresses.

Serial Line Internet Protocol (SLIP)

Enables IP over serial lines. See slip(ADMP) and ``Configuring the Serial Line Internet Protocol (SLIP)''.

Simple Mail Transport Protocol (SMTP)

Used by MMDF to send mail via TCP/IP.

Simple Network Management Protocol (SNMP)

Performs distributed network management functions via TCP/IP. See ``Configuring the Simple Network Management Protocol (SNMP)''.

Network servers and databases

In the UNIX system, most server programs are started by a super server, called the ``Internet daemon''. The Internet daemon, /etc/inetd, acts as a master server for programs specified in its configuration file, /etc/inetd.conf. It listens for service requests for these servers, and starts up the appropriate program whenever a request is received. The configuration file includes information about:

• a service name (as found in /etc/services)

• the type of socket the server expects (for example, stream or dgram)

• the protocol used with the socket (as found in /etc/protocols)

• whether to wait for each server to complete before starting up another

• the user name under which the server should run

• the server program's name

• up to five arguments to pass to the server program

   daytime stream  tcp     nowait  root    internal

Network database files

File	Manual reference	Use
/etc/hosts	hosts(SFF)	host names
/etc/networks	networks(SFF)	network names
/etc/services	services(SFF)	list of known services
/etc/protocols	protocols(SFF)	protocol names
/etc/hosts.equiv	rshd(ADMN)	list of ``trusted'' hosts
/etc/ftpusers	ftpd(ADMN)	list of ``unwelcome'' ftp users
/etc/inetd.conf	inetd(ADMN)	list of servers started by inetd

The files distributed are set up for Internet hosts. Local networks and hosts should be added to the databases to describe the local configuration.

TCP/IP end-user commands

Several TCP/IP commands, described in detail on their manual pages, provide end users with networking capabilities:

TCP/IP Commands

Command	Purpose
ftp	file transfer between machines running TCP/IP (these machines may or may not run the same operating system)
rcmd	remote command execution on another UNIX system
rcp	file copying between two UNIX systems
rlogin	remote login on another UNIX system
ruptime	status display of local network machines
rwho	display list of users logged on to local network machines.
telnet	remote login on a machine running TCP/IP (these machines may or may not run the same operating system)

Configuring TCP/IP

To configure TCP/IP, you must:

• Establish physical connections (serial or otherwise).

• Configure networking drivers.
  

• Configure the TCP/IP protocol stack by:

  • ensuring that your system name is unique on the network.

  • selecting the appropriate device driver for your networking card, adapter, or serial line.

  • selecting a domain name appropriate to your networking needs.

  • assigning unique IP addresses to each machine on the network.

  • assigning a broadcast address that allows messages to reach all machines on your network.

  • assigning netmasks if you are configuring subnets on your network.

• configure pseudo-ttys.
  

• configure additional protocols listed in ``Other TCP/IP protocols''.

• configure NFS to enable filesharing over TCP/IP.

• overcoming distance limitations in cabling media.

• connecting physically dissimilar networks (for example, Ethernet and Token-Ring).

• simplifying network administration and troubleshooting in large networks.

• connecting groups of users to reflect organizational structure.

• configuring the serial interface with a unique IP address.

• configuring the Ethernet or Token-Ring card with a unique IP address.

• adding entries to the /etc/hosts file for the new hostname and network addresses.

Administering TCP/IP

Basic administration of TCP/IP consists of:

• ``Setting interface parameters'' to directly change the netmask, broadcast address, and IP address used at boot time.

• ``Creating subnets'' to allow several local networks to appear as a single Internet network to off-site hosts.

• ``Establishing user equivalence'' to enable users to log in to their accounts on the local machine from remote machines without a password.

• ``Setting up anonymous ftp'' to enable users without accounts on the local system to transfer files via ftp to and from specified areas of your filesystem.

• ``Configuring UUCP over TCP/IP'' to enable file and mail transfer between two computers that may not be running ftpd, rshd, and smtpd.

• ``Obtaining RFCs from the Internet'' to obtain additional information on TCP/IP protocols and applications from sources on the Internet.

• ``Troubleshooting TCP/IP'' to reduce or eliminate TCP/IP errors.