NET 121b: Essentials of Networking
Chapter 9: TCP/IP Fundamentals
This chapter discusses the history and structure of the TCP/IP suite.
The topics of this chapter are:
- Evolution of TCP/IP
- TCP/IP fundamentals
The Beginning of TCP/IP
The chapter opens with the idea that TCP/IP provides a common communication
method for computers regardless of their brand, their operating system,
or their location. The popularity of the Internet led all the major network
operating system vendors to support TCP/IP on their own networks.
The Internet Protocol suite was developed before the OSI Relational
Model. The graphic below shows how the seven layers of the OSI model
map to the four layers of the Department of Defense (DoD) model.
The Department of Defense was instrumental in the construction of the
along with the Defense Advanced Research Projects Agency (DARPA).
The DoD model is the one that was used to plan and develop the TCP/IP
To compare the models, consider this chart:
DoD and OSI Models
|Upper Layer Processes
The four layers of the DoD model address the topics found
in the OSI model. If you understand the OSI model, you already understand
the DoD model.
- Network Access layer - this layer describes how networks are
laid out, and how devices share access to the network medium
- Internet layer - this layer finds routes from one network
to another, addresses devices and networks, and provides access to
gateways between networks. A short list of protocols that work on this
layer: IP, ICMP, ARP, and RARP
- Host-to-host layer - Your book refers to this as the Transport
layer, which is its name in the OSI model. Traditionally, it is called
the Host-to-host layer in the DoD model. The purpose of this layer
is to provide reliable delivery of packets across networks. The protocol
that provides this function is TCP. Another protocol, UDP, also functions
on this layer, but UDP is considered connectionless.
- Application layer - this layer provides access to network
services for applications that run on a client workstation. Some protocols
that function on this layer: HyperText Transfer Protocol (HTTP) for
the World Wide Web, Simple Mail Transfer Protocol (SMTP) for
e-mail, and File Transfer Protocol (FTP) for uploading and downloading
The TCP/IP suite does not address topics at the Network
Access layer (DoD model). This makes it independent of any networking
topology. Another way of saying this is that the TCP/IP suite will
work regardless of the physical nature of a network.
Requests for Comment
The Internet is not managed by any one country or organization. When
committees wish to propose changes to protocols, they publish documents
called Requests for Comments. These are invitations to Internet
users in general to discuss proposals, and reach concensus. To access
current RFCs, go to www.rfc-editor.org/rfc.html.
RFCs concern new protocols, Internet standards, and reference material
about the Internet.
The text continues to discuss protocols associated with particular
layers. As noted above, no specific protocols are associated with the
Network Access layer of the DoD model.
Internet Layer Protocols
- Internet Protocol (IP) is a connectionless protocol
that supports routing, fragmentation, and reassembly.
Note: if you have to decide if a protocol is connectionless (IP,
IPX) or connection-oriented (TCP, SPX), it is connectionless if
it starts with a vowel. This trick only applies to protocols
that you need to categorize this way.
- Internet Control Message Protocol (ICMP) is used to send error
and control messages to Upper Layer Protocols (because they are concerned
with error and flow control).
- Address Resolution Protocol (ARP) is used on IP networks to
resolve an IP address (4 bytes) to a MAC address (6 bytes). You may
wish to review the fact that MAC addresses are typically shown as 12-character
Hexadecimal strings. One byte can be expressed as two Hex characters. Reverse
Address Resolution Protocol (RARP) is the reverse of ARP: it is
used to resolve a known MAC address to an unknown IP address. ARP
tables are constructed by sending broadcast ARP requests to the
network, and recording the responses as IP address/MAC address pairs.
- Reverse Address Resolution Protocol (RARP) and BOOTP.
These protocols are designed to give an IP address to a workstation
on boot. They are typically used on diskless workstations, which have
no medium on which to record their IP addresses.
Transport (Host-to-host) Layer Protocols
- Transmission Control Protocol (TCP), which is connection-oriented.
This is the protocol that makes the TCP/IP suite reliable.
- User Datagram Protocol (UDP) operates in the Host-to-Host
Layer, but it is connectionless, so it is faster than TCP.
Application Layer Protocols
- File Transfer Protocol (FTP) - allows users to copy files
as though using local devices. It supports the use of user IDs and
- Trivial File Transfer Protocol (TFTP) - also allows users
to copy files, but does not support User IDs and passwords
- Hypertext Transfer Protocol (HTTP) - the file transfer protocol
used on the World Wide Web
- Simple Mail Transfer Protocol (SMTP) - this is the standard
e-mail transport protocol for TCP/IP stacks. It depends on TCP for
- Simple Network Management Protocol (SNMP) - a basic network
management tool, it requires that you run an SNMP manager, which
allows you to manage resources running SNMP agents.
- Telnet - a protocol for connecting to a different computer,
and making your workstation a terminal to that other computer
- Gopher - previously discussed, a protocol for conducting text
based search and retrieval from Internet data files
- Dynamic Host Configuration Protocol (DHCP) - DHCP provides
not only the IP address, but also configuration settings for
the host. DHCP requests are broadcast requests. Broadcast requests
are not forwarded by routers, so a DHCP server must be on the
same network segment as the device making the request.
- Domain Name System (DNS) - discussed in a previous
chapter, converts a registered domain name to an IP address
- Windows Internet Naming Service (WINS) - works like
DNS, but it takes NetBIOS names used on Microsoft networks, and converts
them to IP addresses
The word port is used in networking to mean several things. In
this chapter, a port is a named section of memory, typically in a server,
in which a particular program or service is running. This kind of port
is also called a socket, or a service.
You have already learned that an IP packet can be addressed to a particular
network, and to a computer on that network. A port number can be added
to the address, to send the packet to a specific program running on that
A port number can be any number from 1 through 65535. Ports 1
through 1023 are called Well-known
ports. Several port numbers are assigned to specific services
through conventions established by ICANN. (A listing may be found in
- 20 - FTP, data
- 21 - FTP, control
- 23 - Telnet
- 25 - SMTP
- 53 - DNS
- 443 - HTTPS
- 80 - HTTP
- 110 - POP3
Ports 1024 through 65535 are Registered ports. ICANN does not assign
these port numbers.