NET 121b: Essentials of Networking

Chapter 6: TCP/IP Architecture


This chapter discusses specific aspects of the TCP/IP suite. The objectives important to this chapter are:

  1. TCP/IP, as used in a mixed environment
  2. Internet architecture
  3. Using TCP/IP
  4. TCP/IP protocols
  5. Port, as used by TCP/IP protocols
  6. RFCs

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 on page 6-5 shows 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 Internet, 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 suite.

To compare the models, consider this chart:

DoD and OSI Models
DoD Layers OSI Layers
Upper Layer Processes Process/Application Application
Reliable Connections Host-to-host Transport
Internetwork Connections Internet Network
Network Access Data-Link

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 files. .

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.

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

RFCs concern new protocols, Internet standards, and reference material about the Internet.

The text continues to discuss protocols associated with particualar 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.
  • 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 passwords.
  • 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 message routing.
  • 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 computer.

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 RFC 1700.)

  • 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.