The Application Protocols

The following 12 applications were built on top of the TCP/IP protocol suite and are available on most implementations.

Simple Network Management Protocol (SNMP)

SNMP allows network administrators to collect information about the network. It is a communications protocol for collecting information about devices on the network, including hubs, routers, and bridges. Each piece of information to be collected about a device is defined in a Management Information Base (MIB). SNMP uses UDP to send and receive messages on the network.

File Transfer Protocol (FTP)

FTP provides a mechanism for single or multiple file transfers between computer systems; when written in lowercase as “ftp,” it is also the name of the client software used to access the FTP server running on the remote host. The FTP package provides all the tools needed to look at files and directories, change to other directories, and transfer text and binary files from one system to another. FTP uses TCP to actually move the files. We’ll look at how to transfer files using FTP in detail in the next chapter.

Trivial File Transfer Protocol (TFTP)

TFTP is a “stripped down” version of FTP, primarily used to boot diskless workstations and to transfer boot images to and from routers. It uses a reduced feature set (fewer commands and a smaller overall program size). In addition to its reduced size, it also uses UDP instead of TCP, which makes for faster transfers, but with less reliability.

Simple Mail Transfer Protocol (SMTP)

SMTP allows for a simple e-mail service and is responsible for moving messages from one e-mail server to another. The e-mail servers run either Post Office Protocol (POP) or Internet Mail Access Protocol (IMAP) to distribute e-mail messages to users.

Post Office Protocol (POP)

POP provides a storage mechanism for incoming mail; the latest version of the standard is known as POP3. When a client connects to a POP3 server, all the messages addressed to that client are downloaded; there is no way to download messages selectively. Once the messages are downloaded, the user can delete or modify messages without further interaction with the server. In some locations, POP3 is being replaced by another standard, IMAP.

Internet Mail Access Protocol (IMAP)

IMAP allows users to download mail selectively, look at the message header, download just a part of a message, store messages on the e-mail server in a hierarchical structure, and link to documents and Usenet newsgroups. Search commands are also available so that users can locate messages based on their subject, header, or content. IMAP has strong authentication features and supports the Kerberos authentication scheme originally developed at MIT.

Telnet

Telnet is a terminal emulation package that provides a remote logon to another host over the network.

Internet Control Message Protocol (ICMP)

ICMP works at the IP Network layer level and provides the functions used for Network layer management and control. Routers send ICMP messages to respond to undeliverable datagrams by placing an ICMP message in an IP datagram and then sending the datagram back to the original source. The Ping command—used in network troubleshooting and described in Chapter 5, “Major Network Operating Systems”—uses ICMP.

Hypertext Transfer Protocol (HTTP)

HTTP is the command and control protocol used to manage communications between a web browser and a web server. When you access a web page on the Internet or on a corporate intranet, you see a mixture of text, graphics, and links to other documents or other Internet resources. HTTP is the mechanism that opens the related document when you select a link, no matter where that document is actually located.

Note 

Secure Hypertext Transfer Protocol (which you will see abbreviated as SHTTP, S-HTTP, or even HTTPS) is a secure version of HTTP that provides a variety of security mechanisms to the transactions between a web browser and the server. S-HTTP allows browsers and servers to sign, authenticate, and encrypt an HTTP network packet.

Address Resolution Protocol (ARP)

ARP helps to reference the physical hardware address of a network node to its IP address. Under ARP, a network interface card (NIC) contains a table (known as the address resolution cache) that maps logical addresses to the hardware addresses of nodes on the network. When a node needs to send a packet, it first checks the address resolution cache to see if the physical address information is already present. If so, that address is used, and network traffic is reduced; otherwise, a normal ARP request is made to determine the address. See Chapter 5 for more on ARP.

Network Time Protocol (NTP)

NTP, originally developed by Professor David Mills at the University of Delaware, is used to synchronize (or set) computer clocks to some standard time source, which is usually a nuclear clock. This protocol (along with synchronization utilities) keeps all computers on a network set to the same time. Time synchronization is important because many transactions are time and date stamped (in a database, for example). If the time on a server is out of synchronization with the time on two different computers, even by just a few seconds, the server will get confused. For example, one computer can seemingly enter a transaction, but the server will indicate that it occurred before it actually did. Because this time problem will crash the database server, it is important that these servers (and workstations) use NTP.

User Datagram Protocol (UDP)

UDP is a Transport layer connectionless protocol that does not provide the reliability services available with TCP. UDP gives applications a direct interface with IP and the ability to address a specific application process running on a host via a port number without setting up a connection session. UDP also uses IP to deliver its packets.

Figure 3.4 shows how some of these components fit together.

Figure 3.4: The components in a TCP/IP block diagram

The Novell NetWare IPX/SPX Protocol Suite

The Novell NetWare proprietary protocol suite consists of two main parts:

• Internetwork Packet eXchange (IPX)

• Sequenced Packet eXchange (SPX)

IPX is based on the Xerox Network System (XNS) protocol developed in the 1970s and is an internetworking protocol that provides datagram services in the Network layer and also provides routing services. IPX is very efficient and uses a simple addressing scheme that is based on a 4-byte network number, a 6-byte node number, and a 2-byte socket number. A network number is assigned to each segment in the network. The node number or hardware address identifies a specific network interface card or device, and the socket number identifies a particular process in the computer.

IPX packets consist of a 30-byte header that includes the network, node, and socket addresses for the source and the destination, followed by the data area, which can be from 30 bytes (just the header) to 65,535 bytes in length. Most networks impose a more realistic maximum packet size of about 1500 bytes.

The IPX packet header contains the following fields:

Checksum  For data integrity checking.
Packet Length  Length of the packet in bytes.
Transport Control  Number of routers a packet can cross before being discarded.
Packet Type  The service that created the packet.
Destination Network  Network address of the destination network.
Destination Node  Media access control (MAC) address of the destination node.
Destination Socket  Address of the process running on the destination node. Source Network  Network address of the source network.
Source Node  MAC address of the source node.
Source Socket  Address of the process running on the source node.

The other part of the protocol suite, SPX, works at the Transport layer and guarantees packet delivery by making the destination node verify that the data was received correctly. If no response is received within a specified time, SPX retransmits the packet. If several retransmissions fail to return an acknowledgment, SPX assumes the connection has failed and informs the outside world of the error condition. All packets in the transmission are sent in sequence, and they all take the same path to their destination.

If we compare the IPX/SPX protocol suite to the TCP/IP family, IP and IPX are connectionless datagram protocols, and SPX and TCP are connectionoriented protocols. IPX provides routing and internetwork services similar to IP, and SPX provides Transport layer services similar to TCP. Novell NetWare uses two routing protocols:

• Routing Information Protocol (RIP)

• NetWare Link Services Protocol (NLSP)

NLSP is more efficient at maintaining routing information and adapting to changes in the network configuration and allows large or small networks to be connected without causing routing inefficiencies. This is because NLSP doesn’t determine a route based on the number of routers, but rather on the individual route’s “cost” (a value determined by several factors like speed, available bandwidth, etc.).

NetWare Core Protocol (NCP) is the main protocol used to manage service requests between a client and a server. It includes routines for logon requests, for manipulating files and directories, for opening semaphores, for printing, and for creating and destroying service connections. NCP was designed with the assumption that client and server would be physically close; once a router is added to the system, and connections are made over a wide area link, NCP creates network traffic congestion.