The Internet Encyclopedia (Volume 3)

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INTERNETAPPLICATIONS AND THEIRPROTOCOLS 325

Network Dependent

Protocols

Line

IP

TCP UDP

Application

1

Application

n

Application

n+1

RTCP RTP

ARP RARP

Transport

Layer

Network

Layer

Figure 6: Relative levels of common Internet protocols.

Functionally, it is a connectionless protocol and relies on

best-effort service. Although this provides a simple solu-

tion to a short message system over very reliable networks,

it has problems when operating over public services.

Associated with the IP are several well-known service-

providing protocols that operate at and above the network

layer as defined by the ISO 7-layer model. They are the

transmission control protocol (TCP), user datagram pro-

tocol (UDP), real-time transport protocol (RTP), and the

real-time transport control protocol (RTCP). See Figure 6

for their relative positions in the reference model.

Transmission control protocol is offered as a

connection-orientated service to the user at the transport

layer. The user believes they have a permanent connec-

tion to the desired endpoint. The purpose of TCP is to

provide application protocols with network-independent

information exchange services.

User datagram protocol provides a connectionless

service to application protocols. It is particularly useful

for short message reporting.

The advent of the Web and multimedia content has

provided a demand for real-time services across the Inter-

net and hence via IP. The real-time transport protocol is

able to transfer data types such as audio and video where

real-time characteristics are required; however, it does not

guarantee sequence delivery, error correction, or quality

of service (QoS).

The real-time transport control protocol was designed

for control of processes involved in conference calls. It can

provide synchronization, framing, error detection, and

source identification. It can also monitor the QoS. Both

the RTP and RTCP protocols may be considered comple-

mentary and use separate ports; hence, they may be used

together.

The Internet has grown as a collection of cooper-

ative networks, each initially with its own addressing

scheme. These became unified and where necessary cor-

respondences between hardware addresses and Internet

addresses are resolved and stored using the address res-

olution protocols (ARP) and the reverse address resolu-

tion protocols (RARP). Ultimately these addresses must

be communicated to other networks so that data can be

transferred. Thus devices known as routers are required.

The routers calculate the most efficient path to a given

address using the interior gateway protocol (IGP) within

local groups. In order to check the availability of other

routers outside the local group an exterior gateway pro-

tocol (EGP) is used.

Multicast and Unicast Control Protocols

It is often a requirement that more than one recipient re-

ceive a message or that more than one user are involved in

a connection, e.g., conference calls. One such protocol is

the Internet group management protocol (IGMP). IGMP

is defined in RFC 1112. It is used to allow joining and

leaving of multicast groups. Routers involved in multi-

casts periodically send membership queries to determine

who is on the Net for that group. If a response is received,

in the form of a report from a host detailing membership

of groups, this is recorded for the forwarding of group

messages.

IPv6 and the Future

It is only in the past ten years that the home user has be-

come a major source of traffic on the Internet. This has

led to an explosive growth in Internet address requests.

The IETF has defined in RFCs 1883–7 a new version of

the Internet protocol, IPv6, to replace the current version,

IPv4. This has increased the address space to 128 bits from

32 bits. This new version addresses several other areas of

concern. Amongst these are the introduction of improved

security and data integrity, the ability of a host to acquire

automatically an IP address without human assistance

(this is particularly important to mobile applications), and

better quality of service guarantees for multimedia appli-

cations.

INTERNET APPLICATIONS

AND THEIR PROTOCOLS

The Internet is often seen as synonymous with the TCP/IP

protocol suite and closely related protocols; however,

many other protocols exist at various layers for specific

purposes. This section looks at a few not covered else-

where. It is interesting to note that DARPA in 1973 put

the number of networks at over 7500. Currently accor-

ding to figures obtained from the May 2002 Netcraft

surveys (Netcraft, 2002) the number of active server sites

now stands at 15.5 million. This level of use would not

have occurred without the many applications that became

available, the most famous of which is the WWW.

File Access (FTP, Telnet, NFS)

The Internet could not be the success it is without the

early file access and transfer utilities and a supporting file

structure standard known as the network file structure

(NFS). Details of these application and support protocols

may be found in RFCs 1122 and 1123. The file transfer

protocol (FTP) is still considered the primary standard

for file transfers on the Internet (as opposed to Web page

delivery, which does not provide a local copy explicitly).

Remote access of files presupposes the ability to log

onto the remote machine. This is catered for by Telnet,

which is specified in RFC 854.Other log-in protocols such