TCP/IP PROTOCOL SUITE:
The
TCP/IP protocol suite was developed prior to the OSI model. Therefore, the
layers in the TCP/IP protocol suite do not exactly match those in the OSI
model.
The
original TCP/IP protocol suite was defined as having four layers:
host-to-network, internet, transport, and application.
The
TCP/IP protocol suite is made of five layers: physical, data link, network,
transport, and application. The first four layers provide physical standards,
network interfaces, internetworking, and transport functions that correspond to
the first four layers of the OSI model. The three topmost layers in the OSI
model, however, are represented in TCP/IP by a single layer called the
application layer (see Figure 1.23).
1. Physical and Data Link Layers:
At the
physical and data link layers, TCP/IP does not define any specific protocol. It
supports all the standard and proprietary protocols. A network in a TCP/IP
internetwork can be a local-area network or a wide-area network.
2. Network Layer:
At the
network layer (or, more accurately, the internetwork layer), TCP/IP supports
the Internetworking Protocol. IP, in turn, uses four supporting protocols: ARP,
RARP, ICMP, and IGMP.
a. Internetworking Protocol (IP)
The Internetworking Protocol
(IP) is the
transmission mechanism used
by the TCP/IP protocols. It is an unreliable and
connectionless protocol-a best-effort delivery service. The term best effort
means that IP provides no error checking or tracking.
b. Address Resolution Protocol
The
Address Resolution Protocol (ARP) is used to associate a logical address with a
physical address. On a typical physical network, such as a LAN, each device on
a link is identified by a physical or station address, usually imprinted on the
network interface card (NIC). ARP is used to find the physical address of the
node when its Internet address is known.
c. Reverse Address Resolution Protocol
The
Reverse Address Resolution Protocol (RARP)
allows a host to discover its Internet address when it knows only its
physical address. It is used when a computer is connected to a network for the
first time or when a diskless computer is booted.
d. Internet Control Message Protocol
The
Internet Control Message Protocol (ICMP) is a mechanism used by hosts and
gateways to send notification of datagram problems back to the sender. ICMP
sends query and error reporting messages.
e. Internet Group Message Protocol
The Internet
Group Message Protocol
(IGMP) is used
to facilitate the
simultaneous transmission of a message to a group of recipients.
3. Transport Layer:
Traditionally
the transport layer was represented in TCP/IP by two protocols: TCP and UDP. IP
is a host-to-host protocol, meaning that it can deliver a packet from one
physical device to another. UDP and TCP are transport level protocols
responsible for delivery of a message from a process (running program) to
another process. A new transport layer protocol, SCTP, has been devised to meet
the needs of some newer applications.
a. User Datagram Protocol
The User
Datagram Protocol (UDP) is the simpler of the two standard TCP/IP transport
protocols.
It is a process-to-process protocol that adds only port addresses, checksum
error control, and length information to the data from the upper layer.
b. Transmission Control Protocol
The Transmission
Control Protocol (TCP)
provides full transport-layer services
to applications. TCP is a reliable stream transport protocol. The term
stream, in this context, means connection-oriented: A connection must be
established between both ends of a transmission before either can transmit
data.
c. Stream Control Transmission Protocol
The
Stream Control Transmission Protocol (SCTP) provides support for newer
applications
such as voice
over the Internet. It is a transport layer protocol that combines the best
features of UDP and TCP.
4. Application Layer:
The
application layer in TCP/IP is equivalent to the combined session, presentation
and application layers in the OSI model. Many protocols are defined at this
layer.
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