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LAYERS 425(MAC) sub-layer, which controls how a computer on the
network gains access to the data and permission to trans-
mit it, and the logical link control (LLC) sublayer, which
controls frame synchronization, flow control, and error
checking. This layer describes the specification of inter-
face cards to specific types of networks, e.g., Ethernet, To-
ken Ring, etc. Protocols from the TCP/IP suite that occupy
this layer are SLIP and PPP.
Thenetworklayer accepts messages from the source
host, converts them into packets of bytes, and sends them
through the data link. This layer deals with how a route
from the source to the destination is determined. This
layer also deals with congestion control. The IP, address
resolution protocol (ARP), reverse ARP (RARP), Internet
control message protocol (ICMP), and IGMP belong to
this layer.
Thetransportlayer transfers data, and is responsible
for end-to-end error recovery and flow control. The TCP
and user datagram protocol (UDP) belong to this layer.
Thesessionlayer establishes, manages, and terminates
connections between the programs on the two hosts that
are communicating. The concepts of ports and connec-
tions belong to this layer.
Thepresentationlayer provides independence from
possibly different data representations of the host ma-
chines. The HTTP (hypertext transfer protocol), FTP (file
transfer protocol), telnet, DNS, SNMP (simple network
management protocol), NFS (network file system), etc.
belong to this layer.
Theapplicationlayer supports the end-user invoked
programs. FTP, HTTP, IMAP (Internet message access
protocol), NTP (network time protocol), POP3 (post office
protocol version 3), rlogin (remote login), SMTP (simple
mail transfer protocol), SNMP, SOCKS, telnet, X-Window,
Web services, etc. are part of this layer.
The OSI model is officially recognized by the ISO (In-
ternational Standards Organization). The practical world
of TCP/IP networking was in full use by the time the OSI
model was formulated. Its unofficial model, referred to as
theTCP/IP model,the U.S. Department of DefenseDoD
model,or even more simply theInternet model,organizes
the networks into the following five layers:Thephysicallayer, matching the OSI physical layer.
Thelinklayer, similar to the OSI link layer. This is also
called the network access layer. It defines the network
hardware and device drivers.
TheIPlayer, also called the Internet protocol layer. This
layer deals with identifying individual hosts and rout-
ing. It is similar in functionality to the OSI network
layer.
Thetransportlayer, containing the UDP and TCP. The UDP
provides “connectionless service” and TCP provides
“connection-oriented service.” UDP does not guaran-
tee, unlike the OSI transport layer, reliable delivery.
Theapplicationlayer, handling the responsibilities of the
session, presentation, and application layers of the OSI
model.The data unit of each layer isencapsulated by the
layer below it. An application data unit [AP] is encap-TCP
UDP
ICMP other
IP layer IP layer IP layer IP layer
Physical Physical Physical Physical Physical
Figure 1: Protocol stack.sulated by the TCP layer, which prefixes a TCP header
TCPH as [TCPH [AP]]. The IP layer encapsulates it as
[IPH [TCPH [AP]]], where IPH is the IP header. Assum-
ing the Ethernet, it encapsulates it as [EH [IPH [TCPH
[AP]]] FSC], where EH is the Ethernet header and FSC is
a frame check sequence, generated by the Ethernet hard-
ware.Protocol Stack
The computer network literature talks of protocol stacks.
Each item in the stack is a layer of software that im-
plements a collection of protocols. In Fig. 1, the relative
heights indicate the level of functionality and the depen-
dency of the layers. Except for the layer marked “Physical”
all others are software layers. The protocol stack is an in-
tegral component in a modern operating system.Lower Layers
In this section, we describe the layers that support the IP.
The TCP/IP protocol family runs over a variety of network
media including IEEE 802.3 (Ethernet) and 802.5 ( Token
Ring) LANs, X.25 lines, satellite links, and serial lines.Ethernet
Ethernet can support IP and other protocols simultane-
ously. Ethernet was invented in 1972 at Xerox PARC by
Metcalfe and his colleagues. They named it so as to em-
phasize the capability of the physical medium to carry
bits to all hosts, analogous to the “luminiferous ether” of
old physics. The word Ethernet now refers to 10 mega-
bits-per-second (Mbps) transmission speed, Fast Ethernet
refers to 100 Mbps, and Gigabit Ethernet refers to 1000
Mbps. The media varieties include the current twisted-
pair (10baseT, with RJ45 connectors), the original thick
coaxial system (10base5), thin coaxial (10base2), and fiber
optic systems (10basesF). Ethernet has been standardized
by the Institute of Electrical and Electronics Engineers as
IEEE 802.3.
All hosts attached to an Ethernet are connected to a
shared signaling medium. Ethernet signals are transmit-
ted serially, one bit at a time, over the shared medium that
every attached host can observe. Ethernet is a broadcast
medium. That is, when a frame is sent out on the Ether-
net, every controller on the local unswitched network can
see the frame. To send data, a host waits for the channel to
become idle, and transmits its frame. All hosts on the net-
work contend equally for the transmission opportunity.
Access to the shared medium is governed by the MAC
mechanism based on the Carrier Sense Multiple Access
with Collision Detection (CSMA/CD) system. This ensures
that access to the network channel is fair, and that no sin-
gle host can lock out other hosts. If two or more devices
try to transmit at the same instant, a transmit collision is