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Lecture-4
TCP/IP-Overview:
The history goes back to ARPANET – a research network sponsored by DoD US Govt. It
eventually connected hundreds of universities and govt installations, using leased telephone
lines.
When satellite and radio networks were later added, the existing protocol had a trouble
interworking with them, so a new reference architecture was needed, thus the ability to connect
multiple networks in a seamless way was one of the major design goals. The architecture became
known as TCP/IP Reference Model. It was first described by Cerf and Kahn in 1974
Design Goal:
The network should survive loss of subnet hardware, without existing conversation being broken,
as a result of attack from Soviet Union.
Since the applications with divergent requirements were envisioned, ranging from transmitting
files to real time speech transmission, a flexible architecture was needed.
Link Layer:
The physical layer and data link layer of OSI reference model is combined together to form a
link layer.
Purpose:
It describes what links such as serial lines and classical Ethernet must do to meet the
needs of the connectionless internet layer and describes how data should be
physically generated and transmitted over the physical medium by the network
devices.
Following are the functions and responsibilities of link layer :
At physical level
 At physical level data consist of a stream of bits (sequence of 0's and 1's).
 Defines how the bits are encoded into electrical signals by the network devices.
 Synchronizing the sender and receiver at the bit level.
 Line configuration - point-to-point or point-to-multipoint
 Transmission mode - simplex, half duplex or full duplex
 Topology - star, ring, tree, bus etc.
 Mechanical specification of cables and connectors.
 Specification of electronic factors like voltage, frequency, impedance etc.
At logical level
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At logical level data consists of frames.
Encapsulates IP packets or message received from the higher layer into frames.
Synchronization of frames.
Flow control, error detection and correction of frames at LLC sub-layer.
Physical addressing, access control at MAC sub layer.
Protocols used: Ethernet, Token ring, X.25, Frame relay, FDDI, GPRS.
Network Devices : Network adaptor, Modem, Hub, Switch.
The internet layer
Purpose
 Permit host to inject packets into any network and have them travel independently to
destination (on different network).
 Packets may arrive out of order, in which case it is the duty of higher layers to arrange
them.
 Internet layer defines an official packet format and protocol called IP(internet protocol),
plus a companion protocol called ICMP(Internet control Message Protocol) that help it
function
 Routing packet to correct address
 Unicast and multicast routing – the routing protocol do not take part in routing, but they
creates forwarding tables for routers to help them in the routing process
IP is a connectionless protocol that provides no flow control, no error control, and no congestion
control services. If any of these services is required by any application, the application should
rely only on the Transport layer protocol.
Routing layers has auxiliary protocols that help IP in its delivery and routing task. These are:
IP (IPv4,IPv6)
ICMP: internet control message protocol—helps IP to report some problems when routing
IGMP: Internet Group Management protocol – helps IP in multitasking
DHCP: Dynamic host Configuration protocol – help IP to get network layer address for hosts
ARP: Address resolution protocol—help IP to find link-layer address for hosts or router given
the network address
RARP
DHCP
Network devices: Router, Gateway.
Transport Layer:
It provides service to application layer, get a message from application program running on
source host and delivers it to corresponding program on the application layer of destination host.
It is logical end-to-end connection between two hosts at the transport layer. It gets the message
from the application layer, encapsulates it in transport layer packet(called a segment or user
datagram in different protocols) and sends it, through logical connection, to transport layer on
destination.
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Core Protocols used by transport layer - TCP and UDP; In TCP packets are called as
segments, while in UDP it is called as datagrams.
TCP- It is connection oriented protocol, that first establishes a logical connection between
transport layer at two hosts before transferring data. It creates a logical pipe between two TCPs
for transferring a stream of bytes. TCP is:
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Connection-oriented - Establishes a logical connection between sender and receiver before
sending any data packets.
Reliable - Receiver sends acknowledgement of received packet to the sender.
Stateful - Both sender and receiver keeps track of the state of communication session.
It provides:
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Flow control - Matching data rate of two hosts
ii.
Error control – To ensure segment arrive without error, resending the corrupt one
iii.
Congestion control to reduce loss of segments due to congestion
UDP(User datagram protocol) – is connection-less protocol that transmits user datagram without
first creating the logical connection. In UDP each datagram is an independent entity without
being related to previous or next one.
UDP does not provide flow, error, congestion control, Its simplicity is small overhead, used for
sending short messages cannot afford retransmission of packet involved in TCP when message is
corrupt or lost. New protocol.
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Unreliable - No acknowledgement of received packet.
Stateless - Doesn't keep track of the state, works on timeout criteria.
Used in application like streaming media (video, audio), where reliability, error recovery
and flow control is not important.
Has less overhead.
New protocols Datagram Congestion Control Protocol (DCCP), Stream Control
Transmission Protocol (SCTP) etc are used.
It is the new emerging protocol used for multimedia
JPEG, MIDI- Musical Instrument Digital Interface, MPEG- Moving Picture Experts Group,
PICT, TIFF- Tagged Image File Format
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Network devices used : Router, Gateway.
Application Layer:
Communication at application layer is between two processes(two programs running at this
layers)To communicate, a process send a request to other processes and receive a response.
Process-to-Process communication is the duty of application layer. Protocols at this layers are:
HTTP – is vehicle for accepting www
SMTP- for email service
SNMP- Simple Network Management Protocol
FTP- for transferring file
TFTP- Trivial File Transfer Protocol – simple than FTP
TELNET – terminal Network
PING
SSH – Secure Shell- used for accessing sites remotely
DNS
IMAP - Internet Message Access Protocol
LDAP - Lightweight Directory Access Protocol
NTP - Network Time Protocol
POP3
RTP - Real-time Transport Protocol
RTSP
SIP
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Some common application layer functions are :
User interacts directly with the application software, where application process creates user
data and transmits this data to other application process on same or another host via one of the
transport layer protocol (TCP or UDP).
Network application software's uses specific protocol and socket or port number for
communication like web browser - http protocol - 80 port, file transfer - ftp protocol - 21 port,
Logging on remote computer - telnet protocol - 23 port etc
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Following are the functions of application layer :
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Formatting data - Text format may be in American Standard Code for Information
Interchange (ASCII) or Extended Binary Coded Decimal Interchange Code (EBCDIC).
 Representing data - Defines how data is to be represented to the end user.
 Process-to-process communication - E.g. client and server application software.
 Creating sessions - E.g. a web browser with one or more tabs, then its application layer
responsibility to create and manage separate sessions for each tab.
 Provides services like browsing, e-mail, file transfer, chatting, directory etc.
Working of the different Layers – A real Example
Without going into a great deal of technical detail, I will describe a general example of how these
layers work in real life. Assuming that the protocol stack being used is TCP/IP and the user is
going to use an FTP client program to get or send files from/to a FTP server the following will
essentially happen:
1. The user will start the FTP client
program on the sending computer.
2. The user will select the address (If
the user selected a name, a
description of DNS would need to
be described complicating this
scenario) and port of the server.
3. The user will indicate to the FTP
client program that they want to
connect to the server.
4. The application layer will send information through the presentation layer to the session
layer telling it to open a connection to the other computer at a specific address and port.
The presentation layer will not do much at this time, and the presentation layer is actually
handled by the FTP program.
5. The session layer will negociate through to the FTP server for a connection. There are
several synchronization signals sent between the client and server computers just to
establish the connection. This is a description of the sending of a signal from the client to
the server:
a) The session layer of the client will send a data packet (SYN) signal to the
transport layer.
b) The transport layer will add a header (TCP header) to the packet indicating what
the source port is and what the destination port is. There are also some other
flags and information that will not be discussed here to minimize complexity of
this explanation.
c) The network layer will add source IP address and destination IP address along
with other information in a IP header.
d) The datalink layer will determine (using ARP and routing information which is
not discussed here for brevity) the hardware address of the computer the data is
being sent to. An additional header (ethernet) will be added at this layer which
indicates the hardware address to receive the message along with other
information.
e) The information will be transmitted across the physical wire (hardware layer)
until the signal reaches the network card of the server computer. The signal may
go through several hubs or repeaters.
f) The FTP server will normally only look for ethernet frames that are matching its
own hardware address.
g) The FTP server will see the ethernet frame matching its address and strip the
ethernet header information and send it to the network layer.
h) The network layer will examine the IP address information, strip the IP header,
and if the IP address matches its own, will send the information to the transport
layer.
i) The transport layer will look at the TCP port number and based on the port
number and services being run, will strip the TCP header and send the
information to the appropriate program which is servicing the requested port.
j) At this point, the session layer in the FTP program will conduct a series of data
exchanges between itself through all the lower layers to the client computer until
a session is established.
6. At this point information may be sent through several FTP commands between the client
and the server. Every transmission passes through the network layers from the application
layer down to the hardware layer and back up the layers on the receiving computer.
7. When the client decides to terminate the session layer will be informed by the higher
layers and will negotiates for the closing of the connection.