Computer Communication Network

1. 1
TCP/IP Protocol Suite
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 interface, 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.
The topics discussed in this section include:
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer

2. 2
OSI and TCP/IP Protocol Stack
OSI Model TCP/IP Hierarchy Protocols
7th
Application Layer
6th
Presentation Layer
5th
Session Layer
4th
Transport Layer
3rd
Network Layer
2nd
Link Layer
1st
Physical Layer
Application Layer
Transport Layer
Network Layer
Link Layer
Link Layer : includes device driver and network interface card
Network Layer : handles the movement of packets, i.e. Routing
Transport Layer : provides a reliable flow of data between two hosts
Application Layer : handles the details of the particular application

3. Packet Encapsulation
 The data is sent down the protocol stack
 Each layer adds to the data by prepending headers
22Bytes 20Bytes 20Bytes 4Bytes
64 to 1500 Bytes

4. IP-IPV4 Datagram
Vers Header Len Types Of Service Datagram Length(bytes)
16-bit Identifier Flags 13-bits Fragmentation Offset
Time-to-Live Protocol Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options(if Any)...
Data...
0 4 8 16 19 24 31
Field Purpose
Vers IP version number
Header Len Length of IP header (4 bits)
TOS Type of Service
T. Length Length of entire datagram
Ident. IP datagram ID (for frag/reassembly)
Flags Don’t/More fragments
Frag Off Fragment Offset
Field Purpose
TTL Time To Live - Max # of hops
Protocol Higher level protocol (1=ICMP,
6=TCP, 17=UDP)
Checksum Checksum for the IP header
Source IA Originator’s Internet Address
Dest. IA Final Destination Internet Address
Options Source route, time stamp, etc.
Data... Higher level protocol data

5. Cont….Version No: These 4 bit specify the IP protocol version of the data gram, By looking into
datagram version, the routers can determine how to interpret the remainder of the IP datagram .
Header length: These 4 bits are needed to determine where IP datagram the actually begins.
Types of service: Type of service bits were included in IPV4 header r to allow different types of
IP datagram ( for example, datagram particulalarly requiring Low Delay, High throughput , or
reliability) to be distinguished from each other.
Data gram length: This is total length of the IP datagram(header plus data)measured in bytes .
Since this field is 16 bits long, theoretically max size of IP datagram is 65,535 bytes
Identifier, flag, fragmentation offset:
•Identification:IP datagram ID (for frag/reassembly)
•Flags: Don’t/More fragments
•Frag. Off: Fragment Offset
Time-to-live: TTL field is included to ensure that datagram do not circulate forever in the
network. This field is decremented by 1each time the data gram is processed by the routers. If
TTL field is reaches to 0, the datagram must be dropped.
Protocol: This field is used only when an IP datagram reaches to its final destination. The value
of this field indicates the specific transport-layer protocol to which the data portion of this IP
datagram should be passed
Header checksum: The header checksum aids a router in detecting bits errors in a receiving IP
Datagram.
Source & destination IP addresses: When a source creates a datagram, it inserts its IP address
into the source IP address field and inserts the address of the ultimate destination into destination
IP address field. Often the source host determine the destination address via a DNS lookup.
Option: The option fields allow an IP header to be executed.
Data(Payload): Data field of IP datagram contains the transport-layer segments(TCP or UDP) to
be delivered to the destination, However, the data field can carry other types of data, such as
ICMP messages

6. Address Resolution
• Address resolution is the mapping of one address to
another
• It is generally a mapping between a Layer 3
network address (logical) and a Layer 2 hardware
address (physical)
• The reverse process is also address resolution

7. Address Resolution
• Address resolution is accomplished in one of
the following ways:
– Table lookup is a rarely used method of address
resolution
– Closed-form computation is only used in very
specific networks and is time consuming to
configure
– Dynamic message exchange is the most
common and involves an exchange of
information between two hosts

8. Address Resolution
• Address Resolution Protocol (ARP) is used when
an IP host has a known destination IP address
(Layer 3) and it needs to retrieve the
corresponding Layer 2 MAC address from the
destination host
• The ARP cache is used to further reduce the need
for broadcasts by storing the IP-to- MAC mapping
in memory for a specified duration

9. 9
Figure 1: ARP operation

10. Address Resolution
The ARP Process:
Client A sends out an ARP broadcast
All clients receive and process the broadcast
frame but only Machine B responds
Client A receives the response and places
Machine B’s MAC address in its ARP cache

11. 11
Figure 2: Four cases using ARP