UNIT –IV
TRANSPORT LAYER
Next Layer in OSI Model is recognized as Transport Layer (Layer-4). All modules and
procedures pertaining to transportation of data or data stream are categorized into this
layer. As all other layers, this layer communicates with its peer Transport layer of the
remote host.
Transport layer offers peer-to-peer and end-to-end connection between two processes
on remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and
then breaks it into smaller size segments, numbers each byte, and hands over to lower
layer (Network Layer) for delivery.
Functions
This Layer is the first one which breaks the information data, supplied by
Application layer in to smaller units called segments. It numbers every byte in the
segment and maintains their accounting.
This layer ensures that data must be received in the same sequence in which it
was sent.
This layer provides end-to-end delivery of data between hosts which may or may
not belong to the same subnet.
All server processes intend to communicate over the network are equipped with
well-known Transport Service Access Points (TSAPs) also known as port
numbers.
End-to-End Communication
A process on one host identifies its peer host on remote network by means of TSAPs,
also known as Port numbers. TSAPs are very well defined and a process which is trying
to communicate with its peer knows this in advance.
,For example, when a DHCP client wants to communicate with remote DHCP server, it
always requests on port number 67. When a DNS client wants to communicate with
remote DNS server, it always requests on port number 53 (UDP).
The two main Transport layer protocols are:
Transmission Control Protocol
It provides reliable communication between two hosts.
User Datagram Protocol
It provides unreliable communication between two hosts.
The transmission Control Protocol (TCP) is one of the most important protocols of
Internet Protocols suite. It is most widely used protocol for data transmission in
communication network such as internet.
Features
TCP is reliable protocol. That is, the receiver always sends either positive or
negative acknowledgement about the data packet to the sender, so that the
sender always has bright clue about whether the data packet is reached the
destination or it needs to resend it.
TCP ensures that the data reaches intended destination in the same order it was
sent.
TCP is connection oriented. TCP requires that connection between two remote
points be established before sending actual data.
TCP provides error-checking and recovery mechanism.
, TCP provides end-to-end communication.
TCP provides flow control and quality of service.
TCP operates in Client/Server point-to-point mode.
TCP provides full duplex server, i.e. it can perform roles of both receiver and
sender.
Header
The length of TCP header is minimum 20 bytes long and maximum 60 bytes.
Source Port (16-bits) - It identifies source port of the application process on the
sending device.
Destination Port (16-bits) - It identifies destination port of the application process
on the receiving device.
Sequence Number (32-bits) - Sequence number of data bytes of a segment in a
session.
Acknowledgement Number (32-bits) - When ACK flag is set, this number
contains the next sequence number of the data byte expected and works as
acknowledgement of the previous data received.
Data Offset (4-bits) - This field implies both, the size of TCP header (32-bit words)
and the offset of data in current packet in the whole TCP segment.
Reserved (3-bits) - Reserved for future use and all are set zero by default.
Flags (1-bit each)
NS - Nonce Sum bit is used by Explicit Congestion Notification signaling
process.
CWR - When a host receives packet with ECE bit set, it sets Congestion
TRANSPORT LAYER
Next Layer in OSI Model is recognized as Transport Layer (Layer-4). All modules and
procedures pertaining to transportation of data or data stream are categorized into this
layer. As all other layers, this layer communicates with its peer Transport layer of the
remote host.
Transport layer offers peer-to-peer and end-to-end connection between two processes
on remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and
then breaks it into smaller size segments, numbers each byte, and hands over to lower
layer (Network Layer) for delivery.
Functions
This Layer is the first one which breaks the information data, supplied by
Application layer in to smaller units called segments. It numbers every byte in the
segment and maintains their accounting.
This layer ensures that data must be received in the same sequence in which it
was sent.
This layer provides end-to-end delivery of data between hosts which may or may
not belong to the same subnet.
All server processes intend to communicate over the network are equipped with
well-known Transport Service Access Points (TSAPs) also known as port
numbers.
End-to-End Communication
A process on one host identifies its peer host on remote network by means of TSAPs,
also known as Port numbers. TSAPs are very well defined and a process which is trying
to communicate with its peer knows this in advance.
,For example, when a DHCP client wants to communicate with remote DHCP server, it
always requests on port number 67. When a DNS client wants to communicate with
remote DNS server, it always requests on port number 53 (UDP).
The two main Transport layer protocols are:
Transmission Control Protocol
It provides reliable communication between two hosts.
User Datagram Protocol
It provides unreliable communication between two hosts.
The transmission Control Protocol (TCP) is one of the most important protocols of
Internet Protocols suite. It is most widely used protocol for data transmission in
communication network such as internet.
Features
TCP is reliable protocol. That is, the receiver always sends either positive or
negative acknowledgement about the data packet to the sender, so that the
sender always has bright clue about whether the data packet is reached the
destination or it needs to resend it.
TCP ensures that the data reaches intended destination in the same order it was
sent.
TCP is connection oriented. TCP requires that connection between two remote
points be established before sending actual data.
TCP provides error-checking and recovery mechanism.
, TCP provides end-to-end communication.
TCP provides flow control and quality of service.
TCP operates in Client/Server point-to-point mode.
TCP provides full duplex server, i.e. it can perform roles of both receiver and
sender.
Header
The length of TCP header is minimum 20 bytes long and maximum 60 bytes.
Source Port (16-bits) - It identifies source port of the application process on the
sending device.
Destination Port (16-bits) - It identifies destination port of the application process
on the receiving device.
Sequence Number (32-bits) - Sequence number of data bytes of a segment in a
session.
Acknowledgement Number (32-bits) - When ACK flag is set, this number
contains the next sequence number of the data byte expected and works as
acknowledgement of the previous data received.
Data Offset (4-bits) - This field implies both, the size of TCP header (32-bit words)
and the offset of data in current packet in the whole TCP segment.
Reserved (3-bits) - Reserved for future use and all are set zero by default.
Flags (1-bit each)
NS - Nonce Sum bit is used by Explicit Congestion Notification signaling
process.
CWR - When a host receives packet with ECE bit set, it sets Congestion
