here discussed about each topic of network layer like congestion control mechanism , error control , flow, tcp , udp, etc

1. transport layer
lecturer
Mr. Arvind Pandey

2. Connectionless Transport: UDP (User
Datagram Protocol)
 UDP is the connectionless transport protocol in the TCP/IP protocol
stack.
 UDP is a simple protocol that exchanges datagram without guaranteed
delivery.
 It relies on higher layer protocols to handle error and retransmit data.
 UDP doesn’t use window or Asks reliability is provided by application
layer protocols.
 UDP is designed for applications that do not need to put sequence of
segments together.
 The following application layer protocols use UDP: TFTP, SNMP, DHCP,
and DNS

3. contd.
 Used in transport layer
 Offers unreliable connectionless service
 Provides faster service than that of TCP.
 Offers minimum error checking mechanism.
 Supports multicasting because connectionless.
 Offers minimum flow control mechanism.
 Also used by SNMP (Simple Network Management Protocol)

7. congestion
 congestion is a network may occur if the load on
the network is greater that capacity of network .
 eg. so many person are downloading files on
same network because of network will be slow .

8. Causes of Congestion
 When there are more input lines and less or single output lines.
 When there is slow router i.e., if routers CPU’s, are slow
 If the router has no free buffers i.e., insufficient memory to hold queue of
packets.
 If the components used in subnet (link, router, switches, etc) have different
traffics carrying and switching capacities, then congestion occurs.
 If the bandwidths of the lines are low, it can’t carry large volume of packets and
caused congestion. Hence, congestion cannot be eradicated but can be
controlled.

10. Open Loop Congestion Control
 Open loop congestion control policies are applied to
prevent congestion before it happens.
 The congestion control is handled either by the
source or the destination.
Policies adopted by open loop congestion control –
A. Retransmission Policy
B. Window Policy
C. Discarding Policy
D. Acknowledgment Policy
E. Admission Policy

11. Closed Loop Congestion Control
 Closed loop congestion control technique is used to treat or
alleviate congestion after it happens.
Several techniques are used by different protocols; some of them
are:
A. Backpressure
B. Choke Packet Technique
C. Implicit Signaling
D. Explicit Signaling

12. Congestion Control Algorithms
 Leaky Bucket Algorithm
 Token Bucket Algorithm
 Choke Bucket Algorithm

13. leaky bucket algorithm

14. leaky bucket algorithm contd.

16. Token Bucket Algorithm
 The Token Bucket Algorithm compare to Leaky Bucket
Algorithm allow the output rate vary depending on the size of
burst.
 In this algorithm the buckets holds token to transmit a
packet, the host must capture and destroy one token.
 Tokens are generated by a clock at the rate of one token
every ∆t sec.
 Idle hosts can capture and save up tokens (up to the max.
size of the bucket) in order to send larger bursts later

17. Some advantage of token Bucket over
leaky bucket
 If bucket is full in token Bucket , token are discard
not packets. While in leaky bucket, packets are
discarded.
 Token Bucket can send Large bursts can faster rate
while leaky bucket always sends packets at
constant rate.

18. difference between token bucket and leakey
bucket algorithm

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Relation between transport layer and networklayer
• The Network layer and transport layer are responsible for moving messages from end to end in
anetwork.
• Theyare soclosely tied together that they are usuallydiscussed together.
• The transport layer performs three functions: linking the application layer to the
network, segmenting (breaking long messages into smaller packets for transmission), and session
management (establishing an end-to-end connection between the sender and receiver).
• The network layer performs two functions: routing (determining the next computer
to which the message should be sent to reach the final destination) and addressing (finding
the addressof thatnext computer).
• There are several standard transport and network layer protocols that specify how packets
are to be organized, in the sameway that there are standards for data link layer packets,

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RELIABLE DATATRANSFER PROTOCOL
 Located on the Transport Layer like TCP or UDP.
 RDT protocol ensure delivery of all packets and enable the receiver to deliver the
packets in order to its application layer.
 RDT protocol can be designed using some basic tools Known as a stop-and-wait protocol.
 Stop-and-wait has poor performance in a long-distance connection.
 To improve transmission rate, RDT protocol must use pipelining.
 TCP ensures all parts of a message to reach the destination undamaged.
 But sometimes parts of a message fails to reach the destination or damaged message deliver.
 Thus, TCP has to detect and recover from lost or damaged message , Recovering from errors is
called reliable data transfer.

24. Pipelined ReliableData
TransferProtocols
• Thisprotocolallowsfor multiple datapackets to
be sent while waiting for ACK.Results better
networkutilization
• senderandreceiverneedbuffersto hold
multiple packets
• packetsneedsequencenumbersin order to
identify them
• anacknowledgement needs to refer to
corresponding sequencenumber
• retransmissioncangiveriseto duplicate
packets
• sequencenumbersin packetsallowreceiver
to detect duplicates

25. Protocol pipelining is a technique in which multiple requests are written out to a
single socket without waiting for the corresponding responses. Pipelining can be used
in various application layer network protocols, like HTTP/1.1, SMTP and FTP.

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ADVANTAGE
• much better than stop-and-wait
DISADVANTAGE
• More complicated to deal with reliability issues, e.g., corrupted, lost, out of orderdata.
• Two generic approaches to solving this
• Go-Back-N protocols
• Selective repeat protocols

27. Flow Control
 When a data frame (Layer-2 data) is sent from one host to another
over a single medium, it is required that the sender and receiver
should work at the same speed.
 That is, sender sends at a speed on which the receiver can process
and accept the data.
 What if the speed (hardware/software) of the sender or receiver
differs? If sender is sending too fast the receiver may be overloaded
and data may be lost.
Two types of mechanisms can be deployed to control the flow:
Stop and Wait
Sliding Window

28. Stop and Wait
This flow control mechanism forces the sender after transmitting a data frame to
stop and wait until the acknowledgement of the data-frame sent is received.

29. Sliding Window
 In this flow control mechanism, both sender and receiver agree on the
number of data-frames after which the acknowledgement should be sent.
 As we learnt, stop and wait flow control mechanism wastes resources, this
protocol tries to make use of underlying resources as much as possible.

30. Error Control
 When data-frame is transmitted, there is a probability that data-frame may
be lost in the transit or it is received corrupted.
 In both cases, the receiver does not receive the correct data-frame and
sender does not know anything about any loss.
 In such case, both sender and receiver are equipped with some protocols
which helps them to detect transit errors such as loss of data-frame.
 Hence, either the sender retransmits the data-frame or the receiver may
request to resend the previous data-frame.

31. Requirements for error control mechanism:
 Error detection - The sender and receiver, either both or any, must confirm
that there is some error in the transit.
 Positive ACK - When the receiver receives a correct frame, it should
acknowledge it.
 Negative ACK - When the receiver receives a damaged frame or a duplicate
frame, it sends a NACK back to the sender and the sender must retransmit the
correct frame.
 Retransmission: The sender maintains a clock and sets a timeout period.
 If an acknowledgement of a data-frame previously transmitted does not arrive
before the timeout ,the sender retransmits the frame, thinking that the
frame or it’s acknowledgement is lost in transit
There are three types of techniques available to control the errors by
Automatic Repeat Requests (ARQ):

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• Stop and wait ARQ mechanism does not utilize the resources at their best.
• When the acknowledgement is received, the sender sits idle and does nothing.
• In Go-Back-N ARQ method, both sender and receiver maintain a window.
Go-Back-N(GBN)

36.  The sending-window size enables the sender to send multiple frames without
receiving the acknowledgement of the previous ones.
 The receiving-window enables the receiver to receive multiple frames and
acknowledge them.
 The receiver keeps track of incoming frame’s sequence number.
 When the sender sends all the frames in window, it checks up to what sequence
number it has received positive acknowledgement.
 If all frames are positively acknowledged, the sender sends next set of frames.
 If sender finds that it has received NACK or has not receive any ACK for a
particular frame, it retransmits all the frames after which it does not receive any
positive ACK.

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Advantage over Go-Back-N:
• Fewer Retransmissions.
Disadvantages:
• More complexity at sender and receiver
• Each frame must be acknowledged individually
• Receiver may receive frames out of sequence

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Selective Repeat
• In Go-back-N ARQ, it is assumed that
the receiver does not have any buffer
space for its window size and has to
process each frame as it comes.
• This enforces the sender to retransmit
all the frames which are not
acknowledged.
In Selective-Repeat ARQ, the receiver while keeping
track of sequence numbers, buffers the frames in
memory and sends NACK for only frame which is missing
or damaged.
The sender in this case, sends only packet for which
NACK is received.

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ADVANTAGE
• The sender only retransmits frames, for which a NAK is received.
• This will increase the efficiency of the protocol.
DISADVANTAGE
• More complexity at sender and receiver
• Each packet must be acknowledged individual
• Receiver may receive packets out of sequence