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TCP
TCP Congestion Control
& Quality of Service
Presenting By:Aditya Kumar Dakua
Mitali Samal
Rahul Kumar Nayak
Sribishnu Das
Omm Prasad Routray
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Contents
TCP/IP Model
Protocols using in TCP/IP model.
Layers description
Difference between TCP/IP & OSI.
TCP(Transmission Control Protocol)
TCP Segment format
Congestion Control
Open-Loop
Closed-Loop
Causes of congestion
General Policies of congestion control
QOS(Quality Of Service)
 Techniques for achieving good quality of service.
TCP/IP MODEL
TCP/IP
 The TCP/IP protocol suite was developed prior to the
OSI model.
 The original TCP/IP protocol suite was defined as
having four layers: host to network, internet, transport
and application.
 But as compared to OSI model TCP/IP has five layers.
APPLICATION
PRESENTATION
Application layer
SMTP
FTP
HTTP
DNS
SNMP
TELNET
SESSION
TRANSPORT
NETWORK
SCTP
ICMP
TCP
IGMP
UDP
IP
RARP
DATA LINK
Host-to-network
PHYSICAL
ARP
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PROTOCOLS USING IN TCP/IP
MODEL
SMTP- Simple Mail Transfer Protocol
FTP- File Transfer Protocol
HTTP- Hyper Text Transfer Protocol
DNS- Domain Name Service/System
SNMP- Simple Network Management Protocol
TELNET- Terminal Network
SCTP- Stream Control Transmission Protocol
TCP- Transmission Control Protocol
UDP- User Datagram Protocol
ICMP- Internet Control Message Protocol
IGMP- Internet Group Message Protocol
RARP- Reverse Address Resolution Protocol
ARP- Address Resolution Protocol
IP- Internet Protocol
LAYERS DESCRIPTION
PHYSICAL AND DATA LINK LAYER
 TCP/IP does not define any specific protocol.
 A network in TCP/IP internetwork can be local-area network
or a wide-area network.
NETWORK LAYER
 TCP/IP supports the internetworking protocol.
 IP in turn uses four supporting protocols: ARP,RARP,ICMP
and IGMP.
TRANSPORT LAYER
 It is represented in TCP/IP by two protocols.
 IP is host to host protocol.
 UDP and TCP are transport level protocols.
APPLICATION LAYER
 It is equivalent to combined session, presentation and
application layer in the OSI model.
DIFFERENCE BETWEEN OSI & TCP/IP
TCP/IP
o Basically It has 4 Layers.
Such as Application,
Transport, Internet, Network
access Layer.
o Application layer of TCP/IP
model is combination of 3 layers,
i.e Application, Presentation,
Session Layer
o The data link & physical layer
are combinely to form Network
access layer.
o When TCP/IP compared with the
OSI model then it acts as 5 layer.
i.e Application, Transport,
Internet, Data link & Physical
Layer.
OSI
o It has 7 Layers
Such as Application,
Presentation, Session,
Transport, Network, Data
link, Physical Layer.
o In this model each layer
perform a specific task by the
respective layer within their
bounded area.
TCP (Transmission control protocol)
 TCP is a transport layer protocol which provides
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connection-oriented service.
It is a process-to-process or program-to-program protocol.
It creates a virtual connection between two TCPs to send
data.
This protocol provides error recovery , flow control &
reliability which are not available in UDP.
TCP is the protocol used by major Internet applications
such as the world wide web, email, remote administration
& file transfer.
Cont…
 It is a sliding window protocol that provides handling for
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both timeouts and retransmissions.
TCP establishes a full duplex virtual connection between
two endpoints. Each endpoint is defined by an IP address
and a TCP port number.
The operation of TCP is implemented as a finite state
machine.
It does not support multicasting & broadcasting.
TCP maintains a checksum on its header & data.
A TCP connection is a byte stream not a message stream.
A Stream of 8-bit bytes is exchanged across the TCP
connection between the two applications.
HEADER FORMAT OF TCP (TCP SEGMENT FORMAT)
Header
Source port address
16 bits
Data
Destination port
address 16 bits
Sequence number 32 bits
Acknowledgement number
32 bits
Hlen
4
bits
Reser
ved 6
bits
U A P
R C S
G K H
R
S
T
S
Y
N
Check sum 16 bits
Options & Padding
F
I Window
N size
16 bits
Urgent pointer
16 bits
Source port address
• This is a 16-bit field that defines the port number of
the application program in the host that is sending the
segment.
Destination port address
• This is a 16-bit field that defines the port number of
the application program in the host that is receiving
the segment.
Sequence number
• This 32-bit field defines the number assigned to the
first byte of the segment which indicates the sequence
number of that sequence.
Acknowledgement Number
• 32-bit field.
• It defines the byte number that the receiver of the
segment is expecting to receive from the sender.
HLEN(Header length)
• This 4-bit field indicates the number of 4 byte words
in the TCP header.
Reserved
• This is a 6-bit field reserved for future use.
Controlled field
 This field defines six different control bits or flags.
 URG
 ACK
 PSH
 RST
 SYN
 FIN
URG(Urgent)
• Urgent pointer valid flag.
• If it is set to 1 the urgent pointer field valid.
ACK(Acknowledgement)
 Acknowledgment number valid flag.
• If the ACK flag is set to 1 the acknowledgement number field
is valid.
PSH(Push)
 Push flag
 If the field is one then it will push the data segment for maximum
range.
RST(Reset)
 Reset connection flag.
• If this flag value is 1 then the connection is reset or reestablished.
SYN(Synchronized)
 Synchronize sequence numbers flag.
• It is used for synchronized sequence number during connection.
FIN
 Terminate the connection.
 End of data flag.
 For termination of the connection this field is selected to 1.
Congestion Control
 It refers to the technology & mechanism that can
either used to prevent congestion before it happens or
removes congestion after it happens.
 This can be broadly defined into two category.
->Open Loop
->Closed Loop
Congestion Control
Open Loop
(Before Congestion)
Closed Loop
(After Congestion)
Retransmission
Back pressure
Window Policy
Choke Packet
Acknowledgement
Implicit Signaling
Discarding Policy
Explicit Signaling
Admission Policy
Backward
Forward
Open-Loop
 In open-loop congestion control policies are applied to
prevent congestion before it happens .in this mechanism
congestion control is handled either the source or the
destination.
Retransmission Policy
 It is sometimes unavoidable. If the sender feels that a send
packet is lost or corrupted the packet needs to be
retransmitted.
Window Policy
 According to this policy the type of window at the sender may
also affect congestion . The selective repeat window is better
than the Go-Back-N window for congestion control.
Acknowledgement Policy
 This policy imposed by the receiver may also affect
congestion .if the receiver does not acknowledge every
packet it receives, it may slow down the sender & prevent
congestion.
Discarding policy
 A good discarding policy by the routers may prevent congestion
& at the same time may not harm the integrity of the
transmission.
Admission Policy
 An admission policy ,which is a quality of service mechanism
can also prevent congestion in virtual circuit networks
switches in a flow first check the resource requirement of flow
before admitting to the network.
Closed loop
 This congestion control mechanisms try to alleviate
congestion after it happens . several mechanisms have been
used by different protocols.
Back Pressure
 It refers to a congestion control mechanisms in which a
congested node stops receiving data from the immediate
upstream node or nodes . this may causes the upstream node
or nodes to become congested.
 In turn reject data from their upstream node or nodes . this
technique can be applied only to virtual circuit networks in
which each node knows the upstream node from which a flow
of data is coming.
Backpressure method for alleviating congestion
Choke Packet
•It is a packet sent by a node to the source to inform it of
congestion. In this method the warning is from the router,
which has encountered congestion, to the source station directly.
The intermediate nodes through which the packet has traveled
are not warned.
Choke Packet
Implicit signaling
•There is no communication between the congested node or nodes
& the source . The source guesses that there is congestion some
where in the network from other symptoms.
Explicit Signaling
•In explicit signaling method the signal is included in the
packets that carry data . It can occur in either the forward or the
backward direction.
Backward Signaling
•In this signaling a bit can be set in a packet moving in the
direction opposite to the congestion.
Forward Signaling
•In this a bit can be set in a packet moving in the direction
of the congestion.
Causes of Congestion
 If the rate at which packets arrive and queue of exceeds the
rate at which packets can be transmitted the queue size
grows , the delay experienced by a packet goes to infinite
 The other causes of congestion is slow processor speed . if
the router’s CPU speed is slow and performing task like
queuing buffers table updating.queues are built up even
through the line capacity is not fully utilised.
 The bandwidth of the links is also important in congestion .
The links to be used must be of high bandwidth to avoid the
congestion.
 Any mismatch between parts of the system also causes
congestion , up grading the processor’s speed but not
changing the links or vice-versa will causes the congestion.
General Policies of Congestion
Control
 General policies of congestion control can be divided into
two groups ->Open Loop
->Closed Loop
Open Loop
 Open loop solution attempt to solve the problem by good
design in essence to make sure it dose not occur in the first
place .once the system is up and running, midcourse
corrections are not made.
 Tools for doing open loop control include deciding when to
accept new traffic, deciding when to discard packets and
which ones ,and making scheduling decisions at various
points in the network.
Closed Loop
 Closed loop solutions are based on the concept of a feedback
loop this approach has three parts when applied to congestion
control.
 Monitor the system to detect when and where congestion
occurs.
 Pass this information to place where action can be taken.
 Adjust system operation to correct the problem.
 Metrics to monitor the subnet for congestion are the average
queue lengths , percentage of all packets discarded for lack of
buffer space , average packed delay, the number of packets that
time out are retransmitted etc.
Quality Of Service(QOS)
 QOS is an internetworking issue that has been discussed more
than defined . we can roughly defined QOS as something that a
flow seeks to attain.
 It requires a stream of packet from a source to a destination
which is known as flow & application with their quality of
service requirement.
Techniques for achieving good
quality of service
There are 4 methods for improving the QOS
 Scheduling.
 Traffic shaping.
 Admission Control.
 Resources Reservation.
Scheduling
•The intermediate device like router switches
processes the packets which are coming from different
source host . so to process the packets to their
corresponding receiver router should follow a good
scheduling mechanism.
It is divided into 3 categories:•FIFO Queue
•Priority Queue
•Weighted Queue
FIFO queue
•In FIFO(First-in first-out) queuing packets wait in a
buffer until the node is ready to process them . if the
average arrival rate is higher than the average
processing rate . the queue will fill up & new packets
will be discarded.
Priority queuing
•In priority queuing , packets are first assigned to a priority class
each priority class has its own queue . The packets in the
highest –priority queue are processed last . The system dose not
stop serving a queue until it is empty .
•A priority queue can provide better QOS than FIFO queue.
Weighted fair queuing
•A better scheduling method is weighted pair queuing . In this
technique the turning switch selects three packets from first
queue then two packets from second queue &one packet from
the third queue this cycle repeats until all the queues are
empty.
Traffic Shaping
•It is a mechanism to control the amount & the rate of
the traffic sent to the network.
•There are two techniques exists to shape traffic.
Leaky Bucket
Token Bucket
Leaky bucket
Leaky bucket implementation
•A leaky bucket algorithm shapes brusty traffic
into fixed-rate traffic by averaging the data rate. It
may drop the packets if the bucket is full.
Token bucket
•Token bucket is same as leaky bucket algorithm but it
allows the flow of brusty traffic at a regulated maximum
rate.
Admission Control
•Admission control refers to the mechanism used by
a router , or a switch to accept or reject a flow based
on predefined parameters called flow specifications.
Resource Reservation
•A flow of data needs resources such as buffer, bandwidth
,CPU time & so on.
The QOS is improved if these resources are reserved before
hand.