The document discusses flow control in TCP. It explains that TCP uses a sliding window mechanism for flow control to balance the sender's transmission rate with the receiver's reception rate. The size of the sliding window is determined by the receiver's advertised window size and the congestion window size. TCP provides reliable, in-order delivery of data segments through error control methods like checksums, acknowledgements, and retransmissions. It also addresses problems like delayed ACKs, silly window syndrome, and solutions to improve TCP transmission efficiency.

1. Presented By
Anurag Jagetiya
Astt. Prof.
MLV Textile & Engineering College, Bhilwara
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TCP: Flow Control

2. • Introduction
• Flow Control in TCP
• Sliding Window
– Introduction
– Terminology
– Flow Control Through Sliding Window
• Problems with TCP transmission
– Delay ACK
– Silly Window Syndrome
– Solutions to Silly Window Syndrome
• TCP Error Control
– Checksum
– Ack
– Retransmission
• References
Outline
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TCP: Flow Control

3. • Flow Control balances the Sender’s rate (lightly
loaded) of sending data with the Receiver’s rate
(heavily loaded) of properly receiving data.
• Otherwise:
– Receiver will lost the data  Sender has to
retransmit the data  unnecessarily Consume
resources  Cost increases  Performance
decreases  User Frustration…
Introduction
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TCP: Flow Control

4. • Sender can not overrun receiver with lots of information.
• Allows receiver to restrict transmission until it has
sufficient buffer space to accommodate more data
• Flow control in TCP is similar to Data Link layer’s Go
Back N and Selective Repeat in following manners:
.
Flow Control in TCP
Data Link Layer
Protocol
Similarities with TCP Flow Control
Go Back N It does not use NAK
Selective Repeat Holds out of order segments until missing one
arrives
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TCP: Flow Control

5. • But, there are differences as well in flow control mechanism
of TCP and Data Link layer
• TCP uses Sliding Window to handle flow control.
Contd…
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TCP: Flow Control
TCP Flow Control Data Link Flow Control
Byte oriented Uses Frames
TCP sliding window is of variable
size
Fixed Size window

6. • Introduction
– All packets inside the window can be transmitted.
– When an acknowledgment for packet 1 is received
the window slides to the right and allows the
transmission of packet 9
Sliding Window in TCP
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TCP: Flow Control
1 2 3 4 5 6 7 8 9 10
Window
Figure 1: SlidingWindow

7. • Conceptually partitions the window into three
classes:
– Sent and acknowledged [left side, out side the window]
– Being Transmitted [inside the window]
– Waiting to be transmitted [right side, out side the window]
Contd..
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TCP: Flow Control
Figure 2: SlidingWindow Representation

8. • Only unacknowledged packets are retransmitted
– TCP keeps separate timer for each packet.
• Performance:
– Depends on the window size and the speed of the
underlying network.
– Increase window size so that sender is transmitting
packets as fast as the network can carry them.
– Eliminates network idle time.
Contd…
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TCP: Flow Control

9. Terminology used in Sliding Window:
– Imaginary window with two walls: Left and right
– The window can perform three activities namely:
– Opening, Closing, and Shrinking
Contd…
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TCP: Flow Control
NOTE: These activities are controlled by the Receiver and depend
on the congestion in the Network
Figure 3: SlidingWindow terminology

10. • Opening a window
– Moving the right wall to the right
– Allows more new bytes in the buffer that are eligible for
sending
• Closing a window
– Moving left wall to right
– Some bytes have been acknowledged
• Shrinking a window
– Moving right wall to left.
– Removing eligibility of some packets.
– [Warning: Strongly discouraged and not allowed in some
implementation ]
Contd…
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TCP: Flow Control
NOTE: Left wall cannot move to left, as this will revoke some of
previously sent Acknowledges (ACKs)

11. • Sliding window takes the help of two other windows
namely:
– Receiver window (rwnd)
– Congestion window (cwnd)
• Receiver window (rwnd)
– Value advertised by the opposite end in a segment
containing ACK.
– It is number of Bytes which can be accepted by the other
end before its buffer overflows.
Contd…
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TCP: Flow Control

12. • Congestion window (cwnd)
– Value determined by the Network (MTU) to avoid
congestion.
• The size of sliding window at one end is determined
by the lesser of two values:
i.e. Window_Size = Min (rwnd, cwnd)
• TCP window can be open/ close by the receiver. But
should not shrunk.
Contd…
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TCP: Flow Control

13. • Each window can vary in size over time
– Each ACK contains a window advisement
– Specifies how many additional Bytes of data the receiver
is willing to accept
– Sender increases or decreases sending window sized
based on the receiver’s advise
• Provides end-to-end flow control while Data link layer
provides
Flow Control through Sliding Window
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TCP: Flow Control
NOTE: In reality the window size is thousands of Bytes

14. Example of Flow Control
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TCP: Flow Control
In this
example, It is
assumed that
there is only
unidirectional
communicatio
n between
client and
server
Figure 4:
Flow Control [1]

15. • Sender will send the data based upon Receiver’s
Advertized Window Size (Free space in Buffer)
• If, this Window is Zero, sender will not send data.
And, what if Receiver’s advertisements are getting
lost??-------------Deadlock
• Sender may send a One byte segment to make the
receiver re-announce its next byte expected and
window size
Problems with TCP Transmission
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TCP: Flow Control

16. • Sending data in very small segments.
– E.g. Telnet connection to an interactive editor that reacts
to every key-stroke
– Worst Case: When a single character is arrived at TCP,
TCP creates a 21 Bytes segment and further, IP creates 41
Bytes Datagram…..
– Receiver, on the other hand, acknowledge by 40 Bytes
ACK. --------------------Wastage of scarce Band width
– Solution:
• Delay Acknowledgement and window advertisement
Contd…
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TCP: Flow Control

17. •The receiver waits until there is decent amount of
space in its incoming buffer before acknowledging
the arrived segments
•The delayed acknowledgement can be piggyback
on the returning data
• Reduces traffic
•Disadvantage: it may force the sender to
retransmit the unacknowledged segments
•To balance: should not be delayed by more than
500ms
Delayed Acknowledgement
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TCP: Flow Control

18. • Sending application program creates data slowly or
the receiving application program consumes data
slowly, or both. (e.g. 1 byte at a time)
• Server application asks client to create data slowly
if it goes continue, window size goes smaller and
smaller.
• At a time, it becomes smaller than its header size,
Making data transmission extremely inefficient.
Silly Window Syndrome
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TCP: Flow Control

19. • In the given figure:
• The receiving TCP announces a window size of 1 byte.
• The sending TCP sends only 1 byte.
Contd…
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TCP: Flow Control
Figure 5: Receiver’s Silly
Window [2]

20. • Clark’s Solution: For receiving application
– Do not send a window update of 1 byte until there is
enough space to accommodate a segment of max. size
or until half of the buffer is empty.
• Nagle’s Solution: For sending application
– When data come into sender one byte at a time.
– Send first byte and buffer all the rest until the outgoing
byte is acknowledged.
Solution to Silly Window Syndrome
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TCP: Flow Control
GOAL: Sender should not send small segment and the
receiver not ask for them

21. • Alternative method
– Receiving TCP will buffer data, and it can BLOCK a
READ request from application until it has large chunk
of data.
– This will reduce number of calls to TCP and Overhead.
– Trade-Off: Response time will also increase
– Acceptable in Non-Interactive applications like: file
transfer.
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TCP: Flow Control

22. • NET-SEAL
[Teaching Computer NETwork Through
Simulation Experiment and Animation Library]
Simulation of TCP Flow Control
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TCP: Flow Control

23. • TCP is reliable so it will deliver entire stream to the
destination without error/ loss/ duplicity.
• TCP error control provides mechanism for detecting
corrupted/ lost/ out of order and duplicate segment.
• Error detection and correction in TCP is achieved
through:
– Checksum
– Acknowledgement (ACK)
– Time out
Error Control in TCP
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TCP: Flow Control

24. • Checksum is used to find out whether the segments are
corrupted or not.
• TCP uses 16 bit checksum: Mandatory in every segment.
• E.g. 8 bit checksum
Checksum
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TCP: Flow Control
Calculating Checksum Verifying Checksum
Sum
Checksum
10101001
00111001
-----------
11100010
00011101 Sum
Complement
10101001
00111001
00011101
-----------
11111111
0000000

25. • TCP uses ACK to confirm receipt of data segments.
• Control segments that carry no data but consume a
sequence number are also Acknowledged.
• An ACK do not consume sequence number and it is
never Acknowledged.
Acknowledgement (ACK)
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TCP: Flow Control

26. • When a segment is corrupted, lost, or delayed, it is
retransmitted.
• TCP maintains a Retransmission Timer for each
connection
– The timer is started during a transmission. A timeout
causes a retransmission.
Retransmission
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TCP: Flow Control
Q: How to set this time out value for the RetransmissionTimer?

27. • Round Trip Time measurement
– The RTT is based on time difference between segment
transmission and ACK.
– But, TCP does not ACK each segment Each connection
has only one timer.
Retransmission (Contd…)
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TCP: Flow Control
Segment 1
Segment 4
ACK for Segment 1
Segment 2
Segment 3
ACK for Segment 2 + 3
Segment 5
ACK for Segment 4
ACK for Segment 5
RTT
#1
RTT
#2
RTT
#3
Figure 6: RTT [1]

28. • The setting of the retransmission timer is crucial for
efficiency
– Timeout value too small
• Results in unnecessary retransmissions
– Timeout value too large
• long waiting time before a retransmission can be
issued
• A problem is that the delays in the network are not
fixed.
• Therefore, the retransmission timers must be
Dynamic.
Retransmission (Contd…)
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TCP: Flow Control

29. • Retransmission Time Out (RTO)
• If the timer of any segment is expired, that segment is
simply re sent to the destination
• Fast Retransmission:
– If Retransmission time is large, one segment is lost and
receiver received so many out of order segments
– Store out of order segments in buffer
– Worse Case: Limited Buffer Size 
– Remedy: Three duplicate ACKs rule
Retransmission (Contd…)
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TCP: Flow Control

30. • If three or more duplicate ACKs are received in a
row, the TCP sender believes that a segment has
been lost.
• Then TCP performs a retransmission of what seems
to be the missing segment, without waiting for a
timeout to happen.
Fast Retransmission (3 ACK Rule)
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TCP: Flow Control

31. Example of Fast Retransmission
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TCP: Flow Control
Figure 7: 3 ACK [1]

32. 1. Behrouz A Forouzan, “Transport Layer”, in Data Communication and
Networking, 4th Edition, Tata McGraw Hill, Special Indian Edition, 2006.
2. Andrew S. Tanenbaum, “The Transport Layer”, in Computer Network, 4th
Edition, Prentice Hall India, 2005
3. www.Net-seal.net
References
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