1TCP: Flow Control

Presented ByAnurag Jagetiya

Astt. Prof. MLV Textile & Engineering College, Bhilwara

Flow Control in TCP

Anurag Jagetiya

2TCP: Flow Control

• 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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3TCP: Flow Control

• 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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4TCP: Flow Control

• 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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5TCP: Flow Control

• 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 Data Link Flow Control

Byte oriented Uses Frames

TCP sliding window is of variable size

Fixed Size window

6TCP: Flow Control

• 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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1 2 3 4 5 6 7 8 9 10

WindowFigure 1: Sliding Window

7TCP: Flow Control

• 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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Figure 2: Sliding Window Representation

8TCP: Flow Control

• 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…

Anurag Jagetiya

9TCP: Flow Control

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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NOTE: These activities are controlled by the Receiver and depend on the congestion in the Network

Figure 3: Sliding Window terminology

10TCP: Flow Control

• 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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NOTE: Left wall cannot move to left, as this will revoke some of previously sent Acknowledges (ACKs)

11TCP: Flow Control

• 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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12TCP: Flow Control

• 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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13TCP: Flow Control

• 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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NOTE: In reality the window size is thousands of Bytes

14TCP: Flow Control

Example of Flow Control

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In this example, It is assumed that there is only unidirectional communication between client and serverFigure 4:

Flow Control [1]

15TCP: Flow Control

• 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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16TCP: Flow Control

• 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…

Anurag Jagetiya

17TCP: Flow Control

•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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18TCP: Flow Control

• 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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19TCP: Flow Control

• 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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Figure 5: Receiver’s Silly Window [2]

20TCP: Flow Control

• 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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GOAL: Sender should not send small segment and the receiver not ask for them

21TCP: Flow Control

• 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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22TCP: Flow Control

• NET-SEAL

[Teaching Computer NETwork Through

Simulation Experiment and Animation Library]

Simulation of TCP Flow Control

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23TCP: Flow Control

• 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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24TCP: Flow Control

• 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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Calculating Checksum Verifying Checksum

SumChecksum

1010100100111001-----------1110001000011101 Sum

Complement

101010010011100100011101-----------111111110000000

25TCP: Flow Control

• 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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26TCP: Flow Control

• 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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Q: How to set this time out value for the Retransmission Timer?

27TCP: Flow Control

• 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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RT

T #1

RT

T #2

RT

T #3

Figure 6: RTT [1]

28TCP: Flow Control

• 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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29TCP: Flow Control

• 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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30TCP: Flow Control

• 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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31TCP: Flow Control

Example of Fast Retransmission

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Figure 7: 3 ACK [1]

32TCP: Flow Control

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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33TCP: Flow ControlAnurag Jagetiya

Queries…Thank you…