1. CS4470 Computer Networking Protocols
4/5/2019
CS Computer Networking Protocols
17. TCP 4
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline
Flow control
TCP congestion control
17. TCP CS4470

3. 4/5/2019
TCP Flow Control
Define the amount of data the sender can send before receiving an Ack from the destination
Two extreme cases
Send one byte and wait for Ack
Extremely slow
Send all of the data without waiting for Acks
Speed up the process
May overwhelm the receiver
TCP uses a window
Send as much data as defined by the window
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4. TCP segment structure
Flow control
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5. TCP Sliding Window How to determine the window size?
Flow control: rwnd
Congestion Control: cwnd
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6. TCP Sliding Window The window size changes
Controlled by the receiver and the congestion in the network, not the sender
The receiver window is the number of bytes the other end can accept before its buffer overflows and data is discarded
The congestion window is a value determined by the network to avoid congestion
TCP’s sliding windows are byte oriented
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7. TCP Flow Control
Receiver’s side of TCP connection has a receiving buffer:
App. process may be slow at reading from buffer
Two variables
LastByteRead: the number of the last byte in the data stream read from the buffer by the application process
LastByteRcvd: the number of the last byte that has arrived
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8. TCP Flow control: how it works
Spare room in the receiver’s buffer
RcvWnd
= RcvBuffer-[LastByteRcvd - LastByteRead]
Receiver advertises spare room by including value of RcvWnd in segments
Sender limits unACKed data to RcvWindow
LastByteSent-LastByteAcked<=RcvWindow
guarantees receive buffer doesn’t overflow
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9. Exercise 1
What is the value of the receiver window (rwnd) for host A if the receiver, host B, has a buffer size of 5,000 bytes and 1,000 bytes of received and unprocessed data?
Solution
The value of rwnd = 5,000 − 1,000 = 4,000.
Host B can receive only 4,000 bytes of data before overflowing its buffer.
Host B advertises this value in its next segment to A.
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10. Exercise 2
What is the size of the window for host A if the value of rwnd is 3,000 bytes and the value of cwnd is 3,500 bytes?
Solution The size of the window is the smaller of rwnd and cwnd, which is 3,000 bytes.
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11. Exercise 3
The sender has sent bytes up to 202. We assume that cwnd is 20
The receiver has sent an acknowledgment number of 200 with an rwnd of 9 bytes
The size of the sender window is the minimum of rwnd and cwnd or 9 bytes.
Bytes 200 to 202 are sent, but not acknowledged.
Bytes 203 to 208 can be sent without worrying about acknowledgment.
Bytes 209 and above cannot be sent.
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12. Exercise 3
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13. Exercise 4
Following the previous exercise, the sender receives a packet with an acknowledgment value of 202 and an rwnd of 9. The sender has already sent bytes 203, 204, and 205. The value of cwnd is still 20. Show the new window.
Solution
The window closes from the left and opens from the right by an equal number of bytes, so the size of the window has not been changed.
The acknowledgment value, 202, declares that bytes 200 and 201 have been received and the sender needs not worry about them; the window can slide over them.
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14. Exercise 4
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15. Exercise 5
Following the previous exercise, the sender receives a packet with an acknowledgment value of 206 and an rwnd of 12. The sender has not sent any new bytes. The value of cwnd is still 20. Show the new window.
Solution
The value of rwnd is less than cwnd, so the size of the window is 12.
Note that the window has been opened from the right by 7 and closed from the left by 4; the size of the window has increased.
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16. Exercise 5
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17. MSS Maximum segment size (MSS) Why MSS?
The maximum amount of application data (in bytes) that can be put in a TCP segment
Set based on the path MTU (Maximum Transmission Unit, link layer)
Path MTU: the largest link layer frame that can be sent on all links from the source to destination
Why MSS?
Ensure that a single TCP segment (encapsulated in an IP datagram) can fit into a single link layer frame
IP datagram fragmentation
Fragment the data in the IP datagram into two or more smaller IP datagrams
TCP prevents a segment from being fragmented by IP
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18. Congestion Network congestion How congestion happens
The number of packets sent to the network is greater than the number of packets a network can handle
How congestion happens
Each interface of a router has an input and output buffer
Input buffer overflow
Arrival rate > processing rate
Output buffer overflow
Processing rate > departure rate
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19. Router queues
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20. Congestion Control What is congestion control How?
Prevent congestion before it happens
Remove congestion after it has happened
How?
Adjust the congestion window size: cwnd
Let’s now ignore recWindow
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21. TCP Congestion Control
Three phases
Slow start
Congestion avoidance
Congestion detection
The algorithms in each phase are independent
Some important variables
Cwnd
Threshold
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22. Slow start algorithm
The slow start phase is the phase right after the connection is established.
The algorithm
Cwnd starts with 1MSS
Each time one new ACK arrives, cwnd is increased by 1MSS
cwnd is effectively doubled per RTT “epoch”
Cwnd keeps growing until
Cwnd reaches threshold
Or congestion occurs
Cwnd starts slowly, but grows exponentially
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23. TCP Slow Start Slow Start Add one MSS per ACK 17. TCP 4 CS4470 Source
Destination
Slow Start
Add one MSS
per ACK
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24. Congestion avoidance The phase after the slow start phase
Slow down the exponential growth
Avoid congestion before it happens
When congestion avoidance phase starts?
cwnd >= Threshold
The algorithm
Each time the WHOLE WINDOW of packets is acknowledged, Cwin is increased by 1MSS
cwnd is increased by 1/cwnd for each acknowledged segment (Additive increase)
Cwin keeps growing LINEALLY until congestion happens
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25. Congestion avoidance
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26. Congestion detection Timeout 3 duplicate ACKs
Strong possibility of congestion
A segment has probably been dropped
No news about following segments
TCP reacts strongly
3 duplicate ACKs
Weaker possibility of congestion
A segment may have been dropped
Some segments after that may have arrived safely
TCP applies fast recovery algorithm
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27. Timeout Retransmit the lost packet Threshold = ½ cwnd Cwnd = 1
NOT threshold = ½ threshold!
Cwnd = 1
Starts the slow start phase
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28. 17. TCP CS4470

29. Figure: Behavior of TCP Congestion Control
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30. Fast Retransmit
Coarse timeouts remained a problem, and Fast retransmit was added.
Since the receiver responds every time a packet arrives, this implies the sender will see duplicate ACKs.
Basic Idea: use duplicate ACKs to signal lost packet
Fast recovery: upon receipt of three duplicate ACKs, the TCP Sender retransmits the lost packet.
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31. Fast Retransmit Based on three duplicate ACKs 2 3 3 3 3 7
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32. Fast recovery
When fast retransmit detects three duplicate ACKs, start the recovery process
After retransmit
Threshold = ½ cwnd
Cwnd = threshold (multiplicative decrease)
Starts the congestion avoidance phase
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33. Summary: TCP Congestion Control
3 dup
ACKs
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34. Congestion control example
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35. TCP Congestion Window Trace
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36. Exercise
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37. Exercise Slow start time intervals Congestion avoidance interval
4/5/2019
Exercise
Slow start time intervals
Congestion avoidance interval
At time 4, what happened? At time 4, if 3 duplicate acks are received, what’s the value of cwnd, threshold?
Slow start time intervals
0-4, 5-8
Congestion avoidance interval 8-
At time 4, what happened? Timeout or 3 duplicate acks are received?
At time 4, if 3 duplicate acks are received, the value of cwnd, threshold?
Cwnd=8, threshold=8
17. TCP CS4470