1. 5. End-to-end protocols (part 2)
Rocky K. C. Chang
Department of Computing
The Hong Kong Polytechnic University
10 April 2017

2. 1. Sliding window algorithm in TCP
The classical TCP employs a sliding window protocol with +ve acknowledgment and without selective repeat.
TCP uses sliding window algorithm to
guarantee the reliable delivery of data,
ensure the data is delivered in order,
enforce flow control between sender and receiver, and
perform congestion control (Chapter 6.3 in the text)

3. 1.1 TCP send and receive buffers
Sender buffer size: MaxSendBuffer
Receive buffer size: MaxRcvBuffer
Sending application
Receiving application
TCP
TCP
LastByteWritten
LastByteRead
LastByteAcked
LastByteSent
NextByteExpected
LastByteRcvd
(a)
(b)

4. 1.2 TCP send and receive states
Sender:
LastByteAcked <= LastByteSent
LastByteSent <= LastByteWritten
Data, for which LastByteAcked < SN <= LastByteWritten, must be buffered.
LastByteWritten - LastByteAcked <= MaxSendBuffer
Receiver:
LastByteRead < NextByteExpected
NextByteExpected <= LastByteRcvd + 1

5. 1.3 TCP flow control Flow control (performed by the receiver)
Data, for which LastByteRead < SN <= LastByteRcvd, must be buffered.
LastByteRcvd - LastByteRead <= MaxRcvBuffer
Flow control (performed by the receiver)
AdvertisedWindow = MaxRcvBuffer - (LastByteRcvd - LastByteRead), the amount of free space left in the receiver’s buffer.
The receiver sends AdvertisedWindow in the AdvertisedWindow field in the TCP header.

6. 1.4 Window size for TCP sender
MaxWindow is the window size used in the sliding window protocol.
MaxWindow is jointly determined by receiver and sender.
MaxWindow is dynamically adjusted.
MaxWindow = min{AdvertisedWindow, CongestionWindow}
CongestionWindow is determined by the sender.

7. 1.5 Acknowledgment strategies
An acceptable ACK is one for which
LastByteAcked < acknowledgment field  LastByteSent + 1
An acknowledgment field = LastByteAcked is a duplicate ACK.
Acknowledge every segment received vs. delayed acknowledgment.
Whenever a out-of-ordered segment is received, a TCP receiver responds with an ACK for SN = NextByteExpected (a duplicate ACK).

8. 2. Retransmission strategies
A sender may retransmit the segment with SN = LastByteAcked + 1
Upon retransmission timeout or
Upon receiving the third duplicate ACK (Jacobson’s fast retransmission)

9. 3. Estimating the round trip delay
Problem: How does a TCP sender determine its timeout value?
If over-estimate the timeout value, delay the retransmission.
If under-estimate the timeout value, inject duplicate packets into the network.
TCP uses an adaptive transmission algorithm to accommodate varying delays in the Internet:
A TCP sender monitors the round trip time (RTT), either in coarse-grain or fine-grain measurement.
Exponential backoff

10. 3.1 RTT measurements and timeout
Given a new RTT measurement M, TCP updates an estimate of the average RTT by R = R + (1 )M.
 is a filter gain constant (0 <  < 1), determining how much the new measurement contributes to the estimate.
 is usually set to 0.9.
The timeout value RTO is set to   R.
 accounts for the variation in the RTT.
 is usually set to 2.

11. 3.1 RTT measurements and timeout
Solid line: Retransmit timer
Dotted line: A packet’s RTT

12. 3.2 A better estimator
Estimate the variation in the RTT by Diff = M  R.
R = R + (  Diff)
D = D +   (|Diff|  D).
 is a fraction between 0 and 1.
The timeout value is now given by RTO =   R +   D.
 is typically set to 1 and  to 4.
How does the initialization of the parameters affect the estimator?

13. 3.2 A better estimator Solid line: Retransmit timer
Dotted line: A packet’s RTT

14. 3.3 Retransmission ambiguity
Problem: When retransmission occurs, how would the sender know whether the received ACK is for the original segment or the retransmitted segment?
The answer to the question above affects the timeout estimate.
One simple solution is to exclude the RTT measurements for retransmitted segments.
Does this solution work?

15. 3.4 Karn/Partridge algorithm
When a timeout or retransmission occurs,
Do not update the RTT estimator when the ACK for the retransmitted segment arrives.
Moreover, set RTO =  RTO,  > 1 (backoff RTO).
Calculate a new RTO when an ACK is received for a segment that was not retransmitted.

16. Acknowledgments The two RTT diagrams are taken from
Van Jacobson, “Congestion avoidance and control,” Proc. SIGCOMM, vol. 18, no. 4, Aug