1. CS4470 Computer Networking Protocols
12/1/2018
CS Computer Networking Protocols
16. TCP 3
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline TCP reliable data transfer TCP timeout value 16. TCP 3 CS4470
12/1/2018
Outline
TCP reliable data transfer
TCP timeout value
16. TCP CS4470

3. TCP reliable data transfer
TCP creates reliable data transfer service on top of IP’s unreliable service
Use sliding window
Limit the number of bytes that can be sent
Cumulative acks
TCP uses single timer
When the timer expires, only retransmits the oldest segment
16. TCP CS4470

4. TCP reliable data transfer
Retransmissions are triggered by:
timeout events
duplicate acks
Initially consider simplified TCP sender:
ignore duplicate acks
ignore flow control, congestion control
16. TCP CS4470

5. TCP sender (simplified)
NextSeqNum = InitialSeqNum
SendBase = InitialSeqNum
loop (forever) {
switch(event)
event: data received from application above
create TCP segment with sequence number NextSeqNum
if (timer currently not running)
start timer
pass segment to IP
NextSeqNum = NextSeqNum + length(data)
event: timer timeout
retransmit not-yet-acknowledged segment with
smallest sequence number
event: ACK received, with ACK field value of y
if (y > SendBase) {
SendBase = y
if (there are currently not-yet-acknowledged segments) }
} /* end of loop forever */
TCP sender (simplified)
Comment:
SendBase-1: last
cumulatively ack’ed byte
Example:
SendBase-1 = 71; y= 73, so the rcvr wants 73+ ; y > SendBase, so that new data is acked
16. TCP CS4470

6. TCP retransmission scenarios
Host A
Host B
Seq=92, 8 bytes data
loss
ACK=100 X
timeout
Seq=100, 20 bytes data
ACK=120
SendBase
= 120
time
Cumulative ACK scenario
16. TCP CS4470

7. TCP: retransmission scenarios
12/1/2018
TCP: retransmission scenarios
Host A
Seq=92, 8 bytes data
ACK=100
loss
timeout
lost ACK scenario
Host B X
time
Host A
Host B
Seq=92 timeout
Seq=92, 8 bytes data
Seq=100, 20 bytes data
ACK=100
Sendbase
= 100
SendBase
= 120
ACK=120
Seq=92, 8 bytes data
ACK=120
SendBase
= 120
Seq=92 timeout
SendBase
= 100
premature timeout
time
16. TCP CS4470

8. Fast Retransmit Time-out period often relatively long:
long delay before resending lost packet
Detect lost segments via duplicate ACKs.
Sender often sends many segments back-to-back
If segment is lost, there will likely be many duplicate ACKs.
If sender receives 3 duplicate(4 identical) ACKs for the same data, it supposes that segment was lost:
fast retransmit: resend segment before timer expires
16. TCP CS4470

9. Fast retransmit algorithm:
event: ACK received, with ACK field value of y
if (y > SendBase) {
SendBase = y
if (there are currently not-yet-acknowledged segments)
start timer }
else {
increment count of dup ACKs received for y
if (count of dup ACKs received for y = 3) {
resend segment with sequence number y
a duplicate ACK for
already ACKed segment
fast retransmit
16. TCP CS4470

10. Fast retransmit example
Host A
Host B
Seq=92, 8 bytes data
Seg=100, 10 bytes
Seg=110
Seg=120
ACK=100
Seg=130
ACK=100
Seq92 timeout
ACK=100
ACK=100
Seq=100, 10bytes
Fast
retransmit
time
16. TCP CS4470

11. TCP ACK generation [RFC 1122, RFC 2581]
Event at Receiver
Arrival of in-order segment with
expected seq #. All data up to
expected seq # already ACKed
expected seq #. One other
segment has ACK pending
Arrival of out-of-order segment
higher-than-expect seq. # .
Gap detected
Arrival of segment that
partially or completely fills gap
TCP Receiver action
Delayed ACK. Wait up to 500ms
for next segment. If no next segment,
send ACK
Immediately send single cumulative
ACK, ACKing both in-order segments
Immediately send duplicate ACK,
indicating seq. # of next expected byte
Immediate send ACK, provided that
segment starts at lower end of gap
16. TCP CS4470

12. Example: normal operation
16. TCP CS4470

13. Example: Lost segment
16. TCP CS4470

14. Example: fast retransmission
16. TCP CS4470

15. Example: Lost acknowledgment
16. TCP CS4470

16. Example: Lost acknowledgment corrected by resending a segment
16. TCP CS4470

17. TCP Round Trip Time and Timeout
TCP uses timeout/retransmit mechanism to recover from lost segments
Set TCP timeout value?
longer than RTT
how much longer?
too short: premature timeout
unnecessary retransmissions
too long: slow reaction to segment loss
TCP has an algorithm to compute the timeout value
16. TCP CS4470

18. Timeout value Measured RTT, RTTM
The time required for the segment to reach the destination and be acknowledged
The ack may include other segments
Several segments may be acked together
In TCP, only one RTT measurement can be in progress at any time
If an RTT measurement is started, no other measurement starts until the value of this RTT is finalized.
16. TCP CS4470

19. Timeout value Smoothed RTT To calculate RTTS
RTTM changes for round trip.
The fluctuation is so high in today’s Inernet that it cannot be used for retransmission time-out purposes
A smoothed RTT, called RTTS, is a weighted average of RTTM and the previous RTTS
To calculate RTTS
Original: no value
After first measurement: RTTS = RTTM
After any other measurement: RTTS = (1-α)*RTTS+ α * RTTM
The value of α is implementation dependent, but it’s usually set to 1/8
16. TCP CS4470

20. Timeout value RTT deviation, RTTD To calculate RTTD
It’s based on RTTS and RTTM
To calculate RTTD
Original: no value
After first measurement: RTTD = RTTM/2
After any other measurement:
RTTD = (1-β)*RTTD+ β *| RTTS - RTTM|
The value of β is also implementation dependent, but it’s usually set to 1/4
16. TCP CS4470

21. Timeout value Retransmission Timeout (RTO) To calculate RTO
RTO is based on RTTS and RTTD
To calculate RTO
Original: intial value
After any measurement: RTO = RTTS + 4*RTTD
16. TCP CS4470

22. Example
16. TCP CS4470

23. Example
When the SYN segment is sent, there is no value for RTTM, RTTS, or RTTD
When the SYN+ACK segment arrives, RTTM is measured to 1.5s
The following shows the values of these variables
RTTM = 1.5
RTTS = 1.5
RTTD = 1.5/2 = 0.75
RTO = * 0.75 = 4.5
16. TCP CS4470

24. Example
When the 1st segment is sent, a new RTT measurement starts. Note:
Sender doesn’t start an RTT measurement when it sends the ACK segment
No RTT measurement starts for the 2nd segment because a measurement is already in progress
Though the last ACK acks two data segments, its arrival finalize the value of RTTM for the first segment
The following shows the values of these variables
RTTM = 2.5
RTTS = 7/8(1.5) + 1/7(2.5) = 1.625
RTTD = ¾ (0.75) + ¼ |1.625 – 2.5| = 0.78
RTO = * 0.78 = 4.74
16. TCP CS4470

25. Timeout value (cont.)
TCP does not consider the RTT of a retransmitted segment in its calculation of a new RTO.
When a segment is retransmitted, the current RTO value is doubled.
16. TCP CS4470

26. Example
16. TCP CS4470

27. Example(cont.) The first segment in the figure is sent, but lost.
The RTO timer expires after 4.74 seconds. The segment is retransmitted and the timer is set to 9.48, twice the previous value of RTO.
This time an ACK is received before the time-out. We wait until we send a new segment and receive the ACK for it before recalculating the RTO
16. TCP CS4470