TCP timers.ppt

tcp

1. Malathi Veeraraghavan
Originals by Jörg Liebeherr
1
•Error Control
•Congestion Control
•Timers

2. Malathi Veeraraghavan
Originals by Jörg Liebeherr
2
Background on ARQ Error Control 1
•Two types of errors:
–Lost packets
–Damaged packets
•Error control schemes that involve error detection and
retransmission of lost or corrupted frames are referred to
as Automatic Repeat reQuest (ARQ) error control
•Most Error Control techniques are based on:
1. Error Detection Scheme (Parity checks, CRC).
2. Retransmission Scheme.

3. Malathi Veeraraghavan
Originals by Jörg Liebeherr
3
Background on ARQ Error Control 2
•All retransmission schemes use all or a subset of the
following procedures:
–Positive acknowledgments (ACK)
–Negative acknowledgment (NACK)
–Selective acknowledgment (SACK)
–All retransmission schemes (using ACK, NACK, SACK or
all) rely on the use of timers
•The most common ARQ retransmission schemes are:
Stop-and-Wait ARQ
Go-Back-N ARQ
Selective Repeat ARQ

4. Malathi Veeraraghavan
Originals by Jörg Liebeherr
4
Error Control in TCP
•TCP maintains multiple timers for each connection
•TCP couples error control and congestion control (I.e., it
assumes that errors are caused by congestion)

5. Malathi Veeraraghavan
Originals by Jörg Liebeherr
5
TCP Timers
•TCP maintains multiple timers:
–Retransmission Timer:
•The timer is started during a transmission. A timeout causes a
retransmission
–Persist Timer
•Ensures that window size information is transmitted even if no data
is transmitted
–Keepalive Timer
•Detects crashes on the other end of the connection
–Other timers
•Delayed ACK timer, timeout of connection setup, abort timeout
(total timeout -keeps retransmitting till this timeout, then it kills the
connection), 2MSL timeout (when closing connection)

6. Malathi Veeraraghavan
Originals by Jörg Liebeherr
6
TCP Retransmission Timer
•Retransmission Timer:
–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 adaptive

7. Malathi Veeraraghavan
Originals by Jörg Liebeherr
7
Measuring TCP Retransmission Timers aida.poly.edu rigoletto.poly.edu
ftp session
from aida
to rigoletto
•Transfer file from aida to rigoletto
•Unplug Ethernet cable in the middle of file transfer

8. Malathi Veeraraghavan
Originals by Jörg Liebeherr
8
tcpdump Trace
10:42:01.704681aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:42:01.705603 aida.40001 > rigoletto.ftp-data: . 162649:164109(1460) ack 1 win 17520
10:42:01.706753 aida.40001 > rigoletto.ftp-data: . 164109:165569(1460) ack 1 win 17520
10:42:02.741764aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:42:05.741788aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:42:11.741828aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:42:23.741951aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:42:47.742176aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:43:35.742587aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:44:39.743140aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:45:43.743702aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:46:47.744271aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:47:51.752138aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:48:55.745547aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:49:59.746123aida.40001 > rigoletto.ftp-data: . 161189:162649(1460) ack 1 win 17520
10:51:03.745839aida.40001 > rigoletto.ftp-data: R 165569:165569(0) ack 1 win 17520

9. Malathi Veeraraghavan
Originals by Jörg Liebeherr
9
Interpreting the Measurements
•The interval between retransmission
attempts in seconds is:
1.03, 3, 6, 12, 24, 48, 64, 64, 64,
64, 64, 64, 64.
•Time between retransmissions is
doubled each time (Exponential
Backoff Algorithm)
•Timer is not increased beyond 64
seconds
•TCP gives up after 13th attempt
and 9 minutes (total timeout,
tcp_ip_abort_interval is 2 mins in
Solaris and can be programmed by
administrator -9 mins is the
commonly used old timeout value)0
100
200
300
400
500
600
Seconds
0 2 4 6 8
10 12
Transmission Attempts

10. Malathi Veeraraghavan
Originals by Jörg Liebeherr
10
TCP timers
•First timeout occurs based on when timer was intialized.
•This explains why the first timeout occurs at 1.03 sec and not 1.5.
•If the base timer clock is 500 ms, the first timeout occurs after 3 timer ticks. This
happens to occur at 1.03 sec after first segment was sent. Subsequent
retransmissions occur at 3 sec, 6 sec, 12 sec, etc.somewhere
here TCP sends
first segment
500 ms
per tick
Retransmission timer
expires after three
ticks (<1.5 sec; in this
case it happens to be
1.03 sec)
Retransmission timer
expires after six ticks
(3 sec)
4312 65 7 121098 11

11. Malathi Veeraraghavan
Originals by Jörg Liebeherr
11
Adaptive mechanism
•The retransmission mechanism of TCP is adaptive
•The retransmission timers are set based on round-trip time (RTT) measurements
that TCP performsSegment 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
•The RTT is based on time
difference between segment
transmission and ACK
•But:
–TCP does not ACK each
segment
–Can’t start a second RTT
measurement if timing on
one segment is in progress
–Each connection has only
one timer

12. Malathi Veeraraghavan
Originals by Jörg Liebeherr
12
Computation of RTO in adaptive scheme
•Retransmission timer is set to a Retransmission Timeout (RTO) value.
•RTO is calculated based on the RTTmeasurements.
•The RTT measurements are smoothed by the following estimators A (mean RTT value) and
D (smoothed mean deviation of RTT):
Err = M -A
AA+ g Err=A(1-g)+gM
D D+ h (|Err|-D)=D(1-h)+ h|Err|
RTO = A + 4D (latest formula)
(book also says A+2D for initial value; we’ll use A+4D)
The gains are set toh=1/4 and g=1/8
–In the formula for computing the new smoothed mean RTT A, 0.125
times the newly measured value (M) is added to 0.875 times the old smoothed
value of A

13. Malathi Veeraraghavan
Originals by Jörg Liebeherr
13
Example of RTO computation (adaptive)
•Assume A=1, D=1 (initial values)
•Err = 2 -1 =1 (since M, the measured RTT is 2)
•A = 1 + 0.125×1= 1.125; D = 1+0.25 (1-1)=1
•RTO = A+4D=1.125+4 = 5.125
•This is why in the figure below when segment 2 is lost, it is
retransmitted after 5.125 sec.Segment 1
ACK for Segment 1
Segment 2
Segment 2 (retransmitted)
ACK for Segment 2 + 3
RTT =2
X (packet lost)
RTO
=5.125

14. Malathi Veeraraghavan
Originals by Jörg Liebeherr
14
Karn’s Algorithm
•If an ACK for a retransmitted
segment is received, the sender
cannot tell if the ACK belongs to
the original or the
retransmission.segment
ACK
retransmission
of segment
Timeout !
RTT ?
RTT ?
•Karn’s Algorithm:
–Don’t update A or D on any segments that have been
retransmitted.

15. Malathi Veeraraghavan
Originals by Jörg Liebeherr
15
RTO Calculation: Example
•At t
1: RTO = 6 sec
•At t
2: RTO= 2 * 6 = 12 sec
(exponential backoff)
•At t
3: RTO is not updated
(Due to Karn’s algorithm)RTT #1 RTT #3
Segment 1
ACK for Segment 1
SYN
SYN
Time-
out !
SYN + ACK
Segment 2 Segment 3
ACK for Segment 2
ACK for
Segment 3
RTT #2
Segment 4 .
Segment 5 .
Segment 6 . ACK for
Segment 4
t
1
t
2
t
3
t
4
t
5
t
6
t
7
t
8
ACK
t
9

16. Malathi Veeraraghavan
Originals by Jörg Liebeherr
16
Congestion control (Second topic of this
lecture)
•Most often, a packet loss in a network is due to an overflow at
a congested router (rather than due to a transmission error)
•A sender can detect lost packets through a:
•Timeout of a retransmission timer
•Receipt of a duplicate ACK
•TCP assumes that a packet loss is caused by congestion and
reduces the size of the sending window (cwnd)
•Algorithms that reduce and then reopen the sending window
as packets are lost:
–Congestion Avoidance
–Fast retransmit and Fast recovery

17. Malathi Veeraraghavan
Originals by Jörg Liebeherr
17
Recall Slow Start / Congestion Avoidance
•Here we give a recap of the normal operationof Slow Start
and Congestion Avoidance
Ifcwnd <= ssthreshthen
/* Slow Start Phase*/
Each time an ACK is received:
cwnd = cwnd +segsize
else /* cwnd > ssthresh*/
/* Congestion Avoidance Phase*/
Each time an ACK is received:
cwnd = cwnd + segsize* segsize/ cwnd + segsize/ 8
endif

18. Malathi Veeraraghavan
Originals by Jörg Liebeherr
18
Congestion Avoidance Algorithm
•When congestion occurs (indicated by timeout or receipt of
duplicate ACK),
–ssthreshis set to half the current window size (the
minimum of the advertised window (AW) and cwnd):
ssthresh = min(cwnd,AW) / 2but at least 2 segments
–cwndis changed according to:
cwnd = 1 segsize = 1 MSS bytes (in case of timeout only)
•When new data is acknowledged,cwnd is increased according
to whether it is in slow start or CA

19. Malathi Veeraraghavan
Originals by Jörg Liebeherr
19
Slow Start / Congestion Avoidance
•A typical plot of cwndfor a TCP connection (segsize = 1500
bytes) :

20. Malathi Veeraraghavan
Originals by Jörg Liebeherr
20
Accelerated retransmissions (Fast retransmit)
•TCP allows accelerated retransmissions (Fast Retransmit)
–If receiver gets a segment out of order, it sends an ack with
the expected sequence number. If sender receives one or
two duplicate ACKs, it thinks segments are misordered.
When expected segment is received at receiver, it sends
the correct ACK. But if the third duplicate ACK is received
at sender, it assumes lost segments and retransmits
immediately without waiting for expiry of retransmission
timer. Hence it is called fast retransmit.

21. Malathi Veeraraghavan
Originals by Jörg Liebeherr
21
Fast Retransmit and Fast Recovery
•After the third duplicate ACK
(meaning fourth ACK) is received
by the sender, it transmits a single
segment without waiting for a
timeout to expire.Data (100:200)
ACK 100
ACK 100
ACK 100
Data (100:200)
ACK 100
•If 3rd duplicate ACK (this means fourth ACK with same ack no.) is received:
ssthresh = min(cwnd, receiver’s advertised window)/2
cwnd = ssthresh + 3 segsize; then retransmit segment
Reason: TCP receiver has to issue an ACK every time it receives a new segment.
Therefore when the sender receives 3 duplicate ACKs it implies that three
segments got through the network successfully; Therefore it inflates the cwnd.
•For each additional duplicate ACK received:
cwnd = cwnd + segsize
and transmit a segment if allowed by new value of cwnd
•When an ACK arrives that acknowledges new data set cwnd = ssthresh; (this should
be the ACK for the retransmission from step 1); additionally, it will ack intermediate
segments between lost packet and receipt of third duplicate ACK, so set cwnd = cwnd
+ segsize; now in CA phase

22. Malathi Veeraraghavan
Originals by Jörg Liebeherr
22
Example of slow start and congestion avoidance (MSS=512
bytes; advertised window =5120 bytes)
•Normal operationPSH 1:513 (512) ack 10
ack 513
cwnd=512; ssthresh=2560
cwnd=1024 cwnd=1536; ssthresh=2560
PSH 513:1025 (512) ack 10
ack 1025
cwnd=2048
PSH 1025:1537 (512) ack 10
ack 1537 PSH 1537:2049 (512) ack 10
ack 3073
cwnd=2560; ssthresh=2560
cwnd=3072; ssthresh=2560
PSH 2049:2561(512) ack 10
ack 2049
cwnd=3222; ssthresh=2560
PSH 2561:3073(512) ack 10
ack 2561
Enter congestion avoidance

23. Malathi Veeraraghavan
Originals by Jörg Liebeherr
23
Example: computation of cwnd on previous
slide
•Upto and including ack 2561, this TCP connection is in slow
start, and cwnd is increased by 1 MSS bytes each time an
ACK is received.
•Note that when cwnd = ssthresh, slow start is still applied.
Hence when ack 2561 is received, cwnd = 2560+512 = 3072.
•When the last ack shown on the previous slide is received,
the TCP connection is in congestion avoidance since cwnd is
> ssthresh. Therefore, cwnd = cwnd + MSS×MSS/ cwnd +
MSS/ 8 = 3072 + 512 ×512/3072+512/8=3222

24. Malathi Veeraraghavan
Originals by Jörg Liebeherr
24
Example: RTO timeout (see congestion
avoidance algorithm)
•Example of a retransmit based on a timeout
•When segment is retransmitted, ssthresh is dropped to half of the
minimum of the cwnd and advertised window. Since advertised window is
5120 bytes for this example, half of 3222 is 1611, but this is rounded down
to the next multiple of the MSS (see page 316 for this rounding down
concept). PSH
3073:3585(512)
ack 10
cwnd=3222; ssthresh=2560
cwnd=512; ssthresh=1536
X
PSH 3073:3585(512) ack 10
RTO expiry

25. Malathi Veeraraghavan
Originals by Jörg Liebeherr
25
Example: duplicate ACKs
(congestion avoidance algorithm and fast retransmit/recovery algorithm)
•In case of duplicate ACKs, both congestion avoidance algorithm and fast
retransmit/recovery algorithms applycwnd=3222; ssthresh=2560
PSH 3073:3585
(512) ack 10
X
PSH 3585:4097 (512) ack 10
PSH 4097:4609 (512) ack 10
PSH 4609:5121 (512) ack 10
ack 3073
ack 3073
ack 3073
ack 3073
cwnd=3222; ssthresh=1536
cwnd=3222; ssthresh=1536
cwnd=1536+3*512=3072; ssthresh=1536
PSH 3073:3585 (512) ack 10
ack 5121cwnd=ssthresh=1536; ssthresh=1536;
cwnd=2048
•For reason for last cwnd increase to 2048, see last case in Fig. 21.11

26. Malathi Veeraraghavan
Originals by Jörg Liebeherr
26
Repacketization
•When TCP does a retransmission, it can send the missing data in
differently sized segments
•Increase segment size (if allowed by MSS limit) to improve efficiency
(new data arrives after first transmitted segment was lost)Data (1:100)
ACK 100
ACK 300
Data (100:200)
lost
Data (100:300)
new data arrives from
application (100 bytes)
before the retransmission
timer times out

27. Malathi Veeraraghavan
Originals by Jörg Liebeherr
27
Persist Timer in TCP
•Assume the window size goes down to zero and the ACK that
opens the window gets lost3K
2KSeqNo=0
Receiver
Buffer
0 4K
2K
AckNo=2048 Win=2048
ACK is
lost
2KSeqNo=2048
4K
AckNo=4096 Win=0
AckNo=4096 Win=1024
Sender blocked
•If ACK (see figure) is
lost, both sides are
blocked.
•Persist Timer:
Forces the sender to
periodically query the
receiver about its window
size (window probes)

28. Malathi Veeraraghavan
Originals by Jörg Liebeherr
28
Persist Timer
•The persist timer is started by the sender when the sliding window is zero
•Persist timer uses exponential backoff (initial value is 1.5 seconds), but it
is bounded to the range [5 sec, 60sec]
•So the time interval between timeouts are at:
5, 5, 6, 12, 24, 48, 60, 60, …
–The first two are 5 because the first two timer values, 1.5 and 3,
are both increased to be within bound [5, 60]
•The window probe packet contains one byte of data
•TCP allows sender to send one byte beyond close of receiver window
•Persist timer never gives up (till connection gets aborted)

29. Malathi Veeraraghavan
Originals by Jörg Liebeherr
29
Persist Timer1 byte
SeqNo=4096
AckNo=4096 Win=0
AckNo=4096 Win=0
1 byte
SeqNo=4096
AckNo=4096 Win=0
Timeout
(5 sec)
Timeout
(5 sec)
1 byte
SeqNo=4096
AckNo=4096 Win=1024
Timeout
(6 sec)
1 KB
SeqNo=5120
Probe
Probe
Probe

30. Malathi Veeraraghavan
Originals by Jörg Liebeherr
30
Keepalive Timer in TCP
•When a TCP connection has been idle for a long time, a
Keepalive timerreminds a station to check if the other side is
still there.
•A probe packet is sent if the connection has been idle for 2
hours
•Assume a probe has been sent from Ato B:
(1) Bis up and running: Bresponds with an ACK
(2) Bhas crashed and is down: Awill send 10 more probes, each 75
seconds apart. If Adoes not get a
response, it will close the connection
(3)Bhas rebooted: Bwill send a RST segment
(4) Bis up, but unreachable: Looks toA the same as (2)

31. Malathi Veeraraghavan
Originals by Jörg Liebeherr
31
TCP Summary
•TCP Header -fields
•TCP connection open/close (SYN/FIN)
•Interactive TCP data transfer:
–Delayed ACKs
–Nagle’s algorithm

32. Malathi Veeraraghavan
Originals by Jörg Liebeherr
32
TCP Summary Contd.
•Bulk TCP data transfer:
–Flow control: sliding window (receiver paces sender)
–Error control: time-outs and retransmissions
•exponential backoff (in case of retransmits)
•RTO changing adaptively to measured RTTs
•Karn’s algorithm
–Congestion control: congestion window (sender has window)
•Slow start and congestion avoidance phases (normal operation)
•Lost packets (timeout or duplicate ACKs)
–congestion avoidance algorithm
–fast retransmit and fast recovery algorithm
•Because of the congestion recovery schemes, TCP’s ARQ scheme is Go-
back-N if an error (loss) is detected by a retransmission time-out
occurs but selective repeat if an error (loss) is detected by triple
duplicate ACKs.
–Repacketization
•Persist and Keep-alive timers

33. Malathi Veeraraghavan
Originals by Jörg Liebeherr
33
Different schemes for determining RTO
•Exponential backoff if a segment is retransmitted
•adaptive RTO as a function of RTT (A+4D)
–RTT measurement is in progress and a new segment sent
then no RTT measurement is taken for new segment
•Karn’s algorithm
–no RTT measurement on retransmitted segment