TCP uses congestion control algorithms to manage network congestion. These algorithms include slow start, which allows TCP to determine the network capacity by exponentially increasing the congestion window size each round trip time. This allows TCP to quickly find the maximum capacity without causing congestion. However, once the congestion window size reaches the slow start threshold, TCP enters congestion avoidance phase where it increases the window size more slowly to operate close to the maximum capacity without overshooting. The goal of congestion control is to maximize throughput while minimizing congestion through additive increase and multiplicative decrease of the transmission rate.

1. TCP (No-IP) – Part 2
Congestion Management

2. Disclaimer
• This Presentation contains a mixture of self made slides and material
already out on the Internet.
• The goal of this presentation/talk is to provide TCP refresher.
• At the end you wont know everything about TCP.
• The goal here is to introduce you to the Mountains by climbing on top
of a hill. How to climb the Mountains is an exercise left for the
audience/reader .
• My hope is that you will get at least something new out of this but, if
you already knew everything then at least you got the free Lunch .

3. TCP Congestion Control
• As we noticed that TCP strategy to combat packet loss is
retransmission induced by either a retransmission timer expiry or Fast
Retransmit.
• Now assume for a moment that multiple TCP connections sharing a
Path simply retransmitting more packets when that Network path is
experiencing congestion.
• This is only going to make things worse and is known as pouring
Gasoline on a fire and we all know how that will go

4. Motivation
• It’s a mind-boggling unintuitive fact that quickest way to transmit data is
not always sending the data as fast as you can.
• Observe the traffic on the Highway
• Traffic will move faster if the if the traffic load is high but not too high
• When the load is higher then what a highway can handle, bottleneck will cause
the whole highway to slow down
• It is in fact beneficial to a user by restricting one’s own transmission rate
if the current rate is more than a network can handle. This “sacrificial
act” of restricting one’s own transmission rate is known as “congestion
control”
• On the other hand if the network capability increases then its “natural”
one would increase one’s transmission rate.
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

5. Motivation
• Congestion Control is one of a few application of “Communism” that will actually work
because it benefit oneself if one will do it (as long as everyone else will do it also).
• Communist principle fails in general because it doesn't take into account the “self-
beneficial” motives of humanoids. Ordinary humans would only do something if it benefits
oneself. Marx had this crazy fantasy that everyone is a Saint (He must be smoking
something)
• Ref: http://www.philosophybasics.com/branch_communism.html
“Communism is a socio-economic structure that promotes the establishment of a classless, stateless society
based on common ownership of the means of production. It encourages the formation of a proletarian state in
order to overcome the class structures and alienation of labour that characterize capitalistic societies, and
their legacy of imperialism and nationalism. Communism holds that the only way to solve these problems is for
the working class (or proletariat) to replace the wealthy ruling class (or bourgeoisie), through revolutionary
action, in order to establish a peaceful, free society, without classes or government.
Communism, then,] is the idea of a free society with no division or alienation, where humanity is free
from oppression and scarcity, and where there is no need for governments or countries and no class divisions.
It envisages a world in which each person gives according to their abilities, and receives according to
their needs. Its proponents claim it to be the only means to the full realization of human freedom.”
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

6. What is TCP Congestion Control ?
• It’s a set of behaviors determined by algorithms that each TCP
implements in an attempt to prevent Network from being
overwhelmed by too large of an aggregate offered traffic load.
network
congestion

7. TCP Congestion Control
• So in order to deal with congestion, we want TCP to slow down when
congestion occurs or about to occur
• When the congestion is subsided, detect and use appropriate amount
of new bandwidth as it becomes available.
• As you can see that how this simple task can be quite complicated as
TCP doesn't know the state of the Intermediate routers.
• TCP sender has to somehow react to congestion after detecting
congestion.

8. Basic Principle
• The basic Congestion control principle is simple:
• Decrease your transmission rate when you detect congestion
• Increase your transmission rate if you detect no congestion.
• There are two problems which need to be solved when designing a Congestion
Method
• How do I detect an onset of presence of Congestion
• How fast do you increase transmission rate when no congestion is detected and how fast you decrease
transmission rate when you do detect a congestion.
• Congestion Detection:
• Explicit signals from the Routers. (present routers don’t send signals. QOS Policy with ECN ??)
• Packet Loss:
• This can be detected by the source if receiver uses NACK
• If the transport uses only +ve ACK, a packet loss can be detected through RTO.
• This is the worst form of feedback signal because that means the congestion has already occurred and it dropped the
packet.
• Because of that very reason, TCP takes drastic steps when it detects packet loss
• End-to-End Delay:
• This is much better option compared to Packet loss but its harder to use.
• By measuring the end-to-end delay, source can detect an impeding collapse rather than detecting a collapse which has
already happened.
• End-to-End Delay increases/decreases when the Queuing delay increases/decreases
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

9. End-to-End Delay increase
Fast Router Collapse
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

10. End-to-End Delay increase
Slow Router did not collapse
As you can see its not easy to detect by just looking at the Increase in delay. More can be found on this
in TCP Vegas and Raj Jain paper
http://www.mathcs.emory.edu/~cheung/Courses/558a/Papers/Jain-delay-based.pdf
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

11. AIMD is at the heart of Congestion Control
• Assume that you have to obtain a binary congestion indication signal from the network
0 – No Congestion
1 – Congestion
• Next question is
• How much to increase transmission rate if there is no congestion
• How much to decrease transmission rate if there is congestion
• Design considerations:
• Is this method fair ? Will everyone gets a fair share of available BW
• Is this method stable ? Does this converge to a solution or the system state jump back and forth wildly ?
• What's the efficiency and distributedness.
• Raj Jain paper study’s about the linear increase and decrease for Congestion Avoidance
• http://www.mathcs.emory.edu/~cheung/Courses/558a/Papers/Jain-AIMD.pdf
• AIMD – Additive Increase and Multiplicative Decrease, is a special function of linear increase and decrease
• Congestion control in TCP uses AIMD as congestion control.
• Legacy of Jain’s research.
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

12. AIMD
• Distributed, fair and efficient
• Packet loss is seen as sign of congestion and results in a multiplicative rate decrease
• Factor of 2
• TCP periodically probes for available bandwidth by increasing its rate

13. TCP Window-Recap
• Transmit Window Size :
• It is the amount of data that TCP can have "outstanding" (that TCP does not
know whether the data has been received or not)
• Advertised Window Size:
• It is the amount of data that receiver is willing to buffer when data arrived out
of order.
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

14. TCP Window- Congestion Window
• Congestion Window Size is the amount of data that the sender can inject
into the network without causing congestion in the network.
• The amount of data varies over time (it depends on the current network
status - it changes like the weather and it is just as unpredictable....)
• Relationship between Transmit window and Congestion Window:
• Sender must NEVER transmit more data than amount that will cause Network
congestion.
• Transmit Window Size <= Congestion Window Size (CWND)
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

15. TCP Window- Congestion Window
• Advertised Window Size (AWS) = amount of data that the receiver will
buffer. AWS is negotiated at connection establishment and
remains unchanged afterwards
• Congestion Window Size (CWS) = window size imposed by the TCP
congestion mechanism to avoid causing congestion in the network
CWS changes over time !!!
• Transmit Window Size (TWS) = the amount of unacknowledged data, i.e.,
data that TCP transmits in a burst without receiving any indication on
what happened to the data.
• TWS = min (AWS, CWS)
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

16. How TCP controls the Transmission rate ?
• TCP will transmit using a transmit window size equal to:
• TWS = min (AWS, CWS)
• The TCP congestion control algorithm will adjust the value
of CWS according to signals/events from the network
• A change in CWS can change the transmission window size and indirectly
change the transmission data rate....
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

17. TCP Phases
• TCP always operates in two phases:
• Slow Start Phase: the period when TCP has no information about
the current network status.
• In particular, TCP does not know how much traffic the network can handle safely (i.e., without
causing congestion).
• Congestion Avoidance Phase: the period when TCP knows that it is transmitting at
a data rate that is very close to a rate that can cause congestion.
• TCP will update CWS differently in different phases.
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

18. How does TCP know that its near Congestion?
• It’s like playing a Video Game without a cheat sheet
• How do you know when you must be very careful ?
• You walk in a trap in the previous game and died..
• And when you restart the game, and after arriving to the same situation(Again), you progress
very carefully.
• Life of TCP is a like a never ending Video Game
• When TCP detects congestion (through a packet loss), it records the current
transmit window size used.(The current window size indirectly determines how
fast TCP transmits data)
• After detecting congestion, TCP will use a smaller transmit window size (this
will reduce the transmission rate)
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

19. How does TCP know that its near Congestion?
• Immediately after the window size reduction, TCP will first try
to increase the transmit window size aggressively (because it's goal is to
transmit the data as fast as possible, but without causing congestion).This
phase is the slow start phase
• When TCP reach a window size that is close to the one that had caused
congestion (and loss), it will increase the window size much less
aggressively This phase is the congestion avoidance phase
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

20. TCP Phases
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

21. TCP Phases - Goals
• Goal of TCP is to get the highest possible throughput
• This is not achieved by sending as fast as possible. It is achieved by
sending at a data rate that is close to the available network bandwidth.
(Remember the Highway Analogy)
• Available Network bandwidth changes constantly
• TCP finds the available bandwidth by remembering when it dropped a
packet last time and proceeds carefully starting from half this
bandwidth.
NOTE: the first time that TCP starts, it has no idea what the network capacity is and the only thing that it can do is
to estimate it as well as it can - it increases the data rate very rapidly to find the bottle neck in a very short amount
of time.
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

22. TCP Congestion Control
• TCP doesn't’t implement's a single Congestion avoidance mechanism but many.
• TCP Will try to guess what the situation is and many times it can guess wrongly!!!
• TCP Congestion avoidance mechanism employed have very sexy sounding names.
• Tahoe (Jacobson 1988)
• Slow Start
• Congestion Avoidance
• Fast Retransmit
• Reno (Jacobson 1990)
• Fast Recovery
• Vegas (Brakmo & Peterson 1994)
• New Congestion Avoidance
• RED (Floyd & Jacobson 1993)
• Probabilistic marking
• REM (Athuraliya & Low 2000)
• Clear buffer, match rate
• Others…
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

23. • If we have large actual window, should we send data in one shot?
• No, use acks to clock sending new data
• Arrival of an ACK, called ACK clock, triggers the system to take the
action of sending another packet.
Self-Clocking: ACK Clock

24. • The period when the slow start mechanism is used is called a slow
start phase.
• Operation of the slow start mechanism:
• Initialization: set CWS = MSS (i.e., 1 full packet)
• When TCP receives an ACK, it increases the CWS by MSS (i.e., one full packet)
Slow Start
W = 2^k where k = round trip time
K = log2 W RTTs to reach W

25. Slow Start
• Start with cwnd = 1 (slow start)
• On each successful ACK increment cwnd
cwnd  cnwd + 1
• Exponential growth of cwnd
each RTT: cwnd  2 x cwnd
• Enter CA when cwnd >= ssthresh

26. Slow Start
data packet
ACK
receiversender
1 RTT
cwnd
1
2
3
4
5
6
7
8
cwnd  cwnd + 1 (for each ACK)

27. • What is Slow Start trying to achieve ?
• Slow start can determine an estimate for the current network capacity very
rapidly
• By increasing the congestion window size exponentially, TCP will be able to
find a congestion window size that causes packet loss
• since the routers in the Internet does not return any indication about their
state, the only way to see find out if a certain data rate will cause congestion
is to "actually try it" :-)
Slow Start
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

28. • When to BEGIN to use Slow Start ?
• The slow start mechanism is used when the sender does not know the current
network state - i.e., it does not know how much traffic will cause the network to
become congested.
• Slow start is used in 2 situation:
• when a TCP connection is first establish. Obviously, when the sender have never
transmitted any data, it could not know what the current network situation is....
• When TCP has detected a packet loss, TCP will also use slow start. The reason for
this is a bit more subtle. There can be many reason that cause a packet loss, one
of it is someone started a new TCP connection which added more traffic to the
network. Since the sender does not know how fast the new connection is
injecting data into the network, it cannot know what the current network
capacity is.
Slow Start
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

29. • When to STOP using Slow Start ?
• TCP contains a special variable called SSThresh - "Slow Start Threshold".
• SSThresh defines the upper limit of the range of TWS where TCP will use slow
start, i.e.:if TWS <= SSThresh, TCP is in the slow start phase
• Consequently, if TWS > SSThresh, TCP is in the congestion avoidance phase -
i.e., TCP transmitting at a rate that may cause congestion. TCP will still try to
increase its data rate, but not exponentially.
Note:SSThresh can change over time (because network status changes)
Slow Start
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

30. • We saw that Slow Start increases
exponentially. Then why on earth we
are calling an exponential increase a
Slow Start ??
• Answer: Its in the history.
• Prior to Jacobson's work, TCP Sender will
send data equal to the amount of
advertised window of the receiver. This
caused network collapse.
• So compared to sending all AWS packets,
this way of start transmitting slower and
growing gradually is indeed slower and
hence Slow start.
Slow Start- Million $ question
Van Jacobson

31. • The period outside the slow start phase is congestion avoidance phase.
• Operation of TCP in the congestion avoidance phase:
• Starts when cwnd  ssthresh
• On each successful ACK:
• cwnd  cwnd + 1/cwnd
• Linear growth of cwnd
each RTT: cwnd  cwnd + 1
Congestion Avoidance

32. Congestion Avoidance

33. Congestion Avoidance
cwnd
1
2
3
1 RTT
4
data packet
ACK
cwnd  cwnd + 1 (for each cwnd ACKS)
receiversender

34. Congestion Avoidance
What is Congestion Avoidance trying to achieve ?
• As you have already seen, TCP begins a congestion avoidance phase when it reaches a "near congestion
point”
• Why would TCP not keep CWS constant ? Would you not cause a congestion if you keep increasing CWS ???
• The answer is TCP does not really know what the current capacity of network is... and network condition
keeps changing.
• The goal of TCP is to transfer data as fast as possible. If TCP would stop increasing CWS, it would not be
true to its goal.
• So during the congestion avoidance period, TCP is testing the tolerance of the network: after it has
successfully transferring CWS amount of data, it adds one more packet to the congestion window: CWS +
MSS and retest the network.
• This technique should sound very familiar to all of us.... - kids have perfected this skill when they ask their
parents for favors :-)
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

35. Congestion Avoidance
When does the Congestion Avoidance phase end ?
• Eventually, TCP will push the congestion window too far and cause some packet
drop.
• When a packet loss occurs, it can cause the sending TCP to timeout
• When a timeout occurs, TCP will begin a slow start phase:
TCP first set SSThresh = CWS/2.
• The reason is quite obvious: TCP caused a problem (packet loss, timeout) while
using the congestion window size CWS.
• It remembers when it went wrong by setting SSThresh to half that value.
• Then TCP invokes the slow start mechanism which will set CWS = 1 x MSS (i.e., 1
packet worth of data) and increases CWS at an exponential rate
towards SSThresh (that was set to half of the value that caused problems).
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

36. assume ssthresh = 8*MSS
Example:Slow Start/Congestion Avoidance
cwnd = 10
cwnd = 4
cwnd = 2
cnwd = 8
cwnd = 1
cwnd = 9
cwnd = 11
0
2
4
6
8
10
12
1 2 3 4 5 6 7
transmission number
congestionwindowsize
(inMSS)
ssthresh
S R

37. Slow Start & Congestion Avoidance
ssthresh
• Initally:
- cwnd = 1*MSS
- ssthresh = very high
• If a new ACK comes:
- if cwnd < ssthresh  update
cwnd according to slow start
- if cwnd > ssthresh  update
cwnd according to congestion
avoidance
- If cwnd = ssthresh  either
• If timeout (i.e. loss) :
- ssthresh = flight size/2;
- cwnd = 1*MSS
time
cwnd
Loss, e.g. timeout
slow start – in green
congestion avoidance – in blue
(initial) ssthresh

38. Slow Start & Congestion Avoidance

39. Fast Retransmit
Behavior without Fast Retransmit
Wait for a timeout is quite long
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

40. Fast Retransmit
Behavior with Fast Retransmit
• Immediately retransmit after 3 dup ACKs without waiting for timeout
• Adjusts ssthresh
• Enter Slow Start (cwnd = 1)
• Why 3 DUP ACK’s ? Hint: Increases the Loss Probability
Ref: http://www.mathcs.emory.edu/~cheung/Courses/558a/Syllabus/6-transport/TCP.html

41. t=xr
t=(x+1)r
Fast Retransmit Packet
Ack
At a random point
in the transfer
3 dup acks
TCP
Receiver
TCP
Sender
Fast
Retransmit

42. Fast Recovery
• Fast recovery is a relatively small improvement made to TCP after some research discovered that most fast
retransmit actions occur during mild congestions - i.e., only a few routers are congested and the congestion
can be cleared very quickly.
• Idea: each dupACK represents a packet having left the pipe (successfully received)
• Instead of forcing TCP to perform a slow start (which reduces CWS down to 1 x MSS), research found that TCP
can use a larger congestion window - that will let TCP send at a faster rate than when CWS = 1 x MSS
• Fast recovery rule:
• When TCP performs a fast restransmit (and did not timeout), then set SSThresh = CWS/2 and set CWS =
SSThresh + 3 * MSS.
• Use congestion avoidance with the new settings of SSThresh and CWS. (I.e., do not use (the very
drastic) slow start
• Each time another duplicate ACK arrives, set cwnd = cwnd + 1. Then, send a new data segment if allowed
by the value of cwnd.
• Once receive a new ACK (an ACK which acknowledges all intermediate segments sent between the lost
packet and the receipt of the first duplicate ACK), exit fast recovery. Set CWS = SSThresh and then,
continue with linear increasing due to congestion avoidance algorithm.

43. Fast Recovery
• Fast recovery algorithm (avoiding initial slow start phase)
1. When the third duplicate ACK is received,
Set ssthresh = cwnd / 2;
Retransmit the missing segment;
cwnd = ssthresh + 3 ;
2. Each time another duplicate ACK arrives,
Increment cwnd by the segment size;
Transmit a new segment (if allowed by the new cwnd value);
3. When the next ACK arrives that acknowledges new data,
cwnd = ssthresh ;
cwnd = cwnd + 1 every roundtrip time ;

44. Fast Recovery

45. Fast Recovery
cwnd
Slow Start Congestion Avoidance
Time
“inflating” cwnd with dupACKs
“deflating” cwnd with a new ACK
(initial) ssthresh
new ACK
fast-retransmit
fast-retransmit
new ACK
timeout
Concept:
• After fast retransmit,
reduce cwnd by half, and
continue sending segments
at this reduced level.
Problems:
• Sender has too many
outstanding segments.
• How does sender transmit
packets on a dupACK?
Need to use a “trick” -
inflate cwnd.

46. • After receiving 3 dupACKS:
1. Retransmit the lost segment.
2. Set ssthresh = flight size/2.
3. Set cwnd = ssthresh, and ndupacks = 3.
N.B. In Reno: send_win = min ( rwnd, cwnd + ndupacks ).
• If dupACK arrives:
• ++ ndupacks
• Transmit new segment, if allowed.
• If new ACK arrives:
• ndupacks = 0
• Exit fast recovery.
• If RTO expires:
• ndupacks = 0
• Perform slow-start - ( ssthresh = flight size/2, cwnd = 1 )
Fast Retransmit & Fast Recovery

47. 0
1
2
Ack(1)
3
4
5
6
Ack(1)
Ack(1)
Ack(1)
7
8
Initial state
cwnd=7
Slow start
cwnd=8
Fast Retransmit
cwnd=8/2+3=7
ssthresh=8/2=4
=> Fast Recovery
Ack(1)
Ack(1)
Ack(1)
Ack(1)
Ack(9)cwnd=8
cwnd=9 9
cwnd=10 10
cwnd=11 11
Exit Fast Recovery
cwnd=ssthresh=4
=> Congestion Avoidance
Ack(10)
1
12
Fast Retransmit and
Recovery

48. Limitation of Previous Algorithm
• If cwnd size is too small (smaller than 4 packets) then it’s not possible to get 3 duplicate
acks and run the algorithm
• The algorithm can not manage a loss of multiple packets from a single window of data
• It will cause a use of retransmission time out
• The algorithm doesn’t manage a loss of packets during the Fast Recovery stage
• Not a loss of the retransmitted packet
• There is no recursive run of the Fast Retransmit

49. Sender Receiver
0
1
2
Ack(1)
3
4
5
6
Ack(1)
Ack(1)
7
8
Initial State
cwnd=7
Slow Start
cwnd=8
1
Fast Retransmit
cwnd=8/2+3=7
ssthresh=8/2=4
=> Fast Recovery
Ack(1)
Ack(1)
Ack(1)
Ack(3)
cwnd=8
Exit Fast Recovery
cwnd=ssthresh=4
=> Congestion Avoidance
Ack(3)
Flight Size =No. of
Unacknowledged
segments
Flight Size > cwnd
=> No new segments
cwnd=9 9
The algorithm doesn’t
know which segments
were acknowledged
What was happen
if this packet was lost?

50. Improvements to Fast Recovery
• The Idea: If the sender remembers the number of the last segment that was sent before
entering the Fast Retransmit phase
• Then it can deal with a situation when a “new” ACK (which is not duplicate ACK)
does not cover the last remembered segment (“partial ACK”)
• This is a situation when more packets were lost before entering the Fast Retransmit.
• After discovering such situation the sender will retransmit the new lost packet too and
will stay at the Fast Recovery stage
• The sender will finish the Fast Recovery stage when it will get ACK that covers last
segment sent before the Fast Retransmit

51. • Set ssthresh to max (FlightSize / 2, 2*MSS)
• Record to “Recovery” variable the highest sequence number transmitted
• Retransmit the lost segment and set cwnd to ssthresh + 3*MSS.
• The congestion window is increased by the number of segments (three) that were
sent and buffered by the receiver.
• For each additional duplicate ACK received, increment cwnd by MSS.
• Thus, the congestion window reflects the additional segment that has left the
network.
• Transmit a segment, if allowed by the new value of cwnd and the receiver's advertised
window.
Improvements to Fast Recovery (I)

52. • When a partial ACK is received
• retransmit the first unacknowledged segment
• deflate the congestion window by the amount of new acknowledged data,
then add back one MSS
• send a new segment if permitted by the new value of cwnd
• When an acknowledge of all of the data up to and including "recover“ arrives:
• In our example: Set cwnd to ssthresh
Improvements to Fast Recovery (II)

53. Sender Receiver
0
1
2
Ack(1)
3
4
5
Ack(1)
Ack(1)
Initial State
cwnd=5
Slow Start
cwnd=6
1
Fast Retransmit
cwnd=6/2+3=6
ssthresh=6/2=3
Recover=6
=> Fast Recovery
Ack(1)
Ack(3)cwnd=7
Recover < Ack
Exit Fast Recovery
cwnd=ssthresh=3
=> Congestion Avoidance
Ack(8)
7
6
Ack(1)
Ack(3)
3
Recover >= Ack
Partial Ack
cwnd=7-(3-1)+1=6 cwnd=7 8
Ack(9)

54. TCP SACK
• Basic problem is that cumulative acks provide little information.
• If there are multiple packet loss, it forces the sender to either
• wait a roundtrip time to find out about each lost packet
• Or unnecessarily retransmit segments which have been correctly received.
• SACK can help in these situations.

55. TCP SACK
Saves from
RTT time.

56. sender
receiver
fast retransmit
sender
receiver
fast retransmit
TCP without SACK TCP with SACK
Without SACK and With SACK
Saves from Unnecessary data retransmit

57. TCP Options(SACK)
• How many SACK Blocks can be present in a SACK option ?
• A SACK block is represented by a pair of 32 bit Seq Nos.
• If there are n SACK blocks then SACK option is (8 n + 2) bytes long. 2
bytes are used to hold the Kind and length of the SACK option.
• Due to limited option space (40 bytes) in a TCP header, max no. of
SACK blocks which can be sent in a single segment is THREE
(Assuming Timestamp option is used).

58. sender
receiver
100 299
Receiver Buffer
500300 699
699300 500 900 1099
Another SACK Example

59. sender
receiver
699300 500 900 1099
699300 500 900 1099
700300 500 900 1099
1100
Another SACK Example

60. Evolution of TCP
1975 1980 1985 1990
1981
TCP & IP
RFC 793 & 791
1974
TCP described by
Vint Cerf, Bob Kahn
In IEEE Trans Comm
1983
BSD Unix 4.2
supports TCP/IP
1984
Nagel’s algorithm
to reduce overhead
of small packets;
predicts congestion
collapse
1987
Karn’s algorithm
to better estimate
round-trip time
1986
Congestion
collapse
1st observed
1990
4.3BSD Reno
fast recovery
delayed ACK’s
1975
Three-way handshake
Ray Tomlinson
In SIGCOMM 75 1988
Van Jacobson’s
algorithms
slow start,
congestion
avoidance, fast
retransmit (all
implemented in
4.3BSD Tahoe)
SIGCOMM 88

61. Evolution of TCP
1993 1994 1996
1994
ECN
Explicit
Congestion
Notification
(Floyd)
1993
TCP Vegas(not
implemented)
real congestion
avoidance
(Brakmo et al)
1994
T/TCP
Transaction TCP
(Braden)
1996
NewReno
modified fast
recovery
SACK TCP
Selective Ack
(Floyd et al)
1996
Improving TCP
startup
(Hoe)
1996
FACK TCP
Forward Ack
extension to SACK
(Mathis et al)

62. Summary- Packet Loss Management
• TCP Reno (RFC 2581) can manage a loss of at most one packet from a single
window of data
• TCP NewReno (RFC 2582) can manage a loss of more than one packet without
changing the TCP message structure
• TCP SACK (RFC 2018) enables to cope with a loss of more than one packet by
changing message structure (using TCP options)

63. Summary of TCP Behavior
• When entering slow start, if connection is new,
ssthresh = arbitrarily large value
cwnd = 1.
else,
ssthresh = max(flight size/2, 2*MSS)
cwnd = 1.
• In slow start ++cwnd on new ACK
TCP
Variation
Response to 3
dupACK’s
Response to Partial ACK
of Fast Retransmission
Response to “full” ACK of
Fast Retransmission
Tahoe
Do fast retransmit,
enter slow start
++cwnd ++cwnd
Reno
Do fast retransmit,
enter fast recovery
Exit fast recovery, deflate
window, enter congestion
avoidance
Exit fast recovery, deflate
window, enter congestion
avoidance
NewReno
Do fast retransmit,
enter modified fast
recovery
Fast retransmit and deflate
window – remain in
modified fast recovery
Exit modified fast recovery,
deflate window, enter
congestion avoidance
• When entering either fast recovery or modified
fast recovery,
ssthresh = max(flight size/2, 2*MSS)
cwnd = ssthresh.
• In congestion avoidance
cwnd += 1*MSS per RTT

64. TCP Throughput – Derivation
Matthew Mathis equation
• TCP Throughput =
𝑑𝑎𝑡𝑎 𝑝𝑒𝑟 𝑐𝑦𝑐𝑙𝑒
𝑡𝑖𝑚𝑒 𝑝𝑒𝑟 𝑐𝑦𝑐𝑙𝑒
• Each cycle we deliver 𝑊
2 Packets so the total time taken at each cycle will be = RTT ×( 𝑊
2)
• The data per cycle we will be sending is equal to the Area of the shaded region. Which is Height ×
𝐵𝑟𝑒𝑎𝑑𝑡ℎ. Area of the Orange + Area of the RED.
𝑤
2
2
+
1
2
𝑤
2
2
=
𝟑𝒘 𝟐
𝟖

65. TCP Throughput – Derivation
Probability Basics
• Scenario: If I play 5 times and I loose once, what is the probability that I loose ?
P =
1
5
• If a packet is dropped after sending a given amount of packets p, what is the probability of a drop ?
P =
1
𝑝

66. TCP Throughput – Derivation
Matthew Mathis equation
• So we concluded that total no. of packets delivered in the shaded area =
𝟑𝒘 𝟐
𝟖
packets per cycle.
• By probability we know we can deliver
1
𝑝
of packets before we drop:
A =
1
𝑝
where Area A =
𝟑𝒘 𝟐
𝟖
p =
8
3𝑤2 , which means w =
8
3𝑝
• Going back to the original formula of throughput :
𝑑𝑎𝑡𝑎 𝑝𝑒𝑟 𝑐𝑦𝑐𝑙𝑒
𝑡𝑖𝑚𝑒 𝑝𝑒𝑟 𝑐𝑦𝑐𝑙𝑒
=
𝑀𝑆𝑆 ×
𝟑𝒘 𝟐
𝟖
RTT × ( 𝑊
2)
=
𝑀𝑆𝑆 × 𝟑𝑾 𝟐× 𝟐
RTT × 𝑊× 8
=
𝑀𝑆𝑆 × 𝟔𝑾
RTT× 8
=
𝑀𝑆𝑆 × 𝟑𝑾
RTT× 4
We know the value of W =
8
3𝑝
, So substituting this in the above formula.
𝑀𝑆𝑆 × 𝟑𝑾 × 2×√2
𝑅𝑇𝑇× 4× 3𝑝
=
𝑀𝑆𝑆 × √3
𝑅𝑇𝑇× 2𝑝
=
𝑀𝑆𝑆 × C
𝑅𝑇𝑇× 𝑝
= where C =
3
2

67. Questions ?