1. The Transport Layer (TCP)
Lecture 7:
The Transport Layer (TCP)

2. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

3. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

4. Bob Alice process process app layer socket socket receive buf.
send buffer
transport layer
TCP
TCP
network layer

5. Alice
Bob
process
process
socket
socket
TCP
TCP

6. Alice
Bob
process
process
socket
socket
TCP
TCP

7. Bob Alice process process socket socket recv. buf. send buf.
TCP
TCP

8. What is a TCP connection?
Sockets
pass data between app-layer process & TCP
Buffers
store sent/received data
(bidirectional or “full-duplex” communication)
Variables
will discuss in a moment
A set of resources
allocated at the end-systems

9. connection established
Alice
Bob
connection request
request acknowledgment
ack. of request ack.
connection established

10. How is it established? 3-way handshake between end-systems
“client” = the initiating process
“server” = the other process
(but data may flow both directions)

11. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

12. (connection established)
Alice
Bob
(connection established)
data=byte #1
ACK=2

13. (connection established)
Alice
Bob
(connection established)
SEQ=1, data=bytes #1 to #100
ACK=101

14. X Bob Alice cumulative ACK SEQ=1, data=byte #1 ACK=2

15. SEQ & ACK numbers (TCP payload = sequence of numbered bytes)
Sequence number (SEQ)
provided by data sender
# of first byte of data
Acknowledgment number (ACK)
provided by data receiver
# of oldest byte expected by receiver (cumulative)

16. Bob Alice Alice’s last ACK=200 Bob’s last SEQ= , ACK= , 10 ACK=10 200
data=”?”
data=”!” 11
SEQ= , ACK= ,
200
SEQ= , ACK= 11
201
SEQ= , ACK= , 11
201
data=”?”

17. Bob Alice Alice’s last ACK=200 Bob’s last SEQ= , ACK= , 10 ACK=10 200
data=”hello”
data=”hey” 15
SEQ= , ACK= ,
200
SEQ= , ACK= 15
203
SEQ= , ACK= , 15
203
data=”hello”

18. SEQ & ACK numbers Always present Sequence number Acknowledgment number
even when they are not needed
Sequence number
# of first byte of data
(if no data, pretend there is some)
Acknowledgment number
# of oldest byte expected by receiver

19. X Bob Alice timeout ignores the data SEQ=1, data=byte #1 ACK=2

20. Bob Alice timeout ignores the data SEQ=1, data=byte #1
ACK=2
ACK=3
timeout
SEQ=1, data=byte #1
ignores the data
ACK=3

21. Timeout & retransmit Sender times out
segment corrupted, lost or delayed
ACK corrupted, lost or delayed
Sender retransmits the segment with oldest un-ACKed sequence number

22. Timeout = ? A bit longer than the round-trip time
allow enough time for segment to reach receiver and ACK to reach sender
How can the sender predict the RTT?

23. Bob Alice 200 msec sampleRTT = 200 msec 2 sec sampleRTT = 2 sec
SEQ=1, data=byte #1
200 msec
ACK=2
sampleRTT = 200 msec
SEQ=2, data=byte #2
2 sec
ACK=3
sampleRTT = 2 sec

24. Empirical, conservative prediction of RTT
Timeout calculation
EstimatedRTT = EstimatedRTT SampleRTT
DevRTT = function(RTT variance)
Timeout = EstimatedRTT + 4 DevRTT
Empirical, conservative prediction of RTT

25. X Bob Alice fast retransmit SEQ=1, data=byte #1 SEQ=2, data=byte #2
ACK=2
SEQ=3, data=byte #3
SEQ=4, data=byte #4
ACK=2
SEQ=5, data=byte #5
ACK=2
ACK=2
fast retransmit
SEQ=2, data=byte #2

26. Fast retransmit Sender receives 3 duplicate ACKs
segment lost or delayed
Sender retransmits segment with oldest un-ACKed sequence number

27. Retransmission events
Sender times out
segment corrupted, lost or delayed
ACK corrupted, lost or delayed
Sender receives 3 duplicate ACKs

28. Is TCP Go-Back-N or SR? Go-Back-N element Selective Repeat element
receiver sends cumulative ACKs
does not ACK individual out-of-order segments
Selective Repeat element
sender retransmits only one segment (with the oldest un-ACK-ed sequence number)
It is a hybrid

29. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

30. Bob Alice process process socket socket receive buf. send buffer
spare room
TCP
TCP

31. Bob Alice receiver window=100 bytes SEQ=1, data=bytes #1 to #100
ACK=101, receiver window=80 bytes

32. Bob Alice receiver window=4000 bytes SEQ=1, data=bytes #1 to #1500
ACK=4001, receiver window=0 bytes

33. Bob Alice ACK=4001, receiver window=0 bytes SEQ=4001

34. Flow control Receiver provides receiver window
equal to free space in TCP receive buffer
specifies how many bytes it can receive
Sender sends up to this # of bytes
must wait for receiver window to “open”
Slows down the sender

35. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

36. Bob Alice SYN=1,SEQ=14527657 SYN segment
SYN=1,SEQ= , ACK=
SYNACK segment
SYN=0,SEQ= , ACK= , data=...

37. Bob Alice timeout connection closed FIN=1,SEQ=145287942 ACK=145287943

38. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

39. Alice discards Bob’s data
Jack (the hijacker)
Alice
Bob
SEQ=1, ACK=1, data=http GET request
SEQ=1,ACK=21,data=evil html file
SEQ=1,ACK=21,data=html file
Alice discards Bob’s data

40. Jack (the hijacker) Alice Bob
SEQ= , ACK= , data= GET req.
SEQ=??,ACK=??,data=evil html file ??
SEQ= ,
ACK= ,data= html file

41. Make segment content unpredictable
TCP hijacking
Attack: impersonate one of the parties
& provide fake content
Defense: randomize sequence numbers
Make segment content unpredictable

42. Denis Bob SYN segment SYN segment SYN segment SYN segment
SYNACK segment

43. Bob Alice process process socket socket SYN from Denis SYN from Denis
incomplete connections
incomplete connections
SYN from Denis
SYN from Denis
SYN from Denis
SYN from Denis

44. Bob Alice SYN from Alice SYN segment SYNACK segment
non-forgeable ticket
non-forgeable ticket

45. Pass the state to the TCP client
SYN flooding
Attack: exhaust the SYN buffer
Defense: get rid of the SYN buffer
instead use non-forgeable ticket
Pass the state to the TCP client

46. Outline TCP connection Reliability Flow control (Loose ends) Security
Congestion control

47. Alice’s max throughput is R
bottleneck link
transmission rate R
Alice
Bob
Alice’s max throughput is R

48. Alice’s max throughput is R/2
bottleneck link
transmission rate R
Alice
Bob
Alice’s max throughput is R/2

49. If Alice’s transport transmits at rate R/2,
bottleneck link
transmission rate R
Alice
Bob
If Alice’s transport transmits at rate R/2,
she experiences high queuing delay

50. If Alice’s transport transmits at rate R/2,
bottleneck link
transmission rate R
Alice
Bob 4 4 2 3 1
If Alice’s transport transmits at rate R/2,
part of that rate is spent on retransmissions,
so, her effective throughput is < R/2

51. If Alice times out prematurely, and needlessly (re)transmits packets,
bottleneck link
transmission rate R
Alice
Bob 4 4
If Alice times out prematurely,
and needlessly (re)transmits packets,
the switch performs useless transmissions

52. The network uses resources to
Alice
Bob
The network uses resources to
transmit packets that will later be dropped

53. Bad congestion effects
Long queuing delays
Resource waste
sender has to retransmit
routers transmit duplicate packets
routers transmit packets that will be dropped

54. Congestion-control approaches
At the network layer
packet switches signal congestion to end-hosts
At the transport layer
end-hosts signal congestion to each other

55. Congestion window
The number of unacknowledged bytes that the sender may transmit...
... so as to avoid “creating congestion”

56. Alice Bob RTT R bps x RTT sec bandwidth delay product seq# 0 seq# 100
ACK 100
seq# 200
seq# 300
R bps x
RTT sec
bandwidth
delay
product

57. Bandwidth-delay product
The max amount of traffic that the sender can transmit until he gets the first ACK
= the maximum sender window size

58. X X Alice Bob seq# 0 100 bytes ACK 100 seq# 100 200 bytes seq# 200
timeout!
100 bytes
seq# 100

59. Self-clocking
Inferring the “right” congestion window based on the ACKs
ACK = no congestion, increase window
No ACK = congestion, decrease window

60. Alice Bob seq# 0 100 bytes 0 - 99 ACK 100 200 bytes seq# 100 100 - 199
ACK 200
300 bytes
ACK 300
400 bytes

61. Alice
Bob
100 bytes
200 bytes
300 bytes
400 bytes

62. Alice
Bob
100 bytes
RTT
200 bytes
RTT
300 bytes
RTT
400 bytes

63. Increase window size Exponentially by 1 MSS for every ACKed MSS
when we do not expect congestion

64. Alice
Bob
400 bytes X
timeout!
100 bytes

65. Alice
Bob
100 bytes
200 bytes
300 bytes

66. Increase window size Exponentially Linearly
by 1 MSS for every ACKed MSS
when we do not expect congestion
Linearly
by 1 MSS every RTT
when we expect congestion

67. X Alice Bob 100 bytes 200 bytes 400 bytes timeout! 100 bytes
threshold = 200

68. Alice
Bob
100 bytes
200 bytes
300 bytes
threshold = 200

69. Basic algorithm Start with exponential increase
when sender times out, it resets window to MSS
sets congestion threshold to last window size / 2
Transition to linear increase, when:
window reaches congestion threshold

70. X Alice Bob 400 bytes seq# 500 500 - 599 600 - 699 700 - 799 800 - 899
ACK 500
ACK 500
fast
retransmit!
ACK 500
seq# 500
500 bytes
seq# 900
threshold = 200

71. exponential increase linear increase new ACK new ACK
window = window + MSS
window = window + MSS *(MSS/window)
window >= threshold
exponential increase
linear increase
timeout
threshold = window/2
timeout
window = MSS
threshold = window/2
retransmit
window = MSS
retransmit

72. exponential increase linear increase fast recovery
threshold = window/2
new ACK
window = window + MSS
exponential increase
linear increase
timeout
window = MSS
window >= threshold
window = window + MSS *(MSS/window)
3 duplicate ACKs
retransmit
new ACK
threshold = window/2
window = threshold + 3 MSS
timeout
fast retransmit
retransmit
fast recovery
3 duplicate ACKs
duplicate ACK
threshold = window/2
window = window + MSS
window = threshold + 3 MSS
fast retransmit

73. Basic algorithm Start with exponential increase
when sender times out, it resets window to MSS
sets congestion threshold to last window size / 2
Transition to linear increase, when:
window reaches congestion threshold
sender receives 3 duplicate ACKs

74. TCP terminology Exponential increase = slow start
when sender times out, it resets window to MSS
sets congestion threshold to last window size / 2
Linear increase = congestion avoidance
window reaches congestion threshold
sender receives 3 duplicate ACKs