TCP"
Reno, NewReno, SACK
4 September 2009
CS5229, Semester 1, 2009/10
1

TCP Reno
4 September 2009
CS5229, Semester 1, 2009/10
2

14
cwnd
ssthresh
12
10
8
6
4
2
0
4 September 2009
CS5229, Semester 1, 2009/10
3

new ack:"
if (cwnd < sstresh)"
cwnd += 1"
else"
cwnd += 1/cwnd
4 September 2009
CS5229, Semester 1, 2009/10
5

timeout:"
retransmit 1st unacked"
ssthresh = cwnd/2"
cwnd = 1"
4 September 2009
CS5229, Semester 1, 2009/10
6

3rd duplicate ACK:"
fast retransmission"
(ie, retransmit 1st unack) "
fast recovery"
(details today)"
ssthresh = cwnd = cwnd/2
4 September 2009
CS5229, Semester 1, 2009/10
7

TCP’s rule"
send more packets if "
L + cwnd > H"
[ L .. H-1 ] are outstanding packets
4 September 2009
CS5229, Semester 1, 2009/10
10

4 September 2009
CS5229, Semester 1, 2009/10
11

Data Packets
ACK Packets
(received up to this seq number)
Congestion Window
Sequence
Number
Time
4 September 2009
CS5229, Semester 1, 2009/10
12

3rd dup ack:"
retransmit 1st unacked"
ssthresh = cwnd/2"
cwnd = cwnd/2
4 September 2009
CS5229, Semester 1, 2009/10
13

Incorrectly clamping the congestion window
too soon (without fast recovery)
X
4 September 2009
CS5229, Semester 1, 2009/10
14

fast recovery:"
keep the pipe occupied"
4 September 2009
CS5229, Semester 1, 2009/10
15

X
4 September 2009
CS5229, Semester 1, 2009/10
16

4 September 2009
CS5229, Semester 1, 2009/10
17

X
X
4 September 2009
CS5229, Semester 1, 2009/10
18

TCP Reno timeout with
multiple losses in a window
4 September 2009
CS5229, Semester 1, 2009/10
19

TCP NewReno
4 September 2009
CS5229, Semester 1, 2009/10
20

X
X
4 September 2009
CS5229, Semester 1, 2009/10
21

4 September 2009
CS5229, Semester 1, 2009/10
22

3rd dup ack:"
retransmit 1st unacked"
ssthresh = cwnd/2"
cwnd = cwnd/2 + 3"
remember highest
4 September 2009
CS5229, Semester 1, 2009/10
25

“complete” ack:"
(all are acked) "
cwnd = ssthresh
4 September 2009
CS5229, Semester 1, 2009/10
26

“partial” ack:"
(acknowledge n packets)"
retransmit"
cwnd = cwnd - n + 1
4 September 2009
CS5229, Semester 1, 2009/10
27

Note: RFC2581/RFC2582 give the
accurate/gory details. Simplified version
is presented here (eg. cwnd vs FlightSize,
update of cwnd upon partial ACK).
4 September 2009
CS5229, Semester 1, 2009/10
29

What does a dup
ACK tell us?
4 September 2009
CS5229, Semester 1, 2009/10
30

“Coarse Feedback”
4 September 2009
CS5229, Semester 1, 2009/10
31

TCP SACK
4 September 2009
CS5229, Semester 1, 2009/10
32

Use TCP header
options to report
received segments.
4 September 2009
CS5229, Semester 1, 2009/10
33

SACK Blocks:"
1st block - report most recently
received segments"
subsequent blocks - repeat most
recent previous blocks
4 September 2009
CS5229, Semester 1, 2009/10
34

pipe: num of outstanding
packets in the pipe."
send only if pipe < cwnd
4 September 2009
CS5229, Semester 1, 2009/10
35

scoreboard: which
packets have been
received?
4 September 2009
CS5229, Semester 1, 2009/10
36

X
X
4 September 2009
CS5229, Semester 1, 2009/10
37

3rd dup ack:"
pipe = cwnd - 3"
retransmit 1st unacked"
ssthresh = cwnd/2"
cwnd = cwnd/2 + 3
4 September 2009
CS5229, Semester 1, 2009/10
38

subsequent dup ack:"
cwnd++"
pipe--"
if pipe < cwnd"
send packet, pipe++
4 September 2009
CS5229, Semester 1, 2009/10
39

“partial” ack:"
retransmit"
cwnd = cwnd - n + 1"
pipe -= 2"
if pipe < cwnd"
send packet, pipe++
4 September 2009
CS5229, Semester 1, 2009/10
40

Idea of SACK:"
Which packet has left the network?"
Where is the gap?"
Decouple when to send and what to
send.
4 September 2009
CS5229, Semester 1, 2009/10
43

TCP SACK recovers faster
than NewReno with
multiple losses in a window.
4 September 2009
CS5229, Semester 1, 2009/10
44

Deployment
4 September 2009
CS5229, Semester 1, 2009/10
57

Feb 2004
NewReno
Reno
8%
Tahoe
15%
77%
4 September 2009
CS5229, Semester 1, 2009/10
58

70% "
SACK capable
4 September 2009
CS5229, Semester 1, 2009/10
59

TCP BIC/CUBIC"
Linux 2.6.x
4 September 2009
CS5229, Semester 1, 2009/10
63

Compound TCP "
MS Windows Vista
4 September 2009
CS5229, Semester 1, 2009/10
64

TFRC"
equation-based"
congestion control
4 September 2009
CS5229, Semester 1, 2009/10
65

Not everyone uses TCP
4 September 2009
CS5229, Semester 1, 2009/10
66

UDP:"
Media streaming"
Gaming"
VoIP
4 September 2009
CS5229, Semester 1, 2009/10
67

Why not congestion controlled?"
1. UDP has low delay, no need
full reliability. UDP "
is not congestion controlled.
4 September 2009
CS5229, Semester 1, 2009/10
68

Why not congestion controlled?"
2. No incentive. OTOH, there
are incentives NOT to use
congestion control.
4 September 2009
CS5229, Semester 1, 2009/10
69

Sally Floyd, h+p://www.icir.org/floyd/tcp_unfriendly.html
4 September 2009
CS5229, Semester 1, 2009/10
70

“Unresponsive Flows”
4 September 2009
CS5229, Semester 1, 2009/10
71

Bad: lead to unfairness
and congestion collapse.
4 September 2009
CS5229, Semester 1, 2009/10
72

Unfairness: unresponsive
flows consume more bandwidth
than congestion controlled
flows.
4 September 2009
CS5229, Semester 1, 2009/10
73

Congestion Collapse:
wasting bandwidth by sending
packets that will be dropped
4 September 2009
CS5229, Semester 1, 2009/10
77

Today: "
a TCP-friendly"
unreliable protocol
4 September 2009
CS5229, Semester 1, 2009/10
78

Idea: "
send at a rate that a TCP
flow would send
4 September 2009
CS5229, Semester 1, 2009/10
79

we can do the AIMD-thing"
at the source, or
4 September 2009
CS5229, Semester 1, 2009/10
80

4 September 2009
CS5229, Semester 1, 2009/10
81

equation-based "
congestion control
4 September 2009
CS5229, Semester 1, 2009/10
82

steady"
fair"
responsive
4 September 2009
CS5229, Semester 1, 2009/10
83

how to determine"
RTT"
tRTO"
p
4 September 2009
CS5229, Semester 1, 2009/10
84

p is not packet loss rate"
but "
loss event rate
4 September 2009
CS5229, Semester 1, 2009/10
85

4 September 2009
CS5229, Semester 1, 2009/10
86

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CS5229, Semester 1, 2009/10
87

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CS5229, Semester 1, 2009/10
88

RTT can fluctuates
4 September 2009
CS5229, Semester 1, 2009/10
89

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CS5229, Semester 1, 2009/10
90

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91

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CS5229, Semester 1, 2009/10
92

4 September 2009
CS5229, Semester 1, 2009/10
93

how to initialize?
4 September 2009
CS5229, Semester 1, 2009/10
94

slow start"
(until loss occur)"
Tnow = min(2Tprev, 2Trecv)
4 September 2009
CS5229, Semester 1, 2009/10
95

no loss history, how?"
solve p given T, RTT
4 September 2009
CS5229, Semester 1, 2009/10
96

4 September 2009
CS5229, Semester 1, 2009/10
97

TFRC is now part of
DCCP
4 September 2009
CS5229, Semester 1, 2009/10
98