Transcript Part I: Introduction

TCP Plots
r Slides originally from Williamson at Calgary
r Minor modifications are made
CPSC 601.38
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Tutorial: TCP 101
r The Transmission Control Protocol (TCP) is
r
r
r
r
the protocol that sends your data reliably
Used for email, Web, ftp, telnet, p2p,…
Makes sure that data is received correctly:
right data, right order, exactly once
Detects and recovers from any problems
that occur at the IP network layer
Mechanisms for reliable data transfer:
sequence numbers, acknowledgements,
timers, retransmissions, flow control...
CPSC 601.38
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TCP 101 (Cont’d)
r TCP is a connection-oriented protocol
SYN
SYN/ACK
GET URL
ACK
Web Client
Web Server
YOUR DATA HERE
FIN
ACK
FIN/ACK
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TCP 101 (Cont’d)
r TCP slow-start and congestion avoidance
ACK
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TCP 101 (Cont’d)
r TCP slow-start and congestion avoidance
ACK
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TCP 101 (Cont’d)
r TCP slow-start and congestion avoidance
ACK
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TCP 101 (Cont’d)
r This (exponential growth) “slow start”
process continues until either:
m
m
m
packet loss: after a brief recovery phase, you
enter a (linear growth) “congestion avoidance”
phase based on slow-start threshold found
limit reached: slow-start threshold, or
maximum advertised receive window size
all done: terminate connection and go home
CPSC 601.38
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Tutorial: TCP 201
r There is a beautiful way to plot and
visualize the dynamics of TCP behaviour
r Called a “TCP Sequence Number Plot”
r Plot packet events (data and acks) as
points in 2-D space, with time on the
horizontal axis, and sequence number on
the vertical axis
r Example: Consider a 14-packet transfer
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Time
CPSC 601.38
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So What?
r What can it tell you?
r Everything!!!
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SeqNum
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Key: X Data Packet
+ Ack Packet
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RTT
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Time
CPSC 601.38
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SeqNum
TCP
Seg.
Size
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Key: X Data Packet
+ Ack Packet
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Time
CPSC 601.38
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Time
TCP Connection Duration
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Num
Bytes
Sent
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Time
CPSC 601.38
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SeqNum
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Bytes
Key: X Data Packet
+ Ack Packet
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Sec
Time
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SeqNum
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Key: X Data Packet
+ Ack Packet
Access
Network
Bandwidth
(Bytes/Sec)
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Time
CPSC 601.38
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Sender’s
Flow Control
Window Size
Time
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SeqNum
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Key: X Data Packet
+ Ack Packet
TCP
Slow
Start
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Time
CPSC 601.38
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Delayed ACK
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Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
Packet
Loss
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++
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Duplicate
ACK
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Time
CPSC 601.38
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SeqNum
Cumulative ACK
Key: X Data Packet
+ Ack Packet
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++
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Retransmit
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
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++
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Time
RTO
(Retranmission Timeout)
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TCP 201 (Cont’d)
r What happens when a packet loss occurs?
r Quiz Time...
m Consider a 14-packet Web document
m For simplicity, consider only a single packet loss
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Time
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SeqNum
?
Key: X Data Packet
+ Ack Packet
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Time
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SeqNum
Key: X Data Packet
+ Ack Packet
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Time
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
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?
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
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++
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Time
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
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+++
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Time
CPSC 601.38
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SeqNum
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Key: X Data Packet
+ Ack Packet
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
?
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Time
CPSC 601.38
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SeqNum
Key: X Data Packet
+ Ack Packet
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X
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++
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Time
CPSC 601.38
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TCP 201 (Cont’d)
r Main observation:
m
“Not all packet losses are created equal”
r Losses early in the transfer have a huge
adverse impact on the transfer latency
r Losses near the end of the transfer always
cost at least a retransmit timeout
r Losses in the middle may or may not hurt,
depending on congestion window size at the
time of the loss
CPSC 601.38
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Congratulations!
r You are now a TCP expert!
r Or you are so confused now that you will
finally take my advice and buy the TCP
book and read through some chapters
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