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CSEE W4140Networking Laboratory
Lecture 7: TCP flow control and congestion control
Jong Yul Kim03.22.2010
Short review of TCP from last time
Characteristics of the IP network Delivers packets from host to host May lose packets (discarded by routers) Doesn’t care about packet order
TCP is a reliable, in-order, byte-stream service (delivers data from application to application)
Short review of TCP from last time TCP is a reliable, in-order, byte-stream
service The beauty of TCP is that it works without
explicit support from the network Each end of the connection cooperate to make
sure packets are delivered reliably and in order
Techniques used by TCP Sequence numbers Acknowledgements (and numbers) Retransmissions Timer
Today’s lecture
TCP Flow Control Throttling the rate of sender so that the
receiver’s buffer does not overflow
TCP Congestion Control Throttling the rate of sender in the face
of network congestion
TCP Flow Control (Receive Window) When a connection is established, the
receiver allocates a receive buffer. Incoming packets are stored in the buffer
so that the application can read data from the buffer.
TCP Flow Control (Receiver behavior)
The receiver lets the sender know: how much space is
left in the buffer= RcvWindow
by placing that value in the window size field
in every segment that it sends to the sender
IP header TCP header TCP data
Sequence number (32 bits)
DATA
20 bytes 20 bytes
0 15 16 31
Source Port Number Destination Port Number
Acknowledgement number (32 bits)
window sizeheaderlength
0 Flags
Options (if any)
TCP checksum urgent pointer
20 bytes
TCP Flow Control (Sender behavior)
Sender can fill up the spare room in the receiver’s buffer by sending more data
Sender maintains the size of data that has already been sent but unacknowledged = bytes_unACKed
Makes sure that bytes_unACKed ≤ RcvWindow
Network Congestion Why does congestion occur?
Too many senders sending at high rate Routers dropping packets due to overflowing
buffers
What are the symptoms? Packet loss Packet queuing delay More retransmission more packet loss Link bandwidth wasted on retransmissions
TCP Congestion Control
Remember: TCP has no support from the network about congestions Need to use end-to-end congestion
control
TCP relies on perceived network congestion and throttles the sending rate accordingly
How does TCP know there is congestion in the network?
Answer: packet loss Timeout Three duplicate
ACKs shown in diagram on right
How does TCP limit sending rate?
Using a variable called congestion window = CongWin
Size of unacknowledged data must be less than CongWin bytes_unACKed ≤ CongWin
Sending rate is roughly CongWin/RTT bytes/sec
We can throttle the sending rate by controlling CongWin
TCP Congestion Control Algorithm Is an algorithm that controls CongWin
Simply stated: Packet loss (=congestion) decrease CongWin All is well increase CongWin
Three main parts Additive-Increase, Multiplicative Decrease Slow Start Reaction to timeout events
Additive Increase, Multiplicative Decrease (AIMD)
Additive Increase Increase CongWin by 1 MSS every RTT
while there is no packet loss
Multiplicative Decrease Decrease CongWin by half when packet
is lost
Additive Increase, Multiplicative Decrease (AIMD)
Congestion Avoidance phase
Slow Start (SS)
When connection is established CongWin is set to 1 MSS Increase CongWin by 1 MSS every ACK
Different from Additive Increase (+1 MSS every RTT)
Until a loss occurs
Result is an exponentially fast growth in sending rate
Slow Start (SS)
Example of Slow Start/Congestion Avoidance
Assume that ssthresh = 8
cwnd = 1
cwnd = 2
cwnd = 4
cwnd = 8
cwnd = 9
cwnd = 10
0
2
4
6
8
10
12
14
t=0
t=2
t=4
t=6
Roundtrip times
Cw
nd
(in
seg
men
ts)
ssthresh
Reaction to Timeouts We already discussed that TCP perceives
congestion through packet loss
Packet is considered lost when: Timeout occurs Three duplicate ACKs are received
But timeouts and three duplicate ACKs are different. What do they tell about the severity of
congestion?
Reaction to Timeouts Let’s do something different for
timeout. Reset CongWin to 1 MSS Begin again from Slow Start Slow start (exponential increase) until
when? New variable called Threshold
Threshold is set to ½ CongWin After Threshold, do AIMD instead of SS
Lab 5 part 2 in the lab manual
PC1
R1 R2
R3 R4
PC210.0.3.0/24
10.0.2.0/2410.0.1.0/24
10.0.5.0/24
10.0.4.0/24
Serial WAN connection
Lab 5 part 2 – our version
PC1
R1 R2
PC2
10.0.3.0/24 10.0.2.0/2410.0.1.0/24
10.0.5.0/24
Use Table 5.3 and Table 5.4 to configure PC1, PC2, Router1, and Router2.
Do parts 5(B), 6(A), 6(C), and 8.
For part 8, configure PC1 and PC2 so that all packets travel along PC1 – R1 – R2 – PC2 path.
Main Points of Lab 5 Parts 5~8
More about TCP Interactive applications over TCP Data transfer applications over TCP Congestion Control
Prelab and lab reports
Prelab 6 is due this Friday.
Lab report 5.1 is due this week before your labs.
Lab report 5.2 + BGP lab is due next week before your labs.