CSEE W4140Networking Laboratory

Lecture 7: TCP congestion control

Jong Yul Kim03.04.2009

Announcements Midterm next week in class(1:10~2:25 pm)

Problems from the lab exercises, quiz Subnets Network protocols

Similar to ARP exercises we did in class For example, learn how TCP sequence numbers,

ACK numbers work Problems about debugging networks

Lab 5 part 2 after Spring Break Lab report for lab 5 part 1 is due after spring

break

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 Until a loss occurs

Result is an exponentially fast growth in sending rate

Slow Start (SS)

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

Three-Way Handshake

Why is two-way handshake not enough?

Main Points of Lab 5 Parts 5~8

More about TCP Interactive applications over TCP Data transfer applications over TCP Retransmissions Congestion Control

Parts of Lab 5 may not be done because of the use of serial WAN link