CMPT 371

© Janice Regan, CMPT 128, 2007-2012

CMPT 371Data Communications and Networking

Congestion Control

1

Simple Congestion Scenario Two pairs hosts share a connection through a

common router. Host A and host B are sending data at

bytes/sec through a shared link (through the router) with capacity R

The router has a buffer (infinite) which can queue packets if they arrive faster that the link to hosts B and C can accept them

Large queuing delays result

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ni

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Fig 3.43 text Fig 3.44.text

From Kurose and Ross 6ed

Another Scenario Now assume the buffer is finite, packets arriving

at a full buffer will be dropped Dropped packets will eventually be

retransmitted Host A and host B are sending data at

. bytes/sec through a shared link (through the router) with capacity R

is known as the offered load

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retrannn ii'

ni'

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Fig 3.45 text


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Scenariosa) Offered load = application sending rate = R/2

Host A transmits only when buffer is available (by chance) so no packets are lost

b) Offered load = application sending rate + retransmission rate = R/2 Retransmission occurs only when

packet is definitely lost. (e.g. expiry of very long RTT timer)

One out of two original packets sent is retransmitted

ADDITIONAL COST: RETRANSMISSION OF PACKETS THAT ARE DROPPED DUE TO CONGESTION

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Scenariosc) As for B but retransmission occurs when a packet may

be lost.

c) Retransmission rate now includes some duplicate transmissions. Duplicate transmissions are transmissions of packets that were delayed rather than lost

d) ADDITIONAL COST: RETRANSMISSION OF DUPLICATE PACKETS

e) In the diagram each packet assumed to be forwarded twice

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Fig 3.46 text


TCP congestion window Congestion window

Usually smaller than the RecWindow (size of sliding window) for sliding windows

Allows reduction of the size of the sliding window to deal with congestion

Sliding windows will use smallest of RecWindow and CongWindow

Consider RTT to be the time transmission of CongWindow begins until all of CongWindow is acknowledged

Send rate is about CongWindow length / RTT

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TCP: detecting congestion Two types of ‘loss events’ indicate congestion is occuring

Third copy of ACK arrives causing retransmission Transmission timer expires

These events indicate high levels of congestion because They are caused by packets being lost Packets are lost when queuing buffers fill due to congestion and

overflow

When acknowledgements arrive as expected, and packets are not lost TCP assumes there is no congestion. Faster arrival of ACKS indicates possible available bandwidth Slower arrival of ACKS indicates possible congestion (low levels)

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Managing the CongWindow Slow Start

Begins with a CongWindow length of 1 Maximum Segment Size Initial throughput is MSS / RTT If data is transmitted successfully double the length of the

CongWindow (or increase to RecWindow if increase is smaller) Continue doubling the CongWindow each RTT until the

RecWindow is reached or a loss event occurs If a loss event (triple duplicate ACK) occurs cut the CongWindow

in half, exit the slow start procedure, moving to CA mode (fast recovery). CongWindow will not be decreased past 1 MSS

If a loss event (timeout) occurs reinitialize SS mode, and continues in SS mode until the CongWindow reaches a Threshold size equal to half the size when the loss occured

Effect of Slow Start

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Managing the CongWindow AIMD: Additive increase multiplicative decrease

If data is transmitted successfully increase the length of the CongWindow by one MSS (or increase to RecWindow if increase is smaller)

Continue increasing the CongWindow each RTT until the RecWindow is reached or a loss event occurs

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Managing the CongWindow AIMD: Additive increase multiplicative decrease

If a loss event ( in particular a third duplicate ACK) occurs cut the CongWindow in half (and add 3 RTT) then move to fast recovery state (resume linear increases)

If a loss event (timeout) occurs enter SS mode, and continues in SS mode until the CongWindow reaches a Threshold size equal to half the size when the loss occured

While running this algorithm TCP is in collision avoidance (CA) mode

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Figure 3.51.text

Janice Regan 2012From Kurose and Ross 6ed


Fairness (1) Consider N connections through a single router to the

same host. Both connections have the same MSS and RTT

If TCP is fairall connections should end up with the same throughput

For simplicity consider 2 connections both of the connections is operating in CA mode

If both connections are operating below R/2 (capacity of connection is R) then loss should not occur and both connections will increase their CongWindow at the same rate


Fairness (2) At some point the combined load will

become larger than R, queues will fill and a packet will be lost

This will reduce the combined offered load.

When the load has been reduced (one or multiple reductions) until packets are no longer lost then the combined offered load will be less than R

This cycle of increase followed by decrease will continue


Fairness (3) For any given packet arriving at the router

the probability that it will be dropped is the same

If the offered load of connection 1 is larger than connection 2, then more packets are arriving from connection 1 and is more likely that the packet from connection 1 will be dropped

This will mean the connection with the larger throughput is more likely to have its CongWindow reduced. The net effect is that the load of the two connections will over time converge.


Fig 3.54 text Fig 3.55 text


Fairness: UDP UDP does not have built in congestion control

mechanisms. ICMP choke packets may be used to control congestion

is ICMP is activated at all points along the path (ICMP is commonly disabled by hosts/routers as a security measure)

A protocol that uses UDP will be able to continue pumping data into the Internet at a rate as large as its maximum transmission rate.

Thus, it is possible for UDP traffic to fill the available bandwidth of the connection and to prevent the transmission of TCP traffic.


Fairness: TCP an UDP The discussion of fairness and TCP

assumes that each user is making one connection.

Many TCP applications make multiple TCP connections to increase the throughput of data.

This means that the per user fairness is skewed.

CMPT 371

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