Download congestion control

Congestion
Dr. Abdulaziz Almulhem
Congestion
It occurs when network resources are
becoming scarce
High demand
Over utilized
Offered load exceeds throughput
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Taxonomy
Router-Centric vs. Host-Centric
Routers takes responsibility for forwarding
decisions.
 Host observes the network behavior and make
decisions.

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Taxonomy
Reservation-Based vs. FeedbackBased

Resources are allocated based on request
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Congestion Control
Proactive vs. Reactive
Backpressure
 Choke packets
 Implicit congestion control
 Explicit congestion control
 Policing

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Taxonomy
Window-Based vs. Rate-Based

Same terminology applies to flow control;
however congestion control deals with resource
allocation.
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TCP Congestion Control
Congestion control in TCP/IP is
complex and difficult, because:
IP is connectionless: no way to provision
congestion
 TCP is end-to-end: deduces indirectly
presence of congestion
 TCP entities cannot cooperate to maintain
certain level of flow (greedy way)

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TCP Flow Control
The rate at which TCP source can send data
is determined by the rate at which
acknowledgements are received.

Rate of Ack arrival is determined by the bottleneck
in the roundtrip path.
TCP automatically senses the network
bottleneck and regulates its flow accordingly

TCP’s self-clocking
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Network Congestion
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Destination Congestion
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Improving Performance
Retransmission timer management:

The value has a critical effect on TCP
reaction to congestion:
RTT variance estimation
 Exponential RTO backoff
 Karn’s algorithm

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Improving Performance
Window management

Managing the size of the send window has
a critical effect on TCP
Slow start
 Dynamic window sizing on congestion
 Fast retransmit
 Fast recovery

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Slow start
To avoid reaching congestion levels
quickly, the window size is incremented
progressively.
Upon receiving an Ack for a specific
segment, the window is incremented by
2.
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Dynamic Window Sizing
Slow start is effective for initializing a
connection
This can grow infinitely however at
some point in time resources will fill and
a segment will get lost (congestion)

What to do?
Reset window to 1 and start all over (not
practical, congestion may repeat)
 Caution could be possible

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Congestion detection
Congestion avoidance
Congestion recovery
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Fast Retransmit
Instead of reacting to lost segments by
setting the window to 1
Ack for missing segment will be received;
however sender will continue sending
subsequent segments.
 If three duplicate Acks are received, that
segment is retransmitted regardless of time
out.

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Fast Recovery
Since duplicate Ack are getting through
fairly regularly, then there is no point to
set window to 1.
Better to cut window to half
 Proceed with increasing window size
linearly (congestion avoidance)

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