Congestion Avoidance and Control Van Jacobson

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Congestion Avoidance and ControlVan Jacobson

• Jonghyun Kim

April 1, 2004
E-mail netgod77_at_hanmail.net
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Introduction

• To achieve network stability, TCP entity should
  obey a packet conservation principle, which is
  borrowed from physics.
• What is packet conservation principle?
• A new packet is not put into the network until
  an old packet leaves( i.e. conservation) while
  maintaining equilibrium in which a connection
  runs stably by effectively using the bandwidth
  available on the path.

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• Three failures of packet conservation
• 1. The connection does not get to equilibrium
• 2. A sender injects a new packet before an old
  packet
• has exited
• 3. The equilibrium cant be reached because of
  
• resource limits along the path
• 1, 3 Equilibrium problem 2
  Conservation problem
• To fix those failures, three algorithm are
  introduced
• Slow-start algorithm for 1.
• Retransmit timeout estimation algorithm
  for 2.
• Congestion avoidance algorithm for 3.

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Slow-start algorithm

• To understand slow-start algorithm, we should
  understand self-clocking system

• Things for sender to do
• 1. Sender finds the bandwidth of the slowest link
  in the path and sends to the receiver the bursts
  of packets based on that bandwidth (This is the
  core of slow-start algorithm)
• 2. After that, if a new packet is sent whenever
  receiving an ACK (i.e. self-clocking), this
  connection will get to equilibrium without
  wasting the bandwidth.

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• How to find the bandwidth of the slowest link in
  the path

If timeout occurs, we roughly know the bandwidth
bandwidth The last bursts of sent
packets/2
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• Slow-start algorithm (Pseudo-code based on C)
• cwnd congestion window
• rwnd receivers advertised window
• cwnd 1
• while(1)
• send packets of min (cwnd, rwnd) //
  bursts of packets
• wait until receiving all ACKs for the
  previous sent packets
• if ( timeout occurs )
• cwnd 1
• else
• cwnd 2cwnd

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• Startup behavior of TCP with Slow-start

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• Startup behavior of TCP without Slow-start

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Retransmit timeout estimation algorithm

• To estimate retransmit timeout correctly, we
  should know the mean and variance of round trip
  time

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• The algorithm of RFC 793
• SRTT aSRTT (1- a)RTT
• RTO ßSRTT
• -SRTT Smoothed RTT (mean) -RTT
  Recently measured RTT
• -a Smoothing factor (0.9) -ß
  Delay variance (2)
• -RTO Retransmit timeout interval
• The problem of this algorithm
• Since ß is fixed value, RTO is not
  proportional to the variance

• The algorithm of Jacobson
• The idea is to make ß above roughly
  proportional to the standard deviation of RTT by
  using mean deviation
• Standard deviation (sdev)
• Mean deviation (mdev)
• mdev 1.25sdev
• SRTT aSRTT (1- a)RTT
• SMD ßSMD (1- ß)(SRTT-RTT)
• RTO SRTT 4SMD (a
  0.125, ß 0.25)
• -SMD smoothed mean deviation - a ,ß
  smoothing factor

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• Performance of the algorithm of RFC793

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• Performance of the algorithm of Jacobson

D1gtD2. It means that Jacobsons algorithm
estimates RTO more flexibly because RTO varies
proportionally to the mean deviation
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Congestion avoidance algorithm

• We always want to use the full bandwidth
  available on the subnet, keeping stable network.
  In order to do so, the congestion window size is
  increased slowly (i.e. additive increase) to
  probe for more bandwidth becoming available (e.g.
  when a host closes its connection). However, on
  any timeout, the congestion window size is
  decreased fast (i.e. multiplicative decrease) to
  avoid congestion
• On no congestion (to probe for more
  bandwidth)
• cwnd cwnd 1
• On congestion (to avoid congestion)
• cwnd cwnd/2

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• Illustration of congestion avoidance algorithm

To probe more bandwidth
To avoid congestion
To find current bandwidth
To probe more bandwidth
To avoid congestion
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Timeout
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48
Congestion window (Kbyte)
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Timeout
40
36
32

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Threshold
24
20
16
12
8
4
0
2
21
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25
26
27
28
29
24
30
31
3
4
5
1
7
8
9
10
6
11
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15
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20
14
32
Transmission number
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• Congestion avoidance algorithm (pseudo-code based
  on C)

cwnd 1 while(1) send packets of min
(cwnd, rwnd) // burst wait
until receiving all ACKs for the previous sent
packets slow-start if ( timeout
occurs ) break else cwnd
2cwnd threshold cwnd/2 cwnd
1 while(1) if ( cwnd lt threshold )
send packets of min (cwnd, rwnd)
//burst wait until receiving all ACKs for
the previous sent packets slow-start
if ( timeout occurs ) threshold cwnd/2
cwnd 1 else cwnd 2cwnd
if ( cwnd threshold cwnd gt threshold )
send packets of min (threshold, rwnd)
//burst else if ( cwnd gt threshold )
send a new packet whenever an ACK is received
//self-clocking if ( Sender receives all
ACKs for the previous sent packets )
cwnd cwnd 1 send a new packet //to
probe more bandwidth if ( timeout
occurs) threshold cwnd/2 cwnd
1 //to avoid congestion
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• Multiple conversation test setup

When receiving more than 50 packets/sec, the
routers will drop packets
Experimental Parameters Local bandwidth
1MB/s P2P link bandwidth
25KB/s Receivers window size 16KB Packet
size 512 Byte 50-packet size
25KB
Since four connections can send 64KB (416KB) at
a time, they will cause congestion easily.
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• Multiple, simultaneous TCPs with no congestion
  avoidance

Result -4,000 (36) of 11,000 packets were
retransmitted
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• Multiple, simultaneous TCPs with congestion
  avoidance

Result -89 (1) of 8281packets were
retransmitted
Big retransmission means big congestion
occurrence. On the other hand, small
retransmission means small congestion occurrence
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• Total bandwidth used by old and new TCPs

Thin line old TCP Thick line new TCP
Bandwidth negotiation among four connection
Reaching equilibrium
Bandwidth renegotiation due to connection one
shutting down
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• Effective bandwidth of old and new TCPs

Thin line old TCP Thick line new TCP
Old TCP uses 75 of the link bandwidth New TCP
uses almost 100 of the link bandwidth
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Summary
We can think of network system as a linear
system. To make network system stable, TCP should
conform a packet conservation principle. There
exist three failures of packet conservation in
the network system. To fix those failures, three
algorithms are introduced. (Slow-start,
retransmit timeout estimation, congestion
avoidance) This paper introduced only
host-centric congestion control, but we also need
to learn router-centric congestion control.
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Reference

• JACOBSON, V. Congestion avoidance and control.
  Proceedings of the ACM SIGCOMM conference on
  applications, technologies, architectures, and
  protocols for computer communication (Stanford,
  California, USA), 18, 4, August 1988, 314-329.
• ANDREW S. TANENBAUM, Computer Networks, 2003
• RFC 793