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Principles of Congestion Control. Chapter 3.6 Computer Networking : A top-down approach. Principles of Congestion Control. Congestion: informally: “too many sources sending too much data too fast for network to handle” manifestations: lost packets (buffer overflow at routers)

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principles of congestion control

Principles of Congestion Control

Chapter 3.6

Computer Networking: A top-down approach

principles of congestion control1
Principles of Congestion Control

Congestion:

  • informally: “too many sources sending too much data too fast for network to handle”
  • manifestations:
    • lost packets (buffer overflow at routers)
    • long delays (queuing in router buffers)
  • different from flow control!
  • a top-10 problem!
  • 3 examples of cause and costs of congestion control

Transport Layer

causes costs of congestion scenario 1
Causes/costs of congestion: scenario 1

lout

lin : original data

unlimited shared output link buffers

Host A

Host B

Host C

  • large delays when congested
  • maximum achievable throughput
  • two senders, two receivers
  • one router, infinite buffers
  • no retransmission

Host D

Transport Layer

causes costs of congestion scenario 2
Causes/costs of congestion: scenario 2
  • one router, finite buffers
  • sender retransmission of lost packet

Host A

lout

lin : original data

Host C

l\'in : original data, plus retransmitted data

Host B

finite shared output link buffers

Transport Layer

causes costs of congestion scenario 21
Causes/costs of congestion: scenario 2

l

l

l

>

=

l

l

l

C/2

in

in

in

C/2

C/2

out

out

out

C/3

lout

lout

lout

C/4

C/2

C/2

C/2

a.

lin

lin

lin

b.

c.

  • always: (goodput)
  • “perfect” retransmission only when loss:
  • retransmission of delayed (not lost) packet makes larger (than perfect case) for same

“costs” of congestion:

  • more work needed for given “goodput” (retransmission)
  • unneeded retransmissions: link carries multiple copies of packets

Transport Layer

causes costs of congestion scenario 3
Causes/costs of congestion: scenario 3

Host A

Host D

  • four senders
  • multihop paths
  • timeout/retransmit

lin: original data

lout

l\'in: original data, plus retransmitted data

Host B

finite shared output link buffers

R1

R2

Host C

Transport Layer

causes costs of congestion scenario 31
Causes/costs of congestion: scenario 3

Host B

Host A

lout

Another “cost” of congestion:

  • when packet dropped, any “upstream transmission capacity used for that packet was wasted!

Transport Layer

approaches towards congestion control
Approaches towards congestion control

Two broad approaches towards congestion control:

Network-assisted congestion control:

  • routers provide feedback to end systems
    • single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)
    • explicit rate sender should send at

End-to-end congestion control:

  • no explicit feedback from network
  • congestion inferred from end-system observed loss, delay
  • approach taken by TCP

Transport Layer

network assisted congestion control
Network-assistedcongestioncontrol
  • Feedback in twoways
    • Direct feedback – network router  sender (choke packet)
    • Network feedback via receiver – Router marks a packet and receiver notifysender
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