Congestion

• Informally: “too many sources sending too much data too fast for network to handle”

• Manifestations:

  • Long delays (queueing in router buffers)

  • Packet loss (buffer overt routers)

• Different from flow control

Causes/Costs

Scenario 1

Simplest scenario:

• 1 router, infinite buffers

• Input, output link capacity: R

• 2 flows

Scenario 2

• 1 router, finite buffers

• Sender retransmits lost, timed-out packets

  • Application-layer input = application-layer output: λin = λout

  • Transport-layer input includes retransmissions: λ’in >= λin

Idealisation

Perfect knowledge

• Sender sends only when router buffers available
  Some perfect knowledge

• Packets can be lost (dropped at router) due to full buffers

• Sender knows when packet has been dropped: only resends if packet known to be lost

Realistic Scenario

Un-needed duplicates

• Packets can be lost, dropped at router due to full buffers - requiring retransmissions

• But sender times can time out prematurely, sending two copies, both of which are delivered

“Costs” of Congestion

• More work (retransmission) for given receiver throughput

• Unneeded retransmissions - link carries multiple copies of a packet

• Decreasing maximum achievable throughput

Scenario 3

• 4 senders

• Multi-hop paths

• Timeout/retransmit

Another “Cost” of Congestion

When packet dropped, any upstream transmission capacity and buffering used for that packet was wasted

Insights

• Throughput can never exceed capacity

• Delay increases as capacity approached

• Un-needed duplicates further decreases effective throughput

• Loss/retransmission decreases effective throughput

• Upstream transmission capacity / buffering wasted for packets lost downstream