Informally, congestion is when too many sources are sending too much data at a rate too fast for the network to handle.

This can manifest in several ways:

• Long delays ( from queueing in router buffers)
• Packet loss (from buffer overflow at routers)

This is a different problem flow control (congestion is more of an overall issue with the network).

• Flow control ⇒ one sender is sending too fast for one receiver
• Congestion control ⇒ too many senders are sending too fast
• Note that this is also one of the top 10 problems to solve for the internet

The causes/costs of congestion include:

• Premature timeout ⇒ packets are retransmitted ⇒ unnecessary duplicates are sent
  • When packets are lost (i.e. dropped at router due to full buffer), they must be retransmitted
  • Sender timer can time out prematurely ⇒ 2 copies are sent, both of which are delivered

• In practice, we often send more data than can be handled (i.e. packets will be retransmited, leading to duplicates) - higher rate than $\lambda_{in}$ ⇒ actual rate is $\lambda_{in}'$
• Throughput for output is not higher, as duplicates are not considered
  • $\lambda_{out} < \lambda_{in}$
• Hosts will assume timeout in practice ⇒ unnecessary retransmissions lead to slowdown of network
• While routers may be operating at maximum speed, this may not be enough to match the throughput of links
• More work is needed for given receiver throughput due to retransmission
• Unneeded retransmsissions occur, as links carry multiple copies of a packet (lower max achievable throughput)

For example, consider the following case: