3.6 – Principles of Congestion Control

3.6.2 – Approaches to Congestion control

• Congestion-control approaches:
  • End-to-end congestion control:
    • The network layer provides no explicit support to the transport layer for congestion-control purposes. The presence of network congestion must be inferred by the end system based only on observed network behavior. TCP segment loss is taken as an indication of network congestion, and TCP decreases its window size accordingly (It is also proposed that increasing round trip segment delay as an indicator of network congestion)
  • Network-assisted congestion control:
    • Routers provide explicit feedback to the sender and/or receiver regarding the congestion state of the network. This feedback may be as simple as a single bit indication congestion at a link.
    • ATM Available Bite Rate (ABR) congestion control:
      • A router informs the sender of the maximum host sending rate it can support on an outgoing link.
• The Internet-default versions of IP and TCP adopt an end-to-end approach towards congestion control. However, more recently IP and TCP may also optionally implement network-assisted congestion control.
• For network-assisted congestion control, congestion information is typically fed back from the network to the sender in one of two ways:
  • Direct feedback may be sent from a network router to the sender. This form of notification typically takes the form of a choke packet.
  • When a router marks/updates a field in a packet flowing form sender to receiver to indicate congestion. Upon receipt of a marked packet, the receiver then notifies the sender of the congestion indication.