When one part of the subnet (e.g. one or more routers in an area) becomes overloaded, congestion results.
Because routers are receiving packets faster than they can forward them, one of two things must happen:
The subnet must prevent additional packets from entering the congested region until those already present can be processed.
The congested routers can discard queued packets to make room for those that are arriving
Packet arrival rate exceeds the outgoing link capacity.
Insufficient memory to store arriving packets
Bursty traffic
Slow processor
Congestion control is a global issue – involves every router and host within the subnet
Congestion control keeps network operating at full capacity, but minimizes packet loss maximize “goodput”
Flow control – scope is point-to-point; involves just sender and receiver
Congestion Control is concerned with efficiently using a network at high load.
Several techniques can be employed. These include:
Warning bit
Choke packets
Load shedding
Random early discard
Traffic shaping
The first 3 deal with congestion detection and recovery. The last 2 deal with congestion avoidance.
A special bit in the packet header is set by the router to warn the source when congestion is detected.
The bit is copied and piggy-backed on the ACK and sent to the sender.
The sender monitors the number of ACK packets it receives with the warning bit set and adjusts its transmission rate accordingly.
A more direct way of telling the source to slow down.
A choke packet is a control packet generated at a congested node and transmitted to restrict traffic flow.
The source, on receiving the choke packet must reduce its transmission rate by a certain percentage.
An example of a choke packet is the ICMP Source Quench Packet.
Over long distances or at high speeds choke packets are not very effective.
A more efficient method is to send to choke packets hop-by-hop.
This requires each hop to reduce its transmission even before the choke packet arrive at the source.
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