Computer network : CONGESTION CONTROL (Part 2)

Load Shedding

When buffers become full, routers simply discard packets.
Which packet is chosen to be the victim depends on the application and on the error strategy used in the data link layer.
For a file transfer, for, e.g. cannot discard older packets since this will cause a gap in the received data.
For real-time voice or video it is probably better to
    throw away old data and keep new packets.
Get the application to mark packets with discard priority.


Random Early Detection (RED)
This is a proactive approach in which the router discards one or more packets before the buffer becomes completely full.
Each time  a packet arrives, the RED algorithm computes the average queue length, avg.
If avg is lower than some lower threshold, congestion is assumed to be minimal or non-existent and the packet is queued.
If avg is greater than some upper threshold, congestion is assumed to be serious and the packet is discarded.
If avg is between the two thresholds, this might indicate the onset of congestion. The probability of congestion is then calculated.


Problems that can happen to packets as they travel from origin to destination
Low throughput
Due to varying load from disparate users sharing the same network resources, the bit rate that can be provided to a certain data stream may be too low
Dropped packets
The routers might fail to deliver (drop) some packets if their data loads are corrupted, or the packets arrive when the router buffers are already full.
Errors
Sometimes packets are corrupted due to bit errors caused by noise and interference, especially in wireless communications
Latency
It might take a long time for each packet to reach its destination, because it gets held up in long queues, or it takes a less direct route to avoid congestion.


Jitter
Packets from the source will reach the destination with different delays. The variations in the packet arrival time is called jitter.
Out-of-order delivery
When a collection of related packets is routed through a network, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order than they were sent.


Techniques for achieving good QoS
Overprovisioning
Provide so much router capacity, buffer space and bandwidth– expensive
Telephone system is overprovisioned