Generally speaking, when there are too many packets in the communication subnet, the network performance will decrease, which is called congestion.

1 essence: demand for resources >: available resources-congestion indicates that the load exceeds the capacity of resources.

2. Causes of blockage

The main reason is that traffic is often sudden.

Multiple inputs correspond to one output;

Slow processor;

Low bandwidth line.

N3。 solution

N The solution to a certain factor can only slightly improve the network performance, or even just transfer the bottleneck that affects the performance;

All the factors need to be considered comprehensively.

There are two obvious ways to overcome 1: increasing resources and reducing load. (Denied some services)

Methods to manage (avoid as much as possible) congestion: the host can send information at a constant rate; Traffic shaping (forcing packets to be transmitted at a more predictable rate).

4, n the difference between congestion control and flow control

N Congestion control needs to ensure that the communication subnet can carry the traffic submitted by users, which is a global problem involving hosts, routers and many other factors.

N-flow control is related to point-to-point traffic, which mainly solves the problems of fast sender and slow receiver. This is a local problem and is usually controlled according to feedback.

Second, the basic principle of congestion control

According to cybernetics, congestion control methods are divided into two categories.

N 1, open-loop control

N Solve problems through good design and avoid congestion;

The current state of the network is not considered in congestion control;

N2, closed-loop control

N based on feedback mechanism;

N3, working process

N monitoring system to find out when and where congestion occurs;

N send a congestion message to the site that can take action;

N adjust the system operation and solve the problem.

N4。 A parameter to measure whether the network is congested.

N packet loss rate caused by insufficient buffer;

N average queue length;

N Number of packets retransmitted in a period of time;

N average packet delay;

N packet delay variation (jitter).

N5, feedback method

N, sending an alarm packet to a load generation source;

A bit or field is reserved in the N packet structure to indicate the occurrence of congestion. Once congestion occurs, the router will set all outgoing packets and alert neighbors.

N The host or router actively and periodically sends a probe to ask whether congestion occurs.

6. n Congestion Prevention Strategy-traffic shaping.

open-loop control