What is flow control and congestion control?

See Computer Science and Technology Textbook Computer Networks, Fourth Edition, by Xie Xiren.

1. The concept is different.

Flow control is end-to-end control, for example, A sends data to B through the network, and A sends it too fast to cause B to fail to receive it (B buffer window is too small or processing is too slow), then the control is flow control, and the principle is to change the size of the sliding window.

Congestion control is caused by network congestion between A and B, resulting in slow transmission or packet loss, and no time for transmission. Prevent too much data from being injected into the network, so that the routers or links in the network are not overloaded. Congestion control is a global process that involves all hosts, routers, and all factors associated with degrading network performance.

2. The mechanism is different.

Flow Control Mechanism:

Let host A send data to host B. The window value determined by both parties is 400In addition, each packet segment should be 100 bytes long, and the initial value of the sequence number should be seq=1, with uppercase ack indicating that the message in the header is considered to be ack, and lowercase ack indicating the value of the acknowledgment field.

Host B on the receiver has three flow controls. The first time the window is set to rwind=300, the second time it is reduced to rwind=100 and finally to rwind=0, that is, the sender is not allowed to send data again. This state of pausing the sender will continue until host B re-emits a new window value.

Congestion Control Mechanism:

Slow start and congestion avoidance.

The determination of the rate of the transmission packet segment should be based on the receiving capacity of the receiving end and the overall consideration of not congesting the network, which is determined by the two state quantities of the receiving window and the congestion window.

The receiver window is the latest window value promised by the receiver based on the current receive cache size, and is the flow control from the receiver. The congestion window is the window value set by the sender according to its own estimated network congestion degree, and is the traffic control from the sender.

Congestion control is closely related to flow control, but there are also differences: It can be understood in this way that congestion control is the ability of the network to withstand the existing network load and is a global variable; Flow control, on the other hand, is often just a point-to-point control of traffic.

Congestion control: Prevents too much data from being injected into the network, so that the routers or links in the network are not overloaded. There is a premise for congestion control:

The network is able to withstand the existing network load. Congestion control is a global process that involves all hosts, routers, and all factors related to degrading network performance.

Flow control: Refers to the control of point-to-point traffic, which is an end-to-end positive problem. What flow control does is to suppress the rate at which the sender sends data so that the receiver has time to receive it.

The difference between congestion control and flow control before flutter is:

1. The task of congestion control is to ensure that the subnet can carry the traffic that the known hole fiber arrives, which is a global problem that involves all aspects of behavior, including all hosts, all routers, the storage process inside the router, and all other factors that may weaken the carrying capacity of the subnet;

2. Flow control is only related to the point-to-point traffic between a specific sender and a specific receiver, its task is to ensure that a fast imitation sender does not continue to transmit data at a rate that exceeds the receiver's absorption capacity, flow control usually involves the practice that the receiver provides some kind of direct feedback to the sender in order to tell the sender what is going on on the other side.

The difference between congestion control and flow control is:

1. The task of congestion control is to ensure that the subnet can carry the traffic that arrives in the differential zone, which is a global problem that involves all aspects of behavior, including all hosts, all routers, the storage process inside the router, and all other factors that may weaken the carrying capacity of the subnet;

2. Flow control is only related to the point-to-point traffic between a specific sender and a specific receiver, and its task is to ensure that a fast sender does not continue to transmit data at a rate that exceeds the receiver's absorption capacity.

First, the characteristics of the two are different:

1. The characteristics of congestion control: congestion control is based on the resource control of the terminal, which only needs to set a rule, which can limit the upper limit of bandwidth usage of each terminal, and at the same time can set the number of sessions of each terminal to prevent the exhaustion of network resources caused by a virus and other original prisoners.

2. Characteristics of flow control: flow control can identify sessions based on content through high-speed deep protocol analysis, identify the application to which each network session belongs, and can intercept or formulate corresponding bandwidth allocation policies for a certain protocol, while traditional network equipment such as routers and firewalls can only be identified at the lowest level according to the port.

Second, the main methods of the two are different:

1. The main methods of congestion control: the main control methods of congestion control include buffer area allocation method, group discard method, and quota control beam waxing method.

2. The main method of flow control: The main method of flow control is to introduce the concept of QoS, from the identification of different types of network packets to determine the priority of packet traffic.

3. The essence of the two is different:

1. The essence of congestion control: the number of packets reaching a certain part of the communication subnet is too large, so that the part of the network has no time to deal with it, resulting in the phenomenon of degrading the performance of this part or even the entire network, and even causing the network communication service to come to a standstill, that is, deadlock.

2. The essence of flow control: a kind of software or hardware method to control the flow of computer network. It is used to control the data flow between the modem and the computer, and has the ability to prevent data loss caused by the mismatch of hail speed at the communication between the computer and the modem.