TCP global synchronization Analysis

TCP global synchronization analysis with the popularization of the network, information exchange and information sharing have become an essential part of people's daily life. The increasing information (data packets) in the network will inevitably lead to network congestion. Therefore, congestion prevention is particularly important. The mechanism of congestion prevention widely used on the Internet is DropTail: when the length of the queue reaches the maximum length, all incoming messages are discarded.

This discard policy will lead to the global synchronization of tcp. Because of the burstable nature of data on the Internet (Traffic), packets arriving at the router are often abrupt. If the queue is full or almost full, a large number of packets will be dropped continuously in a short period of time. However, TCP streams have Adaptiveness. When data packets are lost at the source end, the transmission window (cwnd) is greatly reduced, and the arrival rate of data packets decreases rapidly, as a result, network congestion is eliminated. Www.2cto.com

However, when the source end learns that the network is no longer congested, it starts to increase the transmission speed and eventually causes network congestion. This phenomenon is often repeated, thus, the Network is in a low Network Utilization rate (Network Utilization) for a period of time, reducing the overall throughput (throughput). This is the so-called "TCP global synchronization" phenomenon. If the end is dropped, the bandwidth allocated between TCP traffic is not balanced. Some "greedy" traffic will occupy most of the bandwidth, while the common TCP traffic cannot be allocated and the traffic will be starved to death ".

In particular, when both TCP and UDP traffic exist in the network, the TCP traffic is released because of the window mechanism (Sliding Window cwnd decreases due to tail loss), and UDP traffic has no window mechanism, as a result, UDP traffic quickly occupies the bandwidth released by TCP, resulting in UDP traffic occupying all the bandwidth, and TCP traffic "starved to death" due to no bandwidth allocation ". Author: pythonlqd