TCP Global Synchronization

Translated from: http://blog.chinaunix.net/uid-24765042-id-3198715.html

With the popularization of network, information exchange and information sharing become an indispensable part of people's daily life. The increasing of information (data packet) in the network inevitably causes network congestion. As a result, congestion avoidance is particularly important, and now the wider use of congestion avoidance mechanisms on the Internet is Lost (Droptail): When the length of the queue reaches the specified maximum length, all incoming messages are discarded. This discard policy raises the TCP global synchronization phenomenon-packets arriving at the router are often burst due to the burst nature of the data on the Internet (traffic). If the queue is full or almost full, it causes a large number of packets to be dropped in a short period of time. And the TCP flow has the adaptive characteristic (adaptiveness), the source side discovers the packet loss to reduce the sending window (congestion Window,cwnd) sharply, the packet arrival rate will drop rapidly, thus the network congestion is relieved. But the source side learned that the network is no longer congested and began to increase the speed of transmission, and eventually caused the network congestion, and this phenomenon will often go on and on, so that in a period of time the network in Network utilization (NET utilization) very low status, reduce the overall throughput ( Throughput), this is known as the "TCP Global Synchronization" phenomenon.

The loss of the tail will cause the allocation of bandwidth imbalance between TCP traffic, some "greedy" traffic will occupy most of the bandwidth, and ordinary TCP traffic can not allocate bandwidth and "starve". In particular, the network has both TCP and UDP traffic, TCP traffic due to the window mechanism (lost tail caused by the sliding window CWnd reduced) and the release of bandwidth, UDP traffic has no window mechanism, so UDP traffic will quickly occupy TCP freed bandwidth, The result is that UDP traffic consumes all of the bandwidth and TCP traffic is "starved" for lack of bandwidth allocation.

TCP Global Synchronization