Active Queue Management

As previously mentioned, dropping or marking schemes for packets that are waiting in a queue can significantly influence TCP’s behavior on the end devices. These schemes are called Active Queue Management (AQM). 如前所述，针对在队列中等待的数据包的丢弃或标记方案会极大地影响 TCP 在终端设备上的行为。这些方案被称为主动队列管理（AQM）。

Tail Drop

The tail drop scheme drops newly arriving packets when a queue is full. Essentially, this scheme drops the “tail” of a queue which are the packets that have most recently arrived. Calling tail drop an Active Queue Management mechanism is a misnomer because dropping the packets due to the lack of buffer space is the default behavior and there is no “active” scheme needed for it. 尾部丢弃方案会在队列满时丢弃新到达的数据包。从本质上讲，这种方案会丢弃队列的 "尾部"，即最近到达的数据包。将尾部丢弃称为主动队列管理机制是一种误解，因为因缓冲空间不足而丢弃数据包是默认行为，并不需要 "主动 "方案。

Tail drop has an interesting effect on TCP performance. In a case where a single TCP flow arrives when the queue is full, a tail drop results in dropping many packets for that flow, resulting in a significant rate reduction of only that flow. 尾部丢弃对 TCP 性能有一个有趣的影响。在单个 TCP 流量到达时队列已满的情况下，尾部丢弃会丢弃该流量的许多数据包，从而导致仅该流量的速率显著降低。

Another case is when many TCP flows arrive simultaneously. Tail drop results in dropping packets from all of them. As a result, all these flows reduce their rates simultaneously and then increase their rates simultaneously. This results in a cyclic underutilization and over-utilization pattern, called TCP global synchronization. 另一种情况是许多 TCP 流量同时到达。尾部丢弃会导致丢弃所有这些流量的数据包。因此，所有这些流量会同时降低速率，然后又同时提高速率。这就形成了一种周期性的利用不足和利用过度模式，称为 TCP 全局同步。

Random Early Detect (RED)

To avoid TCP global synchronization, packets already queued are dropped (or ECN marked) randomly before the queue is full. This allows only a few TCP connections to reduce their rate. 为避免 TCP 全局同步，已排队的数据包会在队列满之前被随机丢弃（或 ECN 标记）。这样，只有少数 TCP 连接可以降低速率。

Weighted Random Early Detection (WRED)

WRED combines the capabilities of RED along with traffic priority, although the implementation may be different on different platforms. WRED 结合了 RED 的功能和流量优先级，但在不同平台上的实现方式可能不同。

As Figure 8-13 shows, when the egress queue utilizatio