(翻译)ACK-compression ACK压缩

原文见http://web.opalsoft.net/qos/default.php?p=tcp-70

7.0.- ACK-compression

In 1992, J.C.Mogul did some investigation observing TCP dynamics in real networks specially about a phenomenom called ACK-compression. Here is a summary of this work.The essence of the congestion avoidance algorithm is the observation that data packets arrive at the receiving host at the rate that the bottleneck link will support. If the receiver's ACK arrive at the sender with the same spacing, then by sending new data packets at the same rate the sender can avoid overruning the bottleneck link. It is by correctly exploting this self-clocking property of TCP that congestion may be avoided.Zhang, Shenker & Clark studied the slightly more complex case of a single link with TCP data flowing in both directions at once. Their simulations showed that several surprising phenomena could arise in such a situation, even when Jacobson's algorithms were employed.On such phenomenom they called ACK-compression. A TCP sender's self-clocking depends on the arrival of ACKs at the same spacing with which the receiver generated them. If these ACKs spend any time sitting in queues during their transit through the network, however,their spacing may be altered. When ACKs arrive closer together than they were sent, the sender might be misled into sending more data than the network can accept, which could lead to congestion and loss of efficiency.The Mogul's paper describes the result of a trace-based study of large numbers of uncontrived connections through the Internet. These were obtained by monitoring the packets flowing in and out of a busy gateway system, widely used by sites all over the Internet for electronic mail and similar protocols. The experiments show that ACK-compression can indeed be detected automatically from packet traces, and that it does happen in real networks. Other phenomena besides ACK-compression, such as synchronization of losses between several connections, out-of-order packet delivery, and some forms of improper TCP behavior, can also be automatically detected from traces, as will be described in this paper. Several simulation studies have identified other phenomena that might be visible to a trace-based analisys. For example, Shenker found that when multiple connections use the same path, the packets from a given connection tend to be clustered together, rather than interleaved with those of other connections.

就翻译一下关于ACK-Compression的部分

Zhang, Shenker & Clark 一次进行了一项较为复杂的案例研究，内容为在一个单链路双向网络中TCP数据流的问题。他们的模拟展现了出现这种情况的现象，尽管也使用了Jacobson的算法。

这种现象被他们称之为ACK-Compression。一个TCP发送者的 自计时取决于到来的，由接收机按照相同时间间隔生成的ACK。如果这些的ACK通过网络过境期间存在一些开销在队列中，但是，它们的间隔可能会改变。当ACK的到达间距小于它们发送的间距，发送者可能会被误导，发送比网络可以接受的更多的数据，这可能导致堵塞和效率损失。