确定网络中的非对称延迟 [英] Determine asymmetric latencies in a network
问题描述
假设您在异构网络环境中的许多主机上有许多群集服务器，因此服务器之间的连接可能具有极大变化的延迟和带宽.您希望通过在服务器之间传输数据来建立服务器之间连接的映射.
Imagine you have many clustered servers, across many hosts, in a heterogeneous network environment, such that the connections between servers may have wildly varying latencies and bandwidth. You want to build a map of the connections between servers by transferring data between them.
当然，随着网络拓扑的变化，此地图可能会随着时间的推移而变得陈旧-但现在让我们忽略这些复杂性，并假设网络相对静态.
Of course, this map may become stale over time as the network topology changes - but lets ignore those complexities for now and assume the network is relatively static.
鉴于此主机图中节点之间的等待时间，计算带宽是相对简单的计时练习.我在延迟方面遇到了更多困难-但是.要获取往返时间，只需对从本地主机到远程主机的回程ping进行计时很简单-两个计时事件(开始，停止)都在本地主机上发生.
Given the latencies between nodes in this host graph, calculating the bandwidth is a relative simply timing exercise. I'm having more difficulty with the latencies - however. To get round-trip time, it is a simple matter of timing a return-trip ping from the local host to a remote host - both timing events (start, stop) occur on the local host.
如果在两个方向的等待时间都不相等的情况下我想单向旅行怎么办?假设各个主机上的时钟未精确同步(至少它们的错误与所涉及的延迟具有相同的幅度)-如何计算单向延迟?
What if I want one-way times under the assumption that the latency is not equal in both directions? Assuming that the clocks on the various hosts are not precisely synchronized (at least that their error is of the the same magnitude as the latencies involved) - how can I calculate the one-way latency?
在一个相关问题中，这种不对称等待时间(一条链路比另一条链路的方向更快)在实践中是否常见?由于什么原因/硬件配置?当然，我知道带宽不对称的情况，尤其是在最后一英里的消费者链路(例如DSL和Cable)上，但是我不确定延迟.
In a related question - is this asymmetric latency (where a link is quicker in direction than the other) common in practice? For what reasons/hardware configurations? Certainly I'm aware of asymmetric bandwidth scenarios, especially on last-mile consumer links such as DSL and Cable, but I'm not so sure about latency.
Added: After considering the comment below, the second portion of the question is probably better off on serverfault.
推荐答案
据我所知，不对称延迟(尤其是最后一英里"不对称)无法自动确定，因为任何网络时间同步协议都会受到同样的影响具有相同的不对称性，因此您没有评估不对称性的参照点.
To the best of my knowledge, asymmetric latencies -- especially "last mile" asymmetries -- cannot be automatically determined, because any network time synchronization protocol is equally affected by the same asymmetry, so you don't have a point of reference from which to evaluate the asymmetry.
例如，如果每个端点都有其自己的GPS时钟，那么您将有一个参考点可用于工作.
If each endpoint had, for example, its own GPS clock, then you'd have a reference point to work from.
在 快速测量LogP参数 对于消息传递平台 ，作者注意到延迟测量需要在被测系统外部进行时钟同步. (粗体字突出显示，斜体显示在原始文本中.)
In Fast Measurement of LogP Parameters for Message Passing Platforms, the authors note that latency measurement requires clock synchronization external to the system being measured. (Boldface emphasis mine, italics in original text.)
非对称等待时间只能通过发送带有时间戳 t s 的消息，然后让接收方从 t r -t s ，其中 t r 是接收时间.这需要发送方和接收方之间的时钟同步.如果没有外部时钟同步(例如使用GPS接收器或诸如网络时间协议，NTP的专用软件)，则时钟只能同步到往返的粒度两台主机之间的时间 [10]，对于测量网络延迟没有用.
Asymmetric latency can only be measured by sending a message with a timestamp ts, and letting the receiver derive the latency from tr - ts, where tr is the receive time. This requires clock synchronization between sender and receiver. Without external clock synchronization (like using GPS receivers or specialized software like the network time protocol, NTP), clocks can only be synchronized up to a granularity of the roundtrip time between two hosts [10], which is useless for measuring network latency.
但是，基于网络的算法(例如NTP)不会消除最后一英里链路的问题，因为算法的每个输入本身都会统一遵守最后一英里链路的性能特征，因此不会外部"在上述意义上. (我相信可以构造一个证明，但是我现在没有时间构造一个证明.)
No network-based algorithm (such as NTP) will eliminate last-mile link issues, though, since every input to the algorithm will itself be uniformly subject to the performance characteristics of the last-mile link and is therefore not "external" in the sense given above. (I'm confident it's possible to construct a proof, but I don't have time to construct one right now.)
这篇关于确定网络中的非对称延迟的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
