确定网络中的不对称延迟 [英] Determine asymmetric latencies in a network

问题描述

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.

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.

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.

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?

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.

解决方案

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.

If each endpoint had, for example, its own GPS clock, then you'd have a reference point to work from.

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.)

Asymmetric latency can only be measured by sending a message with a timestamp t_s, and letting the receiver derive the latency from t_r - t_s, where t_r 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.

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.)

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