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I was reading through the Wikipedia entry on NTP, and noticed this:

The NTP synchronization is correct when both the incoming and outgoing routes between the client and the server have symmetrical nominal delay. If the routes do not have a common nominal delay, the synchronization has a systematic bias of half the difference between the forward and backward travel times.

Does anyone know how often this actually comes up, where the incoming and outgoing routes don't have approximately the same latency?

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Strictly speaking, it is extremely rare for latency on a non-trivial path to NOT be asynchronous.

The only way for latency in both directions to be both equal and stable is if the bit rate of all links in the path are identical, the IP routing in both directions is identical, the traffic flow in both directions is identical, the routing policies at every device in the path in both directions is identical, and the host systems at both ends are behaving identically.

In practice, some paths will be "more" synchronous than others. Latency in two directions on a path may be approximately equal some of time if the various factors happen to balance at that moment. But that is not a stable condition as it depends on a lot more than just path itself.

Not only can latency vary by the path and traffic, but also on how you measure it. ICMP, UDP, and TCP packets may be routed differently, receive different queuing priorities, or be subject to different firewall rules. Latency is also effected by what goes on in the sending and receiving nodes.

I have written an article on the vagaries of network statistics which may be helpful: Loss, Latency, and Speed

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Well, any system with different upstream and downstream bitrates will necessarily have different latency, because it takes a different amount of time to send a packet of the same size. So normal ADSL counts for this, although the difference will be small.

Many systems which don't use wires will have asymmetric latency, such as 3G networks or satellite internet.

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Good point on the satellite networks. I suppose those older residential satellite systems that use a modem for upload would have a tremendous asymmetry. –  Curtis May 15 '12 at 13:28
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