Is latency equivalent to RTT?
Has it become common usage to define latency in an IP network as the round trip time in that network? I've always considered latency to be a one-way measure of delay and RTT to be the sum of the latencies in both directions. When I tried to find something to back up this view, I discovered that a number of companies define latency as equivalent to RTT in their SLAs. Assuming that one has measuring devices in every PoP, do you think it is harder to measure a full matrix of one way latency compared to measuring a full matrix of RTT? Does it even make sense to measure a full matrix of RTT when the measurement of A to B to A should be equivalent to the measurement of B to A to B? --Michael Dillon
On Wed, 14 May 2003 Michael.Dillon@radianz.com wrote:
Has it become common usage to define latency in an IP network as the round trip time in that network?
I've always considered latency to be a one-way measure of delay and RTT to be the sum of the latencies in both directions. When I tried to find something to back up this view, I discovered that a number of companies define latency as equivalent to RTT in their SLAs.
Assuming that one has measuring devices in every PoP, do you think it is harder to measure a full matrix of one way latency compared to measuring a full matrix of RTT?
Does it even make sense to measure a full matrix of RTT when the measurement of A to B to A should be equivalent to the measurement of B to A to B?
Ah but its not is it, so much of the Internet is asymmetric now this is important especially for troubleshooting But generally performance relates to how well say your web pages download from a site which is a 2 way tcp connection to you may as well do rtt and get both directions. Your problem with 1-way is finding a way to do it thats reliable, for rtt ping is about as simple as you can get and works a treat but for 1-way you have to be a bit more creative.... Steve
On Wed, May 14, 2003 at 03:01:06PM +0100, Michael.Dillon@radianz.com wrote:
Has it become common usage to define latency in an IP network as the round trip time in that network?
I've always considered latency to be a one-way measure of delay and RTT to be the sum of the latencies in both directions. When I tried to find something to back up this view, I discovered that a number of companies define latency as equivalent to RTT in their SLAs.
Could this be because customers believe that RTT more closely represents a network characteristic that they (or their customers) actually need? Perhaps they find it easier to digest like this because they're already familiar with the concept (because someone let them have 'ping'). -- Andrew Bangs andrewb@demon.net
On Wed, 14 May 2003 15:01:06 +0100 Michael.Dillon@radianz.com wrote:
Has it become common usage to define latency in an IP network as the round trip time in that network?
It probably depends on the context.
Assuming that one has measuring devices in every PoP, do you think it is harder to measure a full matrix of one way latency compared to measuring a full matrix of RTT?
Yes probably, because of the potential assymetric paths, changing paths and synchronization issues. See below.
Does it even make sense to measure a full matrix of RTT when the measurement of A to B to A should be equivalent to the measurement of B to A to B?
I think so. It would probably be nice to do both if you could though. RTT may be particularly nice to measure, because the path between A to B or between B to A may not be stable. In addition, the endpoints may not be stable either (e.g. if some type of load balancing is happening). The problem with measuring one way latency is that the receiver has to be synchronized with the sender. The receiver also has to either perform the collection of the data or return at minimum the received timestamp. This is all a little more complicated than a simple 'echo' or 'ack' service used for RTT measurements. John
On Wed, 14 May 2003 Michael.Dillon@radianz.com wrote:
Has it become common usage to define latency in an IP network as the round trip time in that network?
I've always considered latency to be a one-way measure of delay and RTT to be the sum of the latencies in both directions. When I tried to find something to back up this view, I discovered that a number of companies define latency as equivalent to RTT in their SLAs.
Assuming that one has measuring devices in every PoP, do you think it is harder to measure a full matrix of one way latency compared to measuring a full matrix of RTT?
The problem is buying and installing the equipment, even if you buy an off the shelf product like RIPE NCC's TTM :-). Once installed, these products will just provide you with the numbers.
Does it even make sense to measure a full matrix of RTT when the measurement of A to B to A should be equivalent to the measurement of B to A to B?
If you are sure that the path taken for A-B-A is equal to B-A-B, then no, measuring only A-B-A is sufficient. Henk ------------------------------------------------------------------------------ Henk Uijterwaal Email: henk.uijterwaal@ripe.net RIPE Network Coordination Centre WWW: http://www.ripe.net/home/henk P.O.Box 10096 Singel 258 Phone: +31.20.5354414 1001 EB Amsterdam 1016 AB Amsterdam Fax: +31.20.5354445 The Netherlands The Netherlands Mobile: +31.6.55861746 ------------------------------------------------------------------------------ That problem that we weren't having yesterday, is it better? (Big ISP NOC)
Perhaps folks would be interested in the products of the IPPM WG of the IETF. Although there's a lot of theory to wade through, there are also open source tools referenced in the email archive. No need to re-invent. -- Barney Wolff http://www.databus.com/bwresume.pdf I'm available by contract or FT, in the NYC metro area or via the 'Net.
participants (6)
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Andrew Bangs -
Barney Wolff -
Henk Uijterwaal (RIPE-NCC) -
John Kristoff -
Michael.Dillon＠radianz.com -
Stephen J. Wilcox