Mike Yes, the idea for real cases would be employ the usual filtering algorithms for the end-point pair round-trip times, and to use these results in the optimization procedure.
You need to be able to specify that the path from A to B went through C and D (in that order) in order to be able to say that the time from A to B equals the time from A to C plus the time from C to D plus the time from D to B. This additivity is the major assumption. However, there is no assumption of symmetry and no need to use intermediate timestamps. Instead we find the asymmetric time delays, and then can directly compute the ToD corrections. Y(J)S From: [email protected] [mailto:[email protected]] Sent: Tuesday, January 12, 2010 13:20 To: Yaakov Stein; [email protected]; [email protected] Subject: RE: [TICTOC] interesting article on a global mechanism for one-way delay measurement What do we mean by optimize the timing paths? is this least hops, minimised jitter? When we say the paths are known and controlled i am assuming we need to know the underlying architecture in detail and minimise the hop / jitter etc. Regards, Mike ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Yaakov Stein Sent: 11 January 2010 07:49 To: Doug Arnold; [email protected] Subject: Re: [TICTOC] interesting article on a global mechanism for one-way delay measurement Doug Yes, the topology has be known, although it can change (reroute events) as long as we are informed of this. For ToD distribution it is a bit less elegant than running full CTP, but it gives you a better feeling for what is happening. I think it could go well with MPLS-TP, or even better with MPLS and a PCE box (which not only knows the topology, but could optimize the timing paths). Y(J)S From: Doug Arnold [mailto:[email protected]] Sent: Wednesday, January 06, 2010 20:22 To: Yaakov Stein; [email protected] Subject: RE: [TICTOC] interesting article on a global mechanism for one-way delay measurement Thanks Yaakov, This is an interesting idea. It does require the that complete paths be known and controlled. Perhaps it could be used in conjunction with MPLS. //Doug From: [email protected] [mailto:[email protected]] On Behalf Of Yaakov Stein Sent: Wednesday, January 06, 2010 9:16 AM To: [email protected] Subject: [TICTOC] interesting article on a global mechanism for one-way delay measurement Hi all, I have recently been working time distribution in the presence of strong asymmetry, and have come across a method that helps in certain cases. I am sure that you all remember the CTP algorithm that I have brought up before (and presented at IETF-74). The same academic group has an earlier article that I had previously overlooked. I am talking about : Gurewitz O, Sidi M. Estimating One-Way Delays from Cyclic Path Delay Measurements. 16th IEEE INFOCOM 2001, Anchorage, Alaska. [PDF<http://www.owlnet.rice.edu/~gurewitz/4.pdf>] This article gives a procedure for determining one-way delays based purely on round-trip delay measurements (i.e., what we would call T4-T1), knowledge of topology, and the assumption of additivity of propagation delays. The idea is that nodes measure round-trip times to various other nodes, knowing which nodes are traversed. For example, assume three nodes connected in a triangle 1 / \ / \ 2 -------- 3 and we measure the times for the following paths 1 2 3 2 3 2 3 1 3 1 2 3 1 We thus have 4 equations for 6 variables (since the links are not assumed symmetric, the variables are D1-2, D2-1, D2-3, D3-2, D1-3, D3-1 ). Using additivity and non-negativity it turns out that one can solve an optimization problem which minimizes the error of these equations. The solution requires a centralized server to do the math PCE-style, but solves a problem that I don't know any other way to solve. Comments ? Y(J)S
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