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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