Subject: [OPSAWG] Discussion about draft-tempia-opsawg-p3m-00

From: Tempia Bonda Alberto <[email-masked]>
Date: Tue, 29 Mar 2011 17:27:50 +0200

Hi All,

after the presentation of draft-tempia-opsawg-p3m-00 in the OPSAWG session on Monday, there were several comments/questions but not time to address them. I'll try to recap and comment those of them that I can remember. If there is something I've missed, please let me know.

- What happens in case of load balancing among several parallel paths?
ATB> As described in the draft, the method can perform flow-based or link-based measurement. Load balancing affects only the former case because the latter refers to a measurement on a single link. In case the flows we are monitoring follow different paths, but share the same ingress and egress routers, we can perform E2E monitoring on the aggregated traffic, while on each intermediate router, where only a subset of the flows may be present (because of load balancing) we can only perform a link-by-link measurement (but we cannot compare intermediate counters with ingress and egress counters).

- How to monitor several flows that share the same ingress point but have different egress routers?
ATB> In this case we cannot have a single E2E measurement for the set of flows aggregated together, we can perform only link-by-link measurement or we have to perform E2E measurement on each flow separately.

- What happens in case of link failure and re-routing?
ATB> When a link fails and traffic is rerouted, synchronization is lost on the failed link and there is a transitory during which correlations between counters cannot be made on that link. However, measurements will be valid again as soon as the network converges. In addition, E2E measurement is not affected by failures on intermediate links/nodes and is indeed able to measure how many packets were lost during the failure.

- What happens in case of out-of-sequence packets arriving in the wrong block?
ATB> Out-of-sequence packets are not a problem because we don't need to compare counters of a block as soon as the block ends, but we can read them after a while. For example let's imagine each block lasts 3 minutes: Block1 starts at time 0:00 and ends at time 3:00, Block2 ends at time 6:00, Block3 ends at time 9:00 and so on. We don't need to count the number of packets of Block1 exactly at time 3:00, we can do it at time 3:15, so that we are sure to include in the counter also delayed packets. It doesn't matter if some packets of Block1 arrive in Block2 because they have a different color and cannot be confused. It would be a problem if packets of Block1 arrived in Block3 (since we use only 2 colors, Block1 and Block3 have the same color), but this can be avoided by choosing block duration long enough.

- The amount of data routers need to process can be very high.
ATB> The information a router needs to collect is just a counter (actually 2 counters, one for each color) for each interface where the monitoring is activated and for each flow (or set of flows) we want to monitor. Its number depends therefore on the number of flows we are monitoring on that node, however the router is not required to make all the correlations, it can just send the values to an external system which collects data, makes correlation and visualizes results. With a similar approach we have tested the monitoring with 300 counters involved in a single router and we haven't seen any impact on the router's CPU and memory.

Hope to have answered to your points.

Regards,
Alberto

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