Re: Sample's sequence number

From: Bao <>
Date: 03/15/05
Message-ID: <>

Neil McKee wrote:
> Hello Bao,
> On Mar 15, 2005, at 4:43 AM, Bao wrote:
>> Hi everyone,
>> For example, we have one of our border routers connected to a
>> peering point at London (Linx) via our Gigabit port 2/3 (IfIndex : 67).
>> I would like to analyze the in/out traffic of this port via sFlow.
>> As Neil McKee suggested, the right algorithm for estimating the frames
>> from a source X is :
>> effective_sampling_rate = delta(sample_pool) / samples_received
>> frames(X) = samples_received(X) * effective_sampling_rate
>> Now, in order to get the samples_received, I try to use the sample's
>> sequence number, making the difference between the current sequence
>> number and the last value of it.
> Actually the samples_received value should be the number of samples that
> you really did receive. In other words, just count them one at a time
> as they come in.
>> We know that the sample sequence number is a 32bits counter which is
>> incremented with each flow sample generated by a designated source_id.
>> But the problem is, when I look at the lower 3 bits of the source_id
>> (unsigned int), I see that it always gets the same value as the "input
>> port" field so, for example, I can determine how much traffic is
>> coming in through this port 67 (IfIndex). How can I do to determine
>> how much traffic is going out through this same port 67 ? Do I have to
>> calculate it by using the "sample rate" field ?
> It sounds like your router is only sampling inbound packets (that's
> quite common). To estimate outbound traffic on interface 67 you need
> to enable sFlow on all interfaces, and then look at all the samples
> whose output_port was ifIndex 67. Of course the other source_ids will
> have different values for sample_pool and samples_received (and may even
> have different sampling rates) so you need to scale those estimates
> separately before you combine them.

        Actually I would like to be absolutly clear on two things :

                - There is one 32bits counter corresponding to the sample sequence
number for each source_id, right ?

                - The 3 lower bytes of the source_id field in the struct flow_sample
represents the IfIndex of the Input port, right ?

        Kind regards

Received on Tue Mar 15 09:30:15 2005

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