Subject: Re: [ippm] Composition of Metrics

From: Maurizio Molina <[email-masked]>
Date: Tue, 02 Aug 2005 14:20:01 +0100

Lei Liang wrote:

>Hi, Al,
>  I just got time to go through the composition metrics draft. Got some
>comments here. 
>  The inaccuracy might be also caused by interdomain routing choices
>additional to the intro-domain ones. Doing the sub-path measurement for
>each domain might conceal the complete path when there are multipath to
>choose from the source to the final destination via several domains. for
>example, the packet can go to the final destination via domain 1, 2 to
>3. it may also travel through domain 1, 4 to 3. Therefore, the
>sub-measurement carried out at the same time interval does not solve the
>path problem. 
>Also, the gateway functions between domains may also address inaccuracy.
>E.g., multiplexing, translation, deconding and encoding will all put
>costs to the complete measurement result.
>  I also got a question here that you might help. Why don't just send
>the packets from the source to the final destination and collect the
>sub-path results on the way?
>  
>
To my knowledge, there isn't any widely implemented protocol that would
allow you to do so, still ensuring that the performance of the test
packet that collects the sub-path results on the way has similar
performances to a "true" source-destination packet.
Cheers,
Maurizio

>Cheers,
>Lei

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---

From: "L.Liang" <[email-masked]>
Date: Thu, 4 Aug 2005 23:05:21 +0100

________________________________

From: Liang L Mr (PG/R - CCSR)
Sent: Thu 04/08/2005 9:57
To: Maurizio Molina
Subject: RE: [ippm] Composition of Metrics

Ok,Maurizio, please see inline comments. 

________________________________

From: [email-masked] on behalf of Maurizio Molina
Sent: Tue 02/08/2005 14:20
To: Lei Liang
Cc: Al Morton; [email-masked]
Subject: Re: [ippm] Composition of Metrics

Lei Liang wrote:

>>allow you to do so, 

No, there is no protocol defined to do so. That's why we can do it here. 

>>still ensuring that the performance of the test

This is absolutely right. The point is it will not be composition but decomposition. As I said in my previous comments, the method described in the I-D will have problem of lacking knowledge of the complete path on which the test packet may go through. Therefore, the information provided by each domain of ISP can only be reference for inter-domain path computation. It will not be sufficient to tell performance from source A to destination B. The inacurracy will not be predictable by using the composition performance because we don't know if the complete path will be the same as the sum of sub-path or how similar they will be. Therefore, I suggest either now explicitly define the measurement methods or we make a composition and decomposition I-D to address the methods for different puppose. 

Cheers,

Lei