Re: [aqm] [Bloat] ping loss "considered harmful"

"Fred Baker (fred)" <fred@cisco.com> Tue, 03 March 2015 18:00 UTC

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From: "Fred Baker (fred)" <fred@cisco.com>
To: Wesley Eddy <wes@mti-systems.com>
Thread-Topic: [aqm] [Bloat] ping loss "considered harmful"
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Date: Tue, 3 Mar 2015 18:00:14 +0000
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Cc: bloat <bloat@lists.bufferbloat.net>, Dave Taht <dave.taht@gmail.com>, "cerowrt-devel@lists.bufferbloat.net" <cerowrt-devel@lists.bufferbloat.net>, "aqm@ietf.org" <aqm@ietf.org>
Subject: Re: [aqm] [Bloat] ping loss "considered harmful"
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> On Mar 3, 2015, at 9:29 AM, Wesley Eddy <wes@mti-systems.com> wrote:
> 
> On 3/3/2015 12:20 PM, Fred Baker (fred) wrote:
>> 
>>> On Mar 1, 2015, at 7:57 PM, Dave Taht <dave.taht@gmail.com
>>> <mailto:dave.taht@gmail.com>> wrote:
>>> 
>>> How can we fix this user perception, short of re-prioritizing ping in
>>> sqm-scripts?
>> 
>> IMHO, ping should go at the same priority as general traffic - the
>> default class, DSCP=0. When I send one, I am asking whether a random
>> packet can get to a given address and get a response back. I can imagine
>> having a command-line parameter to set the DSCP to another value of my
>> choosing.
> 
> I generally agree, however ...
> 
> The DSCP of the response isn't controllable though, and likely the DSCP
> that is ultimately received will not be the one that was sent, so it
> can't be as simple as echoing back the same one.  Ping doesn't tell you
> latency components in the forward or return path (some other protocols
> can do this though).
> 
> So, setting the DSCP on the outgoing request may not be all that useful,
> depending on what the measurement is really for.

Note that I didn’t say “I demand”… :-)

I share the perception that ping is useful when it’s useful, and that it is at best an approximation. If I can get a packet to the destination and a response back, and I know the time I sent it and the time I received the response, I know exactly that - messages went out and back and took some amount of total time. I don’t know anything about the specifics of the path, of buffers en route, or delay time in the target. Traceroute tells me a little more, at the cost of a more intense process. In places I use ping, I tend to send a number of them over a period of time and observe on the statistics that result, not a single ping result.