[LARTC] formula for minimum latency of connections on a loaded link

[LARTC] formula for minimum latency of connections on a loaded link

[bert hubert]

I saw some discussions regarding QoS settings not influencing latency on a
loaded link, or not influencing it enough.

The following formula may help clarify things:

                     avgpacketbytes*5000
msavglatencyadded=   ------------------- 
                           bpsoflink


So for a 56k6 modem, this boils down to 132ms on average *added* to the
existing latency. Why is this so? There is a baseline latency, which is a
given. Then there is a queue of packets that are destined to go out before
yours.

In the absence of QoS control, that queue may be huge. Perfect QoS would
lead to a zero sized queue ahead of your packet. However, as packets can't
be subdivided or interrupted (generally), your modem will already be busy
sending one packet when yours gets queued for transmission.

On average, this packet will be halfway when your packet arrives. So, the
additional latency in msec is the amount of time it takes your link to
transmit half a typical packet.

The peak additional latency is twice that calculated above, so you could
also see 264ms additional delay.

This is an artifact of having packets which take a long time to transmit.
You can ameliorate the problem by setting a lower MTU to make sure your
packets go out quickly.

Regards.

bert

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Re: [LARTC] formula for minimum latency of connections on a loaded

[Martin Devera]

>                      avgpacketbytes*5000
> msavglatencyadded=   ------------------- 
>                            bpsoflink

Hi Bert,

where came the 5000 from ? I'd expect 1000 here ... :-o

devik
 

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Re: [LARTC] formula for minimum latency of connections on a loaded link

[bert hubert]

On Sat, Feb 16, 2002 at 08:33:54PM +0100, Martin Devera wrote:
> >                      avgpacketbytes*5000
> > msavglatencyadded=   ------------------- 
> >                            bpsoflink
> 
> Hi Bert,
> 
> where came the 5000 from ? I'd expect 1000 here ... :-o

'bps' should be read as 'bits per second'. So:

                      avgpacketbytes*10*1000
msavglatencyadded =   ---------------------- * 0.5
                        bitspersecondoflink

And this becomes '*5000'.

Regards,

bert

-- 
http://www.PowerDNS.com          Versatile DNS Software & Services
http://www.tk                              the dot in .tk
Netherlabs BV / Rent-a-Nerd.nl           - Nerd Available -
Linux Advanced Routing & Traffic Control: http://ds9a.nl/lartc
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