[Sis-SCPS-INTEREST] SCPS with jitter

[Feighery, Patrick D.]

I have worked with NistNet on other projects,  My understands is as
Keith mentioned, when you add jitter with NistNet, or Dummynet for that
matter, it does not keep packet sequence.  Therefore when packets
arrive out of order SCPS or most reliable transport protocols for that
matter with react adversely.  If you look at the packet traces with
either ethereal or tcpdump/tcptrace/xplot, you can easily verify this.

On my previous project, I had to modify a home grown satellite emulator
called spanner to keep packet order.

SCPS does provide an option that might mitigate or hopefully solve this
problem for you.  This option was identified and added to the SCPS RI
when were testing striping data over multiple interfaces.  SCPS have
the option to add a delay from when hole is packet sequence has been
identified (either due to actually packet or out of sequence issue) and
when it is acted upon.

If we assume the B Interface is the RF side,  then you can set the
directive BIF_SNACK_DELAY (units are in microseconds not milliseconds)
on both PEPs how long SCPS would wait before acting on indication of
packet loss.

For example if you set the BIF_SNACK_DELAY to 100000 or 150000, you
should be in better shape.

Best Regards

	Pat




>>-----Original Message-----
>>From: Scott, Keith L. 
>>Sent: Monday, January 23, 2006 9:01 AM
>>To: 'frankm at klasonline.com'; 'sis-scps-interest at mailman.ccsds.org'
>>Cc: Feighery, Patrick D.
>>Subject: Re: [Sis-SCPS-INTEREST] SCPS with jitter
>>
>>I wonder if NISTNET delivers packets out of order when the 
>>jitter is high enough?  That could be bad, but shouldn't be 
>>too bad with pure rate control.
>>
>>With pure rate control (no congestion control) there's no way 
>>to get stuck in slow start.
>>
>>Can you tcpdump both ends of the inter-gateway interface?
>>
>>  --keith
>>
>>
>>-----Original Message-----
>>From: sis-scps-interest-bounces at mailman.ccsds.org 
>><sis-scps-interest-bounces at mailman.ccsds.org>
>>To: sis-scps-interest at mailman.ccsds.org 
>><sis-scps-interest at mailman.ccsds.org>
>>Sent: Mon Jan 23 05:15:45 2006
>>Subject: [Sis-SCPS-INTEREST] SCPS with jitter
>>
>>All,
>>I am currently testing 2 SCPS gateways in the configuration below:
>>
>>PC1->SCPS GW1---->Satellite simulator--->SCPS Gw2----->PC2
>>
>>I use nistnet as a sat sim with a round-trip time of 1s and a 
>>symmetric 
>>bandwidth of 512K. I'm using FTP to test performance and as it is a 
>>dedicated link I'm using pure rate control for BIF_CC. The 
>>results have 
>>been good and I get to wire-speed very quickly with this setup. I've 
>>injected errors and the performance is still very good.
>>
>>Problems arise however when I simulate jitter on the sat sim. 
>>I've set 
>>the jitter to have a standard deviation of 100ms and on 2 in 3 ftp 
>>uploads I get wire speed as before but on the other I get average 
>>bandwidths of about 30 kbits/s. The speed never builds up so I'm 
>>guessing that we get stuck in slow start because an ack gets 
>>delayed at 
>>startup. Is this behaviour to be expected with pure rate control?
>>
>>I switched to Vegas but got similar results. I read in one of 
>>the FAQs 
>>that jitter *can* be interpreted as congestion so played 
>>around with the 
>>alpha/betta/gamma parameters without things improving but 
>>there was an 
>>amount of trial and error about this. Any advice on how to tune these

>>parameters would be great.
>>
>>Regards,
>>
>>-- 
>>Frank Murray
>>KlasRouter Project Manager
>>Klas Ltd
>>
>>-------------------------------------------
>>mail to : frankm at klasonline.com
>>
>>Tel: International +353-1-662-4270
>>Tel: Direct line +353-1-4361768
>>Tel: VoIP 3123 at blueface.ie
>>
>>
>>
>>www.klasonline.com - 'making PC encryption easier'
>>
>>
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