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[LDM #GUQ-669331]: Both Radar and Satellite feeds stopped downloading abruptly

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  • Subject: [LDM #GUQ-669331]: Both Radar and Satellite feeds stopped downloading abruptly
  • Date: Thu, 29 Aug 2019 14:50:24 -0600
John,

> I see, thanks.  The slowdown today occurred around 1400Z.

Yes. NEXRAD3 latency went through the roof while IDS|DDPLUS latency was near 
zero.

This is the signature of throttling.

> If that is the case, I should see a ton of TCP Retransmissions and eventual 
> failures in Wireshark during that time.

You wouldn't see that if the throttling is implemented by discarding TCP 
packets. You would simply see regular TCP packets arrive relatively slowly.

> I do see a few "TCP Previous segment not captured" and "TCP Fast 
> Retransmissions" and a ton of "Duplicate ACK"

> Also, ldmping to your server was never interrupted nor slowed in response 
> time.

An ldmping(1) wouldn't be affected because its TCP connection is temporary and 
very low volume.

> Could y'all do a 30 second capture from your similar to this and send it to 
> me to help prove the point to our network folks?

We could do that, but it wouldn't show anything conclusive. From the 
perspective of each endpoint, throttling just looks like a network with lower 
than advertised bandwidth and the TCP layers adjust accordingly.

The best evidence for throttling is the difference between time-series plots of 
latency for of a high-vplume feed (NEXRAD3) and a low-volume feed (IDS|DDPLUS).

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: GUQ-669331
Department: Support LDM
Priority: Normal
Status: Closed
===================
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