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

John,

> So going from .0175 to .03 seconds between packets in a TCP conversation is 
> enough to cause the latency we are seeing?

According to 
<http://rtstats.unidata.ucar.edu/cgi-bin/rtstats/rtstats_summary_volume?uni14.unidata.ucar.edu>,
 about 6e9 bytes arrive every hour on the NIMAGE feed. From that, the rate of 
packet arrival can be approximated:
    (3600 seconds)/(6e9 bytes)(3000 bytes/packet) = 0.0018 seconds/packet

If the time *between* packets is 0.03 seconds, then the network is too slow to 
keep up.

> If you think that packet to packet (time between packets in a 388 
> conversation) time has nothing to do with the latency we are seeing (and 
> wouldn't necessarily be seen while we are seeing the LDM latency), please 
> explain the physics to me because I'm seriously lost.  I cannot for the life 
> of me understand how this latency could be occurring without me seeing the 
> packet to packet times increase dramatically.

> I'm aware that networks can drop packets in a bad connection (where I'll see 
> Retransmits and Duplicates), block packets altogether (where there would be 
> RST ACKs most likely), or sequester/buffer/queue packets.   The third I have 
> not seen in a packet capture and could only guess what it looks like (i.e. 
> many Retransmits with eventual new SYN requests as seen in a web server's 
> denial of service instances).

If a packet-shaping system were discarding packets, then you wouldn't see any 
retransmissions because the TCP layers constantly recompute the 
retransmission-timeout (RTO) parameter based on the round-trip time (RTT). They 
do this to avoid retransmissions. A packet-shaper would, effectively, increase 
RTT and, hence, the RTO used by the TCP layer.

> The packet capture you sent me had no traffic originating from mammatus.  Did 
> you filter that out or was that a delay of 3 seconds?  It looks filtered out.

It was filtered out. What I sent only contained packets from us to you for the 
NIMAGE feed.

Regards,
Steve Emmerson

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