[j-nsp] duplicate acks, EX3300 VC

[Chris Cappuccio]

those TCP statistics have nothing to do with traffic passing through the switch ports, that's for traffic to the control plane (FreeBSD)

Mike Williams [mike.williams at comodo.com] wrote:
> Hey all,
> 
> Before I punt this to JTAC, has anyone had any experience with 
> poor/highly-variable TCP throughput from a small stack of EX3300s?
> 
> We've got a stack of 3, one 48 port, and two 24 ports, and since they went in 
> we can't get reliable TCP transfers transatlantic.
> Linux-Linux can go really fast, but involve Windows and we get a pityful 
> ~100KBps, regardless of tuning done.
> Junos is 11.4R2.14.
> 
> It's taken us *forever* to hone in on the issue possible being the EXs, 
> because who'd have thought a switch couldn't handle packets at a few 10s of 
> megabytes per second (10-20k PPS x 3).
> 
> To cut a looooooooong story short;
> <internet><srx650><ex3300><linux firewall><same ex3300><server>
> Linux firewall sees the 2 initial TCP packets correctly, but the server 
> generally only gets the second one, or if it gets the first it's after the 
> second. Then we're into a bazillion duplicate acks, out-of-order packets, and 
> TCP retransmissions.
> 
> I found the 'show system statistics tcp' command a short while ago and it's, 
> well, "interesting".
> 
> 
> > show system statistics tcp
> fpc0:
> --------------------------------------------------------------------------
> Tcp:
>          84769061 packets sent
>                  16676437 data packets (2039615568 bytes)
>                  1416 data packets retransmitted (1526176 bytes)
>                  0 resends initiated by MTU discovery
>                  67141526 ack only packets (23539653 packets delayed)
>                  0 URG only packets
>                  0 window probe packets
>                  22 window update packets
>                  3468634 control packets
>          125994683 packets received
>                  15916504 acks(for 2039560634 bytes)
>                  82630576 duplicate acks
>                  0 acks for unsent data
>                  25574925  packets received in-sequence(3702132560 bytes)
>                  43149892 completely duplicate packets(5824 bytes)
>                  10 old duplicate packets
>                  5 packets with some duplicate data(2140 bytes duped)
>                  0 out-of-order packets(0 bytes)
>                  0 packets of data after window(0 bytes)
>                  0 window probes
>                  24585 window update packets
>                  23 packets received after close
>                  0 discarded for bad checksums
>                  0 discarded for bad header offset fields
>                  0 discarded because packet too short
> 
> 
> fpc1 and fpc2 have similar numbers, even though these packets have no need to 
> leave fpc0. There aren't even any active servers off fpc1/2 yet.
> fpc0 has been up 33 days, so has seen almost 30 duplicate acks per second 
> since it booted.
> 
> -- 
> Mike Williams
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