Mailing List Archive: nsp: foundry

TCP retransmissions

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foundry-nsp at foureleven

Jul 16, 2012, 5:03 PM

Post #1 of 2 (346 views)
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TCP retransmissions

Greetings Foundry-NSP,

We have an internal ServerIron ADX 12.3 sandwiched between a couple
linux servers, one being the client of the other.

The client reuses TCP source-ports as expected. tcp_tw_re(use|cycle)
is disabled. I have observed however, that when my linux client
reuses the source-port of a recently closed (8-12 seconds old) TCP
connection, the ServerIron does not acknowledge the SYN.

The linux client will retransmit using the same src-port after 3
seconds, and again after 6 seconds (9 seconds total) if necessary.

I believe the "3 second retransmission" and subsequent doubling is
defined in RFC1122, but the 8-12 second window is just an observation.

My question(s):

How do I configure the ServerIron to either accept/acknowledge this TCP flow?

Or, how do I configure the ServerIron to send a NACK or RST to the
client, so that we can abort the 3/6/12 second retransmission timers?

Am I naive to expect the ServerIron session entry (or whatever) to
expire faster than the 8-12 seconds (<< 2*MSL)?

Thank you,
lyle

Here's a sample flow where 10.0.0.3 is the source/client, and
10.0.210.92 is the virtual-server/ServerIron. The 3-second and
6-second retransmissions can be found at 17:29:43 and 17:29:49. The
ServerIron (10.0.210.92) ignores the first two SYNs, then finally ACKs
the third (the second retransmission):

17:29:36.738326 IP 10.0.210.92.80 > 10.0.0.3.44368: . ack 2076 win 9612
17:29:36.738330 IP 10.0.0.3.44368 > 10.0.210.92.80: R
3731308505:3731308505(0) win 0
17:29:40.831336 IP 10.0.0.3.44368 > 10.0.210.92.80: S
3736246299:3736246299(0) win 17920
17:29:43.830840 IP 10.0.0.3.44368 > 10.0.210.92.80: S
3736246299:3736246299(0) win 17920
17:29:49.830005 IP 10.0.0.3.44368 > 10.0.210.92.80: S
3736246299:3736246299(0) win 17920
17:29:49.830081 IP 10.0.210.92.80 > 10.0.0.3.44368: S
1833587:1833587(0) ack 3736246300 win 8000
17:29:49.830098 IP 10.0.0.3.44368 > 10.0.210.92.80: . ack 1 win 17920
17:29:49.830190 IP 10.0.0.3.44368 > 10.0.210.92.80: P 1:159(158) ack 1 win 17920
17:29:49.830680 IP 10.0.210.92.80 > 10.0.0.3.44368: . ack 159 win 6432
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fligor at illinois

Jul 17, 2012, 2:00 PM

Post #2 of 2 (320 views)
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Re: TCP retransmissions [In reply to]

Hi Lyle,

On Jul 16, 2012, at 19:03, Lyle Tagawa wrote:

> Greetings Foundry-NSP,
>
> We have an internal ServerIron ADX 12.3 sandwiched between a couple
> linux servers, one being the client of the other.
>
> The client reuses TCP source-ports as expected. tcp_tw_re(use|cycle)
> is disabled. I have observed however, that when my linux client
> reuses the source-port of a recently closed (8-12 seconds old) TCP
> connection, the ServerIron does not acknowledge the SYN.
>
> The linux client will retransmit using the same src-port after 3
> seconds, and again after 6 seconds (9 seconds total) if necessary.
>
> I believe the "3 second retransmission" and subsequent doubling is
> defined in RFC1122, but the 8-12 second window is just an observation.
>
>
> My question(s):
>
> How do I configure the ServerIron to either accept/acknowledge this TCP flow?
>
> Or, how do I configure the ServerIron to send a NACK or RST to the
> client, so that we can abort the 3/6/12 second retransmission timers?

I don't know how to do this, but the ADXes hold on to a session in the table for the amount of time specified by the "server msl" setting. The default is 8 seconds. If you want sessions to leave faster, you can change that setting to something smaller (although not smaller than 1 second). That might help with your port reuse issue here. We had a meeting last week with an ADX specialist and he mentioned we might want to turn this down from the default.

You can find details on page 122 of the ADX Server Load Balancing Guide for 12.3.01

--
-debbie
Debbie Fligor, n9dn Lead Network Engineer for CITES @ Univ. of Il
email: fligor [at] illinois <http://www.uiuc.edu/ph/www/fligor>
"Every keystroke can be monitored. And the computers never forget."

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