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I am going bonkers trying to explain some TCP connection issues I am seeing. Can you pls help?

Consider this scenario:

Server A has IP:

Client B has IP: 50:60:70:80

A is hidden behind an F5 Virtual IP: 

Client B uses the Virtual IP when it wants to connect to Server A. F5 does not translate client B's IP, so it is visible as is to Server A.

When a connection is established, here is what netstat shows:



But this connection fails often and I am trying to determine why.

Here is my theory:

Right now A's keep_alive_interval < F5's keep_alive_interval.

So, as soon as A's interval expires, A sends a keep alive probe to B directly at (because that it what it sees) totally bypassing F5. B ignores this probe because as far as it is concerned it has no connection with A at So Server A drops the connection, client B does not know about it until later when all hell breaks loose.

Solution: Making F5's keep alive interval the lowest among the three, will make sure that it mediates the connection between A and B and keep them content.

Does the theory make sense at all?

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migrated from stackoverflow.com Oct 6 '11 at 8:35

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Nope, that makes no sense. If the keep alive probe can't get through, neither could data. The keep alive probe is basically just the same packet that would carry data, just with no data. –  David Schwartz Oct 6 '11 at 10:15
Yup, you are right. Back to the drawing board for me. Unless: I am wondering if IP packets have a source as well as a reply-to address fields. –  dvlpr Oct 6 '11 at 15:55
TCP packets have both source and destination IP addresses and ports. –  David Schwartz Oct 6 '11 at 15:59
@David 'If the keep alive probe can't get through, neither could data'. Does this mean that only requests to server go through F5/Big IP and responses from server always go to client directly? –  dvlpr Oct 6 '11 at 17:28
Answer: Page 3 - Basic Load Balancing in this doc: link @David: Thanks for all your responses. I will now pursue other options. –  dvlpr Oct 6 '11 at 18:36

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