LTM :: Zero Window Server Side :: TCP Profiles
We have a virtual server setup for our receiving mail system, and it has been configured as-is for quite some time (measured in years). Never before has an issue arisen, but recently a particular client has been having problems sending attachments to us (and as far as we are aware, ONLY that client). What they claim to see is that the connection is terminated. Normal email works fine. Small file attachments work fine. However when they send us attachments that are Mb in size, the connection will not be successful. On our side, we see the window size slowly creep down until it hits zero. The BIG-IP probes the mail system, the mail system acks the probe, but keeps the window size at zero. It does this until the zero window timeout is reached on the BIG-IP and the connection is terminated by the BIG-IP (TCP RST). This is what the window decrease looks like on the client side (tcp.stream eq 3 and ip.src eq [the mail system]): This is what the window decrease looks like on the server side (tcp.stream eq 2 and ip.src eq [the VIP]): Client side end of the connection: Server side end of the connection: My impression initially was that this is not a BIG-IP problem... but when we remove the BIG-IP from the path, the connection works fine regardless of attachment size. Again, works fine for everyone else as far as we know regardless of if the BIG-IP is in the path... which is perplexing. Things I've tried: * Switching-out TCP profiles (lan optimized, wan optimized, client and server matching and different in combinations of the above). Now on mptcp-mobile-optimized with defaults. * Moving TLS off of the F5 * Resetting TLS profile to defaults * Different mail systems (of same type/configuration) Current configuration: * VIP on port 25 * TCP profile with mptcp-mobile-optimized w/defaults * SSL Profile (defaults w/cert, optional SSL, allowed cipher suites) * SMTPS Profile (allows TLS) * Pool w/single mail system * iRule w/VIP bounceback * Source IP Persistence VIP bounceback iRule: when LB_SELECTED { if {[IP::addr "[IP::client_addr]/24" equals "[LB::server addr]/24"]} { snat automap } else { snat none } } Any ideas/thoughts/suggestions all welcome. Thanks for taking the time.
