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I have a very odd issue with our cisco firewall,

If on our osx machines we run the following to disable window scaling support:

sudo sysctl -w net.inet.tcp.rfc1323=0

We see near 40% improvement in download speeds (tested using varying linux iso image mirrors).

If I sit my machine directly onto the outside modem (Manually setting up an external ip adress within the range etc), the bandwidth remains constant with rfc1323 enabled and disabled, so it has to be the cisco.

Something else to note:

  • None of our linux devices suffer this problem

Has anyone come across this issue before? I seem to find some vague references in old mailing lists detailing PIX workarounds which are no longer applicable to cisco ios.

Thanks in advance.

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  • What version FOS are you running?
    – Paul Ackerman
    Dec 2, 2011 at 15:10
  • Are you blocking inbound ICMP in the firewall? Can you do a tcpdump to figure out what's different between the two connections?
    – Shane Madden
    Dec 2, 2011 at 18:52
  • @ShaneMadden I am blocking ICMP
    – Oneiroi
    Dec 5, 2011 at 11:15
  • @PaulAckerman ASA Version 8.2(1)
    – Oneiroi
    Dec 5, 2011 at 11:16
  • I am running 8.3 and cannot replicate this problem. although I did find this in the release notes for 8.3: CSCsz11180 TCP Proxy mis-calculates TCP window causing connectivity problems. Perhaps you could upgrade and see if the problem remains?
    – Paul Ackerman
    Dec 5, 2011 at 20:27

1 Answer 1

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I am running 8.3 and cannot replicate this problem. although I did find this in the release notes for 8.3: "CSCsz11180 TCP Proxy mis-calculates TCP window causing connectivity problems." Perhaps you could upgrade and see if the problem remains?

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