Duplicate ACKs are fairly common, so they don't always indicate a problem. During normal congestion you will receive duplicate ACKs if the far end has not received a TCP segment it believes it should have. It also can be used to keep alive a connection. However if you get dup ACKs consistently at the start of a connection it might mean that there is some sort of firewall ACL blocking traffic. As Stephen said, it is important to know where this occurs during the 3 way handshake at the beginning of a connection or later on.
Martin Visser Technology Consultant Consulting & Integration Technology Solutions Group - HP Services 410 Concord Road Rhodes NSW 2138 Australia Mobile: +61-411-254-513 Fax: +61-2-9022-1800 E-mail: martin.visserAThp.com This email (including any attachments) is intended only for the use of the individual or entity named above and may contain information that is confidential, proprietary or privileged. If you are not the intended recipient, please notify HP immediately by return email and then delete the email, destroy any printed copy and do not disclose or use the information in it. ________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Ciccone Sent: Saturday, 19 May 2007 7:57 AM To: wireshark-users@wireshark.org Subject: [Wireshark-users] Help with Output "TCP Dup ACK3#2 1320 > 22 ACK Hello, I am having a problem with SSH. I can ssh from some server but not others. I verified that there are no access-lists blocking from doing this. When I ran Wireshark on my pc and tried to ssh to the server I get the following line that could be telling me what the problem is. However, I don't understand it and was hoping some out there could explain it to me. Here is the line: [TCP Dup ACK 3#2] 1320 > 22 [ACK] Seq=1 Ack=1 win=65535 Len=0 Does this mean anything to anyone? I'm guessing my problem lies here. If I ssh to a server that works... I don't see this line Thanks Mike C.
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