Hello all, my first post so forgive me if I omit any info.
I am observing a FTP timeout on our network that I am hoping to pin down using
Wireshark.
The network is an 18 node Frame Relay WAN. Nodes are connected via
point-to-point T1 using Cisco 2600s to a central hub (Site A) which provides
our connection out to the Internet. Our IT Department is located in two sites
(Site B) and (Site C).
I first came across this issue when our payroll dept. complained that they
could not upload a file to the payroll company that cuts our checks (oh no not
payroll!). I found out that the problem has been occurring over the course of a
couple weeks. There is no way for me to tell what has changed over the last
couple weeks!
Using Filezilla in passive mode on Ubuntu Gutsy I am trying to upload a 100MB
file to a private FTP server on the Internet. I am able to recreate the timeout
at Site B and Site C. The System Administrator at Site A is not experiencing
the upload timeout.
There is an ISA proxy server that sits between Site B & C and has been
configured to allow FTP traffic. To be on the safe side I am bypassing proxy
altogether. The problem persists when bypassing proxy as well. Windows
firewalls are disabled via Group Policy. Hmmmm...
I fired up Wireshark and filtered out the following: FTP & FTP-DATA. The
FTP-DATA packets are show a lot (about 50%) of retransmission packets. FTP
shows a <retransmission request> packet; the TCP checksum field states that
the problem may be a TCP checksum offload. If I may assume that the problem is
at Layer 4 and that there is a TCP segment sequencing error originating on our
network. What steps have I missed and where so I look next in the
troubleshooting process?
Thank you in advance.
Luis Pena
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