A certain number of TCP Dup Acks are common, usually no more than one or three per packet in any particular session. In our case we were seeing sixty or over a hundred for individual packets – this is enough to cause significant errors which brought down large-file-size FTP sessions for our customer. We replaced the XENPAK optics on the 10 Gig links in some of our switches, and that seems to have alleviated the issue. I’ve seen bad NIC cards cause this issue, but this is the first time I saw the optics on a fiber link cause this.
For community reference, the optics in question were manufactured by Intel. We got new ones manufactured by Opnext, and they seem to be functioning much better. The Intel ones can be distinguished by their significant weight difference (Intel’s are heavier); I’m not certain, but I believe the Intel XENPAKs were somewhat older models.
Data Access/Datapatch, Inc.
(201) 843-5468 x7032