ISA Server 2006 TCP Retransmits

Health Checks

I perform ISA Server Health Checks for Premier Support (via Premier Field Engineering) as part of my role.

I’ve seen something a few times recently that I thought it might be helpful to call out, while poking around in the Performance Monitor TCPv4 counter area.

The Problem

In short: Lots of TCP retransmissions per second.

Like, lots. More than 1% is annoying; any more than 5% and you pretty surely have a problem.

Recently, I’ve been seeing 20% .

That’s right, kids, according to Perfmon’s statistics, one in five TCP packets requires retransmission. But! That doesn't necessarily jive with what's seen on the wire, suggesting it might be an internal driver or hardware problem.

If your ISA Server seems like it might be a bit slow, and you haven’t looked yet, go look. I’ll wait. You’re interested in the TCPv4 object, specifically the Segments/sec and Segments Retransmitted/sec counters.

What I’ve seen looks like this:

The green area is TCPv4\Segments/sec. The red area is TCPv4\Segments Retransmitted/sec. They’re using the same scale.

Notice that the retransmission figures track with the overall volume.

This 20% figure has been seen across HP and Broadcom (and possibly Intel) server NICs, so I don’t think it’s specific to either vendor.

Fixing It

In at least one of the places I found this, a simple driver upgrade to the latest version available looked like it fixed the problem.

Otherwise, it could indicate a NIC issue, or a hardware issue with the switch.

If you find yourself in this situation, and do resolve it, please do post details in the comments section below.