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.

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 Intel and Broadcom 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.