Tips from the Windows Pros: Pinging with Larger Packets

I have a DSL connection in my office, and one night it appeared that my Internet connection had stopped working. After a closer look, I saw that only downstream communication was affected, meaning my browser could contact Web sites, but information from the Web wasn't reaching my computer.

I first tried pinging my ISP at the gateway address of my DSL modem. It worked just fine. In fact, I could ping any site in the entire Internet but still could not view a single Web page. I called my Internet service provider and they found out that pings from their network into my LAN worked, too. The guy I spoke to suggested that I must have a software problem.

That didn't make sense to me, especially because everything was working fine just minutes before. Then I had a hunch. Ping, by default, sends very small packets: 32 bytes each, plus a few bytes of IP packet packaging. Requests for Web pages are very small, too (maybe 100 bytes). However, responses from Web servers are big and come in the largest packets possibleabout 1500 bytes each. This meant the problem might not be the direction the data was taking. Instead, it could be the size of the data that was causing the problem: If there was a lot of interference on my DSL line, it could be that small packets would likely make it through between bursts of electrical interference, but larger packets would be much more likely to be garbled.

I vaguely remembered that ping has a bunch of command-line options, so I looked up "ping" in Windows Help and saw that I could increase the size of its packets with the -l option. Typing

 ping -l 300 www.someplace.com 

tells ping to send 300-byte packets. Aha! I found that only about 50 percent of these packets made the roundtrip. When I sent 500-byte packets, the success rate dropped to 10 percent. When I called tech support with this news, the guy at my ISP tried the same test, and got the same result when he tried to ping my computer from his network. Now, we knew that there was a physical problem that the ISP was responsible for fixing.

That problem was eventually fixed, but still every month or so my Internet connection seems to bog down. Usually, this lasts only a minute or two due to random Internet happenings, but sometimes it slows down and stays slow. When this happens, I ping the gateway address to see what the round-trip time is. For my connection, this is normally about 20 msec. For reasons I still don't understand, occasionally this jumps up to over 1000 msec (over a second!) and stays there. I have to power-cycle my DSL modem to get back to normal response times.

The moral of this story is that it pays to be familiar with your friendly neighborhood command-line utilities.