For those who don't know, I recently noticed that X-box Live causes messages about illegal ICMP broadcast responses to show up in my network logs. Not a big deal, and you can turn off the logging, but Google didn't have anything linking X-box Live titles to this error, so I figured I'd write it up so perhaps someone else gets a quick answer. Now return to our regularly scheduled programming.

It's possible they wrote their own custom protocol/ICMP variant.

WRT to the X-Box Live voice comments in your link: developers can choose how they implement voice for each title (within the limits of MS's TCR's).

When we first got voice working for an Xbox Live Launch title, everyone had a good laugh with the voice masking, but soon after most of the devs switched it off (or were pressured by other devs to switch it off).

Off-topic:
If you are into cars/racing, check out Forza Motorsport on the XBox (does indeed beat GT4 in handling and sound). For example, the Z06 Corvette in GT4 has a four cylinder engine sound, and in Forza has a decent repro of a stock 2002 Z06 (pretty cool). Handling in both games is fun (over-damped for playability), but doesn't match the real cars (again, comparing the real Z06 to the simulation versions). At the high end, the Audi R8 looks and sounds cool, though the sound and handling don't match the real car (compared to SpeedTV video). Forza also supports Xbox Live.

I've heard mention of funny packets generated by Xboxen before - at least one person I know has had to get a new router because it saw the 'corrupted' packets and just silently dropped them.

About the xbox voice chat, depending on the game it's different. For instance, in halo 2 you can only hear players close to you. Although, you will always be able to hear your teammates it will only come over the communicator if you hit the white button (or are close? i don't recal, i always hit white).

From memory, which is not perfect, it's part of the discovery and NAT defeating process used by XBox because it has its own internal encrypted address information contained in some packets.