MCI has a tool to track spoofed packets, which is the orgin of
any DoS today.
You can get it from ftp.mci.net:/pub/security/dostrack742812.tar
That will trace it to the edge of your network, then you need to
work with the other providers to track it past that point.
If it's within your network, you should filter it either by
using ip verify unicast reverse-path on the serial interfaces or
ip verify unicast reverse-path is a feature in the fib images
by cisco. Most large nsp's run these images, and they're now mainline,
so fire up your cco accounts and grab them.
It drops packets that don't have the same return path, which works
for most customers that are single homed.