Paul,

...

I agree that ever having a source or destination IP that's RFC1918 outside the domain is a very bad thing.

I don't see anyone here disagreeing with that, but apparently a number of ISP's did not consider the ICMP case when they gave numbers to their T1's, and so it's a question of definition rather than of intent. Transit nets are public, not private, and so they have to have public, not private, addresses.

I want to respectfully disagree. I do run internal routing protocols that can't handle VLSM or CIDRization permitting cutting up a class C into 64 disconnected pieces. , igrp in particular. Because of this I would burn too many network numbers by having to use public network numbers for all my T1's. I never permit a case where both sides of a router have RFC1918 address space so there is no confusion in a traceroute at to where to address questions about routing issues. Purity of addresses is valuable but I am willing to compromise on this in this instance. Walt