Why does the Microsoft VPN client/server system (in Windows XP and
probably 2000) use GRE packets?  This makes it difficult for
masquerading firewalls to forward packets correctly since they must
first decode the GRE packet.  Even out of the box the Windows XP
Professional systems cannot route multiple VPN connections that are
behind it's own Internet Connection Sharing service (at least I
haven't seen it able to).  Almost all home routers now based on Linux
(Linksys and others) have the GRE/PPTP patches compiled in
specifically because of this software.

I have a rough understanding of how it works; the question is what
could the motivation be?  It'd be easier just to use a standard TCP
socket connection, wouldn't it?