DirecPC Protocols

Crist J. Clark crist.clark at attbi.com
Thu Nov 14 22:53:59 UTC 2002

I've been looking for some technical descriptions on how DirecPC works
from a TCP/IP point of view. Does anyone out there have some
references? I have not been able to find anything too detailed, and
from what I have been told, they are not too forthcoming when
contacted directly.

I know the rough outline. The customer sends out traffic over a normal
PPP link (since the customer has no uplink to the DirecPC satellite)
to a separate ISP. The traffic has a spoofed source address set to
some DirecPC server at their ground site(s). Thus, the third-party
target's response goes to DirecPC who send it over their satellite
link back to the customer using the wide satellite pipe instead of the
narrow PPP pipe.

But I'm curious about the details. First things first, anyone know how
the DirecPC link is established? That is, how the customer tells
DirecPC what his IP address is? I assume this must all happen over the
PPP-link, or at least the two-way PPP-link is used for bootstrapping.
Now once things are going, what kind of predictive ACKing games does
DirecPC play?

The reason I am curious is that I'm trying to figure out what kinds of
things ISPs, or any Internet access provider (say the user is dialing
into a corporate RAS), can do to break or accomodate DirecPC. The
obvious things that come to mind are egress filtering and NAT. In
addition to any technical information that can help me figure this out
for myself, experience others have had with these same problems would
also be helpful.

Thanks a lot for any help.
Crist J. Clark                     |     cjclark at alum.mit.edu
                                   |     cjclark at jhu.edu
http://people.freebsd.org/~cjc/    |     cjc at freebsd.org

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