[pptp-server] pptp without ppp

Josh Massie Josh at pollstar.com
Wed Sep 20 14:11:37 CDT 2000

Here's from RFC 1548:


   The Point-to-Point Protocol (PPP) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   is comprised of three main components:

      1. A method for encapsulating multi-protocol datagrams.

      2. A Link Control Protocol (LCP) for establishing, configuring,
         and testing the data-link connection.

      3. A family of Network Control Protocols (NCPs) for establishing
         and configuring different network-layer protocols.

   This document defines the PPP organization and methodology, and the
   PPP encapsulation, together with an extensible option negotiation
   mechanism which is able to negotiate a rich assortment of
   configuration parameters and provides additional management
   functions.  The PPP Link Control Protocol (LCP) is described in terms
   of this mechanism.

   This document is the product of the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the ietf-ppp at ucdavis.edu mailing list.

PPP is a huge multiheaded monster (just peruse the RFC :-), and is sort of the "garbage can" that a bunch of other stuff has been kicked into.  In the horrid world of encapsulation, an IP packet can be encapsulated in a PPP packet, which can be encapsulated in an IP packet (and on to infinity, depending on how many tunnels you have).  So it doesn't matter what your riding on (serial cable, X.25, ethernet, T-X, DS-X, yelling out the door)  Here's an example:

Office A has an ethernet based LAN using IP internal space, and is linked to Office B via primnet.  Office B is also in internal address land, and has a fiber link to the main office C, where the internet connection (and NAT box) is.  Office D, in Switzerland (NATed as well), is linked in via a GRE VPN over the Internet.  And in addition to data, they have a VOIP PBX at each location.  You can pick up an handset at office A and get local Swiss dialtone via the PBX at office D, no long distance charges.

I may be wrong, but I think that PPTP is a host based implementation of the GRE tunneling protocol.  Lots of folk use GRE, including Cisco and Lucent (the old Ascend VPN stuff is all GRE).  That's why you allow protocol 47 (GRE) through or to the firewall.  The PPTP service is responsible for encrypting and encapsulating the information, but uses PPP to deliver it.  Again, this last bit is pretty much my conjecture...

josh massie
extranet administrator

email:   josh at pollstar.com 
phone: (559) 271-7977 x 4477
fax:      (559) 271-7979


>>> "Lillian Kulhanek" <Lillian.Kulhanek at energy.on.ca> 09/20/00 11:23AM >>>
> Most of the documentation I've read about assumes the pptp server to be
> the same machine as the machine that connects to the internet.  If it's
> behind this machine, is ppp still necessary, ie. does pptp require it?

Yes, your missing the "true" functionality of PPP, it is Point-To-Point.  A
VPN is Point-To-Point,  pppd really has nothing to do with modems or dialing
(it lets "chat" handle that).  PPP is used for DSL lines, Cisco routers,
other type of VPNs, etc....


Yes, when I say out loud the words that make up the pptp and ppp
abbreviations, it does seem like a silly question.  ;)
I was thinking more from a hardware perspective - ppp is required for a
serial connection, e.g. between two modems, to transmit IP.  I was wondering
if you remove the serial device, is it still necessary to have the protocol
for the serial devices?  But since pptp needs ppp to run, and ppp is indeed
used on media with non-serial interfaces, the point seems moot.

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