RE: PPP

At 03:12 AM 3/4/2002, you wrote:
> I couldn't say it shorter and more clearly than Vint : PPP does NOT belong 
> to the TCP/IP protocol suite.
Other than it was designed for IP and the other stuff came along for the 
ride.  PPP was a relatively early product of the IETF and specifically 
designed for IP.
> It's a Layer 2(*) protocol, intended to carry multiple Layer 3 protocols 
> (like IP, IPX,...) over a point-to-point connection (like PSTN, ISDN,...).
> PPP succeeded SLIP by bringing extended features : SLIP could only 
> encapsulate IP while PPP can encapsulate several protocols, PPP supports 
> authentication while SLIP didn't, etc.
> Remember that TCP/IP only covers Layer 3 up to Layer 7 : it's designed to 
> be implemented on existing lower layers (1 and 2) :  LAN (Ethernet, Token 
> Ring, Wireless Lans,...) or WAN (ISDN, ATM, Frame Relay,...).
This is a common misconception.  The "lower layers (1 and 2)" that you 
mention are often completely routable networks in and of themselves.  You 
can even encapsulate IP within IP therefore IP is operating at layer 2 from 
that interpretation.  Ethernet is regularly routed now (people call it 
switching but a rose by any other name ...).  So all of these, including 
PPP, exist at layers 1-2 in the TCP/IP model (link, network, internetwork, 
transport, application) or layers 1-3b in the ISORM.
This problem plagues developers working with PPP for the first time because 
they keep thinking in terms of PPP being only a link-layer protocol.  If 
they would remember that PPP operates at the network layer then they would 
stop making stupid mistakes like a badly-designed L2TP.
> E.T.
>
> (*) Those layers always refer to the OSI model. Think of a Layer 2 or 3 or 
> 4 Switch : it doesn't depend on the protocol suite above, so we always 
> refer to the vendor- technology- protocol-independent OSI reference model.
I love watching people slavishly adhere to this or that model of 
layering.  Layering is a convenience, not a religion.  (Actually, I got 
that backwards.)  With the widespread use of encapsulating one networking 
or internetworking protocol in another, the whole concept of rigid layering 
goes out the window.  The cry of, "its a network layer; its a link layer," 
should be right up there with, "its a dessert topping; its a floor wax!"

> --- The basic answer ends here ---
>
> Now a small yet technical recall : when data comes from an application to 
> be transported on a physical medium (copper cable, fiber optics, radio 
> waves, infra-red,...), on its way from Layer 7 to Layer 1 it reaches IP 
> (Layer 3)
ISO spent a lot of time trying to sell the 7-layer model and then didn't 
know how to backtrack when they discovered that there were really two 
network layers when you interconnect dissimilar networks using an 
internetworking protocol.  ATM, FR, Ethernet, etc., are all routable 
layer-3 protocols in their own regard so they opted to break layer three 
into three sublayers. (It is really three layers by their reckoning but ISO 
already had so much invested in the "ISO Seven Layer Reference Model [tm]" 
that they couldn't really switch to the "ISO Nine Layer Reference Model 
Formerly Known As The Seven Layer Reference Model [tm].")
> that encapsulates it in a datagram/packet and specifies the destination 
> network+host address. Then it's forwarded to PPP (Layer 2) that 
> encapsulates it in a frame and specifies the way bits are organized to 
> travel through the physical medium. Then it's forwarded to some Layer 1 
> technology that converts the bits into a specific signal using a specific 
> encoding scheme (V.90 on PSTN, I.430 on ISDN BRI,...) and finally reaches 
> the physical medium to be physically transported through the network.
To some extent you are right but your model needs to accommodate things 
like L2TP which tunnels traffic at layer 1|2 (depending on the model of the 
day) in a layer 4 (transport) protocol, or IP tunneled in IP.  It is 
probably better to be able to keep the concept of duality in your mind, 
i.e. when you hold you tongue one way it looks like a link protocol but 
when you hold your tongue a different way it looks like a transport 
protocol.  I suspect that something like this gave early physicists fits 
when they were faced with the duality of nature.
So trying to be rigid in your categorization of any protocol is likely to 
cause you heartburn down the road (ask ISO).  It is far better to 
understand where it makes sense to put interfaces and then perform the 
functions that need to be performed.

> --- The extended answer ends here ---
>
> -----Original Message-----
> From: vint cerf [mailto:vinton(_dot_)g(_dot_)cerf(_at_)wcom(_dot_)com]
>
> IP is encapsulated in PPP for all practical purposes. PPP can support
> multiple protocols on a single point to point link in the same way
> ethernet can support multiple protocols
And, no, as the above quote shows, Vint did not say that PPP does not 
belong to the TCP/IP protocol suite.  He just says that PPP usually 
encapsulates/transports IP datagrams as its payload.  His comment that PPP 
and Ethernet are functionally equivalent is interesting if you mull over 
all the ramifications of that statement.

Brian Lloyd
brian(_at_)lloyd(_dot_)com
+1.530.676.1113 - voice
+1.360.838.9669 - fax