Re: PPP



Brian Lloyd wrote:


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.


I don't see how classification of PPP as a layer 2, layer 3, or any other layer
would have had an affect on how we designed L2TP (perhaps it would have affected
the name of the protocol though). Layers aside, PPP was already deployed and it
was pretty obvious what we wanted to do - make it run over IP without the
installed base of PPP clients being made aware of it. How would you have done
this that is substantially different than L2TP? (As an aside, of the list of
obscene things we did have to do to make L2TP work, the worst were more due to
badly implemented PPP stacks than anything else.)

Tunneling, particularly L2 tunneling, is by its very definition a "layer
violation". The perfect world where this is not necessary or desirable does not
exist beyond textbooks and laboratories. So here we are in the real world,
tunneling not just PPP but a plethora of L2 or L2-like layers. 

- Mark

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