Cell Relay Archive

[James Carlson <carlson@ironbridgenetworks.com>]

albert.e.manfredi@boeing.com writes:
> Please see RFC 1171:

1171 is quite obsolete.  Try RFC 1661.

> PPP in essence provides the overhead functions, over a serial link, that
> Ethernet or FDDI overhead provide over their media. One of which is to
> identify the protocol used by the data packet. You will note that AAL5 does
> not do this.

That's almost correct.  AAL5 is like Frame Relay -- it provides an
addressing structure (the VPI+VCI pair) that can be used to segregate
multiple streams.  This can be done entirely by prior arrangement --
no PPP needed at all -- and the VCs are just used as packet-oriented
pipes between routers, as Frame Relay PVCs are today.

> If you're using ATM AAL5, which would be the logical choice, then by itself,
> AAL5 only provides you with a packet envelope and a CRC.

Plus a VC.  That's not unimportant.

> So the encapsulation
> feature of PPP would not be of added value, but the link layer of PPP would
> be needed. If you're using other AALs, e.g. AAL1 for any reason, you would
> need to have something encapsulate the data packets.

Huh?  That doesn't really make sense.  The "encapsulation" that PPP
provides gives negotiated multiprotocol support over that single VC.
PPP *IS* the link layer, so I don't know what you mean by having the
PPP link layer but not the encapsulation.

Please read RFC 2364 (PPP over AAL5).

> PPP also provides a link quality check, set up and tear down the link, and
> protocol configuration options (but assigning of addresses is not specified in
> PPP).

Wrong.  Network layer addresses are assigned by the various NCPs.
IPCP (RFC 1332), for instance, negotiates IP addresses between the
peers.

> So if you have PPP over the ATM VC, in principle you could terminate the
> IP session and start something else, without having to tear down the
> VC.

You can do much, much more.  You can run multiple simultaneous
protocols over the same VC if you're running PPP.  If you don't run
PPP, then you have to disambiguate the protocols either by using VCs,
or (*shudder*) by adding LLC/SNAP headers.

> As far as I know, DHCP is used over PPP to provide address assignments. PPP
> does not do this by itself.

Read RFC 1332.

(DHCP over PPP is a good thing to do, however since MS doesn't
understand it, there are few implementations that the common folk will
see.)

-- 
James Carlson, Consulting S/W Engineer  <carlson@ironbridgenetworks.com>
IronBridge Networks / 55 Hayden Avenue  71.246W    Vox:  +1 781 372 8132
Lexington MA  02421-7996 / USA          42.423N    Fax:  +1 781 372 8090
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp