Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <8690ibj855.fsf@ironbridgenetworks.com>,
  James Carlson <carlson@ironbridgenetworks.com> wrote:
> albert.e.manfredi@boeing.com writes:

[ ... ]

> > When you use ATM as an access link to an ISP, for example, you are using ATM
> > as if it were a better telephone connection. Which is really a very
> > appropriate use of ATM, in my opinion. But now you need to provide a means
to
> > differentiate between IP packets. I think this might be what you're after?
> > SLIP ought to work as well. The ATM connection does not react to differences
> > in each IP packet sent. Something else must.
>
> That doesn't make sense.
>
> There's no particular reason to be distiguishing among IP packets from
> a given source, and that's not what PPP does, and that's not why you'd
> run PPP over ATM, and SLIP won't reasonably run on this type of
> connection, and even if it did, you wouldn't want that.

Okay, I should have said packets.

Please see RFC 1171:

"Abstract

   The Point-to-Point Protocol (PPP) provides a method for transmitting
   datagrams over serial point-to-point links.  PPP is composed of three
   parts:

      1. A method for encapsulating datagrams over serial links.

      2. An extensible Link Control Protocol (LCP).

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

   This document defines the encapsulation scheme, the basic LCP, and an
   NCP for establishing and configuring the Internet Protocol (IP)
   (called the IP Control Protocol, IPCP)."

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.

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. 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.

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). 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.

> ATM provides Virtual Circuit (VC) connections from one point to
> another.  Using AAL-5, you can transfer packets from one point to
> another with no problem.  If you already have addresses and routes
> statically defined, and if you trust the security of ATM's VC
> connection model (the provisioning of PVCs or signaling of SVCs), then
> you can run IP directly over ATM VCs.
>
> If you need or want either separate user authentication, IP address
> assignment, or protocol use negotiation, then PPP provides that on an
> ATM VC with very little overhead.

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

Bert
manfredi@arl.bna.boeing.com

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