Subject: RE: IPCP IP address

From: Joe Frazier <[email-masked]>
Date: Thu, 18 Dec 1997 17:19:14 -0500

Hi Jim,

      Some comments below:

-----Original Message-----
From:	James Carlson [SMTP:[email-masked]]
Sent:	Thursday, December 18, 1997 3:55 PM
To:	Joe Frazier
Cc:	[email-masked]; [email-masked]
Subject:	Re: IPCP IP address

Hey, smokin' Joe, I'd wondered where you'd gone.

I had responded to Ohad's mail with this:
[...]
> Personally, I treat the remote address on a PPP link as a host 
route
> ("subnet" mask 255.255.255.255) and use any available routing
> protocol
> (or static routes) to set up routes to any networks reachable over
> the
> link.  The subnet mask isn't meaningful, so, for those 
administrators
> who insist on having such things, I generate a separate static 
route
> to represent it.

Joe responds:
> It definitely is a good idea to use host routes in this situation, 
and
> your explanation about the fundamental definition of a PPP link 
really
> drives the point home, Jim.
>
> However, your last sentence does open the door for a little more
> discussion. I'm not sure that a static route is beneficial in a
> "generic" configuration, or an administrators need to insist it.

Some administrators think of all interfaces of subnets, whether or 
not
there actually is a subnet in existence.  No, there's no need to
insist on it.  But, then, there's no need for carriers to insist on
ATM, either ...

I think I see your point.

> PPP links may be host specific, but the interfaces they use don't 
have
> to be. All PPP configuration packages do include the subnet mask as 
> part of the configuration.

Wrongly, in my opinion.

Since PPP interfaces are non broadcast - you probably are correct.

> If an administrator wants to set up a network interface for a PPP
> link, (he,she) can configure to do so. Of course, the subnet mask is 
> not communicated over the link in IPCP, so both sides of the link
> would need to be configured to use the same subnet masks.

Wrong.  The subnet mask on my end of the link represents my belief
about the size of your other attached broadcast medium.  The subnet
mask on your end of the link represents your belief about the size of
my attached broadcast medium.  They are the same if and only if we
both have attached broadcast media (such as Ethernet) with configured
subnet masks that are equal.  (Or if we're trying to aggregate to the
same level ... ugh.)

I was under the impression that IP interfaces were more logical, 
meaning they don't care about the underlying medium. In the case of 
PPP never being done via broadcast media, I see your point.

> However, since these PPP link "interfaces" are usually 
non-permanent,
> a host node configuration would permit a "generic" configuration. 
And
> if both sides of the PPP link are capable of routing, then all the
> routing info can be retrieved via the PPP interface, just like any
> other interface,

I couldn't parse that paragraph.  If you mean that PPP with IP is
perfectly capable of running RIP and other routing protocols, then,
yes, that's the right way to do it.

I just meant here that a PPP Interface in most cases is not permanent. 
By using a host subnet mask, there really needs to be no special 
subnet configuration (i.e. all PPP interfaces will always have the 
same host subnet configuration) . As you say, the mask would be 
meaningless.  Any subnet issues for networks that the hosts reside on, 
will be dealt with by the routing protocol, just like with any other 
interface type.

Thanks again for the dialogue. As we both know, you pretty much have 
answered Ohad's question with the first email.

Thanks,

Joe Frazier

--
James Carlson, Consulting Engineer 
               <[email-masked]>
IronBridge Networks / 5 Corporate Drive            Vox:  +1 978 691 
4644
Andover MA  01810-2448 / USA                       Fax:  +1 978 691 
6300