Subject: RE: what is a site?

From: David Borman <[email-masked]>
Date: Thu, 29 Mar 2001 10:23:02 -0600 (CST)

> From: "Joris Dobbelsteen" <[email-masked]>
> Subject: RE: what is a site?
> Date: Thu, 29 Mar 2001 16:43:03 +0200
>
> >-----Original Message-----
> >[mailto:[email-masked]]On Behalf Of Tomohide Nagashima
> >Sent: Wednesday, 28 March 2001 9:52
> >Subject: Re: what is a site?
...
> >At least, it is clear that "Site" is a set of links. But is
> >the subset of Site
> >the Site ?
...
> >I begin to believe this answer is YES.If else, what
> >do we call that ?
> >I belive this will be a key of definition of "Site".
>
> So a site can be hierarchical and can a site also be a part of two (totally)
> different sites?
> Wouldn't this get quite messy...

From the definition of a site-local address in RFC 2373, pg. 10:

   Site-Local addresses have the following format:
   
   |   10     |
   |  bits    |   38 bits   |  16 bits  |         64 bits            |  
   +----------+-------------+-----------+----------------------------+
   |1111111011|    0        | subnet ID |       interface ID         | 
   +----------+-------------+-----------+----------------------------+

   Site-Local addresses are designed to be used for addressing inside of
   a site without the need for a global prefix.

   Routers must not forward any packets with site-local source or
   destination addresses outside of the site.

The address format does not currently allow for hierarchical
"site-local" addressing.

Just as a machine can be attached to more than one link, with a
link-local address on each link, a machine can be attached to more
than one site, and thus have a site-local address in each site.
The machine has to keep track of what site-local addressess are
associated with each site, just as it has to keep track of which
link-local addresses are associated with each link.  I would expect
that the machine would have a (logical) interface for each site to
which it is attached.

So, one definition of a "site" might be a collection of machines
and networks that are all interconnected and accessable via the same
set of site-local addresses (ignoring administrative barriers...).

You could have a "site within a site", but it wouldn't be a sub-site.
The transition points into and out of the "sub-site" would block all
site-local addresses from crossing the boundry.  So, what you really
have is two sites, and what it looks like just depends on how you draw
the picture.

If the desire was to allow Big Sites to encompass multiple Small
Sites, then we'd have to change the addressing format, say:

   |   10     |
   |  bits    |   38 bits   |  16 bits  |         64 bits            |  
   +----------+-------------+-----------+----------------------------+
   |1111111011|  Site ID    | subnet ID |       interface ID         | 
   +----------+-------------+-----------+----------------------------+

But if a host was to be part of both the big and the small site, it
would have to have a site-local address for each site.  So, I guess this
would be more to allow overlapping sites, as opposed to sub-sites.  It
just seems like a sub-site when the bigger site totally overlaps the
smaller site.  But the "Site ID" would still not be guaranteed unique,
a machine could still be attached to two Sites that each have the same
Site ID.  And with the addition of a "Site ID", you'd give people a
whole lot more rope to build an administrative nightmare of overlapping
sites.  Maybe we better first figure out how to use the currently defined
site-local addresses before delving into adding a "Site ID"...

		-David Borman, [email-masked]
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