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From: braden@BRADEN.ISI.EDU.UUCP
Newsgroups: comp.protocols.tcp-ip
Subject: Re: A fuzzy proposal
Message-ID: <8706191739.AA09161@braden.isi.edu>
Date: Fri, 19-Jun-87 13:39:00 EDT
Article-I.D.: braden.8706191739.AA09161
Posted: Fri Jun 19 13:39:00 1987
Date-Received: Sat, 20-Jun-87 09:51:18 EDT
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Dave Mills,

This is a delayed reply to your "fuzzy proposal" for a way to represent
in HOSTS.TXT the Fuzzballs and other gateways which follow the
gateway-half model. We too have been grappling with this problem, in the
process of gathering a database specifying the connectivity of NSFnet. We
have arrived at a scheme which differs from yours.

We think that your scheme does not make the topology readily
apparent. Consider your example:  
    
        GATEWAY : 35.1.1.1, 192.5.8.* : UMICH1
        GATEWAY : 192.5.8.6, 35.*.*.* : SWAMPRAT 

Now, it may be possible to infer from this pair of entries that UMICH1 is
connected to SWAMPRAT.  One can construct more complex cases where this
is ambiguous. For example, suppose we have two parallel connections:
    
        GATEWAY : 35.1.1.1, 192.5.8.* : UMICH1
        GATEWAY : 192,5,8,6, 35.*.*.* : SWAMPRAT1 
        GATEWAY : 35.1.1.1, 192.5.8.* : UMICH2
        GATEWAY : 192.5.8.6, 35.*.*.* : SWAMPRAT2 
        
Which one is connected to which? Another example, with two serial
lines from UMICH1 to two different gateway-halves on 192.5.8, might
be:
    
        GATEWAY : 35.1.1.1, 192.5.8.*, 192.5.8.* : UMICH1
        GATEWAY : 192.5.8.6, 35.*.*.* : SWAMPRAT1 
        GATEWAY : 192.5.8.6, 35.*.*.* : SWAMPRAT2 

We suggest that the primary purpose for the databases like the NIC
HOSTS.TXT is to show how things are connected together; derivation
of routing rules would be a secondary activity.  Your scheme, which
is based on routing rules rather than actual connectivity, may
hide what we most want to be explicit. 

The scheme which we adopted is to use symbolic interface addresses for
the un-numbered serial lines between gateway-halves.  Specifically, we
chose names of the form:

    <Network name>-<GWhalf 1>-<GWhalf 2>
    
between two gateway halves with names <GWhalf 1> and <GWhalf 2>. For
consistency, the two GWhalf names are always written in alphabetical
order.  We believe, by the way, that for consistency every network ought
to have both a name and a number assigned, even if the number is never
explicitly used because the network is comprised of half gateways
connected with serial lines.  In the absence of a network name, you
may think of the first component as an Autonomous System name.

For example, we might list your first example as:

    GATEWAY: 35.1.1.1, SWAMP-SWAMPRAT-UMICH1 : UMICH1
    GATEWAY:  192.5.8.6, SWAMP-SWAMPRAT-UMICH1 : SWAMPRAT
  
    
Suppose one tries to draw a network map based on your entries, without
doing the implied telescoping of entries; one ends up with a
set of two one-way connections between each network pair.  For
example:

                                                          -------
   -------          |----------------------------------->(       )
  (       )      --------                  --------      ( net   )
  (  net  )<----| umich1 |                |swamprat|---->(192.5.8)
  (  35   )      --------                  --------      (       )
  (       )<----------------------------------|           -------
   -------          


Using our symbolic names, one gets directly the correct topology,
without any telescoping:

   ------- 
  (       )     --------    
  ( net 35)<---| umich1 |
  (       )     -------- 
   -------         |       -------
                   |----> ( SWAMP )  <----|             -------
                           --------       |            (       )
                                        ---------      ( net   )
                                       | swamprat|---->(192.5.8)
                                        ---------      (       )
                                                        -------


In short, HOSTS.TXT is one of the main sources of information on network
configuration that is generally available, and we would like to see it
contain clear connectivity information on the fuzzballs and any other
gateway-halves.
   
By the way, your discussion of the use of the notation for hiding the
internal structure of an autonmous system should not be relevant to
databases such as HOSTS.TXT.  We want this to contain as much information
as possible, not to hide information.

Neither scheme, yours or ours, seems quite right. Maybe we are solving
the wrong problem?
  
    Bob Braden
    Annette DeSchon