What is multihoming was (design of a real routing v. endpoint id seperation)

Tue Oct 25 18:09:59 UTC 2005

Robert Bonomi wrote:
>>From owner-nanog at merit.edu  Mon Oct 24 15:33:02 2005
>>Date: Mon, 24 Oct 2005 13:31:17 -0700
>>Subject: Re: What is multihoming was (design of a real routing v. endpoint id
>> seperation)
>>
>>Stephen Sprunk wrote:
>>[snip]
>>
>>
>>>>Other people use this term in very different ways. To some people
>>>>it means using having multiple IP addresses bound to a single
>>>>network interface. To others it means multiple websites on one
>>>>server.
>>>
>>>
>>>That is virtual hosting in a NANOG context.  Some undereducated MCSEs 
>>>might call it multihoming, but let's not endorse that here.
>>
>>Unfortunately, this is a common and "standards blessed" way to refer to
>>any host with multiple interfaces/addresses (real or virtual). For example,
>>from the "Terminology" section, 1.1.3, of RFC1122, "Requirements for
>>Internet Hosts -- Communication Layers," says,
>>
>>          Multihomed
>>               A host is said to be multihomed if it has multiple IP
>>               addresses.  For a discussion of multihoming, see Section
>>               3.3.4 below.
>>
> 
> 
> *sigh*  Multi-homing simply means 'having external connections to more than 
> one network' -- be it a network with multiple, disjoint, ingress/egress paths,
> or a host with interfaces (real or virtual) on distinct LAN subnets (even if
> those subnets are agregated into a single net somewhere upstream.
> 
> A host with multiple adresses utilizing the _same_ netblock/netmask _should_
> _not_ be called multi-homed (because there is only one path to that host), it
> is simply a single-homed host with multiple identities.  might be called
> "poly-ip-any" or some such.  <grin>

Depends who you ask. Again, RFC1122 says (section 1.1.1),

          A host is generally said to be multihomed if it has more than
          one interface to the same or to different networks.

And also section 3.3.4.1,

             A multihomed host has multiple IP addresses, which we may
             think of as "logical interfaces".  These logical interfaces
             may be associated with one or more physical interfaces, and
             these physical interfaces may be connected to the same or
             different networks.

As far as a "multihomed host" is concerned, RFC1122 sure seems to call
anything with multiple IPs multihomed. Multihomed is a trait of the host
independent of any network topology around the host.

But whatever. It just means people need to be clear what they are talking
about when they say "multihomed." As is clear from this thread, there is
not clear agreement on what the precise meaning is.
-- 
Crist J. Clark                               crist.clark at globalstar.com
Globalstar Communications                                (408) 933-4387