RFC 2101 (rfc2101) - Page 3 of 13

IPv4 Address Behaviour Today

RFC 2101              IPv4 Address Behavior Today          February 1997


   unique with respect to each set of inter-communicating routers (which
   we will call a routing "realm"). While locators must be unique within
   a given routing realm, this uniqueness (but not routability) could
   extend to more than one realm.  Thus we can further distinguish
   between a set of realms with unique locators versus a set of realms
   with non-unique (overlapping) locators.

   Both identifiers and locators have requirements of lifetime, but
   these requirements are different. Identifiers must be valid for at
   least the maximum lifetime of a communication between two hosts.
   Locators must be valid only as long as the routing mechanisms so
   require (which could be shorter or longer than the lifetime of a
   communication).

   It will be noted that it is a contingent fact of history that the
   same address space and the same fields in the IP header (source and
   destination addresses) are used by RFC 791 and RFC 793 for both
   identifiers and locators, and that in the traditional Internet a
   host's identifier is identical to its locator, as well as being
   spatially unique (unambiguous) and temporally unique (constant).

   These uniqueness conditions had a number of consequences for design
   assumptions of routing (the infrastructure that IPv4 locators enable)
   and transport protocols (that which depends on the IP connectivity).
   Spatial uniqueness of an address meant that it served as both an
   interface identifier and a host identifier, as well as the key to the
   routing table.  Temporal uniqueness of an address meant that there
   was no need for TCP implementations to maintain state regarding
   identity of the far end, other than the IP address. Thus IP addresses
   could be used both for end-to-end IP security and for binding upper
   layer sessions.

   Generally speaking, the use of IPv4 addresses as locators has been
   considered more important than their use as identifiers, and whenever
   there has been a conflict between the two uses, the use as a locator
   has prevailed. That is, it has been considered more useful to deliver
   the packet, then worry about how to identify the end points, than to
   provide identity in a packet that cannot be delivered. In other
   words, there has been intensive work on routing protocols and little
   concrete work on other aspects of address usage.

3. Ideal properties.

   Whatever the constraints mentioned above, it is easy to see the ideal
   properties of identifiers and locators. Identifiers should be
   assigned at birth, never change, and never be re-used. Locators
   should describe the host's position in the network's topology, and
   should change whenever the topology changes.



Carpenter, et. al.           Informational