RFC 2101 (rfc2101) - Page 3 of 13
IPv4 Address Behaviour Today
Alternative Format: Original Text Document
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