SIPP Transition Mechanisms
The two key objectives of the SIPP transition mechanisms are to
provide global interoperability between SIPP and IPv4 hosts, and to
allow the user population to adopt SIPP in an a highly diffuse
fashion. The transition must be incremental, performed at the user's
own pace, with few or no critical interdependencies, if it is to
succeed.
The SIPP transition mechanisms provides a number of features:
Global IPv4/SIPP Interoperability
IPv4 and SIPP hosts anywhere in the Internet can interoperate up until
the time when IPv4 addresses "run out". They can continue to
interoperate within a limited scope for an indefinitely period after
this time. This feature protects for a very long time the huge
investment users have made in IPv4. Hosts that need only a limited
connectivity range (e.g. printers) need never be upgraded to SIPP.
Incremental Deployment
Deployment of new SIPP hosts and routers, and upgrade of existing IPv4
hosts and routers to SIPP can be done "one at a time". A site does
not need to upgrade all hosts or all routers at the same time.
No Upgrade Dependencies
Hosts and routers can upgrade to SIPP with very few dependencies on
other systems. Hosts can be upgraded to SIPP before any routers are
upgraded, and routers can be upgraded to SIPP before any hosts are
upgraded. Individual hosts and routers within a site can be upgraded
without other hosts and routers being upgraded.
The incremental upgrade and "no dependencies" features allow the host
and router vendors to integrate SIPP into their product lines at their
own pace. Sites with equipment from multiple vendors can upgrade
systems as soon as SIPP becomes available in each product. They do
not need to wait until SIPP becomes available in all products.
Two Hosts on a Wire
An IPv4 hosts and a SIPP host on the same subnet can directly
interoperate with each other, without the assistance of a third
machine. The ability of "two hosts on a wire" to interoperate is a
key feature of IPv4, and essential to maintain for the transition.
No New Addresses
Hosts and routers do not need to change addresses when they upgrade.
Their SIPP address is composed using their existing IPv4 address, with
a fixed prefix prepended. The same prefix is shared by all hosts and
routers within a domain.
Flexible Upgrade Timing
Users can upgrade their systems to SIPP at almost any time between now
and the time when IPv4 addresses run out. Hosts and routers that do
not need global connectivity need never be upgraded. By giving users
a long window of time, they can schedule the upgrade at their own
convenience.
The SIPP transition uses a collection of mechanisms, some of which are
dependent on each other. The three key elements of the transition
are:
IPv4-Compatible SIPP Addresses
The 64-bit "IPv4-compatible SIPP address format" is an extension to
the existing 32-bit IPv4 address format. It encodes an IPv4 address
in the low-order 32-bits and additional addressing information in the
high-order 31-bits. The high-order bit has special significance for
the transition: It indicates whether the node the address identifies is
an IPV4 node or a SIPP node. This bit, which is termed the C-bit
(compatibility bit), allows 64-bit SIPP addresses to be assigned to
IPv4 hosts and listed in the DNS.
A SIPP addressing plan that is based on the existing IPv4 addressing
plan will be developed. This plan will make use of a hierarchical
delegation of address blocks, like the CIDR plan. The plan will provide
for more levels of hierarchy than IPv4, however, and this will relieve
the Internet route scaling problems. The low-order 32-bits of addresses
assigned by this plan will be maintained globally unique for use as IPv4
addresses for as long as possible. Since the IPv4-compatible SIPP
address format includes an IPv4 address, SIPP hosts do not need
different IPv4 and SIPP addresses. They may use the low-order 32-bits
of their SIPP address as an IPv4 address.
Existing IPv4 nodes do not need to change their addresses when
upgraded to SIPP. They are assigned SIPP addresses with their
existing IPv4 address as the low-order 32-bits.
SIPP-in-IPv4 Encapsulation
SIPP traffic can be "tunneled" across regions of IPv4 topology using
the SIPP-in-IPv4 encapsulation mechanism. This allows the existing
IPv4 infrastructure to be utilized to build the SIPP infrastructure.
The tunneling mechanism makes use of the IPv4-compatible SIPP address
format. Since an IPv4 address is encoded within every IPv4-compatible
SIPP address, the SIPP-in-IPv4 tunnels are automatically configured.
The IPv4 address of the "tunnel endpoint" is always the low-order
32-bits of the SIPP address of the tunnel endpoint.
SIPP-to-IPv4 and IPv4-to-SIPP Header Translation
IPv4 traffic sent by IPv4 hosts can be translated into SIPP form so that
it can be routed by SIPP routers. This feature will allow IPv4 routing
over the Internet backbone to eventually be de-commissioned. Since SIPP
addresses encode more layers of hierarchy than IPv4, this will ease the
IPv4 route scaling problem. The translation technique makes use of
"SIPP translators", which can be a co-resident function in SIPP
routers, to translate IPv4 traffic into SIPP, and SIPP traffic into
IPv4. The translators make use of the C-bit and IPv4 address
information encoded in the IPv4-compatible SIPP address to determine
which packets to translate, and what addresses to place in the
translated packets.
The IPv4-to-SIPP translators require external configuration information
in the form of an address mapping table in order to generate
64-bit SIPP source and destination addresses from the 32-bit addresses
in the IPv4 packets being translated. The mapping table information
must be maintained globally. Its characteristics as is a growing, but
rarely changing, database, make management of the mapping tables on a
global scale tractable. The SIPP-to-IPv4 translators require no
external configuration information; All of the information they need is
contained in the packets that they are translating.
The translators only modify the Internet headers of packets they
translate. The transport layer headers and data are never changed by the
translators.
While some of the SIPP transition mechanisms are inter-dependent, some
elements can be omitted from the transition if better solutions to the
problems they address are found.
For more information on the SIPP transition mechanisms see the
SIPP Transition Mechanisms Specification
.
This page was developed by Robert Gilligan
gilligan@eng.sun.com of Sun Microsystems. It was last updated on
April 18, 1994.