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Internet Draft                                                      D. Crocker
                                                               The Branch Office
                                                                       R. Hinden
                                                                Sun Microsystems
                                                               November 11, 1992

                    IP Address Encapsulation (IPAE):
                  A Mechanism for Introducing a New IP


This document is an Internet Draft.  Internet Drafts are working documents of
the Internet Engineering Task Force (IETF), its Areas, and its Working Groups.
Note that other groups may also distribute working documents as Internet

Internet Drafts are draft documents valid for a maximum of six months.
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The Internet seeks to increase the amount of IP address space that is
available for hosts and to decrease the amount of table storage that is
required by routers.  Ultimately, the current IP (IP version 4) and any
replacement are inherently incompatible and movement to the new version
requires very substantial change to the IP installed base.  This specification
describes an approach which retains as much software, operations and training
as possible, and minimizes overall operational disruption by allowing subsets
of the Internet to carry the new-format Internet datagrams inside old-style
IPv4 datagrams, using a technique called "IP Address Encapsulation" (IPAE).
This permits incremental and asynchronous introduction and makes transition
entirely optional for portions of the Internet infrastructure.  It further
permits early reduction to routing table size.

This specification emphasizes extreme ease of transition to a new version of
IP that is as close to the existing version as is appropriate.  Hence, this

IP Address Encapsulation (IPAE)

specification is notable for attempting to fix only the current and urgent
problems and for neither solving nor precluding solution to less well-
understood problems.  By utilizing a separately-defined protocol for the final
deployment, IPAE can be viewed as a transition technology, rather than as a
protocol, itself.


IPAE has had the benefit of on-going contribution by many members of the
Internet community, with very substantial changes and enhancements ensuing
from the collaborative group engineering process.  Bob Hinden originally
suggested encapsulating one IP datagram inside another [HIND92b].  Dave
Crocker & Bob Hinden brought a modified idea -- to encapsulate new, global
addresses inside normal IP datagrams, using the IP address fields for "next
hop" carriage [HIND92a] -- to the IPAE working group.

While permitting very smooth transition, this left an end-state for IPAE which
was not a very clean, since the IPAE header is a hybrid.  Bob Gilligan
observed similarities between IPAE's "mini-layer" and Deering's SIP design,
resulting in IPAE focus on SIP.  Craig Partridge developed a very early code
analysis for implementing the original IPAE in the Berkeley kernel, permitting
much more concrete understanding of the implementation impact of the
encapsulation scheme and the general effects of increasing the IP address
size.  Recent implementation work by Erik Nordmark and Bob Gilligan has added
to the base of empirical data.

The working group has included contributions from:  Craig Partridge of BBN,
Tom Kessler, Erik Nordmark, and Bob Gilligan of Sun Microsystems, Steve
Deering of Xerox PARC, Greg Chesson, Ronald Jacoby,and and Rob Warnock of
Silicon Graphics, Hon Wah Chin and Dave Newman of Protocol Engines, Ross
Callon, Mike Reilly, Virgil Champlin and Geoff Mulligan of Digital Equipment,
Michael Conn of MCI, John Moy of Proteon, John Veizades of Apple, Jeffrey
Burgan of NASA, and Vince Fuller of BARRNET.

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IP Address Encapsulation (IPAE)


2.1.    Address characteristics
2.2.    Timing of benefits
2.3.    Preservation of Installed Base
2.4.    Transition Ease & Independence
3.1.    Encapsulation
3.2.    Address Format
3.3.    Interoperation
3.4.    Translation
3.4.1   Transit Net(s) Only
3.4.2   IPAE Host to IPAE Host
3.4.3   IP Host to IPAE Host
3.4.4   IPAE Host to SIP Host
3.4.5   IP Host to SIP Host
4.1.    SIP Address Format
4.2.    Datagram Formats
4.2.1   SIP Format
4.2.2   IPAE Format
4.3.    ICMP & IGMP
4.4.    Routing Protocol(s)
4.5.    Transport & Above
4.5.1   Pseudo header checksum
4.5.2   TCP Connection ID
4.6.    Subnetwork & Below
4.7.    Network Management
5.1.    SIP Derived from IP Datagrams
5.2.    IP Derived from SIP Datagrams
5.3.    Receipt of IPAE Datagrams
6.1.    Hosts
6.2.    Interior Routers
6.3.    Border Routers
8.1.    Initial Deployment of IPAE  (Milestone I)
8.2.    IPAE Deployment in Hosts  (Milestone H)
8.3.    Internet Runs Out of 32-Bit IPv4 Network Numbers (Milestone R)
8.4.    Administrative Domains Fully Convert to SIP (Milestone S)
10.     CONTACTS

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The Internet is experiencing explosive growth, usually described as doubling
every 12 months.  There is no indication that this growth will reduce and
development of IP use into mass markets would create an even steeper growth
curve.  The result is a crisis in IP router table storage and use and in near-
term exhaustion of available IP network numbers.  A variety of extensions to
current IP service also are desired, but they are not part of the current
operational crisis.

IP routers which record routes to all networks in the Internet must maintain
an entry for each such network, since the IP network address space is flat.
That is, neighboring networks do not necessarily have any similarity in the IP
network portion of their address.  A new address structure is required,
probably with a hierarchical scheme, so as to allow route-aggregation in table
entries to neighboring networks.

While the 32-bits of the IP address space theoretically can reference about 4
billion nodes, the bits have been partitioned to faciliate assignment and
aggregation into networks, thereby reducing the realistic maximum number of
nodes and -- more importantly -- networks.  While estimates vary considerably,
it is possible that IP network number exhaustion will occur within the next
few years.  For example, IP could begin to make penetrations in mass markets.
Since the damage caused by being unable to assign new IP network numbers is so
great, it is prudent to pursue an urgent path to increasing the number of
bits.  Again, there is debate about the total number of bits that is
necessary, with proposals ranging from 64-bits to 160-bits (as well as the
suggestion that there be no limit.)  In any event, the Internet needs to
determine the size and format for a new, larger IP address.

Various additional enhancements to IP are being discussed, such as provision
for real-time data delivery (i.e., guaranteed throughput and delay), network-
level security features, and support for mobile hosts.  While there is much
discussion about these, and other, issues they represent topics of interest,
rather than of concrete and stable experience and agreement.  No proposals
have achieved concensus in the Internet and none have acquired a substantial
operational base.  Hence, they must be deemed "research" for current purposes.


This section describes the the characteristics of the required solution that
are held to be primary by this specification.  While not attempting to exclude
additional features and consideration, the specification does place particular
emphasis on its concern for the installed base of IP users, while providing
relief from the current technical and operational difficulties:

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2.1.    Address characteristics

The new version of IP must provide a hierarachical address, sufficient to
permit distributed assignment and administration and sufficient to permit
route-aggregation in IP forwarding devices (routers).  A hierarchy which
recognizes country boundaries appears to be the most viable, administratively.
The address space must permit 10**9 networks.

2.2.    Timing of benefits

The need for route aggregation is immediate.  A specification must permit
reductions in route table size in the near term.  It is preferred that a
router achieve this benefit as soon as the new version of IP is implemented in
a router.

2.3.    Preservation of Installed Base

A specification for a new IP must consider the current installed base of
software and people.  IP has an enormous installed base of operational
systems, as well as an installed base of people who provide, support and use
such systems.  Each change to the existing technology carries the expense of
changes to people, software and operations.  Given the size of the installed
base, the aggregate expense of each such change can be quite large,
particularly if it causes reduced utility to the Internet.

To this end, a specification must justify each and every change to formats,
terminology, services, software and operations, with respect to the current IP
Internet.  This applies to IP, itself, as well as to related components, such
as the addressing framework, internetwork control, subnetwork interfaces and
transport-layer interfaces.

2.4.    Transition Ease & Independence

Movement from the existing IP to its replacement will be accomplished over an
extended period of time.  It must not require a highly coordinated change
throughout the Internet community, since the diversity of network
administrations and operations does not permit such coordination.  Further,
movement to the new IP must be accomplished in a manner which permits network-
layer interoperation between users of the existing and new versions, for as
long as is operationally practical.   Hence, this specification emphasizes
smooth and voluntary transitions, attempting to provide incremental benefit

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when adopted, and imposes no changes that are not of fundamental and well-
understood benefit.


This specification seeks to solve the technical problems of immediate concern
to the Internet and does not attempt to provide other, desired functional and
operational enhancements.  Neither does it appear to preclude such
enhancements and there is some indication that its design will prove entirely
adequate for later inclusion of such enhancements.

While a variety of new protocols may satisfy the urgent addressing and routing
table concerns for the Internet, this specifications views concerns for
protecting the installed base as requiring the new protocol to have as little
difference from current IP as possible.  Consequently, this specification
focusses on  Simple IP (SIP), by Steve Deering, as the appropriate choice for
the new IP [DEER92a, DEER92b].  While the general IPAE transition scheme,
using encapsulation of the new protocol inside the old, can be applied to
other candidates, it appears that a number of transition benefits will not
accrue with those candidates.  In particular, the packet and address
similarities to IPv4 greatly facilitate IPv4-SIP translation, which permits
combinations of interoperability tht permit an extremely wide range of
transition options.

At its simplest, IPAE envisions three phases to the adoption of a new IP:

                IP  ->  IPAE  ->  SIP

Initially, all nodes use only the old IP.  The goal is to operate all nodes
only with SIP.  While it is expected that this end-state will take a very long
time to reach for the whole Internet, portions of the Internet may achieve
"pure" SIP operation rather quickly.  Between the current use of IP and the
eventual use of SIP, there will be an extended period of partial adoption and
therefore requiring techniques for smooth transition and interoperation.  IPAE
specifically attends to those requirements.

IPAE permits continued operation of IP-only hosts and IP-only routers.  It
further supports SIP-knowledgeable hosts and routers.  A host may operate SIP-
only, without any IP support, and/or may be able to encapsulate SIP within IP
datagrams, in an IPAE mode.  Routers which support IPAE generally will serve
as "border" routers between IP and IPAE or SIP domains.  The border routing
function can operate at a logical routing level above IP, using IP as a type
of subnetwork-layer service.  A border router also can take native IP
datagrams and turn them into IPAE hybrid datagrams, when the router is acting
as an internetwork-layer gateway translation service.

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3.1.    Encapsulation

This specification defines a technique that is called "IP Address
Encapsulation" (IPAE) because it moves the Internet over to a new version IP
by allowing hosts and routers to upgrade, to the new addressing and datagram
format,  in a manner that is transparent to nodes which have not yet made the
transition.  The technique for accomplishing this is to encapsulate the new
protocol inside the old.  Hence, users of the enhanced addressing scheme may
"tunnel" their datagrams over the existing IP infrastructure.  Neighbors which
have completed conversion to the new IP can interoperate without
encapsulation, whenever that is convenient.

IPAE packets stacks as:

                        | Transport |
                        |    SIP    |
                        |    IP     |
                        |   Link    |

3.2.    Address Format

Support of interoperation between nodes using the old and new versions of IP
is best accomplished by defining the new IP address to contain old IP
addresses in its lower 32-bits.  This is accomplished by conceptually
extending the network field or by adding one or more fields "above" the IP
Network field.  Preserving the definition of the existing 32-bits permits
local operations to default the new bits, if they desire, and faciliates
translation between old and new IP datagrams.

Further the address, itself, must contain specification of the IP version (old
versus new) that is used by the referenced node.  This feature eliminates the
need for routers at the border of an administrative domain to maintain state
information about their user nodes.  Such reduced router and operations
complexity greatly facilitates transition to SIP.

Any new address format will dictate a change to the Domain Name Service and to
any protocol modules which are cognizant of IP addresses.  While fundamental
to the success of an Internet transition, the details of such changes are
beyond the scope of this specification.  Further, the details appear to be
identical for any IP replacement that is envisioned.

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3.3.    Interoperation

This specification supports interoperation between hosts supporting only the
old IP and those supporting only the new IP, through the use of a translation
service.  Translation usually is a hand-crafted and operated service, with
limited ability to scale to large operations.  However, this specification
supports general-purpose network-level translation between IP and SIP, as
desired.  This  is possible by ensuring that:

a)  Host operating mode (old versus new IP) is detected easily,

b)  Address formats map well and algorithmically, and

c)  Header field semantics map well and algorithmically.

3.4.    Translation

Translation can be provided by special routers, or by nodes themselves.  The
latter facility is expected to permit interoperation between hosts operating
on the same network, particularly prior to conversion of the interior routers
for a network.  Hence, nodes operating with the new version of IP may have an
implementation which treats the two versions of IP as entirely independent
network modules.  Alternatively, a node may operate as if all datagrams are
coded as new IP datagrams and then treat translation to/from old IP as a
special case.

Transmission of new IP datagrams can be direct in their native form,
translated into an old IP format, or encapsulated with the new inside the old.
The first form is, of course, preferred.  But it is expected that few nodes
will be able to utilize the new IP initially.  Hence, the choice usually will
be between translation and encapsulation.  If the next SIP hop is the
recipient (final) host, and that host supports only old IP, then the SIP
datagram must be translated into the format of an old IP datagram.  If the
next SIP hop is an intermediate router and that router supports only old IP,
then the SIP datagram needs to be encapsulated in an IP datagram.

Note:  In the following examples, "IPa", "IPb", and RIPcS represent three,
different IP headers, generated independently.  For example, IPa might be
generated by the originating host, but might be discarded when the datagram is
translated into a "pure" SIP datagram.  Later, a border router might need to
have the datagram transit an IP network and would create a fresh IP header
(IPb) derived from the SIP header.

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3.4.1   Transit Net(s) Only

IPAE can be deployed without awareness of hosts:

  Orig      Router       Router      Router        Router      Dest
  IP        IPAE         IP          IPAE          IP          IP
  |         |  |         |  |         |  |         |  |         |
  -----------  -----------  -----------  -----------  -----------
    +-----+      +-----+      +-----+      +-----+      +-----+
    | IPa |      | SIP |      | SIP |      | IPa |      | IPa |
    +-----+      +-----+      +-----+      +-----+      +-----+
                 | IPb |      | IPb |
                 +-----+      +-----+

This permits transit networks to employ IPAE for reduction of routing tables,
without waiting for host-level deployment, since routing can be based on the
structured SIP address rather than the flat IP address.

3.4.2   IPAE Host to IPAE Host

IPAE packets which move through the Internet between two IPAE hosts may
undergro modification, such as:

  Orig      Router       Router       Router       Router      Dest
  IPAE      IP           IP           IPAE         IP          IPAE
  |         |  |         |  |         |  |         |  |         |
  -----------  -----------  -----------  -----------  -----------
    +-----+      +-----+      +-----+      +-----+      +-----+
    | SIP |      | SIP |      | SIP |      | SIP |      | SIP |
    +-----+      +-----+      +-----+      +-----+      +-----+
    | IPa |      | IPa |      | IPa |      | IPb |      | IPb |
    +-----+      +-----+      +-----+      +-----+      +-----+

In this case, two IPAE hosts go through an Internet in which IPAE routers are
rare.  This IPAE router discards the IP header it receives (IPa) and generates
a new one (IPb) just as it does with media frame headers, when relaying
between local area networks.

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3.4.3   IP Host to IPAE Host

Common to early stages of deployment, a pure-IP host can talk with an IPAE or
pure-SIP host via an IP-accessible router:

  Orig      Router       Router       Router       Router      Dest
  IP        IP           IPAE         IP           IPAE        IPAE
  |         |  |         |  |         |  |         |  |         |
  -----------  -----------  -----------  -----------  -----------
    +-----+      +-----+      +-----+      +-----+      +-----+
    | IPa |      | IPa |      | SIP |      | SIP |      | SIP |
    +-----+      +-----+      +-----+      +-----+      +-----+
                              | IPb |      | IPb |      | IPc |
                              +-----+      +-----+      +-----+

This is generally identical to the previous example, except that the first
IPAE router also serves as an IP-to-IPAE translation gateway, deriving the
necessary SIP header from the IP header.

3.4.4   IPAE Host to SIP Host

As localities begin to support "pure" SIP operation, the following scenario
will occur:

  Orig      Router       Router       Router       Router      Dest
  IPAE      IP           IP           IPAE         SIP         SIP
  |         |  |         |  |         |  |         |  |         |
  -----------  -----------  -----------  -----------  -----------
    +-----+      +-----+      +-----+      +-----+      +-----+
    | SIP |      | SIP |      | SIP |      | SIP |      | SIP |
    +-----+      +-----+      +-----+      +-----+      +-----+
    | IP  |      | IP  |      | IP  |
    +-----+      +-----+      +-----+

In this case, the IP header simply is stripped from the datagram and the SIP
portion continues on, to a pure-SIP host.

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3.4.5   IP Host to SIP Host

Use of IPAE can be helpful even in only one router, to map between to
internetworks which employ IP-only and SIP-only:

  Orig      Router       Router       Router       Dest
  IP        IP           IPAE         SIP          SIP
  |         |  |         |  |         |  |         |
  -----------  -----------  -----------  -----------
    +-----+      +-----+      +-----+      +-----+
    | IP  |      | IP  |      | SIP |      | SIP |
    +-----+      +-----+      +-----+      +-----+

A key point, here, is that such pure gatewaying is a straightforward side-
effect of the IPAE principles, when coupled with similarity between IP and SIP
semantics.  The gateway router behaves identically with a configuration of
IPAE border router which treats IP encapsulation as a side-effect and which
supports translation of IP into SIP.


The primary change in IPAE is to the Internet layer.  Other layers must be
changed whenever they use Internet addresses.  In particular, some change is
necessary at the transport and application layers.

The IPAE proposal introduces two new Internet layer packet formats that we
refer to as SIP and IPAE.  The SIP header is a revised version of the IPv4
header with larger addresses and streamlined functionallity.  The IPAE header
is composed of a SIP header encapsulated within an IPv4 header.  The IPAE
packet format is used to transit regions connected with IPv4 routers, while
the SIP packet format is used between directly connected IPAE/SIP hosts and

SIP is specified separately [DEER92a, DEER92b], but details are summarized in
this specification, to facilitate discussion.

4.1.    SIP Address Format

The the basic requirement driving the current effort is to provide enough
additional bits to support global administration, with 2 or more levels of
hierarchy above the existing IP address portion, and the top level referencing

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Given the goal of facilitating transition to SIP, IPAE-related operation
requires enhancements to the current IP address, beyond the base requirement
for a larger address space.  These requirements are:

a)  A bit indicating whether the referenced host is using old or new IP; this
eliminates the need for routers that provide translation gatewaying to
maintain state tables about the capabilities of specific hosts; and

b)  Preservation of the existing format for 32-bit IP addresses, within the
new format,  to faciliate local interoperation between old and new IP hosts.

The resulting SIP address specification is:

      1    15           32            16
     | |Country+|Site / Subscriber|  Local |
     | |/ Metro |        |   IP Address    |
      C  Region      Attachment      Local

      C (IP Compatibility) bit is 0 for IPAE/SIP destination, 1
        for IP destination

      Multicast is encoded by a special country code.

The C-bit is used to indicate the capabilities of the referenced node.  If the
bit is zero, then the node supports SIP, and possibly IPAE.  (In reality, IPAE
is viewed as a per-hop encapsulation technique, so that its use depends upon
the characteristics of the next-hop node, rather than upon the end-system.)

The Region field is used for global routing and includes the top of the
addressing hierarchy which is the country having assignment authority.
Subordinate to this is specification either of the service provider for the
referenced subscriber or the metropolitan area of the site's attachment to the

Service providers assign Subscriber IDs.  Locations operating with coordinated
metropolitan interchanges (MIX) may use Country+Metro code and then assign
Site IDs.

The last 4 bytes of an address function as a IPv4 address.  However, the
actual boundary between IP address and Site/Subscriber information varies.  In
effect, Site/Subscriber is an extension to the network portion of the IP

Until IP network number space is exhausted, 32-bit IP addresses will be
assigned according to current practise, so that providers (and MIX operations)

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will assign only the upper 32 bits of a SIP address.  Later, they also will
assign those bits currently used to specify the IP Network field.

Hence, the transition model permits an independent network site (an
"administrative domain", or AD) to operate internally with 32-bit IP addresses
and to pre-pend an additional 32 bits for global routing.  If an AD network is
multi-homed to the Internet, then the bottom-half of its two SIP addresses
will be the same, during the transition period.  After IP network number
exhaustion, the current specification of SIP means that an AD may have a
single "host" field (subnetwork and host) as currently used by IP, but
different network addresses for each point of attachment and one more for IP
internal exchanges.

4.2.    Datagram Formats

IPAE/SIP hosts may transmit packets using any of the three packet formats:
IPv4, IPAE, or SIP.  IPAE/SIP hosts must be able to receive packets in all
three packet formats.

4.2.1   SIP Format

The SIP header is a revision of IP version 4 header format SIP shares all of
the datalink layer encapsulations that have been defined for IPv4.  A new IP
version number differentiates SIP from IP version 4. This is the format of the
SIP header:

           0 3           15        23        31
          +---+------------+---------+---------+ 0
          |V=6|          RESERVED              |
          +---+------------+---------+---------+ 4
          |   DATA LENGTH  | HOP LMT | PAYLOAD |
          +----------------+---------+---------+ 8
          |                                    |
          +      Global IP Source Address      +
          |                                    |
          +------------------------------------+ 12
          |                                    |
          +    Global IP Destination Address   +
          |                                    |
          +------------------------------------+ 16
          |            Data                    |
          |                                    |

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IP Address Encapsulation (IPAE)

4.2.2   IPAE Format

The IPAE packet format uses the SIP header encapsulated within an IPv4 header.
This is the format of the IPAE header, with relevant portions of the IP header

           0 3   7       15        23        31
--------- +---+---+--------+---------+---------+  0
          |V=4|            |      Length       |
          +---+------------+-------------------+  4
          |                                    |
 Current  +-------+--------+-------------------+  8
   IP     |  TTL  |PTCL=SIP|                   |
  Header  +-------+--------+-------------------+ 12
          |        Source IP Address           |
          +------------------------------------+ 16
          |       Destination IP Address       |
          +------------------------------------+ 20
          |      IP Options (if present)       |
          |                                    |
--------- +---+------------+---------+---------+ 20+n0
          |V=6|          RESERVED              |
          +---+------------+---------+---------+ 24+n0
          |   DATA LENGTH  | HOP LMT | PAYLOAD |
 Extended +-------+--------+-------------------+ 28+nO
   IP     |                                    |
  Header  +      Global IP Source Address      +
          |                                    |
  (SIP)   +------------------------------------+ 36+nO
          |                                    |
          +    Global IP Destination Address   +
          |                                    |
 -------- +------------------------------------+ 44+nO
          |            Data                    |
          |                                    |

        Note:   nO is size of IP options in octets.

When the IPAE format is used, the destination and source IP addresses located
in the IPv4 header target the next hop IPAE/SIP system along the path to the
destination.  In both the SIP and IPAE formats, the global IP source and
destination addresses identify the end systems. It is the global source and
destination addresses located in the SIP header that are used by UDP and TCP
to uniquely identify transport endpoints.

Most of the features of IPv4, including options and fragmentation and

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reassembly, are available in SIP.  Thus it is possible to map an IPv4 packet
into an IPAE or SIP packet and vice versa.  Options are carried in SIP by
using defined Payload values and the last SIP Option header contains the
actual transport Payload value.  When Options are present, the Global
destination address references the host that is to process the option.

4.3.    ICMP & IGMP

The SIP specification defines any required ICMP and IGMP changes or
extensions.  Use of IPAE adds no further requirements, except for relaying
ICMP messages from IP-only routers to IPAE/SIP source hosts.

An IPAE datagram that traverses an IP network may result in having an IP-only
interior router generate an ICMP error message.  The router will specify a
destination for the ICMP datagram which is, in reality, the previous IPAE
border router, rather than the originating IPAE (or SIP) host.  The border
router must function as an ICMP relaying service, when possible, as discussed
in the section on Border Routers, below.

4.4.    Routing Protocol(s)

IPAE uses SIP and IP routing mechanisms.  An administrative domain is free to
use any acceptible IP routing protocol, among is interior routers, with an
appropriate rule for deriving 32-bit addresses from the larger SIP addresses.

4.5.    Transport & Above

IPAE imposes no special requirements on applications or transport, except for
pseudo-header checksum calculation.  Any development of a larger IP address
will directly affect programming interfaces and some application protocols,
since they use current IP addresses directly.  Primarily, enhancements appears
to be straightforward and simply need to handle the larger string, often
simply as an uninterpreted string.

A node may implement IPAE (i.e., SIP over IP) as a special network-level
interface or may make it equivalent to a subnetwork-/link-level option for
SIP.  In the former case, the transport layer needs to select IPAE from the
set of alternatives, including IP and SIP.  In the latter case, the transport
layer selects a generic internet layer which, in turn, chooses IP, SIP or IPAE
to get to the next (or final) SIP node.

TCP and UDP must be modified somewhat in order to be layered above IPAE and

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SIP.  Changes are needed to the "pseudo-header" used in the checksum
algorithm, and to TCP's connection or socket identification algorithm.

4.5.1   Pseudo header checksum

The TCP spec [RFC793] and the UDP spec [RFC768] both define a checksum that
covers the data portion of the segment along with a 96-bit "pseudo header"
that includes the IP source and destination addresses, protocol ID and length
fields from the IP header.  Including this pseudo header in the transport
checksum protects the transport layer against misrouted segments.

The pseudo header used in the transport checksum when TCP and UDP are layered
above IP can viewed logically as this:

             0      7       15                31
            |          source address           |
            |        destination address        |
            |  zero  |protocol|       length    |

Inclusion of a pseudo header that covers the addresses in the checksum of TCP
and UDP layered above IPAE and SIP is even more important since the SIP header
is not checksummed.

When communicating with other IPv4/IPAE/SIP hosts, the TCP and UDP pseudo
header includes the complete global source and destination addresses.  But
since the IPAE mechanism allows IPv4 packets to be translated into IPAE or SIP
and vice-versa, the IPv4/IPAE/SIP host must implement a compatible pseudo-
header checksum when the packets it receives originated from, or the packets
it is sending are destined for, an IPv4 host.  The IPv4/IPAE/SIP pseudo header
checksum algorithm must cover only the low-order 32 bits of the global
addresses when it is communicating with an IPv4 host.  When transmitting, the
IPv4/IPAE/SIP host can use the C-bit of the global destination address to
determine whether the peer is an IPv4 host.  When receiving, the C-bit of the
global source address can be used.

When communicating with an IPv4 host using the IPv4 packet format directly
(e.g., an IPv4 host on the same subnet), the 96 bit pseudo header shown above
is used.

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When sending or receiving packets in the IPAE or SIP format to another
IPv4/IPAE/SIP host the following pseudo header is used when the processing
node is:

1)  Transmitting and C-bit of Global Destination Address is 0, or

2)  Receiving and C-bit of Global Source Address is 0

             0      7       15                31
            |  zero  |protocol|       length    |
            |                                   |
            +    Global source address          +
            |                                   |
            |                                   |
            +    Global destination address     +
            |                                   |

                  IPAE/SIP peer pseudo header

When sending or receiving packets in the IPAE or SIP format to an IPv4 host
the following pseudo header is used when the processing node is:

1)  Transmitting and C-bit of Global Destination Address is 1, or

2)  Receiving and C-bit of Global Destination Address is 1.

             0      7       15                31
            |  zero  |protocol|       length    |
            |               zero                |
            |   32-LSB of global source addr    |
            |               zero                |
            | 32-LSB of global destination addr |

                  IPAE/SIP - IPv4 compatible

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4.5.2   TCP Connection ID

TCP uses the concatenation of local and remote IP address with local and
remote port number to uniquely identify a connection.  TCP uses the term
"socket" to identify one endpoint of a connection.  The local socket is
identified by the local IP address and local port number, while the remote
socket is identified by the remote IP address and remote port number.

In processing received segments and ICMP error messages, TCP must use the
destination IP address and port number -- and possibly the source IP address
and port number -- from the received segment in a lookup in its list of
connection blocks in order to find a matching socket or connection.  When
communicating with another IPv4/IPAE/SIP host, TCP host must use the full
global source and destination addresses to identify connections and sockets.
But when communicating with IPv4 hosts using the IPAE or SIP packet formats,
TCP must use only the low order 32 bits of global source and destination to
identify the connection.

This requires a change to TCP's connection block lookup for received segments.
Assuming that TCP keeps a single list of connection blocks identifying
connections to both IPv4 hosts and IPv4/IPAE/SIP hosts, the change can be
summarized like this:

1)  If the format of the received packet is IPv4,
       use the source and destination IP addresses in the
       received packet to compare with the low-order 32 bits of
       the global source and destination address in TCP's
       connection block list,

2)  If the format of the received packet is IPAE or SIP,

    2a)  If the C-bit of the global source address is 1,
            use the low-order 32 bits of the global source and
            destination addresses in the packet to compare the
            low-order 32 bits of the global source and
            destination address in the connection block list,

    2b)  If the C-bit of the global source address is 0,
            use the full global source and destination addresses
            in the packet to compare with the full global source
            and destination addresses in the connection block

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IP Address Encapsulation (IPAE)

4.6.    Subnetwork & Below

IPAE, as SIP-over-IP, employs standard IP-over-media capabilities.  SIP-over-
media will use the same mechanisms as IP currently uses.  Hence, there are no
changes to use of ARP, or the like.  Note that host references internal to an
administrative domain will require 32-bits or less, as they do now, so that
the upper 32-bits of SIP address space are not required for subnetwork

4.7.    Network Management

Management within an AD can continue to operate, without change, since nodes
retain their IP addresses.  For global management, MIB specifications must be
enhanced to accomodate the larger addresses.  There is no expectation of
additional protocol changes.


By keeping the semantic details of the new IP and the old IP closely related,
it is possible to map between the header fields of either, greatly
facilitating support of internetwork-level translation gateways.  Three cases
need to be supported:  Turning an IP datagram into a SIP or IPAE (SIP-over-IP)
datagram, generation of an IP header from a SIP header, and general handling
of a received IPAE (SIP-over-IP) datagram.

5.1.    SIP Derived from IP Datagrams

If a router supports IPAE for hosts that use only IP, then the router is a
border IPAE router that also provides internetwork-level translation
gatewaying.  That is, it turns IP datagrams into SIP datagrams.  It may also
then send the SIP datagram onward, within an IP encapsulation, but this is a
separate function from gateway translation.

On receipt of an IP datagram that is determined to be destined for a SIP host
or that is otherwise in need of a SIP header, the gateway module will map IP
fields into SIP fields as follows:

Hop Limit:  The value of the IP Time To Live field shall be copied into SIP
Hop Limit field.  (This presumes that the router has already performed its own
decrement to the IP TTL field.)  Though TTL has slightly different semantics,

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there is no need to perform a more complicated translation.

Payload:  The value of the IP Protocol field shall be copied into the SIP
Payload field.

Source Address:  The value of the Source IP Address shall be mapped to a SIP
Global Source Address, as appropriate.  Usually, this will require pre-pending
a constant value to the Source's IP address.

Destination Address:  The value of the Source IP Address shall be mapped to
the SIP Global Destination Address, as appropriate.  Border routers will
support a table for such mappings.  While IP addresses remain globally unique,
this table can be maintained through the Domain Name Service or other
coordinated Internet service.

Options:  If the IP datagram contains options, then each shall be mapped to
the appropriate SIP option, as appropriate.  Options which do not map are
dropped.  Note that creation of SIP options alters the value of the SIP
Payload value, placing the actual value into the Payload field of the SIP
Option mini-layer.

Other fields:  All remaining IP fields are ignored.

5.2.    IP Derived from SIP Datagrams

When a border router needs to create an IP header, either for the purpose of
IPAE encapsulation to achieve transit through an IP-only domain, or to
translate the SIP header for receipt by an IP-only host, the router shall
perform the following mappings:

TTL:  The value of the SIP Hop Limit field shall be copied into the IP Time To
Live field.  (This presumes that the router has already performed its own
decrement to the SIP Hop Limit field.)

Protocol:  The value of the SIP Payload field shall be copied into the IP
Protocol field.

Source Address:  The value of the SIP Global Source Address shall be mapped to
the IP address of the source, if the router has determined that the datagram
is traversing its final IPAE hop, that is, if it being delivered to the
recipient host, or if it is being fully converted to an IP-only datagram.
While IP addresses remain globally unique, the value of the IP Source Address
field is the value of the lower 32-bits of the SIP Global Source Address.

If the IP header is to serve an encapsulation function, with the SIP header
being retained, then the Source Address shall contain the IP address of the

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border router that is creating the IP header.

Destination Address:  The value of the SIP Global Destination Address shall be
mapped to the IP address of the destination, if the router has determined that
the datagram is traversing its final IPAE hop, that is, if it is being
delivered to the recipeint host, or if it is being fully converted to an IP-
only datagram.  While IP addresses remain globally unique, the value of the IP
Destination Address field is the value of the lower 32-bits of the SIP Global
Destination Address.

If the IP headers is to serve an encapsulation function, with the SIP header
being retained, then the Destination Address shall contain the IP address of
the next IPAE-knowledgeable hop.

5.3.    Receipt of IPAE Datagrams

An IPAE datagram contains a SIP header and an IP header.  The IP header is for
the purpose of permitting transit of the SIP datagram through IP-only
networks.  Hence, the IP header should not be viewed as containing the end-to-
end information.  However, the similarity between SIP/IP relationship and
IP/Link relationship has one significant difference:  The SIP header needs to
reflect changes to the IP header that occur during transit.

In particular, an IPAE router needs to update the SIP header fields in the
following manner:

Hop Limit:  The value of the IP Time to Live field is copied into the SIP Hop
Limit field.  Hence, the SIP Hop Limit count must be adequate to count both
SIP-knowledgeable and IP-only hops.

Other fields:  Values in all other IP fields may be ignored.


6.1.    Hosts

An IPAE host supports IP, SIP and SIP-over-IP.  An originating IPAE host must
be able to derive an IP header from the SIP header, in order to create the
SIP/IP encapsulation.

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IP Address Encapsulation (IPAE)

6.2.    Interior Routers

For IPAE operation, it is presumed that the interior routers of an
administrative domain are not IPAE-aware and hence need no modification, since
they will transmit the IPAE datagram on the basis of the IP encapsulating
datagram.  However, administrative domains are expected to convert their
routers over to use of IPAE and/or SIP, eventually, in which case their
behavior for doing IP/SIP translation, address mapping, and the like will be
similar to that of a border router.

6.3.    Border Routers

An IPAE/SIP border router performs the following basic functions:

a)  Typical router store-and-forward relaying, for SIP datagrams,

b)  Participation in local IP infrastuctures, sufficiently to appear to other
IP routers as if the border router is also an IP router,

c)  IP-base encapsulation/decapsulation of SIP datagrams, and

d)  Translation gateway creation of a SIP header based on the contents of an
IP header, and vice-versa.

SIP routing functions are discussed in [DEER92b].

Encapsulation/decapsulation functions are to be performed using the derivation
rules specified in the previous section.

It is expected that conversion of IP datagrams to SIP or IPAE datagrams will
permit the converting border router in an AD to maintain smaller routing
tables, immediately.  For IP, router tables include a source information base
with a copy of the information about all Internet nets, for each of the
router's neighbors.  For SIP, the table still needs to maintain a copy of the
information received from each neighbor, but it can be reduced to a portion of
the global hierarchy, such as all countries, as well as all "local" networks
of interest.

Currently for IPv4,  the real-time information base used to make direct
datagram forwarding decisions needs to have an entry for each network on the
Internet.  For SIP, it needs to have only the relevant portions of the
hierarchy, so that the table information about Internet neighbors can share
the same entry.

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IP Address Encapsulation (IPAE)


The relationship between IP and SIP addressing, particularly when both are
present in an IPAE datagram, appears to be the greatest source of confusion
concerning IPAE dynamics.  This section provides an extended example of the
end-to-end handling of both types of addresses as a datagram moves through the

Notation:  IP addresses are represented in the usual, four decimal fields,
separated by periods.  SIP addresses also use a decimal dotted notation,
showing Country.Metro/Provider.Site/Subscriber, followed by the site's IP
address.  The SIP portion is separated from the IP portion by a slash.
Reference to IP or SIP networks, without referencing specific nodes on those
networks, uses only the network portion of the address.  For example, a Class
C IP network would be shown as the first 3 octets of its address, such as
192.3.4.  A SIP network address would show only the first 4 octets of the 64-
bit address, indicating only the Region and Metro/Provider.

IPAE forms a set of logical networks "above" the set of IP networks, so that
one or more IP networks are the equivalent of IPAE subnetworks.  Hence, IPAE
network boundaries occur only at IPAE routers.  In this example, note that the
last system of the first line (Router IPAEx) is the same as the first system
on the second line.

This shows one intermediate IPAE hop, with an intermediate IP hop between the
originating host and the IPAE router, and another between the IPAE router and
the destination host.  The transit sequence for the datagram is:

SIP Net                        <- 1.213.77. ->         ||
IP Net                    <- 192.3.4. |  <- 128.1. ->  ||
                                      |                ||
Node:               H(IP-O)          R(IPa)           R(IPAEx)...
                     |1:213:77:5/    |  |  1:213:77:5/|
                     |      |  ||
                     |               |  |             |
                     ||  |    |
                     |               |  |             |
                     -------->--------  ------>--------
Datagram:            SIP                SIP
                     SRC:1:213:77:5:    SRC:1:213:77:5:
                     DST:1:2:2:8:       DST:1:2:2:8:
                     -----------------  -----------------
                     IP                 IP
                     SRC:     SRC:
                     DST:     DST:

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IP Address Encapsulation (IPAE)

SIP Net             ||          <- 1.002.2. ->
IP Net              ||         <- 10. | 201.5.7. ->
                    ||                |
Node:          ...R(IPAEx)          R(IPb)           H(IPAE-D)
                     |      |  ||
                     |       |  ||
                     |               |  |             |
                     ||  |    |
                     |               |  |             |
                     -------->--------  ------->-------

Datagram:            SIP                SIP
                     SRC:1:213:77:5:    SRC:1:213:77:5:
                     DST:1:2:2:8:       DST:1:2:2:8:
                     -----------------  ----------------
                     IP                 IP
                     SRC:       SRC:
                     DST:    DST:

The pattern of manipulation to observe is that the SIP source and destination
address fiels are unchanged.  The IP source and destination addresses show the
addresses of the IPAE nodes for the specific SIP hop and are unchanged when
passing through an IP-only router.


The transition plan for IPAE is based around a timeline which has a number of
milestones.  The timeline is as follows:

|                                                          |
|                                                          |

TODAY  ->  I         H          R           S    ->   FUTURE

Milestone I is the initial deployment of IPAE in selected Border Routers.
Milestone H is large scale deployment of IPAE in Hosts.  Milestone R is when
the Internet runs out of the current 32-bit IP addresses.  Milestone S
indicates large scale conversion of administrative domains to pure SIP
operations.  Each milestone in the transition plan is discussed in the

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following  sections.

The first objective of the transition plan is to install IPAE in critical
router locations.   This will reduce the size of routing tables and the load
of routing computation in the Internet.  The authors of this proposal believe
this is very important to keep the Internet growing.  It will be done without
any changes to hosts.

The second objective is to install IPAE in hosts before the current 32-bit IP
network addresses are all allocated.  This will permit an orderly transition
to a global routing and addressing scheme with a minimum of disruption to the
existing internet.

As an administrative domain comes to use IPAE universally, then it may decide
to permit pure SIP exchanges, without IPAE encapsulation.

8.1.    Initial Deployment of IPAE  (Milestone I)

The first step of IPAE deployment is to define the first SIP administrative
domains.   Initially these should be large areas of the Internet, such as
portions of a single continent.  The goal is to reduce the amount of routing
traffic and the size of routing tables by an order of magnitude or more.

The first deployment of IPAE will occur in border routers.  The border routers
will create the first SIP administrative domains.  However, at that time the
current 32-bit IP addresses will still be globally unique.  This will allow
the border routers to look up the global IP addresses from the 32-bit IP
addresses and build packets with extended IP headers for the hosts.  The hosts
will not have to implement IPAE in the first stage of deployment.  The benefit
of this stage of the deployment will be to reduce the size of routing tables
in border routers and to reduce the routing computation load on these routers.
The functions performed by the border routers are primarily of
inserting/removing SIP headers and looking up Global IP addresses based 32-bit

Several approaches are possible for the procedure to lookup the Destination
Global IP Address.  One approach is a static table kept in the border routers.
The mappings in this table would be based on the number of networks in the
Internet.  While this would be a large table towards the time the existing 32-
bit IP Address space runs out, even then it would be possible to maintain this
table within the small set of border routers.

Another approach is to use the Internet's Domain Name System to maintain this
mapping table.  The DNS is well-suited to this task and it would be a
straightforward extension to add the information required.  This approach has
the border routers periodically performa a DNS lookup to obtain recent

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additions to the routing table.  (That is, the DNS would provide a means of
maintained a distributed copy of the Internet address mappings.)  This would
require careful engineering of the border router/DNS interactions to prevent
their becoming a bottleneck.  It is expected that the size of the resulting
cache in border routers will be much smaller in size than with current
operations because most Internet traffic tends to stay in its own IPAC and
only a small percentage of total networks will pass through a border router.

It should be noted that any requirements for address mapping, whether static
or via the DNS, are in no way specific to the IPAE proposal.  Any scheme which
wishes to provide communication among old-style hosts and new-style hosts
having a different address format will require some type of address
translation.  IPAE simply makes the mapping process easier than would be
required if the new address format were entirely unrelated to old-style IP

8.2.    IPAE Deployment in Hosts  (Milestone H)

The second phase of IPAE deployment is deployment to Internet hosts.  Hosts
will need to be able to support both IPv4 Headers format and IPAE/SIP Header
format.  They will use IPv4 Headers when communicating within their
administrative domain and will use SIP Headers when communicating with hosts
in different domains.

The goal for this phase of IPAE deployment it to have the majority of Internet
hosts implement IPAE before the Internet runs out of 32-bit IP addresses.
While it is not possible to have every host implement IPAE, there is
sufficient time before Milestone R occurs that it can be implemented by the
vast majority, since the amount of new software, documentation and training
required will be quite small.  This will reduce the need for address
translation support in the next phase of deployment.

8.3.    Internet Runs Out of 32-Bit IPv4 Network Numbers (Milestone R)

At the third phase of the IPAE deployment, hosts which do not implement IPAE
will be able to communicate directly only with hosts in their own
administrative domain, via IP.  There are several possible methods to extend
their connectvity if it is necessary to provide these hosts with global
Internet service.  One straightforward approach is to stage their
communication through a host which supports IPAE.  This would permit services
such as electronic mail and news to operate transparently.  Even in today's
Internet, mail service often operates in this fashion. Other services such as
Telnet and FTP would require an application-level forwarding agent or double

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Another approach is to perform automatic address mapping.  Various schemes are
possible to support this and IPAE imposes no special constraints on the

8.4.    Administrative Domains Fully Convert to SIP (Milestone S)

The final stage of IPAE deployment is the demise of IPAE usage within
individual administrative domains.  As neighboring domains support pure SIP,
then the border routers between them can be configured to omit the IPAE SIP-
over-IP encapsulation for inter-domain exchanges.  It is important to note,
however, that decisions to permit pure SIP operation are entirely at the
discretion of the local administrations, rather than requiring larger,
Internet coordination.


[BRAD89a]       Braden, R.T., RFC 1127:  Perspective on the Host Requirements RFCs.
1989 October.

[BRAD89b]       Braden, R.T., ed., RFC 1122:  Requirements for Internet hosts -
communication layers.    1989 October.

[BRAD89c]       Braden, R.T.,ed., RFC 1123:  Requirements for Internet hosts -
application and support.    1989 October.

[DEER92a]       Deering, S.  Simple Internet  Protocol (SIP) Specification.  1992

[DEER92b]       Deering, S.  Simple Internet  Protocol (SIP) Addressing and
Routing.  1992 November.

[E.163] CCITT, Numbering Plan for the International Telephone Services.

[HIND92a]       Hinden, B., "New Scheme for Internet Routing and Addressing
(ENCAPS)", Email message to Big-Internet mailing list, March 16, 1992

[HIND92b]       A Proposal for IP Address Encapsulation (IPAE):  A Compatible
Version of IP with Large Addresses; (draft, June 1992)

[WOOD92]        Woodburn, R & D. Mills,  "A Scheme for an Internet Encapsulation
Protocol:  Version 1", RFC 1241.  July 1991.

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10.     CONTACTS

David H. Crocker  <dcrocker@mordor.stanford.edu>

The Branch Office
675 Spruce Dr.
Sunnyvale, CA  94086

+1 408 246 8253

Robert Hinden  <hinden@eng.sun.com>

Sun Microsystems, Inc.
2550 Garcia Avenue
Mountain View, CA  94043-1100

+1 415 336 2082

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