<pre>Network Working Group                                        R. Ullmann
Request for Comments: 1475                 Process Software Corporation
                                                              June 1993


                        <span class="h1">TP/IX: The Next Internet</span>

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard.  Discussion and
   suggestions for improvement are requested.  Please refer to the
   current edition of the "IAB Official Protocol Standards" for the
   standardization state and status of this protocol.  Distribution of
   this memo is unlimited.

Abstract

   The first version of this memo, describing a possible next generation
   of Internet protocols, was written by the present author in the
   summer and fall of 1989, and circulated informally, including to the
   IESG, in December 1989.  A further informal note on the addressing,
   called "Toasternet Part II", was circulated on the IETF mail list
   during March of 1992.

Table of Contents

   <a href="#section-1">1</a>.       Introduction . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
   <a href="#section-1.1">1.1</a>       Objectives  . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
   <a href="#section-1.2">1.2</a>       Philosophy  . . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
   <a href="#section-2">2</a>.       Internet numbers . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
   <a href="#section-2.1">2.1</a>       Is 64 Bits Enough?  . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
   <a href="#section-2.2">2.2</a>       Why version 7?  . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
   <a href="#section-2.3">2.3</a>       The version 7 IP address  . . . . . . . . . . . . . <a href="#page-7">7</a>
   <a href="#section-2.4">2.4</a>       AD numbers  . . . . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
   <a href="#section-2.5">2.5</a>       Mapping of version 4 numbers  . . . . . . . . . . . <a href="#page-8">8</a>
   <a href="#section-3">3</a>.       IP: Internet datagram protocol . . . . . . . . . . . <a href="#page-9">9</a>
   <a href="#section-3.1">3.1</a>       IP datagram header format . . . . . . . . . . . .  <a href="#page-10">10</a>
   <a href="#section-3.1.1">3.1.1</a>       Version . . . . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
   <a href="#section-3.1.2">3.1.2</a>       Header length . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
   <a href="#section-3.1.3">3.1.3</a>       Time to live  . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
   <a href="#section-3.1.4">3.1.4</a>       Total datagram length . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3.1.5">3.1.5</a>       Forward route identifier  . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3.1.6">3.1.6</a>       Destination . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3.1.7">3.1.7</a>       Source  . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3.1.8">3.1.8</a>       Protocol  . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3.1.9">3.1.9</a>       Checksum  . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-3.1.10">3.1.10</a>      Options . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>



<span class="grey">Ullmann                                                         [Page 1]</span></pre>
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<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   <a href="#section-3.2">3.2</a>       Option Format . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-3.2.1">3.2.1</a>       Class (C) . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-3.2.2">3.2.2</a>       Copy on fragmentation (F) . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-3.2.3">3.2.3</a>       Type  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-3.2.4">3.2.4</a>       Length  . . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-3.2.5">3.2.5</a>       Option data . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-3.3">3.3</a>       IP options  . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-3.3.1">3.3.1</a>       Null  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-3.3.2">3.3.2</a>       Fragment  . . . . . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
   <a href="#section-3.3.3">3.3.3</a>       Last Fragment . . . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
   <a href="#section-3.3.4">3.3.4</a>       Don't Fragment  . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-3.3.5">3.3.5</a>       Don't Convert . . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-3.4">3.4</a>       Forward route identifier  . . . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-3.4.1">3.4.1</a>       Procedure description . . . . . . . . . . . . .  <a href="#page-15">15</a>
   <a href="#section-3.4.2">3.4.2</a>       Flows . . . . . . . . . . . . . . . . . . . . .  <a href="#page-17">17</a>
   <a href="#section-3.4.3">3.4.3</a>       Mobile hosts  . . . . . . . . . . . . . . . . .  <a href="#page-17">17</a>
   <a href="#section-4">4</a>.       TCP: Transport protocol  . . . . . . . . . . . . .  <a href="#page-18">18</a>
   <a href="#section-4.1">4.1</a>       TCP segment header format . . . . . . . . . . . .  <a href="#page-18">18</a>
   <a href="#section-4.1.1">4.1.1</a>       Data offset . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
   <a href="#section-4.1.2">4.1.2</a>       MBZ . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
   <a href="#section-4.1.3">4.1.3</a>       Flags . . . . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
   <a href="#section-4.1.4">4.1.4</a>       Checksum  . . . . . . . . . . . . . . . . . . .  <a href="#page-19">19</a>
   <a href="#section-4.1.5">4.1.5</a>       Source port . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.1.6">4.1.6</a>       Destination port  . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.1.7">4.1.7</a>       Sequence  . . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.1.8">4.1.8</a>       Acknowledgement . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.1.9">4.1.9</a>       Window  . . . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.1.10">4.1.10</a>      Options . . . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.2">4.2</a>       Port numbers  . . . . . . . . . . . . . . . . . .  <a href="#page-20">20</a>
   <a href="#section-4.3">4.3</a>       TCP options . . . . . . . . . . . . . . . . . . .  <a href="#page-21">21</a>
   <a href="#section-4.3.1">4.3.1</a>       Option Format . . . . . . . . . . . . . . . . .  <a href="#page-21">21</a>
   <a href="#section-4.3.2">4.3.2</a>       Null  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-21">21</a>
   <a href="#section-4.3.3">4.3.3</a>       Maximum Segment Size  . . . . . . . . . . . . .  <a href="#page-21">21</a>
   <a href="#section-4.3.4">4.3.4</a>       Urgent Pointer  . . . . . . . . . . . . . . . .  <a href="#page-21">21</a>
   <a href="#section-4.3.5">4.3.5</a>       32 Bit rollover . . . . . . . . . . . . . . . .  <a href="#page-21">21</a>
   <a href="#section-5">5</a>.       UDP: User Datagram protocol  . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1">5.1</a>       UDP header format . . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1.1">5.1.1</a>       Data offset . . . . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1.2">5.1.2</a>       MBZ . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1.3">5.1.3</a>       Checksum  . . . . . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1.4">5.1.4</a>       Source port . . . . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1.5">5.1.5</a>       Destination port  . . . . . . . . . . . . . . .  <a href="#page-22">22</a>
   <a href="#section-5.1.6">5.1.6</a>       Options . . . . . . . . . . . . . . . . . . . .  <a href="#page-23">23</a>
   <a href="#section-6">6</a>.       ICMP . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-23">23</a>
   <a href="#section-6.1">6.1</a>       ICMP header format  . . . . . . . . . . . . . . .  <a href="#page-23">23</a>
   <a href="#section-6.2">6.2</a>       Conversion failed ICMP message  . . . . . . . . .  <a href="#page-23">23</a>
   <a href="#section-7">7</a>.       Notes on the domain system . . . . . . . . . . . .  <a href="#page-25">25</a>
   <a href="#section-7.1">7.1</a>       A records . . . . . . . . . . . . . . . . . . . .  <a href="#page-25">25</a>



<span class="grey">Ullmann                                                         [Page 2]</span></pre>
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<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   <a href="#section-7.2">7.2</a>       PTR zone  . . . . . . . . . . . . . . . . . . . .  <a href="#page-25">25</a>
   <a href="#section-8">8</a>.       Conversion between version 4 and version 7 . . . .  <a href="#page-25">25</a>
   <a href="#section-8.1">8.1</a>       Version 4 IP address extension option . . . . . .  <a href="#page-26">26</a>
   <a href="#section-8.1.1">8.1.1</a>     Option format . . . . . . . . . . . . . . . . . .  <a href="#page-26">26</a>
   <a href="#section-8.2">8.2</a>      Fragmented datagrams . . . . . . . . . . . . . . .  <a href="#page-26">26</a>
   <a href="#section-8.3">8.3</a>      Where does the conversion happen?  . . . . . . . .  <a href="#page-27">27</a>
   <a href="#section-8.4">8.4</a>      Hybrid IPv4 systems  . . . . . . . . . . . . . . .  <a href="#page-28">28</a>
   <a href="#section-8.5">8.5</a>      Maximum segment size in TCP  . . . . . . . . . . .  <a href="#page-28">28</a>
   <a href="#section-8.6">8.6</a>      Forwarding and redirects . . . . . . . . . . . . .  <a href="#page-28">28</a>
   <a href="#section-8.7">8.7</a>      Design considerations  . . . . . . . . . . . . . .  <a href="#page-28">28</a>
   <a href="#section-8.8">8.8</a>      Conversion from IPv4 to IPv7 . . . . . . . . . . .  <a href="#page-29">29</a>
   <a href="#section-8.9">8.9</a>      Conversion from IPv7 to IPv4 . . . . . . . . . . .  <a href="#page-30">30</a>
   <a href="#section-8.10">8.10</a>     Conversion from TCPv4 to TCPv7 . . . . . . . . . .  <a href="#page-31">31</a>
   <a href="#section-8.11">8.11</a>     Conversion from TCPv7 to TCPv4 . . . . . . . . . .  <a href="#page-32">32</a>
   <a href="#section-8.12">8.12</a>     ICMP conversion  . . . . . . . . . . . . . . . . .  <a href="#page-33">33</a>
   <a href="#section-9">9</a>.       Postscript . . . . . . . . . . . . . . . . . . . .  <a href="#page-33">33</a>
   <a href="#section-10">10</a>.      References . . . . . . . . . . . . . . . . . . . .  <a href="#page-34">34</a>
   <a href="#section-11">11</a>.      Security Considerations  . . . . . . . . . . . . .  <a href="#page-35">35</a>
   <a href="#section-12">12</a>.      Author's Address . . . . . . . . . . . . . . . . .  <a href="#page-35">35</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This memo presents the specification for version 7 of the Internet
   Protocol, as well as version 7 of the TCP and the user datagram
   protocol.  Version 7 has been designed to address several major
   problems that have arisen as version 4 has evolved and been deployed,
   and to make a major step forward in the datagram switching and
   forwarding architecture of the Internet.

   The major problems are threefold.  First, the address space of
   version 4 is now seen to be too small.  While it was viewed as being
   almost impossibly large when version 4 was designed, two things have
   occurred to create a problem.  The first is a success crisis:  the
   internet protocols have been more widely used and accepted than their
   designers anticipated.  Also, technology has moved forward, putting
   microprocessors into devices not anticipated except as future dreams
   a decade ago.

   The second major problem is a perceived routing explosion.  The
   present routing architecture of the internet calls for routing each
   organization's network independently.  It is becoming increasingly
   clear that this does not scale to a universal internet.  While it is
   possible to route several billion networks in a flat, structureless
   domain, it is not desireable.

   There is also the political administrative issue of assigning network
   numbers to organizations.  The version 4 administrative system calls
   for organizations to request network assignments from a single



<span class="grey">Ullmann                                                         [Page 3]</span></pre>
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<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   authority.  While to some extent this has been alleviated by
   reserving blocks to delegated assignments, the address space is not
   large enough to do this in the necessary general case, with large
   blocks allocated to (e.g.) national authority.

   The third problem is the increasing bandwidth of the networks and of
   the applications possible on the network.  The TCP, while having
   proven useful on an unprecedented range of network speeds, is now the
   limiting factor at the highest speeds, due to restrictions of window
   size, sequence-space, and port numbers.  These limitations can all be
   addressed by increasing the sizes of the relevant fields.  See
   [<a href="./rfc1323" title="&quot;TCP Extensions for High Performance&quot;">RFC1323</a>].

   There is also an opportunity to move the technology forward, and take
   advantage of a combination of the best features of the hop-by-hop
   connectionless forwarding of version 4 (and CLNP) as well as the
   pre-established paths of version 5 (and, e.g., the OSI CONS).

   Internet Version 7 includes four major areas of improvement, while at
   the same time retaining interoperation with version 4 with a small
   amount of conversion knowledge imposed on version 7 hosts and
   routers.

      o  It increases the address fields to 64 bits, with sufficient
         space for visible future expansion of the internet.

      o  It adds a numbering layer for administrations, above the
         organization or network layer, as well as providing more
         space for subnetting within organizations.

      o  It increases the range of speeds and network path delays over
         which the TCP will operate satisfactorily, as well as the
         number of transactions in bounded time that can be served by
         a host.

      o  Finally, it provides a forward route identifier in each
         datagram, to support extremely fast path, circuit, or
         flow-based forwarding, or any desired combination, while
         preserving hop-by-hop connectivity.

   The result is not just a movement sideways, deploying a new network
   layer protocol to patch current problems.  It is a significant step
   forward for network layer technology,








<span class="grey">Ullmann                                                         [Page 4]</span></pre>
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<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>  Objectives</span>

   The following are some of the objectives of the design.

  o  Use what has been learned from the IP version 4 protocol, fixing
     things that are troublesome, and not fixing that which is not
     broken.

  o  Retain the essential "look and feel" of the Internet protocol
     suite.  It has been very successful, and one doesn't argue with
     success.

  o  Not introduce concepts that the Internet has shown do not belong
     in the protocol definition.  Best example:  we do not want to add
     any kind of routing information into the addressing, other than
     the administrative hierarchy that has sometimes proved useful.
     Note that the one feature in version 4 addressing (the class
     system) designed to aid routing is now the most serious single
     problem.

  o  Allow current hosts to interoperate, if not universally, at least
     within an organization or larger area for the indefinite future.
     There will be version 4 hosts for 10-15 years into the future,
     the Internet must remain on good terms with them.

  o  Likewise, we must not impose the new version, telling sites they
     must convert to stay connected.  People resist imposed solutions.
     It must not be marketed as something different from IPv4; the
     differences must be down-played at every opportunity.

  o  The design must allow individual hosts and routers to be upgraded
     effectively at random, with no transition plan constraints.

  o  The design must not require renumbering the Internet.  The
     administrative work already accomplished is immense, if it is to
     be done again it will be in assigning NSAPs.

  o  It must allow IPv4 hosts to interoperate without any reduction in
     function, without any modification to their software or
     configuration.  (Universal connectivity will be lost by IPv4
     hosts, but they must be able to continue operating within their
     organization at least.)

  o  It must permit network layer state-free translation of datagrams
     between IPv4 and IPv7; this is important to the previous point,
     and essential to early testing and transitional deployment.

  o  It must be a competent alternative to CLNP.



<span class="grey">Ullmann                                                         [Page 5]</span></pre>
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<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


  o  It must not involve changing the semantics of the network layer
     service in any way that invalidates the huge amount of work that
     has gone into understanding how TCP (for example) functions in
     the net, and the implementation of that understanding.

  o  It must be defined Real Soon; the window of opportunity is almost
     closed.  It will take vendors 3 years to deploy from the time the
     standard is rock-solid concrete.

   I believe all of these are accomplishable in a consistent, well-
   engineered solution, and all are essential to the survival of the
   Internet.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>  Philosophy</span>

   Protocols should become simpler as they evolve.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Internet numbers</span>

   The version 4 numbering system has proven to be very flexible,
   (mostly) expandable, and simple.  In short:  it works.  There are two
   problems, neither serious when this specification was first developed
   in 1988 and 1989, but have as expected become more serious:

      o  The division into network, and then subnet, is insufficient.
         Almost all sites need a network assignment large enough to
         subnet.  At the top of the hierarchy, there is a need to
         assign administrative domains.

      o  As bit-packing is done to accomplish the desired network
         structure, the 32 bit limit causes more and more aggravation.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>  Is 64 Bits Enough?</span>

   Consider:  (thought experiment) 32 bits presently numbers "all" of
   the computers in the world, and another 32 bits could be used to
   number all of the bytes of on-line storage on each computer.  (Most
   have a lot less than 4 gigabytes on-line, the ones that have more
   could be notionally assigned more than one address.)

   So: 64 bits is enough to number every byte of online storage in
   existence today, in a hierarchical structured numbering plan.

   Another way of looking at 64 bits:  it is more than 2 billion
   addresses for each person on the planet.  Even if I have
   microprocessors in my shirt buttons I'm not going to have that many.
   32 bits, on the other hand, was never going to be sufficient:  there
   are more than 2^32 people.



<span class="grey">Ullmann                                                         [Page 6]</span></pre>
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<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>  Why version 7?</span>

   It was clearly recognized at the start of this project in 1988 that
   making the address 64 bits implies a new IP header format, which was
   called either "TP/IX" or "IP version 7"; there wasn't anything magic
   about the number 7, I made it up.  Version 4 is the familiar current
   version of IP.  Version 5 is the experimental ST (Stream) protocol.
   ST-II, a newer version of ST, uses the same version number, something
   I was not aware of until recently; I suspected it might have been
   allocated 6.  Besides, I liked 7.

   Apparently (as reported by Bob Braden) the IAB followed much the same
   logic, and may have had the idea planted by the mention of version 7
   in the "Toasternet Part II" memo.  The IAB in June 1992 floated a
   proposal that CLNP, or a CLNP-based design, be Internet Version 7.
   (And promptly got themselves toasted.) However, close inspection of
   the bits shows that CLNP is clearly version 8.

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>  The version 7 IP address</span>

   The Version 7 IP 64 bit address looks like:

    +-------+-------+-------+-------+-------+-------+-------+-------+
    |      Admin Domain     |        Network        |     Host      |
    +-------+-------+-------+-------+-------+-------+-------+-------+

   Note:  the boundary between "network" and "host" is no more fixed
   than it is today; each (sub)network will have its own mask.  Just as
   the mask today can be anywhere from FF00 0000 (8/24) to FFFF FFFC
   (30/2), the mask for the 64 bit address can reasonably be FFFF FF00
   0000 0000 (24/40) to FFFF FFFF FFFF FFFC (62/2).

   The AD (Administrative Domain), identifies an administration which
   may be a service provider, a national administration, or a large
   multi-organization (e.g.  a government).  The idea is that there
   should not be more than a few hundred of these at first, and
   eventually thousands or tens of thousands at most.  (But note that we
   do not introduce a hard limit of 2^16 here; this estimate may be off
   by a few orders of magnitude.) Since only 1/4th of the address space
   is initially used (first two bits are 01), the remainder can then be
   allocated in the future with more information available.

   Most individual organizations would not be ADs.  In the short term,
   ADs are known to the "core routing"; it pays to keep the number
   smallish, a few thousand given current routing technology.  In the
   long term, this is not necessary.  Big administrations (i.e., with
   tens of millions of networks) get small blocks where needed, or
   additional single AD numbers when needed.



<span class="grey">Ullmann                                                         [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   While the AD may be used for last resort routing (with a 24/40 mask),
   it is primarily only an administrative device.  Most routing will be
   done on the entire 48 bit AD+network number, or sub and super-sets of
   those numbers.  (I.e., masks between about 32/32 and 56/8.)

   Some ADs (e.g., NSF) may make permanent assignments; others (such as
   a telephone company defining a network number for each subscriber
   line) may tie the assignment to such a subscription.  But in no case
   does this require traffic to be routed via the AD.

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>  AD numbers</span>

   AD numbers are allocated out of the same numbering space as network
   numbers.  This means that a version 4 address can be distinguished
   from the first 32 bits of a version 7 address.  This is useful to
   help prevent the inadvertent use of the first half of the longer
   address by a version 4 host.

   There is a non-trivial amount of software that assumes that an "int"
   is the same size and shape as an IP address, and does things like
   "ipaddr = *(int *)ptr".  This usage has always been incorrect, but
   does occur with disturbing frequency.  As IPv7 8 byte addresses
   appear in the application layers, this software will find those
   addresses unreachable; this is preferable to interacting with a
   random host.

   One possible method would be to allocate ADs in the range 96.0.0 to
   192.255.255, using the top 1/4 of the version 4 class A space.  It is
   probably best to allocate the first component downwards from 192, so
   that the boundary between class A and AD can be moved if desired
   later.  This initial allocation provides for 2031616 ADs, many more
   than there should be even in full deployment.

   Eventually, both AD and network will use the full 24 bit space
   available to them.  Knowledge of the AD range should not be coded
   into software.  If it was coded in, that software would break when
   the entire 24 bit space is used for ADs.  (This lesson should have
   been learned from CIDR.)

<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>  Mapping of version 4 numbers</span>

   Initially, all existing Internet numbers are defined as belonging to
   the NSF/Internet AD, number 192.0.0.








<span class="grey">Ullmann                                                         [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   The mapping from/to version 4 IP addresses:

    +-------+-------+-------+-------+-------+-------+-------+-------+
    |      Admin Domain     |        Network        |     Host      |
    +-------+-------+-------+-------+-------+-------+-------+-------+
     [  fixed at A0 00 00  ] [ 1st 24 bits of V4 IP]   [1]   [last 8]

   So, for example, 192.42.95.15 (V4) becomes 192.0.0.192.42.95.1.15.

   And the "standard" loopback interface address becomes
   192.0.0.127.0.0.1.1 (I can see explaining that in 2015 to someone
   born in 1995.)

   The present protocol multicast (192.0.0.224.x.y.1.z) and loopback
   addresses are permanently allocated in the NSF AD.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  IP:  </span>Internet datagram protocol

   The Internet datagram protocol is revised to expand some fields (most
   notably the addresses), while removing and relegating to options all
   fields not universally useful (imperative) in every datagram in every
   environment.

   This results in some simplification, a length less than twice the
   size of IPv4 even though most fields are doubled in size, and an
   expanded space for options.

   There is also a change in the option philosophy from IPv4:  it
   specified that implementation of options was not optional, what was
   optional was the existence of options in any given datagram.  This is
   changed in IPv7:  no option need be implemented to be fully
   conformant.  However, implementations must understand the option
   classes; and a future Host Requirements specification for hosts and
   routers used in the "connected Internet" may require some options in
   its profile, e.g., Fragment would probably be required.

   Digression:  In IPv4, options are often "considered harmful".  It is
   the opinion of the present author that this is because they are
   rarely needed, and not designed to be processed rapidly on most
   architectures.  This leads to little or no attempt to improve
   performance in implementations, while at the same time enormous
   effort is dedicated to optimization of the no-option case.  IPv7 is
   expected to be different on both counts.

   Fields are always aligned on their own size; the 64 bit fields on 64
   bit intervals from the start of the datagram.

   Options are all 32 bit aligned, and the null option can be used to



<span class="grey">Ullmann                                                         [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   push a subsequent option (or the transport layer header) into 64 bit
   or 64+32 off-phase alignment as desired.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>  IP datagram header format</span>

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |version|     header length     |        time to live           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        total datagram length                                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +        forward route identifier                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +        destination address                                    +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +        source address                                         +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        protocol               |           checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        options                                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A description of each field follows.

<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>  Version</span>

   This document describes version 7 of the protocol.

<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>  Header length</span>

   The header length is a 12 bit count of the number of 32 bit words in
   the IPv7 header.  This allows a header to be (theoretically at least)
   up to 16380 bytes in length.

<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>  Time to live</span>

   The time to live is a 16 bit count, nominally in 1/16 seconds.  Each
   hop is required to decrement TTL by at least one.

   This definition should allow continuation of the useful (even though
   not entirely valid) interpretation of TTL as a hop count, while we



<span class="grey">Ullmann                                                        [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   move to faster networks and routers.  (The most familiar use is by
   "traceroute", which really ought to be directly implemented by one or
   more ICMP messages.)

   The scale factor converts the usual version 4 default TTL into a
   larger number of hops.  This is desireable because the forward route
   architecture of version 7 enables the construction of simpler, faster
   switches, and this may cause the network diameter to increase.

<span class="h4"><a class="selflink" id="section-3.1.4" href="#section-3.1.4">3.1.4</a>  Total datagram length</span>

   The 32 bit length of the entire datagram in octets.  A datagram can
   therefore be up to 4294967295 bytes in overall length.  Particular
   networks will normally impose lower limits.

<span class="h4"><a class="selflink" id="section-3.1.5" href="#section-3.1.5">3.1.5</a>  Forward route identifier</span>

   The identifier from the routing protocol to be used by the next hop
   router to find its next hop.  (A more complete description is given
   below.)

<span class="h4"><a class="selflink" id="section-3.1.6" href="#section-3.1.6">3.1.6</a>  Destination</span>

   The 64 bit IPv7 destination address.

<span class="h4"><a class="selflink" id="section-3.1.7" href="#section-3.1.7">3.1.7</a>  Source</span>

   The 64 bit IPv7 source address.

<span class="h4"><a class="selflink" id="section-3.1.8" href="#section-3.1.8">3.1.8</a>  Protocol</span>

   The transport layer protocol, e.g., TCP is 6.  The present code space
   for this layer of demultiplexing is about half full.  Expanding it to
   16 bits, allowing 65535 registered "transport" layers seems prudent.

<span class="h4"><a class="selflink" id="section-3.1.9" href="#section-3.1.9">3.1.9</a>  Checksum</span>

   The checksum is a 16 bit checksum of the entire IP header, using the
   familiar algorithm used in IPv4.

<span class="h4"><a class="selflink" id="section-3.1.10" href="#section-3.1.10">3.1.10</a>  Options</span>

   Options may follow.  They are variable length, and always 32 bit
   aligned, as discussed previously.







<span class="grey">Ullmann                                                        [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>  Option Format</span>

   Each option begins with a 32 bit header:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | C |F|    type                 |   length                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        option data                 ...          |   padding   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A description of each field:

<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>  Class (C)</span>

   This field tells implementations what to do with datagrams containing
   options they do not understand.  No implementation is required to
   implement (i.e., understand) any given option by the TCP/IP
   specification itself.

   Classes:

       0        use or forward and include this option unmodified
       1        use this datagram, but do not forward the datagram
       2        discard, or forward and include this option unmodified
       3        discard this datagram

   A host receiving a datagram addressed to itself will use it if there
   are no unknown options of class 2 or 3.  A router receiving a
   datagram not addressed to it will forward the datagram if and only if
   there are no unknown options of class 1 or 3.  (The astute reader
   will note that the bits can also be seen as having individual
   interpretations, one allowing use even if unknown, one allowing
   forwarding if unknown.)

   Note that classes 0 and 2 are imperative:  if the datagram is
   forwarded, the unknown option must be included.

   Class and type are entirely orthogonal, different implementations
   might use different classes for the same option, except where
   restricted by the option definition.

   Also note that for options that are known (implemented by) the host
   or router, the class has no meaning; the option definition totally
   determines the behavior.  (Although it should be noted that the
   option might explicitly define a class dependent behavior.)




<span class="grey">Ullmann                                                        [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>  Copy on fragmentation (F)</span>

   If the F bit is set, this option must be copied into all fragments
   when a datagram is fragmented.  If the F bit is reset (zero), the
   option must only be copied into the first (zero-offset) fragment.

<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>  Type</span>

   The type field identifies the particular option, types being
   registered as well known values in the internet.  A few of the
   options with their types are described below.

<span class="h4"><a class="selflink" id="section-3.2.4" href="#section-3.2.4">3.2.4</a>  Length</span>

   Length of the option data, in bytes.

<span class="h4"><a class="selflink" id="section-3.2.5" href="#section-3.2.5">3.2.5</a>  Option data</span>

   Variable length specified by the length field, plus 0-3 bytes of
   zeros to pad to a 32 bit boundary.  Fields within the option data
   that are 64 bits long are normally placed on the assumption that the
   option header is off-phase aligned, the usual case when the option is
   the only one present, and immediately follows the IP header.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>  IP options</span>

   The following sections describe the options defined to emulate IPv4
   features, or necessary in the basic structure of the protocol.

<span class="h4"><a class="selflink" id="section-3.3.1" href="#section-3.3.1">3.3.1</a>  Null</span>

   The null option, type 0, provides for a space filler in the option
   area.  The data may be of any size, including 0 bytes (perhaps the
   most useful case.)

   It may be used to change alignment of the following options or to
   replace an option being deleted, by setting type to 0 and class to 0,
   leaving the length and content of the data unmodified.  (Note that
   this implies that options must not contain "secret" data, relying on
   class 3 to prevent the data from leaving the domain of routers that
   understand the option.)

   Null is normally class 0, and need not be implemented to serve its
   function.







<span class="grey">Ullmann                                                        [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h4"><a class="selflink" id="section-3.3.2" href="#section-3.3.2">3.3.2</a>  Fragment</span>

   Fragment (type 1) indicates that the datagram is part of a complete
   IP datagram.  It is always class 2.

   The data consists of (one of) the 64 bit IP address(es) of the router
   doing the fragmentation, a 64 bit datagram ID generated by that
   router, and a 32 bit fragment offset.  The IDs should be generated so
   as to be very likely unique over a period of time larger than the TCP
   MSL (maximum segment lifetime).  (The TCP ISN (initial sequence
   number) generator might be used to initialize the ID generator in a
   router.)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | C |F|    type                 |   length                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +          fragmenting router IP address                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +          datagram ID                                          +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          offset                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If a datagram must be refragmented, the original 128 bit address+ID
   is preserved, so that the datagram can be reassembled from any
   sufficient set of the resulting fragments.  The 64 bits fields are
   positioned so that they are aligned in the usual case of the fragment
   option following the IP header.

   A router implementing Fragment (doing fragmentation) must recognize
   the Don't Fragment option.

<span class="h4"><a class="selflink" id="section-3.3.3" href="#section-3.3.3">3.3.3</a>  Last Fragment</span>

   Last Fragment (type 2) has the same format as Fragment, but implies
   that this datagram is the last fragment needed to reassemble the
   original datagram.

   Note that an implementation can reasonably add arriving datagrams
   with Fragment to a cache, and then attempt a reassembly when a
   datagram with Last Fragment arrives (and the the total length is
   known); this will work well when datagrams are not reordered in the



<span class="grey">Ullmann                                                        [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   network.

<span class="h4"><a class="selflink" id="section-3.3.4" href="#section-3.3.4">3.3.4</a>  Don't Fragment</span>

   This option (type 3, class 0) indicates that the datagram may not be
   fragmented.  If it can not be forwarded without fragmentation, it is
   discarded, and the appropriate ICMP message sent.  (Unless, of
   course, the datagram is an ICMP message.) There is no data present.

<span class="h4"><a class="selflink" id="section-3.3.5" href="#section-3.3.5">3.3.5</a>  Don't Convert</span>

   The Don't Convert option prohibits conversion from IPv7 to IPv4
   protocol, requiring instead that the datagram be discarded and an
   ICMP message sent (conversion failed/don't convert set).  It is type
   4, usually class 0, and must be implemented by any router
   implementing conversion.  A host is under no such constraint; like
   any protocol specification, only the "bits on the wire" can be
   specified, the host receiving the datagram may convert it as part of
   its procedure.  There is no data present in this option.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>  Forward route identifier</span>

   Each IP datagram carries a 64 bit field, called "forward route
   identifier", that is updated (if the information is available) at
   each hop.  This field's value is derived from the routing protocol
   (e.g., RAP [<a href="./rfc1476" title="&quot;RAP: Internet Route Access Protocol&quot;">RFC1476</a>]).  It is used to expedite routing decisions by
   preserving knowledge where possible between consecutive routers.  It
   can also be used to make datagrams stay within reserved flows and
   mobile-host tunnels where required.

<span class="h4"><a class="selflink" id="section-3.4.1" href="#section-3.4.1">3.4.1</a>  Procedure description</span>

   Consider 3 routers, A, B, and C.  Traffic is passing through them,
   between two other hosts (or networks), X and Y, packets are going
   XABCY and YCBAX.  Consider only one direction:  routing info flowing
   from C to A, to provide a route from A to C.  The same thing will be
   happening in the other direction.

   An explanation of the notation:

     R(r,d,i,h)    A route that means: "from router r, to go toward
                   final destination d, replace the forward route
                   identifier in the packet with i, and take next
                   hop h."

     Ri(r,d)       An opaque (outside of router r) identifier, that can
                   be used by r to find R(r,d,...).




<span class="grey">Ullmann                                                        [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


     Flowi(r,rt)   An opaque (outside of router r) identifier, that
                   router r can use to find a flow or tunnel with which
                   the datagram is associated, and from that the route
                   rt on which the flow or tunnel is built, as well as
                   the Flowi() for the subsequent hop.

     Ri(Dgram)     The forward route identifier in a datagram.

   Router C announces a route R(C,Y,0,Y) to router B.  It includes in it
   an identifier Ri(C,Y) internal to C, that will allow C to find the
   route rapidly.  (A table index, or an actual memory address.)

   Router B creates a route R(B,Y,Ri(C,Y),C) via router C, it announces
   it to A, including an identifier Ri(B,Y), internal to B, and used by
   A as an opaque object.

   Router A creates a route R(A,Y,Ri(B,Y),B) via router B.  It has no
   one to announce it to.

   Now:  X originates a datagram addressed to Y.  It has no routing
   information, and sets Ri(Dgram) to zero.  It forwards the datagram to
   router A (X's default gateway).

   A finds no valid Ri(Dgram), and looks up the destination (Y) in its
   routing tables.  It finds R(A,Y,Ri(B,Y),B), sets Ri(Dgram) &lt;-
   Ri(B,Y), and forwards the datagram to B.

   Router B looks at Ri(Dgram) which directly identifies the next hop
   route R(B,Ri(C,Y),C), sets Ri(Dgram) &lt;- Ri(C,Y) and forwards it to
   router C.

   Router C looks at Ri(Dgram) which directly locates R(C,0,Y), sets
   Ri(Dgram) &lt;- 0 and forwards to Y.

   Y recognizes its own address in Dest(Dgram), ignores Ri(Dgram).

   Of course, the routers will validate the Ri's received, particularily
   if they are memory addresses (e.g., M(a) &lt; Ri &lt; M(b), Ri mod N == 0),
   and probably check that the route in fact describes the destination
   of the datagram.  If the Ri is invalid, the router must use the
   ordinary method of finding a route (i.e., what it would have done if
   Ri was 0), and silently ignore the invalid Ri.

   When a route has been aggregated at some router, implicitly or
   explicitly, it will find that the incoming Ri(Dgram) at most can
   identify the aggregation, and it must make a decision; the forwarded
   datagram then contains the Ri for the specific route.  (Note this may
   happen well upstream of the point at which the routes actually



<span class="grey">Ullmann                                                        [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   diverge.)

   This allows all cooperating routers to make immediate forwarding
   decisions, without any searching of tables or caches once the
   datagram has entered the routing domain.  If the host participates in
   the routing, at least to the extent of acquiring the initial Ri
   required from the first router, then only routers that have done
   aggregations need make decisions.  (If the routing changes with
   datagrams in flight, some router will be required to make a decision
   to re-rail each datagram.)

<span class="h4"><a class="selflink" id="section-3.4.2" href="#section-3.4.2">3.4.2</a>  Flows</span>

   If a "flow" is to be set up, the identifiers are replaced by
   Flowi(router,route), where each router's structure for the flow
   contains a pointer to the route on which the flow is built.
   Datagrams can drop out of the flow at some point, and can be inserted
   either by the originating host or by a cooperating router near the
   originator.  Since the forward route identifier field is opaque to
   the sending router, and implicitly meaningful only to the next hop
   router, use for flows (or similar optimizations) need not be
   otherwise defined by the protocol.  (One presumes that a router
   issuing both Ri's and Flowi's will take care to make sure that it can
   distinguish them by some private method.)

   If a flow has been set up by a restricted target RAP route
   announcement, it is no different from a route in the implementation.
   If this announcement originates from the host itself, the Ri in
   incoming datagrams can be used to determine whether they followed the
   flow, or to optimize delivery of the datagrams to the next layer
   protocol.

<span class="h4"><a class="selflink" id="section-3.4.3" href="#section-3.4.3">3.4.3</a>  Mobile hosts</span>

   First, a definition:  A "mobile host" is a host that can move around,
   connecting via different networks at different times, while
   maintaining open TCP connections.  It is distinguished from a
   "portable host", which is simply a host that can appear in various
   places in the net, without continuity.  A portable host can be
   implemented by assigning a new address for each location (more or
   less automatically), and arranging to update the domain system.
   Supporting truly mobile hosts is the more interesting problem.

   To implement mobile host support in a general way, either some layer
   of the protocol suite must provide network-wide routing, or the
   datagrams must be tunnelled from the "home" network of the host to
   its present location.  In the real network, some combination of these
   is probable:  most of the net will forward datagrams toward the home



<span class="grey">Ullmann                                                        [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   network, and then the datagrams will follow a specific host route to
   the mobile host.

   The requirement on the routing system is that it must be able to
   propagate a host route at least to the home network; any other
   distribution is useful optimization.  When a host route is propagated
   by RAP as a targeted route, and the routers use the resulting Ri's,
   the datagram follows an effective tunnel to the mobile host.  (Not a
   real tunnel, in the strict sense; the datagrams are following an
   actual route at the network protocol layer.)

   As explained in RAP [RFC14XX-RAP], a targeted route can be issued
   when desired; in particular, it can be triggered by the establishment
   of a TCP connection or by the arrival of datagrams that do not carry
   an Ri indicating that they have followed a (non-tunnel) route.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  TCP:  </span>Transport protocol

   Internet version 7 expands the sizes of the sequence and
   acknowledgement fields, the window, and the port numbers.  This is to
   remove limitations in version 4 that begin to restrict throughput at
   (for example) the bandwidth of FDDI and round trip delay of more than
   60 milliseconds.  At gigabit speeds and delays typical of
   international links, the version 4 TCP would be a serious limitation.
   See [<a href="./rfc1323" title="&quot;TCP Extensions for High Performance&quot;">RFC1323</a>].

   The port numbers are also expanded.  This alleviates the problem of
   going through the entire port number range with a rapid sequence of
   transactions in less than the lifetime of datagrams in the network.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>  TCP segment header format</span>

   The 64 bit fields (sequence and acknowledgement) in the TCP header
   are off-phase aligned, in anticipation of the usual case of the TCP
   header following the 9 32-bit word IP header.  If IP options add up
   to an odd number of 32 bit words, a null option may be added to push
   the transport header to off-phase alignment.














<span class="grey">Ullmann                                                        [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  data offset  | MBZ |A|P|R|S|F|           checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        source port                                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        destination port                                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +        sequence number                                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +        acknowledgement number                                 +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        window                                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        options                          ...                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A description of each field:

<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>  Data offset</span>

   An 8 bit count of the number of 32 bit words in the TCP header,
   including any options.

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>  MBZ</span>

   Reserved bits, must be zero, and must be ignored.

<span class="h4"><a class="selflink" id="section-4.1.3" href="#section-4.1.3">4.1.3</a>  Flags</span>

   These are the protocol state flags, use exactly as in TCPv4, except
   that there is no urgent data flag.

<span class="h4"><a class="selflink" id="section-4.1.4" href="#section-4.1.4">4.1.4</a>  Checksum</span>

   This is a 16 bit checksum of the segment.  The pseudo-header used in
   the checksum consists of the destination address, the source address,
   the protocol field (constant 6 for TCP), and the 32 bit length of the
   TCP segment.







<span class="grey">Ullmann                                                        [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h4"><a class="selflink" id="section-4.1.5" href="#section-4.1.5">4.1.5</a>  Source port</span>

   The source port number, a 32 bit identifier.  See the section on port
   numbers below.

<span class="h4"><a class="selflink" id="section-4.1.6" href="#section-4.1.6">4.1.6</a>  Destination port.</span>

   The 32 bit destination port number.

<span class="h4"><a class="selflink" id="section-4.1.7" href="#section-4.1.7">4.1.7</a>  Sequence</span>

   A 64 bit sequence number, the sequence number of the first octet of
   user data in the segment.

   The ISN (Initial Sequence Number) generator used in TCPv4 is used in
   TCPv7, with the 32 bit value loaded into both the high and low 32
   bits of the TCPv7 sequence number.  This provides reasonable behavior
   when the 32 bit rollover option is used (see below) for TCPv4
   interoperation.  V7 hosts must implement the full 64 bit sequence
   number rollover.

<span class="h4"><a class="selflink" id="section-4.1.8" href="#section-4.1.8">4.1.8</a>  Acknowledgement</span>

   The 64 bit acknowledgement number, acknowledging receipt of octets up
   to but not including the octet identified.  Valid if the A flag is
   set, if A is reset (0), this field should be zero, and must be
   ignored.

<span class="h4"><a class="selflink" id="section-4.1.9" href="#section-4.1.9">4.1.9</a>  Window</span>

   The 32 bit offered window.

<span class="h4"><a class="selflink" id="section-4.1.10" href="#section-4.1.10">4.1.10</a>  Options</span>

   TCP options, some of which are documented below.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>  Port numbers</span>

   Port numbers are divided into several ranges:  (all numbers are
   decimal)

    0             reserved
    1-32767       Internet registered ("well-known") protocols
    32768-98303   reserved, to allow TCPv7-TCPv4 conversion
    98304 up      dynamic assignment

   It must also be remembered that hosts are free to dynamically assign
   for active connections any port not actually in use by that host:



<span class="grey">Ullmann                                                        [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   hosts must not reject connections because the "client" port is in the
   registered range.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>  TCP options</span>

<span class="h4"><a class="selflink" id="section-4.3.1" href="#section-4.3.1">4.3.1</a>  Option Format</span>

   Each option begins with a 32 bit header:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        type                   |   length                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        option data                 ...          |   padding   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<span class="h4"><a class="selflink" id="section-4.3.2" href="#section-4.3.2">4.3.2</a>  Null</span>

   The null option (type = 0), is to be ignored.

<span class="h4"><a class="selflink" id="section-4.3.3" href="#section-4.3.3">4.3.3</a>  Maximum Segment Size</span>

   Maximum segment size (type = 1) specifies the largest segment that
   the other TCP should send, in terms of the number of data octets.
   When sent on a SYN segment, it is mandatory; if sent on any other
   segment it is advisory.

   Data is one 32 bit word specifying the size in octets.

<span class="h4"><a class="selflink" id="section-4.3.4" href="#section-4.3.4">4.3.4</a>  Urgent Pointer</span>

   The urgent pointer (type = 2) emulates the urgent field in TCPv4.
   Its presence is equivalent to the U flag being set.  The data is a 64
   bit sequence number identifying the last octet of urgent data.  (Not
   an offset, as in v4.)

<span class="h4"><a class="selflink" id="section-4.3.5" href="#section-4.3.5">4.3.5</a>  32 Bit rollover</span>

   The 32 bit rollover option (type = 3) indicates that only the low
   order 32 bits of the sequence and acknowledgement packets are
   significant in the packet.

   This is necessary because a converting internet layer gateway has no
   retained state, and cannot properly set the high order bits.  This
   option must be implemented by version 7 hosts that want to
   interoperate with version 4 hosts.




<span class="grey">Ullmann                                                        [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  UDP:  </span>User Datagram protocol

   The user datagram protocol is also expanded to include larger port
   numbers, for reasons similar to the TCP.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>  UDP header format</span>

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  data offset  |     MBZ       |           checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        source port                                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        destination port                                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        options                          ...                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A description of each field:

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>  Data offset</span>

   An 8 bit count of the number of 32 bit words in the UDP header,
   including any options.

<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>  MBZ</span>

   Reserved bits, must be zero, and must be ignored.

<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>  Checksum</span>

   This is a 16 bit checksum of the datagram.  The pseudo-header used in
   the checksum consists of the destination address, the source,
   address, and the protocol field (constant 17 for UDP), and the 32 bit
   length of the user datagram.

<span class="h4"><a class="selflink" id="section-5.1.4" href="#section-5.1.4">5.1.4</a>  Source port</span>

   The source port number, a 32 bit identifier.  See the section on TCP
   port numbers above.

<span class="h4"><a class="selflink" id="section-5.1.5" href="#section-5.1.5">5.1.5</a>  Destination port.</span>

   The 32 bit destination port number.






<span class="grey">Ullmann                                                        [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h4"><a class="selflink" id="section-5.1.6" href="#section-5.1.6">5.1.6</a>  Options</span>

   UDP options, none are presently defined.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  ICMP</span>

   The ICMP protocol is very similar to ICMPv4, in some cases not
   requiring any conversion.

   The complication is that IP datagrams are nested within ICMP
   messages, and must be converted.  This is discussed later.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>  ICMP header format</span>

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     type      |     code      |           checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        type-specific parameter                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        type-specific data               ...                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type and code are well-known values, defined in [<a href="./rfc792" title="&quot;Internet Control Message Protocol - DARPA Internet Program Protocol Specification&quot;">RFC792</a>].  The codes
   have meaning only within a particular type, they are not orthogonal.

   The next 32 bit word is usually defined for the specific type,
   sometimes it is unused.

   For many types, the data consists of a nested IP datagram, usually
   truncated, which is a copy of the datagram causing the event being
   reported.  In IPv4, the nested datagram consists of the IP header,
   and another 64 bits (at least) of the original datagram.

   For IPv7, the nested datagram must include the IP header plus 96 bits
   of the remaining datagram (thus including the port numbers within TCP
   and UDP), and should include the first 256 bytes of the datagram.
   I.e., in most cases where the original datagram was not large, it
   will return the entire datagram.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>  Conversion failed ICMP message</span>

   The introduction of network layer conversion requires a new message
   type, to report conversion errors.  Note that an invalid datagram
   should result in the sending of some other ICMP message (e.g.,
   parameter problem) or the silent discarding of the datagram.  This
   message is only sent when a valid datagram cannot be converted.



<span class="grey">Ullmann                                                        [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   Note:  implementations are not expected to, and should not, check the
   validity of incoming datagrams just to accomplish this; it simply
   means that an error detected during conversion that is known to be an
   actual error in the incoming datagram should be reported as such, not
   as a conversion failure.

   Note that the conversion failed ICMP message may be sent in either
   the IPv4 or IPv7 domain; it is a valid ICMP message type for IPv4.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     type      |     code      |           checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        pointer to problem area                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        copy of datagram that could not be converted ....      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The type for Conversion Failed is 31.

   The codes are:

        0       Unknown/unspecified error
        1       Don't Convert option present
        2       Unknown mandatory option present
        3       Known unsupported option present
        4       Unsupported transport protocol
        5       Overall length exceeded
        6       IP header length exceeded
        7       Transport protocol &gt; 255
        8       Port conversion out of range
        9       Transport header length exceeded
        10      32 Bit Rollover missing and ACK set
        11      Unknown mandatory transport option present

   The use of code 0 should be avoided, any other condition found by
   implementors should be assigned a new code requested from IANA.  When
   code 0 is used, it is particularily important that the pointer be set
   properly.

   The pointer is an offset from the start of the original datagram to
   the beginning of the offending field.

   The data is part of the datagram that could not be converted.  It
   must be at least the IP and transport headers, and must include the
   field pointed to by the previous parameter.  For code 4, the
   transport header is probably not identifiable; the data should



<span class="grey">Ullmann                                                        [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   include 256 bytes of the original datagram.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Notes on the domain system</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>  A records</span>

   Address records will be added to the IN (Internet) zone with IPv7
   addresses for all hosts as IPv7 is deployed.  Eventually the IPv4
   addresses will be removed.  As mentioned above, the AD
   (Administrative Domain) space is initially assigned so that the first
   4 octets of a v7 address cannot be confused with a v4 address (or,
   rather, the confusion will be to no effect.)

   For example:

   DELTA.Process.COM.      A       192.42.95.68
                           A       192.0.0.192.42.95.1.68

   It is important that the A record be used, to avoid the cache
   consistancy problem that would arise when different records had
   different remaining TTLs.

   Note that if an unmodified version of the more popular public domain
   nameserver is a secondary for a zone containing IPv7 addresses, it
   will erroneously issue RRs with only the first four bytes.  (I.e.,
   192.0.0.192 in the example.) This is another reason to ensure that
   the AD numbers are initially reserved out of the IPv4 network number
   space.  Eventually, zones with IPv7 addresses would be expected to be
   served only by upgraded servers.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>  PTR zone</span>

   The inverse (PTR) zone is .#, with the IPv7 address (reversed).
   I.e., just like .IN-ADDR.ARPA, but with .# instead.

   This respects the difference in actual authority:  the NSF/DDN NIC is
   the authority for the entire space rooted in .IN-ADDR.ARPA.  in the
   v4 Internet, while in the new Internet it holds the authority only
   for the AD 0.0.192.#.  (Plus, of course, any other ADs assigned to it
   over time.)

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Conversion between version 4 and version 7</span>

   As noted in the description of datagram format, it is possible to
   provide a mostly-transparent bridge between version 4 and version 7.

   This discusses TCP and ICMP at the session/transport layer; UDP is a
   subset of the TCP conversion.  Most protocols at this layer will



<span class="grey">Ullmann                                                        [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   probably need no translation; however it will probably be necessary
   to specify exactly which will have translations done.

   New protocols at the session/transport layer defined over IPv7 should
   have protocol numbers greater than 255, and will not be translated to
   IPv4.

   Most of the translations should consist of copying various fields,
   verifying fixed values in the datagram being translated, and setting
   fixed values in the datagram being produced.  In general, the
   checksum must be verified first, and then a new checksum computed for
   the generated datagram.

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>  Version 4 IP address extension option</span>

   A new option is defined for IP version 4, to carry the extended
   addresses of IPv7.  This will be particularily useful in the initial
   testing of IPv7, during a time when most of the fabric of the
   internet is IPv4.  An IPv7 host will be able to connect to another
   IPv7 host anywhere in the internet even though most of the paths and
   routers are IPv4, and still use the full addressing.  This will
   continue to work until non-unique network numbers are assigned, by
   which time most of the infrastructure should be IPv7.

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>  Option format</span>

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  type (147)   | length = 10   |     source IPv7 AD number     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  ...          | src 7th octet |     destination IPv7 AD       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  number ...   | dst 7th octet |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The source and destination are in IPv4 order (source first), for
   consistancy.  The type code is 147.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>  Fragmented datagrams</span>

   Datagrams that have been fragmented must be reassembled by the
   converting host or router before conversion.  Where the conversion is
   being done by the destination host (i.e.,  the case of a v7 host
   receiving v4 datagrams), this is similar to the present fragmentation
   model.

   When it is being done by an intermediate router (acting as an
   internetwork layer gateway) the router should use all of source,
   destination, and datagram ID for identification of IPv4 fragments;



<span class="grey">Ullmann                                                        [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   note that destination is used implicitly in the usual reassembly at
   the destination.  When reassembling an IPv7 datagram, the 128 bit
   fragment ID is used as usual.

   If the fragments take different paths through the net, and arrive at
   different conversion points, the datagram is lost.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>  Where does the conversion happen?</span>

   The objective of conversion is to be able to upgrade systems, both
   hosts and routers, in whatever order desired by their owners.
   Organizations must be able to upgrade any given system without
   reconfiguration or modification of any other; and IPv4 hosts must be
   able to interoperate essentially forever.  (IPv4 routers will
   probably be effectively eliminated at some point, except where they
   exist in their own remote or isolated corners.)

   Each TCP/IP v7 system, whether host or router, must be able to
   recognize adjacent systems in the topology that are (only) v4, and
   call the appropriate conversion routine just before sending the
   datagram.

   Digression:  I believe v7 hosts will get much better performance by
   doing everything internally in v7, and using conversion to filter
   datagrams when necessary.  This keeps the usual code path simple,
   with only a "hook" right after receiving to convert incoming IPv4
   datagrams, and just before sending to convert to IPv4.  Routers may
   prefer to keep datagrams in their incoming version, at least until
   after the routing decision is made, and then doing the conversion
   only if necessary.  In either case, this is an implementation
   specific decision.

   It must be noted that any forwarding system may convert datagrams to
   IPv7, then back to IPv4, even if that loses information such as
   unknown options.  The reverse is not acceptable:  a system that
   receives an IPv7 datagram should not convert it to IPv4, then back to
   IPv7 on forwarding.

   The preferred method for identifying which hosts require conversion
   is to ARP first for the IPv7 address, and then again if no response
   is received, for the IPv4 address.  The reservation of ADs out of the
   v4 network number space is useful again here, protecting hosts that
   fail to properly use the ARP address length fields.

   On networks where ARP is not normally used, the method is to assume
   that a remote system is v7.  If an IPv7 datagram is received from it,
   the assumption is confirmed.  If, after a short time, no IPv7
   datagram is received, a v7 ICMP echo is sent.  If a reply is received



<span class="grey">Ullmann                                                        [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   (in either version) the assumption is confirmed.

   If no reply is recieved, the remote system is assumed not to
   understand IPv7, and datagrams are converted to IPv4 just before
   transmitting them.

   Implementations should also provide for explicit configuration where
   desired.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>  Hybrid IPv4 systems</span>

   In the course of implementing IPv7, especially in constrained
   environments such as small terminal servers, it may be useful to
   implement the IPv4 address extension option directly, thereby
   regaining universal connectivity.

   This may also be a useful interim step for vendors not prepared to do
   a major rework of an implementation; but it is important not to get
   stalled in this step.

   A hybrid IPv4 + address extension system does not have to implement
   the conversion, it places this onus on its neighbors.  It may itself
   have an address with the subnet extension (7th byte) not equal to 1.

   The implication of hybrid systems is that it is not valid to assume
   that a host with a IPv7 address is a native IPv7 implementation.

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>  Maximum segment size in TCP</span>

   It is probably advisable for IPv4 implementations to reduce the MSS
   offered by a small amount where possible, to avoid fragmentation when
   datagrams are converted to IPv7.  This arises when IPv4 hosts are
   communicating through an IPv7 infrastructure, with the same MTU as
   the local networks of the hosts.

<span class="h3"><a class="selflink" id="section-8.6" href="#section-8.6">8.6</a>  Forwarding and redirects</span>

   It may be important for a router to not send ICMP redirects when it
   finds that it must do a conversion as part of forwarding the
   datagram.  In this case, the hosts involved may not be able to
   interact directly.  The IPv7 host could ignore the redirect, but this
   results in an unpleasant level of noise as the sequence continually
   recurs.

<span class="h3"><a class="selflink" id="section-8.7" href="#section-8.7">8.7</a>  Design considerations</span>

   The conversion is designed to be fairly efficient in implementation,
   especially on RISC architectures, assuming they can either do a



<span class="grey">Ullmann                                                        [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   conditional move (or store), or do a short forward branch without
   losing the instruction cache.  The other conditional branches in the
   body of the code are usually not-taken out to the failure/discard
   case.

   Handling options does involve a loop and a dispatch (case) operation.
   The options in IPv4 are more difficult to handle, not being designed
   for speed on a 32 bit aligned RISCish architecture, but they do not
   occur often, except perhaps the address extension option.

   For CISC machines, the same considerations will lead to fairly
   efficient code.

   The conversion code must be extremely careful to be robust when
   presented with invalid input; in particular, it may be presented with
   truncated transport layer headers when called recursively from the
   ICMP conversion.

<span class="h3"><a class="selflink" id="section-8.8" href="#section-8.8">8.8</a>  Conversion from IPv4 to IPv7</span>

   Individual steps in the conversion; the order is in most cases not
   significant.

      o  Verify checksum.

      o  Verify fragment offset is 0, MF flag is 0.

      o  Verify version is 4.

      o  Extend TTL to 16 bits, multiply by 16.

      o  Set forward route identifier to 0.

      o  Set first 3 octets of destination to AD (i.e., 192.0.0), copy
         first three octets from v4 address, set next octet to 1, copy
         last octet.  (This can be done with shift/mask/or operations
         on most architectures.)

      o  Do the same translation on source address.

      o  Copy protocol, set high 8 bits to zero.

      o  If DF flag set, add Don't Fragment option.

      o  If Address Extension option present, copy ADs and subnet
         extension numbers into destination and source.

      o  Convert other options where possible.  If an unknown option



<span class="grey">Ullmann                                                        [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


         with copy-on-fragment is found, fail.  If copy-on-fragment is
         not set, ignore the option.  I.e., the flag is (ab)used as an
         indicator of whether the option is mandatory.

      o  Compute new IP header length.

      o  Convert session/transport layer (TCP) header and data.

      o  Compute new overall datagram length.

      o  Calculate IPv7 checksum.

<span class="h3"><a class="selflink" id="section-8.9" href="#section-8.9">8.9</a>  Conversion from IPv7 to IPv4</span>

   The steps to convert IPv7 to IPv4 follow.  Note that the converting
   router or host is partly in the role of destination host; it checks
   both bits of class in IP options, and (as in the other direction)
   must reassemble fragmented datagrams.

      o  Verify checksum.

      o  Verify version is 7

      o  Set type-of-service to 0 (there may be an option defined,
         that will be handled later).

      o  If length is greater than (about) 65563, fail.  (That number
         is not a typographical error.  Note that the IPv7+TCPv7
         headers add up to 28 bytes more than the corresponding v4
         headers in the usual case.) This check is only to avoid
         useless work, the precise check is later.

      o  Generate an ID (using an ISN based sequence generator,
         possibly also based on destination or source or both).

      o  Set flags and fragment field to 0.

      o  Divide TTL by 16, if zero, fail (send ICMP Time Exceeded).
         If greater that 255, set to 255.

      o  If next layer protocol is greater than 255, fail.  Else copy.

      o  Copy first 3 octets and 8th octet of destination to
         destination address.

      o  Same for source address.

      o  Generate v4 address extension option.  (If enabled; this



<span class="grey">Ullmann                                                        [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


         probably should be a configuration option, should default to
         on.)

      o  Process v7 options.  If any unknown options of class not 0
         found, fail.

      o  If Don't Fragment option found, set DF flag.

      o  If Don't Convert option found, fail.

      o  Convert other options where possible, or fail.

      o  Compute new IP header length.  This may fail (too large),
         fail conversion if so.

      o  Convert session/transport layer (e.g., TCP).

      o  Compute new overall datagram length.  If greater than 65535,
         fail.

      o  Compute IPv4 checksum.

<span class="h3"><a class="selflink" id="section-8.10" href="#section-8.10">8.10</a>  Conversion from TCPv4 to TCPv7</span>

      o  Subtract header words from v4 checksum.  (Note that this is
         actually done with one's complement addition.)

      o  Copy flags (except for Urgent).

      o  If source port is less than 32768 (a sign condition test will
         suffice on most architectures), copy it.  If equal or
         greater, add 65536.

      o  Same operation on destination port.

      o  Copy sequence to low 32 bits, set high to 0.

      o  Copy acknowledgement to low 32 bits, set high to 0.

      o  Copy window.  (The TCPv4 performance extension [<a href="./rfc1323" title="&quot;TCP Extensions for High Performance&quot;">RFC1323</a>]
         window-scale cannot be used, as it would require state; we
         use the basic window offered.)

      o  Add 32 bit rollover option.

      o  Convert maximum segment size option if present.

      o  Compute data offset and copy data.



<span class="grey">Ullmann                                                        [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


      o  Add header words into saved checksum.  It is important not to
         recompute the checksum over the data; it must remain an
         end-to-end checksum.

      o  Return to IP layer conversion.

<span class="h3"><a class="selflink" id="section-8.11" href="#section-8.11">8.11</a>  Conversion from TCPv7 to TCPv4</span>

      o  Subtract header from v7 checksum.

      o  If source port is greater than 65535, subtract 65536.  If
         result is still greater than 65535, fail.  (Send ICMP
         conversion failed/port conversion out of range.  The sending
         host may then reset its port number generator to 98304.)

      o  Same translation for destination port.

      o  Copy low 32 bits of sequence number.

      o  If A bit set, copy low 32 bits of acknowledgement.

      o  Copy flags.

      o  If window is greater than 61440, set it to 24576.  If less,
         copy it unchanged.  (Rationale for the 24K figure:  this has
         been found to be a good default for IPv4 hosts.  If the IPv7
         host is offering a very large window, the IPv4 host probably
         isn't prepared to play at that level.)

      o  Process options.  If 32 Bit Rollover is not present, and A
         flag is set, fail.  (Send ICMP conversion failed/32 bit
         Rollover missing.)

      o  If Urgent is present, compute offset.  If in segment, set U
         flag and offset field.  If not, ignore.

      o  Convert Maximum Segment Size option.  If greater than 16384,
         set to 16384.

      o  Compute new data offset.

      o  Add header words into v4 checksum.

      o  Return to IP layer conversion.







<span class="grey">Ullmann                                                        [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


<span class="h3"><a class="selflink" id="section-8.12" href="#section-8.12">8.12</a>  ICMP conversion</span>

   ICMP messages are converted by copying the type and code into the new
   packet, and copying the other type-specific fields directly.

   If the message contains an encapsulated, and usually truncated, IP
   datagram, the conversion routine is called recursively to translate
   it as far as possible.  There are some special considerations:

      o  The encapsulated datagram is less likely to be valid, given
         that it did generate an error of some kind.

      o  The conversion should attempt to complete all fields
         available, even if some would cause failures in the general
         case.  Note, in particular, that in the course of converting
         a datagram, when a failure occurs, an ICMP message
         (conversion failed) is sent; this message itself may
         immediately require conversion.  Part of that conversion will
         involve converting the original datagram.

      o  Conditions such as overall datagram length too large are not
         checked.

      o  The AD and subnet byte assumed in the nested conversion may
         not be sensible if the IPv4 address extension option is not
         present and the datagram has strayed from the expected AD.
         (Not unlikely, given that we know a priori that some error
         occured.)

      o  The conversion must be very sure not to make another
         recursive call if the nested datagram is an ICMP message.
         (This should not occur, but obviously may.)

      o  It is probably impossible to generate a correct transport
         layer checksum in the nested datagram.  The conversion may
         prefer to just zero the checksum field.  Likewise, validating
         the original checksum is pointless.

   It may be best in a given implementation to have a separate code path
   for the nested conversion, that handles these issues out of the
   optimized usual path.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Postscript</span>

   The present version of TCP/IP has been a success partly by accident,
   for reasons that weren't really designed in.  Perhaps the most
   significant is the low level of network integration required to make
   it work.



<span class="grey">Ullmann                                                        [Page 33]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   We must be careful to retain the successful ingredients, even where
   we may be unaware of them.  Tread lightly, and use all that we have
   learned, especially about not changing things that work.

   This document has described a fairly conservative step forward, with
   clear extensibility for future developments, but without jumping into
   the abyss.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  References</span>

   [<a id="ref-RFC768">RFC768</a>]    Postel, J., "User Datagram Protocol", STD 6, <a href="./rfc768">RFC 768</a>,
               USC/Information Sciences Institute, August 1980.

   [<a id="ref-RFC791">RFC791</a>]    Postel, J., "Internet Protocol - DARPA Internet Program
               Protocol Specification", STD 5, <a href="./rfc791">RFC 791</a>, DARPA,
               September 1981.

   [<a id="ref-RFC792">RFC792</a>]    Postel, J., "Internet Control Message Protocol -
               DARPA Internet Program Protocol Specification"
               STD 5, <a href="./rfc792">RFC 792</a>, USC/Information Sciences Institute,
               September 1981.

   [<a id="ref-RFC793">RFC793</a>]    Postel, J., "Transmission Control Protocol - DARPA
               Internet Program Protocol Specification", STD 7, <a href="./rfc793">RFC 793</a>,
               USC/Information Sciences Institute, September 1981.

   [<a id="ref-RFC801">RFC801</a>]    Postel, J., "NCP/TCP Transition Plan", USC/Information
               Sciences Institute, November 1981.

   [<a id="ref-RFC1287">RFC1287</a>]   Clark, D., Chapin, L., Cerf, V., Braden, R., and
               R. Hobby, "Towards the Future Internet Architecture", <a href="./rfc1287">RFC</a>
               <a href="./rfc1287">1287</a>, MIT, BBN, CNRI, ISI, UCDavis, December 1991.

   [<a id="ref-RFC1323">RFC1323</a>]   Jacobson, V., Braden, R, and D. Borman, "TCP Extensions
               for High Performance", <a href="./rfc1323">RFC 1323</a>, LBL, USC/Information
               Sciences Institute, Cray Research, May 1992.

   [<a id="ref-RFC1335">RFC1335</a>]   Wang, Z., and J. Crowcroft, A Two-Tier Address Structure
               for the Internet: A Solution to the Problem of Address
               Space Exhaustion", <a href="./rfc1335">RFC 1335</a>, University College London,
               May 1992.

   [<a id="ref-RFC1338">RFC1338</a>]   Fuller, V., Li, T., Yu, J., and K. Varadhan,
               "Supernetting: an Address Assignment and Aggregation
               Strategy", <a href="./rfc1338">RFC 1338</a>, BARRNet, cicso, Merit, OARnet,
               June 1992.





<span class="grey">Ullmann                                                        [Page 34]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-35" ></span>
<span class="grey"><a href="./rfc1475">RFC 1475</a>                         TP/IX                         June 1993</span>


   [<a id="ref-RFC1347">RFC1347</a>]   Callon, R., "TCP and UDP with Bigger Addresses (TUBA),
               A Simple Proposal for Internet Addressing and Routing",
               <a href="./rfc1347">RFC 1347</a>, DEC, June 1992.

   [<a id="ref-RFC1476">RFC1476</a>]   Ullmann, R., "RAP: Internet Route Access Protocol",
               <a href="./rfc1476">RFC 1476</a>, Process Software Corporation, June 1993.

   [<a id="ref-RFC1379">RFC1379</a>]   Braden, R., "Extending TCP for Transactions -- Concepts",
               <a href="./rfc1379">RFC 1379</a>, USC/Information Sciences Institute,
               November 1992.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Security Considerations</span>

   Security issues are not discussed in this memo.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Author's Address</span>

   Robert Ullmann
   Process Software Corporation
   959 Concord Street
   Framingham, MA 01701
   USA

   Phone: +1 508 879 6994 x226
   Email: Ariel@Process.COM


























Ullmann                                                        [Page 35]
</pre>