





Network Working Group:                                        R. Carlson
Request for Comments: 1705                                           ANL
Category: Informational                                     D. Ficarella
                                                                Motorola
                                                            October 1994


                    Six Virtual Inches to the Left:
                         The Problem with IPng

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document was submitted to the IETF IPng area in response to RFC
   1550.  Publication of this document does not imply acceptance by the
   IPng area of any ideas expressed within.  Comments should be
   submitted to the big-internet@munnari.oz.au mailing list.

Overview

   This RFC suggests that a new version of TCP (TCPng), and UDP, be
   developed and deployed.  It proposes that a globally unique address
   (TA) be assigned to Transport layer protocol (TCP/UDP).  The purpose
   of this TA is to uniquely identify an Internet node without
   specifying any routing information.  A new version of TCP, and UDP,
   will need to be developed describing the new header fields and
   formats.  This new version of TCP would contain support for high
   bandwidth-delay networks.  Support for multiple network layer
   (Internet Protocol) protocols is also possible with this new TCP.
   Assigning an address to TCP/UDP would allow for the support of
   multiple network layer protocols (IPng's).  The TA would identify the
   location of an Internet node.  The IPng layer would provide routing
   information to the Internet.  Separating the location and routing
   functions will greatly increase the versatility of the Internet.












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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Historical perspective . . . . . . . . . . . . . . . . . . . .  3
        2.1  OSI and the 7 layer model  . . . . . . . . . . . . . . .  3
        2.2  Internet Evolution . . . . . . . . . . . . . . . . . . .  4
        2.3  The Reasons for Change . . . . . . . . . . . . . . . . .  4
              2.3.1  Class-B Address Exhaustion . . . . . . . . . . .  4
              2.3.2  Routing Table Growth . . . . . . . . . . . . . .  5
   3.  The Problems with Change . . . . . . . . . . . . . . . . . . .  5
        3.1  TCP/UDP Implementations  . . . . . . . . . . . . . . . .  6
        3.2  User Applications  . . . . . . . . . . . . . . . . . . .  6
        3.3  The Entrenched Masses  . . . . . . . . . . . . . . . . .  6
   4.  Making TCP & UDP Protocol Independent  . . . . . . . . . . . .  7
        4.1  Transport Addressing . . . . . . . . . . . . . . . . . .  7
        4.2  TCPng  . . . . . . . . . . . . . . . . . . . . . . . . . 12
        4.3  Mandatory Options  . . . . . . . . . . . . . . . . . . . 15
        4.4  Optional Options . . . . . . . . . . . . . . . . . . . . 16
        4.5  Compatibility Issues . . . . . . . . . . . . . . . . . . 16
              4.5.1  Backward Compatibility . . . . . . . . . . . . . 17
        4.6  Level 4 Gateways . . . . . . . . . . . . . . . . . . . . 17
        4.7  Error Conditions . . . . . . . . . . . . . . . . . . . . 18
   5.  Advantages and Disadvantages of this approach  . . . . . . . . 18
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 19
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   Appendix C . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Appendix D . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Security Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27

1.  Introduction

   For more than a decade, network engineers have understood the
   benefits of a multi-layer protocol stack. However, during its
   development, the Transmission Control Protocol (TCP) was strongly
   linked to the Internet Protocol (IP) [Postel, 1981a]. When the TCP/IP
   protocol suite was developed, two important ideas were implemented.
   The first was that each host would be uniquely identified by a
   network layer number (i.e., IP number = 192.0.2.1). The second was to
   identify an application with a transport layer port number (i.e., TCP
   DNS number = 53). For host-to-host communications, the IP and port
   numbers would be concatenated to form a socket (i.e., 192.0.2.1.53).
   While this has lead to a very efficient and streamlined TCP layer, it
   has tightly coupled the TCP and IP layers. So much so, in fact, that
   it is nearly impossible to run TCP over any network layer except for



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   IP.

   The motivation for writing this paper resulted from research into the
   various Internet Protocol Next Generation (IPng) proposals put forth
   by various IETF working groups. Each of the IPng proposals strives to
   solve the impending IP address exhaustion problem by increasing the
   size of the address field. They all allude to modifications to TCP
   and User Datagram Protocol (UDP) to make them capable of supporting a
   new network layer IPng protocol. The authors of this paper feel that
   this points to an inherent TCP/IP design flaw. The flaw is namely
   that the transport (TCP) and network (IP) layers are not protocol
   independent. In this paper, we will propose a new TCP and UDP
   implementation that will make the transport and protocol layers
   independent and thus allow for any of the IPng protocols to operate
   on the same internet without any further modification to the higher
   layer protocols.  TCP, and UDP would become extremely powerful
   Application Programming Interfaces (APIs) that operate effectively
   over multiple network layer technologies.

2.  Historical perspective

2.1  OSI and the 7 layer model

   Present day computer and communication systems have become
   increasingly heterogeneous in both their software and hardware
   complexity, as well as their intended functionality. Prior to the
   establishment of computer communications industry standards,
   proprietary standards followed by particular software and hardware
   manufacturers prevented communication and information exchange
   between different manufacturers  products and therefore lead to many
   "closed systems" [Halsal, 1992] incapable of readily sharing
   information. With the proliferation of these types of systems in the
   mid 1970s, the potential advantages of "open systems" where
   recognized by the computer industry and a range of standards started
   to be introduced [Halsal, 1992].

   The first and perhaps most important of these standards was the
   International Standards Organization (ISO) reference model for Open
   Systems Interconnection standard (OSI), describing the complete
   communication subsystem within each computer. The goal of this
   standard model was to "allow an application process in any computer
   that supports a particular set of standards to communicate freely
   with an application process in any other computer that supports the
   same standards, irrespective of its origin of manufacture" [Halsal,
   1992].  The last statement above describes the OSI 7-layer model
   which has now, in concept, become the fundamental building block of
   computer networks.  Though there are arguably no present day
   computers and networks completely compliant to all 7 layers of the



Carlson & Ficarella                                             [Page 3]

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   OSI protocol stack, most protocol stacks do embrace the fundamental
   concept of independent layers, thus allowing the flexibility for
   computers operating with dissimilar protocol stacks to communicate
   with one another.

   Take for example, the datalink layers as supported by TCP/IP.  TCP/IP
   will run equally well in either the local area network (LAN) or wide
   area network (WAN) environments. Even though the LAN may use Ethernet
   802.3 and the WAN may use T1 serial links. This function was designed
   to present a "standardized set of network functions (i.e., a logical
   network)", to the upper network layer, "regardless of the exact
   details of the lower level implementations" [Meyer, Zobrist, 1990].

2.2  Internet Evolution

   "The internet architecture, the grand plan behind the TCP/IP protocol
   suite" was developed and tested in the late 1970s, [Braden, et al,
   1991] and but for the addition of subnetting, autonomous systems, and
   the domain name system in the early 1980s and the more recent IP
   multicasting implementation, stands today essentially unchanged. Even
   with the understood benefits of a multi-layer protocol stack, all
   steps taken to enhance the internet and its services have been very
   incremental and narrowly focused.

2.3  The Reasons for Change

   The reasons for change from IP to IPng can be described in terms of
   problems for which the current IP will simply become inadequate and
   unusable in the near future (~2-4 years). These problems are the
   exhaustion of IP class B address space, the exhaustion of IP address
   space in general, and the non-hierarchical nature of address
   allocation leading to a flat routing space [Dixon, 1993].

2.3.1  Class-B Address Exhaustion

   One of the fundamental causes of this problem is the lack of a class
   of network address appropriate for a mid-sized organization. The
   class-C address, with a maximum of 254 unique host addresses is to
   small, while class-B, with a maximum of in excess of 65 thousand
   unique host addresses is to large [Fuller, et al, 1992].  As a
   result, class-B addresses get assigned even though nowhere near the
   number of available addresses will ever get used. This fact, combined
   with a doubling of class-B address allocation on a yearly basis lead
   the Internet Engineering Steering Group (IESG) to conclude in
   November, 1992 that the class-B address space would be completely
   exhausted within 2 years time.  At that point, class-C addresses
   would have to be assigned, sometimes in multiples, to organizations
   needing more than the 254 possible host addresses from a single



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   class-C address [Almquist, Gross, 1992].

2.3.2  Routing Table Growth

   Based on research conducted by the IESG in November 1992, definite
   routing table size explosion problems were identified. Namely, it was
   determined that current router technology at that point could support
   a maximum of 16,000 routes, which in turn could support the internet
   for an additional 12 to 18 months (~May, 1994) at the then twofold
   annual network growth rate. However, vendor router maximum
   capabilities were in the process of being increased to 64,000 routes,
   which at the two-fold annual network growth rate, could bring us an
   additional 2 years of lead time, (at best bringing us to May, 1996,
   and at worst to November, 1995) assuming the class-B address
   exhaustion problem mentioned above could be solved in the interim
   [Almquist, Gross, 1992].

   As a short term, incremental solution to this routing table growth
   problem, and to aid in the class-B address exhaustion problem the
   IESG endorsed the CIDR supernetting strategy proposal (see RFC-1338
   for full details of this proposal). However, this strategy was
   estimated to have a viability of approximately 3 years, at which
   point the internet would run out of all classes of IP addresses in
   general. Hence, it is clear that even CIDR only offers temporary
   relief. However, if implemented immediately, CIDR can afford the
   Internet community time to develop and deploy an approach to
   addressing and routing which allows scaling to orders of magnitude
   larger than the current architecture (IPng).

3.  The Problems with Change

   There are many problems, both philosophical and technical, which
   greatly contribute to the difficulties associated with a large scale
   change such as the one proposed in the conversion from IP to IPng.
   These problems range from having to rewrite highly utilized and
   entrenched user applications, such as NFS, RPC, etc, to potentially
   having to invest additional capital to purchase hardware that
   supports the new protocol(s). This proposal solves the urgent
   internet problems listed above, while simultaneously limiting the
   amounts of retraining and re- investing that the user community would
   have to undertake. The TCP layer will once and for all be changed to
   support a multiprotocol internet.  The net affect is that while
   administrators will necessarily be trained in the operations and
   details of this new TCP, the much larger operator and end user
   community will experience no perceptible change in service and
   network usage.





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3.1  TCP/UDP Implementations

   Both TCP and UDP are highly dependent on the IPv4 network layer for 2
   very low level reasons.  1) a TCP/UDP socket is formed by
   concatenating a network layer address (IP address) and the transport
   layer TCP/UDP port number.  2) included in the TCP/UDP checksum
   calculation are the IP layer source and destination addresses
   mentioned above which are transferred across the TCP/IP [Postel,
   1981b] or UDP/IP [Postel, 1980] interfaces as procedure call
   arguments. It should be noted at this point that the reason for such
   strong dependence between the transport and network layers in TCP/IP
   or UDP/IP is to insure a globally unique TCP/UDP layer address, such
   that a unique connection could be identified by a pair of sockets.
   The authors of this paper propose that the IP address requirement
   with TCP and UDP be replaced with a globally unique transport address
   (TA) concatenated with a transport layer port address. This solution
   offers the capability to still maintain a globally unique address and
   host unique port number with the added benefit of eliminating the
   transport and network layer dependence on one another.

3.2  User Applications

   In addition to TCP and UDP, there are a large number of firmly
   entrenched higher level applications that use the IP network layer
   address embedded internally, and would therefore require modification
   for use with the proposed IPng network layer schemes. These
   applications include, but are not limited to Network File System
   (NFS), Remote Procedure Call (RPC), and File Transfer Protocol (FTP).
   All of these applications should be modified to use the Internet
   Domain name to identify the remote node, and not an embedded,
   protocol dependent IP address.

3.3  The Entrenched Masses

   Will users voluntarily give up their IPv4 systems to move to IPng?
   It seems likely that many users will resist the change.  They will
   perceive no benefit and will not install the new software.  Making
   the local Internet contact responsible may not be feasible or
   practical in all cases. Another issue is backward compatibility
   issues.  If a host needs to run IPng and IPv4 to support old hosts,
   then 1) where is the address savings IPng promised.  2) Why change if
   the host you are talking to has IPv4 anyway?

   On the other hand, replacing the existing TCP (TCPv6) with this new
   version (TCPng) will benefit users in several ways.  1) Users will be
   able to connect to unmodified TCPv6 hosts.  2) As nodes upgrade to
   TCPng, new features will be enabled allowing TCP to communicate
   effectively over high bandwidth*delay network links.  3) System



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   administrators will be able to incrementally upgrade nodes as needed
   or as local conditions demand.  4) Upgraded nodes may return their
   IPv4 address and use an IPng address and TCP transporter function,
   described later, to communicate with IPv4 hosts.

4.  Making TCP & UDP Protocol Independent

   The OSI 7 layer model specifies that each layer be independent of the
   adjacent layers. What is specified is the interface between layers.
   This allows layers to be replaced and/or modified without making
   changes to the other layers.  As was pointed out previously, the TCP
   and UDP transport layers violate this precept.  In the following
   discussion, when we refer to TCP we mean both the TCP and UDP
   protocols.  The generic term transport layer and TCP will be used
   interchangeably.

   Overcoming TCP's dependence on IP will require changes to the
   structure of the TCP header.  The developers and implementors will
   also have to change the way they think about TCP and IP.  End users
   will also have to change the way they view the Internet.  Gone will
   be the days when Internet node names and IP addresses can be used
   interchangeably.  The goal of this change is to allow end users to
   migrate from the current IPv4 network layer to an IPng layer.  What
   this IPng protocol is will be left to the Internet Architecture
   Board/Internet Engineering Steering Group/Internet Engineering Task
   Force (IAB/IESG/IETF) to decide.  By adopting this proposal, the
   migration will be greatly enhanced.

   One of the stated goals of the IAB is to promote a single Internet
   protocol suite [Leiner, Rekhter, 1993].  While this is a laudable
   goal, we should not be blinded by it. The addition of a Transport
   layer address (TA) does not invalidate the IAB's stated goals.  It
   merely brings the implementation into compliance with standard
   networking practices.  The historical reasons for concatenating TCP
   port numbers to IP numbers has long since passed. The increasing
   throughput of transmission lines and the negligible effect of packet
   overhead (see appendix A) prove this.  The details of assigning and
   using TA's are discussed in the next few sections.

4.1  Transport Addressing

   A Transport Address (TA) will be assigned to the TCP transport layer
   on each Internet node.  The purpose of this address is to allow a TCP
   on one node to communicate with a TCP on a remote node.  Some of the
   goals defined in developing this address are:

        1.  Fixed size -- A fixed size will make parsing easier for
            decoding stations.



Carlson & Ficarella                                             [Page 7]

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        2.  Minimum impact on TCP packet size -- This information
            will need to be carried each TCP packet.

        3.  Global Uniqueness -- It is desirable (required) to have a
            globally unique Transport Address.

        4.  Automatic Registration -- To reduce implementation
            problems, an automatic registration of the TA is
            desirable.

   The TA will be used when an Internet node attempts to communicate
   with another Internet node.  Conceptually you can view the TA as
   replacing the IP number in every instance it now appears in the
   transport layer (i.e., a socket would change from IP#.Port# to
   TA#.Port#).  A connection setup would thus be:

        1.  A user starts an application on Node-A and requests
            service from Node-B.  The user identifies Node-B by
            referencing it's Internet Domain Name.

        2.  The TCP on Node-A makes a Domain Name Service (DNS) call
            to determine the TA of Node-B.

        3.  Node-A constructs a TCP packet using the header Src = TA-
            A.port and Dest = TA-B.port and passes this packet down to
            the network layer.

        4.  The IP on Node-A makes a DNS call to determine the IP
            address of Node-B.  The IP will cache this TA/IP pair for
            later use.

        5.  Node-A constructs an IP packet using the header Src = IP-A
            and Dest = IP-B and passes this packet down to the Media
            Access layer.

        6.  Delivery of the packet is identical to the delivery of an
            existing Internet IP packet.

        7.  The IP on Node-B examines the IP Dest address and if it
            matches it's own, strips off the header and passes the
            data portion up to the TCP.  (Note: the packet may have
            passed through several IP routers between the source and
            destination hosts.)

        8.  The TCP on Node-B examines the header to determine if the
            Dest TA is it's own, if so it passes the data to the
            application specified by the port address.  If not it
            determines if it should perform the transporter function.



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            The packet will be forwarded toward the destination or an
            error message will be returned.

   The above steps represent a quick synopsis of how user applications
   may pass data between different Internet nodes.  The exact structure
   of the network is hidden from the application, allowing the network
   to be modified and improved as needed.  Using the transporter
   function, several different network layers may be traversed when
   moving from source to destination (several examples are provided in
   appendix D).

   One of the underlying assumptions is that the user application must
   refrain from making assumptions about the network structure.  As
   pointed out in section 3, this is not the case for the current
   Internet network.  User applications that are deployed with this new
   TCP must be capable of making this assumption.  This means that the
   user application should store the Internet Domain Name in it's
   internal structure instead of the IPng network number.  The domain
   name is globally unique and provides enough information to the system
   to find the transport and network layer addresses.  The user
   application must pass the following parameters down to TCP:

      1.  Destination domain name  (Text string)
      2.  Pointer to data buffer
      3.  Quality of service indicators
      4.  Options

   When the user application writes data to the network, TCP will return
   a nonzero integer to indicate an error condition, or a zero integer
   to indicate success. When the user application reads data from the
   network, TCP will deliver a pointer to a data buffer back to the
   application.

   TCP will receive the users request and it will make a DNS call to
   determine the destination nodes TA.  If DNS returns a TA, TCP will
   build a Transmission Control Block (TCB) (see the paragraph below)
   and call the network layer.   Otherwise, TCP will make a DNS call
   looking for the destination nodes IPv4 address.  If an address is
   returned, TCP will takes the steps listed below in building a TCB,
   and call the proper network layer.  If DNS returns a host unknown
   indication, exit back to the user with a "host unknown" error.  TCP
   should maintain a cache of domain names and addresses in lieu of
   making repeated DNS calls.  This feature is highly recommended, but
   not required.

   The state information needed to keep track of a TCP connection is
   kept in the Transmission Control Block (TCB).  Currently this
   structure has fields for the TCP parameters, source port, destination



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   port, window size, sequence number, acknowledgment number, and any
   TCP options.  The network layer source and destination IP numbers are
   also stored here.  Finally, the status of the connection (LISTEN,
   ESTABLISHED, CLOSING, of the TCP parameters and include the new
   source and destination Transport addresses.  The existing space for
   the IPv4 addresses will be left in place to allow for backward
   compatibility.  The IPv4 fields will be used if the source is
   communicating directly with an unmodified TCP/IP host.

   The existing status indicators will remain with their meaning
   unchanged.  Connection setup will retain the current 3-way handshake.
   When performing transporter functions, TCP will NOT build a TCB,
   unless the destination is an unmodified IPv4 host (see appendix D).
   The TCP connection remains an end-to-end reliable transport service,
   regardless of the number of intermediate transporter nodes.

   TCP will build an old or new header (defined below) placing the user
   application data in the data field.  If TCP is communicating directly
   with an unmodified IPv4 host, the existing TCP header (STD 7, RFC
   793) will be used for comparability reasons.  If the destination host
   is an unmodified host, and an intermediate transporter node is being
   used, this new TCP header must be used with the 'C' bit set to 1.
   The destination TA will be set to the IPv4 address, and the packet
   will be delivered to the transporter node.  If the destination host
   is modified with this new TCP, the destination address will be set to
   the TA and the packet will be delivered, possibly through a
   transporter, to the remote host.

   TCP will communicate with it's underlying network layer(s) to deliver
   packets to remote hosts.  The Internet Assigned Number Authority
   (IANA) will assign unique identifiers to each network layer TCP will
   support.  TCP will maintain a cache of TA's and IANA network layers
   numbers, to allow support of multiple network layers.  When TCP
   wishes to send data, it will consult this cache to determine which
   network to send the packet to.  If the destination TA is not in this
   cache, TCP will send a request to each of it's network layer(s)
   asking if they know how to deliver data to this TA.  All of the
   network layers supported by the sending host will be probed, in an
   order defined by the system administrator, until one responds 'yes'
   or they all have said 'no'.  The first layer to say 'yes' will be
   used.  If no path exists, an error message will be returned to the
   user application.  Once a network layer is identified, TCP will
   communicate with it by passing the following parameters:

          1)  Destination address (TA or IPv4).
          2)  A pointer to the data buffer.
          3)  Options.




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   The network layer will use the destination address as an index into a
   cache to determine the network address to send to.  In the entry is
   not in the cache, it will make a DNS call to determine the network
   address and a cache entry will be build (see appendix D).  It is
   mandatory that a cache be maintained.  If a host is attached to
   several different networks (i.e., a transporter) each layer will
   maintain it's own cache.

   When IP receives a data packet from a remote node, it will strip off
   the IP header and pass a pointer to the data buffer up to TCP.  IP
   will also supply TCP with it's IANA network layer number.  TCP may
   use the source TA and the IANA number to update it's cache.

   The structure of a TA is to concatenate a unique manufacture code
   with a manufacturer defined variable to form a unique 64 bit number.
   The unique manufacture code will be a 24 bit number, possibly the
   same code as the IEEE 802.3 MAC address code.  The remaining 40 bits
   will be supplied by the manufacture to uniquely identify the TCP.  It
   is recommended that this field be built by encoding the
   manufacturer's serial number.  An integer serial number will be
   viewed as an integer number and converted into it's hexadecimal
   equivalent, left padded with 00 octets if necessary.  If a serial
   number contains Alpha characters, these alpha characters will be
   converted into octets using the international standard ASCII code.
   The integer values will then be converted to their hexadecimal
   equivalent and the 2 values will be concatenated to form the unique
   identifier.  These structure will allow 2^24 (16,777,216)
   manufactures to build 2^40 (1,099,511,627,776) transport addressable
   entities. Each of these entities may have 1 or more network
   interfaces using IPv4, IPng, or any other network layer protocol.

   The current growth of the Internet may indicate that this amount of
   address space is inadequate.  A larger fixed space (i.e., 96 or 128
   bits) or a variable length field may be required.  The disadvantage
   is that this address must be transmitted in every packet.
















Carlson & Ficarella                                            [Page 11]

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4.2  TCPng

                      The new TCP header is as shown.
                        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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Destination TA                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Source TA                                  +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Port Number            |  ver  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Source Port Number                 |  QoS  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Window Size                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Sequence Number                            +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Acknowledgment Number                      +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  data offset  |X|X|C|A|P|R|S|F|     Checksum                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                    Variable length option 1                   /
   \                             :                                 \
   /                             :                                 /
   \                   Variable length Option n                    \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                                 Figure 1


   Destination TA:  64 bits.
           The Destination Transport Address.  The concatenation of
           the 24 bit IEEE assigned Ethernet address and the 40 bit
           representation of the machines serial number for the
           remote node.





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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


   Source TA:  64 Bits.
           The Source Transport Address.  The concatenation of the
           24 bit IEEE assigned Ethernet address and the 40 bit
           representation of the machines serial number for the
           local node.

   Destination Port Number:  28 Bits.
           Identifies the specific application on the remote node.

   Ver:  4 bits.
           Version number.  This is TCPng.  RFC 793
           references 9 earlier editions of ARPA TCP.  The current
           TCP is version 10.

   Source Port Number:  28 Bits.
           Identifies the specific application on the local node.

   QoS:  4 bits.
           The Quality of Service parameter may be set by the user
           application and passed down to a network layer that
           supports different levels of service.

   Window:  32 Bits.
           The number of data octets beginning with the one
           indicated in the acknowledgment field which the sender
           of this segment is willing to accept.

   Sequence Number:  64 Bits.
         The sequence number of the first data octet in this segment
         (accept when the S bit is present). If S bit is on, the
         sequence number is the initial sequence number (ISN) and
         the first data octet is ISN+1.  (The ISN is computed using
         the existing algorithm).

   Acknowledgment Number:  64 Bits.
           If the A bit is set, this field contains the value of
           the next sequence number the sender of this segment is
           expecting to receive.  Once a connection is established,
           this is always sent.

   Data Offset:  8 Bits.
         This is the number of 32 bit words in the TCP header.  This
         indicates where the data begins. The TCP header is an
         integral number of 32 bit words long.  The minimum value is
         12 and the maximum is 256.  If options are used, they must
         pad out to a 32 bit boundary.





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   Flags:  8 Bits.
           The A, P, R, S, and F flags carry the same meaning as in
           the current version of TCP.  They are:

         1.  A = Ack, and acknowledgment field significant
         2.  P = Push, the push function
         3.  R = Reset, reset the connection
         4.  S = Sync, synchronize sequence numbers
         5.  F = Fin, No more data from sender

         The C bit, C = Compatibility,  is used to indicate that one
         end of the connection is an unmodified TCP/IP host.  When
         the C bit is set, all header values must conform to the
         TCPv6 specifications.  The source port, destination port,
         and window size must be 16 bits and the Sequence and
         Acknowledgment numbers must be 32 bits.  These values are
         stored in the lower half of the proper area with null octet
         pads filling out the rest of the field.

         The 2 X bits, X = Reserved,  are not defined and must be
         ignored by a receiving TCP.

   Checksum:  16 Bits.
         The checksum field has the same meaning as in the current
         version of TCP.  The current 96 bit pseudo header is NOT
         used in calculating the checksum.  The checksum covers only
         the information present in this header.  The checksum field
         itself is set to zero for the calculation.

   Variable Length Options:
         There are two types of options, mandatory and optional.  A
         TCP must implement all known mandatory options.  It must
         also be capable of ignoring all optional options it does
         not know about.  This will allow new options to be
         introduced without the fear of damage caused by unknown
         options.  An option field must end on a 32 bit boundary.
         If not, null octet pad characters will be appended to the
         right of the option.  The structure of an option is shown
         in figure 2 below:












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                        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                            |      pad      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

4.3  Mandatory Options

   There are three mandatory options defined by this implementation of
   TCP. Each of these options is implemented using the structure
   pictured in figure 2 above.

   A description of each field follows:

   Type: 16 bits
               The type field identifies the particular option.

   Length: 16 bits
               The length field represents the size of the option
               data to follow, in octets.

   Option Data: Variable Length
               The option data is of variable length specified by
               the length field, plus 0-3 bytes of zeros to pad to a
               32-bit boundary.

   The following are the 3 mandatory options that must be implemented:

   Null: 8 bits
         The null option, (type=0) is represented by the bit
         sequence [00000000], preceded by an additional 8, zero
         padding bits to fill out the full 16-bit type field. The
         data may be of any size, including 0 bytes. The option may
         be used to force an option to be ignored.

   Maximum Segment Size: 8 bits
         The maximum segment size option, (type=1) is represented by
         the bit sequence [00000001] preceded by an additional 8,
         zero padding bits to fill out the full 16-bit type field.
         If this option is present, then it communicates the maximum
         receive segment size at the TCP which sends this segment.
         This potion is mandatory if sent in the initial connection
         request (SYN). If it is sent on any other segment it is
         advisory. The data is a 32-bit word specifying the segment



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         size in octets [Ullmann, 1993].

   Urgent Pointer: 8 bits
         The urgent pointer, (type=2) is represented by the bit
         sequence [00000010] preceded by an additional 8, zero
         padding bits to fill out the full 16-bit type field. This
         option emulates the urgent field in TCPv6. The data is a
         64-bit sequence number identifying the last octet of urgent
         data within the segment.

4.4  Optional Options

   This version of TCP must be capable of accepting any unknown options.
   This is to guarantee that when presented with an unrecognized option,
   TCP will not crash, however it must not reject or ignore any option.

4.5  Compatibility Issues

   The Internet community has a large installed base of IP users.  The
   resources required to operate this network,  both people and machine,
   is enormous.  These resources will need to be preserved.  The last
   time a change like this took place, moving from NCP to TCP, there
   were a few 100 nodes to deal with [Postel, 1981c].  A small close
   knit group of engineers managed the network and mandated a one year
   migration strategy.  Today there are millions of nodes and thousands
   of administrators.  It will be impossible to convert any portion of
   the Internet to a new protocol without effecting the rest of the
   community.

   In the worst case, users will lose communications with their peers as
   some systems upgrade and others do not.  In the current global
   environment, this will not be tolerated.  Any attempt to simply
   replace the current IPv4 protocol with a new IPng protocol that does
   not address compatibility issues is doomed to failure.  This
   reasoning has recently been realized by Ullmann (CATNIP) and he
   attempts to use translators to convert from one protocol to another
   (i.e., CATNIP to IPv4).  The problem is what to do when incompatible
   parameters are encountered.  Also CATNIP would need to be replaced
   every time a new network layer protocol was developed.

   This proposal attempts to solve these problems by decoupling the
   transport and network protocols.  By allowing TCP to operate over
   different network layer protocols, we will create a more stable
   environment.  New network layer protocols could be developed and
   implemented without requiring changes that are visible to the user
   community.  As TCP packets flow from host-to-host they may use
   several different network layers, allowing users to communicate
   without having to worry about how the data is moved across the



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   underlying network.

4.5.1  Backward Compatibility

   It may be said that the maturity of a software package can be
   determined by how much code is required to maintain compatibility
   with previous versions.  With the current growth of the Internet,
   backward compatibility issues can not be dismissed or added in as an
   after thought.  This version of TCP was designed with backward
   compatibility in mind. When the TCP communicates with an unmodified
   IPv4 TCP/IP, it takes steps to insure compatibility.  First off it
   sets a bit in the header indicating that the TCP parameters (ack,
   seq, port numbers, and window size) use the TCPv6 values.  When
   communicating directly with an unmodified host the existing TCP/IP
   header is used.  Only existing TCP options may be sent as well.

   The advantage of this approach is that TCP transporter nodes will not
   have to make decisions about how to modify packets just passing
   through.  It is up to the source node to build a header that is
   compatible before sending it.  This approach will allow any new TCP
   to contact and communicate with any unmodified IPv4 host.  The source
   host may have an IPv4 address, or it may send data to a transporter
   for delivery.  The decision will be made based on the source and
   destination addresses.  During connection setup, the location of the
   destination node is determined and the proper network layer is used
   to send data.

   An existing IPv4 host will be capable of opening a connection to any
   new TCPng host that is directly connected to the network with an IPv4
   protocol stack.  If the TCPng host only has an IPng stack, the
   connection attempt will fail.  Some existing batch style services
   (i.e., Simple Mail Transfer Protocol - SMTP) will continue to work
   with the help of transporters.  Interactive sessions (i.e., Telnet)
   will fail.  Thus, our new TCP is backward compatible, but the
   existing IPv4 hosts are not forward compatible.

4.6  Level 4 Gateways

   The ability to allow hosts with differing network layer protocols to
   communicate will be accomplished by using a transport layer gateway
   (called transporter in this paper).  The transporter works just like
   an IP router, receiving TCP packets from one network layer and
   transporting them over to another.  This switching is done by
   examining the packets source and destination TA's.  If a TCP packet
   arrives with a destination TA that differs from this hosts TA, and
   the transporter functionality is enabled, the packet should be
   transported to another network layer.  In some cases, the receiving
   node is a host and not a transporter (i.e., transporter functionality



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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


   disabled).  In this case the host will discard the packet and return
   a TCMP (see below) error message.

   A transporter is not responsible for reading or formatting the TCP
   header of packets it receives.  The header is simply examined to
   determine where to deliver the packet.  When forwarding, the packet
   is sent to any of the network layers the transporter supports.  The
   exception is that the packet may not be presented back to the network
   it was received from. It is the responsibility of the network layer
   to destroy undeliverable packets.  If a transporter is unable to
   determine which network the packet should be forwarded to, the packet
   is discarded and a TCMP message is generated and returned to the
   original source host.  Several examples of how transporting works are
   presented in appendix D.

4.7  Error Conditions

   It is recognized that from time to time certain error conditions will
   occur at some intermediate transporter that will need to be
   communicated back to the source host.  To accomplish this a Transport
   Control Message Protocol (TCMP) service facility will need to be
   developed.  This protocol will model itself after the Internet
   Control Message Protocol (ICMP).  The operational details are
   discussed in a separate TCMP document.

5.  Advantages and Disadvantages of this approach

   This proposal offers the Internet community several advantages.
   First, TCPng will operate over multiple network layer protocol
   stacks.  Users will be able to select the stack(s) that meets their
   needs.  The problem of IPv4 address exhaustion will be postponed as
   sites move from IPv4 to IPng protocol stacks. Future IP3g protocol
   stacks may be designed and deployed without major service
   disruptions.  The increased size of the sequence, acknowledge, and
   window fields will allow applications to run effectively over high
   bandwidth-delay network links.  Lastly, TCPng will allow applications
   to specify certain Quality of Service (QoS) parameters which may be
   used by some network layer protocols (i.e., Asynchronous Transfer
   Mode - ATM).

   This protocol is not without it's share of design compromises.  Among
   these are a large packet header increased in size from 5 to 12 long
   words.  The addition of a TA means that network administrators must
   deal with yet another network number that must be globally
   maintained.  Multiple network protocols may add to the complexity of
   a site's network.  Lastly, is the TA address space large enough so we
   will not have to rebuild TCP.




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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


6.  Conclusions

   In this paper, we have reviewed the current status of the Internet
   society s IPng initiative.  We were struck by the enormity of the
   changes required by those proposals.  We felt that a different
   approach was needed to allow change to occur in a controlled manner.
   This approach calls for replacing the current TCP protocol with one
   that does not require a specific IP layer protocol.  Once this is in
   place, various IPng protocols may be developed and deployed as sites
   require them.  Communications between IPv4 and IPng hosts will be
   maintained throughout the transition period.  Modified hosts will be
   able to remove their IPv4 protocol stacks, while maintaining
   communications with unmodified hosts by using a TCP transporter.

   The title of this paper "Six Virtual Inches to the Left" comes from a
   talk the author once heard.  In this talk an engineer from Control
   Data Corporation (CDC) told a story of CDC's attempt to build a
   cryogenically cooled super computer.  The idea being that the power
   consumption of such a computer would be far lower then that of a
   conventional super computer.  As the story goes, everyone thought
   this was a great idea until someone pointed out what the power
   requirements of the cryo system were.  The result was that all the
   assumed power savings were consumed by the cryo system.  The
   implication being that all the power requirements were not saved but
   simply moved 6 feet from the CPU to the support equipment.  The moral
   being that the entire system should be analyzed instead of just one
   small piece.

References

   [Postel, 1981a] Postel, J., "Transmission Control Protocol - DARPA
   Internet Program Protocol Specification", STD 7, RFC 793, DARPA,
   September 1981.

   [Halsal, 1992] Data Communications, Computer Networks, and Open
   Systems.

   [Meyer, Zobrist, 1990] TCP/IP versus OSI, The Battle of the
   Network Standards, IEEE Potentials.

   [Braden, et al, 1991] Clark, D., Chapin, L., Cer, V., Braden, R., and
   R. Hobby, "Towards the Future Internet Architecture", RFC 1287,
   MIT, BBN, CNRI, ISI, UCDavis, December 1991.

   [Dixon, 1993] Dixon, T., "Comparison of Proposals for Next Version of
   IP", RFC 1454, RARE, May 1993.





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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


   [Fuller, et al, 1992] Fuller, V., Li, T., Yu, J., and K. Varadhan,
   "Supernetting: an Address Assignment and Aggregation Strategy",
   RFC 1338, BARRNet, cicso, Merit, OARnet, June 1992.

   [Almquist, Gross, 1992] Gross, P., and P. Almquist, "IESG
   Deliberations on Routing and Addressing", RFC 1380, IESG Chair,
   IESG Internet AD, November 1992.

   [Postel, 1981b] Postel, J., "Transmission Control Protocol - DARPA
   Internet Program Protocol Specification", STD 7, RFC 793, DARPA,
   September 1981.

   [Postel, 1980] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
   USC/Information Sciences Institute, August 1980.

   [Postel, 1981c] Postel, J., "NCP/TCP Transition Plan", RFC 801,
   USC/Information Sciences Institute, November 1981.

   [Leiner, Rekhter, 1993] Leiner, B., and Y. Rekhter, "The
   Multi-Protocol Internet" RFC 1560, USRA, IBM, December 1993.

   [Ullmann, 1993] Ullmann, R., "TP/IX: The Next Internet", RFC 1475,
   Process Software Corporation, June 1993.

Bibliography

   Gilligan, Nordmark, and Hinden, "The SIPP Interoperability and
   Transition Mechanism", IPAE, 1993.

   Jacobson, V., and R. Braden, "TCP Extensions for Long-Delay Paths",
   RFC 1072, LBL, USC/Information Sciences Institute, October 1988.

   Jacobson, V., Braden, R., and D. Borman, "TCP Extensions for High
   Performance", RFC 1323, LBL, USC/Information Sciences Institute, Cray
   Research, May 1992.

   Jacobson, V., Braden, R., and L. Zhang, "TCP Extension for High-Speed
   Paths", RFC 1185, LBL, USC/Information Sciences Institute, PARC,
   October 1990.

   Leiner, B., and Y. Rekhter, "The Multiprotocol Internet", RFC 1560,
   USRA, IBM, December 1993.

   O'Malley, S., and L. Peterson, "TCP Extensions Considered Harmful",
   RFC 1263, University of Arizona, October 1991.

   Westine, A., Smallberg, D., and J. Postel, "Summary of Smallberg
   Surveys", RFC 847, USC/Information Sciences Institute, February 1983.



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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


Appendix A

   The minimum size of an ethernet frame is 64 bytes.  With the existing
   TCP/IP protocol, a minimum size frame is 18 bytes (ethernet header &
   trailer) + 20 bytes (IP header) + 20 bytes (TCP header) for a total
   of 58 bytes.  The transmitting station must add 6 null pad characters
   to this frame to make it conform to the 64 byte minimum.  This new
   TCP will increase the size of the TCP header to 48 bytes.
   Subtracting 26 bytes (the old header and pad characters) we are left
   with 22 bytes or 176 bits.  The time it takes to transmit these
   additional bits is the impact of this new TCP.  The transmission time
   for several types of media currently used is shown in the table
   below.  You will note that the increased times are all under 20
   micro-seconds for anything over T1 speeds.  User traffic patterns
   vary of course but it is generally agreed that 80% of the traffic
   stays at the local site.  If this is true then the increased header
   size has a negligible impact on communications.


      Media       Speed (Mbps)      Rate  (nsec/bit)  time (usec)
      ------      ------------      ---------------   ----------
        T1            1.544            647.7            144.00
        T3           44.736             22.4              3.91
        Enet         10.00             100.0             17.60
        FDDI        100.00              10.0              1.76
        OC-1         51.84              19.3              3.40
        OC-3        155.52               6.4              1.13

Appendix B

   In order to support the TA, new DNS entries will need to be created.
   It is hoped that this function will be accomplished automatically.
   When a station is installed, the local DNS server is defined.  On
   power up, the station will contact this server and send it it's TA
   and domain name.  A server process will be listening for this type of
   information, and it will collect the data, run an authorization
   check, and install the TA into the DNS server.  The following entry
   will be made.

   node.sub.domain.name    IN     TA   xx.yy.zz.aa.bb.cc.dd.ee

   ee.dd.cc.bb.aa.zz.yy.aa.in-addr.tcp IN  PTR node.sub.domain.name.

   Using these entries, along with the existing DNS A records, a
   requesting node can determine where the remote node is located.  The
   format xx.yy.zz is the IEEE assigned portion and aa.bb.cc.dd.ee is
   the encoded machine serial number as described in section 4.1.




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RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


Appendix C

                          Proposed UDP Header


                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Destination TA                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Source TA                                  +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Port Number            |  ver  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Source Port Number                 |  QoS  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Length               |        Checksum               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                             Data                              /
   \                             :                                 \
   /                             :                                 /
   \                             :                                 \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Destination TA:  64 bits.
         The Destination Transport Address.  The concatenation of
         the 24 bit IEEE assigned Ethernet address and the 40 bit
         representation of the machines serial number for the remote
         node.

   Source TA:  64 Bits.
         The Source Transport Address.  The concatenation of the 24
         bit IEEE assigned Ethernet address and the 40 bit
         representation of the machines serial number for the local
         node.

   Destination Port Number:  28 Bits.
         Identifies the specific application on the remote node.

   Ver:  4 bits.

         This parameter the UDP version number in use within this
         packet.




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   Source Port Number:  28 Bits.
         Identifies the specific application on the local node.

   QoS:  4 bits.
         The Quality of Service parameter may be set by the user
         application and passed down to a network layer that
         supports different levels of service.

   Length:  16 bits
         The length parameter represents the length of the data area
         in octets.  This value will be set to zero if no data is
         sent within this packet.

   Checksum:  16 bits
         The checksum parameter has the same meaning as in the
         current version of UDP.  The current 96 bit pseudo header
         is NOT used in calculating the checksum.  The checksum
         covers only the information present in this header.  The
         checksum field itself is set to zero for the calculation.

   Data: Variable
         This is the area in which the data for the datagram will be
         sent.  The length of this data in octets is specified by
         the length parameter above.



























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Appendix D


          ______                         ______
         |      |                       |      |
         |  H1  |                       |  H2  |
         |      |                       |      |
         |______|                       |______|
              \                          /    \
               \                        /      \
            =========================  /        \
           "                         "/         |
           "       (SIPP)            "          |
           "                         "          |
           "========================="          |
                                                |
                                   ====================
                ______            "                    "
               |      |           "       CLNP         "
               |  H4  |           "                    "
               |      |           "===================="
               |______|                    |
                     \                     |
                      \                    |
             ===================        ___|___
            "                  "       |       |
            "                  "-------|  H3   |
            "     IPv4         "       |       |
            "                  "       |_______|
            "=================="


   Example 1: H1 Wishes to Establish Communication with H4 (Refer to the
   figure above.)

      1.  A user on host H1 attempts to communicate with a user
          on host H4 by referencing H4 s fully qualified domain name.

      2.  The TCP on H1 makes a DNS call to determine the TA
          address of H4.

      3.  The DNS call returns only the IPv4 address since H4 is
          determined to be an IPv4 only host.

      4.  The H1 TCP builds a transmission control block (TCB)
          setting the C-Bit (compatibility) "ON" since H4 is an IPv4
          host.  Included in the TCB will also be DA = IP-H4, SA =
          TA1, DP = 1234, SP = 5000 and any state parameters



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          describing the connection (port numbers are for example
          purposes only).

      5.  The IP on H1 makes a DNS call to determine the network
          IP address of H4 and correspondingly caches both the TA
          address from the TCP as well as the network IP address for
          later use.

      6.  The packet is now routed using standard SIPP procedures
          to H2 this is the only path H1 has to H4.

      7.  H2 receives the packet from H1.  The TCP on H2 checks
          the destination TA of the packet and compares it to its
          own.  In this case it does not match, therefore the packet
          should be forwarded.

      8.  H2 s TCP will interrogate the supported network
          layer(s) and determines the packet must be forwarded to H3.

      9.  The TCP must now pass the packet the CLNP network
          layer.  The network layer checks its cache to determine if
          there is a route specified for DA = IP-H4 already in the
          cache.  If so the cache entry is used, if not an entry is
          created.  H2 then routes the packet to H3 via NA3a, which
          is the network layer address for IP-H4.

      10.  H3 receives the packet from H2. The TCP on H3 checks
           the destination TA of the packet and compares it to its
           own. Once again, it does not match.

      11.  H3, realizing that the destination address is an IPv4
           host, and knowing that it itself is directly connected to
           the IPv4 network constructs an IPv4 compatible header.  H3
           also constructs a TCB to manage the IPv4 connection.

      12.  The packet is sent down to be routed to the IP using
           standard IP routing procedures.

      13.  H4 receives the packet at which point the IP on it
           determines that the destination address is its own and thus
           proceeds to strip off the IP header and pass the packet up
           to the TCP layer.

      14.  The TCP layer than opens the corresponding IPV4_DP
           port (2311) which forms the first half of the connection to
           the application.





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      15.  H4 will now reply with a connection accept message,
           sending the packet back to H3.

      16.  H3 s TCP receives the packet and based on information
           in the TCB determines the packet should be delivered to H1.
           H3 uses the steps outlined above to route the packet back
           through the network structure.

   Example 2: H2 Wishes to Establish Communication with H3 (Refer to the
   figure above.)

      1.  A user on host H2 attempts to communicate with a user
          on host H3 by referencing H3 s fully qualified domain name.

      2.  The TCP on H2 makes a DNS call to determine the TA
          address of H3.

      3.  The DNS call returns the TA address for H3.

      4.  The H2 TCP builds a transmission control block (TCB)
          setting the C-Bit (compatibility) "OFF" since H3 is an IPng
          host.  Included in the TCB will also be DA = TA3, SA = TA2,
          DP = 1111, SP = 2222 and any state parameters describing
          the connection (port numbers are for example purposes
          only).

      5.  The IPng on H2 makes a DNS call to determine the
          network IPng address of H3 and correspondingly caches both
          the TA address from the TCP as well as the network IPng
          address for later use.

      6.  The packet is now routed to H3 over the IPng supported
          on that network.

      7.  H3 receives the packet from H2.  The TCP on H3 checks
          the destination TA of the packet and compares it to its
          own.  In this case it matches.

      8.  H3 s TCP will construct a TCB and respond with an open
          accept message.

      9.  H3 s TCP will interrogate the supported network
          layer(s) to determine the packet must be delivered to H2
          using NA2b which is specified in its cache.







Carlson & Ficarella                                            [Page 26]

RFC 1705     Six Virtual Inches to the Left: IPng Problems  October 1994


Security Considerations

   Security issues are not discussed in this memo.

Authors' Addresses

   Richard Carlson
   Argonne National Laboratory
   Electronics and Computing Technologies
   Argonne,  IL  60439

   Phone:  (708) 252-7289
   EMail:  RACarlson@anl.gov


   Domenic Ficarella
   Motorola

   Phone:  (708) 632-4029
   EMail:  ficarell@cpdmfg.cig.mot.com































Carlson & Ficarella                                            [Page 27]

