INTERNET-DRAFT                           Erik Nordmark, Sun Microsystems
July 30,
November 20, 1997

                    Site prefixes in Neighbor Discovery

                 <draft-ietf-ngtrans-header-trans-00.txt>

                    Stateless IP/ICMP Translator (SIIT)

                 <draft-ietf-ngtrans-header-trans-01.txt>

Status of this Memo

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
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   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

   Distribution of this memo is unlimited.

   This Internet Draft expires January 30, May 20, 1998.

Abstract

   This document specifies a transition mechanism in addition to those
   already specified in RFC 1933.  The new mechanism can be used as part
   of a solution that allows IPv6 hosts that do not have a permanently
   assigned IPv4 address to communication with IPv4-only hosts.

Acknowledgements

   Some text has been pulled from an old Internet Draft titled "IPAE:
   The SIPP Interoperability and Transition Mechanism" authored by R.
   Gilligan, E. Nordmark, and B. Hinden.  George Tsirtsis provides the
   figures for Section 1.

Contents

   Status of this Memo..........................................    1

   1.  INTRODUCTION AND MOTIVATION..............................    2    3
      1.1.  Applicability and Limitations.......................    4    5
      1.2.  Impact Outside the Network Layer....................    5    6

   2.  TERMINOLOGY..............................................    6    7
      2.1.  Requirements........................................    6    8

   3.  OVERVIEW.................................................    6    8
      3.1.  Assumptions.........................................    7    8

   4.  TRANSLATING FROM IPv4 TO IPv6............................    7    9
      4.1.  Translating IPv4 Headers............................    8   10
      4.2.  Translating ICMPv4..................................   10   12
      4.3.  Translating ICMPv4 Error Messages...................   12   14
      4.4.  Knowing when to Translate...........................   13   14

   5.  TRANSLATING FROM IPv6 TO IPv4............................   13   15
      5.1.  Translating IPv6 Headers............................   14   16
      5.2.  Translating ICMPv6..................................   16   18
      5.3.  Translating ICMPv6 Error Messages...................   17   19
      5.4.  Knowing when to Translate...........................   18   20

   6.  SECURITY CONSIDERATIONS..................................   18   20

   7.  OPEN ISSUES..............................................   20

   REFERENCES...................................................   19   21

   AUTHOR'S ADDRESS.............................................   20   22

   APPENDIX A: CHANGES SINCE PREVIOUS DRAFT.....................   22

1.  INTRODUCTION AND MOTIVATION

   The transition mechanisms specified in [TRANS-MECH] handle the case
   of dual IPv4/IPv6 hosts interoperating with both dual hosts and
   IPv4-only hosts which is needed early in the transition to IPv6.  The
   dual hosts are assigned both an IPv4 and one or more IPv6 addresses.
   As the pool of globally unique IPv4 addresses becomes smaller and
   smaller as the Internet grows there will be a desire to take
   advantage of the large IPv6 address and not require that every new
   Internet node have a permanently assigned IPv4 address.

   There are several different scenarios where there might be IPv6-only
   hosts that need to communicate with IPv4-only hosts. [Note: the
   terminology is somewhat unclear is this area.  Is a node with that
   implements both IPv4 and  These IPv6 but has no
   hosts might be IPv4-capable, i.e. include an IPv4 address implementation but
   not be assigned to any
   of its interfaces a dual host an IPv4 address, or they might not even include an IPv6-only host?]
   IPv4 implementation.

    - A completely new network with new devices that all support IPv6.
      In this case it might be beneficial to not have to configure the
      routers within the new network to route IPv4 since none of the
      hosts in the new network are configured with IPv4 addresses.  But
      these new IPv6 devices might occasionally need to communicate with
      some IPv4 nodes out on the Internet.

    - An existing network where a large number of IPv6 devices are
      added.  The IPv6 devices might have both an IPv4 and an IPv6
      protocol stack but there is not enough global IPv4 address space
      to give each one of them a permanent IPv4 address.  In this case
      it is more likely that the routers in the network already route
      IPv4 and are upgraded to dual routers.

   If there is no IPv4 routing inside the network i.e., the cloud that
   contains the new devices, some possible solutions are to either use
   the translators specified in this document at the boundary of the
   cloud, or to use Application Layer Gateways (ALG) on dual nodes at
   the cloud's boundary.  The ALG solution is less flexible in that it
   is application protocol specific and it is also less robust since a
   the ALG box is likely to be a single point of failure for a
   connection using that box.

   If there IPv4 routing is supported inside the cloud and the
   implementations support both IPv6 and IPv4 it might suffice to have a
   mechanism for allocating temporary IPv4 and use IPv4 end to end when
   communicating with IPv4-only nodes.  However, it would seem that such
   a solution would require the pool of temporary IPv4 addresses to be
   partitioned across all the subnets in the cloud which would either
   require a larger pool of IPv4 addresses or result in cases where
   communication would fail due to no available IPv4 address for the
   node's subnet.

   This document specifies a mechanism by which IPv6-only nodes can
   interoperate with IPv4-only nodes by having the IPv6-only nodes
   somehow acquire a temporary IPv4 address.  That IPv4 address will be
   used as an IPv4-compatible IPv6 address and the packets will travel
   through a stateless IP header IP/ICMP translator that will translate the packet
   headers between IPv4 and IPv6 and translate the addresses in those
   headers between IPv4 addresses on one side and IPv4-compatible or
   IPv4-mapped IPv6 addresses on the other side.

   This specification does not cover how an IPv6 node can acquire a
   temporary IPv4 address and how such a temporary address be registered
   in the DNS.  The DHCP protocol, perhaps with some extensions, could
   probably be used to acquire temporary addresses with short leases but
   that is outside the scope of this document.  The mechanism for
   routing this temporary IPv4 address (or the IPv4-compatible IPv6
   address) in the site is currently not specified in this document.

   The figures below show how the Stateless IP/ICMP Translator (SIIT)
   can be used initially for small networks (e.g., a single subnet) and
   later for a site which has IPv6-only hosts in a dual IPv4/IPv6
   network.  This use assumes a mechanism for the IPv6 nodes to acquire
   an temporary address from the pool of IPv4 addresses.  Note that SIIT
   is not likely to be useful later during transition when most of the
   Internet is IPv6 and there are only small islands of IPv4 nodes,
   since such use would either require the IPv6 nodes to acquire
   temporary IPv4 addresses from a "distant" SIIT box operated by a
   different administration, or require that the IPv6 routing contain
   routes for IPv6-mapped addresses.  (The latter is known to be a very
   bad idea.)

                                     ___________
                                    /           \
      [IPv6 Host]---[SIIT]---------< IPv4 network>--[IPv4 Host]
                       |            \___________/
                (pool of v4 addresses)

   Figure 1.  Using SIIT for a single IPv6-only subnet.

                     ___________              ___________
                    /           \            /           \
      [IPv6 Host]--< Dual network>--[SIIT]--< IPv4 network>--[IPv4 Host]
                    \___________/      |     \___________/
                             (pool of v4 addresses)

   Figure 2.  Using SIIT for an IPv6-only or dual cloud (e.g. a site)
   which contains some IPv6-only hosts as well as IPv4 hosts.

1.1.  Applicability and Limitations

   The IPv6 protocol [IPv6] has been designed so that the transport
   pseudo-header checksums are not affected by such a translation thus
   the translator does not need to modify TCP and UDP headers.  However,
   ICMPv6 include a pseudo-header checksum but it is not present in
   ICMPv4 thus the checksum in ICMP messages need to be modified by the
   translator.  In addition, ICMP error messages contain an IP header as
   part of the payload thus the translator need to rewrite those parts
   of the packets to make the receiver be able to understand the
   included IP header.  However, all of the translators operations,
   including path MTU discovery, are stateless in the sense that the
   translator operates independently of each packet and does not retain
   any state from one packet to another.  This allows redundant
   translator boxes without any coordination and a given TCP connection
   can have the two directions of packets go through different
   translator boxes.

   The translating function as specified in this document does not
   translate any IPv4 options and it does not translate IPv6 routing
   headers, hop-by-hop extension headers, or destination options
   headers.  It could be possible to define a translation between source
   routing in IPv4 and IPv6.  However such a translation would not be
   semantically correct since the IPv4 source routing option performs a
   "record route" function as the nodes listed in the source route are
   traversed and the IPv6 routing header does not include the record
   route aspect.  Also, the usefulness of source routing when going
   through a header translator might be limited since all the routers
   would need to have an IPv4 address (or an IPv4-compatible IPv6
   address) since the IPv4-only node will send a source option
   containing only IPv4 addresses.

   At first sight it might appear that the IPsec functionality [IPv6-SA,
   IPv6-ESP, IPv6-AH] can not be carried across the translator.
   However, since the translator does not modify any headers above the
   logical IP layer (IP headers, IPv6 fragment headers, and ICMP
   messages) packets encrypted using ESP in Transport-mode can be
   carried through the translator.  [Note that this assumes that the key
   management can operate between the IPv6-only and the IPv4-only node.]
   The use of AH headers is more complex since the AH computation covers
   most of the fields in the IP header.  Should it be possible for the
   IPv6 node to predict the value of all the IPv4 header fields on the
   other side of the translator then the IPv6 node could calculate the
   authentication data using an IPv4 header instead of the IPv6 header
   even though it is sending and receiving IPv6 packets.  [Currently
   this is not possible since the IP fragment identification field is
   not carried end-to-end through the translator in all cases.] cases.  This
   could be resolved by changing AH to not include the fragment
   identification field in the AH computation for either IPv4 or IPv6.]
   For ESP Tunnel-mode the IPv6 node would have to be able to parse and
   generate "inner" IPv4 headers since the inner IP will be encrypted
   together with the transport protocol.

   IPv4 multicast addresses can not be mapped to IPv6 multicast
   addresses.  For instance, ::ffff:224.1.2.3 is an IPv4 mapped IPv6
   address with a class D address, however it is not an IPv6 multicast
   address.  While the IP/ICMP translation aspect of this draft works
   for multicast packets this address mapping limitation makes it hard
   to the techniques in this draft for multicast traffic.

1.2.  Impact Outside the Network Layer

   The potential existence of header stateless IP/ICMP translators is already
   taken care of from a protocol perspective in [IPv6].  However, an
   IPv6 node that wants to be able to use translators need some
   additional logic in the network layer.

   The network layer in an IPv6-only node when presented with either an
   IPv4 destination address or an IPv4-mapped IPv6 destination address
   by the application is likely to drop the packet and return some error
   message to the application.  In order to take advantage of
   translators such a node should instead send an IPv6 packet where the
   destination address is the IPv4-mapped address and the source address
   is the nodes temporarily assigned IPv4-compatible address.  If the
   node does not have a temporarily assigned IPv4-compatible address it
   should acquire one using mechanisms that are not discussed in this
   document.

   Note that the above also applies to a dual implementation node which
   is not configured with any IPv4 address.

   There are no extra changes needed to applications to operate through
   a translator.  The applications that have been modified to work on a
   dual node already have the mechanisms to determine whether they are
   communicating with an IPv4 or an IPv6 peer.  Thus if the applications
   need to modify their behavior depending on the type of the peer, such
   as ftp determining which flavor of PORT command to use, they already
   need to do that when running on dual nodes and the presense of
   translators does not add anything.  For example, when using the
   socket API [RFC 2133] the applications know that the peer is IPv6 if
   they get an AF_INET6 address from the name service and the address is
   not an IPv4-mapped address (i.e., IN6_IS_ADDR_V4MAPPED returns
   false).  If this is not the case, i.e., the address is AF_INET or an
   IPv4-mapped IPv6 address, the peer is IPv4.

   One way of viewing the translator, which might help clarify why
   applications do not need to know that a translator is used, is to
   look at the information that is passed from the transport layer to
   the network layer.  If the transport passes down an IPv4 address
   (whether or not is in the IPv4-mapped encoding) this means that at
   some point there will be IPv4 packets generated.  In a dual node the
   generation of the IPv4 packets takes place in the sending node.  In
   an IPv6-only node conceptually the only difference is that the IPv4
   packet is generated by the translator - all the information that the
   transport layer passed
   transport layer passed to the network layer will be conveyed to the
   translator in some form.  That form just "happens" to be in the form
   of an IPv6 header.

2.  TERMINOLOGY

   This documents uses the terminology defined in [IPv6] and [TRANS-
   MECH] with these clarifications:

           IPv4 capable node:

                   A node which has an IPv4 protocol stack.  In order
                   for the stack to be usable the node must be assigned
                   one or more IPv4 addresses.

           IPv4 enabled node:  A node which has an IPv4 protocol stack
                   and is assigned one or more IPv4 addresses.  Both
                   IPv4-only and IPv6/IPv4 nodes are IPv4 enabled.

           IPv6 capable node:

                   A node which has an IPv6 protocol stack.  In order
                   for the stack to the network layer will be conveyed to usable the
   translator in some form.  That form just "happens" to node must be in the form
   of assigned
                   one or more IPv6 addresses.

           IPv6 enabled node:  A node which has an IPv6 header.

2.  TERMINOLOGY

   This documents uses the terminology defined in [IPv6] protocol stack
                   and [TRANS-
   MECH]. is assigned one or more IPv6 addresses.  Both
                   IPv6-only and IPv6/IPv4 nodes are IPv6 enabled.

2.1.  Requirements

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [KEYWORDS].

3.  OVERVIEW

   The protocol translators are assumed to fit around some piece of
   topology that includes some IPv6-only nodes and can also include IPv4
   nodes and dual nodes.  There has to be a translator on each path in
   and out of this cloud to ensure that the packets always get
   translated.  This does not require a translator at every physical
   connection between the cloud and the rest of the Internet since the
   routing can be used to deliver the packets to the translator.

   For outbound packets i.e., packets that need to be translated from
   IPv6 to IPv4, it is sufficient to have a route for the IPv4-mapped
   address prefix (::ffff:0:0/96) injected in the internal IPv6 routing
   tables.  This route will deliver packets to the translator since all
   IPv6 packets that need translation will have an IPv4-mapped IPv6
   destination address.

   Inbound IPv4 packets needing translation are likely to have some
   temporary IPv4 address that is drawn from a pool of such addresses.
   Thus the internal IPv4 routing tables could have one or more routes
   for the whole pool that direct the packets to the translator.

3.1.  Assumptions

   The IPv6 nodes using the translator must have an IPv4-compatible IPv6
   address while it is communicating with IPv4 nodes.

4.  TRANSLATING FROM IPv4 TO IPv6

   When an IPv4-to-IPv6 translator receives an IPv4 datagram addressed
   to a destination that lies outside of the attached IPv4 island, it
   translates the IPv4 header of that packet into an IPv6 header.  It
   then forwards the packet based on the IPv6 destination address.  The
   original IPv4 header on the packet is removed and replaced by a IPv6
   header.  Except for ICMP packets the transport layer header and data
   portion of the packet are left unchanged.

        +-------------+                 +-------------+
        |    IPv4     |                 |    IPv6     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |  Transport  |                 |  Fragment   |
        |   Layer     |      ===>       |   Header    |
        |   Header    |                 |(not always) |
        +-------------+                 +-------------+
        |             |                 |  Transport  |
        ~    Data     ~                 |   Layer     |
        |             |                 |   Header    |
        +-------------+                 +-------------+
                                        |             |
                                        ~    Data     ~
                                        |             |
                                        +-------------+

                    IPv4-to-IPv6 Header Translation

   One of the differences between IPv4 and IPv6 is that in IPv6 path MTU
   discovery is mandatory but it is optional in IPv4.  This implies that
   IPv6 routers will never fragment a packet - only the sender can do
   fragmentation.

   When the IPv4 node performs path MTU discovery (by setting the DF bit
   in the header) the path MTU discovery can operate end-to-end i.e.
   across the translator.  In this case either IPv4 or IPv6 routers
   might send back ICMP "packet too big" messages to the sender.  When
   these ICMP errors are sent by the IPv6 routers they will pass through
   a translator which will translate the ICMP error to a form that the
   IPv4 sender can understand.  In this case an IPv6 fragment header is
   only included if the IPv4 packet is already fragmented.

   However, when the IPv4 sender does not perform path MTU discovery the
   translator has to ensure that the packet does not exceed the path MTU
   on the IPv6 side.  This is done by fragmenting the IPv4 packet so
   that it fits in 576 byte IPv6 packet since IPv6 guarantees that 576
   byte packets never need to be fragment.  Also, when the IPv4 sender
   does not perform path MTU discovery the translator MUST always
   include an IPv6 fragment header to indicate that the sender allows
   fragmentation.  That is needed should the packet pass through an
   IPv6-to-IPv4 translator.

   The above rules ensure that when packets are fragmented either by the
   sender or by IPv4 routers that the low-order 16 bits of the fragment
   identification is carried end-end to ensure that packets are
   correctly reassembled.  In addition, the rules use the presence of an
   IPv6 fragment header to indicate that the sender might not be using
   path MTU discovery i.e. the packet should not have the DF flag set
   should it later be translated back to IPv4.

   Other than the special rules for handling fragments and path MTU
   discovery the actual translation of the packet header consists of a
   simple mapping as defined below.  Note that ICMP packets require
   special handling in order to translate the content of ICMP error
   message and also to add the ICMP pseudo-header checksum.

4.1.  Translating IPv4 Headers

   If the DF flag is not set and the IPv4 packet will result in an IPv6
   packet larger than 576 bytes the IPv4 packet MUST be fragmented prior
   to translating it.  Since IPv4 packets with DF not set will always
   result in a fragment header being added to the packet the IPv4
   packets must be fragmented so that their length, excluding the IPv4
   header, is at most 528 bytes (576 minus 40 for the IPv6 header and 8
   for the Fragment header).  The resulting fragments are then
   translated independently using the logic described below.

   If the DF bit is set and the packet is not a fragment (i.e., the MF
   flag is not set and the Fragment Offset is zero) then there is no
   need to add a fragment header to the packet.  The IPv6 header fields
   are set as follows:

           Version:
                   6

           Flow ID:
                   0 (all zero bits)

           Payload Length:
                   Total length value from IPv4 header, minus the size
                   of the IPv4 header and IPv4 options, if present.

           Payload Type:
                   Protocol field copied from IPv4 header

           Hop Limit:
                   TTL value copied from IPv4 header, decremented by
                   one.

           Source Address:
                   The low-order 32 bits is the IPv4 source address.
                   The high-order 96 bits is the IPv4-mapped prefix
                   (::ffff:0:0/96)

           Destination Address:
                   The low-order 32 bits is the IPv4 destination
                   address.  The high-order 96 bits is the IPv4-
                   compatible prefix (0::0/96)

   If IPv4 options are present in the IPv4 packet, they are ignored
   i.e., there is no attempt to translate them.
   [Discussion: Should the packet be dropped and an ICMP error be
   generated instead?]

   If there is need to add a fragment header (the DF bit is not set or
   the packet is a fragment) the header fields are set as above with the
   following exceptions:

       IPv6 fields:

           Payload Length:
                   Total length value from IPv4 header, plus 8 for the
                   fragment header, minus the size of the IPv4 header
                   and IPv4 options, if present.

           Payload Type:
                   Fragment Header (44).

       Fragment header fields:

           Next Header:
                   Protocol field copied from IPv4 header.

           Fragment Offset:
                   Fragment Offset copied from the IPv4 header.

           M flag:
                   More Fragments bit copied from the IPv4 header.

           Identification:
                   The low-order 16 bits copied from the Identification
                   field in the IPv4 header.  The high-order 16 bits set
                   to zero.

4.2.  Translating ICMPv4

   All ICMP messages that are to be translated require that the ICMP
   checksum field be updated as part of the translation since ICMPv6 ICMPv6,
   unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.

   In addition all ICMP packets needs to have the Type value translated
   and for ICMP error messages the included IP header also needs
   translation.

   The actions needed to translate various ICMPv4 messages are:

      ICMPv4 query messages:

        Echo and Echo Reply (Type 8 and Type 0)
           Adjust the type to 128 and 129, respectively, and adjust the
           ICMP checksum both take the type change into account and to
           include the ICMPv6 pseudo-header.

        Information Request/Reply (Type 15 and Type 16)
           Obsoleted in ICMPv4.  Silently drop.

        Timestamp and Timestamp Reply (Type 13 and Type 14)
           Obsoleted in ICMPv6.  Silently drop.

        Address Mask Request/Reply (Type 17 and Type 18)
           Obsoleted in ICMPv6.  Silently drop.

        ICMP Router Advertisement (Type 9)
           Single hop message.  Silently drop.

        ICMP Router Solicitation (Type 10)
           Single hop message.  Silently drop.

        Unknown ICMPv4 types
           Silently drop.

      IGMP messages:

           While there are ICMPv6 counterparts for the IGMP messages all
           the "normal" IGMP messages are single-hop messages and should
           be silently dropped by the translator.  Other IGMP messages
           might be used by multicast routing protocols and, since it
           would be a configuration error to try to have router
           adjacencies across IPv4/IPv6 translators those packets should
           also be silently dropped.

      ICMPv4 error messages:

        Destination Unreachable (Type 3)
           For all that are not explicitly listed below set the Type to
           1.

           Translate the code field as follows:
              Code 0, 1: Set Code to 0 (no route to destination).

              Code 2: Translate to an ICMPv6 Parameter Problem (Type 4,
              Code 1) and make the Pointer point to the IPv6 Payload
              Type field.

              Code 3: Set Code to 4 (port unreachable).

              Code 4: Translate to an ICMPv6 Packet Too Big message
              (Type 2) with code 0.  The MTU field needs to be adjusted
              for the difference between the IPv4 and IPv6 header sizes.
              [TBD: What
              Note that if the IPv4 router did not set the MTU field
              i.e. the router does not implement [PMTUv4].  Should [PMTUv4], then the
              translator must use the plateau values as specified?] specified in
              [PMTUv4] to determine a likely path MTU and include that
              path MTU in the ICMPv6 packet. (Use the greatest plateau
              value that is less than the returned Total Length field.)

              Code 5: Set Code to 2 (not a neighbor).

              Code 6,7: Set Code to 0 (no route to destination).

              Code 8: Set Code to 0 (no route to destination).

              Code 9, 10: Set Code to 1 (communication with destination
              administratively prohibited)

              Code 11, 12: Set Code to 0 (no route to destination).

        Redirect (Type 5)
           Single hop message.  Silently drop.

        Source Quench (Type 4)
           Obsoleted in ICMPv6.  Silently drop.

        Time Exceeded (Type 11)
           Set the Type field to 3.  The Code field is unchanged.

        Parameter Problem (Type 12)
           Set the Type field to 4.  The Pointer needs to be updated to
           point to the corresponding field in the translated include IP
           header.

4.3.  Translating ICMPv4 Error Messages

   There are some differences between the IPv4 and the IPv6 ICMP error
   message formats as detailed above.  In addition, the ICMP error
   messages contain the IP header for the packet in error which needs to
   be translated just like a normal IP header.  This translated is
   likely to change the length of the datagram thus the Payload Length
   field in the outer IPv6 header might need to be updated.

        +-------------+                 +-------------+
        |    IPv4     |                 |    IPv6     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |   ICMPv4    |                 |   ICMPv6    |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |    IPv4     |      ===>       |    IPv6     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |   Partial   |                 |   Partial   |
        |  Transport  |                 |  Transport  |
        |   Layer     |                 |   Layer     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+

                    IPv4-to-IPv6 ICMP Error Header Translation

   The translation of the inner IP header can be done by recursively
   invoking the function that translated the outer IP headers.

4.4.  Knowing when to Translate

   The translator is assumed to know the pool(s) of IPv4 address that
   are used to represent the internal IPv6-only nodes.  Thus if the
   destination address falls in these configured sets of prefixes the
   packet needs to be translated to IPv6.

5.  TRANSLATING FROM IPv6 TO IPv4

   When an IPv6-to-IPv4 translator receives an IPv6 datagram addressed
   to an IPv4-mapped IPv6 address, it translates the IPv6 header of that
   packet into an IPv7 IPv6 header.  It then forwards the packet based on the
   IPv4 destination address.  The original IPv6 header on the packet is
   removed and replaced by a IPv4 header.  Except for ICMP packets the
   transport layer header and data portion of the packet are left
   unchanged.

        +-------------+                 +-------------+
        |    IPv6     |                 |    IPv4     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |  Fragment   |                 |  Transport  |
        |   Header    |      ===>       |   Layer     |
        |(if present) |                 |   Header    |
        +-------------+                 +-------------+
        |  Transport  |                 |             |
        |   Layer     |                 ~    Data     ~
        |   Header    |                 |             |
        +-------------+                 +-------------+
        |             |
        ~    Data     ~
        |             |
        +-------------+

                    IPv6-to-IPv4 Header Translation

   There are some differences between IPv6 and IPv4 in the area of
   fragmentation and the minimum link MTU that effect the translation.
   An IPv6 link has to have an MTU of 576 bytes or greater.  The
   corresponding limit for IPv4 is 68 bytes.  Thus, unless there were
   special measures, it would not be possible to do end-to-end path MTU
   discovery when the path includes an IPv6-to-IPv4 translator since the
   IPv6 node might receive ICMP "packet too big" messages originated by
   an IPv4 router that report an MTU less than 576.  However, [IPv6]
   requires IPv6 nodes to handle such ICMP "packet too big" message by
   reducing the path MTU to 576 and include an IPv6 fragment header with
   each packet.  This allows end-to-end path MTU discovery across the
   translator as long as the path MTU is 576 bytes or greater and when
   the path MTU drops below that limit IPv6 sender will originate 576
   byte packets that will be fragmented by IPv4 routers along the path
   after being translated to IPv4.

   The only drawback with this scheme is that it is not possible to use
   PMTU to do optimal UDP fragmentation at sender.  The presence of an
   IPv6 Fragment header is interpreted that is it OK to fragment the
   packet on the IPv4 side thus if the Fragment header is present
   because UDP wants to send e.g. 8k packets even though the path MTU is
   smaller the path MTU discovery will not be end-to-end but only up to
   and including the translator.

   Other than the special rules for handling fragments and path MTU
   discovery the actual translation of the packet header consists of a
   simple mapping as defined below.  Note that ICMP packets require
   special handling in order to translate the content of ICMP error
   message and also to add the ICMP pseudo-header checksum.

5.1.  Translating IPv6 Headers

   If there is no IPv6 Fragment header the IPv4 header fields are set as
   follows:

           Version:
                   4

           Internet Header Length:
                   5 (no IPv4 options)

           Type of Service:
                   All zero.
                   [Discussion: Should this value be derived from the
                   IPv6 priority field?]

           Total Length:
                   Payload length value from IPv6 header, plus the size
                   of the IPv4 header.

           Identification:
                   All zero.
                   [Discussion: In order to make IPv4 header compression
                   work better for translated packets it might make
                   sense to make this an incrementing counter.  There is
                   no need for the counter to be correct since the
                   packets will not be fragmented.]

           Flags:
                   The More Fragments flag is set to zero.  The Don't
                   Fragments flag is set to one.

           Fragment Offset:
                   All zero.

           Time to Live:
                   Hop Limit value copied from IPv6 header, decremented
                   by one.

           Protocol:
                   Payload Type field copied from IPv6 header.

           Header Checksum:
                   Computed once the IPv4 header has been created.

           Source Address:
                   If the IPv6 source address is an IPv4-compatible or
                   an IPv4-mapped address then the low-order 32 bits of
                   the IPv6 source address is copied to the IPv4 source
                   address.  Otherwise, the source address is set to
                   127.0.0.1.
                   [Discussion: An alternative could be to drop the
                   packet.  However, it would be useful if a traceroute
                   from the IPv4 node showed something for the IPv6
                   router hops.  Thus either using 127.0.0.1 or 0.0.0.0
                   seem like reasonable alternatives.]

           Destination Address:
                   IPv6 packets that are translated have a destination
                   address that is either an IPv4-compatible or an
                   IPv4-mapped address.  Thus the low-order 32 bits of
                   the IPv6 destination address is copied to the IPv4
                   source address.

   If any of an IPv6 hop-by-hop options header, destination options
   header, or routing header are present in the IPv6 packet, they are
   ignored i.e., there is no attempt to translate them.  However, the
   Total Length field and the Protocol field would have to be adjusted
   to "skip" these extension headers.
   [Discussion: Should the packet be dropped and an ICMP error be
   generated instead?]

   If the IPv6 packet contains a Fragment header the header fields are
   set as above with the following exceptions:

           Total Length:
                   Payload length value from IPv6 header, minus 8 for
                   the Fragment header, plus the size of the IPv4
                   header.

           Identification:
                   Copied from the low-order 16-bits in the
                   Identification field in the Fragment header.

           Flags:
                   The More Fragments flag is copied from the M flag in
                   the Fragment header.  The Don't Fragments flag is set
                   to zero allowing this packet to be fragmented by IPv4
                   routers.

           Fragment Offset:
                   Copied from the Fragment Offset field in the Fragment
                   Header.

           Protocol:
                   Next Header value copied from Fragment header.

5.2.  Translating ICMPv6

   All ICMP messages that are to be translated require that the ICMP
   checksum field be updated as part of the translation since ICMPv6 ICMPv6,
   unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.

   In addition all ICMP packets needs to have the Type value translated
   and for ICMP error messages the included IP header also needs
   translation.

   The actions needed to translate various ICMPv6 messages are:

      ICMPv6 informational messages:

        Echo Request and Echo Reply (Type 128 and 129)
           Adjust the type to 0 and 8, respectively, and adjust the ICMP
           checksum both take the type change into account and to
           exclude the ICMPv6 pseudo-header.

        Group Membership Query/Report/Reduction (Type 130, 131, 132)
           Single hop message.  Silently drop.

        Neighbor Discover messages (Type 133 through 137)
           Single hop message.  Silently drop.

        Unknown informational messages
           Silently drop.

      ICMPv6 error messages:

        Destination Unreachable (Type 1)
           Set the Type field to 3.  Translate the code field as
           follows:
              Code 0: Set Code to 1 (host unreachable).

              Code 1: Set Code to 10 (communication with destination
              host administratively prohibited).

              Code 2: Set Code to 5 (source route failed).

              Code 3: Set Code to 1 (host unreachable).

              Code 4: Set Code to 3 (port unreachable).

        Packet Too Big (Type 2)
           Translate to an ICMPv4 Destination Unreachable with code 4.
           The MTU field needs to be adjusted for the difference between
           the IPv4 and IPv6 header sizes taking into account whether or
           not the packet in error includes a Fragment header.

        Time Exceeded (Type 3)
           Set the Type to 11.  The Code field is unchanged.

        Parameter Problem (Type 4)
           If the Code is 1 translate this to an ICMPv4 protocol
           unreachable (Type 3, Code 2).  Otherwise set the Type to 12
           and the Code to zero.  The Pointer needs to be updated to
           point to the corresponding field in the translated include IP
           header.

        Unknown error messages
           Silently drop.

5.3.  Translating ICMPv6 Error Messages

   There are some differences between the IPv4 and the IPv6 ICMP error
   message formats as detailed above.  In addition, the ICMP error
   messages contain the IP header for the packet in error which needs to
   be translated just like a normal IP header.  This translated is
   likely to change the length of the datagram thus the Payload Length
   field in the outer IPv6 header might need to be updated.

        +-------------+                 +-------------+
        |    IPv6     |                 |    IPv4     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |   ICMPv6    |                 |   ICMPv4    |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |    IPv6     |      ===>       |    IPv4     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+
        |   Partial   |                 |   Partial   |
        |  Transport  |                 |  Transport  |
        |   Layer     |                 |   Layer     |
        |   Header    |                 |   Header    |
        +-------------+                 +-------------+

                    IPv6-to-IPv4 ICMP Error Header Translation

   The translation of the inner IP header can be done by recursively
   invoking the function that translated the outer IP headers.

5.4.  Knowing when to Translate

   When the translator receives a IPv6 packet with an IPv4-mapped
   destination address the packet will be translated to IPv4.

6.  SECURITY CONSIDERATIONS

   TBD

   The use of stateless IP/ICMP translators does not introduce any new
   security issues beyond the security issues that are already present
   in the IPv4 and IPv6 protocols and in the routing protocols which are
   used to make the packets reach the translator.

7.  OPEN ISSUES

    - Can/should we make AH work through a translator e.g. by removing
      the fragment ID field from the AH computation for both IPv4 and
      IPv6?

    - For IPv6 to IPv4 should the Identification be something other than
      zero to be more friendly to header compression?  These packets
      have DF set thus it is possible to choose e.g. a local
      incrementing counter without adverse effects on reassembly.

    - IPv6 to IPv4 when source is pure IPv6 address.  What should the
      IPv4 source address be set to in order to make traceroute usable?
      If such packets are not translated an IPv4 traceroute would show
      no responses from IPv6 routers.  We could use 127.0.0.1 or
      0.0.0.0.

REFERENCES

     [KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, March 1997.

     [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version
             6 (IPv6) Specification", RFC 1883, January 1996.

     [IPv4] J. Postel, "Internet Protocol", RFC 791, September 1981.

     [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6
             Addressing Architecture", RFC 1884, January 1996.

     [TRANS-MECH] R. Gilligan, E. Nordmark, "Transition Mechanisms for
             IPv6 Hosts and Routers", RFC 1933, April 1996.

     [DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 1970, August 1996.

     [IPv6-SA] R. Atkinson.  "Security Architecture for the Internet
             Protocol".  RFC 1825, August 1995.

     [IPv6-AUTH] R. Atkinson.  "IP Authentication Header", RFC 1826,
             August 1995.

     [IPv6-ESP] R. Atkinson.  "IP Encapsulating Security Payload (ESP)",
             RFC 1827, August 1995.

     [ICMPv4] J. Postel, "Internet Control Message Protocol", RFC 792,
             September 1981.

     [ICMPv6] A. Conta, S. Deering, "Internet Control Message Protocol
             (ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC
             1885, January 1996.

     [IGMP] S. Deering, "Host extensions for IP multicasting", RFC 1112,
             August 1989.

     [PMTUv4] J. Mogul, S. Deering, "Path MTU Discovery", RFC 1191,
             November 1990.

     [PMTUv6] J. McCann, S. Deering, J. Mogul, "Path MTU Discovery for
             IP version 6", RFC 1981, August 1996.

AUTHOR'S ADDRESS

        Erik Nordmark
        Sun Microsystems, Inc.
        901 San Antonio Road
        Palo Alto, CA 94303
        USA

        phone: +1 415 650 786 5166
        fax:   +1 415 650 786 5896
        email: nordmark@sun.com

APPENDIX A: CHANGES SINCE PREVIOUS DRAFT

   The following changes have been made since version 00 of the draft.

    o Added clarification about applicability for multicast.
       o Clarified the terminology with IPv4 capable vs. enabled etc.
          o Changed the title to provide an acronym (SIIT).
             o Added some figures to explain the applicability.