IPvB translation header

This was the IPvB (nee original IPv6) *translation* header.

Note that it was cleverly designed to translate from IPv4.
Most of the fields are in exactly the same place. Especially,
the 32-bit Source IP address is in exactly the same place, hoping
that filters could operate on both stacks.

We were implementing on then new i386 32-bit machines, but also
tested on i286 16-bit machines. We knew there would be 64-bit
machines (such as the DEC Alpha), so it was carefully designed
for multiple environments.

3.1. Translation Header Format

    >Version> IHV | Service | Minimal Length |
    > Identification | Next Header | Hop Limit |
    > >
    + Source +
    > >
    > >
    + Destination +
    > >

    Version 4 bits: constant 0xB (1011 binary).

                     Indicates the format of the internet header. This
                     document describes version 11.

                                      The IPv4 Version is 0x4 (0100
                                      binary). IPvB is the complement
                                      (binary inverse) of IPv4.

                                      Although this field is always
                                      present, and may be used internally
                                      by systems, different headers MUST
                                      be distinguished at the link layer.
                                      Some implementations of IPv4 failed
                                      to correctly check (or set) the
                                      IPv4 Version.

    IHV 4 bits. Internet Header Variant.

                        Invalid; MUST be silently discarded.

                        IPv4 header translation. The least significant
                        bit here reflects the most significant IPv4 Flags
                        bit, as some systems used that bit in the past.
                        (See "IPvB Translation for IPv4" [].)

                        Flow header (see below).

                        Reserved for future use.

                     The IPvB header is a fixed minimum size. However,
                     the Version field is a scarce resource Therefore,
                     larger values are used for format variants,
                     transient indications, and other purposes.

                                      For IPv4, this field was the
                                      Internet Header Length (IHL) in
                                      32‐bit words. The minimum value
                                      for a matching IPvB header is 6

    Service 8 bits: default 0.

                     Contains the IPv4 Type of Service (ToS).

    Minimal Length 16 bits: minimum 64 (bytes). Smaller values MUST be
                     silently discarded.

                     The length of the datagram, including internet
                     header(s) and payload data. All nodes must be
                     prepared to accept datagrams of up to 1480 octets.
                     It is recommended that hosts send datagrams larger
                     than 1480 octets only after assurance that the
                     destination is prepared to accept the larger

                     The number 1480 is selected to allow a reasonable
                     sized data block to be transmitted in addition to
                     the required header information, and to allow
                     typical lower‐layer encapsulations room for their
                     respective headers. Over time, memory limitations
                     have eased considerably, and there have been some
                     indications that a larger minimum datagram size
                     throughout the internet would be beneficial.

                                      IPv4 has minimum required 576 and
                                      maximum 65,535 octet datagrams.
                                      Translation to IPv4 requires that
                                      the IPvB limits are only applicable
                                      to nodes indicating the presence of
                                      IPvB. (See "IPvB Neighbor
                                      Discovery" [] and "IPvB Translation
                                      for IPv4" [].)

    Identification 16 bits: default 0.

                     Assigned by the sender. Originally used in IPv4 to
                     aid in assembling the fragments of a datagram.
                     However, IPvB uses a Fragmentation Header instead,
                     and treats all IPv4 datagrams as Don’t Fragment

    Next Header 8 bits.

                     Identifies the type of header immediately following
                     this header. Uses the same values as the IPv4
                     Protocol field. (See [RFC‐1700] et seq.)

    Hop Limit 8 bits: initially 255.

                     Indicates the maximum time the datagram is allowed
                     to remain in the internet routing system. The
                     intent is to cause undeliverable datagrams to be
                     discarded, and to bound the maximum datagram
                     lifetime. The time is measured in units of seconds,
                     but is only an upper bound on the time a datagram
                     may exist.

                     This field is modified in internet header
                     processing. Every node that routes the datagram
                     between interfaces MUST decrement the value by at
                     least one. For large bandwidth‐delay paths, the
                     value SHOULD decrease by more than one, and MAY
                     decrease by one for each anticipated second in
                     transit (including queuing delay) over the outgoing

                     If this field contains the value zero (0) before
                     sending, or some higher limit upon receipt as
                     configured for each protocol, then the datagram MUST
                     be discarded.

                                      IPv4 defaults to an initial value
                                      of 64 []. (See also "IPvB
                                      Translation for IPv4" [].)

                                      Security considerations require
                                      that the initial value be well
                                      known, so that protocol
                                      implementations can detect the
                                      number of hops from its source.

    Source 64 bits.

                     The location of the node originating the datagram.

                                      The least significant 32 bits are
                                      aligned with the IPv4 Source field
                                      to permit serendipitous comparisons
                                      for network egress and ingress
                                      filtering of both header versions

    Destination 64 bits.

                     The location of the routing area where an interface
                     of the intended communication peer might be found.

    The IPvB header maintains 64‐bit alignment. Together with an IPvB
    Encapsulating Security Protocol (ESP), the header combination
    maintains 128‐bit and 256‐bit alignment. Serendipitous alignment
    allows simple loads and stores, instead of slower byte by byte