+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |  Hdr Ext Len  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   .                                                               .
   .                            Options                            .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Next Header          8-bit selector.  Identifies the type of header
                        immediately following the Destination Options
                        header.  Uses the same values as the IPv4
                        Protocol field [RFC-1700 et seq.].

   Hdr Ext Len          8-bit unsigned integer.  Length of the
                        Destination Options header in 8-octet units,
                        not including the first 8 octets.

   Options              Variable-length field, of length such that the
                        complete Destination Options header is an
                        integer multiple of 8 octets long.  Contains
                        one or  more TLV-encoded options, as described
                        in section 4.2.


   The only destination options defined in this document are the Pad1
   and PadN options specified in section 4.2.

   Note that there are two possible ways to encode optional destination
   information in an IPv6 packet: either as an option in the Destination
   Options header, or as a separate extension header.  The Fragment
   header and the Authentication header are examples of the latter
   approach.  Which approach can be used depends on what action is
   desired of a destination node that does not understand the optional
   information:

      o  if the desired action is for the destination node to discard
         the packet and, only if the packet's Destination Address is not
         a multicast address, send an ICMP Unrecognized Type message to
         the packet's Source Address, then the information may be
         encoded either as a separate header or as an option in the



 
RFC 1883                   IPv6 Specification              December 1995


         Destination Options header whose Option Type has the value 11
         in its highest-order two bits.  The choice may depend on such
         factors as which takes fewer octets, or which yields better
         alignment or more efficient parsing.

      o  if any other action is desired, the information must be encoded
         as an option in the Destination Options header whose Option
         Type has the value 00, 01, or 10 in its highest-order two bits,
         specifying the desired action (see section 4.2).



4.7 No Next Header

   The value 59 in the Next Header field of an IPv6 header or any
   extension header indicates that there is nothing following that
   header.  If the Payload Length field of the IPv6 header indicates the
   presence of octets past the end of a header whose Next Header field
   contains 59, those octets must be ignored, and passed on unchanged if
   the packet is forwarded.