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= ROOT|Technical|RFC|rfc2460.txt =

page 6 of 22

RFC 2460                   IPv6 Specification              December 1998

   in the packet, for any option whose data may change en-route, its
   entire Option Data field must be treated as zero-valued octets when
   computing or verifying the packet's authenticating value.

      0 - Option Data does not change en-route

      1 - Option Data may change en-route

   The three high-order bits described above are to be treated as part
   of the Option Type, not independent of the Option Type.  That is, a
   particular option is identified by a full 8-bit Option Type, not just
   the low-order 5 bits of an Option Type.

   The same Option Type numbering space is used for both the Hop-by-Hop
   Options header and the Destination Options header.  However, the
   specification of a particular option may restrict its use to only one
   of those two headers.

   Individual options may have specific alignment requirements, to
   ensure that multi-octet values within Option Data fields fall on
   natural boundaries.  The alignment requirement of an option is
   specified using the notation xn+y, meaning the Option Type must
   appear at an integer multiple of x octets from the start of the
   header, plus y octets.  For example:

      2n    means any 2-octet offset from the start of the header.
      8n+2  means any 8-octet offset from the start of the header,
            plus 2 octets.

   There are two padding options which are used when necessary to align
   subsequent options and to pad out the containing header to a multiple
   of 8 octets in length.  These padding options must be recognized by
   all IPv6 implementations:

   Pad1 option  (alignment requirement: none)

      |       0       |

      NOTE! the format of the Pad1 option is a special case -- it does
            not have length and value fields.

      The Pad1 option is used to insert one octet of padding into the
      Options area of a header.  If more than one octet of padding is
      required, the PadN option, described next, should be used, rather
      than multiple Pad1 options.

RFC 2460                   IPv6 Specification              December 1998

   PadN option  (alignment requirement: none)

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
      |       1       |  Opt Data Len |  Option Data
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

      The PadN option is used to insert two or more octets of padding
      into the Options area of a header.  For N octets of padding, the
      Opt Data Len field contains the value N-2, and the Option Data
      consists of N-2 zero-valued octets.

   Appendix B contains formatting guidelines for designing new options.

4.3  Hop-by-Hop Options Header

   The Hop-by-Hop Options header is used to carry optional information
   that must be examined by every node along a packet's delivery path.
   The Hop-by-Hop Options header is identified by a Next Header value of
   0 in the IPv6 header, and has the following format:

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

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

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