receiver must not, for example, scan through a packet looking for a   particular kind of extension header and process that header prior to   processing all preceding ones.Deering & Hinden            Standards Track                     [Page 6]RFC 2460                   IPv6 Specification              December 1998   The exception referred to in the preceding paragraph is the Hop-by-   Hop Options header, which carries information that must be examined   and processed by every node along a packet's delivery path, including   the source and destination nodes.  The Hop-by-Hop Options header,   when present, must immediately follow the IPv6 header.  Its presence   is indicated by the value zero in the Next Header field of the IPv6   header.   If, as a result of processing a header, a node is required to proceed   to the next header but the Next Header value in the current header is   unrecognized by the node, it should discard the packet and send an   ICMP Parameter Problem message to the source of the packet, with an   ICMP Code value of 1 ("unrecognized Next Header type encountered")   and the ICMP Pointer field containing the offset of the unrecognized   value within the original packet.  The same action should be taken if   a node encounters a Next Header value of zero in any header other   than an IPv6 header.   Each extension header is an integer multiple of 8 octets long, in   order to retain 8-octet alignment for subsequent headers.  Multi-   octet fields within each extension header are aligned on their   natural boundaries, i.e., fields of width n octets are placed at an   integer multiple of n octets from the start of the header, for n = 1,   2, 4, or 8.   A full implementation of IPv6 includes implementation of the   following extension headers:           Hop-by-Hop Options           Routing (Type 0)           Fragment           Destination Options           Authentication           Encapsulating Security Payload   The first four are specified in this document; the last two are   specified in [RFC-2402] and [RFC-2406], respectively.4.1  Extension Header Order   When more than one extension header is used in the same packet, it is   recommended that those headers appear in the following order:           IPv6 header           Hop-by-Hop Options header           Destination Options header (note 1)           Routing header           Fragment headerDeering & Hinden            Standards Track                     [Page 7]RFC 2460                   IPv6 Specification              December 1998           Authentication header (note 2)           Encapsulating Security Payload header (note 2)           Destination Options header (note 3)           upper-layer header           note 1: for options to be processed by the first destination                   that appears in the IPv6 Destination Address field                   plus subsequent destinations listed in the Routing                   header.           note 2: additional recommendations regarding the relative                   order of the Authentication and Encapsulating                   Security Payload headers are given in [RFC-2406].           note 3: for options to be processed only by the final                   destination of the packet.   Each extension header should occur at most once, except for the   Destination Options header which should occur at most twice (once   before a Routing header and once before the upper-layer header).   If the upper-layer header is another IPv6 header (in the case of IPv6   being tunneled over or encapsulated in IPv6), it may be followed by   its own extension headers, which are separately subject to the same   ordering recommendations.   If and when other extension headers are defined, their ordering   constraints relative to the above listed headers must be specified.   IPv6 nodes must accept and attempt to process extension headers in   any order and occurring any number of times in the same packet,   except for the Hop-by-Hop Options header which is restricted to   appear immediately after an IPv6 header only.  Nonetheless, it is   strongly advised that sources of IPv6 packets adhere to the above   recommended order until and unless subsequent specifications revise   that recommendation.Deering & Hinden            Standards Track                     [Page 8]RFC 2460                   IPv6 Specification              December 19984.2  Options   Two of the currently-defined extension headers -- the Hop-by-Hop   Options header and the Destination Options header -- carry a variable   number of type-length-value (TLV) encoded "options", of the following   format:      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -      |  Option Type  |  Opt Data Len |  Option Data      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -      Option Type          8-bit identifier of the type of option.      Opt Data Len         8-bit unsigned integer.  Length of the Option                           Data field of this option, in octets.      Option Data          Variable-length field.  Option-Type-specific                           data.   The sequence of options within a header must be processed strictly in   the order they appear in the header; a receiver must not, for   example, scan through the header looking for a particular kind of   option and process that option prior to processing all preceding   ones.   The Option Type identifiers are internally encoded such that their   highest-order two bits specify the action that must be taken if the   processing IPv6 node does not recognize the Option Type:      00 - skip over this option and continue processing the header.      01 - discard the packet.      10 - discard the packet and, regardless of whether or not the           packet's Destination Address was a multicast address, send an           ICMP Parameter Problem, Code 2, message to the packet's           Source Address, pointing to the unrecognized Option Type.      11 - discard the packet and, only if the packet's Destination           Address was not a multicast address, send an ICMP Parameter           Problem, Code 2, message to the packet's Source Address,           pointing to the unrecognized Option Type.   The third-highest-order bit of the Option Type specifies whether or   not the Option Data of that option can change en-route to the   packet's final destination.  When an Authentication header is presentDeering & Hinden            Standards Track                     [Page 9]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.Deering & Hinden            Standards Track                    [Page 10]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.].   Hdr Ext Len          8-bit unsigned integer.  Length of the Hop-by-                        Hop Options header in 8-octet units, not                        including the first 8 octets.   Options              Variable-length field, of length such that the                        complete Hop-by-Hop 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 hop-by-hop options defined in this document are the Pad1 and   PadN options specified in section 4.2.Deering & Hinden            Standards Track                    [Page 11]RFC 2460                   IPv6 Specification              December 19984.4  Routing Header   The Routing header is used by an IPv6 source to list one or more   intermediate nodes to be "visited" on the way to a packet's   destination.  This function is very similar to IPv4's Loose Source   and Record Route option.  The Routing header is identified by a Next   Header value of 43 in the immediately preceding header, and has the   following format:    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |  Next Header  |  Hdr Ext Len  |  Routing Type | Segments Left |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                       type-specific data                      .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Next Header          8-bit selector.  Identifies the type of header                        immediately following the Routing header.  Uses                        the same values as the IPv4 Protocol field                        [RFC-1700 et seq.].   Hdr Ext Len          8-bit unsigned integer.  Length of the Routing                        header in 8-octet units, not including the first                        8 octets.   Routing Type         8-bit identifier of a particular Routing header                        variant.   Segments Left        8-bit unsigned integer.  Number of route                        segments remaining, i.e., number of explicitly                        listed intermediate nodes still to be visited                        before reaching the final destination.