<pre>Internet Engineering Task Force (IETF)                        S. Deering
Request for Comments: 8200                                       Retired
STD: 86                                                        R. Hinden
Obsoletes: <a href="./rfc2460">2460</a>                                     Check Point Software
Category: Standards Track                                      July 2017
ISSN: 2070-1721


           <span class="h1">Internet Protocol, Version 6 (IPv6) Specification</span>

Abstract

   This document specifies version 6 of the Internet Protocol (IPv6).
   It obsoletes <a href="./rfc2460">RFC 2460</a>.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc8200">http://www.rfc-editor.org/info/rfc8200</a>.























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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   <a href="#section-1">1</a>.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
   <a href="#section-2">2</a>.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-5">5</a>
   <a href="#section-3">3</a>.  IPv6 Header Format  . . . . . . . . . . . . . . . . . . . . .   <a href="#page-6">6</a>
   <a href="#section-4">4</a>.  IPv6 Extension Headers  . . . . . . . . . . . . . . . . . . .   <a href="#page-7">7</a>
     <a href="#section-4.1">4.1</a>.  Extension Header Order  . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
     <a href="#section-4.2">4.2</a>.  Options . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-4.3">4.3</a>.  Hop-by-Hop Options Header . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
     <a href="#section-4.4">4.4</a>.  Routing Header  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
     <a href="#section-4.5">4.5</a>.  Fragment Header . . . . . . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>
     <a href="#section-4.6">4.6</a>.  Destination Options Header  . . . . . . . . . . . . . . .  <a href="#page-23">23</a>
     <a href="#section-4.7">4.7</a>.  No Next Header  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-24">24</a>
     <a href="#section-4.8">4.8</a>.  Defining New Extension Headers and Options  . . . . . . .  <a href="#page-24">24</a>
   <a href="#section-5">5</a>.  Packet Size Issues  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-25">25</a>
   <a href="#section-6">6</a>.  Flow Labels . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-26">26</a>
   <a href="#section-7">7</a>.  Traffic Classes . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-26">26</a>
   <a href="#section-8">8</a>.  Upper-Layer Protocol Issues . . . . . . . . . . . . . . . . .  <a href="#page-27">27</a>
     <a href="#section-8.1">8.1</a>.  Upper-Layer Checksums . . . . . . . . . . . . . . . . . .  <a href="#page-27">27</a>
     <a href="#section-8.2">8.2</a>.  Maximum Packet Lifetime . . . . . . . . . . . . . . . . .  <a href="#page-28">28</a>
     <a href="#section-8.3">8.3</a>.  Maximum Upper-Layer Payload Size  . . . . . . . . . . . .  <a href="#page-29">29</a>
     <a href="#section-8.4">8.4</a>.  Responding to Packets Carrying Routing Headers  . . . . .  <a href="#page-29">29</a>
   <a href="#section-9">9</a>.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  <a href="#page-29">29</a>
   <a href="#section-10">10</a>. Security Considerations . . . . . . . . . . . . . . . . . . .  <a href="#page-30">30</a>
   <a href="#section-11">11</a>. References  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-32">32</a>
     <a href="#section-11.1">11.1</a>.  Normative References . . . . . . . . . . . . . . . . . .  <a href="#page-32">32</a>
     <a href="#section-11.2">11.2</a>.  Informative References . . . . . . . . . . . . . . . . .  <a href="#page-33">33</a>
   <a href="#appendix-A">Appendix A</a>.  Formatting Guidelines for Options  . . . . . . . . .  <a href="#page-36">36</a>
   <a href="#appendix-B">Appendix B</a>.  Changes Since <a href="./rfc2460">RFC 2460</a> . . . . . . . . . . . . . . .  <a href="#page-39">39</a>
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-42">42</a>
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-42">42</a>




















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<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   IP version 6 (IPv6) is a new version of the Internet Protocol (IP),
   designed as the successor to IP version 4 (IPv4) [<a href="./rfc791" title="&quot;Internet Protocol&quot;">RFC791</a>].  The
   changes from IPv4 to IPv6 fall primarily into the following
   categories:

      o  Expanded Addressing Capabilities

         IPv6 increases the IP address size from 32 bits to 128 bits, to
         support more levels of addressing hierarchy, a much greater
         number of addressable nodes, and simpler autoconfiguration of
         addresses.  The scalability of multicast routing is improved by
         adding a "scope" field to multicast addresses.  And a new type
         of address called an "anycast address" is defined; it is used
         to send a packet to any one of a group of nodes.

      o  Header Format Simplification

         Some IPv4 header fields have been dropped or made optional, to
         reduce the common-case processing cost of packet handling and
         to limit the bandwidth cost of the IPv6 header.

      o  Improved Support for Extensions and Options

         Changes in the way IP header options are encoded allows for
         more efficient forwarding, less stringent limits on the length
         of options, and greater flexibility for introducing new options
         in the future.

      o  Flow Labeling Capability

         A new capability is added to enable the labeling of sequences
         of packets that the sender requests to be treated in the
         network as a single flow.

      o  Authentication and Privacy Capabilities

         Extensions to support authentication, data integrity, and
         (optional) data confidentiality are specified for IPv6.

   This document specifies the basic IPv6 header and the initially
   defined IPv6 extension headers and options.  It also discusses packet
   size issues, the semantics of flow labels and traffic classes, and
   the effects of IPv6 on upper-layer protocols.  The format and
   semantics of IPv6 addresses are specified separately in [<a href="./rfc4291" title="&quot;IP Version 6 Addressing Architecture&quot;">RFC4291</a>].
   The IPv6 version of ICMP, which all IPv6 implementations are required
   to include, is specified in [<a href="./rfc4443" title="&quot;Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification&quot;">RFC4443</a>].



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   The data transmission order for IPv6 is the same as for IPv4 as
   defined in <a href="./rfc791#appendix-B">Appendix&nbsp;B of [RFC791]</a>.

   Note: As this document obsoletes [<a href="./rfc2460" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC2460</a>], any document referenced
   in this document that includes pointers to <a href="./rfc2460">RFC 2460</a> should be
   interpreted as referencing this document.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Terminology</span>

   node         a device that implements IPv6.

   router       a node that forwards IPv6 packets not explicitly
                addressed to itself.  (See Note below.)

   host         any node that is not a router.  (See Note below.)

   upper layer  a protocol layer immediately above IPv6.  Examples are
                transport protocols such as TCP and UDP, control
                protocols such as ICMP, routing protocols such as OSPF,
                and internet-layer or lower-layer protocols being
                "tunneled" over (i.e., encapsulated in) IPv6 such as
                Internetwork Packet Exchange (IPX), AppleTalk, or IPv6
                itself.

   link         a communication facility or medium over which nodes can
                communicate at the link layer, i.e., the layer
                immediately below IPv6.  Examples are Ethernets (simple
                or bridged); PPP links; X.25, Frame Relay, or ATM
                networks; and internet-layer or higher-layer "tunnels",
                such as tunnels over IPv4 or IPv6 itself.

   neighbors    nodes attached to the same link.

   interface    a node's attachment to a link.

   address      an IPv6-layer identifier for an interface or a set of
                interfaces.

   packet       an IPv6 header plus payload.

   link MTU     the maximum transmission unit, i.e., maximum packet size
                in octets, that can be conveyed over a link.

   path MTU     the minimum link MTU of all the links in a path between
                a source node and a destination node.






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   Note: it is possible for a device with multiple interfaces to be
   configured to forward non-self-destined packets arriving from some
   set (fewer than all) of its interfaces and to discard non-self-
   destined packets arriving from its other interfaces.  Such a device
   must obey the protocol requirements for routers when receiving
   packets from, and interacting with neighbors over, the former
   (forwarding) interfaces.  It must obey the protocol requirements for
   hosts when receiving packets from, and interacting with neighbors
   over, the latter (non-forwarding) interfaces.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  IPv6 Header Format</span>

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| Traffic Class |           Flow Label                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Payload Length        |  Next Header  |   Hop Limit   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                         Source Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Version             4-bit Internet Protocol version number = 6.

      Traffic Class       8-bit Traffic Class field.  See <a href="#section-7">Section 7</a>.

      Flow Label          20-bit flow label.  See <a href="#section-6">Section 6</a>.

      Payload Length      16-bit unsigned integer.  Length of the IPv6
                          payload, i.e., the rest of the packet
                          following this IPv6 header, in octets.  (Note
                          that any extension headers (see <a href="#section-4">Section 4</a>)
                          present are considered part of the payload,
                          i.e., included in the length count.)





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      Next Header         8-bit selector.  Identifies the type of header
                          immediately following the IPv6 header.  Uses
                          the same values as the IPv4 Protocol field
                          [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>].

      Hop Limit           8-bit unsigned integer.  Decremented by 1 by
                          each node that forwards the packet.  When
                          forwarding, the packet is discarded if Hop
                          Limit was zero when received or is decremented
                          to zero.  A node that is the destination of a
                          packet should not discard a packet with Hop
                          Limit equal to zero; it should process the
                          packet normally.

      Source Address      128-bit address of the originator of the
                          packet.  See [<a href="./rfc4291" title="&quot;IP Version 6 Addressing Architecture&quot;">RFC4291</a>].

      Destination Address 128-bit address of the intended recipient of
                          the packet (possibly not the ultimate
                          recipient, if a Routing header is present).
                          See [<a href="./rfc4291" title="&quot;IP Version 6 Addressing Architecture&quot;">RFC4291</a>] and <a href="#section-4.4">Section 4.4</a>.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  IPv6 Extension Headers</span>

   In IPv6, optional internet-layer information is encoded in separate
   headers that may be placed between the IPv6 header and the upper-
   layer header in a packet.  There is a small number of such extension
   headers, each one identified by a distinct Next Header value.

   Extension headers are numbered from IANA IP Protocol Numbers
   [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>], the same values used for IPv4 and IPv6.  When processing a
   sequence of Next Header values in a packet, the first one that is not
   an extension header [<a href="#ref-IANA-EH" title="&quot;IPv6 Extension Header Types&quot;">IANA-EH</a>] indicates that the next item in the
   packet is the corresponding upper-layer header.  A special "No Next
   Header" value is used if there is no upper-layer header.
















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<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   As illustrated in these examples, an IPv6 packet may carry zero, one,
   or more extension headers, each identified by the Next Header field
   of the preceding header:

   +---------------+------------------------
   |  IPv6 header  | TCP header + data
   |               |
   | Next Header = |
   |      TCP      |
   +---------------+------------------------

   +---------------+----------------+------------------------
   |  IPv6 header  | Routing header | TCP header + data
   |               |                |
   | Next Header = |  Next Header = |
   |    Routing    |      TCP       |
   +---------------+----------------+------------------------

   +---------------+----------------+-----------------+-----------------
   |  IPv6 header  | Routing header | Fragment header | fragment of TCP
   |               |                |                 |  header + data
   | Next Header = |  Next Header = |  Next Header =  |
   |    Routing    |    Fragment    |       TCP       |
   +---------------+----------------+-----------------+-----------------

   Extension headers (except for the Hop-by-Hop Options header) are not
   processed, inserted, or deleted by any node along a packet's delivery
   path, until the packet reaches the node (or each of the set of nodes,
   in the case of multicast) identified in the Destination Address field
   of the IPv6 header.

   The Hop-by-Hop Options header is not inserted or deleted, but may be
   examined or processed by any node along a packet's delivery path,
   until the packet reaches the node (or each of the set of nodes, in
   the case of multicast) identified in the Destination Address field of
   the IPv6 header.  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.

   NOTE: While [<a href="./rfc2460" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC2460</a>] required that all nodes must examine and
   process the Hop-by-Hop Options header, it is now expected that nodes
   along a packet's delivery path only examine and process the
   Hop-by-Hop Options header if explicitly configured to do so.








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   At the destination node, normal demultiplexing on the Next Header
   field of the IPv6 header invokes the module to process the first
   extension header, or the upper-layer header if no extension header is
   present.  The contents and semantics of each extension header
   determine whether or not to proceed to the next header.  Therefore,
   extension headers must be processed strictly in the order they appear
   in the packet; a 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.

   If, as a result of processing a header, the destination 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
      Fragment
      Destination Options
      Routing
      Authentication
      Encapsulating Security Payload

   The first four are specified in this document; the last two are
   specified in [<a href="./rfc4302" title="&quot;IP Authentication Header&quot;">RFC4302</a>] and [<a href="./rfc4303" title="&quot;IP Encapsulating Security Payload (ESP)&quot;">RFC4303</a>], respectively.  The current list
   of IPv6 extension headers can be found at [<a href="#ref-IANA-EH" title="&quot;IPv6 Extension Header Types&quot;">IANA-EH</a>].











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<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Extension Header Order</span>

   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 header
      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 [<a href="./rfc4303" title="&quot;IP Encapsulating Security Payload (ESP)&quot;">RFC4303</a>].

      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.






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<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Options</span>

   Two of the currently defined extension headers specified in this
   document -- the Hop-by-Hop Options header and the Destination Options
   header -- carry a variable number of "options" that are type-length-
   value (TLV) encoded in 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 2 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 present





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   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 that 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 1 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.




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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   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.

   <a href="#appendix-A">Appendix A</a> contains formatting guidelines for designing new options.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Hop-by-Hop Options Header</span>

   The Hop-by-Hop Options header is used to carry optional information
   that may be examined and processed 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 [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>].

      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 <a href="#section-4.2">Section 4.2</a>.

   The only hop-by-hop options defined in this document are the Pad1 and
   PadN options specified in <a href="#section-4.2">Section 4.2</a>.



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Routing Header</span>

   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 [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>].

      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.

      type-specific data  Variable-length field, of format determined by
                          the Routing Type, and of length such that the
                          complete Routing header is an integer multiple
                          of 8 octets long.











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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   If, while processing a received packet, a node encounters a Routing
   header with an unrecognized Routing Type value, the required behavior
   of the node depends on the value of the Segments Left field, as
   follows:

      If Segments Left is zero, the node must ignore the Routing header
      and proceed to process the next header in the packet, whose type
      is identified by the Next Header field in the Routing header.

      If Segments Left is non-zero, the node must discard the packet and
      send an ICMP Parameter Problem, Code 0, message to the packet's
      Source Address, pointing to the unrecognized Routing Type.

   If, after processing a Routing header of a received packet, an
   intermediate node determines that the packet is to be forwarded onto
   a link whose link MTU is less than the size of the packet, the node
   must discard the packet and send an ICMP Packet Too Big message to
   the packet's Source Address.

   The currently defined IPv6 Routing Headers and their status can be
   found at [<a href="#ref-IANA-RH" title="&quot;Routing Types&quot;">IANA-RH</a>].  Allocation guidelines for IPv6 Routing Headers
   can be found in [<a href="./rfc5871" title="&quot;IANA Allocation Guidelines for the IPv6 Routing Header&quot;">RFC5871</a>].

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Fragment Header</span>

   The Fragment header is used by an IPv6 source to send a packet larger
   than would fit in the path MTU to its destination.  (Note: unlike
   IPv4, fragmentation in IPv6 is performed only by source nodes, not by
   routers along a packet's delivery path -- see <a href="#section-5">Section 5</a>.)  The
   Fragment header is identified by a Next Header value of 44 in the
   immediately preceding header and has the following format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |   Reserved    |      Fragment Offset    |Res|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Next Header         8-bit selector.  Identifies the initial header
                          type of the Fragmentable Part of the original
                          packet (defined below).  Uses the same values
                          as the IPv4 Protocol field [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>].

      Reserved            8-bit reserved field.  Initialized to zero for
                          transmission; ignored on reception.






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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


      Fragment Offset     13-bit unsigned integer.  The offset, in
                          8-octet units, of the data following this
                          header, relative to the start of the
                          Fragmentable Part of the original packet.

      Res                 2-bit reserved field.  Initialized to zero for
                          transmission; ignored on reception.

      M flag              1 = more fragments; 0 = last fragment.

      Identification      32 bits.  See description below.

   In order to send a packet that is too large to fit in the MTU of the
   path to its destination, a source node may divide the packet into
   fragments and send each fragment as a separate packet, to be
   reassembled at the receiver.

   For every packet that is to be fragmented, the source node generates
   an Identification value.  The Identification must be different than
   that of any other fragmented packet sent recently* with the same
   Source Address and Destination Address.  If a Routing header is
   present, the Destination Address of concern is that of the final
   destination.

      *  "recently" means within the maximum likely lifetime of a
         packet, including transit time from source to destination and
         time spent awaiting reassembly with other fragments of the same
         packet.  However, it is not required that a source node knows
         the maximum packet lifetime.  Rather, it is assumed that the
         requirement can be met by implementing an algorithm that
         results in a low identification reuse frequency.  Examples of
         algorithms that can meet this requirement are described in
         [<a href="./rfc7739" title="&quot;Security Implications of Predictable Fragment Identification Values&quot;">RFC7739</a>].


















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   The initial, large, unfragmented packet is referred to as the
   "original packet", and it is considered to consist of three parts, as
   illustrated:

   original packet:

   +------------------+-------------------------+---//----------------+
   |  Per-Fragment    | Extension &amp; Upper-Layer |   Fragmentable      |
   |    Headers       |       Headers           |      Part           |
   +------------------+-------------------------+---//----------------+

      The Per-Fragment headers must consist of the IPv6 header plus any
      extension headers that must be processed by nodes en route to the
      destination, that is, all headers up to and including the Routing
      header if present, else the Hop-by-Hop Options header if present,
      else no extension headers.

      The Extension headers are all other extension headers that are not
      included in the Per-Fragment headers part of the packet.  For this
      purpose, the Encapsulating Security Payload (ESP) is not
      considered an extension header.  The Upper-Layer header is the
      first upper-layer header that is not an IPv6 extension header.
      Examples of upper-layer headers include TCP, UDP, IPv4, IPv6,
      ICMPv6, and as noted ESP.

      The Fragmentable Part consists of the rest of the packet after the
      upper-layer header or after any header (i.e., initial IPv6 header
      or extension header) that contains a Next Header value of No Next
      Header.

   The Fragmentable Part of the original packet is divided into
   fragments.  The lengths of the fragments must be chosen such that the
   resulting fragment packets fit within the MTU of the path to the
   packet's destination(s).  Each complete fragment, except possibly the
   last ("rightmost") one, is an integer multiple of 8 octets long.
















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   The fragments are transmitted in separate "fragment packets" as
   illustrated:

   original packet:

   +-----------------+-----------------+--------+--------+-//-+--------+
   |  Per-Fragment   |Ext &amp; Upper-Layer|  first | second |    |  last  |
   |    Headers      |    Headers      |fragment|fragment|....|fragment|
   +-----------------+-----------------+--------+--------+-//-+--------+

   fragment packets:

   +------------------+---------+-------------------+----------+
   |  Per-Fragment    |Fragment | Ext &amp; Upper-Layer |  first   |
   |    Headers       | Header  |   Headers         | fragment |
   +------------------+---------+-------------------+----------+

   +------------------+--------+-------------------------------+
   |  Per-Fragment    |Fragment|    second                     |
   |    Headers       | Header |   fragment                    |
   +------------------+--------+-------------------------------+
                         o
                         o
                         o
   +------------------+--------+----------+
   |  Per-Fragment    |Fragment|   last   |
   |    Headers       | Header | fragment |
   +------------------+--------+----------+

   The first fragment packet is composed of:

      (1)  The Per-Fragment headers of the original packet, with the
           Payload Length of the original IPv6 header changed to contain
           the length of this fragment packet only (excluding the length
           of the IPv6 header itself), and the Next Header field of the
           last header of the Per-Fragment headers changed to 44.

      (2)  A Fragment header containing:

              The Next Header value that identifies the first header
              after the Per-Fragment headers of the original packet.

              A Fragment Offset containing the offset of the fragment,
              in 8-octet units, relative to the start of the
              Fragmentable Part of the original packet.  The Fragment
              Offset of the first ("leftmost") fragment is 0.

              An M flag value of 1 as this is the first fragment.



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


              The Identification value generated for the original
              packet.

      (3)  Extension headers, if any, and the Upper-Layer header.  These
           headers must be in the first fragment.  Note: This restricts
           the size of the headers through the Upper-Layer header to the
           MTU of the path to the packet's destinations(s).

      (4)  The first fragment.

   The subsequent fragment packets are composed of:

      (1)  The Per-Fragment headers of the original packet, with the
           Payload Length of the original IPv6 header changed to contain
           the length of this fragment packet only (excluding the length
           of the IPv6 header itself), and the Next Header field of the
           last header of the Per-Fragment headers changed to 44.

      (2)  A Fragment header containing:

              The Next Header value that identifies the first header
              after the Per-Fragment headers of the original packet.

              A Fragment Offset containing the offset of the fragment,
              in 8-octet units, relative to the start of the
              Fragmentable Part of the original packet.

              An M flag value of 0 if the fragment is the last
              ("rightmost") one, else an M flag value of 1.

              The Identification value generated for the original
              packet.

      (3)  The fragment itself.

   Fragments must not be created that overlap with any other fragments
   created from the original packet.














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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   At the destination, fragment packets are reassembled into their
   original, unfragmented form, as illustrated:

   reassembled original packet:

   +---------------+-----------------+---------+--------+-//--+--------+
   | Per-Fragment  |Ext &amp; Upper-Layer|  first  | second |     | last   |
   |    Headers    |     Headers     |frag data|fragment|.....|fragment|
   +---------------+-----------------+---------+--------+-//--+--------+

   The following rules govern reassembly:

      An original packet is reassembled only from fragment packets that
      have the same Source Address, Destination Address, and Fragment
      Identification.

      The Per-Fragment headers of the reassembled packet consists of all
      headers up to, but not including, the Fragment header of the first
      fragment packet (that is, the packet whose Fragment Offset is
      zero), with the following two changes:

         The Next Header field of the last header of the Per-Fragment
         headers is obtained from the Next Header field of the first
         fragment's Fragment header.

         The Payload Length of the reassembled packet is computed from
         the length of the Per-Fragment headers and the length and
         offset of the last fragment.  For example, a formula for
         computing the Payload Length of the reassembled original packet
         is:

            PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.last

            where
            PL.orig  =  Payload Length field of reassembled packet.
            PL.first =  Payload Length field of first fragment packet.
            FL.first =  length of fragment following Fragment header of
                        first fragment packet.
            FO.last  =  Fragment Offset field of Fragment header of last
                        fragment packet.
            FL.last  =  length of fragment following Fragment header of
                        last fragment packet.

         The Fragmentable Part of the reassembled packet is constructed
         from the fragments following the Fragment headers in each of
         the fragment packets.  The length of each fragment is computed
         by subtracting from the packet's Payload Length the length of
         the headers between the IPv6 header and fragment itself; its



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


         relative position in Fragmentable Part is computed from its
         Fragment Offset value.

         The Fragment header is not present in the final, reassembled
         packet.

         If the fragment is a whole datagram (that is, both the Fragment
         Offset field and the M flag are zero), then it does not need
         any further reassembly and should be processed as a fully
         reassembled packet (i.e., updating Next Header, adjust Payload
         Length, removing the Fragment header, etc.).  Any other
         fragments that match this packet (i.e., the same IPv6 Source
         Address, IPv6 Destination Address, and Fragment Identification)
         should be processed independently.

   The following error conditions may arise when reassembling fragmented
   packets:

      o  If insufficient fragments are received to complete reassembly
         of a packet within 60 seconds of the reception of the first-
         arriving fragment of that packet, reassembly of that packet
         must be abandoned and all the fragments that have been received
         for that packet must be discarded.  If the first fragment
         (i.e., the one with a Fragment Offset of zero) has been
         received, an ICMP Time Exceeded -- Fragment Reassembly Time
         Exceeded message should be sent to the source of that fragment.

      o  If the length of a fragment, as derived from the fragment
         packet's Payload Length field, is not a multiple of 8 octets
         and the M flag of that fragment is 1, then that fragment must
         be discarded and an ICMP Parameter Problem, Code 0, message
         should be sent to the source of the fragment, pointing to the
         Payload Length field of the fragment packet.

      o  If the length and offset of a fragment are such that the
         Payload Length of the packet reassembled from that fragment
         would exceed 65,535 octets, then that fragment must be
         discarded and an ICMP Parameter Problem, Code 0, message should
         be sent to the source of the fragment, pointing to the Fragment
         Offset field of the fragment packet.

      o  If the first fragment does not include all headers through an
         Upper-Layer header, then that fragment should be discarded and
         an ICMP Parameter Problem, Code 3, message should be sent to
         the source of the fragment, with the Pointer field set to zero.






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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


      o  If any of the fragments being reassembled overlap with any
         other fragments being reassembled for the same packet,
         reassembly of that packet must be abandoned and all the
         fragments that have been received for that packet must be
         discarded, and no ICMP error messages should be sent.

         It should be noted that fragments may be duplicated in the
         network.  Instead of treating these exact duplicate fragments
         as overlapping fragments, an implementation may choose to
         detect this case and drop exact duplicate fragments while
         keeping the other fragments belonging to the same packet.

   The following conditions are not expected to occur frequently but are
   not considered errors if they do:

      The number and content of the headers preceding the Fragment
      header of different fragments of the same original packet may
      differ.  Whatever headers are present, preceding the Fragment
      header in each fragment packet, are processed when the packets
      arrive, prior to queueing the fragments for reassembly.  Only
      those headers in the Offset zero fragment packet are retained in
      the reassembled packet.

      The Next Header values in the Fragment headers of different
      fragments of the same original packet may differ.  Only the value
      from the Offset zero fragment packet is used for reassembly.

      Other fields in the IPv6 header may also vary across the fragments
      being reassembled.  Specifications that use these fields may
      provide additional instructions if the basic mechanism of using
      the values from the Offset zero fragment is not sufficient.  For
      example, <a href="./rfc3168#section-5.3">Section&nbsp;5.3 of [RFC3168]</a> describes how to combine the
      Explicit Congestion Notification (ECN) bits from different
      fragments to derive the ECN bits of the reassembled packet.

















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>.  Destination Options Header</span>

   The Destination Options header is used to carry optional information
   that need be examined only by a packet's destination node(s).  The
   Destination Options header is identified by a Next Header value of 60
   in the immediately preceding 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 Destination Options
                          header.  Uses the same values as the IPv4
                          Protocol field [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>].

      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 <a href="#section-4.2">Section 4.2</a>.

   The only destination options defined in this document are the Pad1
   and PadN options specified in <a href="#section-4.2">Section 4.2</a>.

   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



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


         Destination Options header whose Option Type has the value 11
         in its highest-order 2 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 2 bits,
         specifying the desired action (see <a href="#section-4.2">Section 4.2</a>).

<span class="h3"><a class="selflink" id="section-4.7" href="#section-4.7">4.7</a>.  No Next Header</span>

   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.

<span class="h3"><a class="selflink" id="section-4.8" href="#section-4.8">4.8</a>.  Defining New Extension Headers and Options</span>

   Defining new IPv6 extension headers is not recommended, unless there
   are no existing IPv6 extension headers that can be used by specifying
   a new option for that IPv6 extension header.  A proposal to specify a
   new IPv6 extension header must include a detailed technical
   explanation of why an existing IPv6 extension header can not be used
   for the desired new function.  See [<a href="./rfc6564" title="&quot;A Uniform Format for IPv6 Extension Headers&quot;">RFC6564</a>] for additional
   background information.

   Note: New extension headers that require hop-by-hop behavior must not
   be defined because, as specified in <a href="#section-4">Section 4</a> of this document, the
   only extension header that has hop-by-hop behavior is the Hop-by-Hop
   Options header.

   New hop-by-hop options are not recommended because nodes may be
   configured to ignore the Hop-by-Hop Options header, drop packets
   containing a Hop-by-Hop Options header, or assign packets containing
   a Hop-by-Hop Options header to a slow processing path.  Designers
   considering defining new hop-by-hop options need to be aware of this
   likely behavior.  There has to be a very clear justification why any
   new hop-by-hop option is needed before it is standardized.

   Instead of defining new extension headers, it is recommended that the
   Destination Options header is used to carry optional information that
   must be examined only by a packet's destination node(s), because they
   provide better handling and backward compatibility.





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<span class="grey"><a href="./rfc8200">RFC 8200</a>                   IPv6 Specification                  July 2017</span>


   If new extension headers are defined, they need to use the following
   format:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  |  Hdr Ext Len  |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    .                                                               .
    .                  Header-Specific Data                         .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Next Header           8-bit selector.  Identifies the type of
                            header immediately following the extension
                            header.  Uses the same values as the IPv4
                            Protocol field [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>].

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

      Header Specific Data  Variable-length field.  Fields specific to
                            the extension header.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Packet Size Issues</span>

   IPv6 requires that every link in the Internet have an MTU of 1280
   octets or greater.  This is known as the IPv6 minimum link MTU.  On
   any link that cannot convey a 1280-octet packet in one piece, link-
   specific fragmentation and reassembly must be provided at a layer
   below IPv6.

   Links that have a configurable MTU (for example, PPP links [<a href="./rfc1661" title="&quot;The Point-to-Point Protocol (PPP)&quot;">RFC1661</a>])
   must be configured to have an MTU of at least 1280 octets; it is
   recommended that they be configured with an MTU of 1500 octets or
   greater, to accommodate possible encapsulations (i.e., tunneling)
   without incurring IPv6-layer fragmentation.

   From each link to which a node is directly attached, the node must be
   able to accept packets as large as that link's MTU.

   It is strongly recommended that IPv6 nodes implement Path MTU
   Discovery [<a href="./rfc8201" title="&quot;Path MTU Discovery for IP version 6&quot;">RFC8201</a>], in order to discover and take advantage of path
   MTUs greater than 1280 octets.  However, a minimal IPv6
   implementation (e.g., in a boot ROM) may simply restrict itself to
   sending packets no larger than 1280 octets, and omit implementation
   of Path MTU Discovery.



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   In order to send a packet larger than a path's MTU, a node may use
   the IPv6 Fragment header to fragment the packet at the source and
   have it reassembled at the destination(s).  However, the use of such
   fragmentation is discouraged in any application that is able to
   adjust its packets to fit the measured path MTU (i.e., down to 1280
   octets).

   A node must be able to accept a fragmented packet that, after
   reassembly, is as large as 1500 octets.  A node is permitted to
   accept fragmented packets that reassemble to more than 1500 octets.
   An upper-layer protocol or application that depends on IPv6
   fragmentation to send packets larger than the MTU of a path should
   not send packets larger than 1500 octets unless it has assurance that
   the destination is capable of reassembling packets of that larger
   size.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Flow Labels</span>

   The 20-bit Flow Label field in the IPv6 header is used by a source to
   label sequences of packets to be treated in the network as a single
   flow.

   The current definition of the IPv6 Flow Label can be found in
   [<a href="./rfc6437" title="&quot;IPv6 Flow Label Specification&quot;">RFC6437</a>].

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Traffic Classes</span>

   The 8-bit Traffic Class field in the IPv6 header is used by the
   network for traffic management.  The value of the Traffic Class bits
   in a received packet or fragment might be different from the value
   sent by the packet's source.

   The current use of the Traffic Class field for Differentiated
   Services and Explicit Congestion Notification is specified in
   [<a href="./rfc2474" title="&quot;Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers&quot;">RFC2474</a>] and [<a href="./rfc3168" title="&quot;The Addition of Explicit Congestion Notification (ECN) to IP&quot;">RFC3168</a>].
















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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Upper-Layer Protocol Issues</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Upper-Layer Checksums</span>

   Any transport or other upper-layer protocol that includes the
   addresses from the IP header in its checksum computation must be
   modified for use over IPv6, to include the 128-bit IPv6 addresses
   instead of 32-bit IPv4 addresses.  In particular, the following
   illustration shows the TCP and UDP "pseudo-header" for IPv6:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                         Source Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Upper-Layer Packet Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      zero                     |  Next Header  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      o  If the IPv6 packet contains a Routing header, the Destination
         Address used in the pseudo-header is that of the final
         destination.  At the originating node, that address will be in
         the last element of the Routing header; at the recipient(s),
         that address will be in the Destination Address field of the
         IPv6 header.

      o  The Next Header value in the pseudo-header identifies the
         upper-layer protocol (e.g., 6 for TCP or 17 for UDP).  It will
         differ from the Next Header value in the IPv6 header if there
         are extension headers between the IPv6 header and the upper-
         layer header.







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      o  The Upper-Layer Packet Length in the pseudo-header is the
         length of the upper-layer header and data (e.g., TCP header
         plus TCP data).  Some upper-layer protocols carry their own
         length information (e.g., the Length field in the UDP header);
         for such protocols, that is the length used in the pseudo-
         header.  Other protocols (such as TCP) do not carry their own
         length information, in which case the length used in the
         pseudo-header is the Payload Length from the IPv6 header, minus
         the length of any extension headers present between the IPv6
         header and the upper-layer header.

      o  Unlike IPv4, the default behavior when UDP packets are
         originated by an IPv6 node is that the UDP checksum is not
         optional.  That is, whenever originating a UDP packet, an IPv6
         node must compute a UDP checksum over the packet and the
         pseudo-header, and, if that computation yields a result of
         zero, it must be changed to hex FFFF for placement in the UDP
         header.  IPv6 receivers must discard UDP packets containing a
         zero checksum and should log the error.

      o  As an exception to the default behavior, protocols that use UDP
         as a tunnel encapsulation may enable zero-checksum mode for a
         specific port (or set of ports) for sending and/or receiving.
         Any node implementing zero-checksum mode must follow the
         requirements specified in "Applicability Statement for the Use
         of IPv6 UDP Datagrams with Zero Checksums" [<a href="./rfc6936" title="&quot;Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums&quot;">RFC6936</a>].

   The IPv6 version of ICMP [<a href="./rfc4443" title="&quot;Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification&quot;">RFC4443</a>] includes the above pseudo-header
   in its checksum computation; this is a change from the IPv4 version
   of ICMP, which does not include a pseudo-header in its checksum.  The
   reason for the change is to protect ICMP from misdelivery or
   corruption of those fields of the IPv6 header on which it depends,
   which, unlike IPv4, are not covered by an internet-layer checksum.
   The Next Header field in the pseudo-header for ICMP contains the
   value 58, which identifies the IPv6 version of ICMP.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Maximum Packet Lifetime</span>

   Unlike IPv4, IPv6 nodes are not required to enforce maximum packet
   lifetime.  That is the reason the IPv4 "Time-to-Live" field was
   renamed "Hop Limit" in IPv6.  In practice, very few, if any, IPv4
   implementations conform to the requirement that they limit packet
   lifetime, so this is not a change in practice.  Any upper-layer
   protocol that relies on the internet layer (whether IPv4 or IPv6) to
   limit packet lifetime ought to be upgraded to provide its own
   mechanisms for detecting and discarding obsolete packets.





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<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Maximum Upper-Layer Payload Size</span>

   When computing the maximum payload size available for upper-layer
   data, an upper-layer protocol must take into account the larger size
   of the IPv6 header relative to the IPv4 header.  For example, in
   IPv4, TCP's Maximum Segment Size (MSS) option is computed as the
   maximum packet size (a default value or a value learned through Path
   MTU Discovery) minus 40 octets (20 octets for the minimum-length IPv4
   header and 20 octets for the minimum-length TCP header).  When using
   TCP over IPv6, the MSS must be computed as the maximum packet size
   minus 60 octets, because the minimum-length IPv6 header (i.e., an
   IPv6 header with no extension headers) is 20 octets longer than a
   minimum-length IPv4 header.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>.  Responding to Packets Carrying Routing Headers</span>

   When an upper-layer protocol sends one or more packets in response to
   a received packet that included a Routing header, the response
   packet(s) must not include a Routing header that was automatically
   derived by "reversing" the received Routing header UNLESS the
   integrity and authenticity of the received Source Address and Routing
   header have been verified (e.g., via the use of an Authentication
   header in the received packet).  In other words, only the following
   kinds of packets are permitted in response to a received packet
   bearing a Routing header:

      o  Response packets that do not carry Routing headers.

      o  Response packets that carry Routing headers that were NOT
         derived by reversing the Routing header of the received packet
         (for example, a Routing header supplied by local
         configuration).

      o  Response packets that carry Routing headers that were derived
         by reversing the Routing header of the received packet IF AND
         ONLY IF the integrity and authenticity of the Source Address
         and Routing header from the received packet have been verified
         by the responder.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  IANA Considerations</span>

   <a href="./rfc2460">RFC 2460</a> is referenced in a number of IANA registries.  These
   include:

      o  Internet Protocol Version 6 (IPv6) Parameters [<a href="#ref-IANA-6P" title="&quot;Internet Protocol Version 6 (IPv6) Parameters&quot;">IANA-6P</a>]

      o  Assigned Internet Protocol Numbers [<a href="#ref-IANA-PN" title="&quot;Protocol Numbers&quot;">IANA-PN</a>]




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      o  ONC RPC Network Identifiers (netids) [<a href="#ref-IANA-NI" title="&quot;ONC RPC Network Identifiers (netids)&quot;">IANA-NI</a>]

      o  Network Layer Protocol Identifiers (NLPIDs) of Interest
         [<a href="#ref-IANA-NL" title="&quot;Network Layer Protocol Identifiers (NLPIDs) of Interest&quot;">IANA-NL</a>]

      o  Protocol Registries [<a href="#ref-IANA-PR" title="&quot;Protocol Registries&quot;">IANA-PR</a>]

   The IANA has updated these references to point to this document.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Security Considerations</span>

   IPv6, from the viewpoint of the basic format and transmission of
   packets, has security properties that are similar to IPv4.  These
   security issues include:

      o  Eavesdropping, where on-path elements can observe the whole
         packet (including both contents and metadata) of each IPv6
         datagram.
      o  Replay, where the attacker records a sequence of packets off of
         the wire and plays them back to the party that originally
         received them.
      o  Packet insertion, where the attacker forges a packet with some
         chosen set of properties and injects it into the network.
      o  Packet deletion, where the attacker removes a packet from the
         wire.
      o  Packet modification, where the attacker removes a packet from
         the wire, modifies it, and reinjects it into the network.
      o  Man-in-the-middle (MITM) attacks, where the attacker subverts
         the communication stream in order to pose as the sender to
         receiver and the receiver to the sender.
      o  Denial-of-service (DoS) attacks, where the attacker sends large
         amounts of legitimate traffic to a destination to overwhelm it.

   IPv6 packets can be protected from eavesdropping, replay, packet
   insertion, packet modification, and MITM attacks by use of the
   "Security Architecture for the Internet Protocol" [<a href="./rfc4301" title="&quot;Security Architecture for the Internet Protocol&quot;">RFC4301</a>].  In
   addition, upper-layer protocols such as Transport Layer Security
   (TLS) or Secure Shell (SSH) can be used to protect the application-
   layer traffic running on top of IPv6.

   There is not any mechanism to protect against DoS attacks.  Defending
   against these type of attacks is outside the scope of this
   specification.

   IPv6 addresses are significantly larger than IPv4 addresses making it
   much harder to scan the address space across the Internet and even on
   a single network link (e.g., Local Area Network).  See [<a href="./rfc7707" title="&quot;Network Reconnaissance in IPv6 Networks&quot;">RFC7707</a>] for
   more information.



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   IPv6 addresses of nodes are expected to be more visible on the
   Internet as compared with IPv4 since the use of address translation
   technology is reduced.  This creates some additional privacy issues
   such as making it easier to distinguish endpoints.  See [<a href="./rfc7721" title="&quot;Security and Privacy Considerations for IPv6 Address Generation Mechanisms&quot;">RFC7721</a>] for
   more information.

   The design of IPv6 extension header architecture, while adding a lot
   of flexibility, also creates new security challenges.  As noted
   below, issues relating to the Fragment extension header have been
   resolved, but it's clear that for any new extension header designed
   in the future, the security implications need to be examined
   thoroughly, and this needs to include how the new extension header
   works with existing extension headers.  See [<a href="./rfc7045" title="&quot;Transmission and Processing of IPv6 Extension Headers&quot;">RFC7045</a>] for more
   information.

   This version of the IPv6 specification resolves a number of security
   issues that were found with the previous version [<a href="./rfc2460" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC2460</a>] of the
   IPv6 specification.  These include:

      o  Revised the text to handle the case of fragments that are whole
         datagrams (i.e., both the Fragment Offset field and the M flag
         are zero).  If received, they should be processed as a
         reassembled packet.  Any other fragments that match should be
         processed independently.  The Fragment creation process was
         modified to not create whole datagram fragments (Fragment
         Offset field and the M flag are zero).  See [<a href="./rfc6946" title="&quot;Processing of IPv6 &quot;">RFC6946</a>] and
         [<a href="./rfc8021" title="&quot;Generation of IPv6 Atomic Fragments Considered Harmful&quot;">RFC8021</a>] for more information.

      o  Removed the paragraph in <a href="#section-5">Section 5</a> that required including a
         Fragment header to outgoing packets if an ICMP Packet Too Big
         message reporting a Next-Hop MTU is less than 1280.  See
         [<a href="./rfc6946" title="&quot;Processing of IPv6 &quot;">RFC6946</a>] for more information.

      o  Changed the text to require that IPv6 nodes must not create
         overlapping fragments.  Also, when reassembling an IPv6
         datagram, if one or more of its constituent fragments is
         determined to be an overlapping fragment, the entire datagram
         (and any constituent fragments) must be silently discarded.
         Includes clarification that no ICMP error message should be
         sent if overlapping fragments are received.  See [<a href="./rfc5722" title="&quot;Handling of Overlapping IPv6 Fragments&quot;">RFC5722</a>] for
         more information.

      o  Revised the text to require that all headers through the first
         upper-layer header are in the first fragment.  See [<a href="./rfc7112" title="&quot;Implications of Oversized IPv6 Header Chains&quot;">RFC7112</a>]
         for more information.






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      o  Incorporated the updates from [<a href="./rfc5095" title="&quot;Deprecation of Type 0 Routing Headers in IPv6&quot;">RFC5095</a>] and [<a href="./rfc5871" title="&quot;IANA Allocation Guidelines for the IPv6 Routing Header&quot;">RFC5871</a>] to remove
         the description of the Routing Header type 0 (RH0), that the
         allocations guidelines for Routing headers are specified in <a href="./rfc5871">RFC</a>
         <a href="./rfc5871">5871</a>, and removed RH0 from the list of required extension
         headers.

   Security issues relating to other parts of IPv6 including addressing,
   ICMPv6, Path MTU Discovery, etc., are discussed in the appropriate
   specifications.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative References</span>

   [<a id="ref-RFC791">RFC791</a>]   Postel, J., "Internet Protocol", STD 5, <a href="./rfc791">RFC 791</a>,
              DOI 10.17487/RFC0791, September 1981,
              &lt;<a href="http://www.rfc-editor.org/info/rfc791">http://www.rfc-editor.org/info/rfc791</a>&gt;.

   [<a id="ref-RFC2474">RFC2474</a>]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", <a href="./rfc2474">RFC 2474</a>,
              DOI 10.17487/RFC2474, December 1998,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2474">http://www.rfc-editor.org/info/rfc2474</a>&gt;.

   [<a id="ref-RFC3168">RFC3168</a>]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              <a href="./rfc3168">RFC 3168</a>, DOI 10.17487/RFC3168, September 2001,
              &lt;<a href="http://www.rfc-editor.org/info/rfc3168">http://www.rfc-editor.org/info/rfc3168</a>&gt;.

   [<a id="ref-RFC4291">RFC4291</a>]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", <a href="./rfc4291">RFC 4291</a>, DOI 10.17487/RFC4291, February
              2006, &lt;<a href="http://www.rfc-editor.org/info/rfc4291">http://www.rfc-editor.org/info/rfc4291</a>&gt;.

   [<a id="ref-RFC4443">RFC4443</a>]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              <a href="./rfc4443">RFC 4443</a>, DOI 10.17487/RFC4443, March 2006,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4443">http://www.rfc-editor.org/info/rfc4443</a>&gt;.

   [<a id="ref-RFC6437">RFC6437</a>]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", <a href="./rfc6437">RFC 6437</a>,
              DOI 10.17487/RFC6437, November 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6437">http://www.rfc-editor.org/info/rfc6437</a>&gt;.








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<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

   [<a id="ref-Err2541">Err2541</a>]  RFC Errata, Erratum ID 2541, <a href="./rfc2460">RFC 2460</a>.

   [<a id="ref-Err4279">Err4279</a>]  RFC Errata, Erratum ID 4279, <a href="./rfc2460">RFC 2460</a>.

   [<a id="ref-Err4657">Err4657</a>]  RFC Errata, Erratum ID 4657, <a href="./rfc2460">RFC 2460</a>.

   [<a id="ref-Err4662">Err4662</a>]  RFC Errata, Erratum ID 4662, <a href="./rfc2460">RFC 2460</a>.

   [<a id="ref-IANA-6P">IANA-6P</a>]  IANA, "Internet Protocol Version 6 (IPv6) Parameters",
              &lt;<a href="https://www.iana.org/assignments/ipv6-parameters">https://www.iana.org/assignments/ipv6-parameters</a>&gt;.

   [<a id="ref-IANA-EH">IANA-EH</a>]  IANA, "IPv6 Extension Header Types",
              &lt;<a href="https://www.iana.org/assignments/ipv6-parameters">https://www.iana.org/assignments/ipv6-parameters</a>&gt;.

   [<a id="ref-IANA-NI">IANA-NI</a>]  IANA, "ONC RPC Network Identifiers (netids)",
              &lt;<a href="https://www.iana.org/assignments/rpc-netids">https://www.iana.org/assignments/rpc-netids</a>&gt;.

   [<a id="ref-IANA-NL">IANA-NL</a>]  IANA, "Network Layer Protocol Identifiers (NLPIDs) of
              Interest", &lt;<a href="https://www.iana.org/assignments/nlpids">https://www.iana.org/assignments/nlpids</a>&gt;.

   [<a id="ref-IANA-PN">IANA-PN</a>]  IANA, "Protocol Numbers",
              &lt;<a href="https://www.iana.org/assignments/protocol-numbers">https://www.iana.org/assignments/protocol-numbers</a>&gt;.

   [<a id="ref-IANA-PR">IANA-PR</a>]  IANA, "Protocol Registries", &lt;<a href="https://www.iana.org/protocols">https://www.iana.org/</a>
              <a href="https://www.iana.org/protocols">protocols</a>&gt;.

   [<a id="ref-IANA-RH">IANA-RH</a>]  IANA, "Routing Types", &lt;<a href="https://www.iana.org/assignments/ipv6-parameters">https://www.iana.org/assignments/</a>
              <a href="https://www.iana.org/assignments/ipv6-parameters">ipv6-parameters</a>&gt;.

   [<a id="ref-RFC1661">RFC1661</a>]  Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
              STD 51, <a href="./rfc1661">RFC 1661</a>, DOI 10.17487/RFC1661, July 1994,
              &lt;<a href="http://www.rfc-editor.org/info/rfc1661">http://www.rfc-editor.org/info/rfc1661</a>&gt;.

   [<a id="ref-RFC2460">RFC2460</a>]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", <a href="./rfc2460">RFC 2460</a>, DOI 10.17487/RFC2460,
              December 1998, &lt;<a href="http://www.rfc-editor.org/info/rfc2460">http://www.rfc-editor.org/info/rfc2460</a>&gt;.

   [<a id="ref-RFC4301">RFC4301</a>]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", <a href="./rfc4301">RFC 4301</a>, DOI 10.17487/RFC4301,
              December 2005, &lt;<a href="http://www.rfc-editor.org/info/rfc4301">http://www.rfc-editor.org/info/rfc4301</a>&gt;.

   [<a id="ref-RFC4302">RFC4302</a>]  Kent, S., "IP Authentication Header", <a href="./rfc4302">RFC 4302</a>,
              DOI 10.17487/RFC4302, December 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4302">http://www.rfc-editor.org/info/rfc4302</a>&gt;.





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   [<a id="ref-RFC4303">RFC4303</a>]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              <a href="./rfc4303">RFC 4303</a>, DOI 10.17487/RFC4303, December 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4303">http://www.rfc-editor.org/info/rfc4303</a>&gt;.

   [<a id="ref-RFC5095">RFC5095</a>]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", <a href="./rfc5095">RFC 5095</a>,
              DOI 10.17487/RFC5095, December 2007,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5095">http://www.rfc-editor.org/info/rfc5095</a>&gt;.

   [<a id="ref-RFC5722">RFC5722</a>]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              <a href="./rfc5722">RFC 5722</a>, DOI 10.17487/RFC5722, December 2009,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5722">http://www.rfc-editor.org/info/rfc5722</a>&gt;.

   [<a id="ref-RFC5871">RFC5871</a>]  Arkko, J. and S. Bradner, "IANA Allocation Guidelines for
              the IPv6 Routing Header", <a href="./rfc5871">RFC 5871</a>, DOI 10.17487/RFC5871,
              May 2010, &lt;<a href="http://www.rfc-editor.org/info/rfc5871">http://www.rfc-editor.org/info/rfc5871</a>&gt;.

   [<a id="ref-RFC6564">RFC6564</a>]  Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
              M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
              <a href="./rfc6564">RFC 6564</a>, DOI 10.17487/RFC6564, April 2012,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6564">http://www.rfc-editor.org/info/rfc6564</a>&gt;.

   [<a id="ref-RFC6936">RFC6936</a>]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              <a href="./rfc6936">RFC 6936</a>, DOI 10.17487/RFC6936, April 2013,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6936">http://www.rfc-editor.org/info/rfc6936</a>&gt;.

   [<a id="ref-RFC6946">RFC6946</a>]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              <a href="./rfc6946">RFC 6946</a>, DOI 10.17487/RFC6946, May 2013,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6946">http://www.rfc-editor.org/info/rfc6946</a>&gt;.

   [<a id="ref-RFC7045">RFC7045</a>]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", <a href="./rfc7045">RFC 7045</a>,
              DOI 10.17487/RFC7045, December 2013,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7045">http://www.rfc-editor.org/info/rfc7045</a>&gt;.

   [<a id="ref-RFC7112">RFC7112</a>]  Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", <a href="./rfc7112">RFC 7112</a>,
              DOI 10.17487/RFC7112, January 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7112">http://www.rfc-editor.org/info/rfc7112</a>&gt;.

   [<a id="ref-RFC7707">RFC7707</a>]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", <a href="./rfc7707">RFC 7707</a>, DOI 10.17487/RFC7707, March 2016,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7707">http://www.rfc-editor.org/info/rfc7707</a>&gt;.







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   [<a id="ref-RFC7721">RFC7721</a>]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              <a href="./rfc7721">RFC 7721</a>, DOI 10.17487/RFC7721, March 2016,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7721">http://www.rfc-editor.org/info/rfc7721</a>&gt;.

   [<a id="ref-RFC7739">RFC7739</a>]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", <a href="./rfc7739">RFC 7739</a>, DOI 10.17487/RFC7739,
              February 2016, &lt;<a href="http://www.rfc-editor.org/info/rfc7739">http://www.rfc-editor.org/info/rfc7739</a>&gt;.

   [<a id="ref-RFC8021">RFC8021</a>]  Gont, F., Liu, W., and T. Anderson, "Generation of IPv6
              Atomic Fragments Considered Harmful", <a href="./rfc8021">RFC 8021</a>,
              DOI 10.17487/RFC8021, January 2017,
              &lt;<a href="http://www.rfc-editor.org/info/rfc8021">http://www.rfc-editor.org/info/rfc8021</a>&gt;.

   [<a id="ref-RFC8201">RFC8201</a>]  McCann, J., Deering, S., Mogul, J., and R. Hinden, "Path
              MTU Discovery for IP version 6", STD 87, <a href="./rfc8201">RFC 8201</a>,
              DOI 10.17487/RFC8201, July 2017,
              &lt;<a href="http://www.rfc-editor.org/info/rfc8201">http://www.rfc-editor.org/info/rfc8201</a>&gt;.

































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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Formatting Guidelines for Options</span>

   This appendix gives some advice on how to lay out the fields when
   designing new options to be used in the Hop-by-Hop Options header or
   the Destination Options header, as described in <a href="#section-4.2">Section 4.2</a>.  These
   guidelines are based on the following assumptions:

      o  One desirable feature is that any multi-octet fields within the
         Option Data area of an option be aligned on their natural
         boundaries, i.e., fields of width n octets should be placed at
         an integer multiple of n octets from the start of the
         Hop-by-Hop or Destination Options header, for n = 1, 2, 4, or
         8.

      o  Another desirable feature is that the Hop-by-Hop or Destination
         Options header take up as little space as possible, subject to
         the requirement that the header be an integer multiple of 8
         octets long.

      o  It may be assumed that, when either of the option-bearing
         headers are present, they carry a very small number of options,
         usually only one.

   These assumptions suggest the following approach to laying out the
   fields of an option: order the fields from smallest to largest, with
   no interior padding, then derive the alignment requirement for the
   entire option based on the alignment requirement of the largest field
   (up to a maximum alignment of 8 octets).  This approach is
   illustrated in the following examples:

   Example 1

   If an option X required two data fields, one of length 8 octets and
   one of length 4 octets, it would be laid out as follows:

                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   | Option Type=X |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         8-octet field                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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   Its alignment requirement is 8n+2, to ensure that the 8-octet field
   starts at a multiple-of-8 offset from the start of the enclosing
   header.  A complete Hop-by-Hop or Destination Options header
   containing this one option would look as follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         8-octet field                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Example 2

   If an option Y required three data fields, one of length 4 octets,
   one of length 2 octets, and one of length 1 octet, it would be laid
   out as follows:

                                                   +-+-+-+-+-+-+-+-+
                                                   | Option Type=Y |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Opt Data Len=7 | 1-octet field |         2-octet field         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Its alignment requirement is 4n+3, to ensure that the 4-octet field
   starts at a multiple-of-4 offset from the start of the enclosing
   header.  A complete Hop-by-Hop or Destination Options header
   containing this one option would look as follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=1 | Pad1 Option=0 | Option Type=Y |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Opt Data Len=7 | 1-octet field |         2-octet field         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PadN Option=1 |Opt Data Len=2 |       0       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   Example 3

   A Hop-by-Hop or Destination Options header containing both options X
   and Y from Examples 1 and 2 would have one of the two following
   formats, depending on which option appeared first:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=3 | Option Type=X |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         8-octet field                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PadN Option=1 |Opt Data Len=1 |       0       | Option Type=Y |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Opt Data Len=7 | 1-octet field |         2-octet field         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PadN Option=1 |Opt Data Len=2 |       0       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=3 | Pad1 Option=0 | Option Type=Y |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Opt Data Len=7 | 1-octet field |         2-octet field         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PadN Option=1 |Opt Data Len=4 |       0       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |       0       | Option Type=X |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         8-octet field                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Changes Since <a href="./rfc2460">RFC 2460</a></span>

   This memo has the following changes from <a href="./rfc2460">RFC 2460</a>.

   o  Removed IP Next Generation from the Abstract.

   o  Added text in <a href="#section-1">Section 1</a> that the data transmission order is the
      same as IPv4 as defined in <a href="./rfc791">RFC 791</a>.

   o  Clarified the text in <a href="#section-3">Section 3</a> about decrementing the Hop Limit.

   o  Clarified that extension headers (except for the Hop-by-Hop
      Options header) are not processed, inserted, or deleted by any
      node along a packet's delivery path.

   o  Changed requirement for the Hop-by-Hop Options header to a "may",
      and added a note to indicate what is expected regarding the
      Hop-by-Hop Options header.

   o  Added a paragraph to <a href="#section-4">Section 4</a> to clarify how extension headers
      are numbered and which are upper-layer headers.

   o  Added a reference to the end of <a href="#section-4">Section 4</a> to the "IPv6 Extension
      Header Types" IANA registry.

   o  Incorporated the updates from RFCs 5095 and 5871 to remove the
      description of RH0, that the allocations guidelines for routing
      headers are specified in <a href="./rfc5871">RFC 5871</a>, and removed RH0 from the list
      of required extension headers.

   o  Revised <a href="#section-4.5">Section 4.5</a> on IPv6 fragmentation based on updates from
      RFCs 5722, 6946, 7112, and 8021.  This includes:

      -  Revised the text to handle the case of fragments that are whole
         datagrams (i.e., both the Fragment Offset field and the M flag
         are zero).  If received, they should be processed as a
         reassembled packet.  Any other fragments that match should be
         processed independently.  The revised Fragment creation process
         was modified to not create whole datagram fragments (Fragment
         Offset field and the M flag are zero).

      -  Changed the text to require that IPv6 nodes must not create
         overlapping fragments.  Also, when reassembling an IPv6
         datagram, if one or more its constituent fragments is
         determined to be an overlapping fragment, the entire datagram
         (and any constituent fragments) must be silently discarded.
         Includes a clarification that no ICMP error message should be
         sent if overlapping fragments are received.



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      -  Revised the text to require that all headers through the first
         Upper-Layer header are in the first fragment.  This changed the
         text describing how packets are fragmented and reassembled and
         added a new error case.

      -  Added text to the Fragment header process on handling exact
         duplicate fragments.

      -  Updated the Fragmentation header text to correct the inclusion
         of an Authentication Header (AH) and noted No Next Header case.

      -  Changed terminology in the Fragment header section from
         "Unfragmentable Headers" to "Per-Fragment headers".

      -  Removed the paragraph in <a href="#section-5">Section 5</a> that required including a
         Fragment header to outgoing packets if an ICMP Packet Too Big
         message reports a Next-Hop MTU less than 1280.

      -  Changed the text to clarify MTU restriction and 8-byte
         restrictions, and noted the restriction on headers in the first
         fragment.

   o  In <a href="#section-4.5">Section 4.5</a>, added clarification noting that some fields in the
      IPv6 header may also vary across the fragments being reassembled,
      and that other specifications may provide additional instructions
      for how they should be reassembled.  See, for example, <a href="./rfc3168#section-5.3">Section&nbsp;5.3
      of [RFC3168]</a>.

   o  Incorporated the update from <a href="./rfc6564">RFC 6564</a> to add a new <a href="#section-4.8">Section 4.8</a>
      that describes recommendations for defining new extension headers
      and options.

   o  Added text to <a href="#section-5">Section 5</a> to define "IPv6 minimum link MTU".

   o  Simplified the text in <a href="#section-6">Section 6</a> about Flow Labels and removed
      what was <a href="#appendix-A">Appendix A</a> ("Semantics and Usage of the Flow Label
      Field"); instead, pointed to the current specifications of the
      IPv6 Flow Label field in [<a href="./rfc6437" title="&quot;IPv6 Flow Label Specification&quot;">RFC6437</a>] and the Traffic Class field in
      [<a href="./rfc2474" title="&quot;Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers&quot;">RFC2474</a>] and [<a href="./rfc3168" title="&quot;The Addition of Explicit Congestion Notification (ECN) to IP&quot;">RFC3168</a>].

   o  Incorporated the update made by <a href="./rfc6935">RFC 6935</a> ("IPv6 and UDP Checksums
      for Tunneled Packets") in <a href="#section-8">Section 8</a>.  Added an exception to the
      default behavior for the handling of UDP packets with zero
      checksums for tunnels.

   o  Added instruction to <a href="#section-9">Section 9</a>, "IANA Considerations", to change
      references to <a href="./rfc2460">RFC 2460</a> to this document.




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   o  Revised and expanded <a href="#section-10">Section 10</a>, "Security Considerations".

   o  Added a paragraph to the Acknowledgments section acknowledging the
      authors of the updating documents.

   o  Updated references to current versions and assigned references to
      normative and informative.

   o  Made changes to resolve the errata on <a href="./rfc2460">RFC 2460</a>.  These are:

         Erratum ID 2541 [<a href="#ref-Err2541" title="Erratum ID 2541">Err2541</a>]: This erratum notes that <a href="./rfc2460">RFC 2460</a>
         didn't update <a href="./rfc2205">RFC 2205</a> when the length of the flow label was
         changed from 24 to 20 bits from <a href="./rfc1883">RFC 1883</a>.  This issue was
         resolved in <a href="./rfc6437">RFC 6437</a> where the flow label is defined.  This
         specification now references <a href="./rfc6437">RFC 6437</a>.  No change is required.

         Erratum ID 4279 [<a href="#ref-Err4279" title="Erratum ID 4279">Err4279</a>]: This erratum noted that the
         specification doesn't handle the case of a forwarding node
         receiving a packet with a zero Hop Limit.  This is fixed in
         <a href="#section-3">Section 3</a> of this specification.

         Erratum ID 4657 [<a href="#ref-Err4657" title="Erratum ID 4657">Err4657</a>]: This erratum proposed text that
         extension headers must never be inserted by any node other than
         the source of the packet.  This was resolved in <a href="#section-4">Section 4</a>,
         "IPv6 Extension Headers".

         Erratum ID 4662 [<a href="#ref-Err4662" title="Erratum ID 4662">Err4662</a>]: This erratum proposed text that
         extension headers, with one exception, are not examined,
         processed, modified, inserted, or deleted by any node along a
         packet's delivery path.  This was resolved in <a href="#section-4">Section 4</a>, "IPv6
         Extension Headers".

         Erratum ID 2843: This erratum is marked "Rejected".  No change
         was made.

















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Acknowledgments

   The authors gratefully acknowledge the many helpful suggestions of
   the members of the IPng Working Group, the End-to-End Protocols
   research group, and the Internet community at large.

   The authors would also like to acknowledge the authors of the
   updating RFCs that were incorporated in this document to move the
   IPv6 specification to Internet Standard.  They are Joe Abley, Shane
   Amante, Jari Arkko, Manav Bhatia, Ronald P. Bonica, Scott Bradner,
   Brian Carpenter, P.F. Chimento, Marshall Eubanks, Fernando Gont,
   James Hoagland, Sheng Jiang, Erik Kline, Suresh Krishnan, Vishwas
   Manral, George Neville-Neil, Jarno Rajahalme, Pekka Savola, Magnus
   Westerlund, and James Woodyatt.

Authors' Addresses

   Stephen E. Deering
   Retired
   Vancouver, British Columbia
   Canada


   Robert M. Hinden
   Check Point Software
   959 Skyway Road
   San Carlos, CA  94070
   United States of America

   Email: bob.hinden@gmail.com





















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