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RFC 8200

 
 
 

Internet Protocol, Version 6 (IPv6) Specification

Part 2 of 2, p. 25 to 42
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5.  Packet Size Issues

   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 [RFC1661])
   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 [RFC8201], 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.

6.  Flow Labels

   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
   [RFC6437].

7.  Traffic Classes

   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
   [RFC2474] and [RFC3168].

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8.  Upper-Layer Protocol Issues

8.1.  Upper-Layer Checksums

   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" [RFC6936].

   The IPv6 version of ICMP [RFC4443] 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.

8.2.  Maximum Packet Lifetime

   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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8.3.  Maximum Upper-Layer Payload Size

   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.

8.4.  Responding to Packets Carrying Routing Headers

   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.

9.  IANA Considerations

   RFC 2460 is referenced in a number of IANA registries.  These
   include:

      o  Internet Protocol Version 6 (IPv6) Parameters [IANA-6P]

      o  Assigned Internet Protocol Numbers [IANA-PN]

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      o  ONC RPC Network Identifiers (netids) [IANA-NI]

      o  Network Layer Protocol Identifiers (NLPIDs) of Interest
         [IANA-NL]

      o  Protocol Registries [IANA-PR]

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

10.  Security Considerations

   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" [RFC4301].  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 [RFC7707] 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 [RFC7721] 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 [RFC7045] for more
   information.

   This version of the IPv6 specification resolves a number of security
   issues that were found with the previous version [RFC2460] 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 [RFC6946] and
         [RFC8021] for more information.

      o  Removed the paragraph in Section 5 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
         [RFC6946] 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 [RFC5722] for
         more information.

      o  Revised the text to require that all headers through the first
         upper-layer header are in the first fragment.  See [RFC7112]
         for more information.

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      o  Incorporated the updates from [RFC5095] and [RFC5871] to remove
         the description of the Routing Header type 0 (RH0), that the
         allocations guidelines for Routing headers are specified in RFC
         5871, 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.

11.  References

11.1.  Normative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

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

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

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

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

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

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11.2.  Informative References

   [Err2541]  RFC Errata, Erratum ID 2541, RFC 2460.

   [Err4279]  RFC Errata, Erratum ID 4279, RFC 2460.

   [Err4657]  RFC Errata, Erratum ID 4657, RFC 2460.

   [Err4662]  RFC Errata, Erratum ID 4662, RFC 2460.

   [IANA-6P]  IANA, "Internet Protocol Version 6 (IPv6) Parameters",
              <https://www.iana.org/assignments/ipv6-parameters>.

   [IANA-EH]  IANA, "IPv6 Extension Header Types",
              <https://www.iana.org/assignments/ipv6-parameters>.

   [IANA-NI]  IANA, "ONC RPC Network Identifiers (netids)",
              <https://www.iana.org/assignments/rpc-netids>.

   [IANA-NL]  IANA, "Network Layer Protocol Identifiers (NLPIDs) of
              Interest", <https://www.iana.org/assignments/nlpids>.

   [IANA-PN]  IANA, "Protocol Numbers",
              <https://www.iana.org/assignments/protocol-numbers>.

   [IANA-PR]  IANA, "Protocol Registries", <https://www.iana.org/
              protocols>.

   [IANA-RH]  IANA, "Routing Types", <https://www.iana.org/assignments/
              ipv6-parameters>.

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

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

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <http://www.rfc-editor.org/info/rfc4302>.

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   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <http://www.rfc-editor.org/info/rfc4303>.

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

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, DOI 10.17487/RFC5722, December 2009,
              <http://www.rfc-editor.org/info/rfc5722>.

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

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

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

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013,
              <http://www.rfc-editor.org/info/rfc6946>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <http://www.rfc-editor.org/info/rfc7045>.

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

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <http://www.rfc-editor.org/info/rfc7707>.

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

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <http://www.rfc-editor.org/info/rfc7739>.

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

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

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Appendix A.  Formatting Guidelines for Options

   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 Section 4.2.  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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Appendix B.  Changes Since RFC 2460

   This memo has the following changes from RFC 2460.

   o  Removed IP Next Generation from the Abstract.

   o  Added text in Section 1 that the data transmission order is the
      same as IPv4 as defined in RFC 791.

   o  Clarified the text in Section 3 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 Section 4 to clarify how extension headers
      are numbered and which are upper-layer headers.

   o  Added a reference to the end of Section 4 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 RFC 5871, and removed RH0 from the list
      of required extension headers.

   o  Revised Section 4.5 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 Section 5 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 Section 4.5, 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, Section 5.3
      of [RFC3168].

   o  Incorporated the update from RFC 6564 to add a new Section 4.8
      that describes recommendations for defining new extension headers
      and options.

   o  Added text to Section 5 to define "IPv6 minimum link MTU".

   o  Simplified the text in Section 6 about Flow Labels and removed
      what was Appendix A ("Semantics and Usage of the Flow Label
      Field"); instead, pointed to the current specifications of the
      IPv6 Flow Label field in [RFC6437] and the Traffic Class field in
      [RFC2474] and [RFC3168].

   o  Incorporated the update made by RFC 6935 ("IPv6 and UDP Checksums
      for Tunneled Packets") in Section 8.  Added an exception to the
      default behavior for the handling of UDP packets with zero
      checksums for tunnels.

   o  Added instruction to Section 9, "IANA Considerations", to change
      references to RFC 2460 to this document.

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   o  Revised and expanded Section 10, "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 RFC 2460.  These are:

         Erratum ID 2541 [Err2541]: This erratum notes that RFC 2460
         didn't update RFC 2205 when the length of the flow label was
         changed from 24 to 20 bits from RFC 1883.  This issue was
         resolved in RFC 6437 where the flow label is defined.  This
         specification now references RFC 6437.  No change is required.

         Erratum ID 4279 [Err4279]: 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
         Section 3 of this specification.

         Erratum ID 4657 [Err4657]: 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 Section 4,
         "IPv6 Extension Headers".

         Erratum ID 4662 [Err4662]: 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 Section 4, "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