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rfc:rfc1883

Network Working Group S. Deering, Xerox PARC Request for Comments: 1883 R. Hinden, Ipsilon Networks Category: Standards Track December 1995

                Internet Protocol, Version 6 (IPv6)
                           Specification

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document specifies version 6 of the Internet Protocol (IPv6),
 also sometimes referred to as IP Next Generation or IPng.

Deering & Hinden Standards Track [Page 1] RFC 1883 IPv6 Specification December 1995

Table of Contents

 1. Introduction..................................................3
 2. Terminology...................................................4
 3. IPv6 Header Format............................................5
 4. IPv6 Extension Headers........................................6
     4.1 Extension Header Order...................................8
     4.2 Options..................................................9
     4.3 Hop-by-Hop Options Header...............................11
     4.4 Routing Header..........................................13
     4.5 Fragment Header.........................................19
     4.6 Destination Options Header..............................24
     4.7 No Next Header..........................................25
 5. Packet Size Issues...........................................26
 6. Flow Labels..................................................28
 7. Priority.....................................................30
 8. Upper-Layer Protocol Issues..................................31
     8.1 Upper-Layer Checksums...................................31
     8.2 Maximum Packet Lifetime.................................32
     8.3 Maximum Upper-Layer Payload Size........................32
 Appendix A. Formatting Guidelines for Options...................33
 Security Considerations.........................................36
 Acknowledgments.................................................36
 Authors' Addresses..............................................36
 References......................................................37

Deering & Hinden Standards Track [Page 2] RFC 1883 IPv6 Specification December 1995

1. Introduction

 IP version 6 (IPv6) is a new version of the Internet Protocol,
 designed as a successor to IP version 4 (IPv4) [RFC-791].  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 auto-configuration 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, 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 packets
       belonging to particular traffic "flows" for which the sender
       requests special handling, such as non-default quality of
       service or "real-time" service.
    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 priority, and the
 effects of IPv6 on upper-layer protocols.  The format and semantics
 of IPv6 addresses are specified separately in [RFC-1884].  The IPv6
 version of ICMP, which all IPv6 implementations are required to
 include, is specified in [RFC-1885].

Deering & Hinden Standards Track [Page 3] RFC 1883 IPv6 Specification December 1995

2. Terminology

 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 or lower-layer protocols being "tunneled"
               over (i.e., encapsulated in) IPv6 such as 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 (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 in one piece over
               a link.
 path MTU    - the minimum link MTU of all the links in a path between
               a source node and a destination node.
 Note: it is possible, though unusual, 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.

Deering & Hinden Standards Track [Page 4] RFC 1883 IPv6 Specification December 1995

3. IPv6 Header Format

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Version| Prio. |                   Flow Label                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Payload Length        |  Next Header  |   Hop Limit   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                                                               +
 |                                                               |
 +                         Source Address                        +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                                                               +
 |                                                               |
 +                      Destination Address                      +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Version              4-bit Internet Protocol version number = 6.
 Prio.                4-bit priority value.  See section 7.
 Flow Label           24-bit flow label.  See section 6.
 Payload Length       16-bit unsigned integer.  Length of payload,
                      i.e., the rest of the packet following the
                      IPv6 header, in octets.  If zero, indicates that
                      the payload length is carried in a Jumbo Payload
                      hop-by-hop option.
 Next Header          8-bit selector.  Identifies the type of header
                      immediately following the IPv6 header.  Uses
                      the same values as the IPv4 Protocol field
                      [RFC-1700 et seq.].
 Hop Limit            8-bit unsigned integer.  Decremented by 1 by
                      each node that forwards the packet. The packet
                      is discarded if Hop Limit is decremented to
                      zero.
 Source Address       128-bit address of the originator of the
                      packet.  See [RFC-1884].

Deering & Hinden Standards Track [Page 5] RFC 1883 IPv6 Specification December 1995

 Destination Address  128-bit address of the intended recipient
                      of the packet (possibly not the ultimate
                      recipient, if a Routing header is present).
                      See [RFC-1884] and section 4.4.

4. IPv6 Extension Headers

 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 are a small number of such extension
 headers, each identified by a distinct Next Header value.  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       |
 +---------------+----------------+-----------------+-----------------
 With one exception, extension headers are not 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.
 There, 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

Deering & Hinden Standards Track [Page 6] RFC 1883 IPv6 Specification December 1995

 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.
 The exception referred to in the preceding paragraph is the Hop-by-
 Hop Options header, which carries information that must be examined
 and processed by every node along a packet's delivery path, including
 the source and destination nodes.  The Hop-by-Hop Options header,
 when present, must immediately follow the IPv6 header.  Its presence
 is indicated by the value zero in the Next Header field of the IPv6
 header.
 If, as a result of processing a header, a node is required to proceed
 to the next header but the Next Header value in the current header is
 unrecognized by the node, it should discard the packet and send an
 ICMP Parameter Problem message to the source of the packet, with an
 ICMP Code value of 2 ("unrecognized Next Header type encountered")
 and the ICMP Pointer field containing the offset of the unrecognized
 value within the original packet.  The same action should be taken if
 a node encounters a Next Header value of zero in any header other
 than an IPv6 header.
 Each extension header is an integer multiple of 8 octets long, in
 order to retain 8-octet alignment for subsequent headers.  Multi-
 octet fields within each extension header are aligned on their
 natural boundaries, i.e., fields of width n octets are placed at an
 integer multiple of n octets from the start of the header, for n = 1,
 2, 4, or 8.
 A full implementation of IPv6 includes implementation of the
 following extension headers:
         Hop-by-Hop Options
         Routing (Type 0)
         Fragment
         Destination Options
         Authentication
         Encapsulating Security Payload
 The first four are specified in this document; the last two are
 specified in [RFC-1826] and [RFC-1827], respectively.

Deering & Hinden Standards Track [Page 7] RFC 1883 IPv6 Specification December 1995

4.1 Extension Header Order

 When more than one extension header is used in the same packet, it is
 recommended that those headers appear in the following order:
         IPv6 header
         Hop-by-Hop Options header
         Destination Options header (note 1)
         Routing header
         Fragment 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 [RFC-1827].
         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 extensions headers, which are separately subject to the same
 ordering recommendations.
 If and when other extension headers are defined, their ordering
 constraints relative to the above listed headers must be specified.
 IPv6 nodes must accept and attempt to process extension headers in
 any order and occurring any number of times in the same packet,
 except for the Hop-by-Hop Options header which is restricted to
 appear immediately after an IPv6 header only.  Nonetheless, it is
 strongly advised that sources of IPv6 packets adhere to the above
 recommended order until and unless subsequent specifications revise
 that recommendation.

Deering & Hinden Standards Track [Page 8] RFC 1883 IPv6 Specification December 1995

4.2 Options

 Two of the currently-defined extension headers -- the Hop-by-Hop
 Options header and the Destination Options header -- carry a variable
 number of type-length-value (TLV) encoded "options", of the following
 format:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
    |  Option Type  |  Opt Data Len |  Option Data
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
    Option Type          8-bit identifier of the type of option.
    Opt Data Len         8-bit unsigned integer.  Length of the Option
                         Data field of this option, in octets.
    Option Data          Variable-length field.  Option-Type-specific
                         data.
 The sequence of options within a header must be processed strictly in
 the order they appear in the header; a receiver must not, for
 example, scan through the header looking for a particular kind of
 option and process that option prior to processing all preceding
 ones.
 The Option Type identifiers are internally encoded such that their
 highest-order two bits specify the action that must be taken if the
 processing IPv6 node does not recognize the Option Type:
    00 - skip over this option and continue processing the header.
    01 - discard the packet.
    10 - discard the packet and, regardless of whether or not the
         packets'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
 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.

Deering & Hinden Standards Track [Page 9] RFC 1883 IPv6 Specification December 1995

    0 - Option Data does not change en-route
    1 - Option Data may change en-route
 Individual options may have specific alignment requirements, to
 ensure that multi-octet values within Option Data fields fall on
 natural boundaries.  The alignment requirement of an option is
 specified using the notation xn+y, meaning the Option Type must
 appear at an integer multiple of x octets from the start of the
 header, plus y octets.  For example:
     2n    means any 2-octet offset from the start of the header.
     8n+2  means any 8-octet offset from the start of the header,
           plus 2 octets.
 There are two padding options which are used when necessary to align
 subsequent options and to pad out the containing header to a multiple
 of 8 octets in length.  These padding options must be recognized by
 all IPv6 implementations:
 Pad1 option  (alignment requirement: none)
     +-+-+-+-+-+-+-+-+
     |       0       |
     +-+-+-+-+-+-+-+-+
     NOTE! the format of the Pad1 option is a special case -- it does
           not have length and value fields.
     The Pad1 option is used to insert one octet of padding into the
     Options area of a header.  If more than one octet of padding is
     required, the PadN option, described next, should be used,
     rather than multiple Pad1 options.
 PadN option  (alignment requirement: none)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
     |       1       |  Opt Data Len |  Option Data
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
     The PadN option is used to insert two or more octets of padding
     into the Options area of a header.  For N octets of padding,
     the Opt Data Len field contains the value N-2, and the Option
     Data consists of N-2 zero-valued octets.
 Appendix A contains formatting guidelines for designing new options.

Deering & Hinden Standards Track [Page 10] RFC 1883 IPv6 Specification December 1995

4.3 Hop-by-Hop Options Header

 The Hop-by-Hop Options header is used to carry optional information
 that must be examined by every node along a packet's delivery path.
 The Hop-by-Hop Options header is identified by a Next Header value of
 0 in the IPv6 header, and has the following format:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Next Header  |  Hdr Ext Len  |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 |                                                               |
 .                                                               .
 .                            Options                            .
 .                                                               .
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next Header          8-bit selector.  Identifies the type of header
                      immediately following the Hop-by-Hop Options
                      header.  Uses the same values as the IPv4
                      Protocol field [RFC-1700 et seq.].
 Hdr Ext Len          8-bit unsigned integer.  Length of the
                      Hop-by-Hop Options header in 8-octet units,
                      not including the first 8 octets.
 Options              Variable-length field, of length such that the
                      complete Hop-by-Hop Options header is an integer
                      multiple of 8 octets long.  Contains one or
                      more TLV-encoded options, as described in
                      section 4.2.
 In addition to the Pad1 and PadN options specified in section 4.2,
 the following hop-by-hop option is defined:
 Jumbo Payload option  (alignment requirement: 4n + 2)
                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |      194      |Opt Data Len=4 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Jumbo Payload Length                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     The Jumbo Payload option is used to send IPv6 packets with
     payloads longer than 65,535 octets.  The Jumbo Payload Length is
     the length of the packet in octets, excluding the IPv6 header but
     including the Hop-by-Hop Options header; it must be greater than
     65,535.  If a packet is received with a Jumbo Payload option
     containing a Jumbo Payload Length less than or equal to 65,535,

Deering & Hinden Standards Track [Page 11] RFC 1883 IPv6 Specification December 1995

     an ICMP Parameter Problem message, Code 0, should be sent to the
     packet's source, pointing to the high-order octet of the invalid
     Jumbo Payload Length field.
     The Payload Length field in the IPv6 header must be set to zero
     in every packet that carries the Jumbo Payload option.  If a
     packet is received with a valid Jumbo Payload option present and
     a non-zero IPv6 Payload Length field, an ICMP Parameter Problem
     message, Code 0, should be sent to the packet's source, pointing
     to the Option Type field of the Jumbo Payload option.
     The Jumbo Payload option must not be used in a packet that
     carries a Fragment header.  If a Fragment header is encountered
     in a packet that contains a valid Jumbo Payload option, an ICMP
     Parameter Problem message, Code 0, should be sent to the packet's
     source, pointing to the first octet of the Fragment header.
     An implementation that does not support the Jumbo Payload option
     cannot have interfaces to links whose link MTU is greater than
     65,575 (40 octets of IPv6 header plus 65,535 octets of payload).

Deering & Hinden Standards Track [Page 12] RFC 1883 IPv6 Specification December 1995

4.4 Routing Header

 The Routing header is used by an IPv6 source to list one or more
 intermediate nodes to be "visited" on the way to a packet's
 destination.  This function is very similar to IPv4's Source Route
 options.  The Routing header is identified by a Next Header value of
 43 in the immediately preceding header, and has the following format:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Next Header  |  Hdr Ext Len  |  Routing Type | Segments Left |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 .                                                               .
 .                       type-specific data                      .
 .                                                               .
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next Header          8-bit selector.  Identifies the type of header
                      immediately following the Routing header.
                      Uses the same values as the IPv4 Protocol field
                      [RFC-1700 et seq.].
 Hdr Ext Len          8-bit unsigned integer.  Length of the
                      Routing header in 8-octet units, not including
                      the first 8 octets.
 Routing Type         8-bit identifier of a particular Routing
                      header variant.
 Segments Left        8-bit unsigned integer.  Number of route
                      segments remaining, i.e., number of explicitly
                      listed intermediate nodes still to be visited
                      before reaching the final destination.
 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.

Deering & Hinden Standards Track [Page 13] RFC 1883 IPv6 Specification December 1995

 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.

Deering & Hinden Standards Track [Page 14] RFC 1883 IPv6 Specification December 1995

 The Type 0 Routing header has the following format:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Next Header  |  Hdr Ext Len  | Routing Type=0| Segments Left |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Reserved    |             Strict/Loose Bit Map              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                                                               +
 |                                                               |
 +                           Address[1]                          +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                                                               +
 |                                                               |
 +                           Address[2]                          +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 .                               .                               .
 .                               .                               .
 .                               .                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                                                               +
 |                                                               |
 +                           Address[n]                          +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next Header          8-bit selector.  Identifies the type of header
                      immediately following the Routing header.
                      Uses the same values as the IPv4 Protocol field
                      [RFC-1700 et seq.].
 Hdr Ext Len          8-bit unsigned integer.  Length of the
                      Routing header in 8-octet units, not including
                      the first 8 octets.  For the Type 0 Routing
                      header, Hdr Ext Len is equal to two times the
                      number of addresses in the header, and must
                      be an even number less than or equal to 46.
 Routing Type         0.

Deering & Hinden Standards Track [Page 15] RFC 1883 IPv6 Specification December 1995

 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.
                      Maximum legal value = 23.
 Reserved             8-bit reserved field.  Initialized to zero for
                      transmission; ignored on reception.
 Strict/Loose Bit Map
                      24-bit bit-map, numbered 0 to 23, left-to-right.
                      Indicates, for each segment of the route, whether
                      or not the next destination address must be a
                      neighbor of the preceding address: 1 means strict
                      (must be a neighbor), 0 means loose (need not be
                      a neighbor).
 Address[1..n]        Vector of 128-bit addresses, numbered 1 to n.
 Multicast addresses must not appear in a Routing header of Type 0, or
 in the IPv6 Destination Address field of a packet carrying a Routing
 header of Type 0.
 If bit number 0 of the Strict/Loose Bit Map has value 1, the
 Destination Address field of the IPv6 header in the original packet
 must identify a neighbor of the originating node.  If bit number 0
 has value 0, the originator may use any legal, non-multicast address
 as the initial Destination Address.
 Bits numbered greater than n, where n is the number of addresses in
 the Routing header, must be set to 0 by the originator and ignored by
 receivers.
 A Routing header is not examined or processed until it reaches the
 node identified in the Destination Address field of the IPv6 header.
 In that node, dispatching on the Next Header field of the immediately
 preceding header causes the Routing header module to be invoked,
 which, in the case of Routing Type 0, performs the following
 algorithm:

Deering & Hinden Standards Track [Page 16] RFC 1883 IPv6 Specification December 1995

 if Segments Left = 0 {
    proceed to process the next header in the packet, whose type is
    identified by the Next Header field in the Routing header
 }
 else if Hdr Ext Len is odd or greater than 46 {
       send an ICMP Parameter Problem, Code 0, message to the Source
       Address, pointing to the Hdr Ext Len field, and discard the
       packet
 }
 else {
    compute n, the number of addresses in the Routing header, by
    dividing Hdr Ext Len by 2
    if Segments Left is greater than n {
       send an ICMP Parameter Problem, Code 0, message to the Source
       Address, pointing to the Segments Left field, and discard the
       packet
    }
    else {
       decrement Segments Left by 1;
       compute i, the index of the next address to be visited in
       the address vector, by subtracting Segments Left from n
       if Address [i] or the IPv6 Destination Address is multicast {
          discard the packet
       }
       else {
          swap the IPv6 Destination Address and Address[i]
          if bit i of the Strict/Loose Bit map has value 1 and the
          new Destination Address is not the address of a neighbor
          of this node {
             send an ICMP Destination Unreachable -- Not a Neighbor
             message to the Source Address and discard the packet
          }
          else if the IPv6 Hop Limit is less than or equal to 1 {
             send an ICMP Time Exceeded -- Hop Limit Exceeded in
             Transit message to the Source Address and discard the
             packet
          }
          else {
             decrement the Hop Limit by 1
             resubmit the packet to the IPv6 module for transmission
             to the new destination
          }
       }
    }
 }

Deering & Hinden Standards Track [Page 17] RFC 1883 IPv6 Specification December 1995

 As an example of the effects of the above algorithm, consider the
 case of a source node S sending a packet to destination node D, using
 a Routing header to cause the packet to be routed via intermediate
 nodes I1, I2, and I3.  The values of the relevant IPv6 header and
 Routing header fields on each segment of the delivery path would be
 as follows:
 As the packet travels from S to I1:
      Source Address = S                  Hdr Ext Len = 6
      Destination Address = I1            Segments Left = 3
                                          Address[1] = I2
      (if bit 0 of the Bit Map is 1,      Address[2] = I3
       S and I1 must be neighbors;        Address[3] = D
       this is checked by S)
 As the packet travels from I1 to I2:
      Source Address = S                  Hdr Ext Len = 6
      Destination Address = I2            Segments Left = 2
                                          Address[1] = I1
      (if bit 1 of the Bit Map is 1,      Address[2] = I3
       I1 and I2 must be neighbors;       Address[3] = D
       this is checked by I1)
 As the packet travels from I2 to I3:
      Source Address = S                  Hdr Ext Len = 6
      Destination Address = I3            Segments Left = 1
                                          Address[1] = I1
      (if bit 2 of the Bit Map is 1,      Address[2] = I2
       I2 and I3 must be neighbors;       Address[3] = D
       this is checked by I2)
 As the packet travels from I3 to D:
      Source Address = S                  Hdr Ext Len = 6
      Destination Address = D             Segments Left = 0
                                          Address[1] = I1
      (if bit 3 of the Bit Map is 1,      Address[2] = I2
       I3 and D must be neighbors;        Address[3] = I3
       this is checked by I3)

Deering & Hinden Standards Track [Page 18] RFC 1883 IPv6 Specification December 1995

4.5 Fragment Header

 The Fragment header is used by an IPv6 source to send packets larger
 than would fit in the path MTU to their destinations.  (Note: unlike
 IPv4, fragmentation in IPv6 is performed only by source nodes, not by
 routers along a packet's delivery path -- see section 5.)  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 [RFC-1700 et seq.].
 Reserved             8-bit reserved field.  Initialized to zero for
                      transmission; ignored on reception.
 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

Deering & Hinden Standards Track [Page 19] RFC 1883 IPv6 Specification December 1995

      awaiting reassembly with other fragments of the same packet.
      However, it is not required that a source node know the maximum
      packet lifetime.  Rather, it is assumed that the requirement can
      be met by maintaining the Identification value as a simple, 32-
      bit, "wrap-around" counter, incremented each time a packet must
      be fragmented.  It is an implementation choice whether to
      maintain a single counter for the node or multiple counters,
      e.g., one for each of the node's possible source addresses, or
      one for each active (source address, destination address)
      combination.
 The initial, large, unfragmented packet is referred to as the
 "original packet", and it is considered to consist of two parts, as
 illustrated:
 original packet:
 +------------------+----------------------//-----------------------+
 |  Unfragmentable  |                 Fragmentable                  |
 |       Part       |                     Part                      |
 +------------------+----------------------//-----------------------+
    The Unfragmentable Part consists 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 Fragmentable Part consists of the rest of the packet, that is,
    any extension headers that need be processed only by the final
    destination node(s), plus the upper-layer header and data.
 The Fragmentable Part of the original packet is divided into
 fragments, each, except possibly the last ("rightmost") one, being an
 integer multiple of 8 octets long.  The fragments are transmitted in
 separate "fragment packets" as illustrated:
 original packet:
 +------------------+--------------+--------------+--//--+----------+
 |  Unfragmentable  |    first     |    second    |      |   last   |
 |       Part       |   fragment   |   fragment   | .... | fragment |
 +------------------+--------------+--------------+--//--+----------+

Deering & Hinden Standards Track [Page 20] RFC 1883 IPv6 Specification December 1995

 fragment packets:
 +------------------+--------+--------------+
 |  Unfragmentable  |Fragment|    first     |
 |       Part       | Header |   fragment   |
 +------------------+--------+--------------+
 +------------------+--------+--------------+
 |  Unfragmentable  |Fragment|    second    |
 |       Part       | Header |   fragment   |
 +------------------+--------+--------------+
                       o
                       o
                       o
 +------------------+--------+----------+
 |  Unfragmentable  |Fragment|   last   |
 |       Part       | Header | fragment |
 +------------------+--------+----------+
 Each fragment packet is composed of:
    (1) The Unfragmentable Part 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 Unfragmentable Part changed to 44.
    (2) A Fragment header containing:
             The Next Header value that identifies the first header of
             the Fragmentable Part 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 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.
 The lengths of the fragments must be chosen such that the resulting
 fragment packets fit within the MTU of the path to the packets'
 destination(s).

Deering & Hinden Standards Track [Page 21] RFC 1883 IPv6 Specification December 1995

 At the destination, fragment packets are reassembled into their
 original, unfragmented form, as illustrated:
 reassembled original packet:
 +------------------+----------------------//------------------------+
 |  Unfragmentable  |                 Fragmentable                   |
 |       Part       |                     Part                       |
 +------------------+----------------------//------------------------+
 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 Unfragmentable Part 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 Unfragmentable
       Part 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 Unfragmentable Part 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 relative
    position in Fragmentable Part is computed from its Fragment Offset
    value.

Deering & Hinden Standards Track [Page 22] RFC 1883 IPv6 Specification December 1995

    The Fragment header is not present in the final, reassembled
    packet.
 The following error conditions may arise when reassembling fragmented
 packets:
    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.
    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.
    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.
 The following conditions are not expected to occur, 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.

Deering & Hinden Standards Track [Page 23] RFC 1883 IPv6 Specification December 1995

4.6 Destination Options Header

 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 [RFC-1700 et seq.].
 Hdr Ext Len          8-bit unsigned integer.  Length of the
                      Destination Options header in 8-octet units,
                      not including the first 8 octets.
 Options              Variable-length field, of length such that the
                      complete Destination Options header is an
                      integer multiple of 8 octets long.  Contains
                      one or  more TLV-encoded options, as described
                      in section 4.2.
 The only destination options defined in this document are the Pad1
 and PadN options specified in section 4.2.
 Note that there are two possible ways to encode optional destination
 information in an IPv6 packet: either as an option in the Destination
 Options header, or as a separate extension header.  The Fragment
 header and the Authentication header are examples of the latter
 approach.  Which approach can be used depends on what action is
 desired of a destination node that does not understand the optional
 information:
    o  if the desired action is for the destination node to discard
       the packet and, only if the packet's Destination Address is not
       a multicast address, send an ICMP Unrecognized Type message to
       the packet's Source Address, then the information may be
       encoded either as a separate header or as an option in the

Deering & Hinden Standards Track [Page 24] RFC 1883 IPv6 Specification December 1995

       Destination Options header whose Option Type has the value 11
       in its highest-order two bits.  The choice may depend on such
       factors as which takes fewer octets, or which yields better
       alignment or more efficient parsing.
    o  if any other action is desired, the information must be encoded
       as an option in the Destination Options header whose Option
       Type has the value 00, 01, or 10 in its highest-order two bits,
       specifying the desired action (see section 4.2).

4.7 No Next Header

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

Deering & Hinden Standards Track [Page 25] RFC 1883 IPv6 Specification December 1995

5. Packet Size Issues

 IPv6 requires that every link in the internet have an MTU of 576
 octets or greater.  On any link that cannot convey a 576-octet packet
 in one piece, link-specific fragmentation and reassembly must be
 provided at a layer below IPv6.
  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.  Links that
 have a configurable MTU (for example, PPP links [RFC-1661]) must be
 configured to have an MTU of at least 576 octets; it is recommended
 that a larger MTU be configured, to accommodate possible
 encapsulations (i.e., tunneling) without incurring fragmentation.
 It is strongly recommended that IPv6 nodes implement Path MTU
 Discovery [RFC-1191], in order to discover and take advantage of
 paths with MTU greater than 576 octets.  However, a minimal IPv6
 implementation (e.g., in a boot ROM) may simply restrict itself to
 sending packets no larger than 576 octets, and omit implementation of
 Path MTU Discovery.
 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 576
 octets).
 A node must be able to accept a fragmented packet that, after
 reassembly, is as large as 1500 octets, including the IPv6 header.  A
 node is permitted to accept fragmented packets that reassemble to
 more than 1500 octets.  However, a node must not send fragments that
 reassemble to a size greater than 1500 octets unless it has explicit
 knowledge that the destination(s) can reassemble a packet of that
 size.
 In response to an IPv6 packet that is sent to an IPv4 destination
 (i.e., a packet that undergoes translation from IPv6 to IPv4), the
 originating IPv6 node may receive an ICMP Packet Too Big message
 reporting a Next-Hop MTU less than 576.  In that case, the IPv6 node
 is not required to reduce the size of subsequent packets to less than
 576, but must include a Fragment header in those packets so that the
 IPv6-to-IPv4 translating router can obtain a suitable Identification
 value to use in resulting IPv4 fragments.  Note that this means the
 payload may have to be reduced to 528 octets (576 minus 40 for the
 IPv6 header and 8 for the Fragment header), and smaller still if
 additional extension headers are used.

Deering & Hinden Standards Track [Page 26] RFC 1883 IPv6 Specification December 1995

      Note: Path MTU Discovery must be performed even in cases where a
      host "thinks" a destination is attached to the same link as
      itself.
      Note: Unlike IPv4, it is unnecessary in IPv6 to set a "Don't
      Fragment" flag in the packet header in order to perform Path MTU
      Discovery; that is an implicit attribute of every IPv6 packet.
      Also, those parts of the RFC-1191 procedures that involve use of
      a table of MTU "plateaus" do not apply to IPv6, because the IPv6
      version of the "Datagram Too Big" message always identifies the
      exact MTU to be used.

Deering & Hinden Standards Track [Page 27] RFC 1883 IPv6 Specification December 1995

6. Flow Labels

 The 24-bit Flow Label field in the IPv6 header may be used by a
 source to label those packets for which it requests special handling
 by the IPv6 routers, such as non-default quality of service or
 "real-time" service.  This aspect of IPv6 is, at the time of writing,
 still experimental and subject to change as the requirements for flow
 support in the Internet become clearer.  Hosts or routers that do not
 support the functions of the Flow Label field are required to set the
 field to zero when originating a packet, pass the field on unchanged
 when forwarding a packet, and ignore the field when receiving a
 packet.
 A flow is a sequence of packets sent from a particular source to a
 particular (unicast or multicast) destination for which the source
 desires special handling by the intervening routers.  The nature of
 that special handling might be conveyed to the routers by a control
 protocol, such as a resource reservation protocol, or by information
 within the flow's packets themselves, e.g., in a hop-by-hop option.
 The details of such control protocols or options are beyond the scope
 of this document.
 There may be multiple active flows from a source to a destination, as
 well as traffic that is not associated with any flow.  A flow is
 uniquely identified by the combination of a source address and a
 non-zero flow label.  Packets that do not belong to a flow carry a
 flow label of zero.
 A flow label is assigned to a flow by the flow's source node.  New
 flow labels must be chosen (pseudo-)randomly and uniformly from the
 range 1 to FFFFFF hex.  The purpose of the random allocation is to
 make any set of bits within the Flow Label field suitable for use as
 a hash key by routers, for looking up the state associated with the
 flow.
 All packets belonging to the same flow must be sent with the same
 source address, destination address, priority, and flow label.  If
 any of those packets includes a Hop-by-Hop Options header, then they
 all must be originated with the same Hop-by-Hop Options header
 contents (excluding the Next Header field of the Hop-by-Hop Options
 header).  If any of those packets includes a Routing header, then
 they all must be originated with the same contents in all extension
 headers up to and including the Routing header (excluding the Next
 Header field in the Routing header).  The routers or destinations are
 permitted, but not required, to verify that these conditions are
 satisfied.  If a violation is detected, it should be reported to the
 source by an ICMP Parameter Problem message, Code 0, pointing to the
 high-order octet of the Flow Label field (i.e., offset 1 within the
 IPv6 packet).

Deering & Hinden Standards Track [Page 28] RFC 1883 IPv6 Specification December 1995

 Routers are free to "opportunistically" set up flow-handling state
 for any flow, even when no explicit flow establishment information
 has been provided to them via a control protocol, a hop-by-hop
 option, or other means.  For example, upon receiving a packet from a
 particular source with an unknown, non-zero flow label, a router may
 process its IPv6 header and any necessary extension headers as if the
 flow label were zero.  That processing would include determining the
 next-hop interface, and possibly other actions, such as updating a
 hop-by-hop option, advancing the pointer and addresses in a Routing
 header, or deciding on how to queue the packet based on its Priority
 field.  The router may then choose to "remember" the results of those
 processing steps and cache that information, using the source address
 plus the flow label as the cache key.  Subsequent packets with the
 same source address and flow label may then be handled by referring
 to the cached information rather than examining all those fields
 that, according to the requirements of the previous paragraph, can be
 assumed unchanged from the first packet seen in the flow.
 Cached flow-handling state that is set up opportunistically, as
 discussed in the preceding paragraph, must be discarded no more than
 6 seconds after it is established, regardless of whether or not
 packets of the same flow continue to arrive.  If another packet with
 the same source address and flow label arrives after the cached state
 has been discarded, the packet undergoes full, normal processing (as
 if its flow label were zero), which may result in the re-creation of
 cached flow state for that flow.
 The lifetime of flow-handling state that is set up explicitly, for
 example by a control protocol or a hop-by-hop option, must be
 specified as part of the specification of the explicit set-up
 mechanism; it may exceed 6 seconds.
 A source must not re-use a flow label for a new flow within the
 lifetime of any flow-handling state that might have been established
 for the prior use of that flow label.  Since flow-handling state with
 a lifetime of 6 seconds may be established opportunistically for any
 flow, the minimum interval between the last packet of one flow and
 the first packet of a new flow using the same flow label is 6
 seconds.  Flow labels used for explicitly set-up flows with longer
 flow-state lifetimes must remain unused for those longer lifetimes
 before being re-used for new flows.
 When a node stops and restarts (e.g., as a result of a "crash"), it
 must be careful not to use a flow label that it might have used for
 an earlier flow whose lifetime may not have expired yet.  This may be
 accomplished by recording flow label usage on stable storage so that
 it can be remembered across crashes, or by refraining from using any
 flow labels until the maximum lifetime of any possible previously
 established flows has expired (at least 6 seconds; more if explicit

Deering & Hinden Standards Track [Page 29] RFC 1883 IPv6 Specification December 1995

 flow set-up mechanisms with longer lifetimes might have been used).
 If the minimum time for rebooting the node is known (often more than
 6 seconds), that time can be deducted from the necessary waiting
 period before starting to allocate flow labels.
 There is no requirement that all, or even most, packets belong to
 flows, i.e., carry non-zero flow labels.  This observation is placed
 here to remind protocol designers and implementors not to assume
 otherwise.  For example, it would be unwise to design a router whose
 performance would be adequate only if most packets belonged to flows,
 or to design a header compression scheme that only worked on packets
 that belonged to flows.

7. Priority

 The 4-bit Priority field in the IPv6 header enables a source to
 identify the desired delivery priority of its packets, relative to
 other packets from the same source.  The Priority values are divided
 into two ranges:  Values 0 through 7 are used to specify the priority
 of traffic for which the source is providing congestion control,
 i.e., traffic that "backs off" in response to congestion, such as TCP
 traffic.  Values 8 through 15 are used to specify the priority of
 traffic that does not back off in response to congestion, e.g.,
 "real-time" packets being sent at a constant rate.
 For congestion-controlled traffic, the following Priority values are
 recommended for particular application categories:
       0 - uncharacterized traffic
       1 - "filler" traffic (e.g., netnews)
       2 - unattended data transfer (e.g., email)
       3 - (reserved)
       4 - attended bulk transfer (e.g., FTP, NFS)
       5 - (reserved)
       6 - interactive traffic (e.g., telnet, X)
       7 - internet control traffic (e.g., routing protocols, SNMP)
 For non-congestion-controlled traffic, the lowest Priority value (8)
 should be used for those packets that the sender is most willing to
 have discarded under conditions of congestion (e.g., high-fidelity
 video traffic), and the highest value (15) should be used for those
 packets that the sender is least willing to have discarded (e.g.,
 low-fidelity audio traffic).  There is no relative ordering implied
 between the congestion-controlled priorities and the non-congestion-
 controlled priorities.

Deering & Hinden Standards Track [Page 30] RFC 1883 IPv6 Specification December 1995

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                      +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Payload Length                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      zero                     |  Next Header  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    o  If the 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.
    o  The Payload Length used in the pseudo-header is the length of
       the upper-layer packet, including the upper-layer header.  It
       will be less than the Payload Length in the IPv6 header (or in

Deering & Hinden Standards Track [Page 31] RFC 1883 IPv6 Specification December 1995

       the Jumbo Payload option) if there are extension headers
       between the IPv6 header and the upper-layer header.
    o  Unlike IPv4, when UDP packets are originated by an IPv6 node,
       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.
 The IPv6 version of ICMP [RFC-1885] 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.

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 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.

Deering & Hinden Standards Track [Page 32] RFC 1883 IPv6 Specification December 1995

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                         +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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

Deering & Hinden Standards Track [Page 33] RFC 1883 IPv6 Specification December 1995

 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       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Deering & Hinden Standards Track [Page 34] RFC 1883 IPv6 Specification December 1995

 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                         +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Deering & Hinden Standards Track [Page 35] RFC 1883 IPv6 Specification December 1995

Security Considerations

 This document specifies that the IP Authentication Header [RFC-1826]
 and the IP Encapsulating Security Payload [RFC-1827] be used with
 IPv6, in conformance with the Security Architecture for the Internet
 Protocol [RFC-1825].

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.

Authors' Addresses

 Stephen E. Deering                   Robert M. Hinden
 Xerox Palo Alto Research Center      Ipsilon Networks, Inc.
 3333 Coyote Hill Road                2191 E. Bayshore Road, Suite 100
 Palo Alto, CA 94304                  Palo Alto, CA 94303
 USA                                  USA
 Phone: +1 415 812 4839               Phone: +1 415 846 4604
 Fax:   +1 415 812 4471               Fax:   +1 415 855 1414
 EMail: deering@parc.xerox.com        EMail: hinden@ipsilon.com

Deering & Hinden Standards Track [Page 36] RFC 1883 IPv6 Specification December 1995

References

 [RFC-1825]   Atkinson, R., "Security Architecture for the Internet
              Protocol", RFC 1825, Naval Research Laboratory, August
              1995.
 [RFC-1826]   Atkinson, R., "IP Authentication Header", RFC 1826,
              Naval Research Laboratory, August 1995.
 [RFC-1827]   Atkinson, R., "IP Encapsulating Security Protocol
              (ESP)", RFC 1827, Naval Research Laboratory, August
              1995.
 [RFC-1885]   Conta, A., and S. Deering, "Internet Control Message
              Protocol (ICMPv6) for the Internet Protocol Version 6
              (IPv6) Specification", RFC 1885, Digital Equipment
              Corporation, Xerox PARC, December 1995.
 [RFC-1884]   Hinden, R., and S. Deering, Editors, "IP Version 6
              Addressing Architecture", RFC 1884, Ipsilon Networks,
              Xerox PARC, December 1995.
 [RFC-1191]   Mogul, J., and S. Deering, "Path MTU Discovery", RFC
              1191, DECWRL, Stanford University, November 1990.
 [RFC-791]    Postel, J., "Internet Protocol", STD 5, RFC 791,
              USC/Information Sciences Institute, September 1981.
 [RFC-1700]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
              RFC 1700, USC/Information Sciences Institute, October
              1994.
 [RFC-1661]   Simpson, W., Editor, "The Point-to-Point Protocol
              (PPP)", STD 51, RFC 1661, Daydreamer, July 1994.

Deering & Hinden Standards Track [Page 37]

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