path: root/doc/rfc/rfc3542.txt

Network Working Group                                         W. Stevens
Request for Comments: 3542                                     M. Thomas
Obsoletes: 2292                                               Consultant
Category: Informational                                      E. Nordmark
                                                                     Sun
                                                               T. Jinmei
                                                                 Toshiba
                                                                May 2003


     Advanced Sockets Application Program Interface (API) for IPv6

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document provides sockets Application Program Interface (API) to
   support "advanced" IPv6 applications, as a supplement to a separate
   specification, RFC 3493.  The expected applications include Ping,
   Traceroute, routing daemons and the like, which typically use raw
   sockets to access IPv6 or ICMPv6 header fields.  This document
   proposes some portable interfaces for applications that use raw
   sockets under IPv6.  There are other features of IPv6 that some
   applications will need to access: interface identification
   (specifying the outgoing interface and determining the incoming
   interface), IPv6 extension headers, and path Maximum Transmission
   Unit (MTU) information.  This document provides API access to these
   features too.  Additionally, some extended interfaces to libraries
   for the "r" commands are defined.  The extension will provide better
   backward compatibility to existing implementations that are not
   IPv6-capable.












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Table of Contents

   1.   Introduction ..............................................  3
   2.   Common Structures and Definitions .........................  5
        2.1  The ip6_hdr Structure ................................  6
             2.1.1  IPv6 Next Header Values .......................  6
             2.1.2  IPv6 Extension Headers ........................  7
             2.1.3  IPv6 Options ..................................  8
        2.2  The icmp6_hdr Structure .............................. 10
             2.2.1  ICMPv6 Type and Code Values ................... 10
             2.2.2  ICMPv6 Neighbor Discovery Definitions ......... 11
             2.2.3  Multicast Listener Discovery Definitions ...... 14
             2.2.4  ICMPv6 Router Renumbering Definitions ......... 14
        2.3  Address Testing Macros ............................... 16
        2.4  Protocols File ....................................... 16
   3.   IPv6 Raw Sockets .......................................... 17
        3.1  Checksums ............................................ 18
        3.2  ICMPv6 Type Filtering ................................ 19
        3.3  ICMPv6 Verification of Received Packets .............. 22
   4.   Access to IPv6 and Extension Headers ...................... 22
        4.1  TCP Implications ..................................... 24
        4.2  UDP and Raw Socket Implications ...................... 25
   5.   Extensions to Socket Ancillary Data ....................... 26
        5.1  CMSG_NXTHDR .......................................... 26
        5.2  CMSG_SPACE ........................................... 26
        5.3  CMSG_LEN ............................................. 27
   6.   Packet Information ........................................ 27
        6.1  Specifying/Receiving the Interface ................... 28
        6.2  Specifying/Receiving Source/Destination Address ...... 29
        6.3  Specifying/Receiving the Hop Limit ................... 29
        6.4  Specifying the Next Hop Address ...................... 30
        6.5  Specifying/Receiving the Traffic Class value ......... 31
        6.6  Additional Errors with sendmsg() and setsockopt() .... 32
        6.7  Summary of Outgoing Interface Selection .............. 32
   7.   Routing Header Option ..................................... 33
        7.1  inet6_rth_space ...................................... 35
        7.2  inet6_rth_init ....................................... 35
        7.3  inet6_rth_add ........................................ 36
        7.4  inet6_rth_reverse .................................... 36
        7.5  inet6_rth_segments ................................... 36
        7.6  inet6_rth_getaddr .................................... 36
   8.   Hop-By-Hop Options ........................................ 37
        8.1  Receiving Hop-by-Hop Options ......................... 38
        8.2  Sending Hop-by-Hop Options ........................... 38
   9.   Destination Options ....................................... 39
        9.1  Receiving Destination Options ........................ 39
        9.2  Sending Destination Options .......................... 39
   10.  Hop-by-Hop and Destination Options Processing ............. 40



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        10.1  inet6_opt_init ...................................... 41
        10.2  inet6_opt_append .................................... 41
        10.3  inet6_opt_finish .................................... 42
        10.4  inet6_opt_set_val ................................... 42
        10.5  inet6_opt_next ...................................... 42
        10.6  inet6_opt_find ...................................... 43
        10.7  inet6_opt_get_val ................................... 43
   11.  Additional Advanced API Functions ......................... 44
        11.1  Sending with the Minimum MTU ........................ 44
        11.2  Sending without Fragmentation ....................... 45
        11.3  Path MTU Discovery and UDP .......................... 46
        11.4  Determining the Current Path MTU .................... 47
   12.  Ordering of Ancillary Data and IPv6 Extension Headers ..... 48
   13.  IPv6-Specific Options with IPv4-Mapped IPv6 Addresses ..... 50
   14.  Extended interfaces for rresvport, rcmd and rexec ......... 51
        14.1  rresvport_af ........................................ 51
        14.2  rcmd_af ............................................. 51
        14.3  rexec_af ............................................ 52
   15.  Summary of New Definitions ................................ 52
   16.  Security Considerations ................................... 56
   17.  Changes from RFC 2292 ..................................... 57
   18.  References ................................................ 59
   19.  Acknowledgments ........................................... 59
   20.  Appendix A: Ancillary Data Overview ....................... 60
        20.1  The msghdr Structure ................................ 60
        20.2  The cmsghdr Structure ............................... 61
        20.3  Ancillary Data Object Macros ........................ 62
              20.3.1  CMSG_FIRSTHDR ............................... 63
              20.3.2  CMSG_NXTHDR ................................. 64
              20.3.3  CMSG_DATA ................................... 65
              20.3.4  CMSG_SPACE .................................. 65
              20.3.5  CMSG_LEN .................................... 65
   21.  Appendix B: Examples Using the inet6_rth_XXX() Functions .. 65
        21.1  Sending a Routing Header ............................ 65
        21.2  Receiving Routing Headers ........................... 70
   22.  Appendix C: Examples Using the inet6_opt_XXX() Functions .. 72
        22.1  Building Options .................................... 72
        22.2  Parsing Received Options ............................ 74
   23.  Authors' Addresses ........................................ 76
   24.  Full Copyright Statement .................................. 77

1.  Introduction

   A separate specification [RFC-3493] contains changes to the sockets
   API to support IP version 6.  Those changes are for TCP and UDP-based
   applications.  This document defines some of the "advanced" features
   of the sockets API that are required for applications to take
   advantage of additional features of IPv6.



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   Today, the portability of applications using IPv4 raw sockets is
   quite high, but this is mainly because most IPv4 implementations
   started from a common base (the Berkeley source code) or at least
   started with the Berkeley header files.  This allows programs such as
   Ping and Traceroute, for example, to compile with minimal effort on
   many hosts that support the sockets API.  With IPv6, however, there
   is no common source code base that implementors are starting from,
   and the possibility for divergence at this level between different
   implementations is high.  To avoid a complete lack of portability
   amongst applications that use raw IPv6 sockets, some standardization
   is necessary.

   There are also features from the basic IPv6 specification that are
   not addressed in [RFC-3493]: sending and receiving Routing headers,
   Hop-by-Hop options, and Destination options, specifying the outgoing
   interface, being told of the receiving interface, and control of path
   MTU information.

   This document updates and replaces RFC 2292.  This revision is based
   on implementation experience of RFC 2292, as well as some additional
   extensions that have been found to be useful through the IPv6
   deployment.  Note, however, that further work on this document may
   still be needed.  Once the API specification becomes mature and is
   deployed among implementations, it may be formally standardized by a
   more appropriate body, such as has been done with the Basic API
   [RFC-3493].

   This document can be divided into the following main sections.

   1. Definitions of the basic constants and structures required for
      applications to use raw IPv6 sockets.  This includes structure
      definitions for the IPv6 and ICMPv6 headers and all associated
      constants (e.g., values for the Next Header field).

   2. Some basic semantic definitions for IPv6 raw sockets.  For
      example, a raw ICMPv4 socket requires the application to calculate
      and store the ICMPv4 header checksum.  But with IPv6 this would
      require the application to choose the source IPv6 address because
      the source address is part of the pseudo header that ICMPv6 now
      uses for its checksum computation.  It should be defined that with
      a raw ICMPv6 socket the kernel always calculates and stores the
      ICMPv6 header checksum.

   3. Packet information: how applications can obtain the received
      interface, destination address, and received hop limit, along with
      specifying these values on a per-packet basis.  There are a class
      of applications that need this capability and the technique should
      be portable.



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   4. Access to the optional Routing header, Hop-by-Hop options, and
      Destination options extension headers.

   5. Additional features required for improved IPv6 application
      portability.

   The packet information along with access to the extension headers
   (Routing header, Hop-by-Hop options, and Destination options) are
   specified using the "ancillary data" fields that were added to the
   4.3BSD Reno sockets API in 1990.  The reason is that these ancillary
   data fields are part of the Posix standard [POSIX] and should
   therefore be adopted by most vendors.

   This document does not address application access to either the
   authentication header or the encapsulating security payload header.

   Many examples in this document omit error checking in favor of
   brevity and clarity.

   We note that some of the functions and socket options defined in this
   document may have error returns that are not defined in this
   document.  Some of these possible error returns will be recognized
   only as implementations proceed.

   Datatypes in this document follow the Posix format: intN_t means a
   signed integer of exactly N bits (e.g., int16_t) and uintN_t means an
   unsigned integer of exactly N bits (e.g., uint32_t).

   Note that we use the (unofficial) terminology ICMPv4, IGMPv4, and
   ARPv4 to avoid any confusion with the newer ICMPv6 protocol.

2.  Common Structures and Definitions

   Many advanced applications examine fields in the IPv6 header and set
   and examine fields in the various ICMPv6 headers.  Common structure
   definitions for these protocol headers are required, along with
   common constant definitions for the structure members.

   This API assumes that the fields in the protocol headers are left in
   the network byte order, which is big-endian for the Internet
   protocols.  If not, then either these constants or the fields being
   tested must be converted at run-time, using something like htons() or
   htonl().

   Two new header files are defined: <netinet/ip6.h> and
   <netinet/icmp6.h>.





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   When an include file is specified, that include file is allowed to
   include other files that do the actual declaration or definition.

2.1.  The ip6_hdr Structure

   The following structure is defined as a result of including
   <netinet/ip6.h>.  Note that this is a new header.

      struct ip6_hdr {
        union {
          struct ip6_hdrctl {
            uint32_t ip6_un1_flow; /* 4 bits version, 8 bits TC, 20 bits
                                      flow-ID */
            uint16_t ip6_un1_plen; /* payload length */
            uint8_t  ip6_un1_nxt;  /* next header */
            uint8_t  ip6_un1_hlim; /* hop limit */
          } ip6_un1;
          uint8_t ip6_un2_vfc;     /* 4 bits version, top 4 bits
                                      tclass */
        } ip6_ctlun;
        struct in6_addr ip6_src;   /* source address */
        struct in6_addr ip6_dst;   /* destination address */
      };

      #define ip6_vfc   ip6_ctlun.ip6_un2_vfc
      #define ip6_flow  ip6_ctlun.ip6_un1.ip6_un1_flow
      #define ip6_plen  ip6_ctlun.ip6_un1.ip6_un1_plen
      #define ip6_nxt   ip6_ctlun.ip6_un1.ip6_un1_nxt
      #define ip6_hlim  ip6_ctlun.ip6_un1.ip6_un1_hlim
      #define ip6_hops  ip6_ctlun.ip6_un1.ip6_un1_hlim

2.1.1.  IPv6 Next Header Values

   IPv6 defines many new values for the Next Header field.  The
   following constants are defined as a result of including
   <netinet/in.h>.

      #define IPPROTO_HOPOPTS   0   /* IPv6 Hop-by-Hop options */
      #define IPPROTO_IPV6     41   /* IPv6 header */
      #define IPPROTO_ROUTING  43   /* IPv6 Routing header */
      #define IPPROTO_FRAGMENT 44   /* IPv6 fragment header */
      #define IPPROTO_ESP      50   /* encapsulating security payload */
      #define IPPROTO_AH       51   /* authentication header */
      #define IPPROTO_ICMPV6   58   /* ICMPv6 */
      #define IPPROTO_NONE     59   /* IPv6 no next header */
      #define IPPROTO_DSTOPTS  60   /* IPv6 Destination options */





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   Berkeley-derived IPv4 implementations also define IPPROTO_IP to be 0.
   This should not be a problem since IPPROTO_IP is used only with IPv4
   sockets and IPPROTO_HOPOPTS only with IPv6 sockets.

2.1.2.  IPv6 Extension Headers

   Six extension headers are defined for IPv6.  We define structures for
   all except the Authentication header and Encapsulating Security
   Payload header, both of which are beyond the scope of this document.
   The following structures are defined as a result of including
   <netinet/ip6.h>.

      /* Hop-by-Hop options header */
      struct ip6_hbh {
        uint8_t  ip6h_nxt;        /* next header */
        uint8_t  ip6h_len;        /* length in units of 8 octets */
          /* followed by options */
      };

      /* Destination options header */
      struct ip6_dest {
        uint8_t  ip6d_nxt;        /* next header */
        uint8_t  ip6d_len;        /* length in units of 8 octets */
          /* followed by options */
      };

      /* Routing header */
      struct ip6_rthdr {
        uint8_t  ip6r_nxt;        /* next header */
        uint8_t  ip6r_len;        /* length in units of 8 octets */
        uint8_t  ip6r_type;       /* routing type */
        uint8_t  ip6r_segleft;    /* segments left */
          /* followed by routing type specific data */
      };

      /* Type 0 Routing header */
      struct ip6_rthdr0 {
        uint8_t  ip6r0_nxt;       /* next header */
        uint8_t  ip6r0_len;       /* length in units of 8 octets */
        uint8_t  ip6r0_type;      /* always zero */
        uint8_t  ip6r0_segleft;   /* segments left */
        uint32_t ip6r0_reserved;  /* reserved field */
          /* followed by up to 127 struct in6_addr */
      };







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      /* Fragment header */
      struct ip6_frag {
        uint8_t   ip6f_nxt;       /* next header */
        uint8_t   ip6f_reserved;  /* reserved field */
        uint16_t  ip6f_offlg;     /* offset, reserved, and flag */
        uint32_t  ip6f_ident;     /* identification */
      };

      #if     BYTE_ORDER == BIG_ENDIAN
      #define IP6F_OFF_MASK       0xfff8  /* mask out offset from
                                             ip6f_offlg */
      #define IP6F_RESERVED_MASK  0x0006  /* reserved bits in
                                             ip6f_offlg */
      #define IP6F_MORE_FRAG      0x0001  /* more-fragments flag */
      #else   /* BYTE_ORDER == LITTLE_ENDIAN */
      #define IP6F_OFF_MASK       0xf8ff  /* mask out offset from
                                             ip6f_offlg */
      #define IP6F_RESERVED_MASK  0x0600  /* reserved bits in
                                             ip6f_offlg */
      #define IP6F_MORE_FRAG      0x0100  /* more-fragments flag */
      #endif

2.1.3.  IPv6 Options

   Several options are defined for IPv6, and we define structures and
   macro definitions for some of them below.  The following structures
   are defined as a result of including <netinet/ip6.h>.

      /* IPv6 options */
      struct ip6_opt {
        uint8_t  ip6o_type;
        uint8_t  ip6o_len;
      };

      /*
       * The high-order 3 bits of the option type define the behavior
       * when processing an unknown option and whether or not the option
       * content changes in flight.
       */
      #define IP6OPT_TYPE(o)        ((o) & 0xc0)
      #define IP6OPT_TYPE_SKIP      0x00
      #define IP6OPT_TYPE_DISCARD   0x40
      #define IP6OPT_TYPE_FORCEICMP 0x80
      #define IP6OPT_TYPE_ICMP      0xc0
      #define IP6OPT_MUTABLE        0x20

      #define IP6OPT_PAD1           0x00  /* 00 0 00000 */
      #define IP6OPT_PADN           0x01  /* 00 0 00001 */



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      #define IP6OPT_JUMBO          0xc2  /* 11 0 00010 */
      #define IP6OPT_NSAP_ADDR      0xc3  /* 11 0 00011 */
      #define IP6OPT_TUNNEL_LIMIT   0x04  /* 00 0 00100 */
      #define IP6OPT_ROUTER_ALERT   0x05  /* 00 0 00101 */

      /* Jumbo Payload Option */
      struct ip6_opt_jumbo {
        uint8_t  ip6oj_type;
        uint8_t  ip6oj_len;
        uint8_t  ip6oj_jumbo_len[4];
      };
      #define IP6OPT_JUMBO_LEN   6

      /* NSAP Address Option */
      struct ip6_opt_nsap {
        uint8_t  ip6on_type;
        uint8_t  ip6on_len;
        uint8_t  ip6on_src_nsap_len;
        uint8_t  ip6on_dst_nsap_len;
          /* followed by source NSAP */
          /* followed by destination NSAP */
      };

      /* Tunnel Limit Option */
      struct ip6_opt_tunnel {
        uint8_t  ip6ot_type;
        uint8_t  ip6ot_len;
        uint8_t  ip6ot_encap_limit;
      };

      /* Router Alert Option */
      struct ip6_opt_router {
        uint8_t  ip6or_type;
        uint8_t  ip6or_len;
        uint8_t  ip6or_value[2];
      };

      /* Router alert values (in network byte order) */
      #ifdef _BIG_ENDIAN
      #define IP6_ALERT_MLD      0x0000
      #define IP6_ALERT_RSVP     0x0001
      #define  IP6_ALERT_AN      0x0002
      #else
      #define IP6_ALERT_MLD      0x0000
      #define IP6_ALERT_RSVP     0x0100
      #define IP6_ALERT_AN       0x0200
      #endif




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2.2.  The icmp6_hdr Structure

   The ICMPv6 header is needed by numerous IPv6 applications including
   Ping, Traceroute, router discovery daemons, and neighbor discovery
   daemons.  The following structure is defined as a result of including
   <netinet/icmp6.h>.  Note that this is a new header.

      struct icmp6_hdr {
        uint8_t     icmp6_type;   /* type field */
        uint8_t     icmp6_code;   /* code field */
        uint16_t    icmp6_cksum;  /* checksum field */
        union {
          uint32_t  icmp6_un_data32[1]; /* type-specific field */
          uint16_t  icmp6_un_data16[2]; /* type-specific field */
          uint8_t   icmp6_un_data8[4];  /* type-specific field */
        } icmp6_dataun;
      };

      #define icmp6_data32    icmp6_dataun.icmp6_un_data32
      #define icmp6_data16    icmp6_dataun.icmp6_un_data16
      #define icmp6_data8     icmp6_dataun.icmp6_un_data8
      #define icmp6_pptr      icmp6_data32[0]  /* parameter prob */
      #define icmp6_mtu       icmp6_data32[0]  /* packet too big */
      #define icmp6_id        icmp6_data16[0]  /* echo request/reply */
      #define icmp6_seq       icmp6_data16[1]  /* echo request/reply */
      #define icmp6_maxdelay  icmp6_data16[0]  /* mcast group
                                                  membership */

2.2.1.  ICMPv6 Type and Code Values

   In addition to a common structure for the ICMPv6 header, common
   definitions are required for the ICMPv6 type and code fields.  The
   following constants are also defined as a result of including
   <netinet/icmp6.h>.

      #define ICMP6_DST_UNREACH             1
      #define ICMP6_PACKET_TOO_BIG          2
      #define ICMP6_TIME_EXCEEDED           3
      #define ICMP6_PARAM_PROB              4

      #define ICMP6_INFOMSG_MASK  0x80    /* all informational
                                             messages */

      #define ICMP6_ECHO_REQUEST          128
      #define ICMP6_ECHO_REPLY            129

      #define ICMP6_DST_UNREACH_NOROUTE     0 /* no route to
                                                 destination */



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      #define ICMP6_DST_UNREACH_ADMIN       1 /* communication with
                                                 destination */
                                              /* admin. prohibited */
      #define ICMP6_DST_UNREACH_BEYONDSCOPE 2 /* beyond scope of source
                                                 address */
      #define ICMP6_DST_UNREACH_ADDR        3 /* address unreachable */
      #define ICMP6_DST_UNREACH_NOPORT      4 /* bad port */

      #define ICMP6_TIME_EXCEED_TRANSIT     0 /* Hop Limit == 0 in
                                                 transit */
      #define ICMP6_TIME_EXCEED_REASSEMBLY  1 /* Reassembly time out */

      #define ICMP6_PARAMPROB_HEADER        0 /* erroneous header
                                                 field */
      #define ICMP6_PARAMPROB_NEXTHEADER    1 /* unrecognized
                                                 Next Header */
      #define ICMP6_PARAMPROB_OPTION        2 /* unrecognized
                                                 IPv6 option */

   The five ICMP message types defined by IPv6 neighbor discovery (133-
   137) are defined in the next section.

2.2.2.  ICMPv6 Neighbor Discovery Definitions

   The following structures and definitions are defined as a result of
   including <netinet/icmp6.h>.

      #define ND_ROUTER_SOLICIT           133
      #define ND_ROUTER_ADVERT            134
      #define ND_NEIGHBOR_SOLICIT         135
      #define ND_NEIGHBOR_ADVERT          136
      #define ND_REDIRECT                 137

      struct nd_router_solicit {     /* router solicitation */
        struct icmp6_hdr  nd_rs_hdr;
          /* could be followed by options */
      };

      #define nd_rs_type               nd_rs_hdr.icmp6_type
      #define nd_rs_code               nd_rs_hdr.icmp6_code
      #define nd_rs_cksum              nd_rs_hdr.icmp6_cksum
      #define nd_rs_reserved           nd_rs_hdr.icmp6_data32[0]









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      struct nd_router_advert {      /* router advertisement */
        struct icmp6_hdr  nd_ra_hdr;
        uint32_t   nd_ra_reachable;   /* reachable time */
        uint32_t   nd_ra_retransmit;  /* retransmit timer */
          /* could be followed by options */
      };

      #define nd_ra_type               nd_ra_hdr.icmp6_type
      #define nd_ra_code               nd_ra_hdr.icmp6_code
      #define nd_ra_cksum              nd_ra_hdr.icmp6_cksum
      #define nd_ra_curhoplimit        nd_ra_hdr.icmp6_data8[0]
      #define nd_ra_flags_reserved     nd_ra_hdr.icmp6_data8[1]
      #define ND_RA_FLAG_MANAGED       0x80
      #define ND_RA_FLAG_OTHER         0x40
      #define nd_ra_router_lifetime    nd_ra_hdr.icmp6_data16[1]

      struct nd_neighbor_solicit {   /* neighbor solicitation */
        struct icmp6_hdr  nd_ns_hdr;
        struct in6_addr   nd_ns_target; /* target address */
          /* could be followed by options */
      };

      #define nd_ns_type               nd_ns_hdr.icmp6_type
      #define nd_ns_code               nd_ns_hdr.icmp6_code
      #define nd_ns_cksum              nd_ns_hdr.icmp6_cksum
      #define nd_ns_reserved           nd_ns_hdr.icmp6_data32[0]

      struct nd_neighbor_advert {    /* neighbor advertisement */
        struct icmp6_hdr  nd_na_hdr;
        struct in6_addr   nd_na_target; /* target address */
          /* could be followed by options */
      };

      #define nd_na_type               nd_na_hdr.icmp6_type
      #define nd_na_code               nd_na_hdr.icmp6_code
      #define nd_na_cksum              nd_na_hdr.icmp6_cksum
      #define nd_na_flags_reserved     nd_na_hdr.icmp6_data32[0]
      #if     BYTE_ORDER == BIG_ENDIAN
      #define ND_NA_FLAG_ROUTER        0x80000000
      #define ND_NA_FLAG_SOLICITED     0x40000000
      #define ND_NA_FLAG_OVERRIDE      0x20000000
      #else   /* BYTE_ORDER == LITTLE_ENDIAN */
      #define ND_NA_FLAG_ROUTER        0x00000080
      #define ND_NA_FLAG_SOLICITED     0x00000040
      #define ND_NA_FLAG_OVERRIDE      0x00000020
      #endif





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      struct nd_redirect {           /* redirect */
        struct icmp6_hdr  nd_rd_hdr;
        struct in6_addr   nd_rd_target; /* target address */
        struct in6_addr   nd_rd_dst;    /* destination address */
          /* could be followed by options */
      };

      #define nd_rd_type               nd_rd_hdr.icmp6_type
      #define nd_rd_code               nd_rd_hdr.icmp6_code
      #define nd_rd_cksum              nd_rd_hdr.icmp6_cksum
      #define nd_rd_reserved           nd_rd_hdr.icmp6_data32[0]

      struct nd_opt_hdr {         /* Neighbor discovery option header */
        uint8_t  nd_opt_type;
        uint8_t  nd_opt_len;      /* in units of 8 octets */
          /* followed by option specific data */
      };

      #define  ND_OPT_SOURCE_LINKADDR       1
      #define  ND_OPT_TARGET_LINKADDR       2
      #define  ND_OPT_PREFIX_INFORMATION    3
      #define  ND_OPT_REDIRECTED_HEADER     4
      #define  ND_OPT_MTU                   5

      struct nd_opt_prefix_info {    /* prefix information */
        uint8_t   nd_opt_pi_type;
        uint8_t   nd_opt_pi_len;
        uint8_t   nd_opt_pi_prefix_len;
        uint8_t   nd_opt_pi_flags_reserved;
        uint32_t  nd_opt_pi_valid_time;
        uint32_t  nd_opt_pi_preferred_time;
        uint32_t  nd_opt_pi_reserved2;
        struct in6_addr  nd_opt_pi_prefix;
      };

      #define ND_OPT_PI_FLAG_ONLINK        0x80
      #define ND_OPT_PI_FLAG_AUTO          0x40

      struct nd_opt_rd_hdr {         /* redirected header */
        uint8_t   nd_opt_rh_type;
        uint8_t   nd_opt_rh_len;
        uint16_t  nd_opt_rh_reserved1;
        uint32_t  nd_opt_rh_reserved2;
          /* followed by IP header and data */
      };






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      struct nd_opt_mtu {            /* MTU option */
        uint8_t   nd_opt_mtu_type;
        uint8_t   nd_opt_mtu_len;
        uint16_t  nd_opt_mtu_reserved;
        uint32_t  nd_opt_mtu_mtu;
      };

   We note that the nd_na_flags_reserved flags have the same byte
   ordering problems as we showed with ip6f_offlg.

2.2.3.  Multicast Listener Discovery Definitions

   The following structures and definitions are defined as a result of
   including <netinet/icmp6.h>.

      #define MLD_LISTENER_QUERY          130
      #define MLD_LISTENER_REPORT         131
      #define MLD_LISTENER_REDUCTION      132

      struct mld_hdr {
        struct icmp6_hdr  mld_icmp6_hdr;
        struct in6_addr   mld_addr; /* multicast address */
      };
      #define mld_type                 mld_icmp6_hdr.icmp6_type
      #define mld_code                 mld_icmp6_hdr.icmp6_code
      #define mld_cksum                mld_icmp6_hdr.icmp6_cksum
      #define mld_maxdelay             mld_icmp6_hdr.icmp6_data16[0]
      #define mld_reserved             mld_icmp6_hdr.icmp6_data16[1]

2.2.4.  ICMPv6 Router Renumbering Definitions

   The following structures and definitions are defined as a result of
   including <netinet/icmp6.h>.

      #define ICMP6_ROUTER_RENUMBERING    138   /* router renumbering */

      struct icmp6_router_renum {  /* router renumbering header */
        struct icmp6_hdr  rr_hdr;
        uint8_t           rr_segnum;
        uint8_t           rr_flags;
        uint16_t          rr_maxdelay;
        uint32_t          rr_reserved;
      };
      #define rr_type                  rr_hdr.icmp6_type
      #define rr_code                  rr_hdr.icmp6_code
      #define rr_cksum                 rr_hdr.icmp6_cksum
      #define rr_seqnum                rr_hdr.icmp6_data32[0]




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      /* Router renumbering flags */
      #define ICMP6_RR_FLAGS_TEST        0x80
      #define ICMP6_RR_FLAGS_REQRESULT   0x40
      #define ICMP6_RR_FLAGS_FORCEAPPLY  0x20
      #define ICMP6_RR_FLAGS_SPECSITE    0x10
      #define ICMP6_RR_FLAGS_PREVDONE    0x08


      struct rr_pco_match {    /* match prefix part */
        uint8_t          rpm_code;
        uint8_t          rpm_len;
        uint8_t          rpm_ordinal;
        uint8_t          rpm_matchlen;
        uint8_t          rpm_minlen;
        uint8_t          rpm_maxlen;
        uint16_t         rpm_reserved;
        struct in6_addr  rpm_prefix;
      };

      /* PCO code values */
      #define RPM_PCO_ADD              1
      #define RPM_PCO_CHANGE           2
      #define RPM_PCO_SETGLOBAL        3

      struct rr_pco_use {    /* use prefix part */
        uint8_t          rpu_uselen;
        uint8_t          rpu_keeplen;
        uint8_t          rpu_ramask;
        uint8_t          rpu_raflags;
        uint32_t         rpu_vltime;
        uint32_t         rpu_pltime;
        uint32_t         rpu_flags;
        struct in6_addr  rpu_prefix;
      };
      #define ICMP6_RR_PCOUSE_RAFLAGS_ONLINK   0x20
      #define ICMP6_RR_PCOUSE_RAFLAGS_AUTO     0x10

      #if BYTE_ORDER == BIG_ENDIAN
      #define ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME 0x80000000
      #define ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME 0x40000000
      #elif BYTE_ORDER == LITTLE_ENDIAN
      #define ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME 0x80
      #define ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME 0x40
      #endif







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      struct rr_result {    /* router renumbering result message */
        uint16_t         rrr_flags;
        uint8_t          rrr_ordinal;
        uint8_t          rrr_matchedlen;
        uint32_t         rrr_ifid;
        struct in6_addr  rrr_prefix;
      };

      #if BYTE_ORDER == BIG_ENDIAN
      #define ICMP6_RR_RESULT_FLAGS_OOB        0x0002
      #define ICMP6_RR_RESULT_FLAGS_FORBIDDEN  0x0001
      #elif BYTE_ORDER == LITTLE_ENDIAN
      #define ICMP6_RR_RESULT_FLAGS_OOB        0x0200
      #define ICMP6_RR_RESULT_FLAGS_FORBIDDEN  0x0100
      #endif

2.3.  Address Testing Macros

   The basic API ([RFC-3493]) defines some macros for testing an IPv6
   address for certain properties.  This API extends those definitions
   with additional address testing macros, defined as a result of
   including <netinet/in.h>.

      int  IN6_ARE_ADDR_EQUAL(const struct in6_addr *,
                              const struct in6_addr *);

   This macro returns non-zero if the addresses are equal; otherwise it
   returns zero.

2.4.  Protocols File

   Many hosts provide the file /etc/protocols that contains the names of
   the various IP protocols and their protocol number (e.g., the value
   of the protocol field in the IPv4 header for that protocol, such as 1
   for ICMP).  Some programs then call the function getprotobyname() to
   obtain the protocol value that is then specified as the third
   argument to the socket() function.  For example, the Ping program
   contains code of the form

      struct protoent  *proto;

      proto = getprotobyname("icmp");

      s = socket(AF_INET, SOCK_RAW, proto->p_proto);

   Common names are required for the new IPv6 protocols in this file, to
   provide portability of applications that call the getprotoXXX()
   functions.



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   We define the following protocol names with the values shown.  These
   are taken under http://www.iana.org/numbers.html.

      hopopt           0    # hop-by-hop options for ipv6
      ipv6            41    # ipv6
      ipv6-route      43    # routing header for ipv6
      ipv6-frag       44    # fragment header for ipv6
      esp             50    # encapsulating security payload for ipv6
      ah              51    # authentication header for ipv6
      ipv6-icmp       58    # icmp for ipv6
      ipv6-nonxt      59    # no next header for ipv6
      ipv6-opts       60    # destination options for ipv6

3.  IPv6 Raw Sockets

   Raw sockets bypass the transport layer (TCP or UDP).  With IPv4, raw
   sockets are used to access ICMPv4, IGMPv4, and to read and write IPv4
   datagrams containing a protocol field that the kernel does not
   process.  An example of the latter is a routing daemon for OSPF,
   since it uses IPv4 protocol field 89.  With IPv6 raw sockets will be
   used for ICMPv6 and to read and write IPv6 datagrams containing a
   Next Header field that the kernel does not process.  Examples of the
   latter are a routing daemon for OSPF for IPv6 and RSVP (protocol
   field 46).

   All data sent via raw sockets must be in network byte order and all
   data received via raw sockets will be in network byte order.  This
   differs from the IPv4 raw sockets, which did not specify a byte
   ordering and used the host's byte order for certain IP header fields.

   Another difference from IPv4 raw sockets is that complete packets
   (that is, IPv6 packets with extension headers) cannot be sent or
   received using the IPv6 raw sockets API.  Instead, ancillary data
   objects are used to transfer the extension headers and hoplimit
   information, as described in Section 6.  Should an application need
   access to the complete IPv6 packet, some other technique, such as the
   datalink interfaces BPF or DLPI, must be used.

   All fields except the flow label in the IPv6 header that an
   application might want to change (i.e., everything other than the
   version number) can be modified using ancillary data and/or socket
   options by the application for output.  All fields except the flow
   label in a received IPv6 header (other than the version number and
   Next Header fields) and all extension headers that an application
   might want to know are also made available to the application as
   ancillary data on input.  Hence there is no need for a socket option





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   similar to the IPv4 IP_HDRINCL socket option and on receipt the
   application will only receive the payload i.e., the data after the
   IPv6 header and all the extension headers.

   This API does not define access to the flow label field, because
   today there is no standard usage of the field.

   When writing to a raw socket the kernel will automatically fragment
   the packet if its size exceeds the path MTU, inserting the required
   fragment headers.  On input the kernel reassembles received
   fragments, so the reader of a raw socket never sees any fragment
   headers.

   When we say "an ICMPv6 raw socket" we mean a socket created by
   calling the socket function with the three arguments AF_INET6,
   SOCK_RAW, and IPPROTO_ICMPV6.

   Most IPv4 implementations give special treatment to a raw socket
   created with a third argument to socket() of IPPROTO_RAW, whose value
   is normally 255, to have it mean that the application will send down
   complete packets including the IPv4 header.  (Note: This feature was
   added to IPv4 in 1988 by Van Jacobson to support traceroute, allowing
   a complete IP header to be passed by the application, before the
   IP_HDRINCL socket option was added.)  We note that IPPROTO_RAW has no
   special meaning to an IPv6 raw socket (and the IANA currently
   reserves the value of 255 when used as a next-header field).

3.1.  Checksums

   The kernel will calculate and insert the ICMPv6 checksum for ICMPv6
   raw sockets, since this checksum is mandatory.

   For other raw IPv6 sockets (that is, for raw IPv6 sockets created
   with a third argument other than IPPROTO_ICMPV6), the application
   must set the new IPV6_CHECKSUM socket option to have the kernel (1)
   compute and store a checksum for output, and (2) verify the received
   checksum on input, discarding the packet if the checksum is in error.
   This option prevents applications from having to perform source
   address selection on the packets they send.  The checksum will
   incorporate the IPv6 pseudo-header, defined in Section 8.1 of [RFC-
   2460].  This new socket option also specifies an integer offset into
   the user data of where the checksum is located.

      int  offset = 2;
      setsockopt(fd, IPPROTO_IPV6, IPV6_CHECKSUM, &offset,
                 sizeof(offset));





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   By default, this socket option is disabled.  Setting the offset to -1
   also disables the option.  By disabled we mean (1) the kernel will
   not calculate and store a checksum for outgoing packets, and (2) the
   kernel will not verify a checksum for received packets.

   This option assumes the use of the 16-bit one's complement of the
   one's complement sum as the checksum algorithm and that the checksum
   field is aligned on a 16-bit boundary.  Thus, specifying a positive
   odd value as offset is invalid, and setsockopt() will fail for such
   offset values.

   An attempt to set IPV6_CHECKSUM for an ICMPv6 socket will fail.
   Also, an attempt to set or get IPV6_CHECKSUM for a non-raw IPv6
   socket will fail.

   (Note: Since the checksum is always calculated by the kernel for an
   ICMPv6 socket, applications are not able to generate ICMPv6 packets
   with incorrect checksums (presumably for testing purposes) using this
   API.)

3.2.  ICMPv6 Type Filtering

   ICMPv4 raw sockets receive most ICMPv4 messages received by the
   kernel.  (We say "most" and not "all" because Berkeley-derived
   kernels never pass echo requests, timestamp requests, or address mask
   requests to a raw socket.  Instead these three messages are processed
   entirely by the kernel.)  But ICMPv6 is a superset of ICMPv4, also
   including the functionality of IGMPv4 and ARPv4.  This means that an
   ICMPv6 raw socket can potentially receive many more messages than
   would be received with an ICMPv4 raw socket: ICMP messages similar to
   ICMPv4, along with neighbor solicitations, neighbor advertisements,
   and the three multicast listener discovery messages.

   Most applications using an ICMPv6 raw socket care about only a small
   subset of the ICMPv6 message types.  To transfer extraneous ICMPv6
   messages from the kernel to user can incur a significant overhead.
   Therefore this API includes a method of filtering ICMPv6 messages by
   the ICMPv6 type field.

   Each ICMPv6 raw socket has an associated filter whose datatype is
   defined as

      struct icmp6_filter;

   This structure, along with the macros and constants defined later in
   this section, are defined as a result of including the
   <netinet/icmp6.h>.




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   The current filter is fetched and stored using getsockopt() and
   setsockopt() with a level of IPPROTO_ICMPV6 and an option name of
   ICMP6_FILTER.

   Six macros operate on an icmp6_filter structure:

      void ICMP6_FILTER_SETPASSALL (struct icmp6_filter *);
      void ICMP6_FILTER_SETBLOCKALL(struct icmp6_filter *);

      void ICMP6_FILTER_SETPASS ( int, struct icmp6_filter *);
      void ICMP6_FILTER_SETBLOCK( int, struct icmp6_filter *);

      int  ICMP6_FILTER_WILLPASS (int,
                                  const struct icmp6_filter *);
      int  ICMP6_FILTER_WILLBLOCK(int,
                                  const struct icmp6_filter *);

   The first argument to the last four macros (an integer) is an ICMPv6
   message type, between 0 and 255.  The pointer argument to all six
   macros is a pointer to a filter that is modified by the first four
   macros and is examined by the last two macros.

   The first two macros, SETPASSALL and SETBLOCKALL, let us specify that
   all ICMPv6 messages are passed to the application or that all ICMPv6
   messages are blocked from being passed to the application.

   The next two macros, SETPASS and SETBLOCK, let us specify that
   messages of a given ICMPv6 type should be passed to the application
   or not passed to the application (blocked).

   The final two macros, WILLPASS and WILLBLOCK, return true or false
   depending whether the specified message type is passed to the
   application or blocked from being passed to the application by the
   filter pointed to by the second argument.

   When an ICMPv6 raw socket is created, it will by default pass all
   ICMPv6 message types to the application.

   As an example, a program that wants to receive only router
   advertisements could execute the following:

      struct icmp6_filter  myfilt;

      fd = socket(AF_INET6, SOCK_RAW, IPPROTO_ICMPV6);

      ICMP6_FILTER_SETBLOCKALL(&myfilt);
      ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &myfilt);
      setsockopt(fd, IPPROTO_ICMPV6, ICMP6_FILTER, &myfilt,



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                 sizeof(myfilt));

   The filter structure is declared and then initialized to block all
   messages types.  The filter structure is then changed to allow router
   advertisement messages to be passed to the application and the filter
   is installed using setsockopt().

   In order to clear an installed filter the application can issue a
   setsockopt for ICMP6_FILTER with a zero length.  When no such filter
   has been installed, getsockopt() will return the kernel default
   filter.

   The icmp6_filter structure is similar to the fd_set datatype used
   with the select() function in the sockets API.  The icmp6_filter
   structure is an opaque datatype and the application should not care
   how it is implemented.  All the application does with this datatype
   is allocate a variable of this type, pass a pointer to a variable of
   this type to getsockopt() and setsockopt(), and operate on a variable
   of this type using the six macros that we just defined.

   Nevertheless, it is worth showing a simple implementation of this
   datatype and the six macros.

      struct icmp6_filter {
        uint32_t  icmp6_filt[8];  /* 8*32 = 256 bits */
      };

      #define ICMP6_FILTER_WILLPASS(type, filterp) \
        ((((filterp)->icmp6_filt[(type) >> 5]) & \
          (1 << ((type) & 31))) != 0)
      #define ICMP6_FILTER_WILLBLOCK(type, filterp) \
        ((((filterp)->icmp6_filt[(type) >> 5]) & \
          (1 << ((type) & 31))) == 0)
      #define ICMP6_FILTER_SETPASS(type, filterp) \
        ((((filterp)->icmp6_filt[(type) >> 5]) |= \
          (1 << ((type) & 31))))
      #define ICMP6_FILTER_SETBLOCK(type, filterp) \
        ((((filterp)->icmp6_filt[(type) >> 5]) &= \
          ~(1 << ((type) & 31))))
      #define ICMP6_FILTER_SETPASSALL(filterp) \
        memset((filterp), 0xFF, sizeof(struct icmp6_filter))
      #define ICMP6_FILTER_SETBLOCKALL(filterp) \
        memset((filterp), 0, sizeof(struct icmp6_filter))

   (Note: These sample definitions have two limitations that an
   implementation may want to change.  The first four macros evaluate
   their first argument two times.  The second two macros require the
   inclusion of the <string.h> header for the memset() function.)



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3.3.  ICMPv6 Verification of Received Packets

   The protocol stack will verify the ICMPv6 checksum and discard any
   packets with invalid checksums.

   An implementation might perform additional validity checks on the
   ICMPv6 message content and discard malformed packets.  However, a
   portable application must not assume that such validity checks have
   been performed.

   The protocol stack should not automatically discard packets if the
   ICMP type is unknown to the stack.  For extensibility reasons
   received ICMP packets with any type (informational or error) must be
   passed to the applications (subject to ICMP6_FILTER filtering on the
   type value and the checksum verification).

4.  Access to IPv6 and Extension Headers

   Applications need to be able to control IPv6 header and extension
   header content when sending as well as being able to receive the
   content of these headers.  This is done by defining socket option
   types which can be used both with setsockopt and with ancillary data.
   Ancillary data is discussed in Appendix A.  The following optional
   information can be exchanged between the application and the kernel:

   1. The send/receive interface and source/destination address,
   2. The hop limit,
   3. Next hop address,
   4. The traffic class,
   5. Routing header,
   6. Hop-by-Hop options header, and
   7. Destination options header.

   First, to receive any of this optional information (other than the
   next hop address, which can only be set) on a UDP or raw socket, the
   application must call setsockopt() to turn on the corresponding flag:

      int  on = 1;

      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO,  &on, sizeof(on));
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR,    &on, sizeof(on));
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS,  &on, sizeof(on));
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS,  &on, sizeof(on));
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVTCLASS,   &on, sizeof(on));






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   When any of these options are enabled, the corresponding data is
   returned as control information by recvmsg(), as one or more
   ancillary data objects.

   This document does not define how to receive the optional information
   on a TCP socket.  See Section 4.1 for more details.

   Two different mechanisms exist for sending this optional information:

   1. Using setsockopt to specify the option content for a socket.
      These are known "sticky" options since they affect all transmitted
      packets on the socket until either a new setsockopt is done or the
      options are overridden using ancillary data.

   2. Using ancillary data to specify the option content for a single
      datagram.  This only applies to datagram and raw sockets; not to
      TCP sockets.

   The three socket option parameters and the three cmsghdr fields that
   describe the options/ancillary data objects are summarized as:

      opt level/    optname/          optval/
      cmsg_level    cmsg_type         cmsg_data[]
      ------------  ------------      ------------------------
      IPPROTO_IPV6  IPV6_PKTINFO      in6_pktinfo structure
      IPPROTO_IPV6  IPV6_HOPLIMIT     int
      IPPROTO_IPV6  IPV6_NEXTHOP      socket address structure
      IPPROTO_IPV6  IPV6_RTHDR        ip6_rthdr structure
      IPPROTO_IPV6  IPV6_HOPOPTS      ip6_hbh structure
      IPPROTO_IPV6  IPV6_DSTOPTS      ip6_dest structure
      IPPROTO_IPV6  IPV6_RTHDRDSTOPTS ip6_dest structure
      IPPROTO_IPV6  IPV6_TCLASS       int

      (Note: IPV6_HOPLIMIT can be used as ancillary data items only)

   All these options are described in detail in Section 6, 7, 8 and 9.
   All the constants beginning with IPV6_ are defined as a result of
   including <netinet/in.h>.

   Note: We intentionally use the same constant for the cmsg_level
   member as is used as the second argument to getsockopt() and
   setsockopt() (what is called the "level"), and the same constant for
   the cmsg_type member as is used as the third argument to getsockopt()
   and setsockopt() (what is called the "option name").







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   Issuing getsockopt() for the above options will return the sticky
   option value i.e., the value set with setsockopt().  If no sticky
   option value has been set getsockopt() will return the following
   values:

   -  For the IPV6_PKTINFO option, it will return an in6_pktinfo
      structure with ipi6_addr being in6addr_any and ipi6_ifindex being
      zero.

   -  For the IPV6_TCLASS option, it will return the kernel default
      value.

   -  For other options, it will indicate the lack of the option value
      with optlen being zero.

   The application does not explicitly need to access the data
   structures for the Routing header, Hop-by-Hop options header, and
   Destination options header, since the API to these features is
   through a set of inet6_rth_XXX() and inet6_opt_XXX() functions that
   we define in Section 7 and Section 10.  Those functions simplify the
   interface to these features instead of requiring the application to
   know the intimate details of the extension header formats.

   When specifying extension headers, this API assumes the header
   ordering and the number of occurrences of each header as described in
   [RFC-2460].  More details about the ordering issue will be discussed
   in Section 12.

4.1.  TCP Implications

   It is not possible to use ancillary data to transmit the above
   options for TCP since there is not a one-to-one mapping between send
   operations and the TCP segments being transmitted.  Instead an
   application can use setsockopt to specify them as sticky options.
   When the application uses setsockopt to specify the above options it
   is expected that TCP will start using the new information when
   sending segments.  However, TCP may or may not use the new
   information when retransmitting segments that were originally sent
   when the old sticky options were in effect.

   It is unclear how a TCP application can use received information
   (such as extension headers) due to the lack of mapping between
   received TCP segments and receive operations.  In particular, the
   received information could not be used for access control purposes
   like on UDP and raw sockets.






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   This specification therefore does not define how to get the received
   information on TCP sockets.  The result of the IPV6_RECVxxx options
   on a TCP socket is undefined as well.

4.2.  UDP and Raw Socket Implications

   The receive behavior for UDP and raw sockets is quite
   straightforward.  After the application has enabled an IPV6_RECVxxx
   socket option it will receive ancillary data items for every
   recvmsg() call containing the requested information.  However, if the
   information is not present in the packet the ancillary data item will
   not be included.  For example, if the application enables
   IPV6_RECVRTHDR and a received datagram does not contain a Routing
   header there will not be an IPV6_RTHDR ancillary data item.  Note
   that due to buffering in the socket implementation there might be
   some packets queued when an IPV6_RECVxxx option is enabled and they
   might not have the ancillary data information.

   For sending the application has the choice between using sticky
   options and ancillary data.  The application can also use both having
   the sticky options specify the "default" and using ancillary data to
   override the default options.

   When an ancillary data item is specified in a call to sendmsg(), the
   item will override an existing sticky option of the same name (if
   previously specified).  For example, if the application has set
   IPV6_RTHDR using a sticky option and later passes IPV6_RTHDR as
   ancillary data this will override the IPV6_RTHDR sticky option and
   the routing header of the outgoing packet will be from the ancillary
   data item, not from the sticky option.  Note, however, that other
   sticky options than IPV6_RTHDR will not be affected by the IPV6_RTHDR
   ancillary data item; the overriding mechanism only works for the same
   type of sticky options and ancillary data items.

   (Note: the overriding rule is different from the one in RFC 2292.  In
   RFC 2292, an ancillary data item overrode all sticky options
   previously defined.  This was reasonable, because sticky options
   could only be specified as a set by a single socket option.  However,
   in this API, each option is separated so that it can be specified as
   a single sticky option.  Additionally, there are much more ancillary
   data items and sticky options than in RFC 2292, including ancillary-
   only one.  Thus, it should be natural for application programmers to
   separate the overriding rule as well.)

   An application can also temporarily disable a particular sticky
   option by specifying a corresponding ancillary data item that could
   disable the sticky option when being used as an argument for a socket
   option.  For example, if the application has set IPV6_HOPOPTS as a



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   sticky option and later passes IPV6_HOPOPTS with a zero length as an
   ancillary data item, the packet will not have a Hop-by-Hop options
   header.

5.  Extensions to Socket Ancillary Data

   This specification uses ancillary data as defined in Posix with some
   compatible extensions, which are described in the following
   subsections.  Section 20 will provide a detailed overview of
   ancillary data and related structures and macros, including the
   extensions.

5.1.  CMSG_NXTHDR

      struct cmsghdr *CMSG_NXTHDR(const struct msghdr *mhdr,
                                  const struct cmsghdr *cmsg);

   CMSG_NXTHDR() returns a pointer to the cmsghdr structure describing
   the next ancillary data object.  Mhdr is a pointer to a msghdr
   structure and cmsg is a pointer to a cmsghdr structure.  If there is
   not another ancillary data object, the return value is NULL.

   The following behavior of this macro is new to this API: if the value
   of the cmsg pointer is NULL, a pointer to the cmsghdr structure
   describing the first ancillary data object is returned.  That is,
   CMSG_NXTHDR(mhdr, NULL) is equivalent to CMSG_FIRSTHDR(mhdr).  If
   there are no ancillary data objects, the return value is NULL.

5.2.  CMSG_SPACE

   socklen_t CMSG_SPACE(socklen_t length);

   This macro is new with this API.  Given the length of an ancillary
   data object, CMSG_SPACE() returns an upper bound on the space
   required by the object and its cmsghdr structure, including any
   padding needed to satisfy alignment requirements.  This macro can be
   used, for example, when allocating space dynamically for the
   ancillary data.  This macro should not be used to initialize the
   cmsg_len member of a cmsghdr structure; instead use the CMSG_LEN()
   macro.











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5.3.  CMSG_LEN

   socklen_t CMSG_LEN(socklen_t length);

   This macro is new with this API.  Given the length of an ancillary
   data object, CMSG_LEN() returns the value to store in the cmsg_len
   member of the cmsghdr structure, taking into account any padding
   needed to satisfy alignment requirements.

   Note the difference between CMSG_SPACE() and CMSG_LEN(), shown also
   in the figure in Section 20.2: the former accounts for any required
   padding at the end of the ancillary data object and the latter is the
   actual length to store in the cmsg_len member of the ancillary data
   object.

6.  Packet Information

   There are five pieces of information that an application can specify
   for an outgoing packet using ancillary data:

      1.  the source IPv6 address,
      2.  the outgoing interface index,
      3.  the outgoing hop limit,
      4.  the next hop address, and
      5.  the outgoing traffic class value.

   Four similar pieces of information can be returned for a received
   packet as ancillary data:

      1.  the destination IPv6 address,
      2.  the arriving interface index,
      3.  the arriving hop limit, and
      4.  the arriving traffic class value.

   The first two pieces of information are contained in an in6_pktinfo
   structure that is set with setsockopt() or sent as ancillary data
   with sendmsg() and received as ancillary data with recvmsg().  This
   structure is defined as a result of including <netinet/in.h>.

      struct in6_pktinfo {
        struct in6_addr ipi6_addr;    /* src/dst IPv6 address */
        unsigned int    ipi6_ifindex; /* send/recv interface index */
      };

   In the socket option and cmsghdr level will be IPPROTO_IPV6, the type
   will be IPV6_PKTINFO, and the first byte of the option value and
   cmsg_data[] will be the first byte of the in6_pktinfo structure.  An
   application can clear any sticky IPV6_PKTINFO option by doing a



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   "regular" setsockopt with ipi6_addr being in6addr_any and
   ipi6_ifindex being zero.

   This information is returned as ancillary data by recvmsg() only if
   the application has enabled the IPV6_RECVPKTINFO socket option:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO, &on, sizeof(on));

   (Note: The hop limit is not contained in the in6_pktinfo structure
   for the following reason.  Some UDP servers want to respond to client
   requests by sending their reply out the same interface on which the
   request was received and with the source IPv6 address of the reply
   equal to the destination IPv6 address of the request.  To do this the
   application can enable just the IPV6_RECVPKTINFO socket option and
   then use the received control information from recvmsg() as the
   outgoing control information for sendmsg().  The application need not
   examine or modify the in6_pktinfo structure at all.  But if the hop
   limit were contained in this structure, the application would have to
   parse the received control information and change the hop limit
   member, since the received hop limit is not the desired value for an
   outgoing packet.)

6.1.  Specifying/Receiving the Interface

   Interfaces on an IPv6 node are identified by a small positive
   integer, as described in Section 4 of [RFC-3493].  That document also
   describes a function to map an interface name to its interface index,
   a function to map an interface index to its interface name, and a
   function to return all the interface names and indexes.  Notice from
   this document that no interface is ever assigned an index of 0.

   When specifying the outgoing interface, if the ipi6_ifindex value is
   0, the kernel will choose the outgoing interface.

   The ordering among various options that can specify the outgoing
   interface, including IPV6_PKTINFO, is defined in Section 6.7.

   When the IPV6_RECVPKTINFO socket option is enabled, the received
   interface index is always returned as the ipi6_ifindex member of the
   in6_pktinfo structure.










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6.2.  Specifying/Receiving Source/Destination Address

   The source IPv6 address can be specified by calling bind() before
   each output operation, but supplying the source address together with
   the data requires less overhead (i.e., fewer system calls) and
   requires less state to be stored and protected in a multithreaded
   application.

   When specifying the source IPv6 address as ancillary data, if the
   ipi6_addr member of the in6_pktinfo structure is the unspecified
   address (IN6ADDR_ANY_INIT or in6addr_any), then (a) if an address is
   currently bound to the socket, it is used as the source address, or
   (b) if no address is currently bound to the socket, the kernel will
   choose the source address.  If the ipi6_addr member is not the
   unspecified address, but the socket has already bound a source
   address, then the ipi6_addr value overrides the already-bound source
   address for this output operation only.

   The kernel must verify that the requested source address is indeed a
   unicast address assigned to the node.  When the address is a scoped
   one, there may be ambiguity about its scope zone.  This is
   particularly the case for link-local addresses.  In such a case, the
   kernel must first determine the appropriate scope zone based on the
   zone of the destination address or the outgoing interface (if known),
   then qualify the address.  This also means that it is not feasible to
   specify the source address for a non-binding socket by the
   IPV6_PKTINFO sticky option, unless the outgoing interface is also
   specified.  The application should simply use bind() for such
   purposes.

   IPV6_PKTINFO can also be used as a sticky option for specifying the
   socket's default source address.  However, the ipi6_addr member must
   be the unspecified address for TCP sockets, because it is not
   possible to dynamically change the source address of a TCP
   connection.  When the IPV6_PKTINFO option is specified for a TCP
   socket with a non-unspecified address, the call will fail.  This
   restriction should be applied even before the socket binds a specific
   address.

   When the in6_pktinfo structure is returned as ancillary data by
   recvmsg(), the ipi6_addr member contains the destination IPv6 address
   from the received packet.

6.3.  Specifying/Receiving the Hop Limit

   The outgoing hop limit is normally specified with either the
   IPV6_UNICAST_HOPS socket option or the IPV6_MULTICAST_HOPS socket
   option, both of which are described in [RFC-3493].  Specifying the



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   hop limit as ancillary data lets the application override either the
   kernel's default or a previously specified value, for either a
   unicast destination or a multicast destination, for a single output
   operation.  Returning the received hop limit is useful for IPv6
   applications that need to verify that the received hop limit is 255
   (e.g., that the packet has not been forwarded).

   The received hop limit is returned as ancillary data by recvmsg()
   only if the application has enabled the IPV6_RECVHOPLIMIT socket
   option:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));

   In the cmsghdr structure containing this ancillary data, the
   cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
   IPV6_HOPLIMIT, and the first byte of cmsg_data[] will be the first
   byte of the integer hop limit.

   Nothing special need be done to specify the outgoing hop limit: just
   specify the control information as ancillary data for sendmsg().  As
   specified in [RFC-3493], the interpretation of the integer hop limit
   value is

      x < -1:        return an error of EINVAL
      x == -1:       use kernel default
      0 <= x <= 255: use x
      x >= 256:      return an error of EINVAL

   This API defines IPV6_HOPLIMIT as an ancillary-only option, that is,
   the option name cannot be used as a socket option.  This is because
   [RFC-3493] has more fine-grained socket options; IPV6_UNICAST_HOPS
   and IPV6_MULTICAST_HOPS.

6.4.  Specifying the Next Hop Address

   The IPV6_NEXTHOP ancillary data object specifies the next hop for the
   datagram as a socket address structure.  In the cmsghdr structure
   containing this ancillary data, the cmsg_level member will be
   IPPROTO_IPV6, the cmsg_type member will be IPV6_NEXTHOP, and the
   first byte of cmsg_data[] will be the first byte of the socket
   address structure.

   This is a privileged option.  (Note: It is implementation defined and
   beyond the scope of this document to define what "privileged" means.
   Unix systems use this term to mean the process must have an effective
   user ID of 0.)




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   This API only defines the case where the socket address contains an
   IPv6 address (i.e., the sa_family member is AF_INET6).  And, in this
   case, the node identified by that address must be a neighbor of the
   sending host.  If that address equals the destination IPv6 address of
   the datagram, then this is equivalent to the existing SO_DONTROUTE
   socket option.

   This option does not have any meaning for multicast destinations.  In
   such a case, the specified next hop will be ignored.

   When the outgoing interface is specified by IPV6_PKTINFO as well, the
   next hop specified by this option must be reachable via the specified
   interface.

   In order to clear a sticky IPV6_NEXTHOP option the application must
   issue a setsockopt for IPV6_NEXTHOP with a zero length.

6.5.  Specifying/Receiving the Traffic Class value

   The outgoing traffic class is normally set to 0.  Specifying the
   traffic class as ancillary data lets the application override either
   the kernel's default or a previously specified value, for either a
   unicast destination or a multicast destination, for a single output
   operation.  Returning the received traffic class is useful for
   programs such as a diffserv debugging tool and for user level ECN
   (explicit congestion notification) implementation.

   The received traffic class is returned as ancillary data by recvmsg()
   only if the application has enabled the IPV6_RECVTCLASS socket
   option:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVTCLASS, &on, sizeof(on));

   In the cmsghdr structure containing this ancillary data, the
   cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
   IPV6_TCLASS, and the first byte of cmsg_data[] will be the first byte
   of the integer traffic class.

   To specify the outgoing traffic class value, just specify the control
   information as ancillary data for sendmsg() or using setsockopt().
   Just like the hop limit value, the interpretation of the integer
   traffic class value is

      x < -1:        return an error of EINVAL
      x == -1:       use kernel default
      0 <= x <= 255: use x
      x >= 256:      return an error of EINVAL



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   In order to clear a sticky IPV6_TCLASS option the application can
   specify -1 as the value.

   There are cases where the kernel needs to control the traffic class
   value and conflicts with the user-specified value on the outgoing
   traffic.  An example is an implementation of ECN in the kernel,
   setting 2 bits of the traffic class value.  In such cases, the kernel
   should override the user-specified value.  On the incoming traffic,
   the kernel may mask some of the bits in the traffic class field.

6.6.  Additional Errors with sendmsg() and setsockopt()

   With the IPV6_PKTINFO socket option there are no additional errors
   possible with the call to recvmsg().  But when specifying the
   outgoing interface or the source address, additional errors are
   possible from sendmsg() or setsockopt().  Note that some
   implementations might only be able to return this type of errors for
   setsockopt().  The following are examples, but some of these may not
   be provided by some implementations, and some implementations may
   define additional errors:

   ENXIO         The interface specified by ipi6_ifindex does not exist.

   ENETDOWN      The interface specified by ipi6_ifindex is not enabled
                 for IPv6 use.

   EADDRNOTAVAIL ipi6_ifindex specifies an interface but the address
                 ipi6_addr is not available for use on that interface.

   EHOSTUNREACH  No route to the destination exists over the interface
                 specified by ipi6_ifindex.

6.7.  Summary of Outgoing Interface Selection

   This document and [RFC-3493] specify various methods that affect the
   selection of the packet's outgoing interface.  This subsection
   summarizes the ordering among those in order to ensure deterministic
   behavior.

   For a given outgoing packet on a given socket, the outgoing interface
   is determined in the following order:

   1. if an interface is specified in an IPV6_PKTINFO ancillary data
      item, the interface is used.

   2. otherwise, if an interface is specified in an IPV6_PKTINFO sticky
      option, the interface is used.




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   3. otherwise, if the destination address is a multicast address and
      the IPV6_MULTICAST_IF socket option is specified for the socket,
      the interface is used.

   4. otherwise, if an IPV6_NEXTHOP ancillary data item is specified,
      the interface to the next hop is used.

   5. otherwise, if an IPV6_NEXTHOP sticky option is specified, the
      interface to the next hop is used.

   6. otherwise, the outgoing interface should be determined in an
      implementation dependent manner.

   The ordering above particularly means if the application specifies an
   interface by the IPV6_MULTICAST_IF socket option (described in [RFC-
   3493]) as well as specifying a different interface by the
   IPV6_PKTINFO sticky option, the latter will override the former for
   every multicast packet on the corresponding socket.  The reason for
   the ordering comes from expectation that the source address is
   specified as well and that the pair of the address and the outgoing
   interface should be preferred.

   In any case, the kernel must also verify that the source and
   destination addresses do not break their scope zones with regard to
   the outgoing interface.

7.  Routing Header Option

   Source routing in IPv6 is accomplished by specifying a Routing header
   as an extension header.  There can be different types of Routing
   headers, but IPv6 currently defines only the Type 0 Routing header
   [RFC-2460].  This type supports up to 127 intermediate nodes (limited
   by the length field in the extension header).  With this maximum
   number of intermediate nodes, a source, and a destination, there are
   128 hops.

   Source routing with the IPv4 sockets API (the IP_OPTIONS socket
   option) requires the application to build the source route in the
   format that appears as the IPv4 header option, requiring intimate
   knowledge of the IPv4 options format.  This IPv6 API, however,
   defines six functions that the application calls to build and examine
   a Routing header, and the ability to use sticky options or ancillary
   data to communicate this information between the application and the
   kernel using the IPV6_RTHDR option.







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   Three functions build a Routing header:

      inet6_rth_space()    - return #bytes required for Routing header
      inet6_rth_init()     - initialize buffer data for Routing header
      inet6_rth_add()      - add one IPv6 address to the Routing header

   Three functions deal with a returned Routing header:

      inet6_rth_reverse()  - reverse a Routing header
      inet6_rth_segments() - return #segments in a Routing header
      inet6_rth_getaddr()  - fetch one address from a Routing header

   The function prototypes for these functions are defined as a result
   of including <netinet/in.h>.

   To receive a Routing header the application must enable the
   IPV6_RECVRTHDR socket option:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR, &on, sizeof(on));

   Each received Routing header is returned as one ancillary data object
   described by a cmsghdr structure with cmsg_type set to IPV6_RTHDR.
   When multiple Routing headers are received, multiple ancillary data
   objects (with cmsg_type set to IPV6_RTHDR) will be returned to the
   application.

   To send a Routing header the application specifies it either as
   ancillary data in a call to sendmsg() or using setsockopt().  For the
   sending side, this API assumes the number of occurrences of the
   Routing header as described in [RFC-2460].  That is, applications can
   only specify at most one outgoing Routing header.

   The application can remove any sticky Routing header by calling
   setsockopt() for IPV6_RTHDR with a zero option length.

   When using ancillary data a Routing header is passed between the
   application and the kernel as follows: The cmsg_level member has a
   value of IPPROTO_IPV6 and the cmsg_type member has a value of
   IPV6_RTHDR.  The contents of the cmsg_data[] member is implementation
   dependent and should not be accessed directly by the application, but
   should be accessed using the six functions that we are about to
   describe.

   The following constant is defined as a result of including the
   <netinet/in.h>:

      #define IPV6_RTHDR_TYPE_0    0 /* IPv6 Routing header type 0 */



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   When a Routing header is specified, the destination address specified
   for connect(), sendto(), or sendmsg() is the final destination
   address of the datagram.  The Routing header then contains the
   addresses of all the intermediate nodes.

7.1.  inet6_rth_space

      socklen_t inet6_rth_space(int type, int segments);

   This function returns the number of bytes required to hold a Routing
   header of the specified type containing the specified number of
   segments (addresses).  For an IPv6 Type 0 Routing header, the number
   of segments must be between 0 and 127, inclusive.  The return value
   is just the space for the Routing header.  When the application uses
   ancillary data it must pass the returned length to CMSG_SPACE() to
   determine how much memory is needed for the ancillary data object
   (including the cmsghdr structure).

   If the return value is 0, then either the type of the Routing header
   is not supported by this implementation or the number of segments is
   invalid for this type of Routing header.

   (Note: This function returns the size but does not allocate the space
   required for the ancillary data.  This allows an application to
   allocate a larger buffer, if other ancillary data objects are
   desired, since all the ancillary data objects must be specified to
   sendmsg() as a single msg_control buffer.)

7.2.  inet6_rth_init

      void *inet6_rth_init(void *bp, socklen_t bp_len, int type,
                           int segments);

   This function initializes the buffer pointed to by bp to contain a
   Routing header of the specified type and sets ip6r_len based on the
   segments parameter.  bp_len is only used to verify that the buffer is
   large enough.  The ip6r_segleft field is set to zero; inet6_rth_add()
   will increment it.

   When the application uses ancillary data the application must
   initialize any cmsghdr fields.

   The caller must allocate the buffer and its size can be determined by
   calling inet6_rth_space().

   Upon success the return value is the pointer to the buffer (bp), and
   this is then used as the first argument to the inet6_rth_add()
   function.  Upon an error the return value is NULL.



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7.3.  inet6_rth_add

      int inet6_rth_add(void *bp, const struct in6_addr *addr);

   This function adds the IPv6 address pointed to by addr to the end of
   the Routing header being constructed.

   If successful, the segleft member of the Routing Header is updated to
   account for the new address in the Routing header and the return
   value of the function is 0.  Upon an error the return value of the
   function is -1.

7.4.  inet6_rth_reverse

      int inet6_rth_reverse(const void *in, void *out);

   This function takes a Routing header extension header (pointed to by
   the first argument) and writes a new Routing header that sends
   datagrams along the reverse of that route.  The function reverses the
   order of the addresses and sets the segleft member in the new Routing
   header to the number of segments.  Both arguments are allowed to
   point to the same buffer (that is, the reversal can occur in place).

   The return value of the function is 0 on success, or -1 upon an
   error.

7.5.  inet6_rth_segments

      int inet6_rth_segments(const void *bp);

   This function returns the number of segments (addresses) contained in
   the Routing header described by bp.  On success the return value is
   zero or greater.  The return value of the function is -1 upon an
   error.

7.6.  inet6_rth_getaddr

      struct in6_addr *inet6_rth_getaddr(const void *bp, int index);

   This function returns a pointer to the IPv6 address specified by
   index (which must have a value between 0 and one less than the value
   returned by inet6_rth_segments()) in the Routing header described by
   bp.  An application should first call inet6_rth_segments() to obtain
   the number of segments in the Routing header.

   Upon an error the return value of the function is NULL.





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8.  Hop-By-Hop Options

   A variable number of Hop-by-Hop options can appear in a single Hop-
   by-Hop options header.  Each option in the header is TLV-encoded with
   a type, length, and value.  This IPv6 API defines seven functions
   that the application calls to build and examine a Hop-by_Hop options
   header, and the ability to use sticky options or ancillary data to
   communicate this information between the application and the kernel.
   This uses the IPV6_HOPOPTS for a Hop-by-Hop options header.

   Today several Hop-by-Hop options are defined for IPv6.  Two pad
   options, Pad1 and PadN, are for alignment purposes and are
   automatically inserted by the inet6_opt_XXX() routines and ignored by
   the inet6_opt_XXX() routines on the receive side.  This section of
   the API is therefore defined for other (and future) Hop-by-Hop
   options that an application may need to specify and receive.

   Four functions build an options header:

      inet6_opt_init()     - initialize buffer data for options header
      inet6_opt_append()   - add one TLV option to the options header
      inet6_opt_finish()   - finish adding TLV options to the options
                             header
      inet6_opt_set_val()  - add one component of the option content to
                             the option

      Three functions deal with a returned options header:

      inet6_opt_next()     - extract the next option from the options
                             header
      inet6_opt_find()     - extract an option of a specified type from
                             the header
      inet6_opt_get_val()  - retrieve one component of the option
                             content

   Individual Hop-by-Hop options (and Destination options, which are
   described in Section 9 and are very similar to the Hop-by-Hop
   options) may have specific alignment requirements.  For example, the
   4-byte Jumbo Payload length should appear on a 4-byte boundary, and
   IPv6 addresses are normally aligned on an 8-byte boundary.  These
   requirements and the terminology used with these options are
   discussed in Section 4.2 and Appendix B of [RFC-2460].  The alignment
   of first byte of each option is specified by two values, called x and
   y, written as "xn + y".  This states that the option must appear at
   an integer multiple of x bytes from the beginning of the options
   header (x can have the values 1, 2, 4, or 8), plus y bytes (y can
   have a value between 0 and 7, inclusive).  The Pad1 and PadN options
   are inserted as needed to maintain the required alignment.  The



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   functions below need to know the alignment of the end of the option
   (which is always in the form "xn," where x can have the values 1, 2,
   4, or 8) and the total size of the data portion of the option.  These
   are passed as the "align" and "len" arguments to inet6_opt_append().

   Multiple Hop-by-Hop options must be specified by the application by
   placing them in a single extension header.

   Finally, we note that use of some Hop-by-Hop options or some
   Destination options, might require special privilege.  That is,
   normal applications (without special privilege) might be forbidden
   from setting certain options in outgoing packets, and might never see
   certain options in received packets.

8.1.  Receiving Hop-by-Hop Options

   To receive a Hop-by-Hop options header the application must enable
   the IPV6_RECVHOPOPTS socket option:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS, &on, sizeof(on));

   When using ancillary data a Hop-by-hop options header is passed
   between the application and the kernel as follows: The cmsg_level
   member will be IPPROTO_IPV6 and the cmsg_type member will be
   IPV6_HOPOPTS.  These options are then processed by calling the
   inet6_opt_next(), inet6_opt_find(), and inet6_opt_get_val()
   functions, described in Section 10.

8.2.  Sending Hop-by-Hop Options

   To send a Hop-by-Hop options header, the application specifies the
   header either as ancillary data in a call to sendmsg() or using
   setsockopt().

   The application can remove any sticky Hop-by-Hop options header by
   calling setsockopt() for IPV6_HOPOPTS with a zero option length.

   All the Hop-by-Hop options must be specified by a single ancillary
   data object.  The cmsg_level member is set to IPPROTO_IPV6 and the
   cmsg_type member is set to IPV6_HOPOPTS.  The option is normally
   constructed using the inet6_opt_init(), inet6_opt_append(),
   inet6_opt_finish(), and inet6_opt_set_val() functions, described in
   Section 10.

   Additional errors may be possible from sendmsg() and setsockopt() if
   the specified option is in error.




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9.  Destination Options

   A variable number of Destination options can appear in one or more
   Destination options headers.  As defined in [RFC-2460], a Destination
   options header appearing before a Routing header is processed by the
   first destination plus any subsequent destinations specified in the
   Routing header, while a Destination options header that is not
   followed by a Routing header is processed only by the final
   destination.  As with the Hop-by-Hop options, each option in a
   Destination options header is TLV-encoded with a type, length, and
   value.

9.1.  Receiving Destination Options

   To receive Destination options header the application must enable the
   IPV6_RECVDSTOPTS socket option:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS, &on, sizeof(on));

   Each Destination options header is returned as one ancillary data
   object described by a cmsghdr structure with cmsg_level set to
   IPPROTO_IPV6 and cmsg_type set to IPV6_DSTOPTS.

   These options are then processed by calling the inet6_opt_next(),
   inet6_opt_find(), and inet6_opt_get_value() functions.

9.2.  Sending Destination Options

   To send a Destination options header, the application specifies it
   either as ancillary data in a call to sendmsg() or using
   setsockopt().

   The application can remove any sticky Destination options header by
   calling setsockopt() for IPV6_RTHDRDSTOPTS/IPV6_DSTOPTS with a zero
   option length.

   This API assumes the ordering about extension headers as described in
   [RFC-2460].  Thus, one set of Destination options can only appear
   before a Routing header, and one set can only appear after a Routing
   header (or in a packet with no Routing header).  Each set can consist
   of one or more options but each set is a single extension header.

   Today all destination options that an application may want to specify
   can be put after (or without) a Routing header.  Thus, applications
   should usually need IPV6_DSTOPTS only and should avoid using
   IPV6_RTHDRDSTOPTS whenever possible.




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   When using ancillary data a Destination options header is passed
   between the application and the kernel as follows: The set preceding
   a Routing header are specified with the cmsg_level member set to
   IPPROTO_IPV6 and the cmsg_type member set to IPV6_RTHDRDSTOPTS.  Any
   setsockopt or ancillary data for IPV6_RTHDRDSTOPTS is silently
   ignored when sending packets unless a Routing header is also
   specified.  Note that the "Routing header" here means the one
   specified by this API.  Even when the kernel inserts a routing header
   in its internal routine (e.g., in a mobile IPv6 stack), the
   Destination options header specified by IPV6_RTHDRDSTOPTS will still
   be ignored unless the application explicitly specifies its own
   Routing header.

   The set of Destination options after a Routing header, which are also
   used when no Routing header is present, are specified with the
   cmsg_level member is set to IPPROTO_IPV6 and the cmsg_type member is
   set to IPV6_DSTOPTS.

   The Destination options are normally constructed using the
   inet6_opt_init(), inet6_opt_append(), inet6_opt_finish(), and
   inet6_opt_set_val() functions, described in Section 10.

   Additional errors may be possible from sendmsg() and setsockopt() if
   the specified option is in error.

10.  Hop-by-Hop and Destination Options Processing

   Building and parsing the Hop-by-Hop and Destination options is
   complicated for the reasons given earlier.  We therefore define a set
   of functions to help the application.  These functions assume the
   formatting rules specified in Appendix B in [RFC-2460] i.e., that the
   largest field is placed last in the option.

   The function prototypes for these functions are defined as a result
   of including <netinet/in.h>.

   The first 3 functions (init, append, and finish) are used both to
   calculate the needed buffer size for the options, and to actually
   encode the options once the application has allocated a buffer for
   the header.  In order to only calculate the size the application must
   pass a NULL extbuf and a zero extlen to those functions.










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10.1.  inet6_opt_init

      int inet6_opt_init(void *extbuf, socklen_t extlen);

   This function returns the number of bytes needed for the empty
   extension header i.e., without any options.  If extbuf is not NULL it
   also initializes the extension header to have the correct length
   field.  In that case if the extlen value is not a positive (i.e.,
   non-zero) multiple of 8 the function fails and returns -1.

   (Note: since the return value on success is based on a "constant"
   parameter, i.e., the empty extension header, an implementation may
   return a constant value.  However, this specification does not
   require the value be constant, and leaves it as implementation
   dependent.  The application should not assume a particular constant
   value as a successful return value of this function.)

10.2.  inet6_opt_append

      int inet6_opt_append(void *extbuf, socklen_t extlen, int offset,
                           uint8_t type, socklen_t len, uint_t align,
                           void **databufp);

   Offset should be the length returned by inet6_opt_init() or a
   previous inet6_opt_append().  This function returns the updated total
   length taking into account adding an option with length 'len' and
   alignment 'align'.  If extbuf is not NULL then, in addition to
   returning the length, the function inserts any needed pad option,
   initializes the option (setting the type and length fields) and
   returns a pointer to the location for the option content in databufp.
   If the option does not fit in the extension header buffer the
   function returns -1.

   Type is the 8-bit option type.  Len is the length of the option data
   (i.e., excluding the option type and option length fields).

   Once inet6_opt_append() has been called the application can use the
   databuf directly, or use inet6_opt_set_val() to specify the content
   of the option.

   The option type must have a value from 2 to 255, inclusive.  (0 and 1
   are reserved for the Pad1 and PadN options, respectively.)

   The option data length must have a value between 0 and 255,
   inclusive, and is the length of the option data that follows.

   The align parameter must have a value of 1, 2, 4, or 8.  The align
   value can not exceed the value of len.



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10.3.  inet6_opt_finish

      int inet6_opt_finish(void *extbuf, socklen_t extlen, int offset);

   Offset should be the length returned by inet6_opt_init() or
   inet6_opt_append().  This function returns the updated total length
   taking into account the final padding of the extension header to make
   it a multiple of 8 bytes.  If extbuf is not NULL the function also
   initializes the option by inserting a Pad1 or PadN option of the
   proper length.

   If the necessary pad does not fit in the extension header buffer the
   function returns -1.

10.4.  inet6_opt_set_val

      int inet6_opt_set_val(void *databuf, int offset, void *val,
                            socklen_t vallen);

   Databuf should be a pointer returned by inet6_opt_append().  This
   function inserts data items of various sizes in the data portion of
   the option.  Val should point to the data to be inserted.  Offset
   specifies where in the data portion of the option the value should be
   inserted; the first byte after the option type and length is accessed
   by specifying an offset of zero.

   The caller should ensure that each field is aligned on its natural
   boundaries as described in Appendix B of [RFC-2460], but the function
   must not rely on the caller's behavior.  Even when the alignment
   requirement is not satisfied, inet6_opt_set_val should just copy the
   data as required.

   The function returns the offset for the next field (i.e., offset +
   vallen) which can be used when composing option content with multiple
   fields.

10.5.  inet6_opt_next

      int inet6_opt_next(void *extbuf, socklen_t extlen, int offset,
                         uint8_t *typep, socklen_t *lenp,
                         void **databufp);

   This function parses received option extension headers returning the
   next option.  Extbuf and extlen specifies the extension header.
   Offset should either be zero (for the first option) or the length
   returned by a previous call to inet6_opt_next() or inet6_opt_find().
   It specifies the position where to continue scanning the extension
   buffer.  The next option is returned by updating typep, lenp, and



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   databufp.  Typep stores the option type, lenp stores the length of
   the option data (i.e., excluding the option type and option length
   fields), and databufp points the data field of the option.  This
   function returns the updated "previous" length computed by advancing
   past the option that was returned.  This returned "previous" length
   can then be passed to subsequent calls to inet6_opt_next().  This
   function does not return any PAD1 or PADN options.  When there are no
   more options or if the option extension header is malformed the
   return value is -1.

10.6.  inet6_opt_find

      int inet6_opt_find(void *extbuf, socklen_t extlen, int offset,
                         uint8_t type, socklen_t *lenp,
                         void **databufp);

   This function is similar to the previously described inet6_opt_next()
   function, except this function lets the caller specify the option
   type to be searched for, instead of always returning the next option
   in the extension header.

   If an option of the specified type is located, the function returns
   the updated "previous" total length computed by advancing past the
   option that was returned and past any options that didn't match the
   type.  This returned "previous" length can then be passed to
   subsequent calls to inet6_opt_find() for finding the next occurrence
   of the same option type.

   If an option of the specified type is not located, the return value
   is -1.  If the option extension header is malformed, the return value
   is -1.

10.7.  inet6_opt_get_val

      int inet6_opt_get_val(void *databuf, int offset, void *val,
                            socklen_t vallen);

   Databuf should be a pointer returned by inet6_opt_next() or
   inet6_opt_find().  This function extracts data items of various sizes
   in the data portion of the option.  Val should point to the
   destination for the extracted data.  Offset specifies from where in
   the data portion of the option the value should be extracted; the
   first byte after the option type and length is accessed by specifying
   an offset of zero.

   It is expected that each field is aligned on its natural boundaries
   as described in Appendix B of [RFC-2460], but the function must not
   rely on the alignment.



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   The function returns the offset for the next field (i.e., offset +
   vallen) which can be used when extracting option content with
   multiple fields.

11.  Additional Advanced API Functions

11.1.  Sending with the Minimum MTU

   Unicast applications should usually let the kernel perform path MTU
   discovery [RFC-1981], as long as the kernel supports it, and should
   not care about the path MTU.  Some applications, however, might not
   want to incur the overhead of path MTU discovery, especially if the
   applications only send a single datagram to a destination.  A
   potential example is a DNS server.

   [RFC-1981] describes how path MTU discovery works for multicast
   destinations.  From practice in using IPv4 multicast, however, many
   careless applications that send large multicast packets on the wire
   have caused implosion of ICMPv4 error messages.  The situation can be
   worse when there is a filtering node that blocks the ICMPv4 messages.
   Though the filtering issue applies to unicast as well, the impact is
   much larger in the multicast cases.

   Thus, applications sending multicast traffic should explicitly enable
   path MTU discovery only when they understand that the benefit of
   possibly larger MTU usage outweighs the possible impact of MTU
   discovery for active sources across the delivery tree(s).  This
   default behavior is based on the today's practice with IPv4 multicast
   and path MTU discovery.  The behavior may change in the future once
   it is found that path MTU discovery effectively works with actual
   multicast applications and network configurations.

   This specification defines a mechanism to avoid path MTU discovery by
   sending at the minimum IPv6 MTU [RFC-2460].  If the packet is larger
   than the minimum MTU and this feature has been enabled the IP layer
   will fragment to the minimum MTU.  To control the policy about path
   MTU discovery, applications can use the IPV6_USE_MIN_MTU socket
   option.

   As described above, the default policy should depend on whether the
   destination is unicast or multicast.  For unicast destinations path
   MTU discovery should be performed by default.  For multicast
   destinations path MTU discovery should be disabled by default.  This
   option thus takes the following three types of integer arguments:

   -1: perform path MTU discovery for unicast destinations but do not
       perform it for multicast destinations.  Packets to multicast
       destinations are therefore sent with the minimum MTU.



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   0: always perform path MTU discovery.

   1: always disable path MTU discovery and send packets at the minimum
       MTU.

   The default value of this option is -1.  Values other than -1, 0, and
   1 are invalid, and an error EINVAL will be returned for those values.

   As an example, if a unicast application intentionally wants to
   disable path MTU discovery, it will add the following lines:

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_USE_MIN_MTU, &on, sizeof(on));

   Note that this API intentionally excludes the case where the
   application wants to perform path MTU discovery for multicast but to
   disable it for unicast.  This is because such usage is not feasible
   considering a scale of performance issues around whether to do path
   MTU discovery or not.  When path MTU discovery makes sense to a
   destination but not to a different destination, regardless of whether
   the destination is unicast or multicast, applications either need to
   toggle the option between sending such packets on the same socket, or
   use different sockets for the two classes of destinations.

   This option can also be sent as ancillary data.  In the cmsghdr
   structure containing this ancillary data, the cmsg_level member will
   be IPPROTO_IPV6, the cmsg_type member will be IPV6_USE_MIN_MTU, and
   the first byte of cmsg_data[] will be the first byte of the integer.

11.2.  Sending without Fragmentation

   In order to provide for easy porting of existing UDP and raw socket
   applications IPv6 implementations will, when originating packets,
   automatically insert a fragment header in the packet if the packet is
   too big for the path MTU.

   Some applications might not want this behavior.  An example is
   traceroute which might want to discover the actual path MTU.

   This specification defines a mechanism to turn off the automatic
   inserting of a fragment header for UDP and raw sockets.  This can be
   enabled using the IPV6_DONTFRAG socket option.

      int on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_DONTFRAG, &on, sizeof(on));






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   By default, this socket option is disabled.  Setting the value to 0
   also disables the option i.e., reverts to the default behavior of
   automatic inserting.  This option can also be sent as ancillary data.
   In the cmsghdr structure containing this ancillary data, the
   cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
   IPV6_DONTFRAG, and the first byte of cmsg_data[] will be the first
   byte of the integer.  This API only specifies the use of this option
   for UDP and raw sockets, and does not define the usage for TCP
   sockets.

   When the data size is larger than the MTU of the outgoing interface,
   the packet will be discarded.  Applications can know the result by
   enabling the IPV6_RECVPATHMTU option described below and receiving
   the corresponding ancillary data items.  An additional error EMSGSIZE
   may also be returned in some implementations.  Note, however, that
   some other implementations might not be able to return this
   additional error when sending a message.

11.3.  Path MTU Discovery and UDP

   UDP and raw socket applications need to be able to  determine the
   "maximum send transport-message size" (Section 5.1 of [RFC-1981]) to
   a given destination so that those applications can participate in
   path MTU discovery.  This lets those applications send smaller
   datagrams to the destination, avoiding fragmentation.

   This is accomplished using a new ancillary data item (IPV6_PATHMTU)
   which is delivered to recvmsg() without any actual data.  The
   application can enable the receipt of IPV6_PATHMTU ancillary data
   items by setting the IPV6_RECVPATHMTU socket option.

      int  on = 1;
      setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPATHMTU, &on, sizeof(on));

   By default, this socket option is disabled.  Setting the value to 0
   also disables the option.  This API only specifies the use of this
   option for UDP and raw sockets, and does not define the usage for TCP
   sockets.

   When the application is sending packets too big for the path MTU
   recvmsg() will return zero (indicating no data) but there will be a
   cmsghdr with cmsg_type set to IPV6_PATHMTU, and cmsg_len will
   indicate that cmsg_data is sizeof(struct ip6_mtuinfo) bytes long.
   This can happen when the sending node receives a corresponding ICMPv6
   packet too big error, or when the packet is sent from a socket with
   the IPV6_DONTFRAG option being on and the packet size is larger than
   the MTU of the outgoing interface.  This indication is considered as
   an ancillary data item for a separate (empty) message.  Thus, when



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   there are buffered messages (i.e., messages that the application has
   not received yet) on the socket the application will first receive
   the buffered messages and then receive the indication.

   The first byte of cmsg_data[] will point to a struct ip6_mtuinfo
   carrying the path MTU to use together with the IPv6 destination
   address.

      struct ip6_mtuinfo {
        struct sockaddr_in6 ip6m_addr; /* dst address including
                                          zone ID */
        uint32_t            ip6m_mtu;  /* path MTU in host byte order */
      };

   This cmsghdr will be passed to every socket that sets the
   IPV6_RECVPATHMTU socket option, even if the socket is non-connected.
   Note that this also means an application that sets the option may
   receive an IPV6_MTU ancillary data item for each ICMP too big error
   the node receives, including such ICMP errors caused by other
   applications on the node.  Thus, an application that wants to perform
   the path MTU discovery by itself needs to keep history of
   destinations that it has actually sent to and to compare the address
   returned in the ip6_mtuinfo structure to the history.  An
   implementation may choose not to delivery data to a connected socket
   that has a foreign address that is different than the address
   specified in the ip6m_addr structure.

   When an application sends a packet with a routing header, the final
   destination stored in the ip6m_addr member does not necessarily
   contain complete information of the entire path.

11.4.  Determining the Current Path MTU

   Some applications might need to determine the current path MTU e.g.,
   applications using IPV6_RECVPATHMTU might want to pick a good
   starting value.

   This specification defines a get-only socket option to retrieve the
   current path MTU value for the destination of a given connected
   socket.  If the IP layer does not have a cached path MTU value it
   will return the interface MTU for the interface that will be used
   when sending to the destination address.

   This information is retrieved using the IPV6_PATHMTU socket option.
   This option takes a pointer to the ip6_mtuinfo structure as the
   fourth argument, and the size of the structure should be passed as a
   value-result parameter in the fifth argument.




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      struct ip6_mtuinfo mtuinfo;
      socklen_t infolen = sizeof(mtuinfo);

      getsockopt(fd, IPPROTO_IPV6, IPV6_PATHMTU, &mtuinfo, &infolen);

   When the call succeeds, the path MTU value is stored in the ip6m_mtu
   member of the ip6_mtuinfo structure.  Since the socket is connected,
   the ip6m_addr member is meaningless and should not be referred to by
   the application.

   This option can only be used for a connected socket, because a non-
   connected socket does not have the information of the destination and
   there is no way to pass the destination via getsockopt().  When
   getsockopt() for this option is issued on a non-connected socket, the
   call will fail.  Despite this limitation, this option is still useful
   from a practical point of view, because applications that care about
   the path MTU tend to send a lot of packets to a single destination
   and to connect the socket to the destination for performance reasons.
   If the application needs to get the MTU value in a more generic way,
   it should use a more generic interface, such as routing sockets
   [TCPIPILLUST].

12.  Ordering of Ancillary Data and IPv6 Extension Headers

   Three IPv6 extension headers can be specified by the application and
   returned to the application using ancillary data with sendmsg() and
   recvmsg(): the Routing header, Hop-by-Hop options header, and
   Destination options header.  When multiple ancillary data objects are
   transferred via recvmsg() and these objects represent any of these
   three extension headers, their placement in the control buffer is
   directly tied to their location in the corresponding IPv6 datagram.
   For example, when the application has enabled the IPV6_RECVRTHDR and
   IPV6_RECVDSTOPTS options and later receives an IPv6 packet with
   extension headers in the following order:

      The IPv6 header
      A Hop-by-Hop options header
      A Destination options header (1)
      A Routing header
      An Authentication header
      A Destination options header (2)
      A UDP header and UDP data









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   then the application will receive three ancillary data objects in the
   following order:

      an object with cmsg_type set to IPV6_DSTOPTS, which represents
      the destination options header (1)
      an object with cmsg_type set to IPV6_RTHDR, which represents the
      Routing header
      an object with cmsg_type set to IPV6_DSTOPTS, which represents the
      destination options header (2)

   This example follows the header ordering described in [RFC-2460], but
   the receiving side of this specification does not assume the
   ordering.  Applications may receive any numbers of objects in any
   order according to the ordering of the received IPv6 datagram.

   For the sending side, however, this API imposes some ordering
   constraints according to [RFC-2460].  Applications using this API
   cannot make a packet with extension headers that do not follow the
   ordering.  Note, however, that this does not mean applications must
   always follow the restriction.  This is just a limitation in this API
   in order to give application programmers a guideline to construct
   headers in a practical manner.  Should an application need to make an
   outgoing packet in an arbitrary order about the extension headers,
   some other technique, such as the datalink interfaces BPF or DLPI,
   must be used.

   The followings are more details about the constraints:

   -  Each IPV6_xxx ancillary data object for a particular type of
      extension header can be specified at most once in a single control
      buffer.

   -  IPV6_xxx ancillary data objects can appear in any order in a
      control buffer, because there is no ambiguity of the ordering.

   -  Each set of IPV6_xxx ancillary data objects and sticky options
      will be put in the outgoing packet along with the header ordering
      described in [RFC-2460].

   -  An ancillary data object or a sticky option of IPV6_RTHDRDSTOPTS
      will affect the outgoing packet only when a Routing header is
      specified as an ancillary data object or a sticky option.
      Otherwise, the specified value for IPV6_RTHDRDSTOPTS will be
      ignored.







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   For example, when an application sends a UDP datagram with a control
   data buffer containing ancillary data objects in the following order:

      an object with cmsg_type set to IPV6_DSTOPTS
      an object with cmsg_type set to IPV6_RTHDRDSTOPTS
      an object with cmsg_type set to IPV6_HOPOPTS

   and the sending socket does not have any sticky options, then the
   outgoing packet would be constructed as follows:

      The IPv6 header
      A Hop-by-Hop options header
      A Destination options header
      A UDP header and UDP data

   where the destination options header corresponds to the ancillary
   data object with the type IPV6_DSTOPTS.

   Note that the constraints above do not necessarily mean that the
   outgoing packet sent on the wire always follows the header ordering
   specified in this API document.  The kernel may insert additional
   headers that break the ordering as a result.  For example, if the
   kernel supports Mobile IPv6, an additional destination options header
   may be inserted before an authentication header, even without a
   routing header.

   This API does not provide access to any other extension headers than
   the supported three types of headers.  In particular, no information
   is provided about the IP security headers on an incoming packet, nor
   can be specified for an outgoing packet.  This API is for
   applications that do not care about the existence of IP security
   headers.

13.  IPv6-Specific Options with IPv4-Mapped IPv6 Addresses

   The various socket options and ancillary data specifications defined
   in this document apply only to true IPv6 sockets.  It is possible to
   create an IPv6 socket that actually sends and receives IPv4 packets,
   using IPv4-mapped IPv6 addresses, but the mapping of the options
   defined in this document to an IPv4 datagram is beyond the scope of
   this document.

   In general, attempting to specify an IPv6-only option, such as the
   Hop-by-Hop options, Destination options, or Routing header on an IPv6
   socket that is using IPv4-mapped IPv6 addresses, will probably result
   in an error.  Some implementations, however, may provide access to





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   the packet information (source/destination address, send/receive
   interface, and hop limit) on an IPv6 socket that is using IPv4-mapped
   IPv6 addresses.

14.  Extended interfaces for rresvport, rcmd and rexec

   Library functions that support the "r" commands hide the creation of
   a socket and the name resolution procedure from an application.  When
   the libraries return an AF_INET6 socket to an application that do not
   support the address family, the application may encounter an
   unexpected result when, e.g., calling getpeername() for the socket.
   In order to support AF_INET6 sockets for the "r" commands while
   keeping backward compatibility, this section defines some extensions
   to the libraries.

14.1.  rresvport_af

   The rresvport() function is used by the rcmd() function, and this
   function is in turn called by many of the "r" commands such as
   rlogin.  While new applications are not being written to use the
   rcmd() function, legacy applications such as rlogin will continue to
   use it and these will be ported to IPv6.

   rresvport() creates an IPv4/TCP socket and binds a "reserved port" to
   the socket.  Instead of defining an IPv6 version of this function we
   define a new function that takes an address family as its argument.

      #include <unistd.h>

      int  rresvport_af(int *port, int family);

   This function behaves the same as the existing rresvport() function,
   but instead of creating an AF_INET TCP socket, it can also create an
   AF_INET6 TCP socket.  The family argument is either AF_INET or
   AF_INET6, and a new error return is EAFNOSUPPORT if the address
   family is not supported.

   (Note: There is little consensus on which header defines the
   rresvport() and rcmd() function prototypes.  4.4BSD defines it in
   <unistd.h>, others in <netdb.h>, and others don't define the function
   prototypes at all.)

14.2.  rcmd_af

   The existing rcmd() function can not transparently use AF_INET6
   sockets since an application would not be prepared to handle AF_INET6
   addresses returned by e.g., getpeername() on the file descriptor
   created by rcmd().  Thus a new function is needed.



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      int rcmd_af(char **ahost, unsigned short rport,
                  const char *locuser, const char *remuser,
                  const char *cmd, int *fd2p, int af)

   This function behaves the same as the existing rcmd() function, but
   instead of creating an AF_INET TCP socket, it can also create an
   AF_INET6 TCP socket.  The family argument is AF_INET, AF_INET6, or
   AF_UNSPEC.  When either AF_INET or AF_INET6 is specified, this
   function will create a socket of the specified address family.  When
   AF_UNSPEC is specified, it will try all possible address families
   until a connection can be established, and will return the associated
   socket of the connection.  A new error EAFNOSUPPORT will be returned
   if the address family is not supported.

14.3.  rexec_af

   The existing rexec() function can not transparently use AF_INET6
   sockets since an application would not be prepared to handle AF_INET6
   addresses returned by e.g., getpeername() on the file descriptor
   created by rexec().  Thus a new function is needed.

      int rexec_af(char **ahost, unsigned short rport, const char *name,
                   const char *pass, const char *cmd, int *fd2p, int af)

   This function behaves the same as the existing rexec() function, but
   instead of creating an AF_INET TCP socket, it can also create an
   AF_INET6 TCP socket.  The family argument is AF_INET, AF_INET6, or
   AF_UNSPEC.  When either AF_INET or AF_INET6 is specified, this
   function will create a socket of the specified address family.  When
   AF_UNSPEC is specified, it will try all possible address families
   until a connection can be established, and will return the associated
   socket of the connection.  A new error EAFNOSUPPORT will be returned
   if the address family is not supported.

15.  Summary of New Definitions

   The following list summarizes the constants and structure,
   definitions discussed in this memo, sorted by header.

      <netinet/icmp6.h> ICMP6_DST_UNREACH
      <netinet/icmp6.h> ICMP6_DST_UNREACH_ADDR
      <netinet/icmp6.h> ICMP6_DST_UNREACH_ADMIN
      <netinet/icmp6.h> ICMP6_DST_UNREACH_BEYONDSCOPE
      <netinet/icmp6.h> ICMP6_DST_UNREACH_NOPORT
      <netinet/icmp6.h> ICMP6_DST_UNREACH_NOROUTE
      <netinet/icmp6.h> ICMP6_ECHO_REPLY
      <netinet/icmp6.h> ICMP6_ECHO_REQUEST
      <netinet/icmp6.h> ICMP6_INFOMSG_MASK



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      <netinet/icmp6.h> ICMP6_PACKET_TOO_BIG
      <netinet/icmp6.h> ICMP6_PARAMPROB_HEADER
      <netinet/icmp6.h> ICMP6_PARAMPROB_NEXTHEADER
      <netinet/icmp6.h> ICMP6_PARAMPROB_OPTION
      <netinet/icmp6.h> ICMP6_PARAM_PROB
      <netinet/icmp6.h> ICMP6_ROUTER_RENUMBERING
      <netinet/icmp6.h> ICMP6_RR_FLAGS_FORCEAPPLY
      <netinet/icmp6.h> ICMP6_RR_FLAGS_PREVDONE
      <netinet/icmp6.h> ICMP6_RR_FLAGS_REQRESULT
      <netinet/icmp6.h> ICMP6_RR_FLAGS_SPECSITE
      <netinet/icmp6.h> ICMP6_RR_FLAGS_TEST
      <netinet/icmp6.h> ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME
      <netinet/icmp6.h> ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME
      <netinet/icmp6.h> ICMP6_RR_PCOUSE_RAFLAGS_AUTO
      <netinet/icmp6.h> ICMP6_RR_PCOUSE_RAFLAGS_ONLINK
      <netinet/icmp6.h> ICMP6_RR_RESULT_FLAGS_FORBIDDEN
      <netinet/icmp6.h> ICMP6_RR_RESULT_FLAGS_OOB
      <netinet/icmp6.h> ICMP6_TIME_EXCEEDED
      <netinet/icmp6.h> ICMP6_TIME_EXCEED_REASSEMBLY
      <netinet/icmp6.h> ICMP6_TIME_EXCEED_TRANSIT
      <netinet/icmp6.h> MLD_LISTENER_QUERY
      <netinet/icmp6.h> MLD_LISTENER_REDUCTION
      <netinet/icmp6.h> MLD_LISTENER_REPORT
      <netinet/icmp6.h> ND_NA_FLAG_OVERRIDE
      <netinet/icmp6.h> ND_NA_FLAG_ROUTER
      <netinet/icmp6.h> ND_NA_FLAG_SOLICITED
      <netinet/icmp6.h> ND_NEIGHBOR_ADVERT
      <netinet/icmp6.h> ND_NEIGHBOR_SOLICIT
      <netinet/icmp6.h> ND_OPT_MTU
      <netinet/icmp6.h> ND_OPT_PI_FLAG_AUTO
      <netinet/icmp6.h> ND_OPT_PI_FLAG_ONLINK
      <netinet/icmp6.h> ND_OPT_PREFIX_INFORMATION
      <netinet/icmp6.h> ND_OPT_REDIRECTED_HEADER
      <netinet/icmp6.h> ND_OPT_SOURCE_LINKADDR
      <netinet/icmp6.h> ND_OPT_TARGET_LINKADDR
      <netinet/icmp6.h> ND_RA_FLAG_MANAGED
      <netinet/icmp6.h> ND_RA_FLAG_OTHER
      <netinet/icmp6.h> ND_REDIRECT
      <netinet/icmp6.h> ND_ROUTER_ADVERT
      <netinet/icmp6.h> ND_ROUTER_SOLICIT

      <netinet/icmp6.h> struct icmp6_filter{};
      <netinet/icmp6.h> struct icmp6_hdr{};
      <netinet/icmp6.h> struct icmp6_router_renum{};
      <netinet/icmp6.h> struct mld_hdr{};
      <netinet/icmp6.h> struct nd_neighbor_advert{};
      <netinet/icmp6.h> struct nd_neighbor_solicit{};
      <netinet/icmp6.h> struct nd_opt_hdr{};



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      <netinet/icmp6.h> struct nd_opt_mtu{};
      <netinet/icmp6.h> struct nd_opt_prefix_info{};
      <netinet/icmp6.h> struct nd_opt_rd_hdr{};
      <netinet/icmp6.h> struct nd_redirect{};
      <netinet/icmp6.h> struct nd_router_advert{};
      <netinet/icmp6.h> struct nd_router_solicit{};
      <netinet/icmp6.h> struct rr_pco_match{};
      <netinet/icmp6.h> struct rr_pco_use{};
      <netinet/icmp6.h> struct rr_result{};

      <netinet/in.h>    IPPROTO_AH
      <netinet/in.h>    IPPROTO_DSTOPTS
      <netinet/in.h>    IPPROTO_ESP
      <netinet/in.h>    IPPROTO_FRAGMENT
      <netinet/in.h>    IPPROTO_HOPOPTS
      <netinet/in.h>    IPPROTO_ICMPV6
      <netinet/in.h>    IPPROTO_IPV6
      <netinet/in.h>    IPPROTO_NONE
      <netinet/in.h>    IPPROTO_ROUTING
      <netinet/in.h>    IPV6_CHECKSUM
      <netinet/in.h>    IPV6_DONTFRAG
      <netinet/in.h>    IPV6_DSTOPTS
      <netinet/in.h>    IPV6_HOPLIMIT
      <netinet/in.h>    IPV6_HOPOPTS

      <netinet/in.h>    IPV6_NEXTHOP
      <netinet/in.h>    IPV6_PATHMTU
      <netinet/in.h>    IPV6_PKTINFO
      <netinet/in.h>    IPV6_RECVDSTOPTS
      <netinet/in.h>    IPV6_RECVHOPLIMIT
      <netinet/in.h>    IPV6_RECVHOPOPTS
      <netinet/in.h>    IPV6_RECVPKTINFO
      <netinet/in.h>    IPV6_RECVRTHDR
      <netinet/in.h>    IPV6_RECVTCLASS
      <netinet/in.h>    IPV6_RTHDR
      <netinet/in.h>    IPV6_RTHDRDSTOPTS
      <netinet/in.h>    IPV6_RTHDR_TYPE_0
      <netinet/in.h>    IPV6_RECVPATHMTU
      <netinet/in.h>    IPV6_TCLASS
      <netinet/in.h>    IPV6_USE_MIN_MTU
      <netinet/in.h>    struct in6_pktinfo{};
      <netinet/in.h>    struct ip6_mtuinfo{};

      <netinet/ip6.h>   IP6F_MORE_FRAG
      <netinet/ip6.h>   IP6F_OFF_MASK
      <netinet/ip6.h>   IP6F_RESERVED_MASK
      <netinet/ip6.h>   IP6OPT_JUMBO
      <netinet/ip6.h>   IP6OPT_JUMBO_LEN



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      <netinet/ip6.h>   IP6OPT_MUTABLE
      <netinet/ip6.h>   IP6OPT_NSAP_ADDR
      <netinet/ip6.h>   IP6OPT_PAD1
      <netinet/ip6.h>   IP6OPT_PADN
      <netinet/ip6.h>   IP6OPT_ROUTER_ALERT
      <netinet/ip6.h>   IP6OPT_TUNNEL_LIMIT
      <netinet/ip6.h>   IP6OPT_TYPE_DISCARD
      <netinet/ip6.h>   IP6OPT_TYPE_FORCEICMP
      <netinet/ip6.h>   IP6OPT_TYPE_ICMP
      <netinet/ip6.h>   IP6OPT_TYPE_SKIP
      <netinet/ip6.h>   IP6_ALERT_AN
      <netinet/ip6.h>   IP6_ALERT_MLD
      <netinet/ip6.h>   IP6_ALERT_RSVP
      <netinet/ip6.h>   struct ip6_dest{};
      <netinet/ip6.h>   struct ip6_frag{};
      <netinet/ip6.h>   struct ip6_hbh{};
      <netinet/ip6.h>   struct ip6_hdr{};
      <netinet/ip6.h>   struct ip6_opt{};
      <netinet/ip6.h>   struct ip6_opt_jumbo{};
      <netinet/ip6.h>   struct ip6_opt_nsap{};
      <netinet/ip6.h>   struct ip6_opt_router{};
      <netinet/ip6.h>   struct ip6_opt_tunnel{};
      <netinet/ip6.h>   struct ip6_rthdr{};
      <netinet/ip6.h>   struct ip6_rthdr0{};

   The following list summarizes the function and macro prototypes
   discussed in this memo, sorted by header.

      <netinet/icmp6.h> void ICMP6_FILTER_SETBLOCK(int, struct
                                               icmp6_filter *);
      <netinet/icmp6.h> void
                        ICMP6_FILTER_SETBLOCKALL(struct icmp6_filter *);
      <netinet/icmp6.h> void
                        ICMP6_FILTER_SETPASS(int,
                                             struct icmp6_filter *);
      <netinet/icmp6.h> void
                        ICMP6_FILTER_SETPASSALL(struct icmp6_filter *);
      <netinet/icmp6.h> int  ICMP6_FILTER_WILLBLOCK(int,
                                           const struct icmp6_filter *);
      <netinet/icmp6.h> int  ICMP6_FILTER_WILLPASS(int,
                                           const struct icmp6_filter *);

      <netinet/in.h>    int IN6_ARE_ADDR_EQUAL(const struct in6_addr *,
                                           const struct in6_addr *);

      <netinet/in.h>    int inet6_opt_append(void *, socklen_t, int,
                                             uint8_t, socklen_t, uint_t,
                                             void **);



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      <netinet/in.h>    int inet6_opt_get_val(void *, int, void *,
                                              socklen_t);
      <netinet/in.h>    int inet6_opt_find(void *, socklen_t,
                                           int, uint8_t ,
                                           socklen_t *, void **);
      <netinet/in.h>    int inet6_opt_finish(void *, socklen_t, int);
      <netinet/in.h>    int inet6_opt_init(void *, socklen_t);
      <netinet/in.h>    int inet6_opt_next(void *, socklen_t,
                                           int, uint8_t *,
                                           socklen_t *, void **);
      <netinet/in.h>    int inet6_opt_set_val(void *, int,
                                              void *, socklen_t);

      <netinet/in.h>    int inet6_rth_add(void *,
                                          const struct in6_addr *);
      <netinet/in.h>    struct in6_addr inet6_rth_getaddr(const void *,
                                                          int);
      <netinet/in.h>    void *inet6_rth_init(void *, socklen_t,
                                             int, int);
      <netinet/in.h>    int inet6_rth_reverse(const void *, void *);
      <netinet/in.h>    int inet6_rth_segments(const void *);
      <netinet/in.h>    soccklen_t inet6_rth_space(int, int);

      <netinet/ip6.h>   int  IP6OPT_TYPE(uint8_t);

      <sys/socket.h>    socklen_t CMSG_LEN(socklen_t);
      <sys/socket.h>    socklen_t CMSG_SPACE(socklen_t);

      <unistd.h>        int rresvport_af(int *, int);
      <unistd.h>        int rcmd_af(char **, unsigned short,
                                    const char *, const char *,
                                    const char *, int *, int);
      <unistd.h>        int rexec_af(char **, unsigned short,
                                     const char *, const char *,
                                     const char *, int *, int);

16.  Security Considerations

   The setting of certain Hop-by-Hop options and Destination options may
   be restricted to privileged processes.  Similarly some Hop-by-Hop
   options and Destination options may not be returned to non-privileged
   applications.

   The ability to specify an arbitrary source address using IPV6_PKTINFO
   must be prevented; at least for non-privileged processes.






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17.  Changes from RFC 2292

   Significant changes that affect the compatibility to RFC 2292:

   -  Removed the IPV6_PKTOPTIONS socket option by allowing sticky
      options to be set with individual setsockopt() calls.

   -  Removed the ability to be able to specify Hop-by-Hop and
      Destination options using multiple ancillary data items.  The
      application, using the inet6_opt_xxx() routines (see below), is
      responsible for formatting the whole extension header.

   -  Removed the support for the loose/strict Routing header since that
      has been removed from the IPv6 specification.

   -  Loosened the constraints for jumbo payload option that this option
      was always hidden from applications.

   -  Disabled the use of the IPV6_HOPLIMIT sticky option.

   -  Removed ip6r0_addr field from the ip6_rthdr structure.

   -  Intentionally unspecified how to get received packet's information
      on TCP sockets.

   New features:

   -  Added IPV6_RTHDRDSTOPTS to specify a Destination Options header
      before the Routing header.

   -  Added separate IPV6_RECVxxx options to enable the receipt of the
      corresponding ancillary data items.

   -  Added inet6_rth_xxx() and inet6_opt_xxx() functions to deal with
      routing or IPv6 options headers.

   -  Added extensions of libraries for the "r" commands.

   -  Introduced additional IPv6 option definitions such as IP6OPT_PAD1.

   -  Added MLD and router renumbering definitions.

   -  Added MTU-related socket options and ancillary data items.

   -  Added options and ancillary data items to manipulate the traffic
      class field.





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   -  Changed the name of ICMPv6 unreachable code 2 to be "beyond scope
      of source address."  ICMP6_DST_UNREACH_NOTNEIGHBOR was removed
      with this change.

   Clarifications:

   -  Added clarifications on extension headers ordering; for the
      sending side, assume the recommended ordering described in RFC
      2460.  For the receiving side, do not assume any ordering and pass
      all headers to the application in the received order.

   -  Added a summary about the interface selection rule.

   -  Clarified the ordering between IPV6_MULTICAST_IF and the
      IPV6_PKTINFO sticky option for multicast packets.

   -  Clarified how sticky options and the ICMPv6 filter are turned off
      and that getsockopt() of a sticky option returns what was set with
      setsockopt().

   -  Clarified that IPV6_NEXTHOP should be ignored for a multicast
      destination, that it should not contradict with the specified
      outgoing interface, and that the next hop should be a sockaddr_in6
      structure.

   -  Clarified corner cases of IPV6_CHECKSUM.

   -  Aligned with the POSIX standard.

   Editorial changes:

   -  Replaced MUST with must (since this is an informational document).

   -  Revised abstract to be more clear and concise, particularly
      concentrating on differences from RFC 2292.

   -  Made the URL of assigned numbers less specific so that it would be
      more robust for future changes.

   -  Updated the reference to the basic API.

   -  Added a reference to the latest POSIX standard.

   -  Moved general specifications of ancillary data and CMSG macros to
      the appendix.






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18.  References

   [RFC-1981]    McCann, J., Deering, S. and J. Mogul, "Path MTU
                 Discovery for IP version 6", RFC 1981, August 1996.

   [RFC-2460]    Deering, S. and R. Hinden, "Internet Protocol, Version
                 6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC-3493]    Gilligan, R., Thomson, S., Bound, J., McCann, J.  and
                 W. Stevens, "Basic Socket Interface Extensions for
                 IPv6", RFC 3493, March 2003.

   [POSIX]       IEEE Std. 1003.1-2001 Standard for Information
                 Technology -- Portable Operating System Interface
                 (POSIX). Open group Technical Standard: Base
                 Specifications, Issue 6, December 2001. ISO/IEC
                 9945:2002. http://www.opengroup.org/austin

   [TCPIPILLUST] Wright, G., Stevens, W., "TCP/IP Illustrated, Volume 2:
                 The Implementation", Addison Wesley, 1994.

19.  Acknowledgments

   Matt Thomas and Jim Bound have been working on the technical details
   in this document for over a year.  Keith Sklower is the original
   implementor of ancillary data in the BSD networking code.  Craig Metz
   provided lots of feedback, suggestions, and comments based on his
   implementing many of these features as the document was being
   written.  Mark Andrews first proposed the idea of the
   IPV6_USE_MIN_MTU option.  Jun-ichiro Hagino contributed text for the
   traffic class API from a document of his own.

   The following provided comments on earlier drafts: Pascal Anelli,
   Hamid Asayesh, Ran Atkinson, Karl Auerbach, Hamid Asayesh, Don
   Coolidge, Matt Crawford, Sam T. Denton, Richard Draves, Francis
   Dupont, Toerless Eckert, Lilian Fernandes, Bob Gilligan, Gerri
   Harter, Tim Hartrick, Bob Halley, Masaki Hirabaru, Michael Hunter,
   Yoshinobu Inoue, Mukesh Kacker, A. N. Kuznetsov, Sam Manthorpe, Pedro
   Marques, Jack McCann, der Mouse, John Moy, Lori Napoli, Thomas
   Narten, Atsushi Onoe, Steve Parker, Charles Perkins, Ken Powell, Tom
   Pusateri, Pedro Roque, Sameer Shah, Peter Sjodin, Stephen P.
   Spackman, Jinmei Tatuya, Karen Tracey, Sowmini Varadhan, Quaizar
   Vohra, Carl Williams, Steve Wise, Eric Wong, Farrell Woods, Kazu
   Yamamoto, Vladislav Yasevich, and Yoshifuji Hideaki.







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20.  Appendix A: Ancillary Data Overview

   4.2BSD allowed file descriptors to be transferred between separate
   processes across a UNIX domain socket using the sendmsg() and
   recvmsg() functions.  Two members of the msghdr structure,
   msg_accrights and msg_accrightslen, were used to send and receive the
   descriptors.  When the OSI protocols were added to 4.3BSD Reno in
   1990 the names of these two fields in the msghdr structure were
   changed to msg_control and msg_controllen, because they were used by
   the OSI protocols for "control information", although the comments in
   the source code call this "ancillary data".

   Other than the OSI protocols, the use of ancillary data has been
   rare.  In 4.4BSD, for example, the only use of ancillary data with
   IPv4 is to return the destination address of a received UDP datagram
   if the IP_RECVDSTADDR socket option is set.  With Unix domain sockets
   ancillary data is still used to send and receive descriptors.

   Nevertheless the ancillary data fields of the msghdr structure
   provide a clean way to pass information in addition to the data that
   is being read or written.  The inclusion of the msg_control and
   msg_controllen members of the msghdr structure along with the cmsghdr
   structure that is pointed to by the msg_control member is required by
   the Posix sockets API standard.

20.1.  The msghdr Structure

   The msghdr structure is used by the recvmsg() and sendmsg()
   functions.  Its Posix definition is:

      struct msghdr {
        void      *msg_name;        /* ptr to socket address
                                       structure */
        socklen_t  msg_namelen;     /* size of socket address
                                       structure */
        struct iovec  *msg_iov;     /* scatter/gather array */
        int        msg_iovlen;      /* # elements in msg_iov */
        void      *msg_control;     /* ancillary data */
        socklen_t  msg_controllen;  /* ancillary data buffer length */
        int        msg_flags;       /* flags on received message */
      };

   The structure is declared as a result of including <sys/socket.h>.

   (Note: Before Posix the two "void *" pointers were typically "char
   *", and the two socklen_t members were typically integers.  Earlier
   drafts of Posix had the two socklen_t members as size_t, but it then
   changed these to socklen_t to simplify binary portability for 64-bit



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   implementations and to align Posix with X/Open's Networking Services,
   Issue 5.  The change in msg_control to a "void *" pointer affects any
   code that increments this pointer.)

   Most Berkeley-derived implementations limit the amount of ancillary
   data in a call to sendmsg() to no more than 108 bytes (an mbuf).
   This API requires a minimum of 10240 bytes of ancillary data, but it
   is recommended that the amount be limited only by the buffer space
   reserved by the socket (which can be modified by the SO_SNDBUF socket
   option).  (Note: This magic number 10240 was picked as a value that
   should always be large enough.  108 bytes is clearly too small as the
   maximum size of a Routing header is 2048 bytes.)

20.2.  The cmsghdr Structure

   The cmsghdr structure describes ancillary data objects transferred by
   recvmsg() and sendmsg().  Its Posix definition is:

      struct cmsghdr {
        socklen_t  cmsg_len;   /* #bytes, including this header */
        int        cmsg_level; /* originating protocol */
        int        cmsg_type;  /* protocol-specific type */
                   /* followed by unsigned char cmsg_data[]; */
      };

   This structure is declared as a result of including <sys/socket.h>.

   (Note: Before Posix the cmsg_len member was an integer, and not a
   socklen_t.  See the Note in the previous section for why socklen_t is
   used here.)

   As shown in this definition, normally there is no member with the
   name cmsg_data[].  Instead, the data portion is accessed using the
   CMSG_xxx() macros, as described in Section 20.3.  Nevertheless, it is
   common to refer to the cmsg_data[] member.

   When ancillary data is sent or received, any number of ancillary data
   objects can be specified by the msg_control and msg_controllen
   members of the msghdr structure, because each object is preceded by a
   cmsghdr structure defining the object's length (the cmsg_len member).
   Historically Berkeley-derived implementations have passed only one
   object at a time, but this API allows multiple objects to be passed
   in a single call to sendmsg() or recvmsg().  The following example
   shows two ancillary data objects in a control buffer.







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|<--------------------------- msg_controllen ------------------------->|
|                                 OR                                   |
|<--------------------------- msg_controllen ---------------------->|
|                                                                      |
|<----- ancillary data object ----->|<---- ancillary data object ----->|
|<------ min CMSG_SPACE() --------->|<----- min CMSG_SPACE() --------->|
|                                   |                                  |
|<---------- cmsg_len ---------->|  |<-------- cmsg_len ----------->|  |
|<--------- CMSG_LEN() --------->|  |<------- CMSG_LEN() ---------->|  |
|                                |  |                               |  |
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+----------+--+
|cmsg_|cmsg_|cmsg_|XX|   cmsg_   |XX|cmsg_|cmsg_|cmsg_|XX|  cmsg_   |XX|
|len  |level|type |XX|   data[]  |XX|len  |level|type |XX|  data[]  |XX|
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+----------+--+
^
|
msg_control
points here

   The fields shown as "XX" are possible padding, between the cmsghdr
   structure and the data, and between the data and the next cmsghdr
   structure, if required by the implementation.  While sending an
   application may or may not include padding at the end of last
   ancillary data in msg_controllen and implementations must accept both
   as valid.  On receiving a portable application must provide space for
   padding at the end of the last ancillary data as implementations may
   copy out the padding at the end of the control message buffer and
   include it in the received msg_controllen.  When recvmsg() is called
   if msg_controllen is too small for all the ancillary data items
   including any trailing padding after the last item an implementation
   may set MSG_CTRUNC.

20.3.  Ancillary Data Object Macros

   To aid in the manipulation of ancillary data objects, three macros
   from 4.4BSD are defined by Posix: CMSG_DATA(), CMSG_NXTHDR(), and
   CMSG_FIRSTHDR().  Before describing these macros, we show the
   following example of how they might be used with a call to recvmsg().

      struct msghdr   msg;
      struct cmsghdr  *cmsgptr;

      /* fill in msg */

      /* call recvmsg() */






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      for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
           cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
          if (cmsgptr->cmsg_len == 0) {
              /* Error handling */
           break;
          }
          if (cmsgptr->cmsg_level == ... &&
              cmsgptr->cmsg_type == ... ) {
              u_char  *ptr;

              ptr = CMSG_DATA(cmsgptr);
              /* process data pointed to by ptr */
          }
      }

   We now describe the three Posix macros, followed by two more that are
   new with this API: CMSG_SPACE() and CMSG_LEN().  All these macros are
   defined as a result of including <sys/socket.h>.

20.3.1.  CMSG_FIRSTHDR

      struct cmsghdr *CMSG_FIRSTHDR(const struct msghdr *mhdr);

   CMSG_FIRSTHDR() returns a pointer to the first cmsghdr structure in
   the msghdr structure pointed to by mhdr.  The macro returns NULL if
   there is no ancillary data pointed to by the msghdr structure (that
   is, if either msg_control is NULL or if msg_controllen is less than
   the size of a cmsghdr structure).

   One possible implementation could be

      #define CMSG_FIRSTHDR(mhdr) \
          ( (mhdr)->msg_controllen >= sizeof(struct cmsghdr) ? \
            (struct cmsghdr *)(mhdr)->msg_control : \
            (struct cmsghdr *)NULL )

   (Note: Most existing implementations do not test the value of
   msg_controllen, and just return the value of msg_control.  The value
   of msg_controllen must be tested, because if the application asks
   recvmsg() to return ancillary data, by setting msg_control to point
   to the application's buffer and setting msg_controllen to the length
   of this buffer, the kernel indicates that no ancillary data is
   available by setting msg_controllen to 0 on return.  It is also
   easier to put this test into this macro, than making the application
   perform the test.)






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20.3.2.  CMSG_NXTHDR

   As described in Section 5.1, CMSG_NXTHDR has been extended to handle
   a NULL 2nd argument to mean "get the first header".  This provides an
   alternative way of coding the processing loop shown earlier:

      struct msghdr  msg;
      struct cmsghdr  *cmsgptr = NULL;

      /* fill in msg */

      /* call recvmsg() */

      while ((cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) != NULL) {
          if (cmsgptr->cmsg_len == 0) {
              /* Error handling */
           break;
          }
          if (cmsgptr->cmsg_level == ... &&
              cmsgptr->cmsg_type == ... ) {
              u_char  *ptr;

              ptr = CMSG_DATA(cmsgptr);
              /* process data pointed to by ptr */
          }
      }

   One possible implementation could be:

      #define CMSG_NXTHDR(mhdr, cmsg) \
        (((cmsg) == NULL) ? CMSG_FIRSTHDR(mhdr) : \
         (((u_char *)(cmsg) + ALIGN_H((cmsg)->cmsg_len) \
                            + ALIGN_D(sizeof(struct cmsghdr)) > \
           (u_char *)((mhdr)->msg_control) + (mhdr)->msg_controllen) ? \
          (struct cmsghdr *)NULL : \
          (struct cmsghdr *)((u_char *)(cmsg) + \
                                        ALIGN_H((cmsg)->cmsg_len))))

   The macros ALIGN_H() and ALIGN_D(), which are implementation
   dependent, round their arguments up to the next even multiple of
   whatever alignment is required for the start of the cmsghdr structure
   and the data, respectively.  (This is probably a multiple of 4 or 8
   bytes.)  They are often the same macro in implementations platforms
   where alignment requirement for header and data is chosen to be
   identical.






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20.3.3.  CMSG_DATA

      unsigned char *CMSG_DATA(const struct cmsghdr *cmsg);

   CMSG_DATA() returns a pointer to the data (what is called the
   cmsg_data[] member, even though such a member is not defined in the
   structure) following a cmsghdr structure.

   One possible implementation could be:

      #define CMSG_DATA(cmsg) ( (u_char *)(cmsg) + \
                                ALIGN_D(sizeof(struct cmsghdr)) )

20.3.4.  CMSG_SPACE

   CMSG_SPACE is new with this API (see Section 5.2).  It is used to
   determine how much space needs to be allocated for an ancillary data
   item.

   One possible implementation could be:

      #define CMSG_SPACE(length) ( ALIGN_D(sizeof(struct cmsghdr)) + \
                                   ALIGN_H(length) )

20.3.5.  CMSG_LEN

   CMSG_LEN is new with this API (see Section 5.3).  It  returns the
   value to store in the cmsg_len member of the cmsghdr structure,
   taking into account any padding needed to satisfy alignment
   requirements.

   One possible implementation could be:

      #define CMSG_LEN(length) ( ALIGN_D(sizeof(struct cmsghdr)) + \
                                 length )

21.  Appendix B: Examples Using the inet6_rth_XXX() Functions

   Here we show an example for both sending Routing headers and
   processing and reversing a received Routing header.

21.1.  Sending a Routing Header

   As an example of these Routing header functions defined in this
   document, we go through the function calls for the example on p. 17
   of [RFC-2460].  The source is S, the destination is D, and the three
   intermediate nodes are I1, I2, and I3.




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              S -----> I1 -----> I2 -----> I3 -----> D

      src:    *    S         S         S         S   S
      dst:    D   I1        I2        I3         D   D
      A[1]:  I1   I2        I1        I1        I1  I1
      A[2]:  I2   I3        I3        I2        I2  I2
      A[3]:  I3    D         D         D        I3  I3
      #seg:   3    3         2         1         0   3

   src and dst are the source and destination IPv6 addresses in the IPv6
   header.  A[1], A[2], and A[3] are the three addresses in the Routing
   header.  #seg is the Segments Left field in the Routing header.

   The six values in the column beneath node S are the values in the
   Routing header specified by the sending application using sendmsg()
   of setsockopt().  The function calls by the sender would look like:

      void  *extptr;
      socklen_t   extlen;
      struct msghdr  msg;
      struct cmsghdr  *cmsgptr;
      int   cmsglen;
      struct sockaddr_in6  I1, I2, I3, D;

      extlen = inet6_rth_space(IPV6_RTHDR_TYPE_0, 3);
      cmsglen = CMSG_SPACE(extlen);
      cmsgptr = malloc(cmsglen);
      cmsgptr->cmsg_len = CMSG_LEN(extlen);
      cmsgptr->cmsg_level = IPPROTO_IPV6;
      cmsgptr->cmsg_type = IPV6_RTHDR;

      extptr = CMSG_DATA(cmsgptr);
      extptr = inet6_rth_init(extptr, extlen, IPV6_RTHDR_TYPE_0, 3);

      inet6_rth_add(extptr, &I1.sin6_addr);
      inet6_rth_add(extptr, &I2.sin6_addr);
      inet6_rth_add(extptr, &I3.sin6_addr);

      msg.msg_control = cmsgptr;
      msg.msg_controllen = cmsglen;

      /* finish filling in msg{}, msg_name = D */
      /* call sendmsg() */

   We also assume that the source address for the socket is not
   specified (i.e., the asterisk in the figure).





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   The four columns of six values that are then shown between the five
   nodes are the values of the fields in the packet while the packet is
   in transit between the two nodes.  Notice that before the packet is
   sent by the source node S, the source address is chosen (replacing
   the asterisk), I1 becomes the destination address of the datagram,
   the two addresses A[2] and A[3] are "shifted up", and D is moved to
   A[3].

   The columns of values that are shown beneath the destination node are
   the values returned by recvmsg(), assuming the application has
   enabled both the IPV6_RECVPKTINFO and IPV6_RECVRTHDR socket options.
   The source address is S (contained in the sockaddr_in6 structure
   pointed to by the msg_name member), the destination address is D
   (returned as an ancillary data object in an in6_pktinfo structure),
   and the ancillary data object specifying the Routing header will
   contain three addresses (I1, I2, and I3).  The number of segments in
   the Routing header is known from the Hdr Ext Len field in the Routing
   header (a value of 6, indicating 3 addresses).

   The return value from inet6_rth_segments() will be 3 and
   inet6_rth_getaddr(0) will return I1, inet6_rth_getaddr(1) will return
   I2, and inet6_rth_getaddr(2) will return I3,

   If the receiving application then calls inet6_rth_reverse(), the
   order of the three addresses will become I3, I2, and I1.

   We can also show what an implementation might store in the ancillary
   data object as the Routing header is being built by the sending
   process.  If we assume a 32-bit architecture where sizeof(struct
   cmsghdr) equals 12, with a desired alignment of 4-byte boundaries,
   then the call to inet6_rth_space(3) returns 68: 12 bytes for the
   cmsghdr structure and 56 bytes for the Routing header (8 + 3*16).

   The call to inet6_rth_init() initializes the ancillary data object to
   contain a Type 0 Routing header:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_len = 20                                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_level = IPPROTO_IPV6                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_type = IPV6_RTHDR                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hdr Ext Len=6 | Routing Type=0|  Seg Left=0   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The first call to inet6_rth_add() adds I1 to the list.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_len = 36                                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_level = IPPROTO_IPV6                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_type = IPV6_RTHDR                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hdr Ext Len=6 | Routing Type=0|  Seg Left=1   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                           Address[1] = I1                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   cmsg_len is incremented by 16, and the Segments Left field is
   incremented by 1.



























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   The next call to inet6_rth_add() adds I2 to the list.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_len = 52                                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_level = IPPROTO_IPV6                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_type = IPV6_RTHDR                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hdr Ext Len=6 | Routing Type=0|  Seg Left=2   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                           Address[1] = I1                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                           Address[2] = I2                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   cmsg_len is incremented by 16, and the Segments Left field is
   incremented by 1.



















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   The last call to inet6_rth_add() adds I3 to the list.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_len = 68                                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_level = IPPROTO_IPV6                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       cmsg_type = IPV6_RTHDR                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hdr Ext Len=6 | Routing Type=0|  Seg Left=3   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                           Address[1] = I1                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                           Address[2] = I2                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                           Address[3] = I3                     +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   cmsg_len is incremented by 16, and the Segments Left field is
   incremented by 1.

21.2.  Receiving Routing Headers

   This example assumes that the application has enabled IPV6_RECVRTHDR
   socket option.  The application prints and reverses a source route
   and uses that to echo the received data.





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      struct sockaddr_in6     addr;
      struct msghdr           msg;
      struct iovec            iov;
      struct cmsghdr          *cmsgptr;
      socklen_t               cmsgspace;
      void                    *extptr;
      int                     extlen;

      int                     segments;
      int                     i;
      char                    databuf[8192];

      segments = 100;        /* Enough */
      extlen = inet6_rth_space(IPV6_RTHDR_TYPE_0, segments);
      cmsgspace = CMSG_SPACE(extlen);
      cmsgptr = malloc(cmsgspace);
      if (cmsgptr == NULL) {
              perror("malloc");
              exit(1);
      }
      extptr = CMSG_DATA(cmsgptr);

      msg.msg_control = cmsgptr;
      msg.msg_controllen = cmsgspace;
      msg.msg_name = (struct sockaddr *)&addr;
      msg.msg_namelen = sizeof (addr);
      msg.msg_iov = &iov;
      msg.msg_iovlen = 1;
      iov.iov_base = databuf;
      iov.iov_len = sizeof (databuf);
      msg.msg_flags = 0;
      if (recvmsg(s, &msg, 0) == -1) {
              perror("recvmsg");
              return;
      }
      if (msg.msg_controllen != 0 &&
          cmsgptr->cmsg_level == IPPROTO_IPV6 &&
          cmsgptr->cmsg_type == IPV6_RTHDR) {
              struct in6_addr *in6;
              char asciiname[INET6_ADDRSTRLEN];
              struct ip6_rthdr *rthdr;

              rthdr = (struct ip6_rthdr *)extptr;
              segments = inet6_rth_segments(extptr);
              printf("route (%d segments, %d left): ",
                  segments, rthdr->ip6r_segleft);
              for (i = 0; i < segments; i++) {
                      in6 = inet6_rth_getaddr(extptr, i);



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                      if (in6 == NULL)
                              printf("<NULL> ");
                      else
                              printf("%s ", inet_ntop(AF_INET6,
                                  (void *)in6->s6_addr,
                                  asciiname, INET6_ADDRSTRLEN));
              }
              if (inet6_rth_reverse(extptr, extptr) == -1) {
                      printf("reverse failed");
                      return;
              }
      }
      iov.iov_base = databuf;
      iov.iov_len = strlen(databuf);
      if (sendmsg(s, &msg, 0) == -1)
              perror("sendmsg");
      if (cmsgptr != NULL)
              free(cmsgptr);

   Note: The above example is a simple illustration.  It skips some
   error checks, including those involving the MSG_TRUNC and MSG_CTRUNC
   flags.  It also leaves some type mismatches in favor of brevity.

22.  Appendix C: Examples Using the inet6_opt_XXX() Functions

   This shows how Hop-by-Hop and Destination options can be both built
   as well as parsed using the inet6_opt_XXX() functions.  These
   examples assume that there are defined values for OPT_X and OPT_Y.

   Note: The example is a simple illustration.  It skips some error
   checks and leaves some type mismatches in favor of brevity.

22.1.  Building Options

   We now provide an example that builds two Hop-by-Hop options using
   the example in Appendix B of [RFC-2460].

      void *extbuf;
      socklen_t extlen;
      int currentlen;
      void *databuf;
      int offset;
      uint8_t value1;
      uint16_t value2;
      uint32_t value4;
      uint64_t value8;

      /* Estimate the length */



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      currentlen = inet6_opt_init(NULL, 0);
      if (currentlen == -1)
              return (-1);
      currentlen = inet6_opt_append(NULL, 0, currentlen, OPT_X,
                                    12, 8, NULL);
      if (currentlen == -1)
              return (-1);
      currentlen = inet6_opt_append(NULL, 0, currentlen, OPT_Y,
                                    7, 4, NULL);
      if (currentlen == -1)
              return (-1);
      currentlen = inet6_opt_finish(NULL, 0, currentlen);
      if (currentlen == -1)
              return (-1);
      extlen = currentlen;

      extbuf = malloc(extlen);
      if (extbuf == NULL) {
              perror("malloc");
              return (-1);
      }
      currentlen = inet6_opt_init(extbuf, extlen);
      if (currentlen == -1)
              return (-1);

      currentlen = inet6_opt_append(extbuf, extlen, currentlen,
          OPT_X, 12, 8, &databuf);
      if (currentlen == -1)
              return (-1);
      /* Insert value 0x12345678 for 4-octet field */
      offset = 0;
      value4 = 0x12345678;
      offset = inet6_opt_set_val(databuf, offset,
                                 &value4, sizeof (value4));
      /* Insert value 0x0102030405060708 for 8-octet field */
      value8 = 0x0102030405060708;
      offset = inet6_opt_set_val(databuf, offset,
                                 &value8, sizeof (value8));

      currentlen = inet6_opt_append(extbuf, extlen, currentlen,
          OPT_Y, 7, 4, &databuf);
      if (currentlen == -1)
              return (-1);
      /* Insert value 0x01 for 1-octet field */
      offset = 0;
      value1 = 0x01;
      offset = inet6_opt_set_val(databuf, offset,
                                 &value1, sizeof (value1));



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      /* Insert value 0x1331 for 2-octet field */
      value2 = 0x1331;
      offset = inet6_opt_set_val(databuf, offset,
                                 &value2, sizeof (value2));
      /* Insert value 0x01020304 for 4-octet field */
      value4 = 0x01020304;
      offset = inet6_opt_set_val(databuf, offset,
                                 &value4, sizeof (value4));

      currentlen = inet6_opt_finish(extbuf, extlen, currentlen);
      if (currentlen == -1)
              return (-1);
      /* extbuf and extlen are now completely formatted */

22.2.  Parsing Received Options

   This example parses and prints the content of the two options in the
   previous example.

      int
      print_opt(void *extbuf, socklen_t extlen)
      {
              struct ip6_dest *ext;
              int currentlen;
              uint8_t type;
              socklen_t len;
              void *databuf;
              int offset;
              uint8_t value1;
              uint16_t value2;
              uint32_t value4;
              uint64_t value8;

              ext = (struct ip6_dest *)extbuf;
              printf("nxt %u, len %u (bytes %d)\n", ext->ip6d_nxt,
                  ext->ip6d_len, (ext->ip6d_len + 1) * 8);

              currentlen = 0;
              while (1) {
                      currentlen = inet6_opt_next(extbuf, extlen,
                                                  currentlen, &type,
                                                  &len, &databuf);
                      if (currentlen == -1)
                              break;
                      printf("Received opt %u len %u\n",
                          type, len);
                      switch (type) {
                      case OPT_X:



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                              offset = 0;
                              offset =
                                  inet6_opt_get_val(databuf, offset,
                                                    &value4,
                                                    sizeof (value4));
                              printf("X 4-byte field %x\n", value4);
                              offset =
                                  inet6_opt_get_val(databuf, offset,
                                                    &value8,
                                                    sizeof (value8));
                              printf("X 8-byte field %llx\n", value8);
                              break;
                      case OPT_Y:
                              offset = 0;
                              offset =
                                  inet6_opt_get_val(databuf, offset,
                                                    &value1,
                                                    sizeof (value1));
                              printf("Y 1-byte field %x\n", value1);
                              offset =
                                  inet6_opt_get_val(databuf, offset,
                                                    &value2,
                                                    sizeof (value2));
                              printf("Y 2-byte field %x\n", value2);
                              offset =
                                  inet6_opt_get_val(databuf, offset,
                                                    &value4,
                                                    sizeof (value4));
                              printf("Y 4-byte field %x\n", value4);
                              break;
                      default:
                              printf("Unknown option %u\n", type);
                              break;
                      }
              }
              return (0);
      }














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23. Authors' Addresses

   W. Richard Stevens (deceased)


   Matt Thomas
   3am Software Foundry
   8053 Park Villa Circle
   Cupertino, CA 95014

   EMail: matt@3am-software.com


   Erik Nordmark
   Sun Microsystems Laboratories, Europe
   180, avenue de l'Europe
   38334 SAINT ISMIER Cedex, France

   Phone: +33 (0)4 74 18 88 03
   Fax:   +33 (0)4 76 18 88 88
   EMail: Erik.Nordmark@sun.com


   Tatuya JINMEI
   Corporate Research & Development Center, Toshiba Corporation
   1 Komukai Toshiba-cho, Kawasaki-shi
   Kanagawa 212-8582, Japan

   EMail: jinmei@isl.rdc.toshiba.co.jp






















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RFC 3542             Advanced Sockets API for IPv6              May 2003


24. Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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