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rfc:rfc3542

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.

Stevens, et al. Informational [Page 1] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 2] RFC 3542 Advanced Sockets API for IPv6 May 2003

      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.

Stevens, et al. Informational [Page 3] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 4] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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>.

Stevens, et al. Informational [Page 5] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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 */

Stevens, et al. Informational [Page 6] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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 */
    };

Stevens, et al. Informational [Page 7] RFC 3542 Advanced Sockets API for IPv6 May 2003

    /* 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 */

Stevens, et al. Informational [Page 8] RFC 3542 Advanced Sockets API for IPv6 May 2003

    #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

Stevens, et al. Informational [Page 9] RFC 3542 Advanced Sockets API for IPv6 May 2003

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 */

Stevens, et al. Informational [Page 10] RFC 3542 Advanced Sockets API for IPv6 May 2003

    #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]

Stevens, et al. Informational [Page 11] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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

Stevens, et al. Informational [Page 12] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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 */
    };

Stevens, et al. Informational [Page 13] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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]

Stevens, et al. Informational [Page 14] RFC 3542 Advanced Sockets API for IPv6 May 2003

    /* 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

Stevens, et al. Informational [Page 15] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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.

Stevens, et al. Informational [Page 16] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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

Stevens, et al. Informational [Page 17] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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));

Stevens, et al. Informational [Page 18] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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>.

Stevens, et al. Informational [Page 19] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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,

Stevens, et al. Informational [Page 20] RFC 3542 Advanced Sockets API for IPv6 May 2003

               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.)

Stevens, et al. Informational [Page 21] RFC 3542 Advanced Sockets API for IPv6 May 2003

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));

Stevens, et al. Informational [Page 22] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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").

Stevens, et al. Informational [Page 23] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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:
  1. For the IPV6_PKTINFO option, it will return an in6_pktinfo

structure with ipi6_addr being in6addr_any and ipi6_ifindex being

    zero.
  1. For the IPV6_TCLASS option, it will return the kernel default

value.

  1. 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.

Stevens, et al. Informational [Page 24] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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

Stevens, et al. Informational [Page 25] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 26] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 27] RFC 3542 Advanced Sockets API for IPv6 May 2003

 "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.

Stevens, et al. Informational [Page 28] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 29] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.)

Stevens, et al. Informational [Page 30] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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

Stevens, et al. Informational [Page 31] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 32] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 33] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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 */

Stevens, et al. Informational [Page 34] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 35] RFC 3542 Advanced Sockets API for IPv6 May 2003

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.

Stevens, et al. Informational [Page 36] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 37] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 38] RFC 3542 Advanced Sockets API for IPv6 May 2003

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.

Stevens, et al. Informational [Page 39] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 40] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 42] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 43] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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. 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.

Stevens, et al. Informational [Page 44] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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));

Stevens, et al. Informational [Page 45] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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

Stevens, et al. Informational [Page 46] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 47] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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

Stevens, et al. Informational [Page 48] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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:
  1. Each IPV6_xxx ancillary data object for a particular type of

extension header can be specified at most once in a single control

    buffer.
  1. IPV6_xxx ancillary data objects can appear in any order in a

control buffer, because there is no ambiguity of the ordering.

  1. 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].
  1. 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.

Stevens, et al. Informational [Page 49] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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

Stevens, et al. Informational [Page 50] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 51] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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

Stevens, et al. Informational [Page 52] RFC 3542 Advanced Sockets API for IPv6 May 2003

    <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{};

Stevens, et al. Informational [Page 53] RFC 3542 Advanced Sockets API for IPv6 May 2003

    <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

Stevens, et al. Informational [Page 54] RFC 3542 Advanced Sockets API for IPv6 May 2003

    <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 **);

Stevens, et al. Informational [Page 55] RFC 3542 Advanced Sockets API for IPv6 May 2003

    <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.

Stevens, et al. Informational [Page 56] RFC 3542 Advanced Sockets API for IPv6 May 2003

17. Changes from RFC 2292

 Significant changes that affect the compatibility to RFC 2292:
  1. Removed the IPV6_PKTOPTIONS socket option by allowing sticky

options to be set with individual setsockopt() calls.

  1. 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.
  1. Removed the support for the loose/strict Routing header since that

has been removed from the IPv6 specification.

  1. Loosened the constraints for jumbo payload option that this option

was always hidden from applications.

  1. Disabled the use of the IPV6_HOPLIMIT sticky option.
  1. Removed ip6r0_addr field from the ip6_rthdr structure.
  1. Intentionally unspecified how to get received packet's information

on TCP sockets.

 New features:
  1. Added IPV6_RTHDRDSTOPTS to specify a Destination Options header

before the Routing header.

  1. Added separate IPV6_RECVxxx options to enable the receipt of the

corresponding ancillary data items.

  1. Added inet6_rth_xxx() and inet6_opt_xxx() functions to deal with

routing or IPv6 options headers.

  1. Added extensions of libraries for the "r" commands.
  1. Introduced additional IPv6 option definitions such as IP6OPT_PAD1.
  1. Added MLD and router renumbering definitions.
  1. Added MTU-related socket options and ancillary data items.
  1. Added options and ancillary data items to manipulate the traffic

class field.

Stevens, et al. Informational [Page 57] RFC 3542 Advanced Sockets API for IPv6 May 2003

  1. 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:
  1. 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.
  1. Added a summary about the interface selection rule.
  1. Clarified the ordering between IPV6_MULTICAST_IF and the

IPV6_PKTINFO sticky option for multicast packets.

  1. Clarified how sticky options and the ICMPv6 filter are turned off

and that getsockopt() of a sticky option returns what was set with

    setsockopt().
  1. 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.
  1. Clarified corner cases of IPV6_CHECKSUM.
  1. Aligned with the POSIX standard.
 Editorial changes:
  1. Replaced MUST with must (since this is an informational document).
  1. Revised abstract to be more clear and concise, particularly

concentrating on differences from RFC 2292.

  1. Made the URL of assigned numbers less specific so that it would be

more robust for future changes.

  1. Updated the reference to the basic API.
  1. Added a reference to the latest POSIX standard.
  1. Moved general specifications of ancillary data and CMSG macros to

the appendix.

Stevens, et al. Informational [Page 58] RFC 3542 Advanced Sockets API for IPv6 May 2003

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.

Stevens, et al. Informational [Page 59] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 60] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 61] RFC 3542 Advanced Sockets API for IPv6 May 2003

←————————– 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_ XXcmsg_cmsg_cmsg_XX cmsg_ XX
len leveltype XX data[] XXlen leveltype 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() */

Stevens, et al. Informational [Page 62] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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.)

Stevens, et al. Informational [Page 63] RFC 3542 Advanced Sockets API for IPv6 May 2003

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.

Stevens, et al. Informational [Page 64] RFC 3542 Advanced Sockets API for IPv6 May 2003

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.

Stevens, et al. Informational [Page 65] RFC 3542 Advanced Sockets API for IPv6 May 2003

            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).

Stevens, et al. Informational [Page 66] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 67] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 68] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 69] RFC 3542 Advanced Sockets API for IPv6 May 2003

 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.

Stevens, et al. Informational [Page 70] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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);

Stevens, et al. Informational [Page 71] RFC 3542 Advanced Sockets API for IPv6 May 2003

                    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 */

Stevens, et al. Informational [Page 72] RFC 3542 Advanced Sockets API for IPv6 May 2003

    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));

Stevens, et al. Informational [Page 73] RFC 3542 Advanced Sockets API for IPv6 May 2003

    /* 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:

Stevens, et al. Informational [Page 74] RFC 3542 Advanced Sockets API for IPv6 May 2003

                            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);
    }

Stevens, et al. Informational [Page 75] RFC 3542 Advanced Sockets API for IPv6 May 2003

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

Stevens, et al. Informational [Page 76] 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.

Stevens, et al. Informational [Page 77]

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