path: root/doc/rfc/rfc2292.txt

Network Working Group                                        W. Stevens
Request for Comments: 2292                                   Consultant
Category: Informational                                       M. Thomas
                                                              AltaVista
                                                          February 1998



                     Advanced Sockets 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 (1998).  All Rights Reserved.

Abstract

   Specifications are in progress for changes to the sockets API to
   support IP version 6 [RFC-2133].  These changes are for TCP and UDP-
   based applications and will support most end-user applications in use
   today: Telnet and FTP clients and servers, HTTP clients and servers,
   and the like.

   But another class of applications exists that will also be run under
   IPv6.  We call these "advanced" applications and today this includes
   programs such as Ping, Traceroute, routing daemons, multicast routing
   daemons, router discovery daemons, and the like.  The API feature
   typically used by these programs that make them "advanced" is a raw
   socket to access ICMPv4, IGMPv4, or IPv4, along with some knowledge
   of the packet header formats used by these protocols.  To provide
   portability for applications that use raw sockets under IPv6, some
   standardization is needed for the advanced API features.

   There are other features of IPv6 that some applications will need to
   access: interface identification (specifying the outgoing interface
   and determining the incoming interface) and IPv6 extension headers
   that are not addressed in [RFC-2133]: Hop-by-Hop options, Destination
   options, and the Routing header (source routing).  This document
   provides API access to these features too.






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Table of Contents

    1.  Introduction ................................................3
    2.  Common Structures and Definitions ...........................5
       2.1.  The ip6_hdr Structure ..................................5
            2.1.1.  IPv6 Next Header Values .........................6
            2.1.2.  IPv6 Extension Headers ..........................6
       2.2.  The icmp6_hdr Structure ................................8
            2.2.1.  ICMPv6 Type and Code Values .....................8
            2.2.2.  ICMPv6 Neighbor Discovery Type and Code Values ..9
       2.3.  Address Testing Macros .................................12
       2.4.  Protocols File .........................................12
    3.  IPv6 Raw Sockets ............................................13
       3.1.  Checksums ..............................................14
       3.2.  ICMPv6 Type Filtering ..................................14
    4.  Ancillary Data ..............................................17
       4.1.  The msghdr Structure ...................................18
       4.2.  The cmsghdr Structure ..................................18
       4.3.  Ancillary Data Object Macros ...........................19
            4.3.1.  CMSG_FIRSTHDR ...................................20
            4.3.2.  CMSG_NXTHDR .....................................22
            4.3.3.  CMSG_DATA .......................................22
            4.3.4.  CMSG_SPACE ......................................22
            4.3.5.  CMSG_LEN ........................................22
       4.4.  Summary of Options Described Using Ancillary Data ......23
       4.5.  IPV6_PKTOPTIONS Socket Option ..........................24
            4.5.1.  TCP Sticky Options ..............................25
            4.5.2.  UDP and Raw Socket Sticky Options ...............26
    5.  Packet Information ..........................................26
       5.1.  Specifying/Receiving the Interface .....................27
       5.2.  Specifying/Receiving Source/Destination Address ........27
       5.3.  Specifying/Receiving the Hop Limit .....................28
       5.4.  Specifying the Next Hop Address ........................29
       5.5.  Additional Errors with sendmsg() .......................29
    6.  Hop-By-Hop Options ..........................................30
       6.1.  Receiving Hop-by-Hop Options ...........................31
       6.2.  Sending Hop-by-Hop Options .............................31
       6.3.  Hop-by-Hop and Destination Options Processing ..........32
            6.3.1.  inet6_option_space ..............................32
            6.3.2.  inet6_option_init ...............................32
            6.3.3.  inet6_option_append .............................33
            6.3.4.  inet6_option_alloc ..............................33
            6.3.5.  inet6_option_next ...............................34
            6.3.6.  inet6_option_find ...............................35
            6.3.7.  Options Examples ................................35
    7.  Destination Options .........................................42
       7.1.  Receiving Destination Options ..........................42
       7.2.  Sending Destination Options ............................43



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    8.  Routing Header Option .......................................43
       8.1.  inet6_rthdr_space ......................................44
       8.2.  inet6_rthdr_init .......................................45
       8.3.  inet6_rthdr_add ........................................45
       8.4.  inet6_rthdr_lasthop ....................................46
       8.5.  inet6_rthdr_reverse ....................................46
       8.6.  inet6_rthdr_segments ...................................46
       8.7.  inet6_rthdr_getaddr ....................................46
       8.8.  inet6_rthdr_getflags ...................................47
       8.9.  Routing Header Example .................................47
    9.  Ordering of Ancillary Data and IPv6 Extension Headers .......53
   10.  IPv6-Specific Options with IPv4-Mapped IPv6 Addresses .......54
   11.  rresvport_af ................................................55
   12.  Future Items ................................................55
       12.1.  Flow Labels ...........................................55
       12.2.  Path MTU Discovery and UDP ............................56
       12.3.  Neighbor Reachability and UDP .........................56
   13.  Summary of New Definitions ..................................56
   14.  Security Considerations .....................................59
   15.  Change History ..............................................59
   16.  References ..................................................65
   17.  Acknowledgments .............................................65
   18.  Authors' Addresses ..........................................66
   19.  Full Copyright Statement ....................................67

1.  Introduction

   Specifications are in progress for changes to the sockets API to
   support IP version 6 [RFC-2133].  These changes are for TCP and UDP-
   based applications.  The current document defines some the "advanced"
   features of the sockets API that are required for applications to
   take advantage of additional features of IPv6.

   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 headers.  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.







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   There are also features from the basic IPv6 specification that are
   not addressed in [RFC-2133]: sending and receiving Hop-by-Hop
   options, Destination options, and Routing headers, specifying the
   outgoing interface, and being told of the receiving interface.

   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.

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

   5.  Additional features required for IPv6 application portability.

   The packet information along with access to the extension headers
   (Hop-by-Hop options, Destination options, and Routing header) 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.1g standard (which should be
   approved in 1997) 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.

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







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

   Datatypes in this document follow the Posix.1g 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 headers are required, along with common
   constant definitions for the structure members.

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

   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;   /* 24 bits of 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, 4 bits priority */
      } 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



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    #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 fragmentation 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 */

   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 */
   /* XXX should we pad it to force alignment on an 8-byte boundary? */
   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 */
   /* XXX should we pad it to force alignment on an 8-byte boundary? */
   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 {



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     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 */
     uint8_t  ip6r0_reserved;  /* reserved field */
     uint8_t  ip6r0_slmap[3];  /* strict/loose bit map */
     struct in6_addr  ip6r0_addr[1];  /* up to 23 addresses */
   };

   /* 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 _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 _offlg */
   #define IP6F_RESERVED_MASK  0x0600  /* reserved bits in ip6f_offlg */
   #define IP6F_MORE_FRAG      0x0100  /* more-fragments flag */
   #endif

   Defined constants for fields larger than 1 byte depend on the byte
   ordering that is used.  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().

   (Note: We show an implementation that supports both big-endian and
   little-endian byte ordering, assuming a hypothetical compile-time #if
   test to determine the byte ordering.  The constant that we show,





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   BYTE_ORDER, with values of BIG_ENDIAN and LITTLE_ENDIAN, are for
   example purposes only.  If an implementation runs on only one type of
   hardware it need only define the set of constants for that hardware's
   byte ordering.)

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



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#define ICMP6_MEMBERSHIP_QUERY      130
#define ICMP6_MEMBERSHIP_REPORT     131
#define ICMP6_MEMBERSHIP_REDUCTION  132

#define ICMP6_DST_UNREACH_NOROUTE     0 /* no route to destination */
#define ICMP6_DST_UNREACH_ADMIN       1 /* communication with */
                                        /* destination */
                                        /* administratively */
                                        /* prohibited */
#define ICMP6_DST_UNREACH_NOTNEIGHBOR 2 /* not a neighbor */
#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 Type and Code Values

   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]

   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 */



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       /* 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

   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 */



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   };

   #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 */
   };

   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;
   };




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

2.3.  Address Testing Macros

   The basic API ([RFC-2133]) 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 *);

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.

   We define the following protocol names with the values shown.  These
   are taken from ftp://ftp.isi.edu/in-notes/iana/assignments/protocol-
   numbers.

       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





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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 typically used the host's byte order.

   Another difference from IPv4 raw sockets is that complete packets
   (that is, IPv6 packets with extension headers) cannot be read or
   written using the IPv6 raw sockets API.  Instead, ancillary data
   objects are used to transfer the extension headers, as described
   later in this document.  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 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 in a received IPv6 header (other
   than the version number and Next Header fields) and all extension
   headers are also made available to the application as ancillary data
   on input.  Hence there is no need for a socket option similar to the
   IPv4 IP_HDRINCL socket option.

   When writing to a raw socket the kernel will automatically fragment
   the packet if its size exceeds the path MTU, inserting the required
   fragmentation 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 PF_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.  We note that this value has no special meaning to
   an IPv6 raw socket (and the IANA currently reserves the value of 255



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   when used as a next-header field).  (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.)

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-
   1883].  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));

   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.

   (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 group membership messages.





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   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> header.

   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 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.




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   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(PF_INET6, SOCK_RAW, IPPROTO_ICMPV6);

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

   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))





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   (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.)

4.  Ancillary Data

   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.1g sockets API standard (which should be completed during
   1997).

   In this document ancillary data is used to exchange the following
   optional information between the application and the kernel:

       1.  the send/receive interface and source/destination address,
       2.  the hop limit,
       3.  next hop address,
       4.  Hop-by-Hop options,
       5.  Destination options, and
       6.  Routing header.

   Before describing these uses in detail, we review the definition of
   the msghdr structure itself, the cmsghdr structure that defines an
   ancillary data object, and some functions that operate on the
   ancillary data objects.






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4.1.  The msghdr Structure

   The msghdr structure is used by the recvmsg() and sendmsg()
   functions.  Its Posix.1g 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 */
      size_t     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.1g the two "void *" pointers were typically "char
   *", and the two socklen_t members and the size_t member were
   typically integers.  Earlier drafts of Posix.1g had the two socklen_t
   members as size_t, but Draft 6.6 of Posix.1g, apparently the final
   draft, changed these to socklen_t to simplify binary portability for
   64-bit implementations and to align Posix.1g 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 Type 0 Routing header is 376 bytes.)

4.2.  The cmsghdr Structure

   The cmsghdr structure describes ancillary data objects transferred by
   recvmsg() and sendmsg().  Its Posix.1g 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>.



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   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 shortly.  Nevertheless, it is common
   to refer to the cmsg_data[] member.

   (Note: Before Posix.1g 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.)

   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.

|<--------------------------- msg_controllen -------------------------->|
|                                                                       |
|<----- ancillary data object ----->|<----- ancillary data object ----->|
|<---------- CMSG_SPACE() --------->|<---------- CMSG_SPACE() --------->|
|                                   |                                   |
|<---------- cmsg_len ---------->|  |<--------- cmsg_len ----------->|  |
|<--------- CMSG_LEN() --------->|  |<-------- CMSG_LEN() ---------->|  |
|                                |  |                                |  |
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
|cmsg_|cmsg_|cmsg_|XX|           |XX|cmsg_|cmsg_|cmsg_|XX|           |XX|
|len  |level|type |XX|cmsg_data[]|XX|len  |level|type |XX|cmsg_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.

4.3.  Ancillary Data Object Macros

   To aid in the manipulation of ancillary data objects, three macros
   from 4.4BSD are defined by Posix.1g: 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;



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    /* fill in msg */

    /* call recvmsg() */

    for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
         cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
        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.1g 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>.

4.3.1.  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 the by msghdr structure (that
   is, if either msg_control is NULL or if msg_controllen is less than
   the size of a cmsghdr structure).

   One possible implementation could be

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

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







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4.3.2.  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.  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_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((cmsg)->cmsg_len) \
                             + ALIGN(sizeof(struct cmsghdr)) > \
            (u_char *)((mhdr)->msg_control) + (mhdr)->msg_controllen) ? \
           (struct cmsghdr *)NULL : \
           (struct cmsghdr *)((u_char *)(cmsg) + ALIGN((cmsg)->cmsg_len))) )

   The macro ALIGN(), which is implementation dependent, rounds its
   argument up to the next even multiple of whatever alignment is
   required (probably a multiple of 4 or 8 bytes).




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4.3.3.  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(sizeof(struct cmsghdr)) )

4.3.4.  CMSG_SPACE

       unsigned int CMSG_SPACE(unsigned int length);

   This macro is new with this API.  Given the length of an ancillary
   data object, CMSG_SPACE() returns 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, to
   allocate 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.

   One possible implementation could be:

       #define CMSG_SPACE(length) ( ALIGN(sizeof(struct cmsghdr)) + \
                                    ALIGN(length) )

4.3.5.  CMSG_LEN

       unsigned int CMSG_LEN(unsigned int 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.

   One possible implementation could be:

       #define CMSG_LEN(length) ( ALIGN(sizeof(struct cmsghdr)) + length
       )








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   Note the difference between CMSG_SPACE() and CMSG_LEN(), shown also
   in the figure in Section 4.2: 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.

4.4.  Summary of Options Described Using Ancillary Data

   There are six types of optional information described in this
   document that are passed between the application and the kernel using
   ancillary data:

       1.  the send/receive interface and source/destination address,
       2.  the hop limit,
       3.  next hop address,
       4.  Hop-by-Hop options,
       5.  Destination options, and
       6.  Routing header.

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

       int  on = 1;

       setsockopt(fd, IPPROTO_IPV6, IPV6_PKTINFO,  &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_HOPLIMIT, &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_HOPOPTS,  &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_DSTOPTS,  &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_RTHDR,    &on, sizeof(on));

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

   Nothing special need be done to send any of this optional
   information; the application just calls sendmsg() and specifies one
   or more ancillary data objects as control information.

   We also summarize the three cmsghdr fields that describe the
   ancillary data objects:

       cmsg_level    cmsg_type      cmsg_data[]               #times
       ------------  ------------   ------------------------  ------
       IPPROTO_IPV6  IPV6_PKTINFO   in6_pktinfo structure     once
       IPPROTO_IPV6  IPV6_HOPLIMIT  int                       once
       IPPROTO_IPV6  IPV6_NEXTHOP   socket address structure  once
       IPPROTO_IPV6  IPV6_HOPOPTS   implementation dependent  mult.



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       IPPROTO_IPV6  IPV6_DSTOPTS   implementation dependent  mult.
       IPPROTO_IPV6  IPV6_RTHDR     implementation dependent  once

   The final column indicates how many times an ancillary data object of
   that type can appear as control information.  The Hop-by-Hop and
   Destination options can appear multiple times, while all the others
   can appear only one time.

   All these options are described in detail in following sections.  All
   the constants beginning with IPV6_ are defined as a result of
   including the <netinet/in.h> header.

   (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").  This is
   consistent with the existing use of ancillary data in 4.4BSD:
   returning the destination address of an IPv4 datagram.)

   (Note: It is up to the implementation what it passes as ancillary
   data for the Hop-by-Hop option, Destination option, and Routing
   header option, since the API to these features is through a set of
   inet6_option_XXX() and inet6_rthdr_XXX() functions that we define
   later.  These functions serve two purposes: to simplify the interface
   to these features (instead of requiring the application to know the
   intimate details of the extension header formats), and to hide the
   actual implementation from the application.  Nevertheless, we show
   some examples of these features that store the actual extension
   header as the ancillary data.  Implementations need not use this
   technique.)

4.5.  IPV6_PKTOPTIONS Socket Option

   The summary in the previous section assumes a UDP socket.  Sending
   and receiving ancillary data is easy with UDP: the application calls
   sendmsg() and recvmsg() instead of sendto() and recvfrom().

   But there might be cases where a TCP application wants to send or
   receive this optional information.  For example, a TCP client might
   want to specify a Routing header and this needs to be done before
   calling connect().  Similarly a TCP server might want to know the
   received interface after accept() returns along with any Destination
   options.







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   A new socket option is defined that provides access to the optional
   information described in the previous section, but without using
   recvmsg() and sendmsg().  Setting the socket option specifies any of
   the optional output fields:

       setsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, len);

   The fourth argument points to a buffer containing one or more
   ancillary data objects, and the fifth argument is the total length of
   all these objects.  The application fills in this buffer exactly as
   if the buffer were being passed to sendmsg() as control information.

   The options set by calling setsockopt() for IPV6_PKTOPTIONS are
   called "sticky" options because once set they apply to all packets
   sent on that socket.  The application can call setsockopt() again to
   change all the sticky options, or it can call setsockopt() with a
   length of 0 to remove all the sticky options for the socket.

   The corresponding receive option

       getsockopt(fd, IPPROTO_IPV6, IPV6_PKTOPTIONS, &buf, &len);

   returns a buffer with one or more ancillary data objects for all the
   optional receive information that the application has previously
   specified that it wants to receive.  The fourth argument points to
   the buffer that is filled in by the call.  The fifth argument is a
   pointer to a value-result integer: when the function is called the
   integer specifies the size of the buffer pointed to by the fourth
   argument, and on return this integer contains the actual number of
   bytes that were returned.  The application processes this buffer
   exactly as if the buffer were returned by recvmsg() as control
   information.

   To simplify this document, in the remaining sections when we say "can
   be specified as ancillary data to sendmsg()" we mean "can be
   specified as ancillary data to sendmsg() or specified as a sticky
   option using setsockopt() and the IPV6_PKTOPTIONS socket option".
   Similarly when we say "can be returned as ancillary data by
   recvmsg()" we mean "can be returned as ancillary data by recvmsg() or
   returned by getsockopt() with the IPV6_PKTOPTIONS socket option".

4.5.1.  TCP Sticky Options

   When using getsockopt() with the IPV6_PKTOPTIONS option and a TCP
   socket, only the options from the most recently received segment are
   retained and returned to the caller, and only after the socket option
   has been set.  That is, TCP need not start saving a copy of the
   options until the application says to do so.



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   The application is not allowed to specify ancillary data in a call to
   sendmsg() on a TCP socket, and none of the ancillary data that we
   describe in this document is ever returned as control information by
   recvmsg() on a TCP socket.

4.5.2.  UDP and Raw Socket Sticky Options

   The IPV6_PKTOPTIONS socket option can also be used with a UDP socket
   or with a raw IPv6 socket, normally to set some of the options once,
   instead of with each call to sendmsg().

   Unlike the TCP case, the sticky options can be overridden on a per-
   packet basis with ancillary data specified in a call to sendmsg() on
   a UDP or raw IPv6 socket.  If any ancillary data is specified in a
   call to sendmsg(), none of the sticky options are sent with that
   datagram.

5.  Packet Information

   There are four 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, and
       4.  the next hop address.

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

       1.  the destination IPv6 address,
       2.  the arriving interface index, and
       3.  the arriving hop limit.

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

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

   In the cmsghdr structure containing this ancillary data, the
   cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
   IPV6_PKTINFO, and the first byte of cmsg_data[] will be the first
   byte of the in6_pktinfo structure.



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   This information is returned as ancillary data by recvmsg() only if
   the application has enabled the IPV6_PKTINFO socket option:

       int  on = 1;
       setsockopt(fd, IPPROTO_IPV6, IPV6_PKTINFO, &on, sizeof(on));

   Nothing special need be done to send this information: just specify
   the control information as ancillary data for sendmsg().

   (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_PKTINFO 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.)

5.1.  Specifying/Receiving the Interface

   Interfaces on an IPv6 node are identified by a small positive
   integer, as described in Section 4 of [RFC-2133].  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.  If the application
   specifies an outgoing interface for a multicast packet, the interface
   specified by the ancillary data overrides any interface specified by
   the IPV6_MULTICAST_IF socket option (described in [RFC-2133]), for
   that call to sendmsg() only.

   When the IPV6_PKTINFO socket option is enabled, the received
   interface index is always returned as the ipi6_ifindex member of the
   in6_pktinfo structure.

5.2.  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



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

5.3.  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-2133].  Specifying the
   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 programs
   such as Traceroute and 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_HOPLIMIT socket option:

       int  on = 1;
       setsockopt(fd, IPPROTO_IPV6, IPV6_HOPLIMIT, &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-2133], the interpretation of the integer hop limit
   value is



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

5.4.  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.)

   If the socket address structure contains an IPv6 address (e.g., the
   sin6_family member is AF_INET6), then 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.

5.5.  Additional Errors with sendmsg()

   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().  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 ifi6_ifindex.







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6.  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.

   Today only three Hop-by-Hop options are defined for IPv6 [RFC-1883]:
   Jumbo Payload, Pad1, and PadN, although a proposal exists for a
   router-alert Hop-by-Hop option.  The Jumbo Payload option should not
   be passed back to an application and an application should receive an
   error if it attempts to set it.  This option is processed entirely by
   the kernel.  It is indirectly specified by datagram-based
   applications as the size of the datagram to send and indirectly
   passed back to these applications as the length of the received
   datagram.  The two pad options are for alignment purposes and are
   automatically inserted by a sending kernel when needed and ignored by

   the receiving kernel.  This section of the API is therefore defined
   for future Hop-by-Hop options that an application may need to specify
   and receive.

   Individual Hop-by-Hop options (and Destination options, which are
   described shortly, and which are 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 A of [RFC-1883].  The alignment
   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.  Whatever code builds
   either a Hop-by-Hop options header or a Destination options header
   must know the values of x and y for each option.

   Multiple Hop-by-Hop options can be specified by the application.
   Normally one ancillary data object describes all the Hop-by-Hop
   options (since each option is itself TLV-encoded) but the application
   can specify multiple ancillary data objects for the Hop-by-Hop
   options, each object specifying one or more options.  Care must be
   taken designing the API for these options since

   1.   it may be possible for some future Hop-by-Hop options to be
        generated by the application and processed entirely by the
        application (e.g., the kernel may not know the alignment
        restrictions for the option),



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   2.   it must be possible for the kernel to insert its own Hop-by-Hop
        options in an outgoing packet (e.g., the Jumbo Payload option),

   3.   the application can place one or more Hop-by-Hop options into a
        single ancillary data object,

   4.   if the application specifies multiple ancillary data objects,
        each containing one or more Hop-by-Hop options, the kernel must
        combine these a single Hop-by-Hop options header, and

   5.   it must be possible for the kernel to remove some Hop-by-Hop
        options from a received packet before returning the remaining
        Hop-by-Hop options to the application.  (This removal might
        consist of the kernel converting the option into a pad option of
        the same length.)

   Finally, we note that access to some Hop-by-Hop options or to 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.

6.1.  Receiving Hop-by-Hop Options

   To receive Hop-by-Hop options the application must enable the
   IPV6_HOPOPTS socket option:

       int  on = 1;
       setsockopt(fd, IPPROTO_IPV6, IPV6_HOPOPTS, &on, sizeof(on));

   All the Hop-by-Hop options are returned as one ancillary data object
   described by a cmsghdr structure.  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_option_next() and
   inet6_option_find() functions, described shortly.

6.2.  Sending Hop-by-Hop Options

   To send one or more Hop-by-Hop options, the application just
   specifies them as ancillary data in a call to sendmsg().  No socket
   option need be set.

   Normally all the Hop-by-Hop options are specified by a single
   ancillary data object.  Multiple ancillary data objects, each
   containing one or more Hop-by-Hop options, can also be specified, in
   which case the kernel will combine all the Hop-by-Hop options into a
   single Hop-by-Hop extension header.  But it should be more efficient
   to use a single ancillary data object to describe all the Hop-by-Hop



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   options.  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_option_init(), inet6_option_append(), and
   inet6_option_alloc() functions, described shortly.

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

6.3.  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.  The function prototypes for
   these functions are all in the <netinet/in.h> header.

6.3.1.  inet6_option_space

       int inet6_option_space(int nbytes);

   This function returns the number of bytes required to hold an option
   when it is stored as ancillary data, including the cmsghdr structure
   at the beginning, and any padding at the end (to make its size a
   multiple of 8 bytes).  The argument is the size of the structure
   defining the option, which must include any pad bytes at the
   beginning (the value y in the alignment term "xn + y"), the type
   byte, the length byte, and the option data.

   (Note: If multiple options are stored in a single ancillary data
   object, which is the recommended technique, this function
   overestimates the amount of space required by the size of N-1 cmsghdr
   structures, where N is the number of options to be stored in the
   object.  This is of little consequence, since it is assumed that most
   Hop-by-Hop option headers and Destination option headers carry only
   one option (p. 33 of [RFC-1883]).)

6.3.2.  inet6_option_init

       int inet6_option_init(void *bp, struct cmsghdr **cmsgp, int
       type);

   This function is called once per ancillary data object that will
   contain either Hop-by-Hop or Destination options.  It returns 0 on
   success or -1 on an error.

   bp is a pointer to previously allocated space that will contain the
   ancillary data object.  It must be large enough to contain all the
   individual options to be added by later calls to
   inet6_option_append() and inet6_option_alloc().



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   cmsgp is a pointer to a pointer to a cmsghdr structure.  *cmsgp is
   initialized by this function to point to the cmsghdr structure
   constructed by this function in the buffer pointed to by bp.

   type is either IPV6_HOPOPTS or IPV6_DSTOPTS.  This type is stored in
   the cmsg_type member of the cmsghdr structure pointed to by *cmsgp.

6.3.3.  inet6_option_append


       int inet6_option_append(struct cmsghdr *cmsg, const uint8_t *typep,
                               int multx, int plusy);

   This function appends a Hop-by-Hop option or a Destination option
   into an ancillary data object that has been initialized by
   inet6_option_init().  This function returns 0 if it succeeds or -1 on
   an error.

   cmsg is a pointer to the cmsghdr structure that must have been
   initialized by inet6_option_init().

   typep is a pointer to the 8-bit option type.  It is assumed that this
   field is immediately followed by the 8-bit option data length field,
   which is then followed immediately by the option data.  The caller
   initializes these three fields (the type-length-value, or TLV) before
   calling this function.

   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.

   multx is the value x in the alignment term "xn + y" described
   earlier.  It must have a value of 1, 2, 4, or 8.

   plusy is the value y in the alignment term "xn + y" described
   earlier.  It must have a value between 0 and 7, inclusive.

6.3.4.  inet6_option_alloc

       uint8_t *inet6_option_alloc(struct cmsghdr *cmsg, int datalen,
                                    int multx, int plusy);








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   This function appends a Hop-by-Hop option or a Destination option
   into an ancillary data object that has been initialized by
   inet6_option_init().  This function returns a pointer to the 8-bit
   option type field that starts the option on success, or NULL on an
   error.

   The difference between this function and inet6_option_append() is
   that the latter copies the contents of a previously built option into
   the ancillary data object while the current function returns a
   pointer to the space in the data object where the option's TLV must
   then be built by the caller.

   cmsg is a pointer to the cmsghdr structure that must have been
   initialized by inet6_option_init().

   datalen is the value of the option data length byte for this option.
   This value is required as an argument to allow the function to
   determine if padding must be appended at the end of the option.  (The
   inet6_option_append() function does not need a data length argument
   since the option data length must already be stored by the caller.)

   multx is the value x in the alignment term "xn + y" described
   earlier.  It must have a value of 1, 2, 4, or 8.

   plusy is the value y in the alignment term "xn + y" described
   earlier.  It must have a value between 0 and 7, inclusive.

6.3.5.  inet6_option_next

       int inet6_option_next(const struct cmsghdr *cmsg, uint8_t
       **tptrp);

   This function processes the next Hop-by-Hop option or Destination
   option in an ancillary data object.  If another option remains to be
   processed, the return value of the function is 0 and *tptrp points to
   the 8-bit option type field (which is followed by the 8-bit option
   data length, followed by the option data).  If no more options remain
   to be processed, the return value is -1 and *tptrp is NULL.  If an
   error occurs, the return value is -1 and *tptrp is not NULL.

   cmsg is a pointer to cmsghdr structure of which cmsg_level equals
   IPPROTO_IPV6 and cmsg_type equals either IPV6_HOPOPTS or
   IPV6_DSTOPTS.

   tptrp is a pointer to a pointer to an 8-bit byte and *tptrp is used
   by the function to remember its place in the ancillary data object
   each time the function is called.  The first time this function is
   called for a given ancillary data object, *tptrp must be set to NULL.



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   Each time this function returns success, *tptrp points to the 8-bit
   option type field for the next option to be processed.

6.3.6.  inet6_option_find

       int inet6_option_find(const struct cmsghdr *cmsg, uint8_t *tptrp,
                             int type);

   This function is similar to the previously described
   inet6_option_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 ancillary data object.  cmsg is a
   pointer to cmsghdr structure of which cmsg_level equals IPPROTO_IPV6
   and cmsg_type equals either IPV6_HOPOPTS or IPV6_DSTOPTS.

   tptrp is a pointer to a pointer to an 8-bit byte and *tptrp is used
   by the function to remember its place in the ancillary data object
   each time the function is called.  The first time this function is
   called for a given ancillary data object, *tptrp must be set to NULL.

   This function starts searching for an option of the specified type
   beginning after the value of *tptrp.  If an option of the specified
   type is located, this function returns 0 and *tptrp points to the 8-
   bit option type field for the option of the specified type.  If an
   option of the specified type is not located, the return value is -1
   and *tptrp is NULL.  If an error occurs, the return value is -1 and
   *tptrp is not NULL.

6.3.7.  Options Examples

   We now provide an example that builds two Hop-by-Hop options.  First
   we define two options, called X and Y, taken from the example in
   Appendix A of [RFC-1883].  We assume that all options will have
   structure definitions similar to what is shown below.

        /* option X and option Y are defined in [RFC-1883], pp. 33-34 */
#define IP6_X_OPT_TYPE       X   /* replace X with assigned value */
#define IP6_X_OPT_LEN       12
#define IP6_X_OPT_MULTX      8   /* 8n + 2 alignment */
#define IP6_X_OPT_OFFSETY    2

struct ip6_X_opt {
  uint8_t   ip6_X_opt_pad[IP6_X_OPT_OFFSETY];
  uint8_t   ip6_X_opt_type;
  uint8_t   ip6_X_opt_len;
  uint32_t  ip6_X_opt_val1;
  uint64_t  ip6_X_opt_val2;
};



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#define IP6_Y_OPT_TYPE       Y   /* replace Y with assigned value */
#define IP6_Y_OPT_LEN        7
#define IP6_Y_OPT_MULTX      4   /* 4n + 3 alignment */
#define IP6_Y_OPT_OFFSETY    3

struct ip6_Y_opt {
  uint8_t   ip6_Y_opt_pad[IP6_Y_OPT_OFFSETY];
  uint8_t   ip6_Y_opt_type;
  uint8_t   ip6_Y_opt_len;
  uint8_t   ip6_Y_opt_val1;
  uint16_t  ip6_Y_opt_val2;
  uint32_t  ip6_Y_opt_val3;
};

   We now show the code fragment to build one ancillary data object
   containing both options.

struct msghdr  msg;
struct cmsghdr  *cmsgptr;
struct ip6_X_opt  optX;
struct ip6_Y_opt  optY;

msg.msg_control = malloc(inet6_option_space(sizeof(optX) +
                                            sizeof(optY)));

inet6_option_init(msg.msg_control, &cmsgptr, IPV6_HOPOPTS);

optX.ip6_X_opt_type = IP6_X_OPT_TYPE;
optX.ip6_X_opt_len  = IP6_X_OPT_LEN;
optX.ip6_X_opt_val1 = <32-bit value>;
optX.ip6_X_opt_val2 = <64-bit value>;
inet6_option_append(cmsgptr, &optX.ip6_X_opt_type,
                    IP6_X_OPT_MULTX, IP6_X_OPT_OFFSETY);

optY.ip6_Y_opt_type = IP6_Y_OPT_TYPE;
optY.ip6_Y_opt_len  = IP6_Y_OPT_LEN;
optY.ip6_Y_opt_val1 = <8-bit value>;
optY.ip6_Y_opt_val2 = <16-bit value>;
optY.ip6_Y_opt_val3 = <32-bit value>;
inet6_option_append(cmsgptr, &optY.ip6_Y_opt_type,
                    IP6_Y_OPT_MULTX, IP6_Y_OPT_OFFSETY);

msg.msg_controllen = cmsgptr->cmsg_len;

   The call to inet6_option_init() builds the cmsghdr structure in the
   control buffer.





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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = CMSG_LEN(0) = 12                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_HOPOPTS                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Here we assume a 32-bit architecture where sizeof(struct cmsghdr)
   equals 12, with a desired alignment of 4-byte boundaries (that is,
   the ALIGN() macro shown in the sample implementations of the
   CMSG_xxx() macros rounds up to a multiple of 4).

   The first call to inet6_option_append() appends the X option.  Since
   this is the first option in the ancillary data object, 2 bytes are
   allocated for the Next Header byte and for the Hdr Ext Len byte.  The
   former will be set by the kernel, depending on the type of header
   that follows this header, and the latter byte is set to 1.  These 2
   bytes form the 2 bytes of padding (IP6_X_OPT_OFFSETY) required at the
   beginning of this option.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = 28                                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_HOPOPTS                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         4-octet field                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                         8-octet field                         +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The cmsg_len member of the cmsghdr structure is incremented by 16,
   the size of the option.

   The next call to inet6_option_append() appends the Y option to the
   ancillary data object.









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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = 44                                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_HOPOPTS                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  | Hdr Ext Len=3 | Option Type=X |Opt Data Len=12|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         4-octet field                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                         8-octet field                         +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PadN Option=1 |Opt Data Len=1 |       0       | Option Type=Y |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Opt Data Len=7 | 1-octet field |         2-octet field         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         4-octet field                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PadN Option=1 |Opt Data Len=2 |       0       |       0       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   16 bytes are appended by this function, so cmsg_len becomes 44.  The
   inet6_option_append() function notices that the appended data
   requires 4 bytes of padding at the end, to make the size of the
   ancillary data object a multiple of 8, and appends the PadN option
   before returning.  The Hdr Ext Len byte is incremented by 2 to become
   3.

   Alternately, the application could build two ancillary data objects,
   one per option, although this will probably be less efficient than
   combining the two options into a single ancillary data object (as
   just shown).  The kernel must combine these into a single Hop-by-Hop
   extension header in the final IPv6 packet.

       struct msghdr  msg;
       struct cmsghdr  *cmsgptr;
       struct ip6_X_opt  optX;
       struct ip6_Y_opt  optY;

       msg.msg_control = malloc(inet6_option_space(sizeof(optX)) +
                                inet6_option_space(sizeof(optY)));

       inet6_option_init(msg.msg_control, &cmsgptr, IPPROTO_HOPOPTS);

       optX.ip6_X_opt_type = IP6_X_OPT_TYPE;



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       optX.ip6_X_opt_len  = IP6_X_OPT_LEN;
       optX.ip6_X_opt_val1 = <32-bit value>;
       optX.ip6_X_opt_val2 = <64-bit value>;
       inet6_option_append(cmsgptr, &optX.ip6_X_opt_type,
                           IP6_X_OPT_MULTX, IP6_X_OPT_OFFSETY);
       msg.msg_controllen = CMSG_SPACE(sizeof(optX));

       inet6_option_init((u_char *)msg.msg_control + msg.msg_controllen,
                         &cmsgptr, IPPROTO_HOPOPTS);

       optY.ip6_Y_opt_type = IP6_Y_OPT_TYPE;
       optY.ip6_Y_opt_len  = IP6_Y_OPT_LEN;
       optY.ip6_Y_opt_val1 = <8-bit value>;
       optY.ip6_Y_opt_val2 = <16-bit value>;
       optY.ip6_Y_opt_val3 = <32-bit value>;
       inet6_option_append(cmsgptr, &optY.ip6_Y_opt_type,
                           IP6_Y_OPT_MULTX, IP6_Y_OPT_OFFSETY);
       msg.msg_controllen += cmsgptr->cmsg_len;

   Each call to inet6_option_init() builds a new cmsghdr structure, and
   the final result looks like the following:






























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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = 28                                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_HOPOPTS                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         4-octet field                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                         8-octet field                         +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = 28                                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_HOPOPTS                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  | Hdr Ext Len=1 | Pad1 Option=0 | Option Type=Y |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Opt Data Len=7 | 1-octet field |         2-octet field         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         4-octet field                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PadN Option=1 |Opt Data Len=2 |       0       |       0       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   When the kernel combines these two options into a single Hop-by-Hop
   extension header, the first 3 bytes of the second ancillary data
   object (the Next Header byte, the Hdr Ext Len byte, and the Pad1
   option) will be combined into a PadN option occupying 3 bytes.

   The following code fragment is a redo of the first example shown
   (building two options in a single ancillary data object) but this
   time we use inet6_option_alloc().

uint8_t  *typep;
struct msghdr  msg;
struct cmsghdr  *cmsgptr;
struct ip6_X_opt  *optXp;  /* now a pointer, not a struct */
struct ip6_Y_opt  *optYp;  /* now a pointer, not a struct */

msg.msg_control = malloc(inet6_option_space(sizeof(*optXp) +
                                            sizeof(*optYp)));




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inet6_option_init(msg.msg_control, &cmsgptr, IPV6_HOPOPTS);

typep = inet6_option_alloc(cmsgptr, IP6_X_OPT_LEN,
                           IP6_X_OPT_MULTX, IP6_X_OPT_OFFSETY);
optXp = (struct ip6_X_opt *) (typep - IP6_X_OPT_OFFSETY);
optXp->ip6_X_opt_type = IP6_X_OPT_TYPE;
optXp->ip6_X_opt_len  = IP6_X_OPT_LEN;
optXp->ip6_X_opt_val1 = <32-bit value>;
optXp->ip6_X_opt_val2 = <64-bit value>;

typep = inet6_option_alloc(cmsgptr, IP6_Y_OPT_LEN,
                           IP6_Y_OPT_MULTX, IP6_Y_OPT_OFFSETY);
optYp = (struct ip6_Y_opt *) (typep - IP6_Y_OPT_OFFSETY);
optYp->ip6_Y_opt_type = IP6_Y_OPT_TYPE;
optYp->ip6_Y_opt_len  = IP6_Y_OPT_LEN;
optYp->ip6_Y_opt_val1 = <8-bit value>;
optYp->ip6_Y_opt_val2 = <16-bit value>;
optYp->ip6_Y_opt_val3 = <32-bit value>;

msg.msg_controllen = cmsgptr->cmsg_len;

   Notice that inet6_option_alloc() returns a pointer to the 8-bit
   option type field.  If the program wants a pointer to an option
   structure that includes the padding at the front (as shown in our
   definitions of the ip6_X_opt and ip6_Y_opt structures), the y-offset
   at the beginning of the structure must be subtracted from the
   returned pointer.

   The following code fragment shows the processing of Hop-by-Hop
   options using the inet6_option_next() function.

    struct msghdr   msg;
    struct cmsghdr  *cmsgptr;

    /* fill in msg */

    /* call recvmsg() */

    for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
         cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
        if (cmsgptr->cmsg_level == IPPROTO_IPV6 &&
            cmsgptr->cmsg_type == IPV6_HOPOPTS) {

            uint8_t  *tptr = NULL;

            while (inet6_option_next(cmsgptr, &tptr) == 0) {
                if (*tptr == IP6_X_OPT_TYPE) {
                    struct ip6_X_opt  *optXp;



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                    optXp = (struct ip6_X_opt *) (tptr - IP6_X_OPT_OFFSETY);
                    <do whatever with> optXp->ip6_X_opt_val1;
                    <do whatever with> optXp->ip6_X_opt_val2;

                } else if (*tptr == IP6_Y_OPT_TYPE) {
                    struct ip6_Y_opt  *optYp;

                    optYp = (struct ip6_Y_opt *) (tptr - IP6_Y_OPT_OFFSETY);
                    <do whatever with> optYp->ip6_Y_opt_val1;
                    <do whatever with> optYp->ip6_Y_opt_val2;
                    <do whatever with> optYp->ip6_Y_opt_val3;
                }
            }
            if (tptr != NULL)
                <error encountered by inet6_option_next()>;
        }
    }

7.  Destination Options

   A variable number of Destination options can appear in one or more
   Destination option headers.  As defined in [RFC-1883], 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 appearing after 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.

   Today no Destination options are defined for IPv6 [RFC-1883],
   although proposals exist to use Destination options with mobility and
   anycasting.

7.1.  Receiving Destination Options

   To receive Destination options the application must enable the
   IPV6_DSTOPTS socket option:

       int  on = 1;
       setsockopt(fd, IPPROTO_IPV6, IPV6_DSTOPTS, &on, sizeof(on));

   All the Destination options appearing before a Routing header are
   returned as one ancillary data object described by a cmsghdr
   structure and all the Destination options appearing after a Routing
   header are returned as another ancillary data object described by a
   cmsghdr structure.  For these ancillary data objects, the cmsg_level





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   member will be IPPROTO_IPV6 and the cmsg_type member will be
   IPV6_HOPOPTS.  These options are then processed by calling the
   inet6_option_next() and inet6_option_find() functions.

7.2.  Sending Destination Options

   To send one or more Destination options, the application just
   specifies them as ancillary data in a call to sendmsg().  No socket
   option need be set.

   As described earlier, one set of Destination options can appear
   before a Routing header, and one set can appear after a Routing
   header.  Each set can consist of one or more options.

   Normally all the Destination options in a set are specified by a
   single ancillary data object, since each option is itself TLV-
   encoded.  Multiple ancillary data objects, each containing one or
   more Destination options, can also be specified, in which case the
   kernel will combine all the Destination options in the set into a
   single Destination extension header.  But it should be more efficient
   to use a single ancillary data object to describe all the Destination
   options in a set.  The cmsg_level member is set to IPPROTO_IPV6 and
   the cmsg_type member is set to IPV6_DSTOPTS.  The option is normally
   constructed using the inet6_option_init(), inet6_option_append(), and
   inet6_option_alloc() functions.

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

8.  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-1883].  This type supports up to 23 intermediate nodes.  With
   this maximum number of intermediate nodes, a source, and a
   destination, there are 24 hops, each of which is defined as a strict
   or loose hop.

   Source routing with 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 eight
   functions that the application calls to build and examine a Routing
   header.  Four functions build a Routing header:

   inet6_rthdr_space()    - return #bytes required for ancillary data
   inet6_rthdr_init()     - initialize ancillary data for Routing header



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   inet6_rthdr_add()      - add IPv6 address & flags to Routing header
   inet6_rthdr_lasthop()  - specify the flags for the final hop

   Four functions deal with a returned Routing header:

    inet6_rthdr_reverse()  - reverse a Routing header
    inet6_rthdr_segments() - return #segments in a Routing header
    inet6_rthdr_getaddr()  - fetch one address from a Routing header
    inet6_rthdr_getflags() - fetch one flag from a Routing header

   The function prototypes for these functions are all in the
   <netinet/in.h> header.

   To receive a Routing header the application must enable the
   IPV6_RTHDR socket option:

       int  on = 1;
       setsockopt(fd, IPPROTO_IPV6, IPV6_RTHDR, &on, sizeof(on));

   To send a Routing header the application just specifies it as
   ancillary data in a call to sendmsg().

   A Routing header is passed between the application and the kernel as
   an ancillary data object.  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 eight functions that we are about to describe.

   The following constants are defined in the <netinet/in.h> header:

    #define IPV6_RTHDR_LOOSE     0 /* this hop need not be a neighbor */
    #define IPV6_RTHDR_STRICT    1 /* this hop must be a neighbor */

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

   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.

8.1.  inet6_rthdr_space

       size_t inet6_rthdr_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




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   segments (addresses).  For an IPv6 Type 0 Routing header, the number
   of segments must be between 1 and 23, inclusive.  The return value
   includes the size of the cmsghdr structure that precedes the Routing
   header, and any required padding.

   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.)

8.2.  inet6_rthdr_init

       struct cmsghdr *inet6_rthdr_init(void *bp, int type);

   This function initializes the buffer pointed to by bp to contain a
   cmsghdr structure followed by a Routing header of the specified type.
   The cmsg_len member of the cmsghdr structure is initialized to the
   size of the structure plus the amount of space required by the
   Routing header.  The cmsg_level and cmsg_type members are also
   initialized as required.

   The caller must allocate the buffer and its size can be determined by
   calling inet6_rthdr_space().

   Upon success the return value is the pointer to the cmsghdr
   structure, and this is then used as the first argument to the next
   two functions.  Upon an error the return value is NULL.

8.3.  inet6_rthdr_add

    int inet6_rthdr_add(struct cmsghdr *cmsg,
                        const struct in6_addr *addr, unsigned int flags);

   This function adds the address pointed to by addr to the end of the
   Routing header being constructed and sets the type of this hop to the
   value of flags.  For an IPv6 Type 0 Routing header, flags must be
   either IPV6_RTHDR_LOOSE or IPV6_RTHDR_STRICT.

   If successful, the cmsg_len member of the cmsghdr structure 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.




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8.4.  inet6_rthdr_lasthop

       int inet6_rthdr_lasthop(struct cmsghdr *cmsg,
                               unsigned int flags);

   This function specifies the Strict/Loose flag for the final hop of a
   Routing header.  For an IPv6 Type 0 Routing header, flags must be
   either IPV6_RTHDR_LOOSE or IPV6_RTHDR_STRICT.

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

   Notice that a Routing header specifying N intermediate nodes requires
   N+1 Strict/Loose flags.  This requires N calls to inet6_rthdr_add()
   followed by one call to inet6_rthdr_lasthop().

8.5.  inet6_rthdr_reverse

    int inet6_rthdr_reverse(const struct cmsghdr *in, struct cmsghdr *out);

   This function takes a Routing header that was received as ancillary
   data (pointed to by the first argument) and writes a new Routing
   header that sends datagrams along the reverse of that route.  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.

8.6.  inet6_rthdr_segments

       int inet6_rthdr_segments(const struct cmsghdr *cmsg);

   This function returns the number of segments (addresses) contained in
   the Routing header described by cmsg.  On success the return value is
   between 1 and 23, inclusive.  The return value of the function is -1
   upon an error.

8.7.  inet6_rthdr_getaddr

    struct in6_addr *inet6_rthdr_getaddr(struct cmsghdr *cmsg, int
    index);

   This function returns a pointer to the IPv6 address specified by
   index (which must have a value between 1 and the value returned by
   inet6_rthdr_segments()) in the Routing header described by cmsg.  An
   application should first call inet6_rthdr_segments() to obtain the
   number of segments in the Routing header.



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

8.8.  inet6_rthdr_getflags

       int inet6_rthdr_getflags(const struct cmsghdr *cmsg, int index);

   This function returns the flags value specified by index (which must
   have a value between 0 and the value returned by
   inet6_rthdr_segments()) in the Routing header described by cmsg.  For
   an IPv6 Type 0 Routing header the return value will be either
   IPV6_RTHDR_LOOSE or IPV6_RTHDR_STRICT.

   Upon an error the return value of the function is -1.

   (Note: Addresses are indexed starting at 1, and flags starting at 0,
   to maintain consistency with the terminology and figures in [RFC-
   1883].)

8.9.  Routing Header Example

   As an example of these Routing header functions, we go through the
   function calls for the example on p. 18 of [RFC-1883].  The source is
   S, the destination is D, and the three intermediate nodes are I1, I2,
   and I3.  f0, f1, f2, and f3 are the Strict/Loose flags for each hop.

                   f0        f1        f2        f3
               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

       check: f0        f1        f2        f3

   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.
   check indicates which bit of the Strict/Loose Bit Map (0 through 3,
   specified as f0 through f3) that node checks.

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





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       void  *ptr;
       struct msghdr  msg;
       struct cmsghdr  *cmsgptr;
       struct sockaddr_in6  I1, I2, I3, D;
       unsigned int  f0, f1, f2, f3;

       ptr = malloc(inet6_rthdr_space(IPV6_RTHDR_TYPE_0, 3));
       cmsgptr = inet6_rthdr_init(ptr, IPV6_RTHDR_TYPE_0);

       inet6_rthdr_add(cmsgptr, &I1.sin6_addr, f0);
       inet6_rthdr_add(cmsgptr, &I2.sin6_addr, f1);
       inet6_rthdr_add(cmsgptr, &I3.sin6_addr, f2);
       inet6_rthdr_lasthop(cmsgptr, f3);

       msg.msg_control = ptr;
       msg.msg_controllen = cmsgptr->cmsg_len;

       /* 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).

   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].  If f0 is IPV6_RTHDR_STRICT, then I1 must be a neighbor of S.

   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_PKTINFO and IPV6_RTHDR 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) and four flags (f0, f1, f2, and f3).
   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_rthdr_segments() will be 3 and
   inet6_rthdr_getaddr(1) will return I1, inet6_rthdr_getaddr(2) will
   return I2, and inet6_rthdr_getaddr(3) will return I3, The return





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   value from inet6_rthdr_flags(0) will be f0, inet6_rthdr_flags(1) will
   return f1, inet6_rthdr_flags(2) will return f2, and
   inet6_rthdr_flags(3) will return f3.

   If the receiving application then calls inet6_rthdr_reverse(), the
   order of the three addresses will become I3, I2, and I1, and the
   order of the four Strict/Loose flags will become f3, f2, f1, and f0.

   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_rthdr_space(3) returns 68: 12 bytes for the
   cmsghdr structure and 56 bytes for the Routing header (8 + 3*16).

   The call to inet6_rthdr_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=0 | Routing Type=0|  Seg Left=0   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |             Strict/Loose Bit Map              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first call to inet6_rthdr_add() adds I1 to the list.




















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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = 36                                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_RTHDR                                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  | Hdr Ext Len=2 | Routing Type=0|  Seg Left=1   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |X|           Strict/Loose Bit Map              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[1] = I1                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bit 0 of the Strict/Loose Bit Map contains the value f0, which we
   just mark as X.  cmsg_len is incremented by 16, the Hdr Ext Len field
   is incremented by 2, and the Segments Left field is incremented by 1.

   The next call to inet6_rthdr_add() adds I2 to the list.


























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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_len = 52                                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_level = IPPROTO_IPV6                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       cmsg_type = IPV6_RTHDR                                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  | Hdr Ext Len=4 | Routing Type=0|  Seg Left=2   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |X|X|         Strict/Loose Bit Map              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[1] = I1                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[2] = I2                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The next bit of the Strict/Loose Bit Map contains the value f1.
   cmsg_len is incremented by 16, the Hdr Ext Len field is incremented
   by 2, and the Segments Left field is incremented by 1.

   The last call to inet6_rthdr_add() adds I3 to the list.


















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

   The next bit of the Strict/Loose Bit Map contains the value f2.
   cmsg_len is incremented by 16, the Hdr Ext Len field is incremented
   by 2, and the Segments Left field is incremented by 1.

   Finally, the call to inet6_rthdr_lasthop() sets the next bit of the
   Strict/Loose Bit Map to the value specified by f3.  All the lengths
   remain unchanged.








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9.  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(): Hop-by-Hop options, Destination options, and the Routing
   header.  When multiple ancillary data objects are transferred via
   sendmsg() or 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.  This API
   imposes some ordering constraints when using multiple ancillary data
   objects with sendmsg().

   When multiple IPv6 Hop-by-Hop options having the same option type are
   specified, these options will be inserted into the Hop-by-Hop options
   header in the same order as they appear in the control buffer.  But
   when multiple Hop-by-Hop options having different option types are
   specified, these options may be reordered by the kernel to reduce
   padding in the Hop-by-Hop options header.  Hop-by-Hop options may
   appear anywhere in the control buffer and will always be collected by
   the kernel and placed into a single Hop-by-Hop options header that
   immediately follows the IPv6 header.

   Similar rules apply to the Destination options: (1) those of the same
   type will appear in the same order as they are specified, and (2)
   those of differing types may be reordered.  But the kernel will build
   up to two Destination options headers: one to precede the Routing
   header and one to follow the Routing header.  If the application
   specifies a Routing header then all Destination options that appear
   in the control buffer before the Routing header will appear in a
   Destination options header before the Routing header and these
   options might be reordered, subject to the two rules that we just
   stated.  Similarly all Destination options that appear in the control
   buffer after the Routing header will appear in a Destination options
   header after the Routing header, and these options might be
   reordered, subject to the two rules that we just stated.

   As an example, assume that an application specifies control
   information to sendmsg() containing six ancillary data objects: the
   first containing two Hop-by-Hop options, the second containing one
   Destination option, the third containing two Destination options, the
   fourth containing a Routing header, the fifth containing a Hop-by-Hop
   option, and the sixth containing two Destination options.  We also
   assume that all the Hop-by-Hop options are of different types, as are
   all the Destination options.  We number these options 1-9,
   corresponding to their order in the control buffer, and show them on
   the left below.





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   In the middle we show the final arrangement of the options in the
   extension headers built by the kernel.  On the right we show the four
   ancillary data objects returned to the receiving application.

          Sender's                                       Receiver's
       Ancillary Data      -->   IPv6 Extension   -->  Ancillary Data
          Objects                    Headers              Objects
       ------------------        ---------------       --------------
       HOPOPT-1,2 (first)        HOPHDR(J,7,1,2)       HOPOPT-7,1,2
       DSTOPT-3                  DSTHDR(4,5,3)         DSTOPT-4,5,3
       DSTOPT-4,5                RTHDR(6)              RTHDR-6
       RTHDR-6                   DSTHDR(8,9)           DSTOPT-8,9
       HOPOPT-7
       DSTOPT-8,9 (last)

   The sender's two Hop-by-Hop ancillary data objects are reordered, as
   are the first two Destination ancillary data objects.  We also show a
   Jumbo Payload option (denoted as J) inserted by the kernel before the
   sender's three Hop-by-Hop options.  The first three Destination
   options must appear in a Destination header before the Routing
   header, and the final two Destination options must appear in a
   Destination header after the Routing header.

   If Destination options are specified in the control buffer after a
   Routing header, or if Destination options are specified without a
   Routing header, the kernel will place those Destination options after
   an authentication header and/or an encapsulating security payload
   header, if present.

10.  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
   the packet information (source/destination address, send/receive
   interface, and hop limit) on an IPv6 socket that is using IPv4-mapped
   IPv6 addresses.






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11.  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 IPv4/TCP socket, it can also create an
   IPv6/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.)

   (Note: We define this function only, and do not define something like
   rcmd_af() or rcmd6().  The reason is that rcmd() calls
   gethostbyname(), which returns the type of address: AF_INET or
   AF_INET6.  It should therefore be possible to modify rcmd() to
   support either IPv4 or IPv6, based on the address family returned by
   gethostbyname().)

12.  Future Items

   Some additional items may require standardization, but no concrete
   proposals have been made for the API to perform these tasks.  These
   may be addressed in a later document.

12.1.  Flow Labels

   Earlier revisions of this document specified a set of
   inet6_flow_XXX() functions to assign, share, and free IPv6 flow
   labels.  Consensus, however, indicated that it was premature to
   specify this part of the API.





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12.2.  Path MTU Discovery and UDP

   A standard method may be desirable for a UDP application to determine
   the "maximum send transport-message size" (Section 5.1 of [RFC-1981])
   to a given destination.  This would let the UDP application send
   smaller datagrams to the destination, avoiding fragmentation.

12.3.  Neighbor Reachability and UDP

   A standard method may be desirable for a UDP application to tell the
   kernel that it is making forward progress with a given peer (Section
   7.3.1 of [RFC-1970]).  This could save unneeded neighbor
   solicitations and neighbor advertisements.

13.  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_NOPORT
     <netinet/icmp6.h> ICMP6_DST_UNREACH_NOROUTE
     <netinet/icmp6.h> ICMP6_DST_UNREACH_NOTNEIGHBOR
     <netinet/icmp6.h> ICMP6_ECHO_REPLY
     <netinet/icmp6.h> ICMP6_ECHO_REQUEST
     <netinet/icmp6.h> ICMP6_INFOMSG_MASK
     <netinet/icmp6.h> ICMP6_MEMBERSHIP_QUERY
     <netinet/icmp6.h> ICMP6_MEMBERSHIP_REDUCTION
     <netinet/icmp6.h> ICMP6_MEMBERSHIP_REPORT
     <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_TIME_EXCEEDED
     <netinet/icmp6.h> ICMP6_TIME_EXCEED_REASSEMBLY
     <netinet/icmp6.h> ICMP6_TIME_EXCEED_TRANSIT
     <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



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     <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 nd_neighbor_advert{};
     <netinet/icmp6.h> struct nd_neighbor_solicit{};
     <netinet/icmp6.h> struct nd_opt_hdr{};
     <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/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_DSTOPTS
     <netinet/in.h>    IPV6_HOPLIMIT
     <netinet/in.h>    IPV6_HOPOPTS
     <netinet/in.h>    IPV6_NEXTHOP
     <netinet/in.h>    IPV6_PKTINFO
     <netinet/in.h>    IPV6_PKTOPTIONS
     <netinet/in.h>    IPV6_RTHDR
     <netinet/in.h>    IPV6_RTHDR_LOOSE
     <netinet/in.h>    IPV6_RTHDR_STRICT
     <netinet/in.h>    IPV6_RTHDR_TYPE_0
     <netinet/in.h>    struct in6_pktinfo{};

     <netinet/ip6.h>   IP6F_OFF_MASK
     <netinet/ip6.h>   IP6F_RESERVED_MASK
     <netinet/ip6.h>   IP6F_MORE_FRAG
     <netinet/ip6.h>   struct ip6_dest{};
     <netinet/ip6.h>   struct ip6_frag{};
     <netinet/ip6.h>   struct ip6_hbh{};



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     <netinet/ip6.h>   struct ip6_hdr{};
     <netinet/ip6.h>   struct ip6_rthdr{};
     <netinet/ip6.h>   struct ip6_rthdr0{};

     <sys/socket.h>    struct cmsghdr{};
     <sys/socket.h>    struct msghdr{};

   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>    uint8_t *inet6_option_alloc(struct cmsghdr *,
                                               int, int, int);
<netinet/in.h>    int inet6_option_append(struct cmsghdr *,
                                          const uint8_t *, int, int);
<netinet/in.h>    int inet6_option_find(const struct cmsghdr *,
                                        uint8_t *, int);
<netinet/in.h>    int inet6_option_init(void *, struct cmsghdr **, int);
<netinet/in.h>    int inet6_option_next(const struct cmsghdr *,
                                        uint8_t **);
<netinet/in.h>    int inet6_option_space(int);

<netinet/in.h>    int inet6_rthdr_add(struct cmsghdr *,
                                      const struct in6_addr *,
                                      unsigned int);
<netinet/in.h>    struct in6_addr inet6_rthdr_getaddr(struct cmsghdr *,
                                                      int);
<netinet/in.h>    int inet6_rthdr_getflags(const struct cmsghdr *, int);
<netinet/in.h>    struct cmsghdr *inet6_rthdr_init(void *, int);
<netinet/in.h>    int inet6_rthdr_lasthop(struct cmsghdr *,
                                                          unsigned int);
<netinet/in.h>    int inet6_rthdr_reverse(const struct cmsghdr *,
                                          struct cmsghdr *);
<netinet/in.h>    int inet6_rthdr_segments(const struct cmsghdr *);
<netinet/in.h>    size_t inet6_rthdr_space(int, int);




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<sys/socket.h>    unsigned char *CMSG_DATA(const struct cmsghdr *);
<sys/socket.h>    struct cmsghdr *CMSG_FIRSTHDR(const struct msghdr *);
<sys/socket.h>    unsigned int CMSG_LEN(unsigned int);
<sys/socket.h>    struct cmsghdr *CMSG_NXTHDR(const struct msghdr *mhdr,
                                              const struct cmsghdr *);
<sys/socket.h>    unsigned int CMSG_SPACE(unsigned int);

<unistd.h>        int rresvport_af(int *, int);

14.  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 nonprivileged
   applications.

15.  Change History

   Changes from the June 1997 Edition (-03 draft)

    -  Added a note that defined constants for multibyte fields are in
       network byte order.  This affects the ip6f_offlg member of the
       Fragment header (Section 2.1.2) and the nd_na_flags_reserved
       member of the nd_neighbor_advert structure (Section 2.2.2).

    -  Section 5: the ipi6_ifindex member of the in6_pktinfo structure
       should be "unsigned int" instead of "int", for consistency with
       the interface indexes in [RFC-2133].

    -  Section 6.3.7: the three calls to inet6_option_space() in the
       examples needed to be arguments to malloc().  The final one of
       these was missing the "6" in the name "inet6_option_space".

    -  Section 8.6: the function prototype for inet6_rthdr_segments()
       was missing the ending semicolon.

   Changes from the March 1997 Edition (-02 draft)

    -  In May 1997 Draft 6.6 of Posix 1003.1g (called Posix.1g herein)
       passed ballot and will be forwarded to the IEEE Standards Board
       later in 1997 for final approval.  Some changes made for this
       final Posix draft are incorporated into this Internet Draft,
       specifically the datatypes mentioned in Section 1 (and used
       throughout the text), and the socklen_t datatype used in Section
       4.1 and 4.2.

    -  Section 1: Added the intN_t signed datatypes, changed the
       datatype u_intN_t to uintN_t (no underscore after the "u"), and



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       removed the datatype u_intNm_t, as per Draft 6.6 of Posix.1g.

    -  Name space issues for structure and constant names in Section 2:
       Many of the structure member names and constant names were
       changed so that the prefixes are the same.  The following
       prefixes are used for structure members: "ip6_", "icmp6_", and
       "nd_".  All constants have the prefixes "ICMP6_" and "ND_".

    -  New definitions: Section 2.1.2: contains definitions for the IPv6
       extension headers, other than AH and ESP.  Section 2.2.2:
       contains additional structures and constants for the neighbor
       discovery option header and redirected header.

    -  Section 2.2.2: the enum for the neighbor discovery option field
       was changed to be a set of #define constants.

    -  Changed the word "function" to "macro" for references to all the
       uppercase names in Sections 2.3 (IN6_ARE_ADDR_EQUAL), 3.2
       (ICMPV6_FILTER_xxx), and 4.3 (CMSG_xxx).

    -  Added more protocols to the /etc/protocols file (Section 2.4) and
       changed the name of "icmpv6" to "ipv6-icmp".

    -  Section 3: Made it more explicit that an application cannot read
       or write entire IPv6 packets, that all extension headers are
       passed as ancillary data.  Added a sentence that the kernel
       fragments packets written to an IPv6 raw socket when necessary.
       Added a note that IPPROTO_RAW raw IPv6 sockets are not special.

    -  Section 3.1: Explicitly stated that the checksum option applies
       to both outgoing packets and received packets.

    -  Section 3.2: Changed the array name within the icmp6_filter
       structure from "data" to "icmp6_filt".  Changes the prefix for
       the filter macros from "ICMPV6_" to "ICMP6_", for consistency
       with the names in Section 2.2.  Changed the example from a ping
       program to a program that wants to receive only router
       advertisements.

    -  Section 4.1: Changed msg_namelen and msg_controllen from size_t
       to the Posix.1g socklen_t datatype.  Updated the Note that
       follows.

    -  Section 4.2: Changed cmsg_len from size_t to the Posix.1g
       socklen_t datatype.  Updated the Note that follows.






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    -  Section 4.4: Added a Note that the second and third arguments to
       getsockopt() and setsockopt() are intentionally the same as the
       cmsg_level and cmsg_type members.

    -  Section 4.5: Reorganized the section into a description of the
       option, followed by the TCP semantics, and the UDP and raw socket
       semantics.  Added a sentence on how to clear all the sticky
       options.  Added a note that TCP need not save the options from
       the most recently received segment until the application says to
       do so.  Added the statement that ancillary data is never passed
       with sendmsg() or recvmsg() on a TCP socket.  Simplified the
       interaction of the sticky options with ancillary data for UDP or
       raw IP: none of the sticky options are sent if ancillary data is
       specified.

    -  Final paragraph of Section 5.1: ipi6_index should be
       ipi6_ifindex.

    -  Section 5.4: Added a note on the term "privileged".

    -  Section 5.5: Noted that the errors listed are examples, and the
       actual errors depend on the implementation.

    -  Removed Section 6 ("Flow Labels") as the consensus is that it is
       premature to try and specify an API for this feature.  Access to
       the flow label field in the IPv6 header is still provided through
       the sin6_flowinfo member of the IPv6 socket address structure in
       [RFC-2133].  Added a subsection to Section 13 that this is a
       future item.

       All remaining changes are identified by their section number in
       the previous draft.  With the removal of Section 6, the section
       numbers are decremented by one.

    -  Section 7.3.7: the calls to malloc() in all three examples should
       be calls to inet6_option_space() instead.  The two calls to
       inet6_option_append() in the third example should be calls to
       inet6_option_alloc().  The two calls to CMSG_SPACE() in the first
       and third examples should be calls to CMSG_LEN().  The second
       call to CMSG_SPACE() in the second example should be a call to
       CMSG_LEN().

    -  Section 7.3.7: All the opt_X_ and opt_Y_ structure member names
       were changed to be ip6_X_opt_ and ip6_Y_opt_.  The two structure
       names ipv6_opt_X and ipv6_opt_Y were changed to ip6_X_opt and
       ip6_Y_opt.  The constants beginning with IPV6_OPT_X_ and
       IPV6_OPT_Y_ were changed to begin with IP6_X_OPT_ and IP6_Y_OPT_.




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    -  Use the term "Routing header" throughout the draft, instead of
       "source routing".  Changed the names of the eight
       inet6_srcrt_XXX() functions in Section 9 to inet6_rthdr_XXX().
       Changed the name of the socket option from IPV6_SRCRT to
       IPV6_RTHDR, and the names of the three IPV6_SRCRT_xxx constants
       in Section 9 to IPV6_RTHDR_xxx.

    -  Added a paragraph to Section 9 on how to receive and send a
       Routing header.

    -  Changed inet6_rthdr_add() and inet6_rthdr_reverse() so that they
       return -1 upon an error, instead of an Exxx errno value.

    -  In the description of inet6_rthdr_space() in Section 9.1, added
       the qualifier "For an IPv6 Type 0 Routing header" to the
       restriction of between 1 and 23 segments.

    -  Refer to final function argument in Sections 9.7 and 9.8 as
       index, not offset.

    -  Updated Section 14 with new names from Section 2.

    -  Changed the References from "[n]" to "[RFC-abcd]".

   Changes from the February 1997 Edition (-01 draft)

    -  Changed the name of the ip6hdr structure to ip6_hdr (Section 2.1)
       for consistency with the icmp6hdr structure.  Also changed the
       name of the ip6hdrctl structure contained within the ip6_hdr
       structure to ip6_hdrctl (Section 2.1).  Finally, changed the name
       of the icmp6hdr structure to icmp6_hdr (Section 2.2).  All other
       occurrences of this structure name, within the Neighbor Discovery
       structures in Section 2.2.1, already contained the underscore.

    -  The "struct nd_router_solicit" and "struct nd_router_advert"
       should both begin with "nd6_". (Section 2.2.2).

    -  Changed the name of in6_are_addr_equal to IN6_ARE_ADDR_EQUAL
       (Section 2.3) for consistency with basic API address testing
       functions.  The header defining this macro is <netinet/in.h>.

    -  getprotobyname("ipv6") now returns 41, not 0 (Section 2.4).

    -  The first occurrence of "struct icmpv6_filter" in Section 3.2
       should be "struct icmp6_filter".

    -  Changed the name of the CMSG_LENGTH() macro to CMSG_LEN()
       (Section 4.3.5), since LEN is used throughout the <netinet/*.h>



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       headers.

    -  Corrected the argument name for the sample implementations of the
       CMSG_SPACE() and CMSG_LEN() macros to be "length" (Sections 4.3.4
       and 4.3.5).

    -  Corrected the socket option mentioned in Section 5.1 to specify
       the interface for multicasting from IPV6_ADD_MEMBERSHIP to
       IPV6_MULTICAST_IF.

    -  There were numerous errors in the previous draft that specified
       <netinet/ip6.h> that should have been <netinet/in.h>.  These have
       all been corrected and the locations of all definitions is now
       summarized in the new Section 14 ("Summary of New Definitions").

    Changes from the October 1996 Edition (-00 draft)

    -  Numerous rationale added using the format (Note: ...).

    -  Added note that not all errors may be defined.

    -  Added note about ICMPv4, IGMPv4, and ARPv4 terminology.

    -  Changed the name of <netinet/ip6_icmp.h> to <netinet/icmp6.h>.

    -  Changed some names in Section 2.2.1: ICMPV6_PKT_TOOBIG to
       ICMPV6_PACKET_TOOBIG, ICMPV6_TIME_EXCEED to ICMPV6_TIME_EXCEEDED,
       ICMPV6_ECHORQST to ICMPV6_ECHOREQUEST, ICMPV6_ECHORPLY to
       ICMPV6_ECHOREPLY, ICMPV6_PARAMPROB_HDR to
       ICMPV6_PARAMPROB_HEADER, ICMPV6_PARAMPROB_NXT_HDR to
       ICMPV6_PARAMPROB_NEXTHEADER, and ICMPV6_PARAMPROB_OPTS to
       ICMPV6_PARAMPROB_OPTION.

    -  Prepend the prefix "icmp6_" to the three members of the
       icmp6_dataun union of the icmp6hdr structure (Section 2.2).

    -  Moved the neighbor discovery definitions into the
       <netinet/icmp6.h> header, instead of being in their own header
       (Section 2.2.1).

    -  Changed Section 2.3 ("Address Testing").  The basic macros are
       now in the basic API.

    -  Added the new Section 2.4 on "Protocols File".

    -  Added note to raw sockets description that something like BPF or
       DLPI must be used to read or write entire IPv6 packets.




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    -  Corrected example of IPV6_CHECKSUM socket option (Section 3.1).
       Also defined value of -1 to disable.

    -  Noted that <netinet/icmp6.h> defines all the ICMPv6 filtering
       constants, macros, and structures (Section 3.2).

    -  Added note on magic number 10240 for amount of ancillary data
       (Section 4.1).

    -  Added possible padding to picture of ancillary data (Section
       4.2).

    -  Defined <sys/socket.h> header for CMSG_xxx() functions (Section
       4.2).

    -  Note that the data returned by getsockopt(IPV6_PKTOPTIONS) for a
       TCP socket is just from the optional headers, if present, of the
       most recently received segment.  Also note that control
       information is never returned by recvmsg() for a TCP socket.

    -  Changed header for struct in6_pktinfo from <netinet.in.h> to
       <netinet/ip6.h> (Section 5).

    -  Removed the old Sections 5.1 and 5.2, because the interface
       identification functions went into the basic API.

    -  Redid Section 5 to support the hop limit field.

    -  New Section 5.4 ("Next Hop Address").

    -  New Section 6 ("Flow Labels").

    -  Changed all of Sections 7 and 8 dealing with Hop-by-Hop and
       Destination options.  We now define a set of inet6_option_XXX()
       functions.

    -  Changed header for IPV6_SRCRT_xxx constants from <netinet.in.h>
       to <netinet/ip6.h> (Section 9).

    -  Add inet6_rthdr_lasthop() function, and fix errors in description
       of Routing header (Section 9).

    -  Reworded some of the Routing header descriptions to conform to
       the terminology in [RFC-1883].

    -  Added the example from [RFC-1883] for the Routing header (Section
       9.9).




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    -  Expanded the example in Section 10 to show multiple options per
       ancillary data object, and to show the receiver's ancillary data
       objects.

    -  New Section 11 ("IPv6-Specific Options with IPv4-Mapped IPv6
       Addresses").

    -  New Section 12 ("rresvport_af").

    -  Redid old Section 10 ("Additional Items") into new Section 13
       ("Future Items").

16.  References

   [RFC-1883]  Deering, S., and R. Hinden, "Internet Protocol, Version 6
               (IPv6), Specification", RFC 1883, December 1995.

   [RFC-2133]  Gilligan, R., Thomson, S., Bound, J., and W. Stevens,
               "Basic Socket Interface Extensions for IPv6", RFC 2133,
               April 1997.

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

   [RFC-1970]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
               Discovery for IP Version 6 (IPv6)", RFC 1970, August
               1996.

17.  Acknowledgments

   Matt Thomas and Jim Bound have been working on the technical details
   in this draft 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.

   The following provided comments on earlier drafts: Pascal Anelli,
   Hamid Asayesh, Ran Atkinson, Karl Auerbach, Hamid Asayesh, Matt
   Crawford, Sam T. Denton, Richard Draves, Francis Dupont, Bob
   Gilligan, Tim Hartrick, Masaki Hirabaru, Yoshinobu Inoue, Mukesh
   Kacker, A. N. Kuznetsov, Pedro Marques, Jack McCann, der Mouse, John
   Moy, Thomas Narten, Erik Nordmark, Steve Parker, Charles Perkins, Tom
   Pusateri, Pedro Roque, Sameer Shah, Peter Sjodin, Stephen P.
   Spackman, Jinmei Tatuya, Karen Tracey, Quaizar Vohra, Carl Williams,
   Steve Wise, and Kazu Yamamoto.




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18.  Authors' Addresses

   W. Richard Stevens
   1202 E. Paseo del Zorro
   Tucson, AZ  85718

   EMail: rstevens@kohala.com


   Matt Thomas
   AltaVista Internet Software
   LJO2-1/J8
   30 Porter Rd
   Littleton, MA  01460
   EMail: matt.thomas@altavista-software.com




































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19.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  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.
























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