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Versions: (RFC 2292) 01 02 03 04 05 06 07 08 RFC 3542

INTERNET-DRAFT                           W. Richard Stevens (Consultant)
Expires: December 24, 1999                      Matt Thomas (Consultant)
Obsoletes RFC 2292                                   Erik Nordmark (Sun)
                                                           Oct 22, 1999


                     Advanced Sockets API for IPv6
                   <draft-ietf-ipngwg-rfc2292bis-01.txt>



Abstract

   A separate specification [RFC-2553] contain changes to the sockets
   API to support IP version 6.  Those 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-2553]: The Routing header (source
   routing), Hop-by-Hop options, and Destination options.  This document
   provides API access to these features too.

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet Draft expires April 22, 2000.









































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Table of Contents

    1.  Introduction ....................................................  6

    2.  Common Structures and Definitions ...............................  7
       2.1.  The ip6_hdr Structure ......................................  8
            2.1.1.  IPv6 Next Header Values .............................  8
            2.1.2.  IPv6 Extension Headers ..............................  9
            2.1.3.  IPv6 Options ........................................ 10
       2.2.  The icmp6_hdr Structure .................................... 12
            2.2.1.  ICMPv6 Type and Code Values ......................... 13
            2.2.2.  ICMPv6 Neighbor Discovery Type and Code Values ...... 14
            2.2.3.  Multicast Listener Discovery Type and Code Values ... 16
            2.2.4.  ICMPv6 Router Renumbering Type and Code Values ...... 17
       2.3.  Address Testing Macros ..................................... 19
       2.4.  Protocols File ............................................. 19

    3.  IPv6 Raw Sockets ................................................ 20
       3.1.  Checksums .................................................. 21
       3.2.  ICMPv6 Type Filtering ...................................... 21

    4.  Access to IPv6 and Extension Headers ............................ 24
       4.1.  TCP Implications ........................................... 26
       4.2.  UDP and Raw Socket Implications ............................ 27

    5.  Extensions to Socket Ancillary Data ............................. 27

    6.  Packet Information .............................................. 28
       6.1.  Specifying/Receiving the Interface ......................... 29
       6.2.  Specifying/Receiving Source/Destination Address ............ 29
       6.3.  Specifying/Receiving the Hop Limit ......................... 30
       6.4.  Specifying the Next Hop Address ............................ 31
       6.5.  Additional Errors with sendmsg() and setsockopt() .......... 31

    7.  Routing Header Option ........................................... 32
       7.1.  inet6_rth_space ............................................ 33
       7.2.  inet6_rth_init ............................................. 33
       7.3.  inet6_rth_add .............................................. 34
       7.4.  inet6_rth_reverse .......................................... 34
       7.5.  inet6_rth_segments ......................................... 35
       7.6.  inet6_rth_getaddr .......................................... 35

    8.  Hop-By-Hop Options .............................................. 35
       8.1.  Receiving Hop-by-Hop Options ............................... 36
       8.2.  Sending Hop-by-Hop Options ................................. 37

    9.  Destination Options ............................................. 37
       9.1.  Receiving Destination Options .............................. 37



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       9.2.  Sending Destination Options ................................ 38

   10.  Hop-by-Hop and Destination Options Processing ................... 39
       10.1.  inet6_opt_init ............................................ 39
       10.2.  inet6_opt_append .......................................... 39
       10.3.  inet6_opt_finish .......................................... 40
       10.4.  inet6_opt_set_val ......................................... 41
       10.5.  inet6_opt_next ............................................ 41
       10.6.  inet6_opt_find ............................................ 41
       10.7.  inet6_opt_get_val ......................................... 42

   11.  Additional Advanced API Functions ............................... 42
       11.1.  Sending with the Minimum MTU .............................. 42
       11.2.  Path MTU Discovery and UDP ................................ 43
       11.3.  Neighbor Reachability and UDP ............................. 43

   12.  Ordering of Ancillary Data and IPv6 Extension Headers ........... 44

   13.  IPv6-Specific Options with IPv4-Mapped IPv6 Addresses ........... 44

   14.  Extended interfaces for rresvport, rcmd and rexec ............... 45
       14.1.  rresvport_af .............................................. 45
       14.2.  rcmd_af ................................................... 45
       14.3.  rexec_af .................................................. 46

   15.  Summary of New Definitions ...................................... 46

   16.  Security Considerations ......................................... 49

   17.  Change History .................................................. 49

   18.  TODO and Open Issues ............................................ 52

   19.  References ...................................................... 52

   20.  Acknowledgments ................................................. 53

   21.  Authors' Addresses .............................................. 53

   22.  Appendix A: Ancillary Data Overview ............................. 54
       22.1.  The msghdr Structure ...................................... 54
       22.2.  The cmsghdr Structure ..................................... 55
       22.3.  Ancillary Data Object Macros .............................. 57
            22.3.1.  CMSG_FIRSTHDR ...................................... 57
            22.3.2.  CMSG_NXTHDR ........................................ 58
            22.3.3.  CMSG_DATA .......................................... 59
            22.3.4.  CMSG_SPACE ......................................... 60
            22.3.5.  CMSG_LEN ........................................... 60



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   23.  Appendix B: Examples using the inet6_rth_XXX() functions ........ 61
       23.1.  Sending a Routing Header .................................. 61
       23.2.  Receiving Routing Headers ................................. 66

   24.  Appendix C: Examples using the inet6_opt_XXX() functions ........ 68
       24.1.  Building options .......................................... 68
       24.2.  Parsing received options .................................. 70












































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1.  Introduction

   A separate specification [RFC-2553] contain changes to the sockets
   API to support IP version 6.  Those changes are for TCP and UDP-based
   applications.  This 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 header files.  This allows programs such as
   Ping and Traceroute, for example, to compile with minimal effort on
   many hosts that support the sockets API.  With IPv6, however, there
   is no common source code base that implementors are starting from,
   and the possibility for divergence at this level between different
   implementations is high.  To avoid a complete lack of portability
   amongst applications that use raw IPv6 sockets, some standardization
   is necessary.

   There are also features from the basic IPv6 specification that are
   not addressed in [RFC-2553]: sending and receiving Routing headers,
   Hop-by-Hop options, and Destination options, 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 Routing header, Hop-by-Hop, and



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       Destination extension headers.

   5.  Additional features required for improved IPv6 application
       portability.

   The packet information along with access to the extension headers
   (Routing header, Hop-by-Hop options, and Destination options) are
   specified using the "ancillary data" fields that were added to the
   4.3BSD Reno sockets API in 1990.  The reason is that these ancillary
   data fields are part of the Posix.1g standard 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.

   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 protocol headers are required, along with
   common constant definitions for the structure members.

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

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

   When an include file is specified, that include file is allowed to



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   include other files that do the actual declaration or definition.


2.1.  The ip6_hdr Structure

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

       struct ip6_hdr {
         union {
           struct ip6_hdrctl {
             uint32_t ip6_un1_flow; /* 4 bits version, 8 bits TC, 24 bits flow-ID */
             uint16_t ip6_un1_plen; /* payload length */
             uint8_t  ip6_un1_nxt;  /* next header */
             uint8_t  ip6_un1_hlim; /* hop limit */
           } ip6_un1;
           uint8_t ip6_un2_vfc;     /* 4 bits version, top 4 bits tclass */
         } ip6_ctlun;
         struct in6_addr ip6_src;   /* source address */
         struct in6_addr ip6_dst;   /* destination address */
       };

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



2.1.1.  IPv6 Next Header Values

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

       #define IPPROTO_HOPOPTS        0 /* IPv6 Hop-by-Hop options */
       #define IPPROTO_IPV6          41 /* IPv6 header */
       #define IPPROTO_ROUTING       43 /* IPv6 Routing header */
       #define IPPROTO_FRAGMENT      44 /* IPv6 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.



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   This should not be a problem since IPPROTO_IP is used only with IPv4
   sockets and IPPROTO_HOPOPTS only with IPv6 sockets.


2.1.2.  IPv6 Extension Headers

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

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

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

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

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

       /* Fragment header */
       struct ip6_frag {
         uint8_t   ip6f_nxt;       /* next header */



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         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



2.1.3.  IPv6 Options

   Eleven options are defined for IPv6 at the time of writing this
   document.  We define structures for all except the unspecified EID
   option.  The following structures are defined as a result of
   including <netinet/ip6.h>.

       /* IPv6 options */
       structip6_opt {
           uint8_tip6o_type;
           uint8_tip6o_len;
       };

       /*
        * The high-order 3 bits of the option type define the behavior
        * when processing an unknown option and whether or not the option
        * content changes in flight.
        */
       #defineIP6OPT_TYPE(o)((o) & 0xc0)
       #defineIP6OPT_TYPE_SKIP0x00
       #defineIP6OPT_TYPE_DISCARD0x40
       #defineIP6OPT_TYPE_FORCEICMP0x80
       #defineIP6OPT_TYPE_ICMP0xc0
       #defineIP6OPT_MUTABLE0x20

       #defineIP6OPT_PAD10x00/* 00 0 00000 */
       #defineIP6OPT_PADN0x01/* 00 0 00001 */
       #defineIP6OPT_JUMBO0xc2/* 11 0 00010 = 194 */
       #defineIP6OPT_NSAP_ADDR0xc3/* 11 0 00011 */
       #defineIP6OPT_TUNNEL_LIMIT0x04/* 00 0 00100 */
       #defineIP6OPT_ROUTER_ALERT0x05/* 00 0 00101 */



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       #defineIP6OPT_BINDING_UPDATE0xc6/* 11 0 00110 */
       #defineIP6OPT_BINDING_ACK0x07/* 00 0 00111 */
       #defineIP6OPT_BINDING_REQ0x08/* 00 0 01000 */
       #defineIP6OPT_HOME_ADDRESS0xc9/* 11 0 01001 */
       #defineIP6OPT_EID0x8a/* 10 0 01010 */

       /* Jumbo Payload Option */
       structip6_opt_jumbo {
           uint8_tip6oj_type;
           uint8_tip6oj_len;
           uint8_t ip6oj_jumbo_len[4];
       };
       #defineIP6OPT_JUMBO_LEN6

       /* NSAP Address Option */
       structip6_opt_nsap {
           uint8_tip6on_type;
           uint8_tip6on_len;
           uint8_t ip6on_src_nsap_len;
           uint8_t ip6on_dst_nsap_len;
             /* followed by source NSAP */
             /* followed by destination NSAP */
       };

       /* Tunnel Limit Option */
       structip6_opt_tunnel {
           uint8_tip6ot_type;
           uint8_tip6ot_len;
           uint8_t ip6ot_encap_limit;
       };

       /* Router Alert Option */
       structip6_opt_router {
           uint8_tip6or_type;
           uint8_tip6or_len;
           uint8_t ip6or_value[2];
       };

       /* Router alert values (in network byte order) */
       #ifdef _BIG_ENDIAN
       #defineIP6_ALERT_MLD0x0000
       #defineIP6_ALERT_RSVP0x0001
       #defineIP6_ALERT_AN0x0002
       #else
       #defineIP6_ALERT_MLD0x0000
       #defineIP6_ALERT_RSVP0x0100
       #defineIP6_ALERT_AN0x0200
       #endif



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       /* Binding Update Option */
       structip6_opt_binding_update {
           uint8_tip6ou_type;
           uint8_tip6ou_len;
           uint8_t ip6ou_flags;
           uint8_t ip6ou_prefixlen;
           uint8_t ip6ou_seqno[2];
           uint8_t ip6ou_lifetime[4];
           uint8_t ip6ou_coa[16];/* Optional based on flags */
             /* followed by sub-options */
       };

       /* Binding Update Flags */
       #defineIP6_BUF_ACK0x80/* Request a binding ack */
       #defineIP6_BUF_HOME0x40/* Home Registration */
       #defineIP6_BUF_COA0x20/* Care-of-address present in option */
       #defineIP6_BUF_ROUTER0x10/* Sending mobile node is a router */

       /* Binding Ack Option */
       structip6_opt_binding_ack {
           uint8_tip6oa_type;
           uint8_tip6oa_len;
           uint8_t ip6oa_status;
           uint8_t ip6oa_seqno[2];
           uint8_t ip6oa_lifetime[4];
           uint8_t ip6oa_refresh[4];
             /* followed by sub-options */
       };

       /* Binding Request Option */
       structip6_opt_binding_request {
           uint8_tip6or_type;
           uint8_tip6or_len;
             /* followed by sub-options */
       };

       /* Home Address Option */
       structip6_opt_home_address {
           uint8_tip6oh_type;
           uint8_tip6oh_len;
           uint8_t ip6oh_addr[16];/* Home Address */
             /* followed by sub-options */
       };



2.2.  The icmp6_hdr Structure




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

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

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



2.2.1.  ICMPv6 Type and Code Values

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

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

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

       #define ICMP6_ECHO_REQUEST          128
       #define ICMP6_ECHO_REPLY            129
       #define ICMP6_MEMBERSHIP_QUERY      130
       #define ICMP6_MEMBERSHIP_REPORT     131
       #define ICMP6_MEMBERSHIP_REDUCTION  132

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



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

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

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

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


2.2.2.  ICMPv6 Neighbor Discovery 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 */
           /* could be followed by options */
       };

       #define nd_ra_type               nd_ra_hdr.icmp6_type
       #define nd_ra_code               nd_ra_hdr.icmp6_code



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       #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 */
       };

       #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



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       #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;
       };

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


2.2.3.  Multicast Listener Discovery Type and Code Values



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

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



2.2.4.  ICMPv6 Router Renumbering Type and Code Values

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

       struct icmp6_router_renum {  /* router renumbering header */
         struct icmp6_hdr  rr_hdr;
         u_int8_t          rr_segnum;
         u_int8_t          rr_flags;
         u_int16_t         rr_maxdelay;
         u_int32_t         rr_reserved;
       };
       #define rr_type                  rr_hdr.icmp6_type
       #define rr_code                  rr_hdr.icmp6_code
       #define rr_cksum                 rr_hdr.icmp6_cksum
       #define rr_seqnum                rr_hdr.icmp6_data32[0]

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


       struct rr_pco_match {    /* match prefix part */
         u_int8_t          rpm_code;
         u_int8_t          rpm_len;
         u_int8_t          rpm_ordinal;
         u_int8_t          rpm_matchlen;
         u_int8_t          rpm_minlen;
         u_int8_t          rpm_maxlen;
         u_int16_t         rpm_reserved;



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         struct in6_addr   rpm_prefix;
       };

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

       struct rr_pco_use {    /* use prefix part */
         u_int8_t          rpu_uselen;
         u_int8_t          rpu_keeplen;
         u_int8_t          rpu_ramask;
         u_int8_t          rpu_raflags;
         u_int32_t         rpu_vltime;
         u_int32_t         rpu_pltime;
         u_int32_t         rpu_flags;
         struct in6_addr   rpu_prefix;
       };
       #define ICMP6_RR_PCOUSE_RAFLAGS_ONLINK   0x20
       #define ICMP6_RR_PCOUSE_RAFLAGS_AUTO     0x10

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

       struct rr_result {    /* router renumbering result message */
         u_int16_t         rrr_flags;
         u_int8_t          rrr_ordinal;
         u_int8_t          rrr_matchedlen;
         u_int32_t         rrr_ifid;
         struct in6_addr   rrr_prefix;
       };

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







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2.3.  Address Testing Macros

   The basic API ([RFC-2553]) defines some macros for testing an IPv6
   address for certain properties.  This API extends those definitions
   with additional address testing macros, defined as a result of
   including <netinet/in.h>.

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

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


2.4.  Protocols File

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

       struct protoent  *proto;

       proto = getprotobyname("icmp");

       s = socket(PF_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 used the host's byte order for certain IP header fields.

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

   All fields 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 and on receipt the application will
   only receive the payload i.e. the data after the IPv6 header and all
   the extension headers.

   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



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


3.1.  Checksums

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

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

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

   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.

   An attempt to set IPV6_CHECKSUM for an ICMPv6 socket will fail.

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


3.2.  ICMPv6 Type Filtering

   ICMPv4 raw sockets receive most ICMPv4 messages received by the
   kernel.  (We say "most" and not "all" because Berkeley-derived
   kernels never pass echo requests, timestamp requests, or address mask
   requests to a raw socket.  Instead these three messages are processed
   entirely by the kernel.)  But ICMPv6 is a superset of ICMPv4, also



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   including the functionality of IGMPv4 and ARPv4.  This means that an
   ICMPv6 raw socket can potentially receive many more messages than
   would be received with an ICMPv4 raw socket: ICMP messages similar to
   ICMPv4, along with neighbor solicitations, neighbor advertisements,
   and the three multicast listener discovery messages.

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

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

       struct icmp6_filter;

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

   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



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   messages of a given ICMPv6 type should be passed to the application
   or not passed to the application (blocked).

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

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

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

       struct icmp6_filter  myfilt;

       fd = socket(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.














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

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

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


4.  Access to IPv6 and Extension Headers

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

       1.  The send/receive interface and source/destination address,
       2.  The hop limit,
       3.  Next hop address,
       4.  Routing header.
       5.  Hop-by-Hop options, and
       6.  Destination options (both before and after a 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:








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       int  on = 1;

       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO,  &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR,    &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS,  &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS,  &on, sizeof(on));
       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDRDSTOPTS,
                  &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.

   Two different mechanisms exist for sending this optional information:

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

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


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

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


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

   Issuing getsockopt() for the above options will return the sticky



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   option value i.e. the value set with setsockopt().

   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").

   The application does not explicitly need to access the data
   structures for the Routing header option, Hop-by-Hop option, and
   Destination options, since the API to these features is through a set
   of inet6_rth_XXX() and inet6_opt_XXX() functions that we define in
   Section 8 and Section 10.  Those functions simplify the interface to
   these features instead of requiring the application to know the
   intimate details of the extension header formats.


4.1.  TCP Implications

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

   Applications using TCP can use ancillary data (after enabling the
   desired IPV6_RECVxxx options) to receive the IPv6 and extension
   header information.  However, since there is not a one-to-one mapping
   between received TCP segments and recv operations seen by the
   application, when different TCP segments have different IPv6 and
   extension headers the application might not be able to observe all
   received headers.  For efficiency reasons it is recommended that a
   TCP implementation not send ancillary data items with every received
   segment but instead try to detect the points in the data stream when
   the requested IPv6 and extension header content changes and only send
   a single ancillary data item at the time of the change.  Also, TCP
   should send ancillary data items at the start of the connection and
   when the application enables a new IPV6_RECVxxx option.

   For example, assume an application has enabled IPV6_RECVHOPLIMIT
   before a connection is established.  Then the first recvmsg() would
   have an IPV6_HOPLIMIT item indicating the hop limit in the first data
   segment.  Should the hoplimit in the received data segment change a
   subsequent recvmsg() will also have an IPV6_HOPLIMIT item.  However,



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   the application must be prepared to handle ancillary data items even
   though the hop limit did not change.  Note that should the hop limit
   in received ACK-only segments be different than the hop limit in data
   segments the application might only be able to observe the hop limit
   in the received data segments.

   The above example was for hop limit but the application should be
   prepared to handle the corresponding behavior for the other option
   information.

   The above recvmsg() behavior allows the application to detect changes
   in the received IPv6 and extension headers without resorting to
   periodic getsockopt() calls.


4.2.  UDP and Raw Socket Implications

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

   For sending the application has the choice between using sticky
   options and ancillary data.  The application can also use both having
   the sticky options specify the "default" and using ancillary data to
   override the default options.  Note that if any ancillary data is
   specified in a call to sendmsg(), all of the sticky options are
   overridden for that datagram.  For example, if the application has
   set IPV6_RTHDR using a sticky option and later passes IPV6_HOPLIMIT
   as ancillary data this will override the IPV6_RTHDR sticky option and
   no Routing header will be sent with that datagram.


5.  Extensions to Socket Ancillary Data

   This specification uses ancillary data as defined in Posix.1g which
   the following compatible extensions:

    -  CMSG_NEXTHDR has been extended to handle a NULL 2nd argument to
       mean "get the first header".  See Section 22.3.2.




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    -  A new CMSG_SPACE macro is defined.  It is used to determine how
       much space need to be allocated for an ancillary data item.  See
       Section 22.3.4.

    -  A new CMSG_LEN macro is defined.  It returns the value to store
       in the cmsg_len member of the cmsghdr structure, taking into
       account any padding needed to satisfy alignment requirements.
       See Section 22.3.5.


6.  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 set with setsockopt() or sent as ancillary data
   with sendmsg() and received as ancillary data with recvmsg().  This
   structure is defined as a result of including the <netinet/in.h>.

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

   In the socket option and cmsghdr level will be IPPROTO_IPV6, the type
   will be IPV6_PKTINFO, and the first byte of the option value and
   cmsg_data[] will be the first byte of the in6_pktinfo structure.  An
   application can clear any sticky IPV6_PKTINFO option by either doing
   a setsockopt for option with optlen being zero, or by doing a
   "regular" setsockopt with ipi6_addr being in6addr_any and
   ipi6_ifindex being zero.

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



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


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


6.1.  Specifying/Receiving the Interface

   Interfaces on an IPv6 node are identified by a small positive
   integer, as described in Section 4 of [RFC-2553].  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-2553]), 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.


6.2.  Specifying/Receiving Source/Destination Address

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



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

   The kernel must verify that the requested source address is indeed a
   unicast address assigned to the node.

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


6.3.  Specifying/Receiving the Hop Limit

   The outgoing hop limit is normally specified with either the
   IPV6_UNICAST_HOPS socket option or the IPV6_MULTICAST_HOPS socket
   option, both of which are described in [RFC-2553].  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_RECVHOPLIMIT socket
   option:

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

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

   Nothing special need be done to specify the outgoing hop limit: just
   specify the control information as ancillary data for sendmsg() or
   using setsockopt().  As specified in [RFC-2553], 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



6.4.  Specifying the Next Hop Address

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

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

   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.


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

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

   ENXIO         The interface specified by ipi6_ifindex does not exist.

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

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




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   EHOSTUNREACH  No route to the destination exists over the interface
                 specified by ifi6_ifindex.


7.  Routing Header Option

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

   Source routing with 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, and the ability to use sticky options or ancillary data to
   communicate this information between the application and the kernel
   using the IPV6_RTHDR option.

   Three functions build a Routing header:

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

   Three functions deal with a returned Routing header:

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

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

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

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

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

   The application can remove any sticky Routing header by calling



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   setsockopt() for IPV6_RTHDR with a zero option length.

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

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

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

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


7.1.  inet6_rth_space


       size_t inet6_rth_space(int type, int segments);

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

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

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


7.2.  inet6_rth_init




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

   This function initializes the buffer pointed to by bp to contain a
   Routing header of the specified type and sets ip6r0_len based on the
   segments parameter. bp_len is only used to verify that the buffer is
   large enough.  The ip6r0_segleft field is set to zero;
   inet6_rth_add() will increment it.

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

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

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


7.3.  inet6_rth_add


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

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

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


7.4.  inet6_rth_reverse


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

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





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   error.


7.5.  inet6_rth_segments


       int inet6_rth_segments(const void *bp);

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


7.6.  inet6_rth_getaddr


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

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

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


8.  Hop-By-Hop Options

   A variable number of Hop-by-Hop options can appear in a single Hop-
   by-Hop options header.  Each option in the header is TLV-encoded with
   a type, length, and value.

   Today only four Hop-by-Hop options are defined for IPv6 [RFC-2460]:
   Jumbo Payload, Pad1, PadN, and router-alert.  The two pad options are
   for alignment purposes and are automatically inserted by the
   inet6_opt_XXX() routines and ignored by the inet6_opt_XXX() routines
   on the receive side.

   This section of the API is therefore defined for future Hop-by-Hop
   options (as well as destination options) that an application may need
   to specify and receive.  This IPv6 API defines seven functions that
   the application calls to build and examine a Hop-by_Hop options
   header, and the ability to use sticky options or ancillary data to
   communicate this information between the application and the kernel.
   This uses the IPV6_HOPOPTS for hob-by-hop options.




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   Four functions build a options header:

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

   Three functions deal with a returned options header:

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


   Individual Hop-by-Hop options (and Destination options, which are
   described in Section 9 and are very similar to the Hop-by-Hop
   options) may have specific alignment requirements.  For example, the
   4-byte Jumbo Payload length should appear on a 4-byte boundary, and
   IPv6 addresses are normally aligned on an 8-byte boundary.  These
   requirements and the terminology used with these options are
   discussed in Section 4.2 and Appendix B of [RFC-2460].  The alignment
   of first byte of each option is specified by two values, called x and
   y, written as "xn + y".  This states that the option must appear at
   an integer multiple of x bytes from the beginning of the options
   header (x can have the values 1, 2, 4, or 8), plus y bytes (y can
   have a value between 0 and 7, inclusive).  The Pad1 and PadN options
   are inserted as needed to maintain the required alignment.  The
   functions below need to know the alignment of the end of the option
   (which is always in the form "xn," where x can have the values 1, 2,
   4, or 8) and the total size of the data portion of the option.  These
   are passed as the "align" and "len" arguments to inet6_opt_append().

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

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


8.1.  Receiving Hop-by-Hop Options

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





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

   When using ancillary data a Hop-by-hop options is passed between the
   application and the kernel as follows: The cmsg_level member will be
   IPPROTO_IPV6 and the cmsg_type member will be IPV6_HOPOPTS.  These
   options are then processed by calling the inet6_opt_next(),
   inet6_opt_find(), and inet6_opt_get_val() functions, described in
   Section 10.6.


8.2.  Sending Hop-by-Hop Options

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

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

   All the Hop-by-Hop options must specified by a single ancillary data
   object.  The cmsg_level member is set to IPPROTO_IPV6 and the
   cmsg_type member is set to IPV6_HOPOPTS.  The option is normally
   constructed using the inet6_opt_init(), inet6_opt_append(),
   inet6_opt_finish(), and inet6_set_val() functions, described in
   Section 10.

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


9.  Destination Options

   A variable number of Destination options can appear in one or more
   Destination option headers.  As defined in [RFC-2460], a Destination
   options header appearing before a Routing header is processed by the
   first destination plus any subsequent destinations specified in the
   Routing header, while a Destination options header 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-2460],
   although proposals exist to use Destination options with Mobile IPv6.


9.1.  Receiving Destination Options



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   To receive Destination options appearing after a Routing header (or
   in a packet without a Routing header) the application must enable the
   IPV6_RECVDSTOPTS socket option:

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

   To receive Destination options appearing before a Routing header the
   application must enable the IPV6_RECVRTHDRDSTOPTS socket option:

       int  on = 1;
       setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDRDSTOPTS,
                  &on, sizeof(on));

   All the Destination options appearing before a Routing header are
   returned as one ancillary data object described by a cmsghdr
   structure (with cmsg_type set to IPV6_RTHDRDSTOPTS) and all the
   Destination options appearing after a Routing header (or in a packet
   without a Routing header) are returned as another ancillary data
   object described by a cmsghdr structure (with cmsg_type set to
   IPV6_DSTOPTS).  For all these ancillary data objects, the cmsg_level
   member will be IPPROTO_IPV6.

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


9.2.  Sending Destination Options

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

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

   As described in Section 6 one set of Destination options can appear
   before a Routing header, and one set can appear after a Routing
   header (or in a packet with no Routing header).  Each set can consist
   of one or more options but each set is a single extension header.

   When using ancillary data a Destination options header is passed
   between the application and the kernel as follows: The set preceding
   a Routing header are specified with the cmsg_level member is set to
   IPPROTO_IPV6 and the cmsg_type member is set to IPV6_RTHDRDSTOPTS.
   Any setsockopt or ancillary data for IPV6_RTHDRDSTOPTS is silently
   ignore when sending packets unless a Routing header is also



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   specified.

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

   The Destination options are normally constructed using the
   inet6_opt_init(), inet6_opt_append(), inet6_opt_finish(), and
   inet6_set_val() functions, described in Section 10.

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


10.  Hop-by-Hop and Destination Options Processing

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

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

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


10.1.  inet6_opt_init


       int inet6_opt_init(void *extbuf, size_t extlen);

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


10.2.  inet6_opt_append





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

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

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

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

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

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

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


10.3.  inet6_opt_finish


       int inet6_opt_finish(void *extbuf, size_t extlen, int prevlen);

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

   If the necessary pad does not fit in the extension header buffer the






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   function returns -1.


10.4.  inet6_opt_set_val


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

   Databuf should be a pointer returned by inet6_opt_append().  This
   function inserts data items of various sizes (1, 2, 4, or 8 bytes) in
   the data portion of the option. val should point to the data to be
   inserted.  Offset specifies where in the data portion of the option
   the value should be inserted; the first byte after the option type
   and length is accessed by specifying an offset of zero.

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


10.5.  inet6_opt_next


       int inet6_opt_next(void *extbuf, size_t extlen, int prevlen,
                          uint8_t *typep, size_t *lenp,
                          void **databufp);

   This function parses received extension headers returning the next
   option.  Extbuf and extlen specifies the extension header.  Prevlen
   should either be zero (for the first option) or the length returned
   by a previous call to inet6_opt_next() or inet6_opt_find().  It
   specifies the position where to continue scanning the extension
   buffer.  The next option is returned by updating typep, lenp, and
   databufp.  This function returns the updated "previous" length
   computed by advancing past the option that was returned.  This
   returned "previous" length can then be passed to subsequent calls to
   inet6_opt_next().  This function does not return any PAD1 or PADN
   options.  When there are no more options the return value is -1.


10.6.  inet6_opt_find


       int inet6_opt_find(void *extbuf, size_t extlen, int prevlen,
                          uint8_t type, size_t *lenp,
                          void **databufp);




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

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

   If an option of the specified type is not located, the return value
   is -1.  If an error occurs, the return value is -1.


10.7.  inet6_opt_get_val


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

   Databuf should be a pointer returned by inet6_opt_next() or
   inet6_opt_find().  This function extracts data items of various sizes
   (1, 2, 4, or 8 bytes) in the data portion of the option. val should
   point to the destination for the extracted data.  Offset specifies
   from where in the data portion of the option the value should be
   extracted; the first byte after the option type and length is
   accessed by specifying an offset of zero.

   The function returns the offset for the next field (i.e., offset +
   vallen) which can be used when extracting option content with
   multiple fields.  XXX Perhaps we should add a note to point out that
   robust receivers should verify alignment before calling
   inet6_opt_get_val().  XXX Or check alignment and fail by returning
   -1?


11.  Additional Advanced API Functions



11.1.  Sending with the Minimum MTU

   Some applications might not want to incur the overhead of path MTU
   discovery, especially if the applications only send a single datagram
   to a destination.  A potential example is a DNS server.




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   This specification defines a mechanism to avoid fragmentation by
   sending at the minimum IPv6 MTU (1280 bytes).  This can be enabled
   using the IPV6_USE_MIN_MTU socket option.

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

   By default, this socket option is disabled.  Setting the value to 0
   also disables the option.  This option can also be sent as ancillary
   data.


11.2.  Path MTU Discovery and UDP

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

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

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

   By default, this socket option is disabled.  Setting the value to 0
   also disables the option.

   When the application is sending packets too big for the path MTU
   recvmsg will return zero (indicating no data) but there will be a
   cmsghdr with cmsg_type set to IPV6_PATHMTU, and cmsg_len will
   indicate that cmsg_data is 4 bytes long.  CMSG_DATA will point to an
   integer carrying the path MTU to use.


11.3.  Neighbor Reachability and UDP

   UDP and raw socket application might know that communication is
   making forward progress i.e. that the path from the node to the next
   hop is working.  In such a case the applications, similarly to TCP as
   specified in [RFC-2461], has the option indicate to the internet
   layer that the neighbor is reachable.  See section 7.3.1 of
   [RFC-2461].  This could save unneeded neighbor solicitation and
   neighbor advertisement messages.




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   This is done by including an ancilary data item with cmsg_type being
   IPV6_REACHCONF and with no attached CMSG_DATA.


12.  Ordering of Ancillary Data and IPv6 Extension Headers

   Three IPv6 extension headers can be specified by the application and
   returned to the application using ancillary data with sendmsg() and
   recvmsg(): the Routing header, Hop-by-Hop options, and Destination
   options.  When multiple ancillary data objects are transferred via
   recvmsg() and these objects represent any of these three extension
   headers, their placement in the control buffer is directly tied to
   their location in the corresponding IPv6 datagram.  This API imposes
   some ordering constraints for using these ancillary data objects with
   sendmsg().

   All Hop-by-Hop options must be specified in a single ancillary data
   object.  Should multiple hop-by-hop ancillary data objects be
   specified the implementation might choose an arbitrary one or drop
   the packet.

   All Destination options that precede a Routing header must be
   specified in a single ancillary data object.  If there is no Routing
   header ancillary data object the IPV6_RTHDRDSTOPTS object will be
   silently ignored.

   All Destination options that follow a Routing header (or are used
   without a Routing header) must be specified in a single ancillary
   data object.

   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.


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

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

   In general, attempting to specify an IPv6-only option, such as the
   Hop-by-Hop options, Destination options, or Routing header on an IPv6



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   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.


14.  Extended interfaces for rresvport, rcmd and rexec

   TBD


14.1.  rresvport_af

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

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

       #include <unistd.h>

       int  rresvport_af(int *port, int family);

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

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


14.2.  rcmd_af

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





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

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


14.3.  rexec_af

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

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

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


15.  Summary of New Definitions

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

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



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     <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
     <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_RECVDSTOPTS
     <netinet/in.h>    IPV6_RECVHOPLIMIT
     <netinet/in.h>    IPV6_RECVHOPOPTS
     <netinet/in.h>    IPV6_RECVPKTINFO



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     <netinet/in.h>    IPV6_RECVRTHDR
     <netinet/in.h>    IPV6_RECVRTHDRDSTOPTS
     <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_RTHDR
     <netinet/in.h>    IPV6_RTHDRDSTOPTS
     <netinet/in.h>    IPV6_RTHDR_TYPE_0
     <netinet/in.h>    IPV6_USE_MIN_MTU
     <netinet/in.h>    IPV6_RECVPATHMTU
     <netinet/in.h>    IPV6_PATHMTU
     <netinet/in.h>    IPV6_REACHCONF
     <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{};
     <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>    int inet6_opt_append(void *, size_t, int,
                                            uint8_t, size_t, uint_8, void **);
     <netinet/in.h>    int inet6_opt_get_val(void *, size_t, void *, int);



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

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

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



16.  Security Considerations

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

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


17.  Change History

   Changes from RFC 2292:

    -  Removed the IPV6_PKTOPTIONS socket option by allowing sticky



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       options to be set with individual setsockopt calls.  This
       simplifies the protocol stack implementation by not having to
       handle options within options and also clarifies the failure
       semantics when some option is incorrectly formatted.

    -  Added the IPV6_RTHDRDSTOPTS for a Destination header before the
       Routing header.  This is necessary to allow setting these
       Destination headers without IPV6_PKTOPTIONS.

    -  Removed the ability to be able to specify Hop-by-Hop and
       Destination options using multiple ancillary data items.  The
       application, using the inet6_option_*() routines, is responsible
       for formatting the whole extension header.  This removes the need
       for the protocol stack to somehow guess the alignment
       restrictions on options when concatenating them together.

    -  Added separate IPV6_RECVxxx options to enable the receipt of the
       corresponding ancillary data items.  This makes the API cleaner
       since it allows the application to retrieve with getsockopt the
       sticky options it has set with setsockopt.

    -  Clarified how sticky options are turned off.

    -  Clarified how and when TCP returns ancillary data.

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

    -  Modified the inet6_rthdr_XXX() functions to not assume a cmsghdr
       structure in order to work with both sticky options and ancillary
       data.  Renamed the functions to inet6_rth_XXX() to allow
       implementations to provide both the old and new functions.

    -  Modified the inet6_option_XXX() functions to not assume a cmsghdr
       structure in order to work with both sticky options and ancillary
       data.  Renamed the functions to inet6_opt_XXX() to allow
       implementations to provide both the old and new functions.

    -  The new inet6_opt_XXX() functions were made different that the
       old as to not require structure declarations but instead use
       functions to add the individual fields to the option.

    -  Changed inet6_rthdr_getaddr() to operate on index O through N-1
       (used to be 1 through N).

    -  Changed the comments in the struct ip6_hdr from "priority" to
       "traffic class".




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    -  Clarified the alignment issues involving ancillary data to allow
       for separate alignment of cmsghdr structures and the data.  Made
       CMSG_SPACE() return an upper bound on the needed space.

    -  Added rcmd_af() and rexec_af().

   Changes since -00:

    -  Changed ICMP unreachable code 2 name to be "beyond scope of
       source address".

    -  Added motivation for rcmd_af() and rexec_af().

    -  Added option definitions (IP6OPT_PAD1 etc) to ip6.h.

    -  Added MLD and router renumbering definitions to icmp6.h

    -  Removed ip6r0_addr field - replaced by a comment.

    -  Made the content of IPV6_RTHDR, IPV6_HOPOPTS etc be specified as
       the extension header format (struct ip6_rthdr etc) instead of the
       previous "implementation dependent".

    -  Removed attempt at RFC 2292 compatibility.

    -  Excluded pad options from inet6_opt_next().

    -  Added IPV6_USE_MIN_MTU socket option for applications to avoid
       fragmentation by sending at the minimum IPv6 MTU.

    -  Added MTU notification so that UDP and raw socket applications
       can participate in path MTU discovery.

    -  Added Reachability confirmation for UDP and raw socket
       applications.

    -  Clarified that if the application asks for e.g., IPV6_RTHDR and a
       received datagram does not contain a Routing header an
       implementation will exclude the IPV6_RTHDR ancillary data item.

    -  Removed the constraints for jumbo option.

    -  Moved the new CMSG macros and changes from the appendix.

    -  Add text about inet6_opt_ depending on 2260 appendix B formatting
       rules i.e.  largest field last in the option.

    -  Specified that getsockopt() of a sticky option returns what was



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       set with setsockopt().


18.  TODO and Open Issues

   Items left to do:

    -  Add credits for UDP MTU stuff

    -  Move information about mapping from inet6_opt to setsockopt and
       cmsg.

    -  Clarify IPV6_RTHDRDSTOPTS's interaction with IPV6_RTHDR.

    -  Make the new inet6_opt_set_val() and inet6_opt_get_val() check
       the length of the data item.

   Open issues:

    -  Anything special for mobility options?  Restrict setting at API?
       Filter out on receipt?  If received what does the home address
       option contain?

    -  Specify "change" for TCP especially when there are multiple HBH
       option headers etc.

    -  Specify binding to scope-id => implies filtering of addresses
       with that scope if the address you are bound to is link-local
       etc.  What about conflicts with bound scope-id and sendto/connect
       scope-id?

    -  Specify order for ifindex selection.  Put in separate section.
       Different cases for sending to link-local (scope_id including
       nexthop scope_id) and global.  Is multicast different?

    -  bind() and sin6_scope_id.  Should have been in Basic API.  Error
       checks when bind/sendto sin6_scope_id does not match?

    -  Specify notion of default multicast interface?  In Basic API?

    -  What about site names and site ids?  Need for interfaces to map?
       Requires that site-prefixes pass name - does name need to use DNS
       format to handle character sets?


19.  References





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

   [RFC-2553]  Gilligan, R. E., Thomson, S., Bound, J., Stevens, W.,
               "Basic Socket Interface Extensions for IPv6", RFC 2553,
               March 1999.

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

   [RFC-2461]  Narten, T., Nordmark, E., Simpson, W., "Neighbor
               Discovery for IP Version 6 (IPv6)", RFC 2461, Dec. 1998.


20.  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, Jim Bound,
   Don Coolidge, Matt Crawford, Sam T. Denton, Richard Draves, Francis
   Dupont, Bob Gilligan, Jun-ichiro Hagino, Gerri Harter, Tim Hartrick,
   Bob Halley, Masaki Hirabaru, Yoshinobu Inoue, Mukesh Kacker, A. N.
   Kuznetsov, Sam Manthorpe, Pedro Marques, Jack McCann, der Mouse, John
   Moy, Thomas Narten, Steve Parker, Charles Perkins, Ken Powell, Tom
   Pusateri, Pedro Roque, Sameer Shah, Peter Sjodin, Stephen P.
   Spackman, Jinmei Tatuya, Karen Tracey, Sowmini Varadhan, Quaizar
   Vohra, Carl Williams, Steve Wise, Eric Wong, Farrell Woods, Kazu
   Yamamoto, and Vlad Yasevich.


21.  Authors' Addresses

    W. Richard Stevens
    1202 E. Paseo del Zorro
    Tucson, AZ  85718
    Email: rstevens@kohala.com









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    Matt Thomas
    3am Software Foundry
    8053 Park Villa Circle
    Cupertino, CA 95014
    Email: matt@3am-software.com


    Erik Nordmark
    Sun Microsystems, Inc.
    901 San Antonio Road
    Palo Alto, CA 94303, USA
    Email: erik.nordmark@eng.sun.com



22.  Appendix A: Ancillary Data Overview

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

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

   Nevertheless the ancillary data fields of the msghdr structure
   provide a clean way to pass information in addition to the data that
   is being read or written.  The inclusion of the msg_control and
   msg_controllen members of the msghdr structure along with the cmsghdr
   structure that is pointed to by the msg_control member is required by
   the Posix.1g sockets API standard.



22.1.  The msghdr Structure

   The msghdr structure is used by the recvmsg() and sendmsg()
   functions.  Its Posix.1g definition is:




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       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.)

   (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.)

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


22.2.  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[]; */
       };




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

   As shown in this definition, normally there is no member with the
   name cmsg_data[].  Instead, the data portion is accessed using the
   CMSG_xxx() macros, as described in Section 22.3.  Nevertheless, it is
   common to refer to the cmsg_data[] member.

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

   |<--------------------------- msg_controllen -------------------------->|
   |                                  OR                                   |
   |<--------------------------- msg_controllen ----------------------->|
   |                                                                       |
   |<----- ancillary data object ----->|<----- ancillary data object ----->|
   |<------ min CMSG_SPACE() --------->|<------ min CMSG_SPACE() --------->|
   |                                   |                                   |
   |<---------- cmsg_len ---------->|  |<--------- cmsg_len ----------->|  |
   |<--------- CMSG_LEN() --------->|  |<-------- CMSG_LEN() ---------->|  |
   |                                |  |                                |  |
   +-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
   |cmsg_|cmsg_|cmsg_|XX|           |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.  While sending an
   application may or may not include padding at the end of last
   ancillary data in msg_controllen and implementations must accept both
   as valid.  On receiving a portable application must provide space for
   padding at the end of the last ancillary data as implementations may
   copy out the padding at the end of the control message buffer and
   include it in the received msg_controllen.  When recvmsg() is called
   if msg_controllen is too small for all the ancillary data items
   including any trailing padding after the last item an implementation




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   may set MSG_CTRUNC.


22.3.  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;

       /* 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>.


22.3.1.  CMSG_FIRSTHDR


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

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

   One possible implementation could be







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       #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.)


22.3.2.  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:
















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       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_H((cmsg)->cmsg_len) \
                             + ALIGN_D(sizeof(struct cmsghdr)) > \
            (u_char *)((mhdr)->msg_control) + (mhdr)->msg_controllen) ? \
           (struct cmsghdr *)NULL : \
           (struct cmsghdr *)((u_char *)(cmsg) + ALIGN_H((cmsg)->cmsg_len))))

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


22.3.3.  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:




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



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

   One possible implementation could be:

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



22.3.5.  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_D(sizeof(struct cmsghdr)) + length )

   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






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   object.


23.  Appendix B: Examples using the inet6_rth_XXX() functions

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


23.1.  Sending a Routing Header

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

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

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

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

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



















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

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

       optptr = CMSG_DATA(cmsgptr);
       optptr = inet6_rth_init(optptr, optlen, IPV6_RTHDR_TYPE_0, 3);

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

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

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

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

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

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



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   header (a value of 6, indicating 3 addresses).

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

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

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

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

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

   The first call to inet6_rth_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=6 | Routing Type=0|  Seg Left=1   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[1] = I1                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   The next call to inet6_rth_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=6 | Routing Type=0|  Seg Left=2   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[1] = I1                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[2] = I2                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   The last call to inet6_rth_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                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[1] = I1                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[2] = I2                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                           Address[3] = I3                     +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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


23.2.  Receiving Routing Headers

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

       struct sockaddr_in6     addr;



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       struct msghdr           msg;
       struct iovec            iov;
       struct cmsghdr          *cmsgptr;
       size_t                  cmsgspace;
       void                    *optptr;
       int                     optlen;

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

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

       msg.msg_control = (char *)cmsgptr;
       msg.msg_controllen = cmsgspace;
       msg.msg_name = (struct sockaddr *)&addr;
       msg.msg_namelen = sizeof (addr);
       msg.msg_iov = &iov;
       msg.msg_iovlen = 1;
       iov.iov_base = databuf;
       iov.iov_len = sizeof (databuf);
       msg.msg_flags = 0;
       if (recvmsg(s, &msg, 0) == -1) {
               perror("recvmsg");
               return;
       }
       if (msg.msg_controllen != 0 &&
           cmsgptr->cmsg_level == IPPROTO_IPV6 &&
           cmsgptr->cmsg_type == IPV6_RTHDR) {
               struct in6_addr *in6;
               char asciiname[INET6_ADDRSTRLEN];
               struct ip6_rthdr0 *rthdr;

               rthdr = (struct ip6_rthdr0 *)optptr;
               segments = inet6_rth_segments(optptr);
               printf("route (%d segments, %d left): ",
                   segments, rthdr->ip6r0_segleft);
               for (i = 0; i < segments; i++) {
                       in6 = inet6_rth_getaddr(optptr, i);
                       if (in6 == NULL)



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

   Note: The above example is a simple illustration.  It skips some
   error checks involving the MSG_TRUNC and MSG_CTRUNC flags.


24.  Appendix C: Examples using the inet6_opt_XXX() functions

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


24.1.  Building options

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

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

       /* Estimate the length */
       currentlen = inet6_opt_init(NULL, 0);
       if (currentlen == -1)
               return (-1);



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

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

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

       currentlen = inet6_opt_append(extbuf, extlen, currentlen,
           OPT_Y, 7, 4, &databuf);
       if (currentlen == -1)
               return (-1);
       /* Insert value 0x01 for 1-octet field */
       offset = 0;
       value1 = 0x01;
       offset = inet6_opt_set_val(databuf, offset, &value1, sizeof (value1));
       /* Insert value 0x1331 for 2-octet field */
       value2 = 0x1331;
       offset = inet6_opt_set_val(databuf, offset, &value2, sizeof (value2));
       /* Insert value 0x01020304 for 4-octet field */
       value4 = 0x01020304;
       offset = inet6_opt_set_val(databuf, offset, &value4, sizeof (value4));

       currentlen = inet6_opt_finish(extbuf, extlen, currentlen);



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



24.2.  Parsing received options

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

       int
       print_opt(void *extbuf, size_t extlen)
       {
               ip6_dest_t *ext;
               int currentlen;
               uint8_t type;
               size_t len;
               void *databuf;
               size_t offset;
               uint8_t value1;
               uint16_t value2;
               uint32_t value4;
               uint64_t value8;

               ext = (ip6_dest_t *)extbuf;
               printf("nxt %u, len %u (bytes %d)\n", ext->ip6d_nxt,
                   ext->ip6d_len, (ext->ip6d_len + 1) * 8);

               currentlen = 0;
               while (1) {
                       currentlen = inet6_opt_next(extbuf, extlen, currentlen,
                           &type, &len, &databuf);
                       if (currentlen == -1)
                               break;
                       printf("Received opt %u len %u\n",
                           type, len);
                       switch (type) {
                       case OPT_X:
                               offset = 0;
                               offset = inet6_opt_get_val(databuf, offset,
                                   &value4, sizeof (value4));
                               printf("X 4-byte field %x\n", value4);
                               offset = inet6_opt_get_val(databuf, offset,
                                   &value8, sizeof (value8));
                               printf("X 8-byte field %llx\n", value8);
                               break;
                       case OPT_Y:



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

































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