💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc3542.txt captured on 2023-06-14 at 17:42:09.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Network Working Group W. Stevens
Request for Comments: 3542 M. Thomas
Obsoletes: 2292 Consultant
Category: Informational E. Nordmark
Sun
T. Jinmei
Toshiba
May 2003
Advanced Sockets Application Program Interface (API) for IPv6
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document provides sockets Application Program Interface (API) to
support "advanced" IPv6 applications, as a supplement to a separate
specification, RFC 3493. The expected applications include Ping,
Traceroute, routing daemons and the like, which typically use raw
sockets to access IPv6 or ICMPv6 header fields. This document
proposes some portable interfaces for applications that use raw
sockets under IPv6. There are other features of IPv6 that some
applications will need to access: interface identification
(specifying the outgoing interface and determining the incoming
interface), IPv6 extension headers, and path Maximum Transmission
Unit (MTU) information. This document provides API access to these
features too. Additionally, some extended interfaces to libraries
for the "r" commands are defined. The extension will provide better
backward compatibility to existing implementations that are not
IPv6-capable.
Stevens, et al. Informational [Page 1]
RFC 3542 Advanced Sockets API for IPv6 May 2003
Table of Contents
1. Introduction .............................................. 3
2. Common Structures and Definitions ......................... 5
2.1 The ip6_hdr Structure ................................ 6
2.1.1 IPv6 Next Header Values ....................... 6
2.1.2 IPv6 Extension Headers ........................ 7
2.1.3 IPv6 Options .................................. 8
2.2 The icmp6_hdr Structure .............................. 10
2.2.1 ICMPv6 Type and Code Values ................... 10
2.2.2 ICMPv6 Neighbor Discovery Definitions ......... 11
2.2.3 Multicast Listener Discovery Definitions ...... 14
2.2.4 ICMPv6 Router Renumbering Definitions ......... 14
2.3 Address Testing Macros ............................... 16
2.4 Protocols File ....................................... 16
3. IPv6 Raw Sockets .......................................... 17
3.1 Checksums ............................................ 18
3.2 ICMPv6 Type Filtering ................................ 19
3.3 ICMPv6 Verification of Received Packets .............. 22
4. Access to IPv6 and Extension Headers ...................... 22
4.1 TCP Implications ..................................... 24
4.2 UDP and Raw Socket Implications ...................... 25
5. Extensions to Socket Ancillary Data ....................... 26
5.1 CMSG_NXTHDR .......................................... 26
5.2 CMSG_SPACE ........................................... 26
5.3 CMSG_LEN ............................................. 27
6. Packet Information ........................................ 27
6.1 Specifying/Receiving the Interface ................... 28
6.2 Specifying/Receiving Source/Destination Address ...... 29
6.3 Specifying/Receiving the Hop Limit ................... 29
6.4 Specifying the Next Hop Address ...................... 30
6.5 Specifying/Receiving the Traffic Class value ......... 31
6.6 Additional Errors with sendmsg() and setsockopt() .... 32
6.7 Summary of Outgoing Interface Selection .............. 32
7. Routing Header Option ..................................... 33
7.1 inet6_rth_space ...................................... 35
7.2 inet6_rth_init ....................................... 35
7.3 inet6_rth_add ........................................ 36
7.4 inet6_rth_reverse .................................... 36
7.5 inet6_rth_segments ................................... 36
7.6 inet6_rth_getaddr .................................... 36
8. Hop-By-Hop Options ........................................ 37
8.1 Receiving Hop-by-Hop Options ......................... 38
8.2 Sending Hop-by-Hop Options ........................... 38
9. Destination Options ....................................... 39
9.1 Receiving Destination Options ........................ 39
9.2 Sending Destination Options .......................... 39
10. Hop-by-Hop and Destination Options Processing ............. 40
Stevens, et al. Informational [Page 2]
RFC 3542 Advanced Sockets API for IPv6 May 2003
10.1 inet6_opt_init ...................................... 41
10.2 inet6_opt_append .................................... 41
10.3 inet6_opt_finish .................................... 42
10.4 inet6_opt_set_val ................................... 42
10.5 inet6_opt_next ...................................... 42
10.6 inet6_opt_find ...................................... 43
10.7 inet6_opt_get_val ................................... 43
11. Additional Advanced API Functions ......................... 44
11.1 Sending with the Minimum MTU ........................ 44
11.2 Sending without Fragmentation ....................... 45
11.3 Path MTU Discovery and UDP .......................... 46
11.4 Determining the Current Path MTU .................... 47
12. Ordering of Ancillary Data and IPv6 Extension Headers ..... 48
13. IPv6-Specific Options with IPv4-Mapped IPv6 Addresses ..... 50
14. Extended interfaces for rresvport, rcmd and rexec ......... 51
14.1 rresvport_af ........................................ 51
14.2 rcmd_af ............................................. 51
14.3 rexec_af ............................................ 52
15. Summary of New Definitions ................................ 52
16. Security Considerations ................................... 56
17. Changes from RFC 2292 ..................................... 57
18. References ................................................ 59
19. Acknowledgments ........................................... 59
20. Appendix A: Ancillary Data Overview ....................... 60
20.1 The msghdr Structure ................................ 60
20.2 The cmsghdr Structure ............................... 61
20.3 Ancillary Data Object Macros ........................ 62
20.3.1 CMSG_FIRSTHDR ............................... 63
20.3.2 CMSG_NXTHDR ................................. 64
20.3.3 CMSG_DATA ................................... 65
20.3.4 CMSG_SPACE .................................. 65
20.3.5 CMSG_LEN .................................... 65
21. Appendix B: Examples Using the inet6_rth_XXX() Functions .. 65
21.1 Sending a Routing Header ............................ 65
21.2 Receiving Routing Headers ........................... 70
22. Appendix C: Examples Using the inet6_opt_XXX() Functions .. 72
22.1 Building Options .................................... 72
22.2 Parsing Received Options ............................ 74
23. Authors' Addresses ........................................ 76
24. Full Copyright Statement .................................. 77
1. Introduction
A separate specification [RFC-3493] contains changes to the sockets
API to support IP version 6. Those changes are for TCP and UDP-based
applications. This document defines some of the "advanced" features
of the sockets API that are required for applications to take
advantage of additional features of IPv6.
Stevens, et al. Informational [Page 3]
RFC 3542 Advanced Sockets API for IPv6 May 2003
Today, the portability of applications using IPv4 raw sockets is
quite high, but this is mainly because most IPv4 implementations
started from a common base (the Berkeley source code) or at least
started with the Berkeley header files. This allows programs such as
Ping and Traceroute, for example, to compile with minimal effort on
many hosts that support the sockets API. With IPv6, however, there
is no common source code base that implementors are starting from,
and the possibility for divergence at this level between different
implementations is high. To avoid a complete lack of portability
amongst applications that use raw IPv6 sockets, some standardization
is necessary.
There are also features from the basic IPv6 specification that are
not addressed in [RFC-3493]: sending and receiving Routing headers,
Hop-by-Hop options, and Destination options, specifying the outgoing
interface, being told of the receiving interface, and control of path
MTU information.
This document updates and replaces RFC 2292. This revision is based
on implementation experience of RFC 2292, as well as some additional
extensions that have been found to be useful through the IPv6
deployment. Note, however, that further work on this document may
still be needed. Once the API specification becomes mature and is
deployed among implementations, it may be formally standardized by a
more appropriate body, such as has been done with the Basic API
[RFC-3493].
This document can be divided into the following main sections.
1. Definitions of the basic constants and structures required for
applications to use raw IPv6 sockets. This includes structure
definitions for the IPv6 and ICMPv6 headers and all associated
constants (e.g., values for the Next Header field).
2. Some basic semantic definitions for IPv6 raw sockets. For
example, a raw ICMPv4 socket requires the application to calculate
and store the ICMPv4 header checksum. But with IPv6 this would
require the application to choose the source IPv6 address because
the source address is part of the pseudo header that ICMPv6 now
uses for its checksum computation. It should be defined that with
a raw ICMPv6 socket the kernel always calculates and stores the
ICMPv6 header checksum.
3. Packet information: how applications can obtain the received
interface, destination address, and received hop limit, along with
specifying these values on a per-packet basis. There are a class
of applications that need this capability and the technique should
be portable.
Stevens, et al. Informational [Page 4]
RFC 3542 Advanced Sockets API for IPv6 May 2003
4. Access to the optional Routing header, Hop-by-Hop options, and
Destination options extension headers.
5. Additional features required for improved IPv6 application
portability.
The packet information along with access to the extension headers
(Routing header, Hop-by-Hop options, and Destination options) are
specified using the "ancillary data" fields that were added to the
4.3BSD Reno sockets API in 1990. The reason is that these ancillary
data fields are part of the Posix standard [POSIX] and should
therefore be adopted by most vendors.
This document does not address application access to either the
authentication header or the encapsulating security payload header.
Many examples in this document omit error checking in favor of
brevity and clarity.
We note that some of the functions and socket options defined in this
document may have error returns that are not defined in this
document. Some of these possible error returns will be recognized
only as implementations proceed.
Datatypes in this document follow the Posix format: intN_t means a
signed integer of exactly N bits (e.g., int16_t) and uintN_t means an
unsigned integer of exactly N bits (e.g., uint32_t).
Note that we use the (unofficial) terminology ICMPv4, IGMPv4, and
ARPv4 to avoid any confusion with the newer ICMPv6 protocol.
2. Common Structures and Definitions
Many advanced applications examine fields in the IPv6 header and set
and examine fields in the various ICMPv6 headers. Common structure
definitions for these protocol headers are required, along with
common constant definitions for the structure members.
This API assumes that the fields in the protocol headers are left in
the network byte order, which is big-endian for the Internet
protocols. If not, then either these constants or the fields being
tested must be converted at run-time, using something like htons() or
htonl().
Two new header files are defined: <netinet/ip6.h> and
<netinet/icmp6.h>.
Stevens, et al. Informational [Page 5]
RFC 3542 Advanced Sockets API for IPv6 May 2003
When an include file is specified, that include file is allowed to
include other files that do the actual declaration or definition.
2.1. The ip6_hdr Structure
The following structure is defined as a result of including
<netinet/ip6.h>. Note that this is a new header.
struct ip6_hdr {
union {
struct ip6_hdrctl {
uint32_t ip6_un1_flow; /* 4 bits version, 8 bits TC, 20 bits
flow-ID */
uint16_t ip6_un1_plen; /* payload length */
uint8_t ip6_un1_nxt; /* next header */
uint8_t ip6_un1_hlim; /* hop limit */
} ip6_un1;
uint8_t ip6_un2_vfc; /* 4 bits version, top 4 bits
tclass */
} ip6_ctlun;
struct in6_addr ip6_src; /* source address */
struct in6_addr ip6_dst; /* destination address */
};
#define ip6_vfc ip6_ctlun.ip6_un2_vfc
#define ip6_flow ip6_ctlun.ip6_un1.ip6_un1_flow
#define ip6_plen ip6_ctlun.ip6_un1.ip6_un1_plen
#define ip6_nxt ip6_ctlun.ip6_un1.ip6_un1_nxt
#define ip6_hlim ip6_ctlun.ip6_un1.ip6_un1_hlim
#define ip6_hops ip6_ctlun.ip6_un1.ip6_un1_hlim
2.1.1. IPv6 Next Header Values
IPv6 defines many new values for the Next Header field. The
following constants are defined as a result of including
<netinet/in.h>.
#define IPPROTO_HOPOPTS 0 /* IPv6 Hop-by-Hop options */
#define IPPROTO_IPV6 41 /* IPv6 header */
#define IPPROTO_ROUTING 43 /* IPv6 Routing header */
#define IPPROTO_FRAGMENT 44 /* IPv6 fragment header */
#define IPPROTO_ESP 50 /* encapsulating security payload */
#define IPPROTO_AH 51 /* authentication header */
#define IPPROTO_ICMPV6 58 /* ICMPv6 */
#define IPPROTO_NONE 59 /* IPv6 no next header */
#define IPPROTO_DSTOPTS 60 /* IPv6 Destination options */
Stevens, et al. Informational [Page 6]
RFC 3542 Advanced Sockets API for IPv6 May 2003
Berkeley-derived IPv4 implementations also define IPPROTO_IP to be 0.
This should not be a problem since IPPROTO_IP is used only with IPv4
sockets and IPPROTO_HOPOPTS only with IPv6 sockets.
2.1.2. IPv6 Extension Headers
Six extension headers are defined for IPv6. We define structures for
all except the Authentication header and Encapsulating Security
Payload header, both of which are beyond the scope of this document.
The following structures are defined as a result of including
<netinet/ip6.h>.
/* Hop-by-Hop options header */
struct ip6_hbh {
uint8_t ip6h_nxt; /* next header */
uint8_t ip6h_len; /* length in units of 8 octets */
/* followed by options */
};
/* Destination options header */
struct ip6_dest {
uint8_t ip6d_nxt; /* next header */
uint8_t ip6d_len; /* length in units of 8 octets */
/* followed by options */
};
/* Routing header */
struct ip6_rthdr {
uint8_t ip6r_nxt; /* next header */
uint8_t ip6r_len; /* length in units of 8 octets */
uint8_t ip6r_type; /* routing type */
uint8_t ip6r_segleft; /* segments left */
/* followed by routing type specific data */
};
/* Type 0 Routing header */
struct ip6_rthdr0 {
uint8_t ip6r0_nxt; /* next header */
uint8_t ip6r0_len; /* length in units of 8 octets */
uint8_t ip6r0_type; /* always zero */
uint8_t ip6r0_segleft; /* segments left */
uint32_t ip6r0_reserved; /* reserved field */
/* followed by up to 127 struct in6_addr */
};
Stevens, et al. Informational [Page 7]
RFC 3542 Advanced Sockets API for IPv6 May 2003
/* Fragment header */
struct ip6_frag {
uint8_t ip6f_nxt; /* next header */
uint8_t ip6f_reserved; /* reserved field */
uint16_t ip6f_offlg; /* offset, reserved, and flag */
uint32_t ip6f_ident; /* identification */
};
#if BYTE_ORDER == BIG_ENDIAN
#define IP6F_OFF_MASK 0xfff8 /* mask out offset from
ip6f_offlg */
#define IP6F_RESERVED_MASK 0x0006 /* reserved bits in
ip6f_offlg */
#define IP6F_MORE_FRAG 0x0001 /* more-fragments flag */
#else /* BYTE_ORDER == LITTLE_ENDIAN */
#define IP6F_OFF_MASK 0xf8ff /* mask out offset from
ip6f_offlg */
#define IP6F_RESERVED_MASK 0x0600 /* reserved bits in
ip6f_offlg */
#define IP6F_MORE_FRAG 0x0100 /* more-fragments flag */
#endif
2.1.3. IPv6 Options
Several options are defined for IPv6, and we define structures and
macro definitions for some of them below. The following structures
are defined as a result of including <netinet/ip6.h>.
/* IPv6 options */
struct ip6_opt {
uint8_t ip6o_type;
uint8_t ip6o_len;
};
/*
* The high-order 3 bits of the option type define the behavior
* when processing an unknown option and whether or not the option
* content changes in flight.
*/
#define IP6OPT_TYPE(o) ((o) & 0xc0)
#define IP6OPT_TYPE_SKIP 0x00
#define IP6OPT_TYPE_DISCARD 0x40
#define IP6OPT_TYPE_FORCEICMP 0x80
#define IP6OPT_TYPE_ICMP 0xc0
#define IP6OPT_MUTABLE 0x20
#define IP6OPT_PAD1 0x00 /* 00 0 00000 */
#define IP6OPT_PADN 0x01 /* 00 0 00001 */
Stevens, et al. Informational [Page 8]
RFC 3542 Advanced Sockets API for IPv6 May 2003
#define IP6OPT_JUMBO 0xc2 /* 11 0 00010 */
#define IP6OPT_NSAP_ADDR 0xc3 /* 11 0 00011 */
#define IP6OPT_TUNNEL_LIMIT 0x04 /* 00 0 00100 */
#define IP6OPT_ROUTER_ALERT 0x05 /* 00 0 00101 */
/* Jumbo Payload Option */
struct ip6_opt_jumbo {
uint8_t ip6oj_type;
uint8_t ip6oj_len;
uint8_t ip6oj_jumbo_len[4];
};
#define IP6OPT_JUMBO_LEN 6
/* NSAP Address Option */
struct ip6_opt_nsap {
uint8_t ip6on_type;
uint8_t ip6on_len;
uint8_t ip6on_src_nsap_len;
uint8_t ip6on_dst_nsap_len;
/* followed by source NSAP */
/* followed by destination NSAP */
};
/* Tunnel Limit Option */
struct ip6_opt_tunnel {
uint8_t ip6ot_type;
uint8_t ip6ot_len;
uint8_t ip6ot_encap_limit;
};
/* Router Alert Option */
struct ip6_opt_router {
uint8_t ip6or_type;
uint8_t ip6or_len;
uint8_t ip6or_value[2];
};
/* Router alert values (in network byte order) */
#ifdef _BIG_ENDIAN
#define IP6_ALERT_MLD 0x0000
#define IP6_ALERT_RSVP 0x0001
#define IP6_ALERT_AN 0x0002
#else
#define IP6_ALERT_MLD 0x0000
#define IP6_ALERT_RSVP 0x0100
#define IP6_ALERT_AN 0x0200
#endif
Stevens, et al. Informational [Page 9]
RFC 3542 Advanced Sockets API for IPv6 May 2003
2.2. The icmp6_hdr Structure
The ICMPv6 header is needed by numerous IPv6 applications including
Ping, Traceroute, router discovery daemons, and neighbor discovery
daemons. The following structure is defined as a result of including
<netinet/icmp6.h>. Note that this is a new header.
struct icmp6_hdr {
uint8_t icmp6_type; /* type field */
uint8_t icmp6_code; /* code field */
uint16_t icmp6_cksum; /* checksum field */
union {
uint32_t icmp6_un_data32[1]; /* type-specific field */
uint16_t icmp6_un_data16[2]; /* type-specific field */
uint8_t icmp6_un_data8[4]; /* type-specific field */
} icmp6_dataun;
};
#define icmp6_data32 icmp6_dataun.icmp6_un_data32
#define icmp6_data16 icmp6_dataun.icmp6_un_data16
#define icmp6_data8 icmp6_dataun.icmp6_un_data8
#define icmp6_pptr icmp6_data32[0] /* parameter prob */
#define icmp6_mtu icmp6_data32[0] /* packet too big */
#define icmp6_id icmp6_data16[0] /* echo request/reply */
#define icmp6_seq icmp6_data16[1] /* echo request/reply */
#define icmp6_maxdelay icmp6_data16[0] /* mcast group
membership */
2.2.1. ICMPv6 Type and Code Values
In addition to a common structure for the ICMPv6 header, common
definitions are required for the ICMPv6 type and code fields. The
following constants are also defined as a result of including
<netinet/icmp6.h>.
#define ICMP6_DST_UNREACH 1
#define ICMP6_PACKET_TOO_BIG 2
#define ICMP6_TIME_EXCEEDED 3
#define ICMP6_PARAM_PROB 4
#define ICMP6_INFOMSG_MASK 0x80 /* all informational
messages */
#define ICMP6_ECHO_REQUEST 128
#define ICMP6_ECHO_REPLY 129
#define ICMP6_DST_UNREACH_NOROUTE 0 /* no route to
destination */
Stevens, et al. Informational [Page 10]
RFC 3542 Advanced Sockets API for IPv6 May 2003
#define ICMP6_DST_UNREACH_ADMIN 1 /* communication with
destination */
/* admin. prohibited */
#define ICMP6_DST_UNREACH_BEYONDSCOPE 2 /* beyond scope of source
address */
#define ICMP6_DST_UNREACH_ADDR 3 /* address unreachable */
#define ICMP6_DST_UNREACH_NOPORT 4 /* bad port */
#define ICMP6_TIME_EXCEED_TRANSIT 0 /* Hop Limit == 0 in
transit */
#define ICMP6_TIME_EXCEED_REASSEMBLY 1 /* Reassembly time out */
#define ICMP6_PARAMPROB_HEADER 0 /* erroneous header
field */
#define ICMP6_PARAMPROB_NEXTHEADER 1 /* unrecognized
Next Header */
#define ICMP6_PARAMPROB_OPTION 2 /* unrecognized
IPv6 option */
The five ICMP message types defined by IPv6 neighbor discovery (133-
137) are defined in the next section.
2.2.2. ICMPv6 Neighbor Discovery Definitions
The following structures and definitions are defined as a result of
including <netinet/icmp6.h>.
#define ND_ROUTER_SOLICIT 133
#define ND_ROUTER_ADVERT 134
#define ND_NEIGHBOR_SOLICIT 135
#define ND_NEIGHBOR_ADVERT 136
#define ND_REDIRECT 137
struct nd_router_solicit { /* router solicitation */
struct icmp6_hdr nd_rs_hdr;
/* could be followed by options */
};
#define nd_rs_type nd_rs_hdr.icmp6_type
#define nd_rs_code nd_rs_hdr.icmp6_code
#define nd_rs_cksum nd_rs_hdr.icmp6_cksum
#define nd_rs_reserved nd_rs_hdr.icmp6_data32[0]
Stevens, et al. Informational [Page 11]
RFC 3542 Advanced Sockets API for IPv6 May 2003
struct nd_router_advert { /* router advertisement */
struct icmp6_hdr nd_ra_hdr;
uint32_t nd_ra_reachable; /* reachable time */
uint32_t nd_ra_retransmit; /* retransmit timer */
/* could be followed by options */
};
#define nd_ra_type nd_ra_hdr.icmp6_type
#define nd_ra_code nd_ra_hdr.icmp6_code
#define nd_ra_cksum nd_ra_hdr.icmp6_cksum
#define nd_ra_curhoplimit nd_ra_hdr.icmp6_data8[0]
#define nd_ra_flags_reserved nd_ra_hdr.icmp6_data8[1]
#define ND_RA_FLAG_MANAGED 0x80
#define ND_RA_FLAG_OTHER 0x40
#define nd_ra_router_lifetime nd_ra_hdr.icmp6_data16[1]
struct nd_neighbor_solicit { /* neighbor solicitation */
struct icmp6_hdr nd_ns_hdr;
struct in6_addr nd_ns_target; /* target address */
/* could be followed by options */
};
#define nd_ns_type nd_ns_hdr.icmp6_type
#define nd_ns_code nd_ns_hdr.icmp6_code
#define nd_ns_cksum nd_ns_hdr.icmp6_cksum
#define nd_ns_reserved nd_ns_hdr.icmp6_data32[0]
struct nd_neighbor_advert { /* neighbor advertisement */
struct icmp6_hdr nd_na_hdr;
struct in6_addr nd_na_target; /* target address */
/* could be followed by options */
};
#define nd_na_type nd_na_hdr.icmp6_type
#define nd_na_code nd_na_hdr.icmp6_code
#define nd_na_cksum nd_na_hdr.icmp6_cksum
#define nd_na_flags_reserved nd_na_hdr.icmp6_data32[0]
#if BYTE_ORDER == BIG_ENDIAN
#define ND_NA_FLAG_ROUTER 0x80000000
#define ND_NA_FLAG_SOLICITED 0x40000000
#define ND_NA_FLAG_OVERRIDE 0x20000000
#else /* BYTE_ORDER == LITTLE_ENDIAN */
#define ND_NA_FLAG_ROUTER 0x00000080
#define ND_NA_FLAG_SOLICITED 0x00000040
#define ND_NA_FLAG_OVERRIDE 0x00000020
#endif
Stevens, et al. Informational [Page 12]
RFC 3542 Advanced Sockets API for IPv6 May 2003
struct nd_redirect { /* redirect */
struct icmp6_hdr nd_rd_hdr;
struct in6_addr nd_rd_target; /* target address */
struct in6_addr nd_rd_dst; /* destination address */
/* could be followed by options */
};
#define nd_rd_type nd_rd_hdr.icmp6_type
#define nd_rd_code nd_rd_hdr.icmp6_code
#define nd_rd_cksum nd_rd_hdr.icmp6_cksum
#define nd_rd_reserved nd_rd_hdr.icmp6_data32[0]
struct nd_opt_hdr { /* Neighbor discovery option header */
uint8_t nd_opt_type;
uint8_t nd_opt_len; /* in units of 8 octets */
/* followed by option specific data */
};
#define ND_OPT_SOURCE_LINKADDR 1
#define ND_OPT_TARGET_LINKADDR 2
#define ND_OPT_PREFIX_INFORMATION 3
#define ND_OPT_REDIRECTED_HEADER 4
#define ND_OPT_MTU 5
struct nd_opt_prefix_info { /* prefix information */
uint8_t nd_opt_pi_type;
uint8_t nd_opt_pi_len;
uint8_t nd_opt_pi_prefix_len;
uint8_t nd_opt_pi_flags_reserved;
uint32_t nd_opt_pi_valid_time;
uint32_t nd_opt_pi_preferred_time;
uint32_t nd_opt_pi_reserved2;
struct in6_addr nd_opt_pi_prefix;
};
#define ND_OPT_PI_FLAG_ONLINK 0x80
#define ND_OPT_PI_FLAG_AUTO 0x40
struct nd_opt_rd_hdr { /* redirected header */
uint8_t nd_opt_rh_type;
uint8_t nd_opt_rh_len;
uint16_t nd_opt_rh_reserved1;
uint32_t nd_opt_rh_reserved2;
/* followed by IP header and data */
};
Stevens, et al. Informational [Page 13]
RFC 3542 Advanced Sockets API for IPv6 May 2003
struct nd_opt_mtu { /* MTU option */
uint8_t nd_opt_mtu_type;
uint8_t nd_opt_mtu_len;
uint16_t nd_opt_mtu_reserved;
uint32_t nd_opt_mtu_mtu;
};
We note that the nd_na_flags_reserved flags have the same byte
ordering problems as we showed with ip6f_offlg.
2.2.3. Multicast Listener Discovery Definitions
The following structures and definitions are defined as a result of
including <netinet/icmp6.h>.
#define MLD_LISTENER_QUERY 130
#define MLD_LISTENER_REPORT 131
#define MLD_LISTENER_REDUCTION 132
struct mld_hdr {
struct icmp6_hdr mld_icmp6_hdr;
struct in6_addr mld_addr; /* multicast address */
};
#define mld_type mld_icmp6_hdr.icmp6_type
#define mld_code mld_icmp6_hdr.icmp6_code
#define mld_cksum mld_icmp6_hdr.icmp6_cksum
#define mld_maxdelay mld_icmp6_hdr.icmp6_data16[0]
#define mld_reserved mld_icmp6_hdr.icmp6_data16[1]
2.2.4. ICMPv6 Router Renumbering Definitions
The following structures and definitions are defined as a result of
including <netinet/icmp6.h>.
#define ICMP6_ROUTER_RENUMBERING 138 /* router renumbering */
struct icmp6_router_renum { /* router renumbering header */
struct icmp6_hdr rr_hdr;
uint8_t rr_segnum;
uint8_t rr_flags;
uint16_t rr_maxdelay;
uint32_t rr_reserved;
};
#define rr_type rr_hdr.icmp6_type
#define rr_code rr_hdr.icmp6_code
#define rr_cksum rr_hdr.icmp6_cksum
#define rr_seqnum rr_hdr.icmp6_data32[0]
Stevens, et al. Informational [Page 14]
RFC 3542 Advanced Sockets API for IPv6 May 2003
/* Router renumbering flags */
#define ICMP6_RR_FLAGS_TEST 0x80
#define ICMP6_RR_FLAGS_REQRESULT 0x40
#define ICMP6_RR_FLAGS_FORCEAPPLY 0x20
#define ICMP6_RR_FLAGS_SPECSITE 0x10
#define ICMP6_RR_FLAGS_PREVDONE 0x08
struct rr_pco_match { /* match prefix part */
uint8_t rpm_code;
uint8_t rpm_len;
uint8_t rpm_ordinal;
uint8_t rpm_matchlen;
uint8_t rpm_minlen;
uint8_t rpm_maxlen;
uint16_t rpm_reserved;
struct in6_addr rpm_prefix;
};
/* PCO code values */
#define RPM_PCO_ADD 1
#define RPM_PCO_CHANGE 2
#define RPM_PCO_SETGLOBAL 3
struct rr_pco_use { /* use prefix part */
uint8_t rpu_uselen;
uint8_t rpu_keeplen;
uint8_t rpu_ramask;
uint8_t rpu_raflags;
uint32_t rpu_vltime;
uint32_t rpu_pltime;
uint32_t rpu_flags;
struct in6_addr rpu_prefix;
};
#define ICMP6_RR_PCOUSE_RAFLAGS_ONLINK 0x20
#define ICMP6_RR_PCOUSE_RAFLAGS_AUTO 0x10
#if BYTE_ORDER == BIG_ENDIAN
#define ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME 0x80000000
#define ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME 0x40000000
#elif BYTE_ORDER == LITTLE_ENDIAN
#define ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME 0x80
#define ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME 0x40
#endif
Stevens, et al. Informational [Page 15]
RFC 3542 Advanced Sockets API for IPv6 May 2003
struct rr_result { /* router renumbering result message */
uint16_t rrr_flags;
uint8_t rrr_ordinal;
uint8_t rrr_matchedlen;
uint32_t rrr_ifid;
struct in6_addr rrr_prefix;
};
#if BYTE_ORDER == BIG_ENDIAN
#define ICMP6_RR_RESULT_FLAGS_OOB 0x0002
#define ICMP6_RR_RESULT_FLAGS_FORBIDDEN 0x0001
#elif BYTE_ORDER == LITTLE_ENDIAN
#define ICMP6_RR_RESULT_FLAGS_OOB 0x0200
#define ICMP6_RR_RESULT_FLAGS_FORBIDDEN 0x0100
#endif
2.3. Address Testing Macros
The basic API ([RFC-3493]) defines some macros for testing an IPv6
address for certain properties. This API extends those definitions
with additional address testing macros, defined as a result of
including <netinet/in.h>.
int IN6_ARE_ADDR_EQUAL(const struct in6_addr *,
const struct in6_addr *);
This macro returns non-zero if the addresses are equal; otherwise it
returns zero.
2.4. Protocols File
Many hosts provide the file /etc/protocols that contains the names of
the various IP protocols and their protocol number (e.g., the value
of the protocol field in the IPv4 header for that protocol, such as 1
for ICMP). Some programs then call the function getprotobyname() to
obtain the protocol value that is then specified as the third
argument to the socket() function. For example, the Ping program
contains code of the form
struct protoent *proto;
proto = getprotobyname("icmp");
s = socket(AF_INET, SOCK_RAW, proto->p_proto);
Common names are required for the new IPv6 protocols in this file, to
provide portability of applications that call the getprotoXXX()
functions.
Stevens, et al. Informational [Page 16]
RFC 3542 Advanced Sockets API for IPv6 May 2003
We define the following protocol names with the values shown. These
are taken under http://www.iana.org/numbers.html.
hopopt 0 # hop-by-hop options for ipv6
ipv6 41 # ipv6
ipv6-route 43 # routing header for ipv6
ipv6-frag 44 # fragment header for ipv6
esp 50 # encapsulating security payload for ipv6
ah 51 # authentication header for ipv6
ipv6-icmp 58 # icmp for ipv6
ipv6-nonxt 59 # no next header for ipv6
ipv6-opts 60 # destination options for ipv6
3. IPv6 Raw Sockets
Raw sockets bypass the transport layer (TCP or UDP). With IPv4, raw
sockets are used to access ICMPv4, IGMPv4, and to read and write IPv4
datagrams containing a protocol field that the kernel does not
process. An example of the latter is a routing daemon for OSPF,
since it uses IPv4 protocol field 89. With IPv6 raw sockets will be
used for ICMPv6 and to read and write IPv6 datagrams containing a
Next Header field that the kernel does not process. Examples of the
latter are a routing daemon for OSPF for IPv6 and RSVP (protocol
field 46).
All data sent via raw sockets must be in network byte order and all
data received via raw sockets will be in network byte order. This
differs from the IPv4 raw sockets, which did not specify a byte
ordering and used the host's byte order for certain IP header fields.
Another difference from IPv4 raw sockets is that complete packets
(that is, IPv6 packets with extension headers) cannot be sent or
received using the IPv6 raw sockets API. Instead, ancillary data
objects are used to transfer the extension headers and hoplimit
information, as described in Section 6. Should an application need
access to the complete IPv6 packet, some other technique, such as the
datalink interfaces BPF or DLPI, must be used.
All fields except the flow label in the IPv6 header that an
application might want to change (i.e., everything other than the
version number) can be modified using ancillary data and/or socket
options by the application for output. All fields except the flow
label in a received IPv6 header (other than the version number and
Next Header fields) and all extension headers that an application
might want to know are also made available to the application as
ancillary data on input. Hence there is no need for a socket option
Stevens, et al. Informational [Page 17]
RFC 3542 Advanced Sockets API for IPv6 May 2003
similar to the IPv4 IP_HDRINCL socket option and on receipt the
application will only receive the payload i.e., the data after the
IPv6 header and all the extension headers.
This API does not define access to the flow label field, because
today there is no standard usage of the field.
When writing to a raw socket the kernel will automatically fragment
the packet if its size exceeds the path MTU, inserting the required
fragment headers. On input the kernel reassembles received
fragments, so the reader of a raw socket never sees any fragment
headers.
When we say "an ICMPv6 raw socket" we mean a socket created by
calling the socket function with the three arguments AF_INET6,
SOCK_RAW, and IPPROTO_ICMPV6.
Most IPv4 implementations give special treatment to a raw socket
created with a third argument to socket() of IPPROTO_RAW, whose value
is normally 255, to have it mean that the application will send down
complete packets including the IPv4 header. (Note: This feature was
added to IPv4 in 1988 by Van Jacobson to support traceroute, allowing
a complete IP header to be passed by the application, before the
IP_HDRINCL socket option was added.) We note that IPPROTO_RAW has no
special meaning to an IPv6 raw socket (and the IANA currently
reserves the value of 255 when used as a next-header field).
3.1. Checksums
The kernel will calculate and insert the ICMPv6 checksum for ICMPv6
raw sockets, since this checksum is mandatory.
For other raw IPv6 sockets (that is, for raw IPv6 sockets created
with a third argument other than IPPROTO_ICMPV6), the application
must set the new IPV6_CHECKSUM socket option to have the kernel (1)
compute and store a checksum for output, and (2) verify the received
checksum on input, discarding the packet if the checksum is in error.
This option prevents applications from having to perform source
address selection on the packets they send. The checksum will
incorporate the IPv6 pseudo-header, defined in Section 8.1 of [RFC-
2460]. This new socket option also specifies an integer offset into
the user data of where the checksum is located.
int offset = 2;
setsockopt(fd, IPPROTO_IPV6, IPV6_CHECKSUM, &offset,
sizeof(offset));
Stevens, et al. Informational [Page 18]
RFC 3542 Advanced Sockets API for IPv6 May 2003
By default, this socket option is disabled. Setting the offset to -1
also disables the option. By disabled we mean (1) the kernel will
not calculate and store a checksum for outgoing packets, and (2) the
kernel will not verify a checksum for received packets.
This option assumes the use of the 16-bit one's complement of the
one's complement sum as the checksum algorithm and that the checksum
field is aligned on a 16-bit boundary. Thus, specifying a positive
odd value as offset is invalid, and setsockopt() will fail for such
offset values.
An attempt to set IPV6_CHECKSUM for an ICMPv6 socket will fail.
Also, an attempt to set or get IPV6_CHECKSUM for a non-raw IPv6
socket will fail.
(Note: Since the checksum is always calculated by the kernel for an
ICMPv6 socket, applications are not able to generate ICMPv6 packets
with incorrect checksums (presumably for testing purposes) using this
API.)
3.2. ICMPv6 Type Filtering
ICMPv4 raw sockets receive most ICMPv4 messages received by the
kernel. (We say "most" and not "all" because Berkeley-derived
kernels never pass echo requests, timestamp requests, or address mask
requests to a raw socket. Instead these three messages are processed
entirely by the kernel.) But ICMPv6 is a superset of ICMPv4, also
including the functionality of IGMPv4 and ARPv4. This means that an
ICMPv6 raw socket can potentially receive many more messages than
would be received with an ICMPv4 raw socket: ICMP messages similar to
ICMPv4, along with neighbor solicitations, neighbor advertisements,
and the three multicast listener discovery messages.
Most applications using an ICMPv6 raw socket care about only a small
subset of the ICMPv6 message types. To transfer extraneous ICMPv6
messages from the kernel to user can incur a significant overhead.
Therefore this API includes a method of filtering ICMPv6 messages by
the ICMPv6 type field.
Each ICMPv6 raw socket has an associated filter whose datatype is
defined as
struct icmp6_filter;
This structure, along with the macros and constants defined later in
this section, are defined as a result of including the
<netinet/icmp6.h>.
Stevens, et al. Informational [Page 19]
RFC 3542 Advanced Sockets API for IPv6 May 2003
The current filter is fetched and stored using getsockopt() and
setsockopt() with a level of IPPROTO_ICMPV6 and an option name of
ICMP6_FILTER.
Six macros operate on an icmp6_filter structure:
void ICMP6_FILTER_SETPASSALL (struct icmp6_filter *);
void ICMP6_FILTER_SETBLOCKALL(struct icmp6_filter *);
void ICMP6_FILTER_SETPASS ( int, struct icmp6_filter *);
void ICMP6_FILTER_SETBLOCK( int, struct icmp6_filter *);
int ICMP6_FILTER_WILLPASS (int,
const struct icmp6_filter *);
int ICMP6_FILTER_WILLBLOCK(int,
const struct icmp6_filter *);
The first argument to the last four macros (an integer) is an ICMPv6
message type, between 0 and 255. The pointer argument to all six
macros is a pointer to a filter that is modified by the first four
macros and is examined by the last two macros.
The first two macros, SETPASSALL and SETBLOCKALL, let us specify that
all ICMPv6 messages are passed to the application or that all ICMPv6
messages are blocked from being passed to the application.
The next two macros, SETPASS and SETBLOCK, let us specify that
messages of a given ICMPv6 type should be passed to the application
or not passed to the application (blocked).
The final two macros, WILLPASS and WILLBLOCK, return true or false
depending whether the specified message type is passed to the
application or blocked from being passed to the application by the
filter pointed to by the second argument.
When an ICMPv6 raw socket is created, it will by default pass all
ICMPv6 message types to the application.
As an example, a program that wants to receive only router
advertisements could execute the following:
struct icmp6_filter myfilt;
fd = socket(AF_INET6, SOCK_RAW, IPPROTO_ICMPV6);
ICMP6_FILTER_SETBLOCKALL(&myfilt);
ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &myfilt);
setsockopt(fd, IPPROTO_ICMPV6, ICMP6_FILTER, &myfilt,
Stevens, et al. Informational [Page 20]
RFC 3542 Advanced Sockets API for IPv6 May 2003
sizeof(myfilt));
The filter structure is declared and then initialized to block all
messages types. The filter structure is then changed to allow router
advertisement messages to be passed to the application and the filter
is installed using setsockopt().
In order to clear an installed filter the application can issue a
setsockopt for ICMP6_FILTER with a zero length. When no such filter
has been installed, getsockopt() will return the kernel default
filter.
The icmp6_filter structure is similar to the fd_set datatype used
with the select() function in the sockets API. The icmp6_filter
structure is an opaque datatype and the application should not care
how it is implemented. All the application does with this datatype
is allocate a variable of this type, pass a pointer to a variable of
this type to getsockopt() and setsockopt(), and operate on a variable
of this type using the six macros that we just defined.
Nevertheless, it is worth showing a simple implementation of this
datatype and the six macros.
struct icmp6_filter {
uint32_t icmp6_filt[8]; /* 8*32 = 256 bits */
};
#define ICMP6_FILTER_WILLPASS(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) & \
(1 << ((type) & 31))) != 0)
#define ICMP6_FILTER_WILLBLOCK(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) & \
(1 << ((type) & 31))) == 0)
#define ICMP6_FILTER_SETPASS(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) |= \
(1 << ((type) & 31))))
#define ICMP6_FILTER_SETBLOCK(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) &= \
~(1 << ((type) & 31))))
#define ICMP6_FILTER_SETPASSALL(filterp) \
memset((filterp), 0xFF, sizeof(struct icmp6_filter))
#define ICMP6_FILTER_SETBLOCKALL(filterp) \
memset((filterp), 0, sizeof(struct icmp6_filter))
(Note: These sample definitions have two limitations that an
implementation may want to change. The first four macros evaluate
their first argument two times. The second two macros require the
inclusion of the <string.h> header for the memset() function.)
Stevens, et al. Informational [Page 21]
RFC 3542 Advanced Sockets API for IPv6 May 2003
3.3. ICMPv6 Verification of Received Packets
The protocol stack will verify the ICMPv6 checksum and discard any
packets with invalid checksums.
An implementation might perform additional validity checks on the
ICMPv6 message content and discard malformed packets. However, a
portable application must not assume that such validity checks have
been performed.
The protocol stack should not automatically discard packets if the
ICMP type is unknown to the stack. For extensibility reasons
received ICMP packets with any type (informational or error) must be
passed to the applications (subject to ICMP6_FILTER filtering on the
type value and the checksum verification).
4. Access to IPv6 and Extension Headers
Applications need to be able to control IPv6 header and extension
header content when sending as well as being able to receive the
content of these headers. This is done by defining socket option
types which can be used both with setsockopt and with ancillary data.
Ancillary data is discussed in Appendix A. The following optional
information can be exchanged between the application and the kernel:
1. The send/receive interface and source/destination address,
2. The hop limit,
3. Next hop address,
4. The traffic class,
5. Routing header,
6. Hop-by-Hop options header, and
7. Destination options header.
First, to receive any of this optional information (other than the
next hop address, which can only be set) on a UDP or raw socket, the
application must call setsockopt() to turn on the corresponding flag:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVTCLASS, &on, sizeof(on));
Stevens, et al. Informational [Page 22]
RFC 3542 Advanced Sockets API for IPv6 May 2003
When any of these options are enabled, the corresponding data is
returned as control information by recvmsg(), as one or more
ancillary data objects.
This document does not define how to receive the optional information
on a TCP socket. See Section 4.1 for more details.
Two different mechanisms exist for sending this optional information:
1. Using setsockopt to specify the option content for a socket.
These are known "sticky" options since they affect all transmitted
packets on the socket until either a new setsockopt is done or the
options are overridden using ancillary data.
2. Using ancillary data to specify the option content for a single
datagram. This only applies to datagram and raw sockets; not to
TCP sockets.
The three socket option parameters and the three cmsghdr fields that
describe the options/ancillary data objects are summarized as:
opt level/ optname/ optval/
cmsg_level cmsg_type cmsg_data[]
------------ ------------ ------------------------
IPPROTO_IPV6 IPV6_PKTINFO in6_pktinfo structure
IPPROTO_IPV6 IPV6_HOPLIMIT int
IPPROTO_IPV6 IPV6_NEXTHOP socket address structure
IPPROTO_IPV6 IPV6_RTHDR ip6_rthdr structure
IPPROTO_IPV6 IPV6_HOPOPTS ip6_hbh structure
IPPROTO_IPV6 IPV6_DSTOPTS ip6_dest structure
IPPROTO_IPV6 IPV6_RTHDRDSTOPTS ip6_dest structure
IPPROTO_IPV6 IPV6_TCLASS int
(Note: IPV6_HOPLIMIT can be used as ancillary data items only)
All these options are described in detail in Section 6, 7, 8 and 9.
All the constants beginning with IPV6_ are defined as a result of
including <netinet/in.h>.
Note: We intentionally use the same constant for the cmsg_level
member as is used as the second argument to getsockopt() and
setsockopt() (what is called the "level"), and the same constant for
the cmsg_type member as is used as the third argument to getsockopt()
and setsockopt() (what is called the "option name").
Stevens, et al. Informational [Page 23]
RFC 3542 Advanced Sockets API for IPv6 May 2003
Issuing getsockopt() for the above options will return the sticky
option value i.e., the value set with setsockopt(). If no sticky
option value has been set getsockopt() will return the following
values:
- For the IPV6_PKTINFO option, it will return an in6_pktinfo
structure with ipi6_addr being in6addr_any and ipi6_ifindex being
zero.
- For the IPV6_TCLASS option, it will return the kernel default
value.
- For other options, it will indicate the lack of the option value
with optlen being zero.
The application does not explicitly need to access the data
structures for the Routing header, Hop-by-Hop options header, and
Destination options header, since the API to these features is
through a set of inet6_rth_XXX() and inet6_opt_XXX() functions that
we define in Section 7 and Section 10. Those functions simplify the
interface to these features instead of requiring the application to
know the intimate details of the extension header formats.
When specifying extension headers, this API assumes the header
ordering and the number of occurrences of each header as described in
[RFC-2460]. More details about the ordering issue will be discussed
in Section 12.
4.1. TCP Implications
It is not possible to use ancillary data to transmit the above
options for TCP since there is not a one-to-one mapping between send
operations and the TCP segments being transmitted. Instead an
application can use setsockopt to specify them as sticky options.
When the application uses setsockopt to specify the above options it
is expected that TCP will start using the new information when
sending segments. However, TCP may or may not use the new
information when retransmitting segments that were originally sent
when the old sticky options were in effect.
It is unclear how a TCP application can use received information
(such as extension headers) due to the lack of mapping between
received TCP segments and receive operations. In particular, the
received information could not be used for access control purposes
like on UDP and raw sockets.
Stevens, et al. Informational [Page 24]
RFC 3542 Advanced Sockets API for IPv6 May 2003
This specification therefore does not define how to get the received
information on TCP sockets. The result of the IPV6_RECVxxx options
on a TCP socket is undefined as well.
4.2. UDP and Raw Socket Implications
The receive behavior for UDP and raw sockets is quite
straightforward. After the application has enabled an IPV6_RECVxxx
socket option it will receive ancillary data items for every
recvmsg() call containing the requested information. However, if the
information is not present in the packet the ancillary data item will
not be included. For example, if the application enables
IPV6_RECVRTHDR and a received datagram does not contain a Routing
header there will not be an IPV6_RTHDR ancillary data item. Note
that due to buffering in the socket implementation there might be
some packets queued when an IPV6_RECVxxx option is enabled and they
might not have the ancillary data information.
For sending the application has the choice between using sticky
options and ancillary data. The application can also use both having
the sticky options specify the "default" and using ancillary data to
override the default options.
When an ancillary data item is specified in a call to sendmsg(), the
item will override an existing sticky option of the same name (if
previously specified). For example, if the application has set
IPV6_RTHDR using a sticky option and later passes IPV6_RTHDR as
ancillary data this will override the IPV6_RTHDR sticky option and
the routing header of the outgoing packet will be from the ancillary
data item, not from the sticky option. Note, however, that other
sticky options than IPV6_RTHDR will not be affected by the IPV6_RTHDR
ancillary data item; the overriding mechanism only works for the same
type of sticky options and ancillary data items.
(Note: the overriding rule is different from the one in RFC 2292. In
RFC 2292, an ancillary data item overrode all sticky options
previously defined. This was reasonable, because sticky options
could only be specified as a set by a single socket option. However,
in this API, each option is separated so that it can be specified as
a single sticky option. Additionally, there are much more ancillary
data items and sticky options than in RFC 2292, including ancillary-
only one. Thus, it should be natural for application programmers to
separate the overriding rule as well.)
An application can also temporarily disable a particular sticky
option by specifying a corresponding ancillary data item that could
disable the sticky option when being used as an argument for a socket
option. For example, if the application has set IPV6_HOPOPTS as a
Stevens, et al. Informational [Page 25]
RFC 3542 Advanced Sockets API for IPv6 May 2003
sticky option and later passes IPV6_HOPOPTS with a zero length as an
ancillary data item, the packet will not have a Hop-by-Hop options
header.
5. Extensions to Socket Ancillary Data
This specification uses ancillary data as defined in Posix with some
compatible extensions, which are described in the following
subsections. Section 20 will provide a detailed overview of
ancillary data and related structures and macros, including the
extensions.
5.1. CMSG_NXTHDR
struct cmsghdr *CMSG_NXTHDR(const struct msghdr *mhdr,
const struct cmsghdr *cmsg);
CMSG_NXTHDR() returns a pointer to the cmsghdr structure describing
the next ancillary data object. Mhdr is a pointer to a msghdr
structure and cmsg is a pointer to a cmsghdr structure. If there is
not another ancillary data object, the return value is NULL.
The following behavior of this macro is new to this API: if the value
of the cmsg pointer is NULL, a pointer to the cmsghdr structure
describing the first ancillary data object is returned. That is,
CMSG_NXTHDR(mhdr, NULL) is equivalent to CMSG_FIRSTHDR(mhdr). If
there are no ancillary data objects, the return value is NULL.
5.2. CMSG_SPACE
socklen_t CMSG_SPACE(socklen_t length);
This macro is new with this API. Given the length of an ancillary
data object, CMSG_SPACE() returns an upper bound on the space
required by the object and its cmsghdr structure, including any
padding needed to satisfy alignment requirements. This macro can be
used, for example, when allocating space dynamically for the
ancillary data. This macro should not be used to initialize the
cmsg_len member of a cmsghdr structure; instead use the CMSG_LEN()
macro.
Stevens, et al. Informational [Page 26]
RFC 3542 Advanced Sockets API for IPv6 May 2003
5.3. CMSG_LEN
socklen_t CMSG_LEN(socklen_t length);
This macro is new with this API. Given the length of an ancillary
data object, CMSG_LEN() returns the value to store in the cmsg_len
member of the cmsghdr structure, taking into account any padding
needed to satisfy alignment requirements.
Note the difference between CMSG_SPACE() and CMSG_LEN(), shown also
in the figure in Section 20.2: the former accounts for any required
padding at the end of the ancillary data object and the latter is the
actual length to store in the cmsg_len member of the ancillary data
object.
6. Packet Information
There are five pieces of information that an application can specify
for an outgoing packet using ancillary data:
1. the source IPv6 address,
2. the outgoing interface index,
3. the outgoing hop limit,
4. the next hop address, and
5. the outgoing traffic class value.
Four similar pieces of information can be returned for a received
packet as ancillary data:
1. the destination IPv6 address,
2. the arriving interface index,
3. the arriving hop limit, and
4. the arriving traffic class value.
The first two pieces of information are contained in an in6_pktinfo
structure that is set with setsockopt() or sent as ancillary data
with sendmsg() and received as ancillary data with recvmsg(). This
structure is defined as a result of including <netinet/in.h>.
struct in6_pktinfo {
struct in6_addr ipi6_addr; /* src/dst IPv6 address */
unsigned int ipi6_ifindex; /* send/recv interface index */
};
In the socket option and cmsghdr level will be IPPROTO_IPV6, the type
will be IPV6_PKTINFO, and the first byte of the option value and
cmsg_data[] will be the first byte of the in6_pktinfo structure. An
application can clear any sticky IPV6_PKTINFO option by doing a
Stevens, et al. Informational [Page 27]
RFC 3542 Advanced Sockets API for IPv6 May 2003
"regular" setsockopt with ipi6_addr being in6addr_any and
ipi6_ifindex being zero.
This information is returned as ancillary data by recvmsg() only if
the application has enabled the IPV6_RECVPKTINFO socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO, &on, sizeof(on));
(Note: The hop limit is not contained in the in6_pktinfo structure
for the following reason. Some UDP servers want to respond to client
requests by sending their reply out the same interface on which the
request was received and with the source IPv6 address of the reply
equal to the destination IPv6 address of the request. To do this the
application can enable just the IPV6_RECVPKTINFO socket option and
then use the received control information from recvmsg() as the
outgoing control information for sendmsg(). The application need not
examine or modify the in6_pktinfo structure at all. But if the hop
limit were contained in this structure, the application would have to
parse the received control information and change the hop limit
member, since the received hop limit is not the desired value for an
outgoing packet.)
6.1. Specifying/Receiving the Interface
Interfaces on an IPv6 node are identified by a small positive
integer, as described in Section 4 of [RFC-3493]. That document also
describes a function to map an interface name to its interface index,
a function to map an interface index to its interface name, and a
function to return all the interface names and indexes. Notice from
this document that no interface is ever assigned an index of 0.
When specifying the outgoing interface, if the ipi6_ifindex value is
0, the kernel will choose the outgoing interface.
The ordering among various options that can specify the outgoing
interface, including IPV6_PKTINFO, is defined in Section 6.7.
When the IPV6_RECVPKTINFO socket option is enabled, the received
interface index is always returned as the ipi6_ifindex member of the
in6_pktinfo structure.
Stevens, et al. Informational [Page 28]
RFC 3542 Advanced Sockets API for IPv6 May 2003
6.2. Specifying/Receiving Source/Destination Address
The source IPv6 address can be specified by calling bind() before
each output operation, but supplying the source address together with
the data requires less overhead (i.e., fewer system calls) and
requires less state to be stored and protected in a multithreaded
application.
When specifying the source IPv6 address as ancillary data, if the
ipi6_addr member of the in6_pktinfo structure is the unspecified
address (IN6ADDR_ANY_INIT or in6addr_any), then (a) if an address is
currently bound to the socket, it is used as the source address, or
(b) if no address is currently bound to the socket, the kernel will
choose the source address. If the ipi6_addr member is not the
unspecified address, but the socket has already bound a source
address, then the ipi6_addr value overrides the already-bound source
address for this output operation only.
The kernel must verify that the requested source address is indeed a
unicast address assigned to the node. When the address is a scoped
one, there may be ambiguity about its scope zone. This is
particularly the case for link-local addresses. In such a case, the
kernel must first determine the appropriate scope zone based on the
zone of the destination address or the outgoing interface (if known),
then qualify the address. This also means that it is not feasible to
specify the source address for a non-binding socket by the
IPV6_PKTINFO sticky option, unless the outgoing interface is also
specified. The application should simply use bind() for such
purposes.
IPV6_PKTINFO can also be used as a sticky option for specifying the
socket's default source address. However, the ipi6_addr member must
be the unspecified address for TCP sockets, because it is not
possible to dynamically change the source address of a TCP
connection. When the IPV6_PKTINFO option is specified for a TCP
socket with a non-unspecified address, the call will fail. This
restriction should be applied even before the socket binds a specific
address.
When the in6_pktinfo structure is returned as ancillary data by
recvmsg(), the ipi6_addr member contains the destination IPv6 address
from the received packet.
6.3. Specifying/Receiving the Hop Limit
The outgoing hop limit is normally specified with either the
IPV6_UNICAST_HOPS socket option or the IPV6_MULTICAST_HOPS socket
option, both of which are described in [RFC-3493]. Specifying the
Stevens, et al. Informational [Page 29]
RFC 3542 Advanced Sockets API for IPv6 May 2003
hop limit as ancillary data lets the application override either the
kernel's default or a previously specified value, for either a
unicast destination or a multicast destination, for a single output
operation. Returning the received hop limit is useful for IPv6
applications that need to verify that the received hop limit is 255
(e.g., that the packet has not been forwarded).
The received hop limit is returned as ancillary data by recvmsg()
only if the application has enabled the IPV6_RECVHOPLIMIT socket
option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));
In the cmsghdr structure containing this ancillary data, the
cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
IPV6_HOPLIMIT, and the first byte of cmsg_data[] will be the first
byte of the integer hop limit.
Nothing special need be done to specify the outgoing hop limit: just
specify the control information as ancillary data for sendmsg(). As
specified in [RFC-3493], the interpretation of the integer hop limit
value is
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
This API defines IPV6_HOPLIMIT as an ancillary-only option, that is,
the option name cannot be used as a socket option. This is because
[RFC-3493] has more fine-grained socket options; IPV6_UNICAST_HOPS
and IPV6_MULTICAST_HOPS.
6.4. Specifying the Next Hop Address
The IPV6_NEXTHOP ancillary data object specifies the next hop for the
datagram as a socket address structure. In the cmsghdr structure
containing this ancillary data, the cmsg_level member will be
IPPROTO_IPV6, the cmsg_type member will be IPV6_NEXTHOP, and the
first byte of cmsg_data[] will be the first byte of the socket
address structure.
This is a privileged option. (Note: It is implementation defined and
beyond the scope of this document to define what "privileged" means.
Unix systems use this term to mean the process must have an effective
user ID of 0.)
Stevens, et al. Informational [Page 30]
RFC 3542 Advanced Sockets API for IPv6 May 2003
This API only defines the case where the socket address contains an
IPv6 address (i.e., the sa_family member is AF_INET6). And, in this
case, the node identified by that address must be a neighbor of the
sending host. If that address equals the destination IPv6 address of
the datagram, then this is equivalent to the existing SO_DONTROUTE
socket option.
This option does not have any meaning for multicast destinations. In
such a case, the specified next hop will be ignored.
When the outgoing interface is specified by IPV6_PKTINFO as well, the
next hop specified by this option must be reachable via the specified
interface.
In order to clear a sticky IPV6_NEXTHOP option the application must
issue a setsockopt for IPV6_NEXTHOP with a zero length.
6.5. Specifying/Receiving the Traffic Class value
The outgoing traffic class is normally set to 0. Specifying the
traffic class as ancillary data lets the application override either
the kernel's default or a previously specified value, for either a
unicast destination or a multicast destination, for a single output
operation. Returning the received traffic class is useful for
programs such as a diffserv debugging tool and for user level ECN
(explicit congestion notification) implementation.
The received traffic class is returned as ancillary data by recvmsg()
only if the application has enabled the IPV6_RECVTCLASS socket
option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVTCLASS, &on, sizeof(on));
In the cmsghdr structure containing this ancillary data, the
cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
IPV6_TCLASS, and the first byte of cmsg_data[] will be the first byte
of the integer traffic class.
To specify the outgoing traffic class value, just specify the control
information as ancillary data for sendmsg() or using setsockopt().
Just like the hop limit value, the interpretation of the integer
traffic class value is
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
Stevens, et al. Informational [Page 31]
RFC 3542 Advanced Sockets API for IPv6 May 2003
In order to clear a sticky IPV6_TCLASS option the application can
specify -1 as the value.
There are cases where the kernel needs to control the traffic class
value and conflicts with the user-specified value on the outgoing
traffic. An example is an implementation of ECN in the kernel,
setting 2 bits of the traffic class value. In such cases, the kernel
should override the user-specified value. On the incoming traffic,
the kernel may mask some of the bits in the traffic class field.
6.6. Additional Errors with sendmsg() and setsockopt()
With the IPV6_PKTINFO socket option there are no additional errors
possible with the call to recvmsg(). But when specifying the
outgoing interface or the source address, additional errors are
possible from sendmsg() or setsockopt(). Note that some
implementations might only be able to return this type of errors for
setsockopt(). The following are examples, but some of these may not
be provided by some implementations, and some implementations may
define additional errors:
ENXIO The interface specified by ipi6_ifindex does not exist.
ENETDOWN The interface specified by ipi6_ifindex is not enabled
for IPv6 use.
EADDRNOTAVAIL ipi6_ifindex specifies an interface but the address
ipi6_addr is not available for use on that interface.
EHOSTUNREACH No route to the destination exists over the interface
specified by ipi6_ifindex.
6.7. Summary of Outgoing Interface Selection
This document and [RFC-3493] specify various methods that affect the
selection of the packet's outgoing interface. This subsection
summarizes the ordering among those in order to ensure deterministic
behavior.
For a given outgoing packet on a given socket, the outgoing interface
is determined in the following order:
1. if an interface is specified in an IPV6_PKTINFO ancillary data
item, the interface is used.
2. otherwise, if an interface is specified in an IPV6_PKTINFO sticky
option, the interface is used.
Stevens, et al. Informational [Page 32]
RFC 3542 Advanced Sockets API for IPv6 May 2003
3. otherwise, if the destination address is a multicast address and
the IPV6_MULTICAST_IF socket option is specified for the socket,
the interface is used.
4. otherwise, if an IPV6_NEXTHOP ancillary data item is specified,
the interface to the next hop is used.
5. otherwise, if an IPV6_NEXTHOP sticky option is specified, the
interface to the next hop is used.
6. otherwise, the outgoing interface should be determined in an
implementation dependent manner.
The ordering above particularly means if the application specifies an
interface by the IPV6_MULTICAST_IF socket option (described in [RFC-
3493]) as well as specifying a different interface by the
IPV6_PKTINFO sticky option, the latter will override the former for
every multicast packet on the corresponding socket. The reason for
the ordering comes from expectation that the source address is
specified as well and that the pair of the address and the outgoing
interface should be preferred.
In any case, the kernel must also verify that the source and
destination addresses do not break their scope zones with regard to
the outgoing interface.
7. Routing Header Option
Source routing in IPv6 is accomplished by specifying a Routing header
as an extension header. There can be different types of Routing
headers, but IPv6 currently defines only the Type 0 Routing header
[RFC-2460]. This type supports up to 127 intermediate nodes (limited
by the length field in the extension header). With this maximum
number of intermediate nodes, a source, and a destination, there are
128 hops.
Source routing with the IPv4 sockets API (the IP_OPTIONS socket
option) requires the application to build the source route in the
format that appears as the IPv4 header option, requiring intimate
knowledge of the IPv4 options format. This IPv6 API, however,
defines six functions that the application calls to build and examine
a Routing header, and the ability to use sticky options or ancillary
data to communicate this information between the application and the
kernel using the IPV6_RTHDR option.
Stevens, et al. Informational [Page 33]
RFC 3542 Advanced Sockets API for IPv6 May 2003
Three functions build a Routing header:
inet6_rth_space() - return #bytes required for Routing header
inet6_rth_init() - initialize buffer data for Routing header
inet6_rth_add() - add one IPv6 address to the Routing header
Three functions deal with a returned Routing header:
inet6_rth_reverse() - reverse a Routing header
inet6_rth_segments() - return #segments in a Routing header
inet6_rth_getaddr() - fetch one address from a Routing header
The function prototypes for these functions are defined as a result
of including <netinet/in.h>.
To receive a Routing header the application must enable the
IPV6_RECVRTHDR socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR, &on, sizeof(on));
Each received Routing header is returned as one ancillary data object
described by a cmsghdr structure with cmsg_type set to IPV6_RTHDR.
When multiple Routing headers are received, multiple ancillary data
objects (with cmsg_type set to IPV6_RTHDR) will be returned to the
application.
To send a Routing header the application specifies it either as
ancillary data in a call to sendmsg() or using setsockopt(). For the
sending side, this API assumes the number of occurrences of the
Routing header as described in [RFC-2460]. That is, applications can
only specify at most one outgoing Routing header.
The application can remove any sticky Routing header by calling
setsockopt() for IPV6_RTHDR with a zero option length.
When using ancillary data a Routing header is passed between the
application and the kernel as follows: The cmsg_level member has a
value of IPPROTO_IPV6 and the cmsg_type member has a value of
IPV6_RTHDR. The contents of the cmsg_data[] member is implementation
dependent and should not be accessed directly by the application, but
should be accessed using the six functions that we are about to
describe.
The following constant is defined as a result of including the
<netinet/in.h>:
#define IPV6_RTHDR_TYPE_0 0 /* IPv6 Routing header type 0 */
Stevens, et al. Informational [Page 34]
RFC 3542 Advanced Sockets API for IPv6 May 2003
When a Routing header is specified, the destination address specified
for connect(), sendto(), or sendmsg() is the final destination
address of the datagram. The Routing header then contains the
addresses of all the intermediate nodes.
7.1. inet6_rth_space
socklen_t inet6_rth_space(int type, int segments);
This function returns the number of bytes required to hold a Routing
header of the specified type containing the specified number of
segments (addresses). For an IPv6 Type 0 Routing header, the number
of segments must be between 0 and 127, inclusive. The return value
is just the space for the Routing header. When the application uses
ancillary data it must pass the returned length to CMSG_SPACE() to
determine how much memory is needed for the ancillary data object
(including the cmsghdr structure).
If the return value is 0, then either the type of the Routing header
is not supported by this implementation or the number of segments is
invalid for this type of Routing header.
(Note: This function returns the size but does not allocate the space
required for the ancillary data. This allows an application to
allocate a larger buffer, if other ancillary data objects are
desired, since all the ancillary data objects must be specified to
sendmsg() as a single msg_control buffer.)
7.2. inet6_rth_init
void *inet6_rth_init(void *bp, socklen_t bp_len, int type,
int segments);
This function initializes the buffer pointed to by bp to contain a
Routing header of the specified type and sets ip6r_len based on the
segments parameter. bp_len is only used to verify that the buffer is
large enough. The ip6r_segleft field is set to zero; inet6_rth_add()
will increment it.
When the application uses ancillary data the application must
initialize any cmsghdr fields.
The caller must allocate the buffer and its size can be determined by
calling inet6_rth_space().
Upon success the return value is the pointer to the buffer (bp), and
this is then used as the first argument to the inet6_rth_add()
function. Upon an error the return value is NULL.
Stevens, et al. Informational [Page 35]
RFC 3542 Advanced Sockets API for IPv6 May 2003
7.3. inet6_rth_add
int inet6_rth_add(void *bp, const struct in6_addr *addr);
This function adds the IPv6 address pointed to by addr to the end of
the Routing header being constructed.
If successful, the segleft member of the Routing Header is updated to
account for the new address in the Routing header and the return
value of the function is 0. Upon an error the return value of the
function is -1.
7.4. inet6_rth_reverse
int inet6_rth_reverse(const void *in, void *out);
This function takes a Routing header extension header (pointed to by
the first argument) and writes a new Routing header that sends
datagrams along the reverse of that route. The function reverses the
order of the addresses and sets the segleft member in the new Routing
header to the number of segments. Both arguments are allowed to
point to the same buffer (that is, the reversal can occur in place).
The return value of the function is 0 on success, or -1 upon an
error.
7.5. inet6_rth_segments
int inet6_rth_segments(const void *bp);
This function returns the number of segments (addresses) contained in
the Routing header described by bp. On success the return value is
zero or greater. The return value of the function is -1 upon an
error.
7.6. inet6_rth_getaddr
struct in6_addr *inet6_rth_getaddr(const void *bp, int index);
This function returns a pointer to the IPv6 address specified by
index (which must have a value between 0 and one less than the value
returned by inet6_rth_segments()) in the Routing header described by
bp. An application should first call inet6_rth_segments() to obtain
the number of segments in the Routing header.
Upon an error the return value of the function is NULL.
Stevens, et al. Informational [Page 36]
RFC 3542 Advanced Sockets API for IPv6 May 2003
8. Hop-By-Hop Options
A variable number of Hop-by-Hop options can appear in a single Hop-
by-Hop options header. Each option in the header is TLV-encoded with
a type, length, and value. This IPv6 API defines seven functions
that the application calls to build and examine a Hop-by_Hop options
header, and the ability to use sticky options or ancillary data to
communicate this information between the application and the kernel.
This uses the IPV6_HOPOPTS for a Hop-by-Hop options header.
Today several Hop-by-Hop options are defined for IPv6. Two pad
options, Pad1 and PadN, are for alignment purposes and are
automatically inserted by the inet6_opt_XXX() routines and ignored by
the inet6_opt_XXX() routines on the receive side. This section of
the API is therefore defined for other (and future) Hop-by-Hop
options that an application may need to specify and receive.
Four functions build an options header:
inet6_opt_init() - initialize buffer data for options header
inet6_opt_append() - add one TLV option to the options header
inet6_opt_finish() - finish adding TLV options to the options
header
inet6_opt_set_val() - add one component of the option content to
the option
Three functions deal with a returned options header:
inet6_opt_next() - extract the next option from the options
header
inet6_opt_find() - extract an option of a specified type from
the header
inet6_opt_get_val() - retrieve one component of the option
content
Individual Hop-by-Hop options (and Destination options, which are
described in Section 9 and are very similar to the Hop-by-Hop
options) may have specific alignment requirements. For example, the
4-byte Jumbo Payload length should appear on a 4-byte boundary, and
IPv6 addresses are normally aligned on an 8-byte boundary. These
requirements and the terminology used with these options are
discussed in Section 4.2 and Appendix B of [RFC-2460]. The alignment
of first byte of each option is specified by two values, called x and
y, written as "xn + y". This states that the option must appear at
an integer multiple of x bytes from the beginning of the options
header (x can have the values 1, 2, 4, or 8), plus y bytes (y can
have a value between 0 and 7, inclusive). The Pad1 and PadN options
are inserted as needed to maintain the required alignment. The
Stevens, et al. Informational [Page 37]
RFC 3542 Advanced Sockets API for IPv6 May 2003
functions below need to know the alignment of the end of the option
(which is always in the form "xn," where x can have the values 1, 2,
4, or 8) and the total size of the data portion of the option. These
are passed as the "align" and "len" arguments to inet6_opt_append().
Multiple Hop-by-Hop options must be specified by the application by
placing them in a single extension header.
Finally, we note that use of some Hop-by-Hop options or some
Destination options, might require special privilege. That is,
normal applications (without special privilege) might be forbidden
from setting certain options in outgoing packets, and might never see
certain options in received packets.
8.1. Receiving Hop-by-Hop Options
To receive a Hop-by-Hop options header the application must enable
the IPV6_RECVHOPOPTS socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS, &on, sizeof(on));
When using ancillary data a Hop-by-hop options header is passed
between the application and the kernel as follows: The cmsg_level
member will be IPPROTO_IPV6 and the cmsg_type member will be
IPV6_HOPOPTS. These options are then processed by calling the
inet6_opt_next(), inet6_opt_find(), and inet6_opt_get_val()
functions, described in Section 10.
8.2. Sending Hop-by-Hop Options
To send a Hop-by-Hop options header, the application specifies the
header either as ancillary data in a call to sendmsg() or using
setsockopt().
The application can remove any sticky Hop-by-Hop options header by
calling setsockopt() for IPV6_HOPOPTS with a zero option length.
All the Hop-by-Hop options must be specified by a single ancillary
data object. The cmsg_level member is set to IPPROTO_IPV6 and the
cmsg_type member is set to IPV6_HOPOPTS. The option is normally
constructed using the inet6_opt_init(), inet6_opt_append(),
inet6_opt_finish(), and inet6_opt_set_val() functions, described in
Section 10.
Additional errors may be possible from sendmsg() and setsockopt() if
the specified option is in error.
Stevens, et al. Informational [Page 38]
RFC 3542 Advanced Sockets API for IPv6 May 2003
9. Destination Options
A variable number of Destination options can appear in one or more
Destination options headers. As defined in [RFC-2460], a Destination
options header appearing before a Routing header is processed by the
first destination plus any subsequent destinations specified in the
Routing header, while a Destination options header that is not
followed by a Routing header is processed only by the final
destination. As with the Hop-by-Hop options, each option in a
Destination options header is TLV-encoded with a type, length, and
value.
9.1. Receiving Destination Options
To receive Destination options header the application must enable the
IPV6_RECVDSTOPTS socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS, &on, sizeof(on));
Each Destination options header is returned as one ancillary data
object described by a cmsghdr structure with cmsg_level set to
IPPROTO_IPV6 and cmsg_type set to IPV6_DSTOPTS.
These options are then processed by calling the inet6_opt_next(),
inet6_opt_find(), and inet6_opt_get_value() functions.
9.2. Sending Destination Options
To send a Destination options header, the application specifies it
either as ancillary data in a call to sendmsg() or using
setsockopt().
The application can remove any sticky Destination options header by
calling setsockopt() for IPV6_RTHDRDSTOPTS/IPV6_DSTOPTS with a zero
option length.
This API assumes the ordering about extension headers as described in
[RFC-2460]. Thus, one set of Destination options can only appear
before a Routing header, and one set can only appear after a Routing
header (or in a packet with no Routing header). Each set can consist
of one or more options but each set is a single extension header.
Today all destination options that an application may want to specify
can be put after (or without) a Routing header. Thus, applications
should usually need IPV6_DSTOPTS only and should avoid using
IPV6_RTHDRDSTOPTS whenever possible.
Stevens, et al. Informational [Page 39]
RFC 3542 Advanced Sockets API for IPv6 May 2003
When using ancillary data a Destination options header is passed
between the application and the kernel as follows: The set preceding
a Routing header are specified with the cmsg_level member set to
IPPROTO_IPV6 and the cmsg_type member set to IPV6_RTHDRDSTOPTS. Any
setsockopt or ancillary data for IPV6_RTHDRDSTOPTS is silently
ignored when sending packets unless a Routing header is also
specified. Note that the "Routing header" here means the one
specified by this API. Even when the kernel inserts a routing header
in its internal routine (e.g., in a mobile IPv6 stack), the
Destination options header specified by IPV6_RTHDRDSTOPTS will still
be ignored unless the application explicitly specifies its own
Routing header.
The set of Destination options after a Routing header, which are also
used when no Routing header is present, are specified with the
cmsg_level member is set to IPPROTO_IPV6 and the cmsg_type member is
set to IPV6_DSTOPTS.
The Destination options are normally constructed using the
inet6_opt_init(), inet6_opt_append(), inet6_opt_finish(), and
inet6_opt_set_val() functions, described in Section 10.
Additional errors may be possible from sendmsg() and setsockopt() if
the specified option is in error.
10. Hop-by-Hop and Destination Options Processing
Building and parsing the Hop-by-Hop and Destination options is
complicated for the reasons given earlier. We therefore define a set
of functions to help the application. These functions assume the
formatting rules specified in Appendix B in [RFC-2460] i.e., that the
largest field is placed last in the option.
The function prototypes for these functions are defined as a result
of including <netinet/in.h>.
The first 3 functions (init, append, and finish) are used both to
calculate the needed buffer size for the options, and to actually
encode the options once the application has allocated a buffer for
the header. In order to only calculate the size the application must
pass a NULL extbuf and a zero extlen to those functions.
Stevens, et al. Informational [Page 40]
RFC 3542 Advanced Sockets API for IPv6 May 2003
10.1. inet6_opt_init
int inet6_opt_init(void *extbuf, socklen_t extlen);
This function returns the number of bytes needed for the empty
extension header i.e., without any options. If extbuf is not NULL it
also initializes the extension header to have the correct length
field. In that case if the extlen value is not a positive (i.e.,
non-zero) multiple of 8 the function fails and returns -1.
(Note: since the return value on success is based on a "constant"
parameter, i.e., the empty extension header, an implementation may
return a constant value. However, this specification does not
require the value be constant, and leaves it as implementation
dependent. The application should not assume a particular constant
value as a successful return value of this function.)
10.2. inet6_opt_append
int inet6_opt_append(void *extbuf, socklen_t extlen, int offset,
uint8_t type, socklen_t len, uint_t align,
void **databufp);
Offset should be the length returned by inet6_opt_init() or a
previous inet6_opt_append(). This function returns the updated total
length taking into account adding an option with length 'len' and
alignment 'align'. If extbuf is not NULL then, in addition to
returning the length, the function inserts any needed pad option,
initializes the option (setting the type and length fields) and
returns a pointer to the location for the option content in databufp.
If the option does not fit in the extension header buffer the
function returns -1.
Type is the 8-bit option type. Len is the length of the option data
(i.e., excluding the option type and option length fields).
Once inet6_opt_append() has been called the application can use the
databuf directly, or use inet6_opt_set_val() to specify the content
of the option.
The option type must have a value from 2 to 255, inclusive. (0 and 1
are reserved for the Pad1 and PadN options, respectively.)
The option data length must have a value between 0 and 255,
inclusive, and is the length of the option data that follows.
The align parameter must have a value of 1, 2, 4, or 8. The align
value can not exceed the value of len.
Stevens, et al. Informational [Page 41]
RFC 3542 Advanced Sockets API for IPv6 May 2003
10.3. inet6_opt_finish
int inet6_opt_finish(void *extbuf, socklen_t extlen, int offset);
Offset should be the length returned by inet6_opt_init() or
inet6_opt_append(). This function returns the updated total length
taking into account the final padding of the extension header to make
it a multiple of 8 bytes. If extbuf is not NULL the function also
initializes the option by inserting a Pad1 or PadN option of the
proper length.
If the necessary pad does not fit in the extension header buffer the
function returns -1.
10.4. inet6_opt_set_val
int inet6_opt_set_val(void *databuf, int offset, void *val,
socklen_t vallen);
Databuf should be a pointer returned by inet6_opt_append(). This
function inserts data items of various sizes in the data portion of
the option. Val should point to the data to be inserted. Offset
specifies where in the data portion of the option the value should be
inserted; the first byte after the option type and length is accessed
by specifying an offset of zero.
The caller should ensure that each field is aligned on its natural
boundaries as described in Appendix B of [RFC-2460], but the function
must not rely on the caller's behavior. Even when the alignment
requirement is not satisfied, inet6_opt_set_val should just copy the
data as required.
The function returns the offset for the next field (i.e., offset +
vallen) which can be used when composing option content with multiple
fields.
10.5. inet6_opt_next
int inet6_opt_next(void *extbuf, socklen_t extlen, int offset,
uint8_t *typep, socklen_t *lenp,
void **databufp);
This function parses received option extension headers returning the
next option. Extbuf and extlen specifies the extension header.
Offset should either be zero (for the first option) or the length
returned by a previous call to inet6_opt_next() or inet6_opt_find().
It specifies the position where to continue scanning the extension
buffer. The next option is returned by updating typep, lenp, and
Stevens, et al. Informational [Page 42]
RFC 3542 Advanced Sockets API for IPv6 May 2003
databufp. Typep stores the option type, lenp stores the length of
the option data (i.e., excluding the option type and option length
fields), and databufp points the data field of the option. This
function returns the updated "previous" length computed by advancing
past the option that was returned. This returned "previous" length
can then be passed to subsequent calls to inet6_opt_next(). This
function does not return any PAD1 or PADN options. When there are no
more options or if the option extension header is malformed the
return value is -1.
10.6. inet6_opt_find
int inet6_opt_find(void *extbuf, socklen_t extlen, int offset,
uint8_t type, socklen_t *lenp,
void **databufp);
This function is similar to the previously described inet6_opt_next()
function, except this function lets the caller specify the option
type to be searched for, instead of always returning the next option
in the extension header.
If an option of the specified type is located, the function returns
the updated "previous" total length computed by advancing past the
option that was returned and past any options that didn't match the
type. This returned "previous" length can then be passed to
subsequent calls to inet6_opt_find() for finding the next occurrence
of the same option type.
If an option of the specified type is not located, the return value
is -1. If the option extension header is malformed, the return value
is -1.
10.7. inet6_opt_get_val
int inet6_opt_get_val(void *databuf, int offset, void *val,
socklen_t vallen);
Databuf should be a pointer returned by inet6_opt_next() or
inet6_opt_find(). This function extracts data items of various sizes
in the data portion of the option. Val should point to the
destination for the extracted data. Offset specifies from where in
the data portion of the option the value should be extracted; the
first byte after the option type and length is accessed by specifying
an offset of zero.
It is expected that each field is aligned on its natural boundaries
as described in Appendix B of [RFC-2460], but the function must not
rely on the alignment.
Stevens, et al. Informational [Page 43]
RFC 3542 Advanced Sockets API for IPv6 May 2003
The function returns the offset for the next field (i.e., offset +
vallen) which can be used when extracting option content with
multiple fields.
11. Additional Advanced API Functions
11.1. Sending with the Minimum MTU
Unicast applications should usually let the kernel perform path MTU
discovery [RFC-1981], as long as the kernel supports it, and should
not care about the path MTU. Some applications, however, might not
want to incur the overhead of path MTU discovery, especially if the
applications only send a single datagram to a destination. A
potential example is a DNS server.
[RFC-1981] describes how path MTU discovery works for multicast
destinations. From practice in using IPv4 multicast, however, many
careless applications that send large multicast packets on the wire
have caused implosion of ICMPv4 error messages. The situation can be
worse when there is a filtering node that blocks the ICMPv4 messages.
Though the filtering issue applies to unicast as well, the impact is
much larger in the multicast cases.
Thus, applications sending multicast traffic should explicitly enable
path MTU discovery only when they understand that the benefit of
possibly larger MTU usage outweighs the possible impact of MTU
discovery for active sources across the delivery tree(s). This
default behavior is based on the today's practice with IPv4 multicast
and path MTU discovery. The behavior may change in the future once
it is found that path MTU discovery effectively works with actual
multicast applications and network configurations.
This specification defines a mechanism to avoid path MTU discovery by
sending at the minimum IPv6 MTU [RFC-2460]. If the packet is larger
than the minimum MTU and this feature has been enabled the IP layer
will fragment to the minimum MTU. To control the policy about path
MTU discovery, applications can use the IPV6_USE_MIN_MTU socket
option.
As described above, the default policy should depend on whether the
destination is unicast or multicast. For unicast destinations path
MTU discovery should be performed by default. For multicast
destinations path MTU discovery should be disabled by default. This
option thus takes the following three types of integer arguments:
-1: perform path MTU discovery for unicast destinations but do not
perform it for multicast destinations. Packets to multicast
destinations are therefore sent with the minimum MTU.
Stevens, et al. Informational [Page 44]
RFC 3542 Advanced Sockets API for IPv6 May 2003
0: always perform path MTU discovery.
1: always disable path MTU discovery and send packets at the minimum
MTU.
The default value of this option is -1. Values other than -1, 0, and
1 are invalid, and an error EINVAL will be returned for those values.
As an example, if a unicast application intentionally wants to
disable path MTU discovery, it will add the following lines:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_USE_MIN_MTU, &on, sizeof(on));
Note that this API intentionally excludes the case where the
application wants to perform path MTU discovery for multicast but to
disable it for unicast. This is because such usage is not feasible
considering a scale of performance issues around whether to do path
MTU discovery or not. When path MTU discovery makes sense to a
destination but not to a different destination, regardless of whether
the destination is unicast or multicast, applications either need to
toggle the option between sending such packets on the same socket, or
use different sockets for the two classes of destinations.
This option can also be sent as ancillary data. In the cmsghdr
structure containing this ancillary data, the cmsg_level member will
be IPPROTO_IPV6, the cmsg_type member will be IPV6_USE_MIN_MTU, and
the first byte of cmsg_data[] will be the first byte of the integer.
11.2. Sending without Fragmentation
In order to provide for easy porting of existing UDP and raw socket
applications IPv6 implementations will, when originating packets,
automatically insert a fragment header in the packet if the packet is
too big for the path MTU.
Some applications might not want this behavior. An example is
traceroute which might want to discover the actual path MTU.
This specification defines a mechanism to turn off the automatic
inserting of a fragment header for UDP and raw sockets. This can be
enabled using the IPV6_DONTFRAG socket option.
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_DONTFRAG, &on, sizeof(on));
Stevens, et al. Informational [Page 45]
RFC 3542 Advanced Sockets API for IPv6 May 2003
By default, this socket option is disabled. Setting the value to 0
also disables the option i.e., reverts to the default behavior of
automatic inserting. This option can also be sent as ancillary data.
In the cmsghdr structure containing this ancillary data, the
cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
IPV6_DONTFRAG, and the first byte of cmsg_data[] will be the first
byte of the integer. This API only specifies the use of this option
for UDP and raw sockets, and does not define the usage for TCP
sockets.
When the data size is larger than the MTU of the outgoing interface,
the packet will be discarded. Applications can know the result by
enabling the IPV6_RECVPATHMTU option described below and receiving
the corresponding ancillary data items. An additional error EMSGSIZE
may also be returned in some implementations. Note, however, that
some other implementations might not be able to return this
additional error when sending a message.
11.3. Path MTU Discovery and UDP
UDP and raw socket applications need to be able to determine the
"maximum send transport-message size" (Section 5.1 of [RFC-1981]) to
a given destination so that those applications can participate in
path MTU discovery. This lets those applications send smaller
datagrams to the destination, avoiding fragmentation.
This is accomplished using a new ancillary data item (IPV6_PATHMTU)
which is delivered to recvmsg() without any actual data. The
application can enable the receipt of IPV6_PATHMTU ancillary data
items by setting the IPV6_RECVPATHMTU socket option.
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPATHMTU, &on, sizeof(on));
By default, this socket option is disabled. Setting the value to 0
also disables the option. This API only specifies the use of this
option for UDP and raw sockets, and does not define the usage for TCP
sockets.
When the application is sending packets too big for the path MTU
recvmsg() will return zero (indicating no data) but there will be a
cmsghdr with cmsg_type set to IPV6_PATHMTU, and cmsg_len will
indicate that cmsg_data is sizeof(struct ip6_mtuinfo) bytes long.
This can happen when the sending node receives a corresponding ICMPv6
packet too big error, or when the packet is sent from a socket with
the IPV6_DONTFRAG option being on and the packet size is larger than
the MTU of the outgoing interface. This indication is considered as
an ancillary data item for a separate (empty) message. Thus, when
Stevens, et al. Informational [Page 46]
RFC 3542 Advanced Sockets API for IPv6 May 2003
there are buffered messages (i.e., messages that the application has
not received yet) on the socket the application will first receive
the buffered messages and then receive the indication.
The first byte of cmsg_data[] will point to a struct ip6_mtuinfo
carrying the path MTU to use together with the IPv6 destination
address.
struct ip6_mtuinfo {
struct sockaddr_in6 ip6m_addr; /* dst address including
zone ID */
uint32_t ip6m_mtu; /* path MTU in host byte order */
};
This cmsghdr will be passed to every socket that sets the
IPV6_RECVPATHMTU socket option, even if the socket is non-connected.
Note that this also means an application that sets the option may
receive an IPV6_MTU ancillary data item for each ICMP too big error
the node receives, including such ICMP errors caused by other
applications on the node. Thus, an application that wants to perform
the path MTU discovery by itself needs to keep history of
destinations that it has actually sent to and to compare the address
returned in the ip6_mtuinfo structure to the history. An
implementation may choose not to delivery data to a connected socket
that has a foreign address that is different than the address
specified in the ip6m_addr structure.
When an application sends a packet with a routing header, the final
destination stored in the ip6m_addr member does not necessarily
contain complete information of the entire path.
11.4. Determining the Current Path MTU
Some applications might need to determine the current path MTU e.g.,
applications using IPV6_RECVPATHMTU might want to pick a good
starting value.
This specification defines a get-only socket option to retrieve the
current path MTU value for the destination of a given connected
socket. If the IP layer does not have a cached path MTU value it
will return the interface MTU for the interface that will be used
when sending to the destination address.
This information is retrieved using the IPV6_PATHMTU socket option.
This option takes a pointer to the ip6_mtuinfo structure as the
fourth argument, and the size of the structure should be passed as a
value-result parameter in the fifth argument.
Stevens, et al. Informational [Page 47]
RFC 3542 Advanced Sockets API for IPv6 May 2003
struct ip6_mtuinfo mtuinfo;
socklen_t infolen = sizeof(mtuinfo);
getsockopt(fd, IPPROTO_IPV6, IPV6_PATHMTU, &mtuinfo, &infolen);
When the call succeeds, the path MTU value is stored in the ip6m_mtu
member of the ip6_mtuinfo structure. Since the socket is connected,
the ip6m_addr member is meaningless and should not be referred to by
the application.
This option can only be used for a connected socket, because a non-
connected socket does not have the information of the destination and
there is no way to pass the destination via getsockopt(). When
getsockopt() for this option is issued on a non-connected socket, the
call will fail. Despite this limitation, this option is still useful
from a practical point of view, because applications that care about
the path MTU tend to send a lot of packets to a single destination
and to connect the socket to the destination for performance reasons.
If the application needs to get the MTU value in a more generic way,
it should use a more generic interface, such as routing sockets
[TCPIPILLUST].
12. Ordering of Ancillary Data and IPv6 Extension Headers
Three IPv6 extension headers can be specified by the application and
returned to the application using ancillary data with sendmsg() and
recvmsg(): the Routing header, Hop-by-Hop options header, and
Destination options header. When multiple ancillary data objects are
transferred via recvmsg() and these objects represent any of these
three extension headers, their placement in the control buffer is
directly tied to their location in the corresponding IPv6 datagram.
For example, when the application has enabled the IPV6_RECVRTHDR and
IPV6_RECVDSTOPTS options and later receives an IPv6 packet with
extension headers in the following order:
The IPv6 header
A Hop-by-Hop options header
A Destination options header (1)
A Routing header
An Authentication header
A Destination options header (2)
A UDP header and UDP data
Stevens, et al. Informational [Page 48]
RFC 3542 Advanced Sockets API for IPv6 May 2003
then the application will receive three ancillary data objects in the
following order:
an object with cmsg_type set to IPV6_DSTOPTS, which represents
the destination options header (1)
an object with cmsg_type set to IPV6_RTHDR, which represents the
Routing header
an object with cmsg_type set to IPV6_DSTOPTS, which represents the
destination options header (2)
This example follows the header ordering described in [RFC-2460], but
the receiving side of this specification does not assume the
ordering. Applications may receive any numbers of objects in any
order according to the ordering of the received IPv6 datagram.
For the sending side, however, this API imposes some ordering
constraints according to [RFC-2460]. Applications using this API
cannot make a packet with extension headers that do not follow the
ordering. Note, however, that this does not mean applications must
always follow the restriction. This is just a limitation in this API
in order to give application programmers a guideline to construct
headers in a practical manner. Should an application need to make an
outgoing packet in an arbitrary order about the extension headers,
some other technique, such as the datalink interfaces BPF or DLPI,
must be used.
The followings are more details about the constraints:
- Each IPV6_xxx ancillary data object for a particular type of
extension header can be specified at most once in a single control
buffer.
- IPV6_xxx ancillary data objects can appear in any order in a
control buffer, because there is no ambiguity of the ordering.
- Each set of IPV6_xxx ancillary data objects and sticky options
will be put in the outgoing packet along with the header ordering
described in [RFC-2460].
- An ancillary data object or a sticky option of IPV6_RTHDRDSTOPTS
will affect the outgoing packet only when a Routing header is
specified as an ancillary data object or a sticky option.
Otherwise, the specified value for IPV6_RTHDRDSTOPTS will be
ignored.
Stevens, et al. Informational [Page 49]
RFC 3542 Advanced Sockets API for IPv6 May 2003
For example, when an application sends a UDP datagram with a control
data buffer containing ancillary data objects in the following order:
an object with cmsg_type set to IPV6_DSTOPTS
an object with cmsg_type set to IPV6_RTHDRDSTOPTS
an object with cmsg_type set to IPV6_HOPOPTS
and the sending socket does not have any sticky options, then the
outgoing packet would be constructed as follows:
The IPv6 header
A Hop-by-Hop options header
A Destination options header
A UDP header and UDP data
where the destination options header corresponds to the ancillary
data object with the type IPV6_DSTOPTS.
Note that the constraints above do not necessarily mean that the
outgoing packet sent on the wire always follows the header ordering
specified in this API document. The kernel may insert additional
headers that break the ordering as a result. For example, if the
kernel supports Mobile IPv6, an additional destination options header
may be inserted before an authentication header, even without a
routing header.
This API does not provide access to any other extension headers than
the supported three types of headers. In particular, no information
is provided about the IP security headers on an incoming packet, nor
can be specified for an outgoing packet. This API is for
applications that do not care about the existence of IP security
headers.
13. IPv6-Specific Options with IPv4-Mapped IPv6 Addresses
The various socket options and ancillary data specifications defined
in this document apply only to true IPv6 sockets. It is possible to
create an IPv6 socket that actually sends and receives IPv4 packets,
using IPv4-mapped IPv6 addresses, but the mapping of the options
defined in this document to an IPv4 datagram is beyond the scope of
this document.
In general, attempting to specify an IPv6-only option, such as the
Hop-by-Hop options, Destination options, or Routing header on an IPv6
socket that is using IPv4-mapped IPv6 addresses, will probably result
in an error. Some implementations, however, may provide access to
Stevens, et al. Informational [Page 50]
RFC 3542 Advanced Sockets API for IPv6 May 2003
the packet information (source/destination address, send/receive
interface, and hop limit) on an IPv6 socket that is using IPv4-mapped
IPv6 addresses.
14. Extended interfaces for rresvport, rcmd and rexec
Library functions that support the "r" commands hide the creation of
a socket and the name resolution procedure from an application. When
the libraries return an AF_INET6 socket to an application that do not
support the address family, the application may encounter an
unexpected result when, e.g., calling getpeername() for the socket.
In order to support AF_INET6 sockets for the "r" commands while
keeping backward compatibility, this section defines some extensions
to the libraries.
14.1. rresvport_af
The rresvport() function is used by the rcmd() function, and this
function is in turn called by many of the "r" commands such as
rlogin. While new applications are not being written to use the
rcmd() function, legacy applications such as rlogin will continue to
use it and these will be ported to IPv6.
rresvport() creates an IPv4/TCP socket and binds a "reserved port" to
the socket. Instead of defining an IPv6 version of this function we
define a new function that takes an address family as its argument.
#include <unistd.h>
int rresvport_af(int *port, int family);
This function behaves the same as the existing rresvport() function,
but instead of creating an AF_INET TCP socket, it can also create an
AF_INET6 TCP socket. The family argument is either AF_INET or
AF_INET6, and a new error return is EAFNOSUPPORT if the address
family is not supported.
(Note: There is little consensus on which header defines the
rresvport() and rcmd() function prototypes. 4.4BSD defines it in
<unistd.h>, others in <netdb.h>, and others don't define the function
prototypes at all.)
14.2. rcmd_af
The existing rcmd() function can not transparently use AF_INET6
sockets since an application would not be prepared to handle AF_INET6
addresses returned by e.g., getpeername() on the file descriptor
created by rcmd(). Thus a new function is needed.
Stevens, et al. Informational [Page 51]
RFC 3542 Advanced Sockets API for IPv6 May 2003
int rcmd_af(char **ahost, unsigned short rport,
const char *locuser, const char *remuser,
const char *cmd, int *fd2p, int af)
This function behaves the same as the existing rcmd() function, but
instead of creating an AF_INET TCP socket, it can also create an
AF_INET6 TCP socket. The family argument is AF_INET, AF_INET6, or
AF_UNSPEC. When either AF_INET or AF_INET6 is specified, this
function will create a socket of the specified address family. When
AF_UNSPEC is specified, it will try all possible address families
until a connection can be established, and will return the associated
socket of the connection. A new error EAFNOSUPPORT will be returned
if the address family is not supported.
14.3. rexec_af
The existing rexec() function can not transparently use AF_INET6
sockets since an application would not be prepared to handle AF_INET6
addresses returned by e.g., getpeername() on the file descriptor
created by rexec(). Thus a new function is needed.
int rexec_af(char **ahost, unsigned short rport, const char *name,
const char *pass, const char *cmd, int *fd2p, int af)
This function behaves the same as the existing rexec() function, but
instead of creating an AF_INET TCP socket, it can also create an
AF_INET6 TCP socket. The family argument is AF_INET, AF_INET6, or
AF_UNSPEC. When either AF_INET or AF_INET6 is specified, this
function will create a socket of the specified address family. When
AF_UNSPEC is specified, it will try all possible address families
until a connection can be established, and will return the associated
socket of the connection. A new error EAFNOSUPPORT will be returned
if the address family is not supported.
15. Summary of New Definitions
The following list summarizes the constants and structure,
definitions discussed in this memo, sorted by header.
<netinet/icmp6.h> ICMP6_DST_UNREACH
<netinet/icmp6.h> ICMP6_DST_UNREACH_ADDR
<netinet/icmp6.h> ICMP6_DST_UNREACH_ADMIN
<netinet/icmp6.h> ICMP6_DST_UNREACH_BEYONDSCOPE
<netinet/icmp6.h> ICMP6_DST_UNREACH_NOPORT
<netinet/icmp6.h> ICMP6_DST_UNREACH_NOROUTE
<netinet/icmp6.h> ICMP6_ECHO_REPLY
<netinet/icmp6.h> ICMP6_ECHO_REQUEST
<netinet/icmp6.h> ICMP6_INFOMSG_MASK
Stevens, et al. Informational [Page 52]
RFC 3542 Advanced Sockets API for IPv6 May 2003
<netinet/icmp6.h> ICMP6_PACKET_TOO_BIG
<netinet/icmp6.h> ICMP6_PARAMPROB_HEADER
<netinet/icmp6.h> ICMP6_PARAMPROB_NEXTHEADER
<netinet/icmp6.h> ICMP6_PARAMPROB_OPTION
<netinet/icmp6.h> ICMP6_PARAM_PROB
<netinet/icmp6.h> ICMP6_ROUTER_RENUMBERING
<netinet/icmp6.h> ICMP6_RR_FLAGS_FORCEAPPLY
<netinet/icmp6.h> ICMP6_RR_FLAGS_PREVDONE
<netinet/icmp6.h> ICMP6_RR_FLAGS_REQRESULT
<netinet/icmp6.h> ICMP6_RR_FLAGS_SPECSITE
<netinet/icmp6.h> ICMP6_RR_FLAGS_TEST
<netinet/icmp6.h> ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME
<netinet/icmp6.h> ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME
<netinet/icmp6.h> ICMP6_RR_PCOUSE_RAFLAGS_AUTO
<netinet/icmp6.h> ICMP6_RR_PCOUSE_RAFLAGS_ONLINK
<netinet/icmp6.h> ICMP6_RR_RESULT_FLAGS_FORBIDDEN
<netinet/icmp6.h> ICMP6_RR_RESULT_FLAGS_OOB
<netinet/icmp6.h> ICMP6_TIME_EXCEEDED
<netinet/icmp6.h> ICMP6_TIME_EXCEED_REASSEMBLY
<netinet/icmp6.h> ICMP6_TIME_EXCEED_TRANSIT
<netinet/icmp6.h> MLD_LISTENER_QUERY
<netinet/icmp6.h> MLD_LISTENER_REDUCTION
<netinet/icmp6.h> MLD_LISTENER_REPORT
<netinet/icmp6.h> ND_NA_FLAG_OVERRIDE
<netinet/icmp6.h> ND_NA_FLAG_ROUTER
<netinet/icmp6.h> ND_NA_FLAG_SOLICITED
<netinet/icmp6.h> ND_NEIGHBOR_ADVERT
<netinet/icmp6.h> ND_NEIGHBOR_SOLICIT
<netinet/icmp6.h> ND_OPT_MTU
<netinet/icmp6.h> ND_OPT_PI_FLAG_AUTO
<netinet/icmp6.h> ND_OPT_PI_FLAG_ONLINK
<netinet/icmp6.h> ND_OPT_PREFIX_INFORMATION
<netinet/icmp6.h> ND_OPT_REDIRECTED_HEADER
<netinet/icmp6.h> ND_OPT_SOURCE_LINKADDR
<netinet/icmp6.h> ND_OPT_TARGET_LINKADDR
<netinet/icmp6.h> ND_RA_FLAG_MANAGED
<netinet/icmp6.h> ND_RA_FLAG_OTHER
<netinet/icmp6.h> ND_REDIRECT
<netinet/icmp6.h> ND_ROUTER_ADVERT
<netinet/icmp6.h> ND_ROUTER_SOLICIT
<netinet/icmp6.h> struct icmp6_filter{};
<netinet/icmp6.h> struct icmp6_hdr{};
<netinet/icmp6.h> struct icmp6_router_renum{};
<netinet/icmp6.h> struct mld_hdr{};
<netinet/icmp6.h> struct nd_neighbor_advert{};
<netinet/icmp6.h> struct nd_neighbor_solicit{};
<netinet/icmp6.h> struct nd_opt_hdr{};
Stevens, et al. Informational [Page 53]
RFC 3542 Advanced Sockets API for IPv6 May 2003
<netinet/icmp6.h> struct nd_opt_mtu{};
<netinet/icmp6.h> struct nd_opt_prefix_info{};
<netinet/icmp6.h> struct nd_opt_rd_hdr{};
<netinet/icmp6.h> struct nd_redirect{};
<netinet/icmp6.h> struct nd_router_advert{};
<netinet/icmp6.h> struct nd_router_solicit{};
<netinet/icmp6.h> struct rr_pco_match{};
<netinet/icmp6.h> struct rr_pco_use{};
<netinet/icmp6.h> struct rr_result{};
<netinet/in.h> IPPROTO_AH
<netinet/in.h> IPPROTO_DSTOPTS
<netinet/in.h> IPPROTO_ESP
<netinet/in.h> IPPROTO_FRAGMENT
<netinet/in.h> IPPROTO_HOPOPTS
<netinet/in.h> IPPROTO_ICMPV6
<netinet/in.h> IPPROTO_IPV6
<netinet/in.h> IPPROTO_NONE
<netinet/in.h> IPPROTO_ROUTING
<netinet/in.h> IPV6_CHECKSUM
<netinet/in.h> IPV6_DONTFRAG
<netinet/in.h> IPV6_DSTOPTS
<netinet/in.h> IPV6_HOPLIMIT
<netinet/in.h> IPV6_HOPOPTS
<netinet/in.h> IPV6_NEXTHOP
<netinet/in.h> IPV6_PATHMTU
<netinet/in.h> IPV6_PKTINFO
<netinet/in.h> IPV6_RECVDSTOPTS
<netinet/in.h> IPV6_RECVHOPLIMIT
<netinet/in.h> IPV6_RECVHOPOPTS
<netinet/in.h> IPV6_RECVPKTINFO
<netinet/in.h> IPV6_RECVRTHDR
<netinet/in.h> IPV6_RECVTCLASS
<netinet/in.h> IPV6_RTHDR
<netinet/in.h> IPV6_RTHDRDSTOPTS
<netinet/in.h> IPV6_RTHDR_TYPE_0
<netinet/in.h> IPV6_RECVPATHMTU
<netinet/in.h> IPV6_TCLASS
<netinet/in.h> IPV6_USE_MIN_MTU
<netinet/in.h> struct in6_pktinfo{};
<netinet/in.h> struct ip6_mtuinfo{};
<netinet/ip6.h> IP6F_MORE_FRAG
<netinet/ip6.h> IP6F_OFF_MASK
<netinet/ip6.h> IP6F_RESERVED_MASK
<netinet/ip6.h> IP6OPT_JUMBO
<netinet/ip6.h> IP6OPT_JUMBO_LEN
Stevens, et al. Informational [Page 54]
RFC 3542 Advanced Sockets API for IPv6 May 2003
<netinet/ip6.h> IP6OPT_MUTABLE
<netinet/ip6.h> IP6OPT_NSAP_ADDR
<netinet/ip6.h> IP6OPT_PAD1
<netinet/ip6.h> IP6OPT_PADN
<netinet/ip6.h> IP6OPT_ROUTER_ALERT
<netinet/ip6.h> IP6OPT_TUNNEL_LIMIT
<netinet/ip6.h> IP6OPT_TYPE_DISCARD
<netinet/ip6.h> IP6OPT_TYPE_FORCEICMP
<netinet/ip6.h> IP6OPT_TYPE_ICMP
<netinet/ip6.h> IP6OPT_TYPE_SKIP
<netinet/ip6.h> IP6_ALERT_AN
<netinet/ip6.h> IP6_ALERT_MLD
<netinet/ip6.h> IP6_ALERT_RSVP
<netinet/ip6.h> struct ip6_dest{};
<netinet/ip6.h> struct ip6_frag{};
<netinet/ip6.h> struct ip6_hbh{};
<netinet/ip6.h> struct ip6_hdr{};
<netinet/ip6.h> struct ip6_opt{};
<netinet/ip6.h> struct ip6_opt_jumbo{};
<netinet/ip6.h> struct ip6_opt_nsap{};
<netinet/ip6.h> struct ip6_opt_router{};
<netinet/ip6.h> struct ip6_opt_tunnel{};
<netinet/ip6.h> struct ip6_rthdr{};
<netinet/ip6.h> struct ip6_rthdr0{};
The following list summarizes the function and macro prototypes
discussed in this memo, sorted by header.
<netinet/icmp6.h> void ICMP6_FILTER_SETBLOCK(int, struct
icmp6_filter *);
<netinet/icmp6.h> void
ICMP6_FILTER_SETBLOCKALL(struct icmp6_filter *);
<netinet/icmp6.h> void
ICMP6_FILTER_SETPASS(int,
struct icmp6_filter *);
<netinet/icmp6.h> void
ICMP6_FILTER_SETPASSALL(struct icmp6_filter *);
<netinet/icmp6.h> int ICMP6_FILTER_WILLBLOCK(int,
const struct icmp6_filter *);
<netinet/icmp6.h> int ICMP6_FILTER_WILLPASS(int,
const struct icmp6_filter *);
<netinet/in.h> int IN6_ARE_ADDR_EQUAL(const struct in6_addr *,
const struct in6_addr *);
<netinet/in.h> int inet6_opt_append(void *, socklen_t, int,
uint8_t, socklen_t, uint_t,
void **);
Stevens, et al. Informational [Page 55]
RFC 3542 Advanced Sockets API for IPv6 May 2003
<netinet/in.h> int inet6_opt_get_val(void *, int, void *,
socklen_t);
<netinet/in.h> int inet6_opt_find(void *, socklen_t,
int, uint8_t ,
socklen_t *, void **);
<netinet/in.h> int inet6_opt_finish(void *, socklen_t, int);
<netinet/in.h> int inet6_opt_init(void *, socklen_t);
<netinet/in.h> int inet6_opt_next(void *, socklen_t,
int, uint8_t *,
socklen_t *, void **);
<netinet/in.h> int inet6_opt_set_val(void *, int,
void *, socklen_t);
<netinet/in.h> int inet6_rth_add(void *,
const struct in6_addr *);
<netinet/in.h> struct in6_addr inet6_rth_getaddr(const void *,
int);
<netinet/in.h> void *inet6_rth_init(void *, socklen_t,
int, int);
<netinet/in.h> int inet6_rth_reverse(const void *, void *);
<netinet/in.h> int inet6_rth_segments(const void *);
<netinet/in.h> soccklen_t inet6_rth_space(int, int);
<netinet/ip6.h> int IP6OPT_TYPE(uint8_t);
<sys/socket.h> socklen_t CMSG_LEN(socklen_t);
<sys/socket.h> socklen_t CMSG_SPACE(socklen_t);
<unistd.h> int rresvport_af(int *, int);
<unistd.h> int rcmd_af(char **, unsigned short,
const char *, const char *,
const char *, int *, int);
<unistd.h> int rexec_af(char **, unsigned short,
const char *, const char *,
const char *, int *, int);
16. Security Considerations
The setting of certain Hop-by-Hop options and Destination options may
be restricted to privileged processes. Similarly some Hop-by-Hop
options and Destination options may not be returned to non-privileged
applications.
The ability to specify an arbitrary source address using IPV6_PKTINFO
must be prevented; at least for non-privileged processes.
Stevens, et al. Informational [Page 56]
RFC 3542 Advanced Sockets API for IPv6 May 2003
17. Changes from RFC 2292
Significant changes that affect the compatibility to RFC 2292:
- Removed the IPV6_PKTOPTIONS socket option by allowing sticky
options to be set with individual setsockopt() calls.
- Removed the ability to be able to specify Hop-by-Hop and
Destination options using multiple ancillary data items. The
application, using the inet6_opt_xxx() routines (see below), is
responsible for formatting the whole extension header.
- Removed the support for the loose/strict Routing header since that
has been removed from the IPv6 specification.
- Loosened the constraints for jumbo payload option that this option
was always hidden from applications.
- Disabled the use of the IPV6_HOPLIMIT sticky option.
- Removed ip6r0_addr field from the ip6_rthdr structure.
- Intentionally unspecified how to get received packet's information
on TCP sockets.
New features:
- Added IPV6_RTHDRDSTOPTS to specify a Destination Options header
before the Routing header.
- Added separate IPV6_RECVxxx options to enable the receipt of the
corresponding ancillary data items.
- Added inet6_rth_xxx() and inet6_opt_xxx() functions to deal with
routing or IPv6 options headers.
- Added extensions of libraries for the "r" commands.
- Introduced additional IPv6 option definitions such as IP6OPT_PAD1.
- Added MLD and router renumbering definitions.
- Added MTU-related socket options and ancillary data items.
- Added options and ancillary data items to manipulate the traffic
class field.
Stevens, et al. Informational [Page 57]
RFC 3542 Advanced Sockets API for IPv6 May 2003
- Changed the name of ICMPv6 unreachable code 2 to be "beyond scope
of source address." ICMP6_DST_UNREACH_NOTNEIGHBOR was removed
with this change.
Clarifications:
- Added clarifications on extension headers ordering; for the
sending side, assume the recommended ordering described in RFC
2460. For the receiving side, do not assume any ordering and pass
all headers to the application in the received order.
- Added a summary about the interface selection rule.
- Clarified the ordering between IPV6_MULTICAST_IF and the
IPV6_PKTINFO sticky option for multicast packets.
- Clarified how sticky options and the ICMPv6 filter are turned off
and that getsockopt() of a sticky option returns what was set with
setsockopt().
- Clarified that IPV6_NEXTHOP should be ignored for a multicast
destination, that it should not contradict with the specified
outgoing interface, and that the next hop should be a sockaddr_in6
structure.
- Clarified corner cases of IPV6_CHECKSUM.
- Aligned with the POSIX standard.
Editorial changes:
- Replaced MUST with must (since this is an informational document).
- Revised abstract to be more clear and concise, particularly
concentrating on differences from RFC 2292.
- Made the URL of assigned numbers less specific so that it would be
more robust for future changes.
- Updated the reference to the basic API.
- Added a reference to the latest POSIX standard.
- Moved general specifications of ancillary data and CMSG macros to
the appendix.
Stevens, et al. Informational [Page 58]
RFC 3542 Advanced Sockets API for IPv6 May 2003
18. References
[RFC-1981] McCann, J., Deering, S. and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
[RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC-3493] Gilligan, R., Thomson, S., Bound, J., McCann, J. and
W. Stevens, "Basic Socket Interface Extensions for
IPv6", RFC 3493, March 2003.
[POSIX] IEEE Std. 1003.1-2001 Standard for Information
Technology -- Portable Operating System Interface
(POSIX). Open group Technical Standard: Base
Specifications, Issue 6, December 2001. ISO/IEC
9945:2002. http://www.opengroup.org/austin
[TCPIPILLUST] Wright, G., Stevens, W., "TCP/IP Illustrated, Volume 2:
The Implementation", Addison Wesley, 1994.
19. Acknowledgments
Matt Thomas and Jim Bound have been working on the technical details
in this document for over a year. Keith Sklower is the original
implementor of ancillary data in the BSD networking code. Craig Metz
provided lots of feedback, suggestions, and comments based on his
implementing many of these features as the document was being
written. Mark Andrews first proposed the idea of the
IPV6_USE_MIN_MTU option. Jun-ichiro Hagino contributed text for the
traffic class API from a document of his own.
The following provided comments on earlier drafts: Pascal Anelli,
Hamid Asayesh, Ran Atkinson, Karl Auerbach, Hamid Asayesh, Don
Coolidge, Matt Crawford, Sam T. Denton, Richard Draves, Francis
Dupont, Toerless Eckert, Lilian Fernandes, Bob Gilligan, Gerri
Harter, Tim Hartrick, Bob Halley, Masaki Hirabaru, Michael Hunter,
Yoshinobu Inoue, Mukesh Kacker, A. N. Kuznetsov, Sam Manthorpe, Pedro
Marques, Jack McCann, der Mouse, John Moy, Lori Napoli, Thomas
Narten, Atsushi Onoe, Steve Parker, Charles Perkins, Ken Powell, Tom
Pusateri, Pedro Roque, Sameer Shah, Peter Sjodin, Stephen P.
Spackman, Jinmei Tatuya, Karen Tracey, Sowmini Varadhan, Quaizar
Vohra, Carl Williams, Steve Wise, Eric Wong, Farrell Woods, Kazu
Yamamoto, Vladislav Yasevich, and Yoshifuji Hideaki.
Stevens, et al. Informational [Page 59]
RFC 3542 Advanced Sockets API for IPv6 May 2003
20. Appendix A: Ancillary Data Overview
4.2BSD allowed file descriptors to be transferred between separate
processes across a UNIX domain socket using the sendmsg() and
recvmsg() functions. Two members of the msghdr structure,
msg_accrights and msg_accrightslen, were used to send and receive the
descriptors. When the OSI protocols were added to 4.3BSD Reno in
1990 the names of these two fields in the msghdr structure were
changed to msg_control and msg_controllen, because they were used by
the OSI protocols for "control information", although the comments in
the source code call this "ancillary data".
Other than the OSI protocols, the use of ancillary data has been
rare. In 4.4BSD, for example, the only use of ancillary data with
IPv4 is to return the destination address of a received UDP datagram
if the IP_RECVDSTADDR socket option is set. With Unix domain sockets
ancillary data is still used to send and receive descriptors.
Nevertheless the ancillary data fields of the msghdr structure
provide a clean way to pass information in addition to the data that
is being read or written. The inclusion of the msg_control and
msg_controllen members of the msghdr structure along with the cmsghdr
structure that is pointed to by the msg_control member is required by
the Posix sockets API standard.
20.1. The msghdr Structure
The msghdr structure is used by the recvmsg() and sendmsg()
functions. Its Posix definition is:
struct msghdr {
void *msg_name; /* ptr to socket address
structure */
socklen_t msg_namelen; /* size of socket address
structure */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data */
socklen_t msg_controllen; /* ancillary data buffer length */
int msg_flags; /* flags on received message */
};
The structure is declared as a result of including <sys/socket.h>.
(Note: Before Posix the two "void *" pointers were typically "char
*", and the two socklen_t members were typically integers. Earlier
drafts of Posix had the two socklen_t members as size_t, but it then
changed these to socklen_t to simplify binary portability for 64-bit
Stevens, et al. Informational [Page 60]
RFC 3542 Advanced Sockets API for IPv6 May 2003
implementations and to align Posix with X/Open's Networking Services,
Issue 5. The change in msg_control to a "void *" pointer affects any
code that increments this pointer.)
Most Berkeley-derived implementations limit the amount of ancillary
data in a call to sendmsg() to no more than 108 bytes (an mbuf).
This API requires a minimum of 10240 bytes of ancillary data, but it
is recommended that the amount be limited only by the buffer space
reserved by the socket (which can be modified by the SO_SNDBUF socket
option). (Note: This magic number 10240 was picked as a value that
should always be large enough. 108 bytes is clearly too small as the
maximum size of a Routing header is 2048 bytes.)
20.2. The cmsghdr Structure
The cmsghdr structure describes ancillary data objects transferred by
recvmsg() and sendmsg(). Its Posix definition is:
struct cmsghdr {
socklen_t cmsg_len; /* #bytes, including this header */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by unsigned char cmsg_data[]; */
};
This structure is declared as a result of including <sys/socket.h>.
(Note: Before Posix the cmsg_len member was an integer, and not a
socklen_t. See the Note in the previous section for why socklen_t is
used here.)
As shown in this definition, normally there is no member with the
name cmsg_data[]. Instead, the data portion is accessed using the
CMSG_xxx() macros, as described in Section 20.3. Nevertheless, it is
common to refer to the cmsg_data[] member.
When ancillary data is sent or received, any number of ancillary data
objects can be specified by the msg_control and msg_controllen
members of the msghdr structure, because each object is preceded by a
cmsghdr structure defining the object's length (the cmsg_len member).
Historically Berkeley-derived implementations have passed only one
object at a time, but this API allows multiple objects to be passed
in a single call to sendmsg() or recvmsg(). The following example
shows two ancillary data objects in a control buffer.
Stevens, et al. Informational [Page 61]
RFC 3542 Advanced Sockets API for IPv6 May 2003
|<--------------------------- msg_controllen ------------------------->|
| OR |
|<--------------------------- msg_controllen ---------------------->|
| |
|<----- ancillary data object ----->|<---- ancillary data object ----->|
|<------ min CMSG_SPACE() --------->|<----- min CMSG_SPACE() --------->|
| | |
|<---------- cmsg_len ---------->| |<-------- cmsg_len ----------->| |
|<--------- CMSG_LEN() --------->| |<------- CMSG_LEN() ---------->| |
| | | | |
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+----------+--+
|cmsg_|cmsg_|cmsg_|XX| cmsg_ |XX|cmsg_|cmsg_|cmsg_|XX| cmsg_ |XX|
|len |level|type |XX| data[] |XX|len |level|type |XX| data[] |XX|
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+----------+--+
^
|
msg_control
points here
The fields shown as "XX" are possible padding, between the cmsghdr
structure and the data, and between the data and the next cmsghdr
structure, if required by the implementation. While sending an
application may or may not include padding at the end of last
ancillary data in msg_controllen and implementations must accept both
as valid. On receiving a portable application must provide space for
padding at the end of the last ancillary data as implementations may
copy out the padding at the end of the control message buffer and
include it in the received msg_controllen. When recvmsg() is called
if msg_controllen is too small for all the ancillary data items
including any trailing padding after the last item an implementation
may set MSG_CTRUNC.
20.3. Ancillary Data Object Macros
To aid in the manipulation of ancillary data objects, three macros
from 4.4BSD are defined by Posix: CMSG_DATA(), CMSG_NXTHDR(), and
CMSG_FIRSTHDR(). Before describing these macros, we show the
following example of how they might be used with a call to recvmsg().
struct msghdr msg;
struct cmsghdr *cmsgptr;
/* fill in msg */
/* call recvmsg() */
Stevens, et al. Informational [Page 62]
RFC 3542 Advanced Sockets API for IPv6 May 2003
for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
if (cmsgptr->cmsg_len == 0) {
/* Error handling */
break;
}
if (cmsgptr->cmsg_level == ... &&
cmsgptr->cmsg_type == ... ) {
u_char *ptr;
ptr = CMSG_DATA(cmsgptr);
/* process data pointed to by ptr */
}
}
We now describe the three Posix macros, followed by two more that are
new with this API: CMSG_SPACE() and CMSG_LEN(). All these macros are
defined as a result of including <sys/socket.h>.
20.3.1. CMSG_FIRSTHDR
struct cmsghdr *CMSG_FIRSTHDR(const struct msghdr *mhdr);
CMSG_FIRSTHDR() returns a pointer to the first cmsghdr structure in
the msghdr structure pointed to by mhdr. The macro returns NULL if
there is no ancillary data pointed to by the msghdr structure (that
is, if either msg_control is NULL or if msg_controllen is less than
the size of a cmsghdr structure).
One possible implementation could be
#define CMSG_FIRSTHDR(mhdr) \
( (mhdr)->msg_controllen >= sizeof(struct cmsghdr) ? \
(struct cmsghdr *)(mhdr)->msg_control : \
(struct cmsghdr *)NULL )
(Note: Most existing implementations do not test the value of
msg_controllen, and just return the value of msg_control. The value
of msg_controllen must be tested, because if the application asks
recvmsg() to return ancillary data, by setting msg_control to point
to the application's buffer and setting msg_controllen to the length
of this buffer, the kernel indicates that no ancillary data is
available by setting msg_controllen to 0 on return. It is also
easier to put this test into this macro, than making the application
perform the test.)
Stevens, et al. Informational [Page 63]
RFC 3542 Advanced Sockets API for IPv6 May 2003
20.3.2. CMSG_NXTHDR
As described in Section 5.1, CMSG_NXTHDR has been extended to handle
a NULL 2nd argument to mean "get the first header". This provides an
alternative way of coding the processing loop shown earlier:
struct msghdr msg;
struct cmsghdr *cmsgptr = NULL;
/* fill in msg */
/* call recvmsg() */
while ((cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) != NULL) {
if (cmsgptr->cmsg_len == 0) {
/* Error handling */
break;
}
if (cmsgptr->cmsg_level == ... &&
cmsgptr->cmsg_type == ... ) {
u_char *ptr;
ptr = CMSG_DATA(cmsgptr);
/* process data pointed to by ptr */
}
}
One possible implementation could be:
#define CMSG_NXTHDR(mhdr, cmsg) \
(((cmsg) == NULL) ? CMSG_FIRSTHDR(mhdr) : \
(((u_char *)(cmsg) + ALIGN_H((cmsg)->cmsg_len) \
+ ALIGN_D(sizeof(struct cmsghdr)) > \
(u_char *)((mhdr)->msg_control) + (mhdr)->msg_controllen) ? \
(struct cmsghdr *)NULL : \
(struct cmsghdr *)((u_char *)(cmsg) + \
ALIGN_H((cmsg)->cmsg_len))))
The macros ALIGN_H() and ALIGN_D(), which are implementation
dependent, round their arguments up to the next even multiple of
whatever alignment is required for the start of the cmsghdr structure
and the data, respectively. (This is probably a multiple of 4 or 8
bytes.) They are often the same macro in implementations platforms
where alignment requirement for header and data is chosen to be
identical.
Stevens, et al. Informational [Page 64]
RFC 3542 Advanced Sockets API for IPv6 May 2003
20.3.3. CMSG_DATA
unsigned char *CMSG_DATA(const struct cmsghdr *cmsg);
CMSG_DATA() returns a pointer to the data (what is called the
cmsg_data[] member, even though such a member is not defined in the
structure) following a cmsghdr structure.
One possible implementation could be:
#define CMSG_DATA(cmsg) ( (u_char *)(cmsg) + \
ALIGN_D(sizeof(struct cmsghdr)) )
20.3.4. CMSG_SPACE
CMSG_SPACE is new with this API (see Section 5.2). It is used to
determine how much space needs to be allocated for an ancillary data
item.
One possible implementation could be:
#define CMSG_SPACE(length) ( ALIGN_D(sizeof(struct cmsghdr)) + \
ALIGN_H(length) )
20.3.5. CMSG_LEN
CMSG_LEN is new with this API (see Section 5.3). It returns the
value to store in the cmsg_len member of the cmsghdr structure,
taking into account any padding needed to satisfy alignment
requirements.
One possible implementation could be:
#define CMSG_LEN(length) ( ALIGN_D(sizeof(struct cmsghdr)) + \
length )
21. Appendix B: Examples Using the inet6_rth_XXX() Functions
Here we show an example for both sending Routing headers and
processing and reversing a received Routing header.
21.1. Sending a Routing Header
As an example of these Routing header functions defined in this
document, we go through the function calls for the example on p. 17
of [RFC-2460]. The source is S, the destination is D, and the three
intermediate nodes are I1, I2, and I3.
Stevens, et al. Informational [Page 65]
RFC 3542 Advanced Sockets API for IPv6 May 2003
S -----> I1 -----> I2 -----> I3 -----> D
src: * S S S S S
dst: D I1 I2 I3 D D
A[1]: I1 I2 I1 I1 I1 I1
A[2]: I2 I3 I3 I2 I2 I2
A[3]: I3 D D D I3 I3
#seg: 3 3 2 1 0 3
src and dst are the source and destination IPv6 addresses in the IPv6
header. A[1], A[2], and A[3] are the three addresses in the Routing
header. #seg is the Segments Left field in the Routing header.
The six values in the column beneath node S are the values in the
Routing header specified by the sending application using sendmsg()
of setsockopt(). The function calls by the sender would look like:
void *extptr;
socklen_t extlen;
struct msghdr msg;
struct cmsghdr *cmsgptr;
int cmsglen;
struct sockaddr_in6 I1, I2, I3, D;
extlen = inet6_rth_space(IPV6_RTHDR_TYPE_0, 3);
cmsglen = CMSG_SPACE(extlen);
cmsgptr = malloc(cmsglen);
cmsgptr->cmsg_len = CMSG_LEN(extlen);
cmsgptr->cmsg_level = IPPROTO_IPV6;
cmsgptr->cmsg_type = IPV6_RTHDR;
extptr = CMSG_DATA(cmsgptr);
extptr = inet6_rth_init(extptr, extlen, IPV6_RTHDR_TYPE_0, 3);
inet6_rth_add(extptr, &I1.sin6_addr);
inet6_rth_add(extptr, &I2.sin6_addr);
inet6_rth_add(extptr, &I3.sin6_addr);
msg.msg_control = cmsgptr;
msg.msg_controllen = cmsglen;
/* finish filling in msg{}, msg_name = D */
/* call sendmsg() */
We also assume that the source address for the socket is not
specified (i.e., the asterisk in the figure).
Stevens, et al. Informational [Page 66]
RFC 3542 Advanced Sockets API for IPv6 May 2003
The four columns of six values that are then shown between the five
nodes are the values of the fields in the packet while the packet is
in transit between the two nodes. Notice that before the packet is
sent by the source node S, the source address is chosen (replacing
the asterisk), I1 becomes the destination address of the datagram,
the two addresses A[2] and A[3] are "shifted up", and D is moved to
A[3].
The columns of values that are shown beneath the destination node are
the values returned by recvmsg(), assuming the application has
enabled both the IPV6_RECVPKTINFO and IPV6_RECVRTHDR socket options.
The source address is S (contained in the sockaddr_in6 structure
pointed to by the msg_name member), the destination address is D
(returned as an ancillary data object in an in6_pktinfo structure),
and the ancillary data object specifying the Routing header will
contain three addresses (I1, I2, and I3). The number of segments in
the Routing header is known from the Hdr Ext Len field in the Routing
header (a value of 6, indicating 3 addresses).
The return value from inet6_rth_segments() will be 3 and
inet6_rth_getaddr(0) will return I1, inet6_rth_getaddr(1) will return
I2, and inet6_rth_getaddr(2) will return I3,
If the receiving application then calls inet6_rth_reverse(), the
order of the three addresses will become I3, I2, and I1.
We can also show what an implementation might store in the ancillary
data object as the Routing header is being built by the sending
process. If we assume a 32-bit architecture where sizeof(struct
cmsghdr) equals 12, with a desired alignment of 4-byte boundaries,
then the call to inet6_rth_space(3) returns 68: 12 bytes for the
cmsghdr structure and 56 bytes for the Routing header (8 + 3*16).
The call to inet6_rth_init() initializes the ancillary data object to
contain a Type 0 Routing header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Stevens, et al. Informational [Page 67]
RFC 3542 Advanced Sockets API for IPv6 May 2003
The first call to inet6_rth_add() adds I1 to the list.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[1] = I1 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cmsg_len is incremented by 16, and the Segments Left field is
incremented by 1.
Stevens, et al. Informational [Page 68]
RFC 3542 Advanced Sockets API for IPv6 May 2003
The next call to inet6_rth_add() adds I2 to the list.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 52 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[1] = I1 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[2] = I2 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cmsg_len is incremented by 16, and the Segments Left field is
incremented by 1.
Stevens, et al. Informational [Page 69]
RFC 3542 Advanced Sockets API for IPv6 May 2003
The last call to inet6_rth_add() adds I3 to the list.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 68 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[1] = I1 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[2] = I2 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[3] = I3 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cmsg_len is incremented by 16, and the Segments Left field is
incremented by 1.
21.2. Receiving Routing Headers
This example assumes that the application has enabled IPV6_RECVRTHDR
socket option. The application prints and reverses a source route
and uses that to echo the received data.
Stevens, et al. Informational [Page 70]
RFC 3542 Advanced Sockets API for IPv6 May 2003
struct sockaddr_in6 addr;
struct msghdr msg;
struct iovec iov;
struct cmsghdr *cmsgptr;
socklen_t cmsgspace;
void *extptr;
int extlen;
int segments;
int i;
char databuf[8192];
segments = 100; /* Enough */
extlen = inet6_rth_space(IPV6_RTHDR_TYPE_0, segments);
cmsgspace = CMSG_SPACE(extlen);
cmsgptr = malloc(cmsgspace);
if (cmsgptr == NULL) {
perror("malloc");
exit(1);
}
extptr = CMSG_DATA(cmsgptr);
msg.msg_control = cmsgptr;
msg.msg_controllen = cmsgspace;
msg.msg_name = (struct sockaddr *)&addr;
msg.msg_namelen = sizeof (addr);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
iov.iov_base = databuf;
iov.iov_len = sizeof (databuf);
msg.msg_flags = 0;
if (recvmsg(s, &msg, 0) == -1) {
perror("recvmsg");
return;
}
if (msg.msg_controllen != 0 &&
cmsgptr->cmsg_level == IPPROTO_IPV6 &&
cmsgptr->cmsg_type == IPV6_RTHDR) {
struct in6_addr *in6;
char asciiname[INET6_ADDRSTRLEN];
struct ip6_rthdr *rthdr;
rthdr = (struct ip6_rthdr *)extptr;
segments = inet6_rth_segments(extptr);
printf("route (%d segments, %d left): ",
segments, rthdr->ip6r_segleft);
for (i = 0; i < segments; i++) {
in6 = inet6_rth_getaddr(extptr, i);
Stevens, et al. Informational [Page 71]
RFC 3542 Advanced Sockets API for IPv6 May 2003
if (in6 == NULL)
printf("<NULL> ");
else
printf("%s ", inet_ntop(AF_INET6,
(void *)in6->s6_addr,
asciiname, INET6_ADDRSTRLEN));
}
if (inet6_rth_reverse(extptr, extptr) == -1) {
printf("reverse failed");
return;
}
}
iov.iov_base = databuf;
iov.iov_len = strlen(databuf);
if (sendmsg(s, &msg, 0) == -1)
perror("sendmsg");
if (cmsgptr != NULL)
free(cmsgptr);
Note: The above example is a simple illustration. It skips some
error checks, including those involving the MSG_TRUNC and MSG_CTRUNC
flags. It also leaves some type mismatches in favor of brevity.
22. Appendix C: Examples Using the inet6_opt_XXX() Functions
This shows how Hop-by-Hop and Destination options can be both built
as well as parsed using the inet6_opt_XXX() functions. These
examples assume that there are defined values for OPT_X and OPT_Y.
Note: The example is a simple illustration. It skips some error
checks and leaves some type mismatches in favor of brevity.
22.1. Building Options
We now provide an example that builds two Hop-by-Hop options using
the example in Appendix B of [RFC-2460].
void *extbuf;
socklen_t extlen;
int currentlen;
void *databuf;
int offset;
uint8_t value1;
uint16_t value2;
uint32_t value4;
uint64_t value8;
/* Estimate the length */
Stevens, et al. Informational [Page 72]
RFC 3542 Advanced Sockets API for IPv6 May 2003
currentlen = inet6_opt_init(NULL, 0);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_append(NULL, 0, currentlen, OPT_X,
12, 8, NULL);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_append(NULL, 0, currentlen, OPT_Y,
7, 4, NULL);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_finish(NULL, 0, currentlen);
if (currentlen == -1)
return (-1);
extlen = currentlen;
extbuf = malloc(extlen);
if (extbuf == NULL) {
perror("malloc");
return (-1);
}
currentlen = inet6_opt_init(extbuf, extlen);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_append(extbuf, extlen, currentlen,
OPT_X, 12, 8, &databuf);
if (currentlen == -1)
return (-1);
/* Insert value 0x12345678 for 4-octet field */
offset = 0;
value4 = 0x12345678;
offset = inet6_opt_set_val(databuf, offset,
&value4, sizeof (value4));
/* Insert value 0x0102030405060708 for 8-octet field */
value8 = 0x0102030405060708;
offset = inet6_opt_set_val(databuf, offset,
&value8, sizeof (value8));
currentlen = inet6_opt_append(extbuf, extlen, currentlen,
OPT_Y, 7, 4, &databuf);
if (currentlen == -1)
return (-1);
/* Insert value 0x01 for 1-octet field */
offset = 0;
value1 = 0x01;
offset = inet6_opt_set_val(databuf, offset,
&value1, sizeof (value1));
Stevens, et al. Informational [Page 73]
RFC 3542 Advanced Sockets API for IPv6 May 2003
/* Insert value 0x1331 for 2-octet field */
value2 = 0x1331;
offset = inet6_opt_set_val(databuf, offset,
&value2, sizeof (value2));
/* Insert value 0x01020304 for 4-octet field */
value4 = 0x01020304;
offset = inet6_opt_set_val(databuf, offset,
&value4, sizeof (value4));
currentlen = inet6_opt_finish(extbuf, extlen, currentlen);
if (currentlen == -1)
return (-1);
/* extbuf and extlen are now completely formatted */
22.2. Parsing Received Options
This example parses and prints the content of the two options in the
previous example.
int
print_opt(void *extbuf, socklen_t extlen)
{
struct ip6_dest *ext;
int currentlen;
uint8_t type;
socklen_t len;
void *databuf;
int offset;
uint8_t value1;
uint16_t value2;
uint32_t value4;
uint64_t value8;
ext = (struct ip6_dest *)extbuf;
printf("nxt %u, len %u (bytes %d)\n", ext->ip6d_nxt,
ext->ip6d_len, (ext->ip6d_len + 1) * 8);
currentlen = 0;
while (1) {
currentlen = inet6_opt_next(extbuf, extlen,
currentlen, &type,
&len, &databuf);
if (currentlen == -1)
break;
printf("Received opt %u len %u\n",
type, len);
switch (type) {
case OPT_X:
Stevens, et al. Informational [Page 74]
RFC 3542 Advanced Sockets API for IPv6 May 2003
offset = 0;
offset =
inet6_opt_get_val(databuf, offset,
&value4,
sizeof (value4));
printf("X 4-byte field %x\n", value4);
offset =
inet6_opt_get_val(databuf, offset,
&value8,
sizeof (value8));
printf("X 8-byte field %llx\n", value8);
break;
case OPT_Y:
offset = 0;
offset =
inet6_opt_get_val(databuf, offset,
&value1,
sizeof (value1));
printf("Y 1-byte field %x\n", value1);
offset =
inet6_opt_get_val(databuf, offset,
&value2,
sizeof (value2));
printf("Y 2-byte field %x\n", value2);
offset =
inet6_opt_get_val(databuf, offset,
&value4,
sizeof (value4));
printf("Y 4-byte field %x\n", value4);
break;
default:
printf("Unknown option %u\n", type);
break;
}
}
return (0);
}
Stevens, et al. Informational [Page 75]
RFC 3542 Advanced Sockets API for IPv6 May 2003
23. Authors' Addresses
W. Richard Stevens (deceased)
Matt Thomas
3am Software Foundry
8053 Park Villa Circle
Cupertino, CA 95014
EMail: matt@3am-software.com
Erik Nordmark
Sun Microsystems Laboratories, Europe
180, avenue de l'Europe
38334 SAINT ISMIER Cedex, France
Phone: +33 (0)4 74 18 88 03
Fax: +33 (0)4 76 18 88 88
EMail: Erik.Nordmark@sun.com
Tatuya JINMEI
Corporate Research & Development Center, Toshiba Corporation
1 Komukai Toshiba-cho, Kawasaki-shi
Kanagawa 212-8582, Japan
EMail: jinmei@isl.rdc.toshiba.co.jp
Stevens, et al. Informational [Page 76]
RFC 3542 Advanced Sockets API for IPv6 May 2003
24. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Stevens, et al. Informational [Page 77]