.\"	$OpenBSD: inet6_option_space.3,v 1.15 2005/01/06 03:50:46 itojun Exp $
.\"	$KAME: inet6_option_space.3,v 1.11 2005/01/05 03:00:44 itojun Exp $
.\"
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.Dd December 23, 2004
.Dt INET6_OPTION_SPACE 3
.Os
.\"
.Sh NAME
.Nm inet6_option_space ,
.Nm inet6_option_init ,
.Nm inet6_option_append ,
.Nm inet6_option_alloc ,
.Nm inet6_option_next ,
.Nm inet6_option_find
.Nd IPv6 Hop-by-Hop and Destination Option Manipulation
.\"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.Ft "int"
.Fn inet6_option_space "int nbytes"
.Ft "int"
.Fn inet6_option_init "void *bp" "struct cmsghdr **cmsgp" "int type"
.Ft "int"
.Fn inet6_option_append "struct cmsghdr *cmsg" "const u_int8_t *typep" "int multx" "int plusy"
.Ft "u_int8_t *"
.Fn inet6_option_alloc "struct cmsghdr *cmsg" "int datalen" "int multx" "int plusy"
.Ft "int"
.Fn inet6_option_next "const struct cmsghdr *cmsg" "u_int8_t **tptrp"
.Ft "int"
.Fn inet6_option_find "const struct cmsghdr *cmsg" "u_int8_t **tptrp" "int type"
.\"
.Sh DESCRIPTION
.\"
Manipulating and parsing IPv6's Hop-by-Hop and Destination options is
complicated by the need to properly align and pad data as well as the
need to manipulate ancillary information that is not part of the data
stream.
RFC2292 defines a set of functions, which are implemented as
part of the Kame libraries, to support help developers create, change,
and parse Hop-by-Hope and Destination options.
All of the prototypes
for the option functions are defined in the
.In netinet/in.h
header file.
.\"
.Ss inet6_option_space
In order to determine the amount of space necessary to hold any option
the
.Fn inet6_option_space
function is called.
It returns the number of bytes required to hold
an option when it is stored as ancillary data, including the
.Li cmsghdr
structure at the beginning, and any necessary padding at the end.
The
.Li nbytes
argument indicates the size of the structure defining the option,
and must include any pad bytes at the beginning (the value
.Li y
in the alignment term
.Dq Li "xn + y" ) ,
the type byte, the length byte, and the option data.
.Pp
Note: If multiple options are stored in a single ancillary data
object, which is the recommended technique, the
.Fn inet6_option_space
function overestimates the amount of space required by the size of
.Li N-1
.Li cmsghdr
structures, where
.Li N
is the number of options to be stored in the object.
Usually this has
no impact because it is assumed that most Hop-by-Hop and Destination
option headers carry only one option as indicated in appendix B of RFC2460.
.\"
.Ss inet6_option_init
The
.Fn inet6_option_init
function is called to initialize any ancillary data object that will contain
a Hop-by-Hop or Destination option.
It returns
.Li 0
on success and
.Li -1
when an error occurs.
.Pp
The
.Fa bp
argument points to a previously allocated area of memory which must be
large enough to contain all the arguments that the application indents
to add later via
.Fn inet6_option_append
and
.Fn inet6_option_alloc
routines.
.Pp
The
.Fa cmsgp
argument is a pointer to a pointer to a
.Li cmsghdr
structure.
The
.Fa *cmsgp
argument
points to a
.Li cmsghdr
structure which is constructed by this function and stored in the
area of memory pointed to by
.Fa bp .
.Pp
The
.Fa type
is either
.Dv IPV6_HOPOPTS
or
.Dv IPV6_DSTOPTS
and is stored in the
.Li cmsg_type
member of the
.Li cmsghdr
structure mentioned above.
.\"
.Ss inet6_option_append
This function appends a Hop-by-Hop option or a Destination option into
an ancillary data object previously initialized by a call to
.Fn inet6_option_init .
The
.Fn inet6_option_append function returns
.Li 0
if it succeeds or
.Li -1
when an error occurs.
.Pp
The
.Fa cmsg
argument is a pointer to the
.Li cmsghdr
structure that was initialized by a call to
.Fn inet6_option_init .
.Pp
The
.Fa typep
argument is a pointer to the 8-bit option type.
All options are
encoded as type-length-value tuples and it is assumed that
the
.Fa typep
field is immediately followed by the 8-bit option data length field,
which is then followed by the option data.
.Pp
The option types of
.Li 0
and
.Li 1 are reserved for the
.Li Pad1
and
.Li PadN
options respectively.
All other values from
.Li 2
through
.Li 255
are available for applications to use.
.Pp
The option data length, since it is stored in 8 bites, must have a
value between
.Li 0
and
.Li 255 ,
inclusive.
.Pp
The
.Fa multx
argument
is the value
.Li x
in the alignment term
.Dq Li xn + y
and indicates the byte alignment necessary for the data.
Alignments may be specified as
.Li 1 ,
.Li 2 ,
.Li 4 ,
or
.Li 8
bytes, which is no alignment, 16 bit, 32 bit and 64 bit alignments
respectively.
.Pp
The
.Fa plusy
argument
is the value
.Li y
in the alignment term
.Dq Li xn + y
and must have a value between
.Li 0
and
.Li 7 ,
inclusive, indicating the amount of padding that is necessary for an
option.
.\"
.Ss inet6_option_alloc
The
.Fn inet6_option_alloc
function appends a Hop-by-Hop option or a Destination option into an
ancillary data object that has previously been initialized by a call to
.Fn inet6_option_init .
The
.Fn inet6_option_alloc
function returns a pointer to the 8-bit option type field that at the
beginning of the allocated the option on success, or
.Dv NULL
on an error.
.Pp
The difference between the
.Fn inet6_option_alloc
and
.Fn inet6_option_append
functions is that the latter copies the contents of a previously built
option into the ancillary data object while the former returns a
pointer to the place in the data object where the option's TLV must
then be built by the application.
.Pp
The
.Fa cmsg
argument is a pointer to a
.Li cmsghdr
structure that was initialized by
.Fn inet6_option_init .
.Pp
The
.Fa datalen
argument is the value of the option data length byte for this option.
This value is required as an argument to allow the function to
determine if padding must be appended at the end of the option.
(The
.Fn inet6_option_append
function does not need a data length argument
since the option data length must already be stored by the caller)
.Pp
The
.Fa multx
and
.Fa plusy
arguments
are identical to the arguments of the same name described in the
.Fn inet6_option_init
function above.
.\"
.Ss inet6_option_next
The
.Fn inet6_option_next
function is used to process Hop-by-Hop and Destination options that
are present in an ancillary data object.
When an option remains to
be processed, the return value of the
.Fn inet6_option_next
function is
.Li 0
and the
.Fa *tptrp
argument points to the 8-bit option type field, which is followed by
the 8-bit option data length, and then the option data.
When no more
options remain to be processed, the return value is
.Li -1
and
.Fa *tptrp
is
.Dv NULL
and when an error occurs, the return value is
.Li -1
but the
.Fa *tptrp
argument is not
.Dv NULL .
This set of return values allows a program to easily loop through all
the options in an ancillary data object, checking for the error and
end of stream conditions along the way.
.Pp
When a valid option is returned the
.Fa cmsg
argument points to a
.Li cmsghdr
where the
.Li cmsg_level
equals
.Dv IPPROTO_IPV6
and
.Li cmsg_type
is either
.Dv IPV6_HOPOPTS
or
.Dv IPV6_DSTOPTS .
.Pp
The
.Fa tptrp
argument is a pointer to a pointer to an 8-bit byte and
.Fa *tptrp
is used by the function to remember its place in the ancillary data
object each time the function is called.
When the
.Fn inet6_option_next
function is called for the first time on a given ancillary data object,
.Fa *tptrp
must be set to
.Dv NULL .
.Pp
Each time the function returns success,
the
.Fa *tptrp
argument points to the 8-bit option type field for the next option to
be processed.
.\"
.Ss inet6_option_find
The
.Fn inet6_option_find
function allows an application to search for a particular option type
in an ancillary data object.
The
.Fa cmsg
argument is a pointer to
.Li cmsghdr
structure in which the
.Li cmsg_level
element equals
.Dv IPPROTO_IPV6
and the
.Li cmsg_type
element is either
.Dv IPV6_HOPOPTS
or
.Dv IPV6_DSTOPTS .
.Pp
The
.Fa tptrp
argument is handled exactly as in the
.Fn inet6_option_next
function described above.
.Pa
The
.fn inet6_option_find
function starts searching for an option of the specified type
beginning after the value of
.Fa *tptrp .
.\"
.Sh DIAGNOSTICS
The
.Fn inet6_option_init
and
.Fn inet6_option_append
functions return
.Li 0
on success or
.Li -1
on an error.
.Pp
The
.Fn inet6_option_alloc
function returns
.Dv NULL
on an error.
.Pp
When
.Fn inet6_option_next
or
.Fn inet6_option_find
detect an error they return
.Li -1
setting
.Fa *tptrp
to non
.Dv NULL
value.
.\"
.Sh EXAMPLES
RFC2292 gives comprehensive examples in chapter 6.
.\"
.Sh SEE ALSO
.Rs
.%A W. Stevens
.%A M. Thomas
.%T "Advanced Sockets API for IPv6"
.%N RFC2292
.%D February 1998
.Re
.Rs
.%A S. Deering
.%A R. Hinden
.%T "Internet Protocol, Version 6 (IPv6) Specification"
.%N RFC2460
.%D December 1998
.Re
.\"
.Sh HISTORY
The implementation first appeared in KAME advanced networking kit.
.\"
.Sh STANDARDS
The functions are documented in
.Dq Advanced Sockets API for IPv6
(RFC2292).
.\"