INET6_OPTION_SPACE(3)

NAME
inet6_option_space, inet6_option_init, inet6_option_append, inet6_option_alloc, inet6_option_next, inet6_option_find - IPv6 Hop-by-Hop and Destination Option Manipulation

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>
int
inet6_option_space(int nbytes);
int
inet6_option_init(void *bp, struct cmsghdr **cmsgp, int type);
int
inet6_option_append(struct cmsghdr *cmsg, const u_int8_t *typep, int multx, int plusy);
u_int8_t *
inet6_option_alloc(struct cmsghdr *cmsg, int datalen, int multx, int plusy);
int
inet6_option_next(const struct cmsghdr *cmsg, u_int8_t **tptrp);
int
inet6_option_find(const struct cmsghdr *cmsg, u_int8_t **tptrp, int type);

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 <netinet/in.h> header file.

  inet6_option_space
In order to determine the amount of space necessary to hold any option the 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 cmsghdr structure at the beginning, and any necessary padding at the end. The nbytes argument indicates the size of the structure defining the option, and must include any pad bytes at the beginning (the value y in the alignment term “xn + y”), the type byte, the length byte, and the option data.

Note: If multiple options are stored in a single ancillary data object, which is the recommended technique, the inet6_option_space() function overestimates the amount of space required by the size of N-1 cmsghdr structures, where N is the number of options to be stored in the object. 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.

  inet6_option_init
The inet6_option_init() function is called to initialize any ancillary data object that will contain a Hop-by-Hop or Destination option. It returns 0 on success and -1 when an error occurs.

The 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 inet6_option_append() and inet6_option_alloc() routines.

The cmsgp argument is a pointer to a pointer to a cmsghdr structure. The *cmsgp argument points to a cmsghdr structure which is constructed by this function and stored in the area of memory pointed to by bp.

The type is either IPV6_HOPOPTS or IPV6_DSTOPTS and is stored in the cmsg_type member of the cmsghdr structure mentioned above.

  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 inet6_option_init(). The inet6_option_append(function, returns) 0 if it succeeds or -1 when an error occurs.

The cmsg argument is a pointer to the cmsghdr structure that was initialized by a call to inet6_option_init().

The 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 typep field is immediately followed by the 8-bit option data length field, which is then followed by the option data.

The option types of 0 and 1 are reserved for the Pad1 and PadN options respectively. All other values from 2 through 255 are available for applications to use.

The option data length, since it is stored in 8 bites, must have a value between 0 and 255, inclusive.

The multx argument is the value x in the alignment term “xn + y” and indicates the byte alignment necessary for the data. Alignments may be specified as 1, 2, 4, or 8 bytes, which is no alignment, 16 bit, 32 bit and 64 bit alignments respectively.

The plusy argument is the value y in the alignment term “xn + y” and must have a value between 0 and 7, inclusive, indicating the amount of padding that is necessary for an option.

  inet6_option_alloc
The 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 inet6_option_init(). The 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 NULL on an error.

The difference between the inet6_option_alloc() and 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.

The cmsg argument is a pointer to a cmsghdr structure that was initialized by inet6_option_init().

The 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 inet6_option_append() function does not need a data length argument since the option data length must already be stored by the caller)

The multx and plusy arguments are identical to the arguments of the same name described in the inet6_option_init() function above.

  inet6_option_next
The 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 inet6_option_next() function is 0 and the *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 -1 and *tptrp is NULL and when an error occurs, the return value is -1 but the *tptrp argument is not 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.

When a valid option is returned the cmsg argument points to a cmsghdr where the cmsg_level equals IPPROTO_IPV6 and cmsg_type is either IPV6_HOPOPTS or IPV6_DSTOPTS.

The tptrp argument is a pointer to a pointer to an 8-bit byte and *tptrp is used by the function to remember its place in the ancillary data object each time the function is called. When the inet6_option_next() function is called for the first time on a given ancillary data object, *tptrp must be set to NULL.

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

  inet6_option_find
The inet6_option_find() function allows an application to search for a particular option type in an ancillary data object. The cmsg argument is a pointer to cmsghdr structure in which the cmsg_level element equals IPPROTO_IPV6 and the cmsg_type element is either IPV6_HOPOPTS or IPV6_DSTOPTS.

The tptrp argument is handled exactly as in the inet6_option_next() function described above. ~ The function starts searching for an option of the specified type beginning after the value of *tptrp.

DIAGNOSTICS
The inet6_option_init() and inet6_option_append() functions return 0 on success or -1 on an error.

The inet6_option_alloc() function returns NULL on an error.

When inet6_option_next() or inet6_option_find() detect an error they return -1 setting *tptrp to non NULL value.

EXAMPLES
RFC2292 gives comprehensive examples in chapter 6.

SEE ALSO
W. Stevens, M. Thomas, Advanced Sockets API for IPv6, RFC2292, February 1998. S. Deering, R. Hinden, Internet Protocol, Version 6 (IPv6) Specification, RFC2460, December 1998.

HISTORY
The implementation first appeared in KAME advanced networking kit.

STANDARDS
The functions are documented in “Advanced Sockets API for IPv6” (RFC2292).