rfc2133.txt

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   The sin6_family field identifies this as a sockaddr_in6 structure.
   This field overlays the sa_family field when the buffer is cast to a
   sockaddr data structure.  The value of this field must be AF_INET6.






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   The sin6_port field contains the 16-bit UDP or TCP port number.  This
   field is used in the same way as the sin_port field of the
   sockaddr_in structure.  The port number is stored in network byte
   order.

   The sin6_flowinfo field is a 32-bit field that contains two pieces of
   information: the 24-bit IPv6 flow label and the 4-bit priority field.
   The contents and interpretation of this member is unspecified at this
   time.

   The sin6_addr field is a single in6_addr structure (defined in the
   previous section).  This field holds one 128-bit IPv6 address.  The
   address is stored in network byte order.

   The ordering of elements in this structure is specifically designed
   so that the sin6_addr field will be aligned on a 64-bit boundary.
   This is done for optimum performance on 64-bit architectures.

   Notice that the sockaddr_in6 structure will normally be larger than
   the generic sockaddr structure.  On many existing implementations the
   sizeof(struct sockaddr_in) equals sizeof(struct sockaddr), with both
   being 16 bytes.  Any existing code that makes this assumption needs
   to be examined carefully when converting to IPv6.

3.4.  Socket Address Structure for 4.4BSD-Based Systems

   The 4.4BSD release includes a small, but incompatible change to the
   socket interface.  The "sa_family" field of the sockaddr data
   structure was changed from a 16-bit value to an 8-bit value, and the
   space saved used to hold a length field, named "sa_len".  The
   sockaddr_in6 data structure given in the previous section cannot be
   correctly cast into the newer sockaddr data structure.  For this
   reason, the following alternative IPv6 address data structure is
   provided to be used on systems based on 4.4BSD:

       #include <netinet/in.h>

       #define SIN6_LEN

       struct sockaddr_in6 {
           u_char          sin6_len;       /* length of this struct */
           u_char          sin6_family;    /* AF_INET6 */
           u_int16m_t      sin6_port;      /* transport layer port # */
           u_int32m_t      sin6_flowinfo;  /* IPv6 flow information */
           struct in6_addr sin6_addr;      /* IPv6 address */
       };





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   The only differences between this data structure and the 4.3BSD
   variant are the inclusion of the length field, and the change of the
   family field to a 8-bit data type.  The definitions of all the other
   fields are identical to the structure defined in the previous
   section.

   Systems that provide this version of the sockaddr_in6 data structure
   must also declare SIN6_LEN as a result of including the
   <netinet/in.h> header.  This macro allows applications to determine
   whether they are being built on a system that supports the 4.3BSD or
   4.4BSD variants of the data structure.

3.5.  The Socket Functions

   Applications call the socket() function to create a socket descriptor
   that represents a communication endpoint.  The arguments to the
   socket() function tell the system which protocol to use, and what
   format address structure will be used in subsequent functions.  For
   example, to create an IPv4/TCP socket, applications make the call:

       s = socket(PF_INET, SOCK_STREAM, 0);

   To create an IPv4/UDP socket, applications make the call:

       s = socket(PF_INET, SOCK_DGRAM, 0);

   Applications may create IPv6/TCP and IPv6/UDP sockets by simply using
   the constant PF_INET6 instead of PF_INET in the first argument.  For
   example, to create an IPv6/TCP socket, applications make the call:

       s = socket(PF_INET6, SOCK_STREAM, 0);

   To create an IPv6/UDP socket, applications make the call:

       s = socket(PF_INET6, SOCK_DGRAM, 0);

   Once the application has created a PF_INET6 socket, it must use the
   sockaddr_in6 address structure when passing addresses in to the
   system.  The functions that the application uses to pass addresses
   into the system are:

       bind()
       connect()
       sendmsg()
       sendto()






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   The system will use the sockaddr_in6 address structure to return
   addresses to applications that are using PF_INET6 sockets.  The
   functions that return an address from the system to an application
   are:

          accept()
          recvfrom()
          recvmsg()
          getpeername()
          getsockname()

   No changes to the syntax of the socket functions are needed to
   support IPv6, since all of the "address carrying" functions use an
   opaque address pointer, and carry an address length as a function
   argument.

3.6.  Compatibility with IPv4 Applications

   In order to support the large base of applications using the original
   API, system implementations must provide complete source and binary
   compatibility with the original API.  This means that systems must
   continue to support PF_INET sockets and the sockaddr_in address
   structure.  Applications must be able to create IPv4/TCP and IPv4/UDP
   sockets using the PF_INET constant in the socket() function, as
   described in the previous section.  Applications should be able to
   hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
   sockets simultaneously within the same process.

   Applications using the original API should continue to operate as
   they did on systems supporting only IPv4.  That is, they should
   continue to interoperate with IPv4 nodes.

3.7.  Compatibility with IPv4 Nodes

   The API also provides a different type of compatibility: the ability
   for IPv6 applications to interoperate with IPv4 applications.  This
   feature uses the IPv4-mapped IPv6 address format defined in the IPv6
   addressing architecture specification [2].  This address format
   allows the IPv4 address of an IPv4 node to be represented as an IPv6
   address.  The IPv4 address is encoded into the low-order 32 bits of
   the IPv6 address, and the high-order 96 bits hold the fixed prefix
   0:0:0:0:0:FFFF.  IPv4-mapped addresses are written as follows:

       ::FFFF:<IPv4-address>

   These addresses are often generated automatically by the
   gethostbyname() function when the specified host has only IPv4
   addresses (as described in Section 6.1).



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   Applications may use PF_INET6 sockets to open TCP connections to IPv4
   nodes, or send UDP packets to IPv4 nodes, by simply encoding the
   destination's IPv4 address as an IPv4-mapped IPv6 address, and
   passing that address, within a sockaddr_in6 structure, in the
   connect() or sendto() call.  When applications use PF_INET6 sockets
   to accept TCP connections from IPv4 nodes, or receive UDP packets
   from IPv4 nodes, the system returns the peer's address to the
   application in the accept(), recvfrom(), or getpeername() call using
   a sockaddr_in6 structure encoded this way.

   Few applications will likely need to know which type of node they are
   interoperating with.  However, for those applications that do need to
   know, the IN6_IS_ADDR_V4MAPPED() macro, defined in Section 6.6, is
   provided.

3.8.  IPv6 Wildcard Address

   While the bind() function allows applications to select the source IP
   address of UDP packets and TCP connections, applications often want
   the system to select the source address for them.  With IPv4, one
   specifies the address as the symbolic constant INADDR_ANY (called the
   "wildcard" address) in the bind() call, or simply omits the bind()
   entirely.

   Since the IPv6 address type is a structure (struct in6_addr), a
   symbolic constant can be used to initialize an IPv6 address variable,
   but cannot be used in an assignment.  Therefore systems provide the
   IPv6 wildcard address in two forms.

   The first version is a global variable named "in6addr_any" that is an
   in6_addr structure.  The extern declaration for this variable is
   defined in <netinet/in.h>:

       extern const struct in6_addr in6addr_any;

















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   Applications use in6addr_any similarly to the way they use INADDR_ANY
   in IPv4.  For example, to bind a socket to port number 23, but let
   the system select the source address, an application could use the
   following code:

       struct sockaddr_in6 sin6;
        . . .
       sin6.sin6_family = AF_INET6;
       sin6.sin6_flowinfo = 0;
       sin6.sin6_port = htons(23);
       sin6.sin6_addr = in6addr_any;  /* structure assignment */
        . . .
       if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
               . . .

   The other version is a symbolic constant named IN6ADDR_ANY_INIT and
   is defined in <netinet/in.h>.  This constant can be used to
   initialize an in6_addr structure:

       struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

   Note that this constant can be used ONLY at declaration time.  It can
   not be used to assign a previously declared in6_addr structure.  For
   example, the following code will not work:

       /* This is the WRONG way to assign an unspecified address */
       struct sockaddr_in6 sin6;
        . . .
       sin6.sin6_addr = IN6ADDR_ANY_INIT; /* will NOT compile */

   Be aware that the IPv4 INADDR_xxx constants are all defined in host
   byte order but the IPv6 IN6ADDR_xxx constants and the IPv6
   in6addr_xxx externals are defined in network byte order.

3.9.  IPv6 Loopback Address

   Applications may need to send UDP packets to, or originate TCP
   connections to, services residing on the local node.  In IPv4, they
   can do this by using the constant IPv4 address INADDR_LOOPBACK in
   their connect(), sendto(), or sendmsg() call.

   IPv6 also provides a loopback address to contact local TCP and UDP
   services.  Like the unspecified address, the IPv6 loopback address is
   provided in two forms -- a global variable and a symbolic constant.







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   The global variable is an in6_addr structure named
   "in6addr_loopback."  The extern declaration for this variable is
   defined in <netinet/in.h>:

       extern const struct in6_addr in6addr_loopback;

   Applications use in6addr_loopback as they would use INADDR_LOOPBACK
   in IPv4 applications (but beware of the byte ordering difference
   mentioned at the end of the previous section).  For example, to open
   a TCP connection to the local telnet server, an application could use
   the following code:

       struct sockaddr_in6 sin6;
        . . .
       sin6.sin6_family = AF_INET6;
       sin6.sin6_flowinfo = 0;
       sin6.sin6_port = htons(23);
       sin6.sin6_addr = in6addr_loopback;  /* structure assignment */
        . . .
       if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
               . . .

   The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is defined
   in <netinet/in.h>.  It can be used at declaration time ONLY; for
   example:

       struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;

   Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
   to a previously declared IPv6 address variable.

4.  Interface Identification