Internet DRAFT - draft-gilligan-ipv6-bsd-api

draft-gilligan-ipv6-bsd-api




Internet Engineering Task Force                   R. E. Gilligan (Sun)
INTERNET-DRAFT                                   S. Thomson (Bellcore)
                                                    J. Bound (Digital)


                                                    October 5, 1994

                IPv6 Program Interfaces for BSD Systems
                  <draft-gilligan-ipv6-bsd-api-00.txt>

Abstract

In order to implement the version 6 Internet Protocol (IPv6) [1] in an
operating system based on Berkely Unix (4.x BSD), changes must be made
to the application program interface (API).  TCP/IP applications written
for BSD-based operating systems have in the past enjoyed a high degree
of portability because most of the systems derived from BSD provide the
same API, known informally as "the socket interface".  We would like to
have the same portability to be possible with IPv6 applications.  This
memo presents a set of extensions to the BSD socket API to support IPv6.
The changes include a new data structure to carry IPv6 addresses, new
name to address translation library functions, new address conversion
functions, and some new setsockopt() options.  The changes are designed
to be minimal and to provide source and binary compatibility for
existing applications.



Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

Internet Drafts are draft documents valid for a maximum of six months.
This Internet Draft expires on April 5, 1995.  Internet Drafts may be
updated, replaced, or obsoleted by other documents at any time. It is
not appropriate to use Internet Drafts as reference material or to cite
them other than as a "working draft" or "work in progress."

To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
munnari.oz.au.

Distribution of this memo is unlimited.




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1.  Introduction.

While IPv4 addresses are 32 bits long, IPv6 nodes are identified by
128-bit addresses [2].  The socket interface API make the size of an IP
address quite visible to an application; virtually all TCP/IP
applications for BSD-based systems have knowledge of the size of an IP
address.  Those parts of the API that expose the addresses need to be
extended to accommodate the larger IPv6 address size.  This paper
presents an attempt to define the API extensions needed to support IPv6
in BSD systems.  This specification is preliminary.  The API extensions
are expected to evolve as we gain more implementation experience.


2.  Design considerations

There are a number of important considerations in designing changes to
this well-worn API:

   -    The extended API should provide both source and binary
        compatibility for programs written to the original API.  That
        is, existing program binaries should continue to operate when
        run on a system supporting the new API.  In addition, existing
        applications that are re-compiled and run on a system supporting
        the new API should continue to operate.  Simply put, the API
        changes for IPv6 should not break existing programs.

   -    The changes to the API should be as small as possible in order
        to simplify the task of converting existing IPv4 applications to
        IPv6.

   -    Where possible, applications should be able to use the extended
        API to interoperate with both IPv6 and IPv4 hosts.  Applications
        should not need know which form of host they are communicating
        with.  The IPv6 transition mechanisms [3] provide a way to
        represent addresses of IPv4 nodes as IPv6 addresses.  If
        possible, this encoding should be used.

   -    IPv6 addresses carried in data structures should be 64-bit
        aligned.  This is necessary in order to obtain optimum
        performance on 64-bit machine architectures.


2.1.  What needs to be changed

The socket interface API consists of a few distinct components:

   -    Core socket functions
   -    Address data structures



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   -    Name-to-address translation functions
   -    Address conversion functions

The core socket functions -- those functions that deal with such things
as setting up and tearing down TCP connections, and sending and
receiving UDP packets -- were designed to be transport independent.
Where protocol addresses are passed as function arguments, they are
carried as opaque pointers.  A protocol specific address data structure
is defined for each protocol that the socket functions support.
Applications must cast these protocol specific address structures into
the generic "sockaddr" data type when using the socket functions.  These
functions need not change for IPv6, but a new IPv6 specific address data
structure is needed.

The "sockaddr_in" structure is the protocol specific data structure for
IPv4.  This data structure actually includes 8-bytes of unused space,
and it is tempting to try to use this space to adapt the sockaddr_in
structure to IPv6.  Unfortunately, the sockaddr_in structure is not
large enough to hold the 16-byte IPv6 address as well as the other
information (2-byte address family and 2-byte port number) that is
needed.  So a new address data structure must be defined for IPv6.

The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr().  Gethostbyname() does not provide
enough flexibility to accommodate more than one protocol family.  To
solve this problem, we introduced a new name-to-address translation
function which is analogous to gethostbyname(), but supports addresses
in both the IPv4 and IPv6 address families.  Gethostbyaddr() does not,
strictly speaking, need to be replaced since it carries an address
family argument and can be extended to support both address families
without introducing compatibility problems.  However, we have chosen to
introduce a new function to maintain symmetry with the replacement to
gethostbyname().  The new functions both carry an address family
parameter, so they can be extended to operate with other protocol
families in addition to IPv4 and IPv6.

The address conversion functions -- inet_ntoa() and inet_addr() --
convert IPv4 addresses between binary and printable form.  These
functions are quite specific to 32-bit IPv4 addresses.  We have designed
two analogous functions which convert both IPv4 and IPv6 addresses, and
carry an address type parameter so that they can be extended to other
protocol families as well.

Finally, a few miscellaneous features are needed to support IPv6.  A new
interface is needed in order to support the IPv6 flow label.  New
interfaces are needed in order to receive IPv6 multicast packets and
control the sending of multicast packets.  And an interface is necessary
in order to pass IPv6 source route information between the application



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and the system.


3.  Implementation experience

Prototype implementations of IPv6 exposed a few issues in designing a
compatible application interface.

First, since the IPv6 transition mechanisms provide a way for IPv4
addresses to be represented as IPv6 addresses, applications using the
extended API can interoperate with IPv4 nodes.  For example, a client
application can open a TCP connection to an IPv4 server by giving the
IPv6 address of the IPv4 server in the connect() call.  Most
applications do not even need to know whether the peer is an IPv4 or
IPv6 node.  Such applications can simply treat IPv6 addresses as opaque
values; They need not understand the "structure" by which IPv4 addresses
are encoded within IPv6 addresses.  Yet the structure can be decoded by
those applications that do need to know whether the peer is IPv6 or
IPv4.  This should prove to be a significant simplification since most
applications will need to interoperate with both IPv4 and IPv6 nodes for
some time to come.

Second, to a limited degree, existing applications written to the IPv4
API can interoperate with IPv6 nodes.  This is generally only possible
if the application does not "look at" the peer address that is provided
by the API.  (e.g. the source address provided by the recvfrom()
function when a UDP packet is received, or the client address returned
by the accept() function.)

Third, the common application practice of passing open socket
descriptors between processes across an exec() call can cause problems.
It is possible, for example, for an application using the extended API
to pass an open socket to an older application using the original API.
The old application could be confused if the socket functions return
IPv6 address structures to it.  The solution designed was to provide a
mechanism by which applications could have explicit control over what
form of addresses are returned.


4.  Interface Specification


4.1.  New Address Family

We have defined a address family macro in <sys/socket.h>:

        #define AF_INET6        24      /* Internet Protocol version 6 */




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The AF_INET6 definition is used to distinguish between the original
sockaddr_in address data structure, and the new sockaddr_in6 data
structure.

We have also defined a new protocol family macro in <sys/socket.h>.
Like most of the other protocol family macros, it is identical with the
corresponding address family macro:

        #define PF_INET6        AF_INET6

The PF_INET6 is used in the first argument to the socket() function to
indicate that an IPv6 socket is being created.

4.2.  Socket Address Structure

The sockaddr_in structure is used to pass IPv4 addresses between
applications and the system in the socket functions.  We have defined a
new structure to carry IPv6 addresses:

        struct sockaddr_in6 {
                u_short         sin6_family;    /* AF_INET6 */
                u_short         sin6_port;      /* Transport layer port # */
                u_long          sin6_flowlabel; /* IPv6 flow label */
                u_long          sin6_addr[4];   /* IPv6 address */
        };

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

The sin6_port field is used to store the UDP or TCP port number.  This
field is used in the same way as the sin_port field of the sockaddr_in
structure.

The sin6_flowlabel is used to store the IPv6 flow label.  The use of
this field is explained in sec 4.8.

The sin6_addr field is used to store the 128-bit IPv6 address.  It is
used in the same manner as the sin_addr field of the sockaddr_in
structure.

All fields of the sockaddr_in6 structure are 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.



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4.3.  The Socket Functions

Applications use 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 which the application uses to pass addresses
into the system are:

           bind()
           connect()
           sendto()

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()
           getpeername()
           getsockname()

None of the core socket functions themselves need to be changed, since
the all of the "address carrying" functions use an opaque address
pointer, and carry an address length as a function argument.





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4.4.  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 addresses
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.  It is not clear, though, how, or even if,
those IPv4 applications should interoperate with IPv6 nodes.  The
problem primarily has to do with the how these applications represent
addresses of IPv6 nodes.  What address should be returned to the
application when an IPv6/UDP packet is received, or an IPv6/TCP
connection is accepted?  The peer's address could be any arbitrary
128-bit IPv6 address.  But the application is only equipped to deal with
32-bit IPv4 addresses encoded in sockaddr_in data structures.

We have not discovered any solution that provides complete transparent
interoperability with IPv6 nodes for applications using the original
IPv4 API.  However, we did develop two techniques that partially solve
the problem:

   1)   Prohibit communication between IPv4 applications and IPv6 nodes.
        Only UDP packets received from IPv4 nodes would be passed up to
        the application, and only TCP connections received from IPv4
        nodes would be accepted.  UDP packets from IPv6 nodes would be
        dropped, and TCP connections from IPv6 nodes would be refused.

   2)   The implementation could generate a local 32-bit cookie to
        represent the full 128-bit IPv6 address, and pass this value to
        the application.  The implementation would maintain a mapping
        from cookie value into the 128-bit IPv6 address that it
        represents.  When the application passed a cookie back into the
        system (for example, in a sendto() or connect() call) the
        implementation would use the 128-bit IPv6 address that the
        cookie represents.

        The cookie would have to be chosen so as to be an invalid IPv4
        address (e.g. an address on net 127.0.0.0), and the
        implementation would have to make sure that these cookie values
        did not escape into the Internet as the source or destination
        addresses of IPv4 packets.



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Both of these techniques have drawbacks.  System implementors may use
one of these techniques or implement another solution.  This is an area
for further study.


4.5.  Compatibility with IPv4 Nodes

The API also provides a different type of compatibility: the ability for
applications using the extended API to interoperate with IPv4 nodes.
This feature is possible because it is possible to to represent IPv4
addresses as IPv6 addresses.  The encoding of IPv4 addresses as IPv6
addresses is defined in the Simple IPv6 Transition (SIT) specifications
[3].  This encoding places the 32-bit IPv4 address in the low-order
32-bits of a 128-bit IPv6 address, and sets the high-order 96-bits to
zero.  An IPv4 address encoded as an IPv6 address is written like this:

        0:0:0:0:0:0:<IPv4-address>

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 IPv6 address, and passing the resulting
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.


4.6.  Sockets Passed Across exec()

Unix allows open sockets to be passed across an exec() call.  It is a
relatively common application practice to pass open sockets across
exec() calls.  Because of this, it is possible for an application using
the original API to pass an open PF_INET socket to an application using
the extended API that is expecting to receive a PF_INET6 socket.
Similarly, it is possible for an application using the extended API to
pass an open PF_INET6 socket to an application using the original API
that is only equipped to deal with PF_INET sockets.  Either of these
cases could cause problems, because the application which is passed the
open socket might not know how to decode the address structures returned
in subsequent socket functions.

To remedy this problems, we have defined a new setsockopt() option that
allows an application to "transform" a PF_INET6 socket into a PF_INET
socket and vice-versa.

An IPv6 application that is passed an open socket from an unknown



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process may use the IP_ADDRFORM setsockopt() option to "convert" the
socket to PF_INET6.  Once that has been done, the system will return
sockaddr_in6 address structures in subsequent socket functions.
Similarly, an IPv6 application that is about to pass an open PF_INET6
socket to a program that may not be IPv6 capable may "downgrade" the
socket to PF_INET before calling exec().  After that, the system will
return sockaddr_in address structures to the application that was
exec()'ed.

The macro definition for this new option in <netinet/in.h> is:

        #define IP_ADDRFORM     0x16    /* get/set form of returned addrs */

The IP_ADDRFORM option is at the IPPROTO_IP level.  The only valid
option values are PF_INET6 and PF_INET.  For example, to convert a
PF_INET6 socket to PF_INET, a program would call:

        int addrform = PF_INET;

        if (setsockopt(s, IPPROTO_IP, IP_ADDRFORM, (char *) &addrform,
                sizeof(addrform)) == -1)
                perror("setsockopt IP_ADDRFORM");

An application may use IP_ADDRFORM in the getsckopt() function to learn
whether an open socket is a PF_INET of PF_INET6 socket.


4.7.  Flow Label

The IPv6 header has a 28-bit field to hold a "flow label".  Applications
have control over what flow label value is used in packets that they
originate, and have access to the flow label value of packets that they
send.

The sin6_flowlabel field of the sockaddr_in6 structure is used to carry
the flow label between the application and the system.  An application
may specify a flow label to use in the transmitted packets of an
actively opened TCP connection by setting the sin6_flowlabel field of
the sockaddr_in6 structure passed in the connect() function.  An
application may specify the flow label to use in transmitted UDP packets
by setting the sin6_flowlabel field of the sockaddr_in6 structure passed
in the sendto() function.  If an application does not care what flow
label is used, it should set the flowlabel value to zero.

An application may specify the flow label to use in transmitted packets
of a passively accepted TCP connection, by setting the sin6_flowlabel
field of the address passed in the bind() function.




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The flow label that appeared in received UDP packets is passed up to the
application in the sin6_flowlabel field of the sockaddr_in6 structure
that is returned in the recvfrom() call.  The flow label that appeared
in the received SYN segment of a passively accepted TCP connection is
returned to the application in the sin6_flowlabel field of the
sockaddr_in6 structure that is passed in the accept() call.


4.8.  Handling IPv6 Source Routes

IPv6 makes more use of the source routing mechanism than IPv4.  In order
for source routing to operate properly, the node receiving a request
packet that bears a source route must reverse that source route when
sending the reply.  In the case of TCP, the reversal can be done in the
transport protocol implementation transparently to the application.  But
in the case of UDP, the application must perform the reversal.  The
transport protocol code can not perform the reversal for UDP packets
because a UDP application may receive a number of requests and generate
replies asynchronously.  A "reply" sent by an application may not match
the "request" most recently passed up to the application.

The API for source routing has two components: providing a source route
to be used with actively opened TCP connections being originated and UDP
packets being sent, and receiving the source route that arrived with
passively accepted TCP connections and received UDP packets.  An
application may always provide a source route with TCP connections being
originated and UDP packets being sent.  But to receive source routes,
the application must enable an option.

To provide a source route, an application simply provides an array of
sockaddr_in6 data structures in the address argument of the sendto()
function (when sending a UDP packet), or the connect() function (when
actively opening a TCP connection).  The length argument of the function
is the total length, in bytes, of the array.  The elements of the array
represent the full source route, including both source and destination
identifying address.  The elements of the array are ordered from
destination to source.  That is, the first element of the array
represents the destination identifying address, and the last element of
the array represents the source identifying address.  If the application
provides a source route, the source identifying address can not be
omitted.  The sin6_addr field of the source identifying address may be
set to zero, however, in which case the system will select an
appropriate source address.  The sin6_port field of the destination
identifying address must be assigned.  The sin_port field of the source
identifying address may be set to zero, in which case the system will
select an appropriate source port number.  The sin6_port and sin6_flowid
fields of the intermediate addresses must be set to zero.




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The arrangement of the address structures in the address buffer passed
to connect() or sendto() is shown in the figure below:

        +--------------------+
        |                    |
        |  sockaddr_in6[0]   |  Destination Identifying Address
        |                    |
        +--------------------+
        |                    |
        |  sockaddr_in6[1]   |  Last Source-Route Hop Address
        |                    |
        +--------------------+
        .                    .
        .                    .
        .                    .
        +--------------------+
        |                    |
        | sockaddr_in6[N-1]  |  First Source-Route Hop Address
        |                    |
        +--------------------+
        |                    |
        |  sockaddr_in6[N]   |  Source Identifying Address
        |                    |
        +--------------------+

               Address buffer when sending a source route

The IP_RCVSRCRT setsockopt() option controls the reception of source
routes.  The option is disabled by default.  Applications must
explicitly enable the option using the setsockopt() function in order to
receive source routes.

The macro definition for this new option in <netinet/in.h> is:

        #define IP_RCVSRCRT     0x17    /* Enable/Disable reception of
                                           IPv6 source routes */

The IP_RCVSRCRT option is at the IPPROTO_IP level.  An example of how an
application might use this option is:

        int on = 1;             /* value == 1 means enable the option */

        if (setsockopt(s, IPPROTO_IP, IP_RCVSRCRT, (char *) &on,
                sizeof(on)) == -1)
                perror("setsockopt IP_RCVSRCRT");

When the IP_RCVSRCRT option is disabled, only a single sockaddr_in6
address structure is returned to applications in the address argument



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of the recvfrom() and accept() functions.  This address represents the
source identifying address of the UDP packet received or the TCP
connection accepted.

When the IP_RCVSRCRT option is enabled, the address argument of the
recvfrom() function (when receiving UDP packets) and the accept()
functions (when passively accepting TCP connections) carry an array of
sockaddr_in6 structures.  The array will hold two elements -- source and
destination address -- when the received UDP packet or TCP SYN packet
does not carry a source route.  The array will hold more than two
elements when the received packet carries a source route.

The addresses in the array are ordered from source to destination.  That
is, the first element of the array holds source identifying address of
the received packet.  Following this in the array are the intermediary
hops.  And the last element of the array holds the destination
identifying address.  Note that this is the opposite of the order
specified for sending.  This ordering was chosen so that the address
array received in a recvfrom() call can be used in a subsequent sendto()
call without requiring the application to re-order the addresses in the
array.  Similarly, the address array received in an accept() call can be
used unchanged in a subsequent connect() call.

The address length argument of the recvfrom() and accept() functions
indicate the length, in bytes, of the full address array.  This argument
is a value-result parameter.  The application sets the maximum size of
the address buffer when it makes the call, and the system modifies the
value to return the actual size of the buffer to the application.

The sin6_port field of the first and last array elements (source and
destination identifying address) will hold the source and destination
UDP or TCP port number of the received packet.  The sin6_port field of
the intermediate elements of the array will be zero.

The address buffer returned to the application in the recvfrom() or
accept() functions when the IP_RCVSRCRT option is enabled is shown
below:














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        +--------------------+
        |                    |
        |  sockaddr_in6[0]   |  Source Identifying Address
        |                    |
        +--------------------+
        |                    |
        |  sockaddr_in6[1]   |  First Source-Route Hop Address
        |                    |
        +--------------------+
        .                    .
        .                    .
        .                    .
        +--------------------+
        |                    |
        | sockaddr_in6[N-1]  |  Last Source-Route Hop Address
        |                    |
        +--------------------+
        |                    |
        |  sockaddr_in6[N]   |  Destination Identifying Address
        |                    |
        +--------------------+

              Address buffer when receiving a source route

4.9.  Sending and Receiving Multicast Packets

IPv6 applications may send UDP multicast packets by simply specifying an
IPv6 multicast address in the address argument of the sendto() function.

A few setsockopt options at the IPPROTO_IP layer are used to control
some of the parameters of sending multicast packets.  These options are
optional: applications may send multicast packets without using these
options.  The setsockopt() options for controlling the sending of
multicast packets are summarized below:

        IP_MULTICAST_IF         Set the interface to use for outgoing
                                multicast packets.

        IP_MULTICAST_TTL        Set the hop count to use for outgoing
                                multicast packets.  (Note a separate
                                option - IP_TTL - is provided to set the
                                hop count to use for outgoing unicast
                                packets.)

        IP_MULTICAST_LOOP       Controls whether outgoing multicast
                                packets sent should be delivered back to
                                the local application.  A toggle.




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The reception of multicast packets is controlled by the two setsockopt()
options summarized below:

        IP_ADD_MEMBERSHIP       Join a multicast group.  Requests
                                that multicast packets sent to a
                                particular multicast address
                                be delivered to this socket.

        IP_DROP_MEMBERSHIP      Leave a multicast group.  Requests that
                                multicast packets sent to a particular
                                multicast address no longer be delivered
                                to this socket.

Additional details of multicast part of the API will be added later.

4.10.  Name-to-Address Translation Functions

We have defined two new functions analogous to gethostbyname() and
gethostbyaddr() which support addresses in both the IPv4 and IPv6
address families.  The names of the new functions are hostname2addr()
and addr2hostname().  These functions were designed to have semantics
similar to gethostbyname() and gethostbyaddr(), so that existing IPv4
applications can be easily ported to IPv6.

Hostname2addr() is defined similarly to gethostbyname(), but enables
applications to specify the type of address to be looked up:

          struct hostent *hostname2addr(const char *name, int af);

This new function looks up the given name in the name service and
returns the completed hostent structure if the lookup succeeds, and NULL
otherwise.  The name argument is the domain name of the host to look up.
The af argument specifies the type of the address -- IPv4 (AF_INET) or
IPv6 (AF_INET6) -- to return to the caller in the h_addr_list field of
the hostent structure.

If the af argument is AF_INET, hostname2addr() queries the name service
for IPv4 addresses and, if any are found, returns a hostent structure
that includes an array of IPv4 addresses.  Each IPv4 address is encoded
in network byte order.

If the af argument is AF_INET6, the processing is as follows: the
hostname2addr() function first queries the name service for IPv6
addresses. If IPv6 addresses are found, they are returned in an array in
the hostent structure.  If no IPv6 addresses are found, the function
queries the name service for IPv4 addresses. If IPv4 addresses are
found, they are returned as 128-bit IPv6 addresses by prepending the
96-bit all-zeros prefix that indicates the address is for an IP-only



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host[2].  As in IPv4, each IPv6 address returned in struct hostent is
encoded in network byte order.

The second new function, called addr2hostname(), is defined in exactly
the same way as the gethostbyaddr() function, except that it now
supports both the IPv4 and IPv6 address families:

        struct hostent *addr2hostname(const void *addr, int len, int af);

addr2hostname() performs an inverse lookup in the name service on the
address specified, returning a completed hostent structure if the lookup
succeeds, or NULL, if the lookup fails. This function supports both the
AF_INET and AF_INET6 address families. If the af argument is AF_INET,
then len must be specified to be 4 octets and addr must refer to an IPv4
address.  If af is AF_INET6, then len must be specified as 16 octets and
addr must refer to an IPv6 address.  If the addr argument is an IPv4
address in IPv6 form (the high-order 96-bit prefix indicates this[2]),
addr2hostname() performs an inverse lookup on the low-order 32 bits of
the address only.

A new name-to-address translation library function is now under
development at Berkeley [4].  This new function, named getconninfo(),
will subsume the functionality of gethostbyname(), hostname2addr(), as
well as the getservbyname() and getservbyport() functions.  The new
function is specifically designed to be "transport independent", so it
can be used by IPv6 applications.

System implementations should provide the addr2hostname() and
hostname2addr() functions in order to simplify the porting of existing
IPv4 applications to IPv6.  System implementations may also provide the
getconninfo() function, once it is defined, so that newly written
applications can be transport independent.

The getconninfo() function is expected to be published as a separate
specification document, not included in this spec.

Implementations must retain the BSD gethostbyname() and gethostbyaddr()
functions in order to provide source and binary compatibility for
existing applications.


4.11.  Address Conversion Functions

BSD Unix provides two functions, inet_addr() and inet_ntoa(), to convert
an IPv4 address between binary and printable form.  IPv6 applications
need similar functions.  We have defined the following two functions to
convert both IPv6 and IPv4 addresses:




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        int ascii2addr(int af, const char *cp, void *ap);

and

        char *addr2ascii(int af, const void *ap, char *cp);

The first function converts an ascii string to an address in the address
family specified by the af argument.  Currently AF_INET and AF_INET6
address families are supported.  The cp argument points to the ascii
string being passed in.  The ap argument points to a buffer that is to
be used by the function to return the address.  Ascii2addr() returns the
length of the address in octets if the conversion succeeds, and -1
otherwise. The function does not modify the storage pointed to by ap if
the conversion fails. The application must ensure that the buffer
referred to by ap is large enough to hold the converted address.

If the af argument is AF_INET, the function accepts a string in the
standard IPv4 dotted decimal form:

        ddd.ddd.ddd.ddd

where ddd is a one to three digit decimal number between 0 and 255.

If the af argument is AF_INET6, then the function accepts a string in
one of the two standard IPv6 printing forms[2]:

        xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:ddd.ddd.ddd.ddd

or

        xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx

where xxxx is a 1 to 4 digit hex value and ddd is a 1 to 3 digit decimal
number between 0 and 255.

The second function converts an address into a printable string.  The af
argument specifies the form of the address.  This can be AF_INET or
AF_INET6.  The ap argument points to a buffer holding an IPv4 address if
the af argument is AF_INET, and an IPv6 address if the af argument is
AF_INET6.  The cp argument points to a buffer that the function can use
to store the ascii string.  If the cp argument is NULL, the function
uses its own private static buffer.  If the application specifies a cp
argument, it must be large enough to hold the ascii conversion of the
address specified as an argument, including the terminating null byte.
For IPv6 addresses, the buffer must be at least 40 bytes.  For IPv4
addresses, the buffer must be at least 16 bytes.

The addr2ascii() function returns a pointer to the buffer containing the



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ascii string if the conversion succeeds, and NULL otherwise.  The
function does not modify the storage pointed to by buf if the conversion
fails.

5.  Security Considerations

Security issues are not discussed in this document.


6. Open Issues

A few open issues for IPv6 socket interface API spec remain, including:

   -    The multicast API needs to be flushed out some more.

   -    Security.  Should we do anything to support the security
        options?  If so, what?

   -    What should the system do if the application has enabled the
        IP_RCVSRCRT option, but the size of the buffer passed in to
        recvfrom() or accept() is not large enough to hold the full
        source route?  The options include: fail the call; omit the
        "intermediate hops" but include both source and destination
        addresses; omit all addresses except the source.

   -    4.4 BSD sa_len compatibility.  We need to write a section about
        how the sockaddr_in6 can be used on systems based on 4.4 BSD
        that include the sa_len field in the sockaddr struct.

Acknowledgments

Thanks to the many people who made suggestions and provided feedback to
earlier revisions of this document.  Comments were provided by: Richard
Stevens, Dan McDonnald, Christian Huitema, Steve Deering, Andrew
Cherenson, Charles Lynn, Ran Atkinson, Erik Nordmark, Glenn Trewitt,
Fred Baker, Robert Elz, and Dean D. Throop.  Craig Partridge suggested
the addr2ascii() and ascii2addr() functions.

Ramesh Govindan made a number of contributions and co-authored an
earlier version of this paper.



References

  [1]   S. Deering, "IPv6 Protocol Specification", Internet Draft in
        progress.




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  [2]   S. Deering, P. Francis and R. Govindan, "IPv6 Routing and
        Addressing Overview", Internet Draft in progress.

  [3]   R. Gilligan, "Simple IPv6 Transition (SIT) Overview", Internet
        Draft in progress.

  [4]   K. Sklower, Private communication.


Authors' Address

        Jim Bound
        Digital Equipment Corporation
        110 Spitbrook Road ZK3-3/U14
        Nashua, NH 03062-2698
        Phone: +1 603 881 0400
        Email: bound@zk3.dec.com

        Susan Thomson
        Bell Communications Research
        MRE 2P-343, 445 South Street
        Morristown, NJ 07960
        Telephone: +1 201 829 4514
        Email: set@thumper.bellcore.com

        Robert E. Gilligan
        Sun Microsystems, Inc.
        2550 Garcia Avenue
        Mailstop UMTV05-44
        Mountain View, CA 94043-1100
        Phone: +1 415 336 1012
        Email: bob.gilligan@eng.sun.com



















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