Internet Engineering Task Force R. E. Gilligan (Sun)
INTERNET-DRAFT S. Thomson (Bellcore)
J. Bound (Digital)
June 21, 1995
IPv6 Program Interfaces for BSD Systems
<draft-ietf-ipngwg-bsd-api-01.txt>
Abstract
In order to implement the version 6 Internet Protocol (IPv6) [1] in an
operating system based on Berkeley 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 the
same portability with IPv6. 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 extensions are designed to provide access to
IPv6 features, while introducing a minimum of change into the system and
providing complete compatibility for existing IPv4 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 December 20, 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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INTERNET-DRAFT IPv6 BSD API Spec June 1995
1. Introduction.
While IPv4 addresses are 32-bits long, IPv6 nodes are identified by
128-bit addresses. 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
defines a set of extensions to the socket interface API to support IPv6.
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 to know which type of host they are
communicating with.
- 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.
Because of the importance of providing IPv4 compatibility in the API,
our extensions are explicitly designed to operate on machines that
provide complete support for both IPv4 and IPv6. A subset of this API
could probably be designed for operation on systems that support only
IPv6. However, this is not addressed in this document.
2.1. Overview of Changes
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The socket interface API consists of a few distinct components:
- Core socket functions.
- Address data structures.
- 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 via 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-octets 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-octet IPv6 address as well as the other
information (2-octet address family and 2-octet 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
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INTERNET-DRAFT IPv6 BSD API Spec June 1995
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 and priority
header fields. 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 and the system.
3. Implementation Experience
A few issues exposed in experimenting with prototype implementations of
IPv6 helped to guide the design of this API:
First, we discovered that, by providing a way to represent the addresses
of IPv4 nodes as IPv6 addresses, we could greatly simplify the
applications' task of providing IPv4 compatibility. New applications
could interoperate with IPv4 nodes by using the new API and expressing
the addresses of IPv4 nodes they interoperate with as IPv6 addresses.
For example, a client application could open a TCP connection to an IPv4
server by giving the IPv6 representation of the server's IPv4 address 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, we learned that existing applications written to the IPv4 API
could be made to interoperate with IPv6 nodes to a limited degree. This
technique does not work for all applications, but does for certain
applications, such as those that do 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, we learned that 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.
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4. Interface Specification
This section specifies the interface changes for IPv6.
The data types of the structure elements given in the following section
are intended to be examples, not absolute requirements. System
implementations may use other types if they are appropriate. In some
cases, such as when a field of a data structure holds a protocol value,
the structure field must be of some minimum size. These size
requirements are noted in the text. For example, since the UDP and TCP
port values are 16-bit quantities, the sin6_port field must be at least
a 16-bit data types. We specify the sin6_port field as a u_short type,
but an implementation may use any data type that is at least 16-bits
long.
4.1. New Address Family
A new address family macro, named AF_INET6, is defined in
<sys/socket.h>. The AF_INET6 definition is used to distinguish between
the original sockaddr_in address data structure, and the new
sockaddr_in6 data structure.
A new protocol family macro, named PF_INET6, is defined in
<sys/socket.h>. Like most of the other protocol family macros, this
will usually be defined to have the same value as 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. IPv6 Address Data Structure
A new data structure to hold a single IPv6 address is defined in
<netinet/in.h>:
struct in_addr6 {
u_char s6_addr[16]; /* IPv6 address */
}
This data structure contains an array of sixteen 8-bit elements, which
make up one 128-bit IPv6 address.
The IPv6 address is stored in network byte order.
4.3. Socket Address Structure for 4.3 BSD-Based Systems
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In the socket interface, a different protocol-specific data structure
is defined to carry the addresses for each of the protocol suite.
Each protocol-specific data structure is designed so it can be cast
into a protocol-independent data structure -- the "sockaddr"
structure. Each has a "family" field which overlays the "sa_family"
of the sockaddr data structure. This field can be used to identify
the type of the data structure.
The sockaddr_in structure is the protocol-specific address data
structure for IPv4. It is used to pass addresses between applications
and the system in the socket functions. We have defined the following
structure in <netinet/in.h> to carry IPv6 addresses:
struct sockaddr_in6 {
u_short sin6_family; /* AF_INET6 */
u_short sin6_port; /* Transport layer port # */
u_long sin6_flowinfo; /* IPv6 flow information */
struct in_addr6 sin6_addr; /* IPv6 address */
};
This structure is designed to be compatible with the sockaddr data
structure used in the 4.3 BSD release.
The sin6_family field is used to identify this 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 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 is used to store the
24-bit IPv6 flow label, and the 4-bit priority field. The IPv6 flow
label is represented as the low-order 24-bits of the 32-bit field, and
the priority is represented in the next 4-bits above this. The
high-order 4 bits of this field are reserved. The sin6_flowinfo field
is stored in network byte order. The use of this field is explained in
sec 4.8.
The sin6_addr field is a single in_addr6 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.
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4.4. Socket Address Structure for 4.4 BSD-Based Systems
The 4.4 BSD 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 can not be
correctly cast into the newer sockaddr data structure. For this
reason, we have defined the following alternative IPv6 address data
structure to be used on systems based on 4.4 BSD:
#define SIN6_LEN
struct sockaddr_in6 {
u_char sin6_len; /* length of this struct */
u_char sin6_family; /* AF_INET6 */
u_short sin6_port; /* Transport layer port # */
u_long sin6_flowinfo; /* IPv6 flow information */
struct in_addr6 sin6_addr; /* IPv6 address */
};
This structure is defined in the <netinet/in.h> header file. The only
differences between this data structure and the 4.3 BSD 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 4.3 BSD variant defined in the previous section.
Systems that provide this version of the sockaddr_in6 data structure
must include the SIN6_LEN macro definition in <netinet/in.h>. This
macro allows applications to determine whether they are being built on
a system that supports the 4.3 BSD or 4.4 BSD variants of the data
structure. Applications can be written to run on both systems by
simply making their assignments and use of the sin6_len field
conditional on the SIN6_LEN field. For example, to fill in an IPv6
address structure in an application, one might write:
struct sockaddr_in6 sin6;
bzero((char *) &sin6, sizeof(struct sockaddr_in6));
#ifdef SIN6_LEN
sin6.sin6_len = sizeof(struct sockaddr_in6);
#endif
sin6.sin6_family = AF_INET6;
sin6.sin6_port = 23;
Note that the size of the sockaddr_in6 structure is larger than the
size of the sockaddr structure. Applications that use the
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sockaddr_in6 structure need to be aware that they can not use
sizeof(sockaddr) to allocate a buffer to hold a sockaddr_in6
structure. They should use sizeof(sockaddr_in6) instead.
4.5. 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()
sendmsg()
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()
recvmsg()
getpeername()
getsockname()
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No changes to the syntax of the socket functions are needed to support
IPv6, since the all of the "address carrying" functions use an opaque
address pointer, and carry an address length as a function argument.
4.6. 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 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 open
issues section (section 7) discusses some of the alternatives.
4.7. 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 uses the IPv4-mapped IPv6 address format defined in the
IPv6 addressing architecture specification [3]. 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>
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.
We expect that few applications will need to know which type of node
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they are interoperating with. However, for those applications that do
need to know, the following function is provided:
int is_ipv4_addr (const struct in_addr6 *ap);
The "ap" argument to this function points to a buffer holding an IPv6
address in network byte order. The function returns true (non-zero)
if that address is an IPv4-mapped address, and returns 0 otherwise.
When an application using the extended API accepts a TCP connection,
or receives a UDP packet, it may determine whether the peer is an IPv4
node by applying the is_ipv4_addr() function to the address returned
by accept() or recvfrom().
4.8. 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 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,
which would be equipped only 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 problem, 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
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 IP_ADDRFORM is in <netinet/in.h>.
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;
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INTERNET-DRAFT IPv6 BSD API Spec June 1995
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. For example:
int addrform;
int len = sizeof(int);
if (getsockopt(s, IPPROTO_IP, IP_ADDRFORM, (char *) &addrform,
&len) == -1)
perror("getsockopt IP_ADDRFORM");
if (addrform == PF_INET)
printf("This is an IPv4 socket.\n");
else if (addrform == PF_INET6)
printf("This is an IPv6 socket.\n");
else
printf("This system is broken.\n");
4.9. Flow Information
The IPv6 header has a 24-bit field to hold a "flow label", and a 4-bit
field to hold a "priority". Applications have control over what values
for these fields are used in packets that they originate, and have
access to the field values of packets that they receive.
The sin6_flowinfo field of the sockaddr_in6 structure is used to carry
the flow information between the application and the system. An
application may specify a flow label and priority to use in the
transmitted packets of an actively opened TCP connection by setting the
sin6_flowinfo field of the destination address sockaddr_in6 structure
passed in the connect() function. An application may specify the flow
label and priority to use in transmitted UDP packets by setting the
sin6_flowinfo field of the destination address sockaddr_in6 structure
passed in the sendto() function. If an application does not care what
values are used, it should set the flowinfo value to zero.
An application may specify the flow label and priority to use in
transmitted packets of a passively accepted TCP connection, by setting
the sin6_flowinfo field of the address passed in the bind() function.
The flow label and priority that appeared in received UDP packets are
passed up to the application in the sin6_flowinfo field of the source
address sockaddr_in6 structure that is returned in the recvfrom() call.
The flow information that appeared in the received SYN segment of a
passively accepted TCP connection is returned to the application in the
source address sin6_flowinfo field of the sockaddr_in6 structure that is
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passed in the accept() call.
4.10. 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 itself.
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 originated traffic -- actively opened TCP connections
and UDP packets being sent -- and retrieving the source route of
received traffic -- 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 octets, 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_flowinfo fields of the intermediate addresses must be set to zero.
The arrangement of the address structures in the address buffer passed
to connect() or sendto() is shown in the figure below:
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+--------------------+
| |
| 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 IP_RCVSRCRT is in <netinet/in.h>.
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
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()
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functions (when passively accepting TCP connections) points to an array
of sockaddr_in6 structures. When the function returns, 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 octets, 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
Since IPv6 allows the number of elements in a source route to be very
large, it is impractical for all applications that have enabled the
reception of source routes to provide buffer space to hold the maximum
number of elements. Some applications may choose a buffer size that is
appropriate for their own use. This means that it is possible that a
received source route may be too large to fit into the buffer provided
by the application. In this circumstance, the system should return only
a single address element -- the source identifying address -- to the
application. This case is clearly distinguishable to the application
because in all other cases, the system returns at least two address
elements -- the source and destination identifying addresses.
4.11. Unicast Hop Limit
A new setsockopt() option is used to control the hop limit used in
outgoing unicast IPv6 packets. The name of this option is
IP_UNICAST_HOPS, and it is used at the IPPROTO_IP layer. The macro
definition for IP_UNICAST_HOPS resides in the <netinet/in.h> header
file. The following example illustrates how it is used:
int hoplimit = 10;
if (setsockopt(s, IPPROTO_IP, IP_UNICAST_HOPS, (char *) &hoplimit,
sizeof(hoplimit)) == -1)
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perror("setsockopt IP_UNICAST_HOPS);
When the IP_UNICAST_HOPS option is set with setsockopt(), the option
value given is used as the hop limit for all subsequent unicast packets
sent via that socket. If the option is not set, the system selects a
default value.
The IP_UNICAST_HOPS option may be used in the getsockopt() function to
determine the hop limit value that the system will use for subsequent
unicast packets sent via that socket. For example:
int hoplimit;
int len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IP, IP_UNICAST_HOPS, (char *) &hoplimit,
&len) == -1)
perror("getsockopt IP_UNICAST_HOPS);
else
printf("Using %d for hop limit.\n", hoplimit);
4.12. 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_HOPS Set the hop limit to use for outgoing
multicast packets. (Note a separate
option - IP_UNICAST_HOPS - is provided
to set the hop limit 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.
The reception of multicast packets is controlled by the two setsockopt()
options summarized below:
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INTERNET-DRAFT IPv6 BSD API Spec June 1995
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.
4.13. 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 IPv4-mapped IPv6 addresses. As in IPv4,
each IPv6 address returned in the hostent structure 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:
draft-ietf-ipngwg-bsd-api-01.txt [Page 17]
INTERNET-DRAFT IPv6 BSD API Spec June 1995
struct hostent *addr2hostname(const void *addr, int len, int af);
addr2hostname() performs an address-to-name lookup 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-mapped IPv6 address, an IPv4 address-to-name lookup is performed on
the embedded IPv4 address.
A new name-to-address translation library function is now under
development at Berkeley [2]. 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
should be directly usable 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.
4.14. 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:
int ascii2addr(int af, const char *cp, void *ap);
and
char *addr2ascii(int af, const void *ap, int len, 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
draft-ietf-ipngwg-bsd-api-01.txt [Page 18]
INTERNET-DRAFT IPv6 BSD API Spec June 1995
address families are supported. The cp argument points to the ascii
string being passed in. The ap argument points to a buffer into which
the function stores 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 standard IPv6 printing forms defined in the addressing
architecture specification [3].
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 len field specifies the length in octets of the address
pointed to by ap, and must be 4 if af is AF_INET, or 16 if af 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 octet.
For IPv6 addresses, the buffer must be at least 46-octets. For IPv4
addresses, the buffer must be at least 16-octets.
The addr2ascii() function returns a pointer to the buffer containing the
ascii string if the conversion succeeds, and NULL otherwise. The
function does not modify the storage pointed to by cp if the conversion
fails.
5. Security Considerations
IPv6 provides a number of new security mechanisms, many of which need to
be accessible to applications. A companion document detailing the
extensions to the socket interfaces to support IPv6 security is being
written [4]. At some point in the future, that document and this one
may be merged into a single API specification.
6. Change History
draft-ietf-ipngwg-bsd-api-01.txt [Page 19]
INTERNET-DRAFT IPv6 BSD API Spec June 1995
Changes from the March 1995 Edition
- Changed the definition of the ipv6_addr structure to be an array
of sixteen chars instead of four longs. This change is
necessary to support machines which implement the socket
interface, but do not have a 32-bit addressable word. Virtually
all machines which provide the socket interface do support an
8-bit addressable data type.
- Added a more detailed explanation that the data types defined in
this documented are not intended to be hard and fast
requirements. Systems may use other data types if they wish.
- Added a note flagging the fact that the sockaddr_in6 structure
is not the same size as the sockaddr structure.
- Changed the sin6_flowlabel field to sin6_flowinfo to accommodate
the addition of the priority field to the IPv6 header.
Changes from the October 1994 Edition
- Added variant of sockaddr_in6 for 4.4 BSD-based systems (sa_len
compatibility).
- Removed references to SIT transition specification, and added
reference to addressing architecture document, for definition of
IPv4-mapped addresses.
- Added a solution to the problem of the application not providing
enough buffer space to hold a received source route.
- Moved discussion of IPv4 applications interoperating with IPv6
nodes to open issues section.
- Added length parameter to addr2ascii() function to be consistent
with addr2hostname().
- Changed IP_MULTICAST_TTL to IP_MULTICAST_HOPS to match IPv6
terminology, and added IP_UNICAST_HOPS option to match
IP_MULTICAST_HOPS.
- Removed specification of numeric values for AF_INET6,
IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on
different implementations.
- Added a definition for the in_addr6 IPv6 address data
structure. Added this so that applications could use
sizeof(struct in_addr6) to get the size of an IPv6 address,
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INTERNET-DRAFT IPv6 BSD API Spec June 1995
and so that a structured type could be used in the
is_ipv4_addr().
7. Open Issues
A few open issues for IPv6 socket interface API specification remain,
including:
- The multicast API needs to be documented in more detail.
- Should we add a timeout parameter to hostname2addr() and
addr2hostname()? DNS lookups need to be given some finite
timeout interval, so it might be nice to let the application
specify that interval.
- Can existing IPv4 applications interoperate with IPv6 nodes?
7.1. IPv4 Applications Interoperating with IPv6 Nodes
This problem primarily has to do with the how IPv4 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, two techniques that partially solve the problem are:
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 system could generate a local 32-bit cookie to represent the
full 128-bit IPv6 address, and pass this value to the
application. The system 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 system 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 system would
have to make sure that these cookie values did not escape into
the Internet as the source or destination addresses of IPv4
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INTERNET-DRAFT IPv6 BSD API Spec June 1995
packets.
Both of these techniques have drawbacks. This is an area for further
study. System implementors may use one of these techniques or implement
another solution.
Acknowledgments
Thanks to the many people who made suggestions and provided feedback to
to the numerous revisions of this document, including: Dave Borman, Mark
Hasson, Alan Cox, Wan-Yen Hsu, Alex Conta, Richard Stevens, Dan
McDonald, Alan Lloyd, Christian Huitema, Steve Deering, Andrew
Cherenson, Charles Lynn, Ran Atkinson, Erik Nordmark, Josh Osborne,
Glenn Trewitt, Fred Baker, Robert Elz, Dean D. Throop, and Francis
Dupont. 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] R. Hinden. "Internet Protocol, Version 6 (IPv6) Specification".
Internet Draft. June 1995.
[2] K. Sklower. Private communication.
[3] R. Hinden., S. Deering. "IP Version 6 Addressing Architecture".
Internet Draft. June 1995.
[4] D. McDonald. "IPv6 Security API for BSD Sockets". Internet
Draft. January 1995.
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
draft-ietf-ipngwg-bsd-api-01.txt [Page 22]
INTERNET-DRAFT IPv6 BSD API Spec June 1995
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
draft-ietf-ipngwg-bsd-api-01.txt [Page 23]
