draft-ietf-ipngwg-bsd-api-04.txt   draft-ietf-ipngwg-bsd-api-05.txt 
Internet Engineering Task Force R. E. Gilligan (Sun) Internet Engineering Task Force R. E. Gilligan (Sun)
INTERNET-DRAFT S. Thomson (Bellcore) INTERNET-DRAFT S. Thomson (Bellcore)
J. Bound (Digital) J. Bound (Digital)
April 18, 1996
January 9, 1996 Basic Socket Interface Extensions for IPv6
<draft-ietf-ipngwg-bsd-api-05.txt>
IPv6 Program Interfaces for BSD Systems
<draft-ietf-ipngwg-bsd-api-04.txt>
Abstract Abstract
In order to implement the version 6 Internet Protocol (IPv6) [1] in an In order to implement the version 6 Internet Protocol (IPv6) [1] in
operating system based on Berkeley Unix (4.x BSD), changes must be made an operating system based on Berkeley Unix (4.x BSD), changes must be
to the application program interface (API). TCP/IP applications written made to the application program interface (API). TCP/IP applications
for BSD-based operating systems have in the past enjoyed a high degree written for BSD-based operating systems have in the past enjoyed a
of portability because most of the systems derived from BSD provide the high degree of portability because most of the systems derived from
same API, known informally as "the socket interface". We would like the BSD provide the same API, known informally as "the socket interface".
same portability with IPv6. This memo presents a set of extensions to We would like the same portability with IPv6. This memo presents a
the BSD socket API to support IPv6. The changes include a new data basic set of extensions to the BSD socket API to support IPv6. The
structure to carry IPv6 addresses, new name-to-address translation changes include a new data structure to carry IPv6 addresses, new
library functions, new address conversion functions, and some new address conversion functions, and some new setsockopt() options. The
setsockopt() options. The extensions are designed to provide access to extensions are designed to provide access to IPv6 features, while
IPv6 features, while introducing a minimum of change into the system and introducing a minimum of change into the system and providing
providing complete compatibility for existing IPv4 applications. complete compatibility for existing IPv4 applications. Additional
extensions for new IPv6 features may be added at a later time.
Status of this Memo Status of this Memo
This document is an Internet Draft. Internet Drafts are working This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts. working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts are draft documents valid for a maximum of six
This Internet Draft expires on July 6, 1996. Internet Drafts may be months. This Internet Draft expires on October 18, 1996. Internet
updated, replaced, or obsoleted by other documents at any time. It is Drafts may be updated, replaced, or obsoleted by other documents at
not appropriate to use Internet Drafts as reference material or to cite any time. It is not appropriate to use Internet Drafts as reference
them other than as a "working draft" or "work in progress." 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 To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow 1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
munnari.oz.au. munnari.oz.au.
Distribution of this memo is unlimited. Distribution of this memo is unlimited.
1. Introduction. 1. Introduction
While IPv4 addresses are 32-bits long, IPv6 nodes are identified by While IPv4 addresses are 32-bits long, IPv6 nodes are identified by
128-bit addresses. The socket interface make the size of an IP address 128-bit addresses. The socket interface make the size of an IP
quite visible to an application; virtually all TCP/IP applications for address quite visible to an application; virtually all TCP/IP
BSD-based systems have knowledge of the size of an IP address. Those applications for BSD-based systems have knowledge of the size of an
parts of the API that expose the addresses need to be extended to IP address. Those parts of the API that expose the addresses need to
accommodate the larger IPv6 address size. IPv6 also introduces new be extended to accommodate the larger IPv6 address size. IPv6 also
features, some of which must be made visible to applications via the introduces new features, some of which must be made visible to
API. This paper defines a set of extensions to the socket interface to applications via the API. This paper defines a set of extensions to
support the larger address size and new features of IPv6. the socket interface to support the larger address size and new
features of IPv6.
This specification is preliminary. These API extensions are expected to This specification is preliminary. These API extensions are expected
evolve as we gain more implementation experience. to evolve as we gain more implementation experience.
2. Design Considerations 2. Design Considerations
There are a number of important considerations in designing changes to There are a number of important considerations in designing changes
this well-worn API: to this well-worn API:
- The extended API should provide both source and binary - The extended API should provide both source and binary
compatibility for programs written to the original API. That compatibility for programs written to the original API. That is,
is, existing program binaries should continue to operate when existing program binaries should continue to operate when run on
run on a system supporting the new API. In addition, existing a system supporting the new API. In addition, existing
applications that are re-compiled and run on a system supporting applications that are re-compiled and run on a system supporting
the new API should continue to operate. Simply put, the API the new API should continue to operate. Simply put, the API
changes for IPv6 should not break existing programs. changes for IPv6 should not break existing programs.
- The changes to the API should be as small as possible in order - The changes to the API should be as small as possible in order to
to simplify the task of converting existing IPv4 applications to simplify the task of converting existing IPv4 applications to
IPv6. IPv6.
- Where possible, applications should be able to use the extended - Where possible, applications should be able to use the extended
API to interoperate with both IPv6 and IPv4 hosts. Applications API to interoperate with both IPv6 and IPv4 hosts. Applications
should not need to know which type of host they are should not need to know which type of host they are communicating
communicating with. with.
- IPv6 addresses carried in data structures should be 64-bit - IPv6 addresses carried in data structures should be 64-bit
aligned. This is necessary in order to obtain optimum aligned. This is necessary in order to obtain optimum
performance on 64-bit machine architectures. performance on 64-bit machine architectures.
Because of the importance of providing IPv4 compatibility in the API, Because of the importance of providing IPv4 compatibility in the API,
these extensions are explicitly designed to operate on machines that these extensions are explicitly designed to operate on machines that
provide complete support for both IPv4 and IPv6. A subset of this API provide complete support for both IPv4 and IPv6. A subset of this
could probably be designed for operation on systems that support only API could probably be designed for operation on systems that support
IPv6. However, this is not addressed in this document. only IPv6. However, this is not addressed in this document.
2.1. What Needs to be Changed 2.1. What Needs to be Changed
The socket interface API consists of a few distinct components: The socket interface API consists of a few distinct components:
- Core socket functions. - Core socket functions.
- Address data structures. - Address data structures.
- Name-to-address translation functions. - Name-to-address translation functions.
- Address conversion functions. - Address conversion functions.
skipping to change at page 3, line 17 skipping to change at page 3, line 14
The socket interface API consists of a few distinct components: The socket interface API consists of a few distinct components:
- Core socket functions. - Core socket functions.
- Address data structures. - Address data structures.
- Name-to-address translation functions. - Name-to-address translation functions.
- Address conversion functions. - Address conversion functions.
The core socket functions -- those functions that deal with such things The core socket functions -- those functions that deal with such
as setting up and tearing down TCP connections, and sending and things as setting up and tearing down TCP connections, and sending
receiving UDP packets -- were designed to be transport independent. and receiving UDP packets -- were designed to be transport
Where protocol addresses are passed as function arguments, they are independent. Where protocol addresses are passed as function
carried via opaque pointers. A protocol specific address data structure arguments, they are carried via opaque pointers. A protocol specific
is defined for each protocol that the socket functions support. address data structure is defined for each protocol that the socket
Applications must cast these protocol specific address structures into functions support. Applications must cast these protocol specific
the generic "sockaddr" data type when using the socket functions. These address structures into the generic "sockaddr" data type when using
functions need not change for IPv6, but a new IPv6 specific address data the socket functions. These functions need not change for IPv6, but
structure is needed. a new IPv6 specific address data structure is needed.
The "sockaddr_in" structure is the protocol specific data structure for The "sockaddr_in" structure is the protocol specific data structure
IPv4. This data structure actually includes 8-octets of unused space, for IPv4. This data structure actually includes 8-octets of unused
and it is tempting to try to use this space to adapt the sockaddr_in space, and it is tempting to try to use this space to adapt the
structure to IPv6. Unfortunately, the sockaddr_in structure is not sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in
large enough to hold the 16-octet IPv6 address as well as the other structure is not large enough to hold the 16-octet IPv6 address as
information (2-octet address family and 2-octet port number) that is well as the other information (2-octet address family and 2-octet
needed. So a new address data structure must be defined for IPv6. 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 The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr(). Gethostbyname() does not provide gethostbyname() and gethostbyaddr(). Gethostbyname() does not
enough flexibility to accommodate more than one protocol family. To provide enough flexibility to accommodate protocols other than IPv4.
solve this problem, we introduced a new name-to-address translation POSIX, in its 1003.g draft specification, has proposed a new hostname
function which is analogous to gethostbyname(), but supports addresses to address translation function which is protocol independent. This
in both the IPv4 and IPv6 address families. Gethostbyaddr() does not, function can be used with IPv6, so no new function is defined here.
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() -- The address conversion functions -- inet_ntoa() and inet_addr() --
convert IPv4 addresses between binary and printable form. These convert IPv4 addresses between binary and printable form. These
functions are quite specific to 32-bit IPv4 addresses. We have designed 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.
two analogous functions which convert both IPv4 and IPv6 addresses, and Finally, a few miscellaneous features are needed to support IPv6. A
carry an address type parameter so that they can be extended to other new interface is needed in order to support the IPv6 flow label and
protocol families as well. priority header fields. New interfaces are needed in order to
receive IPv6 multicast packets and control the sending of multicast
packets.
Finally, a few miscellaneous features are needed to support IPv6. A new The socket interface may be further extended in the future to provide
interface is needed in order to support the IPv6 flow label and priority access to other IPv6 features. These extensions will be made in
header fields. New interfaces are needed in order to receive IPv6 separate documents.
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. Socket Interface 3. Socket Interface
This section specifies the socket interface changes for IPv6. This section specifies the socket interface changes for IPv6.
The data types of the structure elements given in the following section The data types of the structure elements given in the following
are intended to be examples, not absolute requirements. System section are intended to be examples, not absolute requirements.
implementations may use other types if they are appropriate. In some System implementations may use other types if they are appropriate.
cases, such as when a field of a data structure holds a protocol value, In some cases, such as when a field of a data structure holds a
the structure field must be of some minimum size. These size protocol value, the structure field must be of some minimum size.
requirements are noted in the text. For example, since the UDP and TCP These size requirements are noted in the text. For example, since
port values are 16-bit quantities, the sin6_port field must be at least the UDP and TCP port values are 16-bit quantities, the sin6_port
a 16-bit data types. The sin6_port field is specified as a u_int16_t field must be at least a 16-bit data types. The sin6_port field is
type, but an implementation may use any data type that is at least specified as a u_int16m_t type, but an implementation may use any
16-bits long. data type that is at least 16-bits long.
3.1. New Address Family 3.1. New Address Family
A new address family macro, named AF_INET6, is defined in A new address family macro, named AF_INET6, is defined in
<sys/socket.h>. The AF_INET6 definition is used to distinguish between <sys/socket.h>. The AF_INET6 definition is used to distinguish
the original sockaddr_in address data structure, and the new between the original sockaddr_in address data structure, and the new
sockaddr_in6 data structure. sockaddr_in6 data structure.
A new protocol family macro, named PF_INET6, is defined in A new protocol family macro, named PF_INET6, is defined in
<sys/socket.h>. Like most of the other protocol family macros, this <sys/socket.h>. Like most of the other protocol family macros, this
will usually be defined to have the same value as the corresponding will usually be defined to have the same value as the corresponding
address family macro: address family macro:
#define PF_INET6 AF_INET6 #define PF_INET6 AF_INET6
The PF_INET6 is used in the first argument to the socket() function to The PF_INET6 is used in the first argument to the socket() function
indicate that an IPv6 socket is being created. to indicate that an IPv6 socket is being created.
3.2. IPv6 Address Data Structure 3.2. IPv6 Address Data Structure
A new data structure to hold a single IPv6 address is defined as A new data structure to hold a single IPv6 address is defined as
follows: follows:
struct in6_addr { struct in6_addr {
u_char s6_addr[16]; /* IPv6 address */ u_char s6_addr[16]; /* IPv6 address */
} }
This data structure contains an array of sixteen 8-bit elements, which This data structure contains an array of sixteen 8-bit elements,
make up one 128-bit IPv6 address. The IPv6 address is stored in network which make up one 128-bit IPv6 address. The IPv6 address is stored
byte order. in network byte order.
Applications obtain the declaration for this structure by including Applications obtain the declaration for this structure by including
the system header file <netinet/in.h>. the system header file <netinet/in.h>.
3.3. Socket Address Structure for 4.3 BSD-Based Systems 3.3. Socket Address Structure for 4.3 BSD-Based Systems
In the socket interface, a different protocol-specific data structure is In the socket interface, a different protocol-specific data structure
defined to carry the addresses for each of the protocol suite. Each is defined to carry the addresses for each of the protocol suite.
protocol-specific data structure is designed so it can be cast into a Each protocol-specific data structure is designed so it can be cast
protocol-independent data structure -- the "sockaddr" structure. Each into a protocol-independent data structure -- the "sockaddr"
has a "family" field which overlays the "sa_family" of the sockaddr data structure. Each has a "family" field which overlays the "sa_family"
structure. This field can be used to identify the type of the data of the sockaddr data structure. This field can be used to identify
structure. the type of the data structure.
The sockaddr_in structure is the protocol-specific address data The sockaddr_in structure is the protocol-specific address data
structure for IPv4. It is used to pass addresses between applications structure for IPv4. It is used to pass addresses between
and the system in the socket functions. The following structure is applications and the system in the socket functions. The following
defined to carry IPv6 addresses: structure is defined to carry IPv6 addresses:
struct sockaddr_in6 { struct sockaddr_in6 {
u_int16_t sin6_family; /* AF_INET6 */ u_int16m_t sin6_family; /* AF_INET6 */
u_int16_t sin6_port; /* Transport layer port # */ u_int16m_t sin6_port; /* Transport layer port # */
u_int32_t sin6_flowinfo; /* IPv6 flow information */ u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */ struct in6_addr sin6_addr; /* IPv6 address */
}; };
This structure is designed to be compatible with the sockaddr data This structure is designed to be compatible with the sockaddr data
structure used in the 4.3 BSD release. structure used in the 4.3 BSD release.
The sin6_family field is used to identify this as a sockaddr_in6 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 structure. This field is designed to overlay the sa_family field
the buffer is cast to a sockaddr data structure. The value of this when the buffer is cast to a sockaddr data structure. The value of
field must be AF_INET6. this field must be AF_INET6.
The sin6_port field is used to store the 16-bit UDP or TCP port 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 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 the sockaddr_in structure. The port number is stored in network byte
order. order.
The sin6_flowinfo field is a 32-bit field that is used to store three The sin6_flowinfo field is a 32-bit field that is used to store two
pieces of information: the 24-bit IPv6 flow label, the 4-bit priority pieces of information: the 24-bit IPv6 flow label and the 4-bit
priority field. The IPv6 flow label is represented as the low-order
field, and a 1-bit loose/strict source routing flag. The IPv6 flow 24-bits of the 32-bit field. The priority is represented in the next
label is represented as the low-order 24-bits of the 32-bit field. The 4-bits above this. The high-order 4 bits of this field are reserved.
priority is represented in the next 4-bits above this, and the The sin6_flowinfo field is stored in network byte order. The use of
loose/strict flag is the 1 bit above this. The high-order 3 bits of the flow label and priority fields are explained in sec 4.9.
this field are reserved. The sin6_flowinfo field is stored in network
byte order. The use of the flow label and priority fields are explained
in sec 4.9. The use of the loose/strict flag is discussed in section
4.10.
The sin6_addr field is a single in6_addr structure (defined in the The sin6_addr field is a single in6_addr structure (defined in the
previous section). This field holds one 128-bit IPv6 address. The previous section). This field holds one 128-bit IPv6 address. The
address is stored in network byte order. address is stored in network byte order.
The ordering of elements in this structure is specifically designed so The ordering of elements in this structure is specifically designed
that the sin6_addr field will be aligned on a 64-bit boundary. This is so that the sin6_addr field will be aligned on a 64-bit boundary.
done for optimum performance on 64-bit architectures. This is done for optimum performance on 64-bit architectures.
Applications obtain the declaration of the sockaddr_in6 structure by Applications obtain the declaration of the sockaddr_in6 structure by
including the system header file <netinet/in.h>. including the system header file <netinet/in.h>.
3.4. Socket Address Structure for 4.4 BSD-Based Systems 3.4. Socket Address Structure for 4.4 BSD-Based Systems
The 4.4 BSD release includes a small, but incompatible change to the The 4.4 BSD release includes a small, but incompatible change to the
socket interface. The "sa_family" field of the sockaddr data structure socket interface. The "sa_family" field of the sockaddr data
was changed from a 16-bit value to an 8-bit value, and the space saved structure was changed from a 16-bit value to an 8-bit value, and the
used to hold a length field, named "sa_len". The sockaddr_in6 data space saved used to hold a length field, named "sa_len". The
structure given in the previous section can not be correctly cast into sockaddr_in6 data structure given in the previous section can not be
the newer sockaddr data structure. For this reason, following correctly cast into the newer sockaddr data structure. For this
alternative IPv6 address data structure is provided to be used on reason, following alternative IPv6 address data structure is provided
systems based on 4.4 BSD: to be used on systems based on 4.4 BSD:
#define SIN6_LEN #define SIN6_LEN
struct sockaddr_in6 { struct sockaddr_in6 {
u_char sin6_len; /* length of this struct */ u_char sin6_len; /* length of this struct */
u_char sin6_family; /* AF_INET6 */ u_char sin6_family; /* AF_INET6 */
u_int16_t sin6_port; /* Transport layer port # */ u_int16m_t sin6_port; /* Transport layer port # */
u_int32_t sin6_flowinfo; /* IPv6 flow information */ u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */ struct in6_addr sin6_addr; /* IPv6 address */
}; };
The only differences between this data structure and the 4.3 BSD variant The only differences between this data structure and the 4.3 BSD
are the inclusion of the length field, and the change of the family variant are the inclusion of the length field, and the change of the
field to a 8-bit data type. The definitions of all the other fields are family field to a 8-bit data type. The definitions of all the other
identical to the 4.3 BSD variant defined in the previous section. 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 Systems that provide this version of the sockaddr_in6 data structure
must also declare the SIN6_LEN as a result of including the must also declare the SIN6_LEN as a result of including the
<netinet/in.h> header file. This macro allows applications to
<netinet/in.h> header file. This macro allows applications to determine determine whether they are being built on a system that supports the
whether they are being built on a system that supports the 4.3 BSD or 4.3 BSD or 4.4 BSD variants of the data structure. Applications can
4.4 BSD variants of the data structure. Applications can be written to be written to run on both systems by simply making their assignments
run on both systems by simply making their assignments and use of the and use of the sin6_len field conditional on the SIN6_LEN field. For
sin6_len field conditional on the SIN6_LEN field. For example, to fill example, to fill in an IPv6 address structure in an application, one
in an IPv6 address structure in an application, one might write: might write:
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
bzero((char *) &sin6, sizeof(struct sockaddr_in6)); bzero((char *) &sin6, sizeof(struct sockaddr_in6));
#ifdef SIN6_LEN #ifdef SIN6_LEN
sin6.sin6_len = sizeof(struct sockaddr_in6); sin6.sin6_len = sizeof(struct sockaddr_in6);
#endif #endif
sin6.sin6_family = AF_INET6; sin6.sin6_family = AF_INET6;
sin6.sin6_port = htons(23); sin6.sin6_port = htons(23);
Note that the size of the sockaddr_in6 structure is larger than the size Note that the size of the sockaddr_in6 structure is larger than the
of the sockaddr structure. Applications that use the sockaddr_in6 size of the sockaddr structure. Applications that use the
structure need to be aware that they can not use sizeof(sockaddr) to sockaddr_in6 structure need to be aware that they can not use
allocate a buffer to hold a sockaddr_in6 structure. They should use sizeof(sockaddr) to allocate a buffer to hold a sockaddr_in6
sizeof(sockaddr_in6) instead. structure. They should use sizeof(sockaddr_in6) instead.
3.5. The Socket Functions 3.5. The Socket Functions
Applications use the socket() function to create a socket descriptor Applications use the socket() function to create a socket descriptor
that represents a communication endpoint. The arguments to the socket() that represents a communication endpoint. The arguments to the
function tell the system which protocol to use, and what format address socket() function tell the system which protocol to use, and what
structure will be used in subsequent functions. For example, to create format address structure will be used in subsequent functions. For
an IPv4/TCP socket, applications make the call: example, to create an IPv4/TCP socket, applications make the call:
s = socket (PF_INET, SOCK_STREAM, 0); s = socket (PF_INET, SOCK_STREAM, 0);
To create an IPv4/UDP socket, applications make the call: To create an IPv4/UDP socket, applications make the call:
s = socket (PF_INET, SOCK_DGRAM, 0); s = socket (PF_INET, SOCK_DGRAM, 0);
Applications may create IPv6/TCP and IPv6/UDP sockets by simply using 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 the constant PF_INET6 instead of PF_INET in the first argument. For
example, to create an IPv6/TCP socket, applications make the call: example, to create an IPv6/TCP socket, applications make the call:
s = socket (PF_INET6, SOCK_STREAM, 0); s = socket (PF_INET6, SOCK_STREAM, 0);
To create an IPv6/UDP socket, applications make the call: To create an IPv6/UDP socket, applications make the call:
s = socket (PF_INET6, SOCK_DGRAM, 0); s = socket (PF_INET6, SOCK_DGRAM, 0);
Once the application has created a PF_INET6 socket, it must use the Once the application has created a PF_INET6 socket, it must use the
sockaddr_in6 address structure when passing addresses in to the
sockaddr_in6 address structure when passing addresses in to the system. system. The functions which the application uses to pass addresses
The functions which the application uses to pass addresses into the into the system are:
system are:
bind() bind()
connect() connect()
sendmsg() sendmsg()
sendto() sendto()
The system will use the sockaddr_in6 address structure to return The system will use the sockaddr_in6 address structure to return
addresses to applications that are using PF_INET6 sockets. The addresses to applications that are using PF_INET6 sockets. The
functions that return an address from the system to an application functions that return an address from the system to an application
are: are:
accept() accept()
recvfrom() recvfrom()
recvmsg() recvmsg()
getpeername() getpeername()
getsockname() getsockname()
No changes to the syntax of the socket functions are needed to support No changes to the syntax of the socket functions are needed to
IPv6, since the all of the "address carrying" functions use an opaque support IPv6, since the all of the "address carrying" functions use
address pointer, and carry an address length as a function argument. an opaque address pointer, and carry an address length as a function
argument.
3.6. Compatibility with IPv4 Applications 3.6. Compatibility with IPv4 Applications
In order to support the large base of applications using the original In order to support the large base of applications using the original
API, system implementations must provide complete source and binary API, system implementations must provide complete source and binary
compatibility with the original API. This means that systems must compatibility with the original API. This means that systems must
continue to support PF_INET sockets and the sockaddr_in addresses continue to support PF_INET sockets and the sockaddr_in addresses
structure. Applications must be able to create IPv4/TCP and IPv4/UDP structure. Applications must be able to create IPv4/TCP and IPv4/UDP
sockets using the PF_INET constant in the socket() function, as sockets using the PF_INET constant in the socket() function, as
described in the previous section. Applications should be able to hold described in the previous section. Applications should be able to
a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP sockets hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
simultaneously within the same process. sockets simultaneously within the same process.
Applications using the original API should continue to operate as they Applications using the original API should continue to operate as
did on systems supporting only IPv4. That is, they should continue to they did on systems supporting only IPv4. That is, they should
interoperate with IPv4 nodes. It is not clear, though, how, or even if, continue to interoperate with IPv4 nodes. It is not clear, though,
those IPv4 applications should interoperate with IPv6 nodes. The open how, or even if, those IPv4 applications should interoperate with
issues section (section 9) discusses some of the alternatives. IPv6 nodes. The open issues section (section 9) discusses some of
the alternatives.
3.7. Compatibility with IPv4 Nodes 3.7. Compatibility with IPv4 Nodes
The API also provides a different type of compatibility: the ability for The API also provides a different type of compatibility: the ability
applications using the extended API to interoperate with IPv4 nodes. for applications using the extended API to interoperate with IPv4
nodes. This feature uses the IPv4-mapped IPv6 address format defined
This feature uses the IPv4-mapped IPv6 address format defined in the in the IPv6 addressing architecture specification [2]. This address
IPv6 addressing architecture specification [3]. This address format format allows the IPv4 address of an IPv4 node to be represented as
allows the IPv4 address of an IPv4 node to be represented as an IPv6 an IPv6 address. The IPv4 address is encoded into the low-order 32-
address. The IPv4 address is encoded into the low-order 32-bits of the bits of the IPv6 address, and the high-order 96-bits hold the fixed
IPv6 address, and the high-order 96-bits hold the fixed prefix prefix 0:0:0:0:0:FFFF. IPv4-mapped addresses are written as follows:
0:0:0:0:0:FFFF. IPv4-mapped addresses are written as follows:
::FFFF:<IPv4-address> ::FFFF:<IPv4-address>
Applications may use PF_INET6 sockets to open TCP connections to IPv4 Applications may use PF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination's IPv4 address as an IPv4-mapped IPv6 address, and passing destination's IPv4 address as an IPv4-mapped IPv6 address, and
that address, within a sockaddr_in6 structure, in the connect() or passing that address, within a sockaddr_in6 structure, in the
sendto() call. When applications use PF_INET6 sockets to accept TCP connect() or sendto() call. When applications use PF_INET6 sockets
connections from IPv4 nodes, or receive UDP packets from IPv4 nodes, the to accept TCP connections from IPv4 nodes, or receive UDP packets
system returns the peer's address to the application in the accept(), from IPv4 nodes, the system returns the peer's address to the
recvfrom(), or getpeername() call using a sockaddr_in6 structure encoded application in the accept(), recvfrom(), or getpeername() call using
this way. a sockaddr_in6 structure encoded this way.
Few applications will likely need to know which type of node they are Few applications will likely need to know which type of node they are
interoperating with. However, for those applications that do need to interoperating with. However, for those applications that do need to
know, the inet6_isipv4addr() function, defined in section 6.3, is know, the inet6_isipv4addr() function, defined in section 6.3, is
provided. provided.
3.8. Flow Information 3.8. Flow Information
The IPv6 header has a 24-bit field to hold a "flow label", and a 4-bit The IPv6 header has a 24-bit field to hold a "flow label", and a 4-
field to hold a "priority" value. Applications have control over what bit field to hold a "priority" value. Applications have control over
values for these fields are used in packets that they originate, and what values for these fields are used in packets that they originate,
have access to the field values of packets that they receive. and have access to the field values of packets that they receive.
The sin6_flowinfo field of the sockaddr_in6 structure encodes three
pieces of information: IPv6 flow label, IPv6 priority, and a
strict/loose source routing flag which is discussed in section 4.2.
Applications use this field to set the flow label and priority in IPv6
headers of packets they generate, and to retrieve the flow label and
priority from the packets they receive. The header fields of an
actively opened TCP connection are set by assigning in the sin6_flowinfo
field of the destination address sockaddr_in6 structure passed in the
connect() function. The same technique can be used with the
sockaddr_in6 structure passed in to the sendto() or sendmsg() function
to set the flow label and priority fields of UDP packets. Similarly,
the flow label and priority values of received UDP packets and accepted
TCP connections are reflected in the sin6_flowinfo field of the
sockaddr_in6 structure returned to the application by the recvfrom(),
recvmsg(), and accept() functions. And 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 sin6_flowinfo field of the sockaddr_in6 structure encodes two
pieces of information: IPv6 flow label and IPv6 priority.
Applications use this field to set the flow label and priority in
IPv6 headers of packets they generate, and to retrieve the flow label
and priority from the packets they receive. The header fields of an
actively opened TCP connection are set by assigning in the
sin6_flowinfo field of the destination address sockaddr_in6 structure
passed in the connect() function. The same technique can be used
with the sockaddr_in6 structure passed in to the sendto() or
sendmsg() function to set the flow label and priority fields of UDP
packets. Similarly, the flow label and priority values of received
UDP packets and accepted TCP connections are reflected in the
sin6_flowinfo field of the sockaddr_in6 structure returned to the
application by the recvfrom(), recvmsg(), and accept() functions.
And 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.
Implementations provide two bitmask constant declarations to help Implementations provide two bitmask constant declarations to help
applications select out the flow label and priority fields. These applications select out the flow label and priority fields. These
constants are: constants are:
IPV6_FLOWINFO_FLOWLABEL IPV6_FLOWINFO_FLOWLABEL
IPV6_FLOWINFO_PRIORITY IPV6_FLOWINFO_PRIORITY
These constants can be applied to the sin6_flowinfo field of addresses These constants can be applied to the sin6_flowinfo field of
returned to the application, for example: addresses returned to the application, for example:
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
recvfrom(s, buf, buflen, flags, (struct sockaddr *) &sin6, &fromlen); recvfrom(s, buf, buflen, flags, (struct sockaddr *) &sin6, &fromlen);
. . . . . .
received_flowlabel = sin6.sin6_flowinfo & IPV6_FLOWINFO_FLOWLABEL; received_flowlabel = sin6.sin6_flowinfo & IPV6_FLOWINFO_FLOWLABEL;
received_priority = sin6.sin6_flowinfo & IPV6_FLOWINFO_PRIORITY; received_priority = sin6.sin6_flowinfo & IPV6_FLOWINFO_PRIORITY;
On the sending side, applications are responsible for selecting the flow On the sending side, applications are responsible for selecting the
label value. The system provides constant declarations for the IPv6 flow label value. The system provides constant declarations for the
priority values defined in the IPv6 specification [1]. These constants IPv6 priority values defined in the IPv6 specification [1]. These
are: constants are:
IPV6_PRIORITY_UNCHARACTERIZED IPV6_PRIORITY_UNCHARACTERIZED
IPV6_PRIORITY_FILLER IPV6_PRIORITY_FILLER
IPV6_PRIORITY_UNATTENDED IPV6_PRIORITY_UNATTENDED
IPV6_PRIORITY_RESERVED1 IPV6_PRIORITY_RESERVED1
IPV6_PRIORITY_BULK IPV6_PRIORITY_BULK
IPV6_PRIORITY_RESERVED2 IPV6_PRIORITY_RESERVED2
IPV6_PRIORITY_INTERACTIVE IPV6_PRIORITY_INTERACTIVE
IPV6_PRIORITY_CONTROL IPV6_PRIORITY_CONTROL
IPV6_PRIORITY_8 IPV6_PRIORITY_8
skipping to change at page 11, line 4 skipping to change at page 10, line 49
IPV6_PRIORITY_14 IPV6_PRIORITY_14
IPV6_PRIORITY_15 IPV6_PRIORITY_15
Applications can use these constants along with the flow label they Applications can use these constants along with the flow label they
selected to assign the sin6_flowinfo field, for example: selected to assign the sin6_flowinfo field, for example:
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
send_flowlabel = . . . ; send_flowlabel = . . . ;
. . . . . .
sin6.sin6_flowinfo = IPV6_PRIORITY_UNATTENDED | sin6.sin6_flowinfo = IPV6_PRIORITY_UNATTENDED |
(IPV6_FLOWINFO_FLOWLABEL & send_flowlabel); (IPV6_FLOWINFO_FLOWLABEL & send_flowlabel);
The macro declarations for these constants are obtained by including The macro declarations for these constants are obtained by including
the header file <netinet/in.h>. the header file <netinet/in.h>.
3.9. Binding to System-Selected Address 3.9. Binding to System-Selected Address
While the bind() function allows applications to select the source IP While the bind() function allows applications to select the source IP
address of UDP packets and TCP connections, applications often wish to address of UDP packets and TCP connections, applications often wish
let the system select the source address for them. In IPv4, this is to let the system select the source address for them. In IPv4, this
done by specifying the IPv4 address represented by the symbolic is done by specifying the IPv4 address represented by the symbolic
constant INADDR_ANY in the bind() call, or by simply by skipping the constant INADDR_ANY in the bind() call, or by simply by skipping the
bind() entirely. bind() entirely.
Since the IPv6 address type is a structure (struct in6_addr), a Since the IPv6 address type is a structure (struct in6_addr), a
symbolic constant can be used to initialize an IPv6 address variable, symbolic constant can be used to initialize an IPv6 address variable,
but can not be used in an assignment. Therefore systems provide the but can not be used in an assignment. Therefore systems provide the
IPv6 address value that can be used to instruct the system to select IPv6 address value that can be used to instruct the system to select
the source IPv6 address in two forms. the source IPv6 address in two forms.
The first version is a global variable named "ipv6addr_any" which is The first version is a global variable named "in6addr_any" which is
an in6_addr type structure. The extern declaration for this variable an in6_addr type structure. The extern declaration for this variable
is: is:
extern struct in6_addr ipv6addr_any; extern const struct in6_addr in6addr_any;
Applications use ipv6addr_any similarly to the way they use INADDR_ANY 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 in IPv4. For example, to bind a socket to port number 23, but let
system select the source address, an application could use the the system select the source address, an application could use the
following code: following code:
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
sin6.sin6_family = AF_INET6; sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0; sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23); sin6.sin6_port = htons(23);
sin6.sin6_addr = ipv6addr_any; sin6.sin6_addr = in6addr_any;
. . . . . .
if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1) if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . . . . .
The other version is a symbolic constant named IPV6ADDR_ANY_INIT. The other version is a symbolic constant named IN6ADDR_ANY_INIT.
This constant can be used to initialize an in6_addr structure: This constant can be used to initialize an in6_addr structure:
struct in6_addr anyaddr = IPV6ADDR_ANY_INIT; struct in6_addr anyaddr = IN6ADDR_ANY_INIT;
Note that this constant can be used ONLY at declaration type. It can Note that this constant can be used ONLY at declaration type. It can
not be used assign a previously declared in6_addr structure. For not be used assign a previously declared in6_addr structure. For
example, the following code will not work: example, the following code will not work:
/* This is the WRONG way to assign an unspecified address */ /* This is the WRONG way to assign an unspecified address */
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
sin6.sin6_addr = IPV6ADDR_ANY_INIT; /* Will NOT compile */ sin6.sin6_addr = IN6ADDR_ANY_INIT; /* Will NOT compile */
The extern declaration for in6addr_any and the macro declaration for
IN6ADDR_ANY_INIT are obtained by including <netinet/in.h>.
3.10. Communicating with Local Services 3.10. Communicating with Local Services
Applications may need to send UDP packets to, or originate TCP Applications may need to send UDP packets to, or originate TCP
connections to, services residing on the local node. In IPv4, they connections to, services residing on the local node. In IPv4, they
can do this by using the constant IPv4 address INADDR_LOOPBACK in can do this by using the constant IPv4 address INADDR_LOOPBACK in
their connect(), sendto(), or sendmsg() call. their connect(), sendto(), or sendmsg() call.
IPv6 also provides a loopback address which can be used to contact IPv6 also provides a loopback address which can be used to contact
local TCP and UDP services. Like the unspecified address, the IPv6 local TCP and UDP services. Like the unspecified address, the IPv6
loopback address is provided in two forms -- a global variable and a loopback address is provided in two forms -- a global variable and a
symbolic constant. symbolic constant.
The global variable is an in6_addr type structure named The global variable is an in6_addr type structure named
"ipv6addr_loopback." The extern declaration for this variable is: "in6addr_loopback." The extern declaration for this variable is:
extern struct in6_addr ipv6addr_loopback; extern const struct in6_addr in6addr_loopback;
Applications use ipv6addr_loopback as they would use INADDR_LOOPBACK Applications use in6addr_loopback as they would use INADDR_LOOPBACK
in IPv4 applications. For example, to open a TCP connection to the in IPv4 applications. For example, to open a TCP connection to the
local telnet server, an application could use the following code: local telnet server, an application could use the following code:
struct sockaddr_in6 sin6; struct sockaddr_in6 sin6;
. . . . . .
sin6.sin6_family = AF_INET6; sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0; sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23); sin6.sin6_port = htons(23);
sin6.sin6_addr = ipv6addr_loopback; sin6.sin6_addr = in6addr_loopback;
. . . . . .
if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1) if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . . . . .
The symbolic constant is named IPV6ADDR_LOOPBACK_INIT. It can be used The symbolic constant is named IN6ADDR_LOOPBACK_INIT. It can be used
at declaration time ONLY; for example: at declaration time ONLY; for example:
struct in6_addr loopbackaddr = IPV6ADDR_LOOPBACK_INIT; struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;
Like IPV6ADDR_ANY_INIT, this constant can not be used in an assignment Like IN6ADDR_ANY_INIT, this constant can not be used in an assignment
to a previously declared IPv6 address variable. to a previously declared IPv6 address variable.
The extern declaration for in6addr_loopback and the macro declaration
for IN6ADDR_LOOPBACK_INIT are obtained by including <netinet/in.h>.
4. Socket Options 4. Socket Options
A number of new socket options are defined for IPv6. All of these new A number of new socket options are defined for IPv6. All of these
options are at the IPPROTO_IPV6 level. That is, the "level" parameter new options are at the IPPROTO_IPV6 level. That is, the "level"
in the getsockopt() and setsockopt() call is IPPROTO_IPV6 when using parameter in the getsockopt() and setsockopt() call is IPPROTO_IPV6
these options. The constant name prefix IPV6_ is used in all of the new when using these options. The constant name prefix IPV6_ is used in
socket options. This serves to clearly identify these options as all of the new socket options. This serves to clearly identify these
applying to IPv6. options as applying to IPv6.
The macro declaration for IPPROTO_IPV6, the new IPv6 socket options, and The macro declaration for IPPROTO_IPV6, the new IPv6 socket options,
related constants defined in this section are obtained by including the and related constants defined in this section are obtained by
header file <netinet/in.h> including the header file <netinet/in.h>
4.1 Changing Socket Type 4.1 Changing Socket Type
Unix allows open sockets to be passed between processes via the exec() Unix allows open sockets to be passed between processes via the
call and other means. It is a relatively common application practice to exec() call and other means. It is a relatively common application
pass open sockets across exec() calls. Thus it is possible for an practice to pass open sockets across exec() calls. Thus it is
application using the original API to pass an open PF_INET socket to an possible for an application using the original API to pass an open
application that is expecting to receive a PF_INET6 socket. Similarly, PF_INET socket to an application that is expecting to receive a
it is possible for an application using the extended API to pass an open PF_INET6 socket. Similarly, it is possible for an application using
PF_INET6 socket to an application using the original API, which would be the extended API to pass an open PF_INET6 socket to an application
equipped only to deal with PF_INET sockets. Either of these cases could using the original API, which would be equipped only to deal with
cause problems, because the application which is passed the open socket PF_INET sockets. Either of these cases could cause problems, because
might not know how to decode the address structures returned in the application which is passed the open socket might not know how to
subsequent socket functions. decode the address structures returned in subsequent socket
functions.
To remedy this problem, a new setsockopt() option is defined that allows To remedy this problem, a new setsockopt() option is defined that
an application to "transform" a PF_INET6 socket into a PF_INET socket allows an application to "transform" a PF_INET6 socket into a PF_INET
and vice-versa. socket and vice-versa.
An IPv6 application that is passed an open socket from an unknown An IPv6 application that is passed an open socket from an unknown
process may use the IPV6_ADDRFORM setsockopt() option to "convert" the process may use the IPV6_ADDRFORM setsockopt() option to "convert"
socket to PF_INET6. Once that has been done, the system will return the socket to PF_INET6. Once that has been done, the system will
sockaddr_in6 address structures in subsequent socket functions. return sockaddr_in6 address structures in subsequent socket
Similarly, an IPv6 application that is about to pass an open PF_INET6 functions. Similarly, an IPv6 application that is about to pass an
socket to a program that may not be IPv6 capable may "downgrade" the open PF_INET6 socket to a program that may not be IPv6 capable may
socket to PF_INET before calling exec(). After that, the system will "downgrade" the socket to PF_INET before calling exec(). After that,
return sockaddr_in address structures to the application that was the system will return sockaddr_in address structures to the
exec()'ed. application that was exec()'ed.
The IPV6_ADDRFORM option is at the IPPROTO_IP level. The only valid The IPV6_ADDRFORM option is valid at both the IPPROTO_IP and
option values are PF_INET6 and PF_INET. For example, to convert a IPPROTO_IPV6 levels. The only valid option values are PF_INET6 and
PF_INET6 socket to PF_INET, a program would call: PF_INET. For example, to convert a PF_INET6 socket to PF_INET, a
program would call:
int addrform = PF_INET; int addrform = PF_INET;
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM, (char *) &addrform, if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM, (char *) &addrform,
sizeof(addrform)) == -1) sizeof(addrform)) == -1)
perror("setsockopt IPV6_ADDRFORM"); perror("setsockopt IPV6_ADDRFORM");
An application may use IPV6_ADDRFORM in the getsockopt() function to An application may use IPV6_ADDRFORM in the getsockopt() function to
learn whether an open socket is a PF_INET of PF_INET6 socket. For learn whether an open socket is a PF_INET of PF_INET6 socket. For
example: example:
int addrform; int addrform;
int len = sizeof(int); size_t len = sizeof(int);
if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM, (char *) &addrform, if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM, (char *) &addrform,
&len) == -1) &len) == -1)
perror("getsockopt IPV6_ADDRFORM"); perror("getsockopt IPV6_ADDRFORM");
if (addrform == PF_INET) if (addrform == PF_INET)
printf("This is an IPv4 socket.\n"); printf("This is an IPv4 socket.\n");
else if (addrform == PF_INET6) else if (addrform == PF_INET6)
printf("This is an IPv6 socket.\n"); printf("This is an IPv6 socket.\n");
else else
printf("This system is broken.\n"); printf("This system is broken.\n");
4.2. Handling IPv6 Source Routes 4.2. Unicast Hop Limit
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 msg_name field of the msghdr
structure of a sendmsg() function, or the address argument of the
sendto() function (when sending a UDP packet), or the address argument
of the connect() function (when actively opening a TCP connection). For
sendto() and connect(), the length argument of the function is the total
length, in octets, of the array. For sendmsg(), the msg_namelen field
of the msghdr structure specifies the total length of the array. The
elements of the array represent the full source route, including both
source and destination endpoint address. The elements of the array are
ordered from destination to source. That is, the first element of the
array represents the destination endpoint address, and the last element
of the array represents the source endpoint address. If the application
provides a source route, the source endpoint address can not be omitted.
The sin6_addr field of the source endpoint address may be set to
ipv6addr_any, however, in which case the system will select an
appropriate source address. The sin6_port field of the destination
endpoint address must be assigned. The sin6_port field of the source
endpoint address may be set to zero, in which case the system will
select an appropriate source port number. The sin6_port fields of the
intermediate addresses must be set to zero.
The application also has control over the loose/strict source routing
flag that is defined in the IPv6 specification [1]. It does this by
setting or clearing the loose/strict flag contained in the sin6_flowinfo
field of the destination and intermediate addresses. On the receive
side, the implementation uses the loose/strict flag in the address array
returned to the application to indicate the loose/strict status of each
hop.
The implementation provides a set of constant definitions to simplify
getting and setting the loose/strict flag for each of the hops of a
source route. The following constant is used to select the loose/strict
flag from the sin6_flowinfo field:
IPV6_FLOWINFO_SRFLAG
In addition, two constants are provided which represent the two states
of this flag:
IPV6_SRFLAG_STRICT
IPV6_SRFLAG_LOOSE
These constants can be used to inspect the source route flags of
received addresses, for example:
struct sockaddr_in6 sin6[3];
. . .
if ((sin6[0].sin6_flowinfo & IPV6_FLOWINFO_SRFLAG) ==
IPV6_SRFLAG_STRICT)
. . .
And they can also be used to set the source route flags:
struct sockaddr_in6 sin6[3];
. . .
sin6[0].sin6_flowinfo =
(sin6[0].sin6_flowinfo & ~IPV6_FLOWINFO_SRFLAG) |
IPV6_SRFLAG_STRICT;
The flow label and priority sub-fields of the sin6_flowinfo field of the
destination endpoint address may be set, but the these fields must be
set to zero in the intermediate and source endpoint addresses.
The arrangement of the address structures in the address buffer passed
to sendmsg(), connect() or sendto() is shown in the figure below:
+--------------------+
| |
| sockaddr_in6[0] | Destination Endpoint Address
| |
+--------------------+
| |
| sockaddr_in6[1] | Last Source-Route Hop Address
| |
+--------------------+
. .
. .
. .
+--------------------+
| |
| sockaddr_in6[N-1] | First Source-Route Hop Address
| |
+--------------------+
| |
| sockaddr_in6[N] | Source Endpoint Address
| |
+--------------------+
Address buffer when sending a source route
The IPV6_RECVSRCRT 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 IPV6_RECVSRCRT option is at the IPPROTO_IPV6 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_IPV6, IPV6_RECVSRCRT, (char *) &on,
sizeof(on)) == -1)
perror("setsockopt IPV6_RECVSRCRT");
When the IPV6_RECVSRCRT 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 endpoint address of the UDP packet received or the TCP connection
accepted.
When the IPV6_RECVSRCRT option is enabled, the msg_name field of the
msghdr of the recvmsg() function, or the address argument of the
recvfrom() function (when receiving UDP packets) and the accept()
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 endpoint address of the
received packet. Following this in the array are the intermediate hops
in the order in which they were visited. The last element of the array
holds the destination endpoint address. Note that this is the opposite
of the order specified for sending. This ordering was chosen so that
the address array returned in a recvmsg() or recvfrom() call can be used
in a subsequent sendmsg() or 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,
and the msg_namelen field of the msghdr field in the recvmsg() function,
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 endpoint 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 flow label and priority sub-fields of the sin6_flowinfo field of the
source endpoint address will hold the flow label and priority values of
the received packet. The flow label and priority sub-fields of the
intermediate addresses and the destination endpoint address will be
zero. The loose/strict flag of the sin6_flowinfo field of the source
endpoint address and the intermediate addresses will be set according to
the flags in the received packet. The macros defined above can be used
to inspect the loose/strict flag of each hop.
The address buffer returned to the application in the recvfrom() or
accept() functions when the IPV6_RECVSRCRT option is enabled is shown
below:
+--------------------+
| |
| sockaddr_in6[0] | Source Endpoint Address
| |
+--------------------+
| |
| sockaddr_in6[1] | First Source-Route Hop Address
| |
+--------------------+
. .
. .
. .
+--------------------+
| |
| sockaddr_in6[N-1] | Last Source-Route Hop Address
| |
+--------------------+
| |
| sockaddr_in6[N] | Destination Endpoint Address
| |
+--------------------+
Address buffer when receiving a source route
IPv6 allows a source route with up to 23 intermediate hops. Since the
it must also receive the source and destination endpoint addresses, the
application must provide a buffer capable of holding 25 addresses to
receive such a source route. Implementations provide the following
constant declaration in order to allow applications to simply declare
storage for the largest possible source route:
IPV6_SR_MAXADDR
Applications can use this constant like this:
struct sockaddr_in6 sin6[IPV6_SR_MAXADDR];
It may be impractical for some applications to allocate space to hold
the largest possible source route. Thus 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 endpoint 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 endpoint addresses.
4.3. Receiving Interface Determination
Some applications run on multi-homed hosts need to determine which
interface UDP packets were received on or TCP connections are bound to.
While the source routing interface described in the previous section
returns the destination address of the packet, this does not necessarily
identify the receiving interface. Some cases where it does not are:
- When the received packet is multicast. The destination address
in this case is an IPv6 multicast address, not the address of an
interface.
- When the node is operating as an IPv6 router. The node may
receive packets on interfaces other than the one they are
addressed to.
- When the received packet is sent to the node's link-local
address which is being used on multiple interfaces.
The address of the receiving interface is returned to the application
similarly to the way that source routes are returned. A new
setsockopt() option named IPV6_RECVIF is provided at the IPPROTO_IPV6
level. If this option is enabled, the system returns an additional
sockaddr_in6 structure to the application, holding the IPv6 address of
the receiving interface, in the recvfrom(), recvmsg(), or accept()
functions.
The option is enabled like this:
int on = 1; /* value == 1 means enable the option */
if (setsockopt(s, IPPROTO_IPV6, IPV6_RECVIF, (char *) &on,
sizeof(on)) == -1)
perror("setsockopt IPV6_RECVIF");
This option can be used in conjunction with the IP_RECVSRCRT option.
When the IPV6_RECVIF option is enabled, the buffer returned to the
application is structured like this:
+--------------------+ - - - - - - - - - - - - - - -
| |
| sockaddr_in6[0] |
| |
+--------------------+
. . Source Address, or
. . Full Source Route
. .
+--------------------+
| |
| sockaddr_in6[N-1] |
| |
+--------------------+ - - - - - - - - - - - - - - -
| |
| sockaddr_in6[N] | Receiving Interface Address
| |
+--------------------+ - - - - - - - - - - - - - - -
Address buffer with receiving interface address
The last address in the array is an IPv6 address of the receiving
interface. Since interfaces in IPv6 may have more than one address, and
some addresses (e.g. link-local addresses) may be used on more than one
interface, the system should select an address that uniquely identifies
the interface.
As when receiving source routes, the system returns a single
sockaddr_in6 structure holding the source endpoint address if the buffer
supplied by the application is too small to hold the receiving interface
address.
4.4. Sending Interface Specification
Applications may also need to specify the outgoing interface that
originated UDP or TCP packets should use. This is accomplished like
source route selection. The application may provide an additional
sockaddr_in6 structure in its sendto(), sendmsg() or connect() call
specifying the address of the outgoing interface. Unlike source route
selection, the outgoing interface address can only be included if a new
option is enabled. The new option is needed so that the system can
differentiate between the application's specification of an outgoing
interface address and a source route.
The new option option is named IPV6_SENDIF and is at the IPPROTO_IPV6
level. It can be enabled like this:
int on = 1; /* value == 1 means enable the option */
if (setsockopt(s, IPPROTO_IPV6, IPV6_SENDIF, (char *) &on,
sizeof(on)) == -1)
perror("setsockopt IPV6_SENDIF");
This option can be used in conjunction with source route specification.
If this option is enabled, the application passes in an address array
structured as follows:
+--------------------+ - - - - - - - - - - - - - - -
| |
| sockaddr_in6[0] |
| |
+--------------------+
. . Destination Address, or
. . Full Source Route
. .
+--------------------+
| |
| sockaddr_in6[N-1] |
| |
+--------------------+ - - - - - - - - - - - - - - -
| |
| sockaddr_in6[N] | Sending Interface Address
| |
+--------------------+ - - - - - - - - - - - - - - -
Address buffer with sending interface address
The last address in the array is an IPv6 address of the sending
interface. Applications should use an address that uniquely identifies
the interface to use.
4.5. Unicast Hop Limit
A new setsockopt() option is used to control the hop limit used in A new setsockopt() option is used to control the hop limit used in
outgoing unicast IPv6 packets. The name of this option is outgoing unicast IPv6 packets. The name of this option is
IPV6_UNICAST_HOPS, and it is used at the IPPROTO_IPV6 layer. The IPV6_UNICAST_HOPS, and it is used at the IPPROTO_IPV6 layer. The
following example illustrates how it is used: following example illustrates how it is used:
int hoplimit = 10; int hoplimit = 10;
if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit, if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit,
sizeof(hoplimit)) == -1) sizeof(hoplimit)) == -1)
perror("setsockopt IPV6_UNICAST_HOPS"); perror("setsockopt IPV6_UNICAST_HOPS");
When the IPV6_UNICAST_HOPS option is set with setsockopt(), the option When the IPV6_UNICAST_HOPS option is set with setsockopt(), the
value given is used as the hop limit for all subsequent unicast packets 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
sent via that socket. If the option is not set, the system selects a system selects a default value.
default value.
The IPV6_UNICAST_HOPS option may be used in the getsockopt() function to The IPV6_UNICAST_HOPS option may be used in the getsockopt() function
determine the hop limit value that the system will use for subsequent to determine the hop limit value that the system will use for
unicast packets sent via that socket. For example: subsequent unicast packets sent via that socket. For example:
int hoplimit; int hoplimit;
int len = sizeof(hoplimit); int len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit, if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit,
&len) == -1) &len) == -1)
perror("getsockopt IPV6_UNICAST_HOPS"); perror("getsockopt IPV6_UNICAST_HOPS");
else else
printf("Using %d for hop limit.\n", hoplimit); printf("Using %d for hop limit.\n", hoplimit);
4.6. Sending and Receiving Multicast Packets 4.3. Sending and Receiving Multicast Packets
IPv6 applications may send UDP multicast packets by simply specifying an IPv6 applications may send UDP multicast packets by simply specifying
IPv6 multicast address in the address argument of the sendto() function. an IPv6 multicast address in the address argument of the sendto()
function.
A few setsockopt options at the IPPROTO_IPV6 layer are used to control A few setsockopt options at the IPPROTO_IPV6 layer are used to
some of the parameters of sending multicast packets. These options are control some of the parameters of sending multicast packets. These
optional: applications may send multicast packets without using these options are optional: applications may send multicast packets without
options. The setsockopt() options for controlling the sending of using these options. The setsockopt() options for controlling the
multicast packets are summarized below: sending of multicast packets are summarized below:
IPV6_MULTICAST_IF IPV6_MULTICAST_IF
Set the interface to use for outgoing multicast packets. Set the interface to use for outgoing multicast packets. The
The argument is an IPv6 address of the interface to use. argument is an IPv6 address of the interface to use.
Argument type: struct in6_addr Argument type: struct in6_addr
IPV6_MULTICAST_HOPS IPV6_MULTICAST_HOPS
Set the hop limit to use for outgoing multicast packets. Set the hop limit to use for outgoing multicast packets.
(Note a separate option - IPV6_UNICAST_HOPS - is (Note a separate option - IPV6_UNICAST_HOPS - is provided to
provided to set the hop limit to use for outgoing set the hop limit to use for outgoing unicast packets.)
unicast packets.)
Argument type: unsigned int Argument type: unsigned int
IPV6_MULTICAST_LOOP IPV6_MULTICAST_LOOP
Controls whether outgoing multicast packets sent should Controls whether outgoing multicast packets sent should be
be delivered back to the local application. A toggle. delivered back to the local application. A toggle. If the
If the option is set to 1, multicast packets are looped option is set to 1, multicast packets are looped back. If it
back. If it is set to 0, they are not. is set to 0, they are not.
Argument type: unsigned int Argument type: unsigned int
The reception of multicast packets is controlled by the two setsockopt() The reception of multicast packets is controlled by the two
options summarized below: setsockopt() options summarized below:
IPV6_ADD_MEMBERSHIP IPV6_ADD_MEMBERSHIP
Join a multicast group. Requests that multicast packets Join a multicast group. Requests that multicast packets sent
sent to a particular multicast address be delivered to to a particular multicast address be delivered to this
this socket. The argument is the IPv6 multicast address socket. The argument is the IPv6 multicast address of the
of the group to join. group to join.
Argument type: struct ipv6_mreq Argument type: struct ipv6_mreq
IPV6_DROP_MEMBERSHIP IPV6_DROP_MEMBERSHIP
Leave a multicast group. Requests that multicast Leave a multicast group. Requests that multicast packets
packets sent to a particular multicast address no longer sent to a particular multicast address no longer be delivered
be delivered to this socket. The argument is the IPv6 to this socket. The argument is the IPv6 multicast address
multicast address of the group to join. of the group to join.
Argument type: struct ipv6_mreq Argument type: struct ipv6_mreq
The argument type of both of these options is the ipv6_mreq structure, The argument type of both of these options is the ipv6_mreq
structure, which is defined as follows:
which is defined as follows:
struct ipv6_mreq { struct ipv6_mreq {
/* IPv6 multicast address of group */ /* IPv6 multicast address of group */
struct in6_addr ipv6mr_multiaddr; struct in6_addr ipv6mr_multiaddr;
/* local IPv6 address of interface */ /* local IPv6 address of interface */
struct in6_addr ipv6mr_interface; struct in6_addr ipv6mr_interface;
}; };
5. Library Functions 5. Library Functions
New library functions are needed to lookup IPv6 addresses in the name New library functions are needed to perform a variety of operations
service, and to manipulate IPv6 addresses. with IPv6 addresses. Functions are needed to lookup IPv6 addresses
in the Domain Name System (DNS). Both forward lookup (hostname to
5.1. Name-to-Address Translation Functions address translation) and reverse lookup (address to hostname
translation) need to be supported. Functions are also needed to
Two new functions analogous to gethostbyname() and gethostbyaddr() have convert IPv6 addresses between their binary and textual form.
been defined which support both IPv4 and IPv6 addresses. 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.
The new functions differ from the old in one important way that is not
related to IPv6: The old functions could not safely be used by
multi-threaded applications, while the new ones can. There are two
multi-threading problems with the old functions. First, the return
value of the old functions is a pointer to a single static buffer
belonging to the library. The new functions return a dynamically
allocated buffer, and a third new function, named freehostent(), is
provided to free that storage. Second, the old functions returned their
error code in a global variable (h_errno). The new functions carry a
pointer that allows the library to return the error code into storage
provided by the caller.
The hostname2addr() function is similar to gethostbyname(), but enables
applications to specify the type of address to be looked up:
struct hostent *hostname2addr(
const char *name,
int af,
int *error);
This function looks up the hostname argument name in the name service
and, if the lookup succeeds, returns a completed hostent structure. If
the lookup fails, the function returns NULL and an error code is
returned in the buffer pointed to by the argument error. The af argument
specifies the type of the address -- IPv4 (AF_INET) or IPv6 (AF_INET6)
structure.
If the af argument is AF_INET, hostname2addr() behaves much like
gethostbyname. It 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, hostname2addr() queries the name service
for IPv6 addresses. The function may also query the name service for
IPv4 records. If this is done, any IPv4 addresses found are returned to
the application encoded as IPv4-compatible IPv6 addresses. The
determination of whether to query for IPv4 addresses is system specific.
Systems that support querying for IPv4 addresses should provide a
system-wide configuration switch allowing the system administrator to
enable or disable that feature.
IPv6 addresses returned by the hostname2addr() function are encoded in
network byte order.
The second new function, called addr2hostname(), is like the
gethostbyaddr() function, but supports both the IPv4 and IPv6 address
families:
struct hostent *addr2hostname(
const void *addr,
int addrlen,
int af,
int *error);
The addr2hostname() function performs an address-to-name lookup on the
address specified by the addr argument, returning a completed hostent
structure if the lookup succeeds. If the lookup fails, the function
returns NULL and an error code is returned in the buffer pointed to by
the argument error.
The addrlen argument specifies the length of the address (in octets)
pointed to by the addr argument.
The af argument specifies the address family of the addr argument. This
function supports both the AF_INET and AF_INET6 address families. If
the af argument is AF_INET, then addr refers to an IPv4 address and
addrlen must have the value 4. If af is AF_INET6, addr represents an
IPv6 address and addrlen must have the value 16. In the latter case,
the caller may present an IPv4-mapped IPv6 address in the addr argument.
If this is done, an IPv4 address-to-name lookup is performed on the
embedded IPv4 address.
IPv6 addresses are returned by hostname2addr() and addr2hostname() via
the hostent structure. This h_addr_list element of this structure
points to an array of ipv6_hostent_addr type structures. This
structure is declared as follows:
struct ipv6_hostent_addr {
/* amount of time in seconds that address is valid for */
u_int32_t iha_lifetime;
/* the IPv6 address being returned */
struct in6_addr iha_addr;
};
The hostent and address structures returned by both of these functions 5.1. Hostname to Address Translation
is allocated by the library. Applications use the freehostent()
function to return this storage to the library after they are done using
it:
void freehostent( A new hostname to address translation function is being defined by
struct hostent *hp); the Institute of Electrical and Electronic Engineers (IEEE) as part
of the POSIX 1003.1g (Protocol Independent Interfaces) draft
specification [4]. This function, named getaddrinfo(), has been
designed to be protocol independent, so it can be used without change
to lookup IPv6 addresses.
Applications may not access the hostent structure after they have As discussed in the "Transition Mechanisms for IPv6 Hosts and
returned it to the library. Routers" specification [5], systems may provide the ability to
transparently query for IPv4 address records when the application
requests an IPv6 lookup. The getaddrinfo() function can implement
this by automatically performing a query for IPv4 records if its
initial query for IPv6 records finds none. Or it may elect to always
query for both IPv6 and IPv4 records on all lookups. (Many DNS
implementations do not support querying for multiple record types in
a single request, so the IPv6 and IPv4 lookups can not be performed
simultaneously.) If IPv4 records are found, the addresses can be
returned to the application as IPv4-mapped IPv6 addresses. Systems
that support transparent querying for IPv4 address records should
provide a system-wide configuration switch to allow the system
administrator to enable or disable that feature.
Another new name-to-address translation library function is now under 5.2. Address to Hostname Translation
development at Berkeley. 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 The POSIX 1003.1g specification includes no function to perform a
hostname2addr() functions in order to simplify the porting of existing reverse DNS lookup (query based on IPv6 address). Therefore, we have
IPv4 applications to IPv6. System implementations may also provide the defined the following function:
getconninfo() function, once it is defined, so that newly written
applications can be transport independent.
The specification of the getconninfo() function is published as a int getnameinfo(
separate document [2], not included in this spec. const struct sockaddr *sa,
size_t addrlen,
char *host,
size_t hostlen,
char *serv,
size_t servlen);
Implementations must retain the BSD gethostbyname() and gethostbyaddr() This function looks up an IP address and port number provided by the
functions in order to provide source and binary compatibility for caller in the DNS and system-specific database, and returns text
existing applications. strings for both in buffers provided by the caller. The first
argument, sa, points to either a sockaddr_in structure (for IPv4) or
a sockaddr_in6 structure (for IPv6) which holds the IP address and
port number. The addrlen argument gives the length of the
sockaddr_in or sockaddr_in6 structure. The function returns the
hostname associated with the IP address in the buffer pointed to by
the host argument. The caller provides the size of this buffer via
the hostlen argument. The service name associated with the port
number is returned in the buffer pointed to by serv, and the servlen
argument gives the length of this buffer. The caller may instruct
the function not to return either string by providing a zero value
for the hostlen or servlen arguments. Otherwise, the caller must
provide buffers large enough to hold the fully qualified domain
hostname, and the full service name, including the terminating null
character. The function indicates successful completion by a zero
return value; a non-zero return value indicates failure.
Applications obtain the function prototype declarations for Applications obtain the function prototype declaration for
hostname2addr() and addr2hostname() by including the header file getnameinfo() by including the header file <netdb.h>.
<netdb.h>.
5.3. Address Conversion Functions 5.3. Address Conversion Functions
BSD Unix provides two functions, inet_addr() and inet_ntoa(), to convert BSD Unix provides two functions, inet_addr() and inet_ntoa(), to
an IPv4 address between binary and printable form. IPv6 applications convert an IPv4 address between binary and text form. IPv6
need similar functions. The following two functions convert both IPv6 applications need similar functions. The following two functions
and IPv4 addresses: convert both IPv6 and IPv4 addresses:
int ascii2addr( ssize_t inet_pton(
int af, int af,
const char *cp, const char *cp,
void *ap); void *ap);
and and
char *addr2ascii( char *inet_ntop(
int af, int af,
const void *ap, const void *ap,
int len, size_t len,
char *cp); char *cp);
The first function converts an ascii string to an address in the address The first function converts an address in its standard text
family specified by the af argument. Currently AF_INET and AF_INET6 presentation form into its numeric binary form. The af argument
address families are supported. The cp argument points to the ascii specifies the family of the address. Currently AF_INET and AF_INET6
string being passed in. The ap argument points to a buffer into which address families are supported. The cp argument points to the string
the function stores the address. Ascii2addr() returns the length of the being passed in. The ap argument points to a buffer into which the
address in octets if the conversion succeeds, and -1 otherwise. The function stores the numeric address. The address is returned in
function does not modify the storage pointed to by ap if the conversion network byte order. Inet_pton() returns the length of the address in
fails. The application must ensure that the buffer referred to by ap is octets if the conversion succeeds, and -1 otherwise. The function
large enough to hold the converted address. does not modify the buffer pointed to by ap if the conversion fails.
The calling 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 If the af argument is AF_INET, the function accepts a string in the
standard IPv4 dotted decimal form: standard IPv4 dotted decimal form:
ddd.ddd.ddd.ddd ddd.ddd.ddd.ddd
where ddd is a one to three digit decimal number between 0 and 255. 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 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 one of the standard IPv6 text forms defined in the addressing
architecture specification [3]. architecture specification [2].
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 The second function converts a numeric address into a text string
uses its own private static buffer. If the application specifies a cp suitable for presentation. The af argument specifies the family of
argument, it must be large enough to hold the ascii conversion of the the address. This can be AF_INET or AF_INET6. The ap argument
address specified as an argument, including the terminating null octet. points to a buffer holding an IPv4 address if the af argument is
AF_INET, or 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.
This 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
text string. If the cp argument is NULL, the function uses its own
private static buffer. If the application specifies a non-NULL cp
argument, the buffer must be large enough to hold the text
representation of the address, including the terminating null octet.
For IPv6 addresses, the buffer must be at least 46-octets. For IPv4 For IPv6 addresses, the buffer must be at least 46-octets. For IPv4
addresses, the buffer must be at least 16-octets. In order to allow addresses, the buffer must be at least 16-octets. In order to allow
applications to easily declare buffers of the proper size to store IPv4 applications to easily declare buffers of the proper size to store
and IPv6 addresses in string form, implementations should provide the IPv4 and IPv6 addresses in string form, implementations should
following constants, made available to applications that include provide the following constants, made available to applications that
<netinet/in.h>: include <netinet/in.h>:
#define INET_ADDRSTRLEN 16 #define INET_ADDRSTRLEN 16
#define INET6_ADDRSTRLEN 46 #define INET6_ADDRSTRLEN 46
The addr2ascii() function returns a pointer to the buffer containing the The inet_ntop() function returns a pointer to the buffer containing
ascii string if the conversion succeeds, and NULL otherwise. The the text string if the conversion succeeds, and NULL otherwise. The
function does not modify the storage pointed to by cp if the conversion function does not modify the storage pointed to by cp if the
fails. conversion fails.
Applications obtain the prototype declarations for addr2ascii() and Applications obtain the prototype declarations for inet_ntop() and
ascii2addr() by including the header file <arpa/inet.h>. inet_pton() by including the header file <arpa/inet.h>.
5.3. Embedded IPv4 Addresses 5.4. Embedded IPv4 Addresses
The IPv4-mapped IPv6 address format is used to represent IPv4 addresses The IPv4-mapped IPv6 address format is used to represent IPv4
as IPv6 addresses. Most applications should be able to to manipulate addresses as IPv6 addresses. Most applications should be able to to
IPv6 addresses as opaque 16-octet quantities, without needing to know manipulate IPv6 addresses as opaque 16-octet quantities, without
whether they represent IPv4 addresses. However, a few applications may needing to know whether they represent IPv4 addresses. However, a
need to determine whether an IPv6 address is an IPv4-mapped address or few applications may need to determine whether an IPv6 address is an
not. The following function is provided for those applications: IPv4-mapped address or not. The following function is provided for
those applications:
int inet6_isipv4addr (const struct in6_addr *addr); int inet6_isipv4addr (const struct in6_addr *addr);
The "addr" argument to this function points to a buffer holding an IPv6 The "addr" argument to this function points to a buffer holding an
address in network byte order. The function returns true (non-zero) if IPv6 address in network byte order. The function returns true (non-
that address is an IPv4-mapped address, and returns 0 otherwise. zero) if that address is an IPv4-mapped address, and returns 0
otherwise.
This function could be used by server applications to determine whether This function could be used by server applications to determine
the peer is an IPv4 node or an IPv6 node. After accepting a TCP whether the peer is an IPv4 node or an IPv6 node. After accepting a
connection via accept(), or receiving a UDP packet via recvfrom(), the TCP connection via accept(), or receiving a UDP packet via
application can apply the inet6_isipv4addr() function to the returned recvfrom(), the application can apply the inet6_isipv4addr() function
address. to the returned address.
Applications obtain the prototype for this function by including the Applications obtain the prototype for this function by including the
header file <arpa/inet.h>. header file <arpa/inet.h>.
6. Security Considerations 6. Security Considerations
IPv6 provides a number of new security mechanisms, many of which need to IPv6 provides a number of new security mechanisms, many of which need
be accessible to applications. A companion document detailing the to be accessible to applications. A companion document detailing the
extensions to the socket interfaces to support IPv6 security is being extensions to the socket interfaces to support IPv6 security is being
written [4]. At some point in the future, that document and this one written [3]. At some point in the future, that document and this one
may be merged into a single API specification. may be merged into a single API specification.
7. Change History 7. Change History
Changes from the January 1996 Edition
- Eliminated source routing and interface identification features
in order to simplify the spec. API features to provide this
functionallity can be defined at a later time.
- Eliminated definitions of hostname2addr() and addr2hostname().
Added reference to POSIX getaddrinfo() function to provide
functionallity previously provided by hostname2addr(). Added
definition of getnameinfo() function to provide functionallity of
addr2hostname().
- Changed name of addr2ascii() and ascii2addr() functions to
inet_ntop() and inet_pton() to be more consistent with BSD
function naming conventions.
- Changed some type definitions to align with POSIX.
Changes from the November 1995 Edition Changes from the November 1995 Edition
- Added the symbolic constants IPV6ADDR_ANY_INIT and - Added the symbolic constants IPV6ADDR_ANY_INIT and
IPV6ADDR_LOOPBACK_INIT for applications to use for IPV6ADDR_LOOPBACK_INIT for applications to use for
initializations. initializations.
- Eliminated restrictions on the value of ipv6addr_any. Systems - Eliminated restrictions on the value of ipv6addr_any. Systems
may now choose any value, including all-zeros. may now choose any value, including all-zeros.
- Added a mechanism for returning time to live with the address in - Added a mechanism for returning time to live with the address in
skipping to change at page 30, line 5 skipping to change at page 21, line 33
ipv6_isipv4addr() reside. ipv6_isipv4addr() reside.
- Clarified the include file requirements. Say that the structure - Clarified the include file requirements. Say that the structure
definitions are defined as a result of including the header file definitions are defined as a result of including the header file
<netinet/in.h>, not that the structures are necessarily defined <netinet/in.h>, not that the structures are necessarily defined
there. there.
- Removed underscore chars from is_ipv4_addr() function name for - Removed underscore chars from is_ipv4_addr() function name for
BSD compatibility. BSD compatibility.
- Added inet6_ prefix to is_ipv4_addr() function name to avoid - Added inet6_ prefix to is_ipv4_addr() function name to avoid name
name space conflicts. space conflicts.
- Changes setsockopt option naming convention to use IPV6_ prefix - Changes setsockopt option naming convention to use IPV6_ prefix
instead of IP_ so that there is clearly no ambiguity with IPv4 instead of IP_ so that there is clearly no ambiguity with IPv4
options. Also, use level IPPROTO_IPV6 for these options. options. Also, use level IPPROTO_IPV6 for these options.
- Made hostname2addr() and addr2hostname() functions thread-safe. - Made hostname2addr() and addr2hostname() functions thread-safe.
- Added support for sendmsg() and recvmsg() in source routing - Added support for sendmsg() and recvmsg() in source routing
section. section.
- Changed in_addr6 to in6_addr for consistency. - Changed in_addr6 to in6_addr for consistency.
- Re-structured document into sub-sections. - Re-structured document into sub-sections.
- Deleted the implementation experience section. It was too - Deleted the implementation experience section. It was too wordy.
wordy.
- Added argument types to multicast socket options. - Added argument types to multicast socket options.
- Added constant for largest source route array buffer. - Added constant for largest source route array buffer.
- Added the freehostent() function. - Added the freehostent() function.
- Added receiving interface determination and sending interface - Added receiving interface determination and sending interface
selection options. selection options.
- Added definitions of ipv6addr_any and ipv6addr_loopback. - Added definitions of ipv6addr_any and ipv6addr_loopback.
- Added text making the lookup of IPv4 addresses by - Added text making the lookup of IPv4 addresses by hostname2addr()
hostname2addr() optional. optional.
Changes from the June 1995 Edition Changes from the June 1995 Edition
- Added capability for application to select loose or strict - Added capability for application to select loose or strict source
source routing. routing.
Changes from the March 1995 Edition Changes from the March 1995 Edition
- Changed the definition of the ipv6_addr structure to be an array - Changed the definition of the ipv6_addr structure to be an array
of sixteen chars instead of four longs. This change is of sixteen chars instead of four longs. This change is necessary
necessary to support machines which implement the socket to support machines which implement the socket interface, but do
interface, but do not have a 32-bit addressable word. Virtually not have a 32-bit addressable word. Virtually all machines which
all machines which provide the socket interface do support an provide the socket interface do support an 8-bit addressable data
8-bit addressable data type. type.
- Added a more detailed explanation that the data types defined in - Added a more detailed explanation that the data types defined in
this documented are not intended to be hard and fast this documented are not intended to be hard and fast
requirements. Systems may use other data types if they wish. requirements. Systems may use other data types if they wish.
- Added a note flagging the fact that the sockaddr_in6 structure - Added a note flagging the fact that the sockaddr_in6 structure is
is not the same size as the sockaddr structure. not the same size as the sockaddr structure.
- Changed the sin6_flowlabel field to sin6_flowinfo to accommodate - Changed the sin6_flowlabel field to sin6_flowinfo to accommodate
the addition of the priority field to the IPv6 header. the addition of the priority field to the IPv6 header.
Changes from the October 1994 Edition Changes from the October 1994 Edition
- Added variant of sockaddr_in6 for 4.4 BSD-based systems (sa_len - Added variant of sockaddr_in6 for 4.4 BSD-based systems (sa_len
compatibility). compatibility).
- Removed references to SIT transition specification, and added - Removed references to SIT transition specification, and added
skipping to change at page 31, line 39 skipping to change at page 23, line 19
with addr2hostname(). with addr2hostname().
- Changed IP_MULTICAST_TTL to IP_MULTICAST_HOPS to match IPv6 - Changed IP_MULTICAST_TTL to IP_MULTICAST_HOPS to match IPv6
terminology, and added IP_UNICAST_HOPS option to match terminology, and added IP_UNICAST_HOPS option to match
IP_MULTICAST_HOPS. IP_MULTICAST_HOPS.
- Removed specification of numeric values for AF_INET6, - Removed specification of numeric values for AF_INET6,
IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on IP_ADDRFORM, and IP_RCVSRCRT, since they need not be the same on
different implementations. different implementations.
- Added a definition for the in_addr6 IPv6 address data - Added a definition for the in_addr6 IPv6 address data structure.
structure. Added this so that applications could use Added this so that applications could use sizeof(struct in_addr6)
sizeof(struct in_addr6) to get the size of an IPv6 address, to get the size of an IPv6 address, and so that a structured type
and so that a structured type could be used in the could be used in the is_ipv4_addr().
is_ipv4_addr().
8. Open Issues 8. Open Issues
A few open issues for IPv6 socket interface API specification remain, A few open issues for IPv6 socket interface API specification remain,
including: including:
- Should we add a timeout parameter to hostname2addr() and - An API should be provided to allocate and free a flow label that
addr2hostname()? DNS lookups need to be given some finite meets the uniqueness and randomness requirements outlined in the
IPv6 protocol spec.
- Should we add a timeout parameter to the hostname/address
translation functions? DNS lookups need to be given some finite
timeout interval, so it might be nice to let the application timeout interval, so it might be nice to let the application
specify that interval. specify that interval.
- Can the IPV6_ADDRFORM option really be implemented? - Can the IPV6_ADDRFORM option really be implemented?
- An interface is needed to allocate flow labels. Should one be
defined in this spec, or left for another document?
- Can existing IPv4 applications interoperate with IPv6 nodes? - Can existing IPv4 applications interoperate with IPv6 nodes?
This issue is discussed in more detail in the following section. This issue is discussed in more detail in the following section.
8.1. IPv4 Applications Interoperating with IPv6 Nodes 8.1. IPv4 Applications Interoperating with IPv6 Nodes
This problem primarily has to do with the how IPv4 applications This problem primarily has to do with the how IPv4 applications
represent addresses of IPv6 nodes. What address should be returned to represent addresses of IPv6 nodes. What address should be returned
the application when an IPv6/UDP packet is received, or an IPv6/TCP to the application when an IPv6/UDP packet is received, or an
connection is accepted? The peer's address could be any arbitrary IPv6/TCP connection is accepted? The peer's address could be any
128-bit IPv6 address. But the application is only equipped to deal with arbitrary 128-bit IPv6 address. But the application is only equipped
32-bit IPv4 addresses encoded in sockaddr_in data structures. to deal with 32-bit IPv4 addresses encoded in sockaddr_in data
structures.
We have not discovered any solution that provides complete transparent We have not discovered any solution that provides complete
interoperability with IPv6 nodes for applications using the original transparent interoperability with IPv6 nodes for applications using
IPv4 API. However, two techniques that partially solve the problem are: the original IPv4 API. However, two techniques that partially solve
the problem are:
1) Prohibit communication between IPv4 applications and IPv6 nodes. 1) Prohibit communication between IPv4 applications and IPv6 nodes.
Only UDP packets received from IPv4 nodes would be passed up to Only UDP packets received from IPv4 nodes would be passed up to
the application, and only TCP connections received from IPv4 the application, and only TCP connections received from IPv4
nodes would be accepted. UDP packets from IPv6 nodes would be nodes would be accepted. UDP packets from IPv6 nodes would be
dropped, and TCP connections from IPv6 nodes would be refused. dropped, and TCP connections from IPv6 nodes would be refused.
2) The system could generate a local 32-bit cookie to represent the 2) The system could generate a local 32-bit cookie to represent the
full 128-bit IPv6 address, and pass this value to the full 128-bit IPv6 address, and pass this value to the
application. The system would maintain a mapping from cookie application. The system would maintain a mapping from cookie
value into the 128-bit IPv6 address that it represents. When value into the 128-bit IPv6 address that it represents. When the
the application passed a cookie back into the system (for application passed a cookie back into the system (for example, in
example, in a sendto() or connect() call) the system would use a sendto() or connect() call) the system would use the 128-bit
the 128-bit IPv6 address that the cookie represents. IPv6 address that the cookie represents.
The cookie would have to be chosen so as to be an invalid IPv4 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 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 have to make sure that these cookie values did not escape into
the Internet as the source or destination addresses of IPv4 the Internet as the source or destination addresses of IPv4
packets. packets.
Both of these techniques have drawbacks. This is an area for further Both of these techniques have drawbacks. This is an area for further
study. System implementors may use one of these techniques or implement study. System implementors may use one of these techniques or
another solution. implement another solution.
Acknowledgments Acknowledgments
Thanks to the many people who made suggestions and provided feedback to Thanks to the many people who made suggestions and provided feedback
to the numerous revisions of this document, including: Ran Atkinson, to to the numerous revisions of this document, including: Werner
Fred Baker, Dave Borman, Andrew Cherenson, Alex Conta, Alan Cox, Steve Almesberger, Ran Atkinson, Fred Baker, Dave Borman, Andrew Cherenson,
Deering, Francis Dupont, Robert Elz, Marc Hasson, Tom Herbert, Christian Alex Conta, Alan Cox, Steve Deering, Francis Dupont, Robert Elz, Marc
Huitema, Wan-Yen Hsu, Alan Lloyd, Charles Lynn, Dan McDonald, Craig Hasson, Tom Herbert, Christian Huitema, Wan-Yen Hsu, Alan Lloyd,
Metz, Erik Nordmark, Josh Osborne, Richard Stevens, Matt Thomas, Dean D. Charles Lynn, Dan McDonald, Craig Metz, Erik Nordmark, Josh Osborne,
Throop, Glenn Trewitt, David Waitzman, and Carl Williams. Craig Craig Partridge, Richard Stevens, Matt Thomas, Dean D. Throop, Glenn
Partridge suggested the addr2ascii() and ascii2addr() functions. Trewitt, Paul Vixie, David Waitzman, and Carl Williams. The
getnameinfo() function was based on the getinfobysockaddr() function
defined by Keith Sklower.
Ramesh Govindan made a number of contributions and co-authored an Ramesh Govindan made a number of contributions and co-authored an
earlier version of this paper. earlier version of this paper.
References References
[1] R. Hinden. "Internet Protocol, Version 6 (IPv6) Specification". [1] S. Deering, R. Hinden. "Internet Protocol, Version 6 (IPv6)
Internet Draft. June 1995. Specification". RFC 1883. December 1995.
[2] Keith Sklower. "Getconninfo(): An alternative to Gethostbyname()"
Internet Draft. June 1995.
[3] R. Hinden., S. Deering. "IP Version 6 Addressing Architecture". [2] R. Hinden, S. Deering. "IP Version 6 Addressing Architecture".
Internet Draft. June 1995. RFC 1884. December 1995.
[4] D. McDonald. "IPv6 Security API for BSD Sockets". Internet [3] D. McDonald. "IPv6 Security API for BSD Sockets". Internet
Draft. January 1995. Draft. January 1995.
[4] IEEE, "Protocol Independent Interfaces", IEEE Std 1003.1g, DRAFT
6.3. November 1995.
[5] R. Gilligan, E. Nordmark. "Transition Mechanisms for IPv6 Hosts
and Routers". RFC 1933. April 1996.
Authors' Address Authors' Address
Jim Bound Jim Bound
Digital Equipment Corporation Digital Equipment Corporation
110 Spitbrook Road ZK3-3/U14 110 Spitbrook Road ZK3-3/U14
Nashua, NH 03062-2698 Nashua, NH 03062-2698
Phone: +1 603 881 0400 Phone: +1 603 881 0400
Email: bound@zk3.dec.com Email: bound@zk3.dec.com
Susan Thomson Susan Thomson
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