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Network Working Group E. Nordmark
Request for Comments: 5014 Sun Microsystems, Inc.
Category: Informational S. Chakrabarti
Azaire Networks
J. Laganier
DoCoMo Euro-Labs
September 2007
IPv6 Socket API for Source Address Selection
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
The IPv6 default address selection document (RFC 3484) describes the
rules for selecting source and destination IPv6 addresses, and
indicates that applications should be able to reverse the sense of
some of the address selection rules through some unspecified API.
However, no such socket API exists in the basic (RFC 3493) or
advanced (RFC 3542) IPv6 socket API documents. This document fills
that gap partially by specifying new socket-level options for source
address selection and flags for the getaddrinfo() API to specify
address selection based on the source address preference in
accordance with the socket-level options that modify the default
source address selection algorithm. The socket API described in this
document will be particularly useful for IPv6 applications that want
to choose between temporary and public addresses, and for Mobile IPv6
aware applications that want to use the care-of address for
communication. It also specifies socket options and flags for
selecting Cryptographically Generated Address (CGA) or non-CGA source
addresses.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . 5
3. Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Design Alternatives . . . . . . . . . . . . . . . . . . . . . 6
5. Address Preference Flags . . . . . . . . . . . . . . . . . . . 7
6. Additions to the Socket Interface . . . . . . . . . . . . . . 9
7. Additions to the Protocol-Independent Nodename Translation . . 10
8. Application Requirements . . . . . . . . . . . . . . . . . . . 11
9. Usage Example . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 13
11. Mapping to Default Address Selection Rules . . . . . . . . . . 14
12. IPv4-Mapped IPv6 Addresses . . . . . . . . . . . . . . . . . . 16
13. Validating Source Address Preferences . . . . . . . . . . . . 16
14. Summary of New Definitions . . . . . . . . . . . . . . . . . . 19
15. Security Considerations . . . . . . . . . . . . . . . . . . . 19
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
17.1. Normative References . . . . . . . . . . . . . . . . . . 20
17.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Per-Packet Address Selection Preference . . . . . . . 21
Appendix B. Intellectual Property Statement . . . . . . . . . . . 22
1. Introduction
[RFC3484] specifies the default address selection rules for IPv6
[RFC2460]. This document defines socket API extensions that allow
applications to override the default choice of source address
selection. It therefore indirectly affects the destination address
selection through getaddrinfo(). Privacy considerations [RFC3041]
have introduced "public" and "temporary" addresses. IPv6 Mobility
[RFC3775] introduces "home address" and "care-of address" definitions
in the mobile systems.
The default address selection rules in [RFC3484], in summary, are
that a public address is preferred over a temporary address, that a
mobile IPv6 home address is preferred over a care-of address, and
that a larger scope address is preferred over a smaller scope
address. Although it is desirable to have default rules for address
selection, an application may want to reverse certain address
selection rules for efficiency and other application-specific
reasons.
Currently, IPv6 socket API extensions provide mechanisms to choose a
specific source address through simple bind() operation or
IPV6_PKTINFO socket option [RFC3542]. However, in order to use
bind() or IPV6_PKTINFO socket option, the application itself must
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make sure that the source address is appropriate for the destination
address (e.g., with respect to the interface used to send packets to
the destination). The application also needs to verify the
appropriateness of the source address scope with respect to the
destination address and so on. This can be quite complex for the
application, since in effect, it needs to implement all the default
address selection rules in order to change its preference with
respect to one of the rules.
The mechanism presented in this document allows the application to
specify attributes of the source addresses it prefers while still
having the system perform the rest of the address selection rules.
For instance, if an application specifies that it prefers to use a
care-of address over a home address as the source address and if the
host has two care-of addresses, one public and one temporary, then
the host would select the public care-of address by following the
default address selection rule for preferring a public over a
temporary address.
A socket option has been deemed useful for this purpose, as it
enables an application to specify address selection preferences on a
per-socket basis. It can also provide the flexibility of enabling
and disabling address selection preferences in non-connected (UDP)
sockets. The socket option uses a set of flags for specifying
address selection preferences. Since the API should not assume a
particular implementation method of the address selection [RFC3484]
in the network layer or in getaddrinfo(), the corresponding set of
flags are also defined for getaddrinfo(), as it depends on the source
address selection.
As a result, this document introduces several flags for address
selection preferences that alter the default address selection
[RFC3484] for a number of rules. It analyzes the usefulness of
providing API functionality for different default address selection
rules; it provides API to alter only those rules that are possibly
used by certain classes of applications. In addition, it also
considers CGA [RFC3972] and non-CGA source addresses when CGA
addresses are available in the system. In the future, more source
flags may be added to expand the API as the needs may arise.
The approach in this document is to allow the application to specify
preferences for address selection and not to be able to specify hard
requirements. For instance, an application can set a flag to prefer
a temporary source address, but if no temporary source addresses are
available at the node, a public address would be chosen instead.
Specifying hard requirements for address selection would be
problematic for several reasons. The major one is that, in the vast
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majority of cases, the application would like to be able to
communicate even if an address with the 'optimal' attributes is not
available. For instance, an application that performs very short,
e.g., UDP, transactional exchanges (e.g., DNS queries), might prefer
to use a care-of address when running on a mobile host that is away
from home since this provides a short roundtrip time in many cases.
But if the application is running on a mobile host that is at home,
or running on a host that isn't providing Mobile IPv6, then it
doesn't make sense for the application to fail due to no care-of
address being available. Also, in particular, when using UDP sockets
and the sendto() or sendmsg() primitives, the use of hard
requirements would have been problematic, since the set of available
IP addresses might very well have changed from when the application
called getaddrinfo() until it called sendto() or sendmsg(), which
would introduce new failure modes.
For the few applications that have hard requirements on the
attributes of the IP addresses they use, this document defines a
verification function that allows such applications to properly fail
to communicate when their address selection requirements are not met.
Furthermore, the approach is to define two flags for each rule that
can be modified so that an application can specify its preference for
addresses selected as per the rule, the opposite preference (i.e., an
address selected as per the rule reverted), or choose not to set
either of the flags relating to that rule and leave it up to the
system default (Section 4). This approach allows different
implementations to have different system defaults, and works with
getaddrinfo() as well as setsockopt(). (For setsockopt, a different
approach could have been chosen, but that would still require the
same approach for getaddrinfo.)
Note that this document does not directly modify the destination
address selection rules described in [RFC3484]. An analysis has been
done to see which destination address rules may be altered by the
applications. Rule number 4(prefer home address), 8(prefer smaller
scope), 7(prefer native interfaces) of default address selection
document [RFC3484] were taken into consideration for destination
address alteration. But as of this writing, there was not enough
practical usage for applications to alter destination address
selection rules directly by applying the setsockopt() with a
preferred destination type of address flag. However, this document
does not rule out any possibility of adding flags for preferred
destination address selection. However, [RFC3484] destination
address selection rules are dependent on source address selections,
thus by altering the default source address selection by using the
methods described in this document, one indirectly influences the
choice of destination address selection. Hence, this document
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explains how getaddrinfo() can be used to select the destination
address while taking the preferred source addresses into
consideration (Section 11).
This document specifies extensions only to the Basic IPv6 socket API
specified in [RFC3493]. The intent is that this document serves as a
model for expressing preferences for attributes of IP addresses that
also need to be expressible in other networking API, such as those
found in middleware systems and the Java environment. A similar
model is also applicable for other socket families.
2. Definition Of Terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Address preference flag:
A flag expressing a preference for a particular type of address
(e.g., temporary, public).
Opposite flags:
Each flag expressing an address preference has an "opposite flag"
expressing the opposite preference:
* Home address preference flag is the opposite of the care-of
address preference flag.
* Temporary address preference flag is the opposite of the public
address preference flag.
* CGA address preference flag is the opposite of the non-CGA
address preference flag.
Contradictory flags:
Any combination of flags including both a flag expressing a given
address preference and a flag expressing the opposite preference
constitutes contradictory flags. Such flags are contradictory by
definition of their usefulness with respect to source address
selection. For example, consider a set of flags, including both
the home address preference flag and the care-of address
preference flag. When considering source address selection, the
selected address can be a home address, or a care-of address, but
it cannot be both at the same time. Hence, to prefer an address
that is both a home address and a care-of address is
contradictory.
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3. Usage Scenario
The examples discussed here are limited to applications supporting
Mobile IPv6, IPv6 Privacy Extensions, and Cryptographically Generated
Addresses. Address selection document [RFC3484] recommends that home
addresses should be preferred over care-of address when both are
configured. However, a mobile node may want to prefer a care-of
address as the source address for a DNS query in the foreign network,
as it normally means a shorter and local return path compared to the
route via the mobile node's home-agent when the query contains a home
address as the source address. Another example is the IKE
application, which requires a care-of address as its source address
for the initial security association pair with a Home Agent [RFC3775]
while the mobile node boots up at the foreign network and wants to do
the key exchange before a successful home-registration. Also, a
Mobile IPv6 aware application may want to toggle between the home
address and care-of address, depending on its location and state of
the application. It may also want to open different sockets and use
the home address as the source address for one socket and a care-of
address for the others.
In a non-mobile environment, an application may similarly prefer to
use a temporary address as the source address for certain cases. By
default, the source address selection rule selects "public" address
when both are available. For example, an application supporting Web
browser and mail-server may want to use a "temporary" address for the
former and a "public" address for the mail-server, as a mail-server
may require a reverse path for DNS records for anti-spam rules.
Similarly, a node may be configured to use Cryptographically
Generated Addresses [RFC3972] by default, as in Secure Neighbor
Discovery [RFC3971], but an application may prefer not to use it; for
instance, fping [FPING], a debugging tool that tests basic
reachability of multiple destinations by sending packets in parallel.
These packets may end up initiating neighbor discovery signaling that
uses SEND if used with a CGA source address. SEND performs some
cryptographic operations to prove ownership of the said CGA address.
If the application does not require this feature, it would like to
use a non-CGA address to avoid potentially expensive computations
performed by SEND. On the other hand, when a node is not configured
for CGA as default, an application may prefer using CGA by setting
the corresponding preference.
4. Design Alternatives
Some suggested to have per-application flags instead of per-socket
and per-packet flags. However, this design stays with per-socket and
per-packet flags for the following reasons:
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o While some systems have per-environment/application flags (such as
environment variables in Unix systems) this might not be available
in all systems that implement the socket API.
o When an application links with some standard library, that library
might use the socket API while the application is unaware of that
fact. Mechanisms that would provide per-application flags may
affect not only the application itself but also the libraries,
hence, creating risks of unintended consequences.
Instead of the pair of 'flag' and 'opposite flag' for each rule that
can be modified, the socket option could have been defined to use a
single 'flag' value for each rule. This would still have allowed
different implementations to have different default settings as long
as the applications were coded to first retrieve the default setting
(using getsockopt()), and then clear or set the 'flag' according to
their preferences, and finally set the new value with setsockopt().
But such an approach would not be possible for getaddrinfo() because
all the preferences would need to be expressible in the parameters
that are passed with a single getaddrinfo() call. Hence, for
consistency, the 'flag' and 'opposite flag' approach is used for both
getaddrinfo() and setsockopt().
Thus, in this API document, an application has three choices on
source address selection:
a) The application wants to use an address with flag X: Set flag
X; unset opposite/contradictory flags of X if they are set before.
b) The application wants to use an address with 'opposite' or
contradictory flag of X: Set opposite or contradictory flag of X;
unset flag X, if already set.
c) The application does not care about the presence of flag X and
would like to use default: No need to set any address preference
flags through setsockopt() or getaddrinfo(); unset any address
preference flags if they are set before by the same socket.
5. Address Preference Flags
The following flags are defined to alter or set the default rule of
source address selection rules discussed in default address selection
specification [RFC3484].
IPV6_PREFER_SRC_HOME /* Prefer Home address as source */
IPV6_PREFER_SRC_COA /* Prefer Care-of address as source */
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IPV6_PREFER_SRC_TMP /* Prefer Temporary address as source */
IPV6_PREFER_SRC_PUBLIC /* Prefer Public address as source */
IPV6_PREFER_SRC_CGA /* Prefer CGA address as source */
IPV6_PREFER_SRC_NONCGA /* Prefer a non-CGA address as source */
These flags can be combined together in a flag-set to express more
complex address preferences. However, such combinations can result
in a contradictory flag-set, for example:
IPV6_PREFER_SRC_PUBLIC | IPV6_PREFER_SRC_TMP
IPV6_PREFER_SRC_HOME | IPV6_PREFER_SRC_COA
IPV6_PREFER_SRC_HOME | IPV6_PREFER_SRC_COA | IPV6_PREFER_SRC_TMP
IPV6_PREFER_SRC_CGA | IPV6_PREFER_SRC_NONCGA
Etc.
Examples of valid combinations of address selection flags are given
below:
IPV6_PREFER_SRC_HOME | IPV6_PREFER_SRC_PUBLIC
IPV6_PREFER_SRC_HOME | IPV6_PREFER_SRC_CGA
IPV6_PREFER_SRC_COA | IPV6_PREFER_SRC_PUBLIC | IPV6_PREFER_SRC_CGA
IPV6_PREFER_SRC_HOME | IPV6_PREFER_SRC_NONCGA
If a flag-set includes a combination of 'X' and 'Y', and if 'Y' is
not applicable or available in the system, then the selected address
has attribute 'X' and system default for the attribute 'Y'. For
example, on a system that has only public addresses, the valid
combination of flags:
IPV6_PREFER_SRC_TMP | IPV6_PREFER_SRC_HOME
would result in the selected address being a public home address,
since no temporary addresses are available.
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6. Additions to the Socket Interface
The IPv6 Basic Socket API [RFC3493] defines socket options for IPv6.
To allow applications to influence address selection mechanisms, this
document adds a new socket option at the IPPROTO_IPV6 level. This
socket option is called IPV6_ADDR_PREFERENCES. It can be used with
setsockopt() and getsockopt() calls to set and get the address
selection preferences affecting all packets sent via a given socket.
The socket option value (optval) is a 32-bit unsigned integer
argument. The argument consists of a number of flags where each flag
indicates an address selection preference that modifies one of the
rules in the default address selection specification.
The following flags are defined to alter or set the default rule of
source address selection rules discussed in default address selection
specification [RFC3484]. They are defined as a result of including
the <netinet/in.h> header:
IPV6_PREFER_SRC_HOME /* Prefer Home address as source */
IPV6_PREFER_SRC_COA /* Prefer Care-of address as source */
IPV6_PREFER_SRC_TMP /* Prefer Temporary address as source */
IPV6_PREFER_SRC_PUBLIC /* Prefer Public address as source */
IPV6_PREFER_SRC_CGA /* Prefer CGA address as source */
IPV6_PREFER_SRC_NONCGA /* Prefer a non-CGA address as source */
NOTE: No source preference flag for the longest matching prefix is
defined here because it is believed to be handled by the policy table
defined in the default address selection specification.
When the IPV6_ADDR_PREFERENCES is successfully set with setsockopt(),
the option value given is used to specify the address preference for
any connection initiation through the socket and all subsequent
packets sent via that socket. If no option is set, the system
selects a default value as per default address selection algorithm or
by some other equivalent means.
Setting contradictory flags at the same time results in the error
EINVAL.
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7. Additions to the Protocol-Independent Nodename Translation
Section 8 of the Default Address Selection [RFC3484] document
indicates possible implementation strategies for getaddrinfo()
[RFC3493]. One of them suggests that getaddrinfo() collects
available source/destination pairs from the network layer after being
sorted at the network layer with full knowledge of source address
selection. Another strategy is to call down to the network layer to
retrieve source address information and then sort the list in the
context of getaddrinfo().
This implies that getaddrinfo() should be aware of the address
selection preferences of the application, since getaddrinfo() is
independent of any socket the application might be using.
Thus, if an application alters the default address selection rules by
using setsockopt() with the IPV6_ADDR_PREFERENCES option, the
application should also use the corresponding address selection
preference flags with its getaddrinfo() call.
For that purpose, the addrinfo data structure defined in Basic IPV6
Socket API Extension [RFC3493] has been extended with an extended
"ai_eflags" flag-set field to provide the designers freedom from
adding more flags as necessary without crowding the valuable bit
space in the "ai_flags" flag-set field. The extended addrinfo data
structure is defined as a result of including the <netdb.h> header:
struct addrinfo {
int ai_flags; /* input flags */
int ai_family; /* protocol family for socket */
int ai_socktype; /* socket type */
int ai_protocol; /* protocol for socket */
socklen_t ai_addrlen; /* length of socket address */
char *ai_canonname; /* canonical name for hostname */
struct sockaddr *ai_addr; /* socket address for socket */
struct addrinfo *ai_next; /* pointer to next in list */
int ai_eflags; /* Extended flags for special usage */
};
Note that the additional field for extended flags are added at the
bottom of the addrinfo structure to preserve binary compatibility of
the new functionality with the old applications that use the existing
addrinfo data structure.
A new flag (AI_EXTFLAGS) is defined for the "ai_flags" flag-set field
of the addrinfo data structure to tell the system to look for the
"ai_eflags" extended flag-set field in the addrinfo structure. It is
defined in the <netdb.h> header:
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AI_EXTFLAGS /* extended flag-set present */
If the AI_EXTFLAGS flag is set in "ai_flags" flag-set field of the
addrinfo data structure, then the getaddrinfo() implementation MUST
look for the "ai_eflags" values stored in the extended flag-set field
"ai_eflags" of the addrinfo data structure. The flags stored in the
"ai_eflags" field are only meaningful if the AI_EXTFLAGS flag is set
in the "ai_flags" flag-set field of the addrinfo data structure. By
default, AI_EXTFLAGS is not set in the "ai_flags" flag-set field. If
AI_EXTFLAGS is set in the "ai_flags" flag-set field, and the
"ai_eflags" extended flag-set field is 0 (zero) or undefined, then
AI_EXTFLAGS is ignored.
The IPV6 source address preference values (IPV6_PREFER_SRC_*) defined
for the IPV6_ADDR_PREFERENCES socket option are also defined as
address selection preference flags for the "ai_eflags" extended flag-
set field of the addrinfo data structure, so that getaddrinfo() can
return matching destination addresses corresponding to the source
address preferences expressed by the caller application.
Thus, an application passes source address selection hints to
getaddrinfo by setting AI_EXTFLAGS in the "ai_flags" field of the
addrinfo structure, and the corresponding address selection
preference flags (IPV6_PREFER_SRC_*) in the "ai_eflags" field.
Currently, AI_EXTFLAGS is defined for the AF_INET6 socket protocol
family only. But its usage should be extendable to other socket
protocol families -- such as AF_INET or as appropriate.
If contradictory flags, such as IPV6_PREFER_SRC_HOME and
IPV6_PREFER_SRC_COA, are set in ai_eflags, the getaddrinfo() fails
and return the value EAI_BADEXTFLAGS, defined as a result of
including the <netdb.h> header. This error value MUST be interpreted
into a descriptive text string when passed to the gai_strerror()
function [RFC3493].
8. Application Requirements
An application should call getsockopt() prior to calling setsockopt()
if the application needs to be able to restore the socket back to the
system default preferences. Note that this is suggested for
portability. An application that does not have this requirement can
just use getaddrinfo() while specifying its preferences, followed by:
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uint32_t flags = IPV6_PREFER_SRC_TMP;
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
(void *) &flags, sizeof (flags)) == -1) {
perror("setsockopt IPV6_ADDR_REFERENCES");
}
An application that needs to be able to restore the default settings
on the socket would instead do this:
uint32_t save_flags, flags;
int optlen = sizeof (save_flags);
/* Save the existing IPv6_ADDR_PREFERENCE flags now */
if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
(void *) &save_flags, &optlen) == -1 {
perror("getsockopt IPV6_ADDR_REFERENCES");
}
/* Set the new flags */
flags = IPV6_PREFER_SRC_TMP;
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
(void *) &flags, sizeof (flags)) == -1) {
perror("setsockopt IPV6_ADDR_REFERENCES");
}
/*
*
* Do some work with the socket here.
*
*/
/* Restore the flags */
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
(void *) &save_flags, sizeof (save_flags)) == -1) {
perror("setsockopt IPV6_ADDR_REFERENCES");
}
Applications should not set contradictory flags at the same time.
In order to allow different implementations to do different parts of
address selection in getaddrinfo() and in the protocol stack, this
specification requires that applications set the semantically
equivalent flags when calling getaddrinfo() and setsockopt(). For
example, if the application sets the IPV6_PREFER_SRC_COA flag, it
MUST use the same for the "ai_eflag" field of the addrinfo data
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structure when calling getaddrinfo(). If applications are not
setting the semantically equivalent flags, the behavior of the
implementation is undefined.
9. Usage Example
An example of usage of this API is given below:
struct addrinfo hints, *ai, *ai0;
uint32_t preferences;
preferences = IPV6_PREFER_SRC_TMP;
hints.ai_flags |= AI_EXTFLAGS;
hints.ai_eflags = preferences; /* Chosen address preference flag */
/* Fill in other hints fields */
getaddrinfo(....,&hints,. &ai0..);
/* Loop over all returned addresses and do connect */
for (ai = ai0; ai; ai = ai->ai_next) {
s = socket(ai->ai_family, ...);
setsockopt(s, IPV6_ADDR_PREFERENCES, (void *) &preferences,
sizeof (preferences));
if (connect(s, ai->ai_addr, ai->ai_addrlen) == -1){
close (s);
s = -1;
continue;
}
break;
}
freeaddrinfo(ai0);
10. Implementation Notes
o Within the same application, if a specific source address is set
by either bind() or IPV6_PKTINFO socket option, while at the same
time an address selection preference is expressed with the
IPV6_ADDR_PREFERENCES socket option, then the source address
setting carried by bind() or IPV6_PKTINFO takes precedence over
the address selection setting.
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o setsockopt() and getaddrinfo() should silently ignore any address
preference flags that are not supported in the system. For
example, a host that does not implement Mobile IPv6, should not
fail setsockopt() or getaddrinfo() that specify preferences for
home or care-of addresses. The socket option calls should return
error (-1) and set errno to EINVAL when contradictory flags values
are passed to them.
o If an implementation supports both stream and datagram sockets, it
should implement the address preference mechanism API described in
this document on both types of sockets.
o An implementation supporting this API MUST implement both
getaddrinfo() extension flags and socket option flags processing
for portability of applications.
o The following flags are set as default values on a system (which
is consistent with [RFC3484] defaults):
IPV6_PREFER_SRC_HOME
IPV6_PREFER_SRC_PUBLIC
IPV6_PREFER_SRC_CGA
11. Mapping to Default Address Selection Rules
This API defines only those flags that are deemed to be useful by the
applications to alter default address selection rules. Thus, we
discuss the mapping of each set of flags to the corresponding rule
number in the address selection document [RFC3484].
Source address selection rule #4 (prefer home address):
IPV6_PREFER_SRC_HOME (default)
IPV6_PREFER_SRC_COA
Source address selection rule #7 (prefer public address):
IPV6_PREFER_SRC_PUBLIC (default)
IPV6_PREFER_SRC_TMP
At this time, this document does not define flags to alter source
address selection rule #2 (prefer appropriate scope for destination)
and destination address selection rule #8 (prefer smaller scope), as
the implementers felt that there were no practical applications that
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can take advantage of reverting the scoping rules of IPv6 default
address selection. Flags altering other destination address
selection rules (#4, prefer home address and #7, prefer native
transport) could have applications, but the problem is that the local
system cannot systematically determine whether a destination address
is a tunnel address for destination rule #7 (although it can when the
destination address is one of its own, or can be syntactically
recognized as a tunnel address, e.g., a 6-to-4 address.) The flags
defined for source address selection rule #4 (prefer home address)
should also take care of destination address selection rule #4.
Thus, at this point, it was decided not to define flags for these
destination rules.
Also, note that there is no corresponding destination address
selection rule for source address selection rule #7 (prefer public
addresses) of default address selection document [RFC3484]. However,
this API provides a way for an application to make sure that the
source address preference set in setsockopt() is taken into account
by the getaddrinfo() function. Let's consider an example to
understand this scenario. DA and DB are two global destination
addresses and the node has two global source addresses SA and SB
through interface A and B respectively. SA is a temporary address
while SB is a public address. The application has set
IPV6_PREFER_SRC_TMP in the setsockopt() flag. The route to DA points
to interface A and the route to DB points to interface B. Thus, when
AI_EXTFLAGS in ai_flags and IPV6_PREFER_SRC_TMP in ai_eflags are set,
getaddrinfo() returns DA before DB in the list of destination
addresses and thus, SA will be used to communicate with the
destination DA. Similarly, getaddrinfo() returns DB before DA when
AI_EXTFLAGS and ai_eflags are set to IPV6_PREFER_SRC_PUBLIC. Thus,
the source address preference is taking effect into destination
address selection as well as source address selection by the
getaddrinfo() function.
The following numerical example clarifies the above further.
Imagine a host with two addresses:
1234::1:1 public
9876::1:2 temporary
The destination has the following two addresses:
1234::9:3
9876::9:4
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By default, getaddrinfo() will return the destination addresses in
the following order:
1234::9:3
9876::9:4
because the public source is preferred and 1234 matches more bits
with the public source address. On the other hand, if ai_flags is
set to AI_EXTFLAGS and ai_eflags to IPV6_PREFER_SRC_TMP, getaddrinfo
will return the addresses in the reverse order since the temporary
source address will be preferred.
Other source address rules (that are not mentioned here) were also
deemed not applicable for changing its default on a per-application
basis.
12. IPv4-Mapped IPv6 Addresses
IPv4-mapped IPv6 addresses for AF_INET6 sockets are supported in this
API. In some cases, the application of IPv4-mapped addresses are
limited because the API attributes are IPv6 specific. For example,
IPv6 temporary addresses and cryptographically generated addresses
have no IPv4 counterparts. Thus, the IPV6_PREFER_SRC_TMP or
IPV6_PREFER_SRC_CGA are not directly applicable to an IPv4-mapped
IPv6 address. However, the IPv4-mapped address support may be useful
for mobile-IPv4 applications shifting the source address between the
home address and the care-of address. Thus, the IPV6_PREFER_SRC_COA
and IPV6_PREFER_SRC_HOME are applicable to an IPv4-mapped IPv6
address. At this point, it is not well understood whether this
particular API has any value to IPv4 addresses or AF_INET family of
sockets, but a similar model still applies to AF_INET socket family
if corresponding address flags are defined.
13. Validating Source Address Preferences
Sometimes an application may have a requirement to only use addresses
with some particular attribute, and if no such address is available,
the application should fail to communicate instead of communicating
using the 'wrong' address. In that situation, address selection
preferences do not guarantee that the application requirements are
met. Instead, the application has to use a new call that binds a
socket to the source address that would be selected to communicate
with a given destination address, according to its preferences, and
then explicitly verify that the chosen address satisfies its
requirements using a validation function. Such an application would
go through the following steps:
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1. The application specifies one or more IPV6_PREFER_SRC_* flags and
AI_EXTFLAGS ai_flags with getaddrinfo().
2. The application specifies the same IPV6_PREFER_SRC_* flags with
setsockopt().
3. The application calls the stack to select a source address to
communicate with the specified destination address, according to
the expressed address selection preferences. This is achieved
with a connect() call, or a bind2addrsel() call as specified
below. The connect() function must not be used when the
application uses connection-oriented communication (e.g., TCP)
and want to ensure that no single packet (e.g., TCP SYN) is sent
before the application could verify that its requirements were
fulfilled. Instead, the application must use the newly
introduced bind2addrsel() call, which binds a socket to the
source address that would be selected to communicate with a given
destination address, according to the application's preferences.
For datagram-oriented communications (e.g., UDP), the connect()
call can be used since it results in the stack selecting a source
address without sending any packets.
4. Retrieve the selected source address using the getsockname() API
call.
5. Verify with the validation function that the retrieved address is
satisfactory as specified below. If not, abort the
communication, e.g., by closing the socket.
The binding of the socket to the address that would be selected to
communicate with a given destination address, according to the
application preferences, is accomplished via a new binding function
defined for this purpose:
#include <netinet/in.h>
int bind2addrsel(int s, const struct sockaddr *dstaddr,
socklen_t dstaddrlen);
where s is the socket that source address selection preferences have
been expressed by the application, the dstaddr is a non-NULL pointer
to a sockaddr_in6 structure initialized as follows:
o sin6_addr is a 128-bit IPv6 destination address with which the
local node wants to communicate;
o sin6_family MUST be set to AF_INET6;
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o sin6_scope_id MUST be set if the address is link-local;
and dstaddrlen is the size of the sockaddr structure passed as
argument.
The bind2addrsel() call is defined to return the same values as the
bind() call, i.e., 0 if successful, -1 otherwise while the global
variable errno is set to indicate the error. The bind2addrsel() call
fails for the same reasons that the bind() call.
The verification of temporary vs. public, home vs. care-of, CGA vs.
not, are performed by a new validation function defined for this
purpose:
#include <netinet/in.h>
short inet6_is_srcaddr(struct sockaddr_in6 *srcaddr,
uint32_t flags);
where the flags contain the specified IPV6_PREFER_SRC_* source
preference flags, and the srcaddr is a non-NULL pointer to a
sockaddr_in6 structure initialized as follows:
o sin6_addr is a 128-bit IPv6 address of the local node.
o sin6_family MUST be set to AF_INET6.
o sin6_scope_id MUST be set if the address is link-local.
inet6_is_srcaddr() is defined to return three possible values (0, 1,
-1): The function returns true (1) when the IPv6 address corresponds
to a valid address in the node and satisfies the given preference
flags. If the IPv6 address input value does not correspond to any
address in the node or if the flags are not one of the valid
preference flags, it returns a failure (-1). If the input address
does not match an address that satisfies the preference flags
indicated, the function returns false (0.)
This function can handle multiple valid preference flag combinations
as its second parameter, for example, IPV6_PREFER_SRC_COA |
IPV6_PREFER_SRC_TMP, which means that all flags MUST be satisfied for
the result to be true. Contradictory flag values result in a false
return value.
The function will return true for IPV6_PREFER_SRC_HOME even if the
host is not implementing mobile IPv6, as well as for a mobile node
that is at home (i.e., does not have any care-of address).
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14. Summary of New Definitions
The following list summarizes the constants, structure, and extern
definitions discussed in this memo, sorted by header.
<netdb.h> AI_EXTFLAGS
<netdb.h> IPV6_PREFER_SRC_HOME
<netdb.h> IPV6_PREFER_SRC_COA
<netdb.h> IPV6_PREFER_SRC_TMP
<netdb.h> IPV6_PREFER_SRC_PUBLIC
<netdb.h> IPV6_PREFER_SRC_CGA
<netdb.h> IPV6_PREFER_SRC_NONCGA
<netdb.h> EAI_BADEXTFLAGS
<netdb.h> struct addrinfo{};
<netinet/in.h> IPV6_PREFER_SRC_HOME
<netinet/in.h> IPV6_PREFER_SRC_COA
<netinet/in.h> IPV6_PREFER_SRC_TMP
<netinet/in.h> IPV6_PREFER_SRC_PUBLIC
<netinet/in.h> IPV6_PREFER_SRC_CGA
<netinet/in.h> IPV6_PREFER_SRC_NONCGA
<netinet/in.h> short inet6_is_srcaddr(struct sockaddr_in6 *,
uint32_t);
<netinet/in.h> int bind2addrsel(int, const struct sockaddr *,
socklen_t);
15. Security Considerations
This document conforms to the same security implications as specified
in the Basic IPv6 socket API [RFC3493] and address selection rules
[RFC3484]. Allowing applications to specify a preference for
temporary addresses provides per-application (and per-socket) ability
to use the privacy benefits of the temporary addresses. The setting
of certain address preferences (e.g., not using a CGA address, or not
using a temporary address) may be restricted to privileged processes
because of security implications.
16. Acknowledgments
The authors like to thank members of Mobile-IP and IPV6 working
groups for useful discussion on this topic. Richard Draves and Dave
Thaler suggested that getaddrinfo also needs to be considered along
with the new socket option. Gabriel Montenegro suggested that CGAs
may also be considered in this document. Thanks to Alain Durand,
Renee Danson, Alper Yegin, Francis Dupont, Keiichi Shima, Michael
Hunter, Sebastien Roy, Robert Elz, Pekka Savola, Itojun, Jim Bound,
Jeff Boote, Steve Cipolli, Vlad Yasevich, Mika Liljeberg, Ted Hardie,
Vidya Narayanan, and Lars Eggert for useful discussions and
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RFC 5014 Socket API for Source Address Selection September 2007
suggestions. Thanks to Remi Denis-Courmont, Brian Haberman, Brian
Haley, Bob Gilligan, Jack McCann, Jim Bound, Jinmei Tatuya, Suresh
Krishnan, Hilarie Orman, Geoff Houston, Marcelo Bungulo, and Jari
Arkko for the review of this document and suggestions for
improvement.
17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
17.2. Informative References
[FPING] "Fping - a program to ping hosts in parallel", Online web
site http://www.fping.com.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
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Appendix A. Per-Packet Address Selection Preference
This document discusses setting source address selection preferences
on a per-socket basis with the new IPV6_ADDR_PREFERENCES socket
option used in setsockopt(). The document does not encourage setting
the source address selection preference on a per-packet basis through
the use of ancillary data objects with sendmsg(), or setsockopt()
with unconnected datagram sockets.
Per-packet source address selection is expensive, as the system will
have to determine the source address indicated by the application
preference before sending each packet, while setsockopt() address
preference on a connected socket makes the selection once and uses
that source address for all packets transmitted through that socket
endpoint, as long as the socket option is set.
However, this document provides guidelines for those implementations
that like to have an option on implementing transmit-side ancillary
data object support for altering default source address selection.
Therefore, if an application chooses to use the per-packet source
address selection, then the implementation should process at the
IPPROTO_IPV6 level (cmsg_level) ancillary data object of type
(cmsg_type) IPV6_ADDR_PREFERENCES containing as data (cmsg_data[]) a
32-bit unsigned integer encoding the source address selection
preference flags (e.g., IPV6_PREFER_SRC_COA | IPV6_PREFER_SRC_PUBLIC)
in a fashion similar to the advanced IPV6 Socket API [RFC3542]. This
address selection preference ancillary data object may be present
along with other ancillary data objects.
The implementation processing the ancillary data object is
responsible for the selection of the preferred source address as
indicated in the ancillary data object. Thus, an application can use
sendmsg() to pass an address selection preference ancillary data
object to the IPv6 layer. The following example shows usage of the
ancillary data API for setting address preferences:
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void *extptr;
socklen_t extlen;
struct msghdr msg;
struct cmsghdr *cmsgptr;
int cmsglen;
struct sockaddr_in6 dest;
uint32_t flags;
extlen = sizeof(flags);
cmsglen = CMSG_SPACE(extlen);
cmsgptr = malloc(cmsglen);
cmsgptr->cmsg_len = CMSG_LEN(extlen);
cmsgptr->cmsg_level = IPPROTO_IPV6;
cmsgptr->cmsg_type = IPV6_ADDR_PREFERENCES;
extptr = CMSG_DATA(cmsgptr);
flags = IPV6_PREFER_SRC_COA;
memcpy(extptr, &flags, extlen);
msg.msg_control = cmsgptr;
msg.msg_controllen = cmsglen;
/* finish filling in msg{} */
msg.msg_name = dest;
sendmsg(s, &msg, 0);
Thus, when an IPV6_ADDR_PREFERENCES ancillary data object is passed
to sendmsg(), the value included in the object is used to specify
address preference for the packet being sent by sendmsg().
Appendix B. Intellectual Property Statement
This document only defines a source preference flag to choose
Cryptographically Generated Address (CGA) as the source address when
applicable. CGAs are obtained using public keys and hashes to prove
address ownership. Several IPR claims have been made about such
methods.
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Authors' Addresses
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Menlo Park, CA 94025
USA
EMail: Erik.Nordmark@Sun.com
Samita Chakrabarti
Azaire Networks
3121 Jay Street, Suite 210
Santa Clara, CA 95054
USA
EMail: samitac2@gmail.com
Julien Laganier
DoCoMo Euro-Labs
Landsbergerstrasse 312
D-80687 Muenchen
Germany
EMail: julien.IETF@laposte.net
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
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