Internet Engineering Task Force (IETF) F. Gont

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 8981 SI6 Networks
Obsoletes: 4941 S. Krishnan
Category: Standards Track Kaloom
ISSN: 2070-1721 T. Narten

R. Draves
Microsoft Research
February 2021

Temporary Address Extensions for Stateless Address Autoconfiguration in
IPv6

Abstract

This document describes an extension to IPv6 Stateless Address
Autoconfiguration that causes hosts to generate temporary addresses
with randomized interface identifiers for each prefix advertised with
autoconfiguration enabled. Changing addresses over time limits the
window of time during which eavesdroppers and other information
collectors may trivially perform address-based network-activity
correlation when the same address is employed for multiple
transactions by the same host. Additionally, it reduces the window
of exposure of a host as being accessible via an address that becomes
revealed as a result of active communication. This document
obsoletes RFC 4941.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8981.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
1.1. Terminology
1.2. Problem Statement
2. Background
2.1. Extended Use of the Same Identifier
2.2. Possible Approaches
3. Protocol Description
3.1. Design Guidelines
3.2. Assumptions
3.3. Generation of Randomized IIDs
3.3.1. Simple Randomized IIDs
3.3.2. Generation of IIDs with Pseudorandom Functions
3.4. Generating Temporary Addresses
3.5. Expiration of Temporary Addresses
3.6. Regeneration of Temporary Addresses
3.7. Implementation Considerations
3.8. Defined Protocol Parameters and Configuration Variables
4. Implications of Changing IIDs
5. Significant Changes from RFC 4941
6. Future Work
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

[RFC4862] specifies Stateless Address Autoconfiguration (SLAAC) for
IPv6, which typically results in hosts configuring one or more
"stable" IPv6 addresses composed of a network prefix advertised by a
local router and a locally generated interface identifier (IID). The
security and privacy implications of such addresses have been
discussed in detail in [RFC7721], [RFC7217], and [RFC7707]. This
document specifies an extension to SLAAC for generating temporary
addresses that can help mitigate some of the aforementioned issues.
This document is a revision of RFC 4941 and formally obsoletes it.
Section 5 describes the changes from [RFC4941].

The default address selection for IPv6 has been specified in
[RFC6724]. In some cases, the determination as to whether to use
stable versus temporary addresses can only be made by an application.
For example, some applications may always want to use temporary
addresses, while others may want to use them only in some
circumstances or not at all. An Application Programming Interface
(API) such as that specified in [RFC5014] can enable individual
applications to indicate a preference for the use of temporary
addresses.

Section 2 provides background information. Section 3 describes a
procedure for generating temporary addresses. Section 4 discusses
implications of changing IIDs. Section 5 describes the changes from
[RFC4941].

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

The terms "public address", "stable address", "temporary address",
"constant IID", "stable IID", and "temporary IID" are to be
interpreted as specified in [RFC7721].

The term "global-scope addresses" is used in this document to
collectively refer to "Global unicast addresses" as defined in
[RFC4291] and "Unique local addresses" as defined in [RFC4193], and
not to "globally reachable addresses" as defined in [RFC8190].

1.2. Problem Statement

Addresses generated using SLAAC [RFC4862] contain an embedded
interface identifier, which may remain stable over time. Anytime a
fixed identifier is used in multiple contexts, it becomes possible to
correlate seemingly unrelated activity using this identifier.

The correlation can be performed by:

* An attacker who is in the path between the host in question and
the peer(s) to which it is communicating, who can view the IPv6
addresses present in the datagrams.

* An attacker who can access the communication logs of the peers
with which the host has communicated.

Since the identifier is embedded within the IPv6 address, it cannot
be hidden. This document proposes a solution to this issue by
generating interface identifiers that vary over time.

Note that an attacker, who is on path, may be able to perform
significant correlation based on:

* The payload contents of unencrypted packets on the wire.

* The characteristics of the packets, such as packet size and
timing.

Use of temporary addresses will not prevent such correlation, nor
will it prevent an on-link observer (e.g., the host's default router)
from tracking all the host's addresses.

2. Background

This section discusses the problem in more detail, provides context
for evaluating the significance of the concerns in specific
environments, and makes comparisons with existing practices.

2.1. Extended Use of the Same Identifier

The use of a non-changing IID to form addresses is a specific
instance of the more general case where a constant identifier is
reused over an extended period of time and in multiple independent
activities. Anytime the same identifier is used in multiple
contexts, it becomes possible for that identifier to be used to
correlate seemingly unrelated activity. For example, a network
sniffer placed strategically on a link traversed by all traffic to/
from a particular host could keep track of which destinations a host
communicated with and at what times. In some cases, such information
can be used to infer things, such as what hours an employee was
active, when someone is at home, etc. Although it might appear that
changing an address regularly in such environments would be desirable
to lessen privacy concerns, it should be noted that the network-
prefix portion of an address also serves as a constant identifier.
All hosts at, say, a home would have the same network prefix, which
identifies the topological location of those hosts. This has
implications for privacy, though not at the same granularity as the
concern that this document addresses. Specifically, all hosts within
a home could be grouped together for the purposes of collecting
information. If the network contains a very small number of hosts --
say, just one -- changing just the IID will not enhance privacy,
since the prefix serves as a constant identifier.

One of the requirements for correlating seemingly unrelated
activities is the use (and reuse) of an identifier that is
recognizable over time within different contexts. IP addresses
provide one obvious example, but there are more. For example:

* Many hosts also have DNS names associated with their addresses, in
which case, the DNS name serves as a similar identifier. Although
the DNS name associated with an address is more work to obtain (it
may require a DNS query), the information is often readily
available. In such cases, changing the address on a host over
time would do little to address the concerns raised in this
document, unless the DNS name is also changed at the same time
(see Section 4).

* Web browsers and servers typically exchange "cookies" with each
other [RFC6265]. Cookies allow web servers to correlate a current
activity with a previous activity. One common usage is to send
back targeted advertising to a user by using the cookie supplied
by the browser to identify what earlier queries had been made
(e.g., for what type of information). Based on the earlier
queries, advertisements can be targeted to match the (assumed)
interests of the end user.

The use of a constant identifier within an address is of special
concern, because addresses are a fundamental requirement of
communication and cannot easily be hidden from eavesdroppers and
other parties. Even when higher layers encrypt their payloads,
addresses in packet headers appear in the clear. Consequently, if a
mobile host (e.g., laptop) accessed the network from several
different locations, an eavesdropper might be able to track the
movement of that mobile host from place to place, even if the upper-
layer payloads were encrypted.

Changing addresses over time limits the time window over which
eavesdroppers and other information collectors may trivially
correlate network activity when the same address is employed for
multiple transactions by the same host. Additionally, it reduces the
window of exposure during which a host is accessible via an address
that becomes revealed as a result of active communication.

The security and privacy implications of IPv6 addresses are discussed
in detail in [RFC7721], [RFC7707], and [RFC7217].

2.2. Possible Approaches

One approach, compatible with the SLAAC architecture, would be to
change the IID portion of an address over time. Changing the IID can
make it more difficult to look at the IP addresses in independent
transactions and identify which ones actually correspond to the same
host, both in the case where the routing-prefix portion of an address
changes and when it does not.

Many hosts function as both clients and servers. In such cases, the
host would need a name (e.g., a DNS domain name) for its use as a
server. Whether the address stays fixed or changes has little impact
on privacy, since the name remains constant and serves as a constant
identifier. However, when acting as a client (e.g., initiating
communication), such a host may want to vary the addresses it uses.
In such environments, one may need multiple addresses: a stable
address associated with the name, which is used to accept incoming
connection requests from other hosts, and a temporary address used to
shield the identity of the client when it initiates communication.

On the other hand, a host that functions only as a client may want to
employ only temporary addresses for public communication.

To make it difficult to make educated guesses as to whether two
different IIDs belong to the same host, the algorithm for generating
alternate identifiers must include input that has an unpredictable
component from the perspective of the outside entities that are
collecting information.

3. Protocol Description

The following subsections define the procedures for the generation of
IPv6 temporary addresses.

3.1. Design Guidelines

Temporary addresses observe the following properties:

1. Temporary addresses are typically employed for initiating
outgoing sessions.

2. Temporary addresses are used for a short period of time
(typically hours to days) and are subsequently deprecated.
Deprecated addresses can continue to be used for established
connections but are not used to initiate new connections.

3. New temporary addresses are generated over time to replace
temporary addresses that expire (i.e., become deprecated and
eventually invalidated).

4. Temporary addresses must have a limited lifetime (limited "valid
lifetime" and "preferred lifetime" from [RFC4862]). The lifetime
of an address should be further reduced when privacy-meaningful
events (such as a host attaching to a different network, or the
regeneration of a new randomized Media Access Control (MAC)
address) take place. The lifetime of temporary addresses must be
statistically different for different addresses, such that it is
hard to predict or infer when a new temporary address is
generated or correlate a newly generated address with an existing
one.

5. By default, one address is generated for each prefix advertised
by SLAAC. The resulting interface identifiers must be
statistically different when addresses are configured for
different prefixes or different network interfaces. This means
that, given two addresses, it must be difficult for an outside
entity to infer whether the addresses correspond to the same host
or network interface.

6. It must be difficult for an outside entity to predict the
interface identifiers that will be employed for temporary
addresses, even with knowledge of the algorithm/method employed
to generate them and/or knowledge of the IIDs previously employed
for other temporary addresses. These IIDs must be semantically
opaque [RFC7136] and must not follow any specific patterns.

3.2. Assumptions

The following algorithm assumes that, for a given temporary address,
an implementation can determine the prefix from which it was
generated. When a temporary address is deprecated, a new temporary
address is generated. The specific valid and preferred lifetimes for
the new address are dependent on the corresponding lifetime values
set for the prefix from which it was generated.

Finally, this document assumes that, when a host initiates outgoing
communications, temporary addresses can be given preference over
stable addresses (if available), when the device is configured to do
so. [RFC6724] mandates that implementations provide a mechanism that
allows an application to configure its preference for temporary
addresses over stable addresses. It also allows an implementation to
prefer temporary addresses by default, so that the connections
initiated by the host can use temporary addresses without requiring
application-specific enablement. This document also assumes that an
API will exist that allows individual applications to indicate
whether they prefer to use temporary or stable addresses and override
the system defaults (see, for example, [RFC5014]).

3.3. Generation of Randomized IIDs

The following subsections specify example algorithms for generating
temporary IIDs that follow the guidelines in Section 3.1 of this
document. The algorithm specified in Section 3.3.1 assumes a
pseudorandom number generator (PRNG) is available on the system. The
algorithm specified in Section 3.3.2 allows for code reuse by hosts
that implement [RFC7217].

3.3.1. Simple Randomized IIDs

One approach is to select a pseudorandom number of the appropriate
length. A host employing this algorithm should generate IIDs as
follows:

1. Obtain a random number from a PRNG that can produce random
numbers of at least as many bits as required for the IID (please
see the next step). [RFC4086] specifies randomness requirements
for security.

2. The IID is obtained by taking as many bits from the random number
obtained in the previous step as necessary. See [RFC7136] for
the necessary number of bits (i.e., the length of the IID). See
also [RFC7421] for a discussion of the privacy implications of
the IID length. Note: there are no special bits in an IID
[RFC7136].

3. The resulting IID MUST be compared against the reserved IPv6 IIDs
[RFC5453] [IANA-RESERVED-IID] and against those IIDs already
employed in an address of the same network interface and the same
network prefix. In the event that an unacceptable identifier has
been generated, a new IID should be generated by repeating the
algorithm from the first step.

3.3.2. Generation of IIDs with Pseudorandom Functions

The algorithm in [RFC7217] can be augmented for the generation of
temporary addresses. The benefit of this is that a host could employ
a single algorithm for generating stable and temporary addresses by
employing appropriate parameters.

Hosts would employ the following algorithm for generating the
temporary IID:

1. Compute a random identifier with the expression:

RID = F(Prefix, Net_Iface, Network_ID, Time, DAD_Counter,
secret_key)

Where:

RID:
Random Identifier

F():
A pseudorandom function (PRF) that MUST NOT be computable from
the outside (without knowledge of the secret key). F() MUST
also be difficult to reverse, such that it resists attempts to
obtain the secret_key, even when given samples of the output
of F() and knowledge or control of the other input parameters.
F() SHOULD produce an output of at least as many bits as
required for the IID. BLAKE3 (256-bit key, arbitrary-length
output) [BLAKE3] is one possible option for F().
Alternatively, F() could be implemented with a keyed-hash
message authentication code (HMAC) [RFC2104]. HMAC-SHA-256
[FIPS-SHS] is one possible option for such an implementation
alternative. Note: use of HMAC-MD5 [RFC1321] is considered
unacceptable for F() [RFC6151].

Prefix:
The prefix to be used for SLAAC, as learned from an ICMPv6
Router Advertisement message.

Net_Iface:
The MAC address corresponding to the underlying network-
interface card, in the case the link uses IEEE 802 link-layer
identifiers. Employing the MAC address for this parameter
(over the other suggested options in [RFC7217]) means that the
regeneration of a randomized MAC address will result in a
different temporary address.

Network_ID:
Some network-specific data that identifies the subnet to which
this interface is attached -- for example, the IEEE 802.11
Service Set Identifier (SSID) corresponding to the network to
which this interface is associated. Additionally, "Simple
Procedures for Detecting Network Attachment in IPv6" ("Simple
DNA") [RFC6059] describes ideas that could be leveraged to
generate a Network_ID parameter. This parameter SHOULD be
employed if some form of "Network_ID" is available.

Time:
An implementation-dependent representation of time. One
possible example is the representation in UNIX-like systems
[OPEN-GROUP], which measure time in terms of the number of
seconds elapsed since the Epoch (00:00:00 Coordinated
Universal Time (UTC), 1 January 1970). The addition of the
"Time" argument results in (statistically) different IIDs over
time.

DAD_Counter:
A counter that is employed to resolve the conflict where an
unacceptable identifier has been generated. This can be
result of Duplicate Address Detection (DAD), or step 3 below.

secret_key:
A secret key that is not known by the attacker. The secret
key SHOULD be of at least 128 bits. It MUST be initialized to
a pseudorandom number (see [RFC4086] for randomness
requirements for security) when the operating system is
"bootstrapped". The secret_key MUST NOT be employed for any
other purpose than the one discussed in this section. For
example, implementations MUST NOT employ the same secret_key
for the generation of stable addresses [RFC7217] and the
generation of temporary addresses via this algorithm.

2. The IID is finally obtained by taking as many bits from the RID
value (computed in the previous step) as necessary, starting from
the least significant bit. See [RFC7136] for the necessary
number of bits (i.e., the length of the IID). See also [RFC7421]
for a discussion of the privacy implications of the IID length.
Note: there are no special bits in an IID [RFC7136].

3. The resulting IID MUST be compared against the reserved IPv6 IIDs
[RFC5453] [IANA-RESERVED-IID] and against those IIDs already
employed in an address of the same network interface and the same
network prefix. In the event that an unacceptable identifier has
been generated, the DAD_Counter should be incremented by 1, and
the algorithm should be restarted from the first step.

3.4. Generating Temporary Addresses

[RFC4862] describes the steps for generating a link-local address
when an interface becomes enabled, as well as the steps for
generating addresses for other scopes. This document extends
[RFC4862] as follows. When processing a Router Advertisement with a
Prefix Information option carrying a prefix for the purposes of
address autoconfiguration (i.e., the A bit is set), the host MUST
perform the following steps:

1. Process the Prefix Information option as specified in [RFC4862],
adjusting the lifetimes of existing temporary addresses, with the
overall constraint that no temporary addresses should ever remain
"valid" or "preferred" for a time longer than
(TEMP_VALID_LIFETIME) or (TEMP_PREFERRED_LIFETIME -
DESYNC_FACTOR), respectively. The configuration variables
TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME correspond to the
maximum valid lifetime and the maximum preferred lifetime of
temporary addresses, respectively.

Note:
DESYNC_FACTOR is the value computed when the address was
created (see step 4 below).

2. One way an implementation can satisfy the above constraints is to
associate with each temporary address a creation time (called
CREATION_TIME) that indicates the time at which the address was
created. When updating the preferred lifetime of an existing
temporary address, it would be set to expire at whichever time is
earlier: the time indicated by the received lifetime or
(CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR). A
similar approach can be used with the valid lifetime.

Note:
DESYNC_FACTOR is the value computed when the address was
created (see step 4 below).

3. If the host has not configured any temporary address for the
corresponding prefix, the host SHOULD create a new temporary
address for such prefix.

Note:
For example, a host might implement prefix-specific policies
such as not configuring temporary addresses for the Unique
Local IPv6 Unicast Addresses (ULAs) [RFC4193] prefix.

4. When creating a temporary address, DESYNC_FACTOR MUST be computed
and associated with the newly created address, and the address
lifetime values MUST be derived from the corresponding prefix as
follows:

* Its valid lifetime is the lower of the Valid Lifetime of the
prefix and TEMP_VALID_LIFETIME.

* Its preferred lifetime is the lower of the Preferred Lifetime
of the prefix and TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR.

5. A temporary address is created only if this calculated preferred
lifetime is greater than REGEN_ADVANCE time units. In
particular, an implementation MUST NOT create a temporary address
with a zero preferred lifetime.

6. New temporary addresses MUST be created by appending a randomized
IID to the prefix that was received. Section 3.3 of this
document specifies some sample algorithms for generating the
randomized IID.

7. The host MUST perform DAD on the generated temporary address. If
DAD indicates the address is already in use, the host MUST
generate a new randomized IID and repeat the previous steps as
appropriate (starting from step 4), up to TEMP_IDGEN_RETRIES
times. If, after TEMP_IDGEN_RETRIES consecutive attempts, the
host is unable to generate a unique temporary address, the host
MUST log a system error and SHOULD NOT attempt to generate a
temporary address for the given prefix for the duration of the
host's attachment to the network via this interface. This allows
hosts to recover from occasional DAD failures or otherwise log
the recurrent address collisions.

3.5. Expiration of Temporary Addresses

When a temporary address becomes deprecated, a new one MUST be
generated. This is done by repeating the actions described in
Section 3.4, starting at step 4). Note that, in normal operation,
except for the transient period when a temporary address is being
regenerated, at most one temporary address per prefix should be in a
nondeprecated state at any given time on a given interface. Note
that if a temporary address becomes deprecated as result of
processing a Prefix Information option with a zero preferred
lifetime, then a new temporary address MUST NOT be generated (in
response to the same Prefix Information option). To ensure that a
preferred temporary address is always available, a new temporary
address SHOULD be regenerated slightly before its predecessor is
deprecated. This is to allow sufficient time to avoid race
conditions in the case where generating a new temporary address is
not instantaneous, such as when DAD must be performed. The host
SHOULD start the process of address regeneration REGEN_ADVANCE time
units before a temporary address is deprecated.

As an optional optimization, an implementation MAY remove a
deprecated temporary address that is not in use by applications or
upper layers, as detailed in Section 6.

3.6. Regeneration of Temporary Addresses

The frequency at which temporary addresses change depends on how a
device is being used (e.g., how frequently it initiates new
communication) and the concerns of the end user. The most egregious
privacy concerns appear to involve addresses used for long periods of
time (from weeks to years). The more frequently an address changes,
the less feasible collecting or coordinating information keyed on
IIDs becomes. Moreover, the cost of collecting information and
attempting to correlate it based on IIDs will only be justified if
enough addresses contain non-changing identifiers to make it
worthwhile. Thus, having large numbers of clients change their
address on a daily or weekly basis is likely to be sufficient to
alleviate most privacy concerns.

There are also client costs associated with having a large number of
addresses associated with a host (e.g., in doing address lookups, the
need to join many multicast groups, etc.). Thus, changing addresses
frequently (e.g., every few minutes) may have performance
implications.

Hosts following this specification SHOULD generate new temporary
addresses over time. This can be achieved by generating a new
temporary address REGEN_ADVANCE time units before a temporary address
becomes deprecated. As described above, this produces addresses with
a preferred lifetime no larger than TEMP_PREFERRED_LIFETIME. The
value DESYNC_FACTOR is a random value computed when a temporary
address is generated; it ensures that clients do not generate new
addresses at a fixed frequency and that clients do not synchronize
with each other and generate new addresses at exactly the same time.
When the preferred lifetime expires, a new temporary address MUST be
generated using the algorithm specified in Section 3.4 (starting at
step 4).

Because the frequency at which it is appropriate to generate new
addresses varies from one environment to another, implementations
SHOULD provide end users with the ability to change the frequency at
which addresses are regenerated. The default value is given in
TEMP_PREFERRED_LIFETIME and is one day. In addition, the exact time
at which to invalidate a temporary address depends on how
applications are used by end users. Thus, the suggested default
value of two days (TEMP_VALID_LIFETIME) may not be appropriate in all
environments. Implementations SHOULD provide end users with the
ability to override both of these default values.

Finally, when an interface connects to a new (different) link,
existing temporary addresses for the corresponding interface MUST be
removed, and new temporary addresses MUST be generated for use on the
new link, using the algorithm in Section 3.4. If a device moves from
one link to another, generating new temporary addresses ensures that
the device uses different randomized IIDs for the temporary addresses
associated with the two links, making it more difficult to correlate
addresses from the two different links as being from the same host.
The host MAY follow any process available to it to determine that the
link change has occurred. One such process is described by "Simple
DNA" [RFC6059]. Detecting link changes would prevent link down/up
events from causing temporary addresses to be (unnecessarily)
regenerated.

3.7. Implementation Considerations

Devices implementing this specification MUST provide a way for the
end user to explicitly enable or disable the use of temporary
addresses. In addition, a site might wish to disable the use of
temporary addresses in order to simplify network debugging and
operations. Consequently, implementations SHOULD provide a way for
trusted system administrators to enable or disable the use of
temporary addresses.

Additionally, sites might wish to selectively enable or disable the
use of temporary addresses for some prefixes. For example, a site
might wish to disable temporary-address generation for ULA [RFC4193]
prefixes while still generating temporary addresses for all other
prefixes advertised via PIOs for address configuration. Another site
might wish to enable temporary-address generation only for the
prefixes 2001:db8:1::/48 and 2001:db8:2::/48 while disabling it for
all other prefixes. To support this behavior, implementations SHOULD
provide a way to enable and disable generation of temporary addresses
for specific prefix subranges. This per-prefix setting SHOULD
override the global settings on the host with respect to the
specified prefix subranges. Note that the per-prefix setting can be
applied at any granularity, and not necessarily on a per-subnet
basis.

3.8. Defined Protocol Parameters and Configuration Variables

Protocol parameters and configuration variables defined in this
document include:

TEMP_VALID_LIFETIME
Default value: 2 days. Users should be able to override the
default value.

TEMP_PREFERRED_LIFETIME
Default value: 1 day. Users should be able to override the
default value. Note: The TEMP_PREFERRED_LIFETIME value MUST be
smaller than the TEMP_VALID_LIFETIME value, to avoid the
pathological case where an address is employed for new
communications but becomes invalid in less than 1 second,
disrupting those communications.

REGEN_ADVANCE
2 + (TEMP_IDGEN_RETRIES * DupAddrDetectTransmits * RetransTimer /
1000)

| Rationale: This parameter is specified as a function of other
| protocol parameters, to account for the time possibly spent in
| DAD in the worst-case scenario of TEMP_IDGEN_RETRIES. This
| prevents the pathological case where the generation of a new
| temporary address is not started with enough anticipation, such
| that a new preferred address is generated before the currently
| preferred temporary address becomes deprecated.
|
| RetransTimer is specified in [RFC4861], while
| DupAddrDetectTransmits is specified in [RFC4862]. Since
| RetransTimer is specified in units of milliseconds, this
| expression employs the constant "1000", such that REGEN_ADVANCE
| is expressed in seconds.

MAX_DESYNC_FACTOR
0.4 * TEMP_PREFERRED_LIFETIME. Upper bound on DESYNC_FACTOR.

| Rationale: Setting MAX_DESYNC_FACTOR to 0.4
| TEMP_PREFERRED_LIFETIME results in addresses that have
| statistically different lifetimes, and a maximum of three
| concurrent temporary addresses when the default values
| specified in this section are employed.

DESYNC_FACTOR
A random value within the range 0 - MAX_DESYNC_FACTOR. It is
computed each time a temporary address is generated, and is
associated with the corresponding address. It MUST be smaller
than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE).

TEMP_IDGEN_RETRIES
Default value: 3

4. Implications of Changing IIDs

The desire to protect individual privacy can conflict with the desire
to effectively maintain and debug a network. Having clients use
addresses that change over time will make it more difficult to track
down and isolate operational problems. For example, when looking at
packet traces, it could become more difficult to determine whether
one is seeing behavior caused by a single errant host or a number of
them.

It is currently recommended that network deployments provide multiple
IPv6 addresses from each prefix to general-purpose hosts [RFC7934].
However, in some scenarios, use of a large number of IPv6 addresses
may have negative implications on network devices that need to
maintain entries for each IPv6 address in some data structures (e.g.,
SAVI [RFC7039]). For example, concurrent active use of multiple IPv6
addresses will increase Neighbor Discovery traffic if Neighbor Caches
in network devices are not large enough to store all addresses on the
link. This can impact performance and energy efficiency on networks
on which multicast is expensive (see e.g., [MCAST-PROBLEMS]).
Additionally, some network-security devices might incorrectly infer
IPv6 address forging if temporary addresses are regenerated at a high
rate.

The use of temporary addresses may cause unexpected difficulties with
some applications. For example, some servers refuse to accept
communications from clients for which they cannot map the IP address
into a DNS name. That is, they perform a DNS PTR query to determine
the DNS name corresponding to an IPv6 address, and may then also
perform a AAAA query on the returned name to verify it maps back into
the same address. Consequently, clients not properly registered in
the DNS may be unable to access some services. However, a host's DNS
name (if non-changing) would serve as a constant identifier. The
wide deployment of the extension described in this document could
challenge the practice of inverse-DNS-based "validation", which has
little validity, though it is widely implemented. In order to meet
server challenges, hosts could register temporary addresses in the
DNS using random names (for example, a string version of the random
address itself), albeit at the expense of increased complexity.

In addition, some applications may not behave robustly if an address
becomes invalid while it is still in use by the application or if the
application opens multiple sessions and expects them to all use the
same address.

[RFC4941] employed a randomized temporary IID for generating a set of
temporary addresses, such that temporary addresses configured at a
given time for multiple SLAAC prefixes would employ the same IID.
Sharing the same IID among multiple addresses allowed a host to join
only one solicited-node multicast group per temporary address set.

This document requires that the IIDs of all temporary addresses on a
host are statistically different from each other. This means that
when a network employs multiple prefixes, each temporary address of a
set will result in a different solicited-node multicast address, and,
thus, the number of multicast groups that a host must join becomes a
function of the number of SLAAC prefixes employed for generating
temporary addresses.

Thus, a network that employs multiple prefixes may require hosts to
join more multicast groups than in the case of implementations of RFC
4941. If the number of multicast groups were large enough, a host
might need to resort to setting the network interface card to
promiscuous mode. This could cause the host to process more packets
than strictly necessary and might have a negative impact on battery
life and system performance in general.

We note that since this document reduces the default
TEMP_VALID_LIFETIME from 7 days (in [RFC4941]) to 2 days, the number
of concurrent temporary addresses per SLAAC prefix will be smaller
than for RFC 4941 implementations; thus, the number of multicast
groups for a network that employs, say, between 1 and 3 prefixes,
will be similar to the number of such groups for RFC 4941
implementations.

Implementations concerned with the maximum number of multicast groups
that would be required to join as a result of configured addresses,
or the overall number of configured addresses, should consider
enforcing implementation-specific limits on, e.g., the maximum number
of configured addresses, the maximum number of SLAAC prefixes that
are employed for autoconfiguration, and/or the maximum ratio for
TEMP_VALID_LIFETIME/TEMP_PREFERRED_LIFETIME (which ultimately
controls the approximate number of concurrent temporary addresses per
SLAAC prefix). Many of these configuration limits are readily
available in SLAAC and RFC 4941 implementations. We note that these
configurable limits are meant to prevent pathological behaviors (as
opposed to simply limiting the usage of IPv6 addresses), since IPv6
implementations are expected to leverage the usage of multiple
addresses [RFC7934].

5. Significant Changes from RFC 4941

This section summarizes the substantive changes in this document
relative to RFC 4941.

Broadly speaking, this document introduces the following changes:

* Addresses a number of flaws in the algorithm for generating
temporary addresses. The aforementioned flaws include the use of
MD5 for computing the temporary IIDs, and reusing the same IID for
multiple prefixes (see [RAID2015] and [RFC7721] for further
details).

* Allows hosts to employ only temporary addresses. [RFC4941]
assumed that temporary addresses were configured in addition to
stable addresses. This document does not imply or require the
configuration of stable addresses; thus, implementations can now
configure both stable and temporary addresses or temporary
addresses only.

* Removes the recommendation that temporary addresses be disabled by
default. This is in line with BCP 188 ([RFC7258]) and also with
BCP 204 ([RFC7934]).

* Reduces the default maximum valid lifetime for temporary addresses
(TEMP_VALID_LIFETIME). TEMP_VALID_LIFETIME has been reduced from
1 week to 2 days, decreasing the typical number of concurrent
temporary addresses from 7 to 3. This reduces the possible stress
on network elements (see Section 4 for further details).

* DESYNC_FACTOR is computed each time a temporary address is
generated and is associated with the corresponding temporary
address, such that each temporary address has a statistically
different preferred lifetime, and thus temporary addresses are not
generated at any specific frequency.

* Changes the requirement to not try to regenerate temporary
addresses upon TEMP_IDGEN_RETRIES consecutive DAD failures from
"MUST NOT" to "SHOULD NOT".

* The discussion about the security and privacy implications of
different address generation techniques has been replaced with
references to recent work in this area ([RFC7707], [RFC7721], and
[RFC7217]).

* This document incorporates errata submitted (at the time of
writing) for [RFC4941] by Jiri Bohac and Alfred Hoenes.

6. Future Work

An implementation might want to keep track of which addresses are
being used by upper layers so as to be able to remove a deprecated
temporary address from internal data structures once no upper-layer
protocols are using it (but not before). This is in contrast to
current approaches, where addresses are removed from an interface
when they become invalid [RFC4862], independent of whether or not
upper-layer protocols are still using them. For TCP connections,
such information is available in control blocks. For UDP-based
applications, it may be the case that only the applications have
knowledge about what addresses are actually in use. Consequently, an
implementation generally will need to use heuristics in deciding when
an address is no longer in use.

7. IANA Considerations

This document has no IANA actions.

8. Security Considerations

If a very small number of hosts (say, only one) use a given prefix
for extended periods of time, just changing the interface-identifier
part of the address may not be sufficient to mitigate address-based
network-activity correlation, since the prefix acts as a constant
identifier. The procedures described in this document are most
effective when the prefix is reasonably nonstatic or used by a fairly
large number of hosts. Additionally, if a temporary address is used
in a session where the user authenticates, any notion of "privacy"
for that address is compromised for the party or parties that receive
the authentication information.

While this document discusses ways to limit the lifetime of interface
identifiers to reduce the ability of attackers to perform address-
based network-activity correlation, the method described is believed
to be ineffective against sophisticated forms of traffic analysis.
To increase effectiveness, one may need to consider the use of more
advanced techniques, such as onion routing [ONION].

Ingress filtering has been and is being deployed as a means of
preventing the use of spoofed source addresses in Distributed Denial
of Service (DDoS) attacks. In a network with a large number of
hosts, new temporary addresses are created at a fairly high rate.
This might make it difficult for ingress-/egress-filtering mechanisms
to distinguish between legitimately changing temporary addresses and
spoofed source addresses, which are "in-prefix" (using a
topologically correct prefix and nonexistent interface identifier).
This can be addressed by using access-control mechanisms on a per-
address basis on the network ingress point -- though, as noted in
Section 4, there are corresponding costs for doing so.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.

[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.

[RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers",
RFC 5453, DOI 10.17487/RFC5453, February 2009,
<https://www.rfc-editor.org/info/rfc5453>.

[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.

[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[BLAKE3] O'Connor, J., Aumasson, J. P., Neves, S., and Z. Wilcox-
O'Hearn, "BLAKE3: one function, fast everywhere", 2020,
<https://blake3.io/>.

[FIPS-SHS] NIST, "Secure Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.

[IANA-RESERVED-IID]
IANA, "Reserved IPv6 Interface Identifiers",
<https://www.iana.org/assignments/ipv6-interface-ids>.

[MCAST-PROBLEMS]
Perkins, C. E., McBride, M., Stanley, D., Kumari, W., and
J. C. Zuniga, "Multicast Considerations over IEEE 802
Wireless Media", Work in Progress, Internet-Draft, draft-
ietf-mboned-ieee802-mcast-problems-13, 4 February 2021,
<https://tools.ietf.org/html/draft-ietf-mboned-ieee802-
mcast-problems-13>.

[ONION] Reed, M.G., Syverson, P.F., and D.M. Goldschlag, "Proxies
for Anonymous Routing", Proceedings of the 12th Annual
Computer Security Applications Conference,
DOI 10.1109/CSAC.1996.569678, December 1996,
<https://doi.org/10.1109/CSAC.1996.569678>.

[OPEN-GROUP]
The Open Group, "The Open Group Base Specifications Issue
7", Section 4.16 Seconds Since the Epoch, IEEE Std 1003.1,
2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
contents.html>.

[RAID2015] Ullrich, J. and E.R. Weippl, "Privacy is Not an Option:
Attacking the IPv6 Privacy Extension", International
Symposium on Recent Advances in Intrusion Detection
(RAID), 2015, <https://publications.sba-
research.org/publications/Ullrich2015Privacy.pdf>.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.

[RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
DOI 10.17487/RFC5014, September 2007,
<https://www.rfc-editor.org/info/rfc5014>.

[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
DOI 10.17487/RFC6059, November 2010,
<https://www.rfc-editor.org/info/rfc6059>.

[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, DOI 10.17487/RFC7039, October 2013,
<https://www.rfc-editor.org/info/rfc7039>.

[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.

[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.

[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
<https://www.rfc-editor.org/info/rfc7421>.

[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<https://www.rfc-editor.org/info/rfc7707>.

[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.

[RFC7934] Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
"Host Address Availability Recommendations", BCP 204,
RFC 7934, DOI 10.17487/RFC7934, July 2016,
<https://www.rfc-editor.org/info/rfc7934>.

[RFC8190] Bonica, R., Cotton, M., Haberman, B., and L. Vegoda,
"Updates to the Special-Purpose IP Address Registries",
BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017,
<https://www.rfc-editor.org/info/rfc8190>.

Acknowledgments

Fernando Gont was the sole author of this document (a revision of RFC
4941). He would like to thank (in alphabetical order) Fred Baker,
Brian Carpenter, Tim Chown, Lorenzo Colitti, Roman Danyliw, David
Farmer, Tom Herbert, Bob Hinden, Christian Huitema, Benjamin Kaduk,
Erik Kline, Gyan Mishra, Dave Plonka, Alvaro Retana, Michael
Richardson, Mark Smith, Dave Thaler, Pascal Thubert, Ole Troan,
Johanna Ullrich, Eric Vyncke, Timothy Winters, and Christopher Wood
for providing valuable comments on earlier draft versions of this
document.

This document incorporates errata submitted for RFC 4941 by Jiri
Bohac and Alfred Hoenes (at the time of writing).

Suresh Krishnan was the sole author of RFC 4941 (a revision of RFC
3041). He would like to acknowledge the contributions of the IPv6
Working Group and, in particular, Jari Arkko, Pekka Nikander, Pekka
Savola, Francis Dupont, Brian Haberman, Tatuya Jinmei, and Margaret
Wasserman for their detailed comments.

Rich Draves and Thomas Narten were the authors of RFC 3041. They
would like to acknowledge the contributions of the IPv6 Working Group
and, in particular, Ran Atkinson, Matt Crawford, Steve Deering,
Allison Mankin, and Peter Bieringer.

Authors' Addresses

Fernando Gont
SI6 Networks
Segurola y Habana 4310, 7mo Piso
Villa Devoto
Ciudad Autonoma de Buenos Aires
Argentina

Email: [email protected]
URI: https://www.si6networks.com

Suresh Krishnan
Kaloom

Email: [email protected]

Thomas Narten

Email: [email protected]

Richard Draves
Microsoft Research
One Microsoft Way
Redmond, WA
United States of America

Email: [email protected]