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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" obsoletes="4941" ipr="trust200902"
docName="draft-ietf-6man-rfc4941bis-12" number="8981" updates="" submissionType="IETF"
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<front>
<title abbrev="Temporary Address Extensions to Autoconf">Temporary Address Extensions for Stateless Address Autoconfiguration in
IPv6</title>
<seriesInfo name="RFC" value="8981"/>
<author fullname="Fernando Gont" initials="F." surname="Gont">
<organization abbrev="SI6 Networks">SI6 Networks</organization>
<address>
<postal>
<street>Segurola y Habana 4310, 7mo Piso</street>
<city>Villa Devoto</city>
<region>Ciudad Autonoma de Buenos Aires</region>
<country>Argentina</country>
</postal>
<email>fgont@si6networks.com</email>
<uri>https://www.si6networks.com</uri>
</address>
</author>
<author fullname="Suresh Krishnan" initials="S." surname="Krishnan">
<organization>Kaloom</organization>
<address>
<postal>
<street/>
<city/>
<region/>
<code/>
<country/>
</postal>
<email>suresh@kaloom.com</email>
</address>
</author>
<author initials="T." surname="Narten" fullname="Thomas Narten">
<address>
<email>narten@cs.duke.edu</email>
</address>
</author>
<author initials="R." surname="Draves" fullname="Richard Draves">
<organization>Microsoft Research</organization>
<address>
<postal>
<street>One Microsoft Way</street>
<city>Redmond</city>
<region>WA</region>
<country>United States of America</country>
</postal>
<email>richdr@microsoft.com</email>
</address>
</author>
<date year="2021" month="February" />
<area>Internet</area>
<workgroup>IPv6 Maintenance (6man) Working Group</workgroup>
<keyword>privacy</keyword>
<keyword>anonymity</keyword>
<keyword>unlinkability</keyword>
<keyword>crypto-based address changing</keyword>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="intro" numbered="true" toc="default">
<name>Introduction</name>
<t><xref target="RFC4862" format="default"/> 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 <xref target="RFC7721" format="default"/>, <xref target="RFC7217" format="default"/>, and <xref target="RFC7707" format="default"/>. 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. <xref target="changes" format="default"/> describes the changes from <xref target="RFC4941" format="default"/>.</t>
<t>The default address selection for IPv6 has been specified in <xref target="RFC6724" format="default"/>. 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 <xref target="RFC5014" format="default"/> can enable
individual applications to indicate a preference for the use of temporary addresses.
</t>
<t>
<xref target="SECTION2" format="default"/> provides background information. <xref target="SECTION3" format="default"/> describes a procedure for
generating temporary addresses.
<xref target="SECTION4" format="default"/> discusses implications of changing
IIDs. <xref target="changes" format="default"/> describes the changes from <xref target="RFC4941" format="default"/>.
</t>
<section anchor="term" numbered="true" toc="default">
<name>Terminology</name>
<t>
The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
<t>The terms "public address", "stable address", "temporary address", "constant IID", "stable IID", and "temporary IID" are to be
interpreted as specified in <xref target="RFC7721" format="default"/>.</t>
<t>The term "global-scope addresses" is
used in this document to collectively refer to "Global
unicast addresses" as defined in
<xref target="RFC4291" format="default"/> and "Unique local addresses" as
defined in
<xref target="RFC4193" format="default"/>, and not to "globally reachable addresses" as defined in <xref target="RFC8190" format="default"/>.</t>
</section>
<section numbered="true" toc="default">
<name>Problem Statement</name>
<t>Addresses generated using SLAAC
<xref target="RFC4862" format="default"/> 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.</t>
<t>The correlation can be performed by:
</t>
<ul spacing="normal">
<li>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.</li>
<li>An attacker who can access the communication logs of
the peers with which the host has communicated.</li>
</ul>
<t>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.</t>
<t>Note that an attacker, who is on path, may be able to
perform significant correlation based on:
</t>
<ul spacing="normal">
<li>The payload contents of unencrypted packets on the wire.</li>
<li>The characteristics of the packets, such as packet size
and timing.</li>
</ul>
<t>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.</t>
</section>
</section>
<section anchor="SECTION2" numbered="true" toc="default">
<name>Background</name>
<t>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.</t>
<section numbered="true" toc="default">
<name>Extended Use of the Same Identifier</name>
<t>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.</t>
<t>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:
</t>
<ul spacing="normal">
<li>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
<xref target="SECTION4" format="default"/>).</li>
<li>Web browsers and servers typically exchange "cookies"
with each other
<xref target="RFC6265" format="default"/>. 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.</li>
</ul>
<t>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.</t>
<t>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.</t>
<t>The security and privacy implications of IPv6 addresses are discussed in
detail in <xref target="RFC7721" format="default"/>, <xref target="RFC7707" format="default"/>, and <xref target="RFC7217" format="default"/>.</t>
</section>
<section numbered="true" toc="default">
<name>Possible Approaches</name>
<t>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.</t>
<t>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.
</t>
<t>On the other hand, a host that functions only as a
client may want to employ only temporary addresses for
public communication.</t>
<t>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.</t>
</section>
</section>
<section anchor="SECTION3" numbered="true" toc="default">
<name>Protocol Description</name>
<t>The following subsections define the procedures for the generation of IPv6 temporary addresses.</t>
<section anchor="design" numbered="true" toc="default">
<name>Design Guidelines</name>
<t>Temporary addresses observe the following properties:</t>
<ol spacing="normal" type="1"><li>Temporary addresses are typically employed for initiating
outgoing sessions.</li>
<li>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.</li>
<li>New
temporary addresses are generated over time to replace
temporary addresses that expire (i.e., become deprecated and
eventually invalidated).</li>
<li>Temporary addresses must have a limited lifetime (limited "valid lifetime" and "preferred lifetime" from <xref target="RFC4862" format="default"/>). 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.</li>
<li>
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.
</li>
<li>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 <xref target="RFC7136" format="default"/> and must not follow any specific patterns.</li>
</ol>
</section>
<section numbered="true" toc="default">
<name>Assumptions</name>
<t>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.</t>
<t>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.
<xref target="RFC6724" format="default"/> 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, <xref target="RFC5014" format="default"/>).
</t>
</section>
<section anchor="SECTION3_2" numbered="true" toc="default">
<name>Generation of Randomized IIDs</name>
<t>The following subsections specify example algorithms for generating temporary IIDs that follow the guidelines in <xref target="design" format="default"/> of this document. The algorithm specified in <xref target="randomized-IIDs" format="default"/> assumes a pseudorandom number generator (PRNG) is available on the system. The algorithm specified in <xref target="RFC-7217" format="default"/> allows for code reuse by hosts that implement <xref target="RFC7217" format="default"/>.
</t>
<section anchor="randomized-IIDs" numbered="true" toc="default">
<name>Simple Randomized IIDs</name>
<t>One approach is to select a pseudorandom number of the appropriate length. A host employing this algorithm should generate IIDs as follows:
</t>
<ol spacing="normal" type="1">
<li>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).
<xref target="RFC4086" format="default"/> specifies randomness requirements for security.</li>
<li>The IID is obtained by taking as many bits from the random number obtained in the previous step as necessary. See <xref target="RFC7136" format="default"/> for the necessary number of bits (i.e., the length of the IID). See also <xref target="RFC7421" format="default"/> for a discussion of the privacy implications of the IID length. Note: there are no special bits in an IID <xref target="RFC7136" format="default"/>.
</li>
<li>
The resulting IID <bcp14>MUST</bcp14> be compared against the reserved IPv6 IIDs <xref target="RFC5453" format="default"/> <xref target="IANA-RESERVED-IID" format="default"/> 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.
</li>
</ol>
</section>
<section anchor="RFC-7217" numbered="true" toc="default">
<name>Generation of IIDs with Pseudorandom Functions</name>
<t>The algorithm in <xref target="RFC7217" format="default"/> 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.</t>
<t>Hosts would employ the following algorithm for generating the temporary IID:
</t>
<ol spacing="normal" type="1"><li>
<t>
Compute a random identifier with the expression:
</t>
<t>
RID = F(Prefix, Net_Iface, Network_ID, Time, DAD_Counter,
secret_key)
</t>
<t>
Where:
</t>
<dl newline="true" spacing="normal">
<dt>RID:</dt>
<dd>Random Identifier</dd>
<dt>F():</dt>
<dd>A pseudorandom function (PRF) that <bcp14>MUST NOT</bcp14> be computable from the outside (without knowledge of the secret key). F() <bcp14>MUST</bcp14> 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() <bcp14>SHOULD</bcp14> produce an output of at least as many bits as required for the IID.
BLAKE3 (256-bit key, arbitrary-length output) <xref target="BLAKE3" format="default"/> is one possible option for F(). Alternatively, F() could be implemented with a keyed-hash message authentication code (HMAC) <xref target="RFC2104" format="default"/>. HMAC-SHA-256 <xref target="FIPS-SHS" format="default"/> is one possible option for such an implementation alternative. Note: use of HMAC-MD5 <xref target="RFC1321" format="default"/> is considered unacceptable for F() <xref target="RFC6151" format="default"/>.</dd>
<dt>Prefix:</dt>
<dd>The prefix to be used for SLAAC, as learned from an ICMPv6 Router Advertisement message.</dd>
<dt>Net_Iface:</dt>
<dd>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 <xref target="RFC7217" format="default"/>) means that the regeneration of a randomized MAC address will result in a different temporary address.</dd>
<dt>Network_ID:</dt>
<dd>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") <xref target="RFC6059" format="default"/> describes ideas that could be leveraged to generate a Network_ID parameter. This parameter <bcp14>SHOULD</bcp14> be employed if some form of "Network_ID" is available.</dd>
<dt>Time:</dt>
<dd>An implementation-dependent representation of time. One possible example is the representation in UNIX-like systems <xref target="OPEN-GROUP" format="default"/>, 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.</dd>
<dt>DAD_Counter:</dt>
<dd>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.
</dd>
<dt>secret_key:</dt>
<dd>A secret key that is not known by the attacker. The secret key <bcp14>SHOULD</bcp14> be of at least 128 bits. It <bcp14>MUST</bcp14> be initialized to a pseudorandom number (see <xref target="RFC4086" format="default"/> for randomness requirements for security) when the operating system is "bootstrapped". The secret_key <bcp14>MUST NOT</bcp14> be employed for any other purpose than the one discussed in this section. For example, implementations <bcp14>MUST NOT</bcp14> employ the same secret_key for the generation of stable addresses <xref target="RFC7217" format="default"/> and the generation of temporary addresses via this algorithm.</dd>
</dl>
</li>
<li>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 <xref target="RFC7136" format="default"/> for the necessary number of bits (i.e., the length of the IID). See also <xref target="RFC7421" format="default"/> for a discussion of the privacy implications of the IID length. Note: there are no special bits in an IID <xref target="RFC7136" format="default"/>.
</li>
<li>
The resulting IID <bcp14>MUST</bcp14> be compared against the reserved IPv6 IIDs <xref target="RFC5453" format="default"/> <xref target="IANA-RESERVED-IID" format="default"/> 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.
</li>
</ol>
</section>
</section>
<section anchor="SECTION3_3" numbered="true" toc="default">
<name>Generating Temporary Addresses</name>
<t>
<xref target="RFC4862" format="default"/> 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
<xref target="RFC4862" format="default"/> 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 <bcp14>MUST</bcp14>
perform the following steps:</t>
<t>
</t>
<ol spacing="normal" type="1">
<li><t>Process the Prefix Information option as specified in <xref target="RFC4862" format="default"/>, 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.
</t>
<dl newline="true" spacing="normal">
<dt>Note:</dt>
<dd>DESYNC_FACTOR is the value computed when the address was created (see step 4 below).</dd>
</dl>
</li>
<li><t>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.
</t>
<dl newline="true" spacing="normal">
<dt>Note:</dt>
<dd>DESYNC_FACTOR is the value computed when the address was created (see step 4 below).</dd>
</dl>
</li>
<li>
<t>If the host has not configured any temporary address for the corresponding prefix, the host <bcp14>SHOULD</bcp14> create
a new temporary address for such prefix.
</t>
<dl newline="true" spacing="normal">
<dt>Note:</dt>
<dd>For example, a host might implement prefix-specific policies such as
not configuring temporary addresses for the Unique Local IPv6 Unicast
Addresses (ULAs) <xref target="RFC4193" format="default"/> prefix.</dd>
</dl>
</li>
<li>
<t>When creating a temporary address, DESYNC_FACTOR <bcp14>MUST</bcp14> be
computed and associated with the newly created address, and the address lifetime
values <bcp14>MUST</bcp14> be derived from the corresponding prefix as
follows:
</t>
<ul spacing="normal">
<li>Its valid lifetime is the lower of the Valid
Lifetime of the prefix and TEMP_VALID_LIFETIME.</li>
<li>Its preferred lifetime is the lower of the
Preferred Lifetime of the prefix and
TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR.</li>
</ul>
</li>
<li>A temporary address is created only if this
calculated preferred lifetime is greater than
REGEN_ADVANCE time units. In particular, an
implementation <bcp14>MUST NOT</bcp14> create a temporary address with
a zero preferred lifetime.</li>
<li>New temporary addresses <bcp14>MUST</bcp14> be created by appending
a randomized IID to the prefix that was received. <xref target="SECTION3_2" format="default"/> of this document specifies some sample algorithms for generating the randomized IID.</li>
<li>The host <bcp14>MUST</bcp14> perform DAD
on the generated temporary address. If DAD
indicates the address is already in use, the host <bcp14>MUST</bcp14>
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 <bcp14>MUST</bcp14> log
a system error and <bcp14>SHOULD NOT</bcp14> 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.</li>
</ol>
</section>
<section anchor="SECTION3_4" numbered="true" toc="default">
<name>Expiration of Temporary Addresses</name>
<t>When a temporary address becomes deprecated, a new one
<bcp14>MUST</bcp14> be generated. This is done by repeating the actions
described in
<xref target="SECTION3_3" format="default"/>, 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 <bcp14>MUST NOT</bcp14> be generated (in response to the same Prefix Information
option). To ensure that a preferred
temporary address is always available, a new temporary
address <bcp14>SHOULD</bcp14> 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 <bcp14>SHOULD</bcp14>
start the process of address regeneration REGEN_ADVANCE time
units before a temporary address is
deprecated.</t>
<t>As an optional optimization, an implementation <bcp14>MAY</bcp14>
remove a deprecated temporary address that is not in use by
applications or upper layers, as detailed in
<xref target="SECTION6" format="default"/>.</t>
</section>
<section anchor="REGEN" numbered="true" toc="default">
<name>Regeneration of Temporary Addresses</name>
<t>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.</t>
<t>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.</t>
<t>
Hosts following this specification <bcp14>SHOULD</bcp14> 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 <bcp14>MUST</bcp14> be generated
using the algorithm specified in <xref target="SECTION3_3" format="default"/> (starting at step 4).</t>
<t>Because the frequency at which it is appropriate
to generate new addresses varies from one environment to
another, implementations <bcp14>SHOULD</bcp14> 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 <bcp14>SHOULD</bcp14> provide end users with
the ability to override both of these default values.</t>
<t>Finally, when an interface connects to a new (different) link, existing temporary addresses for the corresponding interface <bcp14>MUST</bcp14> be removed, and new temporary addresses <bcp14>MUST</bcp14> be generated for use on the new link, using the algorithm in <xref target="SECTION3_3" format="default"/>.
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 <bcp14>MAY</bcp14>
follow any process available to it to determine that the
link change has occurred. One such process is described by "Simple DNA" <xref target="RFC6059" format="default"/>. Detecting link changes would prevent link down/up events from causing temporary addresses to be (unnecessarily) regenerated.</t>
</section>
<section numbered="true" toc="default">
<name>Implementation Considerations</name>
<t>Devices implementing this specification <bcp14>MUST</bcp14> 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 <bcp14>SHOULD</bcp14> provide a way for trusted system
administrators to enable or disable the use of temporary
addresses.</t>
<t>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
<xref target="RFC4193" format="default"/> 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 <bcp14>SHOULD</bcp14> provide a way to enable and disable
generation of temporary addresses for specific prefix
subranges. This per-prefix setting <bcp14>SHOULD</bcp14> 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.</t>
</section>
<section anchor="constants" numbered="true" toc="default">
<name>Defined Protocol Parameters and Configuration Variables</name>
<t>Protocol parameters and configuration variables defined in this document include:</t>
<dl newline="true">
<dt>TEMP_VALID_LIFETIME</dt><dd>Default value: 2 days. Users should
be able to override the default value.</dd>
<dt>TEMP_PREFERRED_LIFETIME</dt><dd>Default value: 1 day. Users
should be able to override the default value. Note: The TEMP_PREFERRED_LIFETIME value <bcp14>MUST</bcp14> 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.</dd>
</dl>
<dl newline="true">
<dt>REGEN_ADVANCE</dt><dd>2 + (TEMP_IDGEN_RETRIES * DupAddrDetectTransmits * RetransTimer / 1000)</dd>
</dl>
<aside>
<t>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.</t>
<t>RetransTimer is specified in
<xref target="RFC4861" format="default"/>, while DupAddrDetectTransmits is specified in <xref target="RFC4862" format="default"/>. Since RetransTimer is specified in units of milliseconds, this expression employs the constant "1000", such that
REGEN_ADVANCE is expressed in seconds.
</t>
</aside>
<dl newline="true">
<dt>MAX_DESYNC_FACTOR</dt><dd>0.4 * TEMP_PREFERRED_LIFETIME. Upper bound on DESYNC_FACTOR.</dd>
</dl>
<aside>
<t>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.</t>
</aside>
<dl newline="true">
<dt>DESYNC_FACTOR</dt><dd>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).</dd>
<dt>TEMP_IDGEN_RETRIES</dt><dd>Default value: 3</dd>
</dl>
</section>
</section>
<section anchor="SECTION4" numbered="true" toc="default">
<name>Implications of Changing IIDs</name>
<t>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.</t>
<t>It is currently recommended that network deployments provide multiple IPv6 addresses from each prefix to general-purpose hosts <xref target="RFC7934" format="default"/>. 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 <xref target="RFC7039" format="default"/>). 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., <xref target="I-D.ietf-mboned-ieee802-mcast-problems" format="default"/>). Additionally, some network-security devices might incorrectly infer IPv6 address forging if temporary addresses are regenerated at a high rate.</t>
<t>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 an AAAA
query on the returned name to verify it maps back into the 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.</t>
<t>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.</t>
<t>
<xref target="RFC4941" format="default"/> 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.
</t>
<t>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.</t>
<t>
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.</t>
<t>
We note that since this document reduces the default TEMP_VALID_LIFETIME from 7 days (in <xref target="RFC4941" format="default"/>) 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.</t>
<t>
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 <xref target="RFC7934" format="default"/>.
</t>
</section>
<section anchor="changes" numbered="true" toc="default">
<name>Significant Changes from RFC 4941</name>
<t>This section summarizes the substantive changes in this document
relative to RFC 4941.</t>
<t>Broadly speaking, this document introduces the following changes:
</t>
<ul spacing="normal">
<li>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 <xref target="RAID2015" format="default"/> and <xref target="RFC7721" format="default"/> for further details).</li>
<li>
Allows hosts to employ only temporary addresses. <xref target="RFC4941" format="default"/> 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.
</li>
<li>
Removes the recommendation that temporary addresses be disabled by default. This is in line with BCP 188 (<xref target="RFC7258" format="default"/>) and also with BCP 204 (<xref target="RFC7934" format="default"/>).
</li>
<li>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 <xref target="SECTION4"/> for further
details).</li>
<li>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.</li>
<li>Changes the requirement to not try to regenerate temporary addresses upon TEMP_IDGEN_RETRIES consecutive DAD failures from "<bcp14>MUST NOT</bcp14>" to "<bcp14>SHOULD NOT</bcp14>".</li>
<li>The discussion about the security and privacy implications of different address generation techniques has been replaced with references to recent work in this area (<xref target="RFC7707" format="default"/>, <xref target="RFC7721" format="default"/>, and <xref target="RFC7217" format="default"/>).
</li>
<li><t>This document incorporates errata submitted (at the time of writing) for <xref target="RFC4941" format="default"/> by <contact fullname="Jiri Bohac"/> and <contact fullname="Alfred Hoenes"/>.</t></li>
</ul>
</section>
<section anchor="SECTION6" numbered="true" toc="default">
<name>Future Work</name>
<t>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
<xref target="RFC4862" format="default"/>, 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.</t>
</section>
<section anchor="iana-cons" numbered="true" toc="default">
<name>IANA Considerations</name>
<t>This document has no IANA actions.</t>
</section>
<section numbered="true" toc="default">
<name>Security Considerations</name>
<t>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.</t>
<t>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
<xref target="ONION" format="default"/>.</t>
<t>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 <xref target="SECTION4" format="default"/>, there are corresponding costs
for doing so.</t>
</section>
</middle>
<back>
<displayreference target="I-D.ietf-mboned-ieee802-mcast-problems" to="MCAST-PROBLEMS"/>
<references>
<name>References</name>
<references>
<name>Normative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6724.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4086.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5453.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7136.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4193.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4291.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4862.xml"/>
</references>
<references>
<name>Informative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7217.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4941.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8190.xml"/>
<!-- [I-D.ietf-mboned-ieee802-mcast-problems] IESG state IESG Evaluation::Revised I-D Needed -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-mboned-ieee802-mcast-problems.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7934.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7039.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7421.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7258.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5014.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2104.xml"/>
<reference anchor="IANA-RESERVED-IID" target="https://www.iana.org/assignments/ipv6-interface-ids">
<front>
<title>Reserved IPv6 Interface Identifiers</title>
<author>
<organization>IANA</organization>
</author>
<date/>
</front>
</reference>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1321.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6151.xml"/>
<reference anchor="RAID2015" target="https://publications.sba-research.org/publications/Ullrich2015Privacy.pdf">
<front>
<title>Privacy is Not an Option: Attacking the IPv6 Privacy Extension</title>
<author fullname="Johanna Ullrich" initials="J." surname="Ullrich">
</author>
<author fullname="Edgar R. Weippl" initials="E.R." surname="Weippl">
</author>
<date year="2015"/>
</front>
<seriesInfo name="" value="International Symposium on Recent Advances in Intrusion Detection (RAID)"/>
</reference>
<reference anchor="FIPS-SHS" target="https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf">
<front>
<title>Secure Hash Standard (SHS)</title>
<author>
<organization>NIST</organization>
</author>
<date month="August" year="2015"/>
</front>
<seriesInfo name="FIPS PUB" value="180-4"/>
<seriesInfo name="DOI" value="10.6028/NIST.FIPS.180-4"/>
</reference>
<reference anchor="BLAKE3" target="https://blake3.io/">
<front>
<title>BLAKE3: one function, fast everywhere</title>
<author initials="J." surname="O'Connor" fullname="Jack O'Connor">
<organization></organization>
</author>
<author initials="J. P." surname="Aumasson" fullname="Jean-Philippe Aumasson">
<organization>NAGRA</organization>
</author>
<author initials="S." surname="Neves" fullname="Samuel Neves">
<organization></organization>
</author>
<author initials="Z." surname="Wilcox-O'Hearn" fullname="Zooko Wilcox-O'Hearn">
<organization></organization>
</author>
<date year="2020"/>
</front>
</reference>
<reference anchor="OPEN-GROUP" target="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/contents.html">
<front>
<title>The Open Group Base Specifications Issue 7</title>
<author>
<organization>The Open Group</organization>
</author>
<date year="2016"/>
</front>
<seriesInfo name="Section 4.16" value="Seconds Since the Epoch"/>
<seriesInfo name="IEEE Std" value="1003.1"/>
</reference>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6265.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7721.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7707.xml"/>
<reference anchor="ONION">
<front>
<title>Proxies for Anonymous Routing</title>
<author initials="M.G." surname="Reed" fullname="Michael G. Reed">
<organization/>
</author>
<author initials="P.F." surname="Syverson" fullname="Paul F. Syverson">
<organization/>
</author>
<author initials="D.M." surname="Goldschlag" fullname="David M. Goldschlag">
<organization/>
</author>
<date month="December" year="1996"/>
</front>
<seriesInfo name="Proceedings of the" value="12th Annual Computer Security Applications Conference"/>
<seriesInfo name="DOI" value="10.1109/CSAC.1996.569678" />
</reference>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6059.xml"/>
</references>
</references>
<section numbered="false" toc="default">
<name>Acknowledgments</name>
<t>Fernando Gont was the sole author of this document (a revision of RFC 4941). He would like to thank (in alphabetical order) <contact fullname="Fred Baker"/>, <contact fullname="Brian Carpenter"/>, <contact fullname="Tim Chown"/>, <contact fullname="Lorenzo Colitti"/>, <contact fullname="Roman Danyliw"/>, <contact fullname="David Farmer"/>, <contact fullname="Tom Herbert"/>, <contact fullname="Bob Hinden"/>, <contact fullname="Christian Huitema"/>, <contact fullname="Benjamin Kaduk"/>, <contact fullname="Erik Kline"/>, <contact fullname="Gyan Mishra"/>, <contact fullname="Dave Plonka"/>, <contact fullname="Alvaro Retana"/>, <contact fullname="Michael Richardson"/>, <contact fullname="Mark Smith"/>, <contact fullname="Dave Thaler"/>, <contact fullname="Pascal Thubert"/>, <contact fullname="Ole Troan"/>, <contact fullname="Johanna Ullrich"/>, <contact fullname="Eric Vyncke"/>, <contact fullname="Timothy Winters"/>, and <contact fullname="Christopher Wood"/> for providing valuable comments on earlier draft versions of this document.</t>
<t>This document incorporates errata submitted for RFC 4941 by <contact fullname="Jiri Bohac"/> and <contact fullname="Alfred Hoenes"/> (at the time of writing).</t>
<t><contact fullname="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, <contact fullname="Jari Arkko"/>, <contact fullname="Pekka Nikander"/>, <contact fullname="Pekka Savola"/>, <contact fullname="Francis Dupont"/>, <contact fullname="Brian Haberman"/>, <contact fullname="Tatuya Jinmei"/>, and <contact fullname="Margaret Wasserman"/>
for their detailed comments.</t>
<t>
<contact fullname="Rich Draves"/> and <contact fullname="Thomas Narten"/> were the authors of RFC 3041. They
would like to acknowledge the contributions of the IPv6 Working Group
and, in particular, <contact fullname="Ran Atkinson"/>, <contact fullname="Matt Crawford"/>, <contact fullname="Steve Deering"/>, <contact fullname="Allison Mankin"/>, and <contact fullname="Peter Bieringer"/>.
</t>
</section>
</back>
</rfc>