Network Working Group T. Narten Request for Comments: 4941 IBM Corporation Obsoletes: 3041 R. Draves Category: Standards Track Microsoft Research S. Krishnan Ericsson Research September 2007 Privacy Extensions for Stateless Address Autoconfiguration in IPv6 Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
AbstractNodes use IPv6 stateless address autoconfiguration to generate addresses using a combination of locally available information and information advertised by routers. Addresses are formed by combining network prefixes with an interface identifier. On an interface that contains an embedded IEEE Identifier, the interface identifier is typically derived from it. On other interface types, the interface identifier is generated through other means, for example, via random number generation. This document describes an extension to IPv6 stateless address autoconfiguration for interfaces whose interface identifier is derived from an IEEE identifier. Use of the extension causes nodes to generate global scope addresses from interface identifiers that change over time, even in cases where the interface contains an embedded IEEE identifier. Changing the interface identifier (and the global scope addresses generated from it) over time makes it more difficult for eavesdroppers and other information collectors to identify when different addresses used in different transactions actually correspond to the same node.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions Used in This Document . . . . . . . . . . . . 4 1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 4 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Extended Use of the Same Identifier . . . . . . . . . . . 5 2.2. Address Usage in IPv4 Today . . . . . . . . . . . . . . . 6 2.3. The Concern with IPv6 Addresses . . . . . . . . . . . . . 7 2.4. Possible Approaches . . . . . . . . . . . . . . . . . . . 8 3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 9 3.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Generation of Randomized Interface Identifiers . . . . . . 10 3.2.1. When Stable Storage Is Present . . . . . . . . . . . . 11 3.2.2. In The Absence of Stable Storage . . . . . . . . . . . 12 3.2.3. Alternate Approaches . . . . . . . . . . . . . . . . . 12 3.3. Generating Temporary Addresses . . . . . . . . . . . . . . 13 3.4. Expiration of Temporary Addresses . . . . . . . . . . . . 14 3.5. Regeneration of Randomized Interface Identifiers . . . . . 15 3.6. Deployment Considerations . . . . . . . . . . . . . . . . 16 4. Implications of Changing Interface Identifiers . . . . . . . . 17 5. Defined Constants . . . . . . . . . . . . . . . . . . . . . . 18 6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 8. Significant Changes from RFC 3041 . . . . . . . . . . . . . . 19 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 10.1. Normative References . . . . . . . . . . . . . . . . . . . 20 10.2. Informative References . . . . . . . . . . . . . . . . . . 20
ADDRCONF] defines how an IPv6 node generates addresses without the need for a Dynamic Host Configuration Protocol for IPv6 (DHCPv6) server. Some types of network interfaces come with an embedded IEEE Identifier (i.e., a link-layer MAC address), and in those cases, stateless address autoconfiguration uses the IEEE identifier to generate a 64-bit interface identifier [ADDRARCH]. By design, the interface identifier is likely to be globally unique when generated in this fashion. The interface identifier is in turn appended to a prefix to form a 128-bit IPv6 address. Note that an IPv6 identifier does not necessarily have to be 64 bits in length, but the algorithm specified in this document is targeted towards 64-bit interface identifiers. All nodes combine interface identifiers (whether derived from an IEEE identifier or generated through some other technique) with the reserved link-local prefix to generate link-local addresses for their attached interfaces. Additional addresses can then be created by combining prefixes advertised in Router Advertisements via Neighbor Discovery [DISCOVERY] with the interface identifier. Not all nodes and interfaces contain IEEE identifiers. In such cases, an interface identifier is generated through some other means (e.g., at random), and the resultant interface identifier may not be globally unique and may also change over time. The focus of this document is on addresses derived from IEEE identifiers because tracking of individual devices, the concern being addressed here, is possible only in those cases where the interface identifier is globally unique and non-changing. The rest of this document assumes that IEEE identifiers are being used, but the techniques described may also apply to interfaces with other types of globally unique and/or persistent identifiers. This document discusses concerns associated with the embedding of non-changing interface identifiers within IPv6 addresses and describes extensions to stateless address autoconfiguration that can help mitigate those concerns for individual users and in environments where such concerns are significant. Section 2 provides background information on the issue. Section 3 describes a procedure for generating alternate interface identifiers and global scope addresses. Section 4 discusses implications of changing interface identifiers. The term "global scope addresses" is used in this document to collectively refer to "Global unicast addresses" as defined in [ADDRARCH] and "Unique local addresses" as defined in [ULA].
RFC2119]. ADDRCONF] contain an embedded interface identifier, which remains constant 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 o An attacker who is in the path between the node in question and the peer(s) to which it is communicating, and who can view the IPv6 addresses present in the datagrams. o An attacker who can access the communication logs of the peers with which the node has communicated. Since the identifier is embedded within the IPv6 address, which is a fundamental requirement of communication, it cannot be easily 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 o The payload contents of the packets on the wire o The characteristics of the packets such as packet size and timing Use of temporary addresses will not prevent such payload-based correlation.
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. DHCP]. In theory, the address a client gets via DHCP can change over time, but in practice servers often return the same address to the same client (unless addresses are in such short supply that they are reused immediately by a different node when they become free). Thus, even within sites using DHCP, clients frequently end up using the same address for weeks to months at a time. For home users accessing the Internet over dial-up lines, the situation is generally different. Such users do not have permanent connections and are often assigned temporary addresses each time they connect to their ISP. Consequently, the addresses they use change frequently over time and are shared among a number of different users. Thus, an address does not reliably identify a particular device over time spans of more than a few minutes. A more interesting case concerns always-on connections (e.g., cable modems, ISDN, DSL, etc.) that result in a home site using the same address for extended periods of time. This is a scenario that is just starting to become common in IPv4 and promises to become more of a concern as always-on Internet connectivity becomes widely available. Finally, it should be noted that nodes that need a (non-changing) DNS name generally have static addresses assigned to them to simplify the configuration of DNS servers. Although Dynamic DNS [DDNS] can be used to update the DNS dynamically, it may not always be available depending on the administrative policy. In addition, changing an
address but keeping the same DNS name does not really address the underlying concern, since the DNS name becomes a non-changing identifier. Servers generally require a DNS name (so clients can connect to them), and clients often do as well (e.g., some servers refuse to speak to a client whose address cannot be mapped into a DNS name that also maps back into the same address). Section 4 describes one approach to this issue.
In summary, IPv6 addresses on a given interface generated via Stateless Autoconfiguration contain the same interface identifier, regardless of where within the Internet the device connects. This facilitates the tracking of individual devices (and thus, potentially, users). The purpose of this document is to define mechanisms that eliminate this issue in those situations where it is a concern. DHCPV6] for obtaining addresses. Section 12 of [DHCPV6] discusses the use of DHCPv6 for the assignment and management of "temporary addresses", which are never renewed and provide the same property of temporary addresses described in this document with regards to the privacy concern. Another approach, compatible with the stateless address autoconfiguration architecture, would be to change the interface identifier portion of an address over time and generate new addresses from the interface identifier for some address scopes. Changing the interface identifier can make it more difficult to look at the IP addresses in independent transactions and identify which ones actually correspond to the same node, both in the case where the routing prefix portion of an address changes and when it does not. Many machines function as both clients and servers. In such cases, the machine would need a DNS name for its use as a server. Whether the address stays fixed or changes has little privacy implication since the DNS name remains constant and serves as a constant identifier. When acting as a client (e.g., initiating communication), however, such a machine may want to vary the addresses it uses. In such environments, one may need multiple addresses: a "public" (i.e., non-secret) server address, registered in the DNS, that is used to accept incoming connection requests from other machines, and a "temporary" address used to shield the identity of the client when it initiates communication. These two cases are roughly analogous to telephone numbers and caller ID, where a user may list their telephone number in the public phone book, but disable the display of its number via caller ID when initiating calls. To make it difficult to make educated guesses as to whether two different interface identifiers belong to the same node, 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. Picking identifiers from a pseudo-random sequence suffices, so long as the specific sequence cannot be determined by an outsider examining
information that is readily available or easily determinable (e.g., by examining packet contents). This document proposes the generation of a pseudo-random sequence of interface identifiers via an MD5 hash. Periodically, the next interface identifier in the sequence is generated, a new set of temporary addresses is created, and the previous temporary addresses are deprecated to discourage their further use. The precise pseudo-random sequence depends on both a random component and the globally unique interface identifier (when available), to increase the likelihood that different nodes generate different sequences. ADDRCONF]. 2. Create additional addresses based on a random interface identifier for the purpose of initiating outgoing sessions. These "random" or temporary addresses would be used for a short period of time (hours to days) and would then be deprecated. Deprecated address can continue to be used for already established connections, but are not used to initiate new connections. New temporary addresses are generated periodically to replace temporary addresses that expire, with the exact time between address generation a matter of local policy. 3. Produce a sequence of temporary global scope addresses from a sequence of interface identifiers that appear to be random in the sense that it is difficult for an outside observer to predict a future address (or identifier) based on a current one, and it is difficult to determine previous addresses (or identifiers) knowing only the present one. 4. By default, generate a set of addresses from the same (randomized) interface identifier, one address for each prefix for which a global address has been generated via stateless address autoconfiguration. Using the same interface identifier to generate a set of temporary addresses reduces the number of IP multicast groups a host must join. Nodes join the solicited-node multicast address for each unicast address they support, and solicited-node addresses are dependent only on the low-order bits of the corresponding address. This default behavior was made to address the concern that a node that joins a large number of multicast groups may be required to put its interface into promiscuous mode, resulting in possible reduced performance.
A node highly concerned about privacy MAY use different interface identifiers on different prefixes, resulting in a set of global addresses that cannot be easily tied to each other. For example a node MAY create different interface identifiers I1, I2, and I3 for use with different prefixes P1, P2, and P3 on the same interface. Section 3.5. The algorithm also 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 node initiates outgoing communication, temporary addresses can be given preference over public addresses when the device is configured to do so. [ADDR_SELECT] mandates implementations to provide a mechanism, which allows an application to configure its preference for temporary addresses over public addresses. It also allows for an implementation to prefer temporary addresses by default, so that the connections initiated by the node 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 public addresses and override the system defaults.
The random interface identifier generation algorithm, as described in this document, uses MD5 as the hash algorithm. The node MAY use another algorithm instead of MD5 to produce the random interface identifier. RANDOM]. Whenever a new interface identifier is generated, a value generated by the computation is saved in the history value for the next iteration of the algorithm. A randomized interface identifier is created as follows: 1. Take the history value from the previous iteration of this algorithm (or a random value if there is no previous value) and append to it the interface identifier generated as described in [ADDRARCH]. 2. Compute the MD5 message digest [MD5] over the quantity created in the previous step. 3. Take the leftmost 64-bits of the MD5 digest and set bit 6 (the leftmost bit is numbered 0) to zero. This creates an interface identifier with the universal/local bit indicating local significance only. 4. Compare the generated identifier against a list of reserved interface identifiers and to those already assigned to an address on the local device. In the event that an unacceptable identifier has been generated, the node MUST restart the process at step 1 above, using the rightmost 64 bits of the MD5 digest obtained in step 2 in place of the history value in step 1. 5. Save the generated identifier as the associated randomized interface identifier. 6. Take the rightmost 64-bits of the MD5 digest computed in step 2) and save them in stable storage as the history value to be used in the next iteration of the algorithm.
MD5 was chosen for convenience, and because its particular properties were adequate to produce the desired level of randomization. The node MAY use another algorithm instead of MD5 to produce the random interface identifier In theory, generating successive randomized interface identifiers using a history scheme as above has no advantages over generating them at random. In practice, however, generating truly random numbers can be tricky. Use of a history value is intended to avoid the particular scenario where two nodes generate the same randomized interface identifier, both detect the situation via DAD, but then proceed to generate identical randomized interface identifiers via the same (flawed) random number generation algorithm. The above algorithm avoids this problem by having the interface identifier (which will often be globally unique) used in the calculation that generates subsequent randomized interface identifiers. Thus, if two nodes happen to generate the same randomized interface identifier, they should generate different ones on the follow-up attempt. RANDOM] for suggestions on good sources for obtaining random numbers. Note that even though machines may not have stable storage for storing a history value, they will in many cases have configuration information that differs from one machine to another (e.g., user identity, security keys, serial numbers, etc.). One approach to generating a random initial history value in such cases is to use the configuration information to generate some data bits (which may remain constant for the life of the machine, but will vary from one machine to another), append some random data, and compute the MD5 digest as before. CGA], which generate a random interface identifier based on the public key of the node. The goal of CGAs is to prove ownership of an address and to prevent spoofing and stealing of existing IPv6 addresses. They are used for securing neighbor discovery using [SEND]. The CGA random interface identifier generation algorithm may not be suitable for privacy addresses because of the following properties:
o It requires the node to have a public key. This means that the node can still be identified by its public key. o The random interface identifier process is computationally intensive and hence discourages frequent regeneration. ADDRCONF] 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 [ADDRCONF] as follows. When processing a Router Advertisement with a Prefix Information option carrying a global scope prefix for the purposes of address autoconfiguration (i.e., the A bit is set), the node MUST perform the following steps: 1. Process the Prefix Information Option as defined in [ADDRCONF], either creating a new public address or adjusting the lifetimes of existing addresses, both public and temporary. If a received option will extend the lifetime of a public address, the lifetimes of temporary addresses should be extended, subject to 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 approximate target lifetimes for temporary addresses. 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. 3. When a new public address is created as described in [ADDRCONF], the node SHOULD also create a new temporary address. 4. When creating a temporary address, the lifetime values MUST be derived from the corresponding prefix as follows: * Its Valid Lifetime is the lower of the Valid Lifetime of the public address or TEMP_VALID_LIFETIME.
* Its Preferred Lifetime is the lower of the Preferred Lifetime of the public address or 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 the interface's current randomized interface identifier to the prefix that was received. 7. The node MUST perform duplicate address detection (DAD) on the generated temporary address. If DAD indicates the address is already in use, the node MUST generate a new randomized interface identifier as described in Section 3.2 above, and repeat the previous steps as appropriate up to TEMP_IDGEN_RETRIES times. If after TEMP_IDGEN_RETRIES consecutive attempts no non-unique address was generated, the node MUST log a system error and MUST NOT attempt to generate temporary addresses for that interface. Note that DAD MUST be performed on every unicast address generated from this randomized interface identifier. Section 3.3, starting at step 3). Note that, except for the transient period when a temporary address is being regenerated, in normal operation at most one temporary address per prefix should be in a non-deprecated 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. 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 duplicate address detection must be run. The node SHOULD start the address regeneration process REGEN_ADVANCE time units before a temporary address would actually be 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.
Finally, when an interface connects to a new link, a new randomized interface identifier SHOULD be generated immediately together with a new set of temporary addresses. If a device moves from one ethernet to another, generating a new set of temporary addresses from a different randomized interface identifier ensures that the device uses different randomized interface identifiers 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 node. The node MAY follow any process available to it, to determine that the link change has occurred. One such process is described by Detecting Network Attachment [DNA]. ULA] prefixes while still generating temporary addresses for all other global prefixes. Another site might wish to enable temporary address generation only for the prefixes 2001::/16 and 2002::/16, 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 node with respect to the specified prefix subranges. Note that the pre-prefix setting can be applied at any granularity, and not necessarily on a per-subnet basis. The use of temporary addresses may cause unexpected difficulties with some applications. As described below, some servers refuse to accept communications from clients for which they cannot map the IP address into a DNS name. In addition, some applications may not behave robustly if temporary addresses are used and an address expires before the application has terminated, or if it opens multiple sessions, but expects them to all use the same addresses. Consequently, the use of temporary addresses SHOULD be disabled by default in order to minimize potential disruptions. Individual applications, which have specific knowledge about the normal duration of connections, MAY override this as appropriate.
If a very small number of nodes (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 ensure privacy, since the prefix acts as a constant identifier. The procedures described in this document are most effective when the prefix is reasonably non static or is used by a fairly large number of nodes.
ADDRCONF], 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. The determination as to whether to use public versus temporary addresses can in some cases 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. Suitable API extensions will likely need to be developed to enable individual applications to indicate with sufficient granularity their needs with regards to the use of temporary addresses. Recommendations on DNS practices to avoid the problem described in Section 4 when reverse DNS lookups fail may be needed. [DNSOP] contains a more detailed discussion of the DNS- related issues.
While this document discusses ways of obscuring a user's permanent IP address, the method described is believed to be ineffective against sophisticated forms of traffic analysis. To increase effectiveness, one may need to consider use of more advanced techniques, such as Onion Routing [ONION]. RFC 3041 that an implementer of RFC 3041 should be aware of. 1. Excluded certain interface identifiers from the range of acceptable interface identifiers. Interface IDs such as those for reserved anycast addresses [RFC2526], etc. 2. Added a configuration knob that provides the end user with a way to enable or disable the use of temporary addresses on a per- prefix basis. 3. Added a check for denial of service attacks using low valid lifetimes in router advertisements. 4. DAD is now run on all temporary addresses, not just the first one generated from an interface identifier. 5. Changed the default setting for usage of temporary addresses to be disabled. 6. The node is now allowed to generate different interface identifiers for different prefixes, if it so desires. 7. The algorithm used for generating random interface identifiers is no longer restricted to just MD5.
8. Reduced default number of retries to 3 and added a configuration variable. 9. Router advertisement (RA) processing algorithm is no longer included in the document, and is replaced by a reference to [ADDRCONF]. 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. Suresh Krishnan was the sole author of this version of the document. 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. [ADDRARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [ADDRCONF] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. [DISCOVERY] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [ADDR_SELECT] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003. [CGA] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005.
[COOKIES] Kristol, D. and L. Montulli, "HTTP State Management Mechanism", RFC 2965, October 2000. [DDNS] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [DHCPV6] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [DNA] Choi, JH. and G. Daley, "Goals of Detecting Network Attachment in IPv6", RFC 4135, August 2005. [DNSOP] Durand, A., Ihren, J., and P. Savola, "Operational Considerations and Issues with IPv6 DNS", RFC 4472, April 2006. [ONION] Reed, MGR., Syverson, PFS., and DMG. Goldschlag, "Proxies for Anonymous Routing", Proceedings of the 12th Annual Computer Security Applications Conference, San Diego, CA, December 1996. [RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast Addresses", RFC 2526, March 1999. [SEND] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [ULA] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, October 2005.
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