DHCPv6 Extension Practices and Considerations

Document Type Active Internet-Draft (dhc WG)
Authors Ren Gang  , Lin He  , Ying Liu 
Last updated 2021-05-18
Replaces draft-ren-dhc-problem-statement-of-mredhcpv6
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Dynamic Host Configuration (DHC)                                  G. Ren
Internet-Draft                                                     L. He
Intended status: Informational                                    Y. Liu
Expires: November 19, 2021                           Tsinghua University
                                                            May 18, 2021

             DHCPv6 Extension Practices and Considerations


   IP addresses assume an increasing number of attributes as
   communication identifiers to meet different requirements.  Privacy
   protection, accountability, security, and manageability of networks
   can be supported by extending the DHCPv6 protocol as required.  This
   document provides current extension practices and typical DHCPv6
   server software in terms of extensions, defines a general model of
   DHCPv6, discusses some extension points, and presents extension

Status of This Memo

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Current Extension Practices . . . . . . . . . . . . . . . . .   4
     3.1.  Standardized and Non-standardized DHCPv6 Extension Cases    4
     3.2.  Current DHCPv6 Server Software Cases  . . . . . . . . . .   4
   4.  Extension Discussion  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  DHCPv6 General Model  . . . . . . . . . . . . . . . . . .   5
     4.2.  Extension Points  . . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Messages  . . . . . . . . . . . . . . . . . . . . . .   5
       4.2.2.  Options . . . . . . . . . . . . . . . . . . . . . . .   6
       4.2.3.  Message Processing Functions  . . . . . . . . . . . .   6
       4.2.4.  Address Generation Mechanisms . . . . . . . . . . . .   7
     4.3.  Extension Principles  . . . . . . . . . . . . . . . . . .   8
   5.  Extension Cases . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   IP addresses play an essential role in communication over the
   Internet.  Their generation and assignment are also closely linked to
   the privacy protection, accountability, security, and manageability
   of the network [draft-gont-v6ops-ipv6-addressing-considerations].
   The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC8415]
   is an important network protocol that can be used to dynamically
   provide IPv6 addresses and other network configuration parameters to
   IPv6 nodes.  DHCPv6 can be continuously extended and improved through
   new options DHCPv6 can be continually extended and improved with new
   options, protocols, and message processing mechanisms.

   IP addresses assume an increasing number of properties as
   communication identifiers to meet different requirements.  For
   example, APNA [APNA] and PAVI [PAVI] use addresses to enhance source
   responsibility and privacy protection.  These requirements often need
   to be reflected by IP address assignment protocols such as DHCPv6.
   Therefore, extensions to DHCPv6 are made to meet a wide variety of

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   requirements, which is referred to as multi-requirement extensions to
   DHCPv6.  However, it is not easy to extend DHCPv6 to meet a variety
   of requirements.  Although DHCPv6 offers increasingly comprehensive
   functionality and DHCPv6 server software provides extension
   interfaces that allow administrators to change and customize the way
   they process and respond to DHCPv6 messages, there is still a lack of
   comprehensive understanding of where and how to extend in DHCPv6
   effectively.  Therefore, a detailed analysis is needed to clarify the
   issues and design principles and extract and unify design
   specifications to help better address the multi-demand scaling

   In summary, multi-requirement extensions for DHCPv6 can be made to
   support administrator-defined features or to meet application
   requirements.  Since DHCPv6 is an important and useful protocol
   related to IPv6 address generation, it can provide more extensions
   and flexible features to meet administrator or application
   requirements.  Based on careful design principles, interfaces can be
   defined to support more customized multi-requirement extensions
   without sacrificing the stability of DHCPv6.

   Some people would suggest that administrators modify the open-source
   DHCPv6 server to solve their problems.  However, it takes
   considerable time to understand the code of an open-source DHCPv6
   server, not to mention the time-consuming task of debugging errors,
   failures, or system crashes caused by modifying complex modules.
   Another problem is that as open-source software evolves, the source
   code of the server software may change (new features or bug fixes).
   Once the latest version of the open-source server software comes out
   [kea_dhcp_hook_developers_guide], users may need to rewrite their
   code.  Therefore, the multi-requirement extensions to DHCPv6 to
   address the specific issues of administrators are essential and

   This document provides a survey of current extension practices and
   typical DHCPv6 server softwares on extensions and gives DHCPv6
   extension considerations by defining a DHCPv6 general model,
   discussing the extension problems, and presenting extension cases.

2.  Terminology

   Familiarity with DHCPv6 and its terminology, as defined in [RFC8415],
   is assumed.

   Multi-requirement extensions:  According to the administrator or
      application requirements, DHCPv6 extensions can cover several
      aspects, e.g., privacy protection, accountability, and security.
      These extensions can be accomplished by defining new messages,

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      options, message processing functions, or address generation
      mechanisms for DHCPv6.

3.  Current Extension Practices

3.1.  Standardized and Non-standardized DHCPv6 Extension Cases

   Many documents attempt to extend DHCPv6.  They can be classified into
   three categories.

   Extended options    Most extensions for DHCPv6 are implemented in
                       this way.  New-defined options carry specific
                       parameters in DHCPv6 messages, which helps DHCPv6
                       clients or servers know the detailed situation
                       with each other.

   Extended messages   Some documents define new protocols that aim to
                       achieve specific goals, e.g., active leasequery
                       [RFC7653], General Address Generation and
                       Management System [GAGMS].

   Extended entities   Some documents introduce third-party entities
                       into the communications of DHCPv6 to achieve
                       specific goals and provide better services, e.g.,
                       authentication [RFC7037].

3.2.  Current DHCPv6 Server Software Cases

   A lot of commercial and open source DHCPv6 servers exist, including
   Cisco Prime Network Registrar (CPNR) DHCP [CPNR], DHCP Broadband
   [ISC_DHCP], Kea DHCP [Kea_DHCP], Microsoft DHCP [Microsoft_DHCP],
   Nominum DHCP [Nominum_DHCP], VitalQIP [VitalQIP], and WIDE DHCPv6
   [WIDE_DHCPv6].  Commercial and open-source DHCPv6 software often
   considers the extensions of DHCPv6 servers because they cannot always
   meet the requirements that the administrators want.  For example,
   CPNR DHCP server provides extension APIs and allows administrators to
   write extensions and functions to alter and customize how it handles
   and responds to DHCP requests.  A network operator usually decides
   what packet process to modify, how to modify, and which extension
   point to attach the extension.  Then the network operator writes the
   extension and adds the well-written extension to the extension point
   of the DHCP server.  Finally, the network operator reloads the DHCP
   server and debugs whether the server runs as it expects.  Similarly,
   Kea DHCP provides hook mechanisms, a well-designed interface for
   third-party code, to solve the problem that the DHCP server does not
   quite do what a network operator require.

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4.  Extension Discussion

   This section elaborates multi-requirement extensions for DHCPv6.
   Section 4.1 describes the general model of DHCPv6, while Section 4.2
   analyzes the extension points and requirements.

4.1.  DHCPv6 General Model

   Figure 1 summarizes the DHCPv6 general model and its possible
   extensions: messages, options, message processing functions, and
   address generation mechanisms.

+-----------------+                  +----------------+
|  DHCPv6 client  | DHCPv6 messages  |  DHCPv6 relay  |
| +-------------+ |   with options   | +------------+ | External inputs
| |  Message    | |<---------------->| |  Message   | |<----------------
| | processing  | |                  | | relaying   | | e.g., RADIUS
| | functions   | |                  | | functions  | | option [RFC7037]
| +-------------+ |                  | +------------+ |
+-----------------+                  +----------------+
                              DHCPv6 messages |
                                with options  |
+-----------------+               +----------------------------+
|                 |   Extended    |        DHCPv6 server       |
|                 |   messages    | +-----------+ +----------+ |
|External entities|<------------->| |  Address  | | Message  | |
|                 |  e.g., Active | | generation| |processing| |
|                 |  leasequery   | | mechanisms| |functions | |
|                 |  [RFC7653]    | +-----------+ +----------+ |
+-----------------+               +----------------------------+

        Figure 1: DHCPv6 general model and its possible extensions.

4.2.  Extension Points

4.2.1.  Messages

   On the one hand, new messages can be designed and added to the DHCPv6
   protocol to enrich its functionalities.  For example, [RFC5007]
   defines new leasequery messages to allow a requestor to retrieve
   information on the bindings for a client from one or more servers.
   [RFC5460] expands on the Leasequery protocol by defines new messages
   and allowing for bulk transfer of DHCPv6 binding data via TCP.
   [RFC7653] defines active leasequery messages to keep the requestor up
   to date with DHCPv6 bindings.  [RFC8156] defines failover messages to

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   provide a mechanism for running two servers with the capability for
   either server to take over clients' leases in case of server failure
   or network partition.

   On the other hand, people are concerned about the security and
   privacy issues of the DHCPv6 protocol.  [RFC7824] describes the
   privacy issues associated with the use of DHCPv6, respectively.
   DHCPv6 does not provide privacy protection on messages and options.
   Other nodes can see the options transmitted in DHCPv6 messages
   between DHCPv6 clients and servers.  Extended messages can be
   designed to secure exchanges between DHCPv6 entities.

4.2.2.  Options

   DHCPv6 allows defining options to transmit parameters between DHCPv6
   entities for common requirements, e.g., DNS configurations [RFC3646],
   NIS configurations [RFC3898], SNTP configurations [RFC4075], relay
   agent subscriber-id [RFC4580], relay agent remote-id [RFC4649], FQDN
   configurations [RFC4704], relay agent echo request [RFC4994], network
   boot [RFC5970], Relay-Supplied Options [RFC6422], virtual subnet
   selection [RFC6607], client link-layer address [RFC6939], and
   softwire source binding prefix hint [RFC8539].  Also, these
   parameters may come from external entities.  For example, [RFC7037]
   defines RADIUS option to exchange authorization and identification
   information between the DHCPv6 relay agent and DHCPv6 server.

   In other cases, network operators may require DHCPv6 messages to
   transmit some self-defined options between clients and servers.
   Currently, the vendor-specific information option allows clients and
   servers to exchange vendor-specific information.  Therefore,
   administrative domains can define and use the sub-options of the
   vendor-specific information option to serve their private purposes.
   The content of the self-defined options may come from two sources:
   devices and users.  If the content of self-defined options comes from
   users, two methods can be used to solve the problem.  The first one
   is that the clients provide related interfaces to receive such
   information, which is currently merely supported.  The second one is
   that DHCPv6 relays obtain such information and add it to the clients'
   requests.  But this always depends on other protocols to allow DHCPv6
   relays to get the information first.

4.2.3.  Message Processing Functions

   Although current commercial or open-source DHCPv6 server softwares
   provide comprehensive functionalities, they still cannot meet all
   customers' requirements of processing DHCPv6 requests.  Therefore,
   they will offer interfaces that customers can use to write their
   specific extensions to affect the way how DHCPv6 servers handle and

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   respond to DHCP requests.  For example, a network operator may want
   his DHCPv6 server to communicate with external servers.  Thus, he may
   alter his DHCPv6 server through the given extensions to achieve such
   a goal.  However, not all DHCPv6 software considers this extension.

4.2.4.  Address Generation Mechanisms

   Currently, the DHCPv6 servers assign addresses, prefixes and other
   configuration options according to their configured policies.
   Generally, different networks may prefer different address generation
   mechanisms.  Several address generation mechanisms for SLAAC
   [RFC4862] (e.g., IEEE 64-bit EUI-64 [RFC2464], Constant, semantically
   opaque [Microsoft], Temporary [RFC4941], and Stable, semantically
   opaque [RFC7217]) proposed for different requirements can be utilized
   in DHCPv6 protocol as well.  Note that [RFC7943] is the DHCPv6
   version of Stable, semantically opaque [RFC7217].  The many types of
   IPv6 address generation mechanisms available have brought about
   flexibility and diversity.  Therefore, corresponding interfaces could
   be open and defined to allow other address generation mechanisms to
   be configured.

   Moreover, several basic operations are defined to support the design
   of IPv6 addresses generation mechanisms.  A new IPv6 address
   generation mechanism can be made up of the combination of the
   following basic operations.  Also, new basic operations can be
   defined to support new functions.

   Invert(x, n)           invert bit n of input x.

   Insert(x, n, s)        insert s after bit n of input x.

   Concatenate(x, y, ...) concatenate input [x, y, ...] sequentially.

   Replace(x, n, m, s)    change from bit n to bit m of input x into s.
                          Note that the length of s must be equal to
                          m-n+1.  When n=m, change only one bit of input

   Truncate(x, n, m)      truncate from bit n to bit m of input x as the

   Encrypt(x, k)          use some specific encryption algorithm to
                          encrypt input x with key k.  Encryption
                          algorithms can be IDEA, AES, RSA, etc.

   Hash(x)                calculate the hash digest value of input x.
                          Hash algorithms can be MD5, SHA1, SHA256, etc.

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   For example, temporary addresses in [RFC4941] can be expressed as
   tempAddr(eui64, history) = Replace(Truncate(Hash(Concatenate(eui64,
   history)), 0, 63), 6, 6, 0), where eui64 means the EUI-64 identifier
   defined in [RFC2464] and history means a history value defined in

4.3.  Extension Principles

   The principles used to conduct multi-requirement extensions for
   DHCPv6 are summarized as follows:

      1) Do not change the basic design of DHCPv6.

      2) Use simpler interfaces to define and support more extensions.

5.  Extension Cases

   Administrative domains may enforce local policies according to their
   requirements, e.g., authentication, accountability.  Several kinds of
   multi-requirement extensions are presented in this section, including
   configurations in current DHCPv6 software, option definition and
   server modification, and message definition between DHCPv6 entities
   and third-party entities.

   Currently, many DHCPv6 servers provide administrative mechanisms,
   e.g., host reservation and client classification.  Host reservation
   is often used to assign certain parameters (e.g., IP addresses) to
   specific devices.  Client classification is often used to
   differentiate between different types of clients and treat them
   accordingly in certain cases.

   More complicated extensions of DHCPv6 are needed to meet specific
   requirements.  For example, considering such a requirement that
   DHCPv6 servers assign IPv6 addresses generated by user identifiers to
   the clients in a network to hold users accountable, two extensions
   should be fulfilled to meet this requirement.  The first one is that
   clients send their user identifiers to servers.  This can be achieved
   by defining and using sub-options of vendor-specific information
   option.  The second one is that servers use user identifiers to
   generate IP addresses.  To achieve this goal, extension mechanisms
   provided by the server software such as extension points in CPNR
   [CPNR] and hook mechanisms in Kea DHCP [Kea_DHCP] can be used.

   Some extensions for DHCPv6 may need the support of third-party
   entities.  For example, [RFC7037] introduces RADIUS entities into the
   message exchanges between DHCPv6 entities for better service
   provision.  The authentication in [RFC7037] can also be used to meet
   the accountability requirement mentioned above because it is

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   important to authenticate users first before assigning IP addresses
   generated from user identifiers.  Usually, this kind of extension
   requires the definition of messages communicated between DHCPv6
   entities and third-party entities, e.g., active leasequery [RFC7653].

   IPv6 addresses are related to manageability, security, traceability,
   and accountability of networks.  As DHCPv6 assigns IPv6 addresses to
   IPv6 nodes, it is important that DHCPv6 provides interfaces to allow
   administrative domains to conduct extensions to meet their multi-

6.  Security Considerations

   Security issues related with DHCPv6 are described in Section 22 of

7.  IANA Considerations

   This document does not include an IANA request.

8.  Acknowledgements

   The authors would like to thank Bernie Volz, Tomek Mrugalski, Sheng
   Jiang, and Jinmei Tatuya for their comments and suggestions that
   improved the [draft-ren-dhc-mredhcpv6].  Some ideas and thoughts of
   [draft-ren-dhc-mredhcpv6] are contained in this document.

9.  References

9.1.  Normative References

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,

9.2.  Informative References

   [APNA]     Lee, T., Pappas, C., Barrera, D., Szalachowski, P., and A.
              Perrig, "Source Accountability with Domain-brokered
              Privacy", December 2016.

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   [CPNR]     Cisco, "Cisco Prime Network Registrar", 2018,

              Weird Solutions, "DHCP Broadband", 2018,

              Gont, F. and G. Gont, "IPv6 Addressing Considerations",
              February 2021.

              Ren, G., He, L., and Y. Liu, "Multi-requirement Extensions
              for Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)", March 2017.

              FreeRADIUS, "FreeRADIUS DHCP", 2017,

   [GAGMS]    Liu, Y., He, L., and G. Ren, "GAGMS: A Requirement-Driven
              General Address Generation and Management System",
              November 2017.

              Internet System Consortium, "ISC DHCP", 2018,

              Internet System Consortium, "Kea DHCP", 2018,

              Internet Systems Consortium, "Hook Developer's Guide",
              2018, <https://jenkins.isc.org/job/Kea_doc/doxygen/df/d46/

              Microsoft, "IPv6 interface identifiers", 2013, <https://ww

              Microsoft, "Microsoft DHCP", 2008,

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              Nominum, "Nominum DHCP", 2012,

   [PAVI]     He, L., Ren, G., Liu, Y., and J. Yang, "PAVI:
              Bootstrapping Accountability and Privacy to IPv6
              Internet", April 2021.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              DOI 10.17487/RFC3646, December 2003,

   [RFC3898]  Kalusivalingam, V., "Network Information Service (NIS)
              Configuration Options for Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6)", RFC 3898,
              DOI 10.17487/RFC3898, October 2004,

   [RFC4075]  Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
              Configuration Option for DHCPv6", RFC 4075,
              DOI 10.17487/RFC4075, May 2005,

   [RFC4580]  Volz, B., "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6) Relay Agent Subscriber-ID Option", RFC 4580,
              DOI 10.17487/RFC4580, June 2006,

   [RFC4649]  Volz, B., "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6) Relay Agent Remote-ID Option", RFC 4649,
              DOI 10.17487/RFC4649, August 2006,

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, DOI 10.17487/RFC4704, October 2006,

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   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC4994]  Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
              "DHCPv6 Relay Agent Echo Request Option", RFC 4994,
              DOI 10.17487/RFC4994, September 2007,

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, DOI 10.17487/RFC5007,
              September 2007, <https://www.rfc-editor.org/info/rfc5007>.

   [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
              DOI 10.17487/RFC5460, February 2009,

   [RFC5970]  Huth, T., Freimann, J., Zimmer, V., and D. Thaler, "DHCPv6
              Options for Network Boot", RFC 5970, DOI 10.17487/RFC5970,
              September 2010, <https://www.rfc-editor.org/info/rfc5970>.

   [RFC6422]  Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
              RFC 6422, DOI 10.17487/RFC6422, December 2011,

   [RFC6607]  Kinnear, K., Johnson, R., and M. Stapp, "Virtual Subnet
              Selection Options for DHCPv4 and DHCPv6", RFC 6607,
              DOI 10.17487/RFC6607, April 2012,

   [RFC6939]  Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
              Address Option in DHCPv6", RFC 6939, DOI 10.17487/RFC6939,
              May 2013, <https://www.rfc-editor.org/info/rfc6939>.

   [RFC7037]  Yeh, L. and M. Boucadair, "RADIUS Option for the DHCPv6
              Relay Agent", RFC 7037, DOI 10.17487/RFC7037, October
              2013, <https://www.rfc-editor.org/info/rfc7037>.

   [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,

   [RFC7653]  Raghuvanshi, D., Kinnear, K., and D. Kukrety, "DHCPv6
              Active Leasequery", RFC 7653, DOI 10.17487/RFC7653,
              October 2015, <https://www.rfc-editor.org/info/rfc7653>.

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   [RFC7824]  Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              Considerations for DHCPv6", RFC 7824,
              DOI 10.17487/RFC7824, May 2016,

   [RFC7943]  Gont, F. and W. Liu, "A Method for Generating Semantically
              Opaque Interface Identifiers (IIDs) with the Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 7943,
              DOI 10.17487/RFC7943, September 2016,

   [RFC8156]  Mrugalski, T. and K. Kinnear, "DHCPv6 Failover Protocol",
              RFC 8156, DOI 10.17487/RFC8156, June 2017,

   [RFC8539]  Farrer, I., Sun, Q., Cui, Y., and L. Sun, "Softwire
              Provisioning Using DHCPv4 over DHCPv6", RFC 8539,
              DOI 10.17487/RFC8539, March 2019,

              Nokia, "Nokia VitalQIP", 2017,

              KAME project, "WIDE DHCPv6", 2008,

Authors' Addresses

   Gang Ren
   Tsinghua University
   Beijing  100084

   Phone: +86-010 6260 3227
   Email: rengang@cernet.edu.cn

   Lin He
   Tsinghua University
   Beijing  100084

   Email: he-l14@mails.tsinghua.edu.cn

Ren, et al.             Expires November 19, 2021              [Page 13]
Internet-Draft       problem statement of mredhcpv6             May 2021

   Ying Liu
   Tsinghua University
   Beijing  100084

   Email: liuying@cernet.edu.cn

Ren, et al.             Expires November 19, 2021              [Page 14]