Network Working Group                                      S. Jiang, Ed.
Internet-Draft                                                    F. Xia
Intended status: Standards Track                             B. Sarikaya
Expires: March 3, August 29, 2013                             Huawei Technologies
                                                         August 30, 2012
                                                       February 25, 2013

                      Prefix Assignment in DHCPv6


   This document introduces a generic host-oriented prefix assignment
   mechanism using DHCPv6.  In this new address configuration procedure,
   the prefix is assigned from a DHCPv6 server to hosts through DHCPv6
   message exchanging while the interface identifiers are independently
   generated by the hosts.  It enables both integral address assignment
   and self-generated addresses in one single mechanism, DHCPv6.  It
   also enables stateless address configuration without RA attendance.
   The technique described in this document can be used in networks
   which assign IPv6 addresses using DHCPv6, e.g.  WiMAX.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 3, August 29, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Address Auto-configuration . . . . . . . . . . . . . . . . . .  5
   5.  DHCPv6 Operation . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  DHCPv6 IA_PA Option  . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Identity Association for Prefix Assignment Option  . . . .  7
     6.2.  IA_PA Prefix Option  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA consideration . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2. Informative references . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   A host IPv6 address is combined by a prefix and an interface
   identifier.  Currently, there are two mechanisms to configure a host
   IPv6 address.  [RFC3315] describes the operation of address
   assignment by a DHCPv6 server.  The operation assumes that the server
   is responsible for the assignment of an integral address which
   includes both prefix and interface identifier parts as described in
   [RFC4291].  In the Stateless Address Autoconfiguration (SLACC,
   [RFC4862]) model, the interface Identifier is generated by the host
   itself while the prefix is configured through Router Advertisement
   message defined in [RFC4861].

   However, in a DHCPv6-managed network, assigning 128-bit address is
   insufficient.  Some hosts may want to use self-generated address,
   which are combined by prefixes obtained from network configuration
   and interface identifiers generated by hosts.  The applicable user
   cases include CGA [RFC3972], modified EUI-64 interface identifier
   [EUI-64], temporary addresses for privacy [RFC4941] and etc.

   In these scenarios, the address configuration precedure has to be
   splitted in two motheds: integral address assignment through DHCPv6
   and prefix announcement by RA advertisement.  Some ISPs desire to
   manage address configuration using one set of protocol, rather than
   mixture of DHCPv6 and Neighbor Discovery.

   There are also some network environments in that perfix annoucement
   through RAs may not be the best choice.  For example, hosts may
   connect through tunnels, either layer 2 tunnels or layer 3 tunnels.

   While a RA is only able to announce prefix on a single link, DHCPv6
   configuration can be used to manage multiple links by setup DHCPv6

   Up to now, there is no mechanism for host-oriented prefix assignment
   in DHCPv6.  [RFC3633] defines Prefix Delegation options providing a
   mechanism for automated delegation of IPv6 prefixes using the DHCPv6.
   This mechanism is intended for delegating a long-lived prefix from a
   delegating router to a requesting router.  This mechanism "is not
   bound to the assignment of IP addresses or other configuration
   information to hosts" [RFC3633].  It delegates prefixes to a routable
   device for itself use only.  It does not support the host-generated
   interface identifiers model, in which prefix(es) need to be
   propagated to hosts.

   This document introduces a generic prefix assignment mechanism using
   DHCPv6.  In this new address configuration procedure, the prefix is
   propagated from a DHCPv6 server to hosts through DHCPv6 message
   exchanging while the interface identifiers are independently
   generated by the hosts.  It enables both integral address assignment
   and self-generated addresses in one single mechanism, DHCPv6.  Note,
   in many scenarios, Neighbor Discovery [RFC4861] is still needed for
   routing and reachability.  In other scenarios, this mechanism enables
   stateless address configuration while RA absents.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The terminology in this document is mainly based on the definitions
   in [RFC3315] and [RFC3633].

   Prefix assignment: a DHCPv6 server propagates prefix information to
   hosts in unicast model.

3.  Applicability

   In point-to-point link model, DHCPv6 operation with host-generated
   interface identifier, described in this document, may be used.
   [RFC4968] provides different IPv6 link models that are suitable for
   802.16 based networks and a point-to-point link model is recommended.
   Also, 3GPP and 3GPP2 have earlier adopted the point-to-point link
   model based on the recommendations in [RFC3314].  In this model, one
   prefix can only be assigned to one interface of a host (mobile
   station) and different hosts (mobile stations) can't share a prefix.
   The unique prefix can be used to identify the host.  It is not
   necessary for a DHCPv6 server to generate an interface identifier for
   the host.  The host may generate its interface identifier as
   described in [RFC4941].  An interface identifier could even be
   generated via random number generation.

   [RFC3972] defines Cryptographically Generated Addresses (CGA), which
   is generated from a giving prefix and a public signature key.  For
   security reasons, it is only proper to be generated the user, the
   host itself.  It requests a prefix before the interface identifier
   can be computed.

   Modified EUI-64 interface identifier [EUI-64] is also typically
   generated by hosts.  [RFC4941] has defined temporary addresses for
   privacy purposes.  The temporary addresses is also generated by hosts
   using random algorithm.

   The DHCPv6 operations defined in this document supports
   abovementioned address methods, and the host-generated addresses that
   may defined in the future.

4.  Address Auto-configuration

   Router Advertisements in ND [RFC4861] allow routers to inform hosts
   how to perform Address Auto-configuration.  For example, routers can
   specify whether hosts should use DHCPv6 and/or stateless address
   configuration.  In Router Advertisement message, M and O bits are
   used for indication of address auto-configuration mode.

   Whatever address auto-configuration mode a host uses, the following
   two parts are necessary for the host to formulate it's IPv6 address:

   o  A prefix.  "A bit string that consists of some number of initial
      bits of an address" [RFC4861].  The prefixes can be announced
      through Router Advertisement message.  Prefix assignment from a
      DHCPv6 server is not currently support.
   o  An interface identifier.  "From address autoconfiguration's
      perspective, an interface identifier is a bit string of known
      length" [RFC4862].  Modified EUI-64 interface identifier [EUI-64]
      is a widely-used host generated interface identifier.  It
      generates interface identifier from the host MAC address.  The
      interface identifier of CGA [RFC3972] is generated by computing a
      preifx that will be used to form the CGA and a cryptographic hash
      of a public key of a host.  The host is responsible for interface
      identifier generation.

   In the ND-managed environment, RA is used to assign the prefix.

   So far, there is no mechanism to support the scenario that prefixes
   are managed by a DHCPv6 server.  This document targets to meet this
   gap.  The DHCPv6 operation defined in this document enables the
   DHCPv6 server to assign a prefix, rather than a integral address, to
   the host, so that the host can obtain an IPv6 address by combining
   the prefix with its own generated interface identifier.  It enables
   the auto address configuration through DHCPv6.

5.  DHCPv6 Operation

   Figure 1 shows the operation of separating prefix assignment and
   interface identifier generation in the DHCPv6.

        +------------+         +-------------+
        |Host(Client)|         |DHCPv6 Server|
        +------------+         +-------------+
              |  1 Solicit/Request    |
              |---------------------> |
              |  2 Reply with IA_PA   |
              |<--------------------- |
     3 Combination of Prefix          |
     and Interface Identifier         |
              |                       |

                        Figure 1: DHCPv6 Operation

   1.  A host uses a Solicit message to discover DHCPv6 servers.
       Indications of information requests can be included in the
       Solicit message or a Request message after discovery procedure.
       If a host that wants to use host generated addresses, it SHOULD
       request prefix assignment explicitly by including an IA_PA in a
       Solicit or a Request message, in which an IAID is provided by the
   2.  The DHCPv6 server assigns one or more prefixes to the host in the
       Reply messages responding to the prefix requests from the hosts.
       A server MUST return the same set of prefixes for the same IA_PA
       (as identified by the IAID) as long as those prefixes are still
       valid.  After the lifetimes of the prefixes in an IA_TA have
       expired, the IAID may be reused to identify a new IA_PA with new
       prefix.  If there is not a proper prefix available, a
       NoPrefixAvail (defined in [RFC3633]) status-code is returned to
       the host and the procedure is terminated.
   3.  The host generates an interface identifier and formulates a
       combined IPv6 address by concatenating the assigned prefix and
       the self-generated interface identifier.

   After the host generates an IPv6 address using the above procedure,
   the host may send a Request message to the DHCPv6 server in order to
   confirm the usage of the new address.  The confirmation procedure may
   be completed together with the address registration procedure
   [I-D.ietf-dhc-addr-registration].  However, the confirmation
   procedure is out of scope.

   When the host reaches T1 or T2 defined in Section 6.1, it SHOULD use
   the same message exchanges, as described in section 18, "DHCP Client-
   Initiated Configuration Exchange" of [RFC3315], to obtain or update
   prefix(es) from a DHCPv6 server.

   A DHCPv6 server MAY initiatively send a reconfiguration message to
   the host, as described in section 19, "DHCP Server-Initiated
   Configuration Exchange" of [RFC3315], to cause prefix(es) information

   If an IA_PA capable client connects to a network, and the DHCPv6
   server is not IA_PA capable, the Solicit or Request message with
   IA_PA Option will result in no Reply, Reply without IA_PAs, or Reply
   with a Status Code containing UnspecFail.  The client MAY decide the
   network does not support IA_PA immediately or after a period of
   soliciting (with limited retransmissions times).  Then, it MAY
   "failover" to IA_NA/IA_TA requests.

6.  DHCPv6 IA_PA Option

   In this section, one new option is defined, Identity Association for
   Prefix Assignment Option .  The format of this new DHCPv6 IA_PA
   Option has been deliberately designed to be the same with IA_PD
   option[RFC3633].  The IA_PD Prefix and IA Address sub-options from
   IA_PD option are also reused.  However, the two options are different
   on the semantics and usage models.

   Comparing with Prefix Information Option in ND, Section 4.6.2 of
   [RFC4861], the IA_PA option does not provide L flag and A flag.  The
   A (autonomous address-configuration flag) isn't need obviously
   because the IA_PA is implicit for stateless address configuration.
   Because the IA_PA is only address relevant, it does not relevant to
   reachability or routing and the DHCPv6 server may not sure the on-
   link state.  So L (on-Link) flag is not include.  The DHCPv6 client
   should treat the prefix as same as L flag not set, which makes no
   statement about on-link or off-link properties of the prefix.

6.1.  Identity Association for Prefix Assignment Option

   The IA_PA option is used to carry a prefix assignment identity
   association, the parameters associated with the IA_PA and the
   prefixes associated with it.

   The format of the IA_PA option is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |         OPTION_IA_PA          |         option-length         |
       |                         IAID (4 octets)                       |
       |                              T1                               |
       |                              T2                               |
       .                                                               .
       .                          IA_PA-options                        .
       .                                                               .
   option-code:      OPTION_IA_PA (TBA1)

   option-length:    12 + length of IA_PA-options field.

   IAID:             The unique identifier for this IA_PA; the IAID must
                     be unique among the identifiers for all of this
                     host's IA_PAs. The number space for IA_PA IAIDs is
                     separate from the number spaces for IA_TA and IA_NA

   T1:               The time at which the host should
                     contact the DHCPv6 server from which the
                     prefixes in the IA_PA were obtained to extend the
                     lifetimes of the prefixes assigned to the IA_PA;
                     T1 is a time duration relative to the current time
                     expressed in units of seconds.

   T2:               The time at which the host should
                     contact any available DHCPv6 server to extend
                     the lifetimes of the prefixes assigned to the
                     IA_PA; T2 is a time duration relative to the
                     current time expressed in units of seconds.

   IA_PA-options:    Options associated with this IA_PA.

   The details of the fields are similar to the IA_PD option description
   in [RFC3633].  The difference is here a DHCPv6 server and a host
   involved, while a delegating router and requesting router involved in

6.2.  IA_PA Prefix Option

   OPTION_IAPREFIX (26) "IA_PD Prefix Option" defined in Section 10 of
   [RFC3633] is reused.

   Originally, the option is used for conveying prefix information
   between a delegating router and a requesting router.  Here the IA_PD
   Prefix option is used to specify IPv6 address prefixes associated
   with an IA_PA in Section 6.1.  The IA_PD Prefix option must be
   encapsulated in the IA_PA-options field of an IA_PA option.

   Note, the PD_EXCLUDE option [RFC6603] SHOULD NOT be encapsulated in
   the IAPREFIX options that are encapsulated in an IA_PA.

7.  IANA consideration

   This document defines a new DHCPv6 [RFC3315] option, which must be
   assigned Option Type values within the option numbering space for
   DHCPv6 messages:

   The OPTION_IA_PA Option (TBA1), described in Section 6.1.

8.  Security Considerations

   Security considerations in DHCPv6 are described in [RFC3315].

   To guard against attacks through prefix assignment, a host and a
   DHCPv6 server SHOULD use DHCPv6 authentication as described in
   Section 21, "Authentication of DHCP messages" of [RFC3315] or Secure
   DHCPv6 [I-D.ietf-dhc-secure-dhcpv6] .

9.  Acknowledgements

   The authors would like to thanks Suresh Krishnan, Ted Lemon, Bing
   Liu, Andre Kostur, Gaurav Halwasia, Bernie Volz and other members of
   DHC WG for their valuable comments.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

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

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

   [RFC6603]  Korhonen, J., Savolainen, T., Krishnan, S., and O. Troan,
              "Prefix Exclude Option for DHCPv6-based Prefix
              Delegation", RFC 6603, May 2012.

10.2.  Informative references

   [RFC3314]  Wasserman, M., "Recommendations for IPv6 in Third
              Generation Partnership Project (3GPP) Standards",
              RFC 3314, September 2002.

   [RFC4968]  Madanapalli, S., "Analysis of IPv6 Link Models for 802.16
              Based Networks", RFC 4968, August 2007.

              Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs",
              draft-ietf-dhc-secure-dhcpv6-07 (work in progress),
              September 2012.

              Jiang, S. and G. S., Chen, G., and S. Krishnan, "A Generic IPv6
              Addresses Registration Solution Using DHCPv6",
              draft-ietf-dhc-addr-registration-01 (work in progress),
              October 2012.

   [EUI-64]   "Guidelines for 64-bit Global Identifier (EUI-64)
              Registration Authority",
              regauth/oui/tutorials/EUI64.html", March 1997.

Authors' Addresses

   Sheng Jiang (editor)
   Huawei Technologies
   Q14, Huawei Campus, No.156, BeiQing Road
   Hai-Dian District, Beijing  100095
   P.R. China


   Frank Xia
   Huawei Technologies
   1700 Alma Dr. Suite 500
   Plano, TX  75075


   Behcet Sarikaya
   Huawei Technologies
   1700 Alma Dr. Suite 500
   Plano, TX  75075