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rfc:rfc6653

Independent Submission B. Sarikaya Request for Comments: 6653 F. Xia Category: Informational Huawei USA ISSN: 2070-1721 T. Lemon

                                                               Nominum
                                                             July 2012
   DHCPv6 Prefix Delegation in Long-Term Evolution (LTE) Networks

Abstract

 As interest in IPv6 deployment in cellular networks increases,
 several migration issues have been being raised; IPv6 prefix
 management is the issue addressed in this document.  Based on the
 idea that DHCPv6 servers can manage prefixes, we use DHCPv6 Prefix
 Delegation to address such prefix management issues as an access
 router offloading delegation of prefixes and release tasks to a
 DHCPv6 server.  The access router first requests a prefix for an
 incoming mobile node from the DHCPv6 server.  The access router may
 next do stateless or stateful address allocation to the mobile node,
 e.g., with a Router Advertisement or using DHCP.  We also describe
 prefix management using Authentication, Authorization, and Accounting
 (AAA) servers.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6653.

Sarikaya, et al. Informational [Page 1] RFC 6653 Prefix Delegation July 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.

Table of Contents

 1. Introduction ....................................................3
 2. Terminology and Acronyms ........................................4
 3. Prefix Delegation Using DHCPv6 ..................................5
    3.1. Prefix Request Procedure for Stateless Address
         Configuration ..............................................5
    3.2. Prefix Request Procedure for Stateful Address
         Configuration ..............................................7
    3.3. The MN as Requesting Router in Prefix Delegation ...........8
    3.4. Prefix Release Procedure ...................................9
    3.5. Miscellaneous Considerations ...............................9
         3.5.1. How to Generate an IAID .............................9
         3.5.2. Policy to Delegate Prefixes ........................10
 4. Prefix Delegation Using RADIUS and Diameter ....................10
 5. Security Considerations ........................................11
 6. Acknowledgements ...............................................12
 7. Informative References .........................................12

Sarikaya, et al. Informational [Page 2] RFC 6653 Prefix Delegation July 2012

1. Introduction

 Figure 1 illustrates the key elements of a typical cellular access
 network.  In a Long-Term Evolution (LTE) network, the Access Router
 (AR) is the Packet Data Network (PDN) Gateway [3GPP-23401].
                                    +-------------+
                                    |   +------+  |
                                    |   |DHCP  |  |

+—–+ +—–+ +——+ +——+ | |Server| | +——+ | MN |–| BS |–+Access+–+Access+-+ +——+ +-+Border| +—–+ +—–+ | GW | |Router| |IP Network(s)| |Router+-Internet

                 +--+---+  +--+---+ |             | +------+
                    |         |     +-------------+

+—–+ +—–+ | | +——+ | MN |–| BS |—–+ | |AAA | +—–+ +—–+ +— |Server|

                                   +------+
         Figure 1: Key Elements of a Typical Cellular Network
 The Mobile Node (MN) attaches to a Base Station (BS) through an LTE
 air interface.  A BS manages connectivity of User Equipment (UE) and
 extends connections to an Access Gateway (GW), e.g., the Serving
 Gateway (S-GW) in an LTE network.  The access GW and the AR are
 connected via an IP network.  The AR is the first-hop router of the
 MNs and is in charge of address/prefix management.
 The AR is connected to an IP network that is owned by the operator;
 this network is connected to the public Internet via a border router.
 The network contains servers for subscriber management, including
 Quality of Service, billing, and accounting, as well as a Dynamic
 Host Configuration Protocol (DHCP) server [RFC6342].
 With IPv6 addressing, because mobile network links are point-to-point
 (P2P), the per-MN interface prefix model is used [RFC3314] [RFC3316].
 In the per-MN interface prefix model, prefix management is an issue.
 When an MN attaches to an AR, the AR requests one or more prefixes
 for the MN.  When the MN detaches from the AR, the prefixes should be
 released.  When the MN becomes idle, the AR should keep (i.e., not
 release) the allocated prefixes.
 This document describes how to use DHCPv6 Prefix Delegation
 (DHCPv6-PD) in mobile networks, such as networks based on standards
 developed by the 3rd Generation Partnership Project (3GPP) and it
 could easily be adopted by the Worldwide Interoperability for
 Microwave Access (WiMAX) Forum as well.  In view of migration to

Sarikaya, et al. Informational [Page 3] RFC 6653 Prefix Delegation July 2012

 IPv6, the number of MNs connected to the network at a given time may
 become very high.  Traditional techniques such as prefix pools are
 not scalable.  In such cases, DHCPv6-PD becomes the viable approach
 to take.
 The techniques described in this document have not been approved by
 the IETF or the 3GPP, except for those techniques described below in
 Section 3.3.  This document is not a Standard or Best Current
 Practice.  This document is published only for possible consideration
 by operators.
 This document is useful when address space needs to be managed by
 DHCPv6-PD.  There are obviously other means of managing address
 space, including having the AR track internally what address space is
 used by what mobile.

2. Terminology and Acronyms

 3GPP - 3rd Generation Partnership Project
 AAA - Authentication, Authorization, and Accounting
 AR - Access Router
 BS - Base Station
 DHCP - Dynamic Host Configuration Protocol
 E-UTRAN - Evolved Universal Terrestrial Radio Access Network
 GPRS - General Packet Radio Service
 LTE - Long-Term Evolution
 MN - Mobile Node
 P2P - Point-to-Point
 PD - Prefix Delegation
 PDN - Packet Data Network
 S-GW - Serving Gateway
 WiMAX - Worldwide Interoperability for Microwave Access

Sarikaya, et al. Informational [Page 4] RFC 6653 Prefix Delegation July 2012

3. Prefix Delegation Using DHCPv6

 "Access router" refers to the cellular network entity that has a DHCP
 client.  According to [3GPP-23401], the DHCP client is located in the
 PDN Gateway, and so the AR is the PDN Gateway in the LTE
 architecture.

3.1. Prefix Request Procedure for Stateless Address Configuration

 There are two function modules in the AR: the DHCP client and the
 DHCP relay.  DHCP messages should be relayed if the AR and a DHCP
 server are not directly connected; otherwise, the DHCP relay function
 in the AR is not necessary.  Figure 2 illustrates a scenario in which
 the AR and the DHCP server aren't directly connected:
   +-------+               +----------------------+    +-----------+
   |  MN   |               |       AR             |    |DHCP Server|
   +-------+               |DHCP     |  Relay     |    +-----------+
       |                   |Client   |  Agent     |          |
       |                   +----------------------+          |
       |1 Initial NW entry |                                 |
       |or attach procedure|                                 |
       |<----------------->|                                 |
       |                   |2 Solicit                        |
       |                   |--------->         Relay-forward |
       |                   |                 --------------->|
       |                   |                   3 Relay-reply |
       |                   |Advertise        <---------------|
       |                   |<--------                        |
       |                   |4 Request                        |
       |                   |--------->         Relay-forward |
       |                   |                 --------------->|
       |                   |                   5 Relay-reply |
       |                   |Reply            <---------------|
       |                   |<--------                        |
       |6  Attach          |                                 |
       | Completed         |                                 |
       |<----------------->|                                 |
       |7 Router           |                                 |
       |  Solicitation     |                                 |
       |------------------>|                                 |
       | 8 Router          |                                 |
       |  Advertisement    |                                 |
       |<------------------|                                 |
                       Figure 2: Prefix Request

Sarikaya, et al. Informational [Page 5] RFC 6653 Prefix Delegation July 2012

 1.  An MN (also referred to as UE, or User Equipment, by the 3GPP)
     performs initial network entry and authentication procedures,
     a.k.a. the attach procedure.
 2.  On successful completion of Step 1, the AR initiates the DHCP
     Solicit procedure to request prefixes for the MN.  The DHCP
     client in the AR creates and transmits a Solicit message as
     described in Sections 17.1.1 ("Creation of Solicit Messages") and
     17.1.2 ("Transmission of Solicit Messages") of [RFC3315].  The
     DHCP client in an AR that supports DHCPv6 Prefix Delegation
     [RFC3633] creates an Identity Association for Prefix Delegation
     (IA_PD) and assigns it an Identity Association IDentifier (IAID).
     The client must include the IA_PD option in the Solicit message.
     The DHCP client as Requesting Router (RR) must set the
     prefix-length field to a value less than, e.g., 48 or equal to 64
     to request a /64 prefix.  Next, the relay agent in the AR sends
     to the DHCP server a Relay-forward message in which a Solicit
     message is encapsulated.
 3.  The DHCP server sends an Advertise message to the AR in the same
     way as that described in Section 17.2.2 ("Creation and
     Transmission of Advertise Messages") of [RFC3315].  An Advertise
     message with the IA_PD shows that the DHCP server is capable of
     delegating prefixes.  This message is received encapsulated in a
     Relay-reply message by the relay agent in the AR and is sent as
     an Advertise message to the DHCP client in the AR.
 4.  The AR (DHCP client and relay agent) uses the same message
     exchanges as those described in Section 18 ("DHCP Client-
     Initiated Configuration Exchange") of [RFC3315] and in [RFC3633]
     to obtain or update prefixes from the DHCP server.  The AR (DHCP
     client and relay agent) and the DHCP server use the IA_PD Prefix
     option to exchange information about prefixes in much the same
     way as IA Address options are used for assigned addresses.  This
     is accomplished by the AR sending a DHCP Request message and the
     DHCP server sending a DHCP Reply message.
 5.  The AR stores the prefix information it received in the Reply
     message.
 6.  A connection between the MN and AR is established, and the link
     becomes active.  This step completes the Packet Data Protocol
     (PDP) Context Activation Procedure in Universal Mobile
     Telecommunications System (UMTS) and PDN connection establishment
     in LTE networks.

Sarikaya, et al. Informational [Page 6] RFC 6653 Prefix Delegation July 2012

 7.  The MN may send a Router Solicitation message to solicit the AR
     to send a Router Advertisement (RA) message.
 8.  The AR advertises the prefixes received in the IA_PD option to
     the MN via an RA once the PDP Context/PDN connection is
     established, or in response to a Router Solicitation message sent
     from the MN.
 The 4-way exchange between the AR as RR and the DHCP server as
 Delegating Router (DR), as shown in Figure 2, may be reduced to a
 two-message exchange by using the Rapid Commit option [RFC3315].  The
 DHCP client in the AR acting as RR includes a Rapid Commit option in
 the Solicit message.  The DR then sends a Reply message containing
 one or more prefixes.

3.2. Prefix Request Procedure for Stateful Address Configuration

 Stateful address configuration requires a different architecture than
 that shown in Figure 2; in this type of configuration, there are two
 function modules in the AR: the DHCP server and the DHCP client.
 After the initial attach is completed, a connection to the AR is
 established for the MN.  The DHCP client function at the AR as RR and
 the DHCP server as DR follow Steps 2 through 5 of the procedure shown
 in Figure 2 to get the new prefix for this interface of the MN from
 the IA_PD option exchange defined in [RFC3633].
 The DHCPv6 client at the MN sends the DHCP Request to the AR.  The
 DHCP server function at the AR must use the IA_PD option received in
 the DHCPv6-PD exchange to assign an address to the MN.  The IA_PD
 option must contain the prefix.  The AR sends to the MN a DHCP Reply
 message containing the IA address option (IAADDR).  Figure 3 shows
 the message sequence.
 The MN configures its interface with the address assigned by the DHCP
 server in the DHCP Reply message.

Sarikaya, et al. Informational [Page 7] RFC 6653 Prefix Delegation July 2012

 In Figure 3, the AR may be the home gateway of a fixed network to
 which the MN gets connected during the MN's handover.
  +----------+             +--------------+             +-----------+
  |  MN      |             |    AR        |             |DHCP Server|
  |   |DHCP  |             | DHCP |DHCP   |             +-----------+
  |   |Client|             |Server|Client |
  +----------+             +--------------+
      |  Initial NW entry     |                           |
      |or attach procedure    |                           |
      |<----------------->    |                           |
      |                       |      DHCPv6-PD exchange   |
      |                       |      similar to Steps 2-5 |
      |   Solicit             |      of Figure 2 (IA_PD)  |
      |---------------------->|                           |
      |   Advertise           |                           |
      |<----------------------|                           |
      |    Request            |                           |
      |---------------------->|                           |
      |                       |                           |
      |                       |                           |
      |                       | Use prefix in IA_PD       |
      |    Reply              | to assign IAADDR          |
      |<--------------------- |                           |
         Figure 3: Stateful Address Configuration Following PD

3.3. The MN as Requesting Router in Prefix Delegation

 The AR may use a DHCPv6 Prefix Delegation exchange to get a delegated
 prefix shorter than /64 by setting the prefix-length field to a value
 less than 64, e.g., 56 to get a /56 prefix.  Each newly attaching MN
 first goes through the steps in Figure 2, in which the AR requests a
 shorter prefix to establish a default connection with the MN.
 The MN may next request additional prefixes (/64 or shorter) from the
 AR using DHCPv6 Prefix Delegation, where the MN is the RR and the AR
 is the DR (see [RFC6459] and Section 5.3.1.2.6 of [3GPP-23401]).  In
 this case, the call flow is similar to that shown in Figure 3.  The
 Solicit message must include the IA_PD option with the prefix-length
 field set to 64.  The MN may request more than one /64 prefix.  The
 AR as DR must delegate these prefixes, excluding the prefix assigned
 to the default connection.

Sarikaya, et al. Informational [Page 8] RFC 6653 Prefix Delegation July 2012

3.4. Prefix Release Procedure

 Prefixes can be released in two ways: via prefix aging, or via the
 DHCP release procedure.  In prefix aging, a prefix should not be used
 by an MN when the prefix ages, and the DHCP server can delegate it to
 another MN.  A prefix lifetime is delivered from the DHCPv6 server to
 the MN via the DHCP IA_PD Prefix option [RFC3633] and the RA Prefix
 Information option [RFC4861].  Figure 4 illustrates how the AR
 releases prefixes to a DHCP server that isn't directly connected to
 the AR:
 1.  A signal that an MN has detached, such as switch-off or handover,
     triggers the prefix release procedure.
 2.  The AR initiates a Release message to give the prefixes back to
     the DHCP server.
 3.  The server responds with a Reply message.  The prefixes can then
     be reused by other MNs.
     +-------+               +-------+             +-----------+
     |  MN   |               |  AR   |             |DHCP Server|
     +-------+               +-------+             +-----------+
         |                       |                       |
         |  1 De-registration    |                       |
         |  handover, or other   |                       |
         |<--------------------->|                       |
         |                       |2 Relay-forward/Release|
         |                       |---------------------->|
         |                       |                       |
         |                       |3 Relay-reply/Reply    |
         |                       |<--------------------- |
         |                       |                       |
         |                       |                       |
                       Figure 4: Prefix Release

3.5. Miscellaneous Considerations

3.5.1. How to Generate an IAID

 The IAID is 4 bytes in length and should be unique in the scope of an
 AR.  The prefix table should be maintained; this table contains the
 IAID, the Media Access Control (MAC) address, and the prefix(es)
 assigned to the MN.  In LTE networks, the International Mobile
 Equipment Identity (IMEI) uniquely identifies the MN's interface and

Sarikaya, et al. Informational [Page 9] RFC 6653 Prefix Delegation July 2012

 thus corresponds to the MAC address.  The MAC address of the
 interface should be stored in the prefix table and is used as the key
 for searching the table.
 The IAID should be set to Start_IAID; Start_IAID is an integer of
 4 octets.  The following algorithm is used to generate the IAID:
 1.  Set this IAID value in the IA_PD Prefix option.  Request a prefix
     for this MN as described in Section 3.1 or Section 3.2.
 2.  Store the IAID, MAC address, and received prefix(es) in the next
     entry of the prefix table.
 3.  Increment the IAID.
 A prefix table entry for an MN that hands over to another AR must be
 removed.  The IAID value is released and can then be reused.

3.5.2. Policy to Delegate Prefixes

 In P2P links, if /64 prefixes of all MNs connected to one or more ARs
 are broadcast dynamically upstream as route information, high
 routing-protocol traffic (IGP, OSPF, etc.) due to per-MN interface
 prefixes will result.  There are two solutions to this problem.  One
 solution is to use static configuration, which would be preferable in
 many cases.  No routing protocols are needed, because each AR has a
 known piece of address space.  If the DHCP servers also know that
 address space, then they will assign to a particular AR a prefix from
 that space.
 The other solution is to use route aggregation.  For example, each AR
 can be assigned a /48 or /32 prefix (an aggregate prefix, a.k.a
 service provider common prefix), while each interface of an MN can be
 assigned a /64 prefix.  The /64 prefix is an extension of the /48
 prefix -- for example, an AR's /48 prefix is 2001:db8:0::/48 -- while
 an interface of the MN is assigned a 2001:db8:0:2::/64 prefix.  The
 border router in Figure 1 may be manually configured to broadcast
 only an individual AR's /48 or /32 prefix information to the
 Internet.

4. Prefix Delegation Using RADIUS and Diameter

 In the initial network entry procedure shown in Figure 2, the AR as
 Remote Authentication Dial In User Service (RADIUS) client sends an
 Access-Request message with MN information to the RADIUS server.  If
 the MN passes the authentication, the RADIUS server may send an
 Access-Accept message with prefix information to the AR using the
 Framed-IPv6-Prefix attribute.  The AAA server also provides routing

Sarikaya, et al. Informational [Page 10] RFC 6653 Prefix Delegation July 2012

 information to be configured for the MN on the AR using the
 Framed-IPv6-Route attribute.  Using such a process, the AR can handle
 initial prefix assignments to MNs, but managing the lifetime of the
 prefixes is totally left to the AR.  The Framed-IPv6-Prefix is not
 designed to support delegation of IPv6 prefixes.  For this situation,
 the Delegated-IPv6-Prefix attribute, which is discussed below, can be
 used.
 [RFC4818] defines a RADIUS attribute, Delegated-IPv6-Prefix, which
 carries an IPv6 prefix to be delegated.  This attribute is usable
 within either RADIUS or Diameter.  [RFC4818] recommends that the DR
 use the AAA server to receive the prefixes to be delegated, by using
 the Delegated-IPv6-Prefix attribute/Attribute-Value Pair (AVP).
 The DHCP server as DR, as shown in Figure 2, may send an
 Access-Request packet containing the Delegated-IPv6-Prefix attribute
 to the RADIUS server to request prefixes.  In the Access-Request
 message, the DR may provide a hint that it would prefer a prefix --
 for example, a /48 prefix.  As the RADIUS server is not required to
 honor the hint, the server may delegate a longer prefix -- e.g., /56
 or /64 -- in an Access-Accept message containing the
 Delegated-IPv6-Prefix attribute [RFC4818].  The attribute can appear
 multiple times when the RADIUS server delegates multiple prefixes to
 the DR.  The DR sends the prefixes to the RR using the IA_PD option,
 and the AR as RR uses them for MNs, as described in Section 3.
 When Diameter is used, the DHCP server as DR, as shown in Figure 2,
 sends an AA-Request message.  The AA-Request message may contain a
 Delegated-IPv6-Prefix AVP.  The Diameter server replies with an
 AA-Answer message.  The AA-Answer message may contain a
 Delegated-IPv6-Prefix AVP.  The AVP can appear multiple times when
 the Diameter server assigns multiple prefixes to an MN.  The
 Delegated-IPv6-Prefix AVP may appear in an AA-Request packet as a
 hint from the AR to the Diameter server that it would prefer a
 prefix -- for example, a /48 prefix.  The Diameter server may
 delegate in the AA-Answer message a /64 prefix, which is an extension
 of the /48 prefix.  As in the case of RADIUS, the DR sends the
 prefixes to the RR using the IA_PD option, and the AR as RR uses them
 for the MNs as described in Section 3.

5. Security Considerations

 This document does not introduce any additional message types and
 therefore does not introduce any additional threats.  The security
 procedures for DHCPv6 [RFC3633], RADIUS [RFC2865], and Diameter
 [RFC3588] apply.

Sarikaya, et al. Informational [Page 11] RFC 6653 Prefix Delegation July 2012

6. Acknowledgements

 We are grateful to Suresh Krishnan, Hemant Singh, Qiang Zhao, Ole
 Troan, Qin Wu, Jouni Korhonen, Cameron Byrne, Brian Carpenter, Jari
 Arkko, and Jason Lin, whose in-depth reviews of this document led to
 several improvements.

7. Informative References

 [3GPP-23401]
            3GPP, "General Packet Radio Service (GPRS) enhancements
            for Evolved Universal Terrestrial Radio Access Network
            (E-UTRAN) access (Release 11)", TS 23.401 V11.0.0,
            December 2011.
 [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
            "Remote Authentication Dial In User Service (RADIUS)",
            RFC 2865, June 2000.
 [RFC3314]  Wasserman, M., "Recommendations for IPv6 in Third
            Generation Partnership Project (3GPP) Standards",
            RFC 3314, September 2002.
 [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.
 [RFC3316]  Arkko, J., Kuijpers, G., Soliman, H., Loughney, J., and J.
            Wiljakka, "Internet Protocol Version 6 (IPv6) for Some
            Second and Third Generation Cellular Hosts", RFC 3316,
            April 2003.
 [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
            Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
 [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
            Host Configuration Protocol (DHCP) version 6", RFC 3633,
            December 2003.
 [RFC4818]  Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
            Attribute", RFC 4818, April 2007.
 [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
            "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
            September 2007.

Sarikaya, et al. Informational [Page 12] RFC 6653 Prefix Delegation July 2012

 [RFC6342]  Koodli, R., "Mobile Networks Considerations for IPv6
            Deployment", RFC 6342, August 2011.
 [RFC6459]  Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
            T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
            Partnership Project (3GPP) Evolved Packet System (EPS)",
            RFC 6459, January 2012.

Authors' Addresses

 Behcet Sarikaya
 Huawei USA
 5340 Legacy Dr.
 Plano, TX  75074
 EMail: sarikaya@ieee.org
 Frank Xia
 Huawei USA
 1700 Alma Drive, Suite 500
 Plano, TX  75075
 Phone: +1 972-509-5599
 EMail: xiayangsong@huawei.com
 Ted Lemon
 Nominum
 2000 Seaport Blvd.
 Redwood City, CA  94063
 EMail: mellon@nominum.com

Sarikaya, et al. Informational [Page 13]

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