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rfc:rfc4433

Network Working Group M. Kulkarni Request for Comments: 4433 A. Patel Category: Standards Track K. Leung

                                                    Cisco Systems Inc.
                                                            March 2006
          Mobile IPv4 Dynamic Home Agent (HA) Assignment

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.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 Mobile IPv4 (RFC 3344) uses the home agent (HA) to anchor sessions of
 a roaming mobile node (MN).  This document proposes a messaging
 mechanism for dynamic home agent assignment and HA redirection.  The
 goal is to provide a mechanism to assign an optimal HA for a Mobile
 IP session while allowing any suitable method for HA selection.

Kulkarni, et al. Standards Track [Page 1] RFC 4433 Dynamic HA Assignment March 2006

Table of Contents

 1. Introduction ....................................................3
 2. Requirements Terminology ........................................3
 3. Problem Statement ...............................................5
    3.1. Scope ......................................................5
    3.2. Dynamic Home Agent Discovery in Mobile IPv4 ................5
    3.3. NAI Usage and Dynamic HA Assignment ........................6
    3.4. Dynamic HA Extension .......................................6
         3.4.1. Requested HA Extension ..............................7
         3.4.2. Redirected HA Extension .............................7
 4. Messaging Mechanism for Dynamic HA Assignment/Redirection .......7
    4.1. Messaging for Dynamic HA Assignment ........................7
         4.1.1. Example with Message Flow Diagram ...................8
    4.2. Messaging for HA Redirection ..............................10
         4.2.1. Example with Message Flow Diagram ..................12
 5. Mobility Agent Considerations ..................................14
    5.1. Mobile Node Considerations ................................14
         5.1.1. MN Using FA CoA ....................................14
         5.1.2. MN Using Co-Located CoA ............................15
         5.1.3. Refreshing Assigned HA Address on Mobile Node ......16
    5.2. Foreign Agent Considerations ..............................16
    5.3. Home Agent Considerations .................................17
         5.3.1. Assigned Home Agent Considerations .................17
 6. Requested Home Agent Selection .................................19
 7. Error Values ...................................................20
 8. IANA Considerations ............................................20
 9. Security Considerations ........................................20
 10. Backward-Compatibility Considerations .........................21
 11. Acknowledgements ..............................................23
 12. Normative References ..........................................23

Kulkarni, et al. Standards Track [Page 2] RFC 4433 Dynamic HA Assignment March 2006

1. Introduction

 This document adds to the Mobile IP protocol [1], by proposing a
 messaging mechanism for dynamic home agent assignment and home agent
 redirection during initial registration.  The goal is to assign an
 optimal HA for a Mobile IP session.  The mobile node MUST use the
 Network Access Identifier (NAI) extension [2] when requesting a
 dynamically assigned HA.
 The MN requests a dynamically assigned HA by setting the HA field in
 the initial Registration Request to ALL-ZERO-ONE-ADDR (defined in
 Section 2).  If the request is accepted, the HA sends a successful
 Registration Reply containing the HA's own address.  The requested HA
 can suggest an alternate HA and if so, the Registration Reply is
 rejected with a new error code REDIRECT-HA-REQ and the alternate HA
 address is specified in a new extension (Redirected HA Extension).
 This document also defines a new Requested HA Extension for use in
 Registration Requests when the HA field is set to ALL-ZERO-ONE-ADDR.
 The Requested HA address is a hint to the network about the MN's
 preferred HA.
 The messaging mechanism is defined in this document so that the MN
 can request and receive a dynamic HA address in Mobile IP messages.
 However, the mechanism by which the network selects an HA for
 assignment to the MN is outside the scope of this document.  For
 example, the selection may be made by any network node that receives
 the Registration Request (or information about the Registration
 Request), such as a Foreign Agent, AAA server, or home agent.  The
 node that selects the HA may select one based on a number of
 criteria, including but not limited to HA load-balancing,
 geographical proximity, administrative policy, etc.

2. Requirements Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [6].
 The Mobile-IP-related terminology described in RFC 3344 [1] is used
 in this document.  In addition, the following terms are used:
 ALL-ZERO-ONE-ADDR: IP address 0.0.0.0 or 255.255.255.255.  An
                    address of 255.255.255.255 indicates a preference
                    for an HA in the home domain.  An address of
                    0.0.0.0 indicates no preference for home vs.
                    visited domain.

Kulkarni, et al. Standards Track [Page 3] RFC 4433 Dynamic HA Assignment March 2006

 Requested HA:      Destination IP address of home agent that the
                    Registration Request is sent to.  Must be a
                    unicast IP address.  This address can be
                    obtained as described in Section 6.
                    Note that this specification defines a new
                    "Requested HA Extension" in Section 3.4, which
                    is different from the term "Requested HA".
 Assigned HA:       The HA that accepts an MN's Registration Request
                    and returns a successful Registration Reply.
 Redirected HA:     If the registration is rejected with error code
                    REDIRECT-HA-REQ, the HA being referred to is
                    specified in a new extension (Redirected HA
                    Extension).
 AAA server:        Authentication, Authorization, and Accounting
                    Server.
 DNS:               Domain Name System.
 DHCP:              Dynamic Host Configuration Protocol.
 MN:                Mobile node as defined in Mobile IPv4 [1].
 HA:                Home agent as defined in Mobile IPv4 [1].
 FA:                Foreign Agent as defined in Mobile IPv4 [1].
 CoA:               Care-of Address.
 CCoA:              Co-located Care-of Address.
 MN HoA:            Mobile node's home address.
 NAI:               Network Access Identifier [2].
 Src IP:            Source IP address of the packet.
 Dest IP:           Destination IP address of the packet.
 RRQ:               Registration Request.

Kulkarni, et al. Standards Track [Page 4] RFC 4433 Dynamic HA Assignment March 2006

3. Problem Statement

 The Mobile IPv4 NAI Extension for IPv4 [2] introduced the concept of
 identifying an MN by the NAI and enabling dynamic home address
 assignment.  When the home address is dynamically assigned, it is
 desirable to discover the home agent dynamically or inform the MN
 about an optimal HA to use for a multitude of reasons, such as:
  1. If the distance between the visited network and the home network of

the mobile node is large, the signaling delay for these

   registrations may be long.  In such a case, the MN will be anchored
   to its distant home agent, resulting in tunneled traffic traveling
   a long distance between home agent and the mobile node.  When a
   Mobile IP session initiates, if the mobile node can be assigned a
   home agent that is close to the mobile node it can drastically
   reduce the latency between the home agent and mobile node.
  1. In a large-scale Mobile IP deployment, it is cumbersome to

provision MNs with multiple HA addresses.

  1. It is desirable to achieve some form of load balancing between

multiple HAs in the network. Dynamic HA assignment and/or HA

   redirection lets the network select the optimal HA from among a set
   of HAs and thus achieve load balancing among a group of HAs.
  1. Local administrative policies.

3.1. Scope

 This specification does not address the problem of distributing a
 security association between the MN and HA, and it can either be
 statically preconfigured or dynamically distributed using other
 mechanisms [7].
 The document introduces the terms Requested/Assigned/Redirected HA
 (Section 6).  The discovery of candidate HA addresses for insertion
 into the Redirected HA Extension can be accomplished through various
 means that are network and/or deployment specific and hence are
 outside the scope of this specification.
 The MN MAY request dynamic HA assignment when it is not aware of any
 HA address and even when it is aware of at least one HA address.

3.2. Dynamic Home Agent Discovery in Mobile IPv4

 Mobile IPv4 [1] specifies the mechanism for discovering the mobile
 node's home agent using subnet-directed broadcast IP address in the
 home agent field of the Registration Request.  This mechanism was

Kulkarni, et al. Standards Track [Page 5] RFC 4433 Dynamic HA Assignment March 2006

 designed for mobile nodes with a static home address and subnet
 prefix, anchored on fixed home network.  However, using subnet-
 directed broadcast as the destination IP address of the Registration
 Request, it is unlikely that the Registration Request will reach the
 home subnet because routers will drop these packets by default.  See
 CERT Advisory CA-1998-01 Smurf IP Denial-of-Service Attacks [3].

3.3. NAI Usage and Dynamic HA Assignment

 The Mobile IPv4 NAI Extension for IPv4 [2] introduced the concept of
 identifying an MN by the NAI and enabling dynamic home address
 assignment.  This document requires that while using dynamic HA
 assignment, MN MUST use the NAI and obtain a home address.  MN can
 still suggest a static home address in the Registration Request, but
 must take the address in the Registration Reply as the home address
 for the session.  This is compatible with the procedures documented
 in the NAI specification [2].

3.4. Dynamic HA Extension

 The Dynamic HA Extension, shown in Figure 1, contains the address of
 the HA.  This is a generic extension and can be used in Registration
 Request and Reply messages.  It is a skippable extension.
 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |   Subtype     |           Length              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           HA-Address                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 1: The Dynamic HA Address Extension
    Type         DYNAMIC-HA-ADDRESS (skippable) 139 is the type,
                 which specifies the dynamic HA address.
    Subtype      Defines the use of this extension as:
                 subtype 1 = Requested HA Extension
                         2 = Redirected HA Extension
    Length       Indicates the length of the extension not
                 including the type, subtype, and length fields.
                 Length is always 4 bytes.
    HA-Address   Address of the home agent.

Kulkarni, et al. Standards Track [Page 6] RFC 4433 Dynamic HA Assignment March 2006

3.4.1. Requested HA Extension

 The Requested HA Extension is a Dynamic HA Extension of subtype 1.
 The MN may include the Requested HA Extension in the Registration
 Request as a hint to the network where it wishes to be anchored.
 This extension contains the address of the HA.  A valid unicast IP
 address MUST be used as HA address in this extension.
 In absence of an FA, the Registration Request is forwarded to this
 HA.  In presence of an FA, the FA MUST forward the Registration
 Request to the HA address in this extension.

3.4.2. Redirected HA Extension

 The Redirected HA Extension is a Dynamic HA Extension of subtype 2.
 The Redirected HA Extension contains the address of the HA where the
 MN should attempt the next registration.  The HA receiving a
 Registration Request can suggest an alternate HA and, if so, the
 Registration Reply is sent with a new error code REDIRECT-HA-REQ and
 the alternate HA address is specified in this extension.
 The Redirected HA Extension MUST be included in Registration Reply
 when the reply code is REDIRECT-HA-REQ.

4. Messaging Mechanism for Dynamic HA Assignment/Redirection

 This specification presents two alternatives for home agent
 assignment:
 (a) Dynamic HA assignment (described in Section 4.1) and
 (b) HA redirection (described in Section 4.2).

4.1. Messaging for Dynamic HA Assignment

 The following sequence of events occurs when the MN requests dynamic
 home agent assignment:
 1.  The MN sets the Home Agent address field in the Registration
     Request to ALL-ZERO-ONE-ADDR.  If the MN is aware of a desired HA
     address, it can add that address in the Requested HA Extension in
     the Registration Request.  If the HA does not support the
     Requested HA Extension, see step 2 below.

Kulkarni, et al. Standards Track [Page 7] RFC 4433 Dynamic HA Assignment March 2006

 2.  This step is applicable, in lieu of step 1, for an MN that is
     aware of the HA address and desires dynamic HA assignment.  Also,
     the MN follows this (when aware of a HA address) when it
     discovers a legacy FA in the path or if the known HA does not
     support the Requested HA Extension (see Section 10).
     The MN sets the Home Agent address field in the Registration
     Request to the HA address (instead of setting it to ALL-ZERO-
     ONE-ADDR).  The MN also adds the same HA address in the Requested
     HA Extension in the Registration Request.
 3.  The MN (if using co-located CoA and registering directly with the
     HA) or the FA (if the MN is registering via the FA) sends the
     Registration Request to the "Requested HA".  If the Requested HA
     Extension is present, Requested HA is specified in the "HA
     Address" of this extension.
     Per Section 10, in case of a legacy FA, legacy FA forwards the
     Registration Request to the address in the HA field of the
     request (thus, MN uses step 2 above in case of legacy FA instead
     of step 1).
 4.  The "Requested HA" is the home agent that processes the
     Registration Request in accordance with Mobile IPv4 [1] and as
     per the specification in this document.  It creates mobility
     binding for a successful Registration Request.  It also sends a
     Registration Reply to the MN.
 5.  The MN obtains an "Assigned HA" address from the HA field in the
     successful Registration Reply and uses it for the remainder of
     the session.  (Note that the "Assigned HA" will be the same as
     the "Requested HA".)
 6.  Subsequent Registration Request messages for renewal are sent to
     the Assigned HA.
 Section 5.3.1 describes the Assigned HA in detail.  Some ideas on how
 to select the Requested HA are briefly covered in Section 6.

4.1.1. Example with Message Flow Diagram

 Detailed explanation of this alternative is best described with the
 help of a message flow diagram and description.
 Figure 2 shows one specific example of a mobile node using an
 FA-located Care-of Address (FA CoA) and FA understands the Requested
 HA Extension per this specification.

Kulkarni, et al. Standards Track [Page 8] RFC 4433 Dynamic HA Assignment March 2006

 Other scenarios such as when the mobile node uses a co-located care
 of address and presence of a legacy HA or FA are not described below,
 but the behavior is similar.
              MN            FA        Requested/Assigned HA
              |      1      |                |
              |------------>|       2        |
              |             |--------------->|
              |             |                |
              |             |                |
              |             |       3        |
              |      4      |<---------------|
              |<------------|                |
              |             |                |
              |             |       5        |
              |----------------------------->|
              |             |                |
  Figure 2: Example Message Flow for Dynamic HA Assignment
 1.  The MN sets the Home Agent address field in the Registration
     Request to ALL-ZERO-ONE-ADDR.  Since the MN is using FA CoA in
     this example, it sends the Registration Request to the FA.  The
     Registration Request is formatted as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |  MN    |    FA      |         | ALL-ZERO-ONE-ADDR |FA CoA |
     +-----------------------------------------------------------+
     If the MN is aware of a desired HA address, it can add that
     address in the Requested HA Extension in Registration Request as
     a hint.  That extension is not shown above.
 2.  The FA sends the Registration Request to the Requested HA.  If
     the Requested HA Extension is present, Requested HA is the HA
     address in this extension.  If the Requested HA Extension is not
     present, the FA determines the Requested HA through means outside
     the scope of this specification.  The Registration Request is
     formatted as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |  FA    |Requested HA|         | ALL-ZERO-ONE-ADDR |FA CoA |
     +-----------------------------------------------------------+

Kulkarni, et al. Standards Track [Page 9] RFC 4433 Dynamic HA Assignment March 2006

     (If MN includes the Requested HA Extension, the FA copies that
     extension.  The FA then forwards the Registration Request, along
     with the Requested HA Extension, to the HA address specified in
     Requested HA Extension.)
 3.  The HA processes the Registration Request in accordance with
     Mobile IPv4 [1] and the messaging defined in this document.  The
     HA creates mobility binding for successful request and becomes
     the Assigned HA.  The HA then sends a Registration Reply to the
     FA, which is formatted as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |Assigned| Src IP of  |         |    Assigned HA    |FA CoA/|
     |   HA   | the RRQ    |         |                   |       |
     +-----------------------------------------------------------+
 4.  The FA relays the Registration Reply to the MN, as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |  FA    |    MN      |         |    Assigned HA    |FA CoA/|
     +-----------------------------------------------------------+
 5.  The MN obtains the Assigned HA address from the HA field in the
     successful Registration Reply and uses it for the remainder of
     the session.  The MN sends subsequent Re-Registration or
     De-Registration Requests for the remainder session directly to
     the Assigned HA.  The Home Agent address field in this
     Registration Request is set to ALL-ZERO-ONE-ADDR.  Note that the
     Assigned HA is the same as the Requested HA.

4.2. Messaging for HA Redirection

     This section describes the events that occur when the Requested
     HA does not accept the Registration Request and redirects the
     mobile node to another HA (aka Redirected HA) instead.  This
     behavior is not exhibited by a legacy HA and so is not referred
     in the description below.  In presence of a legacy FA, please
     refer to Section 4.1 for the specific field in the Registration
     Request.
 1.  The MN sets the Home Agent address field in the Registration
     Request to ALL-ZERO-ONE-ADDR.

Kulkarni, et al. Standards Track [Page 10] RFC 4433 Dynamic HA Assignment March 2006

 2.  The MN (if using co-located CoA and registering directly with the
     HA) or FA (if the MN is registering via the FA) sends the
     Registration Request to the "Requested HA".  If the MN is aware
     of an HA address, it can add that address in the Requested HA
     Extension in the Registration Request.
 3.  When the HA receives the Registration Request, if the HA field is
     set to ALL-ZERO-ONE-ADDR, the HA may reject the request with
     Reply code REDIRECT-HA-REQ and suggest an alternate HA.
     The HA may reject the request for a number of reasons, which are
     outside the scope of this specification.  If the HA rejects the
     Request, the HA field in the Reply is set to this HA's address.
     The IP address of the HA that is the target of the redirection is
     specified in Redirected HA Extension.  The presence of this
     extension is mandatory when the reply code is set to REDIRECT-
     HA-REQ.  HA sends the Reply to the FA/MN.
 4.  FA sends the Reply to the MN.
 5.  If the error code is set to REDIRECT-HA-REQ, the MN obtains the
     HA address from Redirected HA Extension.  The MN then sends a
     Registration Request to Redirected HA.  The MN may choose to add
     Requested HA Extension in this new Registration Request.  If a
     registration loop occurs (the case when the Redirected HA is an
     HA that had already directed the MN to register elsewhere), then
     the MN stops sending any further Registration Request and
     provides an indication that the loop event was detected.  The
     number of consecutive Redirected HAs remembered by the MN for
     loop detection is an implementation parameter.

Kulkarni, et al. Standards Track [Page 11] RFC 4433 Dynamic HA Assignment March 2006

4.2.1. Example with Message Flow Diagram

 Figure 3 shows one specific example of a mobile node using FA-located
 Care-of Address, where the FA is not a legacy FA.
    MN           FA          Requested HA    Redirected HA
    |      1      |                |               |
    |------------>|       2        |               |
    |             |--------------->|               |
    |             |                |               |
    |             |                |               |
    |             |       3        |               |
    |      4      |<---------------|               |
    |<------------|                |               |
    |             |                |               |
    |             |       5        |               |
    |--------------------------------------------->|
    |             |                |               |
    Figure 3: Example Message Flow for HA Redirection
 1.  The MN sets the Home Agent address field in the Registration
     Request to ALL-ZERO-ONE-ADDR.  Since the MN is using FA CoA in
     this example, it sends the Registration Request to the FA.  The
     Registration Request is formatted as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |  MN    |    FA      |         | ALL-ZERO-ONE-ADDR |FA CoA |
     +-----------------------------------------------------------+
     If the MN is aware of an HA address, it can add that address in
     the Requested HA Extension in the Registration Request as a hint.
     That extension is not shown above.
 2.  The FA sends the Registration Request to the Requested HA.  If
     Requested HA Extension is present, Requested HA is the HA address
     in this extension.  If the Requested HA Extension is not present,
     the FA determines the Requested HA through means outside the
     scope of this specification.  The Registration Request is
     formatted as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |  FA    |Requested HA|         | ALL-ZERO-ONE-ADDR |FA CoA |
     +-----------------------------------------------------------+

Kulkarni, et al. Standards Track [Page 12] RFC 4433 Dynamic HA Assignment March 2006

 3.  The HA processes the Registration Request in accordance with
     Mobile IPv4 [1] and the messaging defined in this specification.
     If the registration is successful, but local
     configuration/administrative policy, etc., directs the HA to
     refer the MN to another HA, the HA rejects the request with error
     code REDIRECT-HA-REQ.  The HA fills in the address of the
     Redirected HA in the Redirected HA Extension.  The HA then sends
     Registration Reply reject to the FA, which is formatted as
     follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |        | Src IP of  |         |       HA          |FA CoA |
     |   HA   | the RRQ    |         |                   |       |
     +-----------------------------------------------------------+
     | Redirected HA Extension ...                               |
     +-----------------------------------------------------------+
 4.  The FA relays the Registration Reply to the MN, as follows:
     +-----------------------------------------------------------+
     | Src IP=| Dest IP =  | MN HoA  |    HA Address =   | CoA = |
     |  FA    |    MN      |         |       HA          |FA CoA/|
     +-----------------------------------------------------------+
     | Redirected HA Extension ...                               |
     +-----------------------------------------------------------+
 5.  If the MN can authenticate the Reply, the MN extracts the HA
     address from the Redirected HA Extension.  The MN then sends a
     Registration Request to the Redirected HA, unless it has already
     received a redirection response from that HA while processing the
     Registration Request.  The MN may choose to add Requested HA
     Extension in this new Registration Request.

Kulkarni, et al. Standards Track [Page 13] RFC 4433 Dynamic HA Assignment March 2006

5. Mobility Agent Considerations

 The following sections describe the behavior of each mobility agent
 in detail.

5.1. Mobile Node Considerations

 The mobile node MUST use the NAI extension for home address
 assignment when using the messaging mechanism in this document.
 Since MN uses the NAI extension, the Home Address field is set to
 0.0.0.0.
 While dynamic HA assignment is in progress and the MN has not
 successfully anchored at a home agent, the MN MUST set the Home Agent
 field in the Registration Request to an ALL-ZERO-ONE-ADDR, which is
 either 255.255.255.255 or 0.0.0.0.
 The Registration Request MUST be protected by a valid authenticator
 as specified in Mobile IPv4 [1] or Mobile IPv4 Challenge/Response
 Extensions [5].  Configuring security associations is deployment
 specific and hence outside the scope of this specification.  The
 security associations between an MN and an individual HA may also be
 dynamically derived during the dynamic HA assignment, based on a
 shared secret between MN and AAA infrastructure [7].
 The mobile node MUST maintain the remaining Mobile IP session with
 the Assigned HA.
 As mentioned in the Security Considerations (Section 9), there is a
 possibility of more than one HA creating a mobility binding entry for
 a given MN, if a rogue node in the middle captures the Registration
 Request and forwards it to other home agents.  The MN can mitigate
 such condition by using a short lifetime (e.g., 5 seconds) in the
 Registration Request with the Home Agent field set to ALL-ZERO-ONE-
 ADDR.
 The following sections describe MN behavior in FA CoA mode and co-
 located CoA mode.

5.1.1. MN Using FA CoA

 When a mobile node initiates a Mobile IP session requesting dynamic
 HA assignment, it MUST set the home agent address field in the
 Registration Request to ALL-ZERO-ONE-ADDR.  The destination IP
 address of the Registration Request is the FA.  The FA will determine
 the Requested HA and forward the Registration Request to the
 Requested HA.  Registration Request processing takes place on the
 Requested HA as per the specification in this document.

Kulkarni, et al. Standards Track [Page 14] RFC 4433 Dynamic HA Assignment March 2006

 The Registration Request MUST be appropriately authenticated for the
 HA to validate the Request.
 If a successful Registration Reply is received, the MN obtains the
 Assigned HA from the HA field of Reply.  The Assigned HA address will
 be the same as the Requested HA Extension, if it was included in the
 Registration Request by the MN.
 If a Registration Reply is received with code REDIRECT-HA-REQ, the MN
 MUST authenticate the Reply based on HA address in HA field of Reply
 and attempt Registration with the HA address specified in the
 Redirected HA Extension.  The MN MUST put the Redirected HA address
 as the Requested HA Extension of the new Registration Request.
 In some cases, for the first Registration Request the MN may want to
 hint to the network to be anchored at a specific HA.  The MN SHOULD
 put that address in the HA address of the Requested HA Extension.

5.1.2. MN Using Co-Located CoA

 An MN in co-located CoA mode requesting dynamic HA assignment MUST
 set the home agent address field in the Registration Request to ALL-
 ZERO-ONE-ADDR.  The destination IP address of the Registration
 Request is the Requested HA.  Some ideas on how to select a Requested
 HA are briefly covered in Section 6.
 If a successful Reply is received, the MN obtains the Assigned HA
 address from the successful Registration Reply.  The Assigned HA will
 be the same as Requested HA to which the Registration Request was
 sent.  The MN MUST cache the Assigned HA address for the length of
 the Mobile IP session.  The mobile node then MUST use this previously
 cached Assigned HA address as the home agent address in subsequent
 Re-Registration and De-Registration Request(s).  This will make sure
 that for the duration of the Mobile IP session, the mobile node will
 always be anchored to the assigned home agent with which it was
 initially registered.
 If a Registration Reply is received with code REDIRECT-HA-REQ, the MN
 MUST authenticate the Reply based on HA address in HA field of Reply
 and attempt Registration with the HA address specified in the
 Redirected HA Extension.  The MN MUST put the Redirected HA in the
 Requested HA Extension of the new Registration Request.
 In some cases, for the first Registration Request MN may want to hint
 to the network to be anchored at a specific HA and the MN SHOULD put
 that address in the HA address of the Requested HA Extension.

Kulkarni, et al. Standards Track [Page 15] RFC 4433 Dynamic HA Assignment March 2006

 While requesting dynamic HA assignment and registering directly with
 an HA, the Requested HA Extension MUST be included and MUST contain
 the address of the HA to which the Registration Request is sent.
 When using co-located CoA but registering via a legacy FA, the HA
 field in the Registration Request may be set to Requested HA.
 If the Registration Request contains the Requested HA Extension, the
 HA address in that extension MUST match the destination IP of the
 Request.

5.1.3. Refreshing Assigned HA Address on Mobile Node

 When the Mobile IP session terminates, the mobile node MAY clear the
 Assigned HA address cached as the home agent address.  It MAY request
 a new HA address for the new Mobile IP session by not including the
 Requested HA Extension.  The advantage of this approach is that the
 mobile node will be always anchored to an optimal home agent from
 where it initiated the Mobile IP session.
 Alternately, the MN may save the Assigned HA address and use it in
 the Requested HA Extension along with ALL-ZERO-ONE-ADDR HA address in
 Registration Request for a new Mobile IP session.

5.2. Foreign Agent Considerations

 When the mobile node is using an FA CoA, it always registers via the
 FA.  When the MN is using a co-located CoA, it may register through
 an FA or it may register directly with an HA, unless the R bit is set
 in the FA's agent advertisement, in which case it always registers
 through the FA.
 When the FA receives a Registration Request with HA address field set
 to ALL-ZERO-ONE-ADDR that doesn't contain the Requested HA Extension,
 the FA obtains the Requested HA address to forward the Registration
 Request using means outside the scope of this specification.  Some
 ideas on how to select a Requested HA are briefly covered in Section
 6.
 If the FA cannot obtain the Requested HA to which to forward a
 Registration Request from the MN, it MUST reject request with error
 code NONZERO-HA-REQD.
 If the MN has included the Requested HA Extension, the FA MUST
 forward the Registration Request to the address in this extension.
 If the HA address in this extension is not a routable unicast
 address, the FA MUST reject the request with error code NONZERO-HA-
 REQD.

Kulkarni, et al. Standards Track [Page 16] RFC 4433 Dynamic HA Assignment March 2006

 If the Registration Request contains the Requested HA Extension, the
 FA uses that address as the destination for the relayed Registration
 Request.
 Backward-compatibility issues related to the mobility agents are
 addressed in Section 10.

5.3. Home Agent Considerations

 A home agent can process an incoming Registration Request in one of
 the following two ways:
 1.  The MN or FA sends the Registration Request to the Requested HA.
     The term Requested HA has meaning in the context of a
     Registration Request message.  When the Requested HA successfully
     processes the Registration Request and creates a binding and
     sends a Reply with its address, it becomes the Assigned HA.  The
     term Assigned HA is meaningful in the context of a Registration
     Reply message.
 2.  A home agent receiving a Registration Request with HA field set
     to ALL-ZERO-ONE-ADDR MAY reject the request even if successfully
     authenticated and suggest an alternate HA address in Reply.  In
     such a case, the HA puts its own address in HA field of Reply and
     sets the Reply code to REDIRECT-HA-REQ and adds the Redirected HA
     Extension.
 If the Registration Request contains the Requested HA Extension, the
 HA address in that extension must match the destination IP of the
 Request.  If it does not match, the Requested HA MUST reject the
 Registration Request with error code 136.

5.3.1. Assigned Home Agent Considerations

 The HA that processes the incoming Registration Request fully in
 accordance with Mobile IPv4 [1] and this specification becomes the
 Assigned HA.  The Registration Request terminates at the Assigned HA.
 The Assigned HA creates one mobility binding per MN and sends the
 Registration Reply to the MN by copying its address in the Home Agent
 field and as the source IP address of the Reply.
 The following table summarizes the behavior of the Assigned HA, based
 on the value of the destination IP address and Home Agent field of
 the Registration Request.

Kulkarni, et al. Standards Track [Page 17] RFC 4433 Dynamic HA Assignment March 2006

 Dest IP Addr   HA field      Processing at Assigned HA
 ------------  ------------ ----------------------------------
 Unicast       non-unicast  Mobile IPv4 [1]: There is no change
                            in handling for this case from
 (Must be                   Mobile IPv4.  It is mentioned here
 equal to the               for reference only.
 HA receiving               HA denies the registration with
 the RRQ)                   error code 136 and sets HA field to
                            its own IP address in the reply as
                            per Section 3.8.3.2 in [1].
               ALL-ZERO-    New Behavior: Accept the RRQ as per
               ONE-ADDR     this specification.  Authenticate
                            the RRQ and create mobility binding
                            if the HA is acting as Assigned HA.
                            Set HA field to its own IP address
                            in the Registration Reply.
                                       OR
                            New Behavior: If authentication is
                            successful, reject RRQ with a new
                            error code REDIRECT-HA-REQ.  HA
                            puts its address in HA address
                            field of Reject.  HA suggests an
                            alternate HA to use in the new
                            Redirected HA Extension.
   Table 1: Registration Request Handling at Assigned HA
 As per the messaging proposed here, the mobile node (or the foreign
 agent) sends the Registration Request to the Requested HA address,
 which is a unicast address.  Therefore, this document does not
 specify any new behavior for the case where the HA receives a subnet
 directed broadcast Registration Request as specified in Section
 3.8.2.1 of the Mobile IPv4 specification [1].  Although the Home
 Agent field in the Registration Request is not a unicast address, the
 destination IP address is a unicast address.  This avoids the problem
 associated with subnet-directed broadcast destination IP address that
 may result in multiple HAs responding.  Thus, there is no need to
 deny the registration as stated in Mobile IPv4 [1] Section 3.8.3.2.
 When the destination IP address is a unicast address and the Home
 Agent field is ALL-ZERO-ONE-ADDR, the HA accepts/denies registration
 and sets the HA field to its own IP address in the reply (i.e., the
 registration is not rejected with error code 136).

Kulkarni, et al. Standards Track [Page 18] RFC 4433 Dynamic HA Assignment March 2006

 The HA can reject the request with the error code REDIRECT-HA-REQ and
 suggest an alternate HA.  This redirection can be used for load
 balancing, geographical proximity based on Care-of Address, or other
 reasons.  The HA puts its own address in the HA field of the
 Registration Reply message and puts the address of the redirected HA
 in the Redirected HA Extension.  If the HA accepts the Request, it
 sets the HA field in the Registration Reply to its own address.
 The Requested HA always performs standard validity checks on the
 Registration Request.  If there is any error, the Registration
 Request is rejected with error codes specified in Mobile IPv4 [1].

6. Requested Home Agent Selection

 When dynamic HA assignment is requested, the MN (or FA in the case of
 registration via FA) sends the Registration Request to the Requested
 HA.  This address MUST be a unicast IP address.  If the MN has
 included a Requested HA Extension in the Registration Request, the HA
 address in this extension is the Requested HA.
 Some examples of methods by which the MN or the FA may select the
 Requested HA are briefly described below:
 DHCP:
    The MN performs DHCP to obtain an IP address on the visited
    network.  The Requested HA is learned from the DHCP Mobile IP Home
    Agent Option 68 [4].  The MN sends the Registration Request
    directly to this HA and receives the Assigned HA to be used for
    the remainder of the Mobile IP session.
 AAA:
    MN performs challenge/response [5] with the FA.  The FA retrieves
    the Requested HA from the AAA server and forwards the Registration
    Request directly to this HA.  The Assigned HA sends a Registration
    Reply to the FA, which relays it to the MN.  MN uses the Assigned
    HA for the remainder of the Mobile IP session.
 DNS:
    In this case, the hostname of the HA is configured on the MN or
    obtained by some other means, e.g., using a service location
    protocol.  The MN performs DNS lookup on the HA hostname.  The DNS
    infrastructure provides a resource record with information to
    identify the optimal HA to the MN.  The MN sends a Registration
    Request directly to the HA and receives the Assigned HA to be used
    for the remainder of the Mobile IP session.

Kulkarni, et al. Standards Track [Page 19] RFC 4433 Dynamic HA Assignment March 2006

 Static configuration:
    The HA address is statically configured on the MN.  The MN sends
    the Registration Request to the configured address.  The Requested
    HA may then redirect the MN to a Redirected HA.

7. Error Values

 Each entry in the following table contains the name and value for the
 error code to be returned in a Registration Reply.  It also includes
 the section in which the error code is first mentioned in this
 document.
 Error Name       Value  Section  Description
 ---------------  -----  -------  -----------------------------
 NONZERO-HA-REQD   90     5.2     Non-zero HA address required
                                  in Registration Request.
 REDIRECT-HA-REQ   143    5.3     Re-register with redirected HA.

8. IANA Considerations

 The code value NONZERO-HA-REQD is a Mobile IP response code [1] taken
 from the range of values associated with rejection by the foreign
 agent (i.e., value in the range 64-127).
 The code value REDIRECT-HA-REQ is a Mobile IP response code [1] taken
 from the range of values associated with rejection by the home agent
 (i.e., value in the range 128-192).
 The Dynamic HA Extension is assigned from the range of values
 associated with skippable extensions at the home agent (i.e., value
 in the range 128-255).
 IANA has recorded the values as defined in Sections 7 and 3.4.

9. Security Considerations

 This specification assumes that a security configuration has been
 preconfigured between the MN and the HA or is configured along with
 the initial Registration Request/Registration Reply as per [7].
 There is a possibility of more than one HA creating a mobility
 binding entry for a given MN, if a man in the middle captures the
 Registration Request with the HA field set to ALL-ZERO-ONE-ADDR and
 forwards it to other HAs.  This scenario assumes that the rogue node
 can find out the addresses of the HAs that are able to authenticate
 the Registration Request.  It also assumes that the rogue node has
 the capability to store, duplicate, and send packets to the other HAs

Kulkarni, et al. Standards Track [Page 20] RFC 4433 Dynamic HA Assignment March 2006

 within the limited time of the replay window.  Otherwise, these HAs
 will reject the Registration Requests anyway.  In addition, this type
 of attack is only possible when the Requested HA Extension is not
 included in the registration message.  The mobile node can minimize
 the duration of this condition by using a short lifetime (e.g., 5
 seconds) in the Registration Request.
 This specification does not change the security model established in
 Mobile IPv4 [1].  Mobile nodes are often connected to the network via
 wireless links, which may be more prone to passive eavesdropping or
 replay attacks.  Such an attack might lead to bogus registrations or
 redirection of traffic or denial of service.
 As per the messaging in this document, the Assigned Home Agent will
 process the incoming Registration Request as per Mobile IPv4 [1].
 Hence the Assigned Home Agent will have the same security concerns as
 those of the home agent in Mobile IPv4 [1].  They are addressed in
 Section 5, "Security Considerations", of Mobile IPv4 [1].
 The Registration Request and Registration Reply messages are
 protected by a valid authenticator as specified in Mobile IPv4 [1].
 Configuring security associations is a deployment-specific issue and
 is covered by other Mobile IP specifications.  There can be many ways
 of configuring security associations, but this specification does not
 require any specific way.
 An example is where the security association between an MN and an
 individual HA (Requested or Assigned) is dynamically derived during
 the registration process based on a shared secret between MN and AAA
 infrastructure, as defined in [7].  The Registration Request is
 protected with MN-AAA Authentication Extension, and Registration
 Reply is protected with MN-HA Authentication Extension.  Because the
 security association is shared between MN and AAA, any dynamically
 assigned HA in the local domain can proxy authenticate the MN using
 AAA as per [7].
 The Assigned Home Agent authenticates each Registration Request from
 the mobile node as specified in Mobile IPv4 [1] and/or RFC 3012.  The
 MN also authenticates the Registration Reply from the Assigned HA;
 thus, the existing trust model in Mobile IPv4 [1] is maintained.

10. Backward-Compatibility Considerations

 In this section, we examine concerns that may arise when using this
 specification in a mixed environment where some nodes implement the
 specification and others do not.  In each of the examples below, we
 consider the case where one node is a "legacy" node, which does not
 implement the specification in the context of other nodes that do.

Kulkarni, et al. Standards Track [Page 21] RFC 4433 Dynamic HA Assignment March 2006

 Legacy Home Agent:
 Legacy home agents may reject the Registration Request with error
 code 136 because the Home Agent field is not a unicast address.
 However, some legacy HA implementations may coincidentally process
 the Registration Request in accordance with this document, when the
 HA field in Registration Request is set to ALL-ZERO-ONE-ADDR.
 Legacy Foreign Agent:
 Legacy foreign agents may forward a Registration Request with home
 agent field set to ALL-ZERO-ONE-ADDR by setting the destination IP
 address to ALL-ZERO-ONE-ADDR.  This will result in the packet being
 dropped or incidentally handled by a next-hop HA, adjacent to the FA.
 The MN may not be aware of the dropped Registration Request and may
 probably retry registration, thereby increasing the delay in
 registration.
 To reduce the delay in registration, the MN should take the following
 steps:
 1.  The MN should send the Registration Request as specified in this
     specification.  In other words, the MN should set the Home Agent
     field in the Registration Request to ALL-ZERO-ONE-ADDR and also
     add the Requested HA Extension.
 2.  If the MN does not receive a Registration Reply within some time
     and/or after sending a few Registration Requests, it can assume
     that the Registration Request(s) has been dropped, either by a
     legacy FA or an incorrect HA.  In addition, if the registration
     is denied with error code 70 (poorly formed Request), the MN can
     assume that the legacy FA cannot process this message.  In either
     case, the MN should fall back to a recovery mechanism.  The MN
     should quickly send a new Registration Request as mentioned in
     Section 4.1 step 2.  This step will ensure that a legacy FA will
     forward the Registration Request to the home agent thereby making
     dynamic HA assignment possible.
 Legacy Mobile Node:
 An MN that sends a Registration Request to an FA that can do dynamic
 HA assignment, but does not set the HA field to ALL-ZERO-ONE-ADDR
 will continue to be registered with its statically configured HA,
 exactly according to RFC 3344.

Kulkarni, et al. Standards Track [Page 22] RFC 4433 Dynamic HA Assignment March 2006

11. Acknowledgements

 The authors would like to thank Pete McCann for thorough review,
 suggestions on security considerations, and definition of ALL-ZERO-
 ONE-ADDR.  Thanks to Kuntal Chowdhury for extensive review and
 comments on this document.  Also thanks to Henrik Levkowetz for
 detailed reviews and suggestions.  Thomas Narten highlighted issues
 for legacy FA considerations.  Thanks to Ahmad Muhanna for pointing
 out scenario of multiple bindings on HAs, documented in the Security
 Considerations section.
 The authors would like to thank Mike Andrews, Madhavi Chandra, and
 Yoshi Tsuda for their review and suggestions.

12. Normative References

 [1]  Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
      2002.
 [2]  Calhoun, P. and C. Perkins, "Mobile IP Network Access Identifier
      Extension for IPv4", RFC 2794, March 2000.
 [3]  Senie, D., "Changing the Default for Directed Broadcasts in
      Routers", BCP 34, RFC 2644, August 1999.
 [4]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
      Extensions", RFC 2132, March 1997.
 [5]  Perkins, C. and P. Calhoun, "Mobile IPv4 Challenge/Response
      Extensions", RFC 3012, November 2000.
 [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [7]  Perkins, C. and P. Calhoun, "Authentication, Authorization, and
      Accounting (AAA) Registration Keys for Mobile IPv4", RFC 3957,
      March 2005.

Kulkarni, et al. Standards Track [Page 23] RFC 4433 Dynamic HA Assignment March 2006

Authors' Addresses

 Milind Kulkarni
 Cisco Systems Inc.
 170 W.  Tasman Drive,
 San Jose, CA 95134
 USA
 Phone: +1 408-527-8382
 EMail: mkulkarn@cisco.com
 Alpesh Patel
 Cisco Systems Inc.
 170 W.  Tasman Drive,
 San Jose, CA 95134
 USA
 Phone: +1 408-853-9580
 EMail: alpesh@cisco.com
 Kent Leung
 Cisco Systems Inc.
 170 W.  Tasman Drive,
 San Jose, CA 95134
 USA
 Phone: +1 408-526-5030
 EMail: kleung@cisco.com

Kulkarni, et al. Standards Track [Page 24] RFC 4433 Dynamic HA Assignment March 2006

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Kulkarni, et al. Standards Track [Page 25]

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