





Network Working Group                                           B. Haley
Request for Comments: 5142                               Hewlett-Packard
Category: Standards Track                                 V. Devarapalli
                                                         Azaire Networks
                                                                 H. Deng
                                                            China Mobile
                                                                J. Kempf
                                                         DoCoMo USA Labs
                                                            January 2008


               Mobility Header Home Agent Switch Message

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.

Abstract

   This document specifies a new Mobility Header message type that can
   be used between a home agent and mobile node to signal to a mobile
   node that it should acquire a new home agent.

























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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Scenarios .......................................................3
      3.1. Overloaded .................................................3
      3.2. Load Balancing .............................................3
      3.3. Maintenance ................................................3
      3.4. Functional Load Balancing ..................................3
      3.5. Home Agent Renumbering .....................................4
   4. Home Agent Switch Message .......................................4
   5. Home Agent Operation ............................................6
      5.1. Sending Home Agent Switch Messages .........................6
      5.2. Retransmissions ............................................7
      5.3. Mobile Node Errors .........................................7
   6. Mobile Node Operation ...........................................8
      6.1. Receiving Home Agent Switch Messages .......................8
      6.2. Selecting a Home Agent .....................................9
   7. Operational Considerations ......................................9
   8. Protocol Constants .............................................10
   9. IANA Considerations ............................................10
   10. Security Considerations .......................................10
   11. References ....................................................11
      11.1. Normative References .....................................11
      11.2. Informative References ...................................11
   Acknowledgments ...................................................11

1.  Introduction

   RFC 3775 [RFC3775] contains no provision to allow a home agent to
   inform a mobile node that it needs to stop acting as the home agent
   for the mobile node.  For example, a home agent may wish to handoff
   some of its mobile nodes to another home agent because it has become
   overloaded or it is going offline.

   This protocol describes a signaling message, called the Home Agent
   Switch message, that can be used to send a handoff notification
   between a home agent and mobile node.

   The Home Agent Switch message does not attempt to solve all general
   problems related to changing the home agent of a mobile node.  In
   particular, this protocol does not attempt to solve:

      o  The case where the Home Address of a mobile node must change in
         order to switch to a new home agent.  This operation should be
         avoided using this message.





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      o  Determining when a home agent should actively move mobile nodes
         to another home agent.  This decision should be made by a
         backend protocol, for example, as described in [hareliability].

2.  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 [RFC2119].

3.  Scenarios

   Here are some example scenarios where a home agent signaling message
   would be useful.

3.1.  Overloaded

   There are a number of reasons a home agent might be considered
   overloaded.  One might be that it is at, or near, its limit on the
   number of home bindings it is willing to accept.  Another is that it
   has reached a pre-determined level of system resource usage --
   memory, cpu cycles, etc.  In either case, it would be desirable for a
   home agent to reduce the number of home bindings before a failure
   occurs.

3.2.  Load Balancing

   A home agent might know of other home agents that are not as heavily
   loaded as itself, learned through some other mechanism outside the
   scope of this document.  An operator may wish to try and balance this
   load so that a failure would disrupt a smaller percentage of mobile
   nodes.

3.3.  Maintenance

   Most operators do periodic maintenance in order to maintain
   reliability.  If a home agent is being shutdown for maintenance, it
   would be desirable to inform mobile nodes so they do not lose
   mobility service.

3.4.  Functional Load Balancing

   A Mobile IPv6 home agent provides mobile nodes with two basic
   services.  It acts as a rendezvous server where correspondent nodes
   can find the current care-of address for the mobile node, and as an
   overlay router to tunnel traffic to/from the mobile node at its
   current care-of address.




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   A mobility service provider could have two sets of home agents to
   handle the two functions.  The rendezvous function could be handled
   by a machine specialized for high-speed transaction processing, while
   the overlay router function could be handled by a machine with high
   data throughput.

   A mobile node would start on the rendezvous server home agent and
   stay there if it does route optimization.  However, if the original
   home agent detects that the mobile node is not doing route
   optimization, but instead reverse-tunneling traffic, it could
   redirect the mobile node to a home agent with better data throughput.

3.5.  Home Agent Renumbering

   Periodically, a mobility service provider may want to shut-down home
   agent services at a set of IPv6 addresses and bring service back up
   at a new set of addresses.  Note that this may not involve anything
   as complex as IPv6 network renumbering [RFC4192]; it may just involve
   changing the addresses of the home agents.  With a signaling message,
   the service provider could inform mobile nodes to look for a new home
   agent.

4.  Home Agent Switch Message

   The Home Agent Switch message is used by the home agent to signal to
   the mobile node that it needs to stop acting as the home agent for
   the mobile node, and that it should acquire a new home agent.  Home
   Agent Switch messages are sent as described in Section 5.

   The message described below follows the Mobility Header format
   specified in Section 6.1 of [RFC3775]:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Payload Proto |  Header Len   |   MH Type     |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Checksum            |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                                               |
      .                                                               .
      .                       Message Data                            .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   The Home Agent Switch Message uses the MH Type value (12).  When this
   value is indicated in the MH Type field, the format of the Message
   Data field in the Mobility Header is as follows:

       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
                                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                      |# of Addresses |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      .                                                               .
      .                      Home Agent Addresses                     .
      .                                                               .
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      .                                                               .
      .                        Mobility Options                       .
      .                                                               .
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   # of Addresses

      An 8-bit unsigned integer indicating the number of IPv6 home agent
      addresses in the message.  If set to zero, the mobile node MUST
      perform home agent discovery.

   Reserved

      An 8-bit field reserved for future use.  The value MUST be
      initialized to zero by the sender, and MUST be ignored by the
      receiver.

   Home Agent Addresses

      A list of alternate home agent addresses for the mobile node.  The
      number of addresses present in the list is indicated by the "# of
      Addresses" field in the Home Agent Switch message.








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   Mobility Options

      A Variable-length field of such length that the complete Mobility
      Header is an integer multiple of 8 octets long.  This field
      contains zero or more TLV-encoded mobility options.  The encoding
      and format of defined options MUST follow the format specified in
      Section 6.2 of [RFC3775].  The receiver MUST ignore and skip any
      options that it does not understand.

      The Binding Refresh Advice mobility option defined in Section
      6.2.4 of [RFC3775] is valid for the Home Agent Switch message.

   If no home agent addresses and no options are present in this
   message, no padding is necessary and the Header Len field in the
   Mobility Header will be set to zero.

5.  Home Agent Operation

5.1.  Sending Home Agent Switch Messages

   When sending a Home Agent Switch message, the sending node constructs
   the packet as it would any other Mobility Header, except:

      o  The MH Type field MUST be set to (12).

      o  If alternative home agent addresses are known, the sending home
         agent SHOULD include them in the list of suggested alternate
         home agents.  The home agent addresses field should be
         constructed as described in Section 10.5.1 of [RFC3775], which
         will randomize addresses of the same preference in the list.

      o  The "# of Addresses" field MUST be filled-in corresponding to
         the number of home agent addresses included in the message.  If
         no addresses are present, the field MUST be set to zero,
         forcing the mobile node to perform home agent discovery by some
         other means.

      o  If the home agent is able to continue offering services to the
         mobile node for some period of time, it MAY include a Binding
         Refresh Advice mobility option indicating the time (in units of
         4 seconds) until the binding will be deleted.

   The Home Agent Switch message MUST use the home agent to mobile node
   IPsec ESP (Encapsulating Security Payload) authentication SA
   (Security Association) for integrity protection.






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   A home agent SHOULD send a Home Agent Switch message when a known
   period of unavailability is pending so the mobile node has sufficient
   time to find another suitable home agent.

   The sending node does not need to be the current home agent for the
   mobile node, for example as described in [hareliability], but it MUST
   have a security association with the mobile node so the message is
   not rejected.  In this case, the Home Agent Switch message SHOULD
   only contain the address of the home agent sending the message in the
   Home Agent Addresses field, which implies that the mobile node should
   switch to using the sender as its new home agent.

5.2.  Retransmissions

   If the home agent does not receive a response from the mobile node --
   either a Binding Update message to delete its home binding if it is
   the current home agent, or a Binding Update message to create a home
   binding if it is not the current home agent -- then it SHOULD
   retransmit the message until a response is received.  The initial
   value for the retransmission timer is INITIAL-HA-SWITCH-TIMEOUT.

   The retransmissions by the home agent MUST use an exponential back-
   off mechanism, in which the timeout period is doubled upon each
   retransmission, until either the home agent gets a response from the
   mobile node to delete its binding, or the timeout period reaches the
   value MAX-HA-SWITCH-TIMEOUT.  The home agent MAY continue to send
   these messages at this slower rate indefinitely.

   If the home agent included a Binding Refresh Advice mobility option,
   then it SHOULD delay any retransmissions until at least one half of
   the time period has expired, or INITIAL-HA-SWITCH-TIMEOUT, whichever
   value is less.

5.3.  Mobile Node Errors

   If a mobile node does not understand how to process a Home Agent
   Switch message, it will send a Binding Error message as described in
   Section 6.1.

   If a mobile node is unreachable, in other words, it still has a home
   binding with the home agent after reaching the timeout period of MAX-
   HA-SWITCH-TIMEOUT, the home agent SHOULD NOT make any conclusions
   about its status.

   In either case, the home agent SHOULD attempt to continue providing
   services until the lifetime of the binding expires.





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   Attempts by the mobile node to extend the binding lifetime with a
   Binding Update message SHOULD be rejected, and a Binding
   Acknowledgement SHOULD be returned with status value 129
   (Administratively prohibited) as specified in Section 6.1.8 of
   [RFC3775].

6.  Mobile Node Operation

6.1.  Receiving Home Agent Switch Messages

   Upon receiving a Home Agent Switch message, the Mobility Header MUST
   be verified as specified in [RFC3775], specifically:

      o  The Checksum, MH type, Payload Proto, and Header Len fields
         MUST meet the requirements of Section 9.2 of [RFC3775].

      o  The packet MUST be covered by the home agent to mobile node
         IPsec ESP authentication SA for integrity protection.

   If the packet is dropped due to the above tests, the receiving node
   MUST follow the processing rules as Section 9.2 of [RFC3775] defines.
   For example, it MUST send a Binding Error message with the Status
   field set to 2 (unrecognized MH Type value) if it does not support
   the message type.

   Upon receipt of a Home Agent Switch message, the mobile node MUST
   stop using its current home agent for services and MUST delete its
   home binding by sending a Binding Update message as described in
   Section 11.7.1 of [RFC3775].  This acts as an acknowledgement of the
   Home Agent Switch message.  Alternately, if the sender of the message
   is not the current home agent, sending a Binding Update message to
   create a home binding will act as an acknowledgement of the Home
   Agent Switch message.  Retransmissions of Binding Update messages
   MUST use the procedures described in Section 11.8 of [RFC3775].

   If a Binding Refresh Advice mobility option is present, the mobile
   node MAY delay the deletion of its home binding and continue to use
   its current home agent until the calculated time period has expired.

   If the Home Agent Switch message contains a list of alternate home
   agent addresses, the mobile node SHOULD select a new home agent as
   described in Section 6.2, and establish the necessary IPsec security
   associations with the new home agent by whatever means required as
   part of the mobile node/home agent bootstrapping protocol for the
   home agent's mobility service provider.  If no alternate home agent
   addresses are included in the list, the mobile node MUST first
   perform home agent discovery.




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6.2.  Selecting a Home Agent

   In most cases, the home agent addresses in the Home Agent Switch
   message will be of other home agents on the home link of the mobile
   node (the computed prefix is the same).  In this case, the mobile
   node SHOULD select a new home agent from the addresses as they are
   ordered in the list.  If the first address in the list is unable to
   provide service, then the subsequent addresses in the list should be
   tried in-order.

   In the case that the home agent addresses in the Home Agent Switch
   message are not all home agents on the home link of the mobile node
   (the computed prefix is different), the mobile node SHOULD select one
   with its home network prefix first, if available, followed by home
   agents with other prefixes.  Choosing a home agent with a different
   prefix might require a change of the home address for the mobile
   node, which could cause a loss of connectivity for any connections
   using the current home address.

7.  Operational Considerations

   This document does not specify how an operator might use the Home
   Agent Switch message in its network.  However, the following
   requirements are placed on its usage:

      o  The use of this message needs to take into account possible
         signaling overhead, congestion, load from the mechanism itself,
         and the resulting registration to another home agent.  A home
         agent may provide service for thousands, if not millions, of
         mobile nodes.  Careless application of the Home Agent Switch
         message may cause the new home agent, or some other parts of
         the network, to suffer.  As a result, it is REQUIRED that
         applications of this message either employ a feedback loop
         between resources of the new home agent and the sending of
         additional Home Agent Switch messages, or apply a maximum rate
         at which mobile nodes can be informed of the switch that is far
         below the designated capacity of new registrations that the set
         of home agents can process.  If no other information is
         available, this maximum rate should default to MAX-HA-SWITCH-
         TRANSMIT-RATE.

      o  In general, switching the home agent of a mobile node should
         only be done when absolutely necessary, since it might cause a
         service disruption if the switch to a new home agent fails, the
         new home agent is itself under an overload condition, or the
         network connection of the new home agent is congested.





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         Similarly, the path characteristics via the new home agent may
         be different, which may cause temporary difficulties for end-
         to-end transport layer operation.

      o  If this message is being used for load-balancing between a set
         of home agents, they should all be configured with the same set
         of prefixes so a home agent switch does not require a change of
         the home address for a mobile node.  That operation is NOT
         RECOMMENDED and should be avoided.

8.  Protocol Constants

   INITIAL-HA-SWITCH-TIMEOUT             5 seconds
   MAX-HA-SWITCH-TIMEOUT                 20 seconds
   MAX-HA-SWITCH-TRANSMIT-RATE           1 per second

9.  IANA Considerations

   IANA has assigned a new Mobility Header type for the following new
   message described in Section 4:

      (12) Home Agent Switch message

10.  Security Considerations

   As with other messages in [RFC3775], the Home Agent Switch message
   MUST use the home agent to mobile node ESP encryption SA for
   confidentiality protection, and MUST use the home agent to mobile
   node ESP authentication SA for integrity protection.

   The Home Agent Switch message MAY use the IPsec ESP SA in place for
   Binding Updates and Acknowledgements, as specified in Section 5.1 of
   [RFC3775], in order to reduce the number of configured security
   associations.  This also gives the message authenticity protection.

   Some operators may not want to reveal the list of home agents to on-
   path listeners.  In such a case, the Home Agent Switch message should
   use the home agent to mobile node IPsec ESP encryption SA for
   confidentiality protection.












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11.  References

11.1.  Normative References

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

   [RFC3775]       Johnson, D., Perkins, C., and J. Arkko, "Mobility
                   Support in IPv6", RFC 3775, June 2004.

11.2.  Informative References

   [RFC4192]       Baker, F., Lear, E., and R. Droms, "Procedures for
                   Renumbering an IPv6 Network without a Flag Day", RFC
                   4192, September 2005.

   [hareliability] Wakikawa, R., Ed., "Home Agent Reliability Protocol",
                   Work in Progress, November 2007.

Acknowledgments

   We would like to thank the authors of a number of previous documents
   that contributed content to this RFC:

      o Ryuji Wakikawa, Pascal Thubert, and Vijay Devarapalli,
        "Inter Home Agents Protocol Specification", March 2006.

      o Hui Deng, Brian Haley, Xiaodong Duan, Rong Zhang, and Kai Zhang,
        "Load Balance for Distributed Home Agents in Mobile IPv6",
        October 2004.

      o James Kempf, "Extension to RFC 3775 for Alerting the Mobile Node
        to Home Agent Unavailability", October 2005.

      o Brian Haley and Sri Gundavelli, "Mobility Header Signaling
        Message", September 2007.

   Thanks also to Kilian Weniger, Jixing Liu, Alexandru Petrescu, Jouni
   Korhonen, and Wolfgang Fritsche for their review and feedback.












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Author's Addresses

   Brian Haley
   Hewlett-Packard Company
   110 Spitbrook Road
   Nashua, NH 03062, USA
   EMail: brian.haley@hp.com

   Vijay Devarapalli
   Azaire Networks
   3121 Jay Street
   Santa Clara, CA 95054 USA
   EMail: vijay.devarapalli@azairenet.com

   James Kempf
   DoCoMo USA Labs
   181 Metro Drive
   Suite 300
   San Jose, CA 95110 USA
   EMail: kempf@docomolabs-usa.com

   Hui Deng
   China Mobile
   53A, Xibianmennei Ave.
   Xuanwu District
   Beijing 100053
   China
   EMail: denghui@chinamobile.com























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