





Network Working Group                                          R. Koodli
Request for Comments: 4988                                    C. Perkins
Category: Experimental                            Nokia Siemens Networks
                                                            October 2007


                       Mobile IPv4 Fast Handovers

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   This document adapts the Mobile IPv6 Fast Handovers to improve delay
   and packet loss resulting from Mobile IPv4 handover operations.
   Specifically, this document addresses movement detection, IP address
   configuration, and location update latencies during a handover.  For
   reducing the IP address configuration latency, the document proposes
   that the new Care-of Address is always made to be the new access
   router's IP address.



























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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Factors Affecting Handover ......................................5
   4. Protocol ........................................................6
      4.1. Overview ...................................................6
      4.2. Operation ..................................................7
   5. Message Formats ................................................10
      5.1. Fast Binding Update (FBU) .................................10
      5.2. Fast Binding Acknowledgment (FBAck) .......................12
      5.3. Router Solicitation for Proxy Advertisement (RtSolPr) .....13
      5.4. Proxy Router Advertisement (PrRtAdv) ......................14
      5.5. Handover Initiate (HI) ....................................17
      5.6. Handover Acknowledge (HAck) ...............................19
   6. Option Formats .................................................20
      6.1. Link-Layer Address Option Format ..........................20
      6.2. New IPv4 Address Option Format ............................22
      6.3. New Router Prefix Information Option ......................22
   7. Security Considerations ........................................23
   8. IANA Considerations ............................................24
   9. Acknowledgments ................................................25
   10. References ....................................................25
      10.1. Normative References .....................................25
      10.2. Informative References ...................................26


























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1.  Introduction

   This document adapts the fast handover specification [rfc4068] to
   IPv4 networks.  The fast handover protocol specified in this document
   is particularly interesting for operation over links such as IEEE 802
   wireless links.  Fast handovers are not typically needed for wired
   media due to the relatively large delays attributable to establishing
   new connections in today's wired networks.  Mobile IPv4 [rfc3344]
   registration messages are reused (with new type numbers) in this
   document to enable faster implementation using existing Mobile IPv4
   software.  This document does not require link-layer triggers for
   protocol operation, but performance will typically be enhanced by
   using the appropriate triggers when they are available.  This
   document assumes that the reader is familiar with the basic operation
   and terminology of Mobile IPv4 [rfc3344] and Fast Handovers for
   Mobile IPv6 [rfc4068].

   The active agents that enable continued packet delivery to a mobile
   node (MN) are the access routers on the networks that the mobile node
   connects to.  Handover means that the mobile node changes its network
   connection, and we consider the scenario in which this change means
   change in access routers.  The mobile node utilizes the access
   routers as default routers in the normal sense, but also as partners
   in mobility management.  Thus, when the mobile node moves to a new
   network, it processes handover-related signaling in order to identify
   and develop a relationship with a new access router.  In this
   document, we call the previous access router PAR and the new access
   router NAR, consistent with the terminology in [rfc4068].  Unless
   otherwise mentioned, a PAR is also a Previous Foreign Agent (PFA) and
   a NAR is also a New Foreign Agent (NFA).

   On a particular network, a mobile node may obtain its IP address via
   DHCP [rfc2131] (i.e., Co-located Care-of Address) or use the Foreign
   Agent CoA.  During a handover, the new CoA (NCoA) is always made to
   be that of NAR.  This allows a mobile node to receive and send
   packets using its previous CoA (PCoA), so that delays resulting from
   IP configuration (such as DHCP address acquisition delay) subsequent
   to attaching to the new link are disengaged from affecting the
   existing sessions.

   Unlike in Mobile IPv6, a Mobile IPv4 host may rely on its Foreign
   Agent to provide a Care-of Address.  Using the protocol specified in
   this document, the binding at the PAR is always established between
   the on-link address the mobile node is using and a new CoA that it
   can use on the NAR's link.  When FA-CoA is used, the on-link address
   is the MN's home address, not the FA-CoA itself, which needs to be





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   bound to the NCoA.  So, when we say "a binding is established between
   PCoA and NCoA", it is actually the home address of the mobile node
   that is bound to the NCoA in the FA-CoA mode.

   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 [RFC2119].

2.  Terminology

   The terminology used in this document in based on [rfc4068] and
   [rfc3344].  We provide some definitions below for convenience.

      Mobile Node (MN): A Mobile IPv4 host.

      Access Point (AP): A Layer 2 device connected to an IP subnet that
      offers wireless connectivity to an MN.  An Access Point Identifier
      (AP-ID) refers to the AP's L2 address.  Sometimes, AP-ID is also
      referred to as a Base Station Subsystem ID (BSSID).

      Access Router (AR): The MN's default router.

      Previous Access Router (PAR): The MN's default router prior to its
      handover.

      New Access Router (NAR): The MN's default router subsequent to its
      handover.

      Previous CoA (PCoA): The IP address of the MN valid on PAR's
      subnet.

      New CoA (NCoA): The MN's Care-of Address valid on NAR's subnet.

      Handover: A process of terminating existing connectivity and
      obtaining new IP connectivity.

      (AP-ID, AR-Info) tuple: Contains an access router's L2 and IP
      addresses, and the prefix valid on the interface to which the
      Access Point (identified by AP-ID) is attached.  The triplet
      [Router's L2 address, Router's IP address, Prefix] is called
      "AR-Info".










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3.  Factors Affecting Handover

   Both link-layer operations and IP-layer procedures affect the
   perceived handover performance.  However, the overall performance is
   also (always) a function of specific implementation of the technology
   as well as the system configuration.  This document only specifies IP
   layer protocol operations.  The purpose of this section is to provide
   an illustration of events that affect handover performance, but it is
   purely informative.

   The IP-layer handover delay and packet loss are influenced by
   latencies due to movement detection, IP address configuration, and
   the Mobile IP registration procedure.  Movement detection latency
   comes from the need to reliably detect movement to a new subnet.
   This is a function of the frequency of router advertisements as well
   as default agent reachability.  IP address configuration latency
   depends on the particular IP CoA being used.  If co-located mode with
   DHCP is used, the latency is quite likely going to be higher and
   potentially unacceptable for real-time applications such as Voice
   over IP.  Finally, the Mobile IP registration procedure introduces a
   round-trip of delay between the Mobile Node and its Home Agent over
   the Internet.  This delay is incurred after the mobile node performs
   movement detection and IP configuration.

   Underlying the IP operations are link-layer procedures.  These are
   technology-specific.  For instance, in IEEE 802.11, the handover
   operation typically involves scanning access points over all
   available channels, selecting a suitable access point, and
   associating with it.  It may also involve performing access control
   operations such as those specified in IEEE 802.1X [ieee-802.1x].
   These delays contribute to the handover performance.  See [fh-ccr]
   and Chapters 20 and 22 in [mi-book].  Optimizations are being
   proposed for standardization in IEEE; for instance, see
   [ieee-802.11r] and [ieee-802.21].  Together with appropriate
   implementation techniques, these optimizations can provide the
   required level of delay support at the link-layer for real-time
   applications.














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4.  Protocol

4.1.  Overview

   The design of the protocol is the same as for Mobile IPv6 [rfc4068].
   Readers should consult [rfc4068] for details; here we provide a
   summary.

   The protocol avoids the delay due to movement detection and IP
   configuration and disengages Mobile IP registration delay from the
   time-critical path.  The protocol provides the surrounding network
   neighborhood information so that a mobile node can determine whether
   it is moving to a new subnet even before the handover.  The
   information provided and the signaling exchanged between the local
   mobility agents allow the mobile node to send and receive packets
   immediately after handover.  In order to disengage the Mobile IP
   registration latency, the protocol provides routing support for the
   continued use of a mobile node's previous CoA.

   After a mobile node obtains its IPv4 Care-of Address, it builds a
   neighborhood access point and subnet map using the Router
   Solicitation for Proxy Advertisement (RtSolPr) and Proxy Router
   Advertisement (PrRtAdv) messages.  The mobile node may scan for
   access points (APs) based on the configuration policy in operation
   for its wireless network interface.  If a scan detects a new AP, the
   mobile node resolves the corresponding AP Identifier to subnet
   information using the RtSolPr and PrRtAdv messages mentioned above.

   At some point, the mobile node decides to undergo handover.  It sends
   a Fast Binding Update (FBU) message to PAR from the previous link or
   from the new link.  An FBU message enables creation of a binding
   between the mobile node's previous CoA and the new CoA.

   The coordination between the access routers is done by way of the
   Handover Initiate (HI) and Handover Acknowledge (HAck) messages
   defined in [rfc4068].  After these signals have been exchanged
   between the previous and new access routers (PAR and NAR), data
   arriving at PAR will be tunneled to NAR for delivery to the newly
   arrived mobile node.  The purpose of HI is to securely deliver the
   routing parameters for establishing this tunnel.  The tunnel is
   created by the access routers in response to the delivery of the FBU
   from the mobile node.









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4.2.  Operation

   In response to a handover trigger or indication, the mobile node
   sends a Fast Binding Update message to the Previous Access Router
   (PAR) (see Section 5.1).  Depending on the Mobile IP mode of
   operation, the source IP address is either the Home Address (in FA
   CoA mode) or co-located CoA (in CCoA mode).  The FBU message SHOULD
   (when possible) be sent while the mobile node is still connected to
   PAR.  When sent in this "predictive" mode, the fields in the FBU MUST
   be set as follows:

      The Home Address field is either the Home Address or the co-
      located CoA whenever the mobile node has a co-located CoA.

      The Home Agent field is set to PAR's IP address.

      The Care-of Address field is the NAR's IP address (as discovered
      via a PrRtAdv message).

      The fields in the IP header MUST be set as follows:

      The Destination IP address is PAR's IP address.

      The Source IP address is either the Home Address or the co-located
      CoA whenever the mobile node has a co-located CoA.

   As a result of processing the FBU, PAR creates a binding between the
   address given by the mobile node in the Home Address field and NAR's
   IP address in its routing table.  The PAR sends an FBack message (see
   Section 5.2) as a response to the mobile node.

   The timeline for the predictive mode of operation (adapted from
   [rfc4068]) is shown in Figure 1.


















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             MN                    PAR                  NAR
              |                     |                    |
              |------RtSolPr------->|                    |
              |<-----PrRtAdv--------|                    |
              |                     |                    |
              |------FBU----------->|--------HI--------->|
              |                     |<------HAck---------|
              |          <--FBack---|--FBack--->         |
              |                     |                    |
           disconnect             forward                |
              |                   packets===============>|
              |                     |                    |
              |                     |                    |
          connect                   |                    |
              |                     |                    |
              |--------- FBU --------------------------->|
              |<=================================== deliver packets
              |                     |              (including FBack)
              |                     |<-----FBU-----------|

                 Figure 1: Predictive Fast Handover

   The mobile node sends the FBU, regardless of its previous
   transmission, when attachment to a new link is detected.  This
   minimally allows NAR to detect the mobile node's attachment, but also
   the retransmission of FBU when an FBack has not been received yet.
   When sent in this "reactive" mode, the Destination IP address in the
   IP header MUST be NAR's IP address; the rest of the fields in the FBU
   are the same as in the "predictive" case.

   When NAR receives FBU, it may already have processed the HI message
   and created a host route entry for the mobile node, using either the
   home address or the co-located care-of address as provided by PAR.
   In that case, NAR SHOULD immediately forward arriving and buffered
   packets as well as the FBAck message.  In any case, NAR MUST forward
   the contents of the FBU message, starting from the Type field, to
   PAR; the Source and Destination IP addresses in the new packet now
   contain the IP addresses of NAR and PAR, respectively.

   The reactive mode of operation (adapted from [rfc4068]) is
   illustrated in Figure 2.  Even though the Figure does not show the HI
   and HAck messages illustrated in Figure 1, these messages could
   already have been exchanged (in the case when the PAR has already
   processed the FBU sent from the previous link); if not, the PAR sends
   a HI message to the NAR.  The FBack packet is forwarded by the NAR to
   the MN along with the data packets.





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                 MN                    PAR                  NAR
                  |                     |                    |
                  |------RtSolPr------->|                    |
                  |<-----PrRtAdv--------|                    |
                  |                     |                    |
               disconnect               |                    |
                  |                     |                    |
                  |                     |                    |
               connect                  |                    |
                  |-----------FBU-------|------------------->|
                  |                     |<-----FBU-----------|
                  |                     |------FBack-------->|
                  |                   forward                |
                  |                   packets===============>|
                  |                     |                    |
                  |<=================================== deliver packets
                  |                                    (including FBack)
                  |                                          |

                       Figure 2: Reactive Fast Handover

   The Handover Initiate (HI) and Handover Acknowledge (HAck) messages
   serve to establish a bidirectional tunnel between the routers to
   support packet forwarding for PCoA.  The tunnel itself is established
   as a response to the FBU message.  The PAR sends the HI message with
   Code = 0 when it receives FBU with source IP address set to PCoA.
   The PAR sends HI with Code = 1 when it receives FBU with source IP
   address not set to PCoA (i.e., when received from NAR).  This allows
   NAR to disambiguate HI message processing sent as a response to
   predictive and reactive modes of operation.  If NAR receives a HI
   message with Code = 1, and it has already set up a host route entry
   and a reverse tunnel for PCoA, it SHOULD still respond with a HAck
   message, using an appropriate Code value defined in Section 5.6.

   The protocol provides an option for NAR to return NCoA for use by the
   mobile node.  When NAR can provide an NCoA for exclusive use of the
   mobile node, the address is supplied in the HAck message.  The PAR
   includes this NCoA in FBack.  Exactly how NAR manages the address
   pool from which it supplies NCoA is not specified in this document.
   Nevertheless, the MN should be prepared to use this address instead
   of performing DHCP or similar operations to obtain an IPv4 address.

   Even though the mobile node can obtain this NCoA from the NAR, it is
   unaware of the address at the time it sends an FBU.  Hence, it binds
   PCoA to NAR's IP address as before.






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5.  Message Formats

   This section specifies the formats for messages used in this
   protocol.  The Code values below are the same as those in [rfc4068],
   and do not require any assignment from IANA.

5.1.  Fast Binding Update (FBU)

   The FBU format is bitwise identical to the Registration Request
   format in [rfc3344].  The same destination port number, 434, is used,
   but the FBU and FBAck messages in this specification have new message
   type numbers.

       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      |x|x|D|M|G|r|T|x| reserved  |     Lifetime      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Home Address                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Home Agent                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Care-of Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                         Identification                        +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Extensions ...
      +-+-+-+-+-+-+-+-

                Figure 3: Fast Binding Update (FBU) Message

      IP Fields:

         Source address: The interface address from which the message is
         sent.  Either PCoA (co-located or Home Address), or NAR's IP
         address (when forwarded from NAR to PAR).

         Destination Address: The IP address of the Previous Access
         Router (PAR) or the New Access Router (NAR).

         Source Port: variable

         Destination port: 434






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      Message Fields:

         Type: 20

         Flags: See [rfc3344].  The 'S' and 'B' flags in [rfc3344] are
         sent as zero, and ignored on reception.

         reserved: Sent as zero, ignored on reception

         Lifetime: The number of seconds remaining before the binding
         expires.  This value MUST NOT exceed 10 seconds.

         Home Address: MUST be either the co-located CoA or the Home
         Address itself (in FA-CoA mode)

         Home Agent: The Previous Access Router's global IP address

         Care-of Address: The New Access Router's global IP address.
         Even when a New CoA is provided to the MN (see Section 5.4),
         NAR's IP address MUST be used for this field.

         Identification: a 64-bit number used for matching an FBU with
         FBack.  Identical to usage in [rfc3344]

         Extensions: MUST contain the MN-PAR Authentication Extension
         (see Section 8)

   The MN-PAR Authentication Extension is the Generalized Mobile IP
   Authentication Extension in [rfc4721] with a new Subtype for MN-PAR
   Authentication.  The Authenticator field in the Generalized Mobile IP
   Authentication Extension is calculated using a shared key between the
   MN and the PAR.  However, the key distribution itself is beyond the
   scope of this document, and is assumed to be performed by other means
   (for example, using [rfc3957]).

















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5.2.  Fast Binding Acknowledgment (FBAck)

   The FBAck format is bitwise identical to the Registration Reply
   format in [rfc3344].

       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      |     Code      | reserved  |     Lifetime      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Home Address                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Home Agent                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                         Identification                        +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Extensions ...
      +-+-+-+-+-+-+-+-

              Figure 4: Fast Binding Acknowledgment (FBAck)

      IP Fields:

         Message Source address: Typically copied from the destination
         address of the FBU message

         Destination Address: Copied from the Source IP address in FBU
         message

         Source Port: variable

         Destination port: Copied from the source port in FBU message

      Message Fields:

         Type: 21

         Code: Indicates the result of processing FBU message.

            0: FBU Accepted
            1: FBU Accepted, NCoA supplied
            128: FBU Not Accepted, reason unspecified
            129: Administratively prohibited
            130: Insufficient resources

         reserved: Sent as zero, ignored on reception



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         Lifetime: The granted number of seconds remaining before
         binding expires.

         Home Address: either the co-located CoA or the Home Address
         itself (in FA-Coa mode)

         Home Agent: The Previous Access Router's global IP address

         Identification: a 64-bit number used for matching FBU.  Copied
         from the field in FBU for which this FBack is a reply.

         Extensions: The MN-PAR Authentication extension MUST be present
         (see Section 8).  In addition, a New IPv4 Address Option, with
         Option-Code 2, MUST be present when NAR supplies the NCoA (see
         Section 6.2).

5.3.  Router Solicitation for Proxy Advertisement (RtSolPr)

   Mobile Nodes send Router Solicitation for Proxy Advertisement in
   order to prompt routers for Proxy Router Advertisements.  All the
   link-layer address options have the format defined in Section 6.1.
   The message format and processing rules are identical to those
   defined in [rfc4068].

      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      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Subtype     |   Reserved    |          Identifier           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

      Figure 5: Router Solicitation for Proxy Advertisement (RtSolPr)
                                  Message

      IP Fields:

         Source Address: An IP address assigned to the sending interface

         Destination Address: The address of the Access Router or the
         all routers multicast address.

         Time-to-Live: At least 1.  See [rfc1256].






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      ICMP Fields:

         Type: 41.  See Section 3 in [rfc4065].

         Code: 0

         Checksum: The 16-bit one's complement of the one's complement
         sum of the ICMP message, starting with the ICMP Type.  For
         computing the checksum, the Checksum and the Reserved fields
         are set to 0.  See [rfc1256].

         Subtype: 6

         Reserved: MUST be set to zero by the sender and ignored by the
         receiver.

         Identifier: MUST be set by the sender so that replies can be
         matched to this Solicitation.

      Valid Options:

         New Access Point Link-layer Address: The link-layer address or
         identification of the access point for which the MN requests
         routing advertisement information.  It MUST be included in all
         RtSolPr messages.  More than one such address or identifier can
         be present.  This field can also be a wildcard address (see
         Section 6.1).

5.4.  Proxy Router Advertisement (PrRtAdv)

   Access routers send out a Proxy Router Advertisement message
   gratuitously if the handover is network-initiated or as a response to
   RtSolPr message from a mobile node, providing the link-layer address,
   IP address, and subnet prefixes of neighboring access routers.  All
   the link-layer address options have the format defined in Section
   6.1.

   The message format and processing rules are identical to those
   defined in [rfc4068].












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      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      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Subtype     |   Reserved    |          Identifier           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

          Figure 6: Proxy Router Advertisement (PrRtAdv) Message

      IP Fields:

         Source Address: An IP address assigned to the sending interface

         Destination Address: The Source Address of an invoking Router
         Solicitation for Proxy Advertisement or the address of the node
         the Access Router is instructing to handover.

         Time-to-Live: At least 1.  See [rfc1256].

      ICMP Fields:

         Type: 41.  See Section 3 in [rfc4065].

         Code 0, 1, 2, 3, or 4.  See below.

         Checksum: The 16-bit one's complement of the one's complement
         sum of the ICMP message, starting with the ICMP Type.  For
         computing the checksum, the Checksum and the Reserved fields
         are set to 0.  See [rfc1256].

         Subtype: 7

         Reserved: MUST be set to zero by the sender and ignored by the
         receiver.

         Identifier: Copied from Router Solicitation for Proxy
         Advertisement or set to Zero if unsolicited.

      Valid Options in the following order:

         New Access Point Link-layer Address: The link-layer address
         (LLA) or identification of the access point.  When there is no
         wildcard in RtSolPr, this is copied from the LLA (for which the
         router is supplying the [AP-ID, AR-Info] tuple) present in




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         RtSolPr.  When a wildcard is present in RtSolPr, PAR uses its
         neighborhood information to populate this field.  This option
         MUST be present.

         New Router's Link-layer Address: The link-layer address of the
         Access Router for which this message is proxied.  This option
         MUST be included when Code is 0 or 1.

         New Router's IP Address: The IP address of NAR.  This option
         MUST be included when Code is 0 or 1.

         New Router Prefix Information Option: The number of leading
         bits that define the network number of the corresponding
         Router's IP Address option (see above).

         New CoA Option: MAY be present, typically when PrRtAdv is sent
         unsolicited.  PAR MAY compute new CoA by communicating with the
         NAR or by means not specified in this document.  In any case,
         the MN should be prepared to use this address instead of
         performing DHCP or similar operations to obtain an IPv4
         address.  Even when it uses the New CoA provided, the MN MUST
         bind its current on-link address (PCoA) to that of NAR in the
         FBU message.

   A PrRtAdv with Code 0 means that the MN should use the [AP-ID,
   AR-Info] tuple present in the options above.  In this case, the
   Option-Code field (see Section 6.1) in the New AP LLA option is 1,
   reflecting the LLA of the access point for which the rest of the
   options are related, and the Option-Code for the New Router's LLA
   option is 3.  Multiple tuples may be present.

   A PrRtAdv with Code 1 means that the message is sent unsolicited.  If
   a New IPv4 option (see Figure 10) is present following the New Router
   Prefix Information option (see Section 6.3), the MN SHOULD use the
   supplied NCoA and send the FBU immediately or else stand to lose
   service.  This message acts as a network-initiated handover trigger.
   The Option-Code field (see Section 6.1) in the New AP LLA option in
   this case is 1 reflecting the LLA of the access point for which the
   rest of the options are related.

   A Proxy Router Advertisement with Code 2 means that no new router
   information is present.  The LLA option contains an Option-Code value
   that indicates a specific reason (see Section 6.1).

   A Proxy Router Advertisement with Code 3 means that new router
   information is only present for a subset of access points requested.
   The Option-Code values in the LLA option distinguish different
   outcomes (see Section 6.1).



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   A Proxy Router Advertisement with Code 4 means that the subnet
   information regarding neighboring access points is sent unsolicited,
   but the message is not a handover trigger, unlike when the message is
   sent with Code 1.  Multiple tuples may be present.

   When a wildcard AP identifier is supplied in the RtSolPr message, the
   PrRtAdv message should include all available [Access Point
   Identifier, Link-Layer Address option, Prefix Information Option]
   tuples corresponding to the PAR's neighborhood.

   The New CoA option may also be used when the PrRtAdv is sent as a
   response to a RtSolPr message.  However, the solicited RtSolPr and
   PrRtAdv exchange for neighborhood discovery is logically decoupled
   from the actual handover phase involving the FBU and FBack messages
   (above) as well as HI and HAck messages (see below).  This means the
   access routers have to carefully manage the supplied address due to
   the relative scarcity of addresses in IPv4.

5.5.  Handover Initiate (HI)

   The Handover Initiate (HI) is an ICMP message sent by an Access
   Router (typically PAR) to another Access Router (typically NAR) to
   initiate the process of a mobile node's handover.

   The message format and processing rules are identical to those
   defined in [rfc4068].

      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      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Subtype     |S|U| Reserved  |          Identifier           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

                 Figure 7:  Handover Initiate (HI) Message

      IP Fields:

         Source Address: The IP address of the PAR

         Destination Address: The IP address of the NAR

         Time-to-Live: At least 1.  See [rfc1256].





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      ICMP Fields:

         Type: 41.  See Section 3 in [rfc4065].

         Code: 0 or 1.  See below

         Checksum: The 16-bit one's complement of the one's complement
         sum of the ICMP message, starting with the ICMP Type.  For
         computing the checksum, the Checksum and the Reserved fields
         are set to 0.  See [rfc1256].

         Subtype: 8

         S: Assigned address configuration flag.  When set, this message
         requests a new CoA to be returned by the destination.  May be
         set when Code = 0.  MUST be 0 when Code = 1.

         U: Buffer flag.  When set, the destination SHOULD buffer any
         packets towards the node indicated in the options of this
         message.  Used when Code = 0, SHOULD be set to 0 when Code = 1.

         Reserved: MUST be set to zero by the sender and ignored by the
         receiver.

         Identifier: MUST be set by the sender so replies can be matched
         to this message.

      Valid Options:

         Link-layer address of MN: The link-layer address of the MN that
         is undergoing handover to the destination (i.e., NAR).  This
         option MUST be included so that the destination can recognize
         the MN.

         Previous Care-of Address: The IP address used by the MN while
         attached to the originating router.  This option MUST be
         included so that a host route can be established on the NAR.

         New Care-of Address: This option MAY be present when the MN
         wishes to use a new IP address when connected to the
         destination.  When the 'S' bit is set, NAR MAY provide this
         address in HAck, in which case the MN should be prepared to use
         this address instead of performing DHCP or similar operations
         to obtain an IPv4 address.

   PAR uses Code = 0 when it processes the FBU received with PCoA as
   source IP address.  PAR uses Code = 1 when the FBU is received with
   NAR's IP address as the source IP address.



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5.6.  Handover Acknowledge (HAck)

   The Handover Acknowledgment message is a new ICMP message that MUST
   be sent (typically by NAR to PAR) as a reply to the Handover Initiate
   (HI) (see Section 5.5) message.

   The message format and processing rules are identical to those
   defined in [rfc4068].

      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      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Subtype     |    Reserved   |          Identifier           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

                 Figure 8:  Handover Acknowledge (HAck) Message

      IP Fields:

         Source Address: Copied from the destination address of the
         Handover Initiate Message to which this message is a response.

         Destination Address: Copied from the source address of the
         Handover Initiate Message to which this message is a response.

         Time-to-Live: At least 1.  See [rfc1256].

      ICMP Fields:

         Type: 41.  See Section 3 in [rfc4065].

         Code:

            0: Handover Accepted
            1: Handover Accepted, NCoA not valid
            2: Handover Accepted, NCoA in use
            3: Handover Accepted, NCoA assigned (used in Assigned
               addressing)
            4: Handover Accepted, NCoA not assigned
            128: Handover Not Accepted, reason unspecified
            129: Administratively prohibited
            130: Insufficient resources





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         Checksum: The 16-bit one's complement of the one's complement
         sum of the ICMP message, starting with the ICMP Type.  For
         computing the checksum, the Checksum and the Reserved fields
         are set to 0.  See [rfc1256].

         Subtype: 9

         Reserved: MUST be set to zero by the sender and ignored by the
         receiver.

         Identifier: Copied from the corresponding field in the Handover
         Initiate message this message is in response to.

      Valid Options:

         New Care-of Address: If the 'S' flag in the HI message is set,
         this option MUST be used to provide NCoA the MN should use when
         connected to this router.  This option MAY be included even
         when 'S' bit is not set, e.g., Code 2 above.  The MN should be
         prepared to use this address instead of performing DHCP or
         similar operations to obtain an IPv4 address.

   The Code 0 is the expected average case of a handover being accepted
   and the routing support provided for the use of PCoA.  The rest of
   the Code values pertain to the use of NCoA (which is common in
   [rfc4068]).  Code values 1 and 2 are for cases when the MN proposes
   an NCoA and the NAR provides a response.  Code 3 is when the NAR
   provides NCoA (which could be the same as that proposed by the MN).
   Code 4 is when the NAR does not provide NCoA, but instead provides
   routing support for PCoA.

6.  Option Formats

   The options in this section are specified as extensions for the HI
   and HAck messages, as well as for the PrRtSol and PrRtAdv messages.
   The Option-Code values below are the same as those in [rfc4068], and
   do not require any assignment from IANA.

6.1.  Link-Layer Address Option Format

      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      |    Length     |  Option-Code  |     LLA ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 9: Link-Layer Address Option Format




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      Fields:

         Type: 20

         Option-Code:

            0: Wildcard requesting resolution for all nearby access
               points
            1: Link-Layer Address of the New Access Point
            2: Link-Layer Address of the MN
            3: Link-Layer Address of the NAR
            4: Link-Layer Address of the source of the RtSolPr or
               PrRtAdv message
            5: The access point identified by the LLA belongs to the
               current interface of the router
            6: No prefix information available for the access point
               identified by the LLA
            7: No fast handovers support available for the access point
               identified by the LLA

         Length: The length of the option (including the Type, Length
         and Option-Code fields) in units of 8 octets.

         Link-Layer Address: The variable-length link-layer address.
         The content and format of this field (including byte and bit
         ordering) depends on the specific link-layer in use.

   There is no length field for the LLA itself.  Implementations MUST
   determine the length of the LLA based on the specific link technology
   where the protocol is run.  The total size of the LLA option itself
   MUST be a multiple of 8 octets.  Hence, padding may be necessary
   depending on the size of the LLA used.  In such a case, the padN
   option [rfc2460] MUST be used.  As an example, when the LLA is 6
   bytes (meaning 7 bytes of padding is necessary to bring the LLA
   option length to 2), the padN option will have a length field of 5
   and 5 bytes of zero-valued octets (see [rfc2460]).















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6.2.  New IPv4 Address Option Format

   This option is used to provide the new router's IPv4 address or the
   NCoA in PrRtAdv, as well as PCoA and NCoA in HI and HAck messages.

      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      |    Length     |  Option-Code  |    Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      New IPv4 Address                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 10: New IPv4 Address Option Format

      Fields:

         Type: 21

         Length: The length of the option (including the Type, Length
         and Option-Code fields) in units of 8 octets.

         Option-Code:

            1: Previous CoA
            2: New CoA
            3: NAR's IP Address

         Reserved: Set to zero.

         New IPv4 Address: NAR's IPv4 address or the NCoA assigned by
         NAR.

6.3.  New Router Prefix Information Option

   This option is used in the PrRtAdv message.

      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      |    Length     |  Option-Code  | Prefix-Length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 11: New Router Prefix Information Option Format





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      Fields:

         Type: 22

         Length: The length of the option (including the Type, Length
         and Option-Code fields) in units of 8 octets.

         Option-Code: 0

         Prefix-Length The number of leading bits that define the
         network number of the corresponding Router's IP Address option.

         Reserved: Set to zero.

7.  Security Considerations

   As outlined in [rfc4068], the following vulnerabilities are
   identified and the solutions mentioned.

   Insecure FBU:

   Failure to protect the FBU message could result in packets meant for
   an address being stolen or redirected to some unsuspecting node.
   This concern is similar to that in Mobile Node and Home Agent
   relationship.

   Hence, the FBU and FBack messages MUST be protected using a security
   association shared between a mobile node and its access router.  In
   particular, the MN-PAR Authentication Extension MUST be present in
   each of these messages.  This document does not specify how the
   security association is established between an MN and the AR/FA.

   Secure FBU, malicious or inadvertent redirection:

   Even if the MN-PAR authentication extension is present in an FBU, an
   MN may inadvertently or maliciously attempt to bind its PCoA to an
   unintended address on NAR's link, and cause traffic flooding to an
   unsuspecting node.

   This vulnerability is avoided by always binding the PCoA to the NAR's
   IP address, even when the NAR supplies an NCoA to use for the MN.  It
   is still possible to jam NAR's buffer with redirected traffic.
   However, the handover state corresponding to the MN's PCoA has a
   finite lifetime, and can be configured to be a few multiples of the
   anticipated handover latency.  Hence, the extent of this
   vulnerability is small.  It is possible to trace the culprit MN with
   an established security association at the access router.




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   Communication between the access routers:

   The access routers communicate using HI and HAck messages in order to
   establish a temporary routing path for the MN undergoing handover.
   This message exchange needs to be secured to ensure routing updates
   take place as intended.

   The HI and HAck messages need to be secured using a preexisting
   security association between the access routers to ensure at least
   message integrity and authentication, and SHOULD also include
   encryption.  IPsec ESP SHOULD be used.

8.  IANA Considerations

   The IANA assignments made for messages, extensions, and options
   specified in this document are described in the following paragraphs.

   This document defines two new messages that use the Mobile IPv4
   control message format [rfc3344].  These message details are as
   follows:

                   +------+-------------+-------------+
                   | Type | Description |  Reference  |
                   +------+-------------+-------------+
                   |  20  |     FBU     | Section 5.1 |
                   |  21  |    FBAck    | Section 5.2 |
                   +------+-------------+-------------+

   This document defines four new experimental ICMP messages that use
   the ICMP Type 41 for IPv4.  See Section 3 in [rfc4065].  The new
   messages specified in this document have been assigned Subtypes from
   the registry in [rfc4065]:

                  +---------+-------------+-------------+
                  | Subtype | Description |  Reference  |
                  +---------+-------------+-------------+
                  |    6    |   RtSolPr   | Section 5.3 |
                  |    7    |   PrRtAdv   | Section 5.4 |
                  |    8    |      HI     | Section 5.5 |
                  |    9    |     HAck    | Section 5.6 |
                  +---------+-------------+-------------+

   This document defines three new options that have been assigned Types
   from the Mobile IP Extensions for ICMP Router Discovery messages
   [rfc3344].  These options are as follows:






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                 +------+------------------+-------------+
                 | Type |    Description   |  Reference  |
                 +------+------------------+-------------+
                 |  20  |        LLA       | Section 6.1 |
                 |  21  | New IPv4 Address | Section 6.2 |
                 |  22  |  NAR Prefix Info | Section 6.3 |
                 +------+------------------+-------------+

   The MN-PAR Authentication Extension described in Sections 5.1 and 5.2
   is a Generalized Mobile IP Authentication Extension defined in
   Section 5 of [rfc4721].  The MN-PAR Authentication has been assigned
   a Subtype from the registry specified in [rfc4721].  The Extension
   details are as follows:

      +---------+-----------------------+--------------------------+
      | Subtype |      Description      |         Reference        |
      +---------+-----------------------+--------------------------+
      |    4    | MN-PAR Auth Extension |        Section 5.1       |
      +---------+-----------------------+--------------------------+

9.  Acknowledgments

   Thanks to all those who expressed interest in having a Fast Handovers
   for Mobile IPv4 protocol along the lines of [rfc4068].  Thanks to
   Vijay Devarapalli, Kent Leung, and Domagoj Premec for their review
   and input.  Kumar Viswanath and Uday Mohan implemented an early
   version of this protocol.  Many thanks to Alex Petrescu for his
   thorough review that improved this document.  Thanks to Pete McCann
   for the proofreading, and to Jari Arkko for the review, which have
   helped improve this document.  Thanks to Francis Dupont and Hannes
   Tschofenig for the GEN-ART and TSV-DIR reviews.

   Sending FBU from the new link (i.e., reactive mode) is similar to
   using the extension defined in [mip4-ro]; however, this document also
   addresses movement detection and router discovery latencies.

10.  References

10.1.  Normative References

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

   [rfc1256]      Deering, S., Ed., "ICMP Router Discovery Messages",
                  RFC 1256, September 1991.

   [rfc2460]      Deering, S. and R. Hinden, "Internet Protocol, Version
                  6 (IPv6) Specification", RFC 2460, December 1998.



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   [rfc3344]      Perkins, C., Ed., "IP Mobility Support for IPv4", RFC
                  3344, August 2002.

   [rfc4065]      Kempf, J., "Instructions for Seamoby and Experimental
                  Mobility Protocol IANA Allocations", RFC 4065, July
                  2005.

   [rfc4068]      Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC
                  4068, July 2005.

   [rfc4721]      Perkins, C., Calhoun, P., and J. Bharatia, "Mobile
                  IPv4 Challenge/Response Extensions (Revised)", RFC
                  4721, January 2007.

10.2.  Informative References

   [fh-ccr]       R. Koodli and C. E. Perkins, "Fast Handovers and
                  Context Transfers in Mobile Networks", ACM Computer
                  Communications Review Special Issue on Wireless
                  Extensions to the Internet, October 2001.

   [ieee-802.11r] IEEE, "IEEE Standard for Local and Metropolitan Area
                  Networks:  Fast Roaming/Fast BSS Transition, IEEE Std
                  802.11r", September 2006.

   [ieee-802.1x]  IEEE, "IEEE Standards for Local and Metropolitan Area
                  Networks: Port-based Network Access Control, IEEE Std
                  802.1X-2001", June 2001.

   [ieee-802.21]  The IEEE 802.21 group, http://www.ieee802.org/21.

   [mi-book]      R. Koodli and C. E. Perkins, "Mobile Internetworking
                  with IPv6: Concepts, Principles and Practices", John
                  Wiley & Sons, June 2007.

   [mip4-ro]      Perkins, C. and D. Johnson, "Route Optimization in
                  Mobile IP", Work in Progress, September 2001.

   [rfc2131]      Droms, R., "Dynamic Host Configuration Protocol", RFC
                  2131, March 1997.

   [rfc3957]      Perkins, C. and P. Calhoun, "Authentication,
                  Authorization, and Accounting (AAA) Registration Keys
                  for Mobile IPv4", RFC 3957, March 2005.







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Authors' Addresses

   Rajeev Koodli
   Nokia Siemens Networks
   313 Fairchild Driive
   Mountain View, CA 94043
   USA

   EMail: rajeev.koodli@nokia.com


   Charles Perkins
   Nokia Siemens Networks
   313 Fairchild Driive
   Mountain View, CA 94043
   USA

   EMail: charles.perkins@nokia.com

































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
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   on the procedures with respect to rights in RFC documents can be
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












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