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Network Working Group P. Nikander Request for Comments: 5206 Ericsson Research NomadicLab Category: Experimental T. Henderson, Ed.

                                                    The Boeing Company
                                                               C. Vogt
                                                              J. Arkko
                                          Ericsson Research NomadicLab
                                                            April 2008
 End-Host Mobility and Multihoming with the Host Identity Protocol

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.


 This document defines mobility and multihoming extensions to the Host
 Identity Protocol (HIP).  Specifically, this document defines a
 general "LOCATOR" parameter for HIP messages that allows for a HIP
 host to notify peers about alternate addresses at which it may be
 reached.  This document also defines elements of procedure for
 mobility of a HIP host -- the process by which a host dynamically
 changes the primary locator that it uses to receive packets.  While
 the same LOCATOR parameter can also be used to support end-host
 multihoming, detailed procedures are left for further study.

Table of Contents

 1.  Introduction and Scope . . . . . . . . . . . . . . . . . . . .  2
 2.  Terminology and Conventions  . . . . . . . . . . . . . . . . .  4
 3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.1.  Operating Environment  . . . . . . . . . . . . . . . . . .  5
     3.1.1.  Locator  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.2.  Mobility Overview  . . . . . . . . . . . . . . . . . .  8
     3.1.3.  Multihoming Overview . . . . . . . . . . . . . . . . .  8
   3.2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .  9
     3.2.1.  Mobility with a Single SA Pair (No Rekeying) . . . . .  9
     3.2.2.  Mobility with a Single SA Pair (Mobile-Initiated
             Rekey) . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.2.3.  Host Multihoming . . . . . . . . . . . . . . . . . . . 11
     3.2.4.  Site Multihoming . . . . . . . . . . . . . . . . . . . 13
     3.2.5.  Dual host multihoming  . . . . . . . . . . . . . . . . 14
     3.2.6.  Combined Mobility and Multihoming  . . . . . . . . . . 14

Nikander, et al. Experimental [Page 1] RFC 5206 HIP Mobility and Multihoming April 2008

     3.2.7.  Using LOCATORs across Addressing Realms  . . . . . . . 14
     3.2.8.  Network Renumbering  . . . . . . . . . . . . . . . . . 15
     3.2.9.  Initiating the Protocol in R1 or I2  . . . . . . . . . 15
   3.3.  Other Considerations . . . . . . . . . . . . . . . . . . . 16
     3.3.1.  Address Verification . . . . . . . . . . . . . . . . . 16
     3.3.2.  Credit-Based Authorization . . . . . . . . . . . . . . 17
     3.3.3.  Preferred Locator  . . . . . . . . . . . . . . . . . . 18
     3.3.4.  Interaction with Security Associations . . . . . . . . 18
 4.  LOCATOR Parameter Format . . . . . . . . . . . . . . . . . . . 21
   4.1.  Traffic Type and Preferred Locator . . . . . . . . . . . . 23
   4.2.  Locator Type and Locator . . . . . . . . . . . . . . . . . 23
   4.3.  UPDATE Packet with Included LOCATOR  . . . . . . . . . . . 24
 5.  Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 24
   5.1.  Locator Data Structure and Status  . . . . . . . . . . . . 24
   5.2.  Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 25
   5.3.  Handling Received LOCATORs . . . . . . . . . . . . . . . . 28
   5.4.  Verifying Address Reachability . . . . . . . . . . . . . . 30
   5.5.  Changing the Preferred Locator . . . . . . . . . . . . . . 31
   5.6.  Credit-Based Authorization . . . . . . . . . . . . . . . . 32
     5.6.1.  Handling Payload Packets . . . . . . . . . . . . . . . 32
     5.6.2.  Credit Aging . . . . . . . . . . . . . . . . . . . . . 33
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 34
   6.1.  Impersonation Attacks  . . . . . . . . . . . . . . . . . . 35
   6.2.  Denial-of-Service Attacks  . . . . . . . . . . . . . . . . 36
     6.2.1.  Flooding Attacks . . . . . . . . . . . . . . . . . . . 36
     6.2.2.  Memory/Computational-Exhaustion DoS Attacks  . . . . . 36
   6.3.  Mixed Deployment Environment . . . . . . . . . . . . . . . 37
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
 8.  Authors and Acknowledgments  . . . . . . . . . . . . . . . . . 38
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   9.1.  Normative references . . . . . . . . . . . . . . . . . . . 38
   9.2.  Informative references . . . . . . . . . . . . . . . . . . 38

1. Introduction and Scope

 The Host Identity Protocol [RFC4423] (HIP) supports an architecture
 that decouples the transport layer (TCP, UDP, etc.) from the
 internetworking layer (IPv4 and IPv6) by using public/private key
 pairs, instead of IP addresses, as host identities.  When a host uses
 HIP, the overlying protocol sublayers (e.g., transport layer sockets
 and Encapsulating Security Payload (ESP) Security Associations (SAs))
 are instead bound to representations of these host identities, and
 the IP addresses are only used for packet forwarding.  However, each
 host must also know at least one IP address at which its peers are
 reachable.  Initially, these IP addresses are the ones used during
 the HIP base exchange [RFC5201].

Nikander, et al. Experimental [Page 2] RFC 5206 HIP Mobility and Multihoming April 2008

 One consequence of such a decoupling is that new solutions to
 network-layer mobility and host multihoming are possible.  There are
 potentially many variations of mobility and multihoming possible.
 The scope of this document encompasses messaging and elements of
 procedure for basic network-level mobility and simple multihoming,
 leaving more complicated scenarios and other variations for further
 study.  More specifically:
    This document defines a generalized LOCATOR parameter for use in
    HIP messages.  The LOCATOR parameter allows a HIP host to notify a
    peer about alternate addresses at which it is reachable.  The
    LOCATORs may be merely IP addresses, or they may have additional
    multiplexing and demultiplexing context to aid the packet handling
    in the lower layers.  For instance, an IP address may need to be
    paired with an ESP Security Parameter Index (SPI) so that packets
    are sent on the correct SA for a given address.
    This document also specifies the messaging and elements of
    procedure for end-host mobility of a HIP host -- the sequential
    change in the preferred IP address used to reach a host.  In
    particular, message flows to enable successful host mobility,
    including address verification methods, are defined herein.
    However, while the same LOCATOR parameter is intended to support
    host multihoming (parallel support of a number of addresses), and
    experimentation is encouraged, detailed elements of procedure for
    host multihoming are left for further study.
 While HIP can potentially be used with transports other than the ESP
 transport format [RFC5202], this document largely assumes the use of
 ESP and leaves other transport formats for further study.
 There are a number of situations where the simple end-to-end
 readdressing functionality is not sufficient.  These include the
 initial reachability of a mobile host, location privacy, simultaneous
 mobility of both hosts, and some modes of NAT traversal.  In these
 situations, there is a need for some helper functionality in the
 network, such as a HIP rendezvous server [RFC5204].  Such
 functionality is out of the scope of this document.  We also do not
 consider localized mobility management extensions (i.e., mobility
 management techniques that do not involve directly signaling the
 correspondent node); this document is concerned with end-to-end
 mobility.  Finally, making underlying IP mobility transparent to the
 transport layer has implications on the proper response of transport
 congestion control, path MTU selection, and Quality of Service (QoS).
 Transport-layer mobility triggers, and the proper transport response
 to a HIP mobility or multihoming address change, are outside the
 scope of this document.

Nikander, et al. Experimental [Page 3] RFC 5206 HIP Mobility and Multihoming April 2008

2. Terminology and Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [RFC2119].
 LOCATOR.  The name of a HIP parameter containing zero or more Locator
    fields.  This parameter's name is distinguished from the Locator
    fields embedded within it by the use of all capital letters.
 Locator.  A name that controls how the packet is routed through the
    network and demultiplexed by the end host.  It may include a
    concatenation of traditional network addresses such as an IPv6
    address and end-to-end identifiers such as an ESP SPI.  It may
    also include transport port numbers or IPv6 Flow Labels as
    demultiplexing context, or it may simply be a network address.
 Address.  A name that denotes a point-of-attachment to the network.
    The two most common examples are an IPv4 address and an IPv6
    address.  The set of possible addresses is a subset of the set of
    possible locators.
 Preferred locator.  A locator on which a host prefers to receive
    data.  With respect to a given peer, a host always has one active
    Preferred locator, unless there are no active locators.  By
    default, the locators used in the HIP base exchange are the
    Preferred locators.
 Credit Based Authorization.  A host must verify a mobile or
    multihomed peer's reachability at a new locator.  Credit-Based
    Authorization authorizes the peer to receive a certain amount of
    data at the new locator before the result of such verification is

Nikander, et al. Experimental [Page 4] RFC 5206 HIP Mobility and Multihoming April 2008

3. Protocol Model

 This section is an overview; more detailed specification follows this

3.1. Operating Environment

 The Host Identity Protocol (HIP) [RFC5201] is a key establishment and
 parameter negotiation protocol.  Its primary applications are for
 authenticating host messages based on host identities, and
 establishing security associations (SAs) for the ESP transport format
 [RFC5202] and possibly other protocols in the future.
  +--------------------+                       +--------------------+
  |                    |                       |                    |
  |   +------------+   |                       |   +------------+   |
  |   |    Key     |   |         HIP           |   |    Key     |   |
  |   | Management | <-+-----------------------+-> | Management |   |
  |   |  Process   |   |                       |   |  Process   |   |
  |   +------------+   |                       |   +------------+   |
  |         ^          |                       |         ^          |
  |         |          |                       |         |          |
  |         v          |                       |         v          |
  |   +------------+   |                       |   +------------+   |
  |   |   IPsec    |   |        ESP            |   |   IPsec    |   |
  |   |   Stack    | <-+-----------------------+-> |   Stack    |   |
  |   |            |   |                       |   |            |   |
  |   +------------+   |                       |   +------------+   |
  |                    |                       |                    |
  |                    |                       |                    |
  |     Initiator      |                       |     Responder      |
  +--------------------+                       +--------------------+
                    Figure 1: HIP Deployment Model
 The general deployment model for HIP is shown above, assuming
 operation in an end-to-end fashion.  This document specifies
 extensions to the HIP protocol to enable end-host mobility and basic
 multihoming.  In summary, these extensions to the HIP base protocol
 enable the signaling of new addressing information to the peer in HIP
 messages.  The messages are authenticated via a signature or keyed
 hash message authentication code (HMAC) based on its Host Identity.
 This document specifies the format of this new addressing (LOCATOR)
 parameter, the procedures for sending and processing this parameter
 to enable basic host mobility, and procedures for a concurrent
 address verification mechanism.

Nikander, et al. Experimental [Page 5] RFC 5206 HIP Mobility and Multihoming April 2008

  1. ——–

| TCP | (sockets bound to HITs)

  1. ——–


  1. ——–
  2. —> | ESP | {HIT_s, HIT_d} ↔ SPI

| ———

    |         |
  ----    ---------
 | MH |-> | HIP   |  {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI}
  ----    ---------
          |  IP   |
     Figure 2: Architecture for HIP Mobility and Multihoming (MH)
 Figure 2 depicts a layered architectural view of a HIP-enabled stack
 using the ESP transport format.  In HIP, upper-layer protocols
 (including TCP and ESP in this figure) are bound to Host Identity
 Tags (HITs) and not IP addresses.  The HIP sublayer is responsible
 for maintaining the binding between HITs and IP addresses.  The SPI
 is used to associate an incoming packet with the right HITs.  The
 block labeled "MH" is introduced below.
 Consider first the case in which there is no mobility or multihoming,
 as specified in the base protocol specification [RFC5201].  The HIP
 base exchange establishes the HITs in use between the hosts, the SPIs
 to use for ESP, and the IP addresses (used in both the HIP signaling
 packets and ESP data packets).  Note that there can only be one such
 set of bindings in the outbound direction for any given packet, and
 the only fields used for the binding at the HIP layer are the fields
 exposed by ESP (the SPI and HITs).  For the inbound direction, the
 SPI is all that is required to find the right host context.  ESP
 rekeying events change the mapping between the HIT pair and SPI, but
 do not change the IP addresses.
 Consider next a mobility event, in which a host is still single-homed
 but moves to another IP address.  Two things must occur in this case.
 First, the peer must be notified of the address change using a HIP
 UPDATE message.  Second, each host must change its local bindings at
 the HIP sublayer (new IP addresses).  It may be that both the SPIs
 and IP addresses are changed simultaneously in a single UPDATE; the
 protocol described herein supports this.  However, simultaneous
 movement of both hosts, notification of transport layer protocols of
 the path change, and procedures for possibly traversing middleboxes
 are not covered by this document.

Nikander, et al. Experimental [Page 6] RFC 5206 HIP Mobility and Multihoming April 2008

 Finally, consider the case when a host is multihomed (has more than
 one globally routable address) and has multiple addresses available
 at the HIP layer as alternative locators for fault tolerance.
 Examples include the use of (possibly multiple) IPv4 and IPv6
 addresses on the same interface, or the use of multiple interfaces
 attached to different service providers.  Such host multihoming
 generally necessitates that a separate ESP SA is maintained for each
 interface in order to prevent packets that arrive over different
 paths from falling outside of the ESP anti-replay window [RFC4303].
 Multihoming thus makes it possible that the bindings shown on the
 right side of Figure 2 are one to many (in the outbound direction,
 one HIT pair to multiple SPIs, and possibly then to multiple IP
 addresses).  However, only one SPI and address pair can be used for
 any given packet, so the job of the "MH" block depicted above is to
 dynamically manipulate these bindings.  Beyond locally managing such
 multiple bindings, the peer-to-peer HIP signaling protocol needs to
 be flexible enough to define the desired mappings between HITs, SPIs,
 and addresses, and needs to ensure that UPDATE messages are sent
 along the right network paths so that any HIP-aware middleboxes can
 observe the SPIs.  This document does not specify the "MH" block, nor
 does it specify detailed elements of procedure for how to handle
 various multihoming (perhaps combined with mobility) scenarios.  The
 "MH" block may apply to more general problems outside of HIP.
 However, this document does describe a basic multihoming case (one
 host adds one address to its initial address and notifies the peer)
 and leave more complicated scenarios for experimentation and future

3.1.1. Locator

 This document defines a generalization of an address called a
 "locator".  A locator specifies a point-of-attachment to the network
 but may also include additional end-to-end tunneling or per-host
 demultiplexing context that affects how packets are handled below the
 logical HIP sublayer of the stack.  This generalization is useful
 because IP addresses alone may not be sufficient to describe how
 packets should be handled below HIP.  For example, in a host
 multihoming context, certain IP addresses may need to be associated
 with certain ESP SPIs to avoid violating the ESP anti-replay window.
 Addresses may also be affiliated with transport ports in certain
 tunneling scenarios.  Locators may simply be traditional network
 addresses.  The format of the locator fields in the LOCATOR parameter
 is defined in Section 4.

Nikander, et al. Experimental [Page 7] RFC 5206 HIP Mobility and Multihoming April 2008

3.1.2. Mobility Overview

 When a host moves to another address, it notifies its peer of the new
 address by sending a HIP UPDATE packet containing a LOCATOR
 parameter.  This UPDATE packet is acknowledged by the peer.  For
 reliability in the presence of packet loss, the UPDATE packet is
 retransmitted as defined in the HIP protocol specification [RFC5201].
 The peer can authenticate the contents of the UPDATE packet based on
 the signature and keyed hash of the packet.
 When using ESP Transport Format [RFC5202], the host may at the same
 time decide to rekey its security association and possibly generate a
 new Diffie-Hellman key; all of these actions are triggered by
 including additional parameters in the UPDATE packet, as defined in
 the base protocol specification [RFC5201] and ESP extension
 When using ESP (and possibly other transport modes in the future),
 the host is able to receive packets that are protected using a HIP
 created ESP SA from any address.  Thus, a host can change its IP
 address and continue to send packets to its peers without necessarily
 rekeying.  However, the peers are not able to send packets to these
 new addresses before they can reliably and securely update the set of
 addresses that they associate with the sending host.  Furthermore,
 mobility may change the path characteristics in such a manner that
 reordering occurs and packets fall outside the ESP anti-replay window
 for the SA, thereby requiring rekeying.

3.1.3. Multihoming Overview

 A related operational configuration is host multihoming, in which a
 host has multiple locators simultaneously rather than sequentially,
 as in the case of mobility.  By using the LOCATOR parameter defined
 herein, a host can inform its peers of additional (multiple) locators
 at which it can be reached, and can declare a particular locator as a
 "preferred" locator.  Although this document defines a basic
 mechanism for multihoming, it does not define detailed policies and
 procedures, such as which locators to choose when more than one pair
 is available, the operation of simultaneous mobility and multihoming,
 source address selection policies (beyond those specified in
 [RFC3484]), and the implications of multihoming on transport
 protocols and ESP anti-replay windows.  Additional definitions of
 HIP-based multihoming are expected to be part of future documents.

Nikander, et al. Experimental [Page 8] RFC 5206 HIP Mobility and Multihoming April 2008

3.2. Protocol Overview

 In this section, we briefly introduce a number of usage scenarios for
 HIP mobility and multihoming.  These scenarios assume that HIP is
 being used with the ESP transform [RFC5202], although other scenarios
 may be defined in the future.  To understand these usage scenarios,
 the reader should be at least minimally familiar with the HIP
 protocol specification [RFC5201].  However, for the (relatively)
 uninitiated reader, it is most important to keep in mind that in HIP
 the actual payload traffic is protected with ESP, and that the ESP
 SPI acts as an index to the right host-to-host context.  More
 specification details are found later in Section 4 and Section 5.
 The scenarios below assume that the two hosts have completed a single
 HIP base exchange with each other.  Both of the hosts therefore have
 one incoming and one outgoing SA.  Further, each SA uses the same
 pair of IP addresses, which are the ones used in the base exchange.
 The readdressing protocol is an asymmetric protocol where a mobile or
 multihomed host informs a peer host about changes of IP addresses on
 affected SPIs.  The readdressing exchange is designed to be
 piggybacked on existing HIP exchanges.  The majority of the packets
 on which the LOCATOR parameters are expected to be carried are UPDATE
 packets.  However, some implementations may want to experiment with
 sending LOCATOR parameters also on other packets, such as R1, I2, and
 The scenarios below at times describe addresses as being in either an
 ACTIVE, VERIFIED, or DEPRECATED state.  From the perspective of a
 host, newly-learned addresses of the peer must be verified before put
 into active service, and addresses removed by the peer are put into a
 deprecated state.  Under limited conditions described below
 (Section 5.6), an UNVERIFIED address may be used.  The addressing
 states are defined more formally in Section 5.1.
 Hosts that use link-local addresses as source addresses in their HIP
 handshakes may not be reachable by a mobile peer.  Such hosts SHOULD
 provide a globally routable address either in the initial handshake
 or via the LOCATOR parameter.

3.2.1. Mobility with a Single SA Pair (No Rekeying)

 A mobile host must sometimes change an IP address bound to an
 interface.  The change of an IP address might be needed due to a
 change in the advertised IPv6 prefixes on the link, a reconnected PPP
 link, a new DHCP lease, or an actual movement to another subnet.  In
 order to maintain its communication context, the host must inform its
 peers about the new IP address.  This first example considers the

Nikander, et al. Experimental [Page 9] RFC 5206 HIP Mobility and Multihoming April 2008

 case in which the mobile host has only one interface, IP address, a
 single pair of SAs (one inbound, one outbound), and no rekeying
 occurs on the SAs.  We also assume that the new IP addresses are
 within the same address family (IPv4 or IPv6) as the first address.
 This is the simplest scenario, depicted in Figure 3.
   Mobile Host                         Peer Host
     Figure 3: Readdress without Rekeying, but with Address Check
 The steps of the packet processing are as follows:
 1.  The mobile host is disconnected from the peer host for a brief
     period of time while it switches from one IP address to another.
     Upon obtaining a new IP address, the mobile host sends a LOCATOR
     parameter to the peer host in an UPDATE message.  The UPDATE
     message also contains an ESP_INFO parameter containing the values
     of the old and new SPIs for a security association.  In this
     case, the OLD SPI and NEW SPI parameters both are set to the
     value of the preexisting incoming SPI; this ESP_INFO does not
     trigger a rekeying event but is instead included for possible
     parameter-inspecting middleboxes on the path.  The LOCATOR
     parameter contains the new IP address (Locator Type of "1",
     defined below) and a locator lifetime.  The mobile host waits for
     this UPDATE to be acknowledged, and retransmits if necessary, as
     specified in the base specification [RFC5201].
 2.  The peer host receives the UPDATE, validates it, and updates any
     local bindings between the HIP association and the mobile host's
     destination address.  The peer host MUST perform an address
     verification by placing a nonce in the ECHO_REQUEST parameter of
     the UPDATE message sent back to the mobile host.  It also
     includes an ESP_INFO parameter with the OLD SPI and NEW SPI
     parameters both set to the value of the preexisting incoming SPI,
     and sends this UPDATE (with piggybacked acknowledgment) to the
     mobile host at its new address.  The peer MAY use the new address
     immediately, but it MUST limit the amount of data it sends to the
     address until address verification completes.

Nikander, et al. Experimental [Page 10] RFC 5206 HIP Mobility and Multihoming April 2008

 3.  The mobile host completes the readdress by processing the UPDATE
     ACK and echoing the nonce in an ECHO_RESPONSE.  Once the peer
     host receives this ECHO_RESPONSE, it considers the new address to
     be verified and can put the address into full use.
 While the peer host is verifying the new address, the new address is
 marked as UNVERIFIED in the interim, and the old address is
 DEPRECATED.  Once the peer host has received a correct reply to its
 UPDATE challenge, it marks the new address as ACTIVE and removes the
 old address.

3.2.2. Mobility with a Single SA Pair (Mobile-Initiated Rekey)

 The mobile host may decide to rekey the SAs at the same time that it
 notifies the peer of the new address.  In this case, the above
 procedure described in Figure 3 is slightly modified.  The UPDATE
 message sent from the mobile host includes an ESP_INFO with the OLD
 SPI set to the previous SPI, the NEW SPI set to the desired new SPI
 value for the incoming SA, and the KEYMAT Index desired.  Optionally,
 the host may include a DIFFIE_HELLMAN parameter for a new Diffie-
 Hellman key.  The peer completes the request for a rekey as is
 normally done for HIP rekeying, except that the new address is kept
 as UNVERIFIED until the UPDATE nonce challenge is received as
 described above.  Figure 4 illustrates this scenario.
   Mobile Host                         Peer Host
            Figure 4: Readdress with Mobile-Initiated Rekey

3.2.3. Host Multihoming

 A (mobile or stationary) host may sometimes have more than one
 interface or global address.  The host may notify the peer host of
 the additional interface or address by using the LOCATOR parameter.
 To avoid problems with the ESP anti-replay window, a host SHOULD use
 a different SA for each interface or address used to receive packets
 from the peer host when multiple locator pairs are being used
 simultaneously rather than sequentially.

Nikander, et al. Experimental [Page 11] RFC 5206 HIP Mobility and Multihoming April 2008

 When more than one locator is provided to the peer host, the host
 SHOULD indicate which locator is preferred (the locator on which the
 host prefers to receive traffic).  By default, the addresses used in
 the base exchange are preferred until indicated otherwise.
 In the multihoming case, the sender may also have multiple valid
 locators from which to source traffic.  In practice, a HIP
 association in a multihoming configuration may have both a preferred
 peer locator and a preferred local locator, although rules for source
 address selection should ultimately govern the selection of the
 source locator based on the destination locator.
 Although the protocol may allow for configurations in which there is
 an asymmetric number of SAs between the hosts (e.g., one host has two
 interfaces and two inbound SAs, while the peer has one interface and
 one inbound SA), it is RECOMMENDED that inbound and outbound SAs be
 created pairwise between hosts.  When an ESP_INFO arrives to rekey a
 particular outbound SA, the corresponding inbound SA should be also
 rekeyed at that time.  Although asymmetric SA configurations might be
 experimented with, their usage may constrain interoperability at this
 time.  However, it is recommended that implementations attempt to
 support peers that prefer to use non-paired SAs.  It is expected that
 this section and behavior will be modified in future revisions of
 this protocol, once the issue and its implications are better
 Consider the case between two hosts, one single-homed and one
 multihomed.  The multihomed host may decide to inform the single-
 homed host about its other address.  It is RECOMMENDED that the
 multihomed host set up a new SA pair for use on this new address.  To
 do this, the multihomed host sends a LOCATOR with an ESP_INFO,
 indicating the request for a new SA by setting the OLD SPI value to
 zero, and the NEW SPI value to the newly created incoming SPI.  A
 Locator Type of "1" is used to associate the new address with the new
 SPI.  The LOCATOR parameter also contains a second Type "1" locator,
 that of the original address and SPI.  To simplify parameter
 processing and avoid explicit protocol extensions to remove locators,
 each LOCATOR parameter MUST list all locators in use on a connection
 (a complete listing of inbound locators and SPIs for the host).  The
 multihomed host waits for an ESP_INFO (new outbound SA) from the peer
 and an ACK of its own UPDATE.  As in the mobility case, the peer host
 must perform an address verification before actively using the new
 address.  Figure 5 illustrates this scenario.

Nikander, et al. Experimental [Page 12] RFC 5206 HIP Mobility and Multihoming April 2008

   Multi-homed Host                    Peer Host
                 Figure 5: Basic Multihoming Scenario
 In multihoming scenarios, it is important that hosts receiving
 UPDATEs associate them correctly with the destination address used in
 the packet carrying the UPDATE.  When processing inbound LOCATORs
 that establish new security associations on an interface with
 multiple addresses, a host uses the destination address of the UPDATE
 containing the LOCATOR as the local address to which the LOCATOR plus
 ESP_INFO is targeted.  This is because hosts may send UPDATEs with
 the same (locator) IP address to different peer addresses -- this has
 the effect of creating multiple inbound SAs implicitly affiliated
 with different peer source addresses.

3.2.4. Site Multihoming

 A host may have an interface that has multiple globally routable IP
 addresses.  Such a situation may be a result of the site having
 multiple upper Internet Service Providers, or just because the site
 provides all hosts with both IPv4 and IPv6 addresses.  The host
 should stay reachable at all or any subset of the currently available
 global routable addresses, independent of how they are provided.
 This case is handled the same as if there were different IP
 addresses, described above in Section 3.2.3.  Note that a single
 interface may experience site multihoming while the host itself may
 have multiple interfaces.
 Note that a host may be multihomed and mobile simultaneously, and
 that a multihomed host may want to protect the location of some of
 its interfaces while revealing the real IP address of some others.
 This document does not presently specify additional site multihoming
 extensions to HIP; further alignment with the IETF shim6 working
 group may be considered in the future.

Nikander, et al. Experimental [Page 13] RFC 5206 HIP Mobility and Multihoming April 2008

3.2.5. Dual host multihoming

 Consider the case in which both hosts would like to add an additional
 address after the base exchange completes.  In Figure 6, consider
 that host1, which used address addr1a in the base exchange to set up
 SPI1a and SPI2a, wants to add address addr1b.  It would send an
 UPDATE with LOCATOR (containing the address addr1b) to host2, using
 destination address addr2a, and a new set of SPIs would be added
 between hosts 1 and 2 (call them SPI1b and SPI2b -- not shown in the
 figure).  Next, consider host2 deciding to add addr2b to the
 relationship.  Host2 must select one of host1's addresses towards
 which to initiate an UPDATE.  It may choose to initiate an UPDATE to
 addr1a, addr1b, or both.  If it chooses to send to both, then a full
 mesh (four SA pairs) of SAs would exist between the two hosts.  This
 is the most general case; it often may be the case that hosts
 primarily establish new SAs only with the peer's Preferred locator.
 The readdressing protocol is flexible enough to accommodate this
  1. ← SPI1a – – SPI2a →-

host1 < > addr1a ←–> addr2a < > host2

  1. >- SPI2a – – SPI1a -←
                           addr1b <---> addr2a  (second SA pair)
                           addr1a <---> addr2b  (third SA pair)
                           addr1b <---> addr2b  (fourth SA pair)
  Figure 6: Dual Multihoming Case in Which Each Host Uses LOCATOR to
                         Add a Second Address

3.2.6. Combined Mobility and Multihoming

 It looks likely that in the future, many mobile hosts will be
 simultaneously mobile and multihomed, i.e., have multiple mobile
 interfaces.  Furthermore, if the interfaces use different access
 technologies, it is fairly likely that one of the interfaces may
 appear stable (retain its current IP address) while some other(s) may
 experience mobility (undergo IP address change).
 The use of LOCATOR plus ESP_INFO should be flexible enough to handle
 most such scenarios, although more complicated scenarios have not
 been studied so far.

3.2.7. Using LOCATORs across Addressing Realms

 It is possible for HIP associations to migrate to a state in which
 both parties are only using locators in different addressing realms.
 For example, the two hosts may initiate the HIP association when both

Nikander, et al. Experimental [Page 14] RFC 5206 HIP Mobility and Multihoming April 2008

 are using IPv6 locators, then one host may loose its IPv6
 connectivity and obtain an IPv4 address.  In such a case, some type
 of mechanism for interworking between the different realms must be
 employed; such techniques are outside the scope of the present text.
 The basic problem in this example is that the host readdressing to
 IPv4 does not know a corresponding IPv4 address of the peer.  This
 may be handled (experimentally) by possibly configuring this address
 information manually or in the DNS, or the hosts exchange both IPv4
 and IPv6 addresses in the locator.

3.2.8. Network Renumbering

 It is expected that IPv6 networks will be renumbered much more often
 than most IPv4 networks.  From an end-host point of view, network
 renumbering is similar to mobility.

3.2.9. Initiating the Protocol in R1 or I2

 A Responder host MAY include a LOCATOR parameter in the R1 packet
 that it sends to the Initiator.  This parameter MUST be protected by
 the R1 signature.  If the R1 packet contains LOCATOR parameters with
 a new Preferred locator, the Initiator SHOULD directly set the new
 Preferred locator to status ACTIVE without performing address
 verification first, and MUST send the I2 packet to the new Preferred
 locator.  The I1 destination address and the new Preferred locator
 may be identical.  All new non-preferred locators must still undergo
 address verification once the base exchange completes.
          Initiator                                Responder
                            R1 with LOCATOR
 record additional addresses
 change responder address
                   I2 sent to newly indicated preferred address
                                                   (process normally)
 (process normally, later verification of non-preferred locators)
                   Figure 7: LOCATOR Inclusion in R1
 An Initiator MAY include one or more LOCATOR parameters in the I2
 packet, independent of whether or not there was a LOCATOR parameter
 in the R1.  These parameters MUST be protected by the I2 signature.
 Even if the I2 packet contains LOCATOR parameters, the Responder MUST
 still send the R2 packet to the source address of the I2.  The new

Nikander, et al. Experimental [Page 15] RFC 5206 HIP Mobility and Multihoming April 2008

 Preferred locator SHOULD be identical to the I2 source address.  If
 the I2 packet contains LOCATOR parameters, all new locators must
 undergo address verification as usual, and the ESP traffic that
 subsequently follows should use the Preferred locator.
          Initiator                                Responder
                           I2 with LOCATOR
                                                   (process normally)
                                           record additional addresses
                     R2 sent to source address of I2
 (process normally)
                   Figure 8: LOCATOR Inclusion in I2
 The I1 and I2 may be arriving from different source addresses if the
 LOCATOR parameter is present in R1.  In this case, implementations
 simultaneously using multiple pre-created R1s, indexed by Initiator
 IP addresses, may inadvertently fail the puzzle solution of I2
 packets due to a perceived puzzle mismatch.  See, for instance, the
 example in Appendix A of [RFC5201].  As a solution, the Responder's
 puzzle indexing mechanism must be flexible enough to accommodate the
 situation when R1 includes a LOCATOR parameter.

3.3. Other Considerations

3.3.1. Address Verification

 When a HIP host receives a set of locators from another HIP host in a
 LOCATOR, it does not necessarily know whether the other host is
 actually reachable at the claimed addresses.  In fact, a malicious
 peer host may be intentionally giving bogus addresses in order to
 cause a packet flood towards the target addresses [RFC4225].
 Likewise, viral software may have compromised the peer host,
 programming it to redirect packets to the target addresses.  Thus,
 the HIP host must first check that the peer is reachable at the new
 An additional potential benefit of performing address verification is
 to allow middleboxes in the network along the new path to obtain the
 peer host's inbound SPI.
 Address verification is implemented by the challenger sending some
 piece of unguessable information to the new address, and waiting for
 some acknowledgment from the Responder that indicates reception of
 the information at the new address.  This may include the exchange of

Nikander, et al. Experimental [Page 16] RFC 5206 HIP Mobility and Multihoming April 2008

 a nonce, or the generation of a new SPI and observation of data
 arriving on the new SPI.

3.3.2. Credit-Based Authorization

 Credit-Based Authorization (CBA) allows a host to securely use a new
 locator even though the peer's reachability at the address embedded
 in the locator has not yet been verified.  This is accomplished based
 on the following three hypotheses:
 1.  A flooding attacker typically seeks to somehow multiply the
     packets it generates for the purpose of its attack because
     bandwidth is an ample resource for many victims.
 2.  An attacker can often cause unamplified flooding by sending
     packets to its victim, either by directly addressing the victim
     in the packets, or by guiding the packets along a specific path
     by means of an IPv6 Routing header, if Routing headers are not
     filtered by firewalls.
 3.  Consequently, the additional effort required to set up a
     redirection-based flooding attack (without CBA and return
     routability checks) would pay off for the attacker only if
     amplification could be obtained this way.
 On this basis, rather than eliminating malicious packet redirection
 in the first place, Credit-Based Authorization prevents
 amplifications.  This is accomplished by limiting the data a host can
 send to an unverified address of a peer by the data recently received
 from that peer.  Redirection-based flooding attacks thus become less
 attractive than, for example, pure direct flooding, where the
 attacker itself sends bogus packets to the victim.
 Figure 9 illustrates Credit-Based Authorization: Host B measures the
 amount of data recently received from peer A and, when A readdresses,
 sends packets to A's new, unverified address as long as the sum of
 the packet sizes does not exceed the measured, received data volume.
 When insufficient credit is left, B stops sending further packets to
 A until A's address becomes ACTIVE.  The address changes may be due
 to mobility, multihoming, or any other reason.  Not shown in Figure 9
 are the results of credit aging (Section 5.6.2), a mechanism used to
 dampen possible time-shifting attacks.

Nikander, et al. Experimental [Page 17] RFC 5206 HIP Mobility and Multihoming April 2008

         +-------+                        +-------+
         |   A   |                        |   B   |
         +-------+                        +-------+
             |                                |
     address |------------------------------->| credit += size(packet)
      ACTIVE |                                |
             |------------------------------->| credit += size(packet)
             |<-------------------------------| do not change credit
             |                                |
             + address change                 |
             + address verification starts    |
     address |<-------------------------------| credit -= size(packet)
  UNVERIFIED |------------------------------->| credit += size(packet)
             |<-------------------------------| credit -= size(packet)
             |                                |
             |<-------------------------------| credit -= size(packet)
             |                                X credit < size(packet)
             |                                | => do not send packet!
             + address verification concludes |
     address |                                |
      ACTIVE |<-------------------------------| do not change credit
             |                                |
                    Figure 9: Readdressing Scenario

3.3.3. Preferred Locator

 When a host has multiple locators, the peer host must decide which to
 use for outbound packets.  It may be that a host would prefer to
 receive data on a particular inbound interface.  HIP allows a
 particular locator to be designated as a Preferred locator and
 communicated to the peer (see Section 4).
 In general, when multiple locators are used for a session, there is
 the question of using multiple locators for failover only or for
 load-balancing.  Due to the implications of load-balancing on the
 transport layer that still need to be worked out, this document
 assumes that multiple locators are used primarily for failover.  An
 implementation may use ICMP interactions, reachability checks, or
 other means to detect the failure of a locator.

3.3.4. Interaction with Security Associations

 This document specifies a new HIP protocol parameter, the LOCATOR
 parameter (see Section 4), that allows the hosts to exchange
 information about their locator(s) and any changes in their
 locator(s).  The logical structure created with LOCATOR parameters

Nikander, et al. Experimental [Page 18] RFC 5206 HIP Mobility and Multihoming April 2008

 has three levels: hosts, Security Associations (SAs) indexed by
 Security Parameter Indices (SPIs), and addresses.
 The relation between these levels for an association constructed as
 defined in the base specification [RFC5201] and ESP transform
 [RFC5202] is illustrated in Figure 10.
  1. ← SPI1a – – SPI2a →-

host1 < > addr1a ←–> addr2a < > host2

  1. >- SPI2a – – SPI1a -←
               Figure 10: Relation between Hosts, SPIs,
                  and Addresses (Base Specification)
 In Figure 10, host1 and host2 negotiate two unidirectional SAs, and
 each host selects the SPI value for its inbound SA.  The addresses
 addr1a and addr2a are the source addresses that the hosts use in the
 base HIP exchange.  These are the "preferred" (and only) addresses
 conveyed to the peer for use on each SA.  That is, although packets
 sent to any of the hosts' interfaces may be accepted on the inbound
 SA, the peer host in general knows of only the single destination
 address learned in the base exchange (e.g., for host1, it sends a
 packet on SPI2a to addr2a to reach host2), unless other mechanisms
 exist to learn of new addresses.
 In general, the bindings that exist in an implementation
 corresponding to this document can be depicted as shown in Figure 11.
 In this figure, a host can have multiple inbound SPIs (and, not
 shown, multiple outbound SPIs) associated with another host.
 Furthermore, each SPI may have multiple addresses associated with it.
 These addresses that are bound to an SPI are not used to lookup the
 incoming SA.  Rather, the addresses are those that are provided to
 the peer host, as hints for which addresses to use to reach the host
 on that SPI.  The LOCATOR parameter is used to change the set of
 addresses that a peer associates with a particular SPI.

Nikander, et al. Experimental [Page 19] RFC 5206 HIP Mobility and Multihoming April 2008

                 SPI1   - address12
              /           address21
         host -- SPI2   <
              \           address22
                 SPI3   - address31
 Figure 11: Relation between Hosts, SPIs, and Addresses (General Case)
 A host may establish any number of security associations (or SPIs)
 with a peer.  The main purpose of having multiple SPIs with a peer is
 to group the addresses into collections that are likely to experience
 fate sharing.  For example, if the host needs to change its addresses
 on SPI2, it is likely that both address21 and address22 will
 simultaneously become obsolete.  In a typical case, such SPIs may
 correspond with physical interfaces; see below.  Note, however, that
 especially in the case of site multihoming, one of the addresses may
 become unreachable while the other one still works.  In the typical
 case, however, this does not require the host to inform its peers
 about the situation, since even the non-working address still
 logically exists.
 A basic property of HIP SAs is that the inbound IP address is not
 used to lookup the incoming SA.  Therefore, in Figure 11, it may seem
 unnecessary for address31, for example, to be associated only with
 SPI3 -- in practice, a packet may arrive to SPI1 via destination
 address address31 as well.  However, the use of different source and
 destination addresses typically leads to different paths, with
 different latencies in the network, and if packets were to arrive via
 an arbitrary destination IP address (or path) for a given SPI, the
 reordering due to different latencies may cause some packets to fall
 outside of the ESP anti-replay window.  For this reason, HIP provides
 a mechanism to affiliate destination addresses with inbound SPIs,
 when there is a concern that anti-replay windows might be violated.
 In this sense, we can say that a given inbound SPI has an "affinity"
 for certain inbound IP addresses, and this affinity is communicated
 to the peer host.  Each physical interface SHOULD have a separate SA,
 unless the ESP anti-replay window is loose.
 Moreover, even when the destination addresses used for a particular
 SPI are held constant, the use of different source interfaces may
 also cause packets to fall outside of the ESP anti-replay window,
 since the path traversed is often affected by the source address or

Nikander, et al. Experimental [Page 20] RFC 5206 HIP Mobility and Multihoming April 2008

 interface used.  A host has no way to influence the source interface
 on which a peer sends its packets on a given SPI.  A host SHOULD
 consistently use the same source interface and address when sending
 to a particular destination IP address and SPI.  For this reason, a
 host may find it useful to change its SPI or at least reset its ESP
 anti-replay window when the peer host readdresses.
 An address may appear on more than one SPI.  This creates no
 ambiguity since the receiver will ignore the IP addresses during SA
 lookup anyway.  However, this document does not specify such cases.
 When the LOCATOR parameter is sent in an UPDATE packet, then the
 receiver will respond with an UPDATE acknowledgment.  When the
 LOCATOR parameter is sent in an R1 or I2 packet, the base exchange
 retransmission mechanism will confirm its successful delivery.
 LOCATORs may experimentally be used in NOTIFY packets; in this case,
 the recipient MUST consider the LOCATOR as informational and not
 immediately change the current preferred address, but can test the
 additional locators when the need arises.  The use of the LOCATOR in
 a NOTIFY message may not be compatible with middleboxes.

4. LOCATOR Parameter Format

 The LOCATOR parameter is a critical parameter as defined by
 [RFC5201].  It consists of the standard HIP parameter Type and Length
 fields, plus zero or more Locator sub-parameters.  Each Locator sub-
 parameter contains a Traffic Type, Locator Type, Locator Length,
 Preferred locator bit, Locator Lifetime, and a Locator encoding.  A
 LOCATOR containing zero Locator fields is permitted but has the
 effect of deprecating all addresses.

Nikander, et al. Experimental [Page 21] RFC 5206 HIP Mobility and Multihoming April 2008

      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             |
     | Traffic Type   | Locator Type | Locator Length | Reserved   |P|
     |                       Locator Lifetime                        |
     |                            Locator                            |
     |                                                               |
     |                                                               |
     |                                                               |
     .                                                               .
     .                                                               .
     | Traffic Type   | Locator Type | Locator Length | Reserved   |P|
     |                       Locator Lifetime                        |
     |                            Locator                            |
     |                                                               |
     |                                                               |
     |                                                               |
                  Figure 12: LOCATOR Parameter Format
 Type:  193
 Length:  Length in octets, excluding Type and Length fields, and
    excluding padding.
 Traffic Type:  Defines whether the locator pertains to HIP signaling,
    user data, or both.
 Locator Type:  Defines the semantics of the Locator field.
 Locator Length:  Defines the length of the Locator field, in units of
    4-byte words (Locators up to a maximum of 4*255 octets are
 Reserved:  Zero when sent, ignored when received.

Nikander, et al. Experimental [Page 22] RFC 5206 HIP Mobility and Multihoming April 2008

 P: Preferred locator.  Set to one if the locator is preferred for
    that Traffic Type; otherwise, set to zero.
 Locator Lifetime:  Locator lifetime, in seconds.
 Locator:  The locator whose semantics and encoding are indicated by
    the Locator Type field.  All Locator sub-fields are integral
    multiples of four octets in length.
 The Locator Lifetime indicates how long the following locator is
 expected to be valid.  The lifetime is expressed in seconds.  Each
 locator MUST have a non-zero lifetime.  The address is expected to
 become deprecated when the specified number of seconds has passed
 since the reception of the message.  A deprecated address SHOULD NOT
 be used as a destination address if an alternate (non-deprecated) is
 available and has sufficient scope.

4.1. Traffic Type and Preferred Locator

 The following Traffic Type values are defined:
 0:  Both signaling (HIP control packets) and user data.
 1:  Signaling packets only.
 2:  Data packets only.
 The "P" bit, when set, has scope over the corresponding Traffic Type.
 That is, when a "P" bit is set for Traffic Type "2", for example, it
 means that the locator is preferred for data packets.  If there is a
 conflict (for example, if the "P" bit is set for an address of Type
 "0" and a different address of Type "2"), the more specific Traffic
 Type rule applies (in this case, "2").  By default, the IP addresses
 used in the base exchange are Preferred locators for both signaling
 and user data, unless a new Preferred locator supersedes them.  If no
 locators are indicated as preferred for a given Traffic Type, the
 implementation may use an arbitrary locator from the set of active

4.2. Locator Type and Locator

 The following Locator Type values are defined, along with the
 associated semantics of the Locator field:

Nikander, et al. Experimental [Page 23] RFC 5206 HIP Mobility and Multihoming April 2008

 0:  An IPv6 address or an IPv4-in-IPv6 format IPv4 address [RFC4291]
     (128 bits long).  This locator type is defined primarily for non-
     ESP-based usage.
 1:  The concatenation of an ESP SPI (first 32 bits) followed by an
     IPv6 address or an IPv4-in-IPv6 format IPv4 address (an
     additional 128 bits).  This IP address is defined primarily for
     ESP-based usage.

4.3. UPDATE Packet with Included LOCATOR

 A number of combinations of parameters in an UPDATE packet are
 possible (e.g., see Section 3.2).  In this document, procedures are
 defined only for the case in which one LOCATOR and one ESP_INFO
 parameter is used in any HIP packet.  Furthermore, the LOCATOR SHOULD
 list all of the locators that are active on the HIP association
 (including those on SAs not covered by the ESP_INFO parameter).  Any
 UPDATE packet that includes a LOCATOR parameter SHOULD include both
 an HMAC and a HIP_SIGNATURE parameter.  The relationship between the
 announced Locators and any ESP_INFO parameters present in the packet
 is defined in Section 5.2.  The sending of multiple LOCATOR and/or
 ESP_INFO parameters is for further study; receivers may wish to
 experiment with supporting such a possibility.

5. Processing Rules

 This section describes rules for sending and receiving the LOCATOR
 parameter, testing address reachability, and using Credit-Based
 Authorization (CBA) on UNVERIFIED locators.

5.1. Locator Data Structure and Status

 In a typical implementation, each outgoing locator is represented by
 a piece of state that contains the following data:
 o  the actual bit pattern representing the locator,
 o  the lifetime (seconds),
 o  the Traffic Type scope of the locator, and
 o  whether the locator is preferred for any particular scope.

Nikander, et al. Experimental [Page 24] RFC 5206 HIP Mobility and Multihoming April 2008

 The status is used to track the reachability of the address embedded
 within the LOCATOR parameter:
 UNVERIFIED  indicates that the reachability of the address has not
    been verified yet,
 ACTIVE  indicates that the reachability of the address has been
    verified and the address has not been deprecated,
 DEPRECATED  indicates that the locator lifetime has expired.
 The following state changes are allowed:
 UNVERIFIED to ACTIVE  The reachability procedure completes
 UNVERIFIED to DEPRECATED  The locator lifetime expires while the
    locator is UNVERIFIED.
 ACTIVE to DEPRECATED  The locator lifetime expires while the locator
    is ACTIVE.
 ACTIVE to UNVERIFIED  There has been no traffic on the address for
    some time, and the local policy mandates that the address
    reachability must be verified again before starting to use it
 DEPRECATED to UNVERIFIED  The host receives a new lifetime for the
 A DEPRECATED address MUST NOT be changed to ACTIVE without first
 verifying its reachability.
 Note that the state of whether or not a locator is preferred is not
 necessarily the same as the value of the Preferred bit in the Locator
 sub-parameter received from the peer.  Peers may recommend certain
 locators to be preferred, but the decision on whether to actually use
 a locator as a preferred locator is a local decision, possibly
 influenced by local policy.

5.2. Sending LOCATORs

 The decision of when to send LOCATORs is basically a local policy
 issue.  However, it is RECOMMENDED that a host send a LOCATOR
 whenever it recognizes a change of its IP addresses in use on an
 active HIP association, and assumes that the change is going to last
 at least for a few seconds.  Rapidly sending LOCATORs that force the
 peer to change the preferred address SHOULD be avoided.

Nikander, et al. Experimental [Page 25] RFC 5206 HIP Mobility and Multihoming April 2008

 When a host decides to inform its peers about changes in its IP
 addresses, it has to decide how to group the various addresses with
 SPIs.  The grouping should consider also whether middlebox
 interaction requires sending the same LOCATOR in separate UPDATEs on
 different paths.  Since each SPI is associated with a different
 Security Association, the grouping policy may also be based on ESP
 anti-replay protection considerations.  In the typical case, simply
 basing the grouping on actual kernel level physical and logical
 interfaces may be the best policy.  Grouping policy is outside of the
 scope of this document.
 Note that the purpose of announcing IP addresses in a LOCATOR is to
 provide connectivity between the communicating hosts.  In most cases,
 tunnels or virtual interfaces such as IPsec tunnel interfaces or
 Mobile IP home addresses provide sub-optimal connectivity.
 Furthermore, it should be possible to replace most tunnels with HIP
 based "non-tunneling", therefore making most virtual interfaces
 fairly unnecessary in the future.  Therefore, virtual interfaces
 SHOULD NOT be announced in general.  On the other hand, there are
 clearly situations where tunnels are used for diagnostic and/or
 testing purposes.  In such and other similar cases announcing the IP
 addresses of virtual interfaces may be appropriate.
 Hosts MUST NOT announce broadcast or multicast addresses in LOCATORs.
 Link-local addresses MAY be announced to peers that are known to be
 neighbors on the same link, such as when the IP destination address
 of a peer is also link-local.  The announcement of link-local
 addresses in this case is a policy decision; link-local addresses
 used as Preferred locators will create reachability problems when the
 host moves to another link.  In any case, link-local addresses MUST
 NOT be announced to a peer unless that peer is known to be on the
 same link.
 Once the host has decided on the groups and assignment of addresses
 to the SPIs, it creates a LOCATOR parameter that serves as a complete
 representation of the addresses and affiliated SPIs intended for
 active use.  We now describe a few cases introduced in Section 3.2.
 We assume that the Traffic Type for each locator is set to "0" (other
 values for Traffic Type may be specified in documents that separate
 the HIP control plane from data plane traffic).  Other mobility and
 multihoming cases are possible but are left for further
 1.  Host mobility with no multihoming and no rekeying.  The mobile
     host creates a single UPDATE containing a single ESP_INFO with a
     single LOCATOR parameter.  The ESP_INFO contains the current
     value of the SPI in both the OLD SPI and NEW SPI fields.  The
     LOCATOR contains a single Locator with a "Locator Type" of "1";

Nikander, et al. Experimental [Page 26] RFC 5206 HIP Mobility and Multihoming April 2008

     the SPI must match that of the ESP_INFO.  The Preferred bit
     SHOULD be set and the "Locator Lifetime" is set according to
     local policy.  The UPDATE also contains a SEQ parameter as usual.
     This packet is retransmitted as defined in the HIP protocol
     specification [RFC5201].  The UPDATE should be sent to the peer's
     preferred IP address with an IP source address corresponding to
     the address in the LOCATOR parameter.
 2.  Host mobility with no multihoming but with rekeying.  The mobile
     host creates a single UPDATE containing a single ESP_INFO with a
     single LOCATOR parameter (with a single address).  The ESP_INFO
     contains the current value of the SPI in the OLD SPI and the new
     value of the SPI in the NEW SPI, and a KEYMAT Index as selected
     by local policy.  Optionally, the host may choose to initiate a
     Diffie Hellman rekey by including a DIFFIE_HELLMAN parameter.
     The LOCATOR contains a single Locator with "Locator Type" of "1";
     the SPI must match that of the NEW SPI in the ESP_INFO.
     Otherwise, the steps are identical to the case in which no
     rekeying is initiated.
 3.  Host multihoming (addition of an address).  We only describe the
     simple case of adding an additional address to a (previously)
     single-homed, non-mobile host.  The host SHOULD set up a new SA
     pair between this new address and the preferred address of the
     peer host.  To do this, the multihomed host creates a new inbound
     SA and creates a new SPI.  For the outgoing UPDATE message, it
     inserts an ESP_INFO parameter with an OLD SPI field of "0", a NEW
     SPI field corresponding to the new SPI, and a KEYMAT Index as
     selected by local policy.  The host adds to the UPDATE message a
     LOCATOR with two Type "1" Locators: the original address and SPI
     active on the association, and the new address and new SPI being
     added (with the SPI matching the NEW SPI contained in the
     ESP_INFO).  The Preferred bit SHOULD be set depending on the
     policy to tell the peer host which of the two locators is
     preferred.  The UPDATE also contains a SEQ parameter and
     optionally a DIFFIE_HELLMAN parameter, and follows rekeying
     procedures with respect to this new address.  The UPDATE message
     SHOULD be sent to the peer's Preferred address with a source
     address corresponding to the new locator.
 The sending of multiple LOCATORs, locators with Locator Type "0", and
 multiple ESP_INFO parameters is for further study.  Note that the
 inclusion of LOCATOR in an R1 packet requires the use of Type "0"
 locators since no SAs are set up at that point.

Nikander, et al. Experimental [Page 27] RFC 5206 HIP Mobility and Multihoming April 2008

5.3. Handling Received LOCATORs

 A host SHOULD be prepared to receive a LOCATOR parameter in the
 following HIP packets: R1, I2, UPDATE, and NOTIFY.
 This document describes sending both ESP_INFO and LOCATOR parameters
 in an UPDATE.  The ESP_INFO parameter is included when there is a
 need to rekey or key a new SPI, and is otherwise included for the
 possible benefit of HIP-aware middleboxes.  The LOCATOR parameter
 contains a complete map of the locators that the host wishes to make
 or keep active for the HIP association.
 In general, the processing of a LOCATOR depends upon the packet type
 in which it is included.  Here, we describe only the case in which
 ESP_INFO is present and a single LOCATOR and ESP_INFO are sent in an
 UPDATE message; other cases are for further study.  The steps below
 cover each of the cases described in Section 5.2.
 The processing of ESP_INFO and LOCATOR parameters is intended to be
 modular and support future generalization to the inclusion of
 multiple ESP_INFO and/or multiple LOCATOR parameters.  A host SHOULD
 first process the ESP_INFO before the LOCATOR, since the ESP_INFO may
 contain a new SPI value mapped to an existing SPI, while a Type "1"
 locator will only contain a reference to the new SPI.
 When a host receives a validated HIP UPDATE with a LOCATOR and
 ESP_INFO parameter, it processes the ESP_INFO as follows.  The
 ESP_INFO parameter indicates whether an SA is being rekeyed, created,
 deprecated, or just identified for the benefit of middleboxes.  The
 host examines the OLD SPI and NEW SPI values in the ESP_INFO
 1.  (no rekeying) If the OLD SPI is equal to the NEW SPI and both
     correspond to an existing SPI, the ESP_INFO is gratuitous
     (provided for middleboxes) and no rekeying is necessary.
 2.  (rekeying) If the OLD SPI indicates an existing SPI and the NEW
     SPI is a different non-zero value, the existing SA is being
     rekeyed and the host follows HIP ESP rekeying procedures by
     creating a new outbound SA with an SPI corresponding to the NEW
     SPI, with no addresses bound to this SPI.  Note that locators in
     the LOCATOR parameter will reference this new SPI instead of the
     old SPI.
 3.  (new SA) If the OLD SPI value is zero and the NEW SPI is a new
     non-zero value, then a new SA is being requested by the peer.
     This case is also treated like a rekeying event; the receiving
     host must create a new SA and respond with an UPDATE ACK.

Nikander, et al. Experimental [Page 28] RFC 5206 HIP Mobility and Multihoming April 2008

 4.  (deprecating the SA) If the OLD SPI indicates an existing SPI and
     the NEW SPI is zero, the SA is being deprecated and all locators
     uniquely bound to the SPI are put into the DEPRECATED state.
 If none of the above cases apply, a protocol error has occurred and
 the processing of the UPDATE is stopped.
 Next, the locators in the LOCATOR parameter are processed.  For each
 locator listed in the LOCATOR parameter, check that the address
 therein is a legal unicast or anycast address.  That is, the address
 MUST NOT be a broadcast or multicast address.  Note that some
 implementations MAY accept addresses that indicate the local host,
 since it may be allowed that the host runs HIP with itself.
 The below assumes that all locators are of Type "1" with a Traffic
 Type of "0"; other cases are for further study.
 For each Type "1" address listed in the LOCATOR parameter, the host
 checks whether the address is already bound to the SPI indicated.  If
 the address is already bound, its lifetime is updated.  If the status
 of the address is DEPRECATED, the status is changed to UNVERIFIED.
 If the address is not already bound, the address is added, and its
 status is set to UNVERIFIED.  Mark all addresses corresponding to the
 SPI that were NOT listed in the LOCATOR parameter as DEPRECATED.
 As a result, at the end of processing, the addresses listed in the
 LOCATOR parameter have either a state of UNVERIFIED or ACTIVE, and
 any old addresses on the old SA not listed in the LOCATOR parameter
 have a state of DEPRECATED.
 Once the host has processed the locators, if the LOCATOR parameter
 contains a new Preferred locator, the host SHOULD initiate a change
 of the Preferred locator.  This requires that the host first verifies
 reachability of the associated address, and only then changes the
 Preferred locator; see Section 5.5.
 If a host receives a locator with an unsupported Locator Type, and
 when such a locator is also declared to be the Preferred locator for
 the peer, the host SHOULD send a NOTIFY error with a Notify Message
 Type of LOCATOR_TYPE_UNSUPPORTED, with the Notification Data field
 containing the locator(s) that the receiver failed to process.
 Otherwise, a host MAY send a NOTIFY error if a (non-preferred)
 locator with an unsupported Locator Type is received in a LOCATOR

Nikander, et al. Experimental [Page 29] RFC 5206 HIP Mobility and Multihoming April 2008

5.4. Verifying Address Reachability

 A host MUST verify the reachability of an UNVERIFIED address.  The
 status of a newly learned address MUST initially be set to UNVERIFIED
 unless the new address is advertised in a R1 packet as a new
 Preferred locator.  A host MAY also want to verify the reachability
 of an ACTIVE address again after some time, in which case it would
 set the status of the address to UNVERIFIED and reinitiate address
 A host typically starts the address-verification procedure by sending
 a nonce to the new address.  For example, when the host is changing
 its SPI and sending an ESP_INFO to the peer, the NEW SPI value SHOULD
 be random and the value MAY be copied into an ECHO_REQUEST sent in
 the rekeying UPDATE.  However, if the host is not changing its SPI,
 it MAY still use the ECHO_REQUEST parameter in an UPDATE message sent
 to the new address.  A host MAY also use other message exchanges as
 confirmation of the address reachability.
 Note that in the case of receiving a LOCATOR in an R1 and replying
 with an I2 to the new address in the LOCATOR, receiving the
 corresponding R2 is sufficient proof of reachability for the
 Responder's preferred address.  Since further address verification of
 such an address can impede the HIP-base exchange, a host MUST NOT
 separately verify reachability of a new Preferred locator that was
 received on an R1.
 In some cases, it MAY be sufficient to use the arrival of data on a
 newly advertised SA as implicit address reachability verification as
 depicted in Figure 13, instead of waiting for the confirmation via a
 HIP packet.  In this case, a host advertising a new SPI as part of
 its address reachability check SHOULD be prepared to receive traffic
 on the new SA.
   Mobile host                                   Peer host
                                                 prepare incoming SA
                    NEW SPI in ESP_INFO (UPDATE)
 switch to new outgoing SA
                         data on new SA
                                                 mark address ACTIVE
           Figure 13: Address Activation Via Use of a New SA
 When address verification is in progress for a new Preferred locator,
 the host SHOULD select a different locator listed as ACTIVE, if one

Nikander, et al. Experimental [Page 30] RFC 5206 HIP Mobility and Multihoming April 2008

 such locator is available, to continue communications until address
 verification completes.  Alternatively, the host MAY use the new
 Preferred locator while in UNVERIFIED status to the extent Credit-
 Based Authorization permits.  Credit-Based Authorization is explained
 in Section 5.6.  Once address verification succeeds, the status of
 the new Preferred locator changes to ACTIVE.

5.5. Changing the Preferred Locator

 A host MAY want to change the Preferred outgoing locator for
 different reasons, e.g., because traffic information or ICMP error
 messages indicate that the currently used preferred address may have
 become unreachable.  Another reason may be due to receiving a LOCATOR
 parameter that has the "P" bit set.
 To change the Preferred locator, the host initiates the following
 1.  If the new Preferred locator has ACTIVE status, the Preferred
     locator is changed and the procedure succeeds.
 2.  If the new Preferred locator has UNVERIFIED status, the host
     starts to verify its reachability.  The host SHOULD use a
     different locator listed as ACTIVE until address verification
     completes if one such locator is available.  Alternatively, the
     host MAY use the new Preferred locator, even though in UNVERIFIED
     status, to the extent Credit-Based Authorization permits.  Once
     address verification succeeds, the status of the new Preferred
     locator changes to ACTIVE and its use is no longer governed by
     Credit-Based Authorization.
 3.  If the peer host has not indicated a preference for any address,
     then the host picks one of the peer's ACTIVE addresses randomly
     or according to policy.  This case may arise if, for example,
     ICMP error messages that deprecate the Preferred locator arrive,
     but the peer has not yet indicated a new Preferred locator.
 4.  If the new Preferred locator has DEPRECATED status and there is
     at least one non-deprecated address, the host selects one of the
     non-deprecated addresses as a new Preferred locator and
     continues.  If the selected address is UNVERIFIED, the address
     verification procedure described above will apply.

Nikander, et al. Experimental [Page 31] RFC 5206 HIP Mobility and Multihoming April 2008

5.6. Credit-Based Authorization

 To prevent redirection-based flooding attacks, the use of a Credit-
 Based Authorization (CBA) approach is mandatory when a host sends
 data to an UNVERIFIED locator.  The following algorithm meets the
 security considerations for prevention of amplification and time-
 shifting attacks.  Other forms of credit aging, and other values for
 the CreditAgingFactor and CreditAgingInterval parameters in
 particular, are for further study, and so are the advanced CBA
 techniques specified in [CBA-MIPv6].

5.6.1. Handling Payload Packets

 A host maintains a "credit counter" for each of its peers.  Whenever
 a packet arrives from a peer, the host SHOULD increase that peer's
 credit counter by the size of the received packet.  When the host has
 a packet to be sent to the peer, and when the peer's Preferred
 locator is listed as UNVERIFIED and no alternative locator with
 status ACTIVE is available, the host checks whether it can send the
 packet to the UNVERIFIED locator.  The packet SHOULD be sent if the
 value of the credit counter is higher than the size of the outbound
 packet.  If the credit counter is too low, the packet MUST be
 discarded or buffered until address verification succeeds.  When a
 packet is sent to a peer at an UNVERIFIED locator, the peer's credit
 counter MUST be reduced by the size of the packet.  The peer's credit
 counter is not affected by packets that the host sends to an ACTIVE
 locator of that peer.
 Figure 14 depicts the actions taken by the host when a packet is
 received.  Figure 15 shows the decision chain in the event a packet
 is sent.
        |       +----------------+               +---------------+
        |       |    Increase    |               |    Deliver    |
        +-----> | credit counter |-------------> |   packet to   |
                | by packet size |               |  application  |
                +----------------+               +---------------+
     Figure 14: Receiving Packets with Credit-Based Authorization

Nikander, et al. Experimental [Page 32] RFC 5206 HIP Mobility and Multihoming April 2008

      |          _________________
      |         /                 \                 +---------------+
      |        /  Is the preferred \       No       |  Send packet  |
      +-----> | destination address |-------------> |  to preferred |
               \    UNVERIFIED?    /                |    address    |
                \_________________/                 +---------------+
                         | Yes
                /                 \                 +---------------+
               /   Does an ACTIVE  \      Yes       |  Send packet  |
              | destination address |-------------> |   to ACTIVE   |
               \       exist?      /                |    address    |
                \_________________/                 +---------------+
                         | No
                /                 \                 +---------------+
               /   Credit counter  \       No       |               |
              |          >=         |-------------> |  Drop packet  |
               \    packet size?   /                |               |
                \_________________/                 +---------------+
                         | Yes
                 +---------------+                  +---------------+
                 | Reduce credit |                  |  Send packet  |
                 |  counter by   |----------------> | to preferred  |
                 |  packet size  |                  |    address    |
                 +---------------+                  +---------------+
      Figure 15: Sending Packets with Credit-Based Authorization

5.6.2. Credit Aging

 A host ensures that the credit counters it maintains for its peers
 gradually decrease over time.  Such "credit aging" prevents a
 malicious peer from building up credit at a very slow speed and using
 this, all at once, for a severe burst of redirected packets.

Nikander, et al. Experimental [Page 33] RFC 5206 HIP Mobility and Multihoming April 2008

 Credit aging may be implemented by multiplying credit counters with a
 factor, CreditAgingFactor (a fractional value less than one), in
 fixed time intervals of CreditAgingInterval length.  Choosing
 appropriate values for CreditAgingFactor and CreditAgingInterval is
 important to ensure that a host can send packets to an address in
 state UNVERIFIED even when the peer sends at a lower rate than the
 host itself.  When CreditAgingFactor or CreditAgingInterval are too
 small, the peer's credit counter might be too low to continue sending
 packets until address verification concludes.
 The parameter values proposed in this document are as follows:
    CreditAgingFactor        7/8
    CreditAgingInterval      5 seconds
 These parameter values work well when the host transfers a file to
 the peer via a TCP connection and the end-to-end round-trip time does
 not exceed 500 milliseconds.  Alternative credit-aging algorithms may
 use other parameter values or different parameters, which may even be
 dynamically established.

6. Security Considerations

 The HIP mobility mechanism provides a secure means of updating a
 host's IP address via HIP UPDATE packets.  Upon receipt, a HIP host
 cryptographically verifies the sender of an UPDATE, so forging or
 replaying a HIP UPDATE packet is very difficult (see [RFC5201]).
 Therefore, security issues reside in other attack domains.  The two
 we consider are malicious redirection of legitimate connections as
 well as redirection-based flooding attacks using this protocol.  This
 can be broken down into the following:
    Impersonation attacks
  1. direct conversation with the misled victim
  1. man-in-the-middle attack
    DoS attacks
  1. flooding attacks (== bandwidth-exhaustion attacks)
  • tool 1: direct flooding
  • tool 2: flooding by zombies
  • tool 3: redirection-based flooding

Nikander, et al. Experimental [Page 34] RFC 5206 HIP Mobility and Multihoming April 2008

  1. memory-exhaustion attacks
  1. computational-exhaustion attacks
 We consider these in more detail in the following sections.
 In Section 6.1 and Section 6.2, we assume that all users are using
 HIP.  In Section 6.3 we consider the security ramifications when we
 have both HIP and non-HIP users.  Security considerations for Credit-
 Based Authorization are discussed in [SIMPLE-CBA].

6.1. Impersonation Attacks

 An attacker wishing to impersonate another host will try to mislead
 its victim into directly communicating with them, or carry out a man-
 in-the-middle (MitM) attack between the victim and the victim's
 desired communication peer.  Without mobility support, both attack
 types are possible only if the attacker resides on the routing path
 between its victim and the victim's desired communication peer, or if
 the attacker tricks its victim into initiating the connection over an
 incorrect routing path (e.g., by acting as a router or using spoofed
 DNS entries).
 The HIP extensions defined in this specification change the situation
 in that they introduce an ability to redirect a connection (like
 IPv6), both before and after establishment.  If no precautionary
 measures are taken, an attacker could misuse the redirection feature
 to impersonate a victim's peer from any arbitrary location.  The
 authentication and authorization mechanisms of the HIP base exchange
 [RFC5201] and the signatures in the UPDATE message prevent this
 attack.  Furthermore, ownership of a HIP association is securely
 linked to a HIP HI/HIT.  If an attacker somehow uses a bug in the
 implementation or weakness in some protocol to redirect a HIP
 connection, the original owner can always reclaim their connection
 (they can always prove ownership of the private key associated with
 their public HI).
 MitM attacks are always possible if the attacker is present during
 the initial HIP base exchange and if the hosts do not authenticate
 each other's identities.  However, once the opportunistic base
 exchange has taken place, even a MitM cannot steal the HIP connection
 anymore because it is very difficult for an attacker to create an
 UPDATE packet (or any HIP packet) that will be accepted as a
 legitimate update.  UPDATE packets use HMAC and are signed.  Even
 when an attacker can snoop packets to obtain the SPI and HIT/HI, they
 still cannot forge an UPDATE packet without knowledge of the secret

Nikander, et al. Experimental [Page 35] RFC 5206 HIP Mobility and Multihoming April 2008

6.2. Denial-of-Service Attacks

6.2.1. Flooding Attacks

 The purpose of a denial-of-service attack is to exhaust some resource
 of the victim such that the victim ceases to operate correctly.  A
 denial-of-service attack can aim at the victim's network attachment
 (flooding attack), its memory, or its processing capacity.  In a
 flooding attack, the attacker causes an excessive number of bogus or
 unwanted packets to be sent to the victim, which fills their
 available bandwidth.  Note that the victim does not necessarily need
 to be a node; it can also be an entire network.  The attack basically
 functions the same way in either case.
 An effective DoS strategy is distributed denial of service (DDoS).
 Here, the attacker conventionally distributes some viral software to
 as many nodes as possible.  Under the control of the attacker, the
 infected nodes, or "zombies", jointly send packets to the victim.
 With such an 'army', an attacker can take down even very high
 bandwidth networks/victims.
 With the ability to redirect connections, an attacker could realize a
 DDoS attack without having to distribute viral code.  Here, the
 attacker initiates a large download from a server, and subsequently
 redirects this download to its victim.  The attacker can repeat this
 with multiple servers.  This threat is mitigated through reachability
 checks and credit-based authorization.  Both strategies do not
 eliminate flooding attacks per se, but they preclude: (i) their use
 from a location off the path towards the flooded victim; and (ii) any
 amplification in the number and size of the redirected packets.  As a
 result, the combination of a reachability check and credit-based
 authorization lowers a HIP redirection-based flooding attack to the
 level of a direct flooding attack in which the attacker itself sends
 the flooding traffic to the victim.

6.2.2. Memory/Computational-Exhaustion DoS Attacks

 We now consider whether or not the proposed extensions to HIP add any
 new DoS attacks (consideration of DoS attacks using the base HIP
 exchange and updates is discussed in [RFC5201]).  A simple attack is
 to send many UPDATE packets containing many IP addresses that are not
 flagged as preferred.  The attacker continues to send such packets
 until the number of IP addresses associated with the attacker's HI
 crashes the system.  Therefore, there SHOULD be a limit to the number
 of IP addresses that can be associated with any HI.  Other forms of
 memory/computationally exhausting attacks via the HIP UPDATE packet
 are handled in the base HIP document [RFC5201].

Nikander, et al. Experimental [Page 36] RFC 5206 HIP Mobility and Multihoming April 2008

 A central server that has to deal with a large number of mobile
 clients may consider increasing the SA lifetimes to try to slow down
 the rate of rekeying UPDATEs or increasing the cookie difficulty to
 slow down the rate of attack-oriented connections.

6.3. Mixed Deployment Environment

 We now assume an environment with both HIP and non-HIP aware hosts.
 Four cases exist.
 1.  A HIP host redirects its connection onto a non-HIP host.  The
     non-HIP host will drop the reachability packet, so this is not a
     threat unless the HIP host is a MitM that could somehow respond
     successfully to the reachability check.
 2.  A non-HIP host attempts to redirect their connection onto a HIP
     host.  This falls into IPv4 and IPv6 security concerns, which are
     outside the scope of this document.
 3.  A non-HIP host attempts to steal a HIP host's session (assume
     that Secure Neighbor Discovery is not active for the following).
     The non-HIP host contacts the service that a HIP host has a
     connection with and then attempts to change its IP address to
     steal the HIP host's connection.  What will happen in this case
     is implementation dependent but such a request should fail by
     being ignored or dropped.  Even if the attack were successful,
     the HIP host could reclaim its connection via HIP.
 4.  A HIP host attempts to steal a non-HIP host's session.  A HIP
     host could spoof the non-HIP host's IP address during the base
     exchange or set the non-HIP host's IP address as its preferred
     address via an UPDATE.  Other possibilities exist, but a simple
     solution is to prevent the use of HIP address check information
     to influence non-HIP sessions.

7. IANA Considerations

 This document defines a LOCATOR parameter for the Host Identity
 Protocol [RFC5201].  This parameter is defined in Section 4 with a
 Type of 193.
 This document also defines a LOCATOR_TYPE_UNSUPPORTED Notify Message
 Type as defined in the Host Identity Protocol specification
 [RFC5201].  This parameter is defined in Section 5.3 with a value of

Nikander, et al. Experimental [Page 37] RFC 5206 HIP Mobility and Multihoming April 2008

8. Authors and Acknowledgments

 Pekka Nikander and Jari Arkko originated this document, and Christian
 Vogt and Thomas Henderson (editor) later joined as co-authors.  Greg
 Perkins contributed the initial draft of the security section.  Petri
 Jokela was a co-author of the initial individual submission.
 The authors thank Miika Komu, Mika Kousa, Jeff Ahrenholz, and Jan
 Melen for many improvements to the document.

9. References

9.1. Normative references

 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3484]     Draves, R., "Default Address Selection for Internet
               Protocol version 6 (IPv6)", RFC 3484, February 2003.
 [RFC4291]     Hinden, R. and S. Deering, "IP Version 6 Addressing
               Architecture", RFC 4291, February 2006.
 [RFC4303]     Kent, S., "IP Encapsulating Security Payload (ESP)",
               RFC 4303, December 2005.
 [RFC4423]     Moskowitz, R. and P. Nikander, "Host Identity Protocol
               (HIP) Architecture", RFC 4423, May 2006.
 [RFC5201]     Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
               Henderson, "Host Identity Protocol", RFC 5201,
               April 2008.
 [RFC5202]     Jokela, P., Moskowitz, R., and P. Nikander, "Using the
               ESP Transport Format with the Host Identity Protocol
               (HIP)", RFC 5202, April 2008.
 [RFC5204]     Laganier, J. and L. Eggert, "Host Identity Protocol
               (HIP) Rendezvous Extension", RFC 5204, April 2008.

9.2. Informative references

 [CBA-MIPv6]   Vogt, C. and J. Arkko, "Credit-Based Authorization for
               Mobile IPv6 Early Binding Updates", Work in Progress,
               February 2005.

Nikander, et al. Experimental [Page 38] RFC 5206 HIP Mobility and Multihoming April 2008

 [RFC4225]     Nikander, P., Arkko, J., Aura, T., Montenegro, G., and
               E. Nordmark, "Mobile IP Version 6 Route Optimization
               Security Design Background", RFC 4225, December 2005.
 [SIMPLE-CBA]  Vogt, C. and J. Arkko, "Credit-Based Authorization for
               Concurrent Reachability Verification", Work
               in Progress, February 2006.

Authors' Addresses

 Pekka Nikander
 Ericsson Research NomadicLab
 JORVAS  FIN-02420
 Phone: +358 9 299 1
 Thomas R. Henderson (editor)
 The Boeing Company
 P.O. Box 3707
 Seattle, WA
 Christian Vogt
 Ericsson Research NomadicLab
 Hirsalantie 11
 JORVAS  FIN-02420
 Jari Arkko
 Ericsson Research NomadicLab
 JORVAS  FIN-02420
 Phone: +358 40 5079256

Nikander, et al. Experimental [Page 39] RFC 5206 HIP Mobility and Multihoming April 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 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

Intellectual Property

 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
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 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
 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

Nikander, et al. Experimental [Page 40]

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