Network Working Group                                  T. Henderson, Ed.
Internet-Draft                                  University of Washington
Intended status: Standards Track                                 C. Vogt
Expires: August 3, October 9, 2016                                        J. Arkko
                                            Ericsson Research NomadicLab
                                                        January 31,
                                                           April 7, 2016

            Host Multihoming with the Host Identity Protocol
                     draft-ietf-hip-multihoming-07
                     draft-ietf-hip-multihoming-08

Abstract

   This document defines host multihoming extensions to the Host
   Identity Protocol (HIP), by leveraging protocol components defined
   for host mobility.

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Table of Contents

   1.  Introduction and Scope  . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Conventions . . . . . . . . . . . . . . . . .   3
   3.  Protocol Model  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Host Multihoming  Background  . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Site Multihoming  . . . . . .   4
     4.2.  Multiple Addresses  . . . . . . . . . . . . . .   6
     4.3.  Dual host multihoming . . . . .   6
     4.3.  Multiple Security Associations  . . . . . . . . . . . . .   7   6
     4.4.  Combined Mobility and  Host Multihoming . . for Fault Tolerance  . . . . . . . . . .   7
     4.5.  Initiating the Protocol in R1 or I2  Host Multihoming for Load Balancing . . . . . . . . . . .   8
     4.6.  Using LOCATOR_SETs across Addressing Realms  Site Multihoming  . . . . . . .   9
   5.  Other Considerations . . . . . . . . . . . . .   9
     4.7.  Dual Host Multihoming . . . . . . .   9
     5.1.  Address Verification . . . . . . . . . . .  10
     4.8.  Combined Mobility and Multihoming . . . . . . .   9
     5.2.  Preferred Locator . . . . .  11
     4.9.  Initiating the Protocol in R1, I2, or R2  . . . . . . . .  11
     4.10. Using LOCATOR_SETs across Addressing Realms . . . . . . .  10
     5.3.  12
     4.11. Interaction with Security Associations  . . . . . . . . .  10
   6.  13
   5.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  13
     5.1.  Sending LOCATOR_SETs  . . . . . . . . . . . . . . . . . .  12
     6.2.  13
     5.2.  Handling Received LOCATOR_SETs  . . . . . . . . . . . . .  14
     6.3.  15
     5.3.  Verifying Address Reachability  . . . . . . . . . . . . .  16
     6.4.  17
     5.4.  Changing the Preferred Locator  . . . . . . . . . . . . .  16
   7.  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  18
   8.  Authors and Acknowledgments . . . . . . . . . . . . . . . . .  17
   10.  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  19
     9.1.  Normative references  . . . . . . . . . . . . . . . . . .  17
     10.2.  19
     9.2.  Informative references  . . . . . . . . . . . . . . . . .  18  20
   Appendix A.  Document Revision History  . . . . . . . . . . . . .  19  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19  21

1.  Introduction and Scope

   The Host Identity Protocol [RFC7401] (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.

   One consequence of such a decoupling is that new solutions to
   network-layer mobility and host multihoming are possible.  Basic host
   mobility is defined in [I-D.ietf-hip-rfc5206-bis] and covers the case
   in which a host has a single address and changes its network point-
   of-attachment while desiring to preserve the HIP-enabled security
   association.  Host multihoming is somewhat of a dual case to host
   mobility, in that a host may simultaneously have more than one
   network point-of-attachment.  There are potentially many variations
   of host multihoming possible.  [I-D.ietf-hip-rfc5206-bis] specifies
   the format of the HIP parameter (LOCATOR_SET parameter) used to
   convey IP addressing information between peers, the procedures for
   sending and processing this parameter to enable basic host mobility,
   and procedures for an address verification mechanism.  The scope of
   this document encompasses messaging and elements of procedure for
   some basic host multihoming scenarios of interest.

   Another variation of multihoming that has been heavily studied is
   site multihoming.  Solutions for site multihoming in IPv6 networks
   have been specified by the IETF shim6 working group.  The shim6
   protocol [RFC5533] bears many architectural similarities to HIP but
   there are differences in the security model and in the protocol.

   While HIP can potentially be used with transports other than the ESP
   transport format [RFC7402], this document largely assumes the use of
   ESP and leaves other transport formats for further study.

   Finally, making underlying IP multihoming 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 multihoming address change, are outside the scope of this
   document.

2.  Terminology and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The following terms used in this document are defined in
   [I-D.ietf-hip-rfc5206-bis]: LOCATOR_SET, Locator, Address, Preferred
   locator, and Credit Based Authorization.

3.  Protocol Model

   The protocol model for HIP support of host multihoming extends the
   model for host mobility described in Section 3 of
   [I-D.ietf-hip-rfc5206-bis].  This section only highlights the
   differences.

   In host multihoming, a host has multiple locators simultaneously
   rather than sequentially, as in the case of mobility.  By using the
   LOCATOR_SET parameter defined in [I-D.ietf-hip-rfc5206-bis], a host
   can inform its peers of additional (multiple) locators at which it
   can be reached.  When multiple locators are available and announced
   to the peer, a host can designate a particular locator as a
   "preferred" locator, meaning that the host prefers that its peer send
   packets to the designated address before trying an alternative
   address.  Although this document defines a basic mechanism for
   multihoming, it does not define all possible 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. protocols.

4.  Protocol Overview

   In this section, we briefly introduce a number of usage scenarios for
   HIP multihoming.  These scenarios assume that HIP is being used with
   the ESP transform [RFC7402], 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 [RFC7401]. [RFC7401], the use of the ESP transport format
   [RFC7402], and the HIP mobility specification
   [I-D.ietf-hip-rfc5206-bis].  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.

4.1.  Background

   The multihoming scenarios below assume that can be explained in contrast to the non-
   multihoming case described in the two hosts have completed a single
   HIP base exchange protocol specification.  We
   review the pertinent details here.  In the base specification when
   used with each other.  Both of the hosts therefore have
   one incoming and one outgoing SA.  Further, each SA uses ESP transport format, the same
   pair of HIP base exchange will set up
   a single SA in each direction.  The IP addresses, which addresses associated with the
   SAs are the ones same as those used in to convey the base exchange.

   The readdressing protocol is an asymmetric protocol where HIP packets.  For data
   traffic, 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 security policy database (SPD) and security association
   database (SAD) will likely exist, following the IPsec architecture.
   One distinction between HIP exchanges.  The majority of and IPsec, however, is that the packets
   on which host IDs,
   and not the LOCATOR_SET parameters are expected to be carried IP addresses, are
   UPDATE packets.  However, some implementations may want to experiment
   with sending LOCATOR_SET parameters also on other packets, such as
   R1, I2, and NOTIFY.

   The scenarios below at times describe addresses conceptually used as being selectors in either an
   ACTIVE, VERIFIED, or DEPRECATED state.  From the perspective of
   SPD.  In the outbound direction, as a
   host, newly-learned addresses result of SPD processing, when
   an outbound SA is selected, the correct IP destination address for
   the peer must also be verified before put
   into active service, and addresses removed by assigned.  Therefore, outbound SAs are
   conceptually associated with the peer are put into a
   deprecated state.  Under limited conditions described in
   [I-D.ietf-hip-rfc5206-bis], an UNVERIFIED IP address may be used.

   Hosts that use link-local addresses must be used as source addresses in their
   the destination IP address below the HIP
   handshakes layer.  In the inbound
   direction, the IP addresses may not be reachable by a mobile peer.  Such hosts SHOULD
   provide a globally routable address either used as selectors in the initial handshake
   or via SAD to
   look up the LOCATOR_SET parameter.

4.1.  Host Multihoming

   A (mobile or stationary) host SA, but they are not strictly required; the ESP SPI may sometimes have more than
   be used alone.  To summarize, in the non-multihoming case, there is
   only one
   interface or global address. source IP address, one destination IP address, one inbound
   SA, and one outbound SA.

   The HIP readdressing protocol [I-D.ietf-hip-rfc5206-bis] is an
   asymmetric protocol in which a mobile or multihomed host may notify the informs a
   peer host about changes of
   the additional interface or IP addresses on affected SPIs.  IP address by using
   and ESP SPI information is carried in Locator data structures in a
   HIP parameter called a LOCATOR_SET.  The HIP mobility specification
   [I-D.ietf-hip-rfc5206-bis] describes how the LOCATOR_SET
   parameter. is carried
   in a HIP UPDATE packet.

   To avoid problems with summarize the ESP anti-replay window, a mobility elements of procedure, as background for
   multihoming, the basic idea of host
   SHOULD use mobility is to communicate a different SA for each interface or
   local IP address used change to
   receive packets from the peer host when multiple locator pairs active HIP-maintained SAs
   are
   being used simultaneously rather than sequentially.

   When more than one locator is provided to in use.  To do so, the peer host, IP address must be conveyed, any
   association between the host
   SHOULD indicate which locator IP address and an inbound SA (via the SPI
   index) may be conveyed, and protection against flooding attacks must
   be ensured.  The association of an IP address with an SPI is preferred (the locator on
   performed by a Locator of type 1, which the
   host prefers to receive traffic).  By default, the is a concatenation of an ESP
   SPI with an IP address.

   An address verification method is specified in
   [I-D.ietf-hip-rfc5206-bis].  It is expected that addresses used learned in
   the base exchange are preferred until indicated otherwise.

   In the
   multihoming case, the sender may scenarios also have multiple valid
   locators from which are subject to source traffic.  In practice, a HIP
   association the same verification
   rules.  The scenarios at times describe addresses as being in either
   an ACTIVE, VERIFIED, or DEPRECATED state.  From the perspective of a multihoming configuration may have both a preferred
   host, newly-learned addresses of the peer locator must be verified before put
   into active service, and addresses removed by the peer are put into a preferred local locator, although rules for source
   deprecated state.  Under limited conditions described in
   [I-D.ietf-hip-rfc5206-bis], an UNVERIFIED address selection should ultimately govern the selection of the
   source locator based on the destination locator.

   Although the protocol may allow for configurations be used.

   With this background, we next describe additional protocol to
   facilitate scenarios in which there one or both hosts have multiple IP
   addresses available.  Increasingly, this is
   an asymmetric number of SAs between the common case with
   network-connected hosts (e.g., one host has two
   interfaces and two inbound SAs, while on the peer has one interface Internet.

4.2.  Multiple Addresses

   Hosts may have multiple IP addresses within different address
   families (IPv4 and
   one inbound SA), it is RECOMMENDED that inbound IPv6) and outbound SAs be
   created pairwise between hosts.  When an ESP_INFO arrives scopes available to rekey support HIP
   messaging and HIP-enabled SAs.  The multiple addresses may be on a
   particular outbound SA,
   single or multiple network interfaces.  It is outside of 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 scope of
   this
   time.  However, it is recommended that implementations attempt to
   support peers that prefer document to use non-paired SAs.

   Consider the case between two hosts, one single-homed and one
   multihomed.  The multihomed specify how a host decides which of possibly
   multiple addresses may decide be used to inform support a HIP association.  Some IP
   addresses may be held back from usage due to privacy, security, or
   cost considerations.

   When multiple IP addresses are shared with a peer, the single-
   homed host about its other address.  It is RECOMMENDED that procedures
   described in the
   multihomed HIP mobility specification
   [I-D.ietf-hip-rfc5206-bis] allow for a host to set up a new SA pair Preferred bit,
   requesting that one of the multiple addresses be preferred for use on this new address.  To
   do this,
   control- or data-plane traffic.  It is also permitted to leave the multihomed host sends
   Preferred bit unset for all addresses, allowing the peer to make
   address selection decisions.

   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_SET with an ESP_INFO,
   indicating parameter.

   To support mobility, as described in the request for HIP mobility specification
   [I-D.ietf-hip-rfc5206-bis], the LOCATOR_SET may be sent in a new SA by setting HIP
   UPDATE packet.  To support multihoming, the OLD SPI value LOCATOR_SET may also be
   sent in R1, I2, or R2 packets.  The reason to
   zero, and the NEW SPI value consider to send
   LOCATOR_SET parameters in the newly created incoming SPI.  A
   Locator Type of "1" base exchange packets is used to associate the new address with the new
   SPI.  The LOCATOR_SET parameter also contains convey all
   usable addresses for fault-tolerance or load balancing
   considerations.

4.3.  Multiple Security Associations

   When multiple addresses are available between peer hosts, a second Type "1"
   locator, question
   that arises is whether to use one or multiple SAs.  The intent of
   this specification is to support different use cases but to leave the original address and SPI.  To simplify parameter
   processing
   policy decision to the hosts.

   When one host has n addresses and avoid explicit protocol extensions the other host has m addresses, it
   is possible to remove locators,
   each LOCATOR_SET parameter MUST list all locators set up as many as (n * m) SAs in use on each direction.  In
   such a
   connection (a complete listing case, every combination of inbound locators source 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 1 illustrates this scenario.

     Multi-homed Host                    Peer Host

              UPDATE(ESP_INFO, LOCATOR_SET, SEQ, [DIFFIE_HELLMAN])
        ----------------------------------->
              UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
        <-----------------------------------
              UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->

                   Figure 1: Basic Multihoming Scenario

   In multihoming scenarios, it is important that hosts receiving
   UPDATEs associate them correctly with the destination IP address used in
   the packet carrying
   would have a unique SA, and the UPDATE.  When processing inbound LOCATOR_SETs
   that establish new security associations possibility of reordering of
   datagrams on each SA will be lessened (ESP SAs may have an interface with
   multiple addresses, a host uses anti-
   replay window sensitive to reordering).  However, the destination address downside to
   creating a mesh of SAs is the signaling overhead required (for
   exchanging UPDATE
   containing messages conveying ESP_INFO parameters) and the LOCATOR_SET as
   state maintenance required in the local address SPD/SAD.

   For load balancing, when multiple paths are to which the
   LOCATOR_SET plus ESP_INFO is targeted.  This is because hosts be used in parallel,
   it may
   send UPDATEs with the same (locator) IP address make sense to create different peer
   addresses -- SAs for different paths.  In
   this has the effect use case, while a full mesh of creating multiple inbound 2 * (n * m) SAs
   implicitly affiliated with different peer source addresses.

4.2.  Site Multihoming

   A host may have an interface that has multiple globally routable IP
   addresses.  Such a situation not be
   required, it may be beneficial to create one SA pair per load-
   balanced path to avoid anti-replay window issues.

   For fault tolerance, it is more likely that a result of the site having single SA can be used
   and multiple upper Internet Service Providers, or just because the site
   provides all hosts IP addresses associated with both IPv4 that SA, and IPv6 addresses.  The host
   should stay reachable at all or any subset the
   alternative addresses used only upon failure detection of the currently available
   global routable addresses, independent
   addresses in use.  Techniques for path failure detection are outside
   the scope of this specification.  An implementation may use ICMP
   interactions, reachability checks, or other means to detect the
   failure of a locator.

   In summary, whether and how they are provided.

   This case a host decides to leverage additional
   addresses in a load-balancing or fault-tolerant manner is handled outside the same as if there were different IP
   addresses, described above
   scope of the specification.  However, in Section 4.1.  Note general, this document
   recommends that for fault tolerance, it is likely sufficient to use a
   single
   interface may experience site multihoming while the host itself may
   have multiple interfaces.

   Note that SA pair for all addresses, and for load balancing, to support
   a different SA pair for all active paths being balanced across.

4.4.  Host Multihoming for Fault Tolerance

   A (mobile or stationary) host may be multihomed and mobile simultaneously, and
   that a multihomed have more than one interface or
   global address.  The host may want choose to protect notify the location of some peer host of
   its interfaces while revealing the real IP address of some others.

   This document does not presently
   additional site multihoming
   extensions to HIP; such extensions are for further study.

4.3.  Dual host multihoming

   Consider interface or address by using the case LOCATOR_SET parameter.
   The LOCATOR_SET parameter may be included in which both hosts would like to add an additional
   address I2, R1, or R2 packet,
   or may be conveyed, after the base exchange completes.  In Figure 2, consider
   that host1, which used address addr1a completes in the base exchange to set up
   SPI1a and SPI2a, wants to add address addr1b.  It would send an UPDATE with LOCATOR_SET (containing
   packet.

   When more than one locator is provided to the address addr1b) peer host, the host MAY
   indicate which locator is preferred (the locator on which the host
   prefers to host2,
   using destination address addr2a, and a new set of SPIs would receive traffic).  By default, the addresses used in the
   base exchange are preferred until indicated otherwise.  It may be
   added between hosts 1 and 2 (call them SPI1b and SPI2b -- the
   case that the host does not shown
   in express any preferred locators.

   In the figure).  Next, consider host2 deciding to add addr2b to multihoming case, the
   relationship.  Host2 must select one of host1's addresses towards sender may also have multiple valid
   locators from which to initiate an UPDATE.  It source traffic.  In practice, a HIP
   association in a multihoming configuration may choose to initiate an UPDATE to
   addr1a, addr1b, or both.  If it chooses to send to both, then have both a full
   mesh (four SA pairs) 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 would exist between the hosts (e.g., one host has two hosts.  This
   is
   interfaces and two inbound SAs, while the most general case; peer has one interface and
   one inbound SA), it often may be the case is RECOMMENDED that hosts
   primarily establish new inbound and outbound SAs only with the peer's Preferred locator.
   The readdressing protocol is flexible enough to accommodate this
   choice.

              -<- SPI1a --                         -- SPI2a ->-
      host1 <              > addr1a <---> addr2a <              > host2
              ->- SPI2a --                         -- SPI1a -<-

                             addr1b <---> addr2a  (second SA pair)
                             addr1a <---> addr2b  (third SA pair)
                             addr1b <---> addr2b  (fourth SA pair)

    Figure 2: Dual Multihoming Case in Which Each Host Uses LOCATOR_SET be
   created pairwise between hosts.  When an ESP_INFO arrives to Add rekey a Second Address

4.4.  Combined Mobility and Multihoming

   It looks likely that in
   particular outbound SA, the future, many mobile hosts will corresponding inbound SA should be
   simultaneously mobile and multihomed, i.e., have multiple mobile
   interfaces.  Furthermore, if the interfaces use different access
   technologies, it is fairly likely also
   rekeyed at that one of time.

   Consider the interfaces case of two hosts, one single-homed and one multihomed.
   The multihomed host may
   appear stable (retain decide to inform the single-homed host about
   its current IP address) while some other(s) other address(es).  It may
   experience mobility (undergo IP address change).

   The use of LOCATOR_SET plus ESP_INFO should be flexible enough choose to
   handle most such scenarios, although more complicated scenarios have
   not been studied do so far.

4.5.  Initiating as follows.

   If the Protocol in R1 or I2

   A Responder multihomed host MAY wishes to convey the additional address(es)
   for fault tolerance, it should include a LOCATOR_SET parameter all of its addresses in
   Locator records, indicating the R1 packet
   that Traffic Type, Locator Type, and
   Preferred Locator for each address.  If it sends wishes to the Initiator.  This parameter MUST be protected bind any
   particular address to an existing SPI, it may do so by using a
   Locator of Type 1 as specified in the R1 signature.  If the R1 packet contains LOCATOR_SET parameters
   with a new Preferred locator, HIP mobility specification
   [I-D.ietf-hip-rfc5206-bis].  It does not need to rekey the Initiator SHOULD directly set existing
   SA or request additional SAs at this time.

   Figure 1 illustrates this scenario.

     Multi-homed Host                    Peer Host

              UPDATE(LOCATOR_SET, SEQ)
        ----------------------------------->
              UPDATE(ACK)
        <-----------------------------------

                   Figure 1: Basic Multihoming Scenario

   In this scenario, the
   new Preferred locator to status ACTIVE without performing address
   verification first, and MUST send peer host associates the I2 packet to multiple addresses
   with the new Preferred
   locator.  The I1 destination address SA pair between it and the new Preferred locator multihomed host.  It may be identical.  All new non-preferred locators must still also
   undergo address verification once procedures to transition the base exchange completes.

            Initiator                                Responder

                              R1 with LOCATOR_SET
                  <-----------------------------------
   record additional addresses
   change responder address
                     I2 sent
   to newly indicated preferred ACTIVE statte.  For inbound data traffic, it may choose to use the
   addresses along with the SPI as selectors.  For outbound data
   traffic, it must choose among the available addresses of the
   multihomed host, considering the state of address
                  ----------------------------------->
                                                     (process normally)
                                  R2
                  <-----------------------------------
   (process normally, later verification
   [I-D.ietf-hip-rfc5206-bis] of non-preferred locators)

                   Figure 3: LOCATOR_SET Inclusion in R1

   An Initiator MAY include one or more LOCATOR_SET parameters each address, and also considering
   available information about whether an address is in a working state.

4.5.  Host Multihoming for Load Balancing

   A multihomed host may decide to set up new SA pairs corresponding to
   new addresses, for the I2
   packet, independent purpose of whether or not there was a LOCATOR_SET
   parameter in load balancing.  The decision to
   load balance and the R1.  These parameters MUST mechanism for splitting load across multiple SAs
   is out of scope of this document.  The scenario can be protected supported by
   sending the I2
   signature.  Even if LOCATOR_SET parameter with one or more ESP_INFO
   parameters to initiate new ESP SAs.  To do this, the I2 packet contains multihomed host
   sends a LOCATOR_SET parameters, with an ESP_INFO, indicating the Responder MUST still send request for a
   new SA by setting the R2 packet OLD SPI value to zero, and the source address of
   the I2.  The new Preferred locator SHOULD be identical NEW SPI value to
   the I2
   source address.  If newly created incoming SPI.  A Locator Type of "1" is used to
   associate the I2 packet contains LOCATOR_SET parameters,
   all new locators must undergo address verification as usual, and with the
   ESP traffic new SPI.  The LOCATOR_SET
   parameter also contains a second Type "1" locator, that subsequently follows should use of the Preferred
   locator.

            Initiator                                Responder

                             I2 with LOCATOR_SET
                  ----------------------------------->
                                                     (process normally)
                                             record additional addresses
                       R2 sent to source
   original address of I2
                  <-----------------------------------
   (process normally)

                   Figure 4: LOCATOR_SET Inclusion in I2

   The I1 and I2 may be arriving from different source addresses if the SPI.  To simplify parameter processing and avoid
   explicit protocol extensions to remove locators, each LOCATOR_SET
   parameter is present MUST list all locators 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 use on a perceived puzzle mismatch.  See, connection (a complete
   listing of inbound locators and SPIs for
   instance, the example in Appendix A host).  The multihomed
   host waits for a corresponding ESP_INFO (new outbound SA) from the
   peer and an ACK of [RFC7401]. its own UPDATE.  As a solution, in the
   Responder's puzzle indexing mechanism must be flexible enough to
   accommodate mobility case, the situation when R1 includes a LOCATOR_SET parameter.

4.6.  Using LOCATOR_SETs 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
   are using IPv6 locators, then one peer
   host may loose its IPv6
   connectivity and obtain must perform an IPv4 address verification before actively using the
   new address.  In such a case, some type
   of mechanism

   Figure 2 illustrates this scenario.

     Multi-homed Host                    Peer Host

              UPDATE(ESP_INFO, LOCATOR_SET, SEQ, [DIFFIE_HELLMAN])
        ----------------------------------->
              UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
        <-----------------------------------
              UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->

               Figure 2: Host Multihoming for interworking between Load Balancing

   In multihoming scenarios, it is important that hosts receiving
   UPDATEs associate them correctly with the different realms must be
   employed; such techniques are outside destination address used in
   the scope of packet carrying the present text.
   The basic problem in this example is UPDATE.  When processing inbound LOCATOR_SETs
   that the host readdressing to
   IPv4 does not know establish new security associations on an interface with
   multiple addresses, a corresponding IPv4 address of host uses the peer.  This
   may be handled (experimentally) by possibly configuring this destination address
   information manually or in of the DNS, or UPDATE
   containing the hosts exchange both IPv4
   and IPv6 addresses in LOCATOR_SET as the locator.

5.  Other Considerations

5.1.  Address Verification

   An local address verification method to which the
   LOCATOR_SET plus ESP_INFO is specified in
   [I-D.ietf-hip-rfc5206-bis].  It targeted.  This is expected that addresses learned in
   multihoming scenarios also are subject to because hosts may
   send UPDATEs with the same verification
   rules.

5.2.  Preferred Locator

   When a host (locator) IP address to different peer
   addresses -- this has multiple locators, the peer effect of creating multiple inbound SAs
   implicitly affiliated with different peer source addresses.

4.6.  Site Multihoming

   A host must decide which to
   use for outbound packets.  It may be have an interface that has multiple globally routable IP
   addresses.  Such a host would prefer to
   receive data on a particular inbound interface.  HIP allows a
   particular locator to situation may be designated as a Preferred locator and
   communicated to result of the peer.

   In general, when site having
   multiple locators are used for a session, there is upper Internet Service Providers, or just because the question of using multiple locators for failover only site
   provides all hosts with both IPv4 and IPv6 addresses.  The host
   should stay reachable at all or for
   load-balancing.  Due to any subset of the implications currently available
   global routable addresses, independent of load-balancing on how they are provided.

   This case is handled the
   transport layer same as if there were different IP
   addresses, described above in Section 4.4 and Section 4.5.  Note that still need
   a single interface may have addresses corresponding to site
   multihoming while the host itself may also have multiple network
   interfaces.

   Note that a host may be worked out, this document
   assumes multihomed and mobile simultaneously, and
   that multiple locators are used primarily for failover.  An
   implementation a multihomed host may use ICMP interactions, reachability checks, or
   other means want to detect protect the failure location of a locator.

5.3.  Interaction with Security Associations some of
   its interfaces while revealing the real IP address of some others.

   This document uses does not presently additional site multihoming
   extensions to HIP; such extensions are for further study.

4.7.  Dual Host Multihoming

   Consider the HIP LOCATOR_SET protocol parameter, specified case in [I-D.ietf-hip-rfc5206-bis]), that allows the which both hosts to exchange
   information about their locator(s) are multihomed and any changes would like
   to notify the peer of an additional address after the base exchange
   completes.  It may be the case that both hosts choose to simply
   announce the second address in their
   locator(s).  The logical structure created with a LOCATOR_SET
   parameters has three levels: hosts, Security Associations (SAs)
   indexed by Security Parameter Indices (SPIs), and addresses.

   The relation between these levels for parameter using an association constructed
   UPDATE message exchange.  It may also be the case that one or both
   hosts decide to ask for new SA pairs to be created using the newly
   announced address.  In the case that both hosts request this, the
   result will be a full mesh of SAs as
   defined depicted in Figure 3.  In such a
   scenario, consider that host1, which used address addr1a in the base specification [RFC7401]
   exchange to set up SPI1a and ESP transform
   [RFC7402] is illustrated SPI2a, wants to add address addr1b.  It
   would send an UPDATE with LOCATOR_SET (containing the address addr1b)
   to host2, using destination address addr2a, and a new ESP_INFO, and a
   new set of SPIs would be added between hosts 1 and 2 (call them SPI1b
   and SPI2b; not shown in Figure 5. 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; the protocol is
   flexible enough to accommodate this choice.

              -<- SPI1a --                         -- SPI2a ->-
      host1 <              > addr1a <---> addr2a <              > host2
              ->- SPI2a --                         -- SPI1a -<-

                             addr1b <---> addr2a  (second SA pair)
                             addr1a <---> addr2b  (third SA pair)
                             addr1b <---> addr2b  (fourth SA pair)

    Figure 5: Relation between Hosts, SPIs, and Addresses (Base
                              Specification)

   In Figure 5, host1 3: Dual Multihoming Case in Which Each Host Uses LOCATOR_SET
                          to Add a Second Address

4.8.  Combined Mobility and host2 negotiate two unidirectional SAs, Multihoming

   It looks likely that in the future, many mobile hosts will be
   simultaneously mobile and
   each host selects multihomed, i.e., have multiple mobile
   interfaces.  Furthermore, if the SPI value for interfaces use different access
   technologies, it is fairly likely that one of the interfaces may
   appear stable (retain its inbound SA. current IP address) while some other(s) may
   experience mobility (undergo IP address change).

   The addresses
   addr1a use of LOCATOR_SET plus ESP_INFO should be flexible enough to
   handle most such scenarios, although more complicated scenarios have
   not been studied so far.

4.9.  Initiating the Protocol in R1, I2, or R2

   A Responder host MAY include a LOCATOR_SET 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_SET parameters
   with a new Preferred locator, the Initiator SHOULD directly set the
   new Preferred locator to status ACTIVE without performing address
   verification first, and addr2a are MUST send the source 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.  It is also
   possible for the host to send the LOCATOR_SET without any Preferred
   bits set, in which case the exchange will continue as normal and the
   newly-learned addresses that will be in an UNVERIFIED state at the hosts use
   initiator.

            Initiator                                Responder

                              R1 with LOCATOR_SET
                  <-----------------------------------
   record additional addresses
   change responder address
                     I2 sent to newly indicated preferred address
                  ----------------------------------->
                                                     (process normally)
                                  R2
                  <-----------------------------------
   (process normally, later verification of non-preferred locators)

                   Figure 4: LOCATOR_SET Inclusion in R1

   An Initiator MAY include one or more LOCATOR_SET parameters in the I2
   packet, independent of whether or not there was a LOCATOR_SET
   parameter in the
   base HIP exchange. R1.  These are parameters MUST be protected by the "preferred" (and only) addresses
   conveyed to I2
   signature.  Even if the peer for use on each SA.  That is, although packets
   sent to any of I2 packet contains LOCATOR_SET parameters,
   the hosts' interfaces may be accepted on Responder MUST still send the inbound
   SA, R2 packet to the peer host in general knows source address of only
   the single destination
   address learned in I2.  The new Preferred locator, if set, SHOULD be identical to
   the base exchange (e.g., for host1, it sends a I2 source address.  If the I2 packet on SPI2a to addr2a to reach host2), unless other mechanisms
   exist to learn of contains LOCATOR_SET
   parameters, all new addresses.

   In general, locators must undergo address verification as
   usual, and the bindings ESP traffic that exist in an implementation
   corresponding subsequently follows should use the
   Preferred locator.

            Initiator                                Responder

                             I2 with LOCATOR_SET
                  ----------------------------------->
                                                     (process normally)
                                             record additional addresses
                       R2 sent to this document can source address of I2
                  <-----------------------------------
   (process normally)

                   Figure 5: LOCATOR_SET Inclusion in I2

   The I1 and I2 may be depicted as shown arriving from different source addresses if the
   LOCATOR_SET parameter is present in Figure 6. R1.  In this figure, a host can have multiple inbound SPIs (and, not
   shown, case,
   implementations simultaneously using multiple outbound SPIs) associated with another host.
   Furthermore, each SPI pre-created R1s,
   indexed by Initiator IP addresses, may have multiple addresses associated with it.
   These addresses that are bound inadvertently fail the puzzle
   solution of I2 packets due to an SPI are not used a perceived puzzle mismatch.  See, for
   instance, the example in Appendix A of [RFC7401].  As a solution, the
   Responder's puzzle indexing mechanism must be flexible enough to lookup
   accommodate the
   incoming SA.  Rather, situation when R1 includes a LOCATOR_SET parameter.

   Finally, the addresses are those that are provided R2 may be used to carry the peer host, LOCATOR_SET parameter.  In
   this case, the LOCATOR_SET is covered by the HIP_MAC_2 and
   HIP_SIGNATURE.  Including LOCATOR_SET in R2 as hints for which addresses opposed to use R1 may have
   some advantages when a host prefers not to reach divulge additional
   locators until after the I2 is successfully processed.

   When the host
   on that SPI.  The LOCATOR_SET parameter is used to change sent in an UPDATE packet, then the set of
   addresses that a peer associates
   receiver will respond with a particular SPI.

                            address11
                          /
                   SPI1   - address12
                 /
                /           address21
           host -- SPI2   <
                \           address22
                 \
                   SPI3   - address31
                          \
                            address32

   Figure 6: Relation between Hosts, SPIs, an UPDATE acknowledgment.  When the
   LOCATOR_SET parameter is sent in an R1, I2, or R2 packet, the base
   exchange retransmission mechanism will confirm its successful
   delivery.

4.10.  Using LOCATOR_SETs across Addressing Realms

   It is possible for HIP associations to use these mechanisms to
   migrate their HIP associations 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 from
   addresses into collections that are likely to experience
   fate sharing.  For example, if in the host needs IPv4 addressing realm to change its addresses
   on SPI2, it is likely that IPv6 or vice versa.  It may
   be possible for a state to arise in which both address21 and address22 will
   simultaneously become obsolete.  In hosts are only using
   locators in different addressing realms, but in such a typical case, such SPIs may
   correspond with physical interfaces; see below.  Note, however, that
   especially in the case of site multihoming, one some
   type of mechanism for interworking between the addresses may
   become unreachable while the other one still works.  In different realms must
   be employed; such techniques are outside the typical
   case, however, this does not require scope of the present
   text.

4.11.  Interaction with Security Associations

   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 inform its peers
   about the situation, since even group the non-working address still
   logically exists. addresses into collections that are likely to experience
   fate sharing, or to perform load balancing.

   A basic property of HIP SAs is that the inbound IP address is not
   used to lookup the incoming SA.  Therefore, in Figure 6, 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. extended or disabled.

   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
   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_SET parameter is sent in an UPDATE packet, then the
   receiver will respond with an UPDATE acknowledgment.  When the
   LOCATOR_SET parameter is sent in an R1 or I2 packet, the base
   exchange retransmission mechanism will confirm its successful
   delivery.  LOCATOR_SETs may experimentally be used in NOTIFY packets;
   in this case, the recipient MUST consider the LOCATOR_SET 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_SET in a NOTIFY message may not be compatible
   with middleboxes.

6.

5.  Processing Rules

   Basic processing rules for the LOCATOR_SET parameter are specified in
   [I-D.ietf-hip-rfc5206-bis].  This document focuses on multihoming-
   specific rules.

6.1.

5.1.  Sending LOCATOR_SETs

   The decision of when to send a LOCATOR_SET, and which addresses to
   include, is a local policy issue.  [I-D.ietf-hip-rfc5206-bis]
   recommends that a host send a LOCATOR_SET 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.

   It is possible to delay the exposure of additional locators to the
   peer, and to send data from previously unannounced locators, as might
   arise in certain mobility or multihoming situations.

   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_SET 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.

   Locators corresponding to tunnel interfaces (e.g.  IPsec tunnel
   interfaces or Mobile IP home addresses) or other virtual interfaces
   MAY be announced in a LOCATOR_SET, but implementations SHOULD avoid
   announcing such locators as preferred locators if more direct paths
   may be obtained by instead preferring locators from non-tunneling
   interfaces if such locators provide a more direct path to the HIP
   peer.

   Hosts MUST NOT announce broadcast or multicast addresses in
   LOCATOR_SETs.  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_SET parameter that serves as a
   complete representation of the addresses and affiliated associated SPIs intended
   for active use.  We now describe a few cases introduced in Section 4.
   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
   experimentation.

   1.  Host multihoming (addition of an address).  We only describe the
       simple case of adding an additional
       simple case of adding an additional address to a (previously)
       single-homed, non-mobile host.  The host MAY choose to simply
       announce this address to the peer, for fault tolerance.  To do
       this, the multihomed host creates a LOCATOR_SET parameter
       including the existing address and SPI as a Type "1" Locator, and
       the new address to as a (previously)
       single-homed, non-mobile host. Type "0" Locator.  The host SHOULD sends this in an
       UPDATE message with SEQ parameter, which is acknowledged by the
       peer.

   2.  The host MAY set up a new SA pair between this new address and the preferred an
       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_SET 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 LOCATOR_SETs, locators with Locator Type "0",
   and multiple ESP_INFO parameters LOCATOR_SETs is for further study. unsupported.  Note that the
   inclusion of LOCATOR_SET in an R1 packet requires the use of Type "0"
   locators since no SAs are set up at that point.

6.2.

5.2.  Handling Received LOCATOR_SETs

   A host SHOULD be prepared to receive a LOCATOR_SET parameter in the
   following HIP packets: R1, I2, UPDATE, R2, and NOTIFY. UPDATE.

   This document describes sending both ESP_INFO and LOCATOR_SET
   parameters in an UPDATE.  The ESP_INFO parameter is included when
   there is a need to rekey or key a new SPI, and is can otherwise be
   included for the possible benefit of HIP-aware middleboxes.  The
   LOCATOR_SET 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_SET 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_SET 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 6.1. 5.1.

   The processing of ESP_INFO and LOCATOR_SET parameters is intended to
   be modular and support future generalization to the inclusion of
   multiple ESP_INFO and/or multiple LOCATOR_SET parameters.  A host
   SHOULD first process the ESP_INFO before the LOCATOR_SET, 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_SET 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
   parameter:

   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_SET 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.

   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_SET parameter are processed.  For
   each locator listed in the LOCATOR_SET 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_SET 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_SET
   parameter as DEPRECATED.

   For each Type "0" address listed in the LOCATOR_SET parameter, if the
   status of the address is DEPRECATED, or the address was not
   previously known, the status is changed to UNVERIFIED.  The host MAY
   choose to associate this address with one or more SAs.  The
   association with different SAs is a local policy decision, unless the
   peer has indicated that the address is Preferred, in which case the
   address should be put into use on a SA that is prioritized in the
   security policy database.

   As a result, at the end of processing, the addresses listed in the
   LOCATOR_SET parameter have either a state of UNVERIFIED or ACTIVE,
   and any old addresses on the old SA not listed in the LOCATOR_SET
   parameter have a state of DEPRECATED.

   Once the host has processed the locators, if the LOCATOR_SET
   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 6.4. 5.4.

   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_SET
   parameter.

6.3.

5.3.  Verifying Address Reachability

   Address verification is defined in [I-D.ietf-hip-rfc5206-bis].

   When address verification is in progress for a new Preferred locator,
   the host SHOULD select a different locator listed as ACTIVE, if one
   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 [I-D.ietf-hip-rfc5206-bis].  Once address verification succeeds,
   the status of the new Preferred locator changes to ACTIVE.

6.4.

5.4.  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_SET parameter that has the "P" bit set.

   To change the Preferred locator, the host initiates the following
   procedure:

   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.

7.

6.  Security Considerations

   Security

   No additional security considerations are addressed beyond those outlined in [I-D.ietf-hip-rfc5206-bis].

8.
   [I-D.ietf-hip-rfc5206-bis] have been identified.

7.  IANA Considerations

   This document has no new requests for IANA considerations.

9. actions.

8.  Authors and Acknowledgments

   This document contains content that was originally included in
   RFC5206.  Pekka Nikander and Jari Arkko originated RFC5206, and
   Christian Vogt and Thomas Henderson (editor) later joined as co-
   authors.  Also in RFC5206, Greg Perkins contributed the initial draft
   of the security section, and 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.

10.  Concepts from a paper
   on host multihoming across address families, by Samu Varjonen, Miika
   Komu, and Andrei Gurtov, contributed to this revised version.

9.  References

10.1.

9.1.  Normative references

   [I-D.ietf-hip-rfc5206-bis]
              Henderson, T., Vogt, C., and J. Arkko, "Host Mobility with
              the Host Identity Protocol", draft-ietf-hip-rfc5206-bis-09 draft-ietf-hip-rfc5206-bis-10
              (work in progress), July 2015. January 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484,
              DOI 10.17487/RFC3484, February 2003,
              <http://www.rfc-editor.org/info/rfc3484>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,
              <http://www.rfc-editor.org/info/rfc7401>.

   [RFC7402]  Jokela, P., Moskowitz, R., and J. Melen, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 7402,
              DOI 10.17487/RFC7402, April 2015,
              <http://www.rfc-editor.org/info/rfc7402>.

10.2.

9.2.  Informative references

   [I-D.ietf-hip-rfc5204-bis]
              Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", draft-ietf-hip-rfc5204-bis-07 (work
              in progress), December 2015.

   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
              June 2009, <http://www.rfc-editor.org/info/rfc5533>.

Appendix A.  Document Revision History

   To be removed upon publication

   +----------+--------------------------------------------------------+
   | Revision | Comments                                               |
   +----------+--------------------------------------------------------+
   | draft-00 | Initial version with multihoming text imported from    |
   |          | RFC5206.                                               |
   |          |                                                        |
   | draft-01 | Document refresh; no other changes.                    |
   |          |                                                        |
   | draft-02 | Document refresh; no other changes.                    |
   |          |                                                        |
   | draft-03 | Document refresh; no other changes.                    |
   |          |                                                        |
   | draft-04 | Document refresh; no other changes.                    |
   |          |                                                        |
   | draft-05 | Move remaining multihoming material from RFC5206-bis   |
   |          | to this document                                       |
   |          |                                                        |
   |          | Update lingering references to LOCATOR parameter to    |
   |          | LOCATOR_SET                                            |
   |          |                                                        |
   | draft-06 | Document refresh with updated references.              |
   |          |                                                        |
   | draft-07 | Document refresh; no other changes.                    |
   |          |                                                        |
   | draft-08 | issues 3 and 11:  Address complaints of complexity due |
   |          | to full mesh of SAs for multihoming.                   |
   |          |                                                        |
   |          | issue 5:  Improve draft based on recommendations for   |
   |          | cross-family handovers in paper by Varjonen et. al.    |
   |          |                                                        |
   |          | issue 7:  Clarify and distinguish between load         |
   |          | balancing and fault tolerance use cases.               |
   +----------+--------------------------------------------------------+

Authors' Addresses

   Thomas R. Henderson (editor)
   University of Washington
   Campus Box 352500
   Seattle, WA
   USA

   EMail: tomhend@u.washington.edu
   Christian Vogt
   Ericsson Research NomadicLab
   Hirsalantie 11
   JORVAS  FIN-02420
   FINLAND

   EMail: christian.vogt@ericsson.com

   Jari Arkko
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 40 5079256
   EMail: jari.arkko@ericsson.com