Network Working Group                                        P. Nikander
Internet-Draft                              Ericsson Research NomadicLab
Intended status: Standards Track                       T. Henderson, Ed.
Expires: April 21, 2011                               The Boeing Company
                                                                 C. Vogt
                                                                J. Arkko
                                            Ericsson Research NomadicLab
                                                        October 18, 2010

            Host Multihoming with the Host Identity Protocol


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 21, 2011.

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   Copyright (c) 2010 IETF Trust and the persons identified as the
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
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Table of Contents

   1.  Introduction and Scope . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Conventions  . . . . . . . . . . . . . . . . .  4
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Operating Environment  . . . . . . . . . . . . . . . . . .  5
     3.2.  Multihoming Overview . . . . . . . . . . . . . . . . . . .  7
   4.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Host Multihoming . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Site Multihoming . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Dual host multihoming  . . . . . . . . . . . . . . . . . . 10
     4.4.  Combined Mobility and Multihoming  . . . . . . . . . . . . 10
     4.5.  Initiating the Protocol in R1 or I2  . . . . . . . . . . . 11
   5.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Address Verification . . . . . . . . . . . . . . . . . . . 12
     5.2.  Preferred Locator  . . . . . . . . . . . . . . . . . . . . 12
     5.3.  Interaction with Security Associations . . . . . . . . . . 13
   6.  Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Handling Received LOCATORs . . . . . . . . . . . . . . . . 17
     6.3.  Verifying Address Reachability . . . . . . . . . . . . . . 19
     6.4.  Changing the Preferred Locator . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   9.  Authors and Acknowledgments  . . . . . . . . . . . . . . . . . 20
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative references . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative references . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Document Revision History . . . . . . . . . . . . . . 21

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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].

   One consequence of such a decoupling is that new solutions to
   network-layer mobility and host multihoming are possible.  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.  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 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.  Future
   versions of this draft will summarize the differences more

   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 defined herein is not sufficient.  These
   include the initial reachability of a multihomed 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.
   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).

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   Transport-layer mobility triggers, and the proper transport response
   to a HIP multihoming address change, are outside the scope of this

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].

   Terminology is copied from [I-D.ietf-hip-rfc5206-bis].

   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

3.  Protocol Model

   This section is an overview; more detailed specification follows this

   The overall protocol model is the same as in Section 3 of
   [I-D.ietf-hip-rfc5206-bis]; this section only highlights the

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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.

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            | TCP   |  (sockets bound to HITs)
      ----> | ESP   |  {HIT_s, HIT_d} <-> SPI
      |     ---------
      |         |
    ----    ---------
   | MH |-> | HIP   |  {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI}
    ----    ---------
            |  IP   |

             Figure 2: Architecture for  HIP 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 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

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   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 documents.

3.2.  Multihoming Overview

   In host multihoming, a host has multiple locators simultaneously
   rather than sequentially, as in the case of mobility.  By using the
   LOCATOR 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, 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.

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 [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.

   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

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   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 in
   [I-D.ietf-hip-rfc5206-bis], an UNVERIFIED address may be used.

   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.

4.1.  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.

   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-

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   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 3 illustrates this scenario.

     Multi-homed Host                    Peer Host


                   Figure 3: 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.

4.2.  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

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   addresses, described above in Section 4.1.  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.

4.3.  Dual host multihoming

   Consider the case in which both hosts would like to add an additional
   address after the base exchange completes.  In Figure 4, 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

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

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

    Figure 4: Dual Multihoming Case in Which Each Host Uses LOCATOR to
                           Add a Second Address

4.4.  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

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   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.

4.5.  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 5: 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
   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.

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            Initiator                                Responder

                             I2 with LOCATOR
                                                     (process normally)
                                             record additional addresses
                       R2 sent to source address of I2
   (process normally)

                     Figure 6: 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.

5.  Other Considerations

5.1.  Address Verification

   An address verification method is specified in
   [I-D.ietf-hip-rfc5206-bis].  It is expected that addresses learned in
   multihoming scenarios also are subject to the same verification

5.2.  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.

   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.

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5.3.  Interaction with Security Associations

   This document uses the HIP LOCATOR protocol parameter, specified in
   [I-D.ietf-hip-rfc5206-bis]), 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
   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 7.

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

        Figure 7: Relation between Hosts, SPIs, and Addresses (Base

   In Figure 7, 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 8.
   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.

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

   Figure 8: 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 8, 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

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   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.

6.  Processing Rules

   Processing rules are specified in [I-D.ietf-hip-rfc5206-bis].  Future
   versions of this document will specify multihoming-specific
   processing rules here.

6.1.  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.

   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

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   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 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

   1.  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

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       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.

6.2.  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 6.1.

   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

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       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.

   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 6.4.

   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

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   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

6.3.  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.  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.

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   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.  Security Considerations

   Security considerations are addressed in [I-D.ietf-hip-rfc5206-bis].

8.  IANA Considerations


9.  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.

10.  References

10.1.  Normative references

   [I-D.ietf-hip-rfc5206-bis]  Nikander, P., Henderson, T., Vogt, C.,
                               and J. Arkko, "End-Host Mobility and
                               Multihoming with the Host Identity
                               Protocol", draft-ietf-hip-rfc5206-bis-00
                               (work in progress), August 2010.

   [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.

   [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.

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   [RFC5201]                   Moskowitz, R., Nikander, P., Jokela, P.,
                               and T. Henderson, "Host Identity
                               Protocol", RFC 5201, April 2008.

   [RFC5202]                   Jokela, P., Moskowitz, R., and P.
                               Nikander, "Using the Encapsulating
                               Security Payload (ESP) Transport Format
                               with the Host Identity Protocol (HIP)",
                               RFC 5202, April 2008.

10.2.  Informative references

   [RFC5204]                   Laganier, J. and L. Eggert, "Host
                               Identity Protocol (HIP) Rendezvous
                               Extension", RFC 5204, April 2008.

   [RFC5533]                   Nordmark, E. and M. Bagnulo, "Shim6:
                               Level 3 Multihoming Shim Protocol for
                               IPv6", RFC 5533, June 2009.

Appendix A.  Document Revision History

   To be removed upon publication

   | Revision | Comments                                               |
   | draft-00 | Initial version with multihoming text imported from    |
   |          | RFC5206.                                               |

Authors' Addresses

   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420

   Phone: +358 9 299 1

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   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

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