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rfc:rfc8047

Internet Engineering Task Force (IETF) T. Henderson, Ed. Request for Comments: 8047 University of Washington Category: Standards Track C. Vogt ISSN: 2070-1721 Independent

                                                              J. Arkko
                                                              Ericsson
                                                         February 2017
          Host Multihoming with the Host Identity Protocol

Abstract

 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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8047.

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Henderson, et al. Standards Track [Page 1] RFC 8047 HIP Multihoming February 2017

Table of Contents

 1.  Introduction and Scope  . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology and Conventions . . . . . . . . . . . . . . . . .   4
 3.  Protocol Model  . . . . . . . . . . . . . . . . . . . . . . .   4
 4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
   4.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.2.  Usage Scenarios . . . . . . . . . . . . . . . . . . . . .   6
     4.2.1.  Multiple Addresses  . . . . . . . . . . . . . . . . .   6
     4.2.2.  Multiple Security Associations  . . . . . . . . . . .   6
     4.2.3.  Host Multihoming for Fault Tolerance  . . . . . . . .   7
     4.2.4.  Host Multihoming for Load Balancing . . . . . . . . .   9
     4.2.5.  Site Multihoming  . . . . . . . . . . . . . . . . . .  10
     4.2.6.  Dual-Host Multihoming . . . . . . . . . . . . . . . .  10
     4.2.7.  Combined Mobility and Multihoming . . . . . . . . . .  11
     4.2.8.  Initiating the Protocol in R1, I2, or R2  . . . . . .  11
     4.2.9.  Using LOCATOR_SETs across Addressing Realms . . . . .  13
   4.3.  Interaction with Security Associations  . . . . . . . . .  13
 5.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .  14
   5.1.  Sending LOCATOR_SETs  . . . . . . . . . . . . . . . . . .  14
   5.2.  Handling Received LOCATOR_SETs  . . . . . . . . . . . . .  16
   5.3.  Verifying Address Reachability  . . . . . . . . . . . . .  18
   5.4.  Changing the Preferred Locator  . . . . . . . . . . . . .  18
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
   7.2.  Informative References  . . . . . . . . . . . . . . . . .  21
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

Henderson, et al. Standards Track [Page 2] RFC 8047 HIP Multihoming February 2017

1. Introduction and Scope

 The Host Identity Protocol (HIP) [RFC7401] 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 [RFC8046] 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.  [RFC8046] 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 host multihoming in multihomed 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.

Henderson, et al. Standards Track [Page 3] RFC 8047 HIP Multihoming February 2017

 This specification relies on implementing Sections 4 ("LOCATOR_SET
 Parameter Format") and 5 ("Processing Rules") of [RFC8046] as a
 starting point for this implementation.

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 [RFC8046]:
 LOCATOR_SET, Locator, 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 [RFC8046].  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 [RFC8046], 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 is available, the operation of
 simultaneous mobility and multihoming, source address selection
 policies (beyond those specified in [RFC6724]), and the implications
 of multihoming on transport 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 transport [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], the use of the ESP transport format
 [RFC7402], and the HIP mobility specification [RFC8046].  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 Security Parameter Index (SPI) acts as an index
 to the right host-to-host context.

Henderson, et al. Standards Track [Page 4] RFC 8047 HIP Multihoming February 2017

4.1. Background

 The multihoming scenarios can be explained in contrast to the
 non-multihoming case described in the base protocol specification
 [RFC7401].  We review the pertinent details here.  In the base
 specification, when used with the ESP transport format, the HIP base
 exchange will set up a single SA in each direction.  The IP addresses
 associated with the SAs are the same as those used to convey the HIP
 packets.  For data traffic, a security policy database (SPD) and
 security association database (SAD) will likely exist, following the
 IPsec architecture.  One distinction between HIP and IPsec, however,
 is that the host IDs, and not the IP addresses, are conceptually used
 as selectors in the SPD.  In the outbound direction, as a result of
 SPD processing, when an outbound SA is selected, the correct IP
 destination address for the peer must also be assigned.  Therefore,
 outbound SAs are conceptually associated with the peer IP address
 that must be used as the destination IP address below the HIP layer.
 In the inbound direction, the IP addresses may be used as selectors
 in the SAD to look up the SA, but they are not strictly required; the
 ESP SPI may be used alone.  To summarize, in the non-multihoming
 case, there is only one source IP address, one destination IP
 address, one inbound SA, and one outbound SA.
 The HIP readdressing protocol [RFC8046] is an asymmetric protocol in
 which a mobile or multihomed host informs a peer host about changes
 of IP addresses on affected SPIs.  IP address and ESP SPI information
 is carried in Locator fields in a HIP parameter called a LOCATOR_SET.
 The HIP mobility specification [RFC8046] describes how the
 LOCATOR_SET is carried in a HIP UPDATE packet.
 To summarize the mobility elements of procedure, as background for
 multihoming, the basic idea of host mobility is to communicate a
 local IP address change to the peer when active HIP-maintained SAs
 are in use.  To do so, the IP address must be conveyed, any
 association between the 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
 performed by a Locator Type of "1", which is a concatenation of an
 ESP SPI with an IP address.
 An address verification method is specified in [RFC8046].  It is
 expected that addresses learned in multihoming scenarios also are
 subject to the same verification rules.  At times, the scenarios
 describe addresses as being in either an ACTIVE, VERIFIED, or
 DEPRECATED state.  From the perspective of a host, newly learned
 addresses of the peer must be verified before put into active

Henderson, et al. Standards Track [Page 5] RFC 8047 HIP Multihoming February 2017

 service, and addresses removed by the peer are put into a deprecated
 state.  Under limited conditions described in [RFC8046], an
 UNVERIFIED address may be used.
 With this background, we next describe an additional protocol to
 facilitate scenarios in which one or both hosts have multiple IP
 addresses available.  Increasingly, this is the common case with
 network-connected hosts on the Internet.

4.2. Usage Scenarios

4.2.1. Multiple Addresses

 Hosts may have multiple IP addresses within different address
 families (IPv4 and IPv6) and scopes available to support HIP
 messaging and HIP-enabled SAs.  The multiple addresses may be on a
 single network interface or multiple network interfaces.  It is
 outside of the scope of this document to specify how a host decides
 which of possibly multiple addresses may be used to 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 procedures
 described in the HIP mobility specification [RFC8046] allow for a
 host to set a preferred locator ("P") bit, requesting that one of the
 multiple addresses be preferred for control- or data-plane traffic.
 It is also permitted to leave the 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 parameter.
 To support mobility, as described in the HIP mobility specification
 [RFC8046], the LOCATOR_SET may be sent in a HIP UPDATE packet.  To
 support multihoming, the LOCATOR_SET may also be sent in R1, I2, or
 R2 packets defined in the HIP protocol specification [RFC7401].  The
 reason to consider sending LOCATOR_SET parameters in base exchange
 packets is to convey all usable addresses for fault-tolerance or
 load-balancing considerations.

4.2.2. Multiple Security Associations

 When multiple addresses are available between peer hosts, a 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
 policy decision to the hosts.

Henderson, et al. Standards Track [Page 6] RFC 8047 HIP Multihoming February 2017

 When one host has n addresses and the other host has m addresses, it
 is possible to set up as many as (n * m) SAs in each direction.  In
 such a case, every combination of source and destination IP addresses
 would have a unique SA, and the possibility of the reordering of
 datagrams on each SA will be lessened (ESP SAs may have an anti-
 replay window [RFC4303] sensitive to reordering).  However, the
 downside to creating a mesh of SAs is the signaling overhead required
 (for exchanging UPDATE messages conveying ESP_INFO parameters) and
 the state maintenance required in the SPD/SAD.
 For load balancing, when multiple paths are to be used in parallel,
 it may make sense to create different SAs for different paths.  In
 this use case, while a full mesh of 2 * (n * m) SAs may 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 single SA and multiple
 IP addresses associated with that SA can be used, and the alternative
 addresses can be used only upon failure detection of the 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 a host decides to leverage additional
 addresses in a load-balancing or fault-tolerant manner is outside the
 scope of the specification (although the academic literature on
 multipath TCP schedulers may provide guidance on how to design such a
 policy).  However, in general, this document recommends that for
 fault tolerance, it is likely sufficient to use a single SA pair for
 all addresses, and for load balancing, to support a different SA pair
 for all active paths being balanced across.

4.2.3. Host Multihoming for Fault Tolerance

 A (mobile or stationary) host may have more than one interface or
 global address.  The host may choose to notify the peer host of the
 additional interface or address by using the LOCATOR_SET parameter.
 The LOCATOR_SET parameter may be included in an I2, R1, or R2 packet,
 or it may be conveyed, after the base exchange completes in an UPDATE
 packet.
 When more than one locator is provided to the peer host, the host MAY
 indicate which locator is preferred (the locator on which the host
 prefers to receive traffic).  By default, the address that a host
 uses in the base exchange is its preferred locator (for the address

Henderson, et al. Standards Track [Page 7] RFC 8047 HIP Multihoming February 2017

 family and address scope in use during the base exchange) until
 indicated otherwise.  It may be the case that the host does not
 express any preferred locators.
 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.  The host should try to
 use the peer's preferred locator unless policy or other circumstances
 prevent such usage.  A preferred local locator may be overridden if
 source address selection rules on the destination address (peer's
 preferred locator) suggest the use of a different source address.
 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 suggested 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 also be
 rekeyed at that time.  Section 4.3 discusses the interaction between
 addresses and security associations in more detail.
 Consider the case of two hosts, one single-homed and one multihomed.
 The multihomed host may decide to inform the single-homed host about
 its other address(es).  It may choose to do so as follows.
 If the multihomed host wishes to convey the additional address(es)
 for fault tolerance, it should include all of its addresses in
 Locator fields, indicating the Traffic Type, Locator Type, and
 whether the locator is a preferred locator.  If it wishes to bind any
 particular address to an existing SPI, it may do so by using a
 Locator Type of "1" as specified in the HIP mobility specification
 [RFC8046].  It does not need to rekey the existing SA or request
 additional SAs at this time.
 Figure 1 illustrates this scenario.  Note that the conventions for
 message parameter notations in figures (use of parentheses and
 brackets) is defined in Section 2.2 of [RFC7401].
   Multihomed Host                     Peer Host
            UPDATE(LOCATOR_SET, SEQ)
      ----------------------------------->
            UPDATE(ACK)
      <-----------------------------------
                 Figure 1: Basic Multihoming Scenario

Henderson, et al. Standards Track [Page 8] RFC 8047 HIP Multihoming February 2017

 In this scenario, the peer host associates the multiple addresses
 with the SA pair between it and the multihomed host.  It may also
 undergo address verification procedures to transition the addresses
 to ACTIVE state.  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 verification
 [RFC8046] of each address, and also considering available information
 about whether an address is in a working state.

4.2.4. Host Multihoming for Load Balancing

 A multihomed host may decide to set up new SA pairs corresponding to
 new addresses, for the purpose of load balancing.  The decision to
 load balance and the mechanism for splitting load across multiple SAs
 is out of scope of this document.  The scenario can be supported by
 sending the LOCATOR_SET parameter with one or more ESP_INFO
 parameters to initiate new ESP SAs.  To do this, the multihomed host
 sends a LOCATOR_SET 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_SET
 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_SET
 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 a corresponding 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 2 illustrates this scenario.
   Multihomed 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 Load Balancing
 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 LOCATOR_SETs

Henderson, et al. Standards Track [Page 9] RFC 8047 HIP Multihoming February 2017

 that establish new security associations on an interface with
 multiple addresses, a host uses the destination address of the UPDATE
 containing the LOCATOR_SET as the local address to which the
 LOCATOR_SET 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.

4.2.5. Site Multihoming

 A host may have an interface that has multiple globally routable IP
 addresses.  Such a situation may be a result of the site having
 multiple upper Internet Service Providers, or just because the site
 provides all hosts with both IPv4 and IPv6 addresses.  The host
 should stay reachable at all or any subset of the currently available
 global routable addresses, independent of how they are provided.
 This case is handled the same as if there were different IP
 addresses, described above in Sections 4.2.3 and 4.2.4.  Note that 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 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 present additional site multihoming extensions
 to HIP; such extensions are for further study.

4.2.6. Dual-Host Multihoming

 Consider the case in which both hosts are multihomed and 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 a LOCATOR_SET parameter using an
 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 depicted in Figure 3.  In such a
 scenario, 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_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 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

Henderson, et al. Standards Track [Page 10] RFC 8047 HIP Multihoming February 2017

 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.
  1. ← SPI1a – – SPI2a →-

host1 < > addr1a ←–> addr2a < > host2

  1. >- SPI2a – – SPI1a -←
                           addr1b <---> addr2a  (second SA pair)
                           addr1a <---> addr2b  (third SA pair)
                           addr1b <---> addr2b  (fourth SA pair)
  Figure 3: Dual-Multihoming Case in which Each Host Uses LOCATOR_SET
                        to Add a Second Address

4.2.7. Combined Mobility and Multihoming

 Mobile hosts may be simultaneously mobile and multihomed, i.e., have
 multiple mobile interfaces.  Furthermore, if the interfaces use
 different access technologies, it is fairly likely that one of the
 interfaces may appear stable (retain its current IP address) while
 some others may experience mobility (undergo IP address change).
 The 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.

4.2.8. 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 it 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.  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 will be in an UNVERIFIED state
 at the initiator.

Henderson, et al. Standards Track [Page 11] RFC 8047 HIP Multihoming February 2017

          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 R1.  These parameters MUST be protected by the I2
 signature.  Even if the I2 packet contains LOCATOR_SET parameters,
 the Responder MUST still send the R2 packet to the source address of
 the I2.  The new preferred locator, if set, SHOULD be identical to
 the I2 source address.  If the I2 packet contains LOCATOR_SET
 parameters, all new locators must undergo address verification as
 usual, and the ESP traffic that subsequently follows should use the
 preferred locator.
          Initiator                                Responder
                           I2 with LOCATOR_SET
                ----------------------------------->
                                                   (process normally)
                                           record additional addresses
                     R2 sent to source address of I2
                <-----------------------------------
 (process normally)
                 Figure 5: LOCATOR_SET Inclusion in I2
 The I1 and I2 may be arriving from different source addresses if the
 LOCATOR_SET 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 [RFC7401].  As a solution, the
 Responder's puzzle indexing mechanism must be flexible enough to
 accommodate the situation when R1 includes a LOCATOR_SET parameter.

Henderson, et al. Standards Track [Page 12] RFC 8047 HIP Multihoming February 2017

 Finally, the R2 may be used to carry the 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 opposed to R1 may have
 some advantages when a host prefers not to divulge additional
 locators until after the I2 is successfully processed.
 When the LOCATOR_SET parameter is sent in an UPDATE packet, the
 receiver will respond with 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.

4.2.9. Using LOCATOR_SETs across Addressing Realms

 It is possible for HIP associations to use these mechanisms to
 migrate their HIP associations and security associations from
 addresses in the IPv4 addressing realm to IPv6, or vice versa.  It
 may be possible for a state to arise in which both hosts are only
 using locators in different addressing realms, but in such a case,
 some type of mechanism for interworking between the different realms
 must be employed; such techniques are outside the scope of the
 present text.

4.3. 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 group the 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 look up the incoming SA.  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 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,

Henderson, et al. Standards Track [Page 13] RFC 8047 HIP Multihoming February 2017

 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.

5. Processing Rules

 Basic processing rules for the LOCATOR_SET parameter are specified in
 [RFC8046].  This document focuses on multihoming-specific rules.

5.1. Sending LOCATOR_SETs

 The decision of when to send a LOCATOR_SET, and which addresses to
 include, is a local policy issue.  [RFC8046] 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 [when it] 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.  If hosts are deployed in an operational environment in which
 HIP-aware NATs and firewalls (that may perform parameter inspection)
 exist, and different such devices may exist on different paths, hosts
 may take that knowledge into consideration about how addresses are
 grouped, and may send the same LOCATOR_SET in separate UPDATEs on the
 different paths.  However, more detailed guidelines about how to
 operate in the presence of such HIP-aware NATs and firewalls are a
 topic for further study.  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.  The 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.

Henderson, et al. Standards Track [Page 14] RFC 8047 HIP Multihoming February 2017

 [RFC8046] specifies that 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 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 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 as a Type "0" Locator.  The host sends this in an
     UPDATE message with the SEQ parameter, which is acknowledged by
     the peer.
 2.  The host MAY set up a new SA pair between this new address and 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.

Henderson, et al. Standards Track [Page 15] RFC 8047 HIP Multihoming February 2017

 The sending of multiple LOCATOR_SETs is 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.

5.2. Handling Received LOCATOR_SETs

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

Henderson, et al. Standards Track [Page 16] RFC 8047 HIP Multihoming February 2017

 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.
 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.  If there exist remaining
 addresses corresponding to the SPI that were NOT listed in the
 LOCATOR_SET parameter, the host sets the status of such addresses to
 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 an 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 a state of either 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 5.4.

Henderson, et al. Standards Track [Page 17] RFC 8047 HIP Multihoming February 2017

 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.

5.3. Verifying Address Reachability

 Address verification is defined in [RFC8046].
 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 [RFC8046].  Once address verification succeeds, the status of the
 new preferred locator changes to ACTIVE.

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 preferred 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.

Henderson, et al. Standards Track [Page 18] RFC 8047 HIP Multihoming February 2017

 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.

6. Security Considerations

 This document extends the scope of host mobility solutions defined in
 [RFC8046] to also include host multihoming, and as a result, many of
 the same security considerations for mobility also pertain to
 multihoming.  In particular, [RFC8046] describes how HIP host
 mobility is resistant to different types of impersonation attacks and
 denial-of-service (DoS) attacks.
 The security considerations for this document are similar to those of
 [RFC8046] because the strong authentication capabilities for mobility
 also carry over to end-host multihoming.  [RFC4218] provides a threat
 analysis for IPv6 multihoming, and the remainder of this section
 first describes how HIP host multihoming addresses those previously
 described threats, and then it discusses some additional security
 considerations.
 The high-level threats discussed in [RFC4218] involve redirection
 attacks for the purposes of packet recording, data manipulation, and
 availability.  There are a few types of attackers to consider:
 on-path attackers, off-path attackers, and malicious hosts.
 [RFC4218] also makes the comment that in identifier/locator split
 solutions such as HIP, application security mechanisms should be tied
 to the identifier, not the locator, and attacks on the identifier
 mechanism and on the mechanism binding locators to the identifier are
 of concern.  This document does not consider the former issue
 (application-layer security bindings) to be within scope.  The latter
 issue (locator bindings to identifier) is directly addressed by the
 cryptographic protections of the HIP protocol, in that locators
 associated to an identifier are listed in HIP packets that are signed
 using the identifier key.
 Section 3.1 of [RFC4218] lists several classes of security
 configurations in use in the Internet.  HIP maps to the fourth
 (strong identifier) and fifth ("leap-of-faith") categories, the

Henderson, et al. Standards Track [Page 19] RFC 8047 HIP Multihoming February 2017

 latter being associated with the optional opportunistic mode of HIP
 operation.  The remainder of Section 3 describes existing security
 problems in the Internet and comments that the goal of a multihoming
 solution is not to solve them specifically but rather not to make any
 of them worse.  HIP multihoming should not increase the severity of
 the identified risks.  One concern for both HIP mobility and
 multihoming is the susceptibility of the mechanisms to misuse
 flooding-based redirections due to a malicious host.  The mechanisms
 described in [RFC8046] for address verification are important in this
 regard.
 Regarding the new types of threats introduced by multihoming
 (Section 4 of [RFC4218]), HIP multihoming should not introduce new
 concerns.  Classic and premeditated redirection are prevented by the
 strong authentication in HIP messages.  Third-party DoS attacks are
 prevented by the address verification mechanism.  Replay attacks can
 be avoided via use of replay protection in ESP SAs.  In addition,
 accepting packets from unknown locators is protected by either the
 strong authentication in the HIP control packets or by the ESP-based
 encryption in use for data packets.
 The HIP mechanisms are designed to limit the ability to introduce DoS
 on the mechanisms themselves (Section 7 of [RFC4218]).  Care is taken
 in the HIP base exchange to avoid creating state or performing much
 work before hosts can authenticate one another.  A malicious host
 involved in HIP multihoming with another host might attempt to misuse
 the mechanisms for multihoming by, for instance, increasing the state
 required or inducing a resource limitation attack by sending too many
 candidate locators to the peer host.  Therefore, implementations
 supporting the multihoming extensions should consider avoiding
 accepting large numbers of peer locators and rate limiting any UPDATE
 messages being exchanged.
 The exposure of a host's IP addresses through HIP mobility and
 multihoming extensions may raise the following privacy concern.  The
 administrator of a host may be trying to hide its location in some
 context through the use of a VPN or other virtual interfaces.
 Similar privacy issues also arise in other frameworks such as WebRTC
 and are not specific to HIP.  Implementations SHOULD provide a
 mechanism to allow the host administrator to block the exposure of
 selected addresses or address ranges.
 Finally, some implementations of VPN tunneling have experienced
 instances of 'leakage' of flows that were intended to have been
 protected by a security tunnel but are instead sent in the clear,
 perhaps because some of the addresses used fall outside of the range
 of addresses configured for the tunnel in the security policy or
 association database.  Implementors are advised to take steps to

Henderson, et al. Standards Track [Page 20] RFC 8047 HIP Multihoming February 2017

 ensure that the usage of multiple addresses between hosts does not
 cause accidental leakage of some data session traffic outside of the
 ESP-protected envelope.

7. References

7.1. Normative References

 [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>.
 [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
            "Default Address Selection for Internet Protocol Version 6
            (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
            <http://www.rfc-editor.org/info/rfc6724>.
 [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>.
 [RFC8046]  Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
            with the Host Identity Protocol", RFC 8046,
            DOI 10.17487/RFC8046, February 2017,
            <http://www.rfc-editor.org/info/rfc8046>.

7.2. Informative References

 [RFC4218]  Nordmark, E. and T. Li, "Threats Relating to IPv6
            Multihoming Solutions", RFC 4218, DOI 10.17487/RFC4218,
            October 2005, <http://www.rfc-editor.org/info/rfc4218>.
 [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
            RFC 4303, DOI 10.17487/RFC4303, December 2005,
            <http://www.rfc-editor.org/info/rfc4303>.
 [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>.

Henderson, et al. Standards Track [Page 21] RFC 8047 HIP Multihoming February 2017

Acknowledgments

 This document contains content that was originally included in RFC
 5206.  Pekka Nikander and Jari Arkko originated RFC 5206, and
 Christian Vogt and Thomas Henderson (editor) later joined as
 coauthors.  Also in RFC 5206, Greg Perkins contributed the initial
 draft of the security section, and Petri Jokela was a coauthor of the
 initial individual submission.
 The authors thank Miika Komu, Mika Kousa, Jeff Ahrenholz, and Jan
 Melen for many improvements to the document.  Concepts from a paper
 on host multihoming across address families, by Samu Varjonen, Miika
 Komu, and Andrei Gurtov, contributed to this revised specification.

Authors' Addresses

 Thomas R. Henderson (editor)
 University of Washington
 Campus Box 352500
 Seattle, WA
 United States of America
 Email: tomhend@u.washington.edu
 Christian Vogt
 Independent
 3473 North First Street
 San Jose, CA  95134
 United States of America
 Email: mail@christianvogt.net
 Jari Arkko
 Ericsson
 Jorvas,  FIN-02420
 Finland
 Phone: +358 40 5079256
 Email: jari.arkko@piuha.net

Henderson, et al. Standards Track [Page 22]

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