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rfc:rfc8046

Internet Engineering Task Force (IETF) T. Henderson, Ed. Request for Comments: 8046 University of Washington Obsoletes: 5206 C. Vogt Category: Standards Track Independent ISSN: 2070-1721 J. Arkko

                                                              Ericsson
                                                         February 2017
           Host Mobility with the Host Identity Protocol

Abstract

 This document defines a mobility extension to the Host Identity
 Protocol (HIP).  Specifically, this document defines a "LOCATOR_SET"
 parameter for HIP messages that allows for a HIP host to notify peers
 about alternate addresses at which it may be reached.  This document
 also defines how the parameter can be used to preserve communications
 across a change to the IP address used by one or both peer hosts.
 The same LOCATOR_SET parameter can also be used to support end-host
 multihoming (as specified in RFC 8047).  This document obsoletes RFC
 5206.

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/rfc8046.

Henderson, et al. Standards Track [Page 1] RFC 8046 HIP Host Mobility February 2017

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 2] RFC 8046 HIP Host Mobility February 2017

Table of Contents

 1.  Introduction and Scope  . . . . . . . . . . . . . . . . . . .   4
 2.  Terminology and Conventions . . . . . . . . . . . . . . . . .   4
 3.  Protocol Model  . . . . . . . . . . . . . . . . . . . . . . .   7
   3.1.  Operating Environment . . . . . . . . . . . . . . . . . .   7
     3.1.1.  Locator . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.2.  Mobility Overview . . . . . . . . . . . . . . . . . .   9
   3.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .  10
     3.2.1.  Mobility with a Single SA Pair (No Rekeying)  . . . .  10
     3.2.2.  Mobility with a Single SA Pair (Mobile-Initiated
             Rekey)  . . . . . . . . . . . . . . . . . . . . . . .  12
     3.2.3.  Mobility Messaging through the Rendezvous Server  . .  13
     3.2.4.  Network Renumbering . . . . . . . . . . . . . . . . .  14
   3.3.  Other Considerations  . . . . . . . . . . . . . . . . . .  14
     3.3.1.  Address Verification  . . . . . . . . . . . . . . . .  14
     3.3.2.  Credit-Based Authorization  . . . . . . . . . . . . .  15
     3.3.3.  Preferred Locator . . . . . . . . . . . . . . . . . .  16
 4.  LOCATOR_SET Parameter Format  . . . . . . . . . . . . . . . .  16
   4.1.  Traffic Type and Preferred Locator  . . . . . . . . . . .  18
   4.2.  Locator Type and Locator  . . . . . . . . . . . . . . . .  19
   4.3.  UPDATE Packet with Included LOCATOR_SET . . . . . . . . .  19
 5.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .  19
   5.1.  Locator Data Structure and Status . . . . . . . . . . . .  19
   5.2.  Sending the LOCATOR_SET . . . . . . . . . . . . . . . . .  21
   5.3.  Handling Received LOCATOR_SETs  . . . . . . . . . . . . .  22
   5.4.  Verifying Address Reachability  . . . . . . . . . . . . .  24
   5.5.  Changing the Preferred Locator  . . . . . . . . . . . . .  26
   5.6.  Credit-Based Authorization  . . . . . . . . . . . . . . .  26
     5.6.1.  Handling Payload Packets  . . . . . . . . . . . . . .  27
     5.6.2.  Credit Aging  . . . . . . . . . . . . . . . . . . . .  29
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   6.1.  Impersonation Attacks . . . . . . . . . . . . . . . . . .  30
   6.2.  Denial-of-Service Attacks . . . . . . . . . . . . . . . .  31
     6.2.1.  Flooding Attacks  . . . . . . . . . . . . . . . . . .  31
     6.2.2.  Memory/Computational-Exhaustion DoS Attacks . . . . .  32
   6.3.  Mixed Deployment Environment  . . . . . . . . . . . . . .  32
   6.4.  Privacy Concerns  . . . . . . . . . . . . . . . . . . . .  33
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
 8.  Differences from RFC 5206 . . . . . . . . . . . . . . . . . .  33
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  35
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  35
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  36
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

Henderson, et al. Standards Track [Page 3] RFC 8046 HIP Host Mobility 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 needs to 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.  There are
 potentially many variations of mobility and multihoming possible.
 The scope of this document encompasses messaging and elements of
 procedure for basic network-level host mobility, leaving more
 complicated mobility scenarios, multihoming, and other variations for
 further study.  More specifically, the following are in scope:
    This document defines a LOCATOR_SET parameter for use in HIP
    messages.  The LOCATOR_SET parameter allows a HIP host to notify a
    peer about alternate locators at which it is reachable.  The
    locators may be merely IP addresses, or they may have additional
    multiplexing and demultiplexing context to aid with the packet
    handling in the lower layers.  For instance, an IP address may
    need to be paired with an ESP Security Parameter Index (SPI) so
    that packets are sent on the correct SA for a given address.
    This document also specifies the messaging and elements of
    procedure for end-host mobility of a HIP host.  In particular,
    message flows to enable successful host mobility, including
    address verification methods, are defined herein.
    The HIP rendezvous server (RVS) [RFC8004] can be used to manage
    simultaneous mobility of both hosts, initial reachability of a
    mobile host, location privacy, and some modes of NAT traversal.
    Use of the HIP RVS to manage the simultaneous mobility of both
    hosts is specified herein.

Henderson, et al. Standards Track [Page 4] RFC 8046 HIP Host Mobility February 2017

 The following topics are out of scope:
    While the same LOCATOR_SET parameter supports host multihoming
    (simultaneous use of a number of addresses), procedures for host
    multihoming are out of scope and are specified in [RFC8047].
    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.
    We do not consider localized mobility management extensions (i.e.,
    mobility management techniques that do not involve directly
    signaling the correspondent node); this document is concerned with
    end-to-end mobility.
    Finally, making underlying IP mobility transparent to the
    transport layer has implications on the proper response of
    transport congestion control, path MTU selection, and Quality of
    Service (QoS).  Transport-layer mobility triggers, and the proper
    transport response to a HIP mobility or multihoming address
    change, are outside the scope of this document.
 The main sections of this document are organized as follows.
 Section 3 provides a summary overview of operations, scenarios, and
 other considerations.  Section 4 specifies the messaging parameter
 syntax.  Section 5 specifies the processing rules for messages.
 Section 6 describes security considerations for this specification.

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 [RFC2119].
 LOCATOR_SET.  A HIP parameter containing zero or more Locator fields.
 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.
 Locator.  When capitalized in the middle of a sentence, this term
    refers to the encoding of a locator within the LOCATOR_SET
    parameter (i.e., the 'Locator' field of the parameter).

Henderson, et al. Standards Track [Page 5] RFC 8046 HIP Host Mobility February 2017

 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.  Certain locators are labeled as preferred when a host
    advertises its locator set to its peer.  By default, the locators
    used in the HIP base exchange are the preferred locators.  The use
    of preferred locators, including the scenario where multiple
    address scopes and families may be in use, is defined more in
    [RFC8047] than in this document.
 Credit-Based Authorization (CBA).  A mechanism allowing a host to
    send a certain amount of data to a peer's newly announced locator
    before the result of mandatory address verification is known.

Henderson, et al. Standards Track [Page 6] RFC 8046 HIP Host Mobility February 2017

3. Protocol Model

 This section is an overview; a more detailed specification follows
 this section.

3.1. Operating Environment

 HIP [RFC7401] is a key establishment and parameter negotiation
 protocol.  Its primary applications are for authenticating host
 messages based on host identities and establishing SAs for the ESP
 transport format [RFC7402] 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 an
 extension to HIP to enable end-host mobility.  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 (HI).  This document specifies
 the format of this new addressing (LOCATOR_SET) parameter, the
 procedures for sending and processing this parameter to enable basic
 host mobility, and procedures for a concurrent address verification
 mechanism.

Henderson, et al. Standards Track [Page 7] RFC 8046 HIP Host Mobility February 2017

  1. ——–

| TCP | (sockets bound to HITs)

  1. ——–

|

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

| ———

    |         |
  ----    ---------
 | MH |-> | HIP   |  {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI}
  ----    ---------
             |
          ---------
          |  IP   |
          ---------
     Figure 2: Architecture for HIP Host Mobility and Multihoming
 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" corresponds to the function that manages the
 bindings at the ESP and HIP sublayers for mobility (specified in this
 document) and multihoming (specified in [RFC8047]).
 Consider first the case in which there is no mobility or multihoming,
 as specified in the base protocol specification [RFC7401].  The HIP
 base exchange establishes the HITs in use between the hosts, the SPIs
 to use for ESP, and the IP addresses (used in both the HIP signaling
 packets and ESP data packets).  Note that there can only be one such
 set of bindings in the outbound direction for any given packet, and
 the only fields used for the binding at the HIP layer are the fields
 exposed by ESP (the SPI and HITs).  For the inbound direction, the
 SPI is all that is required to find the right host context.  ESP
 rekeying events change the mapping between the HIT pair and SPI, but
 do not change the IP addresses.
 Consider next a mobility event, in which a host moves to another IP
 address.  Two things need to occur in this case.  First, the peer
 needs to be notified of the address change using a HIP UPDATE
 message.  Second, each host needs to change its local bindings at the
 HIP sublayer (new IP addresses).  It may be that both the SPIs and IP
 addresses are changed simultaneously in a single UPDATE; the protocol
 described herein supports this.  Although internal notification of
 transport-layer protocols regarding the path change (e.g., to reset

Henderson, et al. Standards Track [Page 8] RFC 8046 HIP Host Mobility February 2017

 congestion control variables) may be desired, this specification does
 not address such internal notification.  In addition, elements of
 procedure for traversing network address translators (NATs) and
 firewalls, including NATs and firewalls that may understand HIP, may
 complicate the above basic scenario and are not covered by this
 document.

3.1.1. Locator

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

3.1.2. Mobility Overview

 When a host moves to another address, it notifies its peer of the new
 address by sending a HIP UPDATE packet containing a single
 LOCATOR_SET parameter and a single ESP_INFO parameter.  This UPDATE
 packet is acknowledged by the peer.  For reliability in the presence
 of packet loss, the UPDATE packet is retransmitted as defined in the
 HIP specification [RFC7401].  The peer can authenticate the contents
 of the UPDATE packet based on the signature and keyed hash of the
 packet.
 When using the ESP transport format [RFC7402], the host may, at the
 same time, decide to rekey its security association and possibly
 generate a new Diffie-Hellman key; all of these actions are triggered
 by including additional parameters in the UPDATE packet, as defined
 in the base protocol specification [RFC7401] and ESP extension
 [RFC7402].
 When using ESP (and possibly other transport modes in the future),
 the host is able to receive packets that are protected using a HIP-
 created ESP SA from any address.  Thus, a host can change its IP
 address and continue to send packets to its peers without necessarily
 rekeying.  However, the peers are not able to send packets to these
 new addresses before they can reliably and securely update the set of
 addresses that they associate with the sending host.  Furthermore,

Henderson, et al. Standards Track [Page 9] RFC 8046 HIP Host Mobility February 2017

 mobility may change the path characteristics in such a manner that
 reordering occurs and packets fall outside the ESP anti-replay window
 for the SA, thereby requiring rekeying.

3.2. Protocol Overview

 In this section, we briefly introduce a number of usage scenarios for
 HIP host mobility.  These scenarios assume that HIP is being used
 with the ESP transform [RFC7402], although other scenarios may be
 defined in the future.  To understand these usage scenarios, the
 reader should be at least minimally familiar with the HIP
 specification [RFC7401] and with the use of ESP with HIP [RFC7402].
 According to these specifications, the data traffic in a HIP session
 is protected with ESP, and the ESP SPI acts as an index to the right
 host-to-host context.  More specification details are found later in
 Sections 4 and 5.
 The scenarios below assume that the two hosts have completed a single
 HIP base exchange with each other.  Therefore, both of the hosts 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
 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.  In support of mobility, the LOCATOR_SET
 parameter is carried in UPDATE packets.
 The scenarios below at times describe addresses as being in either an
 ACTIVE, UNVERIFIED, or DEPRECATED state.  From the perspective of a
 host, newly learned addresses of the peer need to be verified before
 put into active service, and addresses removed by the peer are put
 into a deprecated state.  Under limited conditions described below
 (Section 5.6), an UNVERIFIED address may be used.  The addressing
 states are defined more formally in Section 5.1.
 Hosts that use link-local addresses as source addresses in their HIP
 handshakes may not be reachable by a mobile peer.  Such hosts SHOULD
 provide a globally routable address either in the initial handshake
 or via the LOCATOR_SET parameter.

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

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

Henderson, et al. Standards Track [Page 10] RFC 8046 HIP Host Mobility February 2017

 its peers about the new IP address.  This first example considers the
 case in which the mobile host has only one interface, one IP address
 in use within the HIP session, a single pair of SAs (one inbound, one
 outbound), and no rekeying occurring on the SAs.  We also assume that
 the new IP addresses are within the same address family (IPv4 or
 IPv6) as the previous address.  This is the simplest scenario,
 depicted in Figure 3.  Note that the conventions for message
 parameter notations in figures (use of parentheses and brackets) is
 defined in Section 2.2 of [RFC7401].
   Mobile Host                         Peer Host
           UPDATE(ESP_INFO, LOCATOR_SET, SEQ)
      ----------------------------------->
           UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST)
      <-----------------------------------
           UPDATE(ACK, ECHO_RESPONSE)
      ----------------------------------->
      Figure 3: Readdress without Rekeying but with Address Check
 The steps of the packet processing are as follows:
 1.  The mobile host may be disconnected from the peer host for a
     brief period of time while it switches from one IP address to
     another; this case is sometimes referred to in the literature as
     a "break-before-make" case.  The host may also obtain its new IP
     address before losing the old one ("make-before-break" case).  In
     either case, upon obtaining a new IP address, the mobile host
     sends a LOCATOR_SET parameter to the peer host in an UPDATE
     message.  The UPDATE message also contains an ESP_INFO parameter
     containing the values of the old and new SPIs for a security
     association.  In this case, both the OLD SPI and NEW SPI
     parameters are set to the value of the preexisting incoming SPI;
     this ESP_INFO does not trigger a rekeying event but is instead
     included for possible parameter-inspecting firewalls on the path
     ([RFC5207] specifies some such firewall scenarios in which the
     HIP-aware firewall may want to associate ESP flows to host
     identities).  The LOCATOR_SET parameter contains the new IP
     address (embedded in a Locator Type of "1", defined below) and a
     lifetime associated with the locator.  The mobile host waits for
     this UPDATE to be acknowledged, and retransmits if necessary, as
     specified in the base specification [RFC7401].

Henderson, et al. Standards Track [Page 11] RFC 8046 HIP Host Mobility February 2017

 2.  The peer host receives the UPDATE, validates it, and updates any
     local bindings between the HIP association and the mobile host's
     destination address.  The peer host MUST perform an address
     verification by placing a nonce in the ECHO_REQUEST parameter of
     the UPDATE message sent back to the mobile host.  It also
     includes an ESP_INFO parameter with both the OLD SPI and NEW SPI
     parameters set to the value of the preexisting incoming SPI and
     sends this UPDATE (with piggybacked acknowledgment) to the mobile
     host at its new address.  This UPDATE also acknowledges the
     mobile host's UPDATE that triggered the exchange.  The peer host
     waits for its UPDATE to be acknowledged, and retransmits if
     necessary, as specified in the base specification [RFC7401].  The
     peer MAY use the new address immediately, but it MUST limit the
     amount of data it sends to the address until address verification
     completes.
 3.  The mobile host completes the readdress by processing the UPDATE
     ACK and echoing the nonce in an ECHO_RESPONSE, containing the ACK
     of the peer's UPDATE.  This UPDATE is not protected by a
     retransmission timer because it does not contain a SEQ parameter
     requesting acknowledgment.  Once the peer host receives this
     ECHO_RESPONSE, it considers the new address to be verified and
     can put the address into full use.
 While the peer host is verifying the new address, the new address is
 marked as UNVERIFIED (in the interim), and the old address is
 DEPRECATED.  Once the peer host has received a correct reply to its
 UPDATE challenge, it marks the new address as ACTIVE and removes the
 old address.

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

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

Henderson, et al. Standards Track [Page 12] RFC 8046 HIP Host Mobility February 2017

   Mobile 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 4: Readdress with Mobile-Initiated Rekey

3.2.3. Mobility Messaging through the Rendezvous Server

 Section 6.11 of [RFC7401] specifies procedures for sending HIP UPDATE
 packets.  The UPDATE packets are protected by a timer subject to
 exponential backoff and resent UPDATE_RETRY_MAX times.  It may be,
 however, that the peer is itself in the process of moving when the
 local host is trying to update the IP address bindings of the HIP
 association.  This is sometimes called the "double-jump" mobility
 problem; each host's UPDATE packets are simultaneously sent to a
 stale address of the peer, and the hosts are no longer reachable from
 one another.
 The HIP Rendezvous Extension [RFC8004] specifies a rendezvous service
 that permits the I1 packet from the base exchange to be relayed from
 a stable or well-known public IP address location to the current IP
 address of the host.  It is possible to support double-jump mobility
 with this rendezvous service if the following extensions to the
 specifications of [RFC8004] and [RFC7401] are followed.
 1.  The mobile host sending an UPDATE to the peer, and not receiving
     an ACK, MAY resend the UPDATE to an RVS of the peer, if such a
     server is known.  The host MAY try the RVS of the peer up to
     UPDATE_RETRY_MAX times as specified in [RFC7401].  The host MAY
     try to use the peer's RVS before it has tried UPDATE_RETRY_MAX
     times to the last working address (i.e., the RVS MAY be tried in
     parallel with retries to the last working address).  The
     aggressiveness of a host replicating its UPDATEs to multiple
     destinations, to try candidates in parallel instead of serially,
     is a policy choice outside of this specification.
 2.  An RVS supporting the UPDATE forwarding extensions specified
     herein MUST modify the UPDATE in the same manner as it modifies
     the I1 packet before forwarding.  Specifically, it MUST rewrite
     the IP header source and destination addresses, recompute the IP
     header checksum, and include the FROM and RVS_HMAC parameters.

Henderson, et al. Standards Track [Page 13] RFC 8046 HIP Host Mobility February 2017

 3.  A host receiving an UPDATE packet MUST be prepared to process the
     FROM and RVS_HMAC parameters and MUST include a VIA_RVS parameter
     in the UPDATE reply that contains the ACK of the UPDATE SEQ.
 4.  An Initiator receiving a VIA_RVS in the UPDATE reply should
     initiate address reachability tests (described later in this
     document) towards the end host's address and not towards the
     address included in the VIA_RVS.
 This scenario requires that hosts using RVSs also take steps to
 update their current address bindings with their RVS upon a mobility
 event.  [RFC8004] does not specify how to update the RVS with a
 client host's new address.  Section 3.2 of [RFC8003] describes how a
 host may send a REG_REQUEST in either an I2 packet (if there is no
 active association) or an UPDATE packet (if such association exists).
 According to procedures described in [RFC8003], if a mobile host has
 an active registration, it may use mobility updates specified herein,
 within the context of that association, to readdress the association.

3.2.4. Network Renumbering

 It is expected that IPv6 networks will be renumbered much more often
 than most IPv4 networks.  From an end-host point of view, network
 renumbering is similar to mobility, and procedures described herein
 also apply to notify a peer of a changed address.

3.3. Other Considerations

3.3.1. Address Verification

 When a HIP host receives a set of locators from another HIP host in a
 LOCATOR_SET, it does not necessarily know whether the other host is
 actually reachable at the claimed addresses.  In fact, a malicious
 peer host may be intentionally giving bogus addresses in order to
 cause a packet flood towards the target addresses [RFC4225].
 Therefore, the HIP host needs to first check that the peer is
 reachable at the new address.
 Address verification is implemented by the challenger sending some
 piece of unguessable information to the new address and waiting for
 some acknowledgment from the Responder that indicates reception of
 the information at the new address.  This may include the exchange of
 a nonce or the generation of a new SPI and observation of data
 arriving on the new SPI.  More details are found in Section 5.4 of
 this document.

Henderson, et al. Standards Track [Page 14] RFC 8046 HIP Host Mobility February 2017

 An additional potential benefit of performing address verification is
 to allow NATs and firewalls in the network along the new path to
 obtain the peer host's inbound SPI.

3.3.2. Credit-Based Authorization

 CBA allows a host to securely use a new locator even though the
 peer's reachability at the address embedded in the locator has not
 yet been verified.  This is accomplished based on the following three
 hypotheses:
 1.  A flooding attacker typically seeks to somehow multiply the
     packets it generates for the purpose of its attack because
     bandwidth is an ample resource for many victims.
 2.  An attacker can often cause unamplified flooding by sending
     packets to its victim, either by directly addressing the victim
     in the packets or by guiding the packets along a specific path by
     means of an IPv6 Routing header, if Routing headers are not
     filtered by firewalls.
 3.  Consequently, the additional effort required to set up a
     redirection-based flooding attack (without CBA and return
     routability checks) would pay off for the attacker only if
     amplification could be obtained this way.
 On this basis, rather than eliminating malicious packet redirection
 in the first place, CBA prevents amplifications.  This is
 accomplished by limiting the data a host can send to an unverified
 address of a peer by the data recently received from that peer.
 Redirection-based flooding attacks thus become less attractive than,
 for example, pure direct flooding, where the attacker itself sends
 bogus packets to the victim.
 Figure 5 illustrates CBA: Host B measures the amount of data recently
 received from peer A and, when A readdresses, sends packets to A's
 new, unverified address as long as the sum of the packet sizes does
 not exceed the measured, received data volume.  When insufficient
 credit is left, B stops sending further packets to A until A's
 address becomes ACTIVE.  The address changes may be due to mobility,
 multihoming, or any other reason.  Not shown in Figure 5 are the
 results of credit aging (Section 5.6.2), a mechanism used to dampen
 possible time-shifting attacks.

Henderson, et al. Standards Track [Page 15] RFC 8046 HIP Host Mobility February 2017

         +-------+                        +-------+
         |   A   |                        |   B   |
         +-------+                        +-------+
             |                                |
     address |------------------------------->| credit += size(packet)
      ACTIVE |                                |
             |------------------------------->| credit += size(packet)
             |<-------------------------------| do not change credit
             |                                |
             + address change                 |
             + address verification starts    |
     address |<-------------------------------| credit -= size(packet)
  UNVERIFIED |------------------------------->| credit += size(packet)
             |<-------------------------------| credit -= size(packet)
             |                                |
             |<-------------------------------| credit -= size(packet)
             |                                X credit < size(packet)
             |                                | => do not send packet!
             + address verification concludes |
     address |                                |
      ACTIVE |<-------------------------------| do not change credit
             |                                |
                    Figure 5: Readdressing Scenario
 This document does not specify how to set the credit limit value, but
 the goal is to allow data transfers to proceed without much
 interruption while the new address is verified.  A simple heuristic
 to accomplish this, if the sender knows roughly its round-trip time
 (RTT) and current sending rate to the host, is to allow enough credit
 to support maintaining the sending rate for a duration corresponding
 to two or three RTTs.

3.3.3. Preferred Locator

 When a host has multiple locators, the peer host needs to decide
 which to use for outbound packets.  It may be that a host would
 prefer to receive data on a particular inbound interface.  HIP allows
 a particular locator to be designated as a preferred locator and
 communicated to the peer (see Section 4).

4. LOCATOR_SET Parameter Format

 The LOCATOR_SET parameter has a type number value that is considered
 to be a "critical parameter" as per the definition in [RFC7401]; such
 parameter types MUST be recognized and processed by the recipient.
 The parameter consists of the standard HIP parameter Type and Length
 fields, plus zero or more Locator sub-parameters.  Each Locator sub-

Henderson, et al. Standards Track [Page 16] RFC 8046 HIP Host Mobility February 2017

 parameter contains a Traffic Type, Locator Type, Locator Length,
 preferred locator bit ("P" bit), Locator Lifetime, and a Locator
 encoding.  A LOCATOR_SET containing zero Locator fields is permitted
 but has the effect of deprecating all addresses.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Traffic Type   | Locator Type | Locator Length | Reserved   |P|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Locator Lifetime                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Locator                            |
     |                                                               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Traffic Type   | Locator Type | Locator Length | Reserved   |P|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Locator Lifetime                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Locator                            |
     |                                                               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 6: LOCATOR_SET Parameter Format
 Type:  193
 Length:  Length in octets, excluding Type and Length fields, and
    excluding padding.
 Traffic Type:  Defines whether the locator pertains to HIP signaling,
    user data, or both.
 Locator Type:  Defines the semantics of the Locator field.
 Locator Length:  Defines the length of the Locator field, in units of
    4-byte words (Locators up to a maximum of 4*255 octets are
    supported).

Henderson, et al. Standards Track [Page 17] RFC 8046 HIP Host Mobility February 2017

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

4.1. Traffic Type and Preferred Locator

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

Henderson, et al. Standards Track [Page 18] RFC 8046 HIP Host Mobility February 2017

4.2. Locator Type and Locator

 The following Locator Type values are defined, along with the
 associated semantics of the Locator field:
 0:  An IPv6 address or an IPv4-in-IPv6 format IPv4 address [RFC4291]
     (128 bits long).  This Locator Type is defined primarily for
     non-ESP-based usage.
 1:  The concatenation of an ESP SPI (first 32 bits) followed by an
     IPv6 address or an IPv4-in-IPv6 format IPv4 address (an
     additional 128 bits).  This IP address is defined primarily for
     ESP-based usage.

4.3. UPDATE Packet with Included LOCATOR_SET

 A number of combinations of parameters in an UPDATE packet are
 possible (e.g., see Section 3.2).  In this document, procedures are
 defined only for the case in which one LOCATOR_SET and one ESP_INFO
 parameter are used in any HIP packet.  Any UPDATE packet that
 includes a LOCATOR_SET parameter SHOULD include both an HMAC and a
 HIP_SIGNATURE parameter.
 The UPDATE MAY also include a HOST_ID parameter (which may be useful
 for HIP-aware firewalls inspecting the HIP messages for the first
 time).  If the UPDATE includes the HOST_ID parameter, the receiving
 host MUST verify that the HOST_ID corresponds to the HOST_ID that was
 used to establish the HIP association, and the HIP_SIGNATURE MUST
 verify with the public key associated with this HOST_ID parameter.
 The relationship between the announced Locators and any ESP_INFO
 parameters present in the packet is defined in Section 5.2.  This
 document does not support any elements of procedure for sending more
 than one LOCATOR_SET or ESP_INFO parameter in a single UPDATE.

5. Processing Rules

 This section describes rules for sending and receiving the
 LOCATOR_SET parameter, testing address reachability, and using CBA on
 UNVERIFIED locators.

5.1. Locator Data Structure and Status

 Each locator announced in a LOCATOR_SET parameter is represented by a
 piece of state that contains the following data:
 o  the actual bit pattern representing the locator,

Henderson, et al. Standards Track [Page 19] RFC 8046 HIP Host Mobility February 2017

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

Henderson, et al. Standards Track [Page 20] RFC 8046 HIP Host Mobility February 2017

 In addition to state maintained about status and remaining lifetime
 for each locator learned from the peer, an implementation would
 typically maintain similar state about its own locators that have
 been offered to the peer.
 A locator lifetime that is unbounded (does not expire) can be
 signified by setting the value of the lifetime field to the maximum
 (unsigned) value.
 Finally, the locators used to establish the HIP association are by
 default assumed to be the initial preferred locators in ACTIVE state,
 with an unbounded lifetime.

5.2. Sending the LOCATOR_SET

 The decision of when to send the LOCATOR_SET is a local policy issue.
 However, it is RECOMMENDED that a host send a LOCATOR_SET whenever it
 recognizes a change of its IP addresses in use on an active HIP
 association and assumes that the change is going to last at least for
 a few seconds.  Rapidly sending LOCATOR_SETs that force the peer to
 change the preferred address SHOULD be avoided.
 The sending of a new LOCATOR_SET parameter replaces the locator
 information from any previously sent LOCATOR_SET parameter;
 therefore, if a host sends a new LOCATOR_SET parameter, it needs to
 continue to include all active locators.  Hosts MUST NOT announce
 broadcast or multicast addresses in LOCATOR_SETs.
 We now describe a few cases introduced in Section 3.2.  We assume
 that the Traffic Type for each locator is set to "0" (other values
 for Traffic Type may be specified in documents that separate the HIP
 control plane from data-plane traffic).  Other mobility cases are
 possible but are left for further study.
 1.  Host mobility with no multihoming and no rekeying.  The mobile
     host creates a single UPDATE containing a single ESP_INFO with a
     single LOCATOR_SET parameter.  The ESP_INFO contains the current
     value of the SPI in both the OLD SPI and NEW SPI fields.  The
     LOCATOR_SET contains a single Locator with a Locator Type of "1";
     the SPI MUST match that of the ESP_INFO.  The preferred bit
     SHOULD be set and the "Locator Lifetime" is set according to
     local policy.  The UPDATE also contains a SEQ parameter as usual.
     This packet is retransmitted as defined in the HIP specification
     [RFC7401].  The UPDATE should be sent to the peer's preferred IP
     address with an IP source address corresponding to the address in
     the LOCATOR_SET parameter.

Henderson, et al. Standards Track [Page 21] RFC 8046 HIP Host Mobility February 2017

 2.  Host mobility with no multihoming but with rekeying.  The mobile
     host creates a single UPDATE containing a single ESP_INFO with a
     single LOCATOR_SET parameter (with a single address).  The
     ESP_INFO contains the current value of the SPI in the OLD SPI,
     the new value of the SPI in the NEW SPI, and a KEYMAT Index as
     selected by local policy.  Optionally, the host may choose to
     initiate a Diffie-Hellman rekey by including a DIFFIE_HELLMAN
     parameter.  The LOCATOR_SET contains a single Locator with a
     Locator Type of "1"; the SPI MUST match that of the NEW SPI in
     the ESP_INFO.  Otherwise, the steps are identical to the case in
     which no rekeying is initiated.

5.3. Handling Received LOCATOR_SETs

 A host SHOULD be prepared to receive a single LOCATOR_SET parameter
 in a HIP UPDATE packet.  Reception of multiple LOCATOR_SET parameters
 in a single packet, or in HIP packets other than UPDATE, is outside
 of the scope of this specification.
 Because a host sending the LOCATOR_SET may send the same parameter in
 different UPDATE messages to different destination addresses,
 including possibly the RVS of the host, the host receiving the
 LOCATOR_SET MUST be prepared to handle the possibility of duplicate
 LOCATOR_SETs sent to more than one of the host's addresses.  As a
 result, the host MUST detect and avoid reprocessing a LOCATOR_SET
 parameter that is redundant with a LOCATOR_SET parameter that has
 been recently received and processed.
 This document describes sending both ESP_INFO and LOCATOR_SET
 parameters in an UPDATE.  The ESP_INFO parameter is included when
 there is a need to rekey or key a new SPI, and is otherwise included
 for the possible benefit of HIP-aware NATs and firewalls.  The
 LOCATOR_SET parameter contains a complete listing 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 5.2.
 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 Locator Type of "1" will only contain a reference to the new SPI.

Henderson, et al. Standards Track [Page 22] RFC 8046 HIP Host Mobility February 2017

 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 HIP-aware NATs and
 firewalls.  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 HIP-aware NATs and firewalls) and no rekeying is
     necessary.
 2.  (rekeying) If the OLD SPI indicates an existing SPI and the NEW
     SPI is a different non-zero value, the existing SA is being
     rekeyed and the host follows HIP ESP rekeying procedures by
     creating a new outbound SA with an SPI corresponding to the NEW
     SPI, with no addresses bound to this SPI.  Note that locators in
     the LOCATOR_SET parameter will reference this new SPI instead of
     the old SPI.
 3.  (new SA) If the OLD SPI value is zero and the NEW SPI is a new
     non-zero value, then a new SA is being requested by the peer.
     This case is also treated like a rekeying event; the receiving
     host MUST create a new SA and respond with an UPDATE ACK.
 4.  (deprecating the SA) If the OLD SPI indicates an existing SPI and
     the NEW SPI is zero, the SA is being deprecated and all locators
     uniquely bound to the SPI are put into the DEPRECATED state.
 If none of the above cases apply, a protocol error has occurred and
 the processing of the UPDATE is stopped.
 Next, the locators in the LOCATOR_SET parameter are processed.  For
 each locator listed in the LOCATOR_SET parameter, check that the
 address therein is a legal unicast or anycast address.  That is, the
 address MUST NOT be a broadcast or multicast address.  Note that some
 implementations MAY accept addresses that indicate the local host,
 since it may be allowed that the host runs HIP with itself.
 The below assumes that all Locators are of Type "1" with a Traffic
 Type of "0"; other cases are for further study.
 For each Type "1" address listed in the LOCATOR_SET parameter, the
 host checks whether the address is already bound to the SPI
 indicated.  If the address is already bound, its lifetime is updated.
 If the status of the address is DEPRECATED, the status is changed to
 UNVERIFIED.  If the address is not already bound, the address is

Henderson, et al. Standards Track [Page 23] RFC 8046 HIP Host Mobility February 2017

 added, and its status is set to UNVERIFIED.  Mark all addresses
 corresponding to the SPI that were NOT listed in the LOCATOR_SET
 parameter as DEPRECATED.
 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
 verify reachability of the associated address, and only then change
 the preferred locator; see Section 5.5.
 If a host receives a locator with an unsupported Locator Type, and
 when such a locator is also declared to be the preferred locator for
 the peer, the host SHOULD send a NOTIFY error with a Notify Message
 Type of LOCATOR_TYPE_UNSUPPORTED, with the Notification Data field
 containing the locator(s) that the receiver failed to process.
 Otherwise, a host MAY send a NOTIFY error if a (non-preferred)
 locator with an unsupported Locator Type is received in a LOCATOR_SET
 parameter.
 A host MAY add the source IP address of a received HIP packet as a
 candidate locator for the peer even if it is not listed in the peer's
 LOCATOR_SET, but it SHOULD prefer locators explicitly listed in the
 LOCATOR_SET.

5.4. Verifying Address Reachability

 A host MUST verify the reachability of an UNVERIFIED address.  The
 status of a newly learned address MUST initially be set to UNVERIFIED
 unless the new address is advertised in an R1 packet as a new
 preferred locator.  A host MAY also want to verify the reachability
 of an ACTIVE address again after some time, in which case it would
 set the status of the address to UNVERIFIED and reinitiate address
 verification.  A typical verification that is protected by
 retransmission timers is to include an ECHO REQUEST within an UPDATE
 sent to the new address.
 A host typically starts the address-verification procedure by sending
 a nonce to the new address.  A host MAY choose from different message
 exchanges or different nonce values so long as it establishes that
 the peer has received and replied to the nonce at the new address.

Henderson, et al. Standards Track [Page 24] RFC 8046 HIP Host Mobility February 2017

 For example, when the host is changing its SPI and sending an
 ESP_INFO to the peer, the NEW SPI value SHOULD be random and the
 random value MAY be copied into an ECHO_REQUEST sent in the rekeying
 UPDATE.  However, if the host is not changing its SPI, it MAY still
 use the ECHO_REQUEST parameter for verification but with some other
 random value.  A host MAY also use other message exchanges as
 confirmation of the address reachability.
 In some cases, it MAY be sufficient to use the arrival of data on a
 newly advertised SA as implicit address reachability verification as
 depicted in Figure 7, instead of waiting for the confirmation via a
 HIP packet.  In this case, a host advertising a new SPI as part of
 its address reachability check SHOULD be prepared to receive traffic
 on the new SA.
   Mobile Host                                   Peer Host
                UPDATE(ESP_INFO, LOCATOR_SET, ...)
              ---------------------------------->
                                                 prepare incoming SA
                UPDATE(ESP_INFO, ...) with new SPI
              <-----------------------------------
 switch to new outgoing SA
                         data on new SA
              ----------------------------------->
                                                 mark address ACTIVE
                UPDATE(ACK, ECHO_RESPONSE) later arrives
              ----------------------------------->
           Figure 7: Address Activation via Use of a New SA
 When address verification is in progress for a new preferred locator,
 the host SHOULD select a different locator listed as ACTIVE, if one
 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 CBA
 permits.  CBA is explained in Section 5.6.  Once address verification
 succeeds, the status of the new preferred locator changes to ACTIVE.

Henderson, et al. Standards Track [Page 25] RFC 8046 HIP Host Mobility February 2017

5.5. Changing the Preferred Locator

 A host MAY want to change the preferred outgoing locator for
 different reasons, e.g., because traffic information or ICMP error
 messages indicate that the currently used preferred address may have
 become unreachable.  Another reason may be due to receiving a
 LOCATOR_SET parameter that has the "P" bit set.
 To change the preferred locator, the host initiates the following
 procedure:
 1.  If the new preferred locator has an ACTIVE status, the preferred
     locator is changed and the procedure succeeds.
 2.  If the new preferred locator has an 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 CBA permits.  Once address verification
     succeeds, the status of the new preferred locator changes to
     ACTIVE, and its use is no longer governed by CBA.
 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 local 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 a 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.

5.6. Credit-Based Authorization

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

Henderson, et al. Standards Track [Page 26] RFC 8046 HIP Host Mobility February 2017

5.6.1. Handling Payload Packets

 A host maintains a "credit counter" for each of its peers.  Whenever
 a packet arrives from a peer, the host SHOULD increase that peer's
 credit counter by the size of the received packet.  When the host has
 a packet to be sent to the peer, and when the peer's preferred
 locator is listed as UNVERIFIED and no alternative locator with
 status ACTIVE is available, the host checks whether it can send the
 packet to the UNVERIFIED locator.  The packet SHOULD be sent if the
 value of the credit counter is higher than the size of the outbound
 packet.  If the credit counter is too low, the packet MUST be
 discarded or buffered until address verification succeeds.  When a
 packet is sent to a peer at an UNVERIFIED locator, the peer's credit
 counter MUST be reduced by the size of the packet.  The peer's credit
 counter is not affected by packets that the host sends to an ACTIVE
 locator of that peer.
 Figure 8 depicts the actions taken by the host when a packet is
 received.  Figure 9 shows the decision chain in the event a packet is
 sent.
     Inbound
     Packet
        |
        |       +----------------+               +---------------+
        |       |    Increase    |               |    Deliver    |
        +-----> | credit counter |-------------> |   packet to   |
                | by packet size |               |  application  |
                +----------------+               +---------------+
      Figure 8: Receiving Packets with Credit-Based Authorization

Henderson, et al. Standards Track [Page 27] RFC 8046 HIP Host Mobility February 2017

  Outbound
   Packet
      |          _________________
      |         /                 \                 +---------------+
      |        /  Is the preferred \       No       |  Send packet  |
      +-----> | destination address |-------------> |  to preferred |
               \    UNVERIFIED?    /                |    address    |
                \_________________/                 +---------------+
                         |
                         | Yes
                         |
                         v
                 _________________
                /                 \                 +---------------+
               /   Does an ACTIVE  \      Yes       |  Send packet  |
              | destination address |-------------> |   to ACTIVE   |
               \       exist?      /                |    address    |
                \_________________/                 +---------------+
                         |
                         | No
                         |
                         v
                 _________________
                /                 \                 +---------------+
               / Is credit counter \       No       |               |
              |          >=         |-------------> | Drop or       |
               \    packet size?   /                | buffer packet |
                \_________________/                 +---------------+
                         |
                         | Yes
                         |
                         v
                 +---------------+                  +---------------+
                 | Reduce credit |                  |  Send packet  |
                 |  counter by   |----------------> | to preferred  |
                 |  packet size  |                  |    address    |
                 +---------------+                  +---------------+
       Figure 9: Sending Packets with Credit-Based Authorization

Henderson, et al. Standards Track [Page 28] RFC 8046 HIP Host Mobility February 2017

5.6.2. Credit Aging

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

6. Security Considerations

 The HIP mobility mechanism provides a secure means of updating a
 host's IP address via HIP UPDATE packets.  Upon receipt, a HIP host
 cryptographically verifies the sender of an UPDATE, so forging or
 replaying a HIP UPDATE packet is very difficult (see [RFC7401]).
 Therefore, security issues reside in other attack domains.  The two
 we consider are malicious redirection of legitimate connections as
 well as redirection-based flooding attacks using this protocol.  This
 can be broken down into the following:
    1) Impersonation attacks
  1. direct conversation with the misled victim
  1. man-in-the-middle (MitM) attack

Henderson, et al. Standards Track [Page 29] RFC 8046 HIP Host Mobility February 2017

    2) Denial-of-service (DoS) attacks
  1. flooding attacks (== bandwidth-exhaustion attacks)
  • tool 1: direct flooding
  • tool 2: flooding by botnets
  • tool 3: redirection-based flooding
  1. memory-exhaustion attacks
  1. computational-exhaustion attacks
    3) Privacy concerns
 We consider these in more detail in the following sections.
 In Sections 6.1 and 6.2, we assume that all users are using HIP.  In
 Section 6.3, we consider the security ramifications when we have both
 HIP and non-HIP hosts.

6.1. Impersonation Attacks

 An attacker wishing to impersonate another host will try to mislead
 its victim into directly communicating with them or carry out a MitM
 attack between the victim and the victim's desired communication
 peer.  Without mobility support, such attacks are possible only if
 the attacker resides on the routing path between its victim and the
 victim's desired communication peer or if the attacker tricks its
 victim into initiating the connection over an incorrect routing path
 (e.g., by acting as a router or using spoofed DNS entries).
 The HIP extensions defined in this specification change the situation
 in that they introduce an ability to redirect a connection, both
 before and after establishment.  If no precautionary measures are
 taken, an attacker could potentially misuse the redirection feature
 to impersonate a victim's peer from any arbitrary location.  However,
 the authentication and authorization mechanisms of the HIP base
 exchange [RFC7401] and the signatures in the UPDATE message prevent
 this attack.  Furthermore, ownership of a HIP association is securely
 linked to a HIP HI/HIT.  If an attacker somehow uses a bug in the
 implementation to redirect a HIP connection, the original owner can
 always reclaim their connection (they can always prove ownership of
 the private key associated with their public HI).

Henderson, et al. Standards Track [Page 30] RFC 8046 HIP Host Mobility February 2017

 MitM attacks are possible if an on-path attacker is present during
 the initial HIP base exchange and if the hosts do not authenticate
 each other's identities.  However, once such an opportunistic base
 exchange has taken place, a MitM attacker that comes later to the
 path cannot steal the HIP connection because it is very difficult for
 an attacker to create an UPDATE packet (or any HIP packet) that will
 be accepted as a legitimate update.  UPDATE packets use HMAC and are
 signed.  Even when an attacker can snoop packets to obtain the SPI
 and HIT/HI, they still cannot forge an UPDATE packet without
 knowledge of the secret keys.  Also, replay attacks on the UPDATE
 packet are prevented as described in [RFC7401].

6.2. Denial-of-Service Attacks

6.2.1. Flooding Attacks

 The purpose of a DoS attack is to exhaust some resource of the victim
 such that the victim ceases to operate correctly.  A DoS attack can
 aim at the victim's network attachment (flooding attack), its memory,
 or its processing capacity.  In a flooding attack, the attacker
 causes an excessive number of bogus or unwanted packets to be sent to
 the victim, which fills their available bandwidth.  Note that the
 victim does not necessarily need to be a node; it can also be an
 entire network.  The attack functions the same way in either case.
 An effective DoS strategy is distributed denial of service (DDoS).
 Here, the attacker conventionally distributes some viral software to
 as many nodes as possible.  Under the control of the attacker, the
 infected nodes (e.g., nodes in a botnet) jointly send packets to the
 victim.  With such an "army", an attacker can take down even very
 high bandwidth networks/victims.
 With the ability to redirect connections, an attacker could realize a
 DDoS attack without having to distribute viral code.  Here, the
 attacker initiates a large download from a server and subsequently
 uses the HIP mobility mechanism to redirect this download to its
 victim.  The attacker can repeat this with multiple servers.  This
 threat is mitigated through reachability checks and CBA.  When
 conducted using HIP, reachability checks can leverage the built-in
 authentication properties of HIP.  They can also prevent redirection-
 based flooding attacks.  However, the delay of such a check can have
 a noticeable impact on application performance.  To reduce the impact
 of the delay, CBA can be used to send a limited number of packets to
 the new address while the validity of the IP address is still in
 question.  Both strategies do not eliminate flooding attacks per se,
 but they preclude: (i) their use from a location off the path towards
 the flooded victim; and (ii) any amplification in the number and size

Henderson, et al. Standards Track [Page 31] RFC 8046 HIP Host Mobility February 2017

 of the redirected packets.  As a result, the combination of a
 reachability check and CBA lowers a HIP redirection-based flooding
 attack to the level of a direct flooding attack in which the attacker
 itself sends the flooding traffic to the victim.

6.2.2. Memory/Computational-Exhaustion DoS Attacks

 We now consider whether or not the proposed extensions to HIP add any
 new DoS attacks (consideration of DoS attacks using the base HIP
 exchange and updates is discussed in [RFC7401]).  A simple attack is
 to send many UPDATE packets containing many IP addresses that are not
 flagged as preferred.  The attacker continues to send such packets
 until the number of IP addresses associated with the attacker's HI
 crashes the system.  Therefore, a HIP association SHOULD limit the
 number of IP addresses that can be associated with any HI.  Other
 forms of memory/computationally exhausting attacks via the HIP UPDATE
 packet are handled in the base HIP document [RFC7401].
 A central server that has to deal with a large number of mobile
 clients MAY consider increasing the SA lifetimes to try to slow down
 the rate of rekeying UPDATEs or increasing the cookie difficulty to
 slow down the rate of attack-oriented connections.

6.3. Mixed Deployment Environment

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

Henderson, et al. Standards Track [Page 32] RFC 8046 HIP Host Mobility February 2017

 4.  A HIP host attempts to steal a non-HIP host's session.  A HIP
     host could spoof the non-HIP host's IP address during the base
     exchange or set the non-HIP host's IP address as its preferred
     address via an UPDATE.  Other possibilities exist, but a solution
     is to prevent the local redirection of sessions that were
     previously using an unverified address, but outside of the
     existing HIP context, into the HIP SAs until the address change
     can be verified.

6.4. Privacy Concerns

 The exposure of a host's IP addresses through HIP mobility extensions
 may raise privacy concerns.  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.  While this issue may be more relevant in a host multihoming
 scenario in which multiple IP addresses might be exposed [RFC8047],
 it is worth noting also here that mobility events might cause an
 implementation to try to inadvertently use a locator that the
 administrator would rather avoid exposing to the peer host.

7. IANA Considerations

 [RFC5206], obsoleted by this document, specified an allocation for a
 LOCATOR parameter in the "Parameter Types" subregistry of the "Host
 Identity Protocol (HIP) Parameters" registry, with a type value of
 193.  IANA has renamed the parameter to "LOCATOR_SET" and has updated
 the reference from [RFC5206] to this specification.
 [RFC5206], obsoleted by this document, specified an allocation for a
 LOCATOR_TYPE_UNSUPPORTED type in the "Notify Message Types" registry,
 with a type value of 46.  IANA has updated the reference from
 [RFC5206] to this specification.

8. Differences from RFC 5206

 This section summarizes the technical changes made from [RFC5206].
 This section is informational, intended to help implementors of the
 previous protocol version.  If any text in this section contradicts
 text in other portions of this specification, the text found outside
 of this section should be considered normative.

Henderson, et al. Standards Track [Page 33] RFC 8046 HIP Host Mobility February 2017

 This document specifies extensions to the HIP Version 2 protocol,
 while [RFC5206] specifies extensions to the HIP Version 1 protocol.
 [RFC7401] documents the differences between these two protocol
 versions.
 [RFC5206] included procedures for both HIP host mobility and basic
 host multihoming.  In this document, only host mobility procedures
 are included; host multihoming procedures are now specified in
 [RFC8047].  In particular, multihoming-related procedures related to
 the exposure of multiple locators in the base exchange packets; the
 transmission, reception, and processing of multiple locators in a
 single UPDATE packet; handovers across IP address families; and other
 multihoming-related specifications have been removed.
 The following additional changes have been made:
 o  The LOCATOR parameter in [RFC5206] has been renamed to
    LOCATOR_SET.
 o  Specification text regarding the handling of mobility when both
    hosts change IP addresses at nearly the same time (a "double-jump"
    mobility scenario) has been added.
 o  Specification text regarding the mobility event in which the host
    briefly has an active new locator and old locator at the same time
    (a "make-before-break" mobility scenario) has been added.
 o  Specification text has been added to note that a host may add the
    source IP address of a received HIP packet as a candidate locator
    for the peer even if it is not listed in the peer's LOCATOR_SET,
    but that it should prefer locators explicitly listed in the
    LOCATOR_SET.
 o  This document clarifies that the HOST_ID parameter may be included
    in UPDATE messages containing LOCATOR_SET parameters, for the
    possible benefit of HIP-aware firewalls.
 o  The previous specification mentioned that it may be possible to
    include multiple LOCATOR_SET and ESP_INFO parameters in an UPDATE.
    This document only specifies the case of a single LOCATOR_SET and
    ESP_INFO parameter in an UPDATE.
 o  The previous specification mentioned that it may be possible to
    send LOCATOR_SET parameters in packets other than the UPDATE.
    This document only specifies the use of the UPDATE packet.
 o  This document describes a simple heuristic for setting the credit
    value for CBA.

Henderson, et al. Standards Track [Page 34] RFC 8046 HIP Host Mobility February 2017

 o  This specification mandates that a host must be able to receive
    and avoid reprocessing redundant LOCATOR_SET parameters that may
    have been sent in parallel to multiple addresses of the host.

9. References

9.1. 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>.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, DOI 10.17487/RFC4291, February
            2006, <http://www.rfc-editor.org/info/rfc4291>.
 [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>.
 [RFC8003]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
            Registration Extension", RFC 8003, DOI 10.17487/RFC8003,
            October 2016, <http://www.rfc-editor.org/info/rfc8003>.
 [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
            Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
            October 2016, <http://www.rfc-editor.org/info/rfc8004>.

9.2. Informative References

 [CBA-MIPv6]
            Vogt, C. and J. Arkko, "Credit-Based Authorization for
            Mobile IPv6 Early Binding Updates", Work in Progress,
            draft-vogt-mobopts-credit-based-authorization-00, February
            2005.
 [RFC4225]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
            Nordmark, "Mobile IP Version 6 Route Optimization Security
            Design Background", RFC 4225, DOI 10.17487/RFC4225,
            December 2005, <http://www.rfc-editor.org/info/rfc4225>.

Henderson, et al. Standards Track [Page 35] RFC 8046 HIP Host Mobility February 2017

 [RFC5206]  Nikander, P., Henderson, T., Ed., Vogt, C., and J. Arkko,
            "End-Host Mobility and Multihoming with the Host Identity
            Protocol", RFC 5206, DOI 10.17487/RFC5206, April 2008,
            <http://www.rfc-editor.org/info/rfc5206>.
 [RFC5207]  Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
            Firewall Traversal Issues of Host Identity Protocol (HIP)
            Communication", RFC 5207, DOI 10.17487/RFC5207, April
            2008, <http://www.rfc-editor.org/info/rfc5207>.
 [RFC8047]  Henderson, T., Ed., Vogt, C., and J. Arkko, "Host
            Multihoming with the Host Identity Protocol", RFC 8047,
            DOI 10.17487/RFC8047, February 2017,
            <http://www.rfc-editor.org/info/rfc8047>.
 [SIMPLE-CBA]
            Vogt, C. and J. Arkko, "Credit-Based Authorization for
            Concurrent Reachability Verification", Work in Progress,
            draft-vogt-mobopts-simple-cba-00, February 2006.

Acknowledgments

 Pekka Nikander and Jari Arkko originated this document; Christian
 Vogt and Thomas Henderson (editor) later joined as coauthors.  Greg
 Perkins contributed the initial text of the security section.  Petri
 Jokela was a coauthor of the initial individual submission.
 CBA was originally introduced in [SIMPLE-CBA], and portions of this
 document have been adopted from that earlier document.
 The authors thank Jeff Ahrenholz, Baris Boyvat, Rene Hummen, Miika
 Komu, Mika Kousa, Jan Melen, and Samu Varjonen for improvements to
 the document.

Henderson, et al. Standards Track [Page 36] RFC 8046 HIP Host Mobility February 2017

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 37]

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