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RFC 8046

Proposed STD
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Host Mobility with the Host Identity Protocol

Part 1 of 2, p. 1 to 19
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Obsoletes:    5206

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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
                                                           February 2017

             Host Mobility with the Host Identity Protocol


   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

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

[Page 2] 
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
   ( 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.

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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

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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.

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   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",
   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).

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   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.

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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

    +--------------------+                       +--------------------+
    |                    |                       |                    |
    |   +------------+   |                       |   +------------+   |
    |   |    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

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

       Figure 2: Architecture for HIP 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

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   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

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

   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

   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,

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   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

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   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


        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].

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   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

   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.

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     Mobile Host                         Peer Host


              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.

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   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.

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   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

   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.

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           +-------+                        +-------+
           |   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-

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   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

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   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.

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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.

(page 19 continued on part 2)

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