Network Working Group                                  T. Henderson, Ed.
Internet-Draft                                        The Boeing Company
Obsoletes: 5206 (if approved)                                    C. Vogt
Intended status: Standards Track                                J. Arkko
Expires: January 15, 2013                   Ericsson Research NomadicLab
                                                           July 14, 2012

             Host Mobility with the Host Identity Protocol


   This document defines mobility extensions to the Host Identity
   Protocol (HIP).  Specifically, this document defines a general
   "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 elements of procedure for mobility of a
   HIP host -- the process by which a host dynamically changes the
   primary locator that it uses to receive packets.  While the same
   LOCATOR_SET parameter can also be used to support end-host
   multihoming, detailed procedures are out of scope for this document.
   This document obsoletes RFC 5206.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 15, 2013.

Copyright Notice

   Copyright (c) 2012 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

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   ( in effect on the date of
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Table of Contents

   1.  Introduction and Scope . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology and Conventions  . . . . . . . . . . . . . . . . .  5
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Operating Environment  . . . . . . . . . . . . . . . . . .  6
       3.1.1.  Locator  . . . . . . . . . . . . . . . . . . . . . . .  8
       3.1.2.  Mobility Overview  . . . . . . . . . . . . . . . . . .  8
     3.2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Mobility with a Single SA Pair (No Rekeying) . . . . . 10
       3.2.2.  Mobility with a Single SA Pair (Mobile-Initiated
               Rekey) . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.3.  Using LOCATOR_SETs across Addressing Realms  . . . . . 12
       3.2.4.  Network Renumbering  . . . . . . . . . . . . . . . . . 12
     3.3.  Other Considerations . . . . . . . . . . . . . . . . . . . 12
       3.3.1.  Address Verification . . . . . . . . . . . . . . . . . 12
       3.3.2.  Credit-Based Authorization . . . . . . . . . . . . . . 13
       3.3.3.  Preferred Locator  . . . . . . . . . . . . . . . . . . 14
   4.  LOCATOR_SET Parameter Format . . . . . . . . . . . . . . . . . 14
     4.1.  Traffic Type and Preferred Locator . . . . . . . . . . . . 16
     4.2.  Locator Type and Locator . . . . . . . . . . . . . . . . . 16
     4.3.  UPDATE Packet with Included LOCATOR_SET  . . . . . . . . . 17
   5.  Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 17
     5.1.  Locator Data Structure and Status  . . . . . . . . . . . . 17
     5.2.  Sending LOCATOR_SETs . . . . . . . . . . . . . . . . . . . 18
     5.3.  Handling Received LOCATOR_SETs . . . . . . . . . . . . . . 20
     5.4.  Verifying Address Reachability . . . . . . . . . . . . . . 22
     5.5.  Changing the Preferred Locator . . . . . . . . . . . . . . 23
     5.6.  Credit-Based Authorization . . . . . . . . . . . . . . . . 24
       5.6.1.  Handling Payload Packets . . . . . . . . . . . . . . . 24
       5.6.2.  Credit Aging . . . . . . . . . . . . . . . . . . . . . 26
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
     6.1.  Impersonation Attacks  . . . . . . . . . . . . . . . . . . 28
     6.2.  Denial-of-Service Attacks  . . . . . . . . . . . . . . . . 29
       6.2.1.  Flooding Attacks . . . . . . . . . . . . . . . . . . . 29
       6.2.2.  Memory/Computational-Exhaustion DoS Attacks  . . . . . 29
     6.3.  Mixed Deployment Environment . . . . . . . . . . . . . . . 30
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   8.  Authors and Acknowledgments  . . . . . . . . . . . . . . . . . 31
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     9.1.  Normative references . . . . . . . . . . . . . . . . . . . 31
     9.2.  Informative references . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Document Revision History . . . . . . . . . . . . . . 32

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1.  Introduction and Scope

   The Host Identity Protocol [I-D.ietf-hip-rfc4423-bis] (HIP) supports
   an architecture that decouples the transport layer (TCP, UDP, etc.)
   from the internetworking layer (IPv4 and IPv6) by using public/
   private key pairs, instead of IP addresses, as host identities.  When
   a host uses HIP, the overlying protocol sublayers (e.g., transport
   layer sockets and Encapsulating Security Payload (ESP) Security
   Associations (SAs)) are instead bound to representations of these
   host identities, and the IP addresses are only used for packet
   forwarding.  However, each host must also know at least one IP
   address at which its peers are reachable.  Initially, these IP
   addresses are the ones used during the HIP base exchange

   One consequence of such a decoupling is that new solutions to
   network-layer mobility and host multihoming are possible.  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 scenarios and other variations for further study.  More

      This document defines a generalized 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 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 -- the sequential
      change in the preferred IP address used to reach a host.  In
      particular, message flows to enable successful host mobility,
      including address verification methods, are defined herein.

      However, while the same LOCATOR_SET parameter is intended to
      support host multihoming (parallel support of a number of
      addresses), and experimentation is encouraged, detailed elements
      of procedure for host multihoming are out of scope.

   While HIP can potentially be used with transports other than the ESP
   transport format [I-D.ietf-hip-rfc5202-bis], this document largely
   assumes the use of ESP and leaves other transport formats for further

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   There are a number of situations where the simple end-to-end
   readdressing functionality is not sufficient.  These include the
   initial reachability of a mobile host, location privacy, simultaneous
   mobility of both hosts, and some modes of NAT traversal.  In these
   situations, there is a need for some helper functionality in the
   network, such as a HIP rendezvous server [I-D.ietf-hip-rfc5204-bis].
   Such functionality is out of the scope of this document.  We also 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.  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.

2.  Terminology and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   LOCATOR_SET.  The name of a HIP parameter containing zero or more
      Locator fields.  This parameter's name is distinguished from the
      Locator fields embedded within it by the use of all capital

   Locator.  A name that controls how the packet is routed through the
      network and demultiplexed by the end host.  It may include a
      concatenation of traditional network addresses such as an IPv6
      address and end-to-end identifiers such as an ESP SPI.  It may
      also include transport port numbers or IPv6 Flow Labels as
      demultiplexing context, or it may simply be a network address.

   Address.  A name that denotes a point-of-attachment to the network.
      The two most common examples are an IPv4 address and an IPv6
      address.  The set of possible addresses is a subset of the set of
      possible locators.

   Preferred locator.  A locator on which a host prefers to receive
      data.  With respect to a given peer, a host always has one active
      Preferred locator, unless there are no active locators.  By
      default, the locators used in the HIP base exchange are the
      Preferred locators.

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   Credit Based Authorization.  A host must verify a peer host's
      reachability at a new locator.  Credit-Based Authorization
      authorizes the peer to receive a certain amount of data at the new
      locator before the result of such verification is known.

3.  Protocol Model

   This section is an overview; more detailed specification follows this

3.1.  Operating Environment

   The Host Identity Protocol (HIP) [I-D.ietf-hip-rfc5201-bis] is a key
   establishment and parameter negotiation protocol.  Its primary
   applications are for authenticating host messages based on host
   identities, and establishing security associations (SAs) for the ESP
   transport format [I-D.ietf-hip-rfc5202-bis] and possibly other
   protocols in the future.

    +--------------------+                       +--------------------+
    |                    |                       |                    |
    |   +------------+   |                       |   +------------+   |
    |   |    Key     |   |         HIP           |   |    Key     |   |
    |   | Management | <-+-----------------------+-> | Management |   |
    |   |  Process   |   |                       |   |  Process   |   |
    |   +------------+   |                       |   +------------+   |
    |         ^          |                       |         ^          |
    |         |          |                       |         |          |
    |         v          |                       |         v          |
    |   +------------+   |                       |   +------------+   |
    |   |   IPsec    |   |        ESP            |   |   IPsec    |   |
    |   |   Stack    | <-+-----------------------+-> |   Stack    |   |
    |   |            |   |                       |   |            |   |
    |   +------------+   |                       |   +------------+   |
    |                    |                       |                    |
    |                    |                       |                    |
    |     Initiator      |                       |     Responder      |
    +--------------------+                       +--------------------+

                      Figure 1: HIP Deployment Model

   The general deployment model for HIP is shown above, assuming
   operation in an end-to-end fashion.  This document specifies
   extensions to the HIP protocol to enable end-host mobility and basic
   multihoming.  In summary, these extensions to the HIP base protocol
   enable the signaling of new addressing information to the peer in HIP
   messages.  The messages are authenticated via a signature or keyed
   hash message authentication code (HMAC) based on its Host Identity.

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   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.

            | 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 (MH)

   Figure 2 depicts a layered architectural view of a HIP-enabled stack
   using the ESP transport format.  In HIP, upper-layer protocols
   (including TCP and ESP in this figure) are bound to Host Identity
   Tags (HITs) and not IP addresses.  The HIP sublayer is responsible
   for maintaining the binding between HITs and IP addresses.  The SPI
   is used to associate an incoming packet with the right HITs.  The
   block labeled "MH" is introduced below.

   Consider first the case in which there is no mobility or multihoming,
   as specified in the base protocol specification
   [I-D.ietf-hip-rfc5201-bis].  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

   Consider next a mobility event, in which a host moves to another IP
   address.  Two things must occur in this case.  First, the peer must
   be notified of the address change using a HIP UPDATE message.
   Second, each host must change its local bindings at the HIP sublayer

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   (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.  However, simultaneous movement of both hosts,
   notification of transport layer protocols of the path change, and
   procedures for possibly traversing middleboxes 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 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 LOCATOR_SET
   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 protocol specification
   [I-D.ietf-hip-rfc5201-bis].  The peer can authenticate the contents
   of the UPDATE packet based on the signature and keyed hash of the

   When using ESP Transport Format [I-D.ietf-hip-rfc5202-bis], 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 [I-D.ietf-hip-rfc5201-bis]
   and ESP extension [I-D.ietf-hip-rfc5202-bis].

   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 [I-D.ietf-hip-rfc5202-bis], although other
   scenarios may be defined in the future.  To understand these usage
   scenarios, the reader should be at least minimally familiar with the
   HIP protocol specification [I-D.ietf-hip-rfc5201-bis].  However, for
   the (relatively) uninitiated reader, it is most important to keep in
   mind that in HIP the actual payload traffic is protected with ESP,
   and that the ESP SPI acts as an index to the right host-to-host
   context.  More specification details are found later in Section 4 and
   Section 5.

   The scenarios below assume that the two hosts have completed a single
   HIP base exchange with each other.  Both of the hosts therefore have
   one incoming and one outgoing SA.  Further, each SA uses the same
   pair of IP addresses, which are the ones used in the base exchange.

   The readdressing protocol is an asymmetric protocol where a mobile
   host informs a peer host about changes of IP addresses on affected
   SPIs.  The readdressing exchange is designed to be piggybacked on
   existing HIP exchanges.  The majority of the packets on which the
   LOCATOR_SET parameters are expected to be carried are UPDATE packets.
   However, some implementations may want to experiment with sending
   LOCATOR_SET parameters also on other packets, such as R1, I2, and

   The scenarios below at times describe addresses as being in either an
   ACTIVE, VERIFIED, or DEPRECATED state.  From the perspective of a
   host, newly-learned addresses of the peer must be verified before put
   into active service, and addresses removed by the peer are put into a
   deprecated state.  Under limited conditions described 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.

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

   A mobile host must sometimes 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 must inform its
   peers about the new IP address.  This first example considers the
   case in which the mobile host has only one interface, IP address, a
   single pair of SAs (one inbound, one outbound), and no rekeying
   occurs on the SAs.  We also assume that the new IP addresses are
   within the same address family (IPv4 or IPv6) as the first address.
   This is the simplest scenario, depicted in Figure 3.

     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 loosing 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, the OLD SPI and NEW SPI parameters
       both 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 middleboxes on the
       path.  The LOCATOR_SET parameter contains the new IP address
       (Locator Type of "1", defined below) and a locator lifetime.  The
       mobile host waits for this UPDATE to be acknowledged, and
       retransmits if necessary, as specified in the base specification

   2.  The peer host receives the UPDATE, validates it, and updates any
       local bindings between the HIP association and the mobile host's

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       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 the OLD SPI and NEW SPI
       parameters both set to the value of the preexisting incoming SPI,
       and sends this UPDATE (with piggybacked acknowledgment) to the
       mobile host at its new address.  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.  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.

     Mobile Host                         Peer Host


              Figure 4: Readdress with Mobile-Initiated Rekey

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3.2.3.  Using LOCATOR_SETs across Addressing Realms

   It is possible for HIP associations to migrate to a state in which
   both parties are only using locators in different addressing realms.
   For example, the two hosts may initiate the HIP association when both
   are using IPv6 locators, then one host may loose its IPv6
   connectivity and obtain an IPv4 address.  In such a case, some type
   of mechanism for interworking between the different realms must be
   employed; such techniques are outside the scope of the present text.
   The basic problem in this example is that the host readdressing to
   IPv4 does not know a corresponding IPv4 address of the peer.  This
   may be handled (experimentally) by possibly configuring this address
   information manually or in the DNS, or the hosts exchange both IPv4
   and IPv6 addresses in the locator.

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.

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].
   Likewise, viral software may have compromised the peer host,
   programming it to redirect packets to the target addresses.  Thus,
   the HIP host must first check that the peer is reachable at the new

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

   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.

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3.3.2.  Credit-Based Authorization

   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, Credit-Based Authorization 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 Credit-Based Authorization: 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

3.3.3.  Preferred Locator

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

4.  LOCATOR_SET Parameter Format

   The LOCATOR_SET parameter is a critical parameter as defined by
   [I-D.ietf-hip-rfc5201-bis].  It consists of the standard HIP
   parameter Type and Length fields, plus zero or more Locator sub-
   parameters.  Each Locator sub-parameter contains a Traffic Type,
   Locator Type, Locator Length, Preferred locator bit, Locator
   Lifetime, and a Locator encoding.  A LOCATOR_SET containing zero
   Locator fields is permitted but has the effect of deprecating all

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        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

   Reserved:  Zero when sent, ignored when received.

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   P: Preferred locator.  Set to one if the locator is preferred for
      that Traffic Type; otherwise, set to zero.

   Locator Lifetime:  Locator lifetime, in seconds.

   Locator:  The locator whose semantics and encoding are indicated by
      the Locator Type field.  All Locator sub-fields are integral
      multiples of four octets in length.

   The Locator 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 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 locator from the set of active

4.2.  Locator Type and Locator

   The following Locator Type values are defined, along with the
   associated semantics of the Locator field:

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   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

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 is used in any HIP packet.  Furthermore, the LOCATOR_SET
   SHOULD list all of the locators that are active on the HIP
   association (including those on SAs not covered by the ESP_INFO
   parameter).  Any UPDATE packet that includes a LOCATOR_SET parameter
   SHOULD include both an HMAC and a HIP_SIGNATURE parameter.  The
   relationship between the announced Locators and any ESP_INFO
   parameters present in the packet is defined in Section 5.2.  The
   sending of multiple LOCATOR_SET and/or ESP_INFO parameters is for
   further study; receivers may wish to experiment with supporting such
   a possibility.

5.  Processing Rules

   This section describes rules for sending and receiving the
   LOCATOR_SET parameter, testing address reachability, and using
   Credit-Based Authorization (CBA) on UNVERIFIED locators.

5.1.  Locator Data Structure and Status

   In a typical implementation, each outgoing locator is represented by
   a piece of state that contains the following data:

   o  the actual bit pattern representing the locator,

   o  the lifetime (seconds),


   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:

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   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,

   DEPRECATED  indicates that the locator lifetime has expired.

   The following state changes are allowed:

   UNVERIFIED to ACTIVE  The reachability procedure completes

   UNVERIFIED to DEPRECATED  The locator lifetime expires while the
      locator is UNVERIFIED.

   ACTIVE to DEPRECATED  The locator 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 must be verified again before starting to use it

   DEPRECATED to UNVERIFIED  The host receives a new lifetime for the

   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.

5.2.  Sending LOCATOR_SETs

   The decision of when to send LOCATOR_SETs is basically 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.  It is possible to delay the exposure of
   additional locators to the peer, and to send data from previously
   unannounced locators, as might arise in certain mobility situations.
   Rapidly sending LOCATOR_SETs that force the peer to change the
   preferred address SHOULD be avoided.

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   When a host decides to inform its peers about changes in its IP
   addresses, it has to decide how to group the various addresses with
   SPIs.  The grouping should consider also whether middlebox
   interaction requires sending the same LOCATOR_SET in separate UPDATEs
   on different paths.  Since each SPI is associated with a different
   Security Association, the grouping policy may also be based on ESP
   anti-replay protection considerations.  In the typical case, simply
   basing the grouping on actual kernel level physical and logical
   interfaces may be the best policy.  Grouping policy is outside of the
   scope of this document.

   Note that the purpose of announcing IP addresses in a LOCATOR_SET is
   to provide connectivity between the communicating hosts.  In most
   cases, tunnels or virtual interfaces such as IPsec tunnel interfaces
   or Mobile IP home addresses provide sub-optimal connectivity.
   Furthermore, it should be possible to replace most tunnels with HIP
   based "non-tunneling", therefore making most virtual interfaces
   fairly unnecessary in the future.  Therefore, virtual interfaces
   SHOULD NOT be announced in general.  On the other hand, there are
   clearly situations where tunnels are used for diagnostic and/or
   testing purposes.  In such and other similar cases announcing the IP
   addresses of virtual interfaces may be appropriate.

   Hosts MUST NOT announce broadcast or multicast addresses in
   LOCATOR_SETs.  Link-local addresses MAY be announced to peers that
   are known to be neighbors on the same link, such as when the IP
   destination address of a peer is also link-local.  The announcement
   of link-local addresses in this case is a policy decision; link-local
   addresses used as Preferred locators will create reachability
   problems when the host moves to another link.  In any case, link-
   local addresses MUST NOT be announced to a peer unless that peer is
   known to be on the same link.

   Once the host has decided on the groups and assignment of addresses
   to the SPIs, it creates a LOCATOR_SET parameter that serves as a
   complete representation of the addresses and affiliated SPIs intended
   for active use.  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 experimentation.

   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

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       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 protocol
       specification [I-D.ietf-hip-rfc5201-bis].  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.

   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 and
       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
       "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.

   The sending of multiple LOCATOR_SETs, locators with Locator Type "0",
   and multiple ESP_INFO parameters is for further study.  Note that the
   inclusion of LOCATOR_SET in an R1 packet requires the use of Type "0"
   locators since no SAs are set up at that point.

5.3.  Handling Received LOCATOR_SETs

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

   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 middleboxes.  The LOCATOR_SET
   parameter contains a complete map of the locators that the host
   wishes to make or keep active for the HIP association.

   In general, the processing of a LOCATOR_SET depends upon the packet
   type in which it is included.  Here, we describe only the case in
   which ESP_INFO is present and a single LOCATOR_SET and ESP_INFO are
   sent in an UPDATE message; other cases are for further study.  The
   steps below cover each of the cases described in Section 5.2.

   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 Type "1" locator will only contain a reference to the new SPI.

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   When a host receives a validated HIP UPDATE with a LOCATOR_SET and
   ESP_INFO parameter, it processes the ESP_INFO as follows.  The
   ESP_INFO parameter indicates whether an SA is being rekeyed, created,
   deprecated, or just identified for the benefit of middleboxes.  The
   host examines the OLD SPI and NEW SPI values in the ESP_INFO

   1.  (no rekeying) If the OLD SPI is equal to the NEW SPI and both
       correspond to an existing SPI, the ESP_INFO is gratuitous
       (provided for middleboxes) and no rekeying is necessary.

   2.  (rekeying) If the OLD SPI indicates an existing SPI and the NEW
       SPI is a different non-zero value, the existing SA is being
       rekeyed and the host follows HIP ESP rekeying procedures by
       creating a new outbound SA with an SPI corresponding to the NEW
       SPI, with no addresses bound to this SPI.  Note that locators in
       the LOCATOR_SET parameter will reference this new SPI instead of
       the old SPI.

   3.  (new SA) If the OLD SPI value is zero and the NEW SPI is a new
       non-zero value, then a new SA is being requested by the peer.
       This case is also treated like a rekeying event; the receiving
       host must create a new SA and respond with an UPDATE ACK.

   4.  (deprecating the SA) If the OLD SPI indicates an existing SPI and
       the NEW SPI is zero, the SA is being deprecated and all locators
       uniquely bound to the SPI are put into the DEPRECATED state.

   If none of the above cases apply, a protocol error has occurred and
   the processing of the UPDATE is stopped.

   Next, the locators in the LOCATOR_SET parameter are processed.  For
   each locator listed in the LOCATOR_SET parameter, check that the
   address therein is a legal unicast or anycast address.  That is, the
   address MUST NOT be a broadcast or multicast address.  Note that some
   implementations MAY accept addresses that indicate the local host,
   since it may be allowed that the host runs HIP with itself.

   The below assumes that all locators are of Type "1" with a Traffic
   Type of "0"; other cases are for further study.

   For each Type "1" address listed in the LOCATOR_SET parameter, the
   host checks whether the address is already bound to the SPI
   indicated.  If the address is already bound, its lifetime is updated.
   If the status of the address is DEPRECATED, the status is changed to
   UNVERIFIED.  If the address is not already bound, the address is
   added, and its status is set to UNVERIFIED.  Mark all addresses
   corresponding to the SPI that were NOT listed in the LOCATOR_SET

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   parameter as DEPRECATED.

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

   Once the host has processed the locators, if the LOCATOR_SET
   parameter contains a new Preferred locator, the host SHOULD initiate
   a change of the Preferred locator.  This requires that the host first
   verifies reachability of the associated address, and only then
   changes the Preferred locator; see Section 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

   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

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 a 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

   A host typically starts the address-verification procedure by sending
   a nonce to the new address.  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 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 in an UPDATE message sent
   to the new address.  A host MAY also use other message exchanges as
   confirmation of the address reachability.

   Note that in the case of receiving a LOCATOR_SET in an R1 and
   replying with an I2 to the new address in the LOCATOR_SET, receiving

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   the corresponding R2 is sufficient proof of reachability for the
   Responder's preferred address.  Since further address verification of
   such an address can impede the HIP-base exchange, a host MUST NOT
   separately verify reachability of a new Preferred locator that was
   received on an R1.

   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

                                                   prepare incoming SA
                      NEW SPI in ESP_INFO (UPDATE)
   switch to new outgoing SA
                           data on new SA
                                                   mark address ACTIVE

             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 Credit-
   Based Authorization permits.  Credit-Based Authorization is explained
   in Section 5.6.  Once address verification succeeds, the status of
   the new Preferred locator changes to ACTIVE.

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

   1.  If the new Preferred locator has ACTIVE status, the Preferred
       locator is changed and the procedure succeeds.

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   2.  If the new Preferred locator has UNVERIFIED status, the host
       starts to verify its reachability.  The host SHOULD use a
       different locator listed as ACTIVE until address verification
       completes if one such locator is available.  Alternatively, the
       host MAY use the new Preferred locator, even though in UNVERIFIED
       status, to the extent Credit-Based Authorization permits.  Once
       address verification succeeds, the status of the new Preferred
       locator changes to ACTIVE and its use is no longer governed by
       Credit-Based Authorization.

   3.  If the peer host has not indicated a preference for any address,
       then the host picks one of the peer's ACTIVE addresses randomly
       or according to policy.  This case may arise if, for example,
       ICMP error messages that deprecate the Preferred locator arrive,
       but the peer has not yet indicated a new Preferred locator.

   4.  If the new Preferred locator has DEPRECATED status and there is
       at least one non-deprecated address, the host selects one of the
       non-deprecated addresses as a new Preferred locator and
       continues.  If the selected address is UNVERIFIED, the address
       verification procedure described above will apply.

5.6.  Credit-Based Authorization

   To prevent redirection-based flooding attacks, the use of a Credit-
   Based Authorization (CBA) approach is mandatory when a host sends
   data to an UNVERIFIED locator.  The following algorithm meets 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].

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

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   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

          |       +----------------+               +---------------+
          |       |    Increase    |               |    Deliver    |
          +-----> | credit counter |-------------> |   packet to   |
                  | by packet size |               |  application  |
                  +----------------+               +---------------+

        Figure 8: Receiving Packets with Credit-Based Authorization

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        |          _________________
        |         /                 \                 +---------------+
        |        /  Is the preferred \       No       |  Send packet  |
        +-----> | destination address |-------------> |  to preferred |
                 \    UNVERIFIED?    /                |    address    |
                  \_________________/                 +---------------+
                           | Yes
                  /                 \                 +---------------+
                 /   Does an ACTIVE  \      Yes       |  Send packet  |
                | destination address |-------------> |   to ACTIVE   |
                 \       exist?      /                |    address    |
                  \_________________/                 +---------------+
                           | No
                  /                 \                 +---------------+
                 /   Credit counter  \       No       |               |
                |          >=         |-------------> |  Drop packet  |
                 \    packet size?   /                |               |
                  \_________________/                 +---------------+
                           | Yes
                   +---------------+                  +---------------+
                   | Reduce credit |                  |  Send packet  |
                   |  counter by   |----------------> | to preferred  |
                   |  packet size  |                  |    address    |
                   +---------------+                  +---------------+

         Figure 9: Sending Packets with Credit-Based Authorization

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.

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   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
   [I-D.ietf-hip-rfc5201-bis]).  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

      Impersonation attacks

         - direct conversation with the misled victim

         - man-in-the-middle attack

      DoS attacks

         - flooding attacks (== bandwidth-exhaustion attacks)

            * tool 1: direct flooding

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            * tool 2: flooding by zombies

            * tool 3: redirection-based flooding

         - memory-exhaustion attacks

         - computational-exhaustion attacks

   We consider these in more detail in the following sections.

   In Section 6.1 and Section 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 users.  Security considerations for Credit-
   Based Authorization are discussed in [SIMPLE-CBA].

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 man-
   in-the-middle (MitM) attack between the victim and the victim's
   desired communication peer.  Without mobility support, both attack
   types 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 (like
   IPv6), both before and after establishment.  If no precautionary
   measures are taken, an attacker could misuse the redirection feature
   to impersonate a victim's peer from any arbitrary location.  The
   authentication and authorization mechanisms of the HIP base exchange
   [I-D.ietf-hip-rfc5201-bis] 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 or weakness in some protocol 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).

   MitM attacks are always possible if the attacker is present during
   the initial HIP base exchange and if the hosts do not authenticate
   each other's identities.  However, once the opportunistic base
   exchange has taken place, even a MitM cannot steal the HIP connection
   anymore 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

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   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

6.2.  Denial-of-Service Attacks

6.2.1.  Flooding Attacks

   The purpose of a denial-of-service attack is to exhaust some resource
   of the victim such that the victim ceases to operate correctly.  A
   denial-of-service 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 basically
   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, or "zombies", 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
   redirects this download to its victim.  The attacker can repeat this
   with multiple servers.  This threat is mitigated through reachability
   checks and credit-based authorization.  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 of the redirected packets.  As a
   result, the combination of a reachability check and credit-based
   authorization 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 [I-D.ietf-hip-rfc5201-bis]).  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, there SHOULD be a

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   limit to 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

   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 both HIP and non-HIP aware hosts.
   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.

   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 simple
       solution is to prevent the use of HIP address check information
       to influence non-HIP sessions.

7.  IANA Considerations

   This document defines a LOCATOR_SET parameter for the Host Identity
   Protocol [I-D.ietf-hip-rfc5201-bis].  This parameter is defined in
   Section 4 with a Type of 193.

   This document also defines a LOCATOR_TYPE_UNSUPPORTED Notify Message
   Type as defined in the Host Identity Protocol specification

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   [I-D.ietf-hip-rfc5201-bis].  This parameter is defined in Section 5.3
   with a value of 46.

8.  Authors and Acknowledgments

   Pekka Nikander and Jari Arkko originated this document, and Christian
   Vogt and Thomas Henderson (editor) later joined as co-authors.  Greg
   Perkins contributed the initial draft of the security section.  Petri
   Jokela was a co-author of the initial individual submission.

   The authors thank Miika Komu, Mika Kousa, Jeff Ahrenholz, Jan Melen,
   Baris Boyvat, and Samu Varjonen for many improvements to the

9.  References

9.1.  Normative references

   [I-D.ietf-hip-rfc4423-bis]  Moskowitz, R., "Host Identity Protocol
                               draft-ietf-hip-rfc4423-bis-04 (work in
                               progress), July 2012.

   [I-D.ietf-hip-rfc5201-bis]  Moskowitz, R., Heer, T., Jokela, P., and
                               T. Henderson, "Host Identity Protocol
                               Version 2 (HIPv2)",
                               draft-ietf-hip-rfc5201-bis-08 (work in
                               progress), March 2012.

   [I-D.ietf-hip-rfc5202-bis]  Jokela, P., Moskowitz, R., Nikander, P.,
                               and J. Melen, "Using the Encapsulating
                               Security Payload (ESP) Transport Format
                               with the Host Identity Protocol (HIP)",
                               draft-ietf-hip-rfc5202-bis-00 (work in
                               progress), September 2010.

   [I-D.ietf-hip-rfc5204-bis]  Laganier, J. and L. Eggert, "Host
                               Identity Protocol (HIP) Rendezvous
                               Extension", draft-ietf-hip-rfc5204-bis-01
                               (work in progress), March 2011.

   [RFC2119]                   Bradner, S., "Key words for use in RFCs
                               to Indicate Requirement Levels", BCP 14,
                               RFC 2119, March 1997.

   [RFC3484]                   Draves, R., "Default Address Selection
                               for Internet Protocol version 6 (IPv6)",
                               RFC 3484, February 2003.

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   [RFC4291]                   Hinden, R. and S. Deering, "IP Version 6
                               Addressing Architecture", RFC 4291,
                               February 2006.

   [RFC4303]                   Kent, S., "IP Encapsulating Security
                               Payload (ESP)", RFC 4303, December 2005.

9.2.  Informative references

   [CBA-MIPv6]                 Vogt, C. and J. Arkko, "Credit-Based
                               Authorization for Mobile IPv6 Early
                               Binding Updates", Work in Progress,
                               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,
                               December 2005.

   [SIMPLE-CBA]                Vogt, C. and J. Arkko, "Credit-Based
                               Authorization for Concurrent Reachability
                               Verification", Work in Progress,
                               February 2006.

Appendix A.  Document Revision History

   To be removed upon publication

   | Revision | Comments                                               |
   | draft-00 | Initial version from RFC5206 xml (unchanged).          |
   | draft-01 | Remove multihoming-specific text; no other changes.    |
   | draft-02 | Update references to point to -bis drafts; no other    |
   |          | changes.                                               |
   | draft-03 | issue 4: add make before break use case                |
   |          | issue 6: peer locator exposure policies                |
   |          | issue 10: rename LOCATOR to LOCATOR_SET                |
   |          | issue 14: use of UPDATE packet's IP address            |

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Authors' Addresses

   Thomas R. Henderson (editor)
   The Boeing Company
   P.O. Box 3707
   Seattle, WA


   Christian Vogt
   Ericsson Research NomadicLab
   Hirsalantie 11
   JORVAS  FIN-02420


   Jari Arkko
   Ericsson Research NomadicLab
   JORVAS  FIN-02420

   Phone: +358 40 5079256

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