Network Working Group                              T. Henderson (editor)
Internet-Draft                                        The Boeing Company
Expires: January 18, August 28, 2006                               February 24, 2006                                  July 17, 2005

   End-Host Mobility and Multihoming with the Host Identity Protocol
                          draft-ietf-hip-mm-02
                          draft-ietf-hip-mm-03

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Copyright Notice

   Copyright (C) The Internet Society (2005). (2006).

Abstract

   This document defines mobility and multihoming extensions to the Host
   Identity Protocol (HIP).  Specifically, this document defines a
   general "LOCATOR" 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 parameter can also be used to support end-host
   multihoming, detailed procedures are left for further study.

Table of Contents

   1.  Introduction and Scope . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology and Conventions  . . . . . . . . . . . . . . . . .  5  6
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1  7
     3.1.  Operating Environment  . . . . . . . . . . . . . . . . . .  6
       3.1.1  7
       3.1.1.  Locator  . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2  9
       3.1.2.  Mobility overview  . . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.3 10
       3.1.3.  Multihoming overview . . . . . . . . . . . . . . . . . . . . .  7
     3.2 10
     3.2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .  8
       3.2.1 10
       3.2.1.  Mobility with single SA pair (no rekeying) . . . . . . . . . . . . .  8
       3.2.2 11
       3.2.2.  Host multihoming . . . . . . . . . . . . . . . . . . . 10
       3.2.3 13
       3.2.3.  Site multihoming . . . . . . . . . . . . . . . . . . . 12
       3.2.4 15
       3.2.4.  Dual host multihoming  . . . . . . . . . . . . . . . . 12
       3.2.5 15
       3.2.5.  Combined mobility and multihoming  . . . . . . . . . . 13
       3.2.6 16
       3.2.6.  Using LOCATORs across addressing realms  . . . . . . . 13
       3.2.7 16
       3.2.7.  Network renumbering  . . . . . . . . . . . . . . . . . 13
       3.2.8 16
       3.2.8.  Initiating the protocol in R1 or I2  . . . . . . . . . 13
     3.3 16
     3.3.  Other Considerations . . . . . . . . . . . . . . . . . . . 15
       3.3.1 17
       3.3.1.  Address Verification . . . . . . . . . . . . . . . . . 15
       3.3.2 17
       3.3.2.  Credit-Based Authorization . . . . . . . . . . . . . . 15
       3.3.3 18
       3.3.3.  Preferred locator  . . . . . . . . . . . . . . . . . . 16
       3.3.4 19
       3.3.4.  Interaction with Security Associations . . . . . . . . 17 20
   4.  LOCATOR parameter format . . . . . . . . . . . . . . . . . . . 20
     4.1 23
     4.1.  Traffic Type and Preferred Locator . . . . . . . . . . . . 21
     4.2 24
     4.2.  Locator Type and Locator . . . . . . . . . . . . . . . . . 22
     4.3 25
     4.3.  UPDATE packet with included LOCATOR  . . . . . . . . . . . 22 25
   5.  Processing rules . . . . . . . . . . . . . . . . . . . . . . . 23
     5.1 26
     5.1.  Locator data structure and status  . . . . . . . . . . . . 23
     5.2 26
     5.2.  Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 24
     5.3 26
     5.3.  Handling received LOCATORs . . . . . . . . . . . . . . . . 25
     5.4 28
     5.4.  Verifying address reachability . . . . . . . . . . . . . . 26
     5.5 30
     5.5.  Credit-Based Authorization . . . . . . . . . . . . . . . . 28
       5.5.1 31
       5.5.1.  Handling Payload Packets . . . . . . . . . . . . . . . 28
       5.5.2 31
       5.5.2.  Credit Aging . . . . . . . . . . . . . . . . . . . . . 29
     5.6 33
     5.6.  Changing the preferred locator . . . . . . . . . . . . . . 30 34
   6.  Policy considerations  . . . . . . . . . . . . . . . . . . . . 32
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
     7.1 36
     6.1.  Impersonation attacks  . . . . . . . . . . . . . . . . . . 33
     7.2 36
     6.2.  Denial of Service attacks  . . . . . . . . . . . . . . . . 34
       7.2.1 37
       6.2.1.  Flooding Attacks . . . . . . . . . . . . . . . . . . . 34
       7.2.2 37
       6.2.2.  Memory/Computational exhaustion DoS attacks  . . . . . 35
     7.3 38
     6.3.  Mixed deployment environment . . . . . . . . . . . . . . . 35
   8. 38
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   9. 40
   8.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   10. 41
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 39
   11. . 42
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
     11.1 . 43
     10.1. Normative references . . . . . . . . . . . . . . . . . . . 40
     11.2 43
     10.2. Informative references . . . . . . . . . . . . . . . . . . 40
       Author's Address . . . 43
   Appendix A.  Changes from previous versions  . . . . . . . . . . . 44
     A.1.  From nikander-hip-mm-00 to nikander-hip-mm-01  . . . . . . 44
     A.2.  From nikander-hip-mm-01 to nikander-hip-mm-02  . . . 41
   A.  Changes from previous versions . . . 44
     A.3.  From -02 to draft-ietf-hip-mm-00 . . . . . . . . . . . . . 42
     A.1 44
     A.4.  From nikander-hip-mm-00 draft-ietf-hip-mm-00 to nikander-hip-mm-01 -01 . . . . . . 42
     A.2   From nikander-hip-mm-01 to nikander-hip-mm-02 . . . . . . 42
     A.3 . 45
     A.5.  From -02 draft-ietf-hip-mm-01 to draft-ietf-hip-mm-00 -02 . . . . . . . . . . . . . 42
     A.4 45
     A.6.  From draft-ietf-hip-mm-00 draft-ietf-hip-mm-02 to -01 -03 . . . . . . . . . . . . . 43
     A.5   From draft-ietf-hip-mm-01 to -02 45
   Author's Address . . . . . . . . . . . . . 43 . . . . . . . . . . . . 47
   Intellectual Property and Copyright Statements . . . . . . . . 44 . . 48

1.  Introduction and Scope

   The Host Identity Protocol [1] (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 ESP Security Associations) 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 [2].

   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 mobility and simple multihoming,
   leaving more complicated scenarios and other variations for further
   study.  Specifically,

      This document defines a generalized LOCATOR parameter for use in
      HIP messages.  The LOCATOR parameter allows a HIP host to notify a
      peer about alternate addresses 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 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 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 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 left for further study.

   There

   While HIP can potentially be used with transports other than the ESP
   transport format [5], this document largely assumes the use of ESP
   and leaves other transport for further study.

   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, end-host and
   site multihoming with legacy hosts, 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 [3].  Such functionality is
   out of scope of this document.  We also do not consider localized
   mobility management extensions; 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 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC2119 [6].

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

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

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

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

3.  Protocol Model

3.1

3.1.  Operating Environment

   The Host Identity Protocol (HIP) [2] 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 ESP transport format [5]
   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
   multihoming.  In a nutshell, summary, these extensions to the HIP protocol can
   carry new addressing information to the peer and can enable direct
   authentication of the message via a signature or keyed hash message
   authentication code (HMAC) based on its host identity.  This document
   specifies the format of this new addressing (LOCATOR) parameter, the
   procedures for sending and processing this parameter to enable basic
   host mobility, and procedures for a concurrent address verification
   mechanism.

3.1.1  Locator

   This document defines a generalization of an address called a
   "locator".  A locator specifies a point-of-attachment

            ---------
            | TCP   |  (sockets bound 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 HITs)
            ---------
               |
            ---------
      ----> | ESP   |  {HIT_s, HIT_d} <-> SPI
      |     ---------
      |         |
    ----    ---------
   | MH |-> | HIP sublayer of the stack.  This generalization is useful
   because   |  {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI}
    ----    ---------
               |
            ---------
            |  IP addresses alone may not be sufficient to describe how
   packets should be handled below HIP.  For example, in a host   |
            ---------

   Figure 2: Architecture for  HIP mobility and multihoming context, certain IP addresses may need to be associated
   with certain ESP SPIs, to avoid violation

   Figure 2 depicts a layered architectural view of the a HIP-enabled stack
   using ESP anti-replay
   window [4].  Addresses may also be affiliated with transport ports format.  In HIP, upper-layer protocols (including
   TCP and ESP in
   certain tunneling scenarios.  Or locators may merely be traditional
   network addresses.

3.1.2  Mobility

   When a host moves this figure) are bound to another address, it notifies its peer of the new
   address by sending a HITs and not IP addresses.
   The HIP UPDATE packet containing a LOCATOR
   parameter.  This UPDATE packet sublayer is acknowledged by responsible for maintaining the peer, binding between
   HITs and is
   protected by retransmission. IP addresses.  The peer can authenticate the contents
   of the UPDATE packet based on the signature SPI (or other context tag if ESP is not
   used with HIP), and keyed hash of not necessarily the
   packet. IP addresses, is used to
   associate an incoming packet with the right HITs.  The host may at block labeled
   "MH" is introduced below.

   Consider first 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 case in the
   UPDATE packet, which there is no mobility or multihoming,
   as defined specified in the base protocol specification [2].

   When using ESP Transport Format [5],  The HIP base
   exchange establishes the host is able HITs in use between the hosts, the SPIs to receive
   packets that are protected using a
   use for ESP, and the IP addresses (used in the HIP created ESP SA from any
   address.  Thus, a host signaling
   packets).  Note that there can change its IP address only be one such binding in the
   outbound direction for any given packet, and continue to send
   packets to its peers.  However, the peers only selectors for
   the binding at the HIP layer are not able to reply
   before they can reliably the fields exposed by ESP (the SPI
   and securely update HITs).  For the set of addresses inbound direction, the SPI is all that they associate with is
   required to find the sending host.  Furthermore, mobility may right host context.  ESP rekeying events change
   the path characteristics in such a manner that reordering
   occurs mapping between the HIT pair and packets fall outside SPI, but do not change the ESP anti-replay window.

3.1.3  Multihoming

   A related operational configuration is host multihoming, IP
   addresses.

   Consider next a mobility event, in which a host has multiple locators simultaneously rather than sequentially as is still single-homed
   but moves to another IP address.  Two things must occur in the case this case.
   First, the peer must be notified of mobility.  By using the locator parameter defined
   herein, address change using a HIP
   UPDATE message.  Second, each host can inform must change its peers of additional (multiple) locators local bindings at which it can
   the HIP sublayer (new IP addresses).  It may be reached, that both the SPIs
   and can declare a particular locator as IP addresses are changed simultaneously in a
   "preferred" locator.  Although this single UPDATE; the
   protocol described herein supports this.  This document defines a mechanism for
   multihoming, it does not define associated policies specifies the
   messaging and elements of procedure
   details for such as which locators to choose a mobility event.
   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.

   Finally, consider the case when a host is multihomed (has more than
   one pair is
   available, globally routable address) and wants to make these multiple
   addresses available for use by the operation upper layer protocols, for fault
   tolerance.  Examples include the use of simultaneous mobility and multihoming, (possibly multiple) IPv4 and
   IPv6 addresses on the implications same interface, or the use of multiple
   interfaces attached to different service providers.  Such host
   multihoming on transport protocols and generally necessitates that a separate ESP
   anti-replay windows.  Additional definition of HIP-based multihoming SA is expected
   maintained for each interface in order to be part of a future document.

3.2  Protocol Overview

   In this section we briefly introduce a number prevent packets that arrive
   over different paths from falling outside of usage scenarios
   where the HIP mobility and multihoming facility is useful.  These
   scenarios assume ESP replay
   protection window.  Multihoming thus makes possible that HIP is being used with the ESP Transform,
   although other scenarios may be defined in bindings
   shown on the future.  To understand
   these usage scenarios, the reader should be at least minimally
   familiar with right side of Figure 2 are one to many (in the HIP protocol specification [2]. outbound
   direction, one HIT pair to multiple SPIs, and possibly then to
   multiple IP addresses).  However, only one SPI and address can be
   used for any given packet, so the
   (relatively) uninitiated reader it job of the "MH" block depicted
   above is most important to keep in mind
   that in dynamically manipulate these bindings.  Beyond locally
   managing such multiple bindings, the peer-to-peer HIP signaling
   protocol needs to be flexible enough to define the actual payload traffic is protected with ESP, desired mappings
   between HITs, SPIs, and
   that the ESP SPI acts as an index addresses, and needs to ensure that UPDATE
   messages are sent along the right host-to-host context.

   Each of the scenarios below assumes network paths so that any HIP-aware
   middleboxes can observe the HIP base exchange has
   completed, and SPIs.  This document does not specify the hosts each have a single outbound SA
   "MH" block, nor does it specify detailed elements of procedure for
   how to the peer
   host.  Associated handle various multihoming (perhaps combined with mobility)
   scenarios.  However, this outbound SA is document does describe a single destination basic multihoming
   case (one host adds one address of the peer host-- the source to its initial address used by the peer during and notifies
   the base exchange.

   The readdressing protocol is peer) and leave more complicated scenarios for experimentation
   and future documents.

3.1.1.  Locator

   This document defines a generalization of an asymmetric protocol where one host, address called a
   "locator".  A locator specifies a point-of-attachment to the mobile host, informs another host, called network
   but may also include additional end-to-end tunneling or per-host
   demultiplexing context that affects how packets are handled below the peer host,
   about changes of IP addresses on affected SPIs.  The readdressing
   exchange is designed to be piggybacked on existing
   logical HIP exchanges.
   The main packets on which sublayer of the LOCATOR parameters are expected stack.  This generalization is useful
   because IP addresses alone may not be sufficient to describe how
   packets should be
   carried are UPDATE packets.  However, some implementations handled below HIP.  For example, in a host
   multihoming context, certain IP addresses may want need to experiment with sending LOCATOR parameters also on other packets,
   such as R1, I2, and NOTIFY.

3.2.1  Mobility be associated
   with single SA pair

   A mobile host must sometimes change an IP address bound certain ESP SPIs, to an
   interface.  The change avoid violation of an IP address might the ESP anti-replay
   window [4].  Addresses may also be needed due to a
   change affiliated with transport ports in the advertised IPv6 prefixes on the link, a reconnected PPP
   link,
   certain tunneling scenarios.  Or locators may merely be traditional
   network addresses.  In Section 4, a new DHCP lease, generalized HIP LOCATOR parameter
   is defined that can contain one or an actual movement more locators (addresses).

3.1.2.  Mobility overview

   When a host moves to another subnet.  In
   order to maintain its communication context, the host must inform address, it notifies its
   peers about peer of the new IP address.
   address by sending a HIP UPDATE packet containing a LOCATOR
   parameter.  This first example considers the
   case in which UPDATE packet is acknowledged by the mobile host has only one interface, IP address, peer, and
   a single pair of SAs (one inbound, one outbound).

   1.  The mobile host is disconnected from the
   protected by retransmission.  The peer host for a brief
       period can authenticate the contents
   of time while it switches from one IP address to another.
       Upon obtaining a new IP address, the mobile host sends a LOCATOR
       parameter to the peer host in an UPDATE message.  The LOCATOR
       indicates packet based on the new IP address signature and the SPI associated with the new
       IP address by using a Locator Type keyed hash of "1", the locator lifetime,
       and whether
   packet.

   When using ESP Transport Format [5], the new locator is a preferred locator.  The mobile host may optionally send an ESP_INFO at the same time
   decide to create rekey its security association and possibly generate a new inbound SA,
   Diffie-Hellman key; all of these actions are triggered by including
   additional parameters in which case it transitions to state REKEYING.  In this case,
       the Locator contains the new SPI to use.  Otherwise, the existing
       SPI is identified UPDATE packet, as defined in the Locator parameter, base
   protocol specification [2] and ESP extension [5].

   When using ESP (and possibly other transport modes in the host waits
       for its UPDATE to be acknowledged.

   2.  Depending on whether the mobile host initiated a rekey, and on
       whether future),
   the peer host itself wants is able to rekey, a number of
       responses receive packets that are possible.  Figure 2 illustrates an exchange for
       which neither side initiates protected using a HIP
   created ESP SA from any address.  Thus, a rekeying, but for which the peer host performs an can change its IP
   address check.  If the mobile host is rekeying,
       the peer will also rekey, as shown in Figure 3.  If and continue to send packets to its peers without necessarily
   rekeying.  However, the mobile
       host did peers are not decide able to rekey but reply before they can
   reliably and securely update the peer desires to do so, then set of addresses that they associate
   with the sending host.  Furthermore, 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.

3.1.3.  Multihoming overview

   A related operational configuration is host multihoming, in which a
   host has multiple locators simultaneously rather than sequentially as
   in the case of mobility.  By using the LOCATOR parameter defined
   herein, a host can inform its peers of additional (multiple) locators
   at which it can be reached, and can declare a particular locator as a
   "preferred" locator.  Although this document defines a mechanism for
   multihoming, it does not define detailed policies and procedures such
   as which locators to choose when more than one pair is available, the
   operation of simultaneous mobility and multihoming, and the
   implications of multihoming on transport protocols and ESP anti-
   replay windows.  Additional definition of HIP-based multihoming is
   expected to be part of future documents.

3.2.  Protocol Overview

   In this section we briefly introduce a number of usage scenarios for
   HIP mobility and multihoming.  These scenarios assume that HIP is
   being used with the ESP transform [5], 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 [2].  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.

   Each of the scenarios below assumes that the HIP base exchange has
   completed, and the hosts each have a single outbound SA to the peer
   host.  Associated with this outbound SA is a single destination
   address of the peer host-- the source address used by the peer during
   the base exchange.

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

   Hosts that use link-local addresses as source addresses in their HIP
   handshakes may not be reachable by a mobile peer.  Such hosts SHOULD
   provide a globally routable address either in the initial handshake
   or via the LOCATOR parameter.

3.2.1.  Mobility with 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

             UPDATE(ESP_INFO, LOCATOR, SEQ)
        ----------------------------------->
             UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST)
        <-----------------------------------
             UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->

   Figure 3: Readdress without rekeying, but with address check
   1.  The mobile host is disconnected from the peer host for a brief
       period of time while it switches from one IP address to another.
       Upon obtaining a new IP address, the mobile host sends a LOCATOR
       parameter to the peer host in an UPDATE message.  The UPDATE
       message also contains an ESP_INFO parameter with the "Old SPI"
       and "New SPI" parameters both set to the value of the pre-
       existing 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 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].

   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
       hte 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 pre-existing 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 initiates 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 it 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 rekey correct reply to its
   UPDATE challenge, it marks the new address as illustrated ACTIVE and removes the
   old address.

3.2.1.1.  Mobility with single SA pair (mobile-initiated rekey)

   The mobile host may decide to rekey the SAs at the same time that it
   is notifying the peer of the new address.  In this case, the above
   procedure described in Figure 4. 3 is slightly modified.  The UPDATE
       messages
   message sent from the peer back mobile host includes an ESP_INFO with the "Old
   SPI" set to the mobile are sent previous SPI, the "New SPI" set to the
       newly advertised address.

   3.  While desired new
   SPI value for the incoming SA, and the Keymat Index desired.
   Optionally, the peer host may include a DIFFIE_HELLMAN parameter for a new
   Diffie-Hellman key.  The peer completes the request for rekey as is verifying
   normally done for HIP rekeying, except that the new address, the address is
       marked kept
   as UNVERIFIED in until the interim.  Once it has received a
       correct reply to its UPDATE challenge, or optionally, data on the
       new SA, it marks the new address nonce challenge is received as ACTIVE and removes the old
       address.

     Mobile Host                         Peer Host

             UPDATE(ESP_INFO, LOC, SEQ)
        ----------------------------------->
             UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST)
        <-----------------------------------
             UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->
   described above.  Figure 2: Readdress without rekeying, but with address check 4 illustrates this scenario.

     Mobile Host                         Peer Host

             UPDATE(ESP_INFO, LOC, LOCATOR, SEQ, [DIFFIE_HELLMAN])
        ----------------------------------->
             UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
        <-----------------------------------
             UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->

   Figure 3: 4: Readdress with mobile-initiated rekey

3.2.1.2.  Mobility with single SA pair (peer-initiated rekey)

   A second variation of this basic mobility scenario covers the case in
   which the mobile host does not attempt to rekey the existing SAs, but
   the peer host decides to do so.  This typically results in a four
   packet exchange, as shown in Figure 5.  The initial UPDATE packet
   from the mobile host is the same as in the scenario for which there
   is no rekey (Figure 3).  The peer may decide to rekey, however, in
   which case the subsequent three packets follow the normal rekeying
   procedure described in the ESP specification [5], with the addition
   of the ECHO_REQUEST and ECHO_RESPONSE nonce for verification of the
   new address.

     Mobile Host                         Peer Host

             UPDATE(LOC,

             UPDATE(ESP_INFO, LOCATOR, SEQ)
        ----------------------------------->
             UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN], ECHO_REQUEST)
        <-----------------------------------
             UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_RESPONSE)
        ----------------------------------->
                UPDATE(ACK)
        <-----------------------------------

               Figure 4: Readdress with peer-initiated rekey

   Hosts that use link-local addresses as source addresses in their HIP
   handshakes may not be reachable by a mobile peer.  Such hosts SHOULD
   provide a globally routable address either in the initial handshake
   or via the LOCATOR parameter.

3.2.2
             UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_RESPONSE)
        ----------------------------------->
                UPDATE(ACK)
        <-----------------------------------

   Figure 5: Readdress with peer-initiated rekey

3.2.2.  Host multihoming

   A (mobile or stationary) host may sometimes have more than one
   interface.
   interface or global address.  The host may notify the peer host of
   the additional
   interface(s) interface or address by using the LOCATOR parameter.
   To avoid problems with the ESP anti-replay window, a host SHOULD use
   a different SA for each interface or address used to receive packets
   from the peer host.

   When more than one locator is provided to the peer host, the host
   SHOULD indicate which locator is preferred.  By default, the
   addresses used in the base exchange are preferred until indicated
   otherwise.

   Although the protocol may allow for configurations in which there is
   an asymmetric number of SAs between the hosts (e.g., one host has two
   interfaces and two inbound SAs, while the peer has one interface and
   one inbound SA), it is RECOMMENDED that inbound and outbound SAs be
   created pairwise between hosts.  When an ESP_INFO arrives to rekey a
   particular outbound SA, the corresponding inbound SA should be also
   rekeyed at that time.  Although asymmetric SA configurations might be
   experimented with, their usage may constrain interoperability at this
   time.  However, it is recommended that implementations attempt to
   support peers that prefer to use non-paired SAs.  It is expected that
   this section and behavior will be modified in future revisions of
   this protocol, once the issue and its implications are better
   understood.

   To add both

   Consider the case between two single-homed hosts, in which one of the
   host notifies the peer of an additional interface and SA, address.  It is RECOMMENDED
   that the host set up a new SA pair for use on this new address.  To
   do this, the multihomed host sends a LOCATOR with an ESP_INFO.  The host uses the same (new) SPI value in ESP_INFO,
   indicating the
   LOCATOR and both request for a new SA by setting the "Old SPI" value to
   zero, and the "New SPI" values in value to the ESP_INFO--
   this indicates newly created incoming SPI.  A
   Locator Type of "1" is used to associate the peer new address with the new
   SPI.  The LOCATOR parameter also contains a second Type 1 locator:
   that of the SPI is not replacing an existing original address and SPI.  To simplify parameter
   processing and avoid explicit protocol extensions to remove locators,
   each LOCATOR parameter must list all locators in use on a connection
   (a complete listing of inbound locators and SPIs for the host).  The
   multihomed host transitions to state REKEYING, waiting for a ESP_INFO
   (new outbound SA) from the peer and an ACK of its own UPDATE.  As in
   the mobility case, the peer host must perform an address check while it
   is rekeying. verification
   before putting the new address into active use.  Figure 5 6 illustrates
   the basic packet exchange.

     Multi-homed Host                    Peer Host

              UPDATE(ESP_INFO, LOC, SEQ, [DIFFIE_HELLMAN])
        ----------------------------------->
              UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
        <-----------------------------------
              UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->

                   Figure 5: Basic multihoming scenario

   For the case in which multiple locators are advertised in a LOCATOR,
   the peer does not need to send ACK for the UPDATE(LOCATOR) in every
   subsequent message used for the address check procedure of the
   multiple locators.  Therefore, a sample packet exchange might look as
   shown in Figure 6.

     Multi-homed Host                    Peer Host

                UPDATE(LOC(addr_1,addr_2), SEQ)
        ----------------------------------->
                UPDATE(ACK)
        <-----------------------------------

        sent to addr_1:UPDATE(ESP_INFO,

     Multi-homed Host                    Peer Host

              UPDATE(ESP_INFO, LOCATOR, SEQ, ECHO_REQUEST)
        <-----------------------------------
                UPDATE(ACK, ECHO_RESPONSE) [DIFFIE_HELLMAN])
        ----------------------------------->

        sent to addr_2:UPDATE(ESP_INFO,
              UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
        <-----------------------------------
              UPDATE(ACK, ECHO_RESPONSE)
        ----------------------------------->

   Figure 6: LOCATOR with multiple addresses Basic multihoming scenario
   When processing inbound LOCATORs that establish new security
   associations,
   associations on an interface with multiple addresses, a host uses the
   destination address of the UPDATE containing LOCATOR as the local
   address to which the LOC LOCATOR plus ESP_INFO is targeted.  Hosts may
   send LOCATOR UPDATEs with the same IP address in the LOCATOR to different
   peer addresses-- this has the effect of creating multiple inbound SAs
   implicitly affiliated with different peer source addresses.

   When rekeying in a multihoming situation in which there is an
   asymmetric number of SAs between two hosts, a respondent to the
   ESP_INFO/UPDATE procedure may have some ambiguity as to which inbound
   SA it should update in response to the peer's UPDATE.  In such a
   case, the host SHOULD choose an SA corresponding to the inbound
   interface on which the UPDATE was received.

3.2.3

3.2.3.  Site multihoming

   A host may have an interface that has multiple globally reachable IP
   addresses.  Such a situation may be a result of the site having
   multiple upper Internet Service Providers, or just because the site
   provides all hosts with both IPv4 and IPv6 addresses.  It is
   desirable that the host can stay reachable with all or any subset of
   the currently available globally routable addresses, independent on
   how they are provided.

   This case is handled the same as if there were different IP
   addresses, described above in Section 3.2.2.  Note that a single
   interface may experience site multihoming while the host itself may
   have multiple interfaces.

   Note that a host may be multi-homed and mobile simultaneously, and
   that a multi-homed host may want to protect the location of some of
   its interfaces while revealing the real IP address of some others.

   This document does not presently specify additional site multihoming
   extensions to HIP to HIP; further align it alignment with the requirements of the
   multi6 IETF shim6 working group.

3.2.4
   group may be considered in the future.

3.2.4.  Dual host multihoming

   Consider the case in which both hosts would like to add an additional
   address after the base exchange completes.  In Figure 7, consider
   that host1 wants to add address addr1b.  It would send a an UPDATE with
   LOCATOR to host2 located at addr2a, and a new set of SPIs would be
   added between hosts 1 and 2 (call them SPI1b and SPI2b).  Next,
   consider host2 deciding to add addr2b to the relationship. host2 now
   has a choice of to which of host1's addresses to initiate LOCATOR to. an UPDATE.  It
   may choose to initiate a LOCATOR an UPDATE to addr1a, addr1b, or both.  If it
   chooses to send to both, then a full mesh (four SA pairs) of SAs
   would exist between the two hosts.  This is the most general case; it
   often may be often the case that hosts primarily establish new SAs only
   with the peer's preferred locator.  The readdressing protocol is
   flexible enough to accommodate this choice.

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

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

   Figure 7: Dual multihoming case in which each host uses LOCATOR to
   add a second address

3.2.5

3.2.5.  Combined mobility and multihoming

   It looks likely that in the future many mobile hosts will be
   simultaneously mobile and multi-homed, i.e., have multiple mobile
   interfaces.  Furthermore, if the interfaces use different access
   technologies, it is fairly likely that one of the interfaces may
   appear stable (retain its current IP address) while some other(s) may
   experience mobility (undergo IP address change).

   The use of LOCATOR plus ESP_INFO should be flexible enough to handle
   most such scenarios, although more complicated scenarios have not
   been studied so far.

3.2.6

3.2.6.  Using LOCATORs 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.
   If no mechanism exists, then the UPDATE message carrying the new
   LOCATOR will likely not be acknowledged reach the destination anyway, and the HIP
   state may time out.

3.2.7

3.2.7.  Network renumbering

   It is expected that IPv6 networks will be renumbered much more often
   than most IPv4 networks are.  From an end-host point of view, network
   renumbering is similar to mobility.

3.2.8

3.2.8.  Initiating the protocol in R1 or I2

   A Responder host MAY include one or more LOCATOR parameters in the R1
   packet that it sends to the Initiator.  These parameters MUST be
   protected by the R1 signature.  If the R1 packet contains LOCATOR
   parameters with a new preferred locator, the Initiator SHOULD
   directly set the new preferred locator to status ACTIVE without
   performing address verification first, and MUST send the I2 packet to
   the new preferred locator.  The I1 destination address and the new
   preferred locator may be identical.  All new non-preferred locators
   must still undergo address verification.

            Initiator                                Responder

                              R1 with LOCATOR
                  <-----------------------------------
   record additional addresses
   change responder address
                     I2 with new SPI in ESP_INFO parameter sent to newly indicated preferred address
                  ----------------------------------->
                                                     (process normally)
                                  R2
                  <-----------------------------------
   (process normally) normally, later verification of non-preferred locators)

   Figure 8: LOCATOR inclusion in R1

   An Initiator MAY include one or more LOCATOR parameters in the I2
   packet, independent on of whether there was a LOCATOR parameter(s) parameter in the
   R1 or not.  These parameters MUST be protected by the I2 signature.
   Even if the I2 packet contains LOCATOR parameters, the Responder MUST
   still send the R2 packet to the source address of the I2.  The new
   preferred locator SHOULD be identical to the I2 source address.  If
   the I2 packet contains LOCATOR parameters, all new locators must
   undergo address verification as usual.  If any of these locators is a
   new preferred locator, an efficient method to verify this is to
   piggyback an ECHO_REQUEST parameter with some unguessable data to the
   R2 packet.

            Initiator                                Responder

                             I2 with LOCATOR
                  ----------------------------------->
                                                     (process normally)
                                             record additional addresses
                       R2 with new SPI in ESP_INFO parameter sent to source address of I2
                  <-----------------------------------
   (process normally)
                           data on new SA
                  ------------------------------------>
                                                      (process normally)

   Figure 9: LOCATOR inclusion in I2

3.3

3.3.  Other Considerations

3.3.1

3.3.1.  Address Verification

   When a HIP host receives a set of locators from another HIP host in a
   LOCATOR, 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 [10]. [8].  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 address.

   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 exchange of a
   nonce, or generation of a new SPI and observing data arriving on the
   new SPI.

3.3.2

3.3.2.  Credit-Based Authorization

   Credit-Based Authorization allows a host to securely use a new
   locator even though the peer's reachability at the address embedded
   in this 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 itself for the purpose of its attack because
       bandwidth is an ample resource for many attractive victims.

   2.  An attacker can always cause unamplified flooding by sending
       packets to its victim directly.

   3.  Consequently, the additional effort required to set up a
       redirection-based flooding attack 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 any
   amplification that can be reached through it.  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, e.g., pure direct
   flooding, where the attacker itself sends bogus packets to the
   victim.

   Figure 10 illustrates Credit-Based Authorization: Host B measures the
   bytes recently received from peer A and, when A readdresses, sends
   packets to A's new, unverified address as long as the sum of their
   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, due to multihoming, or due to any other reason.

          +-------+                  +-------+
          |   A   |                  |   B   |
          +-------+                  +-------+
              |                          |
      address |------------------------->| credit += size(packet)
       ACTIVE |                          |
              |------------------------->| credit += size(packet)
              |<-------------------------| don't change credit
              |                          |
              + address change           |
      address |<-------------------------| credit -= size(packet)
   UNVERIFIED |------------------------->| credit += size(packet)
              |<-------------------------| credit -= size(packet)
              |                          |
              |<-------------------------| credit -= size(packet)
              |                          X credit < size(packet)=> drop!
              |                          |
              + address change           |
      address |                          |
       ACTIVE |<-------------------------| don't change credit
              |                          |

   Figure 10: Readdressing Scenario

3.3.3

3.3.3.  Preferred locator

   When a host has multiple locators, the peer host must decide upon
   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).

   In general, when multiple locators are used for a session, there is
   the question of using multiple locators for failover only or for
   load-balancing.  Due to the implications of load-balancing on the
   transport layer that still need to be worked out, this draft assumes
   that multiple locators are used primarily for failover.  An
   implementation may use ICMP interactions, reachability checks, or
   other means to detect the failure of a locator.

3.3.4

3.3.4.  Interaction with Security Associations

   This document specifies a new HIP protocol parameter, the LOCATOR
   parameter (see Section 4), that allows the hosts to exchange
   information about their locator(s), and any changes in their
   locator(s).  The logical structure created with LOCATOR parameters
   has three levels: hosts, Security Associations (SAs) indexed by
   Security Parameter Indices (SPIs), and addresses.

   The relation between these entities for an association negotiated as
   defined in the base specification [2] and ESP transform [5] is
   illustrated in Figure 11.

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

   Figure 11: Relation between hosts, SPIs, and addresses (base
   specification)

   In Figure 11, host1 and host2 negotiate two unidirectional SAs, and
   each host selects the SPI value for its inbound SA.  The addresses
   addr1a and addr2a are the source addresses that each host uses in the
   base HIP exchange.  These are the "preferred" (and only) addresses
   conveyed to the peer for each SA; even though packets sent to any of
   the hosts' interfaces can arrive on an inbound SPI, when a host sends
   packets to the peer on an outbound SPI, it knows of a single
   destination address associated with that outbound SPI (for host1, it
   sends a packet on SPI2a to addr2a to reach host2), unless other
   mechanisms exist to learn of new addresses.

   In general, the bindings that exist in an implementation
   corresponding to this draft can be depicted as shown in Figure 12.
   In this figure, a host can have multiple inbound SPIs (and, not
   shown, multiple outbound SPIs) between itself and another host.
   Furthermore, each SPI may have multiple addresses associated with it.
   These addresses bound to an SPI are not used as SA selectors.
   Rather, the addresses are those addresses that are provided to the
   peer host, as hints for which addresses to use to reach the host on
   that SPI.  The LOCATOR parameter allows for IP addresses and SPIs to
   be combined to form generalized locators.  The LOCATOR parameter is
   used to change the set of addresses that a peer associates with a
   particular SPI.

                            address11
                          /
                   SPI1   - address12
                 /
                /           address21
           host -- SPI2   <
                \           address22
                 \
                   SPI3   - address31
                          \
                            address32

   Figure 12: Relation between hosts, SPIs, and addresses (general case)

   A host may establish any number of security associations (or SPIs)
   with a peer.  The main purpose of having multiple SPIs is to group
   the addresses into collections that are likely to experience fate
   sharing.  For example, if the host needs to change its addresses on
   SPI2, it is likely that both address21 and address22 will
   simultaneously become obsolete.  In a typical case, such SPIs may
   correspond with physical interfaces; see below.  Note, however, that
   especially in the case of site multihoming, one of the addresses may
   become unreachable while the other one still works.  In the typical
   case, however, this does not require the host to inform its peers
   about the situation, since even the non-working address still
   logically exists.

   A basic property of HIP SAs is that the inbound IP address is not
   used as a selector for the SA.  Therefore, in Figure 12, it may seem
   unnecessary for address31, for example, to be associated only with
   SPI3-- in practice, a packet may arrive to SPI1 via destination
   address address31 as well.  However, the use of different source and
   destination addresses typically leads to different paths, with
   different latencies in the network, and if packets were to arrive via
   an arbitrary destination IP address (or path) for a given SPI, the
   reordering due to different latencies may cause some packets to fall
   outside of the ESP anti-replay window.  For this reason, HIP provides
   a mechanism to affiliate destination addresses with inbound SPIs, if
   there is a concern that anti-replay windows might be violated
   otherwise.  In this sense, we can say that a given inbound SPI has an
   "affinity" for certain inbound IP addresses, and this affinity is
   communicated to the peer host.  Each physical interface SHOULD have a
   separate SA, unless the ESP anti-replay window is loose.

   Moreover, even if the destination addresses used for a particular SPI
   are held constant, the use of different source interfaces may also
   cause packets to fall outside of the ESP anti-replay window, since
   the path traversed is often affected by the source address or
   interface used.  A host has no way to influence the source interface
   on which a peer uses to send its packets on a given SPI.  Hosts
   SHOULD consistently use the same source interface and address when
   sending to a particular destination IP address and SPI.  For this
   reason, a host may find it useful to change its SPI or at least reset
   its ESP anti-
   replay anti-replay window when the peer host readdresses.

   An address may appear on more than one SPI.  This creates no
   ambiguity since the receiver will ignore the IP addresses as SA
   selectors anyway.

   A single LOCATOR parameter contains data only about one SPI.  To
   simultaneously signal changes on several SPIs, it is necessary to
   send several LOCATOR parameters.  The packet structure supports this.  However, this document does not specify such
   cases.

   If the LOCATOR parameter is sent in an UPDATE packet, then the
   receiver will respond with an UPDATE acknowledgment.  If the LOCATOR
   parameter is sent in a NOTIFY, I2, or R2 packet, then the recipient
   may consider the LOCATOR as informational, and act only when it needs
   to activate a new address.  The use of LOCATOR in a NOTIFY message
   may not be compatible with middleboxes.

4.  LOCATOR parameter format

   The LOCATOR parameter is a critical parameter as defined by [2].  The
   LOCATOR parameter is also abbreviated as "LOC" in the figures herein.  It
   consists of the standard HIP parameter Type and Length fields, plus
   one 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.

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

   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 bytes are
      supported).

   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: 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 bytes 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 an destination address if an alternate (non-deprecated) is
   available and has sufficient scope.

4.1

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 precedes it.  That is, if a "P" bit is set for Traffic Type "2",
   for example, that means that the locator is preferred for data
   packets.  If there is a conflict (for example, if P bit is set for
   both an
   address of Type "0" and a different address of Type "2"), the more
   specific Traffic Type rule applies.  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
   locators.

4.2

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 [7] (128
      bits long).

   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).

4.3

4.3.  UPDATE packet with included LOCATOR

   A number of combinations of parameters in an UPDATE packet are
   possible (e.g., see Section 3.2).  Only one LOCATOR parameter is used
   in any HIP packet, and this LOCATOR SHOULD list all of the locators
   that the host wishes to make available for the HIP association.  Any
   UPDATE packet that includes includes a LOCATOR parameter SHOULD include both
   an HMAC and a HIP_SIGNATURE parameter.

5.  Processing rules

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  lifetime (seconds),

   o  status (UNVERIFIED, ACTIVE, DEPRECATED).

   The status is used to track the reachability of the address embedded
   within the LOCATOR parameter:

   UNVERIFIED indicates that the reachability of the address has not
      been verified yet,

   ACTIVE indicates that the reachability of the address has been
      verified and the address has not been deprecated,

   DEPRECATED indicates that the locator lifetime has expired

   The following state changes are allowed:

   UNVERIFIED to ACTIVE The reachability procedure completes
      successfully.

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

   ACTIVE to DEPRECATED The locator lifetime expires while it 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
      again.

   DEPRECATED to UNVERIFIED The host receives a new lifetime for the
      locator.

   A DEPRECATED address MUST NOT be changed to ACTIVE without first
   verifying its reachability.

5.2.  Sending LOCATORs

   The decision of when to send LOCATORs is basically a local policy
   issue.  However, it is RECOMMENDED that a host sends a LOCATOR parameter SHOULD include both an HMAC and
   whenever it recognizes a HIP_SIGNATURE
   parameter.

5.  Processing rules

   HIP mobility and multihoming is fundamentally based change of its IP addresses in use on the HIP
   architecture [1], where the transport and internetworking layers are
   decoupled from each other by an interposed host identity protocol
   layer.  In the
   active HIP architecture, association, and assumes that the transport layer sockets are
   bound change is going to the Host Identifiers (through HIT or LSI last
   at least for a few seconds.  Rapidly sending conflicting LOCATORs
   SHOULD be avoided.

   When a host decides to inform its peers about changes in the case of
   legacy APIs), and the Host Identifiers are translated its IP
   addresses, it has to decide how to group the actual
   IP address. various addresses with
   SPIs.  The HIP base protocol specification [2] grouping should consider also whether middlebox
   interaction requires sending (the same) LOCATOR in separate UPDATEs
   on different paths.  Since each SPI is expected to be commonly
   used associated with the ESP Transport Format [5] to establish a pair of different
   Security Associations (SA).  The Association, the grouping policy may also be based on ESP SAs are then used to carry
   anti-replay protection considerations.  In the
   actual payload data between typical case, simply
   basing the two hosts, by wrapping TCP, UDP, grouping on actual kernel level physical and
   other upper layer packets into transport mode ESP payloads.  The IP
   header uses logical
   interfaces may be the actual best policy.  Grouping policy is outside of the
   scope of this document.

   Note that the purpose of announcing IP addresses in a LOCATOR is to
   provide connectivity between the network.

   Although HIP may also 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 specified in the future possible to operate replace most tunnels with an
   alternative to ESP providing the per-packet HIP context,
   based "non-tunneling", therefore making most virtual interfaces
   fairly unnecessary in the
   remainder of this document assumes that HIP is being used future.  Therefore, virtual interfaces
   SHOULD NOT be announced in
   conjunction with ESP.  Future documents may extend this document to
   include general.  On the other behaviors when ESP is not used.

   The base specification does not contain any mechanisms hand, there are
   clearly situations where tunnels are used for changing diagnostic and/or
   testing purposes.  In such and other similar cases announcing the IP
   addresses that were used during of virtual interfaces may be appropriate.

   Once the base HIP exchange.  Hence,
   in order to remain connected, any systems that implement only host has decided on the
   base specification groups and nothing else must retain the ability assignment of addresses
   to
   receive packets at their primary IP address; that is, those systems
   cannot change the IP address on which they are using to receive
   packets without causing loss of connectivity until SPIs, it creates a base exchange is
   performed from LOCATOR parameter that serves as a complete
   representation of the new address.

5.1  Locator data structure addresses and status

   In affiliated SPIs intended for
   active use.  We now describe a typical implementation, few cases introduced in Section 3.2.
   We assume that the Traffic Type for each outgoing locator is represented as
   a piece of state set to "0" (other
   values for Traffic Type may be specified in documents that contains the following data:

   o  the actual bit pattern representing the locator,

   o  lifetime (seconds),

   o  status (UNVERIFIED, ACTIVE, DEPRECATED). separate
   HIP control plane from data plane traffic).  Other mobility and
   multihoming cases are possible but are left for further
   experimentation.

   1.  Host mobility with no multihoming and no rekeying.  The status is used to track mobile
       host creates a single UPDATE containing a single ESP_INFO with a
       single LOCATOR parameter.  The ESP_INFO contains the reachability current
       value of the address embedded
   within SPI in both the "Old SPI" and "New SPI" fields.  The
       LOCATOR parameter:

   UNVERIFIED indicates that the reachability contains a single Locator with a "Locator Type" of "1";
       the address has not
      been verified yet,

   ACTIVE indicates SPI must match that the reachability of the address has been
      verified ESP_INFO.  The Preferred bit
       SHOULD be set 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
      successfully.

   UNVERIFIED to DEPRECATED The locator lifetime expires while it "Locator Lifetime" is
      UNVERIFIED.

   ACTIVE set according to DEPRECATED
       local policy.  The locator lifetime expires while it UPDATE also contains a SEQ parameter as usual
       and is ACTIVE.

   ACTIVE protected by retransmission.  The UPDATE should be sent to UNVERIFIED There has been no traffic on
       the peer's preferred IP address for
      some time, and the local policy mandates that the with an IP source address
      reachability must be verified again before starting to use it
      again.

   DEPRECATED
       corresponding to UNVERIFIED the address in the LOCATOR parameter.

   2.  Host mobility with no multihoming but with rekeying.  The mobile
       host receives creates a new lifetime for the
      locator.

   A DEPRECATED address MUST NOT be changed to ACTIVE without first
   verifying its reachability.

5.2  Sending LOCATORs single UPDATE containing a single ESP_INFO with a
       single LOCATOR parameter (with a single address).  The decision ESP_INFO
       contains the current value of when to send LOCATORs is basically 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
   issue.  However, it is RECOMMENDED that a policy.  Optionally, the host sends may choose to
       initiate a Diffie Hellman rekey by including a DIFFIE_HELLMAN
       parameter.  The LOCATOR
   whenever it recognizes contains a change single Locator with "Locator
       Type" of its IP addresses, and assumes "1"; the SPI must match that of the change "New SPI" in the
       ESP_INFO.  Otherwise, the steps are identical to the case when no
       rekeying is going initiated.

   3.  Host multihoming (addition of an address).  We only describe the
       simple case of adding an additional address to last at least for a few seconds.  Rapidly
   sending conflicting LOCATORs single-homed,
       non-mobile host.  The host SHOULD be avoided.

   When set up a host decides to inform its peers about changes in its IP
   addresses, it has to decide how to group the various addresses, new SA pair between
       this new address and
   whether to include any addresses on multiple SPIs.  Since each SPI is
   associated with a different Security Association, the grouping policy
   may be based on ESP anti-replay protection considerations.  In preferred address of the
   typical case, simply basing peer host.  To
       do this, the grouping on actual kernel level
   physical multihomed host creates a new inbound SA and logical interfaces is often the best policy.  Virtual
   interfaces, such as IPsec tunnel interfaces or Mobile IP home
   addresses SHOULD NOT be announced.

   Note that the purpose creates
       a new ESP_INFO parameter with an "Old SPI" parameter of announcing IP addresses in "0", a LOCATOR is
       "New SPI" parameter corresponding to
   provide connectivity between the communicating hosts.  In most cases,
   tunnels (and therefore virtual interfaces) provide sub-optimal
   connectivity.  Furthermore, it should be possible new SPI, and a "Keymat
       Index" as selected by local policy.  The host adds to replace most
   tunnels the UPDATE
       message a LOCATOR with HIP based "non-tunneling", therefore making most virtual
   interfaces fairly unnecessary in two Type "1" Locators: the future.  On original
       address and SPI active on the other hand,
   there are clearly situations where tunnels are used for diagnostic
   and/or testing purposes.  In such association, and other similar cases announcing the IP addresses of virtual interfaces may be appropriate.

   Once new address
       and new SPI being added (with the host has decided SPI matching the "New SPI"
       contained in the ESP_INFO).  The Preferred bit SHOULD be set
       depending on the groups and assignment of addresses policy to tell the SPIs, it creates peer host which of the two
       locators is preferred.  The UPDATE also contains a LOCATOR SEQ parameter for each group.  If there
   are multiple LOCATOR parameters, the parameters MUST
       and optionally a DIFFIE_HELLMAN parameter, and follows rekeying
       procedures with respect to this new address.  The UPDATE message
       SHOULD be ordered so
   that sent to the new peer's preferred locator is in address with a source
       address corresponding to the first LOCATOR parameter.
   Only one locator (the first one, if at all) may be indicated as
   preferred for each distinct Traffic new locator.

   The sending of multiple LOCATORs, locators with Locator Type "0", and
   multiple ESP_INFO parameters is for further study.

5.3.  Handling received LOCATORs

   A host SHOULD be prepared to receive a LOCATOR parameter in the any HIP
   packet, excluding I1.

   This document describes sending both ESP_INFO and LOCATOR parameter.

   If addresses are being added to parameters
   in an existing SPI, the LOCATOR UPDATE.  The ESP_INFO parameter includes is included if there is a need
   to rekey or key a new SPI, and is otherwise included for the full set possible
   benefit of valid addresses for that SPI, each
   using HIP-aware middleboxes.  The LOCATOR parameter contains a Locator Type
   complete map of "1" and each with the same value for SPI.
   Any locators previously ACTIVE on that SPI that are not included in
   the LOCATOR will be set to DEPRECATED by the receiver.

   If a mobile host decides wishes to change the SPI upon a readdress, it sends
   a LOCATOR with make or keep
   active for the SPI field within HIP association.

   In general, the processing of a LOCATOR set to depends upon the new SPI, packet type
   in which it is included and also an upon whether ESP_INFO parameter with the Old SPI field set to is
   included.  Here, we describe only the
   previous SPI case in which ESP_INFO is
   present and the New SPI field set to the new SPI.  If multiple a single LOCATOR and ESP_INFO parameters are included, the ESP_INFO MUST be
   ordered such that they appear sent in the same order as the set of
   corresponding LOCATORs. an UPDATE
   message; other cases are for further study.  The decision as to whether to rekey and send steps below cover
   each of the cases described in Section 5.2.

   When a new Diffie-Hellman parameter while performing readdressing is host receives a
   local policy decision.

   If new addresses and new SPIs are being created, the LOCATOR
   parameter's SPI field contains parameter in a validated HIP packet,
   it first performs the new SPI, and following operations:

   1.  The host checks if the ESP_INFO
   parameter's Old SPI field and New SPI fields are both set to listed in the new
   SPI, indicating that this ESP_INFO is a new and not a replacement SPI.
       one.  If there are multiple LOCATOR parameters leading to a packet size
   that exceeds the MTU, HIP fragmentation rules as described in [2]
   shall apply.

5.3  Handling received LOCATORs

   A host SHOULD be prepared to receive LOCATOR parameters in any HIP
   packets, excluding I1.

   When a host receives it is a LOCATOR parameter, new one, it first performs creates a new inbound SA with that
       SPI that contains no addresses.  If it is an existing one, it
       prepares to change the
   following operations:

   1. address set on the existing SPI.

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

   2.

   3.  For each Type 1 address listed in the LOCATOR parameter, check if
       the address is already bound to the SPI. 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,
       UNVERIFIED.  If the address is not already bound, the address is
       added, and its status is set to UNVERIFIED.  Mark all addresses
       on the SPI that were NOT listed in the LOCATOR parameter as
       DEPRECATED.  As a result, the SPI now contains any addresses
       listed in the LOCATOR parameter either as UNVERIFIED or ACTIVE,
       and any old addresses not listed in the LOCATOR parameter as
       DEPRECATED.

   4.  If the LOCATOR is paired with an ESP_INFO parameter, the ESP_INFO
       parameter is processed as follows:

       1.  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.  Note that
           the Locators in the LOCATOR parameter will use this New SPI
           instead of the Old SPI.

       2.  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 inbound SA and respond with
           an UPDATE ACK.

       3.  If the Old SPI indicates an existing SPI and the New SPI is
           zero, the SPI is being deprecated and all locators uniquely
           bound to the SPI are put into DEPRECATED state.

       4.  If the Old SPI equals the New SPI and both correspond to an
           existing SPI, the address ESP_INFO is added, gratuitous (provided for
           middleboxes) and its status no rekeying is set
       to UNVERIFIED. necessary.

   5.  Mark all addresses locators on the each SPI that were NOT listed in the LOCATOR
       parameter as DEPRECATED.

   As a result, the each SPI now contains any addresses listed in the
   LOCATOR parameter either as UNVERIFIED or ACTIVE, and any old
   addresses not listed in the LOCATOR parameter as DEPRECATED.

   3.  If the LOCATOR is paired with an ESP_INFO parameter, the ESP_INFO
       parameter is processed.  If the LOCATOR is replacing the address
       on an existing SPI, the SPI itself may be changed-- in this case,
       the host proceeds according to HIP rekeying procedures.  This
       case is indicated by the ESP_INFO parameter including an existing
       SPI in the Old SPI field and a new SPI in the New SPI field, and
       the SPI field in the LOCATOR matching the New SPI in the
       ESP_INFO.  If instead the LOCATOR corresponds to a new SPI, the
       ESP_INFO will include the same SPI in both its Old SPI and New
       SPI fields.

   4.  Mark all locators at the address group that were NOT listed in
       the LOCATOR parameter as DEPRECATED.

   Once the host has updated the SPI, if the LOCATOR parameter contains
   a new preferred locator, the host SHOULD initiate a change of the
   preferred locator.  This requires that the host first verifies
   reachability of the associated address, and only then changes the
   preferred locator.  See Section 5.6.

5.4

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
   verification

   A host typically starts the address-verification procedure by sending
   a nonce to the new address.  For example, if the host is changing its
   SPI and is 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.  If the host is not rekeying, 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 on an R1 and replying
   with an I2, receiving the corresponding R2 is sufficient proof of
   reachability for the Responder's preferred address.  Since further
   address verification of such address can impede the HIP base
   exchange, a host MUST NOT verify reachability of a new preferred
   locator that was received on a R1.

   In some cases, it may be sufficient to use the arrival of data on a
   newly advertised SA as implicit address reachability verification,
   instead of waiting for the confirmation via a HIP packet (e.g.,
   Figure 14).  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.  Marking the address ACTIVE as a part of receiving
   data on the SA is an idempotent operation, and does not cause any
   harm.

     Mobile host                                   Peer host

                                                   prepare incoming SA
                      new SPI in R2, or UPDATE
                <-----------------------------------
   switch to new outgoing SA
                           data on new SA
                ----------------------------------->
                                                   mark address ACTIVE

   Figure 14: Address activation via use of 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 5.5.  Once address verification succeeds, the status of
   the new preferred locator changes to ACTIVE.

5.5

5.5.  Credit-Based Authorization

5.5.1

5.5.1.  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, if the peers preferred locator is
   listed as UNVERIFIED and no alternative locator with status ACTIVE is
   available, the host checks whether it can send the packet to the
   UNVERIFIED locator: The packet SHOULD be sent if the value of the
   credit counter is higher than the size of the outbound packet.  If
   the credit counter is too low, the packet MUST be discarded or
   buffered until address verification succeeds.  When a packet is sent
   to a peer at an UNVERIFIED locator, the peer's credit counter MUST be
   reduced by the size of the packet.  The peer's credit counter is not
   affected by packets that the host sends to an ACTIVE locator of that
   peer.

   Figure 15 depicts the actions taken by the host when a packet is
   received.  Figure 16 shows the decision chain in the event a packet
   is sent.

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

   Figure 15: Receiving Packets with Credit-Based Authorization
    Outbound
     packet
        |          _________________
        |         /                 \                 +---------------+
        |        /  Is the preferred \       No       |  Send packet  |
        +-----> | destination address |-------------> |  to preferred |
                 \    UNVERIFIED?    /                |    address    |
                  \_________________/                 +---------------+
                           |
                           | Yes
                           |
                           v
                   _________________
                  /                 \                 +---------------+
                 /   Does an ACTIVE  \      Yes       |  Send packet  |
                | destination address |-------------> |   to ACTIVE   |
                 \       exist?      /                |    address    |
                  \_________________/                 +---------------+
                           |
                           | No
                           |
                           v
                   _________________
                  /                 \                 +---------------+
                 /   Credit counter  \       No       |               |
                |          >=         |-------------> |  Drop packet  |
                 \    packet size?   /                |               |
                  \_________________/                 +---------------+
                           |
                           | Yes
                           |
                           v
                   +---------------+                  +---------------+
                   | Reduce credit |                  |  Send packet  |
                   |  counter by   |----------------> | to preferred  |
                   |  packet size  |                  |    address    |
                   +---------------+                  +---------------+

   Figure 16: Sending Packets with Credit-Based Authorization

5.5.2

5.5.2.  Credit Aging

   A host ensures that the credit counters it maintains for its peers
   gradually decrease over time.  Such "credit aging" prevents a
   malicious peer from building up credit at a very slow speed and using
   this, all at once, for a severe burst of redirected packets.

   Credit aging may be implemented by multiplying credit counters with a
   factor, CreditAgingFactor, 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 exeed exceed 500 milliseconds.  Alternative credit-aging algorithms may
   use other parameter values or different parameters, which may even be
   dynamically established.

5.6

5.6.  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 is may be due to receiving a LOCATOR
   parameter that has the P-bit set.

   To change the preferred locator, the host initiates the following
   procedure:

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

   2.  If the new preferred locator has UNVERIFIED status, the host
       starts to verify its reachability.  The host SHOULD use a
       different locator listed as ACTIVE until address verification
       completes if one such locator is available.  Altervatively,  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 arrive that deprecate the preferred locator,
       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, this includes
       address verification as described above.

6.  Policy considerations

   XXX: This section needs to be written.

   The host may change the status of unused ACTIVE addresses into
   UNVERIFIED after a locally configured period of inactivity.

7.  Security Considerations

   The HIP mobility mechanism provides a secure means of updating a
   host's IP address via HIP REA update UPDATE packets.  Upon receipt, a HIP host
   cryptographically verifies the sender of a REA update, an UPDATE, so forging or
   replaying a HIP update UPDATE packet is very difficult (see [2]).
   Therefore, security issues reside in other attack domains.  The two
   we consider are malicious redirection of legitimate connections as
   well as redirection-based flooding attacks using this protocol.  This
   can be broken down into the following:

      Impersonation attacks

         - direct conversation with the misled victim

         - man-in-the-middle attack

      DoS attacks

         - flooding attacks (== bandwidth-exhaustion attacks)

            * tool 1: direct flooding

            * tool 2: flooding by zombies

            * tool 2: redirection-based flooding

         - memory-exhaustion attacks

         - computational exhaustion attacks

   We consider these in more detail in the following sections.

   In Section 7.1 6.1 and Section 7.2, 6.2, we assume that all users are using
   HIP.  In Section 7.3 6.3 we consider the security ramifications when we
   have both HIP and non-HIP users.

7.1

6.1.  Impersonation attacks

   An attacker wishing to impersonate will try to mislead its victim
   into directly communicating with them, or carry out a man in the
   middle 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 this feature to
   impersonate a victim's peer from any arbitrary location.  The
   authentication and authorization mechanisms of the HIP base exchange
   [2] and the signatures in the new REA update UPDATE message prevent this
   offense. attack.
   Furthermore, ownership of a connection 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, but once the base exchange has taken place
   even a MitM cannot steal a an opportunistic HIP connection because it
   is very difficult for an attacker to create an REA update UPDATE packet (or any
   HIP packet) that will be accepted as a legitimate update.  Update  UPDATE
   packets use HMAC and are signed.  Even when an attacker can snoop
   packets to
   attain obtain the SPI and HIT/HI, they still cannot forge an update
   UPDATE packet without knowledge of the secret keys.

7.2

6.2.  Denial of Service attacks

7.2.1

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 operating to operate correctly.  A
   denial-of-service attack can aim at the victim's network attachment
   (flooding attack), its memory 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 makes a HIP redirection-based flooding attack as
   effective and applicable as a normal, direct flooding attack in which
   the attacker itself sends the flooding traffic to the victim.

   This analysis leads to the following two points.  First, when a
   reachability packet is received received, this nonce packet MUST be ignored if
   the HIT is not one that is currently active.  Second, if the attacker
   is a MitM and can capture this nonce packet then they it can respond to
   it, in which case it is possible for an attacker to redirect their the
   connection.  Note, this attack will always be possible when a
   reachability packet is not sent.

7.2.2

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 [2]).  A simple attack is to
   send many REA update UPDATE packets containing many ip IP addresses that are not
   flagged as preferred.  The attacker continues to send such packets
   until the number of ip IP addresses associated with the
   attackers attacker's HI
   crashes the system.  Therefore, their there SHOULD be a limit to the number
   of ip IP addresses that can be associated with any HI.  Other forms of
   memory/computationally exhausting attacks via the HIP
   update UPDATE packet
   are handled in the base HIP draft [2].

7.3

6.3.  Mixed deployment environment

   We now assume that we have an environment with both HIP and non-HIP aware hosts.
   Four cases exist.

   1.  A HIP user redirects their connection onto a non-HIP user.  The
       non-HIP user will drop the reachability packet so this is not a
       threat unless the HIP user is a MitM and can respond to the
       reachability packet.

   2.  A non-HIP user attempts to redirect their connection onto a HIP
       user.  This falls into IPv4 and IPv6 security concerns, which are
       outside the scope of this document.

   3.  A non-HIP user attempts to steal a HIP user's session (assume
       that SeND Secure Neighbor Discovery is not active for the following).
       The non-HIP user contacts the service that a HIP user has a
       connection with and then attempts to use a IPv6 change of address
       request to steal the HIP user's connection.  What will happen in
       this case is implementation dependent but such a request should
       be ignored/
       dropped. ignored/dropped.  Even if the attack is sucessful, successful, the HIP
       user can reclaim their its connection via HIP.

   4.  A HIP user attempts to steal a non-HIP user's session.  This
       could be problematic since HIP sits 'on top of' layer 3.  A HIP
       user could spoof the non-HIP user's ip IP address during the base
       exhange
       exchange or set the non-HIP user's ip IP address as their preferred
       address via an REA update. UPDATE.  Other possibilities exist but a simple
       solution is to add a check which does not allow any HIP session
       to be moved to or created upon an already existing ip IP address.

7.  IANA Considerations

   This document defines a LOCATOR parameter for the Host Identity
   Protocol [2].  This parameter is defined in Section 4 with a Type of
   193.

8.  IANA Considerations

9.  Authors

   Pekka Nikander originated this Internet Draft.  Tom Henderson, Jari
   Arkko, Greg Perkins, and Christian Vogt have each contributed
   sections to this draft.

10.

9.  Acknowledgments

   The authors thank Mika Kousa Kousa, Jeff Ahrenholz, and Jan Melen for many
   improvements to the draft.

11.

10.  References

11.1

10.1.  Normative references

   [1]  Moskowitz, R., R. and P. Nikander, "Host Identity Protocol
        Architecture",
        draft-ietf-hip-arch-02 draft-ietf-hip-arch-03 (work in progress), January
        August 2005.

   [2]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-03 draft-ietf-hip-base-04
        (work in progress), June October 2005.

   [3]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
        Rendezvous Extension", draft-ietf-hip-rvs-03 draft-ietf-hip-rvs-04 (work in progress),
        July
        October 2005.

   [4]  Kent, S. and R. Atkinson, S., "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. 4303,
        December 2005.

   [5]  Jokela, P., "Using ESP transport format with HIP",
        draft-ietf-hip-esp-00
        draft-ietf-hip-esp-01 (work in progress), July October 2005.

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

   [7]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.

11.2

10.2.  Informative references

   [8]   Bellovin, S., "EIDs, IPsec, and HostNAT", IETF 41th,
         March 1998.

   [9]   Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
         Security Considerations", draft-iab-sec-cons-00 (work in
         progress), August 2002.

   [10]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
        Nordmark, "Mobile IP version Version 6 Route Optimization Security
        Design Background", draft-nikander-mobileip-v6-ro-sec-02 (work
         in progress), RFC 4225, December 2003.

Author's Address

   Tom Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   Email: thomas.r.henderson@boeing.com 2005.

Appendix A.  Changes from previous versions

A.1

A.1.  From nikander-hip-mm-00 to nikander-hip-mm-01

   The actual protocol has been largely revised, based on the new
   symmetric New SPI (NES) design adopted in the base protocol draft
   version -08.  There are no more separate REA, AC or ACR packets, but
   their functionality has been folded into the NES packet.  At the same
   time, it has become possible to send REA parameters in R1 and I2.

   The Forwarding Agent functionality was removed, since it looks like
   that it will be moved to the proposed HIP Research Group.  Hence,
   there will be two other documents related to that, a simple
   Rendezvous server document (WG item) and a Forwarding Agent document
   (RG item).

A.2

A.2.  From nikander-hip-mm-01 to nikander-hip-mm-02

   Alignment with base-00 draft (use of UPDATE and NOTIFY packets).

   The "logical interface" concept was dropped, and the SA/SPI was
   identified as the protocol component to which a HIP association binds
   addresses to.

   The RR was (again) made recommended, not mandatory, able to be
   administratively overridden.

A.3

A.3.  From -02 to draft-ietf-hip-mm-00

   REA parameter type value is now "3" (was TBD before).

   Recommend that in multihoming situations, that inbound/outbound SAs
   are paired to avoid ambiguity when rekeying them.

   Clarified that multihoming scenario for now was intended for failover
   instead of load-balancing, due to transport layer issues.

   Clarified that if HIP negotiates base exchange using link local
   addresses, that a host SHOULD provide its peer with a globally
   reachable address.

   Clarified whether REAs sent for existing SPIs update the full set of
   addresses associated with that SPI, or only perform an incremental
   (additive) update.  REAs for an existing SPI should list all current
   addresses for that SPI, and any addresses previously in use on the
   SPI but not in the new REA parameter should be DEPRECATED.

   Clarified that address verification pertains to *outgoing* addresses.

   When discussing inclusion of REA in I2, the draft stated "The
   Responder MUST make sure that the puzzle solution is valid BOTH for
   the initial IP destination address used for I1 and for the new
   preferred address."  However, this statement conflicted with Appendix
   D of the base specification, so it has been removed for now.

A.4

A.4.  From draft-ietf-hip-mm-00 to -01

   Introduction section reorganized.  Some of the scope of the document
   relating to multihoming was reduced.

   Removed empty appendix "Implementation experiences"

   Renamed REA parameter to LOCATOR and aligned to the discussion on
   redefining this parameter that occurred on the RG mailing list.

   Aligned with decoupling of ESP from base spec.

A.5

A.5.  From draft-ietf-hip-mm-01 to -02

   Aligned with draft-ietf-hip-base-03 and draft-ietf-hip-esp-00

   Address verification is a MUST (C. Vogt, list post on 06/12/05)

   If UPDATE exceeds MTU because of too many locators, do not split into
   multiple UPDATEs, but instead rely on IP fragmentation (C. Vogt, list
   post on 06/12/05)

   New value for LOCATOR parameter type (193), per 05/31/05 discussion
   on the WG list

   Various additions related to Credit-Based Authorization due to C.
   Vogt

   Security section contributed by Greg Perkins, with subsequent editing
   from C. Vogt and P. Nikander

   Reorganization according to RFC 4101 guidance on writing protocol
   models

   Open issue: LOCATOR parameter semantics (implicit/explicit removal)

A.6.  From draft-ietf-hip-mm-02 to -03

   Aligned with draft-ietf-hip-base-05 and draft-ietf-hip-esp-02

   Further clarification that the scope of this draft is primarily
   limited to the case in which ESP is used
   New layered architectural overview in Section 3

   Limited the scope of multihoming description to just a single host
   adding a single new address; other cases left for further study

   Require that ESP_INFO be included on all UPDATE packets relating to
   mobility and multihoming (for middleboxes)

   New convention for use of "Old SPI" and "New SPI" values to signal
   new SPIs (Old SPI == 0, New SPI != 0) and gratuitous ESP_INFOs with
   no rekeying (Old SPI == New SPI != 0).

   Only specify the use of Locator Type of 1 when using ESP, for
   simplicity of receiver processing.

   Removed multiple addresses in LOCATOR example of section 3.2.2,
   because it is not clear that the example is correct (requires further
   study)

   Corrected mention of sending ECHO_REQUEST nonce in R2 (should be sent
   in separate UPDATE because R2 is not an acknowledged packet)

   Removed first four paragraphs of Section 5, which were redundant with
   previous introductory material.

   Rewrote Sections 5.2 and 5.3 on sending and receiving LOCATOR, to
   more explicitly cover the scenario scope of this document.

   Removed unwritten "Policy Considerations" section

Author's Address

   Tom Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   Email: thomas.r.henderson@boeing.com

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