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Internet Engineering Task Force (IETF)                 T. Henderson, Ed.Request for Comments: 8046                      University of WashingtonObsoletes:5206                                                  C. VogtCategory: Standards Track                                    IndependentISSN: 2070-1721                                                 J. Arkko                                                                Ericsson                                                           February 2017Host Mobility with the Host Identity ProtocolAbstract   This document defines a mobility extension to the Host Identity   Protocol (HIP).  Specifically, this document defines a "LOCATOR_SET"   parameter for HIP messages that allows for a HIP host to notify peers   about alternate addresses at which it may be reached.  This document   also defines how the parameter can be used to preserve communications   across a change to the IP address used by one or both peer hosts.   The same LOCATOR_SET parameter can also be used to support end-host   multihoming (as specified inRFC 8047).  This document obsoletesRFC5206.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8046.Henderson, et al.            Standards Track                    [Page 1]

RFC 8046                    HIP Host Mobility              February 2017Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Henderson, et al.            Standards Track                    [Page 2]

RFC 8046                    HIP Host Mobility              February 2017Table of Contents1.  Introduction and Scope  . . . . . . . . . . . . . . . . . . .42.  Terminology and Conventions . . . . . . . . . . . . . . . . .43.  Protocol Model  . . . . . . . . . . . . . . . . . . . . . . .73.1.  Operating Environment . . . . . . . . . . . . . . . . . .73.1.1.  Locator . . . . . . . . . . . . . . . . . . . . . . .93.1.2.  Mobility Overview . . . . . . . . . . . . . . . . . .93.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .103.2.1.  Mobility with a Single SA Pair (No Rekeying)  . . . .10       3.2.2.  Mobility with a Single SA Pair (Mobile-Initiated               Rekey)  . . . . . . . . . . . . . . . . . . . . . . .123.2.3.  Mobility Messaging through the Rendezvous Server  . .133.2.4.  Network Renumbering . . . . . . . . . . . . . . . . .143.3.  Other Considerations  . . . . . . . . . . . . . . . . . .143.3.1.  Address Verification  . . . . . . . . . . . . . . . .143.3.2.  Credit-Based Authorization  . . . . . . . . . . . . .153.3.3.  Preferred Locator . . . . . . . . . . . . . . . . . .164.  LOCATOR_SET Parameter Format  . . . . . . . . . . . . . . . .164.1.  Traffic Type and Preferred Locator  . . . . . . . . . . .184.2.  Locator Type and Locator  . . . . . . . . . . . . . . . .194.3.  UPDATE Packet with Included LOCATOR_SET . . . . . . . . .195.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .195.1.  Locator Data Structure and Status . . . . . . . . . . . .195.2.  Sending the LOCATOR_SET . . . . . . . . . . . . . . . . .215.3.  Handling Received LOCATOR_SETs  . . . . . . . . . . . . .225.4.  Verifying Address Reachability  . . . . . . . . . . . . .245.5.  Changing the Preferred Locator  . . . . . . . . . . . . .265.6.  Credit-Based Authorization  . . . . . . . . . . . . . . .265.6.1.  Handling Payload Packets  . . . . . . . . . . . . . .275.6.2.  Credit Aging  . . . . . . . . . . . . . . . . . . . .296.  Security Considerations . . . . . . . . . . . . . . . . . . .296.1.  Impersonation Attacks . . . . . . . . . . . . . . . . . .306.2.  Denial-of-Service Attacks . . . . . . . . . . . . . . . .316.2.1.  Flooding Attacks  . . . . . . . . . . . . . . . . . .316.2.2.  Memory/Computational-Exhaustion DoS Attacks . . . . .326.3.  Mixed Deployment Environment  . . . . . . . . . . . . . .326.4.  Privacy Concerns  . . . . . . . . . . . . . . . . . . . .337.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .338.  Differences fromRFC 5206 . . . . . . . . . . . . . . . . . .339.  References  . . . . . . . . . . . . . . . . . . . . . . . . .359.1.  Normative References  . . . . . . . . . . . . . . . . . .359.2.  Informative References  . . . . . . . . . . . . . . . . .35   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .36   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .37Henderson, et al.            Standards Track                    [Page 3]

RFC 8046                    HIP Host Mobility              February 20171.  Introduction and Scope   The Host Identity Protocol (HIP) [RFC7401] supports an architecture   that decouples the transport layer (TCP, UDP, etc.) from the   internetworking layer (IPv4 and IPv6) by using public/private key   pairs, instead of IP addresses, as host identities.  When a host uses   HIP, the overlying protocol sublayers (e.g., transport-layer sockets   and Encapsulating Security Payload (ESP) Security Associations (SAs))   are instead bound to representations of these host identities, and   the IP addresses are only used for packet forwarding.  However, each   host needs to also know at least one IP address at which its peers   are reachable.  Initially, these IP addresses are the ones used   during the HIP base exchange.   One consequence of such a decoupling is that new solutions to   network-layer mobility and host multihoming are possible.  There are   potentially many variations of mobility and multihoming possible.   The scope of this document encompasses messaging and elements of   procedure for basic network-level host mobility, leaving more   complicated mobility scenarios, multihoming, and other variations for   further study.  More specifically, the following are in scope:      This document defines a LOCATOR_SET parameter for use in HIP      messages.  The LOCATOR_SET parameter allows a HIP host to notify a      peer about alternate locators at which it is reachable.  The      locators may be merely IP addresses, or they may have additional      multiplexing and demultiplexing context to aid with the packet      handling in the lower layers.  For instance, an IP address may      need to be paired with an ESP Security Parameter Index (SPI) so      that packets are sent on the correct SA for a given address.      This document also specifies the messaging and elements of      procedure for end-host mobility of a HIP host.  In particular,      message flows to enable successful host mobility, including      address verification methods, are defined herein.      The HIP rendezvous server (RVS) [RFC8004] can be used to manage      simultaneous mobility of both hosts, initial reachability of a      mobile host, location privacy, and some modes of NAT traversal.      Use of the HIP RVS to manage the simultaneous mobility of both      hosts is specified herein.Henderson, et al.            Standards Track                    [Page 4]

RFC 8046                    HIP Host Mobility              February 2017   The following topics are out of scope:      While the same LOCATOR_SET parameter supports host multihoming      (simultaneous use of a number of addresses), procedures for host      multihoming are out of scope and are specified in [RFC8047].      While HIP can potentially be used with transports other than the      ESP transport format [RFC7402], this document largely assumes the      use of ESP and leaves other transport formats for further study.      We do not consider localized mobility management extensions (i.e.,      mobility management techniques that do not involve directly      signaling the correspondent node); this document is concerned with      end-to-end mobility.      Finally, making underlying IP mobility transparent to the      transport layer has implications on the proper response of      transport congestion control, path MTU selection, and Quality of      Service (QoS).  Transport-layer mobility triggers, and the proper      transport response to a HIP mobility or multihoming address      change, are outside the scope of this document.   The main sections of this document are organized as follows.Section 3 provides a summary overview of operations, scenarios, and   other considerations.Section 4 specifies the messaging parameter   syntax.Section 5 specifies the processing rules for messages.Section 6 describes security considerations for this specification.2.  Terminology and Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   LOCATOR_SET.  A HIP parameter containing zero or more Locator fields.   locator.  A name that controls how the packet is routed through the      network and demultiplexed by the end host.  It may include a      concatenation of traditional network addresses such as an IPv6      address and end-to-end identifiers such as an ESP SPI.  It may      also include transport port numbers or IPv6 Flow Labels as      demultiplexing context, or it may simply be a network address.   Locator.  When capitalized in the middle of a sentence, this term      refers to the encoding of a locator within the LOCATOR_SET      parameter (i.e., the 'Locator' field of the parameter).Henderson, et al.            Standards Track                    [Page 5]

RFC 8046                    HIP Host Mobility              February 2017   Address.  A name that denotes a point of attachment to the network.      The two most common examples are an IPv4 address and an IPv6      address.  The set of possible addresses is a subset of the set of      possible locators.   Preferred locator.  A locator on which a host prefers to receive      data.  Certain locators are labeled as preferred when a host      advertises its locator set to its peer.  By default, the locators      used in the HIP base exchange are the preferred locators.  The use      of preferred locators, including the scenario where multiple      address scopes and families may be in use, is defined more in      [RFC8047] than in this document.   Credit-Based Authorization (CBA).  A mechanism allowing a host to      send a certain amount of data to a peer's newly announced locator      before the result of mandatory address verification is known.Henderson, et al.            Standards Track                    [Page 6]

RFC 8046                    HIP Host Mobility              February 20173.  Protocol Model   This section is an overview; a more detailed specification follows   this section.3.1.  Operating Environment   HIP [RFC7401] is a key establishment and parameter negotiation   protocol.  Its primary applications are for authenticating host   messages based on host identities and establishing SAs for the ESP   transport format [RFC7402] and possibly other protocols in the   future.    +--------------------+                       +--------------------+    |                    |                       |                    |    |   +------------+   |                       |   +------------+   |    |   |    Key     |   |         HIP           |   |    Key     |   |    |   | Management | <-+-----------------------+-> | Management |   |    |   |  Process   |   |                       |   |  Process   |   |    |   +------------+   |                       |   +------------+   |    |         ^          |                       |         ^          |    |         |          |                       |         |          |    |         v          |                       |         v          |    |   +------------+   |                       |   +------------+   |    |   |   IPsec    |   |        ESP            |   |   IPsec    |   |    |   |   Stack    | <-+-----------------------+-> |   Stack    |   |    |   |            |   |                       |   |            |   |    |   +------------+   |                       |   +------------+   |    |                    |                       |                    |    |                    |                       |                    |    |     Initiator      |                       |     Responder      |    +--------------------+                       +--------------------+                      Figure 1: HIP Deployment Model   The general deployment model for HIP is shown above, assuming   operation in an end-to-end fashion.  This document specifies an   extension to HIP to enable end-host mobility.  In summary, these   extensions to the HIP base protocol enable the signaling of new   addressing information to the peer in HIP messages.  The messages are   authenticated via a signature or keyed Hash Message Authentication   Code (HMAC) based on its Host Identity (HI).  This document specifies   the format of this new addressing (LOCATOR_SET) parameter, the   procedures for sending and processing this parameter to enable basic   host mobility, and procedures for a concurrent address verification   mechanism.Henderson, et al.            Standards Track                    [Page 7]

RFC 8046                    HIP Host Mobility              February 2017            ---------            | TCP   |  (sockets bound to HITs)            ---------               |            ---------      ----> | ESP   |  {HIT_s, HIT_d} <-> SPI      |     ---------      |         |    ----    ---------   | MH |-> | HIP   |  {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI}    ----    ---------               |            ---------            |  IP   |            ---------       Figure 2: Architecture for HIP Host Mobility and Multihoming   Figure 2 depicts a layered architectural view of a HIP-enabled stack   using the ESP transport format.  In HIP, upper-layer protocols   (including TCP and ESP in this figure) are bound to Host Identity   Tags (HITs) and not IP addresses.  The HIP sublayer is responsible   for maintaining the binding between HITs and IP addresses.  The SPI   is used to associate an incoming packet with the right HITs.  The   block labeled "MH" corresponds to the function that manages the   bindings at the ESP and HIP sublayers for mobility (specified in this   document) and multihoming (specified in [RFC8047]).   Consider first the case in which there is no mobility or multihoming,   as specified in the base protocol specification [RFC7401].  The HIP   base exchange establishes the HITs in use between the hosts, the SPIs   to use for ESP, and the IP addresses (used in both the HIP signaling   packets and ESP data packets).  Note that there can only be one such   set of bindings in the outbound direction for any given packet, and   the only fields used for the binding at the HIP layer are the fields   exposed by ESP (the SPI and HITs).  For the inbound direction, the   SPI is all that is required to find the right host context.  ESP   rekeying events change the mapping between the HIT pair and SPI, but   do not change the IP addresses.   Consider next a mobility event, in which a host moves to another IP   address.  Two things need to occur in this case.  First, the peer   needs to be notified of the address change using a HIP UPDATE   message.  Second, each host needs to change its local bindings at the   HIP sublayer (new IP addresses).  It may be that both the SPIs and IP   addresses are changed simultaneously in a single UPDATE; the protocol   described herein supports this.  Although internal notification of   transport-layer protocols regarding the path change (e.g., to resetHenderson, et al.            Standards Track                    [Page 8]

RFC 8046                    HIP Host Mobility              February 2017   congestion control variables) may be desired, this specification does   not address such internal notification.  In addition, elements of   procedure for traversing network address translators (NATs) and   firewalls, including NATs and firewalls that may understand HIP, may   complicate the above basic scenario and are not covered by this   document.3.1.1.  Locator   This document defines a generalization of an address called a   "locator".  A locator specifies a point of attachment to the network   but may also include additional end-to-end tunneling or a per-host   demultiplexing context that affects how packets are handled below the   logical HIP sublayer of the stack.  This generalization is useful   because IP addresses alone may not be sufficient to describe how   packets should be handled below HIP.  For example, in a host   multihoming context, certain IP addresses may need to be associated   with certain ESP SPIs to avoid violating the ESP anti-replay window.   Addresses may also be affiliated with transport ports in certain   tunneling scenarios.  Locators may simply be traditional network   addresses.  The format of the Locator fields in the LOCATOR_SET   parameter is defined inSection 4.3.1.2.  Mobility Overview   When a host moves to another address, it notifies its peer of the new   address by sending a HIP UPDATE packet containing a single   LOCATOR_SET parameter and a single ESP_INFO parameter.  This UPDATE   packet is acknowledged by the peer.  For reliability in the presence   of packet loss, the UPDATE packet is retransmitted as defined in the   HIP specification [RFC7401].  The peer can authenticate the contents   of the UPDATE packet based on the signature and keyed hash of the   packet.   When using the ESP transport format [RFC7402], the host may, at the   same time, decide to rekey its security association and possibly   generate a new Diffie-Hellman key; all of these actions are triggered   by including additional parameters in the UPDATE packet, as defined   in the base protocol specification [RFC7401] and ESP extension   [RFC7402].   When using ESP (and possibly other transport modes in the future),   the host is able to receive packets that are protected using a HIP-   created ESP SA from any address.  Thus, a host can change its IP   address and continue to send packets to its peers without necessarily   rekeying.  However, the peers are not able to send packets to these   new addresses before they can reliably and securely update the set of   addresses that they associate with the sending host.  Furthermore,Henderson, et al.            Standards Track                    [Page 9]

RFC 8046                    HIP Host Mobility              February 2017   mobility may change the path characteristics in such a manner that   reordering occurs and packets fall outside the ESP anti-replay window   for the SA, thereby requiring rekeying.3.2.  Protocol Overview   In this section, we briefly introduce a number of usage scenarios for   HIP host mobility.  These scenarios assume that HIP is being used   with the ESP transform [RFC7402], although other scenarios may be   defined in the future.  To understand these usage scenarios, the   reader should be at least minimally familiar with the HIP   specification [RFC7401] and with the use of ESP with HIP [RFC7402].   According to these specifications, the data traffic in a HIP session   is protected with ESP, and the ESP SPI acts as an index to the right   host-to-host context.  More specification details are found later in   Sections4 and5.   The scenarios below assume that the two hosts have completed a single   HIP base exchange with each other.  Therefore, both of the hosts have   one incoming and one outgoing SA.  Further, each SA uses the same   pair of IP addresses, which are the ones used in the base exchange.   The readdressing protocol is an asymmetric protocol where a mobile   host informs a peer host about changes of IP addresses on affected   SPIs.  The readdressing exchange is designed to be piggybacked on   existing HIP exchanges.  In support of mobility, the LOCATOR_SET   parameter is carried in UPDATE packets.   The scenarios below at times describe addresses as being in either an   ACTIVE, UNVERIFIED, or DEPRECATED state.  From the perspective of a   host, newly learned addresses of the peer need to be verified before   put into active service, and addresses removed by the peer are put   into a deprecated state.  Under limited conditions described below   (Section 5.6), an UNVERIFIED address may be used.  The addressing   states are defined more formally inSection 5.1.   Hosts that use link-local addresses as source addresses in their HIP   handshakes may not be reachable by a mobile peer.  Such hosts SHOULD   provide a globally routable address either in the initial handshake   or via the LOCATOR_SET parameter.3.2.1.  Mobility with a Single SA Pair (No Rekeying)   A mobile host sometimes needs to change an IP address bound to an   interface.  The change of an IP address might be needed due to a   change in the advertised IPv6 prefixes on the link, a reconnected PPP   link, a new DHCP lease, or an actual movement to another subnet.  In   order to maintain its communication context, the host needs to informHenderson, et al.            Standards Track                   [Page 10]

RFC 8046                    HIP Host Mobility              February 2017   its peers about the new IP address.  This first example considers the   case in which the mobile host has only one interface, one IP address   in use within the HIP session, a single pair of SAs (one inbound, one   outbound), and no rekeying occurring on the SAs.  We also assume that   the new IP addresses are within the same address family (IPv4 or   IPv6) as the previous address.  This is the simplest scenario,   depicted in Figure 3.  Note that the conventions for message   parameter notations in figures (use of parentheses and brackets) is   defined inSection 2.2 of [RFC7401].     Mobile Host                         Peer Host             UPDATE(ESP_INFO, LOCATOR_SET, SEQ)        ----------------------------------->             UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST)        <-----------------------------------             UPDATE(ACK, ECHO_RESPONSE)        ----------------------------------->        Figure 3: Readdress without Rekeying but with Address Check   The steps of the packet processing are as follows:   1.  The mobile host may be disconnected from the peer host for a       brief period of time while it switches from one IP address to       another; this case is sometimes referred to in the literature as       a "break-before-make" case.  The host may also obtain its new IP       address before losing the old one ("make-before-break" case).  In       either case, upon obtaining a new IP address, the mobile host       sends a LOCATOR_SET parameter to the peer host in an UPDATE       message.  The UPDATE message also contains an ESP_INFO parameter       containing the values of the old and new SPIs for a security       association.  In this case, both the OLD SPI and NEW SPI       parameters are set to the value of the preexisting incoming SPI;       this ESP_INFO does not trigger a rekeying event but is instead       included for possible parameter-inspecting firewalls on the path       ([RFC5207] specifies some such firewall scenarios in which the       HIP-aware firewall may want to associate ESP flows to host       identities).  The LOCATOR_SET parameter contains the new IP       address (embedded in a Locator Type of "1", defined below) and a       lifetime associated with the locator.  The mobile host waits for       this UPDATE to be acknowledged, and retransmits if necessary, as       specified in the base specification [RFC7401].Henderson, et al.            Standards Track                   [Page 11]

RFC 8046                    HIP Host Mobility              February 2017   2.  The peer host receives the UPDATE, validates it, and updates any       local bindings between the HIP association and the mobile host's       destination address.  The peer host MUST perform an address       verification by placing a nonce in the ECHO_REQUEST parameter of       the UPDATE message sent back to the mobile host.  It also       includes an ESP_INFO parameter with both the OLD SPI and NEW SPI       parameters set to the value of the preexisting incoming SPI and       sends this UPDATE (with piggybacked acknowledgment) to the mobile       host at its new address.  This UPDATE also acknowledges the       mobile host's UPDATE that triggered the exchange.  The peer host       waits for its UPDATE to be acknowledged, and retransmits if       necessary, as specified in the base specification [RFC7401].  The       peer MAY use the new address immediately, but it MUST limit the       amount of data it sends to the address until address verification       completes.   3.  The mobile host completes the readdress by processing the UPDATE       ACK and echoing the nonce in an ECHO_RESPONSE, containing the ACK       of the peer's UPDATE.  This UPDATE is not protected by a       retransmission timer because it does not contain a SEQ parameter       requesting acknowledgment.  Once the peer host receives this       ECHO_RESPONSE, it considers the new address to be verified and       can put the address into full use.   While the peer host is verifying the new address, the new address is   marked as UNVERIFIED (in the interim), and the old address is   DEPRECATED.  Once the peer host has received a correct reply to its   UPDATE challenge, it marks the new address as ACTIVE and removes the   old address.3.2.2.  Mobility with a Single SA Pair (Mobile-Initiated Rekey)   The mobile host may decide to rekey the SAs at the same time that it   notifies the peer of the new address.  In this case, the above   procedure described in Figure 3 is slightly modified.  The UPDATE   message sent from the mobile host includes an ESP_INFO with the OLD   SPI set to the previous SPI, the NEW SPI set to the desired new SPI   value for the incoming SA, and the KEYMAT Index desired.  Optionally,   the host may include a DIFFIE_HELLMAN parameter for a new Diffie-   Hellman key.  The peer completes the request for a rekey as is   normally done for HIP rekeying, except that the new address is kept   as UNVERIFIED until the UPDATE nonce challenge is received as   described above.  Figure 4 illustrates this scenario.Henderson, et al.            Standards Track                   [Page 12]

RFC 8046                    HIP Host Mobility              February 2017     Mobile Host                         Peer Host             UPDATE(ESP_INFO, LOCATOR_SET, SEQ, [DIFFIE_HELLMAN])        ----------------------------------->             UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)        <-----------------------------------             UPDATE(ACK, ECHO_RESPONSE)        ----------------------------------->              Figure 4: Readdress with Mobile-Initiated Rekey3.2.3.  Mobility Messaging through the Rendezvous ServerSection 6.11 of [RFC7401] specifies procedures for sending HIP UPDATE   packets.  The UPDATE packets are protected by a timer subject to   exponential backoff and resent UPDATE_RETRY_MAX times.  It may be,   however, that the peer is itself in the process of moving when the   local host is trying to update the IP address bindings of the HIP   association.  This is sometimes called the "double-jump" mobility   problem; each host's UPDATE packets are simultaneously sent to a   stale address of the peer, and the hosts are no longer reachable from   one another.   The HIP Rendezvous Extension [RFC8004] specifies a rendezvous service   that permits the I1 packet from the base exchange to be relayed from   a stable or well-known public IP address location to the current IP   address of the host.  It is possible to support double-jump mobility   with this rendezvous service if the following extensions to the   specifications of [RFC8004] and [RFC7401] are followed.   1.  The mobile host sending an UPDATE to the peer, and not receiving       an ACK, MAY resend the UPDATE to an RVS of the peer, if such a       server is known.  The host MAY try the RVS of the peer up to       UPDATE_RETRY_MAX times as specified in [RFC7401].  The host MAY       try to use the peer's RVS before it has tried UPDATE_RETRY_MAX       times to the last working address (i.e., the RVS MAY be tried in       parallel with retries to the last working address).  The       aggressiveness of a host replicating its UPDATEs to multiple       destinations, to try candidates in parallel instead of serially,       is a policy choice outside of this specification.   2.  An RVS supporting the UPDATE forwarding extensions specified       herein MUST modify the UPDATE in the same manner as it modifies       the I1 packet before forwarding.  Specifically, it MUST rewrite       the IP header source and destination addresses, recompute the IP       header checksum, and include the FROM and RVS_HMAC parameters.Henderson, et al.            Standards Track                   [Page 13]

RFC 8046                    HIP Host Mobility              February 2017   3.  A host receiving an UPDATE packet MUST be prepared to process the       FROM and RVS_HMAC parameters and MUST include a VIA_RVS parameter       in the UPDATE reply that contains the ACK of the UPDATE SEQ.   4.  An Initiator receiving a VIA_RVS in the UPDATE reply should       initiate address reachability tests (described later in this       document) towards the end host's address and not towards the       address included in the VIA_RVS.   This scenario requires that hosts using RVSs also take steps to   update their current address bindings with their RVS upon a mobility   event.  [RFC8004] does not specify how to update the RVS with a   client host's new address.Section 3.2 of [RFC8003] describes how a   host may send a REG_REQUEST in either an I2 packet (if there is no   active association) or an UPDATE packet (if such association exists).   According to procedures described in [RFC8003], if a mobile host has   an active registration, it may use mobility updates specified herein,   within the context of that association, to readdress the association.3.2.4.  Network Renumbering   It is expected that IPv6 networks will be renumbered much more often   than most IPv4 networks.  From an end-host point of view, network   renumbering is similar to mobility, and procedures described herein   also apply to notify a peer of a changed address.3.3.  Other Considerations3.3.1.  Address Verification   When a HIP host receives a set of locators from another HIP host in a   LOCATOR_SET, it does not necessarily know whether the other host is   actually reachable at the claimed addresses.  In fact, a malicious   peer host may be intentionally giving bogus addresses in order to   cause a packet flood towards the target addresses [RFC4225].   Therefore, the HIP host needs to first check that the peer is   reachable at the new address.   Address verification is implemented by the challenger sending some   piece of unguessable information to the new address and waiting for   some acknowledgment from the Responder that indicates reception of   the information at the new address.  This may include the exchange of   a nonce or the generation of a new SPI and observation of data   arriving on the new SPI.  More details are found inSection 5.4 of   this document.Henderson, et al.            Standards Track                   [Page 14]

RFC 8046                    HIP Host Mobility              February 2017   An additional potential benefit of performing address verification is   to allow NATs and firewalls in the network along the new path to   obtain the peer host's inbound SPI.3.3.2.  Credit-Based Authorization   CBA allows a host to securely use a new locator even though the   peer's reachability at the address embedded in the locator has not   yet been verified.  This is accomplished based on the following three   hypotheses:   1.  A flooding attacker typically seeks to somehow multiply the       packets it generates for the purpose of its attack because       bandwidth is an ample resource for many victims.   2.  An attacker can often cause unamplified flooding by sending       packets to its victim, either by directly addressing the victim       in the packets or by guiding the packets along a specific path by       means of an IPv6 Routing header, if Routing headers are not       filtered by firewalls.   3.  Consequently, the additional effort required to set up a       redirection-based flooding attack (without CBA and return       routability checks) would pay off for the attacker only if       amplification could be obtained this way.   On this basis, rather than eliminating malicious packet redirection   in the first place, CBA prevents amplifications.  This is   accomplished by limiting the data a host can send to an unverified   address of a peer by the data recently received from that peer.   Redirection-based flooding attacks thus become less attractive than,   for example, pure direct flooding, where the attacker itself sends   bogus packets to the victim.   Figure 5 illustrates CBA: Host B measures the amount of data recently   received from peer A and, when A readdresses, sends packets to A's   new, unverified address as long as the sum of the packet sizes does   not exceed the measured, received data volume.  When insufficient   credit is left, B stops sending further packets to A until A's   address becomes ACTIVE.  The address changes may be due to mobility,   multihoming, or any other reason.  Not shown in Figure 5 are the   results of credit aging (Section 5.6.2), a mechanism used to dampen   possible time-shifting attacks.Henderson, et al.            Standards Track                   [Page 15]

RFC 8046                    HIP Host Mobility              February 2017           +-------+                        +-------+           |   A   |                        |   B   |           +-------+                        +-------+               |                                |       address |------------------------------->| credit += size(packet)        ACTIVE |                                |               |------------------------------->| credit += size(packet)               |<-------------------------------| do not change credit               |                                |               + address change                 |               + address verification starts    |       address |<-------------------------------| credit -= size(packet)    UNVERIFIED |------------------------------->| credit += size(packet)               |<-------------------------------| credit -= size(packet)               |                                |               |<-------------------------------| credit -= size(packet)               |                                X credit < size(packet)               |                                | => do not send packet!               + address verification concludes |       address |                                |        ACTIVE |<-------------------------------| do not change credit               |                                |                      Figure 5: Readdressing Scenario   This document does not specify how to set the credit limit value, but   the goal is to allow data transfers to proceed without much   interruption while the new address is verified.  A simple heuristic   to accomplish this, if the sender knows roughly its round-trip time   (RTT) and current sending rate to the host, is to allow enough credit   to support maintaining the sending rate for a duration corresponding   to two or three RTTs.3.3.3.  Preferred Locator   When a host has multiple locators, the peer host needs to decide   which to use for outbound packets.  It may be that a host would   prefer to receive data on a particular inbound interface.  HIP allows   a particular locator to be designated as a preferred locator and   communicated to the peer (seeSection 4).4.  LOCATOR_SET Parameter Format   The LOCATOR_SET parameter has a type number value that is considered   to be a "critical parameter" as per the definition in [RFC7401]; such   parameter types MUST be recognized and processed by the recipient.   The parameter consists of the standard HIP parameter Type and Length   fields, plus zero or more Locator sub-parameters.  Each Locator sub-Henderson, et al.            Standards Track                   [Page 16]

RFC 8046                    HIP Host Mobility              February 2017   parameter contains a Traffic Type, Locator Type, Locator Length,   preferred locator bit ("P" bit), Locator Lifetime, and a Locator   encoding.  A LOCATOR_SET containing zero Locator fields is permitted   but has the effect of deprecating all addresses.        0                   1                   2                   3        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |             Type              |            Length             |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       | Traffic Type   | Locator Type | Locator Length | Reserved   |P|       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |                       Locator Lifetime                        |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |                            Locator                            |       |                                                               |       |                                                               |       |                                                               |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       .                                                               .       .                                                               .       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       | Traffic Type   | Locator Type | Locator Length | Reserved   |P|       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |                       Locator Lifetime                        |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |                            Locator                            |       |                                                               |       |                                                               |       |                                                               |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                  Figure 6: LOCATOR_SET Parameter Format   Type:  193   Length:  Length in octets, excluding Type and Length fields, and      excluding padding.   Traffic Type:  Defines whether the locator pertains to HIP signaling,      user data, or both.   Locator Type:  Defines the semantics of the Locator field.   Locator Length:  Defines the length of the Locator field, in units of      4-byte words (Locators up to a maximum of 4*255 octets are      supported).Henderson, et al.            Standards Track                   [Page 17]

RFC 8046                    HIP Host Mobility              February 2017   Reserved:  Zero when sent, ignored when received.   P: Preferred locator.  Set to one if the locator is preferred for      that Traffic Type; otherwise, set to zero.   Locator Lifetime:  Lifetime of the locator, in seconds.   Locator:  The locator whose semantics and encoding are indicated by      the Locator Type field.  All sub-fields of the Locator field are      integral multiples of four octets in length.   The Locator Lifetime (lifetime) indicates how long the following   locator is expected to be valid.  The lifetime is expressed in   seconds.  Each locator MUST have a non-zero lifetime.  The address is   expected to become deprecated when the specified number of seconds   has passed since the reception of the message.  A deprecated address   SHOULD NOT be used as a destination address if an alternate   (non-deprecated) is available and has sufficient address scope.4.1.  Traffic Type and Preferred Locator   The following Traffic Type values are defined:   0:   Both signaling (HIP control packets) and user data.   1:   Signaling packets only.   2:   Data packets only.   The "P" bit, when set, has scope over the corresponding Traffic Type.   That is, when a "P" bit is set for Traffic Type "2", for example, it   means that the locator is preferred for data packets.  If there is a   conflict (for example, if the "P" bit is set for an address of Type   "0" and a different address of Type "2"), the more specific Traffic   Type rule applies (in this case, "2").  By default, the IP addresses   used in the base exchange are preferred locators for both signaling   and user data, unless a new preferred locator supersedes them.  If no   locators are indicated as preferred for a given Traffic Type, the   implementation may use an arbitrary destination locator from the set   of active locators.Henderson, et al.            Standards Track                   [Page 18]

RFC 8046                    HIP Host Mobility              February 20174.2.  Locator Type and Locator   The following Locator Type values are defined, along with the   associated semantics of the Locator field:   0:  An IPv6 address or an IPv4-in-IPv6 format IPv4 address [RFC4291]       (128 bits long).  This Locator Type is defined primarily for       non-ESP-based usage.   1:  The concatenation of an ESP SPI (first 32 bits) followed by an       IPv6 address or an IPv4-in-IPv6 format IPv4 address (an       additional 128 bits).  This IP address is defined primarily for       ESP-based usage.4.3.  UPDATE Packet with Included LOCATOR_SET   A number of combinations of parameters in an UPDATE packet are   possible (e.g., seeSection 3.2).  In this document, procedures are   defined only for the case in which one LOCATOR_SET and one ESP_INFO   parameter are used in any HIP packet.  Any UPDATE packet that   includes a LOCATOR_SET parameter SHOULD include both an HMAC and a   HIP_SIGNATURE parameter.   The UPDATE MAY also include a HOST_ID parameter (which may be useful   for HIP-aware firewalls inspecting the HIP messages for the first   time).  If the UPDATE includes the HOST_ID parameter, the receiving   host MUST verify that the HOST_ID corresponds to the HOST_ID that was   used to establish the HIP association, and the HIP_SIGNATURE MUST   verify with the public key associated with this HOST_ID parameter.   The relationship between the announced Locators and any ESP_INFO   parameters present in the packet is defined inSection 5.2.  This   document does not support any elements of procedure for sending more   than one LOCATOR_SET or ESP_INFO parameter in a single UPDATE.5.  Processing Rules   This section describes rules for sending and receiving the   LOCATOR_SET parameter, testing address reachability, and using CBA on   UNVERIFIED locators.5.1.  Locator Data Structure and Status   Each locator announced in a LOCATOR_SET parameter is represented by a   piece of state that contains the following data:   o  the actual bit pattern representing the locator,Henderson, et al.            Standards Track                   [Page 19]

RFC 8046                    HIP Host Mobility              February 2017   o  the lifetime (seconds),   o  the status (UNVERIFIED, ACTIVE, DEPRECATED),   o  the Traffic Type scope of the locator, and   o  whether the locator is preferred for any particular scope.   The status is used to track the reachability of the address embedded   within the LOCATOR_SET parameter:   UNVERIFIED:  indicates that the reachability of the address has not      been verified yet,   ACTIVE:  indicates that the reachability of the address has been      verified and the address has not been deprecated, and   DEPRECATED:  indicates that the locator's lifetime has expired.   The following state changes are allowed:   UNVERIFIED to ACTIVE:  The reachability procedure completes      successfully.   UNVERIFIED to DEPRECATED:  The locator's lifetime expires while the      locator is UNVERIFIED.   ACTIVE to DEPRECATED:  The locator's lifetime expires while the      locator is ACTIVE.   ACTIVE to UNVERIFIED:  There has been no traffic on the address for      some time, and the local policy mandates that the address      reachability needs to be verified again before starting to use it      again.   DEPRECATED to UNVERIFIED:  The host receives a new lifetime for the      locator.   A DEPRECATED address MUST NOT be changed to ACTIVE without first   verifying its reachability.   Note that the state of whether or not a locator is preferred is not   necessarily the same as the value of the preferred bit in the Locator   sub-parameter received from the peer.  Peers may recommend certain   locators to be preferred, but the decision on whether to actually use   a locator as a preferred locator is a local decision, possibly   influenced by local policy.Henderson, et al.            Standards Track                   [Page 20]

RFC 8046                    HIP Host Mobility              February 2017   In addition to state maintained about status and remaining lifetime   for each locator learned from the peer, an implementation would   typically maintain similar state about its own locators that have   been offered to the peer.   A locator lifetime that is unbounded (does not expire) can be   signified by setting the value of the lifetime field to the maximum   (unsigned) value.   Finally, the locators used to establish the HIP association are by   default assumed to be the initial preferred locators in ACTIVE state,   with an unbounded lifetime.5.2.  Sending the LOCATOR_SET   The decision of when to send the LOCATOR_SET is a local policy issue.   However, it is RECOMMENDED that a host send a LOCATOR_SET whenever it   recognizes a change of its IP addresses in use on an active HIP   association and assumes that the change is going to last at least for   a few seconds.  Rapidly sending LOCATOR_SETs that force the peer to   change the preferred address SHOULD be avoided.   The sending of a new LOCATOR_SET parameter replaces the locator   information from any previously sent LOCATOR_SET parameter;   therefore, if a host sends a new LOCATOR_SET parameter, it needs to   continue to include all active locators.  Hosts MUST NOT announce   broadcast or multicast addresses in LOCATOR_SETs.   We now describe a few cases introduced inSection 3.2.  We assume   that the Traffic Type for each locator is set to "0" (other values   for Traffic Type may be specified in documents that separate the HIP   control plane from data-plane traffic).  Other mobility cases are   possible but are left for further study.   1.  Host mobility with no multihoming and no rekeying.  The mobile       host creates a single UPDATE containing a single ESP_INFO with a       single LOCATOR_SET parameter.  The ESP_INFO contains the current       value of the SPI in both the OLD SPI and NEW SPI fields.  The       LOCATOR_SET contains a single Locator with a Locator Type of "1";       the SPI MUST match that of the ESP_INFO.  The preferred bit       SHOULD be set and the "Locator Lifetime" is set according to       local policy.  The UPDATE also contains a SEQ parameter as usual.       This packet is retransmitted as defined in the HIP specification       [RFC7401].  The UPDATE should be sent to the peer's preferred IP       address with an IP source address corresponding to the address in       the LOCATOR_SET parameter.Henderson, et al.            Standards Track                   [Page 21]

RFC 8046                    HIP Host Mobility              February 2017   2.  Host mobility with no multihoming but with rekeying.  The mobile       host creates a single UPDATE containing a single ESP_INFO with a       single LOCATOR_SET parameter (with a single address).  The       ESP_INFO contains the current value of the SPI in the OLD SPI,       the new value of the SPI in the NEW SPI, and a KEYMAT Index as       selected by local policy.  Optionally, the host may choose to       initiate a Diffie-Hellman rekey by including a DIFFIE_HELLMAN       parameter.  The LOCATOR_SET contains a single Locator with a       Locator Type of "1"; the SPI MUST match that of the NEW SPI in       the ESP_INFO.  Otherwise, the steps are identical to the case in       which no rekeying is initiated.5.3.  Handling Received LOCATOR_SETs   A host SHOULD be prepared to receive a single LOCATOR_SET parameter   in a HIP UPDATE packet.  Reception of multiple LOCATOR_SET parameters   in a single packet, or in HIP packets other than UPDATE, is outside   of the scope of this specification.   Because a host sending the LOCATOR_SET may send the same parameter in   different UPDATE messages to different destination addresses,   including possibly the RVS of the host, the host receiving the   LOCATOR_SET MUST be prepared to handle the possibility of duplicate   LOCATOR_SETs sent to more than one of the host's addresses.  As a   result, the host MUST detect and avoid reprocessing a LOCATOR_SET   parameter that is redundant with a LOCATOR_SET parameter that has   been recently received and processed.   This document describes sending both ESP_INFO and LOCATOR_SET   parameters in an UPDATE.  The ESP_INFO parameter is included when   there is a need to rekey or key a new SPI, and is otherwise included   for the possible benefit of HIP-aware NATs and firewalls.  The   LOCATOR_SET parameter contains a complete listing of the locators   that the host wishes to make or keep active for the HIP association.   In general, the processing of a LOCATOR_SET depends upon the packet   type in which it is included.  Here, we describe only the case in   which ESP_INFO is present and a single LOCATOR_SET and ESP_INFO are   sent in an UPDATE message; other cases are for further study.  The   steps below cover each of the cases described inSection 5.2.   The processing of ESP_INFO and LOCATOR_SET parameters is intended to   be modular and support future generalization to the inclusion of   multiple ESP_INFO and/or multiple LOCATOR_SET parameters.  A host   SHOULD first process the ESP_INFO before the LOCATOR_SET, since the   ESP_INFO may contain a new SPI value mapped to an existing SPI, while   a Locator Type of "1" will only contain a reference to the new SPI.Henderson, et al.            Standards Track                   [Page 22]

RFC 8046                    HIP Host Mobility              February 2017   When a host receives a validated HIP UPDATE with a LOCATOR_SET and   ESP_INFO parameter, it processes the ESP_INFO as follows.  The   ESP_INFO parameter indicates whether an SA is being rekeyed, created,   deprecated, or just identified for the benefit of HIP-aware NATs and   firewalls.  The host examines the OLD SPI and NEW SPI values in the   ESP_INFO parameter:   1.  (no rekeying) If the OLD SPI is equal to the NEW SPI and both       correspond to an existing SPI, the ESP_INFO is gratuitous       (provided for HIP-aware NATs and firewalls) and no rekeying is       necessary.   2.  (rekeying) If the OLD SPI indicates an existing SPI and the NEW       SPI is a different non-zero value, the existing SA is being       rekeyed and the host follows HIP ESP rekeying procedures by       creating a new outbound SA with an SPI corresponding to the NEW       SPI, with no addresses bound to this SPI.  Note that locators in       the LOCATOR_SET parameter will reference this new SPI instead of       the old SPI.   3.  (new SA) If the OLD SPI value is zero and the NEW SPI is a new       non-zero value, then a new SA is being requested by the peer.       This case is also treated like a rekeying event; the receiving       host MUST create a new SA and respond with an UPDATE ACK.   4.  (deprecating the SA) If the OLD SPI indicates an existing SPI and       the NEW SPI is zero, the SA is being deprecated and all locators       uniquely bound to the SPI are put into the DEPRECATED state.   If none of the above cases apply, a protocol error has occurred and   the processing of the UPDATE is stopped.   Next, the locators in the LOCATOR_SET parameter are processed.  For   each locator listed in the LOCATOR_SET parameter, check that the   address therein is a legal unicast or anycast address.  That is, the   address MUST NOT be a broadcast or multicast address.  Note that some   implementations MAY accept addresses that indicate the local host,   since it may be allowed that the host runs HIP with itself.   The below assumes that all Locators are of Type "1" with a Traffic   Type of "0"; other cases are for further study.   For each Type "1" address listed in the LOCATOR_SET parameter, the   host checks whether the address is already bound to the SPI   indicated.  If the address is already bound, its lifetime is updated.   If the status of the address is DEPRECATED, the status is changed to   UNVERIFIED.  If the address is not already bound, the address isHenderson, et al.            Standards Track                   [Page 23]

RFC 8046                    HIP Host Mobility              February 2017   added, and its status is set to UNVERIFIED.  Mark all addresses   corresponding to the SPI that were NOT listed in the LOCATOR_SET   parameter as DEPRECATED.   As a result, at the end of processing, the addresses listed in the   LOCATOR_SET parameter have a state of either UNVERIFIED or ACTIVE,   and any old addresses on the old SA not listed in the LOCATOR_SET   parameter have a state of DEPRECATED.   Once the host has processed the locators, if the LOCATOR_SET   parameter contains a new preferred locator, the host SHOULD initiate   a change of the preferred locator.  This requires that the host first   verify reachability of the associated address, and only then change   the preferred locator; seeSection 5.5.   If a host receives a locator with an unsupported Locator Type, and   when such a locator is also declared to be the preferred locator for   the peer, the host SHOULD send a NOTIFY error with a Notify Message   Type of LOCATOR_TYPE_UNSUPPORTED, with the Notification Data field   containing the locator(s) that the receiver failed to process.   Otherwise, a host MAY send a NOTIFY error if a (non-preferred)   locator with an unsupported Locator Type is received in a LOCATOR_SET   parameter.   A host MAY add the source IP address of a received HIP packet as a   candidate locator for the peer even if it is not listed in the peer's   LOCATOR_SET, but it SHOULD prefer locators explicitly listed in the   LOCATOR_SET.5.4.  Verifying Address Reachability   A host MUST verify the reachability of an UNVERIFIED address.  The   status of a newly learned address MUST initially be set to UNVERIFIED   unless the new address is advertised in an R1 packet as a new   preferred locator.  A host MAY also want to verify the reachability   of an ACTIVE address again after some time, in which case it would   set the status of the address to UNVERIFIED and reinitiate address   verification.  A typical verification that is protected by   retransmission timers is to include an ECHO REQUEST within an UPDATE   sent to the new address.   A host typically starts the address-verification procedure by sending   a nonce to the new address.  A host MAY choose from different message   exchanges or different nonce values so long as it establishes that   the peer has received and replied to the nonce at the new address.Henderson, et al.            Standards Track                   [Page 24]

RFC 8046                    HIP Host Mobility              February 2017   For example, when the host is changing its SPI and sending an   ESP_INFO to the peer, the NEW SPI value SHOULD be random and the   random value MAY be copied into an ECHO_REQUEST sent in the rekeying   UPDATE.  However, if the host is not changing its SPI, it MAY still   use the ECHO_REQUEST parameter for verification but with some other   random value.  A host MAY also use other message exchanges as   confirmation of the address reachability.   In some cases, it MAY be sufficient to use the arrival of data on a   newly advertised SA as implicit address reachability verification as   depicted in Figure 7, instead of waiting for the confirmation via a   HIP packet.  In this case, a host advertising a new SPI as part of   its address reachability check SHOULD be prepared to receive traffic   on the new SA.     Mobile Host                                   Peer Host                  UPDATE(ESP_INFO, LOCATOR_SET, ...)                ---------------------------------->                                                   prepare incoming SA                  UPDATE(ESP_INFO, ...) with new SPI                <-----------------------------------   switch to new outgoing SA                           data on new SA                ----------------------------------->                                                   mark address ACTIVE                  UPDATE(ACK, ECHO_RESPONSE) later arrives                ----------------------------------->             Figure 7: Address Activation via Use of a New SA   When address verification is in progress for a new preferred locator,   the host SHOULD select a different locator listed as ACTIVE, if one   such locator is available, to continue communications until address   verification completes.  Alternatively, the host MAY use the new   preferred locator while in UNVERIFIED status to the extent CBA   permits.  CBA is explained inSection 5.6.  Once address verification   succeeds, the status of the new preferred locator changes to ACTIVE.Henderson, et al.            Standards Track                   [Page 25]

RFC 8046                    HIP Host Mobility              February 20175.5.  Changing the Preferred Locator   A host MAY want to change the preferred outgoing locator for   different reasons, e.g., because traffic information or ICMP error   messages indicate that the currently used preferred address may have   become unreachable.  Another reason may be due to receiving a   LOCATOR_SET parameter that has the "P" bit set.   To change the preferred locator, the host initiates the following   procedure:   1.  If the new preferred locator has an ACTIVE status, the preferred       locator is changed and the procedure succeeds.   2.  If the new preferred locator has an UNVERIFIED status, the host       starts to verify its reachability.  The host SHOULD use a       different locator listed as ACTIVE until address verification       completes if one such locator is available.  Alternatively, the       host MAY use the new preferred locator, even though in UNVERIFIED       status, to the extent CBA permits.  Once address verification       succeeds, the status of the new preferred locator changes to       ACTIVE, and its use is no longer governed by CBA.   3.  If the peer host has not indicated a preference for any address,       then the host picks one of the peer's ACTIVE addresses randomly       or according to local policy.  This case may arise if, for       example, ICMP error messages that deprecate the preferred locator       arrive, but the peer has not yet indicated a new preferred       locator.   4.  If the new preferred locator has a DEPRECATED status and there is       at least one non-deprecated address, the host selects one of the       non-deprecated addresses as a new preferred locator and       continues.  If the selected address is UNVERIFIED, the address       verification procedure described above will apply.5.6.  Credit-Based Authorization   To prevent redirection-based flooding attacks, the use of a CBA   approach MUST be used when a host sends data to an UNVERIFIED   locator.  The following algorithm addresses the security   considerations for prevention of amplification and time-shifting   attacks.  Other forms of credit aging, and other values for the   CreditAgingFactor and CreditAgingInterval parameters in particular,   are for further study, and so are the advanced CBA techniques   specified in [CBA-MIPv6].Henderson, et al.            Standards Track                   [Page 26]

RFC 8046                    HIP Host Mobility              February 20175.6.1.  Handling Payload Packets   A host maintains a "credit counter" for each of its peers.  Whenever   a packet arrives from a peer, the host SHOULD increase that peer's   credit counter by the size of the received packet.  When the host has   a packet to be sent to the peer, and when the peer's preferred   locator is listed as UNVERIFIED and no alternative locator with   status ACTIVE is available, the host checks whether it can send the   packet to the UNVERIFIED locator.  The packet SHOULD be sent if the   value of the credit counter is higher than the size of the outbound   packet.  If the credit counter is too low, the packet MUST be   discarded or buffered until address verification succeeds.  When a   packet is sent to a peer at an UNVERIFIED locator, the peer's credit   counter MUST be reduced by the size of the packet.  The peer's credit   counter is not affected by packets that the host sends to an ACTIVE   locator of that peer.   Figure 8 depicts the actions taken by the host when a packet is   received.  Figure 9 shows the decision chain in the event a packet is   sent.       Inbound       Packet          |          |       +----------------+               +---------------+          |       |    Increase    |               |    Deliver    |          +-----> | credit counter |-------------> |   packet to   |                  | by packet size |               |  application  |                  +----------------+               +---------------+        Figure 8: Receiving Packets with Credit-Based AuthorizationHenderson, et al.            Standards Track                   [Page 27]

RFC 8046                    HIP Host Mobility              February 2017    Outbound     Packet        |          _________________        |         /                 \                 +---------------+        |        /  Is the preferred \       No       |  Send packet  |        +-----> | destination address |-------------> |  to preferred |                 \    UNVERIFIED?    /                |    address    |                  \_________________/                 +---------------+                           |                           | Yes                           |                           v                   _________________                  /                 \                 +---------------+                 /   Does an ACTIVE  \      Yes       |  Send packet  |                | destination address |-------------> |   to ACTIVE   |                 \       exist?      /                |    address    |                  \_________________/                 +---------------+                           |                           | No                           |                           v                   _________________                  /                 \                 +---------------+                 / Is credit counter \       No       |               |                |          >=         |-------------> | Drop or       |                 \    packet size?   /                | buffer packet |                  \_________________/                 +---------------+                           |                           | Yes                           |                           v                   +---------------+                  +---------------+                   | Reduce credit |                  |  Send packet  |                   |  counter by   |----------------> | to preferred  |                   |  packet size  |                  |    address    |                   +---------------+                  +---------------+         Figure 9: Sending Packets with Credit-Based AuthorizationHenderson, et al.            Standards Track                   [Page 28]

RFC 8046                    HIP Host Mobility              February 20175.6.2.  Credit Aging   A host ensures that the credit counters it maintains for its peers   gradually decrease over time.  Such "credit aging" prevents a   malicious peer from building up credit at a very slow speed and using   this, all at once, for a severe burst of redirected packets.   Credit aging may be implemented by multiplying credit counters with a   factor, CreditAgingFactor (a fractional value less than one), in   fixed-time intervals of CreditAgingInterval length.  Choosing   appropriate values for CreditAgingFactor and CreditAgingInterval is   important to ensure that a host can send packets to an address in   state UNVERIFIED even when the peer sends at a lower rate than the   host itself.  When CreditAgingFactor or CreditAgingInterval are too   small, the peer's credit counter might be too low to continue sending   packets until address verification concludes.   The parameter values proposed in this document are as follows:      CreditAgingFactor        7/8      CreditAgingInterval      5 seconds   These parameter values work well when the host transfers a file to   the peer via a TCP connection, and the end-to-end round-trip time   does not exceed 500 milliseconds.  Alternative credit-aging   algorithms may use other parameter values or different parameters,   which may even be dynamically established.6.  Security Considerations   The HIP mobility mechanism provides a secure means of updating a   host's IP address via HIP UPDATE packets.  Upon receipt, a HIP host   cryptographically verifies the sender of an UPDATE, so forging or   replaying a HIP UPDATE packet is very difficult (see [RFC7401]).   Therefore, security issues reside in other attack domains.  The two   we consider are malicious redirection of legitimate connections as   well as redirection-based flooding attacks using this protocol.  This   can be broken down into the following:      1) Impersonation attacks         - direct conversation with the misled victim         - man-in-the-middle (MitM) attackHenderson, et al.            Standards Track                   [Page 29]

RFC 8046                    HIP Host Mobility              February 2017      2) Denial-of-service (DoS) attacks         - flooding attacks (== bandwidth-exhaustion attacks)            * tool 1: direct flooding            * tool 2: flooding by botnets            * tool 3: redirection-based flooding         - memory-exhaustion attacks         - computational-exhaustion attacks      3) Privacy concerns   We consider these in more detail in the following sections.   In Sections6.1 and6.2, we assume that all users are using HIP.  InSection 6.3, we consider the security ramifications when we have both   HIP and non-HIP hosts.6.1.  Impersonation Attacks   An attacker wishing to impersonate another host will try to mislead   its victim into directly communicating with them or carry out a MitM   attack between the victim and the victim's desired communication   peer.  Without mobility support, such attacks are possible only if   the attacker resides on the routing path between its victim and the   victim's desired communication peer or if the attacker tricks its   victim into initiating the connection over an incorrect routing path   (e.g., by acting as a router or using spoofed DNS entries).   The HIP extensions defined in this specification change the situation   in that they introduce an ability to redirect a connection, both   before and after establishment.  If no precautionary measures are   taken, an attacker could potentially misuse the redirection feature   to impersonate a victim's peer from any arbitrary location.  However,   the authentication and authorization mechanisms of the HIP base   exchange [RFC7401] and the signatures in the UPDATE message prevent   this attack.  Furthermore, ownership of a HIP association is securely   linked to a HIP HI/HIT.  If an attacker somehow uses a bug in the   implementation to redirect a HIP connection, the original owner can   always reclaim their connection (they can always prove ownership of   the private key associated with their public HI).Henderson, et al.            Standards Track                   [Page 30]

RFC 8046                    HIP Host Mobility              February 2017   MitM attacks are possible if an on-path attacker is present during   the initial HIP base exchange and if the hosts do not authenticate   each other's identities.  However, once such an opportunistic base   exchange has taken place, a MitM attacker that comes later to the   path cannot steal the HIP connection because it is very difficult for   an attacker to create an UPDATE packet (or any HIP packet) that will   be accepted as a legitimate update.  UPDATE packets use HMAC and are   signed.  Even when an attacker can snoop packets to obtain the SPI   and HIT/HI, they still cannot forge an UPDATE packet without   knowledge of the secret keys.  Also, replay attacks on the UPDATE   packet are prevented as described in [RFC7401].6.2.  Denial-of-Service Attacks6.2.1.  Flooding Attacks   The purpose of a DoS attack is to exhaust some resource of the victim   such that the victim ceases to operate correctly.  A DoS attack can   aim at the victim's network attachment (flooding attack), its memory,   or its processing capacity.  In a flooding attack, the attacker   causes an excessive number of bogus or unwanted packets to be sent to   the victim, which fills their available bandwidth.  Note that the   victim does not necessarily need to be a node; it can also be an   entire network.  The attack functions the same way in either case.   An effective DoS strategy is distributed denial of service (DDoS).   Here, the attacker conventionally distributes some viral software to   as many nodes as possible.  Under the control of the attacker, the   infected nodes (e.g., nodes in a botnet) jointly send packets to the   victim.  With such an "army", an attacker can take down even very   high bandwidth networks/victims.   With the ability to redirect connections, an attacker could realize a   DDoS attack without having to distribute viral code.  Here, the   attacker initiates a large download from a server and subsequently   uses the HIP mobility mechanism to redirect this download to its   victim.  The attacker can repeat this with multiple servers.  This   threat is mitigated through reachability checks and CBA.  When   conducted using HIP, reachability checks can leverage the built-in   authentication properties of HIP.  They can also prevent redirection-   based flooding attacks.  However, the delay of such a check can have   a noticeable impact on application performance.  To reduce the impact   of the delay, CBA can be used to send a limited number of packets to   the new address while the validity of the IP address is still in   question.  Both strategies do not eliminate flooding attacks per se,   but they preclude: (i) their use from a location off the path towards   the flooded victim; and (ii) any amplification in the number and sizeHenderson, et al.            Standards Track                   [Page 31]

RFC 8046                    HIP Host Mobility              February 2017   of the redirected packets.  As a result, the combination of a   reachability check and CBA lowers a HIP redirection-based flooding   attack to the level of a direct flooding attack in which the attacker   itself sends the flooding traffic to the victim.6.2.2.  Memory/Computational-Exhaustion DoS Attacks   We now consider whether or not the proposed extensions to HIP add any   new DoS attacks (consideration of DoS attacks using the base HIP   exchange and updates is discussed in [RFC7401]).  A simple attack is   to send many UPDATE packets containing many IP addresses that are not   flagged as preferred.  The attacker continues to send such packets   until the number of IP addresses associated with the attacker's HI   crashes the system.  Therefore, a HIP association SHOULD limit the   number of IP addresses that can be associated with any HI.  Other   forms of memory/computationally exhausting attacks via the HIP UPDATE   packet are handled in the base HIP document [RFC7401].   A central server that has to deal with a large number of mobile   clients MAY consider increasing the SA lifetimes to try to slow down   the rate of rekeying UPDATEs or increasing the cookie difficulty to   slow down the rate of attack-oriented connections.6.3.  Mixed Deployment Environment   We now assume an environment with hosts that are both HIP and non-HIP   aware.  Four cases exist:   1.  A HIP host redirects its connection onto a non-HIP host.  The       non-HIP host will drop the reachability packet, so this is not a       threat unless the HIP host is a MitM that could somehow respond       successfully to the reachability check.   2.  A non-HIP host attempts to redirect their connection onto a HIP       host.  This falls into IPv4 and IPv6 security concerns, which are       outside the scope of this document.   3.  A non-HIP host attempts to steal a HIP host's session (assume       that Secure Neighbor Discovery is not active for the following).       The non-HIP host contacts the service that a HIP host has a       connection with and then attempts to change its IP address to       steal the HIP host's connection.  What will happen in this case       is implementation dependent, but such a request should fail by       being ignored or dropped.  Even if the attack were successful,       the HIP host could reclaim its connection via HIP.Henderson, et al.            Standards Track                   [Page 32]

RFC 8046                    HIP Host Mobility              February 2017   4.  A HIP host attempts to steal a non-HIP host's session.  A HIP       host could spoof the non-HIP host's IP address during the base       exchange or set the non-HIP host's IP address as its preferred       address via an UPDATE.  Other possibilities exist, but a solution       is to prevent the local redirection of sessions that were       previously using an unverified address, but outside of the       existing HIP context, into the HIP SAs until the address change       can be verified.6.4.  Privacy Concerns   The exposure of a host's IP addresses through HIP mobility extensions   may raise privacy concerns.  The administrator of a host may be   trying to hide its location in some context through the use of a VPN   or other virtual interfaces.  Similar privacy issues also arise in   other frameworks such as WebRTC and are not specific to HIP.   Implementations SHOULD provide a mechanism to allow the host   administrator to block the exposure of selected addresses or address   ranges.  While this issue may be more relevant in a host multihoming   scenario in which multiple IP addresses might be exposed [RFC8047],   it is worth noting also here that mobility events might cause an   implementation to try to inadvertently use a locator that the   administrator would rather avoid exposing to the peer host.7.  IANA Considerations   [RFC5206], obsoleted by this document, specified an allocation for a   LOCATOR parameter in the "Parameter Types" subregistry of the "Host   Identity Protocol (HIP) Parameters" registry, with a type value of   193.  IANA has renamed the parameter to "LOCATOR_SET" and has updated   the reference from [RFC5206] to this specification.   [RFC5206], obsoleted by this document, specified an allocation for a   LOCATOR_TYPE_UNSUPPORTED type in the "Notify Message Types" registry,   with a type value of 46.  IANA has updated the reference from   [RFC5206] to this specification.8.  Differences fromRFC 5206   This section summarizes the technical changes made from [RFC5206].   This section is informational, intended to help implementors of the   previous protocol version.  If any text in this section contradicts   text in other portions of this specification, the text found outside   of this section should be considered normative.Henderson, et al.            Standards Track                   [Page 33]

RFC 8046                    HIP Host Mobility              February 2017   This document specifies extensions to the HIP Version 2 protocol,   while [RFC5206] specifies extensions to the HIP Version 1 protocol.   [RFC7401] documents the differences between these two protocol   versions.   [RFC5206] included procedures for both HIP host mobility and basic   host multihoming.  In this document, only host mobility procedures   are included; host multihoming procedures are now specified in   [RFC8047].  In particular, multihoming-related procedures related to   the exposure of multiple locators in the base exchange packets; the   transmission, reception, and processing of multiple locators in a   single UPDATE packet; handovers across IP address families; and other   multihoming-related specifications have been removed.   The following additional changes have been made:   o  The LOCATOR parameter in [RFC5206] has been renamed to      LOCATOR_SET.   o  Specification text regarding the handling of mobility when both      hosts change IP addresses at nearly the same time (a "double-jump"      mobility scenario) has been added.   o  Specification text regarding the mobility event in which the host      briefly has an active new locator and old locator at the same time      (a "make-before-break" mobility scenario) has been added.   o  Specification text has been added to note that a host may add the      source IP address of a received HIP packet as a candidate locator      for the peer even if it is not listed in the peer's LOCATOR_SET,      but that it should prefer locators explicitly listed in the      LOCATOR_SET.   o  This document clarifies that the HOST_ID parameter may be included      in UPDATE messages containing LOCATOR_SET parameters, for the      possible benefit of HIP-aware firewalls.   o  The previous specification mentioned that it may be possible to      include multiple LOCATOR_SET and ESP_INFO parameters in an UPDATE.      This document only specifies the case of a single LOCATOR_SET and      ESP_INFO parameter in an UPDATE.   o  The previous specification mentioned that it may be possible to      send LOCATOR_SET parameters in packets other than the UPDATE.      This document only specifies the use of the UPDATE packet.   o  This document describes a simple heuristic for setting the credit      value for CBA.Henderson, et al.            Standards Track                   [Page 34]

RFC 8046                    HIP Host Mobility              February 2017   o  This specification mandates that a host must be able to receive      and avoid reprocessing redundant LOCATOR_SET parameters that may      have been sent in parallel to multiple addresses of the host.9.  References9.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing              Architecture",RFC 4291, DOI 10.17487/RFC4291, February              2006, <http://www.rfc-editor.org/info/rfc4291>.   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.              Henderson, "Host Identity Protocol Version 2 (HIPv2)",RFC 7401, DOI 10.17487/RFC7401, April 2015,              <http://www.rfc-editor.org/info/rfc7401>.   [RFC7402]  Jokela, P., Moskowitz, R., and J. Melen, "Using the              Encapsulating Security Payload (ESP) Transport Format with              the Host Identity Protocol (HIP)",RFC 7402,              DOI 10.17487/RFC7402, April 2015,              <http://www.rfc-editor.org/info/rfc7402>.   [RFC8003]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)              Registration Extension",RFC 8003, DOI 10.17487/RFC8003,              October 2016, <http://www.rfc-editor.org/info/rfc8003>.   [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)              Rendezvous Extension",RFC 8004, DOI 10.17487/RFC8004,              October 2016, <http://www.rfc-editor.org/info/rfc8004>.9.2.  Informative References   [CBA-MIPv6]              Vogt, C. and J. Arkko, "Credit-Based Authorization for              Mobile IPv6 Early Binding Updates", Work in Progress,draft-vogt-mobopts-credit-based-authorization-00, February              2005.   [RFC4225]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.              Nordmark, "Mobile IP Version 6 Route Optimization Security              Design Background",RFC 4225, DOI 10.17487/RFC4225,              December 2005, <http://www.rfc-editor.org/info/rfc4225>.Henderson, et al.            Standards Track                   [Page 35]

RFC 8046                    HIP Host Mobility              February 2017   [RFC5206]  Nikander, P., Henderson, T., Ed., Vogt, C., and J. Arkko,              "End-Host Mobility and Multihoming with the Host Identity              Protocol",RFC 5206, DOI 10.17487/RFC5206, April 2008,              <http://www.rfc-editor.org/info/rfc5206>.   [RFC5207]  Stiemerling, M., Quittek, J., and L. Eggert, "NAT and              Firewall Traversal Issues of Host Identity Protocol (HIP)              Communication",RFC 5207, DOI 10.17487/RFC5207, April              2008, <http://www.rfc-editor.org/info/rfc5207>.   [RFC8047]  Henderson, T., Ed., Vogt, C., and J. Arkko, "Host              Multihoming with the Host Identity Protocol",RFC 8047,              DOI 10.17487/RFC8047, February 2017,              <http://www.rfc-editor.org/info/rfc8047>.   [SIMPLE-CBA]              Vogt, C. and J. Arkko, "Credit-Based Authorization for              Concurrent Reachability Verification", Work in Progress,draft-vogt-mobopts-simple-cba-00, February 2006.Acknowledgments   Pekka Nikander and Jari Arkko originated this document; Christian   Vogt and Thomas Henderson (editor) later joined as coauthors.  Greg   Perkins contributed the initial text of the security section.  Petri   Jokela was a coauthor of the initial individual submission.   CBA was originally introduced in [SIMPLE-CBA], and portions of this   document have been adopted from that earlier document.   The authors thank Jeff Ahrenholz, Baris Boyvat, Rene Hummen, Miika   Komu, Mika Kousa, Jan Melen, and Samu Varjonen for improvements to   the document.Henderson, et al.            Standards Track                   [Page 36]

RFC 8046                    HIP Host Mobility              February 2017Authors' Addresses   Thomas R. Henderson (editor)   University of Washington   Campus Box 352500   Seattle, WA   United States of America   Email: tomhend@u.washington.edu   Christian Vogt   Independent   3473 North First Street   San Jose, CA  95134   United States of America   Email: mail@christianvogt.net   Jari Arkko   Ericsson   Jorvas,  FIN-02420   Finland   Phone: +358 40 5079256   Email: jari.arkko@piuha.netHenderson, et al.            Standards Track                   [Page 37]