Network Working Group
Request for Comments: 5206
Category: Experimental
P. Nikander
Ericsson Research NomadicLab
T. Henderson, Ed.
The Boeing Company
C. Vogt
J. Arkko
Ericsson Research NomadicLab
April 2008

End-Host Mobility and Multihoming with the Host Identity Protocol

Status of This Memo

This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.


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 . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology and Conventions  . . . . . . . . . . . . . . . . .  4
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Operating Environment  . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Locator  . . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.2.  Mobility Overview  . . . . . . . . . . . . . . . . . .  8
       3.1.3.  Multihoming Overview . . . . . . . . . . . . . . . . .  8
     3.2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Mobility with a Single SA Pair (No Rekeying) . . . . .  9
       3.2.2.  Mobility with a Single SA Pair (Mobile-Initiated
               Rekey) . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.3.  Host Multihoming . . . . . . . . . . . . . . . . . . . 11
       3.2.4.  Site Multihoming . . . . . . . . . . . . . . . . . . . 13
       3.2.5.  Dual host multihoming  . . . . . . . . . . . . . . . . 14
       3.2.6.  Combined Mobility and Multihoming  . . . . . . . . . . 14
       3.2.7.  Using LOCATORs across Addressing Realms  . . . . . . . 14
       3.2.8.  Network Renumbering  . . . . . . . . . . . . . . . . . 15
       3.2.9.  Initiating the Protocol in R1 or I2  . . . . . . . . . 15
     3.3.  Other Considerations . . . . . . . . . . . . . . . . . . . 16
       3.3.1.  Address Verification . . . . . . . . . . . . . . . . . 16
       3.3.2.  Credit-Based Authorization . . . . . . . . . . . . . . 17
       3.3.3.  Preferred Locator  . . . . . . . . . . . . . . . . . . 18
       3.3.4.  Interaction with Security Associations . . . . . . . . 18
   4.  LOCATOR Parameter Format . . . . . . . . . . . . . . . . . . . 21
     4.1.  Traffic Type and Preferred Locator . . . . . . . . . . . . 23
     4.2.  Locator Type and Locator . . . . . . . . . . . . . . . . . 23
     4.3.  UPDATE Packet with Included LOCATOR  . . . . . . . . . . . 24
   5.  Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Locator Data Structure and Status  . . . . . . . . . . . . 24
     5.2.  Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 25
     5.3.  Handling Received LOCATORs . . . . . . . . . . . . . . . . 28
     5.4.  Verifying Address Reachability . . . . . . . . . . . . . . 30
     5.5.  Changing the Preferred Locator . . . . . . . . . . . . . . 31
     5.6.  Credit-Based Authorization . . . . . . . . . . . . . . . . 32
       5.6.1.  Handling Payload Packets . . . . . . . . . . . . . . . 32
       5.6.2.  Credit Aging . . . . . . . . . . . . . . . . . . . . . 33
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 34
     6.1.  Impersonation Attacks  . . . . . . . . . . . . . . . . . . 35
     6.2.  Denial-of-Service Attacks  . . . . . . . . . . . . . . . . 36
       6.2.1.  Flooding Attacks . . . . . . . . . . . . . . . . . . . 36
       6.2.2.  Memory/Computational-Exhaustion DoS Attacks  . . . . . 36
     6.3.  Mixed Deployment Environment . . . . . . . . . . . . . . . 37
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   8.  Authors and Acknowledgments  . . . . . . . . . . . . . . . . . 38
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     9.1.  Normative references . . . . . . . . . . . . . . . . . . . 38
     9.2.  Informative references . . . . . . . . . . . . . . . . . . 38

1. Introduction and Scope

The Host Identity Protocol [RFC4423] (HIP) supports an architecture that decouples the transport layer (TCP, UDP, etc.) from the internetworking layer (IPv4 and IPv6) by using public/private key pairs, instead of IP addresses, as host identities. When a host uses HIP, the overlying protocol sublayers (e.g., transport layer sockets and Encapsulating Security Payload (ESP) Security Associations (SAs)) are instead bound to representations of these host identities, and the IP addresses are only used for packet forwarding. However, each host must also know at least one IP address at which its peers are reachable. Initially, these IP addresses are the ones used during the HIP base exchange [RFC5201].

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. More 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 Security Parameter Index (SPI) so that packets are sent on the correct SA for a given address.

This document also specifies the messaging and elements of procedure for end-host mobility of a HIP host -- the sequential change in the preferred IP address used to reach a host. In particular, message flows to enable successful host mobility, including address verification methods, are defined herein.

However, while the same LOCATOR 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.

While HIP can potentially be used with transports other than the ESP transport format [RFC5202], this document largely assumes the use of ESP and leaves other transport formats 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, 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 [RFC5204]. Such functionality is out of the scope of this document. We also do not consider localized mobility management extensions (i.e., mobility management techniques that do not involve directly signaling the correspondent node); this document is concerned with end-to-end mobility. 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.

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 RFC 2119 [RFC2119].

LOCATOR. The name of a HIP parameter containing zero or more Locator

fields. This parameter's name is distinguished from the Locator fields embedded within it by the use of all capital letters.

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 verify a mobile or

multihomed 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

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

3.1. Operating Environment

The Host Identity Protocol (HIP) [RFC5201] is a key establishment and parameter negotiation protocol. Its primary applications are for authenticating host messages based on host identities, and establishing security associations (SAs) for the ESP transport format [RFC5202] and possibly other protocols in the future.

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

Figure 1: HIP Deployment Model

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

            | 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 Mobility and Multihoming (MH)

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

Consider first the case in which there is no mobility or multihoming, as specified in the base protocol specification [RFC5201]. 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 is still single-homed but moves to another IP address. Two things must occur in this case. First, the peer must be notified of the address change using a HIP UPDATE message. Second, each host must change its local bindings at the HIP sublayer (new IP addresses). It may be that both the SPIs and IP addresses are changed simultaneously in a single UPDATE; the protocol described herein supports this. However, simultaneous movement of both hosts, notification of transport layer protocols of the path change, and procedures for possibly traversing middleboxes are not covered by this document.

Finally, consider the case when a host is multihomed (has more than one globally routable address) and has multiple addresses available at the HIP layer as alternative locators for fault tolerance. Examples include the use of (possibly multiple) IPv4 and IPv6 addresses on the same interface, or the use of multiple interfaces attached to different service providers. Such host multihoming generally necessitates that a separate ESP SA is maintained for each interface in order to prevent packets that arrive over different paths from falling outside of the ESP anti-replay window [RFC4303]. Multihoming thus makes it possible that the bindings shown on the right side of Figure 2 are one to many (in the outbound direction, one HIT pair to multiple SPIs, and possibly then to multiple IP addresses). However, only one SPI and address pair can be used for any given packet, so the job of the "MH" block depicted above is to 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 desired mappings between HITs, SPIs, and addresses, and needs to ensure that UPDATE messages are sent along the right network paths so that any HIP-aware middleboxes can observe the SPIs. This document does not specify the "MH" block, nor does it specify detailed elements of procedure for how to handle various multihoming (perhaps combined with mobility) scenarios. The "MH" block may apply to more general problems outside of HIP. However, this document does describe a basic multihoming case (one host adds one address to its initial address and notifies the peer) and leave more complicated scenarios for experimentation and future documents.

3.1.1. Locator

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

3.1.2. Mobility Overview

When a host moves to another address, it notifies its peer of the new address by sending a HIP UPDATE packet containing a LOCATOR parameter. This UPDATE packet is acknowledged by the peer. For reliability in the presence of packet loss, the UPDATE packet is retransmitted as defined in the HIP protocol specification [RFC5201]. The peer can authenticate the contents of the UPDATE packet based on the signature and keyed hash of the packet.

When using ESP Transport Format [RFC5202], 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 [RFC5201] and ESP extension [RFC5202].

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, 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 basic 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, source address selection policies (beyond those specified in [RFC3484]), and the implications of multihoming on transport protocols and ESP anti-replay windows. Additional definitions of HIP-based multihoming are 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 [RFC5202], 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 [RFC5201]. However, for the (relatively) uninitiated reader, it is most important to keep in mind that in HIP the actual payload traffic is protected with ESP, and that the ESP SPI acts as an index to the right host-to-host context. More specification details are found later in Section 4 and Section 5.

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

The readdressing protocol is an asymmetric protocol where a mobile 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 majority of the 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.

The scenarios below at times describe addresses as being in either an ACTIVE, VERIFIED, or DEPRECATED state. From the perspective of a host, newly-learned addresses of the peer must be verified before put into active service, and addresses removed by the peer are put into a deprecated state. Under limited conditions described below (Section 5.6), an UNVERIFIED address may be used. The addressing states are defined more formally in Section 5.1.

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

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

A mobile host must sometimes change an IP address bound to an interface. The change of an IP address might be needed due to a change in the advertised IPv6 prefixes on the link, a reconnected PPP link, a new DHCP lease, or an actual movement to another subnet. In order to maintain its communication context, the host must inform its peers about the new IP address. This first example considers the case in which the mobile host has only one interface, IP address, a single pair of SAs (one inbound, one outbound), and no rekeying occurs on the SAs. We also assume that the new IP addresses are within the same address family (IPv4 or IPv6) as the first address. This is the simplest scenario, depicted in Figure 3.

     Mobile Host                         Peer Host

Figure 3: Readdress without Rekeying, but with Address Check

The steps of the packet processing are as follows:

  1. The mobile host 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 containing the values of the old and new SPIs for a security association. In this case, the OLD SPI and NEW SPI parameters both are set to the value of the preexisting incoming SPI; this ESP_INFO does not trigger a rekeying event but is instead included for possible parameter-inspecting middleboxes on the path. The LOCATOR 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 [RFC5201].
  1. 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 the OLD SPI and NEW SPI parameters both set to the value of the preexisting incoming SPI, and sends this UPDATE (with piggybacked acknowledgment) to the mobile host at its new address. The peer MAY use the new address immediately, but it MUST limit the amount of data it sends to the address until address verification completes.
  1. The mobile host completes the readdress by processing the UPDATE ACK and echoing the nonce in an ECHO_RESPONSE. Once the peer host receives this ECHO_RESPONSE, it considers the new address to be verified and can put the address into full use.

While the peer host is verifying the new address, the new address is marked as UNVERIFIED in the interim, and the old address is DEPRECATED. Once the peer host has received a correct reply to its UPDATE challenge, it marks the new address as ACTIVE and removes the old address.

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

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

     Mobile Host                         Peer Host

Figure 4: Readdress with Mobile-Initiated Rekey

3.2.3. Host Multihoming

A (mobile or stationary) host may sometimes have more than one interface or global address. The host may notify the peer host of the additional 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 multiple locator pairs are being used simultaneously rather than sequentially.

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

In the multihoming case, the sender may also have multiple valid locators from which to source traffic. In practice, a HIP association in a multihoming configuration may have both a preferred peer locator and a preferred local locator, although rules for source address selection should ultimately govern the selection of the source locator based on the destination locator.

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.

Consider the case between two hosts, one single-homed and one multihomed. The multihomed host may decide to inform the single- homed host about its other address. It is RECOMMENDED that the multihomed 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, indicating the request for a new SA by setting the OLD SPI value to zero, and the NEW SPI value to the newly created incoming SPI. A Locator Type of "1" is used to associate the new address with the new SPI. The LOCATOR parameter also contains a second Type "1" locator, that of the 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 waits for an 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 verification before actively using the new address. Figure 5 illustrates this scenario.

     Multi-homed Host                    Peer Host

Figure 5: Basic Multihoming Scenario

In multihoming scenarios, it is important that hosts receiving UPDATEs associate them correctly with the destination address used in the packet carrying the UPDATE. When processing inbound LOCATORs that establish new security associations on an interface with multiple addresses, a host uses the destination address of the UPDATE containing the LOCATOR as the local address to which the LOCATOR plus ESP_INFO is targeted. This is because hosts may send UPDATEs with the same (locator) IP address to different peer addresses -- this has the effect of creating multiple inbound SAs implicitly affiliated with different peer source addresses.

3.2.4. Site Multihoming

A host may have an interface that has multiple globally routable 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. The host should stay reachable at all or any subset of the currently available global routable addresses, independent of how they are provided.

This case is handled the same as if there were different IP addresses, described above in Section 3.2.3. Note that a single interface may experience site multihoming while the host itself may have multiple interfaces.

Note that a host may be multihomed and mobile simultaneously, and that a multihomed 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; further alignment with the IETF shim6 working group may be considered in the future.

3.2.5. Dual host multihoming

Consider the case in which both hosts would like to add an additional address after the base exchange completes. In Figure 6, consider that host1, which used address addr1a in the base exchange to set up SPI1a and SPI2a, wants to add address addr1b. It would send an UPDATE with LOCATOR (containing the address addr1b) to host2, using destination address addr2a, and a new set of SPIs would be added between hosts 1 and 2 (call them SPI1b and SPI2b -- not shown in the figure). Next, consider host2 deciding to add addr2b to the relationship. Host2 must select one of host1's addresses towards which to initiate an UPDATE. It may choose to initiate 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 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 6: Dual Multihoming Case in Which Each Host Uses LOCATOR to

Add a Second Address

3.2.6. Combined Mobility and Multihoming

It looks likely that in the future, many mobile hosts will be simultaneously mobile and multihomed, 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.

3.2.7. 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. The basic problem in this example is that the host readdressing to IPv4 does not know a corresponding IPv4 address of the peer. This may be handled (experimentally) by possibly configuring this address information manually or in the DNS, or the hosts exchange both IPv4 and IPv6 addresses in the locator.

3.2.8. Network Renumbering

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

3.2.9. Initiating the Protocol in R1 or I2

A Responder host MAY include a LOCATOR parameter in the R1 packet that it sends to the Initiator. This parameter 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 once the base exchange completes.

Initiator Responder

                              R1 with LOCATOR
   record additional addresses
   change responder address
                     I2 sent to newly indicated preferred address
                                                     (process normally)
   (process normally, later verification of non-preferred locators)

Figure 7: LOCATOR Inclusion in R1

An Initiator MAY include one or more LOCATOR parameters in the I2 packet, independent of whether or not there was a LOCATOR parameter in the R1. 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, and the ESP traffic that subsequently follows should use the Preferred locator.

Initiator Responder

                             I2 with LOCATOR
                                                     (process normally)
                                             record additional addresses
                       R2 sent to source address of I2
   (process normally)

Figure 8: LOCATOR Inclusion in I2

The I1 and I2 may be arriving from different source addresses if the LOCATOR parameter is present in R1. In this case, implementations simultaneously using multiple pre-created R1s, indexed by Initiator IP addresses, may inadvertently fail the puzzle solution of I2 packets due to a perceived puzzle mismatch. See, for instance, the example in Appendix A of [RFC5201]. As a solution, the Responder's puzzle indexing mechanism must be flexible enough to accommodate the situation when R1 includes a LOCATOR parameter.

3.3. Other Considerations

3.3.1. Address Verification

When a HIP host receives a set of locators from another HIP host in a LOCATOR, it does not necessarily know whether the other host is actually reachable at the claimed addresses. In fact, a malicious peer host may be intentionally giving bogus addresses in order to cause a packet flood towards the target addresses [RFC4225]. Likewise, viral software may have compromised the peer host, programming it to redirect packets to the target addresses. Thus, the HIP host must first check that the peer is reachable at the new 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 the exchange of a nonce, or the generation of a new SPI and observation of data arriving on the new SPI.

3.3.2. Credit-Based Authorization

Credit-Based Authorization (CBA) allows a host to securely use a new locator even though the peer's reachability at the address embedded in the locator has not yet been verified. This is accomplished based on the following three hypotheses:

  1. A flooding attacker typically seeks to somehow multiply the packets it generates for the purpose of its attack because bandwidth is an ample resource for many victims.
  1. 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.
  1. Consequently, the additional effort required to set up a redirection-based flooding attack (without CBA and return routability checks) would pay off for the attacker only if amplification could be obtained this way.

On this basis, rather than eliminating malicious packet redirection in the first place, Credit-Based Authorization prevents amplifications. This is accomplished by limiting the data a host can send to an unverified address of a peer by the data recently received from that peer. Redirection-based flooding attacks thus become less attractive than, for example, pure direct flooding, where the attacker itself sends bogus packets to the victim.

Figure 9 illustrates Credit-Based Authorization: Host B measures the amount of data recently received from peer A and, when A readdresses, sends packets to A's new, unverified address as long as the sum of the packet sizes does not exceed the measured, received data volume. When insufficient credit is left, B stops sending further packets to A until A's address becomes ACTIVE. The address changes may be due to mobility, multihoming, or any other reason. Not shown in Figure 9 are the results of credit aging (Section 5.6.2), a mechanism used to dampen possible time-shifting attacks.

           +-------+                        +-------+
           |   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 9: Readdressing Scenario

3.3.3. Preferred Locator

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

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 document 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. 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 levels for an association constructed as defined in the base specification [RFC5201] and ESP transform [RFC5202] is illustrated in Figure 10.

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

Figure 10: Relation between Hosts, SPIs,

and Addresses (Base Specification)

In Figure 10, 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 the hosts use in the base HIP exchange. These are the "preferred" (and only) addresses conveyed to the peer for use on each SA. That is, although packets sent to any of the hosts' interfaces may be accepted on the inbound SA, the peer host in general knows of only the single destination address learned in the base exchange (e.g., 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 document can be depicted as shown in Figure 11. In this figure, a host can have multiple inbound SPIs (and, not shown, multiple outbound SPIs) associated with another host. Furthermore, each SPI may have multiple addresses associated with it. These addresses that are bound to an SPI are not used to lookup the incoming SA. Rather, the addresses are those that are provided to the peer host, as hints for which addresses to use to reach the host on that SPI. The LOCATOR parameter is used to change the set of addresses that a peer associates with a particular SPI.

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

Figure 11: 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 with a peer 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 to lookup the incoming SA. Therefore, in Figure 11, 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, when there is a concern that anti-replay windows might be violated. 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 when 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 sends its packets on a given SPI. A host 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 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 during SA lookup anyway. However, this document does not specify such cases.

When the LOCATOR parameter is sent in an UPDATE packet, then the receiver will respond with an UPDATE acknowledgment. When the LOCATOR parameter is sent in an R1 or I2 packet, the base exchange retransmission mechanism will confirm its successful delivery. LOCATORs may experimentally be used in NOTIFY packets; in this case, the recipient MUST consider the LOCATOR as informational and not immediately change the current preferred address, but can test the additional locators when the need arises. The use of the 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 [RFC5201]. It consists of the standard HIP parameter Type and Length fields, plus zero or more Locator sub-parameters. Each Locator sub- parameter contains a Traffic Type, Locator Type, Locator Length, Preferred locator bit, Locator Lifetime, and a Locator encoding. A LOCATOR 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 12: LOCATOR Parameter Format

   Type:  193
   Length:  Length in octets, excluding Type and Length fields, and
      excluding padding.
   Traffic Type:  Defines whether the locator pertains to HIP signaling,
      user data, or both.
   Locator Type:  Defines the semantics of the Locator field.
   Locator Length:  Defines the length of the Locator field, in units of
      4-byte words (Locators up to a maximum of 4*255 octets are
   Reserved:  Zero when sent, ignored when received.


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

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

The Locator Lifetime indicates how long the following locator is expected to be valid. The lifetime is expressed in seconds. Each locator MUST have a non-zero lifetime. The address is expected to become deprecated when the specified number of seconds has passed since the reception of the message. A deprecated address SHOULD NOT be used as a destination address if an alternate (non-deprecated) is available and has sufficient scope.

4.1. Traffic Type and Preferred Locator

The following Traffic Type values are defined:

   0:  Both signaling (HIP control packets) and user data.
   1:  Signaling packets only.
   2:  Data packets only.

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

4.2. Locator Type and Locator

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

   0:  An IPv6 address or an IPv4-in-IPv6 format IPv4 address [RFC4291]
       (128 bits long).  This locator type is defined primarily for non-
       ESP-based usage.
   1:  The concatenation of an ESP SPI (first 32 bits) followed by an
       IPv6 address or an IPv4-in-IPv6 format IPv4 address (an
       additional 128 bits).  This IP address is defined primarily for
       ESP-based usage.

4.3. UPDATE Packet with Included LOCATOR

A number of combinations of parameters in an UPDATE packet are possible (e.g., see Section 3.2). In this document, procedures are defined only for the case in which one LOCATOR and one ESP_INFO parameter is used in any HIP packet. Furthermore, the LOCATOR SHOULD list all of the locators that are active on the HIP association (including those on SAs not covered by the ESP_INFO parameter). Any UPDATE packet that includes a LOCATOR parameter SHOULD include both an HMAC and a HIP_SIGNATURE parameter. The relationship between the announced Locators and any ESP_INFO parameters present in the packet is defined in Section 5.2. The sending of multiple LOCATOR and/or ESP_INFO parameters is for further study; receivers may wish to experiment with supporting such a possibility.

5. Processing Rules

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

5.1. Locator Data Structure and Status

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

  • the actual bit pattern representing the locator,
  • the lifetime (seconds),
  • the Traffic Type scope of the locator, and
  • whether the locator is preferred for any particular scope.

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
   UNVERIFIED to DEPRECATED  The locator lifetime expires while the
      locator is UNVERIFIED.
   ACTIVE to DEPRECATED  The locator lifetime expires while the locator
      is ACTIVE.
   ACTIVE to UNVERIFIED  There has been no traffic on the address for
      some time, and the local policy mandates that the address
      reachability must be verified again before starting to use it
   DEPRECATED to UNVERIFIED  The host receives a new lifetime for the

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

Note that the state of whether or not a locator is preferred is not necessarily the same as the value of the Preferred bit in the Locator sub-parameter received from the peer. Peers may recommend certain locators to be preferred, but the decision on whether to actually use a locator as a preferred locator is a local decision, possibly influenced by local policy.

5.2. Sending LOCATORs

The decision of when to send LOCATORs is basically a local policy issue. However, it is RECOMMENDED that a host send a LOCATOR 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 LOCATORs that force the peer to change the preferred address SHOULD be avoided.

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

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

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

Once the host has decided on the groups and assignment of addresses to the SPIs, it creates a LOCATOR parameter that serves as a complete representation of the addresses and affiliated SPIs intended for active use. We now describe a few cases introduced in Section 3.2. We assume that the Traffic Type for each locator is set to "0" (other values for Traffic Type may be specified in documents that separate the HIP control plane from data plane traffic). Other mobility and multihoming cases are possible but are left for further experimentation.

  1. Host mobility with no multihoming and no rekeying. The mobile host creates a single UPDATE containing a single ESP_INFO with a single LOCATOR parameter. The ESP_INFO contains the current value of the SPI in both the OLD SPI and NEW SPI fields. The LOCATOR 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 protocol specification [RFC5201]. The UPDATE should be sent to the peer's preferred IP address with an IP source address corresponding to the address in the LOCATOR parameter.
  1. Host mobility with no multihoming but with rekeying. The mobile host creates a single UPDATE containing a single ESP_INFO with a single LOCATOR parameter (with a single address). The ESP_INFO contains the current value of the SPI in the OLD SPI and the new value of the SPI in the NEW SPI, and a KEYMAT Index as selected by local policy. Optionally, the host may choose to initiate a Diffie Hellman rekey by including a DIFFIE_HELLMAN parameter. The LOCATOR contains a single Locator with "Locator Type" of "1"; the SPI must match that of the NEW SPI in the ESP_INFO. Otherwise, the steps are identical to the case in which no rekeying is initiated.
  1. Host multihoming (addition of an address). We only describe the simple case of adding an additional address to a (previously) single-homed, non-mobile host. The host SHOULD set up a new SA pair between this new address and the preferred address of the peer host. To do this, the multihomed host creates a new inbound SA and creates a new SPI. For the outgoing UPDATE message, it inserts an ESP_INFO parameter with an OLD SPI field of "0", a NEW SPI field corresponding to the new SPI, and a KEYMAT Index as selected by local policy. The host adds to the UPDATE message a LOCATOR with two Type "1" Locators: the original address and SPI active on the association, and the new address and new SPI being added (with the SPI matching the NEW SPI contained in the ESP_INFO). The Preferred bit SHOULD be set depending on the policy to tell the peer host which of the two locators is preferred. The UPDATE also contains a SEQ parameter and optionally a DIFFIE_HELLMAN parameter, and follows rekeying procedures with respect to this new address. The UPDATE message SHOULD be sent to the peer's Preferred address with a source address corresponding to the new locator.

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

5.3. Handling Received LOCATORs

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

This document describes sending both ESP_INFO and LOCATOR parameters in an UPDATE. The ESP_INFO parameter is included when there is a need to rekey or key a new SPI, and is otherwise included for the possible benefit of HIP-aware middleboxes. The LOCATOR parameter contains a complete map of the locators that the host wishes to make or keep active for the HIP association.

In general, the processing of a LOCATOR 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 and ESP_INFO are sent in an UPDATE message; other cases are for further study. The steps below cover each of the cases described in Section 5.2.

The processing of ESP_INFO and LOCATOR parameters is intended to be modular and support future generalization to the inclusion of multiple ESP_INFO and/or multiple LOCATOR parameters. A host SHOULD first process the ESP_INFO before the LOCATOR, since the ESP_INFO may contain a new SPI value mapped to an existing SPI, while a Type "1" locator will only contain a reference to the new SPI.

When a host receives a validated HIP UPDATE with a LOCATOR and ESP_INFO parameter, it processes the ESP_INFO as follows. The ESP_INFO parameter indicates whether an SA is being rekeyed, created, deprecated, or just identified for the benefit of middleboxes. The host examines the OLD SPI and NEW SPI values in the ESP_INFO 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 middleboxes) and no rekeying is necessary.
  1. (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 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.
  1. (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 parameter are processed. 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.

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

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

Once the host has processed the locators, 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 5.5.

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 parameter.

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, when the host is changing its SPI and sending an ESP_INFO to the peer, the NEW SPI value SHOULD be random and the value MAY be copied into an ECHO_REQUEST sent in the rekeying UPDATE. However, if the host is not changing its SPI, it MAY still use the ECHO_REQUEST parameter in an UPDATE message sent to the new address. A host MAY also use other message exchanges as confirmation of the address reachability.

Note that in the case of receiving a LOCATOR in an R1 and replying with an I2 to the new address in the LOCATOR, receiving the corresponding R2 is sufficient proof of reachability for the Responder's preferred address. Since further address verification of such an address can impede the HIP-base exchange, a host MUST NOT separately verify reachability of a new Preferred locator that was received on an R1.

In some cases, it MAY be sufficient to use the arrival of data on a newly advertised SA as implicit address reachability verification as depicted in Figure 13, instead of waiting for the confirmation via a HIP packet. In this case, a host advertising a new SPI as part of its address reachability check SHOULD be prepared to receive traffic on the new SA.

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

Figure 13: Address Activation Via Use of a New SA

When address verification is in progress for a new Preferred locator, the host SHOULD select a different locator listed as ACTIVE, if one such locator is available, to continue communications until address verification completes. Alternatively, the host MAY use the new Preferred locator while in UNVERIFIED status to the extent Credit- Based Authorization permits. Credit-Based Authorization is explained in Section 5.6. Once address verification succeeds, the status of the new Preferred locator changes to ACTIVE.

5.5. Changing the Preferred Locator

A host MAY want to change the Preferred outgoing locator for different reasons, e.g., because traffic information or ICMP error messages indicate that the currently used preferred address may have become unreachable. Another reason may be due to receiving a LOCATOR 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.
  1. If the new Preferred locator has UNVERIFIED status, the host starts to verify its reachability. The host SHOULD use a different locator listed as ACTIVE until address verification completes if one such locator is available. Alternatively, the host MAY use the new Preferred locator, even though in UNVERIFIED status, to the extent Credit-Based Authorization permits. Once address verification succeeds, the status of the new Preferred locator changes to ACTIVE and its use is no longer governed by Credit-Based Authorization.
  1. If the peer host has not indicated a preference for any address, then the host picks one of the peer's ACTIVE addresses randomly or according to policy. This case may arise if, for example, ICMP error messages that deprecate the Preferred locator arrive, but the peer has not yet indicated a new Preferred locator.
  1. If the new Preferred locator has DEPRECATED status and there is at least one non-deprecated address, the host selects one of the non-deprecated addresses as a new Preferred locator and continues. If the selected address is UNVERIFIED, the address verification procedure described above will apply.

5.6. Credit-Based Authorization

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

5.6.1. Handling Payload Packets

A host maintains a "credit counter" for each of its peers. Whenever a packet arrives from a peer, the host SHOULD increase that peer's credit counter by the size of the received packet. When the host has a packet to be sent to the peer, and when the peer's Preferred locator is listed as UNVERIFIED and no alternative locator with status ACTIVE is available, the host checks whether it can send the packet to the UNVERIFIED locator. The packet SHOULD be sent if the value of the credit counter is higher than the size of the outbound packet. If the credit counter is too low, the packet MUST be discarded or buffered until address verification succeeds. When a packet is sent to a peer at an UNVERIFIED locator, the peer's credit counter MUST be reduced by the size of the packet. The peer's credit counter is not affected by packets that the host sends to an ACTIVE locator of that peer.

Figure 14 depicts the actions taken by the host when a packet is received. Figure 15 shows the decision chain in the event a packet is sent.


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

Figure 14: Receiving Packets with Credit-Based Authorization



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

Figure 15: Sending Packets with Credit-Based Authorization

5.6.2. Credit Aging

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

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 [RFC5201]). 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 3: redirection-based flooding
  • memory-exhaustion attacks
  • computational-exhaustion attacks

We consider these in more detail in the following sections.

In Section 6.1 and Section 6.2, we assume that all users are using HIP. In Section 6.3 we consider the security ramifications when we have both HIP and non-HIP users. Security considerations for Credit- Based Authorization are discussed in [SIMPLE-CBA].

6.1. Impersonation Attacks

An attacker wishing to impersonate another host will try to mislead its victim into directly communicating with them, or carry out a man- in-the-middle (MitM) attack between the victim and the victim's desired communication peer. Without mobility support, both attack types are possible only if the attacker resides on the routing path between its victim and the victim's desired communication peer, or if the attacker tricks its victim into initiating the connection over an incorrect routing path (e.g., by acting as a router or using spoofed DNS entries).

The HIP extensions defined in this specification change the situation in that they introduce an ability to redirect a connection (like IPv6), both before and after establishment. If no precautionary measures are taken, an attacker could misuse the redirection feature to impersonate a victim's peer from any arbitrary location. The authentication and authorization mechanisms of the HIP base exchange [RFC5201] and the signatures in the UPDATE message prevent this attack. Furthermore, ownership of a HIP association is securely linked to a HIP HI/HIT. If an attacker somehow uses a bug in the implementation or weakness in some protocol to redirect a HIP connection, the original owner can always reclaim their connection (they can always prove ownership of the private key associated with their public HI).

MitM attacks are always possible if the attacker is present during the initial HIP base exchange and if the hosts do not authenticate each other's identities. However, once the opportunistic base exchange has taken place, even a MitM cannot steal the HIP connection anymore because it is very difficult for an attacker to create an UPDATE packet (or any HIP packet) that will be accepted as a legitimate update. UPDATE packets use HMAC and are signed. Even 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.

6.2. Denial-of-Service Attacks

6.2.1. Flooding Attacks

The purpose of a denial-of-service attack is to exhaust some resource of the victim such that the victim ceases to operate correctly. A denial-of-service attack can aim at the victim's network attachment (flooding attack), its memory, or its processing capacity. In a flooding attack, the attacker causes an excessive number of bogus or unwanted packets to be sent to the victim, which fills their available bandwidth. Note that the victim does not necessarily need to be a node; it can also be an entire network. The attack basically functions the same way in either case.

An effective DoS strategy is distributed denial of service (DDoS). Here, the attacker conventionally distributes some viral software to as many nodes as possible. Under the control of the attacker, the infected nodes, or "zombies", jointly send packets to the victim. With such an 'army', an attacker can take down even very high bandwidth networks/victims.

With the ability to redirect connections, an attacker could realize a DDoS attack without having to distribute viral code. Here, the attacker initiates a large download from a server, and subsequently redirects this download to its victim. The attacker can repeat this with multiple servers. This threat is mitigated through reachability checks and credit-based authorization. Both strategies do not eliminate flooding attacks per se, but they preclude: (i) their use from a location off the path towards the flooded victim; and (ii) any amplification in the number and size of the redirected packets. As a result, the combination of a reachability check and credit-based authorization lowers a HIP redirection-based flooding attack to the level of a direct flooding attack in which the attacker itself sends the flooding traffic to the victim.

6.2.2. Memory/Computational-Exhaustion DoS Attacks

We now consider whether or not the proposed extensions to HIP add any new DoS attacks (consideration of DoS attacks using the base HIP exchange and updates is discussed in [RFC5201]). A simple attack is to send many UPDATE packets containing many IP addresses that are not flagged as preferred. The attacker continues to send such packets until the number of IP addresses associated with the attacker's HI crashes the system. Therefore, there SHOULD be a limit to the number of IP addresses that can be associated with any HI. Other forms of memory/computationally exhausting attacks via the HIP UPDATE packet are handled in the base HIP document [RFC5201].

A central server that has to deal with a large number of mobile clients may consider increasing the SA lifetimes to try to slow down the rate of rekeying UPDATEs or increasing the cookie difficulty to slow down the rate of attack-oriented connections.

6.3. Mixed Deployment Environment

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

  1. A HIP host redirects its connection onto a non-HIP host. The non-HIP host will drop the reachability packet, so this is not a threat unless the HIP host is a MitM that could somehow respond successfully to the reachability check.
  1. 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.
  1. 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.
  1. A HIP host attempts to steal a non-HIP host's session. A HIP host could spoof the non-HIP host's IP address during the base exchange or set the non-HIP host's IP address as its preferred address via an UPDATE. Other possibilities exist, but a simple solution is to prevent the use of HIP address check information to influence non-HIP sessions.

7. IANA Considerations

This document defines a LOCATOR parameter for the Host Identity Protocol [RFC5201]. This parameter is defined in Section 4 with a Type of 193.

This document also defines a LOCATOR_TYPE_UNSUPPORTED Notify Message Type as defined in the Host Identity Protocol specification [RFC5201]. This parameter is defined in Section 5.3 with a value of 46.

8. Authors and Acknowledgments

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

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

9. References

9.1. Normative references

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC3484]     Draves, R., "Default Address Selection for Internet
                 Protocol version 6 (IPv6)", RFC 3484, February 2003.
   [RFC4291]     Hinden, R. and S. Deering, "IP Version 6 Addressing
                 Architecture", RFC 4291, February 2006.
   [RFC4303]     Kent, S., "IP Encapsulating Security Payload (ESP)",
                 RFC 4303, December 2005.
   [RFC4423]     Moskowitz, R. and P. Nikander, "Host Identity Protocol
                 (HIP) Architecture", RFC 4423, May 2006.
   [RFC5201]     Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
                 Henderson, "Host Identity Protocol", RFC 5201,
                 April 2008.
   [RFC5202]     Jokela, P., Moskowitz, R., and P. Nikander, "Using the
                 ESP Transport Format with the Host Identity Protocol
                 (HIP)", RFC 5202, April 2008.
   [RFC5204]     Laganier, J. and L. Eggert, "Host Identity Protocol
                 (HIP) Rendezvous Extension", RFC 5204, April 2008.

9.2. Informative references

   [CBA-MIPv6]   Vogt, C. and J. Arkko, "Credit-Based Authorization for
                 Mobile IPv6 Early Binding Updates", Work in Progress,
                 February 2005.
   [RFC4225]     Nikander, P., Arkko, J., Aura, T., Montenegro, G., and
                 E. Nordmark, "Mobile IP Version 6 Route Optimization
                 Security Design Background", RFC 4225, December 2005.
   [SIMPLE-CBA]  Vogt, C. and J. Arkko, "Credit-Based Authorization for
                 Concurrent Reachability Verification", Work
                 in Progress, February 2006.

Authors' Addresses

   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   Phone: +358 9 299 1

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

   Christian Vogt
   Ericsson Research NomadicLab
   Hirsalantie 11
   JORVAS  FIN-02420
   Jari Arkko
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   Phone: +358 40 5079256

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