Network Working Group T. Henderson (editor) Internet-Draft The Boeing Company Expires: January 18, 2006 July 17, 2005 End-Host Mobility and Multihoming with the Host Identity Protocol draft-ietf-hip-mm-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 18, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document defines mobility and multihoming extensions to the Host Identity Protocol (HIP). Specifically, this document defines a general "LOCATOR" parameter for HIP messages that allows for a HIP host to notify peers about alternate addresses at which it may be reached. This document also defines elements of procedure for mobility of a HIP host-- the process by which a host dynamically changes the primary locator that it uses to receive packets. While the same LOCATOR parameter can also be used to support end-host multihoming, detailed procedures are left for further study. Henderson (editor) Expires January 18, 2006 [Page 1]
Internet-Draft HIP Mobility and Multihoming July 2005 Table of Contents 1. Introduction and Scope . . . . . . . . . . . . . . . . . . . . 4 2. Terminology and Conventions . . . . . . . . . . . . . . . . . 5 3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 Operating Environment . . . . . . . . . . . . . . . . . . 6 3.1.1 Locator . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.2 Mobility . . . . . . . . . . . . . . . . . . . . . . . 7 3.1.3 Multihoming . . . . . . . . . . . . . . . . . . . . . 7 3.2 Protocol Overview . . . . . . . . . . . . . . . . . . . . 8 3.2.1 Mobility with single SA pair . . . . . . . . . . . . . 8 3.2.2 Host multihoming . . . . . . . . . . . . . . . . . . . 10 3.2.3 Site multihoming . . . . . . . . . . . . . . . . . . . 12 3.2.4 Dual host multihoming . . . . . . . . . . . . . . . . 12 3.2.5 Combined mobility and multihoming . . . . . . . . . . 13 3.2.6 Using LOCATORs across addressing realms . . . . . . . 13 3.2.7 Network renumbering . . . . . . . . . . . . . . . . . 13 3.2.8 Initiating the protocol in R1 or I2 . . . . . . . . . 13 3.3 Other Considerations . . . . . . . . . . . . . . . . . . . 15 3.3.1 Address Verification . . . . . . . . . . . . . . . . . 15 3.3.2 Credit-Based Authorization . . . . . . . . . . . . . . 15 3.3.3 Preferred locator . . . . . . . . . . . . . . . . . . 16 3.3.4 Interaction with Security Associations . . . . . . . . 17 4. LOCATOR parameter format . . . . . . . . . . . . . . . . . . . 20 4.1 Traffic Type and Preferred Locator . . . . . . . . . . . . 21 4.2 Locator Type and Locator . . . . . . . . . . . . . . . . . 22 4.3 UPDATE packet with included LOCATOR . . . . . . . . . . . 22 5. Processing rules . . . . . . . . . . . . . . . . . . . . . . . 23 5.1 Locator data structure and status . . . . . . . . . . . . 23 5.2 Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 24 5.3 Handling received LOCATORs . . . . . . . . . . . . . . . . 25 5.4 Verifying address reachability . . . . . . . . . . . . . . 26 5.5 Credit-Based Authorization . . . . . . . . . . . . . . . . 28 5.5.1 Handling Payload Packets . . . . . . . . . . . . . . . 28 5.5.2 Credit Aging . . . . . . . . . . . . . . . . . . . . . 29 5.6 Changing the preferred locator . . . . . . . . . . . . . . 30 6. Policy considerations . . . . . . . . . . . . . . . . . . . . 32 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 7.1 Impersonation attacks . . . . . . . . . . . . . . . . . . 33 7.2 Denial of Service attacks . . . . . . . . . . . . . . . . 34 7.2.1 Flooding Attacks . . . . . . . . . . . . . . . . . . . 34 7.2.2 Memory/Computational exhaustion DoS attacks . . . . . 35 7.3 Mixed deployment environment . . . . . . . . . . . . . . . 35 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 9. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 39 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 40 11.1 Normative references . . . . . . . . . . . . . . . . . . . 40 Henderson (editor) Expires January 18, 2006 [Page 2]
Internet-Draft HIP Mobility and Multihoming July 2005 11.2 Informative references . . . . . . . . . . . . . . . . . . 40 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 41 A. Changes from previous versions . . . . . . . . . . . . . . . . 42 A.1 From nikander-hip-mm-00 to nikander-hip-mm-01 . . . . . . 42 A.2 From nikander-hip-mm-01 to nikander-hip-mm-02 . . . . . . 42 A.3 From -02 to draft-ietf-hip-mm-00 . . . . . . . . . . . . . 42 A.4 From draft-ietf-hip-mm-00 to -01 . . . . . . . . . . . . . 43 A.5 From draft-ietf-hip-mm-01 to -02 . . . . . . . . . . . . . 43 Intellectual Property and Copyright Statements . . . . . . . . 44 Henderson (editor) Expires January 18, 2006 [Page 3]
Internet-Draft HIP Mobility and Multihoming July 2005 1. Introduction and Scope The Host Identity Protocol [1] (HIP) supports an architecture that decouples the transport layer (TCP, UDP, etc.) from the internetworking layer (IPv4 and IPv6) by using public/private key pairs, instead of IP addresses, as host identities. When a host uses HIP, the overlying protocol sublayers (e.g., transport layer sockets and ESP Security Associations) are instead bound to representations of these host identities, and the IP addresses are only used for packet forwarding. However, each host must also know at least one IP address at which its peers are reachable. Initially, these IP addresses are the ones used during the HIP base exchange [2]. This document defines a generalized LOCATOR parameter for use in HIP messages. The LOCATOR parameter allows a HIP host to notify a peer about alternate addresses at which it is reachable. The LOCATORs may be merely IP addresses, or they may have additional multiplexing and demultiplexing context to aid the packet handling in the lower layers. For instance, an IP address may need to be paired with an ESP SPI so that packets are sent on the correct SA for a given address. This document also specifies the messaging and elements of procedure for end-host mobility of a HIP host-- the sequential change in preferred IP address used to reach a host. In particular, message flows to enable successful host mobility, including address verification methods, are defined herein. However, while the same LOCATOR parameter is intended to support host multihoming (parallel support of a number of addresses), and experimentation is encouraged, detailed elements of procedure for host multihoming are left for further study. There are a number of situations where the simple end-to-end readdressing functionality is not sufficient. These include the initial reachability of a mobile host, location privacy, end-host and site multihoming with legacy hosts, simultaneous mobility of both hosts, and NAT traversal. In these situations there is a need for some helper functionality in the network, such as a HIP Rendezvous server [3]. Such functionality is out of scope of this document. Finally, making underlying IP mobility transparent to the transport layer has implications on the proper response of transport congestion control, path MTU selection, and QoS. Transport-layer mobility triggers, and the proper transport response to a HIP mobility or multihoming address change, are outside the scope of this document. Henderson (editor) Expires January 18, 2006 [Page 4]
Internet-Draft HIP Mobility and Multihoming July 2005 2. Terminology and Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [6]. Locator. A name that controls how the packet is routed through the network and demultiplexed by the end host. It may include a concatenation of traditional network addresses such as an IPv6 address and end-to-end identifiers such as an ESP SPI. It may also include transport port numbers or IPv6 Flow Labels as demultiplexing context, or it may simply be a network address. Address. A name that denotes a point-of-attachment to the network. The two most common examples are an IPv4 address and an IPv6 address. The set of possible addresses is a subset of the set of possible locators. Preferred locator. A locator on which a host prefers to receive data. With respect to a given peer, a host always has one active preferred locator, unless there are no active locators. By default, the locators used in the HIP base exchange are the preferred locators. Credit Based Authorization. A host must must verify a mobile or multi-homed 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. Henderson (editor) Expires January 18, 2006 [Page 5]
Internet-Draft HIP Mobility and Multihoming July 2005 3. Protocol Model 3.1 Operating Environment The Host Identity Protocol (HIP) [2] is a key establishment and parameter negotiation protocol. Its primary applications are for authenticating host messages based on host identities, and establishing security associations (SAs) for ESP transport format [5] and possibly other protocols in the future. +--------------------+ +--------------------+ | | | | | +------------+ | | +------------+ | | | Key | | HIP | | Key | | | | Management | <-+-----------------------+-> | Management | | | | Process | | | | Process | | | +------------+ | | +------------+ | | ^ | | ^ | | | | | | | | v | | v | | +------------+ | | +------------+ | | | IPsec | | ESP | | IPsec | | | | Stack | <-+-----------------------+-> | Stack | | | | | | | | | | | +------------+ | | +------------+ | | | | | | | | | | Initiator | | Responder | +--------------------+ +--------------------+ Figure 1: HIP deployment model The general deployment model for HIP is shown above, assuming operation in an end-to-end fashion. This document specifies extensions to the HIP protocol to enable end-host mobility and multihoming. In a nutshell, the HIP protocol can carry new addressing information to the peer and can enable direct authentication of the message via a signature based on its host identity. This document specifies the format of this new addressing (LOCATOR) parameter, the procedures for sending and processing this parameter to enable basic host mobility, and procedures for a concurrent address verification mechanism. 3.1.1 Locator This document defines a generalization of an address called a "locator". A locator specifies a point-of-attachment to the network but may also include additional end-to-end tunneling or per-host Henderson (editor) Expires January 18, 2006 [Page 6]
Internet-Draft HIP Mobility and Multihoming July 2005 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 violation of the ESP anti-replay window [4]. Addresses may also be affiliated with transport ports in certain tunneling scenarios. Or locators may merely be traditional network addresses. 3.1.2 Mobility 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, and is protected by retransmission. The peer can authenticate the contents of the UPDATE packet based on the signature and keyed hash of the packet. 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 [2]. When using ESP Transport Format [5], 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. However, the peers are not able to reply 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. 3.1.3 Multihoming A related operational configuration is host multihoming, in which a host has multiple locators simultaneously rather than sequentially as in the case of mobility. By using the locator parameter defined herein, a host can inform its peers of additional (multiple) locators at which it can be reached, and can declare a particular locator as a "preferred" locator. Although this document defines a mechanism for multihoming, it does not define associated policies and procedure details such as which locators to choose when more than one pair is available, the operation of simultaneous mobility and multihoming, and the implications of multihoming on transport protocols and ESP anti-replay windows. Additional definition of HIP-based multihoming is expected to be part of a future document. Henderson (editor) Expires January 18, 2006 [Page 7]
Internet-Draft HIP Mobility and Multihoming July 2005 3.2 Protocol Overview In this section we briefly introduce a number of usage scenarios where the HIP mobility and multihoming facility is useful. These scenarios assume that HIP is being used with the ESP Transform, although other scenarios may be defined in the future. To understand these usage scenarios, the reader should be at least minimally familiar with the HIP protocol specification [2]. However, for the (relatively) uninitiated reader it is most important to keep in mind that in HIP the actual payload traffic is protected with ESP, and that the ESP SPI acts as an index to the right host-to-host context. Each of the scenarios below assumes that the HIP base exchange has completed, and the hosts each have a single outbound SA to the peer host. Associated with this outbound SA is a single destination address of the peer host-- the source address used by the peer during the base exchange. The readdressing protocol is an asymmetric protocol where one host, called the mobile host, informs another host, called the peer host, about changes of IP addresses on affected SPIs. The readdressing exchange is designed to be piggybacked on existing HIP exchanges. The main packets on which the LOCATOR parameters are expected to be carried are UPDATE packets. However, some implementations may want to experiment with sending LOCATOR parameters also on other packets, such as R1, I2, and NOTIFY. 3.2.1 Mobility with single SA pair 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, and a single pair of SAs (one inbound, one outbound). 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 LOCATOR indicates the new IP address and the SPI associated with the new IP address by using a Locator Type of "1", the locator lifetime, and whether the new locator is a preferred locator. The mobile host may optionally send an ESP_INFO to create a new inbound SA, in which case it transitions to state REKEYING. In this case, the Locator contains the new SPI to use. Otherwise, the existing Henderson (editor) Expires January 18, 2006 [Page 8]
Internet-Draft HIP Mobility and Multihoming July 2005 SPI is identified in the Locator parameter, and the host waits for its UPDATE to be acknowledged. 2. Depending on whether the mobile host initiated a rekey, and on whether the peer host itself wants to rekey, a number of responses are possible. Figure 2 illustrates an exchange for which neither side initiates a rekeying, but for which the peer host performs an address check. If the mobile host is rekeying, the peer will also rekey, as shown in Figure 3. If the mobile host did not decide to rekey but the peer desires to do so, then it initiates a rekey as illustrated in Figure 4. The UPDATE messages sent from the peer back to the mobile are sent to the newly advertised address. 3. While the peer host is verifying the new address, the address is marked as UNVERIFIED in the interim. Once it has received a correct reply to its UPDATE challenge, or optionally, data on the new SA, it marks the new address as ACTIVE and removes the old address. Mobile Host Peer Host UPDATE(ESP_INFO, LOC, SEQ) -----------------------------------> UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> Figure 2: Readdress without rekeying, but with address check Mobile Host Peer Host UPDATE(ESP_INFO, LOC, SEQ, [DIFFIE_HELLMAN]) -----------------------------------> UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> Figure 3: Readdress with mobile-initiated rekey Henderson (editor) Expires January 18, 2006 [Page 9]
Internet-Draft HIP Mobility and Multihoming July 2005 Mobile Host Peer Host UPDATE(LOC, SEQ) -----------------------------------> UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN], ECHO_REQUEST) <----------------------------------- UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_RESPONSE) -----------------------------------> UPDATE(ACK) <----------------------------------- Figure 4: Readdress with peer-initiated rekey Hosts that use link-local addresses as source addresses in their HIP handshakes may not be reachable by a mobile peer. Such hosts SHOULD provide a globally routable address either in the initial handshake or via the LOCATOR parameter. 3.2.2 Host multihoming A (mobile or stationary) host may sometimes have more than one interface. The host may notify the peer host of the additional interface(s) by using the LOCATOR parameter. To avoid problems with the ESP anti-replay window, a host SHOULD use a different SA for each interface used to receive packets from the peer host. When more than one locator is provided to the peer host, the host SHOULD indicate which locator is preferred. By default, the addresses used in the base exchange are preferred until indicated otherwise. Although the protocol may allow for configurations in which there is an asymmetric number of SAs between the hosts (e.g., one host has two interfaces and two inbound SAs, while the peer has one interface and one inbound SA), it is RECOMMENDED that inbound and outbound SAs be created pairwise between hosts. When an ESP_INFO arrives to rekey a particular outbound SA, the corresponding inbound SA should be also rekeyed at that time. Although asymmetric SA configurations might be experimented with, their usage may constrain interoperability at this time. However, it is recommended that implementations attempt to support peers that prefer to use non-paired SAs. It is expected that this section and behavior will be modified in future revisions of this protocol, once the issue and its implications are better understood. To add both an additional interface and SA, the host sends a LOCATOR with an ESP_INFO. The host uses the same (new) SPI value in the LOCATOR and both the "Old SPI" and "New SPI" values in the ESP_INFO-- Henderson (editor) Expires January 18, 2006 [Page 10]
Internet-Draft HIP Mobility and Multihoming July 2005 this indicates to the peer that the SPI is not replacing an existing SPI. The multihomed host transitions to state REKEYING, waiting for a ESP_INFO from the peer and an ACK of its own UPDATE. As in the mobility case, the peer host must perform an address check while it is rekeying. Figure 5 illustrates the basic packet exchange. Multi-homed Host Peer Host UPDATE(ESP_INFO, LOC, SEQ, [DIFFIE_HELLMAN]) -----------------------------------> UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> Figure 5: Basic multihoming scenario For the case in which multiple locators are advertised in a LOCATOR, the peer does not need to send ACK for the UPDATE(LOCATOR) in every subsequent message used for the address check procedure of the multiple locators. Therefore, a sample packet exchange might look as shown in Figure 6. Multi-homed Host Peer Host UPDATE(LOC(addr_1,addr_2), SEQ) -----------------------------------> UPDATE(ACK) <----------------------------------- sent to addr_1:UPDATE(ESP_INFO, SEQ, ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> sent to addr_2:UPDATE(ESP_INFO, SEQ, ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> Figure 6: LOCATOR with multiple addresses When processing inbound LOCATORs that establish new security associations, a host uses the destination address of the UPDATE containing LOCATOR as the local address to which the LOC plus ESP_INFO is targeted. Hosts may send LOCATOR with the same IP address to different peer addresses-- this has the effect of creating multiple inbound SAs implicitly affiliated with different source Henderson (editor) Expires January 18, 2006 [Page 11]
Internet-Draft HIP Mobility and Multihoming July 2005 addresses. When rekeying in a multihoming situation in which there is an asymmetric number of SAs between two hosts, a respondent to the ESP_INFO/UPDATE procedure may have some ambiguity as to which inbound SA it should update in response to the peer's UPDATE. In such a case, the host SHOULD choose an SA corresponding to the inbound interface on which the UPDATE was received. 3.2.3 Site multihoming A host may have an interface that has multiple globally reachable IP addresses. Such a situation may be a result of the site having multiple upper Internet Service Providers, or just because the site provides all hosts with both IPv4 and IPv6 addresses. It is desirable that the host can stay reachable with all or any subset of the currently available globally routable addresses, independent on how they are provided. This case is handled the same as if there were different IP addresses, described above in Section 3.2.2. Note that a single interface may experience site multihoming while the host itself may have multiple interfaces. Note that a host may be multi-homed and mobile simultaneously, and that a multi-homed host may want to protect the location of some of its interfaces while revealing the real IP address of some others. This document does not presently specify additional site multihoming extensions to HIP to further align it with the requirements of the multi6 working group. 3.2.4 Dual host multihoming Consider the case in which both hosts would like to add an additional address after the base exchange completes. In Figure 7, consider that host1 wants to add address addr1b. It would send a LOCATOR to host2 located at addr2a, and a new set of SPIs would be added between hosts 1 and 2 (call them SPI1b and SPI2b). Next, consider host2 deciding to add addr2b to the relationship. host2 now has a choice of which of host1's addresses to initiate LOCATOR to. It may choose to initiate a LOCATOR 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 may be often the case that hosts primarily establish new SAs only with the peer's preferred locator. The readdressing protocol is flexible enough to accommodate this choice. Henderson (editor) Expires January 18, 2006 [Page 12]
Internet-Draft HIP Mobility and Multihoming July 2005 -<- SPI1a -- -- SPI2a ->- host1 < > addr1a <---> addr2a < > host2 ->- SPI2a -- -- SPI1a -<- addr1b <---> addr2b Figure 7: Dual multihoming case in which each host uses LOCATOR to add a second address 3.2.5 Combined mobility and multihoming It looks likely that in the future many mobile hosts will be simultaneously mobile and multi-homed, i.e., have multiple mobile interfaces. Furthermore, if the interfaces use different access technologies, it is fairly likely that one of the interfaces may appear stable (retain its current IP address) while some other(s) may experience mobility (undergo IP address change). The use of LOCATOR plus ESP_INFO should be flexible enough to handle most such scenarios, although more complicated scenarios have not been studied so far. 3.2.6 Using LOCATORs across addressing realms It is possible for HIP associations to migrate to a state in which both parties are only using locators in different addressing realms. For example, the two hosts may initiate the HIP association when both are using IPv6 locators, then one host may loose its IPv6 connectivity and obtain an IPv4 address. In such a case, some type of mechanism for interworking between the different realms must be employed; such techniques are outside the scope of the present text. If no mechanism exists, then the UPDATE message carrying the new LOCATOR will likely not be acknowledged anyway, and the HIP state may time out. 3.2.7 Network renumbering It is expected that IPv6 networks will be renumbered much more often than most IPv4 networks are. From an end-host point of view, network renumbering is similar to mobility. 3.2.8 Initiating the protocol in R1 or I2 A Responder host MAY include one or more LOCATOR parameters in the R1 packet that it sends to the Initiator. These parameters MUST be protected by the R1 signature. If the R1 packet contains LOCATOR parameters with a new preferred locator, the Initiator SHOULD Henderson (editor) Expires January 18, 2006 [Page 13]
Internet-Draft HIP Mobility and Multihoming July 2005 directly set the new preferred locator to status ACTIVE without performing address verification first, and MUST send the I2 packet to the new preferred locator. The I1 destination address and the new preferred locator may be identical. All new non-preferred locators must still undergo address verification. Initiator Responder R1 with LOCATOR <----------------------------------- record additional addresses change responder address I2 with new SPI in ESP_INFO parameter -----------------------------------> (process normally) R2 <----------------------------------- (process normally) Figure 8: LOCATOR inclusion in R1 An Initiator MAY include one or more LOCATOR parameters in the I2 packet, independent on whether there was LOCATOR parameter(s) in the R1 or not. These parameters MUST be protected by the I2 signature. Even if the I2 packet contains LOCATOR parameters, the Responder MUST still send the R2 packet to the source address of the I2. The new preferred locator SHOULD be identical to the I2 source address. If the I2 packet contains LOCATOR parameters, all new locators must undergo address verification as usual. If any of these locators is a new preferred locator, an efficient method to verify this is to piggyback an ECHO_REQUEST parameter with some unguessable data to the R2 packet. Initiator Responder I2 with LOCATOR -----------------------------------> (process normally) record additional addresses R2 with new SPI in ESP_INFO parameter <----------------------------------- (process normally) data on new SA ------------------------------------> (process normally) Figure 9: LOCATOR inclusion in I2 Henderson (editor) Expires January 18, 2006 [Page 14]
Internet-Draft HIP Mobility and Multihoming July 2005 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 [10]. Likewise, viral software may have compromised the peer host, programming it to redirect packets to the target addresses. Thus, the HIP host must first check that the peer is reachable at the new address. An additional potential benefit of performing address verification is to allow middleboxes in the network along the new path to obtain the peer host's inbound SPI. Address verification is implemented by the challenger sending some piece of unguessable information to the new address, and waiting for some acknowledgment from the responder that indicates reception of the information at the new address. This may include exchange of a nonce, or generation of a new SPI and observing data arriving on the new SPI. 3.3.2 Credit-Based Authorization Credit-Based Authorization allows a host to securely use a new locator even though the peer's reachability at the address embedded in this locator has not yet been verified. This is accomplished based on the following three hypotheses: 1. A flooding attacker typically seeks to somehow multiply the packets it generates itself for the purpose of its attack because bandwidth is an ample resource for many attractive victims. 2. An attacker can always cause unamplified flooding by sending packets to its victim directly. 3. Consequently, the additional effort required to set up a redirection-based flooding attack would pay off for the attacker only if amplification could be obtained this way. On this basis, rather than eliminating malicious packet redirection in the first place, Credit-Based Authorization prevents any amplification that can be reached through it. This is accomplished by limiting the data a host can send to an unverified address of a peer by the data recently received from that peer. Redirection-based flooding attacks thus become less attractive than, e.g., pure direct Henderson (editor) Expires January 18, 2006 [Page 15]
Internet-Draft HIP Mobility and Multihoming July 2005 flooding, where the attacker itself sends bogus packets to the victim. Figure 10 illustrates Credit-Based Authorization: Host B measures the bytes recently received from peer A and, when A readdresses, sends packets to A's new, unverified address as long as the sum of their sizes does not exceed the measured, received data volume. When insufficient credit is left, B stops sending further packets to A until A's address becomes ACTIVE. The address changes may be due to mobility, due to multihoming, or due to any other reason. +-------+ +-------+ | A | | B | +-------+ +-------+ | | address |------------------------->| credit += size(packet) ACTIVE | | |------------------------->| credit += size(packet) |<-------------------------| don't change credit | | + address change | address |<-------------------------| credit -= size(packet) UNVERIFIED |------------------------->| credit += size(packet) |<-------------------------| credit -= size(packet) | | |<-------------------------| credit -= size(packet) | X credit < size(packet)=> drop! | | + address change | address | | ACTIVE |<-------------------------| don't change credit | | Figure 10: Readdressing Scenario 3.3.3 Preferred locator When a host has multiple locators, the peer host must decide upon which to use for outbound packets. It may be that a host would prefer to receive data on a particular inbound interface. HIP allows a particular locator to be designated as a preferred locator, and communicated to the peer (see Section 4). In general, when multiple locators are used for a session, there is the question of using multiple locators for failover only or for load-balancing. Due to the implications of load-balancing on the Henderson (editor) Expires January 18, 2006 [Page 16]
Internet-Draft HIP Mobility and Multihoming July 2005 transport layer that still need to be worked out, this draft assumes that multiple locators are used primarily for failover. An implementation may use ICMP interactions, reachability checks, or other means to detect the failure of a locator. 3.3.4 Interaction with Security Associations This document specifies a new HIP protocol parameter, the LOCATOR parameter (see Section 4), that allows the hosts to exchange information about their locator(s), and any changes in their locator(s). The logical structure created with LOCATOR parameters has three levels: hosts, Security Associations (SAs) indexed by Security Parameter Indices (SPIs), and addresses. The relation between these entities for an association negotiated as defined in the base specification [2] and ESP transform [5] is illustrated in Figure 11. -<- SPI1a -- -- SPI2a ->- host1 < > addr1a <---> addr2a < > host2 ->- SPI2a -- -- SPI1a -<- Figure 11: Relation between hosts, SPIs, and addresses (base specification) In Figure 11, host1 and host2 negotiate two unidirectional SAs, and each host selects the SPI value for its inbound SA. The addresses addr1a and addr2a are the source addresses that each host uses in the base HIP exchange. These are the "preferred" (and only) addresses conveyed to the peer for each SA; even though packets sent to any of the hosts' interfaces can arrive on an inbound SPI, when a host sends packets to the peer on an outbound SPI, it knows of a single destination address associated with that outbound SPI (for host1, it sends a packet on SPI2a to addr2a to reach host2), unless other mechanisms exist to learn of new addresses. In general, the bindings that exist in an implementation corresponding to this draft can be depicted as shown in Figure 12. In this figure, a host can have multiple inbound SPIs (and, not shown, multiple outbound SPIs) between itself and another host. Furthermore, each SPI may have multiple addresses associated with it. These addresses bound to an SPI are not used as SA selectors. Rather, the addresses are those addresses that are provided to the peer host, as hints for which addresses to use to reach the host on that SPI. The LOCATOR parameter allows for IP addresses and SPIs to be combined to form generalized locators. The LOCATOR parameter is used to change the set of addresses that a peer associates with a particular SPI. Henderson (editor) Expires January 18, 2006 [Page 17]
Internet-Draft HIP Mobility and Multihoming July 2005 address11 / SPI1 - address12 / / address21 host -- SPI2 < \ address22 \ SPI3 - address31 \ address32 Figure 12: Relation between hosts, SPIs, and addresses (general case) A host may establish any number of security associations (or SPIs) with a peer. The main purpose of having multiple SPIs is to group the addresses into collections that are likely to experience fate sharing. For example, if the host needs to change its addresses on SPI2, it is likely that both address21 and address22 will simultaneously become obsolete. In a typical case, such SPIs may correspond with physical interfaces; see below. Note, however, that especially in the case of site multihoming, one of the addresses may become unreachable while the other one still works. In the typical case, however, this does not require the host to inform its peers about the situation, since even the non-working address still logically exists. A basic property of HIP SAs is that the inbound IP address is not used as a selector for the SA. Therefore, in Figure 12, it may seem unnecessary for address31, for example, to be associated only with SPI3-- in practice, a packet may arrive to SPI1 via destination address address31 as well. However, the use of different source and destination addresses typically leads to different paths, with different latencies in the network, and if packets were to arrive via an arbitrary destination IP address (or path) for a given SPI, the reordering due to different latencies may cause some packets to fall outside of the ESP anti-replay window. For this reason, HIP provides a mechanism to affiliate destination addresses with inbound SPIs, if there is a concern that anti-replay windows might be violated otherwise. In this sense, we can say that a given inbound SPI has an "affinity" for certain inbound IP addresses, and this affinity is communicated to the peer host. Each physical interface SHOULD have a separate SA, unless the ESP anti-replay window is loose. Moreover, even if the destination addresses used for a particular SPI are held constant, the use of different source interfaces may also cause packets to fall outside of the ESP anti-replay window, since the path traversed is often affected by the source address or Henderson (editor) Expires January 18, 2006 [Page 18]
Internet-Draft HIP Mobility and Multihoming July 2005 interface used. A host has no way to influence the source interface on which a peer uses to send its packets on a given SPI. Hosts SHOULD consistently use the same source interface 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 as SA selectors anyway. A single LOCATOR parameter contains data only about one SPI. To simultaneously signal changes on several SPIs, it is necessary to send several LOCATOR parameters. The packet structure supports this. If the LOCATOR parameter is sent in an UPDATE packet, then the receiver will respond with an UPDATE acknowledgment. If the LOCATOR parameter is sent in a NOTIFY, I2, or R2 packet, then the recipient may consider the LOCATOR as informational, and act only when it needs to activate a new address. The use of LOCATOR in a NOTIFY message may not be compatible with middleboxes. Henderson (editor) Expires January 18, 2006 [Page 19]
Internet-Draft HIP Mobility and Multihoming July 2005 4. LOCATOR parameter format The LOCATOR parameter is a critical parameter as defined by [2]. The LOCATOR parameter is also abbreviated as "LOC" in the figures herein. It consists of the standard HIP parameter Type and Length fields, plus one or more Locator sub-parameters. Each Locator sub-parameter contains a Traffic Type, Locator Type, Locator Length, Preferred Locator bit, Locator Lifetime, and a Locator encoding. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Traffic Type | Locator Type | Locator Length | Reserved |P| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Locator Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Locator | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Traffic Type | Locator Type | Locator Length | Reserved |P| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Locator Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Locator | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: 193 Length: Length in octets, excluding Type and Length fields, and excluding padding. Traffic Type: Defines whether the locator pertains to HIP signaling, user data, or both. Henderson (editor) Expires January 18, 2006 [Page 20]
Internet-Draft HIP Mobility and Multihoming July 2005 Locator Type: Defines the semantics of the Locator field. Locator Length: Defines the length of the Locator field, in units of 4-byte words (Locators up to a maximum of 4*255 bytes are supported). Reserved: Zero when sent, ignored when received. P: Preferred locator. Set to one if the locator is preferred for that Traffic Type; otherwise set to zero. Locator Lifetime: Locator lifetime, in seconds. Locator: The locator whose semantics and encoding are indicated by the Locator Type field. All Locator sub-fields are integral multiples of four bytes in length. The Locator Lifetime indicates how long the following locator is expected to be valid. The lifetime is expressed in seconds. Each locator MUST have a non-zero lifetime. The address is expected to become deprecated when the specified number of seconds has passed since the reception of the message. A deprecated address SHOULD NOT be used as an destination address if an alternate (non-deprecated) is available and has sufficient scope. 4.1 Traffic Type and Preferred Locator The following Traffic Type values are defined: 0: Both signaling (HIP control packets) and user data. 1: Signaling packets only. 2: Data packets only. The "P" bit, when set, has scope over the corresponding Traffic Type that precedes it. That is, if a "P" bit is set for Traffic Type "2", for example, that means that the locator is preferred for data packets. If there is a conflict (for example, if P bit is set for both "0" and "2"), the more specific Traffic Type rule applies. By default, the IP addresses used in the base exchange are preferred locators for both signaling and user data, unless a new preferred locator supersedes them. If no locators are indicated as preferred for a given Traffic Type, the implementation may use an arbitrary locator from the set of active locators. Henderson (editor) Expires January 18, 2006 [Page 21]
Internet-Draft HIP Mobility and Multihoming July 2005 4.2 Locator Type and Locator The following Locator Type values are defined, along with the associated semantics of the Locator field: 0: An IPv6 address or an IPv4-in-IPv6 format IPv4 address [7] (128 bits long). 1: The concatenation of an ESP SPI (first 32 bits) followed by an IPv6 address or an IPv4-in-IPv6 format IPv4 address (an additional 128 bits). 4.3 UPDATE packet with included LOCATOR A number of combinations of parameters in an UPDATE packet are possible (e.g., see Section 3.2). Any UPDATE packet that includes a LOCATOR parameter SHOULD include both an HMAC and a HIP_SIGNATURE parameter. Henderson (editor) Expires January 18, 2006 [Page 22]
Internet-Draft HIP Mobility and Multihoming July 2005 5. Processing rules HIP mobility and multihoming is fundamentally based on the HIP architecture [1], where the transport and internetworking layers are decoupled from each other by an interposed host identity protocol layer. In the HIP architecture, the transport layer sockets are bound to the Host Identifiers (through HIT or LSI in the case of legacy APIs), and the Host Identifiers are translated to the actual IP address. The HIP base protocol specification [2] is expected to be commonly used with the ESP Transport Format [5] to establish a pair of Security Associations (SA). The ESP SAs are then used to carry the actual payload data between the two hosts, by wrapping TCP, UDP, and other upper layer packets into transport mode ESP payloads. The IP header uses the actual IP addresses in the network. Although HIP may also be specified in the future to operate with an alternative to ESP providing the per-packet HIP context, the remainder of this document assumes that HIP is being used in conjunction with ESP. Future documents may extend this document to include other behaviors when ESP is not used. The base specification does not contain any mechanisms for changing the IP addresses that were used during the base HIP exchange. Hence, in order to remain connected, any systems that implement only the base specification and nothing else must retain the ability to receive packets at their primary IP address; that is, those systems cannot change the IP address on which they are using to receive packets without causing loss of connectivity until a base exchange is performed from the new address. 5.1 Locator data structure and status In a typical implementation, each outgoing locator is represented as a piece of state that contains the following data: o the actual bit pattern representing the locator, o lifetime (seconds), o status (UNVERIFIED, ACTIVE, DEPRECATED). The status is used to track the reachability of the address embedded within the LOCATOR parameter: Henderson (editor) Expires January 18, 2006 [Page 23]
Internet-Draft HIP Mobility and Multihoming July 2005 UNVERIFIED indicates that the reachability of the address has not been verified yet, ACTIVE indicates that the reachability of the address has been verified and the address has not been deprecated, DEPRECATED indicates that the locator lifetime has expired The following state changes are allowed: UNVERIFIED to ACTIVE The reachability procedure completes successfully. UNVERIFIED to DEPRECATED The locator lifetime expires while it is UNVERIFIED. ACTIVE to DEPRECATED The locator lifetime expires while it is ACTIVE. ACTIVE to UNVERIFIED There has been no traffic on the address for some time, and the local policy mandates that the address reachability must be verified again before starting to use it again. DEPRECATED to UNVERIFIED The host receives a new lifetime for the locator. A DEPRECATED address MUST NOT be changed to ACTIVE without first verifying its reachability. 5.2 Sending LOCATORs The decision of when to send LOCATORs is basically a local policy issue. However, it is RECOMMENDED that a host sends a LOCATOR whenever it recognizes a change of its IP addresses, and assumes that the change is going to last at least for a few seconds. Rapidly sending conflicting LOCATORs SHOULD be avoided. When a host decides to inform its peers about changes in its IP addresses, it has to decide how to group the various addresses, and whether to include any addresses on multiple SPIs. Since each SPI is associated with a different Security Association, the grouping policy may be based on ESP anti-replay protection considerations. In the typical case, simply basing the grouping on actual kernel level physical and logical interfaces is often the best policy. Virtual interfaces, such as IPsec tunnel interfaces or Mobile IP home addresses SHOULD NOT be announced. Note that the purpose of announcing IP addresses in a LOCATOR is to Henderson (editor) Expires January 18, 2006 [Page 24]
Internet-Draft HIP Mobility and Multihoming July 2005 provide connectivity between the communicating hosts. In most cases, tunnels (and therefore virtual interfaces) 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. 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. Once the host has decided on the groups and assignment of addresses to the SPIs, it creates a LOCATOR parameter for each group. If there are multiple LOCATOR parameters, the parameters MUST be ordered so that the new preferred locator is in the first LOCATOR parameter. Only one locator (the first one, if at all) may be indicated as preferred for each distinct Traffic Type in the LOCATOR parameter. If addresses are being added to an existing SPI, the LOCATOR parameter includes the full set of valid addresses for that SPI, each using a Locator Type of "1" and each with the same value for SPI. Any locators previously ACTIVE on that SPI that are not included in the LOCATOR will be set to DEPRECATED by the receiver. If a mobile host decides to change the SPI upon a readdress, it sends a LOCATOR with the SPI field within the LOCATOR set to the new SPI, and also an ESP_INFO parameter with the Old SPI field set to the previous SPI and the New SPI field set to the new SPI. If multiple LOCATOR and ESP_INFO parameters are included, the ESP_INFO MUST be ordered such that they appear in the same order as the set of corresponding LOCATORs. The decision as to whether to rekey and send a new Diffie-Hellman parameter while performing readdressing is a local policy decision. If new addresses and new SPIs are being created, the LOCATOR parameter's SPI field contains the new SPI, and the ESP_INFO parameter's Old SPI field and New SPI fields are both set to the new SPI, indicating that this is a new and not a replacement SPI. If there are multiple LOCATOR parameters leading to a packet size that exceeds the MTU, HIP fragmentation rules as described in [2] shall apply. 5.3 Handling received LOCATORs A host SHOULD be prepared to receive LOCATOR parameters in any HIP packets, excluding I1. When a host receives a LOCATOR parameter, it first performs the following operations: Henderson (editor) Expires January 18, 2006 [Page 25]
Internet-Draft HIP Mobility and Multihoming July 2005 1. For each locator listed in the LOCATOR parameter, check that the address therein is a legal unicast or anycast address. That is, the address MUST NOT be a broadcast or multicast address. Note that some implementations MAY accept addresses that indicate the local host, since it may be allowed that the host runs HIP with itself. 2. For each address listed in the LOCATOR parameter, check if the address is already bound to the SPI. 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 on the SPI that were NOT listed in the LOCATOR parameter as DEPRECATED. As a result, the SPI now contains any addresses listed in the LOCATOR parameter either as UNVERIFIED or ACTIVE, and any old addresses not listed in the LOCATOR parameter as DEPRECATED. 3. If the LOCATOR is paired with an ESP_INFO parameter, the ESP_INFO parameter is processed. If the LOCATOR is replacing the address on an existing SPI, the SPI itself may be changed-- in this case, the host proceeds according to HIP rekeying procedures. This case is indicated by the ESP_INFO parameter including an existing SPI in the Old SPI field and a new SPI in the New SPI field, and the SPI field in the LOCATOR matching the New SPI in the ESP_INFO. If instead the LOCATOR corresponds to a new SPI, the ESP_INFO will include the same SPI in both its Old SPI and New SPI fields. 4. Mark all locators at the address group that were NOT listed in the LOCATOR parameter as DEPRECATED. Once the host has updated the SPI, if the LOCATOR parameter contains a new preferred locator, the host SHOULD initiate a change of the preferred locator. This requires that the host first verifies reachability of the associated address, and only then changes the preferred locator. See Section 5.6. 5.4 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 Henderson (editor) Expires January 18, 2006 [Page 26]
Internet-Draft HIP Mobility and Multihoming July 2005 A host typically starts the address-verification procedure by sending a nonce to the new address. For example, if the host is changing its SPI and is sending an ESP_INFO to the peer, the new SPI value SHOULD be random and the value MAY be copied into an ECHO_REQUEST sent in the rekeying UPDATE. If the host is not rekeying, it MAY still use the ECHO_REQUEST parameter in an UPDATE message sent to the new address. A host MAY also use other message exchanges as confirmation of the address reachability. Note that in the case of receiving a LOCATOR on an R1 and replying with an I2, receiving the corresponding R2 is sufficient proof of reachability for the Responder's preferred address. Since further address verification of such address can impede the HIP base exchange, a host MUST NOT verify reachability of a new preferred locator that was received on a R1. In some cases, it may be sufficient to use the arrival of data on a newly advertised SA as implicit address reachability verification, instead of waiting for the confirmation via a HIP packet (e.g., Figure 14). In this case, a host advertising a new SPI as part of its address reachability check SHOULD be prepared to receive traffic on the new SA. Marking the address ACTIVE as a part of receiving data on the SA is an idempotent operation, and does not cause any harm. Mobile host Peer host prepare incoming SA new SPI in R2, or UPDATE <----------------------------------- switch to new outgoing SA data on new SA -----------------------------------> mark address ACTIVE Figure 14: Address activation via use of new SA When address verification is in progress for a new preferred locator, the host SHOULD select a different locator listed as ACTIVE, if one such locator is available, to continue communications until address verification completes. Alternatively, the host MAY use the new preferred locator while in UNVERIFIED status to the extent Credit- Based Authorization permits. Credit-Based Authorization is explained in Section 5.5. Once address verification succeeds, the status of the new preferred locator changes to ACTIVE. Henderson (editor) Expires January 18, 2006 [Page 27]
Internet-Draft HIP Mobility and Multihoming July 2005 5.5 Credit-Based Authorization 5.5.1 Handling Payload Packets A host maintains a "credit counter" for each of its peers. Whenever a packet arrives from a peer, the host SHOULD increase that peer's credit counter by the size of the received packet. When the host has a packet to be sent to the peer, if the peers preferred locator is listed as UNVERIFIED and no alternative locator with status ACTIVE is available, the host checks whether it can send the packet to the UNVERIFIED locator: The packet SHOULD be sent if the value of the credit counter is higher than the size of the outbound packet. If the credit counter is too low, the packet MUST be discarded or buffered until address verification succeeds. When a packet is sent to a peer at an UNVERIFIED locator, the peer's credit counter MUST be reduced by the size of the packet. The peer's credit counter is not affected by packets that the host sends to an ACTIVE locator of that peer. Figure 15 depicts the actions taken by the host when a packet is received. Figure 16 shows the decision chain in the event a packet is sent. Inbound packet | | +----------------+ +---------------+ | | Increase | | Deliver | +-----> | credit counter |-------------> | packet to | | by packet size | | application | +----------------+ +---------------+ Figure 15: Receiving Packets with Credit-Based Authorization Henderson (editor) Expires January 18, 2006 [Page 28]
Internet-Draft HIP Mobility and Multihoming July 2005 Outbound packet | _________________ | / \ +---------------+ | / Is the preferred \ No | Send packet | +-----> | destination address |-------------> | to preferred | \ UNVERIFIED? / | address | \_________________/ +---------------+ | | Yes | v _________________ / \ +---------------+ / Does an ACTIVE \ Yes | Send packet | | destination address |-------------> | to ACTIVE | \ exist? / | address | \_________________/ +---------------+ | | No | v _________________ / \ +---------------+ / Credit counter \ No | | | >= |-------------> | Drop packet | \ packet size? / | | \_________________/ +---------------+ | | Yes | v +---------------+ +---------------+ | Reduce credit | | Send packet | | counter by |----------------> | to preferred | | packet size | | address | +---------------+ +---------------+ Figure 16: Sending Packets with Credit-Based Authorization 5.5.2 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. Henderson (editor) Expires January 18, 2006 [Page 29]
Internet-Draft HIP Mobility and Multihoming July 2005 Credit aging may be implemented by multiplying credit counters with a factor, CreditAgingFactor, less than one in fixed time intervals of CreditAgingInterval length. Choosing appropriate values for CreditAgingFactor and CreditAgingInterval is important to ensure that a host can send packets to an address in state UNVERIFIED even when the peer sends at a lower rate than the host itself. When CreditAgingFactor or CreditAgingInterval are too small, the peer's credit counter might be too low to continue sending packets until address verification concludes. The parameter values proposed in this document are as follows: CreditAgingFactor 7/8 CreditAgingInterval 5 seconds These parameter values work well when the host transfers a file to the peer via a TCP connection and the end-to-end round-trip time does not exeed 500 milliseconds. Alternative credit-aging algorithms may use other parameter values or different parameters, which may even be dynamically established. 5.6 Changing the preferred locator A host MAY want to change the preferred outgoing locator for different reasons, e.g., because traffic information or ICMP error messages indicate that the currently used preferred address may have become unreachable. Another reason is receiving a LOCATOR parameter that has the P-bit set. To change the preferred locator, the host initiates the following procedure: 1. If the new preferred locator has ACTIVE status, the preferred locator is changed and the procedure succeeds. 2. If the new preferred locator has UNVERIFIED status, the host starts to verify its reachability. The host SHOULD use a different locator listed as ACTIVE until address verification completes if one such locator is available. Altervatively, the host MAY use the new preferred locator, even though in UNVERIFIED status, to the extent Credit-Based Authorization permits. Once address verification succeeds, the status of the new preferred locator changes to ACTIVE and its use is no longer governed by Credit-Based Authorization. 3. If the peer host has not indicated a preference for any address, then the host picks one of the peer's ACTIVE addresses randomly Henderson (editor) Expires January 18, 2006 [Page 30]
Internet-Draft HIP Mobility and Multihoming July 2005 or according to policy. This case may arise if, for example, ICMP error messages arrive that deprecate the preferred locator, but the peer has not yet indicated a new preferred locator. 4. If the new preferred locator has DEPRECATED status and there is at least one non-deprecated address, the host selects one of the non-deprecated addresses as a new preferred locator and continues. If the selected address is UNVERIFIED, this includes address verification as described above. Henderson (editor) Expires January 18, 2006 [Page 31]
Internet-Draft HIP Mobility and Multihoming July 2005 6. Policy considerations XXX: This section needs to be written. The host may change the status of unused ACTIVE addresses into UNVERIFIED after a locally configured period of inactivity. Henderson (editor) Expires January 18, 2006 [Page 32]
Internet-Draft HIP Mobility and Multihoming July 2005 7. Security Considerations The HIP mobility mechanism provides a secure means of updating a host's IP address via HIP REA update packets. Upon receipt, a HIP host cryptographically verifies the sender of a REA update, so forging or replaying a HIP update packet is very difficult (see [2]). Therefore, security issues reside in other attack domains. The two we consider are malicious redirection of legitimate connections as well as redirection-based flooding attacks using this protocol. This can be broken down into the following: Impersonation attacks - direct conversation with the misled victim - man-in-the-middle attack DoS attacks - flooding attacks (== bandwidth-exhaustion attacks) * tool 1: direct flooding * tool 2: flooding by zombies * tool 2: redirection-based flooding - memory-exhaustion attacks - computational exhaustion attacks We consider these in more detail in the following sections. In Section 7.1 and Section 7.2, we assume that all users are using HIP. In Section 7.3 we consider the security ramifications when we have both HIP and non-HIP users. 7.1 Impersonation attacks An attacker wishing to impersonate will try to mislead its victim into directly communicating with them, or carry out a man in the middle attack between the victim and the victim's desired communication peer. Without mobility support, both attack types are possible only if the attacker resides on the routing path between its victim and the victim's desired communication peer, or if the attacker tricks its victim into initiating the connection over an incorrect routing path (e.g., by acting as a router or using spoofed DNS entries). Henderson (editor) Expires January 18, 2006 [Page 33]
Internet-Draft HIP Mobility and Multihoming July 2005 The HIP extensions defined in this specification change the situation in that they introduce an ability to redirect a connection (like IPv6), both before and after establishment. If no precautionary measures are taken, an attacker could misuse this feature to impersonate a victim's peer from any arbitrary location. The authentication and authorization mechanisms of the HIP base exchange [2] and the signatures in the new REA update message prevent this offense. Furthermore, ownership of a connection 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 but once the base exchange has taken place even a MitM cannot steal a HIP connection because it is very difficult for an attacker to create an REA 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 attain the SPI and HIT/HI, they still cannot forge an update packet without knowledge of the secret keys. 7.2 Denial of Service attacks 7.2.1 Flooding Attacks The purpose of a denial-of-service attack is to exhaust some resource of the victim such that the victim ceases operating 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 Henderson (editor) Expires January 18, 2006 [Page 34]
Internet-Draft HIP Mobility and Multihoming July 2005 with multiple servers. This threat is mitigated through reachability checks and credit-based authorization. Both strategies do not eliminate flooding attacks per se, but they preclude: (i) their use from a location off the path towards the flooded victim; and (ii) any amplification in the number and size of the redirected packets. As a result, the combination of a reachability check and credit-based authorization makes a HIP redirection-based flooding attack as effective and applicable as a normal, direct flooding attack in which the attacker itself sends the flooding traffic to the victim. This analysis leads to the following two points. First, when a reachability packet is received this nonce packet MUST be ignored if the HIT is not one that is currently active. Second, if the attacker is a MitM and can capture this nonce packet then they can respond to it, in which case it is possible for an attacker to redirect their connection. Note, this attack will always be possible when a reachability packet is not sent. 7.2.2 Memory/Computational exhaustion DoS attacks We now consider whether or not the proposed extensions to HIP add any new DoS attacks (consideration of DoS attacks using the base HIP exchange and updates is discussed in [2]). A simple attack is to send many REA update 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 attackers HI crashes the system. Therefore, their 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 draft [2]. 7.3 Mixed deployment environment We now assume that we have both HIP and non-HIP aware hosts. Four cases exist. 1. A HIP user redirects their connection onto a non-HIP user. The non-HIP user will drop the reachability packet so this is not a threat unless the HIP user is a MitM and can respond to the reachability packet. 2. A non-HIP user attempts to redirect their connection onto a HIP user. This falls into IPv4 and IPv6 security concerns, which are outside the scope of this document. 3. A non-HIP user attempts to steal a HIP user's session (assume that SeND is not active for the following). The non-HIP user contacts the service that a HIP user has a connection with and Henderson (editor) Expires January 18, 2006 [Page 35]
Internet-Draft HIP Mobility and Multihoming July 2005 then attempts to use a IPv6 change of address request to steal the HIP user's connection. What will happen in this case is implementation dependent but such a request should be ignored/ dropped. Even if the attack is sucessful, the HIP user can reclaim their connection via HIP. 4. A HIP user attempts to steal a non-HIP user's session. This could be problematic since HIP sits 'on top of' layer 3. A HIP user could spoof the non-HIP user's ip address during the base exhange or set the non-HIP user's ip address as their preferred address via an REA update. Other possibilities exist but a simple solution is to add a check which does not allow any HIP session to be moved to or created upon an already existing ip address. Henderson (editor) Expires January 18, 2006 [Page 36]
Internet-Draft HIP Mobility and Multihoming July 2005 8. IANA Considerations Henderson (editor) Expires January 18, 2006 [Page 37]
Internet-Draft HIP Mobility and Multihoming July 2005 9. Authors Pekka Nikander originated this Internet Draft. Tom Henderson, Jari Arkko, Greg Perkins, and Christian Vogt have each contributed sections to this draft. Henderson (editor) Expires January 18, 2006 [Page 38]
Internet-Draft HIP Mobility and Multihoming July 2005 10. Acknowledgments The authors thank Mika Kousa for many improvements to the draft. Henderson (editor) Expires January 18, 2006 [Page 39]
Internet-Draft HIP Mobility and Multihoming July 2005 11. References 11.1 Normative references [1] Moskowitz, R., "Host Identity Protocol Architecture", draft-ietf-hip-arch-02 (work in progress), January 2005. [2] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-03 (work in progress), June 2005. [3] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) Rendezvous Extension", draft-ietf-hip-rvs-03 (work in progress), July 2005. [4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [5] Jokela, P., "Using ESP transport format with HIP", draft-ietf-hip-esp-00 (work in progress), July 2005. [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [7] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998. 11.2 Informative references [8] Bellovin, S., "EIDs, IPsec, and HostNAT", IETF 41th, March 1998. [9] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", draft-iab-sec-cons-00 (work in progress), August 2002. [10] Nikander, P., "Mobile IP version 6 Route Optimization Security Design Background", draft-nikander-mobileip-v6-ro-sec-02 (work in progress), December 2003. Henderson (editor) Expires January 18, 2006 [Page 40]
Internet-Draft HIP Mobility and Multihoming July 2005 Author's Address Tom Henderson The Boeing Company P.O. Box 3707 Seattle, WA USA Email: thomas.r.henderson@boeing.com Henderson (editor) Expires January 18, 2006 [Page 41]
Internet-Draft HIP Mobility and Multihoming July 2005 Appendix A. Changes from previous versions A.1 From nikander-hip-mm-00 to nikander-hip-mm-01 The actual protocol has been largely revised, based on the new symmetric New SPI (NES) design adopted in the base protocol draft version -08. There are no more separate REA, AC or ACR packets, but their functionality has been folded into the NES packet. At the same time, it has become possible to send REA parameters in R1 and I2. The Forwarding Agent functionality was removed, since it looks like that it will be moved to the proposed HIP Research Group. Hence, there will be two other documents related to that, a simple Rendezvous server document (WG item) and a Forwarding Agent document (RG item). A.2 From nikander-hip-mm-01 to nikander-hip-mm-02 Alignment with base-00 draft (use of UPDATE and NOTIFY packets). The "logical interface" concept was dropped, and the SA/SPI was identified as the protocol component to which a HIP association binds addresses to. The RR was (again) made recommended, not mandatory, able to be administratively overridden. A.3 From -02 to draft-ietf-hip-mm-00 REA parameter type value is now "3" (was TBD before). Recommend that in multihoming situations, that inbound/outbound SAs are paired to avoid ambiguity when rekeying them. Clarified that multihoming scenario for now was intended for failover instead of load-balancing, due to transport layer issues. Clarified that if HIP negotiates base exchange using link local addresses, that a host SHOULD provide its peer with a globally reachable address. Clarified whether REAs sent for existing SPIs update the full set of addresses associated with that SPI, or only perform an incremental (additive) update. REAs for an existing SPI should list all current addresses for that SPI, and any addresses previously in use on the SPI but not in the new REA parameter should be DEPRECATED. Clarified that address verification pertains to *outgoing* addresses. Henderson (editor) Expires January 18, 2006 [Page 42]
Internet-Draft HIP Mobility and Multihoming July 2005 When discussing inclusion of REA in I2, the draft stated "The Responder MUST make sure that the puzzle solution is valid BOTH for the initial IP destination address used for I1 and for the new preferred address." However, this statement conflicted with Appendix D of the base specification, so it has been removed for now. A.4 From draft-ietf-hip-mm-00 to -01 Introduction section reorganized. Some of the scope of the document relating to multihoming was reduced. Removed empty appendix "Implementation experiences" Renamed REA parameter to LOCATOR and aligned to the discussion on redefining this parameter that occurred on the RG mailing list. Aligned with decoupling of ESP from base spec. A.5 From draft-ietf-hip-mm-01 to -02 Aligned with draft-ietf-hip-base-03 and draft-ietf-hip-esp-00 Address verification is a MUST (C. Vogt, list post on 06/12/05) If UPDATE exceeds MTU because of too many locators, do not split into multiple UPDATEs, but instead rely on IP fragmentation (C. Vogt, list post on 06/12/05) New value for LOCATOR parameter type (193), per 05/31/05 discussion on the WG list Various additions related to Credit-Based Authorization due to C. Vogt Security section contributed by Greg Perkins, with subsequent editing from C. Vogt and P. Nikander Reorganization according to RFC 4101 guidance on writing protocol models Open issue: LOCATOR parameter semantics (implicit/explicit removal) Henderson (editor) Expires January 18, 2006 [Page 43]
Internet-Draft HIP Mobility and Multihoming July 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Henderson (editor) Expires January 18, 2006 [Page 44]
