Network Working Group                                        E. Nordmark
Request for Comments: 4218                              Sun Microsystems
Category: Informational                                            T. Li
                                                            October 2005

Threats Relating to IPv6 Multihoming Solutions

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

   This document lists security threats related to IPv6 multihoming.
   Multihoming can introduce new opportunities to redirect packets to
   different, unintended IP addresses.

   The intent is to look at how IPv6 multihoming solutions might make
   the Internet less secure; we examine threats that are inherent to all
   IPv6 multihoming solutions rather than study any specific proposed
   solution.  The threats in this document build upon the threats
   discovered and discussed as part of the Mobile IPv6 work.

   1. Introduction ....................................................2
      1.1. Assumptions ................................................3
      1.2. Authentication, Authorization, and Identifier Ownership ....4
   2. Terminology .....................................................5
   3. Today's Assumptions and Attacks .................................6
      3.1. Application Assumptions ....................................6
      3.2. Redirection Attacks Today ..................................8
      3.3. Packet Injection Attacks Today .............................9
      3.4. Flooding Attacks Today ....................................10
      3.5. Address Privacy Today .....................................11
   4. Potential New Attacks ..........................................13
      4.1. Cause Packets to Be Sent to the Attacker ..................13
           4.1.1. Once Packets Are Flowing ...........................13
           4.1.2. Time-Shifting Attack ...............................14
           4.1.3. Premeditated Redirection ...........................14
           4.1.4. Using Replay Attacks ...............................15

      4.2. Cause Packets to Be Sent to a Black Hole ..................15
      4.3. Third Party Denial-of-Service Attacks .....................16
           4.3.1. Basic Third Party DoS ..............................17
           4.3.2. Third Party DoS with On-Path Help ..................18
      4.4. Accepting Packets from Unknown Locators ...................19
      4.5. New Privacy Considerations ................................20
   5. Granularity of Redirection .....................................20
   6. Movement Implications? .........................................22
   7. Other Security Concerns ........................................23
   8. Security Considerations ........................................24
   9. Acknowledgements ...............................................24
   10. Informative References ........................................25
   Appendix A: Some Security Analysis ................................27

1. Introduction

   The goal of the IPv6 multihoming work is to allow a site to take
   advantage of multiple attachments to the global Internet, without
   having a specific entry for the site visible in the global routing
   table.  Specifically, a solution should allow hosts to use multiple
   attachments in parallel, or to switch between these attachment points
   dynamically in the case of failures, without an impact on the
   transport and application layer protocols.

   At the highest level, the concerns about allowing such "rehoming" of
   packet flows can be called "redirection attacks"; the ability to
   cause packets to be sent to a place that isn't tied to the transport
   and/or application layer protocol's notion of the peer.  These
   attacks pose threats against confidentiality, integrity, and
   availability.  That is, an attacker might learn the contents of a
   particular flow by redirecting it to a location where the attacker
   has a packet recorder.  If, instead of a recorder, the attacker
   changes the packets and then forwards them to the ultimate
   destination, the integrity of the data stream would be compromised.
   Finally, the attacker can simply use the redirection of a flow as a
   denial of service attack.

   This document has been developed while considering multihoming
   solutions architected around a separation of network identity and
   network location, whether or not this separation implies the
   introduction of a new and separate identifier name space.  However,
   this separation is not a requirement for all threats, so this
   taxonomy may also apply to other approaches.  This document is not
   intended to examine any single proposed solution.  Rather, it is
   intended as an aid to discussion and evaluation of proposed
   solutions.  By cataloging known threats, we can help to ensure that
   all proposals deal with all of the available threats.

   As a result of not analyzing a particular solution, this document is
   inherently incomplete.  An actual solution would need to be analyzed
   as part of its own threat analysis, especially in the following
   areas:

    1) If the solution makes the split between locators and identifiers,
       then most application security mechanisms should be tied to the
       identifier, not to the locator.  Therefore, work would be needed
       to understand how attacks on the identifier mechanism affect
       security, especially attacks on the mechanism that would bind
       locators to identifiers.

    2) How does the solution apply multihoming to IP multicast?
       Depending on how this is done, there might be specific threats
       relating to multicast that need to be understood.  This document
       does not discuss any multicast-specific threats.

    3) Connection-less transport protocols probably need more attention.
       They are already difficult to secure, even without a
       locator/identifier split.

1.1. Assumptions

   This threat analysis doesn't assume that security has been applied to
   other security relevant parts of the Internet, such as DNS and
   routing protocols; but it does assume that, at some point in time, at
   least parts of the Internet will be operating with security for such
   key infrastructure.  With that assumption, it then becomes important
   that a multihoming solution would not, at that point in time, become
   the weakest link.  This is the case even if, for instance, insecure
   DNS might be the weakest link today.

   This document doesn't assume that the application protocols are
   protected by strong security today or in the future.  However, it is
   still useful to assume that the application protocols that care about
   integrity and/or confidentiality apply the relevant end-to-end
   security measures, such as IPsec, TLS, and/or application layer
   security.

   For simplicity, this document assumes that an on-path attacker can
   see packets, modify packets and send them out, and block packets from
   being delivered.  This is a simplification because there might exist
   ways (for instance, monitoring capability in switches) that allow
   authenticated and authorized users to observe packets without being
   able to send or block the packets.

   In some cases it might make sense to make a distinction between
   on-path attackers, which can monitor packets and perhaps also inject
   packets, without being able to block packets from passing through.

   On-path attackers that only need to monitor might be lucky and find a
   non-switched Ethernet in the path, or use capacitive or inductive
   coupling to listen on a copper wire.  But if the attacker is on an
   Ethernet that is on the path, whether switched or not, the attacker
   can also employ Address Resolution Protocol/Neighbor Discovery
   (ARP/ND) spoofing to get access to the packet flow which allows
   blocking as well.  Similarly, if the attacker has access to the wire,
   the attacker can also place a device on the wire to block.  Other
   on-path attacks would be those that gain control of a router or a
   switch (or gain control of one of the endpoints), and most likely
   those would allow blocking as well.

   So the strongest currently known case where monitoring is easier than
   blocking, is when switches and routers have monitoring capabilities
   (for network management or for lawful intercept) where an attacker
   might be able to bypass the authentication and authorization checks
   for those capabilities.  However, this document makes the simplifying
   assumption treat all on-path attackers the same by assuming that such
   an attacker can monitor, inject, and block packets.  A security
   analysis of a particular proposal can benefit from not making this
   assumption, and look at how on-path attackers with different
   capabilities can generate different attacks perhaps not present in
   today's Internet.

   The document assumes that an off-path attacker can neither see
   packets between the peers (for which it is not on the path) nor block
   them from being delivered.  Off-path attackers can, in general, send
   packets with arbitrary IP source addresses and content, but such
   packets might be blocked if ingress filtering [INGRESS] is applied.
   Thus, it is important to look at the multihoming impact on security
   both in the presence and absence of ingress filtering.

1.2. Authentication, Authorization, and Identifier Ownership

   The overall problem domain can be described using different
   terminology.

   One way to describe it is that it is necessary to first authenticate
   the peer and then verify that the peer is authorized to control the
   locators used for a particular identifier.  While this is correct, it
   might place too much emphasis on the authorization aspect.  In this
   case, the authorization is conceptually very simple; each host (each
   identifier) is authorized to control which locators are used for
   itself.

   Hence, there is a different way to describe the same thing.  If the
   peer can somehow prove that it is the owner of the identifier, and
   the communication is bound to the identifier (and not the locator),
   then the peer is allowed to control the locators that are used with
   the identifier.  This way to describe the problem is used in [OWNER].

   Both ways to look at the problem, hence both sets of terminology, are
   useful when trying to understand the problem space and the threats.

2. Terminology

      link        - a communication facility or medium over which nodes
                    can communicate at the link layer, i.e., the layer
                    immediately below IPv6.  Examples are Ethernets
                    (simple or bridged); PPP links; X.25, Frame Relay,
                    or ATM networks; and Internet (or higher) layer
                    "tunnels", such as tunnels over IPv4 or IPv6 itself.

      interface   - a node's attachment to a link.

      address     - an IP layer name that has both topological
                    significance (i.e., a locator) and identifies an
                    interface.  There may be multiple addresses per
                    interface.  Normally an address uniquely identifies
                    an interface, but there are exceptions:  the same
                    unicast address can be assigned to multiple
                    interfaces on the same node, and an anycast address
                    can be assigned to different interfaces on different
                    nodes.

      locator     - an IP layer topological name for an interface or a
                    set of interfaces.  There may be multiple locators
                    per interface.

      identifier  - an IP layer identifier for an IP layer endpoint
                    (stack name in [NSRG]), that is, something that
                    might be commonly referred to as a "host".  The
                    transport endpoint name is a function of the
                    transport protocol and would typically include the
                    IP identifier plus a port number.  There might be
                    use for having multiple identifiers per stack/per
                    host.

                    An identifier continues to function regardless of
                    the state of any one interface.

      address field
                  - the source and destination address fields in the
                    IPv6 header.  As IPv6 is currently specified, these
                    fields carry "addresses".  If identifiers and
                    locators are separated, these fields will contain
                    locators.

      FQDN        - Fully Qualified Domain Name [FYI18]

3. Today's Assumptions and Attacks

   The two interesting aspects of security for multihoming solutions are
   (1) the assumptions made by the transport layer and application layer
   protocols about the identifiers that they see, and (2) the existing
   abilities to perform various attacks that are related to the
   identity/location relationship.

3.1. Application Assumptions

   In the Internet today, the initiating part of applications either
   starts with a FQDN, which it looks up in the DNS, or already has an
   IP address from somewhere.  For the FQDN to perform IP address
   lookup, the application effectively places trust in the DNS.  Once it
   has the IP address, the application places trust in the routing
   system delivering packets to that address.  Applications that use
   security mechanisms, such as IPSEC or TLS, have the ability to bind
   an address or FQDN to cryptographic keying material.  Compromising
   the DNS and/or routing system can result in packets being dropped or
   delivered to an attacker in such cases, but since the attacker does
   not possess the keying material, the application will not trust the
   attacker, and the attacker cannot decrypt what it receives.

   At the responding (non-initiating) end of communication today, we
   find that the security configurations used by different applications
   fall into approximately five classes, where a single application
   might use different classes of configurations for different types of
   communication.

   The first class is the set of public content servers.  These systems
   provide data to any and all systems and are not particularly
   concerned with confidentiality, as they make their content available
   to all.  However, they are interested in data integrity and denial of
   service attacks.  Having someone manipulate the results of a search
   engine, for example, or prevent certain systems from reaching a
   search engine would be a serious security issue.

   The second class of security configurations uses existing IP source
   addresses from outside of their immediate local site as a means of
   authentication without any form of verification.  Today, with source
   IP address spoofing and TCP sequence number guessing as rampant
   attacks [RFC1948], such applications are effectively opening
   themselves for public connectivity and are reliant on other systems,
   such as firewalls, for overall security.  We do not consider this
   class of configurations in this document because they are in any case
   fully open to all forms of network layer spoofing.

   The third class of security configurations receives existing IP
   source addresses, but attempt some verification using the DNS,
   effectively using the FQDN for access control.  (This is typically
   done by performing a reverse lookup from the IP address, followed by
   a forward lookup and verifying that the IP address matches one of the
   addresses returned from the forward lookup.)  These applications are
   already subject to a number of attacks using techniques like source
   address spoofing and TCP sequence number guessing since an attacker,
   knowing this is the case, can simply create a DoS attack using a
   forged source address that has authentic DNS records.  In general
   this class of security configurations is strongly discouraged, but it
   is probably important that a multihoming solution doesn't introduce
   any new and easier ways to perform such attacks.  However, we note
   that some people think we should treat this class the same as the
   second class of security configurations.

   The fourth class of security configurations uses cryptographic
   security techniques to provide both a strong identity for the peer
   and data integrity with or without confidentiality.  Such systems are
   still potentially vulnerable to denial of service attacks that could
   be introduced by a multihoming solution.

   Finally, the fifth class of security configurations uses
   cryptographic security techniques, but without strong identity (such
   as opportunistic IPsec).  Thus, data integrity with or without
   confidentiality is provided when communicating with an
   unknown/unauthenticated principal.  Just like the first category
   above, such applications can't perform access control based on
   network layer information since they do not know the identity of the
   peer.  However, they might perform access control using higher-level
   notions of identity.  The availability of IPsec (and similar
   solutions) together with channel bindings allows protocols (which, in
   themselves, are vulnerable to man-in-the-middle (MITM) attacks) to
   operate with a high level of confidentiality in the security of the
   identification of the peer.  A typical example is the Remote Direct
   Data Placement Protocol (RDDP), which, when used with opportunistic
   IPsec, works well if channel bindings are available.  Channel

   bindings provide a link between the IP-layer identification and the
   application protocol identification.

   A variant of the fifth class are those that use "leap-of-faith"
   during some initial communication.  They do not provide strong
   identities except where subsequent communication is bound to the
   initial communication, providing strong assurance that the peer is
   the same as during the initial communication.

   The fifth class is important and its security properties must be
   preserved by a multihoming solution.

   The requirement for a multihoming solution is that security be no
   worse than it is today in all situations.  Thus, mechanisms that
   provide confidentiality, integrity, or authentication today should
   continue to provide these properties in a multihomed environment.

3.2. Redirection Attacks Today

   This section enumerates some of the redirection attacks that are
   possible in today's Internet.

   If routing can be compromised, packets for any destination can be
   redirected to any location.  This can be done by injecting a long
   prefix into global routing, thereby causing the longest match
   algorithm to deliver packets to the attacker.

   Similarly, if DNS can be compromised, and a change can be made to an
   advertised resource record to advertise a different IP address for a
   hostname, effectively taking over that hostname.  More detailed
   information about threats relating to DNS are in [DNS-THREATS].

   Any system that is along the path from the source to the destination
   host can be compromised and used to redirect traffic.  Systems may be
   added to the best path to accomplish this.  Further, even systems
   that are on multi-access links that do not provide security can also
   be used to redirect traffic off of the normal path.  For example, ARP
   and ND spoofing can be used to attract all traffic for the legitimate
   next hop across an Ethernet.  And since the vast majority of
   applications rely on DNS lookups, if DNSsec is not deployed, then
   attackers that are on the path between the host and the DNS servers
   can also cause redirection by modifying the responses from the DNS
   servers.

   In general, the above attacks work only when the attacker is on the
   path at the time it is performing the attack.  However, in some cases
   it is possible for an attacker to create a DoS attack that remains at
   least some time after the attacker has moved off the path.  An

   example of this is an attacker that uses ARP or ND spoofing while on
   path to either insert itself or send packets to a black hole (a
   non-existent L2 address).  After the attacker moves away, the ARP/ND
   entries will remain in the caches in the neighboring nodes for some
   amount of time (a minute or so in the case of ARP).  This will result
   in packets continuing to be black-holed until ARP entry is flushed.

   Finally, the hosts themselves that terminate the connection can also
   be compromised and can perform functions that were not intended by
   the end user.

   All of the above protocol attacks are the subject of ongoing work to
   secure them (DNSsec, security for BGP, Secure ND) and are not
   considered further within this document.  The goal for a multihoming
   solution is not to solve these attacks.  Rather, it is to avoid
   adding to this list of attacks.

3.3. Packet Injection Attacks Today

   In today's Internet the transport layer protocols, such as TCP and
   SCTP, which use IP, use the IP addresses as the identifiers for the
   communication.  In the absence of ingress filtering [INGRESS], the IP
   layer allows the sender to use an arbitrary source address, thus the
   transport protocols and/or applications need some protection against
   malicious senders injecting bogus packets into the packet stream
   between two communicating peers.  If this protection can be
   circumvented, then it is possible for an attacker to cause harm
   without necessarily needing to redirect the return packets.

   There are various levels of protection in different transport
   protocols.  For instance, in general TCP packets have to contain a
   sequence that falls in the receiver's window to be accepted.  If the
   TCP initial sequence numbers are random, then it is very hard for an
   off-path attacker to guess the sequence number close enough for it to
   belong to the window, and as a result be able to inject a packet into
   an existing connection.  How hard this is depends on the size of the
   available window, whether the port numbers are also predictable, and
   the lifetime of the connection.  Note that there is ongoing work to
   strengthen TCP's protection against this broad class of attacks
   [TCPSECURE].  SCTP provides a stronger mechanism with the
   verification tag; an off-path attacker would need to guess this
   random 32-bit number.  Of course, IPsec provides cryptographically
   strong mechanisms that prevent attackers, on or off path, from
   injecting packets once the security associations have been
   established.

   When ingress filtering is deployed between the potential attacker and
   the path between the communicating peers, it can prevent the attacker
   from using the peer's IP address as source.  In that case, the packet
   injection will fail in today's Internet.

   We don't expect a multihoming solution to improve the existing degree
   of prevention against packet injection.  However, it is necessary to
   look carefully at whether a multihoming solution makes it easier for
   attackers to inject packets because the desire to have the peer
   present at multiple locators, and perhaps at a dynamic set of
   locators, can potentially result in solutions that, even in the
   presence of ingress filtering, make packet injection easier.

3.4. Flooding Attacks Today

   In the Internet today there are several ways for an attacker to use a
   redirection mechanism to launch DoS attacks that cannot easily be
   traced to the attacker.  An example of this is to use protocols that
   cause reflection with or without amplification [PAXSON01].

   Reflection without amplification can be accomplished by an attacker
   sending a TCP SYN packet to a well-known server with a spoofed source
   address; the resulting TCP SYN ACK packet will be sent to the spoofed
   source address.

   Devices on the path between two communicating entities can also
   launch DoS attacks.  While such attacks might not be interesting
   today, it is necessary to understand them better in order to
   determine whether a multihoming solution might enable new types of
   DoS attacks.

   For example, today, if A is communicating with B, then A can try to
   overload the path from B to A.  If TCP is used, A could do this by
   sending ACK packets for data that it has not yet received (but it
   suspects B has already sent) so that B would send at a rate that
   would cause persistent congestion on the path towards A.  Such an
   attack would seem self-destructive since A would only make its own
   corner of the network suffer by overloading the path from the
   Internet towards A.

   A more interesting case is if A is communicating with B and X is on
   the path between A and B, then X might be able to fool B to send
   packets towards A at a rate that is faster than A (and the path
   between A and X) can handle.  For instance, if TCP is used, then X
   can craft TCP ACK packets claiming to come from A to cause B to use a
   congestion window that is large enough to potentially cause
   persistent congestion towards A.  Furthermore, if X can suppress the
   packets from A to B, it can also prevent A from sending any explicit

   "slow down" packets to B; that is, X can disable any flow or
   congestion control mechanism such as Explicit Congestion Notification
   [ECN].  Similar attacks can presumably be launched using protocols
   that carry streaming media by forging such a protocol's notion of
   acknowledgement and feedback.

   An attribute of this type of attack is that A will simply think that
   B is faulty since its flow and congestion control mechanisms don't
   seem to be working.  Detecting that the stream of ACK packets is
   generated from X and not from A might be challenging, since the rate
   of ACK packets might be relatively low.  This type of attack might
   not be common today because, in the presence of ingress filtering, it
   requires that X remain on the path in order to sustain the DoS
   attack.  And in the absence of ingress filtering an attacker would
   need either to be present on the path initially and then move away,
   or to be able to perform the setup of the communication "blind",
   i.e., without seeing the return traffic (which, in the case of TCP,
   implies guessing the initial sequence number).

   The danger is that the addition of multihoming redirection mechanisms
   might potentially remove the constraint that the attacker remain on
   the path.  And with the current, no-multihoming support, using
   end-to-end strong security at a protocol level at (or below) this
   "ACK" processing would prevent this type of attack.  But if a
   multihoming solution is provided underneath IPsec that prevention
   mechanism would potentially not exist.

   Thus, the challenge for multihoming solutions is to not create
   additional types of attacks in this area, or make existing types of
   attacks significantly easier.

3.5. Address Privacy Today

   In today's Internet there is limited ability to track a host as it
   uses the Internet because in some cases, such as dialup connectivity,
   the host will acquire different IPv4 addresses each time it connects.
   However, with increasing use of broadband connectivity, such as DSL
   or cable, it is becoming more likely that the host will maintain the
   same IPv4 over time.  Should a host move around in today's Internet,
   for instance, by visiting WiFi hotspots, it will be configured with a
   different IPv4 address at each location.

   We also observe that a common practice in IPv4 today is to use some
   form of address translation, whether the site is multihomed or not.
   This effectively hides the identity of the specific host within a
   site; only the site can be identified based on the IP address.

   In the cases where it is desirable to maintain connectivity as a host
   moves around, whether using layer 2 technology or Mobile IPv4, the
   IPv4 address will remain constant during the movement (otherwise the
   connections would break).  Thus, there is somewhat of a choice today
   between seamless connectivity during movement and increased address
   privacy.

   Today when a site is multihomed to multiple ISPs, the common setup is
   that a single IP address prefix is used with all the ISPs.  As a
   result it is possible to track that it is the same host that is
   communication via all ISPs.

   However, when a host (and not a site) is multi-homed to several ISPs
   (e.g., through a General Packet Radio Service (GPRS) connection and a
   wireless hot spot), the host is provided with different IP addresses
   on each interface.  While the focus of the multihoming work is on
   site multihoming, should the solution also be applicable to host
   multihoming, the privacy impact needs to be considered for this case
   as well.

   IPv6 stateless address auto-configuration facilitates IP address
   management, but raises some concerns since the Ethernet address is
   encoded in the low-order 64 bits of the IPv6 address.  This could
   potentially be used to track a host as it moves around the network,
   using different ISPs, etc.  IPv6 specifies temporary addresses
   [RFC3041], which allow applications to control whether they need
   long-lived IPv6 addresses or desire the improved privacy of using
   temporary addresses.

   Given that there is no address privacy in site multihoming setups
   today, the primary concerns for the "do no harm" criteria are to
   ensure that hosts that move around still have the same ability as in
   today's Internet to choose between seamless connectivity and improved
   address privacy, and also that the introduction of multihoming
   support should still provide the same ability as we have in IPv6 with
   temporary addresses.

   When considering privacy threats, it makes sense to distinguish
   between attacks made by on-path entities observing the packets flying
   by, and attacks by the communicating peer.  It is probably feasible
   to prevent on-path entities from correlating the multiple IP
   addresses of the host; but the fact that the peer needs to be told
   multiple IP addresses in order to be able to switch to using
   different addresses, when communication fails, limits the ability of
   the host to prevent correlating its multiple addresses.  However,
   using multiple pseudonyms for a host should be able address this
   case.

4. Potential New Attacks

   This section documents the additional attacks that have been
   discovered that result from an architecture where hosts can change
   their topological connection to the network in the middle of a
   transport session without interruption.  This discussion is again
   framed in the context where the topological locators may be
   independent of the host identifiers used by the transport and
   application layer protocols.  Some of these attacks may not be
   applicable if traditional addresses are used.  This section assumes
   that each host has multiple locators and that there is some mechanism
   for determining the locators for a correspondent host.  We do not
   assume anything about the properties of these mechanisms.  Instead,
   this list will serve to help us derive the properties of these
   mechanisms that will be necessary to prevent these redirection
   attacks.

   Depending on the purpose of the redirection attack, we separate the
   attacks into several different types.

4.1. Cause Packets to Be Sent to the Attacker

   An attacker might want to receive the flow of packets, for instance
   to be able to inspect and/or modify the payload or to be able to
   apply cryptographic analysis to cryptographically protected payload,
   using redirection attacks.

   Note that such attacks are always possible today if an attacker is on
   the path between two communicating parties, and a multihoming
   solution can't remove that threat.  Hence, the bulk of these concerns
   relate to off-path attackers.

4.1.1. Once Packets Are Flowing

   This might be viewed as the "classic" redirection attack.

   While A and B are communicating X might send packets to B and claim:
   "Hi, I'm A, send my packets to my new location." where the location
   is really X's location.

   "Standard" solutions to this include requiring that the host
   requesting redirection somehow be verified to be the same host as the
   initial host that established communication.  However, the burdens of
   such verification must not be onerous, or the redirection requests
   themselves can be used as a DoS attack.

   To prevent this type of attack, a solution would need some mechanism
   that B can use to verify whether a locator belongs to A before B
   starts using that locator, and be able to do this when multiple
   locators are assigned to A.

4.1.2. Time-Shifting Attack

   The term "time-shifting attack" is used to describe an attacker's
   ability to perform an attack after no longer being on the path.
   Thus, the attacker would have been on the path at some point in time,
   snooping and/or modifying packets; and later, when the attacker is no
   longer on the path, it launches the attack.

   In the current Internet, it is not possible to perform such attacks
   to redirect packets.  But for some time after moving away, the
   attacker can cause a DoS attack, e.g., by leaving a bogus ARP entry
   in the nodes on the path, or by forging TCP Reset packets based on
   having seen the TCP Initial Sequence Numbers when it was on the path.

   It would be reasonable to require that a multihoming solution limit
   the ability to redirect and/or DoS traffic to a few minutes after the
   attacker has moved off the path.

4.1.3. Premeditated Redirection

   This is a variant of the above where the attacker "installs" itself
   before communication starts.

   For example, if the attacker X can predict that A and B will
   communicate in the (near) future, then the attacker can tell B: "Hi,
   I'm A and I'm at this location".  When A later tries to communicate
   with B, will B believe it is really A?

   If the solution to the classic redirection attack is based on "prove
   you are the same as initially", then A will fail to prove this to B
   because X initiated communication.

   Depending on details that would be specific to a proposed solution,
   this type of attack could either cause redirection (so that the
   packets intended for A will be sent to X) or they could cause DoS
   (where A would fail to communicate with B since it can't prove it is
   the same host as X).

   To prevent this attack, the verification of whether a locator belongs
   to the peer cannot simply be based on the first peer that made
   contact.

4.1.4. Using Replay Attacks

   While the multihoming problem doesn't inherently imply any
   topological movement, it is useful to also consider the impact of
   site renumbering in combination with multihoming.  In that case, the
   set of locators for a host will change each time its site renumbers,
   and, at some point in time after a renumbering event, the old locator
   prefix might be reassigned to some other site.

   This potentially give an attacker the ability to replay whatever
   protocol mechanism was used to inform a host of a peer's locators so
   that the host would incorrectly be led to believe that the old
   locator (set) should be used even long after a renumbering event.
   This is similar to the risk of replay of Binding Updates in [MIPv6],
   but the time constant is quite different; Mobile IPv6 might see
   movements every second while site renumbering, followed by
   reassignment of the site locator prefix, might be a matter of weeks
   or months.

   To prevent such replay attacks, the protocol used to verify which
   locators can be used with a particular identifier needs some replay
   protection mechanism.

   Also, in this space one needs to be concerned about potential
   interaction between such replay protection and the administrative act
   of reassignment of a locator.  If the identifier and locator
   relationship is distributed across the network, one would need to
   make sure that the old information has been completely purged from
   the network before any reassignment.  Note that this does not require
   an explicit mechanism.  This can instead be implemented by locator
   reuse policy and careful timeouts of locator information.

4.2. Cause Packets to Be Sent to a Black Hole

   This is also a variant of the classic redirection attack.  The
   difference is that the new location is a locator that is nonexistent
   or unreachable.  Thus, the effect is that sending packets to the new
   locator causes the packets to be dropped by the network somewhere.

   One would expect that solutions that prevent the previous redirection
   attacks would prevent this attack as a side effect, but it makes
   sense to include this attack here for completeness.  Mechanisms that
   prevented a redirection attack to the attacker should also prevent
   redirection to a black hole.

4.3. Third Party Denial-of-Service Attacks

   An attacker can use the ability to perform redirection to cause
   overload on an unrelated third party.  For instance, if A and B are
   communicating, then the attacker X might be able to convince A to
   send the packets intended for B to some third node C.  While this
   might seem harmless at first, since X could just flood C with packets
   directly, there are a few aspects of these attacks that cause
   concern.

   The first is that the attacker might be able to completely hide its
   identity and location.  It might suffice for X to send and receive a
   few packets to A in order to perform the redirection, and A might not
   retain any state on who asked for the redirection to C's location.
   Even if A had retained such state, that state would probably not be
   easily available to C, thus C can't determine who the attacker was
   once C has become the victim of a DoS attack.

   The second concern is that, with a direct DoS attack from X to C, the
   attacker is limited by the bandwidth of its own path towards C.  If
   the attacker can fool another host, such as A, to redirect its
   traffic to C, then the bandwidth is limited by the path from A
   towards C.  If A is a high-capacity Internet service and X has slow
   (e.g., dialup) connectivity, this difference could be substantial.
   Thus, in effect, this could be similar to packet amplifying
   reflectors in [PAXSON01].

   The third, and final concern, is that if an attacker only need a few
   packets to convince one host to flood a third party, then it wouldn't
   be hard for the attacker to convince lots of hosts to flood the same
   third party.  Thus, this could be used for Distributed Denial-of-
   Service attacks.

   A third party DoS attack might be against the resources of a
   particular host (i.e., C in the example above), or it might be
   against the network infrastructure towards a particular IP address
   prefix, by overloading the routers or links even though there is no
   host at the address being targeted.

   In today's Internet, the ability to perform this type of attack is
   quite limited.  In order for the attacker to initiate communication,
   it will in most cases need to be able to receive some packets from
   the peer (the potential exception being techniques that combine this
   with TCP-sequence-number-guessing techniques).  Furthermore, to the
   extent that parts of the Internet uses ingress filtering [INGRESS],
   even if the communication could be initiated, it wouldn't be possible
   to sustain it by sending ACK packets with spoofed source addresses
   from an off-path attacker.

   If this type of attack can't be prevented, there might be mitigation
   techniques that can be employed.  For instance, in the case of TCP a
   partial defense can be constructed by having TCP slow-start be
   triggered when the destination locator changes.  (Folks might argue
   that, separately from security, this would be the correct action for
   congestion control since TCP might not have any congestion-relation
   information about the new path implied by the new locator.)
   Presumably the same approach can be applied to other transport
   protocols that perform different forms of (TCP-friendly) congestion
   control, even though some of them might not adapt as rapidly as TCP.
   But since all congestion-controlled protocols probably need to have
   some reaction to the path change implied by a locator change, it
   makes sense to think about 3rd party DoS attacks when designing how
   the specific transport protocols should react to a locator change.
   However, this would only be a partial solution since it would
   probably take several packets and roundtrips before the transport
   protocol would stop transmitting; thus, an attacker could still use
   this as a reflector with packet amplification.  Thus, the multihoming
   mechanism probably needs some form of defense against third party DoS
   attacks, in addition to the help we can get from the transport
   protocols.

4.3.1. Basic Third Party DoS

   Assume that X is on a slow link anywhere in the Internet.  B is on a
   fast link (gigabits; e.g., a media server) and A is the victim.

   X could flood A directly but is limited by its low bandwidth.  If X
   can establish communication with B, ask B to send it a high-speed
   media stream, then X can presumably fake out the
   "acknowledgements/feedback" needed for B to blast out packets at full
   speed.  So far, this only hurts X and the path between X and the
   Internet.  But if X could also tell B "I'm at A's locator", then X
   has effectively used this redirection capability in multihoming to
   amplify its DoS capability, which would be a source of concern.

   One could envision rather simple techniques to prevent such attacks.
   For instance, before sending to a new peer locator, perform a clear
   text exchange with the claimed new locator of the form "Are you X?"
   resulting in "Yes, I'm X.".  This would suffice for the simplest of
   attacks.  However, as we will see below, more sophisticated attacks
   are possible.

4.3.2. Third Party DoS with On-Path Help

   The scenario is as above, but, in addition, the attacker X has a
   friend Y on the path between A and B:

       -----        -----        -----
       | A |--------| Y |--------| B |
       -----        -----        -----
                                /
                               /
                              /
                             /
                            /
                           /
                        -----
                        | X |
                        -----

   With the simple solution suggested in the previous section, all Y
   might need to do is fake a response to the "Are you X?" packet, and
   after that point in time Y might not be needed; X could potentially
   sustain the data flow towards A by generating the ACK packets.  Thus,
   it would be even harder to detect the existence of Y.

   Furthermore, if X is not the actual end system but an attacker
   between some node C and B, then X can claim to be C, and no finger
   can be pointed at X either:

       -----        -----        -----
       | A |--------| Y |--------| B |
       -----        -----        -----
                                /
                               /
                              /
                             /
                            /
                           /
            -----       -----
            | C |-------| X |
            -----       -----

   Thus, with two attackers on different paths, there might be no trace
   of who did the redirection to the 3rd party once the redirection has
   taken place.

   A specific case of this is when X=Y, and X is located on the same LAN
   as B.

   A potential way to make such attacks harder would be to use the last
   received (and verified) source locator as the destination locator.
   That way, when X sends the ACK packets (whether it claims to be X or
   C) the result would be that the packet flow from B would switch back
   towards X/C, which would result in an attack similar to what can be
   performed in today's Internet.

   Another way to make such attacks harder would be to perform periodic
   verifications that the peer is available at the locator, instead of
   just one when the new locator is received.

   A third way that a multihoming solution might address this is to
   ensure that B will only accept locators that can be authenticated to
   be synonymous with the original correspondent.  It must be possible
   to securely ensure that these locators form an equivalence class.  So
   in the first example, not only does X need to assert that it is A,
   but A needs to assert that it is X.

4.4. Accepting Packets from Unknown Locators

   The multihoming solution space does not only affect the destination
   of packets; it also raises the question from which sources packets
   should be accepted.  It is possible to build a multihoming solution
   that allows traffic to be recognized as coming from the same peer
   even if there is a previously unknown locator present in the source
   address field.  The question is whether we want to allow packets from
   unverified sources to be passed on to transport and application layer
   protocols.

   In the current Internet, an attacker can't inject packets with
   arbitrary source addresses into a session if there is ingress
   filtering present, so allowing packets with unverified sources in a
   multihoming solution would fail our "no worse than what we have now"
   litmus test.  However, given that ingress filtering deployment is far
   from universal and ingress filtering typically wouldn't prevent
   spoofing of addresses in the same subnet, requiring rejecting packets
   from unverified locators might be too stringent.

   An example of the current state are the ability to inject RST packets
   into existing TCP connections.  When there is no ingress filtering in
   the network, this is something that the TCP endpoints need to sort
   out themselves.  However, deploying ingress filtering helps in
   today's Internet since an attacker is limited in the set of source
   addresses it can use.

   A factor to take into account to determine the "requirement level"
   for this is that when IPsec is used on top of the multihoming
   solution, then IPsec will reject such spoofed packets.  (Note that
   this is different than in the redirection attack cases where even
   with IPsec an attacker could potentially cause a DoS attack.)

   There might also be a middle ground where arbitrary attackers are
   prevented from injecting packets by using the SCTP verification tag
   type of approach [SCTP].  (This is a clear-text tag which is sent to
   the peer which the peer is expected to include in each subsequent
   packet.)  Such an approach doesn't prevent packet injection from
   on-path attackers (since they can observe the verification tag), but
   neither does ingress filtering.

4.5. New Privacy Considerations

   While introducing identifiers can be helpful by providing ways to
   identify hosts across events when its IP address(es) might change,
   there is a risk that such mechanisms can be abused to track the
   identity of the host over long periods of time, whether using the
   same (set of) ISP(s) or moving between different network attachment
   points.  Designers of solutions to multihoming need to be aware of
   this concern.

   Introducing the multihoming capability inherently implies that the
   communicating peers need to know multiple locators for each other in
   order to be resilient to failures of some paths/locators.  In
   addition, if the multihoming signaling protocol doesn't provide
   privacy, it might be possible for 3rd parties on the path to discover
   many or all the locators assigned to a host, which would increase the
   privacy exposure compared to a multihomed host today.

   For instance, a solution could address this by allowing each host to
   have multiple identifiers at the same time and perhaps even changing
   the set of identifiers that are used over time.  Such an approach
   could be analogous to what is done for IPv6 addresses in [RFC3041].

5. Granularity of Redirection

   Different multihoming solutions might approach the problem at
   different layers in the protocol stack.  For instance, there have
   been proposals for a shim layer inside IP, as well as transport layer
   approaches.  The former would have the capability to redirect an IP
   address while the latter might be constrained to only redirect a
   single transport connection.  This difference might be important when
   it comes to understanding the security impact.

   For instance, premeditated attacks might have quite different impact
   in the two cases.  In an IP-based multihoming solution a successful
   premeditated redirection could be due to the attacker connecting to a
   server and claiming to be 'A', which would result in the server
   retaining some state about 'A', which it received from the attacker.
   Later, when the real 'A' tries to connect to the server, the
   existence of this state might mean that 'A' fails to communicate, or
   that its packets are sent to the attacker.  But if the same scenario
   is applied to a transport-layer approach, then the state created due
   to the attacker would perhaps be limited to the existing transport
   connection.  Thus, while this might prevent the real 'A' from
   connecting to the server while the attacker is connected (if they
   happen to use the same transport port number), most likely it would
   not affect 'A's ability to connect after the attacker has
   disconnected.

   A particular aspect of the granularity question is the direction
   question: will the created state be used for communication in the
   reverse direction of the direction when it was created?  For
   instance, if the attacker 'X' suspects that 'A' will connect to 'B'
   in the near future, can X connect to A and claim to be B, and then
   have that later make A connect to the attacker instead of to the real
   B?

   Note that transport layer approaches are limited to the set of
   "transport" protocols that the implementation makes aware of
   multihoming.  In many cases there would be "transport" protocols that
   are unknown to the multihoming capability of the system, such as
   applications built on top of UDP.  To understand the impact of the
   granularity question on the security, one would also need to
   understand how such applications/protocols would be handled.

   A property of transport granularity is that the amount of work
   performed by a legitimate host is proportional to the number of
   transport connections it creates that uses the multihoming support,
   since each such connection would require some multihoming signaling.
   And the same is true for the attacker.  This means that an attacker
   could presumably do a premeditated attack for all TCP connections to
   port 80 from A to B, by setting up 65,536 (for all TCP source port
   numbers) connections to server B and causing B to think those
   connections should be directed to the attacker and keeping those TCP
   connections open.  Any attempt to make legitimate communication more
   efficient (e.g., by being able to signal for multiple transport
   connections at a time) would provide as much relative benefit for an
   attacker as the legitimate hosts.

   The issue isn't only about the space (granularity), but also about
   the lifetime component in the results of a multihoming request.  In a
   transport-layer approach, the multihoming state would presumably be
   destroyed when the transport state is deleted as part of closing the
   connection.  But an IP-layer approach would have to rely on some
   timeout or garbage collection mechanisms, perhaps combined with some
   new explicit signaling, to remove the multihoming state.  The
   coupling between the connection state and multihoming state in the
   transport-layer approach might make it more expensive for the
   attacker, since it needs to keep the connections open.

   In summary, there are issues we don't yet understand well about
   granularity and reuse of the multihoming state.

6. Movement Implications?

   In the case when nothing moves around, we have a reasonable
   understanding of the security requirements.  Something that is on the
   path can be a MITM in today's Internet, and a multihoming solution
   doesn't need to make that aspect any more secure.

   But it is more difficult to understand the requirements when hosts
   are moving around.  For instance, a host might be on the path for a
   short moment in time by driving by an 802.11 hotspot.  Would we or
   would we not be concerned if such a drive-by (which many call a
   "time-shifting" attack) would result in the temporarily on-path host
   being able to act as a MITM for future communication?  Depending on
   the solution, this might be possible if the attacker causes a
   multihoming state to be created in various peer hosts while the
   attacker was on the path, and that state remained in the peers for
   some time.

   The answer to this question doesn't seem to be obvious even in the
   absence of any new multihoming support.  We don't have much
   experience with hosts moving around that are able to attack things as
   they move.  In Mobile IPv6 [MIPv6] a conservative approach was taken
   that limits the effect of such drive-by attacks to the maximum
   lifetime of the binding, which is set to a few minutes.

   With multihoming support the issue gets a bit more complicated
   because we explicitly want to allow a host to be present at multiple
   locators at the same time.  Thus, there might be a need to
   distinguish between the host moving between different locators, and
   the host sending packets with different source locators because it is
   present at multiple locators without any topological movement.

   Note that the multihoming solutions that have been discussed range
   from such "drive-by" attacks being impossible (for instance, due to a
   strong binding to a separate identifier as in HIP, or due to reliance
   on the relative security of the DNS for forward plus reverse lookups
   in NOID), to systems that are first-come/first-serve (WIMP being an
   example with a separate ID space, a MAST approach with a PBK being an
   example without a separate ID space) that allow the first host that
   uses an ID/address to claim it without any time limit.

7. Other Security Concerns

   The protocol mechanisms added as part of a multihoming solution
   shouldn't introduce any new DoS in the mechanisms themselves.  In
   particular, care must be taken not to:

    - create state on the first packet in an exchange, since that could
      result in state consumption attacks similar to the TCP SYN
      flooding attack.

    - perform much work on the first packet in an exchange (such as
      expensive verification)

   There is a potential chicken-and-egg problem here, because
   potentially one would want to avoid doing work or creating state
   until the peer has been verified, but verification will probably need
   some state and some work to be done.  Avoiding any work does not seem
   possible, but good protocol design can often delay state creation
   until verification has been completed.

   A possible approach that solutions might investigate is to defer
   verification until there appears to be two different hosts (or two
   different locators for the same host) that want to use the same
   identifier.  In such a case one would need to investigate whether a
   combination of impersonation and DoS attack can be used to prevent
   the discovery of the impersonation.

   Another possible approach is to first establish communications, and
   then perform verification in parallel with normal data transfers.
   Redirection would only be permitted after verification was complete,
   but prior to that event, data could transfer in a normal,
   non-multihomed manner.

   Finally, the new protocol mechanisms should be protected against
   spoofed packets, at least from off-path sources, and replayed
   packets.

8. Security Considerations

   In section 3, the document presented existing protocol-based
   redirection attacks.  But there are also non-protocol redirection
   attacks.  An attacker that can gain physical access to one of

    - the copper/fiber somewhere in the path,

    - a router or L2 device in the path, or

    - one of the end systems

   can also redirect packets.  This could be possible, for instance, by
   physical break-ins or by bribing staff that have access to the
   physical infrastructure.  Such attacks are out of scope of this
   discussion, but are worth keeping in mind when looking at the cost
   for an attacker to exploit any protocol-based attacks against
   multihoming solutions; making protocol-based attacks much more
   expensive to launch than break-ins/bribery type of attacks might be
   overkill.

9. Acknowledgements

   This document was originally produced by a MULTI6 design team
   consisting of (in alphabetical order):  Iljitsch van Beijnum, Steve
   Bellovin, Brian Carpenter, Mike O'Dell, Sean Doran, Dave Katz, Tony
   Li, Erik Nordmark, and Pekka Savola.

   Much of the awareness of these threats come from the work on Mobile
   IPv6 [MIPv6, NIKANDER03, AURA02].

   As the document has evolved, the MULTI6 WG participants have
   contributed to the document.  In particular:  Masataka Ohta brought
   up privacy concerns related to stable identifiers.  The suggestion to
   discuss transport versus IP granularity was contributed by Marcelo
   Bagnulo, who also contributed text to Appendix B.  Many editorial
   clarifications came from Jari Arkko.

10. Informative References

   [NSRG]        Lear, E. and R. Droms, "What's In A Name: Thoughts from
                 the NSRG", Work in Progress, September 2003.

   [MIPv6]       Johnson, D., Perkins, C., and J. Arkko, "Mobility
                 Support in IPv6", RFC 3775, June 2004.

   [AURA02]      Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses",
                 Work in Progress, March 2002.

   [NIKANDER03]  Nikander, P., T. Aura, J. Arkko, G. Montenegro, and E.
                 Nordmark, "Mobile IP version 6 Route Optimization
                 Security Design Background", Work in Progress, December
                 2003.

   [PAXSON01]    V. Paxson, "An Analysis of Using Reflectors for
                 Distributed Denial-of-Service Attacks", Computer
                 Communication Review 31(3), July 2001.

   [INGRESS]     Ferguson, P. and D. Senie, "Network Ingress Filtering:
                 Defeating Denial of Service Attacks which employ IP
                 Source Address Spoofing", BCP 38, RFC 2827, May 2000.

   [SCTP]        Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                 Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
                 Zhang, L., and V. Paxson, "Stream Control Transmission
                 Protocol", RFC 2960, October 2000.

   [RFC3041]     Narten, T. and R. Draves, "Privacy Extensions for
                 Stateless Address Autoconfiguration in IPv6", RFC 3041,
                 January 2001.

   [DNS-THREATS] Atkins, D. and R. Austein, "Threat Analysis of the
                 Domain Name System (DNS)", RFC 3833, August 2004.

   [FYI18]       Malkin, G., "Internet Users' Glossary", RFC 1983,
                 August 1996.

   [ECN]         Ramakrishnan, K., Floyd, S., and D. Black, "The
                 Addition of Explicit Congestion Notification (ECN) to
                 IP", RFC 3168, September 2001.

   [OWNER]       Nikander, P., "Denial-of-Service, Address Ownership,
                 and Early Authentication in the IPv6 World", Security
                 Protocols 9th International Workshop, Cambridge, UK,
                 April 25-27 2001, LNCS 2467, pages 12-26, Springer,
                 2002.

   [RFC1948]     Bellovin, S., "Defending Against Sequence Number
                 Attacks", RFC 1948, May 1996.

   [PBK]         Scott Bradner, Allison Mankin, Jeffrey Schiller, "A
                 Framework for Purpose-Built Keys (PBK)", Work in
                 Progress, June 2003.

   [NOID]        Erik Nordmark, "Multihoming without IP Identifiers",
                 Work in Progress, July 2004.

   [HIP]         Pekka Nikander, "Considerations on HIP based IPv6
                 multi-homing", Work in Progress, July 2004.

   [WIMP]        Jukka Ylitalo, "Weak Identifier Multihoming Protocol
                 (WIMP)", Work in Progress, June 2004.

   [CBHI]        Iljitsch van Beijnum, "Crypto Based Host Identifiers",
                 Work in Progress, February 2004.

   [TCPSECURE]   M. Dalal (ed), "Transmission Control Protocol security
                 considerations", Work in Progress, November 2004.

Appendix A: Some Security Analysis

   When looking at the proposals that have been made for multihoming
   solutions and the above threats, it seems like there are two
   separable aspects of handling the redirection threats:

    - Redirection of existing communication

    - Redirection of an identity before any communication

   This seems to be related to the fact that there are two different
   classes of use of identifiers.  One use is for:

    o Initially establishing communication; looking up an FQDN to find
      something that is passed to a connect() or sendto() API call.

    o Comparing whether a peer entity is the same peer entity as in some
      previous communication.

    o Using the identity of the peer for future communication
      ("callbacks") in the reverse direction, or to refer to a 3rd party
      ("referrals").

   The other use of identifiers is as part of being able to redirect
   existing communication to use a different locator.

   The first class of use seems to be related to something about the
   ownership of the identifier; proving the "ownership" of the
   identifier should be sufficient in order to be authorized to control
   which locators are used to reach the identifier.

   The second class of use seems to be related to something more
   ephemeral.  In order to redirect the existing communication to some
   other locator, it seems to be sufficient to prove that the entity is
   the same as the one that initiated the communication.  One can view
   this as proving the ownership of some context, where the context is
   established around the time when the communication is initiated.

   Preventing unauthorized redirection of existing communication can be
   addressed by a large number of approaches that are based on setting
   up some form of security material at the beginning of communication,
   and later using the existence of that material for one end to prove
   to the other that it remains the same.  An example of this is Purpose
   Built Keys [PBK].  One can envision different approaches for such
   schemes with different complexity, performance, and resulting

   security such as anonymous Diffie-Hellman exchange, the reverse hash
   chains presented in [WIMP], or even a clear-text token exchanged at
   the initial communication.

   However, the mechanisms for preventing unauthorized use of an
   identifier can be quite different.  One way to prevent premeditated
   redirection is to simply not introduce a new identifier name space,
   and instead to rely on existing name space(s), a trusted third party,
   and a sufficiently secure way to access the third party, as in
   [NOID].  Such an approach relies on the third party (DNS in the case
   of NOID) as the foundation.  In terms of multihoming state creation,
   in this case premeditated redirection is as easy or as hard as
   redirecting an IP address today.  Essentially, this relies on the
   return-routability check associated with a roundtrip of
   communication, which verifies that the routing system delivers
   packets to the IP address in question.

   Alternatively, one can use the crypto-based identifiers such as in
   [HIP] or crypto-generated addresses as in [CBHI], which both rely on
   public-key crypto, to prevent premeditated attacks.  In some cases it
   is also possible to avoid the problem by having one end of the
   communication use ephemeral identifiers as the initiator does in
   [WIMP].  This avoids premeditated redirection by detecting that some
   other entity is using the same identifier at the peer and switching
   to use another ephemeral ID.  While the ephemeral identifiers might
   be problematic when used by applications, for instance due to
   callbacks or referrals, note that for the end that has the ephemeral
   identifier, one can skirt around the premeditated attacks (assuming
   the solution has a robust way to pick a new identifier when one is in
   use/stolen).

   Assuming the problem can't be skirted by using ephemeral identifiers,
   there seem to be 3 types of approaches that can be used to establish
   some form of identity ownership:

    - A trusted third party, which states that a given identity is
      reachable at a given set of locators, so the endpoint reached at
      one of this locators is the owner of the identity.

    - Crypto-based identifiers or crypto-generated addresses where the
      public/private key pair can be used to prove that the identifier
      was generated by the node knowing the private key (or by another
      node whose public key hashes to the same value)

    - A static binding, as currently defined in IP, where you trust that
      the routing system will deliver the packets to the owner of the
      locator, and since the locator and the identity are one, you prove
      identity ownership as a sub-product.

   A solution would need to combine elements that provide protection
   against both premeditated and ongoing communication redirection.
   This can be done in several ways, and the current set of proposals do
   not appear to contain all useful combinations.  For instance, the HIP
   CBID property could be used to prevent premeditated attacks, while
   the WIMP hash chains could be used to prevent on-going redirection.
   And there are probably other interesting combinations.

   A related, but perhaps separate aspect, is whether the solution
   provides for protection against man-in-the-middle attacks with
   on-path attackers.  Some schemes, such as [HIP] and [NOID] do, but
   given that an on-path attacker can see and modify the data traffic
   whether or not it can modify the multihoming signaling, this level of
   protection seems like overkill.  Protecting against on-path MITM for
   the data traffic can be done separately using IPsec, TLS, etc.

   Finally, preventing third party DoS attacks is conceptually simpler;
   it would suffice to somehow verify that the peer is indeed reachable
   at the new locator before sending a large number of packets to that
   locator.

   Just as the redirection attacks are conceptually prevented by proving
   at some level the ownership of the identifier or the ownership of the
   communication context, third party DoS attacks are conceptually
   prevented by showing that the endpoint is authorized to use a given
   locator.

   The currently known approaches for showing such authorization are:

    - Return routability.  That is, if an endpoint is capable of
      receiving packets at a given locator, it is because he is
      authorized to do so.  This relies on routing being reasonably hard
      to subvert to deliver packets to the wrong place.  While this
      might be the case when routing protocols are used with reasonable
      security mechanisms and practices, it isn't the case on a single
      link where ARP and Neighbor Discovery can be easily spoofed.

    - Trusted third party.  A third party establishes that a given
      identity is authorized to use a given set of locators (for
      instance the DNS).

Authors' Addresses

   Erik Nordmark
   Sun Microsystems, Inc.
   17 Network Circle
   Mountain View, CA 94025
   USA

   Phone: +1 650 786 2921
   Fax:   +1 650 786 5896
   EMail: erik.nordmark@sun.com

   Tony Li
   EMail: Tony.Li@tony.li

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