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RFC 3775


Mobility Support in IPv6

Part 5 of 5, p. 138 to 165
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12.  Protocol Constants

   DHAAD_RETRIES                   4 retransmissions
   INITIAL_BINDACK_TIMEOUT         1 second
   INITIAL_DHAAD_TIMEOUT           3 seconds
   INITIAL_SOLICIT_TIMER           3 seconds
   MAX_BINDACK_TIMEOUT             32 seconds
   MAX_NONCE_LIFETIME              240 seconds
   MAX_TOKEN_LIFETIME              210 seconds
   MAX_RR_BINDING_LIFETIME         420 seconds
   MAX_UPDATE_RATE                 3 times
   PREFIX_ADV_RETRIES              3 retransmissions
   PREFIX_ADV_TIMEOUT              3 seconds

13.  Protocol Configuration Variables

   MaxMobPfxAdvInterval            Default: 86,400 seconds
   MinDelayBetweenRAs              Default: 3 seconds,
                                   Min: 0.03 seconds
   MinMobPfxAdvInterval            Default: 600 seconds
   InitialBindackTimeoutFirstReg   Default: 1.5 seconds

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   Home agents MUST allow the first three variables to be configured by
   system management, and mobile nodes MUST allow the last variable to
   be configured by system management.

   The default value for InitialBindackTimeoutFirstReg has been
   calculated as 1.5 times the default value of RetransTimer [12] times
   the default value of DupAddrDetectTransmits [13].

   The value MinDelayBetweenRAs overrides the value of the protocol
   constant MIN_DELAY_BETWEEN_RAS, as specified in RFC 2461 [12].  This
   variable SHOULD be set to MinRtrAdvInterval, if MinRtrAdvInterval is
   less than 3 seconds.

14.  IANA Considerations

   This document defines a new IPv6 protocol, the Mobility Header,
   described in Section 6.1.  This protocol has been assigned protocol
   number 135.

   This document also creates a new name space "Mobility Header Type",
   for the MH Type field in the Mobility Header.  The current message
   types are described starting from Section 6.1.2, and are the

      0  Binding Refresh Request

      1  Home Test Init

      2  Care-of Test Init

      3  Home Test

      4  Care-of Test

      5  Binding Update

      6  Binding Acknowledgement

      7  Binding Error

   Future values of the MH Type can be allocated using Standards Action
   or IESG Approval [10].

   Furthermore, each mobility message may contain mobility options as
   described in Section 6.2.  This document defines a new name space
   "Mobility Option" to identify these options.  The current mobility
   options are defined starting from Section 6.2.2 and are the

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      0  Pad1

      1  PadN

      2  Binding Refresh Advice

      3  Alternate Care-of Address

      4  Nonce Indices

      5  Authorization Data

   Future values of the Option Type can be allocated using Standards
   Action or IESG Approval [10].

   Finally, this document creates a third new name space "Status Code"
   for the Status field in the Binding Acknowledgement message. The
   current values are described in Section 6.1.8, and are the following:

        0 Binding Update accepted

        1 Accepted but prefix discovery necessary

      128 Reason unspecified

      129 Administratively prohibited

      130 Insufficient resources

      131 Home registration not supported

      132 Not home subnet

      133 Not home agent for this mobile node

      134 Duplicate Address Detection failed

      135 Sequence number out of window

      136 Expired home nonce index

      137 Expired care-of nonce index

      138 Expired nonces

      139 Registration type change disallowed

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   Future values of the Status field can be allocated using Standards
   Action or IESG Approval [10].

   All fields labeled "Reserved" are only to be assigned through
   Standards Action or IESG Approval.

   This document also defines a new IPv6 destination option, the Home
   Address option, described in Section 6.3.  This option has been
   assigned the Option Type value 0xC9.

   This document also defines a new IPv6 type 2 routing header,
   described in Section 6.4.  The value 2 has been allocated by IANA.

   In addition, this document defines four ICMP message types, two used
   as part of the dynamic home agent address discovery mechanism, and
   two used in lieu of Router Solicitations and Advertisements when the
   mobile node is away from the home link.  These messages have been
   assigned ICMPv6 type numbers from the informational message range:

   o  The Home Agent Address Discovery Request message, described in
      Section 6.5;

   o  The Home Agent Address Discovery Reply message, described in
      Section 6.6;

   o  The Mobile Prefix Solicitation, described in Section 6.7; and

   o  The Mobile Prefix Advertisement, described in Section 6.8.

   This document also defines two new Neighbor Discovery [12] options,
   which have been assigned Option Type values within the option
   numbering space for Neighbor Discovery messages:

   o  The Advertisement Interval option, described in Section 7.3; and

   o  The Home Agent Information option, described in Section 7.4.

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15.  Security Considerations

15.1.  Threats

   Any mobility solution must protect itself against misuses of the
   mobility features and mechanisms.  In Mobile IPv6, most of the
   potential threats are concerned with false Bindings, usually
   resulting in Denial-of-Service attacks.  Some of the threats also
   pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and
   Impersonation attacks.  The main threats this protocol protects
   against are the following:

   o  Threats involving Binding Updates sent to home agents and
      correspondent nodes.  For instance, an attacker might claim that a
      certain mobile node is currently at a different location than it
      really is.  If a home agent accepts such spoofed information sent
      to it, the mobile node might not get traffic destined to it.
      Similarly, a malicious (mobile) node might use the home address of
      a victim node in a forged Binding Update sent to a correspondent

      These pose threats against confidentiality, integrity, and
      availability.  That is, an attacker might learn the contents of
      packets destined to another node by redirecting the traffic to
      itself.  Furthermore, an attacker might use the redirected packets
      in an attempt to set itself as a Man-in-the-Middle between a
      mobile and a correspondent node.  This would allow the attacker to
      impersonate the mobile node, leading to integrity and availability

      A malicious (mobile) node might also send Binding Updates in which
      the care-of address is set to the address of a victim node.  If
      such Binding Updates were accepted, the malicious node could lure
      the correspondent node into sending potentially large amounts of
      data to the victim; the correspondent node's replies to messages
      sent by the malicious mobile node will be sent to the victim host
      or network.  This could be used to cause a Distributed Denial-of-
      Service attack.  For example, the correspondent node might be a
      site that will send a high-bandwidth stream of video to anyone who
      asks for it.  Note that the use of flow-control protocols such as
      TCP does not necessarily defend against this type of attack,
      because the attacker can fake the acknowledgements.  Even keeping
      TCP initial sequence numbers secret does not help, because the
      attacker can receive the first few segments (including the ISN) at
      its own address, and only then redirect the stream to the victim's
      address.  These types of attacks may also be directed to networks
      instead of nodes.  Further variations of this threat are described
      elsewhere [27, 34].

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      An attacker might also attempt to disrupt a mobile node's
      communications by replaying a Binding Update that the node had
      sent earlier.  If the old Binding Update was accepted, packets
      destined for the mobile node would be sent to its old location as
      opposed to its current location.

      In conclusion, there are Denial-of-Service, Man-in-the-Middle,
      Confidentiality, and Impersonation threats against the parties
      involved in sending legitimate Binding Updates, and Denial-of-
      Service threats against any other party.

   o  Threats associated with payload packets: Payload packets exchanged
      with mobile nodes are exposed to similar threats as that of
      regular IPv6 traffic.  However, Mobile IPv6 introduces the Home
      Address destination option, a new routing header type (type 2),
      and uses tunneling headers in the payload packets.  The protocol
      must protect against potential new threats involving the use of
      these mechanisms.

      Third parties become exposed to a reflection threat via the Home
      Address destination option, unless appropriate security
      precautions are followed.  The Home Address destination option
      could be used to direct response traffic toward a node whose IP
      address appears in the option.  In this case, ingress filtering
      would not catch the forged "return address" [36, 32].

      A similar threat exists with the tunnels between the mobile node
      and the home agent.  An attacker might forge tunnel packets
      between the mobile node and the home agent, making it appear that
      the traffic is coming from the mobile node when it is not.  Note
      that an attacker who is able to forge tunnel packets would
      typically also be able to forge packets that appear to come
      directly from the mobile node.  This is not a new threat as such.
      However, it may make it easier for attackers to escape detection
      by avoiding ingress filtering and packet tracing mechanisms.
      Furthermore, spoofed tunnel packets might be used to gain access
      to the home network.

      Finally, a routing header could also be used in reflection
      attacks, and in attacks designed to bypass firewalls.  The
      generality of the regular routing header would allow circumvention
      of IP-address based rules in firewalls.  It would also allow
      reflection of traffic to other nodes.  These threats exist with
      routing headers in general, even if the usage that Mobile IPv6
      requires is safe.

   o  Threats associated with dynamic home agent and mobile prefix

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   o  Threats against the Mobile IPv6 security mechanisms themselves: An
      attacker might, for instance, lure the participants into executing
      expensive cryptographic operations or allocating memory for the
      purpose of keeping state.  The victim node would have no resources
      left to handle other tasks.

   As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to
   be deployed in most nodes of the IPv6 Internet.  The above threats
   should therefore be considered as being applicable to the whole

   It should also be noted that some additional threats result from
   movements as such, even without the involvement of mobility
   protocols.  Mobile nodes must be capable to defend themselves in the
   networks that they visit, as typical perimeter defenses applied in
   the home network no longer protect them.

15.2.  Features

   This specification provides a series of features designed to mitigate
   the risk introduced by the threats listed above.  The main security
   features are the following:

   o  Reverse Tunneling as a mandatory feature.

   o  Protection of Binding Updates sent to home agents.

   o  Protection of Binding Updates sent to correspondent nodes.

   o  Protection against reflection attacks that use the Home Address
      destination option.

   o  Protection of tunnels between the mobile node and the home agent.

   o  Closing routing header vulnerabilities.

   o  Mitigating Denial-of-Service threats to the Mobile IPv6 security
      mechanisms themselves.

   The support for encrypted reverse tunneling (see Section 11.3.1)
   allows mobile nodes to defeat certain kinds of traffic analysis.

   Protecting those Binding Updates that are sent to home agents and
   those that are sent to arbitrary correspondent nodes requires very
   different security solutions due to the different situations.  Mobile
   nodes and home agents are naturally expected to be subject to the
   network administration of the home domain.

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   Thus, they can and are supposed to have a security association that
   can be used to reliably authenticate the exchanged messages.  See
   Section 5.1 for the description of the protocol mechanisms, and
   Section 15.3 below for a discussion of the resulting level of

   It is expected that Mobile IPv6 route optimization will be used on a
   global basis between nodes belonging to different administrative
   domains.  It would be a very demanding task to build an
   authentication infrastructure on this scale.  Furthermore, a
   traditional authentication infrastructure cannot be easily used to
   authenticate IP addresses because IP addresses can change often.  It
   is not sufficient to just authenticate the mobile nodes;
   Authorization to claim the right to use an address is needed as well.
   Thus, an "infrastructureless" approach is necessary.  The chosen
   infrastructureless method is described in Section 5.2, and Section
   15.4 discusses the resulting security level and the design rationale
   of this approach.

   Specific rules guide the use of the Home Address destination option,
   the routing header, and the tunneling headers in the payload packets.
   These rules are necessary to remove the vulnerabilities associated
   with their unrestricted use.  The effect of the rules is discussed in
   Section 15.7, Section 15.8, and Section 15.9.

   Denial-of-Service threats against Mobile IPv6 security mechanisms
   themselves concern mainly the Binding Update procedures with
   correspondent nodes.  The protocol has been designed to limit the
   effects of such attacks, as will be described in Section 15.4.5.

15.3.  Binding Updates to Home Agent

   Signaling between the mobile node and the home agent requires message
   integrity.  This is necessary to assure the home agent that a Binding
   Update is from a legitimate mobile node.  In addition, correct
   ordering and anti-replay protection are optionally needed.

   IPsec ESP protects the integrity of the Binding Updates and Binding
   Acknowledgements by securing mobility messages between the mobile
   node and the home agent.

   IPsec can provide anti-replay protection only if dynamic keying is
   used (which may not always be the case).  IPsec does not guarantee
   correct ordering of packets, only that they have not been replayed.
   Because of this, sequence numbers within the Mobile IPv6 messages are
   used to ensure correct ordering (see Section 5.1).  However, if the
   16 bit Mobile IPv6 sequence number space is cycled through, or the
   home agent reboots and loses its state regarding the sequence

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   numbers, replay and reordering attacks become possible.  The use of
   dynamic keying, IPsec anti-replay protection, and the Mobile IPv6
   sequence numbers can together prevent such attacks.  It is also
   recommended that use of non-volatile storage be considered for home
   agents, to avoid losing their state.

   A sliding window scheme is used for the sequence numbers.  The
   protection against replays and reordering attacks without a key
   management mechanism works when the attacker remembers up to a
   maximum of 2**15 Binding Updates.

   The above mechanisms do not show that the care-of address given in
   the Binding Update is correct.  This opens the possibility for
   Denial-of-Service attacks against third parties.  However, since the
   mobile node and home agent have a security association, the home
   agent can always identify an ill-behaving mobile node.  This allows
   the home agent operator to discontinue the mobile node's service, and
   possibly take further actions based on the business relationship with
   the mobile node's owner.

   Note that the use of a single pair of manually keyed security
   associations conflicts with the generation of a new home address [18]
   for the mobile node, or with the adoption of a new home subnet
   prefix.  This is because IPsec security associations are bound to the
   used addresses.  While certificate-based automatic keying alleviates
   this problem to an extent, it is still necessary to ensure that a
   given mobile node cannot send Binding Updates for the address of
   another mobile node.  In general, this leads to the inclusion of home
   addresses in certificates in the Subject AltName field.  This again
   limits the introduction of new addresses without either manual or
   automatic procedures to establish new certificates.  Therefore, this
   specification restricts the generation of new home addresses (for any
   reason) to those situations where a security association or
   certificate for the new address already exists.  (Appendix B.4 lists
   the improvement of security for new addresses as one of the future
   developments for Mobile IPv6.)

   Support for IKE has been specified as optional.  The following should
   be observed about the use of manual keying:

   o  As discussed above, with manually keyed IPsec, only a limited form
      of protection exists against replay and reordering attacks.  A
      vulnerability exists if either the sequence number space is cycled
      through, or if the home agent reboots and forgets its sequence
      numbers (and uses volatile memory to store the sequence numbers).
      Assuming the mobile node moves continuously every 10 minutes, it

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      takes roughly 455 days before the sequence number space has been
      cycled through.  Typical movement patterns rarely reach this high
      frequency today.

   o  A mobile node and its home agent belong to the same domain.  If
      this were not the case, manual keying would not be possible [28],
      but in Mobile IPv6 only these two parties need to know the
      manually configured keys.  Similarly, we note that Mobile IPv6
      employs standard block ciphers in IPsec, and is not vulnerable to
      problems associated with stream ciphers and manual keying.

   o  It is expected that the owner of the mobile node and the
      administrator of the home agent agree on the used keys and other
      parameters with some off-line mechanism.

   The use of IKEv1 with Mobile IPv6 is documented in more detail in
   [21].  The following should be observed from the use of IKEv1:

   o  It is necessary to prevent a mobile node from claiming another
      mobile node's home address.  The home agent must verify that the
      mobile node trying to negotiate the SA for a particular home
      address is authorized for that home address.  This implies that
      even with the use of IKE, a policy entry needs to be configured
      for each home address served by the home agent.

      It may be possible to include home addresses in the Subject
      AltName field of certificate to avoid this.  However,
      implementations are not guaranteed to support the use of a
      particular IP address (care-of address) while another address
      (home address) appears in the certificate.  In any case, even this
      approach would require user-specific tasks in the certificate

   o  If preshared secret authentication is used, IKEv1 main mode cannot
      be used.  Aggressive mode or group preshared secrets need to be
      used with corresponding security implications instead.

      Note that, like many other issues, this is a general IKEv1 issue
      related to the ability to use different IP addresses, and not
      specifically related to Mobile IPv6.  For further information, see
      Section 4.4 in [21].

   o  Due to the problems outlined in Section 11.3.2, IKE phase 1
      between the mobile node and its home agent is established using
      the mobile node's current care-of address.  This implies that when
      the mobile node moves to a new location, it may have to re-
      establish phase 1.  A Key Management Mobility Capability (K) flag

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      is provided for implementations that can update the IKE phase 1
      endpoints without re-establishing phase 1, but the support for
      this behavior is optional.

   o  When certificates are used, IKE fragmentation can occur as
      discussed in Section 7 in [21].

   o  Nevertheless, even if per-mobile node configuration is required
      with IKE, an important benefit of IKE is that it automates the
      negotiation of cryptographic parameters, including the SPIs,
      cryptographic algorithms, and so on.  Thus, less configuration
      information is needed.

   o  The frequency of movements in some link layers or deployment
      scenarios may be high enough to make replay and reordering attacks
      possible, if only manual keying is used.  IKE SHOULD be used in
      such cases.  Potentially vulnerable scenarios involve continuous
      movement through small cells, or uncontrolled alternation between
      available network attachment points.

   o  Similarly, in some deployment scenarios the number of mobile nodes
      may be very large.  In these cases, it can be necessary to use
      automatic mechanisms to reduce the management effort in the
      administration of cryptographic parameters, even if some per-
      mobile node configuration is always needed.  IKE SHOULD also be
      used in such cases.

   o  Other automatic key management mechanisms exist beyond IKEv1, but
      this document does not address the issues related to them.  We
      note, however, that most of the above discussion applies to IKEv2
      [30] as well, at least as it is currently specified.

15.4.  Binding Updates to Correspondent Nodes

   The motivation for designing the return routability procedure was to
   have sufficient support for Mobile IPv6, without creating significant
   new security problems.  The goal for this procedure was not to
   protect against attacks that were already possible before the
   introduction of Mobile IPv6.

   The next sections will describe the security properties of the used
   method, both from the point of view of possible on-path attackers who
   can see those cryptographic values that have been sent in the clear
   (Section 15.4.2 and Section 15.4.3) and from the point of view of
   other attackers (Section 15.4.6).

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15.4.1.  Overview

   The chosen infrastructureless method verifies that the mobile node is
   "live" (that is, it responds to probes) at its home and care-of
   addresses.  Section 5.2 describes the return routability procedure in
   detail.  The procedure uses the following principles:

   o  A message exchange verifies that the mobile node is reachable at
      its addresses, i.e., is at least able to transmit and receive
      traffic at both the home and care-of addresses.

   o  The eventual Binding Update is cryptographically bound to the
      tokens supplied in the exchanged messages.

   o  Symmetric exchanges are employed to avoid the use of this protocol
      in reflection attacks.  In a symmetric exchange, the responses are
      always sent to the same address the request was sent from.

   o  The correspondent node operates in a stateless manner until it
      receives a fully authorized Binding Update.

   o  Some additional protection is provided by encrypting the tunnels
      between the mobile node and home agent with IPsec ESP.  As the
      tunnel also transports the nonce exchanges, the ability of
      attackers to see these nonces is limited.  For instance, this
      prevents attacks from being launched from the mobile node's
      current foreign link, even when no link-layer confidentiality is

      The resulting level of security is in theory the same even without
      this additional protection: the return routability tokens are
      still exposed only to one path within the whole Internet.
      However, the mobile nodes are often found on an insecure link,
      such as a public access Wireless LAN.  Thus, in many cases, this
      addition makes a practical difference.

   For further information about the design rationale of the return
   routability procedure, see [27, 34, 33, 32].  The mechanisms used
   have been adopted from these documents.

15.4.2.  Achieved Security Properties

   The return routability procedure protects Binding Updates against all
   attackers who are unable to monitor the path between the home agent
   and the correspondent node.  The procedure does not defend against
   attackers who can monitor this path.  Note that such attackers are in
   any case able to mount an active attack against the mobile node when

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   it is at its home location.  The possibility of such attacks is not
   an impediment to the deployment of Mobile IPv6 because these attacks
   are possible regardless of whether or not Mobile IPv6 is in use.

   This procedure also protects against Denial-of-Service attacks in
   which the attacker pretends to be mobile, but uses the victim's
   address as the care-of address.  This would cause the correspondent
   node to send the victim some unexpected traffic.  This procedure
   defends against these attacks by requiring at least the passive
   presence of the attacker at the care-of address or on the path from
   the correspondent to the care-of address.  Normally, this will be the
   mobile node.

15.4.3.  Comparison to Regular IPv6 Communications

   This section discusses the protection offered by the return
   routability method by comparing it to the security of regular IPv6
   communications.  We will divide vulnerabilities into three classes:
   (1) those related to attackers on the local network of the mobile
   node, home agent, or the correspondent node, (2) those related to
   attackers on the path between the home network and the correspondent
   node, and (3) off-path attackers, i.e., the rest of the Internet.

   We will now discuss the vulnerabilities of regular IPv6
   communications.  The on-link vulnerabilities of IPv6 communications
   include Denial-of-Service, Masquerading, Man-in-the-Middle,
   Eavesdropping, and other attacks.  These attacks can be launched
   through spoofing Router Discovery, Neighbor Discovery and other IPv6
   mechanisms.  Some of these attacks can be prevented with the use of
   cryptographic protection in the packets.

   A similar situation exists with on-path attackers.  That is, without
   cryptographic protection, the traffic is completely vulnerable.

   Assuming that attackers have not penetrated the security of the
   Internet routing protocols, attacks are much harder to launch from
   off-path locations.  Attacks that can be launched from these
   locations are mainly Denial-of-Service attacks, such as flooding and/
   or reflection attacks.  It is not possible for an off-path attacker
   to become a Man-in-the-Middle.

   Next, we will consider the vulnerabilities that exist when IPv6 is
   used together with Mobile IPv6 and the return routability procedure.
   On the local link, the vulnerabilities are the same as those in IPv6,
   but Masquerade and Man-in-the-Middle attacks can now also be launched
   against future communications, and not just against current
   communications.  If a Binding Update was sent while the attacker was
   present on the link, its effects remain for the lifetime of the

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   binding.  This happens even if the attacker moves away from the link.
   In contrast, an attacker who uses only plain IPv6 generally has to
   stay on the link in order to continue the attack.  Note that in order
   to launch these new attacks, the IP address of the victim must be
   known.  This makes this attack feasible, mainly in the context of
   well-known interface IDs, such as those already appearing in the
   traffic on the link or registered in the DNS.

   On-path attackers can exploit similar vulnerabilities as in regular
   IPv6.  There are some minor differences, however.  Masquerade, Man-
   in-the-Middle, and Denial-of-Service attacks can be launched with
   just the interception of a few packets, whereas in regular IPv6 it is
   necessary to intercept every packet.  The effect of the attacks is
   the same regardless of the method, however.  In any case, the most
   difficult task an attacker faces in these attacks is getting on the
   right path.

   The vulnerabilities for off-path attackers are the same as in regular
   IPv6.  Those nodes that are not on the path between the home agent
   and the correspondent node will not be able to receive the home
   address probe messages.

   In conclusion, we can state the following main results from this

   o  Return routability prevents any off-path attacks beyond those that
      are already possible in regular IPv6.  This is the most important
      result, preventing attackers on the Internet from exploiting any

   o  Vulnerabilities to attackers on the home agent link, the
      correspondent node link, and the path between them are roughly the
      same as in regular IPv6.

   o  However, one difference is that in basic IPv6 an on-path attacker
      must be constantly present on the link or the path, whereas with
      Mobile IPv6, an attacker can leave a binding behind after moving

      For this reason, this specification limits the creation of
      bindings to at most MAX_TOKEN_LIFETIME seconds after the last
      routability check has been performed, and limits the duration of a
      binding to at most MAX_RR_BINDING_LIFETIME seconds.  With these
      limitations, attackers cannot take any practical advantages of
      this vulnerability.

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   o  There are some other minor differences, such as an effect to the
      Denial-of-Service vulnerabilities.  These can be considered to be

   o  The path between the home agent and a correspondent node is
      typically easiest to attack on the links at either end, in
      particular if these links are publicly accessible wireless LANs.

      Attacks against the routers or switches on the path are typically
      harder to accomplish.  The security on layer 2 of the links plays
      then a major role in the resulting overall network security.
      Similarly, security of IPv6 Neighbor and Router Discovery on these
      links has a large impact.  If these were secured using some new
      technology in the future, this could change the situation
      regarding the easiest point of attack.

   For a more in-depth discussion of these issues, see [32].

15.4.4.  Replay Attacks

   The return routability procedure also protects the participants
   against replayed Binding Updates.  The attacker is unable replay the
   same message due to the sequence number which is a part of the
   Binding Update.  It is also unable to modify the Binding Update since
   the MAC verification would fail after such a modification.

   Care must be taken when removing bindings at the correspondent node,
   however.  If a binding is removed while the nonce used in its
   creation is still valid, an attacker could replay the old Binding
   Update.  Rules outlined in Section 5.2.8 ensure that this cannot

15.4.5.  Denial-of-Service Attacks

   The return routability procedure has protection against resource
   exhaustion Denial-of-Service attacks.  The correspondent nodes do not
   retain any state about individual mobile nodes until an authentic
   Binding Update arrives.  This is achieved through the construct of
   keygen tokens from the nonces and node keys that are not specific to
   individual mobile nodes.  The keygen tokens can be reconstructed by
   the correspondent node, based on the home and care-of address
   information that arrives with the Binding Update.  This means that
   the correspondent nodes are safe against memory exhaustion attacks
   except where on-path attackers are concerned.  Due to the use of
   symmetric cryptography, the correspondent nodes are relatively safe
   against CPU resource exhaustion attacks as well.

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   Nevertheless, as [27] describes, there are situations in which it is
   impossible for the mobile and correspondent nodes to determine if
   they actually need a binding or whether they just have been fooled
   into believing so by an attacker.  Therefore, it is necessary to
   consider situations where such attacks are being made.

   Even if route optimization is a very important optimization, it is
   still only an optimization.  A mobile node can communicate with a
   correspondent node even if the correspondent refuses to accept any
   Binding Updates.  However, performance will suffer because packets
   from the correspondent node to the mobile node will be routed via the
   mobile's home agent rather than a more direct route.  A correspondent
   node can protect itself against some of these resource exhaustion
   attacks as follows.  If the correspondent node is flooded with a
   large number of Binding Updates that fail the cryptographic integrity
   checks, it can stop processing Binding Updates.  If a correspondent
   node finds that it is spending more resources on checking bogus
   Binding Updates than it is likely to save by accepting genuine
   Binding Updates, then it may silently discard some or all Binding
   Updates without performing any cryptographic operations.

   Layers above IP can usually provide additional information to help
   decide if there is a need to establish a binding with a specific
   peer.  For example, TCP knows if the node has a queue of data that it
   is trying to send to a peer.  An implementation of this specification
   is not required to make use of information from higher protocol
   layers, but some implementations are likely to be able to manage
   resources more effectively by making use of such information.

   We also require that all implementations be capable of
   administratively disabling route optimization.

15.4.6.  Key Lengths

   Attackers can try to break the return routability procedure in many
   ways.  Section 15.4.2 discusses the situation where the attacker can
   see the cryptographic values sent in the clear, and Section 15.4.3
   discusses the impact this has on IPv6 communications.  This section
   discusses whether attackers can guess the correct values without
   seeing them.

   While the return routability procedure is in progress, 64 bit cookies
   are used to protect spoofed responses.  This is believed to be
   sufficient, given that to blindly spoof a response a very large
   number of messages would have to be sent before success would be

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   The tokens used in the return routability procedure provide together
   128 bits of information.  This information is used internally as
   input to a hash function to produce a 160 bit quantity suitable for
   producing the keyed hash in the Binding Update using the HMAC_SHA1
   algorithm.  The final keyed hash length is 96 bits.  The limiting
   factors in this case are the input token lengths and the final keyed
   hash length.  The internal hash function application does not reduce
   the entropy.

   The 96 bit final keyed hash is of typical size and is believed to be
   secure.  The 128 bit input from the tokens is broken in two pieces,
   the home keygen token and the care-of keygen token.  An attacker can
   try to guess the correct cookie value, but again this would require a
   large number of messages (an the average 2**63 messages for one or
   2**127 for two).  Furthermore, given that the cookies are valid only
   for a short period of time, the attack has to keep a high constant
   message rate to achieve a lasting effect.  This does not appear

   When the mobile node is returning home, it is allowed to use just the
   home keygen token of 64 bits.  This is less than 128 bits, but
   attacking it blindly would still require a large number of messages
   to be sent.  If the attacker is on the path and capable of seeing the
   Binding Update, it could conceivably break the keyed hash with brute
   force.  However, in this case the attacker has to be on the path,
   which appears to offer easier ways for denial-of-service than
   preventing route optimization.

15.5.  Dynamic Home Agent Address Discovery

   The dynamic home agent address discovery function could be used to
   learn the addresses of home agents in the home network.

   The ability to learn addresses of nodes may be useful to attackers
   because brute-force scanning of the address space is not practical
   with IPv6.  Thus, they could benefit from any means which make
   mapping the networks easier.  For example, if a security threat
   targeted at routers or even home agents is discovered, having a
   simple ICMP mechanism to easily find out possible targets may prove
   to be an additional (though minor) security risk.

   Apart from discovering the address(es) of home agents, attackers will
   not be able to learn much from this information, and mobile nodes
   cannot be tricked into using wrong home agents, as all other
   communication with the home agents is secure.

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15.6.  Mobile Prefix Discovery

   The mobile prefix discovery function may leak interesting information
   about network topology and prefix lifetimes to eavesdroppers; for
   this reason, requests for this information has to be authenticated.
   Responses and unsolicited prefix information needs to be
   authenticated to prevent the mobile nodes from being tricked into
   believing false information about the prefixes and possibly
   preventing communications with the existing addresses.  Optionally,
   encryption may be applied to prevent leakage of the prefix

15.7.  Tunneling via the Home Agent

   Tunnels between the mobile node and the home agent can be protected
   by ensuring proper use of source addresses, and optional
   cryptographic protection.  These procedures are discussed in Section

   Binding Updates to the home agents are secure.  When receiving
   tunneled traffic, the home agent verifies that the outer IP address
   corresponds to the current location of the mobile node.  This acts as
   a weak form of protection against spoofing packets that appear to
   come from the mobile node.  This is particularly useful, if no end-
   to-end security is being applied between the mobile and correspondent
   nodes.  The outer IP address check prevents attacks where the
   attacker is controlled by ingress filtering.  It also prevents
   attacks when the attacker does not know the current care-of address
   of the mobile node.  Attackers who know the care-of address and are
   not controlled by ingress filtering could still send traffic through
   the home agent.  This includes attackers on the same local link as
   the mobile node is currently on.  But such attackers could send
   packets that appear to come from the mobile node without attacking
   the tunnel; the attacker could simply send packets with the source
   address set to the mobile node's home address.  However, this attack
   does not work if the final destination of the packet is in the home
   network, and some form of perimeter defense is being applied for
   packets sent to those destinations.  In such cases it is recommended
   that either end-to-end security or additional tunnel protection be
   applied, as is usual in remote access situations.

   Home agents and mobile nodes may use IPsec ESP to protect payload
   packets tunneled between themselves.  This is useful for protecting
   communications against attackers on the path of the tunnel.

   When site local home addresses are used, reverse tunneling can be
   used to send site local traffic from another location.
   Administrators should be aware of this when allowing such home

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   addresses.  In particular, the outer IP address check described above
   is not sufficient against all attackers.  The use of encrypted
   tunnels is particularly useful for these kinds of home addresses.

15.8.  Home Address Option

   When the mobile node sends packets directly to the correspondent
   node, the Source Address field of the packet's IPv6 header is the
   care-of address.  Therefore, ingress filtering [26] works in the
   usual manner even for mobile nodes, as the Source Address is
   topologically correct.  The Home Address option is used to inform the
   correspondent node of the mobile node's home address.

   However, the care-of address in the Source Address field does not
   survive in replies sent by the correspondent node unless it has a
   binding for this mobile node.  Also, not all attacker tracing
   mechanisms work when packets are being reflected through
   correspondent nodes using the Home Address option.  For these
   reasons, this specification restricts the use of the Home Address
   option.  It may only be used when a binding has already been
   established with the participation of the node at the home address,
   as described in Section 5.5 and Section 6.3.  This prevents
   reflection attacks through the use of the Home Address option.  It
   also ensures that the correspondent nodes reply to the same address
   that the mobile node sends traffic from.

   No special authentication of the Home Address option is required
   beyond the above, but note that if the IPv6 header of a packet is
   covered by IPsec Authentication Header, then that authentication
   covers the Home Address option as well.  Thus, even when
   authentication is used in the IPv6 header, the security of the Source
   Address field in the IPv6 header is not compromised by the presence
   of a Home Address option.  Without authentication of the packet, any
   field in the IPv6 header, including the Source Address field or any
   other part of the packet and the Home Address option can be forged or
   modified in transit.  In this case, the contents of the Home Address
   option is no more suspect than any other part of the packet.

15.9.  Type 2 Routing Header

   The definition of the type 2 routing header is described in Section
   6.4.  This definition and the associated processing rules have been
   chosen so that the header cannot be used for what is traditionally
   viewed as source routing.  In particular, the Home Address in the
   routing header will always have to be assigned to the home address of
   the receiving node; otherwise the packet will be dropped.

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   Generally, source routing has a number of security concerns.  These
   include the automatic reversal of unauthenticated source routes
   (which is an issue for IPv4, but not for IPv6).  Another concern is
   the ability to use source routing to "jump" between nodes inside, as
   well as outside a firewall.  These security concerns are not issues
   in Mobile IPv6, due to the rules mentioned above.

   In essence the semantics of the type 2 routing header is the same as
   a special form of IP-in-IP tunneling where the inner and outer source
   addresses are the same.

   This implies that a device which implements the filtering of packets
   should be able to distinguish between a type 2 routing header and
   other routing headers, as required in Section 8.3.  This is necessary
   in order to allow Mobile IPv6 traffic while still having the option
   of filtering out other uses of routing headers.

16.  Contributors

   Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik Nordmark,
   and Michael Thomas worked on the return routability protocols
   eventually led to the procedures used in this protocol.  The
   procedures described in [34] were adopted in the protocol.

   Significant contributions were made by members of the Mobile IPv6
   Security Design Team, including (in alphabetical order) Gabriel
   Montenegro, Erik Nordmark and Pekka Nikander.

17.  Acknowledgements

   We would like to thank the members of the Mobile IP and IPng Working
   Groups for their comments and suggestions on this work.  We would
   particularly like to thank (in alphabetical order) Fred Baker, Josh
   Broch, Samita Chakrabarti, Robert Chalmers, Noel Chiappa, Greg Daley,
   Vijay Devarapalli, Rich Draves, Francis Dupont, Thomas Eklund, Jun-
   Ichiro Itojun Hagino, Brian Haley, Marc Hasson, John Ioannidis, James
   Kempf, Rajeev Koodli, Krishna Kumar, T.J. Kniveton, Joe Lau, Jiwoong
   Lee, Aime Le Rouzic, Vesa-Matti Mantyla, Kevin Miles, Glenn Morrow,
   Thomas Narten, Karen Nielsen, Simon Nybroe, David Oran, Brett
   Pentland, Lars Henrik Petander, Basavaraj Patil, Mohan Parthasarathy,
   Alexandru Petrescu, Mattias Petterson, Ken Powell, Phil Roberts, Ed
   Remmell, Patrice Romand, Luis A. Sanchez, Jeff Schiller, Pekka
   Savola, Arvind Sevalkar, Keiichi Shima, Tom Soderlund, Hesham
   Soliman, Jim Solomon, Tapio Suihko, Dave Thaler, Benny Van Houdt,
   Jon-Olov Vatn, Carl E. Williams, Vladislav Yasevich, Alper Yegin, and

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   Xinhua Zhao, for their detailed reviews of earlier versions of this
   document.  Their suggestions have helped to improve both the design
   and presentation of the protocol.

   We would also like to thank the participants of the Mobile IPv6
   testing event (1999), implementors who participated in Mobile IPv6
   interoperability testing at Connectathons (2000, 2001, 2002, and
   2003), and the participants at the ETSI interoperability testing
   (2000, 2002).  Finally, we would like to thank the TAHI project who
   has provided test suites for Mobile IPv6.

18.  References

18.1.  Normative References

   [1]   Eastlake 3rd., D., Crocker, S. and J. Schiller, "Randomness
         Recommendations for Security", RFC 1750, December 1994.

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

   [3]   Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
         Addressing Architecture", RFC 3513, April 2003.

   [4]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [5]   Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

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

   [7]   Piper, D., "The Internet IP Security Domain of Interpretation
         for ISAKMP", RFC 2407, November 1998.

   [8]   Maughan, D., Schertler, M., Schneider, M. and J. Turner,
         "Internet Security Association and Key Management Protocol
         (ISAKMP)", RFC 2408, November 1998.

   [9]   Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [10]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October

Top      Up      ToC       Page 159 
   [11]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
         Specification", RFC 2460, December 1998.

   [12]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [13]  Thomson, S. and T. Narten, "IPv6 Stateless Address
         Autoconfiguration", RFC 2462, December 1998.

   [14]  Conta, A. and S. Deering, "Internet Control Message Protocol
         (ICMPv6) for the Internet Protocol Version 6 (IPv6)
         Specification", RFC 2463, December 1998.

   [15]  Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
         Specification", RFC 2473, December 1998.

   [16]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
         Addresses", RFC 2526, March 1999.

   [17]  Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
         Discovery (MLD) for IPv6", RFC 2710, October 1999.

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

   [19]  Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced by
         an On-line Database", RFC 3232, January 2002.

   [20]  National Institute of Standards and Technology, "Secure Hash
         Standard", FIPS PUB 180-1, April 1995, <http://>.

   [21]  Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
         Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
         Agents", RFC 3776, June 2004.

18.2.  Informative References

   [22]  Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
         August 2002.

   [23]  Perkins, C., "IP Encapsulation within IP", RFC 2003, October

   [24]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
         October 1996.

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   [25]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [26]  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.

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

   [28]  Bellovin, S., "Guidelines for Mandating Automated Key
         Management", Work in Progress, August 2003.

   [29]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C. and
         M. Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [30]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", Work in
         Progress, April 2003.

   [31]  Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

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

   [33]  Nordmark, E., "Securing MIPv6 BUs using return routability
         (BU3WAY)", Work in Progress, November 2001.

   [34]  Roe, M., Aura, T., O'Shea, G. and J. Arkko, "Authentication of
         Mobile IPv6 Binding Updates and Acknowledgments", Work in
         Progress, March 2002.

   [35]  Savola, P., "Use of /127 Prefix Length Between Routers
         Considered Harmful", RFC 3627, September 2003.

   [36]  Savola, P., "Security of IPv6 Routing Header and Home Address
         Options", Work in Progress, December 2002.

   [37]  Vida, R. and L. Costa, Eds., "Multicast Listener Discovery
         Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

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Appendix A.  Future Extensions

A.1.  Piggybacking

   This document does not specify how to piggyback payload packets on
   the binding related messages.  However, it is envisioned that this
   can be specified in a separate document when issues such as the
   interaction between piggybacking and IPsec are fully resolved (see
   also Appendix A.3).  The return routability messages can indicate
   support for piggybacking with a new mobility option.

A.2.  Triangular Routing

   Due to the concerns about opening reflection attacks with the Home
   Address destination option, this specification requires that this
   option be verified against the Binding Cache, i.e., there must be a
   Binding Cache entry for the Home Address and Care-of Address.

   Future extensions may be specified that allow the use of unverified
   Home Address destination options in ways that do not introduce
   security issues.

A.3.  New Authorization Methods

   While the return routability procedure provides a good level of
   security, there exist methods that have even higher levels of
   security.  Secondly, as discussed in Section 15.4, future
   enhancements of IPv6 security may cause a need to also improve the
   security of the return routability procedure.  Using IPsec as the
   sole method for authorizing Binding Updates to correspondent nodes is
   also possible.  The protection of the Mobility Header for this
   purpose is easy, though one must ensure that the IPsec SA was created
   with appropriate authorization to use the home address referenced in
   the Binding Update.  For instance, a certificate used by IKE to
   create the security association might contain the home address.  A
   future specification may specify how this is done.

A.4.  Dynamically Generated Home Addresses

   A future version of this specification may include functionality that
   allows the generation of new home addresses without requiring pre-
   arranged security associations or certificates even for the new

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A.5.  Remote Home Address Configuration

   The method for initializing a mobile node's home address upon power-
   up or after an extended period of being disconnected from the network
   is beyond the scope of this specification.  Whatever procedure is
   used should result in the mobile node having the same stateless or
   stateful (e.g., DHCPv6) home address autoconfiguration information it
   would have if it were attached to the home network.  Due to the
   possibility that the home network could be renumbered while the
   mobile node is disconnected, a robust mobile node would not rely
   solely on storing these addresses locally.

   Such a mobile node could be initialized by using the following

   1.  Generate a care-of address.

   2.  Query DNS for an anycast address associated with the FQDN of the
       home agent(s).

   3.  Perform home agent address discovery, and select a home agent.

   4.  Configure one home address based on the selected home agent's
       subnet prefix and the interface identifier of the mobile node.

   5.  Create security associations and security policy database entries
       for protecting the traffic between the selected home address and
       home agent.

   6.  Perform a home registration on the selected home agent.

   7.  Perform mobile prefix discovery.

   8.  Make a decision if further home addresses need to be configured.

   This procedure is restricted to those situations where the home
   prefix is 64 bits and the mobile node knows its own interface
   identifier, which is also 64 bits.

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A.6.  Neighbor Discovery Extensions

   Future specifications may improve the efficiency of Neighbor
   Discovery tasks, which could be helpful for fast movements.  One
   factor is currently being looked at: the delays caused by the
   Duplicate Address Detection mechanism.  Currently, Duplicate Address
   Detection needs to be performed for every new care-of address as the
   mobile node moves, and for the mobile node's link-local address on
   every new link.  In particular, the need and the trade-offs of re-
   performing Duplicate Address Detection for the link-local address
   every time the mobile node moves on to new links will need to be
   examined.  Improvements in this area are, however, generally
   applicable and progress independently from the Mobile IPv6

   Future functional improvements may also be relevant for Mobile IPv6
   and other applications.  For instance, mechanisms that would allow
   recovery from a Duplicate Address Detection collision would be useful
   for link-local, care-of, and home addresses.

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Authors' Addresses

   David B. Johnson
   Rice University
   Dept. of Computer Science, MS 132
   6100 Main Street
   Houston  TX 77005-1892


   Charles E. Perkins
   Nokia Research Center
   313 Fairchild Drive
   Mountain View  CA 94043


   Jari Arkko
   02420  Jorvas


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