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rfc:rfc4651

Network Working Group C. Vogt Request for Comments: 4651 Universitaet Karlsruhe (TH) Category: Informational J. Arkko

                                          Ericsson Research NomadicLab
                                                         February 2007
             A Taxonomy and Analysis of Enhancements to
                   Mobile IPv6 Route Optimization

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 IETF Trust (2007).

IESG Note:

 This RFC is a product of the Internet Research Task Force and is not
 a candidate for any level of Internet Standard.  The IRTF publishes
 the results of Internet-related research and development activities.
 These results might not be suitable for deployment.

Abstract

 This document describes and evaluates strategies to enhance Mobile
 IPv6 Route Optimization, on the basis of existing proposals, in order
 to motivate and guide further research in this context.  This
 document is a product of the IP Mobility Optimizations (MobOpts)
 Research Group.

Vogt & Arkko Informational [Page 1] RFC 4651 MIP6 Route Optimization Enhancements February 2007

Table of Contents

 1. Introduction ....................................................3
    1.1. A Note on Public-Key Infrastructures .......................4
    1.2. A Note on Source Address Filtering .........................5
 2. Objectives for Route Optimization Enhancement ...................7
    2.1. Latency Optimizations ......................................8
    2.2. Security Enhancements ......................................8
    2.3. Signaling Optimizations ....................................9
    2.4. Robustness Enhancements ....................................9
 3. Enhancements Toolbox ............................................9
    3.1. IP Address Tests ..........................................10
    3.2. Protected Tunnels .........................................10
    3.3. Optimistic Behavior .......................................11
    3.4. Proactive IP Address Tests ................................11
    3.5. Concurrent Care-of Address Tests ..........................12
    3.6. Diverted Routing ..........................................13
    3.7. Credit-Based Authorization ................................14
    3.8. Heuristic Monitoring ......................................17
    3.9. Crypto-Based Identifiers ..................................18
    3.10. Pre-Configuration ........................................19
    3.11. Semi-Permanent Security Associations .....................20
    3.12. Delegation ...............................................21
    3.13. Mobile Networks ..........................................21
    3.14. Location Privacy .........................................22
 4. Discussion .....................................................22
    4.1. Cross-Layer Interactions ..................................23
    4.2. Experimentation and Measurements ..........................23
    4.3. Future Research ...........................................24
 5. Security Considerations ........................................24
 6. Conclusions ....................................................25
 7. Acknowledgments ................................................25
 8. References .....................................................26
    8.1. Normative References ......................................26
    8.2. Informative References ....................................26

Vogt & Arkko Informational [Page 2] RFC 4651 MIP6 Route Optimization Enhancements February 2007

1. Introduction

 Mobility support for IPv6, or Mobile IPv6, enables mobile nodes to
 migrate active transport connections and application sessions from
 one IPv6 address to another.  The Mobile IPv6 specification, RFC 3775
 [1], introduces a "home agent", which proxies a mobile node at a
 permanent "home address".  A roaming mobile node connects to the home
 agent through a bidirectional tunnel and can so communicate, from its
 local "care-of address", as if it was present at the home address.
 The mobile node keeps the home agent updated on its current care-of
 address via IPsec-protected signaling messages [40].
 In case the correspondent node lacks appropriate mobility support, it
 communicates with the mobile node's home address, and thus all data
 packets are routed via the home agent.  This mode, Bidirectional
 Tunneling, increases packet-propagation delays.  RFC 3775 hence
 defines an additional mode for Route Optimization, which allows peers
 to communicate on the direct path.  It requires that the
 correspondent node can cache a binding between the mobile node's home
 address and current care-of address.  The challenge with Route
 Optimization is that an administrative relationship between the
 mobile node and the correspondent node can generally not be
 presupposed.  So how can the two authenticate and authorize the
 signaling messages that they exchange?
 Mobile IPv6 solves this problem by verifying a routing property of
 the mobile node.  Specifically, the mobile node is checked to be
 reachable at its home address and current care-of address in that it
 must prove the reception of a home and care-of keygen token,
 respectively.  This is called the "return-routability procedure".  It
 takes place right before a mobile node registers a new care-of
 address with a correspondent node and is periodically repeated in
 case the mobile node does not move for a while.
 The advantage of the return-routability procedure is that it is
 lightweight and does not require pre-shared authentication material.
 It also requires no state at the correspondent node.  On the other
 hand, the two reachability tests can lead to a handoff delay
 unacceptable for many real-time or interactive applications such as
 Voice over IP (VoIP) and video conferencing.  Also, the security that
 the return-routability procedure guarantees might not be sufficient
 for security-sensitive applications.  And finally, periodically
 refreshing a registration at a correspondent node implies a hidden
 signaling overhead that may prevent mobile nodes from hibernation
 during times of inactivity.
 Manifold enhancements for Route Optimizations have hence been
 suggested.  This document describes and evaluates various strategies

Vogt & Arkko Informational [Page 3] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 on the basis of existing proposals.  It is meant to provide a
 conceptual framework for further work, which was found to be
 inevitable in the context of Route Optimization.  Many scientists
 volunteered to review this document.  Their names are duly recorded
 in Section 7.  Section 2 analyzes the strengths and weaknesses of
 Route Optimization and identifies potential objectives for
 enhancement.  Different enhancement strategies are discussed, based
 on existing proposals, in Section 3.  Section 4 discusses the
 different approaches and identifies opportunities for further
 research.  Section 5 and Section 6 conclude the document.
 This document represents the consensus of the MobOpts Research Group.
 It has been reviewed by the Research Group members active in the
 specific area of work.  At the request of their chairs, this document
 has been comprehensively reviewed by multiple active contributors to
 the IETF MIP6 Working Group.  At the time of this writing, some of
 the ideas presented in this document have been adopted by the
 Mobility for IP: Performance, Signaling and Handoff Optimization
 (mipshop) Working Group in the IETF.

1.1. A Note on Public-Key Infrastructures

 Mobile IPv6 Route Optimization verifies a mobile node's authenticity
 through a routing property.  An alternative is cryptographic
 authentication, which requires a binding between a node's identity
 and some sort of secret information.  Although some proposals suggest
 installing shared secrets into end nodes when possible (see Section
 3.10), pre-configuration is not an option for general Internet use
 for scalability reasons.  Authentication based on a Public-Key
 Infrastructure (PKI) does not require pair-wise pre-configuration.
 Here, the secret information is the private component of a
 public/private-key pair, and the binding between a node's identity
 and private key exists indirectly through the cryptographic
 properties of public/private-key pairs and a binding between the
 identity and the public key.  An authority trusted by both end nodes
 issues a certificate that effects this latter binding.
 Large-scale use of a PKI, however, was considered unsuitable for
 mobility management due to the following reasons.
 o  There are differing opinions on whether a PKI could scale up to
    hundreds of millions of mobile nodes.  Some people argue they do,
    as there are already examples of certification authorities
    responsible for millions of certificates.  But more important than
    the expected increase in the number of certificates would be a
    shift in application patterns.  Nowadays, public-key cryptography
    is used only for those applications that require strong,
    cryptographic authentication.

Vogt & Arkko Informational [Page 4] RFC 4651 MIP6 Route Optimization Enhancements February 2007

    If it was used for mobility management as well, certificate checks
    would become mandatory for any type of application, leading to
    more checks per user.  Busy servers with mobility support might be
    unwilling to spent the processing resources required for this
    depending on the service they provide.
 o  Revoked certificates are identified on Certificate Revocation
    Lists (CRLs), which correspondent nodes with mobility support
    would have to acquire from certification authorities.  CRLs must
    be kept up to date, requiring periodic downloads.  This and the
    act of checking a certificate against a CRL create overhead that
    some correspondent nodes might be unwilling to spend.
 o  Certificate verification may take some time and hence interrupt
    ongoing applications.  This can be disturbing from the user's
    perspective, especially when Route Optimization starts in the
    middle of a session, or the session is very short-term anyway.
 o  The bigger a PKI grows, the more attractive it becomes as an
    attack target, endangering the Internet as a whole.
 o  There is little experience with using home addresses as
    identifiers in certificates.  Although the home address could
    theoretically be placed into a certificate's Subject Alternate
    Name field, the entities responsible for IP-address assignment and
    certification are usually not the same, and it may not be easy to
    coordinate the two.
 For these reasons, this document does not consider direct
 authentication of mobile nodes based on a PKI.  Nevertheless, it does
 evaluate certificate-based techniques that make the problems
 identified above more tractable (see Section 3.12).

1.2. A Note on Source Address Filtering

 RFC 3775 uses care-of-address tests to probe a mobile node's presence
 at its claimed location.  Alternatively, verification of care-of
 addresses may be based on infrastructure in the mobile node's local
 access network.  For instance, the infrastructure can verify that the
 IP source addresses of all packets leaving the network are correct.
 "Ingress filtering" [38][43] provides this feature to the extent that
 it inspects the prefix of IP source addresses and ensures topological
 correctness.  Network-access providers that use ingress filtering
 normally deploy the technique in their first-hop and site-exit
 routers.  Similarly, ISPs may filter packets originating from a
 downstream network.

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 Ingress filtering may eventually provide a way to replace care-of-
 address tests.  But there are still a number of uncertainties today:
 o  By definition, ingress filtering can prevent source-address
    spoofing only from those networks that do deploy the technique.
    As a consequence, ingress filtering needs to be widely, preferably
    universally, deployed in order to constitute Internet-wide
    protection.  As long as an attacker can get network access without
    filters, all Internet nodes remain vulnerable.
 o  There is little incentive for ISPs to deploy ingress filtering
    other than conscientiousness.  Legal or regulatory prescription as
    well as financial motivation does not exist.  A corrupt ISP might
    even have a financial incentive not to deploy the technique, if
    redirection-based denial-of-service (DoS) attacks using Route
    Optimization ever become possible and are exploited for financial
    gain.  A similar issue was observed with, for example, email spam.
 o  Ingress filtering is most effective, and easiest to configure, at
    the first-hop router.  However, since only prefixes are checked,
    the filters inevitably get less precise the further upstream they
    are enforced.  This issue is inherent in the technique, so the
    best solution is checking packets as close to the originating
    nodes as possible, preferably in the first-hop routers themselves.
 o  A popular implementation of ingress filtering is "Reverse Path
    Forwarding" (RPF).  This technique relies on routes to be
    symmetric, which is oftentimes the case between edge networks and
    ISPs, but far less often between peering ISPs.  Alternatives to
    RPF are either manually configured access lists or dynamic
    approaches that are more relaxed, and thereby less secure, than
    RPF [43].
 o  Another problem with ingress filtering is multi-homing.  When a
    router attempts to forward to one ISP a packet with a source-
    address prefix from another ISP, filters at the second ISP would
    block the packet.  The IETF seeks to find a way around this [39].
    For instance, one could tunnel the packet to the topologically
    correct ISP, or one could allow source-address changes by means of
    a locator-identifier split [45].
 o  Finally, RFC 3775 defines an Alternative Care-of Address option
    that mobile nodes can use to carry a care-of address within a
    Binding Update message outside of the IPv6 header.  Such an
    address is not subject to inspection by ingress filtering and
    would have to be verified through other means [14].

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 Although these problems are expected to get solved eventually, there
 is currently little knowledge on how applicable and deployable, as a
 candidate for care-of-address verification, ingress filtering will
 be.  High investments or administrative hurdles could prevent a
 large, preferably universal deployment of ingress filtering, which
 would hinder Internet-wide protection, as mentioned in the first
 bullet.  For these reasons, this document does not consider ingress
 filtering as a viable alternative to care-of-address tests, although
 things may be different in the future.

2. Objectives for Route Optimization Enhancement

 Wireless environments with frequently moving nodes feature a number
 of salient properties that distinguish them from environments with
 stationary nodes or nodes that move only occasionally.  One important
 aspect is the efficiency of mobility management.  Nodes may not
 bother about a few round-trip times of handoff latency if they do not
 change their point of IP attachment often.  But the negative impact
 that a mobility protocol can have on application performance
 increases with the level of mobility.  Therefore, in order to
 maximize user satisfaction, it is important to reduce the handoff
 latency that the mobility protocol adds to existing delays in other
 places of the network stack.  A related issue is the robustness of
 the mobility protocol, given that temporary outage of mobility
 support can render mobile nodes incapable of continuing to
 communicate.
 Furthermore, the wireless nature of data transmissions makes it
 potentially easier for an attacker to eavesdrop on other nodes' data
 or send data on behalf of other nodes.  While applications can
 usually authenticate and encrypt their payload if need be, similar
 security measures may not be feasible for signaling packets of a
 mobility protocol, in particular if communicating end nodes have no
 pre-existing relationship.
 Given the typically limited bandwidth in a wireless medium, resources
 ought to be spent in an economic matter.  This is especially
 important for the amount of signaling that a mobility protocol
 requires.
 Endeavors to enhance RFC 3775 Route Optimization generally strive for
 reduced handoff latency, higher security, lower signaling overhead,
 or increased protocol robustness.  These objectives are herein
 discussed from a requirements perspective; the technical means to
 reach the objectives is not considered, nor is the feasibility of
 achieving them.

Vogt & Arkko Informational [Page 7] RFC 4651 MIP6 Route Optimization Enhancements February 2007

2.1. Latency Optimizations

 One important objective for improving Route Optimization is to reduce
 handoff latencies.  Assuming that the home-address test dominates the
 care-of-address test in terms of latency, a Mobile IPv6 handoff takes
 one round-trip time between the mobile node and the home agent for
 the home registration, a round-trip time between the mobile node and
 the home agent plus a round-trip time between the home agent and the
 correspondent node for the home-address test, and a one-way time from
 the mobile node to the correspondent node for the propagation of the
 Binding Update message.  The first packet sent to the new care-of
 address requires an additional one-way time to propagate from the
 correspondent node to the mobile node.  The mobile node can resume
 communications right after it has dispatched the Binding Update
 message.  But if it requests a Binding Acknowledgment message from
 the correspondent node, communications are usually delayed until this
 is received.
 These delays are additive and are not subsumed by other delays at the
 IP layer or link layer.  They can cause perceptible quality
 degradations for interactive and real-time applications.  TCP bulk-
 data transfers are likewise affected since long handoff latencies may
 lead to successive retransmission timeouts and degraded throughput.

2.2. Security Enhancements

 The return-routability procedure was designed with the objective to
 provide a level of security that compares to that of today's non-
 mobile Internet [46].  As such, it protects against impersonation,
 denial of service, and redirection-based flooding attacks that would
 not be possible without Route Optimization.  This approach is based
 on an assumption that a mobile Internet cannot become any safer than
 the non-mobile Internet.
 Applications that require a security level higher than what the
 return-routability procedure can provide are generally advised to use
 end-to-end protection such as IPsec or Transport Layer Security
 (TLS).  But even then they are vulnerable to denial of service.  This
 motivates research for stronger Route Optimization security.
 Security enhancements may also become necessary if future
 technological improvements mitigate some of the existing mobility-
 unrelated vulnerabilities.
 One particular issue with Route Optimization is location privacy
 because route-optimized packets carry both home and care-of addresses
 in plaintext.  A standard workaround is to fall back to Bidirectional
 Tunneling when location privacy is needed.  Packets with the care-of
 address are then transferred only between the mobile node and the

Vogt & Arkko Informational [Page 8] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 home agent, where they can be encrypted through IPsec Encapsulating
 Security Payload (ESP) [42].  But even Bidirectional Tunneling
 requires the mobile node to periodically re-establish IPsec security
 associations with the home agent so as to become untraceable through
 Security Parameter Indexes (SPIs).

2.3. Signaling Optimizations

 Route Optimization requires periodic signaling even when the mobile
 node does not move.  The signaling overhead amounts to 7.16 bits per
 second if the mobile node communicates with a stationary node [6].
 It doubles if both peers are mobile.  This overhead may be negligible
 when the nodes communicate, but it can be an issue for mobile nodes
 that are inactive and stay at the same location for a while.  These
 nodes typically prefer to go to standby mode to conserve battery
 power.  Also, the periodic refreshes consume a fraction of the
 wireless bandwidth that one could use more efficiently.
 Optimizations for reduced signaling overhead could mitigate these
 issues.

2.4. Robustness Enhancements

 Route Optimization could conceptually enable continued communications
 during periods of temporary home-agent unavailability.  The protocol
 defined in RFC 3775 does not achieve this independence, however, as
 the home agent plays an active role in the return-routability
 procedure.  Appropriate enhancements could increase the independence
 from the home agent and thus enable robust Route Optimization even in
 the absence of the home agent.

3. Enhancements Toolbox

 A large body of effort has recently gone into improving Mobile IPv6
 Route Optimization.  Some of the proposed techniques are
 modifications to the return-routability procedure, while others
 replace the procedure by alternative mechanisms.  Some of them
 operate end-to-end; others introduce network-side mobility support.
 In most cases, it is the combination of a set of techniques that is
 required to gain a complete -- that is, efficient and secure --
 route-optimization mechanism.

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3.1. IP Address Tests

 RFC 3775 uses IP-address tests to ensure that a mobile node is live
 and on the path to a specific destination address:  The home-address
 test provides evidence that the mobile node is the legitimate owner
 of its home address; the care-of-address test detects spoofed care-of
 addresses and prevents redirection-based flooding attacks.  Both
 tests can be performed in parallel.
 A home-address test should be initiated by the mobile node so that
 the correspondent node can delay state creation until the mobile node
 has authenticated.  The care-of-address test can conceptually be
 initiated by either side.  It originates with the mobile node in RFC
 3775, but with the correspondent node in [16] and [22].  The
 correspondent-node-driven approach suggests itself when
 authentication is done through other means than a home-address test.
 Important advantages of IP-address tests are zero-configurability and
 the independence of ancillary infrastructure.  As a disadvantage,
 IP-address tests can only guarantee that a node is on the path to the
 probed address, not that the node truly owns this address.  This does
 not lead to new security threats, however, because the types of
 attacks that an on-path attacker can do with Route Optimization are
 already possible in the non-mobile Internet [46].

3.2. Protected Tunnels

 RFC 3775 protects certain signaling messages, exchanged between a
 mobile node and its home agent, through an authenticated and
 encrypted tunnel.  This prevents unauthorized nodes on that path,
 including eavesdroppers in the mobile node's wireless access network,
 from listening in on these messages.
 Given that a pre-existing end-to-end security relationship between
 the mobile node and the correspondent node cannot generally be
 assumed, this protection exists only for the mobile node's side.  If
 the correspondent node is immobile, the path between the home agent
 and the correspondent node remains unprotected.  This is a path
 between two stationary nodes, so all types of attacks that a villain
 could wage on this path are already possible in the non-mobile
 Internet.  In case the correspondent node is mobile, it has its own
 home agent, and only the path between the two (stationary) home
 agents remains unprotected.

Vogt & Arkko Informational [Page 10] RFC 4651 MIP6 Route Optimization Enhancements February 2007

3.3. Optimistic Behavior

 Many Mobile IPv6 implementations [29][31] defer a correspondent
 registration until the associated home registration has been
 completed successfully.  In contrast to such "conservative" behavior,
 a more "optimistic" approach is to begin the return-routability
 procedure in parallel with the home registration [52].  Conservative
 behavior avoids a useless return-routability procedure in case the
 home registration fails.  This comes at the cost of additional
 handoff delay when the home registration is successful.  Optimistic
 behavior saves this delay, but the return-routability procedure will
 be in vain should the corresponding home registration be
 unsuccessful.
 While a parallelization of the home registration and the return-
 routability procedure is feasible within the bounds of RFC 3775, the
 specification does not permit mobile nodes to continue with the
 correspondent registration, by sending a Binding Update message to
 the correspondent node, until a Binding Acknowledgment message
 indicating successful home registration has been received.  This is
 usually not a problem because the return-routability procedure is
 likely to take longer than the home registration anyway.  However,
 some optimizations (see Section 3.4) reduce the delay caused by the
 return-routability procedure.  A useful improvement is then to allow
 Binding Update messages to be sent to correspondent nodes even before
 the home registration has been acknowledged.
 The drawback of optimistic behavior is that a lost, reordered, or
 rejected Binding Update message can cause data packets to be
 discarded.  Nevertheless, packet loss would have similar negative
 impacts on conservative approaches, so the mobile node needs to be
 prepared for the possible loss of these packets in any case.

3.4. Proactive IP Address Tests

 The critical handoff phase, during which the mobile node and the
 correspondent node cannot fully communicate, spans the home
 registration and the correspondent registration, including the
 return-routability procedure.  One technique to shorten this phase is
 to accomplish part of the signaling proactively before the handoff.
 In particular, the home-address test can be done in advance without
 violating the specifications of RFC 3775 [52][51].
 In order to have a fresh home keygen token ready for a future
 handoff, the mobile node should initiate a proactive home-address
 test at least once per token lifetime, that is, every 3.5 minutes.
 This does at most double the signaling overhead spent on home-address
 tests given that correspondent registrations must be refreshed every

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 7 minutes even when the mobile node does not move for a while.  An
 optimization is possible where the mobile node's local link layer can
 anticipate handoffs and trigger the home-address test in such a case.
 [6] or [54] reduce the frequency of home-address tests even further.
 Proactive care-of-address tests are possible only if the mobile node
 is capable of attaching to two networks simultaneously.  Dual
 attachment is possible if the link-layer technology enables it with a
 single interface [10], or if the mobile node is endowed with multiple
 interfaces [7].

3.5. Concurrent Care-of Address Tests

 Without the assumption that a mobile node can simultaneously attach
 to multiple networks, proactive care-of-address tests, executed prior
 to handoff, are not an option.  A correspondent node may instead
 authorize a mobile node to defer the care-of-address test until an
 early, tentative binding has been registered [52][51].  This in
 combination with a technique to eliminate the handoff delay of home-
 address tests (see Section 3.4 and Section 3.9) facilitates early
 resumption of bidirectional communications subsequent to handoff.
 The care-of address is called "unverified" during the concurrent
 care-of-address test, and it is said to be "verified" once the
 correspondent node has obtained evidence that the mobile node is
 present at the address.  A tentative binding's lifetime can be
 limited to a few seconds.
 Home-address tests must not be accomplished concurrently, however,
 given that they serve the purpose of authentication.  They guarantee
 that only the legitimate mobile node can create or update a binding
 pertaining to a particular home address.

Vogt & Arkko Informational [Page 12] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 mobile node              home agent          correspondent node
      |                       |                       |
      |                       |                       |
      |--Home Test Init------>|---------------------->|
      |                       |                       |
      |                       |                       |
      |<----------------------|<-----------Home Test--|
      |                       |                       |
      |                                               |
    ~~+~~ handoff                                     |
      |                                               |
      |--Early Binding Update------------------------>| -+-
      |--Care-of Test Init -------------------------->|  |
      |                                               |  |
      |                                               |  | care-of
      |<----------------Early Binding Acknowledgment--|  | address
      |<-------------------------------Care-of Test---|  | unverified
      |                                               |  |
      |                                               |  |
      |--Binding Update------------------------------>| -+-
      |                                               |
      |                                               |
      |<----------------------Binding Acknowledgment--|
      |                                               |
          Figure 1: Concurrent Care-of Address Tests
 Figure 1 illustrates how concurrent care-of-address tests are used in
 [52][51]:  As soon as the mobile node has configured a new care-of
 address after a handoff, it sends to the correspondent node an Early
 Binding Update message.  Only a home keygen token, obtained from a
 proactive home-address test, is required to sign this message.  The
 correspondent node creates a tentative binding for the new,
 unverified care-of address when it receives the Early Binding Update
 message.  This address can be used immediately.  The mobile node
 finally sends a (standard) Binding Update message to the
 correspondent node when the concurrent care-of-address test is
 complete.  Credit-Based Authorization (see Section 3.7) prevents
 misuse of care-of addresses while they are unverified.

3.6. Diverted Routing

 Given that a home registration is faster than a correspondent
 registration in the absence of additional optimizations, the mobile
 node may request its traffic to be routed through the home address
 until a new binding has been set up at the correspondent node
 [52][51].  The performance of such diverted routing depends on the
 propagation properties of the involved routes, however.

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 For packets to be diverted via the home address, signaling is
 necessary with both the home agent and the correspondent node.  The
 home agent must be informed about the new care-of address so that it
 can correctly forward packets intercepted at the home address.  The
 correspondent node continues to send packets to the old care-of
 address until it receives a Binding Update message indicating that
 the current binding is no longer valid and ought to be removed.  This
 request requires authentication through a home-address test in order
 to prevent denial of service by unauthorized nodes.  The test can be
 accomplished in a proactive way (see Section 3.4).
 The mobile node may send packets via the home address as soon as it
 has dispatched the Binding Update message to the home agent.  It may
 send outgoing packets along the direct path once a Binding Update
 message for the new care-of address has been sent to the
 correspondent node.
 It depends on the propagation latency on the end-to-end path via the
 home agent relative to the latency on the direct path for how long
 the correspondent node should continue to send packets to the home
 address.  If the former path is slow, it may be better to queue some
 of the packets until the correspondent registration is complete and
 packets can be sent along the direct route.

3.7. Credit-Based Authorization

 Concurrent care-of-address tests (see Section 3.5) require protection
 against spoofed unverified care-of addresses and redirection-based
 flooding attacks.  Credit-Based Authorization [50] is a technique
 that provides such protection based on the following three
 hypotheses:
 1.  A flooding attacker typically seeks to somehow multiply the
     packets it assembles for the purpose of the attack because
     bandwidth is an ample resource for many attractive victims.
 2.  An attacker can always cause unamplified flooding by generating
     bogus packets itself and sending them to its victim directly.
 3.  Consequently, the additional effort required to set up a
     redirection-based flooding attack pays off for the attacker only
     if amplification can be obtained this way.
 On this basis, rather than eliminating malicious packet redirection
 in the first place, Credit-Based Authorization prevents any
 amplification that can be reached through it.  This is accomplished
 by limiting the data a correspondent node can send to an unverified
 care-of address of a mobile node by the data that the correspondent

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 node has recently received from that mobile node.  (See Section 3.5
 for a definition on when a care-of address is verified and when it is
 unverified.)  A redirection-based flooding attack is thus no more
 attractive than pure direct flooding, where the attacker itself sends
 bogus packets to the victim.  It is actually less attractive given
 that the attacker must keep Mobile IPv6 state to coordinate the
 redirection.
       mobile node           correspondent node
            |                        |
            |                        |
    address |--data----------------->| credit += size(data)
   verified |                        |
            |--data----------------->| credit += size(data)
            |<-----------------data--| don't change credit
            |                        |
    address + address change         |
 unverified |<-----------------data--| credit -= size(data)
            |--data----------------->| credit += size(data)
            |<-----------------data--| credit -= size(data)
            |                        |
            |<-----------------data--| credit -= size(data)
            |                        X credit < size(data)
            |                        |     ==> Do not send!
    address |                        |
   verified |<-----------------data--| don't change credit
            |                        |
       Figure 2: Credit-Based Authorization
 Figure 2 illustrates Credit-Based Authorization for an exemplifying
 exchange of data packets:  The correspondent node measures the bytes
 received from the mobile node.  When the mobile node registers a new
 care-of address, the correspondent node labels this address
 "unverified" and sends packets there as long as the sum of the packet
 sizes does not exceed the measured, received data volume.  A
 concurrent care-of-address test is meanwhile performed.  Once the
 care-of address has been verified, the correspondent node relabels
 the address from "unverified" to "verified".  Packets can then be
 sent to the new care-of address without restrictions.  When
 insufficient credit is left while the care-of address is still
 "unverified", the correspondent node stops sending further packets to
 the address until the verification completes.  The correspondent node
 may drop these packets, direct them to the mobile node's home
 address, or buffer them for later transmission when the care-of
 address is verified.  Figure 2 does not show Mobile IPv6 signaling
 packets.

Vogt & Arkko Informational [Page 15] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 The correspondent node ensures that the mobile node's acquired credit
 gradually decreases over time.  This "aging" prevents the mobile node
 from building up credit over a long time.  A malicious node with a
 slow Internet connection could otherwise provision for a burst of
 redirected packets that does not relate to its own upstream capacity.
 Allocating the mobile node's credit based on the packets that the
 mobile node sends and reducing the credit based on packets that the
 mobile node receives is defined as "Inbound Mode".  (The
 correspondent node is in control of credit allocation, and it
 computes the credit based on inbound packets received from the mobile
 node.)  A nice property of Inbound Mode is that it does not require
 support from the mobile node.  The mobile node neither needs to
 understand that Credit-Based Authorization is effective at the
 correspondent node, nor does it have to have an idea of how much
 credit it has at a particular point in time.
 Inbound Mode works fine with applications that send comparable data
 volumes into both directions.  On the other hand, the mode may
 prevent the mobile node from collecting the amount of credit it needs
 for a handoff when applications with asymmetric traffic patterns are
 in use.  For instance, file transfers and media streaming are
 characterized by high throughput towards the client, typically the
 mobile node, and comparably little throughput towards the serving
 correspondent node.
 An additional "Outbound Mode" was designed to better accommodate
 applications with asymmetric traffic patterns.  In Outbound Mode,
 packets that the correspondent node sends to the mobile node
 determine both, how much the credit increases while the current
 care-of address is verified, and how much the credit shrinks while
 the care-of address is unverified.  This resolves the issue with
 asymmetric traffic patterns.
 The security of Outbound Mode is based on the further hypothesis that
 the mobile node invests comparable effort for packet reception and
 transmission in terms of bandwidth, memory, and processing capacity.
 This justifies why credit, allocated for packets received by the
 mobile node, can be turned into packets that the correspondent node
 sends.  The question is, though, how the correspondent node can
 determine how many of the packets sent to a mobile node are actually
 received and processed by that mobile node.  Relying on transport-
 layer acknowledgments is not an option as such messages can easily be
 faked.  Outbound Mode hence defines its own feedback mechanism,
 Care-of Address Spot Checks, which is robust to spoofing.  The
 correspondent node periodically tags packets that it sends to the
 mobile node with a random, unguessable number, a so-called Spot Check
 Token.  When the mobile node receives a packet with an attached Spot

Vogt & Arkko Informational [Page 16] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 Check Token, it buffers the token until it sends the next packet to
 the correspondent node.  The Spot Check Token is then included in
 this packet.  Upon reception, the correspondent node verifies whether
 the returned Spot Check Token matches a token recently sent to the
 mobile node.  New credit is allocated in proportion to the ratio
 between the number of successfully returned Spot Check Tokens and the
 total number of tokens sent.  This implies that new credit is
 approximately proportional to the fraction of packets that have made
 their way at least up to the mobile node's IP stack.  The preciseness
 of Care-of Address Spot Checks can be traded with overhead through
 the frequency with which packets are tagged with Spot Check Tokens.
 An interesting question is whether Outbound Mode could be misused by
 an attacker with asymmetric Internet connection.  Widespread digital
 subscriber lines (DSL), for example, typically have a much higher
 download rate than upload rate.  The limited upload rate would render
 most denial-of-service attempts through direct flooding meaningless.
 But the attacker could leverage the strong download rate to build up
 credit at one or multiple correspondent nodes.  It could then
 illegitimately spend the credit on a stronger, redirection-based
 flooding attack.  The reason why this has so far not been considered
 an issue is that, in order to accumulate enough credit at the remote
 end, the attacker would first have to expose itself to the same
 packet flood that it could then redirect towards the victim.

3.8. Heuristic Monitoring

 Heuristic approaches to prevent misuse of unverified care-of
 addresses (see Section 3.5) are conceivable as well.  A heuristic,
 implemented at the correspondent node and possibly supplemented by a
 restrictive lifetime limit for tentative bindings, can prevent, or at
 least effectually discourage such misuse.  The challenge here seems
 to be a feasible heuristic:  On one hand, the heuristic must be
 sufficiently rigid to quickly respond to malicious intents at the
 other side.  On the other hand, it should not have a negative impact
 on a fair-minded mobile node's communications.
 Another problem with heuristics is that they are usually reactive.
 The correspondent node can only respond to misbehavior after it
 appeared.  If sanctions are imposed quickly, attacks may simply not
 be worthwhile.  Yet premature measures should be avoided.  One must
 also bear in mind that an attacker may be able to use different home
 addresses, and it is in general impossible for the correspondent node
 to see that the set of home addresses belongs to the same node.  The
 attacker may furthermore exploit multiple correspondent nodes for its
 attack in an attempt to amplify the result.

Vogt & Arkko Informational [Page 17] RFC 4651 MIP6 Route Optimization Enhancements February 2007

3.9. Crypto-Based Identifiers

 A Crypto-Based Identifier (CBID) is an identifier with a strong
 cryptographic binding to the public component of its owner's
 public/private-key pair [33].  This allows the owner to prove its
 claim on the CBID:  It signs a piece of data with its private key and
 sends this to the verifier along with its public key and the
 parameters necessary to recompute the CBID.  The verifier recomputes
 the CBID and checks the owner's knowledge of the corresponding
 private key.
 CBIDs offer three main advantages:  First, spoofing attacks against a
 CBID are much harder than attacks against a non-cryptographic
 identifier like a domain name or a Mobile IPv6 home address.  Though
 an attacker can always create its own CBID, it is unlikely to find a
 public/private-key pair that produces someone else's.  Second, a CBID
 does not depend on a PKI given its inherent binding to the owner's
 public key.  Third, a CBID can be used to bind a public key to an IP
 address, in which case it is called a Cryptographically Generated
 Address (CGA) [44][34][47].  A CGA is syntactically just an ordinary
 IPv6 address.  It has a standard routing prefix and an interface
 identifier generated from a hash on the CGA owner's public key and
 additional parameters.
 Many applications are conceivable where CGAs are advantageous.  In
 Mobile IPv6, CGAs can bind a mobile node's home address to its public
 key [35][5] and so avoid the home-address test in most correspondent
 registrations.  This accelerates the registration process and allows
 the peers to communicate independently of home-agent availability.
 Since only the interface identifier of a CGA is cryptographically
 protected, its network prefix can be spoofed, and flooding attacks
 against networks are still an issue.  An initial home-address test is
 hence required to validate the network prefix even when the home
 address is a CGA.  For the same reason, CGAs are rarely used as
 care-of addresses.
 One limitation of CGAs compared to other types of CBIDs is that the
 cryptographically protected portion is only at most 62 bits long.
 The rest of the address is occupied by a 64-bit network prefix as
 well as the universal/local and individual/group bits.  (The
 specification in [44] further hard-codes a 3-bit security parameter
 into the address, reducing the cryptographically protected portion to
 59 bits.)  A brute-force attack might thus reveal a public/private
 key public/private-key pair that produces a certain CGA.  This
 vulnerability can be contained by including the network prefix in the
 hash computation for the interface identifier so that an attacker, in

Vogt & Arkko Informational [Page 18] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 case it did find the right public/private key public/private-key
 pair, could not form CGAs for multiple networks from it.
 To resolve collisions in generating CGAs, a collision count is part
 of the input to the hash function.  Changing this produces a
 different CGA.  Unfortunately, the collision count also reduces the
 complexity of a brute-force attack against a CGA because it allows
 the same private/public-key pair to be used to generate multiple
 CGAs.  The collision count is therefore limited to a few values only.
 Higher security can be achieved through longer CBIDs.  For example, a
 node's primary identifier in the Host Identity Protocol [21] is a
 128-bit hash on the node's public key.  It is used as an IP-address
 replacement at stack layers above IP.  This CBID is not routable, so
 there needs to be some external localization mechanism if a node
 wants to contact a peer of which it only knows the identifier.

3.10. Pre-Configuration

 Where mobile and correspondent nodes can be pre-configured with a
 shared key, bound to the mobile node's home address, authentication
 through a home-address test can be replaced by a cryptographic
 mechanism.  This has three advantages.  First, cryptography allows
 for stronger authentication than address tests.  Second, strong
 authentication facilitates binding lifetimes longer than the 7-
 minute limit that RFC 3775 defines for correspondent registrations.
 Third, handoff delays are usually shorter with cryptographic
 approaches because the round-trips of the home-address test can be
 spared.  The disadvantage of pre-configuration is its limited
 applicability.
 Two proposals for pre-configuration are currently under discussion
 within the IETF.  [25] endows mobile nodes with the information they
 need to compute home and care-of keygen tokens themselves rather than
 having to obtain them through the return-routability procedure. [15]
 uses the Internet Key Exchange protocol to establish an IPsec
 security association between the peers.
 From a technical standpoint, pre-configuration can only replace a
 home-address test.  A test of the care-of address is still necessary
 to verify the mobile node's presence at that address.  The problem is
 circumvented in [25] by postulating that the correspondent node has
 sufficient trust in the mobile node to believe that the care-of
 address is correct.  This assumption discourages the use of pre-
 configuration in scenarios where such trust is unavailable, however.
 For example, a mobile-phone operator may be able to configure
 subscribers with secret keys for authorization to a particular
 service, but it may not be able to vouch that all subscribers use

Vogt & Arkko Informational [Page 19] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 this service in a responsible manner.  And even if users are
 trustworthy, their mobile nodes may become infected with malware and
 start behaving unreliably.
 Another way to avoid care-of-address verification is to rely on
 access networks to filter out packets with incorrect IP source
 addresses [38][43].  This approach is taken in [15].  The problem
 with local filtering is that it can only protect a network from
 becoming the source of an attack, not from falling victim to an
 attack.  The technique is hence potentially unreliable unless
 deployed in access networks worldwide (see Section 1.2).
 Care-of-address tests facilitate the use of pre-configuration in
 spite of lacking trust relationships or the existence of access
 networks without local filtering techniques.  For increased
 performance, concurrent care-of-address tests can be used in
 combination with Credit-Based Authorization or heuristic monitoring.

3.11. Semi-Permanent Security Associations

 A compromise between the return-routability procedure and pre-
 configuration are semi-permanent security associations.  A semi-
 permanent security association is established between a mobile node
 and a correspondent node upon first contact, and it is used to
 authenticate the mobile node during subsequent correspondent
 registrations.  Semi-permanent security associations eliminate the
 need for periodic home-address tests and permit correspondent
 registrations with lifetimes longer than the 7-minute limit specified
 in RFC 3775.
 It is important to verify a mobile node's home address before a
 security association is bound to it.  An impersonator could otherwise
 create a security association for a victim's IP address and then
 redirect the victim's traffic at will until the security association
 expires.  An initial home-address test mitigates this vulnerability
 because it requires the attacker to be on the path between the victim
 and the victim's peer at least while the security association is
 being established.  Stronger security can be obtained through
 cryptographically generated home addresses (see Section 3.9).
 Semi-permanent security associations alone provide no verification of
 care-of addresses and must therefore be supplemented by care-of-
 address tests.  These may be performed concurrently for reduced
 handoff delays.  Semi-permanent security associations were first
 developed in [8] where they were called "purpose-built keys".

Vogt & Arkko Informational [Page 20] RFC 4651 MIP6 Route Optimization Enhancements February 2007

3.12. Delegation

 Section 1.1 lists numerous problems of PKIs with respect to
 authentication of mobile nodes.  These problems become more
 tractable, however, if correspondent nodes authenticate home agents
 rather than mobile nodes, and the home agents vouch for the
 authenticity and trustworthiness of the mobile nodes [37].  Such
 delegation of responsibilities solves the scalability issue with PKIs
 given that home agents can be expected to be much less numerous than
 mobile nodes.  Certificate revocation becomes less delicate as well
 because home agents are commonly administrated by a mobility provider
 and should as such be more accountable than mobile nodes.
 Another advantage of delegation is that it avoids public-key
 computations at mobile nodes.  On the other hand, the processing
 overhead at correspondent nodes increases.  This may or may not be an
 issue depending on resources available at the correspondent node
 relative to the services that the correspondent node provides.  The
 correspondent node may also be mobile itself, in which case
 cryptographic operations would be problematic.  Furthermore, the
 increased overhead implies a higher risk to resource-exhaustion
 attacks.

3.13. Mobile Networks

 Mobile nodes may move as a group and attach to the Internet via a
 "mobile router" that stays with the group.  This happens, for
 example, in trains or aircraft where passengers communicate via a
 local wireless network that is globally interconnected through a
 satellite link.
 It is straightforward to support such network mobility [41] with a
 single home agent and a tunnel between the mobile router and this
 home agent.  The mobile nodes themselves then do not have to be
 mobility-aware.  However, Route Optimization for moving networks
 [36][26][27][55] is more complicated.  One possibility is to have the
 mobile router handle Route Optimization on behalf of the mobile
 nodes.  This requires the mobile router to modify incoming and
 outgoing packets such that they can be routed on the direct path
 between the end nodes.  The mobile router would also have to perform
 Mobile IPv6 signaling on behalf of the mobile nodes.  Similarly, a
 network of correspondent nodes can communicate with mobile nodes,
 through a "correspondent router", in a route-optimized way without
 providing mobility support themselves.

Vogt & Arkko Informational [Page 21] RFC 4651 MIP6 Route Optimization Enhancements February 2007

3.14. Location Privacy

 RFC 3775 fails to conceal a mobile node's current position as route-
 optimized packets always carry both home and care-of addresses.  Both
 the correspondent node and a third party can therefore track the
 mobile node's whereabouts.  A workaround is to fall back to
 bidirectional tunneling where location privacy is needed.  Packets
 carrying the mobile node's care-of address are thus only transferred
 between the mobile node and the home agent, where they can be
 encrypted through IPsec ESP [42].  But even then should the mobile
 node periodically re-establish its IPsec security associations so as
 to become untraceable through its SPIs.  Early efforts on location
 privacy in Route Optimization include [17][13][24][30].

4. Discussion

 Common to the proposals discussed in Section 3 is that all of them
 affect a trade-off between effectiveness, on one hand, and economical
 deployability, administrative overhead, and wide applicability, on
 the other.  Effectiveness may be equated with low latency, strong
 security, reduced signaling, or increased robustness.  Economy
 implies no, or only moderate requirements in terms of hardware
 upgrades and software modifications.  Administrative overhead relates
 to the amount of manual configuration and intervention that a
 technique needs.
 The standard return-routability procedure avoids costly pre-
 configuration or new network entities.  This minimizes both
 deployment investments as well as administrative expenses.  Variants
 with optimistic behavior and proactive or concurrent IP-address tests
 have these advantages as well.  CBIDs allow for public-key
 authentication without a PKI.  They constitute a more secure
 alternative to home-address tests and are as such most effective when
 combined with concurrent reachability verification.  CBID-based
 authentication may require nodes to be programmed with a mapping
 between human-readable identifiers and the corresponding CBIDs.
 Pre-configuration is another approach to avoid home-address tests.
 It does without computationally expensive public-key algorithms, but
 requires pair-wise credentials and, therefore, administrative
 maintenance.  Where suitable infrastructure is available, end nodes
 may delegate authentication and encryption tasks to trusted network
 entities which, in turn, vouch for the end nodes.  Delegation could
 resurrect the use of certificates for the purpose of mobility
 support.  But it introduces a dependency on the delegatees, adds the
 provisioning costs for new network entities, and is likely to be
 limited to communities of authorized nodes.

Vogt & Arkko Informational [Page 22] RFC 4651 MIP6 Route Optimization Enhancements February 2007

4.1. Cross-Layer Interactions

 The performance of Route Optimization, as evaluated in this document,
 should be put into perspective of handoff-related activities in other
 parts of the network stack.  These include link-layer attachment
 procedures; link-layer security mechanisms such as negotiation,
 authentication, and key agreement; as well as IPv6 router discovery,
 address configuration, and movement detection.  A complete network
 attachment in a typical IEEE 802.11 commercial deployment requires
 over twenty link- and IP-layer messages.  Current protocol stacks
 also have a number of limitations in addition to long attachment
 delays, such as denial-of-service vulnerabilities, difficulties in
 trusting a set of access nodes distributed to physically insecure
 locations, or the inability to retrieve sufficient information for
 making a handoff decision [2].
 A number of proposals have been put forth to improve handoff
 performance on different parts of the network stack, mostly focusing
 on handoff performance.  These include link-layer parameter tuning
 [49] and network-access authentication [18][2][32], as well as IPv6
 router discovery [11][12], address configuration [23], and movement
 detection [19][20].  It is uncertain how far this optimization can be
 taken by only looking at the different parts individually.  An
 integrated approach may eventually become necessary [4][53].

4.2. Experimentation and Measurements

 The number and diversity of mobility-related activities within a
 typical network stack oftentimes render theoretical analyses
 insufficient and call for additional, extensive experimentation or
 simulation.  The following is a non-exhaustive list of areas where
 practical experience is likely to yield valuable insight.
 o  Conception of a set of standard scenarios that can be used as a
    reference for comparable measurements and experimentation.
    Ideally, such standard scenarios ought to be derived from real-
    world environments, and they should include all features that
    would likely be needed in a commercial deployment.  These features
    include link-layer access control, for instance.
 o  Measurements of the performance impacts that existing enhancement
    proposals have on the different parts of the stack.
 o  Comparisons of different implementations that are based on the
    same specification.  For instance, it would be valuable to know
    how much implementations differ with regards to the use of
    parallelism that RFC 3775 allows in home and correspondent
    registrations.

Vogt & Arkko Informational [Page 23] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 o  Measurements of the impact that network conditions such as packet
    loss can have on existing and new optimizations.
 o  Statistical data collection on the behavior of mobile nodes in
    different networks.  Several Route Optimization techniques behave
    differently depending on the degree of mobility.
 o  Measurements of the performance that Route Optimization schemes
    show under different application scenarios, such as the use of
    applications with symmetric vs. asymmetric traffic patterns.

4.3. Future Research

 Future research that goes beyond the techniques discussed in this
 document may consider the following items.
 o  Local mobility support or local route-repair mechanisms that do
    not require expensive configuration.  This includes
    infrastructure-based Route Optimization like [48].
 o  Care-of-address verification mechanisms that are based on Secure
    Neighbor Discovery.
 o  The introduction of optimizations developed in the context of
    Mobile IPv6 to other mobility protocols, such as the Host Identity
    Protocol, the Stream Control Transmission Protocol, the Datagram
    Congestion Control Protocol, or link-layer mobility solutions.
 o  The extension of the developed mobility techniques to full multi-
    addressing, including multi-homing.
 o  Further strategies that are based on "asymmetric cost wars" [3],
    such as Credit-Based Authorization.
 o  Integrated techniques taking into account both link- and IP-layer
    mobility tasks.

5. Security Considerations

 Standard Route Optimization enables mobile nodes to redirect IP
 packets at a remote peer from one IP address to another IP address.
 This ability introduces new security issues, which are explained and
 discussed in depth in [46].  The alternative Route Optimization
 techniques described in this document may introduce new security
 threats that go beyond those identified in [46].  Where such new
 threats exist, they are discussed and analyzed along with the
 description of the respective technique in Section 3.

Vogt & Arkko Informational [Page 24] RFC 4651 MIP6 Route Optimization Enhancements February 2007

6. Conclusions

 Mobile IPv6 Route Optimization reduces packet-propagation latencies
 so as to facilitate interactive and real-time applications in mobile
 environments.  Unfortunately, the end-to-end protocol's high handoff
 latencies hinder exactly these applications.  A large body of effort
 has therefore recently been dedicated to Route Optimization
 improvements.  Some of the proposed techniques operate on an end-to-
 end basis, others require new or extended infrastructure in the
 network; some need pre-configuration, others are zero-configurable.
 This document has compared and evaluated the different strategies
 based on a selected set of enhancement proposals.  It stands out that
 all proposals make a trade-off between effectiveness, on one hand --
 be it in terms of reduced handoff latency, increased security, or
 lower signaling overhead -- and pre-configuration costs or requisite
 network upgrades, on the other.  An optimization's investment
 requirements, in turn, are in relation to its suitability for
 widespread deployment.
 However, the real-life performance of end-to-end mobility does not
 only depend on enhancements of Route Optimization, but ultimately on
 all parts of the protocol stack [2].  Related optimization endeavors
 are in fact gaining momentum, and a comprehensive approach towards
 Route Optimization must incorporate the most suitable solutions
 amongst them [4].  Whichever proposals will eventually reach a
 maturity level sufficient for standardization, any effort should be
 expended to arrive at that point within the foreseeable future.
 Route Optimization requires support from both peers and depends on a
 solid basis of installed implementations in correspondent nodes.
 This should hence be included in emerging IPv6 stacks early on.
 Although IPv6 deployment is yet far away from becoming widespread,
 the sooner efficient Route Optimization will be available, the more
 likely that it will in the end be ubiquitously supported.

7. Acknowledgments

 This document was thoroughly reviewed, in alphabetical order, by
 Samita Chakrabarti, Francis Dupont, Thierry Ernst, Gerardo Giaretta,
 James Kempf, Rajeev Koodli, Gabriel Montenegro, Vidya Narayanan, and
 Fan Zhao.  The authors wish to thank these folks for their valuable
 comments and suggestions.

Vogt & Arkko Informational [Page 25] RFC 4651 MIP6 Route Optimization Enhancements February 2007

8. References

8.1. Normative References

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

8.2. Informative References

 [2]   Alimian, A. and B. Aboba, "Analysis of Roaming Techniques",
       IEEE Contribution 802.11-04/0377r1, March 2004.
 [3]   Arkko, J. and P. Nikander, "Weak Authentication: How to
       Authenticate Unknown Principals without Trusted Parties",
       Proceedings of Security Protocols Workshop 2002, Cambridge, UK,
       April 2002.
 [4]   Arkko, J., Eronen, P., Nikander, P., and V. Torvinen, "Secure
       and Efficient Network Access", Proceedings of the DIMACS
       Workshop on Mobile and Wireless Security, November 2004.
 [5]   Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
       Optimization for Mobile IPv6", Work in Progress, August 2006.
 [6]   Arkko, J. and C. Vogt, "Credit-Based Authorization for Binding
       Lifetime Extension", Work in Progress, May 2004.
 [7]   Bahl, P., Adya, A., Padhye, J., and A. Walman, "Reconsidering
       Wireless Systems With Multiple Radios", ACM SIGCOMM Computer
       Communication Review, ACM Press, Vol. 34, No. 5, October 2004.
 [8]   Bradner, S., Mankin, A., and J. Schiller, "A Framework for
       Purpose-Built Keys (PBK)", Work in Progress, June 2003.
 [9]   Castellucia, C., Montenegro, G., Laganier, J., and C. Neumann,
       "Hindering Eavesdropping via IPv6 Opportunistic Encryption",
       Proceedings of the European Symposium on Research in Computer
       Security, Lecture Notes in Computer Science, Springer-Verlag,
       September 2004.
 [10]  Chandra, R., Bahl, P., and P. Bahl, "MultiNet: Connecting to
       Multiple IEEE 802.11 Networks Using a Single Wireless Card",
       Proceedings of the IEEE INFOCOM, Vol. 2, August 2004.
 [11]  Daley, G., Pentland, B., and R. Nelson, "Effects of Fast
       Routers Advertisement on Mobile IPv6 Handovers", Proceedings of
       the IEEE International Symposium on Computers and
       Communication, Vol. 1, June 2003.

Vogt & Arkko Informational [Page 26] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 [12]  Daley, G., Pentland, B., and R. Nelson, "Movement Detection
       Optimizations in Mobile IPv6", Proceedings of the IEEE
       International Conference on Networks, September 2003.
 [13]  Daley, G., "Location Privacy and Mobile IPv6", Work in
       Progress, January 2004.
 [14]  Dupont, F., "A Note about 3rd Party Bombing in Mobile IPv6",
       Work in Progress, July 2006.
 [15]  Dupont, F. and J. Combes, "Using IPsec between Mobile and
       Correspondent IPv6 Nodes", Work in Progress, August 2004.
 [16]  Dupont, F. and J. Combes, "Care-of Address Test for MIPv6 using
       a State Cookie", Work in Progress, July 2006.
 [17]  Haddad, W., Nordmark, E., Dupont, F., Bagnulo, M., and B.
       Patil, "Privacy for Mobile and Multi-homed Nodes: MoMiPriv
       Problem Statement", Work in Progress, June 2006.
 [18]  "IEEE Standard for Local and Metropolitan Area Networks: Port-
       Based Network Access Control", IEEE Standard 802.1X, December
       2004.
 [19]  Choi, J. and E. Nordmark, "DNA with Unmodified Routers: Prefix
       List Based Approach", Work in Progress, January 2006.
 [20]  Narayanan, S., Ed., "Detecting Network Attachment in IPv6
       Networks (DNAv6)", Work in Progress, October 2006.
 [21]  Moskowitz, R., Nikander, P., Jokela, Ed., P., and T. Henderson,
       "Host Identity Protocol", Work in Progress, June 2006.
 [22]  Henderson, T., Ed., "End-Host Mobility and Multihoming with the
       Host Identity Protocol", Work in Progress, June 2006.
 [23]  Moore, N., "Optimistic Duplicate Address Detection (DAD) for
       IPv6", RFC 4429, April 2006.
 [24]  Koodli, R., "IP Address Location Privacy and Mobile IPv6:
       Problem Statement", Work in Progress, October 2006.
 [25]  Perkins, C., "Securing Mobile IPv6 Route Optimization Using a
       Static Shared Key", RFC 4449, June 2006.
 [26]  Ng, C., Thubert, P., Watari, M., and F. Zhao, "Network Mobility
       Route Optimization Problem Statement", Work in Progress,
       September 2006.

Vogt & Arkko Informational [Page 27] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 [27]  Ng, C., Zhao, F., Watari, M., and P. Thubert, "Network Mobility
       Route Optimization Solution Space Analysis", Work in Progress,
       September 2006.
 [28]  Arbaugh, W. and B. Aboba, "Handoff Extension to RADIUS", Work
       in Progress, October 2003.
 [29]  "Kame-Shisa", Mobile IPv6 for FreeBSD.
 [30]  Koodli, R., Devarapalli, V., Flinck, H., and C. Perkins,
       "Solutions for IP Address Location Privacy in the Presence of
       IP Mobility", Work in Progress, February 2005.
 [31]  Nuorvala, V., Petander, H., and A. Tuominen, "Mobile IPv6 for
       Linux (MIPL)".
 [32]  Mishra, A., Shin, M., Petroni Jr., N., Clancy, C., and W.
       Arbaugh, "Proactive Key Distribution Using Neighbor Graphs",
       IEEE Wireless Communications, Vol. 11, No. 1, February 2004.
 [33]  Montenegro, G. and Claude. Castelluccia, "Crypto-Based
       Identifiers (CBIDs): Concepts and Applications", ACM
       Transactions on Information and System Security Vol.7, No. 1,
       February 2004.
 [34]  Nikander, P., "Denial-of-Service, Address Ownership, and Early
       Authentication in the IPv6 World", Revised papers from the
       International Workshop on Security Protocols, Springer-Verlag,
       April 2002.
 [35]  O'Shea, G. and M. Roe, "Child-proof Authentication for MIPv6",
       ACM SIGCOMM Computer Communication Review, April 2001.
 [36]  Perera, E., Sivaraman, V., and A. Seneviratne, "Survey on
       Network Mobility Support", ACM SIGCOMM Computer Communication
       Review, Vol. 8, No. 2, ACM Press, April 2004.
  [37]  Bao, F., Deng, R., Qiu, Y., and J. Zhou, "Certificate-
       basedBinding Update Protocol (CBU)", Work in Progress, March
       2005.
 [38]  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.
 [39]  Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
       Multihoming Architectures", RFC 3582, August 2003.

Vogt & Arkko Informational [Page 28] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 [40]  Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
       Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
       Agents", RFC 3776, June 2004.
 [41]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
       "Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
       January 2005.
 [42]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
       December 2005.
 [43]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
       Networks", BCP 84, RFC 3704, March 2004.
 [44]  Aura, T., "Cryptographically Generated Addresses (CGA)", RFC
       3972, March 2005.
 [45]  Huston, G., "Architectural Approaches to Multi-homing for
       IPv6", RFC 4177, September 2005.
 [46]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
       Nordmark, "Mobile IP Version 6 Route Optimization Security
       Design Background", RFC 4225, December 2005.
 [47]  Roe, M., Aura, T., O'Shea, G., and J. Arkko, "Authentication of
       Mobile IPv6 Binding Updates and Acknowledgments", Work in
       Progress, February 2002.
 [48]  Vadali, R., Li, J., Wu, Y., and G. Cao, "Agent-Based Route
       Optimization for Mobile IP", Proceedings of the IEEE Vehicular
       Technology Conference, October 2001.
 [49]  Velayos, H. and G. Karlsson, "Techniques to Reduce IEEE 802.11b
       MAC Layer Handoff Time", Laboratory for Communication Networks,
       KTH, Royal Institute of Technology, Stockholm, Sweden, TRITA-
       IMIT-LCN R 03:02, April 2003.
 [50]  Vogt, C., "Credit-Based Authorization for Concurrent IP-Address
       Tests", Proceedings of the IST Mobile and Wireless
       Communications Summit, June 2005.
 [51]  Vogt, C., Bless, R., Doll, M., and T. Kuefner, "Early Binding
       Updates for Mobile IPv6", Proceedings of the IEEE Wireless
       Communications and Networking Conference, IEEE, Vol. 3, March
       2005.

Vogt & Arkko Informational [Page 29] RFC 4651 MIP6 Route Optimization Enhancements February 2007

 [52]  Vogt, C. and M. Doll, "Efficient End-to-End Mobility Support in
       IPv6", Proceedings of the IEEE Wireless Communications and
       Networking Conference, April 2006.
 [53]  Vogt, C., "A Comprehensive Delay Analysis for Reactive and
       Proactive Handoffs with Mobile IPv6 Route Optimization",
       Institute of Telematics, Universitaet Karlsruhe (TH),
       Karlsruhe, Germany, TM-2006-1, January 2006.
 [54]  Zhao, F., Wu, F., and S. Jung, "Extensions to Return
       Routability Test in MIP6", Work in Progress, February 2005.
 [55]  Calderon, M., Bernardos, C., Bagnulo, M., Soto, I., and A. de
       la Oliva, "Design and Experimental Evaluation of a Route
       Optimisation Solution for NEMO", IEEE Journal on Selected Areas
       in Communications, Vol. 24, No. 9, September 2006.

Authors' Addresses

 Christian Vogt
 Institute of Telematics
 Universitaet Karlsruhe (TH)
 P.O. Box 6980
 76128 Karlsruhe
 Germany
 EMail: chvogt@tm.uka.de
 Jari Arkko
 Ericsson Research NomadicLab
 FI-02420 Jorvas
 Finland
 EMail: jari.arkko@ericsson.com

Vogt & Arkko Informational [Page 30] RFC 4651 MIP6 Route Optimization Enhancements February 2007

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Vogt & Arkko Informational [Page 31]

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