Network Working Group J. Arkko
Internet-Draft Ericsson Research NomadicLab
Expires: July 30, 2005 C. Vogt
University of Karlsruhe
January 26, 2005
A Taxonomy and Analysis of Enhancements to Mobile IPv6 Route
Optimization
draft-irtf-mobopts-ro-enhancements-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
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Abstract
The Mobile IPv6 protocol favors sending packets via the minimum
routing path between a mobile node and its correspondent node over
sending them through a home agent. This feature is called route
optimization. Route optimization requires authentication and
authorization of initially unacquainted and untrusted parties--a
requirement that was rather new to the Internet community at the time
Mobile IPv6 was designed. To solve the problem, the so-called
return-routability procedure was built into Mobile IPv6. It lets the
mobile node retrieve from the correspondent node two cryptographic
tokens, which the mobile node can use to authenticate itself and
prove its presence at a claimed new location after movement.
Recently, a number of improvements or optional alternatives have been
suggested to the standard procedure. Many of these improvements
attempt further reduction of signaling messages and latency, but
other improvements such as better security have also been suggested.
This paper summarizes the goals for enhancements to
route-optimization, discusses the security threats that such
enhancements must consider, and categorizes the techniques that one
can use for optimization. The paper highlights the key ideas of
various recent proposals, and it evaluates the performance gain that
such proposals can yield. It also discusses how significant
enhancements to Mobile IPv6 are compared to ongoing optimization work
in other parts of the network stack. Finally, the paper identifies
needs for additional research.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Mobility-Related Security Threats . . . . . . . . . . . . . 6
2.1 Impersonation Attacks . . . . . . . . . . . . . . . . . . 7
2.2 Resource-Exhaustion Attacks . . . . . . . . . . . . . . . 8
2.3 Flooding Attacks . . . . . . . . . . . . . . . . . . . . . 8
3. Mobile IPv6 Route Optimization . . . . . . . . . . . . . . . 10
3.1 Registration Procedure . . . . . . . . . . . . . . . . . . 10
3.2 Goals and Assumptions . . . . . . . . . . . . . . . . . . 13
3.3 Security Analysis . . . . . . . . . . . . . . . . . . . . 16
4. Objectives for Enhancement . . . . . . . . . . . . . . . . . 17
4.1 Latency Optimizations . . . . . . . . . . . . . . . . . . 17
4.2 Signaling Optimizations . . . . . . . . . . . . . . . . . 18
4.3 Security Enhancements . . . . . . . . . . . . . . . . . . 19
4.4 Applicability Enhancements . . . . . . . . . . . . . . . . 20
5. Enhancements Toolbox . . . . . . . . . . . . . . . . . . . . 20
5.1 IP-Address Tests . . . . . . . . . . . . . . . . . . . . . 21
5.2 Protected Tunnels . . . . . . . . . . . . . . . . . . . . 21
5.3 Optimistic Behavior . . . . . . . . . . . . . . . . . . . 22
5.4 Proactive IP-Address Tests . . . . . . . . . . . . . . . . 22
5.5 Concurrent IP-Address Tests . . . . . . . . . . . . . . . 23
5.6 Diverted Routing . . . . . . . . . . . . . . . . . . . . . 24
5.7 Credit-Based Authorization . . . . . . . . . . . . . . . . 25
5.8 Heuristic Monitoring . . . . . . . . . . . . . . . . . . . 27
5.9 Cryptographically Bound Identifiers . . . . . . . . . . . 28
5.10 Pre-Configuration . . . . . . . . . . . . . . . . . . . 29
5.11 Semi-Permanent Security Associations . . . . . . . . . . 30
5.12 Infrastructure . . . . . . . . . . . . . . . . . . . . . 31
5.13 Local Mobility . . . . . . . . . . . . . . . . . . . . . 32
5.14 Local Repair . . . . . . . . . . . . . . . . . . . . . . 33
5.15 Assisted Auto-Configuration . . . . . . . . . . . . . . 33
5.16 Processing Improvements . . . . . . . . . . . . . . . . 34
5.17 Delegation . . . . . . . . . . . . . . . . . . . . . . . 34
6. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6.1 Categorization of Techniques . . . . . . . . . . . . . . . 34
6.2 Evaluation of Recent Proposals . . . . . . . . . . . . . . 35
6.3 Future Research . . . . . . . . . . . . . . . . . . . . . 41
7. Security Considerations . . . . . . . . . . . . . . . . . . 44
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 44
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 50
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50
Intellectual Property and Copyright Statements . . . . . . . 51
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1. Introduction
An IP address traditionally combines identification and location
semantics. The address prefix locates a node's point of network
attachment. It is used by routers to forward IP packets towards the
correct destination. At the same time, existing transport protocols
and applications, commonly termed "upper layers", use the IP address
as part of a session identification. This naturally rules out
mobility: Whenever a mobile node moves from one IP-attachment point
to another, its IP address must change to reflect the new location.
The new "identity", however, causes sessions at upper layers to
abort.
Protocol designers thus had to decide whether to change transport
protocols and applications, or to come up with a new IP-layer
protocol that could separate location from identification
functionality in a way transparent to upper layers. Due to the
prevalence of TCP and the significant base of existing applications,
most people opted for the latter approach. Mobile IPv6 [29], its
IPv4 counterpart [28], and the Host Identity Protocol [9] are three
dominant mobility-management protocols that the IETF has developed to
facilitate the continued use of existing transport protocols and
applications in an Internet with mobility support. This document
focuses on Mobile IPv6.
Mobile IPv6 uses two IP addresses per mobile node in an attempt to
separate location semantics from identification semantics: a
transient "care-of address" is used for the purpose of routing. It
is re-configured whenever the mobile node moves to a new
IP-attachment point. A static "home address" is configured with the
network prefix from a non-mobile "home agent's" network. The home
address doesn't change when the mobile node moves, and it can be used
for session identification at upper layers.
The mobile node keeps the home agent up to date about its current
care-of address. In the event that packets are sent to the mobile
node's home address, the home agent captures them and tunnels them to
the mobile node's care-of address. In the opposite direction, the
mobile node may tunnel packets to the home agent who, in turn,
decapsulates them and forwards them on to the correspondent node.
This behavior was termed "bidirectional tunneling". It works fine
even if the correspondent node is unaware of its peer being mobile.
The correspondent node can just use the home address, and the home
agent will take care that packets find their way to the mobile node's
actual location. Obviously, this entails a lot of routing overhead
in many common scenarios.
For better routing efficiency, Mobile IPv6 defines a second mode,
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"route optimization", that allows two nodes to directly communicate.
Route optimization requires the correspondent node to be aware of the
mobile node's current care-of address. The mobile node informs the
correspondent node whenever its care-of address changes.
All signaling between a mobile node and its home agent is
authenticated, and optionally encrypted, through IPsec. The IPsec
security associations can either be manually configured into the
nodes, or they can be dynamically derived through IKE. The mobile
node and home agent must also be configured with the material they
need to identify themselves, and the home agent must be able to
authorize a mobile node to use a particular home address.
Preconfiguration of home agents and mobile nodes requires
administrative labor, but it is doable, because the association
between a mobile node and its home agent, or set of potential home
agents, is typically known in advance. On the other hand, when route
optimization is used between an arbitrary pair of nodes, there is
generally no relationship between the two nodes prior to
communication. Empowering a node--not necessarily a mobile one--to
redirect packets from one IP address to another hence poses two
questions:
o When the correspondent node receives a command to redirect a
mobile node's packets, how can the correspondent node be sure that
it is the legitimate mobile node, rather than a malicious third
node, which has send this command?
o How can the correspondent node rely on the mobile node actually
being present at the IP address to which packets are to be
redirected?
The first question identifies the need for a mobile node to
authenticate itself during a correspondent registration. Without
such authentication, a malicious node could interfere with a packet
flow of another node, redirecting the flow to its own location for
inspection purposes, or redirecting it to a random IP address for the
purpose of denial of service against the legitimate recipient. The
second question refers to spoofed care-of addresses: Probing a mobile
node's presence at a care-of address is important to prevent
malicious parties to redirect packets to other nodes that neither
expect nor want those packets.
A variety of approaches have been proposed to solve the
above-mentioned issues for the case of route optimization. People
finally elected the "return-routability procedure" as a default
mechanism for Mobile IPv6. The return-routability procedure delivers
a pair of secret tokens to a mobile node's home and care-of
addresses. The mobile node needs these tokens to prove that it is
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the legitimate owner of the home address and that it is reachable at
the care-of address. (Actually, the return-routability procedure is
less strict: It only determines whether a node is on the path towards
the two addresses, rather than that it actually holds the two
addresses. This is a compromise that the procedure accepts.)
The return-routability procedure is run right before a mobile node
registers a new care-of address with a correspondent node. It is
also run periodically in case the mobile node does not move for a
while. The advantage of the return-routability procedure is that it
is lightway and does not require any sort of pre-shared
authentication material. Moreover, it can be implemented in a
stateless way at the correspondent node's side. On the other hand,
the return-routability procedure usually consumes one round-trip
time, which comes into addition to the rest of any pending
registration. This can lead to a handover delay unacceptable for
many real-time or interactive applications like Voice over IP (VoIP)
and audio or video streaming. Also, the periodic repetitions imply a
hidden signaling overhead that may interfere with mobile nodes who
intend to sleep during times of inactivity. Finally, the security
level of the return-routability procedure can be increased. It
limits vulnerabilities to attackers that are on the path from the
correspondent node to the mobile node or to the home agent. The
residual vulnerabilities are similar to those that exist anyway in an
Internet without mobility support. But still, mechanisms that use
stronger, possibly cryptographic authentication can provide a higher
level of security than the return-routability procedure does.
This paper describes and classifies strategies that can enhance or
optimize Mobile IPv6 route optimization. Following this
introduction, Section 2 discusses which new security threats
mobility-management protocols need to take into account. Section 3
explains the current route-optimization protocol, identifies the
goals and assumptions based on which it was developed, and briefly
analyzes its security properties. A number of potential goals for
enhancements (such as reducing latency) are discussed in Section 4.
Section 5 reviews techniques that can be used to enhance or optimize
Mobile IPv6 route optimization. Section 6 discusses how these
techniques are applied in existing enhancement and optimization
proposals, evaluates some of these proposals, and identifies
opportunities for further research. The paper concludes in
Section 8.
2. Mobility-Related Security Threats
Mobile IPv6 allows a node to redirect those packets, that a
correspondent node would otherwise send to one IP address (the home
address), to a second IP address (the care-of address).
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Unfortunately, the ability for redirection can also be misused by a
malicious node for an arbitrary pair of IP addresses unless
appropriate precautions are taken.
Overall, there are three major families of mobility-related threats:
impersonation attacks, resource-exhaustion attacks, and flooding
attacks. The following subsections take a closer look at each of the
categories. Threats are described in the light of Mobile IPv6, but
some of them apply to other mobility-management protocols as well.
2.1 Impersonation Attacks
The probably most obvious issue with mobility is to ensure that only
a mobile node itself has the ability to change its care-of address.
If care-of-address registrations were unauthenticated, an attacker
could easily impersonate an arbitrary victim. For instance, the
attacker could contact the victim's correspondent node and register
its own IP address on behalf of its victim. The correspondent node
would assume that the victim's care-of address has changed, and it
would redirect all packets intended for the victim to the attacker
instead. The attacker could forward the packets to the victim after
analyzing, or even tampering with, their payloads. In a related
offense, the perpetrator could simply cause havoc at its victim by
directing the victim's packets to a random or non-existent IP
address. These attacks are jointly referred to as "impersonation
attacks". Impersonation attacks can be prevented through proper
authentication techniques that keep an outsider from assuming another
node's identity.
It is important to recognize that impersonation attacks not only
impact those nodes that have an interest in mobility. Although the
attacker makes the correspondent node believe that the victim is
mobile, neither the attacker nor the victim do have to be mobile.
Indeed, mobile nodes, non-moving nodes with mobility support, as well
as traditional stationary nodes are potentially endangered because
they all share the same IPv6 identifier namespace. (Actually, even
IPv4 nodes are jeopardized when IPv4-to-IPv6 translation occurs on
the path between these nodes and their correspondent peers.) This
unfolds the need for mandatory protection of mobility-related
signaling in order to safeguard the Internet community as a whole.
Beyond their large group of potential victims, mobility-related
impersonation attacks allow an attacker to choose the location from
where to wage its attack. For example, the impersonator could
position itself at a place where it is easier to inject spoofed
care-of-address registration packets into the network than anywhere
on the direct path between the victims. The attacker may also move
to a place where it can remain unrecognized. In contrast to this, in
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the non-mobile Internet that we have today, an attacker can only
listen to or tamper with packets while it is on the path between its
victims. Similarly, a mobility-management protocol may give the
attacker the possibility to shift the time for its attack. The
attacker might be able to register false care-of addresses even
before its victims' conversation begins, or attack a network long
after it has visited it. In the non-mobile Internet, an attacker
must strike at the same time as its victims communicate. The ability
to choose the location and time for an attack constitutes a dangerous
new degree of freedom for the attacker.
2.2 Resource-Exhaustion Attacks
Mobility support at correspondent nodes can become an issue if it
takes a lot of processing capacity to handle an incoming
care-of-address registration. During times of increased signaling
load, a correspondent node may thus end up having to commit a
significant fraction of its resources to mobility-related
transactions. What is worse, an attacker may take advantage of this
vulnerability. It could swamp the correspondent node with large
quantities of bogus registrations messages, keeping it from doing
useful work. Such denial-of-service attempts are called
"resource-exhaustion attacks". Clearly, if mobility support is to be
implemented on a large basis, handling care-of-address registrations
must be lightweight in order to lessen the susceptibility to resource
exhaustion. Another effective technique is to defer resource
commitment until late in the registration process: Once the
registrant has proven its identity or shown that it is willing to
invest resources itself, it is less likely malicious. As a last
resort, busy Internet servers should limit the resources they devote
to registration processing, and they may give preference to those
mobile nodes they know or have recently had meaningful communications
with.
It is worthwhile to stress the trade-off between effectiveness of
signaling authentication and resilience against increased signaling
load. On one hand, a strong authentication mechanism can effectively
prevent certain impersonation attacks. On the other hand, the
resources a correspondent node must spend on the verification of a
registering node's authenticity increases with the complexity of the
authentication algorithm. The susceptibility to resource exhaustion
thus grows with the level of protection against impersonation
attacks.
2.3 Flooding Attacks
A third mobility-related security threat emanates from
redirection-based flooding attacks. Redirection-based flooding
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attacks are characterized by a victim being bombarded with unwanted
packets at a rate that the victim, and possibly the victim's access
network, cannot handle. As with impersonation attacks, it is
important to compare existing flooding attacks in today's non-mobile
Internet with redirection-based flooding attacks that could be made
possible through an insecure mobility-management protocol.
Three types of flooding attacks can be identified in today's
Internet. The simplest one is a "direct flooding attack". Here, the
attacker itself sends bogus packets to the victim. In an indirect
"reflection attack", the attacker tricks a third node, the
"reflection point", to send the packets. It typically uses a known
protocol vulnerability to make the reflection point generate these
packets [38]. For example, the attacker may send spoofed ICMP Echo
Request packets to the reflection point, using its victim's IP
address in the packets' IPv6 Source Address field. For each such
request, the reflection point generates an ICMP Echo Reply message,
which it sends "back" to the victim. The advantage of a reflection
attack over a direct flooding attack is that the attacker is usually
harder to track when flooding traffic comes from a third node.
Another example for a reflection attack is TCP-SYN flooding. Here,
the attacker sends TCP SYN packets, again with false source
addresses, to the reflection point, which in turn sends TCP SYN-ACK
packets to someone who does not expect these packets. Since most TCP
servers are configured so that they re-send a TCP SYN packet multiple
times when failing to receive an acknowledgement, this reflection
attack can even produce a small amplification. Gaining higher
amplification in today's Internet necessitates more complex
strategies like "distributed flooding attacks". In a distributed
flooding attack, the attacker typically gains control over other
nodes by spreading viral software. Then, at a certain point of time,
infected nodes simultaneously commence a joint flooding attack
against a common victim.
The introduction of mobility support renders amplified flooding
attacks much less complex. Suppose a mobile node is allowed to
change its care-of address without having to evidence that it is
present at the new care-of address. Then, an attacker can subscribe,
through its own IP address, to a large data flow (e.g., a video
stream) offered by some server on the Internet. The attacker can
easily accomplish the initial handshake procedure with the server
while it uses its own IP address. Once data is flowing, the attacker
can redirect the flow to the IP address of an arbitrary victim. The
attacker can use the sequence numbers learned during the initial
handshake procedure in order to spoof acknowledgements for packets
that it assumes the server has sent to the victim. In this attack,
not the attacker, but a faithful server on the Internet can be made
generate packets used for an attack. The server does not have to be
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infected with compromised code, and neither the victim nor the server
has to be mobile. The attacker produces as little as spoofed
feedback information to keep the data flow alive. To make matters
worse, the attacker can redirect data flows from multiple servers to
the victim.
Support for Mobile IPv6 route optimization is recommended to all IPv6
nodes [11]. The base of correspondent nodes that an attacker could
exploit for a redirection-based flooding attack would therefore be
immense. Also note that no distribution of viral software would be
necessary. The severity of this new type of flooding is that it
would provide potentially unbounded amplification at comparably low
cost.
3. Mobile IPv6 Route Optimization
Route optimization requires the mobile node to register its current
care-of address with both its home agent and correspondent node. The
process of doing so is called a "home registration" and a
"correspondent registration", respectively.
When a mobile node begins communicating with a particular
correspondent node after a successful home registration, all packets
are initially routed through the mobile node's home agent, and
bidirectionally tunneled between the home agent and the mobile node's
current attachment point. For increased routing performance, the
mobile node should do a correspondent registration as early as
possible.
This section explains the standard Mobile IPv6 registration procedure
in the case that route optimization is used. The goals and
assumptions based on which this registration process was developed
are presented thereafter. The section concludes with a security
analysis of the Mobile IPv6 registration process.
3.1 Registration Procedure
A mobile node registers its current care-of address with its home
agent and correspondent node. As a result, the home agent and
correspondent node create "bindings" between the mobile node's home
address and current care-of address. The following is a nutshell
presentation of Mobile IPv6 home and correspondent registrations.
Figure 1 illustrates this process. The interested reader is referred
to RFC 3775 [29] for the complete specification.
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Mobile Correspondent
Node Home Agent Node
| | |
| 1. Binding Update (BU) | |
|-------------------------->| |
| | |
| 2. Binding Ack (BA) | |
|<--------------------------| |
| | |
| 3a. Home Test Init (HoTI) | |
|-------------------------->|-------------------------->|
| |
| 3b. Care-of Test Init (CoTI) |
|------------------------------------------------------>|
| |
| | 4a. Home Test (HoT) |
|<--------------------------|<--------------------------|
| | |
| 4b. Care-of Test (CoT) |
|<------------------------------------------------------|
| |
| 5. Binding Update (BU) |
|-------------------------------------------------------|
| |
| 6. Binding Ack (BA) |
|<------------------------------------------------------|
| |
Figure 1: Mobile IPv6 Registration Procedure
When the mobile node detects that it has moved to a different access
network, it configures a new care-of address. The mobile node then
initiates a home registration by sending to the home agent a Binding
Update (BU) message. The BU contains the mobile node's home address,
current care-of address, and some supplementary information. If the
home registration succeeds, the home agents returns a Binding
Acknowledgement (BA) message, informing the mobile node that its home
address is now bound to the new care-of address. The BA also
specifies for how long the binding will stay in place.
RFC 3775 requires that the BU and BA be authenticated, and recommends
that they also be encrypted, through IPsec. The mobile node and home
agent may be pre-configured with the necessary security associations,
or they may dynamically create them through IKE. In the latter case,
the nodes have to be preconfigured with an identifier and the
credentials necessary to prove their identity during the IKE
authentication stage. This could be a preconfigured shared secret or
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a public/private-key pair combined with a certificate that binds the
public key to the identifier. Finally, the home agent needs
sufficient information to authorize a mobile node to use a particular
home address.
The correspondent registration consists of a return-routability
procedure followed by the registration proper. The
return-routability procedure, in turn, is a combination of two
message exchanges, one exchange that goes through the home network
and another direct exchange. The return-routability procedure aims
to determine whether the mobile node is reachable at its home address
and care-of address. The mobile node may initiate the
return-routability procedure at any time after it has sent the BU to
the home agent. It then sends a Home Test Init (HoTI) message and a
Care-of Test Init (CoTI) message to the correspondent node. The HoTI
is tunneled to the home agent, which forwards the message to the
correspondent node. The CoTI is sent to the correspondent node on
the direct path.
When the correspondent node receives the HoTI, it generates a Home
Keygen Token, which it returns to the mobile node's home address in a
Home Test (HoT) message. The mobile node needs the Home Keygen Token
to show that it is the legitimate owner of its home address.
Similarly, when the correspondent node receives a CoTI, it sends a
Care-of Test (CoT) message with a Care-of Keygen Token to the mobile
node's care-of address. The mobile node needs the Care-of Keygen
Token to prove its reachability at the new care-of address. The
tokens are produced based on unpredictable nonces, the mobile node's
home and care-of address, respectively, and some auxiliary data.
Sufficient information is communicated as part of the registration
protocol such that the correspondent node will eventually be able to
recompute both tokens without having to explicitly store either of
them. Note that, although some Mobile IPv6 implementations do the
two message exchanges in sequence, a standard-conform and more
efficient way is doing them in parallel. Home and Care-of Keygen
Tokens are good for 3.5 minutes, so if the mobile node changes its
care-of address again during this period, it may reuse its Home
Keygen Token. The HoTI-HoT and CoTI-CoT exchanges are respectively
called "home-address test" and "care-of-address test".
RFC 3775 recommends that the HoTI and HoT be authenticated, and
optionally encrypted, through an IPsec tunnel between the mobile node
and the home agent. It is the home agent's responsibility to update
the corresponding security association to the new tunnel end point
during the home registration.
Once the mobile node has received the HoT and CoT from the
correspondent node as well as the BU from the home agent, it can send
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a BU to the correspondent node, requesting the correspondent node to
bind its home address to its current care-of address. The mobile
node must compute a message-authentication code keyed with the Home
and Care-of Keygen Tokens. When the correspondent node receives the
BU, it can thus decide that the mobile node, first, owns the home
address mentioned in the BU and, second, is reachable at the care-of
address to which that home address is to be bound. The mobile node
may optionally request the correspondent node to return a BA for
confirmation by setting a flag in the BU. Note that the BA is
mandatory for the home registration.
Bindings at correspondent nodes have a maximum lifetime of seven
minutes. If a binding is not updated within this time, the mobile
node must re-do the correspondent registration. This includes
another run of the return-routability procedure.
3.2 Goals and Assumptions
An important objective for the development of Mobile IPv6 was to
provide for a wide, preferably universal, support for route
optimization. In fact, support for Mobile IPv6 and, thus, route
optimization is recommended in the requirements suite for IPv6 nodes
[11]. It was, and still is, believed that the additional routing
overhead associated with bidirectional tunneling is too much of a
burden on the core Internet given that the number of connected mobile
nodes is expected to grow substantially within the next decades.
The aspiration for wide-scale deployment of route optimization has an
impact on how a correspondent node can authorize a mobile node to use
a particular home address. A mobile node must authenticate itself,
preferably without any pre-configured keys, as the legitimate owner
of the home address packets addressed to which it seeks to redirect
to a certain care-of address. Without such authentication, any
node--not necessarily a mobile one--could redirect any other node's
packets. The challenge here is to bind to a given home address a
property that only the mobile node owning that home address can have.
The return-routability procedure was elected as the default
authentication mechanism for Mobile IPv6 route optimization. It
verifies home-address ownership through a routing property and so
does without any pre-configured authentication material. When a
mobile node shows that it can receive messages sent to its home
address, this is understood as reasonable evidence that the mobile
node is the legitimate home-address owner. Strictly speaking, the
return-routability procedure checks only that the mobile node is
somewhere on the path between the correspondent node and the home
address. An on-path attacker could thus hijack a communications
connection that is not protected otherwise. However, the problem
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with on-path attackers is independent of mobility and already existed
before the introduction of Mobile IPv6. Hence, the
return-routability procedure does not create a new security threat.
Of course, there are alternatives to the return-routability
procedure. Preconfiguring shared secrets into mobile nodes and
correspondent nodes is one, leveraging public-key cryptography is
another. These approaches may in fact do a very good job in certain
scenarios. However, both of them have deficiencies as far as general
applicability goes. The following explains why neither approach was
taken up as the default authentication mechanism in Mobile IPv6.
If a shared secret is pre-configured into a mobile node and its
correspondent node, the correspondent node can authenticate the
mobile node by having it encrypt a piece of random data and comparing
the result with the expected ciphertext. This process is simple and
appealing. The crux is that there is usually no existing
relationship between an arbitrary pair of nodes before the nodes
start communicating. Preconfiguration may hence not be feasible in
many cases. And where preconfiguration does apply will it involve
considerable administrative overhead, which makes the approach
impractical except for some very limited scenarios. Also note that a
security association alone does not show that a node owns a specific
IP address. This property, however, is required for Mobile IPv6
route optimization, so an external mechanism is needed to authorize
an authenticated mobile node to use a specific home address.
Public-key cryptography requires some external binding between a
public key and the identity this public key is supposed to protect.
Certificates issued by a trusted authority can usually do this job,
although there is little experience with using home addresses as
identifiers in the certificates. (E.g., the home address could be
placed into a certificate's Subject AltName field.) Given a
certificate that binds a public key to a home address, the owner of
this home address can authenticate itself as such by signing some
arbitrary piece of data with its private key. Since everybody can
verify the signature with the mobile node's public key, this proves,
in the end, that the mobile node actually knows the private key
complementing the certified public key, and the certificate authority
vouches that the public key, in turn, is associated with the home
address. The issue with public-key cryptography in the case of
Mobile IPv6 is the extraordinarily high number of potential home
addresses. Many experts doubt that one could built a public-key
infrastructure (PKI) of the size appropriate for Mobile IPv6.
Furthermore, making certificates bind home addresses to public keys
is per se an issue, because IP address assignment is typically
handled by other network entities than the PKI nodes. A global PKI
would also constitute an attractive target for attacks, endangering
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the Internet as a whole.
It is important to recognize that some mobility-related attacks can
be prevented through authentication and authorization to use a
particular home address, while others cannot. A dominant threat
uncovered is resource-exhaustion attacks. In fact, the stronger the
authentication algorithms, the easier it is to exploit the resources
of a node running these algorithms. In a resource-exhaustion attack,
an attacker brings down its victim through massively sending it bogus
requests. Protection against malicious resource exhaustion was
another key driver in the Mobile IPv6 security-design process. The
intent was primarily to safeguard those hosts which offer a popular
or critical service without necessarily having to be mobile
themselves. A Mobile IPv6 correspondent registration is robust
against resource-exhaustion attacks in that it is of low complexity
and delays state creation as well as computational tasks at the
correspondent node until the mobile node has shown its credentials.
Redirection-based flooding attacks are another threat that cannot be
encountered by authentication. Mandatory authentication may lessen
the attractiveness of such flooding, but certainly cannot prevent it.
There must hence be a different mechanism that prevents malicious use
of care-of addresses. In Mobile IPv6, a correspondent node probes a
mobile node's new care-of address before it sends packet there. This
verification strategy operates end to end, and it is as such
independent of any support from the network.
An alternative that does require network support is to enforce proper
use of care-of addresses already at the mobile node's point of
network attachment. The correspondent node may then simply believe
in the validity of a care-of address without doing any verification
itself. Many access networks today provide this service through
ingress filtering [34]. However, the crux with verifying a care-of
address at the fringe of the Internet is that an attacker can choose
the location from where to wage a flooding attack. As long as there
are access networks where ingress filtering, or an equivalent
technique, is not deployed, an attacker can always avoid
care-of-address verification. Designers therefore made Mobile IPv6
not to rely on ingress filtering. Certificate-based authorization of
care-of addresses is also infeasible because care-of addresses change
in a typically unpredictable way, whereas certificates are static.
It should be mentioned that care-of-address verification can be
omitted in scenarios where the mobile node is considered trustworthy.
For instance, RFC 3775 is based on the assumption that there is a
trust relationship between mobile nodes and their respective home
agents. The care-of-address test is hence omitted during home
registrations. This is certainly a feasible hypothesis in many
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cases, but one ought to bear in mind that, in some scenarios, it may
be not. As an example, a mobile services provider may not be able to
trust all individuals from its large customer basis, so it may probe
a mobile node's care-of address even during a home registration
irrespective of what RFC 3775 defines.
3.3 Security Analysis
To analyze the security of the return-routability procedure, one
should evaluate its protection against the tree types of attacks
described in section Section 2: impersonation attacks against third
parties, resource-exhaustion attacks against mobile nodes or
correspondent nodes, and flooding attacks against third parties.
In the context of Mobile IPv6, impersonation is an attack in which
the perpetrator claims ownership over a victim's IP address,
pretending that this IP address be its own Mobile IPv6 home address.
The return-routability procedure can prevent such attacks unless the
attacker is on the path from the correspondent node to the victim (in
the case of a stationary victim) or from the correspondent node to
the victim's home agent (if the victim is mobile). However, if an
attacker happens to be on the critical path, it can spoof a HoTI on
behalf of the victim, eavesdrop on the returning HoT, and thus
illegitimately acquire a Home Keygen Token. The impersonator can
produce its own Care-of Keygen Token by sending the victim's
correspondent peer a tailored CoTI with a care-of address through
which the impersonator is itself reachable. Having both tokens
allows the attacker to send an authenticated BU on behalf of the
victim.
The return-routability procedure's susceptibility to attacks from the
routing path conforms with the objective to prevent new attack types
that did not exist before the introduction of Mobile IPv6, but to
disregard existing threats that are independent of whether mobility
is supported or not. For instance, redirecting someone else's
packets from outside those packets' routing path is generally
impossible with plain IP, but a "man in the middle" may well launch a
successful attack from a position on the routing path. This said, it
stands to reason why the return-routability procedure prevents
off-path attacks, but does little to stop on-path attacks.
Similarly, the return-routability procedure does not prevent an
attacker from registering a care-of address which is located such
that the attacker is on the path between the care-of address and the
correspondent node. This attacker is in a position to launch a
redirection-based flooding attack against the node using the target
care-of address (or the entire network this care-of address belongs
to). But here again could the attacker launch a comparable attack
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already without the help of Mobile IPv6, simply by setting up an
upper-layer connection with the victim's IP address. For instance,
an on-path attacker could perform a TCP handshake on behalf of its
victim, initiating, say, a large file download from an FTP server.
With the TCP sequence numbers at hand, the attacker could also send
acknowledgements on behalf of its victim to keep the data flow going
or even accelerate it.
However, reducing the return-routability procedure's vulnerability to
the routing path is insufficient to prevent a related style of attack
that is called "space- and time-shift attacks". In these attacks,
the perpetrator taps the critical wire in order to eavesdrop on or
manipulate return-routability messages, and it then moves to a saver
place and starts an impersonation attack from there. The attacker
may also wait for a better point of time. It may even install a
binding on behalf of a victim before the victim starts communicating.
The mandatory, periodic registration refreshes defined by RFC 3775
mitigate the threat of space- and time-shift attacks.
The return-routability procedure is such that the correspondent node
does not need to explicitly store the Home or Care-of Keygen Tokens
sent to a mobile node. The information communicated in the protocol
is sufficient for the correspondent node to re-calculate the token.
This saves the correspondent node from attacks against its memory.
On the other hand, it may open the door for attacks against the
correspondent node's processing capacity. A token is a SHA-1 hash on
the mobile node's and correspondent node's IP addresses, a random
nonce, and a secret known only to the correspondent node. The
computational overhead required to do the hash is rather moderate,
although a correspondent node should implement its own policies to
manage resources in a situation of increased processing workload.
4. Objectives for Enhancement
This section identifies areas in which route optimization, as
specified in RFC 3775, may be incompatible with the requirements of
certain applications. Objectives to enhance route optimization
usually boil down to optimizations to the return-routability
procedure, or alternative mechanisms that may replace the
return-routability procedure. The enhancement objectives are herein
discussed from a requirements perspective, such as the need for
decreasing latency. The technical means to reach those objectives is
not considered, nor is the feasibility of achieving them.
4.1 Latency Optimizations
A disadvantage of route optimization is that a mobile node must run a
return-routability procedure before it can inform the correspondent
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node about its new care-of address. Therefore, a correspondent
registration consumes, at a minimum, one and a half round-trip times
until the correspondent node receives the BU, assuming that the
mobile node performs the home-address and care-of-address tests in
parallel. An additional one-way time is needed until the first
packet from the correspondent node, and possibly an optional BA,
arrives at the new care-of address. Note that the CoTI, CoT, BU are
transmitted on the direct path between the mobile node and the
correspondent node, whereas the HoTI and HoT go through the home
agent. The actual latency of the return-routability procedure is
governed by the path with a longer round-trip time.
Direct communications to the correspondent node can optimistically
start right after the Binding Update message has been sent (i.e.,
after one round-trip time), but more generally are delayed until the
Binding Acknowledgement message is received (i.e., after two
round-trip times).
Similarly, optimistic mobile nodes are allowed by RFC 3775 to start
their return-routability procedure right after sending a Binding
Update message to their home agent. They can so reduce the latency
for the correspondent registration. But more generally, mobile nodes
wait for the home registration to be completed and acknowledged
before initiating the correspondent registration.
Depending on the type of application, the above delays can be an
issue. Interactive, real-time applications, like voice over IP, are
an example where the delays may cause perceptible quality
degradations. Even fast bulk-data transfer can be affected if the
lack of packets during the movement period is interpreted as
congestion and leads to a new TCP slow start. There appears to be
general consensus that faster mechanisms for route optimization are
needed.
Note that the handover delay from an application's perspective is not
just the latency of the IP mobility mechanism, but also includes
delays at the IP layer and the link layer. In fact, the delays
introduced by the rest of the stack can be significant as well
Section 6.3.1.
4.2 Signaling Optimizations
As mentioned in section Section 3, correspondent registrations have a
maximum lifetime of seven minutes and must be refreshed in case they
are not updated to a different care-of address in the meantime. The
reason for this is to reasonably reduce the window of vulnerability
to time- and space-shift attacks, where an attacker eavesdrops on
unencrypted authentication material exchanged during the
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return-routability procedure and launches an impersonation attack at
a later time and from a different, probably more amenable location.
Periodic re-registrations limit the likelihood and feasibility of
such off-path attacks, since the attacker would have to get back on
path whenever the authentication material is due to be refreshed.
A calculation in [2] shows that the seven-minute refreshment interval
implies a signaling overhead of 7.16 bps when a mobile node
communicates with a stationary node. The overhead doubles if both
peers are mobile. On one hand, this signaling overhead is certainly
negligible when the nodes actually communicate. On the other hand,
it may cause problems for mobile nodes that are inactive and stay at
the same location for a while, but still want to have route
optimization ready with some correspondent node. These nodes
typically prefer to go to standby mode to conserve battery power.
An example where the maintenance of route optimization in the absence
of traffic may be useful is some sort of messenger service that
mobile nodes can subscribe to. Here, having route optimization in
place would allow the correspondent node in charge of sending the
messages to instantaneously create an efficient connection. If the
mobile node used bidirectional tunneling instead, the first packet
that the correspondent node sent would be relayed through the home
agent and trigger a correspondent registration upon arrival at the
mobile node. Since a messenger service is likely to send a few
packets per event only, the belatedly created new correspondent
registration would probably remain completely unused.
Also, the accumulated signaling overhead for route-optimization
maintenance generated by a large customer base may be an issue from a
network provider's point of view. Not only do the signaling packets
have to be routed through the network, part of them must also be
processed by home agents. For example, of the 716 Mbps signaling
overhead generated by 100 million route-optimized mobile nodes, 220
Mbps goes through a home agent.
This discussion shows that there are scenarios in which an
optimization for reduced signaling would be beneficial. These
scenarios are important enough to have an impact on the deployment of
Mobile IPv6.
4.3 Security Enhancements
The return-routability procedure is lightway and prevents
mobility-related attacks reasonably well. In some cases, however,
may better security be useful. One may in particular attempt to
limit what on-path attackers can do. Attackers that operate in the
same networks as one of the communication end points are also a
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threat that one may want to avoid. There are existing proposals that
offer higher security in Mobile IPv6 [24] and in other
mobility-management protocols such as HIP [22].
However, even with better security for mobility management can the
Internet as a whole not become any safer than the non-mobile
Internet. For instance, on-path attackers can cause denial of
service, or inspect and modify cleartext packets, already without
misusing a mobility-management protocol. Applications that require
strong security are therefore generally advised to end-to-end
mechanisms such as IPsec or TLS. But even communications that are
protected on an end-to-end basis are vulnerable to denial of service.
Better route-optimization security may become necessary in the
future, if technological improvements remove some of the existing
mobility-unrelated vulnerabilities of the Internet. For instance,
the use of Secure Neighbor Discovery [20] in a network where one of
the communication end points resides can remove some of the existing
threats.
4.4 Applicability Enhancements
As per RFC 3775, a mobile node's home address and current care-of
address are carried in all route-optimized packets. The course of
the mobile node is therefore trackable, both by the correspondent
node as well as by a third party. This can be an issue in situations
where the mobile node prefers not to reveal its location. Location
privacy, however, is inherently not supported by Mobile IPv6 route
optimization. A workaround is to fall back to bidirectional
tunneling when location privacy becomes an issue. Packets that carry
the mobile node's care-of address are then only transferred between
the mobile node and the home agent, and they can be encrypted through
IPsec ESP [27][10] on that path. However, the mobile node may have
to periodically re-establish its IPsec security associations so that
it cannot be tracked through its SPIs.
Scenarios where location privacy is desired are one example where
Mobile IPv6 proves insufficient. Early improvement efforts have
already started in this area [8][4]. There may also be other
deployment scenarios where the applicability of Mobile IPv6 is
limited and could be extended.
5. Enhancements Toolbox
This section introduces a number of techniques, a "toolbox", that can
be used in the construction of an efficient and secure
route-optimization protocol. The section starts with the standard
mechanisms used in RFC 3775 and continues with additional techniques
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that have been proposed as enhancements or alternatives.
It is important to mention that many enhancements techniques are
insufficient or insecure when applied on their own, because the scope
of each of them is usually limited to a certain sub-issue. It is the
combination of a set of techniques that makes an efficient and secure
route-optimization mechanism possible. Different techniques also
have different trade-offs with respect to, say, universal
applicability versus efficiency.
5.1 IP-Address Tests
An IP-address test can be employed to ensure that the peer is live
and on the path to a specific destination address. RFC 3775 uses
IP-address tests for two purposes: The home-address test provides
evidence that a mobile node owns the home address it wants to use;
the care-of-address test serves to preventing flooding attacks
related to spoofed care-of addresses. As specified in RFC 3775,
IP-address tests can be stateless for the correspondent node, making
their use in denial-of-service attacks harder.
IP-ddress tests are a zero-configuration approach that is independent
of ancillary infrastructure. The subsequent disadvantage is that
IP-address tests can only guarantee that a peer is on the path to the
probed IP address, not that the peer truly owns this IP address. On
the other hand, the types of attacks that an on-path attacker can do
with route optimization are the same that an on-path attacker can
anyway do without route optimization, so there is actually no
significant new threat.
The use of two IP-address tests requires four messages. Both tests
can be performed in parallel, so they can be completed in one
round-trip time.
5.2 Protected Tunnels
An additional technique used in RFC 3775 is the protection of a part
of the signaling communications through an authorized and,
optionally, encrypted tunnel between a mobile node and its home
agent. This prevents other nodes, close to the mobile node, from
seeing a home-address test.
Given the starting point that we cannot assume a pre-existing
end-to-end security relationship between the mobile node and the
correspondent node, this protection exists only for the mobile node's
side. In case the correspondent node is stationary, the path between
the home agent and the correspondent node remains unprotected. An
attacker on that path can still perform attacks, but these attacks
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are similar to those that an on-path attacker can anyway do without
route optimization. So, as with IP-address tests, the intent here is
not to introduce any significant new threat to the Internet. The
same is true in case the correspondent node is mobile. It then has
its own home agent, and it is the path between the two home agents
that stays unprotected.
5.3 Optimistic Behavior
RFC 3775 leaves quite a bit of freedom for a mobile node with respect
to scheduling signaling and data packets. An optimistic mobile node
can initiate the return-routability procedure right after sending the
BU to its home agent, even before it has gotten a BA back.
The mobile node must wait for the home agent's BA before it can send
a BU to the correspondent node. However, the mobile node may start
sending data packets to the correspondent node right after it has
sent this BU without having to wait for a BA.
The drawback of the described optimistic behavior is that a dropped,
re-ordered, or rejected BU can cause data packets to be dropped.
Such packet loss would also have an effect on pessimistic signaling,
however. As a result, further experimentation and simulation may be
needed to quantify to the effects of optimistic techniques under
different conditions.
5.4 Proactive IP-Address Tests
The post-movement time period during which a mobile node and
correspondent node cannot fully communicate is oftentimes called the
"critical phase". Usually, the critical phase spans a home
registration and a correspondent registration including a
return-routability procedure. One technique to shorten the critical
phase is to move some of these tasks to an earlier stage. In
particular, the home-address test can be done proactively before a
handover, instead of doing it afterwards, without violating the base
specification. This is discussed in [25].
A Home Keygen Token is generally valid for 3.5 minutes.
Consequently, the mobile node must initiate a proactive home-address
test at least every 3.5 minutes if it seeks to have available a fresh
Home Keygen Token at all times. This is especially helpful if the
mobile node cannot foresee the next handover. Alternatively, the
mobile node may be able to receive a trigger from its local link
layer indicating that a handover is imminent. In this case, the
mobile node may initiate the home-address test right in time instead
of doing it periodically every 3.5 minutes. Note, however, that the
mobile node must anyway re-initiate the correspondent
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registration--and, thus, the home-address test--after the maximum
binding lifetime of seven minutes. Link-layer triggers can therefore
save the mobile node at most every second home-address test (unless
they are combined with additional techniques such as [2]).
Another optimization possibility is performing a care-of address test
before the movement. This is possible only if the mobile node is
capable of attaching to two networks simultaneously.
5.5 Concurrent IP-Address Tests
If one assumes that a mobile node can attach to only a single network
at a time, executing the care-of-address test proactively before a
handover is not an option. However, one may authorize a mobile node
to start using a new care-of address right after the handover,
without doing the care-of-address test first, with the restriction
that a care-of-address test be initiated rightaway. The peers could
then already exchange packets through the new care-of address while
the test is being executed. In recent literature, one refers to the
care-of address as "unconfirmed" when the correspondent node does not
yet know the result of the concurrent care-of-address test, and one
calls it "confirmed" thereafter. The lifetime of the associated
binding can be limited to a few seconds as long as the care-of
address is unconfirmed, and it can be extended once it becomes
confirmed.
It is important to understand that concurrency is legitimate only for
care-of-address tests. In contrast, home-address tests are done for
mobile-node authentication, which must be done before any signaling
messages are accepted. Authentication guarantees that only the
legitimate mobile node can create or update a binding pertaining to
its home address. However, both IP-address tests are in general
simultaneously performed during the critical handover period, and one
can expect the home-address test to have a longer latency than the
care-of-address test. The full benefit of eliminating the
care-of-address tests from the critical handover period by means of
concurrency can therefore only unfold if some other mechanism is used
to move the home-address tests out of the critical handover period as
well. For instance, one can do the home-address tests proactively
before a handover as suggested in Section 5.4, or one may use
cryptographically generated home addresses as proposed further down
in Section 5.9.
Concurrent care-of-address tests were first proposed in [25] where
they were combined with proactive home-address tests. In [25], as
soon as the mobile node has configured a new care-of address after a
handover, it sends to the correspondent node an Early Binding Update
(EBU) message. The mobile node signs the EBU with a
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message-authentication code keyed only with the Home Keygen Token
that the mobile node has previously retrieved through a proactive
home-address test. Upon reception of the EBU, the correspondent node
creates a tentative binding for the new care-of address, which can
then be used while the care-of-address test is being executed. When
the care-of-address is done, the mobile node sends a standard BU to
the correspondent node, concluding the registration procedure.
From the reception of an EBU to the reception of the corresponding
standard BU, the correspondent node cannot be sure whether the mobile
node is actually present at the claimed new care-of address. A
malicious node may misuse this property to redirect packets to a
third party's IP address during this phase of uncertainty. Under
many circumstances, this will not be acceptable even if the lifetime
for an unconfirmed care-of address is tentative only, and there needs
to be external protection. Techniques like those described in
Section 5.7 or Section 5.8 can help.
5.6 Diverted Routing
Given that the per-movement signaling takes some time, a mobile node
can optionally request its traffic to be routed through its home
address while this signaling is being completed. The performance
impact of this technique depends on the length of the critical phase
as well as on the capacity and latency of the direct path and the
path through the home agent. With respect to the packets that the
correspondent node sends, the following analysis can be made.
The correspondent node does not know that the mobile node has moved
until it has been told about this. It continues to send packets to
the mobile node's old care-of address until that time. These packets
are usually lost and must be transmitted by upper-layer mechanisms.
In addition, even the request to delete or deactivate a binding
requires some security-related signaling to prevent misuse by
unauthorized nodes. Zero packet loss can generally only be achieved
through local repair techniques in the mobile node's access network
(cf Section 5.14), or if the mobile node can simultaneously attach to
two IP networks.
Once the correspondent node knows that the old care-of address is
stale, it can send further packets to the home address. If one
assumes that the correspondent registration for the new care-of
address involves messages through the home agent, it is obvious that
at least some of these packets reach the mobile node before the new
binding is set up. After all, signaling and data packets travel the
same path.
It depends on the capacity and latency of the path through the home
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agent relative to the latency of the direct path for how long the
correspondent node should continue to send packets to the home
address. If the former path has a high latency, it might be better
to queue some of the packets until the correspondent registration is
complete and packets can be directly sent to the mobile node. One
potential research direction is to look at whether the properties of
the paths could be learned during the signaling and then used to
decide the optimal time when the correspondent node should start
queueing packets.
The situation for the packets that the mobile node sends is similar:
Although the mobile node knows immediately that it has moved, RFC
3775 does not allow the mobile node to route-optimize packets from
new care-of address until it has formally updated the correspondent
node about the new care-of address. Of course, the mobile node may
buffer packets until the correspondent registration is done so that
no packets get lost.
Diverted routing appeared originally in [25] and has since been used
also in [6].
5.7 Credit-Based Authorization
As described in Section 5.5, a new care-of address may already be
used while the care-of-address tests is in progress. The
prerequesite is that sufficient protection is provided against
redirection-based third-party flooding. One way of doing this is
authorizing a mobile node to receive packets at a new, unconfirmed
care-of address based on credit that the mobile node has collected
with a previous care-of address. (See Section 5.5 for a definition
on when a care-of address is confirmed and when it is unconfirmed.)
This mechanism has become known as Credit-Based Authorization (CBA)
[26].
CBA limits the data volume and rate that the correspondent node sends
during a concurrent care-of-address test such that it does not exceed
the data volume and rate that the mobile node has sent in the recent
past. The intention here is not so much to prevent redirection-based
flooding attacks altogether as to render impossible any kind of
amplification that can be achieved through redirection. It is this
inherent potential for amplification which constitutes the attraction
to redirection-based flooding: While the attacker simply does an
initial connection setup and a subsequent correspondent registration
for packet redirection, it is the correspondent node which generates
the packets (typically full-sized TCP segments) that the victim ends
up having to receive. Transport-layer acknowledgements can generally
be faked, so the attacker can easily keep the redirected data stream
alive. In the end, the correspondent node spends, unknowingly, much
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more resources on the flooding attack than the attacker itself. CBA
renders such amplification impossible. This makes redirection-based
flooding attacks very unattractive to the attacker because it would
take the attacker less coordinative effort, and be at least equally
effective, if it sent bogus packets to the victim directly.
Technically, CBA works as follows. A CBA-enabled correspondent node
maintains a credit account for each mobile node it communicates with.
The correspondent node increases the mobile node's credit by the size
of each inbound packet received from the mobile node. When the
correspondent node sends a packet to the mobile node, and a
concurrent care-of-address test is in progress, the IP address to
which the packet is sent depends on how much credit is left. If the
credit is higher than the size of the outbound packet, that packet is
directly sent to the mobile node's care-of address. However, in case
the remaining credit is too small, the packet is sent to the mobile
node's home address. Since the home agent has a trust relationship
with the mobile node, it can forward these packets to the mobile
node's care-of address without having to do a reachability check
first. Exponential aging limits the lifetime of collected credit.
This guarantees that the mobile node cannot collect credit over an
extended time period at a very slow speed and use this credit, all at
once, for a short but potent data burst towards a faked care-of
address.
Allocating a 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 CBA mode 1. With applications
that send comparable data volumes into both directions, CBA mode 1
works fine. On the other hand, CBA mode 1 may prevent the mobile
node from collecting the amount of credit it needs for a handover
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.
To better accommodate such applications, a second CBA mode was
designed.
With CBA mode 2, credit allocation is based on packets that the
mobile node receives from the correspondent node rather than on
packets that the mobile node sends. New credit is allocated while
the mobile node's current care-of address is confirmed; existing
credit is used up while the care-of address is unconfirmed. Thus,
its is the data flow from the correspondent node to the mobile node
that is responsible for both credit allocation and reduction,
resolving the issue with applications producing asymmetric traffic
patterns.
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It is less obvious why CBA mode 2 outrules flooding-attack
amplification than it is for CBA mode 1. The key observation is that
a mobile node invests comparable effort for packet reception as for
packet transmission, in terms of bandwidth, memory, and processing
capacity. It is therefore legitimate to give a mobile node credit
for packets that it has received and processed. 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. As mentioned above, relying on transport-layer
acknowledgements is not an option as such messages can easily be
faked. CBA mode 2 hence defines its own feedback mechanism, Care-of
Address Spot Checks, which is much more robust to spoofing. With
Care-of Address Spot Checks, 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 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 have made their way at least
up to the mobile node's IPv6 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.
5.8 Heuristic Monitoring
Heuristic approaches to protect concurrent care-of-address tests are
conceivable as well. For instance, one may consider a lifetime limit
for unconfirmed care-of addresses which, supplemented by a heuristic
for misuse detection, can prevent, or at least effectually
discourage, misuse of such addresses. The challenge here seems to be
a feasible heuristic: On one hand, the heuristic must be sufficiently
rigid to catch any 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 the response comes quickly, attacks may simply not be
worthwhile. But premature actions should be avoided, of course. One
must also bear in mind that an attacker may be able to use different
home addresses, and it in general hard for the correspondent node to
see that the set of home addresses belongs to the same node. The
attacker may also use multiple correspondent nodes for its attack in
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an attempt to amplify the result.
5.9 Cryptographically Bound Identifiers
Cryptographically Generated Addresses (CGA) offer a method for
binding a public key to an IP address. A CGA is an IPv6 address with
a standard routing prefix and an interface identifier generated from
a hash on the CGA owner's public key and some additional parameters.
A CGA allows the owner to assert a claim on its address: It can sign
a to-be-authenticated message with its private key and attach its
public key along the parameters necessary to recompute the CGA. The
recipient of this message can then verify whether the address is
correct.
CGAs offer three main advantages: First, spoofing attacks against a
CGA are much harder than attacks against a normal IP address. Though
an attacker may always create its own CGA, it is unlikely to find a
public/private key pair that produces someone else's CGA. Second,
CGA-based authentication works entirely end-to-end; it does not
depend on a certification infrastructure. Third, CGAs are
syntactically just ordinary IPv6 addresses. Their additional
semantics do not require any upgrade or modification to the Internet.
Many applications are conceivable where CGAs can be advantageous. In
Mobile IPv6, CGAs can bind a mobile node's home or care-of address to
its public key. CGAs were originally described in [36] and in [37],
and they have later been used in [24], [19], [6], and others.
One limitation of CGAs is that the hash on the CGA owner's public key
can only be 62 bits long. The rest of the address is occupied by a
64-bit routing prefix as well as the universal/local and
individual/group bits. A brute-force attacker might thus be able to
find a public/private key pair that produces a certain CGA. It could
then claim ownership over this CGA. The threat can usually be
contained by including the address prefix in the hash computation, so
that an attacker, in case it did find the right public/private key
pair, could not form CGAs for multiple networks from it.
Higher security can be achieved through longer cryptographically
bound identifiers. For instance, a node's primary identifier in the
"Host Identity Protocol" (HIP) [22] is a 128-bit hash on the node's
public key. It is used as an IP-address replacement at stack layers
above IP. On the other hand, a 128-bit identifier is not routable,
so there needs to be some external location mechanism if a node wants
to contact a peer of which it only knows the identifier.
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5.10 Pre-Configuration
The return-routability procedure was designed for communication peers
that do not share an a-priori security association. In order to
thwart off-path attacks nonetheless, the procedure can establish only
very short-living security associations. However, in certain,
restricted scenarios, it may be possible to pre-configure mobile and
correspondent nodes with security associations. Such security
associations can have much longer lifetimes because pre-configuration
is inherently more secure than the plaintext token exchange from the
return-routability procedure.
Pre-configuration has two major benefits. The first one is strong,
cryptographic authentication and encryption, which can be applied to
both signaling and data packets. The second advantage is lower
signaling delay, because the additional round-trip time otherwise
needed for the return-routability procedure can be spared. The
obvious disadvantage of pre-configuration is its limited
applicability.
It is important to recognize the necessity to unambiguously bind a
security association to the home address that it is to protect. With
regards to the return-routability procedure, this binding is realized
by routing the HoTI and HoT through the home address. In the case of
a pre-configured security association, the association must be
related to the home address as part of the configuration. Note that
this affects both secret-key and public-key cryptography.
Two proposals for pre-configuration are currently under discussion in
the IETF as optional enhancements to RFC 3775. [15] re-uses most
from the standard authentication and authorization procedure defined
in RFC 3775. The only difference is that mobile nodes are endowed
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. [5] replaces the standard RFC-3775
concepts with IPsec and the Internet Key Exchange (IKE) protocol.
From a technical standpoint, a pre-configured security association
can only replace a home-address test, not a care-of-address test.
After all, a correspondent node cannot verify, based on the
association alone, whether a mobile node is actually present at the
announced care-of address. The problem can be circumvented by
postulating that the correspondent node has sufficient trust in the
mobile node to believe that the care-of address is correct. However,
this assumption, which is made in [15], discourages the use of
pre-configuration in scenarios where such trust is unavailable. For
instance, a mobile-phone operator may be able to configure
subscribers with secret keys for authorization to a particular
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service, but it may not be able to vouch that all subscribers use
this service in a trustworthy manner.
Another way to avoid the problem of care-of-address verification is
to rely on access networks to filter out packets with incorrect IP
source addresses (cf. Section 5.12). This approach is taken in [5].
However, the problem with local filtering is that it must be deployed
everywhere an attacker may possibly access the Internet in order to
be fully protective. Otherwise, an attacker can always find a place
from where a spoofing attack is possible, endangering IP nodes
anywhere. As things stand, the assumption that deployment of
filtering techniques be universal is speculative.
Both of the above two assumptions can be eliminated through
care-of-address tests, facilitating the use of pre-configuration in
spite of lacking trust relationships or the existence of acess
networks without local filtering techniques. Care-of-address tests
can be made concurrent for higher efficiency.
5.11 Semi-Permanent Security Associations
A middle-way approach in between the return-routatibility procedure's
short-term security associations and pre-configured permanent ones is
to dynamically set up a semi-permanent security association upon
first contact, and to use it to authenticate signaling over a longer
period. Subsequent signaling can then be unambiguously bound to the
initial contact.
On-demand security associations for IPsec are traditionally
established by executing IKE between two peers. This works well when
the negotiated keys are securely bound to the entity that they are to
protect. For instance, in HIP, the guarded entity is a 128-bit
identifier which can be derived from the owner's public key through a
hash function. In Mobile IPv6, however, the to-be-protected entity
is a plain IPv6 home address, which is syntactically
indistinguishable from other IPv6 addresses. Given that no direct
authentication between the peers is generally feasible, there is no
way for a mobile node to prove possession of its home address either.
This would allow an attacker to do the IKE exchange with an arbitrary
victim's IP address, and to discretionarily redirect the victim's
traffic from that time on. Although the attacker would have to be on
the path between the victim and the correspondent node while running
IKE, it could move off the path once the keys have been exchanged.
As the victim lacks the keys, it cannot even re-claim its IP address.
As a result, dynamic semi-permanent security associations must be
bound to the right home addresses through some additional technique
when used in the context of Mobile IPv6. For instance, they can be
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combined with an initial, one-time home-address test, or IKE can be
run through the home address. Another way is using CGAs as proposed
in [6].
No matter how they are secured, dynamic semi-permanent security
associations cannot be leveraged to prove the correctness of a
care-of address. They hence fail to solve the problem with
redirection-based flooding attacks, and should only be applied in
conjunction with care-of-address tests. Semi-permanent security
associations were first developed in [3] where they were called
"Purpose Built Keys" (PBK).
5.12 Infrastructure
Infrastructure can provide assistance by vouching for the
authenticity of a home address, the correctness of a care-of address,
or the trustworthiness of a mobile node. Infrastructure can take
many forms, such as a PKI tailored for route optimization, or an AAA
infrastructure enhanced with the required features.
In its basic form, such an infrastructure hands out home addresses
and associates a key with each home address. It may also produce
certificates that can be used by others to verify the binding between
a key and a home address, or it may provide a query interface where
this verification is performed within the infrastructure.
Setting up this type of infrastructure has generally been considered
impossible for general Internet use. One of the problems is the
current separation of IP-address assignment and security
infrastructures. However, Certificate-based Binding Updates (CBU)
[23] are a useful simplification: A home agent is assigned a
certificate that binds the home-network prefix to the home agent's
public key. Correspondent nodes can trust the home agent based on
the certificate, and the home agent vouches for the trustworthiness
of the mobile nodes it serves. The advantage is that, rather than
having to issue a certificate per mobile node, only a certificate per
home-network prefix is required. This makes the infrastructure
problem more tractable. Furthermore, the home agent may help to
establish an end-to-end security association between the mobile node
and the correspondent node so that subsequent messages can be
securely exchanged on the direct path between the communicating
peers. This allows for improved signaling delay during later
handovers.
The infrastructure can also help to separate the trustworthy mobile
nodes from the non-trustworthy ones. Together with an identifier for
each mobile node, this could be used to retroactively track down
misbehaving nodes.
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AAA architectures are another approach to mobile-node authentication.
A home AAA server, which may or may not be co-located with the home
agent, can then contact a remote AAA server in the correspondent
node's network. Note that this moves some of the authentication
overhead from the correspondent node to the remote AAA server. An
AAA-based approach can also dynamically assign mobile-node requests
to different correspondent nodes while keeping secret authenticating
material local at a single AAA server.
Verification of care-of addresses may be based on infrastructure in
the mobile node's local access network. For instance, as a care-of
address normally appears as a BU's IP source address, the
infrastructure can verify that the IP source addresses of all packets
leaving the network are correct. Ingress filtering [34] provides
this feature to the extent that only the prefix of an outbound
packet's IP source address is inspected. The pertinence of this
check strongly depends on whether it is done directly in the access
router or further up the packet's path.
The problem with verifying a care-of address at the fringe of the
Internet is that there is no way for a remote correspondent node to
decide whether a packet has really undergone a check or not. And
although many access networks today already do ingress filtering, an
attacker can always find a network where such a technique is not
deployed.
A more secure approach to care-of-address verification is hence to
let the correspondent node itself verify a mobile node's care-of
address by querying a piece of infrastructure, located in the mobile
node's access network, about the mobile node's presence at a
particular care-of address. For this, the mobile node would have to
be identifiable by means known to both the correspondent node and the
infrastructure. If the care-of address is cryptographically
generated (cf. Section 5.9) and configured through Secure Neighbor
Discovery [20], the mobile node can be securely identified by the
care-of address alone. Otherwise, if CGAs are unavailable, an
additional PKI or AAA architecture is needed to distribute the
required credentials.
5.13 Local Mobility
Mobile IPv6 handles all mobility on an end-to-end basis. However, it
may sometimes make sense to handle part of a mobile node's movements
entirely within the local access network. This can yield performance
improvements in terms of signaling overhead and handover latency.
Hierarchical Mobile IPv6 [18] is an optimization of RFC 3775 for
local mobility support. It introduces the concept of a regional
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Mobile Anchor Point (MAP) that acts as a local home agent towards
visiting mobile nodes and proxies them towards their home agents and
correspondent nodes. When a mobile node enters a visited network, it
configures an "on-link care-of address", like in RFC 3775, and a
wider-scope "regional care-of address" from a MAP's network. The
mobile node registers a binding between the two care-of addresses
with the MAP, and it uses the regional care-of address for
communicating with nodes outside the local network. When the mobile
node moves to a different network within the same MAP's realm, it
configures a new on-link care-of address, but keeps its regional
care-of address. For the outside world it thus seems as if the
mobile node was stationary within the MAP's network.
The mobile node may in addition register the regional care-of address
with its own home agent and its correspondent nodes. This allows the
mobile node to roam between the domains of different MAPs without
breaking ongoing communication connections.
5.14 Local Repair
When a mobile node moves from one IP-attachment point to another,
some packets are likely to be still in flight towards the old
location. Local repair techniques can be used to forward these
packets to the new IP-attachment point. This can be done through a
tunnel or a host route between the old and new access router.
Local repair usually implies that packets are buffered at the old or
new access network. If the mobile node leaves the old access network
without telling its new care-of address, it must signal this
information back subsequently. In-flight packets arriving at the old
network should in this case be buffered until the mobile node's new
location is known. Alternatively, the mobile node may be able to
proactively determine its new care-of address before it moves.
In-flight packets can then immediately be forwarded to the new
location, where they probably have to be buffered for a little while
until the mobile node arrives. A protocol that supports both modes
is defined in [17].
5.15 Assisted Auto-Configuration
The local network may assist a mobile node in finding a new access
router to which it can handover. For instance, in [17], the mobile
node can search for a suitable access point and ask its current
access router to proxy-advertise the router to which this access
point is attached.
Additionally, the local network may support the mobile node in
configuring a new care-of address from its old link. In [17], after
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the mobile node has determined the access router that it wants to
handover to, it may suggest a new care-of address. The candidate
care-of address is signaled to the prospective access router which
performs DAD on the care-of address and signals the result back to
the mobile node. The new access router also performs Proxy Neighbor
Discovery in case the new care-of address is available.
Assisted auto-configuration can have enormous performance benefits,
especially when combined with local repair techniques. A
disadvantage is the investments for setting up the required
infrastructure. Also, local support must be provided in both the old
and the new access network, so handovers between different
administrative domains may be problematic.
5.16 Processing Improvements
One goal for designing the return-routability procedure was low
computational complexity. The processing overhead for route
optimization should thus be acceptable in general. However,
mechanisms alternative to the return-routability procedure, such as
public-key cryptography, may be more taxing on processing resources.
Here, it may help to replace RSA algorithms with ECC techniques.
Moreover, even the low-complexity cryptographic algorithms used in
the return-routability procedure may be too expensive for very busy
servers.
5.17 Delegation
Given that the home agent does not need to move or conserve battery
energy, it can be used for performing computationally expensive
tasks. It can also be used for parts of the signaling in order to
reduce communications over slow wireless links. Some work relating
to delegation has been done in [24], [23], and [32].
6. Analysis
This section analyzes the techniques presented in Section 5 in
relation to each other and in the context of the enhancement
objectives described in Section 4. The techniques are categorized
first. Some recent proposals for route-optimization enhancement,
which rely on one or combine multiple of these techniques, are
subsequently evaluated. The section concludes with a number of
opportunities for further research in the area of route optimization.
6.1 Categorization of Techniques
Local techniques include support for micro-mobility, route repair,
and assisted auto-configuration. They seek to reduce or eliminate
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long end-to-end round-trip times or delays caused by standard
auto-configuration techniques. Local techniques have traditionally
been implemented in a manner that requires configuration, but there
appears to be no fundamental reason why this would have to be so.
IP-address tests, which may be proactive or concurrent, credit-based
authorization, heuristic monitoring, CGAs, semi-permanent security
associations, and cryptographically bound identifiers are end-to-end
techniques. They are independent of local network support and are
thus very flexible. They may also be inexpensive to deploy. The
cost for these advantages is typically a smaller performance gain
compared to local mechanisms due to global, thus potentially long,
round-trip times.
Zero-configuration techniques require no prior configuration or
assistance from a managed infrastructure. IP-address tests, diverted
routing, credit-based authorization, heuristic monitoring, CGAs,
semi-permanent security associations, cryptographically bound
identifiers, processing improvements, and delegation are
zero-configuration techniques. Mechanisms that require
pre-configuration or some kind of infrastructure are not among
zero-configuration techniques.
6.2 Evaluation of Recent Proposals
6.2.1 Local Assistance
There are currently two proposals in the IETF for local mobility
assistance, Hierarchical Mobile IPv6 and Fast Handovers for Mobile
IPv6. Hierarchical Mobile IPv6 (HMIPv6) [18] screens a mobile node's
movements within a MAP domain from nodes not inside that domain. In
case standard Mobile IPv6 is used end-to-end, this eliminates most of
the global signaling: While its regional care-of address does not
change, a mobile node does not need to update its home agent or
correspondent nodes beyond the mandatory periodic refreshments. Not
having to signal on a global basis also reduces handover latency.
Updates to the home agent and to correspondent nodes are necessary
only when the mobile node leaves the current MAP domain. The mobile
node may then register a new regional care-of address with a
different MAP if one is available. Note that switching MAPs usually
requires the mobile node to signal more than if standard Mobile IPv6
was used alone. Local and end-to-end signaling then comes together
because, as it stands, a mobile node must contact the new MAP
separately from its home agent and correspondent nodes. Due to
dependencies between a MAP registration and a contemporary home or
correspondent registration, a mobile node may want to wait for the
MAP registration to complete before it initiates the standard Mobile
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IPv6 registration procedure. Handover latency is then increased in
addition to the signaling overhead. Future work could thus go into
the integration of MAP registrations with standard Mobile IPv6
signaling.
Another interesting research opportunity seems to be a mechanism that
tells neighboring MAPs from different administrative sites that their
domains overlap. The MAPs could then mutually advertise each other
throughout certain parts of their domains.
The cost for an inter-MAP handover in terms of signaling load and
delay strongly depends on the network topology. In an optimal
layout, a MAP is located somewhere on the path from the mobile node
to its home agent and correspondent nodes. This may be the case if
the MAP is co-located with an Internet gateway. Then, switching MAPs
is cheap. On the other hand, if the MAP is way off the direct path
between a mobile node and its peers, the additional overhead might
become noticeable. Indeed, a good topological layout is crucial for
the performance of HMIPv6.
The second local optimization, Fast Handovers for Mobile IPv6
(FMIPv6) [17], streamlines the router-discovery and
IPv6-address-configuration processes, and it facilitates lossless
handovers. FMIPv6 assumes that access routers are capable of
matching a neighboring access point to the access router to which it
attaches. The capability is a prerequisite for proxy router
discovery. Yet, it is still an open issue how access routers learn
about this information. Manual configuration is one option, though
it can be extremely expensive. More attractive would be an automated
mechanism that allows access routers to dynamically recognize access
points to which mobile nodes may want to switch. A related issue is
how such knowledge can be securely obtained across the borders of
administrative sites. These are opportunities for future research.
Note that Hierarchical Mobile IPv6 is applicable even when Mobile
IPv6 is not used beyond the local domain. I.e., a mobile node may
use its regional care-of address as a temporary home address. The
mobile node would thus appear to a correspondent node as a stationary
node in the MAP's network. This allows the mobile node to keep its
movements private as long as it stays within the same MAP domain.
FMIPv6 can be used in combination with standard Mobile IPv6, HMIPv6,
or both, but it cannot be used without either.
Of course, there are disadvantages with HMIPv6 and FMIPv6. Local
optimizations in general require investments in the access network
and thus imply additional costs for the network operator. Also, as
mentioned, localized mobility support may even cause increased
overhead in certain situations. And local mechanisms are to date
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ineffective for handovers across administrative domains unless
providers have mutual agreements to interconnect their networks.
6.2.2 Pre-Configuration
The Home Keygen Token exchange from the return-routability procedure
is the default authentication technique used in Mobile IPv6. It
facilitates reasonable security even between nodes that have no
pre-existing relationship. On the other hand, nodes that do share a
common secret should be allowed to omit the home-address test. This
can be beneficial in certain scenarios where the home-address test
causes a long handover latency due to packet redirection through the
home agent. Note that a shared secret cannot replace the
care-of-address test. Eliminating the care-of-address test requires
some sort of trust into mobile nodes not to spoof a care-of address.
The pre-configuration approach standardized in the IETF [15] uses a
shared secret between the mobile node and the correspondent node, and
it assumes that mobile nodes are trustworthy.
The assumption that mobile nodes be fair-minded turns out to be quite
far stretching. On on side, it affords the elimination of the entire
return-routability procedure, not just the Home Keygen Token
exchange. As explained in [15], and as it can also be inferred from
figure Figure 1, this cuts the average handover latency in half. On
the other hand, the assumption significantly limits the applicability
of the optimization. There are certainly scenarios where
pre-configuration per se would be possible, but no trust model can be
assumed. For instance, an ISP may configure its media servers with
the keys of its customers. The customers could then use
pre-configured Mobile IPv6 for communications with the media servers,
but some customers might misuse the lack of a care-of-address test to
wage a redirection-based flooding attack against a third party. This
example reveals the difference between a security association for
authentication and a trust relationship for misuse prevention.
In an effort to extend pre-configuration to scenarios where no trust
relationship can be presupposed, one may combine it with
care-of-address tests. Of course, using a care-of-address test
partly vitiates the handover-latency improvement that can be reached
otherwise. But there may still be a positive impact on handover
latency, because pre-configuration eliminates the triangular
home-address test through the home agent, whereas the care-of-address
test uses the direct, and typically faster, path between the
communicating nodes. For increased performance, a concurrent
care-of-address test can be used in combination with credit-based
authorization or heuristic monitoring. It should also be noted that
pre-configuration facilitates stronger authentication mechanisms than
the return-routability procedure, and thus the use of route
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optimization may become more suitable for applications with high
security requirements.
These things said, it seems like a good idea to make the
pre-configuration protocol bendable to different environments. Is
there a small group of people who trust each other? Then, of course,
group members are unlikely to spoof care-of addresses, and the
care-of-address test may be omitted. Or is the group of users large
and users are primarily unknown to each other like the customer base
of a big ISP? Then, proper authentication does usually not imply
trust, and it may not be feasible to use pre-configured keys without
checking a mobile node's reachability. Traceability techniques are
not necessarily a compensation for the missing care-of-address test,
because they are a reactive measure, whereas a care-of-address test
is a proactive one.
6.2.3 CGA-Based Optimizations
CGAs can guarantee that a mobile node is the legitimate owner of its
home address. They provide higher assurance than the pure use of
routing paths. This facilitates a significant reduction in the
number of signaling messages per correspondent registration as well
as the periodicity of these registrations. In addition, the public
keys used in the CGA technique allow packets to be transferred
privately, a feature that can be used for both data encryption and
for other route-optimization enhancements.
CGA may also be used to reduce the risk of flooding attacks via
care-of addresses, as attackers should be unable to generate a
private/public-key pair of which the public key hashes to a
particular victim's IP address. However, only the interface
identifier of a CGA is cryptographically generated, so flooding a
network or a link is still an issue. As a result, CGAs should be
employed together with a care-of-address test in scenarios where
redirection-based flooding attacks are a concern. Similarly, an
initial home-address test is typically required, too, in order to
avoid that the deletion of a binding causes a flood upon a faked home
address.
To resolve collisions in case CGAs are used as care-of addresses, a
collision count is part of the input to the CGA hash function.
Increasing the collision count by one changes the result of the hash
function, so new CGAs can be successively tried until an unused one
is found. On the other hand, the collision count also helps an
attacker in faking a CGA: It may use the same private/public-key pair
to efficiently generate multiple CGAs. For this reason, the
collision count should usually be limited to a few bytes only.
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6.2.4 Credit-Based Authorization
With CBA, a new care-of address can be probed in parallel with
already using it. This eliminates the handover latency that the
reachability check entails when performed during the critical
handover period. Depending on how much credit a mobile node has at
the time of handover, packets are either routed via its home network
or directly to its care-of address. The performance benefits depend
on the relative delay characteristics of the direct path between the
communicating peers and the path through the home agent. They also
depend on how fast the transport-layer protocol in use detects and
retransmits lost packets.
CBA depends on the mobile node being able to collect an amount of
credit high enough to bring it through the next handover. How easy
it is for a mobile node to acquire new credit depends on the CBA
mode, on the credit-aging factor, and on the application's traffic
patterns. (See Section 5.7 for a description of the two CBA modes.)
In general, applications with symmetric traffic patterns make it easy
for the mobile node to get sufficient credit. CBA mode 1, however,
can be problematic when most data is sent from the correspondent node
to the mobile node unless the credit-aging factor is very small. The
reason is that, in this mode, earned credit is proportional to
packets sent to the correspondent node, whereas spent credit is
proportional to packets going into the opposite direction. Also, CBA
does not work well in either mode if the mobile node moves so rapidly
that it does not manage to refill its credit account between two
successive handovers.
Note that CBA is an optimization technique which can be integrated
into any mobility-management protocol that verifies IP addresses
through probing. Protocols that may benefit are, for instance, other
Mobile IPv6 optimization techniques described in this paper such as
CGA-based ones (cf. Section 5.9). Also, when Mobile IPv6 is used
with pre-configured shared keys between mobile nodes and
correspondent nodes (cf. Section 6.2.2), but reachability checks are
still prescribed to ensure that properly authenticated mobile nodes
do not lie about their care-of address, CBA may be applied to
eliminate the additional cost that the reachability checks would
otherwise entail. Moreover, in scenarios where a home agent cannot
trust in the correctness of the registered mobile nodes' care-of
addresses, CBA-based concurrent care-of-address tests could be
proscribed even for home registrations. The same is true for
Hierarchical Mobile IPv6, which, as it stands, assumes that a MAP can
be confident that mobile nodes use correct on-link care-of addresses,
and so gets around the care-of-address test.
Furthermore, CBA may also be used to parallelize reachability checks
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in the Host Identity Protocol (HIP) [22] and current Mobike
proposals. Note, however, that mobility-management protocols in
general do not have an equivalent to Mobile IPv6's static home
addresses through which mobile nodes stay reachable at all times.
Hence, temporarily routing a mobile node's packets through a static
IP address in case this mobile node runs out of credit during a
handover may not always be an option.
A nice property of CBA mode 1 is that it does not require support
from the mobile node. The mobile node neither needs to understand
that CBA is effective at the correspondent node, nor does it have to
have an idea of how much credit it currently has. A legitimate
question is whether this is true even if the correspondent node may
temporarily send packets to the home address of a mobile node that
has run out of credit. After all, RFC 3775 suggests that mobile
nodes interpret the reception of a tunneled packet as a hint to
initiate a new correspondent registration, which would obviously be
inappropriate in the described situation. The answer is that all
correspondent registration are subject to rate limiting, so mobile
nodes can be expected not to misunderstand tunneled packets during
the handover procedure.
Care-of Address Spot Checks must be responded to by a mobile node.
Consequently, CBA mode 2 can only be implemented transparently to the
mobile node in scenarios where packet loss is not an issue and
Care-of Address Spot Checks can be omitted.
Last, but not least, an interesting question is whether CBA mode 2
could be misused by an attacker that has an asymmetric connection to
the Internet. Wide-spread digital subscriber lines (DSL), for
instance, 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 strong
download rate could be misused to illegitimately build up credit at
one or many correspondent nodes. The credit could then be used for a
more potent, redirection-based flooding attack. The reason why this
has so far not been considered an issue is that, in order to build up
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. This is obviously true with respect to data
volume. Since credit is aged over time, it also applies to the data
rate.
6.2.5 Prefix-Based Certificates
The Mobile IPv6 base specification avoids strong authentication
cryptography for signaling between mobile nodes and correspondent
nodes. One reason for this is the impracticality of a global trusted
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PKI that could approve bindings between the mobile nodes' identities
and public keys. Another reason is that limited power resources and
processing capacity at the mobile nodes generally rule out any
complex cryptographic operations. Robustness to resource-exhaustion
attacks requires a similar restrictiveness on the correspondent-node
side.
CBU circumvents the lack of a global PKI. The reduction in the
number of potentially required certificates makes certificate-based
approaches to mobile-node authentication more feasible than it is
today. The approach also avoids heavy computations at mobile nodes
since public-key cryptography is handled by the home agent. On the
other hand, the processing overhead at correspondent nodes actually
increases compared to standard correspondent registrations. This may
not be an issue since resources at stationary correspondent nodes are
usually higher than those of many mobile devices. But it may be an
issue if the correspondent node is a popular web server or other
central resource that cannot afford doing complex cryptographic
operations. One should, however, bear in mind that the increased
overhand implies a higher risk to resource-exhaustion attacks.
CBU does not solve the issue with care-of-address spoofing: A
vouching home agent does not prevent a malicious mobile node from
faking its care-of address. The culprit could cheat its home agent,
or it could cooperate with it. This said, CBU should be combined
with a care-of-address test that rules out redirection-based flooding
attacks. A combination of concurrent care-of-address tests and CBA
(cf. Section 5.7) can be used to keep the signaling delay during
handover as low as it currently is in [23].
There is ongoing work on the integration of AAA with Mobile IPv6
[13]. The current focus is on authentication between mobile nodes
and home agents with the intention to replace the IPsec-based
authentication protocol for home registrations. But the concept of
security proxies proposed in [23] may as well be re-used for
enhancements to the AAA infrastructure.
6.3 Future Research
Mobility-related optimizations are currently actively studied by many
researchers at different protocol layers. The preceding sections
identify ideas and existing proposals for enhancing route
optimization. While some of the basic methods are fairly well
understood and are being deployed, there are a number of interesting,
newer approaches that deserve to be studied in more detail. This
section discusses research directions that appear fruitful, or
necessary in the future, and that go beyond the existing proposals
described so far.
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6.3.1 Research at Other Protocol Layers
The efficiency and security related to movements does not depend on
Mobile IPv6 route optimization alone, even if researchers often pose
their analysis in that light. A movement that is visible at the IP
layer involves all lower layers as well. This includes layer 2
attachment procedures; layer 2 security mechanisms such as
negotiation, authentication, and key agreement; IPv6 Router and
Neighbor Discovery; as well as IPv6 Address Autoconfiguration and
Duplicate Address Detection. A complete network attachment typically
requires over twenty link- and IP-layer messages, assuming that
features necessary for a commercial deployment (such as security) are
turned on.
A significant research question is the performance of the
network-access stack as a whole. Current protocol stacks have a
number of limitations in addition to the long attachment delays [31],
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.
A number attempts are ongoing to improve various parts of the stack,
mostly focusing on handover performance. These include link-layer
enhancements, parameter tuning [39], network-access authentication
mechanisms [1], fast-handover mechanisms [35], AAA architectures
[21], and IP-layer attachment improvements [12]. It is uncertain how
far this optimization can be taken by only looking at the different
parts individually. An integrated approach may be necessary to gain
more significant improvements [32].
It is also unclear at this time which components are the most
critical ones. [31] suggests that mobility-related signaling
contributes only under 10% of the overall delay in an IEEE 802.11
environment. The most significant delays are caused at the link
layer and for IPv6 attachment. However, the results are not
conclusive due to the high deviation between the measurements. The
results can also be affected by a number of conditions, such as the
availability of specific link-layer optimizations, or the type of
security mechanism used for Mobile IPv6 home registrations.
6.3.2 Further Route Optimization Research
The primary driver to improve route optimization appears to be better
efficiency for a few usage scenarios, such as fast movements or the
ability to reduce signaling frequencies for hosts in standby mode.
Ongoing work addresses these aspects already quite well, and many of
the suggested methods are reasonably stable in this regard. It is
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expected that further, perhaps smaller improvements will continue to
be achieved through research and parameter tuning far into the
future. This is particularly true because the optimizations are
often targeted to a specific usage scenario, and may not give the
same improvement in other situations. As a result, the publication
of a few enhanced methods for different scenarios seems more
reasonable than expecting to define a final, all-encompassing method.
The development of an infrastructure-based route-optimization method
is clearly a longer-term project. At this stage, it is not even
clear that such a mechanism is needed. The Certificate-Based Binding
Update Protocol shows some promise in this area, particularly if it
can be combined with other mechanisms that use certificates, such as
Secure Neighbor Discovery [20]. Pre-configuring keys into end hosts
[15] is simple and efficient, but the number of scenarios where it
applies is likely to be very limited only.
The following is a list of interesting ideas for new
route-optimization research.
o Local mobility or local repair optimizations that require no
configuration.
o Care-of-address verification mechanisms that employ lower-layer
assistance or Secure Neighbor Discovery.
o The introduction of optimizations developed in the context of
Mobile IPv6 to HIP or other mobility protocols, or to link-layer
mobility solutions.
o The extension of the developed techniques to full
multi-addressing, including also multi-homing.
o Further development of techniques that are based on "asymmetric
cost wars" [33], such as CBA.
o Integrated techniques taking into account both link and IP layer
mobility tasks.
6.3.3 Experimentation and Simulation
As discussed earlier, the contribution of different stack parts to
the overall movement latency is still unclear. The following is a
list of areas where measurements and experimentation can yield
further, valuable insight.
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o Measurements of a realistic network scenario, enabling all
features that would likely be needed in a commercial deployment.
These features include link-layer access control, for instance.
Similarly, it is necessary to consider support for existing
enhancement proposals.
o Measurements and simulations of the performance impacts that
existing enhancement proposals have on the different parts of the
stack.
o Measurements and 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, or with respect to early packet
transmission before reception of a BA.
o Measurements of the impact that network conditions such as packet
loss can have on existing and new route-optimization mechanisms.
o Statistical data collection on the behavior of mobile nodes in
different networks. Route-optimization techniques behave
differently depending on what the frequency of movements is, or
what traffic streams appear during a mobile node's lifetime.
o Measurements or simulations of the performance that existing
route-optimization schemes show under different application
scenarios, such as the use of applications with symmetric vs.
asymmetric traffic patterns.
7. Security Considerations
Security issues related to route optimization are an integral part of
this paper and are as such discussed throughout the paper.
8. Conclusions
Mobility-related optimizations are currently actively studied by many
researchers. Some of the basic techniques--such as the
return-routability procedure, pre-configured keys, or CGAs--are
either already being deployed or can expected to be in the near
future. A growing number of new proposals are being studied that
attempt to optimize these basic techniques further, or to make them
better applicable to a particular scenario.
Many of the current proposals are mature enough to withstand close
scrutiny. Their relative advantages are rather subjective, however.
For instance, some proposals are very efficient, but have a high cost
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in terms of configuration, whereas others do not require
configuration, but are slower. It hence appears likely that more
than one new method will have to be standardized. Deployment
experience is also important, so publication of a few alternative
methods as RFCs would be desirable.
It is interesting to see that most if not all current proposals had
predecessors that were shown to be insecure. For instance, the
initial return-routability procedure as well as the first versions of
CGAs were published before the threat of flooding attacks was fully
understood. Concurrent care-of-address tests were also first
suggested with insufficient protection against flooding attacks. And
several proposals employing semi-permanent security associations have
initially suffered from impersonation attacks. This shows the need
to reserve a sufficient amount of time for community analysis and
review of new methods.
Another interesting observation is that most mature proposals combine
a number of techniques and do not rely on any single approach. This
is due to the intricate nature of the problem: to build a mechanism
that is efficient and at the same time avoids a quite significant
number of potential security vulnerabilities.
On the other hand, it is also necessary to avoid overly expensive or
complex solutions. For instance, in evaluating the security needs
for route optimization, it is important to compare these needs to
other vulnerabilities, e.g., denial-of-service attacks, that already
exist for on-path attackers in an Internet without mobility support.
Of course, a mobility-management protocol should not make these
vulnerabilities worse. But since the issues already exist, it is not
necessarily a requirement that they be solved by a
mobility-management protocol.
A significant research question is the overall performance of the
network stack in a mobile setting. This includes mobility management
at the IP layer, but is not limited to it. Current network-access
protocol stacks have a number of limitations, such as long attachment
and movement latencies or significant denial-of-service
vulnerabilities. It is uncertain whether further, significant
benefits can be achieved if one continues to look at the different
parts of the network stack individually. Most likely, a more
comprehensive approach is needed. It is also unclear at this time
which components of the network stack are the most critical ones to
optimize.
Other significant research questions include what effect network
conditions such as packet loss can have on current proposals, and to
what degree proposals depend on specific application patterns. Our
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current understanding about the different traffic patterns and their
effects on mobility is limited, and experiments, modelling, and
simulations will be needed.
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9. References
[1] Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access Control",
IEEE Standard 802.1X, September 2001.
[2] Arkko, J. and C. Vogt, "Credit-Based Authorization for Binding
Lifetime Extension",
Internet-Draft draft-arkko-mipv6-binding-lifetime-extension-00,
May 2004.
[3] Bradner, S., Mankin, A. and J. Schiller, "A Framework for
Purpose-Built Keys (PBK)",
Internet-Draft draft-bradner-pbk-frame-06, June 2003.
[4] Daley, G., "Location Privacy and Mobile IPv6",
Internet-Draft draft-daley-mip6-locpriv-00, January 2004.
[5] Dupont, F. and J. Combes, "Using IPsec between Mobile and
Correspondent IPv6 Nodes",
Internet-Draft draft-dupont-mipv6-cn-ipsec-01, June 2004.
[6] Haddad, W., Madour, L., Arkko, J. and F. Dupont, "Applying
Cryptographically Generated Addresses to Optimize MIPv6
(CGA-OMIPv6)", Internet-Draft draft-haddad-mip6-cga-omipv6-02,
June 2004.
[7] Haddad, W. and S. Krishnan, "Optimizing Mobile IPv6 (OMIPv6)",
Internet-Draft draft-haddad-mipv6-omipv6-01, February 2004.
[8] Haddad, W., "Privacy for Mobile and Multi-homed Nodes: MoMiPriv
Problem Statement",
Internet-Draft draft-haddad-momipriv-problem-statement-00,
October 2004.
[9] Moskowitz, R., "Host Identity Protocol",
Internet-Draft draft-ietf-hip-base-00, June 2004.
[10] Kent, S., "IP Encapsulating Security Payload (ESP)",
Internet-Draft draft-ietf-ipsec-esp-v3-08, March 2004.
[11] Loughney, J., "IPv6 Node Requirements",
Internet-Draft draft-ietf-ipv6-node-requirements-11, August
2004.
[12] Moore, N., "Optimistic Duplicate Address Detection for IPv6",
Internet-Draft draft-ietf-ipv6-optimistic-dad-01, June 2004.
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[13] Patel, A., Leung, K., Khalil, M., Akhtar, H. and K. Chowdhury,
"Authentication Protocol for Mobile IPv6",
Internet-Draft draft-ietf-mip6-auth-protocol-00, July 2004.
[14] Patel, A., "Problem Statement for bootstrapping Mobile IPv6",
Internet-Draft draft-ietf-mip6-bootstrap-ps-00, July 2004.
[15] Perkins, C., "Preconfigured Binding Management Keys for Mobile
IPv6", Internet-Draft draft-ietf-mip6-precfgKbm-00, April 2004.
[16] Nikander, P., Arkko, J., Aura, T., Montenegro, G. and E.
Nordmark, "Mobile IP version 6 Route Optimization Security
Design Background", Internet-Draft draft-ietf-mip6-ro-sec-01,
July 2004.
[17] Koodli, R., "Fast Handovers for Mobile IPv6",
Internet-Draft draft-ietf-mipshop-fast-mipv6-02, July 2004.
[18] Soliman, H., Castelluccia, C., Malki, K. and L. Bellier,
"Hierarchical Mobile IPv6 mobility management (HMIPv6)",
Internet-Draft draft-ietf-mipshop-hmipv6-02, June 2004.
[19] Aura, T., "Cryptographically Generated Addresses (CGA)",
Internet-Draft draft-ietf-send-cga-06, April 2004.
[20] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P. Nikander,
"SEcure Neighbor Discovery (SEND)",
Internet-Draft draft-ietf-send-ndopt-06, July 2004.
[21] Arbaugh, W. and B. Aboba, "Experimental Handoff Extension to
RADIUS", Internet-Draft draft-irtf-aaaarch-handoff-04, November
2003.
[22] Moskowitz, R., Nikander, P. and P. Jokela, "Host Identity
Protocol", Internet-Draft draft-moskowitz-hip-09, February
2004.
[23] Bao, F., "Certificate-based Binding Update Protocol (CBU)",
Internet-Draft draft-qiu-mip6-certificated-binding-update-02,
August 2004.
[24] Roe, M., Aura, T., O'Shea, G. and J. Arkko, "Authentication of
Mobile IPv6 Binding Updates and Acknowledgments",
Internet-Draft draft-roe-mobileip-updateauth-02, March 2002.
[25] Vogt, C., Bless, R., Doll, M. and T. Kuefner, "Early Binding
Updates for Mobile IPv6",
Internet-Draft draft-vogt-mip6-early-binding-updates-00,
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February 2004.
[26] Vogt, C., Arkko, J., Bless, R., Doll, M. and T. Kuefner,
"Credit-Based Authorization for Mobile IPv6 Early Binding
Updates",
Internet-Draft draft-vogt-mipv6-credit-based-authorization-00,
May 2004.
[27] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[28] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
2002.
[29] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[30] Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
Agents", RFC 3776, June 2004.
[31] Alimian, A. and B. Aboba, "Analysis of Roaming Techniques",
IEEE Contribution 11-04-0377r1 2004.
[32] Arkko, J., Eronen, P., Nikander, P. and V. Torvinen, "Secure
and Efficient Network Access", Extended abstract to be
presented in the DIMACS workshop, November 2004.
[33] Arkko, J. and P. Nikander, "Weak Authentication: How to
Authenticate Unknown Principals without Trusted Parties",
Proceedings of Security Protocols Workshop 2002, Cambridge, UK,
April 16-19, 2002.
[34] 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.
[35] Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang,
"Proactive Key Distribution to Support Fast and Secure
Roaming", IEEE Contribution 11-03-084r1-I, January 2003.
[36] Nikander, P., "Denial-of-Service, Address Ownership, and Early
Authentication in the IPv6 World", Proceedings of the Cambridge
Security Protocols Workshop, April 2001.
[37] O'Shea, G. and M. Roe, "Child-proof Authentication for MIPv6",
Computer Communications Review, April 2001.
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[38] Paxson, V., "An Analysis of Using Reflectors for Distributed
Denial-of-Service Attacks", Computer Communication Review
31(3)., July 2001.
[39] Velayos, H. and G. Karlsson, "Techniques to Reduce IEEE 802.11b
MAC Layer Handover Time", Laboratory for Communication
Networks, KTH, Royal Institute of Technology, Stockholm,
Sweden, TRITA-IMIT-LCN R 03:02, April 2003.
Authors' Addresses
Jari Arkko
Ericsson Research NomadicLab
FI-02420 Jorvas
Finland
Email: jari.arkko@ericsson.com
Christian Vogt
Institute of Telematics
University of Karlsruhe
P.O. Box 6980
76128 Karlsruhe
Germany
Email: chvogt@tm.uka.de
URI: http://www.tm.uka.de/~chvogt/
Appendix A. Acknowledgements
The authors wish to thank Gabriel Montenegro and Rajeev Koodli for
their support, review, and suggestions related to this paper.
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