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Network Working Group C. Ng
Request for Comments: 4889 Panasonic Singapore Labs
Category: Informational F. Zhao
UC Davis
M. Watari
KDDI R&D Labs
P. Thubert
Cisco Systems
July 2007
Network Mobility Route Optimization Solution Space Analysis
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
With current Network Mobility (NEMO) Basic Support, all
communications to and from Mobile Network Nodes must go through the
Mobile Router and Home Agent (MRHA) tunnel when the mobile network is
away. This results in increased length of packet route and increased
packet delay in most cases. To overcome these limitations, one might
have to turn to Route Optimization (RO) for NEMO. This memo
documents various types of Route Optimization in NEMO and explores
the benefits and tradeoffs in different aspects of NEMO Route
Optimization.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Benefits of NEMO Route Optimization . . . . . . . . . . . . . 4
3. Different Scenarios of NEMO Route Optimization . . . . . . . . 6
3.1. Non-Nested NEMO Route Optimization . . . . . . . . . . . . 6
3.2. Nested Mobility Optimization . . . . . . . . . . . . . . . 8
3.2.1. Decreasing the Number of Home Agents on the Path . . . 8
3.2.2. Decreasing the Number of Tunnels . . . . . . . . . . . 9
3.3. Infrastructure-Based Optimization . . . . . . . . . . . . 9
3.4. Intra-NEMO Optimization . . . . . . . . . . . . . . . . . 10
4. Issues of NEMO Route Optimization . . . . . . . . . . . . . . 11
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RFC 4889 NEMO RO Space Analysis July 2007
4.1. Additional Signaling Overhead . . . . . . . . . . . . . . 11
4.2. Increased Protocol Complexity and Processing Load . . . . 12
4.3. Increased Delay during Handoff . . . . . . . . . . . . . . 12
4.4. Extending Nodes with New Functionalities . . . . . . . . . 13
4.5. Detection of New Functionalities . . . . . . . . . . . . . 14
4.6. Scalability . . . . . . . . . . . . . . . . . . . . . . . 14
4.7. Mobility Transparency . . . . . . . . . . . . . . . . . . 14
4.8. Location Privacy . . . . . . . . . . . . . . . . . . . . . 15
4.9. Security Consideration . . . . . . . . . . . . . . . . . . 15
4.10. Support of Legacy Nodes . . . . . . . . . . . . . . . . . 15
5. Analysis of Solution Space . . . . . . . . . . . . . . . . . . 16
5.1. Which Entities Are Involved? . . . . . . . . . . . . . . . 16
5.1.1. Mobile Network Node and Correspondent Node . . . . . . 16
5.1.2. Mobile Router and Correspondent Node . . . . . . . . . 17
5.1.3. Mobile Router and Correspondent Router . . . . . . . . 17
5.1.4. Entities in the Infrastructure . . . . . . . . . . . . 18
5.2. Who Initiates Route Optimization? When? . . . . . . . . . 18
5.3. How Is Route Optimization Capability Detected? . . . . . . 19
5.4. How is the Address of the Mobile Network Node
Represented? . . . . . . . . . . . . . . . . . . . . . . . 20
5.5. How Is the Mobile Network Node's Address Bound to
Location? . . . . . . . . . . . . . . . . . . . . . . . . 20
5.5.1. Binding to the Location of Parent Mobile Router . . . 21
5.5.2. Binding to a Sequence of Upstream Mobile Routers . . . 23
5.5.3. Binding to the Location of Root Mobile Router . . . . 24
5.6. How Is Signaling Performed? . . . . . . . . . . . . . . . 26
5.7. How Is Data Transmitted? . . . . . . . . . . . . . . . . . 27
5.8. What Are the Security Considerations? . . . . . . . . . . 28
5.8.1. Security Considerations of Address Binding . . . . . . 28
5.8.2. End-to-End Integrity . . . . . . . . . . . . . . . . . 30
5.8.3. Location Privacy . . . . . . . . . . . . . . . . . . . 30
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 32
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . . 33
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RFC 4889 NEMO RO Space Analysis July 2007
1. Introduction
Network Mobility Route Optimization Problem Statement [1] describes
operational limitations and overheads incurred in a deployment of
Network Mobility (NEMO) Basic Support [2], which could be alleviated
by a set of NEMO Route Optimization techniques to be defined. The
term "Route Optimization" is used in a broader sense than already
defined for IPv6 Host Mobility in [3] to loosely refer to any
approach that optimizes the transmission of packets between a Mobile
Network Node and a Correspondent Node.
Solutions that would fit that general description were continuously
proposed since the early days of NEMO, even before the Working Group
was formed. Based on that long-standing stream of innovation, this
document classifies, at a generic level, the solution space of the
possible approaches that could be taken to solve the Route
Optimization-related problems for NEMO. The scope of the solutions,
the benefits, and the impacts to the existing implementations and
deployments are analyzed. This work should serve as a foundation for
the NEMO WG to decide where to focus its Route Optimization effort,
with a deeper understanding of the relative strengths and weaknesses
of each approach.
It should be beneficial for readers to keep in mind the design
requirements of NEMO [4]. A point to note is that since this
document discusses aspects of Route Optimization, the reader may
assume that a mobile network or a mobile host is away when they are
mentioned throughout this document, unless it is explicitly specified
that they are at home.
1.1. Terminology
It is expected that readers are familiar with terminologies related
to mobility in [3] and [5], and NEMO-related terms defined in [6].
In addition, the following Route Optimization-specific terms are used
in this document:
Correspondent Router (CR)
This refers to the router that is capable of terminating a Route
Optimization session on behalf of a Correspondent Node.
Correspondent Entity (CE)
This refers to the entity that a Mobile Router or Mobile Network
Node attempts to establish a Route Optimization session with.
Depending on the Route Optimization approach, the Correspondent
Entity may be a Correspondent Node or Correspondent Router.
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2. Benefits of NEMO Route Optimization
NEMO Route Optimization addresses the problems discussed in [1].
Although a standardized NEMO Route Optimization solution has yet to
materialize, one can expect it to show some of the following
benefits:
o Shorter Delay
Route Optimization involves the selection and utilization of a
lesser-cost (thus generally shorter and faster) route to be taken
for traffic between a Mobile Network Node and its Correspondent
Node. Hence, Route Optimization should improve the latency of the
data traffic between the two end nodes. This may in turn lead to
better overall Quality of Service characteristics, such as reduced
jitter and packet loss.
o Reduced Consumption of Overall Network Resources
Through the selection of a shorter route, the total link
utilization for all links used by traffic between the two end
nodes should be much lower than that used if Route Optimization is
not carried out. This would result in a lighter network load with
reduced congestion.
o Reduced Susceptibility to Link Failure
If a link along the bi-directional tunnel is disrupted, all
traffic to and from the mobile network will be affected until IP
routing recovers from the failure. An optimized route would
conceivably utilize a smaller number of links between the two end
nodes. Hence, the probability of a loss of connectivity due to a
single point of failure at a link should be lower as compared to
the longer non-optimized route.
o Greater Data Efficiency
Depending on the actual solution for NEMO Route Optimization, the
data packets exchanged between two end nodes may not require as
many levels of encapsulation as that in NEMO Basic Support. This
would mean less packet overheads and higher data efficiency. In
particular, avoiding packet fragmentation that may be induced by
the multiple levels of tunneling is critical for end-to-end
efficiency from the viewpoints of buffering and transport
protocols.
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o Reduced Processing Delay
In a nested mobile network, the application of Route Optimization
may eliminate the need for multiple encapsulations required by
NEMO Basic Support, which may result in less processing delay at
the points of encapsulation and decapsulation.
o Avoiding a Bottleneck in the Home Network
NEMO Route Optimization allows traffic to bypass the Home Agents.
Apart from having a more direct route, this also avoids routing
traffic via the home network, which may be a potential bottleneck
otherwise.
o Avoid the Security Policy Issue
Security policy may forbid a Mobile Router from tunneling traffic
of Visiting Mobile Nodes into the home network of the Mobile
Router. Route Optimization can be used to avoid this issue by
forwarding traffic from Visiting Mobile Nodes directly to their
destinations without going through the home network of the Mobile
Router.
However, it should be taken into consideration that a Route
Optimization mechanism may not be an appropriate solution since
the Mobile Router may still be held responsible for illegal
traffic sent from its Mobile Network Nodes even when Route
Optimization is used. In addition, there can be a variety of
different policies that might conflict with the deployment of
Route Optimization for Visiting Mobile Nodes. Being a policy
issue, solving this with a protocol at the policy plane might be
more appropriate.
o Avoid the Instability and Stalemate
[1] described a potential stalemate situation when a Home Agent is
nested within a mobile network. Route Optimization may circumvent
such stalemate situations by directly forwarding traffic upstream.
However, it should be noted that certain Route Optimization
schemes may require signaling packets to be first routed via the
Home Agent before an optimized route can be established. In such
cases, a Route Optimization solution cannot avoid the stalemate.
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3. Different Scenarios of NEMO Route Optimization
There are multiple proposals for providing various forms of Route
Optimization in the NEMO context. In the following sub-sections, we
describe the different scenarios that would require a Route
Optimization mechanism and list the potential solutions that have
been proposed in that area.
3.1. Non-Nested NEMO Route Optimization
The Non-Nested NEMO Route Optimization involves a Mobile Router
sending binding information to a Correspondent Entity. It does not
involve nesting of Mobile Routers or Visiting Mobile Nodes. The
Correspondent Entity can be a Correspondent Node or a Correspondent
Router. The interesting case is when the Correspondent Entity is a
Correspondent Router. With the use of Correspondent Router, Route
Optimization session is terminated at the Correspondent Router on
behalf of the Correspondent Node. As long as the Correspondent
Router is located "closer" to the Correspondent Node than the Home
Agent of the Mobile Router, the route between Mobile Network Node and
the Correspondent Node can be said to be optimized. For this
purpose, Correspondent Routers may be deployed to provide an optimal
route as illustrated in Figure 1.
************************** HAofMR
* #*#
* #*# +---------------------+
CN #*# | LEGEND |
o #*# +---------------------+
o ############### #*# | #: Tunnel |
CR ooooooooooooooo MR | *: NEMO Basic route |
############### | | o: Optimized route |
MNN +---------------------+
Figure 1: MR-CR Optimization
This form of optimization can carry traffic in both directions or
independently for the two directions of traffic:
o From MNN to CN
The Mobile Router locates the Correspondent Router, establishes a
tunnel with that Correspondent Router and sets up a route to the
Correspondent Node via the Correspondent Router over the tunnel.
Traffic to the Correspondent Node would no longer flow through the
Home Agent anymore.
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o From CN to MNN
The Correspondent Router is on the path of the traffic from the
Correspondent Node to the Home Agent. In addition, it has an
established tunnel with the current Care-of Address (CoA) of the
Mobile Router and is aware of the Mobile Network Prefix(es)
managed by the Mobile Router. The Correspondent Router can thus
intercept packets going to the mobile network, and forward them to
the Mobile Router over the established tunnel.
A straightforward approach to Route Optimization in NEMO is for the
Mobile Router to attempt Route Optimization with a Correspondent
Entity. This can be viewed as a logical extension to NEMO Basic
Support, where the Mobile Router would send Binding Updates
containing one or more Mobile Network Prefix options to the
Correspondent Entity. The Correspondent Entity, having received the
Binding Update, can then set up a bi-directional tunnel with the
Mobile Router at the current Care-of Address of the Mobile Router,
and inject a route to its routing table so that packets destined for
addresses in the Mobile Network Prefix will be routed through the bi-
directional tunnel.
The definition of Correspondent Router does not limit it to be a
fixed router. Here we consider the case where the Correspondent
Router is a Mobile Router. Thus, Route Optimization is initiated and
performed between a Mobile Router and its peer Mobile Router. Such
solutions are often posed with a requirement to leave the Mobile
Network Nodes untouched, as with the NEMO Basic Support protocol, and
therefore Mobile Routers handle the optimization management on behalf
of the Mobile Network Nodes. Thus, providing Route Optimization for
a Visiting Mobile Node is often out of scope for such a scenario
because such interaction would require extensions to the Mobile IPv6
protocol. This scenario is illustrated in Figure 2.
HAofCR ********************************** HAofMR
#*# #*#
#*# #*# +---------------------+
#*# #*# | LEGEND |
#*# #*# +---------------------+
#*# ############### #*# | #: Tunnel |
CR ooooooooooooooo MR | *: NEMO Basic route |
| ############### | | o: Optimized route |
MNN2 MNN1 +---------------------+
Figure 2: MR-MR Optimization
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This form of optimization can carry traffic for both directions
identically:
o MNN1 to/from MNN2
The Mobile Router locates the Correspondent Router, establishes a
tunnel with that Correspondent Router, and sets up a route to the
Mobile Network Node via the Correspondent Router over the tunnel.
Traffic to the Mobile Networks Nodes would no longer flow through
the Home Agents.
Examples of this approach include Optimized Route Cache (ORC) [7][8]
and Path Control Header (PCH) [9].
3.2. Nested Mobility Optimization
Optimization in Nested Mobility targets scenarios where a nesting of
mobility management protocols is created (i.e., Mobile IPv6-enabled
host inside a mobile network or multiple Mobile Routers that attach
behind one another creating a nested mobile network). Note that
because Mobile IPv6 defines its own Route Optimization mechanism in
its base protocol suite as a standard, collaboration between this and
NEMO protocols brings various complexities.
There are two main aspects in providing optimization for Nested
Mobility, and they are discussed in the following sub-sections.
3.2.1. Decreasing the Number of Home Agents on the Path
The aim is to remove the sub-optimality of paths caused by multiple
tunnels established between multiple Mobile Nodes and their Home
Agents. Such a solution will seek to minimize the number of Home
Agents along the path, by bypassing some of the Home Agent(s) from
the original path. Unlike the scenario where no nesting is formed
and only a single Home Agent exists along the path, bypassing one of
the many Home Agents can still be effective.
Solutions for Nested Mobility scenarios can usually be divided into
two cases based on whether the nesting involves Mobile IPv6 hosts or
only involves Mobile Routers. Since Mobile IPv6 defines its own
Route Optimization mechanism, providing an optimal path for such
hosts will require interaction with the protocol and may require an
altering of the messages exchanged during the Return Routability
procedure with the Correspondent Node.
An example of this approach include Reverse Routing Header (RRH)
[10].
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3.2.2. Decreasing the Number of Tunnels
The aim is to reduce the amplification effect of nested tunnels due
to the nesting of tunnels between the Visiting Mobile Node and its
Home Agent within the tunnel between the parent Mobile Router and the
parent Mobile Router's Home Agent. Such a solution will seek to
minimize the number of tunnels, possibly by collapsing the amount of
tunnels required through some form of signaling between Mobile Nodes,
or between Mobile Nodes and their Home Agents, or by using routing
headers to route packets through a discovered path. These limit the
consequences of the amplification effect of nested tunnels, and at
best, the performance of a nested mobile network will be the same as
though there were no nesting at all.
Examples of this approach include the Reverse Routing Header (RRH)
[10], Access Router Option (ARO) [11], and Nested Path Info (NPI)
[12].
3.3. Infrastructure-Based Optimization
An infrastructure-based optimization is an approach where
optimization is carried out fully in the infrastructure. One example
is to make use of Mobility Anchor Points (MAPs) such as defined in
HMIPv6 [13] to optimize routes between themselves. Another example
is to make use of proxy Home Agent such as defined in the global Home
Agent to Home Agent (HAHA) protocol [14]. A proxy Home Agent acts as
a Home Agent for the Mobile Node, and acts as a Mobile Node for the
Home Agent, Correspondent Node, Correspondent Router, and other
proxies. In particular, the proxy Home Agent terminates the MRHA
tunnel and the associated encryption, extracts the packets, and re-
encapsulates them to the destination. In this case, proxy Home
Agents are distributed in the infrastructure and each Mobile Router
binds to the closest proxy. The proxy, in turn, performs a primary
binding with a real Home Agent for that Mobile Router. Then, the
proxy might establish secondary bindings with other Home Agents or
proxies in the infrastructure, in order to improve the end-to-end
path. In this case, the proxies discover each other using some form
of Next Hop Resolution Protocol, establish a tunnel and exchange the
relevant Mobile Network Prefix information in the form of explicit
prefix routes.
Alternatively, another approach is to use prefix delegation. Here,
each Mobile Router in a nested mobile network is delegated a Mobile
Network Prefix from the access router using DHCP Prefix Delegation
[15]. Each Mobile Router also autoconfigures its Care-of Address
from this delegated prefix. In this way, the Care-of Addresses of
each Mobile Router are all formed from an aggregatable address space
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starting from the access router. This may be used to eliminate the
multiple tunnels caused by nesting of Mobile Nodes.
3.4. Intra-NEMO Optimization
A Route Optimization solution may seek to improve the communications
between two Mobile Network Nodes within a nested mobile network.
This would avoid traffic being injected out of the nested mobile
network and route them within the nested mobile network. An example
is the optimized route taken between MNN1 and MNN2 in Figure 3 below.
+--------+ +--------+ +--------+ +--------+
| MR2_HA | | MR3_HA | | MR4_HA | | MR5_HA |
+------+-+ +---+----+ +---+----+ +-+------+
\ | | /
+--------+ +------------------------------+
| MR1_HA |----| Internet |-----CN
+--------+ +--------------+---------------+
|
+----+----+
| MR1 |
+----+----+
|
---+-----------+-----------+---
| | |
+---+---+ +---+---+ +---+---+
| MR5 | | MR2 | | MR4 |
+---+---+ +---+---+ +---+---+
| | |
---+--- +---+---+ ---+---
MNN2 | MR3 | MNN3
+---+---+
|
----+----
MNN1
Figure 3: An Example of a Nested Mobile Network
One may be able to extend a well-designed NEMO Route Optimization for
"Nested Mobility Optimization" (see Section 3.2) to provide for such
kind of Intra-NEMO optimization, where, for example in Figure 3, MNN1
is treated as a Correspondent Node by MR5/MNN2, and MNN2 is treated
as a Correspondent Node by MR3/MNN1.
Another possibility is for the "Non-Nested NEMO Route Optimization"
technique (see Section 3.1) to be applied here. Using the same
example of communication between MNN1 and MNN2, both MR3 and MR2 can
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treat MR5 as Correspondent Routers for MNN2, and MR5 treats MR3 and
MR2 as Correspondent Routers for MNN1. An example of this approach
is [16], which has the Mobile Routers announce their Mobile Network
Prefixes to other Mobile Routers in the same nested Mobile Network.
Yet another approach is to flatten any nested Mobile Network so that
all nested Mobile Network Nodes appear to be virtually on the same
link. Examples of such approaches include delegating a single prefix
to the nested Mobile Network, having Mobile Routers to perform
Neighbor Discovery on behalf of their Mobile Network Nodes, and
exposing a single prefix over the entire mobile network using a
Mobile Ad-Hoc (MANET) protocol. In particular, it might prove useful
to develop a new type of MANET, specialized for the NEMO problem, a
MANET for NEMO (MANEMO). The MANEMO will optimize the formation of
the nested NEMO and maintain inner connectivity, whether or not a
connection to the infrastructure can be established.
4. Issues of NEMO Route Optimization
Although Route Optimization can bring benefits as described in
Section 2, the scenarios described in Section 3 do so with some
tradeoffs. This section explores some general issues that may impact
a NEMO Route Optimization mechanism.
4.1. Additional Signaling Overhead
The nodes involved in performing Route Optimization would be expected
to exchange additional signaling messages in order to establish Route
Optimization. The required amount of signaling depends on the
solution, but is likely to exceed the amount required in the home
Binding Update procedure defined in NEMO Basic Support. The amount
of signaling is likely to increase with the increasing number of
Mobile Network Nodes and/or Correspondent Nodes, and may be amplified
with nesting of mobile networks. It may scale to unacceptable
heights, especially to the resource-scarce mobile node, which
typically has limited power, memory, and processing capacity.
This may lead to an issue that impacts NEMO Route Optimization, known
as the phenomenon of "Binding Update Storm", or more generally,
"Signaling Storm". This occurs when a change in point of attachment
of the mobile network is accompanied with a sudden burst of signaling
messages, resulting in temporary congestion, packet delays, or even
packet loss. This effect will be especially significant for wireless
environment where bandwidth is relatively limited.
It is possible to moderate the effect of Signaling Storm by
incorporating mechanisms such as spreading the transmissions burst of
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signaling messages over a longer period of time, or aggregating the
signaling messages.
Even so, the amount of signaling required might be overwhelming,
since large mobile networks (such as those deployed on a train or
plane) may potentially have a large number of flows with a large
number of Correspondent Nodes. This might suggest a need to have
some adaptive behavior that depends on the amount of signaling
required versus the effort needed to tunnel home.
4.2. Increased Protocol Complexity and Processing Load
It is expected that NEMO Route Optimization will be more complicated
than NEMO Basic Support. Thus, complexity of nodes that are required
to incorporate new functionalities to support NEMO Route Optimization
would be higher than those required to provide NEMO Basic Support.
Coupled with the increased complexity, nodes that are involved in the
establishment and maintenance of Route Optimization will have to bear
the increased processing load. If such nodes are mobile, this may
prove to be a significant cost due to the limited power and
processing resources such devices usually have.
4.3. Increased Delay during Handoff
Due to the diversity of locations of different nodes that Mobile
Network Node may signal with and the complexity of NEMO Route
Optimization procedure that may cause several rounds of signaling
messages, a NEMO Route Optimization procedure may take a longer time
to finish its handoff than that in NEMO Basic Support. This may
exacerbate the overall delay during handoffs and further cause
performance degradation of the applications running on Mobile Network
Nodes.
Another NEMO-specific delay during handoff is that in a nested mobile
network, a child Mobile Network Node may need to detect or be
notified of the handoff of its parent Mobile Router so that it can
begin signaling its own Correspondent Entities. Apart from the
compromise of mobility transparency and location privacy (see
Section 4.7 and Section 4.8), this mechanism also increases the delay
during handoffs.
Some of the solutions for Mobile IPv6, such as Fast Handovers for
Mobile IPv6 [17], may be able to alleviate the increase in handoff
delay.
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4.4. Extending Nodes with New Functionalities
In order to support NEMO Route Optimization, some nodes need to be
changed or upgraded. Smaller number of nodes required to be changed
will allow for easier adoption of the NEMO Route Optimization
solution in the Internet and create less impact on existing Internet
infrastructure. The number and the types of nodes involved with new
functionalities also affect how much of the route is optimized. In
addition, it may also be beneficial to reuse existing protocols (such
as Mobile IPv6) as much as possible.
Possible nodes that may be required to change include the following:
o Local Fixed Nodes
It may prove to be difficult to introduce new functionalities at
Local Fixed Nodes, since by definition, any IPv6 node can be a
Local Fixed Node. This might mean that only those Local Fixed
Nodes that are modified can enjoy the benefits of Route
Optimization.
o Visiting Mobile Nodes
Visiting Mobile Nodes in general should already implement Mobile
IPv6 functionalities, and since Mobile IPv6 is a relatively new
standard, there is still a considerable window to allow mobile
devices to implement new functionalities.
o Mobile Routers
It is expected that Mobile Routers will implement new
functionalities in order to support Route Optimization.
o Access Routers
Some approaches require access routers, or nodes in the access
network, to implement some new functionalities. It may prove to
be difficult to do so, since access routers are, in general,
standard IPv6 routers.
o Home Agents
It is relatively easier for new functionalities to be implemented
in Home Agents.
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o Correspondent Nodes
It may prove to be difficult to introduce new functionalities at
Correspondent Nodes, since by definition, any IPv6 node can be a
Correspondent Node. This might mean that only those Correspondent
Nodes that are modified can enjoy the benefits of Route
Optimization.
o Correspondent Routers
Correspondent Routers are new entities introduced for the purpose
of Route Optimization, and therefore new functionalities can be
defined as needed.
4.5. Detection of New Functionalities
One issue that is related to the need for new functionalities as
described in Section 4.4 is the need to detect the existence of such
functionalities. In these cases, a detection mechanism might be
helpful to allow the initiator of Route Optimization to detect
whether support for the new functionalities is available.
Furthermore, it might be advantageous to have a graceful fall back
procedure if the required functionalities are unavailable.
4.6. Scalability
Given the same number of nodes, the number of Route Optimization
sessions would usually be more than the number of NEMO Basic Support
tunnels. If all Route Optimization sessions of a mobile network are
maintained by a single node (such as the Mobile Router), this would
mean that the single node has to keep track of the states of all
Route Optimization sessions. This may lead to scalability issues
especially when that single node is a mobile device with limited
memory and processing resources.
A similar scalability issue may be faced by a Correspondent Entity as
well if it maintains many route-optimized sessions on behalf of a
Correspondent Node(s) with a large number of Mobile Routers.
4.7. Mobility Transparency
One advantage of NEMO Basic Support is that the Mobile Network Nodes
need not be aware of the actual location and mobility of the mobile
network. With some approaches for Route Optimization, it might be
necessary to reveal the point of attachment of the Mobile Router to
the Mobile Network Nodes. This may mean a tradeoff between mobility
transparency and Route Optimization.
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4.8. Location Privacy
Without Route Optimization, the Correspondent Nodes are not aware of
the actual location and mobility of the mobile network and its Mobile
Network Nodes. To achieve Route Optimization, it might be necessary
to reveal the point of attachment of the Mobile Router to the
Correspondent Nodes. This may mean a tradeoff between location
privacy [18] and Route Optimization.
In Mobile IPv6, a mobile node can decide whether or not to perform
Route Optimization with a given Correspondent Node. Thus, the mobile
node is in control of whether to trade location privacy for an
optimized route. In NEMO Route Optimization, if the decision to
perform Router Optimization is made by the Mobile Router, it will be
difficult for Mobile Network Nodes to control the decision of having
this tradeoff.
4.9. Security Consideration
As Mobile Router and Home Agent usually belong to the same
administration domain, it is likely that there exists a security
association between them, which is leveraged by NEMO Basic Support to
conduct the home Binding Update in a secure way. However, NEMO Route
Optimization usually involves nodes from different domains (for
example, Mobile Router and Correspondent Entity); thus, the existence
of such a security association is not a valid assumption in many
deployment scenarios. For this reason, the security protection of
NEMO Route Optimization signaling message is considered "weaker" than
that in NEMO Basic Support. It is expected that some additional
security mechanisms are needed to achieve the same or similar level
of security as in NEMO Basic Support.
When considering security issues of NEMO Route Optimization, it might
be useful to keep in mind some of the security issues considered when
Mobile IPv6 Route Optimization was designed as documented in [19].
4.10. Support of Legacy Nodes
NEMO Basic Support is designed so that all legacy Mobile Network
Nodes (such as those that are not aware of the mobility of the
network they are in, and those that do not understand any mobility
protocols) can still reach and be reached from the Internet. Some
Route Optimization schemes, however, require that all Mobile Routers
implement the same Route Optimization scheme in order for them to
operate. Thus, a nested Mobile Router may not be able to achieve
Route Optimization if it is attached to a legacy Local Fixed Router.
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5. Analysis of Solution Space
As described in Section 3, there are various different approaches to
achieve Route Optimization in Network Mobility Support. In this
section, we attempt to analyze the vast solution space of NEMO Route
Optimization by asking the following questions:
1. Which entities are involved?
2. Who initiates Route Optimization? When?
3. How is Route Optimization capabilities detected?
4. How is the address of the Mobile Network Node represented?
5. How is the Mobile Network Node's address bound to location?
6. How is signaling performed?
7. How is data transmitted?
8. What are the security considerations?
5.1. Which Entities Are Involved?
There are many combinations of entities involved in Route
Optimization. When considering the role each entity plays in Route
Optimization, one has to bear in mind the considerations described in
Section 4.4 and Section 4.5. Below is a list of combinations to be
discussed in the following sub-sections:
o Mobile Network Node and Correspondent Node
o Mobile Router and Correspondent Node
o Mobile Router and Correspondent Router
o Entities in the Infrastructure
5.1.1. Mobile Network Node and Correspondent Node
A Mobile Network Node can establish Route Optimization with its
Correspondent Node, possibly the same way as a Mobile Node
establishes Route Optimization with its Correspondent Node in Mobile
IPv6. This would achieve the most optimal route, since the entire
end-to-end path is optimized. However, there might be scalability
issues since both the Mobile Network Node and the Correspondent Node
may need to maintain many Route Optimization sessions. In addition,
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new functionalities would be required for both the Mobile Network
Node and Correspondent Node. For the Mobile Network Node, it needs
to be able to manage its mobility, and possibly be aware of the
mobility of its upstream Mobile Router(s). For the Correspondent
Node, it needs to be able to maintain the bindings sent by the Mobile
Network Nodes.
5.1.2. Mobile Router and Correspondent Node
Alternatively, the Mobile Router can establish Route Optimization
with a Correspondent Node on behalf of the Mobile Network Node.
Since all packets to and from the Mobile Network Node must transit
the Mobile Router, this effectively achieves an optimal route for the
entire end-to-end path as well. Compared with Section 5.1.1, the
scalability issue here may be remedied since it is possible for the
Correspondent Node to maintain only one session with the Mobile
Router if it communicates with many Mobile Network Nodes associated
with the same Mobile Router. Furthermore, with the Mobile Router
handling Route Optimization, there is no need for Mobile Network
Nodes to implement new functionalities. However, new functionality
is likely to be required on the Correspondent Node. An additional
point of consideration is the amount of state information the Mobile
Router is required to maintain. Traditionally, it has been generally
avoided having state information in the routers to increase
proportionally with the number of pairs of communicating peers.
5.1.3. Mobile Router and Correspondent Router
Approaches involving Mobile Routers and Correspondent Routers are
described in Section 3.1. The advantage of these approaches is that
no additional functionality is required for the Correspondent Node
and Mobile Network Nodes. In addition, location privacy is
relatively preserved, since the current location of the mobile
network is only revealed to the Correspondent Router and not to the
Correspondent Node (please refer to Section 5.8.3 for more
discussions). Furthermore, if the Mobile Router and Correspondent
Router exchange prefix information, this approach may scale well
since a single Route Optimization session between the Mobile Router
and Correspondent Router can achieve Route Optimization between any
Mobile Network Node in the mobile network, and any Correspondent Node
managed by the Correspondent Router.
The main concern with this approach is the need for a mechanism to
allow the Mobile Router to detect the presence of the Correspondent
Router (see Section 5.3 for details), and its security impact. Both
the Mobile Router and the Correspondent Router need some means to
verify the validity of each other. This is discussed in greater
detail in Section 5.8.
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A deployment consideration with respect to the use of Correspondent
Router is the location of the Correspondent Router relative to the
Correspondent Node. On one hand, deploying the Correspondent Router
nearer to the Correspondent Node would result in a more optimal path.
On the other hand, a Correspondent Router that is placed farther away
from the Correspondent Node can perform Route Optimization on behalf
of more Correspondent Nodes.
5.1.4. Entities in the Infrastructure
Approaches using entities in the infrastructure are described in
Section 3.3. The advantages of this approach include, firstly, not
requiring new functionalities to be implemented on the Mobile Network
Nodes and Correspondent Nodes, and secondly, having most of the
complexity shifted to nodes in the infrastructure. However, one main
issue with this approach is how the Mobile Router can detect the
presence of such entities, and why the Mobile Router should trust
these entities. This may be easily addressed if such entity is a
Home Agent of the Mobile Router (such as in the global Home Agent to
Home Agent protocol [14]). Another concern is that the resulting
path may not be a true optimized one, since it depends on the
relative positions of the infrastructure entities with respect to the
mobile network and the Correspondent Node.
5.2. Who Initiates Route Optimization? When?
Having determined the entities involved in the Route Optimization in
the previous sub-section, the next question is which of these
entities should initiate the Route Optimization session. Usually,
the node that is moving (i.e., Mobile Network Node or Mobile Router)
is in the best position to detect its mobility. Thus, in general, it
is better for the mobile node to initiate the Route Optimization
session in order to handle the topology changes in any kind of
mobility pattern and achieve the optimized route promptly. However,
when the mobile node is within a nested mobile network, the detection
of the mobility of upstream Mobile Routers may need to be conveyed to
the nested Mobile Network Node. This might incur longer signaling
delay as discussed in Section 4.3.
Some solution may enable the node on the correspondent side to
initiate the Route Optimization session in certain situations. For
instance, when the Route Optimization state that is already
established on the Correspondent Entity is about to expire but the
communication is still active, depending on the policy, the
Correspondent Entity may initiate a Route Optimization request with
the mobile node side.
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There is also the question of when Route Optimization should be
initiated. Because Route Optimization would certainly incur
tradeoffs of various forms, it might not be desirable for Route
Optimization to be performed for any kind of traffic. This is,
however, implementation specific and policy driven.
A related question is how often signaling messages should be sent to
maintain the Route Optimization session. Typically, signaling
messages are likely to be sent whenever there are topological
changes. The discussion in Section 4.1 should be considered. In
addition, a Lifetime value is often used to indicate the period of
validity for the Route Optimization session. Signaling messages
would have to be sent before the Lifetime value expires in order to
maintain the Route Optimization session. The choice of Lifetime
value needs to balance between different considerations. On one
hand, a short Lifetime value would increase the amount of signaling
overhead. On the other hand, a long Lifetime value may expose the
Correspondent Entity to the risk of having an obsolete binding cache
entry, which creates an opportunity for an attacker to exploit.
5.3. How Is Route Optimization Capability Detected?
The question here is how the initiator of Route Optimization knows
whether the Correspondent Entity supports the functionality required
to established a Route Optimization session. The usual method is for
the initiator to attempt Route Optimization with the Correspondent
Entity. Depending on the protocol specifics, the initiator may
receive (i) a reply from the Correspondent Entity indicating its
capability, (ii) an error message from the Correspondent Entity, or
(iii) no response from the Correspondent Entity within a certain time
period. This serves as an indication of whether or not the
Correspondent Entity supports the required functionality to establish
Route Optimization. This form of detection may incur additional
delay as a penalty when the Correspondent Entity does not have Route
Optimization capability, especially when the Route Optimization
mechanism is using in-band signaling.
When the Correspondent Entity is not the Correspondent Node but a
Correspondent Router, an immediate question is how its presence can
be detected. One approach is for the initiator to send an Internet
Control Message Protocol (ICMP) message containing the address of the
Correspondent Node to a well-known anycast address reserved for all
Correspondent Routers [7][8]. Only the Correspondent Router that is
capable of terminating the Route Optimization session on behalf of
the Correspondent Node will respond. Another way is to insert a
Router Alert Option (RAO) into a packet sent to the Correspondent
Node [9]. Any Correspondent Router en route will process the Router
Alert Option and send a response to the Mobile Router.
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Both approaches need to consider the possibility of multiple
Correspondent Routers responding to the initiator, and both
approaches will generate additional traffic or processing load to
other routers. Furthermore, both approaches have yet to consider how
the initiator can verify the authenticity of the Correspondent
Routers that responded.
5.4. How is the Address of the Mobile Network Node Represented?
Normally, Route Optimization would mean that a binding between the
address of a Mobile Network Node and the location of the mobile
network is registered at the Correspondent Entity. Before exploring
different ways of binding (see Section 5.5), one must first ask how
the address of the Mobile Network Node is represented. Basically,
there are two ways to represent the Mobile Network Node's address:
o inferred by the use of the Mobile Network Prefix, or
o explicitly specifying the address of the Mobile Network Node.
Using the Mobile Network Prefix would usually mean that the initiator
is the Mobile Router, and has the benefit of binding numerous Mobile
Network Nodes with one signaling. However, it also means that if
location privacy is compromised, the location privacy of an entire
Mobile Network Prefix would be compromised.
On the other hand, using the Mobile Network Node's address would mean
that either the initiator is the Mobile Network Node itself or the
Mobile Router is initiating Route Optimization on behalf of the
Mobile Network Node. Initiation by the Mobile Network Node itself
means that the Mobile Network Node must have new functionalities
implemented, while initiation by the Mobile Router means that the
Mobile Router must maintain some Route Optimization states for each
Mobile Network Node.
5.5. How Is the Mobile Network Node's Address Bound to Location?
In order for route to be optimized, it is generally necessary for the
Correspondent Entity to create a binding between the address and the
location of the Mobile Network Node. This can be done in the
following ways:
o binding the address to the location of the parent Mobile Router,
o binding the address to a sequence of upstream Mobile Routers, and
o binding the address to the location of the root Mobile Router.
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These are described in the following sub-sections.
5.5.1. Binding to the Location of Parent Mobile Router
By binding the address of Mobile Network Node to the location of its
parent Mobile Router, the Correspondent Entity would know how to
reach the Mobile Network Node via the current location of the parent
Mobile Router. This can be done by:
o Binding Update with Mobile Network Prefix
This can be viewed as a logical extension to NEMO Basic Support,
where the Mobile Router would send binding updates containing one
or more Mobile Network Prefix options to the Correspondent Entity.
The Correspondent Entity having received the Binding Update, can
then set up a bi-directional tunnel with the Mobile Router at the
current Care-of Address of the Mobile Router, and inject a route
to its routing table so that packets destined for addresses in the
Mobile Network Prefix would be routed through the bi-directional
tunnel.
Note that in this case, the address of the Mobile Network Node is
implied by the Mobile Network Prefix (see Section 5.4).
o Sending Information of Parent Mobile Router
This involves the Mobile Network Node sending the information of
its Mobile Router to the Correspondent Entity, thus allowing the
Correspondent Entity to establish a binding between the address of
the Mobile Network Node to the location of the parent Mobile
Router. An example of such an approach would be [11].
o Mobile Router as a Proxy
Another approach is for the parent Mobile Router to act as a
"proxy" for its Mobile Network Nodes. In this case, the Mobile
Router uses the standard Mobile IPv6 Route Optimization procedure
to bind the address of a Mobile Network Node to the Mobile
Router's Care-of Address. For instance, when the Mobile Network
Node is a Local Fixed Node without Mobile IPv6 Route Optimization
functionality, the Mobile Router may initiate the Return
Routability procedure with a Correspondent Node on behalf of the
Local Fixed Node. An example of such an approach would be
[20][21][22].
On the other hand, if the Mobile Network Node is a Visiting Mobile
Node, it might be necessary for the Visiting Mobile Node to
delegate the rights of Route Optimization signaling to the Mobile
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Router (see [23] for an example of such delegation). With this
delegation, either the Visiting Mobile Network Node or the Mobile
Router can initiate the Return Routability procedure with the
Correspondent Node. For the case where the Return Routability
procedure is initiated by the Visiting Mobile Node, the Mobile
Router will have to transparently alter the content of the Return
Routability signaling messages so that packets sent from the
Correspondent Node to the Visiting Node will be routed to the
Care-of Address of the Mobile Router once Route Optimization is
established. The case where the Return Routability procedure is
initiated by the Mobile Router is similar to the case where the
Mobile Network Node is a Local Fixed Node.
For all of the approaches listed above, when the Mobile Network Node
is deeply nested within a Mobile Network, the Correspondent Entity
would need to gather Binding Updates from all the upstream Mobile
Routers in order to build the complete route to reach the Mobile
Network Node. This increases the complexity of the Correspondent
Entity, as the Correspondent Entity may need to perform multiple
binding cache look-ups before it can construct the complete route.
Other than increasing the complexity of the Correspondent Entity,
these approaches may incur extra signaling overhead and delay for a
nested Mobile Network Node. For instance, every Mobile Router on the
upstream of the Mobile Network Node needs to send Binding Updates to
the Correspondent Entity. If this is done by the upstream Mobile
Routers independently, it may incur additional signaling overhead.
Also, since each Binding Update takes a finite amount of time to
reach and be processed by the Correspondent Entity, the delay from
the time an optimized route is changed till the time the change is
registered on the Correspondent Entity will increase proportionally
with the number of Mobile Routers on the upstream of the Mobile
Network Node (i.e., the level of nesting of the Mobile Network Node).
For "Binding Update with Mobile Network Prefix" and "Sending
Information of Parent Mobile Router", new functionality is required
at the Correspondent Entity, whereas "Mobile Router as a Proxy" keeps
the functionality of the Correspondent Entity the same as a Mobile
IPv6 Correspondent Node. However, this is done at an expense of the
Mobile Routers, since in "Mobile Router as a Proxy", the Mobile
Router must maintain state information for every Route Optimization
session its Mobile Network Nodes have. Furthermore, in some cases,
the Mobile Router needs to look beyond the standard IPv6 headers for
ingress and egress packets, and alter the packet contents
appropriately (this may impact end-to-end integrity, see 5.8.2).
One advantage shared by all the approaches listed here is that only
mobility protocol is affected. In other words, no modification is
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required on other existing protocols (such as Neighbor Discovery).
There is also no additional requirement on existing infrastructure
(such as the access network).
In addition, having upstream Mobile Routers send Binding Updates
independently means that the Correspondent Entity can use the same
binding cache entries of upstream Mobile Routers to construct the
complete route to two Mobile Network Nodes that have common upstream
Mobile Routers. This may translate to lower memory consumption since
the Correspondent Entity need not store one complete route per Mobile
Network Node when it is having Route Optimization sessions with
multiple Mobile Network Nodes from the same mobile network.
5.5.2. Binding to a Sequence of Upstream Mobile Routers
For a nested Mobile Network Node, it might be more worthwhile to bind
its address to the sequence of points of attachment of upstream
Mobile Routers. In this way, the Correspondent Entity can build a
complete sequence of points of attachment from a single transmission
of the binding information. Examples using this approach are [10]
and [12].
Different from Section 5.5.1, this approach constructs the complete
route to a specific Mobile Network Node at the mobile network side,
thus offering the opportunity to reduce the signaling overhead.
Since the complete route is conveyed to the Correspondent Entity in a
single transmission, it is possible to reduce the delay from the time
an optimized route is changed till the time the change is registered
on the Correspondent Entity to its minimum.
One question that immediately comes to mind is how the Mobile Network
Node gets hold of the sequence of locations of its upstream Mobile
Routers. This is usually achieved by having such information
inserted as special options in the Router Advertisement messages
advertised by upstream Mobile Routers. To do so, not only must a
Mobile Router advertise its current location to its Mobile Network
Nodes, it must also relay information embedded in Router
Advertisement messages it has received from its upstream Mobile
Routers. This might imply a compromise of the mobility transparency
of a mobile network (see Section 4.7). In addition, it also means
that whenever an upstream Mobile Router changes its point of
attachment, all downstream Mobile Network Nodes must perform Route
Optimization signaling again, possibly leading to a "Signaling Storm"
(see Section 4.1).
A different method of conveying locations of upstream Mobile Routers
is (such as used in [10]) where upstream Mobile Routers insert their
current point of attachment into a Reverse Routing Header embedded
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within a packet sent by the Mobile Network Node. This may raise
security concerns that will be discussed later in Section 5.8.2.
In order for a Correspondent Entity to bind the address of a Mobile
Network Node to a sequence of locations of upstream Mobile Routers,
new functionalities need to be implemented on the Correspondent
Entity. The Correspondent Entity also needs to store the complete
sequence of locations of upstream Mobile Routers for every Mobile
Network Node. This may demand more memory compared to Section 5.5.1
if the same Correspondent Entity has a lot of Route Optimization
sessions with Mobile Network Nodes from the same nested Mobile
Network. In addition, some amount of modifications or extension to
existing protocols is also required, such as a new type of IPv6
routing header or a new option in the Router Advertisement message.
5.5.3. Binding to the Location of Root Mobile Router
A third approach is to bind the address of the Mobile Network Node to
the location of the root Mobile Router, regardless of how deeply
nested the Mobile Network Node is within a nested Mobile Network.
Whenever the Correspondent Entity needs to forward a packet to the
Mobile Network Node, it only needs to forward the packet to this
point of attachment. The mobile network will figure out how to
forward the packet to the Mobile Network Node by itself. This kind
of approach can be viewed as flattening the structure of a nested
Mobile Network, so that it seems to the Correspondent Entity that
every node in the Mobile Network is attached to the Internet at the
same network segment.
There are various approaches to achieve this:
o Prefix Delegation
Here, each Mobile Router in a nested mobile network is delegated a
Mobile Network Prefix from the access router (such as using
Dynamic Host Configuration Protocol (DHCP) Prefix Delegation
[15]). Each Mobile Router also autoconfigures its Care-of Address
from this delegated prefix. In this way, the Care-of Addresses of
Mobile Routers are all from an aggregatable address space starting
from the access router. A Mobile Network Node with Mobile IPv6
functionality may also autoconfigure its Care-of Address from this
delegated prefix, and use standard Mobile IPv6 mechanism's to bind
its Home Address to this Care-of Address.
Examples of this approach include [24], [25], and [26].
This approach has the advantage of keeping the implementations of
Correspondent Nodes unchanged. However, it requires the access
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router (or some other entity within the access network) and Mobile
Router to possess prefix delegation functionality, and also
maintain information on what prefix is delegated to which node.
How to efficiently assign a subset of Mobile Network Prefix to
child Mobile Routers could be an issue because Mobile Network
Nodes may dynamically join and leave with an unpredictable
pattern. In addition, a change in the point of attachment of the
root Mobile Router will also require every nested Mobile Router
(and possibly Visiting Mobile Nodes) to change their Care-of
Addresses and delegated prefixes. These will cause a burst of
Binding Updates and prefix delegation activities where every
Mobile Router and every Visiting Mobile Node start sending Binding
Updates to their Correspondent Entities.
o Neighbor Discovery Proxy
This approach (such as [27] and [28]) achieves Route Optimization
by having the Mobile Router act as a Neighbor Discovery [29] proxy
for its Mobile Network Nodes. The Mobile Router will configure a
Care-of Address from the network prefix advertised by its access
router, and also relay this prefix to its subnets. When a Mobile
Network Node configures an address from this prefix, the Mobile
Router will act as a Neighbor Discovery proxy on its behalf. In
this way, the entire mobile network and its access network form a
logical multilink subnet, thus eliminating any nesting.
This approach has the advantage of keeping the implementations of
Correspondent Nodes unchanged. However, it requires the root
Mobile Router to act as a Neighbor Discovery proxy for all the
Mobile Network Nodes that are directly or indirectly attached to
it. This increases the processing load of the root Mobile Router.
In addition, a change in the point of attachment of the root
Mobile Router will require every nested Mobile Router (and
possibly Visiting Mobile Nodes) to change their Care-of Addresses.
Not only will this cause a burst of Binding Updates where every
Mobile Router and every Visiting Mobile Node start sending Binding
Updates to their Correspondent Entities, it will also cause a
burst of Duplicate Address Discovery messages to be exchanged
between the mobile network and the access network. Furthermore,
Route Optimization for Local Fixed Nodes is not possible without
new functionalities implemented on the Local Fixed Nodes.
o Hierarchical Registrations
Hierarchical Registration involves Mobile Network Nodes (including
nested Mobile Routers) registering themselves with either their
parent Mobile Routers or the root Mobile Router itself. After
registrations, Mobile Network Nodes would tunnel packets directly
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to the upstream Mobile Router they register with. At the root
Mobile Router, packets tunneled from sub-Mobile Routers or Mobile
Network Nodes are tunneled directly to the Correspondent Entities,
thus avoiding nested tunneling.
One form of such an approach uses the principle of Hierarchical
Mobile IPv6 [13], where the root Mobile Router acts as a Mobility
Anchor Point. It is also possible for each parent Mobile Router
to act as Mobility Anchor Points for its child Mobile Routers,
thus forming a hierarchy of Mobility Anchor Points. One can also
view these Mobility Anchor Points as local Home Agents, thus
forming a cascade of mobile Home Agents. In this way, each Mobile
Router terminates its tunnel at its parent Mobile Router. Hence,
although there are equal numbers of tunnels as the level of
nestings, there is no tunnel encapsulated within another.
Examples of this approach include [30], [31], [32], and [33].
An advantage of this approach is that the functionalities of the
Correspondent Nodes are unchanged.
o Mobile Ad-Hoc Routing
It is possible for nodes within a mobile network to use Mobile Ad-
hoc routing for packet-forwarding between nodes in the same mobile
network. An approach of doing so might involve a router acting as
a gateway for connecting nodes in the mobile network to the global
Internet. All nodes in the mobile network would configure their
Care-of Addresses from one or more prefixes advertised by that
gateway, while their parent Mobile Routers use Mobile Ad-hoc
routing to forward packets to that gateway or other destinations
inside the mobile network.
One advantage that is common to all the approaches listed above is
that local mobility of a Mobile Network Node within a nested mobile
network is hidden from the Correspondent Entity.
5.6. How Is Signaling Performed?
In general, Route Optimization signaling can be done either in-plane,
off-plane, or both. In-plane signaling involves embedding signaling
information into headers of data packets. A good example of in-plane
signaling would be Reverse Routing Header [10]. Off-plane signaling
uses dedicated signaling packets rather than embedding signaling
information into headers of data packets. Proposals involving the
sending of Binding Updates fall into this category.
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The advantage of in-plane signaling is that any change in the mobile
network topology can be rapidly propagated to the Correspondent
Entity as long as there is a continuous stream of data to be
transmitted. However, this might incur a substantial overhead on the
data packets. Off-plane signaling, on the other hand, sends
signaling messages independently from the data packet. This has the
advantage of reducing the signaling overhead in situations where
there are relatively fewer topological changes to the mobile network.
However, data packet transmission may be disrupted while off-plane
signaling takes place.
An entirely different method of signaling makes use of upper-layer
protocols to establish the bindings between the address of a Mobile
Network Node and the location of the mobile network. Such binding
information can then be passed down to the IP layer to insert the
appropriate entry in the Binding Cache or routing table. An example
of such a mechanism is [34], which uses the Session Initiation
Protocol (SIP) to relay binding information.
5.7. How Is Data Transmitted?
With Route Optimization established, one remaining question to be
answered is how data packets can be routed to follow the optimized
route. There are the following possible approaches:
o Encapsulations
One way to route packets through the optimized path is to use IP-
in-IP encapsulations [35]. In this way, the original packet can
be tunneled to the location bound to the address of the Mobile
Network Node using the normal routing infrastructure. Depending
on how the location is bound to the address of the Mobile Network
Node, the number of encapsulations required might vary.
For instance, if the Correspondent Entity knows the full sequence
of points of attachment, it might be necessary for there to be
multiple encapsulations in order to forward the data packet
through each point of attachment. This may lead to the need for
multiple tunnels and extra packet header overhead. It is possible
to alleviate this by using Robust Header Compression techniques
[36][37][38] to compress the multiple tunnel packet headers.
o Routing Headers
A second way to route packets through the optimized path is to use
routing headers. This is useful especially for the case where the
Correspondent Entity knows the sequence of locations of upstream
Mobile Routers (see Section 5.5.2), since a routing header can
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contain multiple intermediate destinations. Each intermediate
destination corresponds to a point of attachment bound to the
address of the Mobile Network Node.
This requires the use of a new Routing Header type, or possibly an
extension of the Type 2 Routing Header as defined by Mobile IPv6
to contain multiple addresses instead of only one.
o Routing Entries in Parent Mobile Routers
Yet another way is for parent Mobile Routers to install routing
entries in their routing table that will route Route Optimized
packets differently, most likely based on source address routing.
This usually applies to approaches described in Section 5.5.3.
For instance, the Prefix Delegation approach [24][25][26] would
require parent Mobile Routers to route packets differently if the
source address belongs to the prefix delegated from the access
network.
5.8. What Are the Security Considerations?
5.8.1. Security Considerations of Address Binding
The most important security consideration in Route Optimization is
certainly the security risks a Correspondent Entity is exposed to by
creating a binding between the address of a Mobile Network Node and
the specified location(s) of the mobile network. Generally, it is
assumed that the Correspondent Entity and Mobile Network Node do not
share any pre-existing security association. However, the
Correspondent Entity must have some ways of verifying the
authenticity of the binding specified, else it will be susceptible to
various attacks described in [19], such as snooping (sending packets
meant for a Mobile Network Node to an attacker) or denial-of-service
(DoS) (flooding a victim with packets meant for a Mobile Network
Node) attacks.
When the binding is performed between the address of the Mobile
Network Node and one Care-of Address (possibly of the Mobile Router;
see Section 5.5.1 and Section 5.5.3), the standard Return Routability
procedure specified in Mobile IPv6 might be sufficient to provide a
reasonable degree of assurance to the Correspondent Entity. This
also allows the Correspondent Entity to re-use existing
implementations. But in other situations, an extension to the Return
Routability procedure might be necessary.
For instance, consider the case where the Mobile Router sends a
Binding Update containing Mobile Network Prefix information to the
Correspondent Entity (see Section 5.5.1). Although the Return
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Routability procedure allows the Correspondent Entity to verify that
the Care-of and Home Addresses of the Mobile Router are indeed
collocated, it does not allow the Correspondent Entity to verify the
validity of the Mobile Network Prefix. If the Correspondent Entity
accepts the binding without verification, it will be exposed to
attacks where the attacker tricks the Correspondent Entity into
forwarding packets destined for a mobile network to the attacker
(snooping) or victim (DoS); [39] discusses this security threat
further.
The need to verify the validity of network prefixes is not
constrained to Correspondent Entities. In approaches that involve
the Correspondent Routers (see Section 5.1.3), there have been
suggestions for the Correspondent Router to advertise the network
prefix(es) of Correspondent Nodes that the Correspondent Router is
capable of terminating Route Optimization on behalf of to Mobile
Network Nodes. In such cases, the Mobile Network Nodes also need a
mechanism to check the authenticity of such claims. Even if the
Correspondent Routers do not advertise the network prefix, the Mobile
Network Nodes also have the need to verify that the Correspondent
Router is indeed a valid Correspondent Router for a given
Correspondent Node.
In Section 5.5.2, the registration signaling involves sending the
information about one or more upstream Mobile Routers. The
Correspondent Entity (or Home Agent) must also have the means to
verify such information. Again, the standard Return Routability
procedure as defined in [3] is inadequate here, as it is not designed
to verify the reachability of an address over a series of upstream
routers. An extension such as attaching a routing header to the
Care-of Test (CoT) message to verify the authenticity of the
locations of upstream Mobile Routers is likely to be needed. The
risk, however, is not confined to Correspondent Entities. The Mobile
Network Nodes are also under the threat of receiving false
information from their upstream Mobile Routers, which they might pass
to Correspondent Entities (this also implies that Correspondent
Entities cannot rely on any security associations they have with the
Mobile Network Nodes to establish the validity of address bindings).
There are some considerations that this kind of on-path threat exists
in the current Internet anyway especially when no (or weak) end-to-
end protection is used.
All these concerns over the authenticity of addresses might suggest
that perhaps a more radical and robust approach is necessary. This
is currently under extensive study in various Working Groups of the
IETF, and many related documents might be of interest here. For
instance, in Secure Neighbor Discovery (SEND) [40], Cryptographically
Generated Addresses (CGAs) [41] could be used to establish the
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ownership of Care-of Addresses. [42] employs the Home Agent to check
the signaling messages sent by Mobile Routers to provide a way for
Correspondent Entities to verify the authenticity of Mobile Network
Prefixes specified. [18] documents various proposed enhancements to
the Mobile IPv6 Route Optimization mechanism that might be applied to
NEMO Route Optimization as well, such as [43], which allows the
Correspondent Entity to authenticate a certain operator's Home Agent
by verifying the associated certificate. The Host Identity Protocol
(HIP) [44] with end-host mobility considerations [45] may be extended
for NEMO Route Optimization as well.
In addition, interested readers might want to refer to [46], which
discussed the general problem of making Route Optimization in NEMO
secure and explored some possible solution schemes. There is also a
proposed mechanism in [23] for Mobile Network Node to delegate some
rights to their Mobile Routers, which may be used to allow the Mobile
Routers to prove their authenticities to Correspondent Entities when
establishing Route Optimization sessions on behalf of the Mobile
Network Nodes.
5.8.2. End-to-End Integrity
In some of the approaches, such as "Mobile Router as a Proxy" in
Section 5.5.1, the Mobile Router sends messages using the Mobile
Network Node's address as the source address. This is done mainly to
achieve zero new functionalities required at the Correspondent
Entities and the Mobile Network Nodes. However, adopting such a
strategy may interfere with existing or future protocols, most
particularly security-related protocols. This is especially true
when the Mobile Router needs to make changes to packets sent by
Mobile Network Nodes. In a sense, these approaches break the end-to-
end integrity of packets. A related concern is that this kind of
approach may also require the Mobile Router to inspect the packet
contents sent to/by Mobile Network Nodes. This may prove to be
difficult or impossible if such contents are encrypted.
The concern over end-to-end integrity arises for the use of a Reverse
Routing Header (see Section 5.5.2) too, since Mobile Routers would
insert new contents to the header of packets sent by downstream
Mobile Network Nodes. This makes it difficult for Mobile Network
Nodes to protect the end-to-end integrity of such information with
security associations.
5.8.3. Location Privacy
Another security-related concern is the issue of location privacy.
This document currently does not consider the location privacy
threats caused by an on-path eavesdropper. For more information on
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RFC 4889 NEMO RO Space Analysis July 2007
that aspect, please refer to [18]. Instead, we consider the
following three aspects to location privacy:
o Revelation of Location to Correspondent Entity
Route optimization is achieved by creating a binding between the
address of the Mobile Network Node and the current location of the
Mobile Network. It is thus inevitable that the location of the
Mobile Network Node be revealed to the Correspondent Entity. The
concern may be alleviated if the Correspondent Entity is not the
Correspondent Node, since this implies that the actual traffic end
point (i.e., the Correspondent Node) would remain ignorant of the
current location of the Mobile Network Node.
o Degree of Revelation
With network mobility, the degree of location exposure varies,
especially when one considers nested mobile networks. For
instance, for approaches that bind the address of the Mobile
Network Node to the location of the root Mobile Router (see
Section 5.5.3), only the topmost point of attachment of the mobile
network is revealed to the Correspondent Entity. For approaches
such as those described in Section 5.5.1 and Section 5.5.2, more
information (such as Mobile Network Prefixes and current locations
of upstream Mobile Routers) is revealed. Techniques such as
exposing only locally-scoped addresses of intermediate upstream
mobile routers to Correspondent Entities may be used to reduce the
degree of revelation.
o Control of the Revelation
When Route Optimization is initiated by the Mobile Network Node
itself, it is in control of whether or not to sacrifice location
privacy for an optimized route. However, if it is the Mobile
Router that initiates Route Optimization (e.g., "Binding Update
with Mobile Network Prefix" and "Mobile Router as a Proxy" in
Section 5.5.1), then control is taken away from the Mobile Network
Node. An additional signaling mechanism between the Mobile
Network Node and its Mobile Router can be used in this case to
prevent the Mobile Router from attempting Route Optimization for a
given traffic stream.
6. Conclusion
The problem space of Route Optimization in the NEMO context is
multifold and can be split into several work areas. It will be
critical, though, that the solution to a given piece of the puzzle be
compatible and integrated smoothly with others. With this in mind,
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RFC 4889 NEMO RO Space Analysis July 2007
this document attempts to present a detailed and in-depth analysis of
the NEMO Route Optimization solution space by first describing the
benefits a Route Optimization solution is expected to bring, then
illustrating the different scenarios in which a Route Optimization
solution applies, and next presenting some issues a Route
Optimization solution might face. We have also asked ourselves some
of the basic questions about a Route Optimization solution. By
investigating different possible answers to these questions, we have
explored different aspects to a Route Optimization solution. The
intent of this work is to enhance our common understanding of the
Route Optimization problem and solution space.
7. Security Considerations
This is an informational document that analyzes the solution space of
NEMO Route Optimization. Security considerations of different
approaches are described in the relevant sections throughout this
document. Particularly, please refer to Section 4.9 for a brief
discussion of the security concern with respect to Route Optimization
in general, and Section 5.8 for a more detailed analysis of the
various Route Optimization approaches.
8. Acknowledgments
The authors wish to thank the co-authors of previous versions from
which this document is derived: Marco Molteni, Paik Eun-Kyoung,
Hiroyuki Ohnishi, Felix Wu, and Souhwan Jung. In addition, sincere
appreciation is also extended to Jari Arkko, Carlos Jesus Bernardos,
Greg Daley, Thierry Ernst, T.J. Kniveton, Erik Nordmark, Alexandru
Petrescu, Hesham Soliman, Ryuji Wakikawa, and Patrick Wetterwald for
their various contributions.
9. References
9.1. Normative References
[1] Ng, C., Thubert, P., Watari, M., and F. Zhao, "Network Mobility
Route Optimization Problem Statement", RFC 4888, July 2007.
[2] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
"Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
January 2005.
[3] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[4] Ernst, T., "Network Mobility Support Goals and Requirements",
RFC 4886, July 2007.
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RFC 4889 NEMO RO Space Analysis July 2007
[5] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[6] Ernst, T. and H-Y. Lach, "Network Mobility Support
Terminology", RFC 4885, July 2007.
9.2. Informative References
[7] Wakikawa, R., Koshiba, S., Uehara, K., and J. Murai, "ORC:
Optimized Route Cache Management Protocol for Network
Mobility", 10th International Conference on Telecommunications,
vol 2, pp 1194-1200, February 2003.
[8] Wakikawa, R. and M. Watari, "Optimized Route Cache Protocol
(ORC)", Work in Progress, November 2004.
[9] Na, J., Cho, S., Kim, C., Lee, S., Kang, H., and C. Koo, "Route
Optimization Scheme based on Path Control Header", Work
in Progress, April 2004.
[10] Thubert, P. and M. Molteni, "IPv6 Reverse Routing Header and
its application to Mobile Networks", Work in Progress,
February 2007.
[11] Ng, C. and T. Tanaka, "Securing Nested Tunnels Optimization
with Access Router Option", Work in Progress, July 2004.
[12] Na, J., Cho, S., Kim, C., Lee, S., Kang, H., and C. Koo,
"Secure Nested Tunnels Optimization using Nested Path
Information", Work in Progress, September 2003.
[13] Soliman, H., Castelluccia, C., El Malki, K., and L. Bellier,
"Hierarchical Mobile IPv6 Mobility Management (HMIPv6)",
RFC 4140, August 2005.
[14] Thubert, P., Wakikawa, R., and V. Devarapalli, "Global HA to HA
protocol", Work in Progress, September 2006.
[15] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host
Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[16] Baek, S., Yoo, J., Kwon, T., Paik, E., and M. Nam, "Routing
Optimization in the same nested mobile network", Work
in Progress, October 2005.
[17] Koodli, R., "Fast Handovers for Mobile IPv6", RFC 4068,
July 2005.
Ng, et al. Informational [Page 33]
RFC 4889 NEMO RO Space Analysis July 2007
[18] Vogt, C. and J. Arkko, "A Taxonomy and Analysis of Enhancements
to Mobile IPv6 Route Optimization", RFC 4651, February 2007.
[19] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
Nordmark, "Mobile IP Version 6 Route Optimization Security
Design Background", RFC 4225, December 2005.
[20] Bernardos, C., Bagnulo, M., and M. Calderon, "MIRON: MIPv6
Route Optimization for NEMO", 4th Workshop on Applications and
Services in Wireless Network,
Online: http://www.it.uc3m.es/cjbc/papers/miron_aswn2004.pdf,
August 2004.
[21] Calderon, M., Bernardos, C., Bagnulo, M., Soto, I., and A.
Oliva, "Design and Experimental Evaluation of a Route
Optimisation Solution for NEMO", IEEE Journal on Selected Areas
in Communications (J-SAC), vol 24, no 9, September 2006.
[22] Bernardos, C., Bagnulo, M., Calderon, M., and I. Soto, "Mobile
IPv6 Route Optimisation for Network Mobility (MIRON)", Work
in Progress, July 2005.
[23] Ylitalo, J., "Securing Route Optimization in NEMO", Workshop
of 12th Network and Distributed System Security Syposuim, NDSS
Workshop 2005, online: http://www.isoc.org/isoc/conferences/
ndss/05/workshop/ylitalo.pdf, February 2005.
[24] Perera, E., Lee, K., Kim, H., and J. Park, "Extended Network
Mobility Support", Work in Progress, July 2003.
[25] Lee, K., Park, J., and H. Kim, "Route Optimization for Mobile
Nodes in Mobile Network based on Prefix Delegation", 58th IEEE
Vehicular Technology Conference, vol 3, pp 2035-2038,
October 2003.
[26] Lee, K., Jeong, J., Park, J., and H. Kim, "Route Optimization
for Mobile Nodes in Mobile Network based on Prefix Delegation",
Work in Progress, February 2004.
[27] Jeong, J., Lee, K., Park, J., and H. Kim, "Route Optimization
based on ND-Proxy for Mobile Nodes in IPv6 Mobile Network",
59th IEEE Vehicular Technology Conference, vol 5, pp 2461-2465,
May 2004.
[28] Jeong, J., Lee, K., Kim, H., and J. Park, "ND-Proxy based Route
Optimization for Mobile Nodes in Mobile Network", Work
in Progress, February 2004.
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RFC 4889 NEMO RO Space Analysis July 2007
[29] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[30] Kang, H., Kim, K., Han, S., Lee, K., and J. Park, "Route
Optimization for Mobile Network by Using Bi-directional Between
Home Agent and Top Level Mobile Router", Work in Progress,
June 2003.
[31] Lee, D., Lim, K., and M. Kim, "Hierarchical FRoute Optimization
for Nested Mobile Network", 18th Int'l Conf on Advance
Information Networking and Applications, vol 1, pp 225-229,
2004.
[32] Takagi, Y., Ohnishi, H., Sakitani, K., Baba, K., and S.
Shimojo, "Route Optimization Methods for Network Mobility with
Mobile IPv6", IEICE Trans. on Comms, vol E87-B, no 3, pp 480-
489, March 2004.
[33] Ohnishi, H., Sakitani, K., and Y. Takagi, "HMIP based Route
optimization method in a mobile network", Work in Progress,
October 2003.
[34] Lee, C., Zheng, J., and C. HUang, "SIP-based Network Mobility
(SIP-NEMO) Route Optimization (RO)", Work in Progress,
October 2006.
[35] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
Specification", RFC 2473, December 1998.
[36] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,
Yoshimura, T., and H. Zheng, "RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001.
[37] Jonsson, L-E., "RObust Header Compression (ROHC): Terminology
and Channel Mapping Examples", RFC 3759, April 2004.
[38] Minaburo, A., Paik, E., Toutain, L., and J. Bonnin, "ROHC
(Robust Header Compression) in NEMO network", Work in Progress,
July 2005.
[39] Ng, C. and J. Hirano, "Extending Return Routability Procedure
for Network Prefix (RRNP)", Work in Progress, October 2004.
[40] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
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RFC 4889 NEMO RO Space Analysis July 2007
[41] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[42] Zhao, F., Wu, F., and S. Jung, "Extensions to Return
Routability Test in MIP6", Work in Progress, February 2005.
[43] Bao, F., Deng, R., Qiu, Y., and J. Zhou, "Certificate-based
Binding Update Protocol (CBU)", Work in Progress, March 2005.
[44] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", Work in Progress, April 2007.
[45] Henderson, T., "End-Host Mobility and Multihoming with the Host
Identity Protocol", Work in Progress, March 2007.
[46] Calderon, M., Bernardos, C., Bagnulo, M., and I. Soto,
"Securing Route Optimization in NEMO", Third International
Symposium on Modeling and Optimization in Mobile, Ad Hoc, and
Wireless Networks, WIOPT 2005, pages 248-254, April 2005.
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RFC 4889 NEMO RO Space Analysis July 2007
Authors' Addresses
Chan-Wah Ng
Panasonic Singapore Laboratories Pte Ltd
Blk 1022 Tai Seng Ave #06-3530
Tai Seng Industrial Estate, Singapore 534415
SG
Phone: +65 65505420
EMail: chanwah.ng@sg.panasonic.com
Fan Zhao
University of California Davis
One Shields Avenue
Davis, CA 95616
US
Phone: +1 530 752 3128
EMail: fanzhao@ucdavis.edu
Masafumi Watari
KDDI R&D Laboratories Inc.
2-1-15 Ohara
Fujimino, Saitama 356-8502
JAPAN
EMail: watari@kddilabs.jp
Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3, Biot - Sophia Antipolis 06410
FRANCE
EMail: pthubert@cisco.com
Ng, et al. Informational [Page 37]
RFC 4889 NEMO RO Space Analysis July 2007
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