Network Working Group C. Ng

Network Working Group C. Ng
Request for Comments: 4888 Panasonic Singapore Labs
Category: Informational P. Thubert
Cisco Systems
M. Watari
KDDI R&D Labs
F. Zhao
UC Davis
July 2007

Network Mobility Route Optimization Problem Statement

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
bi-directional tunnel established between the Mobile Router and Home
Agent when the mobile network is away. This sub-optimal routing
results in various inefficiencies associated with packet delivery,
such as increased delay and bottleneck links leading to traffic
congestion, which can ultimately disrupt all communications to and
from the Mobile Network Nodes. Additionally, with nesting of Mobile
Networks, these inefficiencies get compounded, and stalemate
conditions may occur in specific dispositions. This document
investigates such problems and provides the motivation behind Route
Optimization (RO) for NEMO.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. NEMO Route Optimization Problem Statement . . . . . . . . . . 3
2.1. Sub-Optimality with NEMO Basic Support . . . . . . . . . . 4
2.2. Bottleneck in the Home Network . . . . . . . . . . . . . . 6
2.3. Amplified Sub-Optimality in Nested Mobile Networks . . . . 6
2.4. Sub-Optimality with Combined Mobile IPv6 Route
Optimization . . . . . . . . . . . . . . . . . . . . . . . 8
2.5. Security Policy Prohibiting Traffic from Visiting Nodes . 9
2.6. Instability of Communications within a Nested Mobile
Network . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.7. Stalemate with a Home Agent Nested in a Mobile Network . . 10
3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative Reference . . . . . . . . . . . . . . . . . . . 12
6.2. Informative Reference . . . . . . . . . . . . . . . . . . 12
Appendix A. Various Configurations Involving Nested Mobile
Networks . . . . . . . . . . . . . . . . . . . . . . 13
A.1. CN Located in the Fixed Infrastructure . . . . . . . . . . 13
A.1.1. Case A: LFN and Standard IPv6 CN . . . . . . . . . . . 14
A.1.2. Case B: VMN and MIPv6 CN . . . . . . . . . . . . . . . 14
A.1.3. Case C: VMN and Standard IPv6 CN . . . . . . . . . . . 14
A.2. CN Located in Distinct Nested NEMOs . . . . . . . . . . . 15
A.2.1. Case D: LFN and Standard IPv6 CN . . . . . . . . . . . 16
A.2.2. Case E: VMN and MIPv6 CN . . . . . . . . . . . . . . . 16
A.2.3. Case F: VMN and Standard IPv6 CN . . . . . . . . . . . 16
A.3. MNN and CN Located in the Same Nested NEMO . . . . . . . . 17
A.3.1. Case G: LFN and Standard IPv6 CN . . . . . . . . . . . 18
A.3.2. Case H: VMN and MIPv6 CN . . . . . . . . . . . . . . . 18
A.3.3. Case I: VMN and Standard IPv6 CN . . . . . . . . . . . 19
A.4. CN Located Behind the Same Nested MR . . . . . . . . . . . 19
A.4.1. Case J: LFN and Standard IPv6 CN . . . . . . . . . . . 20
A.4.2. Case K: VMN and MIPv6 CN . . . . . . . . . . . . . . . 20
A.4.3. Case L: VMN and Standard IPv6 CN . . . . . . . . . . . 21
Appendix B. Example of How a Stalemate Situation Can Occur . . . 22

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1. Introduction

With current Network Mobility (NEMO) Basic Support [1], all
communications to and from nodes in a mobile network must go through
the bi-directional tunnel established between the Mobile Router and
its Home Agent (also known as the MRHA tunnel) when the mobile
network is away. Although such an arrangement allows Mobile Network
Nodes to reach and be reached by any node on the Internet,
limitations associated to the base protocol degrade overall
performance of the network and, ultimately, can prevent all
communications to and from the Mobile Network Nodes.

Some of these concerns already exist with Mobile IPv6 [4] and were
addressed by the mechanism known as Route Optimization, which is part
of the base protocol. With Mobile IPv6, Route Optimization mostly
improves the end-to-end path between the Mobile Node and
Correspondent Node, with an additional benefit of reducing the load
of the Home Network, thus its name.

NEMO Basic Support presents a number of additional issues, making the
problem more complex, so it was decided to address Route Optimization
separately. In that case, the expected benefits are more dramatic,
and a Route Optimization mechanism could enable connectivity that
would be broken otherwise. In that sense, Route Optimization is even
more important to NEMO Basic Support than it is to Mobile IPv6.

This document explores limitations inherent in NEMO Basic Support,
and their effects on communications between a Mobile Network Node and
its corresponding peer. This is detailed in Section 2. It is
expected that readers are familiar with general terminologies related
to mobility in [4][2], NEMO-related terms defined in [3], and NEMO
goals and requirements [5].

2. NEMO Route Optimization Problem Statement

Given the NEMO Basic Support protocol, all data packets to and from
Mobile Network Nodes must go through the Home Agent, even though a
shorter path may exist between the Mobile Network Node and its
Correspondent Node. In addition, with the nesting of Mobile Routers,
these data packets must go through multiple Home Agents and several
levels of encapsulation, which may be avoided. This results in
various inefficiencies and problems with packet delivery, which can
ultimately disrupt all communications to and from the Mobile Network
Nodes.

In the following sub-sections, we will describe the effects of a
pinball route with NEMO Basic Support, how it may cause a bottleneck
to be formed in the Home Network, and how these get amplified with

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nesting of mobile networks. Closely related to nesting, we will also
look into the sub-optimality even when Mobile IPv6 Route Optimization
is used over NEMO Basic Support. This is followed by a description
of security policy in the Home Network that may forbid transit
traffic from Visiting Mobile Nodes in mobile networks. In addition,
we will explore the impact of the MRHA tunnel on communications
between two Mobile Network Nodes on different links of the same
mobile network. We will also provide additional motivations for
Route Optimization by considering the potential stalemate situation
when a Home Agent is part of a mobile network.

2.1. Sub-Optimality with NEMO Basic Support

With NEMO Basic Support, all packets sent between a Mobile Network
Node and its Correspondent Node are forwarded through the MRHA
tunnel, resulting in a pinball route between the two nodes. This has
the following sub-optimal effects:

o Longer Route Leading to Increased Delay and Additional
Infrastructure Load

Because a packet must transit from a mobile network to the Home
Agent then to the Correspondent Node, the transit time of the
packet is usually longer than if the packet were to go straight
from the mobile network to the Correspondent Node. When the
Correspondent Node (or the mobile network) resides near the Home
Agent, the increase in packet delay can be very small. However,
when the mobile network and the Correspondent Node are relatively
near to one another but far away from the Home Agent on the
Internet, the increase in delay is very large. Applications such
as real-time multimedia streaming may not be able to tolerate such
increase in packet delay. In general, the increase in delay may
also impact the performance of transport protocols such as TCP,
since the sending rate of TCP is partly determined by the round-
trip time (RTT) perceived by the communication peers.

Moreover, by using a longer route, the total resource utilization
for the traffic would be much higher than if the packets were to
follow a direct path between the Mobile Network Node and
Correspondent Node. This would result in additional load in the
infrastructure.

o Increased Packet Overhead

The encapsulation of packets in the MRHA tunnel results in
increased packet size due to the addition of an outer header.
This reduces the bandwidth efficiency, as an IPv6 header can be

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quite substantial relative to the payload for applications such as
voice samples. For instance, given a voice application using an 8
kbps algorithm (e.g., G.729) and taking a voice sample every 20 ms
(as in RFC 1889 [6]), the packet transmission rate will be 50
packets per second. Each additional IPv6 header is an extra 320
bits per packet (i.e., 16 kbps), which is twice the actual
payload!

o Increased Processing Delay

The encapsulation of packets in the MRHA tunnel also results in
increased processing delay at the points of encapsulation and
decapsulation. Such increased processing may include encryption/
decryption, topological correctness verifications, MTU
computation, fragmentation, and reassembly.

o Increased Chances of Packet Fragmentation

The augmentation in packet size due to packet encapsulation may
increase the chances of the packet being fragmented along the MRHA
tunnel. This can occur if there is no prior path MTU discovery
conducted, or if the MTU discovery mechanism did not take into
account the encapsulation of packets. Packet fragmentation will
result in a further increase in packet delays and further
reduction of bandwidth efficiency.

o Increased Susceptibility to Link Failure

Under the assumption that each link has the same probability of
link failure, a longer routing path would be more susceptible to
link failure. Thus, packets routed through the MRHA tunnel may be
subjected to a higher probability of being lost or delayed due to
link failure, compared to packets that traverse directly between
the Mobile Network Node and its Correspondent Node.

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2.2. Bottleneck in the Home Network

Apart from the increase in packet delay and infrastructure load,
forwarding packets through the Home Agent may also lead to either the
Home Agent or the Home Link becoming a bottleneck for the aggregated
traffic from/to all the Mobile Network Nodes. A congestion at home
would lead to additional packet delay, or even packet loss. In
addition, Home Agent operations such as security check, packet
interception, and tunneling might not be as optimized in the Home
Agent software as plain packet forwarding. This could further limit
the Home Agent capacity for data traffic. Furthermore, with all
traffic having to pass through the Home Link, the Home Link becomes a
single point of failure for the mobile network.

Data packets that are delayed or discarded due to congestion at the
Home Network would cause additional performance degradation to
applications. Signaling packets, such as Binding Update messages,
that are delayed or discarded due to congestion at the Home Network
may affect the establishment or update of bi-directional tunnels,
causing disruption of all traffic flow through these tunnels.

A NEMO Route Optimization mechanism that allows the Mobile Network
Nodes to communicate with their Correspondent Nodes via a path that
is different from the MRHA tunneling and thereby avoiding the Home
Agent may alleviate or even prevent the congestion at the Home Agent
or Home Link.

2.3. Amplified Sub-Optimality in Nested Mobile Networks

By allowing other mobile nodes to join a mobile network, and in
particular mobile routers, it is possible to form arbitrary levels of
nesting of mobile networks. With such nesting, the use of NEMO Basic
Support further amplifies the sub-optimality of routing. We call
this the amplification effect of nesting, where the undesirable
effects of a pinball route with NEMO Basic Support are amplified with
each level of nesting of mobile networks. This is best illustrated
by an example shown in Figure 1.

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+--------+ +--------+ +--------+ +--------+
| MR2_HA | | MR3_HA | | MR4_HA | | MR5_HA |
+------+-+ +---+----+ +---+----+ +-+------+
\ | | /
+--------+ +------------------------------+
| MR1_HA |----| Internet |-----CN1
+--------+ +------------------------------+
|
+---+---+
root-MR | MR1 |
+-------+
| |
+-------+ +-------+
sub-MR | MR2 | | MR4 |
+---+---+ +---+---+
| |
+---+---+ +---+---+
sub-MR | MR3 | | MR5 |
+---+---+ +---+---+
| |
----+---- ----+----
MNN CN2

Figure 1: An Example of a Nested Mobile Network

Using NEMO Basic Support, the flow of packets between a Mobile
Network Node, MNN, and a Correspondent Node, CN1, would need to go
through three separate tunnels, illustrated in Figure 2 below.

----------.
---------/ /----------.
-------/ | | /-------
MNN -----( - - | - - - | - - - | - - - | - - (------ CN1
MR3-------\ | | \-------MR3_HA
MR2--------\ \----------MR2_HA
MR1---------MR1_HA

Figure 2: Nesting of Bi-Directional Tunnels

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This leads to the following problems:

o Pinball Route

Both inbound and outbound packets will flow via the Home Agents of
all the Mobile Routers on their paths within the mobile network,
with increased latency, less resilience, and more bandwidth usage.
Appendix A illustrates in detail the packets' routes under
different nesting configurations of the Mobile Network Nodes.

o Increased Packet Size

An extra IPv6 header is added per level of nesting to all the
packets. The header compression suggested in [7] cannot be
applied because both the source and destination (the intermediate
Mobile Router and its Home Agent) are different hop to hop.

Nesting also amplifies the probability of congestion at the Home
Networks of the upstream Mobile Routers. In addition, the Home Link
of each upstream Mobile Router will also be a single point of failure
for the nested Mobile Router.

2.4. Sub-Optimality with Combined Mobile IPv6 Route Optimization

When a Mobile IPv6 host joins a mobile network, it becomes a Visiting
Mobile Node of the mobile network. Packets sent to and from the
Visiting Mobile Node will have to be routed not only via the Home
Agent of the Visiting Mobile Node, but also via the Home Agent of the
Mobile Router in the mobile network. This suffers the same
amplification effect of nested mobile network mentioned in
Section 2.3.

In addition, although Mobile IPv6 [4] allows a mobile host to perform
Route Optimization with its Correspondent Node in order to avoid
tunneling with its Home Agent, the "optimized" route is no longer
optimized when the mobile host is attached to a mobile network. This
is because the route between the mobile host and its Correspondent
Node is subjected to the sub-optimality introduced by the MRHA
tunnel. Interested readers may refer to Appendix A for examples of
how the routes will appear with nesting of Mobile IPv6 hosts in
mobile networks.

The readers should also note that the same sub-optimality would apply
when the mobile host is outside the mobile network and its
Correspondent Node is in the mobile network.

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2.5. Security Policy Prohibiting Traffic from Visiting Nodes

NEMO Basic Support requires all traffic from visitors to be tunneled
to the Mobile Router's Home Agent. This might represent a breach in
the security of the Home Network (some specific attacks against the
Mobile Router's binding by rogue visitors have been documented in
[8][9]). Administrators might thus fear that malicious packets will
be routed into the Home Network via the bi-directional tunnel. As a
consequence, it can be expected that in many deployment scenarios,
policies will be put in place to prevent unauthorized Visiting Mobile
Nodes from attaching to the Mobile Router.

However, there are deployment scenarios where allowing unauthorized
Visiting Mobile Nodes is actually desirable. For instance, when
Mobile Routers attach to other Mobile Routers and form a nested NEMO,
they depend on each other to reach the Internet. When Mobile Routers
have no prior knowledge of one another (no security association,
Authentication, Authorization, and Accounting (AAA), Public-Key
Infrastructure (PKI), etc.), it could still be acceptable to forward
packets, provided that the packets are not tunneled back to the Home
Networks.

A Route Optimization mechanism that allows traffic from Mobile
Network Nodes to bypass the bi-directional tunnel between a Mobile
Router and its Home Agent would be a necessary first step towards a
Tit for Tat model, where MRs would benefit from a reciprocal
altruism, based on anonymity and innocuousness, to extend the
Internet infrastructure dynamically.

2.6. Instability of Communications within a Nested Mobile Network

Within a nested mobile network, two Mobile Network Nodes may
communicate with each other. Let us consider the previous example
illustrated in Figure 1 where MNN and CN2 are sharing a communication
session. With NEMO Basic Support, a packet sent from MNN to CN2 will
need to be forwarded to the Home Agent of each Mobile Router before
reaching CN2, whereas, a packet following the direct path between
them need not even leave the mobile network. Readers are referred to
Appendix A.3 for detailed illustration of the resulting routing
paths.

Apart from the consequences of increased packet delay and packet
size, which are discussed in previous sub-sections, there are two
additional effects that are undesirable:

o when the nested mobile network is disconnected from the Internet
(e.g., MR1 loses its egress connectivity), MNN and CN2 can no

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longer communicate with each other, even though the direct path
from MNN to CN2 is unaffected;

o the egress link(s) of the root Mobile Router (i.e., MR1) becomes a
bottleneck for all the traffic that is coming in and out of the
nested mobile network.

A Route Optimization mechanism could allow traffic between two Mobile
Network Nodes nested within the same mobile network to follow a
direct path between them, without being routed out of the mobile
network. This may also off-load the processing burden of the
upstream Mobile Routers when the direct path between the two Mobile
Network Nodes does not traverse these Mobile Routers.

2.7. Stalemate with a Home Agent Nested in a Mobile Network

Several configurations for the Home Network are described in [10].
In particular, there is a mobile home scenario where a (parent)
Mobile Router is also a Home Agent for its mobile network. In other
words, the mobile network is itself an aggregation of Mobile Network
Prefixes assigned to (children) Mobile Routers.

A stalemate situation exists in the case where the parent Mobile
Router visits one of its children. The child Mobile Router cannot
find its Home Agent in the Internet and thus cannot establish its
MRHA tunnel and forward the visitor's traffic. The traffic from the
parent is thus blocked from reaching the Internet, and it will never
bind to its own (grandparent) Home Agent. Appendix B gives a
detailed illustration of how such a situation can occur.

Then again, a Route Optimization mechanism that bypasses the nested
tunnel might enable the parent traffic to reach the Internet and let
it bind. At that point, the child Mobile Router would be able to
reach its parent and bind in turn. Additional nested Route
Optimization solutions might also enable the child to locate its Home
Agent in the nested structure and bind regardless of whether or not
the Internet is reachable.

3. Conclusion

With current NEMO Basic Support, all communications to and from
Mobile Network Nodes must go through the MRHA tunnel when the mobile
network is away. This results in various inefficiencies associated
with packet delivery. This document investigates such inefficiencies
and provides the motivation behind Route Optimization for NEMO.

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We have described the sub-optimal effects of pinball routes with NEMO
Basic Support, how they may cause a bottleneck to be formed in the
Home Network, and how they get amplified with nesting of mobile
networks. These effects will also be seen even when Mobile IPv6
Route Optimization is used over NEMO Basic Support. In addition,
other issues concerning the nesting of mobile networks that might
provide additional motivation for a NEMO Route Optimization mechanism
were also explored, such as the prohibition of forwarding traffic
from a Visiting Mobile Node through an MRHA tunnel due to security
concerns, the impact of the MRHA tunnel on communications between two
Mobile Network Nodes on different links of the same mobile network,
and the possibility of a stalemate situation when Home Agents are
nested within a mobile network.

4. Security Considerations

This document highlights some limitations of NEMO Basic Support. In
particular, some security concerns could prevent interesting
applications of the protocol, as detailed in Section 2.5.

Route Optimization for RFC 3963 [1] might introduce new threats, just
as it might alleviate existing ones. This aspect will certainly be a
key criterion in the evaluation of the proposed solutions.

5. 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, Thierry Ernst, Felix Wu, and Souhwan Jung. Early
work by Masafumi Watari on the extracted appendix was written while
still at Keio University. In addition, sincere appreciation is also
extended to Jari Arkko, Carlos Bernardos, Greg Daley, T.J. Kniveton,
Henrik Levkowetz, Erik Nordmark, Alexandru Petrescu, Hesham Soliman,
Ryuji Wakikawa, and Patrick Wetterwald for their various
contributions.

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6. References

6.1. Normative Reference

[1] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
"Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
January 2005.

[2] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.

[3] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
RFC 4885, July 2007.

6.2. Informative Reference

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

[5] Ernst, T., "Network Mobility Support Goals and Requirements",
RFC 4886, July 2007.

[6] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 1889, January 1996.

[7] Deering, S. and B. Zill, "Redundant Address Deletion when
Encapsulating IPv6 in IPv6", Work in Progress, November 2001.

[8] Petrescu, A., Olivereau, A., Janneteau, C., and H-Y. Lach,
"Threats for Basic Network Mobility Support (NEMO threats)",
Work in Progress, January 2004.

[9] Jung, S., Zhao, F., Wu, S., Kim, H-G., and S-W. Sohn, "Threat
Analysis on NEMO Basic Operations", Work in Progress,
July 2004.

[10] Thubert, P., Wakikawa, R., and V. Devarapalli, "Network
Mobility Home Network Models", RFC RFC4887, July 2007.

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

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Appendix A. Various Configurations Involving Nested Mobile Networks

In the following sections, we try to describe different communication
models that involve a nested mobile network and to clarify the issues
for each case. We illustrate the path followed by packets if we
assume nodes only use Mobile IPv6 and NEMO Basic Support mechanisms.
Different cases are considered where a Correspondent Node is located
in the fixed infrastructure, in a distinct nested mobile network as
the Mobile Network Node, or in the same nested mobile network as the
Mobile Network Node. Additionally, cases where Correspondent Nodes
and Mobile Network Nodes are either standard IPv6 nodes or Mobile
IPv6 nodes are considered. As defined in [3], standard IPv6 nodes
are nodes with no mobility functions whatsoever, i.e., they are not
Mobile IPv6 or NEMO enabled. This means that they cannot move around
keeping open connections and that they cannot process Binding Updates
sent by peers.

A.1. CN Located in the Fixed Infrastructure

The most typical configuration is the case where a Mobile Network
Node communicates with a Correspondent Node attached in the fixed
infrastructure. Figure 3 below shows an example of such topology.

+--------+ +--------+ +--------+
| MR1_HA | | MR2_HA | | MR3_HA |
+---+----+ +---+----+ +---+----+
| | |
+-------------------------+
| Internet |----+ CN
+-------------------------+
| |
+---+---+ +--+-----+
root-MR | MR1 | | VMN_HA |
+---+---+ +--------+
|
+---+---+
sub-MR | MR2 |
+---+---+
|
+---+---+
sub-MR | MR3 |
+---+---+
|
----+----
MNN

Figure 3: CN Located at the Infrastructure

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A.1.1. Case A: LFN and Standard IPv6 CN

The simplest case is where both MNN and CN are fixed nodes with no
mobility functions. That is, MNN is a Local Fixed Node, and CN is a
standard IPv6 node. Packets are encapsulated between each Mobile
Router and its respective Home Agent (HA). As shown in Figure 4, in
such a case, the path between the two nodes would go through:

1 2 3 4 3 2 1
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA --- CN
LFN IPv6 Node

The digits represent the number of IPv6 headers.

Figure 4: MNN and CN Are Standard IPv6 Nodes

A.1.2. Case B: VMN and MIPv6 CN

In this second case, both end nodes are Mobile IPv6-enabled mobile
nodes, that is, MNN is a Visiting Mobile Node. Mobile IPv6 Route
Optimization may thus be initiated between the two and packets would
not go through the Home Agent of the Visiting Mobile Node or the Home
Agent of the Correspondent Node (not shown in the figure). However,
packets will still be tunneled between each Mobile Router and its
respective Home Agent, in both directions. As shown in Figure 5, the
path between MNN and CN would go through:

1 2 3 4 3 2 1
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA --- CN
VMN MIPv6

Figure 5: MNN and CN Are MIPv6 Mobile Nodes

A.1.3. Case C: VMN and Standard IPv6 CN

When the communication involves a Mobile IPv6 node either as a
Visiting Mobile Node or as a Correspondent Node, Mobile IPv6 Route
Optimization cannot be performed because the standard IPv6
Correspondent Node cannot process Mobile IPv6 signaling. Therefore,
MNN would establish a bi-directional tunnel with its HA, which causes
the flow to go out the nested NEMO. Packets between MNN and CN would
thus go through MNN's own Home Agent (VMN_HA). The path would
therefore be as shown in Figure 6:

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2 3 4 5 4
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA
VMN |
| 3
1 2 |
CN --- VMN_HA --- MR3_HA
IPv6 Node

Figure 6: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

Providing Route Optimization involving a Mobile IPv6 node may require
optimization among the Mobile Routers and the Mobile IPv6 node.

A.2. CN Located in Distinct Nested NEMOs

The Correspondent Node may be located in another nested mobile
network, different from the one MNN is attached to, as shown in
Figure 7. We define such configuration as "distinct nested mobile
networks".

+--------+ +--------+ +--------+ +--------+
| MR2_HA | | MR3_HA | | MR4_HA | | MR5_HA |
+------+-+ +---+----+ +---+----+ +-+------+
\ | | /
+--------+ +-------------------------+ +--------+
| MR1_HA |----| Internet |----| VMN_HA |
+--------+ +-------------------------+ +--------+
| |
+---+---+ +---+---+
root-MR | MR1 | | MR4 |
+---+---+ +---+---+
| |
+---+---+ +---+---+
sub-MR | MR2 | | MR5 |
+---+---+ +---+---+
| |
+---+---+ ----+----
sub-MR | MR3 | CN
+---+---+
|
----+----
MNN

Figure 7: MNN and CN Located in Distinct Nested NEMOs

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A.2.1. Case D: LFN and Standard IPv6 CN

Similar to Case A, we start off with the case where both end nodes do
not have any mobility functions. Packets are encapsulated at every
Mobile Router on the way out of the nested mobile network,
decapsulated by the Home Agents, and then encapsulated again on their
way down the nested mobile network.

1 2 3 4 3 2
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
LFN |
| 1
1 2 3 2 |
CN --- MR5 --- MR4 --- MR4_HA --- MR5_HA
IPv6 Node

Figure 8: MNN and CN Are Standard IPv6 Nodes

A.2.2. Case E: VMN and MIPv6 CN

Similar to Case B, when both end nodes are Mobile IPv6 nodes, the two
nodes may initiate Mobile IPv6 Route Optimization. Again, packets
will not go through the Home Agent of the MNN or the Home Agent of
the Mobile IPv6 Correspondent Node (not shown in the figure).
However, packets will still be tunneled for each Mobile Router to its
Home Agent and vice versa. Therefore, the path between MNN and CN
would go through:

1 2 3 4 3 2
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
VMN |
| 1
1 2 3 2 |
CN --- MR5 --- MR4 --- MR4_HA --- MR5_HA
MIPv6 Node

Figure 9: MNN and CN Are MIPv6 Mobile Nodes

A.2.3. Case F: VMN and Standard IPv6 CN

Similar to Case C, when the communication involves a Mobile IPv6 node
either as a Visiting Mobile Node or as a Correspondent Node, MIPv6
Route Optimization cannot be performed because the standard IPv6
Correspondent Node cannot process Mobile IPv6 signaling. MNN would

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RFC 4888 NEMO RO Problem Statement July 2007

therefore establish a bi-directional tunnel with its Home Agent.
Packets between MNN and CN would thus go through MNN's own Home Agent
as shown in Figure 10:

2 3 4 5 4 3
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
VMN |
| 2
1 2 3 2 1 |
CN --- MR5 --- MR4 --- MR4_HA --- MR5_HA --- VMN_HA
IPv6 Node

Figure 10: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

A.3. MNN and CN Located in the Same Nested NEMO

Figure 11 below shows the case where the two communicating nodes are
connected behind different Mobile Routers that are connected in the
same nested mobile network, and thus behind the same root Mobile
Router. Route Optimization can avoid packets being tunneled outside
the nested mobile network.

+--------+ +--------+ +--------+ +--------+
| MR2_HA | | MR3_HA | | MR4_HA | | MR5_HA |
+------+-+ +---+----+ +---+----+ +-+------+
\ | | /
+--------+ +-------------------------+ +--------+
| MR1_HA |----| Internet |----| VMN_HA |
+--------+ +-------------------------+ +--------+
|
+---+---+
root-MR | MR1 |
+-------+
| |
+-------+ +-------+
sub-MR | MR2 | | MR4 |
+---+---+ +---+---+
| |
+---+---+ +---+---+
sub-MR | MR3 | | MR5 |
+---+---+ +---+---+
| |
----+---- ----+----
MNN CN

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RFC 4888 NEMO RO Problem Statement July 2007

Figure 11: MNN and CN Located in the Same Nested NEMO

A.3.1. Case G: LFN and Standard IPv6 CN

Again, we start off with the case where both end nodes do not have
any mobility functions. Packets are encapsulated at every Mobile
Router on the way out of the nested mobile network via the root
Mobile Router, decapsulated and encapsulated by the Home Agents, and
then make their way back to the nested mobile network through the
same root Mobile Router. Therefore, the path between MNN and CN
would go through:

1 2 3 4 3 2
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
LFN |
| 1
1 2 3 4 3 2 |
CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR4_HA --- MR5_HA
IPv6 Node

Figure 12: MNN and CN Are Standard IPv6 nodes

A.3.2. Case H: VMN and MIPv6 CN

Similar to Case B and Case E, when both end nodes are Mobile IPv6
nodes, the two nodes may initiate Mobile IPv6 Route Optimization,
which will avoid the packets going through the Home Agent of MNN or
the Home Agent of the Mobile IPv6 CN (not shown in the figure).
However, packets will still be tunneled between each Mobile Router
and its respective Home Agent in both directions. Therefore, the
path would be the same as with Case G and go through:

1 2 3 4 3 2
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
LFN |
| 1
1 2 3 4 3 2 |
CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR4_HA --- MR5_HA
MIPv6 Node

Figure 13: MNN and CN Are MIPv6 Mobile Nodes

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RFC 4888 NEMO RO Problem Statement July 2007

A.3.3. Case I: VMN and Standard IPv6 CN

As for Case C and Case F, when the communication involves a Mobile
IPv6 node either as a Visiting Mobile Node or as a Correspondent
Node, Mobile IPv6 Route Optimization cannot be performed. Therefore,
MNN will establish a bi-directional tunnel with its Home Agent.
Packets between MNN and CN would thus go through MNN's own Home
Agent. The path would therefore be as shown in Figure 14:

2 3 4 5 4 3
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
VMN |
| 2
|
VMN_HA
|
| 1
1 2 3 4 3 2 |
CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR4_HA --- MR5_HA
IPv6 Node

Figure 14: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

A.4. CN Located Behind the Same Nested MR

Figure 15 below shows the case where the two communicating nodes are
connected behind the same nested Mobile Router. The optimization is
required when the communication involves MIPv6-enabled nodes.

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RFC 4888 NEMO RO Problem Statement July 2007

+--------+ +--------+ +--------+ +--------+
| MR2_HA | | MR3_HA | | MR4_HA | | MR5_HA |
+------+-+ +---+----+ +---+----+ +-+------+
\ | | /
+--------+ +-------------------------+ +--------+
| MR1_HA |----| Internet |----| VMN_HA |
+--------+ +-------------------------+ +--------+
|
+---+---+
root-MR | MR1 |
+---+---+
|
+-------+
sub-MR | MR2 |
+---+---+
|
+---+---+
sub-MR | MR3 |
+---+---+
|
-+--+--+-
MNN CN

Figure 15: MNN and CN Located Behind the Same Nested MR

A.4.1. Case J: LFN and Standard IPv6 CN

If both end nodes are Local Fixed Nodes, no special function is
necessary for optimization of their communications. The path between
the two nodes would go through:

1
MNN --- CN
LFN IPv6 Node

Figure 16: MNN and CN Are Standard IPv6 Nodes

A.4.2. Case K: VMN and MIPv6 CN

Similar to Case H, when both end nodes are Mobile IPv6 nodes, the two
nodes may initiate Mobile IPv6 Route Optimization. Although few
packets would go out the nested mobile network for the Return
Routability initialization, however, unlike Case B and Case E,
packets will not get tunneled outside the nested mobile network.
Therefore, packets between MNN and CN would eventually go through:

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RFC 4888 NEMO RO Problem Statement July 2007

1
MNN --- CN
VMN MIPv6 Node

Figure 17: MNN and CN are MIPv6 Mobile Nodes

If the root Mobile Router is disconnected while the nodes exchange
keys for the Return Routability procedure, they may not communicate
even though they are connected on the same link.

A.4.3. Case L: VMN and Standard IPv6 CN

When the communication involves a Mobile IPv6 node either as a
Visiting Mobile Network Node or as a Correspondent Node, Mobile IPv6
Route Optimization cannot be performed. Therefore, even though the
two nodes are on the same link, MNN will establish a bi-directional
tunnel with its Home Agent, which causes the flow to go out the
nested mobile network. The path between MNN and CN would require
another Home Agent (VMN_HA) to go through for this Mobile IPv6 node:

2 3 4 5 4 3
MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
VMN |
| 2
|
VMN_HA
|
| 1
1 2 3 4 3 2 |
CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
IPv6 Node

Figure 18: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

However, MNN may also decide to use its Care-of Address (CoA) as the
source address of the packets, thus avoiding the tunneling with the
MNN's Home Agent. This is particularly useful for a short-term
communications that may easily be retried if it fails. Default
Address Selection [11] provides some mechanisms for controlling the
choice of the source address.

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RFC 4888 NEMO RO Problem Statement July 2007

Appendix B. Example of How a Stalemate Situation Can Occur

Section 2.7 describes the occurrence of a stalemate situation where a
Home Agent of a Mobile Router is nested behind the Mobile Router.
Here, we illustrate a simple example where such a situation can
occur.

Consider a mobility configuration depicted in Figure 19 below. MR1
is served by HA1/BR and MR2 is served by HA2. The 'BR' designation
indicates that HA1 is a border router. Both MR1 and MR2 are at home
in the initial step. HA2 is placed inside the first mobile network,
thus representing a "mobile" Home Agent.

/-----CN
+----------+
home link 1 +--------+ | |
----+-----------------| HA1/BR |---| Internet |
| +--------+ | |
| +----------+
+--+--+ +-----+
| MR1 | | HA2 |
+--+--+ +--+--+
| |
-+--------+-- mobile net 1 / home link 2
|
+--+--+ +--+--+
| MR2 | | LFN |
+--+--+ +--+--+
| |
-+--------+- mobile net 2

Figure 19: Initial Deployment

In Figure 19 above, communications between CN and LFN follow a direct
path as long as both MR1 and MR2 are positioned at home. No
encapsulation intervenes.

In the next step, consider that the MR2's mobile network leaves home
and visits a foreign network, under Access Router (AR) like in
Figure 20 below.

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RFC 4888 NEMO RO Problem Statement July 2007

/-----CN
+----------+
home link 1 +--------+ | |
--+-----------| HA1/BR |---| Internet |
| +--------+ | |
+--+--+ +-----+ +----------+
| MR1 | | HA2 | \
+--+--+ +--+--+ +-----+
| | | AR |
-+--------+- mobile net 1 +--+--+
home link 2 |
+--+--+ +-----+
| MR2 | | LFN |
+--+--+ +--+--+
| |
mobile net 2 -+--------+-

Figure 20: Mobile Network 2 Leaves Home

Once MR2 acquires a Care-of Address under AR, the tunnel setup
procedure occurs between MR2 and HA2. MR2 sends a Binding Update to
HA2 and HA2 replies with a Binding Acknowledgement to MR2. The bi-
directional tunnel has MR2 and HA2 as tunnel endpoints. After the
tunnel MR2HA2 has been set up, the path taken by a packet from CN
towards LFN can be summarized as:

CN->BR->MR1->HA2=>MR1=>BR=>AR=>MR2->LFN.

Non-encapsulated packets are marked "->" while encapsulated packets
are marked "=>".

Consider next the attachment of the first mobile network under the
second mobile network, like in Figure 21 below.

After this movement, MR1 acquires a Care-of Address valid in the
second mobile network. Subsequently, it sends a Binding Update (BU)
message addressed to HA1. This Binding Update is encapsulated by MR2
and sent towards HA2, which is expected to be placed in mobile net 1
and expected to be at home. Once HA1/BR receives this encapsulated
BU, it tries to deliver to MR1. Since MR1 is not at home, and a
tunnel has not yet been set up between MR1 and HA1, HA1 is not able
to route this packet and drops it. Thus, the tunnel establishment
procedure between MR1 and HA1 is not possible, because the tunnel
between MR2 and HA2 had been previously torn down (when the mobile
net 1 moved from home). The communications between CN and LFN stops,
even though both mobile networks are connected to the Internet.

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RFC 4888 NEMO RO Problem Statement July 2007

/-----CN
+----------+
+--------+ | |
| HA1/BR |---| Internet |
+--------+ | |
+----------+
\
+-----+
| AR |
+--+--+
|
+--+--+ +-----+
| MR2 | | LFN |
+--+--+ +--+--+
| |
mobile net 2 -+--------+-
|
+--+--+ +-----+
| MR1 | | HA2 |
+--+--+ +--+--+
| |
mobile net 1 -+--------+-

Figure 21: Stalemate Situation Occurs

If both tunnels between MR1 and HA1, and between MR2 and HA2, were up
simultaneously, they would have "crossed over" each other. If the
tunnels MR1-HA1 and MR2-HA2 were drawn in Figure 21, it could be
noticed that the path of the tunnel MR1-HA1 includes only one
endpoint of the tunnel MR2-HA2 (the MR2 endpoint). Two MR-HA tunnels
are crossing over each other if the IP path between two endpoints of
one tunnel includes one and only one endpoint of the other tunnel
(assuming that both tunnels are up). When both endpoints of one
tunnel are included in the path of the other tunnel, then tunnels are
simply encapsulating each other.

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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: [email protected]

Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3, Biot - Sophia Antipolis 06410
FRANCE

EMail: [email protected]

Masafumi Watari
KDDI R&D Laboratories Inc.
2-1-15 Ohara
Fujimino, Saitama 356-8502
JAPAN

EMail: [email protected]

Fan Zhao
UC Davis
One Shields Avenue
Davis, CA 95616
US

Phone: +1 530 752 3128
EMail: [email protected]

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RFC 4888 NEMO RO Problem Statement July 2007

Full Copyright Statement

Copyright (C) The IETF Trust (2007).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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