Internet Engineering Task Force (IETF) T. Melia, Ed.

Internet Engineering Task Force (IETF) T. Melia, Ed.
Request for Comments: 7847 Kudelski Security
Category: Informational S. Gundavelli, Ed.
ISSN: 2070-1721 Cisco
May 2016

Logical-Interface Support for IP Hosts with Multi-Access Support

Abstract

A logical interface is a software semantic internal to the host
operating system. This semantic is available in all popular
operating systems and is used in various protocol implementations.
Logical-interface support is required on the mobile node attached to
a Proxy Mobile IPv6 domain for leveraging various network-based
mobility management features such as inter-technology handoffs,
multihoming, and flow mobility support. This document explains the
operational details of the logical-interface construct and the
specifics on how link-layer implementations hide the physical
interfaces from the IP stack and from the network nodes on the
attached access networks. Furthermore, this document identifies the
applicability of this approach to various link-layer technologies and
analyzes the issues around it when used in conjunction with various
mobility management features.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7847.

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Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Hiding Link-Layer Technologies -- Approaches and
Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Link-Layer Abstraction -- Approaches . . . . . . . . . . 4
3.2. Link-Layer Support . . . . . . . . . . . . . . . . . . . 5
3.3. Logical Interface . . . . . . . . . . . . . . . . . . . . 6
4. Technology Use Cases . . . . . . . . . . . . . . . . . . . . 6
5. Logical-Interface Functional Details . . . . . . . . . . . . 7
5.1. Configuration of a Logical Interface . . . . . . . . . . 8
5.2. Logical-Interface Conceptual Data Structures . . . . . . 9
6. Logical-Interface Use Cases in Proxy Mobile IPv6 . . . . . . 11
6.1. Multihoming Support . . . . . . . . . . . . . . . . . . . 11
6.2. Inter-technology Handoff Support . . . . . . . . . . . . 12
6.3. Flow Mobility Support . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16

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1. Introduction

Proxy Mobile IPv6 (PMIPv6) [RFC5213] is a network-based mobility
management protocol standardized by IETF. One of the key goals of
the PMIPv6 protocol is to enable a mobile node to perform handovers
across access networks based on different access technologies. The
protocol was also designed with the goal to allow a mobile node to
simultaneously attach to different access networks and perform flow-
based access selection [RFC7864]. The base protocol features
specified in [RFC5213] and [RFC5844] have support for these
capabilities. However, to support these features, the mobile node is
required to be enabled with a specific software configuration known
as logical-interface support. The logical-interface configuration is
essential for a mobile node to perform inter-access handovers without
impacting the IP sessions on the host.

A logical-interface construct is internal to the operating system.
It is an approach of interface abstraction, where a logical link-
layer implementation hides a variety of physical interfaces from the
IP stack. This semantic was used on a variety of operating systems
to implement applications such as Mobile IP client [RFC6275] and
IPsec VPN client [RFC4301]. Many host operating systems have support
for some form of such logical-interface construct. But, there is no
specification that documents the behavior of these logical interfaces
or the requirements of a logical interface for supporting the above-
mentioned mobility management features. This specification attempts
to document these aspects.

The rest of the document provides a functional description of a
logical interface on the mobile node and the interworking between a
mobile node using a logical interface and the network elements in the
Proxy Mobile IPv6 domain. It also analyzes the issues involved with
the use of a logical interface and characterizes the contexts in
which such usage is appropriate.

2. Terminology

All the mobility-related terms used in this document are to be
interpreted as defined in the Proxy Mobile IPv6 specifications
[RFC5213] and [RFC5844]. In addition, this document uses the
following terms:

PIF (Physical Interface): A network interface module on the host
that is used for connecting to an access network. A host
typically has a number of network interface modules, such as
Ethernet, Wireless LAN, LTE, etc. Each of these network
interfaces can support specific link technology.

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LIF (Logical Interface): A virtual interface in the IP stack. A
logical interface appears to the IP stack just as any other
physical interface and provides similar semantics with respect to
packet transmit and receive functions to the upper layers of the
IP stack. However, it is only a logical construct and is not a
representation of an instance of any physical hardware.

SIF (Sub-Interface): A physical or logical interface that is part of
a logical-interface construct. For example, a logical interface
may have been created by abstracting two physical interfaces, LTE
and WLAN. These physical interfaces, LTE and WLAN, are referred
to as sub-interfaces of that logical interface. In some cases, a
sub-interface can also be another logical interface, such as an
IPsec tunnel interface.

3. Hiding Link-Layer Technologies -- Approaches and Applicability

There are several techniques that allow hiding changes in access
technology changes from the host layer. These changes in access
technology are primarily due to the host's movement between access
networks. This section classifies these existing techniques into a
set of generic approaches, according to their most representative
characteristics. Later sections of this document analyze the
applicability of these solution approaches for supporting features,
such as inter-technology handovers and IP flow mobility support for a
mobile node.

3.1. Link-Layer Abstraction -- Approaches

The following generic mechanisms can hide access technology changes
from the host IP layer:

o Link-Layer Support -- Certain link-layer technologies are able to
hide physical media changes from the upper layers. For example,
IEEE 802.11 is able to seamlessly change between IEEE 802.11a/b/g
physical layers. Also, an 802.11 Station (STA) can move between
different access points within the same domain without the IP
stack being aware of the movement. In this case, the IEEE 802.11
Media Access Control (MAC) layer takes care of the mobility,
making the media change invisible to the upper layers. Another
example is IEEE 802.3, which supports changing the rate from 10
Mbps to 100 Mbps and to 1000 Mbps. Another example is the
situation in the 3GPP Evolved Packet System [TS23401] where the
User Equipment (UE) can perform inter-access handovers between
three different access technologies (2G GSM/EDGE Radio Access
Network (GERAN), 3G Universal Terrestrial Radio Access Network
(UTRAN), and 4G Evolved UTRAN (E-UTRAN)) that are invisible to the
IP layer at the UE.

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o A logical interface denotes a mechanism that logically groups
several physical interfaces so they appear to the IP layer as a
single interface (see Figure 1). Depending on the type of access
technologies, it might be possible to use more than one physical
interface at a time -- such that the node is simultaneously
attached via different access technologies -- or just perform
handovers across a variety of physical interfaces. Controlling
the way the different access technologies are used (simultaneous,
sequential attachment, etc.) is not trivial and requires
additional intelligence and/or configuration within the logical-
interface implementation. The configuration is typically handled
via a connection manager, and it is based on a combination of user
preferences on one hand and operator preferences such as those
provisioned by the Access Network Discovery and Selection Function
(ANDSF) [TS23402] on the other hand. The IETF Interfaces MIB
specified in [RFC2863] and the YANG data model for interface
management specified in [RFC7223] treat a logical interface just
like any other type of network interface on the host. This
essentially makes the logical interface a natural operating system
construct.

3.2. Link-Layer Support

Link-layer mobility support applies to cases in which the same link-
layer technology is used and mobility can be fully handled at that
layer. One example is the case where several 802.11 access points
are deployed in the same subnet with a common IP-layer configuration
(DHCP server, default router, etc.). In this case, the handover
across access points need not be hidden to the IP layer since the IP-
layer configuration remains the same after a handover. This type of
scenario is applicable to cases when the different points of
attachment (i.e., access points) belong to the same network domain,
e.g., enterprise, hotspots from same operator, etc.

Since this type of link-layer technology does not typically allow for
simultaneous attachment to different access networks of the same
technology, the logical interface would not be used to provide
simultaneous access for purposes of multihoming or flow mobility.
Instead, the logical interface can be used to provide inter-access
technology handover between this type of link-layer technology and
another link-layer technology, e.g., between IEEE 802.11 and IEEE
802.16.

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3.3. Logical Interface

The use of a logical interface allows the mobile node to provide a
single-interface perspective to the IP layer and its upper layers
(transport and application). Doing so allows inter-access technology
handovers or application flow handovers to be hidden across different
physical interfaces.

The logical interface may support simultaneous attachment in addition
to sequential attachment. It requires additional support at the node
and the network in order to benefit from simultaneous attachment.
For example, special mechanisms are required to enable addressing a
particular interface from the network (e.g., for flow mobility). In
particular, extensions to PMIPv6 are required in order to enable the
network (i.e., the mobile access gateway (MAG) and local mobility
anchor (LMA)) to deal with the logical interface, instead of using
extensions to IP interfaces as currently specified in RFC 5213. RFC
5213 assumes that each physical interface capable of attaching to a
MAG is an IP interface, while the logical-interface solution groups
several physical interfaces under the same IP logical interface.

It is therefore clear that the logical-interface approach satisfies
the requirement of multi-access technology and supports both
sequential and simultaneous access.

4. Technology Use Cases

3GPP has defined the Evolved Packet System (EPS) for heterogeneous
wireless access. A mobile device equipped with 3GPP and non-3GPP
wireless technologies can simultaneously or sequentially connect to
any of the available access networks and receive IP services through
any of them. This document focuses on employing a logical interface
for simultaneous and sequential use of a variety of access
technologies.

As mentioned in the previous sections, the logical-interface
construct is able to hide from the IP layer the specifics of each
technology in the context of network-based mobility (e.g., in multi-
access technology networks based on PMIPv6). The LIF concept can be
used with at least the following technologies: 3GPP access
technologies (3G and LTE), IEEE 802.16 access technology, and IEEE
802.11 access technology.

In some UE implementations, the wireless connection setup is based on
creation of a PPP interface between the IP layer and the wireless
modem that is configured with the IP Control Protocol (IPCP) and IPv6
Control Protocol (IPv6CP) [RFC5072]. In this case, the PPP interface
does not have any layer 2 (L2) addresses assigned. In some other

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implementations, the wireless modem is presented to the IP layer as a
virtual Ethernet interface.

5. Logical-Interface Functional Details

This section identifies the functional details of a logical interface
and provides some implementation considerations.

On most operating systems, a network interface is associated with a
physical device that offers the services for transmitting and
receiving IP packets from the network. In some configurations, a
network interface can also be implemented as a logical interface,
which does not have the inherent capability to transmit or receive
packets on a physical medium, but relies on other physical interfaces
for such services. An example of such configuration is an IP tunnel
interface.

An overview of a logical interface is shown in Figure 1. The logical
interface allows heterogeneous attachment while making changes in the
underlying media transparent to the IP stack. Simultaneous and
sequential network attachment procedures are therefore possible,
enabling inter-technology and flow mobility scenarios.

+----------------------------+
| TCP/UDP |
Session-to-IP +---->| |
Address Binding | +----------------------------+
+---->| IP |
IP Address +---->| |
Binding | +----------------------------+
+---->| Logical Interface |
Logical-to- +---->| IPv4/IPv6 Address |
Physical | +----------------------------+
Interface +---->| L2 | L2 | | L2 |
Binding |(IF#1)|(IF#2)| ..... |(IF#n)|
+------+------+ +------+
| L1 | L1 | | L1 |
| | | | |
+------+------+ +------+

Figure 1: General Overview of Logical Interface

From the perspective of the IP stack and the applications, a logical
interface is just another interface. In fact, the logical interface
is only visible to the IP and upper layers when enabled. A host does
not see any operational difference between a logical and a physical
interface. As with physical interfaces, a logical interface is
represented as a software object to which IP address configuration is

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bound. However, the logical interface has some special properties
that are essential for enabling inter-technology handover and flow-
mobility features. Following are those properties:

1. The logical interface has a relation to a set of physical
interfaces (sub-interfaces) on the host that it is abstracting.
These sub-interfaces can be attached or detached from the logical
interface at any time. The sub-interfaces attached to a logical
interface are not visible to the IP and upper layers.

2. The logical interface may be attached to multiple access
technologies.

3. The Transmit/Receive functions of the logical interface are
mapped to the Transmit/Receive services exposed by the sub-
interfaces. This mapping is dynamic, and any change is not
visible to the upper layers of the IP stack.

4. The logical interface maintains IP flow information for each of
its sub-interfaces. A conceptual data structure is maintained
for this purpose. The host may populate this information based
on tracking each of the sub-interfaces for the active flows.

5.1. Configuration of a Logical Interface

A host may be statically configured with the logical-interface
configuration, or an application such as a connection manager on the
host may dynamically create it. Furthermore, the set of sub-
interfaces that are part of a logical-interface construct may be a
fixed set or may be kept dynamic, with the sub-interfaces getting
added or deleted as needed. The specific details related to these
configuration aspects are implementation specific and are outside the
scope of this document.

The IP layer should be configured with a default router reachable via
the logical interface. The default router can be internal to the
logical interface, i.e., it is a logical router that in turn decides
which physical interface is to be used to transmit packets.

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5.2. Logical-Interface Conceptual Data Structures

Every logical interface maintains a list of sub-interfaces that are
part of that logical-interface construct. This is a conceptual data
structure, called the LIF table. Figure 2 shows an example LIF table
where logical interface LIF-1 has three sub-interfaces, ETH-0,
WLAN-0, and LTE-0, and logical interface LIF-2 has two sub-
interfaces, ETH-1 and WLAN-1. For each LIF entry, the table should
store the associated link status and policy associated with that sub-
interface (e.g., active or not active). The method by which the
routing policies are configured on the host is out of scope for this
document.

+=======================+========================+==================+
| Logical_Interface | Sub_Interface | Status/Policy |
+=======================+========================+==================+
| LIF-1 | ETH-0 | UP |
+=======================+========================+==================+
| LIF-1 | WLAN-0 | DOWN |
+=======================+========================+==================+
| LIF-1 | LTE-0 | UP |
+=======================+========================+==================+
| LIF-2 | ETH-1 | UP |
+=======================+========================+==================+
| LIF-2 | WLAN-1 | UP |
+=======================+========================+==================+

Figure 2: Logical-Interface Table

The logical interface also maintains the list of flows associated
with a given sub-interface, and this conceptual data structure is
called the Flow table. Figure 3 shows an example Flow table, where
flows FID-1, FID-2, FID-3, FID-4, and FID-5 are associated with sub-
interfaces ETH-0, WLAN-0, LTE-0, ETH-1, and WLAN-1, respectively.

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+=======================+========================+
| Flow | Sub_Interface |
+=======================+========================+
| FID-1 | ETH-0 |
+=======================+========================+
| FID-2 | WLAN-0 |
+=======================+========================+
| FID-3 | LTE-0 |
+=======================+========================+
| FID-4 | ETH-1 |
+=======================+========================+
| FID-5 | WLAN-1 |
+=======================+========================+

Figure 3: Flow Table

The Flow table allows the logical interface to properly route each IP
flow over a specific sub-interface. The logical interface can
identify the flows arriving on its sub-interfaces and associate them
to those sub-interfaces. This approach is similar to reflective QoS
performed by the IP routers. For locally generated traffic (e.g.,
unicast flows), the logical interface should perform interface
selection based on the Flow Routing Policies. In case traffic of an
existing flow is suddenly received from the network on a different
sub-interface from the one locally stored, the logical interface
should interpret the event as an explicit flow mobility trigger from
the network, and it should update the corresponding entry in the Flow
table. Similarly, locally generated events from the sub-interfaces
or configuration updates to the local policy rules can cause updates
to the table and hence trigger flow mobility.

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6. Logical-Interface Use Cases in Proxy Mobile IPv6

This section explains how the logical-interface support on the mobile
node can be used for enabling some of the Proxy Mobile IPv6 protocol
features.

6.1. Multihoming Support

Figure 4 shows a mobile node with multiple interfaces attached to a
Proxy Mobile IPv6 domain. In this scenario, the mobile node is
configured to use a logical interface over the physical interfaces
through which it is attached.

LMA Binding Table
+========================+
+----+ | HNP MN-ID CoA ATT |
|LMA | +========================+
+----+ | HNP-1 MN-1 PCoA-1 5 |
//\\ | HNP-1 MN-1 PCoA-2 4 |
+---------//--\\-----------+
( // \\ )
( // \\ )
+------//--------\\--------+
// \\
PCoA-1 // \\ PCoA-2
+----+ +----+
(WLAN) |MAG1| |MAG2| (3GPP)
+----+ +----+
\ /
\ /
\ /
\ /
\ /
+-------+ +-------+
| if_1 | | if_2 |
|(WLAN) | |(3GPP) |
+-------+-+-------+
| Logical |
| Interface |
| (HNP-1) |
+-----------------|
| MN |
+-----------------+

Figure 4: Multihoming Support

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6.2. Inter-technology Handoff Support

The Proxy Mobile IPv6 protocol enables a mobile node with multiple
network interfaces to move between access technologies but still
retain the same address configuration on its attached interface.
Figure 5 shows a mobile node performing an inter-technology handoff
between access networks. The protocol enables a mobile node to
achieve address continuity during handoffs. If the host is
configured to use a logical interface over the physical interface
through which it is attached, following are the related
considerations.

LMA's Binding Table
+==========================+
+----+ | HNP MN-ID CoA ATT |
|LMA | +==========================+
+----+ | HNP-1 MN-1 PCoA-1 5 |
//\\ (pCoA-2)(4) <--change
+---------//--\\-----------+
( // \\ )
( // \\ )
+------//--------\\--------+
// \\
PCoA-1 // \\ PCoA-2
+----+ +----+
(WLAN) |MAG1| |MAG2| (3GPP)
+----+ +----+
\ /
\ Handoff /
\ /
\ /
+-------+ +-------+
| if_1 | | if_2 |
|(WLAN) | |(3GPP) |
+-------+-+-------+
| Logical |
| Interface |
| (HNP-1) |
+-----------------|
| MN |
+-----------------+

Figure 5: Inter-technology Handoff Support

o When the mobile node performs a handoff between if_1 and if_2, the
change will not be visible to the applications of the mobile node.

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o The protocol signaling between the network elements will ensure
the local mobility anchor will switch the forwarding for the
advertised prefix set from MAG1 to MAG2.

6.3. Flow Mobility Support

To support IP flow mobility, there is a need to support vertical
handoff scenarios such as transferring a subset of a prefix(es)
(hence the flows associated to it/them) from one interface to
another. The mobile node can support this scenario by using the
logical-interface support. This scenario is similar to the inter-
technology handoff scenario defined in Section 6.2; only a subset of
the prefixes are moved between interfaces.

Additionally, IP flow mobility in general initiates when the LMA
decides to move a particular flow from its default path to a
different one. The LMA can decide the best MAG to be used to forward
a particular flow when the flow is initiated (e.g., based on
application policy profiles) and/or during the lifetime of the flow
upon receiving a network-based or a mobile-based trigger. However,
the specific details on how the LMA can formulate such flow policy is
outside the scope of this document.

7. Security Considerations

This specification explains the operational details of a logical
interface on an IP host. The logical-interface implementation on the
host is not visible to the network and does not require any special
security considerations.

Different layer 2 interfaces and the access networks to which they
are connected have different security properties. For example, the
layer 2 network security of a Wireless LAN network operated by an end
user is in the control of the home user whereas an LTE operator has
control of the layer 2 security of the LTE access network. An
external entity using lawful means, or through other means, obtains
the security keys from the LTE operator, but the same may not be
possible in the case of a Wireless LAN network operated by a home
user. Therefore, grouping interfaces with such varying security
properties into one logical interface could have negative
consequences in some cases. Such differences, though subtle, are
entirely hidden by logical interfaces and are unknown to the upper
layers.

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8. References

8.1. Normative References

[RFC5213] Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
RFC 5213, DOI 10.17487/RFC5213, August 2008,
<http://www.rfc-editor.org/info/rfc5213>.

[RFC5844] Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
Mobile IPv6", RFC 5844, DOI 10.17487/RFC5844, May 2010,
<http://www.rfc-editor.org/info/rfc5844>.

8.2. Informative References

[RFC2863] McCloghrie, K. and F. Kastenholz, "The Interfaces Group
MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000,
<http://www.rfc-editor.org/info/rfc2863>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.

[RFC5072] Varada, S., Ed., Haskins, D., and E. Allen, "IP Version 6
over PPP", RFC 5072, DOI 10.17487/RFC5072, September 2007,
<http://www.rfc-editor.org/info/rfc5072>.

[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
2011, <http://www.rfc-editor.org/info/rfc6275>.

[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
<http://www.rfc-editor.org/info/rfc7223>.

[RFC7864] Bernardos, CJ., Ed., "Proxy Mobile IPv6 Extensions to
Support Flow Mobility", RFC 7864, DOI 10.17487/RFC7864,
May 2016, <http://www.rfc-editor.org/info/rfc7864>.

[TS23401] 3rd Generation Partnership Project, "Technical
Specification Group Services and System Aspects; General
Packet Radio Service (GPRS) enhancements for Evolved
Universal Terrestrial Radio Access Network (E-UTRAN)
access", TS 23.401, V13.6.0, March 2016.

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[TS23402] 3rd Generation Partnership Project, "Technical
Specification Group Services and System Aspects;
Architecture enhancements for non-3GPP accesses", TS
23.402, V13.5.0, March 2016.

Acknowledgements

The authors would like to acknowledge all the discussions on this
topic in the NETLMM and NETEXT working groups. The authors would
also like to thank Joo-Sang Youn, Pierrick Seite, Rajeev Koodli,
Basavaraj Patil, Peter McCann, Julien Laganier, Maximilian Riegel,
Georgios Karagian, Stephen Farrell, and Benoit Claise for their input
to the document.

Contributors

This document reflects contributions from the following individuals
(listed in alphabetical order):

Carlos Jesus Bernardos Cano
Email: [email protected]

Antonio De la Oliva
Email: [email protected]

Yong-Geun Hong
Email: [email protected]

Kent Leung
Email: [email protected]

Tran Minh Trung
Email: [email protected]

Hidetoshi Yokota
Email: [email protected]

Juan Carlos Zuniga
Email: [email protected]

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Authors' Addresses

Telemaco Melia (editor)
Kudelski Security
Geneva
Switzerland

Email: [email protected]

Sri Gundavelli (editor)
Cisco
170 West Tasman Drive
San Jose, CA 95134
United States

Email: [email protected]

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