Internet Engineering Task Force                             M. Wasserman
Internet-Draft                                    Painless Security, LLC
Intended status: Informational                                  P. Seite
Expires: August 5, 2011                          France Telecom - Orange
                                                        February 1, 2011

             Current Practices for Multiple Interface Hosts


   An increasing number of hosts are operating in multiple-interface
   environments, where different network interfaces are providing
   unequal levels of service or connectivity.  This document summarizes
   current practices in this area, and describes in detail how some
   common operating systems cope with these challenges.

Status of this Memo

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   This Internet-Draft will expire on August 5, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Summary of Current Approaches  . . . . . . . . . . . . . . . .  3
     2.1.  Centralized Connection Management  . . . . . . . . . . . .  3
     2.2.  Per Application Connection Settings  . . . . . . . . . . .  4
     2.3.  Stack-Level Solutions to Specific Problems . . . . . . . .  4
       2.3.1.  DNS Resolution Issues  . . . . . . . . . . . . . . . .  5
       2.3.2.  Routing  . . . . . . . . . . . . . . . . . . . . . . .  5
       2.3.3.  Address Selection Policy . . . . . . . . . . . . . . .  5
   3.  Current Practices in Some Operating Systems  . . . . . . . . .  6
     3.1.  Mobile Handset Operating Systems . . . . . . . . . . . . .  6
       3.1.1.  Nokia S60 3rd Edition, Feature Pack 2  . . . . . . . .  7
       3.1.2.  Microsoft Windows Mobile 2003 Second Edition and
               Windows Phone 7  . . . . . . . . . . . . . . . . . . .  8
       3.1.3.  BlackBerry . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.4.  Google Android . . . . . . . . . . . . . . . . . . . . 10
       3.1.5.  Qualcomm Brew  . . . . . . . . . . . . . . . . . . . . 11
       3.1.6.  Arena Connection Manager . . . . . . . . . . . . . . . 13
       3.1.7.  Current practices for network selection in handsets  . 13
     3.2.  Desktop Operating Systems  . . . . . . . . . . . . . . . . 14
       3.2.1.  Microsoft Windows  . . . . . . . . . . . . . . . . . . 15  Routing  . . . . . . . . . . . . . . . . . . . . . 15  Outbound and Inbound Addresses . . . . . . . . . . 15  DNS Configuration  . . . . . . . . . . . . . . . . 15
       3.2.2.  Linux and BSD-based Operating Systems  . . . . . . . . 16  Routing  . . . . . . . . . . . . . . . . . . . . . 16  Outbound and Inbound Addresses . . . . . . . . . . 17  DNS Configuration  . . . . . . . . . . . . . . . . 17
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   7.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

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1.  Introduction

   Multiple-interface hosts face several challenges not faced by single-
   interface hosts, some of which are described in the MIF problem
   statement, [I-D.ietf-mif-problem-statement].  This document
   summarizes how current implementations deal with the problems
   identified in the MIF problem statement.

   Publicly-available information about the multiple-interface solutions
   implemented in some widely used operating systems, including both
   mobile handset and desktop operating systems, is collected in this
   document, including: Nokia S60 [S60], Microsoft Windows Mobile
   [WINDOWSMOBILE], Blackberry [BLACKBERRY], Google Android [ANDROID],
   Microsoft Windows, Apple Mac OS X, Linux and BSD-based operating

2.  Summary of Current Approaches

   This section summarizes current approaches that are used to resolve
   the multi-interface issues described in the Multiple Interface
   Problem Statement [I-D.ietf-mif-problem-statement].  These approaches
   can be broken down into three major categories:

   o  Centralized connection management

   o  Per-application connection settings

   o  Stack-level solutions to specific problems

2.1.  Centralized Connection Management

   It is a common practice for mobile handset operating systems to use a
   centralized connection manager that performs network interface
   selection based on application or user input.  The information used
   by the connection manager may be programmed into an application or
   provisioned on a handset-wide basis.  When information is not
   available to make an interface selection, the connection manager will
   query the user to choose between available choices.

   Routing tables are not typically used for network interface selection
   when a connection manager is in use, as the criteria for network
   selection is not strictly IP-based but is also dependent on other
   properties of the interface (cost, type, etc.).  Furthermore,
   multiple overlapping private IPv4 address spaces are often exposed to
   a multiple-interface host, making it difficult to make interface
   selection decisions based on prefix matching.

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2.2.  Per Application Connection Settings

   In mobile handsets, applications are often involved in choosing what
   interface and related configuration information should be used.  In
   some cases, the application selects the interface directly, and in
   other cases the application provides more abstract information to a
   connection manager that makes the final interface choice.

2.3.  Stack-Level Solutions to Specific Problems

   In most desktop operating systems, multiple interface problems are
   dealt with in the stack and related components, based on system-
   level configuration information, without the benefit of input from
   applications or users.  These solutions tend to map well to the
   problems listed in the problem statement:

   o  DNS resolution issues

   o  Routing

   o  Address selection policy

   The configuration information for desktop systems comes from one of
   three sources: DHCP, proprietary configuration systems or manual
   configuration.  While these systems universally accept IP address
   assignment on a per-interface basis, they differ in what set of
   information can be assigned on a per-interface basis and what can be
   configured only on a per-system basis.

   When choosing between multiple sets of information provided, these
   systems will typically give preference to information received on the
   "primary" interface.  The mechanism for designating the "primary"
   interface differs by system.

   There is very little commonality in how desktop operating systems
   handle multiple sets of configuration information, with notable
   variations between different versions of the same operating system
   and/or within different software packages built for the same
   operating system.  Although these systems differ widely, it is not
   clear that any of them provide a completely satisfactory user
   experience in multiple-interface environments.

   The following sections discuss some of the solutions used in each of
   the areas raised in the MIF problem statement.

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2.3.1.  DNS Resolution Issues

   There is very little commonality in how desktop operating systems
   handle the DNS server list.  Some systems support per-interface DNS
   server lists, while others only support a single system-wide list.

   On hosts with per-interface DNS server lists, different mechanisms
   are used to determine which DNS server is contacted for a given
   query.  In most cases, the first DNS server listed on the "primary"
   interface is queried first, with back off to other servers if an
   answer is not received.

   Systems that support a single system-wide list differ in how they
   select which DNS server to use in cases where they receive more than
   one DNS server list to configure (e.g. from DHCP on multiple
   interfaces).  Some accept the information received on the "primary"
   interface, while others use either the first or last set DNS server
   list configured.

2.3.2.  Routing

   Routing information is also handled differently on different desktop
   operating systems.  While all systems maintain some sort of routing
   cache, to handle redirects and/or statically configured routes, most
   packets are routed based on configured default gateway information.

   Some systems do allow the configuration of different default router
   lists for different interfaces.  These systems will always choose the
   default gateway on the interface with the lowest routing metric, with
   different behavior when two or more interfaces have the same routing

   Most systems do not allow the configuration of more than one default
   router list, choosing instead to use the first or last default router
   list configured and/or the router list configured on the "primary"

2.3.3.  Address Selection Policy

   There is somewhat more commonality in how desktop hosts handle
   address selection.  Applications typically provide the destination
   address for an outgoing packet, and the IP stack is responsible for
   picking the source address.

   IPv6 specifies a specific source address selection mechanism in
   [RFC3484], and several systems implement this mechanism with similar
   support for IPv4.  However, many systems do not provide any mechanism
   to update this default policy, and there is no standard way to do so.

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   In some cases, the routing decision (including which interface to
   use) is made before source address selection is performed, and a
   source address is chosen from the outbound interface.  In other
   cases, source address selection is performed before, or independently
   from outbound interface selection.

3.  Current Practices in Some Operating Systems

   The following sections briefly describe the current multiple-
   interface host implementations on some widely-used operating systems.
   Please refer to the References section for pointers to original
   documentation on most of these systems, including further details.

3.1.  Mobile Handset Operating Systems

   Cellular devices typically run a variety of applications in parallel,
   each with different requirements for IP connectivity.  A typical
   scenario is shown in figure 1, where a cellular device is utilizing
   WLAN access for web browsing and GPRS access for transferring
   multimedia messages (MMS).  Another typical scenario would be a real-
   time VoIP session over one network interface in parallel with best
   effort web browsing on another network interface.  Yet another
   typical scenario would be global Internet access through one network
   interface and local (e.g. corporate VPN) network access through

        Web server                                       MMS Gateway
             |                                                |
            -+--Internet----            ----Operator network--+-
                    |                          |
                +-------+                  +-------+
                |WLAN AP|                  | GGSN  |
                +-------+                  +-------+
                    |        +--------+        |
                             |device  |

               A cellular device with two network interfaces

                                 Figure 1

   Different network access technologies require different settings.
   For example, WLAN requires Service Set Identifier (SSID) and the GPRS
   network requires the Access Point Name (APN) of the Gateway GPRS

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   Support Node (GGSN), among other parameters.  It is common that
   different accesses lead to different destination networks (e.g. to
   "Internet", "intranet", cellular network services, etc.).

3.1.1.  Nokia S60 3rd Edition, Feature Pack 2

   S60 uses the concept of an Internet Access Point (IAP) [S60] that
   contains all information required for opening a network connection
   using a specific access technology.  A device may have several IAPs
   configured for different network technologies and settings (multiple
   WLAN SSIDs, GPRS APNs, dial-up numbers, and so forth).  There may
   also be 'virtual' IAPs that define parameters needed for tunnel
   establishment (e.g. for VPN).

   For each application, a correct IAP needs to be selected at the point
   when the application requires network connectivity.  This is
   essential, as the wrong IAP may not be able to support the
   application or reach the desired destination.  For example, MMS
   application must use the correct IAP in order to reach the MMS
   Gateway, which typically is not accessible from the public Internet.
   As another example, an application might need to use the IAP
   associated with its corporate VPN in order to reach internal
   corporate servers.  Binding applications to IAPs avoids several
   problems, such as choosing the correct DNS server in the presence of
   split DNS (as an application will use the DNS server list from its
   bound IAP), and overlapping private IPv4 address spaces used for
   different interfaces (as each application will use the default routes
   from its bound IAP).

   If multiple applications utilize the same IAP, the underlying network
   connection can typically be shared.  This is often the case when
   multiple Internet-using applications are running in parallel.

   The IAP for an application can be selected in multiple ways:

   o  Statically: e.g. from a configuration interface, via client
      provisioning/device management system, or at build-time.

   o  Manually by the user: e.g. each time an application starts the
      user may be asked to select the IAP to use.  This may be needed,
      for example, if a user sometimes wishes to access his corporate
      intranet and other times would prefer to access the Internet

   o  Automatically by the system: after the destination network has
      been selected statically or dynamically.

   The static approach is fine for certain applications, like MMS, for

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   which configuration can be provisioned by the network operator and
   does not change often.  Manual selection works, but may be seen as
   troublesome by the user.  An automatic selection mechanism needs to
   have some way of knowing which destination network the user, or an
   application, is trying access.

   S60 3rd Edition, Feature Pack 2, introduces a concept of Service
   Network Access Points (SNAPs) that group together IAPs that lead to
   the same destination.  This enables static or manual selection of the
   destination network for an application and leaves the problem of
   selecting the best of the available IAPs within a SNAP to the
   operating system.

   When SNAPs are used, it is possibly for the operating system to
   notify applications when a preferred IAP, leading to the same
   destination, becomes available (for example, when a user comes within
   range of his home WLAN access point), or when the currently used IAP
   is no longer available and applications have to reconnect via another
   IAP (for example, when a user goes out of range of his home WLAN and
   must move to the cellular network).

   In S60 3.2 does not support RFC 3484 for source address selection
   mechanisms.  Applications are tightly bound the network interface
   selected for them or by them.  E.g. an application may be connected
   to IPv6 3G connection, IPv4 3G connection, WLAN connection, or VPN
   connection.  The application can change between the connections, but
   uses only one at a time.  If the interface happens to be dual-stack,
   then IPv4 is preferred over IPv6.

   DNS configuration is per-interface; an application bounded to an
   interface will always use the DNS settings for that interface.  Hence
   the device itself remembers these pieces of information for each
   interface separately.

   The S60 3.2 manages with totally overlapping addresses spaces.  Each
   interface can even have same IPv4 address configured on it without
   issues.  This is so because interfaces are kept totally separate from
   each other.  This also implies that the interface selection has to be
   done at application layer, as from network layer point of view device
   is not multihomed in the IP-sense.

   Please see the source documentation for more details and screenshots:

3.1.2.  Microsoft Windows Mobile 2003 Second Edition and Windows Phone 7

   A Connection Manager architecture is described in [WINDOWSMOBILE].
   This architecture centralizes and automates network connection

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   establishment and management, and makes it possible to automatically
   select a connection, to dial-in automatically or by user initiation,
   and to optimize connection and shared resource usage.  Connection
   Manager periodically re-evaluates the validity of the connection
   selection.  The Connection Manager uses various attributes such as
   cost, security, bandwidth, error rate, and latency in its decision

   The Connection Manager selects the best possible connection for the
   application based on the destination network the application wishes
   to reach.  The selection is made between available physical and
   virtual connections (e.g.  VPN, GPRS, WLAN, and wired Ethernet) that
   are known to provide connectivity to the destination network, and the
   selection is based on the costs associated with each connection.
   Different applications are bundled to use the same network connection
   when possible, but in conflict situations when a connection cannot be
   shared, higher priority applications take precedence, and the lower
   priority applications lose connectivity until the conflict situation

   During operation, Connection Manager opens new connections as needed,
   and also disconnects unused or idle connections.

   To optimize resource use, such as battery power and bandwidth,
   Connection Manager enables applications to synchronize network
   connection usage by allowing applications to register their
   requirements for periodic connectivity.  An application is notified
   when a suitable connection becomes available for its use.

   In comparison to Windows Mobile 2003 SE, Windows phone 7 brings
   update of the routing functionality in the case where the terminal
   can be attached simultaneously to several interfaces.  Actually,
   Windows Phone 7 routes the traffic through a preferred interface,
   which has a lower metric.  When there are multiple interfaces, the
   applications system will, by default, choose from an ordered list of
   available interfaces.  The default connection policy will prefer
   wired over wireless and WLAN over cellular.  Hence, if an application
   wants to use cellular 3G as the active interface when WLAN is
   available, the application needs to override the default connection
   mapping policy.  An application specific mapping policy can be set
   via API or provisioned by the Mobile Operator.  The application, in
   compliance with the security model, can request connection type by
   interface (WLAN, cellular), by minimum interface speed (x kbps, y
   mbps), or by name (Access Point Name).

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3.1.3.  BlackBerry

   Depending on the network configuration, Java applications in
   BlackBerry devices [BLACKBERRY] can use can use direct TCP/IP
   connectivity or different application proxys to establish connections
   over the wireless network.  For instance, some wireless service
   providers provide an Internet gateway to offer direct TCP/IP
   connectivity to the Internet while some others can provide a WAP
   gateway that allows HTTP connections to occur over the WAP (Wireless
   Application Protocol) protocol.  It is also possible to use the
   BlackBerry Enterprise Server [BLACKBERRY] as a network gateway, The
   BlackBerry Enterprise Server provides an HTTP and TCP/IP proxy
   service to allow the application to use it as a secure gateway for
   managing HTTP and TCP/IP connections to the intranet or the Internet.
   An application connecting to the Internet, can use either the
   BlackBerry Internet Service or the Internet gateway of the wireless
   server provider to manage connections.  The problem of gateway
   selection is supposed to be managed independently by each Java
   application.  For instance, an application can be designed to always
   use the default Internet gateway, while another application can be
   designed to use a preferred proxy when available.

   A BlackBerry device [BLACKBERRY] can be attached to multiple networks
   simultaneously (wireless/wired).  In this case, Multiple network
   interfaces can be associated to a single IP stack or multiple IP
   stacks.  The device, or the application, can select the network
   interface to be used in various ways.  For instance, the device can
   always map the applications to the default network interface (or the
   default access network).  When muliple IP stacks are associated to
   multiple interfaces, the application can select the source address
   correponding to the preferred network interface.  When multiple
   network interfaces are aggregated into a single IP stack, the device
   associates each application to the more appropriate network
   interface.  The selection can be based on cost, type-of-service
   and/or user preference.

3.1.4.  Google Android

   Android is based on a Linux kernel and, in many situations, behaves
   like a Linux device as described in Section 3.2.2.  As per Linux,
   Android can manage multiple routing tables and rely on policy based
   routing associated with packet filtering capabilities (see
   Section for details).  Such a framework can be used to solve
   complex routing issue brought by multiple interfaces terminals, e.g.
   address space overlapping.

   For incoming packets, Android implements the weak host model
   [RFC1122] on both IPv4 and IPv6.  However, Android can also be

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   configured to support the strong host model.

   Regarding DNS configuration, Android does not list the DNS servers in
   the file /etc/resolv.conf, used by Linux.  However, as per Linux, DNS
   configuration is node-scoped, even if DNS configuration can rely on
   the DHCP client.  For instance, the udhcp client [UDHCP], which is
   also available for Linux, can be used on Android.  Each time new
   configuration data is received by the host from a DHCP server,
   regardless of which interface it is received on, the DHCP client
   rewrites the global configuration data with the most recent
   information received.

   Actually, the main difference between Linux and Android is on the
   address selection mechanism.  Android version prior to 2.2 simply
   prefers IPv6 connectivity over IPv4.  Android 2.2 has been updated
   with [ANDROID-RFC3484], which implements some of the address
   selection rules defined in [RFC3484].  All RFC3484 rules are
   supported, except rule 3 (avoid deprecated addresses), 4 (prefer home
   addresses) and 7 (prefer native transport).  Also, rule 9 (use
   longest matching prefix) has been modified so it does not sort IPv4

   The Android reference documentation describes the package
   [ANDROID] and the ConnectivityManager class that applications can use
   to request a route to a specified destination address via a specified
   network interface (3GPP or WLAN).  Applications also ask Connection
   Manager for permission to start using a network feature.  The
   Connectivity Manager monitors changes in network connectivity and
   attempts to failover to another network if connectivity to an active
   network is lost.  When there are changes in network connectivity,
   applications are notified.  Applications are also able to ask for
   information about all network interfaces, including their
   availability, type and other information.

3.1.5.  Qualcomm Brew

   This section describes how multi-interface support is handled by
   Advanced Mobile Station Software (AMSS) that comes with Brew OS for
   all Qualcomm chipsets (e.g., MSM, Snapdragon etc).  AMSS is a low
   level connectivity platform, on top of which manufacturers can build
   to provide the necessary connectivity to applications.  The
   interaction model between AMSS, the Operating System, and the
   applications is not unique and depend on the design chosen by the
   manufacturer.  The Mobile OS can let an application invoke the AMSS
   directly (via API), or provide its own connection manager that will
   request connectivity to the AMSS based on applications needs.  The
   interaction between the OS connection manager and the applications is
   OS dependent.

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   AMSS supports a concept of netpolicy which allows each application to
   specify the type of network connectivity desired.  The netpolicy
   contains parameters such as access technology, IP version type and
   network profile.  Access technology could be a specific technology
   type such as CDMA or WLAN or could be a group of technologies, such
   as ANY_Cellular or ANY_Wireless.  IP version could be one of IPv4,
   IPv6 or Default.  The network profile identifies a type of network
   domain or service within a certain network technology, such as 3GPP
   APN or Mobile IP Home Agent.  It also specifies all the mandatory
   parameters required to connect to the domain such authentication
   credentials and other optional parameters such as QoS attributes.
   Network Profile is technology specific and set of parameters
   contained in the profile could vary for different technologies.

   Two models of network usage are supported:

   o  Applications requiring network connectivity specify an appropriate
      netpolicy in order to select the desired network.  The netpolicy
      may match one or more network interfaces.  AMSS system selection
      module selects the best interface out of the ones that match the
      netpolicy based on various criteria such as cost, speed or other
      provisioned rules.  Application explicitly starts the selected
      network interface and, as a result, the application also gets
      bound to the corresponding network interface.  All outbound
      packets from this application are always routed over this bound
      interface using the source address of the interface.

   o  Applications may rely on a separate connection manager to control
      (e.g. start/stop) the network interface.  In this model,
      applications are not necessarily bound to any one interface.  All
      outbound packets from such applications are routed on one of the
      interfaces that match its netpolicy.  The routing decision is made
      individually for each packet and selects the best interface based
      on the criteria described above and the destination address.
      Source address is always that assigned to the interface used to
      transmit the packet.

   All of the routing/interface selection decisions are based on the
   netpolicy and not just on the destination address to avoid
   overlapping private IPv4 address issue.  This also allows multiple
   interfaces to be configured with the same IP address, for example, to
   handle certain tunnelling scenarios.  Applications that do not
   specify a netpolicy are routed by AMSS to the best possible interface
   using the default netpolicy.  Default netpolicy could be pre-defined
   or provisioned by the administrator or operator.  Hence default
   interface could vary from device to device and also depends upon the
   available networks at any given time.

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   AMSS allows each interface to be configured with its own set of DNS
   configuration parameters (e.g. list of DNS servers, domain names
   etc.).  Interface selected to make a DNS resolution is the one to
   which application making the DNS query is bound.  Applications can
   also specify a different netpolicy as part of DNS request to select
   another interface for DNS resolution.  Regardless, all the DNS
   queries are sent only over this selected interface using the DNS
   configuration from the interface.  DNS resolution is first attempted
   with the primary server configured in the interface.  If a response
   is not received, the queries are sent to all the other servers
   configured in the interface in a sequential manner using a backoff

3.1.6.  Arena Connection Manager

   Arena, a mobile OS based on Linux, provides a Connection Manager,
   which is described in [I-D.zhang-mif-connection-manager-arena] and
   [I-D.yang-mif-connection-manager-impl-req].  The arena connection
   manager provides a means for applications to register their
   connectivity requirement.  The Connection Manager can then choose an
   interface that matches the application's needs while considering
   other factors such as availability, cost and stability.  Also, the
   Connection Manager can handle multi-interface issues such as
   connection sharing.

3.1.7.  Current practices for network selection in handsets

   This section describes the behavior of connection managers in
   presence of multiple points of attachment for a same interface.  The
   section focuses on WLAN interface, it is described how does the
   connection manager deal with the list of preferred SSID and how does
   it select the SSID for attachment.  Current implementation of
   connection managers are considered for the following handsets: LG
   Pathfinder, Android/HTC magic, RIM BlackBerry , iPhone (3G and 3GS).

   When the terminal is under coverage of different WLAN networks with
   different SSIDs:

      connection managers, excepted for the RIM Blackberry, construct
      the list of preferred SSID giving priority to the last SSID on
      which they have managed to attach.  The user is not allowed to
      define its preferred access.  So, if the terminal discovers and
      manages to attach to SSID1, SSID1 becomes the preferred access for
      future attachment.  If the terminal moves out of SSID1 coverage
      and attaches to a new SSID, SSID2.  SSID2 will then be the
      preferred access of the connection manager.  Then, if the terminal
      processes to WLAN attachment within both SSID1 and SSID2 coverage,
      the connection manager will select SSID2 for attachment.  The RIM

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      Blackberry behaves differently, the connection manager selects the
      first SSID on which it has managed to attach in the past.

      All connection managers behave in the same way when the terminals
      fails to attach to the selected SSID: the connection manager
      automatically selects the second SSID in the list of preferred
      SSID.  Fallback come into play at expiration of a timeout from few
      seconds to about 3 minutes.

      When the IP stack fails to obtain an IP address, the handset,
      excepted the iPhone, restarts WLAN attachment selecting the second
      SSID in the list.

   When the terminal receives signals from different point of attachment
   with same SSID:

      The connection manager selects the point of attachment with best
      signal strength; no other criteria (e.g.  MAC address) is taken
      into account.  If the handset fails to attach to the selected
      point of attachment (e.g. due to L2 authentication failure), the
      connection manager selects the point of attachment with lower
      signal strength.  However, this fallback is not supported on the
      LG pathfinder.  If no more points of attachment (corresponding to
      the preferred SSID) are available, the connection manager selects
      the second SSID in the list of preferred SSID.

      Whatever is the handset, fallback on L3 attachment failure is not
      supported if the terminal remains under coverage of the same WLAN
      access point.  Actually, the connection manager always selects the
      most powerful signal strength without considering IP configuration
      results.  In other words, if the terminal is unable to set up the
      IP connectivity on one WLAN access, the connection manager will
      not try to attach to an alternative point of attachment (or SSID)
      as long as the signal strength of the first radio link is the most
      powerful.  Situation is not the same for mobile terminal since the
      signal strength of the alternative point of attachment could
      become better while the terminal is moving.  If so, the terminal
      automatically restarts IP connectivity process (excepted the HTC
      Magic which requires the user to manually restart the L3

3.2.  Desktop Operating Systems

   Multi-interface issues also occur in desktop environments in those
   cases where a desktop host has multiple (logical or physical)
   interfaces connected to networks with different reachability
   properties, such as one interface connected to the global Internet,
   while another interface is connected to a corporate VPN.

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3.2.1.  Microsoft Windows

   The multi-interface functionality currently implemented in Microsoft
   Windows operation systems is described in more detail in
   [I-D.montenegro-mif-multihoming].  Routing

   It is possible, although not often desirable, to configure default
   routers on more than one Windows interface.  In this configuration,
   Windows will use the default route on the interface with the lowest
   routing metric (i.e. the fastest interface).  If multiple interfaces
   share the same metric, the behavior will differ based on the version
   of Windows in use.  Prior to Windows Vista, the packet would be
   routed out of the first interface that was bound to the TCP/IP stack,
   the preferred interface.  In Windows vista, host-to-router load
   sharing [RFC4311] is used for both IPv4 and IPv6.  Outbound and Inbound Addresses

   If the source address of the outgoing packet has not been determined
   by the application, Windows will choose from the addresses assigned
   to its interfaces.  Windows implements [RFC3484] for source address
   selection in IPv6 and, in Windows Vista, for IPv4.  Prior to Windows
   Vista, IPv4 simply chose the first address on the outgoing interface.

   For incoming packets, Windows will check if the destination address
   matches one of the addresses assigned to its interfaces.  Windows has
   implemented the weak host model [RFC1122] on IPv4 in Windows 2000,
   Windows XP and Windows Server 2003.  The strong host model became the
   default for IPv4 in Windows Vista and Windows server 2008, however
   the weak host model is available via per-interface configuration.
   IPv6 has always implemented the strong host model.  DNS Configuration

   Windows largely relies on suffixes to solve DNS resolution issues.
   Suffixes are used for four different purposes that are reminded

   1.  DNS Suffix Search List (aka domain search list): suffix is added
       to non-FQDN names.

   2.  Interface-specific suffix list, which allows sending different
       DNS queries to different DNS servers.

   3.  Suffix to control Dynamic DNS Updates: determine which DNS server
       will receive a dynamic update for a name with a certain suffix.

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   4.  Suffix in the Name Resolution Policy Table [NRPT] to aid in
       identifying a Namespace that requires special handling (feature
       available only after Windows 7 and its server counterpart,
       Windows Server 2008 R2).

   However, this section focuses on the interface-specific suffix list
   since it is the only suffix usage in the scope of this document.

   DNS configuration information can be host-wide or interface specific.
   Host-wide DNS configuration is input via static configuration or, in
   sites that use Active Directory, Microsoft's Group Policy.  Interface
   specific DNS configuration can be input via static configuration or
   via DHCP.

   The host-wide configuration consists of a primary DNS suffix to be
   used for the local host, as well as a list of suffix that can be
   appended to names being queried.  Before Windows Vista and Windows
   Server 2008, there was also a host-wide DNS server list that took
   precedent over per-interface DNS configuration.

   The interface-specific DNS configuration comprises an interface-
   specific suffix list and a list of DNS server IP addresses.

   Windows uses a host-wide "effective" server list for an actual query,
   where the effective server list may be different for different names.
   In the list of DNS server addresses, the first server is considered
   the "primary" server, with all other servers being secondary.

   When a DNS query is performed in Windows, the query is first sent to
   the primary DNS server on the preferred interface.  If no response is
   received in one second, the query is sent to the primary DNS servers
   on all interfaces under consideration.  If no response is received
   for 2 more seconds, the DNS server sends the query to all of the DNS
   servers on the DNS server lists for all interfaces under
   consideration.  If the host still doesn't receive a response after 4
   seconds, it will send to all of the servers again and wait 8 seconds
   for a response.

3.2.2.  Linux and BSD-based Operating Systems  Routing

   In addition to the two commonly used routing tables (the local and
   main routing tables), the kernel can support up to 252 additional
   routing tables which can be added in the file /etc/iproute2/
   rt_tables.  A routing table can contain an arbitrary number of
   routes, the selection of route is classically made according to the
   destination address of the packet.  Linux also provides more flexible

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   routing selection based on the Type of Service, scope, output
   interface.  In addition, since kernel version 2.2, Linux supports
   policy based routing using the multiple routing tables capability and
   a routing policy database.  This database contains routing rules used
   by the kernel.  Using policy based routing, the source address, the
   ToS flags, the interface name and an "fwmark" (a mark carried through
   added in the data structure representing the packet) can be used as
   route selectors.

   Policy based routing can be used in addition to Linux packet
   filtering capabilities, e.g provided by the "iptables" tool.  In a
   multiple interfaces context, this tool can be used to mark the
   packets, i.e assign a number to fwmark, in order to select the
   routing rule according to the type of traffic.  This mark can be
   assigned according to parameters like protocol, source and/or
   destination addresses, port number and so on.

   Such a routing management framework allows to deal with complex
   situation such as address space overlaping.  In this situation, the
   administrator can use packet marking and policy based routing to
   select the correct interface.  Outbound and Inbound Addresses

   By default, source address selection follows the following basics
   rules: the initial source address for an outbound packet can be
   chosen by the application using the bind() call.  Without information
   from the application, the kernel chooses the first address configured
   on the interface which belongs to the same subnet than the
   destination address or the nexthop router.

   Linux also implements [RFC3484] for source address selection for IPv6
   and dual-stack configurations.  However, the address sorting rules
   from [RFC3484] are not always adequate.  For this reason, Linux
   allows the system administrator to dynamically change the sorting.
   This can be achieved with the /etc/gai.conf file.

   For incoming packets, Linux checks if the destination address matches
   one of the addresses assigned to its interfaces then, processes the
   packet according the configured host model.  By default, Linux
   implements the weak host model [RFC1122] on both IPv4 and IPv6.
   However, Linux can also be configured to support the strong host
   model.  DNS Configuration

   Most BSD and Linux distributions rely on their DHCP client to handle
   the configuration of interface-specific information (such as an IP

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   address and netmask), and a set of system-wide configuration
   information, (such a DNS server list, an NTP server list and default
   routes).  Users of these operating systems have the choice of using
   any DHCP client available for their platform, with an operating
   system default.  This section discusses the behavior of several DHCP
   clients that may be used with Linux and BSD distributions.

   The Internet Systems Consortium (ISC) DHCP Client [ISCDHCP] and its
   derivative for OpenBSD [OPENBSDDHCLIENT] can be configured with
   specific instructions for each interface.  However, each time new
   configuration data is received by the host from a DHCP server,
   regardless of which interface it is received on, the DHCP client
   rewrites the global configuration data, such as the default routes
   and the DNS server list (in /etc/resolv.conf) with the most recent
   information received.  Therefore, the last configured interface
   always become the primary one.  The ISC DHCPv6 client behaves

   The Phystech dhcpcd client [PHYSTECHDHCPC] behaves similarly to the
   ISC client.  It replaces the DNS server list in /etc/resolv.conf and
   the default routes each time new DHCP information is received on any
   interface.  However, the -R flag can be used to instruct the client
   to not replace the DNS servers in /etc/resolv.conf.  However, this
   flag is a global flag for the DHCP server, and is therefore
   applicable to all interfaces.  When dhcpd is called with the -R flag,
   the DNS servers are never replaced.

   The pump client [PUMP] also behaves similarly to the ISC client.  It
   replaces the DNS servers in /etc/resolv.conf and the default routes
   each time new DHCP information is received on any interface.
   However, the nodns and nogateway options can be specified on a per
   interface basis, enabling the user to define which interface should
   be used to obtain the global configuration information.

   The udhcp client [UDHCP] is often used in embedded platforms based on
   busybox.  The udhcp client behaves similarly to the ISC client.  It
   rewrites default routes and the DNS server list each time new DHCP
   information is received.

   Redhat-based distributions, such as Redhat, Centos and Fedora have a
   per-interface configuration option (PEERDNS) that indicates that the
   DNS server list should not be updated based on configuration received
   on that interface.

   The most configurable DHCP clients can be set to define a primary
   interface to use only that interface for the global configuration
   data.  However, this is limited, since a mobile host might not always
   have the same set of interfaces available.  Connection managers may

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   help in this situation.

   Some distributions also have a connection manager.  However, most
   connection managers serve as a GUI to the DHCP client, therefore not
   changing the functionality described above.

4.  Acknowledgements

   Authors of the document would like to thank following people for
   their input and feedback: Dan Wing, Hui Deng, Jari Arkko, Julien
   Laganier and Steinar H. Gunderson.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   This document describes current operating system implementations and
   how they handle the issues raised in the MIF problem statement.
   While it is possible that the currently implemented mechanisms
   described in this document may affect the security of the systems
   described, this document merely reports on current practice.  It does
   not attempt to analyze the security properties (or any other
   architectural properties) of the currently implemented mechanisms.

7.  Contributors

   The following people contributed most of the per-Operating System
   information found in this document:

   o  Marc Blanchet, Viagenie

   o  Hua Chen, Leadcoretech, Ltd.

   o  Yan Zhang, Leadcoretech Ltd.

   o  Shunan Fan, Huawei Technology

   o  Jian Yang, Huawei Technology

   o  Gabriel Montenegro, Microsoft Corporation

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   o  Shyam Seshadri, Microsoft Corporation

   o  Dave Thaler, Microsoft Corporation

   o  Kevin Chin, Microsoft Corporation

   o  Teemu Savolainen, Nokia

   o  Tao Sun, China Mobile

   o  George Tsirtsis, Qualcomm.

   o  David Freyermuth, France telecom.

   o  Aurelien Collet, Altran.

   o  Giyeong Son, RIM.

8.  References

8.1.  Normative References

              Blanchet, M. and P. Seite, "Multiple Interfaces and
              Provisioning Domains Problem Statement",
              draft-ietf-mif-problem-statement-09 (work in progress),
              October 2010.

8.2.  Informative References

   [ANDROID]  Google Inc., "Android developers: package",
              2009, <

              Gunderson, S., "RFC 3484 support for Android", 2010, <http

              Research In Motion Limited, "BlackBerry Java Development
              Environment - Fundamentals Guide: Wireless gateways",
              2009, <

              Montenegro, G., Thaler, D., and S. Seshadri, "Multiple

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              Interfaces on Windows",
              draft-montenegro-mif-multihoming-00 (work in progress),
              March 2009.

              Yang, J., Sun, T., and S. Fan, "Multi-interface Connection
              Manager Implementation and Requirements",
              draft-yang-mif-connection-manager-impl-req-00 (work in
              progress), March 2009.

              Zhang, Y., Sun, T., and H. Chen, "Multi-interface Network
              Connection Manager in Arena Platform",
              draft-zhang-mif-connection-manager-arena-00 (work in
              progress), February 2009.

   [ISCDHCP]  Internet Software Consortium, "ISC DHCP", 2009,

   [NRPT]     Windows, "Name Resolution Policy Table", February 2010, <

              OpenBSD, "OpenBSD dhclient", 2009,

              Phystech, "dhcpcd", 2009,

   [PUMP]     RedHat, "PUMP", 2009, <>.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC4311]  Hinden, R. and D. Thaler, "IPv6 Host-to-Router Load
              Sharing", RFC 4311, November 2005.

   [S60]      Nokia Corporation, "S60 Platform: IP Bearer Management",
              2007, <

   [UDHCP]    Busybox, "uDHCP", 2009, <

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              Microsoft Corporation, "SDK Documentation for Windows
              Mobile-Based Smartphones: Connection Manager", 2005,

Authors' Addresses

   Margaret Wasserman
   Painless Security, LLC
   356 Abbott Street
   North Andover, MA  01845

   Phone: +1 781 405-7464

   Pierrick Seite
   France Telecom - Orange
   4, rue du clos courtel BP 91226
   Cesson-Sevigne  35512


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