Internet Engineering Task Force                        M. Wasserman, Ed.
Internet-Draft                                     Sandstorm Enterprises
Intended status: Informational                          October 19, 2009
Expires: April 22, 2010

             Current Practices for Multiple Interface Hosts

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   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

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   current practices in this area, and describes in detail how some
   common operating systems cope with these challenges.

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 . . . . .  8
       3.1.3.  BlackBerry . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.4.  Google Android . . . . . . . . . . . . . . . . . . . .  9
       3.1.5.  Arena Connection Manager . . . . . . . . . . . . . . .  9
     3.2.  Desktop Operating Systems  . . . . . . . . . . . . . . . . 10
       3.2.1.  Microsoft Windows  . . . . . . . . . . . . . . . . . . 10  Routing  . . . . . . . . . . . . . . . . . . . . . 10  Outbound and Inbound Addresses . . . . . . . . . . 10  DNS Configuration  . . . . . . . . . . . . . . . . 10
       3.2.2.  Linux and BSD-based Operating Systems  . . . . . . . . 11
       3.2.3.  Apple Mac OS X . . . . . . . . . . . . . . . . . . . . 12
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16

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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).

   Please see the source documentation for more details and screenshots:

3.1.2.  Microsoft Windows Mobile 2003 Second Edition

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

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   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.

3.1.3.  BlackBerry

   In BlackBerry devices [BLACKBERRY] Java applications can use one of
   two wireless gateways to proxy the connection to the Internet or to a
   corporate network.  The application can be designed to always use the
   default Internet gateway, or to use a more preferred enterprise
   gateway when available.  The intent is to hide connectivity issues
   from users.

   DISCUSS: How does the Blackberry decides when a WLAN interface, a
   cellular interface or some other physical interface is used?

3.1.4.  Google Android

   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 (Mobile or Wifi).  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.

   DISCUSS: Are applications bound to use one network type at a time, or
   can one application use multiple network features in parallel?

3.1.5.  Arena Connection Manager

   The Arena Connection Manager 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 with the Connection Manager.  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

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   issues such as connection sharing.

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.

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

   Host-wide DNS configuration is input via static configuration or, in
   sites that use Active Directory, Microsoft's Group Policy.  The host-

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   wide configuration consists of a DNS suffix to be used for the local
   host, as well as a list of domain names 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.

   Interface specific DNS configuration can be input via static
   configuration or via DHCP.  It includes:

   o  An interface-specific suffix list.

   o  A list of DNS server IP addresses.

   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

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

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   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
   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 .  TODO: Verify all
   connection managers.

   TODO: DHCPv6 clients

3.2.3.  Apple Mac OS X

   This section is based on testing Mac OS X (version 10.5.6).

   When using multiple interfaces on Mac OS X, global configuration data
   such as default routes and the DNS server list are taken from the
   DHCP data received on the primary interface.  Therefore, the order in
   which the interfaces receive their configuration data is not
   relevant.  For example, if the primary interface receives its

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   configuration data first, then the second interface receives its
   configuration data, the interface-specific information for the second
   interface will be configured, but the global configuration
   information such as the DNS server list and default routes is not

4.  Acknowledgements

   Authors of the document would like to thank following people for
   their input and feedback: Hui Deng, Jari Arkko.

   This document was prepared using xml2rfc template and related web-

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.  Change Log

   The following changes were made between
   draft-mrw-mif-current-practices and

   o  Updated operating system info based on new information supplied.

   o  Renamed "Current Solutions" to "Summary of Current Solutions" and
      moved above OS specifics.  Modified this section substantially to
      match problems in problem statement.

   o  Removed "Current Problems" section, as it duplicated information
      in the problem statement.

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   o  Broke OS specifics into separate sections for mobile handsets and
      desktop operating systems.

   The following changes were made between versions -00 and -01:

   o  Number of authors passed five, so moved to Contributors section.

   o  Added information on Microsoft Windows.

   o  Added information on Arena Connection Manager.

8.  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  Shunan Fan, Huawei Technology

   o  Gabriel Montenegro, Microsoft Corporation

   o  Teemu Savolainen, Nokia

   o  Shyam Seshadri, Microsoft Corporation

   o  Tao Sun, China Mobile

   o  Dave Thaler, Microsoft Corporation

   o  Jian Yang, Huawei Technology

   o  Yan Zhang, Leadcoretech Ltd.

9.  References

9.1.  Normative References

              Blanchet, M. and P. Seite, "Multiple Interfaces Problem
              Statement", draft-ietf-mif-problem-statement-00 (work in
              progress), October 2009.

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9.2.  Informative References

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

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

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

              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.

Wasserman                Expires April 22, 2010                [Page 15]

Internet-Draft            MIF Current Practices             October 2009

   [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, <

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

Author's Address

   Margaret Wasserman (editor)
   Sandstorm Enterprises
   14 Summer Street
   Malden, MA  02148

   Phone: +1 781 333 3200

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