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
draft-ietf-mif-current-practices-00
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Abstract
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
3.2.1.1. Routing . . . . . . . . . . . . . . . . . . . . . 10
3.2.1.2. Outbound and Inbound Addresses . . . . . . . . . . 10
3.2.1.3. 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
systems.
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
metric.
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"
interface.
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
another.
Web server MMS Gateway
| |
-+--Internet---- ----Operator network--+-
| |
+-------+ +-------+
|WLAN AP| | GGSN |
+-------+ +-------+
| +--------+ |
+--------|Cellular|--------+
|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
directly.
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:
[S60].
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
making.
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
clears.
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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 android.net 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].
3.2.1.1. 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.
3.2.1.2. 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.
3.2.1.3. 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
similarly.
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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
overwritten.
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-
tool.
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
draft-ietf-mif-current-practices-00.txt.
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
[I-D.ietf-mif-problem-statement]
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 android.net",
2009, <http://developer.android.com/reference/android/net/
ConnectivityManager.html>.
[BLACKBERRY]
Research In Motion Limited, "BlackBerry Java Development
Environment - Fundamentals Guide: Wireless gateways",
2009, <http://na.blackberry.com/eng/deliverables/5827/
Wireless_gateways_447132_11.jsp>.
[I-D.montenegro-mif-multihoming]
Montenegro, G., Thaler, D., and S. Seshadri, "Multiple
Interfaces on Windows",
draft-montenegro-mif-multihoming-00 (work in progress),
March 2009.
[I-D.yang-mif-connection-manager-impl-req]
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.
[I-D.zhang-mif-connection-manager-arena]
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,
<http://www.isc.org/software/dhcp>.
[OPENBSDDHCLIENT]
OpenBSD, "OpenBSD dhclient", 2009,
<http://www.openbsd.org/>.
[PHYSTECHDHCPC]
Phystech, "dhcpcd", 2009,
<http://www.phystech.com/download/dhcpcd.html>.
[PUMP] RedHat, "PUMP", 2009, <http://redhat.com>.
[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.
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[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, <http://www.forum.nokia.com/info/sw.nokia.com/id/
190358c8-7cb1-4be3-9321-f9d6788ecae5/
S60_Platform_IP_Bearer_Management_v1_0_en.pdf.html>.
[UDHCP] Busybox, "uDHCP", 2009, <http://sources.busybox.net/
index.py/trunk/busybox/networking/udhcp/>.
[WINDOWSMOBILE]
Microsoft Corporation, "SDK Documentation for Windows
Mobile-Based Smartphones: Connection Manager", 2005,
<http://msdn.microsoft.com/en-us/library/aa457829.aspx>.
Author's Address
Margaret Wasserman (editor)
Sandstorm Enterprises
14 Summer Street
Malden, MA 02148
USA
Phone: +1 781 333 3200
Email: mrw@lilacglade.org
URI: http://www.sandstorm.net
Wasserman Expires April 22, 2010 [Page 16]
