Internet Engineering Task Force (IETF) M. Blanchet
Request for Comments: 6418 Viagenie
Category: Informational P. Seite
ISSN: 2070-1721 France Telecom - Orange
November 2011 Multiple Interfaces and Provisioning Domains Problem Statement
This document describes issues encountered by a node attached to
multiple provisioning domains. This node receives configuration
information from each of its provisioning domains, where some
configuration objects are global to the node and others are local to
the interface. Issues such as selecting the wrong interface to send
traffic happen when conflicting node-scoped configuration objects are
received and inappropriately used. Moreover, other issues are the
result of simultaneous attachment to multiple networks, such as
domain selection or addressing and naming space overlaps, regardless
of the provisioning mechanism. While multiple provisioning domains
are typically seen on nodes with multiple interfaces, this document
also discusses situations involving single-interface nodes.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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 ....................................................32. Terminology .....................................................43. Scope and Existing Work .........................................43.1. Interactions Below IP ......................................43.2. MIF Node Characterization ..................................53.3. Host Requirements ..........................................53.4. Mobility and Other IP Protocols ............................63.5. Address Selection ..........................................63.6. Finding and Sharing IP Addresses with Peers ................73.7. Provisioning Domain Selection ..............................73.8. Session Management .........................................83.9. Sockets API ................................................94. MIF Issues ......................................................94.1. DNS Resolution Issues ......................................94.2. Node Routing ..............................................124.3. Conflicting Policies ......................................134.4. Session Management ........................................144.5. Single Interface on Multiple Provisioning Domains .........145. Underlying Problems and Causes .................................156. Security Considerations ........................................177. Contributors ...................................................188. Acknowledgements ...............................................189. Informative References .........................................18
A multihomed node may have multiple provisioning domains (via
physical and/or virtual interfaces). For example, a node may be
simultaneously connected to a wired Ethernet LAN, an 802.11 LAN, a 3G
cell network, one or multiple VPN connections, or one or multiple
tunnels (automatic or manual). Current laptops and smartphones
typically have multiple access network interfaces and, thus, are
often connected to different provisioning domains.
A multihomed node receives configuration information from each of its
attached networks, through various mechanisms such as DHCPv4
[RFC2131], DHCPv6 [RFC3315], PPP [RFC1661], and IPv6 Router
Advertisements [RFC4861]. Some received configuration objects are
specific to an interface, such as the IP address and the link prefix.
Others are typically considered by implementations as being global to
the node, such as the routing information (e.g., default gateway),
DNS server IP addresses, and address selection policies, herein
referred to as "node-scoped".
When the received node-scoped configuration objects have different
values from each provisioning domain, such as different DNS server IP
addresses, different default gateways, or different address selection
policies, the node has to decide which one to use or how it will
Other issues are the result of simultaneous attachment to multiple
networks, such as addressing and naming space overlaps, regardless of
the provisioning mechanism.
The following sections define the multiple interfaces (MIF) node and
the scope of this work, describe related work, list issues, and then
summarize the underlying problems.
A companion document, [RFC6419], discusses some current practices of
various implementations dealing with MIF.
A group of hosts, routers, and networks operated and managed by a
single organization [RFC1136].
A set of consistent configuration information (e.g., default
router, network prefixes, DNS) and the corresponding interface.
One administrative domain may have multiple provisioning domains.
Successful attachment to the provisioning domain implies that the
terminal attaches to the corresponding interface with appropriate
Reference to IP version
When a protocol keyword such as IP, PPP, or DHCP is used in this
document without any reference to a specific IP version, then it
implies both IPv4 and IPv6. A specific IP version keyword such as
DHCPv4 or DHCPv6 is meant to be specific to that IP version.
3. Scope and Existing Work
This section describes existing related work and defines the scope of
3.1. Interactions Below IP
Some types of interfaces have link-layer characteristics that may be
used in determining how multiple provisioning domain issues will be
dealt with. For instance, link layers may have authentication and
encryption characteristics that could be used as criteria for
interface selection. However, network discovery and selection on
lower layers as defined by [RFC5113] is out of scope of this
document. Moreover, interoperability with lower-layer mechanisms
such as services defined in IEEE 802.21, which aims at facilitating
handover between heterogeneous networks [MIH], is also out of scope.
Some mechanisms (e.g., based on a virtual IP interface) allow sharing
a single IP address over multiple interfaces to networks with
disparate access technologies. From the IP-stack view on the node,
there is only a single interface and single IP address. Therefore,
this situation is out of scope of this problem statement.
Furthermore, link aggregation done under IP where a single interface
is shown to the IP stack is also out of scope.
3.2. MIF Node Characterization
A MIF node has the following characteristics:
o A MIF node is an [RFC1122] IPv4- and/or [RFC4294] IPv6-compliant
o A MIF node is configured with more than one IP address (excluding
loopback and link-local).
o A MIF node can attach to more than one provisioning domain, as
presented to the IP stack.
o The interfaces may be virtual or physical.
o Configuration objects come from one or more administrative
o The IP addresses may be from the same or different address
families, such as IPv4 and IPv6.
o Communications using these IP addresses may happen simultaneously
o Some communications using these IP addresses are possible on all
the provisioning domains, while some are only possible on a
smaller set of the provisioning domains.
o While the MIF node may forward packets between its interfaces, the
forwarding of packets is not taken into account in this definition
and is out of scope for this document.
3.3. Host Requirements
"Requirements for Internet Hosts -- Communication Layers" [RFC1122]
describes the multihomed node as if it has multiple IP addresses,
which may be associated with one or more physical interfaces
connected to the same or different networks.
Section 18.104.22.168 of [RFC1122] states that the node maintains a route
cache table where each entry contains the local IP address, the
destination IP address, Type(s) of Service (superseded by the
Differentiated Services Code Point [RFC2474]), and the next-hop
gateway IP address. The route cache entry would have data about the
properties of the path, such as the average round-trip delay measured
by a transport protocol. Nowadays, implementations are not caching
[RFC1122] defines two host models:
o The "strong" host model defines a multihomed host as a set of
logical hosts within the same physical host. In this model, a
packet must be sent on an interface that corresponds to the source
address of that packet.
o The "weak" host model describes a host that has some embedded
gateway functionality. In the weak host model, the host can send
and receive packets on any interface.
The multihomed node computes routes for outgoing datagrams
differently, depending on the model. Under the strong model, the
route is computed based on the source IP address, the destination IP
address, and the Differentiated Services Code Point. Under the weak
model, the source IP address is not used; only the destination IP
address and the Differentiated Services Code Point are used.
3.4. Mobility and Other IP Protocols
The scope of this document is only about nodes implementing [RFC1122]
for IPv4 and [RFC4294] for IPv6 without additional features or
special-purpose support for transport layers, mobility, multihoming,
or identifier-locator split mechanisms. Dealing with multiple
interfaces with such mechanisms is related but considered as a
separate problem and is under active study elsewhere in the IETF
[RFC4960] [RFC5206] [RFC5533] [RFC5648] [RFC6182].
When an application is using one interface while another interface
with better characteristics becomes available, the ongoing
application session could be transferred to the newly enabled
interface. However, in some cases, the ongoing session shall be kept
on the current interface while initiating the new session on the new
interface. The problem of interface selection is within the MIF
scope and may leverage specific node functions (Section 3.8).
However, if transfer of an IP session is required, IP mobility
mechanisms, such as [RFC6275], shall be used.
3.5. Address Selection
"Default Address Selection for Internet Protocol version 6 (IPv6)"
[RFC3484] defines algorithms for source and destination IP address
selections. Default address selection as defined in [RFC3484] is
mandatory to implement in IPv6 nodes, which also means dual-stack
nodes. A node-scoped policy table managed by the IP stack is
defined. Mechanisms to update the policy table are defined in
Issues on using default address selection were found in [RFC5220] and
[RFC5221] in the context of multiple prefixes on the same link.
3.6. Finding and Sharing IP Addresses with Peers
Interactive Connectivity Establishment (ICE) [RFC5245] is a technique
for NAT traversal for UDP-based (and TCP-based) media streams
established by the offer/answer model. The multiplicity of IP
addresses, ports, and transport mechanisms in Session Description
Protocol (SDP) offers are tested for connectivity by peer-to-peer
connectivity checks. The result is candidate IP addresses and ports
for establishing a connection with the other peer. However, ICE does
not solve issues when incompatible configuration objects are received
on different interfaces.
Some application protocols do referrals of IP addresses, port
numbers, and transport for further exchanges. For instance,
applications can provide reachability information to themselves or to
a third party. The general problem of referrals is related to the
multiple-interface problem, since, in this context, referrals must
provide consistent information depending on which provisioning domain
is used. Referrals are discussed in [REFERRAL-PS] and
3.7. Provisioning Domain Selection
In a MIF context, the node may simultaneously handle multiple domains
with disparate characteristics, especially when supporting multiple
access technologies. Selection is simple if the application is
restricted to one specific provisioning domain: the application must
start on the default provisioning domain if available; otherwise, the
application does not start. However, if the application can be run
on several provisioning domains, the selection problem can be
There is no standard method for selecting a provisioning domain, but
some recommendations exist while restricting the scope to the
interface selection problem. For example, [TS23.234] proposes a
default mechanism for the interface selection. This method uses the
following information (non-exhaustive list):
o preferences provided by the user
o policies provided by the network operator
o quality of the radio link
o network resource considerations (e.g., available Quality of
Service (QoS), IP connectivity check)
o the application QoS requirements in order to map applications to
the best interface
However, [TS23.234] is designed for a specific multiple-interfaces
use case. A generic way to handle these characteristics is yet to be
3.8. Session Management
Some implementations, especially in the mobile world, rely on a
higher-level session manager, also called a connection manager, to
deal with issues brought by simultaneous attachment to multiple
provisioning domains. Typically, the session manager may deal with
the selection of the interface, and/or the provisioning domain, on
behalf of the applications, or tackle complex issues such as how to
resolve conflicting policies (Section 4.3). As discussed in
Section 3.7, the session manager may encounter difficulties because
of multiple and diverse criteria.
Session managers usually leverage the link-layer interface to gather
information (e.g., lower-layer authentication and encryption methods;
see Section 3.1) and/or for control purposes. Such a link-layer
interface may not provide all required services to make a proper
decision (e.g., interface selection). Some OSes or terminals already
implement session managers [RFC6419], and vendor-specific platforms
sometimes provide a specific sockets API (Section 3.9) that a session
manager can use. However, the generic architecture of a session
manager and its associated API are not currently standardized, so
session manager behavior may differ between OSes and platforms.
Management of multiple interfaces sometimes relies on a virtual
interface. For instance, a virtual interface allows support of
multihoming, inter-technology handovers, and IP flow mobility in a
Proxy Mobile IPv6 network [LOGICAL-IF-SUPPORT]. This virtual
interface allows a multiple-interface node sharing a set of IP
addresses on multiple physical interfaces and can also add benefits
to multi-access scenarios such as Third Generation Partnership
Project (3GPP) Multi Access Packet Data Network (PDN) Connectivity
[TS23.402]. In most cases, the virtual interface will map several
physical network interfaces, and the session manager should control
the configuration of each one of these virtual and physical
interfaces, as well as the mapping between the virtual and
In a situation involving multiple interfaces, active application
sessions should survive path failures. Here, the session manager may
come into play but only relying on existing mechanisms to manage
multipath TCP (MPTCP) [RFC6182] or failover (Mobile IPv6 (MIP6)
[RFC6275], Shim6 [RFC5533]). A description of the interaction
between these mechanisms and the session manager is out of scope of
3.9. Sockets API
An Application Programming Interface (API) may expose objects that
user applications or session managers use for dealing with multiple
interfaces. For example, [RFC3542] defines how an application using
the advanced sockets API specifies the interface or the source IP
address through a simple bind() operation or with the IPV6_PKTINFO
Other APIs have been defined to solve issues similar to MIF. For
instance, [RFC5014] defines an API to influence the default address
selection mechanism by specifying attributes of the source addresses
it prefers. [RFC6316] gives another example, in a multihoming
context, by defining a sockets API enabling interactions between
applications and the multihoming shim layer for advanced locator
management, and access to information about failure detection and
4. MIF Issues
This section describes the various issues when using a MIF node that
has already received configuration objects from its various
provisioning domains, or when multiple interfaces are used and result
in wrong domain selection, addressing, or naming space overlaps.
They occur, for example, when:
1. one interface is on the Internet and one is on a corporate
private network. The latter may be through VPN.
2. one interface is on one access network (i.e., WiFi) and the other
one is on another access network (3G) with specific services.
4.1. DNS Resolution Issues
A MIF node (M1) has an active interface (I1) connected to a network
(N1), which has its DNS servers (S1 as primary DNS server) and
another active interface (I2) connected to a network (N2), which has
its DNS servers (S2 as primary DNS server). S1 serves some private
namespace, "private.example.com". The user or the application uses a
name "a.private.example.com", which is within the private namespace
of S1 and only resolvable by S1. Any of the following situations may
1. The M1 stack, based on its routing table, uses I2 to reach S1 to
resolve "a.private.example.com". M1 never reaches S1. The name
is not resolved.
2. M1 keeps only one set of DNS server addresses from the received
configuration objects. Let us assume that M1 keeps S2's address
as the primary DNS server. M1 sends the forward DNS query for
a.private.example.com to S2. S2 responds with an error for a
nonexistent domain (NXDOMAIN). The name is not resolved. This
issue also arises when performing a reverse DNS lookup. In the
same situation, the reverse DNS query fails.
3. M1 keeps only one set of DNS server addresses from the received
configuration objects. Let us assume that M1 keeps S2's address.
M1 sends the DNS query for a.private.example.com to S2. S2
queries its upstream DNS and gets an IP address for
a.private.example.com. However, the IP address is not the same
one that S1 would have given. Therefore, the application tries
to connect to the wrong destination node, or to the wrong
interface, which may imply security issues or result in lack of
4. S1 or S2 has been used to resolve "a.private.example.com" to an
[RFC1918] address. Both N1 and N2 are [RFC1918]-addressed
networks. If addresses overlap, traffic may be sent using the
wrong interface. This issue is not related to receiving multiple
configuration objects, but to an address overlap between
interfaces or attaching networks.
5. M1 has resolved a Fully Qualified Domain Name (FQDN) to a locally
valid IP address when connected to N1. If the node loses
connection to N1, the node may try to connect, via N2, to the
same IP address as earlier, but as the address was only locally
valid, connection setup fails. Similarly, M1 may have received
NXDOMAIN for an FQDN when connected to N1. After detachment from
N1, the node should not assume the FQDN continues to be
nonexistent on N2.
6. M1 requests a AAAA record from a DNS server on a network that
uses protocol translators and DNS64 [RFC6147]. If M1 receives a
synthesized AAAA record, it is guaranteed to be valid only on the
network from which it was learned. If M1 uses synthesized AAAA
on any other network interface, traffic may be lost, dropped, or
forwarded to the wrong network.
Some networks require the user to authenticate on a captive web
portal before providing Internet connectivity. If this redirection
is achieved by modifying the DNS reply, specific issues may occur.
Consider a MIF node (M1) with an active interface (I1) connected to a
network (N1), which has its DNS server (S1), and another active
interface (I2) connected to a network (N2), which has its DNS server
(S2). Until the user has not authenticated, S1 is configured to
respond to any A or AAAA record query with the IP address of a
captive portal, so as to redirect web browsers to an access control
portal web page. This captive portal can be reached only via I1.
When the user has authenticated to the captive portal, M1 can resolve
an FQDN when connected to N1. However, if the address is only
locally valid on N1, any of the issues described above may occur.
When the user has not authenticated, any of the following situations
1. M1 keeps only one set of DNS server addresses from the received
configuration objects and kept S2 address. M1 sends the forward
DNS query for a.example.com to S2. S2 responds with the correct
answer, R1. M1 attempts to contact R1 by way of I1. The
connection fails. Or, the connection succeeds, bypassing the
security policy on N1, possibly exposing the owner of M1 to
2. M1 keeps only one set of DNS server addresses from the received
configuration objects and kept S1 address. M1 sends the DNS
query for a.example.com to S1. S1 provides the address of its
captive portal. M1 attempts to contact this IP address using I1.
The application fails to connect, resulting in lack of service.
Or, the application succeeds in connecting but connects to the
captive portal rather than the intended destination, resulting in
lack of service (i.e., an IP connectivity check issue, as
described in Section 4.4).
4.2. Node Routing
Consider a MIF node (M1) with an active interface (I1) connected to a
network (N1) and another active interface (I2) connected to a network
(N2). The user or the application is trying to reach an IP address
(IP1). Any of the following situations may occur:
1. For IP1, M1 has one default route (R1) via network (N1). To
reach IP1, the M1 stack uses R1 and sends through I1. If IP1 is
only reachable by N2, IP1 is never reached or is not the right
2. For the IP1 address family, M1 has one default route (R1, R2) per
network (N1, N2). IP1 is reachable by both networks, but the N2
path has better characteristics, such as better round-trip time,
least cost, better bandwidth, etc. These preferences could be
defined by the user, provisioned by the network operator, or
otherwise appropriately configured. The M1 stack uses R1 and
tries to send through I1. IP1 is reached, but the service would
be better via I2.
3. For the IP1 address family, M1 has a default route (R1), a
specific X.0.0.0/8 route R1B (for example, but not restricted to
an [RFC1918] prefix) to N1, and a default route (R2) to N2. IP1
is reachable by N2 only, but the prefix (X.0.0.0/8) is used in
both networks. Because of the most specific route R1B, the M1
stack sends packets through I2, and those packets never reach the
A MIF node may have multiple routes to a destination. However, by
default, it does not have any hint concerning which interface would
be the best to use for that destination. The first-hop selection may
leverage on local routing policy, allowing some actors (e.g., network
operator or service provider) to influence the routing table, i.e.,
make a decision regarding which interface to use. For instance, a
user on such a multihomed node might want a local policy to influence
which interface will be used based on various conditions. Some
Standards Development Organizations (SDOs) have defined policy-based
routing selection mechanisms. For instance, the Access Network
Discovery and Selection Function (ANDSF) [TS23.402] provides
inter-system routing policies to terminals with both a 3GPP interface
and non-3GPP interfaces. However, the routing selection may still be
difficult, due to disjoint criteria as discussed in Section 3.8.
Moreover, information required to make the right decision may not be
available. For instance, interfaces to a lower layer may not provide
all required hints concerning the selection (e.g., information on
A node usually has a node-scoped routing table. However, a MIF node
is connected to multiple provisioning domains; if each of these
domains pushes routing policies to the node, then conflicts between
policies may happen, and the node has no easy way to merge or
On a MIF node, some source addresses are not valid if used on some
interfaces. For example, an [RFC1918] source address might be
appropriate on the VPN interface but not on the public interface of
the MIF node. If the source address is not chosen appropriately,
then packets may be filtered in the path if source address filtering
is in place ([RFC2827], [RFC3704]), and reply packets may never come
back to the source.
4.3. Conflicting Policies
The distribution of configuration policies (e.g., address selection,
routing, DNS selection) to end nodes is being discussed (e.g., ANDSF
in [TS23.402], [DHCPv6-ROUTE-OPTIONS]). If implemented in multiple
provisioning domains, such mechanisms may conflict and create issues
for the multihomed node. Considering a MIF node (M1) with an active
interface (I1) connected to a network (N1) and another active
interface (I2) connected to a network (N2), the following conflicts
1. M1 receives from both networks (N1 and N2) an update of its
default address selection policy. However, the policies are
specific to each network. The policies are merged by the M1
stack. Based on the merged policy, the chosen source address is
from N1, but packets are sent to N2. The source address is not
reachable from N2; therefore, the return packet is lost. Merging
address selection policies may have important impacts on routing.
2. A node usually has a node-scoped routing table. However, each of
the connected provisioning domains (N1 and N2) may push routing
policies to the node; conflicts between policies may then happen,
and the node has no easy way to merge or reconcile them.
3. M1 receives from one of the networks an update of its access
selection policy, e.g., via the 3GPP/ANDSF [TS23.402]. However,
the policy is in conflict with the local policy (e.g., user-
defined or default OS policy). Assuming that the network
provides a list of overloaded access networks, if the policy sent
by the network is ignored, the packet may be sent to an access
network with poor quality of communication.
4.4. Session Management
Consider that a node has selected an interface and managed to
configure it (i.e., the node obtained a valid IP address from the
network). However, Internet connectivity is not available. The
problem could be due to the following reasons:
1. The network requires a web-based authentication (e.g., the access
network is a WiFi hot spot). In this case, the user can only
access a captive portal. For instance, the network may perform
HTTP redirection or modify DNS behavior (Section 4.1) until the
user has not authenticated.
2. The IP interface is configured as active, but Layer 2 is so poor
(e.g., poor radio condition) that no Layer 3 traffic can succeed.
In this situation, the session manager should be able to perform IP
connectivity checks before selecting an interface.
Session issues may also arise when the node discovers a new
provisioning domain. Consider a MIF node (M1) with an active
interface (I1) connected to a network (N1) where an application is
running a TCP session. A new network (N2) becomes available. If N2
is selected (e.g., because of better quality of communication), M1
gets IP connectivity to N2 and updates the routing table priority.
So, if no specific route to the correspondent node is in place, and
if the node implements the weak host model [RFC1122], the TCP
connection breaks as the next hop changes. In order to continue
communicating with the correspondent node, M1 should try to reconnect
to the server via N2. In some situations, it could be preferable to
maintain current sessions on N1 while new sessions start on N2.
4.5. Single Interface on Multiple Provisioning Domains
When a node using a single interface is connected to multiple
networks, such as different default routers, similar issues to those
described above will happen. Even with a single interface, a node
may wish to connect to more than one provisioning domain: that node
may use more than one IP source address and may have more than one
default router. The node may want to access services that can only
be reached using one of the provisioning domains. In this case, it
needs to use the right outgoing source address and default gateway to
reach that service. In this situation, that node may also need to
use different DNS servers to get domain names in those different
5. Underlying Problems and Causes
This section lists the underlying problems, and their causes, that
lead to the issues discussed in the previous section. The problems
can be divided into five categories: 1) configuration, 2) DNS
resolution, 3) routing, 4) address selection, and 5) session
management and APIs. They are shown below:
1. Configuration. In a MIF context, configuration information
specific to a provisioning domain may be ignored because:
A. Configuration objects (e.g., DNS servers, NTP servers) are
node-scoped. So, the IP stack is not able to maintain the
mapping between configuration information and the
corresponding provisioning domain.
B. The same configuration objects (e.g., DNS server addresses,
NTP server addresses) received from multiple provisioning
domains may be overwritten.
C. Host implementations usually do not keep separate network
configurations (such as DNS server addresses) per
2. DNS resolution
A. Some FQDNs can be resolvable only by sending queries to the
right server (e.g., intranet services). However, a DNS query
could be sent to the wrong interface because DNS server
addresses may be node-scoped.
B. A DNS answer may be only valid on a specific provisioning
domain, but applications may not be aware of that mapping
because DNS answers may not be kept with the provisioning
from which the answer comes.
A. In the MIF context, routing information could be specific to
each interface. This could lead to routing issues because,
in current node implementations, routing tables are node-
B. Current node implementations do not take into account the
Differentiated Services Code Point or path characteristics in
the routing table.
C. Even if implementations take into account path
characteristics, the node has no way to properly merge or
reconcile the provisioning domain preferences.
D. A node attached to multiple provisioning domains could be
provided with incompatible selection policies. If the
different actors (e.g., user and network operator) are
allowed to provide their own policies, the node has no way to
properly merge or reconcile multiple selection policies.
E. The problem of first-hop selection could not be solved via
configuration (Section 3.7), and may leverage on
sophisticated and specific mechanisms (Section 3.8).
4. Address selection
A. Default address selection policies may be specific to their
corresponding provisioning domain. However, a MIF node may
not be able to manage address selection policies per
provisioning domain, because default address selection
policies are node-scoped.
B. On a MIF node, some source addresses are not valid if used on
some interfaces or even on some default routers on the same
interface. In this situation, the source address should be
taken into account in the routing table, but current node
implementations do not support such a feature.
C. Source address or address selection policies could be
specified by applications. However, there are no advanced
APIs that support such applications.
5. Session management and APIs
A. Some implementations, especially in the mobile world, have
higher-level APIs and/or session managers (aka connection
managers) to address MIF issues. These mechanisms are not
standardized and do not necessarily behave the same way
across different OSes and/or platforms in the presence of MIF
problems. This lack of consistency is an issue for the user
and operator, who could experience different session manager
behaviors, depending on the terminal.
B. Session managers usually leverage on an interface to the link
layer to gather information (e.g., lower-layer authentication
and encryption methods) and/or for control purposes.
However, such a link-layer interface may not provide all
required services (e.g., may not provide all information that
would allow a proper interface selection).
C. A MIF node can support different session managers, which may
have contradictory ways of solving MIF issues. For instance,
because of different selection algorithms, two different
session managers could select different domains in the same
context. Or, when dealing with different domain selection
policies, one session manager may give precedence to user
policy while another could favor mobile operator policy.
D. When host routing is updated and if the weak host model is
supported, ongoing TCP sessions may break if routes change
for these sessions. When TCP sessions should be bound to the
interface, the strong host model should be used.
E. When provided by different actors (e.g., user, network,
default OS), policies may conflict and, thus, need to be
reconciled at the host level. Policy conflict resolution may
impact other functions (e.g., naming, routing).
F. Even if the node has managed to configure an interface,
Internet connectivity could be unavailable. This could be
due to an access control function coming into play above
Layer 3, or because of poor Layer 2 conditions. An IP
connectivity check should be performed before selecting an
6. Security Considerations
The problems discussed in this document have security implications,
such as when packets sent on the wrong interface might be leaking
some confidential information. Configuration parameters from one
provisioning domain could cause a denial of service on another
provisioning domain (e.g., DNS issues). Moreover, the undetermined
behavior of IP stacks in the multihomed context brings additional
threats where an interface on a multihomed node might be used to
conduct attacks targeted to the networks connected by the other
interfaces. Corrupted provisioning domain selection policy may
induce a node to make decisions causing certain traffic to be
forwarded to the attacker.
Additional security concerns are raised by possible future mechanisms
that provide additional information to the node so that it can make a
more intelligent decision with regards to the issues discussed in
this document. Such future mechanisms may themselves be vulnerable
and may not be easy to protect in the general case.
This document is a joint effort with the authors of the MIF
requirements document [MIF-REQ]. This includes, in alphabetical
order: Jacni Qin, Carl Williams, and Peng Yang.
The documents written prior to the existence of the MIF working
group, and the discussions during the MIF Birds of a Feather (BOF)
meeting and around the MIF charter scope on the mailing list, brought
very good input to the problem statement. This document steals a lot
of text from these discussions and initial documents (e.g.,
[MIF-REQ], [IP-MULTIPLE-CONN], [MIF-DNS-SERVER-SELECT]). Therefore,
the authors would like to acknowledge the following people (in no
specific order), from whom some text has been taken: Jari Arkko,
Keith Moore, Sam Hartman, George Tsirtsis, Scott Brim, Ted Lemon,
Bernie Volz, Giyeong Son, Gabriel Montenegro, Julien Laganier, Teemu
Savolainen, Christian Vogt, Lars Eggert, Margaret Wasserman, Hui
Deng, Ralph Droms, Ted Hardie, Christian Huitema, Remi Denis-
Courmont, Alexandru Petrescu, Zhen Cao, Gaetan Feige, Telemaco Melia,
and Juan-Carlos Zuniga. Apologies to any contributors who have
inadvertently not been named.
9. Informative References
Matsumoto, A., Fujisaki, T., and R. Hiromi, "Solution
approaches for address-selection problems", Work
in Progress, March 2010.
Dec, W., Ed., Mrugalski, T., Sun, T., and B. Sarikaya,
"DHCPv6 Route Options", Work in Progress, September 2011.
Hui, M. and H. Deng, "Problem Statement and Requirement of
Simple IP Multi-homing of the Host", Work in Progress,
Melia, T., Ed., and S. Gundavelli, Ed., "Logical Interface
Support for multi-mode IP Hosts", Work in Progress,
Savolainen, T., Kato, J., and T. Lemon, "Improved DNS
Server Selection for Multi-Interfaced Nodes", Work
in Progress, October 2011.
[MIF-REQ] Yang, P., Seite, P., Williams, C., and J. Qin,
"Requirements on multiple Interface (MIF) of simple IP",
Work in Progress, February 2009.
[MIH] IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Part 21: Media Independent Handover Services",
IEEE LAN/MAN Std. 802.21-2008, January 2009.
Carpenter, B., Jiang, S., and Z. Cao, "Problem Statement
for Referral", Work in Progress, February 2011.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1136] Hares, S. and D. Katz, "Administrative Domains and Routing
Domains: A model for routing in the Internet", RFC 1136,
[RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC4294] Loughney, J., Ed., "IPv6 Node Requirements", RFC 4294,
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
[RFC5113] Arkko, J., Aboba, B., Korhonen, J., Ed., and F. Bari,
"Network Discovery and Selection Problem", RFC 5113,
[RFC5206] Nikander, P., Henderson, T., Ed., Vogt, C., and J. Arkko,
"End-Host Mobility and Multihoming with the Host Identity
Protocol", RFC 5206, April 2008.
[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in
Multi-Prefix Environments: Operational Issues of RFC 3484
Default Rules", RFC 5220, July 2008.
[RFC5221] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Requirements for Address Selection Mechanisms", RFC 5221,
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.
[RFC5648] Wakikawa, R., Ed., Devarapalli, V., Tsirtsis, G., Ernst,
T., and K. Nagami, "Multiple Care-of Addresses
Registration", RFC 5648, October 2009.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, March 2011.
[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, July 2011.
[RFC6316] Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, Ed.,
"Sockets Application Program Interface (API) for
Multihoming Shim", RFC 6316, July 2011.
[RFC6419] Wasserman, M. and P. Seite, "Current Practices for
Multiple-Interface Hosts", RFC 6419, November 2011.
Nordmark, E., "Shim6 Application Referral Issues", Work
in Progress, July 2005.
3GPP, "3GPP system to Wireless Local Area Network (WLAN)
interworking", TS 23.234, December 2009.
3GPP, "Architecture enhancements for non-3GPP accesses",
TS 23.402, December 2010.
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