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Errata Exist
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

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Scope and Existing Work .........................................4
      3.1. Interactions Below IP ......................................4
      3.2. MIF Node Characterization ..................................5
      3.3. Host Requirements ..........................................5
      3.4. Mobility and Other IP Protocols ............................6
      3.5. Address Selection ..........................................6
      3.6. Finding and Sharing IP Addresses with Peers ................7
      3.7. Provisioning Domain Selection ..............................7
      3.8. Session Management .........................................8
      3.9. Sockets API ................................................9
   4. MIF Issues ......................................................9
      4.1. DNS Resolution Issues ......................................9
      4.2. Node Routing ..............................................12
      4.3. Conflicting Policies ......................................13
      4.4. Session Management ........................................14
      4.5. Single Interface on Multiple Provisioning Domains .........14
   5. Underlying Problems and Causes .................................15
   6. Security Considerations ........................................17
   7. Contributors ...................................................18
   8. Acknowledgements ...............................................18
   9. Informative References .........................................18

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

1.  Introduction

   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
   merge them.

   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.

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2.  Terminology

   Administrative domain

      A group of hosts, routers, and networks operated and managed by a
      single organization [RFC1136].

   Provisioning domain

      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
      configuration information.

   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
   the problem.

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.

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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
      and independently.

   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 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
   this information.

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   [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

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   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

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   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

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   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
   this document.

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
   socket option.

   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
   path exploration.

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

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   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.

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   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
   may occur:

   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).

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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
   interface quality).

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   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
   reconcile them.

   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
   may occur:

   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.

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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
   provisioning domains.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

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
           provisioning domain.

   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.

   3.  Routing

       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.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

       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.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

       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.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

   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.

7.  Contributors

   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.

8.  Acknowledgements

   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.,
   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,
              March 2009.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

              Melia, T., Ed., and S. Gundavelli, Ed., "Logical Interface
              Support for multi-mode IP Hosts", Work in Progress,
              October 2011.

              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,
              December 1989.

   [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,
              December 1998.

   [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.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

   [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,
              April 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [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,
              September 2007.

   [RFC5113]  Arkko, J., Aboba, B., Korhonen, J., Ed., and F. Bari,
              "Network Discovery and Selection Problem", RFC 5113,
              January 2008.

   [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,
              July 2008.

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RFC 6418          Multiple Interfaces Problem Statement    November 2011

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [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,
              April 2011.

   [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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RFC 6418          Multiple Interfaces Problem Statement    November 2011

Authors' Addresses

   Marc Blanchet
   2875 boul. Laurier, suite D2-630
   Quebec, QC  G1V 2M2

   EMail: Marc.Blanchet@viagenie.ca
   URI:   http://viagenie.ca

   Pierrick Seite
   France Telecom - Orange
   4, rue du Clos Courtel, BP 91226
   Cesson-Sevigne  35512

   EMail: pierrick.seite@orange.com

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