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Internet Engineering Task Force (IETF)                       M. BlanchetRequest for Comments: 6418                                      ViagenieCategory: Informational                                         P. SeiteISSN: 2070-1721                                  France Telecom - Orange                                                           November 2011Multiple Interfaces and Provisioning Domains Problem StatementAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6418.Blanchet & Seite              Informational                     [Page 1]

RFC 6418          Multiple Interfaces Problem Statement    November 2011Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. 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 .........................................18Blanchet & Seite              Informational                     [Page 2]

RFC 6418          Multiple Interfaces Problem Statement    November 20111.  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.Blanchet & Seite              Informational                     [Page 3]

RFC 6418          Multiple Interfaces Problem Statement    November 20112.  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.Blanchet & Seite              Informational                     [Page 4]

RFC 6418          Multiple Interfaces Problem Statement    November 20113.2.  MIF Node Characterization   A MIF node has the following characteristics:   o  A MIF node is an [RFC1122] IPv4- and/or [RFC4294] IPv6-compliant      node.   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      domains.   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 3.3.1.3 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.Blanchet & Seite              Informational                     [Page 5]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   [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   [ADDR-SELECT-SOL].Blanchet & Seite              Informational                     [Page 6]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   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   [SHIM6-APP-REFER].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   difficult.   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 linkBlanchet & Seite              Informational                     [Page 7]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   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   defined.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 inSection 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;   seeSection 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   sub-interfaces.Blanchet & Seite              Informational                     [Page 8]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   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 privateBlanchet & Seite              Informational                     [Page 9]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   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   occur:   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       service.   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.Blanchet & Seite              Informational                    [Page 10]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   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       prosecution.   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 inSection 4.4).Blanchet & Seite              Informational                    [Page 11]

RFC 6418          Multiple Interfaces Problem Statement    November 20114.2.  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       target.   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       target.   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 inSection 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).Blanchet & Seite              Informational                    [Page 12]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   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.Blanchet & Seite              Informational                    [Page 13]

RFC 6418          Multiple Interfaces Problem Statement    November 20114.4.  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.Blanchet & Seite              Informational                    [Page 14]

RFC 6418          Multiple Interfaces Problem Statement    November 20115.  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-           scoped.       B.  Current node implementations do not take into account the           Differentiated Services Code Point or path characteristics in           the routing table.Blanchet & Seite              Informational                    [Page 15]

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.Blanchet & Seite              Informational                    [Page 16]

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           interface.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.Blanchet & Seite              Informational                    [Page 17]

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.,   [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   [ADDR-SELECT-SOL]              Matsumoto, A., Fujisaki, T., and R. Hiromi, "Solution              approaches for address-selection problems", Work              in Progress, March 2010.   [DHCPv6-ROUTE-OPTIONS]              Dec, W., Ed., Mrugalski, T., Sun, T., and B. Sarikaya,              "DHCPv6 Route Options", Work in Progress, September 2011.   [IP-MULTIPLE-CONN]              Hui, M. and H. Deng, "Problem Statement and Requirement of              Simple IP Multi-homing of the Host", Work in Progress,              March 2009.Blanchet & Seite              Informational                    [Page 18]

RFC 6418          Multiple Interfaces Problem Statement    November 2011   [LOGICAL-IF-SUPPORT]              Melia, T., Ed., and S. Gundavelli, Ed., "Logical Interface              Support for multi-mode IP Hosts", Work in Progress,              October 2011.   [MIF-DNS-SERVER-SELECT]              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.   [REFERRAL-PS]              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.Blanchet & Seite              Informational                    [Page 19]

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 ofRFC 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.Blanchet & Seite              Informational                    [Page 20]

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.   [SHIM6-APP-REFER]              Nordmark, E.,"Shim6 Application Referral Issues", Work              in Progress, July 2005.   [TS23.234]              3GPP, "3GPP system to Wireless Local Area Network (WLAN)              interworking", TS 23.234, December 2009.   [TS23.402]              3GPP, "Architecture enhancements for non-3GPP accesses",              TS 23.402, December 2010.Blanchet & Seite              Informational                    [Page 21]

RFC 6418          Multiple Interfaces Problem Statement    November 2011Authors' Addresses   Marc Blanchet   Viagenie   2875 boul. Laurier, suite D2-630   Quebec, QC  G1V 2M2   Canada   EMail: Marc.Blanchet@viagenie.ca   URI:http://viagenie.ca   Pierrick Seite   France Telecom - Orange   4, rue du Clos Courtel, BP 91226   Cesson-Sevigne  35512   France   EMail: pierrick.seite@orange.comBlanchet & Seite              Informational                    [Page 22]