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Internet Engineering Task Force (IETF)
Request for Comments: 7556
Category: Informational
ISSN: 2070-1721
D. Anipko, Ed.
Unaffiliated
June 2015

Multiple Provisioning Domain Architecture

Abstract

This document is a product of the work of the Multiple Interfaces Architecture Design team. It outlines a solution framework for some of the issues experienced by nodes that can be attached to multiple networks simultaneously. The framework defines the concept of a Provisioning Domain (PvD), which is a consistent set of network configuration information. PvD-aware nodes learn PvD-specific information from the networks they are attached to and/or other sources. PvDs are used to enable separation and configuration consistency in the presence of multiple concurrent connections.

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 http://www.rfc-editor.org/info/rfc7556.

Copyright Notice

Copyright © 2015 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Definitions and Types of PvDs . . . . . . . . . . . . . . . .   5
     2.1.  Explicit PvDs . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Implicit PvDs and Incremental Adoption of Explicit PvDs .   6
     2.3.  Relationship between PvDs and Interfaces  . . . . . . . .   7
     2.4.  PvD Identity/Naming . . . . . . . . . . . . . . . . . . .   8
     2.5.  The Relationship to Dual-Stack Networks . . . . . . . . .   8
   3.  Conveying PvD Information . . . . . . . . . . . . . . . . . .   9
     3.1.  Separate Messages or One Message? . . . . . . . . . . . .   9
     3.2.  Securing PvD Information  . . . . . . . . . . . . . . . .  10
     3.3.  Backward Compatibility  . . . . . . . . . . . . . . . . .  10
     3.4.  Retracting/Updating PvD Information . . . . . . . . . . .  10
     3.5.  Conveying Configuration Information Using IKEv2 . . . . .  10
   4.  Example Network Configurations  . . . . . . . . . . . . . . .  11
     4.1.  A Mobile Node . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  A Node with a VPN Connection  . . . . . . . . . . . . . .  12
     4.3.  A Home Network and a Network Operator with Multiple PvDs   12
   5.  Reference Model for the PvD-Aware Node  . . . . . . . . . . .  13
     5.1.  Constructions and Maintenance of Separate PvDs  . . . . .  13
     5.2.  Consistent Use of PvDs for Network Connections  . . . . .  14
       5.2.1.  Name Resolution . . . . . . . . . . . . . . . . . . .  14
       5.2.2.  Next-Hop and Source Address Selection . . . . . . . .  15
       5.2.3.  Listening Applications  . . . . . . . . . . . . . . .  16
         5.2.3.1.  Processing of Incoming Traffic  . . . . . . . . .  16
       5.2.4.  Enforcement of Security Policies  . . . . . . . . . .  17
     5.3.  Connectivity Tests  . . . . . . . . . . . . . . . . . . .  18
     5.4.  Relationship to Interface Management and Connection
           Managers  . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  PvD Support in APIs . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Basic . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  Intermediate  . . . . . . . . . . . . . . . . . . . . . .  19
     6.3.  Advanced  . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  PvD Trust for PvD-Aware Node  . . . . . . . . . . . . . . . .  20
     7.1.  Untrusted PvDs  . . . . . . . . . . . . . . . . . . . . .  20
     7.2.  Trusted PvDs  . . . . . . . . . . . . . . . . . . . . . .  20
       7.2.1.  Authenticated PvDs  . . . . . . . . . . . . . . . . .  21
       7.2.2.  PvDs Trusted by Attachment  . . . . . . . . . . . . .  21
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  23
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

1. Introduction

Nodes attached to multiple networks may encounter problems from conflicting configuration between the networks or attempts to simultaneously use more than one network. While various techniques are currently used to tackle these problems [RFC6419], in many cases issues may still appear. The Multiple Interfaces Problem Statement document [RFC6418] describes the general landscape and discusses many of the specific issues and scenario details.

Problems, enumerated in [RFC6418], can be grouped into 3 categories:

  1. Lack of consistent and distinctive management of configuration elements associated with different networks.
  1. Inappropriate mixed use of configuration elements associated with different networks during a particular network activity or connection.
  1. Use of a particular network that is not consistent with the intended use of the network, or the intent of the communicating parties, leading to connectivity failure and/or other undesired consequences.

An example of (1) is a single, node-scoped list of DNS server IP addresses learned from different networks leading to failures or delays in resolution of names from particular namespaces; an example of (2) is an attempt to resolve the name of an HTTP proxy server learned from network A using a DNS server learned from network B; and an example of (3) is the use of an employer-provided VPN connection for peer-to-peer connectivity unrelated to employment activities.

This architecture provides solutions to these categories of problems, respectively, by:

  1. Introducing the formal notion of PvDs, including identity for PvDs, and describing mechanisms for nodes to learn the intended associations between acquired network configuration information elements.
  1. Introducing a reference model for PvD-aware nodes that prevents the inadvertent mixed use of configuration information that may belong to different PvDs.
  1. Providing recommendations on PvD selection based on PvD identity and connectivity tests for common scenarios.

2. Definitions and Types of PvDs

Provisioning Domain:

A consistent set of network configuration information. Classically, all of the configuration information available on a single interface is provided by a single source (such as a network administrator) and can therefore be treated as a single provisioning domain. In modern IPv6 networks, multihoming can result in more than one provisioning domain being present on a single link. In some scenarios, it is also possible for elements of the same PvD to be present on multiple links.

Typical examples of information in a provisioning domain learned from the network are:

  • Source address prefixes for use by connections within the provisioning domain
      *  IP address(es) of the DNS server(s)
  • Name of the HTTP proxy server (if available)
  • DNS suffixes associated with the network
  • Default gateway address

PvD-aware node:

A node that supports the association of network configuration information into PvDs and the use of these PvDs to serve requests for network connections in ways consistent with the recommendations of this architecture.

PvD-aware application:

An application that contains code and/or application-specific configuration information explicitly aware of the notion of PvD and/or specific types of PvD elements or properties.

2.1. Explicit PvDs

A node may receive explicit information from the network and/or other sources conveying the presence of PvDs and the association of particular network information with a particular PvD. PvDs that are constructed based on such information are referred to as "explicit" in this document.

Protocol changes or extensions will likely be required to support explicit PvDs through IETF-defined mechanisms. As an example, one could think of one or more DHCP options carrying PvD identity and/or its elements.

A different approach could be the introduction of a DHCP option that only carries the identity of a PvD. Here, the associations between network information elements with the identity is implemented by the respective protocols, for example, with a Router Discovery [RFC4861] option associating an address range with a PvD. Additional discussion can be found in Section 3.

Other examples of a delivery mechanism for PvDs are key exchange or tunneling protocols, such as the Internet Key Exchange Protocol version 2 (IKEv2) [RFC7296] that allows the transport of host configuration information.

Specific, existing, or new features of networking protocols that enable the delivery of PvD identity and association with various network information elements will be defined in companion design documents.

Link-specific and/or vendor-proprietary mechanisms for the discovery of PvD information (differing from IETF-defined mechanisms) can be used by nodes either separate from or in conjunction with IETF- defined mechanisms, providing they allow the discovery of the necessary elements of the PvD(s).

In all cases, nodes must by default ensure that the lifetime of all dynamically discovered PvD configuration is appropriately limited by relevant events. For example, if an interface media state change is indicated, previously discovered information relevant to that interface may no longer be valid and thus needs to be confirmed or re-discovered.

It is expected that the way a node makes use of PvD information is generally independent of the specific mechanism/protocol that the information was received by.

In some network topologies, network infrastructure elements may need to advertise multiple PvDs. Generally, the details of how this is performed will be defined in companion design documents.

2.2. Implicit PvDs and Incremental Adoption of Explicit PvDs

For the foreseeable future, there will be networks that do not advertise explicit PvD information, because deployment of new features in networking protocols is a relatively slow process.

When connected to networks that don't advertise explicit PvD information, a PvD-aware node shall automatically create separate PvDs for received configuration. Such PvDs are referred to in this document as "implicit".

Through the use of implicit PvDs, PvD-aware nodes may still provide benefits to their users (when compared to non-PvD-aware nodes) by following the best practices described in Section 5.

PvD-aware nodes shall treat network information from different interfaces, which is not identified as belonging explicitly to some PvD, as belonging to separate PvDs, one per interface.

Implicit PvDs can also occur in a mixed mode, i.e., where of multiple networks that are available on an attached link, only some advertise PvD information. In this case, the PvD-aware node shall create explicit PvDs from information explicitly labeled as belonging to PvDs. It shall associate configuration information not labeled with an explicit PvD with an implicit PvD(s) created for that interface.

2.3. Relationship between PvDs and Interfaces

By default, implicit PvDs are limited to the network configuration information received on a single interface, and by default, one such PvD is formed for each interface. If additional information is available to the host (through mechanisms out of scope of this document), the host may form implicit PvDs with different granularity. For example, PvDs spanning multiple interfaces such as a home network with a router that has multiple internal interfaces or multiple PvDs on a single interface such as a network that has multiple uplink connections.

In the simplest case, explicit PvDs will be scoped for configuration related only to a specific interface. However, there is no requirement in this architecture for such a limitation. Explicit PvDs may include information related to more than one interface if the node learns the presence of the same PvD on those interfaces and the authentication of the PvD ID meets the level required by the node policy (authentication of a PvD ID may be also required in scenarios involving only one connected interface and/or PvD; for additional discussion of PvD Trust, see Section 7).

This architecture supports such scenarios. Hence, no hierarchical relationship exists between interfaces and PvDs: it is possible for multiple PvDs to be simultaneously accessible over one interface, as well as a single PvD to be simultaneously accessible over multiple interfaces.

2.4. PvD Identity/Naming

For explicit PvDs, the PvD ID is a value that is or has a high probability of being globally unique and is received as part of PvD information. It shall be possible to generate a human-readable form of the PvD ID to present to the end user, either based on the PvD ID itself or using metadata associated with the ID. For implicit PvDs, the node assigns a locally generated ID with a high probability of being globally unique to each implicit PvD.

We say that a PvD ID should be, or should have a high probability of being, globally unique. The purpose of this is to make it unlikely that any individual node will ever accidentally see the same PvD name twice if it is not actually referring to the same PvD. Protection against deliberate attacks involving name clashes requires that the name be authenticated (see Section 7.2.1).

A PvD-aware node may use these IDs to select a PvD with a matching ID for special-purpose connection requests in accordance with node policy, as chosen by advanced applications, or to present a human- readable representation of the IDs to the end user for selection of PvDs.

A single network provider may operate multiple networks, including networks at different locations. In such cases, the provider may chose whether to advertise single or multiple PvD identities at all or some of those networks as it suits their business needs. This architecture does not impose any specific requirements in this regard.

When multiple nodes are connected to the same link with one or more explicit PvDs available, this architecture assumes that the information about all available PvDs is made available by the networks to all the connected nodes. At the same time, connected nodes may have different heuristics, policies, and/or other settings, including their configured sets of trusted PvDs. This may lead to different PvDs actually being used by different nodes for their connections.

Possible extensions whereby networks advertise different sets of PvDs to different connected nodes are out of scope of this document.

2.5. The Relationship to Dual-Stack Networks

When applied to dual-stack networks, the PvD definition allows for multiple PvDs to be created whereby each PvD contains information relevant to only one address family, or for a single PvD containing information for multiple address families. This architecture requires that accompanying design documents describing PvD-related protocol changes must support PvDs containing information from multiple address families. PvD-aware nodes must be capable of creating and using both single-family and multi-family PvDs.

For explicit PvDs, the choice of either of these approaches is a policy decision for the network administrator and/or the node user/ administrator. Since some of the IP configuration information that can be learned from the network can be applicable to multiple address families (for instance, DHCPv6 Address Selection Policy Option [RFC7078]), it is likely that dual-stack networks will deploy single PvDs for both address families.

By default for implicit PvDs, PvD-aware nodes shall include multiple IP families into a single implicit PvD created for an interface. At the time of writing, in dual-stack networks it appears to be common practice for the configuration of both address families to be provided by a single source.

A PvD-aware node that provides an API to use, enumerate, and inspect PvDs and/or their properties shall provide the ability to filter PvDs and/or their properties by address family.

3. Conveying PvD Information

DHCPv6 and Router Advertisements (RAs) are the two most common methods of configuring hosts. To support the architecture described in this document, these protocols would need to be extended to convey explicit PvD information. The following sections describe topics that must be considered before finalizing a mechanism to augment DHCPv6 and RAs with PvD information.

3.1. Separate Messages or One Message?

When information related to several PvDs is available from the same configuration source, there are two possible ways of distributing this information: One way is to send information from each different provisioning domain in separate messages. The second method is combining the information from multiple PvDs into a single message. The latter method has the advantage of being more efficient but could have problems with authentication and authorization, as well as potential issues with accommodating information not tagged with any PvD information.

3.2. Securing PvD Information

DHCPv6 [RFC3315] and RAs [RFC3971] both provide some form of authentication to ensure the identity of the source as well as the integrity of the secured message content. While this is useful, determining authenticity does not tell a node whether the configuration source is actually allowed to provide information from a given PvD. To resolve this, there must be a mechanism for the PvD owner to attach some form of authorization token or signature to the configuration information that is delivered.

3.3. Backward Compatibility

The extensions to RAs and DHCPv6 should be defined in such a manner that unmodified hosts (i.e., hosts not aware of PvDs) will continue to function as well as they did prior to PvD information being added. This could imply that some information may need to be duplicated in order to be conveyed to legacy hosts. Similarly, PvD-aware hosts need to be able to correctly utilize legacy configuration sources that do not provide PvD information. There are also several initiatives that are aimed at adding some form of additional information to prefixes [DHCPv6-CLASS-BASED-PREFIX] [IPv6-PREFIX-PROPERTIES], and any new mechanism should try to consider coexistence with such deployed mechanisms.

3.4. Retracting/Updating PvD Information

After PvD information is provisioned to a host, it may become outdated or superseded by updated information before the hosts would normally request updates. To resolve this requires that the mechanism be able to update and/or withdraw all (or some subset) of the information related to a given PvD. For efficiency reasons, there should be a way to specify that all information from the PvD needs to be reconfigured instead of individually updating each item associated with the PvD.

3.5. Conveying Configuration Information Using IKEv2

IKEv2 [RFC7296] [RFC5739] is another widely used method of configuring host IP information. For IKEv2, the provisioning domain could be implicitly learned from the Identification - Responder (IDr) payloads that the IKEv2 initiator and responder inject during their IKEv2 exchange. The IP configuration may depend on the named IDr. Another possibility could be adding a specific provisioning domain identifying payload extensions to IKEv2. All of the considerations for DHCPv6 and the RAs listed above potentially apply to IKEv2 as well.

4. Example Network Configurations

4.1. A Mobile Node

Consider a mobile node with two network interfaces: one to the mobile network, the other to the Wi-Fi network. When the mobile node is only connected to the mobile network, it will typically have one PvD, implicit or explicit. When the mobile node discovers and connects to a Wi-Fi network, it will have zero or more (typically one) additional PvD(s).

Some existing OS implementations only allow one active network connection. In this case, only the PvD(s) associated with the active interface can be used at any given time.

As an example, the mobile network can explicitly deliver PvD information through the Packet Data Protocol (PDP) context activation process. Then, the PvD-aware mobile node will treat the mobile network as an explicit PvD. Conversely, the legacy Wi-Fi network may not explicitly communicate PvD information to the mobile node. The PvD-aware mobile node will associate network configuration for the Wi-Fi network with an implicit PvD in this case.

The following diagram illustrates the use of different PvDs in this scenario: 

                 <----------- Wi-Fi 'Internet' PvD -------->
        +---------+
        | +-----+ |    +-----+         _   __               _  _
        | |Wi-Fi| |    |     |        ( `    )             ( `   )_
        | |-IF  + |----+     |---------------------------(         `)
        | |     | |    |Wi-Fi|      (         )         (  Internet  )
        | +-----+ |    | AP  |     (           )        (            )
        |         |    |     |    (   Service    )      (            )
        |         |    +-----+    (  Provider's   )     (            )
        |         |               (   Networks    -     (            )
        | +----+  |                `_            )      (            )
        | |CELL|  |                 (          )        (            )
        | |-IF +--|-------------------------------------(            )
        | |    |  |                 (_     __)          (_          _)
        | +----+  |                  `- --               `- __  _) -
        +---------+
                 <------- Mobile 'Internet' PvD ----------->

Figure 1: An Example of PvD Use with Wi-Fi and Mobile Interfaces

4.2. A Node with a VPN Connection

If the node has established a VPN connection, zero or more (typically one) additional PvD(s) will be created. These may be implicit or explicit. The routing to IP addresses reachable within this PvD will be set up via the VPN connection, and the routing of packets to addresses outside the scope of this PvD will remain unaffected. If a node already has N connected PvDs, after the VPN session has been established typically there will be N+1 connected PvDs.

The following diagram illustrates the use of different PvDs in this scenario: 

             <----------- 'Internet' PvD ------>
    +--------+
    | +----+ |    +----+         _   __        _  _
    | |Phy | |    |    |        ( `    )      ( `   )_
    | |-IF +-|----+    |--------------------(         `)
    | |    | |    |    |      (         )  (_ Internet  _)
    | +----+ |    |    |     (           )   `- __  _) -
    |        |    |Home|    (   Service    )      ||
    |        |    |Gate|    (  Provider's   )     ||
    |        |    |-way|    (   Network     -     ||
    | +----+ |    |    |    `_            )  +---------+  +------------+
    | |VPN | |    |    |      (          )   |   VPN   |  |            |
    | |-IF +-|----+    |---------------------+ Gateway |--+  Private   |
    | |    | |    |    |       (_     __)    |         |  |  Services  |
    | +----+ |    +----+         `- --       +---------+  +------------+
    +--------+
             <-------------- Explicit 'VPN' PvD ----->

Figure 2: An Example of PvD Use with VPN

4.3. A Home Network and a Network Operator with Multiple PvDs

An operator may use separate PvDs for individual services that they offer to their customers. These may be used so that services can be designed and provisioned to be completely independent of each other, allowing for complete flexibility in combinations of services that are offered to customers.

From the perspective of the home network and the node, this model is functionally very similar to being multihomed to multiple upstream operators: Each of the different services offered by the service provider is its own PvD with associated PvD information. In this case, the operator may provide a generic/default PvD (explicit or implicit), which provides Internet access to the customer. Additional services would then be provisioned as explicit PvDs for subscribing customers.

The following diagram illustrates this, using video-on-demand as a service-specific PvD: 

                <------ Implicit 'Internet' PvD ------>
           +----+     +-----+        _   __              _  _
           |    |     |     |       ( `    )            ( `   )_
           | PC +-----+     |-------------------------(         `)
           |    |     |     |     (         )        (_ Internet  _)
           +----+     |     |    (           )         `- __  _) -
                      |Home |   (   Service    )
                      |Gate-|   (  Provider's   )
                      |way  |   (   Network     -
           +-----+    |     |   `_            )        +-----------+
           | Set-|    |     |     (          )         |ISP Video- |
           | Top +----+     |--------------------------+on-Demand  |
           | Box |    |     |      (_     __)          | Service   |
           +-----+    +-----+        `- --             +-----------+
                 <-- Explicit 'Video-on-Demand' PvD -->

Figure 3: An Example of PvD Use with Wi-Fi and Mobile Interfaces

In this case, the number of PvDs that a single operator could provision is based on the number of independently provisioned services that they offer. Some examples may include:

  • Real-time packet voice
  • Streaming video
  • Interactive video (n-way video conferencing)
  • Interactive gaming
  • Best effort / Internet access

5. Reference Model for the PvD-Aware Node

5.1. Constructions and Maintenance of Separate PvDs

It is assumed that normally, the configuration information contained in a single PvD shall be sufficient for a node to fulfill a network connection request by an application, and hence there should be no need to attempt to merge information across different PvDs.

Nevertheless, even when a PvD lacks some necessary configuration information, merging of information associated with a different PvD(s) shall not be done automatically as this will typically lead to the issues described in [RFC6418].

A node may use other sources, for example: node local policy, user input, or other mechanisms not defined by the IETF for any of the following:

  • Construction of a PvD in its entirety (analogous to statically configuring IP on an interface)
  • Supplementing some or all learned PvDs with particular configuration elements
  • Merging of information from different PvDs (if this is explicitly allowed by policy)

As an example, a node administrator could configure the node to use a specific DNS resolver on a particular interface, or for a particular named PvD. In the case of a per-interface DNS resolver, this might override or augment the DNS resolver configuration for PvDs that are discovered on that interface. Such creation/augmentation of a PvD(s) could be static or dynamic. The specific mechanism(s) for implementing this is outside the scope of this document. Such a merging or overriding of DNS resolver configuration might be contrary to the policy that applies to a special-purpose connection, such as, for example, those discussed in Sections 5.2.1 and 5.2.4. In such cases, either the special-purpose connection should not be used or the merging/overriding should not be performed.

5.2. Consistent Use of PvDs for Network Connections

PvDs enable PvD-aware nodes to consistently use the correct set of configuration elements to serve specific network requests from beginning to end. This section provides examples of such use.

5.2.1. Name Resolution

When a PvD-aware node needs to resolve the name of the destination for use by a connection request, the node could use one or multiple PvDs for a given name lookup.

The node shall choose a single PvD if, for example, the node policy required the use of a particular PvD for a specific purpose (e.g., to download a Multimedia Messaging Service (MMS) message using a specific Access Point Name (APN) over a cellular connection or to direct traffic of enterprise applications to a VPN connection to the enterprise network). To make this selection, the node could use a match between the PvD DNS suffix and a Fully Qualified Domain Name (FQDN) that is being resolved or a match of the PvD ID, as determined by the node policy.

The node may pick multiple PvDs if, for example, the PvDs are for general purpose Internet connectivity, and the node is attempting to maximize the probability of connectivity similar to the Happy Eyeballs [RFC6555] approach. In this case, the node could perform DNS lookups in parallel, or in sequence. Alternatively, the node may use only one PvD for the lookup, based on the PvD connectivity properties, user configuration of preferred Internet PvD, etc.

If an application implements an API that provides a way of explicitly specifying the desired interface or PvD, that interface or PvD should be used for name resolution (and the subsequent connection attempt), provided that the host's configuration permits this.

In either case, by default a node uses information obtained via a name service lookup to establish connections only within the same PvD as the lookup results were obtained.

For clarification, when it is written that the name service lookup results were obtained "from a PvD", it should be understood to mean that the name service query was issued against a name service that is configured for use in a particular PvD. In that sense, the results are "from" that particular PvD.

Some nodes may support transports and/or APIs that provide an abstraction of a single connection, aggregating multiple underlying connections. Multipath TCP (MPTCP) [RFC6182] is an example of such a transport protocol. For connections provided by such transports/ APIs, a PvD-aware node may use different PvDs for servicing that logical connection, provided that all operations on the underlying connections are performed consistently within their corresponding PvD(s).

5.2.2. Next-Hop and Source Address Selection

For the purpose of this example, let us assume that the preceding name lookup succeeded in a particular PvD. For each obtained destination address, the node shall perform a next-hop lookup among routers associated with that PvD. As an example, the node could determine such associations via matching the source address prefixes / specific routes advertised by the router against known PvDs or by receiving an explicit PvD affiliation advertised through a new Router Discovery [RFC4861] option.

For each destination, once the best next hop is found, the node selects the best source address according to rules defined in [RFC6724], but with the constraint that the source address must belong to a range associated with the used PvD. If needed, the node would use the prefix policy from the same PvD for selecting the best source address from multiple candidates.

When destination/source pairs are identified, they are sorted using the [RFC6724] destination sorting rules and prefix policy table from the used PvD.

5.2.3. Listening Applications

Consider a host connected to several PvDs, running an application that opens a listening socket / transport API object. The application is authorized by the host policy to use a subset of connected PvDs that may or may not be equal to the complete set of the connected PvDs. As an example, in the case where there are different PvDs on the Wi-Fi and cellular interfaces, for general Internet traffic the host could use only one, preferred PvD at a time (and accordingly, advertise to remote peers the host name and addresses associated with that PvD), or it could use one PvD as the default for outgoing connections, while still allowing use of the other PvDs simultaneously.

Another example is a host with an established VPN connection. Here, security policy could be used to permit or deny an application's access to the VPN PvD and other PvDs.

For non-PvD-aware applications, the operating system has policies that determine the authorized set of PvDs and the preferred outgoing PvD. For PvD-aware applications, both the authorized set of PvDs and the default outgoing PvD can be determined as the common subset produced between the OS policies and the set of PvD IDs or characteristics provided by the application.

Application input could be provided on a per-application, per- transport-API-object, or per-transport-API-call basis. The API for application input may have an option for specifying whether the input should be treated as a preference instead of a requirement.

5.2.3.1. Processing of Incoming Traffic

Unicast IP packets are received on a specific IP address associated with a PvD. For multicast packets, the host can derive the PvD association from other configuration information, such as an explicit PvD property or local policy.

The node OS or middleware may apply more advanced techniques for determining the resultant PvD and/or authorization of the incoming traffic. Those techniques are outside the scope of this document.

If the determined receiving PvD of a packet is not in the allowed subset of PvDs for the particular application/transport API object, the packet should be handled in the same way as if there were no listener.

5.2.3.1.1. Connection-Oriented APIs

For connection-oriented APIs, when the initial incoming packet is received, the packet PvD is remembered for the established connection and used for the handling of outgoing traffic for that connection. While typically connection-oriented APIs use a connection-oriented transport protocol, such as TCP, it is possible to have a connection- oriented API that uses a generally connectionless transport protocol, such as UDP.

For APIs/protocols that support multiple IP traffic flows associated with a single transport API connection object (for example, Multipath TCP), the processing rules may be adjusted accordingly.

5.2.3.1.2. Connectionless APIs

For connectionless APIs, the host should provide an API that PvD-aware applications can use to query the PvD associated with the packet. For outgoing traffic on this transport API object, the OS should use the selected outgoing PvDs, determined as described in Sections 5.2.1 and 5.2.2.

5.2.4. Enforcement of Security Policies

By themselves, PvDs do not define, and cannot be used for communication of, security policies. When implemented in a network, this architecture provides the host with information about connected networks. The actual behavior of the host then depends on the host's policies (provisioned through mechanisms out of scope of this document), applied by taking received PvD information into account. In some scenarios, e.g., a VPN, such policies could require the host to use only a particular VPN PvD for some/all of the application's traffic (VPN 'disable split tunneling' also known as 'force tunneling' behavior) or apply such restrictions only to selected applications and allow the simultaneous use of the VPN PvD together with the other connected PvDs by the other or all applications (VPN 'split tunneling' behavior).

5.3. Connectivity Tests

Although some PvDs may appear as valid candidates for PvD selection (e.g., good link quality, consistent connection parameters, etc.), they may provide limited or no connectivity to the desired network or the Internet. For example, some PvDs provide limited IP connectivity (e.g., scoped to the link or to the access network) but require the node to authenticate through a web portal to get full access to the Internet. This may be more likely to happen for PvDs that are not trusted by a given PvD-aware node.

An attempt to use such a PvD may lead to limited network connectivity or application connection failures. To prevent the latter, a PvD- aware node may perform a connectivity test for the PvD before using it to serve application network connection requests. In current implementations, some nodes already implement this, e.g., by trying to reach a dedicated web server (see [RFC6419]).

Section 5.2 describes how a PvD-aware node shall maintain and use multiple PvDs separately. The PvD-aware node shall perform a connectivity test and, only after validation of the PvD, consider using it to serve application connections requests. Ongoing connectivity tests are also required, since during the IP session, the end-to-end connectivity could be disrupted for various reasons (e.g., L2 problems and IP QoS issues); hence, a connectivity monitoring function is needed to check the connectivity status and remove the PvD from the set of usable PvDs if necessary.

There may be cases where a connectivity test for PvD selection may not be appropriate and should be complemented, or replaced, by PvD selection based on other factors. For example, this could be realized by leveraging some 3GPP and IEEE mechanisms, which would allow the exposure of some PvD characteristics to the node (e.g., 3GPP Access Network Discovery and Selection Function (ANDSF) [TS23402], Access Network Query Protocol (ANQP) [IEEE802.11u]).

5.4. Relationship to Interface Management and Connection Managers

Current devices such as mobile handsets make use of proprietary mechanisms and custom applications to manage connectivity in environments with multiple interfaces and multiple sets of network configuration. These mechanisms or applications are commonly known as connection managers [RFC6419].

Connection managers sometimes rely on policy servers to allow a node that is connected to multiple networks to perform network selection. They can also make use of routing guidance from the network (e.g., 3GPP ANDSF [TS23402]). Although connection managers solve some connectivity problems, they rarely address network selection problems in a comprehensive manner. With proprietary solutions, it is challenging to present coherent behavior to the end user of the device, as different platforms present different behaviors even when connected to the same network, with the same type of interface, and for the same purpose. The architecture described in this document should improve the host's behavior by providing the hosts with tools and guidance to make informed network selection decisions.

6. PvD Support in APIs

For all levels of PvD support in APIs described in this chapter, it is expected that the notifications about changes in the set of available PvDs are exposed as part of the API surface.

6.1. Basic

Applications are not PvD aware in any manner and only submit connection requests. The node performs PvD selection implicitly, without any application participation, based purely on node-specific administrative policies and/or choices made by the user from a user interface provided by the operating environment, not by the application.

As an example, PvD selection can be done at the name service lookup step by using the relevant configuration elements, such as those described in [RFC6731]. As another example, PvD selection could be made based on application identity or type (i.e., a node could always use a particular PvD for a Voice over IP (VoIP) application).

6.2. Intermediate

Applications indirectly participate in PvD selection by specifying hard requirements and soft preferences. As an example, a real-time communication application intending to use the connection for the exchange of real-time audio/video data may indicate a preference or a requirement for connection quality, which could affect PvD selection (different PvDs could correspond to Internet connections with different loss rates and latencies).

Another example is the connection of an infrequently executed background activity, which checks for application updates and performs large downloads when updates are available. For such connections, a cheaper or zero-cost PvD may be preferable, even if such a connection has a higher relative loss rate or lower bandwidth. The node performs PvD selection based on applications' inputs and policies and/or user preferences. Some/all properties of the resultant PvD may be exposed to applications.

6.3. Advanced

PvDs are directly exposed to applications for enumeration and selection. Node polices and/or user choices may still override the applications' preferences and limit which PvD(s) can be enumerated and/or used by the application, irrespective of any preferences that the application may have specified. Depending on the implementation, such restrictions (imposed by node policy and/or user choice) may or may not be visible to the application.

7. PvD Trust for PvD-Aware Node

7.1. Untrusted PvDs

Implicit and explicit PvDs for which no trust relationship exists are considered untrusted. Only PvDs that meet the requirements in Section 7.2 are trusted; any other PvD is untrusted.

In order to avoid the various forms of misinformation that could occur when PvDs are untrusted, nodes that implement PvD separation cannot assume that two explicit PvDs with the same identifier are actually the same PvD. A node that makes this assumption will be vulnerable to attacks where, for example, an open Wi-Fi hotspot might assert that it was part of another PvD and thereby attempt to draw traffic intended for that PvD onto its own network.

Since implicit PvD identifiers are synthesized by the node, this issue cannot arise with implicit PvDs.

Mechanisms exist (for example, [RFC6731]) whereby a PvD can provide configuration information that asserts special knowledge about the reachability of resources through that PvD. Such assertions cannot be validated unless the node has a trust relationship with the PvD; therefore, assertions of this type must be ignored by nodes that receive them from untrusted PvDs. Failure to ignore such assertions could result in traffic being diverted from legitimate destinations to spoofed destinations.

7.2. Trusted PvDs

Trusted PvDs are PvDs for which two conditions apply: First, a trust relationship must exist between the node that is using the PvD configuration and the source that provided that configuration; this is the authorization portion of the trust relationship. Second, there must be some way to validate the trust relationship. This is the authentication portion of the trust relationship. Two mechanisms for validating the trust relationship are defined.

It shall be possible to validate the trust relationship for all advertised elements of a trusted PvD, irrespective of whether the PvD elements are communicated as a whole, e.g., in a single DHCP option, or separately, e.g., in supplementary RA options. The feasibility of mechanisms to implement a trust relationship for all PvD elements will be determined in the respective companion design documents.

7.2.1. Authenticated PvDs

One way to validate the trust relationship between a node and the source of a PvD is through the combination of cryptographic authentication and an identifier configured on the node.

If authentication is done using a public key mechanism such as PKI certificate chain validation or DNS-Based Authentication of Named Entities (DANE), authentication by itself is not enough since theoretically any PvD could be authenticated in this way. In addition to authentication, the node would need configuration to trust the identifier being authenticated. Validating the authenticated PvD name against a list of PvD names configured as trusted on the node would constitute the authorization step in this case.

7.2.2. PvDs Trusted by Attachment

In some cases, a trust relationship may be validated by some means other than those described in Section 7.2.1 simply by virtue of the connection through which the PvD was obtained. For instance, a handset connected to a mobile network may know through the mobile network infrastructure that it is connected to a trusted PvD. Whatever mechanism was used to validate that connection constitutes the authentication portion of the PvD trust relationship. Presumably, such a handset would be configured from the factory (or else through mobile operator or user preference settings) to trust the PvD, and this would constitute the authorization portion of this type of trust relationship.

8. Security Considerations

There are at least three different forms of attacks that can be performed using configuration sources that support multiple provisioning domains. 

   Tampering with provided configuration information:  An attacker may
      attempt to modify information provided inside the PvD container
      option.  These attacks can easily be prevented by using message
      integrity features provided by the underlying protocol used to
      carry the configuration information.  For example, SEcure Neighbor

Discovery (SEND) [RFC3971] would detect any form of tampering with the RA contents and the DHCPv6 [RFC3315] AUTH option that would detect any form of tampering with the DHCPv6 message contents. This attack can also be performed by a compromised configuration source by modifying information inside a specific PvD, in which case the mitigations proposed in the next subsection may be helpful. 

   Rogue configuration source:  A compromised configuration source, such
      as a router or a DHCPv6 server, may advertise information about
      PvDs that it is not authorized to advertise.  For example, a
      coffee shop WLAN may advertise configuration information
      purporting to be from an enterprise and may try to attract
      enterprise-related traffic.  This may also occur accidentally if
      two sites choose the same identifier (e.g., "linsky").

In order to detect and prevent this, the client must be able to authenticate the identifier provided by the network. This means that the client must have configuration information that maps the PvD identifier to an identity and must be able to authenticate that identity.

In addition, the network must provide information the client can use to authenticate the identity. This could take the form of a PKI-based or DNSSEC-based trust anchor, or a key remembered from a previous leap-of-faith authentication of the identifier.

Because the PvD-specific information may come to the network infrastructure with which the client is actually communicating from some upstream provider, it is necessary in this case that the PvD container and its contents be relayed to the client unchanged, leaving the upstream provider's signature intact. 

   Replay attacks:  A compromised configuration source or an on-link
      attacker may try to capture advertised configuration information
      and replay it on a different link, or at a future point in time.
      This can be avoided by including a replay protection mechanism
      such as a timestamp or a nonce inside the PvD container to ensure
      the validity of the provided information.

9. Informative References

[DHCPv6-CLASS-BASED-PREFIX] 

              Systems, C., Halwasia, G., Gundavelli, S., Deng, H.,
              Thiebaut, L., Korhonen, J., and I. Farrer, "DHCPv6 class
              based prefix", Work in Progress, draft-bhandari-dhc-class-
              based-prefix-05, July 2013.

[IEEE802.11u] 

              IEEE, "Local and Metropolitan networks - specific
              requirements - Part II: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) specifications: Amendment
              9: Interworking with External Networks", IEEE Std 802.11u,
              <http://standards.ieee.org/findstds/
              standard/802.11u-2011.html>.

[IPv6-PREFIX-PROPERTIES] 

              Korhonen, J., Patil, B., Gundavelli, S., Seite, P., and D.
              Liu, "IPv6 Prefix Mobility Management Properties", Work in
              Progress, draft-korhonen-dmm-prefix-properties-03, October
              2012.
   
   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.
   
   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <http://www.rfc-editor.org/info/rfc3971>.
   
   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.
   
   [RFC5739]  Eronen, P., Laganier, J., and C. Madson, "IPv6
              Configuration in Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 5739, DOI 10.17487/RFC5739, February 2010,
              <http://www.rfc-editor.org/info/rfc5739>.
   
   [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
              <http://www.rfc-editor.org/info/rfc6182>.
   
   [RFC6418]  Blanchet, M. and P. Seite, "Multiple Interfaces and
              Provisioning Domains Problem Statement", RFC 6418,
              DOI 10.17487/RFC6418, November 2011,
              <http://www.rfc-editor.org/info/rfc6418>.
   
   [RFC6419]  Wasserman, M. and P. Seite, "Current Practices for
              Multiple-Interface Hosts", RFC 6419, DOI 10.17487/RFC6419,
              November 2011, <http://www.rfc-editor.org/info/rfc6419>.
   
   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.
   
   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.
   
   [RFC6731]  Savolainen, T., Kato, J., and T. Lemon, "Improved
              Recursive DNS Server Selection for Multi-Interfaced
              Nodes", RFC 6731, DOI 10.17487/RFC6731, December 2012,
              <http://www.rfc-editor.org/info/rfc6731>.
   
   [RFC7078]  Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy Using DHCPv6", RFC 7078,
              DOI 10.17487/RFC7078, January 2014,
              <http://www.rfc-editor.org/info/rfc7078>.
   
   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.
   
   [TS23402]  3GPP, "Technical Specification Group Services and System
              Aspects; Architecture enhancements for non-3GPP accesses",
              Release 12, 3GPP TS 23.402, 2014.

Acknowledgments

The authors would like to thank (in no specific order) Ian Farrer, Markus Stenberg, and Mikael Abrahamsson for their review and comments.

Contributors

   The following individuals contributed to this document (listed in no
   specific order): Alper Yegin (alper.yegin@yegin.org), Aaron Yi Ding
   (yding@cs.helsinki.fi), Zhen Cao (caozhenpku@gmail.com), Dapeng Liu
   (liudapeng@chinamobile.com), Dave Thaler (dthaler@microsoft.com),
   Dmitry Anipko (dmitry.anipko@gmail.com), Hui Deng
   (denghui@chinamobile.com), Jouni Korhonen (jouni.nospam@gmail.com),
   Juan Carlos Zuniga (JuanCarlos.Zuniga@InterDigital.com), Konstantinos
   Pentikousis (k.pentikousis@huawei.com), Marc Blanchet
   (marc.blanchet@viagenie.ca), Margaret Wasserman
   (margaretw42@gmail.com), Pierrick Seite (pierrick.seite@orange.com),
   Suresh Krishnan (suresh.krishnan@ericsson.com), Teemu Savolainen
   (teemu.savolainen@nokia.com), Ted Lemon (ted.lemon@nominum.com), and
   Tim Chown (tjc@ecs.soton.ac.uk).

Author's Address

Dmitry Anipko (editor)
Unaffiliated 

   Phone: +1 425 442 6356
   EMail: dmitry.anipko@gmail.com