| Internet-Draft | MPVD architecture | October 2013 |
| Anipko | Expires 21 April 2014 | [Page] |
This document is a product of the work of MIF architecture design team. It outlines a solution framework for some of the issues, experienced by nodes that can be attached to multiple networks. The framework defines the notion of a Provisioning Domain (PVD) - a consistent set of network configuration information, and PVD-aware nodes - nodes which learn PVDs from the attached network(s) and/or other sources and manage and use multiple PVDs for connectivity separately and consistently.¶
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Nodes attached to multiple networks may encounter problems due to conflict of the networks configuration and/or simultaneous use of the multiple available networks. While existing implementations apply various techniques ([RFC6419]) to tackle such problems, in many cases the issues may still appear. The MIF problem statement document [RFC6418] describes the general landscape as well as discusses many specific issues and scenarios details, and are not listed in this document.¶
Across the layers, problems enumerated in [RFC6418] can be grouped into 3 categories:¶
As an illustration: 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 name from particular namespaces; an example of (2) is use of an attempt to resolve a name of a HTTP proxy server, learned from a network A, with a DNS server, learned from a network B, likely to fail; an example of (3) is a use of employer-sponsored VPN connection for peer-to-peer connections, unrelated to employment activities.¶
This architecture describes a solution to these categories of problems, respectively, by:¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].¶
Provisioning Domain: a consistent set of network configuration information. Classically, the entire set available on a single interface is provided by a single source, such as 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 domain to be present on multiple links.¶
Typical examples of information in a provisioning domain, learned from the network, are: source address prefixes, to be used by connections within the provisioning domain, IP address of DNS server, name of HTTP proxy server if available, DNS suffixes associated with the network etc.¶
PVD-aware node: a node that supports association of network configuration information into PVDs, and using the resultant PVDs to serve requests for network connections in ways, consistent with recommendations of this architecture.¶
A node may receive explicit information from the network and/or other sources, about presence of PVDs and association of particular network information with a particular PVD. PVDs, constructed based on such information, are referred to in this document as "explicit".¶
Protocol changes/extensions will likely be required to support the explicit PVDs by IETF-defined mechanisms. As an example, one could think of one or several DHCP options, carrying PVD identity and / or its elements. A different approach could be to introduce a DHCP option, which only carries identity of a PVD, while the association of network information elements with that identity, is implemented by the respective protocols - such as e.g., with a Router Discovery [RFC4861] option associating an address range with a PVD.¶
Specific, existing or new, features of networking protocols to enable 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 discovery of PVD information, different from the IETF-defined mechanisms, can be used by the nodes separately from or together with IETF-defined mechanisms, as long as they allow to discover necessary elements of the PVD(s). In all cases, by default nodes must ensure that the lifetime of all dynamically discovered PVD configuration is appropriately limited by the relevant events - for example, if an interface media state change was indicated, the previously discovered information may no longer be valid and needs to be re-discovered or confirmed.¶
It shall be possible for networks to communicate that some of their configuration elements could be used within a context of other networks/PVDs. Based on such declaration and their policies, PVD-aware nodes may choose to inject such elements into some or all other PVDs they connect to.¶
In some network topologies, the network infrastructure elements may need to advertise multiple PVDs. The details of how this is done generally will be defined in the individual companion design documents. However, where different design choices are possible, the choice that requires smaller number of packets shall be preferred for efficiency.¶
It is likely that for a long time there may be networks which do not advertise explicit PVD information, since deployment of any new features in networking protocols is a relatively slow process.¶
When connected to networks, which don't advertise explicit PVD information, PVD-aware shall automatically create separate PVDs for configuration received on multiple interfaces. Such PVDs are referred to in this document as "implicit".¶
With implicit PVDs,PVD-aware nodes may still provide benefits to their users, compared to non-PVD aware nodes, by using network information from different interfaces separately and consistently to serve network connection requests, following best practices described in Section 4.¶
In the mixed mode, where e.g., multiple networks are available on the link the interface is attached to, and only some of the networks advertise PVD information, the PVD-aware node shall create explicit PVDs based on explicitly learned PVD information, and associate the rest of the configuration with an implicit PVD created for that interface.¶
Implicit PVDs are limited to network configuration information received on a single interface. Explicit PVDs, in practice will often also be scoped to a configuration related to a particular interface, however per this architecture there is no such requirement or limitation and as defined in this architecture, explicit PVDs may include information related to more than one interfaces, if the node learns presence of the same PVD on those interfaces and the authentication of the PVD ID meets the level required by the node policy.¶
It is an intent of this architecture to support such scenarios among others. Hence, it shall be noted that no hierarchical relationship exists between interfaces and PVDs: it is possible for multiple PVDs to be simultaneously accessible over one interface, as well as single PVD to be simultaneously accessible over multiple interfaces.¶
For explicit PVDs, PVD ID (globally unique ID, that possibly is human-readable) is received as part of that information. For implicit PVDs, the node assigns a locally generated globally unique ID to each implicit PVD.¶
PVD-aware node may use these IDs to choose a PVD with matching ID for special-purpose connection requests, in accordance with node policy or choice by advanced applications, and/or to present human-readable IDs to the end-user for selection of Internet-connected 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 doesn't impose specific requirements in this regard.¶
When multiple nodes are connected to the same link, where one or more explicit PVDs are available, this architecture assumes that the information about all available PVDs is advertized by the networks to all the connected nodes. At the same time, the connected nodes may have different heuristics, policies and/or other settings, including configured set of their trusted PVDs, which may lead to different PVDs actually being used by different nodes for their connections.¶
Possible extensions, where different sets of PVDs may be advertised by the networks to different connected nodes, are out of scope for this document.¶
When applied to dual-stack networks, the PVD definition allows for multiple PVDs to be created, where each PVD contain information for only one address family, or for a single PVD that contains information about multiple address families. This architecture requires that accompanying design documents for accompanying protocol changes must support PVDs containing information from multiple address families. PVD-aware nodes must be capable of dealing with both single-family and multi-family PVDs.¶
For explicit PVDs, the choice of either of the approaches is a policy decision of a network administrator and/or 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 DHCP address selection option [I-D.ietf-6man-addr-select-opt]), it is likely that dual-stack networks will deploy single PVDs for both address families.¶
For implicit PVDs, by default PVD-aware nodes shall including multiple IP families into single implicit PVD created for an interface. At the time of writing of this document in dual-stack networks it appears to be a common practice for configuration of both address families to be provided by a single source.¶
A PVD-aware node that provides API to use / enumerate / inspect PVDs and/or their properties shall provide ability to filter PVDs and/or their properties by address family.¶
It is assumed that normally, 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 lack some parts of the configuration, merging of information from different PVD(s) shall not be done automatically, since typically it would lead to issues described in [RFC6418].¶
A node may use other sources, such as e.g., node local policy, user input or other mechanisms, not defined by IETF, to either construct a PVD entirely (analogously to static IP configuration of an interface), or supplement with particular elements all or some PVDs learned from the network, or potentially merge information from different PVDs, if such merge is known to the node to be safe, based on explicit policies.¶
As an example, node administrator could inject a not ISP-specific DNS server into PVDs for any of the networks the node could become attached to. Such creation / augmentation of PVD(s) could be static or dynamic. The particular implementation mechanisms are outside of the scope of this document.¶
PVDs enable PVD-aware nodes to use consistently a correct set of configuration elements to serve the specific network requests from beginning to end. This section describes specific examples of such consistent use.¶
When PVD-aware node needs to resolve a name of the destination used by a connection request, the node could decide to use one, or multiple PVDs for a given name lookup.¶
The node shall chose one PVD, if e.g., the node policy required to use a particular PVD for a particular purpose (e.g. to download an MMS using a specific APN over a cellular connection). To make the choice, the node could use a match of the PVD DNS suffix or other form of PVD ID, as determined by the node policy.¶
The node may pick multiple PVDs, if e.g., they are general purpose PVDs providing connectivity to the Internet, and the node desires to maximize chances for connectivity in Happy Eyeballs style. In this case, the node could do the lookups in parallel, or in sequence. Alternatively, the node may use for the lookup only one PVD, based on the PVD connectivity properties, user choice of the preferred Internet PVD, etc.¶
In either case, by default the node uses information obtained in a name service lookup to establish connections only within the same PVD from which the lookup results were obtained.¶
For simplicity, when we say that name service lookup results were obtained from a PVD, what we mean is that the name service query was issued against a name service the configuration of which is present in a particular PVD. In that sense, the results are "from" that particular PVD.¶
Some nodes may support transports and/or APIs, which provide an abstraction of a single connection, aggregating multiple underlying connections. MPTCP [RFC6182] is an example of such transport protocol. For the connections provided by such transports/APIs, a PVD-aware node may use different PVDs for servicing of that logical connection, provided that all operations on the underlying connections are done consistently within their corresponding PVD(s).¶
For the purpose of this discussion, let's assume 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, such association could be determined by the node via matching the source address prefixes/specific routes advertized by the router against known PVDs, or receiving explicit PVD affiliation advertized through a new Router Discovery [RFC4861] option.¶
For each destination, once the best next-hop is found, the node selects best source address according to the [RFC6724] rules, but with a 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 the best source address selection among multiple candidates.¶
When destination/source pairs are identified, then they are sorted using the [RFC6724] destination sorting rules and the prefix policy table from the used PVD.¶
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, which are not trusted by the given PVD-aware node.¶
An attempt to use such PVD may lead to limited network connectivity or connection failures for applications. To prevent the latter, a PVD-aware node may perform connectivity test for the PVD, before using it to serve network connection requests of the applications. In current implementations, some nodes do that, for instance, by trying to reach a dedicated web server (e.g., see [RFC6419]).¶
Per Section 4.2, a PVD-aware node shall maintain and use multiple PVDs separately. The PVD-aware node shall perform 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. poor L2, 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 be not appropriate and should be complemented, or replaced, by PVD selection based on other factors. This could be realized e.g., by leveraging some 3GPP and IEEE mechanisms, which would allow to expose some PVD characteristics to the node (e.g. 3GPP Access Network Discovery and Selection Function (ANDSF) [REF ANDSF], IEEE 802.11u/ANQP [REF 802.11u]).¶
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 configurations. These mechanisms or applications are commonly known as connection managers [RFC6419].¶
Connection managers sometimes rely on policy servers to allow the node, connected to multiple networks, perform the network selection. They can also make use of routing guidance from the network (e.g. 3GPP ANDSF [REF ANDSF]). Although connection managers solve some connectivity problems, they rarely address the network selection problems in a comprehensive manner. With proprietary solutions, it is challenging to present a coherent behaviour to the end user of the device, as different platforms present different behaviours even when connected to the same network, with the same type of interface, and for the same purpose.¶
In all cases changes in available PVDs must be somehow exposed, appropriately for each of the approaches.¶
Applications are not PVD-aware in any manner, and only submit connection requests. The node performs PVD selection implicitly, without any otherwise applications participation, and based purely on node-specific administrative policies and/or choices made by the user in a user interface provided by the operating environment, not by the application.¶
As an example, such PVD selection can be done at the name service lookup step, by using the relevant configuration elements, such as e.g., those described in [RFC6731]. As another example, the PVD selection could be done based on application identity or type (i.e., a node could always use a particular PVD for a VOIP application).¶
Applications indirectly participate in selection of PVD by specifying hard requirements and soft preferences. 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.¶
PVDs are directly exposed to applications, for enumeration and selection. Node polices and/or user choices, may still override the application preferences and limit which PVD(s) can be enumerated and/or used by the application, irrespectively of any preferences which application may have specified. Depending on the implementation, such restrictions, imposed per node policy and/or user choice, may or may not be visible to the application.¶
Implicit and explicit PVDs for which no trust relationship exists are considered untrusted. Only PVDs, which meet the requirements in Section 6.2, are trusted; any other PVD is untrusted.¶
In order to avoid various forms of misinformation that can be asserted 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 did make this assumption would be vulnerable to attacks where for example an open Wifi hotspot might assert that it was part of another PVD, and thereby might 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; assertions of this type therefore 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.¶
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, irrespectively 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. Whether or not this is feasible to provide mechanisms to implement trust relationship for all PVD elements, will be determined in the respective companion design documents.¶
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. In some cases, the two could be the same; for example, if authentication is done with a shared secret, the secret would have to be associated with the PVD identifier. Without a (PVD Identifier, shared key) tuple, authentication would be impossible, and hence authentication and authorization are combined.¶
However, if authentication is done using some public key mechanism such as a TLS cert or DANE, authentication by itself isn't enough, since theoretically any PVD could be authenticated in this way. In addition to authentication, the node would need to be configured 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.¶
In some cases a trust relationship may be validated by some means other than described in Section 6.2.1, 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, and 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.¶
This memo includes no request to IANA.¶
All drafts are required to have a security considerations section.¶