| Internet-Draft | PAN questions | August 2020 |
| Trammell | Expires 14 February 2021 | [Page] |
In contrast to the present Internet architecture, a path-aware internetworking architecture has two important properties: it exposes the properties of available Internet paths to endpoints, and provides for endpoints and applications to use these properties to select paths through the Internet for their traffic. This document poses questions in path-aware networking open as of 2019, that must be answered in the design, development, and deployment of path-aware internetworks. It was originally written to frame discussions in the Path Aware Networking proposed Research Group (PANRG), and has been published to snapshot current thinking in this space.¶
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In the current Internet architecture, the network layer provides an unverifiable, best-effort service: an application can assume that a packet with a given destination address will eventually be forwarded toward that destination, but little else. A transport layer protocol such as TCP can provide reliability over this best-effort service, and a protocol above the network layer such as IPsec AH [RFC4302] or TLS [RFC8446] can authenticate the remote endpoint. However, little, if any, explicit information about the path is available, and assumptions about that path often do not hold, sometimes with serious impacts on the application, as in the case with BGP hijacking attacks.¶
By contrast, in a path-aware internetworking architecture, endpoints have the ability to select or influence the path through the network used by any given packet, and the network and transport layers explicitly expose information about the path or paths available from one endpoint to another, and vice versa, to those endpoints and the applications running on them, so that they can make this selection.¶
Path selection provides transparency and control to applications and users of the network. Selection may be made at either the application layer or the transport layer. Path control at the packet level enables the design of new transport protocols that can leverage multipath connectivity across disjoint paths through the Internet, even over a single physical interface. When exposed to applications, or to end-users through a system configuration interface, path control allows the specification of constraints on the paths that traffic should traverse, for instance to confound passive surveillance in the network core [RFC7624].¶
We note that this property of "path awareness" already exists in many Internet-connected networks within single domains. Indeed, much of the practice of network engineering using encapsulation at layer 3 can be said to be "path aware", in that it explicitly assigns traffic at tunnel endpoints to a given path within the network. Path-aware internetworking seeks to extend this awareness across domain boundaries without resorting to overlays, except as a transition technology.¶
For purposes of this document, "path aware networking" describes endpoint discovery of the properties of paths they use for communication, and endpoint reaction to these properties that affects routing and/or transmission; note that this can and already does happen to some extent in the current Internet architecture. Expanding on this definition, a "path aware internetwork" is one in which endpoint discovery of path properties and endpoint selection of paths used by traffic exchanged by the endpoint are explicitly supported, regardless of the specific design of the protocol features which enable this discovery and selection.¶
Research into path aware networking covers any and all aspects of designing, building, and operating path aware internetworks or the networks and endpoints attached to them. This document presents a collection of research questions to address in order to make a path aware Internet a reality.¶
Realizing path-aware networking requires answers to a set of open research questions. This document poses these questions, as a starting point for discussions about how to realize path awareness in the Internet, and to direct future research efforts within the Path Aware Networking Research Group.¶
In order for information about paths to be exposed to an endpoint, and for the endpoint to make use of that information, it is necessary to define a common vocabulary for paths through an internetwork, and properties of those paths. The elements of this vocabulary could include terminology for components of a path and properties defined for these components, for the entire path, or for subpaths of a path. These properties may be relatively static, such as the presence of a given node or service function on the path; as well as relatively dynamic, such as the current values of metrics such as loss and latency.¶
This vocabulary must be defined carefully, as its design will have impacts on the expressiveness of a given path-aware internetworking architecture. This expressiveness also exhibits tradeoffs. For example, a system that exposes node-level information for the topology through each network would maximize information about the individual components of the path at the endpoints, at the expense of making internal network topology universally public, which may be in conflict with the business goals of each network's operator. Furthermore, properties related to individual components of the path may change frequently and may quickly become outdated. However, aggregating the properties of individual components to distill end-to-end properties for the entire path is not trivial.¶
The first question: how are paths and path properties defined and represented?¶
Once endpoints and networks have a shared vocabulary for expressing path properties, the network must have some method for distributing those path properties to the endpoint. Regardless of how path property information is distributed to the endpoints, the endpoints require a method to authenticate the properties - to determine that they originated from and pertain to the path that they purport to.¶
Choices in distribution and authentication methods will have impacts on the scalability of a path-aware architecture. Possible dimensions in the space of distribution methods include in-band versus out-of-band, push versus pull versus publish-subscribe, and so on. There are temporal issues with path property dissemination as well, especially with dynamic properties, since the measurement or elicitation of dynamic properties may be outdated by the time that information is available at the endpoints, and interactions between the measurement and dissemination delay may exhibit pathological behavior for unlucky points in the parameter space.¶
The second question: how do endpoints and applications get access to trustworthy path properties?¶
Access to trustworthy path properties is only half of the challenge in establishing a path-aware architecture. Endpoints must be able to use this information in order to select paths for specific traffic they send. As with the dissemination of path properties, choices made in path selection methods will also have an impact on the tradeoff between scalability and expressiveness of a path-aware architecture. One key choice here is between in-band and out-of-band control of path selection. Another is granularity of path selection (whether per packet, per flow, or per larger aggregate), which also has a large impact on the scalabilty/expressiveness tradeoff. Path selection must, like path property information, be trustworthy, such that the result of a path selection at an endpoint is predictable. Moreover, any path selection mechanism should aim to provide an outcome that is not worse than using a single path, or selecting paths at random.¶
The third question: how can endpoints select paths to use for traffic in a way that can be trusted by the network, the endpoints, and the applications using them?¶
In order for applications to make effective use of a path-aware networking architecture, the control interfaces presented by the network and transport layers must also expose path properties to the application in a useful way, and provide a useful set of paths among which the application can select. Path selection must be possible based not only on the preferences and policies of the application developer, but of end-users as well. Also, the path selection interfaces presented to applications and end users will need to support multiple levels of granularity. Most applications' requirements can be satisfied with the expression path selection policies in terms of properties of the paths, while some applications may need finer-grained, per-path control. These interfaces will need to support incremental development and deployment of applications, and provide sensible defaults, to avoid hindering their adoption.¶
The fourth question: how can interfaces to the transport and application layers support the use of path awareness?¶
In the current Internet, the basic assumption that at a given time all traffic for a given flow will receive the same network treatment and traverse the same path or equivalend paths often holds. In a path aware network, this assumption is more easily violated holds. The weakening of this assumption has implications for the design of protocols above any path-aware network layer.¶
For example, one advantage of multipath communication is that a given end-to-end flow can be "sprayed" along multiple paths in order to confound attempts to collect data or metadata from those flows for pervasive surveillance purposes [RFC7624]. However, the benefits of this approach are reduced if the upper-layer protocols use linkable identifiers on packets belonging to the same flow across different paths. Clients may mitigate linkability by opting to not re-use cleartext connection identifiers, such as TLS session IDs or tickets, on separate paths. The privacy-conscious strategies required for effective privacy in a path-aware Internet are only possible if higher-layer protocols such as TLS permit clients to obtain unlinkable identifiers.¶
The fifth question: how should transport-layer and higher layer protocols be redesigned to work most effectively over a path-aware networking layer?¶
The vision of path-aware networking articulated so far makes an assumption that path properties will be disseminated to endpoints on which applications are running (terminals with user agents, servers, and so on). However, incremental deployment may require that a path-aware network "core" be used to interconnect islands of legacy protocol networks. In these cases, it is the gateways, not the application endpoints, that receive path properties and make path selections for that traffic. The interfaces provided by this gateway are necessarily different than those a path-aware networking layer provides to its transport and application layers, and the path property information the gateway needs and makes available over those interfaces may also be different.¶
The sixth question: how is path awareness (in terms of vocabulary and interfaces) different when applied to tunnel and overlay endpoints?¶
The network operations model in the current Internet architecture assumes that traffic flows are controlled by the decisions and policies made by network operators, as expressed in interdomain and intradomain routing protocols. In a network providing path selection to the endpoints, however, this assumption no longer holds, as endpoints may react to path properties by selecting alternate paths. Competing control inputs from path-aware endpoints and the routing control plane may lead to more difficult traffic engineering or nonconvergent forwarding, especially if the endpoints' and operators' notion of the "best" path for given traffic diverges significantly. The degree of difficulty may depend on the fidelity of information made available to path selection algorithms at the endpoints. Explicit path selection can also specify outbound paths, while BGP policies are expressed in terms of inbound traffic.¶
A concept for path aware network operations will need to have clear methods for the resolution of apparent (if not actual) conflicts of intent between the network's operator and the path selection at an endpoint. It will also need set of safety principles to ensure that increasing path control does not lead to decreasing connectivity; one such safety principle could be "the existence of at least one path between two endpoints guarantees the selection of at least one path between those endpoints."¶
The seventh question: how can a path aware network in a path aware internetwork be effectively operated, given control inputs from the network administrator as well as from the endpoints?¶
The vision presented in the introduction discusses path aware networking from the point of view of the benefits accruing at the endpoints, to designers of transport protocols and applications as well as to the end users of those applications. However, this vision requires action not only at the endpoints but within the interconnected networks offering path aware connectivity. While the specific actions required are a matter of the design and implementation of a specific realization of a path aware protocol stack, it is clear than any path aware architecture will require network operators to give up some control of their networks over to endpoint-driven control inputs.¶
Here the question of apparent versus actual conflicts of intent arises again: certain network operations requirements may appear essential, but are merely accidents of the interfaces provided by current routing and management protocols. Incentives for deployment must show how existing network operations requirements are met through new path selection and property dissemination mechanisms.¶
The incentives for network operators and equipment vendors need to be made clear, in terms of a plan to transition [RFC8170] an internetwork to path-aware operation, one network and facility at a time. This plan to transition must also take into account that the dynamics of path aware networking early in this transition (when few endpoints and flows in the Internet use path selection) may be different than those later in the transition.¶
The eighth question: how can the incentives of network operators and end-users be aligned to realize the vision of path aware networking, and how can the transition from current ("path-oblivious") to path-aware networking be managed?¶
Many thanks to Adrian Perrig, Jean-Pierre Smith, Mirja Kuehlewind, Olivier Bonaventure, Martin Thomson, Shwetha Bhandari, Chris Wood, Lee Howard, Mohamed Boucadair, Thorben Krueger, Gorry Fairhurst, Spencer Dawkins, and Theresa Enghardt for discussions leading to questions in this document, and for feedback on the document itself.¶
This work is partially supported by the European Commission under Horizon 2020 grant agreement no. 688421 Measurement and Architecture for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat for Education, Research, and Innovation under contract no. 15.0268. This support does not imply endorsement.¶