GENWiki



Internet Research Task Force (IRTF) B. Trammell Request for Comments: 9217 Google Switzerland GmbH Category: Informational March 2022 ISSN: 2070-1721

          Current Open Questions in Path-Aware Networking

Abstract

 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 it
 provides for endpoints and applications to use these properties to
 select paths through the Internet for their traffic.  While this
 property of "path awareness" already exists in many Internet-
 connected networks within single domains and via administrative
 interfaces to the network layer, a fully path-aware internetwork
 expands these concepts across layers and across the Internet.

 This document poses questions in path-aware networking, open as of
 2021, 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 Research Group
 (PANRG), and has been published to snapshot current thinking in this
 space.

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 Research Task Force
 (IRTF).  The IRTF publishes the results of Internet-related research
 and development activities.  These results might not be suitable for
 deployment.  This RFC represents the consensus of the Path Aware
 Networking Research Group of the Internet Research Task Force (IRTF).
 Documents approved for publication by the IRSG are not candidates for
 any level of Internet Standard; see Section 2 of RFC 7841.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9217.

Copyright Notice

 Copyright (c) 2022 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
 (https://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.

Table of Contents

 1.  Introduction to Path-Aware Networking
   1.1.  Definitions
 2.  Questions
   2.1.  A Vocabulary of Path Properties
   2.2.  Discovery, Distribution, and Trustworthiness of Path
         Properties
   2.3.  Supporting Path Selection
   2.4.  Interfaces for Path Awareness
   2.5.  Implications of Path Awareness for the Transport and
         Application Layers
   2.6.  What is an Endpoint?
   2.7.  Operating a Path-Aware Network
   2.8.  Deploying a Path-Aware Network
 3.  IANA Considerations
 4.  Security and Privacy Considerations
 5.  Informative References
 Acknowledgments
 Author's Address

1. Introduction to Path-Aware Networking

 In the current Internet architecture, the network layer provides a
 best-effort service to the endpoints using it, without verifiability
 of the properties of the path between the endpoints.  While there are
 network-layer technologies that attempt better-than-best-effort
 delivery, the interfaces to these are generally administrative as
 opposed to endpoint exposed (e.g., Path Computation Element (PCE)
 [RFC4655] and Software-Defined Wide Area Network (SD-WAN) [MEF70]
 approaches), and they are often restricted to single administrative
 domains.  In this architecture, 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 Transport Layer Security (TLS) [RFC8446], can authenticate
 the remote endpoint.  However, little, if any, explicit information
 about the path is available to the endpoints, and any assumptions
 made about that path often do not hold.  These sometimes have serious
 impacts on the application, as in the case with BGP hijacking
 attacks.

 By contrast, in a path-aware internetworking architecture, endpoints
 can select or influence the path(s) through the network used by any
 given packet or flow.  The network and transport layers explicitly
 expose information about the path or paths available to the endpoints
 and to the applications running on them, so that they can make this
 selection.  The Application-Layer Traffic Optimization (ALTO)
 protocol [RFC7285] can be seen as an example of a path-awareness
 approach implemented in transport-layer terms on the present Internet
 protocol stack.

 Path selection provides explicit visibility and control of network
 treatment to applications and users of the network.  This selection
 is available to the application-, transport-, and/or network-layer
 entities at each endpoint.  Path control at the flow and subflow
 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.

 This document presents a snapshot of open questions in this space
 that will need to be answered in order to realize a path-aware
 internetworking architecture; it is published to further frame
 discussions within and outside the Path Aware Networking Research
 Group, and is published with the rough consensus of that group.

1.1. Definitions

 For purposes of this document, "path-aware networking" describes
 endpoint discovery of the properties of paths they use for
 communication across an internetwork, and endpoint reaction to these
 properties that affects routing and/or data transfer.  Note that this
 can and already does happen to some extent in the current Internet
 architecture; this definition expands current techniques of path
 discovery and manipulation to cross administrative domain boundaries
 and up to the transport and application layers at the endpoints.

 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
 that enable this discovery and selection.

 A "path", for the purposes of these definitions, is abstractly
 defined as a sequence of adjacent path elements over which a packet
 can be transmitted, where the definition of "path element" is
 technology dependent.  As this document is intended to pose questions
 rather than answer them, it assumes that this definition will be
 refined as part of the answer to the first two questions it poses
 about the vocabulary of path properties and how they are
 disseminated.

 Research into path-aware internetworking 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.

2. Questions

 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.

2.1. A Vocabulary of Path Properties

 The first question: how are paths and path properties defined and
 represented?

 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 and its representation must be defined carefully, as
 its design will have impacts on the properties (e.g., expressiveness,
 scalability, and security) of a given path-aware internetworking
 architecture.  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.

2.2. Discovery, Distribution, and Trustworthiness of Path Properties

 The second question: how do endpoints and applications get access to
 accurate, useful, and trustworthy path properties?

 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 endpoints.  Regardless of how path
 property information is distributed, the endpoints require a method
 to authenticate the properties in order 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.

2.3. Supporting Path Selection

 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?

 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 trade-
 off 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 scalability/expressiveness trade-off.  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.

 Path selection may be exposed in terms of the properties of the path
 or the identity of elements of the path.  In the latter case, a path
 may be identified at any of multiple layers (e.g., routing domain
 identifier, network-layer address, higher-layer identifier or name,
 and so on).  In this case, care must be taken to present semantically
 useful information to those making decisions about which path(s) to
 trust.

2.4. Interfaces for Path Awareness

 The fourth question: how can interfaces among the network, transport,
 and application layers support the use of path awareness?

 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 of 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.

2.5. Implications of Path Awareness for the Transport and Application

    Layers

 The fifth question: how should transport-layer and higher-layer
 protocols be redesigned to work most effectively over a path-aware
 networking layer?

 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 equivalent paths often holds.  In a
 path-aware network, this assumption is more easily violated.  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 reuse
 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.

2.6. What is an Endpoint?

 The sixth question: how is path awareness (in terms of vocabulary and
 interfaces) different when applied to tunnel and overlay endpoints?

 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.

2.7. Operating a Path-Aware Network

 The seventh question: how can a path-aware network in a path-aware
 internetwork be effectively operated, given control inputs from
 network administrators, application designers, and end users?

 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 non-
 convergent 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 a 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."

2.8. Deploying a Path-Aware Network

 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?

 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 also 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 that 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 operation requirements may appear
 essential but are merely accidents of the interfaces provided by
 current routing and management protocols.  For example, related (but
 adjacent) to path-aware networking, the widespread use of the TCP
 wire image [RFC8546] in network monitoring for DDoS prevention
 appears in conflict with the deployment of encrypted transports, only
 because path signaling [RFC8558] has been implicit in the deployment
 of past transport protocols.

 Similarly, incentives for deployment must show how existing network
 operation 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.

 Aspects of data security and information management in a network that
 explicitly radiates more information about the network's deployment
 and configuration, and implicitly radiates information about endpoint
 configuration and preference through path selection, must also be
 addressed.

3. IANA Considerations

 This document has no IANA actions.

4. Security and Privacy Considerations

 This document poses questions about path-aware internetworking; the
 answers are a matter for future research, and security considerations
 for those answers would be included in the corresponding RFCs that
 describe them.  While each of these questions is to a lesser or
 greater degree relevant to the security and privacy of users of a
 path-aware network, questions of discovery and trustworthiness
 (Section 2.2) are most security-relevant.

5. Informative References

 [MEF70]    MEF, "SD-WAN Service Attributes and Services", MEF
            Standard, MEF 70, July 2019, <https://www.mef.net/wp-
            content/uploads/2019/07/MEF-70.pdf>.

 [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
            Computation Element (PCE)-Based Architecture", RFC 4655,
            DOI 10.17487/RFC4655, August 2006,
            <https://www.rfc-editor.org/info/rfc4655>.

 [RFC7285]  Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
            Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
            "Application-Layer Traffic Optimization (ALTO) Protocol",
            RFC 7285, DOI 10.17487/RFC7285, September 2014,
            <https://www.rfc-editor.org/info/rfc7285>.

 [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
            Trammell, B., Huitema, C., and D. Borkmann,
            "Confidentiality in the Face of Pervasive Surveillance: A
            Threat Model and Problem Statement", RFC 7624,
            DOI 10.17487/RFC7624, August 2015,
            <https://www.rfc-editor.org/info/rfc7624>.

 [RFC8170]  Thaler, D., Ed., "Planning for Protocol Adoption and
            Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
            May 2017, <https://www.rfc-editor.org/info/rfc8170>.

 [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
            Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
            <https://www.rfc-editor.org/info/rfc8446>.

 [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
            Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
            2019, <https://www.rfc-editor.org/info/rfc8546>.

 [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
            RFC 8558, DOI 10.17487/RFC8558, April 2019,
            <https://www.rfc-editor.org/info/rfc8558>.

Acknowledgments

 Many thanks to Adrian Perrig, Jean-Pierre Smith, Mirja Kühlewind,
 Olivier Bonaventure, Martin Thomson, Shwetha Bhandari, Chris Wood,
 Lee Howard, Mohamed Boucadair, Thorben Krüger, Gorry Fairhurst,
 Spencer Dawkins, Reese Enghardt, Laurent Ciavaglia, Stephen Farrell,
 and Richard Yang 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.

Author's Address

 Brian Trammell
 Google Switzerland GmbH
 Gustav-Gull-Platz 1
 CH-8004 Zurich
 Switzerland
 Email: ietf@trammell.ch