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.

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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