Current Open Questions in Path-Aware Networking
RFC 9217

#x27;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   quickly become outdated.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   elicitation of dynamic properties may be outdated by the time that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