1. RFC 9419
Internet Architecture Board (IAB)                               J. Arkko
Request for Comments: 9419                                      Ericsson
Category: Informational                                        T. Hardie
ISSN: 2070-1721                                                    Cisco
                                                                T. Pauly
                                                            M. Kühlewind
                                                               July 2023

Considerations on Application - Network Collaboration Using Path Signals


   This document discusses principles for designing mechanisms that use
   or provide path signals and calls for standards action in specific
   valuable cases.  RFC 8558 describes path signals as messages to or
   from on-path elements and points out that visible information will be
   used whether or not it is intended as a signal.  The principles in
   this document are intended as guidance for the design of explicit
   path signals, which are encouraged to be authenticated and include a
   minimal set of parties to minimize information sharing.  These
   principles can be achieved through mechanisms like encryption of
   information and establishing trust relationships between entities on
   a path.

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 Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB 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

Copyright Notice

   Copyright (c) 2023 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
   2.  Principles
     2.1.  Intentional Distribution
     2.2.  Control of the Distribution of Information
     2.3.  Protecting Information and Authentication
     2.4.  Minimize Information
     2.5.  Limiting Impact of Information
     2.6.  Minimum Set of Entities
     2.7.  Carrying Information
   3.  Further Work
   4.  IANA Considerations
   5.  Security Considerations
   6.  Informative References
   IAB Members at the Time of Approval
   Authors' Addresses

1.  Introduction

   [RFC8558] defines the term "path signals" as signals to or from on-
   path elements.  Today, path signals are often implicit; for example,
   they are derived from cleartext end-to-end information by, e.g.,
   examining transport protocols.  For instance, on-path elements use
   various fields of the TCP header [RFC9293] to derive information
   about end-to-end latency as well as congestion.  These techniques
   have evolved because the information was available and its use
   required no coordination with anyone.  This made such techniques more
   easily deployable than alternative, potentially more explicit or
   cooperative, approaches.

   However, this also means that applications and networks have often
   evolved their interaction without comprehensive design for how this
   interaction should happen or which (minimal) information would be
   needed for a certain function.  This has led to a situation where
   information that happens to be easily available is used instead of
   the information that is actually needed.  As such, that information
   may be incomplete, incorrect, or only indirectly representative of
   the information that is actually needed.  In addition, dependencies
   on information and mechanisms that were designed for a different
   function limit the evolvability of the protocols in question.

   In summary, such unplanned interactions end up having several
   negative effects:

   *  Ossifying protocols by introducing dependencies to unintended
      parties that may not be updating, such as how middleboxes have
      limited the use of TCP options

   *  Creating systemic incentives against deploying more secure or
      otherwise updated versions of protocols

   *  Basing network behavior on information that may be incomplete or

   *  Creating a model where network entities expect to be able to use
      rich information about sessions passing through

   For instance, features such as DNS resolution or TLS setup have been
   used beyond their original intent, such as in name-based filtering.
   Media Access Control (MAC) addresses have been used for access
   control, captive portals have been used to take over cleartext HTTP
   sessions, and so on.  (This document is not about whether those
   practices are good or bad; it is simply stating a fact that the
   features were used beyond their original intent.)

   Many protocol mechanisms throughout the stack fall into one of two
   categories: authenticated private communication that is only visible
   to a very limited set of parties, often one on each "end", and
   unauthenticated public communication that is visible to all network
   elements on a path.

   Exposed information encourages pervasive monitoring, which is
   described in [RFC7258].  It may also be used for commercial purposes
   or to form a basis for filtering that the applications or users do
   not desire.  However, a lack of all path signaling, on the other
   hand, may limit network management, debugging, or the ability for
   networks to optimize their services.  There are many cases where
   elements on the network path can provide beneficial services, but
   only if they can coordinate with the endpoints.  It also affects the
   ability of service providers and others to observe why problems occur

   As such, this situation is sometimes cast as an adversarial trade-off
   between privacy and the ability for the network path to provide
   intended functions.  However, this is perhaps an unnecessarily
   polarized characterization as a zero-sum situation.  Not all
   information passing implies loss of privacy.  For instance,
   performance information or preferences do not require disclosing the
   content being accessed, the user identity, or the application in use.
   Similarly, network congestion status information does not have to
   reveal network topology, the status of other users, and so on.

   Increased deployment of encryption is changing this situation.
   Encryption provides tools for controlling information access and
   protects against ossification by avoiding unintended dependencies and
   requiring active maintenance.  The increased deployment of encryption
   provides an opportunity to reconsider parts of Internet architecture
   that have used implicit derivation of input signals for on-path
   functions rather than explicit signaling, as recommended by

   For instance, QUIC replaces TCP for various applications and protects
   end-to-end signals so that they are only accessible by the endpoints,
   ensuring evolvability [RFC9000].  QUIC does expose information
   dedicated for on-path elements to consume by using explicit signals
   for specific use cases, such as the Spin Bit for latency measurements
   or connection ID that can be used by load balancers [RFC9312].  This
   information is accessible by all on-path devices, but information is
   limited to only those use cases.  Each new use case requires
   additional action.  This points to one way to resolve the adversity:
   the careful design of what information is passed.

   Another extreme is to employ explicit trust and coordination between
   specific entities, endpoints, and network path elements.  VPNs are a
   good example of a case where there is an explicit authentication and
   negotiation with a network path element that is used to gain access
   to specific resources.  Authentication and trust must be considered
   in both directions: how endpoints trust and authenticate signals from
   network path elements and how network path elements trust and
   authenticate signals from endpoints.

   The goal of improving privacy and trust on the Internet does not
   necessarily need to remove the ability for network elements to
   perform beneficial functions.  We should instead improve the way that
   these functions are achieved and design new ways to support explicit
   collaboration where it is seen as beneficial.  As such, our goals
   should be to:

   *  ensure that information is distributed intentionally, not

   *  understand the privacy and other implications of any distributed

   *  ensure that the information distribution is limited to the
      intended parties; and

   *  gate the distribution of information on the participation of the
      relevant parties.

   These goals for exposure and distribution apply equally to senders,
   receivers, and path elements.

   Going forward, new standards work in the IETF needs to focus on
   addressing this gap by providing better alternatives and mechanisms
   for building functions that require some collaboration between
   endpoints and path elements.

   We can establish some basic questions that any new network function
   should consider:

   *  Which entities must consent to the information exchange?

   *  What is the minimum information each entity in this set needs?

   *  What is the effect that new signals should have?

   *  What is the minimum set of entities that need to be involved?

   *  What are the right mechanism and needed level of trust to convey
      this kind of information?

   If we look at ways network functions are achieved today, we find that
   many, if not most of them, fall short of the standard set up by the
   questions above.  Too often, they send unnecessary information, fail
   to limit the scope of distribution, or fail to provide any
   negotiation or consent.

   Designing explicit signals between applications and network elements,
   and ensuring that all information is appropriately protected, enables
   information exchange in both directions that is important for
   improving the quality of experience and network management.  The
   clean separation provided by explicit signals is also more conducive
   to protocol evolvability.

   Beyond the recommendation in [RFC8558], the IAB has provided further
   guidance on protocol design.  Among other documents, [RFC5218]
   provides general advice on incremental deployability based on an
   analysis of successes and failures, and [RFC6709] discusses protocol
   extensibility.  The Internet Technology Adoption and Transition
   (ITAT) workshop report [RFC7305] is also a recommended reading on
   this same general topic.  [RFC9049], an IRTF document, provides a
   catalog of past issues to avoid and discusses incentives for adoption
   of path signals such as the need for outperforming end-to-end
   mechanisms or considering per-connection state.

   This document discusses different approaches for explicit
   collaboration and provides guidance on architectural principles to
   design new mechanisms.  Section 2 discusses principles that good
   design can follow.  This section also provides examples and explores
   the consequences of not following these principles in those examples.
   Section 3 points to topics that need to be looked at more carefully
   before any guidance can be given.

2.  Principles

   This section provides architecture-level principles for protocol
   designers and recommends models to apply for network collaboration
   and signaling.

   While [RFC8558] focuses specifically on communication to "on-path
   elements", the principles described in this document apply
   potentially to:

   *  on-path signaling (in either direction) and

   *  signaling with other elements in the network that are not directly
      on-path but still influence end-to-end connections.

   An example of on-path signaling is communication between an endpoint
   and a router on a network path.  An example of signaling with another
   network element is communication between an endpoint and a network-
   assigned DNS server, firewall controller, or captive portal API
   server.  Note that these communications are conceptually independent
   of the base flow, even if they share a packet; they are coming from
   and going to other parties, rather than creating a multiparty

   Taken together, these principles focus on the inherent privacy and
   security concerns of sharing information between endpoints and
   network elements, emphasizing that careful scrutiny and a high bar of
   consent and trust need to be applied.  Given the known threat of
   pervasive monitoring, the application of these principles is critical
   to ensuring that the use of path signals does not create a
   disproportionate opportunity for observers to extract new data from

2.1.  Intentional Distribution

   The following guideline is best expressed in [RFC8558]:

   |  Fundamentally, this document recommends that implicit signals
   |  should be avoided and that an implicit signal should be replaced
   |  with an explicit signal only when the signal's originator intends
   |  that it be used by the network elements on the path.  For many
   |  flows, this may result in the signal being absent but allows it to
   |  be present when needed.

   The goal is that any information should be provided knowingly, for a
   specific purpose, sent in signals designed for that purpose, and that
   any use of information should be done within that purpose.  In
   addition, an analysis of the security and privacy implications of the
   specific purpose and associated information is needed.

   This guideline applies in the network element to application
   direction as well: a network element should not unintentionally leak
   information.  While this document makes recommendations that are
   applicable to many different situations, it is important to note that
   the above call for careful analysis is key.  Different types of
   information, applications, and directions of communication influence
   the analysis and can lead to very different conclusions about what
   information can be shared and with whom.  For instance, it is easy to
   find examples of information that applications should not share with
   network elements (e.g., content of communications) or that network
   elements should not share with applications (e.g., detailed user
   location in a wireless network).  But, equally, information about
   other things, such as the onset of congestion, should be possible to
   share and can be beneficial information to all parties.

   Intentional distribution is a precondition for explicit collaboration
   that enables each entity to have the highest possible level of
   control about what information to share.

2.2.  Control of the Distribution of Information

   Explicit signals are not enough.  The entities also need to agree to
   exchange the information.  Trust and mutual agreement between the
   involved entities must determine the distribution of information in
   order to give each entity adequate control over the collaboration or
   information sharing.  This can be achieved as discussed below.

   The sender needs to decide that it is willing to send information to
   a specific entity or set of entities.  Any passing of information or
   request for an action needs to be explicit and use signaling
   mechanisms that are designed for the purpose.  Merely sending a
   particular kind of packet to a destination should not be interpreted
   as an implicit agreement.

   At the same time, the recipient of information or the target of a
   request should have the option to agree or deny to receiving the
   information.  It should not be burdened with extra processing if it
   does not have willingness or a need to do so.  This happens naturally
   in most protocol designs, but it has been a problem for some cases
   where "slow path" packet processing is required or implied, and the
   recipient or router is not willing to handle it.  Performance impacts
   like this are best avoided, however.

   In any case, all involved entities must be identified and potentially
   authenticated if trust is required as a prerequisite to share certain

   Many Internet communications are not performed on behalf of the
   applications but are ultimately made on behalf of users.  However,
   not all information that may be shared directly relates to user
   actions or other sensitive data.  All shared information must be
   evaluated carefully to identify potential privacy implications for
   users.  Information that directly relates to the user should not be
   shared without the user's consent.  It should be noted that the
   interests of the user and other parties, such as the application
   developer, may not always coincide; some applications may wish to
   collect more information about the user than the user would like.  As
   discussed in [RFC8890], from an IETF point of view, the interests of
   the user should be prioritized over those of the application
   developer.  The general issue of how to achieve a balance of control
   between the actual user and an application representing a user's
   interest is out of scope for this document.

2.3.  Protecting Information and Authentication

   Some simple forms of information often exist in cleartext form, e.g.,
   Explicit Congestion Notification (ECN) bits from routers are
   generally not authenticated or integrity protected.  This is possible
   when the information exchanges do not carry any significantly
   sensitive information from the parties.  Often, these kinds of
   interactions are also advisory in their nature (see Section 2.5).

   In other cases, it may be necessary to establish a secure signaling
   channel for communication with a specific other party, e.g., between
   a network element and an application.  This channel may need to be
   authenticated, integrity protected, and confidential.  This is
   necessary, for instance, if the particular information or request
   needs to be shared in confidence only with a particular, trusted
   network element or endpoint or if there is danger of an attack where
   someone else may forge messages that could endanger the

   Authenticated integrity protections on signaled data can help ensure
   that data received in a signal has not been modified by other
   parties.  Still, both network elements and endpoints need to be
   careful in processing or responding to any signal.  Whether through
   bugs or attacks, the content of path signals can lead to unexpected
   behaviors or security vulnerabilities if not properly handled.  As a
   result, the advice in Section 2.5 still applies even in situations
   where there's a secure channel for sending information.

   However, it is important to note that authentication does not equal
   trust.  Whether a communication is with an application server or
   network element that can be shown to be associated with a particular
   domain name, it does not follow that information about the user can
   be safely sent to it.

   In some cases, the ability of a party to show that it is on the path
   can be beneficial.  For instance, an ICMP error that refers to a
   valid flow may be more trustworthy than any ICMP error claiming to
   come from an address.

   Other cases may require more substantial assurances.  For instance, a
   specific trust arrangement may be established between a particular
   network and application.  Or technologies, such as confidential
   computing, can be applied to provide an assurance that information
   processed by a party is handled in an appropriate manner.

2.4.  Minimize Information

   Each party should provide only the information that is needed for the
   other parties to perform the task for which collaboration is desired
   and no more.  This applies to information sent by an application
   about itself, sent about users, or sent by the network.  This also
   applies to any information related to flow identification.

   An architecture can follow the guideline from [RFC8558] in using
   explicit signals but still fail to differentiate properly between
   information that should be kept private and information that should
   be shared.  [RFC6973] also outlines this principle of data
   minimization as a mitigation technique to protect privacy and
   provides further guidance.

   In looking at what information can or cannot be easily passed, we
   need to consider both information from the network to the application
   and from the application to the network.

   For the application-to-network direction, user-identifying
   information can be problematic for privacy and tracking reasons.
   Similarly, application identity can be problematic if it might form
   the basis for prioritization or discrimination that the application
   provider may not wish to happen.

   On the other hand, as noted above, information about general classes
   of applications may be desirable to be given by application providers
   if it enables prioritization that would improve service, e.g.,
   differentiation between interactive and non-interactive services.

   For the network-to-application direction, there is similarly
   sensitive information, such as the precise location of the user.  On
   the other hand, various generic network conditions, predictive
   bandwidth and latency capabilities, and so on might be attractive
   information that applications can use to determine, for instance,
   optimal strategies for changing codecs.  However, information given
   by the network about load conditions and so on should not form a
   mechanism to provide a side channel into what other users are doing.

   While information needs to be specific and provided on a per-need
   basis, it is often beneficial to provide declarative information
   that, for instance, expresses application needs and then makes
   specific requests for action.

2.5.  Limiting Impact of Information

   Information shared between a network element and an endpoint of a
   connection needs to have a limited impact on the behavior of both
   endpoints and network elements.  Any action that an endpoint or
   network element takes based on a path signal needs to be considered
   appropriately based on the level of authentication and trust that has
   been established, and it needs to be scoped to a specific network

   For example, an ICMP signal from a network element to an endpoint can
   be used to influence future behavior on that particular network path
   (such as changing the effective packet size or closing a path-
   specific connection) but should not be able to cause a multipath or
   migration-capable transport connection to close.

   In many cases, path signals can be considered advisory information,
   with the effect of optimizing or adjusting the behavior of
   connections on a specific path.  In the case of a firewall blocking
   connectivity to a given host, endpoints should only interpret that as
   the host being unavailable on that particular path; this is in
   contrast to an end-to-end authenticated signal, such as a DNSSEC-
   authenticated denial of existence [RFC7129].

2.6.  Minimum Set of Entities

   It is recommended that a design identifies the minimum number of
   entities needed to share a specific signal required for an identified

   Often, this will be a very limited set, such as when an application
   only needs to provide a signal to its peer at the other end of the
   connection or a host needs to contact a specific VPN gateway.  In
   other cases, a broader set is needed, such as when explicit or
   implicit signals from a potentially unknown set of multiple routers
   along the path inform the endpoints about congestion.

   While it is tempting to consider removing these limitations in the
   context of closed, private networks, each interaction is still best
   considered separately, rather than simply allowing all information
   exchanges within the closed network.  Even in a closed network with
   carefully managed elements, there may be compromised components, as
   evidenced in the most extreme way by the Stuxnet worm that operated
   in an air-gapped network.  Most "closed" networks have at least some
   needs and means to access the rest of the Internet and should not be
   modeled as if they had an impenetrable security barrier.

2.7.  Carrying Information

   There is a distinction between what information is sent and how it is
   sent.  The information that is actually sent may be limited, while
   the mechanisms for sending or requesting information can be capable
   of sharing much more.

   There is a trade-off here between flexibility and ensuring that the
   information is minimal in the future.  The concern is that a fully
   generic data-sharing approach between different layers and parties
   could potentially be misused, e.g., by making the availability of
   some information a requirement for passing through a network, such as
   making it mandatory to identify specific applications or users.  This
   is undesirable.

   This document recommends that signaling mechanisms that send
   information be built to specifically support sending the necessary,
   minimal set of information (see Section 2.4) and no more.  As
   previously noted, flow-identifying information is a path signal in
   itself, and as such, provisioning of flow identifiers also requires
   protocol-specific analysis.

   Further, such mechanisms also need to have the ability to establish
   an agreement (see Section 2.2) and sufficient trust to pass the
   information (see Section 2.3).

3.  Further Work

   This is a developing field, and it is expected that our understanding
   of it will continue to grow.  One recent change is much higher use of
   encryption at different protocol layers.  This obviously impacts the
   field greatly, by removing the ability to use most implicit signals.
   However, it may also provide new tools for secure collaboration and
   force a rethinking of how collaboration should be performed.

   While there are some examples of modern, well-designed collaboration
   mechanisms, the list of examples is not long.  Clearly, more work is
   needed if we wish to realize the potential benefits of collaboration
   in further cases.  This requires a mindset change, a migration away
   from using implicit signals.  And of course we need to choose such
   cases where the collaboration can be performed safely, where it is
   not a privacy concern, and where the incentives of the relevant
   parties are aligned.  It should also be noted that many complex cases
   would require significant developments in order to become feasible.

   Some of the most difficult areas are listed below.  Research on these
   topics would be welcome.  Note that the topics include both those
   dealing directly with on-path network element collaboration and some
   adjacent issues that would influence such collaboration.

   *  Some forms of collaboration may depend on business arrangements,
      which may or may not be easy to put in place.  For instance, some
      quality-of-service mechanisms involve an expectation of paying for
      a service.  This is possible and has been successful within
      individual domains, e.g., users can pay for higher data rates or
      data caps in their ISP networks.  However, it is a business-wise
      proposition that is much harder for end-to-end connections across
      multiple administrative domains [Claffy2015] [RFC9049].

   *  Secure communication with path elements is needed as discussed in
      Section 2.3.  Finding practical ways for this has been difficult,
      both from the mechanics and scalability point of view, partially
      because there is no easy way to find out which parties to trust or
      what trust roots would be appropriate.  Some application-network
      element interaction designs have focused on information (such as
      ECN bits) that is distributed openly within a path, but there are
      limited examples of designs with secure information exchange with
      specific network elements or endpoints.

   *  The use of path signals to reduce the effects of denial-of-service
      attacks, e.g., perhaps modern forms of "source quench" designs,
      could be developed.  The difficulty is finding a solution that
      would be both effective against attacks and would not enable third
      parties from slowing down or censoring someone else's

   *  Work has begun on mechanisms that dissociate the information held
      by servers from knowledge of the user's network location and
      behavior.  Among the solutions that exist for this but are not
      widely deployed are [Oblivious] [PDoT] [DNS-CONFIDENTIAL]
      [HTTP-OBLIVIOUS].  These solutions address specific parts of the
      issue, and more work remains to find ways to limit the spread of
      information about the user's actions.  Host applications currently
      share sensitive information about the user's action with a variety
      of infrastructure and path elements, starting from basic data,
      such as domain names, source and destination addresses, and
      protocol header information.  This can expand to detailed end-user
      identity and other information learned by the servers.  Work to
      protect all of this information is needed.

   *  Work is needed to explore how to increase the deployment of
      mechanisms for sharing information from networks to applications.
      There are some working examples of this, e.g., ECN.  A few other
      proposals have been made (see, e.g.,
      [MOBILE-THROUGHPUT-GUIDANCE]), but very few of those have seen

   *  Additional work on sharing information from applications to
      networks would also be valuable.  There are a few working examples
      of this (see Section 1).  Numerous proposals have been made in
      this space, but most of them have not progressed through standards
      or been deployed for a variety of reasons [RFC9049].  However,
      several current or recent proposals exist, such as

   *  Data privacy regimes generally deal with multiple issues, not just
      whether or not some information is shared with another party.  For
      instance, there may be rules regarding how long information can be
      stored or what purpose that information may be used for.  Similar
      issues may also be applicable to the kind of information sharing
      discussed in this document.

   *  The present work has focused on the technical aspects of making
      collaboration safe and mutually beneficial, but of course,
      deployments need to take into account various regulatory and other
      policy matters.  These include privacy regulation, competitive
      issues, network neutrality aspects, and so on.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   This document has no security considerations.

6.  Informative References

              Claffy, KC. and D. Clark, "Adding Enhanced Services to the
              Internet: Lessons from History", TPRC 43: The 43rd
              Research Conference on Communication, Information and
              Internet Policy Paper, DOI 10.2139/ssrn.2587262, November
              2015, <https://papers.ssrn.com/sol3/

              Arkko, J. and J. Novotny, "Privacy Improvements for DNS
              Resolution with Confidential Computing", Work in Progress,
              Internet-Draft, draft-arkko-dns-confidential-02, 2 July
              2021, <https://datatracker.ietf.org/doc/html/draft-arkko-

              Trammell, B., Ed., "Architectural Considerations for
              Transport Evolution with Explicit Path Cooperation", Work
              in Progress, Internet-Draft, draft-trammell-stackevo-
              explicit-coop-00, 23 September 2015,

              Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
              Progress, Internet-Draft, draft-thomson-http-oblivious-02,
              24 August 2021, <https://datatracker.ietf.org/doc/html/

              Jain, A., Terzis, A., Flinck, H., Sprecher, N.,
              Arunachalam, S., Smith, K., Devarapalli, V., and R. Bar
              Yanai, "Mobile Throughput Guidance Inband Signaling
              Protocol", Work in Progress, Internet-Draft, draft-flinck-
              mobile-throughput-guidance-04, 13 March 2017,

              Yiakoumis, Y., McKeown, N., and F. Sorensen, "Network
              Tokens", Work in Progress, Internet-Draft, draft-
              yiakoumis-network-tokens-02, 21 December 2020,

              Schmitt, P., Edmundson, A., Mankin, A., and N. Feamster,
              "Oblivious DNS: Practical Privacy for DNS Queries",
              Proceedings on Privacy Enhancing Technologies, Volume
              2019, Issue 2, pp. 228-244, DOI 10.2478/popets-2019-0028,
              December 2018, <https://doi.org/10.2478/popets-2019-0028>.

              Arkko, J., "Considerations on Information Passed between
              Networks and Applications", Work in Progress, Internet-
              Draft, draft-arkko-path-signals-information-00, 22
              February 2021, <https://datatracker.ietf.org/doc/html/

   [PDoT]     Nakatsuka, Y., Paverd, A., and G. Tsudik, "PDoT: Private
              DNS-over-TLS with TEE Support", Digital Threats: Research
              and Practice, Volume 2, Issue 1, Article No. 3, pp. 1-22,
              DOI 10.1145/3431171, February 2021,

              Colitti, L. and T. Pauly, "Per-Application Networking
              Considerations", Work in Progress, Internet-Draft, draft-
              per-app-networking-considerations-00, 15 November 2020,

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

   [RFC6709]  Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              DOI 10.17487/RFC6709, September 2012,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of
              Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
              February 2014, <https://www.rfc-editor.org/info/rfc7129>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7305]  Lear, E., Ed., "Report from the IAB Workshop on Internet
              Technology Adoption and Transition (ITAT)", RFC 7305,
              DOI 10.17487/RFC7305, July 2014,

   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,

   [RFC8890]  Nottingham, M., "The Internet is for End Users", RFC 8890,
              DOI 10.17487/RFC8890, August 2020,

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC9049]  Dawkins, S., Ed., "Path Aware Networking: Obstacles to
              Deployment (A Bestiary of Roads Not Taken)", RFC 9049,
              DOI 10.17487/RFC9049, June 2021,

   [RFC9075]  Arkko, J., Farrell, S., Kühlewind, M., and C. Perkins,
              "Report from the IAB COVID-19 Network Impacts Workshop
              2020", RFC 9075, DOI 10.17487/RFC9075, July 2021,

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,

   [RFC9312]  Kühlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", RFC 9312, DOI 10.17487/RFC9312,
              September 2022, <https://www.rfc-editor.org/info/rfc9312>.

IAB Members at the Time of Approval

   Internet Architecture Board members at the time this document was
   approved for publication were:

      Jari Arkko
      Deborah Brungard
      Lars Eggert
      Wes Hardaker
      Cullen Jennings
      Mallory Knodel
      Mirja Kühlewind
      Zhenbin Li
      Tommy Pauly
      David Schinazi
      Russ White
      Qin Wu
      Jiankang Yao


   The authors would like to thank everyone at the IETF, the IAB, and
   our day jobs for interesting thoughts and proposals in this space.
   Fragments of this document were also in [PER-APP-NETWORKING] and
   [PATH-SIGNALS-INFO].  We would also like to acknowledge that similar
   thoughts are presented in [EXPLICIT-COOP].  Finally, the authors
   would like to thank Adrian Farrell, Toerless Eckert, Martin Thomson,
   Mark Nottingham, Luis M. Contreras, Watson Ladd, Vittorio Bertola,
   Andrew Campling, Eliot Lear, Spencer Dawkins, Christian Huitema,
   David Schinazi, Cullen Jennings, Mallory Knodel, Zhenbin Li, Chris
   Box, and Jeffrey Haas for useful feedback on this topic and document.

Authors' Addresses

   Jari Arkko
   Email: jari.arkko@ericsson.com

   Ted Hardie
   Email: ted.ietf@gmail.com

   Tommy Pauly
   Email: tpauly@apple.com

   Mirja Kühlewind
   Email: mirja.kuehlewind@ericsson.com
  1. RFC 9419