RFC9473: A Vocabulary of Path Properties

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Internet Research Task Force (IRTF)                          R. Enghardt
Request for Comments: 9473                                       Netflix
Category: Informational                                    C. Krähenbühl
ISSN: 2070-1721                                               ETH Zürich
                                                          September 2023

                    A Vocabulary of Path Properties


   Path properties express information about paths across a network and
   the services provided via such paths.  In a path-aware network, path
   properties may be fully or partially available to entities such as
   endpoints.  This document defines and categorizes path properties.
   Furthermore, the document identifies several path properties that
   might be useful to endpoints or other entities, e.g., for selecting
   between paths or for invoking some of the provided services.  This
   document is a product of the Path Aware Networking Research Group

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

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.  Terminology
     2.1.  Terminology Usage for Specific Technologies
   3.  Use Cases for Path Properties
     3.1.  Path Selection
     3.2.  Protocol Selection
     3.3.  Service Invocation
   4.  Examples of Path Properties
   5.  Security Considerations
   6.  IANA Considerations
   7.  Informative References
   Authors' Addresses

1.  Introduction

   The current Internet architecture does not explicitly support
   endpoint discovery of forwarding paths through the network nor the
   discovery of properties and services associated with these paths.
   Path-aware networking, as defined in Section 1.1 of [RFC9217],
   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".  This document
   provides a generic definition of path properties, addressing the
   first of the questions in path-aware networking [RFC9217].

   As terms related to paths have been used with different meanings in
   different areas of networking, first, this document provides a common
   terminology to define paths, path elements, and flows.  Based on
   these terms, the document defines path properties.  Then, this
   document provides some examples of use cases for path properties.
   Finally, the document lists several path properties that may be
   useful for the mentioned use cases.  This list is intended to be
   neither exhaustive nor definitive.

   Note that this document does not assume that any of the listed path
   properties are actually available to any entity.  The question of how
   entities can discover and distribute path properties in a trustworthy
   way is out of scope for this document.

   This document represents the consensus of the Path Aware Networking
   Research Group (PANRG).

2.  Terminology

   Entity:  A physical or virtual device or function, or a collection of
      devices or functions, that plays a role related to path-aware
      networking for particular paths and flows.  An entity can be on-
      path or off-path.  On the path, an entity may participate in
      forwarding the flow, i.e., what may be called "data plane
      functionality".  On or off the path, an entity may influence
      aspects of how the flow is forwarded, i.e., what may be called
      "control plane functionality", such as path selection or service
      invocation.  An entity influencing forwarding aspects is usually
      aware of path properties, e.g., by observing or measuring them or
      by learning them from another entity.

   Node:  An on-path entity that processes packets, e.g., sends,
      receives, forwards, or modifies them.  A node may be physical or
      virtual, e.g., a physical device, a service function provided as a
      virtual element, or even a single queue within a switch.  A node
      may also be an entity that consists of a collection of devices or
      functions, e.g., an entire Autonomous System (AS).

   Link:  A medium or communication facility that connects two or more
      nodes with each other.  A link enables a node to send packets to
      other nodes.  Links can be physical, e.g., a Wi-Fi network that
      connects an Access Point to stations, or virtual, e.g., a virtual
      switch that connects two virtual machines hosted on the same
      physical machine.  A link is unidirectional.  As such,
      bidirectional communication can be modeled as two links between
      the same nodes in opposite directions.

   Path element:  Either a node or a link.  For example, a path element
      can be an Abstract Network Element (ANE) as defined in [RFC9275].

   Path:  A sequence of adjacent path elements over which a packet can
      be transmitted, starting and ending with a node.

      Paths are unidirectional and time dependent, i.e., there can be a
      variety of paths from one node to another, and the path over which
      packets are transmitted may change.  A path definition can be
      strict (i.e., the exact sequence of path elements remains the
      same) or loose (i.e., the start and end node remain the same, but
      the path elements between them may vary over time).

      The representation of a path and its properties may depend on the
      entity considering the path.  On the one hand, the representation
      may differ due to entities having partial visibility of path
      elements comprising a path or their visibility changing over time.
      On the other hand, the representation may differ due to treating
      path elements at different levels of abstraction.  For example, a
      path may be given as a sequence of physical nodes and the links
      connecting these nodes, be given as a sequence of logical nodes
      such as a sequence of ASes or an Explicit Route Object (ERO), or
      only consist of a specific source and destination that is known to
      be reachable from that source.

      A multicast or broadcast setting where a packet is sent by one
      node and received by multiple nodes is described by multiple paths
      over which the packet is sent, one path for each combination of
      sending and receiving node; these paths do not have to be disjoint
      as defined by the disjointness path property, see Section 4.

   Endpoint:  The endpoints of a path are the start and end node of the
      path.  For example, an endpoint can be a host as defined in
      [RFC1122], which can be a client (e.g., a node running a web
      browser) or a server (e.g., a node running a web server).

   Reverse Path:  The path that is used by a remote node in the context
      of bidirectional communication.

   Subpath:  Given a path, a subpath is a sequence of adjacent path
      elements of this path.

   Flow:  One or multiple packets to which the traits of a path or set
      of subpaths may be applied in a functional sense.  For example, a
      flow can consist of all packets sent within a TCP session with the
      same five-tuple between two hosts, or it can consist of all
      packets sent on the same physical link.

   Property:  A trait of one or a sequence of path elements, or a trait
      of a flow with respect to one or a sequence of path elements.  An
      example of a link property is the maximum data rate that can be
      sent over the link.  An example of a node property is the
      administrative domain that the node belongs to.  An example of a
      property of a flow with respect to a subpath is the aggregated
      one-way delay of the flow being sent from one node to another node
      over this subpath.  A property is thus described by a tuple
      containing the path element(s), the flow or an empty set if no
      packets are relevant for the property, the name of the property
      (e.g., maximum data rate), and the value of the property (e.g., 1

   Aggregated property:  A collection of multiple values of a property
      into a single value, according to a function.  A property can be
      aggregated over:

      *  multiple path elements (i.e., a subpath), for example, the MTU
         of a path as the minimum MTU of all links on the path,

      *  multiple packets (i.e., a flow), for example, the median one-
         way latency of all packets between two nodes,

      *  or both path elements and packets, for example, the mean of the
         queueing delays of a flow on all nodes along a path.

      The aggregation function can be numerical (e.g., median, sum,
      minimum) or logical (e.g., "true if all are true", "true if at
      least 50% of values are true"), or it can be an arbitrary function
      that maps multiple input values to an output value.

   Observed property:  A property that is observed for a specific path
      element, subpath, or path.  A property may be observed using
      measurements, for example, the one-way delay of a specific packet
      transmitted from node to node.

   Assessed property:  An approximate calculation or assessment of the
      value of a property.  An assessed property includes the
      reliability of the calculation or assessment.  The notion of
      reliability depends on the property.  For example, a path property
      based on an approximate calculation may describe the expected
      median one-way latency of packets sent on a path within the next
      second, including the confidence level and interval.  A non-
      numerical assessment may instead include the likelihood that the
      property holds.

   Target property:  An objective that is set for a property over a path
      element, subpath, or path.  Note that a target property can be set
      for observed properties, such as one-way delay, and also for
      properties that cannot be observed by the entity setting the
      target, such as inclusion of certain nodes on a path.

2.1.  Terminology Usage for Specific Technologies

   The terminology defined in this document is intended to be general
   and applicable to existing and future path-aware technologies.  Using
   this terminology, a path-aware technology can define and consider
   specific path elements and path properties on a specific level of
   abstraction.  For instance, a technology may define path elements as
   IP routers, e.g., in source routing [RFC1940].  Alternatively, it may
   consider path elements on a different layer of the Internet
   architecture [RFC1122] or as a collection of entities not tied to a
   specific layer, such as an AS or ERO.  Even within a single path-
   aware technology, specific definitions might differ depending on the
   context in which they are used.  For example, the endpoints might be
   the communicating hosts in the context of the transport layer, ASes
   that contain the hosts in the context of routing, or specific
   applications in the context of the application layer.

3.  Use Cases for Path Properties

   When a path-aware network exposes path properties to endpoints or
   other entities, these entities may use this information to achieve
   different goals.  This section lists several use cases for path

   Note that this is not an exhaustive list; as with every new
   technology and protocol, novel use cases may emerge, and new path
   properties may become relevant.  Moreover, for any particular
   technology, entities may have visibility of and control over
   different path elements and path properties and consider them on
   different levels of abstraction.  Therefore, a new technology may
   implement an existing use case related to different path elements or
   on a different level of abstraction.

3.1.  Path Selection

   Nodes may be able to send flows via one (or a subset) out of multiple
   possible paths, and an entity may be able to influence the decision
   about which path(s) to use.  Path selection may be feasible if there
   are several paths to the same destination (e.g., in case of a mobile
   device with two wireless interfaces, both providing a path) or if
   there are several destinations, and thus several paths, providing the
   same service (e.g., Application-Layer Traffic Optimization (ALTO)
   [RFC5693], an application layer peer-to-peer protocol allowing
   endpoints a better-than-random peer selection).  Entities can express
   their intent to achieve a specific goal by specifying target
   properties for their paths, e.g., related to performance or security.
   Then, paths can be selected that best meet the target properties,
   e.g., the entity can select these paths from all available paths or
   express the target properties to the network and rely on the network
   to select appropriate paths.

   Target properties relating to network performance typically refer to
   observed properties, such as one-way delay, one-way packet loss, and
   link capacity.  Entities then select paths based on their target
   property and the assessed property of the available paths that best
   match the application requirements.  For such performance-related
   target properties, the observed property is similar to a Service
   Level Indicator (SLI), and the assessed property is similar to a
   Service Level Objective (SLO) for IETF Network Slices
   [NETWORK-SLICES].  As an example path-selection strategy, an entity
   may select a path with a short one-way delay for sending a small
   delay-sensitive query, while it may select a path with high link
   capacities on all links for retrieving a large file.

   It is also possible for an entity to set target properties that it
   cannot (directly) observe, similar to Service Level Expectations
   (SLEs) for IETF Network Slices [NETWORK-SLICES].  This may apply to
   security-related target properties (e.g., to mandate that all
   enterprise traffic goes through a specific firewall) and path
   selection (e.g., to enforce traffic policies by allowing or
   disallowing sending flows over paths that involve specific networks
   or nodes).

   Care needs to be taken when selecting paths based on observed path
   properties, as path properties that were previously measured may not
   be helpful in predicting current or future path properties, and such
   path selection may lead to unintended feedback loops.  Also, there
   may be trade-offs between path properties (e.g., one-way delay and
   link capacity), and entities may influence these trade-offs with
   their choices.  Finally, path selection may impact fairness.  For
   example, if multiple entities concurrently attempt to meet their
   target properties using the same network resources, one entity's
   choices may influence the conditions on the path as experienced by
   flows of another entity.

   As a baseline, a path-selection algorithm should aim to do a better
   job of meeting the target properties, and consequently accommodating
   the user's requirements, than the default case of not selecting a
   path most of the time.

   Path selection can be done either by the communicating node(s) or by
   other entities within the network.  A network (e.g., an AS) can
   adjust its path selection for internal or external routing based on
   path properties.  In BGP, the Multi-Exit Discriminator (MED)
   attribute is used in the decision-making process to select which path
   to choose among those having the same AS path length and origin
   [RFC4271]; in a path-aware network, instead of using this single MED
   value, other properties such as link capacity or link usage could
   additionally be used to improve load balancing or performance

3.2.  Protocol Selection

   Before sending data over a specific path, an entity may select an
   appropriate protocol or configure protocol parameters depending on
   path properties.  For example, an endpoint may cache state if a path
   allows the use of QUIC [RFC9000]; if so, it may first attempt to
   connect using QUIC before falling back to another protocol when
   connecting over this path again.  A video-streaming application may
   choose an (initial) video quality based on the achievable data rate
   or the monetary cost of sending data (e.g., volume-based or flat-rate
   cost model).

3.3.  Service Invocation

   In addition to path or protocol selection, an entity may choose to
   invoke additional functions in the context of Service Function
   Chaining [RFC7665], which may influence which nodes are on the path.
   For example, a 0-RTT Transport Converter [RFC8803] will be involved
   in a path only when invoked by an endpoint; such invocation will lead
   to the use of Multipath TCP (MPTCP) [RFC8684] or tcpcrypt [RFC8548]
   capabilities, while such use is not supported via the default
   forwarding path.  Another example is a connection that is composed of
   multiple streams where each stream has specific service requirements.
   An endpoint may decide to invoke a given service function (e.g.,
   transcoding) only for some streams while others are not processed by
   that service function.

4.  Examples of Path Properties

   This section gives some examples of path properties that may be
   useful, e.g., for the use cases described in Section 3.

   Within the context of any particular technology, available path
   properties may differ as entities have insight into and are able to
   influence different path elements and path properties.  For example,
   an endpoint may have some visibility into path elements that are
   close and on a low level of abstraction (e.g., individual nodes
   within the first few hops), or it may have visibility into path
   elements that are far away and/or on a higher level of abstraction
   (e.g., the list of ASes traversed).  This visibility may depend on
   factors such as the physical or network distance or the existence of
   trust or contractual relationships between the endpoint and the path
   element(s).  A path property can be defined relative to individual
   path elements, a sequence of path elements, or "end-to-end", i.e.,
   relative to a path that comprises of two endpoints and a single
   virtual link connecting them.

   Path properties may be relatively dynamic, e.g., the one-way delay of
   a packet sent over a specific path, or non-dynamic, e.g., the MTU of
   an Ethernet link that only changes infrequently.  Usefulness over
   time differs depending on how dynamic a property is: the merit of a
   momentarily observed dynamic path property may diminish greatly as
   time goes on, e.g., it is possible for the observed values of one-way
   delay to change on timescales that are shorter than the one-way delay
   between the measurement point and an entity making a decision such as
   path selection, which may cause the measurement to be outdated when
   it reaches the decision-making entity.  Therefore, consumers of
   dynamic path properties need to apply caution when using them, e.g.,
   by aggregating them appropriately or applying a dampening function to
   their changes to avoid oscillation.  In contrast, the observed value
   of a less dynamic path property might stay relevant for a longer
   period of time, e.g., a NAT typically stays on a particular path
   during the lifetime of a connection involving packets sent over this

   Access Technology:  The physical- or link-layer technology used for
      transmitting or receiving a flow on one or multiple path elements.
      If known, the access technology may be given as an abstract link
      type, e.g., as Wi-Fi, wired Ethernet, or cellular.  It may also be
      given as a specific technology used on a link, e.g., 3GPP cellular
      or 802.11 Wireless Local Area Network (WLAN).  Other path elements
      relevant to the access technology may include nodes related to
      processing packets on the physical or link layer, such as elements
      of a cellular core network.  Note that there is no common registry
      of possible values for this property.

   Monetary Cost:  The price to be paid to transmit or receive a
      specific flow across a network to which one or multiple path
      elements belong.

   Service Function:  A service function that a path element applies to
      a flow, see [RFC7665].  Examples of abstract service functions
      include firewalls, Network Address Translation (NAT), and TCP
      Performance Enhancing Proxies.  Some stateful service functions,
      such as NAT, need to observe the same flow in both directions,
      e.g., by being an element of both the path and the reverse path.

   Transparency:  When a node performs an action A on a flow F, the node
      is transparent to F with respect to some (meta-)information M if
      the node performs A independently of M.  M can, for example, be
      the existence of a protocol (header) in a packet or the content of
      a protocol header, payload, or both.  For example, A can be
      blocking packets or reading and modifying (other protocol) headers
      or payloads.  Transparency can be modeled using a function f,
      which takes as input F and M and outputs the action taken by the
      node.  If a taint analysis shows that the output of f is not
      tainted (impacted) by M, or if the output of f is constant for
      arbitrary values of M, then the node is considered to be
      transparent.  An IP router could be transparent to transport
      protocol headers such as TCP/UDP but not transparent to IP headers
      since its forwarding behavior depends on the IP headers.  A
      firewall that only allows outgoing TCP connections by blocking all
      incoming TCP SYN packets regardless of their IP address is
      transparent to IP but not to TCP headers.  Finally, a NAT that
      actively modifies IP and TCP/UDP headers based on their content is
      not transparent to either IP or TCP/UDP headers.  Note that
      according to this definition, a node that modifies packets in
      accordance with the endpoints, such as a transparent HTTP proxy,
      as defined in [RFC9110], and a node listening and reacting to
      implicit or explicit signals, see [RFC8558], are not considered

      Transparency only applies to nodes and not to links, as a link
      cannot modify or perform any other actions on the packets by
      itself.  For example, if the content of a packet is altered when
      forwarded over a Generic Routing Encapsulation (GRE) tunnel
      [RFC2784] [RFC7676], per this document the software instances that
      terminate the tunnel are considered nodes over which the actions
      are performed; thus, the transparency definition applies to these

   Administrative Domain:  The identity of an individual or an
      organization that controls access to a path element (or several
      path elements).  Examples of administrative domains are an IGP
      area, an AS, or a service provider network.

   Routing Domain Identifier:  An identifier indicating the routing
      domain of a path element.  Path elements in the same routing
      domain are in the same administrative domain and use a common
      routing protocol to communicate with each other.  An example of a
      routing domain identifier is the globally unique Autonomous System
      Number (ASN) as defined in [RFC1930].

   Disjointness:  For a set of two paths or subpaths, the number of
      shared path elements can be a measure of intersection (e.g.,
      Jaccard coefficient, which is the number of shared elements
      divided by the total number of elements).  Conversely, the number
      of non-shared path elements can be a measure of disjointness
      (e.g., 1 - Jaccard coefficient).  A multipath protocol might use
      disjointness as a metric to reduce the number of single points of
      failure.  As paths can be defined at different levels of
      abstraction, two paths may be disjoint at one level of abstraction
      but not on another.

   Symmetric Path:  Two paths are symmetric if the path and its reverse
      path consist of the same path elements on the same level of
      abstraction, but in reverse order.  For example, a path that
      consists of layer 3 switches and links between them and a reverse
      path with the same path elements but in reverse order are
      considered "routing" symmetric, as the same path elements on the
      same level of abstraction (IP forwarding) are traversed in the
      opposite direction.  Symmetry can depend on the level of
      abstraction on which the path is defined or modeled.  If there are
      two parallel physical links between two nodes, modeling them as
      separate links may result in a flow using asymmetric paths, and
      modeling them as a single virtual link may result in symmetric
      paths, e.g., if the difference between the two physical links is
      irrelevant in a particular context.

   Path MTU:  The maximum size, in octets, of a packet or frame that can
      be transmitted without fragmentation.

   Transport Protocols available:  Whether a specific transport protocol
      can be used to establish a connection over a path or subpath,
      e.g., whether the path is QUIC-capable or MPTCP-capable, based on
      input such as policy, cached knowledge, or probing results.

   Protocol Features available:  Whether a specific protocol feature is
      available over a path or subpath, e.g., Explicit Congestion
      Notification (ECN) or TCP Fast Open.

   Some path properties express the performance of the transmission of a
   packet or flow over a link or subpath.  Such transmission performance
   properties can be observed or assessed, e.g., by endpoints or by path
   elements on the path, or they may be available as cost metrics, see
   [RFC9439].  Transmission performance properties may be made available
   in an aggregated form, such as averages or minimums.  Properties
   related to a path element that constitutes a single layer 2 domain
   are abstracted from the used physical- and link-layer technology,
   similar to [RFC8175].

   Link Capacity:  The link capacity is the maximum data rate at which
      data that was sent over a link can correctly be received at the
      node adjacent to the link.  This property is analogous to the link
      capacity defined in [RFC5136] and [RFC9097] but is not restricted
      to IP-layer traffic.

   Link Usage:  The link usage is the actual data rate at which data
      that was sent over a link is correctly received at the node
      adjacent to the link.  This property is analogous to the link
      usage defined in [RFC5136] and [RFC9097] but is not restricted to
      IP-layer traffic.

   One-Way Delay:  The one-way delay is the delay between a node sending
      a packet and another node on the same path receiving the packet.
      This property is analogous to the one-way delay defined in
      [RFC7679] but is not restricted to IP-layer traffic.

   One-Way Delay Variation:  The variation of the one-way delays within
      a flow.  This property is similar to the one-way delay variation
      defined in [RFC3393], but it is not restricted to IP-layer traffic
      and it is defined for packets on the same flow instead of packets
      sent between a source and destination IP address.

   One-Way Packet Loss:  Packets sent by a node but not received by
      another node on the same path after a certain time interval are
      considered lost.  This property is analogous to the one-way loss
      defined in [RFC7680] but is not restricted to IP-layer traffic.
      Metrics such as loss patterns [RFC3357] and loss episodes
      [RFC6534] can be expressed as aggregated properties.

5.  Security Considerations

   If entities are basing policy or path-selection decisions on path
   properties, they need to rely on the accuracy of path properties that
   other devices communicate to them.  In order to be able to trust such
   path properties, entities may need to establish a trust relationship
   or be able to independently verify the authenticity, integrity, and
   correctness of path properties received from another entity.

   Entities that reveal their target path properties to the network can
   negatively impact their own privacy, e.g., if the target property
   leaks personal information about a user, such as their identity or
   which (type of) application is used.  Such information could then
   allow network operators to block or reprioritize traffic for certain
   users and/or applications.  Conversely, if privacy-enhancing
   technologies, e.g., MASQUE proxies [RFC9298], are used on a path, the
   path may only be partially visible to any single entity.  This may
   diminish the usefulness of path-aware technologies over this path.

   The need for, and potential definition of, security- and privacy-
   related path properties, such as confidentiality and integrity
   protection of payloads, are the subject of ongoing discussion and
   research, for example, see [RFC9049] and [RFC9217].  As the
   discussion of such properties is not mature enough, they are out of
   scope for this document.  One aspect discussed in this context is
   that security-related properties are difficult to characterize since
   they are only meaningful with respect to a threat model that depends
   on the use case, application, environment, and other factors.
   Likewise, properties for trust relations between entities cannot be
   meaningfully defined without a concrete threat model, and defining a
   threat model is out of scope for this document.  Properties related
   to confidentiality, integrity, and trust seem to be orthogonal to the
   path terminology and path properties defined in this document, since
   they are tied to the communicating nodes and the protocols they use
   (e.g., client and server using HTTPS, or client and remote network
   node using a VPN service) as well as additional context, such as
   keying material and who has access to such a context.  In contrast,
   the path as defined in this document is typically oblivious to these
   aspects.  Intuitively, the path describes what function the network
   applies to packets, while confidentiality, integrity, and trust
   describe what function the communicating parties apply to packets.

6.  IANA Considerations

   This document has no IANA actions.

7.  Informative References

              Farrel, A., Drake, J., Rokui, R., Homma, S., Makhijani,
              K., Contreras, L. M., and J. Tantsura, "A Framework for
              Network Slices in Networks Built from IETF Technologies",
              Work in Progress, Internet-Draft, draft-ietf-teas-ietf-
              network-slices-24, 25 August 2023,

              Xu, X., Hegde, S., Talaulikar, K., Boucadair, M., and C.
              Jacquenet, "Performance-based BGP Routing Mechanism", Work
              in Progress, Internet-Draft, draft-ietf-idr-performance-
              routing-03, 21 December 2020,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, DOI 10.17487/RFC1930, March 1996,

   [RFC1940]  Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.
              Zappala, "Source Demand Routing: Packet Format and
              Forwarding Specification (Version 1)", RFC 1940,
              DOI 10.17487/RFC1940, May 1996,

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,

   [RFC3357]  Koodli, R. and R. Ravikanth, "One-way Loss Pattern Sample
              Metrics", RFC 3357, DOI 10.17487/RFC3357, August 2002,

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC5136]  Chimento, P. and J. Ishac, "Defining Network Capacity",
              RFC 5136, DOI 10.17487/RFC5136, February 2008,

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              DOI 10.17487/RFC5693, October 2009,

   [RFC6534]  Duffield, N., Morton, A., and J. Sommers, "Loss Episode
              Metrics for IP Performance Metrics (IPPM)", RFC 6534,
              DOI 10.17487/RFC6534, May 2012,

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,

   [RFC7676]  Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
              for Generic Routing Encapsulation (GRE)", RFC 7676,
              DOI 10.17487/RFC7676, October 2015,

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <https://www.rfc-editor.org/info/rfc7680>.

   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
              Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
              DOI 10.17487/RFC8175, June 2017,

   [RFC8548]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic Protection of TCP Streams
              (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,

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

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <https://www.rfc-editor.org/info/rfc8684>.

   [RFC8803]  Bonaventure, O., Ed., Boucadair, M., Ed., Gundavelli, S.,
              Seo, S., and B. Hesmans, "0-RTT TCP Convert Protocol",
              RFC 8803, DOI 10.17487/RFC8803, July 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,

   [RFC9097]  Morton, A., Geib, R., and L. Ciavattone, "Metrics and
              Methods for One-Way IP Capacity", RFC 9097,
              DOI 10.17487/RFC9097, November 2021,

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [RFC9217]  Trammell, B., "Current Open Questions in Path-Aware
              Networking", RFC 9217, DOI 10.17487/RFC9217, March 2022,

   [RFC9275]  Gao, K., Lee, Y., Randriamasy, S., Yang, Y., and J. Zhang,
              "An Extension for Application-Layer Traffic Optimization
              (ALTO): Path Vector", RFC 9275, DOI 10.17487/RFC9275,
              September 2022, <https://www.rfc-editor.org/info/rfc9275>.

   [RFC9298]  Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
              DOI 10.17487/RFC9298, August 2022,

   [RFC9439]  Wu, Q., Yang, Y., Lee, Y., Dhody, D., Randriamasy, S., and
              L. Contreras, "Application-Layer Traffic Optimization
              (ALTO) Performance Cost Metrics", RFC 9439,
              DOI 10.17487/RFC9439, August 2023,


   Thanks to the Path Aware Networking Research Group for the discussion
   and feedback.  Specifically, thanks to Mohamed Boucadair for the
   detailed review, various text suggestions, and shepherding; thanks to
   Brian Trammell for suggesting the flow definition; and thanks to Luis
   M. Contreras, Spencer Dawkins, Paul Hoffman, Jake Holland, Colin
   Perkins, Adrian Perrig, and Matthias Rost for the reviews, comments,
   and suggestions.  Many thanks to Dave Oran for his careful IRSG

Authors' Addresses

   Reese Enghardt
   Email: ietf@tenghardt.net

   Cyrill Krähenbühl
   ETH Zürich
   Email: cyrill.kraehenbuehl@inf.ethz.ch