Internet Research Task Force (IRTF) R. Enghardt

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

Abstract

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
(PANRG).

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/rfc9473.

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
Acknowledgments
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
Gbps).

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
properties.

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
[PERFORMANCE-ROUTING].

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
path.

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
transparent.

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
nodes.

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

[NETWORK-SLICES]
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,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
ietf-network-slices-24>.

[PERFORMANCE-ROUTING]
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,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
performance-routing-03>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[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,
<https://www.rfc-editor.org/info/rfc1930>.

[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,
<https://www.rfc-editor.org/info/rfc1940>.

[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<https://www.rfc-editor.org/info/rfc2784>.

[RFC3357] Koodli, R. and R. Ravikanth, "One-way Loss Pattern Sample
Metrics", RFC 3357, DOI 10.17487/RFC3357, August 2002,
<https://www.rfc-editor.org/info/rfc3357>.

[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
DOI 10.17487/RFC3393, November 2002,
<https://www.rfc-editor.org/info/rfc3393>.

[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,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC5136] Chimento, P. and J. Ishac, "Defining Network Capacity",
RFC 5136, DOI 10.17487/RFC5136, February 2008,
<https://www.rfc-editor.org/info/rfc5136>.

[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
DOI 10.17487/RFC5693, October 2009,
<https://www.rfc-editor.org/info/rfc5693>.

[RFC6534] Duffield, N., Morton, A., and J. Sommers, "Loss Episode
Metrics for IP Performance Metrics (IPPM)", RFC 6534,
DOI 10.17487/RFC6534, May 2012,
<https://www.rfc-editor.org/info/rfc6534>.

[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.

[RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
for Generic Routing Encapsulation (GRE)", RFC 7676,
DOI 10.17487/RFC7676, October 2015,
<https://www.rfc-editor.org/info/rfc7676>.

[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,
<https://www.rfc-editor.org/info/rfc8175>.

[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,
<https://www.rfc-editor.org/info/rfc8548>.

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

[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,
<https://www.rfc-editor.org/info/rfc8803>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC9049] Dawkins, S., Ed., "Path Aware Networking: Obstacles to
Deployment (A Bestiary of Roads Not Taken)", RFC 9049,
DOI 10.17487/RFC9049, June 2021,
<https://www.rfc-editor.org/info/rfc9049>.

[RFC9097] Morton, A., Geib, R., and L. Ciavattone, "Metrics and
Methods for One-Way IP Capacity", RFC 9097,
DOI 10.17487/RFC9097, November 2021,
<https://www.rfc-editor.org/info/rfc9097>.

[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[RFC9217] Trammell, B., "Current Open Questions in Path-Aware
Networking", RFC 9217, DOI 10.17487/RFC9217, March 2022,
<https://www.rfc-editor.org/info/rfc9217>.

[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,
<https://www.rfc-editor.org/info/rfc9298>.

[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,
<https://www.rfc-editor.org/info/rfc9439>.

Acknowledgments

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
review.

Authors' Addresses

Reese Enghardt
Netflix
Email: [email protected]

Cyrill Krähenbühl
ETH Zürich
Email: [email protected]