Architecture and Requirements for Transport Services
RFC 9621



Internet Engineering Task Force (IETF) T. Pauly, Ed.
Request for Comments: 9621 Apple Inc.
Category: Standards Track B. Trammell, Ed.
ISSN: 2070-1721 Google Switzerland GmbH
 A. Brunstrom
 Karlstad University
 G. Fairhurst
 University of Aberdeen
 C. S. Perkins
 University of Glasgow
 January 2025

 Architecture and Requirements for Transport Services

Abstract

 This document describes an architecture that exposes transport
 protocol features to applications for network communication. The
 Transport Services Application Programming Interface (API) is based
 on an asynchronous, event-driven interaction pattern. This API uses
 Messages for representing data transfer to applications and describes
 how a Transport Services Implementation can use multiple IP
 addresses, multiple protocols, and multiple paths and can provide
 multiple application streams. This document provides the
 architecture and requirements. It defines common terminology and
 concepts to be used in definitions of a Transport Services API and a
 Transport Services Implementation.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in 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/rfc9621.

Copyright Notice

 Copyright (c) 2025 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. Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1. Introduction
 1.1. Background
 1.2. Overview
 1.3. Specification of Requirements
 1.4. Glossary of Key Terms
 2. API Model
 2.1. Event-Driven API
 2.2. Data Transfer Using Messages
 2.3. Flexible Implementation
 2.4. Coexistence
 3. API and Implementation Requirements
 3.1. Provide Common APIs for Common Features
 3.2. Allow Access to Specialized Features
 3.3. Select Between Equivalent Protocol Stacks
 3.4. Maintain Interoperability
 3.5. Support Monitoring
 4. Transport Services Architecture and Concepts
 4.1. Transport Services API Concepts
 4.1.1. Endpoint Objects
 4.1.2. Connections and Related Objects
 4.1.3. Preestablishment
 4.1.4. Establishment Actions
 4.1.5. Data Transfer Objects and Actions
 4.1.6. Event Handling
 4.1.7. Termination Actions
 4.1.8. Connection Groups
 4.2. Transport Services Implementation
 4.2.1. Candidate Gathering
 4.2.2. Candidate Racing
 4.2.3. Separating Connection Contexts
 5. IANA Considerations
 6. Security and Privacy Considerations
 7. References
 7.1. Normative References
 7.2. Informative References
 Acknowledgements
 Authors' Addresses

1. Introduction

 Many Application Programming Interfaces (APIs) to provide transport
 interfaces to networks have been deployed, perhaps the most widely
 known and imitated being the Socket interface (Socket API) [POSIX].
 The naming of objects and functions across these APIs is not
 consistent and varies, depending on the protocol being used. For
 example, the concept of sending and receiving streams of data is the
 same for both an unencrypted Transmission Control Protocol (TCP)
 stream and operating on an encrypted Transport Layer Security (TLS)
 stream [RFC8446] over TCP, but applications cannot use the same
 socket send() and recv() calls on top of both kinds of connections.
 Similarly, terminology for the implementation of transport protocols
 varies based on the context of the protocols themselves: terms such
 as "flow", "stream", "message", and "connection" can take on many
 different meanings. This variety can lead to confusion when trying
 to understand the similarities and differences between protocols and
 how applications can use them effectively.

 The goal of the Transport Services System architecture is to provide
 a flexible and reusable system with a common interface for transport
 protocols. An application uses the Transport Services System through
 an abstract Connection (we use capitalization to distinguish these
 from the underlying connections of, for example, TCP). This provides
 flexible Connection establishment allowing an application to request
 or require a set of Properties.

 As applications adopt this interface, they will benefit from a wide
 set of transport features that can evolve over time and will ensure
 that the system providing the interface can optimize its behavior
 based on the application requirements and network conditions, without
 requiring changes to the applications. This flexibility enables
 faster deployment of new features and protocols.

 This architecture can also support applications by offering racing
 mechanisms (attempting multiple IP addresses, protocols, or network
 paths in parallel), which otherwise need to be implemented in each
 application separately (see Section 4.2.2). Racing selects one or
 more candidates, each with equivalent Protocol Stacks that are used
 to identify an optimal combination of a transport protocol instance
 such as TCP, UDP, or another transport, together with configuration
 of parameters and interfaces. A Connection represents an object
 that, once established, can be used to send and receive Messages. A
 Connection can also be created from another Connection, by cloning,
 and then forms a part of a Connection Group whose Connections share
 Properties.

 This document was developed in parallel with the specification of the
 Transport Services API [RFC9622] and implementation guidelines
 [RFC9623]. Although following the Transport Services Architecture
 does not require all APIs and implementations to be identical, a
 common minimal set of features represented in a consistent fashion
 will enable applications to be easily ported from one implementation
 of the Transport Services System to another.

1.1. Background

 The architecture of the Transport Services System is based on the
 survey of services provided by IETF transport protocols and
 congestion control mechanisms [RFC8095] and the distilled minimal set
 of the features offered by transport protocols [RFC8923]. These
 documents identified common features and patterns across all
 transport protocols developed thus far in the IETF.

 Since transport security is an increasingly relevant aspect of using
 transport protocols on the Internet, this document also considers the
 impact of transport security protocols on the feature set exposed by
 Transport Services [RFC8922].

 One of the key insights to come from identifying the minimal set of
 features provided by transport protocols [RFC8923] was that features
 either (1) require application interaction and guidance (referred to
 in that document as Functional or Optimizing Features) or (2) can be
 handled automatically by an implementation of the Transport Services
 System (referred to as Automatable Features). Among the identified
 Functional and Optimizing Features, some are common across all or
 nearly all transport protocols, while others present features that,
 if specified, would only be useful with a subset of protocols, but
 would not harm the functionality of other protocols. For example,
 some protocols can deliver messages more quickly for applications
 that do not require messages to arrive in the order in which they
 were sent. This functionality needs to be explicitly allowed by the
 application, since reordering messages would be undesirable in many
 cases.

1.2. Overview

 The following sections describe the Transport Services System:

 * Section 2 describes how the Transport Services API model differs
 from that of socket-based APIs. Specifically, it offers
 asynchronous event-driven interaction, the use of Messages for
 data transfer, and the flexibility to use different transport
 protocols and paths without requiring major changes to the
 application.

 * Section 3 explains the fundamental requirements for a Transport
 Services System. These principles are intended to make sure that
 transport protocols can continue to be enhanced and evolve without
 requiring significant changes by application developers.

 * Section 4 presents the Transport Services Implementation and
 defines the concepts that are used by the API [RFC9622] and
 described in the implementation guidelines [RFC9623]. This
 introduces the Preconnection, which allows applications to
 configure Connection Properties.

1.3. Specification of Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

1.4. Glossary of Key Terms

 This subsection provides a glossary of key terms related to the
 Transport Services Architecture. It provides a short description of
 key terms that are defined later in this document.

 Application: An entity that uses the transport layer for end-to-end
 delivery of data across the network [RFC8095].

 Cached State: The state and history that the Transport Services
 Implementation keeps for each set of the associated Endpoints that
 have been used previously.

 Candidate Path: One path that is available to an application and
 conforms to the Selection Properties and System Policy during
 racing.

 Candidate Protocol Stack: One Protocol Stack that can be used by an
 application for a Connection during racing.

 Client: The peer responsible for initiating a Connection.

 Clone: A Connection that was created from another Connection and
 that forms a part of a Connection Group.

 Connection: Shared state of two or more Endpoints that persists
 across Messages that are transmitted and received between these
 Endpoints [RFC8303]. When this document and other Transport
 Services documents use the capitalized "Connection" term, it
 refers to a Connection object that is being offered by the
 Transport Services System, as opposed to more generic uses of the
 word "connection".

 Connection Context: A set of stored Properties across Connections,
 such as cached protocol state, cached path state, and heuristics,
 which can include one or more Connection Groups.

 Connection Group: A set of Connections that share Properties and
 caches.

 Connection Property: A Transport Property that controls per-
 Connection behavior of a Transport Services Implementation.

 Endpoint: An entity that communicates with one or more other
 Endpoints using a transport protocol.

 Endpoint Identifier: An identifier that specifies one side of a
 Connection (local or remote), such as a hostname or URL.

 Equivalent Protocol Stacks: Protocol Stacks that can be safely
 swapped or raced in parallel during establishment of a Connection.

 Event: A primitive that is invoked by an Endpoint [RFC8303].

 Framer: A data translation layer that can be added to a Connection
 to define how application-layer Messages are transmitted over a
 Protocol Stack.

 Local Endpoint: The local Endpoint.

 Local Endpoint Identifier: A representation of the application's
 identifier for itself that it uses for a Connection.

 Message: A unit of data that can be transferred between two
 Endpoints over a Connection.

 Message Property: A property that can be used to specify details
 about Message transmission or obtain details about the
 transmission after receiving a Message.

 Parameter: A value passed between an application and a transport
 protocol by a primitive [RFC8303].

 Path: A representation of an available set of Properties that a
 Local Endpoint can use to communicate with a Remote Endpoint.

 Peer: An Endpoint application party to a Connection.

 Preconnection: An object that represents a Connection that has not
 yet been established.

 Preference: A preference for prohibiting, avoiding, ignoring,
 preferring, or requiring a specific transport feature.

 Primitive: A function call that is used to locally communicate
 between an application and an Endpoint, which is related to one or
 more transport features [RFC8303].

 Protocol Instance: A single instance of one protocol, including any
 state necessary to establish connectivity or send and receive
 Messages.

 Protocol Stack: A set of protocol instances that are used together
 to establish connectivity or send and receive Messages.

 Racing: The attempt to select between multiple Protocol Stacks based
 on the Selection and Connection Properties communicated by the
 application, along with any Security Parameters.

 Remote Endpoint: The peer that a Local Endpoint can communicate with
 when a Connection is established.

 Remote Endpoint Identifier: A representation of the application's
 identifier for a peer that can participate in establishing a
 Connection.

 Rendezvous: The action of establishing a peer-to-peer Connection
 with a Remote Endpoint.

 Security Parameters: Parameters that define an application's
 requirements for authentication and encryption on a Connection.

 Selection Property: A Transport Property that can be set to
 influence the selection of paths between the Local and Remote
 Endpoints.

 Server: The peer responsible for responding to a Connection
 initiation.

 Socket: The combination of a destination IP address and a
 destination port number [RFC8303].

 System Policy: The input from an operating system or other global
 preferences that can constrain or influence how an implementation
 will gather Candidate Paths and Candidate Protocol Stacks and race
 the candidates during establishment of a Connection.

 Transport Feature: A specific end-to-end feature that the transport
 layer provides to an application.

 Transport Property: A property of a transport protocol and the
 services it provides [RFC8095].

 Transport Service: A set of transport features, not associated with
 any given framing protocol, that provides a complete service to an
 application.

 Transport Services API: The abstract interface [RFC9622] to a
 Transport Services Implementation [RFC9623].

 Transport Services Implementation: All objects and protocol
 instances used internally to a system or library to implement the
 functionality needed to provide a transport service across a
 network, as required by the abstract interface.

 Transport Services System: The Transport Services Implementation and
 the Transport Services API.

2. API Model

 The model of using sockets can be represented as follows (see
 Figure 1):

 * Applications create connections and transfer data using the Socket
 API.

 * The Socket API provides the interface to the implementations of
 TCP and UDP (typically implemented in the system's kernel).

 * TCP and UDP in the kernel send and receive data over the available
 network-layer interfaces.

 * Sockets are bound directly to transport-layer and network-layer
 addresses, obtained via a separate resolution step, usually
 performed by a system-provided DNS stub resolver.

 +-----------------------------------------------------+
 | Application |
 +-----------------------------------------------------+
 | | |
 +------------+ +------------+ +--------------+
 | DNS Stub | | Stream API | | Datagram API |
 | Resolver | +------------+ +--------------+
 +------------+ | |
 +---------------------------------+
 | TCP UDP |
 | Kernel Networking Stack |
 +---------------------------------+
 |
 +-----------------------------------------------------+
 | Network-Layer Interface |
 +-----------------------------------------------------+

 Figure 1: Socket API Model

 The architecture of the Transport Services System is an evolution of
 this general model of interaction. It both modernizes the API
 presented to applications by the transport layer and enriches the
 capabilities of the Transport Services Implementation below this API.

 The Transport Services API [RFC9622] defines the interface for an
 application to create Connections and transfer data. It combines
 interfaces for multiple interaction patterns into a unified whole
 (see Figure 2). This offers generic functions and also the protocol-
 specific mappings for TCP, UDP, UDP-Lite, and other protocol layers.
 These mappings are extensible. Future documents could define similar
 mappings for new layers and for other transport protocols, such as
 QUIC [RFC9000].

 +-----------------------------------------------------+
 | Application |
 +-----------------------------------------------------+
 |
 +-----------------------------------------------------+
 | Transport Services API |
 +-----------------------------------------------------+
 |
 +-----------------------------------------------------+
 | Transport Services Implementation |
 | (Using DNS, UDP, TCP, SCTP, DCCP, TLS, QUIC, etc.) |
 +-----------------------------------------------------+
 |
 +-----------------------------------------------------+
 | Network-Layer Interface |
 +-----------------------------------------------------+

 Figure 2: Transport Services API Model

 By combining name resolution with Connection establishment and data
 transfer in a single API, it allows for more flexible implementations
 to provide path and transport protocol agility on the application's
 behalf.

 The Transport Services Implementation [RFC9623] is the component of
 the Transport Services System that implements the transport-layer
 protocols and other functions needed to send and receive data. It is
 responsible for mapping the API to a specific available transport
 Protocol Stack and managing the available network interfaces and
 paths.

 There are key differences between the architecture of the Transport
 Services System and the architecture of the Socket API. The API of
 the Transport Services System:

 * is asynchronous and event-driven;

 * uses Messages for representing data transfer to applications;

 * describes how a Transport Services Implementation can resolve
 Endpoint Identifiers to use multiple IP addresses, multiple
 protocols, and multiple paths and to provide multiple application
 streams.

2.1. Event-Driven API

 Originally, the Socket API presented a blocking interface for
 establishing connections and transferring data. However, most modern
 applications interact with the network asynchronously. Emulation of
 an asynchronous interface using the Socket API can use a try-and-fail
 model: if the application wants to read but data has not yet been
 received from the peer, the call to read will fail. The application
 then waits and can try again later.

 In contrast to the Socket API, all interactions using the Transport
 Services API are expected to be asynchronous. The API is defined
 around an event-driven model (see Section 4.1.6), which models this
 asynchronous interaction. Other forms of asynchronous communication
 could also be available to applications, depending on the platform
 implementing the interface.

 For example, when an application that uses the Transport Services API
 wants to receive data, it issues an asynchronous call to receive new
 data from the Connection. When delivered data becomes available,
 this data is delivered to the application using asynchronous events
 that contain the data. Error handling is also asynchronous,
 resulting in asynchronous error events.

 This API also delivers events regarding the lifetime of a connection
 and changes in the available network links, which were not previously
 made explicit in the Socket API.

 Using asynchronous events allows for a more natural interaction model
 when establishing connections and transferring data. Events in time
 more closely reflect the nature of interactions over networks, as
 opposed to how the Socket API represents network resources as file
 system objects that may be temporarily unavailable.

 Separate from events, callbacks are also provided for asynchronous
 interactions with the Transport Services API that are not directly
 related to events on the network or network interfaces.

2.2. Data Transfer Using Messages

 The Socket API provides a message interface for datagram protocols
 like UDP but provides an unstructured stream abstraction for TCP.
 While TCP has the ability to send and receive data as a byte-stream,
 most applications need to interpret structure within this byte-
 stream. For example, HTTP/1.1 uses character delimiters to segment
 messages over a byte-stream [RFC9112]; TLS record headers carry a
 version, content type, and length [RFC8446]; and HTTP/2 uses frames
 to segment its headers and bodies [RFC9113].

 The Transport Services API represents data as Messages, so that it
 more closely matches the way applications use the network. A
 Message-based abstraction provides many benefits, such as:

 * providing additional information to the Protocol Stack;

 * the ability to associate deadlines with Messages, for applications
 that care about timing;

 * the ability to control reliability, which Messages to retransmit
 when there is packet loss, and how best to make use of the data
 that arrived;

 * the ability to automatically assign Messages and connections to
 underlying transport connections to utilize multistreaming and
 create Pooled Connections.

 Allowing applications to interact with Messages is backward-
 compatible with existing protocols and APIs because it does not
 change the wire format of any protocol. Instead, it provides the
 Protocol Stack with additional information to allow it to make better
 use of modern transport protocols, while simplifying the
 application's role in parsing data. For protocols that inherently
 use a streaming abstraction, Framers (Section 4.1.5) bridge the gap
 between the two abstractions.

2.3. Flexible Implementation

 The Socket API for protocols like TCP is generally limited to
 connecting to a single address over a single interface (IP source
 address). It also presents a single stream to the application.
 Software layers built upon this API often propagate this limitation
 of a single-address single-stream model. The Transport Services
 Architecture is designed to:

 * handle multiple candidate endpoints, protocols, and paths;

 * support candidate protocol racing to select the most optimal stack
 in each situation;

 * support multipath and multistreaming protocols;

 * provide state caching and application control over it.

 A Transport Services Implementation is intended to be flexible at
 Connection establishment time, considering many different options and
 trying to select the most optimal combinations by racing them and
 measuring the results (see Sections 4.2.1 and 4.2.2). This requires
 applications to specify identifiers for the Local and Remote Endpoint
 that are at a higher level than IP addresses, such as a hostname or
 URL. These identifiers are used by a Transport Services
 Implementation for resolution, path selection, and racing. An
 implementation can further implement fallback mechanisms if
 connection establishment for one protocol fails or performance is
 determined to be unsatisfactory.

 Information used in Connection establishment (e.g., cryptographic
 resumption tokens, information about usability of certain protocols
 on the path, results of racing in previous connections) is cached in
 the Transport Services Implementation. Applications have control
 over whether this information is used for a specific establishment,
 in order to allow trade-offs between efficiency and linkability.

 Flexibility after Connection establishment is also important.
 Transport protocols that can migrate between multiple network-layer
 interfaces need to be able to process and react to interface changes.
 Protocols that support multiple application-layer streams need to
 support initiating and receiving new streams using existing
 connections.

2.4. Coexistence

 While the architecture of the Transport Services System is designed
 as an enhanced replacement for the Socket API, it need not replace it
 entirely on a system or platform; indeed, coexistence has been
 recommended for incremental deployability [RFC8170]. The
 architecture is therefore designed such that it can run alongside
 (or, indeed, on top of) an existing Socket API implementation; only
 applications built on the Transport Services API are managed by the
 system's Transport Services Implementation.

3. API and Implementation Requirements

 One goal of the architecture is to redefine the interface between
 applications and transports in a way that allows the transport layer
 to evolve and improve without fundamentally changing the contract
 with the application. This requires careful consideration of how to
 expose the capabilities of protocols. The architecture also
 encompasses system policies that can influence and inform how
 transport protocols use a network path or interface.

 There are several ways the Transport Services System can offer
 flexibility to an application. It can:

 * provide access to transport protocols and protocol features;

 * use these protocols across multiple paths that could have
 different performance and functional characteristics;

 * communicate with different remote systems to optimize performance,
 robustness to failure, or some other metric.

 Beyond these, if the Transport Services API remains the same over
 time, new protocols and features can be added to the Transport
 Services Implementation without requiring changes in applications for
 adoption. Similarly, this can provide a common basis for utilizing
 information about a network path or interface, enabling evolution
 below the transport layer.

 The normative requirements described in this section allow Transport
 Services APIs and Transport Services Implementations to provide this
 functionality without causing incompatibility or introducing security
 vulnerabilities.

3.1. Provide Common APIs for Common Features

 Any functionality that is common across multiple transport protocols
 SHOULD be made accessible through a unified set of calls using the
 Transport Services API. As a baseline, any Transport Services API
 SHOULD allow access to the minimal set of features offered by
 transport protocols [RFC8923]. If that minimal set is updated or
 expanded in the future, the Transport Services API ought to be
 extended to match.

 An application can specify constraints and preferences for the
 protocols, features, and network interfaces it will use via
 Properties. Properties are used by an application to declare its
 preferences for how the transport service should operate at each
 stage in the lifetime of a connection. Transport Properties are
 subdivided into the following:

 * Selection Properties, which specify which paths and Protocol
 Stacks can be used and are preferred by the application;

 * Connection Properties, which inform decisions made during
 Connection establishment and fine-tune the established connection;
 and

 * Message Properties, which can be set on individual Messages.

 It is RECOMMENDED that the Transport Services API offer Properties
 that are common to multiple transport protocols. This enables a
 Transport Services System to appropriately select between protocols
 that offer equivalent features. Similarly, it is RECOMMENDED that
 the Properties offered by the Transport Services API be applicable to
 a variety of network-layer interfaces and paths, to permit racing of
 different network paths without affecting the applications using the
 API. Each is expected to have a default value.

 It is RECOMMENDED that the default values for Properties be selected
 to ensure correctness for the widest set of applications, while
 providing the widest set of options for selection. For example,
 since both applications that require reliability and those that do
 not require reliability can function correctly when a protocol
 provides reliability, reliability ought to be enabled by default. As
 another example, the default value for a Property regarding the
 selection of network interfaces ought to permit as many interfaces as
 possible.

 Applications using the Transport Services API need to be designed to
 be robust to the automated selection provided by the Transport
 Services System. This automated selection is constrained by the
 preferences expressed by the application and requires applications to
 explicitly set Properties that define any necessary constraints on
 protocol, path, and interface selection.

3.2. Allow Access to Specialized Features

 There are applications that will need to control fine-grained details
 of transport protocols to optimize their behavior and ensure
 compatibility with remote systems. It is therefore RECOMMENDED that
 the Transport Services API and the Transport Services Implementation
 permit more specialized protocol features to be used.

 Some specialized features could be needed by an application only when
 using a specific protocol and not when using others. For example, if
 an application is using TCP, it could require control over the User
 Timeout Option for TCP [RFC5482]. Such features would not take
 effect for other transport protocols. In such cases, the API ought
 to expose the features in such a way that they take effect when a
 particular protocol is selected but do not imply that only that
 protocol could be used. For example, if the API allows an
 application to specify a preference for using the User Timeout
 Option, communication would not fail when a protocol such as UDP is
 selected.

 Other specialized features, however, can also be strictly required by
 an application and thus further constrain the set of protocols that
 can be used. For example, if an application requires support for
 automatic handover or failover for a connection, only Protocol Stacks
 that provide this feature are eligible to be used, e.g., Protocol
 Stacks that include a multipath protocol or a protocol that supports
 connection migration. A Transport Services API needs to allow
 applications to define such requirements and constrain the options
 available to a Transport Services Implementation. Since such options
 are not part of the core/common features, it will generally be simple
 for an application to modify its set of constraints and change the
 set of allowable protocol features without changing the core
 implementation.

 To control these specialized features, the application can declare
 its preference: whether the presence of a specific feature is
 prohibited, should be avoided, can be ignored, is preferred, or is
 required in the preestablishment phase. An implementation of a
 Transport Services API would honor this preference and allow the
 application to query the availability of each specialized feature
 after successful establishment.

3.3. Select Between Equivalent Protocol Stacks

 A Transport Services Implementation can attempt to use, and select
 between, multiple Protocol Stacks based on the Selection and
 Connection Properties communicated by the application, along with any
 Security Parameters. The implementation can only attempt to use
 multiple Protocol Stacks when they are "equivalent", which means that
 the stacks can provide the same Transport Properties and interface
 expectations as requested by the application. Equivalent Protocol
 Stacks can be safely swapped or raced in parallel (see Section 4.2.2)
 during Connection establishment.

 The following two examples show non-equivalent Protocol Stacks:

 * If the application requires preservation of Message boundaries, a
 Protocol Stack that runs UDP as the top-level interface to the
 application is not equivalent to a Protocol Stack that runs TCP as
 the top-level interface. A UDP stack would allow an application
 to read out Message boundaries based on datagrams sent from the
 remote system, whereas TCP does not preserve Message boundaries on
 its own but needs a framing protocol on top to determine Message
 boundaries.

 * If the application specifies that it requires reliable
 transmission of data, then a Protocol Stack using UDP without any
 reliability layer on top would not be allowed to replace a
 Protocol Stack using TCP.

 The following example shows equivalent Protocol Stacks:

 * If the application does not require reliable transmission of data,
 then a Protocol Stack that adds reliability could be regarded as
 an equivalent Protocol Stack as long as providing this would not
 conflict with any other application-requested Properties.

 A Transport Services Implementation can race different security
 protocols, e.g., if the System Policy is explicitly configured to
 consider them equivalent. A Transport Services Implementation SHOULD
 only race Protocol Stacks where the transport security protocols
 within the stacks are identical. To ensure that security protocols
 are not incorrectly swapped, a Transport Services Implementation MUST
 only select Protocol Stacks that meet application requirements
 [RFC8922]. A Transport Services Implementation MUST NOT
 automatically fall back from secure protocols to insecure protocols
 or fall back to weaker versions of secure protocols. A Transport
 Services Implementation MAY allow applications to explicitly specify
 which versions of a protocol ought to be permitted, e.g., to allow a
 minimum version of TLS 1.2 if TLS 1.3 is not available.

 A Transport Services Implementation MAY specify security Properties
 relating to how the system operates (e.g., requirements,
 prohibitions, and preferences for the use of DNS Security Extensions
 (DNSSEC) or DNS over HTTPS (DoH)).

3.4. Maintain Interoperability

 It is important to note that neither the Transport Services API
 [RFC9622] nor the guidelines for implementation of the Transport
 Services System [RFC9623] define new protocols or protocol
 capabilities that affect what is communicated across the network. A
 Transport Services System MUST NOT require that a peer on the other
 side of a connection use the same API or implementation. A Transport
 Services Implementation acting as a connection initiator is able to
 communicate with any existing Endpoint that implements the transport
 protocol(s) and all the required Properties selected. Similarly, a
 Transport Services Implementation acting as a Listener can receive
 connections for any protocol that is supported from an existing
 initiator that implements the protocol, independently of whether or
 not the initiator uses the Transport Services System.

 A Transport Services Implementation makes decisions that select
 protocols and interfaces. In normal use, a given version of a
 Transport Services System SHOULD result in consistent protocol and
 interface selection decisions for the same network conditions, given
 the same set of Properties. This is intended to provide predictable
 outcomes to the application using the API.

3.5. Support Monitoring

 The Transport Services API increases the layer of abstraction for
 applications, and it enables greater automation below the API. Such
 increased abstraction comes at the cost of increased complexity when
 application programmers, users, or system administrators try to
 understand why any issues and failures may be happening. A Transport
 Services System should therefore offer monitoring functions that
 provide relevant debug and diagnostics information. For example,
 such monitoring functions could indicate the protocol(s) in use, the
 number of open connections per protocol, and any statistics that
 these protocols may offer.

4. Transport Services Architecture and Concepts

 This section describes the architecture non-normatively and explains
 the operation of a Transport Services Implementation. The concepts
 defined in this document are intended primarily for use in the
 documents and specifications that describe the Transport Services
 System. This includes the architecture, the Transport Services API,
 and the associated Transport Services Implementation. While the
 specific terminology can be used in some implementations, it is
 expected that there will remain a variety of terms used by running
 code.

 The architecture divides the concepts for the Transport Services
 System into two categories:

 1. API concepts, which are intended to be exposed to applications;
 and

 2. System-implementation concepts, which are intended to be
 internally used by a Transport Services Implementation.

 The following diagram summarizes the top-level concepts in a
 Transport Services System and how they relate to one another.

 +-----------------------------------------------------+
 | Application |
 +-+----------------+------^-------+--------^----------+
 | | | | |
 pre- | data | events
 establishment | transfer | |
 | establishment | termination |
 | | | | |
 | +--v------v-------v+ |
 +-v-------------+ Connection(s) +-------+----------+
 | Transport +--------+---------+ |
 | Services | |
 | API | +-------------+ |
 +------------------------+--+ Framer(s) |-----------+
 | +-------------+
 +------------------------|----------------------------+
 | Transport | |
 | System | +-----------------+ |
 | Implementation | | Cached | |
 | | | State | |
 | (Candidate Gathering) | +-----------------+ |
 | | |
 | (Candidate Racing) | +-----------------+ |
 | | | System | |
 | | | Policy | |
 | +----------v-----+ +-----------------+ |
 | | Protocol | |
 +-------------+ Stack(s) +----------------------+
 +-------+--------+
 V
 +-----------------------------------------------------+
 | Network-Layer Interface |
 +-----------------------------------------------------+

 Figure 3: Concepts and Relationships in the Architecture of the
 Transport Services System

 The Transport Services Implementation includes the Cached State and
 System Policy.

 The System Policy provides input from an operating system or other
 global preferences that can constrain or influence how an
 implementation will gather Candidate Paths and Protocol Stacks and
 race the candidates when establishing a Connection. As the details
 of System Policy configuration and enforcement are largely dependent
 on the platform and implementation and do not affect application-
 level interoperability, the Transport Services API [RFC9622] does not
 specify an interface for reading or writing System Policy.

 The Cached State is the state and history that the Transport Services
 Implementation keeps for each set of associated Endpoints that have
 previously been used. An application ought to explicitly request any
 required or preferred Properties via the Transport Services API.

4.1. Transport Services API Concepts

 Fundamentally, a Transport Services API needs to provide Connection
 objects (Section 4.1.2) that allow applications to establish
 communication and then send and receive data. These could be exposed
 as handles or referenced objects, depending on the chosen programming
 language.

 Beyond the Connection objects, there are several high-level groups of
 actions that any Transport Services API needs to provide:

 * Preestablishment (Section 4.1.3) encompasses the Properties that
 an application can pass to describe its intent, requirements,
 prohibitions, and preferences for its networking operations.
 These Properties apply to multiple transport protocols, unless
 otherwise specified. Properties specified during preestablishment
 can have a large impact on the rest of the interface: they modify
 how establishment occurs, influence the expectations around data
 transfer, and determine the set of events that will be supported.

 * Establishment (Section 4.1.4) focuses on the actions that an
 application takes on the Connection objects to prepare for data
 transfer.

 * Data transfer (Section 4.1.5) consists of how an application
 represents the data to be sent and received, the functions
 required to send and receive that data, and how the application is
 notified of the status of its data transfer.

 * Event handling (Section 4.1.6) defines categories of notifications
 that an application can receive during the lifetime of a
 Connection. Events also provide opportunities for the application
 to interact with the underlying transport by querying state or
 updating maintenance options.

 * Termination (Section 4.1.7) focuses on the methods by which data
 transmission is stopped and connection state is torn down.

 The diagram below provides a high-level view of the actions and
 events during the lifetime of a Connection object. Note that some
 actions are alternatives (e.g., whether to initiate a connection or
 listen for incoming connections), while others are optional (e.g.,
 setting Connection and Message Properties in preestablishment) or
 have been omitted for brevity and simplicity.

 Preestablishment : Established : Termination
 ----------------- : ----------- : -----------
 : :
 +-- Local Endpoint : Message :
 +-- Remote Endpoint : Receive() | :
 +-- Transport Properties : Send() | :
 +-- Security Parameters : | :
 | : | :
 | InitiateWithSend() | Close() :
 | +---------------+ Initiate() +-----+------+ Abort() :
 +---+ Preconnection |------------->| Connection |-----------> Closed
 +---------------+ Rendezvous() +------------+ :
 Listen() | : | | :
 | : | v :
 v : | Connection :
 +----------+ : | Ready :
 | Listener |----------------------+ :
 +----------+ Connection Received :
 : :

 Figure 4: The Lifetime of a Connection Object

 In this diagram, the lifetime of a Connection object is divided into
 three phases: preestablishment, the Established state, and
 termination of a Connection.

 Preestablishment is based around a Preconnection object containing
 various sub-objects that describe the Properties and parameters of
 desired Connections (Local and Remote Endpoints, Transport
 Properties, and Security Parameters). A Preconnection can be used to
 start listening for inbound connections -- in which case a Listener
 object is created -- or can be used to establish a new connection
 directly using Initiate (for outbound connections) or Rendezvous (for
 peer-to-peer connections).

 Once a Connection is in the Established state, an application can
 send and receive Message objects and can receive state updates.

 Closing or aborting a Connection, either locally or from the peer,
 can terminate a Connection.

4.1.1. Endpoint Objects

 An Endpoint Identifier specifies one side of a transport connection.
 Endpoints can be Local Endpoints or Remote Endpoints, and the
 Endpoint Identifiers can respectively represent an identity that the
 application uses for the source or destination of a connection. An
 Endpoint Identifier can be specified at various levels of
 abstraction. An Endpoint Identifier at a higher level of abstraction
 (such as a hostname) can be resolved to more concrete identities
 (such as IP addresses). A Remote Endpoint Identifier can also
 represent a multicast group or anycast address. In the case of
 multicast, a multicast transport will be selected for communication.

 Remote Endpoint Identifier: The Remote Endpoint Identifier
 represents the application's identifier for a peer that can
 participate in a transport connection, for example, the
 combination of a DNS name for the peer and a service name/port.

 Local Endpoint Identifier: The Local Endpoint Identifier represents
 the application's identifier for itself that it uses for transport
 connections, for example, a local IP address and port.

4.1.2. Connections and Related Objects

 Connection: A Connection object represents one or more active
 transport protocol instances that can send and/or receive Messages
 between Local and Remote Endpoints. It is an abstraction that
 represents the communication. The Connection object holds state
 pertaining to the underlying transport protocol instances and any
 ongoing data transfers. For example, an active Connection can
 represent a connection-oriented protocol such as TCP, or it can
 represent a fully specified 5-tuple for a connectionless protocol
 such as UDP, where the Connection remains an abstraction at the
 endpoints. It can also represent a pool of transport protocol
 instances, e.g., a set of TCP and QUIC connections to equivalent
 endpoints, or a stream of a multistreaming transport protocol
 instance. Connections can be created from a Preconnection or by a
 Listener.

 Preconnection: A Preconnection object is a representation of a
 Connection that has not yet been established. It has state that
 describes parameters of the Connection: the Local Endpoint
 Identifier from which that Connection will be established, the
 Remote Endpoint Identifier to which it will connect, and Transport
 Properties that influence the paths and protocols a Connection
 will use. A Preconnection can be either fully specified
 (representing a single possible Connection) or partially specified
 (representing a family of possible Connections). The Local
 Endpoint (Section 4.1.3) is required for a Preconnection used to
 Listen for incoming Connections but is optional if it is used to
 Initiate a Connection. The Remote Endpoint Identifier is required
 in a Preconnection that is used to Initiate a Connection but is
 optional if it is used to Listen for incoming Connections. The
 Local Endpoint Identifier and the Remote Endpoint Identifier are
 both required if a peer-to-peer Rendezvous is to occur based on
 the Preconnection.

 Transport Properties: Transport Properties allow the application to
 express requirements, prohibitions, and preferences and configure
 a Transport Services Implementation. There are three kinds of
 Transport Properties:

 Selection Properties (Section 4.1.3): Selection Properties can
 only be specified on a Preconnection.

 Connection Properties (Section 4.1.3): Connection Properties can
 be specified on a Preconnection and changed on the Connection.

 Message Properties (Section 4.1.5): Message Properties can be
 specified as defaults on a Preconnection or a Connection and
 can also be specified during data transfer to affect specific
 Messages.

 Listener: A Listener object accepts incoming transport protocol
 connections from Remote Endpoints and generates corresponding
 Connection objects. It is created from a Preconnection object
 that specifies the type of incoming Connections it will accept.

4.1.3. Preestablishment

 Selection Properties: Selection Properties consist of the Properties
 that an application can set to influence the selection of paths
 between the Local and Remote Endpoints, influence the selection of
 transport protocols, or configure the behavior of generic
 transport protocol features. These Properties can take the form
 of requirements, prohibitions, or preferences. Examples of
 Properties that influence path selection include the interface
 type (such as a Wi-Fi connection or a Cellular LTE connection),
 requirements around the largest Message that can be sent, or
 preferences for throughput and latency. Examples of Properties
 that influence protocol selection and configuration of transport
 protocol features include reliability, multipath support, and
 support for TCP Fast Open.

 Connection Properties: Connection Properties are used to configure
 protocol-specific options and control per-connection behavior of a
 Transport Services Implementation; for example, a protocol-
 specific Connection Property can express that if TCP is used, the
 implementation ought to use the User Timeout Option. Note that
 the presence of such a property does not require that a specific
 protocol be used. In general, these Properties do not explicitly
 determine the selection of paths or protocols but can be used by
 an implementation during Connection establishment. Connection
 Properties are specified on a Preconnection prior to Connection
 establishment and can be modified on the Connection later.
 Changes made to Connection Properties after Connection
 establishment take effect on a best-effort basis.

 Security Parameters: Security Parameters define an application's
 requirements for authentication and encryption on a Connection.
 They are used by transport security protocols (such as those
 described in [RFC8922]) to establish secure Connections. Examples
 of parameters that can be set include local identities, private
 keys, supported cryptographic algorithms, and requirements for
 validating trust of remote identities. Security Parameters are
 primarily associated with a Preconnection object, but Properties
 related to identities can be associated directly with Endpoints.

4.1.4. Establishment Actions

 Initiate: The primary action that an application can take to create
 a Connection to a Remote Endpoint and prepare any required local
 or remote state to enable the transmission of Messages. For some
 protocols, this will initiate a client-to-server-style handshake;
 for other protocols, this will just establish local state (e.g.,
 with connectionless protocols such as UDP). The process of
 identifying options for connecting, such as resolution of the
 Remote Endpoint Identifier, occurs in response to calling
 Initiate.

 Listen: Enables a Listener to accept incoming connections. The
 Listener will then create Connection objects as incoming
 connections are accepted (Section 4.1.6). Listeners by default
 register with multiple paths, protocols, and Local Endpoints,
 unless constrained by Selection Properties and/or the specified
 Local Endpoint Identifier(s). Connections can be accepted on any
 of the available paths or endpoints.

 Rendezvous: The action of establishing a peer-to-peer connection
 with a Remote Endpoint. It simultaneously attempts to initiate a
 connection to a Remote Endpoint while listening for an incoming
 connection from that Endpoint. The process of identifying options
 for the connection, such as resolution of the Remote Endpoint
 Identifier(s), occurs in response to calling Rendezvous. As with
 Listeners, the set of local paths and endpoints is constrained by
 Selection Properties. If successful, calling Rendezvous generates
 and asynchronously returns a Connection object to represent the
 established peer-to-peer connection. The processes by which
 connections are initiated during a Rendezvous action will depend
 on the set of Local and Remote Endpoints configured on the
 Preconnection. For example, if the Local and Remote Endpoints are
 TCP host candidates, then a TCP simultaneous open [RFC9293] might
 be performed. However, if the set of Local Endpoints includes
 server-reflexive candidates, such as those provided by STUN
 (Session Traversal Utilities for NAT) [RFC8489], a Rendezvous
 action will race candidates in the style of the ICE (Interactive
 Connectivity Establishment) algorithm [RFC8445] to perform NAT
 binding discovery and initiate a peer-to-peer connection.

4.1.5. Data Transfer Objects and Actions

 Message: A Message object is a unit of data that can be represented
 as bytes that can be transferred between two endpoints over a
 transport connection. The bytes within a Message are assumed to
 be ordered. If an application does not care about the order in
 which a peer receives two distinct spans of bytes, those spans of
 bytes are considered independent Messages. Messages are sent in
 the payload of IP packets. One packet can carry one or more
 Messages or parts of a Message.

 Message Properties: Message Properties are used to specify details
 about Message transmission. They can be specified directly on
 individual Messages or can be set on a Preconnection or Connection
 as defaults. These Properties might only apply to how a Message
 is sent (such as how the transport will treat prioritization and
 reliability) but can also include Properties that specific
 protocols encode and communicate to the Remote Endpoint. When
 receiving Messages, Message Properties can contain information
 about the received Message, such as metadata generated at the
 receiver and information signaled by the Remote Endpoint. For
 example, a Message can be marked with a Message Property
 indicating that it is the final Message on a Connection.

 Send: The Send action transmits a Message over a Connection to the
 Remote Endpoint. The interface to Send can accept Message
 Properties specific to how the Message content is to be sent. The
 status of the Send action is delivered back to the sending
 application in an event (Section 4.1.6).

 Receive: The Receive action indicates that the application is ready
 to asynchronously accept a Message over a Connection from a Remote
 Endpoint, while the Message content itself will be delivered in an
 event (Section 4.1.6). The interface to Receive can include
 Message Properties specific to the Message that is to be delivered
 to the application.

 Framer: A Framer is a data translation layer that can be added to a
 Connection. Framers allow extending a Connection's Protocol Stack
 to define how to encapsulate or encode outbound Messages and how
 to decapsulate or decode inbound data into Messages. In this way,
 Message boundaries can be preserved when using a Connection
 object, even with a protocol that otherwise presents unstructured
 streams, such as TCP. This is designed based on the fact that
 many of the current application protocols evolved over TCP, which
 does not provide Message boundary preservation, and since many of
 these protocols require Message boundaries to function, each
 application-layer protocol has defined its own framing. For
 example, when an HTTP application sends and receives HTTP Messages
 over a byte-stream transport, it must parse the boundaries of HTTP
 Messages from the stream of bytes.

4.1.6. Event Handling

 The following categories of events can be delivered to an
 application:

 Connection Ready: Signals to an application that a given Connection
 is ready to send and/or receive Messages. If the Connection
 relies on handshakes to establish state between peers, then it is
 assumed that these steps have been taken.

 Connection Closed: Signals to an application that a given Connection
 is no longer usable for sending or receiving Messages. The event
 delivers a reason or error to the application that describes the
 nature of the termination.

 Connection Received: Signals to an application that a given Listener
 has received a Connection.

 Message Received: Delivers received Message content to the
 application, based on a Receive action. To allow an application
 to limit the occurrence of such events, each call to Receive will
 be paired with a single Receive event. This can include an error
 if the Receive action cannot be satisfied, e.g., due to the
 Connection being closed.

 Message Sent: Notifies the application of the status of its Send
 action. This might indicate a failure if the Message cannot be
 sent or might indicate that the Message has been processed by the
 Transport Services System.

 Path Properties Changed: Notifies the application that a Property of
 the Connection has changed that might influence how and where data
 is sent and/or received.

4.1.7. Termination Actions

 Close: The action an application takes on a Connection to indicate
 that it no longer intends to send data or is no longer willing to
 receive data. The protocol should signal this state to the Remote
 Endpoint if the transport protocol permits it. (Note that this is
 distinct from the concept of "half-closing" a bidirectional
 connection, such as when a FIN is sent in one direction of a TCP
 connection [RFC9293]. The end of a stream can also be indicated
 using Message Properties when sending.)

 Abort: The action the application takes on a Connection to indicate
 that the Transport Services System should not attempt to deliver
 any outstanding data and that it should immediately close and drop
 the connection. This is intended for immediate, usually abnormal,
 termination of a connection.

4.1.8. Connection Groups

 A Connection Group is a set of Connections that shares Connection
 Properties and Cached State generated by protocols. A Connection
 Group represents state for managing Connections within a single
 application and does not require end-to-end protocol signaling. For
 transport protocols that support multiplexing, only Connections
 within the same Connection Group are allowed to be multiplexed
 together.

 The API allows a Connection to be created from another Connection.
 This adds the new Connection to the Connection Group. A change to
 one of the Connection Properties on any Connection in the Connection
 Group automatically changes the Connection Property for all others.
 All Connections in a Connection Group share the same set of
 Connection Properties except for the Connection Priority. These
 Connection Properties are said to be entangled.

 Passive Connections can also be added to a Connection Group, e.g.,
 when a Listener receives a new Connection that is just a new stream
 of an already-active multistreaming protocol instance.

 While Connection Groups are managed by the Transport Services
 Implementation, an application can define different Connection
 Contexts for different Connection Groups to explicitly control
 caching boundaries, as discussed in Section 4.2.3.

4.2. Transport Services Implementation

 This section defines the key architectural concepts for the Transport
 Services Implementation within the Transport Services System.

 The Transport Services System consists of the Transport Services
 Implementation and the Transport Services API. The Transport
 Services Implementation consists of all objects and protocol
 instances used internally to a system or library to implement the
 functionality needed to provide a transport service across a network,
 as required by the abstract interface.

 Path: Represents an available set of Properties that a Local
 Endpoint can use to communicate with a Remote Endpoint, such as
 routes, addresses, and physical and virtual network interfaces.

 Protocol Instance: A single instance of one protocol, including any
 state necessary to establish connectivity or send and receive
 Messages.

 Protocol Stack: A set of protocol instances (including relevant
 application, security, transport, or Internet protocols) that are
 used together to establish connectivity or send and receive
 Messages. A single stack can be simple (e.g., one application
 stream carried over TCP running over IP) or complex (e.g,.
 multiple application streams carried over a multipath transport
 protocol using multiple subflows over IP).

 Candidate Path: One path that is available to an application and
 conforms to the Selection Properties and System Policy, of which
 there can be several. Candidate Paths are identified during the
 gathering phase (Section 4.2.1) and can be used during the racing
 phase (Section 4.2.2).

 Candidate Protocol Stack: One Protocol Stack that can be used by an
 application for a connection, for which there can be several
 candidates. Candidate Protocol Stacks are identified during the
 gathering phase (Section 4.2.1) and are started during the racing
 phase (Section 4.2.2).

 System Policy: The input from an operating system or other global
 preferences that can constrain or influence how an implementation
 will gather Candidate Paths and Candidate Protocol Stacks
 (Section 4.2.1) and race the candidates during establishment
 (Section 4.2.2). Specific aspects of the System Policy apply to
 either all Connections or only certain Connections, depending on
 the runtime context and Properties of the Connection.

 Cached State: The state and history that the implementation keeps
 for each set of associated Endpoints that have been used
 previously. This can include DNS results, TLS session state,
 previous success and quality of transport protocols over certain
 paths, as well as other information. This caching does not imply
 that the same decisions are necessarily made for subsequent
 connections; rather, it means that Cached State is used by a
 Transport Services Implementation to inform functions such as
 choosing the candidates to be raced, selecting appropriate
 transport parameters, etc. An application SHOULD NOT rely on
 specific caching behavior; instead, it ought to explicitly request
 any required or preferred Properties via the Transport Services
 API.

4.2.1. Candidate Gathering

 Candidate Path Selection: Candidate Path Selection represents the
 act of choosing one or more paths that are available to use based
 on the Selection Properties and any available Local and Remote
 Endpoint Identifiers provided by the application, as well as the
 policies and heuristics of a Transport Services Implementation.

 Candidate Protocol Selection: Candidate Protocol Selection
 represents the act of choosing one or more sets of Protocol Stacks
 that are available to use based on the Transport Properties
 provided by the application, and the heuristics or policies within
 the Transport Services Implementation.

4.2.2. Candidate Racing

 Connection establishment attempts for a set of candidates may be
 performed simultaneously, synchronously, serially, or using some
 combination of all of these. We refer to this process as racing,
 borrowing terminology from Happy Eyeballs [RFC8305].

 Protocol Option Racing: Protocol Option Racing is the act of
 attempting to establish, or scheduling attempts to establish,
 multiple Protocol Stacks that differ based on the composition of
 protocols or the options used for protocols.

 Path Racing: Path Racing is the act of attempting to establish, or
 scheduling attempts to establish, multiple Protocol Stacks that
 differ based on a selection from the available paths. Since
 different paths will have distinct configurations (see [RFC7556])
 for local addresses and DNS servers, attempts across different
 paths will perform separate DNS resolution steps, which can lead
 to further racing of the resolved Remote Endpoint Identifiers.

 Remote Endpoint Racing: Remote Endpoint Racing is the act of
 attempting to establish, or scheduling attempts to establish,
 multiple Protocol Stacks that differ based on the specific
 representation of the Remote Endpoint Identifier, such as a
 particular IP address that was resolved from a DNS hostname.

4.2.3. Separating Connection Contexts

 A Transport Services Implementation can by default share stored
 Properties across Connections within an application, such as cached
 protocol state, cached path state, and heuristics. This provides
 efficiency and convenience for the application, since the Transport
 Services System can automatically optimize behavior.

 The Transport Services API can allow applications to explicitly
 define Connection Contexts that force separation of Cached State and
 Protocol Stacks. For example, a web browser application could use
 Connection Contexts with separate caches when implementing different
 tabs. Possible reasons to isolate Connections using separate
 Connection Contexts include privacy concerns regarding:

 * reusing cached protocol state, as this can lead to linkability.
 Sensitive state could include TLS session state [RFC8446] and HTTP
 cookies [RFC6265]. These concerns could be addressed using
 Connection Contexts with separate caches, such as for different
 browser tabs.

 * allowing Connections to multiplex together, which can tell a
 Remote Endpoint that all of the Connections are coming from the
 same application. Using Connection Contexts avoids the
 Connections being multiplexed in an HTTP/2 or QUIC stream.

5. IANA Considerations

 This document has no IANA actions.

6. Security and Privacy Considerations

 The Transport Services System does not recommend the use of specific
 security protocols or algorithms. Its goal is to offer ease of use
 for existing protocols by providing a generic security-related
 interface. Each provided interface translates to an existing
 protocol-specific interface provided by supported security protocols.
 For example, trust verification callbacks are common parts of TLS
 APIs; a Transport Services API exposes similar functionality
 [RFC8922].

 As described above in Section 3.3, if a Transport Services
 Implementation races between two different Protocol Stacks, both need
 to use the same security protocols and options. However, a Transport
 Services Implementation can race different security protocols, e.g.,
 if the application explicitly specifies that it considers them
 equivalent.

 The application controls whether information from previous racing
 attempts or other information about past communications that was
 cached by the Transport Services System is used during establishment.
 This allows applications to make trade-offs between efficiency
 (through racing) and privacy (via information that might leak from
 the cache toward an on-path observer). Some applications have
 features (e.g., "incognito mode") that align with this functionality.

 Applications need to ensure that they use security APIs
 appropriately. In cases where applications use an interface to
 provide sensitive keying material, e.g., access to private keys or
 copies of pre-shared keys (PSKs), key use needs to be validated and
 scoped to the intended protocols and roles. For example, if an
 application provides a certificate to only be used as client
 authentication for outbound TLS and QUIC connections, the Transport
 Services System MUST NOT use this automatically in other contexts
 (such as server authentication for inbound connections or in other
 security protocol handshakes that are not equivalent to TLS).

 A Transport Services System MUST NOT automatically fall back from
 secure protocols to insecure protocols or fall back to weaker
 versions of secure protocols (see Section 3.3). For example, if an
 application requests a specific version of TLS but the desired
 version of TLS is not available, its connection will fail. As
 described in Section 3.3, the Transport Services API can allow
 applications to specify minimum versions that are allowed to be used
 by the Transport Services System.

7. References

7.1. Normative References

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/info/rfc2119>.

 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2. Informative References

 [POSIX] "IEEE/Open Group Standard for Information Technology -
 Portable Operating System Interface (POSIX(TM)) Base
 Specifications, Issue 8", IEEE Std 1003.1-2024,
 DOI 10.1109/IEEESTD.2024.10555529, 2024,
 <https://ieeexplore.ieee.org/document/10555529>.

 [RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
 RFC 5482, DOI 10.17487/RFC5482, March 2009,
 <https://www.rfc-editor.org/info/rfc5482>.

 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
 DOI 10.17487/RFC6265, April 2011,
 <https://www.rfc-editor.org/info/rfc6265>.

 [RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
 Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
 <https://www.rfc-editor.org/info/rfc7556>.

 [RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
 Ed., "Services Provided by IETF Transport Protocols and
 Congestion Control Mechanisms", RFC 8095,
 DOI 10.17487/RFC8095, March 2017,
 <https://www.rfc-editor.org/info/rfc8095>.

 [RFC8170] Thaler, D., Ed., "Planning for Protocol Adoption and
 Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
 May 2017, <https://www.rfc-editor.org/info/rfc8170>.

 [RFC8303] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
 Transport Features Provided by IETF Transport Protocols",
 RFC 8303, DOI 10.17487/RFC8303, February 2018,
 <https://www.rfc-editor.org/info/rfc8303>.

 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
 Better Connectivity Using Concurrency", RFC 8305,
 DOI 10.17487/RFC8305, December 2017,
 <https://www.rfc-editor.org/info/rfc8305>.

 [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
 Connectivity Establishment (ICE): A Protocol for Network
 Address Translator (NAT) Traversal", RFC 8445,
 DOI 10.17487/RFC8445, July 2018,
 <https://www.rfc-editor.org/info/rfc8445>.

 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
 <https://www.rfc-editor.org/info/rfc8446>.

 [RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
 D., Mahy, R., and P. Matthews, "Session Traversal
 Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
 February 2020, <https://www.rfc-editor.org/info/rfc8489>.

 [RFC8922] Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
 Wood, "A Survey of the Interaction between Security
 Protocols and Transport Services", RFC 8922,
 DOI 10.17487/RFC8922, October 2020,
 <https://www.rfc-editor.org/info/rfc8922>.

 [RFC8923] Welzl, M. and S. Gjessing, "A Minimal Set of Transport
 Services for End Systems", RFC 8923, DOI 10.17487/RFC8923,
 October 2020, <https://www.rfc-editor.org/info/rfc8923>.

 [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>.

 [RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
 Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
 June 2022, <https://www.rfc-editor.org/info/rfc9112>.

 [RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
 DOI 10.17487/RFC9113, June 2022,
 <https://www.rfc-editor.org/info/rfc9113>.

 [RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
 STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
 <https://www.rfc-editor.org/info/rfc9293>.

 [RFC9622] Trammell, B., Ed., Welzl, M., Ed., Enghardt, R.,
 Fairhurst, G., Kühlewind, M., Perkins, C. S., Tiesel,
 P.S., and T. Pauly, "An Abstract Application Programming
 Interface (API) for Transport Services", RFC 9622,
 DOI 10.17487/RFC9622, January 2025,
 <https://www.rfc-editor.org/info/rfc9622>.

 [RFC9623] Brunstrom, A., Ed., Pauly, T., Ed., Enghardt, R., Tiesel,
 P.S., and M. Welzl, "Implementing Interfaces to Transport
 Services", RFC 9623, DOI 10.17487/RFC9623, January 2025,
 <https://www.rfc-editor.org/info/rfc9623>.

Acknowledgements

 This work has received funding from the European Union's Horizon 2020
 research and innovation programme under grant agreements No. 644334
 (NEAT), No. 688421 (MAMI), and No. 815178 (5GENESIS).

 This work has been supported by:

 * Leibniz Prize project funds from the DFG - German Research
 Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ FE 570/4-1).

 * the UK Engineering and Physical Sciences Research Council under
 grant EP/R04144X/1.

 Thanks to Reese Enghardt, Max Franke, Mirja Kühlewind, Jonathan
 Lennox, and Michael Welzl for the discussions and feedback that
 helped shape the architecture of the system described here.
 Particular thanks are also due to Philipp S. Tiesel and Christopher
 A. Wood, who were both coauthors of this specification as it
 progressed through the Transport Services (TAPS) Working Group.
 Thanks as well to Stuart Cheshire, Josh Graessley, David Schinazi,
 and Eric Kinnear for their implementation and design efforts,
 including Happy Eyeballs, that heavily influenced this work.

Authors' Addresses

 Tommy Pauly (editor)
 Apple Inc.
 One Apple Park Way
 Cupertino, CA 95014
 United States of America
 Email: tpauly@apple.com

 Brian Trammell (editor)
 Google Switzerland GmbH
 Gustav-Gull-Platz 1
 CH-8004 Zurich
 Switzerland
 Email: ietf@trammell.ch

 Anna Brunstrom
 Karlstad University
 Universitetsgatan 2
 651 88 Karlstad
 Sweden
 Email: anna.brunstrom@kau.se

 Godred Fairhurst
 University of Aberdeen
 Fraser Noble Building
 Aberdeen, AB24 3UE
 United Kingdom
 Email: gorry@erg.abdn.ac.uk
 URI: https://erg.abdn.ac.uk/

 Colin S. Perkins
 University of Glasgow
 School of Computing Science
 Glasgow G12 8QQ
 United Kingdom
 Email: csp@csperkins.org