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An Architecture for Transport Services

Document Type Active Internet-Draft (taps WG)
Authors Tommy Pauly , Brian Trammell , Anna Brunstrom , Gorry Fairhurst , Colin Perkins
Last updated 2023-02-03 (Latest revision 2022-10-20)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state Submitted to IESG for Publication
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Sep 2021
Submit document(s) to be published as Proposed Standard to the IESG specifying one or more methods to provide applications with the Transport Services identified by the WG
Document shepherd Michael Welzl
Shepherd write-up Show Last changed 2022-10-21
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TAPS Working Group                                         T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                        B. Trammell, Ed.
Expires: 23 April 2023                           Google Switzerland GmbH
                                                            A. Brunstrom
                                                     Karlstad University
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                              C. Perkins
                                                   University of Glasgow
                                                         20 October 2022

                 An Architecture for Transport Services


   This document describes an architecture for exposing transport
   protocol features to applications for network communication, a
   Transport Services system.  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 implementations can use
   multiple IP addresses, multiple protocols, and multiple paths, and
   provide multiple application streams.  This document further defines
   common terminology and concepts to be used in definitions of a
   Transport Service API and a Transport Services implementation.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 23 April 2023.

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Copyright Notice

   Copyright (c) 2022 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 (
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Specification of Requirements . . . . . . . . . . . . . .   5
   2.  API Model . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Event-Driven API  . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Data Transfer Using Messages  . . . . . . . . . . . . . .   8
     2.3.  Flexible Implementation . . . . . . . . . . . . . . . . .   9
   3.  API and Implementation Requirements . . . . . . . . . . . . .  10
     3.1.  Provide Common APIs for Common Features . . . . . . . . .  10
     3.2.  Allow Access to Specialized Features  . . . . . . . . . .  11
     3.3.  Select Equivalent Protocol Stacks . . . . . . . . . . . .  12
     3.4.  Maintain Interoperability . . . . . . . . . . . . . . . .  13
   4.  Transport Services Architecture and Concepts  . . . . . . . .  13
     4.1.  Transport Services API Concepts . . . . . . . . . . . . .  15
       4.1.1.  Endpoint Objects  . . . . . . . . . . . . . . . . . .  16
       4.1.2.  Connections and Related Objects . . . . . . . . . . .  16
       4.1.3.  Pre-Establishment . . . . . . . . . . . . . . . . . .  18
       4.1.4.  Establishment Actions . . . . . . . . . . . . . . . .  18
       4.1.5.  Data Transfer Objects and Actions . . . . . . . . . .  19
       4.1.6.  Event Handling  . . . . . . . . . . . . . . . . . . .  20
       4.1.7.  Termination Actions . . . . . . . . . . . . . . . . .  21
       4.1.8.  Connection Groups . . . . . . . . . . . . . . . . . .  21
     4.2.  Transport Services Implementation . . . . . . . . . . . .  22
       4.2.1.  Candidate Gathering . . . . . . . . . . . . . . . . .  23
       4.2.2.  Candidate Racing  . . . . . . . . . . . . . . . . . .  23
       4.2.3.  Separating Connection Contexts  . . . . . . . . . . .  24
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   6.  Security and Privacy Considerations . . . . . . . . . . . . .  25
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  26

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     8.2.  Informative References  . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   Many application programming interfaces (APIs) to perform transport
   networking have been deployed, perhaps the most widely known and
   imitated being the BSD Socket [POSIX] interface (Socket API).  The
   naming of objects and functions across these APIs is not consistent,
   and varies depending on the protocol being used.  For example,
   sending and receiving streams of data is conceptually the same for
   both an unencrypted Transmission Control Protocol (TCP) stream and
   operating on an encrypted Transport Layer Security (TLS) [RFC8446]
   stream 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 architecture is to provide a
   flexible and reusable architecture that provides a common interface
   for transport protocols.  As applications adopt this interface, they
   will benefit from a wide set of transport features that can evolve
   over time, and 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.  It 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).

   This document was developed in parallel with the specification of the
   Transport Services API [I-D.ietf-taps-interface] and implementation
   guidelines [I-D.ietf-taps-impl].  Although following the Transport
   Services architecture does not require that all APIs and
   implementations are identical, a common minimal set of features
   represented in a consistent fashion will enable applications to be
   easily ported from one system to another.

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1.1.  Background

   The Transport Services architecture 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 architecture 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 require application interaction and guidance (referred to in
   that document as Functional or Optimizing Features), or else can be
   handled automatically by a system implementing Transport Services
   (referred to as Automatable Features).  Among the identified
   Functional and Optimizing Features, some were common across all or
   nearly all transport protocols, while others could be seen as
   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 faster for
   applications that do not require messages to arrive in the order in
   which they were sent.  However, this functionality needs to be
   explicitly allowed by the application, since reordering messages
   would be undesirable in many cases.

1.2.  Overview

   This document describes the Transport Services architecture in three

   *  Section 2 describes how the API model of Transport Services
      architecture differs from traditional 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.

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   *  Section 4 presents a diagram showing the Transport Services
      architecture and defines the concepts that are used by both the
      API [I-D.ietf-taps-interface] and implementation guidelines
      [I-D.ietf-taps-impl].  The Preconnection allows applications to
      configure Connection Properties.

   *  Section 4 also presents how an abstract Connection is used to
      select a transport protocol instance such as TCP, UDP, or another
      transport.  The Connection represents an object that can be used
      to send and receive messages.

1.3.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  API Model

   The traditional model of using sockets for networking can be
   represented as follows:

   *  Applications create connections and transfer data using the Socket

   *  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 stub resolver.

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   |                    Application                      |
           |                 |                  |
     +------------+     +------------+    +--------------+
     |    stub    |     | Stream API |    | Datagram API |
     |  resolver  |     +------------+    +--------------+
     +------------+          |                  |
                       |    TCP                UDP       |
                       |    Kernel Networking Stack      |
   |               Network Layer Interface               |

                         Figure 1: Socket API Model

   The Transport Services architecture evolves this general model of
   interaction, to both modernize the API surface presented to
   applications by the transport layer and to enrich the capabilities of
   the implementation below the API.

   |                    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

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   The Transport Services API [I-D.ietf-taps-interface] defines the
   interface for an application to create Connections and transfer data.
   It combines interfaces for multiple interaction patterns into a
   unified whole.  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 [I-D.ietf-taps-impl] implements
   the transport layer protocols and other functions needed to send and
   receive data.  It is 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 Transport Services architecture
   and the architecture of the Socket API: the API of the Transport
   Services architecture is asynchronous and event-driven; it uses
   messages for representing data transfer to applications; and it
   describes how implementations can use multiple IP addresses, multiple
   protocols, multiple paths, and 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 generally uses 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 interaction using the Transport
   Services API is expected to be asynchronous.  The API is defined
   around an event-driven model (see Section 4.1.6) in order to model
   this asynchronous interaction, though other forms of asynchronous
   communication may be available to applications depending on the
   platform implementing the interface.

   For example, an application first issues a 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; a failure to
   send data results in an asynchronous error event.

   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.

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   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 represent 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.  Providing
   a message-based abstraction provides many benefits, such as:

   *  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 multi-streaming and
      pooled connections.

   Allowing applications to interact with messages is backwards-
   compatible with existing protocols and APIs because it does not
   change the wire format of any protocol.  Instead, it gives the
   protocol stack additional information to allow it to make better use
   of modern transport services, while simplifying the application's
   role in parsing data.  For protocols which natively use a streaming
   abstraction, framers (Section 4.1.5) bridge the gap between the two

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2.3.  Flexible Implementation

   The Socket API for protocols like TCP is generally limited to
   connecting to a single address over a single interface.  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

   *  to handle multiple candidate endpoints, protocols, and paths;

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

   *  to support multipath and multistreaming protocols;

   *  to 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 Section 4.2.1 and Section 4.2.2).  This
   requires applications to provide higher-level endpoints than IP
   addresses, such as hostnames and URLs, which are used by a Transport
   Services implementation for resolution, path selection, and racing.
   An implementation can further implement fallback mechanisms if
   connection establishment of one protocol fails or performance is
   detected 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) are cached in
   the Transport Services implementation.  Applications have control
   over whether this information is used for a specific establishment,
   in order to allow tradeoffs 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

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3.  API and Implementation Requirements

   A goal of the Transport Services 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 a careful
   consideration of how to expose the capabilities of protocols.  This
   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; it can use these protocols across
   multiple paths that could have different performance and functional
   characteristics; and it can 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 implementation to provide this
   functionality without causing incompatibility or introducing security

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].

   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 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, set on individual Messages.

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

   It is RECOMMENDED that the default values for Properties are 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

   Applications using the Transport Services API are REQUIRED to be
   robust to the automated selection provided by the Transport Services
   implementation.  This automated selection is constrained by the
   properties and preferences expressed by the application and requires
   applications to explictly set properties that define any necssary
   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.

   A specialized feature 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; these options 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 to use the User Timeout Option, communication would not
   fail when a protocol such as QUIC is selected.

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   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

3.3.  Select Equivalent Protocol Stacks

   A Transport Services implementation can select Protocol Stacks based
   on the Selection and Connection Properties communicated by the
   application, along with any security parameters.  If two different
   Protocol Stacks can be safely swapped, or raced in parallel (see
   Section 4.2.2), then they are considered to be "equivalent".
   Equivalent Protocol Stacks are defined as stacks that can provide the
   same Transport Properties and interface expectations as requested by
   the application.

   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

   *  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.

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   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 SHOULD only race Protocol Stacks where the transport
   security protocols within the stacks are identical.  A Transport
   Services implementation MUST NOT automatically fall back from secure
   protocols to insecure protocols, or 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 in case
   TLS 1.3 is not available.

3.4.  Maintain Interoperability

   It is important to note that neither the Transport Services API
   [I-D.ietf-taps-interface] nor the guidelines for the Transport
   Service implementation [I-D.ietf-taps-impl] 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 uses 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, independent of
   whether the initiator uses the Transport Services architecture or

   A Transport Services system 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.

4.  Transport Services Architecture and Concepts

   This section and the remainder of this document describe the
   architecture non-normatively.  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.

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

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

   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 the
   architecture 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)    +----------------------+
                 Network Layer Interface

       Figure 3: Concepts and Relationships in the Transport Services

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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:

   *  Pre-Establishment (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 Pre-
      Establishment can have a large impact on the rest of the
      interface: they modify how establishment occurs, they influence
      the expectations around data transfer, and they 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

   *  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 transport
      objects.  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 state is torn down in the transport.

   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
   to listen for incoming connections), while others are optional (e.g.,
   setting Connection and Message Properties in Pre-Establishment) or
   have been omitted for brevity and simplicity.

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     Pre-Establishment     :       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

4.1.1.  Endpoint Objects

   *  Endpoint: An Endpoint represents an identifier for one side of a
      transport connection.  Endpoints can be Local Endpoints or Remote
      Endpoints, and respectively represent an identity that the
      application uses for the source or destination of a connection.
      An Endpoint can be specified at various levels of abstraction.  An
      Endpoint at a higher level of abstraction (such as a hostname) can
      be resolved to more concrete identities (such as IP addresses).
      An endpoint may also represent a multicast group, in which case it
      selects a multicast transport for communication.

   *  Remote Endpoint: The Remote Endpoint 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: The Local Endpoint 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

   *  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 from

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      which that Connection will be established, the Remote Endpoint
      (Section 4.1.3) 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 it can be 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 optional if it is used to
      Initiate a Connection.  The Remote Endpoint is required in a
      Preconnection that used to Initiate a Connection, but is optional
      if it is used to Listen for incoming Connections.  The Local
      Endpoint and the Remote Endpoint 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 their requirements, prohibitions, and preferences and
      configure a Transport Services system.  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

      -  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

   *  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 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 multi-streaming transport protocol
      instance.  Connections can be created from a Preconnection or by a

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   *  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.  Pre-Establishment

   *  Selection Properties: The Selection Properties consist of the
      properties that an application can set to influence the selection
      of paths between the Local and Remote Endpoints, to influence the
      selection of transport protocols, or to 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 fast
      open support.

   *  Connection Properties: The 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 will 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

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   *  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, occurs in response to the Initiate call.

   *  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(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,
      occurs in response to the Rendezvous call.  As with Listeners, the
      set of local paths and endpoints is constrained by Selection
      Properties.  If successful, the Rendezvous call 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] will be performed.  However, if the
      set of Local Endpoints includes server reflexive candidates, such
      as those provided by STUN, a Rendezvous action will race
      candidates in the style of the ICE 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.

   *  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

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      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 signalled 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 action to transmit 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 operation is delivered back to the sending
      application in an Event (Section 4.1.6).

   *  Receive: An action that 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 to define how application-layer Messages are
      transmitted over a transport stack.  This is particularly relevant
      when using a protocol that otherwise presents unstructured
      streams, such as TCP.

4.1.6.  Event Handling

   The following categories of events can be delivered to an

   *  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.

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   *  Message Received: Delivers received Message content to the
      application, based on a Receive action.  This can include an error
      if the Receive action cannot be satisfied 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 an indication 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, is no longer willing to
      receive data, and that the protocol should signal this state to
      the Remote Endpoint if the transport protocol allows this.  (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.  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 a Close and also indicate that a Transport Services
      system should not attempt to deliver any outstanding data, and
      immediately 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 properties and
   caches.  A Connection Group represents state for managing Connections
   within a single application, and does not require end-to-end protocol
   signaling.  For multiplexing transport protocols, only Connections
   within the same Connection Group are allowed to be multiplexed

   When the API clones an existing Connection, this adds a 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.

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   For multiplexing transport protocols, only Connections within the
   same Connection Group are allowed to be multiplexed together.
   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 multi-streaming protocol instance.

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

4.2.  Transport Services Implementation

   This section defines the key concepts of the Transport Services

   *  Transport Service implementation: This 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.

   *  Transport Service system: This consists of the Transport Service
      implementation and the Transport Services API.

   *  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

   *  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 (a single transport
      protocol instance over IP), or it can be complex (multiple
      application protocol streams going through a single security and
      transport protocol, over IP; or, a multi-path transport protocol
      over multiple transport sub-flows).

   *  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).

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   *  Candidate Protocol Stack: One Protocol Stack that can be used by
      an application for a Connection, 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: Represents the input from an operating system or
      other global preferences that can constrain or influence how an
      implementation will gather candidate paths and Protocol Stacks
      (Section 4.2.1) and race the candidates during establishment
      (Section 4.2.2).  Specific aspects of the System Policy either
      apply to all Connections or only certain ones, 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.

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
      Endpoints 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.

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   *  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 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 Endpoints.

   *  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, 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 about re-using cached protocol state that 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.

   *  Privacy concerns about 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 a HTTP/2 or QUIC

5.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no actions for IANA.

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6.  Security and Privacy Considerations

   The Transport Services architecture does not recommend 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

   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

   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 tradeoffs between efficiency
   (through racing) and privacy (via information that might leak from
   the cache toward an on-path observer).  Some applications have native
   concepts (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
   another security protocol handshake that is not equivalent to TLS).

   A Transport Services system must not automatically fall back from
   secure protocols to insecure protocols, or 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.  The Transport Services API
   MAY allow applications to specify minimum versions that are allowed
   to be used by the Transport Services system.

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7.  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 of DFG -
   German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
   FE 570/4-1).

   This work has been supported by the UK Engineering and Physical
   Sciences Research Council under grant EP/R04144X/1.

   Thanks to Theresa Enghardt, Max Franke, Mirja Kuehlewind, Jonathan
   Lennox, and Michael Welzl for the discussions and feedback that
   helped shape the architecture described here.  Particular thanks is
   also due to Philipp S.  Tiesel and Christopher A.  Wood, who were
   both co-authors of this architecture specification as it progressed
   through the 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.

8.  References

8.1.  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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

8.2.  Informative References

              Brunstrom, A., Pauly, T., Enghardt, R., Tiesel, P. S., and
              M. Welzl, "Implementing Interfaces to Transport Services",
              Work in Progress, Internet-Draft, draft-ietf-taps-impl-13,
              31 August 2022, <

              Trammell, B., Welzl, M., Enghardt, R., Fairhurst, G.,
              K├╝hlewind, M., Perkins, C., Tiesel, P. S., and T. Pauly,

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              "An Abstract Application Layer Interface to Transport
              Services", Work in Progress, Internet-Draft, draft-ietf-
              taps-interface-17, 27 September 2022,

   [POSIX]    "IEEE Std. 1003.1-2008 Standard for Information Technology
              -- Portable Operating System Interface (POSIX).  Open
              group Technical Standard: Base Specifications, Issue 7",

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

   [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,

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

   [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,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [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,

   [RFC8923]  Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for End Systems", RFC 8923, DOI 10.17487/RFC8923,
              October 2020, <>.

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   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <>.

   [RFC9113]  Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,

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

Authors' Addresses

   Tommy Pauly (editor)
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America

   Brian Trammell (editor)
   Google Switzerland GmbH
   Gustav-Gull-Platz 1
   CH- 8004 Zurich

   Anna Brunstrom
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad

   Godred Fairhurst
   University of Aberdeen
   Fraser Noble Building
   Aberdeen, AB24 3UE

Pauly, et al.             Expires 23 April 2023                [Page 28]
Internet-Draft              TAPS Architecture               October 2022

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ
   United Kingdom

Pauly, et al.             Expires 23 April 2023                [Page 29]