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          13 14                                                         
TAPS Working Group                                         T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                        B. Trammell, Ed.
Expires: 1 November 2021                         Google Switzerland GmbH
                                                            A. Brunstrom
                                                     Karlstad University
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                              C. Perkins
                                                   University of Glasgow
                                                               P. Tiesel
                                                                  SAP SE
                                                               C.A. Wood
                                                           30 April 2021

                 An Architecture for Transport Services


   This document describes an architecture for exposing transport
   protocol features to applications for network communication, the
   Transport Services architecture.  The Transport Services Application
   Programming Interface (API) is based on an asynchronous, event-driven
   interaction pattern.  It uses messages for representing data transfer
   to applications, and it 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
   Transport Services APIs and implementations.

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 https://datatracker.ietf.org/drafts/current/.

   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."

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   This Internet-Draft will expire on 1 November 2021.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified 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  . . . . . . . . . . . . . .   7
     2.3.  Flexibile Implementation  . . . . . . . . . . . . . . . .   8
   3.  API and Implementation Requirements . . . . . . . . . . . . .   9
     3.1.  Provide Common APIs for Common Features . . . . . . . . .   9
     3.2.  Allow Access to Specialized Features  . . . . . . . . . .  10
     3.3.  Select Equivalent Protocol Stacks . . . . . . . . . . . .  11
     3.4.  Maintain Interoperability . . . . . . . . . . . . . . . .  12
   4.  Transport Services Architecture and Concepts  . . . . . . . .  12
     4.1.  Transport Services API Concepts . . . . . . . . . . . . .  13
       4.1.1.  Endpoint Objects  . . . . . . . . . . . . . . . . . .  15
       4.1.2.  Connections and Related Objects . . . . . . . . . . .  15
       4.1.3.  Pre-Establishment . . . . . . . . . . . . . . . . . .  16
       4.1.4.  Establishment Actions . . . . . . . . . . . . . . . .  17
       4.1.5.  Data Transfer Objects and Actions . . . . . . . . . .  18
       4.1.6.  Event Handling  . . . . . . . . . . . . . . . . . . .  19
       4.1.7.  Termination Actions . . . . . . . . . . . . . . . . .  20
       4.1.8.  Connection Groups . . . . . . . . . . . . . . . . . .  20
     4.2.  Transport Services Implementation Concepts  . . . . . . .  21
       4.2.1.  Candidate Gathering . . . . . . . . . . . . . . . . .  22
       4.2.2.  Candidate Racing  . . . . . . . . . . . . . . . . . .  22
       4.2.3.  Separating Connection Contexts  . . . . . . . . . . .  23
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24

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   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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
   common, flexible, and reusable 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 and fallback mechanisms, which
   otherwise need to be implemented in each application separately.

   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
      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

   *  Section 3 explains the fundamental requirements for a Transport
      Services API.  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 the Transport Services architecture diagram and
      defines the concepts that are used by both the API and
      implementation documents.  The Preconnection allows applications
      to configure Connection Properties, and 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.

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

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                         Figure 1: Socket API Model

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

   |                    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

   The Transport Services API [I-D.ietf-taps-interface] defines the
   mechanism for an application to create network connections and
   transfer data.  The implementation [I-D.ietf-taps-impl] is
   responsible for mapping the API to the various available transport
   protocols 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 Transport
   Services API 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.

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2.1.  Event-Driven API

   Originally, sockets 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 sockets 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 sockets, all interaction with a Transport Services
   system is expected to be asynchronous, and use an event-driven model
   (see Section 4.1.6).  For example, if the application wants to read,
   its call to read will not complete immediately, but will deliver an
   event containing the received data once it is available.  Error
   handling is also asynchronous; a failure to send results in an
   asynchronous send error as an event.

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

   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 sockets represent network resources as file system
   objects that may be temporarily unavailable.

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

2.2.  Data Transfer Using Messages

   Sockets provide a message interface for datagram protocols like UDP,
   but provide an unstructured stream abstraction for TCP.  While TCP
   does indeed provide the ability to send and receive data as streams,
   most applications need to interpret structure within these streams.
   For example, HTTP/1.1 uses character delimiters to segment messages
   over a stream [RFC7230]; TLS record headers carry a version, content
   type, and length [RFC8446]; and HTTP/2 uses frames to segment its
   headers and bodies [RFC7540].

   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:

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

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

   *  the ability to manage dependencies between messages, when the
      Transport Services system could decide to not deliver a message,
      either following packet loss or because it has missed a deadline.
      In particular, this can avoid (re-)sending data that relies on a
      previous transmission that was never received.

   *  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

2.3.  Flexibile Implementation

   Sockets, for protocols like TCP, are generally limited to connecting
   to a single address over a single interface.  They also present a
   single stream to the application.  Software layers built upon sockets
   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; and support
   multipath and multistreaming protocols.

   Transport Services implementations are meant to be flexible at
   connection establishment time, considering many different options and
   trying to select the most optimal combinations (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.  Transport services implementations can
   further implement fallback mechanisms if connection establishment of
   one protocol fails or performance is detected to be unsatisfactory.

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   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

3.  API and Implementation Requirements

   The 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.

   There are several degrees in which a Transport Services system is
   intended to offer flexibility to an application: it can provide
   access to multiple sets of 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 API for the
   system remains the same over time, new protocols and features can be
   added to the system's implementation without requiring changes in
   applications for adoption.

   The normative requirements described here allow Transport Services
   APIs and Implementations to provide this functionality without
   causing incompatibility or introducing security vulnerabilities.  The
   rest of this document describes the architecture non-normatively.

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 Transport Services
   API calls.  As a baseline, any Transport Services API MUST allow
   access to the minimal set of features offered by transport protocols

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   An application can specify constraints and preferences for the
   protocols, features, and network interfaces it will use via
   Properties.  A Transport Services API SHOULD offer Properties that
   are common to multiple transport protocols, which enables the system
   to appropriately select between protocols that offer equivalent
   features.  Similarly, a Transport Services API SHOULD offer
   Properties that are applicable to a variety of network layer
   interfaces and paths, which permits racing of different network paths
   without affecting the applications using the system.  Each Property
   is expected to have a default value.

   The default values for Properties SHOULD 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 a Transport Services system interface are REQUIRED
   to be robust to the automated selection provided by the system, where
   the automated selection is constrained by the requirements and
   preferences expressed by the application.

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.  A Transport Services system
   therefore SHOULD permit more specialized protocol features to be

   A specialized feature could be required 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.

   Other specialized features, however, can be strictly required by an
   application and thus 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

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   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 system's
   options.  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.

3.3.  Select Equivalent Protocol Stacks

   A Transport Services implementation can select Protocol Stacks based
   on the Properties communicated by the application.  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.

   To ensure that security protocols are not incorrectly swapped,
   Transport Services systems MUST only select Protocol Stacks that meet
   application requirements ([RFC8922]).  Systems SHOULD only race
   Protocol Stacks where the transport security protocols within the
   stacks are identical.  Transport Services systems MUST NOT
   automatically fall back from secure protocols to insecure protocols,

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   or to weaker versions of secure protocols.  A Transport Services
   system MAY allow applications to explicitly specify that fallback to
   a specific other version of a protocol is allowed, e.g., to allow
   fallback to TLS 1.2 if 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 Implementation document
   [I-D.ietf-taps-impl] define new protocols or protocol capabilities
   that affect what is communicated across the network.  Use of 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 system acting as a connection initiator is able to
   communicate with any existing system that implements the transport
   protocol(s) and all the required properties selected by the Transport
   Services system.  Similarly, a Transport Services system acting as a
   listener can receive connections for any protocol that is supported
   by the system from existing initiators that implement the protocol,
   independent of whether the initiator uses a Transport Services system
   or not.

   In normal use, 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

4.  Transport Services Architecture and Concepts

   The concepts defined in this document are intended primarily for use
   in the documents and specifications that describe the Transport
   Services architecture and API.  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 Transport Services into two

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

   2.  System-implementation concepts, which are intended to be
       internally used when building systems that implement Transport

   The following diagram summarizes the top-level concepts in the
   architecture and how they relate to one another.

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     |                    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

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 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.

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      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
      which 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, and
      an Endpoint at a higher level of abstraction (such as a hostname)
      can be resolved to more concrete identities (such as IP

   *  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
      potential Connection.  It has state that describes parameters of a
      Connection that might exist in the future: the Local Endpoint from
      which that Connection will be established, the Remote Endpoint

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      (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 fully specified such that it represents a
      single possible Connection, or it can be partially specified such
      that it represents a family of possible Connections.  The Local
      Endpoint (Section 4.1.3) is required if the Preconnection is used
      to Listen for incoming Connections.  The Local Endpoint is
      optional if it is used to Initiate Connections.  The Remote
      Endpoint is required in the Preconnection that is used to Initiate
      Connections.  The Remote Endpoint 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 the Transport Services system.  There are three kinds of
      Transport Properties:

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

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

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

   *  Connection: A Connection object represents one or more active
      transport protocol instances that can send and/or receive Messages
      between local and remote systems.  It holds state pertaining to
      the underlying transport protocol instances and any ongoing data
      transfers.  This represents, for example, an active Connection in
      a connection-oriented protocol such as TCP, or a fully-specified
      5-tuple for a connectionless protocol such as UDP.  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 Listener.

   *  Listener: A Listener object accepts incoming transport protocol
      connections from remote systems 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

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   *  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 systems, 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 the Transport Services system; 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

   *  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.

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   *  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 [RFC0793] 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 systems
      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.

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   *  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 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 if the
      peer sent a TCP FIN.

   *  Send: The action to transmit a Message over a Connection to the
      remote system.  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
      system, 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 protocol.  This is particularly
      relevant for protocols that otherwise present 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.

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   *  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.  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
      protocol stack.

   *  Path Properties Changed: Notifies the application that some
      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 system 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.  Indicating the end of a stream in
      the Transport Services architecture is possible using Message
      Properties when sending.)

   *  Abort: The action the application takes on a Connection to
      indicate a Close and also indicate that the Transport Services
      system should not attempt to deliver any outstanding data.  This
      is intended for immediate termination of a connection, without
      cleaning up state.

4.1.8.  Connection Groups

   A Connection Group is a set of Connections that shares properties and
   caches.  For multiplexing transport protocols, only Connections
   within the same Connection Group are allowed to be multiplexed
   together.  While Connection Groups are managed by the transport
   system, an application can define Connection Contexts to control
   caching boundaries, as discussed in Section 4.2.3.

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4.2.  Transport Services Implementation Concepts

   This section defines the set of objects 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

   *  Path: Represents an available set of properties that a local
      system can use to communicate with a remote system, 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).

   *  Candidate Protocol Stack: One Protocol Stack that can be used by
      an application for a connection, of which there can be several.
      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.

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   *  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 system.

   *  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 system.

4.2.2.  Candidate Racing

   Connection establishment attempts for a set of candidates may be
   performed simultaneously, synchronously, serially, or 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 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.

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4.2.3.  Separating Connection Contexts

   By default, stored properties of the implementation, such as cached
   protocol state, cached path state, and heuristics, may be shared
   (e.g. across multiple connections in an application).  This provides
   efficiency and convenience for the application, since the Transport
   Services implementation can automatically optimize behavior.

   There are several reasons, however, that an application might want to
   explicitly isolate some Connections.  These reasons include:

   *  Privacy concerns about re-using cached protocol state that can
      lead to linkability.  Sensitive state may include TLS session
      state [RFC8446] and HTTP cookies [RFC6265].

   *  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 (for example, when Connections
      are multiplexed HTTP/2 or QUIC streams).

   *  Performance concerns about Connections introducing head-of-line
      blocking due to multiplexing or needing to share state on a single

   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 might use
   Connection Contexts with separate caches for different tabs in the
   browser to decrease linkability.

5.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no actions for IANA.

6.  Security 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.  Transport Services APIs will expose similar functionality

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

   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.  For
   example, applications ought not to use PSK material created for the
   Encapsulating Security Protocol (ESP, part of IPsec) [RFC4303] with
   QUIC, and applications ought not to use private keys intended for
   server authentication as keys for client authentication.

   Moreover, Transport Services systems 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.  Applications are thus
   responsible for implementing security protocol fallback or version
   fallback by creating multiple Transport Services Connections, if so
   desired.  Alternatively, a Transport Services system MAY allow
   applications to specify that fallback to a specific other version of
   a protocol is allowed.

7.  Acknowledgements

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

   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.  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

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8.1.  Normative References

              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P., Wood, C., Pauly,
              T., and K. Rose, "An Abstract Application Layer Interface
              to Transport Services", Work in Progress, Internet-Draft,
              draft-ietf-taps-interface-12, 9 April 2021,

   [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, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

              Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", Work in
              Progress, Internet-Draft, draft-ietf-taps-impl-08, 2
              November 2020, <https://www.ietf.org/internet-drafts/

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

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

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

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   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [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, <https://www.rfc-editor.org/info/rfc8923>.

Authors' Addresses

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

   Email: tpauly@apple.com

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

   Email: ietf@trammell.ch

   Anna Brunstrom
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad

   Email: anna.brunstrom@kau.se

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

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk/

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

   Email: csp@csperkins.org

   Philipp S. Tiesel
   Konrad-Zuse-Ring 10

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   14469 Potsdam

   Email: philipp@tiesel.net

   Christopher A. Wood
   101 Townsend St
   San Francisco,
   United States of America

   Email: caw@heapingbits.net

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