TAPS Working Group                                         T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                        B. Trammell, Ed.
Expires: January 9, May 7, 2020                                              Google
                                                            A. Brunstrom
                                                     Karlstad University
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                              C. Perkins
                                                   University of Glasgow
                                                               P. Tiesel
                                                               TU Berlin
                                                                 C. Wood
                                                              Apple Inc.
                                                           July 08,
                                                       November 04, 2019

                 An Architecture for Transport Services
                        draft-ietf-taps-arch-04
                        draft-ietf-taps-arch-05

Abstract

   This document provides an overview of the architecture of Transport
   Services, a model for exposing transport protocol features to
   applications for network communication.  In contrast to what is
   provided by most existing Application Programming Interfaces (APIs),
   Transport Services is based on an asynchronous, event-driven
   interaction pattern; it uses messages for representing data transfer
   to applications; and it assumes an implementation that can use
   multiple IP addresses, multiple protocols, and multiple paths, and
   provide multiple application streams.  This document further defines
   the common set of 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."
   This Internet-Draft will expire on January 9, May 7, 2020.

Copyright Notice

   Copyright (c) 2019 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.  Overview  .  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Event-Driven API  Overview  . . . . . . . . . . . . . . . . . . . .   5 . . . .   4
     1.3.  Data Transfer Using Messages  Specification of Requirements . . . . . . . . . . . . . .   5
     1.4.  Flexibile Implementation
   2.  API Model . . . . . . . . . . . . . . . . .   6
   2.  Background . . . . . . . . .   5
     2.1.  Event-Driven API  . . . . . . . . . . . . . . . .   7
     2.1.  Specification of Requirements . . . .   6
     2.2.  Data Transfer Using Messages  . . . . . . . . . . . . . .   7
     2.3.  Flexibile Implementation  . . . . . . . . . . . . . . . .   8
   3.  Design Principles . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Common APIs for Common Features . . . . . . . . . . . . .   8   9
     3.2.  Access to Specialized Features  . . . . . . . . . . . . .   8   9
     3.3.  Scope for API and Implementation Definitions  . . . . . .   9  10
   4.  Transport Services Architecture and Concepts  . . . . . . . .  10  11
     4.1.  Transport Services API Concepts . . . . . . . . . . . . .  11  12
       4.1.1.  Basic  Connection Objects  . . . . . . . . . . . . . . . . . . . .  13  14
       4.1.2.  Pre-Establishment . . . . . . . . . . . . . . . . . .  14  15
       4.1.3.  Establishment Actions . . . . . . . . . . . . . . . .  15  16
       4.1.4.  Data Transfer Objects and Actions . . . . . . . . . .  16  17
       4.1.5.  Event Handling  . . . . . . . . . . . . . . . . . . .  16  18
       4.1.6.  Termination Actions . . . . . . . . . . . . . . . . .  17  18
     4.2.  Transport System Implementation Concepts  . . . . . . . .  17  18
       4.2.1.  Candidate Gathering . . . . . . . . . . . . . . . . .  19  20
       4.2.2.  Candidate Racing  . . . . . . . . . . . . . . . . . .  19  20
       4.2.3.  Protocol Stack Equivalence  . . . . . . . . . . . . .  19  20
       4.2.4.  Separating Connection Groups  . . . . . . . . . . . .  21  22
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21  22
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  21  23
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22  23
   8.  Informative  References  . . . . . . . . . . . . . . . . . . .  23 . . . . . .  24
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24  25

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.  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 does not only enable enables faster
   deployment of new feature features and protocols, but it protocols.  It can also support
   applications with by offering racing and fallback mechanisms mechanisms, which
   otherwise need to be implemented in each application separately.

   This document is 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 of course not mean require that all APIs and
   implementations have to be are identical, a common minimal set of features
   represented in a consistent fashion will enable applications to be
   easily ported from one system to the another.

1.1.  Overview  Background

   The model Transport Services architecture is based on the survey of using sockets for networking can be represented as
   follows: applications create connections
   Services Provided by IETF Transport Protocols and Congestion Control
   Mechanisms [RFC8095], and transfer data using the
   socket API, which provides the interface to the implementations distilled minimal set of
   UDP and TCP (typically implemented in the system's kernel), which features
   offered by transport protocols [I-D.ietf-taps-minset].  These
   documents identified common features and patterns across all
   transport protocols developed thus far in
   turn send data over the available network layer interfaces.

   +-----------------------------------------------------+
   |                    Application                      |
   +-----------------------------------------------------+
              |                             |
   +---------------------+       +-----------------------+
   |  Socket Stream API  |       |  Socket Datagram API  |
   +---------------------+       +-----------------------+
              |                             |
   +-----------------------------------------------------+
   |         TCP                           UDP           |
   |           Kernel Protocol Implementation            |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |               Network Layer Interface               |
   +-----------------------------------------------------+

                       Figure 1: socket() API Model

   The Transport Services architecture maintains this general model IETF.

   Since transport security is an increasingly relevant aspect of
   interaction, but aims to both modernize the API surface exposed for using
   transport protocols and enrich on the capabilities Internet, this architecture also considers
   the impact of transport security protocols on the feature-set exposed
   by transport
   system implementation.

   +-----------------------------------------------------+
   |                    Application                      |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |               Transport Services API                |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |           Transport System Implementation           |
   |       (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 services [I-D.ietf-taps-transport-security].

   One of the
   mechanism for an application key insights to create network connections and
   transfer data.  The Implementation [I-D.ietf-taps-impl] is
   responsible for mapping the API into come from identifying the various available minimal set of
   features provided by transport protocols [I-D.ietf-taps-minset] was
   that features either require application interaction and managing the available network interfaces and paths.

   There are guidance
   (referred to as Functional or Optimizing Features), or else can be
   handled automatically by a few key departures that system implementing Transport Services makes from the
   sockets API: it presents an asynchronous, event-driven API; it uses
   messages for representing data transfer
   (referred to applications; and it
   assumes an implementation that can use multiple IP addresses,
   multiple protocols, multiple paths, as Automatable Features).  Among the Functional and provide multiple application
   streams.

1.2.  Event-Driven API

   Originally, sockets presented
   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 blocking interface subset of protocols, but would
   not harm the functionality of other protocols.  For example, some
   protocols can deliver messages faster for establishing
   connections and transferring data.  However, most modern applications
   interact with that do not
   require messages to arrive in the network asynchronously.  When sockets are presented
   as an asynchronous interface, order in which they generally use a try-and-fail
   model.  If the application wants were sent.
   However, this functionality needs to read, but data has not yet been
   received from be explicitly allowed by the peer,
   application, since reordering messages would be undesirable in many
   cases.

1.2.  Overview

   This document describes the call to read will fail.  The application
   then waits and can try again later.

   All interaction with a Transport Services system is expected to be
   asynchronous, and use an event-driven model unlike sockets architecture in three
   sections:

   o  Section 4.1.5.  For example, if the application wants to read, its
   call to read will not fail, but will deliver an event containing 2 describes how the
   received data once it is available.

   The API model of Transport Services API also delivers events regarding
      differs from traditional socket-based APIs.  Specifically, it
      offers asynchronous event-driven interaction, the
   lifetime use of a connection and changes to available network links,
   which were not previously made explicit in sockets.

   Using asynchronous events allows messages
      for a much simpler interaction model
   when establishing connections data transfer, and transferring data.  Events in time
   more closely reflect the nature of interactions over networks, as
   opposed ability to how sockets represent network resources as file system
   objects easily adopt different
      transport protocols.

   o  Section 3 explains the design principles that may be temporarily unavailable.

1.3.  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 guide 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; TLS record headers carry a version, content type, and
   length; and HTTP/2 uses frames to segment its headers and bodies.

   The Transport
      Services API represents data as messages, so API.  These principles are intended to make sure that it
   more closely matches the way applications use the network.  Messages
   seamlessly work with
      transport protocols that support datagrams or
   records, but can also continue to be used over a stream enhanced and evolve without
      requiring too many changes by defining an
   application-layer framer application developers.

   o  Section 4.1.4.  When framing protocols are
   placed on top of unstructured streams, 4 presents the messages Transport Services architecture diagram and
      defines the concepts that are used in by both the API
   represent the framed messages within the stream.  In and
      implementation documents.  The Preconnection allows applications
      to configure connection properties, and the absence of a
   framer, protocols Connection represents
      an object that deal only can be used to send and receive Messages.

1.3.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in byte streams, such this document are to be interpreted as TCP,
   represent their data described in each direction BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as a single, long message.

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

2.  API Model

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

   o  Applications create connections and transfer data using the ability to associate deadlines with messages, for applications
      that care about timing; socket
      API.

   o  The socket API provides the ability to provide control of reliability, choosing which
      messages interface to retransmit in the event implementations of packet loss,
      TCP and how best
      to make use of UDP (typically implemented in the data that arrived; system's kernel).

   o  TCP and UDP in the ability to manage dependencies between messages, when the
      transport 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 kernel send and receive data that relies on a
      previous transmission that was never received.

   o over the ability to automatically assign messages and connections available
      network layer interfaces.

   +-----------------------------------------------------+
   |                    Application                      |
   +-----------------------------------------------------+
              |                             |
   +---------------------+       +-----------------------+
   |  Socket Stream API  |       |  Socket Datagram API  |
   +---------------------+       +-----------------------+
              |                             |
   +-----------------------------------------------------+
   |         TCP                           UDP           |
   |           Kernel Protocol Implementation            |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |               Network Layer Interface               |
   +-----------------------------------------------------+

                       Figure 1: socket() API Model

   The Transport Services architecture maintains this general model of
   interaction, but aims to
      underlaying both modernize the API surface exposed for
   transport connections to utilize multi-streaming and
      pooled connections.

   Allowing applications to interact with messages is backwards-
   compatible with existings protocols and APIs, as it does not change enrich the wire format capabilities 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.

1.4.  Flexibile
   system implementation.

   +-----------------------------------------------------+
   |                    Application                      |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |               Transport Services API                |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |           Transport System Implementation

   Sockets, for protocols like           |
   |       (UDP, 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 SCTP, DCCP, TLS, QUIC, etc)        |
   +-----------------------------------------------------+
                             |
   +-----------------------------------------------------+
   |               Network Layer Interface               |
   +-----------------------------------------------------+

                  Figure 2: Transport Services architecture is designed to handle
   multiple candidate endpoints, protocols, and paths; and support
   multipath and multistreaming protocols. API Model

   The Transport Services implementations are meant API [I-D.ietf-taps-interface] defines the
   mechanism for an application to be flexible at
   connection establishment time, considering many different options create network connections and
   trying
   transfer data.  The implementation [I-D.ietf-taps-impl] is
   responsible for mapping the API to select the most optimal combinations (Section 4.2.1 various available transport
   protocols and
   Section 4.2.2).  This requires applications to provide higher-level
   endpoints than IP addresses, such as hostnames managing the available network interfaces and URLs, which paths.

   There are
   used by a key differences between the architecture of the Transport
   Services implementation system and the architecture of the sockets API: it presents
   an asynchronous, event-driven API; it uses messages for resolution, path
   selection, representing
   data transfer to applications; and racing.

   Flexibility after connection establishment is also important.
   Transport protocols it assumes an implementation that
   can migrate between use multiple network-layer
   interfaces need IP addresses, multiple protocols, multiple paths,
   and provide multiple application streams.

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.  When sockets are presented
   as an asynchronous interface, they generally use a try-and-fail
   model.  If the application wants to be able read, but data has not yet been
   received from the peer, the call to process read will fail.  The application
   then waits and react to interface changes.
   Protocols that support multiple application-layer streams need can try again later.

   All interaction with a Transport Services system is expected to
   support initiating be
   asynchronous, and receiving new streams using existing
   connections.

2.  Background use an event-driven model unlike sockets
   Section 4.1.5.  For example, if the application wants to read, its
   call to read will not fail, but will deliver an event containing the
   received data once it is available.

   The Transport Services architecture is based on API also delivers events regarding the survey
   lifetime of
   Services Provided by IETF Transport Protocols a connection and Congestion Control
   Mechanisms [RFC8095], changes in the available network links,
   which were not previously made explicit in sockets.

   Using asynchronous events allows for a much simpler interaction model
   when establishing connections and transferring data.  Events in time
   more closely reflect the distilled minimal set nature of the features
   offered by transport interactions over networks, as
   opposed to how sockets represent network resources as file system
   objects that may be temporarily unavailable.

2.2.  Data Transfer Using Messages

   Sockets provide a message interface for datagram protocols [I-D.ietf-taps-minset].  These
   documents identified common features like UDP,
   but provide an unstructured stream abstraction for TCP.  While TCP
   does indeed provide the ability to send and patterns across all 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.  Messages
   seamlessly work with transport protocols developed thus far that support datagrams or
   records, but can also be used over a stream by defining an
   application-layer framer Section 4.1.4.  When framing protocols are
   placed on top of unstructured streams, the messages used in the IETF.

   Since transport security is an increasingly relevant aspect API
   represent the framed messages within the stream.  In the absence of using
   transport a
   framer, protocols on that deal only in byte streams, such as TCP,
   represent their data in each direction as a single, long message.

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

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

   o  the ability to provide control of reliability, choosing which
      messages to retransmit in the event of packet loss, and how best
      to make use of the data that arrived;

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

   o  the impact of ability to automatically assign messages and connections to
      underlaying transport security connections to utilize multi-streaming and
      pooled connections.

   Allowing applications to interact with messages is backwards-
   compatible with existings protocols on and APIs, as it does not change
   the feature-set exposed
   by transport services [I-D.ietf-taps-transport-security].

   One wire format of any protocol.  Instead, it gives the key insights protocol
   stack additional information to come from identifying the minimal set allow it to make better use of
   features provided by modern
   transport services, while simplifying the application's role in
   parsing data.

2.3.  Flexibile Implementation

   Sockets, for protocols [I-D.ietf-taps-minset] was
   that features either require application interaction and guidance
   (referred like TCP, are generally limited to connecting
   to as Functional Features), or else can be handled
   automatically by a system implementing Transport Services (referred single address over a single interface.  They also present a
   single stream to as Automatable Features).  Among the Functional 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 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, but would not harm the
   functionality of other and paths; and support
   multipath and multistreaming 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

   Transport Services implementations are meant to be explicitly allowed by the application,
   since reordering messages would be undesirable in flexible at
   connection establishment time, considering many cases.

2.1.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", different options and
   "OPTIONAL" in this document
   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.

   Flexibility after connection establishment is also important.
   Transport protocols that can migrate between multiple network-layer
   interfaces need to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, able to process and only when, they appear in all
   capitals, as shown here. react to interface changes.
   Protocols that support multiple application-layer streams need to
   support initiating and receiving new streams using existing
   connections.

3.  Design Principles

   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 can 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 could
   be added to the system's implementation without requiring changes in
   applications for adoption.

   The following considerations were used in the design of this
   architecture.

3.1.  Common APIs for Common Features

   Functionality that is common across multiple transport protocols
   SHOULD be accessible through a unified set of API calls.  An
   application ought to be able to implement logic for its basic use of
   transport networking (establishing the transport, and sending and
   receiving data) once, and expect that implementation to continue to
   function as the transports change.

   Any Transport Services API is REQUIRED to allow access to the
   distilled minimal set of features offered by transport protocols
   [I-D.ietf-taps-minset].

3.2.  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 also permit more specialized protocol features to be
   used.  The interface for these specialized options should ought to be
   exposed differently from the common options to ensure flexibility.

   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 UDP, it could require control over the
   checksum or fragmentation behavior for UDP; if it used a protocol to
   frame its data over a byte stream like TCP, it would not need these
   options.  In such cases, the API ought to expose the features in such
   a way that they take effect when a particular protocol is selected,
   but do not imply that only that protocol could be used.  For example,
   if the API allows an application to specify a preference for
   constrained checksum usage, communication would not fail when a
   protocol such as TCP is selected, which uses a checksum covering the
   entire payload.

   Other specialized features, however, could be strictly required by an
   application and thus constrain the set of protocols that can be used.
   For example, if an application requires encryption of its transport
   data, only protocol stacks that include some a transport security
   protocol function
   are eligible to be used.  A Transport Services API MUST 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.  Scope for API and Implementation Definitions

   The Transport Services API is envisioned as the abstract model for a
   family of APIs that share a common way to expose transport features
   and encourage flexibility.  The abstract API definition
   [I-D.ietf-taps-interface] describes this interface and is aimed at how it can be
   exposed to application developers.

   Implementations that provide the Transport Services API
   [I-D.ietf-taps-impl] will vary due to system-specific support and the
   needs of the deployment scenario.  It is expected that all
   implementations of Transport Services will offer the entire mandatory
   API, but that some features will not be functional in certain
   implementations.
   API.  All implementations are REQUIRED to offer an API that is
   sufficient APIs to use the distilled minimal set of features offered by
   transport protocols [I-D.ietf-taps-minset], including API support for
   TCP and UDP transport, but transport.  However, some features provided by this API
   will not be functional in certain implementations.  For example, it
   is possible that some very constrained devices might not have, for example, have a full
   TCP implementation beneath the API.

   To preserve flexibility and compatibility with future protocols, top-
   level features in the Transport Services API SHOULD avoid referencing
   particular transport protocols.  The mappings of these API features
   to specific implementations of each feature is explained in the
   [I-D.ietf-taps-impl] along with the implications of the feature on
   existing protocols.  It is expected that this document [I-D.ietf-taps-interface]
   will be updated and supplemented as new protocols and protocol
   features are developed.

   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] defines define new protocols or protocol capabilities
   that require any changes
   to affect what is communicated across the network.  The Transport
   Services system MUST be deployable on one side only.  A Transport
   Services system acting as a remote connection initiator can communicate with
   any existing system that implements the transport protocol(s)
   selected by the Transport Services system.  The  Similarly, a Transport
   Services system MUST be deployable
   on one side only. acting as a listener can receive connections for any
   protocol that is supported by the system, from existing initiators.

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
   categories:

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

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

   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                   |                            |
     +------------------------|----------------------------+
                              |
     +------------------------|----------------------------+
     |  Transport             |                            |
     |  System                |        +-----------------+ |
     |  Implementation        |        |     Cached      | |
     |                        |        |      State      | |
     |  (Candidate Gathering) |        +-----------------+ |
     |                        |                            |
     |  (Candidate Racing)    |        +-----------------+ |
     |                        |        |     System      | |
     |                        |        |     Policy      | |
     |             +----------v-----+  +-----------------+ |
     |             |    Protocol    |                      |
     +-------------+    Stack(s)    +----------------------+
                   +-------+--------+
                           V
                 Network Layer Interface

      Figure 3: Concepts and Relationships in the Transport Services
                               Architecture

4.1.  Transport Services API Concepts

   Fundamentally, a Transport Services API needs to provide basic connection
   objects (Section 4.1.1) that allow applications to establish
   communication
   communication, and then send and receive data.  These could be
   exposed as handles or referenced objects, depending on the language.

   Beyond the basic connection objects, there are several high-level groups of
   actions that any Transport Services API implementing this
   specification MUST provide:

   o  Pre-Establishment (Section 4.1.2) encompasses the properties that
      an application can pass to describe its intent, requirements,
      prohibitions, and preferences for its networking operations.  For
      any system that provides generic Transport Services, these
      properties SHOULD be defined to apply to multiple transport
      protocols.  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.

   o  Establishment (Section 4.1.3) focuses on the actions that an
      application takes on the basic connection objects to prepare for data
      transfer.

   o  Data Transfer (Section 4.1.4) 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.

   o  Event Handling (Section 4.1.5) defines the set of properties about
      which an application can receive notifications during the lifetime
      of transport objects.  Events MAY also provide opportunities for
      the application to interact with the underlying transport by
      querying state or updating maintenance options.

   o  Termination (Section 4.1.6) 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 taken and
   events during the lifetime of a connection.  Note that some actions
   are alternatives (e.g., whether to initiate a connection or to listen
   for incoming connections), others are optional (e.g., setting
   Connection and Message Properties in Pre-Establishment), or have been
   omitted for brevity.

     Pre-Establishment     :       Established             : Termination
     -----------------     :       -----------             : -----------
                           :                       Close() :
     +---------------+  Initiate()  +------------+ Abort() :
 +-->| Preconnection |----------->| |------------->| Connection |---------------> |-----------> Closed
 |   +---------------+     : Rendezvous() +------------+ Connection: Conn.   :
 |                         :           ^  ^  ^     |    Finished :
 +-- Local Endpoint        :           |     |  |  |             :
 |                         :           |     |  |  +---------+             :
 +-- Remote Endpoint       :  +----+  |  +-----+           |     v             :
 |                         :           |       |Send()  |      | Connection        :
 +-- Selection Properties  :           |  +---------+   |      v   Ready           :
 +-- Connection Properties ---+  | Message | :           |  Message                   :
 +-- Message Properties -------->| to send |    :           |  Received                   :
 |                         :     +---------+           |                   :
 |   +----------+          :           |                   :
 +-->| Listener |------------------------------+ |----------------------+                   :
     +----------+  Connection Received                     :
           ^               :                               :
           |               :                               :
        Listen()           :                               :

                  Figure 4: The lifetime of a connection

4.1.1.  Basic  Connection Objects

   o  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
      (Section 4.1.2) to which it will connect, and Selection Properties
      (Section 4.1.2) that influence the paths and protocols a
      Connection will use.  A Preconnection can be fully specified and
      represent 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 4.1.2) MUST be specified
      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 MUST be specified 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 MUST both
      be specified if a peer-to-peer Rendezvous is to occur based on the
      Preconnection.

      *

   o  Transport Properties: Transport Properties can be specified as
      part of a Preconnection to allow the application to configure the
      Transport System and express their requirements, prohibitions, and
      preferences.  There are three kinds of Transport Properties:

      *  Selection Properties (Section 4.1.2), 4.1.2)

      *  Connection Properties (Section 4.1.2), 4.1.2)

      *  and Message Properties (Section 4.1.4). 4.1.4); note that Message
         Properties can also be specified during data transfer to affect
         specific Messages.

   o  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 instance, e.g., a set of
      TCP and QUIC connections to equivalent endpoints, or a stream of a
      multi-streaming transport protocol instance.

   o  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.

4.1.2.  Pre-Establishment

   o  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, and an Endpoint
      with wider scope (such as a hostname) can be resolved to more
      concrete identities (such as IP addresses).

   o  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.

   o  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.

   o  Selection Properties: The Selection Properties consist of the
      options 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 options can take the
      form of requirements, prohibitions, or preferences.  Examples of
      options that influence path selection include the interface type
      (such as a Wi-Fi Ethernet connection, or a Cellular LTE
      connection), requirements around the Maximum Transmission Unit
      (MTU) or path MTU (PMTU), or preferences for throughput and
      latency properties.  Examples of options that influence protocol
      selection and configuration of transport protocol features include
      reliability, service class, multipath support, and fast open
      support.

   o  Connection Properties: The Connection Properties are used to
      configure protocol-specific options and control per-connection
      behavior of the Transport System.  For example, a protocol-
      specific Connection Property can express that if UDP is used, the
      implementation ought to use checksums.  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 MAY be used in this way
      by an implementation during connection establishment.  Connection
      Properties SHOULD be specified on a Preconnection prior to
      Connection establishment, but MAY be modified later.  Changes made
      to Connection Properties after establishment take effect on a
      best-effort basis.  Such changes do not affect protocol or path
      selection, but only modify the manner in which a connection sends
      and receives data.

4.1.3.  Establishment Actions

   o  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 be able to send and/or receive 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.  The process of identifying options for connecting, such as
      resolution of the Remote Endpoint, occurs in response the Initiate
      call.

   o  Listen: The action of marking a Listener as willing to accept
      incoming Connections.  The Listener will then create Connection
      objects as incoming connections are accepted (Section 4.1.5).

   o  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 whilst listening for an incoming
      connection from that endpoint.  This corresponds, for example, to
      a TCP simultaneous open [RFC0793].  The process of identifying
      options for the connection, such as resolution of the Remote
      Endpoint, occurs during the Rendezvous call.  If successful, the
      rendezvous call returns a Connection object to represent the
      established peer-to-peer connection.

4.1.4.  Data Transfer Objects and Actions

   o  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 within the Message.  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.  If a received Message is incomplete or corrupted, it
      might or might not be usable by certain applications.  Boundaries of a Message might or might not be
      understood or transmitted by transport protocols.  Specifically,
      what one application considers to be two Messages sent on a
      stream-based transport can be treated as a single Message by the
      application on the other side.

   o  Message Properties: Message Properties can be used to annotate
      specific Messages.  These properties can specify might only apply to how
      Message is sent (such as how the transport will send the Message (for treat
      prioritization and reliability), along
      with any per-protocol but can also include properties
      that specific protocols encode and communicate to send with the Message. Remote
      Endpoint.  Message Properties MAY be set on a Preconnection to
      define defaults properties for sending.  When receiving Messages,
      Message Properties can contain per-protocol properties. properties for
      properties that are sent between the endpoints.

   o  Send: The action to transmit a Message or partial Message over a
      Connection to the remote system.  The interface to Send MAY
      include Message Properties specific to how the Message's Message content is
      to be sent.  Status  The status of the Send operation can be delivered
      back to the sending application in an event (Section 4.1.5).

   o  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 4.1.5).  The interface to Receive MAY include
      Message Properties specific to the Message that is to be delivered
      to the application.

   o  Framer: A Framer is a data translation layer that can be added to
      a Connection to define how application-level Messages are
      transmitted over a transport protocol.  This is particularly
      relevant for protocols that otherwise present unstructured
      streams, such as TCP.

4.1.5.  Event Handling

   This list section provides the top-level categories of events events that
   can be delivered to an application application.  This list is not
   exhaustive, but gives the top-level categories of events.  The API
   MAY expand this list. exhaustive.

   o  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.

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

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

   o  Message Received: Delivers received Message content to the
      application, based on a Receive action.  This MAY include an error
      if the Receive action cannot be satisfied due to the Connection
      being closed.

   o  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 Message has been processed by the
      protocol stack.

   o  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.

4.1.6.  Termination Actions

   o  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.

   o  Abort: The action the application takes on a Connection to
      indicate a Close and also indicate that the transport system
      SHOULD NOT attempt to deliver any outstanding data.

4.2.  Transport System Implementation Concepts

   The Transport System Implementation Concepts define

   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
   interface.

   o  Connection Group: A set of Connections that share properties and
      caches.  For multiplexing transport protocols, only Connections
      within the same Connection Group are allowed to be multiplexed
      together.  Applications  An application can use their explicitly defined define Connection Groups
      to control caching boundaries, as discussed in Section 4.2.4.

   o  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.

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

   o  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 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).

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

   o  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).

   o  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.

   o  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.

4.2.1.  Candidate Gathering

   o  Path Selection: Path Selection represents the act of choosing one
      or more paths that are available to use based on the Selection
      Properties provided by the application, the policies and
      heuristics of a Transport Services system.

   o  Protocol Selection: Protocol Selection represents the act of
      choosing one or more sets of protocol options that are available
      to use based on the Transport Properties provided by the
      application, and a the heuristics or policies within the Transport
      Services system's policies and
      heuristics. system.

4.2.2.  Candidate Racing

   o  Protocol Option Racing: Protocol 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.

   o  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 stepss, which can lead to further
      racing of the resolved Remote Endpoints.

   o  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 IP addresses
      resolved from a DNS hostname.

4.2.3.  Protocol Stack Equivalence

   The Transport Services architecture defines a mechanism that allows
   applications to easily use different network paths and Protocol
   Stacks.  Transitioning between different  In some cases, changing which Protocol Stacks is in some
   cases controlled or network
   paths are used will require updating the preferences expressed by properties that only change when the
   application code
   is updated. that uses the Transport Services system.  For example, an
   application can enable the use of a multipath or multistreaming
   transport protocol by modifying the properties in its Pre-Connection
   configuration.  In some cases, however, the Transport Services system
   will be able to automatically change Protocol Stacks without an
   update to the application, either by selecting a new stack entirely,
   or by racing multiple candidate Protocol Stacks during connection
   establishment.  This functionality in the API can be a powerful
   driver of new protocol adoption, but needs to be constrained
   carefully to avoid unexpected behavior that can lead to functional or
   security problems.

   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 need to meet the following
   criteria:

   1.  Both stacks MUST offer the same interface to the application for
       connection establishment and data transmission.  For example, if
       one Protocol Stack has UDP as the top-level interface to the
       application, then it is not equivalent to a Protocol Stack that
       runs TCP as the top-level interface.  Among other differences,
       the 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.

   2.  Both stacks MUST offer the same transport services, as services that are required
       by the application.  For example, if an 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.  However, if the
       application does not require reliability, then a Protocol Stack
       that adds unnecessary reliability might could be allowed regarded as an equivalent Protocol
       Stack as long as it does providing this would not conflict with any other
       application-requested properties.

   3.  Both stacks MUST offer the same security properties.  The
       inclusion of transport security protocols
       [I-D.ietf-taps-transport-security] in a Protocol Stack adds
       additional restrictions to Protocol Stack equivalence.  Security
       features and properties, such as cryptographic algorithms, peer
       authentication, and identity privacy vary across security
       protocols, and across versions of security protocols.  Protocol
       equivalence ought not to be assumed for different protocols or
       protocol versions, even if they offer similar application
       configuration options.  To ensure that security protocols are not
       incorrectly swapped, Transport Services systems SHOULD only
       automatically generate equivalent Protocol Stacks when the
       transport security protocols within the stacks are identical.
       Specifically, a transport system would consider protocols
       identical only if they are of the same type and version.  For
       example, the same version of TLS running over two different
       transport protocol stacks are considered equivalent, whereas TLS
       1.2 and TLS 1.3 [RFC8446] are not considered equivalent.

4.2.4.  Separating Connection Groups

   By default, all stored properties of the Implementation implementation are shared
   within a process, such as cached protocol state, cached path state,
   and heuristics.  This provides efficiency and convenience for the
   application, since the Transport System Implementation implementation can
   automatically optimize behavior.

   There are several reasons, however, that an application might want to
   isolate some Connections within a single process.  These reasons
   include:

   o  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].

   o  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).

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

   The Transport Services API SHOULD allow applications to explicitly
   define Connection Groups that force separation of Cached State and
   Protocol Stacks.  For example, a web browser application might use
   Connection Groups with separate caches for different tabs in the
   browser to decrease linkability.

   The interface to specify these Groups groups MAY expose fine-grained tuning
   for which properties and cached state is allowed to be shared with
   other Connections.  For example, an application might want to allow
   sharing TCP Fast Open cookies across groups, but not TLS session
   state.

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
   [I-D.ietf-taps-transport-security].

   As described above in Section 4.2.3, if a Transport Services system
   races between two different Protocol Stacks, both MUST use the same
   security protocols and options.

   Clients need to ensure that security APIs are used appropriately.  In
   cases where clients 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, clients ought
   not to use PSK material created for the Encapsulating Security
   Protocol (ESP, part of IPsec) [RFC4303] with QUIC, and clients ought
   not to use private keys intended for server authentication as a keys
   for client authentication.

   Moreover, unlike certain transport features such as TCP Fast Open
   (TFO) [RFC7413] or Explicit Congestion Notification (ECN) [RFC3168]
   which can fall back to standard configurations, Transport Services systems MUST prohibit fallback for security NOT automatically fall back
   from secure protocols to insecure protocols, or to weaker versions of
   secure protocols.  For example, if a client requests TLS, yet TLS or but the
   desired version are of TLS is not available, its connection will fail.
   Clients are thus responsible for implementing security protocol
   fallback or version fallback by creating multiple Transport Services
   Connections, if so desired.

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 Stuart Cheshire, Josh Graessley, David Schinazi, and Eric
   Kinnear for their implementation and design efforts, including Happy
   Eyeballs, that heavily influenced this work.

8.  Informative  References

   [I-D.ietf-taps-impl]
              Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", draft-
              ietf-taps-impl-03 (work in progress), March 2019.

8.1.  Normative References

   [I-D.ietf-taps-interface]
              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P., and C. Wood, C., and T.
              Pauly, "An Abstract Application Layer Interface to
              Transport Services", draft-ietf-taps-interface-03 draft-ietf-taps-interface-04 (work in
              progress), July 2019.

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

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

8.2.  Informative References

   [I-D.ietf-taps-impl]
              Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", draft-
              ietf-taps-impl-04 (work in progress), July 2019.

   [I-D.ietf-taps-minset]
              Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for End Systems", draft-ietf-taps-minset-11 (work
              in progress), September 2018.

   [I-D.ietf-taps-transport-security]
              Wood, C., Enghardt, T., Pauly, T., Perkins, C., and K.
              Rose, "A Survey of Transport Security Protocols", draft-
              ietf-taps-transport-security-06
              ietf-taps-transport-security-09 (work in progress), March
              September 2019.

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

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

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

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S.,

   [RFC7230]  Fielding, R., Ed. and A. Jain, "TCP
              Fast Open", J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7413, 7230, DOI 10.17487/RFC7413, December 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7413>.
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

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

   [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>.

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

Authors' Addresses

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

   Email: tpauly@apple.com

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

   Email: ietf@trammell.ch
   Anna Brunstrom
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad
   Sweden

   Email: anna.brunstrom@kau.se

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

   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
   TU Berlin
   Einsteinufer 25
   10587 Berlin
   Germany

   Email: philipp@tiesel.net

   Chris Wood
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: cawood@apple.com