An Architecture for Transport Services
draft-ietf-taps-arch-03

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TAPS Working Group                                         T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                        B. Trammell, Ed.
Expires: September 12, 2019                                       Google
                                                            A. Brunstrom
                                                     Karlstad University
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                              C. Perkins
                                                   University of Glasgow
                                                               P. Tiesel
                                                               TU Berlin
                                                                 C. Wood
                                                              Apple Inc.
                                                          March 11, 2019

                 An Architecture for Transport Services
                        draft-ietf-taps-arch-03

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
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   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 September 12, 2019.

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

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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
   faster deployment of new feature and protocols, but it can also
   support applications with racing and fallback 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 that all APIs and
   implementations have to be 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

   The model of using sockets for networking can be represented as
   follows: applications create connections and transfer data using the
   socket API, which provides the interface to the implementations of
   UDP and TCP (typically implemented in the system's kernel), which in
   turn send data over the available network layer interfaces.

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   +-----------------------------------------------------+
   |                    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 both modernize the API surface exposed for
   transport protocols and enrich the capabilities of the 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 the
   mechanism for an application to create network connections and
   transfer data.  The Implementation [I-D.ietf-taps-impl] is

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   responsible for mapping the API into the various available transport
   protocols and managing the available network interfaces and paths.

   There are a few key departures that Transport Services makes from the
   sockets API: it presents an asynchronous, event-driven API; it uses
   messages for representing data transfer to applications; and it
   assumes an implementation that can use multiple IP addresses,
   multiple protocols, multiple paths, and provide multiple application
   streams.

1.2.  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 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.

   All interaction with a Transport Services system is expected to be
   asynchronous, and 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 API also delivers events regarding the
   lifetime of a connection and changes to 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 nature of interactions over networks, as
   opposed to how sockets represent network resources as file system
   objects 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 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 that it
   more closely matches the way applications use the network.  Messages

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   seamlessly work with transport protocols 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 API
   represent the framed messages within the stream.  In the absence of a
   framer, protocols 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 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 can avoid (re-)sending data that relies on a
      previous transmission that was never received.

   Allowing applications to interact with messages is backwards-
   compatible with existings protocols and APIs, as 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.

1.4.  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.

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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
   connections.

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

   One of the key insights to come from identifying the minimal set of
   features provided by transport protocols [I-D.ietf-taps-minset] was
   that features either require application interaction and guidance
   (referred to as Functional Features), or else can be handled
   automatically by a system implementing Transport Services (referred
   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 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.

2.1.  Specification of Requirements

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

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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
   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 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

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   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 transport security
   protocol 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
   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.  All implementations are REQUIRED to offer
   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 it is possible that some very
   constrained devices might not have, for example, 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 will be
   updated and supplemented as new protocols and protocol features are
   developed.

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   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 new protocols that require any changes
   to a remote system.  The Transport Services system MUST be deployable
   on one side only.

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.

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     +-----------------------------------------------------+
     |                    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
   objects (Section 4.1.1) that allow applications to establish
   communication and send and receive data.  These could be exposed as
   handles or referenced objects, depending on the language.

   Beyond the basic 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,

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      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 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
   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.

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

                  Figure 4: The lifetime of a connection

4.1.1.  Basic 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 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 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.

      *  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,

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         prohibitions, and preferences.  There are three kinds of
         Transport Properties: Selection Properties (Section 4.1.2),
         Connection Properties (Section 4.1.2), and Message Properties
         (Section 4.1.4).  Message Properties can also be specified
         during data transfer to affect specific Messages.

   o  Connection: A Connection object represents an active transport
      protocol instance that can send and/or receive Messages between
      local and remote systems.  It holds state pertaining to the
      underlying transport protocol instance and any ongoing data
      transfer.  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.

   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

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      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.

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 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.

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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 how the transport
      will send the Message (for prioritization and reliability), along
      with any per-protocol properties to send with the Message.
      Message Properties MAY be set on a Preconnection to define
      defaults properties for sending.  When receiving Messages, Message
      Properties can contain per-protocol properties.

   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 content
      is to be sent.  Status of the Send operation can be delivered back
      to the 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 of events that can be delivered to an application is not
   exhaustive, but gives the top-level categories of events.  The API
   MAY expand this list.

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   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 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 be multiplexed

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      together.  Applications can use their explicitly defined
      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.

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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 Transport Services system's policies and
      heuristics.

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 Protocol Stacks is in some
   cases controlled by properties that only change when application code
   is updated.  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
   can be a powerful driver of new protocol adoption, but needs to be

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   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 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 be allowed as an
       equivalent Protocol Stack as long as it does 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.

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4.2.4.  Separating Connection Groups

   By default, all stored properties of the 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 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 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

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   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 protocols.  For example,
   if a client requests TLS, yet TLS or the desired version are 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.

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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-02 (work in progress), October 2018.

   [I-D.ietf-taps-interface]
              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P., and C. Wood, "An
              Abstract Application Layer Interface to Transport
              Services", draft-ietf-taps-interface-02 (work in
              progress), October 2018.

   [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 (work in progress), March
              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>.

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   [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., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [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

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   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
   Marchstrasse 23
   10587 Berlin
   Germany

   Email: philipp@inet.tu-berlin.de

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

   Email: cawood@apple.com

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