TAPS Working Group                                         T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Intended status: Informational Standards Track                        B. Trammell, Ed.
Expires: January 2, April 25, 2019                                       ETH Zurich
                                                            A. Brunstrom
                                                     Karlstad University
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                              C. Perkins
                                                   University of Glasgow
                                                               P. Tiesel
                                                               TU Berlin
                                                                 C. Wood
                                                              Apple Inc.
                                                           July 01,
                                                        October 22, 2018

                 An Architecture for Transport Services


   This document provides an overview of the architecture of Transport
   Services, a system for exposing the features of transport protocols
   to applications.  This architecture serves as a basis for Application
   Programming Interfaces (APIs) and implementations that provide
   flexible transport networking services.  It defines the common set of
   terminology and concepts to be used in more detailed discussion of
   Transport Services.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   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  . . . . . . . . . . . . . . . . . . . . . . . . .   3   6
   3.  Design Principles . . . . . . . . . . . . . . . . . . . . . .   4   7
     3.1.  Common APIs for Common Features . . . . . . . . . . . . .   4   7
     3.2.  Access to Specialized Features  . . . . . . . . . . . . .   4   7
     3.3.  Scope for API and Implementation Definitions  . . . . . .   5   8
   4.  Transport Services Architecture and Concepts  . . . . . . . .   6   9
     4.1.  Transport Services API Concepts . . . . . . . . . . . . .   7  10
       4.1.1.  Basic Objects . . . . . . . . . . . . . . . . . . . .   9  12
       4.1.2.  Pre-Establishment . . . . . . . . . . . . . . . . . .  10  13
       4.1.3.  Establishment Actions . . . . . . . . . . . . . . . .  11  14
       4.1.4.  Data Transfer Objects and Actions . . . . . . . . . .  11  14
       4.1.5.  Event Handling  . . . . . . . . . . . . . . . . . . .  12  15
       4.1.6.  Termination Actions . . . . . . . . . . . . . . . . .  13  16
     4.2.  Transport System Implementation Concepts  . . . . . . . .  13  16
       4.2.1.  Candidate Gathering . . . . . . . . . . . . . . . . .  14  17
       4.2.2.  Candidate Racing  . . . . . . . . . . . . . . . . . .  14  17
     4.3.  Protocol Stack Equivalence  . . . . . . . . . . . . . . .  15  18
       4.3.1.  Transport Security Equivalence  . . . . . . . . . . .  16  19
     4.4.  Message Framing, Parsing, and Serialization . . . . . . .  16  19
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17  20
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17  20
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18  21
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  18  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19  23

1.  Introduction

   Many APIs 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 names and
   functions between these APIs are not consistent, and vary depending
   on the protocol being used.  For example, sending and receiving on a
   stream of data is conceptually the same between operating on an
   unencrypted Transmission Control Protocol (TCP) stream and operating
   on an encrypted Transport Layer Security (TLS) [I-D.ietf-tls-tls13]
   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 protocols offering transport
   services vary based on the context of the protocols themselves.  This
   variety can lead to confusion when trying to understand 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.

   This document is developed in parallel with the specification of the
   Transport Services API [I-D.ietf-taps-interface] and Implementation
   [I-D.ietf-taps-impl] documents.

1.1.  Overview

   The model for 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.

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

   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               |

   The Transport Services API [I-D.ietf-taps-interface] defines the
   mechanism for an application to create and monitor network
   connections, and transfer data.  The Implementation
   [I-D.ietf-taps-impl] is 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 respresenting data transfer to applications; and it
   assumes an implementation that can use multiple IP addresses,
   multiple protocols, multiple paths, and provide multiple application

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 for a notification that it should try again.

   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.
   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.

   In order to more closely match the way applications use the network,
   the Transport Services API respresents data as messages.  Messages
   seamlessly work with transport protocols that support datagrams or
   records, but can also be used over a stream by defining the
   application-layer framing being used Section 4.4.

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 similarities and differences between
   protocols, and how applications can use them effectively.

   The goal application.  Software layers built upon sockets
   often propagate this limitation of the a single-address single-stream
   model.  The Transport Services architecture is designed to provide a
   common, flexible, handle
   multiple candidate endpoints, protocols, and reusable interface for transport paths; and support
   multipath and multistreaming protocols.  As
   applications adopt this interface, they will benefit from a wide set
   of transport features that can evolve over

   Transport Services implementations are meant to be flexible at
   connection establishment time, considering many different options and ensure that the
   system providing the interface can optimize its behavior based on
   trying to select the
   application requirements most optimal combinations (Section 4.2.1 and network conditions.
   Section 4.2.2).  This document is developed in parallel with the specification of the requires applications to provide higher-level
   endpoints than IP addresses, such as hostnames and URLs, which are
   used by a Transport Services API [I-D.ietf-taps-interface] implementation for resolution, path
   selection, and Implementation
   [I-D.ietf-taps-impl] documents. racing.

   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

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].  This work has
   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, or perhaps even a single transport
   protocol, but would not harm the functionality of other protocols.
   For example, some protocols can deliver messages faster for
   applications that do not require them to arrive in the order in which
   they were sent.  However, this functionality must be explicitly
   allowed by the application, since reordering messages would be
   undesirable in many cases.

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 may have different performance
   and functional characteristics, and it can communicate with different
   Remote Endpoints to optimize performance. 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 may be added to the
   system's implementation without requiring changes in applications for

   The following considerations were used in the design of this

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 should 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 must allow access to the distilled minimal
   set of features offered by transport protocols

3.2.  Access to Specialized Features


   There are applications that will often need to control fine-grained details
   of transport protocols to optimize their behavior and ensure
   compatibility with remote peers, a peers,. A Transport Services system will
   therefore also needs to allow 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 may 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 may 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 should 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 may could be used if there are
   equivalent options.

   Other specialized features, however, may 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 should 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 must 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.

   In order to

   To preserve flexibility and compatibility with future protocols, top-level top-
   level features in the Transport Services API should avoid referencing
   particular transport protocols.  Mappings  The mappings of these API features
   to specific implementations of each feature is explained in the Implementation document, on the other hand, must
   [TAPS-IMPL], which also explain the ramifications implications of each the feature on
   provided by existing protocols.  It is expected that the Implementation this document
   will be updated and supplemented as new protocols and protocol
   features are developed.

   It is important to note that neither the Transport Services API nor
   the Implementation document defines new protocols that require any
   changes on to a remote hosts. host.  The Transport Services system must be
   deployable on one side only, as a way to allow an application to make
   better use of available capabilities on a system and protocol
   features that may be supported by peers across the network.

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 may be
   used in some implementations, it is expected that there will remain a
   variety of terms used by running code.

   The architecture divides the concepts for Transport Services into two

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

   2.  System-implementation concepts, which are meant 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-------------+   Basic Objects  +-------+----------+
     |  Transport    +--------+---------+                  |
     |  Services              |                            |
     |  API                   |                            |
     |  Transport             |                            |
     |  System                |        +-----------------+ |
     |  Implementation        |        |     Cached      | |
     |                        |        |      State      | |
     |  (Candidate Gathering) |        +-----------------+ |
     |                        |                            |
     |  (Candidate Racing)    |        +-----------------+ |
     |                        |        |     System      | |
     |                        |        |     Policy      | |
     |             +----------v-----+  +-----------------+ |
     |             |    Protocol    |                      |
     +-------------+    Stack(s)    +----------------------+
                 Network Layer Interface

      Figure 1: Concepts and Relationships in the Transport Services

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 may 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 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 primarily be defined to apply to multiple
      transports.  Properties may have a large impact on the rest of the
      aspects of the interface: they can modify how establishment
      occurs, they can 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

   o  Data Transfer (Section 4.1.4) consists of how an application
      represents 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 can 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.

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

                  Figure 2: 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 to
      which it will connect, and Path Selection Properties, Protocol
      Selection Properties, and Specific Protocol Properties that
      influence the choice of transport that 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
      must be specified if the Preconnection is used to Listen for
      incoming connections, but is optional if it is used to Initiate
      connections.  The Remote Endpoint must be specified in the
      Preconnection is used to Initiate connections, but 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  Connection: A Connection object represents an active transport
      protocol instance that can send and/or receive Messages between a
      Local Endpoint and a Remote Endpoint.  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 Endpoints and generates corresponding
      Connection objects.  It is created from a Preconnection object
      that specifies the type of incoming connections it will accept.

4.1.2.  Pre-Establishment

   o  Endpoint: An Endpoint represents 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 may 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
      name 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
      name for itself that it uses for transport connections.  For
      example, a local IP address and port.

   o  Path Selection Properties: The Path Selection Properties consist
      of the options that an application may set to influence the
      selection of paths between the Local Endpoint and the Remote
      Endpoint.  These options can take the form of requirements,
      prohibitions, or preferences.  Examples of options that may
      influence path selection include the interface type (such as a Wi-
      Fi Ethernet connection, or a Cellular LTE connection),
      characteristics of the path that are locally known like Maximum
      Transmission Unit (MTU) or discovered like Path MTU (PMTU), or
      predicted based on cached information like expected throughput or

   o  Protocol Selection Properties: The Protocol Selection Properties
      consist of the options that an application may set to influence
      the selection of transport protocol, or to configure the behavior
      of generic transport protocol features.  These options can take
      the form of requirements, prohibitions, and preferences.  Examples
      include reliability, service class, multipath support, and fast
      open support.

   o  Specific Protocol Properties: The Specific Protocol Properties
      refer to the subset of Protocol Properties options that apply to a
      single protocol (transport protocol, IP, or security protocol).
      The presence of such Properties does not necessarily require that
      a specific protocol must be used when a Connection is established,
      but that if this protocol is employed, a particular set of options
      should then be used..

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 may initiate a client-to-server style
      handshake; for other protocols, this may just establish local
      state.  The process of identifying options for connecting, such as
      resolution of the Remote Endpoint, occurs in response the Initiate

   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 endpoints
      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
      may or may not be usable by certain applications.  Boundaries of a
      Message may or may not be understood or transmitted by transport
      protocols.  Specifically, what one application considers to be two
      Messages sent on a stream-based transport may be treated as a
      single Message by the application on the other side.

   o  Send: The action to transmit a Message or partial Message over a
      Connection to a Remote Endpoint.  The interface to Send may
      include options specific to how the Message's content is to be
      sent.  Status of the Send operation may 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
      Endpoint, while the Message content itself will be delivered in an
      event (Section 4.1.5).  The interface to Receive may include
      options specific to the Message that is to be delivered to the

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.

   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.
      This should deliver an 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 may be an error 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 may influence how and
      where data is sent and/or received.

4.1.6.  Termination Actions

   o  Close: The action an application may take on a Connection to
      indicate that it no longer intends to send data, is no longer
      willing to receive data, and that the protocol should signal this
      state to the remote endpoint if applicable.

   o  Abort: The action the application may take on a Connection to
      indicate a Close, but with the additional indication that the
      transport system should not attempt to deliver any outstanding

4.2.  Transport System Implementation Concepts

   The Transport System Implementation Concepts define the set of
   objects used internally to a system or library to provide the
   functionality required to provide a transport service across a
   network, as required by the abstract interface.

   o  Connection Group: A set of Connections that share properties.  For
      multiplexing transport protocols, the Connection Group defines the
      set of Connections that can be multiplexed together.

   o  Path: Represents an available set of properties that a Local
      Endpoint may use to send or receive packets with a Remote

   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 may 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 Path Selection Properties and System Policy.
      Candidate Paths are identified during the gathering phase
      (Section 4.2.1) and may be used during the racing phase
      (Section 4.2.2).

   o  Candidate Protocol Stack: One protocol stack that may be used by
      an application for a connection, of which there may be several.

      Candidate Protocol Stacks are identified during the gathering
      phase (Section 4.2.1) and may be 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 may 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

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 Path
      Selection Properties provided by the application, and a Transport
      Services system's policies and heuristics.

   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 Protocol Properties provided by the
      application, and a Transport Services system's policies and

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.

   o  Endpoint Racing: 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 and the Local Endpoint, such as IP addresses
      resolved from a DNS hostname.

4.3.  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 may in some
   cases be controlled by properties that only change when application
   code is updated.  For example, an application may 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 racing multiple candidate
   Protocol Stacks during connection establishment.  This functionality
   can be a powerful driver of new protocol adoption, but must 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 must meet the following

   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 Endpoint,
       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.  See the
       security protocol equivalence section below for futher discussion
       (Section 4.3.1).

4.3.1.  Transport Security Equivalence

   The inclusion of transport security protocols
   [I-D.ietf-taps-transport-security] in a Protocol Stack adds extra
   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 should not 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 system should
   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 may be considered equivalent,
   whereas TLS 1.2 and TLS 1.3 [I-D.ietf-tls-tls13] should not be
   considered equivalent.

4.4.  Message Framing, Parsing, and Serialization

   While some transports expose a byte stream abstraction, most higher
   level protocols impose some structure onto that byte stream.  That
   is, the higher level protocol operates in terms of messages, protocol
   data units (PDUs), rather than using unstructured sequences of bytes,
   with each message being processed in turn.  Protocols are specified
   in terms of state machines acting on semantic messages, with parsing
   the byte stream into messages being a necessary annoyance, rather
   than a semantic concern.  Accordingly, the Transport Services
   architecture exposes messages as the primary abstraction.  Protocols
   that deal only in byte streams, such as TCP, represent their data in
   each direction as a single, long message.  When framing protocols are
   placed on top of byte streams, the messages used in the API represent
   the framed messages within the stream.

   Providing a message-based abstraction also provides:

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

   o  the ability to provide control of reliability, choosing what
      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 some
      messages may not be delivered due to either packet loss or missing
      a deadline, in particular the ability to avoid (re-)sending data
      that relies on a previous transmission that was never received.

   All require explicit message boundaries, and application-level
   framing of messages, to be effective.  Once a message is passed to
   the transport, it can not be cancelled or paused, but prioritization
   as well as lifetime and retransmission management will provide the
   protocol stack with all needed information to send the messages as
   quickly as possible without blocking transmission unnecessarily.  The
   transport services architecture facilitates this by handling
   messages, with known identity (sequence numbers, in the simple case),
   lifetimes, niceness, and antecedents.

   Transport protocols such as SCTP provide a message-oriented API that
   has similar features to those we describe.  Other transports, such as
   TCP, do not.  To support a message oriented API, while still being
   compatible with stream-based transport protocols, implementations of
   the transport services architecture should provide APIs for framing
   and de-framing messages.  That is, we push message framing down into
   the transport services API, allowing applications to send and receive
   complete messages.  This is backwards compatible with existing
   protocols and APIs, since the wire format of messages does not
   change, but gives the protocol stack additional information to allow
   it to make better use of modern transport services.

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 mimics 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.

   Clients must take care to use security APIs appropriately.  In cases
   where clients use said interface to provide sensitive keying
   material, e.g., access to private keys or copies of pre-shared keys
   (PSKs), key use must be validated.  For example, clients should not
   use PSK material created for the Encapsulating Security Protocol
   (ESP, part of IPsec) [RFC4303] with QUIC, and clients must not 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 not permit fallback for security
   protocols.  For example, if a client requests TLS, yet TLS or the
   desired version are not available, its connection must fail.  Clients
   are responsible for implementing protocol or version fallback using a
   Transport Services API if so desired.

7.  Acknowledgements

   This work has received funding from the European Union's Horizon 2020
   research and innovation programme under grant agreement No. 644334

   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

              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-01 (work in progress), May July 2018.

              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-00 draft-ietf-taps-interface-01 (work in
              progress), April July 2018.

              Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for End Systems", draft-ietf-taps-minset-04 draft-ietf-taps-minset-11 (work
              in progress), June September 2018.

              Pauly, T., Perkins, C., Rose, K., and C. Wood, "A Survey
              of Transport Security Protocols", draft-ietf-taps-
              transport-security-02 (work in progress), June 2018.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

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

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

   [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,

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

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [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,

              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-01 (work in progress), July 2018.

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)
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich

   Email: ietf@trammell.ch

   Anna Brunstrom
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad

   Email: anna.brunstrom@kau.se

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

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

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

   Email: csp@csperkins.org
   Philipp S. Tiesel
   TU Berlin
   Marchstrasse 23
   10587 Berlin

   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