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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12         Standards Track
          13                                                            
TAPS Working Group                                      B. Trammell, Ed.
Internet-Draft                                   Google Switzerland GmbH
Intended status: Standards Track                           M. Welzl, Ed.
Expires: 13 January 2022                              University of Oslo
                                                             T. Enghardt
                                                                 Netflix
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                           M. Kuehlewind
                                                                Ericsson
                                                              C. Perkins
                                                   University of Glasgow
                                                               P. Tiesel
                                                                  SAP SE
                                                               C.A. Wood
                                                              Cloudflare
                                                                T. Pauly
                                                              Apple Inc.
                                                                 K. Rose
                                               Akamai Technologies, Inc.
                                                            12 July 2021


     An Abstract Application Layer Interface to Transport Services
                      draft-ietf-taps-interface-13

Abstract

   This document describes an abstract application programming
   interface, API, to the transport layer that enables the selection of
   transport protocols and network paths dynamically at runtime.  This
   API enables faster deployment of new protocols and protocol features
   without requiring changes to the applications.  The specified API
   follows the Transport Services Architecture by providing
   asynchronous, atomic transmission of messages.  It is intended to
   replace the traditional BSD sockets API as the common interface to
   the transport layer, in an environment where endpoints could select
   from multiple interfaces and potential transport protocols.

Status of This Memo

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

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



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

   This Internet-Draft will expire on 13 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology and Notation  . . . . . . . . . . . . . . . .   5
     1.2.  Specification of Requirements . . . . . . . . . . . . . .   7
   2.  Overview of Interface Design  . . . . . . . . . . . . . . . .   7
   3.  API Summary . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Usage Examples  . . . . . . . . . . . . . . . . . . . . .   9
       3.1.1.  Server Example  . . . . . . . . . . . . . . . . . . .  10
       3.1.2.  Client Example  . . . . . . . . . . . . . . . . . . .  10
       3.1.3.  Peer Example  . . . . . . . . . . . . . . . . . . . .  12
   4.  Transport Properties  . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Transport Property Names  . . . . . . . . . . . . . . . .  13
     4.2.  Transport Property Types  . . . . . . . . . . . . . . . .  14
   5.  Scope of the Interface Definition . . . . . . . . . . . . . .  15
   6.  Pre-Establishment Phase . . . . . . . . . . . . . . . . . . .  16
     6.1.  Specifying Endpoints  . . . . . . . . . . . . . . . . . .  17
       6.1.1.  Using Multicast Endpoints . . . . . . . . . . . . . .  18
       6.1.2.  Endpoint Aliases  . . . . . . . . . . . . . . . . . .  18
       6.1.3.  Endpoint Examples . . . . . . . . . . . . . . . . . .  19
     6.2.  Specifying Transport Properties . . . . . . . . . . . . .  20
       6.2.1.  Reliable Data Transfer (Connection) . . . . . . . . .  23
       6.2.2.  Preservation of Message Boundaries  . . . . . . . . .  23
       6.2.3.  Configure Per-Message Reliability . . . . . . . . . .  24
       6.2.4.  Preservation of Data Ordering . . . . . . . . . . . .  24
       6.2.5.  Use 0-RTT Session Establishment with a Safely
               Replayable Message  . . . . . . . . . . . . . . . . .  24



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       6.2.6.  Multistream Connections in Group  . . . . . . . . . .  24
       6.2.7.  Full Checksum Coverage on Sending . . . . . . . . . .  25
       6.2.8.  Full Checksum Coverage on Receiving . . . . . . . . .  25
       6.2.9.  Congestion control  . . . . . . . . . . . . . . . . .  25
       6.2.10. Keep alive  . . . . . . . . . . . . . . . . . . . . .  26
       6.2.11. Interface Instance or Type  . . . . . . . . . . . . .  26
       6.2.12. Provisioning Domain Instance or Type  . . . . . . . .  27
       6.2.13. Use Temporary Local Address . . . . . . . . . . . . .  28
       6.2.14. Multipath Transport . . . . . . . . . . . . . . . . .  28
       6.2.15. Advertisement of Alternative Addresses  . . . . . . .  29
       6.2.16. Direction of communication  . . . . . . . . . . . . .  30
       6.2.17. Notification of ICMP soft error message arrival . . .  30
       6.2.18. Initiating side is not the first to write . . . . . .  31
     6.3.  Specifying Security Parameters and Callbacks  . . . . . .  31
       6.3.1.  Specifying Security Parameters on a Pre-Connection  .  31
       6.3.2.  Connection Establishment Callbacks  . . . . . . . . .  33
   7.  Establishing Connections  . . . . . . . . . . . . . . . . . .  33
     7.1.  Active Open: Initiate . . . . . . . . . . . . . . . . . .  34
     7.2.  Passive Open: Listen  . . . . . . . . . . . . . . . . . .  35
     7.3.  Peer-to-Peer Establishment: Rendezvous  . . . . . . . . .  36
     7.4.  Connection Groups . . . . . . . . . . . . . . . . . . . .  38
   8.  Managing Connections  . . . . . . . . . . . . . . . . . . . .  39
     8.1.  Generic Connection Properties . . . . . . . . . . . . . .  41
       8.1.1.  Required Minimum Corruption Protection Coverage for
               Receiving . . . . . . . . . . . . . . . . . . . . . .  41
       8.1.2.  Connection Priority . . . . . . . . . . . . . . . . .  42
       8.1.3.  Timeout for Aborting Connection . . . . . . . . . . .  42
       8.1.4.  Timeout for keep alive packets  . . . . . . . . . . .  42
       8.1.5.  Connection Group Transmission Scheduler . . . . . . .  43
       8.1.6.  Capacity Profile  . . . . . . . . . . . . . . . . . .  43
       8.1.7.  Policy for using Multipath Transports . . . . . . . .  45
       8.1.8.  Bounds on Send or Receive Rate  . . . . . . . . . . .  46
       8.1.9.  Group Connection Limit  . . . . . . . . . . . . . . .  46
       8.1.10. Isolate Session . . . . . . . . . . . . . . . . . . .  46
       8.1.11. Read-only Connection Properties . . . . . . . . . . .  47
     8.2.  TCP-specific Properties: User Timeout Option (UTO)  . . .  48
       8.2.1.  Advertised User Timeout . . . . . . . . . . . . . . .  48
       8.2.2.  User Timeout Enabled  . . . . . . . . . . . . . . . .  48
       8.2.3.  Timeout Changeable  . . . . . . . . . . . . . . . . .  49
     8.3.  Connection Lifecycle Events . . . . . . . . . . . . . . .  49
       8.3.1.  Soft Errors . . . . . . . . . . . . . . . . . . . . .  49
       8.3.2.  Path change . . . . . . . . . . . . . . . . . . . . .  49
   9.  Data Transfer . . . . . . . . . . . . . . . . . . . . . . . .  49
     9.1.  Messages and Framers  . . . . . . . . . . . . . . . . . .  50
       9.1.1.  Message Contexts  . . . . . . . . . . . . . . . . . .  50
       9.1.2.  Message Framers . . . . . . . . . . . . . . . . . . .  50
       9.1.3.  Message Properties  . . . . . . . . . . . . . . . . .  53
     9.2.  Sending Data  . . . . . . . . . . . . . . . . . . . . . .  58



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       9.2.1.  Basic Sending . . . . . . . . . . . . . . . . . . . .  59
       9.2.2.  Send Events . . . . . . . . . . . . . . . . . . . . .  59
       9.2.3.  Partial Sends . . . . . . . . . . . . . . . . . . . .  60
       9.2.4.  Batching Sends  . . . . . . . . . . . . . . . . . . .  61
       9.2.5.  Send on Active Open: InitiateWithSend . . . . . . . .  61
       9.2.6.  Priority in TAPS  . . . . . . . . . . . . . . . . . .  62
     9.3.  Receiving Data  . . . . . . . . . . . . . . . . . . . . .  63
       9.3.1.  Enqueuing Receives  . . . . . . . . . . . . . . . . .  63
       9.3.2.  Receive Events  . . . . . . . . . . . . . . . . . . .  64
       9.3.3.  Receive Message Properties  . . . . . . . . . . . . .  66
   10. Connection Termination  . . . . . . . . . . . . . . . . . . .  67
   11. Connection State and Ordering of Operations and Events  . . .  69
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  70
   13. Privacy and Security Considerations . . . . . . . . . . . . .  70
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  72
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  72
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  72
     15.2.  Informative References . . . . . . . . . . . . . . . . .  73
   Appendix A.  Implementation Mapping . . . . . . . . . . . . . . .  76
     A.1.  Types . . . . . . . . . . . . . . . . . . . . . . . . . .  76
     A.2.  Events and Errors . . . . . . . . . . . . . . . . . . . .  77
     A.3.  Time Duration . . . . . . . . . . . . . . . . . . . . . .  77
   Appendix B.  Convenience Functions  . . . . . . . . . . . . . . .  77
     B.1.  Adding Preference Properties  . . . . . . . . . . . . . .  77
     B.2.  Transport Property Profiles . . . . . . . . . . . . . . .  78
       B.2.1.  reliable-inorder-stream . . . . . . . . . . . . . . .  78
       B.2.2.  reliable-message  . . . . . . . . . . . . . . . . . .  78
       B.2.3.  unreliable-datagram . . . . . . . . . . . . . . . . .  79
   Appendix C.  Relationship to the Minimal Set of Transport Services
           for End Systems . . . . . . . . . . . . . . . . . . . . .  80
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  83

1.  Introduction

   This document specifies a modern abstract application programming
   interface (API) atop the high-level architecture for transport
   services defined in [I-D.ietf-taps-arch].  The Transport Services
   Architecture supports asynchronous, atomic transmission of messages
   over transport protocols and network paths dynamically selected at
   runtime, in environments where an endpoint selects from multiple
   interfaces and potential transport protocols.

   Applications that adopt this interface will benefit from a wide set
   of transport features that can evolve over time.  This protocol-
   independent API ensures that the system providing the interface can
   optimize its behavior based on the application requirements and
   network conditions, without requiring changes to the applications.
   This flexibility enables faster deployment of new features and



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   protocols, and can support applications by offering racing and
   fallback mechanisms, which otherwise need to be separately
   implemented in each application.

   This API derives specific path and protocol selection properties and
   supported transport features from the analysis provided in [RFC8095],
   [RFC8923], and [RFC8922].  The design encourages implementations
   underneath the interface to dynamically choose a transport protocol
   depending on an application's choices rather than statically binding
   applications to a protocol at compile time.  Nevertheless, the
   Transport Services API also provides applications with a way to
   override transport selection and instantiate a specific stack, e.g.,
   to support servers wishing to listen to a specific protocol.
   However, forcing a specific transport stack choice is discouraged for
   general use, because it can reduce portability.

1.1.  Terminology and Notation

   This API is described in terms of

   *  Objects with which an application can interact;

   *  Actions the application can perform on these Objects;

   *  Events, which an Object can send to an application to be processed
      aynchronously; and

   *  Parameters associated with these Actions and Events.

   The following notations, which can be combined, are used in this
   document:

   *  An Action that creates an Object:

         Object := Action()

   *  An Action that creates an array of Objects:

         []Object := Action()

   *  An Action that is performed on an Object:

         Object.Action()

   *  An Object sends an Event:

         Object -> Event<>




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   *  An Action takes a set of Parameters; an Event contains a set of
      Parameters.  Action and Event parameters whose names are suffixed
      with a question mark are optional.

         Action(param0, param1?, ...) / Event<param0, param1, ...>

   Actions associated with no Object are Actions on the abstract
   interface itself; they are equivalent to Actions on a per-application
   global context.

   Events are sent to the application or application-supplied code (e.g.
   framers, see Section 9.1.2) for processing; the details of event
   processing are platform- and implementation-specific.

   We also make use of the following basic types:

   *  Boolean: Instances take the value "true" or "false".

   *  Integer: Instances take positive or negative numeric integer
      values, or sometimes special non-numeric (symbolic) values.

   *  Numeric: Instances take positive or negative numeric values, or
      sometimes special non-numeric (symbolic) values.

   *  Enumeration: A family of types in which each instance takes one of
      a fixed, predefined set of values specific to a given enumerated
      type.

   *  Tuple: An ordered grouping of multiple value types, represented as
      a comma-separated list in parentheses, e.g., "(Enumeration,
      Preference)".  Instances take a sequence of values each valid for
      the corresponding value type.

   *  Array: Denoted []Type, an instance takes a value for each of zero
      or more elements in a sequence of the given Type.  An array may be
      of fixed or variable length.

   *  Collection: An unordered grouping of one or more values of the
      same type.

   For guidance on how these abstract concepts may be implemented in
   languages in accordance with native design patterns and language and
   platform features, see Appendix A.








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1.2.  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.

2.  Overview of Interface Design

   The design of the interface specified in this document is based on a
   set of principles, themselves an elaboration on the architectural
   design principles defined in [I-D.ietf-taps-arch].  The interface
   defined in this document provides:

   *  Access to a variety of transport protocols, independent of the
      Protocol Stacks that will be used at runtime.  All common features
      of these protocol stacks are made available to the application in
      a transport-independent way to the degree possible.  This enables
      applications written to a single API to make use of transport
      protocols in terms of the features they provide.

   *  A unified interface to datagram and stream-oriented transports,
      allowing use of a common API for connection establishment and
      closing.

   *  Message-orientation, as opposed to stream-orientation, using
      application-assisted framing and deframing where the underlying
      transport does not provide these.

   *  Asynchronous Connection establishment, transmission, and
      reception.  This allows concurrent operations during establishment
      and event-driven application interactions with the transport
      layer, in line with developments in modern platforms and
      programming languages;

   *  Selection between alternate network paths, using additional
      information about the networks over which a connection can operate
      (e.g.  Provisioning Domain (PvD) information [RFC7556]) where
      available.

   *  Explicit support for transport-specific features to be applied,
      should that particular transport be part of a chosen Protocol
      Stack.

   *  Explicit support for security properties as first-order transport
      features.




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   *  Explicit support for configuration of cryptographic identities and
      transport security parameters persistent across multiple
      Connections.

   *  Explicit support for multistreaming and multipath transport
      protocols, and the grouping of related Connections into Connection
      Groups through cloning of Connections.  This allows applications
      to take full advantage of new transport protocols supporting these
      features.

3.  API Summary

   The Transport Services API is the basic common abstract application
   programming interface to the Transport Services Architecture defined
   in the TAPS Architecture [I-D.ietf-taps-arch].

   An application primarily interacts with this API through two Objects:
   Preconnections and Connections.  A Preconnection object (Section 6)
   represents a set of properties and constraints on the selection and
   configuration of paths and protocols to establish a Connection with
   an Endpoint.  A Connection object represents an instance of a
   transport Protocol Stack on which data can be sent to and/or received
   from a Remote Endpoint (i.e., a logical connection that, depending on
   the kind of transport, can be bi-directional or unidirectional, and
   that can use a stream protocol or a datagram protocol).  Connections
   are presented consistently to the application, irrespective of
   whether the underlying transport is connection-less or connection-
   oriented.  Connections can be created from Preconnections in three
   ways:

   *  by initiating the Preconnection (i.e., actively opening, as in a
      client; Section 7.1),

   *  through listening on the Preconnection (i.e., passively opening,
      as in a server Section 7.2),

   *  or rendezvousing on the Preconnection (i.e., peer to peer
      establishment; Section 7.3).













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   Once a Connection is established, data can be sent and received on it
   in the form of Messages.  The interface supports the preservation of
   message boundaries both via explicit Protocol Stack support, and via
   application support through a Message Framer that finds message
   boundaries in a stream.  Messages are received asynchronously through
   event handlers registered by the application.  Errors and other
   notifications also happen asynchronously on the Connection.  It is
   not necessary for an application to handle all Events; some Events
   may have implementation-specific default handlers.  The application
   should not assume that ignoring Events (e.g., Errors) is always safe.

   Section 6, Section 7, Section 9.2, Section 9.3, and Section 10
   describe the details of application interaction with Objects through
   Actions and Events in each phase of a Connection, following the
   phases (Pre-Establishment, Establishment, Data Transfer, and
   Termination) described in Section 4.1 of [I-D.ietf-taps-arch].

3.1.  Usage Examples

   The following usage examples illustrate how an application might use
   the Transport Services Interface to:

   *  Act as a server, by listening for incoming connections, receiving
      requests, and sending responses, see Section 3.1.1.

   *  Act as a client, by connecting to a Remote Endpoint using
      Initiate, sending requests, and receiving responses, see
      Section 3.1.2.

   *  Act as a peer, by connecting to a Remote Endpoint using Rendezvous
      while simultaneously waiting for incoming Connections, sending
      Messages, and receiving Messages, see Section 3.1.3.

   The examples in this section presume that a transport protocol is
   available between the Local and Remote Endpoints that provides
   Reliable Data Transfer, Preservation of data ordering, and
   Preservation of Message Boundaries.  In this case, the application
   can choose to receive only complete messages.

   If none of the available transport protocols provides Preservation of
   Message Boundaries, but there is a transport protocol that provides a
   reliable ordered byte stream, an application could receive this byte
   stream as partial Messages and transform it into application-layer
   Messages.  Alternatively, an application might provide a Message
   Framer, which can transform a sequence of Messages into a byte stream
   and vice versa (Section 9.1.2).





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3.1.1.  Server Example

   This is an example of how an application might listen for incoming
   Connections using the Transport Services Interface, and receive a
   request, and send a response.

   LocalSpecifier := NewLocalEndpoint()
   LocalSpecifier.WithInterface("any")
   LocalSpecifier.WithService("https")

   TransportProperties := NewTransportProperties()
   TransportProperties.Require(preserve-msg-boundaries)
   // Reliable Data Transfer and Preserve Order are Required by default

   SecurityParameters := NewSecurityParameters()
   SecurityParameters.Set(identity, myIdentity)
   SecurityParameters.Set(key-pair, myPrivateKey, myPublicKey)

   // Specifying a Remote Endpoint is optional when using Listen()
   Preconnection := NewPreconnection(LocalSpecifier,
                                     TransportProperties,
                                     SecurityParameters)

   Listener := Preconnection.Listen()

   Listener -> ConnectionReceived<Connection>

   // Only receive complete messages in a Conn.Received handler
   Connection.Receive()

   Connection -> Received<messageDataRequest, messageContext>

   //---- Receive event handler begin ----
   Connection.Send(messageDataResponse)
   Connection.Close()

   // Stop listening for incoming Connections
   // (this example supports only one Connection)
   Listener.Stop()
   //---- Receive event handler end ----

3.1.2.  Client Example

   This is an example of how an application might open two Connections
   to a remote application using the Transport Services Interface, and
   send a request as well as receive a response on each of them.





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   RemoteSpecifier := NewRemoteEndpoint()
   RemoteSpecifier.WithHostname("example.com")
   RemoteSpecifier.WithService("https")

   TransportProperties := NewTransportProperties()
   TransportProperties.Require(preserve-msg-boundaries)
   // Reliable Data Transfer and Preserve Order are Required by default

   SecurityParameters := NewSecurityParameters()
   TrustCallback := NewCallback({
     // Verify identity of the Remote Endpoint, return the result
   })
   SecurityParameters.SetTrustVerificationCallback(TrustCallback)

   // Specifying a local endpoint is optional when using Initiate()
   Preconnection := NewPreconnection(RemoteSpecifier,
                                     TransportProperties,
                                     SecurityParameters)

   Connection := Preconnection.Initiate()
   Connection2 := Connection.Clone()

   Connection -> Ready<>
   Connection2 -> Ready<>

   //---- Ready event handler for any Connection C begin ----
   C.Send(messageDataRequest)

   // Only receive complete messages
   C.Receive()
   //---- Ready event handler for any Connection C end ----

   Connection -> Received<messageDataResponse, messageContext>
   Connection2 -> Received<messageDataResponse, messageContext>

   // Close the Connection in a Receive event handler
   Connection.Close()
   Connection2.Close()

   Preconnections are reusable after being used to initiate a
   Connection.  Hence, for example, after the Connections were closed,
   the following would be correct: ~~~ //.. carry out adjustments to the
   Preconnection, if desire Connection := Preconnection.Initiate() ~~~








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3.1.3.  Peer Example

   This is an example of how an application might establish a connection
   with a peer using Rendezvous(), send a Message, and receive a
   Message.

// Configure local candidates: a port on the Local Endpoint and via a STUN server
HostCandidate := NewLocalEndpoint()
HostCandidate.WithPort(9876)

StunCandidate := NewLocalEndpoint()
StunCandidate.WithStunServer(address, port, credentials)

LocalCandidates = [HostCandidate, StunCandidate]

// Configure transport and security properties
TransportProperties := ...
SecurityParameters  := ...

Preconnection := NewPreconnection(LocalCandidates,
                                  [], // No remote candidates yet
                                  TransportProperties,
                                  SecurityParameters)

// Resolve the LocalCandidates. The Preconnection.Resolve() call
// resolves both local and remote candidates but, since the remote
// candidates have not yet been specified, the ResolvedRemote list
// returned will be empty and is not used.
ResolvedLocal, ResolvedRemote = Preconnection.Resolve()

// ...Send the ResolvedLocal list to peer via signalling channel
// ...Receive a list of RemoteCandidates from peer via signalling channel

Preconnection.AddRemote(RemoteCandidates)
Preconnection.Rendezvous()

Preconnection -> RendezvousDone<Connection>

//---- RendezvousDone event handler begin ----
Connection.Send(messageDataRequest)
Connection.Receive()
//---- RendezvousDone event handler end ----

Connection -> Received<messageDataResponse, messageContext>

// Close the Connection in a Receive event handler
Connection.Close()




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4.  Transport Properties

   Each application using the Transport Services Interface declares its
   preferences for how the transport service should operate using
   properties at each stage of the lifetime of a connection using
   Transport Properties, as defined in [I-D.ietf-taps-arch].

   Transport Properties are divided into Selection, Connection, and
   Message Properties.  Selection Properties (see Section 6.2) can only
   be set during pre-establishment.  They are only used to specify which
   paths and protocol stacks can be used and are preferred by the
   application.  Although Connection Properties (see Section 8.1) can be
   set during pre-establishment, they may be changed later.  They are
   used to inform decisions made during establishment and to fine-tune
   the established connection.  Calling Initiate on a Preconnection
   creates an outbound Connection or a Listener, and the Selection
   Properties remain readable from the Connection or Listener, but
   become immutable.

   The behavior of the selected protocol stack(s) when sending Messages
   is controlled by Message Properties (see Section 9.1.3).

   Selection Properties can be set on Preconnections, and the effect of
   Selection Properties can be queried on Connections and Messages.
   Connection Properties can be set on Connections and Preconnections;
   when set on Preconnections, they act as an initial default for the
   resulting Connections.  Message Properties can be set on Messages,
   Connections, and Preconnections; when set on the latter two, they act
   as an initial default for the Messages sent over those Connections,

   Note that configuring Connection Properties and Message Properties on
   Preconnections is preferred over setting them later.  Early
   specification of Connection Properties allows their use as additional
   input to the selection process.  Protocol Specific Properties, which
   enable configuration of specialized features of a specific protocol,
   see Section 3.2 of [I-D.ietf-taps-arch], are not used as an input to
   the selection process, but only support configuration if the
   respective protocol has been selected.

4.1.  Transport Property Names

   Transport Properties are referred to by property names.  For the
   purposes of this document, these names are alphanumeric strings in
   which words may be separated by hyphens.  These names serve two
   purposes:






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   *  Allowing different components of a TAPS implementation to pass
      Transport Properties, e.g., between a language frontend and a
      policy manager, or as a representation of properties retrieved
      from a file or other storage.

   *  Making the code of different TAPS implementations look similar.
      While individual programming languages may preclude strict
      adherence to the aforementioned naming convention (for instance,
      by prohibiting the use of hyphens in symbols), users interacting
      with multiple implementations will still benefit from the
      consistency resulting from the use of visually similar symbols.

   Transport Property Names are hierarchically organized in the form
   [<Namespace>.]<PropertyName>.

   *  The Namespace component MUST be empty for well-known, generic
      properties, i.e., for properties that are not specific to a
      protocol and are defined in an RFC.

   *  Protocol Specific Properties MUST use the protocol acronym as the
      Namespace, e.g., "tcp" for TCP specific Transport Properties.  For
      IETF protocols, property names under these namespaces SHOULD be
      defined in an RFC.

   *  Vendor or implementation specific properties MUST use a string
      identifying the vendor or implementation as the Namespace.

   Namespaces for each of the keywords provided in the IANA protocol
   numbers registry (see https://www.iana.org/assignments/protocol-
   numbers/protocol-numbers.xhtml), reformatted where necessary to
   conform to an implementation's naming conventions, are reserved for
   Protocol Specific Properties and MUST NOT be used for vendor or
   implementation-specific properties.

4.2.  Transport Property Types

   Each Transport Property has a one of the basic types described in
   Section 1.1.

   Most Selection Properties (see Section 6.2) are of the Enumeration
   type, and use the Preference Enumeration, which takes one of five
   possible values (Prohibit, Avoid, Ignore, Prefer, or Require)
   denoting the level of preference for a given property during protocol
   selection.







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5.  Scope of the Interface Definition

   This document defines a language- and platform-independent interface
   to a Transport Services system.  Given the wide variety of languages
   and language conventions used to write applications that use the
   transport layer to connect to other applications over the Internet,
   this independence makes this interface necessarily abstract.

   There is no interoperability benefit in tightly defining how the
   interface is presented to application programmers across diverse
   platforms.  However, maintaining the "shape" of the abstract
   interface across different platforms reduces the effort for
   programmers who learn the transport services interface to then apply
   their knowledge to another platform.

   We therefore make the following recommendations:

   *  Actions, Events, and Errors in implementations of this interface
      SHOULD use the names given for them in the document, subject to
      capitalization, punctuation, and other typographic conventions in
      the language of the implementation, unless the implementation
      itself uses different names for substantially equivalent objects
      for networking by convention.

   *  Implementations of this interface SHOULD implement each Selection
      Property, Connection Property, and Message Context Property
      specified in this document.  Each interface SHOULD be implemented
      even when in a specific implementation/platform it will always
      result in no operation, e.g. there is no action when the API
      specifies a Property that is not available in a transport protocol
      implemented on a specific platform.  For example, if TCP is the
      only underlying transport protocol, the Message Property
      "msgOrdered" can be implemented (trivially, as a no-op) as
      disabling the requirement for ordering will not have any effect on
      delivery order for Connections over TCP.  Similarly, the "msg-
      lifetime" Message Property can be implemented but ignored, as the
      description of this Property states that "it is not guaranteed
      that a Message will not be sent when its Lifetime has expired".

   *  Implementations may use other representations for Transport
      Property Names, e.g., by providing constants, but should provide a
      straight-forward mapping between their representation and the
      property names specified here.








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6.  Pre-Establishment Phase

   The Pre-Establishment phase allows applications to specify properties
   for the Connections that they are about to make, or to query the API
   about potential Connections they could make.

   A Preconnection Object represents a potential Connection.  It is a
   passive Object (a data structure) that merely maintains the state
   that describes the properties of a Connection that might exist in the
   future.  This state comprises Local Endpoint and Remote Endpoint
   Objects that denote the endpoints of the potential Connection (see
   Section 6.1), the Selection Properties (see Section 6.2), any
   preconfigured Connection Properties (Section 8.1), and the security
   parameters (see Section 6.3):

      Preconnection := NewPreconnection([]LocalEndpoint,
                                        []RemoteEndpoint,
                                        TransportProperties,
                                        SecurityParameters)

   At least one Local Endpoint MUST be specified if the Preconnection is
   used to Listen() for incoming Connections, but the list of Local
   Endpoints MAY be empty if the Preconnection is used to Initiate()
   connections.  If no Local Endpoint is specified, the Transport
   Services system will assign an ephemeral local port to the Connection
   on the appropriate interface(s).  At least one Remote Endpoint MUST
   be specified if the Preconnection is used to Initiate() Connections,
   but the list of Remote Endpoints MAY be empty if the Preconnection is
   used to Listen() for incoming Connections.  At least one Local
   Endpoint and one Remote Endpoint MUST be specified if a peer-to-peer
   Rendezvous() is to occur based on the Preconnection.

   If more than one Local Endpoint is specified on a Preconnection, then
   all the Local Endpoints on the Preconnection MUST represent the same
   host.  For example, they might correspond to different interfaces on
   a multi-homed host, of they might correspond to local interfaces and
   a STUN server that can be resolved to a server reflexive address for
   a Preconnection used to make a peer-to-peer Rendezvous().

   If more than one Remote Endpoint is specified on the Preconnection,
   then all the Remote Endpoints on the Preconnection SHOULD represent
   the same host.  For example, the Remote Endpoints might represent
   various network interfaces of a host, or a server reflexive address
   that can be used to reach a host, or a set of hosts that provide
   equivalent local balanced service.






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   In most cases, it is expected that a single Remote Endpoint will be
   specified by name, and a later call to Initiate() on the
   Preconnection (see Section 7.1) will internally resolve that name to
   a list of concrete endpoints.  Specifying multiple Remote Endpoints
   on a Preconnection allows applications to override this for more
   detailed control.

   If Message Framers are used (see Section 9.1.2), they MUST be added
   to the Preconnection during pre-establishment.

6.1.  Specifying Endpoints

   The transport services API uses the Local Endpoint and Remote
   Endpoint Objects to refer to the endpoints of a transport connection.
   Endpoints can be created as either Remote or Local:

   RemoteSpecifier := NewRemoteEndpoint()
   LocalSpecifier := NewLocalEndpoint()

   A single Endpoint Object represents the identity of a network host.
   That endpoint can be more or less specific depending on which
   identifiers are set.  For example, an Endpoint that only specifies a
   hostname may in fact end up corresponding to several different IP
   addresses on different hosts.

   An Endpoint Object can be configured with the following identifiers:

   *  Hostname (string):

   RemoteSpecifier.WithHostname("example.com")

   *  Port (a 16-bit integer) or a Service (string) that maps to a port:

   RemoteSpecifier.WithPort(443)

   RemoteSpecifier.WithService("https")

   *  IP address (IPv4 or IPv6 address):

   RemoteSpecifier.WithIPv4Address(192.0.2.21)

   RemoteSpecifier.WithIPv6Address(2001:db8:4920:e29d:a420:7461:7073:0a)

   *  Interface name (string):

   LocalSpecifier.WithInterface("en0")





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   An Endpoint cannot have multiple identifiers of a same type set.
   That is, an endpoint cannot have two IP addresses specified.  Two
   separate IP addresses are represented as two Endpoint Objects.  If a
   Preconnection specifies a Remote Endpoint with a specific IP address
   set, it will only establish Connections to that IP address.  If, on
   the other hand, the Remote Endpoint specifies a hostname but no
   addresses, the Connection can perform name resolution and attempt
   using any address derived from the original hostname of the Remote
   Endpoint.

   The Transport Services API resolves names internally, when the
   Initiate(), Listen(), or Rendezvous() method is called to establish a
   Connection.  Privacy considerations for the timing of this resolution
   are given in Section 13.

   The Resolve() action on a Preconnection can be used by the
   application to force early binding when required, for example with
   some Network Address Translator (NAT) traversal protocols (see
   Section 7.3).

6.1.1.  Using Multicast Endpoints

   Specifying a multicast group address on a Local Endpoint will
   indicate to the transport system that the resulting connection will
   be used to receive multicast messages.  The Remote Endpoint can be
   used to filter incoming multicast from specific senders.  Such a
   Preconnection will only support calling Listen(), not Initiate().
   The accepted Connections are receive-only.

   Similarly, specifying a multicast group address on the Remote
   Endpoint will indicate that the resulting connection will be used to
   send multicast messages.

6.1.2.  Endpoint Aliases

   An Endpoint can have an alternative definition when using different
   protocols.  For example, a server that supports both TLS/TCP and QUIC
   may be accessible on two different port numbers depending on which
   protocol is used.

   To support this, Endpoint Objects can specify "aliases".  An Endpoint
   can have multiple aliases set.

   RemoteSpecifier.AddAlias(AlternateRemoteSpecifier)

   In order to scope an alias to a specific transport protocol, an
   Endpoint can specify a protocol identifier.




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   RemoteSpecifier.WithProtocol(QUIC)

   The following example shows a case where "example.com" has a server
   running on port 443, with an alternate port of 8443 for QUIC.

   RemoteSpecifier := NewRemoteEndpoint()
   RemoteSpecifier.WithHostname("example.com")
   RemoteSpecifier.WithPort(443)

   QUICRemoteSpecifier := NewRemoteEndpoint()
   QUICRemoteSpecifier.WithHostname("example.com")
   QUICRemoteSpecifier.WithPort(8443)
   QUICRemoteSpecifier.WithProtocol(QUIC)

   RemoteSpecifier.AddAlias(QUICRemoteSpecifier)

6.1.3.  Endpoint Examples

   The following examples of Endpoints show common usage patterns.

   Specify a Remote Endpoint using a hostname and service name:

   RemoteSpecifier := NewRemoteEndpoint()
   RemoteSpecifier.WithHostname("example.com")
   RemoteSpecifier.WithService("https")

   Specify a Remote Endpoint using an IPv6 address and remote port:

   RemoteSpecifier := NewRemoteEndpoint()
   RemoteSpecifier.WithIPv6Address(2001:db8:4920:e29d:a420:7461:7073:0a)
   RemoteSpecifier.WithPort(443)

   Specify a Remote Endpoint using an IPv4 address and remote port:

   RemoteSpecifier := NewRemoteEndpoint()
   RemoteSpecifier.WithIPv4Address(192.0.2.21)
   RemoteSpecifier.WithPort(443)

   Specify a Local Endpoint using a local interface name and local port:

   LocalSpecifier := NewLocalEndpoint()
   LocalSpecifier.WithInterface("en0")
   LocalSpecifier.WithPort(443)

   As an alternative to specifying an interface name for the Local
   Endpoint, an application can express more fine-grained preferences
   using the "Interface Instance or Type" Selection Property, see
   Section 6.2.11.  However, if the application specifies Selection



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   Properties that are inconsistent with the Local Endpoint, this will
   result in an Error once the application attempts to open a
   Connection.

   Specify a Local Endpoint using a STUN server:

   LocalSpecifier := NewLocalEndpoint()
   LocalSpecifier.WithStunServer(address, port, credentials)

   Specify a Local Endpoint using an Any-Source Multicast group to join
   on a named local interface:

   LocalSpecifier := NewLocalEndpoint()
   LocalSpecifier.WithIPv4Address(233.252.0.0)
   LocalSpecifier.WithInterface("en0")

   Source-Specific Multicast requires setting both a Local and Remote
   Endpoint:

   LocalSpecifier := NewLocalEndpoint()
   LocalSpecifier.WithIPv4Address(232.1.1.1)
   LocalSpecifier.WithInterface("en0")

   RemoteSpecifier := NewRemoteEndpoint()
   RemoteSpecifier.WithIPv4Address(192.0.2.22)

6.2.  Specifying Transport Properties

   A Preconnection Object holds properties reflecting the application's
   requirements and preferences for the transport.  These include
   Selection Properties for selecting protocol stacks and paths, as well
   as Connection Properties for configuration of the detailed operation
   of the selected Protocol Stacks.

   The protocol(s) and path(s) selected as candidates during
   establishment are determined and configured using these properties.
   Since there could be paths over which some transport protocols are
   unable to operate, or remote endpoints that support only specific
   network addresses or transports, transport protocol selection is
   necessarily tied to path selection.  This may involve choosing
   between multiple local interfaces that are connected to different
   access networks.

   When additional information (such as Provisioning Domain (PvD)
   information Path information can include network segment PMTU, set of
   supported DSCPs, expected usage, cost, etc.  The usage of this
   information by the Transport Services System is generally independent
   of the specific mechanism/protocol used to receive the information



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   (e.g. zero-conf, DHCP, or IPv6 RA).[RFC7556]) is available about the
   networks over which an endpoint can operate, this can inform the
   selection between alternate network paths.

   Most Selection Properties are represented as Preferences, which can
   take one of five values:

          +============+========================================+
          | Preference | Effect                                 |
          +============+========================================+
          | Require    | Select only protocols/paths providing  |
          |            | the property, fail otherwise           |
          +------------+----------------------------------------+
          | Prefer     | Prefer protocols/paths providing the   |
          |            | property, proceed otherwise            |
          +------------+----------------------------------------+
          | Ignore     | No preference                          |
          +------------+----------------------------------------+
          | Avoid      | Prefer protocols/paths not providing   |
          |            | the property, proceed otherwise        |
          +------------+----------------------------------------+
          | Prohibit   | Select only protocols/paths not        |
          |            | providing the property, fail otherwise |
          +------------+----------------------------------------+

               Table 1: Selection Property Preference Levels

   The implementation MUST ensure an outcome that is consistent with all
   application requirements expressed using Require and Prohibit.  While
   preferences expressed using Prefer and Avoid influence protocol and
   path selection as well, outcomes can vary given the same Selection
   Properties, because the available protocols and paths can differ
   across systems and contexts.  However, implementations are
   RECOMMENDED to seek to provide a consistent outcome to an
   application, given the same set of Selection Properties.
















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   Note that application preferences can conflict with each other.  For
   example, if an application indicates a preference for a specific path
   by specifying an interface, but also a preference for a protocol, a
   situation might occur in which the preferred protocol is not
   available on the preferred path.  In such cases, applications can
   expect properties that determine path selection to be prioritized
   over properties that determine protocol selection.  The transport
   system SHOULD determine the preferred path first, regardless of
   protocol preferences.  This ordering is chosen to provide consistency
   across implementations, based on the fact that it is more common for
   the use of a given network path to determine cost to the user (i.e.,
   an interface type preference might be based on a user's preference to
   avoid being charged more for a cellular data plan).

   Selection and Connection Properties, as well as defaults for Message
   Properties, can be added to a Preconnection to configure the
   selection process and to further configure the eventually selected
   protocol stack(s).  They are collected into a TransportProperties
   object to be passed into a Preconnection object:

   TransportProperties := NewTransportProperties()

   Individual properties are then set on the TransportProperties Object.
   Setting a Transport Property to a value overrides the previous value
   of this Transport Property.

   TransportProperties.Set(property, value)

   To aid readability, implementations MAY provide additional
   convenience functions to simplify use of Selection Properties: see
   Appendix B.1 for examples.  In addition, implementations MAY provide
   a mechanism to create TransportProperties objects that are
   preconfigured for common use cases as outlined in Appendix B.2.

   Transport Properties for an established connection can be queried via
   the Connection object, as outlined in Section 8.

   A Connection gets its Transport Properties either by being explicitly
   configured via a Preconnection, by configuration after establishment,
   or by inheriting them from an antecedent via cloning; see Section 7.4
   for more.

   Section 8.1 provides a list of Connection Properties, while Selection
   Properties are listed in the subsections below.  Many properties are
   only considered during establishment, and can not be changed after a
   Connection is established; however, they can still be queried.  The
   return type of a queried Selection Property is Boolean, where "true"
   means that the Selection Property has been applied and "false" means



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   that the Selection Property has not been applied.  Note that "true"
   does not mean that a request has been honored.  For example, if
   "Congestion control" was requested with preference level "Prefer",
   but congestion control could not be supported, querying the
   "congestionControl" property yields the value "false".  If the
   preference level "Avoid" was used for "Congestion control", and, as
   requested, the Connection is not congestion controlled, querying the
   "congestionControl" property also yields the value "false".

   An implementation of this interface must provide sensible defaults
   for Selection Properties.  The default values for each property below
   represent a configuration that can be implemented over TCP.  If these
   default values are used and TCP is not supported by a Transport
   Services implementation, then an application using the default set of
   Properties might not succeed in establishing a connection.  Using the
   same default values for independent Transport Services
   implementations can be beneficial when applications are ported
   between different implementations/platforms, even if this default
   could lead to a connection failure when TCP is not available.  If
   default values other than those suggested below are used, it is
   RECOMMENDED to clearly document any differences.

6.2.1.  Reliable Data Transfer (Connection)

   Name:  reliability

   Type:  Preference

   Default:  Require

   This property specifies whether the application needs to use a
   transport protocol that ensures that all data is received at the
   Remote Endpoint without corruption.  When reliable data transfer is
   enabled, this also entails being notified when a Connection is closed
   or aborted.

6.2.2.  Preservation of Message Boundaries

   Name:  preserveMsgBoundaries

   Type:  Preference

   Default:  Ignore

   This property specifies whether the application needs or prefers to
   use a transport protocol that preserves message boundaries.





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6.2.3.  Configure Per-Message Reliability

   Name:  perMsgReliability

   Type:  Preference

   Default:  Ignore

   This property specifies whether an application considers it useful to
   specify different reliability requirements for individual Messages in
   a Connection.

6.2.4.  Preservation of Data Ordering

   Name:  preserveOrder

   Type:  Preference

   Default:  Require

   This property specifies whether the application wishes to use a
   transport protocol that can ensure that data is received by the
   application on the other end in the same order as it was sent.

6.2.5.  Use 0-RTT Session Establishment with a Safely Replayable Message

   Name:  zeroRttMsg

   Type:  Preference

   Default:  Ignore

   This property specifies whether an application would like to supply a
   Message to the transport protocol before Connection establishment
   that will then be reliably transferred to the other side before or
   during Connection establishment.  This Message can potentially be
   received multiple times (i.e., multiple copies of the message data
   may be passed to the Remote Endpoint).  See also Section 9.1.3.4.

6.2.6.  Multistream Connections in Group

   Name:  multistreaming

   Type:  Preference

   Default:  Prefer





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   This property specifies that the application would prefer multiple
   Connections within a Connection Group to be provided by streams of a
   single underlying transport connection where possible.

6.2.7.  Full Checksum Coverage on Sending

   Name:  FullChecksumSend

   Type:  Preference

   Default:  Require

   This property specifies the application's need for protection against
   corruption for all data transmitted on this Connection.  Disabling
   this property could enable later control of the sender checksum
   coverage (see Section 9.1.3.6).

6.2.8.  Full Checksum Coverage on Receiving

   Name:  FullChecksumRecv

   Type:  Preference

   Default:  Require

   This property specifies the application's need for protection against
   corruption for all data received on this Connection.  Disabling this
   property could enable later control of the required minimum receiver
   checksum coverage (see Section 8.1.1).

6.2.9.  Congestion control

   Name:  congestionControl

   Type:  Preference

   Default:  Require














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   This property specifies whether the application would like the
   Connection to be congestion controlled or not.  Note that if a
   Connection is not congestion controlled, an application using such a
   Connection SHOULD itself perform congestion control in accordance
   with [RFC2914] or use a circuit breaker in accordance with [RFC8084],
   whichever is appropriate.  Also note that reliability is usually
   combined with congestion control in protocol implementations,
   rendering "reliable but not congestion controlled" a request that is
   unlikely to succeed.  If the Connection is congestion controlled,
   performing additional congestion control in the application can have
   negative performance implications.

6.2.10.  Keep alive

   Name:  keepAlive

   Type:  Preference

   Default:  Ignore

   This property specifies whether the application would like the
   Connection to send keep-alive packets or not.  Note that if a
   Connection determines that keep-alive packets are being sent, the
   applicaton should itself avoid generating additional keep alive
   messages.  Note that when supported, the system will use the default
   period for generation of the keep alive-packets.  (See also
   Section 8.1.4).

6.2.11.  Interface Instance or Type

   Name:  interface

   Type:  Collection of (Preference, Enumeration)

   Default:  Empty (not setting a preference for any interface)

   This property allows the application to select any specific network
   interfaces or categories of interfaces it wants to "Require",
   "Prohibit", "Prefer", or "Avoid".  Note that marking a specific
   interface as "Require" strictly limits path selection to that single
   interface, and often leads to less flexible and resilient connection
   establishment.

   In contrast to other Selection Properties, this property is a tuple
   of an (Enumerated) interface identifier and a preference, and can
   either be implemented directly as such, or for making one preference
   available for each interface and interface type available on the
   system.



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   The set of valid interface types is implementation- and system-
   specific.  For example, on a mobile device, there may be "Wi-Fi" and
   "Cellular" interface types available; whereas on a desktop computer,
   "Wi-Fi" and "Wired Ethernet" interface types might be available.  An
   implementation should provide all types that are supported on the
   local system, to allow applications to be written generically.  For
   example, if a single implementation is used on both mobile devices
   and desktop devices, it should define the "Cellular" interface type
   for both systems, since an application might wish to always prohibit
   cellular.

   The set of interface types is expected to change over time as new
   access technologies become available.  The taxonomy of interface
   types on a given Transport Services system is implementation-
   specific.

   Interface types should not be treated as a proxy for properties of
   interfaces such as metered or unmetered network access.  If an
   application needs to prohibit metered interfaces, this should be
   specified via Provisioning Domain attributes (see Section 6.2.12) or
   another specific property.

6.2.12.  Provisioning Domain Instance or Type

   Name:  pvd

   Type:  Collection of (Preference, Enumeration)

   Default:  Empty (not setting a preference for any PvD)

   Similar to interface instances and types (see Section 6.2.11), this
   property allows the application to control path selection by
   selecting which specific Provisioning Domain (PvD) or categories of
   PVDs it wants to "Require", "Prohibit", "Prefer", or "Avoid".
   Provisioning Domains define consistent sets of network properties
   that may be more specific than network interfaces [RFC7556].

   As with interface instances and types, this property is a tuple of an
   (Enumerated) PvD identifier and a preference, and can either be
   implemented directly as such, or for making one preference available
   for each interface and interface type available on the system.

   The identification of a specific PvD is implementation- and system-
   specific, because there is currently no portable standard format for
   a PvD identifier.  For example, this identifier might be a string
   name or an integer.  As with requiring specific interfaces, requiring
   a specific PvD strictly limits the path selection.




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   Categories or types of PvDs are also defined to be implementation-
   and system-specific.  These can be useful to identify a service that
   is provided by a PvD.  For example, if an application wants to use a
   PvD that provides a Voice-Over-IP service on a Cellular network, it
   can use the relevant PvD type to require a PvD that provides this
   service, without needing to look up a particular instance.  While
   this does restrict path selection, it is broader than requiring
   specific PvD instances or interface instances, and should be
   preferred over these options.

6.2.13.  Use Temporary Local Address

   Name:  useTemporaryLocalAddress

   Type:  Preference

   Default:  Avoid for Listeners and Rendezvous Connections.  Prefer for
      other Connections.

   This property allows the application to express a preference for the
   use of temporary local addresses, sometimes called "privacy"
   addresses [RFC4941].  Temporary addresses are generally used to
   prevent linking connections over time when a stable address,
   sometimes called "permanent" address, is not needed.  There are some
   caveats to note when specifying this property.  First, if an
   application Requires the use of temporary addresses, the resulting
   Connection cannot use IPv4, because temporary addresses do not exist
   in IPv4.  Second, temporary local addresses might involve trading off
   privacy for performance.  For instance, temporary addresses can
   interfere with resumption mechanisms that some protocols rely on to
   reduce initial latency.

6.2.14.  Multipath Transport

   Name:  multipath

   Type:  Enumeration

   Default:  Disabled for connections created through initiate and
      rendezvous, Passive for listeners

   This property specifies whether and how applications want to take
   advantage of transferring data across multiple paths between the same
   end hosts.  Using multiple paths allows connections to migrate
   between interfaces or aggregate bandwidth as availability and
   performance properties change.  Possible values are:

   Disabled:  The connection will not use multiple paths once



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      established, even if the chosen transport supports using multiple
      paths.

   Active:  The connection will negotiate the use of multiple paths if
      the chosen transport supports this.

   Passive:  The connection will support the use of multiple paths if
      the Remote Endpoint requests it.

   The policy for using multiple paths is specified using the separate
   "multipath-policy" property, see Section 8.1.7 below.  To enable the
   peer endpoint to initiate additional paths towards a local address
   other than the one initially used, it is necessary to set the
   Alternative Addresses property (see Section 6.2.15 below).

   Setting this property to "Active", can have privacy implications: It
   enables the transport to establish connectivity using alternate paths
   that might result in users being linkable across the multiple paths,
   even if the Advertisement of Alternative Addresses property (see
   Section 6.2.15 below) is set to false.

   Note that Multipath Transport has no corresponding Selection Property
   of type Preference.  Enumeration values other than "Disabled" are
   interpreted as a preference for choosing protocols that can make use
   of multiple paths.  The "Disabled" value implies a requirement not to
   use multiple paths in parallel but does not prevent choosing a
   protocol that is capable of using multiple paths, e.g., it does not
   prevent choosing TCP, but prevents sending the "MP_CAPABLE" option in
   the TCP handshake.

6.2.15.  Advertisement of Alternative Addresses

   Name:  advertises-altaddr

   Type:  Boolean

   Default:  False

   This property specifies whether alternative addresses, e.g., of other
   interfaces, should be advertised to the peer endpoint by the protocol
   stack.  Advertising these addresses enables the peer-endpoint to
   establish additional connectivity, e.g., for connection migration or
   using multiple paths.








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   Note that this can have privacy implications because it might result
   in users being linkable across the multiple paths.  Also, note that
   setting this to false does not prevent the local Transport Services
   system from _establishing_ connectivity using alternate paths (see
   Section 6.2.14 above); it only prevents _proactive advertisement_ of
   addresses.

6.2.16.  Direction of communication

   Name:  direction

   Type:  Enumeration

   Default:  Bidirectional

   This property specifies whether an application wants to use the
   connection for sending and/or receiving data.  Possible values are:

   Bidirectional:  The connection must support sending and receiving
      data

   Unidirectional send:  The connection must support sending data, and
      the application cannot use the connection to receive any data

   Unidirectional receive:  The connection must support receiving data,
      and the application cannot use the connection to send any data

   Since unidirectional communication can be supported by transports
   offering bidirectional communication, specifying unidirectional
   communication may cause a transport stack that supports bidirectional
   communication to be selected.

6.2.17.  Notification of ICMP soft error message arrival

   Name:  softErrorNotify

   Type:  Preference

   Default:  Ignore

   This property specifies whether an application considers it useful to
   be informed when an ICMP error message arrives that does not force
   termination of a connection.  When set to true, received ICMP errors
   are available as SoftErrors, see Section 8.3.1.  Note that even if a
   protocol supporting this property is selected, not all ICMP errors
   will necessarily be delivered, so applications cannot rely upon
   receiving them [RFC8085].




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6.2.18.  Initiating side is not the first to write

   Name:  activeReadBeforeSend

   Type:  Preference

   Default:  Ignore

   The most common client-server communication pattern involves the
   client actively opening a connection, then sending data to the
   server.  The server listens (passive open), reads, and then answers.
   This property specifies whether an application wants to diverge from
   this pattern - either by actively opening with Initiate(),
   immediately followed by reading, or passively opening with Listen(),
   immediately followed by writing.  This property is ignored when
   establishing connections using Rendezvous().  Requiring this property
   limits the choice of mappings to underlying protocols, which can
   reduce efficiency.  For example, it prevents the Transport Services
   system from mapping Connections to SCTP streams, where the first
   transmitted data takes the role of an active open signal
   [I-D.ietf-taps-impl].

6.3.  Specifying Security Parameters and Callbacks

   Most security parameters, e.g., TLS ciphersuites, local identity and
   private key, etc., may be configured statically.  Others are
   dynamically configured during connection establishment.  Security
   parameters and callbacks are partitioned based on their place in the
   lifetime of connection establishment.  Similar to Transport
   Properties, both parameters and callbacks are inherited during
   cloning (see Section 7.4).

6.3.1.  Specifying Security Parameters on a Pre-Connection

   Common security parameters such as TLS ciphersuites are known to
   implementations.  Clients should use common safe defaults for these
   values whenever possible.  However, as discussed in [RFC8922], many
   transport security protocols require specific security parameters and
   constraints from the client at the time of configuration and actively
   during a handshake.  These configuration parameters need to be
   specified in the pre-connection phase and are created as follows:

   SecurityParameters := NewSecurityParameters()

   Security configuration parameters and sample usage follow:






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   *  Local identity and private keys: Used to perform private key
      operations and prove one's identity to the Remote Endpoint.
      (Note, if private keys are not available, e.g., since they are
      stored in hardware security modules (HSMs), handshake callbacks
      must be used.  See below for details.)

   SecurityParameters.Set(identity, myIdentity)
   SecurityParameters.Set(key-pair, myPrivateKey, myPublicKey)

   *  Supported algorithms: Used to restrict what parameters are used by
      underlying transport security protocols.  When not specified,
      these algorithms should use known and safe defaults for the
      system.  Parameters include: ciphersuites, supported groups, and
      signature algorithms.  These parameters take a collection of
      supported algorithms as parameter.

SecurityParameters.Set(supported-group, "secp256k1")
SecurityParameters.Set(ciphersuite, "TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256")
SecurityParameters.Set(signature-algorithm, "ed25519")

   *  Pre-Shared Key import: Used to install pre-shared keying material
      established out-of-band.  Each pre-shared keying material is
      associated with some identity that typically identifies its use or
      has some protocol-specific meaning to the Remote Endpoint.

   SecurityParameters.Set(pre-shared-key, key, identity)

   *  Session cache management: Used to tune session cache capacity,
      lifetime, and other policies.

   SecurityParameters.Set(max-cached-sessions, 16)
   SecurityParameters.Set(cached-session-lifetime-seconds, 3600)

   Connections that use Transport Services SHOULD use security in
   general.  However, for compatibility with endpoints that do not
   support transport security protocols (such as a TCP endpoint that
   does not support TLS), applications can initialize their security
   parameters to indicate that security can be disabled, or can be
   opportunistic.  If security is disabled, the Transport Services
   system will not attempt to add transport security automatically.  If
   security is opportunistic, it will allow Connections without
   transport security, but will still attempt to use security if
   available.

   SecurityParameters := NewDisabledSecurityParameters()

   SecurityParameters := NewOpportunisticSecurityParameters()




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   Representation of Security Parameters in implementations should
   parallel that chosen for Transport Property names as sugggested in
   Section 5.

6.3.2.  Connection Establishment Callbacks

   Security decisions, especially pertaining to trust, are not static.
   Once configured, parameters may also be supplied during connection
   establishment.  These are best handled as client-provided callbacks.
   Callbacks block the progress of the connection establishment, which
   distinguishes them from other Events in the transport system.  How
   callbacks and events are implemented is specific to each
   implementation.  Security handshake callbacks that may be invoked
   during connection establishment include:

   *  Trust verification callback: Invoked when a Remote Endpoint's
      trust must be validated before the handshake protocol can
      continue.

   TrustCallback := NewCallback({
     // Handle trust, return the result
   })
   SecurityParameters.SetTrustVerificationCallback(trustCallback)

   *  Identity challenge callback: Invoked when a private key operation
      is required, e.g., when local authentication is requested by a
      Remote Endpoint.

   ChallengeCallback := NewCallback({
     // Handle challenge
   })
   SecurityParameters.SetIdentityChallengeCallback(challengeCallback)

7.  Establishing Connections

   Before a Connection can be used for data transfer, it needs to be
   established.  Establishment ends the pre-establishment phase; all
   transport properties and cryptographic parameter specification must
   be complete before establishment, as these will be used to select
   candidate Paths and Protocol Stacks for the Connection.
   Establishment may be active, using the Initiate() Action; passive,
   using the Listen() Action; or simultaneous for peer-to-peer, using
   the Rendezvous() Action.  These Actions are described in the
   subsections below.







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7.1.  Active Open: Initiate

   Active open is the Action of establishing a Connection to a Remote
   Endpoint presumed to be listening for incoming Connection requests.
   Active open is used by clients in client-server interactions.  Active
   open is supported by this interface through the Initiate Action:

   Connection := Preconnection.Initiate(timeout?)

   The timeout parameter specifies how long to wait before aborting
   Active open.  Before calling Initiate, the caller must have populated
   a Preconnection Object with a Remote Endpoint specifier, optionally a
   Local Endpoint specifier (if not specified, the system will attempt
   to determine a suitable Local Endpoint), as well as all properties
   necessary for candidate selection.

   The Initiate() Action returns a Connection object.  Once Initiate()
   has been called, any changes to the Preconnection MUST NOT have any
   effect on the Connection.  However, the Preconnection can be reused,
   e.g., to Initiate another Connection.

   Once Initiate is called, the candidate Protocol Stack(s) may cause
   one or more candidate transport-layer connections to be created to
   the specified Remote Endpoint.  The caller may immediately begin
   sending Messages on the Connection (see Section 9.2) after calling
   Initiate(); note that any data marked "Safely Replayable" that is
   sent while the Connection is being established may be sent multiple
   times or on multiple candidates.

   The following Events may be sent by the Connection after Initiate()
   is called:

   Connection -> Ready<>

   The Ready Event occurs after Initiate has established a transport-
   layer connection on at least one usable candidate Protocol Stack over
   at least one candidate Path.  No Receive Events (see Section 9.3)
   will occur before the Ready Event for Connections established using
   Initiate.

   Connection -> EstablishmentError<reason?>

   An EstablishmentError occurs either when the set of transport
   properties and security parameters cannot be fulfilled on a
   Connection for initiation (e.g., the set of available Paths and/or
   Protocol Stacks meeting the constraints is empty) or reconciled with
   the Local and/or Remote Endpoints; when the remote specifier cannot
   be resolved; or when no transport-layer connection can be established



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   to the Remote Endpoint (e.g., because the Remote Endpoint is not
   accepting connections, the application is prohibited from opening a
   Connection by the operating system, or the establishment attempt has
   timed out for any other reason).

   Connection establishment and transmission of the first message can be
   combined in a single action Section 9.2.5.

7.2.  Passive Open: Listen

   Passive open is the Action of waiting for Connections from Remote
   Endpoints, commonly used by servers in client-server interactions.
   Passive open is supported by this interface through the Listen Action
   and returns a Listener object:

   Listener := Preconnection.Listen()

   Before calling Listen, the caller must have initialized the
   Preconnection during the pre-establishment phase with a Local
   Endpoint specifier, as well as all properties necessary for Protocol
   Stack selection.  A Remote Endpoint may optionally be specified, to
   constrain what Connections are accepted.

   The Listen() Action returns a Listener object.  Once Listen() has
   been called, any changes to the Preconnection MUST NOT have any
   effect on the Listener.  The Preconnection can be disposed of or
   reused, e.g., to create another Listener.

   Listener.Stop()

   Listening continues until the global context shuts down, or until the
   Stop action is performed on the Listener object.

   Listener -> ConnectionReceived<Connection>

   The ConnectionReceived Event occurs when a Remote Endpoint has
   established a transport-layer connection to this Listener (for
   Connection-oriented transport protocols), or when the first Message
   has been received from the Remote Endpoint (for Connectionless
   protocols), causing a new Connection to be created.  The resulting
   Connection is contained within the ConnectionReceived Event, and is
   ready to use as soon as it is passed to the application via the
   event.

   Listener.SetNewConnectionLimit(value)






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   If the caller wants to rate-limit the number of inbound Connections
   that will be delivered, it can set a cap using
   SetNewConnectionLimit().  This mechanism allows a server to protect
   itself from being drained of resources.  Each time a new Connection
   is delivered by the ConnectionReceived Event, the value is
   automatically decremented.  Once the value reaches zero, no further
   Connections will be delivered until the caller sets the limit to a
   higher value.  By default, this value is Infinite.  The caller is
   also able to reset the value to Infinite at any point.

   Listener -> EstablishmentError<reason?>

   An EstablishmentError occurs either when the Properties and Security
   Parameters of the Preconnection cannot be fulfilled for listening or
   cannot be reconciled with the Local Endpoint (and/or Remote Endpoint,
   if specified), when the Local Endpoint (or Remote Endpoint, if
   specified) cannot be resolved, or when the application is prohibited
   from listening by policy.

   Listener -> Stopped<>

   A Stopped Event occurs after the Listener has stopped listening.

7.3.  Peer-to-Peer Establishment: Rendezvous

   Simultaneous peer-to-peer Connection establishment is supported by
   the Rendezvous() Action:

   Preconnection.Rendezvous()

   A Preconnection Object used in a Rendezvous() MUST have both the
   Local Endpoint candidates and the Remote Endpoint candidates
   specified, along with the transport properties and security
   parameters needed for Protocol Stack selection, before the
   Rendezvous() Action is initiated.

   The Rendezvous() Action listens on the Local Endpoint candidates for
   an incoming Connection from the Remote Endpoint candidates, while
   also simultaneously trying to establish a Connection from the Local
   Endpoint candidates to the Remote Endpoint candidates.

   If there are multiple Local Endpoints or Remote Endpoints configured,
   then initiating a Rendezvous() action will systematically probe the
   reachability of those endpoint candidates following an approach such
   as that used in Interactive Connectivity Establishment (ICE)
   [RFC5245].





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   If the endpoints are suspected to be behind a NAT, Rendezvous() can
   be initiated using Local Endpoints that support a method of
   discovering NAT bindings such as Session Traversal Utilities for NAT
   (STUN) [RFC8489] or Traversal Using Relays around NAT (TURN)
   [RFC5766].  In this case, the Local Endpoint will resolve to a
   mixture of local and server reflexive addresses.  The Resolve()
   action on the Preconnection can be used to discover these bindings:

   []LocalEndpoint, []RemoteEndpoint := Preconnection.Resolve()

   The Resolve() call returns lists of Local Endpoints and Remote
   Endpoints, that represent the concrete addresses, local and server
   reflexive, on which a Rendezvous() for the Preconnection will listen
   for incoming Connections, and to which it will attempt to establish
   connections.

   An application that uses Rendezvous() to establish a peer-to-peer
   connection in the presence of NATs will configure the Preconnection
   object with at least one a Local Endpoint that supports NAT binding
   discovery.  It will then Resolve() the Preconnection, and pass the
   resulting list of Local Endpoint candidates to the peer via a
   signalling protocol, for example as part of an ICE [RFC5245] exchange
   within SIP [RFC3261] or WebRTC [RFC7478].  The peer will then, via
   the same signalling channel, return the Remote Endpoint candidates.
   The set of Remote Endpoint candidates are then configured onto the
   Preconnection:

   Preconnection.AddRemote([]RemoteEndpoint)

   The Rendezvous() Action can be initiated once both the Local Endpoint
   candidates and the Remote Endpoint candidates retrieved from the peer
   via the signalling channel have been added to the Preconnection.

   If successful, the Rendezvous() Action returns a Connection object
   via a RendezvousDone<> Event:

   Preconnection -> RendezvousDone<Connection>

   The RendezvousDone<> Event occurs when a Connection is established
   with the Remote Endpoint.  For Connection-oriented transports, this
   occurs when the transport-layer connection is established; for
   Connectionless transports, it occurs when the first Message is
   received from the Remote Endpoint.  The resulting Connection is
   contained within the RendezvousDone<> Event, and is ready to use as
   soon as it is passed to the application via the Event.  Changes made
   to a Preconnection after Rendezvous() has been called do not have any
   effect on existing Connections.




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   An EstablishmentError occurs either when the Properties and Security
   Parameters of the Preconnection cannot be fulfilled for rendezvous or
   cannot be reconciled with the Local and/or Remote Endpoints, when the
   Local Endpoint or Remote Endpoint cannot be resolved, when no
   transport-layer connection can be established to the Remote Endpoint,
   or when the application is prohibited from rendezvous by policy:

   Preconnection -> EstablishmentError<reason?>

7.4.  Connection Groups

   Connection Groups can be created using the Clone Action:

   Connection := Connection.Clone(framer?)

   Calling Clone on a Connection yields a Connection Group containing
   two Connections: the parent Connection on which Clone was called, and
   a resulting cloned Connection.  The new Connection is actively
   openend, and it will send a Ready Event or an EstablishmentError
   Event.  Calling Clone on any of these Connections adds another
   Connection to the Connection Group.  Connections in a Connection
   Group share all Connection Properties except "Connection Priority"
   (see Section 8.1.2), and these Connection Properties are entangled:
   Changing one of the Connection Properties on one Connection in the
   Connection Group automatically changes the Connection Property for
   all others.  For example, changing "Timeout for aborting Connection"
   (see Section 8.1.3) on one Connection in a Connection Group will
   automatically make the same change to this Connection Property for
   all other Connections in the Connection Group.  Like all other
   Properties, "Connection Priority" is copied to the new Connection
   when calling Clone(), but in this case, a later change to the
   "Connection Priority" on one Connection does not change it on the
   other Connections in the same Connection Group.

   Message Properties are also not entangled.  For example, changing
   "Lifetime" (see Section 9.1.3.1) of a Message will only affect a
   single Message on a single Connection.

   A new Connection created by Clone can have a Message Framer assigned
   via the optional "framer" parameter of the Clone Action.  If this
   parameter is not supplied, the stack of Message Framers associated
   with a Connection is copied to the cloned Connection when calling
   Clone.  Then, a cloned Connection has the same stack of Message
   Framers as the Connection from which they are Cloned, but these
   Framers may internally maintain per-Connection state.






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   It is also possible to check which Connections belong to the same
   Connection Group.  Calling GroupedConnections() on a specific
   Connection returns a set of all Connections in the same group.

   []Connection := Connection.GroupedConnections()

   Connections will belong to the same group if the application
   previously called Clone.  Passive Connections can also be added to
   the same group - e.g., when a Listener receives a new Connection that
   is just a new stream of an already active multi-streaming protocol
   instance.

   If the underlying protocol supports multi-streaming, it is natural to
   use this functionality to implement Clone.  In that case, Connections
   in a Connection Group are multiplexed together, giving them similar
   treatment not only inside endpoints, but also across the end-to-end
   Internet path.

   Note that calling Clone() can result in on-the-wire signaling, e.g.,
   to open a new transport connection, depending on the underlying
   Protocol Stack.  When Clone() leads to the opening of multiple such
   connections, the Transport Services system will ensure consistency of
   Connection Properties by uniformly applying them to all underlying
   connections in a group.  Even in such a case, there are possibilities
   for a Transport Services system to implement prioritization within a
   Connection Group [TCP-COUPLING] [RFC8699].

   Attempts to clone a Connection can result in a CloneError:

   Connection -> CloneError<reason?>

   The "Connection Priority" Connection Property operates on Connections
   in a Connection Group using the same approach as in Section 9.1.3.2:
   when allocating available network capacity among Connections in a
   Connection Group, sends on Connections with lower Priority values
   will be prioritized over sends on Connections with higher Priority
   values.  Capacity will be shared among these Connections according to
   the Connection Group Transmission Scheduler property (Section 8.1.5).
   See Section 9.2.6 for more.

8.  Managing Connections

   During pre-establishment and after establishment, connections can be
   configured and queried using Connection Properties, and asynchronous
   information may be available about the state of the connection via
   Soft Errors.





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   Connection Properties represent the configuration and state of the
   selected Protocol Stack(s) backing a Connection.  These Connection
   Properties may be Generic, applying regardless of transport protocol,
   or Specific, applicable to a single implementation of a single
   transport protocol stack.  Generic Connection Properties are defined
   in Section 8.1 below.

   Protocol Specific Properties are defined in a transport- and
   implementation-specific way, and MUST NOT apply across different
   protocols.  Too much reliance by an application on Protocol Specific
   Properties can significantly reduce the flexibility of a transport
   services implementation.

   The application can set and query Connection Properties on a per-
   Connection basis.  Connection Properties that are not read-only can
   be set during pre-establishment (see Section 6.2), as well as on
   connections directly using the SetProperty action:

   Connection.SetProperty(property, value)

   Note that changing one of the Connection Properties on one Connection
   in a Connection Group will also change it for all other Connections
   of that group; see further Section 7.4.

   At any point, the application can query Connection Properties.

   ConnectionProperties := Connection.GetProperties()
   value := ConnectionProperties.Get(property)
   if ConnectionProperties.Has(boolean_or_preference_property) then ...

   Depending on the status of the connection, the queried Connection
   Properties will include different information:

   *  The connection state, which can be one of the following:
      Establishing, Established, Closing, or Closed.

   *  Whether the connection can be used to send data.  A connection can
      not be used for sending if the connection was created with the
      Selection Property "Direction of Communication" set to
      "unidirectional receive" or if a Message marked as "Final" was
      sent over this connection.  See also Section 9.1.3.5.










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   *  Whether the connection can be used to receive data.  A connection
      cannot be used for reading if the connection was created with the
      Selection Property "Direction of Communication" set to
      "unidirectional send" or if a Message marked as "Final" was
      received.  See Section 9.3.3.3.  The latter is only supported by
      certain transport protocols, e.g., by TCP as half-closed
      connection.

   *  For Connections that are Established, Closing, or Closed:
      Connection Properties (Section 8.1) of the actual protocols that
      were selected and instantiated, and Selection Properties that the
      application specified on the Preconnection.  Selection Properties
      of type "Preference" will be exposed as boolean values indicating
      whether or not the property applies to the selected transport.
      Note that the instantiated protocol stack might not match all
      Protocol Selection Properties that the application specified on
      the Preconnection.

   *  For Connections that are Established, additional properties of the
      path(s) in use.  These properties can be derived from the local
      provisioning domain [RFC7556], measurements by the Protocol Stack,
      or other sources.

8.1.  Generic Connection Properties

   Generic Connection Properties are defined independent of the chosen
   protocol stack and therefore available on all Connections.

   Many Connection Properties have a corresponding Selection Property
   that enables applications to express their preference for protocols
   providing a supporting transport feature.

8.1.1.  Required Minimum Corruption Protection Coverage for Receiving

   Name:  recvChecksumLen

   Type:  Integer (non-negative with special value "Full Coverage")

   Default:  Full Coverage

   This property specifies the minimum number of bytes in a received
   message that need to be covered by a checksum.  A special value of 0
   means that a received packet does not need to have a non-zero
   checksum field.  A receiving endpoint will not forward messages that
   have less coverage to the application.  The application is
   responsible for handling any corruption within the non-protected part
   of the message [RFC8085].




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8.1.2.  Connection Priority

   Name:  connPrio

   Type:  Integer

   Default:  100

   This Property is a non-negative integer representing the relative
   inverse priority (i.e., a lower value reflects a higher priority) of
   this Connection relative to other Connections in the same Connection
   Group.  It has no effect on Connections not part of a Connection
   Group.  As noted in Section 7.4, this property is not entangled when
   Connections are cloned, i.e., changing the Priority on one Connection
   in a Connection Group does not change it on the other Connections in
   the same Connection Group.  No guarantees of a specific behavior
   regarding Connection Priority are given; a Transport Services system
   may ignore this property.  See Section 9.2.6 for more details.

8.1.3.  Timeout for Aborting Connection

   Name:  connTimeout

   Type:  Numeric, with special value "Disabled"

   Default:  Disabled

   This property specifies how long to wait before deciding that an
   active Connection has failed when trying to reliably deliver data to
   the Remote Endpoint.  Adjusting this Property will only take effect
   when the underlying stack supports reliability.  The special value
   "Disabled" means that no timeout is scheduled.

8.1.4.  Timeout for keep alive packets

   Name:  keepAliveTimeout

   Type:  Numeric, with special value "Disabled"

   Default:  Implementation-defined

   A Transport Services system can request a protocol that supports
   sending keep alive packets Section 6.2.10.  This property specifies
   the maximum length of time an idle connection (one for which no
   transport packets have been sent) should wait before the Local
   Endpoint sends a keep-alive packet to the Remote Endpoint.  Adjusting
   this Property will only take effect when the underlying stack
   supports sending keep-alive packets.  Guidance on setting this value



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   for datagram transports is provided in [RFC8085].  A value greater
   than the connection timeout (Section 8.1.3), or the special value
   "Disabled", will disable the sending of keep-alive packets.

8.1.5.  Connection Group Transmission Scheduler

   Name:  connScheduler

   Type:  Enumeration

   Default:  Weighted Fair Queueing (see Section 3.6 in [RFC8260])

   This property specifies which scheduler should be used among
   Connections within a Connection Group, see Section 7.4.  The set of
   schedulers can be taken from [RFC8260].

8.1.6.  Capacity Profile

   Name:  connCapacityProfile

   This property specifies the desired network treatment for traffic
   sent by the application and the tradeoffs the application is prepared
   to make in path and protocol selection to receive that desired
   treatment.  When the capacity profile is set to a value other than
   Default, the Transport Services system SHOULD select paths and
   configure protocols to optimize the tradeoff between delay, delay
   variation, and efficient use of the available capacity based on the
   capacity profile specified.  How this is realized is implementation-
   specific.  The Capacity Profile MAY also be used to set markings on
   the wire for Protocol Stacks supporting this.  Recommendations for
   use with DSCP are provided below for each profile; note that when a
   Connection is multiplexed, the guidelines in Section 6 of [RFC7657]
   apply.

   The following values are valid for the Capacity Profile:

   Default:  The application provides no information about its expected
      capacity profile.  Transport Services system implementations that
      map the requested capacity profile onto per-connection DSCP
      signaling SHOULD assign the DSCP Default Forwarding [RFC2474] Per
      Hop Behaviour (PHB).

   Scavenger:  The application is not interactive.  It expects to send








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      and/or receive data without any urgency.  This can, for example,
      be used to select protocol stacks with scavenger transmission
      control and/or to assign the traffic to a lower-effort service.
      Transport Services system implementations that map the requested
      capacity profile onto per-connection DSCP signaling SHOULD assign
      the DSCP Less than Best Effort [RFC8622] PHB.

   Low Latency/Interactive:  The application is interactive, and prefers
      loss to latency.  Response time should be optimized at the expense
      of delay variation and efficient use of the available capacity
      when sending on this connection.  This can be used by the system
      to disable the coalescing of multiple small Messages into larger
      packets (Nagle's algorithm); to prefer immediate acknowledgment
      from the peer endpoint when supported by the underlying transport;
      and so on.  Transport Services system implementations that map the
      requested capacity profile onto per-connection DSCP signaling
      without multiplexing SHOULD assign a DSCP Assured Forwarding
      (AF41,AF42,AF43,AF44) [RFC2597] PHB.  Inelastic traffic that is
      expected to conform to the configured network service rate could
      be mapped to the DSCP Expedited Forwarding [RFC3246] or [RFC5865]
      PHBs.

   Low Latency/Non-Interactive:  The application prefers loss to
      latency, but is not interactive.  Response time should be
      optimized at the expense of delay variation and efficient use of
      the available capacity when sending on this connection.  Transport
      system implementations that map the requested capacity profile
      onto per-connection DSCP signaling without multiplexing SHOULD
      assign a DSCP Assured Forwarding (AF21,AF22,AF23,AF24) [RFC2597]
      PHB.

   Constant-Rate Streaming:  The application expects to send/receive
      data at a constant rate after Connection establishment.  Delay and
      delay variation should be minimized at the expense of efficient
      use of the available capacity.  This implies that the Connection
      might fail if the Path is unable to maintain the desired rate.  A
      transport can interpret this capacity profile as preferring a
      circuit breaker [RFC8084] to a rate-adaptive congestion
      controller.  Transport system implementations that map the
      requested capacity profile onto per-connection DSCP signaling
      without multiplexing SHOULD assign a DSCP Assured Forwarding
      (AF31,AF32,AF33,AF34) [RFC2597] PHB.

   Capacity-Seeking:  The application expects to send/receive data at







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      the maximum rate allowed by its congestion controller over a
      relatively long period of time.  Transport Services system
      implementations that map the requested capacity profile onto per-
      connection DSCP signaling without multiplexing SHOULD assign a
      DSCP Assured Forwarding (AF11,AF12,AF13,AF14) [RFC2597] PHB per
      Section 4.8 of [RFC4594].

   The Capacity Profile for a selected protocol stack may be modified on
   a per-Message basis using the Transmission Profile Message Property;
   see Section 9.1.3.8.

8.1.7.  Policy for using Multipath Transports

   Name:  multipath-policy

   Type:  Enumeration

   Default:  Handover

   This property specifies the local policy for transferring data across
   multiple paths between the same end hosts if Multipath Transport is
   not set to Disabled (see Section 6.2.14).  Possible values are:

   Handover:  The connection ought only to attempt to migrate between
      different paths when the original path is lost or becomes
      unusable.  The thresholds used to declare a path unusable are
      implementation specific.

   Interactive:  The connection ought only to attempt to minimize the
      latency for interactive traffic patterns by transmitting data
      across multiple paths when this is beneficial.  The goal of
      minimizing the latency will be balanced against the cost of each
      of these paths.  Depending on the cost of the lower-latency path,
      the scheduling might choose to use a higher-latency path.  Traffic
      can be scheduled such that data may be transmitted on multiple
      paths in parallel to achieve a lower latency.  The specific
      scheduling algorithm is implementation-specific.

   Aggregate:  The connection ought to attempt to use multiple paths in
      parallel to maximize available capacity and possibly overcome the
      capacity limitations of the individual paths.  The actual strategy
      is implementation specific.

   Note that this is a local choice - the Remote Endpoint can choose a
   different policy.






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8.1.8.  Bounds on Send or Receive Rate

   Name:  minSendRate / minRecvRate / maxSendRate / maxRecvRate

   Type:  Numeric (with special value "Unlimited") / Numeric (with
      special value "Unlimited") / Numeric (with special value
      "Unlimited") / Numeric (with special value "Unlimited")

   Default:  Unlimited / Unlimited / Unlimited / Unlimited

   This property specifies an upper-bound rate that a transfer is not
   expected to exceed (even if flow control and congestion control allow
   higher rates), and/or a lower-bound rate below which the application
   does not deem it will be useful.  These are specified in bits per
   second.  The special value "Unlimited" indicates that no bound is
   specified.

8.1.9.  Group Connection Limit

   Name:  groupConnLimit

   Type:  Numeric (with special value "Unlimited")

   Default:  Unlimited

   This property controls the number of Connections that can be accepted
   from a peer as new members of the Connection's group.  Similar to
   SetNewConnectionLimit(), this limits the number of ConnectionReceived
   Events that will occur, but constrained to the group of the
   Connection associated with this property.  For a multi-streaming
   transport, this limits the number of allowed streams.

8.1.10.  Isolate Session

   Name:  isolateSession

   Type:  Boolean

   Default:  false

   When set to true, this property will initiate new Connections using
   as little cached information (such as session tickets or cookies) as
   possible from previous connections that are not in the same
   Connection Group.  Any state generated by this Connection will only
   be shared with Connections in the same Connection Group.  Cloned
   Connections will use saved state from within the Connection Group.
   This is used for separating Connection Contexts as specified in
   [I-D.ietf-taps-arch].



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   Note that this does not guarantee no leakage of information, as
   implementations may not be able to fully isolate all caches (e.g.
   RTT estimates).  Note that this property may degrade connection
   performance.

8.1.11.  Read-only Connection Properties

   The following generic Connection Properties are read-only, i.e. they
   cannot be changed by an application.

8.1.11.1.  Maximum Message Size Concurrent with Connection Establishment

   Name:  zeroRttMsgMaxLen

   Type:  Integer

   This property represents the maximum Message size that can be sent
   before or during Connection establishment, see also Section 9.1.3.4.
   It is given in Bytes.

8.1.11.2.  Maximum Message Size Before Fragmentation or Segmentation

   Name:  singularTransmissionMsgMaxLen

   Type:  Integer

   This property, if applicable, represents the maximum Message size
   that can be sent without incurring network-layer fragmentation at the
   sender.  It exposes a value to the application based on the Maximum
   Packet Size (MPS) as described in Datagram PLPMTUD [RFC8899].  This
   can allow a sending stack to avoid unwanted fragmentation at the
   network-layer or segmentation by the transport layer.

8.1.11.3.  Maximum Message Size on Send

   Name:  sendMsgMaxLen

   Type:  Integer

   This property represents the maximum Message size that an application
   can send.

8.1.11.4.  Maximum Message Size on Receive

   Name:  recvMsgMaxLen

   Type:  Integer




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   This numeric property represents the maximum Message size that an
   application can receive.

8.2.  TCP-specific Properties: User Timeout Option (UTO)

   These properties specify configurations for the User Timeout Option
   (UTO), in the case that TCP becomes the chosen transport protocol.
   Implementation is optional and useful only if TCP is implemented in
   the Transport Services system.

   These TCP-specific properties are included here because the feature
   "Suggest timeout to the peer" is part of the minimal set of transport
   services [RFC8923], where this feature was categorized as
   "functional".  This means that when an implementation offers this
   feature, it has to expose an interface to it to the application.
   Otherwise, the implementation might violate assumptions by the
   application, which could cause the application to fail.

   All of the below properties are optional (e.g., it is possible to
   specify "User Timeout Enabled" as true, but not specify an Advertised
   User Timeout value; in this case, the TCP default will be used).
   These properties reflect the API extension specified in Section 3 of
   [RFC5482].

8.2.1.  Advertised User Timeout

   Name:  tcp.userTimeoutValue

   Type:  Integer

   Default:  the TCP default

   This time value is advertised via the TCP User Timeout Option (UTO)
   [RFC5482] at the Remote Endpoint to adapt its own "Timeout for
   aborting Connection" (see Section 8.1.3) value.

8.2.2.  User Timeout Enabled

   Name:  tcp.userTimeout

   Type:  Boolean

   Default:  false

   This property controls whether the UTO option is enabled for a
   connection.  This applies to both sending and receiving.





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8.2.3.  Timeout Changeable

   Name:  tcp.userTimeoutRecv

   Type:  Boolean

   Default:  true

   This property controls whether the "Timeout for aborting Connection"
   (see Section 8.1.3) may be changed based on a UTO option received
   from the remote peer.  This boolean becomes false when "Timeout for
   aborting Connection" (see Section 8.1.3) is used.

8.3.  Connection Lifecycle Events

   During the lifetime of a connection there are events that can occur
   when configured.

8.3.1.  Soft Errors

   Asynchronous introspection is also possible, via the SoftError Event.
   This event informs the application about the receipt and contents of
   an ICMP error message related to the Connection.  This will only
   happen if the underlying protocol stack supports access to soft
   errors; however, even if the underlying stack supports it, there is
   no guarantee that a soft error will be signaled.

   Connection -> SoftError<>

8.3.2.  Path change

   This event notifies the application when at least one of the paths
   underlying a Connection has changed.  Changes occur on a single path
   when the PMTU changes as well as when multiple paths are used and
   paths are added or removed, or a handover has been performed.

   Connection -> PathChange<>

9.  Data Transfer

   Data is sent and received as Messages, which allows the application
   to communicate the boundaries of the data being transferred.









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9.1.  Messages and Framers

   Each Message has an optional Message Context, which allows to add
   Message Properties, identify Send Events related to a specific
   Message or to inspect meta-data related to the Message sent.  Framers
   can be used to extend or modify the message data with additional
   information that can be processed at the receiver to detect message
   boundaries.

9.1.1.  Message Contexts

   Using the MessageContext object, the application can set and retrieve
   meta-data of the message, including Message Properties (see
   Section 9.1.3) and framing meta-data (see Section 9.1.2.2).
   Therefore, a MessageContext object can be passed to the Send action
   and is returned by each Send and Receive related event.

   Message Properties can be set and queried using the Message Context:

   MessageContext.add(scope?, parameter, value)
   PropertyValue := MessageContext.get(scope?, property)

   To get or set Message Properties, the optional scope parameter is
   left empty.  To get or set meta-data for a Framer, the application
   has to pass a reference to this Framer as the scope parameter.

   For MessageContexts returned by send Events (see Section 9.2.2) and
   receive Events (see Section 9.3.2), the application can query
   information about the Local and Remote Endpoint:

   RemoteEndpoint := MessageContext.GetRemoteEndpoint()
   LocalEndpoint := MessageContext.GetLocalEndpoint()

9.1.2.  Message Framers

   Although most applications communicate over a network using well-
   formed Messages, the boundaries and metadata of the Messages are
   often not directly communicated by the transport protocol itself.
   For example, HTTP applications send and receive HTTP messages over a
   byte-stream transport, requiring that the boundaries of HTTP messages
   be parsed from the stream of bytes.

   Message Framers allow extending a Connection's Protocol Stack to
   define how to encapsulate or encode outbound Messages, and how to
   decapsulate or decode inbound data into Messages.  Message Framers
   allow message boundaries to be preserved when using a Connection
   object, even when using byte-stream transports.  This is designed
   based on the fact that many of the current application protocols



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   evolved over TCP, which does not provide message boundary
   preservation, and since many of these protocols require message
   boundaries to function, each application layer protocol has defined
   its own framing.

   To use a Message Framer, the application adds it to its Preconnection
   object.  Then, the Message Framer can intercept all calls to Send()
   or Receive() on a Connection to add Message semantics, in addition to
   interacting with the setup and teardown of the Connection.  A Framer
   can start sending data before the application sends data if the
   framing protocol requires a prefix or handshake (see [RFC8229] for an
   example of such a framing protocol).

     Initiate()   Send()   Receive()   Close()
         |          |         ^          |
         |          |         |          |
    +----v----------v---------+----------v-----+
    |                Connection                |
    +----+----------+---------^----------+-----+
         |          |         |          |
         |      +-----------------+      |
         |      |    Messages     |      |
         |      +-----------------+      |
         |          |         |          |
    +----v----------v---------+----------v-----+
    |                Framer(s)                 |
    +----+----------+---------^----------+-----+
         |          |         |          |
         |      +-----------------+      |
         |      |   Byte-stream   |      |
         |      +-----------------+      |
         |          |         |          |
    +----v----------v---------+----------v-----+
    |         Transport Protocol Stack         |
    +------------------------------------------+

             Figure 1: Protocol Stack showing a Message Framer

   Note that while Message Framers add the most value when placed above
   a protocol that otherwise does not preserve message boundaries, they
   can also be used with datagram- or message-based protocols.  In these
   cases, they add an additional transformation to further encode or
   encapsulate, and can potentially support packing multiple
   application-layer Messages into individual transport datagrams.

   The API to implement a Message Framer can vary depending on the
   implementation; guidance on implementing Message Framers can be found
   in [I-D.ietf-taps-impl].



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9.1.2.1.  Adding Message Framers to Pre-Connections

   The Message Framer object can be added to one or more Preconnections
   to run on top of transport protocols.  Multiple Framers may be added
   to a Preconnection; in this case, the Framers operate as a framing
   stack, i.e. the last one added runs first when framing outbound
   messages, and last when parsing inbound data.

   The following example adds a basic HTTP Message Framer to a
   Preconnection:

   framer := NewHTTPMessageFramer()
   Preconnection.AddFramer(framer)

   Since Message Framers pass from Preconnection to Listener or
   Connection, addition of Framers must happen before any operation that
   may result in the creation of a Connection.

9.1.2.2.  Framing Meta-Data

   When sending Messages, applications can add Framer-specific key/value
   pairs to a MessageContext (Section 9.1.1).  This mechanism can be
   used, for example, to set the type of a Message for a TLV format.
   The namespace of values is custom for each unique Message Framer.

   messageContext := NewMessageContext()
   messageContext.add(framer, key, value)
   Connection.Send(messageData, messageContext)

   When an application receives a MessageContext in a Receive event, it
   can also look to see if a value was set by a specific Message Framer.

   messageContext.get(framer, key) -> value

   For example, if an HTTP Message Framer is used, the values could
   correspond to HTTP headers:

   httpFramer := NewHTTPMessageFramer()
   ...
   messageContext := NewMessageContext()
   messageContext.add(httpFramer, "accept", "text/html")










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9.1.3.  Message Properties

   Applications needing to annotate the Messages they send with extra
   information (for example, to control how data is scheduled and
   processed by the transport protocols supporting the Connection) can
   include this information in the Message Context passed to the Send
   Action.  For other uses of the message context, see Section 9.1.1.

   Message Properties are per-Message, not per-Send if partial Messages
   are sent (Section 9.2.3).  All data blocks associated with a single
   Message share properties specified in the Message Contexts.  For
   example, it would not make sense to have the beginning of a Message
   expire, but allow the end of a Message to still be sent.

   A MessageContext object contains metadata for the Messages to be sent
   or received.

   messageData := "hello"
   messageContext := NewMessageContext()
   messageContext.add(parameter, value)
   Connection.Send(messageData, messageContext)

   The simpler form of Send, which does not take any messageContext, is
   equivalent to passing a default MessageContext without adding any
   Message Properties.

   If an application wants to override Message Properties for a specific
   message, it can acquire an empty MessageContext Object and add all
   desired Message Properties to that Object.  It can then reuse the
   same messageContext Object for sending multiple Messages with the
   same properties.

   Properties can be added to a MessageContext object only before the
   context is used for sending.  Once a messageContext has been used
   with a Send call, it is invalid to modify any of its properties.

   The Message Properties could be inconsistent with the properties of
   the Protocol Stacks underlying the Connection on which a given
   Message is sent.  For example, a Protocol Stack must be able to
   provide ordering if the msgOrdered property of a Message is enabled.
   Sending a Message with Message Properties inconsistent with the
   Selection Properties of the Connection yields an error.

   Connection Properties describe the default behavior for all Messages
   on a Connection.  If a Message Property contradicts a Connection
   Property, and if this per-Message behavior can be supported, it
   overrides the Connection Property for the specific Message.  For
   example, if "Reliable Data Transfer (Connection)" is set to "Require"



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   and a protocol with configurable per-Message reliability is used,
   setting "Reliable Data Transfer (Message)" to "false" for a
   particular Message will allow this Message to be sent without any
   reliability guarantees.  Changing the Reliable Data Transfer property
   on Messages is only possible for Connections that were established
   enabling the Selection Property "Configure Per-Message Reliability".

   The following Message Properties are supported:

9.1.3.1.  Lifetime

   Name:  msgLifetime

   Type:  Numeric

   Default:  infinite

   The Lifetime specifies how long a particular Message can wait to be
   sent to the Remote Endpoint before it is irrelevant and no longer
   needs to be (re-)transmitted.  This is a hint to the Transport
   Services system - it is not guaranteed that a Message will not be
   sent when its Lifetime has expired.

   Setting a Message's Lifetime to infinite indicates that the
   application does not wish to apply a time constraint on the
   transmission of the Message, but it does not express a need for
   reliable delivery; reliability is adjustable per Message via the
   "Reliable Data Transfer (Message)" property (see Section 9.1.3.7).
   The type and units of Lifetime are implementation-specific.

9.1.3.2.  Priority

   Name:  msgPrio

   Type:  Integer (non-negative)

   Default:  100

   This property specifies the priority of a Message, relative to other
   Messages sent over the same Connection.

   A Message with Priority 0 will yield to a Message with Priority 1,
   which will yield to a Message with Priority 2, and so on.  Priorities
   may be used as a sender-side scheduling construct only, or be used to
   specify priorities on the wire for Protocol Stacks supporting
   prioritization.





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   Note that this property is not a per-message override of the
   Connection Priority - see Section 8.1.2.  The Priority properties may
   interact, but can be used independently and be realized by different
   mechanisms; see Section 9.2.6.

9.1.3.3.  Ordered

   Name:  msgOrdered

   Type:  Boolean

   Default:  the queried Boolean value of the Selection Property
      "preserveOrder" (Section 6.2.4)

   The order in which Messages were submitted for transmission via the
   Send Action will be preserved on delivery via Receive<> events for
   all Messages on a Connection that have this Message Property set to
   true.

   If false, the Message is delivered to the receiving application
   without preserving the ordering.  This property is used for protocols
   that support preservation of data ordering, see Section 6.2.4, but
   allow out-of-order delivery for certain messages, e.g., by
   multiplexing independent messages onto different streams.

9.1.3.4.  Safely Replayable

   Name:  safelyReplayable

   Type:  Boolean

   Default:  false

   If true, Safely Replayable specifies that a Message is safe to send
   to the Remote Endpoint more than once for a single Send Action.  It
   marks the data as safe for certain 0-RTT establishment techniques,
   where retransmission of the 0-RTT data may cause the remote
   application to receive the Message multiple times.

   For protocols that do not protect against duplicated messages, e.g.,
   UDP, all messages need to be marked as "Safely Replayable".  To
   enable protocol selection to choose such a protocol, "Safely
   Replayable" needs to be added to the TransportProperties passed to
   the Preconnection.  If such a protocol was chosen, disabling "Safely
   Replayable" on individual messages MUST result in a SendError.






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9.1.3.5.  Final

   Name:  final

   Type:  Boolean

   Default:  false

   If true, this indicates a Message is the last that the application
   will send on a Connection.  This allows underlying protocols to
   indicate to the Remote Endpoint that the Connection has been
   effectively closed in the sending direction.  For example, TCP-based
   Connections can send a FIN once a Message marked as Final has been
   completely sent, indicated by marking endOfMessage.  Protocols that
   do not support signalling the end of a Connection in a given
   direction will ignore this property.

   A Final Message must always be sorted to the end of a list of
   Messages.  The Final property overrides Priority and any other
   property that would re-order Messages.  If another Message is sent
   after a Message marked as Final has already been sent on a
   Connection, the Send Action for the new Message will cause a
   SendError Event.

9.1.3.6.  Sending Corruption Protection Length

   Name:  msgChecksumLen

   Type:  Integer (non-negative with special value "Full Coverage")

   Default:  Full Coverage

   This property specifies the minimum length of the section of a sent
   Message, starting from byte 0, that the application requires to be
   delivered without corruption due to lower layer errors.  It is used
   to specify options for simple integrity protection via checksums.  A
   value of 0 means that no checksum needs to be calculated, and "Full
   Coverage" means that the entire Message needs to be protected by a
   checksum.  Only "Full Coverage" is guaranteed, any other requests are
   advisory, which may result in "Full Coverage" being applied.

9.1.3.7.  Reliable Data Transfer (Message)

   Name:  msgReliable

   Type:  Boolean

   Default:  the queried Boolean value of the Selection Property



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      "reliability" (Section 6.2.1)

   When true, this property specifies that a Message should be sent in
   such a way that the transport protocol ensures all data is received
   on the other side without corruption.  Changing the "Reliable Data
   Transfer" property on Messages is only possible for Connections that
   were established enabling the Selection Property "Configure Per-
   Message Reliability".  When this is not the case, changing
   "msgReliable" will generate an error.

   Disabling this property indicates that the Transport Services system
   may disable retransmissions or other reliability mechanisms for this
   particular Message, but such disabling is not guaranteed.

9.1.3.8.  Message Capacity Profile Override

   Name:  msgCapacityProfile

   Type:  Enumeration

   Default:  inherited from the Connection Property
      "connCapacityProfile" (Section 8.1.6)

   This enumerated property specifies the application's preferred
   tradeoffs for sending this Message; it is a per-Message override of
   the Capacity Profile connection property (see Section 8.1.6).

9.1.3.9.  No Network-Layer Fragmentation

   Name:  noFragmentation

   Type:  Boolean

   Default:  false

   This property specifies that a message should be sent and received
   without network-layer fragmentation, if possible.  It can be used to
   avoid network layer fragmentation when transport segmentation is
   prefered.

   This only takes effect when the transport uses a network layer that
   supports this functionality.  When it does take effect, setting this
   property to true will cause the sender to avoid network-layer source
   frgementation.  When using IPv4, this will result in the Don't
   Fragment bit being set in the IP header.






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   Attempts to send a message with this property that result in a size
   greater than the transport's current estimate of its maximum packet
   size ("singularTransmissionMsgMaxLen") can result in transport
   segmentation when permitted, or in a "SendError".

   Note: noSegmentation should be used when it is desired to only send a
   message within a single network packet.

9.1.3.10.  No Segmentation

   Name:  noSegmentation

   Type:  Boolean

   Default:  false

   When set to true, this property requests the transport layer to not
   provide segmentation of messages larger than the maximum size
   permitted by the network layer, and also to avoid network-layer
   source fragmentation of messages.  When running over IPv4, setting
   this property to true can result in a sending endpount setting the
   Don't Fragment bit in the IPv4 header of packets generated by the
   transport layer.  An attempt to send a message that results in a size
   greater than the transport's current estimate of its maximum packet
   size (singularTransmissionMsgMaxLen) will result in a SendError.
   This only takes effect when the transport and network layer support
   this functionality.

9.2.  Sending Data

   Once a Connection has been established, it can be used for sending
   Messages.  By default, Send enqueues a complete Message, and takes
   optional per-Message properties (see Section 9.2.1).  All Send
   actions are asynchronous, and deliver Events (see Section 9.2.2).
   Sending partial Messages for streaming large data is also supported
   (see Section 9.2.3).

   Messages are sent on a Connection using the Send action:

   Connection.Send(messageData, messageContext?, endOfMessage?)

   where messageData is the data object to send, and messageContext
   allows adding Message Properties, identifying Send Events related to
   a specific Message or inspecting meta-data related to the Message
   sent (see Section 9.1.1).

   The optional endOfMessage parameter supports partial sending and is
   described in Section 9.2.3.



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9.2.1.  Basic Sending

   The most basic form of sending on a connection involves enqueuing a
   single Data block as a complete Message with default Message
   Properties.

   messageData := "hello"
   Connection.Send(messageData)

   The interpretation of a Message to be sent is dependent on the
   implementation, and on the constraints on the Protocol Stacks implied
   by the Connection's transport properties.  For example, a Message may
   be a single datagram for UDP Connections; or an HTTP Request for HTTP
   Connections.

   Some transport protocols can deliver arbitrarily sized Messages, but
   other protocols constrain the maximum Message size.  Applications can
   query the Connection Property "Maximum Message size on send"
   (Section 8.1.11.3) to determine the maximum size allowed for a single
   Message.  If a Message is too large to fit in the Maximum Message
   Size for the Connection, the Send will fail with a SendError event
   (Section 9.2.2.3).  For example, it is invalid to send a Message over
   a UDP connection that is larger than the available datagram sending
   size.

9.2.2.  Send Events

   Like all Actions in this interface, the Send Action is asynchronous.
   There are several Events that can be delivered in response to Sending
   a Message.  Exactly one Event (Sent, Expired, or SendError) will be
   delivered in response to each call to Send.

   Note that if partial Sends are used (Section 9.2.3), there will still
   be exactly one Send Event delivered for each call to Send.  For
   example, if a Message expired while two requests to Send data for
   that Message are outstanding, there will be two Expired events
   delivered.

   The interface should allow the application to correlate which Send
   Action resulted in a particular Send Event.  The manner in which this
   correlation is indicated is implementation-specific.

9.2.2.1.  Sent

   Connection -> Sent<messageContext>






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   The Sent Event occurs when a previous Send Action has completed,
   i.e., when the data derived from the Message has been passed down or
   through the underlying Protocol Stack and is no longer the
   responsibility of this interface.  The exact disposition of the
   Message (i.e., whether it has actually been transmitted, moved into a
   buffer on the network interface, moved into a kernel buffer, and so
   on) when the Sent Event occurs is implementation-specific.  The Sent
   Event contains a reference to the Message Context of the Message to
   which it applies.

   Sent Events allow an application to obtain an understanding of the
   amount of buffering it creates.  That is, if an application calls the
   Send Action multiple times without waiting for a Sent Event, it has
   created more buffer inside the Transport Services system than an
   application that always waits for the Sent Event before calling the
   next Send Action.

9.2.2.2.  Expired

   Connection -> Expired<messageContext>

   The Expired Event occurs when a previous Send Action expired before
   completion; i.e. when the Message was not sent before its Lifetime
   (see Section 9.1.3.1) expired.  This is separate from SendError, as
   it is an expected behavior for partially reliable transports.  The
   Expired Event contains a reference to the Message Context of the
   Message to which it applies.

9.2.2.3.  SendError

   Connection -> SendError<messageContext, reason?>

   A SendError occurs when a Message was not sent due to an error
   condition: an attempt to send a Message which is too large for the
   system and Protocol Stack to handle, some failure of the underlying
   Protocol Stack, or a set of Message Properties not consistent with
   the Connection's transport properties.  The SendError contains a
   reference to the Message Context of the Message to which it applies.

9.2.3.  Partial Sends

   It is not always possible for an application to send all data
   associated with a Message in a single Send Action.  The Message data
   may be too large for the application to hold in memory at one time,
   or the length of the Message may be unknown or unbounded.






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   Partial Message sending is supported by passing an endOfMessage
   boolean parameter to the Send Action.  This value is always true by
   default, and the simpler forms of Send are equivalent to passing true
   for endOfMessage.

   The following example sends a Message in two separate calls to Send.

   messageContext := NewMessageContext()
   messageContext.add(parameter, value)

   messageData := "hel"
   endOfMessage := false
   Connection.Send(messageData, messageContext, endOfMessage)

   messageData := "lo"
   endOfMessage := true
   Connection.Send(messageData, messageContext, endOfMessage)

   All data sent with the same MessageContext object will be treated as
   belonging to the same Message, and will constitute an in-order series
   until the endOfMessage is marked.

9.2.4.  Batching Sends

   To reduce the overhead of sending multiple small Messages on a
   Connection, the application could batch several Send Actions
   together.  This provides a hint to the system that the sending of
   these Messages ought to be coalesced when possible, and that sending
   any of the batched Messages can be delayed until the last Message in
   the batch is enqueued.

   The semantics for starting and ending a batch can be implementation-
   specific, but need to allow multiple Send Actions to be enqueued.

   Connection.StartBatch()
   Connection.Send(messageData)
   Connection.Send(messageData)
   Connection.EndBatch()

9.2.5.  Send on Active Open: InitiateWithSend

   For application-layer protocols where the Connection initiator also
   sends the first message, the InitiateWithSend() action combines
   Connection initiation with a first Message sent:

Connection := Preconnection.InitiateWithSend(messageData, messageContext?, timeout?)





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   Whenever possible, a messageContext should be provided to declare the
   Message passed to InitiateWithSend as "Safely Replayable".  This
   allows the Transport Services system to make use of 0-RTT
   establishment in case this is supported by the available protocol
   stacks.  When the selected stack(s) do not support transmitting data
   upon connection establishment, InitiateWithSend is identical to
   Initiate() followed by Send().

   Neither partial sends nor send batching are supported by
   InitiateWithSend().

   The Events that may be sent after InitiateWithSend() are equivalent
   to those that would be sent by an invocation of Initiate() followed
   immediately by an invocation of Send(), with the caveat that a send
   failure that occurs because the Connection could not be established
   will not result in a SendError separate from the EstablishmentError
   signaling the failure of Connection establishment.

9.2.6.  Priority in TAPS

   The Transport Services interface provides two properties to allow a
   sender to signal the relative priority of data transmission: the
   Priority Message Property Section 9.1.3.2, and the Connection
   Priority Connection Property Section 8.1.2.  These properties are
   designed to allow the expression and implementation of a wide variety
   of approaches to transmission priority in the transport and
   application layer, including those which do not appear on the wire
   (affecting only sender-side transmission scheduling) as well as those
   that do (e.g.  [I-D.ietf-httpbis-priority].

   A Transport Services system gives no guarantees about how its
   expression of relative priorities will be realized.  However, the
   Transport Services system will seek to ensure that performance of
   relatively-prioritized connections and messages is not worse with
   respect to those connections and messages than an equivalent
   configuration in which all prioritization properties are left at
   their defaults.

   The Transport Services interface does order Connection Priority over
   the Priority Message Property.  In the absense of other externalities
   (e.g., transport-layer flow control), a priority 1 Message on a
   priority 0 Connection will be sent before a priority 0 Message on a
   priority 1 Connection in the same group.








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9.3.  Receiving Data

   Once a Connection is established, it can be used for receiving data
   (unless the "Direction of Communication" property is set to
   "unidirectional send").  As with sending, the data is received in
   Messages.  Receiving is an asynchronous operation, in which each call
   to Receive enqueues a request to receive new data from the
   connection.  Once data has been received, or an error is encountered,
   an event will be delivered to complete any pending Receive requests
   (see Section 9.3.2).  If Messages arrive at the Transport Services
   system before Receive requests are issued, ensuing Receive requests
   will first operate on these Messages before awaiting any further
   Messages.

9.3.1.  Enqueuing Receives

   Receive takes two parameters to specify the length of data that an
   application is willing to receive, both of which are optional and
   have default values if not specified.

   Connection.Receive(minIncompleteLength?, maxLength?)

   By default, Receive will try to deliver complete Messages in a single
   event (Section 9.3.2.1).

   The application can set a minIncompleteLength value to indicate the
   smallest partial Message data size in bytes that should be delivered
   in response to this Receive.  By default, this value is infinite,
   which means that only complete Messages should be delivered (see
   Section 9.3.2.2 and Section 9.1.2 for more information on how this is
   accomplished).  If this value is set to some smaller value, the
   associated receive event will be triggered only when at least that
   many bytes are available, or the Message is complete with fewer
   bytes, or the system needs to free up memory.  Applications should
   always check the length of the data delivered to the receive event
   and not assume it will be as long as minIncompleteLength in the case
   of shorter complete Messages or memory issues.

   The maxLength argument indicates the maximum size of a Message in
   bytes that the application is currently prepared to receive.  The
   default value for maxLength is infinite.  If an incoming Message is
   larger than the minimum of this size and the maximum Message size on
   receive for the Connection's Protocol Stack, it will be delivered via
   ReceivedPartial events (Section 9.3.2.2).

   Note that maxLength does not guarantee that the application will
   receive that many bytes if they are available; the interface could
   return ReceivedPartial events with less data than maxLength according



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   to implementation constraints.  Note also that maxLength and
   minIncompleteLength are intended only to manage buffering, and are
   not interpreted as a receiver preference for message reordering.

9.3.2.  Receive Events

   Each call to Receive will be paired with a single Receive Event,
   which can be a success or an error.  This allows an application to
   provide backpressure to the transport stack when it is temporarily
   not ready to receive messages.

   The interface should allow the application to correlate which call to
   Receive resulted in a particular Receive Event.  The manner in which
   this correlation is indicated is implementation-specific.

9.3.2.1.  Received

   Connection -> Received<messageData, messageContext>

   A Received event indicates the delivery of a complete Message.  It
   contains two objects, the received bytes as messageData, and the
   metadata and properties of the received Message as messageContext.

   The messageData object provides access to the bytes that were
   received for this Message, along with the length of the byte array.
   The messageContext is provided to enable retrieving metadata about
   the message and referring to the message, e.g., to send replies and
   map responses to their requests.  See Section 9.1.1 for details.

   See Section 9.1.2 for handling Message framing in situations where
   the Protocol Stack only provides a byte-stream transport.

9.3.2.2.  ReceivedPartial

Connection -> ReceivedPartial<messageData, messageContext, endOfMessage>

   If a complete Message cannot be delivered in one event, one part of
   the Message can be delivered with a ReceivedPartial event.  To
   continue to receive more of the same Message, the application must
   invoke Receive again.

   Multiple invocations of ReceivedPartial deliver data for the same
   Message by passing the same MessageContext, until the endOfMessage
   flag is delivered or a ReceiveError occurs.  All partial blocks of a
   single Message are delivered in order without gaps.  This event does
   not support delivering discontiguous partial Messages.  If, for
   example, Message A is divided into three pieces (A1, A2, A3) and
   Message B is divided into three pieces (B1, B2, B3), and



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   preserveOrder is not Required, the ReceivedPartial may deliver them
   in a sequence like this: A1, B1, B2, A2, A3, B3, because the
   messageContext allows the application to identify the pieces as
   belonging to Message A and B, respectively.  However, a sequence
   like: A1, A3 will never occur.

   If the minIncompleteLength in the Receive request was set to be
   infinite (indicating a request to receive only complete Messages),
   the ReceivedPartial event may still be delivered if one of the
   following conditions is true:

   *  the underlying Protocol Stack supports message boundary
      preservation, and the size of the Message is larger than the
      buffers available for a single message;

   *  the underlying Protocol Stack does not support message boundary
      preservation, and the Message Framer (see Section 9.1.2) cannot
      determine the end of the message using the buffer space it has
      available; or

   *  the underlying Protocol Stack does not support message boundary
      preservation, and no Message Framer was supplied by the
      application

   Note that in the absence of message boundary preservation or a
   Message Framer, all bytes received on the Connection will be
   represented as one large Message of indeterminate length.

9.3.2.3.  ReceiveError

   Connection -> ReceiveError<messageContext, reason?>

   A ReceiveError occurs when data is received by the underlying
   Protocol Stack that cannot be fully retrieved or parsed, and when it
   is useful for the application to be notified of such errors.  For
   example, a ReceiveError can indicate that a Message (identified via
   the MessageContext) that was being partially received previously, but
   had not completed, encountered an error and will not be completed.
   This can be useful for an application, which may want to use this
   error as a hint to remove previously received Message parts from
   memory.  As another example, if an incoming Message does not fulfill
   the Required Minimum Corruption Protection Coverage for Receiving
   property (see Section 8.1.1), an application can use this error as a
   hint to inform the peer application to adjust the Sending Corruption
   Protection Length property (see Section 9.1.3.6).






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   In contrast, internal protocol reception errors (e.g., loss causing
   retransmissions in TCP) are not signalled by this Event.  Conditions
   that irrevocably lead to the termination of the Connection are
   signaled using ConnectionError (see Section 10).

9.3.3.  Receive Message Properties

   Each Message Context may contain metadata from protocols in the
   Protocol Stack; which metadata is available is Protocol Stack
   dependent.  These are exposed through additional read-only Message
   Properties that can be queried from the MessageContext object (see
   Section 9.1.1) passed by the receive event.  The following metadata
   values are supported:

9.3.3.1.  UDP(-Lite)-specific Property: ECN

   When available, Message metadata carries the value of the Explicit
   Congestion Notification (ECN) field.  This information can be used
   for logging and debugging, and for building applications that need
   access to information about the transport internals for their own
   operation.  This property is specific to UDP and UDP-Lite because
   these protocols do not implement congestion control, and hence expose
   this functionality to the application (see [RFC8293], following the
   guidance in [RFC8085])

9.3.3.2.  Early Data

   In some cases it can be valuable to know whether data was read as
   part of early data transfer (before connection establishment has
   finished).  This is useful if applications need to treat early data
   separately, e.g., if early data has different security properties
   than data sent after connection establishment.  In the case of TLS
   1.3, client early data can be replayed maliciously (see [RFC8446]).
   Thus, receivers might wish to perform additional checks for early
   data to ensure it is safely replayable.  If TLS 1.3 is available and
   the recipient Message was sent as part of early data, the
   corresponding metadata carries a flag indicating as such.  If early
   data is enabled, applications should check this metadata field for
   Messages received during connection establishment and respond
   accordingly.











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9.3.3.3.  Receiving Final Messages

   The Message Context can indicate whether or not this Message is the
   Final Message on a Connection.  For any Message that is marked as
   Final, the application can assume that there will be no more Messages
   received on the Connection once the Message has been completely
   delivered.  This corresponds to the Final property that may be marked
   on a sent Message, see Section 9.1.3.5.

   Some transport protocols and peers do not support signaling of the
   Final property.  Applications therefore should not rely on receiving
   a Message marked Final to know that the sending endpoint is done
   sending on a connection.

   Any calls to Receive once the Final Message has been delivered will
   result in errors.

10.  Connection Termination

   A Connection can be terminated i) by the Local Endpoint (i.e., the
   application calls the Close, CloseGroup, Abort or AbortGroup Action),
   ii) by the Remote Endpoint (i.e., the remote application calls the
   Close, CloseGroup, Abort or AbortGroup Action), or iii) because of an
   error (e.g., a timeout).  A local call of the Close Action will cause
   the Connection to either send a Closed Event or a ConnectionError
   Event, and a local call of the CloseGroup Action will cause all of
   the Connections in the group to either send a Closed Event or a
   ConnectionError Event.  A local call of the Abort Action will cause
   the Connection to send a ConnectionError Event, indicating local
   Abort as a reason, and a local call of the AbortGroup Action will
   cause all of the Connections in the group to send a ConnectionError
   Event, indicating local Abort as a reason.

   Remote Action calls map to Events similar to local calls (e.g., a
   remote Close causes the Connection to either send a Closed Event or a
   ConnectionError Event), but, different from local Action calls, it is
   not guaranteed that such Events will indeed be invoked.  When an
   application needs to free resources associated with a Connection, it
   should therefore not rely on the invocation of such Events due to
   termination calls from the Remote Endpoint, but instead use the local
   termination Actions.

   Close terminates a Connection after satisfying all the requirements
   that were specified regarding the delivery of Messages that the
   application has already given to the Transport Services system.  Upon
   successfully satisfying all these requirements, the Connection will
   send the Closed Event.  For example, if reliable delivery was
   requested for a Message handed over before calling Close, the Closed



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   Event will signify that this Message has indeed been delivered.  This
   Action does not affect any other Connection in the same Connection
   Group.

   Connection.Close()

   The Closed Event informs the application that a Close Action has
   successfully completed, or that the Remote Endpoint has closed the
   Connection.  There is no guarantee that a remote Close will be
   signaled.

   Connection -> Closed<>

   Abort terminates a Connection without delivering any remaining
   Messages.  This action does not affect any other Connection that is
   entangled with this one in a Connection Group.  When the Abort Action
   has finished, the Connection will send a ConnectionError Event,
   indicating local Abort as a reason.

   Connection.Abort()

   CloseGroup gracefully terminates a Connection and any other
   Connections in the same Connection Group.  For example, all of the
   Connections in a group might be streams of a single session for a
   multistreaming protocol; closing the entire group will close the
   underlying session.  See also Section 7.4.  All Connections in the
   group will send a Closed Event when the CloseGroup Action was
   successful.  As with Close, any Messages remaining to be processed on
   a Connection will be handled prior to closing.

   Connection.CloseGroup()

   AbortGroup terminates a Connection and any other Connections that are
   in the same Connection Group without delivering any remaining
   Messages.  When the AbortGroup Action has finished, all Connections
   in the group will send a ConnectionError Event, indicating local
   Abort as a reason.

   Connection.AbortGroup()

   A ConnectionError informs the application that: 1) data could not be
   delivered to the peer after a timeout, or 2) the Connection has been
   aborted (e.g., because the peer has called Abort).  There is no
   guarantee that an Abort from the peer will be signaled.

   Connection -> ConnectionError<reason?>





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11.  Connection State and Ordering of Operations and Events

   This interface is designed to be independent of an implementation's
   concurrency model.  The details of how exactly actions are handled,
   and how events are dispatched, are implementation dependent.

   Each transition of connection state is associated with one of more
   events:

   *  Ready<> occurs when a Connection created with Initiate() or
      InitiateWithSend() transitions to Established state.

   *  ConnectionReceived<> occurs when a Connection created with
      Listen() transitions to Established state.

   *  RendezvousDone<> occurs when a Connection created with
      Rendezvous() transitions to Established state.

   *  Closed<> occurs when a Connection transitions to Closed state
      without error.

   *  EstablishmentError<> occurs when a Connection created with
      Initiate() transitions from Establishing state to Closed state due
      to an error.

   *  ConnectionError<> occurs when a Connection transitions to Closed
      state due to an error in all other circumstances.

   The following diagram shows the possible states of a Connection and
   the events that occur upon a transition from one state to another.

                 (*)                               (**)
   Establishing -----> Established -----> Closing ------> Closed
        |                                                   ^
        |                                                   |
        +---------------------------------------------------+
                     EstablishmentError<>

   (*) Ready<>, ConnectionReceived<>, RendezvousDone<>
   (**) Closed<>, ConnectionError<>

                     Figure 2: Connection State Diagram

   The interface provides the following guarantees about the ordering of
   operations:






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   *  Sent<> events will occur on a Connection in the order in which the
      Messages were sent (i.e., delivered to the kernel or to the
      network interface, depending on implementation).

   *  Received<> will never occur on a Connection before it is
      Established; i.e. before a Ready<> event on that Connection, or a
      ConnectionReceived<> or RendezvousDone<> containing that
      Connection.

   *  No events will occur on a Connection after it is Closed; i.e.,
      after a Closed<> event, an EstablishmentError<> or
      ConnectionError<> will not occur on that connection.  To ensure
      this ordering, Closed<> will not occur on a Connection while other
      events on the Connection are still locally outstanding (i.e.,
      known to the interface and waiting to be dealt with by the
      application).

12.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no Actions for IANA.  Later versions of this
   document may create IANA registries for generic transport property
   names and transport property namespaces (see Section 4.1).

13.  Privacy and Security Considerations

   This document describes a generic API for interacting with a
   transport services (TAPS) system.  Part of this API includes
   configuration details for transport security protocols, as discussed
   in Section 6.3.  It does not recommend use (or disuse) of specific
   algorithms or protocols.  Any API-compatible transport security
   protocol ought to work in a TAPS system.  Security considerations for
   these protocols are discussed in the respective specifications.

   The described API is used to exchange information between an
   application and the Transport Services system.  While it is not
   necessarily expected that both systems are implemented by the same
   authority, it is expected that the Transport Services system
   implementation is either provided as a library that is selected by
   the application from a trusted party, or that it is part of the
   operating system that the application also relies on for other tasks.

   In either case, the Transport Services API is an internal interface
   that is used to change information locally between two systems.
   However, as the Transport Services system is responsible for network
   communication, it is in the position to potentially share any
   information provided by the application with the network or another



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   communication peer.  Most of the information provided over the
   Transport Services API are useful to configure and select protocols
   and paths and are not necessarily privacy sensitive.  Still, some
   information could be privacy sensitive because it might reveal usage
   characteristics and habits of the user of an application.

   Of course any communication over a network reveals usage
   characteristics, as all packets, as well as their timing and size,
   are part of the network-visible wire image [RFC8546].  However, the
   selection of a protocol and its configuration also impacts which
   information is visible, potentially in clear text, and which other
   entities can access it.  In most cases, information provided for
   protocol and path selection should not directly translate to
   information that can be observed by network devices on the path.
   However, there might be specific configuration information that is
   intended for path exposure, e.g., a DiffServ codepoint setting, that
   is either provided directly by the application or indirectly
   configured for a traffic profile.

   Applications should be aware that communication attempts can lead to
   more than one connection establishment.  This is the case, for
   example, when the Transport Services system also executes name
   resolution, when support mechanisms such as TURN or ICE are used to
   establish connectivity, if protocols or paths are raised, or if a
   path fails and fallback or re-establishment is supported in the
   Transport Services system.

   Applications should also take care to not assume that all data
   received using the Transport Services API is always complete or well-
   formed.  Specifically, messages that are received partially
   Section 9.3.2.2 could be a source of truncation attacks if
   applications do not distinguish between partial messages and complete
   messages.

   The interface explicitly does not require the application to resolve
   names, though there is a tradeoff between early and late binding of
   addresses to names.  Early binding allows the API implementation to
   reduce connection setup latency, at the cost of potentially limited
   scope for alternate path discovery during Connection establishment,
   as well as potential additional information leakage about application
   interest when used with a resolution method (such as DNS without TLS)
   which does not protect query confidentiality.

   These communication activities are not different from what is used
   today.  However, the goal of a Transport Services system is to
   support such mechanisms as a generic service within the transport
   layer.  This enables applications to more dynamically benefit from
   innovations and new protocols in the transport, although it reduces



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   transparency of the underlying communication actions to the
   application itself.  The TAPS API is designed such that protocol and
   path selection can be limited to a small and controlled set if
   required by the application for functional or security purposes.
   Further, TAPS implementations should provide an interface to poll
   information about which protocol and path is currently in use as well
   as provide logging about the communication events of each connection.

14.  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.

   This work has been supported by the Research Council of Norway under
   its "Toppforsk" programme through the "OCARINA" project.

   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.  Thanks to Laurent Chuat
   and Jason Lee for initial work on the Post Sockets interface, from
   which this work has evolved.  Thanks to Maximilian Franke for asking
   good questions based on implementation experience and for
   contributing text, e.g., on multicast.

15.  References

15.1.  Normative References

   [I-D.ietf-taps-arch]
              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
              Perkins, C., Tiesel, P. S., and C. A. Wood, "An
              Architecture for Transport Services", Work in Progress,
              Internet-Draft, draft-ietf-taps-arch-10, 30 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-taps-arch-
              10.txt>.

   [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>.



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   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers",
              BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
              <https://www.rfc-editor.org/info/rfc8084>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [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>.

   [RFC8303]  Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
              Transport Features Provided by IETF Transport Protocols",
              RFC 8303, DOI 10.17487/RFC8303, February 2018,
              <https://www.rfc-editor.org/info/rfc8303>.

   [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>.

15.2.  Informative References

   [I-D.ietf-httpbis-priority]
              Oku, K. and L. Pardue, "Extensible Prioritization Scheme
              for HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-priority-03, 11 January 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              priority-03.txt>.

   [I-D.ietf-taps-impl]
              Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P. S., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", Work in
              Progress, Internet-Draft, draft-ietf-taps-impl-09, 30
              April 2021, <https://www.ietf.org/archive/id/draft-ietf-
              taps-impl-09.txt>.





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   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597,
              DOI 10.17487/RFC2597, June 1999,
              <https://www.rfc-editor.org/info/rfc2597>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
              Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
              <https://www.rfc-editor.org/info/rfc3246>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <https://www.rfc-editor.org/info/rfc4594>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <https://www.rfc-editor.org/info/rfc5245>.

   [RFC5482]  Eggert, L. and F. Gont, "TCP User Timeout Option",
              RFC 5482, DOI 10.17487/RFC5482, March 2009,
              <https://www.rfc-editor.org/info/rfc5482>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, DOI 10.17487/RFC5865, May 2010,
              <https://www.rfc-editor.org/info/rfc5865>.



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   [RFC7478]  Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use Cases and Requirements", RFC 7478,
              DOI 10.17487/RFC7478, March 2015,
              <https://www.rfc-editor.org/info/rfc7478>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
              <https://www.rfc-editor.org/info/rfc7556>.

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,
              <https://www.rfc-editor.org/info/rfc7657>.

   [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>.

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <https://www.rfc-editor.org/info/rfc8229>.

   [RFC8260]  Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol", RFC 8260,
              DOI 10.17487/RFC8260, November 2017,
              <https://www.rfc-editor.org/info/rfc8260>.

   [RFC8293]  Ghanwani, A., Dunbar, L., McBride, M., Bannai, V., and R.
              Krishnan, "A Framework for Multicast in Network
              Virtualization over Layer 3", RFC 8293,
              DOI 10.17487/RFC8293, January 2018,
              <https://www.rfc-editor.org/info/rfc8293>.

   [RFC8489]  Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
              D., Mahy, R., and P. Matthews, "Session Traversal
              Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
              February 2020, <https://www.rfc-editor.org/info/rfc8489>.

   [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <https://www.rfc-editor.org/info/rfc8546>.

   [RFC8622]  Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
              Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
              June 2019, <https://www.rfc-editor.org/info/rfc8622>.



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   [RFC8699]  Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion
              Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699,
              January 2020, <https://www.rfc-editor.org/info/rfc8699>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

   [RFC8922]  Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
              Wood, "A Survey of the Interaction between Security
              Protocols and Transport Services", RFC 8922,
              DOI 10.17487/RFC8922, October 2020,
              <https://www.rfc-editor.org/info/rfc8922>.

   [RFC8923]  Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for End Systems", RFC 8923, DOI 10.17487/RFC8923,
              October 2020, <https://www.rfc-editor.org/info/rfc8923>.

   [TCP-COUPLING]
              Islam, S., Welzl, M., Hiorth, K., Hayes, D., Armitage, G.,
              and S. Gjessing, "ctrlTCP: Reducing Latency through
              Coupled, Heterogeneous Multi-Flow TCP Congestion Control",
              IEEE INFOCOM Global Internet Symposium (GI) workshop (GI
              2018) , 2018.

Appendix A.  Implementation Mapping

   The way the concepts from this abstract interface map into concrete
   APIs in a given language on a given platform largely depends on the
   features and norms of the language and the platform.  Actions could
   be implemented as functions or method calls, for instance, and Events
   could be implemented via event queues, handler functions or classes,
   communicating sequential processes, or other asynchronous calling
   conventions.

A.1.  Types

   The basic types mentioned in Section 1.1 typically have natural
   correspondences in practical programming languages, perhaps
   constrained by implementation-specific limitations.  For example:

   *  An Integer can typically be represented in C by an "int" or
      "long", subject to the underlying platform's ranges for each.  To
      accommodate special values, a C function that returns a non-
      negative "int" on success may return -1 on failure.  In Python,
      such a function might return "None" or raise an exception.




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   *  In C, a Tuple may be represented as a "struct" with one member for
      each of the value types in the ordered grouping.  In Python, by
      contrast, a Tuple may be represented natively as a "tuple", a
      sequence of dynamically-typed elements.

   *  A Collection may be represented as a "std::set" in C++ or as a
      "set" in Python.  In C, it may be represented as an array or as a
      higher-level data structure with appropriate accessors defined.

   The objects described in Section 1.1 can similarly be represented in
   different ways depending on which programming language is used.
   Objects like Preconnections, Connections, and Listeners can be long-
   lived, and benefit from using object-oriented constructs.  Note that
   in C, these objects may need to provide a way to release or free
   their underlying memory when the application is done using them.  For
   example, since a Preconnection can be used to initiate multiple
   Connections, it is the responsibility of the application to clean up
   the Preconnection memory if necessary.

A.2.  Events and Errors

   This specification treats Events and Errors similarly.  Errors, just
   as any other Events, may occur asynchronously in network
   applications.  However, implementations of this interface may report
   Errors synchronously, according to the error handling idioms of the
   implementation platform, where they can be immediately detected, such
   as by generating an exception when attempting to initiate a
   connection with inconsistent Transport Properties.  An error can
   provide an optional reason to the application with further details
   about why the error occurred.

A.3.  Time Duration

   Time duration types are implementation-specific.  For instance, it
   could be a number of seconds, number of milliseconds, or a "struct
   timeval" in C or a user-defined "Duration" class in C++.

Appendix B.  Convenience Functions

B.1.  Adding Preference Properties

   As Selection Properties of type "Preference" will be set on a
   TransportProperties object quite frequently, implementations can
   provide special actions for adding each preference level i.e,
   "TransportProperties.Set(some_property, avoid)" is equivalent to
   "TransportProperties.Avoid(some_property)":





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   TransportProperties.Require(property)
   TransportProperties.Prefer(property)
   TransportProperties.Ignore(property)
   TransportProperties.Avoid(property)
   TransportProperties.Prohibit(property)

B.2.  Transport Property Profiles

   To ease the use of the interface specified by this document,
   implementations can provide a mechanism to create Transport Property
   objects (see Section 6.2) that are pre-configured with frequently
   used sets of properties; the following are in common use in current
   applications:

B.2.1.  reliable-inorder-stream

   This profile provides reliable, in-order transport service with
   congestion control.  TCP is an example of a protocol that provides
   this service.  It should consist of the following properties:

                    +=======================+=========+
                    | Property              | Value   |
                    +=======================+=========+
                    | reliability           | require |
                    +-----------------------+---------+
                    | preserveOrder         | require |
                    +-----------------------+---------+
                    | congestionControl     | require |
                    +-----------------------+---------+
                    | preserveMsgBoundaries | ignore  |
                    +-----------------------+---------+

                         Table 2: reliable-inorder-
                             stream preferences

B.2.2.  reliable-message

   This profile provides message-preserving, reliable, in-order
   transport service with congestion control.  SCTP is an example of a
   protocol that provides this service.  It should consist of the
   following properties:










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                    +=======================+=========+
                    | Property              | Value   |
                    +=======================+=========+
                    | reliability           | require |
                    +-----------------------+---------+
                    | preserveOrder         | require |
                    +-----------------------+---------+
                    | congestionControl     | require |
                    +-----------------------+---------+
                    | preserveMsgBoundaries | require |
                    +-----------------------+---------+

                         Table 3: reliable-message
                                preferences

B.2.3.  unreliable-datagram

   This profile provides a datagram transport service without any
   reliability guarantee.  An example of a protocol that provides this
   service is UDP.  It consists of the following properties:

                    +=======================+=========+
                    | Property              | Value   |
                    +=======================+=========+
                    | reliability           | avoid   |
                    +-----------------------+---------+
                    | preserveOrder         | avoid   |
                    +-----------------------+---------+
                    | congestionControl     | ignore  |
                    +-----------------------+---------+
                    | preserveMsgBoundaries | require |
                    +-----------------------+---------+
                    | safely replayable     | true    |
                    +-----------------------+---------+

                        Table 4: unreliable-datagram
                                preferences

   Applications that choose this Transport Property Profile would avoid
   the additional latency that could be introduced by retransmission or
   reordering in a transport protocol.

   Applications that choose this Transport Property Profile to reduce
   latency should also consider setting an appropriate Capacity Profile
   Property, see Section 8.1.6 and might benefit from controlling
   checksum coverage, see Section 6.2.7 and Section 6.2.8.





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Appendix C.  Relationship to the Minimal Set of Transport Services for
             End Systems

   [RFC8923] identifies a minimal set of transport services that end
   systems should offer.  These services make all non-security-related
   transport features of TCP, MPTCP, UDP, UDP-Lite, SCTP and LEDBAT
   available that 1) require interaction with the application, and 2) do
   not get in the way of a possible implementation over TCP (or, with
   limitations, UDP).  The following text explains how this minimal set
   is reflected in the present API.  For brevity, it is based on the
   list in Section 4.1 of [RFC8923], updated according to the discussion
   in Section 5 of [RFC8923].  The present API covers all elements of
   this section except "Notification of Excessive Retransmissions (early
   warning below abortion threshold)".  This list is a subset of the
   transport features in Appendix A of [RFC8923], which refers to the
   primitives in "pass 2" (Section 4) of [RFC8303] for further details
   on the implementation with TCP, MPTCP, UDP, UDP-Lite, SCTP and
   LEDBAT.

   *  Connect: "Initiate" Action (Section 7.1).

   *  Listen: "Listen" Action (Section 7.2).

   *  Specify number of attempts and/or timeout for the first
      establishment message: "timeout" parameter of "Initiate"
      (Section 7.1) or "InitiateWithSend" Action (Section 9.2.5).

   *  Disable MPTCP: "Multipath Transport" Property (Section 6.2.14).

   *  Hand over a message to reliably transfer (possibly multiple times)
      before connection establishment: "InitiateWithSend" Action
      (Section 9.2.5).

   *  Change timeout for aborting connection (using retransmit limit or
      time value): "Timeout for Aborting Connection" property, using a
      time value (Section 8.1.3).

   *  Timeout event when data could not be delivered for too long:
      "ConnectionError" Event (Section 10).

   *  Suggest timeout to the peer: "TCP-specific Properties: User
      Timeout Option (UTO)" (Section 8.2).

   *  Notification of ICMP error message arrival: "Notification of ICMP
      soft error message arrival" property (Section 6.2.17).






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   *  Choose a scheduler to operate between streams of an association:
      "Connection Group Transmission Scheduler" property
      (Section 8.1.5).

   *  Configure priority or weight for a scheduler: "Connection
      Priority" property (Section 8.1.2).

   *  "Specify checksum coverage used by the sender" and "Disable
      checksum when sending": "Sending Corruption Protection Length"
      property (Section 9.1.3.6) and "Full Checksum Coverage on Sending"
      property (Section 6.2.7).

   *  "Specify minimum checksum coverage required by receiver" and
      "Disable checksum requirement when receiving": "Required Minimum
      Corruption Protection Coverage for Receiving" property
      (Section 8.1.1) and "Full Checksum Coverage on Receiving" property
      (Section 6.2.8).

   *  "Specify DF field": "No Network-Layer Fragmentation" property
      (Section 9.1.3.9).

   *  Get max. transport-message size that may be sent using a non-
      fragmented IP packet from the configured interface: "Maximum
      Message Size Before Fragmentation or Segmentation" property
      (Section 8.1.11.2).

   *  Get max. transport-message size that may be received from the
      configured interface: "Maximum Message Size on Receive" property
      (Section 8.1.11.4).

   *  Obtain ECN field: "UDP(-Lite)-specific Property: ECN" is a read-
      only Message Property of the MessageContext object
      (Section 9.3.3.1).

   *  "Specify DSCP field", "Disable Nagle algorithm", "Enable and
      configure a "Low Extra Delay Background Transfer"": as suggested
      in Section 5.5 of [RFC8923], these transport features are
      collectively offered via the "Capacity Profile" property
      (Section 8.1.6).  Per-Message control ("Request not to bundle
      messages") is offered via the "Message Capacity Profile Override"
      property (Section 9.1.3.8).

   *  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side: this is offered
      by the "Close" Action with slightly changed semantics in line with
      the discussion in Section 5.2 of [RFC8923] (Section 10).





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   *  "Abort without delivering remaining data, causing an event
      informing the application on the other side" and "Abort without
      delivering remaining data, not causing an event informing the
      application on the other side": this is offered by the "Abort"
      action without promising that this is signaled to the other side.
      If it is, a "ConnectionError" Event will fire at the peer
      (Section 10).

   *  "Reliably transfer data, with congestion control", "Reliably
      transfer a message, with congestion control" and "Unreliably
      transfer a message": data is transferred via the "Send" action
      (Section 9.2).  Reliability is controlled via the "Reliable Data
      Transfer (Connection)" (Section 6.2.1) property and the "Reliable
      Data Transfer (Message)" Message Property (Section 9.1.3.7).
      Transmitting data as a message or without delimiters is controlled
      via Message Framers (Section 9.1.2).  The choice of congestion
      control is provided via the "Congestion control" property
      (Section 6.2.9).

   *  Configurable Message Reliability: the "Lifetime" Message Property
      implements a time-based way to configure message reliability
      (Section 9.1.3.1).

   *  "Ordered message delivery (potentially slower than unordered)" and
      "Unordered message delivery (potentially faster than ordered)":
      these two transport features are controlled via the Message
      Property "Ordered" (Section 9.1.3.3).

   *  Request not to delay the acknowledgement (SACK) of a message:
      should the protocol support it, this is one of the transport
      features the Transport Services system can apply when an
      application uses the "Capacity Profile" Property (Section 8.1.6)
      or the "Message Capacity Profile Override" Message Property
      (Section 9.1.3.8) with value "Low Latency/Interactive".

   *  Receive data (with no message delimiting): "Receive" Action
      (Section 9.3) and "Received" Event (Section 9.3.2.1).

   *  Receive a message: "Receive" Action (Section 9.3) and "Received"
      Event (Section 9.3.2.1), using Message Framers (Section 9.1.2).

   *  Information about partial message arrival: "Receive" Action
      (Section 9.3) and "ReceivedPartial" Event (Section 9.3.2.2).

   *  Notification of send failures: "Expired" Event (Section 9.2.2.2)
      and "SendError" Event (Section 9.2.2.3).





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   *  Notification that the stack has no more user data to send:
      applications can obtain this information via the "Sent" Event
      (Section 9.2.2.1).

   *  Notification to a receiver that a partial message delivery has
      been aborted: "ReceiveError" Event (Section 9.3.2.3).

Authors' Addresses

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

   Email: ietf@trammell.ch


   Michael Welzl (editor)
   University of Oslo
   PO Box 1080 Blindern
   0316  Oslo
   Norway

   Email: michawe@ifi.uio.no


   Theresa Enghardt
   Netflix
   121 Albright Way
   Los Gatos, CA 95032,
   United States of America

   Email: ietf@tenghardt.net


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

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


   Mirja Kuehlewind
   Ericsson
   Ericsson-Allee 1



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   Herzogenrath
   Germany

   Email: mirja.kuehlewind@ericsson.com


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

   Email: csp@csperkins.org


   Philipp S. Tiesel
   SAP SE
   Konrad-Zuse-Ring 10
   14469 Potsdam
   Germany

   Email: philipp@tiesel.net


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

   Email: caw@heapingbits.net


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

   Email: tpauly@apple.com


   Kyle Rose
   Akamai Technologies, Inc.
   145 Broadway
   Cambridge, MA,
   United States of America




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   Email: krose@krose.org


















































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