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Versions: (draft-trammell-taps-interface) 00 01 02 03

TAPS Working Group                                      B. Trammell, Ed.
Internet-Draft                                                ETH Zurich
Intended status: Standards Track                           M. Welzl, Ed.
Expires: April 25, 2019                               University of Oslo
                                                             T. Enghardt
                                                               TU Berlin
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                           M. Kuehlewind
                                                              ETH Zurich
                                                              C. Perkins
                                                   University of Glasgow
                                                               P. Tiesel
                                                               TU Berlin
                                                                 C. Wood
                                                              Apple Inc.
                                                        October 22, 2018


     An Abstract Application Layer Interface to Transport Services
                      draft-ietf-taps-interface-02

Abstract

   This document describes an abstract programming interface to the
   transport layer, following the Transport Services Architecture.  It
   supports the asynchronous, atomic transmission of messages over
   transport protocols and network paths dynamically selected at
   runtime.  It is intended to replace the traditional BSD sockets API
   as the lowest common denominator interface to the transport layer, in
   an environment where endpoints have multiple interfaces and potential
   transport protocols to select from.

Status of This Memo

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

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

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




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   This Internet-Draft will expire on April 25, 2019.

Copyright Notice

   Copyright (c) 2018 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
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   5
   3.  Interface Design Principles . . . . . . . . . . . . . . . . .   6
   4.  API Summary . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Transport Properties  . . . . . . . . . . . . . . . . . .   7
     4.2.  Scope of the Interface Definition . . . . . . . . . . . .   8
   5.  Pre-Establishment Phase . . . . . . . . . . . . . . . . . . .   9
     5.1.  Specifying Endpoints  . . . . . . . . . . . . . . . . . .   9
     5.2.  Specifying Transport Properties . . . . . . . . . . . . .  11
       5.2.1.  Reliable Data Transfer (Connection) . . . . . . . . .  13
       5.2.2.  Configure per-Message reliability . . . . . . . . . .  13
       5.2.3.  Preservation of data ordering . . . . . . . . . . . .  13
       5.2.4.  Use 0-RTT session establishment with an idempotent
               Message . . . . . . . . . . . . . . . . . . . . . . .  13
       5.2.5.  Multistream Connections in Group  . . . . . . . . . .  13
       5.2.6.  Control checksum coverage on sending or receiving . .  13
       5.2.7.  Congestion control  . . . . . . . . . . . . . . . . .  14
       5.2.8.  Interface Instance or Type  . . . . . . . . . . . . .  14
       5.2.9.  Provisioning Domain Instance or Type  . . . . . . . .  15
     5.3.  Specifying Security Parameters and Callbacks  . . . . . .  15
       5.3.1.  Pre-Connection Parameters . . . . . . . . . . . . . .  16
       5.3.2.  Connection Establishment Callbacks  . . . . . . . . .  17
   6.  Establishing Connections  . . . . . . . . . . . . . . . . . .  17
     6.1.  Active Open: Initiate . . . . . . . . . . . . . . . . . .  17
     6.2.  Passive Open: Listen  . . . . . . . . . . . . . . . . . .  18
     6.3.  Peer-to-Peer Establishment: Rendezvous  . . . . . . . . .  19
     6.4.  Connection Groups . . . . . . . . . . . . . . . . . . . .  21
   7.  Sending Data  . . . . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Basic Sending . . . . . . . . . . . . . . . . . . . . . .  22



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     7.2.  Send Events . . . . . . . . . . . . . . . . . . . . . . .  22
       7.2.1.  Sent  . . . . . . . . . . . . . . . . . . . . . . . .  23
       7.2.2.  Expired . . . . . . . . . . . . . . . . . . . . . . .  23
       7.2.3.  SendError . . . . . . . . . . . . . . . . . . . . . .  23
     7.3.  Message Properties  . . . . . . . . . . . . . . . . . . .  24
       7.3.1.  Lifetime  . . . . . . . . . . . . . . . . . . . . . .  24
       7.3.2.  Niceness  . . . . . . . . . . . . . . . . . . . . . .  25
       7.3.3.  Ordered . . . . . . . . . . . . . . . . . . . . . . .  25
       7.3.4.  Idempotent  . . . . . . . . . . . . . . . . . . . . .  25
       7.3.5.  Final . . . . . . . . . . . . . . . . . . . . . . . .  25
       7.3.6.  Corruption Protection Length  . . . . . . . . . . . .  26
       7.3.7.  Reliable Data Transfer (Message)  . . . . . . . . . .  26
       7.3.8.  Transmission Profile  . . . . . . . . . . . . . . . .  26
       7.3.9.  Singular Transmission . . . . . . . . . . . . . . . .  27
     7.4.  Partial Sends . . . . . . . . . . . . . . . . . . . . . .  27
     7.5.  Batching Sends  . . . . . . . . . . . . . . . . . . . . .  28
     7.6.  Send on Active Open: InitiateWithIdempotentSend . . . . .  28
     7.7.  Sender-side Framing . . . . . . . . . . . . . . . . . . .  29
   8.  Receiving Data  . . . . . . . . . . . . . . . . . . . . . . .  29
     8.1.  Enqueuing Receives  . . . . . . . . . . . . . . . . . . .  30
     8.2.  Receive Events  . . . . . . . . . . . . . . . . . . . . .  30
       8.2.1.  Received  . . . . . . . . . . . . . . . . . . . . . .  30
       8.2.2.  ReceivedPartial . . . . . . . . . . . . . . . . . . .  31
       8.2.3.  ReceiveError  . . . . . . . . . . . . . . . . . . . .  32
     8.3.  Message Receive Context . . . . . . . . . . . . . . . . .  32
       8.3.1.  ECN . . . . . . . . . . . . . . . . . . . . . . . . .  32
       8.3.2.  Early Data  . . . . . . . . . . . . . . . . . . . . .  32
       8.3.3.  Receiving Final Messages  . . . . . . . . . . . . . .  33
     8.4.  Receiver-side De-framing over Stream Protocols  . . . . .  33
   9.  Managing Connections  . . . . . . . . . . . . . . . . . . . .  34
     9.1.  Generic Connection Properties . . . . . . . . . . . . . .  35
       9.1.1.  Notification of excessive retransmissions . . . . . .  35
       9.1.2.  Retransmission threshold before excessive
               retransmission notification . . . . . . . . . . . . .  36
       9.1.3.  Notification of ICMP soft error message arrival . . .  36
       9.1.4.  Required minimum coverage of the checksum for
               receiving . . . . . . . . . . . . . . . . . . . . . .  36
       9.1.5.  Niceness (Connection) . . . . . . . . . . . . . . . .  36
       9.1.6.  Timeout for aborting Connection . . . . . . . . . . .  37
       9.1.7.  Connection group transmission scheduler . . . . . . .  37
       9.1.8.  Maximum message size concurrent with Connection
               establishment . . . . . . . . . . . . . . . . . . . .  37
       9.1.9.  Maximum Message size before fragmentation or
               segmentation  . . . . . . . . . . . . . . . . . . . .  37
       9.1.10. Maximum Message size on send  . . . . . . . . . . . .  37
       9.1.11. Maximum Message size on receive . . . . . . . . . . .  37
       9.1.12. Capacity Profile  . . . . . . . . . . . . . . . . . .  38
     9.2.  Soft Errors . . . . . . . . . . . . . . . . . . . . . . .  39



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   10. Connection Termination  . . . . . . . . . . . . . . . . . . .  39
   11. Connection State and Ordering of Operations and Events  . . .  40
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  41
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  41
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  42
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     15.2.  Informative References . . . . . . . . . . . . . . . . .  43
   Appendix A.  Additional Properties  . . . . . . . . . . . . . . .  44
     A.1.  Experimental Transport Properties . . . . . . . . . . . .  45
       A.1.1.  Direction of communication  . . . . . . . . . . . . .  45
       A.1.2.  Suggest a timeout to the Remote Endpoint  . . . . . .  45
       A.1.3.  Abort timeout to suggest to the Remote Endpoint . . .  46
       A.1.4.  Traffic Category  . . . . . . . . . . . . . . . . . .  46
       A.1.5.  Size to be Sent or Received . . . . . . . . . . . . .  46
       A.1.6.  Duration  . . . . . . . . . . . . . . . . . . . . . .  47
       A.1.7.  Send or Receive Bit-rate  . . . . . . . . . . . . . .  47
       A.1.8.  Cost Preferences  . . . . . . . . . . . . . . . . . .  47
   Appendix B.  Sample API definition in Go  . . . . . . . . . . . .  48
   Appendix C.  Relationship to the Minimal Set of Transport
                Services for End Systems . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

1.  Introduction

   The BSD Unix Sockets API's SOCK_STREAM abstraction, by bringing
   network sockets into the UNIX programming model, allowing anyone who
   knew how to write programs that dealt with sequential-access files to
   also write network applications, was a revolution in simplicity.  The
   simplicity of this API is a key reason the Internet won the protocol
   wars of the 1980s.  SOCK_STREAM is tied to the Transmission Control
   Protocol (TCP), specified in 1981 [RFC0793].  TCP has scaled
   remarkably well over the past three and a half decades, but its total
   ubiquity has hidden an uncomfortable fact: the network is not really
   a file, and stream abstractions are too simplistic for many modern
   application programming models.

   In the meantime, the nature of Internet access, and the variety of
   Internet transport protocols, is evolving.  The challenges that new
   protocols and access paradigms present to the sockets API and to
   programming models based on them inspire the design principles of a
   new approach, which we outline in Section 3.

   As a first step to realizing this design, [I-D.ietf-taps-arch]
   describes a high-level architecture for transport services.  This
   document builds a modern abstract programming interface atop this
   architecture, deriving specific path and protocol selection
   properties and supported transport features from the analysis



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   provided in [RFC8095], [I-D.ietf-taps-minset], and
   [I-D.ietf-taps-transport-security].

2.  Terminology and Notation

   This API is described in terms of Objects, which an application can
   interact with; Actions the application can perform on these Objects;
   Events, which an Object can send to an application asynchronously;
   and Parameters associated with these Actions and Events.

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

   o  An Action creates an Object:

   Object := Action()

   o  An Action creates an array of Objects:

   []Object := Action()

   o  An Action is performed on an Object:

   Object.Action()

   o  An Object sends an Event:

   Object -> Event<>

   o  An Action takes a set of Parameters; an Event contains a set of
      Parameters:

   Action(parameter, parameter, ...) / Event<parameter, parameter, ...>

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

   How these abstract concepts map into concrete implementations of this
   API in a given language on a given platform is largely dependent on
   the features of the language and the platform.  Actions could be
   implemented as functions or method calls, for instance, and Events
   could be implemented via callbacks, communicating sequential
   processes, or other asynchronous calling conventions.  The method for
   dispatching and handling Events is left as an implementation detail,
   with the caveat that the interface for receiving Messages must
   require the application to invoke the Connection.Receive() Action
   once per Message to be received (see Section 8).



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   This specification treats Events and errors similarly.  Errors, just
   as any other Events, may occur asynchronously in network
   applications.  However, it is recommended that implementations of
   this interface also return errors immediately, according to the error
   handling idioms of the implementation platform, for errors which can
   be immediately detected, such as inconsistency in Transport
   Properties.

   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.

3.  Interface Design Principles

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

   o  A single interface to a variety of transport protocols to be used
      in a variety of application design patterns, independent of the
      properties of the application and the Protocol Stacks that will be
      used at runtime, such that all common specialized features of
      these protocol stacks are made available to the application as
      necessary in a transport-independent way, to enable applications
      written to a single API to make use of transport protocols in
      terms of the features they provide;

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

   o  Asynchronous Connection establishment, transmission, and
      reception, allowing concurrent operations during establishment and
      supporting event-driven application interactions with the
      transport layer, in line with developments in modern platforms and
      programming languages;

   o  Explicit support for security properties as first-order transport
      features, and for long-term caching of cryptographic identities
      and parameters for associations among endpoints; and

   o  Explicit support for multistreaming and multipath transport
      protocols, and the grouping of related Connections into Connection
      Groups through cloning of Connections, to allow applications to




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      take full advantage of new transport protocols supporting these
      features.

4.  API Summary

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

   An application primarily interacts with this interface through two
   Objects, Preconnections and Connections.  A Preconnection represents
   a set of properties and constraints on the selection and
   configuration of paths and protocols to establish a Connection with a
   remote endpoint.  A Connection represents a transport Protocol Stack
   on which data can be sent to and/or received from a remote endpoint
   (i.e., depending on the kind of transport, connections can be bi-
   directional or unidirectional).  Connections can be created from
   Preconnections in three ways: by initiating the Preconnection (i.e.,
   actively opening, as in a client), through listening on the
   Preconnection (i.e., passively opening, as in a server), or
   rendezvousing on the Preconnection (i.e.  peer to peer
   establishment).

   Once a Connection is established, data can be sent 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 deframing callback which finds message
   boundaries in a stream.  Messages are received asynchronously through
   a callback registered by the application.  Errors and other
   notifications also happen asynchronously on the Connection.

   Section 5, Section 6, Section 7, Section 8, and Section 10 describe
   the details of application interaction with Objects through Actions
   and Events in each phase of a Connection, following the phases
   described in [I-D.ietf-taps-arch].

4.1.  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.  During
   pre-establishment, Selection Properties Section 5.2 are used to
   specify which paths and protocol stacks can be used and are preferred
   by the application, and Connection Properties Section 9.1 can be used
   to fine-tune the eventually established connection.  These Connection
   Properties can also be used to monitor and fine-tune established
   connections.  The behavior of the selected protocol stack(s) when
   sending Messages is controlled by Message Properties Section 7.3.



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   Collectively, Selection, Connection, and Message Properties can be
   referred to as Transport Properties.  All Transport Properties,
   regardless of the phase in which they are used, are organized within
   a single namespace.  This enables setting them as defaults in earlier
   stages and querying them in later stages: - Connection Properties can
   be set on Preconnections - Message Properties can be set on
   Preconnections and Connections - The effect of Selection Properties
   can be queried on Connections and Messages

   Transport Properties can have one of a set of data types:

   o  Boolean: can take the values "true" and "false"; representation is
      implementation-dependent.

   o  Integer: can take positive or negative numeric values; range and
      representation is implementation-dependent.

   o  Enumeration: can take one value of a finite set of values,
      dependent on the property itself.  The representation is
      implementation dependent; however, implementations MUST provide a
      method for the application to determine the entire set of possible
      values for each property.

   o  Preference: can take one of five values (Prohibit, Avoid, Ignore,
      Prefer, Require) for the level of preference of a given property
      during protocol selection; see Section 5.2.

4.2.  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.  While
   there is no interoperability benefit to tightly defining how the
   interface be presented to application programmers in diverse
   platforms, maintaining the "shape" of the abstract interface across
   these platforms reduces the effort for programmers who learn the
   transport services interface to apply their knowledge in multiple
   platforms.  We therefore make the following recommendations:

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




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   o  Implementations of this interface SHOULD implement each Selection
      Property, Connection Property, and Message Context Property
      specified in this document, exclusive of appendices, even if said
      implementation is a non-operation, e.g.  because transport
      protocols implementing a given Property are not available on the
      platform.

5.  Pre-Establishment Phase

   The pre-establishment phase allows applications to specify properties
   for the Connections they are about to make, or to query the API about
   potential connections they could make.

   A Preconnection Object represents a potential Connection.  It has
   state that describes 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 5.1), the Selection Properties (see Section 5.2), any
   preconfigured Connection Properties (Section 9.1), and the security
   parameters (see Section 5.3):

      Preconnection := NewPreconnection(LocalEndpoint,
                                        RemoteEndpoint,
                                        TransportProperties,
                                        SecurityParams)

   The Local Endpoint MUST be specified if the Preconnection is used to
   Listen() for incoming Connections, but is OPTIONAL if it is used to
   Initiate() connections.  The Remote Endpoint MUST be specified if the
   Preconnection is used to Initiate() Connections, but is OPTIONAL if
   it is used to Listen() for incoming Connections.  The Local Endpoint
   and the Remote Endpoint MUST both be specified if a peer-to-peer
   Rendezvous is to occur based on the Preconnection.

   Framers (see Section 7.7) and deframers (see Section 8.4), if
   necessary, should be bound to the Preconnection during pre-
   establishment.

5.1.  Specifying Endpoints

   The transport services API uses the Local Endpoint and Remote
   Endpoint types to refer to the endpoints of a transport connection.
   Subtypes of these represent various different types of endpoint
   identifiers, such as IP addresses, DNS names, and interface names, as
   well as port numbers and service names.






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

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

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

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

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

   Implementations may also support additional endpoint representations
   and provide a single NewEndpoint() call that takes different endpoint
   representations.

   Multiple endpoint identifiers can be specified for each Local
   Endpoint and Remote Endpoint.  For example, a Local Endpoint could be
   configured with two interface names, or a Remote Endpoint could be
   specified via both IPv4 and IPv6 addresses.  These multiple
   identifiers refer to the same transport endpoint.

   The transport services API resolves names internally, when the
   Initiate(), Listen(), or Rendezvous() method is called establish a
   Connection.  The API 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.

   The Resolve() action on 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 6.3).







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5.2.  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.

   Selection properties are represented as preferences, which can have
   one of five preference levels:

   +------------+------------------------------------------------------+
   | Preference | Effect                                               |
   +------------+------------------------------------------------------+
   | Require    | Select only protocols/paths providing the property,  |
   |            | fail otherwise                                       |
   |            |                                                      |
   | Prefer     | Prefer protocols/paths providing the property,       |
   |            | proceed otherwise                                    |
   |            |                                                      |
   | Ignore     | Cancel any system default preference for this        |
   |            | property                                             |
   |            |                                                      |
   | Avoid      | Prefer protocols/paths not providing the property,   |
   |            | proceed otherwise                                    |
   |            |                                                      |
   | Prohibit   | Select only protocols/paths not providing the        |
   |            | property, fail otherwise                             |
   +------------+------------------------------------------------------+

   Internally, the transport system will first exclude all protocols and
   paths that match a Prohibit, then exclude all protocols and paths
   that do not match a Require, then sort candidates according to
   Preferred properties, and then use Avoided properties as a
   tiebreaker.  Selection Properties which select paths take preference
   over those which select protocols.  For example, if an application
   indicates a preference for a specific path by specifying an
   interface, but also a preference for a protocol not available on this




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   path, the transport system will try the path first, ignoring the
   preference.

   Both Selection and Connection 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 added to the TransportProperties
   Object:

   TransportProperties.Add(property, value)

   Selection Properties can be added to a TransportProperties object
   using special actions for each preference level i.e,
   "TransportProperties.Add(some_property, avoid)" is equivalent to
   "TransportProperties.Avoid(some_property)":

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

   For an existing Connection, the Transport Properties can be queried
   any time by using the following call on the Connection Object:

   TransportProperties := Connection.GetTransportProperties()

   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 6.4
   for more.

   Section 9.1 provides a list of Connection Properties, while Selection
   Properties are listed in the subsections below.  Note that many
   properties are only considered during establishment, and can not be
   changed after a Connection is established; however, they can be
   queried.  Querying a Selection Property after establishment yields
   the value Required for properties of the selected protocol and path,
   Avoid for properties avoided during selection, and Ignore for all
   other properties.

   An implementation of this interface must provide sensible defaults
   for Selection Properties.  The recommended defaults given for each



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   property below represent a configuration that can be implemented over
   TCP.  An alternate set of default Protocol Selection Properties would
   represent a configuration that can be implemented over UDP.

5.2.1.  Reliable Data Transfer (Connection)

   This property specifies whether the application needs to use a
   transport protocol that ensures that all data is received on the
   other side without corruption.  This also entails being notified when
   a Connection is closed or aborted.  The recommended default is to
   enable Reliable Data Transfer.

5.2.2.  Configure per-Message reliability

   This property specifies whether an application considers it useful to
   indicate its reliability requirements on a per-Message basis.  This
   property applies to Connections and Connection Groups.  The
   recommended default is to not have this option.

5.2.3.  Preservation of data ordering

   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.  The
   recommended default is to preserve data ordering.

5.2.4.  Use 0-RTT session establishment with an idempotent Message

   This property specifies whether an application would like to supply a
   Message to the transport protocol before Connection establishment,
   which will then be reliably transferred to the other side before or
   during Connection establishment, potentially multiple times.  See
   also Section 7.3.4.  The recommended default is to not have this
   option.

5.2.5.  Multistream Connections in Group

   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.  The
   recommended default is to not have this option.

5.2.6.  Control checksum coverage on sending or receiving

   This property specifies whether the application considers it useful
   to enable, disable, or configure a checksum when sending a Message,
   or configure whether to require a checksum or not when receiving.




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   The recommended default is full checksum coverage without the option
   to configure it, and requiring a checksum when receiving.

5.2.7.  Congestion control

   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].  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.
   The recommended default is that the Connection is congestion
   controlled.

5.2.8.  Interface Instance or Type

   This property allows the application to select which specific network
   interfaces or categories of interfaces it wants to "Require",
   "Prohibit", "Prefer", or "Avoid".

   In contrast to other Selection Properties, this property is 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.

   Note that marking a specific interface as "Required" strictly limits
   path selection to a single interface, and leads to less flexible and
   resilient connection establishment.

   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,
   there may be "Wi-Fi" and "Wired Ethernet" interface types available.
   Implementations should provide all types that are supported on some
   system to all systems, in order to allow applications to write
   generic code.  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 may
   want to always "Prohibit Cellular".  Note that marking a specific
   interface type as "Required" limits path selection to a small set of
   interfaces, and leads to less flexible and resilient connection
   establishment.

   The set of interface types is expected to change over time as new
   access technologies become available.




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   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 5.2.9) or
   another specific property.

5.2.9.  Provisioning Domain Instance or Type

   Similar to interface instances and types (see Section 5.2.8), this
   property allows the application to control path selection by
   selecting which specific Provisioning Domains or categories of
   Provisioning Domains 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 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 Provisioning Domain (PvD) is defined
   to be implementation- and system-specific, since there is not a
   portable standard format for a PvD identitfier.  For example, this
   identifier may be a string name or an integer.  As with requiring
   specific interfaces, requiring a specific PvD strictly limits path
   selection.

   Categories or types of PvDs are also defined to be implementation-
   and system-specific.  These may 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 some 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.

5.3.  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.  Thus, we
   partition security parameters and callbacks based on their place in
   the lifetime of connection establishment.  Similar to Transport
   Properties, both parameters and callbacks are inherited during
   cloning (see Section 6.4).




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5.3.1.  Pre-Connection Parameters

   Common parameters such as TLS ciphersuites are known to
   implementations.  Clients should use common safe defaults for these
   values whenever possible.  However, as discussed in
   [I-D.ietf-taps-transport-security], 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 are created as follows:

   SecurityParameters := NewSecurityParameters()

   Security configuration parameters and sample usage follow:

   o  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.AddIdentity(identity)
   SecurityParameters.AddPrivateKey(privateKey, publicKey)

   o  Supported algorithms: Used to restrict what parameters are used by
      underlying transport security protocols.  When not specified,
      these algorithms should default to known and safe defaults for the
      system.  Parameters include: ciphersuites, supported groups, and
      signature algorithms.

SecurityParameters.AddSupportedGroup(secp256k1)
SecurityParameters.AddCiphersuite(TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256)
SecurityParameters.AddSignatureAlgorithm(ed25519)

   o  Session cache management: Used to tune cache capacity, lifetime,
      re-use, and eviction policies, e.g., LRU or FIFO.  Constants and
      policies for these interfaces are implementation-specific.

   SecurityParameters.SetSessionCacheCapacity(MAX_CACHE_ELEMENTS)
   SecurityParameters.SetSessionCacheLifetime(SECONDS_PER_DAY)
   SecurityParameters.SetSessionCachePolicy(CachePolicyOneTimeUse)

   o  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.AddPreSharedKey(key, identity)




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5.3.2.  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.
   Security handshake callbacks that may be invoked during connection
   establishment include:

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

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

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

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

6.  Establishing Connections

   Before a Connection can be used for data transfer, it must 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.

6.1.  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()

   Before calling Initiate, the caller must have populated a
   Preconnection Object with a Remote Endpoint specifier, optionally a



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   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 consumes the Preconnection.  Once Initiate()
   has been called, no further properties may be added to the
   Preconnection, and no subsequent establishment call may be made on
   the Preconnection.

   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 7) after calling
   Initate(); note that any idempotent data 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 8) will
   occur before the Ready Event for Connections established using
   Initiate.

   Connection -> InitiateError<>

   An InitiateError 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 to the remote
   endpoint (e.g. because the remote endpoint is not accepting
   connections, or the application is prohibited from opening a
   Connection by the operating system).

   See also Section 7.6 to combine Connection establishment and
   transmission of the first message in a single action.

6.2.  Passive Open: Listen

   Passive open is the Action of waiting for Connections from remote
   endpoints, commonly used by servers in client-server interactions.




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   Passive open is supported by this interface through the Listen
   Action:

   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
   consumes the Preconnection.  Once Listen() has been called, no
   further properties may be added to the Preconnection, and no
   subsequent establishment call may be made on the Preconnection.

   Listening continues until the global context shuts down, or until the
   Stop action is performed on the same Preconnection:

   Preconnection.Stop()

   After Stop() is called, the preconnection can be disposed of.

   Preconnection -> ConnectionReceived<Connection>

   The ConnectionReceived Event occurs when a Remote Endpoint has
   established a transport-layer connection to this Preconnection (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.

   Preconnection -> ListenError<>

   A ListenError occurs either when the Preconnection cannot be
   fulfilled for listening, when the Local Endpoint (or Remote Endpoint,
   if specified) cannot be resolved, or when the application is
   prohibited from listening by policy.

   Preconnection -> Stopped<>

   A Stopped event occurs after the Preconnection has stopped listening.

6.3.  Peer-to-Peer Establishment: Rendezvous

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




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   Preconnection.Rendezvous()

   The Preconnection Object must be specified with both a Local Endpoint
   and a Remote Endpoint, and also the transport properties and security
   parameters needed for Protocol Stack selection.

   The Rendezvous() Action causes the Preconnection to listen on the
   Local Endpoint for an incoming Connection from the Remote Endpoint,
   while simultaneously trying to establish a Connection from the Local
   Endpoint to the Remote Endpoint.  This corresponds to a TCP
   simultaneous open, for example.

   The Rendezvous() Action consumes the Preconnection.  Once
   Rendezvous() has been called, no further properties may be added to
   the Preconnection, and no subsequent establishment call may be made
   on the Preconnection.

   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.

   Preconnection -> RendezvousError<msgRef, error>

   An RendezvousError occurs either when the Preconnection cannot be
   fulfilled for listening, 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.

   When using some NAT traversal protocols, e.g., Interactive
   Connectivity Establishment (ICE) [RFC5245], it is expected that the
   Local Endpoint will be configured with some method of discovering NAT
   bindings, e.g., a Session Traversal Utilities for NAT (STUN) server.
   In this case, the Local Endpoint may resolve to a mixture of local
   and server reflexive addresses.  The Resolve() action on the
   Preconnection can be used to discover these bindings:

   []Preconnection := Preconnection.Resolve()

   The Resolve() call returns a list of Preconnection Objects, that
   represent the concrete addresses, local and server reflexive, on
   which a Rendezvous() for the Preconnection will listen for incoming



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   Connections.  These resolved Preconnections will share all other
   Properties with the Preconnection from which they are derived, though
   some Properties may be made more-specific by the resolution process.
   This list can be passed to a peer via a signalling protocol, such as
   SIP [RFC3261] or WebRTC [RFC7478], to configure the remote.

6.4.  Connection Groups

   Groups of Connections can be created using the Clone Action:

   Connection := Connection.Clone()

   Calling Clone on a Connection yields a group of two Connections: the
   parent Connection on which Clone was called, and the resulting cloned
   Connection.  These connections are "entangled" with each other, and
   become part of a Connection Group.  Calling Clone on any of these two
   Connections adds a third Connection to the Connection Group, and so
   on.  Connections in a Connection Group share all Protocol Properties
   that are not applicable to a Message.

   Changing one of these Protocol Properties on one Connection in the
   group changes it for all others.  Per-Message Protocol Properties,
   however, are not entangled.  For example, changing "Timeout for
   aborting Connection" (see Section 9.1.6) on one Connection in a group
   will automatically change this Protocol Property for all Connections
   in the group in the same way.  However, changing "Lifetime" (see
   Section 7.3.1) of a Message will only affect a single Message on a
   single Connection, entangled or not.

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

   If the underlying Protocol Stack does not support cloning, or cannot
   create a new stream on the given Connection, then attempts to clone a
   connection will result in a CloneError:

   Connection -> CloneError<>

   The Protocol Property "Niceness" operates on entangled Connections as
   in Section 7.3.2: when allocating available network capacity among
   Connections in a Connection Group, sends on Connections with higher
   Niceness values will be prioritized over sends on Connections with
   lower Niceness values.  An ideal transport system implementation
   would assign each Connection the capacity share (M-N) x C / M, where
   N is the Connection's Niceness value, M is the maximum Niceness value



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   used by all Connections in the group and C is the total available
   capacity.  However, the Niceness setting is purely advisory, and no
   guarantees are given about the way capacity is shared.  Each
   implementation is free to implement a way to share capacity that it
   sees fit.

7.  Sending Data

   Once a Connection has been established, it can be used for sending
   data.  Data is sent in terms of Messages, which allow the application
   to communicate the boundaries of the data being transferred.  By
   default, Send enqueues a complete Message, and takes optional per-
   Message properties (see Section 7.1).  All Send actions are
   asynchronous, and deliver events (see Section 7.2).  Sending partial
   Messages for streaming large data is also supported (see
   Section 7.4).

7.1.  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.  Message data is created as an array of octets, and the
   resulting object contains both the byte array and the length of the
   array.

   messageData := "hello".octets()
   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 protocol property Maximum Message Size on Send 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 7.2.3).  For example, it is
   invalid to send a Message over a UDP connection that is larger than
   the available datagram sending size.

7.2.  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.



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   Note that if partial Sends are used (Section 7.4), 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.

7.2.1.  Sent

   Connection -> Sent<msgRef>

   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 the implementation 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 an implementation-
   specific reference to 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 system than an application
   that always waits for the Sent Event before calling the next Send
   Action.

7.2.2.  Expired

   Connection -> Expired<msgRef>

   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 7.3.1) expired.  This is separate from SendError, as it
   is an expected behavior for partially reliable transports.  The
   Expired Event contains an implementation-specific reference to the
   Message to which it applies.

7.2.3.  SendError

   Connection -> SendError<msgRef>

   A SendError occurs when a Message could not be 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 an
   implementation-specific reference to the Message to which it applies.



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7.3.  Message Properties

   Applications may need to annotate the Messages they send with extra
   information to control how data is scheduled and processed by the
   transport protocols in the Connection.  A MessageContext object
   contains properties for sending Messages, and can be passed to the
   Send Action.  Note that these properties are per-Message, not per-
   Send if partial Messages are sent (Section 7.4).  All data blocks
   associated with a single Message share properties.  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.

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

   The simpler form of Send that does not take any messageContext is
   equivalent to passing a default MessageContext with not values added.

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

   Message Properties may be inconsistent with the properties of the
   Protocol Stacks underlying the Connection on which a given Message is
   sent.  For example, a Connection must provide reliability to allow
   setting an infinitie value for the lifetime property of a Message.
   Sending a Message with Message Properties inconsistent with the
   Selection Properties of the Connection yields an error.

   The following Message Properties are supported:

7.3.1.  Lifetime

   Type:  Integer

   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.  When a Message's Lifetime is infinite, it must
   be transmitted reliably.  The type and units of Lifetime are
   implementation-specific.



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7.3.2.  Niceness

   Type:  Integer (non-negative)

   This property represents an unbounded hierarchy of priorities.  It
   can specify the priority of a Message, relative to other Messages
   sent over the same Connection.

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

   Note that this property is not a per-message override of the
   connection Niceness - see Section 9.1.5.  Both Niceness properties
   may interact, but can be used independently and be realized by
   different mechanisms.

7.3.3.  Ordered

   Type:  Boolean

   If true, it specifies that the receiver-side transport protocol stack
   only deliver the Message to the receiving application after the
   previous ordered Message which was passed to the same Connection via
   the Send Action, when such a Message exists.  If false, the Message
   may be delivered to the receiving application out of order.  This
   property is used for protocols that support preservation of data
   ordering, see Section 5.2.3, but allow out-of-order delivery for
   certain messages.

7.3.4.  Idempotent

   Type:  Boolean

   If true, it specifies that a Message is safe to send to the remote
   endpoint more than once for a single Send Action.  It is used to mark
   data 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.

7.3.5.  Final

   Type:  Boolean

   If true, this Message is the last one that the application will send
   on a Connection.  This allows underlying protocols to indicate to the



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   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.

   Note that a Final Message must always be sorted to the end of a list
   of Messages.  The Final property overrides Niceness 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.

7.3.6.  Corruption Protection Length

   Type:  Integer (non-negative with -1 as special value)

   This property specifies the length of the section of the 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.  By default,
   the entire Message is protected by a checksum.  A value of 0 means
   that no checksum is required, and a special value (e.g. -1) can be
   used to indicate the default.  Only full coverage is guaranteed, any
   other requests are advisory.

7.3.7.  Reliable Data Transfer (Message)

   Type:  Boolean

   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 if the Connection supports
   reliability.  When this is not the case, changing it will generate an
   error.

7.3.8.  Transmission Profile

   Type:  Enumeration

   This enumerated property specifies the application's preferred
   tradeoffs for sending this Message; it is a per-Message override of
   the Capacity Profile protocol and path selection property (see
   Section 9.1.12).

   The following values are valid for Transmission Profile:



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   Default:  No special optimizations of the tradeoff between delay,
      delay variation, and bandwidth efficiency should be made when
      sending this message.

   Low Latency:  Response time (latency) should be optimized at the
      expense of efficiently using the available capacity when sending
      this message.  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; to signal a
      preference for lower-latency, higher-loss treatment; and so on.

   [TODO: This is inconsistent with {prop-cap-profile}} - needs to be
   fixed]

7.3.9.  Singular Transmission

   Type:  Boolean

   This property specifies that a message should be sent and received as
   a single packet without transport-layer segmentation or network-layer
   fragmentation.  Attempts to send a message with this property set
   with a size greater to the transport's current estimate of its
   maximum transmission segment size will result in a "SendError".  When
   used with transports supporting this functionality and running over
   IP version 4, the Don't Fragment bit will be set.

7.4.  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.

   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.











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

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

   messageData := "lo".octets()
   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.  Once the end of the Message is
   marked, the MessageContext object may be re-used as a new Message
   with identical parameters.

7.5.  Batching Sends

   In order to reduce the overhead of sending multiple small Messages on
   a Connection, the application may want to batch several Send actions
   together.  This provides a hint to the system that the sending of
   these Messages should be coalesced when possible, and that sending
   any of the batched Messages may be delayed until the last Message in
   the batch is enqueued.

   Connection.Batch(
       Connection.Send(messageData)
       Connection.Send(messageData)
   )

7.6.  Send on Active Open: InitiateWithIdempotentSend

   For application-layer protocols where the Connection initiator also
   sends the first message, the InitiateWithIdempotentSend() action
   combines Connection initiation with a first Message sent, provided
   that message is idempotent.

   Without a message context (as in Section 7.1):

   Connection := Preconnection.InitiateWithIdempotentSend(messageData)

   With a message context (as in Section 7.3):

Connection := Preconnection.InitiateWithIdempotentSend(messageData, messageContext)

   The message passed to InitiateWithIdempotentSend() is, as suggested
   by the name, considered to be idempotent (see Section 7.3.4)



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   regardless of declared message properties or defaults.  If protocol
   stacks supporting 0-RTT establishment with idempotent data are
   available on the Preconnection, then 0-RTT establishment may be used
   with the given message when establishing candidate connections.  For
   a non-idemponent initial message, or when the selected stack(s) do
   not support 0-RTT establishment, InitiateWithIdempotentSend is
   identical to Initiate() followed by Send().

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

   The Events that may be sent after InitiateWithIdempotentSend() are
   equivalent to those that would be sent by an invocation of Initate()
   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
   InitiateError signaling the failure of Connection establishment.

7.7.  Sender-side Framing

   Sender-side framing allows a caller to provide the interface with a
   function that takes a Message of an appropriate application-layer
   type and returns an array of octets, the on-the-wire representation
   of the Message to be handed down to the Protocol Stack.  It consists
   of a Framer Object with a single Action, Frame.  Since the Framer
   depends on the protocol used at the application layer, it is bound to
   the Preconnection during the pre-establishment phase:

   Preconnection.FrameWith(Framer)

   OctetArray := Framer.Frame(messageData)

   Sender-side framing is a convenience feature of the interface, for
   parity with receiver-side framing (see Section 8.4).

8.  Receiving Data

   Once a Connection is established, it can be used for receiving data.
   As with sending, data is received in terms of 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 the Receive request (see Section 8.2).

   As with sending, the type of the Message to be passed is dependent on
   the implementation, and on the constraints on the Protocol Stacks
   implied by the Connection's transport parameters.




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8.1.  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 8.2.1).

   The application can set a minIncompleteLength value to indicates 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 8.2.2 and Section 8.4 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 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 8.2.2).

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

8.2.  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.

8.2.1.  Received

   Connection -> Received<messageData, messageContext>





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   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.
   See {#receive-context} for details about the received context.

   The messageData object provides access to the bytes that were
   received for this Message, along with the length of the byte array.

   See Section 8.4 for handling Message framing in situations where the
   Protocol Stack provides octet-stream transport only.

8.2.2.  ReceivedPartial

Connection -> ReceivedPartial<messageData, messageContext, endOfMessage>

   If a complete Message cannot be delivered in one event, one part of
   the Message may be delivered with a ReceivedPartial event.  In order
   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 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:

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

   o  the underlying Protocol Stack does not support message boundary
      preservation, and the deframer (see Section 8.4) cannot determine
      the end of the message using the buffer space it has available; or

   o  the underlying Protocol Stack does not support message boundary
      preservation, and no deframer was supplied by the application

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






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8.2.3.  ReceiveError

   Connection -> ReceiveError<messageContext>

   A ReceiveError occurs when data is received by the underlying
   Protocol Stack that cannot be fully retrieved or deframed, or when
   some other indication is received that reception has failed.  Such
   conditions that irrevocably lead to the termination of the Connection
   are signaled using ConnectionError instead (see Section 10).

   The ReceiveError event passes an optional associated MessageContext.
   This may indicate that a Message that was being partially received
   previously, but had not completed, encountered an error and will not
   be completed.

8.3.  Message Receive Context

   Each Received Message Context may contain metadata from protocols in
   the Protocol Stack; which metadata is available is Protocol Stack
   dependent.  The following metadata values are supported:

8.3.1.  ECN

   When available, Message metadata carries the value of the Explicit
   Congestion Notification (ECN) field.  This information can be used
   for logging and debugging purposes, and for building applications
   which need access to information about the transport internals for
   their own operation.

8.3.2.  Early Data

   In some cases it may 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
   [I-D.ietf-tls-tls13]).  Thus, receivers may wish to perform
   additional checks for early data to ensure it is idempotent or not
   replayed.  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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8.3.3.  Receiving Final Messages

   The Received 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 Section 7.3.5.

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

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

8.4.  Receiver-side De-framing over Stream Protocols

   The Receive Event is intended to be fired once per application-layer
   Message sent by the remote endpoint; i.e., it is a desired property
   of this interface that a Send at one end of a Connection maps to
   exactly one Receive on the other end.  This is possible with Protocol
   Stacks that provide message boundary preservation, but is not the
   case over Protocol Stacks that provide a simple octet stream
   transport.

   For preserving message boundaries over stream transports, this
   interface provides receiver-side de-framing.  This facility is based
   on the observation that, since many of our current application
   protocols 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.  A Deframer allows an application to push this de-
   framing down into the interface, in order to transform an octet
   stream into a sequence of Messages.

   Concretely, receiver-side de-framing allows a caller to provide the
   interface with a function that takes an octet stream, as provided by
   the underlying Protocol Stack, reads and returns a single Message of
   an appropriate type for the application and platform, and leaves the
   octet stream at the start of the next Message to deframe.  It
   consists of a Deframer Object with a single Action, Deframe.  Since
   the Deframer depends on the protocol used at the application layer,
   it is bound to the Preconnection during the pre-establishment phase:






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   Preconnection.DeframeWith(Deframer)

   {messageData} := Deframer.Deframe(OctetStream, ...)

9.  Managing Connections

   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.

   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 9.1 below.  Specific Protocol Properties are defined in a
   transport- and implementation-specific way, and must not be assumed
   to apply across different protocols.  Attempts to set Specific
   Protocol Properties on a protocol stack not containing that specific
   protocol are simply ignored, and do not raise an error; however, too
   much reliance by an application on Specific Protocol Properties may
   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 5.2), as well as on
   connections directly using the SetProperty action: ~~~
   Connection.SetProperty(property, value) ~~~

   At any point, the application can query Connection Properties.  ~~~
   ConnectionProperties := Connection.GetProperties() ~~~

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

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

   o  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 Section 7.3.5.

   o  Whether the connection can be used to receive data.  A connection
      can not be used for reading if the connection was created with the
      Selection Property "Direction of Communication" set to



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      "unidirectional send" or if a Message marked as "Final" was
      received, see Section 8.3.3.  The latter is only supported by
      certain transport protocols, e.g., by TCP as half-closed
      connection.

   o  For Connections that are Establishing: Transport Properties that
      the application specified on the Preconnection, see Section 5.2.

   o  For Connections that are Established, Closing, or Closed:
      Selection (Section 5.2) and Connection Properties (Section 9.1) of
      the actual protocols that were selected and instantiated.
      Selection Properties indicate whether or not the Connection has or
      offers a certain Selection Property.  Note that the actually
      instantiated protocol stack may not match all Protocol Selection
      Properties that the application specified on the Preconnection.
      For example, a certain Protocol Selection Property that an
      application specified as Preferred may not actually be present in
      the chosen protocol stack because none of the currently available
      transport protocols had this feature.

   o  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.

9.1.  Generic Connection Properties

   The Connection Properties defined as independent, and available on
   all Connections are defined in the subsections below.

   Note that many protocol properties have a corresponding selection
   property, which prefers protocols providing a specific transport
   feature that controlled by that protocol property.  [EDITOR'S NOTE:
   todo: add these cross-references up to Section 5.2]

9.1.1.  Notification of excessive retransmissions

   Type:  Boolean

   This property specifies whether an application considers it useful to
   be informed in case sent data was retransmitted more often than a
   certain threshold.  When set to true, the effect is twofold: The
   application may receive events in case excessive retransmissions.  In
   addition, the transport system considers this as a preference to use
   transports stacks that can provide this notification.  This is not a
   strict requirement.  If set to false, no notification of excessive
   retransmissions will be sent and this transport feature is ignored
   for protocol selection.



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   The recommended default is to have this option.

9.1.2.  Retransmission threshold before excessive retransmission
        notification

   Type:  Integer

   This property specifies after how many retransmissions to inform the
   application about "Excessive Retransmissions".

9.1.3.  Notification of ICMP soft error message arrival

   Type:  Boolean

   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
   will be available as SoftErrors.  Note that even if a protocol
   supporting this property is selected, not all ICMP errors will
   necessarily be delivered, so applications cannot rely on receiving
   them.  Setting this option also implies a preference to prefer
   transports stacks that can provide this notification.  If not set, no
   events will be sent for ICMP soft error message and this transport
   feature is ignored for protocol selection.

   This property applies to Connections and Connection Groups.  The
   recommended default is not to have this option.

9.1.4.  Required minimum coverage of the checksum for receiving

   Type:  Integer

   This property specifies the part of the received data that needs to
   be covered by a checksum.  It is given in Bytes.  A value of 0 means
   that no checksum is required, and a special value (e.g., -1)
   indicates full checksum coverage.

9.1.5.  Niceness (Connection)

   Type:  Integer

   This Property is a non-negative integer representing the relative
   inverse 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 6.4, this property is not
   entangled when Connections are cloned.





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9.1.6.  Timeout for aborting Connection

   Type:  Integer

   This property specifies how long to wait before aborting a Connection
   during establishment, or before deciding that a Connection has failed
   after establishment.  It is given in seconds.

9.1.7.  Connection group transmission scheduler

   Type:  Enum

   This property specifies which scheduler should be used among
   Connections within a Connection Group, see Section 6.4.  The set of
   schedulers can be taken from [I-D.ietf-tsvwg-sctp-ndata].

9.1.8.  Maximum message size concurrent with Connection establishment

   Type:  Integer (read only)

   This property represents the maximum Message size that can be sent
   before or during Connection establishment, see also Section 7.3.4.
   It is given in Bytes.

9.1.9.  Maximum Message size before fragmentation or segmentation

   Type:  Integer (read only)

   This property, if applicable, represents the maximum Message size
   that can be sent without incurring network-layer fragmentation or
   transport layer segmentation at the sender.

9.1.10.  Maximum Message size on send

   Type:  Integer (read only)

   This property represents the maximum Message size that can be sent.

9.1.11.  Maximum Message size on receive

   Type:  Integer (read only)

   This numeric property represents the maximum Message size that can be
   received.







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9.1.12.  Capacity Profile

   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 system should select paths and profiles to
   optimize for the capacity profile specified.  The following values
   are valid for the Capacity Profile:

   Default:  The application makes no representation about its expected
      capacity profile.  No special optimizations of the tradeoff
      between delay, delay variation, and bandwidth efficiency should be
      made when selecting and configuring transport protocol stacks.
      Transport system implementations that map the requested capacity
      profile onto per-connection DSCP signaling without multiplexing
      SHOULD assign the DSCP Default Forwarding [RFC2474] PHB; when the
      Connection is multiplexed, the guidelines in section 6 of
      [RFC7657] apply.

   Scavenger:  The application is not interactive.  It expects to send
      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 system implementations that map the requested capacity
      profile onto per-connection DSCP signaling without multiplexing
      SHOULD assign the DSCP Less than Best Effort [LE-PHB] PHB; when
      the Connection is multiplexed, the guidelines in section 6 of
      [RFC7657] apply.

   Low Latency/Interactive:  The application is interactive, and prefers
      loss to latency.  Response time should be optimized at the expense
      of bandwidth efficiency and delay variation 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 system implementations that map the requested capacity
      profile onto per-connection DSCP signaling without multiplexing
      SHOULD assign the DSCP Expedited Forwarding [RFC3246] PHB; when
      the Connection is multiplexed, the guidelines in section 6 of
      [RFC7657] apply.

   Low Latency/Non-Interactive:  The application prefers loss to latency
      but is not interactive.  Response time should be optimized at the
      expense of bandwidth efficiency and delay variation when sending
      on this connection.Transport system implementations that map the
      requested capacity profile onto per-connection DSCP signaling



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      without multiplexing SHOULD assign a DSCP Assured Forwarding
      (AF21,AF22,AF23,AF24) [RFC2597] PHB; when the Connection is
      multiplexed, the guidelines in section 6 of [RFC7657] apply.

   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 bandwidth
      efficiency.  This implies that the Connection may fail if the
      desired rate cannot be maintained across the Path.  A transport
      may 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; when the Connection is multiplexed, the guidelines
      in section 6 of [RFC7657] apply.

   High Throughput Data:  The application expects to send/receive data
      at the maximum rate allowed by its congestion controller over a
      relatively long period of time.  Transport 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].  When the Connection is multiplexed, the guidelines in
      section 6 of [RFC7657] apply.

   The Capacity Profile for a selected protocol stack may be modified on
   a per-Message basis using the Transmission Profile Message Property;
   see Section 7.3.8.

9.2.  Soft Errors

   Asynchronous introspection is also possible, via the SoftError Event.
   This event informing the application about the receipt 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<>

10.  Connection Termination

   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 system.  For example,
   if reliable delivery was requested for a Message handed over before
   calling Close, the transport system will ensure that this Message is



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   indeed delivered.  If the Remote Endpoint still has data to send, it
   cannot be received after this call.

   Connection.Close()

   The Closed Event can inform the application that the Remote Endpoint
   has closed the Connection; however, there is no guarantee that a
   remote Close will indeed be signaled.

   Connection -> Closed<>

   Abort terminates a Connection without delivering remaining data:

   Connection.Abort()

   A ConnectionError can inform the application that the other side has
   aborted the Connection; however, there is no guarantee that an Abort
   will indeed be signaled.

   Connection -> ConnectionError<>

11.  Connection State and Ordering of Operations and Events

   As this interface is designed to be independent of an
   implementation's concurrency model, the details of how exactly
   actions are handled, and on which threads/callbacks events are
   dispatched, are implementation dependent.

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

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

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

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

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

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





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

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

   o  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).

   o  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.

   o  No events will occur on a Connection after it is Closed; i.e.,
      after a Closed<> event, an InitiateError<> or ConnectionError<> 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).  ConnectionError<> may occur
      after Closed<>, but the interface must gracefully handle all cases
      where application ignores these errors.

12.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no Actions for IANA.

13.  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 5.3.  It does not recommend use (or disuse) of specific
   algorithms or protocols.  Any API-compatible transport security
   protocol should work in a TAPS system.

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).



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   This work has been supported by the UK Engineering and Physical
   Sciences Research Council under grant EP/R04144X/1.

   Thanks to Stuart Cheshire, Josh Graessley, David Schinazi, and Eric
   Kinnear for their implementation and design efforts, including Happy
   Eyeballs, that heavily influenced this work.  Thanks to Laurent Chuat
   and Jason Lee for initial work on the Post Sockets interface, from
   which this work has evolved.

15.  References

15.1.  Normative References

   [I-D.ietf-taps-arch]
              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
              Perkins, C., Tiesel, P., and C. Wood, "An Architecture for
              Transport Services", draft-ietf-taps-arch-01 (work in
              progress), July 2018.

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

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

   [I-D.ietf-tsvwg-rtcweb-qos]
              Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
              Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
              qos-18 (work in progress), August 2016.

   [I-D.ietf-tsvwg-sctp-ndata]
              Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol", draft-ietf-tsvwg-
              sctp-ndata-13 (work in progress), September 2017.

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

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



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15.2.  Informative References

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

   [LE-PHB]   Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
              draft-ietf-tsvwg-le-phb-06 (work in progress), October
              2018.

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

   [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>.

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

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., 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>.





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   [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>.

   [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>.

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

   [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>.

Appendix A.  Additional Properties

   The interface specified by this document represents the minimal
   common interface to an endpoint in the transport services
   architecture [I-D.ietf-taps-arch], based upon that architecture and
   on the minimal set of transport service features elaborated in
   [I-D.ietf-taps-minset].  However, the interface has been designed
   with extension points to allow the implementation of features beyond
   those in the minimal common interface: Protocol Selection Properties,
   Path Selection Properties, and Message Properties are open sets.
   Implementations of the interface are free to extend these sets to
   provide additional expressiveness to applications written on top of
   them.

   This appendix enumerates a few additional properties that could be
   used to enhance transport protocol and/or path selection, or the
   transmission of messages given a Protocol Stack that implements them.
   These are not part of the interface, and may be removed from the



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   final document, but are presented here to support discussion within
   the TAPS working group as to whether they should be added to a future
   revision of the base specification.

A.1.  Experimental Transport Properties

   The following Transport Properties might be made available in
   addition to those specified in Section 5.2, Section 9.1, and
   Section 7.3.

A.1.1.  Direction of communication

   Classification:  Selection Property, Control Property [TODO: Discuss]

   Type:  Enumeration

   Applicability:  Preconnection, Connection (read only)

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

   Bidirectional (default):  The connection must support sending and
      receiving data

   unidirectional send:  The connection must support sending data.

   unidirectional receive:  The connection must support receiving data

   In case a unidirectional connection is requested, but unidirectional
   connections are not supported by the transport protocol, the system
   should fall back to bidirectional transport.

A.1.2.  Suggest a timeout to the Remote Endpoint

   Classification:  Selection Property

   Type:  Preference

   Applicability:  Preconnection

   This property specifies whether an application considers it useful to
   propose a timeout until the Connection is assumed to be lost.  The
   default is to have this option.

   [EDITOR'S NOTE: For discussion of this option, see
   https://github.com/taps-api/drafts/issues/109]





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A.1.3.  Abort timeout to suggest to the Remote Endpoint

   Classification:  Protocol Property

   Type:  Integer

   Applicability:  Preconnection, Connection

   This numeric property specifies the timeout to propose to the Remote
   Endpoint.  It is given in seconds.

   [EDITOR'S NOTE: For discussion of this property, see
   https://github.com/taps-api/drafts/issues/109]

A.1.4.  Traffic Category

   Classification:  Intent

   Type:  Enumeration

   Applicability:  Preconnection

   This property specifies what the application expects the dominating
   traffic pattern to be.  Possible values are:

   Query:  Single request / response style workload, latency bound

   Control:  Long lasting low bandwidth control channel, not bandwidth
      bound

   Stream:  Stream of data with steady data rate

   Bulk:  Bulk transfer of large Messages, presumably bandwidth bound

   The default is to not assume any particular traffic pattern.  Most
   categories suggest the use of other intents to further describe the
   traffic pattern anticipated, e.g., the bulk category suggesting the
   use of the Message Size intents or the stream category suggesting the
   Stream Bitrate and Duration intents.

A.1.5.  Size to be Sent or Received

   Classification:  Intent

   Type:  Integer

   Applicability:  Preconnection, Message




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   This property specifies how many bytes the application expects to
   send (Size to be Sent) or how many bytes the application expects to
   receive in response (Size to be Received).

A.1.6.  Duration

   Classification:  Intent

   Type:  Integer

   Applicability:  Preconnection

   This Intent specifies what the application expects the lifetime of a
   Connection to be.  It is given in milliseconds.

A.1.7.  Send or Receive Bit-rate

   Classification:  Intent

   Type:  Integer

   Applicability:  Preconnection, Message

   This Intent specifies what the application expects the bit-rate of a
   transfer to be.  It is given in Bytes per second.

   On a Message, this property specifies at what bitrate the application
   wishes the Message to be sent.  A transport system supporting this
   feature will not exceed the requested Send Bitrate even if flow-
   control and congestion control allow higher bitrates.  This helps to
   avoid a bursty traffic pattern on busy streaming video servers.

A.1.8.  Cost Preferences

   Classification:  Intent

   Type:  Enumeration

   Applicability:  Preconnection, Message

   This property describes what an application prefers regarding
   monetary costs, e.g., whether it considers it acceptable to utilize
   limited data volume.  It provides hints to the transport system on
   how to handle trade-offs between cost and performance or reliability.

   Possible values are:

   No Expense:  Avoid transports associated with monetary cost



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   Optimize Cost:  Prefer inexpensive transports and accept service
      degradation

   Balance Cost:  Use system policy to balance cost and other criteria

   Ignore Cost:  Ignore cost, choose transport solely based on other
      criteria

   The default is "Balance Cost".

Appendix B.  Sample API definition in Go

   This document defines an abstract interface.  To illustrate how this
   would map concretely into a programming language, an API interface
   definition in Go is available online at https://github.com/mami-
   project/postsocket.  Documentation for this API - an illustration of
   the documentation an application developer would see for an instance
   of this interface - is available online at
   https://godoc.org/github.com/mami-project/postsocket.  This API
   definition will be kept largely in sync with the development of this
   abstract interface definition.

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

   [I-D.ietf-taps-minset] identifies a minimal set of transport services
   that end systems should offer.  These services make all transport
   features offered by 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, this uses the list in
   Section 4.1 of [I-D.ietf-taps-minset], updated according to the
   discussion in Section 5 of [I-D.ietf-taps-minset].

   [EDITOR'S NOTE: This is early text.  In the future, this section will
   contain backward references, which we currently avoid because things
   are still being moved around and names / categories etc. are
   changing.  Also, clearly, the intention is for the full minset to be
   reflected by the API at some point.]

   o  Connect:
      "Initiate" Action.

   o  Listen:
      "Listen" Action.





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   o  Specify number of attempts and/or timeout for the first
      establishment message:
      TODO.

   o  Disable MPTCP:
      TODO.

   o  Hand over a message to reliably transfer (possibly multiple times)
      before connection establishment:
      "InitiateWithIdempotentSend" Action.

   o  Hand over a message to reliably transfer during connection
      establishment:
      TODO.

   o  Change timeout for aborting connection (using retransmit limit or
      time value):
      "Timeout for aborting Connection" property, using a time value in
      seconds.

   o  Timeout event when data could not be delivered for too long:
      TODO: this should probably be covered by the "ConnectionError"
      Event, but the text above it currently reads: "...can inform the
      application that the other side has aborted the Connection".  In
      this case, it is the local side.

   o  Suggest timeout to the peer:
      "Suggest a timeout to the Remote Endpoint" and "Abort timeout to
      suggest to the Remote Endpoint" Selection property.  [EDITOR'S
      NOTE: For discussion of this option, see https://github.com/taps-
      api/drafts/issues/109].

   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold):
      "Notification of excessive retransmissions" property.

   o  Notification of ICMP error message arrival:
      "Notification of ICMP soft error message arrival" property.

   o  Choose a scheduler to operate between streams of an association:
      "Connection group transmission scheduler" property.

   o  Configure priority or weight for a scheduler:
      "Niceness (Connection)" property.

   o  "Specify checksum coverage used by the sender" and "Disable
      checksum when sending":
      "Corruption Protection Length" property (value 0 to disable).



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   o  "Specify minimum checksum coverage required by receiver" and
      "Disable checksum requirement when receiving":
      "Required minimum coverage of the checksum for receiving" property
      (value 0 to disable).

   o  "Specify DF" field and "Request not to bundle messages:"
      The "Singular Transmission" Message property combines both of
      these requests, i.e. if a request not to bundle messages is made,
      this also turns off DF in case of protocols that allow this (only
      UDP and UDP-Lite, which cannot bundle messages anyway).

   o  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.

   o  Get max. transport-message size that may be received from the
      configured interface:
      "Maximum Message size on receive" property.

   o  Obtain ECN field:
      "ECN" is a defined metadata value as part of the Message Receive
      Context.

   o  "Specify DSCP field", "Disable Nagle algorithm", "Enable and
      configure a 'Low Extra Delay Background Transfer'":
      As suggested in Section 5.5 of [I-D.ietf-taps-minset], these
      transport features are collectively offered via the "Capacity
      profile" property.

   o  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
      [I-D.ietf-taps-minset].

   o  "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.

   o  "Reliably transfer data, with congestion control", "Reliably
      transfer a message, with congestion control" and "Unreliably
      transfer a message":




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      Reliability is controlled via the "Reliable Data Transfer
      (Message)" Message property.  Transmitting data without delimiters
      is done by not using a Framer.  The choice of congestion control
      is provided via the "Congestion control" property.

   o  Configurable Message Reliability:
      The "Lifetime" Message Property implements a time-based way to
      configure message reliability.

   o  "Ordered message delivery (potentially slower than unordered)" and
      "Unordered message delivery (potentially faster than ordered)":
      The two transport features are controlled via the Message property
      "Ordered".

   o  Request not to delay the acknowledgement (SACK) of a message:
      Should the protocol support it, this is one of the transport
      features the transport system can use when an application uses the
      Capacity Profile Property with value "Low Latency/Interactive".

   o  Receive data (with no message delimiting):
      "Received" Event without using a Deframer.

   o  Receive a message:
      "Received" Event.  Section 5.1 of [I-D.ietf-taps-minset] discusses
      how messages can be obtained from a bytestream in case of
      implementation over TCP.  Here, this is dealt with by Framers and
      Deframers.

   o  Information about partial message arrival:
      "ReceivedPartial" Event.

   o  Notification of send failures:
      "Expired" and "SendError" Events.

   o  Notification that the stack has no more user data to send:
      Applications can obtain this information via the "Sent" Event.

   o  Notification to a receiver that a partial message delivery has
      been aborted:
      "ReceiveError" Event.

Authors' Addresses









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   Brian Trammell (editor)
   ETH Zurich
   Gloriastrasse 35
   8092 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
   TU Berlin
   Marchstrasse 23
   10587 Berlin
   Germany

   Email: theresa@inet.tu-berlin.de


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

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


   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch







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

   Email: csp@csperkins.org


   Philipp S. Tiesel
   TU Berlin
   Marchstrasse 23
   10587 Berlin
   Germany

   Email: philipp@inet.tu-berlin.de


   Chris Wood
   Apple Inc.
   1 Infinite Loop
   Cupertino, California 95014
   United States of America

   Email: cawood@apple.com


























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