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Versions: 00

TAPS Working Group                                      B. Trammell, Ed.
Internet-Draft                                                ETH Zurich
Intended status: Informational                             M. Welzl, Ed.
Expires: September 2, 2018                            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.
                                                          March 01, 2018


     An Abstract Application Layer Interface to Transport Services
                    draft-trammell-taps-interface-00

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 September 2, 2018.

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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  Interface Design Principles . . . . . . . . . . . . . . . . .   4
   4.  API Summary . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Pre-Establishment Phase . . . . . . . . . . . . . . . . . . .   6
     5.1.  Specifying Endpoints  . . . . . . . . . . . . . . . . . .   6
     5.2.  Specifying Transport Parameters . . . . . . . . . . . . .   7
       5.2.1.  Transport Parameters Object . . . . . . . . . . . . .  11
     5.3.  Specifying Security Parameters and Callbacks  . . . . . .  12
   6.  Establishing Connections  . . . . . . . . . . . . . . . . . .  13
     6.1.  Active Open: Initiate . . . . . . . . . . . . . . . . . .  13
     6.2.  Passive Open: Listen  . . . . . . . . . . . . . . . . . .  14
     6.3.  Peer-to-Peer Establishment: Rendezvous  . . . . . . . . .  15
     6.4.  Connection Groups . . . . . . . . . . . . . . . . . . . .  16
   7.  Sending Data  . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Send Parameters . . . . . . . . . . . . . . . . . . . . .  19
       7.1.1.  Lifetime  . . . . . . . . . . . . . . . . . . . . . .  19
       7.1.2.  Niceness  . . . . . . . . . . . . . . . . . . . . . .  19
       7.1.3.  Ordered . . . . . . . . . . . . . . . . . . . . . . .  20
       7.1.4.  Idempotent  . . . . . . . . . . . . . . . . . . . . .  20
       7.1.5.  Corruption Protection Length  . . . . . . . . . . . .  20
       7.1.6.  Immediate Acknowledgement . . . . . . . . . . . . . .  20
       7.1.7.  Instantaneous Capacity Profile  . . . . . . . . . . .  21
     7.2.  Sender-side Framing . . . . . . . . . . . . . . . . . . .  21
   8.  Receiving Data  . . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Receiver-side De-framing over Stream Protocols  . . . . .  23
   9.  Setting and Querying of Connection Properties . . . . . . . .  24
     9.1.  Protocol Properties . . . . . . . . . . . . . . . . . . .  25
   10. Connection Termination  . . . . . . . . . . . . . . . . . . .  27



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   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     14.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Appendix A.  Additional Properties  . . . . . . . . . . . . . . .  29
     A.1.  Protocol and Path Selection Properties  . . . . . . . . .  29
       A.1.1.  Application Intents . . . . . . . . . . . . . . . . .  30
     A.2.  Protocol Properties . . . . . . . . . . . . . . . . . . .  32
     A.3.  Send Parameters . . . . . . . . . . . . . . . . . . . . .  32
   Appendix B.  Sample API definition in Go  . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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.  It
   would not be an overstatement to say that this simple API is the
   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, [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 provided in [RFC8095]
   and [I-D.ietf-taps-minset].

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.




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

   o  An Action creates an Object: ~~~ 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 callback passing or other asynchronous
   calling conventions.  The method for registering callbacks and
   handlers 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).

   This specification treats Events and errors similarly, as 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
   parameters.

3.  Interface Design Principles

   We begin with the architectural design principles defined in
   [TAPS-ARCH]; from these, we derive and elaborate a set of principles
   on which the design of the interface is based.  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



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

   o  Asynchronous Connection establishment, transmission, and
      reception, allowing most application interactions with the
      transport layer to be Event-driven, in line with developments in
      modern platforms and programming languages;

   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
      take full advantage of new transport protocols supporting these
      features; and

   o  Atomic transmission of data, using application-assisted framing
      and deframing where the underlying transport does not provide
      these.

4.  API Summary

   The Transport Services Interface is the basic common abstract
   application programming interface to the Transport Services
   Architecture defined in [TAPS-ARCH].  An application primarily
   interacts with this interface through two Objects, Preconnections and
   Connections.  A Preconnection represents a set of parameters 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 received from a remote endpoint.  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.





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   In the following sections, we describe the details of application
   interaction with Objects through Actions and Events in each phase of
   a Connection, following the phases described in [TAPS-ARCH].

5.  Pre-Establishment Phase

   The pre-establishment phase allows applications to specify parameters
   for the Connections they're 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 parameters 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 transport parameters (see Section 5.2), and the
   security parameters (see Section 5.3):

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

   Framers (see Section 7.2) and deframers (see Section 8.1), if
   necessary, should be bound to the Preconnection during pre-
   establishment.

   Preconnections, as Connections, can be cloned, in order to establish
   Connection groups before Connection initiation; see Section 6.4 for
   details.

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 RemoteEndoint.  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.  The multiple identifiers
   refer to the same endpoint.

   The transport services API will resolve names internally, when the
   Initiate(), Listen(), or Rendezvous() method is called establish a
   Connection.  The API does not need the application to resolve names,
   and premature name resolution can damage performance by limiting the
   scope for alternate path discovery during Connection establishment.
   The Resolve() method is, however, provided to resolve a Local
   Endpoint or a Remote Endpoint in cases where this is required, for
   example with some NAT traversal protocols (see Section 6.3).

5.2.  Specifying Transport Parameters

   A Preconnection Object holds parameters reflecting the application's
   requirements and preferences for the transport.  These include
   protocol and path selection parameters, as well as Generic and
   Specific Protocol Properties for configuration of the detailed
   operation of the selected Protocol Stacks.

   All Transport Parameters are organized within a single namespace
   shared with Send Parameters (see Section 7.1).  All transport



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   parameters take paremeter-specific values.  Protocol and Path
   Selection properties additionally take one of five preference levels,
   though not all preference levels make sense with all such properties.
   Note that it is possible for a set of specified transport parameters
   to be internally inconsistent, or for preferences to be inconsistent
   with the later use of the API by the application.  Application
   developers should reduce inconsistency by only using the most
   stringent preference levels when failure to meet a preference would
   break the application's functionality (e.g.  the Reliable Data
   Transfer preference, which is a core assumption of many application
   protocols).  Implementations of this interface should also raise
   errors in configuration as early as possible, to help ensure these
   inconsistencies are caught early in the development process.

   The protocol(s) and path(s) selected as candidates during Connection
   establishment are determined by a set of 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.

   To reflect the needs of an individual Connection, they can be
   specified with five different 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 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                             |
   +------------+------------------------------------------------------+

   An implementation of this interface must provide sensible defaults
   for protocol and path selection properties.  The defaults given for
   each 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.



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   The following properties can be used during Protocol and Path
   selection:

   o  Reliable Data Transfer: This boolean property specifies whether
      the application needs the transport protocol to ensure that data
      is received completely and without corruption on the other side.
      This also entails being notified when a Connection is closed or
      aborted.  This property applies to Connections and Connection
      Groups.  This is a strict requirement.  The default is to enable
      Reliable Data Transfer.

   o  Preservation of data ordering: This boolean property specifies
      whether the application needs the transport protocol to assure
      that data is received by the application on the other end in the
      same order as it was sent.  This property applies to Connections
      and Connection Groups.  This is a strict requirement.  The default
      is to preserve data ordering.

   o  Configure reliability on a per-Message basis: This boolean
      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.  This
      is not a strict requirement.  The default is to not have this
      option.

   o  Use 0-RTT session establishment with an idempotent Message: This
      boolean 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 7.1.4.  This is a strict
      requirement.  The default is to not have this option.

   o  Multiplex Connections: This boolean property specifies that the
      application would prefer multiple Connections between the same
      endpoints within a Connection Group to be multiplexed onto a
      single underlying transport connection where possible, for reasons
      of efficiency.  This is not a strict requirement.  The default is
      to not have this option.

   o  Notification of excessive retransmissions: This boolean property
      specifies whether an application considers it useful to be
      informed in case sent data was retransmitted more often than a
      certain threshold.  This property applies to Connections and
      Connection Groups.  This is not a strict requirement.  The default
      is to have this option.





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   o  Notification of ICMP error message arrival: This boolean property
      specifies whether an application considers it useful to be
      informed when an ICMP error message arrives.  This property
      applies to Connections and Connection Groups.  This is not a
      strict requirement.  The default is to have this option.

   o  Control checksum coverage on sending or receiving: This boolean
      property specifies whether the application considers it useful to
      enable / disable / configure a checksum when sending data, or
      decide whether to require a checksum or not when receiving data.
      This property applies to Connections and Connection Groups.  This
      is not a strict requirement, as it signifies a reduction in
      reliability.  The default is full checksum coverage without being
      able to change it, and requiring a checksum when receiving.

   o  Interface Type: This enumerated property specifies which kind of
      access network interface, e.g., WiFi, Ethernet, or LTE, to prefer
      over others for this Connection, in case they are available.  In
      general, Interface Types should be used only with the "Prefer" and
      "Prohibit" preference level.  Specifically, using the "Require"
      preference level for Interface Type may limit path selection in a
      way that is detrimental to connectivity.  The default is to use
      the default interface configured in the system policy.

   o  Capacity Profile: This enumerated property specifies the
      application's expectation of the dominating traffic pattern for
      this Connection.  The Capacity Profile should only be used with
      the "Prefer" preference level; other preference levels make no
      sense for profiles.  The following values are valid for 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
         stacks.

      Interactive/Low Latency:  The application is interactive.
         Response time (latency) should be optimized at the expense of
         bandwidth efficiency and delay variation.  This can be used by
         the system to disable the coalescing of multiple small Messages
         into larger packets (Nagle's algorithm), to prefer lower-
         latency paths, signal a preference for lower-latency, higher-
         loss treatment, and so on.

      Constant Rate:  The application expects to send/receive data at a
         constant rate after Connection establishment.  Delay and delay
         variation should be optimized at the expense of bandwidth



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         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.

      Scavenger/Bulk:  The application is not interactive.  It expects
         to send/receive a large amount of data, without any urgency.
         This can be used to select protocol stacks with scavenger
         transmission control, to signal a preference for less-than-
         best-effort treatment, and so on.

   In addition to protocol and path selection properties, the transport
   parameters may also contain Generic and/or Specific Protocol
   Properties (see Section 9.1).  These properties will be passed to the
   selected candidate Protocol Stack(s) to configure them before
   candidate Connection establishment.

5.2.1.  Transport Parameters Object

   All transport parameters used in the pre-establishment phase are
   collected in a TransportParameters Object that is passed to the
   Preconnection Object.

   TransportParameters := NewTransportParameters()

   The Individual parameters are then added to the TransportParameters
   Object.  While Protocol Properties use the "add" call, Transport
   Preferences use special calls for the levels defined in Section 5.2.

   TransportParameters.Add(parameter, value)

   TransportParameters.Require(preference)
   TransportParameters.Prefer(preference)
   TransportParameters.Ignore(preference)
   TransportParameters.Avoid(preference)
   TransportParameters.Prohibit(preference)

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

   TransportParameters := Connection.GetTransportParameters()

   Note that most properties are only considered for Connection
   establishment and can not be changed after a Connection is
   established; however, they can be queried.  See Section 9.






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   A Connection gets its Transport Parameters either by being explicitly
   configured via a Preconnection, or by inheriting them from an
   antecedent via cloning; see Section 6.4 for more.

5.3.  Specifying Security Parameters and Callbacks

   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.pauly-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 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(22)    // secp256k1
SecurityParameters.AddCiphersuite(0xCCA9)   // TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
SecurityParameters.AddSignatureAlgorithm(7) // ed25519

   o  Session cache: Used to tune cache capacity, lifetime, re-use, and
      eviction policies, e.g., LRU or FIFO.

   SecurityParameters.SetSessionCacheCapacity(1024)     // 1024 elements
   SecurityParameters.SetSessionCacheLifetime(24*60*60) // 24 hours
   SecurityParameters.SetSessionCacheReuse(1)           // One-time use

   o  Pre-shared keying material: 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.



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   SecurityParameters.AddPreSharedKey(key, identity)

   Security decisions, especially pertaining to trust, are not static.
   Thus, once configured, parameters must also be supplied during live
   handshakes.  These are best handled as client-provided callbacks.
   Security handshake callbacks 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)

   Like transport parameters, security parameters are inherited during
   cloning (see Section 6.4).

6.  Establishing Connections

   Before a Connection can be used for data transfer, it must be
   established.  Establishment ends the pre-establishment phase; all
   transport and cryptographic parameter specification must be complete
   before establishment, as these parameters 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()




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   Before calling Initiate, the caller must have populated a
   Preconnection Object with a Remote Endpoint specifier, optionally a
   Local Endpoint specifier (if not specified, the system will attempt
   to determine a suitable Local Endpoint), as well as all parameters
   necessary for candidate selection.  After calling Initiate, no
   further parameters may be bound to the Connection.  The Initiate()
   call consumes the Preconnection and creates a Connection Object.  A
   Preconnection can only be initiated once.

   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 and
   cryptographic 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).

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

   Preconnection.Listen()



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   Before calling Listen, the caller must have initialized the
   Preconnection during the pre-establishment phase with a Local
   Endpoint specifier, as well as all parameters 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 parameters may be bound to the Preconnection, and no
   subsequent establishment call may be made on the Preconnection.

   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.

6.3.  Peer-to-Peer Establishment: Rendezvous

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

   Preconnection.Rendezvous()

   The Preconnection Object must be specified with both a Local Endpoint
   and a Remote Endpoint, and also the transport 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 parameters may be bound to
   the Preconnection, and no subsequent establishment call may be made
   on the Preconnection.

   Preconnection -> RendezvousDone<Connection>



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

   PreconnectionBindings := 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
   Connections.  This list can be passed to a peer via a signalling
   protocol, such as SIP or WebRTC, to configure the remote.

6.4.  Connection Groups

   Groups of Preconnections or Connections can be created using the
   Clone Action:

   Preconnection := Preconnection.Clone()

   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 clone
   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 group, and so on.
   Connections in a Connection Group share all their properties, and
   changing the properties on one Connection in the group changes the
   property for all others.



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   Calling Clone on a Preconnection yields a Preconnection with the same
   parameters, which is entangled with the parent Preconnection: all the
   Connections created from entangled Preconnections will be entangled
   as if they had been cloned, and will belong to the same Connection
   Group.

   Establishing a Connection from a cloned Preconnection will not cause
   Connections for other entangled Preconnections to be established;
   each such Connection must be established separately.  Changes to the
   parameters of a Preconnection entangled with a Preconnection from
   which a Connection has already been established will fail.  Calling
   Clone on a Preconnection may be taken by the system an implicit
   signal that Protocol Stacks supporting multiplexed Connections for
   efficient Connection Grouping are preferred by the application.

   There is only one Protocol Property that is not entangled, i.e., it
   is a separate per-Connection Property for individual Connections in
   the group: niceness.  Niceness works as in Section 7.1.2: 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 the
   Connection the capacity share (M-N) x C / M, where N is the
   Connection's Niceness value, M is the maximum Niceness value 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 capacity allocation and each implementation is free
   to implement exact capacity allocation as it sees fit.

7.  Sending Data

   Once a Connection has been established, it can be used for sending
   data.  Data is sent by passing a Message Object and additional
   parameters Section 7.1 to the Send Action on an established
   Connection:

   Connection.Send(Message, sendParameters)

   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.  It may itself contain an
   array of octets to be transmitted in the transport protocol payload,
   or be transformable to an array of octets by a sender-side framer
   (see Section 7.2).

   Some transport protocols can deliver arbitrarily sized Messages, but
   other protocols constrain the maximum Message size.  Applications can




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   query the protocol property Maximum Message Size on Send to determine
   the maximum size.

   There may also be system and Protocol Stack dependent limits on the
   size of a Message which can be transmitted atomically.  For that
   reason, the Message object passed to the Send action may also be a
   partial Message, either representing the whole data object and
   information about the range of bytes to send from it, or an object
   referring back to the larger whole Message.  The details of partial
   Message sending are implementation-dependent.

   If Send is called on a Connection which has not yet been established,
   an Initiate Action will be implicitly performed simultaneously with
   the Send.  Used together with the Idempotent property (see
   Section 7.1.4), this can be used to send data during establishment
   for 0-RTT session resumption on Protocol Stacks that support it.

   Like all Actions in this interface, the Send Action is asynchronous.

   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 when the Sent Event occurs is specific to
   the implementation and the constraints on the Protocol Stacks implied
   by the Connection's transport parameters.  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 only issues a Send after this Event fires.

   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 7.1.1) 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.

   Connection -> SendError<msgRef>





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   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 send parameters not consistent with the
   Connection's transport parameters.  The SendError contains an
   implementation-specific reference to the Message to which it applies.

7.1.  Send Parameters

   The Send Action takes per-Message send parameters which control how
   the contents will be sent down to the underlying Protocol Stack and
   transmitted.

   If Send Parameters should be overridden for a specific Message, an
   empty sent parameter Object can be acquired and all desired Send
   Parameters can be added to that Object.  A sendParameters Object can
   be reused for sending multiple contents with the same properties.

   SendParameters := NewSendParameters()
   SendParameters.Add(parameter, value)

   The Send Parameters share a single namespace with the Transport
   Parameters (see Section 5.2).  This allows the specification of
   Protocol Properties that can be overridden on a per-Message basis.

   Send Parameters may be inconsistent with the properties of the
   Protocol Stacks underlying the Connection on which a given Message is
   sent.  For example, infinite Lifetime is not possible on a Message
   over a Connection not providing reliability.  Sending a Message with
   Send Properties inconsistent with the Transport Preferences on the
   Connection yields an error.

   The following send parameters are supported:

7.1.1.  Lifetime

   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.

7.1.2.  Niceness

   Niceness represents an unbounded hierarchy of priorities of Messages,
   relative to other Messages sent over the same Connection and/or
   Connection Group (see Section 6.4).  It is most naturally represented
   as a non-negative integer.  A Message with Niceness 0 will yield to a



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   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 inversion of normal schemes for expressing priority
   has a convenient property: priority increases as both Niceness and
   Lifetime decrease.

7.1.3.  Ordered

   Ordered is a boolean property.  If true, this Message should be
   delivered after the last Message passed to the same Connection via
   the Send Action; if false, this Message may be delivered out of
   order.

7.1.4.  Idempotent

   Idempotent is a boolean property.  If true, the application-layer
   entity in the 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.

7.1.5.  Corruption Protection Length

   This numeric property specifies the length of the section of the
   Message, starting from byte 0, that the application assumes will be
   received 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 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.

7.1.6.  Immediate Acknowledgement

   This boolean property specifies, if true, that an application wants
   this Message to be acknowledged immediately by the receiver.  In case
   of reliable transmission, this informs the transport protocol on the
   sender side faster that it can remove the Message from its buffer;
   therefore this property can be useful for latency-critical
   applications that maintain tight control over the send buffer (see
   Section 7).






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7.1.7.  Instantaneous Capacity Profile

   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 5.2).

   The following values are valid for Instantaneous Capacity Profile:

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

   Interactive/Low Latency:  Response time (latency) should be optimized
      at the expense of bandwidth efficiency and delay variation 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 signal a preference for lower-latency,
      higher-loss treatment, and so on.

   Constant Rate:  Delay and delay variation should be optimized at the
      expense of bandwidth efficiency.

   Scavenger/Bulk:  This Message may be sent at the system's leisure.
      This can be used to signal a preference for less-than-best-effort
      treatment, to delay sending until lower-cost paths are available,
      and so on.

7.2.  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(Message)

   Sender-side framing is a convenience feature of the interface, for
   parity with receiver-side framing (see Section 8.1).







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8.  Receiving Data

   Once a Connection is established, Messages may be received on it.
   The application can indicate that it is ready to receive Messages by
   calling Receive() on the Connection.

   Connection.Receive(ReceiveHandler, maxLength)

   Receive takes a ReceiveHandler, which can handle the Received Event
   and the ReceiveError error.  Each call to Receive will result in at
   most one Received event being sent to the handler, though
   implementations may provide convenience functions to indicate
   readiness to receive a larger but finite number of Messages with a
   single call.  This allows an application to provide backpressure to
   the transport stack when it is temporarily not ready to receive
   messages.

   Receive also takes an optional maxLength argument, the maximum size
   (in bytes of data) Message 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 received as a partial Message.  Note that maxLength does
   not guarantee that the application will receive that many bytes if
   they are available; the interface may return partial Messages smaller
   than maxLength according to implementation constraints.

   Connection -> Received<Message>

   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.  The Message may
   also contain metadata from protocols in the Protocol Stack; which
   metadata is available is Protocol Stack dependent.  In particular,
   when this information is available, the value of the Explicit
   Congestion Notification (ECN) field is contained in such metadata.
   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.

   The Message Object must provide some method to retrieve an octet
   array containing application data, corresponding to a single message
   within the underlying Protocol Stack's framing.  See Section 8.1 for
   handling framing in situations where the Protocol Stack provides
   octet-stream transport only.

   The Message Object passed to Received is complete and atomic, unless
   one of the following conditions holds:



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   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 8.1) 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

   The Message Object passed to Received will indicate one of the
   following:

   1.  this is a complete message;

   2.  this is a partial message containing a section of a message with
       a known message boundary (made partial for local buffering
       reasons, either by the underlying Protocol Stack or the
       deframer).  In this case, the Message Object passed to Received
       may contain the byte offset of the data in the partial Message
       within the full Message, an indication whether this is the last
       (highest-offset) partial Message in the full Message, and an
       optional reference to the full Message it belongs to; or

   3.  this is a partial message containing data with no definite
       message boundary, i.e. the only known message boundary is given
       by termination of the Connection

   Note that in the absence of message boundary preservation and without
   deframing, the entire Connection is represented as one large message
   of indeterminate length.

   Connection -> ReceiveError<>

   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 the the termination of the
   Connection are signaled using ConnectionError instead (see
   Section 10).

8.1.  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



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   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:

   Preconnection.DeframeWith(Deframer)

   Message := Deframer.Deframe(OctetStream, ...)

9.  Setting and Querying of Connection Properties

   At any point, the application can set and query the properties of a
   Connection.  Depending on the phase the Connection is in, the
   Connection properties will include different information.

   ConnectionProperties := Connection.GetProperties()

   Connection.SetProperties()

   Connection properties include:

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

   o  Transport Features of the protocols that conform to the Required
      and Prohibited Transport Preferences, which might be selected by
      the transport system during Establishment.  These features
      correspond to the properties given in Section 5.2 and can only be
      queried.




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   o  Transport Features of the Protocol Stacks that were selected and
      instantiated, once the Connection has been established.  These
      features correspond to the properties given in Section 5.2 and can
      only be queried.  Instead of preference levels, these features
      have boolean values indicating whether or not they were selected.
      Note that these transport features may not fully reflect the
      specified parameters given in the pre-establishment phase.  For
      example, a certain Protocol Selection Property that an application
      specified as Preferred may not actually be present in the chosen
      Protocol Stack Instances because none of the currently available
      transport protocols had this feature.

   o  Protocol Properties of the Protocol Stack in use (see Section 9.1
      below).  These can be set or queried.  Certain specific procotol
      queries may be read-only, on a protocol- and property-specific
      basis.

   o  Path Properties of the path(s) in use, once the Connection has
      been established.  These properties can be derived from the local
      provisioning domain, measurements by the Protocol Stack, or other
      sources.  They can only be queried.

9.1.  Protocol Properties

   Protocol Properties represent the configuration of the selected
   Protocol Stacks backing a Connection.  Some properties apply
   generically across multiple transport protocols, while other
   properties only apply to specific protocols.  The default settings of
   these properties will vary based on the specific protocols being used
   and the system's configuration.

   Note that Protocol Properties are also set during pre-establishment,
   as transport parameters, to preconfigure Protocol Stacks during
   establishment.

   Generic Protocol Properties include:

   o  Relative niceness: This numeric property is similar to the
      Niceness send property (see Section 7.1.2), 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.

   o  Timeout for aborting Connection: This numeric property specifies
      how long to wait before aborting a Connection during




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      establishment, or before deciding that a Connection has failed
      after establishment.  It is given in seconds.

   o  Retransmission threshold before excessive retransmission
      notification: This numeric property specifies after how many
      retransmissions to inform the application about "Excessive
      Retransmissions".

   o  Required minimum coverage of the checksum for receiving: This
      numeric 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.

   o  Connection group transmission scheduler: This enumerated property
      specifies which scheduler should be used among Connections within
      a Connection Group.  It applies to Connection Groups; the set of
      schedulers can be taken from [I-D.ietf-tsvwg-sctp-ndata].

   o  Maximum message size concurrent with Connection establishment:
      This numeric property represents the maximum Message size that can
      be sent before or during Connection establishment, see also
      Section 7.1.4.  It is given in Bytes.  This property is read-only.

   o  Maximum Message size before fragmentation or segmentation: This
      numeric property, if applicable, represents the maximum Message
      size that can be sent without incurring network-layer
      fragmentation and/or transport layer segmentation at the sender.
      This property is read-only.

   o  Maximum Message size on send: This numeric property represents the
      maximum Message size that can be sent.  This property is read-
      only.

   o  Maximum Message size on receive: This numeric property represents
      the maximum Message size that can be received.  This property is
      read-only.

   In order to specify Specific Protocol Properties, Transport System
   implementations may offer applications to attach a set of options to
   the Preconnection Object, associated with a specific protocol.  For
   example, an application could specify a set of TCP Options to use if
   and only if TCP is selected by the system.  Such properties 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.





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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
   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 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 be signaled:

   Connection -> ConnectionError<>

11.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no Actions for IANA.

12.  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 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.

13.  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).



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   This work has been supported by Leibniz Prize project funds of DFG -
   German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
   FE 570/4-1).

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

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

14.  References

14.1.  Normative References

   [I-D.ietf-taps-minset]
              Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for TAPS Systems", draft-ietf-taps-minset-02
              (work in progress), February 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.

   [TAPS-ARCH]
              Pauly, T., Ed., Trammell, B., Ed., Brunstrom, A.,
              Fairhurst, G., Perkins, C., Tiesel, P., and C. Wood, "An
              Architecture for Transport Services", n.d..

14.2.  Informative References

   [I-D.pauly-taps-transport-security]
              Pauly, T., Rose, K., and C. Wood, "A Survey of Transport
              Security Protocols", draft-pauly-taps-transport-
              security-01 (work in progress), January 2018.

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



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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>.

   [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 [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 options on Message send 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 parameters and 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 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.  Protocol and Path Selection Properties

   The following protocol and path selection properties might be made
   available in addition to those specified in Section 5.2:

   o  Suggest a timeout to the Remote Endpoint: This boolean property
      specifies whether an application considers it useful to propose a
      timeout until the Connection is assumed to be lost.  This property
      applies to Connections and Connection Groups.  This is not a
      strict requirement.  The default is to have this option.



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

   o  Request not to delay acknowledgment of Message: This boolean
      property specifies whether an application considers it useful to
      request for Message that its acknowledgment be sent out as early
      as possible instead of potentially being bundled with other
      acknowledgments.  This property applies to Connections and
      Connection groups.  This is not a strict requirement.  The default
      is to not have this option.  [EDITOR'S NOTE: For discussion of
      this option, see https://github.com/taps-api/drafts/issues/90]

A.1.1.  Application Intents

   Application Intents are a group of transport properties expressing
   what an application wants to achieve, knows, assumes or prefers
   regarding its communication.  They are not strict requirements.  In
   particular, they should not be used to express any Quality of Service
   expectations that an application might have.  Instead, an application
   should express its intentions and its expected traffic
   characteristics in order to help the transport system make decisions
   that best match it, but on a best-effort basis.  Even though
   Application Intents do not represent Quality of Service requirements,
   a transport system may use them to determine a DSCP value, e.g.
   similar to Table 1 in [I-D.ietf-tsvwg-rtcweb-qos].

   Application Intents can influence protocol selection, protocol
   configuration, path selection, and endpoint selection.  For example,
   setting the "Timeliness" Intent to "Interactive" may lead the
   transport system to disable the Nagle algorithm for a Connection,
   while setting the "Timeliness" to "Background" may lead it to setting
   the DSCP value to "scavenger".  If the "Size to be Sent" Intent is
   set on an individual Message, it may influence path selection.

   Specifying Application Intents is not mandatory.  An application can
   specify any combination of Application Intents.  If specified,
   Application Intents are defined as parameters passed to the
   Preconnection Object, and may influence the Connection established
   from that Preconnection.  If a Connection is cloned to form a
   Connection Group, and associated Application Intents are cloned along
   with the other transport parameters.  Some Intents have also
   corresponding Message Properties, similar to the properties in
   Section 7.1.

   Application Intents can be added to this interface as Transport
   Preferences with the "Prefer" preference level.





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A.1.1.1.  Traffic Category

   This Intent specifies what the application expect the dominating
   traffic pattern to be.

   Possible Category 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.1.1.2.  Size to be Sent / Received

   This Intent specifies what the application expects the size of a
   transfer to be.  It is a numeric property and given in Bytes.

A.1.1.3.  Duration

   This Intent specifies what the application expects the lifetime of a
   transfer to be.  It is a numeric property and given in milliseconds.

A.1.1.4.  Send / Receive Bit-rate

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

A.1.1.5.  Cost Preferences

   This Intent 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.  This
   Intent can also apply to an individual Messages.

   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".

A.2.  Protocol Properties

   The following protocol properties might be made available in addition
   to those in Section 9.1:

   o  Abort timeout to suggest to the Remote Endpoint: 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.3.  Send Parameters

   The following send parameters might be made available in addition to
   those specified in Section 7.1:

   o  Immediate: Immediate is a boolean property.  If true, the caller
      prefers immediacy to efficient capacity usage for this Message.
      For example, this means that the Message should not be bundled
      with other Message into the same transmission by the underlying
      Protocol Stack.

   o  Send Bitrate: This numeric property in Bytes per second specifies
      at what bitrate the application wishes the Message to be sent.  A
      transport supporting this feature will not exceed the requested
      Send Bitrate even if flow-control and congestion control allow
      higher bitrates.  This helps to avid bursty traffic pattern on
      busy video streaming servers.

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




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   definition will be kept largely in sync with the development of this
   abstract interface definition.

Authors' Addresses

   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/











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   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

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


   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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