<pre>Internet Engineering Task Force (IETF)                          M. Welzl
Request for Comments: 8303                            University of Oslo
Category: Informational                                        M. Tuexen
ISSN: 2070-1721                         Muenster Univ. of Appl. Sciences
                                                              N. Khademi
                                                      University of Oslo
                                                           February 2018


                   <span class="h1">On the Usage of Transport Features</span>
                  <span class="h1">Provided by IETF Transport Protocols</span>

Abstract

   This document describes how the transport protocols Transmission
   Control Protocol (TCP), MultiPath TCP (MPTCP), Stream Control
   Transmission Protocol (SCTP), User Datagram Protocol (UDP), and
   Lightweight User Datagram Protocol (UDP-Lite) expose services to
   applications and how an application can configure and use the
   features that make up these services.  It also discusses the service
   provided by the Low Extra Delay Background Transport (LEDBAT)
   congestion control mechanism.  The description results in a set of
   transport abstractions that can be exported in a transport services
   (TAPS) API.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="https://www.rfc-editor.org/info/rfc8303">https://www.rfc-editor.org/info/rfc8303</a>.











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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>) 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

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Terminology .....................................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. Pass 1 ..........................................................<a href="#page-6">6</a>
      <a href="#section-3.1">3.1</a>. Primitives Provided by TCP .................................<a href="#page-6">6</a>
           <a href="#section-3.1.1">3.1.1</a>. Excluded Primitives or Parameters ...................<a href="#page-9">9</a>
      <a href="#section-3.2">3.2</a>. Primitives Provided by MPTCP ..............................<a href="#page-10">10</a>
      <a href="#section-3.3">3.3</a>. Primitives Provided by SCTP ...............................<a href="#page-11">11</a>
           <a href="#section-3.3.1">3.3.1</a>. Excluded Primitives or Parameters ..................<a href="#page-18">18</a>
      <a href="#section-3.4">3.4</a>. Primitives Provided by UDP and UDP-Lite ...................<a href="#page-18">18</a>
      <a href="#section-3.5">3.5</a>. The Service of LEDBAT .....................................<a href="#page-19">19</a>
   <a href="#section-4">4</a>. Pass 2 .........................................................<a href="#page-20">20</a>
      <a href="#section-4.1">4.1</a>. CONNECTION-Related Primitives .............................<a href="#page-21">21</a>
      <a href="#section-4.2">4.2</a>. DATA-Transfer-Related Primitives ..........................<a href="#page-38">38</a>
   <a href="#section-5">5</a>. Pass 3 .........................................................<a href="#page-41">41</a>
      <a href="#section-5.1">5.1</a>. CONNECTION-Related Transport Features .....................<a href="#page-41">41</a>
      <a href="#section-5.2">5.2</a>. DATA-Transfer-Related Transport Features ..................<a href="#page-47">47</a>
           <a href="#section-5.2.1">5.2.1</a>. Sending Data .......................................<a href="#page-47">47</a>
           <a href="#section-5.2.2">5.2.2</a>. Receiving Data .....................................<a href="#page-48">48</a>
           <a href="#section-5.2.3">5.2.3</a>. Errors .............................................<a href="#page-49">49</a>
   <a href="#section-6">6</a>. IANA Considerations ............................................<a href="#page-49">49</a>
   <a href="#section-7">7</a>. Security Considerations ........................................<a href="#page-49">49</a>
   <a href="#section-8">8</a>. References .....................................................<a href="#page-50">50</a>
      <a href="#section-8.1">8.1</a>. Normative References ......................................<a href="#page-50">50</a>
      <a href="#section-8.2">8.2</a>. Informative References ....................................<a href="#page-52">52</a>
   <a href="#appendix-A">Appendix A</a>. Overview of RFCs Used as Input for Pass 1 .............<a href="#page-54">54</a>
   <a href="#appendix-B">Appendix B</a>. How This Document Was Developed .......................<a href="#page-54">54</a>
   Acknowledgements ..................................................<a href="#page-56">56</a>
   Authors' Addresses ................................................<a href="#page-56">56</a>






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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This specification describes how transport protocols offer transport
   services, such that applications using them are no longer directly
   tied to a specific protocol.  Breaking this strict connection can
   reduce the effort for an application programmer, yet attain greater
   transport flexibility by pushing complexity into an underlying
   transport services (TAPS) system.

   This design process has started with a survey of the services
   provided by IETF transport protocols and congestion control
   mechanisms [<a href="./rfc8095" title="&quot;Services Provided by IETF Transport Protocols and Congestion Control Mechanisms&quot;">RFC8095</a>].  The present document and [<a href="./rfc8304" title="&quot;Transport Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP- Lite)&quot;">RFC8304</a>] complement
   this survey with an in-depth look at the defined interactions between
   applications and the following unicast transport protocols:
   Transmission Control Protocol (TCP), MultiPath TCP (MPTCP), Stream
   Control Transmission Protocol (SCTP), User Datagram Protocol (UDP),
   and Lightweight User Datagram Protocol (UDP-Lite).  We also define a
   primitive to enable/disable and configure the Low Extra Delay
   Background Transport (LEDBAT) unicast congestion control mechanism.
   For UDP and UDP-Lite, the first step of the protocol analysis -- a
   discussion of relevant RFC text -- is documented in [<a href="./rfc8304" title="&quot;Transport Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP- Lite)&quot;">RFC8304</a>].

   This snapshot in time of the IETF transport protocols is published as
   an RFC to document the analysis by the authors and the TAPS Working
   Group; this generates a set of transport abstractions that can be
   exported in a TAPS API.  It provides the basis for the minimal set of
   transport services that end systems supporting TAPS should implement
   [<a href="#ref-TAPS-MINSET">TAPS-MINSET</a>].

   The list of primitives, events, and transport features in this
   document is strictly based on the parts of protocol specifications
   that describe what the protocol provides to an application using it
   and how the application interacts with it.  Transport protocols
   provide communication between processes that operate on network
   endpoints, which means that they allow for multiplexing of
   communication between the same IP addresses, and this multiplexing is
   achieved using port numbers.  Port multiplexing is therefore assumed
   to be always provided and not discussed in this document.

   Parts of a protocol that are explicitly stated as optional to
   implement are not covered.  Interactions between the application and
   a transport protocol that are not directly related to the operation
   of the protocol are also not covered.  For example, there are various
   ways for an application to use socket options to indicate its
   interest in receiving certain notifications [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].  However, for
   the purpose of identifying primitives, events, and transport
   features, the ability to enable or disable the reception of
   notifications is irrelevant.  Similarly, "one-to-many style sockets"



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   [<a id="ref-RFC6458">RFC6458</a>] just affect the application programming style, not how the
   underlying protocol operates, and they are therefore not discussed
   here.  The same is true for the ability to obtain the unchanged value
   of a parameter that an application has previously set (e.g., via
   "get" in get/set operations [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>]).

   The document presents a three-pass process to arrive at a list of
   transport features.  In the first pass (pass 1), the relevant RFC
   text is discussed per protocol.  In the second pass (pass 2), this
   discussion is used to derive a list of primitives and events that are
   uniformly categorized across protocols.  Here, an attempt is made to
   present or -- where text describing primitives or events does not yet
   exist -- construct primitives or events in a slightly generalized
   form to highlight similarities.  This is, for example, achieved by
   renaming primitives or events of protocols or by avoiding a strict
   1:1 mapping between the primitives or events in the protocol
   specification and primitives or events in the list.  Finally, the
   third pass (pass 3) presents transport features based on pass 2,
   identifying which protocols implement them.

   In the list resulting from the second pass, some transport features
   are missing because they are implicit in some protocols, and they
   only become explicit when we consider the superset of all transport
   features offered by all protocols.  For example, TCP always carries
   out congestion control; we have to consider it together with a
   protocol like UDP (which does not have congestion control) before we
   can consider congestion control as a transport feature.  The complete
   list of transport features across all protocols is therefore only
   available after pass 3.

   Some protocols are connection oriented.  Connection-oriented
   protocols often use an initial call to a specific primitive to open a
   connection before communication can progress and require
   communication to be explicitly terminated by issuing another call to
   a primitive (usually called 'Close').  A "connection" is the common
   state that some transport primitives refer to, e.g., to adjust
   general configuration settings.  Connection establishment,
   maintenance, and termination are therefore used to categorize
   transport primitives of connection-oriented transport protocols in
   pass 2 and pass 3.  For this purpose, UDP is assumed to be used with
   "connected" sockets, i.e., sockets that are bound to a specific pair
   of addresses and ports [<a href="./rfc8304" title="&quot;Transport Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP- Lite)&quot;">RFC8304</a>].









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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Terminology</span>

   Transport Feature:  a specific end-to-end feature that the transport
      layer provides to an application.  Examples include
      confidentiality, reliable delivery, ordered delivery, message-
      versus-stream orientation, etc.

   Transport Service:  a set of transport features, without an
      association to any given framing protocol, which provides a
      complete service to an application.

   Transport Protocol:  an implementation that provides one or more
      transport services using a specific framing and header format on
      the wire.

   Transport Protocol Component:  an implementation of a transport
      feature within a protocol.

   Transport Service Instance:  an arrangement of transport protocols
      with a selected set of features and configuration parameters that
      implement a single transport service, e.g., a protocol stack (RTP
      over UDP).

   Application:  an entity that uses the transport layer for end-to-end
      delivery of data across the network (this may also be an upper-
      layer protocol or tunnel encapsulation).

   Endpoint:  an entity that communicates with one or more other
      endpoints using a transport protocol.

   Connection:  shared state of two or more endpoints that persists
      across messages that are transmitted between these endpoints.

   Primitive:  a function call that is used to locally communicate
      between an application and a transport endpoint.  A primitive is
      related to one or more transport features.

   Event:  a primitive that is invoked by a transport endpoint.

   Parameter:  a value passed between an application and a transport
      protocol by a primitive.

   Socket:  the combination of a destination IP address and a
      destination port number.

   Transport Address:  the combination of an IP address, transport
      protocol, and the port number used by the transport protocol.




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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Pass 1</span>

   This first iteration summarizes the relevant text parts of the RFCs
   describing the protocols, focusing on what each transport protocol
   provides to the application and how it is used (abstract API
   descriptions, where they are available).  When presenting primitives,
   events, and parameters, the use of lower- and upper-case characters
   is made uniform for the sake of readability.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Primitives Provided by TCP</span>

   The initial TCP specification [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>] states:

      The Transmission Control Protocol (TCP) is intended for use as a
      highly reliable host-to-host protocol between hosts in packet-
      switched computer communication networks, and in interconnected
      systems of such networks.

   <a href="./rfc0793#section-3.8">Section&nbsp;3.8 of [RFC0793]</a> further specifies the interaction with the
   application by listing several transport primitives.  It is also
   assumed that an Operating System provides a means for TCP to
   asynchronously signal the application; the primitives representing
   such signals are called 'events' in this section.  This section
   describes the relevant primitives.

   Open:  This is either active or passive, to initiate a connection or
      listen for incoming connections.  All other primitives are
      associated with a specific connection, which is assumed to first
      have been opened.  An active open call contains a socket.  A
      passive open call with a socket waits for a particular connection;
      alternatively, a passive open call can leave the socket
      unspecified to accept any incoming connection.  A fully specified
      passive call can later be made active by calling 'Send'.
      Optionally, a timeout can be specified, after which TCP will abort
      the connection if data has not been successfully delivered to the
      destination (else a default timeout value is used).  A procedure
      for aborting the connection is used to avoid excessive
      retransmissions, and an application is able to control the
      threshold used to determine the condition for aborting; this
      threshold may be measured in time units or as a count of
      retransmission [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>].  This indicates that the timeout could
      also be specified as a count of retransmission.

      Also optional, for multihomed hosts, the local IP address can be
      provided [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>].  If it is not provided, a default choice will
      be made in case of active open calls.  A passive open call will
      await incoming connection requests to all local addresses and then
      maintain usage of the local IP address where the incoming



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      connection request has arrived.  Finally, the 'options' parameter
      allows the application to specify IP options such as Source Route,
      Record Route, or Timestamp [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>].  It is not stated on which
      segments of a connection these options should be applied, but
      probably on all segments, as this is also stated in a
      specification given for the usage of the Source Route IP option
      (<a href="./rfc1122#section-4.2.3.8">Section&nbsp;4.2.3.8 of [RFC1122]</a>).  Source Route is the only non-
      optional IP option in this parameter, allowing an application to
      specify a source route when it actively opens a TCP connection.

      Master Key Tuples (MKTs) for authentication can optionally be
      configured when calling 'Open' (<a href="./rfc5925#section-7.1">Section&nbsp;7.1 of [RFC5925]</a>).  When
      authentication is in use, complete TCP segments are authenticated,
      including the TCP IPv4 pseudoheader, TCP header, and TCP data.

      TCP Fast Open (TFO) [<a href="./rfc7413" title="&quot;TCP Fast Open&quot;">RFC7413</a>] allows applications to immediately
      hand over a message from the active open to the passive open side
      of a TCP connection together with the first message establishment
      packet (the SYN).  This can be useful for applications that are
      sensitive to TCP's connection setup delay.  [<a href="./rfc7413" title="&quot;TCP Fast Open&quot;">RFC7413</a>] states that
      "TCP implementations MUST NOT use TFO by default, but only use TFO
      if requested explicitly by the application on a per-service-port
      basis."  The size of the message sent with TFO cannot be more than
      TCP's maximum segment size (minus options used in the SYN).  For
      the active open side, it is recommended to change or replace the
      connect() call in order to support a user data buffer argument
      [<a href="./rfc7413" title="&quot;TCP Fast Open&quot;">RFC7413</a>].  For the passive open side, the application needs to
      enable the reception of Fast Open requests, e.g., via a new
      TCP_FASTOPEN setsockopt() socket option before listen().  The
      receiving application must be prepared to accept duplicates of the
      TFO message, as the first data written to a socket can be
      delivered more than once to the application on the remote host.

   Send:  This is the primitive that an application uses to give the
      local TCP transport endpoint a number of bytes that TCP should
      reliably send to the other side of the connection.  The 'urgent'
      flag, if set, states that the data handed over by this send call
      is urgent and this urgency should be indicated to the receiving
      process in case the receiving application has not yet consumed all
      non-urgent data preceding it.  An optional timeout parameter can
      be provided that updates the connection's timeout (see 'Open').
      Additionally, optional parameters allow the ability to indicate
      the preferred outgoing MKT (current_key) and/or the preferred
      incoming MKT (rnext_key) of a connection (<a href="./rfc5925#section-7.1">Section&nbsp;7.1 of
      [RFC5925]</a>).






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   Receive:  This primitive allocates a receiving buffer for a provided
      number of bytes.  It returns the number of received bytes provided
      in the buffer when these bytes have been received and written into
      the buffer by TCP.  The application is informed of urgent data via
      an 'urgent' flag: if it is on, there is urgent data; if it is off,
      there is no urgent data or this call to 'Receive' has returned all
      the urgent data.  The application is also informed about the
      current_key and rnext_key information carried in a recently
      received segment via an optional parameter (<a href="./rfc5925#section-7.1">Section&nbsp;7.1 of
      [RFC5925]</a>).

   Close:  This primitive closes one side of a connection.  It is
      semantically equivalent to "I have no more data to send" but does
      not mean "I will not receive any more", as the other side may
      still have data to send.  This call reliably delivers any data
      that has already been given to TCP (and if that fails, 'Close'
      becomes 'abort').

   Abort:  This primitive causes all pending 'Send' and 'Receive' calls
      to be aborted.  A TCP "RESET" message is sent to the TCP endpoint
      on the other side of the connection [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>].

   Close Event:  TCP uses this primitive to inform an application that
      the application on the other side has called the 'Close'
      primitive, so the local application can also issue a 'Close' and
      terminate the connection gracefully.  See <a href="./rfc0793#section-3.5">[RFC0793], Section&nbsp;3.5</a>.

   Abort Event:  When TCP aborts a connection upon receiving a "RESET"
      from the peer, it "advises the user and goes to the CLOSED state."
      See <a href="./rfc0793#section-3.4">[RFC0793], Section&nbsp;3.4</a>.

   User Timeout Event:  This event is executed when the user timeout
      (<a href="./rfc0793#section-3.9">Section&nbsp;3.9 of [RFC0793]</a>) expires (see the definition of 'Open'
      in this section).  All queues are flushed, and the application is
      informed that the connection had to be aborted due to user
      timeout.

   Error_Report event:  This event informs the application of "soft
      errors" that can be safely ignored [<a href="./rfc5461" title="&quot;TCP's Reaction to Soft Errors&quot;">RFC5461</a>], including the
      arrival of an ICMP error message or excessive retransmissions
      (reaching a threshold below the threshold where the connection is
      aborted).  See <a href="./rfc1122#section-4.2.4.1">Section&nbsp;4.2.4.1 of [RFC1122]</a>.

   Type-of-Service:  <a href="#section-4.2.4.2">Section 4.2.4.2</a> of the requirements for Internet
      hosts [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>] states that "The application layer MUST be able to
      specify the Type-of-Service (TOS) for segments that are sent on a
      connection."  The application should be able to change the TOS
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      to the IP layer unchanged.  Since then, the TOS field has been
      redefined.  The Differentiated Services (Diffserv) model [<a href="./rfc2475" title="&quot;An Architecture for Differentiated Services&quot;">RFC2475</a>]
      [<a href="./rfc3260" title="&quot;New Terminology and Clarifications for Diffserv&quot;">RFC3260</a>] replaces this field in the IP header, assigning the six
      most significant bits to carry the Differentiated Services Code
      Point (DSCP) field [<a href="./rfc2474" title="&quot;Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers&quot;">RFC2474</a>].

   Nagle:  The Nagle algorithm delays sending data for some time to
      increase the likelihood of sending a full-sized segment
      (<a href="./rfc1122#section-4.2.3.4">Section&nbsp;4.2.3.4 of [RFC1122]</a>).  An application can disable the
      Nagle algorithm for an individual connection.

   User Timeout Option:  The User Timeout Option (UTO) [<a href="./rfc5482" title="&quot;TCP User Timeout Option&quot;">RFC5482</a>] allows
      one end of a TCP connection to advertise its current user timeout
      value so that the other end of the TCP connection can adapt its
      own user timeout accordingly.  In addition to the configurable
      value of the user timeout (see 'Send'), there are three per-
      connection state variables that an application can adjust to
      control the operation of the UTO: 'adv_uto' is the value of the
      UTO advertised to the remote TCP peer (default: system-wide
      default user timeout); 'enabled' (default false) is a boolean-type
      flag that controls whether the UTO option is enabled for a
      connection.  This applies to both sending and receiving.
      'changeable' is a boolean-type flag (default true) that controls
      whether the user timeout may be changed based on a UTO option
      received from the other end of the connection. 'changeable'
      becomes false when an application explicitly sets the user timeout
      (see 'Send').

   Set/Get Authentication Parameters:  The preferred outgoing MKT
      (current_key) and/or the preferred incoming MKT (rnext_key) of a
      connection can be configured.  Information about current_key and
      rnext_key carried in a recently received segment can be retrieved
      (<a href="./rfc5925#section-7.1">Section&nbsp;7.1 of [RFC5925]</a>).

<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>.  Excluded Primitives or Parameters</span>

   The 'Open' primitive can be handed optional precedence or security/
   compartment information [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>], but this was not included here
   because it is mostly irrelevant today [<a href="./rfc7414" title="&quot;A Roadmap for Transmission Control Protocol (TCP) Specification Documents&quot;">RFC7414</a>].

   The 'Status' primitive was not included because the initial TCP
   specification describes this primitive as "implementation dependent"
   and states that it "could be excluded without adverse effect"
   [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>].  Moreover, while a data block containing specific
   information is described, it is also stated that not all of this
   information may always be available.  While [<a href="./rfc5925" title="&quot;The TCP Authentication Option&quot;">RFC5925</a>] states that
   'Status' "SHOULD be augmented to allow the MKTs of a current or
   pending connection to be read (for confirmation)", the same



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   information is also available via 'Receive', which, following
   [<a href="./rfc5925" title="&quot;The TCP Authentication Option&quot;">RFC5925</a>], "MUST be augmented" with that functionality.  The 'Send'
   primitive includes an optional 'push' flag which, if set, requires
   data to be promptly transmitted to the receiver without delay
   [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>]; the 'Receive' primitive described in can (under some
   conditions) yield the status of the 'push' flag.  Because "push"
   functionality is optional to implement for both the 'Send' and
   'Receive' primitives [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>], this functionality is not included
   here.  The requirements for Internet hosts [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>] also introduce
   keep-alives to TCP, but these are optional to implement and hence not
   considered here.  The same document also describes that "some TCP
   implementations have included a FLUSH call", indicating that this
   call is also optional to implement; therefore, it is not considered
   here.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Primitives Provided by MPTCP</span>

   MPTCP is an extension to TCP that allows the use of multiple paths
   for a single data stream.  It achieves this by creating different so-
   called TCP subflows for each of the interfaces and scheduling the
   traffic across these TCP subflows.  The service provided by MPTCP is
   described as follows in [<a href="./rfc6182" title="&quot;Architectural Guidelines for Multipath TCP Development&quot;">RFC6182</a>]:

      Multipath TCP MUST follow the same service model as TCP [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>]:
      in-order, reliable, and byte-oriented delivery.  Furthermore, a
      Multipath TCP connection SHOULD provide the application with no
      worse throughput or resilience than it would expect from running a
      single TCP connection over any one of its available paths.

   Further, there are some constraints on the API exposed by MPTCP, as
   stated in [<a href="./rfc6182" title="&quot;Architectural Guidelines for Multipath TCP Development&quot;">RFC6182</a>]:

      A multipath-capable equivalent of TCP MUST retain some level of
      backward compatibility with existing TCP APIs, so that existing
      applications can use the newer transport merely by upgrading the
      operating systems of the end hosts.

   As such, the primitives provided by MPTCP are equivalent to the ones
   provided by TCP.  Nevertheless, the MPTCP RFCs [<a href="./rfc6824" title="&quot;TCP Extensions for Multipath Operation with Multiple Addresses&quot;">RFC6824</a>] and
   [<a href="./rfc6897" title="&quot;Multipath TCP (MPTCP) Application Interface Considerations&quot;">RFC6897</a>] clarify some parts of TCP's primitives with respect to
   MPTCP and add some extensions for better control on MPTCP's subflows.
   Hereafter is a list of the clarifications and extensions the above-
   cited RFCs provide to TCP's primitives.








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   Open:  "An application should be able to request to turn on or turn
      off the usage of MPTCP" [<a href="./rfc6897" title="&quot;Multipath TCP (MPTCP) Application Interface Considerations&quot;">RFC6897</a>].  This functionality can be
      provided through a socket option called 'tcp_multipath_enable'.
      Further, MPTCP must be disabled in case the application is binding
      to a specific address [<a href="./rfc6897" title="&quot;Multipath TCP (MPTCP) Application Interface Considerations&quot;">RFC6897</a>].

   Send/Receive:  The sending and receiving of data does not require any
      changes to the application when MPTCP is being used [<a href="./rfc6824" title="&quot;TCP Extensions for Multipath Operation with Multiple Addresses&quot;">RFC6824</a>].
      The MPTCP-layer will take one input data stream from an
      application, and split it into one or more subflows, with
      sufficient control information to allow it to be reassembled and
      delivered reliably and in order to the recipient application.

      The use of the Urgent Pointer is special in MPTCP [<a href="./rfc6824" title="&quot;TCP Extensions for Multipath Operation with Multiple Addresses&quot;">RFC6824</a>], which
      states: "a TCP subflow MUST NOT use the Urgent Pointer to
      interrupt an existing mapping."

   Address and Subflow Management:  MPTCP uses different addresses and
      allows a host to announce these addresses as part of the protocol.
      The MPTCP API Considerations RFC [<a href="./rfc6897" title="&quot;Multipath TCP (MPTCP) Application Interface Considerations&quot;">RFC6897</a>] says "An application
      should be able to restrict MPTCP to binding to a given set of
      addresses" and thus allows applications to limit the set of
      addresses that are being used by MPTCP.  Further, "An application
      should be able to obtain information on the pairs of addresses
      used by the MPTCP subflows."

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Primitives Provided by SCTP</span>

   TCP has a number of limitations that SCTP removes (<a href="./rfc4960#section-1.1">Section&nbsp;1.1 of
   [RFC4960]</a>).  The following three removed limitations directly
   translate into transport features that are visible to an application
   using SCTP: 1) it allows for preservation of message delimiters; 2)
   it does not provide in-order or reliable delivery unless the
   application wants that; 3) multihoming is supported.  In SCTP,
   connections are called "associations" and they can be between not
   only two (as in TCP) but multiple addresses at each endpoint.

   <a href="#section-10">Section 10</a> of the SCTP base protocol specification [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>]
   specifies the interaction with the application (which SCTP calls the
   "Upper-Layer Protocol (ULP)").  It is assumed that the Operating
   System provides a means for SCTP to asynchronously signal the
   application; the primitives representing such signals are called
   'events' in this section.  Here, we describe the relevant primitives.
   In addition to the abstract API described in <a href="./rfc4960#section-10">Section&nbsp;10 of [RFC4960]</a>,
   an extension to the sockets API is described in [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].  This
   covers the functionality of the base protocol [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>] and some of
   its extensions [<a href="./rfc3758" title="&quot;Stream Control Transmission Protocol (SCTP) Partial Reliability Extension&quot;">RFC3758</a>] [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>] [<a href="./rfc5061" title="&quot;Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration&quot;">RFC5061</a>].  For other protocol
   extensions [<a href="./rfc6525" title="&quot;Stream Control Transmission Protocol (SCTP) Stream Reconfiguration&quot;">RFC6525</a>] [<a href="./rfc6951" title="&quot;UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication&quot;">RFC6951</a>] [<a href="./rfc7053" title="&quot;SACK- IMMEDIATELY Extension for the Stream Control Transmission Protocol&quot;">RFC7053</a>] [<a href="./rfc7496" title="&quot;Additional Policies for the Partially Reliable Stream Control Transmission Protocol Extension&quot;">RFC7496</a>] [<a href="./rfc7829" title="&quot;SCTP-PF: A Quick Failover Algorithm for the Stream Control Transmission Protocol&quot;">RFC7829</a>]



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   [<a href="./rfc8260" title="&quot;Stream Schedulers and User Message Interleaving for the Stream Control Transmission Protocol&quot;">RFC8260</a>], the corresponding extensions of the sockets API are
   specified in these protocol specifications.  The functionality
   exposed to the ULP through all these APIs is considered here.

   The abstract API contains a 'SetProtocolParameters' primitive that
   allows elements of a parameter list [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>] to be adjusted; it is
   stated that SCTP implementations "may allow ULP to customize some of
   these protocol parameters", indicating that none of the elements of
   this parameter list are mandatory to make ULP configurable.  Thus, we
   only consider the parameters in the abstract API that are also
   covered in one of the other RFCs listed above, which leads us to
   exclude the parameters 'RTO.Alpha', 'RTO.Beta', and 'HB.Max.Burst'.
   For clarity, we also replace 'SetProtocolParameters' itself with
   primitives that adjust parameters or groups of parameters that fit
   together.

   Initialize:  Initialize creates a local SCTP instance that it binds
      to a set of local addresses (and, if provided, a local port
      number) [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>].  Initialize needs to be called only once per
      set of local addresses.  A number of per-association
      initialization parameters can be used when an association is
      created, but before it is connected (via the primitive 'Associate'
      below): the maximum number of inbound streams the application is
      prepared to support, the maximum number of attempts to be made
      when sending the INIT (the first message of association
      establishment), and the maximum retransmission timeout (RTO) value
      to use when attempting an INIT [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].  At this point, before
      connecting, an application can also enable UDP encapsulation by
      configuring the remote UDP encapsulation port number [<a href="./rfc6951" title="&quot;UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication&quot;">RFC6951</a>].

   Associate:  This creates an association (the SCTP equivalent of a
      connection) that connects the local SCTP instance and a remote
      SCTP instance.  To identify the remote endpoint, it can be given
      one or multiple (using "connectx") sockets (<a href="./rfc6458#section-9.9">Section&nbsp;9.9 of
      [RFC6458]</a>).  Most primitives are associated with a specific
      association, which is assumed to first have been created.
      Associate can return a list of destination transport addresses so
      that multiple paths can later be used.  One of the returned
      sockets will be selected by the local endpoint as the default
      primary path for sending SCTP packets to this peer, but this
      choice can be changed by the application using the list of
      destination addresses.  Associate is also given the number of
      outgoing streams to request and optionally returns the number of
      negotiated outgoing streams.  An optional parameter of 32 bits,
      the adaptation layer indication, can be provided [<a href="./rfc5061" title="&quot;Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration&quot;">RFC5061</a>].  If
      authenticated chunks are used, the chunk types required to be sent
      authenticated by the peer can be provided [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>].  An
      'SCTP_Cant_Str_Assoc' notification is used to inform the



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      application of a failure to create an association [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].  An
      application could use sendto() or sendmsg() to implicitly set up
      an association, thereby handing over a message that SCTP might
      send during the association setup phase [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].  Note that this
      mechanism is different from TCP's TFO mechanism: the message would
      arrive only once, after at least one RTT, as it is sent together
      with the third message exchanged during association setup, the
      COOKIE-ECHO chunk).

   Send:  This sends a message of a certain length in bytes over an
      association.  A number can be provided to later refer to the
      correct message when reporting an error, and a stream id is
      provided to specify the stream to be used inside an association
      (we consider this as a mandatory parameter here for simplicity: if
      not provided, the stream id defaults to 0).  A condition to
      abandon the message can be specified (for example limiting the
      number of retransmissions or the lifetime of the user message).
      This allows control of the partial reliability extension [<a href="./rfc3758" title="&quot;Stream Control Transmission Protocol (SCTP) Partial Reliability Extension&quot;">RFC3758</a>]
      [<a href="./rfc7496" title="&quot;Additional Policies for the Partially Reliable Stream Control Transmission Protocol Extension&quot;">RFC7496</a>].  An optional maximum lifetime can specify the time
      after which the message should be discarded rather than sent.  A
      choice (advisory, i.e., not guaranteed) of the preferred path can
      be made by providing a socket, and the message can be delivered
      out-of-order if the 'unordered' flag is set.  An advisory flag
      indicates that the peer should not delay the acknowledgement of
      the user message provided [<a href="./rfc7053" title="&quot;SACK- IMMEDIATELY Extension for the Stream Control Transmission Protocol&quot;">RFC7053</a>].  Another advisory flag
      indicates whether the application prefers to avoid bundling user
      data with other outbound DATA chunks (i.e., in the same packet).
      A payload protocol-id can be provided to pass a value that
      indicates the type of payload protocol data to the peer.  If
      authenticated chunks are used, the key identifier for
      authenticating DATA chunks can be provided [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>].

   Receive:  Messages are received from an association, and optionally a
      stream within the association, with their size returned.  The
      application is notified of the availability of data via a 'Data
      Arrive' notification.  If the sender has included a payload
      protocol-id, this value is also returned.  If the received message
      is only a partial delivery of a whole message, a 'partial' flag
      will indicate so, in which case the stream id and a stream
      sequence number are provided to the application.

   Shutdown:  This primitive gracefully closes an association, reliably
      delivering any data that has already been handed over to SCTP.  A
      parameter lets the application control whether further receive or
      send operations or both are disabled when the call is issued.  A
      return code informs about success or failure of this procedure.





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   Abort:  This ungracefully closes an association, by discarding any
      locally queued data and informing the peer that the association
      was aborted.  Optionally, an abort reason to be passed to the peer
      may be provided by the application.  A return code informs about
      success or failure of this procedure.

   Change Heartbeat / Request Heartbeat:  This allows the application to
      enable/disable heartbeats and optionally specify a heartbeat
      frequency as well as requesting a single heartbeat to be carried
      out upon a function call, with a notification about success or
      failure of transmitting the HEARTBEAT chunk to the destination.

   Configure Max. Retransmissions of an Association:  The parameter
      'Association.Max.Retrans' [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>] (called "sasoc_maxrxt" in the
      SCTP sockets API extensions [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>]) allows the configuration of
      the number of unsuccessful retransmissions after which an entire
      association is considered as failed; this should invoke a
      'Communication Lost' notification.

   Set Primary:  This allows the ability to set a new primary default
      path for an association by providing a socket.  Optionally, a
      default source address to be used in IP datagrams can be provided.

   Change Local Address / Set Peer Primary:  This allows an endpoint to
      add/remove local addresses to/from an association.  In addition,
      the peer can be given a hint for which address to use as the
      primary address [<a href="./rfc5061" title="&quot;Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration&quot;">RFC5061</a>].

   Configure Path Switchover:  The abstract API contains a primitive
      called 'Set Failure Threshold' [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>].  This configures the
      parameter 'Path.Max.Retrans', which determines after how many
      retransmissions a particular transport address is considered as
      unreachable.  If there are more transport addresses available in
      an association, reaching this limit will invoke a path switchover.
      An extension called "SCTP-PF" adds a concept of "Potentially
      Failed (PF)" paths to this method [<a href="./rfc7829" title="&quot;SCTP-PF: A Quick Failover Algorithm for the Stream Control Transmission Protocol&quot;">RFC7829</a>].  When a path is in PF
      state, SCTP will not entirely give up sending on that path, but it
      will preferably send data on other active paths if such paths are
      available.  Entering the PF state is done upon exceeding a
      configured maximum number of retransmissions.  Thus, for all paths
      where this mechanism is used, there are two configurable error
      thresholds: one to decide that a path is in PF state, and one to
      decide that the transport address is unreachable.

   Set/Get Authentication Parameters:  This allows an endpoint to add/
      remove key material to/from an association.  In addition, the
      chunk types being authenticated can be queried [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>].




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   Add/Reset Streams, Reset Association:  This allows an endpoint to add
      streams to an existing association or to reset them individually.
      Additionally, the association can be reset [<a href="./rfc6525" title="&quot;Stream Control Transmission Protocol (SCTP) Stream Reconfiguration&quot;">RFC6525</a>].

   Status:  The 'Status' primitive returns a data block with information
      about a specified association, containing: an association
      connection state; a destination transport address list;
      destination transport address reachability states; current local
      and peer receiver window sizes; current local congestion window
      sizes; number of unacknowledged DATA chunks; number of DATA chunks
      pending receipt; a primary path; the most recent Smoothed Round-
      Trip Time (SRTT) on a primary path; RTO on a primary path; SRTT
      and RTO on other destination addresses [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>]; and an MTU per
      path [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Enable/Disable Interleaving:  This allows the negotiation of user
      message interleaving support for future associations to be enabled
      or disabled.  For existing associations, it is possible to query
      whether user message interleaving support was negotiated or not on
      a particular association [<a href="./rfc8260" title="&quot;Stream Schedulers and User Message Interleaving for the Stream Control Transmission Protocol&quot;">RFC8260</a>].

   Set Stream Scheduler:  This allows the ability to select a stream
      scheduler per association, with a choice of: First-Come, First-
      Served; Round-Robin; Round-Robin per Packet; Priority-Based; Fair
      Bandwidth; and Weighted Fair Queuing [<a href="./rfc8260" title="&quot;Stream Schedulers and User Message Interleaving for the Stream Control Transmission Protocol&quot;">RFC8260</a>].

   Configure Stream Scheduler:  This allows the ability to change a
      parameter per stream for the schedulers: a priority value for the
      Priority-Based scheduler and a weight for the Weighted Fair
      Queuing scheduler.

   Enable/Disable NoDelay:  This turns on/off any Nagle-like algorithm
      for an association [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Configure Send Buffer Size:  This controls the amount of data SCTP
      may have waiting in internal buffers to be sent or retransmitted
      [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Configure Receive Buffer Size:  This sets the receive buffer size in
      octets, thereby controlling the receiver window for an association
      [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Configure Message Fragmentation:  If a user message causes an SCTP
      packet to exceed the maximum fragmentation size (which can be
      provided by the application and is otherwise the Path MTU (PMTU)
      size), then the message will be fragmented by SCTP.  Disabling
      message fragmentation will produce an error instead of fragmenting
      the message [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].



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   Configure Path MTU Discovery:  Path MTU Discovery (PMTUD) can be
      enabled or disabled per peer address of an association
      (<a href="./rfc6458#section-8.1.12">Section&nbsp;8.1.12 of [RFC6458]</a>).  When it is enabled, the current
      Path MTU value can be obtained.  When it is disabled, the Path MTU
      to be used can be controlled by the application.

   Configure Delayed SACK Timer:  The time before sending a SACK can be
      adjusted; delaying SACKs can be disabled; and the number of
      packets that must be received before a SACK is sent without
      waiting for the delay timer to expire can be configured [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Set Cookie Life Value:  The cookie life value can be adjusted
      (<a href="./rfc6458#section-8.1.2">Section&nbsp;8.1.2 of [RFC6458]</a>).  'Valid.Cookie.Life' is also one of
      the parameters that is potentially adjustable with
      'SetProtocolParameters' [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>].

   Set Maximum Burst:  The maximum burst of packets that can be emitted
      by a particular association (default 4, and values above 4 are
      optional to implement) can be adjusted (<a href="./rfc6458#section-8.1.2">Section&nbsp;8.1.2 of
      [RFC6458]</a>).  'Max.Burst' is also one of the parameters that is
      potentially adjustable with 'SetProtocolParameters' [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>].

   Configure RTO Calculation:  The abstract API contains the following
      adjustable parameters: 'RTO.Initial'; 'RTO.Min'; 'RTO.Max';
      'RTO.Alpha'; and 'RTO.Beta'.  Only the initial, minimum and
      maximum RTOs are also described as configurable in the SCTP
      sockets API extensions [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Set DSCP Value:  The DSCP value can be set per peer address of an
      association (<a href="./rfc6458#section-8.1.12">Section&nbsp;8.1.12 of [RFC6458]</a>).

   Set IPv6 Flow Label:  The flow label field can be set per peer
      address of an association (<a href="./rfc6458#section-8.1.12">Section&nbsp;8.1.12 of [RFC6458]</a>).

   Set Partial Delivery Point:  This allows the ability to specify the
      size of a message where partial delivery will be invoked.  Setting
      this to a lower value will cause partial deliveries to happen more
      often [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Communication Up Notification:  When a lost communication to an
      endpoint is restored or when SCTP becomes ready to send or receive
      user messages, this notification informs the application process
      about the affected association, the type of event that has
      occurred, the complete set of sockets of the peer, the maximum
      number of allowed streams, and the inbound stream count (the
      number of streams the peer endpoint has requested).  If
      interleaving is supported by both endpoints, this information is
      also included in this notification.



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   Restart Notification:  When SCTP has detected that the peer has
      restarted, this notification is passed to the upper layer
      [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Data Arrive Notification:  When a message is ready to be retrieved
      via the 'Receive' primitive, the application is informed by this
      notification.

   Send Failure Notification / Receive Unsent Message / Receive
      Unacknowledged Message: When a message cannot be delivered via an
      association, the sender can be informed about it and learn whether
      the message has just not been acknowledged or (e.g., in case of
      lifetime expiry) if it has not even been sent.  This can also
      inform the sender that a part of the message has been successfully
      delivered.

   Network Status Change Notification:  This informs the application
      about a socket becoming active/inactive [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>] or "Potentially
      Failed" [<a href="./rfc7829" title="&quot;SCTP-PF: A Quick Failover Algorithm for the Stream Control Transmission Protocol&quot;">RFC7829</a>].

   Communication Lost Notification:  When SCTP loses communication to an
      endpoint (e.g., via heartbeats or excessive retransmission) or
      detects an abort, this notification informs the application
      process of the affected association and the type of event (failure
      OR termination in response to a shutdown or abort request).

   Shutdown Complete Notification:  When SCTP completes the shutdown
      procedures, this notification is passed to the upper layer,
      informing it about the affected association.

   Authentication Notification:  When SCTP wants to notify the upper
      layer regarding the key management related to authenticated chunks
      [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>], this notification is passed to the upper layer.

   Adaptation Layer Indication Notification:  When SCTP completes the
      association setup and the peer provided an adaptation layer
      indication, this is passed to the upper layer [<a href="./rfc5061" title="&quot;Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration&quot;">RFC5061</a>] [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Stream Reset Notification:  When SCTP completes the procedure for
      resetting streams [<a href="./rfc6525" title="&quot;Stream Control Transmission Protocol (SCTP) Stream Reconfiguration&quot;">RFC6525</a>], this notification is passed to the
      upper layer, informing it about the result.

   Association Reset Notification:  When SCTP completes the association
      reset procedure [<a href="./rfc6525" title="&quot;Stream Control Transmission Protocol (SCTP) Stream Reconfiguration&quot;">RFC6525</a>], this notification is passed to the
      upper layer, informing it about the result.






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   Stream Change Notification:  When SCTP completes the procedure used
      to increase the number of streams [<a href="./rfc6525" title="&quot;Stream Control Transmission Protocol (SCTP) Stream Reconfiguration&quot;">RFC6525</a>], this notification is
      passed to the upper layer, informing it about the result.

   Sender Dry Notification:  When SCTP has no more user data to send or
      retransmit on a particular association, this notification is
      passed to the upper layer [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].

   Partial Delivery Aborted Notification:  When a receiver has begun to
      receive parts of a user message but the delivery of this message
      is then aborted, this notification is passed to the upper layer
      (<a href="./rfc6458#section-6.1.7">Section&nbsp;6.1.7 of [RFC6458]</a>).

<span class="h4"><a class="selflink" id="section-3.3.1" href="#section-3.3.1">3.3.1</a>.  Excluded Primitives or Parameters</span>

   The 'Receive' primitive can return certain additional information,
   but this is optional to implement and therefore not considered.  With
   a 'Communication Lost' notification, some more information may
   optionally be passed to the application (e.g., identification to
   retrieve unsent and unacknowledged data).  SCTP "can invoke" a
   'Communication Error' notification and "may send" a 'Restart'
   notification, making these two notifications optional to implement.
   The list provided under 'Status' includes "etc.", indicating that
   more information could be provided.  The primitive 'Get SRTT Report'
   returns information that is included in the information that 'Status'
   provides and is therefore not discussed.  The 'Destroy SCTP Instance'
   API function was excluded: it erases the SCTP instance that was
   created by 'Initialize' but is not a primitive as defined in this
   document because it does not relate to a transport feature.  The
   'Shutdown' event informs an application that the peer has sent a
   SHUTDOWN, and hence no further data should be sent on this socket
   (<a href="./rfc6458#section-6.1">Section&nbsp;6.1 of [RFC6458]</a>).  However, if an application would try to
   send data on the socket, it would get an error message anyway; thus,
   this event is classified as "just affecting the application
   programming style, not how the underlying protocol operates" and is
   not included here.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Primitives Provided by UDP and UDP-Lite</span>

   The set of pass 1 primitives for UDP and UDP-Lite is documented in
   [<a href="./rfc8304" title="&quot;Transport Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP- Lite)&quot;">RFC8304</a>].










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<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  The Service of LEDBAT</span>

   The service of the LEDBAT congestion control mechanism is described
   as follows:

      LEDBAT is designed for use by background bulk-transfer
      applications to be no more aggressive than standard TCP congestion
      control (as specified in <a href="./rfc5681">RFC 5681</a>) and to yield in the presence of
      competing flows, thus limiting interference with the network
      performance of competing flows [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>].

   LEDBAT does not have any primitives, as LEDBAT is not a transport
   protocol.  According to its specification [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>]:

      LEDBAT can be used as part of a transport protocol or as part of
      an application, as long as the data transmission mechanisms are
      capable of carrying timestamps and acknowledging data frequently.
      LEDBAT can be used with TCP, Stream Control Transmission Protocol
      (SCTP), and Datagram Congestion Control Protocol (DCCP), with
      appropriate extensions where necessary; and it can be used with
      proprietary application protocols, such as those built on top of
      UDP for peer-to-peer (P2P) applications.

   At the time of writing, the appropriate extensions for TCP, SCTP, or
   DCCP do not exist.

   A number of configurable parameters exist in the LEDBAT
   specification: TARGET, which is the queuing delay target at which
   LEDBAT tries to operate, must be set to 100 ms or less.
   'allowed_increase' (should be 1, must be greater than 0) limits the
   speed at which LEDBAT increases its rate. 'gain', which according to
   [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>] "MUST be set to 1 or less" to avoid a faster ramp-up than
   TCP Reno, determines how quickly the sender responds to changes in
   queueing delay.  Implementations may divide 'gain' into two
   parameters: one for increase and a possibly larger one for decrease.
   We call these parameters 'Gain_Inc' and 'Gain_Dec' here.
   'Base_History' is the size of the list of measured base delays, and,
   according to [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>], "SHOULD be 10".  This list can be filtered
   using a 'Filter' function, which is not prescribed [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>], that
   yields a list of size 'Current_Filter'.  The initial and minimum
   congestion windows, 'Init_CWND' and 'Min_CWND', should both be 2.

   Regarding which of these parameters should be under control of an
   application, the possible range goes from exposing nothing on the one
   hand to considering everything that is not prescribed with a "MUST"
   in the specification as a parameter on the other hand.  Function
   implementations are not provided as a parameter to any of the
   transport protocols discussed here; hence, we do not regard the



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   'Filter' function as a parameter.  However, to avoid unnecessarily
   limiting future implementations, we consider all other parameters
   above as tunable parameters that should be exposed.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Pass 2</span>

   This pass categorizes the primitives from pass 1 based on whether
   they relate to a connection or to data transmission.  Primitives are
   presented following the nomenclature
   "CATEGORY.[SUBCATEGORY].PRIMITIVENAME.PROTOCOL".  The CATEGORY can be
   CONNECTION or DATA.  Within the CONNECTION category, ESTABLISHMENT,
   AVAILABILITY, MAINTENANCE, and TERMINATION subcategories can be
   considered.  The DATA category does not have any SUBCATEGORY.  The
   PROTOCOL name "UDP(-Lite)" is used when primitives are equivalent for
   UDP and UDP-Lite; the PROTOCOL name "TCP" refers to both TCP and
   MPTCP.  We present "connection" as a general protocol-independent
   concept and use it to refer to, e.g., TCP connections (identifiable
   by a unique pair of IP addresses and TCP port numbers), SCTP
   associations (identifiable by multiple IP address and port number
   pairs), as well UDP and UDP-Lite connections (identifiable by a
   unique socket pair).

   Some minor details are omitted for the sake of generalization --
   e.g., SCTP's 'Close' [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>] returns success or failure and lets
   the application control whether further receive or send operations,
   or both, are disabled [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>].  This is not described in the same
   way for TCP [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>], but these details play no significant role for
   the primitives provided by either TCP or SCTP (for the sake of being
   generic, it could be assumed that both receive and send operations
   are disabled in both cases).

   The TCP 'Send' and 'Receive' primitives include usage of an 'urgent'
   parameter.  This parameter controls a mechanism that is required to
   implement the "synch signal" used by telnet [<a href="./rfc0854" title="&quot;Telnet Protocol Specification&quot;">RFC0854</a>], but [<a href="./rfc6093" title="&quot;On the Implementation of the TCP Urgent Mechanism&quot;">RFC6093</a>]
   states that "new applications SHOULD NOT employ the TCP urgent
   mechanism."  Because pass 2 is meant as a basis for the creation of
   future systems, the "urgent" mechanism is excluded.  This also
   concerns the notification 'Urgent Pointer Advance' in the
   'Error_Report' (<a href="./rfc1122#section-4.2.4.1">Section&nbsp;4.2.4.1 of [RFC1122]</a>).

   Since LEDBAT is a congestion control mechanism and not a protocol, it
   is not currently defined when to enable/disable or configure the
   mechanism.  For instance, it could be a one-time choice upon
   connection establishment or when listening for incoming connections,
   in which case it should be categorized under CONNECTION.ESTABLISHMENT
   or CONNECTION.AVAILABILITY, respectively.  To avoid unnecessarily





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   limiting future implementations, it was decided to place it under
   CONNECTION.MAINTENANCE, with all parameters that are described in the
   specification [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>] made configurable.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  CONNECTION-Related Primitives</span>

   ESTABLISHMENT:

   Active creation of a connection from one transport endpoint to one or
   more transport endpoints.  Interfaces to UDP and UDP-Lite allow both
   connection-oriented and connection-less usage of the API [<a href="./rfc8085" title="&quot;UDP Usage Guidelines&quot;">RFC8085</a>].

   o  CONNECT.TCP:

      Pass 1 primitive/event: 'Open' (active) or 'Open' (passive) with
      socket, followed by 'Send'

      Parameters: 1 local IP address (optional); 1 destination transport
      address (for active open; else the socket and the local IP address
      of the succeeding incoming connection request will be maintained);
      timeout (optional); options (optional); MKT configuration
      (optional); and user message (optional)

      Comments: if the local IP address is not provided, a default
      choice will automatically be made.  The timeout can also be a
      retransmission count.  The options are IP options to be used on
      all segments of the connection.  At least the Source Route option
      is mandatory for TCP to provide.  'MKT configuration' refers to
      the ability to configure MKTs for authentication.  The user
      message may be transmitted to the peer application immediately
      upon reception of the TCP SYN packet.  To benefit from the lower
      latency this provides as part of the experimental TFO mechanism,
      its length must be at most the TCP's maximum segment size (minus
      TCP options used in the SYN).  The message may also be delivered
      more than once to the application on the remote host.

   o  CONNECT.SCTP:

      Pass 1 primitive/event: 'Initialize', followed by 'Enable/Disable
      Interleaving' (optional), followed by 'Associate'

      Parameters: list of local SCTP port number / IP address pairs
      ('Initialize'); one or several sockets (identifying the peer);
      outbound stream count; maximum allowed inbound stream count;
      adaptation layer indication (optional); chunk types required to be
      authenticated (optional); request interleaving on/off; maximum





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      number of INIT attempts (optional); maximum init.  RTO for INIT
      (optional); user message (optional); and remote UDP port number
      (optional)

      Returns: socket list or failure

      Comments: 'Initialize' needs to be called only once per list of
      local SCTP port number / IP address pairs.  One socket will
      automatically be chosen; it can later be changed in MAINTENANCE.
      The user message may be transmitted to the peer application
      immediately upon reception of the packet containing the
      COOKIE-ECHO chunk.  To benefit from the lower latency this
      provides, its length must be limited such that it fits into the
      packet containing the COOKIE-ECHO chunk.  If a remote UDP port
      number is provided, SCTP packets will be encapsulated in UDP.

   o  CONNECT.MPTCP:

      This is similar to CONNECT.TCP except for one additional boolean
      parameter that allows the ability to enable or disable MPTCP for a
      particular connection or socket (default: enabled).

   o  CONNECT.UDP(-Lite):

      Pass 1 primitive/event: 'Connect' followed by 'Send'

      Parameters: 1 local IP address (default (ANY) or specified); 1
      destination transport address; 1 local port (default (OS chooses)
      or specified); and 1 destination port (default (OS chooses) or
      specified).

      Comments: associates a transport address creating a UDP(-Lite)
      socket connection.  This can be called again with a new transport
      address to create a new connection.  The CONNECT function allows
      an application to receive errors from messages sent to a transport
      address.

   AVAILABILITY:

   Preparing to receive incoming connection requests.

   o  LISTEN.TCP:

      Pass 1 primitive/event: 'Open' (passive)

      Parameters: 1 local IP address (optional); 1 socket (optional);
      timeout (optional); buffer to receive a user message (optional);
      and MKT configuration (optional)



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      Comments: if the socket and/or local IP address is provided, this
      waits for incoming connections from only and/or to only the
      provided address.  Else this waits for incoming connections
      without this/these constraint(s).  ESTABLISHMENT can later be
      performed with 'Send'.  If a buffer is provided to receive a user
      message, a user message can be received from a TFO-enabled sender
      before the TCP's connection handshake is completed.  This message
      may arrive multiple times.  'MKT configuration' refers to the
      ability to configure MKTs for authentication.

   o  LISTEN.SCTP:

      Pass 1 primitive/event: 'Initialize', followed by the
      'Communication Up' or 'Restart' notification and possibly the
      'Adaptation Layer' notification

      Parameters: list of local SCTP port number / IP address pairs
      (initialize)

      Returns: socket list; outbound stream count; inbound stream count;
      adaptation layer indication; chunks required to be authenticated;
      and interleaving supported on both sides yes/no

      Comments: 'Initialize' needs to be called only once per list of
      local SCTP port number / IP address pairs.  'Communication Up' can
      also follow a 'Communication Lost' notification, indicating that
      the lost communication is restored.  If the peer has provided an
      adaptation layer indication, an 'Adaptation Layer' notification is
      issued.

   o  LISTEN.MPTCP:

      This is similar to LISTEN.TCP except for one additional boolean
      parameter that allows the ability to enable or disable MPTCP for a
      particular connection or socket (default: enabled).

   o  LISTEN.UDP(-Lite):

      Pass 1 primitive/event: 'Receive'

      Parameters: 1 local IP address (default (ANY) or specified); 1
      destination transport address; local port (default (OS chooses) or
      specified); and destination port (default (OS chooses) or
      specified)

      Comments: the 'Receive' function registers the application to
      listen for incoming UDP(-Lite) datagrams at an endpoint.




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   MAINTENANCE:

   Adjustments made to an open connection, or notifications about it.
   These are out-of-band messages to the protocol that can be issued at
   any time, at least after a connection has been established and before
   it has been terminated (with one exception: CHANGE_TIMEOUT.TCP can
   only be issued for an open connection when DATA.SEND.TCP is called).
   In some cases, these primitives can also be immediately issued during
   ESTABLISHMENT or AVAILABILITY, without waiting for the connection to
   be opened (e.g., CHANGE_TIMEOUT.TCP can be done using TCP's 'Open'
   primitive).  For UDP and UDP-Lite, these functions may establish a
   setting per connection but may also be changed per datagram message.

   o  CHANGE_TIMEOUT.TCP:

      Pass 1 primitive/event: 'Open' or 'Send' combined with unspecified
      control of per-connection state variables

      Parameters: timeout value (optional); adv_uto (optional); boolean
      uto_enabled (optional, default false); and boolean changeable
      (optional, default true)

      Comments: when sending data, an application can adjust the
      connection's timeout value (the time after which the connection
      will be aborted if data could not be delivered).  If 'uto_enabled'
      is true, the 'timeout value' (or, if provided, the value
      'adv_uto') will be advertised for the TCP on the other side of the
      connection to adapt its own user timeout accordingly.
      'uto_enabled' controls whether the UTO option is enabled for a
      connection.  This applies to both sending and receiving.
      'changeable' controls whether the user timeout may be changed
      based on a UTO option received from the other end of the
      connection; it becomes false when the 'timeout value' is used.

   o  CHANGE_TIMEOUT.SCTP:

      Pass 1 primitive/event: 'Change Heartbeat' combined with
      'Configure Max. Retransmissions of an Association'

      Parameters: 'Change Heartbeat': heartbeat frequency and 'Configure
      Max. Retransmissions of an Association': Association.Max.Retrans

      Comments: 'Change Heartbeat' can enable/disable heartbeats in SCTP
      as well as change their frequency.  The parameter
      'Association.Max.Retrans' defines after how many unsuccessful
      transmissions of any packets (including heartbeats) the





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      association will be terminated; thus, these two primitives/
      parameters together can yield a similar behavior for SCTP
      associations as CHANGE_TIMEOUT.TCP does for TCP connections.

   o  DISABLE_NAGLE.TCP:

      Pass 1 primitive/event: not specified

      Parameters: one boolean value

      Comments: the Nagle algorithm delays data transmission to increase
      the chance of sending a full-sized segment.  An application must
      be able to disable this algorithm for a connection.

   o  DISABLE_NAGLE.SCTP:

      Pass 1 primitive/event: 'Enable/Disable NoDelay'

      Parameters: one boolean value

      Comments: Nagle-like algorithms delay data transmission to
      increase the chance of sending a full-sized packet.

   o  REQUEST_HEARTBEAT.SCTP:

      Pass 1 primitive/event: 'Request Heartbeat'

      Parameters: socket

      Returns: success or failure

      Comments: requests an immediate heartbeat on a path, returning
      success or failure.

   o  ADD_PATH.MPTCP:

      Pass 1 primitive/event: not specified

      Parameters: local IP address and optionally the local port number

      Comments: the application specifies the local IP address and port
      number that must be used for a new subflow.









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   o  ADD_PATH.SCTP:

      Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'

      Parameters: local IP address

   o  REM_PATH.MPTCP:

      Pass 1 primitive/event: not specified

      Parameters: local IP address; local port number; remote IP
      address; and remote port number

      Comments: the application removes the subflow specified by the IP/
      port-pair.  The MPTCP implementation must trigger a removal of the
      subflow that belongs to this IP/port-pair.

   o  REM_PATH.SCTP:

      Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'

      Parameters: local IP address

   o  SET_PRIMARY.SCTP:

      Pass 1 primitive/event: 'Set Primary'

      Parameters: socket

      Returns: result of attempting this operation

      Comments: update the current primary address to be used, based on
      the set of available sockets of the association.

   o  SET_PEER_PRIMARY.SCTP:

      Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'

      Parameters: local IP address

      Comments: this is only advisory for the peer.










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   o  CONFIG_SWITCHOVER.SCTP:

      Pass 1 primitive/event: 'Configure Path Switchover'

      Parameters: primary max retrans (number of retransmissions after
      which a path is considered inactive) and PF max retrans (number of
      retransmissions after which a path is considered to be
      "Potentially Failed", and others will be preferably used)
      (optional)

   o  STATUS.SCTP:

      Pass 1 primitive/event: 'Status', 'Enable/Disable Interleaving',
      and 'Network Status Change' notification

      Returns: data block with information about a specified
      association, containing: association connection state; destination
      transport address list; destination transport address reachability
      states; current local and peer receiver window sizes; current
      local congestion window sizes; number of unacknowledged DATA
      chunks; number of DATA chunks pending receipt; primary path; most
      recent SRTT on primary path; RTO on primary path; SRTT and RTO on
      other destination addresses; MTU per path; and interleaving
      supported yes/no

      Comments: the 'Network Status Change' notification informs the
      application about a socket becoming active/inactive; this only
      affects the programming style, as the same information is also
      available via 'Status'.

   o  STATUS.MPTCP:

      Pass 1 primitive/event: not specified

      Returns: list of pairs of tuples of IP address and TCP port number
      of each subflow.  The first of the pair is the local IP and port
      number, while the second is the remote IP and port number.

   o  SET_DSCP.TCP:

      Pass 1 primitive/event: not specified

      Parameters: DSCP value

      Comments: this allows an application to change the DSCP value for
      outgoing segments.





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   o  SET_DSCP.SCTP:

      Pass 1 primitive/event: 'Set DSCP value'

      Parameters: DSCP value

      Comments: this allows an application to change the DSCP value for
      outgoing packets on a path.

   o  SET_DSCP.UDP(-Lite):

      Pass 1 primitive/event: 'Set_DSCP'

      Parameter: DSCP value

      Comments: this allows an application to change the DSCP value for
      outgoing UDP(-Lite) datagrams.  [<a href="./rfc7657" title="&quot;Differentiated Services (Diffserv) and Real-Time Communication&quot;">RFC7657</a>] and [<a href="./rfc8085" title="&quot;UDP Usage Guidelines&quot;">RFC8085</a>] provide
      current guidance on using this value with UDP.

   o  ERROR.TCP:

      Pass 1 primitive/event: 'Error_Report'

      Returns: reason (encoding not specified) and subreason (encoding
      not specified)

      Comments: soft errors that can be ignored without harm by many
      applications; an application should be able to disable these
      notifications.  The reported conditions include at least: ICMP
      error message arrived and excessive retransmissions.

   o  ERROR.UDP(-Lite):

      Pass 1 primitive/event: 'Error_Report'

      Returns: Error report

      Comments: this returns soft errors that may be ignored without
      harm by many applications; an application must connect to be able
      receive these notifications.











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   o  SET_AUTH.TCP:

      Pass 1 primitive/event: not specified

      Parameters: current_key and rnext_key

      Comments: current_key and rnext_key are the preferred outgoing MKT
      and the preferred incoming MKT, respectively, for a segment that
      is sent on the connection.

   o  SET_AUTH.SCTP:

      Pass 1 primitive/event: 'Set/Get Authentication Parameters'

      Parameters: key_id; key; and hmac_id

   o  GET_AUTH.TCP:

      Pass 1 primitive/event: not specified

      Parameters: current_key and rnext_key

      Comments: current_key and rnext_key are the preferred outgoing MKT
      and the preferred incoming MKT, respectively, that were carried on
      a recently received segment.

   o  GET_AUTH.SCTP:

      Pass 1 primitive/event: 'Set/Get Authentication Parameters'

      Parameters: key_id and chunk_list

   o  RESET_STREAM.SCTP:

      Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'

      Parameters: sid and direction

   o  RESET_STREAM-EVENT.SCTP:

      Pass 1 primitive/event: 'Stream Reset' notification

      Parameters: information about the result of RESET_STREAM.SCTP

      Comments: this is issued when the procedure for resetting streams
      has completed.





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   o  RESET_ASSOC.SCTP:

      Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'

      Parameters: information related to the extension, as defined in
      [<a href="./rfc3260" title="&quot;New Terminology and Clarifications for Diffserv&quot;">RFC3260</a>]

   o  RESET_ASSOC-EVENT.SCTP:

      Pass 1 primitive/event: 'Association Reset' notification

      Parameters: information about the result of RESET_ASSOC.SCTP

      Comments: this is issued when the procedure for resetting an
      association has completed.

   o  ADD_STREAM.SCTP:

      Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'

      Parameters: number of outgoing and incoming streams to be added

   o  ADD_STREAM-EVENT.SCTP:

      Pass 1 primitive/event: 'Stream Change' notification

      Parameters: information about the result of ADD_STREAM.SCTP

      Comments: this is issued when the procedure for adding a stream
      has completed.

   o  SET_STREAM_SCHEDULER.SCTP:

      Pass 1 primitive/event: 'Set Stream Scheduler'

      Parameters: scheduler identifier

      Comments: choice of First-Come, First-Served; Round-Robin; Round-
      Robin per Packet; Priority-Based; Fair Bandwidth; and Weighted
      Fair Queuing.











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   o  CONFIGURE_STREAM_SCHEDULER.SCTP:

      Pass 1 primitive/event: 'Configure Stream Scheduler'

      Parameters: priority

      Comments: the priority value only applies when Priority-Based or
      Weighted Fair Queuing scheduling is chosen with
      SET_STREAM_SCHEDULER.SCTP.  The meaning of the parameter differs
      between these two schedulers, but in both cases, it realizes some
      form of prioritization regarding how bandwidth is divided among
      streams.

   o  SET_FLOWLABEL.SCTP:

      Pass 1 primitive/event: 'Set IPv6 Flow Label'

      Parameters: flow label

      Comments: this allows an application to change the IPv6 header's
      flow label field for outgoing packets on a path.

   o  AUTHENTICATION_NOTIFICATION-EVENT.SCTP:

      Pass 1 primitive/event: 'Authentication' notification

      Returns: information regarding key management

   o  CONFIG_SEND_BUFFER.SCTP:

      Pass 1 primitive/event: 'Configure Send Buffer Size'

      Parameters: size value in octets

   o  CONFIG_RECEIVE_BUFFER.SCTP:

      Pass 1 primitive/event: 'Configure Receive Buffer Size'

      Parameters: size value in octets

      Comments: this controls the receiver window.










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   o  CONFIG_FRAGMENTATION.SCTP:

      Pass 1 primitive/event: 'Configure Message Fragmentation'

      Parameters: one boolean value (enable/disable) and maximum
      fragmentation size (optional; default: PMTU)

      Comments: if fragmentation is enabled, messages exceeding the
      maximum fragmentation size will be fragmented.  If fragmentation
      is disabled, trying to send a message that exceeds the maximum
      fragmentation size will produce an error.

   o  CONFIG_PMTUD.SCTP:

      Pass 1 primitive/event: 'Configure Path MTU Discovery'

      Parameters: one boolean value (PMTUD on/off) and PMTU value
      (optional)

      Returns: PMTU value

      Comments: this returns a meaningful PMTU value when PMTUD is
      enabled (the boolean is true), and the PMTU value can be set if
      PMTUD is disabled (the boolean is false).

   o  CONFIG_DELAYED_SACK.SCTP:

      Pass 1 primitive/event: 'Configure Delayed SACK Timer'

      Parameters: one boolean value (delayed SACK on/off); timer value
      (optional); and number of packets to wait for (default 2)

      Comments: if delayed SACK is enabled, SCTP will send a SACK either
      upon receiving the provided number of packets or when the timer
      expires, whatever occurs first.

   o  CONFIG_RTO.SCTP:

      Pass 1 primitive/event: 'Configure RTO Calculation'

      Parameters: init (optional); min (optional); and max (optional)

      Comments: this adjusts the initial, minimum, and maximum RTO
      values.







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   o  SET_COOKIE_LIFE.SCTP:

      Pass 1 primitive/event: 'Set Cookie Life Value'

      Parameters: cookie life value

   o  SET_MAX_BURST.SCTP:

      Pass 1 primitive/event: 'Set Maximum Burst'

      Parameters: max burst value

      Comments: not all implementations allow values above the default
      of 4.

   o  SET_PARTIAL_DELIVERY_POINT.SCTP:

      Pass 1 primitive/event: 'Set Partial Delivery Point'

      Parameters: partial delivery point (integer)

      Comments: this parameter must be smaller or equal to the socket
      receive buffer size.

   o  SET_CHECKSUM_ENABLED.UDP:

      Pass 1 primitive/event: 'Checksum_Enabled'

      Parameters: 0 when zero checksum is used at sender, 1 for checksum
      at sender (default)

   o  SET_CHECKSUM_REQUIRED.UDP:

      Pass 1 primitive/event: 'Require_Checksum'

      Parameter: 0 to allow zero checksum, 1 when a non-zero checksum is
      required (default) at the receiver

   o  SET_CHECKSUM_COVERAGE.UDP-Lite:

      Pass 1 primitive/event: 'Set_Checksum_Coverage'

      Parameters: coverage length at sender (default maximum coverage)








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   o  SET_MIN_CHECKSUM_COVERAGE.UDP-Lite:

      Pass 1 primitive/event: 'Set_Min_Coverage'

      Parameter: coverage length at receiver (default minimum coverage)

   o  SET_DF.UDP(-Lite):

      Pass 1 primitive event: 'Set_DF'

      Parameter: 0 when DF is not set (default) in the IPv4 header, 1
      when DF is set

   o  GET_MMS_S.UDP(-Lite):

      Pass 1 primitive event: 'Get_MM_S'

      Comments: this retrieves the maximum transport-message size that
      may be sent using a non-fragmented IP packet from the configured
      interface.

   o  GET_MMS_R.UDP(-Lite):

      Pass 1 primitive event: 'Get_MMS_R'

      Comments: this retrieves the maximum transport-message size that
      may be received from the configured interface.

   o  SET_TTL.UDP(-Lite) (IPV6_UNICAST_HOPS):

      Pass 1 primitive/event: 'Set_TTL' and 'Set_IPV6_Unicast_Hops'

      Parameters: IPv4 TTL value or IPv6 Hop Count value

      Comments: this allows an application to change the IPv4 TTL of
      IPv6 Hop Count value for outgoing UDP(-Lite) datagrams.

   o  GET_TTL.UDP(-Lite) (IPV6_UNICAST_HOPS):

      Pass 1 primitive/event: 'Get_TTL' and 'Get_IPV6_Unicast_Hops'

      Returns: IPv4 TTL value or IPv6 Hop Count value

      Comments: this allows an application to read the IPv4 TTL of the
      IPv6 Hop Count value from a received UDP(-Lite) datagram.






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   o  SET_ECN.UDP(-Lite):

      Pass 1 primitive/event: 'Set_ECN'

      Parameters: ECN value

      Comments: this allows a UDP(-Lite) application to set the Explicit
      Congestion Notification (ECN) code point field for outgoing
      UDP(-Lite) datagrams.  It defaults to sending '00'.

   o  GET_ECN.UDP(-Lite):

      Pass 1 primitive/event: 'Get_ECN'

      Parameters: ECN value

      Comments: this allows a UDP(-Lite) application to read the ECN
      code point field from a received UDP(-Lite) datagram.

   o  SET_IP_OPTIONS.UDP(-Lite):

      Pass 1 primitive/event: 'Set_IP_Options'

      Parameters: options

      Comments: this allows a UDP(-Lite) application to set IP options
      for outgoing UDP(-Lite) datagrams.  These options can at least be
      the Source Route, Record Route, and Timestamp option.

   o  GET_IP_OPTIONS.UDP(-Lite):

      Pass 1 primitive/event: 'Get_IP_Options'

      Returns: options

      Comments: this allows a UDP(-Lite) application to receive any IP
      options that are contained in a received UDP(-Lite) datagram.

   o  CONFIGURE.LEDBAT:

      Pass 1 primitive/event: N/A

      Parameters: enable (boolean); target; allowed_increase; gain_inc;
      gain_dec; base_history; current_filter; init_cwnd; and min_cwnd

      Comments: 'enable' is a newly invented parameter that enables or
      disables the whole LEDBAT service.




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   TERMINATION:

   Gracefully or forcefully closing a connection or being informed about
   this event happening.

   o  CLOSE.TCP:

      Pass 1 primitive/event: 'Close'

      Comments: this terminates the sending side of a connection after
      reliably delivering all remaining data.

   o  CLOSE.SCTP:

      Pass 1 primitive/event: 'Shutdown'

      Comments: this terminates a connection after reliably delivering
      all remaining data.

   o  ABORT.TCP:

      Pass 1 primitive/event: 'Abort'

      Comments: this terminates a connection without delivering
      remaining data and sends an error message to the other side.

   o  ABORT.SCTP:

      Pass 1 primitive/event: 'Abort'

      Parameters: abort reason to be given to the peer (optional)

      Comments: this terminates a connection without delivering
      remaining data and sends an error message to the other side.

   o  ABORT.UDP(-Lite):

      Pass 1 primitive event: 'Close'

      Comments: this terminates a connection without delivering
      remaining data.  No further UDP(-Lite) datagrams are sent/received
      for this transport service instance.









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   o  TIMEOUT.TCP:

      Pass 1 primitive/event: 'User Timeout' event

      Comments: the application is informed that the connection is
      aborted.  This event is executed on expiration of the timeout set
      in CONNECTION.ESTABLISHMENT.CONNECT.TCP (possibly adjusted in
      CONNECTION.MAINTENANCE.CHANGE_TIMEOUT.TCP).

   o  TIMEOUT.SCTP:

      Pass 1 primitive/event: 'Communication Lost' event

      Comments: the application is informed that the connection is
      aborted.  This event is executed on expiration of the timeout that
      should be enabled by default (see the beginning of <a href="./rfc4960#section-8.3">Section&nbsp;8.3 in
      [RFC4960]</a>) and was possibly adjusted in
      CONNECTION.MAINTENANCE.CHANGE_TIMEOOUT.SCTP.

   o  ABORT-EVENT.TCP:

      Pass 1 primitive/event: not specified

   o  ABORT-EVENT.SCTP:

      Pass 1 primitive/event: 'Communication Lost' event

      Returns: abort reason from the peer (if available)

      Comments: the application is informed that the other side has
      aborted the connection using CONNECTION.TERMINATION.ABORT.SCTP.

   o  CLOSE-EVENT.TCP:

      Pass 1 primitive/event: not specified

   o  CLOSE-EVENT.SCTP:

      Pass 1 primitive/event: 'Shutdown Complete' event

      Comments: the application is informed that
      CONNECTION.TERMINATION.CLOSE.SCTP was successfully completed.









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<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  DATA-Transfer-Related Primitives</span>

   All primitives in this section refer to an existing connection, i.e.,
   a connection that was either established or made available for
   receiving data (although this is optional for the primitives of
   UDP(-Lite)).  In addition to the listed parameters, all sending
   primitives contain a reference to a data block, and all receiving
   primitives contain a reference to available buffer space for the
   data.  Note that CONNECT.TCP and LISTEN.TCP in the ESTABLISHMENT and
   AVAILABILITY categories also allow to transfer data (an optional user
   message) before the connection is fully established.

   o  SEND.TCP:

      Pass 1 primitive/event: 'Send'

      Parameters: timeout (optional); current_key (optional); and
      rnext_key (optional)

      Comments: this gives TCP a data block for reliable transmission to
      the TCP on the other side of the connection.  The timeout can be
      configured with this call (see also
      CONNECTION.MAINTENANCE.CHANGE_TIMEOUT.TCP). 'current_key' and
      'rnext_key' are authentication parameters that can be configured
      with this call (see also CONNECTION.MAINTENANCE.SET_AUTH.TCP).

   o  SEND.SCTP:

      Pass 1 primitive/event: 'Send'

      Parameters: stream number; context (optional); socket (optional);
      unordered flag (optional); no-bundle flag (optional); payload
      protocol-id (optional); pr-policy (optional) pr-value (optional);
      sack-immediately flag (optional); and key-id (optional)

      Comments: this gives SCTP a data block for transmission to the
      SCTP on the other side of the connection (SCTP association).  The
      'stream number' denotes the stream to be used.  The 'context'
      number can later be used to refer to the correct message when an
      error is reported.  The 'socket' can be used to state which path
      should be preferred, if there are multiple paths available (see
      also CONNECTION.MAINTENANCE.SETPRIMARY.SCTP).  The data block can
      be delivered out of order if the 'unordered' flag is set.  The
      'no-bundle flag' can be set to indicate a preference to avoid
      bundling.  The 'payload protocol-id' is a number that will, if
      provided, be handed over to the receiving application.  Using
      pr-policy and pr-value, the level of reliability can be
      controlled.  The 'sack-immediately' flag can be used to indicate



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      that the peer should not delay the sending of a SACK corresponding
      to the provided user message.  If specified, the provided key-id
      is used for authenticating the user message.

   o  SEND.UDP(-Lite):

      Pass 1 primitive/event: 'Send'

      Parameters: IP address and port number of the destination endpoint
      (optional if connected)

      Comments: this provides a message for unreliable transmission
      using UDP(-Lite) to the specified transport address.  The IP
      address and port number may be omitted for connected UDP(-Lite)
      sockets.  All CONNECTION.MAINTENANCE.SET_*.UDP(-Lite) primitives
      apply per message sent.

   o  RECEIVE.TCP:

      Pass 1 primitive/event: 'Receive'

      Parameters: current_key (optional) and rnext_key (optional)

      Comments: 'current_key' and 'rnext_key' are authentication
      parameters that can be read with this call (see also
      CONNECTION.MAINTENANCE.GET_AUTH.TCP).

   o  RECEIVE.SCTP:

      Pass 1 primitive/event: 'Data Arrive' notification, followed by
      'Receive'

      Parameters: stream number (optional)

      Returns: stream sequence number (optional) and partial flag
      (optional)

      Comments: if the 'stream number' is provided, the call to receive
      only receives data on one particular stream.  If a partial message
      arrives, this is indicated by the 'partial flag', and then the
      'stream sequence number' must be provided such that an application
      can restore the correct order of data blocks that comprise an
      entire message.








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   o  RECEIVE.UDP(-Lite):

      Pass 1 primitive/event: 'Receive'

      Parameters: buffer for received datagram

      Comments: all CONNECTION.MAINTENANCE.GET_*.UDP(-Lite) primitives
      apply per message received.

   o  SENDFAILURE-EVENT.SCTP:

      Pass 1 primitive/event: 'Send Failure' notification, optionally
      followed by 'Receive Unsent Message' or 'Receive Unacknowledged
      Message'

      Returns: cause code; context; and unsent or unacknowledged message
      (optional)

      Comments: 'cause code' indicates the reason of the failure, and
      'context' is the context number if such a number has been provided
      in DATA.SEND.SCTP, for later use with 'Receive Unsent Message' or
      'Receive Unacknowledged Message', respectively.  These primitives
      can be used to retrieve the unsent or unacknowledged message (or
      part of the message, in case a part was delivered) if desired.

   o  SEND_FAILURE.UDP(-Lite):

      Pass 1 primitive/event: 'Send'

      Comments: this may be used to probe for the effective PMTU when
      using in combination with the 'MAINTENANCE.SET_DF' primitive.

   o  SENDER_DRY-EVENT.SCTP:

      Pass 1 primitive/event: 'Sender Dry' notification

      Comments: this informs the application that the stack has no more
      user data to send.

   o  PARTIAL_DELIVERY_ABORTED-EVENT.SCTP:

      Pass 1 primitive/event: 'Partial Delivery Aborted' notification

      Comments: this informs the receiver of a partial message that the
      further delivery of the message has been aborted.






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<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Pass 3</span>

   This section presents the superset of all transport features in all
   protocols that were discussed in the preceding sections, based on the
   list of primitives in pass 2 but also on text in pass 1 to include
   transport features that can be configured in one protocol and are
   static properties in another (congestion control, for example).
   Again, some minor details are omitted for the sake of generalization
   -- e.g., TCP may provide various different IP options, but only
   source route is mandatory to implement, and this detail is not
   visible in the pass 3 transport feature "Specify IP options".  As
   before, "UDP(-Lite)" represents both UDP and UDP-Lite, and "TCP"
   refers to both TCP and MPTCP.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  CONNECTION-Related Transport Features</span>

   ESTABLISHMENT:
   Active creation of a connection from one transport endpoint to one or
   more transport endpoints.

   o  Connect
      Protocols: TCP, SCTP, and UDP(-Lite)

   o  Specify which IP options must always be used
      Protocols: TCP and UDP(-Lite)

   o  Request multiple streams
      Protocols: SCTP

   o  Limit the number of inbound streams
      Protocols: SCTP

   o  Specify number of attempts and/or timeout for the first
      establishment message
      Protocols: TCP and SCTP

   o  Obtain multiple sockets
      Protocols: SCTP

   o  Disable MPTCP
      Protocols: MPTCP










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   o  Configure authentication
      Protocols: TCP and SCTP
      Comments: with TCP, this allows the configuration of MKTs.  In
      SCTP, this allows the specification of which chunk types must
      always be authenticated.  DATA, ACK, etc., are different 'chunks'
      in SCTP; one or more chunks may be included in a single packet.

   o  Indicate an Adaptation Layer (via an adaptation code point)
      Protocols: SCTP

   o  Request to negotiate interleaving of user messages
      Protocols: SCTP

   o  Hand over a message to reliably transfer (possibly multiple times)
      before connection establishment
      Protocols: TCP

   o  Hand over a message to reliably transfer during connection
      establishment
      Protocols: SCTP

   o  Enable UDP encapsulation with a specified remote UDP port number
      Protocols: SCTP

   AVAILABILITY:

   Preparing to receive incoming connection requests.

   o  Listen, 1 specified local interface
      Protocols: TCP, SCTP, and UDP(-Lite)

   o  Listen, N specified local interfaces
      Protocols: SCTP

   o  Listen, all local interfaces
      Protocols: TCP, SCTP, and UDP(-Lite)

   o  Obtain requested number of streams
      Protocols: SCTP

   o  Limit the number of inbound streams
      Protocols: SCTP

   o  Specify which IP options must always be used
      Protocols: TCP and UDP(-Lite)






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   o  Disable MPTCP
      Protocols: MPTCP

   o  Configure authentication
      Protocols: TCP and SCTP
      Comments: with TCP, this allows the configuration of MKTs.  In
      SCTP, this allows the specification of which chunk types must
      always be authenticated.  DATA, ACK, etc., are different 'chunks'
      in SCTP; one or more chunks may be included in a single packet.

   o  Indicate an Adaptation Layer (via an adaptation code point)
      Protocols: SCTP

   MAINTENANCE:

   Adjustments made to an open connection, or notifications about it.

   o  Change timeout for aborting connection (using retransmit limit or
      time value)
      Protocols: TCP and SCTP

   o  Suggest timeout to the peer
      Protocols: TCP

   o  Disable Nagle algorithm
      Protocols: TCP and SCTP

   o  Request an immediate heartbeat, returning success/failure
      Protocols: SCTP

   o  Notification of excessive retransmissions (early warning below
      abortion threshold)
      Protocols: TCP

   o  Add path
      Protocols: MPTCP and SCTP
      MPTCP Parameters: source-IP; source-Port; destination-IP; and
      destination-Port
      SCTP Parameters: local IP address

   o  Remove path
      Protocols: MPTCP and SCTP
      MPTCP Parameters: source-IP; source-Port; destination-IP; and
      destination-Port
      SCTP Parameters: local IP address






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   o  Set primary path
      Protocols: SCTP

   o  Suggest primary path to the peer
      Protocols: SCTP

   o  Configure Path Switchover
      Protocols: SCTP

   o  Obtain status (query or notification)
      Protocols: SCTP and MPTCP
      SCTP parameters: association connection state; destination
      transport address list; destination transport address reachability
      states; current local and peer receiver window sizes; current
      local congestion window sizes; number of unacknowledged DATA
      chunks; number of DATA chunks pending receipt; primary path; most
      recent SRTT on primary path; RTO on primary path; SRTT and RTO on
      other destination addresses; MTU per path; and interleaving
      supported yes/no
      MPTCP parameters: subflow-list (identified by source-IP;
      source-Port; destination-IP; and destination-Port)

   o  Specify DSCP field
      Protocols: TCP, SCTP, and UDP(-Lite)

   o  Notification of ICMP error message arrival
      Protocols: TCP and UDP(-Lite)

   o  Change authentication parameters
      Protocols: TCP and SCTP

   o  Obtain authentication information
      Protocols: TCP and SCTP

   o  Reset Stream
      Protocols: SCTP

   o  Notification of Stream Reset
      Protocols: STCP

   o  Reset Association
      Protocols: SCTP

   o  Notification of Association Reset
      Protocols: STCP

   o  Add Streams
      Protocols: SCTP



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   o  Notification of Added Stream
      Protocols: STCP

   o  Choose a scheduler to operate between streams of an association
      Protocols: SCTP

   o  Configure priority or weight for a scheduler
      Protocols: SCTP

   o  Specify IPv6 flow label field
      Protocols: SCTP

   o  Configure send buffer size
      Protocols: SCTP

   o  Configure receive buffer (and rwnd) size
      Protocols: SCTP

   o  Configure message fragmentation
      Protocols: SCTP

   o  Configure PMTUD
      Protocols: SCTP

   o  Configure delayed SACK timer
      Protocols: SCTP

   o  Set Cookie life value
      Protocols: SCTP

   o  Set maximum burst
      Protocols: SCTP

   o  Configure size where messages are broken up for partial delivery
      Protocols: SCTP

   o  Disable checksum when sending
      Protocols: UDP

   o  Disable checksum requirement when receiving
      Protocols: UDP

   o  Specify checksum coverage used by the sender
      Protocols: UDP-Lite

   o  Specify minimum checksum coverage required by receiver
      Protocols: UDP-Lite




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   o  Specify DF field
      Protocols: UDP(-Lite)

   o  Get max. transport-message size that may be sent using a non-
      fragmented IP packet from the configured interface
      Protocols: UDP(-Lite)

   o  Get max. transport-message size that may be received from the
      configured interface
      Protocols: UDP(-Lite)

   o  Specify TTL/Hop Count field
      Protocols: UDP(-Lite)

   o  Obtain TTL/Hop Count field
      Protocols: UDP(-Lite)

   o  Specify ECN field
      Protocols: UDP(-Lite)

   o  Obtain ECN field
      Protocols: UDP(-Lite)

   o  Specify IP options
      Protocols: UDP(-Lite)

   o  Obtain IP options
      Protocols: UDP(-Lite)

   o  Enable and configure "Low Extra Delay Background Transfer"
      Protocols: A protocol implementing the LEDBAT congestion control
      mechanism

   TERMINATION:

   Gracefully or forcefully closing a connection, or being informed
   about this event happening.

   o  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side
      Protocols: TCP and SCTP
      Comments: a TCP endpoint locally only closes the connection for
      sending; it may still receive data afterwards.

   o  Abort without delivering remaining data, causing an event that
      informs the application on the other side
      Protocols: TCP and SCTP




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      Comments: in SCTP, a reason can optionally be given by the
      application on the aborting side, which can then be received by
      the application on the other side.

   o  Abort without delivering remaining data, not causing an event that
      informs the application on the other side
      Protocols: UDP(-Lite)

   o  Timeout event when data could not be delivered for too long
      Protocols: TCP and SCTP
      Comments: the timeout is configured with CONNECTION.MAINTENANCE
      "Change timeout for aborting connection (using retransmit limit or
      time value)".

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  DATA-Transfer-Related Transport Features</span>

   All transport features in this section refer to an existing
   connection, i.e., a connection that was either established or made
   available for receiving data.  Note that TCP allows the transfer of
   data (a single optional user message, possibly arriving multiple
   times) before the connection is fully established.  Reliable data
   transfer entails delay -- e.g., for the sender to wait until it can
   transmit data or due to retransmission in case of packet loss.

<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>.  Sending Data</span>

   All transport features in this section are provided by DATA.SEND from
   pass 2.  DATA.SEND is given a data block from the application, which
   here we call a "message" if the beginning and end of the data block
   can be identified at the receiver, and "data" otherwise.

   o  Reliably transfer data, with congestion control
      Protocols: TCP

   o  Reliably transfer a message, with congestion control
      Protocols: SCTP

   o  Unreliably transfer a message, with congestion control
      Protocols: SCTP

   o  Unreliably transfer a message, without congestion control
      Protocols: UDP(-Lite)

   o  Configurable Message Reliability
      Protocols: SCTP






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   o  Choice of stream
      Protocols: SCTP

   o  Choice of path (destination address)
      Protocols: SCTP

   o  Ordered message delivery (potentially slower than unordered)
      Protocols: SCTP

   o  Unordered message delivery (potentially faster than ordered)
      Protocols: SCTP and UDP(-Lite)

   o  Request not to bundle messages
      Protocols: SCTP

   o  Specifying a 'payload protocol-id' (handed over as such by the
      receiver)
      Protocols: SCTP

   o  Specifying a key identifier to be used to authenticate a message
      Protocols: SCTP

   o  Request not to delay the acknowledgement (SACK) of a message
      Protocols: SCTP

<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>.  Receiving Data</span>

   All transport features in this section are provided by DATA.RECEIVE
   from pass 2.  DATA.RECEIVE fills a buffer provided by the
   application, with what here we call a "message" if the beginning and
   end of the data block can be identified at the receiver, and "data"
   otherwise.

   o  Receive data (with no message delimiting)
      Protocols: TCP

   o  Receive a message
      Protocols: SCTP and UDP(-Lite)

   o  Choice of stream to receive from
      Protocols: SCTP

   o  Information about partial message arrival
      Protocols: SCTP
      Comments: in SCTP, partial messages are combined with a stream
      sequence number so that the application can restore the correct
      order of data blocks an entire message consists of.




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<span class="h4"><a class="selflink" id="section-5.2.3" href="#section-5.2.3">5.2.3</a>.  Errors</span>

   This section describes sending failures that are associated with a
   specific call to DATA.SEND from pass 2.

   o  Notification of an unsent (part of a) message
      Protocols: SCTP and UDP(-Lite)

   o  Notification of an unacknowledged (part of a) message
      Protocols: SCTP

   o  Notification that the stack has no more user data to send
      Protocols: SCTP

   o  Notification to a receiver that a partial message delivery has
      been aborted
      Protocols: SCTP

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  IANA Considerations</span>

   This document does not require any IANA actions.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   Authentication, confidentiality protection, and integrity protection
   are identified as transport features [<a href="./rfc8095" title="&quot;Services Provided by IETF Transport Protocols and Congestion Control Mechanisms&quot;">RFC8095</a>].  These transport
   features are generally provided by a protocol or layer on top of the
   transport protocol; none of the transport protocols considered in
   this document provides these transport features on its own.
   Therefore, these transport features are not considered in this
   document, with the exception of native authentication capabilities of
   TCP and SCTP for which the security considerations in [<a href="./rfc5925" title="&quot;The TCP Authentication Option&quot;">RFC5925</a>] and
   [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>] apply.

   Security considerations for the use of UDP and UDP-Lite are provided
   in the referenced RFCs.  Security guidance for application usage is
   provided in the UDP Guidelines [<a href="./rfc8085" title="&quot;UDP Usage Guidelines&quot;">RFC8085</a>].














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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  References</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Normative References</span>

   [<a id="ref-RFC0793">RFC0793</a>]  Postel, J., "Transmission Control Protocol", STD 7,
              <a href="./rfc793">RFC 793</a>, DOI 10.17487/RFC0793, September 1981,
              &lt;<a href="https://www.rfc-editor.org/info/rfc793">https://www.rfc-editor.org/info/rfc793</a>&gt;.

   [<a id="ref-RFC1122">RFC1122</a>]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, <a href="./rfc1122">RFC 1122</a>,
              DOI 10.17487/RFC1122, October 1989,
              &lt;<a href="https://www.rfc-editor.org/info/rfc1122">https://www.rfc-editor.org/info/rfc1122</a>&gt;.

   [<a id="ref-RFC3758">RFC3758</a>]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", <a href="./rfc3758">RFC 3758</a>,
              DOI 10.17487/RFC3758, May 2004,
              &lt;<a href="https://www.rfc-editor.org/info/rfc3758">https://www.rfc-editor.org/info/rfc3758</a>&gt;.

   [<a id="ref-RFC4895">RFC4895</a>]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", <a href="./rfc4895">RFC 4895</a>, DOI 10.17487/RFC4895, August
              2007, &lt;<a href="https://www.rfc-editor.org/info/rfc4895">https://www.rfc-editor.org/info/rfc4895</a>&gt;.

   [<a id="ref-RFC4960">RFC4960</a>]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              <a href="./rfc4960">RFC 4960</a>, DOI 10.17487/RFC4960, September 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4960">https://www.rfc-editor.org/info/rfc4960</a>&gt;.

   [<a id="ref-RFC5061">RFC5061</a>]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", <a href="./rfc5061">RFC 5061</a>,
              DOI 10.17487/RFC5061, September 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5061">https://www.rfc-editor.org/info/rfc5061</a>&gt;.

   [<a id="ref-RFC5482">RFC5482</a>]  Eggert, L. and F. Gont, "TCP User Timeout Option",
              <a href="./rfc5482">RFC 5482</a>, DOI 10.17487/RFC5482, March 2009,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5482">https://www.rfc-editor.org/info/rfc5482</a>&gt;.

   [<a id="ref-RFC5925">RFC5925</a>]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", <a href="./rfc5925">RFC 5925</a>, DOI 10.17487/RFC5925,
              June 2010, &lt;<a href="https://www.rfc-editor.org/info/rfc5925">https://www.rfc-editor.org/info/rfc5925</a>&gt;.

   [<a id="ref-RFC6182">RFC6182</a>]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", <a href="./rfc6182">RFC 6182</a>, DOI 10.17487/RFC6182, March 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6182">https://www.rfc-editor.org/info/rfc6182</a>&gt;.





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   [<a id="ref-RFC6458">RFC6458</a>]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", <a href="./rfc6458">RFC 6458</a>,
              DOI 10.17487/RFC6458, December 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6458">https://www.rfc-editor.org/info/rfc6458</a>&gt;.

   [<a id="ref-RFC6525">RFC6525</a>]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              <a href="./rfc6525">RFC 6525</a>, DOI 10.17487/RFC6525, February 2012,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6525">https://www.rfc-editor.org/info/rfc6525</a>&gt;.

   [<a id="ref-RFC6817">RFC6817</a>]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", <a href="./rfc6817">RFC 6817</a>,
              DOI 10.17487/RFC6817, December 2012,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6817">https://www.rfc-editor.org/info/rfc6817</a>&gt;.

   [<a id="ref-RFC6824">RFC6824</a>]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", <a href="./rfc6824">RFC 6824</a>, DOI 10.17487/RFC6824, January 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6824">https://www.rfc-editor.org/info/rfc6824</a>&gt;.

   [<a id="ref-RFC6897">RFC6897</a>]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", <a href="./rfc6897">RFC 6897</a>, DOI 10.17487/RFC6897,
              March 2013, &lt;<a href="https://www.rfc-editor.org/info/rfc6897">https://www.rfc-editor.org/info/rfc6897</a>&gt;.

   [<a id="ref-RFC6951">RFC6951</a>]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", <a href="./rfc6951">RFC 6951</a>,
              DOI 10.17487/RFC6951, May 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6951">https://www.rfc-editor.org/info/rfc6951</a>&gt;.

   [<a id="ref-RFC7053">RFC7053</a>]  Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
              IMMEDIATELY Extension for the Stream Control Transmission
              Protocol", <a href="./rfc7053">RFC 7053</a>, DOI 10.17487/RFC7053, November 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7053">https://www.rfc-editor.org/info/rfc7053</a>&gt;.

   [<a id="ref-RFC7413">RFC7413</a>]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", <a href="./rfc7413">RFC 7413</a>, DOI 10.17487/RFC7413, December 2014,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7413">https://www.rfc-editor.org/info/rfc7413</a>&gt;.

   [<a id="ref-RFC7496">RFC7496</a>]  Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
              "Additional Policies for the Partially Reliable Stream
              Control Transmission Protocol Extension", <a href="./rfc7496">RFC 7496</a>,
              DOI 10.17487/RFC7496, April 2015,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7496">https://www.rfc-editor.org/info/rfc7496</a>&gt;.






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   [<a id="ref-RFC7829">RFC7829</a>]  Nishida, Y., Natarajan, P., Caro, A., Amer, P., and K.
              Nielsen, "SCTP-PF: A Quick Failover Algorithm for the
              Stream Control Transmission Protocol", <a href="./rfc7829">RFC 7829</a>,
              DOI 10.17487/RFC7829, April 2016,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7829">https://www.rfc-editor.org/info/rfc7829</a>&gt;.

   [<a id="ref-RFC8085">RFC8085</a>]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", <a href="https://www.rfc-editor.org/bcp/bcp145">BCP 145</a>, <a href="./rfc8085">RFC 8085</a>, DOI 10.17487/RFC8085,
              March 2017, &lt;<a href="https://www.rfc-editor.org/info/rfc8085">https://www.rfc-editor.org/info/rfc8085</a>&gt;.

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

   [<a id="ref-RFC8304">RFC8304</a>]  Fairhurst, G. and T. Jones, "Transport Features of the
              User Datagram Protocol (UDP) and Lightweight UDP (UDP-
              Lite)", <a href="./rfc8304">RFC 8304</a>, DOI 10.17487/RFC8304, February 2018,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8304">https://www.rfc-editor.org/info/rfc8304</a>&gt;.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Informative References</span>

   [<a id="ref-RFC0854">RFC0854</a>]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, <a href="./rfc854">RFC 854</a>, DOI 10.17487/RFC0854, May
              1983, &lt;<a href="https://www.rfc-editor.org/info/rfc854">https://www.rfc-editor.org/info/rfc854</a>&gt;.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;.

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

   [<a id="ref-RFC2475">RFC2475</a>]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", <a href="./rfc2475">RFC 2475</a>, DOI 10.17487/RFC2475, December 1998,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2475">https://www.rfc-editor.org/info/rfc2475</a>&gt;.

   [<a id="ref-RFC3260">RFC3260</a>]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", <a href="./rfc3260">RFC 3260</a>, DOI 10.17487/RFC3260, April 2002,
              &lt;<a href="https://www.rfc-editor.org/info/rfc3260">https://www.rfc-editor.org/info/rfc3260</a>&gt;.





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   [<a id="ref-RFC5461">RFC5461</a>]  Gont, F., "TCP's Reaction to Soft Errors", <a href="./rfc5461">RFC 5461</a>,
              DOI 10.17487/RFC5461, February 2009,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5461">https://www.rfc-editor.org/info/rfc5461</a>&gt;.

   [<a id="ref-RFC6093">RFC6093</a>]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", <a href="./rfc6093">RFC 6093</a>, DOI 10.17487/RFC6093,
              January 2011, &lt;<a href="https://www.rfc-editor.org/info/rfc6093">https://www.rfc-editor.org/info/rfc6093</a>&gt;.

   [<a id="ref-RFC7414">RFC7414</a>]  Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
              Zimmermann, "A Roadmap for Transmission Control Protocol
              (TCP) Specification Documents", <a href="./rfc7414">RFC 7414</a>,
              DOI 10.17487/RFC7414, February 2015,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7414">https://www.rfc-editor.org/info/rfc7414</a>&gt;.

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

   [<a id="ref-RFC8095">RFC8095</a>]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", <a href="./rfc8095">RFC 8095</a>,
              DOI 10.17487/RFC8095, March 2017,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8095">https://www.rfc-editor.org/info/rfc8095</a>&gt;.

   [<a id="ref-TAPS-MINSET">TAPS-MINSET</a>]
              Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for TAPS Systems", Work in Progress, <a href="./draft-ietf-taps-minset-01">draft-ietf-</a>
              <a href="./draft-ietf-taps-minset-01">taps-minset-01</a>, February 2018.






















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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Overview of RFCs Used as Input for Pass 1</span>

   TCP:        [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>], [<a href="./rfc1122" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">RFC1122</a>], [<a href="./rfc5482" title="&quot;TCP User Timeout Option&quot;">RFC5482</a>], [<a href="./rfc5925" title="&quot;The TCP Authentication Option&quot;">RFC5925</a>], and
               [<a href="./rfc7413" title="&quot;TCP Fast Open&quot;">RFC7413</a>].

   MPTCP:      [<a href="./rfc6182" title="&quot;Architectural Guidelines for Multipath TCP Development&quot;">RFC6182</a>], [<a href="./rfc6824" title="&quot;TCP Extensions for Multipath Operation with Multiple Addresses&quot;">RFC6824</a>], and [<a href="./rfc6897" title="&quot;Multipath TCP (MPTCP) Application Interface Considerations&quot;">RFC6897</a>].

   SCTP:       RFCs without a sockets API specification:
               [<a href="./rfc3758" title="&quot;Stream Control Transmission Protocol (SCTP) Partial Reliability Extension&quot;">RFC3758</a>], [<a href="./rfc4895" title="&quot;Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)&quot;">RFC4895</a>], [<a href="./rfc4960" title="&quot;Stream Control Transmission Protocol&quot;">RFC4960</a>], and [<a href="./rfc5061" title="&quot;Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration&quot;">RFC5061</a>].

               RFCs that include a sockets API specification:
               [<a href="./rfc6458" title="&quot;Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)&quot;">RFC6458</a>], [<a href="./rfc6525" title="&quot;Stream Control Transmission Protocol (SCTP) Stream Reconfiguration&quot;">RFC6525</a>], [<a href="./rfc6951" title="&quot;UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication&quot;">RFC6951</a>], [<a href="./rfc7053" title="&quot;SACK- IMMEDIATELY Extension for the Stream Control Transmission Protocol&quot;">RFC7053</a>], [<a href="./rfc7496" title="&quot;Additional Policies for the Partially Reliable Stream Control Transmission Protocol Extension&quot;">RFC7496</a>],
               and [<a href="./rfc7829" title="&quot;SCTP-PF: A Quick Failover Algorithm for the Stream Control Transmission Protocol&quot;">RFC7829</a>].

   UDP(-Lite): See [<a href="./rfc8304" title="&quot;Transport Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP- Lite)&quot;">RFC8304</a>].

   LEDBAT:     [<a href="./rfc6817" title="&quot;Low Extra Delay Background Transport (LEDBAT)&quot;">RFC6817</a>].

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  How This Document Was Developed</span>

   This section gives an overview of the method that was used to develop
   this document.  It was given to contributors for guidance, and it can
   be helpful for future updates or extensions.

   This document is only concerned with transport features that are
   explicitly exposed to applications via primitives.  It also strictly
   follows RFC text: if a transport feature is truly relevant for an
   application, the RFCs should say so, and they should describe how to
   use and configure it.  Thus, the approach followed for developing
   this document was to identify the right RFCs, then analyze and
   process their text.

   Primitives that "MAY" be implemented by a transport protocol were
   excluded.  To be included, the minimum requirement level for a
   primitive to be implemented by a protocol was "SHOULD".  Where style
   requirement levels as described in [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>] are not used, primitives
   were excluded when they are described in conjunction with statements
   like, e.g., "some implementations also provide" or "an implementation
   may also".  Excluded primitives or parameters were briefly described
   in a dedicated subsection.

   Pass 1: This began by identifying text that talks about primitives.
   An API specification, abstract or not, obviously describes primitives
   -- but we are not *only* interested in API specifications.  The text
   describing the 'Send' primitive in the API specified in [<a href="./rfc0793" title="&quot;Transmission Control Protocol&quot;">RFC0793</a>],






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   for instance, does not say that data transfer is reliable.  TCP's
   reliability is clear, however, from this text in <a href="./rfc0793#section-1">Section&nbsp;1 of
   [RFC0793]</a>:

      The Transmission Control Protocol (TCP) is intended for use as a
      highly reliable host-to-host protocol between hosts in packet-
      switched computer communication networks, and in interconnected
      systems of such networks.

   Some text for the pass 1 subsections was developed by copying and
   pasting all the relevant text parts from the relevant RFCs then
   adjusting the terminology to match that in <a href="#section-2">Section 2</a> and shortening
   phrasing to match the general style of the document.  An effort was
   made to formulate everything as a primitive description such that the
   primitive descriptions became as complete as possible (e.g., the
   'SEND.TCP' primitive in pass 2 is explicitly described as reliably
   transferring data); text that is relevant for the primitives
   presented in this pass but still does not fit directly under any
   primitive was used in a subsection's introduction.

   Pass 2: The main goal of this pass is unification of primitives.  As
   input, only text from pass 1 was used (no exterior sources).  The
   list in pass 2 is not arranged by protocol (i.e., "first protocol X,
   here are all the primitives; then protocol Y, here are all the
   primitives, ...") but by primitive (i.e., "primitive A, implemented
   this way in protocol X, this way in protocol Y, ...").  It was a goal
   to obtain as many similar pass 2 primitives as possible.  For
   instance, this was sometimes achieved by not always maintaining a 1:1
   mapping between pass 1 and pass 2 primitives, renaming primitives,
   etc.  For every new primitive, the already-existing primitives were
   considered to try to make them as coherent as possible.

   For each primitive, the following style was used:

   o  PRIMITIVENAME.PROTOCOL:
      Pass 1 primitive/event:
      Parameters:
      Returns:
      Comments:

   The entries "Parameters", "Returns", and "Comments" were skipped when
   a primitive had no parameters, no described return value, or no
   comments seemed necessary, respectively.  Optional parameters are
   followed by "(optional)".  When known, default values were provided.

   Pass 3: The main point of this pass is to identify transport features
   that are the result of static properties of protocols, for which all
   protocols have to be listed together; this is then the final list of



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<span class="grey"><a href="./rfc8303">RFC 8303</a>                   Transport Services              February 2018</span>


   all available transport features.  This list was primarily based on
   text from pass 2, with additional input from pass 1 (but no external
   sources).

Acknowledgements

   The authors would like to thank (in alphabetical order) Bob Briscoe,
   Spencer Dawkins, Aaron Falk, David Hayes, Karen Nielsen, Tommy Pauly,
   Joe Touch, and Brian Trammell for providing valuable feedback on this
   document.  We especially thank Christoph Paasch for providing input
   related to Multipath TCP and Gorry Fairhurst and Tom Jones for
   providing input related to UDP(-Lite).  This work has received
   funding from the European Union's Horizon 2020 research and
   innovation programme under grant agreement No. 644334 (NEAT).

Authors' Addresses

   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316
   Norway

   Email: michawe@ifi.uio.no


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   Steinfurt  48565
   Germany

   Email: tuexen@fh-muenster.de


   Naeem Khademi
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316
   Norway

   Email: naeemk@ifi.uio.no









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